Revealing the mechanism of indirect ammonia electrooxidation

The governing mechanism of indirect ammonia electrooxidation has been often described similarly to breakpoint chlorination. However, comparison of the chloramine concentrations which develop in batch indirect ammonia electrolysis and classical breakpoint chlorination experiments (performed under similar conditions) suggests that the governing reactions are different. Three experimental sets were carried out with excess-ammonia solutions, with the aim of elucidating the mechanism of indirect electrochemical ammonia oxidation: (1) chloramination with Cl2(g) and NaOCl; (2) batch- and (3) single-pass electrolysis experiments. Based on the results we propose a new mechanism for indirect ammonia electrooxidation, according to which trichloramine, rather than monochloramine, is the initial and primary product. NCl3 apparently forms from a reaction between NH4+ and Cl2(aq), which occurs in the near anode area where pH is <2 and the bulk Cl− concentration is high. At such conditions Cl2(aq) is the dominant active chlorine species in the anode vicinity. Upon formation in the near anode area NCl3 decomposes to N2, NH2Cl and NHCl2 in the bulk solution or/and close to the cathode surface area, where pH > 12. Under batch operation and/or single-pass electrolysis characterized by long contact times both NH2Cl and NHCl2 that form in the bulk electrolyte are oxidized to NCl3 by Cl2(aq) upon return to the near-anode zone.