Degradation Of Mixed Volatile Organic Compounds (Toluene, Acetone And Ethyl Acetate) By Catalyst And Dielectric Barrier Plasma

VOCs is one of the frontier issues of air pollution control.Non-thermal plasma（NTP）synergistic catalytic degradation technology,as an emerging technology,has the advantages of simple equipment,rapid response and strong applicability.It has a potential wide application prospect in industrial production.Researches in recent 20 years have shown that the NTP catalytic synergistic degradation of VOCs can remove all kinds of VOCs quickly and effectively.However,it still has some problems such as:unclear degradation synergistic mechanism,many by-products,low CO₂selectivity and so on.The selection and optimization of catalyst and its support carrier need to be further studied.At present,relevant studies were only carried out for VOCs of single component,while the exhaust emission in real industrial production usually contained multi-component VOCs.Although a small amount of works on mixed VOCs have been carried out,the conclusion was still not clear.Therefore,research on NTP synergistic catalytic degradation of multi-component VOCs is necessary to explore the interaction influence and the mechanism of multi-component VOCs degradation.In order to study and explore the new phenomena and laws of NTP synergistic catalytic degradation of multi-component VOCs,improve the degradation efficiency,carbon balance,CO₂selectivity,reduce by-products and understand the reaction path,three representative VOCs（toluene,acetone and ethyl acetate）with high emissions in the coating industry were selected for the NTP synergistic catalytic degradation of mixed VOCs in a double dielectric barrier discharge（DDBD）reactor,so as to study the synergistic effect between mixed VOCs and the catalytic selectivity of catalysts;The transition metal oxide catalyst was selected and optimized to achieve the best degradation effect;The catalysts were characterized and the ozone adsorptions were analyzed by density functional theory（DFT）in order to understand the catalytic characteristics and catalytic mechanism of the catalyst.The main work and conclusions are as follows:（1）Comparison of degradation characteristics of VOCs in single degradation and mixed degradation with or without catalysis,and analysis of catalytic selectivity of the catalyst.NTP synergistic catalytic degradation was carried out with three representative VOCs（toluene,acetone and ethyl acetate）with high emissions in the coating industry.The degradation rate of acetone was significantly lower than that in single degradation;Compared with the degradation of three VOCs alone,the ozone production and carbon balance of mixed VOCs’s degradation decreased;The formation of ozone was inhibited and the degradation rate and carbon balance under various conditions were improved by Mn₂O₃/γ-Al₂O₃catalyst;In mixed VOCs,the higher the difficulty of VOCs degradation,the higher the improvement of Mn₂O₃/γ-Al₂O₃catalyst on its degradation rate.（2）Comparative study on catalytic performance of transition metal catalysts（MnO_x/γ-Al₂O₃、Fe O_x/γ-Al₂O₃、Cu O_x/γ-Al₂O₃、Ce O_x/γ-Al₂O₃and La O_x/γ-Al₂O₃）..MnO_x/γ-Al₂O₃、Fe O_x/γ-Al₂O₃、Cu O_x/γ-Al₂O₃、Ce O_x/γ-Al₂O₃and La O_x/γ-Al₂O₃catalysts were prepared for NTP synergistic catalytic degradation of mixed VOCs.It was found that the improvement effect of each catalyst on the degradation rate of toluene,ethyl acetate and acetone can be arranged from high to low as follows:Mn₂O₃/γ-Al₂O₃>Fe₂O₃/γ-Al₂O₃>Cu O/γ-Al₂O₃>Ce O₂/γ-Al₂O₃>La₂O₃/γ-Al₂O₃>γ-Al₂O₃。The degradation rates were all greatly improved in Mn₂O₃/γ-Al₂O₃+DDBD system;La₂O₃/γ-Al₂O₃catalyst had a poor effect on the degradation rate of VOCs but good effect on carbon balance and CO₂selectivity;The adsorptions of ozone on five catalysts were simulated by DFT,and the results were in good agreement with the experimental results.In addition to Ce O₂,the adsorption energy of other transition metal oxides could be used as a reference for catalytic effect to a certain extent.The absolute value of adsorption energy of each metal oxides were arranged as follows:Mn₂O₃>Fe₂O₃>Cu O>La₂O₃.The higher the absolute value of adsorption energy,the greater the improvement of VOCs degradation rate of the catalyst.Among the five transition metal oxides,Mn₂O₃had the largest absolute value of adsorption energy.At the same time,it also changed the structure of O₃,an isolated oxygen atom was produced as a strong electron donor and two adsorbed oxygen atoms were produced as electron donor,which greatly enhanced its oxidizability,making Mn₂O₃/γ-Al₂O₃catalyst the more effective catalyst among the five catalysts.