稀有金属(英文版) 2019,38(03),210-220

Catalytic activity of Cu-SSZ-13 prepared with different methods for NH₃-SCR reaction

Meng-Jie Han Yun-Lei Jiao Chun-Hong Zhou Yang-Long Guo Yun Guo Guan-Zhong Lu Li Wang Wang-Cheng Zhan

Key Laboratory for Advanced Materials and Research Institute of Industrial Catalysis, East China University of Science and Technology

作者简介：*Wang-Cheng Zhan,e-mail: zhanwc@ecust.edu.cn;

收稿日期：27 October 2017

基金：financially supported by the National Key Research and Development Program of China (No. 2016YFC0204300);the National Natural Science Foundation of China (Nos. 21577034 and 21333003);the Science and Technology Commission of Shanghai Municipality (No. 16ZR1407900);

Catalytic activity of Cu-SSZ-13 prepared with different methods for NH₃-SCR reaction

Meng-Jie Han Yun-Lei Jiao Chun-Hong Zhou Yang-Long Guo Yun Guo Guan-Zhong Lu Li Wang Wang-Cheng Zhan

Key Laboratory for Advanced Materials and Research Institute of Industrial Catalysis, East China University of Science and Technology

Abstract：

A series of Cu-SSZ-13 catalysts with the same Cu loading were prepared by different methods of incipient wetness impregnation [Cu-SSZ-13(IWI)], ion exchange[Cu-SSZ-13(IE)] and hydro-thermal synthesis [Cu-SSZ-13(HTS)]. Their activity for selective catalytic reduction of nitrogen oxides(NO_x) with NH3 was determined. The results show that the Cu-SSZ-13(HTS) catalyst exhibits a better ammonia selective catalytic reduction(NH3-SCR)activity compared with the other two catalysts, over which more than 90% NO conversion is obtained at 215-600℃under the space velocity of 180,000 h^-1. The characterization results reveal that the Cu-SSZ-13(HTS) catalyst possesses more amount of stable Cu²⁺ in the six-membered ring and high ability for NH3 and NO adsorption, leading to its high NH3-SCR activity, although this catalyst has low surface area. On the other hand, the activity of Cu-SSZ-13(IE) catalyst is almost the same as that of Cu-SSZ-13(IWI) catalyst at the temperature lower than 400 ℃, but the activity of the former is much higher than that of the latter at > 400 ℃ due to the high activity of Cu-SSZ-13(IWI) catalyst for NH3 oxidation.

Keyword：

SSZ-13 zeolite; Selective catalytic reduction; Nitrogen oxides; Preparation method; Cu²⁺;

Received： 27 October 2017

1 Introduction

Nitrogen oxides (NO_x) are one of the main atmospheric pollutants that harm the ecological environment and human health.Controlling and minimizing NO_x emissions,which is a very challenging scientific problem,has become one of the most important subjects in the environmental protection [ 1, 2, 3, 4, 5] .Ammonia selective catalytic reduction (NH₃-SCR)is considered to be one of the most effective methods of removing nitrogen oxides [ 6, 7, 8, 9, 10, 11] .V₂O₅/WO₃/TiO₂ catalyst with high activity,high selectivity and good anti-sulfur performance is early used in SCR after-treatment converters to meet the diesel NO_x emission standards in the world.However,the application of this catalyst is limited to a great extent due to the relatively narrow active window(350-450℃),the poor activity in the low temperature range,the poor stability in the high temperature range and the toxicity of vanadium to human health and the ecoenvironment [ 12, 13, 14, 15] .Modern research indicates that zeolite-supported base metal (e.g.,Cu,Fe) catalysts are also the promising denitration catalysts [ 16, 17, 18, 19, 20] .In generally,Fe-ZSM-5 and Cu-ZSM-5 exhibit high SCR activity in the high temperature range and low temperature range,respectively.However,there are still some serious problems in the industrial application of Fe-ZSM-5 and CuZSM-5 catalysts.Firstly,temperature window in the catalytic activity of Fe-ZSM-5 and Cu-ZSM-5 need to be further extended [ 21, 22, 23] .Secondly,their hydrothermal stability needs to be further increased because their activity always decreases significantly in the presence of water vapor at high temperatures due to the dealumination and the zeolite structure collapse [ 24, 25] .

