Direct synthesis of Pt nanoparticles-containing MCM-41 using surfactant stabilized Pt nanoparticles

834

Studies in Surface Science and Catalysis, volume 154 E. van Steen, L.H. Callanan and M. Claeys (Editors) © 2004 Elsevier B.V. All rights reserved.

DIRECT SYNTHESIS OF Pt NANOPARTICLES-CONTAINING MCM.41 USING SURFACTANT STABILIZED Pt NANOPARTICLES Oumi, Y., Oka, H., Kanehara, M., Teranishi, T. and Sano, T. School of Materials Science, Japan Advanced Institute of Science and Technology, Tatsunokuchi, Ishikawa 923-1292, Japan. Fax: +81-761-51-1625. E-mail: oumi(g)iaist.ac.ip ABSTRACT Pt nanoparticles-containing mesoporous silica MCM-41 (Pt/MCM-41) was directly synthesized using surfactant stabilized Pt nanoparticles after gel filtration. The obtained Pt/MCM-41 composite materials were characterized with XRD, UV-Vis, TEM and N2 adsorption. It was found that the Pt nanoparticles are located inside the pores of MCM-41 even after calcination at 500 °C, suggesting the high thermal stability of Pt nanoparticles within the mesopores.

INTRODUCTION Metal nanoparticles have attracted much attention due to their unique properties such as size-dependent catalytic, nonlinear optic and magnetic characteristics [1-6]. Since these properties may be dependent upon the dimension and size of metal nanoparticles, it is very important to control the size and shape of metal nanoparticles uniformly. Mesoporous materials seem to be ideal for forming a scaffold in which a three-dimensional dispersion of metal nanoparticles could be supported. There are many reports concerning preparation of metal nanoparticles inside mesoporous structures by several different methods: (i) ion -exchange of transition or noble metal into the previously formed silicate and then the reducing of the precursor to form metal particles within the mesoporous structure [7], (ii) chemical vapor deposition of volatile metal compounds and their subsequent decomposition in the porous structure [8] and (iii) wet preparation techniques such as incipient wetness and impregnation [9,10]. Although these methods have been successftil in introducing a large metal content into the mesopores, the control level of the size and shape of metal nanoparticles is not necessarily high and metal nanoparticles are usually formed both inside and outside the mesopores. Recently, several researchers have indicated the high potential of "host-guest" direct synthesis method with the surfactant stabilized metal nanoparticles for control of the size and shape of metal nanoparticles [11-14]. The surfactant stabilized metal nanoparticles have a particle diameter with the same order of magnitude as the pore size of the mesopoous host material. However, the formation of metal particles outside the mesopores is still pointed out due to coexistence of the metal-fi-ee surfactants. Namely, as the metal-free surfactants are mainly used for synthesis of mesoporous materials, the surfactant stabilized metal nanoparticles adsorbed outside the mesopores aggregate to form the bigger metal particles during calcination process for removal of surfactants. From such a viewpoint, in this paper, we tried to prepare metal especially Pt nanoparticles within the pores of mesoporous silica MCM-41 by the direct synthesis with using the surfactant stabilized Pt nanoparticles collected by gel filtration. EXPERIMENTAL Preparation of surfactant stabilized Pt nanoparticles and Pt nanoparticles-containing MCIVI-41 The surfactant stabilized Pt nanoparticles were prepared by photoreduction (365 nm, 500W, 24h) of H2PtCl6 (Wako Pure Chemical, Japan, 98.5%) using various quaternary ammonium surfactants (CnH2n+iN(CH3)3Br, CnTAB, n=8~18, Tokyo Kasei Chemical, Japan, 99%) as a protective agent. After photoreduction, the surfactant stabilized Pt nanoparticles were collected by gel filtration. Pt nanoparticles-containing MCM-41 (Pt/MCM-41) was synthesized according to the conventional synthesis procedure of MCM-41 except for the use of Pt nanoparticles instead of the micelle forming surfactant [15]. The chemical composition of the starting gel was as follows : 6Si02 : CnTAB : 1.5Na20 : 0.15(NH4)2O : 25OH2O. The products obtained were filtered

835 off, washed thoroughly with deionized water and dried at 120°C for 12 h. The Pt/MCM-41 samples were calcined at 500°C for 10 h to decompose the surfactant. Characterization The identification of the products was carried out by X-ray diffraction (Rigaku, RINT2000). Transmission electron microscopy (TEM) images were obtained using a Hitachi H-8000 125KV microscope. Textural properties (BET surface area, pore diameter, pore volume) were evaluated by nitrogen adsorption at -196°C (Bel Japan, Belsorp 28SA). Elemental compositions were determined by X-ray fluorescence (Philips, PW2400). The UV-Vis spectra of the aqueous solution of Ft nanoparticles-surfactant after photoreduction were recorded on a Hitachi U-3310 spectrophotometer at ambient temperature. RESULTS AND DISCUSSION Preparation of surfactant stabilized Pt nanoparticles Figure 1 shows the UV-Vis spectra of the aqueous solution of H2PtCl6 before and after photoreduction in the presence of C12TAB as a protective agent. The adsorption peak assigned to Pt ions was clearly observed at ca. 260 nm in the UV-Vis spectrum before photoreduction, whereas no peaks in the spectrum after photoreduction for 24 h. This strongly indicates the completion of reduction of Pt ions. The TEM image of the surfactant stabilized Pt nanoparticles obtained is shown in Figure 2. The Pt nanoparticles with 1-10 nm in diameter were observed. To clarify an influence of the carbon number of alky 1 chain of surfactant, the surfactant stabilized Pt nanoparticles were prepared using various surfactants. Table 1 lists the average diameter of Pt nanoparticles obtained. The average diameter of Pt nanoparticles was hardly dependent upon the carbon number of alkyl chain, expect for the Pt nanoparticles stabiHzed with CgTAB.

