Catalytic Performance And Reaction Mechanism Study Of Synergistic Elimination Of No_x And Chloroaromatics On CeWo_x Catalysts

In waste incineration,iron ore sintering,electric arc furnace steelmaking,non-ferrous metal smelting and other industries,their flue gases contain large amounts of nitrogen oxides（NO_x）and low concentration of chlorinated aromatic compounds and other pollutants,among which the chlorinated aromatic compounds mainly involve dioxins,polychlorinated biphenyls and other highly toxic substances.In this thesis,the reaction process of CeWO_x catalysts for the synergistic elimination of NO_x and chlorobenzene（CB）was studied.The catalytic performance and reaction characteristics of different Ce/W ratio catalysts were investigated.The interaction mechanism between ammonia（NH₃）selective catalytic reduction（SCR）denitrification and chlorobenzene catalytic oxidation（CBCO）reaction was explored,so as to provide theoretical basis for industrial synergistic catalytic removal of NO_x and chloroaromatic compounds on the practical guidance.Firstly,the synergistic catalytic performance of Ce/W ratio（Ce/W ratio from 1:2to 10:1）for the elimination of NO_x and chlorobenzene over the CeWO_x catalysts was studied.The results showed that Ce₈W₁O_x catalyst had the best catalytic activity.The NO_x and chlorobenzene conversion rate reached 100%and 87%at 300℃,respectively.The conversion of both NO_x and chlorobenzene reached 100%at 350℃,but the maximum CO₂ selectivity was only 70%,which indicated that oxidation capacity of this catalytic system needed to be further improved.A series of characterizations revealed that Ce₈W₁O_x catalyst had the best redox ability and stronger acidity than the Ce O₂,WO₃ and Ce₁W₂O_x,which is conducive to synergistic reaction.Additionally,comparing the catalytic activities of SCR,CBCO and synergistic elimination reaction（SCR+CBCO）,it was found that NO_x and CB transformation of synergistic reaction were both inhibited,among which the CBCO reaction was inhibited more obviously by the SCR process.In order to explore the synergistic reaction mechanism,competitive adsorption experiments were carried out on the CeWO_x catalysts.The results of NH₃-CB-TPD and DRIFT analyses showed that competitive adsorption between the NH₃ and chlorobenzene was observed,where the presence of NH₃ significantly inhibited the adsorption of chlorobenzene on catalyst surface,resulting in the decrease in CBCO activity.The results of TPSR analyses revealed that chlorine generated by the CB dissociation on the CeWO_x catalysts was activated by oxygen vacancy to form Cl·free radicals,which led to electrophilic chlorination reaction,thus promoting the formation of polychlorobenzene.Furthermore,desorption temperature of the HCl in the synergistic reaction was shown to be lower than that in CBCO reaction,indicating that the SCR reaction provided additional hydrogen for HCl generation due to the process of NH₃ adsorbing on the Lewis acid site and dehydrogenating to amide species（NH₂^-/NH^-）by reactive oxygen species on oxygen vacancy;this promoted the separation of Cl·/Cl^-from the catalyst surface to form HCl.Notably,the presence of nitrogen-containing by-products such as benzoonitrile was found in reaction byproducts analyses,indicating that the intermediates of the SCR and CBCO reactions were coupled with each other.Finally,the stability of Ce₈W₁O_x catalyst in the synergistic reaction was elevated,which revealed that there was the deactivation at 250℃,mainly caused by the deposition of chlorine and NH₄Cl on the catalytic surface.However,the synergistic catalytic activity of this catalyst can be maintained at 300℃ and 350℃.