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.