Fuel Processing Technology 92 (2011) 1718–1724

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Fuel Processing Technology

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Influence of preparation conditions to structure property, NOx and SO2 sorptionbehavior of the BaFeO3−x perovskite catalyst

Hui Xian a, Feng-Li Li a, Xin-Gang Li a,⁎, Xing-Wen Zhang a, Ming Meng a,⁎,Tian-Yong Zhang a, Noritatsu Tsubaki b

a Tianjin Key Laboratory of Applied Catalysis Science & Technology, School of Chemical Engineering & Technology, Tianjin University, Tianjin, 300072, PR Chinab Department of Applied Chemistry, School of Engineering, University of Toyama, Gofuku 3190, Toyama city, Toyama 930-8555, Japan

⁎ Corresponding authors. Tel./fax: +86 22 27892275E-mail addresses: xingang_li@tju.edu.cn (X.-G. Li), m

0378-3820/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.fuproc.2011.04.021

a b s t r a c t

a r t i c l e i n f o

Article history:Received 27 December 2010Received in revised form 8 April 2011Accepted 12 April 2011Available online 13 May 2011

Keywords:PerovskiteBaFeO3−x

NOx storageSulfur-resistanceRegeneration

A series of the BaFeO3−x perovskite catalysts was synthesized by a sol-gel method using citric acid and/orEDTA as complexants with a purpose to improve their sulfur-resistance by forming a uniform perovskitestructure at a low calcination temperature, i.e. 750 °C. The thermogravimetry results show that almost nocarbonate was formed after calcination of the xerogel precursor with the complexants' molar ratio of CA/EDTA≤1.5, which was convinced by the in situ DRIFT spectra results of the Ba–Fe-1 catalyst during the SO2/O2

sorption. It indicates that, after adding EDTA into the complexants, the metal ions of the rawmaterial could bemixed homogeneously and react stoichiometrically by calcination at 750 °C to form a uniform perovskitestructure. Accordingly, the obtained Ba–Fe-1 perovskite presented a performed sulfur-resistance. Moreover,the seriously damaged structure of the BaFeO3−x perovskite by reduction could be in situ regenerated bycalcination under lean conditions at 400 °C, which is within the operating temperature zone of theaftertreatment system of diesel to meet the real commercial demands.

.engm@tju.edu.cn (M. Meng).

ll rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Lean-burn technique is an enfficient methodology to reducegreenhouse effect and bring a high economic efficiency [1]. However,NOx is hardly removed from exhaust under an excessive O2

atmosphere. The NOx storage-reduction (NSR) method [2] is apromising way to overcome this problem. Many studies [2–5] focuson the Pt/BaO/Al2O3 NSR catalyst. Other type NSR catalysts, such ashydrotalcite-like compounds [6,7] and electrochemical devices [8,9],are also investigated.

However, the noble metal is very expensive, and easily sinteredand lost although they present a high catalytic activity. To solve thisproblem, perovskite oxides in ABO3 formula are deemed as one of thepotential substitutes for noble metals to remove NOx released fromlean-burn engine because of their prior redox properties, stablestructures and low cost [10,11]. Their catalytic properties are mainlydue to the oxygen vacancy or the abnormal valence of the B siteelement, which is induced by the electric neutrality principle.Presently, the studies using perovskites as NSR catalysts were mainlyfocusing on the perovskite that the alkali earthmetal element acted asthe A site element, such as Ba, Sr, or Ca [12,13]. Among these

perovskites, the BaSnO3 catalyst possessed the best NOx absorptionability [12], but poor sulfur-resistance [14].

It is reported that the presence of Fe2O3 in the Pt/BaO/γ-Al2O3

catalyst could inhibit the growth of the neighboring BaSO4 duringsulfation, and lower the removal temperature of BaSO4 under thereducing condition [15], which improved the catalyst's sulfur-resistance significantly. Therefore, we employed the BaFeO3− x

perovskite as a NSR catalyst, on which the iron ions are highlydispersed around the barium ions. Our previous results show that thiscatalyst was an efficient NOx storage material [16–18] with a highsulfur-resistance [16,17]. The catalyst with the best sulfur-resistanceproperty was prepared by calcination at 950 °C in static air. The lowercalcination temperature produced a lot of carbonate on the perov-skite, which would increase its NOx storage capacity (NSC) butsuppress its sulfur-resistance greatly [17]. It is due to the presence of alarge amount of carbonate, which can enhance the NOx storage butalso be easily poisoned by SO2. If the precursor was calcined in flowingair, the perovskite structure could be easily formed even at 750 °C.However, the presence of the carbonate on the perovskite was stillconfirmed by in situ DRIFT spectra recorded during NOx storageprocess [18], which would suppress the perovskite catalyst's sulfur-resistance.

