NH3-SCR反应过程中NH3和NOx在Cu_SAPO-34分子筛催化剂上的吸附特性和作用

NH 3-SCR 反应过程中NH 3和NO x 在Cu/SAPO-34分子筛催化剂上的

吸附特性和作用

石

琳1

于

铁1王欣全1王军1沈美庆1,2,*

(1天津大学化工学院,绿色合成与转化教育部重点实验室,天津300072;

2

天津大学内燃机国家重点实验室,天津300072)

摘要:通过离子交换法制得Cu/SAPO-34菱沸石分子筛催化剂,同时研究了NH 3和NO x (NO 和NO 2)在该催化

剂上的吸附位、吸附强度、吸附量和吸附速率,得到了不同反应气氛在Cu/SAPO-34上的吸附性能及其在NH 3选择性催化还原(NH 3-SCR)反应中的作用.研究采用瞬态实验、程序升温脱附(TPD)和漫反射傅里叶变换红外光谱(DRIFTS)等方法进行表征实验.瞬态实验结果表明NH 3是吸附性气体.程序升温脱附实验和红外漫反射实验结果表明NH 3可以吸附在布朗斯特和路易斯酸性位上形成不同的NH 3物种,它们显示不同的SCR 活性.NH 3在Cu 2+上的吸附速率最快,且键强最强.NO x 可以氧化并以硝酸盐/亚硝酸盐的形式吸附在Cu 物种上.最后,本文讨论了NH 3选择性催化还原反应过程中在Cu 物种上的中间物种并推测反应机理.关键词:

NH 3选择性催化还原;NO x (NO 和NO 2);Cu/SAPO-34;漫反射傅里叶变换红外光谱;吸附物种

中图分类号:

O643

Properties and Roles of Adsorbed NH 3and NO x over Cu/SAPO-34

Zeolite Catalyst in NH 3-SCR Process

SHI Lin 1

YU Tie 1

WANG Xin-Quan 1

WANG Jun 1

SHEN Mei-Qing 1,2,*

(1Key Laboratory for Green Chemical Technology of State Education Ministry,School of Chemical Engineering &Technology,Tianjin University,Tianjin 300072,P .R.China ;2State Key Laboratory of Engines,Tianjin University,Tianjin 300072,P .R.China )

Abstract:To investigate the adsorption properties and roles of different feed gases in the selective catalytic reduction by ammonia (NH 3-SCR),the adsorption sites,strength,and amount as well as reaction rates of NH 3and NO x (a mixture of NO and NO 2)on exchanged Cu/SAPO-34(chabazite zeolite)catalyst were studied.Transient response,temperature programmed desorption (TPD),and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS)experiments were performed to characterize the catalyst.Transient response experiments showed that NH 3was adsorbed by the catalyst.TPD and DRIFTS indicated that NH 3can be adsorbed at both Br ?nsted and Lewis acid sites to form various NH 3adsorption species that show different SCR activities.The adsorption rate of NH 3by Cu 2+cations was the fastest and the adsorption bond strength of NH 3-Cu 2+was the strongest between NH 3and the Cu/SAPO-34catalyst.NO x can be oxidized and stored as nitrates and nitrites on the Cu catalyst.The intermediate species formed at Cu active sites during the NH 3-SCR reaction are discussed,allowing SCR reaction mechanisms to be inferred.

Key Words:Ammonia selective catalytic reduction;NO x (NO and NO 2);Cu/SAPO-34;

Diffuse

reflectance infrared Fourier transform spectrum;Adsorption species

[Article]

doi:10.3866/PKU.WHXB201304283

https://www.360docs.net/doc/614605106.html,

物理化学学报(Wuli Huaxue Xuebao )

Acta Phys.-Chim.Sin .2013,29(7),1550-1557

July

Received:February 15,2013;Revised:April 27,2013;Published on Web:April 28,2013.?

Corresponding author.Email:mqshen@https://www.360docs.net/doc/614605106.html,;Tel:+86-22-27892301.

