Mechanistic Study of Carbon Dioxide Reforming with Methane over

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Energy & Fuels 1998, 12, 1114-1120

Mechanistic Study of Carbon Dioxide Reforming with Methane over Supported Nickel Catalysts Zi-Feng Yan,* Rong-Gang Ding, Lin-Hua Song, and Ling Qian State Key Laboratory for Heavy Oil Processing, University of Petroleum, Dongying 257062, China Received May 5, 1998. Revised Manuscript Received August 17, 1998

Carbon dioxide reforming with methane to synthesis gas has been investigated in terms of the effects of active metal loading, feed ratio, and reaction temperature on catalyst activity and stability employing supported Ni, Co, and Fe catalysts. Temperature-programmed desorption, temperature-programmed surface reaction, and X-ray photoelectron spectroscopy experiments were conducted to elucidate the reaction mechanism. An interesting observation made was that the direct dissociation of methane without the involvement of adsorbed or gas-phase CO2 most likely occurs and that the formation of hydrogen and surface carbon species, which are both primary products of the decomposition of methane, is the key step for the reforming reaction. Carbon monoxide might be the secondary product formed by interaction between surface carbon species and gas-phase or adsorbed CO2. Three surface carbon species CR, Cβ and Cγ, which are produced by the decomposition of methane, showed various mobility, thermal stability, and reactivity. In the meantime, the residual partial oxidative NiOx species that were not thoroughly reduced could not migrate on the catalyst surface and be stepwise reduced during the reaction.

Introduction In recent years, new political and environmental concerns over the obligatory mitigation of greenhouse gases have added to commercial interests in carbon dioxide reforming with methane. Catalytic reforming of carbon dioxide with methane to synthesis gas has been proposed as one of the most promising technologies for the utilization of these two greenhouse gases as carbon-containing materials, since both methane and carbon dioxide are greenhouse gases. Unfortunately, no commercial catalyst is yet available for the reforming of methane with carbon dioxide. However, there has been increasing interest worldwide to develop processes for methane conversion and mitigation of greenhouse gases, especially carbon dioxide. As far as the utilization of methane is concerned, this catalytic process is considered to be more feasible than the oxidative coupling of methane. Probably the most notable is that this route produces synthesis gas with a suitable CO/ H2 ratio for the production of hydrocarbons and oxygenated derivatives. Supported noble metals give promising catalytic performance in terms of activity, selectivity, and resistance to coke formation.1-11 Nevertheless, consideration of the high cost and limited availability * Author to whom correspondence should be addressed. E-mail: [email protected]. (1) Solymosi, F.; Kustan, G.; Erdohelyi, A. Catal. Lett. 1991, 11, 144. (2) Vennon, P. D. F.; Green, H. M. Catal. Today 1992, 13, 417. (3) Richardson, J. T.; Paripatyadar, S. A. Appl. Catal. 1990, 61, 293. (4) Solymosi, F.; Kustan, G., Erdohelyi, A. Catal. Lett. 1991, 11, 149. (5) Erdohelyi, A.; Cserenyi, J.; Solymosi F. J. Catal. 1993, 141, 287. (6) Erdohelyi, A.; Cserenyi, J.; Solymosi, F. Appl. Catal. A 1994, 108, 205. (7) Nkamura, J.; Akkawa, K.; Sato, K.; Uchijima, T. Catal. Lett. 1994, 25, 165. (8) Tsipouriari, V. A.; Efstathiou, A. M.; Zhang, Z. L.; Verykios X. E. Catal. Today 1994, 21, 579.

of noble metals makes the development of a nickel-based catalyst more feasible for industrial practice. The major obstacle encountered in this process is rapid catalyst deactivation by carbon deposition on a nickel-based catalyst surface. The main reason for this is the formation of carbon via the reaction

CH4 f C + 2H2 (∆H°298 ) 74.9 kJ/mol) which, in the absence of steam, quickly deactivates nickel-based catalysts. A similar difficulty arises with the methanation catalysts, because the low H2/CO ratio generated by the reforming reaction leads to carbon formation via the disproportionation reaction of carbon monoxide

