Elimination Of Chloroaromatic Congeners On A Commercial V₂O₅-WO₃/TiO₂ Catalyst:The Effect Of Heavy Metal Pb And As

With the rapid growth of economy and the acceleration of urbanization,the generation of municipal solid waste?MSW?in China has been significantly increased.Incineration as an effective method for MSW disposal has been widely applied,which could achieve harmless,decrement and resourceful disposal of the refuse.However,the composition of municipal solid waste is complex,usually containing certain chlorine source.The presence of these chlorines causes a large number of chlorinated volatile organic compounds?VOCs?and even dioxins during the incineration process,resulting in a serious secondary pollution.As an efficient and cost-effective treatment technology,catalytic combustion can induce the efficient degradation of chloroaromatics at ralatively low temperatures.V₂O₅-WO₃/TiO₂ catalyst,as an efficient catalyst in industrial flue gas denitrification and synergistic denitrification,have been also used as the catalysts for the treatment of chloroaromatics.However,the deactivation of catalysts,particulalry posinoned by heavy metals has severely hindered their industrial applications.The composition of municipal solid waste?MSW?is complex,which contains a large number of heavy metals,the presence of which not only poses a huge risk to environment and human health,but also has a inhibition impact on the process of chlorinated VOCs elimination.Among the heavy metals,lead and arsenic have the most serious impact.Although studies of the heavy metal on the effect of V₂O₅-WO₃/Ti O₂ catalyst have been extensively explored,the majority of work focus on the influence on SCR process,and only few studies investigated such influences on the catalytic destruction of chloroaromatics.In this paper,a commercial V₂O₅-WO₃/TiO₂ catalyst was used and the lead poisoning process in the catalytic destruction of chlorobenzene was similated by using an impregnation method.It was found that the addition of small amount of lead?1wt%?had little effect on the catalytic activity and CO₂ selectivity of V₂O₅-WO₃/TiO₂ catalyst.When a small amount of lead was added,the surface acidity and redox performance of the catalyst decreased,the surface oxygen vacancy increased,the removal temperature of chlorine and hydrogen chloride increased,and the dechlorination performance decreased.Especially,due to the increase of oxygen vacancy on the surface of catalyst,electrophilic chlorination was inclined to occur,so there are more polychlorinated by-products in the off-gases With the increase of lead content,when it reached 8wt%,the catalyst had obvious deactivation and the selectivity of CO₂ is greatly reduced.The addition of a large amount of lead occupied the acid site of the catalyst,which caused the change of negative electric region around the vanadium species and inhibited the regeneration of acid site on the catalyst surface.Both of the active and acid sites on the catalyst surface were occupied by lead,and chlorobenzene could not be deeply oxidized over the vanadium species,resulting in the deactivation of V₂O₅-WO₃/TiO₂ catalyst.However,due to the high content of lead,many chlorine adsorbed on the vacancy will be fixed to form chemically stable lead chloride,thus inhibiting the polychlorinated by-products by electrophilic chlorination.The arsenic poisoning process of V₂O₅-WO₃/TiO₂ catalyst was simulated by immersion in arsenic standard solution.It is found that the addition of a small amount of arsenic?molar ratio As/V=0.05 and As/V=0.2?made arsenic mainly exist in the form of trivalent arsenic?As₂O₃?,which was inclined to enter into the crystal lattice of the catalyst and promoted the formation of oxygen vacancy on the catalyst surface.When the amount of arsenic doping increased?molar ratio As/V=0.5?,it mainly existed in the form of pentavalent arsenic?As₂O₅?,and formed an arsenic coating on the surface of the catalyst,which inhibitd the regeneration of surface acid site,thus hindering the catalytic destruction of chlorobenzene.The As/V=0.5 catalyst showed the lowest activity and CO₂ selectivity,and the temperature for the conversion rate approching 90%was close to 300?.Although the presence of a small amount of arsenic increased the oxygen vacancy and promoted the redox ability of the catalyst,it promoted the electrophilic chlorination reaction and formed more toxic polychlorinated by-products.The redox ability of the catalyst was severely inhibited by the addition of high amounts of arsenic,where the chlorobenzene was difficult to be mineralized on such catalysts.