Steam Reforming on Transition-Metal Carbides from Density-Functional Theory
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Steam Reforming on Transition-Metal Carbides from Density-Functional Theory Aleksandra Vojvodic
Received: 9 September 2011 / Accepted: 31 March 2012 / Published online: 21 April 2012 Ó Springer Science+Business Media, LLC 2012
Abstract A screening study of the steam reforming reaction on clean and oxygen covered early transitionmetal carbides surfaces is performed by means of densityfunctional theory calculations. It is found that carbides provide a wide spectrum of reactivities, from too reactive via suitable to too inert. Several molybdenum-based systems are identified as possible steam reforming catalysts. The findings suggest that carbides provide a playground for reactivity tuning, comparable to the one for pure metals. Keywords DFT Steam reforming Carbides Surfaces Reactivity trends
1 Introduction Steam reforming is an important industrial process, where natural gas (CH4) is converted into synthesis gas (CO and H2) according to the overall reaction CH4 þ H2 O CO þ 3H2 :
ð1Þ
The synthesis gas is subsequently transformed into more valuable chemicals, such as ammonia, methanol, and A. Vojvodic Department of Applied Physics, Chalmers University of Technology, Go¨teborg 412 96, Sweden A. Vojvodic (&) SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USA e-mail: [email protected] A. Vojvodic Department of Chemical Engineering, Stanford University, Stanford, CA 94305, USA
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diesel. Since steam reforming acts as a source of hydrogen, it is also potentially important for any emerging hydrogen economy. For a detailed review of the steam reforming process and its applications see for example Ref. [1]. Commercially the steam reforming reaction is conducted over a Ni-based catalyst due to the relatively low cost and good activity of nickel. This reaction has been studied in detail on the close-packed Ni(111) surface and on the stepped Ni(211) surface by means of density-functional theory (DFT) [2]. The major technological challenge for Ni catalysts is the formation of carbonaceous deposits, termed coke, that lead to catalyst deactivation. In Refs. [2, 3] it was established that the step edges on Ni surfaces act as growth centers for graphite. Other transition metals (TM), such as Ru, Rh, Pd, Ir, and Pt also show high activity and selectivity towards steam reforming [4] and have a high resistance against carbon formation. However, these materials are scarce in nature and expensive. Therefore new materials that are resistant to carbon formation are needed. Transition-metal carbides (TMC’s) have gained quite some attention since Levy and Boudart reported that they have ‘‘platinum-like behavior’’ for certain reactions [5]. The starting material for the production of carbides is cheap and abundant and therefore it has been suggested that they can replace the noble metals in catalysis. The main problem with the carbides has been to produce materials with sufficiently high surface area for them to be interesting for catalytic
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