Catalytic wet air oxidation of ammonia over M/CeO2 catalysts in the treatment of nitrogen-containing pollutants

Ammonia, a well-known by-product of chemical, fertiliser and metallurgy industries, is also the most refractory product of nitrogen-containing compound oxidation. Consequently, NH₄⁺ is a key component of waste disposal of conventional processes like anaerobic digestion or nitrification/denitrification. Catalytic wet air oxidation (CWAO) process, able to eliminate organic matter with non toxic by-product formation, was investigated for ammonium ions removal from wastewater. Oxidation of aniline and of ammonia were carried out on mono- and bimetallic noble metal catalysts (Pt, Ru, Pd, etc.) prepared by impregnation and supported on cerium oxides. In liquid phase, at high temperature (150–250 °C) and high pressure of oxygen (20 bar), a Ru/CeO₂ catalyst is able to achieve the elimination of refractory nitrogenous organic products like aniline. The greatest interest of CWAO compared to the classical biological one, is that the selectivity towards molecular nitrogen is much higher (>90%). Indeed, in this process, ammonium ions give essentially N₂, via hydroxylamine and below 200 °C. At higher temperatures the rate of conversion is extremely high but nitrite and nitrate ions appear in the effluent. On a RuPd/CeO₂ catalyst, the optimal temperature for ammonia conversion is then 200 °C. In these conditions, the N₂ selectivity is up to 90%.