JOURNAL OF CATALYSIS 180, 85–100 (1998) ARTICLE NO. CA 982260 New Catalysts for the Conversion of Methane to Synthesis Gas: Molybdenum and Tungsten Carbide John B. Claridge, 1 Andrew P. E. York, Attila J. Brungs, Carlos Marquez-Alvarez, 2 Jeremy Sloan, Shik Chi Tsang, 3 and Malcolm L. H. Green 4 The Catalysis Centre, Inorganic Chemistry L aboratory, University of Oxford, South Parks Road, Oxford OX 1 3QR, United Kingdom Received June 12, 1998; revised August 17, 1998; accepted August 18, 1998 High-surface-area molybdenum and tungsten carbide materials, synthesised by the temperature programming reduction of the rele- vant metal oxide with methane/hydrogen, are highly efficient cata- lysts for the conversion of methane to synthesis gas, via the steam reforming, dry reforming, orpartial oxidation processes. The activi- ties of the carbides were found to be comparable to those of elemental iridium and ruthenium (well known to be active noble metal cata- lysts forthe reforming of methane), and the conversion and product distribution were in accord with those calculated from the thermo- dynamic equilibria. At ambient pressure the carbides deactivated, in all the processes, due to the oxidation of the catalyst to MO 2 , while operation at elevated pressure (8 bar) resulted in stabilisation of the carbide and no catalyst deactivation for the duration of the experi- ments (72 h). HRTEManalysis showed that no macroscopic carbon was deposited on the catalysts during the catalytic reactions. The deactivation rate of the carbides reflected the strength of the oxidant used: oxygen > water ∼ = carbon dioxide. A deactivation mechanism, via the insertion of O ∗ resulting in oxide terraces is discussed, and two possible mechanisms for the production of synthesis gas by the methane dry reforming reaction overmetal carbides are proposed: noble metal type and redox type. c  1998 Academic Press INTRODUCTION Natural gas, the main component of which is methane, is an abundant fossil fuel resource found all over the world and is predicted to outlast oil reserves by a significant mar- gin (1). Most of these reserves, however, are situated in areas remote from the centres of highest energy consump- tion (2), and the costs of compression, transportation and storage make methane an unattractive proposition as an energy source (3). To make methane economically more 1 Current address: Department of Chemistry and Biochemistry, Uni- versity of South Carolina, Columbia, SC 29208. 2 On leave from Instituto de Cat´ alisis y Petroleoqu´ ımica, CSIC, Cam- pus Cantoblanco, 28049 Madrid, Spain. 3 Current address: The Catalysis Research Centre, Department of Chemistry,UniversityofReading,Whiteknights,ReadingRG66AD,UK. 4 To whom correspondence should be addressed. E-mail: malcolm. green@chem.ox.ac.uk. viable, a large amount of research into the conversion of methane to liquids or higher hydrocarbons has been car- ried out. For example, the direct oxidative conversions of methane into methanol(4–7),formaldehyde (8,9),benzene and other aromatics (10, 11), and propanal (12, 13) have all been demonstrated, but unfortunately the yields achieved are too low or the processes too costly. The most promis- ing of the direct conversion routes is the oxidative coupling of methane to ethane and ethene. However, an inherent mechanistically imposed limit to the maximum C 2 yield ap- pears to exist, limiting the maximum yield to about 30% (14, 15); in fact, the highest reported yields in the literature are lower than this, at around 20–25% (16–20), and do not meet industrialrequirementsat the current price ofoil(21). Current industrial use of methane as a chemical feed- stock proceeds by initial conversion to carbon monoxide andhydrogen(synthesisgas)byeithersteamreforming(22) [1] (the most widespread process) or by dry reforming [2]. In addition, while currently limited as an industrial process, partial oxidation [3] has recently attracted much attention due to significant inherent advantages(e.g.,exothermicity). The synthesisgasproduced isthen used in downstream pro- cesses, such as methanol synthesis, Fischer–Tropsch synthe- sis, or ammonia synthesis. CH 4 + H 2 O = CO + 3H 2 H θ 298 =+206kJ mol −1 [1] CH 4 + CO 2 = 2CO + 2H 2 H θ 298 =+247kJ mol −1 [2] CH 4 + 1 2 O 2 = CO + 2H 2 H θ 298 =−38 kJ mol −1 [3] Nickel catalysts are used commercially for both methane steam and dryreformingreactions(22),while partialoxida- tion iscarried out autothermally(23).However,nickelalso catalyses carbon formation, via methane decomposition [4] and CO disproportionation (Boudouard reaction) [5], both of which lead to catalyst deactivation and plugging of the reformer tubesbycarbon deposits.Excessesofoxidants are used in order to prevent carbon deposition, but this in- creasesthe H 2 /CO and/or CO 2 /CO ratios;low H 2 /CO and CO 2 /CO ratiosare required for optimalfurther conversion 85 0021-9517/98 $25.00 Copyright c  1998 by Academic Press All rights of reproduction in any form reserved.