Alkaline-earth metal-oxide overlayers on TiO2: application toward CO2 photoreduction

Converting CO2 into valuable C-1 products such as CO, methanol, and methane using photocatalysts is an attractive way to recycle atmospheric CO2 into fine chemicals and fuels. The most commonly studied photocatalyst, TiO2, however, suffers from poor initial adsorption of CO2. To overcome this problem, it has been proposed that a thin overlayer of a basic oxide might promote CO2 adsorption and thus improve the reactivity of TiO2 for photoreduction of CO2. In this work, we investigated CO2 adsorption on the (100) surfaces of a series of basic, alkaline-earth metal oxides (MgO, CaO, SrO, BaO). Using periodic density functional theory (DFT) calculations, we found that CO2 adsorption becomes significantly more favorable in the order MgO < CaO < SrO < BaO, and we attribute this order to the more suitable lattice parameter of BaO compared to MgO. To understand the effect of a thin layer of basic oxide on TiO2 for CO2 photoreduction, SrO on TiO2 was investigated as a model system. A dramatic improvement in CO2 adsorption and activation was observed on SrO/TiO2 compared to the bare TiO2, and dissociated water was found to be thermodynamically more favorable than intact water on the SrO/TiO2 surface. A possible reaction route for the photocatalytic reduction of CO2 to CO on the bare and SrO-modified TiO2 surfaces was further investigated. Although the reaction is slightly more favorable on the TiO2 surface than on the 0.5 ML SrO-covered TiO2, the SrO half layer helps activate CO2 and favors desorption of CO, which are challenging steps for CO2 reduction on pure TiO2. Therefore, our results suggest that < 1 ML SrO overlayer might be a promising candidate for further experimental exploration.