2CO f C + CO2 (∆H°298 ) -172 kJ/mol) The carbon formation sequence for various metals, reported by Rostrup-Nilsen,11 is Ni . Pt > Ru. This paper aimed to investigate the possible carbon dioxide reforming mechanism for ultimate solution of the catalyst deactivation using TPD (temperature-programmed desorption), TPSR (temperature-programmed surface reaction), and XPS (X-ray photoelectron spectroscopy) studies. Experimental Section Catalyst Preparation. Three series of 20-60 mesh γ-alumina supported Ni, Co, and Fe catalysts were prepared by the incipient wetness impregnation method with aqueous solutions (9) Nkamura, J.; Akkawa, K.; Sato, K.; Uchijima, T. J. Jpn. Pet. Inst. 1993, 97, 38. (10) Ashcroft, A. T.; Cheetman, A. K.; Green, M. L. H.; Vernon, P. D. F. Science 1991, 352, 225. (11) Rostrup-Nielson, J. R.; Back Hansen, J. H. J. Catal. 1993, 144, 38.

10.1021/ef980105b CCC: $15.00 © 1998 American Chemical Society Published on Web 10/09/1998

Carbon Dioxide Reforming with Methane of nitrates as metal precursors. The solids were dried overnight in air at 393 K, then calcined at 773 K in air for 6 h for complete decomposition of the nitrate. After this treatment, the catalyst was reduced at 873 K in a stream of H2 for at least 2 h. The gas used was ultrahigh purity, 99.995%. Catalytic Reaction. The reforming reaction was carried out in a continuous-flow quartz fixed-bed reactor (i.d. ) 6 mm) under atmospheric pressure at various reaction temperatures and feed ratios of the reactant mixture. One portion of the catalyst (40 mg) was diluted with up to 6 portions of R-Al2O3 so as to reduce the temperature gradient along the catalyst bed. The solid mixture was pelletized and then crushed and sieved to the size range from 20 to 40 mesh. Rate limitations by external or internal mass transfer were proven to be negligible by applying suitable criteria under the experimental conditions. The catalyst was reduced again in situ at the reaction temperature in the H2 flow for 1 h. The flow rates of the feed gases were controlled by mass flow meters (Matheson Mass Flow Controller model 8240). The temperature of the catalyst bed was measured by a chromel-alumel thermocouple, and it was kept constant within (1 °C. The composition of the reactants/products mixture was analyzed with an on-line SP-3420 gas chromatograph equipped with a TCD and a Porapak QS column. The methane and CO2 used were ultrahigh purity (99.9995%). The impurities in methane were determined by HP5890 and SP-3420 GC. The results show that no detectable ethane or other higher hydrocarbons and CO2, CO, and H2 exist in the methane gas flow. Hydrogen, methane, or higher hydrocarbons could not be detected in the carbon dioxide feed either. The carbon quantity deposited on the catalyst is below 0.02 wt % of the total carbon feed (CH4 + CO2) according to the carbon deposition test. So the carbon deposition reaction is ignored in the carbon balance. The conversion and yield are calculated by the carbon and hydrogen balances, which are determined by the reforming reaction and water-gas shift reverse reaction. The yield of CO and hydrogen is defined as follows

YCO (%) ) YH2 (%) )

XCH4 + XCO2 2

3XCH4 - XCO2 2

× 100% × 100%

Catalyst Characterization. The reaction mechanism of carbon dioxide reforming with methane on supported nickel catalyst was investigated by means of TPD, TPSR, and XPS techniques. TPD experiments were conducted at a constant heating rate (10 K/min), using ultrahigh purity helium as the carrier gas, at a flow rate of 40 mL/min. The catalyst sample was first reduced in the H2 flow at 1023 K for 2 h. After purging with He for 20 min, the sample was cooled under He flow. When the required adsorption temperature was reached, the He flow was switched to CO or CH4 flow. The He flow was switched again to flush the reactor after cooling the sample to room temperature in the adsorption gas flow. Then temperature programming was initiated and the analysis of the desorbed gases was performed with an online ion trap detector (ITD) (Finnigan MAT 700).12 In the TPD process, the helium gas was dried with Mg(ClO4)2 and deoxygenated with 402 deoxygenating reagent. The residual oxygen that flowed over the catalyst was removed by using a liquid nitrogen cold trap before flowing into the reactor. Leak tests on the reaction system were also strictly performed to exclude the possibility of the oxidation of surface carbon. (12) Yan, Z. F.; Xue, J. Z.; Shen, S. K. Chem. Hong Kong 1998, 1 (2), 64.

