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Catalysis Today 184 (2012) 145– 152

Contents lists available at SciVerse ScienceDirect

Catalysis Today

jou rn al h om epage: www.elsev ier .com/ locate /ca t tod

nfluence of calcination temperature on Fe/HBEA catalyst for the selectiveatalytic reduction of NOx with NH3

ei Ma, Junhua Li ∗, Hamidreza Arandiyan, Wenbo Shi, Caixia Liu, Lixin Futate Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China

r t i c l e i n f o

rticle history:eceived 20 August 2011eceived in revised form 4 October 2011ccepted 4 October 2011vailable online 1 November 2011

eywords:eNOx

eolite HBEA

a b s t r a c t

A serious of Fe/HBEA catalysts were prepared by impregnation methods under different calcination tem-peratures, and their catalytic activity in selective catalytic reduction (SCR) of NOx with ammonia weretested. The influence of calcination temperature on the microstructures of zeolite support, iron species,redox ability and reactant adsorption ability was investigated in detail by various characterization meth-ods. Results showed that calcination temperature showed significantly influence on catalytic activityand selectivity of serial Fe/HBEA catalysts in NH3-SCR reaction. Some agglomeration of Fe/HBEA catalystoccurred at high calcination temperature (especially above 850 ◦C), which result in drastic decrease ofsurface area and pore volume of Fe/HBEA catalysts. Meanwhile, the surface Lewis and Brönsted acidity of

◦

hermal stabilityalcination temperature

ron migrationron oxide cluster

zeolite did not change when the calcination temperature was below 750 C, but significantly decreasedabove 850 ◦C. Furthermore, high temperature aging led to some reduction migration of iron ions out ofion exchange sites on the Fe/HBEA catalyst. Consequently, iron oxide clusters are mainly formed uponFe migration, which contributed to the SCR activity above 450 ◦C. The deactivation of Fe/HBEA catalystscalcined at high temperatures was ascribed to surface area and pore volume decrease, Lewis and Brönstedacid sites inhibition, and the loss of iron ions and formation of iron oxide clusters.

Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.

. Introduction

Nitrogen oxides (NO, NO2, N2O) in the exhaust gases from com-ustion of fossil fuels are a major cause of photochemical smog, acidain, and ozone depletion. It is known that over 50% of NOx emis-ions are from automotive sources, such as gasoline cars and dieselngine trucks, and over 40% of NOx are from stationary sources, suchs power plants fueled by fossil fuel combustion. Diesel enginesave many benefits over gasoline engines, such as better fuel econ-my and less CO2 production, but they also emit more NOx. Withollution regulations tightening, it appears urgent to eliminate NOx

missions [1,2].Iron-containing zeolite catalysts have attracted much attention

or selective catalytic reduction (SCR) of NOx with ammonia (or ureahich decompose into ammonia). Comparing to the other SCR cata-

ysts, one important reason that Fe-exchanged zeolite SCR catalystsave been considered for NH3-SCR is its remarkable DeNOx activityt higher temperatures [3,4]. Because of high activity and durabil-

ty, Fe-exchanged zeolite catalysts [5–7], especially Fe-ZSM-5, wastudied extensively in the past decade. In addition to zeolite ZSM-, zeolite HBEA, a 12-ring aperture (6.6 A × 6.7 A and 5.6 A × 5.6 A)

∗ Corresponding author. Tel.: +86 10 62771093.E-mail address: lijunhua@tsinghua.edu.cn (J. Li).

920-5861/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rioi:10.1016/j.cattod.2011.10.007

three dimensional high-silica zeolite, have also received muchattention as a potential catalyst in numerous reactions [8]. Com-pared to zeolite ZSM-5, zeolite HBEA showed more stable andhigher hydrothermal ability. In recent years, some researchers havestudied activity of Fe/HBEA catalyst in the SCR reaction, and resultsshowed that it was a potential SCR catalyst for NOx removal fromthe automotive emissions [2,9–14]. It is reported that heavy dutyvehicles in DPF regeneration usually provide the maximum tem-peratures up to 800 ◦C. DeNOx activity over the Fe/HBEA catalystshould be examined after aging at different temperature, especiallyat high temperatures, which might result in alteration of zeolitestructure and iron species property upon aging. Therefore, it isvery necessary to study the calcination temperature effect on theFe/HBEA catalyst, and characterize the property of the Fe/HBEA cat-alyst after thermal treatment and discuss the cause for the thermaldeactivation of Fe/HBEA catalyst.

