The Oxidation of Hydroxylamine on Gold Electrodes in Mildly Acidic Aqueous Electrolytes: Electrochemical and In Situ Differential Reflectance Studies

Abstract

The oxidation of hydroxylamine ͑HAM͒ on polycrystalline Au electrodes has been examined in aqueous acetate buffer ͑pH = 4͒ using electrochemical and in situ reflectance spectroscopic techniques. Cyclic voltammograms recorded under quiescent conditions in the potential region negative to the onset of Au oxidation were characterized by two clearly defined peaks centered at ca. 0.42 V and ca. 0.76 V versus SCE. Corresponding polarization curves obtained with a Au rotating disk electrode ͑RDE͒ as a function of rotation rate, , yielded two rather well defined plateaus. However, plots of the limiting currents, i lim , versus ͱ, and particularly of i lim versus the concentration of HAM at fixed , were found to be non-linear pointing to complexities in the reaction mechanism. Experiments involving dual electrode techniques, including rotating ring-disk electrodes, afforded evidence that the two sequential redox waves are associated primarily with the oxidation of HAM to nitrite and nitrate, respectively. Normal incidence differential reflectance spectroscopy measurements, ⌬R/R, on a Au RDE performed either at high or large ͓HAM͔ failed to detect the presence of Au oxide during HAM oxidation in the potential region in which Au is known to undergo oxidation. This behavior is analogous with that found earlier in our laboratories for the oxidation of bisulfite on Au. The electrochemical properties of hydroxylamine ͑HAM͒ in aqueous electrolytes have received increased attention over the past few years.

Oxidation of Hydroxylamine on Gold Electrodes in Aqueous Electrolytes: Rotating Ring-Disk and In Situ Infrared Reflection Absorption Spectroscopy Studies

“…The much lower currents observed experimentally for the highest rotation rate are in all likelihood due to the gradual accumulation of adsorbed anionic impurities on the surface, in all likelihood chloride, at potentials positive to the point of zero charge, for which the rate would increase as ω increases. This effect would also be responsible for the lack of linearity of the Levich plots, i.e., i peak vs ω 1/2 , in Figure 14 observed in our laboratory for acetate buffer solutions pH 4 6 and also for the slightly smaller Levich slope than that which would be predicted theoretically for a three-electron transfer reaction. Within these limitations, which would amount to a difference of ca.…”

“…Our interests have focused on the use of Au electrodes for this process, a metal that displays no affinity for NO leading in principle to a much simpler reaction mechanism. Indeed voltammetric experiments performed in acetate buffer solution (pH 4) yielded two clearly defined peaks in the double layer region of Au . Furthermore, experiments involving rotating ring-disk electrode (RRDE) techniques afforded evidence that the two consecutive redox waves are associated primarily with oxidation of NH 2 OH to nitrite and nitrate, respectively.…”

“…Indeed voltammetric experiments performed in acetate buffer solution (pH 4) yielded two clearly defined peaks in the double layer region of Au. 6 Furthermore, experiments involving rotating ring-disk electrode (RRDE) techniques afforded evidence that the two consecutive redox waves are associated primarily with oxidation of NH 2 OH to nitrite and nitrate, respectively. Analysis of the data collected with a rotating disk electrode (RDE) yielded nonlinear Levich plots pointing to complications in the overall mechanism.…”

Pronounced Surface Sensitivity of Hydroxylamine Oxidation on Gold Single-Crystal Electrodes in Acidic and Neutral Aqueous Solutions

“…Most of the emphasis has been focused on metals such as Pt, which are known to exhibit high affinity for a variety of species. , Rather surprisingly, in view of its general inertness, Au has also been found to exhibit strong electrocatalytic effects as a function of the crystal plane exposed to the electrolyte. − Among the most striking illustrations is the reduction of dioxygen on Au(100) in alkaline media, a process that displays an onset potential far more positive than that found for the other two low index faces. , Far more intriguing is the higher activity of Au(100) for the oxidation of hydrazine in aqueous acidic electrolytes reported by the Alicante group about a decade ago . This brief contribution describes the role of the surface orientation on the oxidation of NH 2 OH on Au, and represents an extension of electrochemical and spectroscopic studies carried out in our laboratories involving polycrystalline (poly) rotating Au disk electrodes (RDE) in aqueous media. − Hydroxylamine, NH 2 OH, is a reducing agent used most commonly in the synthesis of oximes from aldehydes and ketones. The oxime of cyclohexanone, for example, can be converted to ε-caprolactam, the precursor to the manufacturing of Nylon-6.…”

Quantitative Correlations between the Normal Incidence Differential Reflectance and the Coverage of Adsorbed Bromide on a Polycrystalline Platinum Rotating Disk Electrode

“…Efforts in our laboratory have focused on the development and implementation of optical techniques under forced convection as a means of overcoming many of these limitations. Studies involving near normal incidence reflectance spectroscopy at a rotating disk electrode (RDE), for example, revealed an interesting interplay between the oxidation of bisulfite 13 and, more recently, hydroxylamine, 14 on Au RDEs in aqueous solutions, and the extent of oxide formation and the rate of mass transport of the reactant toward the electrode surface. More specifically, the amount of oxide formed on the surface, as measured optically, was found to decrease significantly when the concentration of the reactant was increased at constant rotation rate, ω, or ω was increased at constant concentration of the reactant in solution.…”