Abstract
The role of catalyst and the reason for the formation of N2 as a preferential product in the oxidation reaction of aqueous ammonia (NH3(aq.)) over Ru/TiO2 catalyst was elucidated. The catalyst was responsible only for the oxidation of aqueous NH3, finally giving a molecule of nitrous acid. An oxidation reaction pathway was proposed as follows: Oxidation of ammonia in water was initiated by the reaction of NH3(aq.) with catalytically activated oxygen. After undergoing further successive oxidation reactions with activated oxygen, NH3(aq.) was finally converted to a molecule of nitrous acid, which, in turn, dissociated into a nitrite ion and a proton. The concentration of NH4+ in solution increased in accordance with the decrease of the solution pH by the HNO2-dissociated protons. Molecular nitrogen was formed from the homogeneous aqueous phase reaction between NH4+ and NO2- ions such produced. Further reaction of NO2- with the catalytically activated oxygen led to the formation of nitrate ion as another final product. The ionic reaction of NO2- with NH4+ was accelerated above 100°C, and kinetically more rapid than the oxidation reaction of NO2- with the catalytically activated oxygen as long as NH4+ was available in the solution.