The project was supported by the National High-Tech Research and Development Program of China (863)(2011AA03A405).国家高技术研究发展计划项目(863)(2011AA03A405)资助

?Editorial office of Acta Physico-Chimica Sinica

1550

SHI Lin et al.:Properties and Roles of Adsorbed NH3and NO x over Cu/SAPO-34Zeolite Catalyst in NH3-SCR Process No.7

1Introduction

Selective catalytic reduction with ammonia(NH3-SCR)or with hydrocarbons(HC-SCR)has been extensively studied for lean NO x(NO and NO2)control.Due to the high decrease NO x efficiency,NH3-SCR has been mainly used for NO x control in stationary sources1and diesel vehicle emissions.2-9Many re-searches have reported that transition metal ion exchanged zeo-lite catalysts,such as Cu/Y,Cu/ZSM-5,and Cu/beta,show su-perior activity and N2selectivity.10-12However the narrow activ-ity window and hydrothermal deterioration have restricted their commercial applications.13,14

Recently,many researches have reported that Cu2+ion-exchanged SAPO-34(Cu/SAPO-34)shows high NO x conver-sion,N2selectivity(φ(N2O)<1×10-5(volume fraction)),and ex-cellent hydrothermal stability in NH3-SCR.15,16And,it has been reported that the Cu2+species are the active sites for the NH3-SCR reaction.17-19However,the NH3and NO x adsorption behav-iors on Cu/SAPO-34have been less researched.

In this report,the adsorption properties of NH3and NO x in the NH3-SCR reaction were investigated.The adsorption sites, amounts,strength were tested through transient response exper-iments,temperature programmed desorption(TPD),and dif-fuse reflectance infrared Fourier transform spectra(DRIFTS). The intermediate species,activity sites,and reaction mecha-nism of NH3-SCR were discussed.

2Experimental

2.1Catalyst preparation

Cu/SAPO-34catalyst was prepared by liquid-phase ion-ex-change method using H-SAPO-34,which was supplied by Tianjin Chemist Scientific Limited.The procedure contains two steps:NH+4species exchange and Cu2+species exchange. The detailed procedures were described in our previous study.20 Cu loading was2.25%(w)tested by inductively coupled plas-ma and atomic emission spectrometry(ICP-AES).In addition, it was not seen the diffraction peaks of copper species(CuO, Cu2O)in XRD patterns of Cu/SAPO-34,18,20meaning that cop-per was well-dispersed or existed as isolated ions at the ex-change sites.

2.2Transient response experiment

Transient response experiments were tested in a stainless steel reactor using0.5g catalysts mixed with1.5g quartz at 300°C.The catalyst was sealed in the tube with quartz wool. The temperature was obtained by a type K thermocouple insert-ed into the center of the powder catalyst.The inlet composition was controlled by mass flow controllers.FTIR-Nicolet IS10 was used to measure the concentration of NO,NO2,N2O,and NH3.Prior to all the following experiments,the samples were pretreated at500°C in10%(φ)O2for30min to remove ad-sorbed H2O or other gases.After the sample was cooled to 300°C in N2,4×10-4(φ)NO(or NH3)and10%(φ)O2were in-troduced in until the concentration got stabilization.Then NH3 (or NO)was cut in an instant.After the SCR reaction reached equilibrium,NH3(or NO)was turned off.The gas flow and the space velocity in all experiments were controlled at1000mL·min-1and35000h-1,respectively.

2.3TPD

Temperature programmed desorption(TPD)experiments were performed in a stainless steel reactor.Firstly,5×10-4(φ) NH3(or NO or NO+10%(φ)O2)/N2was adsorbed until the out-let NH3(or NO or NO+10%(φ)O2)concentration was stable. Then,the catalysts were purged with N2to remove any weakly adsorbed species until NH3(or NO or NO+10%(φ)O2)concen-tration was lower than1×10-5(φ).At last,the catalysts were heated from30to600°C at a ramping rate of10°C·min-1. The total flow rate was1000mL·min-1and the balance gas was N2.

