2590 Phys. Chem. Chem. Phys., 2011, 13, 2590–2602 This journal is c the Owner Societies 2011
Cite this: Phys. Chem. Chem. Phys., 2011, 13, 2590–2602
Geometric and electronic effects on hydrogenation of cinnamaldehyde
over unsupported Pt-based nanocrystalsw
William O. Oduro,z Nick Cailuo, Kai Man K. Yu, Hongwei Yangy andShik Chi Tsang*
Received 16th September 2010, Accepted 16th December 2010
DOI: 10.1039/c0cp01832e
It is reported that catalytic hydrogenation of cinnamaldehyde to cinnamyl alcohol is a structural
sensitive reaction dependent on size and type of metal doper of unsupported platinum
nanocrystals used. Smaller sizes of platinum nanocrystals are found to give lower selectivity to
cinnamyl alcohol, which suggests the high index Pt sites are undesirable for the terminal aldehyde
hydrogenation. A plot of reaction selectivity across the first row of transition metals as dopers
gives a typical volcano shape curve, the apex of which depicts that a small level of cobalt on
platinum nanocrystals can greatly promote the reaction selectivity. The selectivity towards
cinnamyl alcohol over the cobalt doped Pt nanocrystals can reach over 99.7%, following the
optimization in reaction conditions such as temperature, pressure and substrate concentration.
Detailed studies of XRD, CO chemisorption (for FTIR), TEM, SEM, AES and XPS of the
nanostructure catalyst clearly reveal that the decorated cobalt atoms not only block the high index
sites of Pt nanocrystals (sites for Co deposition) but also exert a strong electronic influence on
reaction pathways. The d-band centre theory is invoked to explain the volcano plot of selectivity
versus metal doper.
Introduction
Hydrogenation of organic compounds plays a very important
role in chemical manufacturing processes. Among all
the hydrogenation reactions reported, the hydrogenation of
a,b-unsaturated aldehydes to their corresponding unsaturated
alcohols draws the most attention as the hydrogenation of
these compounds is of both fundamental and industrial
importance.1,2 There has been much recent interest in
synthesizing uniform metallic and bimetallic nanocrystals as
new heterogeneous catalysts because of appropriate metal
particle size and optimised geometric and/or electronic effects
in metallic and bimetallic nanostructures which may allow the
nanocrystals with tuneable catalytic properties 3,4 to overcome
thermodynamic favourable CQC hydrogenation over CQO
hydrogenation. Thus, this approach is especially important for
nowadays’ catalyst development for a high performance catalyst
material in terms of activity (to increase productivity), selectivity
(to reduce the needs in product separation) and energy
considerations (to reduce energy consumption).
We have investigated hydrogenation of cinnamaldehyde to
cinnamyl alcohol over unsupported Pt nanocrystals and its
transition metal doped (bimetallic) nanoparticles. The substrate
molecule contains three reducible groups (terminal aldehyde,
double bond at a-b carbon position and benzene ring) as a
chemical probe for this investigation. Cinnamaldehyde is one
interesting model compound for hydrogenation because a
number of partially hydrogenated products can be synthesized,
depending on the selectivity of the hydrogenation reaction
(see Scheme 1). In addition, the economic importance of
selective hydrogenation of a,b-unsaturated aldehyde is
particularly denoted,1,2 because the cinnamyl alcohol can be
used as pharmaceuticals, fragrances, and perfumes.5 From the
literature, selective hydrogenation of this compound is one of the
most widely studied reactions. A wide range of catalysts,
including promoted and unpromoted metals/alloys,6–8 metal
oxides,9,10 microporous supports,11 and polymer fibre catalysts12 in both liquid2,13–15 and vapour16,17 phases was systematically
investigated. It has been empirically shown that the selectivity of
the reaction can depend on some key parameters, including the
nature of the metal and particle size,18 catalyst support,19–21 and
type of promoters/additives21–23 used. There were postulations
on the importance of structural and electronic properties of
Wolfson Catalysis Centre, Department of Chemistry, University ofOxford, Oxford, UK OX1 3QR. E-mail: [email protected];Fax: +44 1865 272600; Tel: +44 1865 282610w Electronic supplementary information (ESI) available. See DOI:10.1039/c0cp01832ez Contact address: Institute of Industrial Research- CSIR. P. O. BoxLG 576 Legon, Accra, Ghana.y Contact address: State Key Laboratory of Physical Chemistry ofSolid Surfaces, National Engineering Laboratory for Green ChemicalProductions of Alcohols-Ethers-Esters, College of Chemistry ofChemical Engineering, Xiamen University, Xiamen 361005, Fujian,P. R. China.
PCCP Dynamic Article Links
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This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 2590–2602 2591
metal catalysts as the main underlying factors.1,4,24,25 The studies
of platinum particle size 1,4 and shape24 on activity and selectivity
also gave circumstantial evidence that the particle geometry plays
a key role in the hydrogenation reaction. However, a fundamental
understanding in reaction selectivity by these structural
parameters leading to tuneable properties has yet to be achieved.
Fourier transmission Infrared Spectroscopy, FTIR, is
commonly employed to study the surface chemistry of metal-
substrate interaction in catalytic systems, which provides a
means of observing the different types of adsorption emanating
from the different geometric arrangement of atoms on
the surface of a particle. Carbon monoxide-metal interaction
is one of such matrices that can act as useful interrogating
tools in elucidating surface properties because of the
strong CO-metal bond and also the extensive background
information in literature on the types of adsorbed
species.26–34 Three types of adsorption modes of CO on Pt
single crystal surfaces have been previously observed. A
vibration frequency u(CO) between 2090–2040 cm�1 is
assigned to linearly adsorbed CO mode on a Pt site and
1860–1780 cm�1 to bridged mode adsorption26 whilst a
strong vibration frequency u(CO) of ca. 1950–1925 cm�1
prevalent in small size particles (o5.0 nm) is attributed to
the stretching mode of multicarbonyl species.4,27 In additional,
the degree of back bonding of adsorbed CO on Pt gives
progressive red shift in adsorption frequencies that can
reflect the electron density of the metal nanoparticles. Thus,
in this paper, the technique is particularly employed to obtain
the surface feature relationship with respect to catalytic
performance and to provide a mechanistic understanding on
the substrate interaction with metal surface.