All of the BaFeO3−x catalysts mentioned earlier were preparedonly using citric acid (CA) as the complexant. In this study, a series ofthe perovskite type BaFeO3−x catalysts was synthesized with variouscontents of the CA and EDTA complexants to explore a way that the

1719H. Xian et al. / Fuel Processing Technology 92 (2011) 1718–1724

perovskite structure could be well constructed at a low temperaturesuch as 750 °C with less impurities, i.e. carbonate and spinel. Thiswould realize a perfect dispersion between Ba and Fe ions to achieve ahigh sulfur-resistance of the perovskite catalyst. Furthermore, theinfluence of the complexants to the structure and catalytic activity ofthe perovskite was also evaluated. The structure of the perovskitecatalysts was characterized by X-ray diffraction (XRD), BET surfacearea measurement, thermogravimetry (TG), field emission scanningelectron microscopy (FE-SEM), and O2 Temperature-programmedDesorption (O2-TPD). The NOx storage behavior and the sulfur-resistance of the perovskite catalysts were also investigated anddiscussed through the XRD, H2 Temperature-programmed Reduction(H2-TPR), in situ Diffuse Reflectance Infrared Fourier Transform(DRIFT) spectroscopy, and X-ray Photoelectron Spectroscopy (XPS)experiments. Moreover, the influence of the various treating condi-tions on the perovskite, i.e. lean/rich atmosphere and NOx/SO2

adsorption process, was also evaluated and discussed.

2. Experimental

2.1. Catalyst preparation

A series of BaFeO3−x catalysts was synthesized through a sol-gelroute. Briefly, the required amount of Ba(NO3)2 (Tianjin University,Kewei Co.) and Fe(NO3)3·9H2O (Tianjin University, Kewei Co.) wasdissolved together, and then the solution was ultrasonic-treated for0.5 h, followed by addition of 0.05 mol L−1 of citric acid (CA, TianjinUniversity, Kewei Co.) and Ethylenediaminetetraacetic acid (EDTA,Tianjin University, Kewei Co.) solution. The detailed composition ofthe raw material for each sample is listed in Table 1. The pH value ofthe aqueous solution was maintained at 6.0 with the adjustment of28% NH3·H2O solution (Tianjin University, Kewei Co.) at 80 °C. Aftervigorous stirring and evaporation, a transparent brown gel wasformed, which was then dried at 120 °C overnight. The obtainedxerogel precursor was firstly calcined from room temperature to350 °C in air with a rate of 1 °C min−1, stayed for 2 h, and was thencalcined at 750 °C for 6 h with a rate of 4 °C min−1. The wholecalcination process was conducted in a pipe furnacewith a continuousair flow (50 mL min−1) to eliminate the produced CO2 during CA orEDTA combustion. The obtained catalyst was denoted as Ba–Fe-n(n=1–5) as listed in Table 1.

2.2. Catalyst characterizations

The X-ray diffraction (XRD) measurements were carried out on anX'pert pro rotatory diffractometer, using Co Kα (λ=0.17890 nm) asradiation source. The X-ray tube was operated at 40 kV and 40 mA.

The BET surface area of the catalysts was determined by N2

physisorption using an automatic gas adsorption system (NOVA 2000,Quantachrome Co.) at −196 °C. The sample was outgassed at 300 °Cfor 8 h prior to N2 physisorption.

Table 1Ligand composition and NSC of the samples.

Samples Molar ratio ofBa:Fe:CA:EDTA

NSC of freshsample(μmol g−1)

NSC of sulfatedsample(μmol g−1)

NSC dropafter sulfation(%)

Ba–Fe-1 1:1:3:2 230 204a 11145b 39

Ba–Fe-2 1:1:3:0 405 244a 40Ba–Fe-3 1:1:3:3 226 199a 12Ba–Fe-4 1:1:3:1 298 217a 27Ba–Fe-5 1:1:0:2 312 209a 33

a Sulfated in SO2/O2 flow for 1 h.b Sulfated in SO2/O2 flow for 3 h.

The thermogravimetry (TG) analysis was conducted with a Perkin-Elmer Diamond TG instrument. Approximate 5 mg of the precursor,which was pre-calcined at 350 °C for 2 h, was used. It wasimplemented in an air flow of 100 mL min−1. The temperatureincreased from room temperature to 900 °C at a rate of 10 °C min−1.

