Deciphering CO<sub>2</sub> and H<sub>2</sub> Activation on ZnZrO<sub><i>x</i></sub> Solid-Solution Catalyst: Atomic-Level Insights into Methanol Synthesis
Tongyao Wang, Chizhou Tang, Linhai He, Lanqi Ning, Meiling Guo, Xuebin Liu, Lixin Liang, Rongtan Li, Yi Ji, Kuizhi Chen, Jijie Wang, Qiang Fu, Pan Gao, Guangjin Hou
Abstract
The application of ZnO–ZrO 2 -based oxide catalysts in the CO 2 -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 CO 2 and H 2 molecules on the ZnZrO x solid-solution catalyst, complemented by comparative investigations on supported ZnO/ZrO 2 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 17 O MAS NMR. Notably, the ZnZrO x solid-solution and supported ZnO/tetragonal-ZrO 2 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 CO 2 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 31 P 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/ZrO 2 catalysts during H 2 activation. These Zn–H species, in close proximity to oxygen vacancies, are shown to readily activate CO 2 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 CO 2 hydrogenation on the ZnO–ZrO 2 -based catalysts, and also paves the way for the rational design of more efficient catalysts for methanol production.