In this study, the Fe/HBEA catalysts were prepared by impreg-nation methods under different calcination temperatures, andcharacterized by various methods. The structure property of theFe/HBEA catalysts was investigated by N2 physisorption, powderX-ray diffraction (XRD), and high resolution transmission electron

microscopy (HRTEM). The redox property of Fe/HBEA catalysts wascharacterized by hydrogen temperature programmed reduction(H2-TPR), and oxidation of NO to NO2 test. The reactant adsorp-tion ability was studied by NH3 and NOx adsorption. In addition,

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V–vis diffuse reflectance spectra (UV–vis DRS) was used to char-cterize different iron species on the Fe/HBEA. The experimentalesults in the study will develop characterization of the iron speciesnd bulk properties of the urea SCR catalysts, find the changes ofhese properties induced by aging, and provide a recommendationn developing robust urea SCR systems for automotive exhaust use.

. Experimental

.1. Preparation of catalysts

A series of Fe/HBEA catalysts was prepared by impregnationethods under different calcination temperatures. The starting

eolite was HBEA with SiO2/Al2O3 = 25 from Nankai University cat-lyst Co., Ltd. The required amount of an aqueous Fe(NO3)3·9H2Oolution was dropped onto HBEA to obtain the desired Fe con-ent (2 wt.%). The mixture was stirred for 4 h and the water wasllowed to slowly evaporate at 90 ◦C. The paste obtained was driedvernight at 110 ◦C and then calcined at different temperatures (i.e.50, 650, 750, 850 and 950 ◦C) for 4 h in static air. Finally, the cat-lyst was palletized and crushed to 40–60 mesh for evaluation.he sample coding was Fe/HBEA-X, with X being the calcinationemperature.

.2. Activity test of catalyst

Catalytic activity tests were performed in a fixed-bed quartzube reactor of 9 mm internal diameter containing 100 mg of cata-yst (40–60 mesh). The concentration of NH3 and NOx (NO, NO2 and

2O) in the inlet and outlet gas was measured by a FTIR spectrom-ter (Gasmet FTIR DX4000) made in Finland. At steady state, a gas2 mixture containing 5% H2O (when used), 500 ppm NO, 500 ppmH3 and 5% O2 was introduced into the reactor. Water vapor wasenerated by passing N2 through a heated gas-wash bottle con-aining de-ionized water. In the tests, the total flow rate was fixedt 500 mL/min, which corresponded to a gas hourly space velocityGHSV) of 160,000 h−1.

The performance of the catalysts is presented in terms of con-ersion of NOx (X(NOx)) and selectivity of N2O (S(N2O)) as definedy Eqs. (1) and (2).

(NOx) = [NOx]inlet − [NOx]outlet

[NOx]inlet× 100% with [NOx]

= [NO] + [NO2] (1)

(N2O) = 2[N2O][NOx]inlet − [NOx]outlet

× 100% (2)

Catalyst activity tests were carried out at the temperature rangef 150–550 ◦C. To avoid the impact of gas adsorption on the cata-yst samples, the test data were recorded after the reactions had

aintained stable states for 30 min.

.3. Hydrothermal test

A hydrothermal test of the Fe/HBEA catalyst similar to that men-ioned in Section 2.2 was performed on a laboratory bench, whereresh sample (0.1 g) was taken for the aging. In the hydrothermalxposure, the Fe/HBEA sample calcined at 550 ◦C was treated for0 h at 800 ◦C with a total flow of 500 mL/min consisting of 5% H2ON2 balance).

.4. Characterization of catalyst

A Quantachrome Nova Automated Gas Sorption System wassed to measure the N2 adsorption isotherms of the samples at

184 (2012) 145– 152

liquid N2 temperature (−196 ◦C). The specific surface area wasdetermined from the linear portion of the BET plot. The pore sizedistribution was calculated from the desorption branch of the N2adsorption isotherm using the HK method. Prior to the surfacearea and pore size distribution measurements, the samples weredegassed in vacuum at 300 ◦C for 4 h.

