Hydrogenation of CO₂ to Methanol on CeOx/Cu(111) and ZnO/Cu(111) Catalysts: Role of the Metal–Oxide Interface and Importance of Ce³⁺ Sites

The role of the interface between a metal and oxide (CeO x –Cu and ZnO–Cu) is critical to the production of methanol through the hydrogenation of CO 2 (CO 2 + 3H 2 → CH 3 OH + H 2 O). The deposition of nanoparticles of CeO x or ZnO on Cu(111), θ oxi < 0.3 monolayer, produces highly active catalysts for methanol synthesis. The catalytic activity of these systems increases in the sequence: Cu(111) < ZnO/Cu(111) < CeO x /Cu(111). The apparent activation energy for the CO 2 → CH 3 OH conversion decreases from 25 kcal/mol on Cu(111) to 16 kcal/mol on ZnO/Cu(111) and 13 kcal/mol on CeO x /Cu(111). The surface chemistry of the highly active CeO x –Cu(111) interface was investigated using ambient pressure X-ray photoemission spectroscopy (AP-XPS) and infrared reflection absorption spectroscopy (AP-IRRAS). Both techniques point to the formation of formates (HCOO – ) and carboxylates (CO 2 δ− ) during the reaction. Our results show an active state of the catalyst rich in Ce 3+ sites which stabilize a CO 2 δ− species that is an essential intermediate for the production of methanol. The inverse oxide/metal configuration favors strong metal–oxide interactions and makes possible reaction channels not seen in conventional metal/oxide catalysts.