Theoretical Analysis of Hydrogen Underpotential Deposition on Pt(111) Under Alkaline Conditions

Proton discharge onto an electrode surface, denoted the Volmer step, is one of the most fundamental reactions in electrochemistry and plays an important role in hydrogen production. To probe this fundamental reaction, hydrogen underpotential deposition (UPD) on single-crystal Pt(111) under alkaline conditions has been experimentally studied. This reaction is a concerted proton-coupled electron transfer (PCET) process, involving proton transfer from a water molecule to the electrode surface and electron transfer from the electrode to the localized chemical bond between the proton and the surface. Herein, a PCET theory based on nonadiabatic transitions between diabatic vibronic states is applied to hydrogen UPD on Pt(111) under alkaline conditions. This theory accounts for hydrogen tunneling by treating the transferring hydrogen nucleus quantum mechanically and spans both the nonadiabatic and adiabatic regimes. Periodic density functional theory calculations are employed to compute the input quantities to the general PCET rate constant expression. The transfer coefficients and kinetic isotope effects are compared to cyclic voltammetry experiments on Pt(111) single-crystal surfaces. The calculations corresponding to proton transfer from a side-lying water above a Pt(111) atop site to an fcc-hollow site are in reasonable agreement with the experimental data. A decomposition of the contributions to the adsorption rate constant highlights the importance of hydrogen tunneling and excited vibronic states. Moreover, this process is found to exhibit behavior in between the adiabatic and nonadiabatic limits. This study elucidates the underlying physical principles of hydrogen UPD on Pt(111) and paves the way for theoretical studies of other related processes.