partial oxidation
TRANSCRIPT
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Hydrogen production by partial oxidation of
methane over Co based, Ni and Ru monolithic
catalysts
Halit Eren Figen* , Sema Z. Baykara
Yildiz Technical University, Chemical Engineering Department, Davutpasa Campus, Topkapi 34210, Istanbul,
Turkey
a r t i c l e i n f o
Article history:
Received 30 August 2014
Received in revised form
21 February 2015
Accepted 23 February 2015
Available online xxx
Keywords:
Hydrogen production
Methane
Partial oxidationCPOM
Catalyst
Monolith support
a b s t r a c t
Fossil fuels which supply most of the world energy demand are depletable, and they cause
greenhouse gas emissions which eventually lead to global warming and climate change.
Hydrogen, a clean and versatile energy carrier, can be converted into useful forms of en-
ergy in several ways. Catalytic partial oxidation of methane is a very promising process for
hydrogen and synthesis gas production, besides steam reforming of methane, the leading
technology. In the present work, catalysts for partial oxidation of methane have been
developed and studied in terms of structural properties and chemical performance. For this
purpose Co, CoeNi, CoeRu, CoeNieRu, and Ni catalysts loaded onto cordierite ceramic
monolithic supports were prepared via modified sol-gel-impregnation method. The cata-
lysts were characterized by, SEM-EDS, XRD, BET, and ICP-OES techniques. Activity tests of
the catalysts were performed in a tubular reactor at 450 ml/min total flow rate from 600 Cto 850 C. CoeNieRu was the most successful catalyst, with selectivity values of 93.10% H 2
and 93.81% CO, and CH4 conversion of 98.71%, and hydrogen production efficiency of
95.89% at 850 C. During the activity tests of this catalyst 2.13% CO2 was present in the
product stream.
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
reserved.
Introduction
Production of hydrogen (H2) from sources with established
infrastructures such as natural gas, which is mostly methane
(CH4), via catalytic processes is expected to facilitate transi-
tion to clean energy systems and sustainable development
[1e5]. In recent years, catalyst development and determina-
tion of optimum operating conditions for fuel processors have
been the main areas of investigation contributing to the
progress of catalytic H2 production [6e11]. Catalytic process-
ing of CH4 at large scale is mostly carried out by steamreforming, partial oxidation, and dry reforming [12e14]; where
synthesis gas containing H2 and CO is produced from which
H2 and other fuels can be obtained. Catalytic partial oxidation
of methane (CPOM) yields 2 mol of H2 and ~36 kJ/mol energy
and ~30% cost reduction in comparison to steam reforming.
Moreover, NOx formation is avoided, smaller reactors [6e13]
can be used, and H2 /CO ratio (~2) is suitable for methanol
production and motor fuel synthesis (FischereTropsch) [12].
* Corresponding author. Tel.: þ90 533 311 5112, þ90 212 383 4757; fax: þ90 212 383 4725.
E-mail addresses: [email protected], [email protected] (H.E. Figen).
Available online at www.sciencedirect.com
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http://dx.doi.org/10.1016/j.ijhydene.2015.02.109
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Main reaction of CH4 partial oxidation (R1), accompanied with
full oxidation of CH4 (R2) and with side Reactions R3 and R4,
can be assumed and stated as (at 25 C and 1 atm):
R1: CH4 þ ½ O2/ CO þ 2H2, DH ¼ 35.59 kJ/mol (1)
R2: CH4 þ 2O2/ CO2 þ 2H2O, DH ¼ 802.0 kJ/mol (2)
R3: CH4 þ3∕2 O2/ CO þ 2H2O, DH ¼ 519.33 kJ/mol (3)
R4: CH4 þ O2/ CO2 þ 2H2, DH ¼ 318.66 kJ/mol (4)
Reaction R1 is the main reaction step and it produces
synthesis gas (CO þ H2). Reaction R2 is a full combustion re-
action of CH4 while R3 is a side reaction that increases selec-
tivity of CO and decreases selectivity of H2. On the other hand,
side Reaction R4 increases selectivity of H2 and decreasesselectivity of CO [15e17].
Catalytic partial oxidation of methane reaction has been
studied in presence of heterogeneous catalysts with or
without noble metal constituents. Although noble metal cat-
alysts have high activity and stability, due to their high cost
and low accessibility, non-precious metal catalysts are often
preferred in industrial applications [12,18].Various catalysts
containing metals, noble and otherwise, have been developed
and studied previously, and results involving metals such as
B, Ca, Ce, Co, Ir, La, Ni, Pd, Pt, Rh, Ru, Sr, Th, Y, and Zr are
available [12,18e23] for applications using powder or mono-
lithic catalysts.
