Highly Dispersed ZnO Sites in a ZnO/ZrO₂ Catalyst Promote Carbon Dioxide-to-Methanol Conversion

ZnO/ZrO₂ catalysts have shown better activity in the CO₂ hydrogenation to methanol compared with single component counterparts, but the interaction between ZnO and ZrO₂ is still poorly understood. In particular, the effect of the ZrO₂ support phase (tetragonal vs. monoclinic) was not systematically explored. Here, we have synthesized ZnO/ZrO₂ catalysts supported on tetragonal ZrO₂ (ZnO/ZrO₂-t) and monoclinic ZrO₂ (ZnO/ZrO₂-m), which resulted in the formation of different ZnO_x species, consisting of sub-nanometer ZnO moieties and large-sized ZnO particles, respectively. ZnO/ZrO₂-t exhibited a higher methanol selectivity (81 vs. 39 %) and methanol yield (1.25 vs. 0.67 mmol g^-1 h^-1) compared with ZnO/ZrO₂-m. The difference in performance was attributed to the redox state and degree of dispersion of Zn, based on spectroscopy and microscopy results. ZnO/ZrO₂-t had a high density of ZnO_x-ZrO_y sites, which favored the formation of active HCOO* species and enhanced the yield and selectivity of methanol along the formate pathway. Such ZnO clusters were further dispersed on ZrO₂-t during catalysis, while larger ZnO particles on ZnO/ZrO₂-m remained stable throughout the reaction. This study shows that the phase of ZrO₂ supports can be used to control the dispersion of ZnO and the catalyst surface chemistry, and lead to enhanced catalytic performance.