SSZ-13 is a new type of molecular sieve with the structure of the chabazite.It possesses eight-membered ring elliptical crystal structure which is connected by AlO₄and SiO₄ tetrahedra via orderly arranged oxygen atoms from the head to the tail [ 26] .SSZ-13 has a small pore structure and was expected to be more resistant to hydrocarbon poisoning [ 27] and good thermal stability [ 28] .Furthermore,compared with other zeolites,SSZ-13 can maintain the zeolite structure well and the active species after severe hydrothermal aging,indicating a highly hydrothermal stability [ 29] .These characteristics indicate that SSZ-13 could be a very promising candidate for practical application in NO_x emission control from diesel engines.By now,many methods have been developed to prepare Cu-SSZ-13 with good NH₃-SCR activity,as well as to reduce the cost of Cu-SSZ-13 catalyst.Ren et al. [ 30] designed a one-pot synthesis method of Cu-SSZ-13 catalyst using low-cost copper-tetraethylenepentamine as a novel template.The catalysts exhibited superior catalytic performance on NH₃-SCR reaction due to a high Cu loading and high dispersion of Cu species simultaneously.More recently,He et al.prepared Cu-SSZ-13 samples by a novel one-pot synthesis method,achieving excellent NH₃-SCR performance and high N₂ selectivity from 150 to550℃after ion exchange treatments [ 31] .Meanwhile,many studies have been carried out on the occupied sites of Cu ions in Cu-SSZ-13 and the active sites for SCR reaction.Isolated Cu²⁺species located in the six-membered rings were confirmed to be the active sites in the NH₃-SCR reaction over Cu-SSZ-13 catalyst [ 32, 33] .Although a lot of effort has been made to the improvement of SCR activity and the identification of the active sites of CuSSZ-13 catalysts,the activity needs to be further improved and much more work should be done to investigate this catalyst material in detail before its actual industrial application.

In this paper,Cu-SSZ-13 catalysts were prepared by different methods of ion exchange method,impregnation method and hydrothermal synthesis method,and their catalytic performance in the NH₃-SCR reaction was systemically investigated.Based on the varied characterizations,the relationship among the preparation method,the active site related to Cu species and the catalytic performance for SCR reaction were discussed in detail.

2 Experimental

2.1 Preparation of catalyst

2.1.1 Preparation of H-SSZ-13

H-SSZ-13 was synthesized according to the procedure reported in Refs. [ 34, 35] .The silicon source is sodium silicate (30%),and aluminum source is aluminum sulfate.The concentration of TMAda-OH was 25 wt%.The molar ratio of synthesis gel Al₂O₃:SiO₂:NaO₂:TMAda-OH:H₂O was 1:12:6:2.3:260.In the typical experiment,a certain concentration of sodium hydroxide solution (named as A solution) and aluminum sulfate solution (named as B solution) was prepared firstly.After a certain amount of sodium silicate solution was mixed with A solution and the mixture was stirred for 20 min,the template agent N,N,N-trimethyl-1-adamantyl ammonium hydroxide was added into the mixture.Then,a certain amount of B solution was added,and after stirred for 3 h,the synthesis gel was transferred to a Teflon-lined autoclave and heated to160℃for 4 days.The solid particles produced were collected by filtration,dried overnight at 100℃and calcined in air at 600℃for 6 h.The sample prepared was designated Na-SSZ-13.After Na-SSZ-13 and NH₄N0₃(1 mol-L^-1) were mixed to ion exchange three times at80℃,obtained sample was designated H-SSZ-13.

2.1.2 Cu-SSZ-13 sample prepared by incipient wetness impregnation

The prepared H-SSZ-13 sample was mixed with Cu(N0₃)₂solution,and the mixture was stirred at room temperature for 12 h.Then the solid particles were collected by filtration,dried overnight at 120℃and calcined in air at550℃for 6 h.The sample prepared was labeled as Cu_xSSZ-13(IWI),where x was the Cu content.

2.1.3 Cu-SSZ-13 sample prepared by ion exchange

The prepared H-SSZ-13 sample was mixed with0.08 mol·L^-1 Cu(NO₃)2 solution,and the mixture was stirred in oil bath pan at 80℃.After reacting for 24 h,the solid particles were collected by filtration,washed with deionized water,dried overnight at 120℃and calcined in air at 550℃for 6 h.and then Cu-SSZ-13 powder catalyst was obtained.The sample prepared was labeled as Cu_xSSZ-13(IE),where x was the Cu content.

2.1.4 Cu-SSZ-13 sample prepared by hydrothermal synthesis

The catalyst was synthesized according to a procedure similar to that used for Cu-SSZ-13 as reported by Ren et al. [ 30] .The synthesis gels were adjusted to Al₂O₃:-Si0₂:Na0₂:Cu-TEPA:H₂O mole ratio of 3:36:14.8:4:600,and the dosage of copper nitrate was changed to obtain the catalysts with different copper contents.In the typical experiment,copper nitrate was added to NaAlO₂ solution and stirred for 30 min.Then tetraethylenepentamine(TEPA) was added to the mixture,and after stirred for 30min,a certain amount of NaOH solution and silica sol(30%) was added successively.After stirred for 3 h,the synthesis gel was transferred to a Teflon-lined autoclave and heated to 160℃for 4 days.The original solid samples were collected by filtration.Because Cu content in the initial product was relatively high,an ion exchange method using NH₄NO₃ solution (1 mol·L^-1) was applied to obtain suitable Cu loadings.After ion exchange at 80℃for 6 h 3times,the sample was dried overnight at 120℃and calcined in air at 550℃for 6 h.The sample prepared was labeled as Cu_x-SSZ-13(HTS),where x was the Cu content.