On the other hand, it should be noted that it is very challenging
to disentangle the complex interplays between geometric,
electronic, and steric effects in a working catalytic system (for
example, supported bimetallic catalysts with wide heterogeneities
in size, shape, composition and metal-support interfaces).
In order to shed light on the geometric and electronic
contributions on Pt and bimetallic nanoparticles in the
selective hydrogenation of cinnamaldehyde to cinnamyl
alcohol, we have employed unsupported Pt based nanocrystals
to eliminate the support effect in this paper. In addition, a
solution technique for controlled growth of metallic
nanocrystals of defined size and surface feature by chemical
reduction allowing tailoring of particle size has also been
used. Apart from the using CO-chemisorption (FTIR),
other spectroscopic techniques such X-ray Photoelectron
Spectroscopy (XPS), Auger Electron Spectroscopy (AES)
techniques have been employed to probe the electronic
properties. X-ray Powder Diffraction (XRD), Transmission
Electron Microscopy (TEM), Energy Dispersive X-ray analysis
(EDX) and Scanning Electron Microscopy (SEM) were
conducted in order to examine the structural and chemical
changes for the Pd nanocrystals before and after modification
with a second metal. Experimental parameters such as pressure
of hydrogen, concentration of cinnamaldehyde, temperature,
and reaction time were also studied.
Experimental
Synthesis of Pt nanostructures
The Pt and its bimetallic nanocrystals were synthesized by a
modified polyol process.35–37 Typically, a mixture of bis-(acetyl
acetonato) platinum(II), Pt(acac)2, (Pt 49.49%min. Alfa Aesar,
0.30 g), 1,2-hexadecanediol (90%, Aldrich, 0.20 g), oleic acid
(99+%, Aldrich, 100 mL) and oleylamine (98%, Aldrich,
100 mL) in 6.0 mL of dioctylether (99%, Aldrich) was
refluxed at 250 1C for 40 min in a three necked round
bottom flask in an inert environment by bubbling nitrogen
gas whilst ensuring continuous stirring with a magnetic stirrer.
After 40 min the reaction mixture was cooled and 4.2 nm Pt
nanocrystals were obtained. The effect of varying the amounts
of stabilisers (oleic acid and oleylamine) was investigated using
Scheme 1 Reaction pathways of the cinnamaldehyde hydrogenation.
Table 1 The quantities of metal precursors, surfactants and reducing agents used in the synthesis of unsupported Pt and bimetallic Pt basednanocrystals
Bimetal system Quantity of second metal precursor salt usedQuantity of Pt precursor salt, oleic acid, oleylamine,and hexadecanediol used
Pt Nil 0.30 g Pt(acac)2 ,100 ml, 100 ml and 0.2 gTiPt 0.22 g Ti(IV) isopropoxide (99.999%, Aldrich)MnPt 0.19 g Mn(acac)2 (Aldrich)FePt 0.13 g Fe(Ac)2 (95% Aldrich)CoPt 0.20 g Co(acac)2 (97% Aldrich)NiPt 0.20 g Ni(acac)2 (95% Aldrich)CuPt 0.20 g Cu(acac)2 AldrichZnPt 0.17 g Zn(Ac)2 dihydrate (98.5% BDH GPR)SnPt 0.18 g Sn(Ac)2 AldrichPbPt 0.29 g Pb(Ac)2 trihydrate (99.999%, Aldrich)
NB: acac = acetyl acetonate and Ac = acetate.
2592 Phys. Chem. Chem. Phys., 2011, 13, 2590–2602 This journal is c the Owner Societies 2011
the same process which resulted in a newmethod of controlling
the Pt particle size in the range of 2.8–14 nm by the
modification of the polyol process (see Table 3).4
In the case of 4.2 nm Pt doped with other metallic element,
i.e cobalt, a separate glass vial containing a solution of
cobalt(II) acetyl acetonate, Co(acac)2, (min. 97%, Aldrich,
0.20 g), 5.0 mL of dioctylether (98%, Aldrich), 64 mL of oleic
acid (99+%, Aldrich) and 1,2-hexadecanediol (90%, Aldrich,
0.50 g) was first pre-heated to 100 1C to remove water. This
typical cobalt precursor solution was injected into the 4.2 nm
platinum sol in the refluxing set-up using a glass Pasteur
pipette at 200 1C. The nitrogen gas flow was increased as
a precautionary measure to maintain the oxygen free
environment during the injection process. The temperature
was maintained at 250 1C for 20 min. The reaction mixture
was then allowed to cool to room temperature (20 1C) and
repeatedly washed in portions of an ethanol and hexane
solvents before centrifugation. The collected particles after
the centrifugation were then re-dispersed and stored in
isopropanol (Fisher Analytical Reagent grade). The air dried
particles was directly used for catalysis studies.
The quantities of Pt(acac)2 for the Pt seed formation and
the second metal precursor salt used in the preparation of
the bimetallic systems are listed in Table 1 (metal mole ratio
of ca. 1).
Catalytic Testing of Pt nanostructures
The hydrogenation experiments were carried out under liquid
phase in a stainless steel autoclave with a glass liner fitted
tightly to its inner wall. The autoclave, equipped with a
magnetic stirrer, pressure gauge and a thermocouple, was
heated and regulated by a Parr 4842 temperature controller.
The size of reactor, weight of catalyst used and quantities of
testing mixture were clearly stated in the experiments. Initially,
for the comparison of selectivity, 37 mg of unsupported Pt
nanocrystals, 5 mL of 12.5% vol/vol cinnamaldehyde in
isopropanol (IPA), 20 bar H2, 100 1C for 2 h were placed
in 25 mL stainless steel autoclave. The liquid sample was
analyzed with GC-FID and GC-MS to identify and
quantify all products, which showed a total consumption of
cinnamaldehyde under the reaction conditions. Only two
main products, the unsaturated alcohol (cinnamyl alcohol)
and the saturated aldehyde, the 3-phenylpropionaldehyde
(hydrocinnamaldehyde) were obtained with only trace of
phenylpropanol detected. In order to verify the relationship
between conversion and selectivity, the reaction was prevented
from reaching a complete conversion. Thus, only 1.0 mg of
unsupported metal nanocrystals was then weighed and placed
in the autoclave together with a magnetic stirrer bar. 5.0 mL of
3.0 vol% of trans-cinnamaldehyde (99% Aldrich) in IPA was
then added. The autoclave was purged with pure hydrogen
(99.99%, BOC) for a minute to remove any traces of oxygen. It
was then charged to 20.0 bar of hydrogen (99.99%, BOC) at
20 1C and heated to 100 1C. The reaction was allowed to
process for 2 h. The autoclave was then cooled to room
temperature, depressurised and the liquid content was
analyzed by a Hewlett Packard 5890A GC - FID equipped
with Supelco COWAX 10 polar capillary column. The
parameters to be investigated i.e. reaction pressure, time and
concentration of substrate were varied.
