Highly Dispersed ZnO Sites in a ZnO/ZrO2 Catalyst Promote the CO2-to-Methanol Reaction

ZnO/ZrO2 catalysts have shown excellent 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 catalyst materials supported on tetragonal ZrO2 (ZnO/ZrO2-t) and monoclinic ZrO2 (ZnO/ZrO2-m), which resulted in the formation of different ZnOx species, consisting of sub-nanometer ZnO moieties and large-sized ZnO particles, respectively. ZnO/ZrO2-t exhibited much higher methanol selectivity (81 %) and methanol yield (1.25 mmol/g/h) compared with ZnO/ZrO2-m (39 % and 0.67 mmol/g/h). The difference in performance was attributed to the redox state and degree of dispersion of Zn, based on spectroscopy and microscopy results. Operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and in situ near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) showed that ZnO/ZrO2-t had a high density of ZnO-ZrO sites, which favored the formation of active HCOO* species and enhanced the yield and selectivity of methanol along the formate pathway. Operando UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS) together with Raman spectroscopy and high-angle angular dark field-scanning transmission electron microscopy (HAADF-STEM) combined with energy dispersive X-ray (EDX) analysis revealed that on ZnO/ZrO2-t, ZnO clusters were further dispersed during catalysis at 320 °C, forming high density of ZnO-ZrO sites, and facilitating the hydrogenation of CO2 to methanol, 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 the active phase and lead to enhanced catalytic performance. We believe the insights are relevant for other methanol synthesis catalysts, where metal-support interactions are pivotal for catalyst activity and stability.