Mechanisms of methanol synthesis from carbon dioxide and from carbon monoxide at atmospheric pressure over Cu/ZnO

Methanol synthesis from CO2 and CO was carried out at atmospheric pressure over a Cu/ZnO catalyst (Cu/Zn = 3/7) and the surface species formed were analyzed by diffuse reflectance FT-IR spectroscopy and temperature programmed desorption, It was revealed that CH3OH and CO were produced from CO2-H-2 through parallel pathways, Two types of formate species, HCOO-Cu and HCOO-Cu and HCOO-Zn, and zinc methoxide were formed in the course of methanol synthesis from CO2. HCOO-Cu and HCOO-Zn were hydrogenated to methoxide, A comparison of the rates of hydrogenation with that of methanol synthesis from CO2 suggested that methanol synthesis from CO2 occurred via the hydrogenation of HCOO-Cu, The methanol synthesis from CO proceeded at a rate much slower than that from CO2, HCOO-Zn and zinc methoxide were formed in the course of the CO-H-2 reaction, No HCOO-Cu was detected. The amount of the zinc methoxide formed in the CO-H-2 reaction greatly exceeded that formed in the CO2-H-2, reaction, The rate of the formation of methoxide in the CO-H-2 reaction was in reasonable agreement with that of the hydrogenation of HCOO-Zn. On the basis of these findings, the difference in the mechanisms of methanol synthesis from CO2 and from CO was discussed. (C) 1995 Academic Press, Inc.