Reversibility and irreversibility in the formation and reduction of oxide film states on Co at ambient and low temperatures

Abstract

The transition from submonolayer through monolayer to multilayer surface oxide formation and the growth of such films on Pt and other noble metals can be followed sensitively by means of cyclic voltammetric experiments. However, on baser metals, multilayer oxide film formation proceeds rapidly in aqueous media at room temperature so that the initial stages cannot be observed under normal conditions. In order to examine whether monolayer or submonolayer coverage of surface oxidation can be achieved on Co, cyclic voltammetric experiments were conducted at temperatures down to 240 K in alkaline solutions in an 80 mol% methanol + water mixture. It is shown that surface oxidation coverages down to levels equivalent to a monolayer of Co(OH) 2, or less can be achieved at about 260 K. However, even at low levels of Co oxidation, the oxide formation and reduction processes remain irreversible. When Co(III) and Co(IV) oxides are generated, their formation and reduction shows a transition from complete irreversibility initially to fairly reversible redox behaviour after cycling when a thick oxide film has been generated.