Synthesis Of Manganese Dioxide Catalysts With Different Crystal Types And Their Performances For Catalytic Combustion

Today,the problem of environmental pollution is becoming more and more prominent.A variety of prevention and treatment policies have been applied,among which volatile organic compounds（VOCs）are the main object of treatment.Catalytic combustion is recognized as one of the widely used and effective VOCs elimination technologies,and the core of the technology is an efficient and stable catalyst.In recent years,non-noble metal catalysts with low cost and good activity have become the focus of research,among which MnO₂-based catalysts have attracted extensive attention as a potential VOCs catalytic purification catalyst.In this dissertation,by using chemical synthesis method,there kinds of MnO₂ based catalysts have been prepared,namely,theα-MnO₂@Co₃O₄ composite materials with heterostructure,3D flower-likeδ-MnO₂ with different surface oxygen vacancy defects and 2D lamellar structure ofβ-MnO₂ nanosheet and their catalytic combustion performance for toluene were studied.The main research contents and conclusions are as follows:（1）Heterogeneous structure catalysts often show synergistic catalytic performance.By calcining the precursor of MnO₂@ZIF-67,Co₃O₄ nanoparticles were uniformly loaded onα-MnO₂ nanowires to synthesize 3D composite MnO₂@Co₃O₄ with heterostructure.The effects of the ratio of raw materials,reaction time and calcination temperature on the final morphology and crystal structure of the composite catalyst were discussed.By the analysis of catalyst structure characterization and the toluene catalytic combustion performance,a relatively large specific surface area,more oxygen species adsorbed on the surface and the active heterogeneous interface formed due to the influence of Co₃O₄ contribute to the excellent catalytic performance of MnO₂@Co₃O₄ catalyst.,with a T₁₀ and T₉₀ at 202℃ and 235℃,respectively.（2）Controlling of defect is one of the effective means to improve the catalytic performance of manganese dioxide.The 3D flower-likeδ-MnO₂ with different surface oxygen vacancy defects were synthesize by reducing the 3D flower-likeδ-MnO₂,which was prepared at room temperature,with ethylene glycol under microwave heating to control the defects.The growth mechanism of 3D flower-likeδ-MnO₂ was discussed,and the effects of microwave treatment temperature and time on the morphology and crystal structure of the catalysts were examined.The results showed that theδ-MnO₂-30 catalyst,which treated at 120℃ for 30 min,has the best catalytic activity(T₉₀=212℃).XPS characterization analysis showed that the Mn element on the surface ofδ-MnO₂ was mainly in the form of Mn³⁺after reduction treatment.Mn³⁺can induce the formation of oxygen vacancy defects and the activation of lattice oxygen,which together lead to theδ-MnO₂-30 showed the excellent catalytic performance.（3）Different from the conventional morphology,the non-layered structural materials with two-dimensional lamellar morphology often shows excellent catalytic performance.The tunneledβ-MnO₂ with unusual nanosheet structure was prepared by using the topological transformation strategy.First,a sheet-like precursorβ-MnOOH was synthesized,and then topologically transformed intoβ-MnO₂ nanosheet.Under the weight hourly space velocity（WHSV）of 40,000mL·g^-1·h^-1,the T₁₀ and T₉₀ of β-MnO₂-3 were 198℃ and 235℃,respectively,which were far superior to the typical morphology ofα-MnO₂ nanowire andβ-MnO₂ prism,also superior to theβ-MnO₂-24 with relatively high crystallinity.The order of reaction rate of per unit specific surface area of each catalyst at 235℃ follows:β-MnO₂-3>β-MnO₂-24>α-MnO₂>β-MnO₂-HR.It is shown that the catalytic activity ofβ-MnO₂-3 with the main exposed（101）crystal plane is much higher thanβ-MnO₂-HR with the main exposed（110）crystal plane,and the 2D lamellar structure which is rich in surface defects can significantly enhance the catalytic combustion capacity of toluene ofβ-MnO₂.