Synthesis Gas Production from Methane Using Oxidized-Diamond

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Energy & Fuels 2003, 17, 971-976

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Synthesis Gas Production from Methane Using Oxidized-Diamond-Supported Group VIII Metal Catalysts Kiyoharu Nakagawa,†,‡ Hiroaki Nishimoto,‡ Masaki Kikuchi,‡ Sayaka Egashira,‡ Yuji Enoki,‡ Na-oki Ikenaga,‡ Toshimitsu Suzuki,‡ Mikka Nishitani-Gamo,§ Tetsuhiko Kobayashi,¶ and Toshihiro Ando*,⊥ National Institute for Materials Science (NIMS), 1-1, Namiki, Tsukuba, Ibaraki, 305-0044, Japan, Department of Chemical Engineering, and High Technology Research Center, Kansai University, Suita, Osaka 564-8680, Japan, Institute of Applied Physics and Center for Tsukuba Advanced Research Alliance (TARA), University of Tsukuba, Tsukuba, Ibaraki 305-8577, Japan, Special Division for Green Life Technology, National Institute of Advanced Industrial Science and Technology (AIST), Ikeda, Osaka 563-8577, Japan, and Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Corporation (JST) and National Institute for Materials Science (NIMS), 1-1, Namiki, Tsukuba, Ibaraki 305-0044, Japan Received January 9, 2003

We performed partial oxidation, steam reforming, and CO2 reforming of CH4 to synthesis gas, using oxidized-diamond-supported group VIII metal catalysts, to investigate the properties of oxidized diamond as a support material and to avoid carbon deposition on a metal-loaded catalyst. Nickel (5 wt %)/oxidized diamond afforded the highest CH4 conversion of 24% (CH4/O2 ) 5), giving CO and H2 at 873 K for the partial oxidation of CH4. No carbon deposition was observed with a supported oxidized-diamond catalyst (5 wt % nickel loading level) in the partial oxidation of CH4 at >923 K. In the steam reforming of CH4, ruthenium (5 wt %)/oxidized diamond afforded a CH4 conversion of 63% (CH4/H2O ) 0.33), giving CO and H2 at 873 K. Nickel, palladium, ruthenium, and rhodium (5 wt %)/oxidized diamond showed high catalytic activities for CO2 reforming of CH4 (CH4/CO2 ) 1) at 873 K.

1. Introduction Synthesis gas production from CH4 is indispensable to the chemical utilization of natural gas. Hydrogen and synthesis gas are also important for use in fuel cells, Fischer-Tropsch reaction and methanol synthesis, etc. Thus, the partial oxidation (reaction 1), steam reforming (reaction 2), and CO2 reforming (reaction 3) of CH4 have attracted much attention.

1 CH4 + O2 f CO + 2H2 2

0 (∆H298 ) -36 kJ/mol)

(1) CH4 + H2O a CO + 3H2

0 (∆H298 ) +206 kJ/mol) (2)

CH4 + CO2 a 2CO + 2H2

0 (∆H298 ) +247 kJ/mol) (3)

Catalysts loaded with group VIII transition metals have been widely examined as catalysts for the produc* Author to whom correspondence should be addressed. E-mail: [email protected], [email protected]. † National Insitute for Materials Science. ‡ Kansai University. § University of Tsukuba. ¶ National Institute of Advanced Industrial Science and Technology. ⊥ Japan Science and Technology Corporation and National Institute for Materials Science.