2.2 Characterization of catalyst

X-ray diffraction (XRD) patterns were recorded on a Brook/D8 diffractometer with Cu Kαradiation(λ=0.154056 nm).The N₂ adsorption-desorption isotherms were measured on a Quantachrome NOVA1200surface area and pore size analyzer at-196℃.Prior to measurement,all samples were degassed at 200℃until a stable vacuum of ca.0.665 Pa was reached.Ultravioletvisible (UV-Vis) spectra were recorded on a Varian Cary500 UV-Vis-NIR spectrophotometer at 200-800 nm with BaS0₄ as a reference,and the spectra were converted with the Kubelka-Munk (K-M) function F(R) for comparison.Elemental analysis of the sample was done by inductively coupled plasma-atomic emission spectrometry (ICP-AES,TJA IRIS Advantage 1000).X-ray photoelectron spectroscopy (XPS) spectra were obtained on an ESCALAB250Xi spectrometer with Mg Kαradiation at room temperature under 5.0×10^-8 Pa.Cu 2p binding energy was calibrated with a C 1s band at 284.6 eV from carbon impurity.Peak fitting was done using XPSPEAK 4.1 with a linear background and 20:80 Lorentzian/Gaussian convolution product shapes.

Temperature-programmed reduction of H₂ (H₂-TPR)was carried out on a PX200 apparatus (Tianjin Pengxiang Technology Co.Ltd.) with a thermal conductivity detector(TCD).Fifty milligram sample (250-380μm) was heated from room temperature to 600℃at 10℃·min^-1 in the mixed gas of 5 vol%H₂/N₂ (40 ml·min^-1).The hydrogen consumption was measured quantitatively by TCD.

Temperature-programmed desorption of NH₃ (NH₃-TPD) adsorbed on the catalyst was carried out in a conventional flow system equipped with a TCD.After 0.05 g sample was pretreated at 550℃for 30 min and cooled to room temperature in N₂,it adsorbed NH₃ for 1 h under the mixed gas of 10%NH₃/N₂.After the sample was purged in N₂ for 2 h at 100℃to remove weakly adsorbed NH₃,NH₃-TPD was carried out at 90-700℃in a flow of N₂(30 ml·min^-1) with heating rate of 10℃.min^-1.

Temperature-programmed desorption of NO (NO-TPD)was carried out in a conventional flow system equipped with a NO_x analyzer (Thermo Fisher Model 42i-HL NO-NO_x-chemiluminescence analyzer) as detector.The sample was pretreated in Ar (450 ml·min^-1) at 600℃for 1 h and cooled down to room temperature.Then the sample was exposed to a flow of 500×10^-6 NO/Ar (300 ml·min^-1)for 1 h.After the sample was purged in Ar (300 ml·min^-1)for 1 h,NO-TPD was carried out from room temperature to600℃in a follow of Ar (300 ml·min^-1) with heating rate of 10℃·min^-1.

In the NH₃ oxidation,the reactant gas for NH₃ oxidation was composed of 500×10^-6 NH₃+5%O₂/Ar balanced.Their total flow rate was 300 ml·min^-1 and gas hourly space velocity (GHSV) was about 180,000 h^-1.The concentration of NO and NO₂ in the tail gas was detected by a Thermo Fisher NO-NO_x-chemiluminescence analyzer.

2.3 Catalyst activity testing

The SCR activity of the catalyst was measured in a fixedbed quartz reactor.The Cu-SSZ-13 catalysts were tableted and sieved to 380-830μm.The reactant gas consisted of500×10^-6 NO,500×10^-6 NH₃,5%0₂ and balance Ar.The total flow rate was 300 ml·min^-1 (under ambient condition),and the gas hourly space velocity (GHSV) was180,000 h^-1.The concentrations of NO and NO₂ were continually monitored by a chemiluminescent NO/NO_x analyzer (Thermo-Scientific,Model 42i-HL).NO_x conversion ( ) was calculated as follows:

3 Results and discussion

3.1 Catalyst activity testing

Figure la shows the NH₃-SCR activities of the catalysts prepared by three different methods.Cu content in all the three catalysts is about 2.5%.As shown in Fig.la,Cu_2.4-SSZ-13 (HTS) catalyst exhibits better activity compared with Cu_2.5-SSZ-13 (IWI) and Cu_2.4-SSZ-13 (IE) catalysts,over which more than 90%NO conversion is obtained at215-600℃.Furthermore,Cu_2.4-SSZ-13(HTS) catalyst exhibits a very excellent stability,and the activity can remain unchanged for 50 h,as shown in Fig.lb.As far as Cu_2.5-SSZ-13(IWI) and Cu_2.4-SSZ-13 (IE) catalysts are concerned,the situation is different at different temperature windows for NH₃-SCR reaction.The activity of Cu_2.5-SSZ-13(IE) catalyst is almost the same as that of Cu_2.4-SSZ-13 (IWI) catalyst at<400℃,but there is much difference in the activity of two catalysts at>400℃.The activity of Cu_2.5-SSZ-13(IE) catalyst is much higher than that of Cu_2.4-SSZ-13 (IWI) catalyst at>400℃.

Fig.1 a NO_x conversion as a function of reaction temperature in NH₃-SCR reaction over Cu_2.5-SSZ-13(IWI),Cu_2.4-SSZ-13(IE) and Cu_2.4-SSZ-13(HTS) catalysts;b stability of Cu_2,4-SSZ-13(HTS) catalyst for NH₃-SCR reaction at 350℃(reaction conditions:reactant gas500×10^-6 NO/500×10^-6 NH₃/5%O₂/Ar bal.,GHSV=180,000 h^-1)

In order to highlight the advantages of Cu_2.4-SSZ-13(HTS) catalyst in activity,the comparison of the catalytic activity of Cu_2.4-SSZ-13(HTS) catalyst with those reported in the literature is listed in Table 1.It can be found that operation temperature for achieving>80%NO_x conversion over Cu_2.4-SSZ-13(HTS) catalyst,which is similar to one-pot synthesized Cu_3.8-SSZ-13 [ 31] ,is much wider than that over Cu-modified zeolite catalysts in the references,such as Cu-Beta [ 29] ,Cu-SAPO-34 [ 35] and Cu-ZSM-5 [ 36] catalysts.These results reveal that Cu_2.4-SSZ-13(HTS) catalyst prepared has an excellent SCR activity.

Since Cu content in these three catalysts is the same,the following characterizations were aimed at the nature of Cu in Cu-SSZ-13 catalysts prepared with different methods and the relationship among the preparation method;the nature of Cu species and the catalytic performance for NH₃-SCR reaction were discussed in detail.

3.2 Structure characterization of catalysts

3.2.1 XRD

XRD patterns of the three samples prepared with different methods are shown in Fig.2.All the Cu-SSZ-13 catalysts only exhibit the diffraction peaks of H-SSZ-13,indicating that all catalysts can maintain chabazite (CHA) characteristic structure,no matter what preparation methods are [ 37] .Furthermore,the diffraction peaks for CuO(2θ=35.6°and 38.8°) and Cu₂O (2θ=36.4°) are not observed in XRD patterns of the three samples,indicating that Cu species in the catalysts are highly dispersed on the surface of molecular sieve,or the amount of CuO_x crystal is very low and beyond the determination limit [ 38] .

3.2.2 N2 adsorption-desorption

Figure 3 shows N₂ adsorption-desorption isotherms (inset is pore size distribution) of Cu-SSZ-13 catalysts prepared with different methods.The results of the specific surface areas,pore volumes and pore size are summarized in Table 2.Compared with Cu_2.5-SSZ-13(IWI) and Cu_2.4-SSZ-13(IE) catalysts,the surface area of Cu_2.4-SSZ-13(HTS) catalyst decreases from 433 (458) to 353 m²·g^-1.This decrease may be attributed to the presence of CuO,which can block the pores of SSZ-13 zeolite.Another reason may be the case that the presence of Cu ion in the synthesis gel influences the construction of SSZ-13 structure.According to the following characterizations,such as UV-Vis spectra,the former can be excluded.On the contrary,there is no significant difference in the pore volume and the pore size of the three catalysts.

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Table 1 Comparison of catalytic activity of Cu_2.4-SSZ-13(HTS) catalyst prepared with those reported in literatures

Fig.2 XRD patterns of Cu_2.5-SSZ-13(IWI),Cu_2.4-SSZ-13(IE),Cu_2.4-SSZ-13(HTS) and H-SSZ-13 catalysts

Fig.3 N₂ adsorption-desorption isotherms (inset:pore size distribu-tion,where V being incremental pore volume,w being pore size) of Cu-SSZ-13 catalysts prepared with different methods

3.2.3 UV-Vis spectra

Figure 4 shows UV-Vis spectra of the catalysts prepared with different methods.All catalysts have two different bands:a narrow one centered at 215 nm and a broad band between 550 and 800 nm.The former can be attributed to the charge transfer band related to O→Cu transition from lattice oxygen to isolated Cu²⁺ions [ 33, 39, 40, 41, 42] The adsorption band between 550 and 800 nm is attributed to d-d transition of isolated Cu²⁺ [ 40, 43] .In addition,a new band at 250 nm is also found in the spectra of Cu_2.5-SSZ-13(IWI) and Cu_2.4-SSZ-13(IE) catalysts,attributed to the presence of CuO species [ 42] .Compared with Cu_2.4-SSZ-13(IE) catalyst,the intensity of this band is higher in the spectra of Cu_2.5-SSZ-13 (IWI) catalyst,accompanied by a decrease in the charge transfer band of the isolated Cu²⁺.

