- Email: [email protected]
Preparation and characterization of SBA-15 supported Pd catalyst for CO oxidation
Applied Catalysis B: Environmental 106 (2011) 672–680
Contents lists available at ScienceDirect
Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb
Preparation and characterization of SBA-15 supported Pd catalyst for CO oxidation Houpeng Wang, Chang-jun Liu ∗ Advanced Nanotechnology Center, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China
a r t i c l e
i n f o
Article history: Received 14 March 2011 Received in revised form 29 May 2011 Accepted 21 June 2011 Available online 28 June 2011 Keywords: Mesoporous material SBA-15 Palladium CO oxidation Plasma
a b s t r a c t SBA-15 supported noble metal catalysts for CO oxidation have been extensively investigated. However, no reported work on the SBA-15 supported palladium catalyst for CO oxidation can be found in the literature. In this work, highly dispersed palladium nanoparticles were synthesized within the uniform channels of SBA-15 via a glow discharge plasma reduction treatment. The obtained Pd/SBA-15 catalyst shows a good activity for CO oxidation. The activity can be further improved by the hydrogen reduction thermally. The low-angle X-ray diffraction (XRD) patterns and transmission electron microscope (TEM) indicate that the ordered mesoporous structure was well maintained during the catalyst preparation and reaction processes. The wide-angle XRD patterns and TEM images show that the spherical palladium nanoparticles are highly dispersed within the SBA-15 channels and remain stable after the reaction. The diffuse reflectance infrared Fourier transformation (DRIFT) spectroscopy of adsorbed CO confirms that the structure change caused by different preparation conditions has a significant influence on the activity of the catalyst. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Highly ordered mesoporous silica [1,2] has been extensively applied for the studies of catalysis [3,4], sensors [5] and drug delivery [6,7]. SBA-15 is by far the largest pore-size mesoporous silica material, which has a well ordered two-dimensional hexagonal structure with the uniform pore sizes of 2.0–30.0 nm and high surface area of 600–1000 m2 g−1 , as well as the highly hydrothermal and thermal stabilities [1,8,9]. There has been an increasing research interest in various SBA-15 supported catalysts. The studies explored include oxidation [10–12], desulfurization [13,14], adsorption [15], CO2 reforming [16,17] and many others. Especially, an intense investigation has been conducted on CO oxidation over the SBA-15 supported catalysts. The SBA-15 supported catalysts for CO oxidation provide us an ideal system for the fundamental studies. Among the catalysts investigated, the SBA-15 supported noble metal catalysts show good activities for CO oxidation. The metals tested include Au [18–20], Ag [21,22], Rh [23], and Pt [24]. However, there is still no reported work on the SBA-15 supported palladium catalyst for CO oxidation, although one can easily find many papers on it over the palladium catalysts loaded on other supporting materials, like Al2 O3 [25], CeO2 [26], TiO2 [27], FeOx [28], SAPO-34 [29] and NaZSM-5 [30].
∗ Corresponding author. Tel.: +86 22 27406490; fax: +86 22 27406490. E-mail address: ughg [email protected] (C.-j. Liu). 0926-3373/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2011.06.034
In this work, we attempt to investigate CO oxidation over the SBA-15 supported Pd catalyst. We confirm that the Pd/SBA-15 shows a good activity for CO oxidation. 2. Experimental 2.1. Preparation of catalysts The SBA-15 silica was synthesized in the laboratory of Prof. D.Y. Zhao (Department of Chemistry, Fudan University, P.R. China) [1]. Prior to use, the silica was calcined in air at 500 ◦ C for 3 h to remove the adsorbed water. It was then impregnated with an aqueous solution of palladium nitrate overnight. Considering the difficulty in the loading of metal ions into the pores of SBA-15 by the conventional incipient wetness impregnation, the glow discharge plasma treatment was applied in this work to load the metal in. During the plasma treatment, the metal ions are reduced by the electrons within the plasma. The plasma reduction was operated at room temperature. The glow discharge plasma setup and plasma reduction protocol have been previously described in detail [31–33]. Briefly, the sample (about 0.4 g), loaded on a quartz boat, was placed in a quartz tube (i.d. 35 mm) with two stainless steel electrodes (o.d. 30 mm). When the pressure in the discharge tube was evacuated to about 100 Pa, the glow discharge plasma was generated by applying 1000 V to the electrodes using a high voltage amplifier (Trek, 20/20B). The signal input for the high voltage amplifier was supplied by a function/arbitrary waveform generator (HewlettPackard, model 33120A) with a 100 Hz square wave. Ultra high
H. Wang, C.-j. Liu / Applied Catalysis B: Environmental 106 (2011) 672–680
P-Pd/SBA-15 H-Pd/SBA-15
100
80
CO conversion (%)
pure grade argon (>99.999%) was used as the plasma-forming gas. The plasma reduction was operated for 10 min each time (totally 6 times), with a manual mixing of the sample between the treatments to ensure even exposure to the plasma. The bulk temperature of the plasma was measured by infrared imaging (Ircon modes 100PHT). The result indicated that the heating effect of the glow discharge can be ignored because the temperature of the powder was close to room temperature. The sample reduced via plasma was denoted as P-Pd/SBA-15. The pure SBA-15 treated in this way was designated as P-SBA-15. The hydrogen reduced Pd/SBA-15 has also prepared in order to study the effect of reduction methods. The hydrogen reduced catalyst was prepared in this way: the plasma treated sample (P-Pd/SBA-15) was calcined at 500 ◦ C for 3 h under the flowing oxygen with a flow rate of 30 mL min−1 . Then the calcined sample was further reduced at 500 ◦ C for 4 h using a mixture of hydrogen and helium with a flow rate of 60 mL min−1 (5.11 vol.% H2 ). The obtained sample was designated as H-Pd/SBA15. The palladium loading amount is 2 wt% unless otherwise mentioned.
673
60
40
20
0
20
40
60
80 100 120 140 160 180 200 220 240
Temperature (°C) 2.2. Catalyst characterization N2 adsorption–desorption isotherms were measured at liquid nitrogen temperature using a Micromeritics Tristar 3000 instrument. The samples were degassed at 300 ◦ C for 3 h under vacuum before analysis. The Brunauer–Emmett–Teller (BET) equation (relative pressure between 0.05 and 0.25) was employed to calculate the specific surface areas. The pore size and pore volume were calculated from the adsorption branches of N2 physisorption isotherms and the Barrett–Joyner–Halenda (BJH) model. The X-ray powder diffraction (XRD) patterns of the samples were recorded on a Rigaku D/Max-2500 V/PC diffractometer with ˚ The X-ray source was a Cu K␣ radiation source ( = 1.54056 A). operated at 40 kV and 200 mA. The wide-angle XRD patterns were collected at a scanning speed of 8◦ /min over the 2 range from 10◦ to 90◦ , whereas the low-angle XRD patterns were collected at a scanning speed of 1◦ /min over the 2 range of 0.6–6◦ . The phase identification was made by comparison to the Joint Committee on Powder Diffraction Standards (JCPDSs). Transmission electron microscope (TEM) measurements were performed on a Philips Tecnai G2 F20 system operated at 200 kV. The samples were suspended into the ethanol and dispersed ultrasonically for 15 min. A drop of the suspension was deposited on a copper grid coated with carbon. The size distribution of Pd particles was determined from TEM images by counting more than 150 particles. X-ray photoelectron spectrometer (XPS) analyses were conducted on a Perkin-Elmer PHI-1600 spectrometer with monochromatic Mg K␣ (1253.6 eV) radiation. Binding energies were calibrated using the C1s peak (284.6 eV) as reference. CO adsorbed diffuse reflectance Fourier transform infrared (DRIFT) spectroscopy was obtained on a Tensor 27 spectrometer (Bruker). The instrument was equipped with a liquid nitrogen cooled mercury–cadmium–tellurium (MCT) detector, a diffuse reflectance accessory and a high temperature reaction chamber (Praying Mantis, Harrick). The powder (7 mg) was loaded into the reflectance cell and purged by flowing helium (20 mL min−1 ) at 300 ◦ C for 1 h to remove gas phase and weakly bound water. CO adsorption was carried out in 20 mL min−1 CO (1.11 vol.%)/helium for 30 min at 25 ◦ C. After the cell was flushed with helium (20 mL min−1 ) for another 30 min at the same temperature, the DRIFT spectra were recorded with 64 scans at 4 cm−1 resolution. All spectra were illustrated using Kubelka Munk units, which is linear with the concentration of surface species.
Fig. 1. Effect of the reaction temperature on CO conversion.
2.3. Activity test The temperature-dependent CO oxidation was performed in a quartz tube with an inner diameter of 4 mm under atmospheric pressure. A catalyst sample of 50 mg was packed into the quartz tube. The sample was pretreated in 30 mL min−1 argon at 150 ◦ C to remove the gas phase and weakly bound water. After cooling down to 20 ◦ C, the reaction was started in flowing CO/O2 mixture (total flow = 20 mL min−1 ; 1.0 vol.% CO, 20.0 vol.% O2 , balance N2 ) over the temperature range of 30–220 ◦ C. The effluent was then analyzed online by a gas chromatograph (Agilent 6890) equipped with a TDX01 column and a thermal conductivity detector (TCD). For each reaction temperature, the samples were collected after 45 min on steam to allow the attainment of steady state. And each sample was measured in 16 temperature points, thus the whole reaction time was 12 h. 3. Results and discussion 3.1. CO oxidation CO conversion versus reaction temperature for the samples PPd/SBA-15 and H-Pd/SBA-15 is presented in Fig. 1. Obviously, the conversion of CO of the two samples increases with the increasing temperature. Compared to the sample P-Pd/SBA-15, H-Pd/SBA15 exhibits a higher activity. The catalyst reduction method has a significant effect on the catalytic activity. The conversion over H-Pd/SBA-15 increases more quickly with the temperature during the whole process of the test. The reaction starts at 40 ◦ C over both samples. However, the initial conversion of the sample PPd/SBA-15 is 2.94%, while it is 5.34% over H-Pd/SBA-15. It can be also observed that the samples P-Pd/SBA-15 and H-Pd/SBA-15 exhibit typical light-off (ignition) profiles (Fig. 1), with light-off temperatures of 201 and 133 ◦ C (for 50% conversation). The corresponding 100% conversion temperatures (T100 ) are 206 and 135 ◦ C, respectively. In order to compare the catalytic performance of the 2 wt% HPd/SBA-15 with the other loading catalysts, 1 wt% H-Pd/SBA-15 and 5 wt% H-Pd/SBA-15 samples were prepared using the same procedure. As shown in Fig. 2, the palladium loading has a remarkable effect on the catalytic activity, where the 100% conversion
674
H. Wang, C.-j. Liu / Applied Catalysis B: Environmental 106 (2011) 672–680
( 100)
H-Pd/SBA-15-1% H-Pd/SBA-15-2% H-Pd/SBA-15-5%
100
( 110) ( 200)
SBA-15
Intensity (a.u.)
