3.2 The morphology and microstructures of the catalysts
The morphologies of the as-obtained catalysts were investigated with SEM. Figure 2(a)-(d) showed the SEM images of MnO2, Mn2O3, Mn3O4 and MnO2-H-200, respectively. For all samples, it could be seen that the shape was dominated by one dimensional nanorods with a diameter of tens of nanometers and a length of several micrometers. To study the detailed structural features, the micro-morphologies of four samples were characterized by TEM. Figure 3(b) showed that MnO2possessed the inter-planar distance of 0.31nm, corresponding to the (110) plane of β-MnO2, which is in good agreement with the results of the selected-area electron diffraction (SAED). The MnO2 had excellent crystallinity and displayed oriented growth along the [001] direction, and mainly exposed (110) facets. As shown in Figure 3(d) and (f), Mn2O3and Mn3O4 preferentially exposed (211) and (200) facets, respectively. For MnO2-H-200, the lattice fringe image became more indiscernible than MnO2(Figure 3(h)). The indistinct lattice fringe image of MnO2-H-200 suggested that defects were generated due to oxygen element escaping from the MnO2 lattice under H2/Ar atmosphere.