Defect-Driven Redox Interplay on Anatase TiO2: Surface-Structure Dependent Activation for CO2 Hydrogenation Catalysis

Titanium dioxide (TiO2) is one of the most extensively studied oxides as an active catalyst or catalyst support, particularly in energy and environmental applications, but the atomistic mechanisms governing its dynamic response to reactive environments and their correlation to reactivity remain largely elusive. Using in situ environmental transmission electron microscopy (ETEM), synchrotron X-ray diffraction (XRD), ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), temperature-programmed reduction (TPR), reactivity measurements, and theoretical modeling, we reveal the dynamic interplay between oxygen loss and replenishment of anatase TiO2 under varying reactive conditions. Under H2 exposure, anatase TiO2 undergoes surface reduction via lattice oxygen loss, forming Ti3O5. In contrast, CO2 exposure induces oxygen replenishment, reversing stoichiometry. In mixed H2/CO2 environments, the reverse water–gas shift (RWGS) reaction proceeds selectively on stepped and high-indexed TiO2 surfaces, whereas the thermodynamically stable TiO2(101) surface remains inactive and intact. Critically, H2 pretreatment generates oxygen vacancies on TiO2(101), transforming it into an active Ti3O5 or defect-rich surface that catalyzes RWGS. By correlating surface structure, defect dynamics, and gas-phase interactions, this work deciphers the competition between H2-driven reduction and CO2-driven oxidation pathways at the atomic scale. These insights establish defect engineering as a strategic lever to activate inert TiO2 facets, advancing the design of adaptive catalysts for sustainable fuel synthesis technologies.