Electrochemical Proton-Coupled Electron Transfer at a Metal–Semiconductor–Solution Interface

Interfacial proton-coupled electron transfer (I-PCET) in composite metal–semiconductor systems is important for many electrocatalytic processes. Herein, I-PCET at a metal–semiconductor–solution interface is investigated computationally using a Au-TiO₂ model system, where the proton transfers from the TiO₂ surface to an alcohol acceptor. We studied four possible I-PCET mechanisms that could occur in the system. For each mechanism, the calculated slope of the I-PCET equilibrium constant as a function of applied potential qualitatively agrees with experimental measurements of I-PCET-promoted Brønsted acid catalysis on a Ti-TiO₂ composite system, although the dependence of the calculated results on the thickness of the TiO₂ slab modeled prevents the unambiguous identification of the I-PCET mechanism. Focusing on one specific I-PCET mechanism, our charge analysis indicates that each proton transfer from the TiO₂ surface is coupled to the transfer of n ≈ 0.5 electron from the Au metal to the external circuit. The calculated inverse slope of ∼110 mV is consistent with the Nernstian slope of a 1H⁺-ne^– PCET process. Further analysis shows that the observed slope arises from both electrostatic and capacitive contributions from the interface, together leading to a ∼0.6 eV change in the reaction free energy for deprotonation per 1 V potential change. These analyses reveal distinctive I-PCET reaction characteristics at electrified metal–semiconductor–solution interfaces and provide fundamental insights into how catalyst material properties influence the potential dependence of these elementary steps.