Kinetic modeling and dynamic simulation for the catalytic wet air oxidation of aqueous ammonia to molecular nitrogen

Abstract

A kinetic model for the catalytic wet air oxidation of aqueous ammonia over Ru/TiO₂ catalyst was developed considering the consecutive reaction steps as follows: (i) formation of active oxygen sites O* by the dissociative adsorption of aqueous O₂ on the catalyst, (ii) oxidation of aqueous NH₃ by the reaction with three O* sites to produce HNO₂, (iii) aqueous phase dissociation of HNO₂ into H⁺ and NO ⁻₂ , (iv) formation of NH ⁺₄ by the association of NH₃ with the HNO₂-dissociated H⁺, (v) formation of N₂ by the aqueous phase reaction between NO ⁻₂ and NH ⁺₄ , (vi) formation of NO₃ by the reaction of NO ⁻₂ with an O* site. For each reaction step, a rate equation was derived and its kinetic parameters were optimized by experimental data fitting. Activation energies for the reactions (ii), (v), and (vi) were 123.1, 76.7, and 54.5 kJ/mol, respectively, suggesting that the oxidation reaction of aqueous NH₃ to HNO₂ was a ratedetermining step. From the simulation using the kinetic parameters determined, the initial pH adjustment of the ammonia solution proved to be critical for determining the oxidation product selectivity between desirable N₂ and undesirable NO ⁻₃ as well as the degree of oxidation conversion of ammonia.