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 3 6 ( 2 0 1 1 ) 2 9 6 9 e2 9 7 8

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Combined dry reforming and partial oxidation of methane

to synthesis gas on noble metal catalysts

Behzad Nematollahi a, Mehran Rezaei a,b,*, Majid Khajenoori a

a
Catalyst and Advanced Materials Research Laboratory, Chemical Engineering Department, Faculty of Engineering,
University of Kashan, Kashan, Iran
b
Institute of Nanoscience and Nanotechnology, University of Kashan, Kashan, Iran

article info abstract

Article history: In this paper CO2 reforming of methane combined with partial oxidation of methane to
Received 20 October 2010 syngas over noble metal catalysts (Rh, Ru, Pt, Pd, Ir) supported on alumina-stabilized
Received in revised form magnesia has been studied. The catalysts were characterized by using BET, XRD, SEM, TEM,
28 November 2010 TPR, TPH and H2S chemisorption techniques. The H2S chemisorption analysis showed an
Accepted 2 December 2010 active metal crystallite size in the range of 1.8e4.24 nm for the prepared catalysts. The
Available online 5 January 2011 obtained results revealed that the Rh and Ru catalysts showed the highest activity in
combined reforming and both the dry reforming and partial oxidation of methane. The
Keywords: obtained results also showed a high catalytic stability without any decrease in methane
Combined reforming conversion up to 50 h of reaction. In addition, the H2/CO ratio was around 2 and 0.7 over
Dry reforming different catalysts for catalytic partial oxidation and dry reforming, respectively.
Partial oxidation ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Noble metal catalysts
Syngas

1. Introduction One of the other methods to produce syngas is partial

oxidation of methane. The H2/CO ratio obtained via this
Synthesis gas, a mixture of H2 and CO, is an important reaction is suitable for methanol and alkane synthesis by the
feedstock for several chemical processes operated in the FischereTropsch process and with supported group VIII noble
production of methanol and synthetic fuels through a metal catalysts, optimal performance is achieved without
FischereTropsch synthesis [1]. Reforming of methane with carbon formation [10]. For exothermic processes heat removal
carbon dioxide to produce syngas is one of the methods to from the catalyst may prove challenging, and the formation of
utilize the two major greenhouse contributors [2e5]. This hot spots complicates the evaluation of experimental data and
reaction can produce a suitable H2/CO ratio for use in the elucidation of reaction mechanisms [11e14]. A way to
FischereTropsch synthesis, however, this process has not overcome this problem and minimizing temperature gradi-
reached commercialization level due to the following limita- ents is by coupling partial oxidation with an endothermic
tions: (i) it is an intensively endothermic reaction that reaction, typically CO2 reforming [15]. Accordingly, the com-
consumes much energy [6]; (ii) the used catalysts are inclined bination of CO2 reforming and partial oxidation of methane
to deactivate due to coke deposition on the catalysts surface has been successfully conducted in the past few years and
[7e9]. appears to be a promising process in synthesis gas production

* Corresponding author. Catalyst and Advanced Materials Research Laboratory, Chemical Engineering Department, Faculty of
Engineering, University of Kashan, Kashan, Iran. Tel.: þ98 361 5912469; fax: þ98 361 5559930.
E-mail address: rezaei@kashanu.ac.ir (M. Rezaei).
0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.ijhydene.2010.12.007
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dry reforming process is able to reduce the carbon deposition

on the catalytic surface and to increase the methane conver-
sion, although this can also cause the reduction in the process
selectivity. In this paper noble metal catalysts (Ru, Rh, Ir, Pt,
and Pd) supported on alumina-stabilized magnesia were
employed in combined dry reforming and partial oxidation of
methane for production of synthesis gas.

2. Experimental

2.1. Materials

The starting materials were Pd(NH3)4(NO3)2, Pt(NH3)4(NO3)2, Rh

(NH3)6(NO3)3, IrCl3, and Ru(NO)(NO3)3 as precursors of Pd, Pt,
Rh, Ir, and Ru, respectively. The supported noble metal cata-
lysts were prepared by the impregnating of pellets
(D ¼ 2.56 mm, L ¼ 2.01 mm) of alumina-stabilized magnesia
(the molar ratio of Mg/Al ¼ 7/1) with solutions of metal
precursors to obtain 1wt.% of metals. Before impregnation,
the alumina-stabilized magnesia was calcined at 975  C for
4 h. After impregnation, the pellets were dried at 80  C and
calcined at 450  C for 1 h.

