Electrochemical reduction of molecular oxygen at Hg adatom-modified Au electrodes

O2 reduction on Hg adatom-modified Au electrodes has been examined in acidic, neutral and alkaline solutions using cyclic voltammetry and the rotating ring-disk electrode technique. The half-wave potential for the O2 reduction in 0.1 M NaOH solution was 0.15–0.2 V more positive on the Hg adatom-modified Au electrode than on the bare Au electrode. That is, the O2 reduction was accelerated on the Hg adatom-modified Au electrode surface in the potential range from ca. −0.1 to −0.5 V, compared with the bare Au electrode. Moreover, the accelerated reduction of hydrogen peroxide (HO2−) to OH− could be also observed in the same potential range. Thus, it was found that the O2 reduction on the Hg adatom-modified Au electrode proceeds with the exchange of four electrons in a series of pathways leading to the formation of OH−[O2→HO2−→OH−] in the potential region from ca. −0.1 to −0.5 V. In the potential region from −0.3 to −0.5 V, the reduction current of O2 decreased from the diffusion limiting current corresponding to four-electron reduction of O2 to that for two-electron reduction of O2. The efficiency for the four-electron reduction of O2 depended on the surface coverage (θ) of Hg on the Au electrode surface, and the highest efficiency was obtained on the electrode with θ=0.24. The four-electron reduction current of O2 decreased with decreasing pH of the solution. The reduction of HO2− to OH− ion and also its oxidation to O2 in alkaline media were found to be accelerated on the Hg adatom-modified Au electrode. The observed acceleration of the reduction of O2 is explained satisfactorily by taking account of the potential dependent adsorption of OH− on the Hg adatom-modified Au electrode.