THE CHLORIDE INHIBITION OF ETHANOL ELECTROOXIDATION AT A PLATINUM ELECTRODE IN ACIDIC AND NEUTRAL AQUEOUS SOLUTIONS

The electrooxidation of ethanol at a platinum electrode in aqueous solutions at equal ionic strength of sulfuric acid, nitric acid, perchloric acid, sodium nitrate, sodium perchlorate, and sodium nitrate/sodium sulfate and sodium perchlorate/sodium sulfate mixtures has been investigated by cyclic voltammetry. This study proposes mechanisms for each of the three anodic waves observed in the voltammograms for both acidic and neutral solutions and characterizes the effect of anions.; Sodium chloride at concentrations of 0.1-10 mM in 0.5 M sulfuric acid inhibits all three cyclic voltammetric waves. This inhibition is caused by the adsorption of chloride ions on the platinum electrode, indicating that surface species participate in each wave. Proposed mechanisms are based on results both in this thesis and in the literature. For the two waves observed during the positive potential sweep, the mechanisms involve a chemical reaction between chemisorbed or adsorbed ethanol and some form of the platinum oxide layer, yielding acetaldehyde as a product. The mechanism for the wave seen during the reverse sweep involves both an electrooxidation of adsorbed ethanol to acetaldehyde and formation of a chemisorbed dehydrogenated ethanol radical which is oxidized during the first wave of the positive sweep.; Experiments in the acidic solutions have shown that the ethanol electrooxidation and its inhibition by chloride are influenced by other anions, suggesting a competitive adsorption between the anions, the oxygen species and ethanol. The magnitude of this effect is related to the surface coverages by the anions. Anion surface coverage in the present study increases in the order perchlorate, sulfate and nitrate.; A smaller anion effect is observed in the neutral solutions than in acidic solutions. This is attributed to the expected increase in surface coverage by oxygen species on platinum at the higher pH of these solutions, as well as to changes in the rate-determining steps of the mechanisms. Smaller negative peak potential shifts than expected, based on published reports, are observed in going from acidic to neutral solutions. The results from the neutral solutions are consistent with the mechanisms proposed for the acidic solutions. The participation of the platinum electrode surface states in the electrooxidation suggests the use of the ethanol system as a probe for platinum surface states under various conditions. Additionally, studies of anion adsorption behavior could be made using this system.