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Shanshan Wang
Copper Colloid-Based Catalysts
for Methanol Synthesis
Dissertation zur Erlangung des Grades eines Doktors der Naturwissenschaften
(Dr. rer. nat) der Fakultät für Chemie und Biochemie an der Ruhr-Universität Bochum
Max-Planck-Institut für Kohlenforschung
Mülheim an der Ruhr
CO2 aromatics
CO
CH4
H2
olefin
paraffin
CH3OH
Copper Colloid-Based Catalysts for
Methanol Synthesis
Dissertation
zur Erlangung des Grades
eines Doktors der Naturwissenschaften (Dr. rer. nat.)
der Fakultät für Chemie und Biochemie
an der Ruhr-Universität Bochum
vorgelegt von
Shanshan Wang
aus Beijing
Bochum 2012
Die vorliegende Arbeit wurde in der Zeit von August 2007 bis März 2012
am Max-Planck-Institut für Kohlenforschung in Mülheim an der Ruhr unter
der Leitung von Herrn Prof. Dr. Ferdi Schüth angefertigt.
Referent: Prof. Dr. Ferdi Schüth
Korreferent: Prof. Dr. Martin Muhler
The fear of the LORD is the beginning
of wisdom: and the knowledge of the
holy is understanding.
——Proverbes 9:10
Acknowledgement
VII
Acknowledgement
First of all, I would like to thank my Ph.D. supervisor Prof. Dr. Ferdi Schüth for giving
me such a brilliant opportunity to carry out the research work in his group. His guidance,
his wisdom, as well as his charm of leadership have an important influence on my
personal career development.
I would like to thank Prof. Dr. Martin Muhler for the helpful discussions and suggestions
throughout my study, in particular his agreement on being my examiner.
My gratefulness goes to Prof. Dr. Jan-Dierk Grunwaldt and Dr. Matthias Bauer for the
great cooperation in developing XAS techniques for the investigation of copper colloids.
Very sincere thank is for the Feinmechanik and Druck Technikum. Without their help and
patience, I would have never succeeded in operating the catalytic set-ups. Here are the
names of people I want to address: Dr. Nils Theyssen, Wolfgang Kersten, Knut
Gräfenstein, Dirk Ullner, Sebastian Plankert, Ralf Thomas, Jürgen Majer, Christoph
Maul and Lars Winkel.
Special thank is for Dr. Claudia Weidenthaler, Dr. Wolfgang Schmidt, Prof. Dr.
Christian Lehman and Prof. Dr. Anhui Lu for XRD, N2 sorption and helpful discussion
on analytical techniques, as well as materials preparation during my study. I want to
thank many people from analytical department for their excellent work, including Axel
Dreier (TEM), Bernd Spliethoff (HRTEM), Hans-Joseph Bongard (HRSEM), Silvia
Palm (SEM), Ulrich Häusig (GC installation), Jutta Rosentreter (GC) and Manfred
Scheppat (GC-MS). I also want to thank people in Glassbläserei for making a large
amount of special Schlenk flasks for me.
Many thanks are for those who provided me great help with my research work. They are
Prof. Dr. Oliver Trapp, Dr. Sascha Vukojević, Dr. Christian Baltes, Dr. Jean-Sébastien
Girardon and Dr. Andreas Kempter. I would also like to thank Klaus Schlichte, Manfred
Schwickardi, Udo Richter, Klaus Hauschild and Ursula Wilczok for their help with my
lab work. I am also very grateful for the help from Helga Wasilewski, Angellika Rathofer,
Kirsten Kalische and Krappweis, Annette.
Acknowledgement
VIII
Very special thank is for all the research co-workers in SFB558. The workshops I
participated during the last four years have been very fruitful.
I am especially grateful for all the Chinese colleagues who provide me help with both my
study and life. Thank Dr. Heqing Jiang, Dr. Shutian Zhou, Prof. Dr. Chunjiang Jia and
Dr. Dong Gu for spending massive time going through my thesis and giving me
suggestions. I am equally grateful for Dr. Guanghui Huang, his wife Lili Zheng, Xingyu
Wang, and Dr. Na Ji for their kindness and help with ensuring my health during my
thesis writing. Thank Ivy, Xuxiao, Huiling, Roberto, Flávio and Nuno for their cherish
friendship.
I would like to thank especially all my office mates during different periods, for their
kindness. They are Michael, Yan, Niklas, Lorena, Mario, Kristina. I want to thank
equally all the Schüth group members for the great time spent together, in particular those
previous members. They are Sven, Jens, Joachim, Sabrina, Javier, Liu, Alex, Lily, Piotr,
Joanna, Massimiliano, Uli, Mathias, Bastian, Robert, Guido, and Harun (new again). I
also enjoyed my time very much with the ‘new generation’. They are Kemah, Murhat,
Felix, Tim, Carolina G., Carolina N., Mano, Tobias, Laila, Julia, Valeria and Jean-
Pascal.
Very special thank is for my aunt (Dr. Lijuan Wang), uncle-in-law (Prof. Dr. Wei Shen)
and my cousin Hansen. They have spent so much time polishing my English writing.
Gratefulness also goes to all my brothers and sisters in Chinese Christian Churches in
Aachen and Düsseldorf. I had such wonderful time after work in Germany because of you.
At the end and most importantly, I want to take this chance to thank my parents for their
endless support in my foreign study and career decision making, as well as their
everlasting love which has been encouraging me the whole way of my life. Without you
two, I would not have reached that far. 爸爸妈妈，我爱你们！
Abstract
IX
Abstract
During the Ph.D. project described in this thesis, the Cu colloid-based catalysts were
developed for studying the methanol synthesis reaction in both quasi-homogeneous and
heterogeneous systems. This research should help to address unclear issues concerning
the heterogeneous systems using solid catalysts, concerning the reaction mechanism, the
active sites, the roles of the components, etc.
The Cu colloids were prepared using a Bönnemann route - reductive stabilization. The
Cu(acac)2, as Cu precursor, was simultaneously reduced and stabilized by either
alkylaluminium or alkylzinc in THF solution under Ar protection. The stabilizers applied
could be extended to four different types of alkylaluminium or alkylzinc compounds,
including Al(n-butyl)3, Al(n-octyl)3, Zn(ethyl)2 and Zn(n-butyl)2. The structural
properties of these different Cu colloids were intensively investigated by various
characterization techniques, such as TEM, UV-Vis, XRD and XAS. They all confirmed
that the Cu precursor was well reduced to form Cu nanocrystals and the particle sizes
were in a range of 3-6 nm with a narrow size distribution. However, it was difficult to
further tune the particle size despite of variation of synthesis parameters. The in situ XAS
measurements suggested a possible colloid formation mechanism: Cu(II) was reduced
directly to Cu(0) without going through Cu(I). The Cu colloids were proven to be stable
without agglomeration in storage under Ar protection for a long time.
In order to investigate their catalytic performance in methanol synthesis from synthesis
gas feed, the Cu colloids were tested directly in a quasi-homogeneous phase. They all
exhibited high activity in methanol formation and the methanol productivity reached
values as high as 23.3 molMeOH/(kgCu·h). They were much more active than the
benchmark catalyst -KATALCOJM 51-8 (Johnson Matthey Catalysts, Cu/ZnO/Al2O3)- in
THF suspension tested under the same reaction conditions. Surprisingly, even those Cu
colloids only stabilized by alkylaluminium were highly active without the presence of Zn
species that are usually considered to be necessary in a solid catalyst. Moreover, the use
of Cu colloids favored the methanol synthesis at lower temperature and the methanol
formation already started at 130 °C. An on-line product analysis demonstrated the
formation of methyl formate that is most probably considered as an intermediate rather
Abstract
X
than a by-product. The Cu colloids remained active for up to 40 hours during reaction.
Different techniques were applied to reveal the reason(s) for their activity. It was found
that though the Cu colloids all decomposed, the core of the Cu particles still remained
metallic. In contrast, the metal alkyl shell was oxidized, which formed Cu nanoparticles
supported on ZnO or Al2O3. These components might provide the activity in a THF
suspension for a long time. Furthermore, a series of experiments was designed to explore
the nature of both Cu core and metal alkyl stabilizing shell as well as the structure-
activity relationship of Cu colloids. Among the different systems, Ag colloid, Ni colloid
and non-metal alkyl-stabilized Cu colloids showed no activity in methanol synthesis,
whereas Mg(n-butyl)2-stabilized colloids showed low activity. It was clearly
demonstrated that the activity of the Cu colloids is provided by the synergy of both Cu
core and stabilizing shell, and there were strong interactions of Cu-Al and Cu-Zn,
probably associated with sites on the surface of Cu nanoparticles.
With the aim of maintaining the high activity of the Cu colloids in a heterogeneous
system, supported Cu nanoparticles were prepared by a colloidal deposition method,
using ordered mesoporous materials (SBA-15 and CMK-5) and metal oxides (ZrO2 and
ZnO) as supports. All the supported Cu nanoparticles showed stable activity throughout
the whole gas-phase reaction under similar conditions used in an industrial process. It
was found that the supports did have significant influence on the activity of Cu
nanoparticles, and their interactions with Cu nanoparticles were different. Cu
nanoparticles stabilized on SBA-15 and CMK-5 were much less active, and there was
obvious particle agglomeration. In contrast, some of those stabilized by ZrO2 and ZnO
were nearly as active as the benchmark catalyst and the highest methanol productivity
reached 50.8 molMeOH/(kgCu·h). The high activity might be due to the formation of
Cu/Al2O3/ZrO2(ZnO) systems, similar to the active components in technical catalysts.
The Cu colloid-based catalyst system was established for studying methanol synthesis.
Additional insight in some aspects, including the reaction mechanism and the active sites,
could be obtained. In particular, Cu colloids were proven to be highly active in a quasi-
homogeneous phase at lower temperature. However, the high activity of Cu nanoparticles
could not be well maintained by solid supports in a gas-phase reaction, and their activities
were only at the same level as those of technical catalysts.
Abbreviations
XI
Abbreviations
acac Acetyl Acetonate
AOT bis(2-Ethylhexyl) Sulfosuccinate
ATR Attenuated Total Reflectance
BET Brunauer-Emmett-Teller
BJH Barrett-Joyner-Halenda
CFE Cold Field Emitter
CMK-5 Carbon Mesostructured by KAIST No. 5
CSTR Continuous Stirred-Tank Reactor
DLS Dynamic Light Scattering
DME Dimethyl Ether
DMFC Direct Methanol Fuel Cell
DMT Dimethyl Terephtalate
DRIFTS Diffuse Reflection Infrared Spectroscopy
EDS Energy Dispersive Spectroscopy
EDX Energy Dispersive X-Ray
EELS Electron Energy Loss Spectroscopy
EPC Electronic Pressure Controller
EPR Electron Paramagnetic Resonance Spectroscopy
eV Electron Volt
EXAFS Extended X-ray Absorption Fine Structure
fcc face-centered-cubic
FID Flame Ionization Detector
FT Fischer Tropsch Reaction
FTIR-FPA Fourier-transformed Infrared Focal Plane Array Detector
GC Gas Chromatography
HAD non-Hydroxylamine
HF Hydrofluoric Acid
HPiv Pivalic Acid
HRTEM High Resolution Transmission Electron Microscopy
ICP Inductively Coupled Plasma Analysis
IR Infrared Spectroscopy
LPG Liquefied Petroleum Gas
LSPR Localized Surface Plasmon Resonance
MFC Mass Flow Controller
MMA Methyl Methacrylate
MOCVD Metalorganic Chemical Vapor Deposition
MPS 3-Mercaptopropyltrimethoxysilane
MT Metric Ton
MTA Methanol To Aromatic
MTBE tert-Butyl ether
MTG Methanol To Gasoline
MTO Methanol To Olefins
NEXAFS Near-Edge X-ray Absorption Fine Structure
NMR Nuclear Magnetic Resonance Spectroscopy
Abbreviations
XII
PCA Principal Component Analysis
PFR Plug-Flow Reactor
PPO Poly(2,6-dimethyl-1,4-Phenylene Oxide)
PPO Poly(Propylene Oxide)
PVA Polyvinyl Alcohol
PVP Polyvinylpyrrolidone
QEXAFS Quick Extended X-ray Absorption Fine Structure
RFC Reactive Frontal Chromatography
RWGS Reverse Water-Gas-Shift Reaction
SB N-dodecil-N,N-dimethyl-3-amino-1-propan sulphonate
SBA-15 Santa Barbara No. 15
SEM Scanning Electron Microscopy
SMAD Solvated Metal Atom Dispersed
SMSI Strong Metal-Support Interaction
SSITKA Steady-State Isotopic Transient Kinetic Analysis
STM Scanning Transmission Electron Microscopy
TDS Thermal Desorption Spectroscopy
TEM Transmission Electron Microscopy
THF Tetrahydrofuran
TOF Turnover Frequency
TON Turnover Number
TPD Temperature Programmed Desorption
TTAB Tetradecyltrimethylammonium Bromide
UHV Ultra-High Vacuum
UV-Vis Ultraviolet Visible Spectroscopy
WGS Water-Gas-Shift Reaction
XAFS X-ray Absorption Fine Structure
XANES X-ray Absorption Near Edge Structure
XAS X-ray Absorption Spectroscopy
XPS X-ray photoelectron spectroscopy
XRD X-Ray Diffraction
Content
XIII
Content
1 Introduction ............................................................................................................... 1
1.1 Sociopolitical motivation .................................................................................... 1
1.2 Scientific motivation ........................................................................................... 4
2 State of the art............................................................................................................ 7
2.1 Methanol synthesis .............................................................................................. 7
2.1.1 Introduction ................................................................................................... 7
2.1.2 Industrial methanol production ..................................................................... 8
2.1.3 Copper-based catalysts ................................................................................ 11
2.1.3.1 Introduction .......................................................................................... 11
2.1.3.2 Cu/ZnO and Cu/ZnO/Al2O3 catalyst system ........................................ 11
2.1.3.3 Other Cu/MeOx catalyst systems ......................................................... 15
2.1.4 Reaction mechanisms .................................................................................. 15
2.1.5 Reaction kinetics ......................................................................................... 17
2.1.6 Active sites .................................................................................................. 19
2.2 Metal colloids .................................................................................................... 26
2.2.1 General introduction .................................................................................... 26
2.2.2 Synthesis ...................................................................................................... 26
2.2.3 Stabilization ................................................................................................. 28
2.2.4 Structural property control .......................................................................... 29
2.2.4.1 Particle size control .............................................................................. 29
2.2.4.2 Particle shape control ........................................................................... 30
2.2.5 Applications of metal colloids in catalysis .................................................. 30
2.2.6 Synthesis of supported metal colloids ......................................................... 31
2.2.7 Structure-activity relationship of metal colloid-based catalysts.................. 34
2.2.7.1 Size effect ............................................................................................. 34
2.2.7.2 Shape effect .......................................................................................... 37
2.2.7.3 Support effect ....................................................................................... 38
2.2.8 Copper colloids............................................................................................ 40
2.2.8.1 Different types of copper colloids ........................................................ 41
Content
XIV
2.2.8.2 Copper colloids in methanol synthesis ................................................. 42
3 Results and discussion ............................................................................................. 45
3.1 Metal alkyl-stabilized copper colloids ............................................................... 46
3.1.1 Synthesis ...................................................................................................... 47
3.1.2 Characterization ........................................................................................... 49
3.1.2.1 TEM analysis ........................................................................................ 49
3.1.2.2 UV-Vis measurements ......................................................................... 51
3.1.2.3 XRD measurements .............................................................................. 54
3.1.2.4 XAS analysis ........................................................................................ 56
3.1.3 Copper colloid formation............................................................................. 60
3.1.3.1 In situ XAS measurements at room temperature ................................. 60
3.1.3.2 In situ XAS measurements at low temperature .................................... 62
3.1.4 Copper colloid stability ............................................................................... 66
3.1.5 Summary ...................................................................................................... 67
3.2 Copper colloids in quasi-homogeneous methanol synthesis ............................. 69
3.2.1 Catalytic activity tests .................................................................................. 70
3.2.1.1 Activity ................................................................................................. 70
3.2.1.2 Reaction mechanism and kinetics ........................................................ 75
3.2.2 Change of copper colloid during reaction ................................................... 76
3.2.2.1 TEM analysis ........................................................................................ 77
3.2.2.2 XRD measurements .............................................................................. 81
3.2.2.3 XAS measurement ................................................................................ 83
3.2.2.4 Decomposition of the copper colloids during the reaction .................. 87
3.2.3 Factors affecting the activity of the copper colloids .................................... 91
3.2.3.1 The role of the copper core .................................................................. 91
3.2.3.2 The role of the metal alkyl stabilizing shell ......................................... 93
3.2.3.3 Interaction between core and shell ....................................................... 96
3.2.4 Summary ...................................................................................................... 99
3.3 Supported copper nanoparticles ....................................................................... 101
3.3.1 Synthesis .................................................................................................... 102
3.3.2 Catalytic performance in gas-phase methanol synthesis ........................... 104
Content
XV
3.3.3 Support effect ............................................................................................ 106
3.3.3.1 SBA-15............................................................................................... 107
3.3.3.2 CMK-5 ............................................................................................... 112
3.3.3.3 Metal oxides - ZrO2 and ZnO ............................................................ 117
3.3.4 Mechanism of methanol synthesis over supported Cu nanoparticles........ 120
3.3.5 Summary ................................................................................................... 122
4 Conclusions and outlook ....................................................................................... 125
5 Experimental .......................................................................................................... 129
5.1 Synthesis of metal colloid-base catalysts ........................................................ 129
5.1.1 Metal colloids ............................................................................................ 129
5.1.1.1 Alkylaluminium-stabilized copper colloids ....................................... 129
5.1.1.2 Alkylzinc-stabilized copper colloids .................................................. 130
5.1.1.3 Alkylmagnesium-stabilized copper colloids ...................................... 130
5.1.1.4 Non-metal alkyl-stabilized copper colloids ....................................... 130
5.1.1.5 Alkylalunimium-stabilized silver colloids ......................................... 131
5.1.2 Supported copper nanoparticles ................................................................ 131
5.2 Characterization............................................................................................... 132
5.2.1 TEM and EDX........................................................................................... 132
5.2.2 SEM, HRSEM and EDX ........................................................................... 133
5.2.3 UV-Vis spectroscopy ................................................................................ 133
5.2.4 XRD........................................................................................................... 134
5.2.5 XAS ........................................................................................................... 134
5.2.5.1 Sample preparation ............................................................................ 134
5.2.5.2 XAS measurements of copper colloids .............................................. 135
5.2.5.3 In situ XAS measurement at room temperature ................................. 136
5.2.5.4 In situ XAS measurement at low temperature ................................... 136
5.2.6 Determination of copper, aluminium and zinc concentration ................... 137
5.2.7 Nitrogen sorption ....................................................................................... 137
5.3 Catalytic testing ............................................................................................... 138
5.3.1 Copper colloids in quasi-homogeneous slurry reactor .............................. 138
5.3.2 Supported copper nanoparticles in plug-flow reactor ............................... 140
Content
XVI
6 Bibliography ........................................................................................................... 145
7 Scientific contributions.......................................................................................... 159
7.1 Publications (PhD thesis related) ..................................................................... 159
7.2 Scientific presentations .................................................................................... 159
8 Curriculum Vitae ................................................................................................... 161
Introduction
1
1 Introduction
1.1 Sociopolitical motivation
Methanol is a basic building block for hundreds of essential chemicals that play important
roles in daily life. The largest derivatives of methanol (see Figure 1.1) are formaldehyde,
methyl tert-butyl ether (MTBE), acetic acid, methyl methacrylate (MMA) and dimethyl
terephthalate (DMT), which require approximately 70% of the methanol produced [1]
.
Methanol is also a common laboratory solvent and used in fuel applications. In 2010,
over 45 million metric tons of methanol were consumed around the globe. By 2012,
global demand is expected to reach over 50 million metric tons [2]
.
Figure 1.1 Principle uses of methanol [3]
.
The interest of methanol as element of our energy system is growing. There are two
powerful driving forces that are responsible for the world’s increasing energy
consumption - population and income growth [4]
. The latest world energy report (see
Figure 1.2) stated that the world primary energy consumption grew by 5.6% in 2010, the
strongest growth since 1973 when the oil crisis took place. It is believed that, based on
the current consumption level, the oil in storage is only sufficient for the next few
decades. Natural gas reserves will last longer, while coal is the most abundant resource
and may last for another couple of centuries. However, another problem we must face is
that the energy reserves are not well distributed geographically. The Middle East holds
the largest share of oil and natural gas reserves. Unfortunately unstable geopolitical
Introduction
2
situations in this area make transportation of energy sources unsecure and potentially
dangerous. This could lead to another energy crisis. Although renewables including
biomass, solar energy and wind energy have been intensively investigated and set to
production, they still cannot replace fossil fuels to meet the large energy demand in the
future.
Figure 1.2 World energy consumption (million tons oil equivalent) [5]
.
The diminishing fossil fuel resources and increasing oil price have created an urgent need
to develop new and safe ways to store and produce energy. Methanol, produced from an
array of diverse feedstocks, can be considered as a promising replacement. As the price
of natural gas is lower than that of oil, methanol is mainly produced from synthesis gas,
which is, in turn, generated from natural gas [2]
. Methanol can also be potentially
produced from synthesis gas derived from biomass gasification. Apart from the synthesis
gas feed, methanol from direct methane oxidation appears to be an interesting way, based
on the natural gas feed [6-7]
. Coal, though causing environmental problems, is still used for
methanol synthesis in some countries where coal is abundant. Shenhua Baotou Coal
Chemical Company settled down the world’s largest coal to methanol production plant
using coal derived synthesis gas, where the methanol technology and synthesis catalyst
from Davy Process Technology Ltd. and Johnson Matthey Catalysts are applied [8]
.
Introduction
3
Methanol is an important intermediate for petroleum products and synthetic fuels. The
industrial methanol to hydrocarbons routes (MTH) comprise the process of methanol to
gasoline (MTG), the methanol to olefins (MTO) as well as methanol to propene (MTP)
processes [9-10]
. Among them, the UOP/Hydro MTO process was licensed and has since
2011 been firstly commercialized in China. Methanol is already applied in the biodiesel
production through transesterification reaction [11]
. Furthermore, the use of methanol in
dimethyl ether (DME), as important fuel additive, is increasing due to the fact that China
becomes the main driver of the global market [12-13]
. Apart from petroleum product
generation, the direct methanol fuel cell (DMFC) has attracted much attention, potentially
able to provide power to cellular phones, laptops, cars, buses, etc [14]
.
Methanol itself can be used as motor fuel by being mixed in various ratios with
conventional gasoline and diesel. However, this requires the modification of the existing
car engines due to the lower volumetric energy density of the methanol blended fuel [1]
.
As early as the 1960s, the use of methanol as energy carrier was already advocated by
German researcher F. Asinger, suggesting the synthesis of methanol using the energy of
nuclear high temperature reactors [15-16]
. Then in 2005, Nobel prize winner Geoge A. Olah
and his friend Maximillian Mutzke proposed the ‘methanol economy’ in an essay [17-18]
and they described this concept briefly in a published book [19]
. ‘Methanol economy’ has
been suggested as a future economy, in which methanol could replace fossil fuels as a
means of energy storage, fuel and raw material for other hydrocarbons and energy
production. Methanol is truly a key C1 building block, a bridge connecting starting gas
hydrocarbon (synthesis gas, methane and CO2) and liquid hydrocarbon (aromatics, olefin,
paraffin), as shown in Figure 1.3 [20]
.
CH3OH
Fuel (Gasoline)CO2
Synthesis Gas
Natural GasSynthetic Petroleum Chemicals
(Aromatics, olefin, paraffin)
Figure 1.3 Methanol as key intermediate for chemical and energy production.
Introduction
4
1.2 Scientific motivation
As early as the beginning of the 1920s, the first heterogeneously catalyzed methanol
synthesis process from synthesis gas feed was commercialized by BASF, known as the
‘high-pressure’ process with ZnO/Cr2O3 as catalyst [1, 21]
. Then during the 1960s, ICI
(now Johnson Matthey Catalysts), developed a ‘low-pressure’ process, which was more
economical to the industry, using Cu/ZnO/Al2O3 as catalyst. The following research on
the catalytic methanol synthesis reaction revealed that Cu is the active center; it is
dispersed by ZnO to form the heterogeneous catalysts, while Al2O3 serves as stabilizing
additive [22]
. This ternary Cu/ZnO/Al2O3 system is so far the most active solid catalyst,
which makes the industrial methanol production feasible under mild conditions - lower
pressures (5-10 MPa) and lower temperature (200-300 °C) [23]
.
Due to their high activity and stability, substantial research efforts have been focused on
the methanol synthesis reaction over Cu-based catalysts, mainly using Cu/ZnO as a
model [24]
. This research was based on the fact that the industrial Cu/ZnO/Al2O3 catalysts
are usually prepared via co-precipitation of the corresponding metal salts, and that
metallic Cu nanoparticles formed are dispersed by ZnO [25-26]
. A number of research
groups studied the issues of the nature of the active sites, the reaction mechanism, the
roles of Cu, ZnO and other components in the solid [27-29]
. However, they reached
contradictory conclusions, and the reason could mainly be due to two aspects: first, the
different preparation procedures employed to form the solid heterogeneous catalysts;
second, difficulties encountered in identifying the actual roles of different components in
the catalysts [30]
. Most of the techniques employed for identifying the reaction mechanism
and active sites were not used under realistic conditions due to their incompatibility with
high pressure and/or temperature, so the results acquired are rather technique-dependent.
Therefore, a new break-through is required to study the methanol synthesis reaction, and
a homogeneous system would be expected to overcome some of these difficulties [31]
.
Cu colloids could be applied as potential catalysts in the new homogeneous system based
on several advantages. Generally speaking, a metal colloid system is considered as a
‘micro-heterogeneous reactor’ in the liquid phase, having a large surface area and
sufficient active sites located on the surface of nano-scaled particles [32-33]
. Cu colloids
Introduction
5
resemble well the existing solid Cu catalysts for methanol synthesis, and by designed
synthesis they may contain Cu, Zn, and Al as the same active elements [34-35]
. Those
colloidal Cu nanoparticles are well isolated with well-defined shapes, sizes and
components, in order to provide a clearer insight to the catalytic mechanisms, eliminating
the influence of support. Moreover, Cu colloids, as quasi-molecular catalysts, would
possibly favor the methanol synthesis reaction at lower temperature below the range of
200-300 °C used in industry [23]
. In our previous work, Cu colloids had successfully been
obtained via a chemical reduction, where trialkylaluminium or dibutylzinc served as both
reducing agent and stabilizer [36-37]
. These Cu colloids exhibited high activity in a quasi-
homogeneous methanol synthesis at lower temperatures (140-170 °C). The Cu colloids
stabilized by alkylaluminium were highly active despite of the absence of Zn species,
which are usually considered as a key component. Moreover, some preliminary
mechanistic studies were carried out and methyl formate was proposed as a possible
intermediate [36]
.
The objectives of this thesis are to further develop the Cu colloid-based catalysts and to
evaluate their performances as model catalysts in methanol synthesis from synthesis gas
feed. The main research plan is shown in Figure 1.4, where the whole study is divided
into two main systems. In a quasi-homogeneous system, Cu colloids were directly
applied as model catalysts. In a parallel approach, the Cu colloids were transferred into a
heterogeneous system by supporting them on different solids. For the study of both
systems, the catalysts were first synthesized, characterized and finally tested in methanol
synthesis. Based on their catalytic performances, some of them were selected for more
detailed studies involving also spent catalysts, in order to draw relationships between the
structure and the activity of the catalysts.
As the Cu colloids themselves were initially investigated in this work from a materials
point of view, this part is at first described as a separate section, before going to the parts
on catalysis. The synthesis of Cu colloids was extended to the use of four different
alkylaluminium or alkylzinc stabilizers with the aim of tuning the particle size and to
check the influence of stabilizers on synthesis. The structures of Cu colloids were
identified by various characterization techniques, including TEM, UV-Vis, XRD and
XAS. The formation of Cu colloids was studied in detail by applying in situ XAS
Introduction
6
measurements, which were designed in cooperation with Prof. Dr. Jan-Dierk Grunwaldt
and Dr. Matthias Bauer. The experiments were carried out at room temperature and also
at low temperature (down to -30 °C) in order to explore the change of Cu oxidation state
during the reduction process as well as possibly existing Cu intermediate species. All the
Cu colloids were tested in methanol synthesis in a quasi-homogeneous phase to reveal
their activities via a series of temperature dependent experiments. The changes in the Cu
colloids concerning the particle size, shape and composition were thoroughly investigated
employing the same characterization techniques. Furthermore, the effect of the different
components of the Cu colloids was elucidated by studying the roles of both Cu core and
metal alkyl stabilizing shell. Based on a series of designed experiments on the syntheses
and tests of metal colloids, a final conclusion was drawn on the relation between the
specific structure of Cu colloids and their catalytic activity in methanol synthesis. Finally,
in order to realize the idea of maintaining the activity of Cu colloids in a heterogeneous
system, Cu nanoparticles were deposited onto typical solid supports, using the colloidal
deposition method. This allowed the test of Cu colloids in a gas-phase reaction, and
allowed comparison with the industrial benchmark catalyst. The activities of all the
supported Cu nanoparticles were evaluated in a gas-phase reaction using similar
conditions as in an industrial process. Due to the different activities demonstrated by
those Cu nanoparticles supported on different supports, the support effect was further
evaluated. Based on the performance of the investigated supports, the selection of better
supports could be proposed for future study.
Quasi-Homogeneous Heterogeneous
Characterization
Synthesis
Catalytic test
Copper Colloids Supported Copper
Nanoparticles
TEM
XAS
XRD
DLS
UV-vis
TEM
SEM-EDX
N2 sorption
XRD
Slurry Reaction
in Liquid-Gas Phase
High-Throughput Reactor
& Single-Tube Reactor
in Solid-Gas Phase
Figure 1.4 The research plan for the investigations of Cu colloid-based catalysts in methanol
synthesis.
State of the art
7
2 State of the art
2.1 Methanol synthesis
2.1.1 Introduction
Methanol is one of the most important chemical commodities with several main
applications, i.e. the production of formaldehyde, MTBE, acetic acid, MMA and DMT
and as a solvent. The use of methanol as an energy carrier, such as gasoline blender, is
also increasing [21]
. The latest statistics show that in 2010 global methanol demand was ca.
45.6 million t/a and it is expected to exceed 50 million t/a in 2012. This is largely driven
by increased demand for cleaner energy including the use of methanol as direct
transportation fuel and its conversion to DME as fuel additives [38]
.
Historically, methanol was first obtained in 1661 by Sir R. Boyle through the rectification
of crude wood vinegar over milk of lime. Then the term ‘methyl’ was introduced into
chemistry in 1835, based on the independent work of determining the composition of
methanol which was carried out by J. von Liebig and J. B. A. Dumas. Until 1923, the
only important source of methanol, ‘wood alcohol’, was obtained by the dry distillation
of wood [1]
.
It was French scientist M. Patart who, in 1921, firstly described the heterogeneously
catalyzed production of methanol from synthesis gas [21]
. Soon after, the production of
methanol was advanced by BASF in a large-scale using a sulfur resistant zinc oxide-
chromium oxide catalyst developed by M. Pier and co-workers [1]
. Then the catalytic
synthesis of methanol has been commercially available since 1923, when the first
commercial plant for the synthesis of methanol from synthesis gas was built by BASF
Leuna Works [39]
. This BASF process was performed at high pressure (25-35 MPa) and
temperature (320-450 °C) employing solid ZnO/Cr2O3 catalysts. It was, at that time, the
second large-scale application of catalysis and high-pressure technology to the chemical
industry. Methanol production was usually combined with that of ammonia, due to the
similar technology developed by BASF. This high-pressure process dominated the
industrial production of methanol for nearly half a century. However, during this period,
research work was still continued with catalysts containing different elements. Among
them, Cu was found to be active when added to ZnO. This was also the case with the
State of the art
8
ZnO/Cr2O3, and when CuO was added to it, its activity increased. Then in 1960s, ICI
(now Johnson Matthey Catalysts) developed medium-pressure and low-pressure
processes for methanol synthesis, which was based on the CuO-based catalysts as shown
in Table 2.1. Cu/ZnO/Al2O3 catalysts had much higher activity and enable methanol
synthesis to be carried out under milder conditions at lower temperatures (below 300°C)
and lower pressures (5-10 MPa). The low-pressure process, more efficient and cheaper to
operate than the early high-pressure process, remains the only economical route.
Table 2.1 Methanol production processes [21]
.
Process Conditions Catalyst
High-pressure 25-35 MPa, 320-450 °C ZnO/Cr2O3
Medium pressure 10-25 MPa, 200-300 °C Cu/ZnO/Cr2O3
Low pressure 5-10 MPa, 200-300 °C Cu/ZnO/Al2O3
2.1.2 Industrial methanol production
Industrial methanol production can be subdivided into three main steps: production of
synthesis gas; conversion of synthesis gas into methanol; distillation of crude methanol [1,
23]. Synthesis gas in large-scale production is mainly generated from natural gas through
steam reforming. Additionally, it can be obtained from gasification of coal and biomass,
depending on the abundance of raw materials. The cooled synthesis gas from the
generator needs to go through a gas purification stage in order to remove the sulfur that
poisons the catalysts. A sulfur free synthesis gas mixture containing hydrogen, carbon
monoxide and carbon dioxide is then used for low-pressure methanol production. The
formation and thermodynamics of methanol can be generally described by the following
equilibrium reactions, which are in combination during industrial synthesis [21]
:
CO + 2H2 CH3OH ΔH 298 K, 5 MPa = - 90.7 kJ mol−1
(2.1)
CO2 + 3H2 CH3OH + H2O ΔH 298 K, 5 MPa = - 40.9 kJ mol−1
(2.2)
CO2 + H2 CO + H2O ΔH 298 K, 5 MPa = 49.8 kJ mol−1
(2.3)
State of the art
9
Reaction (2.1) and (2.2) are both exothermic and result in volume decreasing. The
methanol formation is thus favorable when increasing pressure and decreasing
temperature, and the maximum conversion is obtained by low temperature, high pressure
and synthesis gas composition at equilibrium. Apart from to two methanol formation
reactions, reaction (2.3), the reverse water-gas-shift reaction (RWGS), must also be taken
into account, which is endothermic.