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Figure 1. Reactant conversions of the reforming reaction on 8 wt % Ni, Co, and Fe supported on alumina. TPSR experiments were also performed in the quartz fixedbed reactor. The reforming reaction was carried out in situ at a certain temperature, and then the reactor was quickly cooled to room temperature. Subsequently, a 30 mL/min flow of hydrogen was introduced to flush the reactor continuously to take away the mixture of gases and physically adsorbed. After treatment, the surface intermediate species produced in the reforming process were well-characterized by TPSR in a hydrogen flow of 30 mL/min. The desorbed species from the metal surface along with the temperature-programmed process were simultaneously detected by online ITD. XPS for the nickel samples was conducted on an VG ESCALAB 210 X-ray photoelectron spectrometer with Mg KR radiation and a base pressure of 10-8 Pa. The specially designed pretreatment cell makes sure that the transfer of the XPS sample is not exposed to air before XPS experiments. The Si2p level, with a binding energy of 102.7 eV, was used to correct the charge effect as the reference line. The Fisons Eclipse Data System was used to perform the data acquisition and analysis.

Results and Discussion Evaluation of Catalyst Performance. The stoichiometric reforming reaction was conducted at atmospheric pressure and GHSV of 45000 cm3 g-1 h-1 over Ni, Co, and Fe supported on alumina at various temperatures. It was found that reduced Ni/Al2O3 was the optimum reforming catalyst. Indeed, this catalyst provided about 91% and 90% conversions of CO2 and CH4 at about 1000 K (Figure 1), respectively. Although the conversions of CO2 and CH4 over Co/Al2O3 were much lower than that over Ni/Al2O3 at 823 K, the activity of the Co/Al2O3 catalyst approximately approached that of the latter, with the conversions of CH4 and CO2 about 83% and 88.5%, respectively. Very low conversions of CH4 and CO2 at different temperatures gave evidence for the poor activity of Fe/Al2O3 catalyst. Sacco et al.17 studied the thermodynamics of carbon deposition on Fe, Co, and Ni in the gas mixtures of CH4-H2-H2O-CO-CO2 using the phase diagram. The results show that the carbon deposition order is Fe . (13) Gadalla, A. M.; Bower, B. Chem. Eng. Sci. 1988, 42, 3049. (14) Beebe, T. P.; Goodman, D. W.; Kay, B. D.; Yates, J. T. J. Chem. Phys. 1988, 92, 5213. (15) Yan, Z. F.; Xue J. Z.; Shen, S. K. J. Nat. Gas Chem. 1996, 5, 51. (16) Wang, S. B.; Lu, G. Q. Energy Fuel 1996, 10, 896-904. (17) Sacco, A., Jr.; Geurts, F. W. A. H.; Jablonski, G. A.; Lee, S.; Gately, R. A. J. Catal. 1989, 119, 322.

