Reactions of fluid and lattice oxygen mediated by interstitial atoms at the TiO₂(110)-water interface

O₂ interacts with TiO₂ surfaces in numerous aqueous reactions for clean hydrogen production, wastewater cleanup, reduction of CO₂ and N₂, and O₂ sensing. In many cases, these reactions involve reversible exchange of O with the solid, whose participation is usually thought to require oxygen vacancies (V_O). Based on measurements of oxygen isotopic self-diffusion in rutile TiO₂ under water, this work proposes a different perspective centered on O interstitial atoms (O_i). Experiments with varying concentrations of O₂ and mole fractions of ¹⁸O show that the (110) surface facilitates O exchange with both the H₂O liquid and its dissolved O₂. First-principles calculations indicate that on-top and "surface O_i" configurations of adsorbed O participate sequentially in the exchange process. Adsorbed OH appears to provide a single pathway for H₂O and O₂ to contribute oxygen, although fitting the diffusion data to simple models indicates that H₂O contributes more. Because rutile TiO₂ is a prototypical metal oxide, this picture based on O_i probably generalizes in many cases to other oxides - explaining important aspects of their thermal, electrochemical, and photochemical reactions with dissolved O₂.