Figure 2.1 illustrates a simplified flow diagram for methanol production [1]
. As the high
pressure favors the conversion, a low fraction of the converted synthesis gas in each pass
(typically some 10%) is obtained in reaction at low pressure [23]
. Therefore, a recycle
loop is required for the process. Depending upon the process used, the synthesis gas
mixture may be boosted to the desired pressure (5-10 MPa) with a compressor (f) and
heated. Fresh synthesis gas feed is mixed with unconverted recycled synthesis gas and
sent to the reactor (a). Purge gas is usually introduced in order to have the reaction gas
composition meet a certain stoichiometric ratio, as well as to remove the impurities in the
synthesis gas. In a typical ICI process, for example, the gas composition is CO/CO2/H2 =
10/10/80 [22]
. After the reaction, methanol and water are separated in a separator (d),
while remaining synthesis gas must be recycled and then compressed in compressor (e).
The exothermic methanol synthesis reaction takes place in the reactor at 200-300 °C. The
gas going through the reactor (a) carries the heat released during reaction and then
transfers it to the reactant gas mixture through a heat exchanger (b) prior to the reactor
entrance. The mixture is cooled further by a cooler (c).
Due to its exothermic nature, the heat removal is important in the entire process.
Therefore, special reactor designs are in use in order to control the reaction temperature.
Among many reactors that are available, adiabatic (ICI) and quasi-isothermal (Lurgi) are
the most common ones. The ICI process (quench reactor, tube-cooled reactor and ICI-
steam-raising reactor) and the Lurgi process account for 60% and 30% of worldwide
methanol production, respectively [1]
.
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10
Figure 2.1 Simplified flow diagram for methanol synthesis: (a) reactor; (b) heat exchanger; (c)
cooler; (d) separator; (e) recycle gas compressor; (f ) fresh gas compressor [1]
.
In the modern low-pressure methanol synthesis process, most of the typical industrial
catalysts are based on Cu/ZnO/Al2O3, which provides high activity and stability. They are
usually obtained by co-precipitation of aqueous metal salt solutions (e.g., nitrates) with
sodium carbonate solution and they are in the form of metal hydroxycarbonates or
nitrates. There are several factors that affect the quality of the formed catalyst, i.e. the
precipitation temperature, the composition of the metal components, the pH, the sequence
of metal salt additions, aging time, the stirring rate, stirring energy and so on [1, 21, 25-26]
.
Nowadays, the most widely used methanol catalysts are KATALCOJM 51-8 (Johnson
Matthey Catalysts), MegaMax® 700 (Süd Chemie), and S3-86 (BASF). The ratios of the
components vary from one manufacturer to another. Fiedler et al. summarized the
component ratio of the typical copper-based catalysts: the proportion of Cu is 60-75
atom%, Zn is 18-30 atom%, and Al or Cr is 5-12 atom%. MK-121 (Haldor Topsøe)
contains Cr instead of Al and the proportion of Cr is ca. 48 atom% [1]
. Some other
commercial catalysts also contain rare-earth oxide as additives. These solid catalysts are
stable and normally have active lives of 2-5 years. However, impurities from reaction gas
feed (chlorine and sulfur-containing contaminations) and sintering of the active Cu
particle at high reaction temperature cause the deactivation of the catalyst.
State of the art
11
Despite of a rapidly growing number of scientific papers on methanol synthesis over Cu-
based catalysts during the last few decades, there are still divided views on certain issues,
such as the nature and the location of the active site, whether CO2 or CO is the
predominant reactant, the state of the Cu in the working catalysts, the role of ZnO and
Al2O3, and the reaction mechanism [21-22]
.
2.1.3 Copper-based catalysts
2.1.3.1 Introduction
Since the early 1920s, Cu-based catalysts for methanol synthesis have been investigated,
and found to have higher activity than ZnO/Cr2O3 catalysts. The Cu/ZnO catalysts for
methanol production were described by BASF in the 1920s and reaction temperatures as
low as 300 °C could be used [1]
. However, the catalysts containing Cu were not stable and
lost activity fast. They were sensitive to certain impurities, such as hydrogen sulfide and
chlorine compounds from synthesis gas, which caused rapid deactivation. Nevertheless,
since they were promising for methanol production at lower temperature and pressure,
many investigations were carried out on the Cu-containing catalysts for 40 years. A low-
pressure catalyst for methanol synthesis was finally achieved by ICI, which contained
CuO and ZnO stabilized with Al2O3. It was extremely active and enabled the methanol
synthesis to be operated at below 250 °C and 5 MPa. The use of support prevented Cu
from sintering, and had higher selectivity to methanol. Nowadays, the low-pressure
catalysts used all contain Cu/ZnO with other stabilizing additives, such as Al2O3, or
Cr2O3.
2.1.3.2 Cu/ZnO and Cu/ZnO/Al2O3 catalyst system
Cu/ZnO catalysts, stabilized by Al2O3, are common catalysts for methanol synthesis.
Cu/ZnO/Al2O3 catalysts are used predominantly in the industrial low-pressure methanol
synthesis from synthesis gas feed [40-41]
. These catalysts can be prepared via various
methods including co-precipitation of metal salts in solution, kneading metal components,
impregnation of a metal precursor on solid supports and leaching metal components to
form Raney alloy. To date, co-precipitation is considered as the common method for the
preparation of industrial ternary Cu/ZnO/Al2O3 catalysts with the best catalytic
performance, where nitrates of Cu, Zn, and Al and alkali bicarbonates or alkali
State of the art
12
carbonates as basic precipitating agents are usually used [26, 42]
. This classical hydroxy
carbonate route leads to the formation of several mixed-metal hydroxy carbonates,
including aurichalcite ((Cu,Zn)5(CO3)2(OH)6), zincian malachite ((Cu,Zn)2(OH)2CO3),
and a Cu-Zn hydrotalcite-like phase ((Cu,Zn)6Al2(OH)16CO3·4H2O) [43]
. It is then by
calcination that all of these phases are transformed into well dispersed oxidized phases.
The decomposition temperature varies between 250-400 °C [44]
. The final active catalyst
is obtained by reduction of CuO to metallic Cu under a diluted H2 flow before feeding the
synthesis gas mixture [26, 45-46]
. Every single step taking place during the catalyst synthesis
procedure may have a significant influence on the activity of the final catalyst. In
particular, synthesis parameters during precipitation, including precipitation temperature,
pH, aging procedure, etc., play important roles for the activity, stability and selectivity of
the final catalysts.
A study on the preparation parameters was carried out by the Schüth group applying a 49-
fold parallel gas flow reactor system [25-26, 42, 47]
. They prepared a series of ternary
Cu/ZnO/Al2O3 catalysts using a co-precipitation method and pH, precipitation
temperature, and calcination temperature were under strict control. Detailed correlations
between synthesis conditions (precipitation pH and temperature), catalyst texture
(metallic Cu surface area), and catalytic performance (methanol productivity) were
established as shown in Figure 2.2. The catalyst with the highest methanol productivity
was obtained by using the following conditions: precipitation temperature of 70 °C, pH of
6-8, aging time of 20-60 min, and calcination temperature of 300 °C. It was thus
demonstrated that catalysts having higher BET and Cu(0) surface areas give greater
methanol productivity and that the catalyst activity was well correlated to its ‘preparation
history’, leading to varied catalyst structure and morphology. Another study also showed
that precipitations around a neutral pH gave the best catalysts [48]
. Bems and Schlögl,
after having investigated formation and reactivity of the Cu-Zn binary hydroxycarbonates,
confirmed the critical influence of the precipitation process on the structure of the
precipitate precursor - termed the ‘chemical memory’ [27]
.
State of the art
13
Figure 2.2 Influence between the catalyst preparation conditions (pH, precipitation temperature)
to catalyst Cu(0) surface area, catalyst BET surface area and methanol productivity measured at
245 °C and 4.5 MPa respectively [26]
.
The influence of individual parameters during precipitation has been studied and reported
by different groups. The group of Schlögl very recently reported the study of Cu/ZnO-
based catalysts, which were synthesized using a precipitation temperature at 65 °C [49]
,
which was close to 70 °C as reported by Schüth. The aging time of the wet precipitate is
essential for the formation of active catalysts. The changes in composition of the catalyst
precursor before, during, and after ageing are of great significance in the development of
maximum catalytic activity [44, 50]
. Waller and Spencer used IR, XRD, TGA, etc. to study
the ageing effect in the precursor structure and claimed that the maximum catalyst
activity would not be achieved until the ageing time was sufficiently long (30 min) [44, 50]
.
Kiener et al. stated that the pH of the Cu/ZnO system after precipitation varied strongly
State of the art
14
during the ageing of the precursor, and that the properties of the catalyst precursors could
be better defined under strict control of pH during both the precipitation and aging [42]
.
Schlögl et al. [27, 51]
observed the change of pH during ageing and also concluded that the
ageing time in the post-precipitation led to the different microstuctures of the final
catalysts. It was found that longer ageing time caused the decrease in the content of
aurichalcite, as well as the reduction of the amount of nitrate and hydroxides in the
precursors.
The properties of the Cu/Zn precursor system as a factor influencing activity have been
studied based on the work of some groups [48, 50, 52]
. Four precursor phases were seen: a
Zn-containing malachite, a mixed Cu, Zn hydroxycarbonate, an aurichalcite and a Cu-
containing hydrozincite. As a typical example, Figure 2.3 shows a reaction scheme in the
preparation of 2:1 Cu/Zn catalysts. It demonstrated the ageing effect on the formation of
the precursor phases: low-zincian malachite and high-zincian were recrystallized to high-
zincian malachite as the final product after ageing [53]
.
Cu/ZnO catalystCu/ZnO catalyst
Figure 2.3 Reaction scheme for the precipitation, ageing and subsequent stages in the preparation
of 2:1 Cu/Zn catalysts [21, 52-53]
.
State of the art
15
2.1.3.3 Other Cu/MeOx catalyst systems
Other supported Cu catalysts for methanol synthesis were also studied, such as Cu/CeO2
[54], Cu/SiO2
[55-57] and Cu/ZrO2
[58-60]. Among all, Cu/ZrO2 has attracted more attention
and ZrO2 has been investigated as a promoter or support. ZrO2, in a monoclinic phase, is
one of the typical fluorite-type oxides, which have a face-centered-cubic (fcc) crystal
structure. Zr4+
ion is surrounded by eight equivalent nearest O2-
ions. Therefore, ZrO2
provides high oxygen vacancy concentration that serves as an active site for some
reactions, such as methanol synthesis [61]
. Using Cu/ZrO2 as catalyst, the synthesis of
methanol can be achieved via hydrogenation of either CO or CO2, having the advantage
that the CO/CO2 ratio does not need to be adjusted [58, 62]
. Rhodes et al. have studied the
methanol synthesis from CO and H2 over Cu/ZrO2, especially focusing on the role of
ZrO2 [59-60]
. They found that the different phases of ZrO2, either t-ZrO2 or m-ZrO2, had a
strong influence on the activity and selectivity of Cu/ZrO2. CO was adsorbed to ZrO2 as
HCOO-Zr and CH3O-Zr species that participated directly in the reaction. Köppel et al.
reported that efficient Cu/ZrO2 catalysts have high interfacial area of Cu and ZrO2, which
consist of microcrystalline Cu particles that are well dispersed by an amorphous ZrO2
matrix [63]
. The activity for methanol formation using ZrO2 as the support is slightly lower
than when ZnO is used as the support. Even though ZrO2 is still a less preferred support
for the methanol synthesis catalyst, more and more research focuses on to its catalytic
effect because of its special structure and stability.
2.1.4 Reaction mechanisms
The reaction mechanism for the low-temperature synthesis of methanol is still under
debate and there has been no universal agreement on a single proposal as follows:
What are the roles of CO and CO2 and from which is the methanol formed?
What is the state of the Cu in the working catalysts?
Which reaction step is rate-determining?
Klier et al. studied the catalytic methanol synthesis from CO and H2 in the early 1980s,
and this work had a great impact [64-67]
at that time, indicating the CO being adsorbed on
Cu+. Accordingly, the principal reactions based on the direct hydrogenation of CO
include reactions (2.1) and reverse water gas shift (2.3) [39]
. However, as early as 1970s,
State of the art
16
Russian scientists demonstrated that methanol was formed from CO2 rather than CO.
Both kinetic experiments and studies with radioactively labeled carbon oxide isotopes
supported this hypothesis [68-70]
, indicating that the rate of the reaction (2.3) - RWGS was
slower than that of the methanol synthesis reaction and was negligible. Then, more
extensive isotope studies from ICI scientists using Cu/ZnO/Al2O3 catalysts confirmed the
conclusion that CO2 is the principle carbon source in methanol synthesis [71]
. The transient
experiments also showed agreement with the results of former studies which showed that
methanol is formed predominantly from CO2 [72]
.
As previously stated, Cu/ZrO2 catalysts provided methanol formation routes either from
CO or CO2 and the mechanisms remained controversial. Weigel and Baiker et al., based
on the identification of intermediates, considered CO as the precursor to methanol. They
suggested a mechanism that methanol is generated by hydrogenolysis or protolysis of
surface-bound formaldehyde and methylate, which came from the reduction of the
adsorbed CO on the methanol synthesis catalyst [73]
. In contrast, Fisher and Bell et al.
proposed another mechanism that methanol was produced from CO2 that was adsorbed
onto ZrO2 forming bicarbonate species [55]
.
A high number of studies was carried out on the surface adsorption of CO, CO2, H2, H2O,
CH3OH, formaldehyde and methyl formate, applying Cu/ZnO or Cu/ZrO2-based catalysts
[21]. Various techniques were employed, including IR, DRIFTS, TDS, TPD and chemical
trapping. The three most important species found in the experiments are formyl, methoxy
and formate (shown in Figure 2.4). Proved by IR spectroscopy, formyl species were
formed from CO and H2, and they have been detected on ZnO and Cu/ZnO and
Cu/ZnO/Al2O3 [64, 74]
. These formyl species were unstable and could rapidly be
hydrogenated to methoxy species, also found on the surface of the synthesized catalysts
[75], which were more stable than formyl species but less stable than formates. Rasmussen
et al. investigated its formation in detail and found that on Cu(100) formate was detected
as nearly the only existing adsorbed species [76-77]
. Fujitani et al. studied the Zn-deposited
Cu(111) and concluded that the formate species on Zn was stabilized by special sites [78-
80]. They also detected formate species in the cases of Zn/Cu(110), Zn/Cu(100) and
Zn/Cu(111) [80-81]
. The formate, present as bidentate species, was considered to be the
pivotal intermediate for methanol synthesis from CO2 hydrogenation, and the rate
State of the art
17
determining step would be the hydrogenation of these formate structures [82-83]
. This
formate is subsequently hydrogenated through methoxy to methanol, leaving a partially
oxidized Cu [22]
. H2 and CO could also interact at the Cu/ZnO interface to produce a
formate species on the Cu component of the catalyst [75]
.
C OC
O O
H CH3
O
H
Formyl Methoxy Formate
Figure 2.4 Surface species found as intermediates on methanol synthesis catalysts [21]
.
2.1.5 Reaction kinetics
Different kinetic models were proposed for the Cu-based catalysts in methanol synthesis.
Szarawara and Reychman used an industrial Cu/ZnO/Al2O3 catalyst with synthesis gas at
about 5 MPa and 190-260 °C, which was close to the conditions in the low pressure
process [43]
. They analyzed their results in terms of two empirical rate Equations (2.4 and
2.5):
2
3
2H
2
CO1
OHCH
HCO0.5
11PPK
P1PPkR (2.4)
22
3
22
H3
CO2
OHCHH
1.5CO
0.5
22PPK
P1PPkR (2.5)
where k1 and k2 are rate constants while K1 and K2 are equilibrium constants. Besides,
Nattal has reported a detailed study of the methanol synthesis kinetics for the ZnO/Cr2O3
catalyst in the high-pressure methanol synthesis process [22]
. In the temperature range
330-390 °C a rate expression of the form (Equation 2.6):
3OHCH3H2CO1
OHCHH
2
CO
32
3
2
PKPKPK1
K
PPP
R
(2.6)
was used to predict reaction rates [22, 84-85]
.
State of the art
18
A novel approach, the so-called microkinetic approach, has been established by Ovesen
et al. for both the methanol synthesis and WGS reactions [86-88]
. The microkinetic model
is based on a ‘surface redox’ mechanism deduced from surface science studies of well-
defined Cu single crystals. The sixteen elementary steps considered in the model are
shown in Figure 2.5. The first eight steps represent the elementary steps of the redox
mechanism of the WGS reaction that occurs under reaction conditions [88-89]
. Steps 9-13
and steps 14-16 constitute the synthesis of methanol through a formate intermediate and
possible formation of formaldehyde, respectively. The rate-determining step in the kinetic
model was hydrogenation of the adsorbed H2COO* to methoxide and oxide (step 11),
determined from Cu(100) single-crystal experiments under CO2 and H2 gas feed [86]
. In
contrast, the rate determining step of the WGS reaction was either step 2 or step 7,
depending on the water to CO ratio [87-88]
.
1 H2O(g) + * H2O*
2 H2O* + * OH* + H*
3 2OH* H2O* + O*
4 OH* + * O* + H*
5 2H* H2 + 2*
6 CO(g) + * CO*
7 CO* + O* CO2* + *
8 CO2* CO2(g) + *
9 CO2* + H* HCOO* + *
10 HCOO* + H* H2COO* + *
11 H2COO* + H* H3CO* + O*
12 H3CO* + H* CH3OH* + *
13 CH3OH* CH3OH(g) + *
14 H2COO* + * HCHO* + O*
15 HCHO* HCHO(g) + *
16 H2COO* + H* HCHO* + OH*
Figure 2.5 Elementary steps of the microkinetic model of methanol synthesis reaction (The *
represents an empty surface site and X* stands for an adsorbed species.) [86-87]
.
State of the art
19
2.1.6 Active sites
As stated above, some common conclusions were obtained based on the investigations on
the reaction mechanism and kinetics. However, the nature of the active sites that are
responsible for catalyzing the reaction still remains unclear. Since Cu/ZnO based
catalysts are the most active in industrial processes, the active sites in Cu/ZnO have been
studied by scientists for a long time. A variety of different techniques were employed
throughout the last decades, both from the points of view of surface science and technical
catalysis [24]
. The nature and the location of active sites have been investigated by
different research groups and there are two major opinions: one is that the activity is
governed by metallic Cu atoms in the methanol synthesis, whereas the other one is that
besides metallic Cu atoms some other special sites exist.
Early research by Chinchen and Waugh using in situ frontal chromatographic
measurements of Cu surface area showed that there is a linear relationship between the
methanol synthesis activity of Cu/ZnO/Al2O3 and their total Cu surface area [90-91]
. They
also stated that when using other materials as supports, TON of the Cu remained nearly
the same as for Cu/ZnO/Al2O3. The results from the group of Chorkendorff supported the
idea of metallic Cu as the active catalyst, based on the investigation of methanol synthesis
on Cu(100) [76]
. They also proposed that ZnO determines the degree of dispersion and the
distribution of exposed Cu surface planes. The technique for determining surface Cu
atom was further developed by Schüth et al. using a spatially resolving Fourier-
transformed infrared focal plane array detector (FTIR-FPA) system combined with high-
throughput analysis [26, 92]
, which helped to establish catalyst synthesis-property-activity
relationships. The Cu/ZnO/Al2O3 catalysts under investigation had the same composition
but precipitated at different pH and temperatures, and the Cu/Zn/Al molar ratios of most
of the catalysts were ca. 1/0.47/0.3. They were tested at 4.5 MPa and 245 °C with the
technical catalyst from ICI as reference that showed the activity of 30 molMeOH/(kgCu·h).
The linear correlation between the specific Cu surface area and catalytic activity was also
obtained as illustrated in Figure 2.6, a similar correlation as reported in literature.
Various research has been directed to elucidate the special nature of the synergetic effect
between Cu and ZnO, and as a result, other models of the active site were proposed [40, 93]
.
State of the art
20
Figure 2.6 Correlation between Cu surface areas and methanol synthesis activity of different
Cu/ZnO/Al2O3 catalysts compared to a commercial benchmark catalyst [92]
.
(a) Cu ions as the active sites
Klier et al. proposed that Cu could be present in three possible valence states Cu0, Cu
+
and Cu2+
, all incorporated in a ZnO matrix [65]
. Cu+ could be detected on both binary
Cu/ZnO and ternary Cu/ZnO/Al2O3 by different techniques, such as adsorption studies [94]
,
XPS [66, 83]
and IR spectroscopy [95]
. Later, Fujitani et al. supported the model of Klier
employing reactive frontal chromatography (RFC), which investigated the coverage of
oxygen [96]
. They indicated the formation of Cu+ sites at the interface between the Cu
particles and ZnO or other metal oxide supports. Frost, instead, suggested an opposite
theory suggesting the presence of Cu- species. It is caused by the Schottky junction at the
interface, where there is an electron transfer from the semiconducting oxide (ZnO, ThO2
or ZrO2) to Cu (or other metals such as Au, Ag, Pt and Pd) in contact with oxide surface
[97]. Methanol formation over the junction-effect-promoted system is considered to take
place on the oxygen vacancy sites in the oxide with either CO or rather CO2 insertion into
a hydride.
(b) Cu particles dispersed onto ZnO
It has been well accepted that one of the important roles of ZnO is to increase the
dispersion of Cu particles [43, 76, 91]
. A model was supported by Campbell et al., suggesting
that metallic Cu on Cu/ZnO catalysts acts as active site for methanol synthesis. They also
proposed that metallic Cu in ‘ultrathin islands’ was stabilized by ZnO and had a behavior
State of the art
21
that resembled Cu(110) [98]
. The Topsøe group has proposed that the high surface area
and morphologies of Cu crystals are stabilized by ZnO [99-100]
. By investigating the
surface changes by IR using CO as a probe molecule, they concluded that Cu/ZnO
catalysts are dynamic systems and their structures are influenced by the reductive
potential of the reaction gas. Hansen and Topsøe put forward the most convincing proof
of this dynamic model, employing HRTEM [30]
. Figure 2.7 shows HRTEM images of Cu
nanoparticles dispersed on ZnO with particles size between 3 and 6 nm. It is clear that
dynamic shape changes of Cu nanoparticles depended on the changes of the gas
environment. In a pure hydrogen atmosphere (Figure 2.7 A), Cu(111) facets of the Cu
particles appear to be in contact with the ZnO support. A more oxidative atmosphere
(Figure 2.7 C) containing water transforms the Cu crystals into a more spherical
morphology. The same Cu facets exist, while the fraction of Cu(110) obviously increases.
The addition of the more reducing gas CO (Figure 2.7 E) changes the Cu particle shape to
be disc-like, and the exposed Cu facets are mainly Cu(111) and Cu(100). The shape
transformation was therefore reversible due to the change of gases environment. The
oxidative conditions of the gas determine the oxygen contents and oxygen vacancies in
ZnO, thus the interface energy. This study was performed under realistic conditions that
are relevant for the complex surfaces and interfaces of Cu/ZnO during reaction. It is
demonstrated that the relevant active sites are generated during the catalytic reaction.
Figure 2.7 In situ HRTEM studies of the Cu/ZnO catalyst system: (top) A, C, and E of a Cu/ZnO
catalyst in various gas environments together with (bottom) B, D and F corresponding images of
the Wulff constructions of the Cu nanocrystals [30]
.
State of the art
22
(c) Cu-Zn alloy model
Early research by Nakamura and Fujitani et al., based on the surface coverage of oxygen
of Cu/ZnO catalysts using RFC, revealed that the partially reduced ZnOx migrates from
ZnO particles to the surface of Cu particles [96, 101]
. The authors conducted further studies,
employing XRD, EDX and IR, and confirmed the formation of Cu-Zn alloy under
reduction conditions at above 600 K in H2 [102]
. They also stated that it was the ZnOx on
the surface of Cu particles that stabilized the Cu+ as active sites and that promoted higher
activity of the catalysts in methanol synthesis [101-103]
. Later, Topsøe and Topsøe
presented in situ IR measurements of 1 and 5% Cu on ZnO catalysts during methanol
synthesis and the results showed CO band shift and a decrease in the vibrational
frequency of CO under severe reducing conditions [100]
. It suggested that Cu-Zn alloy
structures could be formed due to the migration of reduced ZnO species [100, 104]
. By using
a combination of in situ EELS and in situ HR-TEM, the same metal-support interaction
and a formation of Cu-Zn alloy could be identified [105]
. In addition to the investigations
of the Cu-Zn surface alloy employing the above techniques, this surface phase was also
determined thoroughly in an in situ EXAFS study under varying gas-phase composition
by Grunwaldt and Topsøe, and a wetting/non-wetting model was proposed, as shown in
Figure 2.8 [99, 106]
. Under oxidative conditions (wet synthesis gas), the Cu particles
remained more spherically shaped with higher Cu coordination number and low methanol
formation activity. Under more reductive conditions (dry synthesis gas), the Cu particles
became more disk-like and the Cu coordination number was lower. This wetting
transition occurred when the ZnO surface becomes oxygen deficient and the Cu surface
area was larger. These structure changes are reversible according to the change of the
reduction potential of the gas atmosphere, and this model has also been proved using in
situ TEM by Hansen et al. [30]
as shown before. This effect only took place when using
ZnO as support and this significantly influenced the catalyst activity. Further, a bulk Cu-
Zn alloy was formed under higher temperature and reductive atmosphere.
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Figure 2.8 Schematic model for the wetting/non-wetting transition of Cu particles on a ZnO
support, surface alloying, and bulk alloy formation: (a) round-shaped particle under oxidizing
synthesis gas conditions; (b) disk-like particle under more reducing conditions; (c) surface Zn-Cu
alloying due to stronger reducing conditions; (d) brass alloy formation due to severe reducing
conditions [106]
.
(d) Microstrain of Cu particles
As stated above, Schlögl et al. suggested that the phenomenon of a ‘chemical memory’
exists for the Cu/ZnO catalysts synthesized from precipitation of different Cu and Zn
precursors [27]
. The disorder and strain in Cu were generated at the Cu/ZnO interface and
led to a higher intrinsic activity for methanol synthesis [107-108]
. The same group also
reported the microstructure of Cu/ZnO catalysts which were prepared from
hydroxycarbonate precipitate after different ageing times [109]
. For samples with ageing
times of more than 30 min, the catalytic activity increased and the microstructure of the
samples became more homogeneous. As illustrated in Figure 2.9, the HRTEM study
showed that Cu and ZnO particles are mostly round-shaped. The well crystallized ZnO
particles were found to be located between Cu particles and they might protect the Cu
particles from sintering. Cu particles lacks of degree of order, linked to the increased Cu
ZnO interface area. The authors later on carried out further analysis of the nanostructure
of a series of catalysts, employing TEM and in situ XRD. Some planar defects and strain
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in nanostructured Cu were observed, present as non-equilibrium structures during
synthesis, causing ‘chemical memory’ [110]
. Therefore, the authors confirmed that the high
performance of the Cu/ZnO-based catalysts are due to the Cu lattice strain, and not only
the high dispersion of Cu particles [51]
. A very recent report from the same group
highlighted the nonideal nature of metal copper as microstrian and stated that the
mechanism of strain relaxation leads to the defect [49]
. It was also suggested that the step,
a defect on the surface of Cu particles, had impact on the catalytic property and there
seems to be a trend of intrinsic activity with higher lattice strain.
Figure 2.9 HRTEM images of a Cu/ZnO catalyst obtained from a copper zinc hydroxycarbonate
precipitate aged for 120 min [109]
.
(e) Methanol synthesis over ZnO
ZnO has also been intensively studied, as a catalyst for methanol synthesis. French et al.
in an early theoretical study used a novel solid-state embedding technique to explain the
intermediates and mechanism of methanol formation from a CO2/H2 mixture on a Cu-
free model [111]
. They also proposed that oxygen vacancies are the active sites in methanol
synthesis. Based on it, Muhler et al. carried out more detailed studies using ZnO with
different structures [112]
. It was found that under the high pressure methanol synthesis, the
activities of ZnO were structure-sensitive and they were correlated to the specific surface
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active sites on ZnO. ZnO with better developed crystalline faces provided higher
activities, and polar faces were assumed to play an important role in methanol synthesis.
Later they investigated ZnO based catalysts and found CO as the carbon source, and
hydrogenation of CO is a more energetically favorable reaction pathway [28]
. Polarz and
Muhler reported a study on nanocrystalline ZnO materials obtained by thermolysis of
organometallic heterocubane Zn4O4 precursors at low pressure [113]
. Catalytic tests of
these materials, as shown in Figure 2.10, using CO and H2 as feed gas proved that a
correlation between the catalytic activity and the amount of oxygen vacancies exists. The
study also confirmed that hydrogenation of CO is driven by the oxygen vacancies present
as active sites in ZnO.
Figure 2.10 Correlation between the occurrence of oxygen vacancy sites (determined by EPR
spectroscopy) and the normalized activity for CO hydrogenation at different pressures [113]
.
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2.2 Metal colloids
2.2.1 General introduction
Colloid chemistry is a multidisciplinary field involving chemistry, physics, materials
science, biology, etc. The properties of the colloidal systems are governed by the
colloidal particle size that is in the range of 1-100 nm [114]
. Metal colloids are used for
aesthetic purposes, as pigments, and technological applications, as in catalysis, so they
are of great interest for scientists. Colloidal metal nanoparticles are typically well isolated
particles, being prevented from agglomeration by protecting shells. They can be dispersed
in water (‘hydrosols’) or organic solvents (‘organosols’) [32]
.
2.2.2 Synthesis
Metal colloids are obtained through two main methods as displayed in Figure 2.11:
physical methods (‘top down method’) by the mechanical subdivision of metallic
aggregates, or chemical methods (‘bottom up method’) through the nucleation and
growth of metallic particles [114]
. ‘Solvated metal atom dispersed (SMAD)’ procedure as
one typical physical method was reported by Klabunde et al. [115]
. It allows to achieve
high dispersion of metal without reduction; however, the synthesis is not economical, not
quite reproducible and cannot be scaled up [116]
. Chemical methods provide the most
convenient ways to control the particle size and surface composition as well as to ensure
reproducibility of the synthesis. As summed up in the literature, there are five main
chemical methods [114]
: (1) chemical reduction of transition metal salts; (2) thermal,
photochemical, or sonochemical decomposition; (3) ligand reduction and displacement
from organometallics; (4) metal vapor synthesis; (5) electrochemical reduction. Among
them, chemical reduction of metal salts is the most widely used one, because of its
reproducibility, narrow particle size distribution reached, and possibility to be extended to
a multigram scale [32]
. A large number of compounds can be applied as reducing agents:
sodium borohydride, sodium citrate, hydrogen or carbon monoxide, or even alcohols [114]
.
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Figure 2.11 Schematic illustration of preparative methods of metal nanoparticles [117]
Early in the 1950s, Turkevich et al. were the first who established the reproducible
standard synthesis approach of metal colloids [118-119]
. According to their investigation of
the process of nucleation and growth in colloids, the nuclei were present in a form of
mixed polymer with AuCl4- prior to their reduction
[119]. Then they proposed a
mechanism for the stepwise formation of colloidal nanoparticles based on nucleation,
growth and agglomeration, which was then supported by analytical techniques (in situ
XAS, UV-Vis), thermodynamic and kinetic studies [120-123]
. As illustrated in Figure 2.12,
at first the metal salt is reduced to give zero-valent metal atoms at the initial stage of the
nucleation. The mechanism of particle formation involves two possible paths including
an autocatalytic pathway and a collision of metal atoms. The former describes a process
where metal ions are adsorbed and successively reduced at the zero-valent cluster surface.
This was proved by the formation process of colloidal Cu, where intermediate Cu+ was
detected before the nucleation of particles [121]
. The stepwise reductive formation of Ag3+
and Ag4+ clusters also supported this assumption
[124].
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Nanostructure metal colloid
(TEM micrograph)
Figure 2.12 Formation of nanostructured metal colloids by the ‘salt reduction’ method [32]
.
2.2.3 Stabilization
The stabilization of metal colloids is crucial during colloid synthesis, in order to preserve
their finely dispersed state and small particle size. Quite a few different stabilizing agents
can be used to control the growth of the colloidal particles and prevent them from
agglomeration [32]
. Based on the different stabilizing agents applied, there are mainly four
stabilization procedures: (1) electrostatic stabilization; (2) steric stabilization; (3)
electrosteric stabilization; (4) stabilization by ligand or solvent [114]
. In the electrostatic
stabilization, the stabilizing compounds and their counterions are adsorbed on the
metallic surface. A coulombic repulsion between the particles exists, caused by an
electrical double-layer around the nanoparticles. Halides, carboxylates, or polyoxoanions
are usually involved and some typical examples are the syntheses of colloidal gold and
palladium, prepared via sodium citrate reduced AuCl4- and PdCl2, respectively
[118-119].
Steric stabilization is obtained by surrounding the metal centre with bulky layers of
materials, such as polymer or surfactants [125]
. One example are the NR4+-stabilized metal
organosols, with tetraalkylammonium hydrotriorganoborates as stabilizers in an organic
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solution [126]
. Electrosteric stabilization is a combination of both electrostatic and steric
stabilization, such as in the case of polyoxoanion-stabilized nanoclusters [127]
. Ligand
stabilization relies on stabilization of transition metal colloids by coordination, using
traditional ligands, such as carbon monoxide, phosphines, thiols, amines, etc [114]
.
2.2.4 Structural property control
Much attention has been paid to the control of the structure of metal nanoparticles, in
particular their size and shape, since both factors have significant influence on catalyst
activity and selectivity. This point will be discussed in particular together with the
supported metal nanoparticles in the following Section 2.2.7 [128-130]
. Here only the
general methods for structural control are discussed.
2.2.4.1 Particle size control
The particle size can be controlled by varying several preparation parameters, including
the stabilizer, reducing agent, pH, solvent, concentration, reaction temperature, etc. Jia et
al. and Pachon et al. summarized accordingly the different types of stabilizers that could
tune the metal nanoparticle size [131-132]
. Some typical stabilizers are linear polymers,
dendrimer, surfactants, micelles, ligands, etc. PVP, PVA, N-dodecyl-N, N-dimethyl-3-
amino-1-propan sulfonate (SB) are typically used as protective polymer stabilizers, and
PVP is the most common one among them. The size of various metal nanoparticles
stabilized by PVP, such as Pd [133-134]
, Pt [135]
, Rh [136]
, Ru [137]
, could be successfully
controlled. Miyake et al. studied the size control of Pd nanoparticles through the variation
of the polymer concentration: the higher the PVP concentration, the better the
stabilization and the smaller the particle size [133]
. Tsunoyama et al. reported the size
effect of PVP-stabilized Au nanoparticles in the aerobic oxidation of benzylic alcohols
and found that the smaller Au nanoparticles exhibited higher catalytic activity [138]
. The
use of dendrimers, the three-dimensional macromolecules, as stabilizers provides
nanoparticles with high stability and controllable structure [131, 139-140]
. Crooks and co-
workers synthesized dendrimer-stabilized Pd nanoparticles as catalysts in a size range of
1.3 and 1.9 nm and found that the rate of hydrogenation of allyl alcohol depended on
their diameters [141]
. Water-containing reverse micelles or microemulsions are indeed
interesting media for producing monodispersed metal nanoparticles [131]
. This technique
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allows the production of metal nanoparticles, including Pd, Pt, Au, Rh, Fe, and Cu, with
controlled size and their size effects were studied in catalytic reactions [142-145]
. Various
stabilizing ligands, such as thioether, 1, 10-phenanthroline, polyoxoanions, 2,2’-
bipyridine, tetraalkylammonium salts and dodecylamine, can provide size control over
metal nanoparticles [125, 146-148]
. Reetz et al. reported tetraalkylammonium-stabilized Pd
nanoparticles, where no external reducing agent was used, and the particle size range was
tuned between 1 and 10 nm [149]
.