The morphology of the samples was investigated with a Hitachi S-4800 field emission scanning electron microscopy (FE-SEM, 5 kV).

The in situ Diffuse Reflectance Infrared Fourier Transform (DRIFT)spectra experiments were carried out on a Nexus FT-IR spectrometer(Thermo Nicolet Co.) equippedwith aMCT detector with 32 scans. ForSO2 sorption experiments at 400 °C with various SO2 sorption periods,the catalyst was pretreated in 5% O2/He at 400 °C for 30 min to recordthe background spectrum, and then 200 ppm SO2 and 5% O2 diluted byHe was introduced to the sample cell. The spectra were collected atthe different exposure time from 1 to 60 min. The flow rate of theintroduced gas for all of the DRIFT spectra experiments was50 mL min−1.

The hydrogen temperature-programmed reduction (H2-TPR) andoxygen temperature- programmed desorption (O2-TPD) experimentswereperformedona TPDRO1100SERIES instrument (Thermo-FinniganCo.) with a thermal conductivity detector (TCD). Before the detectionwith the TCD, the introduced gas was purified by a H2O and CO2 trapcontaining CaO and NaOH materials. For either H2-TPR or O2-TPDexperiments, 30 mg of a powdered catalyst was loaded into a quartztube reactor, pretreated in a flowing air (20 mLmin−1) at 400 °C for 1 hto clean the catalyst surface, and cooled down to room temperature. Forthe O2-TPD experiments, the catalyst was purged at room temperatureby a pureN2flow (20 mLmin−1) to get a stable TCD signal baseline, andthen was heated from room temperature to 950 °C. For the H2-TPR test,the catalystwas purged by 5%H2/N2 at room temperature to get a stableTCD signal baseline, and then was heated from room temperature to900 °C in 5% H2/N2 (20 mLmin−1). For both O2-TPD and H2-TPRexperiments, the heating rate was constant at 10 °C min−1.

The X-ray photoelectron spectroscopy (XPS) measurements wereperformedwith a PHI-1600 ESCA spectrometer usingMg Kα radiation(1253.6 eV). The base pressure was 5×10−8 Pa. The recorded spectrawere calibrated using the binding energy (BE) peak at 284.6 eVbelonging to the contaminant carbon in the region of 1 s as standardwith an accuracy of 0.2 eV.

2.3. NOx storage process

The NOx storage capacity (NSC)measurementswere carried out in aconventional fixed bed quartz reactor (i.d.=8 mm) under atmosphericpressure. Our previous studies clearly showed that the optimized NOx

storage temperature for BaFeO3−x perovskite catalysts was 400 °C[17,18]. Therefore, the followed catalyst evaluation concerning the NOx

storage was operated at 400 °C. 0.5 g of the sample was used. After thepretreatmentwith 5%O2 inN2 (50 mLmin−1) at 400 °C for 1 h, and thencoolingdown to thedesired temperature forNOx sorption, a gasmixtureof 800 ppm NO and 5%O2 balanced with N2 passed through the loadedcatalyst at a space velocity of 50,000 h−1. An on-line chemi-luminescenceNOx analyzer (Model 42i-HL, Thermo Scientific) was used to determinethe concentration ofNO,NO2, andNOx. TheNSCvaluewas calculated afterthe NOx sorption for 30 min.

During the sulfationprocess, 0.5 g of the samplewasexposed to a gasmixture of 200 ppm SO2 and 5%O2 balancedwith N2 (400 mLmin−1) at400 °C for 1 or 3 h.

3. Results and discussion

3.1. NOx storage process

The NSC of the catalysts at 400 °C with or without SO2 pretreat-ment is listed in Table 1. Among these fresh catalysts, the Ba–Fe-2catalyst that only CA was used as a complexant during the sol