TEM Characterisation of Pt nanostructures
A few drops of the as-synthesised nanoparticles sol was
dispersed in ethanol using an ultra-sonic bath for 15 min.
Three drops of the uniformly dispersed sol was then added
drop-wise onto a carbon film on a cupper grid (Lacey carbon
film, 300 Mesh Cu, Agar Scientific) and the solvent was dried
off using a 60 watts lamp. The cupper grid was then placed in
an oven at 100 1C to completely dry off the solvent. The
samples were analyzed by JEOL JEM-3000F FEG TEM,
JEOIL JEM-2010 (high resolution TEMs) and Philips CM
20 (analytical TEM).
SEM/EDX Characterisation of Pt nanostructures
The nanoparticle sol was dried on a watch glass and the
powder was then dispersed to cover the entire surface of the
25 mm diameter carbon disc mounted on an aluminium SEM
specimen stubs (Agar Scientific) by a double sided adhesive
tape. The samples were then arranged in the multiple stub
sample holder for analysis using a lower energy, o4 kV
Cambridge 360 Stereoscan, SEM which was equipped with
an Oxford Instruments INCA X-ray analysis system.
AES/XPS Characterisation of Pt nanostructures
The AES/XPS analyses were carried out using the ESCA Lab
II at Johnson Matthey technology centre, Sonning Common,
Reading, UK. The spectra were measured using an Al Ka(1486.6eV) X-ray source and a hemispherical electron energy
analyser. The acceleration voltage is 10 kV and the emission
current is 12 mA for the X-ray source. The spectra were
measured at around 90 degree emission angle. The electron
energy analyser operates in FAT mode (Fixed Analyzer
Transmission), with constant pass energy of 50 eV for survey
(wide) scans and 25 eV for high resolution scans. The overall
resolution of this XPS is around 1.1 eV. The peak fitting was
conducted using CasaXPS, with a Shirley background
subtraction, a lineshape of mixed Lorentzian (30%) and
Gaussian (70%) character. The typical FWHM values
obtained from the fitting are: C1s, O 1s, Pt 4f 7/2, Pt 4f 5/2,
Co 2p3/2, Co 2p 1/2, etc. (the exact number varies from sample
to sample but mostly close to these values).
XRD Characterisation of Pt nanostructures
A portion of the nanocrystals as unsupported nanocatalysts
(powder) were packed into the groove of an aluminium sample
holder and smoothened with a glass slid to ensure a very level
sample with diameter ca. 23 mm. The sample was then loaded
into a Philips DIFF 3, X-ray diffractometer with a Cu-anode
source operating at 40 kV and 30 mA generating the Cu-KaX-ray radiation at wavelengths Cu-Ka1 = 1.5406 A and
Cu-Ka2=1.54439 A. The sample was rotated through
angular acquisition range in 2y of 31–701, step size 0.021, step
speed 0.51 min�1 and at 1.25 s per step. The detector was
equipped with a graphite diffracted beam monochromator set
for Cu-Ka NaI scintillator with pulse height analysis. The
This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 2590–2602 2593
phase search and match were carried out using X’Pert High
Score software (version 1.0d).
FTIR surface characterisation of CO pre-adsorbed Pt
nanostructures
Pure carbon monoxide (obtained from BOC) at 1 atmosphere
was bubbled through the nanoparticles colloid in isopropanol
for 15 min. The colloid sample was placed onto the crystal
surface of the Smart Golden Gate for analysis. Excess solvent
was evaporated by heating the top plate to 60 1C, and
all spectra were obtained from an average of 512 scans using
a Nicolet 6700 FTIR spectrometer equipped with an MCT
detector at a resolution of 4 cm�1 over a range of wavenumber
from 4000 cm�1 to 650 cm�1.
Result and discussion
TEM Imaging
Typically, 4.2 nm Pt nanoparticles of a uniform size
distribution were obtained by our chemical reduction process
in solution, as shown in Fig. 1 (left). The high resolution TEM
images suggested that many of the particles were found to be
highly crystallized in cubooctahedron shape as nanocrystals
under careful examination of the images (Fig. 2, left). We
showed that the effect of varying the amounts of stabilisers
(oleic acid and oleylamine) can also result in controlling the Pt
particle size in the range of 2.8–14 nm (Table 3). Based on
geometric models of size effect of nanocrystals from Boronin
and Poltorak38 and Hardeveld and Hartog,39 smaller particle
sizes (2–4 nm) possess more corners and edges than larger
particle sizes, but the larger particles show more terrace faces
than those smaller particles (Fig. 2, right). As a result, different
particle sizes can give different catalytic properties if the
catalytic process depends on nature of the active sites on the
nanocrystal locations (faces, corners and edges). Experimental
evidence was indeed obtained by Schimpf and co workers40 in
1998, where their gold nanoparticles were characterized by
transmission electron microscopy (TEM) with reference to
catalytic activity. They showed that the increase in particle
size resulted in an increase in the proportion of the surface
atoms in the closely pack (111) plane with a dramatic decrease
in the edge and corner atoms.
Our 4.2 nm Pt nanoparticle sample was later decorated
(doped) with a second metal using subsequent deposition via
the same modified polyol process. It was reported that the
pre-formed metal seed is able to catalyse the reduction of a
second metal on the seed surface.41,42 Generally, surface atoms
with the lower coordination numbers (such as adatom, kink,
edge and step atoms) will be sites for deposition due to their
high surface energy and instability. This process was aimed
to modify the Pt nanocrystal surface with another metal in
order to alter the electronic structure of the Pt. Transmission
electron micrographs of the as-synthesized unsupported Pt
nanocrystals (seed) and the second metal doped Pt showed
no significant deviation in mean particle size within
experimental error (ca. 4.25 � 1.0 nm for Pt seed particles
and 3.40 � 0.42 nm for doped Pt particles), as shown in Fig. 1
(right). This implies that the extent of decoration was very
small degree perhaps only in atomic levels.
