Use of infrared spectroscopy and density functional theory to study the influence of rubidium on alumina-supported molybdenum carbide catalyst for higher alcohol synthesis from syngas

Molybdenum carbide nanoclusters supported on alumina were promoted by Rb2CO3 and tested for higher alcohol synthesis from syngas. At 573 K and 30 bar syngas (H2/CO = 1), the promoted catalyst demonstrated 63% selectivity to alcohols (on a CO2-free basis), with hydrocarbons and ethers as side products. In contrast, the alcohol selectivity of an unpromoted Mo2C/Al2O3 catalyst was very low. DRIFTS of adsorbed CO on Mo2C/Al2O3 revealed a significant red shift in the CO band with added Rb promoter, which correlated to a decrease in hydrocarbon production rate and therefore an increase in alcohol selectivity. A quantum chemical description of CO adsorbed on a model Mo2C surface identified several possible binding modes of the CO. Both reactivity studies and DRIFTS of adsorbed CO demonstrated that mobility of the Rb promoter was enhanced in the presence of water vapor, which facilitates dispersion of Rb across the catalyst surface and effectively neutralizes acid sites that are deleterious to alcohol selectivity.