Controllable Sythesis And Catalytic Property Of A-MnO₂ Nanoparticles

Benzene, toluene, and xylene (BTX) are the major volatile organic compounds (VOCs). They are primarily released during printing, chemical production and mobile emission, and cause serious harm to environment and human health. Complete catalytic oxidation techniques that convert a contaminant into carbon dioxide (CO2) and water are an effective way to remove VOCs. Therefore, preparation of the catalysts with low temperature and high activity, and to be used in the complete catalytic oxidation of BTX is a very interesting research topic.In this paper, Mn₃O₄, Mn₂O₃ andα-,β-,γ-,δ-MnO₂ catalysts were prepared in different ways. Then they were used in complete catalytic oxidation of o-xylene. The effects of manganese oxides composition and crystal structure on catalytic behavior were examined. Subsequently, theα-MnO₂ nanoparticals with better catalytic activity were prepared via a redox-precipitation method with Na2S2O3 and KMnO₄ precursors, and hydrothemal redox method using KMnO₄ and divalent manganese salts as precursors. The results showed that preparation conditions affected the microstructures and activities of theα-MnO₂ catalysts. In addition, our researches studied the reaction mechanism ofα-MnO₂ on complete catalytic oxidation of o-xylene. The X-ray diffraction (XRD), Field Emission Scanning Electron Microscope (FESEM), Brunauer-Emmett-Teller (BET) surface area measurement were used to research the microstructure of the catalysts. The specific studies are as follows:1,α-,β-,γ- ,δ-MnO₂, Mn2O3 and Mn3O4 were prepared by different methods. Catalytic activities of these catalysts were evaluated in terms of both o-xylene conversion and CO2 yeild. MnO₂ exhibited much higher catalytic activities than Mn2O3 and Mn3O4. The catalytic activities of MnO₂ in a sequence is:δ-≈α-﹥γ-﹥β-MnO₂. At 230 oC,δ-MnO₂ can convert 0.06 vol.% o-xylene into carbon dioxide and water.2,α-MnO₂ nanoparticals with an diameter of 25⁵0 nm were prepared via a redox-precipitation method by using Na2S2O3 and KMnO₄ as precursors, HNO3 adjusting pH = 3⁵ and calcined at 400⁷00 oC. Theα-MnO₂ nanoparticals showed good catalytic activity and thermal stability. Among them,α-MnO₂ nanoparticals of 25nm can convert 0.06 vol.% o-xylene into carbon dioxide and water at 220 oC. In a continuous 60 h activity test, the catalyst remained stable.3,The microcrystalα-MnO₂ nanoparticals were prepared via hydrothemal redox process using KMnO₄, divalent manganese salts and alkali modifier as the raw materials, and by changing the preparation time, reaction tempreture and concentrations of the initial reactants. Theα-MnO₂ nanoparticals were used for complete catalytic oxidation of o-xylene. The results showed that theα-MnO₂ catalyst of 25nm can achieve the complete catalytic oxidation of 0.06 vol.% o-xylene at 200 oC. The adsorbed oxygen reaction mechanism is performed overα-MnO₂ nanoparticals in complete catalytic oxidation of o-xylene. Moreover, the catalyst showed good stability after continuous 60h activity test and can be reused three times.