Nickel (Ni) is the most widely used catalyst presently.Although Ni containing catalysts have high activity, the
exothermic CPOM reaction leads to carbon deposition, sin-
tering of Ni due to coke formation; and deactivation of the
catalyst by forming NiAl2O4 phase especially when Ni is used
with Al2O3 support [12,18e23]. Studies with Ni catalysts in the
literature indicate reduction in coke formation and increase in
activity upon addition of Co, Cr, Sn, Mg, Ca, Ce, Gd, Y, Zr and
even trace amounts of noble metals [18e23]. For example, Ce-
NixOy has been prepared by precipitation method and was
used in partial oxidation of CH4 for H2 production; CH4 con-
version and H2 selectivity values of 75% and 52% were ob-
tained at 600 C respectively, while 70% CH4 conversion and
49% H2 selectivity were obtained at 200
C [6]. Catalysts Ni andCu in various compositions were deposited on Al2O3 by
impregnation method and they were tested in CH4 partial
oxidation reactions. When the CH4 to O2 ratio was 2.0 to 1.0,
50% conversion of CH4 was attained with Ni(5%)Cu(5%)/Al2O3
at 300 C [18]. In another work, partial oxidation of CH4 cata-
lyzed by NiO/Al2O3 including PteCeO2 ataC H4 to O2 ratio of 2.0
to 1.0 and at 800 C, hydrogen concentration was 40%; and
when the space velocity was set to 320 l. (h.g cat)1, this value
reached 74%. Catalyst selectivity to H2 was obtained as 87%
under the same conditions [19]. In a study where Ni/a-Al2O3,
NiSn/a-Al2O3, NiMn/a-Al2O3, NiMo/a-Al2O3 catalysts were
prepared by impregnation and tested for partial oxidation of
CH4, the results of the catalytic tests carried out at 800
C
indicated that the most promising catalyst was NiMo/a-Al2O3
with 90.6% CH4 conversion and 93.9% H2 selectivity [20].
Catalysts incorporating 3% (Ni þ Co) and supported on
CaAl2O4 /Al2O3 were prepared via impregnation method. The
most active catalyst in this work was (2%Ni þ 1%Co) on
CaAl2O4 /Al2O3 exhibiting 97% CH4 conversion and 97% H2
selectivity at 800 C [12]. Addition of Co can result in reduction
in the rate of carbon formation [24e
26], however Co catalystscan be strongly affected by factors such as the nature of
support, calcination temperature, and metal loading [27].
The activity of various Group VIII metals for hydrogen
production from methane by steam reforming is known to
follow the relative order: Ruz Rh > Ni > Ir > Pdz Pt >> Coz
Fe. Ruthenium (Ru) and Rhodium (Rh) have better stability on
stream, in the long term than Ni [28]. Furthermore, Ru is a lot
less expensive than other precious metals [29]. Precious
metals like Ru are better catalysts than Ni for methane com-
bustion and partial oxidation reactions as well [28]. Alumina
supported Ru catalysts (1% w/w) have good activity and
selectivity towards partial oxidation of methane [30]. For
example, a catalyst with as little as 0.015% (w/w) Ru on Al2O3
can display higher synthesis gas selectivity than a catalyst
with Ni on SiO2 [28].However, hot spot formation in the cata-
lyst bed and coking (leading to catalyst deactivation) are
possible with supported Group VIII metals (Rh, Pt, Ru, Ni) as
catalysts for partial oxidation of methane [6,12]. Coking is
more likely with Ni based catalysts, although Ni is much
cheaper, and is a good catalyst for synthesis gas production
[12].
In the present work, partial oxidation of CH4 over mono-
lithic catalysts with oxides of Co, CoeNi, CoeRu, CoeNieRu
and Ni for H2 production was investigated. Catalysts were
prepared and tested in terms of structural properties and
chemical performance.
Experimental
Materials and characterization
All reagents used were of analytical grade. Nitrate salts of Co,
Ni and Al, (Co(NO3)2$6H2O, purity:>99%, from Carlo Erba,
Ni(NO3)2$6H2O, purity:>99%, from Carlo Erba, Al(NO3)3$9H2O,
purity: >99%, from Merck), citric acid monohydrate
(C6H8O7$H2O, purity: >99% from Carlo Erba), and Ruthenium
Chloride Ru$Cl2$xH2O(Merck) were used as received. Honey-
comb type cordierite monolithic structures (400 CPSI),48.3 mm 48.3 mm 150 mm blocs, Rauschert Technical
Ceramics) were used as supports in the preparation of cata-
lysts. The Honeycomb type cordierite monoliths are referred
to as M-0 in the manuscript. Instrumental analysis with X-ray
diffraction (XRD), scanning electron microscopy-energy
dispersive spectroscopy (SEM-EDS), inductively coupled
plasma-optical emission spectroscopy (ICP-OES) and Brun-
nereEmmetteTeller (BET) surface analysis techniques were
used for structural study of the samples.
Characterization of crystal structure and determination of
crystallographic parameters of the catalysts were performed
by XRD analyses. Samples were ground in an agate mortarand
settled in an aluminum sample holder and XRD analyses were
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carried out at ambient temperature using a Philips Panalytical
X'Pert-Pro diffractometer in a diffraction angle range of
10e90 with CuKa radiation (l ¼ 0.15418 nm) at operating
parameters of 40 mA and 45 kV with a step size of 0.02 and
speed of 1 /min. Phases were identified with reference to
powder diffraction file (PDF) database.
Specific surface area of the catalysts were characterized by
using BET technique under N2 adsorptive gas and He carriergas at 77 K after outgassing at 0.6 Pa and 473 K, using Quan-
tachrome, Autosorb Instrument.
To quantify metal contents present in the samples ICP-OES
measurements were performed using Perkin Elmer Optima
2100DV. Beforethe ICP-OES readings, a few milligrams of each
catalyst sample was ground into a powder which was dis-
solved in a mixture of certain strong acids (H3PO4, HCl, HNO3,
HF). The sample was then treated in a microwave digester
(Berghof Speedwave 3þ, Microwave Digestion System). During
the ICP-OES analysis; each treated catalyst sample was
divided into three portions, and according to the standard
procedure, 3 parallel readings were obtained and their average
was taken as the final value of the elemental analysis of theloaded metal content for the specific sample [31].
Microstructure and surface morphology of the catalysts
were observed by field-emission gun scanning electron mi-
croscopy (CamScan Apollo 300 FEG-SEM). The samples were
covered with Au and made ready for analysis by fixing to the
device's sample holder with carbon sticky band.
