Study On The Synergistic Degradation Of VOCs By Low-temperature Plasmas Combinning With Multi-element Iron-based Catalysts On The Sepiolite Supporte

With the rapid development of economy,the industrial production is increasing and expanding.The excessive exploitation and utilization of natural resources has led to the increasingly serious emission of volatile organic compounds(VOCs).VOCs are diverse and volatile.Most VOCs have high photochemical reactivity and can easily become precursors of ozone and fine particles in the atmosphere.In addition,VOCs are pathogenic as well,which can pollute the environment and harm to human health.Therefore,the control and reduction the VOCs emission has become an important topic of air pollution control.As an emerging technology for effective degradation of VOCs,Non-thermal plasma(NTP)has attracted extensive attention and research in the field of VOCs treatment because of its simple equipment structure,fast response speed,strong applicability,and can operate at room temperature and pressure.However,the application of single NTP technology in the practical treatment of VOCs still has some disadvantages,such as relatively high energy consumption,un-expected by-products and incomplete oxidative degradation.The synergistic degradation of VOCs by plasma catalysis has become an effective method to solve the above shortcomings.Considerable progress has been made in the research of plasma catalyzed synergistic degradation of VOCs.However,in-depth research and exploration are still needed in the aspects of catalyst optimization,catalyst support selection,synergistic mechanism of catalyst and plasma,so as to optimize the synergistic effect of catalytic degradation and meet the needs of practical industrial applications.In order to improve the degradation efficiency,carbon balance,CO₂ selectivity and reduce the by-products of plasma catalytic synergistic degradation of VOCs,pulse modulated double dielectric barrier discharge(DDBD)was used to degrade VOCs.After comparing and evaluating the catalytic effects of various metal oxides,sepiolite is selected as the supporting substrate,and multi-component iron catalysts were used to study the degradation of VOCs simulated by toluene.The main research work and results are as follows:(1)The basic characteristics of toluene degradation by pulse modulated DDBD plasma were studied.The effect of plasma characteristics on toluene degradation was studied by using pulse modulated power supply and self-made DDBD reactor.It was found that at the same specific input energy,the energy efficiency and toluene degradation efficiency increased first and then decreased with the increase of duty cycle.When the duty cycle is about 40%,the highest energy efficiency and toluene degradation rate could be obtained;the degradation efficiency and energy efficiency of toluene decreased with the increase of flow rate;the degradation efficiency of toluene decreased with the increase of initial concentration,while the energy efficiency was increased.Therefore,when energy efficiency was used as an evaluation index,the flow rate and the initial concentration are needed to assist the evaluation,as to obtain a reference index in practical application.Through the study on the degradation of several common VOCs with different molecular structures,it was found that the molecular structure and ionization energy of VOCs had an impact on the degradation efficiency.The experimental results also showed that pulse modulation can effectively reduce the concentration of NO_x generated in air plasma.When the duty cycle is less than 40%,the NO_x products is almost undetectable.(2)The synergistic plasma catalysis degradation of toluene by a variety of metal oxides and supporting materials was studied.When metal oxides such as Fe Ox,Mn Ox,Cu Ox,Co Ox,Cr Ox and Al Ox were used as catalysts to degrade toluene with DDBD,it was found that the catalytic activities of different metal oxides were significantly different.For example,Fe O_x had higher toluene degradation rate,while Mn O_x had better carbon dioxide selectivity and low ozone residual concentration.Comparing the catalytic performance of Fe O_x supported on alumina,13X molecular sieve and sepiolite(SEP)fiber,it is found that SEP was more suitable for catalyst support than alumina and 13X molecular sieve.SEP has good specific surface