CeO2−Al2O3 Catalyst for Dry

Feb 21, 2007 - Deactivation Studies over Ni−K/CeO2−Al2O3 Catalyst for Dry .... Seema Ghodke , Richa Bobade , Hanmant Gurav , Satyanarayana Chiluku...
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Ind. Eng. Chem. Res. 2007, 46, 1731-1736

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Deactivation Studies over Ni-K/CeO2-Al2O3 Catalyst for Dry Reforming of Methane Nandini A. Pechimuthu, Kamal K. Pant,* and Subhash C. Dhingra Department of Chemical Engineering Indian Institute of Technology - Delhi Hauz Khas, New Delhi -110016, India

The activity of 13.5Ni-2K/10CeO2-Al2O3 catalyst was tested for 60 h time on stream (TOS) for the carbon dioxide reforming of methane at three different temperatures (650, 700, and 750 °C). The amount of coke deposited on the catalyst at different time intervals was estimated by thermogravimetric analysis (TGA). Results suggested that both CH4 cracking and CO disproportionation contribute to coke deposition. No appreciable deactivation was observed for the catalysts at all three temperatures for the 60-h run. The used catalysts were characterized by Brunauer-Emmett-Teller (BET) surface area, X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) analysis to understand the morphology of the coke deposited on the catalyst. XRD patterns showed that carbon formed on 13.5Ni2K/10CeO2-Al2O3 after 60 h TOS at 700 and 750 °C dispersed well and could not be observed, while after 60 h TOS at 650 °C, mainly graphitic carbon (peak at 2θ ) 26.3°) formed. XPS characterization demonstrated the existence of mainly two kinds of carbon species, graphitic (-C-C-) and oxidized carbons (-C-CO-, CO3-). TGA, XRD, and XPS studies revealed that significant amount of coke was deposited on 13.5Ni2K/10CeO2-Al2O3 catalyst at 650 °C. However, the amount of accumulated coke with TOS did not affect the high activity of the catalyst up to 60 h. This suggested that some carbon species formed on the surface of the catalyst must be involved in the reaction to produce CO. TEM results indicated that a large part of the graphitic carbon deposited on the catalyst surface was of the filamentous form with nickel on top of these carbon filaments. This form of graphitic carbon is more active in the reforming reaction of methane, probably because of its close interaction with nickel particles. Therefore, the catalyst had high stability at 650 °C despite the coke deposited. The value of the activation energy for coke oxidation for the 13.5Ni-2K/10CeO2Al2O3 catalyst was estimated to be in the range of 110-160 kJ/mol. 1. Introduction The catalytic process of carbon dioxide reforming of methane into synthesis gas (CH4 + CO2 T 2H2 + 2CO) has attracted many researchers for the chemical utilization of the undesirable greenhouse gases: natural gas and carbon dioxide. This process produces synthesis gas with a H2/CO ratio of about 1, which can be preferentially used for production of Fischer-Tropsch liquid hydrocarbons and oxygenates, and can also be considered for chemical energy transmission systems. Supported metals of groups 8-10 are good catalysts for this reaction. However, the major drawback of this reaction is the problem of serious coking on the catalysts. The deposits deactivate the catalyst either by suppressing active sites or choking the pores or both. The origin of inactive carbon during dry reforming may occur via either CH4 decomposition (CH4 T C + 2H2) or CO disproportionation (2CO T C + CO2). Thermodynamically, CO disproportionation is exothermic; thus, the equilibrium constant decreases with increasing temperature. Conversely, CH4 decomposition is endothermic; thus, the equilibrium constant increases with increasing temperature. Thus, with the aim of improving the coke resistance of Ni-based catalysts, several approaches have been considered such as sulfur passivation of the catalyst; selection of an appropriate support; addition of promoter, metal oxide/metal additives with strong Lewis basicity which increase CO2 adsorption or which preferentially eliminate the large ensembles of metal atoms necessary for carbon deposition; change of reaction conditions and catalyst preparation method; * To whom correspondence should be addressed. Tel.: +91 11 26596172. Fax: +91 11 26581120. E-mail: [email protected].