Elemental analysis
The extent of coverage of the secondmetal on the surface of the
Pt nanocrystals was determined using two analysis techniques,
namely the lower energy, high grazing angle SEM-EDX
(Fig. 3) and the XPS. The results summarized in Table 2
confirm that the decoration was indeed limited to selected
area on the Pt surface as both analyses still gave predominant
Pt signals. The exception was the CuPt which might have
formed bulk alloy under our reaction conditions. Also, in
general, the composition of Pt-M particles obtained from
SEM-EDX analysis is consistently lower than those of XPS
(more closer to near surface analysis). This attributes to the
difference in analytical depth of the two instruments (the
analysis depth for XPS is of about o5 nm while SEM-EDX
can be up to 20 nm) and the presence of small amount of pure
second metal nanoparticles from self-nucleation. It should be
noted that estimation of the surface coverage of sub-10 nm
particles by XPS is very challenging due to the similarity in
particle dimensions and photoelectron escape depths. If the
mean bimetallic nanoparticle dimensions arer4 nm then XPS
will sample the majority of the particles (likely to take place
in this case), hence this technique is not as surface sensitive
as when considering larger nanoparticles. The different
sampling depths for each dopant bimetal could be estimated
from associated inelastic mean free paths of the relevant
Fig. 1 (left) A transmission electron micrograph (magnification of 300 000 X and 200 kV) shows the as-synthesised Pt nanocrystals with particle
size distribution of 4.25� 1.0 nm; (right) TEM image of unsupported CoPt bimetallic nanoparticles before reaction with particle size distribution of
3.4 � 0.42 nm.
2594 Phys. Chem. Chem. Phys., 2011, 13, 2590–2602 This journal is c the Owner Societies 2011
photoelectron excitation monitored. On possible way to assess
the degree of surface versus bulk dopant incorporation is to
compare the atomic% of dopants observed using two different
photon excitation energies, e.g. Mg vs. Al Ka sources. If the
resulting compositions are identical then the dopant is likely
uniformly distributed throughout these 4 nm NPs, while if the
Mg-Ka yields a significantly higher value then surface
segregation is proven and can be subsequently quantified.
Such detailed analysis is yet to be collected. Nevertheless,
taking the small degree of particle size variation and self-
nucleation into account, the determined compositions by
both techniques clearly suggest that the second metal content
on Pt was much lower than the recipe value. Thus, there was
only a small degree of deposition or decoration of the second
Fig. 2 (L) High resolution TEM of typical platinum nanocrystals synthesised using the polyol process, and the corresponding modelled surface
atoms ratio with particles size of cuboctahedal shaped nanocrystals.39
Fig. 3 An SEM micrograph (insert) and EDX spectrum showing the chemical composition of Pt decorated with equal mole of (a) Ni, (b) Co,
(c) Cu and (d) Zn in synthesis recipes.
Table 2 Surface elemental composition analysis of the as-synthesised platinum bimetallic catalysts by SEM-EDX and XPS techniques
Platinum bimetallic catalyst
Atomic % of Pt Atomic % of 2nd metal
SEM-EDX XPS SEM-EDX XPS
FePt 83 88.5 17 11.5CoPt 85 87.2 15 12.8NiPt 85 n.d. 15 n.d.CuPt 63 n.d. 37 n.d.ZnPt 85 93.9 15 6.1TiPt n.d. 85.9 n.d 14.1
This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 2590–2602 2595
metal on specific locations on Pt surface (but no extensive
encapsulation of Pt nanoparticle by the second metal). By
taking the atom response factors into account, the doping
metal contents were found to be below 14% with reference to
the near surface Pt atoms. We believe that these second metal
atoms on Pt nanocrystal surface play important geometric and
electronic roles in affecting catalytic performance of the
crystals.
XRD analysis
It is known that when a secondmetal is deposited (decorated) on
another metal of different lattice parameters, a slight expansion
or contraction of the lattice of top metal exerted by the
underlying metal lattice can be resulted. This could cause a
significant alteration in electronic properties of the metal due to
the structural change (isomorphic effect).43 XRD was therefore
carefully conducted (Fig. 4) in order to examine any possible Pt
lattice alteration before and after the second metal doping.
As seen from Fig. 4, we have only observed the typical Pt
diffraction peaks with no detectable diffraction peak due to
second metal (this may indicate the second metal was either
in amorphous state or as ultra-thin layers where the severe
peaks broadening rendered them indistinguishable from the
background). There is also no observable shift in diffraction
peaks to suggest any formation of M–Pt alloy or lattice
expansion / contraction of the Pt nanocrystal after the
modification of the second metal doper. On the other hand,
owing to the poor sensitivity of XRD to surface change, we still
cannot discount a small degree of geometrical disparity due to
surface alloy formation or lattice parameter alteration of either
the host or the gust metals at the interface. However, it is
interesting to observe from the figure that the diffraction peaks
of all second metal doped Pt samples becomes broadened,
which implies the ensembles of Pt (nanocrystals) are broken up
slightly by the presence of the metal doper.
Catalytic testing and optimisations
Table 3 summarises our earlier communication note4 that Pt
nanocrystals of different sizes made by modified polyol process
showed a very large difference in selectivity towards cinnamyl
alcohol (2.8 nm gave 24.8% selectivity and 14.4 nm gave
85.6%) at a complete cinnamaldehyde conversion, suggesting
that this reaction is very structural sensitive. Clearly, the
smaller Pt sizes with higher proportion of high index sites
(undesirable sites) gave lower selectivity to cinnamyl alcohol
(2.8 nm with 24.3% and 3.3 nm with 44.6%). The effect
appears to be less significant at larger Pt sizes (6.0 nm with
80.8% and 14.4 nm with 85.6%). It is interesting to note that
the selectivity to unsaturated alcohol converged to around
80–85%. Thus, it is consistent with the geometric models
from Boronin and Poltorak38 and Hardeveld and Hartog39
where the change in Pt coordination numbers is rather
insensitive to particle size greater than 5 nm owing to the
small variations. Thus, the supported Pt samples prepared by
conventional means with a generally smaller size than the
present unsupported particles should show greater
attenuation in selectivity. However, with the accurate control
in crystal size by the polyol method this work clearly underpins
the size effect on selectivity of this hydrogenation, which can be
separated from the effects of support and chemical promotion.
