Kinetics and mechanism of the cobalt phthalocyanine catalyzed reduction of nitrite and nitrate by dithionite in aqueous solution

The reaction of sodium nitrite with sodium dithionite was studied in the presence of cobalt(II) tetrasulfophthalocyanine, Co-II(TSPc)(4-), in aqueous alkaline solution. The overall mechanism comprises the reduction of Co-II(TSPc)(4-) by dithionite, followed by the formation of an intermediate complex between Co-I(TSPc)(5-) and nitrite, which undergoes two parallel subsequent reactions with and without nitrite as a reagent. Kinetic parameters for the different reaction steps of the catalytic process were determined. The final product of the reaction was found to be ammonia. Contrary to those found for the catalytic reduction of nitrite, the products of the catalytic reduction of nitrate were found to be dinitrogen and nitrous oxide. The possible catalytic reduction of nitrous oxide was confirmed by independent experiments. The striking differences in the reduction products of nitrite and nitrate are explained in terms of different structures of the intermediate complex between Co-I(TSPc)(5-) and substrate, in which nitrite and nitrate are suggested to coordinate via nitrogen and oxygen, respectively.