BaCO3–Al2O3 used for efficient NOx storage and desulfation: Comparison with conventional impregnated catalyst

Available online at www.sciencedirect.com

Microporous and Mesoporous Materials 113 (2008) 277–285 www.elsevier.com/locate/micromeso

Highly thermo-stable mesoporous catalyst Pt/BaCO3–Al2O3 used for efficient NOx storage and desulfation: Comparison with conventional impregnated catalyst Jin-Yong Luo, Ming Meng *, Xin-Gang Li, Yu-Qing Zha Department of Catalysis Science and Technology, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China Received 17 July 2007; received in revised form 15 November 2007; accepted 22 November 2007 Available online 3 January 2008

Abstract A mesoporous NSR catalyst Pt/BaCO3–Al2O3 was synthesized by using tri-block copolymer P123 as template. Systematic comparative studies on the structural and catalytic performance between the mesoporous catalyst and the conventional impregnated one were performed. N2 physisorption, XRD, TPD were employed for their structural characterization. In situ DRIFTS, TPR, TEM were used for investigation of the catalytic behaviors for NOx and SOx sorption, as well as desulfation. The results of structural characterization show that mesoporous Pt/BaCO3–Al2O3 exhibits high surface area (261 m2 g 1 after calcination at 600 °C), uniform pore size with a diameter of ca. 5 nm and high thermal stability up to 800 °C. The Ba-containing species are highly dispersed in three-dimensions and strongly interacted with Al2O3, and all the BaCO3 presents as LT-BaCO3 (BaCO3 with low thermal stability). By contrast, most of the Ba species in the impregnated sample exist predominantly as HT-BaCO3 (BaCO3 with high thermal stability) and are enriched on the surface. As a result, the mesoporous sample possesses great advantages in serving as NSR catalysts, such as enhanced NOx trapping ability, lower sulfation degree, and higher desulfation extent, as compared with the impregnated one. In addition, after NOx and SOx sorption, no bulk phases of barium nitrates and sulfates were observed in the mesoporous catalyst, while they are evidently formed on the impregnated one. In a word, the mesoporous structure is of great significance in achieving high dispersion of barium species and better performance for NOx storage and regeneration of the catalyst. Ó 2007 Elsevier Inc. All rights reserved. Keywords: Mesoporous structure; Pt/BaCO3–Al2O3; Dispersion; NOx trap; Desulfation

1. Introduction The extensive application of lean-burn engines improves the fuel efficiency and decreases the emission of carbon dioxide. However, due to the presence of excess oxygen in the tail-gas, the conventional three-way catalyst (TWC) can not reduce NOx effectively [1]. Within the last decade, a new concept, namely NOx storage-reduction (NSR) was developed [2,3], and the catalysts mainly consisting of Pt/Ba–Al–O were first put into the markets in Japan. *

Corresponding author. Tel.: +86 (0)22 2789 2275; fax: +86 (0)22 2740 5243. E-mail address: [email protected] (M. Meng). 1387-1811/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.11.025

Since then, the Pt–Ba/Al2O3 system has been extensively investigated, and theoretical models for NOx storage have been developed [4–10]. Recently, great attention has been attracted to the morphology properties of barium species. Two different Ba-containing species on Ba/Al2O3 were identified: a monolayer BaO on Al2O3 surface and nanocrystalline BaO particles on this BaO monolayer [11,12]. Since large amount of CO2 is present in the exhaust, it is thermodynamically more favorable for the formation of BaCO3 [13], rather than BaO. Similarly, two types of BaCO3 species were reported for the Pt–Ba supported NSR catalysts [14–16], namely LT-BaCO3 (barium carbonate with low thermal stability) and HT-BaCO3 (bulk barium carbonate with high thermal stability). In general, the bulk barium species can hardly serve as active phase

