The synthesis of methanol from CO2 and H-2 over YBa2Cu3O7 was studied. The aim was to clarify (i) the nature of the catalyst in the working state, (ii) the redox behaviour of copper in various oxidation states, and (iii) the formation and transformation of precursor species in methanol synthesis. The effects of reaction pressure, temperature, and space velocity on the catalytic performance were also investigated. The optimum reaction conditions were pressure = 3.0 MPa, space velocity = 3600 h(-1), and temperature = 240 degrees C. After H-2 reduction at 250 degrees C, the YBa2Cu3O7 transformed from an orthorhombic to a tetragonal structure, a phase which is active for methanol synthesis. In a Hz-reduced YBa2Cu3O7 sample, there were Cu+ and oxygen vacancies and electrons trapped at the oxygen vacancies. We observed that CO2 adsorption would consume the trapped electrons, resulting in the reoxidation of Cu+ to Cu2+. Intermediate species such as formate, methylenebisoxy, formyl, formaldehyde, and methoxide were observed in in situ-FTIR and FT-Raman studies; they were also captured by CD3I during methanol synthesis. Based on these experimental results, a reaction mechanism for CO2 hydrogenation to methanol over YBa2Cu3O7 was proposed. In this mechanism, hydrogen adsorbs dissociatively at the Cu+ sites of the CuOx planes, whereas CO2 adsorbs at the oxygen vacancies. The spillover of hydrogen atoms from Cu+ to the oxygen atoms and/or carbon atom of adsorbed CO2 leads to the formation of COOH, COHOH, HCOHOH, and H2COHOH species; subsequently, one of the C-O bonds is weakened. Other intermediate species such as formate, methylenebisoxy, formyl, formaldehyde, and methoxide could be formed, and the final products are methanol, bimethyl ether, and CO. (C) 2000 Academic Press.