The powder X-ray diffraction (XRD) measurements were carriedout with a D8 advance system with Cu K� (� = 0.154 nm) radiation.The samples were loaded on a sample holder with a depth of 1 mm.

High resolution transmission electron microscopy (HRTEM) wasperformed on JEM-2010 (Jeol) at an acceleration voltage of 200 kV.Samples were dispersed on a lacey support film.

UV–vis diffuse reflectance spectra (UV–vis DRS) was recorded inair in the wavelength range 200–800 nm on a UV-2450 (Shimadzu).

Hydrogen temperature programmed reduction (H2-TPR) exper-iments were performed on ChemiSorb 2720 (Micromeritics). Ineach experiment, 100 mg of sample was loaded into a quartz reac-tor and then pretreated in N2 (50 mL/min) at 300 ◦C for 1 h. Thesample was then cooled down to the room temperature in N2 flow.The reduction of the sample was carried out from the room temper-ature to 1000 ◦C in a flow of 10% H2/Ar (50 mL/min) at 10 ◦C/min.The consumption of H2 was monitored continuously with a ther-mal conductivity detector. The water produced during reductionwas trapped in U-tube immersed in a cold trap.

Temperature Programmed Desorption (TPD) was conducted byusing 0.1 g catalyst in a quartz reactor. The adsorption was per-formed by passing a gas mixture, containing 500 ppm NH3, or500 ppm NO and 5% O2 with N2 as balance gas, through the sam-ple bed at 25 ◦C for 2 h with the total flow rate of 100 mL/min. Andthen, the adsorption gas was purged with N2 until no NH3 or NOx

was detected in the effluent. TPD measurements were carried outup to 750 ◦C with a heating rate of 10 ◦C/min in flowing N2 with theflow rate of 300 (NH3-TPD) or 100 mL/min (NOx-TPD). The concen-tration of NH3, NO and NO2 was continuously monitored by a FTIRspectrometer (Gasmet FTIR DX4000) equipped with a heated, lowvolume multiple-path gas cell (5 m).

3. Results

3.1. SCR performance of different catalysts

Fig. 1(a) shows the catalytic activity of NH3-SCR on Fe/HBEAcatalysts prepared at different calcination temperatures. It wasobserved that Fe/HBEA-550 was much active in the reaction,and reached nearly 100% NOx conversion at 350–500 ◦C. Thisresult is consistent with the previous result that Fe/HBEA wasactive for the SCR reaction at high temperatures [12]. Fe/HBEA-650 and Fe/HBEA-750 showed almost the same SCR activity asFe/HBEA-550. By comparison, the Fe/HBEA-850 showed relativelylower NOx conversions below 350 ◦C under the same condi-tions. The lowest NOx conversion in the whole temperature wasobserved on Fe/HBEA-950, on which the maximum NOx conver-sion reached only 82%. At high test temperatures (i.e. 450–550 ◦C),DeNOx efficiency of all these samples ranked in the followingsequence: Fe/HBEA-850 > Fe/HBEA-750 > Fe/HBEA-650 > Fe/HBEA-550 > Fe/HBEA-950. The sequence of DeNOx efficiency at thehigh temperatures was much different with that at low tem-peratures. Brandenberger et al. [15] considered that the relativeconcentrations of different iron species were correlated with theirDeNOx efficiencies and NH3 oxidation activity. They suggested thatmonomeric iron sites contributed to SCR activity below 300 ◦C,

but clustered iron sites including dimeric and oligomeric speciescontributed more to the overall SCR activity than monomeric ironsites above 300 ◦C, and the clustered iron sites not only contributedto the SCR activity but also caused nonselective oxidation of NH3

L. Ma et al. / Catalysis Today 184 (2012) 145– 152 147

600500400300200100

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Fe/HBEA-550 Fe/HBEA-650 Fe/HBEA-750 Fe/HBEA-850 Fe/HBEA-950

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a

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NOx c

onve

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n / %

600500400300200100

0

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60 Fe/HBEA-550 Fe/HBEA-650 Fe/HBEA-750 Fe/HBEA-850 Fe/HBEA-950

Temperature / oC

N2O

sele

ctiv

ity /

%

Fig. 1. Catalytic performance for SCR of NOx with ammonia on different Fe/HBEAcatalysts. Reaction conditions: 0.1 g catalyst, 500 ppm NO, 500 ppm NH3, 5% O2, bal-aN

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Fresh Fe/HBEA-550 Fe/HBEA aged 800 oC, 10 h

Temperature / oC

NO

x con

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/ %

Fig. 2. NOx conversions of fresh and aged Fe/HBEA catalysts calcined at 550 ◦C.

