The Study On The Activity Of Transition Metal （Fe, Cu） Modified Beta Catalysts In NH₃-SCR Reaction

Selectve catalytic reduction by ammonia (NH3-SCR) is one of the most efficientmethods to removal of nitrogen oxides under lean-burn conditions. Due to theremarkable hydrothermal stability during SCR process, Fe (Cu)-Beta has been drawngreat attention. In this paper, the catalytic activity of Fe (Cu)-Beta catalysts wereinvestigated and the structure of the catalysts was characterized. Furthermore, theNH3-SCR kinetic experiments of Fe (Cu)-Beta catalysts were conducted andconfirmed the active sites for SCR reaction. It is necessary to provide the basic datafor the development of new zeolite catalyst.A series of Fe-Beta catalysts were prepared by liquid ion-exchange method tostudy the influence of various iron species on the NH3-SCR activity. A combinationof UV-vis and EPR techniques was applied to identify and quantify the iron species.The quantitative analysis indicated that the percentage of tetrahedral Fe3+isolatesdecreased, while the percentage of octahedral Fe3+isolates increased with theincrement of iron loading. In situ EPR results suggested that isolated Fe3+performedexcellent activity for NH3-SCR. The SCR TOF values proved that isolated Fe3+sitesboth in tetrahedral and octahedral coordination were the active sites for NH3-SCRreaction. In addition, the NH3oxidation TOF results indicated that oligomers were theactive sites for NH3oxidation over Fe-Beta catalysts and the contribution of diverseclustered oligomers was unequal.Three Cu-Beta catalysts were synthesized by liquid ion-exchange method. Andthe NH3-SCR activity was investigated over Cu-Beta catalysts. The EPR and H2-TPRmethods were employed to study the structure of the samples. The results indicatedthat the isolated Cu2+, Cu+and CuO clusters coexisted in the catalysts and the contentsof the three copper species improved with the increment of copper loading. The in situEPR results suggested that isolated Cu2+had superior redox ability in NH3-SCRreaction. The TOF values proved that isolated Cu2+were the active sites for NH3-SCRreaction.The Fe-Cu-Beta catalysts were synthesized by mechanical mixing method. TheSCR activity and hydrothermal stability were studied. The results indicated that theSCR activity of Fe-Cu-Beta catalysts at low temperature was better than Fe-Beta andthe SCR activity at high temperature was higher than Cu-Beta. The EPR and H2-TPRresults demonstrated that there were more amounts of isolated Fe3+and isolated Cu2+ in Fe-Cu-Beta catalysts and its redox ability at low temperature was better thanFe-Beta catalyst. The kinetic experiment result indicated that the hydrothermalprocessing could not change the SCR mechanism and the order of activation energywas EaFe-Beta> EaFe-Cu-Beta> EaCu-Beta.