Synthesis Gas Conversion Over Molybdenum-Based Catalysts Promoted by Transition Metals

Catalysts consisting of transition metals (Ni, Co, Cu and Ru) supported on molybdenum oxide synthesized by atomic layer deposition (ALD) on silica were studied for synthesis gas conversion at 523 K and a pressure of 580 psi. Transition-metal-promoted Mo-based catalysts (M/MoO3/SiO2) showed different selectivity patterns from the transition metal supported on silica (M/SiO2). All molybdenum-based catalysts displayed a similar selectivity pattern, consisting of 15%~20% CH4, 30~40% C2+ hydrocarbons, and 35~40% oxygenates. The addition of transition metals to molybdenum oxide promoted the catalytic activity by an order of magnitude. Temperature program reduction indicated hydrogen spillover from the transition metals to molybdenum species. H2-D2 exchange rate measurements showed that the addition of the transition metal enhanced the rate of H2 dissociation on the catalyst. CO chemisorption measurements indicated that transition-metal-promoted molybdenum catalysts consist of similar amount of strong adsorption sites, which may originate from the reduced transition metal, and weak adsorption sites, which may originate from reduced molybdenum oxides. Here, a dual-site mechanism is suggested in which low-valent molybdenum species dissociate CO and generate CHx groups that are hydrogenated to hydrocarbons or react with adsorbed CO on higher-valent Mo sites to form higher alcohols. Ni, Co and Ru are able to generate CHx groups and enhance the production of C2+ oxygenates whereas all of the transition metals studied are able to provide sites for H2 dissociation and H spillover to molybdenum oxide, leading to further enhancement in catalytic activity compared to MoOx/SiO2.