Electrochemical behavior of a number of bispyridyl-substituted porphyrins and their electrocatalytic activity in molecular oxygen reduction reaction

Abstract

ABSTRACT:The investigation of electrochemical properties of active layers containing 5,15-bis(pyrid-3-yl) -3,7,13,17-tetramethyl-2,8,12,18-tetraethylporphine, [5,15-bis(pyrid-3-yl)-3,7,13,17-tetramethyl-2,8,12, 18-tetraethylporphinato]cobalt(II), bispyridine [5,15-bis(pyrid-3-yl)-3,7,13,17-tetramethyl-2,8,12,18-tetraethylporphinato]cobalt(III), [5,15-bis(pyrid-4-yl)-3,7,13,17-tetramethyl-2,8,12,18-tetraethylporphinato]cobalt(II) and bispyridine [5,15-bis(pyrid-4-yl)-3,7,13,17-tetramethyl-2,8,12,18-tetraethylporphinato]cobalt(III) in 0.1 M KOH aqueous solution has been carried out by cyclic voltammetry. The potential ranges of redox processes related to the transformation of the porphyrin macrocycle, pyridyl substituents and central metal have been established. The electrochemical parameters of oxygen electroreduction reaction (the half-wave potential -E 1/2 (O 2 ), the maximum potential -E p (O 2 ), the current density -j, the activation energy -E act ) have been determined. It was shown that all porphyrins exhibit the catalytic activity. The electroreduction process of oxygen on electrode with the porphyrin-ligand occurs via 2-electron mechanism to give hydrogen peroxide ion HO 2 -, and on ones with the cobalt porphyrins -via parallel-sequential pathway including 2-electron and 4-electron processes to give HO 2 -, OH -. The presence of pyridine axial ligands in Co-complexes leads to a significant increase of the electrocatalytic activity in molecular oxygen reduction reaction.

Cobalt Tetrabutano- and Tetrabenzotetraarylporphyrin Complexes: Effect of Substituents on the Electrochemical Properties and Catalytic Activity of Oxygen Reduction Reactions

“…It has long been known that cobalt porphyrins can be used as catalysts for oxygen reduction reactions (ORR) in acid media to generate H 2 O 2 and/or H 2 O via a two- or four-electron-transfer pathway, − and a large number of derivatives have been examined for their catalytic properties for ORRs over the last 3 decades. − The catalytic activity and selectivity of the cobalt porphyrins toward the formation of H 2 O 2 or H 2 O as a product of the ORR will depend upon the type and position of the porphyrin ring substituents and the planarity of the macrocycle. ,,− For example, meso -ferrocenyl-substituted porphyrins have been shown to be highly selective catalysts toward the two-electron reduction of O 2 ( n = 2) and lead almost exclusively to the formation of H 2 O 2 as a reaction product. The low value of n is due to steric hindrance from the bulky ferrocene groups, which prevent π–π interaction between macrocycles, thus hindering the formation of porphyrin dimers and making the four-electron reduction of O 2 less favorable. , In contrast, both cobalt tetramethylporphyrin and cobalt porphine have been shown to catalyze the reduction of O 2 via a four-electron-transfer pathway ( n = 4), giving H 2 O as a final reduction product.…”

Electrochemical Characterization of Bis‐Cobalt Hexaphyrin: A Selective Electrocatalyst for the Two‐Electron Reduction of Oxygen in Acid Media

“…It is well‐known that modifications in the macrocyclic structure of cobalt porphyrins or corroles may affect their activity as catalysts for oxygen reduction reactions (ORR) [37–48] . The ORR may involve a 2‐electron or a 4‐electron pathway or both to give H 2 O 2 and/or H 2 O as a product, the exact amount of which will depend upon the specific catalyst, the solvent and the experimental conditions [38–39,49–54] …”