Compared to 549.6 m2/g for Fe/HBEA-550, the surface area ofFe/HBEA-650 slightly decreased to 510.3 m2/g. As the calcinationtemperature continued to increase (especially above 850 ◦C), sur-face area and pore volume of Fe/HBEA significantly decreased, but

Table 1Comparison of BET surface area and pore structure results of different Fe/HBEAcatalysts.

Sample SBET (m2/g) Pore volume(cm3/g)

Pore diameter(nm)

Fe/HBEA-550 549.6 0.25 0.49

nce N2, and GHSV = 160,000 h−1. (a) NOx conversion on different catalysts; and (b)2O selectivity on different catalysts.

bove 350 ◦C. We speculated that similar phenomenon occurredn these Fe/HBEA catalysts. Different calcination temperature pro-ess caused some changes of the iron species on these samples,hich resulted in the different redox ability of ammonia when the

emperature was above 450 ◦C. Based on impregnation method atifferent calcination temperatures, different content of iron oxidelusters might form on zeolite HBEA surface and channels. Theseron oxide clusters contributed to both SCR activity and ammo-ia oxidation, which resulted in the different DeNOx performancet high temperature. This inference will be further discussed inection 3.2.2.

The N2O selectivity is one of the most important aspects toeal application of catalysts in NH3-SCR reaction. Therefore, theesults of N2O selectivity in the NH3-SCR are tested and given inig. 1(b). The low N2O yields were detected on the three Fe/HBEAatalysts calcined at low temperatures (i.e. 550, 650, and 750 ◦C),ut much more N2O was detected on the other two Fe/HBEA-850nd Fe/HBEA-950 catalysts at low test temperatures (<350 ◦C). Theesults also indicated that some changes of iron species might takelace after the different calcination process. Because the theoryontent of iron species was exactly the same on all the three cat-lysts, the iron ions (active sites for NH3-SCR reaction at low testemperature [15]) might migrate from the ion exchange sites andransform to iron oxide clusters on the zeolite support at high cal-ination temperature (especially above 850 ◦C).

Fig. 2 displays the DeNOx results of hydrothermal aging on

e/HBEA-550 catalyst. After 800 ◦C aging with H2O, Fe/HBEA ledo some decrease in DeNOx efficiency in the whole temperatureange, and showed almost 80% NOx conversion at 350–500 ◦C. The

Reaction conditions are the same as Fig. 1. Hydrothermal aging condition: Fe/HBEAcatalyst was made at 800 ◦C for 10 h with 500 mL/min mixture gases consisting of5% H2O (N2 balance).

hydrothermal test results indicated that Fe/HBEA showed relativelygood hydrothermal stability for heavy duty diesel applications.Because iron species was usually responsible for the redox abilityof Fe/zeolite catalysts, H2-TPR and NO oxidation to NO2 methodswere used to study iron existing state and redox ability on theaged Fe/HBEA sample. Detailed study results of these two meth-ods on Fe/HBEA catalysts calcined at different temperatures will begiven and further discussed in Sections 3.2.3 and 3.2.4. The H2-TPRresults showed that the Fe3+ reduction peak located at 360 ◦C ofaged Fe/HBEA obviously decreased compared to that of Fe/HBEA-550 catalyst (Fig. S1). This indicated detachment of isolated ironfrom the ion exchange sites took place at high temperatures in thepresence of water. In addition, aged Fe/HBEA showed very low NOoxidation activity and reached the maximum NO conversion to NO2of 12.2% at 350 ◦C, which was much lower than 34.3% on Fe/HBEAcalcined at higher temperature of 850 ◦C (Fig. S2). Because oxida-tion ability of NO to NO2 was obviously inhibited on the Fe/HBEAsample aged with water, we speculated that dealumination mighttake place on Fe/HBEA at 800 ◦C in the presence of water. Further-more, hydrothermal aging might cause a reduction of isolated ironions located at ion exchange sites, which led to the formation ofFeOx clusters. The migration of iron was favored in the presence ofwater, which enhances the mobility of cations in the zeolite [16,17].