and addition of steam and use of a catalyst whose structure affects carbon deposition.1-3 Fundamental knowledge concerning the coking process is required to improve the resistance to coking of a nickel-based catalyst for the reforming of CH4 with CO2 to a degree acceptable for industrial application. In particular, the basic studies related to this aspect need to be done. These include studies on carbon deposition and its influence on the stability of the catalyst, effect of metal-support interactions on the kind of deposited carbon and its reactivity, individual role of CH4 and CO2 reaction pathways in the accumulation of adsorbed carbon under reforming reaction conditions, and chemical and morphological properties of the carbon species formed.4 Results of our previous investigation (Nandini et al.5) suggested that stable Ni/Al2O3 catalysts for the carbon dioxide reforming of methane can be prepared by the addition of both potassium and CeO2 as promoters. In the present study, the coke deposition characteristics in the carbon dioxide reforming of methane over 13.5Ni-2K/10CeO2-Al2O3 catalyst were tested at three different temperatures (650, 700, and 750 °C). BrunauerEmmett-Teller (BET) surface area measurements, X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) analysis were used to understand the morphology of the coke deposited on the catalyst. 2. Experimental Section 2.1. Catalyst Preparation. For the 13.5Ni-2K/10CeO2Al2O3 catalyst, a support with CeO2 content of 10 wt % on γ-Al2O3 was prepared by excess solution impregnation using a pelletized γ-Al2O3 support (IPCL, India, average diameter 1 (

10.1021/ie061389n CCC: $37.00 © 2007 American Chemical Society Published on Web 02/21/2007

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Figure 2. Coke deposition profiles at different temperatures for 13.5Ni2K/10CeO2-Al2O3 catalyst. Figure 1. CH4 and CO2 conversion with time over 13.5Ni-2K/10CeO2Al2O3 catalyst (CH4:CO2:N2 ) 1:1:1; W/FCH4,0 ) 2.6 kgcat‚h/kgmethane).

0.2 mm) and Ce(NO3)2·6H2O (CDH, India). The excess water from the slurry was removed in a rotary vacuum evaporator at 80 °C. The residue was then dried at 110 °C for 24 h in an oven and was subsequently calcined at 550 °C in air for 3 h for complete decomposition of the nitrate. Ni and K were then coimpregnated on this support using nickel nitrate salt (MERCK, Germany) and KNO3 (CDH, India) followed by drying and calcination at 550 °C in air for 3 h. 2.2. Catalyst Characterization. The BET surface area of the catalyst was determined by nitrogen adsorption at -196 °C using a surface area analyzer (Micromeritics, ASAP 2010). XRD powder diffraction measurements were performed on an X’Pert PRO PW 3040/60 (PANalytical) powder diffractometer with Cu KR radiation (1.5418 Å) at 40 kV and 30 mA. The nickel particle size was determined using Ni(111) reflectance and the Scherrer equation. TEM investigations were performed on a PHILIPS CM-12 microscope. The samples were prepared as substrate ethanol slurry and were transferred to a carbon-coated copper grid for examination. X-ray photoelectron spectroscopy (XPS) measurements were performed in an ultrahigh vacuum chamber (PHI 1257) with a base pressure of 4 × 10-10 Torr. 2.3. Catalytic Stability Tests. The catalytic carbon dioxide reforming of methane was carried out under atmospheric pressure in a flow system using a vertical quartz tube fixed bed reactor. The details of the experimental setup and experimental procedure are given elsewhere.5,6 The amount of coke deposited on the catalysts during an experimental run was determined by oxidation in air conducted in a thermogravimetric analyzer (TGA) (Seiko TG/DTA 32 SSC 5100). A 10-15 mg sample was loaded in a Pt pan, was heated from room temperature to 110 °C under the flow of air (3 L/h), was kept at that temperature for 0.5 h, and then was heated to 800 °C at the rate of 10 °C/ min. 3. Results and Discussion 3.1. Stability Tests and Coke Content. The activity of 13.5Ni-2K/10CeO2-Al2O3 catalyst for the reforming reaction was tested for 60 h time on stream (TOS) at three different temperatures (650, 700, and 750 °C) using CH4:CO2:N2 ) 1:1:1 and W/FCH4,0 ) 2.6 kgcat‚h/kgmethane. The CH4 and CO2 conversion with time is shown in Figure 1. The corresponding equilibrium CH4 conversion values at 650, 700, and 750 °C

