Catal Lett (2009) 133:354–361 DOI 10.1007/s10562-009-0185-7

Preparation of Promoted Pt/SBA-15 and Effect of Cerium on the Catalytic Activity over Carbon Monoxide Oxidation Conversion Reaction Guoqian Chen • Ying Zheng • Guohui Cai Yong Zheng • Yihong Xiao • Kemei Wei

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Received: 8 August 2009 / Accepted: 4 October 2009 / Published online: 17 October 2009 Ó Springer Science+Business Media, LLC 2009

Abstract Pt/SBA-15 catalysts are modified by cerium dopant and agent. Experimental results show that the catalytic activities of Pt/SBA-15 can be deeply promoted by the introduced cerium species, especially while both cerium dopant and cerium agent are introduced. The promoted catalytic activities are due to the occurrence of new coordination environments, promoted physicochemical properties of Pt loading created by both introduced cerium species, and the strong metal-support interactions created by the cerium loading. Keywords Dopant
Promoter
Carbon monoxide
Catalytic activity

1 Introduction For its excellent performance such as large surface area, large pore volume and well defined pore size etc., ordered mesoporous silica material SBA-15 has received reasonable attentions [1–5]. Many studies concerned with the noble metal catalysts base on SBA-15 are introduced recently, since the prepared catalyst has high activity over carbon monoxide (CO) oxidation reaction with long Electronic supplementary material The online version of this article (doi:10.1007/s10562-009-0185-7) contains supplementary material, which is available to authorized users. G. Chen
Y. Zheng (&) College of Chemistry and Materials Science, Fujian Normal University, 350007 Fuzhou, Fujian, China e-mail: [email protected] G. Cai
Y. Zheng
Y. Xiao
K. Wei National Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou University, 350002 Fuzhou, China

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service life [6, 7]. Promising catalytic characteristics over CO oxidation conversion reaction of these catalysts, paving a way for the material which efficiently applied in the environment protections, gas detectors, petrochemical industries and related theoretical researches etc. However, the deficiency of noble metal resources and the escalation of their prices limit the further use of it. This bewildering reality makes it necessary to modify SBA-15 supported catalysts and to promote the catalytic activities. Up to date, there are some progresses on the promoting of the catalytic activity of SBA-15 supported catalyst. Studies show that cerium (Ce) loading could be made as a suitable agent to promote catalytic performance of SBA-15 supported catalysts. For example, Liu et al. [8] successfully prepared a Ce and ruthenium (Ru) loaded catalyst base on SBA-15 (RuCe/SBA-15) for the selective hydrogenation of benzene to cyclohexene, on which the maximum yield of cyclohexene was promoted to 44.8% from 25%. At the same time, researchers also focus on seeking other effective way, such as the introducing of Ce dopant, to promote the catalytic performance of SBA-15 supported catalysts and to reduce the consumption of noble metals [9–12]. However, the incorporation process is difficult to be conduct since the different sizes of silicon atom and cerium ionic radii. The catalytic activities of these catalysts can not be promoted deeply by the introducing of Ce agent, either the Ce dopant. It is reasonable to propose that the catalytic activity might be further promoted while both Ce agent and dopant are introduced into catalyst. However, to the best of our knowledge, there are no relevant reported studies. The reduplicative influent details on the catalytic activities of both cerium agent and dopant are unclear so far. Additionally, few information is available on the performance of Pt/SBA-15 catalyst over CO oxidation conversion reaction.

Preparation of Promoted Pt/SBA-15 and Effect of Cerium on the Catalytic Activity

In this paper, we prepared modified Pt/SBA-15 catalysts, in which one Ce species is incorporated into the framework of SBA-15 (Ce-SBA-15) with the assistant of microwave procedure, and another Ce species is used as an alternative (promoter) of noble metal (Pt and Ce species loaded on Ce-modified SBA-15). We first report the reduplicative influent of both cerium promoter and dopant on the catalytic activity over CO oxidation conversion reaction. Experimental results indicate that both introduced Ce species have great impact on the catalytic activity over CO oxidation conversion reaction. Contrasting with traditional modified SBA-15 based catalysts, the catalytic activity of as prepared catalyst over CO oxidation conversion reaction can be promoted deeply while both Ce loading and dopant are introduced into the catalyst. Details concerned with the influence on the catalytic activity of different introduced Ce species are mainly introduced in this paper.