tion of synthesis gas from CH4; in some cases, the effects of support material on the catalytic activity have been reported.1-3 We have previously investigated the partial oxidation and CO2 reforming of CH4 using group VIII metalloaded catalysts.4-11 In the partial oxidation of CH4, the reaction pathway of synthesis gas production over the iridium, platinum, and nickel/TiO2 catalysts seemed to proceed primarily via two-step reactions promoted by iridium, platinum, and nickel/TiO2 catalysts, resulting (1) Dissanayake, D.; Rosynek, M. P.; Kharas, K. C. C.; Lunsford, J. H. J. Catal. 1991, 132, 117. (2) Tsang, S. C.; Claridge, J. B.; Green, M. L. H. Catal. Today 1995, 23, 3. (3) Bharadwaj, S. S.; Schmidt, L. D. Fuel Process. Technol. 1995, 42, 109. (4) Nakagawa, K.; Suzuki, T.; Kobayashi, T.; Haruta, M. Chem. Lett. 1996, 1029. (5) Nakagawa, K.; Ikenaga, N.; Suzuki, T.; Kobayashi, T.; Haruta, M. Appl. Catal. A 1998, 169, 281. (6) Nakagawa, K.; Anzai, K.; Matsui, N.; Ikenaga, N.; Suzuki, T.; Teng, Y.; Kobayashi, T.; Haruta, M. Catal. Lett. 1998, 51, 163. (7) Nakagawa, K.; Ikenaga, N.; Suzuki, T.; Teng, Y.; Kobayashi, T. Appl. Catal. A 1999, 180, 183. (8) Nakagawa, K.; Ikenaga, N.; Teng, Y.; Kobayashi, T.; Suzuki, T. J. Catal. 1999, 186, 405. (9) Matsui, N.; Nakagawa, K.; Ikenaga, N.; Suzuki, T. J. Catal. 2000, 194, 115. (10) Nakagawa, K.; Ikenaga, N.; Kobayashi, T.; Suzuki, T. Catal. Today 2001, 64, 31. (11) Nishimoto, H.; Nakagawa, K.; Ikenaga, N.; Suzuki, T. Catal. Lett. 2002, 82, 161.

10.1021/ef0300040 CCC: $25.00 © 2003 American Chemical Society Published on Web 06/20/2003

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in a reaction sequence of the complete oxidation of CH4 to H2O and CO2, and reforming reactions to produce synthesis gas (reactions 2 and 3). In contrast, for rhodium/TiO2 and palladium/TiO2 catalysts, a different reaction pathway for synthesis gas formation could be proposed. Rhodium/TiO2 and palladium/TiO2 catalysts exhibited high catalytic activity in the decomposition of CH4 to give H2 and deposited carbon or CHx, even in the presence of oxygen.7,9 Diamond is one of the most important, interesting, unique, and stable forms of carbonaceous materials.12-16 In particular, oxidized diamond has gained attention as a unique material-phase carbon oxide. Until now, no solid-phase carbon oxide has been found; however, a pseudo-carbon oxide solid is considered to comprise the near surface of the oxidized diamond.17-20 The oxidizeddiamond surface is expected to behave as a carbon oxide for specified surface reactions, such as catalytic and electrochemical reactions. Such reactions must be controlled by the interaction between molecules and surfaces; therefore, the bulk structures of the solids are less important. The aforementioned discussion has led us to use diamond in a novel way. Oxidized diamond has been found to exhibit novel and unique catalytic properties for alkane conversion chemistry.21-26 In particular, oxygen species on the diamond surface play an important role in the catalytic activities that are involved in alkane activation. We will describe detailed herein our studies of the partial oxidation, steam, and CO2 reforming of CH4 over oxidized-diamond-supported catalysts and discuss the effects of supports on synthesis gas production from CH4 over oxidized-diamond-supported catalysts. 2. Experimental Section Catalyst Preparation. To make reactivity for a catalytic reaction and hydrophilicity for impregnating metal oxides as an active material possible, oxidized diamond was prepared by oxidizing commercial, fine-powdered diamond (General Electric Company) at 723 K for 1 h under a stream of an O2(12) Angus, J. C.; Hayman, C. C. Science 1988, 241, 913. (13) May, P. W. Philos. Trans. R. Soc. London, Ser. A 2000, 358, 473. (14) Jhon, P. Science 2001, 292, 1847. (15) Ashfold, M. N. R.; May, P. W.; Petherbridge, J. R.; Rosser, K. N.; Smith, J. A.; Mankelevich, Y. A.; Suetin, N. V. Phys. Chem. Chem. Phys. 2001, 3, 3471. (16) Ferro, S.; De Battisti, A. J. Phys. Chem. B 2002, 106, 2249. (17) Ando, T.; Inoue, S.; Ishii, M.; Kamo, M.; Sato, Y.; Yamada, O.; Nakano, T. J. Chem. Soc., Faraday Trans. 1993, 89, 749. (18) Ando, T.; Ishii, M.; Kamo, M.; Sato, Y. J. Chem. Soc., Faraday Trans. 1993, 89, 1383. (19) Ando, T.; Ishii, M.; Kamo, M.; Sato, Y. J. Chem. Soc., Faraday Trans. 1993, 89, 1783. (20) Ando, T.; Yamamoto, K.; Ishii, M.; Kamo, M.; Sato, Y. J. Chem. Soc., Faraday Trans. 1993, 89, 3635. (21) Nakagawa, K.; Kajita, C.; Ikenaga, N.; Kobayashi, T.; NishitaniGamo, M.; Ando, T.; Suzuki, T. Chem. Lett. 2000, 1100. (22) Nakagawa, K.; Nishimoto, H.; Enoki, Y.; Egashira, S.; Ikenaga, N.; Kobayashi, T.; Nishitani-Gamo, M.; Ando, T.; Suzuki, T. Chem. Lett. 2001, 460. (23) Nakagawa, K.; Hashida, T.; Kajita, C.; Ikenaga, N.; Kobayashi, T.; Nishitani-Gamo, M.; Suzuki, T.; Ando, T. Catal. Lett. 2002, 80, 161. (24) Suzuki, T.; Nakagawa, K.; Ikenaga, N.; Ando, T. Stud. Surf. Sci. Catal. 2002, 143, 1073. (25) Nakagawa, K.; Kajita, C.; Ikenaga, N.; Suzuki, T.; Kobayashi, T.; Nishitani-Gamo, M.; Ando, T. J. Phys. Chem. B 2003, 107, 4048. (26) Nakagawa, K.; Kajita, C.; Ikenaga, N.; Nishitani-Gamo, M.; Ando, T.; Suzuki, T. Catal. Today, manuscript to be submitted.