are 66%, 80%, and 89% respectively. The amount of coke deposited on the catalyst at different time intervals was estimated by TGA analysis, and the result is given in Figure 2. Under the range of reaction conditions investigated (T ) 650-750 °C, CH4/CO2 ) 1.0 mole ratio), carbon deposition is predicted by thermodynamics.7 No appreciable deactivation was observed for the catalysts at all three temperatures for the 60-h run. The adopted operating conditions of high space time imply a close approach to equilibrium condition, which might disguise the occurrence of catalyst deactivation if the catalyst was tested for a very short time span. However, in the present study, the catalyst was tested for 60 h TOS. Moreover, high space time corresponding to high catalyst loading which gives high methane conversion for the reforming reaction would also catalyze the deactivation mechanisms (carbon formation, catalyst sintering) to a larger extent. Hence, in the present study, any masking of the occurrence of catalyst deactivation because of the high space time used is expected to be negligible. At temperature 650 °C, the amount of deposited coke increased almost linearly with TOS. The rate of coke formation increased gradually with TOS at 650 °C, while for reaction at 700 and 750 °C, the rate of coke formation first increased from 0 to 30 h TOS and then decreased up to 45 h TOS, beyond which the rate is reversed corresponding to disappearance of coke previously deposited. The decrease in rate may be due to the reverse Boudouard and methane decomposition reaction.8 The decrease in the rate of coke deposited with TOS beyond 30 h at 700 and 750 °C can also be explained considering that the coke being accumulated produces the blockage of the most active sites for coke formation, hindering methane cracking.3 The amount of coke deposited on the catalyst varied with temperature in the following order: 650 °C > 750 °C > 700 °C. Thus, the minimum amount of coke was deposited at 700 °C. As per the equilibrium calculations performed by Gadalla and Bower,7 coking was expected to reduce with increasing temperature from 650 to 750 °C. As per thermodynamic calculations, methane cracking starts at 550 °C and Boudouard reaction becomes negligible at T > 700 °C. If methane decomposition was the main route for carbon deposition, coke deposited should vary with temperature as 750 °C > 700 °C > 650 °C. The reverse would occur if CO disproportionation was the main route. The existence of coke above 700 °C and the large difference in the amount of coke at 650 °C and 700 °C suggest that both CH4 cracking and CO disproportionation contribute to coke deposition. The extent of contribution of methane decomposition and CO disproportionation need to be

Ind. Eng. Chem. Res., Vol. 46, No. 6, 2007 1733 Table 1. Characteristics of Fresh and Used 13.5Ni-2K/ 10CeO2-Al2O3 Catalysts (Run Time: 60 h) reaction temperature °C/ catalyst

surface area, SBET (m2/g)

pore volume, V (cm3/g)

pore diameter, Dp (A0)

DNia (nm)

coke on catalyst (wt %)

fresh catalyst 650 700 750

127.2 69.0 76.7 69.4

0.289 0.190 0.213 0.210

93.0 112.4 113.8 123.4

16b 15 15 16

13.1 trace 2.7

a

Determined by XRD. b Refers to reduced catalyst.

