Controlled Fabricationand Catalytic Performance Of Regularly Morphological MnO_x

One-dimensional （1-D） nanostructures, such as nanowires, nanorods, and nanotubes,have attracted special interests in recent years. The material of1-D structure has become aresearch hotspot. Manganese oxide becomes an important application material in scienceand technology due to its novel physicochemical properties. Manganese oxide is often usedin ion exchange, adsorption, catalytic materials, electrochemical capacitiors, and otherfields. Up to now, manganese oxides with various morphologies have beeen aynthesized,such as nanoparticles, nanorods/nanobelts/nanowires/nanotubes, mesoporous molecularsieves, dendrogram structure, seaurchin/orchid, and other layered structure. Manganeseoxide nanoparticles exhibit good redox behaviors because of their large surface areas andhigh oxygen vacancy density. Therefore, it is of academic and practical significance toinvestigate the preparation and catalytic properties of manganese oxides with differentmorphologies.In this thesis, manganese oxide materials with regular morphologies have been preparedusing the hydrothermal method. The physicochemical properties of these materials werecharacterized by means of numerous techniques, such as X-ray diffraction （XRD）,thermogravimetric analysis （TGA）, differential scanning calorimetry （DSC）, nitrogenadsorption-desorption （BET）, scanning electron microscopy （SEM）, transmission electronmicroscopy （TEM）, selected-area electron diffraction （SAED）, X-ray photoelectronspectroscopy （XPS）, and hydrogen temperature-programmed reduction （H₂-TPR）. Thecatalytic activities of the manganese oxides were evaluated for the oxidation of toluene.The main results obtained are as follows:（1）1-D nanosized rod-like, wire-like, and tubular-MnO₂and flower-like sphericalMn₂O₃have been prepared using the hydrothermal method at differenttemperatures with KMnO4and/or MnSO4as Mn source and using the CCl4solution method with KMnO4and MnCl2as Mn source, respectively. It is shownthat the1-D-MnO₂nanorods, nanowires, and nanotubes with a surface area of4583m²/g were tetragonal in crystal structure, whereas the flower-like sphericalMn₂O₃sample with a surface area of162m²/g was of cubic crystal structure.There were the presence of surface Mn ions in multiple oxidation states (e.g.,Mn³⁺, Mn⁴⁺or even Mn²⁺) and the formation of surface oxygen vacancies of thewell-defined morphological-MnO₂and Mn₂O₃samples.（2） The oxygen adspecies concentration and low-temperature reducibility decreasedin the order of rod-like-MnO₂> tube-like-MnO₂> flower-like Mn₂O₃> wire-like-MnO₂, in good agreement with the sequence of the catalyticperformance of these samples. The best-performing rod-like-MnO₂catalystcould effectively catalyze the total oxidation of toluene at lower temperatures（T50%=210oC and T90%=225oC at space velocity=20,000mL/（g h））.（3） The rod-like tetragonal-MnO₂, flower-like hexagonal-MnO₂, anddumbbell-like tetragonal-MnO₂were prepared using the hydrothermal orwater-bathing method under different conditions. It is shown that the-MnO₂,-MnO₂, and-MnO₂catalysts possessed a surface area of ca.53,30, and114m²/g, respectively.（4） The oxygen adspecies concentration and low-temperature reducibility decreasedin the order of-MnO₂>-MnO₂>-MnO₂, coinciding with the sequence oftheir catalytic activities for toluene combustion. The well-defined morphologicalMnO₂catalysts performed much better than the bulk counterpart. At a spacevelocity of20,000mL/（g h）, the temperature for90%toluene conversion was238,229, and241oC over the-MnO₂,-MnO₂, and-MnO₂catalysts, respectively.（5） The excellent catalytic performance of the as-fabricated MnOxwas associatedwith their better low-temperature reducibility and higher oxygen adspeciesconcentrations, but the specific surface areas of the MnOxcatalysts were not thekey factors for influencing the catalytic activities of MnOx.