2.4DRIFTS

Diffuse reflectance infrared Fourier transform spectra (DRIFTS)were performed on Nicolet6700FTIR equipped with a MCT detector at a resolution of1cm-1.Ten scans were operated for each spectrum.The total flow rate was100mL·min-1.Before all the experiments,the samples were pretreated with10%(φ)O2in He at500°C for30min and the back-ground spectra were collected under He at the test temperature. For the adsorption experiments,the samples were exposed to4×10-4(φ)of NH3or NO or NO+10%(φ)O2for1h at30, 100,160,and300°C.

For the surface SCR experiments,the catalyst was first purged under4×10-4(φ)NH3(or NO+10%(φ)O2)till the spec-tra were stable.Then,the catalyst was exposed to4×10-4(φ) NO(or NH3)and10%(φ)O2until the stabilization.The experi-ments were performed at100,160,and300°C.

3Results and discussion

3.1NH3adsorbed sites and species

The result of NH3transient response over Cu/SAPO-34is shown in Fig.1.Upon the NH3step addition,the ammonia out-let concentration slowly increases with time,approaching to steady state.However NO concentration fast declines.Upon the ammonia shut-off,the outlet NH3concentration slowly de-creases with time due to the desorption of previously adsorbed

Fig.1Curves of NH3transient response experiment at300°C

over

Cu/SAPO-34

Acta Phys.-Chim.Sin.2013Vol.29

ammonia.The NO concentration slowly increases to the maxi-mum due to the reaction between NO and adsorbed NH 3spe-cies.This clearly indicates that ammonia is involved in adsorp-tion-desorption processes at the catalyst surface.Due to exis-tence of some NH 3oxidation or other reactions at 300°C,the

NH 3concentration could not reach 4×10-4(φ)and has larger de-crease than NO.

NH 3-TPD profiles over Cu/SAPO-34and H-SAPO-34are shown in Fig.2.The TPD profiles show three NH 3desorption peaks centered at 100,250,and 350°C.The total amount of NH 3storage is calculated based on the time integration of NH 3-TPD curves and the results are listed in Table 1.For Cu/SAPO-34,the amount of desorption NH 3at high temperature (HT)and low temperature (LT)is less than that of H-SAPO-34due to the occupation of acid sites in SAPO-34by Cu species.

NH 3has been extensively used as a probe molecule to identi-fy the types of surface acidic sites.Fig.3illustrates the DRIFT spectra of H-SAPO-34and Cu/SAPO-34for NH 3adsorption.As shown in Fig.3A,three distinct regions at 3800-3100,1700-1400,and 1000-800cm -1are appeared,which are as-signed to NH 3species adsorbed on Br ?nsted acid sites and Lewis acid sites over Cu/SAPO-34.The negative peaks at 3737,3670,and 3605cm -1are assigned to the OH vibrations of P-OH,Si-OH,and Al-OH-Si,respectively.19The strong band at 1489cm -1is related to asymmetric vibration of NH +4groups.In addition,four kinds of adsorbed NH 3species on Lewis acid are observed.According to the literature,21-28the peak at 1630cm -1is attributed to N-H deformation,and the bands at 3356,

3275cm -1are assigned as molecule adsorption.The band at 3179cm -1is attributed to NH 3-Cu +species on the exchanged sites.The bands at 899and 858cm -1are related to internal asymmetric framework vibrations perturbed by copper cations to form NH 3-Cu 2+.It should be noted that the intensities of peaks at 899and 858cm -1(NH 3-Cu 2+)reach steady after 1min over Cu/SAPO-34,however other peak intensities reach steady after 10min.It is inferred that NH 3can be adsorbed on Cu 2+cations easily and fast.As shown in Fig.3B,H-SAPO-34has the same NH 3adsorption species as Cu/SAPO-34.However,the intensities of total peaks are higher than that of Cu/SAPO-34and the NH 3species on Cu species are not appeared.As Lewis acid sites,the introduction of Cu cations increases two new types of NH 3adsorption sites (Cu +and Cu 2+species)and reduces the Br ?nsted acid sites.

