Proton-Coupled Electron Transfer Involving Localized Electronic Defects on Solid Surfaces

Abstract

Proton-coupled electron transfer (PCET) is a critical elementary step in many (photo)electrocatalytic transformations. PCET reactions on semiconducting metal oxide surfaces involve proton transfer that is charge compensated by electronic defects, such as electrons or holes. When highly localized as small polarons, these electronic defects are coupled with local bond distortions that accompany the excess or depleted charge. Using anatase TiO2 as a model system, I will describe recent first-principles modeling studies of interfacial PCET on metal oxide surfaces. The PCET reaction free energies involving the cleavage of O–H bonds on TiO2 surfaces varies widely depending on whether the charge-compensating electronic defects are conduction d-band electrons or valence p-band holes. Differences in PCET thermochemistry are accounted for using a Marcus theory framework based on defect energy levels and inner-sphere reorganization energies. The PCET rate constants associated with the cleavage of O–H bonds in the presence of a nitroxyl radical oxidant are calculated using vibronically nonadiabatic PCET theory, where both the transferring electron and proton are treated quantum mechanically. Input parameters to the rate constant expressions are calculated using hybrid periodic density functional theory calculations. These studies highlight the effects of local electronic defects on the PCET reactivity of semiconductor surfaces.