Study On The Modification Mechanism And VOCs Catalytic Oxidation Performance Of Composite Manganese-based Catalysts

With the acceleration of China’s industrialization process,the environmental pollution problems caused by the annual discharge of volatile organic compounds(VOCs)are becoming increasingly serious,and it is urgent to develop an efficient,safe and stable VOCs purification technology.Catalytic oxidation is eco-friendly,safe and widely used,and is a potential VOCs terminal treatment technology,and the reduction of catalytic reaction temperature will greatly reduce the cost of enterprise pollution control and improve the safety of equipment operation,which is of great significance.As the core of catalytic oxidation technology,the activity of the catalysts can help reduce the catalytic reaction temperature,so it is necessary to develop VOCs catalysts with high activity and stability to ensure stable degradation of VOCs at low temperature.Manganese-based catalysts have low cost,strong sulfur and chloride resistance,and flexible and adjustable valence,morphology and crystal form,which have strong modification optimization and industrial application potential.However,conventional single manganese-based catalysts are easy to agglomerate and inactivate,so the low-temperature catalytic activity is poor,and it is necessary to construct composite materials to optimize.Based on this,this paper focuses on the composite modification of manganese-based catalysts,starting from improving the structure and electronic characteristics of catalysts,promoting catalysts dispersion and inducing the formation of reactive oxygen species,thereby improving the catalytic oxidation activity of VOCs and reducing the reaction temperature.Furthermore,the industrial application method of highly active catalyst is developed,the application scope of composite manganese-based catalyst is expanded,and technical support is provided for the scientific governance of industrial VOCs.Specifically,the main research carried out in this paper is as follows:Firstly,MnOx/zeolite composite catalysts were developed and cooperated with ozone to catalyze the oxidation of VOCs at low temperatures.Five composite catalysts,MnOx/MCM-41,MnOx/13X,MnOx/ZSM-5,MnOx/HY,and MnOx/USY,were synthesized by impregnation method,and the synergistic effect of support and active components on the low-temperature catalytic oxidation performance of VOCs was investigated.The experimental results indicate that the support properties have a significant impact on the catalyst structure,with a specific surface area of up to 955 m2/g for MnOx/MCM-41.Its unique pore structure and morphology provide sites for MnOx loading.In addition,MnOx/MCM-41 has the highest concentration of surface adsorbed oxygen species,which is attributed to the presence of Si-O-Mn interface effect enhancing the interfacial electron transfer rate.The activity test showed that MnOx/MCM-41 performed the highest catalytic activity and could achieve 100%degradation of toluene and ozone at room temperature in synergy with ozone.At the same time,MnOx/MCM-41 has excellent catalytic stability and can drive active hydroxyl species to degrade VOCs under high humidity conditions.The mechanism analysis showed that the excellent pore structure and high concentration of surface oxygen vacancies promoted the formation of MnOx/MCM-41 surface adsorbed oxygen species,realized the pre-oxidation of toluene adsorption process,and accelerated the oxidation of VOCs to the final product CO2.Secondly,in order to avoid the potential environmental risks of ozone assistance and further enhance the activity of low-temperature catalytic oxidation of VOCs by manganesebased catalysts,a new 3D snowflake graphene was developed to synthesize graphene-modified manganese-based catalyst rGO@α-MnO2 based on the "anchoring-growth" mechanism.Structural characterization tests show that graphene doping can effectively increase the specific surface area and pore volume of the catalyst,and promote the dispersion of the catalyst grains.Graphene is doped into α-MnO2 grains to construct new electron transfer channels and induce interface effect formation.XPS.Raman,and EPR tests show that the Mn-O-C covalent structure formed after graphene modification introduces a high-speed moving charge into the catalyst.promotes the formation of defects and increases the concentration of surface adsorbed oxygen species,which is conducive to improving the low-temperature catalytic activity.The test results show that 4%rGO@α-MnO2 achieves 90%toluene degradation at 155℃.and has high moisture resistance.In 5 dry and wet cycle tests,160℃ variable humidity life test and 200℃ dry and wet life test.4%rGO@α-MnO2 showed high catalytic activity and stability,and had the potential to replace noble metal-based catalysts.Through in-situ testing and density functional theory calculation,the reaction mechanism was further explored,and it was found that the adsorption energy of toluene could be enhanced by graphene modification,and the exposure defects of the crystal plane could be induced,thereby increasing the concentration of surface adsorbed oxygen species and promoting the degradation of VOCs into final products.Finally,in order to develop highly active industrial VOCs catalysts,a cost-effective"ball milling-assisted impregnation method" was developed,and a monolithic catalyst was prepared by making powder 4%rGO@α-MnO2 into a micron particle suspension and impregnating oxalic acid etched manganese-based cordierite.Comprehensive analysis showed that the monolithic catalyst after four impregnations had the highest loading rate(6.8wt.%),the most developed pore structure,and the surface lattice oxygen concentration was high,which was conducive to the catalytic oxidation of VOCs.Performance tests have shown that the 44GM@iolite monolithic catalyst can achieve 90%toluene degradation rate at 200℃ and retain the high moisture resistance of 4%rGO@α-MnO2.The well-developed pore structure and high concentration of lattice oxygen ensure the catalytic activity and stability of the catalyst at 200℃,enabling it to show high stability in 5 dry and wet cycles and 10 h lifetime test.Furthermore,with the gas hourly space velocity up to 6000 h-1,the monolithic catalyst still maintains stable catalytic activity,which is comparable to the noble metal monolithic catalyst and has great application potential.In summary,this paper develops highly active composite catalysts and industrial application methods,which effectively solve the current technical problems of VOCs treatment and catalyst modification,and finally achieve efficient and stable degradation of VOCs at low temperature.This thesis provides new methods and ideas for the preparation and industrial application of high-activity and high-stability VOCs catalysts,which has high innovative significance and application value.