Energy & Fuels 2007, 21, 3513–3519
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Performance of Sulfur Tolerant Reforming Catalysts for Production of Hydrogen from Jet Fuel Simulants Amanda C. McCoy,† Martin J. Duran,† Abdul-Majeed Azad,† Sudipta Chattopadhyay,‡ and Martin A. Abraham*,† Department of Chemical and EnVironmental Engineering, The UniVersity of Toledo, Toledo, Ohio 43606, and Catacel Corp., 7998 Gotham Road, GarrettsVille, Ohio 44231 ReceiVed March 3, 2007. ReVised Manuscript ReceiVed August 2, 2007
The development of robust desulfurizers and new reforming catalysts is a critical path for the use of jet fuels in powering the commercial growth of fuel cell systems for air and military applications. The presence of high concentrations of sulfur-containing organic compounds leads to rapid deactivation of traditional reforming catalysts, and removal of the sulfur components from the fuel through adsorptive methods is not practical for long term operations. The current work describes the use of several ceria-based catalyst compositions that were studied to assess their performance based on the formation of hydrogen and product yield from a fuel consisting of toluene and thiophene. The effect of noble metals, metal oxide additives, and stabilized ceria supports on the performance of the catalyst was studied. The addition of selected components led to higher yields or greater stability; combinations of these additives were not necessarily synergistic. Interestingly, the presence of sulfur in the fuel was shown to enhance the initial activity of catalysts containing rhodium. Analysis in terms of the kinetic rates of reaction and deactivation illustrated the effects of these additives and provided insight into the design of a more highly stable steam reforming catalyst for production of hydrogen from jet fuel.
Introduction Because of the United States’ dependency on oil, the scarcity of oil resources, and an increasing need for sustainable energy resources, the focus of much research in the past two decades has been finding new power sources that could supply a cleaner and more efficient energy source. Hydrogen is considered a viable energy carrier in various industries but is of particular interest for the automotive industry1,2 operated by fuel cells. Unfortunately, the lack of infrastructure, such as a network of hydrogen refueling stations, has slowed the growth of the hydrogen economy. Reliable and cost effective means of hydrogen production, distribution, and storage are not yet feasible.3 NASA envisions employing solid oxide fuel cells (SOFCs) running on jet fuel reformates for its uninhabited aerial vehicle (UAV) and low emission alternative power (LEAP) missions. Additionally, transatlantic and intercontinental commercial flights could also use power from these fuel cells to run equipment such as refrigeration units, air conditioning, and other auxiliary requirements. In these cases, jet fuel can be reformed on-board, and the power supplied by the processes would be efficient and cost effective. The U.S. military also wants to use fuel cells for similar reasons; however, their motivation is because fuel cells are relatively lightweight and can operate quietly and undetectably (due to zero emissions). Jet fuel, * Correspondingauthor.Phone:+14195304968.E-mail:martin.abraham@ utoledo.edu. † The University of Toledo. ‡ Catacel Corp. (1) Shinnar, R. Technology in Society 2003, 25, 455–476. (2) Rifkin, J. The Hydrogen Economy. The EnVironmental Magazine 2003 (January). (3) Gieger, S. Fuel Cell Today 2003, (February), 1–10.
specifically grades JP-8 (military) and Jet A (commercial), is already on board in many applications, and if it can be reformed and used in the field, military missions can be enhanced and made more effective. Reports indicate that an SOFC operating with jet fuel can achieve an efficiency up to 60–70%.4 The biggest challenge in the reformation of jet fuel arises from the presence of sulfur in high concentrations, which severely poisons the reforming catalysts.4 The U.S. government regulates the level of sulfur in the various types of jet fuels to be a maximum of 3000 ppm.5 If the sulfur is not removed from the fuel stream, it leads to the formation of hydrogen sulfide (H2S), which poisons the anode in the fuel cell stack, leading to low SOFC efficiency and decreased performance. NASA has prioritized two main areas of research with respect to using logistic fuels for fuel cells: the desulfurization of jet fuel and the development of sulfur-tolerant reforming catalysts for jet fuels. One method for converting hydrocarbon fuels into hydrogen is steam reforming (SR) at elevated temperatures (750–850 °C),6,7 usually with nickel catalysts8 and high steam-to-carbon (H2O/C) ratios. The water gas shift reaction is then used to convert additional CO into CO2. Nickel has been the most suitable metal for many years for SR of hydrocarbons.9 Two (4) Song, C. Catal. Today 2002, 77, 17–49. (5) Song, C.; Ma, X. Appl. Catal., B 2003, 41, 207–238. (6) Rostrup-Nielson, J. R.; Rostrup-Nielson, T. Catal. Sci. Technol. 1984, 5, 1. (7) Aasberg-Peterson, K.; Bak Hansen, J. H.; Christensen, T. S.; Dybkjaer, I.; SierChristensen, P.; Stub Neilsen, C.; Winter Madsen, S. E. L.; Rostrup-Nielson, J. R. Appl. Catal., A 2001, 221, 379. (8) Renner, H.-J. Ullman’s Encyclopedia of Industrial Chemistry, Vol. A12, 5th ed.; Wiley-VCH: Weinheim, 1989; pp 204–214. (9) Ming, Q.; Allen, L.; Healey, T.; Irving, P.; Thompson, W. Abstracts of the American Chemical Society 2001, 222, U472–U473; 88-FUEL Part 1, AUG 2001.
10.1021/ef070111k CCC: $37.00 2007 American Chemical Society Published on Web 10/24/2007
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major problems result, however, with nickel catalysts: coke formation10 and sulfur poisoning.11,12 Precious metal-based catalysts, such as ruthenium and rhodium, have been reported to be more effective than nickel for SR because they tend to prevent carbon deposition on the surface.11–13 Palladium and platinum catalysts are also being extensively studied,13 with promising results when Pd is supported on ceria.11 Ru/Al2O3CeO2 catalysts were tested14 with both desulfurized kerosene (C10H22 with 50%) for longer duration than their palladium counterparts; most of the Pd-loaded formulations never attained H2 yields of 50% even when their initial activity was relatively high and the deactivation rate low. Catalytic systems with bimetal loading performed better, in terms of both activity and stability, than those containing either of the two noble metals alone. Catalysts modified with metal oxide additives were shown to have improved performance relative to their unmodified counterparts. The addition of CuO improved hydrogen yield but was not effective in enhancing the stability. On the other hand, Y2O3 increased the catalyst stability, but the overall yields were considerably lower. A combination of oxide promoters gave varying performance depending on the nature of the noble metal with which it was associated. Thus, combining rhodium with two promoters gave an overall performance which was better than that compared to the combination with palladium. Initial activity and deactivation rate constants were calculated assuming first order kinetics and first order deactivation based on an integral packed bed reactor model. Qualitatively, this analysis confirmed that Rh-based catalysts give superior performance in the presence of sulfur rather than in a sulfur-free environment. The initial activities as well as the hydrogen yield are higher (>50%). However, these catalysts also showed signs of enhanced deactivation, although they still maintained high hydrogen generation levels. Palladium-containing catalysts performed well without sulfur but deactivated when sulfur was introduced. The hydrogen yield was very low, and the activities reflected that result. EF070111K
