CO₂-Driven Oxygen Vacancy Diffusion and Healing on TiO₂(110) at Ambient Pressure

Understanding how TiO₂ interacts with CO₂ at the molecular level is crucial in the CO₂ reduction toward value-added energy sources. Here, we report in situ observations of the CO₂ activation process on the reduced TiO₂(110) surface at room temperature using ambient pressure scanning tunneling microscopy. We find that oxygen vacancies (V_o) diffuse dynamically along the bridging oxygen (O_br) rows of the TiO₂(110) surface under ambient CO₂(g) environments. This physical phenomenon exclusively occurs when the oxygen abstracted upon CO₂ dissociation instantly occupies the V_o sites of O_br rows on the TiO₂(110), whereas the TiO₂(110) surface without the V_o only allows CO₂ physisorption on five-fold-coordinated Ti⁴⁺ sites. Synchrotron-based ambient pressure X-ray photoelectron spectroscopy also identifies the changes in surface oxidation states of TiO₂(110) by the healing of V_o sites or the CO₂ physisorption under ambient CO₂(g) conditions. Density functional theory calculations propose a mechanism of the CO₂-driven V_o diffusion and the physisorbed CO₂ configurations. Our combined results unravel the critical role of defect sites on TiO₂ in determining the elementary step of CO₂ activation during chemical reactions.