On the Mechanistic Origins of the pH-Dependency in Au-Catalyzed Glycerol Electro-Oxidation: Insight from First Principles Calculations

Abstract

Electrocatalytic oxidation of glycerol (EOG) is an attractive approach to convert surplus glycerol to value-added products. Experiments have shown that EOG activity and selectivity depend on the electrocatalyst, but also on the electrode potential, the pH, and the electrolyte. For broadly employed gold (Au) electrocatalysts, experiments have demonstrated high EOG activity under alkaline conditions with glyceric acid as a primary product, whereas under acidic and neutral conditions Au is rather inactive producing only small amounts of dihydroxyacetone. In the present computational work, we have performed an extensive mechanistic study to understand the pH- and potential-dependency of Au-catalyzed EOG. Our results show that activity and selectivity are controlled by the presence of surface-bound hydroxyl groups. Under alkaline conditions and close to the experimental onset potential, modest OH coverage is preferred according to our constant potential calculations. This indicates that both Au(OH)ads and Au can be active sites and they cooperatively facilitate the thermodynamically and kinetically feasible formation of glyceric acid thus explaining the experimentally observed high activity and selectivity. Under acidic conditions, hydroxide coverage is negligible and the dihydroxyacetone emerges as the favored product. Calculations predict slow reaction kinetics, however, which explains the low activity and selectivity towards dihydroxyacetone reported in experiments. Overall, our findings highlight that computational studies should explicitly account for pH and coverage effects under alkaline conditions for electrocatalytic oxidation reactions to reliably predict electrocatalytic behaviour.