(Invited) Climbing over the Activity Volcano Correlation by Biomimicking Vitamin B12: A Co Phthalocyanine Pyridine Axial Ligand Coordinated Catalyst for the Reduction of Molecular Oxygen

Abstract

CoN4 macrocyclic complexes like Co phthalocyanines (CoPc) have been extensively studied as electrocatalysts for the ORR but they preferentially promote the 2-e reduction of O 2 to give peroxide. In contrast, vitamin B 12 , a naturally occurring CoN4 macrocyclic molecule alkaline media promotes the 4-electron reduction of O2 to H 2 O. Vitamin B12 possesses an imidazole axial back ligand and this seems to be the reason for its higher activity and selectivity for the 4-e reduction of O 2 . To test this hypothesis, we synthesized a CoPc axially coordinated to pyridine anchored to carbon nanotubes (Co-Py-CNT). The Co center is therefore coordinated to 5 nitrogens as in vitamin B 12 . The modified CoPcPy containing catalytic material was characterized by EPR and XPS spectroscopy. DFT calculations. According to our results the pyridine back ligand increases the Co-O 2 binding energy, making it more similar to VitB12, favoring the splitting of the O-O bond. The back ligand then plays a crucial role in modifying Co-O 2 binding energy which is a well know reactivity descriptor. A further evidence of this dramatic change in reactivity of CoPc by the presence of the "Py" back ligand is that it moves from the weak binding leg of a volcano correlation to the strong leg of the volcano. This might explain why nature uses a back ligand in cytochrome c to catalyze the ORR.

Penta-coordinated transition metal macrocycles as electrocatalysts for the oxygen reduction reaction

“…Cobalt macrocycles presented far higher activity than those containing copper. In 2018, Tasca et al [121], the same experiment was attempted for cobalt phthalocyanine. Two different aryl thiols, 4-aminothiophenol (ATP) and 1-(4-Mercaptophenyl)2,6-diphenyl-4-(4-pyridyl)pyridinium tetrafluoroborate (MDDP), were bound to Au (111) in the form of a self-assembled monolayer (SAM) and then CoPc was bound to them through their nitrogenous group (Fig.…”

“…9 Diagrammatic representation of CoPc bound to Au (111) with 4 ATP (left) and MDDP (right). Adapted with permission from Riquelme et al [121]. Copyright (2018) The Electrochemical Society peroxide production was examined, it was observed that both catalysts show maximum production at 0.35 V which rapidly decreases in the case of CoPc-Py-CNT and takes an additional 0.250 V to reduce in the case of VitB12-CNT.…”

“…[5,[130][131][132][133][134][135]. In the case of MN5 CoPcs, the volcano plot has both an ascending region which corresponds to weak binding energies (2-electron processes) and a descending region which corresponds to strong binding energies (4-electron processes) [31,57,58,64,121]. With strong binding favoring of the 4 electron process, it may be why strong O 2 binding is a prerequisite for the ORR, similarly to gold (weak binding) favoring 2 electron processes [136] and platinum (Strong binding) favoring 4 electron processes [137].…”

A theoretical study on the ORR electrocatalytic activity of axial ligand modified cobalt polyphthalocyanine

“…CoPc and CoPPc were mainly studied in the alkaline medium as electrocatalysts, where they were stable and not poisoned. [26][27][28][41][42][43][44][45][46] CoPc mainly performed the ORR through 2-electron and 4-electron mixed paths; 45,47 however, CoPPc 24 and pentacoordinated CoPc 43,46,48 showed higher activity and selectivity for the 4-electron ORR. So, in this work, the 4-electron reaction mechanism 49 was chosen to study the ORR activity of CoPPc and various CoPPc-L under alkaline conditions (pH = 14), and the Gibbs free energy change (DG) of each reaction step can be described as follows:…”

First-principle calculations study of the ORR/OER electrocatalytic activity of ruthenium polyphthalocyanine axially modified with aliphatic thiol groups

“…[43][44][45] It has been found that the introduction of suitable axial ligands on the central metal atom can effectively improve the catalytic performance of MN 4 complexes, because the axial ligands can change the electronic structure of the central metal atom, thus changing the adsorption strength of the catalyst to the reaction intermediate. 32,[53][54][55][56][57][58][59][60][61][62][63][64][65][66][67][68][69] For instance, the strong interaction between cobalt phthalocyanine and nitrogen-rich carbon nitride leads to the formation of Co-N 5 catalytic sites, which can adjust the electronic structure of cobalt phthalocyanine and improve the electrocatalytic performance of cobalt phthalocyanine for CO 2 reduction. 70 Zhao et al reported that iron polyphthalocyanine axially coordinated by iodide ions exhibited good stability and catalytic performance.…”