Catalytic Performance and Mechanism of Iron meso-Tetra(4-carboxyphenyl)porphine for Oxygen Reduction Reaction

The catalytic performance and mechanism of the prepared iron meso-tetra (4-carboxyphenyl) porphyrin (FeTCPP) on oxygen reduction reaction in alkaline environments were studied. At high concentrations of FeTCPP, the oxygen reduction reaction followed an efficient direct 4-electron transfer mechanism instead of 2+2 electron transfer mechanism. The central iron ion in FeTCPP underwent redox reactions from FeIII to FeII. O2 molecules formed a superoxide anion coordination complex with FeIITCCP, transferring an electron from the central iron ion. Protonation of the complex formed a superoxide hydrogen radical complex, which then acquired another electron, generating a hydroperoxide radical and completing the first 2-electron reduction. The peroxide formed a coordination complex with FeIITCCP to produce a peroxide complex FeIIITCCP:O2– with intramolecular charge transfer. And then it acquired two electrons, completing the second 2-electron reduction and being reduced to OH–. When the concentration of FeIIITCCP was high, the complex FeIIITCCP:O2– could protonate without stepwise reactions, directly acquiring three electrons from the electrode surface and being reduced to OH–. The oxygen reduction reaction exhibited a 4-electron mechanism. Both the 2+2 and 4-electron pathways occurred simultaneously, with the 4-electron pathway becoming more prominent as the concentration of FeTCPP increased.