Structure- And Shape-controllable Synthesis Of Manganese Dioxides By Hydrothermal Method And Their Catalytic Activities

Nowadays, exploring the controllable synthesis of nano-materials and their applications in related areas are the hot research topics in new material field. MnO2 has been widely applied in electrochemistry, catalysis and adsorption owing to its low-cost, unique pore structure and variable oxidation states of Mn. This dissertation focuses on the structure- and shape-controllable synthesis of manganese dioxides and their catalytic properties. On one hand, in order to overcome the key technical difficulties, MnO2 with different structures and morphologies were prepared via the simple hydrothermal method by adjusting the experiment parameters in relatively mild and environment-friendly conditions. On the other hand, in order to provide the experimental basis and achieve the catalytic application of MnO2, valuable investigations have been carried out on the relationship between the crystalline structure and shape of manganese dioxides and their catalytic performances. The details about the experiment contents and results are summarized briefly as follows:(1) ?-MnO2 nanowires with a length about 6-10 ?m and an average diameter of 20 nm were synthesized through a facile hydrothermal process without any templates or surfactants.The products were characterized by X-ray powder diffraction, Raman spectroscopy, field emission scanning electron microscopy,transmission electron microscopy, hydrogen temperature-programmed reduction techniques, X-ray photoelectron spectroscopy and surface area analysis. The effects of the hydrothermal temperature and the concentration of CH3COOH on the crystal phase and morphology of the final products were studied in detail.The hydrothermal temperature and the concentration of CH3COOH play crucial roles in determining the crystal phase and morphology of the products. The possible formation mechanism of the?a-MnO2 nanowires was investigated and discussed. Additionally,the as-prepared ?-MnO2 nanowires showed higher catalytic activity for toluene combustion than the commercial MnO2, suggesting their potential applications in the elimination of volatile organic compounds.(2) Branched ?-MnO2 bipods with novel nanopincer morphology were prepared by a facile one-pot hydrothermal method via a redox reaction between NaClO3 and MnCl2 in sulfuric acid solution without using any surfactants or templates. The products were characterized in detail by various techniques including X-ray powder diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, surface area analyzer, field emission scanning electron microscopy and transmission electron microscopy. Results show that the obtained ?-MnO2 nanopincers consist of two sharp nanorods with a diameter of 100-200 nm and a length of 1-2 ?m. The concentration of H2SO4 solution plays an important role in controlling the crystal phase and morphology of the final product. A possible formation mechanism for the ?-MnO2 nanopincers was proposed. Moreover, these ?-MnO2 nanostructures exhibited better catalytic performance than the commercial MnO2 particles to decompose methyl blue (MB) in the presence of H2O2.(3) Herein, ?-MnO2 catalysts with three well-defined morphologies (nanorod, ultra-long nanowire and microsphere) were rational designed and prepared by the hydrothermal route,and their catalytic activities were evaluated for the dimethyl ether (DME) combustion. These nanostructured ?-MnO2 catalysts were characterized in detail by various analytical techniques: XRD, FESEM, TEM, BET, XPS and H2-TPR. As a result, the a-Mn02 nanorod exhibited the higher catalytic activity (T10= 170 ? and T90 = 238 ? at WHSV = 30, 000 mL·g-1··h-1) than the other two a-Mn02 samples due to its larger specific surface area, higher average oxidation state of Mn, more abundant surface lattice oxygen (Olatt) species and higher reducibility, and there was no obvious decrease after the a-Mn02 nanorod was run for 50 h. Moreover, the transient response technique revealed that the Olatt species of the?-MnO2 nanorod should be the crucial role in the deep oxidation of DME.(4) ?-MnO2 hollow microspheres were synthesized by a facile one-pot hydrothermal method via a redox reaction between Na2S2O8 and MnSO4 in sulfuric acid solution without any surfactants or templates. The effects of synthesis conditions on the structure and morphology of the products were also investigated. The products were characterized in detail by various techniques including X-ray powder diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, surface area analyzer, scanning electron microscopy and transmission electron microscopy. Results indicate that the obtained P-Mn02 hollow microsphere consist of nanorods with a length of ?1.5 ?m. The concentration of H2SO4 solution and reaction temperature plays significant roles in controlling the crystalline structure and morphology of the final product. A possible formation mechanism for the?-MnO2 hollow microsphere was proposed. In addition, the ?-MnO2 hollow microspheres showed better ORR activity and durability than the commercial MnO2 particle.(5) To achieve high-performance fuel cells and metal-air batteries, inexpensive and earth-abundant catalysts with enhanced activity and durability for the oxygen reduction reaction (ORR) are currently sought after. Herein, three-dimensional (3D) ?-MnO2 and?-MnO2 hierarchical star-like architectures with tunable crystal phases and desirable ORR activity were readily prepared by a facile hydrothermal method with no surfactants or templates. The effects of reaction temperature, anion type, and dwell time on the morphologies of the MnO2 products were studied in detail, and the possible formation mechanism of the 3D MnO2 hierarchical stars was proposed. Due to the improved electrical conductivity and O2 adsorption ability, the resulting ?-MnO2 catalyst showed substantially enhanced ORR activity, compared to the ?-MnO2 and bulk MnO2 catalysts, with a more positive onset potential, a larger limiting current density, and better durability. Our results provide a facile chemical route towards the phase-controlled synthesis of 3D MnO2 architectures,which can serve as efficient catalysts for ORR-based applications.