The Study Of Reaction Mechanism For Selective Catalytic Reduction Of No With NH₃ Over Fe-Mn/ZSM-5 Catalyst

Selective catalytic reduction with NH₃（NH₃-SCR）continues to be one of the most efficient and universal technologies to reduce NOx to environmentally amicable nitrogen.And the reaction mechanism for NH₃-SCR of NO over various catalysts has been studied in detail,such as,V-based catalysts,Fe-based catalysts and Mn-based catalysts.However,the mechanism of SCR reaction over Mn-Fe/ZSM-5 owned superior catalytic performance at low temperature has been rarely studied so far.Moreover,there still was controversy for the relationship between catalytic performance and ratios of active metallic ions with different valence.Hence,it is necessary to investigate the reaction mechanism for NH₃-SCR over Fe-Mn/ZSM-5 catalyst and dissect the internal operating mechanism between reaction mechanism and metallic ions ratios.Firstly,xFe-Mn/ZSM-5 catalysts were prepared by co-precipitation method.Their catalytic activities were tested and the results revealed that amount of Fe must be appropriate for the most outstanding catalytic performance at low temperature.Then,N2-adsorption/desorption,XRD,SEM（EDS）,XPS,NH₃-TPD,NO oxidation activities experiments,NO+O₂-TPD and in-situ DRIFTs were conducted to dissect the intrinsic mechanisms.On the basis of the characteristic results and theoretical analysis,many concepts,METB（multifunctional electron-transfer-bridge）,RMETB and DMETB（the rate and direction of multifunctional electron-transfer-bridge,respectively）as two basic characteristics of METB,were initially proposed.Under the relationship between these concepts and reaction mechanism,main conclusions were summarized as follows:（1）Textural properties,surface morphologies,element dispersion might not be as the determining factors to manipulate catalytic performance due to rarely differences in that.However,METB played key role in catalytic performance at low temperature.（2）Although adsorbed NH₃ were activated utilizing METB to produce-NH2 that react with NO（gas）enhancing the activities at low temperature,acidity was just as the secondary role to change performance on account of little differences in acidity over xFe-Mn/ZSM-5（x=10,15,20）.Similarly,the ability of NO oxidation to NO₂ utilizing the METB was also not the main factor to influence activities due to the inferior production and little otherness of NO₂ production at low temperature.（3）Linear nitrites and monodentate nitrates were propitious for activities through the Langmuir-Hinshelwood mechanism utilizing METB,while mono-metal bidentate nitrates were detrimental for NH₃-SCR performances at low temperatures.Moreover,ratios of Mn⁴⁺/Mn³⁺ and Fe³⁺/Fe²⁺ influenced by the amount of Fe could control the METB to determine catalytic performance through acting on the formation of NOx-intermediates.（4）RMETB and DMETB played key role in the production and types of intermediates,respectively.Inferior RMETB was set as limiting factor for relatively rare production of intermediates inducing the worse catalytic activities when the ratios of metallic ions were lower.If the ratios were too high,DMETB made prior promotion in the production of mono-metal bidentate nitrates creating deactive effect in activities,which was deemed as inhibiting factor.（5）Appropriate ratios of metallic ions over 15Fe-Mn/ZSM-5 catalysts stimulated the optimum NH₃-SCR performance at low temperature,which was attributed to the mesoscopic synergistic effects between RMETB and DMETB facilitating the generation of linear nitrites and monodentate nitrates as well as restraining the production of mono-metal bidentate nitrates.（6）SO₂ can react with active metal sites to produce stable sulfates which make Fe and Mn ions in toxic state limiting the RMETB,then induce inferior catalytic activities.When GHSV was too high,the contact time was too short to produce enough reactive intermediates contributing catalytic performance.