i * ^ ^" ' *.' - .

•:*- * > •

•.

25 nm 300 400 Wavelength /nm

500

Figure 1. UV-Vis spectra of aqueous solutions of H2PtCl^ in the presence of C12TAB (a) before and (b) after photoreduction for 24h.

Figure 2. TEM image of Pt nanoparticles stabilized with C12TAB.

Table 1. Average diameters of R nanoparticles stabilized with various surfactants. Surfactant CsTAB CioTAB C12TAB C14TAB C16TAB CigTAB

Average diameter /nm 2.59 3.11 3.07 3.09 2.90 3.57

836 Preparation of Pt/MCM-41 using surfactant stabilized Pt nanoparticles before gel filtration The Pt/MCM-41 samples were prepared using the Pt nanoparticles stabilized with various surfactants. Figure 3 shows a series of the powder XRD patterns for the as-synthesized and calcined Pt/MCM-41 samples. For reference, the XRD pattern of MCM-41 prepared with CieTAB was also shown. Except for the samples prepared using Pt nanoparticles stabilized with CgTAB and CioTAB, all as-synthesized samples exhibited a typical XRD pattern that indicates hexagonal structure. However, the crystallinity of MCM-41 structure was dependent upon the surfactant used. It was also found that no peaks assigned to Pt metals were observed in the XRD pattern. On the other hand, in the XRD patterns of Pt/MCM-41 after calcination at 500°C for 10 h, the peaks assigned to Pt metals as well as the characteristic MCM-41 peaks were clearly observed at higher 26, indicating an existence of bigger Pt particles.

Ut-*1

Figure 3. XRD pattems of (A) as-synthesized and (B) calcined (500°C, 10 h) Pt/MCM-41 prepared using Pt nanoparticles stabilized with various surfactants. Surfactant: (a) CigTAB, (b) CieTAB, (c) C14TAB, (d) C12TAB, (e) CioTAB, (f) CgTAB, (g) MCM-41. The particle size of Pt nanoparticles in MCM-41 was measured by TEM. Figure 4 shows the TEM image of the as-synthesized Pt/MCM-41 prepared using the Pt nanoparticles stabilized with C12TAB. The average diameter of Pt nanoparticles observed was ca. 3.9 nm. As compared with Pt nanoparticles stabilized with surfactant in the aqueous solution, the particle size of Pt was bigger and the particle size distribution became relatively broader. Figure 5 shows the TEM images of various calcined Pt/MCM-41 samples. The average diameter of Pt nanoparticles in the Pt/MCM-41 was 3-20 nm and was considerably larger than the pore size of MCM-41, indicating aggregation of Pt nanoparticles during calcination process. As aggregation of Pt nanoparticles within the mesopores of MCM-41 during calcination process is considered to be not so easy, these facts suggest strongly that the Pt-fi-ee surfactants are mainly used for synthesis of MCM-41 and that the Pt nanoparticles in the as-synthesized Pt/MCM-41 are located outside the mesopores. The surfactant stabilized Pt nanoparticles adsorbed outside the mesopores of MCM-41 may aggregate during calcination process, resulting in formation of Pt particles with bigger sizes. Chi et al. have also reported the similar phenomenon of

837 aggregation of Pt nanoparticles during calcination process of MCM-41, resulting in formation of bigger metal particles [12].

%;

25 nmTI

Figure 4. TEM image of as-synthesized Pt/MCM-41 prepared using Pt nanoparticles stabilized with C12TAB.

Cb)

(a)

^^% 25 nm

CO

25 nm

i^

25 nm

• 251

Figure 5. TEM images of calcined Pt/MCM-41 prepared using Pt nanoparticles stabilized with various surfactants. Surfactant: (a) CigTAB, (b) CieTAB, (c) C14TAB, (d) C12TAB. (b) •-

25 nm

(^

(c)

25 nm

25 nm

Figure 6. TEM images of Pt nanoparticles stabilized with various surfactants after gel filtration. Surfactant: (a) CigTAB, (b) C16TAB, (c) C14TAB, (d) C12TAB.

