Interfacial Engineering of Mn Porphyrin/Au Electrodes for Identifying MnOx as the Active Species under Oxygen Evolution Reactions

Understanding the influence of surface structural features at a molecular level is beneficial in guiding an electrode's mechanistic proposals for electrocatalytic reactions. The relationship between structural stability and catalytic activity significantly impacts reaction performance, even though atomistic knowledge of active sites remains a topic of discussion. In this context, this work presents scanning tunneling microscopy (STM) results for the highly ordered arrangement of manganese porphyrin molecules on a Au(111) surface; STM allows us to monitor active sites at a molecular level to focus on long-standing issues with the electrocatalytic process, especially the exact nature of the real active sites at the interfaces. After water oxidation, manganese porphyrin rapidly decomposes into active catalytic species as bright protrusions. These newly formed active species drastically lost catalytic activity, up to 82%, through only acid treatment, one of the oxide removal methods, not by deionized water and acetone treatments. STM results of the obviated active species on the Au surface by an acidic solution support the forfeited catalytic activity. In addition, it shows a 67% decrement in catalytic activity by adsorption of phosphonic acid, one of the oxide's preferred adsorption materials, compared to the pristine one. Based on these observations, we confirm that the newly formed active species, as water oxidation catalysts, mostly consist of manganese oxides. Notable findings of our work provide molecular evidence for the active sites of Au and modified Au electrodes that spur the future development of water oxidation catalysts.