Heterogeneous proton-coupled electron transfer at nanoparticle and electrode interfaces

Interfacial proton-coupled electron transfer (PCET) reactions play a vital role in a wide range of energy conversion processes. A general theory of PCET that includes the quantum mechanical effects of the active electrons and transferring protons, as well as the motions of the proton donor-acceptor mode and solvent environment, has been developed. This formulation enables the calculation of rate constants and kinetic isotope effects for homogeneous as well as heterogeneous processes at nanoparticles and electrode surfaces. This theory has been applied to experimentally studied photoreduced ZnO nanocrystals reacting by PCET with the nitroxyl radical TEMPO. The calculations indicate that the electron transfers from the conduction band of the ZnO nanocrystal to TEMPO concertedly with proton transfer from a surface oxygen of the ZnO nanocrystal to the oxygen of TEMPO. Proton diffusion from inside the nanocrystal to reactive sites on the surface was found to explain the experimentally observed nonexponential kinetics. This PCET theory has also been applied to experimentally studied proton discharge from triethylammonium to a gold electrode in acetonitrile. These experiments demonstrated an isotope-dependent Tafel slope or, equivalently, a potential-dependent kinetic isotope effect. The calculations explain the potential-dependent kinetic isotope effect in terms of contributions from excited electron-proton vibronic states that depend on both isotope and applied potential. Proton discharge to a gold electrode in acidic and alkaline aqueous solution has also been studied. These applications highlight the importance of using a vibronically nonadiabatic theory that quantizes the transferring proton and includes the effects of hydrogen tunneling and excited electron-proton vibronic states. These studies are also assisting in the interpretation of experimental data and the design of more effective catalysts for energy conversion processes.