Synthesis Of Nanostructured Manganese Oxides And Their Catalytic Applications In Dying Waste Water Treatment

Single-crystalline nanorods of β-MnO₂, Mn2O₃ and Mn3O₄ were successfully synthesized via the heat-treatment of γ-MnOOH nanorods, which were prepared through a hydrothermal method in advance. The calcination process of γ-MnOOH nanorods was studied with the help of Thermogravimetric analysis and X-ray powder diffraction. When the calcinations were conducted in air from 250℃ to 1050℃, the precursor γ-MnOOH was first changed to p-MnO₂, then to Mn2O₃ and finally to Mn3O₄. When calcined in N2 atmosphere, y-MnOOH was directly converted into Mn3O₄ at as low as 500 ℃. Transmission electron microscopy (TEM) and high-resolution TEM were also used to characterize the products. The obtained manganese oxides maintain the one-dimensional morphology similar to the precursor γ-MnOOH nanorods. Further experiments show that the as-prepared nanostructured manganese oxides have catalytic effects on the oxidation and decomposition of the methylene blue (MB) dye with H2O₂.The results indicate that the catalytic activity of β-MnO₂ nanorods, nanorods and Mn3O₄ nanoparticles is higher than that of commercial MnO₂, microspheres and Mn3O₄ nanorods respectively. The influences of reaction conditions such as initial concentration of methylene blue, H2O₂ concentration and catalyst dosage on the decoloration efficiency of methylene blue are studied. When β-MnO₂ nanorods, Mn2O₃ nanorods and Mn3O₄ nanoparticles are used as catalysts, the decoloration rates of methylene blue follow 2.5-order, first-order, first-order kinetics respectively and the reaction rate constants are 1.677 (mol/L)^-1.5 min^-1, 0.00224 min^-1 and 0.00724 min^-1, respectively. It has been proved that methylene blue could be decomposed to final products as CO₂ and other smaller organic compounds by COD, UV-vis, GC, FTIR and other analytical measurements.