3.3 The redox property of the catalysts
The reducibility of the MnOx above has been evaluated by
H2-TPR experiments and the results are shown in Figure
4. All the samples got green after H2-TPR experiments.
It indicated the samples were reduced into MnO. Only one reduction peak
occurred for Mn3O4, which was attributed
to the reduction of Mn3O4 to MnO. While
two peaks were detected over other samples. The lower-temperature peak
was ascribed to the reduction of
MnO2/Mn2O3 to
Mn3O4, and the higher-temperature peak
is attributed to the further reduction of
Mn3O4 to MnO [28, 29]. Moreover, it
should be noted that the low reduction temperature decreased from 306°C
to 295°C for MnO2 and MnO2-H-200.
Generally, low onset reduction temperature corresponds to better
reducibility. As shown in Figure 4, the reducibility lied in the
following sequence: Mn3O4<
Mn2O3<
MnO2< MnO2-H-200, implied that
abundant oxygen vacancies or high Mn valence can enhance reducibility of
MnOx catalyst.