Xiaoling Cheng

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 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.

Crystallization design of MnO2via acid towards better oxygen reduction activity

Manganese dioxide is one of the important electrode materials used for oxygen reduction reaction (ORR) in fuel-cell and metal-air batteries, and its electrochemical activity is greatly influenced by its crystalline structure. To elucidate the crystalline structure effect on the ORR, herein, we designed a facile method to obtain α-MnO2 and γ-MnO2via adjusting the acid under hydrothermal conditions using the same starting materials and reagents. The prepared α-MnO2 and γ-MnO2 have a similar morphology composed of 3D microscopic spheres made up of numerous nanorods. X-ray photoelectron spectroscopy, temperature programmed desorption of oxygen characterization and conductivity measurements demonstrate that the α-MnO2 material has a relatively higher ratio of surface oxygen species, higher oxygen mobility and higher electrical conductivity. When compared with γ-MnO2, α-MnO2 displays superior ORR activity with a higher electron transfer number and lower yield of peroxide species in alkaline solutions. The obtained results shed new light on the crystalline structure-dependent ORR activity of MnO2.

Rational design of MnO2@MnO2 hierarchical nanomaterials and their catalytic activities

Hierarchically structured materials have special properties and possess potential in applications in the catalytic and electrochemical fields. Herein, two kinds of hierarchical core-shell nanostructures, lavender-like α-MnO2@α-MnO2 and balsam pear-like α-MnO2@γ-MnO2, were prepared by a facile room-temperature method using α-MnO2 nanowires as a backbone under acidic and alkaline conditions, respectively. When being used as a catalyst for dimethyl ether combustion, α-MnO2@γ-MnO2 exhibited a better performance than α-MnO2@α-MnO2 (T10 = 171 vs. 196 °C; T90 = 220 vs. 258 °C, SV = 30, 000 mL g-1 h-1). It is concluded that the larger surface area, higher reducibility/oxygen mobility, richer surface oxygen species, and the relatively smaller apparent activation energy are responsible for the superior performance of α-MnO2@γ-MnO2.