Promotional Effects of Silanization on the Hydrothermal Stability of CuCe/BEA Catalyst for Selective Catalytic Reduction of NOxwith NH3

LIU Jing-Ying ,,4,5 ,,4,5 CHEN Yao-Qiang,,4,5

(1Sichuan Provincial Environmental Protection Environmental Catalytic Materials Engineering Technology Center,College of Chemistry,Sichuan University,Chengdu 610064,China)(2State Key Laboratory of Polymer Materials Engineering,Institute of Polymer Science,Sichuan University,Chengdu 610064,China)

(3Institute of New Energy and Low-Carbon Technology,Sichuan University,Chengdu 610064,China)

(4National Engineering Laboratory for Mobile Source Emission Control Technology,China Automotive Technology&Research Center,Tianjin 300300,China)

(5Sichuan University FGD(Flue Gas Desulfurization)State Engineering Research Center,Chengdu 610064,China)

Abstract:The hydrothermal stability of CuCe/BEA catalyst is improved by its silanization for selective catalytic reduction of NOxwith NH3(NH3-SCR).The results indicate that the silanization modification effectively enhances the catalytic activity of CuCe/BEA after the hydrothermal treatment due to significantly inhibit the hydrolysis of Si-O-Al bonds from the BEA skeleton to maintain structural integrity,revealed by X-ray diffraction(XRD),scanning electron microscope(SEM)and27Al nuclear magnetic resonance(27Al NMR).More complete skeleton structure of CuCeSi/BEA catalyst results in the formation of more acid sites after the hydrothermal treatment,confirmed by NH3-temperature programmed desorption(NH3-TPD)and in situ diffuse reflectance infrared transform spectroscopy(in situ DRIFTS).Moreover,H2-temperature programmed reduction(H2-TPR)and X-ray photoelectron spectrum(XPS)show that silanization is beneficial to facilitate the dispersion of active copper ions after the hydrothermal treatment.Therefore,more acid sites and highly dispersed Cu species together contribute to higher hydrothermal stability of silanized CuCeSi/BEA than CuCe/BEA catalyst.

Keywords:silanization;zeolite;hydrothermal stability;structure-activity relationships;heterogeneous catalysis

0 Introduction

With the popularization of diesel vehicles in people's life,the issues of nitrogen oxides(NOx)emission is increasingly serious.NOxhas been an important source of air pollution which could cause significantly harm to both the environment(e.g.acid rain,ozone depletion,photochemical smog,and greenhouse effect)and human health[1-3],the selective catalytic reduction of NOxwith NH3(NH3-SCR)is an efficient NOxelimination technology among all NOxemission control technologies for the diesel exhausts[4-6].

Zeolites(e.g.SAPO-34,BEA,and ZSM-5)as supports with a large specific surface area,channel complex and orderly advantages,can promote the dispersion of active metal species,and abundant surface acid sites from the skeleton of zeolites are beneficial to the adsorption of NH3[7-9].Among the reported materials,Cu/BEA catalysts have shown relatively excellent catalytic performance and lower cost,which have attracted wide attention[10-12].However,poor hydrothermal stability of Cu/BEA limited its commercial application due to dealumination of the zeolite and subsequent formation of the inactive species after the hydrothermal treatment[13].To meet the increasingly stringent regulations on emitted NOx,it is essential to improve the hydrothermal stability of Cu/BEA.Zhao et al.[14]found that Ce and Nb additives could not only improve the NOxconversion of Cu/BEA,but also improve its hydrothermal stability.The hydrothermal stability of Cu-exchanged BEA was obviously improved by the introduction of Fe as the co-active component[15].Our previous work has reported that the hydrothermal stability of Cu/BEA was effectively enhanced by the alkaline metal Ba due tothe formation of more isolated Cu2+species from the enhanced interactions between Cu and other surrounding atoms by the addition of Ba[16].The above works mainly focused on stabilizing the active Cu species,but there is little literature to further stabilize the skeleton structure of BEA to our best knowledge.Considering that the hydrothermal stability of the BEA catalyst has an important relationship with the hydrolysis of Si-OAl bonds in the skeleton of BEA[17].Therefore,the additional silanizing reagent(e.g.ethyl orthosilicate,TEOS)deposited on the surface of BEA would be hydrolyzed firstly to protect against the attack of H2O to Si-O-Al bonds and finally stabilize the structural skeleton of BEA[18].

