Bicatalyst electrocatalytic reaction of sulfur oxoanions by water-soluble iron porphyrins and iron(II) tris(1,10-phenanthroline)

The electrocatalytic reductions of S4O62- to S2O32- by Fe(2-TMPyP) and Fe(3-TMPyP) were investigated in aqueous solution at room temperature at various pH values. The water-soluble iron tetrakis (N-methyl-2-pyridyl) porphyrin (Fe(2-TMPyP)) catalyzed the rapid reduction of tetrathionate after using an electrochemical method to reduce Fe(2-TMPyP) to the Fe-I(2-TMPyP) species. Iron(II) tris(1,10-phenanthroline) performed the electrocatalytic oxidation of thiosulfate via an outer-sphere electron transfer mechanism. The electrocatalytic reduction current of S4O62- by Fe(2-TMPyP) appears when S4O62- is produced from the electrocatalytic oxidation of S2O32- by a catalyst, Fe(II) tris(1,10-phenanthroline) and Fe(2-TMPyP) with the Fe(II/I)(2-TMPyP) redox couple. On the other hand, the electrocatalytic oxidation current of S2O32- by Fe(II) tris(1,10-phenanthroline) appears when S2O32- is produced from the electrocatalytic reduction of S4O62- by the catalyst Fe(2-TMPyP) in the range of pH values in which Fe(n/I) (2-TMPyP) redox couples are present. The electrocatalytic reduction of S4O62- to S2O32- by Fe(3-TMPyP) was also investigated. The electrocatalytic reaction is different to that observed in the case of Fe(2-TMPyP). First, Fe-III(3-TMPyP) reduces to Fe-II(3-TMPyP) by the electrochemical method, then Fen(3-TMPyP) coordinates with S4O62- and electrocatalytically reduces S4O62- to S2O32- to a more negative potential.