Adsorption and Electroreduction of Oxygen on Gold in Acidic Media: In Situ Spectroscopic Identification of Adsorbed Molecular Oxygen and Hydrogen Superoxide

Surface-enhanced infrared absorption spectroscopy was applied to the in situ observations of the surface intermediates during the oxygen reduction reaction (ORR) on quasi-Au(111) thin-film electrode surfaces in contact with 0.5 M perchloric acid aqueous solutions. The spectroscopy revealed the presence of adsorbed molecular oxygen (O₂(ads)) and hydrogen superoxide (HO₂(ads)) during the reaction. These species are not present on the surfaces exposed to the solutions containing no dissolved O₂. In addition, coupled to the onset of the reaction current for the ORR, the surface coverages of the O₂(ads) and the HO₂(ads) showed an abrupt decrease and increase, respectively. These results constitute the first spectroscopic evidence of the proton-coupled one-electron electroreduction of the O₂(ads) to form the HO₂(ads), which has been postulated to occur during the ORR on Au surfaces in acidic media. In addition, the spectroscopy provided for the first time detailed information regarding the structure of the ionic and molecular species formed on the electrode surface in which the O₂(ads) is present; that is, the O₂ hydrated with water molecules is preferentially adsorbed onto the surfaces over the perchlorate anions