Kinetics and Mechanism of the Oxidation of Ammonia by Ferrate(VI) in Aqueous Solutions

Metal oxo complexes are potentially useful for the catalytic oxidation of ammonia to produce nitrite/nitrate or for the construction of ammonia fuel cell. In the work we have studied the kinetics and mechanisms of the oxidation of ammonia by ferrate(VI) (FeO₄^2–), a well-defined metal oxo species. The kinetics was investigated over a pH range of 7.5 to 10, the rate-law is rate=k₂[ferrate][ammonia]. k₂ decrease with increasing pH, the results indicated that at pH 7.5 the reacting species are HFeO₄^– and NH₄⁺, while at pH >9 the reacting species are FeO₄^2– and NH₃. At pH 7.5 the reaction occurs according to the following stoichiometry: 8HFeO₄^– + 3NH₄⁺ + 3H₂O →8Fe(OH)₃ + 3NO₃^– + 2OH^–. Nitrate is also formed as the predominantly product at higher pHs. Based on the experimental results and DFT calculations, the proposed mechanism involves an oxygen-atom transfer from ferrate(VI) to ammonia to produce hydroxylamine, which is then rapidly oxidized by excess ferrate to nitrate.