A Theoretical Study of CO₂ Anions on Anatase (101) Surface

Binding configurations of CO₂ and CO₂⁻ on perfect and oxygen-deficient anatase (101) surfaces were explored using first-principles calculations on both cluster and periodic models. The solvent effect was taken into account via the polarizable continuum model. Analysis of molecular orbitals, charge, and spin density distributions was used to help identify the radical anion CO₂⁻ adsorbed on the surface. On defect-free surfaces, it is found to bind as a bridging bidentate configuration with both oxygens coordinating to the 5-fold Ti ions. Analysis of vibrational frequencies provides a specific signature of the CO₂ anion to distinguish it from other species in experiments. The reduction potential of adsorbed CO₂ on a (101) surface is lower by 0.24 V than the reduction potential of a CO₂ molecule, both in aqueous solution, due to the formation of hybridized orbitals, which facilitates charge transfer to CO₂. The reduced (101) surface of TiO₂ is much more favorable for CO₂ binding with accompanying charge transfer to CO₂.