Wet Air Oxidation of Aqueous Ammonia Solutions Catalyzed by Composite Metal Oxide

Abstract

The wet oxidation of ammonia solutions (400-1,000 mg/L) at a pressure of 4.0 MPa over a Cu/La/Ce (7:2:1) composite catalyst and temperatures of 150, 200, and 230°C, was performed to elucidate the elimination of ammonia. The results indicate that the reduction of ammonia is inefficient when the reaction temperature is below 230°C, at which point ammonia removal reached nearly 95%. The decomposition of ammonia was observed to proceed in two steps during wet oxidation, and the yield of the product was directly proportional to the reaction temperature. Higher reaction temperature was also associated with more efficient wet oxidation. The first-stage reaction was observed to conclude within 20 min, and its reaction rate was markedly higher than that of the second-stage reaction. The activation energy for ammonia decomposition over a Cu/La/Ce (7:2:1) composite catalyst and without the catalyst were calculated as 9.75 and 19.8 kJ mol^-1, respectively. Increasing the partial pressure of oxygen during wet oxidation promotes the decomposition of ammonia; however, it is in inverse proportion to the initial concentration of ammonia in solution. Moreover, the decomposition of ammonia was markedly affected by pH because the conversion reached 95% at a pH of approximately 12.