Carbon deposition on catalysts were detected using the
thermal gravimetric analysis/Fourier transform infrared
spectroscopy (TGA/FTIR) technique, by measuring the CO2
evolved during thermal analysis [32,33]. A thermogravimetric
analyzer (Perkin Elmer Diamond TG/DTA) was coupled with a
Fourier transform infrared spectrometer (Perkin Elmer Spec-
trum One). Placing approximately 10 mg of sample on TGAbalanceand using a purge gas (pure O2) rate of 200ml/min, the
gases from TGA unit were transferred into the IR spectrom-
eter; and the interface was maintained at 200 C. A tempera-
ture program from ambient to 900 C, increasing at a rate of
5 C/min was used. Peaks within 2358e2344 cm1 bands were
investigated for CO2 asymmetric stretching.
Preparation of monolithic catalysts
Monolithic catalysts with oxides of Co, CoeNi, CoeRu,
CoeNieRu, and Ni were prepared by the sol-gel impregnation
technique. Preparation procedure can be divided into three
steps. (1) Preparation of monolithic ceramic supports: Beforecoating with catalysts the ceramic supports were prepared in
cylindrical shape with 13 mm diameter and 20 mm length. (2)
Coating of monolithic ceramic supports by alumina: Before
coating the monoliths with active catalysts, alumina was
wash coated [34] on thesurface of ceramic supports in order to
form a surface which would enable metals to adhere easily.
For this purpose 5 M aluminum nitrate solution was prepared
to begin wash coating of alumina onto the substrates. Weight
gain after each deposition was recorded until desired loadings
were achieved. Monoliths which were impregnated with
alumina nitrate solution were then calcinated at 600 C for3h.
During calcination of the monolithic supports, aluminum ni-
trate decomposes into nitrates and has high surface area after
the formation of cubic crystal gamma-alumina (g-Al2O3)
phase. (3) Coating of monolithic ceramic supports by sol-gel
impregnation method: Ceramic supports which had been
coated with alumina were sol-gel impregnated by using so-
lutions of Al(NO3)3$9H2O, Ni(NO3)2$6H2O, Co(NO3)2$6H2O, and
RuCl2$xH2O according to their composition. Weight gains
were again monitored to obtain desired loadings fallowed by
calcination at 800 C for 5 h. Synthesized catalysts werelabeled as M (Table 1) and were used for referring to support
and catalyst samples.
Performance tests of monolithic catalysts
Monolithic catalysts with oxides of Co, CoeNi, CoeRu,
CoeNieRu, and Ni were tested for H2 production from CH4
partial oxidation in a catalytic fuel processor system (HyGear-
Hexion located in TUBITAK-Marmara Research Center, Energy
Institute) equipped with Agilent 6890 gas chromatography
including a thermal conductivity detector (TCD) and a flame
ionization detector (FID). MolSieve, Plot-Q and GasPro col-
umns were used for separation of gasses. Before the experi-
ments, gas chromatography device was calibrated with a
certificated standard gas mixture suitable for feed andproduct
gases. In terms of determination of gas composition by gas
chromatography (GC), sources of uncertainty described in Ref.
[35] were taken into consideration. In the HyGear experi-
mental set-up the accuracy levels (as provided by the sup-
pliers) of mass flow controllers (Bronkhorst El-Flow),
temperature controller (Enda EUC442 PID Controller) with K-
type thermocouples (NiCreNi, 0e1200 C), and pressure
gauges (Swagelok, BG10) were ±0.5%, ±0.2% of full scale and
±1 C; and ±1.5% respectively.
The partial oxidation reaction was carried out in a tubular
stainless steel reactor with an inside diameter of 1.35 cm,
operated at atmospheric pressure. Reactants CH4 (99.500%),
O2 (99.999%), N2 (99.999%), H2 (99.999%) and calibration gases
for GC were supplied by HABAS and Air Products Company,
respectively. A gas mixture of CH4 and O2 (with a CH4 to O2
ratio of 2.0 to 1.0) was passed through the monolithic cata-
lysts at a gas hourly space velocity (GHSV) of 1 104 h1. The
GHSV was defined as the ratio of the reactant gas flow rate at
25 C and 1 atm to the total volume of the catalyst. The
catalysts were reduced in situ for 8 h at 300 C with hydrogen
gas (1%) before reaction. Water content of the reacting
mixture was condensed. Calculation of conversions and
yields were carried out according to the general equations
below [34,36]:
CH4 conversation : xCH4 ð%Þ ¼
FCO þ FCO2
FCO þ FCO2 þ FCH4
$100 (5)
H2 selectivity : SH2ð%Þ ¼
FH2
2$
FCO þ FCO2
$100 (6)
CO selectivity : SCOð%Þ ¼ ðFCOÞFCO þ FCO2
$100 (7)
H2=CO ratio : H2
CO
¼ FH2
FCO
¼2$SH2
SCO
(8)
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If H2 /CO > 2, partial oxidation process occurs according to
reactions R1, R2 and R4 [34].
R1 ð%Þ ¼ FCO
FCO þ FCO2
$100 ¼ SCO (9)
R2 ð%Þ ¼
FCO þ FCO2
FH2
2
FCO þ FCO2
$100 ¼ 100 SH2 (10)
R4 ð%Þ ¼
FH2
2 FCO
FCO þ FCO2
$
100 ¼ SH2 SCO (11)
If H2 /CO < 2, partial oxidation process occurs according to
reactions R1, R2 and R3 [34].
