Catalytic wet oxidation of ammonia: Why is N2 formed preferentially against NO3-?

The role of catalyst and the reason for the preferential formation of N2 in the catalytic oxidation reaction of ammonia in water over a Ru (3 wt.%)/TiO2 catalyst were elucidated. It was verified that the catalyst in the reaction had no direct relevance to the selective formation of N2, but was responsible only for the oxidation of aqueous ammonia, NH3(aq), finally giving a molecule of nitrous acid. The preferential production of N2 was experimentally demonstrated due to the homogeneous aqueous phase reaction of the nitrous acid-dissociated NO2- with NH4+ ions. Even under the highly oxidizing condition, NO2- was much more likely to react with NH4+ to form N2 than being oxidized over the catalyst to NO3- as long as NH4+ was available in solution.