Biomimicking vitamin B12. A Co phthalocyanine pyridine axial ligand coordinated catalyst for the oxygen reduction reaction

CoN4 complexes like Co phthalocyanines (CoPc) have been extensively studied as electrocatalysts for the oxygen reduction reaction (ORR) but they only promote the 2-electron reduction of O₂ to give peroxide. In contrast, vitamin B₁₂ a Co macrocyclic naturally occurring molecule has attracted the attention of the scientific community because instead of catalysing the 2-electron reduction of O₂ to H₂O₂ like the other Co macrocycles it promotes the 4-electron reduction to H₂O. Vitamin B₁₂ possesses an axial back ligand and this seems to be the reason for its higher activity and selectivity for the 4-electron reduction of O₂. To test this hypothesis, we synthetized a CoPc axially coordinated to pyridine anchored to carbon nano-tubes (CoPc-Py-CNT). The Co centre is therefore coordinated to 5 N as in vitamin B₁₂. The modified CoPc containing catalytic material was characterized by EPR and XPS spectroscopy. Ab initio calculations, Koutecky– Levich extrapolation and Tafel plots well describe the similarities between the 2 complexes and reveal insights into the mechanism of action of Co penta-coordinated complexes. According to our results the pyridine back ligand increases the Co-O₂ binding energy and making it more similar to that of Vitamin B₁₂, favouring the splitting of the O-O bond. The back ligand then plays a crucial role in modifying Co-O₂ binding energy which is a well know reactivity descriptor.