Oxidation of hydroxylamine on gold electrodes in aqueous electrolytes: Rotating ring-disk and in situ infrared reflection absorption spectroscopy studies

The oxidation of hydroxylamine on Au electrodes in aqueous phosphate buffer solutions (pH 7) was examined using electrochemical and in situ infrared reflection absorption spectroscopy techniques. Polarization curves recorded with a rotating Au disk electrode showed that the onset of NH ₂OH oxidation occurs at ca. 0.0 V vs SCE, reaching a well-defined peak at a disk potential, Edisk _peak, ca. 0.2 V vs SCE. Plots of the disk current, i _disk, at Edisk _peak vs ω ^1/2 were linear with a close to zero intercept. Measurements in which E _disk of the rotating ring-disk electrode was scanned, while E _ring was fixed at a value negative enough for solution phase NO to undergo reduction, yielded plots of i _ring vs E _disk, which mirrored the peak found for i _disk and thus was consistent with NO being one of the predominant products of NH ₂OH oxidation. In situ infrared measurements provided evidence for N ₂O being produced at the same onset potential of NH ₂OH oxidation. The disk polarization curves could be reproduced by theoretical simulations involving an EEECE mechanism in which nitrite, one of the products of NH ₂OH oxidation, reacts with NH ₂OH yielding an electrochemically inert species. In accordance with theory, plots of i _disk at Edisk _peak as a function of [NH ₂OH] bent downward as [NH ₂OH] increased.