Reaction Energy for an Electrode Surface Atom Inserting into an R-H Bond and Its Dependence on Electrode Potential: Application to Pt(111)

Abstract

Formulas are derived for calculating the dissociation energies for adsorbed R–H forming adsorbed R and H on Pt(111) electrodes from the reversible potentials for oxidizing R–Hads to Rads + H+aq. Here R is a radical CHxOy· such as CH3·, OH· and HOĊHCH3. When the oxidation reversible potentials lie potential range where in the under potential deposited Hads is present on the surface, which is zero to approximately 0.35 V, on the standard hydrogen scale, the R–H bond strengths are predicted to be zero. For adsorbed molecules with reversible potentials above 0.35 V, the dissociation energy increases linearly as the difference between the reversible potential and 0.35 V and for molecules with reversible potentials below zero volts the dissociation energy becomes negative. Applications to reversible potentials for water and ethanol redox steps show that Hads does not play a role in oxygen reduction to water and for only one of the steps in ethanol oxidation is dissociation forming Hads excluded. For the other ethanol oxidation steps there is currently insufficient information to decide whether R–H dissociates prior to oxidation.