R1 ð%Þ ¼ FH2
2
FCO þ FCO2
$100 ¼ SH2 (12)
R2 ð%Þ ¼
FCO2
FCO þ FCO2
$100 ¼ 100 SCO (13)
R3 ð%Þ ¼
FCO FH2
2
FCO þ FCO2
$100 ¼ SCO SH2 (14)
Based on product compositions, CH4 conversion, H2 and CO
selectivity, and H2 /CO ratio were calculated at different tem-
peratures for Co,CoeNi, CoeRu, CoeNieRu, and Ni monolithic
catalysts and also reaction steps and realization ratios (extent
of completion of the reaction, %) of these reactions (R1, R2, R3
and R4) were determined. Increase in CH4 conversion, H2 and
CO selectivity values were observed with increase in temper-
ature (Table 3).
Results and discussion
Results pertaining to phase and elemental identification,
morphology, surface area, optimum reaction temperature and
activity rating of the catalysts have been obtained.
Structural characterization of monolithic catalysts
By using impregnation method, ceramic supported catalysts
incorporating various Ni, Co and Ru based oxides were pre-
pared. Crystal phase properties, elemental analysis, specific
surface area and microstructure properties were determined
by XRD, ICP-OES, BET and SEM methods.
Fig. 1 e XRD patterns of monolith support and monolithic catalyst: (a) Blank Monolith M-0, (b-1) M-1 Fresh, (b-2) M-1 Spent,
(c-1) M-2 Fresh, (c-2) M-2 Spent (d-1) M-3 Fresh, (d-2) M-3 Spent, (e-1) M-4 Fresh, (e-2) M-4 Spent, (f-1) M-5 Fresh and (f-2) M-5
Spent.
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Fig. 1 shows the XRD patterns of the ceramic monolith and
prepared monolithic catalysts in fresh and spent states. The
ceramic support cordierite (Mg 2Al4Si5O18) has hexagonal
crystalline structure with 01-084-1221 ICDD file number. Ac-
cording to the XRD analysis results, oxides of Co, CoeNi,
CoeRu, CoeNieRu, and Ni monolithic catalysts have cordi-
erite as the main phase. The XRD patterns verified the exis-
tence of Co3O4 (PDF: 01-074-1656) as the metal oxide phase in
all Co containing monolithic catalysts. The detailed crystal
phase analysis results of the other catalysts (fresh and spent)
are listed in Table 1. As expected, following impregnation, Co,
Ni, and Ru metal phases were crystalized as their oxide forms
(Co3O4, NiO, RuO2) on the ceramic support. Based on XRD
analyses of spent catalysts samples elemental Ni was found inM-5 (Fig. 1f, Table 2). Possible phases of CoNiO2, Co2RuO4,
NiAl2O4, CoAl2O4, NiO2, NiRuO2, Ru, Ni, Co were not encoun-
tered in remaining samples M-1 to M-4, indicating that new
metallic and oxide phases were not formed in those samples
within 18 h of exposure.
Crystal sizes of metal oxide phases found on ceramic
supports were calculated using Scherrer equation based on
XRD results [37]. At first, the most intense peaks of metals and
their locations (2q) were determined and the “Full Width at
Half-Maximum (FWHM)” values were specified. After “Fit
Profile” calculations [38], it was seen that Co3O4; NiO; Co3O4
and NiO; Co3O4 and RuO2; Co3O4, NiO and RuO2 oxide crystals
on ceramic supports of Co, Coe
Ni, Coe
Ru, Coe
Nie
Ru, and Ni
monolithic catalysts, had relatively same crystal sizes be-
tween 23.45 nm and 75.56 nm. It can be emphasized that
although each catalyst had different composition, the metal
oxide crystals formed at relatively same size since they were
prepared using the same procedure.
The specific surface areas of monolithic catalysts with
oxides of Co, CoeNi, CoeRu, CoeNieRu, and Ni were
measured to be 29.90 m2 /g, 25.42 m2 /g, 34.76 m2 /g, 38.68 m2 /g,
and 39.90 m2 /g respectively while the specific surface area of
ceramic support was 4.20 m2 /g. Although the monolithic cat-
alysts had quite similar specific surface areas, the sample
containing Ni, Co and Ru oxides had the largest, which is
consistent with literature [39]. Elemental composition of Co,
Coe
Ni, Coe
Ru, Coe
Nie
Ru, and Ni monolithic catalysts weredetermined by ICP-OES analysis. Table 1 gives the metal
content of the metal oxide phases in the catalyst samples.
Fig. 2a(50 magnification) and Fig. 2b (1000 magnifica-
tion) show the SEM images of horizontal cross sections of
interior surfaces of blank ceramic monolith and prepared
monolithic catalysts, respectively. According to SEM image of
ceramic monolith taken at 50 magnification (Fig. 2a1), it is
easily seen that corners of the blank ceramic monolith are
clear. Fig. 2a2e2a6 show increasing metal oxide accumulation
and regions of metal oxide agglomerates at the corners of the
square channels. Similar results are reported in literature [40].
According to SEM images taken at 1000 magnification
(Fig. 2b1e
6), comparison of horizontal cross sectional views of
Fig. 1 e ( continued ).
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the interior channels of blank ceramic supports and metal
oxide loaded supports show that the coating process of poreswith metal oxides by impregnation method fills the pores in
between crystal like structures on the surface. From SEM im-
ages, crystal structures smaller than 1 mm can be observed.
Fig. 3 shows the elemental mapping analysis of the blank
ceramic monolith and prepared monolithic catalysts.
