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.