Fig.4presents the results of NH 3adsorption under 30,100,160,and 300°C over Cu/SAPO-34.It is seen that the peak inten-sities of NH 3adsorption on Br ?nsted acid sites (3800-3500and 1489cm -1)begin to reduce at 100°C and all other peak intensi-ties start to reduce at higher temperature.Whereas those of NH 3-Cu 2+(899,858cm -1)start to reduce at 300°C.It indicates that the bond strengths of NH 3adsorption on Br ?nsted acid sites are

Fig.2NH 3-TPD profiles over Cu/SAPO-34(A)and H-SAPO-34

(B)

3DRIFT spectra of NH 3adsorption over Cu/SAPO-34(A)and H-SAPO-34(B)at 30°C for 0,1,2,5,10

min

Table 1

Quantitative analyses of NH 3-TPD over H-SAPO-34and

1552

SHI Lin et al .:Properties and Roles of Adsorbed NH 3and NO x over Cu/SAPO-34Zeolite Catalyst in NH 3-SCR Process No.7weaker than those of NH 3-Cu 2+species.For H-SAPO-34,with

the temperature increasing,most peak intensities are decrease.However,the intensity of peak at 3605cm -1has some increase and still some NH 3molecules are adsorbed over H-SAPO-34.It is in accordance with the phenomena in Fig.2.3.2NO x adsorption sites and species 3.2.1NO adsorption sites and species

Fig.5illustrates the NO transient response experiment over Cu/https://www.360docs.net/doc/614605106.html,pared with the NH 3transient response ex-periment,the NO response is quick due to the fact that NO is hardly involved in adsorption-desorption processes.

NO-TPD profiles over Cu/SAPO-34and H-SAPO-34are shown in Fig.6.There is only NO 2detected.In addition,the de-sorption peaks of NO 2show two distinct peaks at 80and 350°C,

respectively.The amounts of NO 2desorption are calculated based on the time integration of TPD results.

Quantitative analyses of NO-TPD over H-SAPO-34and Cu/SAPO-34are shown in Table 2,and the total NO 2desorption amounts over H-SAPO-34and Cu/SAPO-34are 0.008122and 0.021952mmol ·g -1,respectively.It confirms that less NO could be adsorbed.

Fig.7illustrates the NO adsorption behaviors over samples.For Cu/SAPO-34,the spectra are mainly divided into two re-

Fig.4DRIFT spectra of NH 3adsorption over Cu/SAPO-34(A)and H-SAPO-34(B)at different

temperatures

5

Curves of NO transient response experiment at 300°C

over

Cu/SAPO-34

6NO-TPD profiles over Cu/SAPO-34and

H-SAPO-34

Table 2

Quantitative analyses of NO-TPD over H-SAPO-34and

Fig.7DRIFT spectra of NO adsorption over Cu/SAPO-34(A)

and H-SAPO-34(B)at 30°C for 0,1,2,5,10,30,60min

1553

Acta Phys.-Chim.Sin.2013Vol.29

gions:1200-1300and 1450-1600cm -1.The peaks at 1291,

1280cm -1are assigned to nitrites.The species at 1595,1575,and 1480cm -1are related to chelate nitrates and monodentate nitrates,respectively.29-35In addition,a peak at 1747cm -1ap-pears,which is assigned to Cu +(NO 2)species.The vibration peaks of NO x adsorption sites over Cu/SAPO-34are summa-rized in Table https://www.360docs.net/doc/614605106.html,pared with the spectra of NO adsorption over Cu/SAPO-34,no Cu +(NO 2)peak exists for H-SAPO-34and all peak intensities of adsorbed NO x species are lower (Fig.7B).It implies that Cu 2+cations can promote NO oxidi-zation and storage of NO 2on Cu sites as nitrates/nitrites.36,37Because no O 2exists in the inlet gas,the NO may be oxidized by the oxygen species ([O]x -species)in samples.1,38Combining the results of DRIFTS and TPD,it can be inferred that Cu species are the active sites for NO oxidation and the amount of stored nitrates/nitrites over catalyst is more than that on H-SAPO-34.

