The role of ion adsorption in surface oxide formation and reduction at noble metals: General features of the surface process

Abstract

Some important general features of the processes of electrochemical surface oxidation of noble metals are identified and related to effects of specific adsorption of anions. The first three stages in metal oxidation occur generally in successive overlay lattices which arise on account of repulsion between metal-oxygen dipoles; they are reversible on metals, such as Ru, Ir, having little tendency to adsorb anions or on Pt in alkaline solutions where anions other than OH − are absent. Irreversibility increases with oxide coverage, but the monolayer film always shows hysteresis due to place-exchange. The next stage of oxidation occurs over a single broad region, with hysteresis between its formation and reduction, decreased by anion adsorption. The anion effects arise on account of: (a) adsorption competitive with OH/O deposition which blocks initial stages of oxidation (Pt in acid or at Au); (b) change of inner-layer field which modifies the field-assisted place-exchange process and (c) lateral repulsion with metal-oxygen dipoles, which facilitates place-exchange (Au and Pt in acid). Cations, on the contrary, stabilize the unrearranged metal-oxygen dipoles, as is shown by the behaviour of Au in Ba(OH) 2 solution. Studies on the reduction kinetics at Au distinguish one reduction stage, which is a simple surface charge-transfer process, from a second, which is preceded by a slow chemical step, probably reverse place-exchange.