Detailed analysis also showed that the selectivity depends on
the coverage of the substrate molecule (the measured selectivity
was higher when higher concentration of the substrate
molecule was used. This effect implies that the higher degree
of surface coverage at higher substrate concentration favours
the terminal aldehyde hydrogenation by creating a steric
effect that inhibits hydrogenation of the CQC bond of
cinnamaldehyde, see Fig. 6).
When the 4.2 nm Pt nanocrystal (100 mL oleic acid and
100 mL oleylamine) was decorated with second metal across
the periodic table, it is interesting to note from Fig. 5 that a
volcano response was observed for the selectivity towards
cinnamyl alcohol (and also the yield) of this hydrogenation
reaction. Only with the exceptionally lower selectivity of Ni
(nickel is well known to be hydrogenation catalyst on its own)
and higher selectivity of Sn (tin can form PtSn alloy) the volcano
response across the transition elements in the periodic table
clearly reflects the changes in electronic properties of Pt
modified by the surface transition metal doper. This will be
discussed in later section. It is worth noting that the selectivity
with respect to the second metal doper reached the maximum
value of >99% when cobalt element was doped on the
Pt nanocrystal. As a result, Co doped Pt was selected
for detailed investigation of experimental parameters on the
reaction selectivity.
Effect of substrate concentration on selectivity and conversion
Fig. 6 shows clearly that the selectivity of the CoPt
nanocatalyst towards the production of cinnamyl alcohol is
linearly dependent on the concentration of cinnamaldehyde
used. The preferential hydrogenation of cinnamaldehyde to
cinnamyl alcohol increased gradually as the concentration
of the substrate increased reaching 100% selectivity at a
concentration of 0.983 mol L�1 (12% v/v cinnamaldehyde in
isopropanol). The notable improvement in selectivity from
94% to virtually 100% as substrate concentration increasing
from 0.04 mol L�1 to 0.983 mol L�1 can be attributed to stericFig. 4 Plots of XRD patterns of the preformed Pt sample and samples
obtained after doping with a second metal by polyol process.
2596 Phys. Chem. Chem. Phys., 2011, 13, 2590–2602 This journal is c the Owner Societies 2011
effects as a result of the coverage of substrate molecules on
catalyst surface.44,45 The high coverage would enable the
cinnamaldehyde molecules to be aligned linearly for closer
surface packing of the adsorbed molecules hence facilitating the
hydrogenation of the terminal aldehyde. Thus, at low coverage,
cinnamaldehyde would likely adsorb with the CQC and CQO
bonds co-planar with the metal surface (i.e. a flat-lying geometry
that facilitates hydrogenation of unsaturated bonds), while
at high coverage, a tilted geometry is likely adopted with the
preferential CQO interaction with the surface as observed those
for alkoxides on Pt. As the concentration of cinnamaldehyde
increased, the conversion was also found to decrease (from ca.
85% to 60%), as seen from Fig. 6.
Effect of hydrogen pressure on selectivity and conversion
Under this liquid phase batch process, reaction parameters
such as 3.0 mg of unsupported CoPt catalyst, 5.0 mL of
3% vol/vol cinnamaldehyde at 100 1C over a reaction time of
2 h were kept constant, which enabled the effect of hydrogen
pressure on conversion and selectivity to be studied (Fig. 7). It
was found that the increasing the hydrogen pressure from
Table 3 Size effect of Pt nanocrystals on reaction selectivity towards cinnmyl alcohol at complete cinnamaldehyde conversion, the main sideproduct is hydrocinnamaldehyde with only trace of phenylpropanol detected4
Stabilizers
Particle size (TEM) Cinnamyl alcohol selectivity (%)Oleic acid (mL) Oleylamine (mL)
40 40 14.4 (XRD) 85.680 80 6.0 80.8120 120 4.8 49.1200 200 3.3 44.6300 300 3.1 30.2400 400 2.8 24.3
Fig. 5 A volcano trend in catalytic performance (selectivity and yield) for 4.2 nm unsupported Pt decorated with second metal. Conditions:
1.0 mg of unsupported Pt nanocrystals, 5.0 mL of 3% vol/vol cinnamaldehyde in isopropanol, 20 bar hydrogen, 100 1C and 2 h. Reactor volume
is 25 mL.
Fig. 6 Effect of cinnamaldehyde concentration on selectivity and
conversion. Conditions: 1.0 mg of unsupported CoPt nanocrystals,
5.0 mL of cinnamaldehyde at various concentrations in isopropanol,
100 1C, 20 bar of hydrogen, 2 h of reaction time. Reactor volume
is 25 mL.
This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 2590–2602 2597
1.5 bar to 10 bar resulted in an increase in the cinnamyl alcohol
selectivity sharply from 70% to 96%. The selectivity was then
levelled between 10 bar and 55 bar. Similarly, the conversion
increased steadily from 21.6% at 1.5 bar to 83% at 10 bar
reaching a plateau at about 20 bar, as shown in Fig. 7. This
trend is similar to the generally proposed trend of activity
trend in many hydrogenation reactions in liquid phase with
respect to hydrogen pressure in the literature.4 The Langmuir
adsorption model is generally adopted for catalytic
hydrogenation, which requires a mass transfer of hydrogen
gas to adsorbed hydrogen over active metal surfaces like Pt,
Pd and Ru.46 Since hydrogen gas solubility is poor at low
applied hydrogen gas pressure in the reactor, an increase in
pressure would therefore favour the dissolution of hydrogen
gas in isopropanol according to the Henry’s law from 1–10 bar
of hydrogen until it reached saturation.47 The important
information from this fit is to derive the saturation hydrogen
pressure, more data point with vigorous statistical treatment is
yet required in order to deduce the Langmuir equilibrium
constant.
Effect of temperature on selectivity and conversion
Fig. 8 shows that the extent of hydrogenation is dependent on
applied temperature while all other reaction conditions are
kept constant. The conversion increases gradually from 13% at
313 K to 19% at 373 K. A similar trend is observed for the
selective towards the unsaturated alcohol (from 88% to 98%
over the same temperature regime).