3.2. Characterization of catalysts

3.2.1. Structure propertyBET results of Fe/HBEA calcined at different temperatures are

summarized in Table 1. For Fe/HBEA-550 and Fe/HBEA-650 cat-alysts, pore volume and pore diameter did not differ from eachother, and were stable at 0.25 cm3/g and 0.49 nm, respectively.

Fe/HBEA-650 510.3 0.25 0.49Fe/HBEA-750 495.3 0.24 0.49Fe/HBEA-850 463.2 0.23 0.49Fe/HBEA-950 304.4 0.14 0.49

148 L. Ma et al. / Catalysis Today

908070605040302010

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Fe/HBEA-650

Fe/HBEA-750

Fe/HBEA-850

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[22,23]. Therefore, the comparison of oxidation activity for NO toNO2 with O2 over different Fe/HBEA catalysts was tested and givenin Fig. 7. The NO conversion on all the Fe/HBEA catalysts increasedfrom 150 to 350 ◦C, and then decreased somewhat from 350 to

Table 2Numerical analysis of UV–vis DRS of different Fe/HBEA catalysts.

Sample Fea (%) Feb (%) Fec (%)

Fe/HBEA-550 88.6 1.2 10.2Fe/HBEA-650 81.2 5.4 13.4Fe/HBEA-750 49.0 47.9 3.1Fe/HBEA-850 33.1 62.1 4.8

Bragg an gle / 2θ

Fig. 3. XRD patterns of different Fe/HBEA catalysts.

he pore diameter was stable without any change. Compared toe/HBEA-550, a clear decrease of BET surface and pore volume areaas observed on Fe/HBEA-950, and decreased by 44.6% and 44.0%,

espectively. It is therefore assumed that, at high calcination tem-erature (especially above 850 ◦C), some agglomeration of HBEAeolite occurred, which result in drastically decrease of surface areand pore volume of Fe/HBEA catalysts. Because the pore diameterid not change any more with the temperature rising, we thoughthe zeolite crystal structure of HBEA would not be destroyed at50 ◦C.

For further confirming the changes of zeolite HBEA structurender different calcination temperatures, the XRD profiles of differ-nt Fe/HBEA catalysts were tested and depicted in Fig. 3. The figurelearly shows that zeolite diffraction peaks (at 7.7 and 22.5◦) mainlyppeared on these catalysts without any obvious iron oxide sig-als. This results further illustrated HBEA crystalline phase did nothange, but HBEA crystallinity would be deteriorated at calcinationemperature of 950 ◦C.

The Fe/HBEA particles were also observed by HRTEM, whichhowed that the particle size was about 0.3 �m for Fe/HBEA-550,e/HBEA-650 and Fe/HBEA-750 (Fig. 4). However, the particle sizeradually increased on Fe/HBEA-850, and increased to about 3 �mn Fe/HBEA-950. This indicated that Fe/HBEA crystal agglomera-ion formed at high temperatures, and resulted in surface area andore volume decreased. It should be noted that all the remainingamples iron oxide entities could not be clearly differentiated fromure HBEA. The evidence of iron oxo species deposited in the zeo-

ite channels is not straightforward, since at high magnificationshe energy of the electron beam is found to destroy the crystallinetructure of the zeolite.

.2.2. UV–vis DRSThe UV–vis DRS results are displayed in Fig. 5. It shows that

pectral shapes depend on the different calcination temperaturesf Fe/HBEA catalysts. To obtain the detailed information of thesee/HBEA catalysts, the sub-bands have been derived and sum-arized in Table 2 using the same method as Schwidder et al.