CO conversion (%)
80
60
P-Pd/SBA-15 H-Pd/SBA-15 P-Pd/SBA-15 after 12h reaction
40
H-Pd/SBA-15 after 12h reaction
20 1.0
1.5
2.0
2.5
3.0
3.5
4.0
Two-theta (deg.) 0
Fig. 4. XRD patterns at low diffraction angles.
20
40
60
80
100
120
140
160
180
Temperature (°C) Fig. 2. Effect of the Pd loading on the activity.
temperatures (T100 ) are 127, 135 and 140 ◦ C, respectively, for the H-Pd/SBA-15 catalysts with the 5%, 2% and 1% loading. From Figs. 1 and 2, it can be concluded that the SBA-15 supported Pd catalysts show a good activity on CO oxidation, compared to the reported works over palladium catalysts supported by other materials. Also, the catalyst reduction method has a significant effect on the catalytic activity. To understand the beneficial effect of the hydrogen reduction thermally, the catalyst characterization has been conducted and will be discussed below. 3.2. N2 adsorption–desorption
3.3. XRD characterization The low-angle XRD patterns of the samples are presented in Fig. 4. The support SBA-15 shows three well-resolved peaks at 2 ranged from 0.6◦ to 2.6◦ , associated with (1 0 0), (1 1 0) and (2 0 0) reflections of P6mm symmetry. These peaks are the characteristic patterns from the hexagonal ordered structure in SBA-15. After loaded with palladium and subsequently treated via glow discharge plasma, the SBA-15 in the sample P-Pd/SBA-15 shows a little change, whereas these three peaks shift slightly to lower angles, indicating that the frameworks of the silica host may be enlarged during the synthesis process via the argon plasma reduction. This result is consistent with the previous reports by our group [35]. On the other hand, after the sample P-Pd/SBA-15 being treated thermally at 500 ◦ C, the (1 0 0) diffraction peak of the sample HPd/SBA-15 shifts to larger 2 degree and the relative intensity of diffraction peaks reduces. This result suggests that the treatment at high temperature may cause some shrinkage of mesopores and
(B)
(A) (d) H-Pd/SBA-15
(d) H-Pd/SBA-15
dV/dr ( cm /g)
-3
Volume Absorbed ( cm /g, STP )
Fig. 3 shows N2 adsorption–desorption isotherms (A) and pore size distributions (B) of the samples. The textural properties of the samples are summarized in Table 1. All the isotherms are of type IV [34], which is a characteristic of mesoporous materials. SBA-15 and the SBA-15 treated via the plasma (Fig. 3A(a) and (b)) exhibit a hysteresis loop of H1 type, which typically features a two-dimensional P6mm structure formed by the open cylindrical mesopores. Additionally, the isotherms of these two samples present a sharp step in the P/P0 range of 0.6–0.78, which is an indicative of the uniformity of the pore size. However, the hysteresis loops of P-Pd/SBA-15 and H-Pd/SBA-15 become smaller (Fig. 3A(c) and (d)), with smaller SBET , VBJH and DBJH values (Table 1), suggesting that (i) the palladium
nanoparticles have been dispersed into the pores of the SBA-15 via the glow discharge plasma reduction; (ii) some channels in the SBA-15 for the sample H-Pd/SBA-15 may undergo a collapse as a result of the calcination at 500 ◦ C, because the loop of the sample H-Pd/SBA-15 is dramatically smaller than the loop of the sample PPd/SBA-15. This result is confirmed by the pore diameters and the surface area of the two samples (Fig. 3(B) and Table 1), where the pore diameter value is 7.7 nm for the two samples but the surface area is reduced from 528 to 472 m2 g−1 .
-3
(c) P-Pd/SBA-15
(b) P-SBA-15
(c) P-Pd/SBA-15
(b) P-SBA-15
(a) SBA-15 0.0
0.2
(a) SBA-15 0.4
0.6
0.8
Relative pressure ( P/P0)
1.0
1
2
3
4
5
6
7
Pore radius (nm)
Fig. 3. N2 adsorption–desorption isotherms (A) and pore size distributions (B).
8
9
10
H. Wang, C.-j. Liu / Applied Catalysis B: Environmental 106 (2011) 672–680
675
Table 1 Pore structure parameters of SBA-15 and Pd/SBA-15. Sample
d1 0 0 (nm)
a (nm)
SBET (m2 g−1 )
VBJH (cm3 g−1 )
DBJH (nm)
SBA-15 P-SBA-15 P-Pd/SBA-15 H-Pd/SBA-15
9.627 9.630 9.633 9.436
11.116 11.120 11.123 10.896
607 600 528 472
0.933 0.900 0.849 0.732
7.8 7.8 7.7 7.7
√ d1 0 0 : the interplanar spacing of the (1 0 0) plane; a: the unit cell parameter, a = 2d1 0 0 / 3; SBET , calculated by BET method at relative pressure of P/P0 = 0.05–0.25; VBJH , DBJH : pore volume and pore size calculated by BJH method.
●
structural disorder, which is similar to the observation reported in the literature [9]. Nevertheless, the sample H-Pd/SBA-15 still present three well-resolved peaks, indexed to (1 0 0), (1 1 0), and (2 0 0) diffraction peaks, indicating that the highly ordered structure is still well maintained after the high temperature treatment. After CO oxidation, there are still three resolved peaks obtained in the samples P-Pd/SBA-15 and H-Pd/SBA-15 after 12 h reaction. This demonstrates that the highly ordered structure is also well maintained during CO oxidation. However, it is interesting to notice that the intensities of the (1 0 0) peak together with two additional peaks decrease dramatically in the sample P-Pd/SBA-15 after 12 h reaction, by comparing the sample P-Pd/SBA-15, which could be ascribed to the heating effect of the reaction. The wide-angle XRD patterns of the samples are shown in Fig. 5. A boarded diffraction peak at 2 degree of ca. 22.4◦ clearly presents in all the samples, which is ascribed to the amorphous framework of the SBA-15. From the patterns of the sample P-Pd/SBA-15 (Fig. 5(b)), four diffraction peaks are clearly observed at 2 = 40.08◦ , 46.62◦ , 68.08◦ , and 82.14◦ , indexed to (1 1 1), (2 0 0), (2 2 0), and (3 1 1) reflections of the face-centered cubic palladium lattice, with the space group assigned to Fm−3m(2 2 5), respectively (JCPDS card, File No. 46-1043). After calcined in O2 and subsequently reduced in H2 at 500 ◦ C, the sample H-Pd/SBA-15 (Fig. 5(d)) still shows four diffraction peaks, indicating that the palladium species remain as metallic states. These above-mentioned results demonstrate that (i) the plasma treatment can reduce the palladium ions into the metallic palladium, (ii) after the sample P-Pd/SBA-15 calcined at
PdO
(111)
●
(200)
(220)
(331)
Intensity (a.u.)
(e)
●
(d)
(c)
(b) (a) 10
20
30
40
50
60
70
80
90
Two-theta (deg.) Fig. 5. XRD patterns at wide diffraction angles of the samples: (a) SBA-15, (b) PPd/SBA-15, (c) P-Pd/SBA-15 after 12 h reaction, (d) H-Pd/SBA-15, and (e) H-Pd/SBA15 after 12 h reaction.
500 ◦ C for 3 h with the following reduction by hydrogen thermally, the palladium species in the sample H-Pd/SBA-15 is still the Pd0 . The samples after 12 h reaction are also investigated by wide-angle XRD (Fig. 5(c) and (e)). For the samples P-Pd/SBA-15 and H-Pd/SBA15 after 12 h reaction, a small peak at 2 = 33.80◦ is clearly visible, indicating that the palladium clusters are partially oxidized into palladium oxide particles. These results would be further confirmed by the XPS analyses. From Fig. 5, by comparing the patterns (b) and (d), the intensity of the Pd reflection peaks in the sample H-Pd/SBA-15 increases and the full-width-at-half maximum (FWHM) decreases slightly, suggesting a larger particle size after the thermal treatment. Based on the calculation using Scherrer equation, the Pd nanoparticle size is about 9.6 and 12.0 nm for the samples P-Pd/SBA-15 and H-Pd/SBA15, respectively. Furthermore, the corresponding intensity of the Pd reflection peaks in the samples P-Pd/SBA-15 and H-Pd/SBA-15 after 12 h reaction remains almost the same, indicating no increase in the particle size of the Pd nanoparticles during the reaction. These results would be further investigated by TEM images.
3.4. TEM analyses As displayed in Fig. 6(a) and (b), the sample P-SBA-15 shows regular hexagonal arrays of cylindrical channels after the plasma reduction, whereas the ordered arrays of channels have an average diameter of about 7–8 nm with a wall thickness of about 3 nm. Furthermore, from Fig. 6(c)–(f), it is clearly visible that the ordered mesoporous channels are well maintained after loaded with the metallic palladium. All these results are consistent with the N2 adsorption–desorption and low-angle XRD characteristics. In addition, for the samples P-Pd/SBA-15 and H-Pd/SBA-15, spherical palladium nanoparticles are homogeneously dispersed in the interior of the SBA-15 channels (Fig. 6(c)–(f)). This confirms that the glow discharge plasma reduction can lead to the formation of highly dispersed metal particles confined within the channels of SBA-15. Fig. 7 shows the particle size distribution derived from the TEM images by surveying 150 particles. In the sample P-Pd/SBA-15, the particle size has a narrow distribution with the largest size of 10.8 nm. The particle size is mostly ranged from 6 to 9 nm, which is very similar to the pore size of the channels in the SBA-15. This result indicates that the diameter of the Pd nanoparticles is limited by the pore diameter of the SBA-15 host. On the other hand, the particle size of the sample H-Pd/SBA-15 has a broader distribution with the largest 12.7 nm. The number-weighted size of the Pd size of nj dj / nj ; nj : number of particles, dj : size of parparticles (dn = ticle j) is 7.83 and 8.24 nm for the P-Pd/SBA-15 and H-Pd/SBA-15 samples, respectively. The increase of the particle size of the sample H-Pd/SBA-15 is resulted from a moderate sintering during the calcination in O2 and subsequent reduction in H2 atmosphere at 500 ◦ C. These bigger particles confined within the channels, may have some negative effect on the channel walls of the SBA-15, resulting in the surface area reduces dramatically. This result is consistent with the conclusion of N2 adsorption–desorption.
