(771a) Synthesis Gas Conversion over Rh/Mo Catalysts Prepared By Atomic Layer Deposition

The catalytic conversion of synthesis gas to value-add products, including oxygenates and C₂₊ hydrocarbons, is a promising process to transform coal, natural gas and biomass into high value chemicals and transportation fuels. However, development of active catalysts with satisfactory selectivity remains challenging and a clear understanding of structure-reactivity relation remains to be explored. A Rh-based catalyst was developed in this work utilizing supports prepared by atomic layer deposition (ALD) of molybdenum and tungsten species on silica. Reaction kinetics studies showed that coating the silica support with molybdenum and tungsten species helped to suppress the methane selectivity and promote the overall conversion rate. Upon coating the silica support with 5 cycles of β-Mo₂C, the methane selectivity decreased from 32% (1%Rh/SiO₂) to 13% (1%Rh/5c-Mo₂C/SiO₂), and the overall product rate increased 33 times from 0.4 mmol/min/g Rh to 12.7 mmol/min/g Rh. CO-FTIR results showed that supporting Rh on silica led to the formation of Rh(211) facets, which favored formation of methane and had higher CO conversion rate. A catalyst formed by depositing Rh on a MoO₃/SiO₂ support prepared by ALD was found to be the most active catalyst while maintaining the suppression of methane selectivity, showing around 60 times higher overall rate than 1%Rh/SiO₂. A reaction pathway is proposed, in which hydrogenation steps are promoted most significantly by Mo and W species, followed by promotion of CO insertion steps for ethanol synthesis and C-C coupling steps for hydrocarbons formation. CO-FTIR results showed that 1%Rh/MoO₃/SiO₂ has the highest proportion of gem-dicarbonyl adsorption sites and the lowest proportion of bridge-bonded CO adsorption sites. The rate of methanol formation shows a positive correlation with the number of sites that form gem-dicarbonyl species.