| Chlorinated Volatile Organic Compounds(CVOCs)are widely used in chemical industry,pesticides,medicine and organic synthesis.They can emit into the atmosphere through volatilization,leakage and exhaust emissions,causing ozne layer destruction and photo chemistry smog.Some of CVOCs have potential hazardous effects on human health because of their high degree of biocumulatiori and carcinogenic.Catalytic combustion is considered to be one of the effective technologies to treat CVOCs because of its high efficiency and low energy consumption.However,the problems of chlorine poisoning and secondary pollution of catalysts greatly restrict the industrial application of this technology.Herein,MnO2,which is widely used in the field of environmental catalysis,was selected as the research object to investigate the reaction characteristics and by-product formation mechanism of three morphological characteristics of MnO2 in the catalytic oxidation of chlorobenzene.The mechanism of surface oxygen vacancies and Mn species in the process of water molecule activation and chlorine adsorption and desorption was clarified,and the important role of their synergy in inhibiting the formation of polychlorine by-products was revealed.The effects of industrial flue gas interferents on the reaction characteristics of MnO2 and the formation of by-products were studied;this is conducted by surface loading of heavy metal lead and sulfation treatment of the MnO2.The morphological characteristics of three catalysts,namely ?-MnO2,?-MnO 2 and ?-MnO2 were investigated and their apparent conversion and CO2 selectivity in the catalytic oxidation of chlorobenzene were revealed.The structure-activity relationship of three catalysts was analyzed using XPS,CB-TPSR,H2-TPR and other characterization techniques.It was found that the redox activity of three MnO2 follows the order of ?-MnO2??-MnO2>?-MnO2,which is consistent with the conversion rate in chlorobenzene oxidation.Among them,90%of chlorobenzene can be converted by ?-MnO2 and ?-MnO2 at 175?,while 210 ? is required for ?-MnO2.In terms of CO2 selectivity,?-MnO2 exhibited the worst activity,and its CO2 selectivity was only 40%at 300?.This is due to the fact that ?-MnO2 had the most Mn4+species on the surface,which were vulnerable to chlorine erosion and formed stable Mn-Cl bonds,resulting in the loss of redox ability of the catalyst.The presence of more toxic polychlorinated by-products was all detected in the effluents and on the surface of three MnO2 catalysts.These by-products mainly involved trichloromethane,tetrachloromethane,trichloroethylene and tetrachloroethylene.In particular,dichlorobenzene was found in the effluent of?-MnO2,which could be further polymerized into dioxin-like substances by nucleophilic substitution,thus posing a high environmental risk.H2O-TPD indicated that ?-MnO2 had the lowest ability to dissociate water molecules;this was attributed to its low specific surface area,less surface oxygen vacancies and Lewis acid-base pairs.The inability of H2O activiation hindered the reaction between H2O and chlorine to produce HCl,and thus promoted the electrophilic chlorination reaction.Finally,the effects of SO2 and Pb2+on the reaction characteristics and by-product formation of ?-MnO2 were investigated.The samples were identified as p?-MnO2 and s?-MnO2,respectively.It was found that p?-MnO2 remained in the form of ?-MnO2,while s?-MnO2 was in a mixed form with catalyst surface being seriously sulfated.Based on surface element analyses,Pb2+mainly existed in the form of PbO and sulfur mainly existed in the form of SO42-.The redox performance of both catalysts decreased significantly,which reduced their apparent activity and CO2 selectivity in the catalytic oxidation of chlorobenzene.Under drying conditions,s?-MnO2 produced significant dichlorobenzene intermediates.This mainly originated from surface sulfation that resulted in increased acidity of the catalyst and the non-synergism in the adsorption and oxidation of chlorobenzene.After the addition of water vapor,the by-products of s?-MnO2 and p?-MnO2 catalysts both decreased significantly. |
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