Mechanisms of Methanol Synthesis from Carbon Dioxide and from Carbon Monoxide at Atmospheric Pressure over Cu/ZnO

Abstract

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-H2 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-H2 reaction. No HCOO-Cu was detected. The amount of the zinc methoxide formed in the CO-H2 reaction greatly exceeded that formed in the CO2-H2 reaction. The rate of the formation of methoxide in the CO-H2 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.