Effect of reaction time on cinnamyl alcohol formation
The concentration of cinnamyl alcohol increases almost
linearly as the reaction progresses with initial experimental
time from 0 to 4 h, as shown in Fig. 9. At the beginning of the
reaction, there was simultaneous formation of both
hydrocinnamaldehyde (i.e. the hydrogenation of the CQC)
and cinnamyl alcohol so the initial selectivity at the first
120 min was as low as 90% (not shown). However, further
experimental time for this batch reaction led to higher
selectivity to cinnamyl alcohol at the expense of the
hydrocinnamaldehyde until the cinnamyl alcohol was almost
exclusively formed after 90 min, a similar observation that was
also made by Breen et al.49 It was believed that the initial
catalyst was comprised of partially oxidised Pt nanoparticles
which was subsequently reduced to active metallic phase over
the 2 h under the reaction conditions.
However, by pre-reducing the catalyst material at 100 1Cwith
5 bar of hydrogen (99.99% BOC) for 1 h before injecting
the substrate concentration (3% vol/vol cinnamaldehyde in
isopropanol), the initial formation of hydrocinnamaldehyde
can indeed be greatly suppressed, as shown in Fig. 10,
giving high selectivity towards cinnamyl alcohol. It is therefore
evident that Pt based catalysts were partially oxidised in air as
previously shown.50–53 The pre-reduction step is important to
restore the metallic surface (it took time for the reduction under
the reaction conditions), which is essential for the production
of cinnamyl alcohol through the terminal adsorption of the
substrate. Partial oxidised Pt surface appears to give a greater
extent of the initial CQC hydrogenation.
Reaction kinetics
Considering the hydrogenation of cinnamaldehyde (CAL) to
its products, the reaction is represented by the equation;
CALþH2 �!k
products
The rate equation can be represented as
R = k[CAL]m[H2]n (1)
From Fig. 11, the concentration of cinnamaldehyde in the
hydrogenation reaction decreases with the progress of the
reaction where the rate constants, k, at different temperatures
can be deduced.
For a first order reaction, plotting ln[CAL] against time (t)
should give a straight line.48,53 Fig. 12 depicts that the
experimental data apparently fit to the first order reaction
with respect to cinnamaldehyde concentration.
Arrhenius equation below was used to estimate the reaction
activation energy. Fig. 13 shows the activation energy
Fig. 7 Langmuirian fit to experimentally determined data illustrating
the effect of hydrogen pressure on selectivity and conversion.
Conditions: 3.0 mg unsupported CoPt catalyst, 5.0 mL of 3% vol/vol
cinnamaldehyde in isopropanol, 100 1C and reaction time 2 h. Reactor
volume is 25 mL.
Fig. 8 A temperature effect on conversion and selectivity in the
hydrogenation of cinnamaldehyde over CoPt; conditions: 3.0 mg
unsupported CoPt, 5.0 mL of 3% vol/vol cinnamaldehyde in
isopropanol, 20 bar of hydrogen and reaction time 2 h. Reactor
volume is 25 mL.
2598 Phys. Chem. Chem. Phys., 2011, 13, 2590–2602 This journal is c the Owner Societies 2011
Fig. 9 The effect of reaction time on the formation of cinnamyl alcohol catalysed by CoPt on carbon. Conditions: 30 mg supported catalyst, 60 mL
of 3% vol/vol cinnamaldehyde in isopropanol, 100 1C and 20 bar hydrogen. Reactor volume is 300 mL.
Fig. 10 Effect of time on cinnamyl alcohol selectivity and
cinnamaldehyde conversion over the pre-reduced CoPt nanocrystals.
Conditions: 30 mg supported catalyst, 60 mL of 3% vol/vol
cinnamaldehyde in isopropanol, 100 1C and 20 bar hydrogen.
Reactor volume is 300 mL.
Fig. 11 A profile of the concentration of cinnamaldehyde dependence on time at various temperatures.
Fig. 12 A fit of the experimental data assuming a first order reaction
with respect to substrate concentration at 373.15 K and 20 bar
hydrogen.
This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 2590–2602 2599
calculated from the slope of the graph.
ln k ¼ lnA� Ea
RTð2Þ
The apparent activation energy of 17.26 kJ mol�1 was
derived in our case. This value agreed with the finding of Li
et al., who obtained their activation energy of 18 kJ mol�1 over
cobalt boron amorphous alloy catalyst.52 Breen et al. estimated
the activation energy to be 37 KJ/mol over iridium-carbon
catalyst.49 All these low value of activation energies for
this catalysed reaction suggest the rates could be somehow
subjected from mass transport or diffusion limited.48
In contrast, in our testing conditions, parameters were
deliberately chosen so no mass or diffusion limitation could
be encountered (used 20 atmosphere of H2, see Fig. 7; high
substrate concentration, see Fig. 6, over 1 mg catalyst
under optimum stirring) only except that in the kinetic rate
law determinations where the considerably lower hydrogen
pressures and lower substrate concentrations were used. They
could affect the accurate measurements in activation energy. It
would be interesting to apply a classical mathematically
modelling to the system using Weisz equation54 in order to
determine the valid experimental parameters that are not prone
to mass limitations in combination with the use of a Rushton
turbine with a hollow agitator for these fast reactions in the
future.
Geometric and electronic effects
Fig. 14 (top) shows a collection of typical FTIR spectra after
CO chemisorption on Pt nanocrystals with different sizes as
compared with a spectrum collected from the cobalt decorated
Pt (4.2 nm) sample. It demonstrates clearly that there is a sharp
increase in multicarbonyl adsorption due to the increase in the
number of highly uncoordinated Pt surface site upon
decreasing the particle size. It is apparent that the cobalt
deposition occurred principally on the highly uncoordinated
Pt sites that blocked the formation of multicarbonyl band
(M peak). As a consequence, the disappearance of the
characteristic band at u(CO) of 1925 cm�1 was evidenced.
Thus, the beneficial geometric modification of Pt nanocrystal
by cobalt atoms is concluded. In addition, there was a red shift
in the linear mode indicative of electronic promotion of Pt
(back donation) by the surface cobalt atoms. Fig. 14 (bottom)
shows a collection of FTIR spectra collected after CO
chemisorption on surface modified 4.2 nm Pt nanocrystals
where a range of transition metals were chosen as the dopant.
