2066 Catal. Sci. Technol., 2012, 2, 2066–2076 This journal is c The Royal Society of Chemistry 2012 Cite this: Catal. Sci. Technol., 2012, 2, 2066–2076 A potassium-promoted Mo carbide catalyst system for hydrocarbon synthesis Dai-Viet N. Vo ab and Adesoji A. Adesina* a Received 6th June 2012, Accepted 7th August 2012 DOI: 10.1039/c2cy20385e Potassium-promoted Mo carbide catalysts prepared by temperature-programmed carburization with H 2 –C 3 H 8 have been evaluated for Fischer–Tropsch synthesis (FTS). Temperature- programmed carburization of MoO 3 –Al 2 O 3 with a mixture of H 2 –C 3 H 8 was a two-stage conversion involving the transformation of Mo oxide to the final carbide phase via an intermediate oxycarbide phase. Compensation effect and isokinetic relationship were observed for both oxycarbide and carbide phase formation suggesting that a similar topotactic mechanism was involved in the solid phase transformation. CO seemed to adsorb more strongly than H 2 as implicated by its higher heat of desorption and site concentration. Both acid and basic centres were detected on the Mo carbide surface. The K-promoter increased site concentration for both weak and strong basic sites. The relationship between FT activity and K loading paralleled the trend in CO 2 heat of desorption with K addition. FT activity attained a maximum at about 3%K loading. CO hydrogenation over the Mo carbide catalyst was most suitably described by an enolic intermediate mechanism. A generalized kinetic model was derived for predicting the reaction rate for individual hydrocarbons as a function of chain growth propagation and olefin-to- paraffin ratio. Introduction Following the original report by Levy and Boudart on the Pt-like characteristics of the Mo carbide catalyst, 1 it has been considered a promising catalyst for the Fischer–Tropsch synthesis (FTS) of higher hydrocarbons. 2,3 Other studies have shown that MoC 1x (0 r x o 1) exhibited high olefin selectivity, carbon and sulphur resilience. 4,5 This makes it an attractive catalyst for natural gas conversion to clean fuels since most gas fields contain sulphur compounds. The common method for synthesizing Mo carbide with high surface area is temperature-programmed carburization between MoO 3 and H 2 –CH 4 at temperature greater than 1073 K. The physicochemical properties and catalytic performance of metal carbide are however influenced by the type of carburizing agent and conditions. 6 The utilization of higher alkanes (C 2+ ) as a carbon source has been shown to reduce the carburization temperature and improve the surface area 7 albeit with a potential for high carbon deposition. H 2 –C 3 H 8 was first used to synthesize an Al 2 O 3 -supported Mo carbide catalyst with a surface area of 92–204 m 2 g cat 1 from a metal sulphide precursor with an optimum of H 2 :C 3 H 8 = 5 : 1 for negligible surface carbon resilience on the catalyst. 8 Conventional FT catalysts are generally promoted by alkali addition. Thus, the possible substitution of expensive Pt noble metal with Mo carbide in FT catalysis may also benefit from similar promotion. Therefore, in this study, the effect of K loading (1–5 wt%) on supported Mo carbide prepared by temperature-programmed carburization using the H 2 –C 3 H 8 mixture has been investigated. Specifically, the objective of this paper was to seek the correlation between K loading and the physicochemical properties as well as FT reaction metrics. Experimental 10%MoC 1x /Al 2 O 3 catalysts promoted with different levels of K (1–5 wt%) were prepared by a co-impregnation method. Calculated amounts of aqueous (NH 4 ) 6 Mo 7 O 24 4H 2 O and K 2 CO 3 solution precursor were mixed with g-Al 2 O 3 (pre-treated at 973 K in air with 5 K min 1 for 6 h to ensure thermal stability) and stirred for 3 h. The resulting slurry was subsequently dried in an oven for 16 h at 403 K. Temperature-programmed carburiza- tion between the alumina-supported MoO 3 and a mixture of H 2 :C 3 H 8 = 5 : 1 at 50 mL min 1 was carried out in a computer-controlled fixed-bed microreactor at 973 K using a heating rate of 10 K min 1 for 2 h. FT evaluation was performed in situ in the same reactor by switching to a H 2 : CO feed mixture (7 different ratios) at 453–473 K and atmospheric pressure. a Reactor Engineering & Technology Group, School of Chemical Engineering, The University of New South Wales, Sydney, 2052, Australia. E-mail: a.adesina@unsw.edu.au; Fax: +61 2 9385 5966; Tel: +61 2 9385 5268 b Chemical Engineering Program, Texas A&M University at Qatar, P.O. Box 23874, Doha, Qatar Catalysis Science & Technology Dynamic Article Links www.rsc.org/catalysis PAPER