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the prepared products, where the conducted Ce/Si = 0.05, 0.10, 0.15, 0.20 respectively in the synthesized process. Preparation of Ce/SBA-15: the synthesized SBA-15 was impregnated with a certain amount of aqueous solution of Ce(NO3)3
6H2O overnight (Ce/Si = 0.005, 0.010, 0.015, 0.025) respectively. Then, the result product was dried at 100 °C and calcinated in static air at 500 °C for 4 h. The prepared 1Ce/SBA-15, 2Ce/SBA-15, 3Ce/SBA-15, 5Ce/ SBA-15 are defined as the support, where the conducted Ce/Si = 0.005, 0.010, 0.015, 0.025, respectively. The Ce/Ce-SBA-15 was prepared using the same method. As for preparation of Pt loaded catalysts, using H2PtCl6 aqueous solution as the Pt source. The impregnation procedure is the same with that of Ce/SBA-15. For all the tested catalysts, the percentage of Pt was around 1% and expressed as the weight ratio of WPt/WSupport. 2.2 Catalyst Characterizations

2 Experimental 2.1 Catalysts Preparation All chemicals employed in this study were analytical reagent except that P123 was purchased from Aldrich (Mw: *5,800). In the synthesis procedure, nonionic triblock co-polymer surfactant EO20PO70EO20 (P123), tetraethyl orthosilicate (TEOS), cerium nitrate (Ce(NO3)3
6H2O), hydrochloric acid (HCl) were used as structure-directing agent, silicon precursor, cerium precursor and acid source, respectively. Preparation of SBA-15 and Ce incorporated SBA-15 (Ce-SBA-15): SBA-15 was prepared as previous report described [13]. Ce incorporated SBA-15 (Ce-SBA-15) was synthesized via microwave assistant method. Firstly, a stirring solution of 1 g P123 and 30 g distilled water were mixed with aqueous HCl solution with pH = 2.50, and stirred at 40 °C for 30 min. Then, a certain amount (Ce/ Si = 0.05, 0.10, 0.15, 0.25) of cerium nitrate was added followed by the addition of 6 mL tetraethyl orthosilicate (TEOS), followed by the addition of NH4F (F/Si = 0.03). After stirring for 1 h, the resulting solution was transfered to an Initiator 8 EXP microwave synthesizer and heated at 40 °C for 3 h with stirring, and aged at 80 °C for 5 h. The obtained solid powder was recovered by filtration, drying (at 100 °C), calcined in N2 flow (at 500 °C for 3 h, heating rate is 60 °C/h) and calcined in air (at 500 °C for 5 h) orderly. The Ce-SBA-15 samples were treated by 0.01 mol/L HCl aqueous solutions in order to remove the surface cerium compounds. Resulting solid was recovered again by filtration, washed with deionized water and dried under ambient conditions. The obtained Ce-SBA-15-1, Ce-SBA-15-2, Ce-SBA-15-3 and Ce-SBA-15-4 are defined as