Elemental mapping image of the blank ceramic monolith
(Fig. 3a) shows that the interior channel surface also reveals
the homogeneous dispersion of Al, Mg and Si oxides in the
cordierite structure. Results from elemental mapping of inte-
rior channels of prepared monolithic catalysts (Fig. 3bef)
indicate homogeneous dispersion of elements over the inte-
rior surfaces and deposition of metal oxides at the corners
along the channel length. This situation is in agreement withSEM images (Fig. 2b).
Aging (sintering) is promoted by prolonged exposure to
high gas-phase temperatures. Rates of metal sintering can be
greatly minimized by choosing reaction temperatures lower
than 0.3-0.5 times the melting point of the metal, since metal
crystallite growth is highly thermally activated [41]. Although
water vapor in the reaction atmosphere may accelerate the
crystallization and structural modification of oxide supports,
it is possible to lower sintering rates by adding thermal sta-
bilizers to the catalyst. Thermal stability of a base metal such
as Ni can be increased via addition of a higher melting noble
metal such as Rh or Ru [42]. In the present study, maximum
reaction temperature was not higher than 850 C. Among the
catalysts used, Ni and Co have relatively lower melting points
(1455 C, 1495 C) compared to those of Ru and Mo (2334 C,2623 C), and the temperature ratios were 0.58, 0.57, 0.36 and
0.32 respectively.
Catalyst performance experiments were carried out in a
temperature range of 650e850 C, using a freshsample in each
run, and the exposure time did not exceed 18 h. Methane
conversions with present catalyst samples were quite close to
the values obtained by thermodynamic calculations [16].
Consequently, possibility of significant levels of sintering
was quite low, especially in samples containing Ru, as it has
been confirmed in Ref. [42].
Carbon or coke results from a balance between the re-
actions that produce atomic carbon or coke precursors and
the reactions of these species with H2, H2O or O2 that removethem from the surface.
Methods for lowering formation rates of precursors of
carbon or coke relative to their gasification rates vary with the
mechanism of formation and the nature of the active catalytic
phase [43]. Formation and growth of species of carbon or coke
on metal surfaces can be minimized by choosing reaction
conditions that minimize their precursors and by introducing
gasifying agents (ie, H2, H2O). Introduction of modifiers which
change surface metal ensemble sizes or which lower the sol-
ubility of carbon (i.e. Pt in Ni) can be effective in minimizing
deactivation due to fouling by carbon and coke [43].
In a previous study [43], investigating carbon deposition on
supported metal catalysts during partial oxidation of methane
Fig. 1 e ( continued ).
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to synthesis gas at 1050 K (777 C) using a CH4 to O2 ratio of 2.0
to 1.0, it was found that the relative rate of carbon deposition
followed the order of Ni > Pd >> Rh, Ru, Irz Pt.
Very little carbon deposition was observed over the noble
metal catalysts, even after 200 h; demonstrating that macro-
scopic carbon deposition was independent of the mechanismfor synthesis gas production and that it was possible to
kinetically avoidcarbon deposition by using suitable catalysts.
In the present work, the exposure time of thecatalysts was
shorter (~18 h maximum) and CH4 conversion was close to
equilibrium.As canbe seen from Table 3, since reactions R1, R3
and R4 involve species (CO2 and H2O) that prevent carbon
deposition, significant deactivation was not expected espe-
cially in Ru containing catalysts. Carbon deposition on the
catalystswas investigatedin thepresent study viaTG-FTIR and
IR absorbancevalues of CO2 gaswithinthe wave numberrange
of 2344e2358 cm1 were obtained in the temperature range
45e680 C. Time scale being proportional to the oxidation
temperature, the amount of CO2 is interpreted in view of Lambert BeerLaw assumingthat thevariation of absorbance of
CO2 is directly proportional to its concentration. The maximum
absorbancevalues (absorbance/g. catalyst) of the catalysts M-1
to M-5 were 0.507 (at 346 C), 0.576 (at 440 C), 0.285 (at 360 C),
0.175 (at316 C)and0.972 (at 501C). Consequently, the relative
rate of carbondeposition on thecatalysts fallowed theorderM-
5 > M-2 > M-1 > M-3 > M-4 (Fig. 4).
Performance tests of monolithic catalysts
Variation of product compositions with temperature for cata-
lysts with Co, CoeNi, CoeRu, CoeNieRu, and Ni can be seen in
Fig. 5. For Co monolithic catalyst, generation of H2 and CO
gasses was not observed until 600 C. The product gas also
contained 29.39% CO2. Hydrogen (H2) generation started at
650 C and increased continuously to 29.65%, 52.43%, 62.486%
and 63.26% at 700 C, 750 C, 800 C and 850 C, respectively.
After 800
C there was no considerable increase in H2 compo-sition. While the percentage of H2 and CO increased, percent-
age of CH4 and CO2 were decreased with increasing
temperature. Values of CH4 conversion, H2 selectivity, CO
selectivity and H2 /CO ratio were calculated based on product
compositions at different temperatures in the range of
650e850 C. In addition, reaction steps and realization ratios of
these reactions were determined. As can be seen in Table 3,
CH4 conversion, H2 selectivity, and CO selectivity increased
with temperature. Methane full combustion reaction took
place only with 100% realization ratio at 600 C, accompanied
by both partial and full oxidation reactions of methane and the
second side Reaction R4, (Table 3)at650 C and 800 C. Methane
partial oxidation reaction (R1) started becoming dominant witha realization ratio of 71.30% at 750 C. The realization ratios of
methane partial oxidation reaction, defined as the main reac-
tion, were determined as 88.69% and 91.51% as temperature
increased to 800 C and 850 C, respectively.
