Open Circuit Potential\nof a Au Catalyst during Selective\nOxidation of Glycerol

It has been of great interest to understand the relationships\nbetween\nthe potential of a metal catalyst and its thermochemical catalytic\nactivity, especially in an aqueous phase environment. In the literature,\nthere are correlations of open circuit potential with reaction rates\nor surface concentrations of the reaction intermediates. In this study,\nwe measured the open circuit potential (OCP) of a Au gauze during\nselective oxidation of glycerol to glyceric acid and hydrogen peroxide\nin a basic solution as a function of hydroxide and alcoholate concentrations.\nIt is found that applying a potential to the Au catalyst has no influence\non the reaction rate. Although a rough correlation appears to exist\nbetween the OCP and reaction rate, the data are better fit to an equation\nwhich assumes that the potential of the metal (i.e., the OCP) is in\nequilibrium with the electrochemical potential of the solution, which\nis defined by the thermodynamic activities of the oxidizing and reducing\nspecies. The equation: OCP = constant + Σ<sub>ox</sub>[(<i>RT</i>/<i>FZ</i><sub>ox</sub>) ln[Ox]] – Σ<sub>red</sub>[(<i>RT</i>/<i>FZ</i><sub>red</sub>)\nln[Red]] implicitly assumes that the Au metal functions as a probing\nelectrode. It is further found that this equation also applies to\nthe Au-catalyzed H<sub>2</sub>O<sub>2</sub> oxidation/decomposition\nreaction in a basic medium and possibly to formic acid dehydrogenation.\nWe postulate that the apparent correlation between OCP and reaction\nrate is due to the fact that the reaction rate is proportional to\nthe concentrations of the reaction products and/or reactants that\ndefine the electrochemical potential of the solution.