Performance Optimization And Mechanism Research On Cu-based SCR Catalysts For NO_x Removal

Catalysts are the key to the promotion and application of selective catalytic reduction with ammonia(NH₃-SCR)technology for denitrification.However,there is no systematic understanding on the role of support in the supported catalysts,and few researches on the molding application of SCR catalysts have been reported.In recent years,the development of novel zeolite catalysts has attracted much attention of scholars at home and abroad,but the researches on traditional zeolites in SCR fields are less.In addition,the commercial V-W(Mo)/Ti based catalysts have some disadvantages,such as extremely high toxicity,narrow active temperature window,and easiness in oxidization of SO₂ to SO₃,which limit their application.Therefore,the development of novel and non-toxic SCR catalysts with high efficiency in wide active temperature window and strong industrial adaptability has become the general trend of research hotspots.In this paper,the simple CuSO₄-TiO₂ system was selected to conduct experiments of phase separation and theoretical calculations to study the role of zeolite support in the supported catalysts and its influence on the adsorption and reaction behavior of gas molecules on the catalyst surface,as well as various characterizations such as FESEM/HRTEM,H₂-TPR/NH₃-TPD,In-situ DRIFTS,etc.The results showed no difference that the NH₃-SCR reactions on the surface of Cu SO₄/Ti O₂ catalyst followed the Eley-Rideal(E-R)mechanism,and that the support in supported catalyst could improve dispersion of each component,surface acidity and redox ability,increase surface active oxygen,as well as promote charge transfer and enhance adsorption and activation of NH₃/NO,effectively reducing the energy barrier of NH₃-SCR reaction and facilitating the occurrence of NH₃-SCR reactions.Various zeolite catalysts were investigated and found that zeolites have high crystallinity,large specific surface area and rich acid sites on the surface,performing superior catalytic activity for NO removal by ion-exchange method.Then,Y-type zeolite with the best performance was selected to compare the influence of two different introduction method of active substances(impregnation and ion-exchange method).The main active components in IE-Cu Y catalysts prepared by ion exchange method were isolated Cu²⁺while that in catalysts prepared by impregnation method were Cu SO₄ and Cu O.The main influencing factors were sequenced as:p H>time>concentration>temperature,and the optimal ion exchange conditions were:C(concentration of Cu SO₄ exchange solution)=0.05 mol/L,p H=10,T(ion exchange temperature)=50°C and t(ion exchange time)=1.5 h.The NO conversion over this catalyst could obtain more than 80%within 120-360°C and almost complete removal of NO within 180-300°C,and the N₂ selectivity in reduction products of NO was found to be more than 80%within 120-340°C.However,a non accidental and sudden change of NH₃-SCR performance was found around 150°C.In view of the abnormal effect of ion exchange time on the activity of IE-Cu Y catalysts,various characterizations such as SEM/TEM were carried out and found that too short or too long ion exchange time would lead to the decrease of catalytic activity.Too short ion exchange time would lead to the inadequate ion exchange without enough active sites obtained.And too long ion exchange time would even destroy the framework structure of zeolite under the condition of hot alkali and active isolated Cu²⁺species agglomerated together.In view of the sudden change in NH₃-SCR performance,In-situ DRIFTS analyses revealed the consistence and temperature dependence in sudden change of performance and mechanisms.The low-temperature(<150°C)NH₃-SCR reactions on Cu Y catalysts should mostly obey the Eley-Rideal(“E-R”)mechanism with Br?nsted acid sites mainly involved and NH₄⁺as the critical intermediate species.The high-temperature(>150°C)NH₃-SCR reactions should take place mainly according to“E-R”mechanisms with Lewis acid sites mainly involved and the Langmuir-Hinshelwood mechanism(“L-H”)could also happen,with the coordinated NH_x and nitrate species possibly as the key active intermediates.A kind of monolithic honeycomb SCR catalyst consisting of active components(powder IE-Cu Y),molding agents and molding aids with light-weight(bulk density of 0.9 g/cm³),high-strength(axial strength of 3.698 MPa),and high efficiency in wide temperature(more than 80%of NO conversion within 120-400°C,and nearly complete removal within 180-320°C)was eventually developed and extruded after adjusting the viscosity and fabricability of green body.Surprisingly,the toxicity of SO₂ poisoning only occurrs in the temperature range of 180-240°C,and the mechanism of sulfur resistance was studied by TG,IR,EPR characterizations and theoretical calculations.It was found that Cu Y catalyst could catalyze the decomposition of ammonium sulfates,and the differences in the easily-adsorbed sites of NH₃ and SO₂ provided the most basic way for its resistance to SO₂.