Anomalous oxidation reactions at noble metal surfaces at low potentials: With particular reference to palladium

Multilayer palladium hydrous oxide films, produced by potential cycling, were more stable in acid solution at 0 degrees C (as compared with room temperature). The electrochemical responses for the reduction of such films were more consistent at the lower temperature and were remarkably similar to those of platinum under similar conditions. Both metals yield a hydrous oxide containing two components, HO1 and HO2. The cyclic voltammogram for the bare metal (Pd or Pt) in acid contains an unusual feature at low potentials in the absorbed hydrogen region. This feature, a low level reversible transition at ca. 0.24 V(RHE) for palladium, was reported in earlier independent work. It is attributed here to a premonolayer oxidation effect, i.e., an active metal atom/incipient hydrous oxide transition. The peak maximum potential (E(p)) for HO2 reduction in multilayer Pd hydrous oxide reduction experiments, on extrapolation to either very low sweep rates or low film thicknesses, gave a similar potential value. The relevance of this result to noble metal electrocatalysis is outlined.