3.1 The crystal structures of the catalysts
The structure of the products by calcining the γ-MnOOH precursors
(Figure S1) under different conditions was studied by XRD measurement.
For clarity, the diffraction peak positions and the relative intensities
of the standard patterns of MnO2,
Mn2O3, and
Mn3O4 are also shown at the bottom of
Figure 1. As can be seen from Figure 1, for the samples being calcined
in air at 350°C, all the peaks of XRD patterns could be indexed to
tetragonal β-MnO2 (JCPDS card no. 72-1984). Further
elevating temperature to 600°C, cubic
α-Mn2O3 (JCPDS card no.71-0636) were
obtained. Replacing the oxidative atmosphere to inert Ar, the obtained
products consisted of tetragonal Mn3O4(JCPDS card no. 80-0382). After
hydrogen reduction, the XRD patterns denoted as
MnO2-H-200 were in good agreement with
β-MnO2. However, the broad and weak XRD features of
MnO2-H-200 indicated that β-MnO2 owned a
poor crystalline phase, which was led by introducing more defects during
hydrogen reduction.