Fundamental Research Of PCDD/Fs Destruction Over Vo_x-CeO_x/TiO₂ Catalyst At Low Temperature

The emission of PCDD/Fs,with teratogenic,carcinogenic and mutagenic effects,from solid waste incineration limits rapid development of the incineration disposal technology.Catalytic destruction technology,completely destroying PCDD/Fs,is considered to the most promising technology for dioxin control at the end of flue gas.Based on transition metal oxides and nano TiO2 support,this paper explores and develops a new VOx-CeOx/TiO2 catalyst for PCDD/Fs destruction,studies low temperature destruction characteristics of PCDD/Fs over this catalyst,couples ozone to increase the destruction efficiency of PCDD/Fs and tests the activity of the new catalyst in actual flue gas.The main conclusions are summarized as follows:(1)Catalyst activity is co-decided by adsorption and oxidation capacity.The increase of temperature enhances the oxidation capacity of catalyst,and also reduces adsorption capacity.Dioxin molecules are first adsorbed on the surface of the catalyst,and then destroyed.Thus,high temperature instead reduces destruction efficiency of PCDD/Fs.Dechlorination of PCDD/Fs happens on nano TiO2 at thermal condition.The highest destruction efficiency is observed over VOx/TiO2 catalyst when vanadium loading is 5 wt.%.Results of kinetics analysis indicate the activation energies of OCDD and OCDF over VOx/TiO2 catalyst are 13.4 and 12.5 kJ/mol,respectively.The application of nano TiO2 and optimum vanadium loading(5 wt.%)decrease reaction activation energy.(2)The order of catalytic activity for PCDD/Fs destruction over transition metal oxides catalyst is shown as follows:VOx>CeOx>MnOx>WOx>MoOx.In addition,when WOx?MoOx?MnOx and CeOx are added into VOx/TiO2 catalyst as the second active components respectively,the order of increase range of destruction efficiency is arranged as follows:CeOx>MnOx>MoOx>WOx.According to the results,the high-performance VOx-CeOx/TiO2 catalyst is developed.At 200?,destruction efficiency of PCDD/Fs over the catalyst can reach 92.5%.(3)At low temperature(180 ?),the destruction efficiency of PCDD/Fs over VOx-CeOx/TiO2 catalyst reaches 73.4%.Oxygen in carrier gas can accelerate the catalytic circulation of active components,and then increase the activity of catalyst.In the absence of oxygen,lattice oxygen,containing in active components,can oxidize a small amount of PCDD/Fs.However,it can't get replenishing without oxygen.Excessive water vapour inhibits the destruction of PCDD/Fs while moderate water vapour has a promoting effect.In the presence of oxygen,an irreversible reaction between VOx and SO2 will happen with the formation of VOSO4,which result in catalyst deactivation.The results of XPS analysis indicate Cevx can promote V3+species oxidized,and then more V5+ species,increasing oxidation capacity of catalyst,are generated.The results of adsorption and desorption research indicate the catalyst,adsorbed PCDD/Fs after catalytic reaction,will be regenerated after 13 h in the presence of oxygen.(4)At 180 ?,the destruction efficiency of PCDD/Fs over the catalyst coupled with ozone(200 ppm)can reach 86.7%,which is 13.3%higher than the use of catalyst alone.The effects of ozone in catalytic reaction can be summarized as three aspects:(a)active oxygen,decomposed by ozone,can directly oxidize dioxin molecules;(b)active oxygen can accelerate catalytic circulation rate between high valent and low valent metals;(c)ozone increases the amount of lattice oxygen,which enhances the catalyst activity.(5)VOx-CeOx/TiO2 catalyst is applied in actual flue gas,generated from waste incineration.The results indicate complex gas component of flue gas will deactivate catalyst.The destruction efficiency is at least 17.7%lower than the results of lab research.When the catalyst is coupled with ozone(500 ppm),destruction efficiencies can reach 94.8%and 97.5%,respectively,at 200 ? and 240 ?.Based on the intermediate products of OCDD destruction,a possible reaction path on PCDD/Fs destruction is speculated.There are two kinds of reaction in the destruction of PCDD/Fs:dechlorination and oxidation.Dechlorination persistently happens until PCDD/Fs transforms into DD and DF without chlorine.Oxidation can decompose dioxin molecules into single benzene ring and double benzene ring organic matters.