Herein,the introduction of extraneous Si into CuCe/BEAattemptstoimprovethehydrothermal stability.27Al nuclear magnetic resonance(27Al NMR),X ray diffraction(XRD),NH3 temperature programmed desorption(NH3-TPD),in situ diffuse reflectance infrared transform spectroscopy(in situ DRIFTS),H2-temperature programmed reduction(H2 TPR),and X ray photoelectron spectrum(XPS)are employed to characterize and compare the effects of Si on the structure and active species of CuCe/BEA catalyst before and after the hydrothermal treatment,with a focus on investigating the enhanced influences of Si on the hydrothermal stability of CuCe/BEA catalyst.

1 Experimental

1.1 Catalyst preparation

The Cu/BEA catalyst was prepared by an incipient wetness impregnation method.Firstly,12.32 g commercial H/BEA(nsi/nAl=25,Nankai University Catalyst Co.,Ltd.,China)was dropped into 9.24 mL solutioncontained 1.13gCu(NO3)2·3H2O(AR,Kelong,Chengdu,China),and then,the mixture was stirred to ensure that the copper ions were dispersed well.Next,the mixture was dried in a water bath to a solid powder and calcined at 550℃for 3 h in the static air in a tubular furnace.Finally,the calcined Cu/BEA powders were coated on the cordierite monolith(2.5 cm3,62 cell per cm2,Corning Ltd.,USA),and the amount of catalyst deposited on the monolith substrate was about 160 g·L-1.The fresh catalyst was denoted as Cu,and the mass fraction of copper was 3%(measured by inductively coupled plasma atomic emission spectrometry,ICP-AES).

CuCe/BEA catalyst was prepared by a two step method.Firstly,10.00 g commercial H/BEA was ionexchanged with 100.00 mL solution contained 4.34 g Ce(NO3)3(AR,Kelong,Chengdu,China),which was then filtered,washed with distilled water and dried to obtain Ce/BEA material.Secondly,9.24 mL solution contained 1.13 g Cu(NO3)3·3H2O was impregnated on 12.32 g Ce/BEA powder by an incipient wetness impregnation method as mentioned above.The fresh catalyst was denoted as CuCe.

CuCeSi/BEA catalyst was obtained by modifying the CuCe/BEA with TEOS(AR,Kelong,Chengdu,China).Firstly,2.25 mL TEOS was added to 20.00 mL solution of equal proportions of water and ethanol.Then,10.00 g CuCe/BEA powder was added,stirred magnetically for 12 h,dried in a water bath,and calcined at 550℃for 3 h in the static air in a tubular furnace.Finally,the coated fresh catalyst was denoted as CuCeSi.

The monolithic Cu,CuCe and CuCeSi catalysts were hydrothermally treated at 700℃for 10 h in a fixed-bed quartz flow reactor using air,containing 10%(volume fraction)water vapor flowing at a space velocity of 30 000 h-1,The hydrothermally treated catalysts were denoted as Cu-HT,CuCe-HT and CuCeSi-HT,respectively.

1.2 Catalyst characterization

27Al NMR experiments were carried out on JEOLECZ600 instrument in order to detect dealuminization in BEA structure before and after the hydrothermal treatment.

The XRD measurements were achieved by the Rigaku D/MAX ray diffractometer using Cu Kα radiation(λ=0.154 18 nm).The X-ray tube was operated at 45 kV and 25 mA.The samples were investigated in the 2θ range of 5°～60°at a scanning speed of 0.02(°)·min-1.

SEM images of the catalyst were observed by field emission scanning electron microscopy(FESEM)on a Hitachi SU8220 instrument operating at 10 kV.

NH3-TPD was also performed on the thermal conductivity detector(Xianquan,TP5076).And approximately 100 mg sample was pretreated under the He gas(30 mL·min-1)at 550℃for 30 min,and then cooled to 120℃in the above atmosphere.Next,the gas was switched into the NH3(30 mL·min-1)until saturation.After that,the catalyst was flushed by He(30 mL·min-1)to remove physical adsorbed NH3.Finally,the NH3 TPD desorption process was started at a linear heating rate of 10℃·min-1from 120 to 800℃under the He flow(30 mL·min-1).

