AN INFRARED STUDY OF METHANOL SYNTHESIS FROM CO2 ON CLEAN AND POTASSIUM-PROMOTED CU SIO2/

Infrared spectroscopy and temperature-programmed reaction (TPR) spectr oscopy were used to study hydrogenation of CO2 and CO2/CO mixtures ove r Cu/SiO2 and potassium-promoted Cu/SiO2 catalysts. Isothermal and tem perature-programmed reactions were conducted at temperatures between 3 03 and 563 K and pressures of 0.1 and 0.72 MPa. The only products obse rved during the reaction of H-2 and CO2 are CO, H2O, and CH3OH. At 0.1 MPa over Cu/SiO2, only 1% of the CO2 reacts to form methanol, the bal ance forming CO via the reverse water-gas shift (RWGS) reaction. Incre asing the total pressure to 0.72 MPa and adding CO to the feed (CO/CO2 = 1) improve the methanol selectivity to 26%. Apparent activation ene rgies measured at 480 K during H-2/CO2/ Ar reaction at 0.72 MPa are 21 .1 kcal/mol for methanol synthesis and 15.2 kcal/mol for the RWGS reac tion. Substitution of CO for Ar in the feed does not affect the appare nt 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 f ormate, H2O, CO, and two forms of carbonate. Small concentrations of m ethanol and methoxy species are also observed. Addition of CO to the H -2/CO2 feed increases both bidentate formate coverage and methanol syn thesis TOF by 40%. Potassium promotion of Cu/SiO2 accelerates the RWGS reaction but hinders methanol synthesis, reducing methanol selectivit y to 1% at 0.72 MPa during the reaction of H-2 and CO2. Infrared obser vations show that potassium promotion stabilizes formate and carboxyla te species. Based on the species observed during TPR-IR experiments an d previous insights from methanol decomposition studies, a mechanism i s proposed for CO2 hydrogenation over Cu. The effects of CO addition t o the feed and potassium promotion of Cu can be explained by the propo sed scheme. (C) 1995 Academic Press, Inc.