CO₂ Hydrogenation to Methanol on a YBa₂Cu₃O₇ Catalyst

The synthesis of methanol from CO₂ and H₂ 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°C. After H₂ reduction at 250°C, the YBa₂Cu₃O₇ transformed from an orthorhombic to a tetragonal structure, a phase which is active for methanol synthesis. In a H₂-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 CuO_x 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.