3.2.4 XPS

Figure 5 shows XPS spectra of the catalysts prepared by different methods.Two peaks centered at 933.6 and953.5 eV are observed in the spectra of all samples,which are assigned to Cu 2p3/2 and 2p_1/2,respectively.In addition,a satellite peak of Cu 2p_3/2 is located at 944.6 eV,indicating that the valence of Cu in Cu-SSZ-13 catalysts is present as+2 [ 44, 45, 46] .Cu 2p3/2 peaks in Fig.5 can be deconvoluted with a linear background and 20:80 of Lorentzian/Gaussian convolution product shape ratio.It has been reported that the peaks below 933.0 eV belong to metallic copper (Cu⁰) and Cu₂O,whereas the peaks above933.0 eV belong to different Cu²⁺ [ 44, 47, 48, 49, 50] .The peaks at 933.4 and 936.2 eV of Cu 2p3/2 are assigned to CuO species and Cu²⁺coordinated to superficial oxygen atoms of the zeolite (Cu-O-Si-O),respectively [ 44, 47, 48, 49, 50, 51] .The relative ratio between the amount of Cu²⁺ions and CuOspecies are calculated according to the areas of the deconvolution peaks,as shown in Table 3.It can be found that Cu_2.4-SSZ-13(HTS) catalyst has a higher Cu²⁺/CuOratio on catalyst surface,which is beneficial to its high SCRactivity of Cu_2.4-SSZ-13(HTS) catalyst.In addition,the Cu²⁺/CuO ratio for Cu_2.5-SSZ-13(IWI) catalyst is slightly lower than that for Cu_2.4-SSZ-13 (IE) catalyst,which coincides well with UV-Vis results.

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Table 2 Physicochemical properties of Cu-SSZ-13

^a Detected by ICP analysis ^b Cu²⁺/CuO ratio on surface of different Cu-SSZ-13 catalysts from XPS results

Fig.4 UV-Vis spectra of Cu_2.5-SSZ-13(IWI),Cu_2.4-SSZ-13(IE)and Cu_2.4-SSZ-13(HTS) catalysts

Fig.5 XPS spectra of Cu 2P of a Cu_2.4-SSZ-13(HTS),b Cu_2.4-SSZ-13(IE) and c Cu_2.5-SSZ-13(IWI) catalysts

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Table 3 Cu²⁺/CuO ratio on surface of different Cu-SSZ-13 catalysts from XPS results

3.3 H2-TPR

Figure 6 shows H₂-TPR profiles of Cu_2.5-SSZ-13(IWI),Cu_2.4-SSZ-13(IE) and Cu_2.4-SSZ-13(HTS) catalysts.All catalysts exhibit two reduction peaks in the temperature range of 100-600℃,i.e.,at 237 and 363℃for Cu_2.5-SSZ-13 (IWI),240 and 350℃for Cu_2.4-SSZ-13(IE),and230 and 363℃for Cu_2.4-SSZ-13(HTS).It has been reported that the reduction peak at the temperature below500℃is assigned to the reduction of isolated Cu²⁺to Cu⁺or highly dispersed CuO species to Cu⁰,while the reduction of Cu⁺to Cu⁰ generally occurred at the temperature of>500℃ [ 31, 52, 53] .Based on UV-Vis spectra and XPS results,the low-temperature peaks at 230,237 and 240℃can be attributed to the reduction of unstable Cu²⁺inside the large cages of CHA structure to Cu⁺and highly dispersed CuO species to Cu⁰,and the peaks at 363 and350℃can be attributed to the reduction of stable Cu²⁺in the six-membered ring to Cu⁺ [ 38, 54, 55] .It can be found that the areas of the corresponding reduction peaks are much different for Cu_2.5-SSZ-13 (IWI),Cu_2.4-SSZ-13(IE)and Cu_2.4-SSZ-13(HTS) catalysts.The order of the areas of the reduction peaks at low temperature is Cu_2.5-SSZ-13(IWI)>Cu_2.4-SSZ-13(HTS)>Cu_2.4-SSZ-13(IE),but that at high temperature is Cu_2.4-SSZ-13 (HTS)>Cu_2.4-SSZ-13(IE)>Cu_2.5-SSZ-13 (IWI).Since Cu²⁺located in six-membered ring of the CHA structure is the active center of catalyst [ 35, 56, 57] ,more Cu²⁺in the sixmembered ring of Cu_2.4-SSZ-13 (HTS) would lead to its higher SCR activity compared with Cu_2.4-SSZ-13(IE) and Cu_2.5-S SZ-13(IWI) catalysts.