determined by isotopic TPO experiments.4,9 There is substantial evidence in the literature to suggest that both CH4 decomposition and CO disproportionation can contribute to the formation of inactive carbon deposits during CO2 reforming of CH4, with the relative contribution of each depending on the reaction conditions.4,8-10 There was a decrease in the coke content beyond 45 h TOS at 700 and 750 °C, which could be attributed to increased reactivity of carbonaceous species with TOS. Goula et al.4 observed that by increasing the time of reaction from 15 min to 2 h the reactivity of carbonaceous species formed increased (i.e., CO2 peak in TPO shifted to a lower temperature) over Ni/CaO/Al2O3 catalyst. The disappearance of coke could be attributed to increased rate of coke removal by gasifying agents (H2, CO2, and H2O) during the course of reaction.11 The oxidant water can be generated by dehydroxylation of Al2O3 or by the reverse water-gas shift reaction under the reaction condition.12 Goula et al.4 in a study on Ni/CaO-Al2O3 catalyst found the amount of carbon formed after 15 min of reaction to be smaller than that corresponding to 5 min of reaction, indicating that the rates of carbon formation and removal depend on TOS and catalyst surface composition. In the present study, runs with increased TOS need to be conducted to verify this decrease in the coke content beyond 45 h TOS. 3.2. Characterization of Used Catalyst. Carbon deposition is a major problem in methane reforming with carbon dioxide. Carbon deposition can cause catalyst deactivation and reactor plugging. Understanding of the chemical and morphological properties of carbon deposits helps us to show how they influence catalytic activity and stability of the catalysts. 3.2.1. BET Surface Area. The characteristics of 13.5Ni2K/10CeO2-Al2O3 catalyst before and after reaction for 60 h TOS at different temperatures are given in Table 1. It is seen that the surface area and pore volume decreased upon reaction, whereas the average pore size showed an increasing trend. Carbon deposits would prevent N2 adsorption on the catalyst surface. The results indicate that the deposited carbon species gradually blocked the catalyst pores. The used catalysts showed a 40-45% reduction in surface area, which can be attributed to coke deposition and sintering, but since XRD showed no increase in Ni particle size after reaction (Table 1), the major contribution may be from coke deposition. In spite of the decrease in surface area of the catalyst, no decrease in its activity was observed. This could be because the overall active metallic surface may have remained unchanged during the reaction and some carbon species formed on the surface of the catalyst must be acting as active sites in the reaction. 3.2.2. X-ray Diffraction Patterns (XRD). XRD patterns of used 13.5Ni-2K/10CeO2-Al2O3 catalyst after reaction are presented in Figure 3. XRD patterns showed that carbon formed on 13.5Ni-2K/10CeO2-Al2O3 after 60 h TOS at 700 and 750 °C dispersed well and could not be observed (no peaks corresponding to carbon were observed) probably because of a very low amount of residual carbon over the catalyst. After 60

Figure 3. XRD patterns of used 13.5Ni-2K/10CeO2-Al2O3 catalysts: (a) T ) 650 °C, 60 h TOS; (b) T ) 700 °C, 45 h TOS; (c) T ) 700 °C, 60 h TOS; (d) T ) 750 °C, 45 h TOS; (e) T ) 750 °C, 60 h TOS.