278

J.-Y. Luo et al. / Microporous and Mesoporous Materials 113 (2008) 277–285

in real operations due to the diffusion limitation during both storage and reduction, whereas the LT-BaCO3 with a higher dispersion is considered as the main active phase for NOx storage and reduction. Furthermore, it has been reported recently that the removal of sulfur species and the catalyst regeneration are much easier for sulfur-poisoned Pt–Ba/Al2O3 catalyst with a lower barium loading [17]. As it is known, the samples with a low barium loading normally have barium species of high dispersion. Therefore, it is extremely important to disperse the BaCO3 phase in Pt–Ba/Al2O3 catalyst, in order to achieve improved performance for NOx storage and sulfur removal. It is well believed that the preparation method plays a crucial role in elevating component dispersion. For the Pt–Ba/Al2O3 NSR catalysts, the most frequently used method is impregnation. In this method, Ba species are two-dimensionally dispersed on Al2O3 surface. In order to achieve higher NOx storage capacity, the Ba-loading must be high enough. However, if the barium loading exceeds the amount required for the monolayer dispersion, excess Ba species tend to form large bulk-like particles, resulting in the inefficient utilization of the storage phase. In order to gain a higher degree of barium dispersion, many other methods are developed, like co-precipitation [18], flame-made synthesis [14,16,19] and sol–gel method [20], etc. It is well known that mesoporous materials generally exhibit high surface area and excellent component dispersion, holding great potential as trap materials. Unfortunately, little research has been conducted in applying mesoporous materials to NOx trap. Recently, by using tri-block copolymer as template, a series of mesoporous materials have been successfully synthesized [21,22]. These materials generally exhibit high resistance to thermal deterioration. Consequently, in this work, by using EO20PO70EO20 poly (alkylene oxide) block polymer Pluronic P123 as structure-directing agent, highly thermo-stable mesostructured oxide BaCO3–Al2O3 was synthesized. Compared with the conventional impregnated sample, this catalyst shows larger surface area, higher component dispersion, and superior performance for NOx storage and desulfation.

2. Experimental 2.1. Catalyst preparation The mesoporous BaCO3–Al2O3 was synthesized as follows: 0.001 mol P123 (5.8 g, BASF) and appropriate Ba(NO3)2 (Tianjin No. 3 Chemical Reagent Factory) were dissolved in 90 ml distilled water under stirring at 60 °C, then 0.05 mol aluminum isopropoxide (AIP, Fuchen Chemical Reagent Factory; Ba/Al = 1:12) was added and stirred for 6 h to allow hydrolysis. After cooling to room temperature, excess amount of (NH4)2CO3 was added dropwise to precipitate Ba2+ into BaCO3. After standing at room temperature for 48 h, the precipitate was filtered

off, washed with water, dried at room temperature and calcined at different temperatures for 4 h. For comparison, another BaCO3–Al2O3 sample was also prepared by wet impregnation method, using Al2O3 (300 m2 g 1, Tianjin Research & Design Institute of Chemical Industry) and Ba(Ac)2 (Ba/Al = 1:12) as raw materials. Ba(Ac)2 is selected because of its high solubility and the desired decomposition product BaCO3. After drying at 110 °C for 12 h, the solid was calcined at 600 °C for 4 h in air. After calcination at 600 °C, the above two kinds of BaCO3–Al2O3 solids were impregnated with chloroplatinic acid to give a 1 wt% Pt loading, followed by drying at 110 °C for 3 h, calcined at 500 °C for 1 h in air and reduced at 450 °C for 0.5 h in pure hydrogen flow. The final mesoporous and impregnated samples are denoted as Pt/Meso600 and Pt/IM-600, respectively. 2.2. Catalyst characterization Surface area, pore volume and pore size distribution of the sample were measured by nitrogen adsorption/desorption at 77 K using a Quantachrome NOVA-2000 instrument. The samples were degassed at 350 °C for 10 h prior to the adsorption experiments. The specific surface area (SBET) was determined by the BET method in the 0–0.3 partial pressure range and the pore size distribution was determined by the Barrett–Joyner–Halenda (BJH) method from the desorption branch of the isotherm. X-ray powder diffraction (XRD) measurement was carried out on an X’pert Pro rotatory diffractometer operating at 40 mA and 40 kV using Co Ka as radiation source (k = 0.1790 nm). The data of 2h from 20° to 90° were collected with the stepsize of 0.017°. Temperature-programmed thermal decomposition of BaCO3 measurements were performed on a Thermo-Finnigan TPDRO 1100 instrument with a thermal conductivity detector (TCD). Two hundred milligram sample was heated from room temperature to 1000 °C at a rate of 10 °C min 1 in pure helium with a flow rate of 20 ml min 1. Before detection by the TCD, the gas was purified by a trap containing Mg(ClO4)2 in order to remove H2O traces. The measurement of temperature-programmed reduction by H2 (H2-TPR) was also performed on the ThermoFinnigan TPDRO 1100 instrument. Before the test, samples were sulfated at 400 °C for 1 h in a gas mixture containing 180 ppm (vol) SO2, 10 vol% O2, and N2 for balance. For the TPR measurement, the reducing gas contains 5 vol% H2 balanced by pure N2, and a flow rate of 20 ml min 1 was used. The quartz tube reactor was loaded with 20 mg sample at a heating rate of 10 °C min 1. Before detection by the TCD, the gas was purified by a trap containing CaO + NaOH materials in order to remove the H2O, H2 S and CO2. Transmission electron microscopy (TEM) images were obtained using a Philips Tecnai G2F20 microscope operating at 200 kV.

