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

Abstract 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) (FeO 4 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= k 2 [ferrate][ammonia]. k 2 decrease with increasing pH, the results indicated that at pH 7.5 the reacting species are HFeO 4 – and NH 4 + , while at pH >9 the reacting species are FeO 4 2– and NH 3 . At pH 7.5 the reaction occurs according to the following stoichiometry: 8HFeO 4 – + 3NH 4 + + 3H 2 O →8Fe(OH) 3 + 3NO 3 – + 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.