Construction Of Fe-Based Catalysts With Novel Structures For NH₃-SCR Of NO

In the past few decades,air pollution has become a worldwide problem due to the burning of coal and the popularization of automobiles.Nitrogen oxide is one of the main pollutants in air pollution,which is one of the main causes of environmental problems such as haze,acid rain and photochemical pollution.At present,the most effective way to remove nitrogen oxides is ammonia selective catalytic reduction（NH₃-SCR）technology.Now vanadium titanium catalyst system is the most commonly used catalyst system for denitration.This catalyst system achives high catalytic activity,but the high temperature window and vanadium species toxicity problems restrict the further application of vanadium titanium catalysts.Therefore,it is of great practical significance to develop a new type of low temperature and high activity,high N₂ selectivity and non biological toxicity denitration catalysts.Manganese and iron oxides have been widely studied because of their environmental friendliness,abundant reserves and good thermal stability.The low temperature catalytic activity and N₂ selectivity of the catalyst can be improved by the morphology control and the structure design.This paper has prepared three kinds of denitration catalyst with special morphology,which improved the low temperature catalytic activity.The theoretical calculation and experimental study illustrates the relation between the active species and the morphology of catalyst.The main contents of this paper are as follows:（1）Anatase TiO₂ nanosheets（TiO₂-NS）and nanospindles（TiO₂-NSP）have been successfully prepared with F^-and glacial acetic acid as structure-directing agents by hydrothermal method,respectively.And then,The Fe₂O₃/TiO₂-NS and Fe₂O₃/TiO₂-NSP nanocatalysts were prepared by a wet incipient impregnation method with a monolayer amount of Fe₂O₃.The facet-activity relationship of Ti O₂ in Fe₂O₃/TiO₂ catalysts for selective catalytic reduction of NO with NH₃ was studied by in situ DRIFTs and DFT methods.The results show that the surface of Fe₂O₃/TiO₂-NS catalyst has more acid sites,oxygen defect sites,reactived oxygen species and easy to adsorb a variety of nitro species,which is conducive to achieving a better NO_x removal effect.This work paves a way for understanding the facet-activity relationship of Fe₂O₃/TiO₂ catalysts in the NH₃-SCR reaction.（2）The monolith denitration catalysts were designed and synthesized by using in situ decorating method with Mn and Fe precursors.By adjusting the ratio of the precursors,we obtained Mn-Fe bi-metal oxides with various morphologies on Fe wire mesh.Impressively,the cube-like Mn-Fe bi-metal oxides structure on Fe wire mesh performed excellent catalytic activity,stability,H₂O tolerance,K⁺poisoning resistance and regeneration performances.The results show that the spinel structure of(Mn_0.37Fe_0.63)₂O₃ is the key factor to improve the adsorption performance and reducibility.The good combination of Mn-Fe bi-metal oxide and Fe wire mesh makes the monolithic catalyst have good stability.The obtained monolith catalysts showed the good resistance to K⁺poisoning and good regeneration performance,which was attributed to the structural stability of Mn-Fe spinel and strong synergistic effect between the support and active species.This new monolithic catalyst prepared by an in-situ technique can be used as a potential substitute for vanadium based denitration catalysts.（3）The Mn-Fe-MOF-74 was first constructed on the surface of Fe wire mesh by solvothermal method.The Mn-Fe-MOF-74@Fe wire mesh was used as the precursor of Mn-Fe@Fe wire mesh（Mn-Fe@Fe）monolith catalyst.By SEM,EDS mapping and XRD characterization,the morphology and composition of the MOF-74 on the wire surface were successfully determined.It is found that the highly dispersed Fe and Mn species on Mn-Fe-MOF-74,which provides a good precondition for the preparation of highly dispersed Mn-Fe oxide species on the surface of Fe wire mesh.Through the high temperature calcination,we obtained the Mn-Fe@Fe monolithic denitration catalyst with Mn-Fe coated.The catalyst showed good catalytic activity,which was mainly attributed to the strong interaction between Mn-Fe species,as well as the excellent mass transfer and heat transfer performance of the metal support.In addition,the catalytic activity of the catalyst was not significantly changed in the process of the stability and water resistance of the Mn-Fe@Fe monolithic denitration catalyst.It is mainly due to the good binding force between the Mn-Fe active component and Fe wire mesh,as well as a large number of acid sites on the surface of Mn-Fe@Fe.The method of in situ growth of MOF materials on the surface of metal has broadened the application of MOF materials,and provided a new idea for the insitu construction of metal based monolithic catalysts.