An Infrared Study of Methanol Synthesis from CO2 on Clean and Potassium-Promoted Cu/SiO2

Infrared spectroscopy and temperature-programmed reaction (TPR) spectroscopy were used to study hydrogenation of CO2 and CO2/CO mixtures over Cu/SiO2 and potassium-promoted Cu/SiO2 catalysts. Isothermal and temperature-programmed reactions were conducted at temperatures between 303 and 563 K and pressures of 0.1 and 0.72 MPa. The only products observed during the reaction of H2 and CO2 are CO, H2O, and CH3OH. At 0.1 MPa over Cu/SiO2, only 1% of the CO2 reacts to form methanol, the balance forming CO via the reverse water-gas shift (RWGS) reaction. Increasing the total pressure to 0.72 MPa and adding CO to the feed (CO/CO2 = 1) improve the methanol selectivity to 26%. Apparent activation energies measured at 480 K during H2/CO2/Ar reaction at 0.72 MPa are 21.1 kcal/mol for methanol synthesis and 15.2 kcal/mol for the RWGS reaction. Substitution of CO for Ar in the feed does not affect the apparent activation barrier for RWGS but lowers that for methanol synthesis to 16.9 kcal/mol. Infrared spectra reveal the following species on the Cu surface during CO2 hydrogenation: bidentate formate, monodentate formate, H2O, CO, and two forms of carbonate. Small concentrations of methanol and methoxy species are also observed. Addition of CO to the H2/CO2 feed increases both bidentate formate coverage and methanol synthesis TOF by 40%. Potassium promotion of Cu/SiO2 accelerates the RWGS reaction but hinders methanol synthesis, reducing methanol selectivity to 1% at 0.72 MPa during the reaction of H2 and CO2. Infrared observations show that potassium promotion stabilizes formate and carboxylate species. Based on the species observed during TPR-IR experiments and previous insights from methanol decomposition studies, a mechanism is proposed for CO2 hydrogenation over Cu. The effects of CO addition to the feed and potassium promotion of Cu can be explained by the proposed scheme.