Fig.6 H₂-TPR profiles of Cu_2.5-SSZ-13(IWI),Cu_2.4-SSZ-13(IE)and Cu_2.4-SSZ-13(HTS) catalysts

3.4 NH3-TPD and NO-TPD

The surface acidity of the catalyst plays an important role in SCR reaction [ 58, 59] .Acid site of molecular sieve is essential to the adsorption and activation of reducing agent of NH₃,no matter Langmuir-Hinshelwood (LH) or EleyRideal (ER) mechanism [ 60] .NH₃-TPD profiles of CuSSZ-13 and H-SSZ-13 catalysts are shown in Fig.7.H-SSZ-13 catalyst exhibits two desorption peaks at 198 and530℃,which can be attributed to the desorption of ammonia species from weak acid (Lewis acid) sites and strong acid (Br nsted acid) sites,respectively [ 51, 56, 61] .

With the introduction of Cu in H-SSZ-13,the desorption peak assigned to the Br nsted acid sites obviously decreases because of the exchange of the Br nsted acid sites (H⁺) in H-SSZ-13 with Cu²⁺.On the contrary,the desorption peak assigned to the weak Lewis acid sites does not significantly change.In addition,compared with H-SSZ-13 catalyst,Cu-SSZ-13 catalyst displays one more desorption peak at about 350℃,which can be attributed to NH₃ adsorbed on the strong Lewis acid adsorption arising from Cu species in the molecular sieve,such as Cu²⁺located in six-membered ring and the large cages of the CHA structure [ 55, 56, 62, 63] .Furthermore,the intensity of the desorption peak at 350℃for Cu_2.4-SSZ-13(HTS)catalyst is stronger than that for Cu_2.5-SSZ-13(IWI) and Cu_2.4-SSZ-13(IE) catalysts.The areas of the desorption peak at 350℃are 2208,2270 and 2630 for Cu-SSZ-13(IWI),Cu-SSZ-13(IE) and Cu-SSZ-13(HTS) catalysts,respectively.On the whole,Cu-SSZ-13(HTS) catalyst has higher ability for NH₃ adsorption than others,which is beneficial for the activation of NH₃ and the improvement in SCR activity.

Fig.7 NH₃-TPD profiles of Cu_2.5-SSZ-13(IWI),Cu_2.4-SSZ-13(IE),Cu_2.4-SSZ-13(HTS) and H-SSZ-13 catalysts

Fig.8 NO-TPD profiles of Cu_2.5-SSZ-13(IWI),Cu_2.4-SSZ-13(IE)and Cu_2.4-SSZ-13 (HTS) catalysts

Figure 8 shows NO-TPD profiles of different Cu-SSZ-13 catalysts.There are two desorption peaks at 100 and350℃for Cu_2.5-SSZ-13(IWI) catalyst,and desorption peaks at 125 and 360℃for Cu_2.4-SSZ-13(IE) catalyst.Besides two main desorption peaks at 80 and 365℃Cu_2.4-SSZ-13(HTS) catalyst exhibits a wide shoulder peak at 285℃.According to Refs [ 64, 65, 66] ,the desorption peak at～100℃can be assigned to the physical adsorption of NO,while the desorption peaks at 285,360 and 365℃can be assigned to the decomposition of nitrate species that come from chemical adsorption of NO.Compared with Cu_2.5-S SZ-13(IWI) and Cu_2.4-S SZ-13 (IE) catalysts,Cu_2.4-SSZ-13(HTS) catalyst shows higher area of the peak assigned to the decomposition of nitrate species,indicating that Cu_2.4-SSZ-13(HTS) catalyst can adsorb more NO to form nitrate species to participate in SCR reaction.As a result,the Cu_2.4-SSZ-13(HTS) catalyst displays better SCR activity compared with other two catalysts.