Fig.6and Fig.7clearly present that NO can be oxidized to NO 2,and O 2exists in real working condition.So the adsorption behaviors of NO+O 2are studied and the results are shown in Figs.8-10and Table https://www.360docs.net/doc/614605106.html,pared with the NO-TPD results (Fig.6),the intensities of peak at LT over Cu/SAPO-34have risen in NO+O 2TPD (Fig.8).It is seen oxygen is favorable for NO oxidation and storage.39In addition,the nitrates/nitrites spe-cies oxidized by oxygen desorb at LT,presents that they are weakly adsorption species.Relativel nitrates/nitrites oxidized by [O]x -species in samples are strong adsorption species.3.2.2NO+O 2adsorption sites and species

The DRIFTS results of NO +O 2adsorption are shown in

https://www.360docs.net/doc/614605106.html,pared with the NO adsorption over Cu/SAPO-34,the intensity of peak at 1480cm -1(monodentate nitrates)is higher due to the existence of O 2.Further,a new peak at 2050cm -1related to Cu 2+(NO 2)appears.However,there is no differ-ence between NO and NO +O 2adsorption over H-SAPO-34.The DRIFTS results of NO+O 2adsorption can indicate that the oxygen in feed gas promotes NO oxidization to NO 2and NO 2could be stored as nitrates/nitrites on Cu species.

Fig.10presents the DRIFT spectra of NO+O 2adsorption un-der different temperatures.As the temperature above 100°C,it is seen that only the peak at 1595cm -1(chelate nitrates)exists.It suggests that the chelate nitrates are the stronger adsorption species than monodentate nitrates and nitrites over Cu/SAPO-34.

3.3Roles of adsorption species in NH 3

-SCR

Fig.8NO+O 2-TPD profiles over Cu/SAPO-34and H-SAPO-34Fig.9DRIFT spectra of NO+O 2adsorption over Cu/SAPO-34(A)and H-SAPO-34(B)at 30°C for 0,1,2,5,10,30,60

min

Table 4

Quantitative analyses of NO+O 2-TPD over H-SAPO-34

Fig.10

DRIFT spectra of NO+O 2adsorption over Cu/SAPO-34

at different temperatures

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SHI Lin et al .:Properties and Roles of Adsorbed NH 3and NO x over Cu/SAPO-34Zeolite Catalyst in NH 3-SCR Process

No.7Fig.11illustrates the adsorbed NH 3species reacting with NO +O 2over Cu/SAPO-34under different temperatures.Line at 0min represents the spectra of NH 3species adsorption satu-ration.At 100°C (Fig.11A),when NO+O 2are cut in,the peak intensities at 899,858cm -1(NH 3-Cu 2+)decrease with time and become a line in 30min (Fig.11A(30min)).Further,the peaks at 1450-1600cm -1(nitrates)and 1200-1300cm -1(nitrites)ap-pear after 6min.As the reaction continues,other peak intensi-ties of adsorbed NH 3species reduce a little (Fig.11A(15min,30min)).As these peak intensities reaching steady state,the NH 3species adsorbed over Cu ions are consumed completely,

while the ones on other adsorbed sites still exist.When the ex-periments are carried out at 160°C (Fig.11B)and 300°C (Fig.11C),the NH 3species adsorbed on Cu 2+cations also take part in SCR reaction firstly.These phenomena indicate that the NH 3species over Cu 2+sites show the best SCR activity and Cu 2+species are activate sites.40Above 100°C,the peak intensi-ties of all others adsorbed NH 3species decrease with time in-creasing.These species are likely to be involved in the SCR re-action upon “migration ”to near-by reactive Cu 2+cations once these sites are available.It is inferred that the NH 3adsorption on other sites,which represents that the major fraction of ad-sorbed NH 3species over Cu/SAPO-34,does not act simply as “spectators ”in the SCR reaction but is involved in the NO con-sumption.

At 160°C,the peaks of nitrates/nitrites appear and intensi-ties reach the highest in 15min (Fig.11B(15min)).Moreover the peak intensities at 1480,1291,1280cm -1reduce as SCR re-action proceeds.It indicates that some NH 4NO x (NH 4NO 2/NH 4NO 3)species are formed due to the reaction of adsorbed NH 3species with nitrates/nitrates on Cu cations.At 300°C,the DRIFT result (Fig.11C)shows no nitrates/nitrites.It suggested that nitrates/nitrites are consumed immediately as formed.

