The oxygen electrode. Part 4. Lowering of the overvoltage for oxygen evolution at noble metal electrodes in the presence of ruthenium salts

The presence of low levels of ruthenium trichloride (ca. 10^-4 M) in aqueous acid solution was found to give a considerable increase in the rate of oxygen evolution from platinum and gold, but not graphite, anodes. The mechanism of this catalytic effect was investigated using potential-step and a.c. impedance techniques, and it was shown that ruthenium species were incorporated into the surface oxide film. Changes in the oxidation state of these ruthenium species are probably responsible for the lowering of the oxygen overvoltage. Both the theoretical and practical aspects of the reaction were complicated by the fact that at constant potential the rates of both the catalysed and the uncatalysed oxygen evolution process exhibit an appreciable, continuous decrease with either time or degree of oxidation of the substrate. The enhancement of the oxygen evolution process in the presence of ruthenium is, however, sufficiently marked to warrant further investigation with substrates containing either this or related metal ions.