Band gap effect of TiO2 on supported Ru single-atom catalysts for CO2 methanation by DFT calculations

The influence of TiO₂ band gap on structure and reactivity of the supported single ruthenium (Ru₁) atom was studied by density functional theory calculations, utilizing Ru₁/2L-TiO₂ and Ru₁/3L-TiO₂ as model catalysts for CO₂ methanation. The supports have band gaps of 2.39 eV and 1.48 eV, respectively. The band gap plays a significant role in electronic metal-support interactions (EMSIs) and the position of the d-band center of the Ru₁ on the TiO₂. The Ru₁/3L-TiO₂, the Ru₁ catalyst supported on the 3L-TiO₂ with a narrower band gap, shows enhanced EMSIs and a d-band center that is positioned farther from Fermi level, leading to lower charge density on the Ru₁ and weaker adsorption of H₂, CO₂, and CO compared to the Ru₁/2L-TiO₂. CO₂ methanation followed CO pathway, with the hydrogenation of CO* to HCO* identified as the rate-determining step on the Ru₁/nL-TiO₂. The Ru₁ catalyst supported on TiO₂ with a narrower band gap is more favorable kinetically and thermodynamically for CO₂ methanation, despite the band gap not altering the reaction pathway. Enhanced hydrogen mobility and a pronounced promotional effect of hydrogen on CO adsorption, due to the narrower band gap support, are key factors facilitating the easier hydrogenation of CO* on the Ru₁/3L-TiO₂.