Rod-, flower-, and dumbbell-like MnO2: Highly active catalysts for the combustion of toluene

The rod-like tetragonal alpha-MnO2, flower-like hexagonal epsilon-MnO2, and dumbbell-like tetragonal beta-MnO2 were prepared using the hydrothermal or water-bathing method under different conditions. It is shown that the alpha-MnO2, epsilon-MnO2, and beta-MnO2 catalysts possessed a surface area of ca. 53, 30, and 114 m(2)/g, respectively. The oxygen adspecies concentration and low-temperature reducibility decreased in the order of alpha-MnO2 > epsilon-MnO2 > beta-MnO2, coinciding with the sequence of their catalytic activities for toluene combustion. The well-defined morphological MnO2 catalysts performed much better than the bulk counterpart. At a space velocity of 20,000 mL/(g h), the temperature for 90% toluene conversion was 238, 229, and 241 degrees C over alpha-MnO2, epsilon-MnO2, and beta-MnO2, respectively. The apparent activation energies of alpha-MnO2, epsilon-MnO2, and beta-MnO2 were in the range of 20-26 kJ/mol. It is concluded that higher oxygen adspecies concentrations and better low-temperature reducibility were responsible for the good catalytic performance of the alpha-MnO2, epsilon-MnO2, and beta-MnO2 materials. (C) 2012 Elsevier B.V. All rights reserved.