Axial ligands tailoring the ORR activity of cobalt porphyrin

Yue Zhou; Yong-Fang Xing; Jing Wen; Hai-Bo Ma; Feng-Bin Wang; Xing-Hua Xia

doi:10.1016/j.scib.2019.07.003

Axial ligands tailoring the ORR activity of cobalt porphyrin

Sci Bull (Beijing). 2019 Aug 30;64(16):1158-1166. doi: 10.1016/j.scib.2019.07.003. Epub 2019 Jul 2.

Authors

Yue Zhou¹, Yong-Fang Xing¹, Jing Wen¹, Hai-Bo Ma², Feng-Bin Wang³, Xing-Hua Xia⁴

Affiliations

¹ School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China.
² School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China. Electronic address: haibo@nju.edu.cn.
³ School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China. Electronic address: fbwang@nju.edu.cn.
⁴ School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China. Electronic address: xhxia@nju.edu.cn.

PMID: 36659687
DOI: 10.1016/j.scib.2019.07.003

Abstract

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 [5,10,15,20-tetrakis(4-methoxyphenyl)porphyrin]cobalt(II) (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(II) 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(II) center and O₂ molecules, which has been suggested to be critical toward the O₂ adsorption and subsequent activation process.

Keywords: Axial ligands; Cobalt porphyrin; Oxygen reduction reaction (ORR); Push effect.