Synthesis Gas Conversion over Rh/Mo Catalysts Prepared by Atomic Layer Deposition

Conversion of synthesis gas into value-added products, including oxygenates and C 2+ hydrocarbons, was studied at 523 K, 580 psi, and a 1/1 CO/H 2 ratio over Rh catalysts on catalyst supports prepared by atomic layer deposition (ALD) of molybdenum and tungsten species on silica. The reactivity measurements showed that coating the silica support with molybdenum and tungsten species helped to suppress the methane selectivity and promote the overall conversion rate. When the silica support was coated with five cycles of β-Mo 2 C, the methane selectivity decreased from 32% (1% Rh/SiO 2 ) to 13% (1% Rh/5c-Mo 2 C/SiO 2 ), and the overall product rate increased 33 times from 0.4 to 12.7 mmol min –1 (g of Rh) −1 . CO-FTIR results showed that supporting Rh on silica led to the formation of Rh(211) facets, which favored the formation of methane and had a higher CO conversion rate. Rh on a MoO 3 /SiO 2 support prepared by ALD was found to be the most active catalyst while maintaining the suppression of methane selectivity, showing an overall rate ∼60 times higher than that of 1% Rh/SiO 2 . 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 hydrocarbon formation. CO-FTIR results showed that 1% Rh/MoO 3 /SiO 2 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.