Nakagawa et al. Ar mixture (1:4 ratio). Before oxidation, to eliminate impurities and to activate the diamond surface, the diamond powder was hydrogenated at 1173 K for 1 h. The catalyst supports used were oxidized diamond (with a Brunauer-Emmett-Teller (BET) surface area of 12.6 m2‚g-1), Al2O3 (JRC-ALO-4; reference catalyst provided by the Catalyst Society of Japan), SiO2 (Wako Pure Chemical), MgO (Ube Industries, Ltd.), TiO2 (Japan Aerosil Co.), La2O3 (Nacalai Tesque, Inc.), and active carbon (Wako Pure Chemical). The supported group VIII metal catalysts were prepared by impregnating an aqueous solution of RuCl3‚nH2O, Pd(NO3)2, IrCl4‚H2O, (NH3)2Pt(NO2)2 (Mitsuwa Pure Chemicals), Fe(NO3)3‚9H2O, Co(NO3)2‚6H2O, Ni(NO3)2‚6H2O, and RhCl3‚H2O (Kishida Chemicals) onto suspended supports, followed by evaporation to dryness. Supported catalysts were calcined at 873 K for 5 h in air prior to the reaction, except for the oxidized-diamond-supported catalysts. Oxidized-diamond-supported catalysts were calcined at 723 K for 5 h in air prior to the reaction. Catalytic Reactions. The reactions were conducted using a fixed-bed flow-type quartz reactor (10 mm inside diameter × 350 mm) at atmospheric pressure. The experiments involved a partial oxidation of CH4, steam reforming of CH4, and CO2 reforming of CH4: (1) The conditions for the partial oxidation of CH4 were as follows: 60 mg of catalyst, 25 mL/min CH4, and 5 mL/min of O2 were introduced at 673-873 K. (2) Steam reforming of CH4 was tested using 100 mg of the catalyst at 873 K with a mixture of CH4:H2O:Ar (1:3:5 ratio) at a total flow rate of 45 mL/min. (3) For CO2 reforming of CH4, using 100 mg of catalyst and 300 mg of silica sand (Merck), 30 mL/min of CH4 and 30 mL/ min of CO2 were introduced at 873 K. Silica sand was used as a heat buffer for a large endothermic reforming reaction. Prior to the reaction, the catalyst was reduced with H2 at 873 K for 1 h. Reaction product gases (H2, CO, CH4, and CO2) were analyzed using a gas chromatograph with a thermal conductivity detector (model M200 chromato analyzer, Nippon Tyran Co.) that can separate H2, CO, CH4, and CO2 within a few hundred seconds for the partial oxidation and CO2 reforming of CH4. For the steam reforming of CH4, the reaction products (CO, CH4, and CO2) were analyzed with a Shimadzu model GC8AIT gas chromatograph (TCD detector) with a 3 mm × 3 m stainless steel column that was packed with an activated carbon (30/60 mesh), using helium as a carrier gas. Analyses of H2 were performed using a Shimadzu GC8AIT gas chromatograph (TCD detector) that was equipped with a 3 mm × 3 m stainless steel column packed with activated carbon (30/ 60 mesh), using a N2 carrier. Characterization of Catalysts. The surface area of the catalyst was measured by the BET method using N2 at 77 K with automatic Micromeritics Gemini model 2375 equipment. X-ray photoelectron spectroscopy (XPS) results were obtained on a X-ray photoelectron spectrometer (model JPS9000MX, JEOL), using Mg KR radiation as the energy source.