h TOS at 650 °C, mainly graphitic carbon (peak at 2θ ) 26.3°) formed. Comparison of the XRD patterns of reduced (not shown here) and used 13.5Ni-2K/10CeO2-Al2O3 catalyst showed no significant change in the intensity of the Ni peaks at 2θ ) 44.5, 51.8, and 76.4 after reaction. Thus, the particle size of metallic nickel calculated from XRD did not change after the reaction (Table 1). This indicates that there is no sintering of nickel particles during the reaction. 3.2.3. Transmission Electron Microscopy (TEM). TEM was used to characterize the surface morphology of reacted catalysts. TEM micrographs (Figure 4) showed that the carbon on 13.5Ni-2K/10CeO2-Al2O3 after reaction for 60 h TOS at 650 °C is filamentous carbon with nickel crystallite at the tip of the carbon filament with its size roughly equal to the diameter of the filament (20-30 nm), whereas no forms of carbon were visible after a run of 60 h at 700 °C. Hence, it can be concluded that filamentous carbon having a graphitic structure dominated the carbon species at 650 °C. 3.2.4. X-ray Photoelectron Spectroscopy (XPS). C 1s spectra over 13.5Ni-2K/10CeO2-Al2O3 reacted at 650 °C and 700 °C for 60 h TOS are shown in Figure 5. There are three peaks deconvoluted from the C 1s spectra. On the basis of the reference values,13 the spectra showed peaks corresponding to graphitic carbon (-C-C-, binding energy (BE) ) 284.8 eV) and oxidized carbon species (-C-CO-, BE ) 287 eV and CO3-, BE ) 288.8 eV). It is seen that two carbon species are generally formed on the catalyst surface. Comparison of the amount of each carbon species on the catalyst reacted at 650 and 700 °C confirms that graphitic carbon is the dominating one at 650 °C and more oxidized carbon species were formed at 700 °C.

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Figure 4. TEM pictures of used 13.5Ni-2K/10CeO2-Al2O3 catalyst after 60 h TOS: (a) T ) 650 °C, (b) T ) 700 °C.

Figure 5. XPS C 1s spectra of 13.5Ni-2K/10CeO2-Al2O3 catalyst after 60 h TOS (a) T ) 650 °C; (b) T ) 700 °C.

TGA, XRD, and XPS measurements have shown that a significant amount of coke deposited on 13.5Ni-2K/10CeO2Al2O3 catalyst at 650 °C. However, the amount of accumulated coke with TOS did not affect the high activity of the catalyst up to 60 h. This suggests that some carbon species formed on the surface of catalysts must be involved in the reaction to produce CO (i.e., acting as active sites in the reaction but not as a deactivating factor). Thus, the deposited coke does not deactivate the Ni active phase, that is, Ni particles are not being blocked by the coke formed and the catalytic activity can be kept with reaction time because the active metal is still exposed to the reactants. Results revealed that a large part of the graphitic carbon deposited on the catalyst surface in the present study is of the filamentous form with nickel on top of these carbon filaments. This form of graphitic carbon is more active in the reforming reaction of methane probably because of its close contact with nickel particles and does not cause the catalyst deactivation.14 These factors (i.e., types of carbon species and their reactivity) can explain both the large amounts of coke builtup on the catalyst surface and the catalyst stability with high

activity for long times on stream at 650 °C. Similar results of high activity and stability despite large amounts of coke deposition have been reported.2,10 3.3. Coke Oxidation Kinetics. Several researches have reported the promotion effect of CeO2 in Ni/Al2O3 catalysts for CO2 reforming of methane.2,5,15 The redox property of ceria and the high mobility of lattice oxygen are among the most important factors which contribute to the catalytic reactivity of CeO2 in an oxidation reaction. According to Shyu et al.,16 below 1000 °C, CeO2 can be reduced by H2 only to Ce2O3. Monteiro et al.17 also showed that the reduction of CeO2 supported on Al2O3 depends on the loading: with lower contents (3% CeO2), the reduction is incomplete, while with higher contents (20% CeO2), it will be reduced completely. Hence, when the CeO2 promoter in 13.5Ni-2K/10CeO2-Al2O3 catalyst was reduced by H2 at 700 °C for 2 h, oxides Ce2O3 and CeO2 might exist. During the reforming process, the reactant CO2 first absorbs base centers and then dissociates on Ce2O3 by transferring electrons to CO2 and forms CO and CeO2. Then, CeO2 reacts with carbon deposited by CH4 dehydrogenation, and CeO2 changes to Ce2O3 again. The reaction process may be represented as follows:

Ce2O3 + CO2 ) 2CeO2 + CO

(1)

4CeO2 + C ) 2Ce2O3 + CO2

(2)

In fact, the above reactions imply a process of transferring oxygen. As CO2 dissociates and forms CO and an absorbed oxygen or oxygen-containing species, the affinity of the absorbed oxygen for the carbon atom of CH4 is responsible for the inhibition of carbon formation of the reaction. Therefore, the presence of CeO2 and Ce2O3 in catalyst will improve the inhibition of the surface carbon formation.18 A strong metal support interaction as in 13.5Ni-2K/10CeO2-Al2O3 would probably enhance oxygen transfer from the support to Ni (where methane decomposition takes place), which is beneficial to prevent carbon formation during the reaction.5 The TG-DTA behavior of 13.5Ni-2K/10CeO2-Al2O3 catalyst after a certain period of TOS at three different temperatures showed an exothermic Differential Thermal Analysis (DTA) peak between 400-600 °C and a corresponding mass loss on the TG between the same temperatures. This TG data corresponding to the oxidation of the coke residue was used to obtain

Ind. Eng. Chem. Res., Vol. 46, No. 6, 2007 1735 Table 2. Activation Energies (E) and Pre-Exponential Factors (A) for the Coke Removal from 13.5Ni-2K/10CeO2-Al2O3 Catalyst reaction run temperature time (°C) (h) 650

700

750

Figure 6. TPO profiles of carbonaceous species on 13.5Ni-2K/10CeO2Al2O3 catalyst after 45 h TOS.

temperature-programmed oxidation (TPO) profiles (weight loss rate vs temperature) at the three temperatures. A typical profile is shown in Figure 6 after 45 TOS. TPO profile on 13.5Ni2K/10CeO2-Al2O3 is asymmetric. The carbon oxidation started at 400 °C and the maximum peak occurred at 515-535 °C. The TPO profile shows that the coke formed at 700 and 750 °C is more reactive than that formed at 650 °C. The rate of reaction, written as dR/dt, is dependent on some function of R (f(R)), where R is the fraction of coke oxidized. Depending on f(R), the rate equation may take the following forms

nth order (Fn) dR/dt ) k(1 - R)n

(3a)

2-D diffusion (D2) dR/dt ) k[1/(-ln(1 - R) )] (3b) n

Avrami-Erofeev (A3) dR/dt ) 3k(1 - R)((-ln(1 - R))2/3) (3c) contracting volume (R3) dR/dt ) 3k(1 - R)2/3 (3d) The rate constant for heterogeneous reactions is assumed to obey the Arrhenius equation, Thus, for the nth order

dR/dt ) A exp(-E/RT)(1 - R)n

(4)

For a non-isothermal experiment, it is useful to convert this to a derivative with respect to temperature, using a (constant) heating rate, β ) dT/dt

dR/dT ) (A/β) exp(-E/RT)(1 - R)

n

(5)

Taking logs, and rearranging the above equation in linear form,

ln(dR/dT) - n ln(1 - R) ) ln(A/β) - E/RT

(6)

A plot of the left-hand side of eq 6 versus the reciprocal of the absolute temperature (1/T) should give a good straight line from which the activation energy E and pre-exponential factor A are obtained. The Arrhenius parameter, A, is a useful indication of the turnover frequency (TOF) for a first-order reaction and, hence, in combination with E yields valuable information on the surface energetics of the reacting site and its density (turnover number, TON ) TOF-1) allowing meaningful comparison between surfaces of the same material subjected to different conditions.19 The mechanistic laws given above as eq 3a-3d were all tested, and all gave reasonably acceptable log function versus

15 30 45 60 15 30 45 60 15 30 45 60

oxidation temperature range (°C)a 447-571 (522.2) 428-576 (520.6) 426-585 (535.2) 400-600 (526.4) 527-581 440-573 (521.9) 446-592 (524.4) 520-527 482-556 (530.8) 414-595 (513.9) 401-588 (519.7) 413-595 (520.9)

