Synergy of oxygen vacancies and surface Zn in tuned ZnO crystal facets for enhanced CO2 cycloaddition to epoxides
Exploring the impact of metal oxide crystal facets on the cycloaddition reaction of CO 2 with epoxides is meaningful because pure metal oxides exposing specific crystal facets show considerable promise as catalysts for industrial applications. In this study, we developed a simple strategy to prepare single-crystal ZnO nanosheet materials with controllable crystal facets through hydrothermal synthesis, followed by pyrolysis under different atmospheres. Studies revealed that the formation of ZnO single-crystal nanosheets involves the decomposition of the precursors and an ordered phase transition. Based on the quantitative correlation between oxygen vacancies, surface Zn, and the catalytic activity of ZnO (002) with varying numbers of oxygen vacancies, together with TPD, Py-IR, in-situ EPR, fluorescence spectroscopy characterizations, and DFT calculations, a synergistic mechanism between oxygen vacancies and surface Zn was proposed. Specifically, oxygen vacancies are the active sites for CO 2 , whereas saturated coordination surface Zn atoms activate epoxides. At 125 °C, 2 MPa, and 4 h, 93.3 % yield of the product was achieved using only 5 mg of the optimal ZnO catalyst. The excellent performance, stability, and applicability of single-crystal nanosheets make them ideal candidates for industrial applications. This study provides a new insight into the catalytic mechanisms of CO 2 cycloaddition over metal oxides.
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