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.