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 Cl _2(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. NCl ₃ apparently forms from a reaction between NH ₄ ⁺ and Cl _2(aq), which occurs in the near anode area where pH is <2 and the bulk Cl ^- concentration is high. At such conditions Cl _2(aq) is the dominant active chlorine species in the anode vicinity. Upon formation in the near anode area NCl ₃ decomposes to N ₂, NH ₂Cl and NHCl ₂ 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 NH ₂Cl and NHCl ₂ that form in the bulk electrolyte are oxidized to NCl ₃ by Cl _2(aq) upon return to the near-anode zone.