Enhanced High-Temperature CO₂-to-Methanol Conversion over the ZnZrO_x Solid Solution Catalyst with Abundant Asymmetric Oxygen Vacancy (Zn-O_v-Zr) Sites

ZnZrO_x solid solution catalysts have gained considerable attention for their effectiveness in CO₂ hydrogenation and the conversion of methanol to C²⁺ products. However, their performance in high-temperature hydrogenation of CO₂ to methanol needs enhancement. In this study, we present a surface engineering approach for these catalysts by thermally treating a ZnZr hydroxide precursor in a reducing atmosphere (H₂/Ar). This method promoted the migration of Zn from the bulk of ZrO₂ to the surface layer, leading to the formation of abundant asymmetric oxygen vacancy (Zn-O_v-Zr) sites. Consequently, the 11.5ZnZr-500H₂/Ar catalyst (treated in H₂/Ar) exhibited a methanol selectivity of 83.0% at 325 °C, which is significantly higher than the 54% selectivity of 11.5ZnZr-500Air (treated in air) at the same temperature and comparable to the 85.1% selectivity of 11.5ZnZr-500Air at 275 °C. By selectively removing Zn at various levels through acid treatment of the catalysts, we have discovered an almost linear correlation between the concentration of asymmetric oxygen vacancies and the yield of methanol production. DRIFTS measurements and DFT calculations demonstrate that these active sites enhance the CO₂ activation and conversion of formate intermediates during the methanol synthesis process. This study is the first to identify the catalytic function of the asymmetric Zn-O_v-Zr sites, paving the way for the subsequent production of C²⁺ products.