J.-Y. Luo et al. / Microporous and Mesoporous Materials 113 (2008) 277–285

2.3. In situ diffuse reflectance infrared spectroscopy (in situ DRIFTS) In situ DRIFTS experiments were performed on a Nicolet Nexus spectrometer equipped with a MCT detector cooled by liquid nitrogen, and a heating chamber allowing samples to be heated up to 600 °C. The DRIFTS spectra were recorded against a background spectrum of the sample purified just prior to introducing the adsorbates. In each run, about 11 mg of the sample was used. The sample was pretreated in a 6 vol% O2/He flow at 300 °C for 0.5 h. After cooling to room temperature, a background spectrum was collected. For the NOx sorption experiment, a gas mixture containing 360 ppm NO, 3 vol% O2 and N2 for balance was introduced. After sorption at room temperature for 30 min, the temperature was increased from 100 °C to 600 °C at a 50 °C interval. The spectra were collected after holding at each temperature for 20 min. By replacing the 360 ppm NO with 100 ppm SO2, the SOx sorption was also carried out. Due to the low concentration of SO2, the spectra were collected after holding at each temperature for 30 min. 3. Results and discussion 3.1. Structural properties

Volume adsorbed / a. u.

dV/dD(cm3g-1nm-1)

Fig. 1 shows the N2 adsorption/desorption isotherms of the samples. It can be seen that after calcination at 600 °C, the BaCO3–Al2O3 sample exhibits a typical IV isotherm [23], with the P/P0 position of the inflection point corresponding to a diameter in the mesoporous range, the BJH pore size distribution of this sample is very narrow,

Pt/Meso-600 Meso-600 Meso-700 Meso-800 Meso-1000

0

10

20

30

40

50

Pore Diameter(nm)

Pt/Meso-600 Meso-600

Meso-700 Meso-800 Meso-1000 0.0

0.2

0.4

0.6

0.8

1.0

P / P0 Fig. 1. Nitrogen adsorption/desorption isotherms of the samples.

279

centered at ca. 5.7 nm (see the insert in Fig. 1). As the calcination temperature increases to 800 °C, no appreciable change in the isotherms takes place, and only a slight shift of pore diameter to larger size and a little widening of BJH pore size distribution are observed. Further increase in the calcination temperature (1000 °C) results in the great decrease in the area of the hysteresis loop, implying the collapse of mesoporous structure. Anyway, the mesostructure could be maintained at least to 800 °C. At the same time, even if the sample was hydrothermally treated at 500 °C for 6 h (4% O2, 20% steam, balance N2), no obvious change in the texture properties can be observed, indicating the high stability of this catalyst and its great potential for real application. Table 1 lists the texture data of these samples, as well as the impregnated Pt/IM-600 sample. From this table, it can be seen that sample Meso-600 displays high surface area of 236 m2 g 1, larger than the samples prepared by sol–gel method (190 m2 g 1) [20]. After thermal treatment at 800 °C, a large surface area of 182 m2 g 1 can still be remained. After Pt deposition, the surface area increases to 261 m2 g 1, much larger than the samples prepared by the impregnation method (192 m2 g 1). Fig. 2 depicts the XRD patterns of the samples. From the patterns of the mesoporous sample calcined at different temperatures as shown in Fig. 2a, it can be seen that in the as-prepared sample calcined at 600 °C, component Ba exists as orthorhombic BaCO3 witherite (ICSD #: 15196). While in samples calcined at 700 and 800 °C, no Ba-containing reflections can be observed, suggesting that barium species exist possibly as amorphous BaO or BaAl2O4, due to the thermal decomposition of BaCO3 and the interaction between Ba species and Al2O3, respectively. After calcination at high temperature 1000 °C, a weak peak at ca. 33° (2h), characteristic of BaAl2O4 appears. Fig. 2b shows the XRD patterns of the samples prepared by different methods, as well as the Pt-supported catalysts. All the samples clearly display reflections of BaCO3. After deposition with Pt, it is interesting that the BaCO3 reflections slightly decrease in the intensity for Meso-600, while increase predominantly for IM-600. This may be resulted from the different existing state of barium species. To make clear this question, the measurement of temperature-programmed thermal decomposition of BaCO3 (TPD) was carried out and the results are shown in Fig. 3. The TPD pattern of Meso-600 presents two types of CO2 evolution peaks, with the peak zenith at 180 and 805 °C, which can be attributed to the desorption of weakly adsorbed CO2 and the decomposition of LT-BaCO3 [14], respectively. Since LT-BaCO3 begins to decompose at ca. 630 °C, after calcination at 700 and 800 °C the barium species has transformed into small XRD-invisible BaO or BaAl2O4 phase, as indicated in Fig. 2a. For the sample prepared by impregnation, a predominant peak appears at high temperature of 970 °C, which can be assigned to the decomposition of bulk HT-BaCO3. Considering the nature of the preparation methods, it can be expected that barium