Fig.9 NO_x concentration for separate NH₃ oxidation reaction of Cu_2.5-SSZ-13(IWI),Cu_2.4-SSZ-13(IE) and Cu_2.4-SSZ-13(HTS)(reaction conditions:0.1 ml catalyst,reactant gas 500×10^-6 NH₃/5%O₂/Ar bal.,GHSV=180,000 h^-1)

3.5 NH3 oxidation

Figure 9 shows the activity of different Cu-SSZ-13 catalysts for NH₃ oxidation.It can be found that the activities of all catalysts for NH₃ oxidation are very low at<400℃.However,ammonia oxidation becomes obvious at>400℃,and NO_x concentration markedly increases with the increase in reaction temperature.Among the prepared catalysts,Cu_2.5-SSZ-13(IWI) catalyst exhibits the highest activity for NH₃ oxidation,while Cu_2.4-SSZ-13(HTS) and Cu_2.4-SSZ-13(IE) catalysts display the similar activity for NH₃ oxidation,which is identical with their SCR activity at high temperature.

3.6 Discussion

Three styles of Cu-doped SSZ-13 catalysts were prepared with incipient wetness impregnation,ion exchange and hydrothermal synthesis,respectively.It is interesting that Cu_2.4-SSZ-13 (HTS) catalyst exhibits better activity for NH₃-SCR reaction compared with Cu_2.5-SSZ-13 (IWI) and Cu_2.4-SSZ-13 (IE) catalysts,over which more than 90%NO conversion is obtained at 215-600℃.However,the activity of Cu_2.5-SSZ-13(IE) catalyst is almost the same as that of Cu_2.4-SSZ-13 (IWI) catalyst at the temperature lower than 400℃C.On the contrary,the activity of Cu_2.5-SSZ-13(IE) catalyst is much higher than that of Cu_2.4-SSZ-13 (IWI) catalyst at the temperature higher than 400℃.Because Cu content in three catalysts is almost the same(2.4 wt%),it can be deduced that the different activities of the three catalysts prepared with different methods are attributed to the nature of Cu in the Cu-SSZ-13 catalysts.

XRD results show that all the Cu-SSZ-13 catalysts can maintain CHA characteristic structure and Cu species are highly dispersed on the surface of the catalysts,or the amount of CuO_x crystal is very low and beyond the determination limit.Meanwhile,N₂ adsorption-desorption isotherms reveal that the surface area of Cu_2.4-SSZ-13(HTS) catalyst is lower than that of Cu_2.5-SSZ-13(IWI)and Cu_2.4-SSZ-13(IE) catalysts,due to the influence of Cu in the synthesis gel in the construction of the SSZ-13structure.UV-Vis Spectra and XPS results indicate that the valence of Cu in Cu-SSZ-13 catalysts is present as+2 and exists as Cu²⁺coordinated to superficial oxygen atoms of the zeolite (Cu-O-Si-O) and dispersed CuO species in the extra-framework position.Furthermore,Cu_2.4-SSZ-13(HTS) catalyst has a much higher Cu²⁺/CuO ratio on the catalyst surface compared with Cu_2.5-SSZ-13(IWI) and Cu_2.4-SSZ-13(IE) catalysts,while Cu_2.5-SSZ-13(IWI)catalyst has a high amount of CuO species.H₂-TPR results further illustrate that there are mainly three styles of Cu species in Cu-SSZ-13 catalysts,which are unstable Cu²⁺inside the large cages of CHA structure (S1),highly dispersed CuO species (S2) and stable Cu²⁺in the sixmembered ring (S3).It can be found that the total content of the former two styles of Cu species (S1+S2) is Cu_2.5-SSZ-13(IWI)>Cu_2.4-SSZ-13(HTS)>Cu_2.4-SSZ-13 (IE),but the content of stable Cu²⁺in the six-membered ring(S3) is Cu_2.4-SSZ-13(HTS)>Cu_2.4-SSZ-13(IE)>Cu_2.5-SSZ-13(IWI).

The previous researches proposed that there are many possible Cu²⁺locations in SSZ-13 zeolite and predicted the SCR activity based on the content of Cu species in different sites [ 31, 63, 67, 68] .Unfortunately,there is no solid reason to assume that certain Cu species are less than the others,except for the relatively high activity on Cu²⁺located in sixmembered ring of CHA structure and relatively low activity on highly dispersed CuO species.In this research,it is impossible to accurately quantify the content of Cu²⁺located in different sites of the CHA structure.Therefore,the relative contents of S1,S2 and S3 were carefully compared for different catalysts based on the results from UV-Vis spectra,XPS and H₂-TPR.Subsequently,the relationship between the nature of Cu species and the catalytic performance for NH₃-SCR reaction was discussed in detail.

