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-H ads to R ads + H + aq . Here R is a radical CH x O y · such as CH 3 · , OH · and HOĊHCH 3 . When the oxidation reversible potentials lie potential range where in the under potential deposited H ads 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 H ads does not play a role in oxygen reduction to water and for only one of the steps in ethanol oxidation is dissociation forming H ads excluded. For the other ethanol oxidation steps there is currently insufficient information to decide whether R-H dissociates prior to oxidation. Electrochemistry is not ordinarily expected to be a tool for measuring bond strengths. In this paper it is shown how to determine R-H bond strengths in adsorbed molecules, such as CH 4ads , H 2 O ads , and HOCH 2 CH 3ads , from standard reversible potentials for oxidation of the bonds to R ads + H + aq + e − . We call this the oxidation into solution reaction. The electrode treated in this paper is platinum, and the close-packed (111) surface is chosen for modeling.The standard reversible potentials employed in this paper were determined previously.1 In Ref. 1 this was done by taking known standard Gibbs reaction energies for solution phase reactions and perturbing them by the adsorption bond strengths of R-H and R to give Gibbs energies for the reactions on the surface. Reversible potentials for the surface reactions were then calculated using these surface reaction Gibbs energies. Except for O-H and HO-H bonds, standard reversible potentials for oxidizing R-H bonds in aqueous molecules treated in this paper are unavailable from experiments. This makes it necessary to calculate the missing values. Because reversible potentials are functions of reaction Gibbs energies, predicting them demands accurate calculations of the Gibbs free energies of the reactants and products. For this, a self-consistent density functional theory (DFT) which includes the electrolyte and its polarization by the aqueous molecules 2 was used in Ref. 1. The details of the DFT will be outlined later in this paper.The R-H oxidation reactions addressed in this paper occur as steps during electrooxidation of ethanol on the anode of a fuel cell. The Pt(111) surface is chosen because it has broad, well-defined under potential deposited (UPD) H ads and double layer potential ranges and because it is well-studied by electrochemists. Knowing the reversible potentials for all the conceivable oxidation into solut...