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 ZnZrO_x solid-solution catalyst, complemented by comparative investigations on supported ZnO/ZrO₂ catalysts. We revealed the intricate surface structure of the ZnZrO_x 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 ZnZrO_x 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 ZnZrO_x 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.