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Energy & Fuels 2005, 19, 1708-1716
Low-Temperature Reforming of Ethanol over Copper-Plated Raney Nickel: A New Route to Sustainable Hydrogen for Transportation David A. Morgenstern* and James P. Fornango Monsanto Company, 800 North Lindbergh Boulevard, St. Louis, Missouri 63167 Received November 29, 2004. Revised Manuscript Received May 4, 2005
Raney nickel can be plated with a high loading of copper (28%) to produce a novel coppernickel catalyst, which retains a Raney-type structure. A simple two-step aqueous procedure was used. The catalyst exhibits high activity for low-temperature (250-300 °C) reforming of ethanol to methane, carbon monoxide, and hydrogen. Stable activity for over 400 h was achieved with no detectable methanation. The catalyst is significantly less active for methanol reforming and has low water-gas shift activity. The kinetics fit a two-step model in which ethanol is dehydrogenated to acetaldehyde in a first-order reaction with an activation energy of 149 kJ/mol followed by the decarbonylation of acetaldehyde, which is also first-order. The low-temperature ethanol reforming pathway has not previously been considered as a route to hydrogen for fuel cell vehicles because it leads to formation of only 2 mol of hydrogen/mol of ethanol versus 6 mol of hydrogen for traditional, high-temperature reforming. We suggest that capturing the energy value of the methane produced by low-temperature reforming in an internal combustion engine, combined with use of the waste heat from the engine to heat the reformer, will close the efficiency gap between the two pathways. Vehicles powered in this way will be less expensive since a much smaller fuel cell unit is required and will benefit from the stability and low cost of low-temperature ethanol reforming. Ethanol is a sustainable fuel, derived from biomass, which will not contribute to global warming. The low-temperature reforming pathway for ethanol may therefore represent a technically and economically attractive pathway to ethanol-fueled vehicles.
Introduction The proton exchange membrane (PEM) fuel cell is the only fuel cell technology currently available that meets the requirements for vehicular power.1 PEM cells have high power densities2 and lower operating temperatures (typically 80 °C) than other fuel cell types.3 The low temperature is required for reasonable startup times. PEM fuel cells, operated on pure hydrogen, exhibit stable operation for thousands of hours.4 Among the most daunting challenges impeding the commercialization of fuel cell vehicles are the development of a sustainable source of hydrogen5 and the cost of the fuel cell unit.4 A Department of Energy (DOE) study placed the cost of PEM fuel cells at $324/kW in 2001.6 Although the DOE target is $45/kW in 2008, a manufacturing cost study funded by the DOE projects * Corresponding author e-mail:
[email protected]; phone: (314)694-8786. (1) EG&G Services; Parsons. Inc.; Science Applications International Corp. Fuel Cell Handbook, 5th ed.; U.S. Department of Energy, Office of Fossil Energy, National Energy Technology Laboratory: Morgantown, WV, 2000; p 3-1. (2) Ballard Power System’s 1.2 kW Nexa module occupies 46 L. See http://www.ballard.com/pdfs/power%20gen/Nexa_Power_Module.pdf. (3) Chem. Eng. Prog. 1999, March, 59-66. Startup time is the most important issue. PEM systems are the fastest, but even so, United Technology’s 50 kW PEM power plant takes 45 min to reach 25% power. See http://www.cartech.doe.gov/research/fuelcells/power-system2.html. (4) Ralph, T. R. Platinum Metals Rev. 1997, 41, 102-113. (5) Schrope, M. Nature 2001, 414, 682-684. (6) Results of a 2002 Arthur D. Little study for the U.S. DOE. See http://www.cartech.doe.gov/research/fuelcells/cost-model.html.
a cost of $170/kW for the stack at high production volume.7 A family sedan is usually assumed to require about 50 kW of fuel cell capacity. Its fuel cell stack would cost $8500 to manufacture. There is also the dilemma of either equipping fuel cell vehicles with pressurized hydrogen tanks or with a fuel processor capable of converting liquid fuels such as methanol to hydrogen. The use of compressed hydrogen limits vehicle range and may entail safety concerns.8,9 On-board reforming of liquid fuels enables fuel cell vehicles to achieve ranges comparable to gasoline-fueled automobiles, but the reformer must be very durable, at least 5 years or 80 000 km.10 Other issues such as size and weight, resistance to vibration, cold start, and transient response complicate the development of practical reformers for fuel cell vehicles.11 In addition, hydrocarbons and ethanol require reforming temperatures in excess of 600 °C, although methanol can be reformed at 250-300 °C. PEM fuel cells are currently constrained to operate at 90 °C or below. The energy lost as heat when cooling the reformate stream imposes (7) Based on a 50 kW power plant. The cost of the entire system including a fuel reformer, air and water loops, and controls was projected at $262/kW. Study by Directed Technology Inc, April 2002. See http://www.cartech.doe.gov/research/fuelcells/mfg-costs.html. (8) Appleby, A. J. Sci Am. 1999, July, 74-79. (9) Wald, M. L. Sci Am. 2004, May, 66-73. (10) See Agrell, J.; Lindstro¨m, B.; Pettersson, L. J.; Ja¨rås, S. G. In CatalysissSpecialist Periodical Reports No. 16; Spivey, J. J., Ed.; Royal Society of Chemistry: Cambridge, 2002; pp 67-132. (11) Pettersson, L. J.; Westerholm, R. Int. J. Hydrogen Energy 2001, 26, 243-264.
10.1021/ef049692t CCC: $30.25 © 2005 American Chemical Society Published on Web 06/25/2005
Reforming of Ethanol over Cu-Plated Raney Ni
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an energy efficiency penalty. The U.S. DOE recently concluded that the efficiency expected from fuel cell vehicles utilizing on-board high-temperature reforming is no better than that for hybrid vehicles and recommended termination of support for research in this area.12 Several other considerations weigh against the use of hydrocarbon fuels as a source of hydrogen, chief among them sustainability and the presence of sulfur, which poisons the fuel cell electrocatalyst. The sulfur specification for PEM cells is
