Transition-Metal-Doped Manganese Oxide Hollow Nanospheres: Synthesis and Catalytic Activityc

! Manganese oxide hollow spheres with approximate MnO2 stoichiometry were synthesized by aqueous precipitation with a redox reaction between Mn(II) and MnO4– in the presence of butyric acid. The butyric acid acts as a structure-directing agent, reducing the average particle size of the materials, while also acting as a soft template for the spheres to form. This is possible because butyric acid forms a thermodynamically stable microemulsion in water, which is unable to be broken up in solution. Without incorporation of the carboxylic acids, the materials have a surface area of 133 m2/g, while with the addition of butyric acid, the surface area is increased to 233 m2/g. By powder X-ray diffraction, these materials are seen to be amorphous, having no long-range order in their structure.! ! Manganese oxide hollow spheres are able to be isomorphously doped with various transition metals, including iron, copper and vanadium. These metals were chosen due to their proximity to manganese on the periodic table, thus having similar size, weight and oxidation states to manganese. Also, they have all been reported to enhance the catalytic activity of manganese oxide materials.! ! The iron-doped manganese oxides exhibited a drastic increase in surface area, up to 434 m2/g. Copper-doped spheres gave up to 350 m2/g, and vanadium-doped spheres brought it to 331 m2/g. The presence of any of the metals increases the surface area of the undoped hollow spheres by at least 100 m2/g, the reason for which is still unknown. At low concentrations of the metal dopant, each systems also exhibits a core-shell structure. In the case of the iron-doped materials, as the iron level increases, the spheres become solid. In the copperand vanadium-doped systems, the