Insights into the Reaction Mechanism of Catalytic Wet Air Oxidation of Ammonia Over Bimetallic Ru–Cu Catalyst

Abstract

The mechanism for catalytic wet air oxidation (CWAO) of ammonia to N2 over Ru–Cu/C catalyst is extensively studied by altering the initial pH, reaction temperature and atmosphere. It is found that N2 formation can be from the catalytically selective oxidation of ammonia or the disproportionation reaction between NH4+ and NO2−. The initial oxidation of ammonia determines the reaction mechanism, the evolution of pH and the distribution of various nitrogen species. Over-oxidation to nitrous acid lowers the pH of the solution due to dissociation of HNO2 to H+ and NO2−. With the decrease of pH, the concentration of NH4+ is increased and rapidly reacts with NO2− to form N2. The relatively lower pH also makes some nitrites be oxidized to NO3−. Enhancing the reaction of selective oxidation of ammonia to N2 increases the selectivity to N2 while limits the pH decrease and NO3− formation, since NO2− is more dominant to HNO2 at high pH and hardly oxidized to NO3−. The reaction temperature is one key factor to determine the reaction mechanism of CWAO of ammonia.