Electrocatalytic oxidation of sulfite at polymeric iron tetra (4-aminophenyl) porphyrin—modified electrode

Abstract

The electro-oxidation of sulfite was studied by using a glassy carbon electrode coated with a polymeric film of Fe-tetra-4-aminophenylporphyrin, in a wide pH range. The polymeric complex-modified electrode catalyses the electro-oxidation of sulfite to sulfate in acid and basic media, but it is more active at pH higher than 8.5. The polymer film coating is obtained by cycling the electrode in a solution containing the complex (monomer). The polymerization process takes place after the oxidation of the amino groups via a radical mechanism. When the modified electrode is obtained, the polymeric system needs to be over-oxidized in order to avoid the further oxidation of the remaining amino groups in the potential range where the oxidation of the sulfite takes place. The over-oxidation process destroys the monomer-modified electrode. Then, the electropolymerization of the iron complex is the only way to obtain a catalyst for this reaction with this porphyrin. The specie postulated for the oxidation is Fe(IV). The polymer-modified electrode is very stable and its activity remains almost constant for more than 500 consecutive cycles. Also, its activity remains unchanged during more than 30 days of exposure to air and light. The oxidation process occurs during the anodic and the cathodic potentiodynamic scans, showing that it is only necessary the recovery of the active sites; the formation of Fe(IV) is probably needed to promote the oxidation. However, this regeneration is low and becomes the slow step of the reaction.