The role of hydrous oxide in the electrochemical behaviour of platinum

Abstract

The role of hydrous oxide species in the behaviour of platinum electrodes in aqueous media has been largely ignored until recently. The material may be produced in thick film form by vigorous anodization or potential cycling techniques. The extent of its formation at low potentials in a single anodic unit sweep is quite limited as special, high energy, metal adatom sites are involved. The electrochemical behaviour of this material is complicated by the interaction between its acid-base and redox properties, plus the fact that the reduced form of the couple (the platinum adatom) is unstable at significant coverage. In electroorganic oxidation reactions the low coverage hydrous material functions as a weak but reactive oxidizing (or oxygen donating) agent — the high activity evidently being due largely to interaction of the organic with the reduced form of the couple. This view of electrocatalysis, which is an extension of earlier work with gold, is supported by the observation that, with precautions taken to minimize poisoning effects, many reactive organics commence oxidation on the anodic sweep at the same potential, i.e. in the region of the adatom/hydrous oxide transition. The influence of thick hydrous oxide layers on the oxygen gas evolution process, as well as the previously unexplained inhibition of certain redox reactions, are discussed. Complications associated with hydrous oxide growth, and reduction, in base are outlined.