Study On The Copper Species And Framework Stability Regulation Of Cu/SAPO-34 For NH₃-SCR Performance

Selective catalytic reduction NO_x with NH₃（NH₃-SCR）technology is currently one of the most mainstream methods for controlling diesel engine nitrogen oxide（NO_x）emissions,and the excellent performance of SCR catalyst is the core of ensuring the performance of diesel engine SCR system.With the implementation of more and more stringent diesel vehicle exhaust emission regulations,traditional commercial SCR vanadium-based catalysts have gradually been eliminated due to the narrower activity window,poor hydrothermal stability and secondary pollution problems,and replaced by a series of environmentally friendly catalysts with excellent catalytic performance.Among them,the Cu/SAPO-34 catalyst has great application potential due to its excellent NO_x conversion,wider active temperature window and higher N₂ selectivity.However,due to the harsh and complex working conditions of diesel engines such as high temperature and high humidity,and toxic substances（SO₂,P and alkali metals）,it is still necessary to further improve the high-temperature hydrothermal stability and anti-poisoning ability of SCR catalysts to meet increasingly stringent emission regulations.Therefore,the development of NH₃-SCR catalysts with excellent comprehensive performance is of great significance to the control of diesel vehicle exhaust NO_x emissions.In this paper,based on the problem of high temperature hydrothermal aging,the methods of doping rare earth elements into Cu/SAPO-34 and adjusting the copper species are adopted to obtain stable active copper species and stable zeolite framework.In addition,based on the problem of chemical poisoning,the chemical poisoning effect of PO₄^3-and SO₂ on Cu/SAPO-34 catalyst is studied.The main research results of this paper are as follows:（1）The NH₃-SCR activity and stability of Cu/SAPO-34（Cu/S-fresh）and rare earth La-modified Cu/SAPO-34（Cu/La-S-fresh）catalysts under different hydrothermal temperature were investigated.It is found that La modification can improve the low-temperature activity and high-temperature hydrothermal stability of Cu/SAPO-34.When the Cu²⁺loading is 5%,the NO_x conversion of Cu/La-S-fresh at 150℃is increased by about 20%compared with Cu/S-fresh.After hydrothermal aging at 650℃,the low-temperature activity of Cu/S-650 and Cu/La-S-650 are significantly improved.However,Cu/S-750 and Cu/S-850 show severe deactivation,while Cu/La-S-750 still shows excellent catalytic performance,and deactivation occurs only when treated at850℃.XRD and Raman results show that the doping of La slows down the collapse of the molecular sieve structure.The H₂-TPR and EPR characterizations indicate that the modification of La inhibited the conversion of active Cu²⁺ions to bulk Cu O in the sample,thereby slowing the deactivation of the catalyst.（2）Based on the enhanced strategy of rare earth to regulate the SCR activity and high temperature hydrothermal stability of Cu/SAPO-34,the exchange amount of Pr is regulated by changing the number of ion exchanges of rare earth Pr on SAPO-34,and Cu/SAPO-34（Cu/SF）and Cu/Pr（x）-SAPO-34（Cu/Pr（x）-F,x=1,2,3）catalysts.Among them,Cu/Pr（1）-F with a Pr ion exchange number of 1 has better low-temperature activity and hydrothermal stability than the other three samples.Especially,the aged Cu/Pr（1）-A at 175℃achieve a NO_x conversion rate of more than 90%.However,Cu/S-A and Cu/Pr（3）-A lost most of the active sites.A small amount of Pr stabilizes the structure of the catalyst and active Cu²⁺ions,so that it can maintain excellent catalytic performance under high temperature hydrothermal environment,while high content of Pr doping destroys the zeolite structure,resulting in the decline of low temperature activity and hydrothermal stability.In situ DRIFTS results show that the SCR reaction on the Cu/SAPO-34 and Pr modified catalysts all follow the L-H and E-R mechanisms.（3）Based on the results brought by different Pr doping levels in Chapter 4,we were inspired to control the distribution of copper species in the catalyst by changing the copper loading in Cu/SAPO-34.The characterization results show that the isolated Cu²⁺ions in fresh samples showed a monotonous increase with the increase of copper loading.Correspondingly,the amount of unstable active Cu²⁺ions in samples with high copper loadings（3Cu-F and 3.5Cu-F）was also high.Therefore,although the catalytic activity of the fresh catalyst increases with the increase of copper loading,the aged3Cu-A and 3.5Cu-A are inactivated due to the conversion of more unstable Cu²⁺ions into Cu O species inducing structural collapse and active Cu²⁺ion loss.When the copper loading is less than 3%,a small amount of unstable Cu²⁺ions on 0.5Cu-A,1Cu-A,1.5Cu-A,2Cu-A and 2.5Cu-A samples will be converted into stable Cu²⁺ions,which is more than that on the correspondingly fresh samples.Therefore,the activity of these aged samples has increased significantly.（4）Explore the anti-PO₄^3-and SO₂ poisoning ability of Cu/SAPO-34 with ideal copper loading.Due to the introduction of P,the copper species in Cu/SAPO-34 exist mainly in stable isolated Cu²⁺ions,so the low temperature activity is improved.However,the presence of non-framework PO₄^3-increases the probability of dealumination of the catalyst in the high-temperature hydrothermal process,which has a slight adverse effect on the hydrothermal stability.In addition,by investigating the effect of SO₂ on the SCR activity of Cu/SAPO-34 and PO₄^3-doped Cu/SAPO-34catalysts,it is found that the introduction of external P sources can improve the SO₂resistance of Cu/SAPO-34 catalysts.The SO₂-TPD and in situ DRIFTS characterization results show that the introduction of PO₄^3-can reduce the ability of SO₂ adsorption on the catalyst and the ability to bind to the active site Cu²⁺,inhibiting the competitive adsorption of SO₂ and NH₃,so that the catalyst can still maintain high catalytic activity.It is found that the Cu/SAPO-34 catalyst has not only good low temperature de NO_xactivity and high temperature hydrothermal stability,but also excellent resistance to chemical poisoning through proper adjustment,which is a kind of material with great application prospect.