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

Cyclic voltammetry has been used to investigate the behaviour of sulph ate and perchlorate anions adsorbed on Ir{111) and PolycrYstalline rho dium. In 0.1 M HClO4, perchlorate anions decompose on polycrystalline rhodium to give a surface intermediate which inhibits hydrogen adsorpt ion. This is in agreement with a previous study by Wieckowski and cowo rkers, who showed that the surface intermediate was, in fact, adsorbed chloride. The voltammetry of the same electrode in sulphate containin g electrolytes depends critically on the concentration of sulphate and the potential sweep rate. We conclude that hydrogen adsorption can on ly occur after significant desorption of sulphate. Hysteresis in the a dsorption and desorption of sulphate results in an asymmetric hydrogen adsorption region with respect to the potential axis. lr{111} is show n also to cause decomposition of perchlorate anions, although, contrar y to the case of rhodium, the process is very much a chemical one in w hich electric currents, arising from perchlorate decomposition, were n ot detected. In sulphuric acid, a strong interaction between anions an d 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.