International Journal of Hydrogen Energy, Vol.45, No.17, 10398-10408, 2020
Electrochemical oxidation of ammonia on nickel oxide nanoparticles
NiO and NiO-TiO2 nano-catalysts were synthetized using solution combustion synthesis (SCS) method and tested toward ammonia oxidation in synthetic and real wastewater. Assynthesized NiO nano-catalyst showed a tightly agglomerated nano-porous spherical structure with sizes ranging from 10 to 50 nm. NiO-TiO2 nano powders have homogenous structure with an average size of 19.5 +/- 0.03 nm and a lattice spacing of 0.22 +/- 0.03 nm corresponding to cubic planes of NiO and 0.25 +/- 0.01 nm corresponding to TiO2. Cyclic voltammetry under alkaline condition at low potential ranging from 0.95 to 1.35 V vs. HgO/Hg improved the electro-chemical activity of the nano-catalysts by the formation of Ni(OH)(2) film on the surface of catalyst as confirmed by XPS measurements. Ammonia electro-oxidation on nano-catalysts occurred at approximately 1.28 V vs. HgO/Hg and was highly pH-dependent. Ammonia removals up to 92.9 and 96.4% were achieved by NiO and NiO-TiO2, respectively. Total nitrogen material balance showed that the electro-chemical oxidation of ammonia produce small amounts of NO2- and NO3 and the balance N-2. Ammonia oxidation at concentration less than 150 mM followed direct electron transfer mechanism, whereas at higher concentrations, the oxidation mechanism shifted to the indirect oxidation regime. Ammonia electro-oxidation kinetics followed zero order reaction at ammonia concentration <= 100 mM and first order kinetics at higher concentrations. More than 93% of ammonia, 35% of organic matter and 40% phosphorous were removed from real wastewater samples using electro-oxidation process confirming the suitability of this technology as advanced wastewater treatment. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.