THE UNUSUAL REDUCTION BEHAVIOR OF THIN, HYDROUS PLATINUM OXIDE-FILMS

Thin, hydrous beta-Pt oxide films were formed electrochemically by the rapid potential cycling method. When these films were partially reduced at constant potential (0.65-0.30 V), and the remaining oxide film was reduced by scanning the potential to 0 V, up to 20% more charge is passed than when the film is reduced exclusively in a potential sweep. Also, the potential and shape of the remaining beta-oxide peak is significantly altered after partial potentiostatic reduction. The excess cathodic charge passed in these experiments has been correlated with the appearance and magnitude of a new peak beta' seen in the next negative scan after a positive sweep into the region of compact Pt oxide formation. These results may indicate that potentiostatic film reduction occurs by a different mechanism, such that no oxide dissolution occurs, as compared to sweep reduction. They may also reflect the electrodeposition of dissolved Pt species at the base of the beta-oxide pores during potential holding. This deposit could inhibit the full reduction of the beta-oxide film in the normal range of potentials, while surface rearrangement brought about by the place exchange process during compact oxide formation/reduction in the next negative cycle allows it to be reduced in the beta' peak.