Axial ligands tailoring the ORR activity of cobalt porphyrin

In an effort to provide visualization and understanding to the electronic push effect of axial ligands on the catalytic activity of cobalt macrocyclic molecules, we design a simple model system involving an 15,10,15,20-tetrakis(4-methoxyphenyl)porphyrin]cobalt(11) (TMMPCo) monolayer axially-coordinated on thiol ligand modified Au electrode and explore the activity of the axial-ligand coordinated TMPPCo toward oxygen reduction reaction (ORR) in acidic medium. Three different ligands, with a decreasing order of coordinating ability as: 4-mercaptopyridine (MPy) > 4-aminothiolphenol (APT) > 4-mercaptobenzonitrile (MBN) are used and a maximum difference in ORR onset potential of 80 mV is observed between the MPy (highest onset potential) and MBN systems (lowest onset potential). The ORR activity of TMPPCo increases with the increase in binding strength of the axial ligand. A detailed mechanism study reveals that ORR on the three ligand coordinated TMPPCo systems shares the same 2-electron mechanism with H2O2 as the terminal product. Theoretical calculation into the structure of the ligand coordinated cobalt porphyrins uncovers the variation in atomic charge of the Co(11) center and altered frontier molecular orbital distribution among the three ligand systems. Both properties have great influence on the back-bonding formation between the Co(11) center and O-2 molecules, which has been suggested to be critical toward the O-2 adsorption and subsequent activation process. (C) 2019 Science China Press. Published by Elsevier B.V. and Science China Press. All rights reserved.