676
H. Wang, C.-j. Liu / Applied Catalysis B: Environmental 106 (2011) 672–680
Fig. 6. TEM images of SBA-15 after plasma treatment and the nanoparticles confined within the channels of SBA-15: (a) and (b) P-SBA-15; (c) and (d) P-Pd/SBA-15; (e) and (f) H-Pd/SBA-15.
40
Frenquency (%)
To further study the atomic structure of the Pd nanoparticles, high resolution TEM images of the particles of the two samples are shown in Fig. 8. The crystallinity of the Pd particle in the sample P-Pd/SBA-15 (Fig. 8(a)) is clearly visible, indicating that the plasma reduction can induce the formation of the crystallinity of the metal particles. In addition, the lattice fringes with d = 0.227 nm are clearly observed, which can be attributed to the Pd(1 1 1) planes. These results suggest that the uniformly crystallized particles have been generated within the pore channels of SBA-15. From Fig. 8(b), the lattice fringes with d = 0.227 nm can be also observed, suggesting that after the calcination at high temperature, the crystallinity remains well in the sample H-Pd/SBA-15. Fig. 9 shows the TEM images of the samples P-Pd/SBA-15 and H-Pd/SBA-15 after 12 h reaction. These images revealed that (i) a well-ordered mesoporous structure is maintained in the support SBA-15 after CO oxidation, which is in agreement with the lowangle XRD results and indicates a good stability during the reaction;
P-Pd/SBA-15
H-Pd/SBA-15
30 20 10 0 5 6
7 8 9 10 11 12
5 6
7 8 9 10 11 12 13
Particle size (nm) Fig. 7. The particle size distribution of the samples.
H. Wang, C.-j. Liu / Applied Catalysis B: Environmental 106 (2011) 672–680
677
Fig. 8. High resolution TEM images of the Pd nanoparticles: (a) P-Pd/SBA-15 and (b) H-Pd/SBA-15.
(ii) spherical Pd nanoparticles are still highly dispersed in the interior of the SBA-15 channels; (iii) the average Pd nanoparticle sizes are determined to be 7.8 and 8.2 nm for the samples P-Pd/SBA-15 and H-Pd/SBA-15 after 12 h reaction, respectively. Obviously, the particle sizes remain unchanged compared to those before the reactions. This result suggests the catalysts are very stable and supports the conclusion from the wide-angle XRD patterns. 3.5. XPS analyses The binding energies (BE) of Si2p, O1s and Pd3d, with the relative surface atomic composition of samples before and after reaction are summarized in Table 2. For the SBA-15 support, the obtained
Si/O ratio is almost constant with the expected SiO2 stoichiometry. The binding energy values of Si2p and O1s remain essentially unchanged in each sample before and after reaction. Fig. 10 presents the spectra of Pd3d. For the sample P-Pd/SBA15, two symmetrical peaks can be observed whereas no other peaks can be found (Fig. 10(a)). The binding energy values of the Pd 3d5/2 and Pd 3d3/2 are 335.4 eV and 340.5 eV, respectively, which can be attributed to Pd0 [36–38]. These results definitely indicate that the plasma treatment can reduce the palladium ions completely into the metallic palladium, which have been previously reported [31,35]. In addition, from Fig. 10(b), only the two sharp and intense peaks can be observed. These two peaks have been attributed to Pd0 species, suggesting that the palladium species of H-Pd/SBA-
Fig. 9. TEM images of the samples: (a) and (b) P-Pd/SBA-15 after 12 h reaction; (c) and (d) H-Pd/SBA-15 after 12 h reaction.
678
H. Wang, C.-j. Liu / Applied Catalysis B: Environmental 106 (2011) 672–680
340.5 eV
335.4 eV
(b) 0
0
Pd 3d5/2
Pd 3d3/2
(a)
344
342
340
338
336
334
332
Binding Energy (eV) Fig. 10. XPS spectra of the samples: (a) P-Pd/SBA-15 and (b) H-Pd/SBA-15.
15 have been totally reduced into the metallic states. However, as shown in Table 2, the surface Pd converge of P-Pd/SBA-15, indicated by Pd/Si = 0.020, is higher than 0.015 of H-Pd/SBA-15. This means that after the thermal treatment, a migration of Pd into the SBA-15 inner surface has occurred. As illustrated by the XPS spectra in Fig. 11 and Table 2, it can be concluded that the metallic palladium in the samples P-Pd/SBA-15 and H-Pd/SBA-15 are partially oxidized to PdO during the CO oxidation, which is in agreement with the conclusions of wide-angle XRD. These results demonstrate that under reaction conditions PdO and Pd co-exist at the surface of the nanoparticles. A redox mechanism is suggested. Further studies are being conducted for a better understanding. 3.6. DRIFT study From the results discussed above, it is still difficult to explain the difference between the activities of P-Pd/SBA-15 and H-Pd/SBA-15. The DRIFT study with CO adsorption has been therefore conducted.
Fig. 12 shows infrared spectra of CO adsorbed at room temperature on P-Pd/SBA-15 and H-Pd/SBA-15 samples. Three main bands are clearly visible in these CO/FTIR spectra, which can be easily assigned on the basis of the surface science literature [39–42]. Briefly, the band in the range 2050–2150 cm−1 is assigned to linear carbonyls on Pd particles with a (1 1 1) face [42]; a sharper band close to 1990 cm−1 in the range 1970–2050 cm−1 is attributed to 2-fold bridged carbonyls on Pd particles with a (1 0 0) face [39,42], while the bonded band in the range 1850–1942 cm−1 is corresponded to 2-fold bridged carbonyls on Pd particles with a (1 1 1) face [40,41]. These three components have been marked as L, B1 , B2 , respectively (Fig. 12). These features mentioned are quite confidently assigned to carbonyls adsorbed on zerovalent Pd species [43]. Additionally, in the spectra after CO desorption for 30 min (the curves j in Fig. 12(a) and (b)), there are no other bands found except these three bands, whereas the carbonyls over Pd2+ species ranged from 2116 to 2160 cm−1 and the carbonyls on Pd+ could be observed between 2140 and 2115 cm−1 according to the literature [41,44]. This means that the state of the Pd species is Pd0 over P-Pd/SBA-15 and H-Pd/SBA-15. This result is supported by the results obtained from XPS spectra and the wide-angle XRD patterns. As shown in Fig. 12(a), the P-Pd/SBA-15 shows the typical spectra expected for Pd particles with both (1 1 1) and (1 0 0) planes. Both L and B bands can be immediately noticed after the sample PPd/SBA-15 is exposed to CO for 1 min, while a pronounced shoulder of the L band is also evident, which is attributed to the gaseous CO. With the increasing time of the sample exposed to CO (from 1 to 20 min), the intensity of the L and B2 bands increases dramatically, owning to the increase in the converge . When the time is prolonged to 30 min, it is easily found that the intensity of all bands remains unchanged, suggesting that the state of CO adsorbed on Pd particles reaches saturated. However, no other bands can be observed except the three typical bands (L, B1 and B2 ) over the sample P-Pd/SBA-15 purged with helium from the 5th min. At the same time, the intensity of all the bands decreases evidently and the maximum of the typical bands tends to shift to a lower frequency with decreasing CO converge. These results are very similar for the sample H-Pd/SBA-15 in the process of CO absorption and desorption on the Pd particles exposing both (1 1 1) and (1 0 0) facets (Fig. 12(b)). Nevertheless, by comparing the CO/FTIR spectra at 30th min during the CO desorption on the two samples, it can be easily found that the maximum of the three bands shifts slightly to a lower frequency (about 3–6 cm−1 ) and the intensity of the B1 and B2 bands increases dramatically for the sample H-Pd/SBA-15. Interestingly, the relative intensity of the three bands has a uniform trend in the CO/FTIR spectra between the sample P-Pd/SBA-15
(a) P-Pd/SBA-15 after 12 h reaction
346
Intensity (a.u.)
Intensity (a.u.)
(b) H-Pd/SBA-15 after 12 h reaction
344
342
340
338
336
334
Binding Energy (eV)
332
330
346
344
342
340
338
336
334
Binding Energy (eV)
Fig. 11. XPS spectra of the samples after CO oxidation.
332
330
H. Wang, C.-j. Liu / Applied Catalysis B: Environmental 106 (2011) 672–680
679
Table 2 Binding energies (BE) of core electrons and surface atomic composition (at.%) of samples before and after reactions. Sample
Si2p
P-Pd/SBA-15 H-Pd/SBA-15 P-Pd/SBA-15 after 12 h reaction H-Pd/SBA-15 after 12 h reaction
O1s
Pd3d5/2
BE (eV)
at.%
Pd0 BE (eV)
Pd2+ BE (eV)
A[Pd0 ] /A[Pd2+ ]
at.%
Pd/Si
103.5 103.6 103.5 103.6
20.4 26.4 25.2 28.0
532.9 532.8 532.8 532.8
43.0 54.0 54.0 58.4
335.4 335.5 335.3 335.2
– – 337.0 336.9
100/0 100/0 71/29 74/26
0.4 0.4 0.5 0.4
0.020 0.015 0.020 0.014
3.7. Discussions In this work, the glow discharge plasma reduction has been employed for the loading of palladium into the pores of SBA-15. The 0.025
L 2090
j
4. Conclusion In this work, we confirm that the SBA-15 supported palladium catalyst is also good for CO oxidation, compared to other noble metal catalysts (like Pt and Rh) supported by SBA-15. Highly dispersed palladium nanoparticles can be produced by the conventional impregnation using the glow discharge plasma treatment, operated at room temperature with argon as the plasma forming gas. The reduction thermally using hydrogen after the calcination of plasma treated catalyst induces a slight increase in the particle size. The particle size is always close to the diameter of the channels of SBA-15. The preparation condition has a significant influence on the catalyst activity. The hydrogen reduced catalyst shows a higher activity compared to the argon plasma reduced catalyst. The structure change by the thermal treatment causes this improvement, 0.025
B1 B2 1993
plasma reduction was conducted at room temperature. An electron reduction mechanism has been presented [31,33,35]. This room temperature plasma reduction is totally different from hydrogen reduction at elevated temperatures in the nucleation and growth kinetics of the metal particles. A high dispersion of metal particles has been achieved with enhanced properties for hydrogen storage [47], electrochemical oxidation [48] and catalytic conversions [33]. The carbon nanotubes generated over the plasma reduced Pd/HZSM-5 catalyst are also different from those obtained over the hydrogen-reduced catalysts [49]. All these changes confirm the change in the catalyst structure induced by the room temperature plasma reduction, as discussed above with the DRIFT analyses. The present study on CO oxidation over Pd/SBA-15 is another excellent example for the structure sensitive catalyst.
