Indirect urea electrooxidation by nickel(III) in alkaline medium: From kinetic and mechanism to reactor modeling

Abstract

This study consists in the determination of two kinetic laws of urea oxidation (UO) and electrooxidation (UEO) in alkaline media on nickel(III). Two kinds of active sites were examined, the first one derived from a Ni(OH)2 powder and the second from a massive nickel electrode. Partial orders of nickel(III) (two for UO and five UEO) enable to conclude about (i) the urea adsorption on two nickel(III) sites and (ii) that a multistep oxidation of urea occurs involving five nickel(III) site electroregenerations. A multipathway mechanism is also proposed to explain UEO facilitated by the nickel(III)/nickel(II) mediation system, and to predict the by‐products' formation previously identified (). At last, a model combining the UEO kinetic law previously established, with diffusive and convective transport phenomena was developed. A consistent correlation (maximum deviation of 6%) between laboratory electrolysis results with the model' predictions was obtained under different operating conditions, enabling the validation of this model.

Unveiling the Role of Iron in the Nickel-Catalyzed Urea Oxidation Reaction

“…Note that contrary to Hopsort et al, , no NH 4 + formation was observed in this work. To clarify whether NH 4 + could be eventually formed and then oxidized electrochemically on Ni, cyclic voltammograms were acquired in the presence of ammonia for the Ni and Ni x Fe 1– x hydroxide electrodes (Figure S18).…”

“…As to the products, early studies reported formation of N 2 , CO 2 , and H 2 O as the only products of the UOR on Ni. − Meanwhile, recent works revealed that depending on the experimental conditions, other species, such as cyanate (NCO – ), ammonium (NH 4 + ), nitrite (NO 2 – ), nitrate (NO 3 – ), and nitrous oxide (N 2 O) can also be formed. − A question thus arises as to whether the UOR products are influenced by the Fe incorporation in Ni­(OH) 2 .…”

New Insights into Urea Electro-Oxidation: Complete Mass-Balances and Proof of Concept with Real-Matrix Effluent

“…− 1 for OCN − and NH 4 + (the formation of these species indirectly implies Ni(II)→Ni(III) oxidation system, as described in a previous work 16 ), − 6 for N 2 and − 6 for NO .…”

“…Unfortunately, in the case of electrolysis with large urea conversion rates, complete mass balances (MB) are still scarcely established, even if complex mechanisms are always suggested. 12,13,15,16 Identifying all the byproducts requires the coupling of several analytical techniques, which is generally not done. Li et al 12 have revealed that the commonly used nickel-based catalysts exhibit a tendency to overoxidize urea into NO 2 − with about 80% of Faraday efficiency (FE).…”

“…Faraday efficiency balances are always reported, but their scope remains limited as, without complete mass balances, the composition of liquid and gaseous phases cannot be fully determined. In previous works, 11,16 we proposed for the first time a complete mass balance in the liquid phase during potentiostatic electrolyzes obtained at high urea conversion rates (>80%); as well as kinetics laws and reaction mechanisms based on a chain of reactional adsorption of urea on Ni (III) active sites. However, in this study, no mass balance in the gas phase has been made, thus pointing out the need of additional investigation for the part of the mechanism leading to the gaseous products.…”