Variation of product compositions with temperature is
given in Fig. 5 for CoeNi catalyst. At 600 C 21.98% CO2 was
present in product gas mixture. Hydrogen production started
at 650 C and its composition increased to 29.81% (700 C),
51.33% (750 C), 59.15% (800 C) and 62.49% (850 C). By 850 C,
H2 and CO amounts increased substantially parallel to the
significant increase in methane conversion. At 600 C, only
methane full combustion reaction took place with 99.48%
Table 1 e Structural properties of fresh monolithic catalysts.
Code Catalyst Crystal phases Crystal sizes (nm) Elemental composition (Weight, %)
M-0 Monolith support Mg 2Al4Si5O18 (01-084-1221) e 35.1e14.4e50.5 (Al2O3eMgOeSiO2)
M-1 Co Co3O4 (01-074-1656) 65.58 2.33
(Co)
M-2 CoeNi NiO (01-089-7130) 34.20 1.22e1.20
Co3O4
(01-076-1802) 35.75 (CoeNi)
M-3 CoeRu Co3O4 (01-078-1969) 39.27 2.44e0.05
RuO2 (00-018-1139) 75.56 (CoeRu)
M-4 CoeNieRu Co3O4 (01-076-1802) 38.07 0.30e0.30e0.02
NiO (01-071-1179) 23.45 (CoeNi e Ru)
RuO2 (00-021-1172) 43.72
M-5 Ni NiO (01-089-3080) 39.85 1.65
(Ni)
Table 2 e Crystal phases of fresh and spent catalyst samples.
Code Catalyst Fresh catalyst crystal phases Spent catalyst crystal phases
M-0 Monolith support Mg 2Al4Si5O18 (01-084-1221) Mg 2Al4Si5O18 (01-084-1221)
M-1 Co Co3O4 (01-074-1656) Co3O4 (01-074-1656)
M-2 CoeNi NiO (01-089-7130) NiO (01-089-7130)
Co3O4 (01-076-1802) Co3O4 (01-076-1802)
M-3 CoeRu Co3O4 (01-078-1969) Co3O4 (01-078-1969)
RuO2 (00-018-1139) RuO2 (00-018-1139)
M-4 CoeNieRu Co3O4 (01-076-1802) Co3O4 (01-076-1802)
NiO (01-071-1179) NiO (01-071-1179)
RuO2 (00-021-1172) RuO2 (00-021-1172)
M-5 Ni NiO (01-089-3080) NiO (01-089-3080)
Ni (01-078-0712)
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realization ratio. At 650 C, both partial and full oxidation of
methane and also second side reaction (R4, Table 3) were in
progress. As from 750 C, methane partial oxidation reaction
(R1) became dominant with a realization ratio of 69.50%. At
temperatures 800 C and 850 C realization ratios of the re-
action were 84.70% and 90.60%. Conversion of methane and
composition of reaction products increased from 800 C to
850 C. While partial oxidation main reaction and full oxida-tion reaction occured along with second side reaction ( R4) up
to 800 C, R4 replaced the first side reaction (R3) at 850 C. Also
selectivity of CO increased and CO2 level decreased apparently
in parallel with the extent of full oxidation reaction at 850 C.
Product compositions at different temperatures for CoeRu
catalyst are given in Fig. 5. In the product mixture at 600 C
29.32% CO2 was found. Starting from 650 C, 23.66%, 50.44%,
62.93% and 63.64% H2 was found in product gas at 700 C,
750 C, 800 C and 850 C respectively. When the temperature
reached 850 C, there was a slight increase in H2 and CO
amounts in parallel with a significant increase in methane
conversion. At 600 C, methane full oxidation reaction was in
progress with 95.02% realization ratio. After 700 C, partialoxidation of methane and second side reaction (R4, Table 3)
started along with full oxidation reaction. From 700 C,
methane partial oxidation became dominant with 62.60%
realization ratio and remained as the main reaction with
realization ratiosof 76.84%,87.86% and91.44% at 750 C,800 C
and 850 C respectively. Methane conversion increaseed sub-
stantially in parallel with hydrogen from 800 C to 850 C.
Partial oxidation main reaction and full oxidation reaction
along with second side reaction R4 continued at 850 C, par-
allel to increase in hydrogen selectivity.
Change in product compositions with temperature for
CoeNieRu catalyst is given in Fig. 5. At 600 C, 23.10% CO2 was
found in product gas mixture. At 650 C, hydrogen productionstarted and became: 18.78% (650 C), 48.03% (700 C), 57.93%
(750 C), 62.52% (800 C) and 63.92% (850 C). There was no
apparent increase in hydrogen amount after 800 C. In parallel
with temperature dependent H2 percentage increase, CO
amount also increased; however, CH4 and CO2 amounts
decreased. At 600 C, only methane full oxidation reaction
occured with 100% realization ratio. From 650 C, partial
oxidation of methane and second side reaction (R4, Table 3)
were in progress along with full oxidation reaction. From
700 C, methane partial oxidation became dominant with
64.30% realization ratio and remained so at 750 C, 800 C and
850 C with realization ratios of 81.92%, 90.85% and 93.10%.
From 800 C to 850 C there was substantial increase inmethane conversion in parallel with hydrogen amount. While
partial oxidation main reaction and full oxidation reaction
continued along with second side reaction (R4)upto800 C, R4
replaceed first side reaction (R3) at 850 C. Selectivity of CO
increased while CO2 level decreased.
