Controlled synthesis of nanostructured manganese oxide: crystalline evolution and catalytic activities

A hydrothermal process has been used to synthesize manganese oxides of various crystalline structures and morphologies, such as alpha-, beta-, gamma-MnO2, MnOOH and Mn3O4. The nanostructured materials were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and hydrogen temperature-programmed reduction (H-2-TPR) techniques. The crystalline evolution mechanism of converting beta-MnO2 or MnOOH to alpha-MnO2 was studied. The crystalline-dependent effect of the nanostructured manganese oxides was explored by using toluene combustion as a probe reaction. Results indicated that the catalytic activity of alpha-MnO2 with ultra-long nanowires is higher than that of manganese oxides with other crystalline structures. The catalytic activity was correlated with the H-2-TPR and XPS results.