rod-, flower-, and dumbbell-like mno2: highly active catalysts for the combustion of toluene

the rod-like tetragonal α-mno₂, flower-like hexagonal ɛ-mno₂, and dumbbell-like tetragonal β-mno₂ were prepared using the hydrothermal or water-bathing method under different conditions. it is shown that the α-mno₂, ɛ-mno₂, and β-mno₂ catalysts possessed a surface area of ca. 53, 30, and 114 m²/g, respectively. the oxygen adspecies concentration and low-temperature reducibility decreased in the order of α-mno₂ > ɛ-mno₂ > β-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 °c over α-mno₂, ɛ-mno₂, and β-mno₂, respectively. the apparent activation energies of α-mno₂, ɛ-mno₂, and β-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 α-mno₂, ɛ-mno₂, and β-mno₂ materials.