280

J.-Y. Luo et al. / Microporous and Mesoporous Materials 113 (2008) 277–285

Table 1 Texture data of the catalysts

800

SBET (m2 g 1)

Pore diameter (nm)

Pore volume (cm3 g 1)

Meso-600 Meso-700 Meso-800 Meso-1000 Pt/Meso-600 Pt/IM-600

236.4 203.7 181.7 102.3 261.0 190.6

5.7 6.2 7.2 9.2 5.0 –

0.467 0.432 0.423 0.301 0.422 –

* *

Intensity / a.u.

40

&

50

60

2θ /

70

90

& Al2O 3 * BaCO3

*

Intensity / a.u.

80

o

*

&

*

& * * *& ** *

& Pt/IM-600

IM-600 Pt/Meso-600 Meso-600 20

30

40

50

60

2θ /

70

100 200 300 400 500 600 700 800 900 1000

Fig. 3. Profiles of temperature-programmed thermal decomposition of BaCO3.

Meso-800

&

805

Temperature / oC

Meso-1000

30

IM-600

0

Meso-700

&

970

Pt-Meso-600

Meso-600

Meso-600

20

637

180

* ** *

# &

Pt-IM-600

& Al2O3 * BaCO3 # BaAl2O4

* *

CO2 desorption / a.u.

Sample

80

90

flame-made samples with relatively high barium loading [14]. The small size and its strong interaction with Al2O3 contribute to the easier decomposition. Meanwhile, the presence of weakly adsorbed CO2 on the mesoporous sample indicates that a relatively larger fraction of the surface is not covered by the barium species, since such CO2 is thought to be adsorbed on the surface of Al2O3 or at the Al–Ba interface, putting further evidence of the dispersion behavior. After Pt deposition, it is noted that the decomposition temperature for both types of BaCO3 is decreased by ca. 170 °C, from 805 °C to 637 °C for LT-BaCO3 and from 970 °C to 800 °C for HT-BaCO3, suggesting that Pt catalyzes this decomposition process. As a consequence, part of the LT-BaCO3 decomposes after Pt deposition, leading to the obvious decrease of BaCO3 reflections in Pt/Meso600. While for IM sample, although the stability of HTBaCO3 is decreased after Pt deposition, these species hardly decompose since the decomposition temperature is still higher than the calcination temperature. In contrast, Pt seems to promote CO2 uptake in IM sample, possibly through the transformation of some invisible BaO in amorphous state into BaCO3 during calcination after Pt deposition, resulting in higher intensity of BaCO3 reflections (Fig. 2b) and larger amount of CO2 evolution (Fig. 3).

o

Fig. 2. XRD patterns of the samples: (a) Mesoporous sample calcined at different temperatures; (b) before and after deposition of Pt.

species are two-dimensionally spread on the surface as bulk BaCO3 in IM-600 sample, whereas three-dimensionally dispersed throughout the sample Meso-600 as small BaCO3 crystalline. It is very similar to the model proposed to describe the dispersion behavior of the impregnated and

3.2. In situ DRIFTS study of NOx and SOx sorption In situ DRIFTS spectra of the samples during exposure to NO + O2 at different temperatures are shown in Fig. 4. The appearance of negative bands at 1466 and 1577 cm 1 is related to the decomposition of surface carbonates or their transformation to nitrites or nitrates during storage process. For Pt/Meso-600 as shown in Fig. 4a, the band at

J.-Y. Luo et al. / Microporous and Mesoporous Materials 113 (2008) 277–285

ionic nitrate

nitrite 0.3

bidentate nitrate

K-M unit

0.2

1231 carbonates 1466

0.1

1545

RT T100 T150 T200 T250 T300 T350 T400 T450 T500 T550

1577 0.0

1413 1317 -0.1 1700

1600

1500

1400

1300

1200

1100

1000

Wavenumber (cm-1)

T200 T250 T300 T350 T400 T450 T500 T550

1317

K-M unit

0.2

0.1

1231 1545 1464 1577

1406

0.0

1700

1600

1500

1400

1300

1200

1100

1000

Wavenumber (cm-1) Fig. 4. DRIFTS spectra of NOx sorption over the catalyst at different temperatures: (a) Pt/Meso-600; (b) Pt/IM-600.

