(Invited) Resolving Catalytic Mechanism at an Electrode Surface: Thermodynamics & Kinetics of Reaction Steps

Abstract

Computationally, often the energetics of intermediate reaction steps differentiate the efficiency of heterogeneous catalysts for product evolution. Yet, when compared to experiment, kinetic models are applied. For example, a material’s activity measured by one rate of product evolution is plotted as a function of the calculated formation energies of intermediate chemical forms. A critically important reaction for which this dichotomy between experiment and theory exists is the oxygen evolution reaction (OER) from water. In the laboratory group, we employ time-resolved optical and vibrational spectroscopy to deconstruct OER into its individual reaction steps on an electrode surface. In the presentation, I will describe the experimental methodology and the chosen model system, the n-doped SrTiO3/aqueous interface. I’ll show how these experiments identify both the rates and energetics of the first reaction step, the release of a proton and electron from an absorbed water species, denoted by OH* or O*. A recent success was to isolate a Langmuir isotherm of the intermediate population arising within < 2 ps on the SrTiO3 surface. The intermediate population is of trapped holes measured by emission from the conduction band into unoccupied states in the middle of the semi-conductor bandgap. The isotherm is tuned by the pH of the electrolyte. The pH-dependent formation of these intermediates are connected to their pH-dependent decay at microsecond timescales, presumably related to later reaction steps of OER. I'll also present work on the side of vibrational spectroscopy that looks at how the potential energy surface of a particular trapped hole, the terminal oxyl radical, arises in time through the spectral kinetics of its normal mode. A growing area is to apply the particular experimental methodology to a surface of similar electronic structure but diverse crystal geometries, namely polymorphs of TiO2.