Study On The Catalytic Degradation Mechanism Of Chlorobenzenes And PCDD/Fs Over Cerium-based Catalysts At Low Temperature

In China,incineration has emerged as the primary method for disposing of municipal solid waste（MSW）.However,it is imperative to note that the control of polychlorinated dibenzo-p-dioxins and dibenzofurans（PCDD/Fs）and nitrogen oxides（NO_x）is crucial to ensure ultra-low emissions.Therefore,developing and enhancing advanced control methods and technologies is urgent to be pursued.Catalytic degradation is a highly promising technology that has the potential to achieve complete oxidative degradation of chlorinated aromatic hydrocarbons（CAHs）to non-toxic substances（i.e.,CO₂ and H₂O）.However,the active temperature（220-300℃）of commercial SCR catalysts is too high and consumes a large amount of steam to reheat the flue gas（～160℃）,which causes poor economy and poses the risk of secondary PCDD/F synthesis.Based on the catalytic removal characteristics of PCDD/Fs and deactivation mechanism of typical SCR catalysts,this study systematically studied the honeycomb catalyst preparation process,developed a new low-temperature catalyst（VO_x-Ce O_x-WO_x/Ti O₂）,thoroughly investigated the catalytic degradation mechanism of CAHs at low temperature,revealed the ensemble relationship of catalyst active components and the interaction mechanism between CAHs and NO_x.The catalytic degradation activity of PCDD/Fs at low temperature was verified by pilot test.The results will promote the subsequent application of catalysts in the MSW incineration industry.The main conclusions are summarized as follows:（1）There were serious PCDD/F regeneration phenomena at the high-temperature（400-420°C）and medium-temperature（220-230℃）SCR systems before the fabric filter,attributing to:1)the suitable temperature and high-dust environment promotes the PCDD/F regeneration;2)fly ash could cover the active sites and prevent PCDD/F adsorption and degradation.In the typical SCR system installed after fabric filter,the severe deactivation occurred in vanadium-based catalysts after 2-3 years.The deactivation mechanism mainly included:1)the destruction of porous structure and Br（?）nsted acid sites（B acid）;2)the inhibition of the redox cycle of VO_x;3)competitive consumption of surface active oxygen;4)blocking of surface active sites,etc.After 2-3 years,the reaction kinetics showed a significant decrease in the catalytic reaction rate（k）of 1,2-DCB at low temperature（150-200℃）from 1.59-2.18 s^-1 to 0.007-0.38 s^-1,and the corresponding reaction activation energy（E_a）increased from 11.01 k J/mol to34.44 and 92.70 k J/mol.（2）The honeycomb catalyst preparation process and key parameters were systematically investigated,and a method for the preparation of cerium-based catalysts was established by conducting qualitative and quantitative studies on the effects of regulating clay plasticity,drying and calcination temperature profiles and atmospheric conditions on structural strength,cracking and catalyst activity of extruded embryos.A new honeycomb catalyst of VO_x-Ce O_x-WO_x/Ti O₂ with high efficiency at low temperature was developed based on the optimization of VO_x proportion and the modification of Ce O_x and WO_x.The drying and calcination process was further improved based on the enterprise production process,and pore parameters（65.3 vs.62.0 m²/g）and mechanical strength（1.88 vs.0.94 MPa）,the 1,2-DCB conversion（88.7-94.5%vs.88.5-91.8%）and CO₂ selectivity（39.8-43.7%vs.37.5-41.6%）were improved.The developed catalyst exhibited stable,efficient,and durable catalytic oxidation performance of CAHs at low temperature,which was significantly better than the beforementioned representative SCR catalysts.（3）Nano-Ti O₂ modification reduced the specific surface area and B-acid amount,which weakened the adsorption and conversion of 1,2-DCB at 160-200℃.Nano-Ti O₂modification significantly enhanced the NO conversion(T₅₀:162→112℃;T₉₀:232→205℃)at low temperature.Based on in-situ DRIFT,the catalytic oxidation of1,2-DCB by VO_x-Ce O_x-WO_x/Ti O₂ mainly followed the M-K and L-H mechanisms,while NH₃-SCR followed the E-R and L-H mechanisms.NH₃ has a significant advantage in the competitive adsorption,leading to an increase in NO conversion with increasing reaction temperature but a decrease in 1,2-DCB conversion.In the synergistic catalytic reaction of CAHs and NO_x,NO can improve the conversion and complete oxidation of 1,2-DCB.The reaction products of HCl and H₂O can enhance the NH₃ chemisorption by promoting the production of HONO and HNO₃ to improve the catalyst acidity,or participate in the NH₃-SCR through the L-H mechanism.（4）The VO_x species in VO_x-Ce O_x-WO_x/Ti O₂ mainly existed in polymeric type,and the modification of Ce O_x and WO_x diluted the surface density of V.Ce O_x was more effective in promoting 1,2-DCB adsorption and C-Cl bond cleavage than WO_x,while WO_x not only protected Ce O_x from Cl-poisoning,but also released its excellent oxygen storage/release capacity.The enhancing mechanism of the modification of Ce O_x and WO_x mainly attributed to:1)increasing the surface acidity,reducing the energy barrier of 1,2-DCB adsorption and C-Cl bond cleavage;2)enhancing the storage capacity of reactive oxygen species（33-53 fold）and mobility;3)extending the VO_x(V⁵⁺（?）V⁴⁺)redox cycle to VO_x(V⁵⁺（?）V⁴⁺（?）V³⁺);4)promoting the electron transfer and replenishment of reactive oxygen species.The main catalytic oxidation pathways of CAHs molecules on the VO_x-Ce O_x-WO_x/Ti O₂ surface are dechlorination,phenolization,quinonization,ring-opening cleavage and deep oxidation.（5）The degradation of gas-phase PCDD/Fs by the VO_x-Ce O_x-WO_x/Ti O₂ catalyst showed that increasing the reaction temperature was beneficial to accelerate the degradation of the adsorbed PCDD/F molecules,and the removal efficiency reached88.5-91.1%at 180-200℃（I-TEQ:88.7-90.0%）;increasing the gas hourly space velocity（GHSV）would weaken the adsorption and reduce the degradation efficiency.The degradation efficiency of PCDD/Fs congeners increased and then decreased with the increase of chlorination degree,and the peaks were Pe CDDs and Pe CDFs.Mutual conversion and competition happened between the toxic and non-toxic congeners of PCDD/Fs,and the highly chlorinated congeners underwent dechlorination to generate toxic congeners such as 2,3,7,8-TCDD and accumulated on the catalyst surface,which reduced the degradation efficiency of toxic PCDD/Fs.A preliminary pilot test of the catalyst was conducted,and the results showed that the catalyst exhibited better removal efficiency at low temperature（160-180℃）,which reached 96.7-98.2%.