Improving Selectivity and Activity of CO2 Reduction Photocatalysts with Oxygen

A highly porous photocatalyst (copper on TiO2 aerogel) was synthesized and applied in aqueous CO2 reduction without using external sacrificial electron donors. For the first time, complete selectivity toward CO and improved catalyst productivity are observed in the presence of oxygen. The optimal activity is achieved in a feed containing 0.5 vol% O2 in CO2. In situ XAS, EPR, and UV-vis measurements suggest, among different Cu oxidation states, Cu(I) to be the most active species in photocatalytic CO2 reduction. Oxygen sensing of the catalyst in the presence of O2/CO2 mixtures indicated an unexpected photoadsorption of oxygen on the titania surface. We propose photooxidation of surface hydroxyl groups to be the electron source for CO2 reduction, which is supported by hydroxyl group consumption, detection of hydroxyl radicals using in situ EPR, and detection of surface peroxide species after the reaction.