Catalysts for the oxygen reduction from heat-treated iron(III) tetramethoxyphenylporphyrin chloride: Structure and stability and active sites

Structure and stability of an iron-based catalyst for the oxygen reduction reaction, prepared by heat treatment of carbon-supported iron(III) tetramethoxyphenylporphyrin chloride (FeTMPP-Cl), were investigated. The oxygen reduction in acid electrolyte was examined with the rotating (ring) disk electrode. The measurements confirmed that H₂O₂ is generated as a byproduct of the oxygen reduction. The structural elucidation of the catalyst showed that the porphyrin decomposes during heat treatment. Nitrogen atoms of the heat-treated porphyrin become bonded at the edge of graphene layers as pyridine- and pyrrole-type nitrogen. Two Fe³⁺ components as well as metallic, carbidic and oxidic iron were detected by Mössbauer spectroscopy. An electrochemical longevity test and two degradation experiments with sulfuric acid and H₂O₂ showed that H₂O₂ causes the degradation of active sites. A 6-fold coordinated Fe³⁺ compound seems to be responsible for the catalytic activity. Only 8% of the primary iron content is present in the active iron component.