Mechanism and structure–activity relationship of H2 and CO2 activation at the ZnO/Cu catalyst interface

Abstract

Cu/ZnO/Al₂O₃ catalysts are the most well-known heterogeneous catalysts for the hydrogenation of CO and CO₂ into methanol. Herein, density functional theory calculations were performed to investigate the mechanism of H₂ activation and the effects of hydrogen spillover on CO₂ adsorption and activation at the interfacial site of the ZnO/Cu model catalyst, which was simulated by loading ZnO ribbons of different sizes on the Cu(111), Cu(100), and Cu(211) surfaces. The ZnO/Cu interface is found to facilitate the formation of H adsorbates from the dissociation of H₂ molecules, which promotes the facile formation of oxygen vacancy (V_O) sites in the ZnO component due to its reducibility and the hydrogen spillover effect. The resulting interfacial structure of the ZnO/Cu model catalyst can contain perfect, hydroxylated, and oxygen-vacancy-present ZnO sites, which may act as the adsorption and activation sites for CO₂. Further calculations show that molecular CO₂ adsorbed at the V_O site can be efficiently activated by direct dissociation or hydrogenation to the HCOO* species. In addition, the smaller ZnO structure and less exposure of the Cu(211) facet facilitate hydrogen spillover and the formation of the interfacial V_O site. This study provides important insights into the structure–activity relationship for the active sites of the ZnO/Cu model catalyst and the mechanisms of CO₂ activation and hydrogenation.