2087
1942
B1 B2
(b) 1936
i h
0.015
K-M units
K-M units
1989
j 0.020
i h
0.015
f COgas
0.010
e
g f COgas
0.010
e
d c
0.005
d c
0.000
a
b 0.000
L
(a)
g
0.005
Atomic ratio
at.%
and H-Pd/SBA-15. Firstly, it is clearly visible that the B2 /B1 area ratio in the CO/FTIR spectra of the sample H-Pd/SBA-15 is bigger than the B2 /B1 area ratio in the CO/FTIR spectra of the sample P-Pd/SBA-15 at the same time of CO adsorption and desorption, which is induced by a change of the metal particle size: the bigger the fcc cubeoctahedron metal crystallites, the larger the ratio of Pd(1 1 1) to Pd(1 0 0) planes [45,46]. This result is supported by the particle size distribution calculated from the TEM images and the wide-angle XRD patterns mentioned above. Secondly, the (B1 + B2 )/L area ratio in the CO/FTIR spectra of the sample H-Pd/SBA-15 is obviously bigger than the (B1 + B2 )/L area ratio of the sample P-Pd/SBA-15, which is also induced by a change of the metal particle size: linear CO is preferentially formed on low-coordinated Pd atoms where smaller particles expose a larger fraction of low-coordinated atoms than larger particles, while the B/L ratio increases with the increasing particle size [45]. These results indicate that during the thermal treatment, the Pd crystallites undergo a further reconstruction, which leads to a slight increase in the particle size, with a minor change in the exposing planes and atoms. It appears that this preferential adsorption of CO at bridge sites induces a geometrically advantageous setup for reaction between CO and O adatoms and changes the reaction rate, with a result of a higher activity for the sample H-Pd/SBA-15 on CO oxidation. This result is similar to the finding reported by Somorjai and his co-workers on the study of Rh nanoparticles [23].
0.020
Pd3d
BE (eV)
b
a
2200
2100
2000
1900 -1
Wavenumber cm
1800
2200
2100
2000
1900
1800
-1
Wavenumber cm
Fig. 12. DRIFT spectra of CO adsorbed on P-Pd/SBA-15 (a) and H-Pd/SBA-15 (b) samples at 25 ◦ C. Sample exposed to 1.1 kPa CO for 1 min, a; 5 min, b; 10 min, c; 20 min, d; 30 min, e; and purged with He for 1 min, f; 5 min, g; 10 min, h; 20 min, i; 30 min, j.
680
H. Wang, C.-j. Liu / Applied Catalysis B: Environmental 106 (2011) 672–680
according to the DRIFT studies. Because of the importance of Pd catalysts, the Pd/SBA-15 catalysts prepared in this work will have many other applications beyond CO oxidation, like the three-waycatalysts (TWC), sensors, drug delivery and optoelectronic. Acknowledgement This work was supported by the National Natural Science Foundation of China (#20990223). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.apcatb.2011.06.034. References [1] D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky, Science 279 (1998) 548–552. [2] C.T. Kresga, M.E. Lenowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710–712. [3] H. Takahashi, B. Li, T. Sasaki, C. Miyazaki, T. Kajino, S. Inagaki, Chem. Mater. 12 (2000) 3301–3305. [4] J. Adam, E. Antonakou, A. Lappas, M. Stöcker, M.H. Nilsen, A. Bouzga, J.E. Hustad, G. Øye, Micropor. Mesorpor. Mater. 96 (2006) 93–101. [5] V.S.-Y. Lin, C.-Y. Lai, J. Huang, S.-A. Song, S. Xu, J. Am. Chem. Soc. 123 (2001) 11510–11511. [6] M.V. Reg´, F. Balas, D. Arcos, Angew. Chem. Int. Ed. 46 (2007) 7548–7558. [7] S.-W. Song, K. Hidajat, S. Kawi, Langmuir 21 (2005) 9568–9575. [8] Q. Lu, F. Gao, S. Komarneni, T.E. Mallouk, J. Am. Chem. Soc. 126 (2004) 8650–8651. [9] F. Zhang, Y. Yan, H. Yang, Y. Meng, C. Yu, B. Tu, D. Zhao, J. Phys. Chem. B 109 (2005) 8723–8732. [10] H. Cui, Y. Zhang, Z. Qiu, L. Zhao, Y. Zhu, Appl. Catal. B: Environ. 101 (2010) 45–53. [11] E. Weber, D. Sirim, T. Schreiber, B. Thomas, J. Pleiss, M. Hunger, R. Gläser, V.B. Urlacher, J. Mol. Catal. B 64 (2010) 29–37. [12] Q. Tang, S. Hu, Y. Chen, Z. Guo, Y. Hu, Y. Chen, Y. Yang, Micropor. Mesorpor. Mater. 132 (2010) 501–509. ´ B. Pawelec, E. Rivera-Munoz, R. Nava, J. Espino, J.L.G. Fierro, [13] R. Huirache-Acuna, Appl. Catal. B: Environ. 92 (2009) 168–184. [14] Y. Wang, R.T. Yang, J.M. Heinzel, Chem. Eng. Sci. 63 (2008) 356–365. [15] Th.A. Steriotis, K.L. Stefanopoulos, F.K. Katsaros, R. Gläser, A.C. Hannon, J.D.F. Ramsay, Phys. Rev. B 78 (2008) 115424.
[16] D. Liu, X.Y. Q, H.H.A. Wah, G. Zeng, Y. Li, Y. Yang, Catal. Today 148 (2009) 243–250. [17] X. Wu, S. Kawi, Catal. Today 148 (2009) 251–259. [18] L. Escamilla-Perea, R. Nava, B. Pawelec, M.G. Rosmaninho, C.L. Peza-Ledesma, J.L.G. Fierro, Appl. Catal. A 381 (2010) 42–53. [19] C.-W. Chiang, A. Wang, B.-Z. Wan, C.-Y. Mou, J. Phys. Chem. B 109 (2005) 18042–18047. [20] H. Zhu, C. Liang, W. Yan, S.H. Overbury, S. Dai, J. Phys. Chem. B 110 (2006) 10842–10848. [21] D. Tian, G. Yong, Y. Dai, X. Yan, S. Liu, Catal. Lett. 130 (2009) 211–216. [22] C. Tu, A. Wang, M. Zheng, Y. Meng, J. Shan, T. Zhang, Chin. J. Catal. 26 (2005) 631–633. [23] M.E. Grass, S.H. Joo, Y. Zhang, G.A. Somorjai, J. Phys. Chem. C 113 (2009) 8616–8623. [24] C.K. Krishnan, K. Nakamura, H. Hirata, M. Ogura, Phys. Chem. Chem. Phys. 12 (2010) 7513–7520. [25] A.S. Ivanova, E.M. Slavinskaya, R.V. Gulyaev, V.I. Zaikovskii, O.A. Stonkus, I.G. Danilova, L.M. Plyasova, I.A. Polukhina, A.I. Boronin, Appl. Catal. B: Environ. 97 (2010) 57–71. [26] S. Hinokuma, H. Fujii, M. Okamoto, K. Ikeue, M. Machida, Chem. Mater. 22 (2010) 6183–6190. [27] Z. Wang, B. Li, M. Chen, W. Weng, H. Wan, Sci. China Chem. 53 (2010) 2047–2056. [28] L. Liu, F. Zhou, L. Wang, X. Qi, F. Shi, Y. Deng, J. Catal. 274 (2010) 1–10. [29] J. Gu, A. Bao, M. Jia, Y. Liu, B. Zhaorigetu, Z. Yuan, Chin. J. Catal. 31 (2010) 322–328. [30] Y. Bi, G. Lu, Appl. Catal. B: Environ. 41 (2003) 279–286. [31] Z.-J. Wang, Y. Zhao, L. Cui, H. Du, P. Yao, C.-J. Liu, Green Chem. 9 (2007) 554–559. [32] Z.-J. Wang, Y. Liu, P. Shi, C.-J. Liu, Y. Liu, Appl. Catal. B: Environ. 90 (2009) 570–577. [33] X. Liang, C.-J. Liu, P. Kuai, Green Chem. 10 (2008) 1318–1322. [34] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquérol, T. Siemieniewska, Pure Appl. Chem. 57 (1985) 603–619. [35] Z.-J. Wang, Y. Xie, C.-J. Liu, J. Phys. Chem. C 112 (2008) 19818–19824. [36] T.L. Barr, J. Phys. Chem. 82 (1978) 1801–1810. [37] Y. Sohn, D. Pradhan, K.T. Leung, ACS Nano 4 (2010) 5111–5120. [38] Y. Li, B.W.L. Jang, Appl. Catal. A 392 (2011) 173–179. [39] G. Agostini, R. Pellegrini, G. Leofanti, L. Bertinetti, S. Bertarione, E. Groppo, A. Zecchina, C. Lamverti, J. Phys. Chem. C 113 (2009) 10485–10492. [40] O. Dulaurent, K. Chandes, C. Bouly, D. Bianchi, J. Catal. 192 (2000) 273–285. [41] V.S. Escribano, L. Arrighi, P. Riani, G. Busca, Langmuir 22 (2006) 9214–9219. [42] J.B. Giorgi, T. Schroeder, M. Bäumer, H.-J. Freund, Surf. Sci. 498 (2002) L71–L77. [43] K. Almusaiteer, S.S.C. Chuang, J. Catal. 184 (1999) 189–201. [44] I. Eswaranoorthi, A.K. Dalai, Int. J. Hydrogen Energy 34 (2009) 2580–2590. [45] G.C. Cabilla, A.L. Bonivardi, M.A. Baltanás, Catal. Lett. 55 (1998) 147–156. [46] R. Van Hardeveld, F. Hartog, Adv. Catal. 22 (1972) 75–113. [47] Y.W. Li, R.T. Yang, C.-J. Liu, Z. Wang, Ind. Eng. Chem. Res. 46 (2007) 8277–8281. [48] Z. Wang, C.-J. Liu, G.L. Zhang, Catal. Commun. 10 (2009) 959–962. [49] Y. Zhao, Y.-X. Pan, L. Cui, C.-J. Liu, Diamond Relat. Mater. 16 (2007) 229–235.