The Nicolet 6700 spectrometer with a high temperature environmental cell which contained a KBr window and a DTGS detector was used to measure the in situ DRIFTS of catalysts.Before each test,the sample was pretreated in the pure N2(300 mL·min-1)flow from 50 to 350℃at a rate of 10℃min-1,and pretreated at 350℃for 30 min.Then,the background spectra were acquired from 350 to 50℃per 50℃.Pure N2 was switched to the adsorption gas(volume fraction of 0.1%NH3,N2as the balance gas)at 50℃.Afterwards,the temperature was raised at the rate of 10℃·min-1 from 50 to 350℃,and the adsorption gas spectra were determined at per 50℃.Finally,spectra of catalysts were measured by the means of subtracting the background diffuse reflection spectra.

The H2 TPR was carried on the thermal conductivity detector(Xianquan,TP5076).And approximately 100 mg sample was pretreated under volume fraction of 10%O2/N2(30 mL·min-1)at 550℃for 30 min.Then,the sample was cooled to 50℃in above atmosphere,and the H2-TPR desorption process was started at a linear heating rate of 10℃·min-1from 50 to 900℃under a volume fraction of 10%H2/N2(30 mL·min-1)flow.

The XPS was carried out on a spectrometer(XSAM-800,KRATOS Co.)with Al Kα radiation under ultra-high vacuum(UHV).The C1s peak(284.6 eV)was used for the calibration of binding energy value.

1.3 NH3-SCR activity measurements

Catalyst activity measurements for NH3-SCR were tested in a fixed bed quartz flow reactor.The monolith substrate loaded into a horizontal quartz microreactor and the reaction temperature was measured by two K-type thermocouples.The feed gases were regulated by mass-flow controllers and composed of volume fraction of 0.02%NO(cNOx,in),0.02%NH3,5%H2O and 10%O2,and N2was used as the balance gas with a gas hourly space velocity of 40 000 h-1.Before each test,the catalyst was pretreated at 550℃in the reaction atmosphere.The concentration of outlet gases(cNOx,out)were monitored by an FTIR spectrometer(Antaris IGS,Thermo Fisher Scientific),and the test temperature was varied between 550～150℃.The NOxconversion was calculated by the followed equation:

“cNOx,out”and“cNOx,in”represent the outlet and inlet reactant concentrations of the reactor,respectively.

2 Results and discussion

2.1 Structural and morphological properties

27Al NMR spectra are employed to characterize the dealuminization of CuCe and CuCeSi before and after the hydrothermal treatment,as shown in Fig.1.All four samples display three peaks:the peak of chemical shift at 54 is ascribed to the tetracoordinated framework aluminum atoms(AlⅣ),the peak of chemical shift at 12 is attributed to the pentacoordinated aluminum atom(AlⅤ),and the peak of chemical shift at-13 isassigned to the hesacoordinated aluminum atom(AlⅥ).BEA molecular sieve is mainly composed of tetrahedral SiO4and AlO4,AlⅣis the main Al species in the structure of BEA molecular sieve,showing the largest NMR peak.AlⅤmay be due to the partial incomplete structure existing in the synthesis of BEA molecular sieve,or the hydrolytic destruction of some BEA zeolite supports occurred during the preparation of the catalyst.AlVIis due to the hydrolysis of Si-O-Al bonds in the hydrothermal process,which results in the recoordination of AlⅣand AlⅤwith H2O molecules to form Al atoms with hesacoordinated[19-21].The relative content of AlⅣover CuCeSi was slightly higher than that in CuCe,as listed in Table 1.AlⅣspecies are significantly decreased due to the dealuminization for both catalysts after the hydrothermal treatment,although CuCeSi-HT catalyst still possesses higher relative content of AlⅣafter the hydrothermal treatment,indicating that the introduction of Si can inhibit the attack of H2O molecules on the Si-O-Al bonds,inhibit the occurrence of dealuminization and protect the structure of BEA during the process of the hydrothermal treatment.

