Methanol oxidation at the electrochemical codeposited Pt-Os composite electrode

The electrocatalytic oxidation of methanol has been investigated in aqueous 0.1 M H2SO4 at Pt-Os composite electrodes, which were electrochemically codeposited on a glassy carbon substrate. Scanning electron microscopy indicates that the resulting Pt-Os composite deposits are adherent microparticles, not a film. X-ray fluorescence analysis gives the bulk composition as Pt-81-Os-19 (atomic ratio). The Pt-Os electrode exhibits a high catalytic activity for methanol oxidation at the potentials beyond 0.2 V and shows a large prepeak with a peak potential of 0.48 V prior to the main peak of 0.67 V corresponding to the methanol oxidation at Pt electrode. The results indicate that its catalytic enhancement over Pt results from a more oxophilic behavior of Os. In addition, the change in the Os valence state may dramatically affect the catalytic behavior. X-ray photoelectron spectroscopy indicates the formation of OsO2 in the applied potentials range, but it cannot identify the potential dependence of the Os valence state change. The chemical probe, bromate, is very useful for revealing the change in Os valence state. The results indicate that the formation of Os4+ and subsequent OsO2 leads to the loss of the active OH species on the Os site of the electrode surface and hence results in the prepeak at 0.48 V for the methanol oxidation, attributed to a potential-gate effect, i.e., the Os valence states and activity are determined by potential. (C) 2001 The Electrochemical Society.