2.2.4.2 Particle shape control
Nanoparticles, composed of single or multicrystals, expose different facets when they are
in different shapes. The control of the particle shape determines which crystal facets
appears on the surface of crystallized nanoparticles, thus influencing both their reactivity
and selectivity in catalysis [150-153]
. Yang et al. in their review stated that in practical
synthesis, the shape control of colloids can be realized by protecting specific crystal
planes with molecular capping agents [154]
. The shape anisotropy thus is generated during
nanocrystal growth by this molecular interaction. Generally, growth is favored when the
bonding is weak and limited in the case of strong bonding. Nanocubes and nanotetrahedra
are two common shapes and they expose (100) and (111) facets for a fcc metal,
respectively [150]
. Studies of Pt single crystals in aromatization reactions showed that the
Pt(111) surface was three to seven times more active than the Pt(100) surface [155]
. El-
Sayed et al. studied both cubic and tetrahedral Pt nanoparticles in nanocatalysis reaction
and found that the catalytic activity of nanoparticles correlated with different shapes and
the fraction of atoms located on corners or edges [156-157]
. However, surface reconstruction
and shape changes did occur in the course of the catalytic reaction [158]
. In the electron-
transfer reaction catalyzed by Pt nanoparticles of different shapes, it was found that a
tetrahedral shape is more sensitive to shape change [157, 159]
.
2.2.5 Applications of metal colloids in catalysis
The catalytic properties of metal colloids have attracted great interest over the last
decades, because of their large surface area and thus a high fraction of atoms located at
the surface [114]
. Colloidal metal nanoparticles should be available for homogeneous
reactions, because they can be well dispersed in either organic or aqueous solution [33]
.
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The solubility offers the possibility to apply various different analytical techniques.
Schmid called the type of homogeneous catalysis using metal clusters a kind of
‘heterogeneous catalysis in solution’ [33]
. Nowadays, reactions catalyzed by metal clusters
are generally considered by Bönnemann as ‘quasi-homogeneous catalytic reactions’ [32]
.
There are indeed some typical classes of reactions catalyzed by metal clusters, such as
hydrogenation, oxidation, hydrosilylation and C-C coupling [114, 160-161]
. In addition,
colloidal metal precursors could be used for the formation of high-performance fuel-cell
catalysts [32, 160]
. Pd colloids are among the most typical metal cluster catalysts which
were applied in various reactions [114]
. Herrmann was the first to describe the use of Pd
nanoparticles to catalyze Heck coupling reaction [162]
. Reetz et al. shortly after reported
Pd or Pd/Ni nanoparticles for Suzuki coupling reaction [163]
. The early work of Moiseev et
al. intensively focused on giant Pd clusters in several catalytic reactions, including
dimerization, isomerization, acetoxylation, oxidative reactions of some olefins and
alcohols [164]
, the hydrogen-transfer reduction of multiple bonds by formic acid [165]
, the
phenol oxidative carbonylation to diphenyl carbonate [166]
. Shiraishi et al. obtained PVP-
stabilized Ag colloids that had higher activity than commercial Ag catalysts in ethane
oxidation [167]
. It was also reported that some bimetallic colloids were active in
hydrogenation reactions, such as Pd/Pt, Au/Pd, Cu/Pt, Pt/Ru, Ru/Pd, etc [32, 114]
. Other
metallic nanoparticles (Pt, Fe, Ru, Co, etc.) that showed catalytic properties have been
covered in some review articles [32, 114, 125, 161]
. Particularly, Cu colloids will be discussed
in more details in the following Section 2.2.8.
2.2.6 Synthesis of supported metal colloids
The recovery of colloids as catalysts from the reaction products is a significant drawback
for a quasi-homogeneous catalysis system, and the agglomeration of colloidal
nanoparticles may also lead to a loss of activity [114]
. Due to these drawbacks, more
attention has been paid to the immobilization of metal nanoparticles on a solid support.
Using supported nanoparticles, the catalysts can be easily recycled and the nanoparticles
are isolated by porous supports so that metal sintering at high temperature is effectively
avoided. Therefore, the supported metal nanoparticles maintain the main advantages of
nanoparticles in reaction, but can be applied in heterogeneous catalysis.
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Jia and Roucoux reviewed the supported metal nanoparticles with respect to their
preparation and application in catalysis [114, 132]
. The adsorption of the metal colloids onto
supports and grafting of the nanoparticles onto supports are the most common preparation
methods for supported metal nanoparticles. Both techniques are based on the deposition
of as-synthesized metal colloids onto the solid supports, which is the so-called ‘precursor
concept’ developed in the 1990s (refer to Figure 2.13) [126, 168-169]
. There are a few
advantages of the ‘precursor concept’, summarized by Bönnemann et al. [32]
. First is that
the size, shape and composition of the metal colloids as precursors can be well
maintained onto the support as how they are dispersed in solution. Second, the metal
nanoparticles may be further modified by being coated by lipophilic or hydrophilic
protecting shells or intermediate layers on the surface, or by using dopants. In general, the
supported metal nanoparticles are simply synthesized by placing the supports into organic
or aqueous colloidal solution at room temperature to adsorb the as-synthesized metal
nanoparticles without post calcination. Various solid materials can be applied as supports,
such as metal oxides, carbon, sillca, and even some low-surface-area materials (e.g.
sapphire, quartz, and highly oriented pyrolitic graphite) [32, 132]
.
Figure 2.13 The precursor concept [126]
.
The adsorption of metal colloids onto a solid support is the most facile approach. It relies
on wet impregnation of a support with a direct colloidal dispersion, and it can be
considered as a direct colloid deposition method [170-172]
. The most widely reported
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supports used to adsorb the colloidal nanoparticles are inorganic solids such as charcoal,
SiO2, Al2O3 or other metal oxides (MgO, TiO2, ZrO2, ZnO, Fe2O3, CeO2, etc.). The
deposition is followed by washing of the solid with filtration or centrifugation. As stated
above, this method is more advantageous compared with the conventional impregnation
followed by reduction, because the original particle shape and size are independent of the
support under mild preparation conditions and it also favors the metal dispersion [114]
.
There are quite a few typical examples of supported metal nanoparticles obtained using
this method [126]
. Bönnemann immobilized metal nanoparticles on charcoal by stirring
nanoparticles (Ti, Zr, V, Nb and Mn) and charcoal in THF suspension [168, 173-174]
. Reetz et
al., using the same procedure, deposited Pd nanoparticles on charcoal or SiO2, and the
authors described different adsorption behaviors, depending on the different stabilizing
agent [175-177]
. Yang et al. deposited the benzyl mercaptan reduced Ag nanoparticles onto
carbon nanotubes (CNTs), where benzyl mercaptan protects the Ag nanoparticles from
agglomeration and ensure their uniform dispersion onto the support [178]
. Comotti et al.
synthesized colloidal Au nanoparticles dispersed on carbon or metal oxide supports
(Al2O3, TiO2, ZrO2, ZnO) [171-172]
. The solid supports were mixed with Au colloidal
solution by mechanic stirring until the adsorption was completed, indicated by the
disappearance of the color of the Au solution. In our present report, the supported Cu
colloids were also prepared through this approach, where the solid supports were pre-
dried under vacuum and the whole synthesis was carried out under Ar (Section 5.1.2).
The obtained catalysts are sometimes calcined to remove the stabilizer and thus to
activate the catalyst. It is also popular to immobilize the metal nanoparticles onto
supports via a wide range of chemical bonds, thus modifying the surface of nanoparticle
and support. The group of Akashi [179-181]
and the groups of Hirai and Toshima [182-185]
carried out research on grafting Pt or Rh colloids onto polymer supports. The surface of
polystyrene micropheres was modified with poly(N-iso-propylacrylamide) (PNIPAAm),
which prevent Pt nanoparticles from agglomeration by steric stabilization and also
immobilize them onto the support [179-181]
. Another method to prepare support metal
nanoparticles is similar to deposition-precipitation used in the synthesis of supported
metal catalysts. An example is Ag nanoparticles supported on hydroxyapatite (Ag-HAp),
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where aqueous Ag(NO3)2 solution was mixed with support and then reduction took place
by using potassium borohydride [186]
.
2.2.7 Structure-activity relationship of metal colloid-based catalysts
The same as has been discussed for the metal colloids used in quasi-homogeneous
reactions, quite a few heterogeneous reactions have been catalyzed by supported metal
nanoparticles, such as CO oxidation [187-188]
, isomerization [178]
, etc. They exhibited
comparable or even better catalytic performance than supported metal catalysts prepared
via conventional routes. It is reported that the catalytic performance of supported Cu
colloids is also determined by the size, shape and composition of the metal nanoparticles
[128, 130]. Besides, the support effect and the residual stabilizer on the surface also have
impact on the reactivity and the selectivity of the catalytic reactions [132]
. In the following
sections, only the main factors (size effect, shape effect and support effect) that affect the
activities of both metal colloids and supported metal nanoparticles will be discussed.
2.2.7.1 Size effect
Van Santen summarized the relation between the particle size regime and particle
reactivity [130]
. The particle size range, in which structure sensitivity of heterogeneous
catalytic reactions is typically observed, is between 2 and 20 nm. In this case, there are
terrace, corner, edge sites, as well as step sites formed on the surface of the particles [189]
.
Three typical types of particle size-reactivity relationship can be distinguished, which is
shown in Figure 2.14 [190-191]
. The activities of metal nanoparticles (normalized to
exposed metal atoms) in some reactions are independent of their size, whereas some
others show strong influence of the particle size. For example, Rh nanocrystals of a size
range of 5-15 nm are used to catalyze ethylene hydrogenation and theirs activities showed
no dependence on particle sizes [192]
. The activities of some reactions increase when the
particle sizes become larger. For example, Pd/SiO2 was used in the liquid phase
hydrogenation of 2,4-dinitrotoluene to form tuluenediamine [193]
. It was shown that the
catalytic activity was higher on larger Pd nanoparticles, since larger particle size favored
the hydrogen adsorption forming β-hydride. For some other reactions, activities of metal
nanoparticles increase with decreasing particle size. In the case of the NO-CO reaction
catalyzed by Pd/MgO, the catalytic activity generally increased with decreasing
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nanoparticle size and the largest particle showed less activity [194]
. However, the author
claimed that the study of particle size effect was complicated, because the particle
morphology needs also to be taken into account. This remarkable dependence of catalytic
activity on dispersion of active particles has been known for nearly half a century and
Boudart referred to this in the terms of ‘structure sensitive’ and ‘structure insensitive’
reactions [195]
.
Figure 2.14 Structure-sensitivity of catalytic reactions [130].
An interesting study was carried out by Narayanan and El-Sayed et al. on how the
catalytic process affects the nanoparticles during the catalytic process [196-197]
. They
investigated thoroughly the Suzuki cross-coupling reaction that was catalyzed by PVP-
stabilized Pd nanoparticles [196-197]
. During the first 12 hours, the particle size and the size
distribution of the nanoparticles increased, due to the Ostwald ripening that explains the
formation of larger nanoparticles by the dissolution of small ones. When the
nanoparticles were recycled, the particle size in solution decreased, since larger particles
were separated from the solution because of aggregation and precipitation. This process
led to the loss of catalytic activity of the nanoparticles during the second cycle. The same
authors also studied the stability and catalytic activity of PVP-stabilized Pt nanoparticles
for the electron-transfer reaction between hexacyanoferrate(III) ions and thiosulfate ions
[198]. The average size and width of the PVP-Pt nanoparticles decreased slightly after the
first and second reaction cycles. Thiosulfate as reactant binds to the particle surface, so it
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prevents the particles from being attacked by hexacyanoferrate. It was also suggested that
the aggregated Pt particles have higher activity.
Some typical structure-sensitive reactions, where the reactions are catalyzed by supported
metal nanopartcles, are illustrated here. deJong and co-workers studied the catalytic
Fischer-Tropsch (FT) reaction (220 °C, H2/CO=2) over carbon nanofiber-supported Co
nanoparticles with sizes ranging from 2.6 to 16 nm [199]
. The coverage and residence
times were detected by steady-state isotopic transient kinetic analysis (SSITKA).
Accordingly, the relationship between the Co particle size and surface-specific activity
(TOF) is shown in Figure 2.15. The TOF is lower when the particle size is below 6 nm,
due to both blocking of edge/corner sites and a lower intrinsic activity at the small
terraces. The authors also proved that the small Co nanoparticles were more selective to
CH4, which was attributed to their higher hydrogen coverage. CO oxidation is another
reaction that has been studied quite often over different supported metal nanoparticles as
catalysts.
Figure 2.15 Comparison of measured (FT, 1 bar, 220 °C, H2/CO = 2) and modeled (SSITKA,
1.85 bar, 210 °C, H2/CO = 10) TOF [199]
.
Grass and Somorjai et al. synthesized Ru nanoparticles with size from 2 to 6 nm, which
were deposited on a silicon wafer [137]
. They found the size effect of Ru nanoparticles in
CO oxidation: larger Ru nanoparticles (6 nm) were much more active than smaller ones
(2 nm). The same group also investigated a series of SBA-15-supported Rh nanoparticles
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stabilized by PVP with different particle size between 1.9 and 11.3 nm [200]
. In the same
catalytic reaction, the catalytic behaviors of Rh particles were opposite to those of Ru
nanoparticles: for the as-synthesized catalysts without post-calcination the TOF increases
as the particle size decreases. Our group investigated TiO2-supported Au nanoparticles
with different calcination temperature leading to particle size from 3.0 nm to 4.9 nm [171]
.
Their activity decreased with an increase in particle size, which was linked to a drop in
active surface area.
2.2.7.2 Shape effect
The shape effect of the supported metal nanoparticles is similar to that of the metal
nanoparticles in quasi-homogeneous catalysis and it plays an important role in tuning
activity and selectivity. For quasi-homogeneous catalysis, the group of Somorjai studied
the shape dependence of the catalytic reduction of NO by CO catalyzed by Rh
nanopolyhedra and nanocubes [201]
. The nanocubes were found to exhibit higher TOF and
lower activation energy than the noanopolydedra. This is consistent with the conclusion
drawn from another work using Pd/MgO as catalyst that the activity depends on the
particle shape [194]
. It is due to the different facet exposed on the surface of crystal that
govern the activity, as discussed in the previous Section 2.2.4.2. For heterogeneous
catalysis, as stated in the previous Section, Lee and Zaera et al. prepared silica xerogel-
stabilized Pt nanoparticles with different shapes that were either cubic or tetrahedral [178]
.
Their catalytic performance was tested in isomerization of both cis-and trans-2-butene
and the kinetic data are shown in Figure 2.16. It shows the ratios of the initial rates for the
conversion of the trans to the cis isomer vs. those of the cis-to-trans isomerization for
catalysts made with tetrahedral and with cubic particles. The cubic particles have
comparable rates between the cis-to-trans and trans-to-cis conversions over the
calcination temperature range. In contrast, for the tetrahedral particles the rates are
different and they vary with the different calcination temperature and a switch in
selectivity is seen at slightly higher than 500 K. There is also an exception that the group
of P. D. Yang found the activity of Pt nanocrystals in ethylene hydrogenation is
independent of their size and shape [202]
.
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Figure 2.16 Kinetic data for the conversion of cis- and trans-2-butenes with hydrogen on
catalysts prepared by impregnation of 1.0 wt% Pt nanoparticles on silica xerogel as a function of
calcination temperature [178]
.
2.2.7.3 Support effect
Some early reviews on the synthesis, structure and catalytic performance of supported
metal clusters as catalysts are reported [190, 203-208]
. They focused particularly on the effect
of the support on their catalytic activity in order to establish a structure-activity
relationship. With the help of modern characterization methods (IR, XPS, XAS, HRTEM,
STM, etc.), the active sites and the morphology and electronic state of supported metals
as well as reaction intermediates are well studied. The main support effects, therefore, are
summarized by Stakheev and Kustov as (1) changes due to metal particle charging, (2)
effects related to variations in metal particle shape and crystallographic structure and, (3)
appearance of specific active sites at the metal-support boundary [209]
. The use of supports
can efficiently separate metal nanoparticles and protect them from aggregation and
sintering, which are fatal issues for the stability of the catalyst performance [128]
.
It has been found that the interaction between metals and oxide supports, so-called metal-
support interactions, are of great importance for heterogeneous catalysis [209]
. In particular,
the SMSI (Strong Metal-Support Interaction) was first suggested by Tauster et al. to
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39
explain the suppression of both H2 and CO chemisorption capacity of noble metal
particles supported on TiO2 which are reduced at high temperatures [210-211]
. Later, SMSI,
governed by both electronic factor and a geometric factor, was widely observed in many
metal/oxide catalyst systems [212-214]
. One example is the study of Pt clusters growing on
the surface of TiO2(110) under ultrahigh vacuum and high temperature using STM, in
order to identify the active sites responsible for SMSI [213]
.
There are several examples for support effects of supported metal catalysts. Sachtler et al.
studied the hydrogenolysis of neopetane (2,2-dimethylpropane) over supported Rh
catalysts using zeolite HY, NaHY and SiO2 as supports [215]
. Rh/HY showed the highest
activity due to the formation of the electron-deficient Rh particles. Researchers from
former ICI (now Johnson Matthey Catalysts) investigated the catalytic properties of Pt
supported on different supports including SiO2, Al2O3 and MoO3 [216]
. In the reaction of
the hydrogenolysis of propane, Pt/Al2O3 catalyst was the most active, whereas Both
Pt/SiO2 and Pt/MoO3 favored isomerization of butanes. The former was attributed to the
strong adsorption of alkane fragments on Pt sites of Pt/Al2O3 exhibiting δ+ polarization.
The special character of MoO3 supported Pt was due to the acidic sites on the support
providing bifunctionality. Apart from many examples using supported metal catalysts
prepared via conventional methods, very few cases were found using the colloid
deposition method to synthesize supported metal nanoparticles, except some studies
carried out in our group. A typical study is on catalytic CO oxidation using Au
nanoparticles, which are supported on different metal oxide supports, such as TiO2, Al2O3,
ZrO2, and ZnO [171]
. The catalytic results obtained by Comotti et al. are illustrated in
Figure 2.17. It is clear that Au/TiO2 is the most active one and the CO conversion reached
100% below 20 °C. Au/Al2O3 unexpectively also showed high activities after the first run,
while Au/ZnO and Au/ZrO2 was less active. Later Jia et al. used Mg(OH)2 as support,
Au/Mg(OH)2 showed high activity and the CO conversion even reached 100% at
temperatures as low as -89 °C [188]
. Another study was carried out by the same authors
using MgFe2O4 as support and it also showed high activity at low temperature [217]
. It is
discovered that pre-treatment in O2 led to the formation of O2-Au/MgFe2O4, providing
active oxygen species. However, the mechanism and support effect to Au nanoparticles
are still not clear for CO oxidation. The same approach was applied for the investigation
State of the art
40
of supported Cu nanoparticles in methanol synthesis as will be described in the following
Chapter 3.3.
a b
cd
Figure 2.17 CO conversion as a function of temperature for supported gold catalysts (a) Au/TiO2,
(b) Au/ZrO2, and (c) Au/γ-Al2O3, (d) Au/ZnO (Figures have been reorganized) [171]
.
2.2.8 Copper colloids
Cu colloids attract great interest for the investigation of the methanol synthesis reaction
from synthesis gas. As stated before in Chapter 2.1, some issues of the heterogeneous
systems still remain under debate, such as the role of Cu, active species, Cu/ZnO
interactions, etc. Therefore, the use of colloids in catalytic reactions, as described earlier,
has several advantages. Cu nanoparticles were synthesized with well-designed shape, size
and composition, which made them interesting and promising model catalyst system for
the methanol synthesis reaction. A homogenous model system for the methanol synthesis,
a quasi-homogeneous methanol synthesis, has been established, relying on the use of Cu
colloidal nanoparticles [35, 218]
. Besides catalysis in solution, Cu colloids can be
heterogenized on solid supports and then applied in the gas-phase reaction.
State of the art
41
2.2.8.1 Different types of copper colloids
Cu nanoparticles, having high thermal and electrical conductivity, are especially
attractive [219]
. As stated in the previous section, metal nanoparticles can be prepared
using different methods, including chemical reduction of metal salt, electrochemical
synthesis, thermal decomposition methods and microemulsion routes [32, 35, 132]
. These
techniques can be applied for the preparation of Cu colloids, and there are quite a few
scientific articles explaining different approaches to synthesize Cu colloids
The ‘wet chemical’ reduction has become the most common method for making metal
nanoparticles, which is a combination of metal salt precursors with both reducing agent
and stabilizing agents [132]
. Chen et al. reported the synthesis of Cu nanoparticles
protected by alkanethiolate monolayers in a one-phase system, where superhydride
(lithium triethylborohydride) was used as the reducing reagent [220]
. The Cu nanoparticles
were spherical with a diameter range of 1-2 nm. Bulky polymers, such as PVP, were
often applied as stabilizers together with reducing agents. Huang et al. synthesized Cu
nanoparticles by reducing Cu acetate with hydrazine in water and 2-ethoxyethanol and
stabilizing them with PVP [221]
. They showed that the average particle size varied from
6.6 to 22.7 nm with different amounts of PVP. The classical approach consists of
reducing transition metal salts in an organic phase. The ‘Bönnemann method’ is typical
for the preparation of metal colloids, involving the reduction of a wide range of Group
VIB, VIIB, VIII, and IB metal halide salts, including Cu(II) by tetraalkylammonium
hydrotriorganoborates (NR4(BEt3H)) in THF solution [32, 34, 114, 126, 168-169, 174, 222]
. The
authors reported the reduction of (N(octyl)4)2CuCl2Br2 in toluene using Li(BEt3H) to
form Cu colloids. The resultant Cu particle, protected by cationic surfactants (NR4+), has
a diameter ranged between 5 and 10 nm [121]
. The use of reverse micelles makes it
possible to produce Cu nanocrystals of various sizes and shapes. Pileni and coworkers
carried out intensive investigations on the reduction of aqueous Cu(II) salts within
inverse micelles [223-227]
. Cu nanoparticles were obtained through the reduction of
Cu(AOT)2 in isooctane, where bis(2-ethylhexyl) sulfosuccinate (AOT) acted as a
surfactant and hydrazine served as reducing agent. By increasing the water content, the
average size of the metallic nanocrystals increased from 3 to 13 nm, and the shape could
also be tuned. For AOT reverse micelles, Cason et al. investigated the influence of the
State of the art
42
bulk solvent type and the addition of cosolvents on the growth rate of Cu nanoparticles
[228]. The Cu particle growth rate was found to increase with increasing water content and
the addition of cosolvent.
The group of Fischer reported the preparation of highly monodispersed Cu nanoparticles
through thermal decomposition of the Cu(II) precursor Cu(OCH(Me)CHNMe2)2 in n-
hexadecylamin (HDA) without further reducing agents in a non-aqueous medium [229]
.
The Cu nanoparticles were confirmed to be well-defined, spherical particles with a
diameter of about 7.5 nm. Cu/CuxO core-shell particles were formed when exposed to air,
which could be then re-reduced by CO, as detected by IR spectroscopy [230]
. Later, in
order to study the Cu/ZnO nanocomposite and SMSI from a new point of view, the
authors established an alloyed colloidal Cu/Zn system - ‘nano brass’, using
Cu(OCH(Me)CH2NMe2)2 and Et2Zn as precursors [231-232]
. The thermolysis in the same
solvent HDA led to the formation of nanoscale, colloidal Cu/Zn alloy nanoparticles with
compositions of Cu:Zn = 95:5, 70:30, and 35:65, respectively. The nanoparticles were
almost spherical, highly monodispersed with a rather broad size distribution (5-10 nm),
depending on Cu/Zn ratio. They further synthesized novel, Cu/Zn (brass) nanoparticles
also under non-aqueous conditions, which were obtained from the co-hydrogenolysis of
the precursors CpCu(PMe3) and ZnCp*2 in mesitylene at 150 °C and 0.3 MPa H2 [232]
.
Hydrogenation of each precursor alone gives elemental Cu or Zn. The co-hydrogenolysis
of both precursors took place in the presence of poly(2,6-dimethyl-1,4-phenylene oxide)
(PPO) as surfactant. The size of the spherical particles was around 10 ± 2 nm.
2.2.8.2 Copper colloids in methanol synthesis
Although there were already reports on catalytic methanol synthesis in homogeneous
phase, using transition metal complexes such as Ru [233-234]
, Ni [235-236]
, or Rh [237]
, none of
them could represent or has any comparison with the commercial catalyst system -
Cu/ZnO/Al2O3. Therefore, it was necessary to produce and to use Cu colloidal
nanoparticles as a more relevant system to the technically used one. So far, the
homogeneous catalysis system for methanol synthesis over Cu nanoparticles has been
established and described by both our group and the groups of Fischer and Muhler [35, 218]
.
As previously mentioned, Bönnemann and co-workers established a wet chemical method
for the preparation of transition metal nanoparticles [168]
. Cu colloids can be prepared via
State of the art
43
the so-called ‘reductive stabilization’ pathway, where Cu precursors (typically
acetylacetonate) in an organic solvent (THF or toluene) are reduced by trialkylaluminium
[32, 34]. In this reaction, the triorganoaluminium compounds are employed as both the
reducing agent and colloid stabilizer [34]
. In a recent report by S. Vukojević, alkylzinc was
also discussed as a suitable stabilizer [35-36]
. Cu nanoparticles stabilized by either stabilizer
were spherical, and their particle size was between 3-6 nm with narrow distribution. The
alkylaluminium-stabilized Cu nanoparticles were all tested in methanol synthesis from
synthesis gas feed with a gas composition of H2/CO/CO2 = 86:10:4. A series of
temperature dependent tests was carried out between 140 °C and 170 °C and the final
total reaction gas pressure at the desired temperature was between 17-22 MPa. Methanol
formation already started, when the temperature increased to 130°C. The results of the
methanol synthesis reaction as determined by on-line product analysis are shown in
Figure 2.18. The productivity of the reaction system (molMeOH/(kgCu·h)) was calculated
from the slope of the linear regression of these data points. Methanol productivities
(PMeOH) reached 25.2 molMeOH/(kgCu·h) at elevated reaction gas pressure and temperature
(170 °C). Furthermore, methyl formate was also observed and the concentration of
methyl formate reached a steady state. It thus indicated that it is an intermediate rather
than a by-product, and it led to the final methanol formation via hydrogenolysis. The Cu
nanoparticles were active even without the presence of Zn species, which are thought to
improve the catalytic activity in the heterogeneous system. Alkylzinc-stabilized Cu
nanoparticles also exhibited activity in methanol synthesis according to preliminary tests
at different reaction temperatures [36]
. When these Cu nanoparticles were supported on
solid supports, they all showed some activity in methanol synthesis in the gas phase,
though the activity was only about 7% of that of the benchmark catalyst [37]
. In particular,
three different supports were applied including CMK-5, CMK-3 and activated carbon-
Rütgers KK 2099. The first two showed obviously higher activity than the latter one.
Meanwhile, Fischer and Muhler et al. reported the preparation of ZnO surface decorated
Cu nanoparticles by sequential co-thermolysis of Cu(OCH(Me)CH2NMe2)2 and
Zn(ethyl)2 in squalane [218]
. The colloidal ZnO/Cu colloids were tested as catalysts in
methanol synthesis under a continuous flow of synthesis gas feed at 220 °C and 2.6 MPa.
Their methanol productivity (ca. 4 molMeOH/(kgCu·h)) reached 84.1% of the activity level
State of the art
44
of reference catalyst Cu/ZnO/Al2O3 tested in a relevant slurry phase under the same
reaction conditions. There was no methyl formate detected, which is different to the
results from our group. The HAD/Cu colloid, prepared in HAD using
Cu(OCH(Me)CHNMe2)2 as precursor without Zn [229]
, only exhibited an activity as low
as 0.009 molMeOH/(kgCu·h). The authors then developed a one-step process to prepare Cu-
Zn colloids via the reduction of Cu and Zn stearates with H2 in a continuously operated
stirred tank reactor (CSTR) [238]
. The Cu particles, stabilized by a Zn stearate, were
spherical, well separated and their particle size was 5-10 nm. The catalytic activity of
these colloids was tested at 493 K in the same CSTR as used for the synthesis. It was
remarkable that the methanol productivity of the Cu-Zn stearate (50:50) colloid at 493 K
and 2.6 MPa reached the same level (ca. 6 molMeOH/(kgCu·h)) as that of the conventional
ternary Cu/ZnO/Al2O3 catalyst, which was applied as a fine powder in the CSTR.
The results of these previous studies from both groups lead us to deeper investigations on
the interesting Cu colloids and their application in methanol synthesis, since many
aspects, such as the structure and formation mechanism of Cu colloids, their structure-
activity relationship, the reaction mechanism, the active sites involved in the catalytic
reaction, etc. are still unclear.
Figure 2.18 Formation of methanol at temperatures between 140 and 170 °C over Al(n-octyl)3-
stabilized Cu colloids. Inset: Arrhenius plot for the determination of the apparent activation
energy [35]
.
Results and discussion
45
3 Results and discussion
Cu colloid-based catalysts were systematically investigated, from their structural and
chemical features to their catalytic proprieties in methanol synthesis from synthesis gas
feed. The results are presented in three main parts:
1. Part one (Chapter 3.1) describes the chemistry and structures of different Cu
colloids. It includes detailed studies on their synthesis, formation mechanism and
stability.
2. Part two (Chapter 3.2) focuses on the catalytic performance of the Cu colloids in a
quasi-homogeneous phase. The structural changes of Cu colloids during reaction
were characterized using various techniques. Furthermore, the results of a series
of experiments to elucidate structure-activity relationships are reported.
3. Part three (Chapter 3.3) concentrates on the heterogeneous system - supported Cu
nanoparticles. The catalytic properties of these supported Cu catalysts were
studied in a gas-phase reaction. A possible influence of the support on the
catalytic activity of Cu nanoparticles was also studied.
Results and discussion
46
3.1 Metal alkyl-stabilized copper colloids
The previous research on Cu colloid catalysis reported the activities of the Cu colloids
that were stabilized by Al(n-octyl)3 and Zn(n-butyl)2. In this thesis, a series of Cu colloids
was successfully prepared via the same chemical reduction pathway. In addition, two new
alkylaluminium or alkylzinc compounds, i.e. Al(n-butyl)3 and Zn(ethyl)2, were used as
stabilizers. The size, shape and composition of all four types of Cu colloids were
characterized by techniques, such as TEM, UV-Vis, XRD, XAS, etc. Moreover, in situ
XAS techniques at both room temperature and low temperature (down to -30 °C) were
employed to follow the formation of Cu colloids during the reduction process; a possible
colloid formation pathway was proposed. Furthermore, the stability of Cu colloids under
Ar storage was also investigated.
Results and discussion
47
3.1.1 Synthesis
The colloid preparation method - Bönnemann route - was established by Bönnemann and
co-workers for the production of stable transition metal nanoparticles, such as Ag, Pt and
Cu [34]
. This method is a single step synthesis via the ‘reductive stabilization’ pathway,
where the metal salt precursors, such as metal acetylacetonate in solution, are reduced by
alkylaluminium. In this method, the alkylaluminium serves as both a reducing agent and a
stabilizer; it eliminates the need of another stabilizing surfactant, which is normally used
for most of the colloid synthesis protocols (see Section 2.2.3).
Cu colloids were successfully synthesized by S. Vukojević using Al(n-octyl)3 and Zn(n-
butyl)2 as stabilizers in the previous study [35-36]
. In order to further investigate the
influence of the stabilizer on the formation of the Cu colloid, the stabilizer type was
extended to similar compounds with different ligand chain length including Al(n-butyl)3
and Zn(ethyl)2. The synthesis procedure is shown in Figure 3.1, where copper
acetylacetonate (Cu(acac)2) was simultaneously reduced and stabilized by
alkylaluminium (Al(n-butyl)3, Al(n-octyl)3) or alkylzinc (Zn(ethyl)2, Zn(n-butyl)2) in
anhydrous THF under Ar protection. During the dropwise addition of the metal alkyls,
the color of the solution changed from blue to deep red. The metal alkyl compounds form
an organometallic protecting shell around the Cu core, and it had been proven in earlier
studies that a direct ligand exchange with the ligands of Cu(acac)2 takes place during
colloid formation [239]
. The syntheses of all the Cu colloids were repeated several times
and found to be highly reproducible.
Figure 3.1 Synthesis of Cu colloids via simultaneous reduction and stabilization of Cu(acac)2 by
metal alkyl compounds.
Results and discussion
48
The Cu colloids are named according to the different stabilizer types employed in the
synthesis as listed in Table 3.1, which do not imply the exact types of ligands contained
in the stabilizing shell of Cu particles after the colloid formation. To ensure the
stabilization of the colloids, based on our previous studies the molar ratios of the
stabilizer to the Cu precursor during the actual preparation were set to be high: in the case
of alkylaluminium-stabilized colloids, the Al to Cu ratio is 3/1, whereas in the case of
alkylzinc, the Zn to Cu ratios are even higher - 10/1-20/1. This is based on our previous
investigation that the Cu colloids were better stabilized when the Zn/Cu ratio was high, as
S. Vukojević had also used a Zn/Cu ratio of 9/1 in his study [36]
. According to the amount
of the Cu precursor in the colloids, the calculated Cu concentration was 0.82 mg/mL for
the alkylaluminium-stabilized colloids and 0.58 mg/mL for the alkylzinc-stabilized ones.
The actual concentrations of Cu, Al and Zn in the Cu colloid solution were determined by
inductively-coupled plasma (ICP) elemental analysis, as listed in Table 3.1.
Table 3.1 Four different Cu colloids and their metal element concentrations.