The platinum contents of the catalysts were analyzed by atomic adsorption spectroscopy (AAS) and are all around 1% (WPt/WSupport). The Si: Ce molar ratios of the obtained samples were analyzed by X-ray fluorescence (XRF) spectrometer on a PANalytical Axios XRF Petro Spectrometer. X-ray powder diffraction (XRD) was performed on a Bruker D8 Advance X-ray diffractometer system ˚ ). The tube voltage using Cu Ka radiation (k = 1.54056 A was 40 kV, and the current was 40 mA. Fourier-transform infrared spectra (FT-IR) was recorded with a Nicolet Model 759 Fourier transform infrared spectrometer, in transmission mode in a KBr pellet. X-ray photoelectron spectroscopy (XPS) measurement was performed using a Physical Electronics Quantum 2000 spectrometer operating with an Al Ka monochromatic X-ray source (hv = 1,486.6 eV). The standard binding energy was regulated by C1 s (284.6 eV). The diffuse reflectance UV–vis spectra (DRS) were collected on Perkin Elmer Lambda 950, using BaSO4 as standard for measurements. The powder sample was loaded into a diffuse reflectance acessory, and the spectrum was collected over the range of 200–800 nm with a resolution of 2 nm. Nitrogen adsorption experiments were performed at -196 °C using nitrogen in a conventional volumetric technique by Quantachrome Nova 4200. All samples were degassed under vacuum to 1 9 10-5 Pa at 300 °C for 3 h. Surface areas of samples were calculated from nitrogen adsorption by Brunauer-Emmett-Teller (BET) method. Nitrogen desorption experiments were performed at -196 °C by controlling pressure (P/P0) using an ASAP 2020 M apparatus which from Micromeritics. The poresize distributions of each samples were determined by desorption isotherm measurements and determined by using Barrett–Joyner–Halenda (BJH) method.

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The chemisorption study was carried out using an AutoChem 2910 instrument (Micromeritics). In typical procedure, the catalyst (ca. 150 mg) was reduced in a H2 stream at 400 °C for 1 h. The sample was subsequently flushed with a helium stream for 1.5 h to remove H2 adsorbed on the surface of the catalyst, and it was finally cooled in a helium stream to 25 °C. CO chemisorption was measured by the pulse method by introducing 5% CO ? 95% He flowing over the sample maintained at 25 °C. The dispersions, specific metal surface areas and sizes of Pt particles were calculated from the cumulative volume of CO adsorbed during pulse chemisorption. Temperature-programmed reduction (TPR) characterization was carried out in a Micromeritics Autochem 2910 Instrument. 150 mg of the fresh sample was packed into a reactor with quartz tubing of 6 mm id., and it was oxidized by oxygen for 1 h at 500 °C, then purged with high purity helium gas at 300 °C for 1 h, and cooled down to room temperature. TPR traces of the sample were then pursued in a reductive flow of 30 mL/min 10% H2-Ar, heated at a rate of 10 °C/min from room temperature to 800 °C. The rate of hydrogen consumption was monitored by TCD of instrument. 2.3 Activity Test 0.100 g catalysts were enclosed in a fixed-bed quartz reactor with an interior diameter of 5 mm. The catalysts were reduced in flowing H2 at 300 °C for 1 h (30 mL/min) and then cooled to the desired reaction temperature. The gas mixture of 1% CO ? 1% O2 ? 98% He passed through the reactor with space velocity of about 900 cm3/ min g. The reactants and the products were analyzed with a Shimadzu GC-14 gas chromatograph equipped with a TCD detector. The conversion efficiencies of CO at different temperatures were recorded and the lowest total conversion temperature of CO was used for characterizing the catalytic activity of catalysts.

