Energy & Fuels 2008, 22, 2183–2187
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Carbon Dioxide Reforming of Methane by Ni/Co Nanoparticle Catalysts Immobilized on Single-Walled Carbon Nanotubes Yongquan Qu, Alexander M. Sutherland, and Ting Guo* Department of Chemistry, UniVersity of California, One Shields AVenue, DaVis, California 95616 ReceiVed June 21, 2007. ReVised Manuscript ReceiVed May 5, 2008
We investigated the catalytic reaction of CO2 reforming of methane (CRM) using as-made Ni/Co nanoparticles believed to be immobilized at the tips of single-walled carbon nanotubes (SWNTs). It was found that under the reforming conditions used in this work the turnover rate of the Ni/Co-SWNT catalytic material for CRM was tens to hundreds of times greater than the best existing CRM catalysts. Combined with the long lifetime of this new catalyst, its total turnover times for CRM were significantly greater than those of any other existing CRM catalysts.
Introduction Carbon dioxide reforming of methane (CRM) is an important process that converts two of the most abundant greenhouse gases in the atmosphere, carbon dioxide and methane, into hydrogen and carbon monoxide, also known as syngas.1 CRM is one of the most effective methods to simultaneously remove these two greenhouse gases in one chemical reaction while generating hydrogen and carbon monoxide with the optimal ratio that can be used in the Fischer-Tropsch synthesis to produce long-chain liquid fuels.2–7 Alternatively, it is possible to use the two products separatelyshydrogen in hydrogen fuel cells and CO in other reactions such as disproportionation reactions to produce carbon nanotubes.8–10 This latter option has the advantage of sequestrating the carbon and thus creating a clean energy source in the future. However, performing CRM with traditionally prepared nanoparticle catalysts such as Ni nanoparticles has met with several * Corresponding author. E-mail:
[email protected]. Tel.: (530) 754-5283. Fax: (530) 752-8995. (1) Ross, J. Natural gas reforming and CO2 mitigation. Catal. Today 2005, 100 (1-2), 151–158. (2) Ashcroft, A.; Cheetham, A.; Green, M.; Vernon, P. Partial Oxidation of Methane to Synthesis Gas-Using Carbon-Dioxide. Nature 1991, 352 (6332), 225–226. (3) Richardson, J.; Paripatyadar, S. Carbon-Dioxide Reforming of Methane with Supported Rhodium. Appl. Catal. 1990, 61 (2), 293–309. (4) Ruckenstein, E.; Hu, Y. Near 100-Percent CO Selectivity in CH4 Direct Catalytic-Oxidation at Low-Temperatures (Less-Than-700-DegreesC) under Unsteady-State Conditions. Catal. Lett. 1995, 35 (3-4), 265– 269. (5) Bradford, M.; Vannice, M. CO2 reforming of CH4. Catal. ReV.sSci. Eng. 1999, 41 (1), 1–42. (6) Wang, S.; Cao, D.; Li, Y.; Wang, J.; Jiao, H. CO2 reforming of CH4 on Ni(111): A density functional theory calculation. J. Phys. Chem. B 2006, 110 (20), 9976–9983. (7) Song, C. Global challenges and strategies for control, conversion and utilization of CO2 for sustainable development involving energy, catalysis, adsorption and chemical processing. Catal. Today 2006, 115 (14), 2–32. (8) Armor, J. Catalysis and the hydrogen economy. Catal. Lett. 2005, 101 (3-4), 131–135. (9) Rostrup-Nielsen, T. Manufacture of hydrogen. Catal. Today 2005, 106 (1-4), 293–296. (10) Bronikowski, M. J.; Willis, P. A.; Colbert, D. T.; Smith, K. A.; Smalley, R. E. Gas-phase production of carbon single-walled nanotubes from carbon monoxide via the HiPco process: A parametric study. J. Vac. Sci. Technol. AsVac. Surf. Films 2001, 19 (4 PT2), 1800–1805.
severe challenges.11–14 First, the commonly used catalysts tend to sinter at the high reaction temperatures. Within a relative short time, Ni or Ni alloy nanoparticles may quickly aggregate and lose their catalytic activity. In addition, some of the substrate particles may deteriorate as well, reacting with other substrate particles or the catalysts.15 Another related issue is carbon deposition on nanoparticle catalysts, which becomes more problematic when the nanoparticles are relatively large, i.e., over 6-7 nm.16 Both CO disproportionation reaction, which is an exothermic reaction and prefers to happen at lower temperatures,17 and methane decomposition reactions, which are endothermic and prefer to occur at higher temperatures, can cause carbon deposition. In either case, carbon deposits cover the surface of nanoparticles to practically destroy the catalytic activity. Several methods have been developed to overcome these two serious problems. Metal oxide substrates, for example, have been used to reduce carbon deposition.18–20 The redox properties and the high mobility of lattice oxygen of these oxides have been (11) Kroll, V.; Swaan, H.; Mirodatos, C. Methane reforming reaction with carbon dioxide over Ni/SiO2 catalyst 0.1. Deactivation studies. J. Catal. 1996, 161 (1), 409–422. (12) Choi, J.; Moon, K.; Kim, Y.; Lee, J.; Kim, C.; Trimm, D. Stable carbon dioxide reforming of methane over modified Ni/Al2O3 catalysts. Catal. Lett. 1998, 52 (1-2), 43–47. (13) Hu, Y.; Ruckenstein, E. An optimum NiO content in the CO2 reforming of CH4 with NiO/MgO solid solution catalysts. Catal. Lett. 1996, 36 (3-4), 145–149. (14) Nakagawa, K.; Anzai, K.; Matsui, N.; Ikenaga, N.; Suzuki, T.; Teng, Y.; Kobayashi, T.; Haruta, M. Effect of support on the conversion of methane to synthesis gas over supported iridium catalysts. Catal. Lett. 1998, 51 (3-4), 163–167. (15) Wang, S. B.; Lu, G. Q. A comprehensive study on carbon dioxide reforming of methane over Ni/gamma-Al2O3 catalysts. Ind. Eng. Chem. Res. 1999, 38 (7), 2615–2625. (16) Kim, J.; Suh, D.; Park, T.; Kim, K. Effect of metal particle size on coking during CO2 reforming of CH4 over Ni-alumina aerogel catalysts. Appl. Catal. AsGen. 2000, 197 (2), 191–200. (17) Souza, M.; Aranda, D.; Schmal, M. Coke formation on Pt/ZrO2/ Al2O3 catalysts during CH4 reforming with CO2. Ind. Eng. Chem. Res. 2002, 41 (18), 4681–4685. (18) Rezaei, M.; Alavi, S.; Sahebdelfar, S.; Yan, Z. Nanocrystalline zirconia as support for nickel catalyst in methane reforming with CO2. Energy Fuels 2006, 20 (3), 923–929. (19) Cui, Y.; Zhang, H.; Xu, H.; Li, W. Kinetic study of the catalytic reforming of CH4 with CO2 to syngas over Ni/alpha-Al2O3 catalyst: The effect of temperature on the reforming mechanism. Appl. Catal. AsGen. 2007, 318, 79–88.
10.1021/ef800130a CCC: $40.75 2008 American Chemical Society Published on Web 06/24/2008
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suggested to prevent carbon formation when these oxides are intimately mixed with nanoparticle catalysts.21 Metal oxide nanoparticle substrates have also been used to counter the sintering problem. In addition, decreasing the size of nanoparticles has been used to reduce the amount of carbon deposition. It has been shown that small nanoparticle catalysts (