2.2. Characterization

The surface areas (BET) were determined by nitrogen adsorp-

tion at 196  C using an automated gas adsorption analyzer
(The Tristar 3000, Micromeritics). The pore size distribution
was calculated from the desorption branch of the isotherm
using the Barrett, Joyner, and Halenda (BJH) method. Powder X-
ray diffraction (XRD) patterns were recorded using a CueKa
monochromatized radiation source and a Ni filter in the range
2q ¼ 5 e70 . The surface area of the metals was determined by
chemisorption of hydrogen sulfide, as described elsewhere [20]
Fig. 1 e Pore size distribution of the reduced catalysts. (conditions: the volume ratio of H2S/H2 ¼ 15  106, 550  C,
100 h). The surface area of the metal was calculated by
assuming a sulfur monolayer of 44.5  109 g S cm2 for noble
metals, which corresponds to 0.5 sulfur atom per noble metal
for several reasons [15e19]. Firstly, the coupling of exothermic atom (S/noble metal ¼ 0.5) [20]. This was assumed to be close to
with endothermic reactions can facilitate heat transfer the composition of the monolayers on the noble metals. In
through the catalyst bed. Thus, reaction can be operated in other words, the surface area of the metal was calculated using
a safe manner and more efficient in terms of energy. Secondly, S0 ¼ 440 wt. ppm, equivalent to 1 m2 g1, S0 being the sulfur
by changing the feed composition, the product ratio of H2/CO capacity of the catalyst (mgS/gmetal). The mean diameter of
and the selectivity for various FischereTropsch synthesis a metal particle can thus be estimated from follow equation:
products can be tailored to the customer’s needs. Thirdly, the dmetal¼(2640  Xmetal)/(S0  rmetal), where dmetal is given in nm
raw material of this process is readily and easily available and Xmetal is the weight percent of metal in the reduced state.
from those abundant natural gas reserves, which contain Temperature-programmed reduction (TPR) was carried out
substantial amounts of CO2. Finally, addition of oxygen to the using an automatic apparatus (ChemBET-3000 TPR/TPD,

Table 1 e Structural properties of the reduced catalysts.

Catalyst BET surface area Pore volume Pore diameter Sulfur capacity Metal area Metal crystallite
(m2 g1) (cm3 g1) (nm) (wt%) (m2/g) Size(nm)

Pt 86.56 0.238 10.66 295 0.67 4.24

Pd 179.10 0.222 4.96 450 1.02 1.53
Rh 159.00 0.276 6.95 1650 3.75 1.28
Ru 120.84 0.253 7.53 700 1.59 3.96
Ir 95.33 0.262 8.95 625 1.40 1.86
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catalyst bed for monitoring the temperature. The reactor was

charged with 150 mg of the prepared catalyst. Prior to reaction,
the different catalysts were reduced with the flow of pure H2
gas (GHSV ¼ 16000 L/kgcat h) at a heating rate of 10  C/min
from room temperature to 600  C and then maintained at
600  C for 2 h. The reactant gas stream consisted of CH4, CO2
and O2 with various molar ratios, controlled by mass flow
controller and the activity tests were carried out at different
temperatures ranging from 500 to 700  C in steps of 50  C. The
reaction effluents were analyzed using a gas chromatograph
(Varian 3400) equipped with a TCD and a carboxen 1000
column.

