Structural evolution and catalytic performance in CO2 hydrogenation reaction of ZnO-ZrO2 composite oxides

Abstract

ZnO-ZrO2 composite oxides are used as catalysts for CO2 hydrogenation reaction. Herein, we prepare a series of ZnO-ZrO2 composite oxides via co-precipitation method and comprehensively study their structural evolution and catalytic performance in CO2 hydrogenation reaction. Via combined spectroscopic and microscopic characterization techniques, we found that, as the Zr content increased, phase structures of ZnO-ZrO2 composite oxides evolved from the mixture of hexagonal ZnO phase and Zn-doped ZrO2 solid solution phases to the pure Zn-doped ZrO2 solid solution phase. Zn-doped ZrO2 solid solution nanoparticles of mixed-phased ZnO-ZrO2 composite oxides were observed to preferentially nucleate and grow within the bulk of ZnO nanoparticles. Surfaces of mixed-phased ZnO-ZrO2 composite oxides are enriched with Zn, while the pure Zn-doped ZrO2 solid solutions exhibit uniform bulk and surface compositions. ZnO-ZrO2 composite oxides are much more catalytic active than pure ZnO, t-ZrO2 and m-ZrO2 in catalyzing CO2 hydrogenation reactions. The pure Zn-doped ZrO2 solid solution phase exhibit high selectivity toward CH3OH production, while pure ZnO, t-ZrO2, m-ZrO2 and mixed-phased ZnO-ZrO2 composite oxides exhibit high selectivity toward CO production. Bidentate formate species and carbonates/bicarbonates species are the key intermediates of the CO2 hydrogenation to methanol and CO, respectively. These results deepen the fundamental understanding of structure–activity relation and reaction mechanisms for ZnO-ZrO2 composite oxides-catalyzed CO2 hydrogenation reaction.