838 Preparation of Pt/MCM-41 using surfactant stabilized Ft nanoparticles after gel filtration To prepare the Pt/MCM-41 composite materials effectively using the surfactant stabilized Pt nanoparticles, the Pt-free surfactants were removed by gel filtration. Figure 6 shows the TEM images of the surfactant stabiHzed Pt nanoparticles collected by gel filtration. The average diameter of Pt nanoparticles was ca. 3 nm and no change in the average diameter was observed between Pt nanoparticles before and after gel filtration. Therefore, the Pt/MCM-41 was prepared using the surfactant stabilized Pt nanoparticles after gel filtration.

Figure 7. XRD patterns of calcined Pt/MCM-41 prepared using Pt nanoparticles stabilized with various surfactants after gel filtration. Surfactant: (a) CigTAB, (b) CigTAB, (c) CHTAB, (d) C12TAB. Figure 7 shows the powder XRD patterns of the calcined Pt/MCM-41 samples prepared using the surfactant stabilized Pt nanoparticles. Except for the Pt/MCM-41 prepared with Pt nanoparticles stabilized with CigTAB, surprisingly, all XRD patterns showed no peaks other than those corresponding to hexagonal structure of MCM-41. In the XRD pattern of Pt/MCM-41 from CigTAB, very weak diffraction peaks assigned to Pt metals were only observed. These results suggest no aggregation of Pt nanoparticles during calcination process. To confirm this, TEM images of the calcined Pt/MCM-41 samples were measured. Figure 8 shows the TEM image of the calcined Pt/MCM-41 prepared using Pt nanoparticles stabilized with C12TAB. The average diameter of Pt nanoparticles in the mesopores of MCM-41 was 3-4 nm and was comparable to the pore diameter of MCM-41. These results strongly indicate that Pt nanoparticles are located inside even after calcination and are highly dispersed in the mesopores.

839 500 (St)

400

e PH

H

e Qegm?c?d^&^^

ft

(c)

9

300

ui I Pt

-e (b)

20 nm

Figure 8. TEM image of calcined Pt/MCM-41 prepared using Pt nanoparticles stabilized with C12TAB after gel filtration.

0.5 Relative pressure (P/Po)

1.0

Figure 9. N2 adsorption isotherms of (a) MCM-41 and Pt/MCM-41 prepared using Pt nanoparticles stabilized with C12TAB (b) before and (c) after gel filtration.

If Pt nanoparticles exist in the mesopores of MCM-41, the adsorption capacity of Pt/MCM-41 might be reduced markedly. Accordingly, nitrogen adsorption isotherms were measured on Pt/MCM-41 and MCM-41 samples. As shown in Figure 9, although all isotherms exhibited the type IV, a large difference in the amount of nitrogen adsorbed was observed. Nitrogen adsorption hardly took place on the Pt/MCM-41 synthesized using the surfactant stabilized Pt nanoparticles before gel filtration, whereas the amount of nitrogen adsorbed for the Pt/MCM-41 prepared using Pt nanoparticles after gel filtration was comparable to that of MCM-41. The isotherms exhibited sharp inflective characteristics of capillary condensation at the relative pressure of ca. 0.25. Table 2 lists characteristics of MCM-41 and Pt/MCM-41 calculated from their isotherms. Table 2. Characteristics of calcined Pt/MCM-41 prepared using Pt nanoparticles stabilized with C12TAB. Sample

BET surface area

Pore volume^^

Pore diameter

d(lOO)

aO^>

Wall thickness^^

/ cmVg

/nm

/nm

/nm

/nm

951

0.65

2.12

3.35

3.87

1.75

892

0.57

2.32

3.35

3.87

1.75

95

0.06

2.12

3.27

3.93

1.81

/m% Parent MCM-41 Pt/MCM-41 (without gel filtration) Pt/MCM-41 (with gel filtration)

1) Determined by Dollimore-Heal (D-H) method. 2) Lattice parameterfi-omXRD data using the formula, ao=2d(100)/V3. 3) Pore wall thickness, ao - pore diameter. The BET specific surface area and pore volume of the Pt/MCM-41 prepared with the surfactant stabilized Pt nanoparticles after gel filtration was approximately ten times as larger as Pt/MCM-41 prepared with Pt nanoparticles without gel filtration. The rapid reduction also exhibited the possibility of the pore blocking of Pt/MCM-41 by Pt nanoparticles inside the mesopores of MCM-41.

CONCLUSIONS Incorporation of Pt nanoparticles into the mesopores of MCM-41 was tried by synthesis of MCM-41 in the presence of surfactant stabilized Pt nanoparticles. In the case of Pt nanoparticles without gel filtration, the incorporation of Pt nanoparticles into the mesopores could not be achieved because of coexistence of Pt-free

840 surfactants, which were mainly used for synthesis of MCM-41. The surfactant stabilized Pt nanoparticles adsorbed outside the mesopores of MCM-41 aggregated during calcination process, resulting in formation of Pt particles with bigger sizes. On the other hand, by using the surfactant stabilized Pt nanoparticles after gel filtration, the Pt nanoparticles-containing MCM-41 were successfully prepared. It was found that the direct synthesis method with gel filtration is very effective for preparation of metal nanoparticles-containing MCM-41.

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