Manganese Oxides with Rod-, Wire-, Tube-, and Flower-Like Morphologies: Highly Effective Catalysts for the Removal of Toluene

Nanosized rod-like, wire-like, and tubular α-MnO(2) and flower-like spherical Mn(2)O(3) have been prepared via the hydrothermal method and the CCl(4) solution method, respectively. The physicochemical properties of the materials were characterized using numerous analytical techniques. The catalytic activities of the catalysts were evaluated for toluene oxidation. It is shown that α-MnO(2) nanorods, nanowires, and nanotubes with a surface area of 45-83 m(2)/g were tetragonal in crystal structure, whereas flower-like spherical Mn(2)O(3) with a surface area of 162 m(2)/g was of cubic crystal structure. There were the presence of surface Mn ions in multiple oxidation states (e.g., Mn(3+), Mn(4+), or even Mn(2+)) and the formation of surface oxygen vacancies. The oxygen adspecies concentration and low-temperature reducibility decreased in the order of rod-like α-MnO(2) > tube-like α-MnO(2) > flower-like Mn(2)O(3) > wire-like α-MnO(2), in good agreement with the sequence of the catalytic performance of these samples. The best-performing rod-like α-MnO(2) catalyst could effectively catalyze the total oxidation of toluene at lower temperatures (T(50%) = 210 °C and T(90%) = 225 °C at space velocity = 20,000 mL/(g h)). It is concluded that the excellent catalytic performance of α-MnO(2) nanorods might be associated with the high oxygen adspecies concentration and good low-temperature reducibility. We are sure that such one-dimensional well-defined morphological manganese oxides are promising materials for the catalytic elimination of air pollutants.