Free-Base Octaethylporphyrin on Au(111) as Heterogeneous Organic Molecular Electrocatalyst for Oxygen Reduction Reaction in Acid Media: An Electrochemical Scanning Tunneling Microscopy and Rotating Ring-Disc Electrode Analyses

The oxygen reduction reaction (ORR) using metal porphyrin catalysts is currently widely explored. Conversely, metal-free molecular systems are much less investigated, and there is limited information available for molecules such as nonmetalated macrocycles capable of catalyzing the ORR or other small mole cules. Herein, the activity and selectivity of a heterogeneous organic molecular electrocatalyst, octaethylporphyrin (H2OEP), adsorbed on Au(111) toward ORR in acidic aqueous electrolyte are investigated. Electrochemical scanning tunneling microscopy (EC-STM) is employed to monitor the molecular layer during the electrochemical process. Additionally, cyclic and linear sweep voltammetries are performed at still and rotating Pt/ring-H2OEP-functionalized Au(111)/disk electrodes to determine the activity and selectivity of the H2OEP monolayer toward ORR on Au(111). Based on EC-STM and computation analysis, dioxygen electroreduction does not follow an inner-sphere electron transfer reduction as seen in metal porphyrins, where a preliminary M─O2 bond has to form, but it follows an outer-sphere mechanism involving the precoordination of O2 induced by the protonated hydrogen of the macrocycle cavity.