1720 H. Xian et al. / Fuel Processing Technology 92 (2011) 1718–1724

formation process had the largest NSC, i.e. 405 μmol g−1. After addingEDTA into the precursor solution during the preparation process, theNSC of the prepared perovskite dropped, and the Ba–Fe-1 and Ba–Fe-3catalysts, whose precursor composition were Ba:Fe:CA:EDTA=1:1:1.5:1 and 1:1:1.5:1.5, respectively, had a similar NSC value, i.e.about 230 μmol g−1. After sulfation, the NSC of the Ba–Fe-1 catalystdecreased only about 11% comparing with the fresh one. Contrarily,although the Ba–Fe-2 catalyst possessed much higher NSC, its NSCdropped seriously about 40% comparing with the fresh catalyst. Ourprevious results show that the poor sulfur-resistance property of theperovskite catalyst is due to the presence of carbonate [17]. Hence, theaddition of EDTA into the precursor solution during the preparationprocess might prohibit the formation of carbonate on the perovskitecatalyst, and enhance the perovskite catalyst's sulfur-resistance. Theoptimized metal ions and complexants composition were Ba:Fe:CA:EDTA=1:1:1.5:1 and 1:1:1.5:1. Herein, it suggests that the complex-ants used during the precursor preparation play a significant role todetermine the sulfur-resistance.

3.2. Characterizations of the catalysts

The TG results of the perovskite precursors are presented in Fig. 1.For comparison, the TG profiles of the commercial Ba(NO3)2 andBaCO3 were also plotted here. It is found that the decomposition of thebulk BaCO3 mainly happened at above 810 °C, and the bulk Ba(NO3)2decomposed from 510 to 720 °C. All of the perovskite precursors arecalcined at 350 °C for 2 h before the TG experiments to coincide withthe synthesis procedure of the perovskite catalyst as described inSection 2.1. Four groups of the DTG peaks were detected for the Ba–Fe-2 precursor. The peak centered at around 440 °C is ascribed to thecombustion of the unburned CA and EDTA. The broad peak from 570

-100

0

100

200

300

Ba-Fe-3

Ba-Fe-1

Ba-Fe-4

Ba-Fe-2

DT

G /

µg m

in-1

DT

G /

µg m

in-1

Temperature / °°C

Temperature / °°C

Ba-Fe-5

(a)

200 300 400 500 600 700 800 900

200 300 400 500 600 700 800 900

0

5

10

15

20

95

96

97

98

99

100

200 300 400 500 600 700 800 900

0

100

200

300

400

0

20

40

60

80

100

(c) Wei

ght

loss

/ %

(b)

Fig. 1. TG analysis of the (a) perovskite catalysts; (b) Ba(NO3)2; and (c) BaCO3.

to 680 °C is corresponded to the decomposition of nitrates to metaloxide and may also be involved in the formation of spinel. The peakcentered at 766 °C is due to the formation of perovskite fromcarbonate, which was produced during the CA combustion, andspinel [17,19]. Therefore, the weight loss of the Ba–Fe-2 catalyst atabove 805 °C was probably due to the decomposition of carbonate.After addition of EDTA into the aqueous precursor solution, thecarbonate in the synthesized perovskite catalyst didn't disappeareduntil the molar ratio of EDTA:CA was larger than 1:1.5 as shown inFig. 1 (a). It indicates that almost no carbonate phase existed on theBa–Fe-1 and Ba–Fe-3 catalyst, which is in good agreement with theNSC results listed in Table 1 that these catalysts had a good sulfur-resistance property.

Moreover, during the precursor preparation, we found theformation of the xerogel presented a different behavior determinedby using EDTA as a complexant or not, as shown in Fig. 2. Obviously,the black colored xerogel of the Ba–Fe-1 catalyst expands greatlycomparing with the Ba–Fe-2 catalyst, indicating that the formermightexperience a better sol-gel process, and the metal ions distributedhomogeneously in the xerogel [20]. The SEM images of the Ba–Fe-1and Ba–Fe-2 catalysts are presented in Fig. 3. After calcination at750 °C, the Ba–Fe-2 catalyst was seriously sintered comparing withthe Ba–Fe-1 catalyst, and some amorphous phase appeared. Itsuggests that the homogeneous distribution of metal ions throughcombination of EDTA and CA as the complexants could inhibit theaggregation of the formed perovskite phases after calcination.Accordingly, a relatively uniform perovskite was formed for the Ba–Fe-1 catalyst.