Contents lists available at ScienceDirect
Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb
Preparation and characterization of SBA-15 supported Pd catalyst for CO oxidation Houpeng Wang, Chang-jun Liu ∗ Advanced Nanotechnology Center, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China
a r t i c l e
i n f o
Article history: Received 14 March 2011 Received in revised form 29 May 2011 Accepted 21 June 2011 Available online 28 June 2011 Keywords: Mesoporous material SBA-15 Palladium CO oxidation Plasma
a b s t r a c t SBA-15 supported noble metal catalysts for CO oxidation have been extensively investigated. However, no reported work on the SBA-15 supported palladium catalyst for CO oxidation can be found in the literature. In this work, highly dispersed palladium nanoparticles were synthesized within the uniform channels of SBA-15 via a glow discharge plasma reduction treatment. The obtained Pd/SBA-15 catalyst shows a good activity for CO oxidation. The activity can be further improved by the hydrogen reduction thermally. The low-angle X-ray diffraction (XRD) patterns and transmission electron microscope (TEM) indicate that the ordered mesoporous structure was well maintained during the catalyst preparation and reaction processes. The wide-angle XRD patterns and TEM images show that the spherical palladium nanoparticles are highly dispersed within the SBA-15 channels and remain stable after the reaction. The diffuse reflectance infrared Fourier transformation (DRIFT) spectroscopy of adsorbed CO confirms that the structure change caused by different preparation conditions has a significant influence on the activity of the catalyst. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Highly ordered mesoporous silica [1,2] has been extensively applied for the studies of catalysis [3,4], sensors [5] and drug delivery [6,7]. SBA-15 is by far the largest pore-size mesoporous silica material, which has a well ordered two-dimensional hexagonal structure with the uniform pore sizes of 2.0–30.0 nm and high surface area of 600–1000 m2 g−1 , as well as the highly hydrothermal and thermal stabilities [1,8,9]. There has been an increasing research interest in various SBA-15 supported catalysts. The studies explored include oxidation [10–12], desulfurization [13,14], adsorption [15], CO2 reforming [16,17] and many others. Especially, an intense investigation has been conducted on CO oxidation over the SBA-15 supported catalysts. The SBA-15 supported catalysts for CO oxidation provide us an ideal system for the fundamental studies. Among the catalysts investigated, the SBA-15 supported noble metal catalysts show good activities for CO oxidation. The metals tested include Au [18–20], Ag [21,22], Rh [23], and Pt [24]. However, there is still no reported work on the SBA-15 supported palladium catalyst for CO oxidation, although one can easily find many papers on it over the palladium catalysts loaded on other supporting materials, like Al2 O3 [25], CeO2 [26], TiO2 [27], FeOx [28], SAPO-34 [29] and NaZSM-5 [30].
∗ Corresponding author. Tel.: +86 22 27406490; fax: +86 22 27406490. E-mail address: ughg [email protected] (C.-j. Liu). 0926-3373/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2011.06.034
In this work, we attempt to investigate CO oxidation over the SBA-15 supported Pd catalyst. We confirm that the Pd/SBA-15 shows a good activity for CO oxidation. 2. Experimental 2.1. Preparation of catalysts The SBA-15 silica was synthesized in the laboratory of Prof. D.Y. Zhao (Department of Chemistry, Fudan University, P.R. China) [1]. Prior to use, the silica was calcined in air at 500 ◦ C for 3 h to remove the adsorbed water. It was then impregnated with an aqueous solution of palladium nitrate overnight. Considering the difficulty in the loading of metal ions into the pores of SBA-15 by the conventional incipient wetness impregnation, the glow discharge plasma treatment was applied in this work to load the metal in. During the plasma treatment, the metal ions are reduced by the electrons within the plasma. The plasma reduction was operated at room temperature. The glow discharge plasma setup and plasma reduction protocol have been previously described in detail [31–33]. Briefly, the sample (about 0.4 g), loaded on a quartz boat, was placed in a quartz tube (i.d. 35 mm) with two stainless steel electrodes (o.d. 30 mm). When the pressure in the discharge tube was evacuated to about 100 Pa, the glow discharge plasma was generated by applying 1000 V to the electrodes using a high voltage amplifier (Trek, 20/20B). The signal input for the high voltage amplifier was supplied by a function/arbitrary waveform generator (HewlettPackard, model 33120A) with a 100 Hz square wave. Ultra high
H. Wang, C.-j. Liu / Applied Catalysis B: Environmental 106 (2011) 672–680
P-Pd/SBA-15 H-Pd/SBA-15
100
80
CO conversion (%)
pure grade argon (>99.999%) was used as the plasma-forming gas. The plasma reduction was operated for 10 min each time (totally 6 times), with a manual mixing of the sample between the treatments to ensure even exposure to the plasma. The bulk temperature of the plasma was measured by infrared imaging (Ircon modes 100PHT). The result indicated that the heating effect of the glow discharge can be ignored because the temperature of the powder was close to room temperature. The sample reduced via plasma was denoted as P-Pd/SBA-15. The pure SBA-15 treated in this way was designated as P-SBA-15. The hydrogen reduced Pd/SBA-15 has also prepared in order to study the effect of reduction methods. The hydrogen reduced catalyst was prepared in this way: the plasma treated sample (P-Pd/SBA-15) was calcined at 500 ◦ C for 3 h under the flowing oxygen with a flow rate of 30 mL min−1 . Then the calcined sample was further reduced at 500 ◦ C for 4 h using a mixture of hydrogen and helium with a flow rate of 60 mL min−1 (5.11 vol.% H2 ). The obtained sample was designated as H-Pd/SBA15. The palladium loading amount is 2 wt% unless otherwise mentioned.
673
60
40
20
0
20
40
60
80 100 120 140 160 180 200 220 240
Temperature (°C) 2.2. Catalyst characterization N2 adsorption–desorption isotherms were measured at liquid nitrogen temperature using a Micromeritics Tristar 3000 instrument. The samples were degassed at 300 ◦ C for 3 h under vacuum before analysis. The Brunauer–Emmett–Teller (BET) equation (relative pressure between 0.05 and 0.25) was employed to calculate the specific surface areas. The pore size and pore volume were calculated from the adsorption branches of N2 physisorption isotherms and the Barrett–Joyner–Halenda (BJH) model. The X-ray powder diffraction (XRD) patterns of the samples were recorded on a Rigaku D/Max-2500 V/PC diffractometer with ˚ The X-ray source was a Cu K␣ radiation source ( = 1.54056 A). operated at 40 kV and 200 mA. The wide-angle XRD patterns were collected at a scanning speed of 8◦ /min over the 2 range from 10◦ to 90◦ , whereas the low-angle XRD patterns were collected at a scanning speed of 1◦ /min over the 2 range of 0.6–6◦ . The phase identification was made by comparison to the Joint Committee on Powder Diffraction Standards (JCPDSs). Transmission electron microscope (TEM) measurements were performed on a Philips Tecnai G2 F20 system operated at 200 kV. The samples were suspended into the ethanol and dispersed ultrasonically for 15 min. A drop of the suspension was deposited on a copper grid coated with carbon. The size distribution of Pd particles was determined from TEM images by counting more than 150 particles. X-ray photoelectron spectrometer (XPS) analyses were conducted on a Perkin-Elmer PHI-1600 spectrometer with monochromatic Mg K␣ (1253.6 eV) radiation. Binding energies were calibrated using the C1s peak (284.6 eV) as reference. CO adsorbed diffuse reflectance Fourier transform infrared (DRIFT) spectroscopy was obtained on a Tensor 27 spectrometer (Bruker). The instrument was equipped with a liquid nitrogen cooled mercury–cadmium–tellurium (MCT) detector, a diffuse reflectance accessory and a high temperature reaction chamber (Praying Mantis, Harrick). The powder (7 mg) was loaded into the reflectance cell and purged by flowing helium (20 mL min−1 ) at 300 ◦ C for 1 h to remove gas phase and weakly bound water. CO adsorption was carried out in 20 mL min−1 CO (1.11 vol.%)/helium for 30 min at 25 ◦ C. After the cell was flushed with helium (20 mL min−1 ) for another 30 min at the same temperature, the DRIFT spectra were recorded with 64 scans at 4 cm−1 resolution. All spectra were illustrated using Kubelka Munk units, which is linear with the concentration of surface species.
Fig. 1. Effect of the reaction temperature on CO conversion.
2.3. Activity test The temperature-dependent CO oxidation was performed in a quartz tube with an inner diameter of 4 mm under atmospheric pressure. A catalyst sample of 50 mg was packed into the quartz tube. The sample was pretreated in 30 mL min−1 argon at 150 ◦ C to remove the gas phase and weakly bound water. After cooling down to 20 ◦ C, the reaction was started in flowing CO/O2 mixture (total flow = 20 mL min−1 ; 1.0 vol.% CO, 20.0 vol.% O2 , balance N2 ) over the temperature range of 30–220 ◦ C. The effluent was then analyzed online by a gas chromatograph (Agilent 6890) equipped with a TDX01 column and a thermal conductivity detector (TCD). For each reaction temperature, the samples were collected after 45 min on steam to allow the attainment of steady state. And each sample was measured in 16 temperature points, thus the whole reaction time was 12 h. 3. Results and discussion 3.1. CO oxidation CO conversion versus reaction temperature for the samples PPd/SBA-15 and H-Pd/SBA-15 is presented in Fig. 1. Obviously, the conversion of CO of the two samples increases with the increasing temperature. Compared to the sample P-Pd/SBA-15, H-Pd/SBA15 exhibits a higher activity. The catalyst reduction method has a significant effect on the catalytic activity. The conversion over H-Pd/SBA-15 increases more quickly with the temperature during the whole process of the test. The reaction starts at 40 ◦ C over both samples. However, the initial conversion of the sample PPd/SBA-15 is 2.94%, while it is 5.34% over H-Pd/SBA-15. It can be also observed that the samples P-Pd/SBA-15 and H-Pd/SBA-15 exhibit typical light-off (ignition) profiles (Fig. 1), with light-off temperatures of 201 and 133 ◦ C (for 50% conversation). The corresponding 100% conversion temperatures (T100 ) are 206 and 135 ◦ C, respectively. In order to compare the catalytic performance of the 2 wt% HPd/SBA-15 with the other loading catalysts, 1 wt% H-Pd/SBA-15 and 5 wt% H-Pd/SBA-15 samples were prepared using the same procedure. As shown in Fig. 2, the palladium loading has a remarkable effect on the catalytic activity, where the 100% conversion
674
H. Wang, C.-j. Liu / Applied Catalysis B: Environmental 106 (2011) 672–680
( 100)
H-Pd/SBA-15-1% H-Pd/SBA-15-2% H-Pd/SBA-15-5%
100
( 110) ( 200)
SBA-15
Intensity (a.u.)
