Kinetic and mechanistic analysis of the reactions in the aqueous system pentacyanoferrate(II)-ammonia-nitrite

摘要

The pentacyanoferrate(II)-ammonia-nitrite system was subjected to a kinetic and mechanistic study as a function of NH_3 and NO_2~- concentration, pH, temperature and pressure. A detailed study was performed on the formation of [Fe~(II)(CN)_5(NO)]~(2-) in the system consisting of NO_2~- and [Fe~(II)(CN)_5L]~(3-) (L = H_2O or NH_3). Two equilibria were taken into consideration, viz. the pH dependent aquation of the ammine complex, and the transformation of the nitro into the nitrosyl complex. Based on the reported activation parameters, the substitution of water or ammonia in [Fe~(II)(CN)_5L]~(3-) (L = H_2O or NH_3) by nitrite follows a limiting dissociative mechanism. The reaction of [Fe~(II)(CN)_5(NO)]~(2-) with ammonia leading to the formation of dinitrogen was investigated in a more qualitative way, and found to depend strongly on pH with a maximum nitrogen yield at a pH of ca. 10.5. The trend was accounted for on the basis that the process involves nucleophilic attack of NH_3 on the N (NO) atom in [Fe~(II)(CN)_5(NO)]~(2-). This reaction was found to be the rate-limiting step in the catalytic cycle occurring in this system and leading to the overall reduction of NO_x to dinitrogen.