2.1 Catalyst preparation
Firstly, γ-MnOOH nanorod was synthesized by a facile redox precipitation process. Then the corresponding β-MnO2, α-Mn2O3 and Mn3O4 were prepared by calcining γ-MnOOH nanorods under different temperature and atmosphere. The details of preparation precedure and chemicals invloved are shown in Supporting information (SI).