Analysis Of Production And Degradation Characteristics Of Chlorobenzene With Catalytic Non-thermal Plasma Reactor

Recently,environmental problems caused by volatile organic compounds?VOCs?have received more and more attention.Biological treatment is widely considered to be a promising technology due to their convenient maintenance,low operating costs and lower likelihood of secondary pollution.However,its application is strongly limited in the case of slow water-soluble and less biodegradable VOCs,so developing high efficiency pre-treatment technology is the hot issues of research.In the present study,a recalcitrant VOC,chlorobenzene,was employed as the target contamination,and plasma-catalysis had been utilized as a pretreatment.The aim of this study was to identify the degradation characteristics and degradation products of chlorobenzene using a catalytic non-thermal plasma reactor,which provide fundamental data to evaluate the possibility of success of the subsequent biodegradation.CuO/MnO₂,CeO₂/HZSM-5,Ag/TiO₂ catalysts were prepared by impregnation and characterized by BET,SEM and XRD.The BET of CuO/MnO₂,CeO₂/HZSM-5 and Ag/TiO₂ catalysts were 34.9687 m²/g,315.8355 m²/g and 83.4404 m²/g,respectively.It is found that the CeO₂/HZSM-5 catalysts exhibited the highest surface area and pore volume,which may be in favour of the adsorption performance toward chlorobenzene.At the same time,the SEM showed that the morphology of CuO/MnO₂ and Ag/TiO₂ was presented ellipticity,and CeO₂/HZSM-5 was blacky square.The XRD showed that the phase composition of synthesized catalysts was consistent with the target ones,expect CuO/MnO₂ catalysts,which contained small amount of Mn₂O₃.The performance of catalysts was evaluated by the removal efficiency,energy efficiency and COx selectivity.The result showed that CeO₂/HZSM-5 exhibited the best catalytic activity,which resulted in greater enhancements of chlorobenzene removal efficiency,COx selectivity and energy efficiency.Using CeO₂/HZSM-5 as a target catalyst,the effects of the inlet concentration,residence time,discharge voltage on decomposition efficiency were investigated and the decomposition products of chlorobenzene were analyzed.The result showed that the removal efficiency of chlorobenzene increased and energy efficiency decreased with an increasing discharge voltage.Increasing the initial concentration of the gas into the plasma reactor resulted in decreasing the removal efficiency of chlorobenzene both with and without catalyst,but removal amount increased.In addition,the highest removal load?653.4 mg/m³?h?was observed with an residence time of 5 s.5 kV,5 s was regarded as the optimum reaction condition in this study.Through the analysis of degradation products,the main products of chlorobenzene degradation were O₃,COx and benzene ring derivatives.Ozone is considered as an unavoidable byproducts produced by air plasma reactor.The concentration of ozone rose at first as the SIE increased,and then decreased with an excess increase input energy,which reached the higest?0.24 mg/L?with the SIE of 9.2 kJ/L.As expect,a noticeable decrease in the O₃ concentration was observed in the presence of chlorobenzene relative to plasma alone.The presence of catalysts could oxidize the by-products such as nitrogenous organics,which leads to a higher CO₂ selectivity.Besides,the pretreatment technology resulted in greater enhancements of solubility and biodegradability of byproducts,which provided the fundamental data to evaluate the possibilities of the combined system.After reaction for a period of time in non-thermal plasma-catalysis system,the chlorobenzene conversion decreases from 78 to 60%after 75 h,which means that the CeO₂/HZSM-5 suffered slightly deactivation.Some organic compounds and chlorine-bearing compoung were adsorbed or deposited on the catalysts surface based on the FT-IR and EDS spectra of catalyst before and after reaction,revealing the cause of catalyst deactivation.The evalution of the intermediate products on the catalyst surfaces were analysed by GC-MS.Main intermediates of plasma catalytic degradation of chlorobenzene were two categories:one was benzene ring derivatives such as phenol and benzoic acid,another was small molecule alkyd,like ethylene glycol,oxalic acid and glycerol.Three possible degradation pathways were established accoding to the intermediate products,the principle processes of the chlorobenzene destruction mainly induced by energetic electrons active radicals and active oxygen atoms.