3. Results and discussion

Fig. 1 shows the pore size distributions of the noble metal

catalysts supported on alumina-stabilized magnesia. As can
be seen, Rh and Pd catalysts showed a narrow pore size
distribution, whereas Ru, Pt and Ir catalysts showed a broad
pore size distribution. Table 1 shows that the Ru, Pt and Ir
catalysts posses a bigger pore size than the Pd and Rh cata-
lysts. The BET measurements of the catalysts showed a higher
specific surface area for the Ru, Rh and Pd catalysts compared
with the Ir and Pt catalysts (Table 1). The obtained results
showed that the pore volume for all the catalysts was higher
than 200 ml/kg. It is noteworthy the specific surface area of the
catalysts after reduction was much higher than that of the
support. The specific surface area of the support was about
36.6 m2 g1. This could be attributed to the transformation of
the pore size distribution after the impregnation of the
support with metal salts and calcination and reduction of
the impregnated catalysts. During the calcination process, the
decomposition of metal salts led to a change in the pore
Fig. 2 e (a) Pore size distribution and (b) XRD pattern of the
structure and also in the pore distribution of the support.
support.
Fig. 2a shows the pore size distribution of the fresh support
before impregnation, which is completely different from the
pore size distribution of the catalysts (Fig. 1a and b). As can be
Quantachrome) equipped with thermal conductivity detector. seen, for the catalysts, the pore size shifted to smaller values
The fresh catalyst (200 mg) was subjected to a heat treatment and the pore volume to higher values, especially for Rh, Ru,
(10  C/min) in a gas flow (30 ml/min) containing a mixture of
H2:Ar (10:90). Before the TPR experiment, the samples were
heat treated under an inert atmosphere at 350  C for 3 h.
Temperature-programmed hydrogenation (TPH) of the spent
catalysts was carried out using the apparatus that was used for
TPR analysis. The spent catalyst (25 mg) was subjected to a heat
treatment (10  C/min up to 1000  C) in a gas flow (80 ml/min)
containing a mixture of H2:Ar (10:90). Before the TPH
experiment, the samples were heat treated under an inert
atmosphere at 300  C for 1 h. Surface morphology of samples
was investigated using scanning and transmission electron
microscopes (SEM, Vega@Tescan and TEM, JEM-2100UHR
instrument).

2.3. Catalytic reaction

The catalytic reaction was carried out in a tubular fixed bed

flow reactor made of quartz (i.d. ¼ 7 mm) under atmospheric
pressure. The thermocouple was inserted in bottom of the Fig. 3 e SEM analysis of the catalyst support.
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Fig. 4 e TEM analysis of reduced catalysts, (a) Rh, (b) Ru, (c) Ir, (d) Pt and (e) Pd.

and Pd catalysts (Table 1); therefore, these catalysts showed

a higher specific surface area compared with the Ir, Pt, and the
fresh support. The metal crystallite sizes, were determined
from hydrogen sulfide chemisorption analysis, showed
a smaller size than the pore size of the catalyst support, Table
1. This indicates that active metals could be introduced into
the pores of the support, which would change the pore
structure and also the pore size distribution of the catalyst
support. This led to a reduction in the values of the pore size
and an increase in the specific surface area of the catalysts,
Table 1. As can be seen in Table 1, the Rh catalyst has the
highest sulfur capacity and consequently the highest active
metal area and the lowest metal crystallite size. The lowest
sulfur capacity was observed for the Pt catalyst and this
catalyst showed the largest metal crystallite size. Fig. 2b
shows the XRD pattern of the support. MgO, Al2O3 and
MgAl2O4 were observed as the major crystallite phases.
Fig. 3 shows the SEM analysis of the fresh support before
impregnation. As can be seen the particles are sintered Fig. 5 e TPR profiles of the noble metal catalysts, Rh(1), Ru
together. In addition the particles are in nanometer scale. (2), Ir(3), Pt(4), and Pd (5).
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Fig. 6 e CH4 conversion and CO2 conversion, (a,b) dry reforming (CH4/CO2 [ 1), (c) partial oxidation (CH4/O2 [ 2), (d,e)
combined reforming, (CH4/CO2/O2 [ 1/1/0.5) on the different noble metal catalysts, GHSV [ 16000 ml/(h gcat).

Fig. 4 shows the TEM analysis of fresh catalysts. As can be reduction peak shifted to higher temperatures. For the Ru and
seen, metal crystals are well dispersed on catalyst support. Ir catalysts, one major peak was observed at about 160 and
The noble metal crystallite sizes in all catalysts are less than 275  C, respectively, which indicates that the major fractions of
5 nm which is in agreement with H2S chemisorption results. Ru and Ir have a low reduction temperature, whereas the other
Fig. 5 shows the temperature-programmed reduction (TPR) catalysts showed several peaks in their TPR profiles. This
profiles of the noble metal catalysts. It was observed that Rh, Ir, showed that the active metal in these catalysts has to be
and Ru catalysts showed the lowest reduction temperature, present in several species with different types of interaction
whereas Pd and Pt catalysts showed the highest interaction with the support and therefore brings about different
with the support, as the maximum temperature of the reducibility.
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Fig. 7 e H2 selectivity and CO selectivity, (a,b) dry reforming (CH4/CO2 [ 1), (c,d) partial oxidation (CH4/O2 [ 2), (e,f) combined
reforming (CH4/CO2/O2 [ 1/1/0.5) on the different noble metal catalysts, GHSV [ 16000 ml/(h gcat).

