Ab initio mechanisms and design principles for photodesorption from TiO2

Photocatalytic reactions often exhibit fast kinetics and high product selectivity, qualities difficult to achieve simultaneously in thermal processes. However, photo-driven mechanisms remain poorly understood due to challenges in realistically modeling catalysts in optically excited states. Here, we apply many-body perturbation theory (MBPT) calculations to gain insight into these mechanisms by studying a prototypical photocatalytic reaction, proton desorption from a rutile TiO₂ (110) surface. Our results reveal dramatic changes upon photoexcitation, including an over 50% reduction in the desorption energy and the emergence of an energy barrier. We rationalize these findings using a generalizable model based on Fano theory, and explain the surprising increase of excitonic effects as the proton detaches from the surface. Our model also connects the alignment of various ionization potentials to the shape of the excited-state potential energy surface. These results, not qualitatively captured by constrained density-functional theory, highlight how MBPT calculations can inform photocatalytic reaction design.