1231 cm 1 due to surface nitrite [5] is formed after exposure to NO + O2 at room temperature. With the temperature increasing, this band increases slightly in the intensity until 150 °C. However, further promotion in the temperature leads to a decrease in its intensity. In parallel, characteristic bands of ionic nitrates at 1317 and 1413 cm 1 [5] appear, suggesting the phase transformation from nitrites to nitrates. This transformation proceeds predominantly at 250 °C and completes at 300 °C. At the same time, a weak band at 1545 cm 1 [5] due to bidentate nitrate (another band at 1300 cm 1 is overlapped) appears, indicating that a small amount of NOx is also stored as bidentate nitrate. When the temperature is increased to 350 °C, although the band of nitrite completely disappears, bands

281

due to ionic and bidentate nitrates still develop obviously, indicative of the continuous NOx storage. On this basis, it is suggested that depending on the temperature, at least two routes exist for NOx storage, namely nitrite and NO2 route, respectively. It is known that NO exposure to Ba/ Al2O3 and Pt/Ba/Al2O3 mainly generates nitrites [5], while NO2 exposure to these catalysts generates nitrates, even at very low temperature without the formation of nitrites [5,8,12]. Therefore, at low temperature (below 300 °C), NO can be directly stored as nitrite, which is then oxidized into nitrates gradually if the temperature is promoted. While at high temperature (above 300 °C), the oxidation of NO to NO2 occurs very quickly, and NO2 can be directly stored as nitrates without the formation of nitrite, possibly through a dismutation pathway. However, as the temperature is further elevated up to 400 °C, a decrease in the nitrates band can be observed, which is due to the thermal decomposition of the stored nitrates. At 550 °C, these nitrates are completely decomposed. Fig. 4b shows the in situ DRIFTS spectra for NOx storage over Pt/IM-600. It is rather similar to Fig. 4(a) for sample Pt/Meso-600, but some difference still exists. Firstly, the band intensity is lower for all these three types of nitrite and nitrates, as compared with Pt/Meso-600, especially in terms of the bidentate nitrate. Secondly, the stored NOx species in sample Pt/IM-600 show higher stability with appreciable amount of nitrates undecomposed even at 550 °C. Fig. 5 shows the in situ DRIFTS spectra of the samples during exposure to SO2 + O2 at different temperatures for 30 min. It can be seen that the bands at 1250 and 1172 cm 1assigned to S@O and S–O vibrations for BaSO4 [24], and that at 1346 cm 1 due to surface Al2(SO4)3 [24] develop gradually with the temperature increasing. The surface aluminum sulfate accounts for a larger portion of the total sulfates in Pt/Meso-600, as compared with catalyst Pt/IM-600, consistent with the TPD results that a larger fraction of the surface of Pt/Meso-600 is uncovered by barium species. Fig. 6 shows the evolution of the intensity of bands at 1317 cm 1 for ionic barium nitrate (as shown in Fig. 4) and 1250 cm 1 for barium sulfate (as shown in Fig. 5) during NOx and SOx sorption at different temperatures. Compared with Pt/IM-600, Pt/Meso-600 can trap larger amount of NOx in the lower temperature range. This is because that all the BaCO3 in this sample presents totally as active LT-BaCO3, which will decrease the diffusion limitation during storage, and increase the utilization of barium species. But it is noted that the stored NOx as nitrates possess lower thermal stability, which is due to the high dispersion state of barium species and their strong interaction with Al2O3. Our previous study [25] shows that the highly dispersed barium nitrate decomposes at ca. 450 °C, much lower than the temperature (570 °C) required for the bulk barium nitrate decomposition. Anyway, both impregnated and mesoporous catalysts achieve the best NOx storage performance at temperature 350 °C, which

282

J.-Y. Luo et al. / Microporous and Mesoporous Materials 113 (2008) 277–285 0.6

Nitrate band (1317 cm-1)

0.3

0.4

1172

K-M unit

1346 (6) 0.2

1577

1458

1.8

Pt/IM-600

Pt/Meso-600 0.2

1.2

Pt/IM-600 0.1

Pt/Meso-600

0.6

Sulfate band (1250 cm-1)

1250

0.0

(1) BaCO3

-0.2 1600

0.0

Surface Al2(SO4)3

1500

1400

BaSO4

0.0 200

300

400

500

600

T / ºC

1300

1200

1100

1000

900

Fig. 6. The evolution of the intensity of bands at 1317 cm 1 for ionic barium nitrate and at 1250 cm 1 for barium sulfate during NOx and SOx sorption over the catalysts as a function of temperature.