（3）Optimization of catalytic performance of MnO_x/γ-Al₂O₃catalyst-study on mass ratio of Mn.MnO_x/γ-Al₂O₃catalysts with different Mn content of 3%,5%,7%and 9%were used for NTP synergistic catalytic degradation of mixed VOCs.5%Mn₂O₃/γ-Al₂O₃catalyst with lower reduction temperature was found has the best effect on VOCs degradation rate,carbon balance,CO₂selectivity and ozone inhibition.The main organic by-products from the degradation of toluene,ethyl acetate and acetone mixtures were benzene,acetic acid,formic anhydride,2-butanone,benzaldehyde,phenol and phenol and 3-methyl-phenol.The introduction of transition metal oxide catalyst did not change the types of organic by-products.（4）Study on catalytic performance of Cu,Fe,Ce,La doped MnO_x/γ-Al₂O₃catalysts.It was found that the improvement of VOCs degradation rate by each catalyst could be arranged from high to low as follows:Mn₂O₃-La/γ-Al₂O₃>Mn₂O₃-Cu/γ-Al₂O₃>Mn₂O₃-Ce/γ-Al₂O₃>Mn₂O₃-Fe/γ-Al₂O₃.he degradation rate of toluene reached saturation in the higher SIE region with the introduction of catalysts;Mn₂O₃-La/γ-Al₂O₃catalyst also showed excellent optimization effect on carbon balance and CO₂selectivity of mixed VOCs’degradation,which was better than the other three composite transition metal oxide catalysts.The results of DFT calculation showed that the crystal sections of each composite transition metal oxide catalyst could effectively adsorb O₃molecules and produce a free oxygen atom with strong oxidation,which was easy to interact with other substances.Compared with other crystal planes,Mn-La crystal plane had the highest absolute value of adsorption energy.The adsorption of O₃molecules was chemical adsorption,and the system structure was the most stable,which could effectively adsorb O₃molecules.After the adsorption of O₃molecules,it could be more effective as an electron donor,making it have strong catalytic performance.Combined with the results of VOCs degradation experiment,Mn₂O₃-La/γ-Al₂O₃catalyst was the most effective catalyst structure,and the DFT calculation results were in good agreement with the experimental results..（5）Study on the optimum La:Mn ratio of Mn₂O₃-La/γ-Al₂O₃catalyst.Mn₂O₃-La/γ-Al₂O₃catalyst with different La:Mn ratios were prepared for NTP synergistic catalytic degradation of mixed VOCs.It was found that Mn₂O₃-La_0.5/γ-Al₂O₃catalyst with La:Mn ratio of 0.5:1 had the best performance in improving degradation rate,carbon balance and CO₂selectivity of mixed VOCs’degradation,and effectively inhibited the formation of ozone.（6）Construction of plasma thermal catalytic oxidation system.Mn₂O₃-La_0.5/γ-Al₂O₃catalyst was used for NTP synergistic thermal catalytic oxidation of mixed VOCs at 50-200℃.It was found that NTP synergistic thermal catalytic oxidation could effectively reduce the window temperature for Mn₂O₃-La_0.5/γ-Al₂O₃catalyst to exert the effect of thermal catalytic oxidation only when the ambient temperature was 100℃.With the increase of the ambient temperature,the degradation rate of acetone and ethyl acetate were further enhanced.When the ambient temperature was 200℃,the degradation rate of each component of mixed VOCs reached saturation at specific input energy（SIE）of 300 J/L,and there was no ozone emission in the tail gas;The increase of ambient temperature could also improve the carbon balance.When the ambient temperature reached 200℃,the carbon balance was almost saturated.Under different ambient temperatures,the CO₂selectivity showed different trends with the increase of SIE.The CO₂selectivity decreased with the increase of SIE when the temperature was lower than 100℃,while increased gently when the temperature was greater than 100℃.When the ambient temperature was greater than 100℃,the degradation rate and carbon balance of each component of mixed VOCs were close to saturation.More active species produced by the rise of SIE did not need to oxidize VOCs molecules or intermediates of VOCs’degradation,but were more used to deeply oxidize CO to CO₂.As a result,the CO₂selective showed an upward trend with the increase of SIE.