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dopants of Ce-SBA-x (x = 1, 2, 3) are increased monotonously as the increased of introduced Ce precursors content in the synthesis procedure. Pt/Ce-SBA-15-2 and Pt2Ce/SBA-15 are selected for the studying and constrasting of potential valences of cerium in catalysts since there have similar Ce contents (Table S1). Figure 1 shows the Ce 3d XP spectra of Pt/Ce-SBA-15-2 and Pt2Ce/SBA-15. According to the literature, the Ce 3d spectra could be assigned 3d3/2 spin–orbit states (labeled U) and 3d5/2 states (labeled V) where composed of 10 various states [14, 15]. Careful analysis of the Ce 3d XP spectra allows distinguish and quantify the relative contribution of the Ce3? and Ce4? compounds. The surface concentration of Ce4? can be determined from the following equation [16, 17]:
Ce4þ ¼ Ce4þ = Ce3þ þ Ce4þ Ce3þ ¼ U0 ð904:0 eVÞ þ U0 ð898:8 eVÞ þ V0 ð885:2 eVÞ þ V0 ð880:5 eVÞ Ce4þ ¼ U000 ð916:9 eVÞ þ U00 ð907:5 eVÞ þ Uð901:1 eVÞ þ V000 ð898:3 eVÞ þ V00 ð888:7 eVÞ þ Vð882:6 eVÞ The fraction of Ce4? can be obtained by solving Eq. above. It can be noticed that comparing with that of Pt2Ce/ SBA-15, more Ce(IV) species appears in the Pt/Ce-SBA15-2. This value was to be approximately 76% in Pt/CeSBA-15-3 and 60% in Pt2Ce/SBA-15. Definitely, Ce 3d XP spectra analysis shows that Ce(IV) and Ce(III) coexist in Pt/Ce-SBA-15-2 and Pt2Ce/SBA-15. However, the ratios of Ce3?/Ce4? in these two catalysts are different with each other. This feature may attribute to the formation of new Ce(III) and Ce(IV) species after incorporation procedure.

3 Results and Discussion Low angle XRD and FT-IR characterizations (see Supporting Information) of as prepared supports indicate that the Ce species can be introduced into the framework of SBA-15 successfully via our procedure. More ordered structure can be obtained after Ce species was incorporated into the SBA-15 with a suitable content. The Ce-SBA-15-1 and Ce-SBA-15-2 have more ordered structure than that of Ce-SBA-15-3. We do not need to discuss the catalyst based on Ce-SBA-15-4 since it is meaningless while the pristine structure of SBA-15 was destroyed almost. From FT-IR results (Fig. S2), together with Si: Ce molar ratio analysis (Table S1), we can conclude that the contents of Ce

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Fig. 1 Ce 3d XP spectra of Pt/Ce-SBA-15-2 and Pt2Ce/SBA-15

Preparation of Promoted Pt/SBA-15 and Effect of Cerium on the Catalytic Activity

3.1 Catalytic Performances of Modified Pt/SBA-15 Catalysts A series of catalysts prepared base on as synthesized supports described above, for studying their catalytic activities over CO oxidation conversion reaction. Table 1 shows the catalytic activities of these samples. Detailed data plot of the dependence of CO oxidation conversion reaction over bare SBA-15 and different prepared catalysts are shown in the supporting information (Fig. S3–5). From these experimental results, the bare SBA-15 exhibits no catalytic activity at the investigated temperatures while the 2Ce/ SBA-15 has a CO conversion of 100% at 350 °C. Much higher catalytic activity can be obtained in case the Pt to be introduced. Comparing with Pt/SBA-15, Pt/Ce-SBA-15-x (x = 1, 2, 3) has much better catalytic activity. Obviously, although there are no significant changes on their complete conversion temperature, Pt/Ce-SBA-15-2 and Pt/Ce-SBA15-3 have 80% conversion at much lower temperature than that of Pt/SBA-15 and Pt/Ce-SBA-15-1. Based on these features, it is clear that the incorporation of Ce species into the framework of SBA-15 can favor for the catalytic activity over CO oxidation conversion reaction. Meaningful, the introduction of cerium loading could to improve the catalytic activity also. Along with the increasing of Ce loading, the 100% CO conversion temperature of PtxCe/SBA-15 (x = 1, 2, 3, 5) declines from 210 to 190 °C, then eventually increases again. Although there are no significant changes on their temperature dependence of CO conversion at 100%. Pt2Ce/SBA-15 is more active, capable of effective converting of CO ([90%). A completely converting CO of Pt2Ce/SBA-15 is at 190 °C. Table 1 Catalytic activities of different catalysts over CO oxidation conversion reaction Catalysts

T100(oC)

