Interfacial Proton-Coupled Electron Transfer via Localized Trap States on Metal Oxide Surfaces

Proton-coupled electron transfer (PCET) reactions on semiconducting metal oxide surfaces often involve charged defects in the form of electron or hole polarons. Herein, vibronically nonadiabatic PCET theory is used to model rate constants for the PCET reaction between a reduced anatase TiO₂(101) surface and 4-MeO-TEMPO, where electron polarons on the TiO₂ surface directly participate in the PCET reaction. This modeling strategy treats the transferring proton as well as all electrons quantum mechanically and includes the effects of excited vibronic states. The rate constant expression depends on the reorganization energy, as well as the reaction free energies and vibronic couplings for different pairs of vibronic states, and accounts for proton donor-acceptor motion. Hybrid functional periodic density functional theory (DFT) is used to calculate the parameters in the rate constant expression, and a Hubbard α-based constrained DFT approach is used to enforce charge constraints consistent with the two electronically diabatic states for the PCET reaction. This modeling strategy is applied to compute the PCET rate constants and kinetic isotope effects for reactions involving five-coordinate and six-coordinate Ti³⁺ defects on the TiO₂(101) surface, showing that excited vibronic states contribute significantly to the rate constant for both defects, especially for deuterium. This study highlights the importance of hydrogen tunneling and excited vibronic states in interfacial PCET reactions. Such modeling strategies can be used to further understand and tailor the reactivity of metal oxide surfaces for energy conversion.