Fig.1 27Al NMR spectra of CuCe and CuCeSi catalysts before and after the hydrothermal treatment

Table 1 Integrated area(A)of AlⅣatoms in CuCe and CuCeSi catalysts

To investigate the effect of Si on the structure ofBEA,XRD patterns of CuCe,CuCeSi,and their hydrothermal samples are shown in Fig.2.Only diffraction peaks ascribed to the typical structure of BEA were detected over all samples and no obvious other diffraction peaks ascribed to CuxOyand SiO2species[22-24](Fig.2a).No peaks assigned to metallic or metal oxides species can be observed in Fig.2a,which could be that they are highly dispersed,or the concentration is too low to meet the detection limitation of XRD,or they do present as isolated metal species in the BEA molecular skeleton[25].The main diffraction peak at 2θ=22.5°was not shifted and no significant difference between the crystallinity of CuCe and CuCe Si indicates little effect of Si on the structures of BEA.Compared with the fresh catalysts,the intensity of the characteristic diffraction peaks of BEA decreased to a certain extent for the HT catalysts,indicating that the structure of catalysts is destroyed during the process of the hydrothermal treat-ment,which is consistent with the27Al NMR results.

In order to further evaluate the sample crystallinity changes before and after the hydrothermal treatment more accurately,it is assumed that the crystallinity of fresh Cu catalyst was 100%,and the relative content changes were calculated through the main diffraction peak of 2θ=22.5°,the results are shown in Fig.2b.Although the crystallinity of both catalysts was affected by the hydrothermal treatment,the crystallinity of CuCe was declined by 15.4%,while that of CuCeSi was only 7.1%.The above phenomena suggest that the modification of Si can prevent the destruction of the skeleton structure of BEA zeolite during the process of the hydrothermal treatment,protect the integrity of the skeleton structure.

Fig.2 XRD patterns of CuCe and CuCeSi catalysts before and after the hydrothermal treatment(a)and crystallinity loss ratio(b)

Fig.3 SEM images of CuCe and CuCeSi catalysts before and after the hydrothermal treatment

The SEM images displayed in Fig.3 show the effect of Si on the morphology of CuCe before and after the hydrothermal treatment.CuCe and CuCeSi havethe characteristic regular morphology of BEA zeolite,but due to the low amount of Si,there was no obvious difference between them.After hydrothermally treated at 700℃for 10 h,the surface morphology of the catalysts are severely damaged,but CuCeSi HT could retain more regular morphology than CuCe-HT[26].Combined with27Al NMR and XRD,the results show that Si modification can effectively inhibit the hydrolysis of Si-O-Al bonds,protect the skeleton and morphological of BEA and improve the hydrothermal stability of CuCe.

2.2 NH3adsorption

The hydrolysis of BEA skeleton will lead to the loss of surface acidity of zeolite during the process of the hydrothermal treatment,and it is well known that the surface acidity of the catalyst plays a crucial role in NH3 SCR reaction.So NH3 TPD was employed to investigate the influence of Si on the surface acidity of CuCe before and after the hydrothermal treatment(Fig.4).Three desorption peaks were detected in the NH3 TPD curves of all samples in Fig.4.The low temperature desorption peak(α)at 150～250℃can be classified as NH3adsorbed on the weak silanol(Si-OH)group,which is the Lewis acid sites generated by the structural defect.The medium-temperature desorption peak(β)at 250～400℃can be attributed to the NH3 species adsorbed on the strongly acidic hydroxyl group,that is,the Brønsted acid sites[27].The high-temperature desorption peak(γ)above 400°C can be attributed to the strong Lewis acid sites generated by the introduced copper ions[28].The surface acidity of CuCe is slightly depressed after introducing Si,which may be causedby the hydrolysis of TEOS at the acid sites of BEA,part of the acid sites of BEA was covered by SiO2after calcined,preventing it from adsorbing NH3.Moreover,the amount of acid sites of the fresh catalysts was decreased significantly after the hydrothermal treatment,which may be caused by H2O molecules attack the Si O Al bonds during the hydrothermal treatment process leading to structural collapse of BEA zeolite,resulting in less acidic sites.Interestingly,Si modification can inhibit the loss of surface acidity of CuCe during the hydrothermal treatment,and the amount of acidic sites of CuCeSi HT was even higher than CuCe HT.NH3 TPD results show that the modification of Si can stabilize the surface acidity of the catalyst and inhibit the loss of surface acidity of catalyst during the hydrothermal treatment.