CO conversion (%)
80
60
P-Pd/SBA-15 H-Pd/SBA-15 P-Pd/SBA-15 after 12h reaction
40
H-Pd/SBA-15 after 12h reaction
20 1.0
1.5
2.0
2.5
3.0
3.5
4.0
Two-theta (deg.) 0
Fig. 4. XRD patterns at low diffraction angles.
20
40
60
80
100
120
140
160
180
Temperature (°C) Fig. 2. Effect of the Pd loading on the activity.
temperatures (T100 ) are 127, 135 and 140 ◦ C, respectively, for the H-Pd/SBA-15 catalysts with the 5%, 2% and 1% loading. From Figs. 1 and 2, it can be concluded that the SBA-15 supported Pd catalysts show a good activity on CO oxidation, compared to the reported works over palladium catalysts supported by other materials. Also, the catalyst reduction method has a significant effect on the catalytic activity. To understand the beneficial effect of the hydrogen reduction thermally, the catalyst characterization has been conducted and will be discussed below. 3.2. N2 adsorption–desorption
3.3. XRD characterization The low-angle XRD patterns of the samples are presented in Fig. 4. The support SBA-15 shows three well-resolved peaks at 2 ranged from 0.6◦ to 2.6◦ , associated with (1 0 0), (1 1 0) and (2 0 0) reflections of P6mm symmetry. These peaks are the characteristic patterns from the hexagonal ordered structure in SBA-15. After loaded with palladium and subsequently treated via glow discharge plasma, the SBA-15 in the sample P-Pd/SBA-15 shows a little change, whereas these three peaks shift slightly to lower angles, indicating that the frameworks of the silica host may be enlarged during the synthesis process via the argon plasma reduction. This result is consistent with the previous reports by our group [35]. On the other hand, after the sample P-Pd/SBA-15 being treated thermally at 500 ◦ C, the (1 0 0) diffraction peak of the sample HPd/SBA-15 shifts to larger 2 degree and the relative intensity of diffraction peaks reduces. This result suggests that the treatment at high temperature may cause some shrinkage of mesopores and
(B)
(A) (d) H-Pd/SBA-15
(d) H-Pd/SBA-15
dV/dr ( cm /g)
-3
Volume Absorbed ( cm /g, STP )
Fig. 3 shows N2 adsorption–desorption isotherms (A) and pore size distributions (B) of the samples. The textural properties of the samples are summarized in Table 1. All the isotherms are of type IV [34], which is a characteristic of mesoporous materials. SBA-15 and the SBA-15 treated via the plasma (Fig. 3A(a) and (b)) exhibit a hysteresis loop of H1 type, which typically features a two-dimensional P6mm structure formed by the open cylindrical mesopores. Additionally, the isotherms of these two samples present a sharp step in the P/P0 range of 0.6–0.78, which is an indicative of the uniformity of the pore size. However, the hysteresis loops of P-Pd/SBA-15 and H-Pd/SBA-15 become smaller (Fig. 3A(c) and (d)), with smaller SBET , VBJH and DBJH values (Table 1), suggesting that (i) the palladium
nanoparticles have been dispersed into the pores of the SBA-15 via the glow discharge plasma reduction; (ii) some channels in the SBA-15 for the sample H-Pd/SBA-15 may undergo a collapse as a result of the calcination at 500 ◦ C, because the loop of the sample H-Pd/SBA-15 is dramatically smaller than the loop of the sample PPd/SBA-15. This result is confirmed by the pore diameters and the surface area of the two samples (Fig. 3(B) and Table 1), where the pore diameter value is 7.7 nm for the two samples but the surface area is reduced from 528 to 472 m2 g−1 .
-3
(c) P-Pd/SBA-15
(b) P-SBA-15
(c) P-Pd/SBA-15
(b) P-SBA-15
(a) SBA-15 0.0
0.2
(a) SBA-15 0.4
0.6
0.8
Relative pressure ( P/P0)
1.0
1
2
3
4
5
6
7
Pore radius (nm)
Fig. 3. N2 adsorption–desorption isotherms (A) and pore size distributions (B).
8
9
10
H. Wang, C.-j. Liu / Applied Catalysis B: Environmental 106 (2011) 672–680
675
Table 1 Pore structure parameters of SBA-15 and Pd/SBA-15. Sample
d1 0 0 (nm)
a (nm)
SBET (m2 g−1 )
VBJH (cm3 g−1 )
DBJH (nm)
SBA-15 P-SBA-15 P-Pd/SBA-15 H-Pd/SBA-15
9.627 9.630 9.633 9.436
11.116 11.120 11.123 10.896
607 600 528 472
0.933 0.900 0.849 0.732
7.8 7.8 7.7 7.7
√ d1 0 0 : the interplanar spacing of the (1 0 0) plane; a: the unit cell parameter, a = 2d1 0 0 / 3; SBET , calculated by BET method at relative pressure of P/P0 = 0.05–0.25; VBJH , DBJH : pore volume and pore size calculated by BJH method.
●
structural disorder, which is similar to the observation reported in the literature [9]. Nevertheless, the sample H-Pd/SBA-15 still present three well-resolved peaks, indexed to (1 0 0), (1 1 0), and (2 0 0) diffraction peaks, indicating that the highly ordered structure is still well maintained after the high temperature treatment. After CO oxidation, there are still three resolved peaks obtained in the samples P-Pd/SBA-15 and H-Pd/SBA-15 after 12 h reaction. This demonstrates that the highly ordered structure is also well maintained during CO oxidation. However, it is interesting to notice that the intensities of the (1 0 0) peak together with two additional peaks decrease dramatically in the sample P-Pd/SBA-15 after 12 h reaction, by comparing the sample P-Pd/SBA-15, which could be ascribed to the heating effect of the reaction. The wide-angle XRD patterns of the samples are shown in Fig. 5. A boarded diffraction peak at 2 degree of ca. 22.4◦ clearly presents in all the samples, which is ascribed to the amorphous framework of the SBA-15. From the patterns of the sample P-Pd/SBA-15 (Fig. 5(b)), four diffraction peaks are clearly observed at 2 = 40.08◦ , 46.62◦ , 68.08◦ , and 82.14◦ , indexed to (1 1 1), (2 0 0), (2 2 0), and (3 1 1) reflections of the face-centered cubic palladium lattice, with the space group assigned to Fm−3m(2 2 5), respectively (JCPDS card, File No. 46-1043). After calcined in O2 and subsequently reduced in H2 at 500 ◦ C, the sample H-Pd/SBA-15 (Fig. 5(d)) still shows four diffraction peaks, indicating that the palladium species remain as metallic states. These above-mentioned results demonstrate that (i) the plasma treatment can reduce the palladium ions into the metallic palladium, (ii) after the sample P-Pd/SBA-15 calcined at
PdO
(111)
●
(200)
(220)
(331)
Intensity (a.u.)
(e)
●
(d)
(c)
(b) (a) 10
20
30
40
50
60
70
80
90
Two-theta (deg.) Fig. 5. XRD patterns at wide diffraction angles of the samples: (a) SBA-15, (b) PPd/SBA-15, (c) P-Pd/SBA-15 after 12 h reaction, (d) H-Pd/SBA-15, and (e) H-Pd/SBA15 after 12 h reaction.
500 ◦ C for 3 h with the following reduction by hydrogen thermally, the palladium species in the sample H-Pd/SBA-15 is still the Pd0 . The samples after 12 h reaction are also investigated by wide-angle XRD (Fig. 5(c) and (e)). For the samples P-Pd/SBA-15 and H-Pd/SBA15 after 12 h reaction, a small peak at 2 = 33.80◦ is clearly visible, indicating that the palladium clusters are partially oxidized into palladium oxide particles. These results would be further confirmed by the XPS analyses. From Fig. 5, by comparing the patterns (b) and (d), the intensity of the Pd reflection peaks in the sample H-Pd/SBA-15 increases and the full-width-at-half maximum (FWHM) decreases slightly, suggesting a larger particle size after the thermal treatment. Based on the calculation using Scherrer equation, the Pd nanoparticle size is about 9.6 and 12.0 nm for the samples P-Pd/SBA-15 and H-Pd/SBA15, respectively. Furthermore, the corresponding intensity of the Pd reflection peaks in the samples P-Pd/SBA-15 and H-Pd/SBA-15 after 12 h reaction remains almost the same, indicating no increase in the particle size of the Pd nanoparticles during the reaction. These results would be further investigated by TEM images.
3.4. TEM analyses As displayed in Fig. 6(a) and (b), the sample P-SBA-15 shows regular hexagonal arrays of cylindrical channels after the plasma reduction, whereas the ordered arrays of channels have an average diameter of about 7–8 nm with a wall thickness of about 3 nm. Furthermore, from Fig. 6(c)–(f), it is clearly visible that the ordered mesoporous channels are well maintained after loaded with the metallic palladium. All these results are consistent with the N2 adsorption–desorption and low-angle XRD characteristics. In addition, for the samples P-Pd/SBA-15 and H-Pd/SBA-15, spherical palladium nanoparticles are homogeneously dispersed in the interior of the SBA-15 channels (Fig. 6(c)–(f)). This confirms that the glow discharge plasma reduction can lead to the formation of highly dispersed metal particles confined within the channels of SBA-15. Fig. 7 shows the particle size distribution derived from the TEM images by surveying 150 particles. In the sample P-Pd/SBA-15, the particle size has a narrow distribution with the largest size of 10.8 nm. The particle size is mostly ranged from 6 to 9 nm, which is very similar to the pore size of the channels in the SBA-15. This result indicates that the diameter of the Pd nanoparticles is limited by the pore diameter of the SBA-15 host. On the other hand, the particle size of the sample H-Pd/SBA-15 has a broader distribution with the largest 12.7 nm. The number-weighted size of the Pd size of nj dj / nj ; nj : number of particles, dj : size of parparticles (dn = ticle j) is 7.83 and 8.24 nm for the P-Pd/SBA-15 and H-Pd/SBA-15 samples, respectively. The increase of the particle size of the sample H-Pd/SBA-15 is resulted from a moderate sintering during the calcination in O2 and subsequent reduction in H2 atmosphere at 500 ◦ C. These bigger particles confined within the channels, may have some negative effect on the channel walls of the SBA-15, resulting in the surface area reduces dramatically. This result is consistent with the conclusion of N2 adsorption–desorption.
