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

Converting CO₂ into valuable C₁ 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.