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Co . Ni. Simultaneously, nickel carbide and carbon are favorably obtained over the catalyst in the dry mixture of methane and CO2. It shows that the suppression of the carbon deposition is very important for the optimization of CO2 reforming with CH4 over supported transition-metal catalysts. Figure 1 shows that nickel is the best nonnoble metallic component as a CO2 reforming catalyst. According to the thermodynamic studies, Ni-containing catalysts for CO2 reforming are prone to carbon formation, resulting in deactivation of catalysts.13 Some reports have shown that it was impossible to avoid carbon formation under low CO2/CH4 ratios using nickel catalysts.10 To prevent carbon deposition, CO2/CH4 ratios above unity should be used. The supported noble metals have been studied in recent years, and results showed that they were less sensitive to carbon deposition. Catalysts based on Ni, Ru, Rh, Pd, Pt, and Ir were compared for CO2 reforming of methane.18-33 Ru and Rh showed high selectivity for carbon-free operation. As usual, the activity of the metal catalyst is affected by the combination of metal, support, and promoters. Recently, a Ni/La2O3 catalyst was developed which exhibited high activity and excellent long-term stability for CO2 reforming of methane. The results reported by Takayasu et al.34 indicated that physically mixed SiO2 with Ni catalyst had a very low carbon deposition. Nickel-based catalysts were investigated in detail with the following experiments considering its excellent performance. For comparison, CO2 reforming of CH4 was carried out over unreduced NiO/Al2O3 catalysts. Results showed that the unreduced NiO/Al2O3 catalysts had very low activities. The initial conversions of CH4 and CO2 were less than 40%. Of interest is that the conversions of reactants and the yields of products gradually increase with reaction time. It is postulated that the gradual reduction of NiO to Ni metal by the product hydrogen, which is the active site of CO2 reforming with CH4, resulted in the increase of the conversions of CH4 and CO2. Metal loadings on supports also affect the activity of a catalyst. Nickel catalysts supported on SiO2 with 40%, 44%, and 60% metal loadings were investigated (18) Dissanayake, D.; Rosynek, M. P,; Kharas, K. C. C.; Lunsford, J. H. J. Catal. 1991, 132, 117. (19) Choudhary, V. R.; Samsare, S. D.; Mammam, A. S. Appl. Catal. 1992, 90, L1. (20) Ashcroft, A. T.; Cheetman, A. K.; Green, M. L. H.; Vernon, P. D. F. Nature 1990, 344, 319. (21) Jones, R. H.; Ashcroft, A. T.; Green, M. L. H.; Thomas, J. M. Catal. Lett. 1991, 8, 169. (22) Hochmuth, J. K. Appl. Catal. B: Environ. 1992, 1, 89. (23) Vernon P. D. F.; Green M. L. H.; Cheetman, A. K.; Ashcroft, A. T. Catal. Lett. 1990, 6, 181. (24) Poirier, M. G.; Trudel, J.; Guay, D. Catal. Lett. 1993, 21, 99. (25) Torniainen, P. M.; Chu, X.; Schmidt, L. D. J. Catal. 1994, 146, 1. (26) Gadalla, A. M.; Sommer, M. E. Chem. Eng. Sci. 1989, 44, 2815. (27) Kim, G. J.; Cho, D. S.; Kim, K. H. Catal. Lett. 1994, 28, 41. (28) Swaan, H. M.; Kroll, V. C. H.; Martin, G. A.; Mirodatos, G. Catal. Today 1994, 21, 571. (29) Chang, J. S.; Park, S. E.; Lee, K. W. Stud. Surf. Sci. Catal. 1994, 84, 1587. (30) Zhang, Z.; Verykois, X. E. J. Chem. Soc., Chem. Commun. 1995, 71. (31) Rostrup-Nielsen, J. R.; Hansen, J. H. B. J. Catal. 1993, 144, 38. (32) Qin, D.; Lapszewicz, J. Catal. Today 1994, 21, 551. (33) RiChardson, J. T.; Paripatyadar, S. A. Appl. Catal. 1990, 61, 293. (34) Takayasu, O.; Takegahara, Y.; Matsuura, I. Energy Convers. Mgmt. 1995, 36, 605.

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Figure 2. Reactant conversions and product yields of reforming reaction as a function of Ni metal loading.

for CO2/CH4 reforming. The conversions of methane were similar for 40% and 60% Ni/SiO2, while 44% Ni/ SiO2 showed the lowest activity.35 Ni/Al2O3 catalysts with varying nickel content, namely 6-10 wt % of nickel over Al2O3, have been studied in the CO2 reforming of methane at 1000 K. Higher loading is required for Ni catalysts in which the metal-support interaction is stronger than in the case of noble metals.16 As shown in Figure 2, conversions of CH4 and CO2 and yields of H2 and CO increased gradually with elevated Ni metal loading, reaching a maximum level at 8 wt % metal loading. However, conversions of reactants and yields of synthesis gas declined slightly when the Ni loading increased to above 8 wt %. This indicates that a nickel loading of 8 wt % on Al2O3 is sufficient for obtaining high activity and selectivity. The accumulation of the metal active sites and the formation of a crystal phase over the catalyst loaded with too much metal may be responsible for the significant reduction in activity of high metal-loading catalysts. High reactant conversions and synthesis gas yields observed for the 8 wt % Ni/ Al2O3 catalyst are a good indication that the appropriate microstructure is formed on the catalyst, which is under further investigation in our group. The key to carbon dioxide reforming with methane is to guard against carbon deposition on the catalyst surface. Carbon-free operation with suitable CO2/CH4 feed could be achieved while the nickel is used as the catalyst component. For various CO2/CH4 feed ratios, the thermodynamic analysis pinpoints diversified critical temperatures above which carbon deposition on the surface of the catalyst is suppressed. Figure 3 shows the equilibrium composition of the reforming reaction at different CO2/CH4 feed ratios at atmospheric pressure and 1023 K. At a CO2/CH4 feed ratio of 1, an approaching equimolar amount of CO and H2 was produced. As the feed ratio increased, the concentration of H2O was increased. The formation of water is unavoidable and responsible for a higher CO/H2 ratio than unity in the reforming reaction. The water formation can be attributed to the reverse water-gas shift reaction. The higher yield of H2O due to the excess amount of CO2 in the feed was compensated for by a higher conversion of CH4. It is observed that CH4 conversion increases as (35) Takano, A.; Tagawa, T.; Goto, S. J. J. Chem. Eng. Jpn. 1994, 27, 723.

