Mechanism and Nature of Active Sites for Methanol Synthesis from CO/CO₂on Cu/CeO₂

CO2 hydrogenation to methanol can play an important role in meeting the sustainability goals of the chemical industry. In this study, we investigated in detail the role of the Cu-CeO2 interactions for methanol synthesis, emphasizing the role of the copper surface and interface sites between copper and ceria for the hydrogenation of CO2 and CO. A combined CO2-N2O titration approach was developed to quantify the exposed metallic copper sites and ceria oxygen vacancies in reduced Cu/CeO2 catalysts. Extensive characterization shows that copper dispersion is strongly enhanced by strong Cu-CeO2 interactions in comparison to Cu/SiO2. CO2 hydrogenation activity data show that the Cu/CeO2 catalysts displayed higher methanol selectivity compared to a reference Cu/SiO2 catalyst. The improved methanol selectivity stems from inhibition of the reverse water-gas-shift activity. The role of CO in CO2-to-methanol conversion was studied by steady-state and transient cofeeding activity measurements together with (quasi) in situ characterization (TPH, XPS, SSITKA, and IR spectroscopy). The Cu-CeO2 interface provides active sites for the direct hydrogenation of CO to methanol via a formyl intermediate. Cofeeding of small amounts of CO2 to a CO/H2 mixture poisons these interfacial sites due to the formation of carbonate-like species. Methanol synthesis proceeds mainly via CO2 hydrogenation in which the metallic Cu surface provides the active sites.