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.