Carbon Dioxide Reforming with Methane

Figure 3. Effect of the CO2/CH4 feed ratio on the equilibrium composition of the dry reforming system.

Figure 4. The reactant conversions and product yields on 8 wt %Ni/Al2O3 as function of temperature.

the CO2/CH4 feed ratio increases from the experimental results. It is postulated that methane would be totally converted at even higher feed ratios, i.e., appreciable amount of hydrogen in CH4 is wasted due to water formation. Researchers16 have reported the equilibrium conversion as a function of temperature at a total pressure of 0.01-0.1atm with CO2/CH4 ) 1. It has been shown that at a fixed temperature, conversions of reactants at lower total pressures are always higher than those at higher total pressures. Gadalla et al.13 recommended that the optimum temperature be between 1143 and 1310 K. The activity and selectivity of the 8 wt % Ni/Al2O3 catalyst was investigated at different temperatures, and the variation of conversions and yields are shown in Figure 4. The conversions of CH4 and CO2 and yields of H2 and CO reached only 10% at relatively low temperature (ca. 700 K). The conversions and yields increase with increasing reaction temperature, and all approach 90% at 1020 K. It appears that the strongly endothermic reforming process should be favored at high temperatures, but the catalyst will suffer from sintering and formation of nickel carbide at too high temperatures. Characterization of the Catalysts. Methane decomposition on a transition-metal surface is a thermalassisted catalytically activated process. The reactive sticking of alkanes on nickel single-crystal surfaces is

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strongly dependent on the surface structure. For example, methane reactivity is seen to increase in the order of Ni(111) < Ni(110) < Ni(100) with prolonged exposure to methane.14 Initial reaction rates for Ni(110) and Ni(100) surfaces are rather similar and are 7-10 times greater than the initial rate for the Ni(111) surface at 450 K. In this study, several questions are addressed. First, how do carbonaceous fragments, in particular the simplest fragment, CHx(ad) species, bond to metal surfaces with a more complex structure than that to single-crystal surfaces? Second, a low-index nickel single-crystal surface should be effective toward the decomposition of methane, but can an aluminasupported nickel catalyst surface be effective in the decomposition of methane? What is the mechanism of the thermal decomposition of methane on aluminasupported nickel catalysts? TPSR, TPD, and XPS techniques are used to identify surface-bound species and gas-phase products of the decomposition of methane adsorbed on alumina-supported nickel catalysts. For the reaction of CO2 reforming of methane, the most probable slow steps are methane activation to CHx(ad) (x ) 0-3) species and the reaction between CHx(ad) species and the oxidant, either in the form of oxygen adatoms originating from CO2 dissociation or CO2 itself. Experiments have confirmed that methane promotes the dissociation of carbon dioxide on the catalysts. The promotion of carbon dioxide is attributed to the effect of hydrogen formed in the decomposition of methane. It seems that the key to elucidation of the nature of the CO2 reforming of methane over Ni/R-Al2O3 catalyst is to reveal the mechanistic scheme of the activation and dissociation of methane. Great effort was made to detect adsorbed carbonaceous CHx(ad) fragments formed in the decomposition of methane by means of sensitive in situ FT-IR spectroscopy. However, no adsorption bands attributable to any of the vibration modes of carbonaceous CHx(ad) species were identified either by in situ measurements or after a sudden cooling of the sample in a continuous methane flow at 700 K. This means that either all the above carbonaceous CHx(ad) species react or decompose too quickly at high temperature or their surface concentrations are below the detection limit. However, the presence of surface carbonaceous CHx(ad) species was well manifested by its reaction with hydrogen. After flushing the reactor with pure helium flow (following methane decomposition at a certain temperature) and switching to a hydrogen flow, the hydrogenation of the surface carbonaceous CHx(ad) species was investigated by TPSR and TPD techniques, which reveals that methane is initially produced at high rates on the catalysts. Figure 5 showed that the decomposition of methane could result in the formation of at least three kinds of surface carbon species on supported nickel catalyst. Generally, the carbon deposition is comprised of various forms of carbon which are different in terms of reactivity. The distribution and features of these carbonaceous species depend sensitively on the nature of the transition metals and the conditions of methane adsorption. These carbonaceous species can be described as completely dehydrogenated carbidic CR type, partially dehydrogenated CHx (1 e x e 3) species, namely Cβ type, and carbidic clusters Cγ type formed