As a probe molecule,NH3can further explore the changes of acidity and acid sites of catalysts before and after the hydrothermal treatment.The in situ DRIFTS results of NH3adsorption on the catalysts after the hydrothermal treatment are shown in Fig.5.Brønsted and Lewis acid sites can be both detected on the two HT catalysts via NH3adsorption.The Brønsted acid sites(3 361,3 282 and 3 178 cm-1)are mainly provided by zeolite supports,while the Lewis acid sites(1 627,1 251 and 1 159 cm-1)are mainly derived from Si-OH in the structure and the introduced active copper species[29-31].It can be observed that the Brønsted acid sites are the dominant acid sites for both catalysts.In order to more intuitively compare the adsorption of NH3between two catalysts and their acidity at the sametemperature,the comparison results of NH3adsorption spectra at 350℃are listed in Fig.5c.It can be clearly seen that,CuCeSi HT has more Brønsted acid sites than CuCe HT at 350℃,which is from the Si O Al bonds on the zeolite skeleton structure,and this result is consistent with the NH3-TPD,indicating that Si modification can protect the surface acid sites.Combined with XRD and27Al NMR results,CuCeSi-HT catalyst with a more complete skeleton structure due to the introduction of Si provides more Brønsted acid sites for adsorption and activation of NH3than CuCe catalyst,which may improve NH3-SCR activity.

Fig.4 NH3-TPD profiles of catalysts before(a)and after(b)the hydrothermal treatment

Fig.5 In situ DRIFTS results of NH3adsorption over CuCe-HT(a)and CuCeSi-HT(b);Comparison of NH3adsorption of catalysts under the hydrothermal treatment at 350℃(c)

2.3 Redox properties

Isolated Cu2+species are the main active species of CuCe for NH3-SCR reaction.As shown in Fig.6,H2-TPR profiles of all samples can be divided into four reduction peaks[32-34]:the first peak(α)centered at 262℃and the third peak(δ)located at 447℃can be assigned to the two-step reduction of isolated Cu2+to Cu0;the second peak(β)at 357℃is related the reduction of CuO to Cu0,and the last peak(γ)at 554℃is relatedto Cu+self-reduction(SCu+)to Cu0.H2consumptions were quantitatively calculated according to the CuO reference sample,and the results are summed up in Table 2.The decrease of isolated Cu2+species was clearly observed in CuCe modified by Si,possibly due to the blocking and covering of the structures and active components by SiO2.H2consumptions of all peaks were decreased after the hydrothermal treatment,but it can be seen from the quantitative results that the amount of isolated Cu2+species in CuCeSi HT was unexpectedly higher than that in CuCe HT,and its amount of CuO was lower than that of CuCe-HT.

Fig.6 H2-TPR spectra of fresh(a)and hydrothermal treated(b)CuCe and CuCeSi catalysts

Table 2 Quantitative analysis of the H2-TPR profiles of the fresh and hydrothermal treated CuCe and CuCeSi catalysts

XPS has been widely used in the semiquantitative evaluation of catalyst surface species.In order to further characterized the chemical state of Cu species,the Cu2p XPS spectra of the catalysts before and after the hydrothermal treatment are shown in Fig.7.All samples show characteristic peaks of Cu2+species,including the main peaks of the Cu2p spin orbit split[35](Cu2p3/2and Cu2p1/2at 933～937 eV and 953～956 eV,respectively)and one satellite peak near 944.6 eVwhich is assigned to CuO[36].Cu2p3/2peaks are deconvoluted to the band at 933.8 and 936.7 eV,which are assigned to CuO and isolated Cu2+species in ion exchanged positions,respectively[37].In order to better analyze the differences in the distribution of surface elements before and after the hydrothermal treatment of catalysts,the quantitative calculation results are shown in Table 3.There are significant differences in the distribution of copper species among different catalysts.For fresh catalysts,the content of isolated Cu2+species on the surface of CuCeSi is lower than that of CuCe due to Si coverage,which is consistent with the TPR results(Table 3).After the hydrothermal treatment,the content of isolated Cu2+is increased for both catalysts,possibly because the copper species in thebulk phase migrated to the surface and mainly existed in isolated Cu2+and CuO forms.Isolated Cu2+of 0.82%was detected on CuCeSi-HT,higher than that of 0.64%in CuCe-HT,which is consistent with H2-TPR.In addition,the ratio of Cu2+/CuO increased from 0.28 to 0.42,indicating that the addition of Si can inhibit the aggregation of isolated Cu2+.Combined with the results of H2-TPR and XPS,it can be concluded that the addition of Si can inhibit the aggregation of isolated Cu2+to form CuO during the process of the hydrothermal treatment,so more isolated Cu2+species could improve the lowtemperature SCR activity,while less CuO species can weaken the oxidation of high-temperature NH3.Therefore,the addition of Si may be beneficial to maintain the NH3-SCR activity of CuCeSi-HT in the entire reac-tion temperature range after hydrothermal treatment.

