Oxidation of ammonia in dilute aqueous solutions over graphite-supported α- and β-lead dioxide electrodes (PbO₂@G)

Graphite-supported α- and β-PbO₂ electrodes (PbO₂@G) were prepared by electrochemical deposition at appropriate potentials with regard to Pb(II)/PbO₂ redox couple under alkaline and acidic conditions, respectively, for studying the direct electro-oxidation of ammonia in aqueous solutions. Results of surface characterization including scanning electron microscopy (SEM), X-ray diffractometer (XRD) and X-ray photoelectron spectroscopy (XPS) indicated the presence of polymorphs of PbO₂@G. Cyclic voltammetry (CV) of the electrolyte containing NH₃ indicated mediation of electron transfer by PbO₂. At the onset potential of ca. +1.0 to +1.45 V (vs. Hg/HgO), a pathway of NH₃ oxidation to nitrogen byproducts, namely, N₂, NO₂⁻, and NO₃⁻ was proposed. The removal efficiency and selective conversion of ammonia (0.1 M Na₂SO₄, pH 11, 25 °C) on PbO₂@G was determined based on controlled potential experiments.