676
H. Wang, C.-j. Liu / Applied Catalysis B: Environmental 106 (2011) 672–680
Fig. 6. TEM images of SBA-15 after plasma treatment and the nanoparticles confined within the channels of SBA-15: (a) and (b) P-SBA-15; (c) and (d) P-Pd/SBA-15; (e) and (f) H-Pd/SBA-15.
40
Frenquency (%)
To further study the atomic structure of the Pd nanoparticles, high resolution TEM images of the particles of the two samples are shown in Fig. 8. The crystallinity of the Pd particle in the sample P-Pd/SBA-15 (Fig. 8(a)) is clearly visible, indicating that the plasma reduction can induce the formation of the crystallinity of the metal particles. In addition, the lattice fringes with d = 0.227 nm are clearly observed, which can be attributed to the Pd(1 1 1) planes. These results suggest that the uniformly crystallized particles have been generated within the pore channels of SBA-15. From Fig. 8(b), the lattice fringes with d = 0.227 nm can be also observed, suggesting that after the calcination at high temperature, the crystallinity remains well in the sample H-Pd/SBA-15. Fig. 9 shows the TEM images of the samples P-Pd/SBA-15 and H-Pd/SBA-15 after 12 h reaction. These images revealed that (i) a well-ordered mesoporous structure is maintained in the support SBA-15 after CO oxidation, which is in agreement with the lowangle XRD results and indicates a good stability during the reaction;
P-Pd/SBA-15
H-Pd/SBA-15
30 20 10 0 5 6
7 8 9 10 11 12
5 6
7 8 9 10 11 12 13
Particle size (nm) Fig. 7. The particle size distribution of the samples.
H. Wang, C.-j. Liu / Applied Catalysis B: Environmental 106 (2011) 672–680
677
Fig. 8. High resolution TEM images of the Pd nanoparticles: (a) P-Pd/SBA-15 and (b) H-Pd/SBA-15.
(ii) spherical Pd nanoparticles are still highly dispersed in the interior of the SBA-15 channels; (iii) the average Pd nanoparticle sizes are determined to be 7.8 and 8.2 nm for the samples P-Pd/SBA-15 and H-Pd/SBA-15 after 12 h reaction, respectively. Obviously, the particle sizes remain unchanged compared to those before the reactions. This result suggests the catalysts are very stable and supports the conclusion from the wide-angle XRD patterns. 3.5. XPS analyses The binding energies (BE) of Si2p, O1s and Pd3d, with the relative surface atomic composition of samples before and after reaction are summarized in Table 2. For the SBA-15 support, the obtained
Si/O ratio is almost constant with the expected SiO2 stoichiometry. The binding energy values of Si2p and O1s remain essentially unchanged in each sample before and after reaction. Fig. 10 presents the spectra of Pd3d. For the sample P-Pd/SBA15, two symmetrical peaks can be observed whereas no other peaks can be found (Fig. 10(a)). The binding energy values of the Pd 3d5/2 and Pd 3d3/2 are 335.4 eV and 340.5 eV, respectively, which can be attributed to Pd0 [36–38]. These results definitely indicate that the plasma treatment can reduce the palladium ions completely into the metallic palladium, which have been previously reported [31,35]. In addition, from Fig. 10(b), only the two sharp and intense peaks can be observed. These two peaks have been attributed to Pd0 species, suggesting that the palladium species of H-Pd/SBA-
Fig. 9. TEM images of the samples: (a) and (b) P-Pd/SBA-15 after 12 h reaction; (c) and (d) H-Pd/SBA-15 after 12 h reaction.
678
H. Wang, C.-j. Liu / Applied Catalysis B: Environmental 106 (2011) 672–680
340.5 eV
335.4 eV
(b) 0
0
Pd 3d5/2
Pd 3d3/2
(a)
344
342
340
338
336
334
332
Binding Energy (eV) Fig. 10. XPS spectra of the samples: (a) P-Pd/SBA-15 and (b) H-Pd/SBA-15.
15 have been totally reduced into the metallic states. However, as shown in Table 2, the surface Pd converge of P-Pd/SBA-15, indicated by Pd/Si = 0.020, is higher than 0.015 of H-Pd/SBA-15. This means that after the thermal treatment, a migration of Pd into the SBA-15 inner surface has occurred. As illustrated by the XPS spectra in Fig. 11 and Table 2, it can be concluded that the metallic palladium in the samples P-Pd/SBA-15 and H-Pd/SBA-15 are partially oxidized to PdO during the CO oxidation, which is in agreement with the conclusions of wide-angle XRD. These results demonstrate that under reaction conditions PdO and Pd co-exist at the surface of the nanoparticles. A redox mechanism is suggested. Further studies are being conducted for a better understanding. 3.6. DRIFT study From the results discussed above, it is still difficult to explain the difference between the activities of P-Pd/SBA-15 and H-Pd/SBA-15. The DRIFT study with CO adsorption has been therefore conducted.
Fig. 12 shows infrared spectra of CO adsorbed at room temperature on P-Pd/SBA-15 and H-Pd/SBA-15 samples. Three main bands are clearly visible in these CO/FTIR spectra, which can be easily assigned on the basis of the surface science literature [39–42]. Briefly, the band in the range 2050–2150 cm−1 is assigned to linear carbonyls on Pd particles with a (1 1 1) face [42]; a sharper band close to 1990 cm−1 in the range 1970–2050 cm−1 is attributed to 2-fold bridged carbonyls on Pd particles with a (1 0 0) face [39,42], while the bonded band in the range 1850–1942 cm−1 is corresponded to 2-fold bridged carbonyls on Pd particles with a (1 1 1) face [40,41]. These three components have been marked as L, B1 , B2 , respectively (Fig. 12). These features mentioned are quite confidently assigned to carbonyls adsorbed on zerovalent Pd species [43]. Additionally, in the spectra after CO desorption for 30 min (the curves j in Fig. 12(a) and (b)), there are no other bands found except these three bands, whereas the carbonyls over Pd2+ species ranged from 2116 to 2160 cm−1 and the carbonyls on Pd+ could be observed between 2140 and 2115 cm−1 according to the literature [41,44]. This means that the state of the Pd species is Pd0 over P-Pd/SBA-15 and H-Pd/SBA-15. This result is supported by the results obtained from XPS spectra and the wide-angle XRD patterns. As shown in Fig. 12(a), the P-Pd/SBA-15 shows the typical spectra expected for Pd particles with both (1 1 1) and (1 0 0) planes. Both L and B bands can be immediately noticed after the sample PPd/SBA-15 is exposed to CO for 1 min, while a pronounced shoulder of the L band is also evident, which is attributed to the gaseous CO. With the increasing time of the sample exposed to CO (from 1 to 20 min), the intensity of the L and B2 bands increases dramatically, owning to the increase in the converge . When the time is prolonged to 30 min, it is easily found that the intensity of all bands remains unchanged, suggesting that the state of CO adsorbed on Pd particles reaches saturated. However, no other bands can be observed except the three typical bands (L, B1 and B2 ) over the sample P-Pd/SBA-15 purged with helium from the 5th min. At the same time, the intensity of all the bands decreases evidently and the maximum of the typical bands tends to shift to a lower frequency with decreasing CO converge. These results are very similar for the sample H-Pd/SBA-15 in the process of CO absorption and desorption on the Pd particles exposing both (1 1 1) and (1 0 0) facets (Fig. 12(b)). Nevertheless, by comparing the CO/FTIR spectra at 30th min during the CO desorption on the two samples, it can be easily found that the maximum of the three bands shifts slightly to a lower frequency (about 3–6 cm−1 ) and the intensity of the B1 and B2 bands increases dramatically for the sample H-Pd/SBA-15. Interestingly, the relative intensity of the three bands has a uniform trend in the CO/FTIR spectra between the sample P-Pd/SBA-15
(a) P-Pd/SBA-15 after 12 h reaction
346
Intensity (a.u.)
Intensity (a.u.)
(b) H-Pd/SBA-15 after 12 h reaction
344
342
340
338
336
334
Binding Energy (eV)
332
330
346
344
342
340
338
336
334
Binding Energy (eV)
Fig. 11. XPS spectra of the samples after CO oxidation.
332
330
H. Wang, C.-j. Liu / Applied Catalysis B: Environmental 106 (2011) 672–680
679
Table 2 Binding energies (BE) of core electrons and surface atomic composition (at.%) of samples before and after reactions. Sample
Si2p
P-Pd/SBA-15 H-Pd/SBA-15 P-Pd/SBA-15 after 12 h reaction H-Pd/SBA-15 after 12 h reaction
O1s
Pd3d5/2
BE (eV)
at.%
Pd0 BE (eV)
Pd2+ BE (eV)
A[Pd0 ] /A[Pd2+ ]
at.%
Pd/Si
103.5 103.6 103.5 103.6
20.4 26.4 25.2 28.0
532.9 532.8 532.8 532.8
43.0 54.0 54.0 58.4
335.4 335.5 335.3 335.2
– – 337.0 336.9
100/0 100/0 71/29 74/26
0.4 0.4 0.5 0.4
0.020 0.015 0.020 0.014
3.7. Discussions In this work, the glow discharge plasma reduction has been employed for the loading of palladium into the pores of SBA-15. The 0.025
L 2090
j
4. Conclusion In this work, we confirm that the SBA-15 supported palladium catalyst is also good for CO oxidation, compared to other noble metal catalysts (like Pt and Rh) supported by SBA-15. Highly dispersed palladium nanoparticles can be produced by the conventional impregnation using the glow discharge plasma treatment, operated at room temperature with argon as the plasma forming gas. The reduction thermally using hydrogen after the calcination of plasma treated catalyst induces a slight increase in the particle size. The particle size is always close to the diameter of the channels of SBA-15. The preparation condition has a significant influence on the catalyst activity. The hydrogen reduced catalyst shows a higher activity compared to the argon plasma reduced catalyst. The structure change by the thermal treatment causes this improvement, 0.025
B1 B2 1993
plasma reduction was conducted at room temperature. An electron reduction mechanism has been presented [31,33,35]. This room temperature plasma reduction is totally different from hydrogen reduction at elevated temperatures in the nucleation and growth kinetics of the metal particles. A high dispersion of metal particles has been achieved with enhanced properties for hydrogen storage [47], electrochemical oxidation [48] and catalytic conversions [33]. The carbon nanotubes generated over the plasma reduced Pd/HZSM-5 catalyst are also different from those obtained over the hydrogen-reduced catalysts [49]. All these changes confirm the change in the catalyst structure induced by the room temperature plasma reduction, as discussed above with the DRIFT analyses. The present study on CO oxidation over Pd/SBA-15 is another excellent example for the structure sensitive catalyst.