3. Results and Discussion Partial Oxidation of Methane. We have previously reported that the support-activity order of nickel-loaded catalysts for the partial oxidation of CH4 at 873 K was as follows: oxidized diamond . La2O3 > Al2O3 > SiO2, MgO, activated carbon, > TiO2.22 Oxidized diamond was expected to be a useful support material for the nickelloaded catalyst. Therefore, we discuss the catalytic activity of oxidized-diamond-supported catalysts for the partial oxidation of CH4 to synthesis gas.

Synthesis Gas Production from Methane

Energy & Fuels, Vol. 17, No. 4, 2003 973 Table 1. Measured Binding Energies of the Nickel Element binding energy/eV sample fresh used reduced

Ni(2p3/2) 853.1 852.7 851.7

854.8 854.5

a The sample was Ni(2p ), and the X-ray source was Mg KR, 3/2 run at 10 mA and 10 kV.

Figure 1. Effects of various oxidized-diamond-supported metal catalysts on the product concentrations and CH4 conversion for the partial oxidation of CH4. Reaction conditions were as follows: temperature, 873 K; reaction time, 0.5 h; metal loading level, 5.0 wt %; CH4:O2 ratio, 25:5 (mL/mL); catalyst amount, 60 mg; SV, 30 000 h-1‚mL‚g-1‚cat-1.

Figure 1 shows the product distribution in partial oxidation of CH4 over oxidized-diamond-supported group VIII metal (5 wt %) catalysts. CH4 conversions were affected by the metal species; their order of activity was Ni > Co > Ru > Rh > Pd > Ir > Pt > Fe. At 873 K, selectivities to CO and H2 were the highest with the nickel and cobalt loaded on oxidized-diamond catalysts with a H2:CO ratio of 2.3-2.4. Carbon deposition was only observed on the nickel (5 wt %)/oxidized-diamond catalyst at 873 K. Above 923 K, however, carbon deposition did not occur on the oxidized-diamondsupported nickel catalyst (5 wt % nickel loading level). Ruthenium, rhodium, and palladium/oxidized-diamond catalysts exhibited moderate activity in the partial oxidation of CH4. However, rapid carbon deposition occurred on the palladium/oxidized-diamond catalyst in the initial stage of the reaction at 873 K, indicating that these catalysts would be deactivated by carbon deposition. The iridium/oxidized-diamond catalyst showed low activity in the partial oxidation of CH4. The high CO2 selectivity is a characteristic feature of platinum/oxidizeddiamond and iron/oxidized-diamond catalysts, indicating that complete oxidation occurred primarily. To produce synthesis gas on the nickel- and cobalt-loaded catalysts, metallic nickel and cobalt are required.2 If nickel or cobalt species are highly dispersed on Al2O3 and La2O3 surfaces, the oxidic nickel or cobalt might be stabilized and difficult to reduce. Even if it is reduced with H2, it will be oxidized with O2.7 However, in the case of oxidized-diamond-supported catalysts, the nickel- and cobalt-loaded oxidized-diamond catalysts produce synthesis gas. These results indicate that oxidic nickel and cobalt species on oxidized diamond could easily be reduced to metallic and/or lower-valency-state nickel and cobalt species under the aforementioned conditions. XPS analyses revealed that the binding energy of the nickel species of the oxidized-diamond-supported nickel catalyst that was reacted at 973 K shifted to a slightly lower binding energy than that of fresh catalyst (Table 1). Moreover, this binding energy was higher than that

