The study of the adsorption/desorption of acetate anions on a Pt(111) electrode and the effect of counter cations in acidic media

The mechanism and kinetics of specific adsorption/desorption of acetate were studied at Pt(111) in acidic solution by the potential step method and also by cyclic voltammetry. A so-called anomalous wave in the cyclic voltammogram appeared with the addition of acetate ions into perchloric acid solution, and was shifted by -60 mV with a 10-fold change of the acetate concentration. Such behavior shows that the anomalous wave is due to acetate ion adsorption/desorption by a one-electron transfer process. The acetate adsorption/desorption of the anomalous wave shifted with the increase of pH by -60 mV per pH unit in low pH solutions at pH < 2 with the effect of cations, whereas the pH dependence of the anomalous wave disappeared at pH > 5. These pH dependent tendencies are discussed as that the adsorbed form of acetate is present on Pt(111) electrode over the pH range studied. The current-time (j-t) curve of acetate adsorption/desorption showed different decay features in the adsorption and desorption directions and in different potential regions. In the case of a 0.2 M acetate solution of pH 5.0, random adsorption without interaction, analyzed as the Langmuir adsorption, took place at lower coverage, while random adsorption with repulsive interaction was observed at higher coverage. In contrast to adsorption, the desorption process did not take place by monotonic decay of the j-t curve at high coverage, and the desorption mechanism changed into random desorption without interaction in the medium coverage potential region. When the surface coverage was even lower, a humped j-t curve was observed again. Square-pulse potential step methods revealed the protuberant j-t curve for desorption of the adsorbed acetate within a few ms after the discharge process takes place by a random adsorption reaction. The addition of K+ decreased the rate of acetate adsorption by 40% in the low coverage potential region. (C) 1999 Elsevier Science S.A. All rights reserved.