Synthesis, Characterization Of Several Manganese Oxides Nanomaterials And Their Application In Catalytic Degradation Of Dyes

Nanomaterials have a great number of novel properties, such as the surface effect, small size effect as well as quantum size effect. Because the composition and morphology of nanomaterials have an important influence on the properties, selective synthesis of nanoscale materials with controllable composition and morphology represents an increasingly important research direction in nanosciences. In addition, the preparation methods of nanomaterials with cheap, easy and environmentally friendly are also the important task of inorganic synthetic chemistry.Manganese oxides nanomaterials render them to be of considerable interests in many technological applications owing to their variable oxidation states such as MnO2, MnOOH, Mn2O3 and Mn3O4 with crystal structures. Moreover, there are wide application prospects in the catalytic oxidation, lithium-ion battery anode materials, new magnetic materials and so on; for the manganese oxides nanomaterials with different compositions and morphologies due to many special physical and chemical properties. With the rapid development of modern society, the environment problem has been aroused more and more attention. The environment pollution treatment by using nanometer material with these special physical and chemical properties is also currently a research hotspot.In this thesis, the manganese oxide nanomaterials with different crystalline structures and morphologies, such as nanosphere, nanowire, nanorod, nanoplate, octahedral and nanoparticle were prepared by different methods and their electrochemical and catalytic properties were investigated. The main results and achievements in this dissertation are given as follows:1. Mn3O4 hexagonal nanoplates and nanoparticles were synthesized via a solvent-assisted hydrothermal oxidation process at low temperature and a solvothermal oxidation method, respectively. The synthesized products were characterized by powder X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), electron diffraction (ED), Fourier transform infrared (FT-IR) spectroscopy. Their capability of catalytic oxidation of formaldehyde to formic acid at room temperature and atmospheric pressure and electrochemical properties by cyclic voltammogram (CV) were compared. The results showed that although the hexagonal nanoplates and nanoparticles modified electrodes blended with carbon black have a higher specific capacitance, Mn3O4 hexagonal nanoplate is a better catalyst.2. MnOOH nanorod and Mn3O4 octahedron-like were synthesized by hydrothermal method based on the redox reaction between MnO4 and CH2O at 120℃and 200℃for 10h, respectively. The (3-MnO2 nanowires were prepared by calcining MnOOH nanorods at 300℃for 3h in air. As-synthesized products were characterized by XRD, FE-SEM, TEM, SA-ED, HR-TEM, Brunauer-Emmett-Teller (BET) and UV-vis diffuse reflectance spectroscopy. Their capability of catalytic degradation of alizarin yellow R with air oxygen in aqueous under visible light irradiation and electrochemical properties of as-prepared nanocrystal modified with carbon paste electrode were comparatively studied. The decolourization of alizarin yellow R and the degradation products were invetigated by UV-vis and GC-MS analysis. In addition, some influences factors, such as pH and temperature on the photocatalytic degradation were also investigated. Among the as-prepared MnOx nanostructures,β-MnO2 nanowire exhibit higher specific capacitance and better catalytic activity than Mn3O4 octahedron-like and MnO(OH) nanorod and it can be degraded 98% of alizarin yellow R within 2 h under irridadtion. The hydroxyl radical mechanism of the photocatalytic degradation of alizarin yellow R was detected and discussed.3. Birnessite-type phase Ko.5Mn2043(H2O)0.5 nanotubes and nanorods have been synthesized via a calcination process of manganese acetate with potassium hydroxide in presence of polyethylene glycol-melamine-formaldehyde. The morphologies of the products were studied by FE-SEM, TEM and HR-TEM. The characterizations of their structures and chemical composition were investigated by XRD, FT-IR, SA-ED and EDX. The thermal behavior and water content of the samples was studied by TGA. The N2 adsorption-desorption isotherms were employed to determine the specific surface area and pore size. The result revealed that the reaction temperature and time play important roles in controlling the morphology of final products. The oriented attachment- thermodynamical process (OA-TD) was suggested to elucidate the tube-rod transformation. As-synthesized K0.5Mn2O4.3(H2O)0.5 nanostructure was tested for the catalytic degradation of safranin O (SO). The result showed that the catalytic efficiency of tubular structure is better than that the nanorod structure.4. A simple one-step hydrothermal reaction route was developed for the synthesis KxCoyMn8-yO16 hollow spheres with high surface area～292.03m2g-1.Characterization by XRD, FT-IR, SA-ED and energy-dispersive X-ray analysis (EDXA) studies proved that cobalt was incorporated into the manganese oxide cryptomelane-type. FE-SEM, TEM and HRTEM results show that poly-crystalline structure KxCoyMn8-yO16 hollow spheres are composed of well-aligned nanoplates. N2 adsorption-desorption isotherms were employed to estimate the specific surface area and pore size. Molar ratios of reactants and reaction time were proved to be key parameters to the morphology of the final products. It has been obviously investigated that the partial Co substitution for the nanostructured manganese oxides causes the improvement of their catalytic activity with respect to rhodoamine B (RhB) and methyl red (MR) degradation at room temperature. Results revealed that the KxCoyMn8-yO16 microspheres prepared with higher molar ratio of [Co+2]:[Mn+7] is better catalyst than the other samples with lower [Co+2]:[Mn+7] molar ratios.