Sample
name
Stabilizer
type
Cu
concentration
Al
concentration
Zn
concentration
(mg/mL) (mg/mL) (mg/mL)
Cu/TBAl Al(n-butyl)3 0.75, 0.76 2.02, 2.08 X
Cu/TOAl Al(n-octyl)3 0.73, 0.79 1.06, 1.10 X
Cu/DEZn Zn(ethyl)2 0.17, 0.21, 0.24 X 5.41, 5.63, 6.66
Cu/DBZn Zn(n-butyl)2 0.54, 0.61, 0.66, 0.70 X 6.81, 6.87, 7.33, 9.00
The actual Cu concentrations in both Cu/TBAl and Cu/TOAl are 0.76 mg/mL in average,
which is quite close to the calculated value of 0.82 mg/mL from the preparation. The Cu
concentration of the Cu/DBZn is 0.63 mg/mL in average, also similar to the calculated
value. Moreover, the actual ratio of Al/Cu in Cu/TBAl and the ratio of Zn/Cu in
Cu/DBZn are 2.7/1 and 11.9/1, respectively. These data suggest that results obtained by
ICP are very close to the calculated values. In contrast, the actual ratio of Al/Cu in
Cu/TOAl is 1.4/1, much lower than the calculated value (3/1). Cu/DEZn has much lower
Results and discussion
49
Cu concentration (0.21 mg/mL in average) than the calculated value, and also the actual
ratio of Zn/Cu in Cu/DEZn is 28.1/1, much higher than the calculated value (10/1). This
is in line with the fact that there were more dark precipitates at the bottom of the flask
after the synthesis was completed.
3.1.2 Characterization
3.1.2.1 TEM analysis
TEM is the most widely used technique for characterizing metal colloids. It provides
direct visual information on the size, shape and dispersion of nanoparticles. The TEM
images in Figure 3.2 show that the Cu colloidal nanoparticles stabilized by different
metal alkyls are all spherical in shape and well dispersed in THF solution. A statistical
analysis of the particle size was carried out by counting ca. 200 particles from the TEM
images of each Cu colloid. It is revealed that each Cu colloid has a narrow size
distribution. The Cu particle size is tuned in the range of 3-6 nm, depending on the
different metal alkyl stabilizers applied. The particle size of Cu/TBAl and Cu/DBZn is 3-
4 nm, smaller than that of Cu/TOAl and Cu/DEZn. Parallel to the work on this thesis,
research by A. Kempter et al. [240]
has attempted to tune the Cu particle size by varying
the preparation parameters, including the reaction temperature, Cu precursor and
alkylaluminium compounds. However, it was shown that the chain length of the
alkylaluminium compound as well as the reaction temperature did not significantly
influence the particle formation. In contrast, the use of different Cu precursors did allow
tuning of the resultant Cu particle size in a narrow range - between 4 and 8 nm. It was
postulated, that the steric demand of the ligands at the Cu precursor has an impact on the
final particle size [240]
.
Results and discussion
50
0
40
80
0
20
40
0
20
40
Fre
qu
en
cy
[%
]
0 2 4 6 8 10
0
40
80
Particle size [nm]
(c)
(d)
(a)
(b) 0
40
80
0
20
40
0
20
40
Fre
qu
en
cy
[%
]
0 2 4 6 8 10
0
40
80
Particle size [nm]
(c)
(d)
(a)
(b)
Figure 3.2 TEM images of Cu colloids stabilized by different metal alkyls and their estimated
particle sizes (taking ca. 200 particles): (a) Cu/TBAl, (b) Cu/TOAl, (c) Cu/DEZn, (d) Cu/DBZn
and their particle size distribution.
HRTEM allows a closer inspection of the Cu nanoparticles. As shown by the HRTEM
images (Figure 3.3 (a) and (b)) of freshly prepared Cu colloids, some of the colloidal Cu
nanoparticles only contain a single crystalline domain, whereas most of the larger
Results and discussion
51
particles are polycrystalline. These Cu nanocrystals are likely to be agglomerates formed
from differently oriented single crystallites. One particle having a single crystalline
domain is focused in Figure 3.3 (c), where the lattice fringe distance is ca. 2.1 Å,
corresponding to the Cu(111) distance. Interestingly, Cu(111) is also one of the possible
exposed Cu surface facets in a solid industrial catalyst for methanol synthesis [241]
.
(a)
(b)
(c)
Figure 3.3 HRTEM images of Cu colloids stabilized by different metal alkyls: (a) Cu/TOAl; (b)
Cu/DBZn; (c) one crystallized particle from Cu/DBZn.
3.1.2.2 UV-Vis measurements
Metal clusters exhibit absorption bands or broad regions of absorption in the ultraviolet-
visible range [242]
, which is due to the excitation of surface plasmons (SP). The frequency
(a)
Results and discussion
52
and intensity of the SP absorption bands are sensitive to the metallic nature, size and
shape of the nanoparticles [242-245]
. Some metal clusters have distinct absorption bands in
the visible region, such as Au, Ag and Cu [223, 227, 244, 246-247]
. The absorption spectra of
these metal nanoparticles are not strongly dependent on the particle size within the 3-20
nm diameter size range [242]
.
Figure 3.4 displays the UV-Vis absorption spectra of Cu/TOAl and Cu/DBZn. The SP
absorption appears at ca. 560 nm for Cu/TOAl, which is characteristic of Cu colloids
containing metal nanoparticles with diameters of between 5 and 10 nm [121, 223, 227, 230, 242]
.
In contrast, the same absorption does not seem to exist for the Cu/DBZn. In the former
study by S. Vukojević [36]
, it was shown that when the Zn/Cu ratio was as low as 4/1, the
absorption for Cu/DBZn existed but was much less pronounced compared to the one for
Cu/TOAl and Cu/TBAl. According to the extended Mie theory, particle size has impact
on the absorption position and peak broadening [248-249]
. For Cu/DBZn, Cu particle size (4
nm) is smaller than that of Cu/TOAl (5 nm), which causes a decrease in intensity of the
plasmon band but an increase in its broadening at 560 nm [223, 227, 248]
. Furthermore,
another study by Kalidindi et al. [250]
demonstrated that ZnO colloids showed no
absorption, whereas Cu@ZnO core-shell nanocomposite, which had similar structure as
the sphere studied here, exhibited a weak absorption at 580 nm. Thus, it seems that the
interaction between Cu and Zn also influences the plasmon band of Cu particles.
Therefore, the lack of absorption of Cu/DBZn could be due to its smaller particle size and
also to the effect of the alkylzinc stabilizing shell on the Cu surface, considering that the
Zn/Cu ratio is much higher at 10/1 than the 4/1 ratio studied previously. Similar
observations were reported by Fischer et al. in the UV-Vis study of ZnO surface
decorated Cu nanoparticles of 1-3 nm size [218]
.
The oxidation behavior of Cu colloids has been studied by exposing the Cu colloids to air
for one hour and the UV-Vis spectra were collected every 5 min. Figure 3.5 shows that
the initial absorption band of Cu/TOAl undergoes a gradual red-shift from 560 nm to ca.
600 nm with increasing oxidation time and remains unchanged after 50 min. Meanwhile,
the color of the colloid turned from deep red to dark green, corresponding to oxidized Cu
particles. The same observation of a shift of the absorption maximum to higher
Results and discussion
53
wavelengths was also recorded by Schröter et al. in an in situ study of the oxidation of Cu
colloids (Cu-HAD and Cu-PPO) [230, 232]
400 450 500 550 600 650 700 750 800
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Ab
so
rpti
on
[a.u
.]
Wavelength [nm]
Cu/TOAl
Cu/DBZn560 nm
Figure 3.4 UV-Vis spectra of Cu/TOAl and Cu/DBZn.
.
400 450 500 550 600 650 700 750 8000.0
0.5
1.0
1.5
2.0
2.5
3.0
Ab
so
rpti
on
[a
.u.]
Wavelength [nm]
0 min
5 min
10 min
15 min
20 min
25 min
30 min
35 min
40 min
45 min
50 min
55 min
60 min
560 nm
(0-5 min)
600 nm
(50 min)
Figure 3.5 UV-Vis spectra of Cu/TOAl during oxidation in air.
In contrast, the absorption band for the alkylzinc-stabilized Cu colloid (see Figure 3.6)
becomes more pronounced with increasing oxidation time. A slight red-shift occurs to the
absorption from ca. 550 nm to ca. 570 nm, and then the band maximum remains constant
Results and discussion
54
after 45 min. During oxidation the color of the Cu colloid changed from deep red to
brownish. The Cu nanoparticles might not have been completely oxidized, but the
alkylzinc surrounding the Cu core could have been oxidized to ZnO. This would explain
the band shift, since in another study, for Cu/ZnO the band position was reported to be at
580 nm [218, 250]
.
It should also be noted that for both Cu colloids during the first 5-10 min of the oxidation
period the absorption band does not change, which demonstrates that the metal alkyl
protecting shell prevents the Cu core from being oxidized for a short time. The results at
longer reaction time suggest that particularly for Cu/TOAl the Cu core is accessible to air
and is eventually oxidized. However, it might also be possible that for the
akylaluminium-stabilized Cu colloid, the stabilizing shell first reacts with oxygen
molecules. In comparison, for Cu/DBZn the alkylzinc might be oxidized at first to form
Cu/ZnO particles, and the ZnO layer could be relatively dense. The alkylzinc-stabilized
Cu colloids would then be more stable in air.
400 450 500 550 600 650 700 750 800
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Ab
so
rpti
on
[a
.u.]
Wavelength [nm]
0 min
5 min
10 min
15 min
20 min
25 min
30 min
35 min
40 min
45 min
50 min
55 min
60 min
570 nm
(45 min)
Figure 3.6 UV-Vis spectra of Cu/DBZn during oxidation in air.
3.1.2.3 XRD measurements
X-ray diffraction can be used to determine the crystallographic structure of the Cu
nanoparticles. The XRD patterns (Figure 3.7) of four Cu colloids show a similar low-
intensity reflection at 2θ = 19.7°. It is the only obvious reflection and it corresponds to
the Cu(111) planes. An advantage of XRD is that it provides a very simple method to
Results and discussion
55
estimate the crystalline domain size from the broadening of the XRD reflections based on
the Scherrer formula (Equation 3.1) [251-252]
:
cos2ω
Kλd (3.1)
where d is the particle size, λ is the wavelength of the radiation, θ is the angle of the
considered Bragg reflection, ω is the width on a 2θ scale, and Κ is a constant close to
unity [253]
. The average size of the single crystalline domain of all the four Cu colloids
based on the (111) reflection was estimated to be 3 ± 1 nm. Due to such small crystalline
domain size, the reflections become significantly broadened and the intensities are low. It
has to be noticed that the reflection in the XRD pattern for Cu/TBAl is broader and shifts
to the right, compared to the other three patterns. This broader reflection could be due to
the overlap of more than one phase, so it may not correspond to the pure Cu [254]
. Due to
the low intensity of the reflection, it is difficult to conclude on the identification of other
possibly existing phases. The results from XRD are roughly consistent with the
observation and the estimation of particle size from the TEM. One should bear in mind
that calculation from XRD only provides the crystalline domain size, which is usually
smaller than the Cu particle size determined by TEM, since most of the Cu nanoparticles
shown by TEM are agglomerates of several single crystals.
15 20 25 30 35 40
(d) Cu/DBZn
(c) Cu/DEZn
(b) Cu/TOAl
Inte
ns
ity
[a
.u.]
(a) Cu/TBAl
Figure 3.7 XRD patterns (Mo-radiation 0.7093Å, transmission mode under Ar protection) of Cu
colloids stabilized by different metal alkyls.
Results and discussion
56
3.1.2.4 XAS analysis
The X-ray absorption fine structure (XAFS) analysis is a powerful tool to investigate the
local structural and chemical properties around the selected type of atom. The X-ray
absorption spectrum is typically divided into two regimes: X-ray absorption near-edge
spectroscopy (XANES) and extended X-ray absorption fine-structure spectroscopy
(EXAFS). The XANES is highly sensitive to oxidation state and coordination
environment of the absorbing atom and is frequently used as a fingerprint of atomic
species. In contrast, the EXAFS is used to determine the distances and coordination
number of the absorbing atom, as well as the species of its neighbors [255]
. In this study,
the X-ray absorption spectroscopic data for all the Cu colloids were recorded at the Cu K-
edge. For those colloids stabilized by alkylzinc, measurements at the Zn K-edge were
also carried out.
The XANES spectra in Figure 3.8 for all the Cu colloids display typical shoulder
absorption (α) in the pre-edge region at ca. 8.981 keV, corresponding to metallic Cu. By
comparing with Cu foil as reference, this feature reveals that Cu species of the different
Cu colloids were all reduced from Cu(II) of the precursor to Cu(0); no CuxOy species
were detected. Besides, the XANES region is also highly sensitive to size and geometry
of nanoparticles, since this range is dominated by multiple scattering effects within the
nearest neighbor coordination [256]
. A double peak feature is marked as (β) and (γ) at the
rising edge, which is unique for zero-valent state Cu foil. This feature is present for
Cu/TOAl, Cu/DEZn and Cu/DBZn. Bazin et al. reported the XANES of Cu clusters of
different sizes and found that the Cu cluster of 13 atoms did not show the resonance (γ)
while those of 55 atoms presented the double peak feature [257]
. Because the double
feature is size dependent and Cu/TBAl, as seen from Figure 3.8, does not show the
double peak feature, it is clear that it has the smallest particle size. In contrast, Cu/DEZn
with higher intensity than the other colloids has larger particle size. This is in good
agreement with the observation from TEM (Figure 3.2) that Cu/TBAl has the smallest
particle size and Cu/DEZn has the largest one.
Figure 3.9 (a) shows the EXAFS χ(k) for all the Cu colloids in comparison with that of
the Cu foil as reference. The same frequency is obtained for all the Cu colloids, but the
amplitudes are lower than that of Cu foil. The atom coordination number is directly
Results and discussion
57
correlated with the EXAFS amplitude [256-257]
. In a small particle, only atoms of inner
shells have the bulk coordination number, and those located in outer shell or at the
particle surface have smaller numbers of neighboring atoms. Therefore, the lower
amplitude of the Cu colloids is a signal of the smaller Cu particle size in the colloids [35,
106, 258]. The amplitude of Cu/TBAl is the lowest corresponding to its smallest particle size.
This conclusion is in agreement with the results from TEM and XRD discussed above.
8.94 8.96 8.98 9.00 9.02 9.04 9.06
0.0
0.4
0.8
1.2
1.6
2.0
Ab
so
rpti
on
[a.u
.]
Cu foil
Cu/TBAl
Cu/TOAl
Cu/DEZn
Cu/DBZn
()
()()
Figure 3.8 XANES at Cu K-edge for Cu colloids with Cu foil as reference.
0 2 4 6 8
0.000
0.001
0.002
0.003
0.004
0.005
0.006
2 4 6 8 10 12
0.0
Cu foil
Cu/TBAl
Cu/TOAl
Cu/DEZn
Cu/DBZn
FT
(k
1-w
eig
hte
d)
R [Å]
Cu foil
Cu/TBAl
Cu/TOAl
Cu/DEZn
Cu/DBZn
k [Å-1]
(k
) [a
.u]
(a) (b)
Figure 3.9 XAS spectra at Cu K-edge for Cu colloids with Cu foil as reference:
(a) EXAFS function in terms of k1-weighted χ(k)-function; (b) radial distribution functions
(Fourier-transformed k1-weighted EXAFS spectra).
E [keV]
Results and discussion
58
The reduced Cu state is confirmed by analysis of the Fourier-transformed EXAFS spectra
shown in Figure 3.9 (b). The spectra provide direct information of the first Cu-Cu
coordination shell and the Cu-Cu distance is ca. 2.53 Å for the Cu colloids, which is
slightly shorter than that of bulk Cu (2.55 Å), a known effect for small particles due to the
particle size effect [259]
. No oxygen shells (Cu-O distance at 1.85 Å) are detected for any
of the colloids, as compared with literature [260-261]
. From the analysis of the EXAFS
information on interatomic distances, structural disorder and number and kind of
neighboring atoms at a given distance can be obtained [262]
. It further provides
information on the size of the cluster. The estimated Cu atom coordination numbers for
all the Cu colloids are listed in Table 3.2, where the errors for the coordination number
and Cu-Cu distance are ± 0.5 and ± 0.005 respectively. The values of Cu coordination
number are in the range of 8 to 12. Except that of Cu/TBAl, the others are all lower than
the coordination number of Cu atoms in the bulk fcc structure, which is 12. The
coordination number range observed corresponds to Cu particle sizes of 2-4 nm [106, 263]
.
The coordination number of Cu/DBZn is smaller than those of the others, in good
agreement with the TEM analysis that the particle size of Cu/DBZn is smaller. The
results from XAS are generally consistent with the range of particle sizes determined by
both TEM and XRD. However, Cu/TBAl is an exception in XAS analysis: here a
coordination number of 12 was extracted from the spectra, which would suggest the
presence of large particles, in contrast to the TEM and XRD data. The reason for this
result could not be clarified.
Table 3.2 Structural parameters obtained from EXAFS data fitting for all the Cu colloids.
Sample
name Stabilizer type
Coordination
number
Cu-Cu
distance (Å)
Cu/TBAl Al(n-butyl)3 12 2.54
Cu/TOAl Al(n-octyl)3 9.8 2.53
Cu/DEZn Zn(ethyl)2 9.6 2.55
Cu/DBZn Zn(n-butyl)2 8.2 2.53
Results and discussion
59
XAS was also performed at the Zn K-edge for the alkylzinc-stabilized Cu colloids in
order to investigate the nature of the stabilizing shell. Both XANES and Fourier-
transformed EXAFS spectra are illustrated in Figure 3.10 with Zn foil, ZnO, as well as
alkylzinc in THF solution as references. The XANES in Figure 3.10 (a) displays
pronounced absorptions at ca. 9.660-9.662 keV in the pre-edge region for both alkylzinc-
stabilized Cu colloids and alkylzinc in THF solution, which for the other references are
absent. In addition, the spectra of alkylzinc-stabilized Cu colloids are different compared
with the spectra of Zn foil and ZnO and resemble significantly the corresponding
alkylzinc precursor, indicating that Zn could be present in a more complex state in the
organometallic protecting shell. Instead of the significant increase of the contribution at
2.9 Å, a shift to 2.7 Å is found in the alkylzinc-stabilized colloids (Figure 3.10 (b)),
which may be due to the contribution of some metallic Zn. However, a closer inspection
shows differences between the alkylzinc stabilizing shell and ZnO: the higher shells of
alkylzinc stabilizing layer are clearly missing. This result is very close to that for the
ZnO/Cu colloid that showed in EXAFS a phase that was close to the ZnO bulk structure
without exhibiting distinct long-range order [218]
. As explained in S. Vukojević’s thesis, it
was suggested by analytical data that organoaluminium groups including Al-CH3, Al-
C2H5 and Al-acac were present, due to the ligand exchange in the protecting shell, and
possibly Al-O also existed [36, 239]
. Therefore, when using alkylzinc as stabilizer, there
could also be Zn-acac forming, according to the similar Zn-O distance as in ZnO, which
is seen in the EXAFS, Figure 3.10 (b), at the Zn K-edge.
A comparable Cu/ZnO nanoparticle system has been investigated by Fischer, Schröter,
Müller et al. [218, 264]
. They stated that Zn2+
was coordinated by O2-
in a tetrahedral fashion
in the first shell, similar to ZnO, but a ZnO phase was not present. Thus, zinc is most
likely to exist in oxidized state. However, more investigations are required to identify the
interactions between metal alkyl stabilizing shell and the Cu core.
Results and discussion
60
9.60 9.65 9.70 9.75 9.80 9.85
0.0
0.4
0.8
1.2
1.6
2.0
2.4
0 1 2 3 4 5 6 7 8
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
Ab
so
rpti
on
[a.u
.]
E [keV]
Cu foil
ZnO
Zn(ethyl)2 in THF
Zn(butyl)2 in THF
Cu/DEZn
Cu/DBZn
FT
(k
3-w
eig
hte
d)
R [Å]
Zn foil
ZnO
Zn(ethyl)2 in THF
Zn(butyl)2 in THF
Cu/DEZn
Cu/DBZn
(a) (b)
Figure 3.10 XAS spectra at Zn K-edge for Cu colloids with Zn foil, ZnO and alkylzinc in THF
solution as references: (a) XANES; (b) radial distribution functions (Fourier-transformed k3-
weighted EXAFS spectra).
3.1.3 Copper colloid formation
Since the structure of the metal alkyl-stabilized Cu colloids is reasonably well understood,
it appeared interesting to further explore their properties by examining their formation
during the reduction process. In situ XAS at room temperature for such processes has
been well developed by J.-D. Grunwaldt. His technique was adapted in our previous
study to alkylaluminium-stabilized Cu colloids, and it was proven to be a suitable tool for
the investigation of Cu colloids [106, 240, 265-266]
. In the following section, in situ XAS to
study the formation of alkylzinc-stabilized Cu colloids is discussed. Furthermore, in situ
XAS experiments at low temperature (down to -30 °C) were designed by M. Bauer [267]
(refer to Section 5.2.5.4) and were used to follow the formation of all four Cu colloids in
order to reveal their formation mechanism and possible intermediates.
3.1.3.1 In situ XAS measurements at room temperature
The formation of the alkylaluminium-stabilized Cu colloid at room temperature by in situ
XAS had already been studied by S. Vukojević and J.-D. Grunwaldt. It was concluded
that at room temperature the Cu(II) was reduced directly to Cu(0); no Cu(I) intermediate
Results and discussion
61
species could be detected [36, 268]
. Following the previous report, in this thesis, in situ XAS
was performed using alkylzinc, typically Zn(n-butyl)2, as stabilizer for a supplemental
study in order to compare with the colloid formation using alkylaluminium as stabilizer.
As displayed in Figure 3.11 (a), with reduction time the whiteline representing Cu(II)
continuously decreases, while the pre-edge absorption at 8.981 keV, attributed to Cu(0),
increases. There does not seem to be any Cu(I) present, similar to using Al(n-octyl)3 in
the previous study [36]
. A linear combination of the in situ XANES data in Figure 3.11 (b)
further confirms that the reduction process only involves Cu(II) and Cu(0) without any
Cu(I) species as intermediates. It shows that when using Zn(n-butyl)2 as stabilizer the
reduction process was completed within only 15 min, which is shorter than ca. 60 min in
the case of using Al(n-octyl)3. At the end of the measurement, the Zn to Cu molar ratio
was ca. 10/1, the same as in normal alkylzinc-stabilized Cu colloids.
Another interesting measurement, using in situ XAS during reduction of solid binary
Cu/ZnO catalysts at higher temperature, was also carried out in our group by Kiener et al.
[269]. It showed that there was a small fraction (<10%) of Cu(I) species during the fast
reduction. However, this study was carried out with solid samples and the Cu(I) species
might be more stable in a solid phase than in the liquid phase. Hence, during reduction in
solution, Cu(I) was not detectable.
8.96 8.98 9.00 9.02 9.04 9.06
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0.0 2.5 5.0 7.5 10.0 12.5 15.00.0
0.2
0.4
0.6
0.8
1.0
Ab
so
rpti
on
[a
.u.]
E [keV]
0 min
2.5 min
5 min
7.5 min
10 min
12.5 min
15 min
Fra
cti
on
Time [min]
Cu(II)
Cu(0)
(a) (b)
pre
-ed
ge
wh
itelin
e
Cu(0)
Cu(II)
Figure 3.11 (a) In situ XANES at the Cu K-edge during reduction of Cu(acac)2 in THF by Zn(n-
butyl)2 under Ar; (b) the corresponding concentrations of oxidized and reduced Cu species in
solution as determined by linear combination analysis.
Results and discussion
62
3.1.3.2 In situ XAS measurements at low temperature
Even though in situ XAS at room temperature did not detect any Cu(I) intermediate
species, the possibility that they might exist during a short period should not be neglected.
This is due to the fact that during the synthesis there was very short moment when the
color of the synthesis solution after adding the reducing agent turned to light yellow and
that during synthesis at very low temperature (-30 °C) this color was maintained for a
longer period [240]
. This could be an indication that there are intermediates during colloid
formation. Therefore, in situ XAS was carried out at low temperature (down to -30 °C)
during the reduction, following the same procedure as for the measurements at room
temperature.
Figure 3.12 to Figure 3.15 display the in situ XANES spectra at the Cu K-edge and the
linear combination of Cu species during the reduction process using the four different
stabilizers at lower temperature. However, the low temperature could not be held for a
long time and it gradually increased to room temperature during the measurement.
Similar to the in situ XANES spectra obtained at room temperature, for all of the
stabilizers, the whiteline for Cu(II) decreases with increasing pre-edge at 8.981 keV,
attributed to Cu(0). The linear combinations of the corresponding XANES data show that
the intermediate spectra can be well reconstructed from the spectra of Cu(II) and Cu(0).
No Cu(I) was detected during the reduction even at such a low temperature, which was
not as expected judging from the color observation during synthesis.
Interesting information obtained from the linear combinations is that different stabilizers
provide different reduction abilities and that the difference between the alkylaluminium
and alkylzinc is evident. For those colloids stabilized by alkylaluminium, shown in
Figure 3.12 (b) and Figure 3.13 (b), the Cu(0) fraction reaches 100% when the
temperatures increase to between 5 °C and 15 °C. It has to be noticed that in Figure 3.13
(b) there seems to be an overlap of measurements, it was due to more measurements
recorded at 5 °C because that temperature was maintained longer. In the case of using
Al(n-butyl)3 as reducing agent, it seems that Cu(II) was reduced to Cu(0) when the
temperature was lower (at ca. 5 °C) and the reduction process also took place faster.
Al(n-butyl)3 is probably more reactive than Al(n-octyl)3 as reducing agent.
Results and discussion
63
8.950 8.975 9.000 9.025 9.050
5
10
15
20
25 0.00
0.29
0.58
0.87
1.16
E [keV]
Ab
so
rptio
n [a
.u.]
-10 -5 0 5 10 15 200.0
0.2
0.4
0.6
0.8
1.0
1.2
Fra
cti
on
Temperature [°C]
Cu(0)
Cu(II)
Sum
(a) (b)
E [keV] Temperature [°C]
Ab
so
rptio
n [a
.u.]
Fra
cti
on
Figure 3.12 (a) In situ XAS spectra at the Cu K-edge during the reduction of Cu(acac)2 in THF
by Al(n-butyl)3 as stabilizer at lower temperatures under Ar; (b) the corresponding concentrations
of Cu species in solution as determined by linear combination analysis.
8.950 8.975 9.000 9.025 9.050
5
10
15
20
25
30
35 0.00
0.46
0.92
1.38
1.84
E [eV]
Ab
so
rptio
n [a
.u.]
-5 0 5 10 15 20
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Fra
cti
on
Temperature [°C]
Cu(0)
Cu(II)
Sum
E [keV] Temperature
Ab
so
rptio
n [a
.u.]
Fra
cti
on
(a) (b)
Figure 3.13 (a) In situ XAS spectra at the Cu K-edge during the reduction of Cu(acac)2 in THF
by Al(n-octyl)3 as stabilizer at lower temperatures under Ar; (b) the corresponding concentrations
of Cu species in solution as determined by linear combination analysis.
In Figure 3.14 (b) and Figure 3.15 (b) the linear combinations are shown for the Cu
colloids using alkylzinc as stabilizers. Apparently, the full Cu(0) composition is already
achieved at the very beginning of the experiments even when the temperature is as low as
-26 °C. This indicates that alkylzinc stabilizers are more reactive for reduction reactions
than alkylaluminium compounds, and the reduction processes using alkylzinc are
completed within a much shorter time.
Results and discussion
64
8.950 8.975 9.000 9.025 9.050
10
20
30
40
50
60 0.00
0.41
0.82
1.23
1.64
Ab
so
rptio
n [a
.u.]
-20 -10 0 10
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Temperature [°C]
Cu(0)
Cu(II)
Sum
(a) (b)
E [keV] Temperature [°C]
Ab
so
rptio
n [a
.u.]
Fra
cti
on
Figure 3.14 (a) In situ XAS spectra at the Cu K-edge during the reduction of Cu(acac)2 in THF
by Zn(ethyl)2 as stabilizer at lower temperatures under Ar; (b) the corresponding concentrations
of Cu species in solution as determined by linear combination analysis.
8.950 8.975 9.000 9.025 9.050
5
10
15
200.00
0.59
1.18
1.77
2.36
-20 -10 0 10 20
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Cu(0)
Cu(II)
Sum
E [keV] Temperature [°C]
Ab
so
rptio
n [a
.u.]
Fra
cti
on
(a) (b)
Figure 3.15 (a) In situ XAS spectra at the Cu K-edge during the reduction of Cu(acac)2 in THF
by Zn(n-butyl)2 as stabilizer at lower temperatures under Ar; (b) the corresponding concentrations
of Cu species in solution as determined by linear combination analysis.
In Figure 3.16 the change of prepeak intensity with increasing temperature is plotted. The
prepeak intensity level, extracted from the XANES spectrum of the formed Cu colloid at
the end of each in situ measurement, is marked in blue in each case. This level is
correlated to the completion of the Cu reduction during the colloid formation. The
marked final prepeak intensities appear at the end for the alkylaluminium-stabilized Cu
colloids, as shown in Figure 3.16 (a) and (b). It means that the Cu(0) was formed when
the temperature was approaching 20 °C. In contrast, for the alkylzinc-stabilized colloids
displayed in Figure 3.16 (c) and (d), the general prepeak intensities are higher throughout
Results and discussion
65
the whole reduction process, while the final prepeak intensities stay at a lower level. This
indicates that, using alkylzinc as stabilizer, the reduction was already completed at much
lower temperatures than in the case of using alkylaluminium. The conclusions obtained
from the prepeak intensities are consistent with these from the linear combination
analysis explained above.
-15
-10
-5
0
5
10
15
20
25
Te
mp
era
ture
[°C
]
0.30
0.35
0.40
0.45
0.50
0.55
Pre
pea
k i
nte
nsit
y [
a.u
.]
-15
-10
-5
0
5
10
15
20
25
Te
mp
era
ture
[°C
]
0.30
0.35
0.40
0.45
0.50
0.55
Pre
pea
k i
nte
nsit
y [
a.u
.]
0.1
0.2
0.3
0.4
0.5
-10
-5
0
5
10
15
20
25
0.1
0.2
0.3
0.4
0.5
-10
-5
0
5
10
15
20
25
0.30
0.35
0.40
0.45
0.50
0.55
0.60
-30
-20
-10
0
10
20
0.30
0.35
0.40
0.45
0.50
0.55
0.60
-30
-20
-10
0
10
20
0.46
0.48
0.50
0.52
0.54
0.56
0.58
0.60
0.62
0.64
-20
-10
0
10
20
0.46
0.48
0.50
0.52
0.54
0.56
0.58
0.60
0.62
0.64
-20
-10
0
10
20
Pre
peak
inte
nsit
y[a
.u.]
Pre
pea
kin
ten
sit
y[a
.u.]
Pre
pea
kin
ten
sit
y[a
.u.]
Pre
peak
inte
nsit
y[a
.u.]
Tem
pera
ture
[°C
]T
em
pera
ture
[°C
]
Te
mp
era
ture
[°C
]T
em
pera
ture
[°C
]
(a) (b)
(c) (d)
Figure 3.16 Prepeak intensity and temperature change during in situ XAS measurements using
four different stabilizers for reduction: (a) Al(n-butyl)3; (b)Al(n-octyl)3; (c)Zn(ethyl)2; (d)Zn(n-
butyl)2. The final prepeak intensity is marked by the blue dashed lines.
Results and discussion
66
There is no Cu(I) species detected during reduction as shown by the results from the in
situ XAS at lower temperatures, which is identical to the conclusion from the in situ
experiments at room temperature. The light yellow color observed during the synthesis at
low temperature could not be linked to the existence of Cu(I) at least not at the detection
level of the analysis. In order to exclude the presence of a Cu(I) intermediate at lower
concentration, more advanced experiments would be required.
3.1.4 Copper colloid stability
Considering that the Cu colloids are highly sensitive to air, they must be stored under the
protection of Ar atmosphere. When in contact with air or water, the colloids are oxidized
or decomposed. Therefore, it is worth evaluating their stability, because their catalytic
activities might be affected by possible oxidation and/or decomposition. After storage for
two months up to two years, the color of the alkylaluminium-stabilized colloids still
remained dark red as that of the freshly prepared ones. This suggests that the Cu colloids
did not change during storage. In contrast, those stabilized by alkylzinc, after a few
months’ storage a trace of dark precipitate appeared, which might be due to the
decomposition of Zn complexes. In order to analyze the colloid stability in more details,
TEM characterization was carried out for Cu/TOAl and Cu/DBZn after one year of
storage. The TEM images in Figure 3.17 show that the Cu colloids are identical to the
freshly prepared ones (displayed in Figure 3.2 (b) and (d)). There is no obvious particle
agglomeration or shape change.
(a) (b)
Figure 3.17 TEM images of Cu colloids stored for one year: (a) Cu/TOAl; (b) Cu/DBZn.
Results and discussion
67
3.1.5 Summary
Following the ‘Bönnemann route’, Cu colloids were successfully prepared via chemical
reduction of transition metal salt with metal alkyls in THF solution under Ar protection.
Cu(acac)2 as precursor was simultaneously reduced and stabilized by alkylaluminium or
alkylzinc that served as both reducing agent and stabilizer. The synthesis was highly
reproducible and could be extended to the use of another two stabilizers - Al(n-butyl)3
and Zn(ethyl)2, besides Al(n-octyl)3 and Zn(n-butyl)2 which had also been investigated in
a previous study. The Cu, Al and Zn concentrations were determined by ICP.
Different characterization techniques were applied for an intensive study of the structure
of different Cu colloids. TEM demonstrated that the colloidal Cu nanoparticles were
spherical. Some of them were single crystallites, while some were agglomerates of
nanocrystals. Their particle size varied between 3 and 6 nm with a narrow size
distribution, which was influenced by the type of stabilizer applied. The sizes of Cu
particles from Cu/TOAl and Cu/DEZn were larger than the other two. However, since the
difference between the particle sizes of each colloid was only 1-3 nm, it was difficult to
establish a clear relation between the stabilizer type and the particle size of the Cu
colloidal nanoparticles. Also variation of synthesis parameters, such as reaction
temperature and Cu precursor ligands, did not allow tuning the Cu particle size within a
wider range. UV-Vis spectra of Cu colloids showed absorptions at ca. 560 nm that were
characteristics for the spherical metallic Cu nanoparticles with rather small particle size.