Wavenumber (cm-1)

1246 1.0

mesoporous catalyst has obvious advantages over the impregnated catalyst.

1169

3.3. Desulfation

K-M unit

(6) 0.5

1341

1467 0.0

(1)

1600

1500

1400

1300

1200

1100

1000

900

Wavenumber (cm-1) Fig. 5. DRIFTS spectra of SOx sorption over the catalyst at different temperatures: (a) Pt/Meso-600; (b) Pt/IM-600. Curve (1)–(6) represent the spectra from 200 to 450 °C at an interval of 50 °C.

is determined by the kinetic and thermodynamic limitation, or a balance between the oxidation rate and stability of the stored nitrates, coupled with the average work temperature in the engines [26]. Around this temperature, mesoporous catalyst exhibits larger NOx storage capacity. Different from the NOx adsorption, no decomposition of sulfates can be observed during sulfation, due to the high stability of these species. Comparison of SOx adsorption over Pt/Meso-600 and Pt/IM-600 as shown in Fig. 5a and b reveals that much larger amount of barium sulfate is formed on catalyst Pt/IM-600, as indicated by the corresponding much higher intensity, which is due to the surface enrichment of bulk BaCO3. Therefore, on the basis of the NOx and SO2 trap performance, it can be seen that the

TPR measurement was also performed for the investigation of the sulfur-poisoning and regeneration of the catalysts. The TPR profiles for samples sulfated at 400 °C are shown in Fig. 7. For the sulfated sample Pt/Meso-600, two reduction peaks can be observed, one at 470 °C due to the reduction of surface aluminum sulfate and the other at 597 °C due to the reduction of barium sulfate [27]. It should be noted that an additional peak appearing at ca. 700 °C is observed for sulfated sample Pt/IM-600, which can be attributed to bulk-like barium sulfate [27]. After peak deconvolution analysis, it is calculated that bulk-like BaSO4 accounts for 11% of the total amount of sulfur in sulfated Pt/IM-600. Therefore, for sample Pt/IM-600, both the NOx storage and sulfation result in bulk barium phases, which is considered to be related to the low dispersion state of barium species and the presence of HT-BaCO3 phase. For figuring out the transformation of sulfur-containing species, the sulfated samples were aged under oxidizing conditions (6 vol% O2/He) at different temperatures (600 or 800 °C), then TPR tests were performed, and the results are shown in Fig. 8. The small peak centered at 382 °C can be assigned to the reduction of platinum oxide. As the oxidation temperature increases, the shoulder reduction peak at 470 °C due to surface aluminum sulfates gradually decreases, suggesting their transformation into more stable barium sulfates. With respect to the reduction patterns for barium sulfate, no remarkable change takes place as the temperature was promoted to 600 °C. Nevertheless, as the temperature was further promoted to 800 °C, the amount of bulk barium sulfate greatly increases in sulfated

J.-Y. Luo et al. / Microporous and Mesoporous Materials 113 (2008) 277–285

H2 Consumption / a.u.

702 470 Pt/IM-600 597

Pt/Meso-600 0

100

200

300

400

500

600

700

800

900

T / ºC Fig. 7. TPR profiles for samples sulfated at 400 °C.

Pt/IM-600, while still no bulk barium sulfate is formed in sulfated Pt/Meso-600. Therefore, the high dispersion state of barium species results in their strong interaction with Al2O3, and therefore, inhibits the aggregation of BaSO4. The interaction can be interpreted as a partial encapsulation of the barium species by Al2O3, which decreases their mobility. For investigating the regeneration behaviors (desulfation) for these two samples, as well as phase transformation of the sulfur-containing species under reducing condition, repeated TPR tests were performed. Between two successive TPR tests, the reduced samples were temperature-programmed oxidized in 6% O2/He flow to

615

400 600 800

1st 2nd 3rd

729

382

597

Pt/IM-600

900 800

800

H2 consumption / a.u.

H2 Consumption / a.u.

