A kinetic study of the redox behaviour of FeIII(TPPS) [TPPS = 5,10,15,20-tetrakis(p-sulfonato)porphyrinate] in the presence of peroxomonosulfate, hydrogen peroxide, and sulfite/oxygen. Direct evidence for multiple redox cycling and suggested mechanisms†

Abstract

The reactions of the water soluble complex Fe^III(TPPS) [TPPS = 5,10,15,20-tetrakis(p-sulfonatophenyl)porphyrinate] with peroxomonosulfate, hydrogen peroxide and sulfite/oxygen have been investigated kinetically as a function of reactant concentration and pH. The spectral changes recorded for the reactions between the Fe^III(TPPS) dimer and peroxomonosulfate and hydrogen peroxide can be interpreted in terms of a redox cycle between (TPPS)Fe^IIIOFe^III(TPPS) and (TPPS)Fe^IIIOFe^IV(O)(TPPS⁺), and in terms of multiple redox cycles also involving Fe^II(TPPS) for the Fe^III(TPPS)–sulfite–oxygen system. In the case of peroxomonosulfate and hydrogen peroxide a slow redox cycle (1000 s) between iron-(III) and -(IV) complexes is observed at low [SO₅^2–] and [H₂O₂]. In the case of sulfite–oxygen the kinetic traces are quite different; the Fe^III/Fe^IV redox cycle is very fast (a few seconds) and is only observed after what appears to be an induction period. Furthermore, it also depends significantly on the selected experimental conditions (pH, sulfite and oxygen concentration). Rapid-scan techniques were used to study these redox cycles. Reaction mechanisms for the redox cycling of the Fe^III(TPPS)–SO₅^2– system, and for the multiple redox cycling of the Fe^III(TPPS)–sulfite–oxygen system, are proposed. They are based on reactions that participate in the suggested mechanism for the iron-catalysed autoxidation of sulfite. In contrast to the Fe^III(TPPS)–HSO₅^– system, which is insensitive to oxygen, oxygen plays an essential role in the multiple redox cycles of the Fe^III(TPPS)–sulfite–oxygen system, which is accounted for in the proposed mechanism. Computer simulations based on the proposed reaction mechanisms are in good agreement with the observed experimental kinetic traces and indicate that for the Fe^III(TPPS)–sulfite–oxygen system the formation of the SO₅˙^– radical is the main oxygen-consuming step during the overall redox process.