Electrocatalytic Reduction of Nitrite to Ammonia Based on a Water-Soluble Iron Porphyrin

Article abstract of DOI:10.1021/ja00279a036

In aqueous solution at either pH 4.5 or 6.7, the water-soluble porphyrin 3- (H2TPPS4- = tetraanonic form of meso-tetrakis(p-sulfonatophenyl)porphyrin) is an effective electrocatalyst for the reduction of nitrite ion to ammonia with hydroxylamine or N2O also appearing as significant products, depending upon the reaction conditions.The reductions proceed via the nitrosyl complex 3- as an intermediate.The nitrosyl complex forms at pH<3.0 by a reaction between the Fe(III) porphyrin and NO arising from disproportionation of HONO or, at 4.0< pH >7.0, following reduction of Fe(III) to Fe(II) (E1/2=-0.23 V vs.SSCE) via an acid-base reaction.Reduction of the nitrosyl complex occurs by sequential 1-electron steps at E1/2=+0.35 and -0.63 V vs.SSCE, the second of which is pH-dependent at pH <2.6.Evidence for a third 1-electron step has been found by differential pulse polarography at pH 2.1.The first two reductions are followed by a multiple-electron wave which is greatly enhanced as the pH is decreased from 4 to 2.6.Although NH3 is the ultimate reduction product at -0.9 V, NH2OH appears to be an intermediate stage in the reduction and electrolysis to the 2-electron stage at -0.63 V gives increasing yields of N2O.The redox properties of the nitrosyl complex and probable mechanism of reduction of N2O^- to NH3 are discussed and compared with earlier results obtained on polypyridyl complexes of Os and Ru.