This spectroscopic feature, however, was less evident for Cu/DBZn, probably due to the
interaction between Cu and Zn. In situ UV-Vis spectroscopy was performed for both
Cu/TOAl and Cu/DEZn to study their oxidation behavior. The latter indicated the
formation of ZnO around the Cu core, and the forming ZnO might protect the Cu core
longer from being oxidized. XRD confirmed that the metallic Cu particles are crystallized
and the main crystalline domain size was ca. 3 nm, based on calculations using the
Scherrer formula. XAS measurements were performed firstly at the Cu K-edge, and they
further proved that Cu in all cases is in the zero-valent state. XAS then at the Zn K-edge
for the alkylzinc-stabilized Cu colloids demonstrated that the Zn state is more complex.
The Zn is oxidized and the first coordination shells resemble those of ZnO.
Results and discussion
68
In order to obtain more insight into the Cu colloid formation, in situ XAS measurements
were designed and applied to detect possible Cu intermediate species during the reduction
process. The in situ XAS experiments were carried out first at room temperature with
Zn(n-butyl)2 as stabilizer. The results suggested that the Cu(II) was directly reduced to
Cu(0), without going through Cu(I). Then in situ XAS experiments at low temperature
(down to -30 °C) were performed, using all four stabilizers. The data further confirmed
that the Cu(II) was reduced to Cu(0) without detectable existence of any Cu(I) species as
intermediates, which was different from what had been expected from the experimental
observation of an intermediate color change to yellow during reduction. The reduction
process in each case took place very fast even at temperatures as low as below -20 °C,
especially in the case of using alkylzinc as stabilizer.
The Cu colloids could be stored under Ar for a few years without agglomeration.
Therefore, these well-defined, stable Cu colloids are potential model catalysts for
methanol synthesis. The study of their catalytic properties will be reported in the next
chapter.
Results and discussion
69
3.2 Copper colloids in quasi-homogeneous methanol synthesis
Previously in this group, a homogeneous model system using Cu colloids as catalysts for
methanol synthesis was established [36, 268]
. Cu colloids stabilized by Al(n-octyl)3 or Zn(n-
butyl)2 were tested in a quasi-homogeneous phase for methanol formation from synthesis
gas feed. The preliminary results showed that both Cu colloids were highly active. As
described in Chapter 3.1, the synthesis of Cu colloids by the ‘Bönnemann route’ could be
extended to the use of four different alkylaluminium or alkylzinc as stabilizer compounds,
and their structures were already clarified. Therefore, further research on the catalytic
performance of these four Cu colloids in quasi-homogeneous methanol synthesis was
carried out in order to establish the relation between their structures and their catalytic
activity. This study could help to address the questions still open for the solid catalysts
for methanol synthesis. This part covers the following three main aspects:
1. The activities of the four Cu colloids (Cu/TBAl, Cu/TOAl, Cu/DEZn and
Cu/DBZn) were determined from their tests in methanol synthesis from synthesis
gas feed.
2. The change of Cu colloids during reaction was investigated as the main focus in
this study. For this purpose, the structural changes of the colloids (size, shape,
composition, etc.) throughout the entire reaction were fully studied with various
techniques (TEM, XRD, XAS, GC-MS, etc) and were compared with freshly
prepared colloids.
3. The roles of the Cu core and the metal alkyl shell in the catalytic reaction were
studied in a series of experiments, where each of the individual factors involved in
the colloid composition was changed, and the performance of the modified
colloids in the methanol synthesis was investigated.
Results and discussion
70
3.2.1 Catalytic activity tests
Cu/ZnO/Al2O3 is the most widely used solid catalyst in industrial methanol production
since the 1960s. However, as stated in Chapter 2.1, there is still a number of issues
remaining unsolved. These include the active sites, reaction mechanism, the role of
carbon oxides in the synthesis, etc. Cu colloids are of great interest for the understanding
of the methanol synthesis reaction in a homogeneous system, due to their stability and
well-defined particle size. Moreover, a wide range of techniques can be applied to
investigate the system in order to gain a better understanding of this catalytic system. In a
previous report [35-36]
, S. Vukojević had established a quasi-homogeneous model system
using Al(n-octyl)3 or Zn(n-butyl)2-stabilized Cu colloids as catalysts, since they bear
some resemblance to the solid catalysts, as they contain the same elements - Cu, Zn and
Al. Both of these two colloids were proven to be highly active in methanol synthesis. In
particular, the Cu colloid stabilized by Al(n-octyl)3 was even very active in spite of the
absence of Zn species, which are usually considered as crucial species providing active
sites in a solid methanol synthesis catalyst [104, 270]
. Based on those preliminary results,
our further work attempted to synthesize a series of Cu colloids with different stabilizers
and investigated this Cu colloid system in depth. The goal of this study was to determine
whether the activity of Cu colloids in methanol synthesis can be extended to those Cu
colloids using other alkylaluminium or alkylzinc stabilizers and to establish the possible
relation between their particle size, stabilizer type and their catalytic activity.
3.2.1.1 Activity
As described in Section 5.3.1, the batch reactor was filled with the desired reaction gas
composition at the approximate molar composition - H2/CO/CO2 = 86:10:4. The reaction
pressures were between 17 and 20 MPa depending on the reaction temperature. A first
test using Cu/TOAl at 130°C was carried out. It was found that the methanol formation
occurred at a temperature as low as 130 °C and after 20 hours reaction time the methanol
productivity reached ca. 0.8 molMeOH/(kgCu·h). This study suggested that the use of Cu
colloids made the methanol synthesis feasible at lower temperature in quasi-
homogeneous phase. This temperature is below the range of 200-300 °C which is used in
an industrial process with the technical catalysts. A subsequent series of temperature
Results and discussion
71
dependent tests for each colloid was performed at reaction temperatures between 140 and
170 °C via a low heating ramp of 0.5 °C/min. Due to the design limitations of
temperature and pressure of the reactor, the maximum temperature and the pressure are
170 °C and 21 MPa, respectively. The reaction pressure increased with the reaction
temperature and reached between 17 and 21 MPa, depending on the exact temperature set
for the reaction. A typical result from the on-line test is shown in Figure 3.18. The figure
shows the increase in the concentrations of both methanol and methyl formate with
reaction time to up to 40 hours, when Cu/DEZn was used as the catalyst. Moreover, with
each colloid, there was always a trace of ethanol detected in the product. The presence of
ethanol in the product is not surprising, since ethanol is a common by-product in
methanol synthesis [21]
. Due to the design restrictions, the on-line sampling caused both
the pressure drop and the blockage of the GC valves, so the on-line sample analysis could
not be further performed after 20 to 40 hours at high temperatures depending on the type
of colloid. During this limited reaction time, all the Cu colloids were active and the
methanol concentration is proportional to reaction time.
In the previous study on Cu/TOAl, the methanol concentration based on each on-line GC
analysis was proportional to the reaction time within the first 12 hours, as seen from
Figure 2.18 in Section 2.2.8.2. The methanol productivity could thus be determined from
the slope of the linear regression of the methanol concentration with reaction time [35-36]
.
In our current study, during 20 up to 40 hours reaction period, the methanol concentration
was still proportional to the reaction time. Furthermore, when the reaction was stopped
and the reactor cooled down naturally, the continuous on-line sample analysis showed
that the concentrations of all the products including methanol and methyl formate
increased strongly. The reason could be that partial amount of products, during reaction at
higher temperatures in gas phase, dissolved back into the liquid phase (the Cu colloids in
the reactor) at room temperature. After removing the reaction gas that was left in the
reactor after reaction, the further off-line sample analysis of the solution confirmed equal
product concentration to those on-line analyzed after reaction at room temperature.
Therefore, in order to obtain more precise product concentrations, off-line sample
analysis were carried out for each of the Cu colloids at different temperatures and the
methanol productivities were thus calculated using the product concentrations at room
Results and discussion
72
temperature and reaction time (see the equation (5.1)). This new calculation would avoid
the error in the calculations of methanol productivity caused by the on-line analysis
leading to pressure drop and loss of catalyst during reaction.
0 5 10 15 20 25 30 35 40
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Co
nce
ntr
atio
n [m
g/m
l]
Time [h]
Methanol
Methyl formate
Ethanol
DEZ stabilized
Figure 3.18 Formation of methanol, methyl formate, and ethanol over Cu/DEZ at 150°C.
The methanol productivity over each colloid at different temperatures was obtained based
on the off-line analysis and the results are summarized in Figure 3.19. The methanol
productivities over different Cu colloids increase with reaction temperature, so they are
clearly temperature dependent. Productivities as high as 9-10 molMeOH/(kgCu·h) were
obtained at 170 °C at a total reaction gas pressure of ca. 20 MPa. The methanol
productivities of three Cu colloids (Cu/TBAl, Cu/TOAl and Cu/DEZn) are at a similar
level at each reaction temperature, whereas that of Cu/DBZn is lower. For each route,
several samples from different synthesis batches, either freshly synthesized or stored
under Ar after several months, were tested. They all showed methanol productivities
within small variations marked in Figure 3.19 - approximately ± 20%, which proved that
the synthesis of Cu colloids are highly reproducible. These results also confirm that the
properties of Cu colloid do not change during long time storage and that they are very
stable.
Results and discussion
73
140 150 160 1700
2
4
6
8
10
12
Me
tha
no
l p
rod
uc
tiv
ity
[m
ol/
kg
Cu
/h]
Temperaure [°C]
Cu/TBAl
Cu/TOAl
Cu/DEZn
Cu/DBZn
Figure 3.19 Methanol productivity of different Cu colloids at reaction temperatures between
140 °C and 170 °C.
Because the molar ratio of the stabilizer to the Cu precursor is varied between 3 and 10,
there is certainly a large excess of stabilizer dispersed in the colloidal solutions which did
not react with the Cu precursor. One previous investigation of Bönnemann’s group [222]
indicated that Al-CH3 at the surface of the colloidal particles reacted with R-OH group.
Therefore, the same type of reaction may thus take place between the metal alkyls and the
alcohol. Since the amount of the stabilizer in all cases was in excess, the ‘free’ stabilizers
might have reacted with the methanol that was formed during the reaction, thus
producing methoxyzinc or methoxyaluminium and the corresponding alkanes. The
possible reactions are displayed in Scheme 3.1. Presumably, some methanol formed
during the catalytic tests could also be bonded to the Al or Zn within the protecting shell
causing a lower methanol concentration than would be observed without this effect.
Al(alkyl)3
Zn(alkyl)2
+ MeOHAl(OMe)3
Zn(OMe)2
+R
R’
Scheme 3.1 Simultaneous side reactions during methanol formation over Cu colloid.
Previously, S. Vukojević in his study [36]
found that upon exposure of the Cu colloids to
air after catalysis a significantly higher methanol amount could be detected in the THF
Results and discussion
74
solution. This observation was also confirmed with the Cu/DBZn after catalysis. In order
to quantify the amount of methanol that had reacted with the extra amount of metal alkyl
stabilizers, experiments were carried out using each Cu colloid without synthesis gas feed
under the same heating process. Methanol was added prior to the heating in the same
amount, which corresponded to the total amount of the ‘free’ stabilizer in each colloid.
The consumed methanol amount after heating was found to be between 50% and 90% of
that added into the colloid. This indicated that there was indeed extra methanol
consumption in the colloids and its amount was close to that of the ‘free’ stabilizer. The
methanol productivity was then recalculated, taking into account this extra amount of
methanol, as shown in Figure 3.20. The calculated methanol productivity increases by a
factor of 2-5, and the highest methanol productivity reaches 23.3 molMeOH/(kgCu·h) using
Cu/TOAl at 170 °C. Moreover, in a previous study the industrial catalyst KATALCOJM
51-8 (Cu/ZnO/Al2O3) as a reference was also tested under the same conditions in THF
suspension at 150 °C. Its methanol productivity was only 5.5 molMeOH/(kgCu·h) [35]
, much
lower than that of any Cu colloid at 150 °C. Therefore, the Cu colloids were further
proven to be highly active in a quasi-homogeneous phase at lower temperatures.
Among the four Cu colloids, the methanol productivities of Cu/TBAl and Cu/DBZn are
obviously lower, only half of the level of the other two colloids. As stated in Section
2.2.7.1, there can be a particle size effect on the activity of the metal nanoparticles. The
particle size effect is a complex factor not only related to the particle surface area but also
to the real concentration of active sites. Depending on the nature of the catalytic reaction,
the catalytic activity of the metal nanoparticles may increase or decrease with decreasing
particle size, or even remain unchanged [130, 132]
. TEM images in Figure 3.2 (a) and (d)
showed that the particle sizes of Cu/TBAl and Cu/DBZn are smaller. The methanol
productivities observed for these two colloids are lower than those of Cu/TOAl and
Cu/DEZn. Therefore, the data suggest that in the case of the Cu colloid catalyzed
methanol synthesis, smaller particle size leads to lower activity. Whether the metal alkyl
stabilizers are alkylaluminium or alkylzinc seems to have no evident influence on the
methanol productivity; rather the particle size itself influences the activity.
Results and discussion
75
140 150 160 1700
5
10
15
20
25
Meth
an
ol p
rod
ucti
vit
y [m
ol/kg
Cu
/h]
Temperature [°C]
Cu/TBAl
Cu/TOAl
Cu/DEZn
Cu/DBZn
KATALCOJM 51-8
Figure 3.20 Methanol productivity of different Cu colloids that are recalculated taking into
account the consumed methanol which reacts with the metal alkyl stabilizers during reaction.
3.2.1.2 Reaction mechanism and kinetics
More information can be extracted from Figure 3.18 on the formation of main products. It
can be seen that the concentration of methyl formate remains nearly constant after an
initial increase at the beginning, meanwhile methanol concentration increases throughout
the entire run. This further confirms mechanism proposed in previous reports that methyl
formate reaches a steady-state concentration and might be considered as an intermediate
rather than a by-product [35-36]
. A methanol formation mechanism over Cu colloids is
proposed in Scheme 3.2. The methyl formate is formed first, and then transformed into
methanol via hydrogenolysis with H2.
HCOOCH3
H2
2CH3OH
Scheme 3.2 Mechanism of methanol formation from synthesis gas over Cu colloid via
hydrogenolysis of methyl formate as intermediate.
Furthermore, no pressure drop throughout the reaction was observed, meaning that the
conversion of the reaction gas (H2, CO and CO2) is very low. Since the total pressure
Results and discussion
76
remained constant throughout the reaction, reaction rate equation (3.2) can be
approximated as follows:
c
CO
b
H
a
CO 22PPPk r ≈ k’ (3.2)
This reaction can be considered as ‘pseudo zero order reaction’, where the reaction rate is
a constant and independent of the reactant gas pressure. This constant corresponds to the
linear regression of the methanol concentration with reaction time, which was used to
calculate the methanol productivity (PMeOH) in the previous study [35]
. Also, seen from
Figure 3.18 methanol concentration is nearly proportional to the reaction time up to 40
hours reaction.
3.2.2 Change of copper colloid during reaction
According to the on-line analysis, the Cu colloids remained quite active throughout the
reaction for at least 20 to up to 40 hours. In order to understand the activities of different
Cu colloids, it would be interesting to obtain a deeper insight into the structure of the Cu
colloids throughout the entire reaction.
All four Cu colloids were investigated after reaction, while Cu/TOAl and Cu/DBZn were
chosen as typical examples for more detailed studies after 5 hours reaction. The naming
of all the colloid samples and the corresponding reaction conditions are summarized in
Table 3.3. The samples were collected under Ar, after the reactor was slowly evacuated to
atmospheric pressure. For all the samples collected under Ar after reaction, gel-like
precipitate was observed at the bottom of the autoclave, whereas the upper layer was
more transparent. The color of the precipitates depended on the stabilizers - dark red for
the alkylaluminium-stabilized colloids and dark grey for the alkylzinc-stabilized ones.
The samples for the following analysis were prepared using the precipitates. The samples
were characterized by the same techniques as used for the freshly prepared Cu colloids
before reaction.
Results and discussion
77
Table 3.3 Cu colloids studied during and after reaction at 150 °C.
Sample name Stabilizer type Reaction time (hour)
Cu/TBAl AR Al(n-butyl)3 20
Cu/TOAl A5h Al(n-octyl)3 5
Cu/TOAl AR Al(n-octyl)3 20
Cu/DEZn AR Zn(ethyl)2 40
Cu/DBZn A5h Zn(n-butyl)2 5
Cu/DBZn AR Zn(n-butyl)2 30
3.2.2.1 TEM analysis
The structure of Cu colloids during reaction was first studied using TEM. As seen from
the TEM image in Figure 3.21 (a) and (b), Cu nanoparticles in both Cu/TOAl A5h and
Cu/DBZn A5h are still well dispersed. However, a small difference is observed: the Cu
particles in Cu/TOAl A5h seem to be embedded in a bulky and ill-defined material with
no clear structure, while this is not the case for Cu/DBZn A5h. HRTEM was also
performed for both samples for more detailed analysis. In Cu/TOAl A5h most of the
particles still remain single crystals, and the crystallite size is larger than 5 nm, as shown
in Figure 3.21 (c). In contrast, in Cu/DBZn A5h as demonstrated in Figure 3.21 (d), it is
difficult to distinguish individual particles. They are all small crystals with diameter <5
nm and they look different from the very well-defined spherical Cu nanoparticles of
Cu/DBZn before reaction (see Figure 3.3 (b)).
Results and discussion
78
(a) (b)
(c) (d)
(a) (b)
(c) (d)
Figure 3.21 TEM images of (a) Cu/TOAl A5h and (b) Cu/DBZn A5h; HRTEM images of (c)
Cu/TOAl A5h and (d) Cu/DBZn A5h.
The situation is different after the whole reaction period up to 40 hours. As illustrated in
Figure 3.22 (a) and (b), compared to the alkylzinc-stabilized colloids after reaction, the
alkylaluminium-stabilized colloidal Cu nanoparticles after reaction are still visible and
well dispersed. The particle size of Cu/TBAl AR ranges from 4 nm to 10 nm. Its size
distribution becomes broader and the particle size is larger than that of the freshly
prepared Cu/TBAl (3 nm). In comparison with the freshly prepared Cu/TOAl and
Cu/TOAl A5h, the particle size of Cu/TOAl AR still remained at 3-4 nm in average,
however, with a broadened particle size distribution from 2 nm to even 9 nm.
Furthermore, the shape of the particles is no longer spherical, but distorted. Similar
observations were obtained with both Cu/TBAl AR and Cu/TOAl AR: the Cu colloidal
particles are embedded in an ill-defined phase. In the case of alkylzinc-stabilized colloids
after reaction (Figure 3.22 (c) and (d)), there are coarser, dark particles, which are much
larger in size, in a hazy phase. The TEM image of Cu/DBZn AR however, is significantly
Results and discussion
79
different from that of Cu/DBZn A5h in Figure 3.21 (b) and (d) where small particles are
still clearly visible.
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 150
5
10
15
20
25
30
Fre
qu
nc
y [
%]
Particle size [nm]
0 1 2 3 4 5 6 7 8 9 100
10
20
30
40
Fre
qu
en
cy
[%
]
Particale [nm]
(a)
(b)
(c) (d)
Figure 3.22 TEM images of Cu colloids stabilized by different metal alkyls after reaction and
their particle size distribution (taking ca. 200 particles): (a) Cu/TBAl AR; (b) Cu/TOAl AR; (c)
Cu/DEZn AR; (d) Cu/DBZn AR.
A more detailed analysis is further carried out by HRTEM for Cu/TOAl AR and
Cu/DBZn AR as examples. Figure 3.23 (a) shows that the Cu nanoparticles contain small
polycrystalline domains in the case of Cu/TOAl AR. The background, corresponding to
the ill-defined material, is present as non-crystallized phase. Further elemental analysis
Results and discussion
80
using EDX (Figure 3.23 (c)) shows that low contrast regions of 1 and 5-7, which are the
ill-defined phases, comprise of mainly Al and Si. Thus, there could be a large amount of
amorphous Al species formed. However, the presence of large amounts of Si (up to ca.
45 wt%) is difficult to explain, and it might come from silicon grease used during the
colloid synthesis to seal the opening of Schlenk flask. Such silicon contamination has
occasionally been observed before in EDX analysis. High contrast regions of 2-4 on the
other hand contain both Cu and Al. Therefore, the alkylaluminium-stabilized Cu
nanoparticles after reaction are embedded in amorphous Al-containing species. In
contrast, the large particles of Cu/DBZn AR shown in Figure 3.23 (b) are multicrystalline,
and it is difficult to identify any Cu particle due to the low contrast between Cu and Zn
species. EDX analysis (Figure 3.23 (d)) on different regions shows that: regions 1, 3 and
7 contain only Cu; regions 6 and 9 contain only Zn; regions 2, 4, 5 and 8 contain both Cu
and Zn. It seems that Cu and Zn species are mostly separated from each other.
(a) (b)
(c) (d)
Figure 3.23 HRTEM images of Cu colloids after reaction: (a) Cu/TOAl AR; (b) Cu/DBZn AR
and EDX: (c) Cu/TOAl AR; (b) Cu/DBZn AR.
Results and discussion
81
3.2.2.2 XRD measurements
All samples analyzed by TEM were also measured by XRD. XRD patterns of the
alkylaluminium-stabilized Cu colloids during and after the reaction are shown in Figure
3.24. The strong reflection at 2θ = 19.7° of Figure 3.24 (a) for Cu/TOAl A5h is attributed
to Cu(111), whereas the weak one at 22.7° is indexed to Cu(200). This pattern is
comparable with that of the Cu/TOAl before reaction, indicating that the structure of the
particles did not change during the first five hours of reaction. However, the mean size of
the crystalline domain of Cu/TOAl A5h calculated from the Scherrer formula is ca. 8 nm,
which is substantially larger than the 3 nm for the fresh Cu/TOAl. In the XRD patterns of
Cu/TOAl AR and Cu/TBAl AR, shown in Figure 3.24 (b) and (c), almost no reflections
are observed, indicating that either the Cu is amorphous, or its size of crystalline domains
is maximum 1 nm [254]
. Since polycrystalline domains are still clearly observed in the
HRTEM images for Cu/TOAl AR (Figure 3.23 (a)), it means that the Cu crystalline
domain becomes much smaller after long time reaction. Both XRD results of Cu/TOAl
A5h and Cu/TOAl AR show that the alkylaluminium-stabilized Cu nanoparticles in the
beginning of the reaction are single crystal particles. After 20 hours reaction, however,
the Cu particles become polycrystalline and each consists of several smaller crystalline
domains. No reflections indicative of aluminium oxides were detected. Thus, if
aluminium oxide species are present, they might be in the form of very small crystallites
(<1-2 nm). They might exist as amorphous Al2O3, methanolate or hydrated mixed species.
There is no reflection corresponding to CuxOy either, which indicates that there might be
negligible amount of CuxOy or Cu is not oxidized at all during reaction. However, since
XRD only detects species with long range order, this statement requires further proof by
XAS at the Cu K-edge, as described in the following section.
Results and discussion
82
14 16 18 20 22 24 26 28 30
(b) Cu/TOAl AR
(c) Cu/TBAl AR
Inte
ns
ity [
a.u
.]
2 [°]
(a) Cu/TOAl A5h
Cu(111)
Cu(200)
14 16 18 20 22 24 26 28 30
(b) Cu/TOAl AR
(c) Cu/TBAl AR
Inte
ns
ity [
a.u
.]
2 [°]
(a) Cu/TOAl A5h
Cu(111)
Cu(200)
Figure 3.24 XRD patterns (Mo-radiation 0.7093Å, transmission mode under Ar protection) of
alkylaluminium-stabilized Cu colloids during and after reaction.
XRD patterns of the alkylzinc-stabilized Cu colloids during and after reaction are shown
in Figure 3.25. Seen from Figure 3.25 (a), the XRD pattern of Cu/DBZn A5h exhibits a
complex series of reflections. They cannot be attributed to Cu, CuxOy or ZnO, instead, the
reflections might be related to some organocopper or organozinc substances formed
during the reaction [254]
. In contrast, both XRD patterns of Cu/DBZn AR and Cu/DEZn
AR shown in Figure 3.25 (b) and (c) present the reflections at 2θ = 19.7° and 22.7°,
corresponding to Cu(111) and Cu(200), respectively. The mean crystalline domain size of
Cu is 9 nm and no CuxOy species are detected. The other reflections in Figure 3.25 (b)
and (c), from left to the right, represent ZnO, including ZnO(100), ZnO(002), ZnO(101),
ZnO(102), ZnO(110), ZnO(103) and Zn(112) reflections. The mean crystalline domain
size of ZnO is 23 nm and ZnO could be formed from oxidation of the alkylzinc
stabilizing shell. The XRD results confirm that both metallic Cu and ZnO exist and they
are crystallized. This result corresponds to the presence of large crystalline particles in
the TEM images of Cu/DBZn AR. Based on these analyses, Cu particles containing
larger crystalline domains might exist, but they are covered or surrounded by large
Results and discussion
83
amounts of ZnO. The information provided by the XRD patterns in Figure 3.25
demonstrates that the change of the alkylzinc-stabilized Cu nanoparticles during reaction
is more complicated than that of those nanoparticles stabilized by alkylaluminium. This is
due to the fact that the oxidation of alkylzinc stabilizer leads to the crystallization of ZnO.
15 20 25 30
(b) Cu/DBZn AR
(c) Cu/DEZn AR
Inte
nsit
y [
a.u
.]
2
(a) Cu/DBZn AR 5h
Cu
Zn
(100)
(002)
(101)
(111)(102)
(200)
(110)(103)
(112)
Figure 3.25 XRD patterns (Mo-radiation 0.7093Å, transmission mode under Ar protection) of
alkylzinc-stabilized Cu colloids during and after reaction.
3.2.2.3 XAS measurement
In addition, XAS measurements were performed to follow the changes of the Cu colloids
during the course of the reaction. Figure 3.26 shows the XANES spectra of all the
samples at the Cu K-edge and Cu foil was used as a reference. All spectra exhibit the
characteristic prepeak, indicating that Cu is still in the metallic state and is not oxidized in
the reaction. This is consistent with the XRD results, which shows no indication of the
presence of CuxOy phases. It should be noticed that the XANES spectra for Cu/TOAl A5h
and Cu/DBZn A5h show an interesting difference to all other spectra: their whiteline does
not show splitting (double peak feature). Since there are still obvious prepeaks for both,
these two samples must contain metallic Cu as well. However, the difference of these two
Results and discussion
84
spectra from others might be due either to the Cu particles being smaller, or to their
possibly different particle geometry [257]
.
8.94 8.96 8.98 9.00 9.02 9.04 9.06
0.0
0.4
0.8
1.2
1.6
2.0
2.4
Ab
so
rpti
on
[a
.u.]
E [keV]
Cu foil
Cu/TOAl A5h
Cu/TOAl AR
Cu/TBAl AR
Cu/DBZn A5h
Cu/DBZn AR
Cu/DEZn AR
Figure 3.26 XANES spectra at the Cu K-edge for Cu colloids recovered during and after reaction
with Cu foil as reference.
Figure 3.27 shows the Fourier-transformed EXAFS spectra at the Cu K-edge using both
Cu foil and fresh Cu colloids as references. Similar information concerning the Cu atom
distance and coordination number can be obtained as for the fresh Cu colloids. In
comparison to Cu foil and fresh colloids, the Cu-Cu distance in the reacted Cu colloids
did not change, and the spectra of the reacted Cu colloids were found to be in agreement
with that of the as-synthesized Cu colloid, which corresponds to Cu in the metallic state.
The backscattering intensity of the first and further shells from the four colloids after
reaction are all higher than for the fresh colloids, indicating the increasing Cu
coordination number and larger particle size. In contrast, the intensity decreases for those
colloids recovered after 5 hours reaction - both Cu/TOAl A5h and Cu/DBZn A5h,
suggesting that their coordination number decreases and that their particle size becomes
smaller. Both of them have a significant intensity at ca. R = 1.5 Å, which, however, is the
artifact of the background subtraction due to the high noise level [271]
. The coordination
number and the particle size of all the samples are summarized in Table 3.4. However,
Results and discussion
85
since the background influence to sample Cu/TOAl A5h could not be easily removed, no
structural data could be calculated for this sample.
0 1 2 3 4 5 6 7 8
0.000
0.001
0.002
0.003
0.004
0.005
0.006
0 1 2 3 4 5 6 7 8
0.000
0.001
0.002
0.003
0.004
0.005
0.006
0.007
FT
(k
1-w
eig
hte
d)
R [Å]
Cu foil
Cu/DBZn
Cu/DBZn A5h
Cu/DBZn AR
Cu/DEZn
Cu/DEZn AR
F
T (
k1
-weig
hte
d)
R [Å]
Cu foil
Cu/TOAl
Cu/TOAl A5h
Cu/TOAl AR
Cu/TBAl
Cu/TBAl AR
Figure 3.27 Fourier-transformed k1-weighted EXAFS spectra at the Cu K-edge for Cu colloids
recovered during and after reaction with Cu foil and fresh Cu colloids as reference: (a)
alkylaluminium-stabilized Cu colloids; (b) alkylzinc-stabilized Cu colloids.
As listed in Table 3.4, the Cu-Cu distance for all the Cu colloids recovered during and
after reaction remains unchanged. The Cu atom coordination number of Cu/DBZn A5R is
slightly larger than that before reaction, but smaller than that after reaction. The Cu atom
coordination numbers of both Cu/TOAl AR and Cu/DBZn AR are likely to be larger than
those before reaction. A coordination number of 11 corresponds to a particle size larger
than 4 nm, according to the same measurement of metal nanoparticle size using XAS
indicated by Grunwaldt et al. [106]
. In contrast, the coordination numbers of the Cu/TBAl
AR or Cu/DEZn AR appear slightly smaller than those before reaction. However, the
variation is still within the error margin of 10-20 %. Moreover, the coordination number
is also influenced by strain, imperfections in the lattice and the averaging over all
particles [271]
. The coordination number of both the alkylaluminium-stabilized particles
after reaction and the alkylzinc-stabilized Cu colloids after reaction is close to the Cu
coordination number in the bulk that is 12.
(a) (b)
Results and discussion
86
Table 3.4 Structural parameters obtained from EXAFS data fitting for all four Cu colloids before,
during and after reaction.
Sample name Stabilizer type Coordination
number
Cu-Cu
distance (Å)
Cu/TBAl Al(n-butyl)3
12 2.54
Cu/TBAl AR 11.2 2.54
Cu/TOAl
Al(n-octyl)3
9.8 2.53
Cu/TOAl A5h - -
Cu/TOAl AR 11.2 2.53
Cu/DEZn Zn(ethyl)2
12 2.56
Cu/DEZn AR 10.8 2.54
Cu/DBZn
Zn(n-butyl)2
8.2 2.53
Cu/DBZn A5h 9.3 2.52
Cu/DBZn AR 11.8 2.54
Finally, XAS at the Zn K-edge was also performed for all the alkylzinc-stabilized
colloids using Zn foil, ZnO and alkylzinc in THF solution as references, as illustrated in
Figure 3.28. The XANES spectra of Cu colloids recovered during and after reaction are
shown in Figure 3.28 (a) and they closely resemble the ZnO spectrum. In the Fourier-
transformed EXAFS spectra from Figure 3.28 (b), when comparing with the fresh Cu
colloids, the intensity of the back scattering peak at 2.9 Å for the colloids after reaction
significantly increases. In contrast, the spectrum of Cu/DBZn A5h has lower intensity at
this position. This suggests that ZnO was formed at the end of the reaction, but might not
have been formed - at least not to such extent - at the beginning of the reaction. This
corresponds very well to the results from XRD (Figure 3.25 (a)) that no ZnO was
detected after 5 hours reaction.
Results and discussion
87
9.60 9.65 9.70 9.75 9.80 9.85
-0.4
0.0
0.4
0.8
1.2
1.6
2.0
2.4
0 1 2 3 4 5 6 7 8
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
Ab
so
rpti
on
[a.u
.]
E [keV]
Zn foil
ZnO
Zn(n-butyl)2 in THF
Cu/DBZn A5h
Cu/DBZn AR
Zn(ethyl)2 in THF
Cu/DEZn AR
FT
(k
3-w
eig
hte
d)
R [Å]
Zn foil
ZnO
Zn(n-butyl)2 in THF
Cu/DBZn A5h
Cu/DBZn AR
Zn(ethyl)2 in THF
Cu/DEZn AR
(a) (b)
Figure 3.28 XAS spectra at the Zn K-edge for Cu colloids with Zn foil, ZnO and alkylzinc in
THF solutions as references: (a) XANES; (b) radial distribution functions (Fourier-transformed
k3-weighted EXAFS spectra at Zn K-edge).
3.2.2.4 Decomposition of the copper colloids during the reaction
The changes in Cu particle sizes of both Cu/TOAl and Cu/DBZn, determined by the
different characterization techniques, during the methanol synthesis reaction are
summarized in Table 3.5. As described above, in general, measurements by TEM provide
direct information on particle size, however, only a small number of nanoparticle can be
characterized. The calculation from XRD gives the mean size of Cu crystalline domains,
whereas the estimation from XAS is based on the Cu atom coordination number. Particle
size calculated from XAS is most precise for very small particles, whereas the reflection
broadening analysis of XRD data gives the most reliable size measurement for
intermediate particle size. Although broadening is sensitive to particle size, it is not
highly pronounced, since reflections often cannot be reliably distinguished from the
background. Overall, the results of particle size obtained by the three characterization
techniques are in good agreement with one another. Seen from Table 3.5, the change of
the Cu particle size of the alkylaluminium-stabilized Cu colloids appears to be different
from that of alkylzinc-stabilized Cu colloids. For Cu/TOAl A5h, the particle size,
estimated from TEM images, does not change significantly. Concerning the crystallinity,
the HRTEM image in Figure 3.21 (c) shows that most of the Cu particles of Cu/TOAl
Results and discussion
88
A5h still remain as single nanocrystals, and the mean domain size increases to ca. 8 nm
based on XRD measurement. After reaction, for these colloids, the particle size
distribution of the particles increases from 2 nm to 8 nm, and the particle shape appears to
be further distorted, as shown in Figure 3.22 (b). As explained in the previous section,
due to the fact that XRD for both alkylaluminium-stabilized Cu colloids after reaction
exhibited no reflection, those particles might be finally polycrystalline containing
crystalline domains with a maximum size of 1 nm. In contrast, the alkylzinc-stabilized Cu
particles appear to be smaller nanocrystals in the initial phase of the reaction, as displayed
in Figure 3.21 (b) and (d). They then agglomerate obviously and form larger crystalline
domains with a size of 9 nm.
