Mathematical modeling of ammonia electrooxidation kinetics in a Polycrystalline Pt rotating disk electrode

A mathematical model for the ammonia electrooxidation kinetics, over polycrystalline Pt in alkaline media, was developed through the micro kinetics approach. Two different mechanisms were evaluated for the development of the model, a combined surface-diffusion controlled process and a surface confined process. Nonlinear multiple parameters estimation was implemented by direct comparison with experimental data, obtained in a rotating disk electrode system during linear sweep voltammetry, to calculate the kinetic parameters for the different elementary steps proposed in the mechanisms. It was demonstrated through modeling discrimination techniques that a surface confined process drives the oxidation of ammonia on the implemented electrode surface. The model validates that the formation of N₂ is controlled by the catalytic dimerization of partially reduced ammonia adsorbates (i.e. NH_2(ads)). Moreover, it confirms that the formation of stronger adsorbed intermediates at high overpotentials is the main cause for the catalyst deactivation.