The XRD pattern of the fresh Ba–Fe-2 catalyst was presented inFig. 4. It clearly shows that the BaFeO3 (PDF number: 75-0426) andBaFeO3−x phases (PDF number: 70-0034 and 70-1321) were formedon the Ba–Fe-2 catalyst, as well as the spinel BaFe2O4 (PDF number:70-2468) and carbonate phases (PDF number: 71-2394). The XRDpattern of the catalyst after NOx storage is also provided in Fig. 4. Afterthe NOx sorption, the intensity of the BaCO3 phase decreasedsignificantly, and a new diffraction peak, which is the strongestBa(NO3)2 diffraction peak (PDF number: 76-1376), appeared at 22.0°,simultaneously. It coincides with the previous studies [17,18] that theBaCO3 phase was one of the active NOx storage sites, and could betransformed to the nitrate species. Moreover, the diffraction peak at36.7° of the fresh Ba–Fe-2 catalyst belonging to BaFeO3 phaseweakened, and two new diffraction peaks at 36.4 (Fig. 4(c)) and 43.0°appeared, whichwere ascribed to the BaFeO3−x phase. It suggests that,during the NOx storage period, the BaFeO3 phase was partiallytransformed into the oxygen defected BaFeO3−x. The oxygen atomslocated in the perovskite structure were involved in the NOx

storage process due to its excellent oxidizing ability. These are

Fig. 2. Photos of the xerogel precursors of the (a) Ba–Fe-1 and (b) Ba–Fe-2 catalysts.

5 m

1 m1 m

5 m

(a) (b)

(c) (d)

Fig. 3. SEM images of the (a, c) Ba–Fe-1 and (b, d) Ba–Fe-2 catalysts.

1721H. Xian et al. / Fuel Processing Technology 92 (2011) 1718–1724

coincided with our previous work [17] that the perovskite phase canoxidize NO forming nitrate, which then can be transferred to theneighboring carbonate for storage. Moreover, the strongest diffraction

(1)

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2000

(3)

(a)

(3)

(2)

(1)

(b)

10 20 30 40 50 60 70 80 90

32.0 32.5 33.0 33.5 34.0 36 37 38

(3)

(2)

(1)

(c)

Fig. 4. XRD patterns of the Ba–Fe-2 catalyst with the various treatments: (1) freshcatalyst; (2) after NOx storage; and (3) sulfated catalyst.

peak of the perovskite shifts to the small diffraction angle, indicatingthat the unit cell of the perovskite expanded after NOx sorption. Aftersulfation, the intensity of the diffraction peaks of the perovskiteweakened a little, and the unit cell of the perovskite also expanded(Fig. 4(c)). Meanwhile, the XRD pattern of the spinel phase had littlechange comparing with the fresh sample, including the shape and theposition of thediffraction peak, but their intensity of thediffraction peakhad a little suppressed after the NOx or SO2 treatment.

The XRD pattern of the Ba–Fe-1 catalyst is depicted in Fig. 5. Thestrong diffraction peaks belonging to the perovskites (BaFeO3 andBaFeO3−x) and the weak diffraction peaks of the spinel were clearlyobserved. The diffraction peak at 30.6° was corresponding to thecharacteristic diffraction peak of the BaFeO3−x phases (PDF number:70-0034 and 70-1321), indicating that a large amount of the defectiveperovskite species was formed. Here, almost no carbonate phase wasdetected, and only a tiny of the BaFe2O4 diffraction peakwas observed.These results coincided with the previous results in Figs. 1 and 2 thatthe addition of EDTA into the aqueous precursor solution made themetal ions, which was fixed by the coordination complexants [20],distribute homogeneously in the gel. Thereafter, the xerogel reactedstoichiometrically forming a relatively uniform perovskite structurewith less impurity. After the NOx sorption, the diffraction peak of Ba(NO3)2 was detected, and the diffraction peak at 36.7° shifted slightlyto the small diffraction angle, the same behavior as that of the Ba–Fe-2catalyst, suggesting that they have the similar NOx storage behavior.The regeneration of the NOx stored catalyst was also characterizedhere. After the H2 reduction at 400 °C for 1 h, the diffraction peak ofnitrate disappeared indicating that it was fully reduced. The intensityof the diffraction peaks of the perovskites were also weakened greatly.Moreover, a new diffraction peak at 35.4° was detected, which is thestrongest diffraction peak of Ba3Fe2O6 phase (PDF number: 25-1477).It suggests that some iron in the perovskite crystal lattice was reducedto lower valence state, and released out of the perovskite structureafter the serious reduction by H2. All of these results indicate that thestructure of the perovskites was partially destroyed under thereducing conditions. Then, the reduced perovskite catalyst was

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10 20 30 40 50 60 70 80 90

32.0 32.5 33.0 33.5 34.0 36 37 38

(c)

(5)

(3)

(2)

(4)

(1)

Fig. 5. XRD patterns of the Ba–Fe-1 catalyst with the various treatments: (1) freshcatalyst; (2) after NOx storage; (3) reduction of catalyst (2); (4) re-oxidization ofcatalyst (3); and (5) sulfated catalyst.

