Urea indirect electrooxidation by nickel(III) in alkaline medium: from kinetics and mechanisms to reactor modeling

This paper presents an innovative approach to determine and model the kinetics of the catalytic oxidation of urea in alkaline medium on nickel(III) sites. Firstly, the kinetic law is established by considering two types of active sites, either from a chemically synthesized Ni-based powder or from a massive nickel electrode. Thus, the electrochemical regeneration kinetics of nickel(III) can be differentiated from the kinetics of the purely chemical pathway of NiOOH solid particles consumption by urea. Secondly, a mechanism for the urea indirect oxidation mediated by the nickel(III)/nickel(II) system is proposed to predict the formation of all the by-products, contained in the liquid phase that have been experimentally identified in our previous work. Finally, a model combining kinetic laws with diffusive and convective transport phenomena is constructed. The robustness and relevance of the latter are proven by comparing the experimental results obtained during laboratory-scale electrolyzes with those predicted by the model.