Figure 2. Activity of oxidized-diamond-supported metal catalysts for the steam reforming of CH4. Reaction conditions were as follows: prior to the reaction, catalysts were reduced with H2 at 873 K for 1 h; temperature, 873 K; reaction time, 0.5 h; metal loading level, 5.0 wt %; CH4:H2O:Ar ratio, 1:3:5; total flow rate, 45 mL/min; catalyst amount, 100 mg; SV, 28 000 h-1‚mL‚g-1‚cat-1.

of the reduced nickel species under hydrogen. It seems to indicate that the role of the oxygen species on the diamond surface is important for the oxidation state of nickel. Steam Reforming of Methane. Figure 2 shows the product distributions with various oxidized-diamondsupported metal catalysts for the steam reforming of CH4 to synthesis gas at 873 K. A much higher CH4 conversion, compared to that observed for the partial oxidation of CH4, was observed in all the catalysts that were employed, although the H2O and CH4 were diluted with argon. The CH4 conversion was affected by the metal species; their order of activity was Ru > Ni > Co > Ir > Rh > Pd > Pt > Fe. Ruthenium- and nickelloaded oxidized-diamond catalysts gave higher CH4 conversion and product concentrations. Figure 3 shows the product distributions with various nickel-loaded catalysts for the steam reforming of CH4 at 873 K. The CH4 conversion was affected by the supports; their order of activity was Al2O3 > oxidized diamond, Y2O3 > TiO2 > MgO > SiO2 > La2O3. A large support effect was observed in the product concentrations and CH4 conversion. The nickel/Al2O3 catalyst is known to have a high activity and is commonly used in industry. The reactivity of steam might be higher than that of CO2. A higher H2:CO ratio, however, was expected from the stoichiometry. The stoichiometric H2: CO ratio is 3 in steam reforming; the higher H2:CO ratios with oxidized-diamond-supported catalysts and nickel-loaded catalysts can be explained by a water-

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Nakagawa et al. Table 3. Effect of Supports on the Conversions and Synthesis Gas Yields over Nickel-Loaded Catalysts for the CO2 Reforming of CH4a

catalyst

surface conversion/% yield/% area/ H2:CO m2‚g-1 CH4 CO2 H2 CO ratio

nickel/oxidized diamond 18.5 nickel/La2O3 6.4 nickel/SiO2 180.4 nickel/MgO 46.9 nickel/Al2O3 156.3 nickel/TiO2 37.8 nickel/active carbon 822

23.3 31.1 25.9 21.6 19.1 12.5 4.2

30.3 43.5 28.5 34.7 31.5 23.2 10.0

19.9 24.9 24.5 15.0 13.3 9.0 1.3

26.8 37.3 27.2 28.2 24.9 18.5 7.1

0.74 0.67 0.90 0.53 0.53 0.49 0.18

a Conditions were as follows: prior to the reaction, catalysts were reduced with H2 at 873 K for 1 h; catalyst amount, 100 mg; silica sand amount, 300 mg; flow rate, 60.0 mL/min (CH4/CO2 ) 1.0); reaction temperature, 873 K; metal loading level, 5.0 wt %; and reaction time, 1 h.

Figure 3. Effect of supports on CH4 conversion and product concentrations for the steam reforming of CH4 over nickelloaded catalysts. Reaction conditions were as follows: temperature, 873 K; reaction time, 0.5 h; nickel loading level, 5.0 wt %; CH4:H2O:Ar ratio, 1:3:5; total flow rate, 45 mL/min; catalyst amount, 100 mg; SV, 28 000 h-1‚mL‚g-1‚cat-1. Table 2. Effect of Various Oxidized-Diamond-Supported Metal Catalysts on the Conversion and Product Concentrations for the CO2 Reforming of CH4a conversion/% nickel/oxidized diamond palladium/oxidized diamond ruthenium/oxidized diamond rhodium/oxidized diamond cobalt/oxidized diamond iridium/oxidized diamond platinum/oxidized diamond iron/oxidized diamond

