Study on the Electronic Structure Control of Mn Oxide Nano-catalysts by Surface Functionalization

Abstract

Nature has selected various types of manganese enzymes due to good catalytic activity resulting from highly redox-active characteristics of manganese. In the meanwhile, multinuclear manganese enzymes are known to mediate reactions with high performance by the cooperative effects of each manganese ion in the structure. Interestingly, in this regards, photosynthetic water oxidase which has a tetranuclear Mn4CaO5 cluster in PS II is known to be the best catalyst for water oxidation among all reported. Inspired by this Mn-Ca cluster in nature, the Calcium EDTA chelates were functionalized on the manganese oxide nanoparticles

so to change the electronic structure of manganese and broaden the applicability of catalyst as a future work. In this work, the removal of myristic acid, the original ligand of manganese oxide nanoparticles, and successful ligand exchange to Ca-EDTA chelates were verified by the FT-IR spectroscopy. Moreover, the local structure on the surface of manganese oxide nanoparticles was suggested by the stretching vibration of COO- and bending vibration of C-N bond

calcium ion and manganese ion are proposed to form pseudo-bridge as a major state and the nitrogen atoms are suggested to chelate the manganese ion on the surface of manganese oxide nanoparticles. Partially, some carboxylate groups of EDTA are considered to be bound as unidentate and bidentate mode as a mixture. Also, electronic structural analysis of calcium EDTA chelated manganese oxide nanoparticles was performed by EPR analysis. The calcium-affected manganese ions are verified to have high axial zero-field-splitting (D ~ 0.023 cm^-1) compared to conventional Mn (II) (D = 0.014 cm^-1) by a multi-frequency EPR simulation and are demonstrated to have weak exchange coupling between manganese ions (J below 1 cm^-1) via temperature dependency analysis of EPR and SQUID. The observation of electronic structural change of manganese, the axial distortion in the ligand field of manganese and weak exchange coupling between manganese ions, in this work will be a pioneering model in heterogeneous system for further application to catalysis and in the meantime for better understanding of the role of calcium ion in Mn-Ca cluster in nature.