Ultrafast Interfacial Proton-Coupled Electron Transfer

Abstract

Interfaces between metallic or semiconducting solids and protic solvent adsorbates or liquids represent one of the most important, and yet hardly explored material environments for proton coupled electron transfer (PCET) processes. PCET mediated dynamical phenomena driven by light, electron, and chemical potentials are central in energy transduction processes of vast economic and environmental importance including the photocatalytic splitting of H₂O, the photo and electrochemical reduction of CO₂, and the conversion of chemical to electrical energy in fuel cells. Experimental and theoretical investigations of the dynamical aspects of PCET at solid surfaces are particularly challenging because relatively localized charges within a solvent couple in the presence of strong interfacial potentials to delocalized states of electronic continua of semiconductor or metal electrodes. Moreover, the localized charges are never the bare protons and electrons that balance chemical equations, but rather are dressed particles with associated polarization clouds inhomogeneously distributed and comprised variously of free electrons, lattice ions, and solvent molecules. The polarization clouds screen the Coulomb potential on the medium specific time scales and impose energetic costs associated with transport through the inhomogeneous region of interfaces between the solid and molecular environments. We introduce some recent theoretical studies aimed at providing an atomisticmore » description on metal-protic solvent interface and modeling of simple processes such as the discharge of H⁺ at a metal interface. Because of the paucity of experimental research and embryonic stage of theory, our goal is to present some key theoretical concepts and early experimental efforts based primarily on a surface science approach to ultrafast electron induced dynamics. In order to introduce some key features of interfacial PCET in the strong and intermediate coupling regimes, we discuss specific examples of photoinduced dissociation of alkanes on metals and photoinduced PCET dynamics of methanol covered TiO₂ surfaces.« less