Extended study of ammonia conversion to N2 using a Ru/0.2TiZrO4 catalyst via catalytic wet air oxidation

Abstract

As one of the key components of waste disposal in conventional water treatment processes, (NH₄)₂SO₄ (NH₃–N concentration = 4390 mg L⁻¹, pH = 5.3) was selected as a model compound. The effectiveness of various heterogeneous catalysts was evaluated by catalytic wet air oxidation (CWAO) in batch reactors. Then, the activity of NH₃–N decomposition was compared systematically in regard the loading amount of noble metal, reaction conditions and concentration of substrate in the continuous reactors. The results showed that under acidic conditions, the elimination of refractory aqueous ammonia can be significantly enhanced upon the addition of a Ru/0.2TiZrO₄ catalyst. After 150 min of reaction, NH₃–N was totally decomposed to N₂ as the final product and no undesirable nitrites or nitrates were detected. The effluent was strongly acidic and H₂SO₄ produced in the effluent could adsorb exhausted NH₃ again to realize green cyclic economy. When combined with density functional theory (DFT), the mechanism of (NH₄)₂SO₄ degradation was assumed showing that NH₄⁺ instead of aqueous ammonia was the necessary active nitrogen-species participating in the CWAO reaction and both the NO₂⁻ and NO₃⁻ generated during the oxidation process can react with NH₄⁺ in the solution and produce N₂. The charts of the related electron cloud were displayed to confirm the possibility and feasibility of the abovementioned reactions according to frontier molecule orbital theory. It can be concluded that CWAO is superior because N₂ was formed as the only final product in our study and can be considered as a potential method to remove large amounts of NH₃–N in wastewater.