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

Abstract

A hydrothermal process has been used to synthesize manganese oxides of various crystalline structures and morphologies, such as α-, β-, γ-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 (H₂-TPR) techniques. The crystalline evolution mechanism of converting β-MnO₂ or MnOOH to α-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 α-MnO₂ with ultra-long nanowires is higher than that of manganese oxides with other crystalline structures. The catalytic activity was correlated with the H₂-TPR and XPS results.