Table 3.5 Cu particle size change throughout reaction
Sample
name
Particle size (nm)
Before reaction After 5 hours reaction After entire reaction
TEM XRD* XAS TEM XRD* XAS TEM XRD* XAS
Cu/TOAl 5 3 3 > 5 8 X 2-8 1 > 4
Cu/DBZn 4 3 3 < 5 X 3 X 9 > 4
*: Mean size of Cu crystalline domain determined by Scherrer formula
X: No useful information could be obtained.
With respect to the changes of the metal alkyl stabilizing shell, XRD and XAS could
provide additional information on the oxidation state and the crystallinity state of all the
relevant components of the stabilizing shell. Though only Cu was detected by XRD,
HRTEM-EDX of Cu/TOAl AR confirmed that the ill-defined phases contain aluminium.
They are thus assumed to be amorphous Al2O3, hydrated or methanolate aluminium
species, formed by oxidation of the alkylaluminium stabilizing shell. For Cu/DBZn AR,
the polycrystalline phase mixture was confirmed by both XRD and XAS to contain ZnO
with much larger mean domain sizes of 23 nm.
In addition, thermal stability of the Cu colloids was evaluated. Each of the fresh Cu
colloids was heated at 200 °C with a heating rate of 0.5 °C/min, a typical temperature
ramp used in catalytic tests. The heating period for each Cu colloid was the same as the
Results and discussion
89
reaction time of each. Figure 3.29 shows the appearance of Cu nanoparticles after being
heated for 20 hours up to 40 hours. In general, the Cu nanoparticles are still observed,
though the particle shape seems distorted and the particles are surrounded by an ill-
defined phase. Particularly for the alkylzinc-stabilized Cu colloids, the particles can be
clearly distinguished, which is completely different from the same colloids after reaction
where there were only bulk phases. Therefore, the observed changes in particle size and
shape during reaction are unlikely to be caused only by high temperature, but rather by
the reactions between the Cu colloids and the reactant, products or even the intermediates.
These reactions eventually lead to complete decomposition of the Cu colloids.
(a) (b)
(c) (d)
Figure 3.29 TEM images of Cu colloids stabilized by different metal alkyls heated at 200 °C: (a)
Cu/TBAl heated for 20 hours; (b) Cu/TOAl heated for 20 hours; (c) Cu/DEZn heated for 40 hours;
(d) Cu/DBZn heated for 30 hours.
In order to further confirm the Cu colloid decomposition, GC-MS analysis was used to
analyze the colloids collected after reaction from the reactor. The collection of the liquid
sample was carried out under Ar protection and the precipitate in the colloid suspension
Results and discussion
90
was removed by filling with quartz wool. The main substances detected in the solution
except methanol, methyl formate and ethanol are listed in Table 3.6. This analysis shows
that there are also relatively large amount of alkanes, alcohols and acids present in
solution, which correspond to the ligand length in the stabilizer ligands. They might be
generated from the alcoholization, oxidation or simply ligand decomposition. During
reaction, it is possible that the metal alkyl compounds could react with H2, CO and CO2
to form these corresponding alkanes and alcohols. However, there are also other
substances formed in relatively large quantity. Surprisingly, a relatively large amount of
diethyl ether was detected from the Cu/TBAl AR, whose formation mechanism is not
clear. There were significant amounts of 2-butanol, 1-propanol and 2-pentanol from the
Cu/DEZn AR. Their formation could be through the hydrogenation of the corresponding
ketones, which derived from the CO insertion into the metal-alkyl bonds in the system.
Table 3.6 Substances formed from decomposition of Cu colloids during reaction.
Sample name Main substances from Cu colloid decomposition
Cu/TBAl n-butanol, diethyl ether
Cu/TOAl octane
Cu/DEZn 2-butanol, 1-propanol, 2-pentanol
Cu/DBZn butane, butanol
The Cu colloids, though decomposed during reaction over 20-40 hours, still maintained
their activity, giving a continuous increase in the methanol concentration (see Figure
3.18). This is probably due to the existence of metallic Cu particles in suspension, which
are mixed with the oxidized species - Al2O3 and ZnO, as have been confirmed by
different characterization techniques. One can assume that decomposed Cu colloids are
present probably as Cu nanoparticles supported on Al2O3 or ZnO, which resemble to
some extent solid catalysts in suspension. These Cu/Al2O3 and Cu/ZnO are the active
components in common technical solid catalysts for methanol synthesis.
Results and discussion
91
3.2.3 Factors affecting the activity of the copper colloids
The structures of the Cu colloids and their activities in quasi-homogeneous methanol
synthesis have already been discussed in the above sections. In this section, the factors
responsible for the catalytic activity will the discussed in more detail. The previous
studies showed that the Cu precursor - Cu(acac)2 in THF solution - has no activity in
methanol synthesis under the same reaction conditions as used in the catalytic tests for Cu
colloids [35-36]
. This indicates that it is the reduced Cu or Cu nanoparticles, instead of a Cu
complex that is active in catalyzing the reaction. The colloidal Cu nanoparticles consist of
two parts - the metallic Cu core and the metal alkyl stabilizing shell. A series of
experiments was designed in order to reveal the roles of the Cu core and metal alkyl
stabilizing shell, respectively, as well as the interaction between them in catalysis. At the
end of this section, possible active species of the Cu colloids will also be discussed.
3.2.3.1 The role of the copper core
The composition of the Cu nanoparticles is rather complex, since their synthesis involves
the reaction and exchange of several different species and ligands in both the Cu
precursor and the stabilizing shell. Due to the Cu particle decomposition during methanol
synthesis, there are some additional substances released, as listed in Table 3.6. The total
amount of carbon contained in the Cu colloids from the synthesis is 3 up to 5 times higher
than that contained in methanol and methyl formate produced in the reaction. The
possibility that the final products are generated from the decomposition of ligands of Cu
precursor or stabilizer therefore cannot be fully excluded. In order to rule out this
possibility, as well as to exclude another possibility that the metal alkyl stabilizers
themselves catalyze the reaction, further experiments needed to be designed. The evident
way is to replace the Cu core with other metals while maintaining an identical stabilizing
shell, and then to test the activity of the new colloids in methanol synthesis under the
same conditions. Those other metals must be inactive for methanol synthesis, so that the
possibility that the stabilizers catalyze the reaction could be explicitly checked.
For this purpose, Ag was chosen as a substitute to replace Cu, since Ag is known to be
inert in methanol synthesis from synthesis gas. Ag can easily coordinate with the ligands
by the same synthesis method [34]
and the synthesis process is demonstrated in Scheme
Results and discussion
92
3.3. In order to keep the same ligands, Ag(acac) was used as the Ag precursor and Al(n-
octyl)3 as stabilizer. The TEM image of colloidal Ag nanoparticles (Figure 3.30) shows
the characteristic morphology of Ag nanoparticles with a particle size of between 2 and
10 nm. When they were tested under the same conditions as used for Cu, no methanol or
methyl formate was detected during 20 hours reaction at 150 °C. This result strongly
suggests that the presence of the Cu core is crucial and that the stabilizer shell alone has
no catalytic activity.
Ag(acac)
Ni(acac)2
+ Al(n-octyl)3
Ag
Nicolloid
THF
Ar
Scheme 3.3 Preparation of Ag and Ni colloids with the ‘Bönnemann route’ as used for the
synthesis of Cu nanoparticles.
Figure 3.30 TEM image of Ag colloid.
Furthermore, S. Vukojević also reported that a Ni colloid, stabilized by alkylaluminium
and synthesized from Ni(acac)2 as precursor (Scheme 3.3), showed no activity in the
same reaction [35-36]
. This further confirmed the crucial role of the Cu core in methanol
synthesis. Therefore, based on the catalytic results after replacing the Cu core with other
metals, two important conclusions can be drawn: (1) The Cu core plays a crucial role in
the catalyzed methanol synthesis reaction and methanol cannot be obtained without it
under our reaction conditions; (2) Methanol is neither generated from the decomposition
Results and discussion
93
of the Cu colloids during reaction, nor is it produced by the catalytic function of the metal
alkyl stabilizers.
3.2.3.2 The role of the metal alkyl stabilizing shell
Although it has been proven that the Cu core has great importance in catalyzing the
methanol synthesis reaction, another question arose: could the Cu nanoparticles work
independently or is the stabilizing shell required for catalytic activity? The functions of
the stabilizers are basically to avoid the aggregation of the nanoparticles in solution and
to control the size and shape of the particles. It would be interesting to determine the
catalytic influence of the stabilizers in a quasi-homogeneous reaction by studying
different stabilizer types. Therefore, another two experiments were carried out: the first
was to replace the metal alkyl stabilizing shell completely by a different stabilizer and use
additional reducing agents; the second was to use a different metal alkyl stabilizer which
does not contain Al or Zn.
a. Non-metal alkyl-stabilized Cu colloids
Sodium borohydride (NaBH4) is widely used as a reducing agent in other metal colloid
syntheses (Ag, Au colloid, etc.) [272-273]
. However, in order to build a stabilizing shell an
additional surfactant, CH3(CH2)10CH2NH2 (dodecylamine), is required [274]
. Bönnemann
also reported the synthesis of Ag colloids using Korantin SH (oleoyl sarcosine) as
stabilizer [34]
. Therefore, two other colloid synthesis routes were developed: one with
dodecylamine and the other with Korantin SH as stabilizer. NaBH4 was applied together
with the surfactants to synthesize Cu colloid using the same Cu precursor - Cu(acac)2.
The preparation method is shown in Scheme 3.4.
Cu(acac)2 +THF
ArCu colloidNaBH4+
Dodecylamine
Korantin SH
Scheme 3.4 Preparation of non-metal alkyl-stabilized Cu colloids.
TEM images (Figure 3.31 (a) and (b)) show that Cu nanoparticles are formed using
dodecyl amine and Korantin SH, respectively. In the case where CH3(CH2)10CH2NH2 was
used as surfactant (Figure 3.31 (a)), the Cu nanoparticles have well-defined particle shape
Results and discussion
94
and the particle size varies from 3 to 5 nm, identical to that of the Cu colloids synthesized
with metal alkyls. In the case of using Korantin SH shown in Figure 3.31 (b), the shape of
the particle is less well-defined. Both colloids were tested under the same conditions in
methanol synthesis as the metal alkyl-stabilized colloids, however, none of the typical
products - methanol, methyl formate or ethanol, could be detected. To exclude the
possibility that methanol formed during reaction is adsorbed or reacts with any
substances in these new colloids, it was be necessary to carry out an additional
experiment by adding methanol into these freshly prepared colloid solutions. By GC
analysis, no loss of methanol was detected. Therefore, the possibility that the Cu colloid
adsorbs or reacts with methanol can be excluded, and the Cu nanoparticles that are
stabilized by non-metal alkyl exhibit no activity in methanol synthesis.
(a) (b)(a) (b)
Figure 3.31 (a) TEM image of Cu colloids synthesized using dodecylamine as surfactant; (b)
TEM image of Cu colloids synthesized using Korantin SE as surfactant.
b. Alkylmagnesium-stabilized Cu colloids
Apart from alkylaluminium and alkylzinc, there are other metal alkyls which could also
possibly serve as reducing agent, such as alkylmagnesium, alkylgallium, alkylboron and
alkyltin compounds. The question was whether they would be able to provide similar
catalytic function in methanol synthesis. Since the alkylmagnesium complex, among all,
is well studied and commercially available, it appeared to be the best option as both
reducing agent and stabilizer.
Results and discussion
95
As commercial Mg(n-butyl)2 in heptane solution has similar reducing power as
alkylaluminium or alkylzinc, it was applied directly as both reducing agent and stabilizer
without any additional surfactant, as shown in Scheme 3.5. The synthesis followed
exactly the same preparation method (Bönnemann route) as with alkylaluminium or
alkylzinc. The reduction took place rapidly and the color of the Cu precursor solution
turned to deep red immediately after adding a few drops of Mg(n-butyl)2 solution.
However, a dark precipitate was clearly observed at the bottom shortly after the reaction
(ca. 15 min). This Mg(n-butyl)2-stabilized Cu colloid is marked as Cu/DBMg. As seen
from Figure 3.32 (a), the Cu nanoparticles are spherical and in a size range of 3-6 nm.
They are comparable with sizes of the Cu particles stabilized by alkylaluminium or
alkylzinc. This Cu colloid was also tested in methanol synthesis from synthesis gas under
the same conditions as the previous metal alkyl-stabilized colloids. There were traces of
methanol and methyl formate detected by GC analysis, which corresponded to a methanol
productivity level as low as only ca. 4 mol/(kgCu·h) even at 170 °C. Furthermore,
stoichiometric amounts of methanol required for methanolysis of Mg(n-butyl)2 were
added into the fresh Cu colloid, but the methanol amount remained constant according to
GC analysis. This excluded the possibility that methanol formed during reaction is
adsorbed or reacts with the Mg(n-butyl)2 in excess.
Further analysis of this Cu/DBMg after reaction was also carried out by TEM. After
reaction the Cu nanoparticles show obvious agglomeration; the particle size increases to
between 5 and 10 nm, as shown in Figure 3.32 (b). However, this might not be the main
reason why this Cu colloid was not very active, since the same also occurred to the Cu
particles stabilized by alkylaluminium or alkylzinc. It should also be considered that
Mg(n-butyl)2 might not have the same interaction with the Cu core as the other two metal
alkyls have.
Cu(acac)2 + Mg(n-butyl)2
THF
ArCu colloid
Scheme 3.5 Preparation of Cu colloid analogous to the same ‘Bönnemann route’, but using
Mg(n-butyl)2 as stabilizer.
Results and discussion
96
(a) (b)
Figure 3.32 TEM images of Mg(n-butyl)2-stabilized Cu colloid: (a) before reaction; (b) after
reaction at 150 °C for 20 hours.
3.2.3.3 Interaction between core and shell
Based on the results discussed in the previous sections on substitution of stabilizers, the
Cu colloids appear to be very active only when being stabilized by alkylaluminium or
alkylzinc. In order to further investigate any possible special synergy between Cu-Al and
Cu-Zn in the catalytic reaction, a different Cu precursor was used to form Cu colloid and
its activity was studied.
Cu2(Piv)4(HPiv)2 (the synthesis of this Cu complex is described in Section 5.1.1.1) was
applied as Cu precursor in the synthesis, as shown in Scheme 3.6. The TEM image
(Figure 3.33 (a)) shows well dispersed Cu nanoparticles with the same particle size
distribution as those synthesized with Cu(acac)2 as precursor. In methanol synthesis
under standard conditions, a methanol productivity higher than 2 molMeOH/(kgCu·h) at
150 °C was observed. This productivity is almost the same as the Cu colloids formed
using Cu(acac)2 as precursor (before correction by taking into account the methanol
reacting with the stabilizer during reaction). After reaction, the Cu particles are still
observable under TEM as shown in Figure 3.33 (b). There is no convincing sign of
particle agglomeration. An ill-defined phase appears, which is also observed for
alkylaluminium-stabilized Cu colloids after reaction. As explained before, those ill-
defined bulky phases could be amorphous Al2O3 or other Al species formed via the
oxidation of the alkylaluminium. In addition, previous investigations by S. Vukojević
showed that Cu colloid synthesized with Cu(hfacac)2 as precursor also exhibited
methanol productivity at a level of 2.0 molMeOH/(kgCu·h) [35-36]
. Although the Cu precursor
Results and discussion
97
varied, as long as the stabilizers remained the same - alkylaluminium, the Cu colloids
always showed comparable activity in methanol synthesis under the same conditions. It
can thus be concluded that strong interactions of Cu-Al and Cu-Zn govern the activity in
the Cu colloid system.
Cu2(Piv)4(HPiv)2 + Al(n-octyl)3
THF
ArCu colloid
Scheme 3.6 Preparation of Cu colloid following the ‘Bönnemann route’, using a different Cu
precursor.
(a) (b)(a) (b)
Figure 3.33 TEM images of Cu colloid synthesized using Cu2(Piv)4(HPiv)2 as Cu precursor:
(a) before reaction; (b) after reaction at 150 °C for 20 hours.
The methanol productivities of the different Cu colloids as well as other metal colloids
involved in this study are summarized in Figure 3.34. For a fair comparison, they were all
obtained from reactions under the same reaction conditions at 150 °C. Seen from Figure
3.34, first, it is clear that the presence of the metallic Cu core is mandatory. Cu(II)
precursor solution, though reduced under H2 at reaction temperature is not active at all.
Ag colloid and Ni colloid, which were reduced and stabilized by the Al(n-octyl)3, are
completely inert in methanol synthesis as well. This means that the methanol is formed
due to the catalytic function of the Cu core but not the stabilizer, and that the methanol
does not result from the decomposition of stabilizer ligands either.
All the results obtained for the Cu colloids synthesized using different stabilizers or
alternative Cu precursors indicate that the metal alkyl stabilizer is an indispensable factor
to induce activity of Cu colloids under the reaction conditions studied. When using non-
Results and discussion
98
metal alkyl surfactant as stabilizers, the Cu colloids are not active. The activity of Cu
colloids is thus clearly ensured by the synergy between Cu colloid and the metal alkyl
stabilizing shell. Therefore, those Cu-M interactions, located on the surface of the Cu
particles, are most probably the locations of the active sites. Among the different systems,
the alkylmagnesium-stabilized Cu colloid shows much lower activity, whereas
alkylaluminium or alkylzinc-stabilized Cu colloids exhibit high activities, regardless of
precursor used. The interactions of Cu-Al and Cu-Zn may thus be stronger, providing
higher activity. Moreover, as discussed in the above Section 3.2.2.4, metal alkyl
stabilizers during reaction were oxidized to Al2O3, methanolate or hydrated Al species
and ZnO. This might lead to the formation of supported Cu nanoparticles in solution in
the form of Cu/ZnO and Cu/Al2O3, the active components contained in the highly active
solid ternary catalysts for methanol synthesis - Cu/ZnO/Al2O3. This might explain why
the Cu colloids, though decomposed under reaction conditions, still maintained activity
for quite a long time.
As stated in Section 2.2.8.2, some related research on Cu nanoparticles in methanol
synthesis was carried out by Fischer and Muhler [218, 229, 231, 238, 275]
. It was found that
ZnO/Cu colloids, as well as Cu-Zn stearate and Cu-Al stearate were all very active in
methanol synthesis. Under the same conditions, the methanol productivity of Cu-Zn
stearate was as high as that of the Cu/ZnO/Al2O3 reference catalysts - ca. 6
molMeOH/(kgCu·h). In contrast, hexadecylamin (HDA)-stabilized Cu colloid only showed
negligible activity [218, 230]
. Their results support the two conclusions from this study. One
is that the ZnO component contained in the technical catalyst might not be absolutely
necessary in Cu colloid to promote high activity for methanol synthesis. Whether the
stabilizer is alkylaluminium or alkylzinc seems to have no obvious influence on the
methanol productivity. The other is that non-metal alkyl-stabilized Cu colloids have no
activity in methanol synthesis. Therefore, the interfaces of either Cu-Zn or Cu-Al, even in
colloidal solutions, have a strong influence on the Cu particle activity in methanol
synthesis, similar as for the solid catalyst.
Results and discussion
99
0
5
10
15
20
Ag c
olloid
Ni c
olloid
Cu(a
cac)
2
Cu/D
BM
g
Cu /T
BAl
Cu/T
OAl
Cu /D
EZn
Cu /D
BZn
Cu c
olloid
(alte
rnat
ive
precur
sor)
Kat
alco
JM 5
8-1
Catalyst type
Meth
an
ol
pro
du
cti
vit
y [
mo
l/K
gC
u/h
]
Cu(a
cac) 2
KATA
LCO JM
51-8
Meth
an
ol
pro
du
cti
vit
y[m
ol M
eO
H/k
gC
u/h
]
Figure 3.34 Summary of methanol productivity of all the metal colloids and reference materials.
3.2.4 Summary
The four Cu colloids stabilized by either alkylaluminium or alkylzinc all exhibited high
activity in methanol synthesis in a quasi-homogeneous phase. The catalytic results of the
temperature dependent tests showed that the highest methanol productivity of Cu/TOAl at
170 °C was 23.3 molMeOH/(kgCu·h). The use of Cu colloids allows methanol synthesis at
lower temperatures and methanol formation already started from 130 °C. Under the same
reaction conditions, the colloids were more active than the benchmark catalyst -
KATALCOJM 51-8 (Cu/ZnO/Al2O3) in THF suspension. The Cu colloids were stable
over relatively long reaction times up to 40 hours. Among the four Cu colloids
synthesized with Cu(acac)2 as Cu precursor, Cu/DEZn and Cu/TOAl, which have larger
particle sizes, show higher methanol productivities than Cu/DBZn and Cu/TBAl. It seems
that the Cu particle size has some influence on the activity; in the case of metal alkyl-
stabilized Cu colloids in methanol synthesis the larger the particle size is the higher is the
activity. An on-line product analysis showed that substantial amounts of methyl formate
were formed, which is rather an intermediate than a by-product. A possible pathway
Results and discussion
100
proceeds via the hydrogenolysis of initially formed methyl formate with H2 to the final
product - methanol.
Changes in morphology and crystallinity of Cu nanoparticles have been observed using
TEM, XRD and XAS. During reaction, the Cu nanoparticles agglomerated and their
particle shape was distorted. The Cu core also became polycrystalline with either larger
or smaller crystalline domains. The alkylaluminium stabilizer was likely to be oxidized to
amorphous Al2O3, methanolate, or hydrated Al species, while alkylzinc stabilizer was
confirmed to be oxidized to crystalline ZnO. GC-MS further confirmed the existence of
substances from the decomposed colloids, corresponding to the ligands in the colloids.
Surprisingly, these decomposed Cu colloids, present as precipitates, still remained active
at a high level in solution for a long time. This might be attributed to possible reformation
of Cu/Al2O3 and Cu/ZnO from the decomposed Cu colloids, similar to the active
components in the solid methanol synthesis catalysts. Thus, they behaved as supported
Cu nanoparticles in THF suspension, which led to a continuous activity level under
reaction conditions.
A series of experiments was designed to reveal the function of Cu colloids in catalysis.
The Cu colloids only stabilized by organic surfactants showed no activity. It was found
that the activity of Cu colloids was provided by the synergy between Cu core and the
metal alkyl stabilizing shell. The interactions of Cu-metal present on the surface of Cu
nanoparticles may provide the active sites. Cu colloids stabilized by alkylaluminium and
alkylzinc showed higher activities than those stabilized by alkylmagnesium. The
interaction of Cu-Al and Cu-Zn are stronger, promoting higher activity, which
corresponds very well to the fact that Cu/ZnO and Cu/Al2O3 are the common active
components in solid catalysts.
The study of the catalytic properties of Cu colloids in quasi-homogeneous phase provides
rich and interesting information on the essential features required for methanol synthesis
activity. It thus seemed worthwhile to attempt immobilization and stabilization of the Cu
nanoparticles on solid supports, so that the high activity of Cu colloids could possibly be
transferred to a gas-phase reaction. The results of these efforts are described in the next
chapter.
Results and discussion
101
3.3 Supported copper nanoparticles
In Chapters 3.1 and 3.2, the properties and the catalytic performance of Cu colloids
stabilized by different stabilizers in quasi-homogenous methanol synthesis were studied
using various techniques. Since the Cu colloids were highly active in a quasi-
homogeneous phase, it would be interesting to see if such high activity could be
transferred to a heterogeneous system where Cu nanoparticles are supported by solid
supports. The solid supports would be expected to stabilize the Cu nanoparticles and
maintain their high activity in a heterogeneous reaction. Moreover, heterogenization
would allow the reactions to be carried out in the gas phase, which is more comparable to
an industrial process for methanol synthesis. In this chapter, Cu colloids supported on
different solids will be studied and their catalytic activity in methanol synthesis will be
discussed. The methanol formation mechanism in the gas-phase reaction is also discussed
in comparison with that in quasi-homogeneous phase.
Results and discussion
102
3.3.1 Synthesis
The supported Cu colloids were prepared by a direct colloidal deposition method via the
‘precursor concept’, as described in Section 5.1.2. [126, 132]
. This direct and facile approach
includes wet impregnation of a solid support with a colloidal solution, followed by the
removal of solvent [171-172]
. Before the impregnation, the solid supports were dried and
outgassed. Then these supports were loaded directly with Cu colloids as Cu precursor
under Ar protection. The THF solvent was subsequently removed under vacuum and
stirring at 30 °C overnight. These two steps of adding colloid and removing solvent were
repeated three times and the amount of loaded Cu was adjusted to be around 1 and 2 wt%,
in order to ensure homogeneous dispersion [128]
. The actual amount of Cu loading
determined by EDX analysis was found to be in agreement with the intended loading
level. Compared with the conventional salt solution impregnation with subsequent
reduction, this method has its advantages, since the preparation conditions help to
maintain similar particle size and shape as for the Cu colloid in solution [114]
. Also, it is
possible to tailor the size and composition of the ‘precursor’ independently using
different Cu colloids.
The most widely reported supports used to adsorb the colloidal nanoparticles are
inorganic solids, such as activated carbon, SiO2, Al2O3, or other metal oxides. In this
work, several typical solid supports were selected and studied. The solid samples are
listed in Table 3.7. The selected supports can be divided into two classes: One is ordered
mesoporous materials, including mesoporous silica (SBA-15) and ordered mesoporous
carbon (CMK-5); the other class comprises conventional metal oxides (ZrO2 and ZnO).
The ordered mesoporous supports may be interesting in various applications, such as
heterogeneous catalysis, adsorption, sensing, separation and energy storage [276-280]
.
Especially ordered mesoporous carbon and silica have very high surface area, large pore
volume, high thermal stability, and they contain almost no impurities. These properties
make them excellent catalyst supports. Since SBA-15 and CMK-5 are inert solid supports
for many reactions, they may stabilize and isolate the Cu nanoparticles without strong
interference with Cu. Therefore, the supported Cu nanoparticles in gas-phase reaction
could be directly compared to Cu nanoparticles in colloidal solutions. Moreover, since
Results and discussion
103
the Cu particle size (3-6 nm) is smaller than the general pore size of the solid materials
(6-8 nm), the Cu nanoparticles are expected to be fixed and separated inside the pore
systems (channels) of the solids, as illustrated in Figure 3.35 [281-282]
. This could further
prevent the Cu nanoparticles from agglomeration under the reaction conditions. ZrO2 and
ZnO are two typical active supports that are widely used to support catalysts for methanol
synthesis. In this study, both these supports were used in order to reveal the interaction
between Cu and one support over the other.
Table 3.7 Overview of the supported catalysts studied.
Sample name Cu colloid Support
Cu/Al@SBA-15 Cu/TOAl SBA-15
Cu/Zn@SBA-15 Cu/DBZn
Cu/Al@CMK-5 Cu/TOAl CMK-5
Cu/Zn@CMK-5 Cu/DBZn
Cu/Al@ZrO2 Cu/TOAl ZrO2
Cu/Zn@ZrO2 Cu/DBZn
Cu/Al@ZnO Cu/TOAl
ZnO
Cu/Zn@ZnO Cu/DBZn
Figure 3.35 The direct colloidal deposition method via ‘precursor concept’ applied in the
synthesis of supported Cu nanoparticles, using ordered mesoporous materials as supports.
Results and discussion
104
3.3.2 Catalytic performance in gas-phase methanol synthesis
All the supported Cu nanoparticles were tested in methanol synthesis in a gas-phase
reaction using a plug-flow reactor (single-tube reactor). The reaction conditions were the
same as previously reported when using the 49-fold high-throughput reactor system,
close to those used in an industrial process [26]
. The total gas pressure was 4.5 MPa with
the molar ratios of H2/CO/CO2 = 70/24/6, and the reaction temperature was 245 °C. Prior
to the tests, the supported Cu nanoparticles were not oxidized or calcined, but were filled
in the reactor under Ar protection in a glove box. Thus, the state of the Cu nanoparticles
on the supports was as close as possible to that in colloidal solutions.
The catalytic results of the methanol synthesis over Cu nanoparticles supported on
different solids are summarized in Figure 3.36, in comparison with those of the
benchmark catalyst (KATALCOJM 51-8). This catalyst is one of the most often used solid
catalysts worldwide for methanol synthesis, and is produced by Johnson Matthey
Catalysts. Most of the reported values are the averages of the methanol productivities
from different synthesis batches and the variation is between 1% and 34% as marked by
the error bar. Because their methanol productivity using the unit molMeOH/(kgCat·h) was
too low to compare in the figure with the benchmark catalyst, which is due to the low Cu
loading at 1-2 wt%, the values of methanol productivity in the gas-phase reaction were all
normalized to molMeOH/(kgCu·h), as it was done in the quasi-homogeneous reaction. As a
reference and industrial catalyst, KATALCOJM 51-8 shows the highest methanol
productivity - 53.9 molMeOH/(kgCu·h). Most of the colloids supported on metal oxides
show comparable activities for methanol synthesis to that of the reference catalyst, except
Cu/Zn@ZnO. The highest methanol productivities of supported Cu nanopaticles
(Cu/Al@ZnO, and Cu/Al@ZrO2) reach 50.8 molMeOH/(kgCu·h). Very surprisingly, Cu
nanoparticles supported on ordered mesoporous materials (SBA-15 and CMK-5) exhibit
very low activity, compared to the other systems. This is opposite to our expectation that
the ordered mesoporous materials, having higher specific surface area and ordered pore
structure, would better stabilize the Cu nanoparticles and thus offer higher activity.
It has also to be mentioned here that, the methanol productivity of Cu/Al@CMK-5
obtained from the plug-flow reactor is 11.1 molMeOH/(kgCu·h). It is close to 7.4
molMeOH/(kgCu·h) that was reported by S. Vukojević in his study using the high-
Results and discussion
105
throughput reactor under the same conditions [36]
. It further confirms reproducibility of
the supported Cu nanoparticles in gas-phase reaction tested by different reactors.
0
10
20
30
40
50
60
Cu/Z
n@SBA-1
5
Cu/A
l@SBA-1
5
Cu/Z
n@CM
K-5
Cu/A
l@CM
K-5
Cu/Z
n@ZnO
Cu/A
l@ZnO
Cu/Z
n@ZrO
2
Cu/A
l@ZrO
2
Kat
alco
JM 5
1-8
Catalyst
Me
tha
no
l p
rod
uc
tiv
ity
[m
ol/k
gC
u/h
]
Cu/A
l@ZrO
2
Cu/Z
n@ZrO
2
KATA
LCO JM
51-8M
eth
an
ol p
rod
ucti
vit
y [
mo
l MeO
H/(
kg
Cu·h
)]
Figure 3.36 Methanol productivities of all the supported Cu nanoparticles in comparison with the
benchmark catalyst (activity normalized to Cu content).
The methanol productivities of each catalyst depending on time on stream are
summarized in Figure 3.37. All different supported Cu colloids were used to catalyze the
methanol synthesis reaction under the same conditions for 10 hours and the products were
analyzed using an on-line GCs every 12 min. The most active benchmark catalyst
KATALCOJM 51-8 shows stable reaction rate throughout the reaction without obvious
drop of productivity. Both Cu/Al@ZrO2 and Cu/Al@ZnO show comparable methanol
productivities to that of the benchmark catalyst. In the case of Cu/Al@ZrO2, it shows the
same performance as the benchmark catalyst at the beginning of the reaction, but a slight
decrease of productivity occurs during reaction. The reason is unclear, but it could be due
to the change of the catalyst structure, such as the agglomeration of particles, formation
of new phases, etc. In contrast, for Cu/Al@ZnO, the methanol productivity slightly
increases with reaction time, and it arrives at the same level as that of the benchmark
catalyst at the end of 10 hours reaction. Very interestingly, in the case of Cu/Zn@ZrO2,
the methanol productivity increases strongly with reaction time, and at the end of reaction
almost reaches the level of benchmark catalyst. This indicates that under reaction
Results and discussion
106
conditions the catalysts might undergo substantial changes. Cu/Zn@ZnO and those Cu
nanoparticles supported on SBA-15 or CMK-5, though less active, all maintain high
stability throughout the entire reaction time, with just a slight decrease.
10 20 30 40 500
10
20
30
40
50
60
Meth
an
ol p
rod
ucti
vit
y [
mo
l/kg
Cu
/h]
Injection
Cu/Zn@SBA-15
Cu/Al@SBA-15
Cu/Zn@CMK-5
Cu/Al@CMK-5
Cu/Zn@ZnO
Cu/Al@ZnO
Cu/Zn@ZrO2
Cu/Al@ZrO2
KATALCOJM
51-8
Figure 3.37 Methanol productivities of all the supported Cu nanoparticles over 10 hours (12 min
per injection).
New questions thus arise for the methanol synthesis in gas-phase reaction: does the
chemistry of the support influence the activity of the Cu nanoparticles? Therefore, it
would be necessary to study the support effect in more detail, and to investigate the
interaction between the support and Cu nanoparticles.
3.3.3 Support effect
The catalytic performance of the supported metal colloids could be influenced by several
factors. Among those factors, the support effect (the nature of the support and the metal-
support interaction) could have direct impact on the reactivity and the selectivity of the
catalytic reactions [128, 130]
. As shown in the above section, after transferring the Cu
colloids to a heterogeneous system, the supported Cu nanoparticles exhibit very different
activities when using different solid supports. Therefore, more detailed studies on the
correlation between the catalyst structure and their activity were required to determine the
nature of the support effect. Since this gas-phase reaction using supported Cu
Results and discussion
107
nanoparticles is comparable to the industrial process, the study may also help to answer
questions about the reaction mechanism and active sites for methanol synthesis.
3.3.3.1 SBA-15
Figure 3.38 (a) and (b) show TEM images of the Cu nanoparticles supported on typical
ordered mesoporous silica (SBA-15). For both Cu/Zn@SBA-15 and Cu/Al@SBA-15, the
Cu nanoparticles do not seem to be uniformly dispersed inside the pore systems of SBA-
15, instead, they are located on the external surface of the support. EDX analysis of
Cu/Zn@SBA-15 detected a loading of Cu of ca. 2.0 wt%, which is very close to the Cu
loading desired in the preparation. This indicates that all the Cu offered was loaded onto
the solid support.
(a) (b)(a) (b)
Figure 3.38 TEM images of (a) Cu/Zn@SBA-15 and (b) Cu/Al@SBA-15.
The N2 sorption measurements give useful information on the textural properties of these
ordered mesoporous solids loaded with Cu nanoparticles. The N2 adsorption-desorption
isotherms (Figure 3.39) of both the original SBA-15 and Cu/Zn@SBA-15 show type-IV
shapes with H2 hysteresis loops in the mesopore range. The shapes of the N2 isotherms of
those supports before and after Cu loading are identical, indicating that the porous
structure of the support is intact after the introduction of Cu nanoparticles.
