(Invited) Controlling the Selectivity and Catalytic Activity of Bio-Inspired Transition Metal Complexes for the Oxygen Reduction Reaction

Abstract

In the search for abundant precious group metal free (PGM-free) catalysts, it was biological systems that inspired researchers over the years. One of the most prominent groups of such bio-inspired catalysts are transition metal/nitrogen/carbon-based catalysts (MNC), among them are heme-like molecular catalysts such as porphyrins and phthalocyanins.Most of the bio-inspired catalysts designed to-date, focused solely on the catalytic center of molecular catalysts found in biological systems, while neglecting the electron mediators and ligands, which seem to have an important role in the catalytic scheme. It was shown that Imidazole for instance, is an important ligands which plays a vital role in biological systems, and can also be used to affect the catalytic activity of molecular catalysts. Some have used imidazoles and their analogues to study such effects, and showed significant control over the overpotential and selectivity upon the addition of such ligands to macrocyclic compounds. Another important molecule in enzymatic processes is quinone and its derivatives which also acts as an electron mediator in electron transport processes. It was shown recently that when incorporating certain axial ligand to the metal center in a metal-phthalocyanine, there is an increase in activity towards ORR, and that it induces a change in the mechanism of the reaction.To enhance the ORR kinetics and selectivity by lowering the energy barriers, it is suggested herein to imbed ligands that have an imidazole group on reduced graphene oxide (rGO) and anchor porphyrins to them via coordinative bonds.In this talk I will present a bio-mimetic catalytic structure as a new approach to increase the oxygen reduction catalytic selectivity of transition metal complexes. As in biological systems, where there is a series of mediators between the active site and the surface, that lowers the activation energy of the overall reaction and increases its selectivity, we used in this study a structure where a molecular catalyst Fe(III)tetrakis(1-methyl-4-pyridyl) porphyrin bonded to a surface via a benzimidazole mediator. Using electron paramagnetic resonance (EPR), UV-vis spectroscopy and electrochemical measurements, we compared the ORR activity of the molecular catalyst with and without benzimidazole and show that this ligand strongly influences the selectivity of the reaction and shifts it from the 2 to the 2+2-electron reduction.