Study On Alkali Metal Modification And Catalytic Performance Of Delta Manganese Dioxide Catalyst For Oxidation Of Toluene

Volatile organic compounds（VOCs）are one of the main sources of atmospheric pollutants,and are also important precursors of secondary organic aerosols（SOA）and ozone（O₃）.Therefore,effective control of VOCs emissions is crucial to solving the problem of air pollution.Among many VOCs treatment technologies,catalytic combustion can achieve complete conversion of VOCs at lower temperatures and is considered to be one of the most promising technologies.What’s more,the catalyst with high activity and low cost is the key to the large-scale application of this technology.Compared with the commercially used precious metal catalysts,Mn O₂has been widely used due to its cheap and easy availability,diverse valence states and environmental friendliness.In particular,manganese dioxide with a special layered crystal structure as a catalyst exhibits good catalytic activity and thermal stability in the removal of volatile organic compounds.The interlayer cations have been confirmed to affect the specific surface area,morphology,element valence and reducibility of the catalyst,which further affects the catalytic performance of the material.Therefore,in this thesis,the effect of surface sodium ion concentration on the catalytic oxidation performance of toluene was firstly investigated.On this basis,the classical alkali metal ion cesium ion was inserted between the layers of manganese dioxide,and a series of structural properties were characterized.The reasons for the effect of different concentrations of cesium ion introduction on the catalytic performance and the intrinsic relationship between the catalyst structure and performance are revealed.The main research contents and findings are as follows:The layered manganese dioxide catalysts（Na-600,Na-1200,Na-2000,Na-3600）with different concentrations of surface Na⁺were successfully prepared by hydrothermal method by controlling the amount of washing water.The toluene catalytic oxidation activity and CO₂selectivity of the four catalysts were investigated and compared,and the results showed that the order of catalytic performance was Na-2000>Na-1200>Na-3600≈Na-600.The moderate surface Na⁺brings higher Mn⁴⁺content,lattice oxygen content and mobility,and also increases the surface chemisorption oxygen content,so it has better toluene catalytic performance.The catalyst Na-2000 under the condition of toluene concentration of 1000 ppm and WHSV of 30000 m L/（g.h）,the low temperature catalytic oxidation temperature T₅₀and T₉₀of toluene are 202°C and 212°C,respectively,and it also has high CO₂selectivity.In the second part,Cs-0.05,Cs-0.10,Cs-0.15,Cs-0.20 layered manganese dioxide catalysts with different Cs⁺concentrations were prepared by adjusting the raw material Cs/Mn by the same hydrothermal method using the best washing water amount in the previous chapter,and discussed the effect of Cs⁺doping on the toluene catalytic performance of the catalyst.The order of toluene catalytic activity of the catalyst doped with Cs⁺ion is:Cs-0.15(T₉₀=188℃)>Cs-0.10(T₉₀=192℃)>Cs-0.05(T₉₀=201℃)>Na-2000(T₉₀=212℃)>Cs-0.20(T₉₀=220℃).XRD results show that the catalyst can maintain the layered structure after Cs-doped.XRD,ICP and Raman characterization results confirmed that Cs⁺was successfully intercalated the layers.After the incorporation of Cs⁺,the catalyst activity increased,and the best activity was when Cs/Mn was 0.15;As the doping amount further increased,the catalytic activity was inhibited.Raman,TEM,XPS and H₂-TPR,O₂-TPD characterization results show that the introduction of a moderate concentration of Cs⁺can affect the crystal structure of theδ-Mn O₂catalyst,resulting in an increase in lattice defects and a decrease in the Mn-O bond strength.The higher content of active lattice oxygen and Mn⁴⁺promoted the redox reaction,thus showing the best catalytic oxidation performance of toluene(T₉₀=188℃).When Cs/Mn was further increased to 0.20,more excess Cs⁺remained on the surface of the catalyst,resulting in a block morphology and a sharp decrease in the S_BETof the Cs-0.20 catalyst,thus inhibiting the catalytic activity.In the third part,the catalytic performance,long-term stability and recyclability of the catalyst for p-toluene under different working conditions（space velocity,toluene concentration,water content,and catalytic reaction time）were investigated.The results show that the water resistance of the Cs-0.15 catalyst is enhanced after doping with Cs⁺,and the catalytic performance decreases after being affected by water vapor.At the same time,the Cs-0.15catalyst can show excellent catalytic activity under a wide range of space velocity and toluene concentration,and it has good long-term stability and recyclability.