SBA-15

No activity

2Ce/SBA-15

350

Pt/SBA-15

255

Pt/Ce-SBA-15-1

212

Pt/Ce-SBA-15-2

210

Pt/Ce-SBA-15-3

215

Pt1Ce/SBA-15

210

Pt2Ce/SBA-15

190

Pt3Ce/SBA-15

200

Pt5Ce/SBA-15

205

Pt1Ce/Ce-SBA-15-3

180

Pt2Ce/Ce-SBA-15-3

120

Pt3Ce/Ce-SBA-15-3

170

T100: temperature of complete conversion

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As far as we know, catalyst with Ce dopant can favor for the catalytic activity. It is difficult to incorporate Ce species into the framework of SBA-15 since the different size of silicon and cerium atoms. Details of characterizations described above indicate that Ce species can be introduced into the framework of SBA-15 via our procedure. It is unclear that whether the introducing of loaded-Ce into Cedoped SBA-15 (Ce-SBA-15) can favor for the catalytic activity over CO oxidation conversion reaction. We prepared Pt and Ce loaded catalyst base on Ce-SBA-15-3 for studying the catalytic performance, since Ce-SBA-15-3 possess both suitable ordered structure and Ce content. The catalytic activity datas of PtxCe/Ce-SBA-15-3 (x = 1, 2, 3) indicate that catalysts with both Ce-doping and Ce-loading have more capable catalytic performance. The catalytic activities of these catalysts are much higher than that of PtxCe/SBA-15 (x = 1, 2, 3, 5) and Pt/Ce-SBA-15-x (x = 1, 2, 3). Especially, Pt2Ce/Ce-SBA-15-3 has the highest catalytic activity over the CO conversion reaction. For this catalyst, the oxidation quickly attained a conversion near 80% at 110 °C and complete conversion at 120 °C. This catalytic activity over CO oxidation conversion reaction is better than that of reported Pt loaded catalysts such as Pt/Al2O3, Pt/CeO2/Al2O3, Pt/CeO2-Al2O3 and Pt/SiO2 [18–22]. We can conclude that both the Ce-doping and loading can enhance the catalytic activity. The better catalytic activity can be obtained while both Ce-doping and Ce-loading are introduced into the catalyst. i.e., the introducing of loaded Ce to Ce-doped SBA-15 could promote the catalytic activity deeply. The prepared Pt2Ce/Ce-SBA15-3 has much better catalytic performance than the others. We interested in the promotion mechanism on the catalytic performance of Ce species since the expected experimental results would be meaningful for deep application of SBAbased catalysts and other industrial catalysts. The influences on the catalytic activity of different introduced Ce species are mainly introduced below. 3.2 Effections on Texural Characteristics of Introduced Ce Species Nitrogen adsorption–desorption isotherms (see Supporting Information) of all studied samples are characteristics of mesoporous solid and can be classified as type IV isotherms with an H1 hysteresis loop, which is similar with that of SBA-15 [23]. This feature indicates that the mesoporous structure of the catalyst is preserved mostly after suitable amount of Ce species introduced. The physicochemical properties of these samples are shown in Table 2. For Ce-SBA-15-x (x = 1, 2, 3), the BET surface areas (SBET) increase slightly from 620 to 683 m2/g after Ce species introduced, variously. This feature can be attribute

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Table 2 Physicochemical properties of SBA-15, Ce-SBA-15-x (x = 1, 2, 3), xCe/SBA-15 (x = 1, 2, 3, 5) Sample

SBET (m2g-1)

VP (cm3g-1)

dP (nm)