0 200 400 600 800

Ba-Fe-1

Ba-Fe-2

Temperature / °C

Fig. 6. O2-TPD profiles of the catalysts.

(a)

60 min

40 min

20 min10 min5 min1 min

981

10761106

1184

1462

Wavenumber / cm-1

0.005

1500 1400 1300 1200 1100 1000 900

Wavenumber / cm-11500 1400 1300 1200 1100 1000 900

(b)

1080

11091188

60 min

40 min

20 min10 min

5 min

1 min

0.01

Fig. 7. DRIFT spectra following the sulfation duration of the (a) Ba–Fe-2, and (b) Ba–Fe-1 catalysts.

1722 H. Xian et al. / Fuel Processing Technology 92 (2011) 1718–1724

treated in air at 400 °C for 1 h. The diffraction peak at 36.7°, which is themain diffraction peak of the perovskite species, recovered again, andother diffractionpeaksalsowellmatchedwith that of theXRDpatternofthe fresh catalyst with the similar position and intensity. This resultdemonstrates that the structure of the perovskite catalyst after theserious reduction in the reducing atmosphere can be regeneratedsuccessively at the real operating temperature zone of the after-treatment system under lean-burn atmosphere, which meets therequirement of the NOx storage-reduction technology indeed. Aftersulfation, we also found the expansion of the unit cell of the perovskiteas the Ba–Fe-2 catalyst, and no sulfated phase was detected here.

The BET surface area of the Ba–Fe-1 and Ba–Fe-2 catalyst is 3.8 and0.9 m2 g−1, respectively. Interestingly, the Ba–Fe-2 perovskite catalysthas a smaller BET surface area, but possesses a larger NSC (Table 1)comparing with the Ba–Fe-2 catalyst. A lot of the carbonate species,which might be in the form of amorphous state and fill in the pore ofthe perovskite structure as shown in Fig. 3, was detected in the Ba–Fe-2 catalyst. So, the Ba–Fe-2 catalyst's BET surface area was suppressed.Nevertheless, NOx could be oxidized on the perovskite phase, andthen transferred to the neighboring carbonates, regenerating theactive sites on the perovskites [17,18]. Therefore, the presence of thecarbonate phase synergistically enhanced the perovskite catalyst'sNSC significantly.

The O2-TPD profiles of the catalysts were shown in Fig. 6. The twocatalysts had the similar oxygen desorption behavior. The first broadO2 desorption peak centered at around 430 °C was due to the oxygenadsorbed from the gas phase or oxygen ions in the superficial crystallattice of the perovskite, while the second one at 807 °C was theoxygen desorption from the bulk. Obviously, more oxygen desorbedfrom the Ba–Fe-1 catalyst below 500 °C. It suggests that this catalysthas a better oxygen storage/release ability, which would affect itsredox property greatly, than that of the Ba–Fe-2 catalyst.

The in situ DRIFT spectra of the catalysts after sulfation are shownin Fig. 7. In Fig. 7(a), the vibration bands at 1184, 1106, 1065, and981 cm−1 corresponded to the sulfate species [17,21,22], and theband at 1184 cm−1 is due to the sulfate formed on perovskite, whilethe bands at 1109 and 1080 cm−1 were due to the sulfation of thecarbonate or iron oxide species [17]. Here, a negative IR peak wasobserved at 1462 cm−1, and the intensity of the negative peak wasstrengthened following the SO2 sorption period. Our previous studyshows that this negative peak can be attributed to the transformationfrom carbonate to nitrate or sulfate [17,20,23]. The presence ofcarbonate on the Ba–Fe-2 catalyst has been confirmed by our XRDresults in Fig. 4. Therefore, in this study, the negative peak should be

1723H. Xian et al. / Fuel Processing Technology 92 (2011) 1718–1724

correlated to the elimination of the BaCO3 phase and the formation ofthe BaSO4 phase. In Fig. 7(b), the sulfate on perovskite and othersulfate species, like presented in Fig. 7(a), also appeared on the Ba–Fe-1 catalyst after the sulfation. Nevertheless, no negative peakconcerning the carbonate phase was observed, which has beenconvinced by the TG and XRD results (Figs. 1, 4 and 5) that almost nocarbonate existed on the Ba–Fe-1 perovskite catalyst. Here, the intensityof the band at 1188 cm−1 was stronger than that of the bands at 1109and 1080 cm−1, which is contrary to the results in Fig. 7(a). It is becausemore perovskite phase existed in the Ba–Fe-1 catalyst. As presented inTable 1, the Ba–Fe-2 catalyst had the high NSC but poor sulfur-resistance, while the Ba–Fe-1 catalyst was on the contrary. These resultscoincide with our previous study that the presence of carbonate couldenhance the NSC of the catalyst [18] with NO and O2 exposure, butsuppress NSC greatly after sulfation [17].

