Atomic-Scale Unveiling of the True Active Phase of Mn Porphyrin Catalysts on Au(111) Using Electrochemical Scanning Tunneling Microscopy

Manganese (Mn)-based molecular complexes, inspired by the MnxCay cluster in photosystem II, have been widely investigated as biomimetic catalysts for the oxygen evolution reaction (OER). However, it remains unclear whether these molecular catalysts maintain their structural integrity under operating conditions or transform into metal oxide species that serve as the true active sites. Here, we visualize the electrochemical restructuring behavior of Mn porphyrin (MnOEP) monolayers on Au(111) using electrochemical scanning tunneling microscopy (EC-STM) in alkaline electrolyte. During repeated redox cycles, the MnOEP adlayer undergoes progressive decomposition, leading to the formation of two-dimensional MnOx nanoislands on the Au surface. We show that in situ formed metal oxides act as the active catalytic species under operating conditions and play a key role in enhancing OER activity. EC-STM images reveal that the repeated MnOEP adsorptions/redox cycles result in increased surface coverage of these activated catalysts, i.e., metal oxide islands. The catalytic activity of the activated electrode is gradually strengthened as the reconstructed MnOx islands grow horizontally and expand within the monolayer, whereas multilayer oxide growth leads to diminished activity. Specifically, the controlled monatomic-high MnOx islands achieve lower overpotential with favorable OER kinetics, showing a remarkable OER current density at 1.82 V versus the reversible hydrogen electrode and surpassing initial MnOEP-catalyzed Au electrode by up to 1.8 times. These notable findings provide molecular-level insights into the active sites of MnOx-modified Au electrodes and spur the future development of water oxidation catalysts.