SBA-15

620

0.89

4.93

Ce-SBA-15-1

683

0.84

4.61

Ce-SBA-15-2

681

0.58

4.26

Ce-SBA-15-3

663

0.49

3.95

1Ce/SBA-15

597

0.53

4.21

2Ce/SBA-15

558

0.51

4.19

3Ce/SBA-15 5Ce/SBA-15

537 503

0.47 0.46

4.18 4.03

to the enhancement in long-rang ordering of the mesoporous structure after Ce species was incorporated into the SBA-15. However, comparing with that of Ce-SBA-15-1 (683 m2/g), BET surface areas of Ce-SBA-15-2 and CeSBA-15-3 decrease to 681 and 663 m2/g respectively. We can find that as increasing of Ce content, the pore volume is lose from 0.89 cm3g-1 (SBA-15) to 0.49 cm3g-1 (Ce-SBA15-3) as well as the pores size. The pore diameters (dP) of Ce-SBA-15-x (x = 1, 2, 3) is a bit smaller than that of SBA-15. While Ce loading is introduced, moderate loss in the surface area (form 620 m2/g of SBA-15 to 503 m2/g of 5Ce/SBA-15), pore volume (form 0.89 cm3g-1 of SBA-15 to 0.46 cm3g-1 of 5Ce/SBA-15) and pore diameter (form 4.93 nm of SBA-15 to 4.03 nm of 5Ce/SBA-15) can be observed. This may attributes to clogging support pores by cerium species makes pores inaccessible for nitrogen adsorption [24]. We can conclude that the introducing of Ce species into SBA-15 would to affect the textural characteristics of catalysts slightly and the mesoporous structure of SBA-15 can be preserved mostly. Although the pore sizes and pore volumes decreased after Ce species introduced, Ce-modified Pt/SBA-15 catalysts have higher catalytic performances than that of the others. The promotions of catalytic performances of modified catalysts should highly depend on other prior factors such as the changes of coordination environment, the physicochemical properties of active metal particles, the metal-support interactions etc. 3.3 Coordination Environment Changes New coordination environments caused by the difference introducing Ce species are considered as influences of catalytic performances. Ce-SBA-15-3 and 2Ce/SBA-15 are selected as typical modified SBA-15 supports in this investigation since the Pt2Ce/Ce-SBA-15-3 has the best catalytic activity among series prepared catalysts. Figure 2 shows the DRS spectra of SBA-15, CeO2, Ce-SBA-15-3, and 2Ce/SBA-15. No electronic transition can be found on

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Fig. 2 DRS spectra of SBA-15, CeO2, 2Ce/SBA-15 and Ce-SBA-15-3

the DRS spectra of pure SBA-15. The DRS spectrum of CeO2 shows two intense absorption bands at 239 and 345 nm, while the DRS spectra of 2Ce/SBA-15 shows a broad absorption band around 300 nm which attributes to the metal charge transfer (O2 ? Ce4?) of CeO2 clusters of several nanometers in size [25]. On the contrary, new charge transfer at 222 and 256 nm appears in the DRS spectra of Ce-SBA-15-3, which can be assigned to ligandto-metal charge transfer O ? Ce4? in isolated Ce4? ions species [26, 27] and the O ? Ce3? charge transfer respectively [28]. No absorption bands at 350–800 nm are observed, indicates that there are well-dispersed isolated Ce3? and Ce4? in the framework of the Ce-SBA-15 or the cerium species present as small nanometer scale [26, 29]. Comparing with Ce/SBA-15, the Ce-SBA-15 is more favor for the dispersion of Ce species, inhibits aggregation particles from formation, and therefore improves the properties of catalyst. 3.4 Physicochemical Properties of Loaded Pt All the XRD patterns of Pt/Ce-SBA-15-x (x = 1, 2, 3) and Pt/SBA-15 (Fig. 3) exhibit a broad diffraction peak between 15–35 degrees, corresponding to the amorphous SiO2 of the supports [30, 31]. There are no diffraction peaks of CeO2 were observed indicates that well-dispersed of Ce species in Ce-doped SBA-15, this result in accord with the DRS analysis result. The XRD patterns of Pt/SBA15 and Pt/Ce-SBA-15-1 show the peak characteristics of Pt at 2h = 39.9°, 46.2°, 67.6° and 81.0°, which are usually assigned to (111), (200), (220) and (311) inter planar spacings of the cubic platinum metal structure, respectively [32, 33]. However, as increasing of the Ce dopant, these peaks broadening and disappearing gradually. It indicates that the Pt nanoparticles are highly dispersed and the

