In-situ electrochemical formation of nickel oxyhydroxide (NiOOH) on metallic nickel foam electrode for the direct oxidation of ammonia in aqueous solution.

A nickel foam-supported Ni(OH) 2 /NiOOH electrode, synthesized in-situ at a specific electrode overpotential, was used to study the oxidation of ammonia in aqueous solution. Results of voltammetric analysis showed the formal potential of Ni(OH) 2 /NiOOH transition at +0.6 V (vs. Hg/HgO, pH 11) at which the current profile was improved by electron transfers of NH 3 in the electrolyte. Selectivity of NH 3 conversion to NO 3 − and N 2 was evaluated by batch constant current experiments. Electrochemical parameters, including solution pH (6–12), temperature (20–40 °C), current density (0.2–3 mA cm −2 ), and initial NH 3 -N concentration (20–450 mg-L −1 ), that may affect ammonia oxidation toward nitrogen selectivity were studied. At constant current density of 1.5 mA cm −2 A, ammonia removal reached 98.5% and NO 3 − was the major product at initial NH 3 -N concentration of 50 mg-L −1 in 7 h. By contrast, N 2 evolution dominated at low current density (<1 mA cm −2 ) and high initial NH 3 -N concentration (i.e., >100 mg-L −1 ). A surface steady-state approach, with NH 3 deprotonation as the rate-limiting step, provided the reaction pathways of NH 3 conversion to molecular nitrogen byproduct. [ABSTRACT FROM AUTHOR]