Fig. 6a,b and d show the CH4 conversions over the noble that the methane conversion on Ru and Rh catalysts
metal catalysts at different reaction temperatures in dry, reached equilibrium in dry and combined reforming at the
partial oxidation and combined reforming, respectively. investigated range of temperatures and for partial oxidation
The results obtained showed an increase in CH4 conversion the equilibrium was reached at 600  C. The lowest CH4
with increasing reaction temperature. It was observed that conversions in all three processes were observed for the Pt
Ru and Rh catalysts showed the highest activity for and Pd catalysts, indicating the lower activity of these
methane reforming with carbon dioxide, combined catalysts in these processes. In addition, it was observed
reforming and partial oxidation. Under these reaction that the methane conversion in combined reforming was
conditions, the following order of activity was observed for higher than those observed in dry reforming and partial
different catalysts: Rh w Ru > Ir > Pt > Pd. It was observed oxidation.
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At the lower temperatures the endothermic reaction of dry so in high temperatures dominated endothermic CO2
reforming is not progressing well and CH4 conversion is low, reforming reaction and CH4 conversion remained high. Fig. 6b
but by increasing the temperature, this reaction is intensity and e shows the influence of temperature on CO2 conversion
and CH4 conversion is increasing. In partial oxidation of in dry and combined reforming processes. Increasing in
methane, combustion of methane is predominant reaction reaction temperature increased the CO2 conversion, indi-
at lower temperatures and conversion is high. However cating an increasing participation of the endothermic dry
this reaction is rather exothermic and not favored to do at reforming reaction. For all the catalysts in dry reforming, the
high temperatures. In combined reforming at low and CO2 conversion was higher than the CH4 conversion due to the
medium temperatures, dominated exothermic reactions are reverse water gas shift reaction. However, the CO2 conversion
combustion and partial oxidation of methane respectively and in combined reforming is lower than the CH4 conversion and it
is much lower than the CO2 conversion in dry reforming.
About their reactions, it was observed that Rh and Ru catalysts
have the highest activities, while Pd and Pt catalysts have the
lowest activities for CO2 conversion. The order of activity for
CO2 reforming observed for different catalysts is similar to
CH4 conversion. It was observed that the CO2 conversion in
combined reforming process is negative at temperatures
below than 550  C, because of the occurrence of methane
combustion. Fig. 7(a,b), (c,d) and (e,f) shows the H2 selectivity

Fig. 8 e H2/CO molar ratio, (a) dry reforming (CH4/CO2 [ 1),

(b) partial oxidation (CH4/O2 [ 2), (c) combined reforming,
(CH4/CO2/O2 [ 1/1/0.5) on different noble metal catalysts, Fig. 9 e (a) Stability of CH4 conversion and (b) H2/CO molar
GHSV [ 16000 ml/(h gcat). ratio at 700  C, GHSV [ 16000 ml/(h gcat)
2976 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 3 6 ( 2 0 1 1 ) 2 9 6 9 e2 9 7 8

Table 2 e Effect of feed ratio on CH4 conversion and H2/CO molar ratio for DR, POM and CR reactions over different noble
metal catalysts at T [ 700  C, GHSV [ 16000 ml/(h gcat)
Catalyst DR CR POM