Preparation of Promoted Pt/SBA-15 and Effect of Cerium on the Catalytic Activity

359

Fig. 4 XRD patterns of PtxCe/SBA-15 (x = 1, 2, 3, 5) Fig. 3 XRD patterns of Pt/SBA-15 and Pt/Ce-SBA-15-x (x = 1, 2, 3)

Table 3 Physicochemical properties of Pt loaded on different supporters Catalysts Pt/SBA-15

DPt (%)a

SPt (m2/g)b

dPt (nm)c

6.21

15.33

18.24

Pt/Ce-SBA-15-1

10.34

25.55

10.95

Pt/Ce-SBA-15-2

23.87

58.96

4.74

Pt/Ce-SBA-15-3 Pt1Ce/SBA-15

30.21 14.91

74.61 36.84

3.75 7.59

Pt2Ce/SBA-15

17.06

42.13

6.64

Pt3Ce/SBA-15

17.92

44.28

6.32

Pt5Ce/SBA-15

21.62

53.41

5.24

Pt1Ce/Ce-SBA-15-3

15.28

37.74

7.41

Pt2Ce/Ce-SBA-15-3

22.75

56.18

4.98

Pt3Ce/Ce-SBA-15-3

24.61

60.36

4.24

a

Dispersions of Pt particles

b

Surface area of Pt particles

c

Particle sizes of Pt

average particle sizes are too small to be detected by XRD. Clearly, the Ce dopant would to promote the dispersions of Pt particles. Table 3 shows the detailed physicochemical properties of different Pt loaded catalysts obtained via CO chemisorption experiment. The Pt dispersion (DPt) and particle size (diameter, dPt) of Pt/SBA-15 is 6.21% and 18.2 nm respectively. However, the particle size of Pt of Pt/Ce-SBA-15-x (x = 1, 2, 3) decreases obviously, while the dispersion and surface area of Pt particles (SPt) increases accordingly. These changes show a trend that is in line with the XRD observations (Fig. 3). The Pt particle of Pt/Ce-SBA-15-3 has the highest dispersion and surface area. This improvement of dispersion can to promote the exposed Pt atom and therefore favor for catalytic activity.

As a result, the Pt/Ce-SBA-15-x (x = 1, 2, 3) behaves higher catalytic performance over CO oxidation conversion reaction than Pt/SBA-15. Similar changes can be found in the XRD patterns of PtxCe/SBA-15 (x = 1, 2, 3, 5, Fig. 4). Table 3 also shows physicochemical properties of these Pt loaded catalysts. The increasing of the Ce loading also can favor for the dispersion of Pt. However, more Ce loading introduced, more intense diffraction peaks of CeO2 can be observed, and indicates that accumulated CeO2 crystal appears gradually. In this case, the catalytic performances of PtxCe/SBA-15 (x = 1, 2, 3, 5) are not dependent on the metal dispersions simply. Comparing with Pt2Ce/SBA-15, Pt3Ce/SBA-15 and Pt5Ce/SBA-15 have higher metal dispersions but lower catalytic performances. It is reasonable for this feature is due to the accumulated CeO2 with bigger dimension and to hinders the catalytic activity. This negative influence on the catalytic activity can also observed in PtxCe/Ce-SBA-15-3 (x = 1, 2, 3) since the accumulated CeO2. Although Pt2Ce/ SBA-15-3 has lower Pt dispersion than that of Pt3Ce/SBA15-3, it has much higher catalytic performance. Thus, we can conclude that the introduced Ce loading should be maintained as an appropriate content (2%). Obviously, both introducing of Ce dopant and loading can favor for the dispersion of Pt particle. The decline of the size of Pt particle could strengthen the bond strength of Pt–O and facilitate the catalytic activity of catalyst. However, it does not mean that the catalytic activity promoted monotonically with the increasing of Pt dispersion. Although the Pt2Ce/Ce-SBA-15-3 has a lower metal dispersion (22.75%) than that of Pt/Ce-SBA-15-2 (23.87%) and Pt/Ce-SBA-15-3 (30.21%), higher catalytic performance is observed. More experiment concerned with this feature such as the interaction between active component platinum and Ce species should be introduced further.