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Figure 5. TPSR spectra of CH4 in H2 flow on fresh 8 wt % Ni/Al2O3 at different adsorption temperatures.

by the agglomeration and conversion of CR and Cβ species under certain conditions. A fraction of the surface carbon species, which might be assigned to carbidic CR (∼461 K), was mainly hydrogenated to methane even below 500 K. Simultaneously, a trace of ethane was also produced in addition to methane. It shows that the carbidic CR species is rather active and thermally unstable on a nickel surface. The carbidic CR species is suggested to be responsible for CO formation.36 A significant amount of surface carbon species was hydrogenated to methane below 600 K and assigned to partially dehydrogenated Cβ (∼583 K) species. A majority of the surface carbon was hydrogenated above 800 K and attributed to carbidic clusters Cγ (∼823 K). The formation of less active Cβ and Cγ species causes the catalyst deactivation.36 It also indicated that the formation of three kinds of surface carbon species with different structures and properties largely depend on the exposure temperature and duration to methane. When the nickel catalyst was exposed to methane above 723 K, the carbidic CR species was not detected and a significant amount of Cβ was transformed into the carbidic clusters Cγ. This shows that the carbidic clusters Cγ species might be the precursor of the surface carbon deposition, which may be produced by the interactions between CR and Cβ species and between CR and Cβ themselves. The carbon species originally produced by methane are believed to be atomic or carbidic carbon. It is known to be a very active and important intermediate in the CO2 reforming of methane. The interaction between the adsorbed or gaseous CO2 and surface carbon species, which are mainly derived from the decomposition of methane, can result in the formation of CO. On the basis of this consideration, the possible reaction processes of CO2 reforming of methane can be inferred as follows: methane is first decomposed into hydrogen and different surface carbon species, then the adsorbed or gas-phase CO2 reacts with surface carbons to form CO. As shown in Figure 6, TPD spectra of methane on fresh Ni/Al2O3 catalyst exhibited three corespondent peaks of CO2. This means that methane decomposition on transition metals truly takes place and that the reactivity of the surface carbon species depends sensi(36) Zhang, Z.; Verykois, X. E. Catal. Today 1994, 21, 589.

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Figure 6. TPD spectra of CH4 in H2 flow on fresh 8 wt % Ni/Al2O3 at different adsorption temperatures.

tively on the various reactions. For TPSR spectra in a flow of hydrogen, desorbed products were mainly methane and traces of higher hydrocarbons. But for TPD spectra in a flow of helium, desorbed species were mainly CO2 and some CO, which means that the TPD process is actually a process of temperature-programmed oxidation (TPO), in which the surface carbon was oxidized to form CO and CO2. The surface oxygen species which resulted in the oxidation of surface carbon might be the residual or remaining surface-bonded oxygen (M-O) on transition metals. The oxygen adsorbed in subsurface and bulk phase of the metal cannot migrate to the surface below 1000 K.15 Furthermore, XPS results show that the crystalline oxygen of alumina support does not interact with the surface carbon at moderate temperatures ( CR > Cβ, with the ∆T value of 0, 5, and 34 K, respectively. This indicates that the carbonaceous species formed by the decomposition of methane are mobile enough and interact with partial metal oxide to form CO2. In the meantime, from Figure 6 another conclusion can be drawn that CR and Cβ species could be transformed into Cγ species and that the transformation could be accelerated with the increasing adsorption temperature, similar to those exhibited in Figure 5. The XPS experiments and Ar+ etching techniques can give more supporting information. The binding energy data of the catalyst calcined in air at 923 K, reduced in H2 at 923 K, and exposed to methane followed by

Carbon Dioxide Reforming with Methane

Figure 7. Deconvoluted XPS of C(1s) of 673 K-adsorbed CH4 over 8 wt % Ni/Al2O3 at 300 K.

