Anodic film formation on osmium electrodes

The formation and reduction of anodic films on Os electrodes in 2 M HCl and HClO4 solutions were studied by anodic and cathodic charging curves. The galvanostatic oxidation of Os in HClO4 shows the formation of OsO2 as an intermediate step to OsO4 that goes in the solution. The cathodic charging curves at Os electrodes previously oxidized at constant potential reveal the anodic film to be made up of a reversibly desorbed oxygen layer and an oxide phase reduced irreversibly. Both layers increase with time under potentiostatic conditions following a logarithmic equation until a constant value is reached. At all times, the content of OsO2 in the anodic film at high potentials is larger than that of chemisorbed oxygen. In HCl solutions only the reversible reduction of an oxygen layer is observed. The growth of this film also complies with a direct logarithmic law before attaining a limiting coverage. The charge involved in the reduction increases linearly with the potential at a given time of formation. The results are discussed in terms of a Temkin-type isotherm and a place-exchange mechanism.