ANION SURFACE INTERACTIONS .1. PERCHLORATE DECOMPOSITION AND SULFATE ADSORPTION HYSTERESIS STUDIED BY VOLTAMMETRY

Cyclic voltammetry has been used to investigate the behaviour of sulphate and perchlorate anions adsorbed on Ir{111) and PolycrYstalline rhodium. In 0.1 M HClO4, perchlorate anions decompose on polycrystalline rhodium to give a surface intermediate which inhibits hydrogen adsorption. This is in agreement with a previous study by Wieckowski and coworkers, who showed that the surface intermediate was, in fact, adsorbed chloride. The voltammetry of the same electrode in sulphate containing electrolytes depends critically on the concentration of sulphate and the potential sweep rate. We conclude that hydrogen adsorption can only occur after significant desorption of sulphate. Hysteresis in the adsorption and desorption of sulphate results in an asymmetric hydrogen adsorption region with respect to the potential axis. lr{111} is shown also to cause decomposition of perchlorate anions, although, contrary to the case of rhodium, the process is very much a chemical one in which electric currents, arising from perchlorate decomposition, were not detected. In sulphuric acid, a strong interaction between anions and the lr{111} surface is observed, which precludes oxide formation at potentials less than 1 V (Pd-H), behaviour characteristic of a Pt{111} surface in the same electrolyte.