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 (Vo) diffuse dynamically along the bridging oxygen (Obr) 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 Vo sites of Obr rows on the TiO2(110), whereas the TiO2(110) surface without the Vo 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 Vo sites or the CO2 physisorption under ambient CO2(g) conditions. Density functional theory calculations propose a mechanism of the CO2-driven Vo 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.