area and certain catalytic activity,which can well disperse and improve the activity of the catalyst.According to the H₂-TPR analysis,it was found that SEP could reduce the reduction temperature of Fe₂O₃ and improve its catalytic performance better.Through the detailed study and comparison of Fe₂O₃ catalyst in plasma catalysis(IPC)and post plasma catalysis(PPC),it is found that the carbon balance of IPC is better than that of PPC,the toluene degradation rate and the carbon dioxide selectivity of IPC were also slightly higher than PPC.However,the residual ozone concentration of IPC is higher than PPC.According to the GC-MS analysis,it was found that the types of organic by-products of IPC and PPC were the same.Compared with PPC,IPC could slightly reduce the emissions of butanone and phenol.Overall,IPC is superior to PPC.The prepared Fe₂O₃/SEP catalyst has good deactivation resistance.After 48 hours of IPC test,the catalytic performance did not weaken significantly.(3)The characteristics of Iron-containing composite catalyst for the degradation of toluene were studied and analyzed.Binary composite catalysts such as Cu Fe and Mn Fe were prepared with SEP as carrier.According to the experimental analysis of catalytic activity,the catalytic activity of Cu Fe catalyst was slightly better than that of single metal oxide,while catalytic activity of Fe Mn catalyst was significantly better than that of single metal oxide.It was indicated that the catalytic activities of Mn O_x and Fe O_x are mutually promoted.After the compound,not only the degradation rate of Fe Ox toluene was higher,but also the carbon balance was higher and the ozone residual concentration of Mn Ox was lower.In the study of Mn loading in Mn-Fe composite catalyst,it was found that the optimal loading of Mn is 6%(mass fraction),and the catalytic activity will not be improved by further increasing the content of manganese.The types of organic by-products of binary composite catalyst are the same as those of single metal oxide catalyst.The characterization of Mn Fe/SEP showed that the catalyst exists on the surface of SEP in the form of tetragonal Fe₂O₃ particles and flower ball Mn O₂ particles.With the increase of Mn loading,the lattice oxygen content first increased significantly,and then tended to be flat.The lattice oxygen content of 6Mn Fe/SEP was significantly higher than 4Mn Fe/SEP,2Mn Fe/SEP and Fe/SEP,and slightly lower than 8Mn Fe/SEP and 10Mn Fe/Sep.The results showed that the high content of lattice oxygen was conducive to the catalytic oxidation of organic compounds,which led to the best degradation efficiency with 6Mn Fe/SEP.Under the action of NTP,toluene was degraded by active oxygen atoms provided by lattice oxygen,and resulting the conversion of Fe3+and Mn4+to Fe2+and Mn3+,respectively.The lost lattice oxygen was supplemented by gas-phase oxygen,and then Fe²⁺and Mn³⁺were oxidized to Fe³⁺and Mn⁴⁺respectively to complete the redox cycle.(4)The characteristics of toluene degradation over Zr Mn Fe/SEP catalyst were studied and analyzed.Zr Mn Fe/SEP catalyst was prepared by impregnation method on the basis of Mn Fe/SEP catalyst.The results showed that the combination of Zr O₂,Mn O₂ and Fe₂O₃ can greatly improve the degradation efficiency,carbon balance and CO₂ selectivity of toluene degradation,and inhibited the production of ozone.The reduction temperature of Zr Mn Fe/SEP catalyst was relatively low,which was conducive to improve the catalytic performance.Compared with NTP alone,Zr Mn Fe/SEP+NTP could improve the degradation efficiency of toluene by 40%,the carbon balance by 26%,the CO₂ selectivity by 31%and reduce the ozone concentration by 92%.The surface of the catalyst was characterized by SEM.It was found that the doping of Zr could inhibit the agglomeration of Fe₂O₃ and Mn O₂,and the formed particles were fine and evenly distributed on the surface of SEP support.The XPS characterization showed that Zr could increase the content of lattice oxygen.Compared with Mn Fe/SEP,Zr Mn Fe/SEP increased the content of lattice oxygen by 28.1%.At the same time,due to the addition of Zr,the reduction temperature of Zr Mn Fe/SEP was decreased,which led to better catalytic performance.The degradation effect and characterization results of toluene degradation showed that Zr Mn Fe/SEP was a catalyst with high efficiency and stable catalytic activity.