18], who got the sub-bands of UV–vis spectrum of Fe/ZSM-5 cat-lysts and ascribed the band at 220 and 285 nm to isolated Fe3+,he band above 400 nm to large Fe2O3 particles, and the bandround 350 nm to oligomeric clusters. With the increase of calci-

ation temperature, the proportion of oligomeric Fex

3+Oy clustersncreased, isolated Fe3+ (220–285 nm) gradually decreased, andarge Fe2O3 particles first decreased and then remained basicallynchanged. Compared to Fe/HBEA-550, the content of isolated

184 (2012) 145– 152

Fe3+ on Fe/HBEA-750 decreased from 88.6% to 49.0%, while theoligomeric iron oxide clusters on Fe/HBEA-750 increased from 1.2%to 47.9%. With the calcination temperature rising to 950 ◦C, the con-tent of isolated Fe3+ and oligomeric iron oxide clusters changed to35.2% and 60.3%, respectively. This indicated that high temperatureaging led to some reduction in the number of isolated iron sitesof the catalyst, which was caused by the migration of active ironions out of ion exchange sites. Consequently, iron oxide clusterswere the main species formed upon migration at high calcinationtemperatures, and contributed to the SCR activity above 450 ◦C [15].

3.2.3. H2-TPRThe H2-TPR profiles are presented in Fig. 6. The reduction pro-

files of these Fe/HBEA samples were very complicated, whichindicated a highly heterogeneous iron species distribution on thesesamples with low iron loading [18]. In the temperature region100–900 ◦C, Fe/HBEA-550 showed four reduction peaks includ-ing one intense peak at 395 ◦C and one broad peak containingthree peaks between 435 and 710 ◦C. According to literature data[16,19,20], Fe3+ bound to O–Al sites of the zeolite framework arereducible to Fe2+ at relatively lower temperatures, while furtherreduction to Fe0 occurs at T > 1000 ◦C. We speculated that thefirst intense reduction peak of Fe/HBEA-550 could be attributedto the reduction peak of iron ions at ion exchange site, and thebroad peak located at relatively higher temperature to the step-wise reduction of Fe2O3 to metallic Fe (Fe2O3 (hematite) → Fe3O4(magnetite) → Fe (metal)) [21]. Fe/HBEA-650 showed almost sim-ilar reduction peak with Fe/HBEA-550. For Fe/HBEA-750, the firstreduction peak attributed to iron ion reduction was found to moveto higher temperature at 428 ◦C, and the following reduction peaksattributed to iron oxide reduction located at 508, 598 and 700 ◦C.But for Fe/HBEA-850 and Fe/HBEA-950, the first hydrogen reduc-tion peak at 395 or 428 ◦C completely disappeared, and two mainreduction peaks around 508 and 700 ◦C were found. The disappear-ance of Fe3+ reduction peak could be attributed to migration of ironions species, which were detached from the ion exchange sites andformed some iron oxide clusters. These results further indicatedthat the redox ability was obviously weakened on Fe/HBEA-850 andFe/HBEA-950, on which NO could not be oxidized to NO2 effectively.The oxidation step of NO to NO2 was usually considered as the keystep in the SCR reaction [22,23]. Because the H2 consumption peakareas of these catalysts were much low, the actual H2 consumptionof these Fe/HBEA samples could not be calculated very accurately.The quantitative evaluation results were not given here.

3.2.4. Oxidation of NO to NO2The NH3-SCR reaction mechanism on iron-exchanged zeolite

has been deeply studied by many researchers [7,22–24]. It isaccepted that oxidation of NO to NO2 on iron ion sites is the ratedetermination step in the SCR reaction over iron-exchanged zeolite

Fe/HBEA-950 35.2 60.3 4.5

a Isolated Fe3+ in tetrahedral and higher coordination.b Oligomeric Fex

3+Oy clusters.c Large Fe2O3 particles.

L. Ma et al. / Catalysis Today 184 (2012) 145– 152 149

f diffe

53pFvC8c

Fig. 4. HRTEM pictures o

50 ◦C. The reason that the highest NO conversion was achieved at50 ◦C is due to NO oxidation being kinetically limited at lower tem-eratures and thermodynamically limited at higher temperatures.e/HBEA-550 and Fe/HBEA-650 showed almost the same NO con-

ersion, and reached the highest NO conversion of 44.1% at 350 ◦C.ompared to the above two samples, Fe-HBEA-750 and Fe/HBEA-50 showed relatively lower activity for NO oxidation, on which NOonversion decreased gradually, and reached maximum conversion

rent Fe/HBEA catalysts.