To elucidate the different sulfur-resistance behaviors of the Ba–Fe-1 and Ba–Fe-2 catalysts, the H2-TPR tests of the fresh and the sulfatedcatalysts were conducted. Here, the H2-TPR profiles of the commercialFe2O3, which was calcined at 750 °C for 6 h, and Fe2(SO4)3[17] werealso presented to identify each reduction peak of the perovskitecatalysts.

As shown in Fig. 8(a), on the profiles of the Fe2O3 sample, threereduction peaks were clearly observed. The three reduction peakswere probably due to the reduction from Fe2O3 to Fe3O4[24], Fe3O4 toFeO, and FeO to metallic Fe [24], respectively. The variation of the H2-TPR profile of the fresh Ba–Fe-2 catalyst was quite similar with that ofthe Fe2O3 sample, except a weak reduction peak at 410 °C. On theprofile of the Ba–Fe-1 catalyst, a weak reduction peak at around410 °C was also observed. It might be due to the reduction of theoxygen atoms stored from the gas phase or the reduction of Fe4+ toFe3+[17,25]. The broad peak from about 550 to 700 °C might be

intensity divided by 50

825

614745

615

410508

Fe2O

3

784640

480

(a)

Ba-Fe-2

Ba-Fe-1

0 150 300 450 600 750 900

intensity divided by 100

(b)

Fe2(SO

4)

3

Ba-Fe-2

Ba-Fe-1

Temperature / °C

0 150 300 450 600 750 900Temperature / °C

Fig. 8. H2-PTR profiles of the Ba–Fe-1 and Ba–Fe-2 catalyst. (a) Fresh catalyst, and(b) after sulfation.

correlated to the reduction of the Fe3+ in the perovskite crystallattices [26], and the reduction peak at above 750 °C was assigned tothe partial reduction of Fe2+ to Fe0[26] from the perovskite. The shiftof the reduction peak to higher temperature than that of the Ba–Fe-2catalyst was because the iron oxide in the bulk perovskite weredifficultly reduced than that of the well dispersed one. Accordingly,the intensity of the reduction peak of the Ba–Fe-1 catalyst was alsomuch smaller than that of the Ba–Fe-2 catalyst. These results are ingood agreement with the XRD results presented in Fig. 5 that theBa3Fe2O6 species could be formed during the reduction, indicatingthat iron in perovskite was hardly reduced to the metallic state.

After the sulfation pretreatment, the H2-TPR profiles of the Ba–Fe-1,Ba–Fe-2 catalysts, and the commercial Fe2(SO4)3 are plotted in Fig. 8(b).The profile of the Fe2(SO4)3 sample presented a broad shoulder peak atthe range from 490 to 780 °C. This peak should include the reduction ofFe3+ to Fe2+ and Fe2+ to Fe0, and the reduction of SO4

2−. The variation ofthe H2-TPR profile of the Ba–Fe-1 catalyst was very similar with that ofthe Fe2(SO4)3 sample at the temperature range from 480 to 760 °C. Itsuggests that the H2 consumption of the Ba–Fe-1 catalyst at thistemperaturewindowwasmainly due to the reduction of the Fe2(SO4)3-like species, and itmight also be partially contributed by the reductionofthe BaSO4. This result is quite similar with our previous study on thesulfated BaFeO3 − x catalyst [17] that the formation of Fe2(SO4)3-likespecies leading to the improved sulfur-resistance. The weak reductionpeak appeared at above 760 °Cwasdue to the reduction of Fe2+ to Fe0 inthe bulk perovskite crystal lattices as discussed in Fig. 8(a). For the Ba–Fe-2 catalyst, a broad peak from 450 to 900 °C was observed. From theresults of the in situDRIFT experiments (Fig. 7(a)), BaCO3 on theBa–Fe-2catalyst could be transformed to BaSO4. Many reactions were involvedhere, such as the reduction of the Fe2(SO4)3, BaSO4, Fe2O3, and so on.Among these overlapped reduction peaks, the strongest reduction peakwas centered at around 776 °C, which is higher about 30 °C than that ofin Fig. 8(a). This reduction peakmight include not only the reduction ofFe2+ to Fe0 but also sulfates, suggesting more hardly reduced bulksulfates were formed on the Ba–Fe-2 catalyst than that of the Ba–Fe-1catalyst. This result is in good agreement with the NSC results that thelatter one has good sulfur-resistance property.

