Performance And Mechanism Of Selective Catalytic Reduction Of NO With C₃H₆ Over Fe-based Catalysts Supported On LDHs

With the acceleration of the industrialization process,NOx emissions have increased rapidly,which is harmful to the environment and human health.Selective catalytic reduction of NO with hydrocarbons（SCR-HC）is a very promising method to remove NOx.The core of SCR-HC lies in the research of catalysts.Due to their high cost-efficiency,high catalytic activity,high N₂ selectivity,superior resistance to H₂O and SO₂,and non-toxic to the environment,catalysts containing ferrite were considered as the potential candidate for SCR-HC catalysts.However,Fe working as active metal alone in SCR-HC suffered from high reaction temperatures（>400°C）.To reduce the reaction temperature,it is necessary to introduce other transition metals to modify,such as Cu,Ni,etc.However,traditional Fe-based catalysts have the problems of uneven distribution of active components and inaccurate load control.Therefore,how to synthesize a Fe-based SCR-HC catalyst with properties of simple fabrication,uniform dispersion of active components,and high efficiency at low temperatures under oxygen-rich conditions,is a challenge to meet the practical application.Layered double hydroxides（LDHs）,commonly known as hydrotalcite,with abundant adjustable chemical composition,are an ideal platform for studying polymetallic catalyst systems.After calcination,the topotactic transformation of LDHs can produce the nanocatalyst LDO（layered double oxides）,which has the characteristics of high specific surface area,small crystallite size,and excellent dispersion at an atomic level of metal ions.At present,there are few reports on the application of LDHs in SCR-HC at home and abroad,and the application of Fe-based LDHs in SCR-HC is even blank.Hence,in this paper,a series of Fe-based LDHs designed by coprecipitation method,optimization and regulation of metal elements,were investigated SCR-C₃H₆ performance and tolerance to poisoning components.Various characterization techniques were employed to explore the structure-activity relationship of the catalyst.The SCR-C₃H₆ reaction mechanism on the catalyst surface was proposed by In situ DRIFTS,and some research results have been obtained.1.Cu_xFe_y-600c catalysts derived from Cu_xFe_y-LDHs prepared by the coprecipitation method,exhibited superior catalytic performance than catalysts containing single metals（Cu O,Fe₂O₃）.Among them,the Cu_0.71Fe_0.29-600c catalyst exhibits the best catalytic performance,achieving the NO conversion of 60%at 300°C.This is due to the strongest synergy effect between Cu and Fe,more Cu Fe₂O₄,more lattice oxygen,and stronger Br（?）nsted acid on the catalyst surface.In situ DRIFTS results show that monodentate,bidentate nitrates,and acetate are the main active intermediates on the catalyst surface.2.A series of Cu_xNi_yFe_z-C catalysts derived from Cu_xNi_yFe_z-LDHs were synthesized by introducing Ni to modify Cu Fe-LDHs via the modified coprecipitation method.The introduction of Ni obviously improved the low-temperature performance of catalysts and widened the reaction temperature window.The most active Cu_0.18Ni_0.49Fe_0.33-C catalyst achieves NO conversion of 61%and N₂ selectivity of 86%at 250°C in the presence of 5%H₂O.According to various characterization results,there exists a strong synergy between Ni and Cu,leading to the formation of Cu_xNi_yFe₂O₄,improvements of active oxygen concentration,specific surface area,redox performance,and acidity.In situ DRIFTS results reveal that monodentate,bidentate,bridge nitrates,acetate,and R-NCO are active species.Moreover,the strong synergy between Ni and Cu promotes the formation of monodentate nitrates,acetate,and R-NCO,thereby promoting the improvement of low-temperature catalytic performance.In addition,based on the TPSR experiment,the reaction mechanism of SCR-C₃H₆ over Cu_xNi_yFe_z-C catalysts has been proposed.3.Considering the existence of poisoning components（H₂O,alkali metal,SO₂）in actual fule gas,based on the preferred Cu_0.18Ni_0.49Fe_0.33-C catalyst,the resistance to poisoning components for the catalyst was investigated.The results show that the catalytic activity is slightly reduced by H₂O and K⁺,due to the blockage of small pores or the creation of large pores,the attenuation of the strong synergy between Ni and Cu,and the drop of reducibility for catalysts.4.The influence of SO₂ on SCR-C₃H₆ performance is relatively complex,featured by a significant decrease at low temperatures and a dramatic increase at high temperatures.Assisted by a variety of characterization techniques,the complex influence mechanism of SO₂ is revealed.Aside from sulfites and sulfates,surface sulphides as the primary sulfide generate after SO₂ adsorption at 30°C,introducing the inert Lewis acid sites and even masking the active sites.As a result,low-temperature activity suffers from inhibition due to minor active nitrates and inert formate.The high-temperature activity is enhanced,profiting from the release of active sites following surface sulphides breakdown,suppression of C₃H₆ combustion,activation of C₃H₆ on extra Br（?）nsted acid sites,plentiful R-NCO,and new active-CN species.Eventually,a possible SCR-C₃H₆ mechanism in the presence of SO₂ was postulated.5.Based on Cu Ni Fe-LDHs,the catalytic performance of the derivative monolithic catalyst prepared from composite oxide microspheres was tested in the presence of H₂O and SO₂.The low-temperature activity is inhibited,while the activity at medium and high temperatures gets a significant promotion.Thereinto,0.5Al₂O₃@LDO catalyst shows the best catalytic performance with the conversion efficiency of NO to N₂ of 82%at 325°C.Several characterization techniques are employed to reveal structure-activity relationship and SCR-C₃H₆reaction mechanism of the catalyst.