烧结烟气中Zn对V_(2)O_(5)-WO_(3)/TiO_(2)催化剂脱除NO_(x)和二噁英性能的影响

Influence of Zn in the iron ore sintering flue gas on the removal of NO_(x) and dioxins by V_(2)O_(5)-WO_(3)/TiO_(2) catalyst

V_(2)O_(5)−WO_(3)/TiO_(2)(VWTi)催化剂可以同时脱除铁矿烧结烟气中的NO_(x)和二噁英,但复杂的烟气成分会导致催化剂失活.本文采用浸渍法对VWTi催化剂进行ZnCl_(2)、ZnO和ZnSO_(4)中毒实验.模拟烧结烟气条件,研究了在VWTi催化剂表面负载不同形态Zn对其同时脱除NO_(x)和二噁英(以氯苯作为模拟物)性能的影响,分析了中毒前后催化剂表面活性物质的理化性质,并对中毒催化剂开展了再生实验.结果表明:不同Zn物种对VWTi催化剂同时脱除NO_(x)和氯苯(CB)均具有失活作用.Zn物种会引起催化剂表面颗粒轻微团聚,表面酸性位点数量减少,表面V的还原性减弱,表面化学吸附氧比例,以及V^(5+)和V^(4+)的物质的量比值降低.再生实验结果表明:酸洗可以在一定程度上恢复中毒催化剂的催化活性,但水洗不能恢复中毒催化剂的活性.研究发现Zn盐中毒作用机理为:Zn^(2+)与催化剂表面酸性位点V=O和V−OH反应形成V−O−Zn,对NH_(3)与CB的吸附产生不利影响,造成催化剂中毒失活,ZnSO_(4)中的SO_(4)^(2-)可以为NH_(3)和CB的吸附转化提供新的酸性位点,减轻中毒效果,ZnCl_(2)中的Cl−会在反应后产生副产物HCl,造成催化剂表面更多活性位点中毒,加深中毒效果.

Iron ore sintering is a process in which fuel,flux,and iron ore powders are mixed and sintered into a block under incomplete melting conditions.The flue gas from iron ore sintering process is one of the largest sources of nitrogen oxide(NO_(x))and dioxin emissions in industries.The V_(2)O_(5)-WO_(3)/TiO_(2)(VWTi)catalyst can simultaneously remove NO_(x) and dioxins,but the presence of the complex flue gas results in the deactivation of the catalysts.In response to this challenge,this study carried out experiments for ZnCl_(2),ZnO,and ZnSO_(4) poisoning over the VWTi catalyst via wet impregnation method.The effects of the different Zn species on the simultaneous removal of NO_(x) and dioxins(chlorobenzene was used as the simulant for dioxins)by the VWTi catalyst were studied under simulated conditions of the iron ore sintering flue gas.The surface physicochemical properties of the fresh and poisoned catalysts were characterized to reveal the deactivation mechanism,and the regeneration experiments of the poisoned catalysts were investigated.Results showed that deactivation through catalytic denitrification and chlorobenzene(CB)catalytic degradation processes could be observed in different Zn-containing catalysts.The poisoning effect was more obvious with the increase of Zn content,and the effects of deactivation were as follows:ZnCl_(2)>ZnO>ZnSO_(4).Results from physical and chemical analyses indicated that Zn species had a significant influence on the chemical environment of the active substances on the surface of the catalysts.Zn species caused a slight agglomeration of particles on the surface of the catalysts,a decrease in the number of surface acid sites,a reduction in the reducibility of surface V species,and a decrease in the chemisorbed oxygen ratio and the molar ratio of n(V^(5+))/n(V^(4+)).The regeneration experiments confirmed that employing the dilute sulfuric acid solution washing method was effective for recovering the catalytic activity,whereas the water washing method failed to restore the catalytic activity.The mechanism of Zn salt poisoning is as follows:Zn^(2+)reacts with the acid sites V=O and V−OH on the surface of the catalyst to form V−O−Zn,which adversely affects the adsorption of NH_(3) and CB,resulting in the catalyst poisoning and deactivation.The SO_(4)^(2-) in ZnSO_(4) provides a new acidic site for the adsorption and transformation of NH_(3) and CB alleviating the poisoning effect.The Cl−in ZnCl_(2) produces HCl as a by-product after the reaction,resulting in more active sites poisoning on the surface of the catalyst and deepening the poisoning effect.