of 39.4% and 34.3% at 350 ◦C, respectively. For Fe/HBEA-950, NOconversion significantly reduced, and only reached maximum valueof 19.4% at 350 ◦C. Based on the results of UV–vis DRS and TPR,we also suggested that the high calcination temperature resulted

in detachment of iron ions from iron exchange sites, and migra-tion of iron at ion exchange site and iron oxide clusters formationinhibited the oxidability. On the other hand, Fe/HBEA-950 catalystshowed high iron oxide cluster content, yet its DeNOx performance

150 L. Ma et al. / Catalysis Today 184 (2012) 145– 152

800700600500400300200Wavenu mber / nm

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Fe/HBEA-550

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Fe/HBEA-750

Fe/HBEA-850

Fe/HBEA-950

Fig. 5. UV–vis DRS spectra of different Fe/HBEA catalysts.

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o

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Temperature / oC

Fig. 6. H2-TPR profiles of different Fe/HBEA catalysts.

as the lowest at higher temperatures in the SCR activity testFig. 1(a)), even lower than the relatively mildly calcined cata-ysts (e.g. 550 ◦C) with very low iron oxide cluster content. This

ight be due to its low surface area, low pore volume and acid

5505004504003503002502001500

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100 Fe/HBEA-550 Fe/HBEA-650 Fe/HBEA-750 Fe/HBEA-850 Fe/HBEA-950 Equilibrium

NO

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ion

to N

O2 /

%

Temperature / oC

ig. 7. Oxidation activity of NO to NO2 by O2 over different Fe/HBEA cata-ysts. Reaction conditions: 0.1 g catalyst, 500 ppm NO, 5% O2, and balance N2,HSV = 160,000 h−1.

Temperature / C

Fig. 8. NH3-TPD profiles over different Fe/HBEA catalysts.

sites inhibition, which were demonstrated by the BET and NH3-TPDresults.

3.2.5. NH3/NOx-TPDFig. 8 shows the NH3-TPD results of different Fe/HBEA catalysts.

In the entire temperature range, Fe/HBEA catalysts calcined at lowtemperature (e.g. Fe/HBEA-550, Fe/HBEA-650 and Fe/HBEA-750)mainly showed two large desorption peaks at 110 and 160 ◦C, andtwo small desorption peaks around 260 and 350 ◦C. The peak at110 ◦C could be assigned to physical adsorbed ammonia on Fe/HBEA[25], and the high temperature desorption peaks at 260 and 350 ◦Ccould be assigned to ammonia desorption on Lewis and Brönstedacid sites, respectively [25,26]. Assignment of the peak at 160 ◦Cwas somewhat controversial. This feature might be attributedto ammonia weakly adsorbed on acid sites [25,27,28]. The totalamount of ammonia desorption was calculated by the integralcalculation of desorption peak area, which showed that the totalamount of ammonia desorption was 1274.9 �mol/g on Fe/HBEA-550. With the calcination temperature increasing to 750 ◦C, thetotal amount of ammonia desorption decreased to 971.6 �mol/g.Because it was observed that there was almost no differenceof ammonia desorption at 260 and 350 ◦C among Fe/HBEA-550,Fe/HBEA-650 and Fe/HBEA-750, we speculated that the surfaceLewis and Brönsted acidity of zeolite would not change when thecalcination temperature was below 750 ◦C. This decrease of ammo-nia adsorption on Fe/HBEA-750 could be mainly ascribed to thephysical or weak acid adsorption of ammonia, which mainly causedby the decrease of surface area and pore volume. As the calcinationtemperature continued to rise, ammonia desorption significantlyreduced on Fe/HBEA-850 and Fe/HBEA-950, on which the totalamount of ammonia desorption was 558.0 and 109.4 �mol/g,respectively. Not only physical or weak acid adsorption of ammo-nia decreased on these two samples, but also ammonia adsorbedon Lewis and Brönsted acidity obviously decreased. The calcinationtemperature above 850 ◦C resulted in not only surface area and porevolume reduction, but also Lewis and Brönsted acid sites inhibition.

