Preparation and characterization of manganese ferrite supported-transition metal doped birnessite nanocomposites for catalytic water oxidation

In the first part of this thesis study; core-shell magnetic composite catalysts were prepared for use in the oxidation of water by coating transition metal-doped layered manganese oxide (MnOx) on manganese ferrite (MnFe2O4) nanoparticles. Spinel manganese ferrite nanoparticles, synthesized by a hydrothermal method, were used as the core material. The shell material "birnessite" was in situ deposited over the manganese ferrite nuclei and was doped in situ and by ion exchange with some first raw transition metal ions (Cr3+, Co2+, Ni2+, Cu2+ and Zn2+). The products were characterized and analyzed by X-Ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FT-IR), Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDS), Transmission Electron Microscopy (TEM), Thermogravimetric Analysis (TGA), magnetization measurements (VSM) and Atomic Absorption Spectroscopy (AAS). It was found that the composites were in the form of core-shell structured, 100-150 nm sized, soft ferromagnetic particles and that the transition metal doping did not cause any structural or morphological change but influenced the thermal stability. Finally; the catalytic activities of these nanocomposites in the chemical oxidation of water were investigated in the presence of a single electron oxidizer (Ce4+). The amount of oxygen evolved by oxidation was determined by Clark electrode and GC-TCD systems. Among the composites doped with transition metals, it was observed that those containing Co2+ and Ni2+ ions exhibited higher catalytic activity than the neat birnessite-manganese ferrite catalysts. The highest amount of oxygen evolution was recorded by in situ Co2+-doped composite catalyst. Co2+ and Ni2+ -doped birnessite-manganese ferrite nanocomposites prepared in this study displayed higher WOC activity than their neat analogues and can be considered as next generation catalysts for the oxidation of the water.