Preparation Of Manganese Oxides By Soft Chemistry Processes. Evaluation Of Their Catalytic Activities And Electrochemical Properties

Due to the nature of low-cost, abundance and environmentally friendly, as well as their unique structures and excellent ion-exchange properties, adsorption properties and redox properties, manganese oxides have been extensively studied in the field of battery, adsorption, magnetism and catalysis, etc. This dissertation is mainly concerned on the synthesis and modification of manganese oxides with different structures. The author focuses on the preparation of manganese oxides or metal-doped manganese oxides with particular structures, and morphologies, and to investigate their catalytic and electrochemical performance for potential applications in aspects of environment and energy.In this work, various manganese oxides, such as birnessite (2×∞), cryptomelane (2×2), pyrolusite (1×1), ramsdellite (1×2), MnOOH, Mn2O3, Mn3O4, and metal-dopped manganese oxides have been synthesized by different soft chemistry processes. Their structure, morphology, BET surface area, oxygen species and other physico-chemical properties have been characterized systematically.Firstly, birnessite (K1.35MnO2) and cryptomelane (K0.27MnO2) has been controlled synthesized by a modified sol-gel process using KMnO4and glucose as precursor. In this sol-gel process, the gel treatment, the reaction time and the calcination temperature have been investigated. Moreover, a plausible formation mechanism for the controllable synthesis of the birnessite and cryptomelane is provided. Results show that the gel treatment appears to be crucial for the controllable preparation of birnessite and cryptomelane. The K+content and the glucose concentration, and the intermediate of the finale products, i.e. MnCO3and MnOx, can be controlled by using different gel treatments, subsequently, birnessite and cryptomelane are controlled synthesized by oxidizing the intermediate. The synthetic parameters such as the concentration of the initial reactant, the calcination temperature and the reaction time seem to have little effect on controlling structure of products. Moreover, the first result of the birnessite to the application of dimethyl ether (DME) catalytic combustion is also investigated. The birnessite catalyst is active for DME combustion, however, the catalytic activity of birnessite seems to be lower than that of the cryptomelane.Secondly, birnessite shows huge flexibility in its layered structure modification, which can be tuned to other interesting structures and properties. Birnessite-H.γ-MnOOH. Mn3O4. Mn2O3, cryptomelane and pyrolusite with irregular sphere-like and rod-like morphologies are synthesized using the birnessite precursor through an acified-hydrothemal-calcination treatment. The catalytic activities towards DME combustion reaction over the catalysts prepared from Birnessite-H and γ-MnOOH are in the following order:Cryptomelane Mn2O3>Mn2O3> Pyrolusite. The catalysts prepared by the hydrothermal treatment exhibits better catalytic activity.Thirdly, a facile hydrothermal method is employed to prepare tunnel structure of manganese oxides with different tunnel size. i.e. cryptomelane. pyrolusite and ramsdellite. Their catalytic activities are mainly dominated by the tunnel side, the crystalline phase, BET suface area and redox properties, which follow the order of (2×2) cryptomelane>(1×2) ramsdellite>(1×1) pyrolusite.Fourthly, to further improve the catalytic activity of birnessite and cryptomelane towards DME combustion. Ce was introduced on the surface of the birnessite (cryptomelane) using the ion exchange and impregnation methods. The relationships between the activities and the structure, the Ce content, and the BET suface area on the one hand and. and on the other hand, the precursors, the preparation methods, as well as the redox properties were discussed. A promote effect of Ce in Ce-MO catalytic has been observed in this research. The Ce-cry-ex catalyst prepared by the ion-exchange method from cryptomelane precursor was the most active catalyst, with a light-off temperature and a full conversion temperature of135℃and145℃, respectively.Fifthly, in order to extend the application range for birnessite, we tried to investigate the electrochemical properties of birnessite. We successfully synthesized birnessite with large specific surface area (137.2m/g) by a facile and mild polyol method. The first electrochemical results show that the first discharge capacity is120mAh/g while the discharge capacity is52mAh/g after19cycles for birnessite. We also extended the polyol method to synthesize LiMn2O4with specific surface area of about329m2/g. Preliminary electrochemical tests shows that the first discharge capacity is170mAh/g for samples and the discharge capacity is70mAh/g after19cycles. Furthermore, a primary study to the synthesis of Mg-doped birnessite using the polyol method is also presented.