The textural parameters of both materials obtained from N2 sorption are listed in Table
3.8. The average pore diameter of SBA-15 before Cu loading calculated from the
desorption branch using the BJH method is 8.04 nm, larger than the particle size of
Cu/DBZn (ca. 4 nm). Therefore, it could be expected that the Cu naoparticles entered the
pore system and that the particles would be stabilized. Moreover, all the textural
Results and discussion
108
parameters decreased, probably due to the loading of Cu onto/into the porous structure.
However, there is no conclusive proof, and whether the Cu nanoparticles were located
inside or outside the pore system remain unclear. Electron tomography or thin sectioning
for TEM could clarify the problem, but the possible result did not seem to justify the
effort.
0.0 0.2 0.4 0.6 0.8 1.00
200
400
600
800
1000
1200
Vo
lum
ead
s [
cm
3/g
]
Relative pressure [p/p0]
SBA-15
Cu/Zn@SBA-15
+400 cm3/g
Figure 3.39 N2 adsorption-desorption isotherms of Cu/Zn@SBA-15 in comparison with SBA-15.
Table 3.8 Textural parameters obtained from N2 sorption for SBA-15 and Cu/Zn@SBA-15.
Sample SBET (m2g
-1) Vtot (cm
3g
-1) Dmax (nm)
SBA-15 476 1.03 8.04
Cu/Zn@SBA-15 396 0.73 5.77
SBET: apparent surface area calculated by BET method; Vtot: total pore volume at P/P0 = 0.97;
Dmax: pore sizes at the maxima of the pore size distribution calculated from the desorption branch
using the BJH method.
After 10 hours reaction, both Cu/Zn@SBA-15 and Cu/Al@SBA-15 were collected in air
and investigated by TEM (Figure 3.40). It is clear that the mesoporous structure of the
SBA-15 is retained, but the Cu particles appear different from those before reaction.
Interestingly, for Cu/Zn@SBA-15, as shown in Figure 3.40 (a), there are pronounced
Results and discussion
109
elongated particles growing on the external surface of the SBA-15. In contrast, for
Cu/Al@SBA-15 as shown in Figure 3.40 (b), large particles in a size range of 10 to 50
nm appear on the surface of SBA-15. These particles are much larger than the original Cu
particles (5 nm), the Cu particles thus agglomerated significantly during the reaction.
Apparently, they might not have been introduced into the pore systems of SBA-15 or they
might also have migrated out during reaction, so that might be the reason why their
particle agglomeration was not limited at all.
(a) (b)(a) (b)
Figure 3.40 TEM images of SBA-15-supported Cu nanoparticles after reaction:
(a) Cu/Zn@SBA-15; (b) Cu/Al@SBA-15.
In order to further explore the changes of the Cu nanoparticles and to analyze the
compositions of Cu nanoparticles after reaction, HRTEM and EDX analysis were applied
to both materials. The HRTEM images and EDX analysis results of Cu/Zn@SBA-15 are
shown in Figure 3.41 and Table 3.9, respectively. Figure 3.41 (a) demonstrates that the
elongated particles are well crystallized and mostly appear to be single crystalline
particles. The region 3 in Figure 3.41 (b) selects only the elongated particles and Table
3.9 shows that they are composed of mainly Zn and very little Cu. Therefore, it can be
concluded that these elongated particles are most likely pure ZnO on the surface of SBA-
15. Within most of the other regions (1, 2, 5-7) focusing on the surface of the solid, the
Cu and Zn are well distributed with a Cu/Zn ratio of between 1/10 and 1/15 as in the
Cu/DBZn precursor. Region 4 has darker contrast, which corresponds to a relatively high
Cu concentration. The SEM-EDX mapping further showed that the Cu amount is ca. 2
wt% and the Zn amount is ca. 20 wt%, which also corresponds to the Zn/Cu ratio of 10/1
that is the same as in the Cu/DBZn precursor. It can be assumed that the alkylzinc
Results and discussion
110
stabilizer was oxidized during gas-phase reaction to form ZnO, similar to what had
occurred to the Cu colloids in a quasi-homogeneous reaction.
(b)(a) (b)(a)
Figure 3.41 HRTEM images of Cu/Zn@SBA-15 after reaction
Table 3.9 EDX analysis of Cu and Zn distribution in Figure 3.41 (b) for Cu/Zn@SBA-15 after
reaction.
Region Cu/Zn@SBA-15
Cu wt% Zn wt%
1 1.2 14.9
2 1.3 12.7
3 1.0 38.6
4 16.2 27.7
5 1.0 13.8
6 1.2 15.9
7 1.0 12.3
Large polycrystalline particles are found in the Cu/Al@SBA-15 after reaction in HRTEM
images, as shown in Figure 3.42. The EDX analysis results for Cu/Al@SBA-15 after
reaction are displayed in Figure 3.42 (b) and Table 3.10. In large regions (1-4), the Al/Cu
ratio is around 3/1, the ratio in the Cu/TOAl precursor. SEM-EDX mapping revealed that
the Cu concentration is ca. 2.5 wt% and the Al concentration is ca. 3 wt%, which is close
to the ratio detected by ICP using the Cu/TOAl. The Cu content in the regions 5 and 7,
Results and discussion
111
focusing on the particles with the dark contrast is much higher, whereas in region 6 only
Al is found. Therefore, the large particles are enriched in Cu. In comparison with the Cu
particle size of the fresh Cu/TOAl (5 nm), the particle size of Cu/Al@SBA-15 has
become evidently larger after reaction. From the counting statistics, most of the Cu
particles are larger than the pore size of SBA-15 and they thus must be located on the
external surface or must have locally destroyed the SBA-15 structure. Therefore, the Cu
nanoparticles either had never entered the pore system of SBA-15, have migrated out of
the channel system, or even damaged the support material during reaction.
(a) (b)(a) (b)
Figure 3.42 HRTEM images of Cu/Al@SBA-15 after reaction.
Table 3.10 EDX analysis of Cu and Al distribution in Figure 3.42 (b) for Cu/Al@SBA-15 after
reaction.
Region Cu/Al@SBA-15
Cu wt% Al wt%
1 1.2 3.3
2 0.7 3.0
3 1.1 3.4
4 1.1 3.3
5 82.7 0
6 0 1.6
7 13.3 3.2
Results and discussion
112
Even though the pore size of SBA-15 (ca. 8 nm) is much larger than that of the Cu
particle size (3-6 nm), due to the hydrophilic surface property [283]
, SBA-15 has low
affinity to the Cu particles that have a hydrophobic surface caused by the metal alkyl
stabilizing shells. Therefore, it might be difficult for the Cu nanoparticles to enter the
pore systems of SBA-15, and they may be preferably adsorbed outside. Therefore, the Cu
nanoparticles are in fact not isolated well by the pore system of SBA-15. This causes
severe agglomeration under reaction conditions, thus resulting in their low methanol
productivity. It should be noticed, however, that even though particle growth existed, the
catalyst activity remained stable throughout the reaction as shown in Figure 3.37. This
may suggest that particles growth occurs already early during the reaction. However, the
low activity of the SBA-15 supported Cu particles may be also due to the fact that SiO2
has no strong interaction with Cu. As has already been reported in the literature, among
Cu catalysts supported by several typical metal oxide supports (SiO2, ZnO, ZrO2, Ga2O3,
Al2O3), SiO2 as support showed the lowest efficiency [284]
.
3.3.3.2 CMK-5
The TEM images of Cu nanoparticles supported on the ordered mesoporous carbon
CMK-5 are shown in Figure 3.43. A similar mesostructure as for SBA-15 is observed,
representing the hexagonal pore arrangement due to the nanocasting synthesis route of
CMK-5 with SBA-15 as template [278, 285]
. It is clearly seen that the Cu particles are well
dispersed and the particle size range is unchanged (3-6 nm). However, it is still difficult
to judge whether the particles are fixed inside the pores of the support or just adsorbed
onto the surface. The EDX mapping for Cu/Zn@CMK-5 confirmed that the Cu loading is
ca. 1.3 wt%.
The N2 adsorption-desorption isotherms of both the CMK-5 and Cu/Zn@CMK-5 are
shown in Figure 3.44, and the texture parameters are listed in Table 3.11. The isotherms
of both original CMK-5 and Cu/Zn@CMK-5 are of type-IV with predominantly H2
hysteresis loops in the mesopore range. The average pore diameters calculated from the
desorption branch by the BJH method for CMK-5 and Cu/Zn@CMK-5 are 3.86 nm and
3.78 nm, respectively. Both are about the same size of Cu/DBZn (ca. 4 nm). It could thus
be possible that the Cu particles are located either inside or outside of the pore systems of
CMK-5.
Results and discussion
113
(a) (b)(a) (b)
Figure 3.43 TEM images of (a) Cu/Zn@CMK-5 and (b) Cu/Al@CMK-5.
0.0 0.2 0.4 0.6 0.8 1.0
250
500
750
1000
1250
1500
1750
2000
CMK-5
Cu/Zn@CMK-5
Vo
lum
ead
s [
cm
3/g
]
Relative pressure [p/p0]
+400 cm3/g
Figure 3.44 N2 adsorption-desorption isotherms of Cu/Zn@CMK-5 in comparison with CMK-5.
Similar to the results of SBA-15, the shapes of the N2 isotherms before and after Cu
loading remain the same, indicating that the porous structure of the support was intact
after the introduction of Cu nanoparticles. With exception of the pore diameter, the other
textural parameters shown in Table 3.11 all decrease strongly upon Cu loading. The
decrease of pore volume and surface area could be mainly due to pore blocking by Cu
particles in the pore system of CMK-5, which suggests that the particles are located
inside the pores of the support. In our measurement, the Cu/Zn@CMK-5 was only
degassed at 80 °C for 10-12 hours in order to avoid the Cu colloid decomposition and
particle agglomeration. At such low degassing temperature, it is possible that there was
still a trace amount of the stabilizer ligands from solvent THF or even the residual THF
Results and discussion
114
inside the pores, causing the decrease in the values of the textual parameters. S.
Vukojević, during his study of Cu/Al@CMK-5, attributed the change of the textual
properties of CMK-5 to the local destruction of the carbon structure by Cu nanoparticles,
but no solid evidence was provided [36]
.
Table 3.11 Textural parameters obtained from N2 sorption for CMK-5 and Cu/Zn@CMK-5.
Sample SBET (m2g
-1) Vtot (cm
3g
-1) Dmax (nm)
CMK-5 1985 2.37 3.86
Cu/Zn@CMK-5 1409 1.64 3.78
SBET: apparent surface area calculated by BET method; Vtot: total pore volume at P/P0 = 0.97;
Dmax: pore sizes at the maxima of the pore size distribution calculated from the desorption branch
using the BJH method.
The Cu nanoparticles supported on CMK-5 after 10 hours reaction were also investigated
by TEM, as shown in Figure 3.45. Different from the SBA-15-supported Cu
nanoparticles, the CMK-5-supported nanoparticles are still small and well dispersed.
Even though their particle sizes did not increase as strongly as for Cu/Al@SBA-15 (10-
50 nm), they still grew to sizes bigger than 10 nm, suggesting the agglomeration of Cu
nanoparticles during the reaction. For Cu/Al@CMK-5, one possible reason could be that
due to the larger Cu particle size of Cu/TOAl, some of the Cu particles could not be
introduced into the pore systems of CMK-5, leading to their agglomeration on the surface
of the support.
(a) (b)(a) (b)
Figure 3.45 TEM images of CMK-5-supported Cu nanoparticles after reaction:
(a) Cu/Zn@CMK-5; (b) Cu/Al@CMK-5.
Results and discussion
115
Cu/Zn@CMK-5 after reaction was chosen for further characterization by HRTEM, as
shown in Figure 3.46, in order to explore how the Cu nanoparticles are located after
reaction. Spherical particles are observed, which seem to be dispersed both in the
framework and on the surface of CMK-5. Due to the low contrast difference between
CuO and ZnO in TEM, it is difficult to differentiate the two phases. In any case, no
elongated ZnO particles are observed after reaction, which is different from the
Cu/Zn@SBA-15 sample. This suggests that for Cu/Zn@CMK-5 the Cu nanoparticles are
most likely located inside the pore system of CMK-5, and their particle growth under
reaction conditions thus is limited.
(a) (b)(a) (b)
Figure 3.46 HRTEM images of Cu/Zn@CMK-5 after reaction.
In order to check the distribution of Cu and Zn, EDX analysis was carried out in different
areas of the sample, as demonstrated in Figure 3.47 and Table 3.12. In the larger regions
(1, 2, 4-7) the amount of Zn is much higher than that of Cu, whereas in the other dark
contrast regions (3, 8-10) the Cu content is much higher. Therefore, Cu particles in this
case mainly agglomerated forming large particles, while Zn is dispersed into/onto the
support but not identified as such a distinct feature as the elongated particles in the case
of Cu/Zn@SBA-15. The SEM-EDX mapping showed that the Cu concentration is ca. 1.7
wt% and the Zn concentration is 5.1 wt%. However, there is no proof on whether the
stabilizer is oxidized to form crystallized ZnO, as in the case of Cu colloid in solution
after reaction.
Results and discussion
116
Figure 3.47 HRTEM-EDX of Cu/Zn@CMK-5 after reaction.
Table 3.12 EDX analysis of Cu and Zn dispersion in Figure 3.47 for Cu/Zn@CMK-5 after
reaction.
Region Cu wt% Zn wt%
1 2.4 55.7
2 1.1 55.0
3 34.5 16.9
4 14.3 48.9
5 1.8 62.9
6 4.8 52.7
7 2.3 53.1
8 28.9 27.7
9 38.4 28.7
10 83.8 6.4
The results described above prove that with CMK-5 as support, the Cu nanoparticles
could be better adsorbed and fixed inside the pore system. However, they still
Results and discussion
117
agglomerated and grew in the pore system, though the agglomeration was less extensive
than in the SBA-15 support. Like the Cu nanoparticles supported on SBA-15, those
supported on CMK-5 showed very low but stable activities throughout the reaction. This
might be due to the weak interactions between the support and the Cu nanoparticles; the
weak interaction does not promote the activity of the Cu nanoparticles in the gas-phase
reaction. However, no explanation could be given for the fact that particle agglomeration
did not affect the activity. Generally, the properties of Cu nanoparticles, supported on
SBA-15 or CMK-5, are close to those of the colloids dispersed in solution, but their high
activity in a quasi-homogeneous phase methanol synthesis cannot be transferred to a gas-
phase reaction.
3.3.3.3 Metal oxides - ZrO2 and ZnO
Both ZrO2 and ZnO have already been reported as good promoters and active supports
for solid catalysts in methanol synthesis from synthesis gas feed [21]
. As already described
above, Cu nanoparticles supported on either ZrO2 or ZnO exhibited much higher
activities than using SBA-15 or CMK-5 as supports; normalized to Cu content, the
activity is even comparable with that of the benchmark catalyst. In order to understand
the support effect better, the samples were analyzed with different techniques, including
TEM, SEM and EDX, for both freshly prepared and spent catalysts. Figure 3.48 displays
the TEM images of the Cu nanoparticles supported on metal oxides (ZrO2 and ZnO)
before reaction. In the TEM images, no Cu particles can be identified on the support
structures. For Cu particles supported on ZnO, it is due to the fact that the contrast
difference between Cu and Zn is very small. In the case of using ZrO2 as support, the
small amount of low scattering Cu cannot be distinguished, because it is inside the highly
scattering ZrO2 matrix which is present as the dark background.
Results and discussion
118
(a) (b)
(c) (d)
(a) (b)
(c) (d)
Figure 3.48 TEM images of Cu nanoparticles supported on metal oxides: (a) Cu/Zn@ZrO2; (b)
Cu/Al@ZrO2 (c) Cu/Zn@ZnO; (d) Cu/Al@ZnO.
The two most active catalysts - Cu/Al@ZrO2 and Cu/Al@ZnO - were selected for more
detailed studies of the samples after reaction. HRTEM images of Cu/Al@ZrO2 after
reaction in Figure 3.49 demonstrate the existence of a multicrystalline phase mixture.
SEM-EDX confirms the Cu loading to be ca. 1.5 wt%. However, none of these images
shows how the particles are located on the ZrO2 support. Since the phases are not well
distinguished from each other, HRSEM mapping was carried out to determine the
distribution of elements and particles. As shown in Figure 3.50, Cu, Al and Zr are clearly
homogeneously dispersed in the solid material. Cu does not exist in the form of
nanoparticles and there is no agglomeration of Cu particles either, which is not the case
as expected. Cu seems to be well dissolved into the support, which resembles the model
proposed in the literature for the Cu/ZnO [64, 97]
. Furthermore, it can be assumed that
under reaction conditions the alkylaluminium protection shell of the Cu particles was
oxidized to generate Al2O3, methanolate or hydrated Al species, similar to what happened
Results and discussion
119
in solution. Accordingly, the catalyst during reaction might form the ternary system
Cu/Al2O3/ZrO2. It has been proven by many studies that ZrO2 is a good carrier for Cu and
Cu/ZrO2 is very active in methanol synthesis [21]
. Therefore, it is not surprising that Cu
nanoparticles supported on ZrO2 also showed high activity in this study.
(a) (b)(a) (b)
Figure 3.49 HRTEM images of Cu/Al@ZrO2 after reaction.
Cu
Al Zr
Figure 3.50 HRSEM-EDX mapping of Cu, Al and Zr for Cu/Al@ZrO2 after reaction.
Results and discussion
120
The same conclusions can be drawn from the study of Cu/Al@ZnO after reaction.
HRTEM images of this sample are shown in Figure 3.51. The Cu loading determined by
SEM-EDX is as low as 2.1 wt%. The multicrystalline phase mixture could be due to the
formation of the ternary system Cu/ZnO/Al2O3, which contains the same active
component as the commercial benchmark catalyst for methanol synthesis, even if it is at
very different concentrations. Therefore, the possible formation of both Cu/Al2O3/ZrO2
and Cu/ZnO/Al2O3 ternary systems during reaction could be the reason why these two
materials (Cu/Al@ZrO2 and Cu/Al@ZnO) were found to be much more active than the
other supported Cu nanoparticles tested in gas-phase methanol synthesis. However, more
detailed studies are required to explore how the Cu species are located in ZnO matrices.
It needs to be mentioned that among the metal oxide-supported Cu nanoparticles,
Cu/Zn@ZnO was the least active, whereas the other three were nearly as active as the
benchmark catalysts under the same conditions. This observation is in agreement with
some published work, which showed that the pure binary Cu/ZnO is much less active
than the ternary system Cu/ZnO/Al2O3 [275, 286]
. Due to the possible oxidation of the
alkylzinc stabilizing shell, Cu/Zn@ZnO during reaction probably only resulted in the
formation of the binary Cu/ZnO system.
(a) (b)(a) (b)
Figure 3.51 HRTEM images of Cu/Al@ZnO after reaction.
3.3.4 Mechanism of methanol synthesis over supported Cu nanoparticles
For almost all the tested catalysts, methanol was the main product of the reaction. The
only exception was Cu/Al@CMK-5, where the reaction generated a substantial amount of
methyl formate, besides methanol. S. Vukojević had also found, in his study, significant
Results and discussion
121
amounts of methyl formate over Cu/Al@CMK-5 [36-37]
. This significant formation of
methyl formate was also observed over Cu colloids in quasi-homogeneous phase, but not
in the case of using the solid Cu/ZnO/Al2O3 ternary catalysts where the formation of
methyl formate was negligible. To understand this interesting difference, kinetic tests
with varied reactant gas flow rate were carried out, where the gas flow was set to 10, 20
and 40 mL/min at the same gas composition. Both methanol and methyl formate
productivities under different reactant flow rate are shown in Figure 3.52.
0 150 300 450
0
5
10
15
20
25
30
Methanol
Methyl formate
Pro
du
cti
vit
y [
mo
l/(K
gC
u•h
)]
Time on stream [min]
10 ml/min
0 150 300 450
20 ml/min
0 150 300 450 600
40 ml/min
Methanol
Methyl formate
Figure 3.52 The productivities of both methanol and methyl formate using Cu/Al@CMK-5
according to different reactant flow rate.
The results show that the higher the reactant gas flow rate is, the higher is the methyl
formate productivity. At lower gas flow rate of 10 mL/min, the methanol productivity is
higher than the methyl formate productivity. Both are at about the same level at a gas
flow rate of 20 mL/min. At higher gas flow rate of 40 mL/min, the methyl formate
productivity exceeds methanol productivity. Thus, at shorter contact time the methyl
formate productivity is higher. Both methanol and methyl formate productivities decrease
with reaction time, but the methyl formate productivity decreases more rapidly than that
of methanol at shorter contact time. The dependence of methyl formate and methanol
formations on contact time is in agreement with the assumption that methyl formate could
be an intermediate, which is further hydrogenolized to form methanol. Therefore, the
Results and discussion
122
mechanism of methanol formation in gas phase using supported Cu nanoparticles as
catalysts might be the same as that over Cu colloids in solution. However, more evidence
would be required to provide details for the methanol formation mechanism in a gas-
phase reaction. It also needs to be mentioned here that, the overall reactivity of this
catalyst seems to increase at the higher flow rate of 40 mL/min. The reason for this is
presently unclear, which might be due to certain errors existing in the measurement
during reaction process.
3.3.5 Summary
Supported Cu nanoparticles were synthesized by a colloidal deposition method via a
‘precursor concept’, using Cu colloids directly as Cu precursors. The Cu nanoparticles
were adsorbed onto different solid supports without further modification, so their
structural properties, including size and shape should not be changed by deposition.
Typical solids were selected as supports, including ordered mesoporous materials (SBA-
15 and CMK-5) and metal oxides (ZrO2 and ZnO). All the Cu nanoparticles supported on
different solid supports were active in gas-phase methanol synthesis under similar
conditions as used in an industrial process. Most of the investigated supported Cu
nanoparticles exhibited relatively stable methanol productivity throughout 10 hours
reaction time, with only a slight decrease in activity. In general, metal oxide-supported
Cu nanoparticles exhibited much higher methanol productivity than those supported on
the ordered mesoporous materials. Some of the metal oxide-supported Cu nanoparticles
(Cu/Al@ZrO2 and Cu/Al@ZnO) were nearly as active as the benchmark catalyst
(normalized to Cu content), and their methanol productivity was as high as 50.8
molMeOH/(kgCu·h), when normalized to the low Cu loading of 1-2 wt%. The differences
between their activities were clearly caused by the support applied, so the support effect
in each case was investigated in detail.
When using SBA-15 as support, the Cu nanoparticles were most likely deposited on its
external surface. This led to severe agglomeration of Cu particles during reaction as well
as to the growth of ZnO nancrystals on the external surface of the SBA-15. For CMK-5,
the Cu nanoparticles seemed to be fixed inside the mesopore system of CMK-5. Although
the particle agglomeration was limited to some extent, particle growth during reaction
Results and discussion
123
still took place. Therefore, Cu nanoparticles are not well isolated or protected by either
SBA-15 or CMK-5. Surprisingly, this did not influence the stability of these materials
and their activities throughout reaction did not decrease. However, probably because
these supports do not interact substantially with Cu particles, methanol productivities
over these samples were very low.
For metal oxide (ZrO2 or ZnO)-supported Cu nanoparticles, due to the limitation of the
TEM technique, the Cu particles could not be distinguished from the support.
Surprisingly, HRSEM-EDX mapping showed that the low amount of Cu was
homogeneously dispersed over the solid support and the Cu nanoparticles did not exist
anymore. The polycrystalline phase mixture could be assigned to Cu particles, metal
oxide support, as well as other oxidized species, which might be generated from the
oxidation of the stabilizers, as observed for the Cu colloids during reaction. Therefore,
the ternary Cu/Al2O3/ZnO (ZrO2) systems may have formed, containing the same active
components as in the technical solid catalysts for methanol synthesis, though at different
concentration. If only Cu/ZnO was present, without Al2O3 or ZrO2 as in the case of
Cu/Zn@ZnO, the methanol productivity was much lower. It had already been described
in the literature that the binary Cu/ZnO is less active than the ternary catalysts [275]
. ZrO2
and ZnO thus have strong interactions with the Cu nanoparticles, not only as supports but
also as promoters, which provide higher activity.
Besides methanol formation, methyl formate was also detected, but only in the case of
Cu/Al@CMK-5 as catalyst, which was the least active one. At a higher reactant flow rate
(shorter gas-catalyst contact time), methyl formate formation was favored, in agreement
with this compound being an intermediate.
Although the supported Cu nanoparticles are active in a gas-phase reaction, their activity,
when taking the whole catalyst amount into account, is still very low. Their highest
activities are just at about the same level as that of the benchmark catalyst. However,
none of the supports investigated in a gas-phase reaction could maintain the high activity
of Cu nanoparticles observed in the quasi-homogeneous reaction. Accordingly, for more
active solid catalysts based on colloidal Cu particles, the search for better supports must
continue. Encapsulation of the Cu colloids in porous shells with pore sizes below the size
of Cu nanoparticles could be a suitable approach, which should be explored in the future.
Conclusions and outlook
125
4 Conclusions and outlook
The main objectives of this work were a deeper study of Cu colloid-based catalysts and
the evaluation of their catalytic performance in methanol synthesis from synthesis gas
feed. This work covered both the investigation of Cu colloids as pure materials and
studies of methanol synthesis reactions using these Cu colloid-based catalysts in both
quasi-homogeneous and heterogeneous systems.
The work started with the preparation of a series of Cu colloids. They were successfully
prepared via a reductive stabilization pathway, where the Cu(acac)2 as Cu precursor could
be simultaneously reduced and stabilized in THF solution under Ar protection. Four
different types of alkylaluminium or alkylzinc served as both stabilizers and reducing
agents, i.e. Al(n-butyl)3, Al(n-octyl)3, Zn(ethyl)2 and Zn(n-butyl)2. Various
characterization techniques (ICP, TEM, UV-Vis, XRD, XAS) identified the structural
properties of these four different Cu colloids, and the results of the different techniques
were in good agreement. It was proven that the colloidal Cu nanoparticles consist of well
crystallized metallic Cu cores with a spherical shape, and particle sizes varied between 3
and 6 nm with a narrow size distribution. The metal cores are covered by stabilizing
shells, and the particle size seems to depend on the type of stabilizer applied. The
colloidal Cu nanoparticles are larger in the case of Al(n-octyl)3 or Zn(ethyl)2 as stabilizer.
However, the difference between the particle sizes was only 1-3 nm, and it was difficult
to tune the particle size further by varying the synthesis parameters.
The formation of the Cu colloids was intensively studied by in situ XAS measurements in
order to elucidate the formation mechanism and to capture possible intermediates during
the reduction process. Based on the results of the in situ experiments at room temperature
with Al(n-octyl)3 or Zn(n-butyl)2 as stabilizers, the in situ XAS experiments were
extended to operation at low temperature (down to -30 °C) using all four different
stabilizers. All the results confirmed that Cu(II) is reduced within a short time directly to
Cu(0); no Cu(I) as intermediate species could be detected. This reduction process already
takes place at temperatures as low as -20 °C. The syntheses of the different Cu colloids
were very facile and highly reproducible. The Cu colloids could be stored for a few years
under Ar protection without agglomeration.
Conclusions and outlook
126
Such Cu colloids with well-defined shape and size could be interesting model catalysts,
and thus their catalytic performance in methanol synthesis from synthesis gas feed was
investigated. All of the four Cu colloids showed high activity in a series of temperature
dependent experiments in quasi-homogeneous phase. The highest methanol productivity
using Cu/TOAl reached 23.3 molMeOH/(kgCu·h) at 170 °C. Any of the Cu colloids was
substantially more active than the benchmark catalyst from ICI (now Johnson Matthey
Catalysts) - KATALCOJM 51-8 (Cu/ZnO/Al2O3) that was tested as reference in THF
suspension under the same reaction conditions. Moreover, the use of Cu colloids made
the methanol synthesis feasible at temperatures substantially lower than the typical range
of 200-300 °C used in an industrial process. Methanol formation under our conditions
already started at 130 °C. However, there was no clear correlation between the stabilizer
type, the particle size and the activity of the Cu colloids, though it appears that Cu/DEZn
and Cu/TOAl with larger particle size are more active. It was quite surprising that those
Cu colloids only stabilized by alkylaluminium were as active as those stabilized by
alkylzinc, despite of the absence of Zn species that are thought to be necessary promoters
in solid catalysts. The reaction to methanol probably proceeds via the hydrogenolysis of
methyl formate that can be considered as an intermediate rather than a by-product. All Cu
colloids remained active under reaction conditions during many hours - at least 20 hours
up to 40 hours.
Different techniques were applied in order to reveal the reason for the activity of Cu
colloids. Based on the changes of all the colloids concerning their shape, size and
composition during reaction, it was proven that the Cu colloids all decomposed during
longer reaction times. The Cu core still consisted of metallic Cu with larger or smaller
crystalline domain sizes and distorted shape after reaction. In contrast, the metal alkyl
stabilizing shell of alkylaluminium was oxidized to form amorphous Al2O3, methanolate
or hydrated Al species, while alkylzinc decomposed to crystallized ZnO. Further studies
on the influence of both Cu core and metal alkyl stabilizers on catalytic activity were
carried out through a series of experiments, in which individual components were
changed. Ag colloid, Ni colloid and non-metal alkyl-stabilized Cu colloids showed no
activity in methanol synthesis under the same conditions, whereas Mg(n-butyl)2-
stabilized Cu colloids exhibited low activity. This series of experiments excluded all
Conclusions and outlook
127
other possible pathways for methanol formation and individual components as being
responsible for activity. The results clearly suggest that Cu core and metal alkyl
stabilizing shell are indispensable factors, and thus the activity of Cu colloids is governed
by a synergy between the Cu core and the stabilizing shell. Among the Cu-metal
interactions, the interactions of Cu-Al and Cu-Zn are stronger, providing higher activities
of the colloids. These interactions are probably associated with the sites located on the
surface of Cu nanoparticles. Moreover, even the decomposed Cu colloids still had
activity under reaction conditions. The reason might be the formation of Cu/Al2O3 and
Cu/ZnO components during reaction, due to the decomposition of colloids in solution,
which bear resemblance to the conventional solid catalyst.
The high activity of Cu colloids in liquid phase led to the idea of attempting to transfer
their properties to a heterogeneous system, so that it would become suitable for the use in
a gas-phase reaction. Therefore, supported Cu nanoparticles were prepared, choosing
typical solids as supports to stabilize the Cu nanoparticles, such as ordered mesoporous
materials (SBA-15 and CMK-5) and metal oxides (ZrO2 and ZnO). The supported Cu
nanoparticles were synthesized by a direct colloidal deposition method to keep the same
shape and size of Cu nanoparticles as in colloidal solution. All of the catalysts exhibited
activity in a gas-phase reaction under similar conditions as used in an industrial process,
and their methanol productivities were stable over 10 hours reaction time. When taking
the only 1-2 wt% Cu loading into account, the Cu nanoparticles supported on ZrO2 and
ZnO were nearly as active as the benchmark catalyst and the highest methanol
productivity of Cu/Al@ZrO2 and Cu/Al@ZnO reached 50.8 molMeOH/(kgCu·h). A
significant amount of methyl formate was detected in the case of Cu/Al@CMK-5. This
indicates that the heterogeneous system using supported Cu nanoparticles might have the
same mechanism of methanol formation as the quasi-homogeneous system using Cu
colloids. Cu nanoparticles supported on different supports showed different activities, so
detailed studies were performed to reveal the nature of the support effect. It was found
that the Cu nanoparticles supported on SBA-15 or CMK-5 all agglomerated during
reaction, though their activities kept stable. Their low activities might be caused by the
low interaction between the support (silica or carbon) and Cu particles. When using ZnO
or ZrO2 as supports, Cu was shown to be homogeneously dispersed on the support
Conclusions and outlook
128
without being in the form of particles. In addition, the oxidation of metal alkyl stabilizer
might occur in a gas-phase reaction as for the Cu colloids in solution. Therefore, there
could be the formation of ternary systems - Cu/Al2O3/ZrO2(ZnO), similar to the active
components in the technical catalysts, which provide the high activity. However, higher
activity of Cu nanoparticles than in the commercial system could not be induced by any
of these supports. The Cu nanoparticles, supported on different solids in a gas-phase
methanol synthesis were not as active as Cu colloids in the quasi-homogenous phase.
In general, the Cu colloid-based catalyst system for studying methanol synthesis offers
several possibilities. The synthesis method of the metal alkyl-stabilized Cu colloid is
facile and the Cu nanoparticles are well-structured. They behave as micro-heterogeneous
catalysts in solution and can favor the methanol formation at low temperatures. This
would make the industrial process more economical and could further provide the
possibility to operate the process at even reduced pressure. However, presently the
decomposition of Cu colloids during reaction and difficulties in separation are obstacles
which certainly prevent their application. The homogeneous model system can help to
address to some extent the unclear issues of the heterogeneous systems using traditional
ternary solid catalysts - Cu/ZnO/Al2O3 - concerning reaction mechanism, active sites and
roles of the components, etc. However, Cu colloids as catalysts are rather complex and
more investigations are required to elucidate the reason for their high activity in a quasi-
homogeneous phase. This would require more specific characterization techniques to
identify the interactions of Cu-Al and Cu-Zn in the colloids during reaction. In situ ATR-
IR and XAS could be appropriate techniques for operation under reaction conditions.
Supported Cu nanoparticles would be promising to solve the problems of the stability of
Cu colloids under harsh reaction conditions, and would make the catalyst recycling
possible in the future. Therefore, in future research, more efforts should be focused on the
selection of supports that can better stabilize the Cu nanoparticles and promote higher
activity in a gas-phase reaction. This would increase Cu loading and provide better
interaction between the Cu particles and the support. Mesoporous metal oxides, such as
ZnO and core/shell systems, could be candidates of choice. In spite of the difficulties
associated with the use of colloidal catalysts, such systems remain interesting for further,
extended studies.
Experimental
129
5 Experimental
5.1 Synthesis of metal colloid-base catalysts
5.1.1 Metal colloids
The synthesis of metal colloids, including both the Cu colloids and the Ag colloid, was
based on the so-called ‘Bönnemann route’, which had been developed by Prof.
Bönnemann and his co-workers [34]
. The metal colloids were all prepared through a
simultaneous reduction and stabilization of Cu(II) and Ag(I) salts by either
alkylaluminium or alkylzinc which act as both reducing agents and stabilizers. All
preparations were carried out in anhydrous tetrahydrofuran (THF) under Ar protection.
The metal colloids were then used as catalysts in the quasi-homogeneous methanol
synthesis.
