Redox potentials of radicals on metal–water interfaces; diffusion-controlled processes in water are sometimes endergonic on metal surfaces

Following the interest in heterogeneous reactions, a method, using DFT, for calculating reduction potentials of adsorbates on a surface, is developed. This method uses only adsorption energies and the published reduction potentials in aqueous solutions, avoiding the problematic computation of H⁺. The focus of this manuscript is on radicals, H., OH., HO₂., CH₃., and reactions in which they are involved. The potentials were calculated on four surfaces: Ag(111), Au(111), Pt(111), Ni(111). The reduction potentials of the radicals adsorbed on the surfaces differ considerably both from their values in aqueous solutions and from one another, indicating that the reactivity of adsorbed radicals is substantially altered relative to their behaviour in solution. The calculated reduction potentials were further used to calculate Gibbs free energies for reactions in which radicals are involved. All the calculated ΔG⁰ values for radical reactions on the surface are higher than in aqueous solutions, indicating the relative stability of radicals adsorbed on metal surfaces.