| At present,the emission of volatile organic compounds(VOCs)in our country is increasing gradually,and a large amount of VOCs is discharged into the environment,which is bound to pose a threat to human health and ecological environment.The new non-thermal plasma(NTP)technology is considered to be one of the promising technologies in the field of VOCs processing,the individual plasma technology,however,low energy efficiency and the production of not expected by-products,restrict its large-area industrial application.The combination of plasma and catalyst can overcome the disadvantage of individual plasma technology,so the research of plasma synergistic catalysis is of great significance to the application of plasma technology in VOCs large area industrial applications.In this study,xylene was studied as a target pollutant in benzene series.The Dielectric barrier Discharge(DBD)combined with Mn catalyst was used to deal with it.The transformation process and catalysis mechanism of xylene in in-plasma catalytic system were studied;the difference in effect and the reasons of them between the in-plasma and post-plasma catalytic system were discussed;and the reason of catalyst deactivation under prolonged discharge was analyzed.The research results were showed that:(1)When Mn catalysts existed,they promoted the conversion of xylene and reduced the amount of O3 and particulate matter,but also reduced the absolute and selectivity of COx.There was a suitable concentration of Mn load,which had the highest rate of xylene removal when the load was 1 wt.%.Increased energy density,facilitated the conversion of xylene and the formation of COx,but produced more O3,without catalyst,the increase of energy density would significantly increase the production of particulate matter,and the largest order of magnitude was 108#/cm3,when there was with catalyst,the amount of particulate matter production showed decreasing trend,maintained in the order of magnitude of 102#/cm3.The increase of temperature is advantageous to the decomposition of 03,without catalyst,although it is beneficial to the removal of xylene,but it is not conducive to CO2 generation,while with catalyst,xylene removal rate and COx selectivity increase,but when the temperature increased,the number of aerosols in the tail gas would increase correspondingly.The increase of the initial concentration was not conducive to xylene conversion,when there was without catalyst,increased the initial concentration,the amount of O3 production decreased,COx absolute amount also decreased,the COx selectivity was basically unchanged,but with catalyst,the increase of the COx absolute amount and selectivity corresponded to the increasing trend of the yield of O3.(2)According to the generation of the byproducts,the mechanism for the removal of xylene catalyzed by plasma combined Mn catalysts was proposed as:p-xylene ? p-methylbenzyl alcohol ? p-methyl benzaldehyde ?p-methylbenzoic acid ? small molecule alcohols,aldehydes and acids ? COx.Because that the active O at ambient temperature could not heavily oxidize methyl benzene methanol and methyl benzaldehyde adsorbed on catalyst,therefore,the absolute amount of COx with catalyst was less than without catalyst.(3)Compared with the post-plasma catalytic system,the removal rate of xylene in in-plasma catalytic system was higher and the amount of aerosol produced was lower,but the O3 concentration was slightly higher,although the absolute and selectivity of COx were higher in the post-plasma catalytic system,but it was mainly due to the contribution of individual plasma effects and the difference was caused by the different mechanisms of removal.Compared with single plasma,the absolute amount of COx was slightly smaller than that of the post-plasma catalytic system at ambient temperature,which further confirmed that the active O atom at ambient temperature mainly oxidized xylene,but it could promote that the CO convert to C02 that adsorbed on catalysts.Because COx was adsorbed on the catalyst,it would produce carbonate and could not be degraded further,result in a COx absolute lower than that of single plasma.Therefore,in the in-plasma and the post-plasma catalytic system,the lower COx was mainly caused by the inability of the intermediate products to deep oxidation together with partial COx transformed into carbonate.(4)Compared with in-plasma catalytic system,the Mn/Al2O3 catalyst had longer catalyst life in post-plasma catalytic system while the Al2O3 carrier was on the contrary.The deactivation of catalyst was mainly due to the fact that the intermediate product was not further degraded on the catalyst and deposited on the catalyst surface,which hindered the catalyst surface reaction;while the valence state of the catalyst active component was very important to the long life of the catalyst,the higher the ratio of Mn4+/(Mn4++Mn3+)and the lattice oxygen,the deeper the degree of degradation of contaminated substances,it could also reduce the coking and prolonged catalyst life. |
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