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3.5 The Interaction Between Loaded Pt Particles and Introduced Ce Species Figure 5 shows the H2-TPR results of different catalysts. H2-TPR spectra of CeO2/SBA-15 exhibits a main reduction peak at 450 °C and shoulder reduction peak at 680 °C, which can be assigned to the reduction of surface oxygen and bulk oxygen reduction in support respectively [34]. The relative high amount of hydrogen consumed in the low temperature reduction peak indicating that the high proportion of CeO2 species presents as nanoparticle in the Ce/SBA-15. In the H2-TPR spectra of Pt/SBA-15, two reduction peaks at 300 and 520 °C can be attributed to the reduction of Pt-oxide species and oxychlor-platinum surface complexes, PtOxCly species, respectively [35]. This may be caused by the presence of comparative large Pt particle in Pt/SBA-15 (dPt = 18.24 nm, listed in Table 3). For Pt/Ce-SBA-15-3, comparing with the H2-TPR spectra of CeO2/SBA-15, the reduction peak of Ce species presents at a higher temperature, and suggests that the Ce species is strong interacted with the support. The low temperature reduction peak at 100 °C of this spectrum is attributed to the reduction of oxidized Pt to Pt0. Pt particle is present on the surface of SBA-15 and no strong metal-support interaction appears in this catalyst. However, for the spectra of Pt2Ce/SBA-15 and Pt2Ce/Ce-SBA-15-3, there are two low temperature reduction peaks are close to each other with temperature maximum at about 100 °C and 160 °C, respectively. The first peak is assigned to the reduction of oxidized Pt to Pt0 which has not interacts with the support. The hydrogen consumption in the broad range from 120 to 300 °C is assigned to surface reduction of CeO2 which in close contact with noble metal particle. This reduction temperature is much lower than that of CeO2/SBA-15 since the reduction of the surface oxygen on ceria is facilitated

G. Chen et al.

by Pt due to hydrogen spill over from the exposed metal surface [36]. Together with the catalytic performances of different catalysts, we can conclude that the existence of strong metal-support interaction in Pt2Ce/SBA-15 and Pt2Ce/CeSBA-15-3 greatly promoted the catalytic activities over CO oxidation conversion reaction. As a result, the Pt2Ce/CeSBA-15-3 has higher catalytic activity than that of Pt/CeSBA-15-2 and Pt/Ce-SBA-15-3 although the loaded Pt has lower dispersion.

4 Conclusions We studied dual promotions of catalytic activity of Ce dopant and Ce loading based on Pt/SBA-15. The textural characteristics of SBA-15 can be preserved mostly, while the introduced Ce dopant was maintained as a suitable content. Experimental results indicate that the catalytic activity of as prepared catalyst over CO oxidation conversion reaction can be promoted deeply while both Ce loading and dopant are introduced into the catalyst. The obviously promoted catalytic activities are not only depend on the increased dispersions of Pt loading which promoted by introduced Ce species, but also depend on the strong metal-support interaction created by the Ce loading. New coordination environments would appear after Ce species introduced and it is favor for the catalytic activity. It is noteworthy that the introduced Ce loading should be counted as a suitable content, for obtaining modified SBA15 based catalyst with high catalytic activity. Superfluous introduced Ce loading would create accumulated CeO2 crystal or big CeO2 particle which would to hinder the catalytic activity. Reported preparing process is suitable for obtaining modified Pt/SBA-15 with high catalytic activity over CO oxidation conversion reaction. The introduced results would provide a new sight to preparing Pt/SBA-15 based catalysts with high catalytic activity over CO oxidation conversion reaction and to seeking a cheaper alternative for noble metal catalysts. Acknowledgments The project was supported by the National Key Technology R&D Program (2007BEA08B01), the Natural Science Foundation of Fujian Province of China (No: E0710004) and Joint Research Program of Fuzhou University (No: DH-548).

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Fig. 5 TPR profiles of different samples

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