2087
1942
B1 B2
(b) 1936
i h
0.015
K-M units
K-M units
1989
j 0.020
i h
0.015
f COgas
0.010
e
g f COgas
0.010
e
d c
0.005
d c
0.000
a
b 0.000
L
(a)
g
0.005
Atomic ratio
at.%
and H-Pd/SBA-15. Firstly, it is clearly visible that the B2 /B1 area ratio in the CO/FTIR spectra of the sample H-Pd/SBA-15 is bigger than the B2 /B1 area ratio in the CO/FTIR spectra of the sample P-Pd/SBA-15 at the same time of CO adsorption and desorption, which is induced by a change of the metal particle size: the bigger the fcc cubeoctahedron metal crystallites, the larger the ratio of Pd(1 1 1) to Pd(1 0 0) planes [45,46]. This result is supported by the particle size distribution calculated from the TEM images and the wide-angle XRD patterns mentioned above. Secondly, the (B1 + B2 )/L area ratio in the CO/FTIR spectra of the sample H-Pd/SBA-15 is obviously bigger than the (B1 + B2 )/L area ratio of the sample P-Pd/SBA-15, which is also induced by a change of the metal particle size: linear CO is preferentially formed on low-coordinated Pd atoms where smaller particles expose a larger fraction of low-coordinated atoms than larger particles, while the B/L ratio increases with the increasing particle size [45]. These results indicate that during the thermal treatment, the Pd crystallites undergo a further reconstruction, which leads to a slight increase in the particle size, with a minor change in the exposing planes and atoms. It appears that this preferential adsorption of CO at bridge sites induces a geometrically advantageous setup for reaction between CO and O adatoms and changes the reaction rate, with a result of a higher activity for the sample H-Pd/SBA-15 on CO oxidation. This result is similar to the finding reported by Somorjai and his co-workers on the study of Rh nanoparticles [23].
0.020
Pd3d
BE (eV)
b
a
2200
2100
2000
1900 -1
Wavenumber cm
1800
2200
2100
2000
1900
1800
-1
Wavenumber cm
Fig. 12. DRIFT spectra of CO adsorbed on P-Pd/SBA-15 (a) and H-Pd/SBA-15 (b) samples at 25 ◦ C. Sample exposed to 1.1 kPa CO for 1 min, a; 5 min, b; 10 min, c; 20 min, d; 30 min, e; and purged with He for 1 min, f; 5 min, g; 10 min, h; 20 min, i; 30 min, j.
680
H. Wang, C.-j. Liu / Applied Catalysis B: Environmental 106 (2011) 672–680
according to the DRIFT studies. Because of the importance of Pd catalysts, the Pd/SBA-15 catalysts prepared in this work will have many other applications beyond CO oxidation, like the three-waycatalysts (TWC), sensors, drug delivery and optoelectronic. Acknowledgement This work was supported by the National Natural Science Foundation of China (#20990223). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.apcatb.2011.06.034. References [1] D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky, Science 279 (1998) 548–552. [2] C.T. Kresga, M.E. Lenowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710–712. [3] H. Takahashi, B. Li, T. Sasaki, C. Miyazaki, T. Kajino, S. Inagaki, Chem. Mater. 12 (2000) 3301–3305. [4] J. Adam, E. Antonakou, A. Lappas, M. Stöcker, M.H. Nilsen, A. Bouzga, J.E. Hustad, G. Øye, Micropor. Mesorpor. Mater. 96 (2006) 93–101. [5] V.S.-Y. Lin, C.-Y. Lai, J. Huang, S.-A. Song, S. Xu, J. Am. Chem. Soc. 123 (2001) 11510–11511. [6] M.V. Reg´, F. Balas, D. Arcos, Angew. Chem. Int. Ed. 46 (2007) 7548–7558. [7] S.-W. Song, K. Hidajat, S. Kawi, Langmuir 21 (2005) 9568–9575. [8] Q. Lu, F. Gao, S. Komarneni, T.E. Mallouk, J. Am. Chem. Soc. 126 (2004) 8650–8651. [9] F. Zhang, Y. Yan, H. Yang, Y. Meng, C. Yu, B. Tu, D. Zhao, J. Phys. Chem. B 109 (2005) 8723–8732. [10] H. Cui, Y. Zhang, Z. Qiu, L. Zhao, Y. Zhu, Appl. Catal. B: Environ. 101 (2010) 45–53. [11] E. Weber, D. Sirim, T. Schreiber, B. Thomas, J. Pleiss, M. Hunger, R. Gläser, V.B. Urlacher, J. Mol. Catal. B 64 (2010) 29–37. [12] Q. Tang, S. Hu, Y. Chen, Z. Guo, Y. Hu, Y. Chen, Y. Yang, Micropor. Mesorpor. Mater. 132 (2010) 501–509. ´ B. Pawelec, E. Rivera-Munoz, R. Nava, J. Espino, J.L.G. Fierro, [13] R. Huirache-Acuna, Appl. Catal. B: Environ. 92 (2009) 168–184. [14] Y. Wang, R.T. Yang, J.M. Heinzel, Chem. Eng. Sci. 63 (2008) 356–365. [15] Th.A. Steriotis, K.L. Stefanopoulos, F.K. Katsaros, R. Gläser, A.C. Hannon, J.D.F. Ramsay, Phys. Rev. B 78 (2008) 115424.
[16] D. Liu, X.Y. Q, H.H.A. Wah, G. Zeng, Y. Li, Y. Yang, Catal. Today 148 (2009) 243–250. [17] X. Wu, S. Kawi, Catal. Today 148 (2009) 251–259. [18] L. Escamilla-Perea, R. Nava, B. Pawelec, M.G. Rosmaninho, C.L. Peza-Ledesma, J.L.G. Fierro, Appl. Catal. A 381 (2010) 42–53. [19] C.-W. Chiang, A. Wang, B.-Z. Wan, C.-Y. Mou, J. Phys. Chem. B 109 (2005) 18042–18047. [20] H. Zhu, C. Liang, W. Yan, S.H. Overbury, S. Dai, J. Phys. Chem. B 110 (2006) 10842–10848. [21] D. Tian, G. Yong, Y. Dai, X. Yan, S. Liu, Catal. Lett. 130 (2009) 211–216. [22] C. Tu, A. Wang, M. Zheng, Y. Meng, J. Shan, T. Zhang, Chin. J. Catal. 26 (2005) 631–633. [23] M.E. Grass, S.H. Joo, Y. Zhang, G.A. Somorjai, J. Phys. Chem. C 113 (2009) 8616–8623. [24] C.K. Krishnan, K. Nakamura, H. Hirata, M. Ogura, Phys. Chem. Chem. Phys. 12 (2010) 7513–7520. [25] A.S. Ivanova, E.M. Slavinskaya, R.V. Gulyaev, V.I. Zaikovskii, O.A. Stonkus, I.G. Danilova, L.M. Plyasova, I.A. Polukhina, A.I. Boronin, Appl. Catal. B: Environ. 97 (2010) 57–71. [26] S. Hinokuma, H. Fujii, M. Okamoto, K. Ikeue, M. Machida, Chem. Mater. 22 (2010) 6183–6190. [27] Z. Wang, B. Li, M. Chen, W. Weng, H. Wan, Sci. China Chem. 53 (2010) 2047–2056. [28] L. Liu, F. Zhou, L. Wang, X. Qi, F. Shi, Y. Deng, J. Catal. 274 (2010) 1–10. [29] J. Gu, A. Bao, M. Jia, Y. Liu, B. Zhaorigetu, Z. Yuan, Chin. J. Catal. 31 (2010) 322–328. [30] Y. Bi, G. Lu, Appl. Catal. B: Environ. 41 (2003) 279–286. [31] Z.-J. Wang, Y. Zhao, L. Cui, H. Du, P. Yao, C.-J. Liu, Green Chem. 9 (2007) 554–559. [32] Z.-J. Wang, Y. Liu, P. Shi, C.-J. Liu, Y. Liu, Appl. Catal. B: Environ. 90 (2009) 570–577. [33] X. Liang, C.-J. Liu, P. Kuai, Green Chem. 10 (2008) 1318–1322. [34] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquérol, T. Siemieniewska, Pure Appl. Chem. 57 (1985) 603–619. [35] Z.-J. Wang, Y. Xie, C.-J. Liu, J. Phys. Chem. C 112 (2008) 19818–19824. [36] T.L. Barr, J. Phys. Chem. 82 (1978) 1801–1810. [37] Y. Sohn, D. Pradhan, K.T. Leung, ACS Nano 4 (2010) 5111–5120. [38] Y. Li, B.W.L. Jang, Appl. Catal. A 392 (2011) 173–179. [39] G. Agostini, R. Pellegrini, G. Leofanti, L. Bertinetti, S. Bertarione, E. Groppo, A. Zecchina, C. Lamverti, J. Phys. Chem. C 113 (2009) 10485–10492. [40] O. Dulaurent, K. Chandes, C. Bouly, D. Bianchi, J. Catal. 192 (2000) 273–285. [41] V.S. Escribano, L. Arrighi, P. Riani, G. Busca, Langmuir 22 (2006) 9214–9219. [42] J.B. Giorgi, T. Schroeder, M. Bäumer, H.-J. Freund, Surf. Sci. 498 (2002) L71–L77. [43] K. Almusaiteer, S.S.C. Chuang, J. Catal. 184 (1999) 189–201. [44] I. Eswaranoorthi, A.K. Dalai, Int. J. Hydrogen Energy 34 (2009) 2580–2590. [45] G.C. Cabilla, A.L. Bonivardi, M.A. Baltanás, Catal. Lett. 55 (1998) 147–156. [46] R. Van Hardeveld, F. Hartog, Adv. Catal. 22 (1972) 75–113. [47] Y.W. Li, R.T. Yang, C.-J. Liu, Z. Wang, Ind. Eng. Chem. Res. 46 (2007) 8277–8281. [48] Z. Wang, C.-J. Liu, G.L. Zhang, Catal. Commun. 10 (2009) 959–962. [49] Y. Zhao, Y.-X. Pan, L. Cui, C.-J. Liu, Diamond Relat. Mater. 16 (2007) 229–235.