Mechanism Study On Catalytic Decomposition Of Gas Phase Dioxins Over Modified Vanadium-based Catalysts

With the increasingly strict environmental protection supervision,pollutants emission from municipal solid waste incineration has attracted much attention.The flue gas contains not only inorganic pollutants such as NOx,HCl and SO2,but also highly toxic dioxins.The Municipal Solid Waste Incineration Contaminants Control Standard(GB18485-2014)limits the emission of dioxin in waste incineration flue gas to 0.1 ng TEQ/m3.The most widely applied technique for the removal of dioxins from flue gas is activated carbon injection combined with baghouse filter,which only transfer the dioxins into fly ash without completely degrading dioxin.Catalytic degradation technology is a technology with great development potential.Especially vanadium-based catalysts have high catalytic activity and good stability,and have been applied to the catalytic degradation of chlorinated organic pollutants.However,previous studies used chlorobenzene as a model molecule for dioxins destruction,and the reaction is only oxygen with nitrogen as balance gas,ignoring the complex components in the flue gas.And this process also neglected the role of the oxygen heterocycle in the molecular structure of dioxins.In this work,different carriers of catalysts and the effect of H₂O,SO₂,NO and NH₃ on the removal efficiency of 1,2-chlorobenzene were studied.The effects of oxygen heterocycles in the catalytic degradation process of dioxin molecular structure were studied experimentally and theoretically.Various characterization methods such as BET,XPS,XRD,H2-TPR and NH3-TPD were used to study the physicochemical properties of the catalyst before and after the reaction to explore the reasons affecting the catalytic activity of the catalyst.The following are the main conclusions:1)The V₂O₅/Ti O₂ catalysts were prepared by using P25 and PC105 and PC500as catalyst carriers respectively.It was found that the catalyst prepared by the acidic Ti O₂ support has better physicochemical properties.The Ts V catalyst prepared by PC500 can completely degrade 1,2-dichlorobenzene at 250°C.Different amount of active component Mo was added to the V2O5/Ti O2 catalyst and it showed better catalytic activity whrn Mo:V was 1.When the proportion increased to 2 ceystals was formed on the surface of the catalyst.And the addition of Mo has a negative effect on the distribution of V on the surface of the catalyst.The degradation of chlorobenzene over a long period of time was observed.The physicochemical properties of the catalyst did not change after the reaction,which proved the stability of the catalyst.2)The degradation of 1,2-chlorobenzene in various atmospheres was studied.It was found that the addition of H₂O showed different effects on different catalysts.When 5 vol.%H₂O was added in the atmosphere,the removal efficiency of chlorobenzene over V₂O₅/Ti O₂ catalyst increased,while that over V₂O₅-Mo Ox/Ti O₂decreased.The addition of Mo reduces the reaction of H₂O with the reacted Cl ions and affects the desorption process.The addition of SO₂ in the atmosphere has a positive effect on degradation efficiency of different catalysts.The SO₂ in the atmosphere generates some acidic sites on the surface of the catalyst,while large number of acidic sites are already present in the acidic Ti O₂ carrier,so the addition of SO₂ only has a significant effect on the activity of the TVMo catalyst.The catalytic activity increased with the concentration of NO increasing.NO reacted with O₂ in the atmosphere to form NO₂,which can oxidize the pollutants.However,NH₃ and chlorobenzene compete on the surface of the catalyst,which reduces the possibility of adsorption of chlorobenzene at the active site,and the catalytic degradation efficiency decreases.The catalyst is tested for the degradation of dioxins in actual waste incineration flue gas.Due to the complex composition of the actual flue gas,the catalytic degradation efficiency of dioxins decreased.3)In the molecular structure of dioxin,there are not only chlorinated benzene rings,but also oxygen heterocyclic structures connecting two benzene rings.The structure of the oxygen heterocyclic ring is similar to that of furan.The difference is that the C-O bond in the dioxin linked to a benzene ring.The catalytic degradation of furan and 2,5-dimethylfuran was studied respectively.Both furan and 2,5-dimethyfuran are easily adsorbed on the surface of the catalyst.The furan was completely degraded at 250°C.The adsorption of furan on the surface of the catalyst hindered the adsorption of chlorobenzene.Only when the furan was completely degraded,chlorobenzene began to adsorb onto the surface of the catalyst.The above results indicate that the oxygen heterocycle is more advantageous in the competition of the adsorption of the C-Cl bond on the adsorption process.The addition of 2,5-dimethylfuran decresed the removal and degradation efficiency of dioxins.The preparation of the catalytic filter material was preliminarily studied.It was found that the spray method can uniformly disperse the catalyst particles in the filter material,and the degradation efficiency can be improved by increasing the catalyst content.4)Using the density functional theory,the V₂O₅(010)model was established to study the adsorption process of 2,3,7,8-TCDD onto the surface of V₂O₅.TCDD was easy to adsorb onto the catalyst because high activity adsorption sites can be obtained on the whole surface.The bond between the oxygen atom of TCDD and the surface vanadium site adsorption exhibit the lowest Ead value,and the shortest O-V adsorption bond.Therefore,O-V is considered to be a stable structure and the oxygen hertecycle is easy to adsorb onto the surface of the catalyst.This is consistent with the experimental results.