Figure 8. Deconvoluted XPS of C(1s) of 673 K-adsorbed CH4 over 8 wt % Ni/Al2O3 after sputtering by Ar+ ions of 1.36 × 10-4 Pa at 300 K and 5 kV for 5 min.

reduction were determined by XPS experiments. The results show that surface nickel exists in the oxidative state (BE ) 856.00 eV) for the calcined sample, partially oxidative state (BE ) 853.64 eV) for a reduced sample in H2, and metallic state (BE ) 852.90 eV) for the methane-exposed sample (Figure 7). The binding energy of metal Ni is 852.30 eV, so it is concluded that the catalyst exposed to methane following reduction exists essentially in the metallic state. This is also consistent with the TPD results above. Figure 7 shows that methane decomposition on supported nickel catalyst can result in three different types of surface carbon, similar to the TPSR results, whose binding energies are 282.00, 282.90, and 284.70 eV, respectively. The deconvoluted XPS spectra of C(1s) of 673 K-adsorbed CH4 after sputtering by Ar+ at 300 K are shown in Figure 8. Comparison of deconvoluted XPS spectra before and after Ar+ etching exhibited that CR (BE ) 282.00 eV) and Cβ (BE ) 282.90 eV) species are thermally unstable and can be transformed into Cγ species (BE ) 284.70 eV) under the experimental conditions. The CO TPD profiles over the fresh Ni/Al2O3 catalyst were obtained following CO adsorption at 300 K (Figure 9). The TPD profiles of used Ni/Al2O3 catalyst after 8 h of the reforming reaction are shown in Figure 10. The response of CO2 formation was recorded to monitor the occurrence of CO disproportionation during the process

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Figure 9. CO-TPD profile over fresh 8 wt % Ni/Al2O3 catalyst.

Figure 10. TPD profile over the used 8 wt % nickel catalyst supported on alumina.

of temperature programming. Compared with CO-TPD profiles over the fresh Ni/Al2O3 catalyst, on which two respective CO2 desorption peaks appear, the TPD profiles on used catalyst are very different. An additional intense CO2 peak at ca. 910 K was observed on used supported nickel catalyst. The CO desorption was also increased with increasing temperature from ca. 810 K. The two CO2 desorption peaks appear on both the COTPD profiles over the fresh catalyst and used catalyst at ca. 410 and 570 K. They seem to correspond to the desorbed CO2 in the form of weakly chemisorbed CO2 on different sites on both catalysts. It is interesting to note that a large quantity of CO and CO2 desorbed at approximately the same temperature from at ca. 800 K, but the increase of CO obviously lagged behind. This indicates that CO might be the secondary product rather than primary one. The interaction of surface carbon with gaseous CO2 would result in the formation of CO. The obvious hysteresis effect of the CO peak with respect to the CO2 peak and the continuously increasing intensity of the CO peak are noteworthy. This could be manifested by the mobility of the surface carbon species and the reactivity of the oxygen species on the nickel catalyst. The mobile surface carbon species can attack the neighboring oxygen adatoms and surface oxygen species to form CO or CO2. It is also possible that the CO2 desorbed from the catalyst readsorbed and then reacted with surface carbonaceous species to produce CO.

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Conclusions The following conclusions can be drawn from the results in this investigation: (1) The nickel-based catalyst exhibits a good performance for the CO2/CH4 reforming reaction, offering a good possibility of industrial application. Appropriate metal loading is favored for optimum catalytic performance. (2) Carbidic CR, carbonaceous Cβ, and carbidic cluster Cγ surface carbon species formed by decomposition of methane showed different surface mobility, thermal stability, and reactivity. CR and Cβ species on the nickel surface are thermally unstable and can be rapidly converted into Cγ species upon increasing the temperature. (3) The possible reaction processes may be as follows: methane is first decomposed into hydrogen and

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different surface carbon species, then the adsorbed or gaseous CO2 reacts with surface carbons to form CO. (4) The TPD process actually undergoes a TPO process involving the surface residual oxygen species. The surface carbon produced on the catalyst surface was oxidized to form CO and CO2 during the TPD process. The residual partial oxidative NiOx species that are not thoroughly reduced cannot migrate on the catalyst surface and can be stepwise reduced during the reaction. Acknowledgment. Financial support by the Young Scientists Award Foundation of Shandong Province and Young Scientists Innovation Foundation of China National Petroleum Corporation are gratefully acknowledged. Sincere thanks are given to G. Q. (Max) Lu for his help in improving the quality of the English. EF980105B