800 °C and held at this temperature for 10 min. The results are presented in Fig. 9. The most predominant feature is the appearance of an additional peak at high temperature 800 °C in the sulfated Pt/IM-600, but not in sulfated Pt/ Meso-600. In the above, the BaSO4 species reducible at ca. 700 °C have been attributed to bulk BaSO4 particles. Here, the reduction peak at 800 °C should be related to the reduction of the much larger particles of bulk BaSO4, which interact very weakly with Al2O3 and are far away from Pt sites. However, these large BaSO4 particles are not formed at the same temperature under oxidizing conditions, as indicated in Fig. 8. It is known that under reducing conditions, part of the BaSO4 is reduced into BaS. It seems that BaS is more easily agglomerated than BaSO4, possibly due to their different physicochemical properties (eg. BaS is cubic and BaSO4 is orthorhombic [27]). Therefore, with respect to the impregnated sample, large BaSO4 particles are transformed from agglomerated BaS after reoxidation. Although BaS is also formed in the mesoporous catalyst, bulk BaSO4 is still not formed, possibly due to the high dispersion of barium species and their strong interaction with Al2O3. Similarly, a partial Al2O3 encapsulation anchors the barium species and decreases their mobility. Furthermore, from the amount of the hydrogen consumption after one or two redox cycles, it is noted that the sulfur in Pt/Meso-600 can be more efficiently removed, as compared with Pt/IM-600. Therefore, it can be concluded that the removal of sulfur is much easier for the highly dispersed barium sulfate. The strong interaction of the barium species with both Al2O3 and Pt sites mainly contributes to the easier and efficient desulfation. On the one hand, the presence of Ba–O–Al bonds in monolayer morphology may modify the Ba–S bonds in a positive way, which contributes more facile (at lower temperature)

700 600

Pt/IM-600

500 400

Temperature / ºC

615

283

300 200

Pt/Meso-600 100

Pt/Meso-600 0

100

200

300

400

500

600

700

800

900

T / ºC Fig. 8. TPR profiles for samples aged under oxidizing condition at 600 °C or 800 °C, and a comparison with those of the samples sulfated at 400 °C (solid line).

0

1000

2000

3000

4000

0 5000

t / second Fig. 9. Repeated TPR profiles for sulfated samples. Between two successive TPR tests, samples were temperature-programmed oxidation to 800 °C and held for 10 min.

284

J.-Y. Luo et al. / Microporous and Mesoporous Materials 113 (2008) 277–285

and more complete desulfation than bulk BaSO4 [17]. On the other hand, the contribution of Pt–Ba interaction can not be excluded. Our previous study has shown that the interaction between Pt and Ba species plays a crucial role in selectively reducing the BaSO4 into H2S but not BaS, which contributes to the sulfur removal [25]. From another point of view, it is considered that the Pt–Ba interaction is also related to the dispersion state of barium species, since if the barium species are highly dispersed, the contact probability between barium species and Pt will be enhanced. To sum up, we conclude that the desulfation is much easier for the highly dispersed barium species. Finally, TEM test was performed for the fresh and sulfated-reduced catalysts. The results are shown in Fig. 10. Fresh sample Pt/Meso-600 exhibits a spongy structure with a large degree of porosity, and such a structure is still maintained even after sulfation and reduction at high temperature 800 °C. While both the fresh and treated sample Pt/IM-600 is characterized by relatively large agglomerated crystals. At the same time, some Pt particles less than 2 nm can be found in fresh samples, as well as in the sulfatedreduced sample Pt/Meso-600, whereas larger Pt particles

of ca. 3–4 nm, can only be identified in the sulfated-reduced Pt/IM-600 samples. The Pt particle growth results in the decreased Pt–Ba interaction, which is possibly another reason for the high reduction temperature of BaSO4 of the Pt/ IM-600 sample. 4. Conclusions A mesoporous mixed oxide BaCO3–Al2O3 was successfully synthesized using tri-block polymer P123 as template, which exhibits high surface area, good thermal stability and excellent dispersion of BaCO3. It is considered that all the BaCO3 phase is three-dimensionally dispersed as LT-BaCO3 and strongly interacted with Al2O3. While in the impregnated sample, most of the barium species are two-dimensionally enriched on the surface, existing as bulk HT-BaCO3. After Pt impregnation, the mesoporous catalyst exhibits higher NOx trapping capacity and lower sulfation ability. At the same tine, it is much easier and more efficient for the sulfur removal of the sulfated mesoporous catalyst. This is due to the strong interaction of the barium sulfates with both Al2O3 and Pt sites in the mesoporous

Fig. 10. TEM images of the samples: (a) fresh Pt/Meso-600, (b) fresh Pt/IM-600, (c) pretreated Pt/Meso-600 and (d) pretreated Pt/IM-600. Pretreated condition: sulfated at 400 °C and reduced at 800 °C.

