Adsorption, Exchange, and Oxidation of Formaldehyde on Gold:  A Radiotracer Study

The adsorption, exchange, and oxidation of formaldehyde have been studied on gold at various potentials, temperatures, concentrations, and pH values by voltammetry and the radiotracer thin-gap method and compartmental analysis. Two adsorption rates were identified in the overall adsorption process at the open circuit potential (OCP): a rapid physisorption and a slow chemisorption. Compartmental analysis of the adsorption kinetic data revealed that the physisorption and chemisorption steps proceed in a catenary (serial), reversible fashion. Approximately linear adsorption isotherms were obtained under most conditions, implying that the Gibbs free energy of the reaction depends only a little on the potential, temperature, and pH (the activation energy was previously seen to be strongly affected by these variables). The strong dependence of the quantity of species adsorbed on the formaldehyde concentration in the solution and the invariance of the adsorption isotherms with increasing potentials support the view that potential gradients on the solution side are more important in the adsorption of species than the potential of the electrode itself. At pH 13, the kinetics of the electro-oxidation of formaldehyde is approximately first order both in the concentration of formaldehyde and in the quantity of species adsorbed at the gold surface. The overall rate of the reaction is determined by chemisorption of the enolate anion at low potentials, by desorption of formate at medium potentials, and by diffusion of reactants and products at the highest potentials. Free energies of adsorption (ΔG a) ranged between −29.1 and −25.3 kJ mol-1, heats of adsorption (ΔH a) between −93.9 and −9.0 kJ mol-1, and entropies of adsorption (ΔS a) between −0.2 and 0.1 kJ mol-1.