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 copper- and vanadium-doped systems, the #1 core-shell structures disappear at higher dopant amount, reverting back to regular hollow spheres.
The catalytic activity of all of these materials was tested by using the common catalytic system of isopropanol oxidation. Being a relatively simple system to use, a lab-mate and I assisted in building an apparatus to test this and combined it with the use of a gas chromatograph to measure the amount of isopropanol that is converted to acetone. This was tested by heating the catalysts up to 200° and 250° C. The undoped hollow spheres showed strong catalytic activity of about 90% conversion of isopropanol to acetone, with the copper and iron dopants having little effect on that efficiency. The vanadium doped materials however lowered the catalytic efficiency to 50-70% conversion. The oxidation of carbon monoxide was also briefly tested, and the copperdoped materials gave 100% conversion at 25° C, while the other systems proved far less efficient. This gives promise to the copper-doped materials at lower temperatures in the isopropanol system.
Carmichael, Jeffrey, "Transition-Metal-Doped Manganese Oxide Hollow Nanospheres: Synthesis and Catalytic Activityc" (2014). Chemistry Honors Papers. 12.
The views expressed in this paper are solely those of the author.