CH4:CO2:O2 1:1:0 1:1:0.3 1:1:0.5 1:1:0.8 1:1:1 1:0:0.5

Rh CH4 conversion (%) 74.48 84.05 91.21 98.31 99.48 74.08

CO2 conversion (%) 76.10 45.99 26.75 e e e
H2/CO (molar ratio) 0.779 0.91 1.09 1.21 1.30 1.89
Ru CH4 conversion (%) 70.48 84.61 91.92 97.58 99.16 73.03
CO2 conversion (%) 72.67 48.40 28.33 0.99 e e
H2/CO (molar ratio) 0.671 0.91 1.02 1.12 1.13 1.91
Ir CH4 conversion (%) 64.06 79.63 87.58 95.59 98.36 72.11
CO2 conversion (%) 66.40 44.97 29.20 3.92 e e
H2/CO (molar ratio) 0.768 0.91 0.98 1.11 1.17 1.86
Pt CH4 conversion (%) 49.91 62.28 73.18 85.23 91.45 68.37
CO2 conversion (%) 55.33 36.21 18.97 e e e
H2/CO (molar ratio) 0.649 0.80 0.87 1.03 1.16 1.93
Pd CH4 conversion (%) 34.71 48.86 56.34 68.08 75.80 59.02
CO2 conversion (%) 39.62 10.61 e e e e
H2/CO (molar ratio) 0.695 0.72 0.81 0.84 0.85 1.94

and CO selectivity over the noble metal catalysts at different the catalysts. These peaks could be related to whisker-like
reaction temperatures in dry reforming, partial oxidation and filamentous carbon with different reactivity towards hydro-
combined reforming, respectively. The obtained results genation. According to previously published reports [22], these
showed an increase in H2 and CO selectivity with increasing species are produced by the adsorbed carbon atoms derived
reaction temperature. from methane decomposition and CO dissociation. Fig. 11b
Fig. 8aec shows the H2/CO molar ratios over the noble shows TPH profiles of the spent catalysts in combined
metal catalysts at different reaction temperatures in dry reforming. The results obtained clearly showed the existence
reforming, partial oxidation and combined reforming, res- of two types of carbonaceous species on the catalysts with
pectively. The results obtained showed the H2/CO ratios different reactivities towards hydrogenation. A first peak is
were around 0.7, 2 and 1 over different catalysts for dry observed at different temperatures between 380  C and 450  C
reforming, partial oxidation and combined reforming, in different catalysts. This peak is related to amorphous
respectively. The reason that H2/CO ratio in combined carbon located at the interior of the active metal particles [21].
reforming achieved unity at temperatures around 700  C is A second peak is observed at temperatures above 800  C,
due to the influence of the dry reforming and the reverse identified as whisker-like filamentous carbon. In addition, the
water gas shift reactions. results clearly showed that the addition of O2 decreased the
Fig. 9 shows the stability of methane conversion value and degree of carbon deposition, Fig. 11b.
H2/CO molar ratio up to 50 h of combined reforming at 700  C.
It is seen that all the catalysts showed a high stability during
the both reactions without any decrease in methane conver-
sion. In addition, the H2/CO ratio was between 0.9 and 1.1 over
different catalysts.
Table 2 presents the effect of feed ratio on CH4 conversion
and H2/CO molar ratio for dry reforming, partial oxidation and
combined reforming reactions over different noble metal
catalysts at 700  C. The results showed that by addition of
oxygen in dry reforming, conversion of CH4 is increasing and
in all of ratios, the Rh and Ru showed the highest activities. In
addition, it was observed that in combined reforming because
of using two strong oxidants, CO2 and O2, the CH4 conversions
are higher than those obtained in both dry reforming and
partial oxidation of methane.
Fig. 10 shows the effect of gas hour space velocity (GHSV)
on the catalytic performance of combined reforming over
different noble metal catalysts. The obtained results showed
that for all catalysts increasing the GHSV leads to decrease in
CH4 conversions. The temperature-programmed hydrogena-
tion (TPH) profiles of the spent catalysts in dry reforming Fig. 10 e Effect of GHSV on CH4 conversions in combined
reaction are shown in Fig. 11a. The results obtained clearly reforming reaction, Reaction conditions: T [ 700  C, CH4/
showed the existence of two types of carbonaceous species on CO2/O2 [ 1/1/0.5.
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Fig. 11 e TPH profiles of the spent catalysts in (a) dry reforming and (b) combined reforming.

4. Conclusion Acknowledgement

Combined dry reforming and partial oxidation of methane to The authors gratefully acknowledge the financial support of
synthesis gas over noble metal catalysts, supported on this work by the National Iranian Oil Refining and Distribution
alumina-stabilized magnesia, has been studied. The BET Company (NIORDC).
measurements of the catalysts showed a higher specific
surface area for the Ru, Rh, and Pd catalysts compared with
the Ir and Pt catalysts. It was observed that the methane references
conversion in combined reforming was higher than of those
observed in dry reforming and partial oxidation. It is seen that
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