yield/%

CH4

CO2

H2

CO

H2:CO ratio

28.0 27.5 26.6 25.8 22.3 16.4 4.2 2.6

31.8 23.1 31.8 30.1 28.8 23.3 5.7 0.3

26.2 13.7 13.7 12.2 8.7 7.2 0.4 0.0

29.9 21.9 32.3 26.6 22.6 20.0 4.3 0.3

0.88 0.65 0.44 0.47 0.39 0.36 0.11 0.03

a Conditions were as follows: prior to the reaction, catalysts were reduced with H2 at 873 K for 1 h; catalyst amount, 100 mg; silica sand amount, 300 mg; flow rate, 60.0 mL/min (CH4/CO2 ) 1.0); reaction temperature, 873 K; metal loading level, 5.0 wt %; and reaction time, 0.5 h.

gas shift reaction (reaction 4), as evidenced by the considerable formation of CO2.

CO + H2O a CO2 + H2

0 (∆H298 ) -41 kJ/mol) (4)

Carbon Dioxide Reforming of Methane. Table 2 shows the effects of loaded metal on the activity order at 873 K, as follows: Ni > Pd g Ru g Rh > Co g Ir > Pt > Fe. This order is the same as that observed in the partial oxidation of CH4, except for cobalt (see Figure 1). Nickel-loaded oxidized-diamond catalysts gave the highest CH4 conversion, with a H2:CO ratio of 1, as expected from the stoichiometry of the reaction (reaction 3). When palladium, ruthenium, rhodium, and cobalt were loaded, comparatively higher CH4 conversions were obtained. H2:CO ratios were lower than the stoichiometric ratio, possibly because of the reverse watergas shift reaction shown below:

CO2 + H2 a CO + H2O

0 (∆H298 ) +41 kJ/mol) (5)

Figure 4. Effect of the nickel loading level of the oxidizeddiamond-supported catalyst on CH4 conversion for the CO2 reforming of CH4. Reaction conditions were as follows: prior to the reaction, catalysts were reduced with H2 at 873 K for 1 h; temperature, 873 K; reaction time, 20 min; CH4:CO2 ratio, 1:1; total flow rate, 60 mL/min; catalyst amount, 100 mg; SV, 36 000 h-1‚mL‚g-1‚cat-1.

The excess CO may have come from this reaction, with H2 being consumed to increase CO2 conversion. This reaction seems to be faster than the CH4-CO2 reaction. Table 3 shows the specific surface areas of the catalysts and the catalytic activities in the CO2 reforming of CH4 over nickel (5 wt %)-loaded catalysts. The order of the catalytic activity in the different supports at 873 K was La2O3 > SiO2, oxidized diamond > MgO > Al2O3 > TiO2 . activated carbon. In the case of Al2O3, oxidized diamond, and Y2O3, CH4 conversions were lower than that of steam reforming. When La2O3, SiO2, and oxidized diamond were used as supports, these catalysts afforded high CH4 conversions to CO and H2. Figure 4 shows the effects of the nickel loading levels on the CO2 reforming of CH4 using a nickel/oxidizeddiamond catalyst. The reaction occurred with a loading level as low as 0.5 wt % and a CH4 conversion of ca. 18%. Further increases in the loading level to 3 wt % increased the CH4 conversion to 33%. Figure 5 shows the temperature dependence of the CO2 reforming of CH4 over the nickel (3 wt %)/oxidizeddiamond catalyst. The reaction did not proceed at 673 K, and the CO2 reforming of CH4 occurred at >723 K. The amounts of CO and H2 increased as the reaction temperatures increased. At 1073 K, nickel (3 wt %)/

Synthesis Gas Production from Methane

Energy & Fuels, Vol. 17, No. 4, 2003 975 Scheme 1. Proposed Reaction Pathway for CO2 Reforming of CH4 over the Oxidized-Diamond-Supported Nickel Catalyst.

Figure 5. Effect of temperature on the CO2 reforming of CH4 over the oxidized-diamond-supported nickel catalyst. Reaction conditions were as follows: temperature, 873 K; reaction time, 20 min; nickel loading level, 3.0 wt %; CH4:CO2 ratio, 1:1; total flow rate, 60 mL/min; catalyst amount, 100 mg; SV, 36 000 h-1‚mL‚g-1‚cat-1.