Fig.12shows the results of adsorbed NO+O 2species react-ing with NH 3+O 2over Cu/SAPO-34under 100and 160°C.At 100°C (Fig.12A),the intensities of peak at 1595cm -1(chelate nitrates)and 2050cm -1Cu +(NO 2)decline immediately when NH 3+O 2is introduced.Then,the peak intensities of all other ni-

Fig.11DRIFT spectra of 4×10-6(φ)NO+O 2/He following exposure of Cu/SAPO-34to 4×10-6(φ)NH 3/He at 100°C for 30min (A),160°C for 40min (B),and 300°C for 35min (C)Fig.12DRIFT spectra of 4×10-6(φ)NH 3+O 2/He following exposure of Cu/SAPO-34to 4×10-6(φ)NO+O 2at 100°C for

30min (A)and 160°C for 30min

(B)

Acta Phys.-Chim.Sin.2013Vol.29

trates/nitrites decrease subsequently.After1-min reaction (Fig.12A(1min)),the peaks of all NH3adsorption species(ex-cept NH3-Cu2+species)appear and reach steady state in3min. At the same time,the peaks of NH3-Cu2+species appear and steady state is achieved until nitrates/nitrites species are con-sumed completely(Fig.12A(30min)).It has verified that NH3 can be adsorbed on Cu2+cations firstly over clean catalyst in Fig.3A.So it indicates that the emergence of NH3-Cu2+species lastly over NO x storage catalyst is due to the immediate reac-tion of nitrates/nitrites with NH3-Cu2+species as formed.

3.4Mechanism discussion

Fig.13displays the proposed mechanism of NH3-SCR reaction over Cu/SAPO-34.Firstly,NH3can be adsorbed on Br?nsted ac-id sites and Lewis acid sites over Cu/SAPO-34to form several NH3adsorption species.Secondly,NO can be oxidized to NO2 by[O]x-species in SAPO-34or O2in feed gas on Cu ions.1,32,41 Thirdly,NO2could be stored as nitrites/nitrates on Cu ions to form NO-2-Cu p+/NO-3-Cu q+,which might be oxidized to NO-2-Cu2+/NO-3-Cu2+.The electrons of NO oxidization may easily come from Cu ions and Cu2+ions reduced to Cu p+or Cu q+.Be-cause low reduction ability of NO,so the ranges of p,q are1 to2.In addition,NH3-Cu2+would react with the nitrites/nitrates on Cu ions to form NH4NO2and NH4NO3.And some NH4NO3 may react with NO to form NH4NO2again.Finally,NH4NO2 would generate N2,H2O,and little NH4NO3would generate lit-tle N2O.

4Conclusions

The transient response experiments show that NH3is the ad-sorption gas over Cu/SAPO-34during NH3-SCR process, while NO could not be adsorbed appreciably on the catalyst surface.H-SAPO-34contains two kinds of acid sites:Br?nsted acid sites and Lewis acid sites.The Br?nsted acid sites may ad-sorb NH3to form M-O-NH+4(M=P,Si,Al)and asymmetric vibra-tion of NH+4groups.The NH3can be adsorbed on Lewis acid sites as N-H deformation and NH3molecule.Over Cu/SAPO-34,NH3can not only be absorbed on the adsorption sites of H-SAPO-34,but also can be adsorbed on Cu+and Cu2+species. In addition,the adsorption rate of NH3on Cu2+cations is the fastest and the adsorption bond strength of NH3-Cu2+is the strongest.

Cu species can promote the oxidization of NO to NO2by the [O]x-species in SAPO-34and O2in feed gas.The NO2could be stored on Cu sites as nitrates/nitrites..

Cu2+species are the active sites for NH3-SCR reaction over Cu/SAPO-34.NH4NO x(NH4NO2/NH4NO3)species stored on Cu ions may be the SCR intermediate species and promote NH3-SCR reaction.

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