Surface Acidity of Oxygen Evolution Intermediates by Excited State Optical Spectroscopy

While the protonation of homogeneous metal-oxide catalysts is quite relevant for mechanisms in aqueous solution, the relevance of surface acidity to catalytic mechanisms of heterogeneous metal oxides is far less understood. Without a distinguishing metric ascribed to transient intermediates, invoking surface acidity is limited to rationalizing pH-dependent product evolution. Recently, the emissive population associated with intermediates of electron and proton transfer at a titanium oxide surface demonstrated an isotherm with pH whose half-rise occurred at pH = 11.8. As detected by a time-resolved visible broadband probe, the intermediates formed within <2 ps upon photoexcitation of the oxygen evolution reaction (OER) at an electron-doped (0.1% Nb) SrTiO₃/aqueous interface. Here, we demonstrate how the pH dependence of the <2 ps population is preserved such that when the intermediate population finally decays with a time constant of 10 μs, it does so with a pH onset at 11.4. The distinctive pH associated with a metastable population ascribes a surface acidity to the first intermediate of water oxidation, interpreted as favoring Ti-OH^• below and Ti-O^•- above an acid dissociation constant (i.e., pK_a). The work shows that the excited state optical spectroscopy is sensitive to protonation over the lifetime of metastable intermediates. Further, it shows that the details of the hydration layer prior to excitation are important for stabilizing intermediate populations of different acidities that then affect the chemical steps of water oxidation catalysis at longer time scales, e.g., bond formation.