Fig.7 Cu2p XPS spectra for the fresh(a)and hydrothermal treated(b)CuCe and CuCeSi catalysts

Table 3 Surface chemical composition and contents over catalysts before and after the hydrothermal treatment

2.4 NH3-SCR activity

The curves of NOxconversion as a function of reaction temperatures in NH3 SCR reaction over Cu,CuCe and CuCeSi catalysts are shown in Fig.8.It can be seen that both CuCe and CuCeSi catalysts show similar NOxconversions except higher NH3-SCR activity of CuCeSi catalyst than that of CuCe catalyst above 500℃in Fig.8a.Their low-temperature NOxconversion both higher than that of Cu catalyst,and their values of T90(the reaction temperature when the NOx conversion rate reaches 90%)are shifted to 186℃from 218℃of Cu catalyst,decreased by about 30℃.After the hydrothermal treatment at 700℃for 10 h,the catalytic activities have a significant loss over all catalysts at the temperature below 500℃(as shown in Fig.8b).However,the maximum NOxconversion over CuCeSi-HT was 85%in the temperature range of 175 to 400℃,which is higher than CuCe HT and Cu HT(80%and 78%,respectively).Furthermore,CuCeSi HT can retain over 80%NOxconversion between 200～550℃,while CuCe HT and Cu HT have over 80%NOxconversion only in the high temperature range,indicating that Si modification can improve the hydrothermal stability of the CuCe catalyst to some extent.

Combined with the characterization results to analyze,for fresh catalysts,the amount of isolated Cu2+and the surface acid sites are slightly reduced in CuCeSi compared with CuCe due to the covering effect of SiO2 generated by TEOS in the calcined process on the catalyst surface.But their NH3-SCR activity are almost the same,probably because CuCeSi possesses enough isolated Cu2+and acid sites to maintain the optimal activity.After the hydrothermal treatment,the attack of H2O molecules on the Si-O-Al bonds causes the collapse of zeolite skeleton structure,resulting in the decrease of isolated Cu2+and surface acid sites content,which leads to the decrease of catalysts activity.However,after the addition of Si,TEOS hydrolyze on the acid sites and cover the surface to play a certain protective role,thus preventing H2O molecules from attacking the Si O Al bonds in the process of hydrothermal treatment,which is conducive to stabilizing the structure of zeolite skeleton.The more complete skeleton structure helps to stabilize the acid sites on the catalyst surface(particularly Brønsted acid sites),and inhibit the aggregation of isolated Cu2+.Therefore,compared with CuCe-HT,CuCeSi-HT has more isolated Cu2+and more surface acid sites,and finally,exhibits better NH3-SCR activity than CuCe-HT.

Fig.8 NOxconversion as a function of the reaction temperature over Cu,CuCe and CuCeSi during NH3-SCR

3 Conclusions

In summary,the hydrothermal stability of CuCe catalyst is effectively improved through the modification of Si.The presence of Si stabilizes the skeletonstructure of the zeolite,which improves the amount of acid sites on the catalyst surface and the dispersion of copper ions,and finally leads to the improvement of the hydrothermal stability of the catalyst.The results of XRD,SEM and27Al NMR reveal that the addition of Sican protect the Si O Al bonds,inhibit the attack of H2O molecules in the process of hydrothermal treatment,so as to protect the skeleton structure of zeolite and improve the stability.The NH3 TPD and in situ DRIFTS results show that more complete skeleton structure is beneficial to protect the structural acid sites on the surface of zeolite and reduce the loss of the amount of acid sites during the process of the hydrothermal treatment.The results of XPS and H2 TPR show that although the modification of Si covered some active sites on the catalyst surface,a more complete skeleton structure is beneficial to improve the dispersion of copper ions and inhibit the migration and aggregation of copper ions to form CuO.Therefore,the modification of Si can provide more acid sites and highly dispersed Cu species which finally improve the hydrothermal stability of CuCe catalyst for NH3 SCR.This study proves the feasibility of adding Si to stabilize zeolite skeleton,which is helpful to improve the hydrothermal stability of catalysts and develop catalysts with excellent activity for NH3-SCR.