It has been suggested that NH₃-SCR primarily occurs on copper sites in the six-membered rings at low temperature(～400℃) [ 35, 56, 57] .Therefore,more Cu²⁺located in six-membered ring of Cu_2.4-SSZ-13(HTS) would lead to its higher SCR activity at low temperature compared with Cu_2.4-SSZ-13(IE) and Cu_2.5-SSZ-13 (IWI) catalysts [ 35, 56, 57] .As for Cu_2.4-SSZ-13(IE) and Cu_2.5-SSZ-13(IWI) catalysts,although the content of Cu²⁺located in six-membered ring of Cu_2.4-SSZ-13(IE) catalyst is higher than that for Cu_2.5-SSZ-13(IWI) catalysts (Fig.6),the content of S1 may be reverse for these two catalysts since they possess similar Cu²⁺/CuO ratio.Consequently,the activity of Cu_2.5-SSZ-13(IE) catalyst is almost the same as that of Cu_2.4-SSZ-13 (IWI) catalyst at<400℃.

NH₃-TPD and NO-TPD results show that Cu-SSZ-13(HTS) catalyst has higher ability for NH₃ and NO adsorption than Cu_2.4-SSZ-13(IE) and Cu_2.5-SSZ-13 (IWI)catalysts.However,Cu_2.4-SSZ-13(IE) and Cu_2.5-SSZ-13(IWI) catalysts exhibit similar ability for NH₃ and NO adsorption.Although the reason for the different adsorption abilities is ambiguous,it is unequivocal that the high ability for NH₃ and NO adsorption is beneficial for the increase in SCR activity [ 10, 69] .Therefore,NH₃-TPD and NO-TPD results are consistent with the high NH₃-SCR activity for Cu-SSZ-13(HTS) catalyst.

As shown in Fig.la,the activity of all catalysts for NH₃-SCR reaction decreases at the reaction temperature higher than 400℃.The SCR activity of Cu_2.5-SSZ-13(IWI) catalyst is much lower than that of Cu-SSZ-13(HTS) and Cu_2.4-SSZ-13(IE) catalysts.However,the SCR activity of the latter two catalysts is very close.It is well known that the reason for the decrease in SCR activity at high temperature is the aggravation of non-selective oxidation of NH₃.In the experiment,as shown in Fig.9,Cu_2.5-SSZ-13(IWI) catalyst exhibits the highest activity for NH₃ oxidation,while Cu_2.4-SSZ-13(HTS) and Cu_2.4-SSZ-13(IE) catalysts display similar activity for NH₃ oxidation.These results are consistent with their different activities for NH₃-SCR at high temperature.It is well known that CuO species are the primary active sites for NH₃ oxidation at high temperature [ 63] .The characterization results from UV-Vis spectra,XPS and H₂-TPR reveal that less CuO species are present on Cu_2.4-SSZ-13(HTS) catalyst compared with Cu_2.5-SSZ-13(IWI) and Cu_2.4-SSZ-13(IE) catalysts,leading to its low activity for NH₃ oxidation at high temperature.On the contrary,Cu_2.5-SSZ-13(IWI) catalyst possesses a high amount of CuO species,leading to its high activity for NH₃ oxidation.

4 Conclusion

Three different kinds of Cu-SSZ-13 catalysts with the same amount of Cu loading were prepared by incipient wetness impregnation,ion exchange and hydrothermal synthesis,respectively.Cu_2.4-SSZ-13(HTS) catalyst displays better NH₃-SCR activity compared with Cu_2.5-SSZ-13 (IWI) and Cu_2.4-SSZ-13(IE) catalysts.Cu_2.5-SSZ-13(IWI) and Cu_2.4-SSZ-13(IE) catalysts show the similar reaction activity at<400℃,whereas the activity of the former is much lower than that of the latter at>400℃.According to the varied characterizations,the relationship among the preparation method,the nature of Cu species and the catalytic performance for NH₃-SCR reaction were discussed in detail.All Cu-SSZ-13 catalysts keep CHA characteristic structure,and Cu species are highly dispersed on the surface of the catalysts,which can be classified to unstable Cu²⁺inside the large cages of the CHA structure(S1),highly dispersed CuO species (S2) and stable Cu²⁺in the six-membered ring (S3).Cu_2.4-SSZ-13(HTS) catalyst possesses more isolated Cu²⁺in six-membered ring of CHA structure,on which NH₃-SCR primarily occurs at low temperature (about 400℃).In addition,Cu_2.4-SSZ-13(HTS) catalyst has higher ability for NH₃ and NO adsorption than Cu_2.4-SSZ-13(IE) and Cu_2.5-SSZ-13(IWI)catalysts.Together these factors determine higher activity of Cu_2.4-SSZ-13(HTS) catalyst than that of other two catalysts.On the other hand,Cu_2.4-SSZ-13(IE) and Cu_2.5-SSZ-13(IWI) catalysts possess the close ratio of(S1+S3)/S2,leading to their similar NH₃-SCR activity at<400℃.However,Cu_2.5-SSZ-13(IWI) catalyst exhibits high activity for NH₃ oxidation at high temperature due to high amount of CuO species,resulting in its low NH₃-SCR activity at>400℃.

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