The Functionality of Surface Hydroxy Groups on the Selectivity and Activity of Carbon Dioxide Reduction over Cuprous Oxide in Aqueous Solutions

Carbon dioxide (CO2 ) reduction in aqueous solutions is an attractive strategy for carbon capture and utilization. Cuprous oxide (Cu2 O) is a promising catalyst for CO2 reduction as it can convert CO2 into valuable hydrocarbons and suppress the side hydrogen evolution reaction (HER). However, the nature of the active sites in Cu2 O remains under debate because of the complex surface structure of Cu2 O under reducing conditions, leading to limited guidance in designing improved Cu2 O catalysts. This paper describes the functionality of surface-bonded hydroxy groups on partially reduced Cu2 O(111) for the CO2 reduction reaction (CO2 RR) by combined density functional theory (DFT) calculations and experimental studies. We find that the surface hydroxy groups play a crucial role in the CO2 RR and HER, and a moderate coverage of hydroxy groups is optimal for promotion of the CO2 RR and suppression of the HER simultaneously. Electronic structure analysis indicates that the charge transfer from hydroxy groups to coordination-unsaturated Cu (CuCUS ) sites stabilizes surface-adsorbed COOH*, which is a key intermediate during the CO2 RR. Moreover, the CO2 RR was evaluated over Cu2 O octahedral catalysts with {111} facets and different surface coverages of hydroxy groups, which demonstrates that Cu2 O octahedra with moderate coverage of hydroxy groups can indeed enhance the CO2 RR and suppress the HER.