Open Circuit Potential of a Au Catalyst during Selective Oxidation of Glycerol

Abstract

It has been of great interest to understand the relationships between the potential of a metal catalyst and its thermochemical catalytic activity, especially in an aqueous phase environment. In the literature, there are correlations of open circuit potential with reaction rates or surface concentrations of the reaction intermediates. In this study, we measured the open circuit potential (OCP) of a Au gauze during selective oxidation of glycerol to glyceric acid and hydrogen peroxide in a basic solution as a function of hydroxide and alcoholate concentrations. It is found that applying a potential to the Au catalyst has no influence on the reaction rate. Although a rough correlation appears to exist between the OCP and reaction rate, the data are better fit to an equation which assumes that the potential of the metal (i.e., the OCP) is in equilibrium with the electrochemical potential of the solution, which is defined by the thermodynamic activities of the oxidizing and reducing species. The equation: OCP = constant + Σ_ox[(RT/FZ_ox) ln[Ox]] – Σ_red[(RT/FZ_red) ln[Red]] implicitly assumes that the Au metal functions as a probing electrode. It is further found that this equation also applies to the Au-catalyzed H₂O₂ oxidation/decomposition reaction in a basic medium and possibly to formic acid dehydrogenation. We postulate that the apparent correlation between OCP and reaction rate is due to the fact that the reaction rate is proportional to the concentrations of the reaction products and/or reactants that define the electrochemical potential of the solution.