To better understand the NOx sulfation processes of the perovskitecatalyst, the XPS measurements were conducted. The elementalanalysis on the Ba–Fe-1 and Ba–Fe-2 catalysts' surface is listed inTable 2. For the fresh Ba–Fe-1 catalyst, its Ba/Fe molar ratio is around1.1. After the NOx storage treatment, the Ba content on the catalystsurface dropped from 11.4 to 7.5 %, and the Ba/Fe molar ratiodecreased to 0.8, which may be due to the formation of bariumnitrates, which has been convinced by the XRD results in Fig. 5.Whereas, after the sulfation treatment, the Fe content on the catalystsurface dropped significantly from 10.3 to 3.3 %, and the Ba/Femolar ratio increased dramatically to 4.1, indicating the formation ofFe2(SO4)3-like species. For the Ba–Fe-1 catalyst, these results revealedthat, during the NOx storage process the NOx molecule mainly bondedwith Ba atoms, while, during the sulfation process the sulfur mainlydeposited on iron forming the Fe2(SO4)3-like species [17]. For the Ba–

Table 2Catalyst composition on the catalysts after the different treatment calculated by the XPSmeasurements.

Catalyst composition (at.%)

Ba 3 d5/2 Fe 2p3/2 O 1s S 2p3/2 Ba/Fe ratio

Fresh Ba–Fe-1 11.4 10.3 78.3 / 1.1Ba–Fe-1 after NOx storage 7.5 9.7 82.8 / 0.8Sulfated Ba–Fe-1 12.3 3.3 70.1 14.3 4.1Fresh Ba–Fe-2 17.8 7.7 74.5 / 2.3Sulfated Ba–Fe-2 10.8 5.4 70.7 13.1 2.6BaSO4

a 14.1 / 67.6 18.3 /Fe2(SO4)3a / 2.7 77.3 20.0 /

a The commercial samples.

1724 H. Xian et al. / Fuel Processing Technology 92 (2011) 1718–1724

Fe-2 catalyst, the Ba content dropped significantly after sulfation from17.8 to 10.8 %, while the Fe content decreased slightly from 7.7 to 5.4%.A lot of BaSO4 phase was formed here. All of these results suggest thatthe Ba and Fe ions arewell dispersed in the perovskite crystal lattice forthe Ba–Fe-1 catalyst, which is mainly composed by the uniformperovskite phase, and accordingly, it presents a better sulfur-resistance property comparing with that of the Ba–Fe-2 catalyst.

4. Conclusions

The complexants used during the xerogel precursor synthesisprocess by the sol-gel method play an important role to affect thecomposition of the BaFeO3 − x perovskite catalyst significantly, as wellas their NOx storage and sulfur-resistance. The xerogel precursorexpanded extremely after drying by addition of EDTA as one of thecomplexants. It makes the metal ions in the xerogel mix homoge-neously and form stoichiometrically the perovskite without carbonateand with a tiny of the spinel phases. The Ba–Fe-1 catalyst, which usedCA and EDTA (CA:EDTA=1.5:1) as complexants, had a uniformperovskite structure and presented an excellent sulfur-resistance.During the NOx storage, NO was mainly bonded with Ba atoms. Thedestroyed perovskite structure of the Ba–Fe-1 catalyst could beregenerated successfully in air at 400 °C after experiencing the seriousreduction. If only CA was used as the complexant, a lot of amorphouscarbonate was formed on the Ba–Fe-2 catalyst, inducing thesuppressed sulfur-resistance property.

Acknowledgement

The financial aids from the National Natural Science Foundation ofChina (No. 20806056), the “863 Program” of the Ministry of Science &Technology of China (No. 2008AA06Z323), the Natural ScienceFoundation of Tianjin (No. 09ZCGHHZ00400, and 11JCYBJC03700), theFoundation of State Key Laboratory of Coal Conversion (No. 10-11-902),and the Doctoral Fund of Ministry of Education of China (No.200800561002 and No. 20090032110013) are greatly appreciated.

This work is also financially supported by the Program for New CenturyExcellent Talents in University of China (NCET-07-0599) and theProgram for Introducing Talents of Discipline to Universities of China(No. B06006).

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