Fundamental Research Of Catalytic Decomposition Of Chlorobenzenes And Dioxins Over Ball Milling Synthesized Vanadium-based Catalysts

In China,the emission control of dioxins in municipal solid waste incineration(MSWI)flue gas has drawn much more attention in recent years,owing to constantly increased generation of municipal solid waste and the implementation of more stringent MSWI emission standards(GB-18485-2014).The most widely applied technique for the removal of dioxins from flue gas is activated carbon(AC)adsorption which only transfers the burden of dioxins to further fly ash treatment.At times,this method even can cause additional dioxin formation.Catalytic decomposition technology can overcome the drawback of AC adsorption and destroy the structure of dioxin molecules,becoming the most potential technology for removing dioxins from flue gas.Compared to traditional catalyst synthesis methods,mechanochemical synthesis attracts wide attentions due to its overall simplicity,low cost and solvent-free conditions.Current environmental concerns over the use of solvents make it now particularly timely to investigate mechanochemical synthesis more extensively than has previously been done.In this paper,we prepared VOx/TiO2 catalysts by ball milling and investigate the effect of ball milling time,precursors of VOx,addition of Fe and its contents upon the physicochemical properties and catalytic activity of the catalysts.The mechanism of dioxin decomposition was also studied.The following are the main conclusions:(1)Compared to catalysts synthesized by sol-gel and wet impregnation methods,ball milling prepared catalysts have relatively large specific surface areas,pore volume and strong acidity and thus exhibit strong adsorption ability for 1,2-dichlorobenzen(1,2-DCBz).Moreover,the high energy created by friction and collision of balls in ball milling process is sufficient to cause the dislodging of surface lattice oxygen(Olatt);surface chemisorbed oxygen(Osur)is generated from the dissociated adsorption of gaseous O2 on the surface oxygen vacancies of the catalysts.Therefore,ball milling prepared catalysts contain plenty of Osur and have strong oxidation ability.(2)Ball milling time has a strong effect on the physicochemical properties of the catalyst.The specific surface area,total pore volume,and surface acidity of catalysts firstly increase with prolonging balling time,then decrease.At 2 h,those parameters reach the maximum,which is beneficial to the adsorption of 1,2-DCBz.Also,balling milling time can affect the concentration of V5+ and Osur in the catalyst and thus influence its redox properties.VOx/TiO2 prepared by using NH4VO3 as precursors(denoted as BM1)showed superior activity than the counterpart using V2O5 as precursors(denoted as BM2),particularly at low temperature.This is mainly attributed to the outstanding adsorption ability of BM1.The high energy generated in ball milling process makes NH4VO3 decompose to H2O and NH3 which facilitate the formation and development of pore structures.Also,BM1 has stronger surface acidity than BM2.In addition,BM1 has larger specific surface area,better dispersion,higher concentration of V5+ and Osurand,therefore,has stronger redox properties than BM2.(3)The catalyst activity greatly enhanced after addition of 3 wt.%Fe,due to the improvement of both adsorption and redox properties of the catalyst.The contents of Fe also have an important effect on the catalyst activity.As the contents of Fe rise,the removal efficiencies of 1,2-DCBz firstly increased,reach the maximum at the content of 3 wt.%,and then decreased.On one hand,the contents of Fe are strongly related with the specific surface area,pore volume,pore distribution and surface acidity of the catalyst.One the other hand,the contents of Fe also have an obvious influence on the redox properties of the catalyst.(4)The removal efficiencies(REs)of PCDD/Fs over ball milling synthesized VOx/TiO2 were all above 97%in the temperature range of 160-300 ?.This is mainly attributed to the good porosity and strong acidity of the catalyst.Consequently,most of the toxic PCDD/F congeners absorb onto the catalyst surface.In the studied temperature range VOx-FeOx/TiO2 showed higher removal efficiencies of PCDDs and PCDFs,especially at high temperatures(>250?),than VOx/TiO2.When tested in flue gas,the removal efficiencies and decomposition efficiencies(DEs)of dioxins over VOx-FeOx/TiO2 at 180 ?,200? 220? are 77.7%?89.0%?90.7%and 61.7%?80.8%?82.4%,respectively.The REs and DEs of dioxins increase with rising the temperature.Compared with those obtained at lab-scale tests,there are no obvious decreases of DEs of dioxins over VOx-FeOx/TiO2 in flue gas.In addition,no catalyst deactivation was observed in the consecutive operation of 240 min.(5)According to the analysis of decomposition products by FTIR and GC-TOFMS,the decomposition mechanism of dioxins was proposed as:1)The dioxins absorb dissociatively onto the catalyst surface by C-Cl abstraction;2)Oxygen heterocycles crack to diphenylmethane compounds;3)The carbon connecting two benzene rings is oxidized to-C=O-,producing benzophenone;4)The C=O cleaved to form benzene derivatives;5)The benzene ring reacts with active oxygen species to produce benzoquinone;6)The carbanyl group breaks the stability of the benzene ring and contributes further oxidation.The benzene ring cracks and the oxygen atom inserts,producing maleate and carboxylate.Finally,the middle products are oxidized to CO2,CO,H20,HCl,etc.