Mechanism and Nature of Active Sites for Methanol Synthesis from CO/CO2 on Cu/CeO2

Abstract

Abstract 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.