As seen from the typical M–Pt spectra (NiPt and FePt), apart
from linear CO adsorption mode of the Pt nanocrystals, they
also show absorbance bands corresponding to linear modes of
M–CO around 2050–1950 cm�1.27,32
The trend observed is that the selective hydrogenation of
CQO is favoured for catalysts in which the multicarbonyl
adsorption band at around 1925 cm�1 is suppressed by the
second metal deposition. This phenomenon also occurred in all
the bimetallic systems but to varying degree of efficiency with
Co metal the most exemplary. The exception to this trend is the
PbPt system (Fig. 14 bottom) in which the linear mode of CO
adsorption almost disappeared. It was observed that there was
a sharp decrease in the intensity for the linearly adsorbed CO
on Pt mode (u(CO) between 2090–2040 cm�1) which suggested
the poison effect of Pb on Pt. Pt decorated with Mn and Co
metals showed similar u(CO) patterns and they both gave a
high selectivity to cinnamyl alcohol above 90%.
It is very interesting to find that second metal atoms
preferentially segregate at sites of low coordination at the
surface of the Pt crystal, which can suppress the formation of
the multi-carbonyl peak (1925 cm�1) despite their low surface
coverage. Cyclic voltammetry can be used to analyse the
surface of platinum crystal structures; the technique can
provide structural information of the materials once they are
impregnated on to an electrode. Using this technique it should
be possible to see if the surface of platinum had been
modification in terms of second metal decoration on the
surface. Cyclic voltammograms for Pt single crystal surfaces
are well known, and Pt/graphite catalysts show four features
that correspond (by analogy with the single crystal data) to
Pt{1 1 1} � {1 1 1} steps, Pt{1 0 0} � {1 1 1} steps, Pt{1 0 0}
terraces and Pt{1 1 1} terraces. Changes in the populations
of these surface features with catalyst treatment can be
investigated and the effects on catalytic activity and
selectivity determined. Wells and co-workers have shown
that it is possible to work out the presence of different sites,
and surface structures of different size crystals.55 Cyclic
voltammetry was therefore conducted on our three samples
CoPt (1 : 1), Commercial JM 5%Pt/C and Pt/C, which was
made by the polyol process. The CoPt/C and Pt/polyol/C
contained 2.4 and 6.1% m/m Pt respectively from ICP
analysis. The JM 5%Pt/C was used as a reference sample in
order to determine the crystal faces. Pt/polyol/C was also used
as a reference sample as the platinum particles were synthesised
by the same method but without the presence of the typical
cobalt, this eliminated any effects of unwashed stabilisers on
the surface that might have been present on the sample, which
could have blocked certain crystal planes thus giving a
false interpretation of the cobalt decoration on the surface
of platinum.
The cyclic voltammograms (Fig. 15) indicated the
disappearance of Pt {111} � {111} and Pt {100} � {111}
steps (distinctive features in CVs of Pt/graphite that
correspond with single crystal data upon the decoration of
Co atoms.55 It is clear that no stabiliser appeared to have
blocked the crystals planes as all the distinctive features of
platinum were visible from the spectra on the Pt/polyol/C
Fig. 13 Arrhenius plot for cinnamaldehyde hydrogenation over CoPt
nanocrystals.
2600 Phys. Chem. Chem. Phys., 2011, 13, 2590–2602 This journal is c the Owner Societies 2011
sample which was synthesised using the same stabilisers as the
CoPt particles. Pulse CO chemisorption also confirmed site
blocking (coverage) on platinum nanoparticle by Co atoms
(Table 4). The CO: metal stoichiometry of 1 : 1 was assumed in
these measurements. The addition of cobalt clearly resulted in
a decrease of the platinum surface area. Thus, the preferential
site-blocking by Co atoms reinforces the earlier postulation of
catalytic deposition on low coordination, but highly reactive
sites upon the subsequent reduction of Co2+ in the presence of
Pt nanocrystals.
Thus, the geometric (site) modification by doper is clearly
demonstrated to play an important role in cinnamyl alcohol
selectivity but the electronic modification on Pt nanocrystals
exerted by metal dopant is not yet clear. On the other hand, the
strength of surface bonding (chemisorption) with adsorbed
atoms and molecules is generally known to depend strongly on
electronic properties of underlying metal or bimetallic surface.
In addition, it has been noted from the CO chemisorption
spectra (Fig. 14) that there is a clear red shift in the linear mode
of uCQO particularly upon the addition of cobalt doper.
Bearing in mind that the Co atoms are specifically decorated
on the low coordination sites at low surface coverage, the
results therefore clearly imply that the exposed Pt atom bearing
a CO ligand must have experienced an increasing degree of
back-bonding of its d-electrons (increasing d- of Pt) to the p* ofthe CO when it is placed closer to the Co atoms on a smaller
size of Pt crystals. X-ray Photoelectron Spectroscopy and
Auger Spectroscopy (surface sensitive techniques) which can
give information about the surface composition and electronic
interactions of the metals, were conducted. Analyses of the
surface compositions of the CoPt with different recipes’ (the
number in () was the recipe Co/Pt atomic ratio) using XPS as
shown in Table 5 confirm that the second metal did not
completely encapsulate but merely decorate the platinum
nanocrystals. Their surface percentages were in line with the
results from the EDX analyses in Table 2. It is however noted
from the XPS spectra that there was no detectable chemical shift
in the binding energies (Co 2p3/2 and 2p1/2 and Pt 4f7/2 and 4f5/2)
of both Pt and Co, presumably these innermost core electrons
might be less sensitive to any change in nearby chemical
environment. Interestingly, there were more significant and
progressive shifts in Pt Auger peaks NNN and Co LMM peaks
than their core peaks (Table 5). Co exerts a greater electronic
influence to Pt on the outermost electrons and vice versa as
reported in PtCo alloy in the literature,56 indicating the local
electronic influence of Pt crystal by increasing Co atoms content.
d-band Center analysis of cinnamaldehyde hydrogenation
With the appreciation of the electronic influence of the
Pt exerted by transition metal doper from the above
characterisations one key question is how important this
electronic effect to catalytic performance. The Density
Functional Theory, DFT summarized by the d-band center
model which has been successfully used in the literature to
model various chemisorption systems over metals, in which the
closer the d-band center is to the Femi level (towards zero), the
higher adsorption energy for adsorbate obtained.57 Here,
d-band center plot is used to examine the correlation
Fig. 14 (top) Plots of ATR-FTIR spectra from CO chemisorption
studies on Pt nanocrystals colloids with different particle sizes showing
different characteristic peaks of CO linear mode u(2060 cm�1, (L)),
multicarbonyl mode u(1925 cm�1, (M)) and bridging mode u(1830cm�1, (B)). The absence of multicarbonyl peak over 4.8 nm Pt
nanocrystal decorated with cobalt is demonstrated;4 (bottom) Plots
of ATR-FTIR spectra from CO chemisorption studies on Pt samples
with a second metal decoration.
