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.