Study On The Selective Catalytic Reduction Of NO_x Over Cu-based SAPO Zeolite Catalysts

Nitrogen oxides（NO_x）emitted from diesel vehicles are the major air pollutant that can lead to a range of environmental problems,such as photochemical smog,acid rain,ozone layer hole,etc.With the increasing of vehicles year by year,environmental problems become increasingly serious.Despite of their small quantity（10%）,diesel vehicles can emit nearly 90%of the total NO_x emissions of vehicles.China has also begun to fully implement the most stringent national VI emission standard,so controlling and reducing NO_x emissions in diesel vehicle exhaust has become an urgent issue.The selective catalytic reduction of NO_x with ammonia（NH₃-SCR）technology is currently the most effective technology to eliminate nitrogen oxides,and high-performance catalyst is the core of NH₃-SCR technology.Zeolite-based catalysts are currently the most promising mobile source denitration catalysts due to their high NO_x conversion,excellent N₂ selectivity and hydrothermal stability.In this thesis,Cu-SAPO-RHO,Cu-SAPO-34 and Fe or Ce modified Cu-SAPO-34 were prepared and used to remove nitrogen oxides from diesel engine exhaust using a simple and low-cost method.XRD,SEM,TEM,BET,ICP-OES,XPS,H₂-TPR,NH₃-TPD and in situ DRIFTS were used to characterize the structure and physicochemical properties of the catalysts,and the possible NH₃-SCR reaction mechanism over these catalysts was proposed.The main conclusions are as follows:（1）A series of Cu-SAPO-RHO catalysts were prepared by template-assisted ion exchange method,and the effects of Si content and Cu content on the catalytic reduction performance of NO_x were investigated.It was found that compared with the catalysts prepared by the conventional ion exchange method,the template-assisted ion exchange method simplified the experimental procedure and the catalysts prepared by the template-assisted ion exchange method had higher crystallinity and catalytic activity.By adjusting the amount of acidic sites and the content of the active Cu species,it was found that the Cu-SAPO-RHO catalyst obtained by Cu ion exchanging with SAPO-RHO（0.8Si）for three times showed excellent catalytic performance,and the conversion of NO_x exceeded 90%in the temperature range of 230-407℃.The in situ DRIFTS results show that the catalyst follows Eley-Rideal（E-R）and Langmuir-Hinshelwood（L-H）reaction mechanism at low temperatures,and L-H reaction path is the main reaction path.This work proposed a more suitable method for the introduction of metal ions into SAPO zeolite,which provides a more efficient and simple modification method for the preparation of metal-based SAPO zeolite catalysts.（2）Cu-SAPO-34 catalysts were synthesized by using morpholine（MA）,propylamine（PA）and diethylamine（DEA）as templates,through template-assisted ion exchange method.The template agent significantly affected the grain size,the distribution of copper species and acid amount of the catalyst,and then affected the NH₃-SCR activity and low temperature hydrothermal stability.Compared with the catalysts synthesized by PA and DEA,the Cu-SAPO-34 synthesized by MA template has the smallest grain size,more amounts of isolated Cu²⁺ions and strong acidic sites,and shows excellent catalytic activity.The NO_x conversion is higher than 90%in the temperature range of 193-550℃.After low temperature hydrothermal aging,the catalytic activity is maintained well,and the NO_xconversion is higher than 80%in the temperature range of 245-590℃.In situ DRIFTS results show that all the fresh catalysts follow L-H and E-R mechanisms.After low temperature hydrothermal aging,the Cu-SAPO-34（MA）-LTH and Cu-SAPO-34（DEA）-LTH catalysts still follow L-H and E-R mechanisms.However,the Cu-SAPO-34（PA）-LTH catalyst mainly follows the E-R path.The effect of template agent on the preparation of Cu-SAPO-34 catalysts by template-assisted ion exchange is investigated in this work,which provides a reference for the design of high catalytic activity SAPO-34 catalysts.（3）Cu Fe-SAPO-34 catalysts were synthesized via template-assisted ion exchange method by simultaneous exchange of Cu and Fe ions.The introduction of appropriate amount of Fe improved the catalytic activity at high temperature and the low temperature hydrothermal stability.The characterization results showed that Fe was beneficial to the dispersal of Cu species,and the presence of Fe could protect Br（?）nsted and Lewis acidic sites and inhibit Cu O formation during low temperature hydrothermal aging.In the presence of water vapor,the NH₃-SCR activity of Cu Fe3-SAPO-34catalyst is maintained well at low temperature and increased at high temperatures.In addition,the presence of Fe species increased the reactivity between adsorbed NH₃species and NO_x in the presence of SO₂,which made Cu Fe3-SAPO-34 catalyst have better resistance to H₂O and SO₂.In situ DRIFTS results show that NH₃-SCR reaction over Cu Fe3-SAPO-34 catalyst follows L-H and E-R mechanism.Template-assisted bimetallic ion exchange method was firstly proposed in this work,which further simplifies the experimental steps and provides a reference for the preparation of high performance bimetallic zeolite catalysts.（4）Cu Ce-SAPO-34 catalyst was synthesized via template-assisted ion exchange method by simultaneous exchange of Cu and Ce ions.Appropriate Ce introduction could improve the NH₃-SCR activity and low temperature hydrothermal stability of Cu-SAPO-34.It was found that the introduction of Ce increased the amount of isolated Cu²⁺ions and acidic sites,and stabilized SAPO-34 structure,protected the acidic sites and reduced the irreversible hydrolysis of active Cu species during low temperature hydrothermal aging.Cu Ce2-SAPO-34 catalyst also has better resistance to H₂O and SO₂.The introduction of Ce can effectively inhibit the competitive adsorption of H₂O on the acid and active sites of the catalyst,and can also prevent the adsorption of SO₂ to a certain extent,reducing the deposition of ammonium bisulfate species on Cu Ce2-SAPO-34 surface and protect the active copper site from acidification.In situ DRIFTS results show that NH₃-SCR reaction over Cu Ce2-SAPO-34 catalyst mainly follows L-H mechanism.Ce introduction is beneficial to the adsorption and activation of NO_x species,and easily reacts with adsorbed NH₃ species to accelerate the L-H reaction rate at low temperature.This work demonstrates that the reasonable introduction and regulation of the ratio of bimetallic Cu and Ce can greatly improve the denitrification performance of the catalyst,and provides a solution to improve the low-temperature hydrothermal stability of Cu-SAPO-34 catalyst.