Profiles of adsorption during the oxidation of small organic molecules: oxidation of formic acid at polycrystalline Pt in acid solutions

Abstract

The electrochemical quartz crystal microbalance has been used to monitor changes in adsorption at Pt electrodes during the oxidation of formic acid in 0.1 mol dm^–3 HClO₄ solutions. This has been achieved through cyclic voltammetric, injection and open-circuit potential decay experiments where mass profiles are recorded alongside the electrochemical response. Adsorption in the H underpotential deposition (UPD) region causes the mass to increase relative to the background electrolyte whereas in the double-layer region of potential, increased coverage of strongly adsorbed intermediates has the reverse effect. Removal of these latter species can be followed from the mass response because it leads to a characteristic mass step. Subsequent to this process there is a region of potential where oxidation of formic acid occurs through consumption of adsorbed OH or PtOH and mass responses reveal that, as concentration increases, there is increased adsorption of organic residues here. The consumption of surface oxy species (OH_ads, PtOH or PtO) by formic acid also results in irreversible oxidation of the electrode surface being shifted to higher potentials with increasing formic acid concentration, since it is only at the higher potentials that the rate of the place exchange process can compete effectively with reaction with formic acid. The mass decrease associated with removal of the surface oxide is also accelerated at higher formic acid concentrations and occurs at higher potentials.