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

In aqueous solution at either pH 4.5 or 6.7, the water-soluble porphyrin [Fe^III(H₂O)(TPPS)]^3- (H₂TPPS^4-= tetraanonic form of meso-tetrakis(p-sulfonatophenyl)porphyrin) is an effective electrocatalyst for the reduction of nitrite ion to ammonia with hydroxylamine or N₂0 also appearing as significant products, depending upon the reaction conditions. The reductions proceed via the nitrosyl complex [Fe^II(NO⁺)(TPPS)]^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) (E_1/2= -0.23 V vs. SSCE) via an acid-base reaction. Reduction of the nitrosyl complex occurs by sequential 1-electron steps at E_1/2= +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 N₂0. The redox properties of the nitrosyl complex and probable mechanism of reduction of NO₂^-to NH₃ are discussed and compared with earlier results obtained on polypyridyl complexes of Os and Ru.