Reaction Characteristics And Byproducts Analyses Over V₂O₅-WO₃/TiO₂ Catalyst In The Synergistic Elimination Of NO_x And Chloroaromatics

In typical industrial processes,such as municipal solid waste incineration,iron ore sintering,and metal smelting and coking industries,the emitted gases involve not only nitrogen oxides?NO_x?,sulfur dioxide?SO₂?and dust,but also other pollutants such as chlorinated volatile organic compounds?Cl-VOCs?.As a typical type of Cl-VOCs,chloroaromatics,together with NO_x,are important precursors for the formation of fine particles and ozone.Currently,the state advocates promoting the synergistic emission reduction of volatile organic compounds?VOCs?,NO_x and other pollutants,which makes the synergistic elimination technology become research frontier in the field of environmental catalysis.In this study,a commercial de NO_x catalyst?V₂O₅-WO₃/Ti O₂?was chosen,and the reaction performance over it for the synergistic elimination of NO_xand chlorobenzene?CB?was investigated.The type and origin of reaction byproducts were analyzed with an aim to access the environmental risk for the application of this technology.Furthermore,the interference effect of SO₂ on the synergistic reaction was also explored.Firstly,the reaction activities of synergistic elimination of NO_x and CB over the V₂O₅-WO₃/Ti O₂ catalyst was studied.Experimental results showed that the V₂O₅-WO₃/Ti O₂ catalyst was able to maintain high purification performance at 300°C with the NO and CB conversion rate at 90%and 100%,respectively and the CO and CO₂selectivity at 41%and 52%,respectively.When the reaction temperature was lower than 250°C,the introduction of CB was found to cause the NO conversion decline.This has been reported ascribed to the reaction between dissociated Cl and NH₃ that formed inactive NH₄Cl and the consumption of NH₃ inhibited the SCR?selective catalytic readuction?reaction in a way.In addition,the existence of SO₂ obviously decreased the effiency of synergistic elimination and the catalyst was deactivated at200°C due to the accumulation of?NH₄?₂SO₄ and NH₄HSO₄ on the surface.The reaction mechanism of synergistic elimination of NO_x and CB on the V₂O₅-WO₃/Ti O₂ catalyst was then studied.In this reaction,CB was found to mainly adsorb on V-OH sites,while some was adsorbed on V=O sites.The SCR reaction was shown in favor of the adsorption and dechlorination of CB at acid sites.The presence of NO promoted the deep oxidaition of CB at the temperatures higher than 250°C,owning to the oxidation of NO that formed NO₂,which assisted the re-oxidation of VO_x species and sped up the redox cycle of V₂O₅-WO₃/Ti O₂ catalysts.In the synergistic reaction,Cl was mainly dissociated in the form of HCl,and the structural hydroxyl group is the main source of H protons for the formation of HCl.NH₃ was found to competitively adsorb with the H₂O on the catalyst surface,which decreased the amount of structural hydroxyl groups and inhibited the formation of HCl.Finally,byproducts in the synergistic elimination were qualitatively and quantitatively analyzed,and the origins of typical byproducts were clarified.A series of aliphatic and aromatic compounds and some polychlorinated organic compounds were detected in the off-gases and on the catalyst surface.Several highly toxic byproducts,such as 3-chlorobenzonitrile,4-chloro-2-nitrophenol and inorganic carbon disulfide?CS₂?,were first discovered in the synergistic reaction.In the time-on-stream test at250°C,the PCDD/Fs collected from the off-gases was measured at 0.0514 ng I-TEQ Nm^-3,but the most toxic dioxins congener,2,3,7,8-TCDD,was not observed.The alkalinity of SCR reaction facilitated the chlorophenol formation,which eventually promoted the dioxins generation.The SO₂ was found conducive to the polychlorinated byproducts generation,but distinctly suppressed the dioxins formation.The study of dioxins in the synergistic elimination process,combined with byproducts analyses,reveals the formation mechanisms of several highly toxic byproducts generated in the reaction,benefits to assess the environmental risks in the application of the synergisitic reaction over V₂O₅-WO₃/Ti O₂ catalysts and can provide guidance for the application of synergisitic elimination technology in industries.