The importance of superficial (adatom) surface oxidation in the electrocatalytic behaviour of noble metals in aqueous media

Abstract

The conventional, activated chemisorption, model of electrocatalysis is unable to explain fully the surprisingly high level of activity of gold anodes in base for certain organic electrooxidation reactions or even the ability of platinum to catalyse certain oxygen insertion reactions, e.g. methanol or formaldehyde oxidation in acid, at low potentials (in the hydrogen or double layer region) where the surface is considered usually to be oxygen-free. It was demonstrated that such behaviour can be rationalized by postulating the involvement of a quite low coverage of hydrous oxide species, generated at low potential at adatom sites, which act as mediators or catalysts in many electrooxidation reactions. The converse situation was also shown to arise, i.e. the reduction of certain species, e.g. HNO3 or NO 3 − , which requires the involvment of adatoms, was shown to be inhibited as long as the latter existed in the oxidized, hydrous oxide, form. An appreciation of hydrous oxide electrochemistry was shown to be very useful in interpreting electrocatalytic effects at noble metal electrodes in aqueous media.