Variation of product compositions with temperature is
given in Fig.5 for Ni catalyst. At 600 C 26.25% CO2 was present
in the product gas mixture. Hydrogen production started at
650 C and its composition increased to 43.39% (700 C),47.54%
(750 C), 56.87% (800 C) and 63.32% (850 C). By 850 C, H2 and
CO amounts increased substantially parallel to the significant
increase of methane conversion. At 600 C, only methane full
oxidation reaction occured with 100% realization ratio. At
Fig. 2 e XRD SEM images of monolith support and
monolithic catalysts: (a) Cross sections at 50X
magnification, a1: M-0, a2: M-1, a3: M-2, a4: M -3, a5: M-4,
a6 : M-5; (b) Inner channels at 1000X magnification, b1: M-
0, b2: M-1, b3: M-2, b4: M-3, b5: M-4, b6 : M-5.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 38
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650 C, both partial and full oxidation of methane and also
second side reaction (R4, Table 3) were taking place. As from
700 C, methane partial oxidation reaction (R1) became
dominant with a realization ratio of 53.41%. At temperatures
750 C, 800 C and 850 C realization ratios of the reaction were
62.99%, 78.75% and 91.42%. Conversion of methane and
composition of reaction products significantly increased from
800 Cto850 C. While partial oxidation main reaction and full
oxidation reaction were in progress along with second side
reaction (R4) up to 800 C, R4 replaces the first side reaction
(R3) at 850 C. Also selectivities of CO and H2 remarkably
increased while CO2 level decreased at 850 C.
Methane (CH4) conversion, H2 selectivity and CO selec-
tivity are the most important parameters for determining
Fig. 3 e Inner channel elemental mapping of monolith support and monolithic catalysts: (a) M-0, (b) M-1, (c) M-2, (d) M -3, (e)
M-4, (f) M-5.
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the catalyst performance. Type of support, promoter mate-
rials, and textural properties of the catalyst, reaction tem-
perature and GSHV affect the performance parameters.
Values of CH4 conversion, H2 selectivity and CO selectivity
reported in this study compared favorably with other re-
ported results for CH4 partial oxidation with monolithic
catalysts (Table 4) [15,33,35e38]. It can be seen that oxides of
Co (M-1), CoeRu (M- 3), CoeNieRu (M- 4), and Ni (M-5)
monolithic catalyst demonstrated high catalytic activity for
CH4 partial oxidation.
The values of CH4 conversion in the present work obtained
with Co, Coe
Ni, Coe
Ru, Coe
Nie
Ru, and Ni, monolithic cata-lysts were higher than the results with a Ni monolith (81.18%)
[45], metallic Ni monolith-Ni/MgAl2O4 (85.30%) [46] and Ni-
aAl2O3 (80.20%) [15]. However these values were lower than
the results with a Rh based monolith (99.80%) [39]. When
compared with Ni monolith (88.71%) [45], metallic Ni mono-
lith-Ni/MgAl2O4 (91.50%) [46] and Rh based monolith (91.60%)
[33] H2 selectivity of the CoeRu (M-3) and CoeNieRu (M-4)
monolithic catalyst was better. On the other hand, Ce eZr/Ni
monolith (98.61%) [45] and Ni-aAl2O3 with Ce promoter
(98.10%) [15] catalysts have shown higher H2 selectivity when
compared with CoeNieRu monolithic catalyst. Table 4 pro-
vides a comparison of values of CH4 conversion, H2 and CO
selectivity obtained in this work with some other publishedvalues [15,39,44e47].
Catalytic partial oxidation of methane (CPOM) includes
several reaction equilibria and the resulting product compo-
sition is defined by the global thermodynamic equilibrium of
all the species involved [16].
Considering the results of a study on thermodynamic
equilibrium of CPOM at 1 bar pressure and a temperature
range of 700e1200 K, with a CH4 to O2 ratio of 2.0 to 1.0. using
HYSYS 3.2 [16]; methane conversion values obtained at 850 C
temperature and 0.36 s space time with the catalysts (M-1 to
M5) developed in the present study were comparable to
equilibrium conversions by 94.34, 93.40, 94.27, 95.98, and
94.28% for Reaction R1 (Table 5). T
a b l e 3 e
R e a l i z a t i o n r a t i o o f r e a c t i o n s ( R 1 , R 2 , R 3 , R 4 ) a t d i f f e r e n t t e m p e r a t u r e f o r C H 4
p a r t i a l o x i d a t i o n o v e r m o
n o l i t h i c c a t a l y s t s .
