Controllable Synthesis?Optimization And Study Of Low-Temperature Catalytic Oxidation On Toluene

In recent years,with the rapid development of China's economy,air pollution has attracted great attention.Volatile Organic Compounds?VOCs?in air pollution can generate haze weather and cause direct damage to respiratory tract,liver and nerves,or even lead to cancer.Long-term exposure to VOCs can lead to canceration,so the governance of VOCs has attracted extensive attention of many scholars.Catalytic oxidation method has been widely studied as an important means to degrade VOCs.The element Mn as a transition metal is widely used to degrade various organic compounds due to its various valence states,in addition,manganese dioxide?MnO₂?has attracted the attention of researchers in the field of catalysis because of its advantages of being environmentally friendly and low cost.The microstructure,valence state and crystal structure of manganese dioxide have important influence on its catalytic performance.And the combination of manganese dioxide and other metals can increase the catalytic activity sites to improve the catalytic activity.In this thesis,the catalytic degradation of toluene by MnO₂?nanotubes,nanowires,and nanocubes?with different morphologies was investigated,and the structure-activity relationship between the morphologies and catalytic properties was discussed.MnO₂ was selected as carrier to construct a series of FeO_x/MnO₂ composites and we considered the Fe-Mn synergistic effect on catalytic performance.Finally,the possible mechanism for catalytic degradation of toluene was proposed on basis of intermediate products of nanotube MnO₂ and 3%FeO_x/MnO₂ catalysts.The key results in this study were summarized as follows:?1?Manganese dioxide with nanotube,nanowire and nanocube morphologies were synthesized successfully via a conventionally hydrothermal process and their catalytic performance was investigated.The results of H₂-TPR showed that nanotube MnO₂ had the best catalytic performance.The catalytic degradation of toluene decreased in the order nanotube MnO₂>nanowire MnO₂>nanocube MnO₂.The T₉₀ of nanotube,nanowire and nanocube MnO₂ were 220�C,239�C and 318�C,respectively,so the nanotube MnO₂possessed the most excellent catalytic property.For the three catalysts,the nanotube MnO₂have larger specific area and can expose more active sites,leading to the strongest catalytic activity.The nanocube MnO₂ exhibits has the lowest catalytic activity due to its beta-crystal phase.?2?We used MnO₂ as carrier to prepare a series of FeO_x/MnO₂ complexes with different load ratios by traditional hydrothermal method.The results of H₂-TPR showed that 3%FeO_x/MnO₂ had the best low temperature catalytic performance,and the catalytic degradation of toluene decreased in the order 3%FeO_x/MnO₂>nanotube MnO₂>1%FeO_x/MnO₂>0.6%FeO_x/MnO₂>5%FeO_x/MnO₂.The results showed that3%FeO_x/MnO₂ displayed the excellent catalytic activity among a series of complexes,that is,catalytic performance achieved the best property when the load ratio of iron oxide was3%.This is attributed to its surface oxygen species and large specific area.Besides,3%FeO_x/MnO₂ and nanotube MnO₂ catalysts showed high stability in the toluene catalytic process.The result of experiment indicated that morphologies of MnO₂,specific surface area,redox properties and content of surface oxygen species could be affected by the loading of iron oxide.The interaction between iron and manganese improved the redox properties of MnO₂ catalyst.The conversion from Fe²⁺to Fe³⁺in the FeO_x/MnO₂ increased the oxygen absorption capacity of the catalysts.?3?The degradation of toluene by 3%FeO_x/MnO₂ catalysts were studied by impregnation method,ultrasonic method or hydrothermal method,followed by carbonization process.The results indicated that the 3%FeO_x/MnO₂ synthesized by different methods could not affect the crystal phase of carrier,but had obvious influence on the morphology.Through the test of toluene catalytic activity,it showed that the catalytic degradation of toluene by 3%FeO_x/MnO₂ prepared via different methods decreased in the order 3%FeO_x/MnO₂>3%FeO_x/MnO₂HT-400>3%FeO_x/MnO₂IM-400>3%FeO_x/MnO₂NM-400.The poor catalytic performance of the impregnation method and ultrasonic method may be attributed to decrease specific surface area and active sites which result from blockage of part of the pore channels of MnO₂ by coating iron oxide,leading to the reduction of catalytic performance.The catalytic performance of3%FeO_x/MnO₂HT-400 decreased through the carbonization process,which may ascribe to the change of morphology,thus affecting its redox properties.Above all,supported catalyst prepared by impregnation and ultrasonic method had poorer catalytic performance than that obtained by hydrothermal method.The hydrothermal method has an advantage over others,and 3%FeO_x/MnO₂ prepared by hydrothermal method has the best toluene degradation ability.?4?The intermediate products of toluene degradation by nanotube MnO₂ and3%FeO_x/MnO₂ catalysts were qualitatively analyzed,and the reaction path was proposed based on the kind of products.The intermediate products of the two catalysts were similar and had so much common intermediates,indicating that the loading of iron oxide may not change the catalytic mechanism.According to the intermediates on catalyst surface,the degradation of toluene may obey the mechanism of MVK?Marse-van Krevelen?.