Influence Mechanism Of Palygorskite On Catalytic Performance Of Ce-based Catalyst For NH₃-SCR Reaction

With the development of industry,air pollution becomes more and more serious,especially the pollution of nitrogen oxide（NO_x）.NO_xmainly comes from fuel combustion and vehicle exhaust emissions.NO_xnot only harms the human body,but also leads to ecological problems such as acid rain,photochemical smog and ozone hole,it even causes global warming and does great harm,so it is particularly important to remove NO_x.Selective catalytic reduction of NH₃（NH₃-SCR）denitration technology,as the main technology of NO_xremoval at present,which has the advantages of high catalytic activity and wide temperature window.The most important parts of the technology are the catalysts.At present,V-based catalysts are the most widely used in industry,but the V-based catalysts are toxic and the active temperature range is narrow,which limit its wide application.Cerium-based catalysts are favored because of their non-toxicity,strong oxygen storage capacity and excellent redox performance.However,cerium-based catalysts have not been widely used,because of their inappropriate carrier,easy sintering at high temperature and poor thermal stability.Therefore,it is necessary to find a suitable carrier for NH₃-SCR reaction.In recent years,the catalysts supported on palygorskite（Pal）have attracted close attention.Because the surface of Pal is different from p H,it can form a double electron layer structure or rich surface functional groups.In detail,the result of isomorphous substitution of metal ions with different valence states makes Pal have a negative structural charge.At the same time,the presence of structural defects and incomplete surface valence bonds in Pal makes it have a negative surface charge.In order to compensate for these negative charge,there are exchangeable cations on the surface of Pal or in the channels,so the surface of Pal has a double-electron layer structure.In addition,moderate acid treatment can increase the number of Si-OH groups on the surface of Pal,thereby improving the ability of Pal to exchange metal ions and complex.Alkali activation under hydrothermal conditions can increase the number of Si-O^-groups on the surface of Pal and significantly improve the adsorption capacity of Pal for heavy metal ions.In addition,Pal is also beneficial to the loading of rare earth active components,and the active materials uniformly dispersed on the surface of Pal,improving the adsorption and activation of reactants,and the exchange or transfer of active species within the catalyst,many active components such as Ni²⁺,Zn²⁺,Co²⁺and Cu²⁺can replace Mg ions in the octahedral framework of Pal.As a result,the reaction efficiency is improved,and the catalytic performance of the catalysts is significantly enhanced,which is expected to achieve industrial application.Therefore,using Pal as the carrier,and adding TiO₂,and loading the active components Ce and W,the CeO₂-WO₃/Pal-TiO₂denitration catalysts were prepared to achieve high denitration performance.Secondly,the surface modification of Pal was further studied.Therefore,the study of Pal carrier and Pal surface modification is of great significance.The main work of this thesis is as follows:the CeO₂-WO₃/Pal-TiO₂catalysts were prepared with different Pal proportions by adjusting the content of Pal,to improve the SCR denitration performance of the catalysts at medium and low-temperature,and the 20%Pal was modified by different methods,a series of different CeO₂-WO₃/20%Pal-TiO₂catalysts were prepared,to improve the SCR activity of the denitration catalysts.On this basis,the effects of different Pal proportions on the activity of CeO₂-WO₃/TiO₂catalysts and the effects of 20%Pal modified by different methods on the activity of CeO₂-WO₃/TiO₂catalysts were studied by the means of SEM,XRD,BET,TEM,H₂-TPR,NH₃-TPD,XPS,Raman and FTIR characterization methods.The results of the study are as follows:The CeO₂-WO₃/Pal-TiO₂ catalysts were prepared with different Pal proportions by adjusting the content of Pal by impregnation method.A series of characterizations and analysis results show that the different Pal proportions have great influence on the activity of the CeO₂-WO₃/TiO₂catalysts.The study on the denitration catalytic activity shows that the CeO₂-WO₃/20%Pal-TiO₂catalyst prepared has the best denitration performance by impregnation method.When the temperature of 240°C,the NO_xconversion could reach 82%,and the NO_xconversion could keep above 92%in the range of 280-400°C.Through a series of characterizations and analysis,the CeO₂-WO₃/20%Pal-TiO₂catalyst has higher Ce³⁺/(Ce³⁺+Ce⁴⁺)surface atomic ratio,more acid sites,and the active species were uniformly dispersed on the surface of Pal,thus improving the SCR performance of the CeO₂-WO₃/20%Pal-TiO₂catalyst.The 20%Pal was modified by different methods,and a series of different CeO₂-WO₃/20%Pal-TiO₂catalysts were prepared by impregnation method.A series of characterizations and analysis results show that the 20%Pal was modified by different methods has a great influence on the activity of the CeO₂-WO₃/TiO₂catalyst,and the catalyst prepared has the best denitration catalytic activity by using the dissociated and acidified 20%Pal.At 240°C,the NO_xconversion could reach88%,and the NO_xconversion could keep above 93%in the range of 280-400°C.Besides,the catalyst also has better resistance to water and sulfur property.After testing at 280°C for 8 h,the NO_xconversion is basically stable and above 80%.In a word,by using the dissociated and acidified 20%Pal,the adsorption performance of the active components was improved,and the interaction was enhanced between the active components and the TiO₂ carrier,so that the catalyst exhibited more excellent denitration activity.