(
C )
M - 1
M - 2
M - 3
M - 4
M - 5
R 1
R 2
R 3
R 4
R 1
R 2
R 3
R 4
R 1
R 2
R 3
R 4
R 1
R 2
R 3
R 4
R 1
R
2
R 3
R 4
6
5 0
1 0 . 6 7
7 8 . 2
9
e
1 1 . 0 4
5 . 3
8
8 8 . 7 4
e
5 . 8
8
1 8 . 9 1
6 1 . 3
2
e
1 9 . 7 7
1 9 . 8 1
6 8 . 3
2
e
1 1 . 8 7
2 8 . 8
8
4 3 . 6
9
e
2 7 . 4 4
7
0 0
3 1 . 4 7
5 2 . 9
9
e
1 5 . 5 4
3 3 . 3
2
5 0 . 7
2
e
1 5 . 9 7
6 2 . 6
0
2 2 . 6
9
e
1 4 . 7 1
6 4 . 3
0
2 5 . 4
2
e
1 0 . 2
8
5 3 . 4 1
3 2 . 9 1
e
1 3 . 6
8
7
5 0
7 1 . 3 1
2 0 . 7 7
e
7 . 9
2
6 9 . 5
0
2 1 . 8 7
e
8 . 6
3
7 6 . 8 4
1 3 . 7
2
e
9 . 4 4
8 1 . 9
2
1 3 . 2 7
e
4 . 8 1
6 2 . 9
9
2 7 . 9
8
e
9 . 0
2
8
0 0
8 8 . 6
9
8 . 8
8
e
2 . 4
3
8 4 . 7
0
1 2 . 7
3
e
2 . 5
6
8 7 . 8
6
7 . 2
9
e
4 . 8 5
9 0 . 8 5
8 . 1
9
e
0 . 9
6
7 8 . 7 5
1 7 . 2
6
e
3 . 9
9
8
5 0
9 1 . 5 1
7 . 2 5
0 . 8 7
e
9 0 . 6
0
8 . 5
0
0 . 8
9
e
9 1 . 4 4
8 . 0
2
e
0 . 5 4
9 3 . 1
0
6 . 1
9
0 . 7 1
e
9 1 . 4
2
7 . 2 5
1 . 3
3
e
Fig. 4 e Study of carbon deposition on the catalysts in
terms of change in absorbance of CO2 with temperature by
TG/FTIR technique.
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Fig. 5 e Product stream composition rates of CH4 partial oxidation over monolithic catalysts: (a) CH4 conversion (%), (b) CO
selectivity (%), (c) H2 selectivity (%), (d) H2 /CO ratio.
Table 4 e CH4 conversion (%), H2 (%) and COselectivity (%) of CH4 partial oxidation over monolithic catalyst with oxides of Co(M-1), CoeNi (M-2), CoeRu (M-3), CoeNieRu (M-4), and Ni (M-5) compared with various monolith supported catalysts inliterature.
Catalyst CH4 conversion,% H2 selectivity,% CO selectivity,% Temperature, C GHSV, h1 Reference
Rh monolith 89.0 90.0 95.0 e e [35]
CeeZr/Ni monolith 91.88 98.61 95.84 800 1*105 [36]
Ni monolith 81.18 88.71 92.38 800 1*105 [36]
Metallic Ni monolith-Ni/MgAl2O4 85.3 91.5 93.0 800 1*105 [37]
Ni-aAl2O3 80.20 92.60 89.2 e 1*105 [15]
Ni-aAl2O3 with La promoter 93.90 100.0 93.70 e 1*105 [15]
Ni-aAl2O3 with Ce promoter 91.80 98.10 91.90 e 1*105 [15]
Pd based metal monolith 90.00 89.00 92.00 700 1*105 [38]
Rh based monolith (non-adiabatic) 99.0 91.6 90.5 ~ 800 1*104 [33]
Rh based monolith (adiabatic) 99.8 91.4 91.9 ~900 1*104 [33]
M-1 94.04 91.12 88.69 800 1*104 In this study
M-2 85.01 87.27 84.70 800 1*104 In this study
M-3 94.34 92.71 87.86 800 1*104 In this study
M-4 93.96 91.81 90.85 800 1*104 In this study
M-5 81.49 82.72 78.75 800 1*104 In this study
Table 5 e Comparison of catalyst performance experimentally obtained in the present study in terms of CH4 conversion byR1 (%) with thermodynamic equilibrium values calculated by HYSYS 3.2 simulation [16] for T: 650e850 C.
T (C) T(K) M-1 (Co) M-2 (CoeNi) M-3 (CoeRu) M-4 (CoeNieRu) M-5 (Ni) Equilibrium (HYSYS 3.2) [16]
650 10.67 5.38 18.91 19.81 28.88 75
923
700 31.47 33.32 62.60 64.30 53.41 84
973
750 71.31 69.50 76.84 81.92 62.99 90
1023
800 88.69 84.70 87.86 90.85 78.75 94
1073
850 91.51 90.60 91.44 93.10 91.42 97
1123
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Conclusion
A significant advantage of CPOM over steam reforming is that
the endothermic heat is generated internally (in situ) in the
monolith thereby minimizing the high endothermic heat that
must be added for steam reforming. This is especially true for
ceramic monoliths which have low heat transfer properties.In the present work, effect of addition of Ni and Ru oxides into
Co based catalysts, and its contribution to hydrogen produc-
tion efficiency have been studied. Performance tests of
monolithic catalysts Co, CoeNi, CoeRu, CoeNieRu, and Ni
oxide were carried out at 600, 650, 700, 750, 800 and 850 C in a
tubular reactor. Complete combustion reaction was dominant
within 600e650 C. After 700 C, partial oxidation reaction
became dominant. Since hydrogen production for CoeNieRu
oxide catalyst had increased by only 0.98% from 800 to 850 C,
the optimum reaction temperature was accepted as 800 C.
Efficiency values of hydrogen production by partial oxidation
of methane at 800 C using Co, CoeNi, CoeRu, CoeNieRu, and
Ni as oxide catalysts have been calculated as 93.73%, 88.72%,94.40%, 93.78%, and 85.30%, which were higher than those
reported in the literature. At 850 C, these values increased to
94.90%, 93.74%, 95.47%, 95.89%, and 94.83%, respectively.
Further tests are planned for the following phase of the study,
to explore the long term stability of the catalysts developed in
the present study.
Acknowledgment
Financial support of the Yildiz Technical University (YTU-
BAPK: 27-07-01-07) and the technical support extended by theEnergy Institute of Marmara Research Center (MRC) of the
Scientific and Technological Research Council of Turkey
(TUBITAK) during performance testing of the catalysts are
gratefully acknowledged.
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