As NOx-TPD results shown in Fig. 9, most of the Fe/HBEA cat-alysts shows two groups of NOx desorption, of which the smallpeaks below 165 ◦C are ascribed to decomposition monodentatenitrate, and the large peaks between 370 and 380 ◦C are attributedto the decomposition of bidentate nitrate [29]. But Fe/HBEA-950shows three groups of NOx desorption peaks at 133, 290, and 490 ◦C,

respectively, which are attributed to the decomposition of differentnitrate species. Because there was no NOx desorption from physicaladsorbed sites on Fe/HBEA, the decrease of surface area and porevolume was not the main cause for decrease of total adsorption

L. Ma et al. / Catalysis Today 184 (2012) 145– 152 151

7006005004003002001000

0

50

100

150

200

250 NO NO2

NOx

Temperature / oC Temperature / oC

Temperature / oC Temperature / oC

Temperature / oC

NO

x con

cent

ratio

n / p

pm

Fe/HBEA-550

165

380

7006005004003002001000

0

50

100

150

200

250 NO NO2

NOx

NO

x con

cent

ratio

n / p

pm

Fe/HBEA-650

138

370

7006005004003002001000

0

50

100

150

200

250 NO NO2

NOx

NO

x con

cent

ratio

n / p

pm

Fe/HBEA-750 377

150

7006005004003002001000

0

20

40

60

80

100

120

140 NO NO2

NOx

NO

x con

cent

ratio

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Fe/HBEA-850 370

140

7006005004003002001000

0

5

10

15

20

25

30 NO NO2

NOx

NO

x con

cent

ratio

n / p

pm

Fe/HBEA-950 290

133

490

ver di

ofiaFtwchaoddw

4

i

Fig. 9. NOx-TPD profiles o

f NOx. The total amount of NOx desorption was 152.3 �mol/gor the Fe/HBEA-550 catalyst. With calcination temperature ris-ng, the total amount of NOx desorption first remained stable,nd then decreased to 86.3 and 23.2 �mol/g for Fe/HBEA-850 ande/HBEA-950, respectively. Based on the accepted mechanism thathe reaction of NO oxidation usually took place on iron sites [22,23],e speculated that NOx adsorption was relevant to the following

auses. Migration of iron from ion exchange sites took place atigh calcination temperature (>850 ◦C), and led to the changes ofdsorbed nitrate species. It should be noted that with the increasingf calcination temperature, the intensity of monodentate nitrateid not reduce or disappear, but the bidentate nitrate obviouslyecreased. The main causes for NOx adsorption reduction was theeakened bidentate nitrate adsorption.

. Conclusions

The calcination temperature of serial Fe/HBEA catalysts dur-ng the preparation progress showed significantly influence on

fferent Fe/HBEA catalysts.

the microstructures of zeolite support, iron species, redox abilityand reactant adsorption ability, which result in different catalyticactivity of NH3-SCR. The DeNOx efficiency and N2O selectivity ofFe/HBEA obviously decreased, when the calcination temperaturewas above 850 ◦C. On the one hand, some agglomeration of Fe/HBEAcatalyst occurred at high calcination temperature (especially above850 ◦C), which result in drastically decrease of surface area andpore volume of Fe/HBEA catalyst. The size of Fe/HBEA particlesremained stable at 0.3 �m at low calcination temperatures, butwould increase to approximately 3 �m on Fe/HBEA-950. Mean-while, the surface Lewis and Brönsted acidity of zeolite would notchange when the calcination temperature below 750 ◦C, but signif-icantly decreased above 850 ◦C. The deactivation of Fe/HBEA-850and Fe/HBEA-950 was ascribed to not only surface area and porestructure decrease, but also Lewis and Brönsted acid sites inhi-

bition. On the other hand, high temperature aging led to somereduction in the number of iron ions on the Fe/HBEA catalyst, whichwas caused by the migration of iron out of ion exchange sites. Con-sequently, iron oxide clusters are mainly formed upon Fe migration,

1 Today

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52 L. Ma et al. / Catalysis

hich contributed to the SCR activity above 450 ◦C. The loss of ironons at ion exchange sites was considered for one of the main causesor low DeNOx efficiency and N2O selectivity at low temperatures.

cknowledgements

This work was financially supported by the National Naturalcience Foundation of China (Grant No. 51078203), the Nationaligh-Tech Research and Development (863) Program of China

Grant No. 2010AA065002) and Ford Motor Company.

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at doi:10.1016/j.cattod.2011.10.007.

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