5.1.1.1 Alkylaluminium-stabilized copper colloids
The synthesis of alkylaluminium-stabilized copper colloids using copper acetylacetonate
(Cu(acac)2, 98.0%, Aldrich) followed the same route as described in S. Vukojević’s
thesis and our previous report [35-36]
. Copper acetylacetonate (Cu(acac)2, 1.45 g, 5.5 mmol,
99.9% Aldrich), which had been stored in a 1000 mL flask, was dried under vacuum at
70 °C overnight prior to the synthesis. It was dissolved by 400 mL anhydrous THF at
room temperature, and the color of the solution was transparent blue. Then the upper
transparent solution was transferred under Ar into a 1000 mL flask while carefully
avoiding to transfer the undissolved solid at the bottom. Trioctylaluminium (Al(n-octyl)3,
7.3 mL, 16.5 mmol, Crompton) or tributylaluminium (Al(n-butyl)3, 6.7 mL, 27.5 mmol,
Crompton) was diluted with 30 mL anhydrous THF. It was then transferred to a dripping
funnel that was connected to the above 1000 mL flask. The alkylaluminium solution was
added dropwise to the Cu(acac)2 solution at room temperature under mechanical stirring
with an addition rate of approximately 2 drops per second. After a few hours, the
formation of the Cu colloid was completed and a deep red color was observed. The
typical Cu to Al molar ratio was 1/3 and 1/5 for trioctylaluminium-stabilized Cu colloid
and tributylaluminium-stabilized Cu colloid, respectively.
Experimental
130
Other Cu(II) precursors were also used to prepare the Cu colloids, such as
Cu2(Piv)4(HPiv)2, which was prepared by Dr. Rainer Weiß following a procedure
reported in the literature [287]
. First, a mixture of 10 mmol Cu (II) acetate monohydrate
and 30 mmol pivalic acid (HPiv) in 10 mL isopropanol was refluxed under stirring. After
cooling, first 10 mL diethylether was added, followed by 20 mL water. The phase
separated, upper organic layer could then be collected, which contained the final green
compound. It was washed with water to leach the rest of acetic acid. The procedure to
prepare Cu colloids from Cu2(Piv)4(HPiv)2 was similar as described above for the
Cu(acac)2 as precursor.
5.1.1.2 Alkylzinc-stabilized copper colloids
The reducing agent dibutylzinc was synthesized in the lab based on the description in the
literature [288]
, while diethylzinc was obtained commercially (Strem Chemicals). The
alkylzinc-stabilized Cu colloids were synthesized following a route analogous to the
alkylaluminium-stabilized Cu colloid synthesis. Cu(acac)2 solution (0.52 g, 2.0 mmol) in
THF (200 mL) was added dropwise into the solution of Zn(ethyl)2 (2 mL, ≈20 mmol) or
Zn(n-butyl)2 (3.5mL, ≈20 mmol) in 20 ml THF. After stirring for a few hours, the color
of the Cu colloid suspension became brownish red. A small amount of dark precipitate
was formed, which was filtered through a glass frit (P4 porosity) and then stored under
Ar. A typical Cu to Zn molar ratio in the Cu colloid was 1/10; it could also be varied
between 1/10 and 1/20, when adding more alkylzinc.
5.1.1.3 Alkylmagnesium-stabilized copper colloids
0.3 mL solution of dibutylmagnesium (3 mmol, 1M in heptan, Aldrich) was added slowly
into 10 ml solution of copper actylacetonate (0.26 g, 1 mmol) in anhydrous THF. The
dark red Cu colloid was obtained with a trace of dark precipitate.
5.1.1.4 Non-metal alkyl-stabilized copper colloids
Prior to the synthesis, three solutions were prepared under Ar: (1) Cu(acac)2 solution
(0.72 g, 2.8 mmol) in 200 mL THF; (2) dodecylamine (CH3(CH2)10CH2NH2, 7.0 g, 38
mmol, 97.0% Aldrich) in 20 mL THF; (3) sodium borohydride (NaBH4, 0.75 g, 19 mmol,
Experimental
131
99% Aldrich) in 50 mL THF. The syntheses of the non-metal alkyl-stabilized copper
colloids were in a smaller scale and sodium borohydide was applied as reducing agent.
When using dodecylamine as surfactant, 6.25 mL of solution (2) (12 mmol dodecyl
amine) was transferred quickly into 50 mL of solution (1) (0.69 mmol copper
acetylacetonate) to form solution (4). Then 6.25 mL of solution (3) (2.5 mmol sodium
borohydride) was added dropwise into solution (4). The color changed slowly from blue
to light green then to dark red with some precipitate at the bottom.
When using Korantin SE as surfactant, Korantin SE (oleoyl sarcosine, 5.3 g, 15 mmol
BASF) was added into 50 mL of solution (1) (0.69 mmol copper actylacetonate) to form
mixture solution (5). Then 6.25 mL of solution (3) (2.5 mmol sodium borohydride) was
added dropwise into solution (5). The color changed from blue to green then to red brown.
5.1.1.5 Alkylalunimium-stabilized silver colloids
Since the Ag salt was light sensitive, the whole process was carried out in darkness
(protection with Al foil) under Ar protection. The Ag colloid was synthesized for
comparison to the alkylaluminium-stabilized Cu colloids, so the method was applied
using the same stabilizer - trioctylaluminium. The silver acetylacetonate (Ag(acac), 0.058
g, 0.27 mmol, 98% Aldrich) was dissolved in 40 mL THF, whereas trioctylaluminium
(0.77 mL, 1.74 mmol) was dissolved in 5 mL THF. The solution of trioctylaluminium
was added dropwise into the solution of silver acetylacetonate. The color changed from
dark grey to transparent and finally became light yellow, indicating the formation of Ag
nanoparticles.
5.1.2 Supported copper nanoparticles
One type of support used for the synthesis of Cu catalysts was CMK-5, which was
prepared based on a ‘nanocasting’ route [278]
. The SBA-15 as hard template was firstly
synthesized following the procedure described in the literature [282]
, while the CMK-5 was
synthesized following a method developed in our group [289]
. The characteristics of these
support samples are discussed in Section 3.3.3. Supported Cu catalysts were prepared by
a typical direct colloidal deposition method via a ‘precursor concept’, using Cu colloid
directly as Cu precursor, as illustrated in Figure 5.1. Typically, 50 mg support (SBA-15,
CMK-5, ZnO, and ZrO2) was added into a Schlenk flask and it was dried at 80 °C
Experimental
132
overnight under vacuum in order to completely remove the adsorbed water. Then the
flask containing the support was filled with Ar. To reach 2 wt% Cu loading, 0.4 mL
trioctylaluminium-stabilized Cu colloid or 0.7 mL dibutylzinc-stabilized Cu colloid was
added to the support under stirring. After a few minutes, the Cu colloid-loaded support
was exposed to vacuum to remove the solvent. The addition of colloid and the removal of
solvent were repeated for another 3 times. In the end, the support was kept under vacuum
at 30 °C overnight until it was completely dry. When using ZnO (Brüggemann Chemical
SP11815) or ZrO2 (99% Riedel-de Haën 96484 Fluka) as support, the amount of the Cu
colloid was reduced to half, thus the Cu loading was 1 wt%.
Solid support
Dry under vacuum
Solid support filled with Ar
Supported Cu nanoparticles
Cu colloids
Solvent removal under vacuum
Figure 5.1 Direct colloidal deposition method via ‘precursor concept’ for preparing supported Cu
nanoparticles under Ar protection
5.2 Characterization
Various characterization methods were used for the determination of the structure and
properties of the Cu colloid-based catalysts. All the measurements were carried out
directly using Cu colloids or supported Cu nanoparticles under strict Ar protection.
5.2.1 TEM and EDX
TEM measurements and high-resolution TEM (HRTEM) measurements were carried out
using a Hitachi 7500 transmission electron microscope and a Hitachi HF-2000
microscope, respectively. The maximum acceleration voltage of the H-7500 microscope
is 120 kV. The HF-2000 instrument was equipped with a cold field emitter (CFE) and
was capable of a maximum acceleration voltage of 200 kV. The EDX measurements for
Experimental
133
this study were performed on the H-7500 TEM instrument equipped with an Oxford Inca
X-ray detector or the HF-2000 TEM instrument with a ThermoNoran (Thermo Electron
Corporation) X-ray detector.
Samples were prepared under Ar in a glove box to prevent the samples from being
exposed to air. In a flask fully filled with Ar, a droplet of the diluted colloidal solution in
THF was placed onto a lacey carbon-coated Cu grid. For EDX measurements of the
samples, a Ni or Au grid was employed. Solid samples were prepared by touching the
solid with the grid.
5.2.2 SEM, HRSEM and EDX
SEM and EDX analyses were performed on a Hitachi S-3500 N instrument equipped
with an Oxford EDX unit (INCA Surveyor Imaging System). SEM was performed at a
maximum acceleration voltage of 25 kV with a working distance of 5 mm. For samples
of Cu colloids, the concentrated colloid was dropped onto Leit-Tab and its surface was
covered by a layer of Au (10 nm thick). The samples of supported Cu colloids were
directly deposited on the sample holder, which was covered either by Al or Si, depending
on the elements which should be analyzed.
HRSEM and EDX analysis were taken using a Hitachi S-5500 system with a cold-field-
emission gun (FEG) and an in-lens detector, operating at 1 and 30 kV, respectively. Thin
sections were prepared by embedding the solid sample (Cu/Al@ZrO2) with the two-step
method in Spurr-resin (hard mixture). After hardening every step overnight, the
embedded sample was prepared for sectioning by trimming the surface-area to around
200 µm square. It was then cut by a Reichert-Jung Ultracut ultramicrotome with a
diamond-knife (35°, 1 mm/sec) stepwise to a thickness of around 30 nm. The sliced
sections were transferred to lacey-film coated Cu-grid.
5.2.3 UV-Vis spectroscopy
The UV-Vis spectra of the Cu colloids were collected on a Varian Cary 5
spectrophotometer. The liquid samples were transferred under Ar into a 1 cm path length
quartz cuvette sealed with a Teflon® plug. THF was used as reference. During the
measurement, the chamber was also filled with Ar. For the in situ measurement to study
Experimental
134
oxidation of Al(n-octyl)3-stabilized or Zn(n-butyl)2-stabilized Cu colloids, the Teflon®
plug was removed and the UV-Vis spectra were recorded in 5 min intervals.
5.2.4 XRD
The XRD measurements were performed on a Stoe STADI P transmission diffractometer
(Mo Kα1: 0.7093 Å), equipped with a primary Ge (111) monochromator and a linear
position sensitive detector. Prior to the XRD measurement, most of the solvent of the Cu
colloids was removed in order to obtain XRD patterns with higher intensity. The samples
were then transferred into class capillaries with 0.7 mm Ø under Ar. The data collection
was carried out at room temperature.
5.2.5 XAS
XAS measurements were carried out in collaboration with Prof. Dr. Jan-Dierk Grunwaldt
and Dr. Matthias Bauer (Karlsruhe Institute of Technology, Department of Chemical
Technology and Polymer Chemistry, Germany) at beamlines both at ANKA
(Forschungszentrum Karlsruhe, Germany) and at HASYLAB (DESY Hamburg,
Germany).
5.2.5.1 Sample preparation
Each liquid sample of Cu colloid was held in a stainless-steel spectroscopic cell, which
was designed by Prof. Dr. Jan-Dierk Grunwaldt. Since the Cu concentration in the colloid
was relatively low, a cell thickness of 0.8 cm was required. Illustrated in Figure 5.2, the
sample chamber was made of two round Kapton® foils whose diameter was ca. 4 cm.
Each side of the foil was fixed by a round stainless-steel spacer ring with a Viton® ring
sealing. Two gas-tight Swagelok® fittings were mounted on top of the spacer, which
served as sample inlet and Ar circulator for protection. The angle between the surface of
the sample chamber and the X-ray beam was 45°. The sample chamber volume was ca. 5
mL, and it was connected to a stainless-steel hollow cylindrical body. This cell allowed
for both transmission and fluorescence mode of XAS measurements. Prior to the
measurement, the cell was dried, evacuated and then filled with Ar. The Cu colloid was
filled into the sample chamber via one inlet, whereas Ar was kept flowing through the
Experimental
135
other one. The samples of Cu colloids collected after reaction were introduced under
stirring in order to avoid the precipitate aggregation.
Figure 5.2 Stainless-steel liquid cell for XAS measurements of Cu colloids (designed by Prof. Dr.
J.-D. Grunwaldt)
5.2.5.2 XAS measurements of copper colloids
XAS data of each colloid sample was collected at ANKA-XAS beamline at the
Angströmquelle Karlsruhe (ANKA, Karlsruhe Institute of Technology, Germany). The
XAS data were obtained in transmission geometry using a Si(111) monochromator and
higher harmonics were eliminated by detuning the second crystal to 60% of the maximum
intensity. Fluorescence EXAFS spectra were recorded using a 5-element solid state
detector [269]
. Kapton® windows in the cell allowed the X-ray transmission, and prior to
each measurement, the cell was dried, evacuated and then filled with Ar. For XAS
measurements at both the Cu K-edge (8.980 keV) and Zn K-edge (9.659 keV) the
incident and transmitted X-rays were recorded with ionization chambers.
Three ionization chambers located before and after the cell as well as after a Cu foil for
energy calibration were used to measure the incident and output X-ray intensities. The
raw EXAFS data were energy-calibrated with the respective Cu foil, background
corrected, normalized, and fitted with WINAXS 3.1 software [290]
. Fourier transformation
was applied in the region 3 to 12.5 Å-1
, and data fitting was performed in R-space using
Experimental
136
theoretical phase and amplitude functions calculated with the FEFF 6.0 code [291]
. R-
values in the Fourier-transformed EXAFS spectra were not phase-shift corrected.
5.2.5.3 In situ XAS measurement at room temperature
The stainless-steel liquid sample cell (Figure 5.2) was also applied in the in situ XAS
measurement and the experiments were also carried out at ANKA. For the in situ
measurement at the Cu K-edge, the sample cell was first filled with Cu(acac)2 dissolved
in THF (0.014 mol/L) with Ar flow. The Zn(n-butyl)2 was diluted by THF and then filled
into a 1 mL syringe. This syringe was fixed onto a syringe pump (KDS100, kdScientific)
and the needle was connected to the liquid cell via a gas-tight Swagelok®
. The solution
introduction rate was controlled at ca. 1 mL/h and the final Cu/Zn ratio after the addition
was 1/10. More detailed instrumental conditions of the in situ measurements were
reported earlier in S. Vukojević’s PhD thesis [36]
.
5.2.5.4 In situ XAS measurement at low temperature
In situ XAS measurements were performed at beamline X4 at the Hamburger
Synchrotron Strahlungslabor (HASYLAB), and the following experiments were designed
by Dr. M. Bauer. A Si(111) double crystal monochromator was used for measurements at
the Cu K-edge (8.979 keV). The second monochromator crystal was tilted for optimal
harmonic rejection. The spectra were recorded in transmission mode with ionization
chambers filled with N2. The XAS scan time was set to be 90 seconds. The individual
pressures were adjusted to optimize the signal to noise ratio.
A low temperature cell, which allows XAS measurement at temperatures down to -30 °C
under inert gas conditions was used. The scheme and picture of the low temperature
measurement cell are displayed in Figure 5.3. The filling of the cell was similar to the
normal liquid cell, as described above. To ensure the low temperature, ethanol as a
cooling liquid was pumped through the wall of the liquid cell. Ethanol was cooled by
isopropanol, which was frozen by liquid N2, down to -30 °C. The wall of the cell was also
equipped with tubes, which were flushed with He to avoid condensation of water on the
windows [267]
. 2 mL Cu(acac)2 solution in THF (0.014 mol/L) was first filled into the cell
under cooling, and then 0.2 mL solution of reducing agent in THF was added quickly.
Experimental
137
The Cu/Al molar ratio was 1/3, while the Cu/Zn molar ratio was 1/10. Both were the
same ratios as used in the synthesis of corresponding Cu colloids.
(a) (b)
Figure 5.3 Low-temperature XAS measurement cell for solutions under inert condition (designed
by A. Pacher) (a): cell sketch; (b): cell in operation.
The in situ XAS data interpretation was completely provided by Dr. M. Bauer and more
detailed information on data interpretation can be found in his review and other related
reports [292-294]
. LC-XANES fits were carried out by a least square fit of the spectra using
the WINXAS program package [295]
.
5.2.6 Determination of copper, aluminium and zinc concentration
The determination of Cu, Al and Zn concentration in the Cu colloids was carried out by
elemental analysis - ICP (Inductively Coupled Plasma) in Microanalytical laboratory
(http://www.mikro-lab.de), located in Mülheim an der Ruhr.
5.2.7 Nitrogen sorption
The measurements of nitrogen adsorption-desorption isotherms were carried out by an
ASAP2010 adsorption analyzer (Micromeritics) with liquid N2 at 77 K. Prior to the
measurements the samples were degassed for around 12 hours at 80 °C. The surface area
was calculated by the Brunauer-Emmett-Teller (BET) method from the adsorption branch
in the relative pressure (P/P0) interval from 0.04 to 0.20. Pore size and pore size
Experimental
138
distribution curves were determined using the Barrett-Joyner-Halenda (BJH) method
from the desorption branch. The total pore volume was estimated from the amount of
adsorbed nitrogen at a relative pressure (P/P0) at 0.97.
5.3 Catalytic testing
Depending on the catalyst systems, the catalytic tests in methanol synthesis were carried
out using two different experimental set-ups, which were both designed and built in-
house. The Cu colloids as well as other metal colloids were tested for their activity in
methanol synthesis using a quasi-homogenous slurry reactor, whereas the supported Cu
nanoparticles were typically measured in the gas phase using a plug-flow reactor (single-
tube reactor). The two set-ups as well as their functions are described in detail in the
following sections.
5.3.1 Copper colloids in quasi-homogeneous slurry reactor
The different types of Cu colloids were directly tested for methanol synthesis from
synthesis gas feed using a slurry reactor, in order to evaluate their catalytic properties in a
quasi-homogeneous phase.
Figure 5.4 shows the quasi-homogeneous methanol synthesis set-up used for methanol
synthesis, which had previously been designed and constructed by Dr. S. Vukojević and
Prof. Dr. O. Trapp [35]
. The detailed descriptions of the function concerning each device
are briefly reported in Dr. S. Vukojević’s thesis [36]
. Only the modifications of some
devices are listed and explained as follows:
Reactor For the sealing of the reactor lid, Teflon® O-rings were used instead of silver
ring sealing. However, high reaction temperature and pressure caused distortion of the
Teflon® O-rings, leading to leaks of the reactor. Therefore, Teflon
® O-rings must be
replaced after 2-3 experiments. Through the two glass windows (Schott) installed
opposite to each other on the reactor chamber (Figure 5.4 (e)), it was possible to observe
the reactant liquid during reaction and check color change and precipitate formation. The
windows are sealed by Teflon® O-ring sealing with a size that fits the window. Both
sealings should be replaced by new ones each time before reaction. The gas outlet
mounted at one side of the reactor lid was replaced by a pressure meter, in order to
observe more precisely the gas pressure inside the reactor. For temperature control a
Experimental
139
Jumo iTRON 16 was used and a Jumo dTRON 316 was used as temperature display.
Both controllers were connected via an analog-digital converter (ICP.CON 7520A, RS-
232 to RS-422/485) to a computer.
Function of set-up Samples were automatically analyzed every 1-2 hours. After the
reaction, the reactor cooled down to room temperature and was vented to ambient
pressure. The rest of the colloid in the reactor after reaction was collected under Ar and
further analyzed by GC, GC-MS, and TEM. The concentration of the products (mg
product per mL solution) and the methanol productivity (mol methanol per kg Cu per
hour) was calculated based on the GC analysis results. After the reaction the whole set-up
needed to be cleaned thoroughly. The reactor was filled with a 25 mL THF/acetic acid
mixture and pressurized with CO2 to ca. 6 MPa. It was then heated under stirring at 50 °C
for 2 hours. The reaction line, including the microvolume valve system, was purged with
this mixture. Afterwards, the whole system was purged with THF several times until no
trace of acetic acid could be detected. At the end the valve system was dried under Ar.
(b)
(c) (d)
(e)
(a)
Figure 5.4 The experimental set-up for methanol synthesis in quasi-homogeneous phase: (a)
H2/CO premixed in 5 L autoclave; (b) CO2 compressor; (c) set-up overview; (d) autoclave filled
with Cu colloid; (e) window autoclave (225 mL stainless-steel batch reactor).
Data processing In a previous report, the colloid activity, expressed as the methanol
productivity, was obtained from the slope of the linear regression of the methanol
concentration over reaction time [35]
. As explained in Section 3.2.1.1, since the colloid is
Experimental
140
active for up to 40 hours with linear increase in methanol concentration (see Figure 3.18),
the calculation of methanol productivity (PMeOH) can be alternatively done as expressed in
Equation 5.1 based on the off-line product analysis:
mol
g
g
mg
kg
mg
321000
1000000
himeReaction tmL
mgion(Cu)Concentrat
mL
mgion(MeOH)Concentrat
hkg
mol P
Cu
MeOH
MeOH
(5.1)
where the reaction time is the entire period of the test, which is different for each colloid
(see Table 3.3). The methanol concentration was taken at the end of this period, after the
reaction cooled down and the rest of the reaction gas was evacuated.
5.3.2 Supported copper nanoparticles in plug-flow reactor
Following the previous work with the high-throughput reactor for methanol synthesis in
gas phase, the 49-channel parallel reactor was replaced with a plug-flow reactor (PFR) -
single-tube reactor. This replacement was necessary, since the 49-channel parallel reactor
had developed leaks. In addition, a single-tube reactor can provide kinetic tests for
individual catalyst. In order to minimize the modification and maintain the configuration
of the catalytic process, the functions of the multiport valve system, electronic pressure
controller, LabVIEW programming, and on-line sample analysis were kept. The detailed
description of the function of the entire set-up was reported in S. Vokujević’s thesis [36]
.
Only the modification concerning the new reactor is introduced here:
Plug-flow reactor A single-tube (plug-flow) reactor was newly designed for testing the
supported Cu nanoparticles in the gas-phase methanol synthesis, as shown in Figure 5.5.
The objective was to have a size of the single-tube reactor identical to one cartridge of the
high-throughput reactor, so that the catalytic tests could be made comparable to those
obtained with the high-throughput reactor previously employed. The internal diameter of
the tube is 7 mm. There is no more frit at the bottom of the catalyst bed, instead the
catalyst bed is supported by quartz wool and is placed in the middle of the tube. The
reactor tube is wrapped by an oven, which controls temperature to up to 245 °C. The
Experimental
141
heating temperatures are controlled using a temperature controller (Jumo dTron 16.1).
Two NiCr/Ni thermocouples are used and each of them has a different function. One
thermocouple is positioned between the reactor and the oven; it is used to control the
reaction temperature. The other is inserted into the catalyst bed from the reactor inlet; it is
used for observing the reaction temperature, which is observed by a temperature display
(night-watch, Jumo iTron 16). The trigger point for the cut-off of the heating power was
set to be 10 °C above the setting temperature. Similar to the design of the high-
throughput reactor, the gas inlet is split into two: one is passed through the back pressure
regulator to the exhaust, and the other to the reactor tube. Accordingly, two pressure
meters are used to watch the reaction pressure in order to check if there is pressure drop
trough the tube: one is placed at the tube inlet and the other one at the tube outlet as
shown in Figure 5.5 (b).
Function of set-up The entire set-up for the gas-phase reaction is displayed in Figure 5.6.
Since the supported Cu colloid was sensitive to air, prior to each measurement the tube
reactor was filled with quartz and catalyst in the glove box. First, the tube was filled with
quartz wool until the middle and then typically with a mixture of around 25 mg solid
catalyst and quartz. The catalyst bed was finally covered with another quartz wool layer.
The tube was mounted in the set-up while keeping N2 going through the tube to avoid
contact to air. After registering the LabView control file, the test started. As the first step,
the catalyst was reduced in situ with a H2 (vol. 5%)/Ar gas mixture and the reduction
process is briefly described in S. Vukojević’s thesis. Eventually, the reaction gas mixture
reached a constant flow of 41.2 nL/h H2, 14.1 nL/h CO and 3.6 nL/h CO2. The pressure
increased to 4.5 MPa and the temperature was kept at 245 °C. The reaction gas flow
through the catalyst bed was then tuned to be 20 mL/min. The reaction gas consisted of
70 vol.% H2, 24 vol.% CO and 6 vol.% CO2. Before on-line sample analysis using the
combined GC system, the catalyst was kept under fixed conditions (temperature, flow
rate, pressure) for another 2 hours. After the measurement program, the reactor was
cooled down to room temperature naturally. After each reaction, the reactor tube was
cleaned with ethanol and flushed by pressurized air.
Experimental
142
P
GC
gas feed
Quartz wool
Oven
Catalyst bed
Thermocouple 1
Back
pressure
Thermocouple 2
Pressuremeter 1
Pressuremeter 2
Temperature controller
Night-watch
P
Multiport
valve box
Exhaust
P
GC
gas feed
Quartz wool
Oven
Catalyst bed
Thermocouple 1
Back
pressure
Thermocouple 2
Pressuremeter 1
Pressuremeter 2
Temperature controller
Night-watch
P
Multiport
valve box
Exhaust
(a)
(b)
Figure 5.5 (a) The schematic drawing of the plug-flow reactor; (b) schematic drawing of reaction
system.
Experimental
143
Figure 5.6 Overview of the set-up for gas-phase reaction.
Data processing The calculation of methanol productivity (PMeOH) was carried out
according to Equation 5.2:
hkg
molP
Cu
MeOH
MeOH
L
mL1000
Mol
L22.4
%
1
1001
h
min60
kg
mg1000000
h
L)feedHfeedCO(feedCOmgmasscatalyst
h
LfeedCO)(feedCO
min
mLflow analytic%carbon eOH)totalfraction(M
22
2
(5.2)
KATALCOJM 51-8 (Cu/ZnO/Al2O3) as benchmark catalyst was tested firstly to confirm
the stability and reproducibility of this new reactor. The results showed that the methanol
productivity measured in the plug-flow reactor was ca. 33 molMeOH/(kgCat.·h) at 245 °C
Experimental
144
and 4.5 MPa. The variation was below 3%, so the reactor function was stable and could
provide reliable results. However, this value of methanol productivity was lower
compared to that from the test using the 49-fold high-throughput reactor of ca. 45
molMeOH/(kgcat.·h). Due to the low Cu loading, the methanol productivity of each Cu
containing catalyst was normalized to molMeOH/(kgCu·h).
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Scientific contributions
159
7 Scientific contributions
7.1 Publications (PhD thesis related)
1. M. Bauer, A. Pacher, S. Wang, F. Schüth, ‘The formation of transition metal colloids
by temperature dependent EXAFS measurement’, HASYLAB Annual Report 2011
2. S. Wang, J.-D. Grunwalldt, C. Weidenthaler, O. Trapp, S. Vokujević, A. Kempter, F.
Schüth, ‘Easy synthesis of metal alkyl-stabilized copper colloids and their high activity
and stability in quasi-homogeneous methanol synthesis’, in preparation
3. A. Kempter, S. Wang, F. Schüth, ‘The effect of variations of the synthetic parameters
in the formation of copper-colloids via reduction with metal alkyls’, in preparation
7.2 Scientific presentations
08/2011 Oral presentation at 10th
EuropaCat (European Congress of Catalysis) in
Glasgow, UK
‘Highly active supported copper nanoparticles in methanol synthesis’, S. Wang,
J.-D. Grunwaldt, C. Weidenthaler, F. Schüth
03/2010 Poster presentation at 12th
JCF-Spring symposium in Erlangen, Germany
‘Copper colloid-based catalysts in methanol synthesis’, S. Wang, J.-D.
Grunwaldt, C. Weidenthaler, S. Vukojević, O. Trapp, A. Kempter, F. Schüth
08/2010 Invited oral presentation as a representative for GDCh in CHED (Division
of Chemical Education) at 240th
ACS (American Chemical Society) Fall
Meeting in Boston, USA
‘Catalysis - as a key technology for a sustainable development’, S. Wang, F.
Schüth
08/2010 Poster presentation in CATL (Division of Catalysis Science and Technology)
at 240th
ACS Meeting in Boston, USA
‘Copper colloid-based catalysts in methanol synthesis’, S. Wang, J.-D.
Grunwaldt, C. Weidenthaler, S. Vukojević, O. Trapp, A. Kempter, F. Schüth
Scientific contributions
160
05/2010 Poster Presentation at IDECAT Conference on Catalysis in Porquerolles,
France
‘Copper colloid-based catalysts in methanol synthesis’, S. Wang, J.-D.
Grunwaldt, C. Weidenthaler, S. Vukojević, O. Trapp, A. Kempter, F. Schüth
03/2010 Oral presentation at 11th
JCF-Spring symposium in Göttingen, Germany
‘Highly active metal alkyl-stabilized copper colloids in quasi-homogeneous
methanol synthesis’, S. Wang, C. Weidenthaler, J.-D. Grunwaldt, O. Trapp, S.
Vukojević, A. Kempter, F. Schüth
03/2010 Poster presentation at 43 Jahrestreffen Deutscher Katalytiker (German
Catalysis Society Annual Meeting) in Weimar, Germany
‘Copper colloid-based catalyst in methanol synthesis’, S. Wang, C.
Weidenthaler, J.-D. Grunwaldt, S. Vukojević, O. Trapp, A. Kempter, F. Schüth
07/2009 Poster presentation at 9th
EuropaCat (European Congress of Catalysis) in
Salamanca, Spain
‘Tailored copper nanoparticles and their catalytic performance in methanol
synthesis’, S. Wang, S. Vukojević, O. Trapp, J.-D. Grunwaldt, F. Schüth
03/2009 Poster presentation at 42 Jahrestreffen Deutscher Katalytiker (German
Catalysis Society Annual Meeting) in Weimar, Germany
‘Copper colloid-based catalysts for methanol synthesis’, S. Wang, S. Vukojević,
O. Trapp, J.-D. Grunwaldt, A. Rittermeier, F. Schüth
06/2008 Oral presentation at SFB558 (German Research Foundation) workshop in
Hennesee, Germany
‘Copper colloid-based catalysts for methanol synthesis’, S. Wang, J.-S.
Girardon, C. Baltes, S. Vukojević, F. Schüth
10/2007 Poster presentation at Joint Symposium of the Collaborative Research
Centers 546 (Berlin) and 558 (Bochum), Erkner, Germany
‘Cu/ZnO/Al2O3 catalysts and copper colloids for methanol synthesis’, C.
Baltes, S. Vukojevic, J.-S. Giradon, S. Wang, F. Schüth
Curriculum Vitae
161
8 Curriculum Vitae
Personal Information
Name: Shanshan Wang
Date of Birth: 6 April 1982
Place of Birth: Beijing, China
Higher Educational
04/2012 – 06/2012 Researcher, Max-Planck-Institut für Kohlenforschung,
Mülheim an der Ruhr, Germany
Topic: Nanocatalysts for Dehydration of D-Glucose (supervisor:
Prof. Walter Leitner and Dr. Nils Theyssen)
08/2007 – 03/2012 PhD, Max-Planck-Institut für Kohlenforschung,
Department of Heterogeneous Catalysis, Mülheim an der
Ruhr, Germany
Thesis: Copper Colloid-based Catalysts for Methanol Synthesis
(supervisor: Prof. Dr. Ferdi Schüth)
Workshop: 11-21/08/2009 one among a group of highly
selective international graduate students in BASF Summer
Courses, BASF SE Ludwigshafen, Germany
09/2006 – 07/2007 Master of Science, University of Lyon 1 and Research
Institute on Catalysis and Environment (IRCELYON),
Lyon, France
Major: Physical Chemistry and Catalysis;
Thesis: Ceramic-LUS Nanocomposite Membrane:
Organophilic Modification and Quality Testing (supervisor: Dr.
Jean-Alain Dalmon and Prof. Laurent Bonneviot)
09/2004 – 09/2007 Engineer’s Diploma, (Equivalent to Master of Science),
Advanced School of Chemistry Physics and Electronics
(ESCPE Lyon), Lyon, France
Major: Chemistry and Process Engineering
Other main courses: Foreign Languages, Marketing, Patent
Law, Finance, Project Management, etc.
Curriculum Vitae
162
09/2000 – 06/2004 Bachelor’s Degree of Science, Institute of Chemistry and
Chemical Engineering, Nanjing University, Nanjing, China
Major: Chemistry
Thesis: Synthesis of Ti and Cu incorporated ordered
mesoporous silica (MCM-41) (supervisor: Prof. Qijie Yan)
Professional Experience
07/2012 – present Research Chemist at BP Chemicals, Conversion Technology
Centre, Kingston upon Hull, United Kingdom
01/2009 – 05/2009 Referee for Journal of Fuel Process Technology
07/2005 – 06/2006 Industrial Placement at Johnson Matthey Catalysts,
Strategic Catalysis Group, Billingham, United Kingdom
Project: Development of new catalysts for biofuel production
Scientific Publications (non-PhD thesis related)
1. C. Jin, T.-C. Nagaiah, W. Xia, B, Spliethoff, S. Wang, M. Bron, W. Schuhmann, M.
Muhler, ‘Metal-free and electrocatalytically active nitrogen-doped carbon nanotubes
synthesized by coating with polyaniline’, Nanoscale 2010, 2, 981-987
2. B. Hamad, A. Alshebani, M. Pera-Titus, S. Wang, M. Torres, B. Albela, L.
Bonneviot, S. Miachon, J.-A. Dalmon, ‘Synthesis and characterization of
nanocomposite MCM-41 ('LUS') ceramic membranes’, Microporous and Mesoporous
Materials 2008, 115, 40-50
3. Y. Kong, S. Jiang, J. Wang, S. Wang, Q. Yan, Y. Lu, ‘Synthesis and
characterization of Ti-Cu-MCM41’, Microporous and Mesoporous Materials 2005,
86, 191-197
Scientific related activity
03/2011 – present Figure image on the new institute official website
27/01/2011 Figure image on DerWesten, ‘Wissenschaft im Ruhrgebiet
hat Nachholbedarf’
06/05/2010 Figure image on NRZ, ‘Planks forsche Erben’
Curriculum Vitae
163
09/2008 – 03/2012 Representative of GDCh-JCF, Germany
Responsibility: industrial liaison for experts from international
companies; key organizer for regular lectures and events
Exchange: 21-28/08/2010 Representative for GDCh-JCF in the
exchange program with Northeastern Section Younger Chemist
Committees (NSYCC) in Boston, USA