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 wer e studied at Pt(111) in acidic solution by the potential step method and al so by cyclic voltammetry. A so-called anomalous wave in the cyclic voltammo gram appeared with the addition of acetate ions into perchloric acid soluti on, and was shifted by -60 mV with a 10-fold change of the acetate concentr ation. Such behavior shows that the anomalous wave is due to acetate ion ad sorption/desorption by a one-electron transfer process. The acetate adsorpt ion/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, w hereas 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-tim e (j-t) curve of acetate adsorption/desorption showed different decay featu res 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 adsorpti on without interaction, analyzed as the Langmuir adsorption, took place at lower coverage, while random adsorption with repulsive interaction was obse rved 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, an d the desorption mechanism changed into random desorption without interacti on in the medium coverage potential region. When the surface coverage was e ven lower, a humped j-t curve was observed again. Square-pulse potential st ep methods revealed the protuberant j-t curve for desorption of the adsorbe d acetate within a few ms after the discharge process takes place by a rand om adsorption reaction. The addition of K+ decreased the rate of acetate ad sorption by 40% in the low coverage potential region. (C) 1999 Elsevier Sci ence S.A. All rights reserved.