E (kJ/mol)

A (s-1)

149.9 ( 2.0 142.6 ( 0.7 155.5 ( 1.2 137.5 ( 0.6

(3.16 ( 0.98) × 107 (9.55 ( 1.03) × 106 (5.24 ( 1.00) × 107 (3.90 ( 0.36) × 106

146.3 ( 3.3 (1.66 ( 0.86) × 107 129.2 ( 2.4 (9.46 ( 3.48) × 105 164.3 ( 4.9 110.6 ( 1.3 117.0 ( 0.5 117.4 ( 1.2

(2.63 ( 1.96) × 108 (6.02 ( 1.20) × 104 (1.71 ( 0.13) × 105 (1.65 ( 0.30) × 105

a Values in brackets represent the temperature corresponding to the maximum weight loss rate.

1/T plots; however, the plots of calculated fraction reacted against T matched the experimental data rather poorly, with the exception of the results for an nth order reaction with n between 0.8 and 1.1. Therefore, re-parametrization was done using n ) 1.0, and a nonlinear regression technique was used to calculate the activation energy and pre-exponential factor with a regression coefficient of 0.99. The results are summarized in Table 2. Over the 13.5Ni-2K/10CeO2-Al2O3 catalyst, the activation energy for coke oxidation ranged between 110 and 160 kJ/mol. Wang and Lu10 calculated the activation energies for the oxidation of carbon species deposited on 5 wt % Ni/γ-Al2O3 after a certain period of reaction TOS at 700 °C on the basis of the Haines method and estimated it to range from 150 to 285 kJ/mol. The oxidation rate reached its maximum at approximately 670 °C for catalysts that reacted for 1 h, and the peak temperature shifted a bit to a higher temperature when the catalyst reacted for a longer period of time. For the 13.5Ni2K/10CeO2-Al2O3 catalyst, the peak temperature corresponding to the maximum weight loss rate was 515-535 °C, a value quite less than that observed by Wang and Lu. The difference in the value of activation energy and peak temperature obtained in the present study and that estimated by Wang and Lu shows that addition of K, CeO2 to Ni/Al2O3 catalyst results in coke which is more active toward oxidation. 4. Conclusion Carbon deposition, or coking, is the major factor that influences catalyst activity and stability and that leads to deactivation. Carbon filaments and amorphous carbon formed on nickel catalysts during carbon dioxide reforming of methane. Several factors, including temperature, CO2/CH4 ratio, catalyst support, metal particle size, metal morphology, and chemical properties, affect the carbon structure and coke growth mechanism. So far, most work on carbon filament growth mechanism and kinetics has been centered on studies of CH4 cracking or CO disproportionation. Additional research on the mechanism and kinetics is required to understand the complex role of different carbon species formed and their influence on catalyst activity and stability. In the present study, the 13.5Ni-2K/10CeO2-Al2O3 catalyst exhibited very stable catalytic performance for the reforming reaction at all three temperatures (650, 700, and 750 °C) for 60 h TOS. The activation energy for coke oxidation ranged between 110 and 160 kJ/mol.

