Studying The Effect Of Metal Active Sites In Zeolite On The Types And Contents Of Surface Denitrification Gases By DRIFT

NO_x from vehicle exhaust and power plants has brought serious harm to the environment and human health.NO_x-SCR is the most widely used denitration technology at present.At present,the metal zeolite catalyst has been widely used in the field of denitrification due to its advantages of high denitrification activity,good selectivity,good thermal stability,stable structure and no corrosion to equipment.In situ diffuse Fourier transform infrared spectroscopy?DRIFT?,which can trace the adsorption state of the reaction on the surface of the catalyst,has attracted more and more attention in the field of catalytic characterization.In this paper,by selecting Fe/Cu metal ion exchange ZSM-5 as the research object,the types of denitrification gases on the surface of the metal molecular sieve catalyst,the thermal stability of adsorbed species,the oxidation state of the metal sites,and the reaction mechanism of denitrification were characterized by DRIFT,so as to explore the role of the active sites of the metal molecular sieve catalyst in the denitrification reaction.Then,by further characterization of coordination adsorption of NH₃ and NO_x species at active sites of Fe/Cu metal molecular sieve and the denitrification activity test of molecular sieve,it was found that the intrinsic properties of Fe and Cu metal sites affected the amount of denitrification gas adsorption and denitrification activity.Finally,starting from the metals Ce,Mn and Co,the correlation between the coordination unsaturated active sites of various metals and the ratio of NO_X/NH₃ adsorption was constructed to further explore the correlation between the ratio of coordination adsorption of NH₃/NO_Xspecies at the metal active sites and the denitrification activity as well as the intrinsic characteristics of the metal.The main research conclusions of this paper are as follows:?1?the metal active site of Fe/Cu-ZSM-5 catalyst is conducive to the adsorption and reaction of denitrification gas.Through the characterization of DRIFT,it was found that there were a large number of coordination unsaturated sites of Cu²⁺ions in Cu-ZSM-5 and a large number of coordination unsaturated sites of Fe³⁺ions in Fe-ZSM-5 molecular sieve.The coordination ratio of Fe and Cu-ZSM-5 metal sites to NH₃/NO_x species is different when calcined at 550?.The different ratios reflect the different intrinsic properties of Cu and Fe metals,that is,the different coordination environments of Cu and Fe metal exchange ions.At the same time,compared with iron based molecular sieve catalysts,copper based molecular sieve catalysts have better NO-SCR reactivity in the whole temperature range under the same metal loading capacity.After calcination at 800?at high temperature,the coordination ratio of Fe-ZSM-5 at 200?was 1.27 for NH₃/NO_x and the coordination ratio of Cu-ZSM-5 at 550?was similar to that of 1.35 for Cu-ZSM-5.The similar ratio reflected the changes in the coordination environment around the Fe active site in Fe-ZSM-5 after calcination at 800?,forming a coordination environment similar to the Cu active site in Cu-ZSM-5.Therefore,the increased denitrification activity of Fe-ZSM-5 by calcination at 800?is similar to that of Cu-ZSM-5.?2?the intrinsic properties of Fe and Cu metals affect the amount of denitrification gas adsorption and denitrification activity.The presence of isolated Cu²⁺and Fe³⁺sites was verified by XRD and UV-Vis characterization of the structure and active sites of the molecular sieve.It was found that the ratio of NH₃/NO_x species adsorbed by Cu active sites to coordination was about 1,while the ratio of NH₃/NO_xspecies adsorbed by Fe active sites was far less than 1.After calcination at 800?,Fe-ZSM-5 improved the coordination adsorption ratio of NH₃/NO_x species at the Fe active site to about 1.Through the denitrification activity test of Fe/Cu molecular sieve,it was found that molecular sieve catalysts with the same loading capacity of different metals had different denitrification activities.Compared with iron molecular sieve catalysts,copper molecular sieve catalysts had better NO-SCR reactivity in the whole temperature range.Similarly,calcination at 800?enhances the denitrification activity of Fe-ZSM-5 catalyst.The change of NH₃/NO_x species ratio and denitration activity at coordination adsorption of Fe and Cu metal active sites was related to the intrinsic characteristics of the metal sites.We concluded that when the NH₃/NO_x species ratio for coordination adsorption of metal active sites was about 1,the activity of the metal-supported molecular sieve NO-SCR was better.?3?The denitrification activity of M-ZSM-5 is related to the ratio ofspecies adsorbed by coordination of metal sites.The ratio of NH₃/NO_X adsorbed by coordination at the active sites of Ce,Mn and Co at 50?was 0.42,0.73 and 0.345,respectively.The different ratios indicate the intrinsic properties of Ce,Mn and Co active sites.The metal sites of ion exchange in the molecular sieve have different Redox capacity and acidity as well as different coordination unsaturated sites,so they have different coordination adsorption capacities for the same gas.However,the ratio of coordination adsorption of NH₃/NO_X at the Ce and Co metal ion sites is similar,indicating that coordination adsorption of denitrification gas at the Ce and Co metal active sites is similar.Ce and Co with good Redox capacity have superior denitrification activity than Mn exchange zeolite.When the ratio of NH₃/NO_X adsorbed by Ce and Co supported molecular sieves was similar,the denitrification activity of the catalyst was similar.This again proves the correlation between the coordination adsorption ratio of NH₃/NO_X at the active metal sites and the denitrification activity.