Ammonia Electrooxidation on NiPd Nanoparticles in Alkaline Media: Effect of pH and Concentration

Abstract

Ammonia electrooxidation on NiPd nanoparticles with 99:1 nominal atomic composition is investigated in NaNO3 at different pH and NH4NO3 concentrations. As-prepared NiPd has very low to no activity towards this reaction, whereas the catalytic activity of NiPd can be significantly improved by building up a layer of Ni(OH)2 by continuous cycling in NaNO3 at pH 9 in -0.95 to +1.35 V vs. HgO/Hg. Cyclic voltammetry shows that ammonia oxidation on Ni(OH)2/NiPd occurs around 1.28 V vs. HgO/Hg and is highly pH-dependent. Direct and indirect electron transfer mechanisms are discussed: at low ammonia concentration (< 100 mM) the direct electron transfer takes place, whereas indirect electron transfer mechanism is believed to occur at higher concentrations. The stability of the electrooxidation reaction is evaluated by chronoamperometry. 12-hours electrolysis at 10 mA.cm-2 showed 22 % of initial ammonia is mainly transformed into nitrogen, with traces of NO2 (1.6 %) and NO3 (0.9 %).