J.-Y. Luo et al. / Microporous and Mesoporous Materials 113 (2008) 277–285

catalyst as a result of the high dispersion state of the barium species, as well as the smaller Pt particle size. Acknowledgments This work is financially supported by the Natural Science Foundation of Tianjin (No. 05YFJMJC09700), the Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20040056028), the ‘‘863 Program” of the Ministry of Science & Technology of China (No. 2006AA06Z348) and the National Natural Science Foundation of China (No. 20676097). The authors are also grateful to the support of the Program for New Century Excellent Talents in University of China (NCET-07-0599) and the Cheung Kong Scholar Program for Innovative Teams of the Ministry of Education (No. IRT0641). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.micromeso.2007.11.025. References [1] S. Matsumoto, CATTECH 4 (2000) 102. [2] H. Shinjoh, N. Takahashi, K. Yokota, M. Sugiura, Appl. Catal. B 15 (1998) 189. [3] S. Matsumoto, Catal. Today 90 (2004) 183. [4] E. Fridell, M. Skoglundh, B. Westerberg, S. Johansson, G. Smedler, J. Catal. 183 (1999) 196. [5] I. Nova, L. Castoldi, L. Lietti, E. Tronconi, P. Forzatti, F. Prinetto, G. Ghiotti, J. Catal. 222 (2004) 377. [6] Z. Liu, J.A. Anderson, J. Catal. 224 (2004) 18.

285

[7] Ch. Sedlmair, K. Seshan, A. Jentys, J.A. Lercher, J. Catal. 214 (2003) 308. [8] F. Prinetto, G. Ghiotti, I. Nova, L. Lietti, E. Tronconi, P. Forzatti, J. Phys. Chem. B 105 (2001) 12732. [9] C.M.L. Scholz, V.R. Gangwal, M.H.J.M. de Croon, J.C. Schouten, J. Catal. 245 (2007) 215. [10] P.J. Schmitz, R.J. Baird, J. Phys. Chem. B 106 (2002) 4172. [11] J. Szanyi, J.H. Kwak, J. Hanson, C. Wang, T. Szailer, C.H.F. Peden, J. Phys. Chem. B 109 (2005) 7339. [12] J. Szanyi, J.H. Kwak, D.H. Kim, S.D. Burton, C.H.F. Peden, J. Phys. Chem. B 109 (2005) 27. [13] P. Broqvist, I. Panas, H. Gro¨nbeck, J. Phys. Chem. B 109 (2005) 9613. [14] M. Piacentini, R. Strobel, M. Maciejewski, S.E. Pratsinis, A. Baiker, J. Catal. 243 (2006) 43. [15] M. Piacentini, M. Maciejewski, A. Baiker, Appl. Catal. B 72 (2006) 105. [16] R. Strobel, F. Krumeich, S.E. Pratsinis, A. Baiker, J. Catal. 243 (2006) 229. [17] D.H. Kim, J. Szanyi, J.H. Kwak, T. Szailer, J. Hanson, C.M. Wang, C.H.F. Peden, J. Phys. Chem. B 110 (2006) 10441. [18] X. Li, M. Meng, P. Lin, Y. Fu, T. Hu, Y. Xie, J. Zhang, Top. Catal. 22 (2003) 111. [19] R. Strobel, L. Ma¨dler, M. Piacentini, M. Maciejewski, A. Baiker, S.E. Pratsinis, Chem. Mater. 18 (2006) 2532. [20] C.K. Narula, S.R. Nakouzi, R. Wu, C.T. Goralski Jr., L.F. Allard Jr., AIChE J. 47 (2001) 744. [21] P. Yang, D. Zhao, D.I. Margolese, B.F. Chmelka, G.D. Stucky, Nature 396 (1998) 152. [22] Q. Liu, A. Wang, X. Wang, P. Gao, X. Wang, T. Zhang, Micropor. Mesopor. Mater. (2007), doi:10.1016/j.micromeso.2007.08.007. [23] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouque´rol, T. Siemieniewska, Pure Appl. Chem. 57 (1985) 603. [24] H. Abdulhamid, E. Fridell, J. Dawody, M. Skoglundh, J. Catal. 241 (2006) 200. [25] J.Y. Luo, M. Meng, Y.Q. Zha, Y.N. Xie, T.D. Hu, J. Zhang, T. Liu, Appl. Catal. B 78 (2008) 38. [26] S. Morandi, F. Prinetto, G. Ghiotti, M. Livi, A. Vaccari, Micropor. Mesopor. Mater. 107 (2008) 31. [27] S. Elbouazzaoui, E.C. Corbos, X. Courtois, P. Marecot, D. Duprez, Appl. Catal. B 61 (2005) 236.