Figure 6 shows the effects of time-on-stream on the CO2 reforming of CH4 at 923 and 973 K over nickel (3 wt %)/oxidized-diamond and nickel (5 wt %)/SiO2 catalysts. The nickel/oxidized-diamond catalyst maintained initial catalytic activity for 10 h with a high CH4 conversion. Above 923 K, carbon deposition was not observed on the oxidized-diamond-supported nickel catalyst. In contrast, the nickel/SiO2 catalyst showed gradual deactivation over a period of 10 h, and carbon deposition was observed after the reaction. According to scanning electron microscopy (SEM), whisker-type carbon deposition occurred on the SiO2-supported nickel catalyst. The nickel catalyst emerged as being the most practical, because of its high activity and low cost. The major technical problem with nickel catalysts is the whisker-type carbon deposition that occurs on the catalysts, which may lead to plugging of the reformer tubes. Various nickel-loaded catalysts have been shown to achieve CO2 reforming of CH4 to synthesis gas, with high conversions occurring at high temperatures, under industrially applicable short residence times. However, the calculated gas compositions neglect consideration of the thermodynamical carbon deposition. Carbon deposition through the Boudouard reaction (reaction 6) or methane decomposition (reaction 7) is thermodynamically favored below ∼1173 K.2 Figure 6. Effect of time-on-stream on CH4 conversion for the CO2 reforming of CH4 over a nickel-loaded catalyst ((a) Ni/ SiO2, 5.0 wt % nickel loading level, and (b) nickel/oxidized diamond, 3.0 wt% nickel loading level). Reaction conditions were as follows: temperature, 873 K; CH4:CO2 ratio, 1:1; total flow rate, 60 mL/min; catalyst amount, 100 mg; SV, 36 000 h-1‚mL‚g-1‚cat-1.

oxidized-diamond catalyst afforded a CH4 conversion of 84%. Only a small amount of carbon deposition was observed at 873 K. However, no carbon deposition was observed at >923 K.

2CO a C + CO2 CH4 a C + 2H2

0 (∆H298 ) -173 kJ/mol) (6) 0 (∆H298 ) +75 kJ/mol)

(7)

The support effects of nickel-loaded catalysts affect not only the CH4 conversion, but also carbon deposition. No carbon deposition was observed in the oxidizeddiamond supported case, with the oxidized-diamond supports exhibiting high activities at >923 K. In contrast, carbon deposition was observed with Al2O3 and SiO2 supports at 873 K.

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Scheme 1 shows plausible interpretations of such a synergism between nickel and oxidized diamond in the CO2 reforming of CH4. Generally, in the CO2 reforming of CH4 over various nickel-based catalysts, the reaction mechanism is as follows: (i) H2 and deposited carbon are formed by the decomposition of CH4 over nickel metal; (ii) deposited carbon is oxidized by CO2, and CO is formed. However, in many nickel-based catalysts, the CH4 decomposition reaction is faster than the CO2 oxidation of deposited carbon, and the carbon deposition cannot be avoided. However, carbon deposition did not occur over the nickel/oxidized-diamond catalyst at >923 K. In the nickel/oxidized-diamond catalyst, an oxygen species with higher reactivity existed near active species and seemed to promote oxidation of the deposited carbon. Oxygen on the oxidized-diamond surface may be supplied by the CO2. Such mutual promotion is believed to take place during the CH4 decomposition reaction on nickel metal and may explain the observed apparent synergism or promoting effect of CO2 activation on the CO2 reforming of CH4.

Nakagawa et al.

Conclusions The partial oxidation, steam reforming, and CO2 reforming of CH4 with nickel-loaded catalysts were strongly affected by support materials. Nickel/oxidized-diamond catalysts exhibited high catalytic activity for the partial oxidation, steam reforming, and CO2 reforming of CH4 to synthesis gas. A synergistic effect between nickel and oxidized diamond seems to prevent carbon deposition. The support effects of nickel-loaded catalysts affect not only the CH4 conversion, but also carbon deposition. When oxidized diamond was used as a catalytic support material, carbon deposition was suppressed in CH4-tosynthesis-gas reactions. Acknowledgment. This work was financially supported by a Grant-in-Aid for Scientific Research (B) (No. 14350429) from the Japan Society for the Promotion of Science (JSPS). K.N. is grateful for his fellowship for Young Scientists from JSPS. EF0300040