Defect-Driven Redox Interplay on Anatase TiO: Surface-Structure Dependent Activation for CO Hydrogenation Catalysis.

Titanium dioxide (TiO) 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 TiO under varying reactive conditions. Under H exposure, anatase TiO undergoes surface reduction via lattice oxygen loss, forming TiO. In contrast, CO exposure induces oxygen replenishment, reversing stoichiometry. In mixed H/CO environments, the reverse water-gas shift (RWGS) reaction proceeds selectively on stepped and high-indexed TiO surfaces, whereas the thermodynamically stable TiO(101) surface remains inactive and intact. Critically, H pretreatment generates oxygen vacancies on TiO(101), transforming it into an active TiO or defect-rich surface that catalyzes RWGS. By correlating surface structure, defect dynamics, and gas-phase interactions, this work deciphers the competition between H-driven reduction and CO-driven oxidation pathways at the atomic scale. These insights establish defect engineering as a strategic lever to activate inert TiO facets, advancing the design of adaptive catalysts for sustainable fuel synthesis technologies.