Fig. 15 Cyclic voltammograms of 5% Pt (synthesised of polyol
reduction) on graphite; JM commercial 5%Pt on graphite and 5%
Co decorated Pt (synthesised of polyol reduction, 1 : 1 CoPt) on
graphite using 0.5 M sulfuric acid as the electrolyte, voltage was
swept between 0 and 0.8 V at a rate of 10 mV s�1.
This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 2590–2602 2601
between electronic effect and the catalytic performance of this
reaction. As seen from Fig. 16, a volcano relationship between
selectivity to cinnamyl alcohol with respect to the position of
d-band center of the particular 1st row transition doper metal
referenced to its Fermi level is obtained. The selectivity reaches
the highest point at the CoPt, but starts to decrease at FePt and
drops sharply at the NiPt catalyst on both side of the apex.
This variation in selectivity is believed due to the difference in
their d-band centre position from the Fermi level when the Pt
nanocrystal surface was electronically modified by the
transition metal. Amazingly, the same volcano response with
the apex on cobalt was presented by Stamenkovic et al. who
investigated the specific activity of bimetallic Pt electro-
catalysts for oxygen reduction reaction (ORR) and the band
center position of the added transition metal.58 They attributed
their best CoPt activity to the proper balance of adsorption
energy of O2 or reaction intermediates (O2�, O2
2�, H2O2) for
optimised surface electro-catalysis. They believed that for
metal surfaces that binded oxygen or intermediates too
strongly, as in the case of Pt, the d-band center was too close
to the Fermi level and the rate of the ORR was limited by the
availability of free Pt sites (poisoning effect). On the other
hand, when the d-band center was too far from the Fermi level,
as in their case of TiPt, the surface binded O2 and intermediates
too weakly to significantly promote the ORR activity.
Although our catalytic hydrogenation reaction was very
different from the ORR in the cathode, the adsorption
strengths of substrate or intermediates in the function of
electronic properties of the underlying bimetallic surfaces
must be closely related to this, giving the same volcano
response. Since cinnamaldehyde adsorption on catalyst
surface can take place with either adsorption of alkene or
terminal carbonyl moieties followed by the formation di-smetal C–C or di-s metal C–O bonds, respectively. As d-band
center moves close to Fermi level, a stronger and more
thermodynamic favourable di-s metal C–C bonds is
preferred over di-s metal C–O on NiPt and Pt surfaces,
which reduce the selectivity in the hydrogenation of the
CQO bond. As the d-band centre moves away from the
Fermi level, it is likely to expect the di-s metal C–C binding
energy to decrease, thus leading to a more selective CQO
hydrogenation pathway. Thus, the selectivity is maximized at
around FePt and CoPt, which indicate that the value of the
d-band centre on these surfaces correspond to the optimal
binding energy of CQO in cinnamaldehyde. However, the shift
in the d-band centre will also reduce the di-s C–O binding
energy and would likely make it too weak for hydrogenation to
occur when the d-band centre is too far away from the Fermi
level, such as that observed with the TiPt catalyst. As evident
from the poor conversion shown in Fig. 5, the drop in cinnamyl
alcohol selectivity at low conversion was due to rising
significance of other side products. Therefore, d-band center
indicates the need for a delicate balance between the strengths
of these two bonding configurations which would lead to a
volcano-type relationship for the production of cinnamyl
alcohol, as illustrated in Fig. 16.
Conclusion
Bimetallic nanostructures are currently used as industrial
catalysts for many important transformations. It has been
postulated that the second metal can play several pivotal
roles within the catalyst, including site blocking and
electronic promotion on the primary metal. However, a poor
understanding of these mechanisms hampers further
development within this area. It is often difficult to disentangle
and optimise these effects, especially using catalysts that have
been prepared using traditional techniques, as invariably side
reactions are promoted. In this paper, we have presented some
new ways to approach these problems. By using simple nano-
chemistry skills, unsupported Pt nanocrystals with tailored sizes
can be decorated with another metal atom in a controlled
Table 4 Pulse CO chemisorption analysis
Catalyst (on graphite) Surface area/g metal (m2 g�1) Atom dispersion (%)
Commercial Pt (JM) 4.8 1.8Pt (Polyol) 13.7 5.0CoPt (Polyol) 9.2 3.4
Table 5 X-ray photoelectron spectroscopy and Auger emission data of Co decorated Pt nanocrystals
Catalyst% surfaceatom ratio (Co/Pt)
B.E Co0
2P3/2B.E Co2+
2P1/2B.E Pt4f7/2
B.E Pt4f5/2
K.E AugerPtNNN
K.E AugerCoLMM2
CoPt (0.24) 0.14 778.15 793.04 70.7 74.0 183.3 775.2CoPt (1.47) 0.22 778.21 793.26 70.8 74.1 184.5 774.7CoPt (5.33) — 778.21 796.74 70.8 74.7 185.2 772.2Co (Lit.) — 778.2 � 0.3 796.4 � 0.05 71.2 � 0.61 74.3 � 0.09 770.70
Fig. 16 Selectivity towards cinnamyl alcohol on the hydrogenation of
cinnamaldehyde as a function of the d-band center of transitionmetal doper.
2602 Phys. Chem. Chem. Phys., 2011, 13, 2590–2602 This journal is c the Owner Societies 2011
manner. Thus, we showed that the blockage of unselective low
coordination metal sites and the optimisation in electronic
influence of the decorated Pt surface, can be independently
studied in the selective hydrogenation of a,b-unsaturatedaldehydes. In this paper, we also show that the terminal CQO
hydrogenation can be achieved in high activity, whilst the
undesirable hydrogenation of the CQC group can be greatly
suppressed by some second metal dopers. In particular, the Co
decorated Pt nanocrystals display the best activity and
selectivity for the formation of cinnamyl alcohol. Our work
clearly demonstrates the advantage in engineering preformed
nanoparticles via a bottom-up construction and illustrates that
this route of catalyst design may lead to improved and greener
manufacturing processes.
Acknowledgements
We thank Dr Richard Smith of Johnson Matthey technology
centre, Sonning Common, Reading, UK for the XPS/AES
studies.
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