Deciphering CO2 and H2 Activation on ZnZrOx Solid-Solution Catalyst: Atomic-Level Insights into Methanol Synthesis

The application of ZnO–ZrO2-based oxide catalysts in the CO2-to-methanol hydrogenation reaction has garnered significant attention; yet, insights into the active site configurations and reaction mechanism remain elusive. In this study, by employing advanced solid-state NMR techniques, we comprehensively investigated the surface active sites and the activation of CO2 and H2 molecules on the ZnZrOx solid-solution catalyst, complemented by comparative investigations on supported ZnO/ZrO2 catalysts. We revealed the intricate surface structure of the ZnZrOx solid-solution catalyst at the atomic level, highlighting the presence of a disordered surface ZnO phase and the Zn–OH–Zr interface, as identified by 17O MAS NMR. Notably, the ZnZrOx solid-solution and supported ZnO/tetragonal-ZrO2 catalysts exhibit strikingly similar surface features, correlating with their comparable catalytic performances. A key breakthrough is the direct identification of active bidentate carbonate species formed through the CO2 interaction with surface oxygen vacancies, specifically at the Zn–[Ov]–Zr interface. Using trimethylphosphine as a probe molecule, the relationship between oxygen vacancies and methanol production was confirmed by 31P NMR. More importantly, NMR analysis provides the direct evidence on the formation of surface zinc hydride (Zn–H) over both ZnZrOx solid-solution and supported ZnO/ZrO2 catalysts during H2 activation. These Zn–H species, in close proximity to oxygen vacancies, are shown to readily activate CO2 even at room temperature, leading to the formation of a surface formate intermediate and thereby facilitating methanol production. This study offers fundamental atomic-level insights into the critical surface active sites and reaction mechanism underlying CO2 hydrogenation on the ZnO–ZrO2-based catalysts, and also paves the way for the rational design of more efficient catalysts for methanol production.