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Acknowledgment The authors thank the Ministry of Human Resources and Development (MHRD), Delhi, for the financial support of the project. Literature Cited (1) Bradford, M. C. J.; Vannice, M. A. Catalytic Reforming of Methane with Carbon Dioxide over Nickel Catalysts I. Catalyst Characterization and Activity. Appl. Catal. A 1996, 142, 73. (2) Wang, S.; Lu, G. Q. M. Role of CeO2 in Ni/CeO2-Al2O3 Catalysts for Carbon Dioxide Reforming of Methane. Appl. Catal. B 1998, 19, 267. (3) Juan-Juan, J.; Roman-Martinez, M. C.; Illan-Gomez, M. J. Catalytic Activity and Characterization of Ni/Al2O3 and NiK/Al2O3 Catalysts for CO2 Methane Reforming. Appl. Catal. A 2004, 264, 169. (4) Goula, M. A.; Lemonidou, A. A.; Efstathiou, A. M. Characterization of Carbonaceous Species Formed during Reforming of CH4 with CO2 over Ni/CaO-Al2O3 Catalysts Studied by Various Transient Techniques. J. Catal. 1996, 161, 626. (5) Nandini, A. P.; Pant, K. K.; Dhingra, S. C. Characterization and Activity of K, CeO2 and Mn Promoted Ni/Al2O3 Catalysts for Carbon Dioxide Reforming of Methane. Ind. Eng. Chem. Res. 2006, 45, 7435. (6) Nandini, A.; Pant, K. K.; Dhingra, S. C. Kinetic Study of the Catalytic Carbon Dioxide Reforming of Methane to Synthesis Gas over Ni-K/CeO2Al2O3 Catalyst. Appl. Catal. A 2006, 308, 119. (7) Gadalla, A. M.; Bower, B. The Role of Catalyst Support on the Activity of Nickel for Reforming Methane with CO2. Chem. Eng. Sci. 1988, 43, 3049. (8) Tomishige, K.; Yamazaki, O.; Chen, Y. G.; Yokoyama, K.; Li, X.; Fujimoto, K. Development of Ultra-Stable Ni Catalysts for CO2 Reforming of Methane. Catal. Today 1998, 45, 35. (9) Swaan, H. M.; Kroll, V. C. H.; Martin, G. A.; Mirodatos, C. Deactivation of Supported Nickel Catalysts during the Reforming of Methane by Carbon Dioxide. Catal. Today 1994, 21, 571.

(10) Wang, S.; Lu, G. Q. M. A Comprehensive Study on Carbon Dioxide Reforming of Methane over Ni/γ-Al2O3 Catalysts. Ind. Eng. Chem. Res. 1999, 38, 2615. (11) Forzatti, P.; Lietti, L. Catalyst Deactivation. Catal. Today 1999, 52, 165. (12) Querini, C. A.; Fung, S. C. Coke Characterization by Temperature Programmed Techniques. Catal. Today 1997, 37, 277. (13) NIST (National Institute of Standards and Technology) X-ray Photoelectron spectroscopy database. http://srdata.nist.gov/XPS. (Accessed Sep 2006). (14) Rostrup-Nielsen, J. R. In Catalysis Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer: Berlin, 1984; Vol. 5, p 3. (15) Wang, C. Y.; Li, S. Y.; Yang, X. R.; Ren, J.; Chen, Y. G. CH4CO2 Reforming Anti-Carbon Deposition Catalyst. Energy ConVers. Manage. 1996, 37, 1357. (16) Shyu, J. Z.; Weber, W. H.; Gandhi, H. S. Surface Characterization of Alumina-Supported Ceria. J. Phys. Chem. 1988, 92, 4964. (17) Monteiro, R. S.; Noronha, F. B.; Dieguez, L. C.; Schmal, M. Characterization of Pd-CeO2 Interaction on Alumina Support and Hydrogenation of 1,3-Butadiene. Appl. Catal. A 1995, 131, 89. (18) Xu, G.; Shi, K.; Gao, Y.; Xu, H.; Wei, Y. Studies of Reforming Natural Gas with Carbon Dioxide to Produce Synthesis Gas X. The Role of CeO2 and MgO Promoters. J. Mol. Catal. A 1999, 147, 47. (19) Hardiman, K. M.; Cooper, C. G.; Adesina, A. A.; Lange, R. PostMortem Characterization of Coke-Induced Deactivated Alumina-Supported Co-Ni Catalysts. Chem. Eng. Sci. 2006, 61, 2565.

ReceiVed for reView October 29, 2006 ReVised manuscript receiVed January 15, 2007 Accepted January 22, 2007 IE061389N