Degradation Of Xylene Via Combined Non-Thermal Plasma And Modified LaMnO₃ Catalysts With Byproducts Control

Non-thermal plasma(NTP)-catalysis technology for volatile organic compounds(VOCs)abatment is commonly used in the treatment of low concentration VOCs or odor.This technology can be ignited quickly and operate at room temperature,which gives this technology unique advantages in the field of VOCs treatment In general NTP equipment,VOCs degrade rapidly but are difficult to degrade completely.Meanwhile,by-products are generated,including small organic molecules,O3,CO,N2O and NOx.Due to the potential pollution of these by-products,the application of NTP-catalysis technology is limited.In this work,the decomposition rates of three xylene isomers during NTP were comparied experimentally and simulatively LaMnO3 perovskite(ABO3)was used as the basement of catalyst.By doping Fe at B-site and Ce at A-site respectively,the effects of A-site and B-site modification on the comprehensive catalytic performance of catalysts in the process of NTP catalytic degradation of o-xylene were explored,such as xylene conversion,CO2 selectivity and by-products control.The doping ratio was optimized.The reaction mechanism was also studied with the characterization of the catalysts.Morever,the B site modification of Co,Zr,Cr and Fe was carried out to study the influence of the different modified elements.The comprehensive catalytic activities of different modified LaMnO3 catalysts were tested in the process of NTP catalytic degradation of m-xylene.The stabilities of these catalysts were also evaluated.Xylene is a kind of typical VOCs,the emission of which is serious.It has three isomers.Through NTP degradation experiments,it is found that the degradation rate sequence is as follows:o-xylene>m-xylene ? p-xylene.With the help of ReaxFF simulation with an external electric field,NTP atmosphere is simulated.In the decomposition process of o-xylene,there is a unique fast ring cleavage pathway.Under the influence of steric hindrance effect,intramolecular hydrogen transfer occurs,which induces supersaturation of hydrogen atom connected with a carbon atom on the benzene ring.Thus,the benzene ring in the o-xyelne molecule opens very fast.This fast reaction pathway explains the experimental phenomenon that the degradation rate of o-xylene is the highest among xylene isomers on the molecular level.In the process of NTP catalytic degradation of o-xylene,the o-xylene conversion,COx selectivity and by-product control can be improved by doping Fe at B site in LaMnO3 catalyst.The comprehensive performance of LaMn0.9Fe0.1O3 catalyst is the best among all tested catalysts herein.When the initial concentration of o-xylene is 300 ppm,the carrier gas flow rate is 200 mL/min,and the specific input energy(SEE)is 744 J/L,LaMn0.9Fe0.1O3 catalyst has the highest o-xylene conversion of 91.3%,high COx selectivity of 88.1%,the highest N2O inhibition efficiency of 61.2%.In addition,H2-TPR results show that Fe3+has stronger reducibility and higher low temperature activity than Mn4+.Fe doping makes Fe3+replace part of Mn4+,accelerating the dissociation process of B-site metal and O species and improving the O3 decomposition ability of LaMnO3 catalyst.XPS results show that LaMn0.9Fe0.1O3 catalyst has highest Fe2+concentration,while Fe2+ plays an important role in the O species exchange between Mn3+/Mn4+and Fe2+/Fe3+during the decomposition process of O3,indicating that LaMn0.9Fe0.1O3 catalyst has excellent O3 decomposition ability.Compared with the unmodified LaMnO3 catalyst,the O3 inhibition efficiency of LaMn0.9Fe0.1O3 catalyst increased from 63.3%to 84.9%.A small amount(less than 20%)of Ce doping at A site in LaMnO3 catalyst can improve the catalytic performance in an NTP-catalysis system.If the doping amount is more than 20%,the structure of LaMnO3 perovskite will be destroyed and the activity will decrease.The optimum Ce doping ratio is 10%.La0.9Ce0.1MnO3 catalyst has best performance of O3 control.When SIE=719.7 J/L,the O3 inhibition efficiency reaches 86.6%.Ce doping can not only adjust the catalytic activity of LaMnO3 catalyst by affecting the valence of B-site metal in perovskite crystal structure,but also directly enhance the O3 inhibition ability of the catalyst by accelerating the electron transport process when O3 dissociates in the oxygen vancacy on the surface of catalyst.However,the Ce modified LaMnO3 catalyst is not as good as the Fe modified LaMnO3 catalyst in terms of the improvement on oxidation ability,which shows the limit of the A-site modification in perovskite catalyst.Four transition metal elements,Co,Zr,Cr and Fe,were selected as doping elements of B site doping LaMnO3 to compare the NTP catalytic degradation of m-xylene.It is found that the xylene conversions of all the modified catalysts are higher than 90%when SIE is 719.7 J/L.Considering O3 control,LaMn0.9Zr0.1O3 catalyst has the best performance with O3 inhibition efficiency reaching 92.8%.The N2O inhibition efficiency of LaMn0.9Zr0.1O3 catalyst is also the highest,reaching 73.1%.The comprehensive catalytic performance of LaMn0.9Fe0.1O3 catalyst is the best.Its xylene conversion is the highest,97.0%;the selectivity of COx and CO2 is the best,90.7%and 85.4%respectively;and the NO2 inhibition efficiency is the highest,84.9%.The O3 inhibition efficiency of LaMn0.9Fe0.1O3 catalyst is also high,reaching 92.6%.In the process of NTP catalytic degradation of m-xylene,the stabilities of B site modified LaMnO3 catalyst were also tested.The effects of initial concentration of xylene,relative humidity of carrier gas,and NTP discharge time were investigated.The results show that the initial concentration of xylene has little effect on the catalytic efficiency of NTP catalytic system,while the relative humidity of carrier gas has a significant effect.Combining the positive effect of the hydroxyl radicals and the negative effect by the deactivation of catalyst,the oxidation ability of NTP-catalysis system is inhibited,the N2O control ability is also inhibited,but the O3 control ability is improved.LaMn0.9Zr0.1O3 catalyst has good ability of water resistance.When the relative humidity of carrier gas was 45%,the LaMn0.9Zr0.1O3 catalyst maintains high xylene conversion and CO2 selectivity,and the O3 emission decreased by 6.0%.In addition,the catalyst which is inactivated with water can be regenerated by heating at 110 ?.After the durability testament of 24h,LaMn0.9Fe0.1O catalyst can maintain high xylene conversion and COx selectivity,with N2O inhibition efficiency of 63.6%and O3 inhibition efficiency of 88.5%,indicating good durability of LaMn0.9Fe0.1O catalyst.This study shows that LaMn0.9Fe0.1O catalyst has the greatest potential for industrial application in NTP-catalytic technology for VOCs abatement.