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.

Addition of Rb promoter to alumina-supported Mo2C catalysts shifted the IR spectrum of adsorbed CO to lower vibrational frequencies, which correlated to an increase in alcohol selectivity during CO hydrogenation. Water vapor facilitated the dispersion of the Rb promoter across the catalyst surface.Figure optionsDownload high-quality image (96 K)Download as PowerPoint slideHighlights
► Rb-promoted Mo2C/Al2O3 exhibited high alcohol selectivity during CO hydrogenation.
► Addition of Rb to supported Mo2C caused a red shift in the IR spectrum of adsorbed CO.
► DFT calculations qualitatively reproduced the trends observed by IR spectroscopy.
► Water vapor facilitated the dispersion of Rb across the catalyst surface.