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Ultrasound-Assisted Polyol Method for the Preparation of SBA-15-Supported Ruthenium Nanoparticles and the Study of Their Catalytic Activity on the Partial Oxidation of Methane Hongliang Li,† Renzhang Wang,† Qi Hong,‡ Luwei Chen,‡ Ziyi Zhong,‡ Yuri Koltypin,† J. Calderon-Moreno,§ and Aharon Gedanken*,† Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel, Institute of Chemical Engineering and Sciences, Ayer Rajah Crescent, Blk28, Unit #02-08, Singapore 139959, and Materials and Structures Laboratory, Tokyo Institute of Technology, 4259, Nagatsuta, Midori-Ku, 226-8503, Yokohama, Japan Received March 18, 2004. In Final Form: June 6, 2004 Metallic Ru nanoparticles have been successfully produced and incorporated into the pores of SBA-15 in situ employing a simple ultrasound-assisted polyol method. The product has been confirmed by X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy, where ultrasound provides both the energy for the reduction of the Ru(III) ion and the driving force for the loading of the Ru0 nanoparticles into the SBA-15 pores. An ultrasound-assisted insertion mechanism has been proposed based on the microjets and shake-wave effect of the collapsed bubbles. The catalytic properties of the SBA-15-supported Ru nanoparticles have been tested by the partial oxidization of methane and show very high activity and high CO selectivity.
Introduction Ruthenium has been known to exhibit a very unique and interesting activity as a catalyst. For example, alumina- or silica-supported ruthenium selectively reduces nitrogen oxide to nitrogen.1,2 Zeolite-supported ruthenium is an excellent catalyst for the water-gas-shift reaction,3,4 and it is also active in the hydrogenation of carbon monoxide.5,6 Recently, the Ru catalyst was found to have the highest catalytic activities for ammonia synthesis.7 The group III transition elements of noble metals (Ru, Rh, Ir, Pt, Pd) and non-noble metals (Co, Ni, Fe) have been reported as active catalysts for the partial oxidization of methane to get synthesis gas.8 Among these elements, Ru and Rh show relative high activity and a resistance to coke.9 However, Rh is much more expensive than Ru, and therefore, in most preparations Ru has been used. There have been a few literature reports on the partial oxidization of methane with Ru-related catalysts, such as Ln2RuO7,10 Ru/Al2O3,11 Ru/SiO2,12 and Ru/Y2O3.8 It has * Corresponding author. E-mail:
[email protected]. Fax: 972-3-5351250. Tel: 972-3-5318315. † Bar-Ilan University. ‡ Institute of Chemical Engineering and Sciences. § Tokyo Institute of Technology. (1) Shelef, M.; Gandhi, H. S. Ind. Eng. Chem. Prod. Res. Dev. 1972, 11, 393. (2) Clausen, C.; Good, M. L. J. Catal. 1977, 46, 58. (3) Verdonck, J. J.; Jacobs, P. A.; Uytterhoeven, J. B. J. Chem. Soc., Chem. Commun. 1979, 191. (4) Chen, Y. W.; Wang, W. Catal. Today 1989, 6, 105. (5) Nijs, H.; Jacobs, P. A.; Uytterhoeven, J. B. J. Chem. Soc., Chem. Commun. 1979, 180. (6) Leith, I. R. J. Catal. 1985, 91, 283. (7) Tennison, S. R. In Catalytic Ammonia Synthesis; Jennings, J. R., Ed.; Plenum: New York, 1991. (8) Nishimoto, H.; Nakagawa, K.; Ikenaga, N.; Suzuki, T. Catal. Lett. 2002, 82, 161. (9) Rostrup-Nielsen, J. R.; Bak-Hansen, J. H. J. Catal. 1993, 44, 38. (10) Ashcroft, A. T.; Cheethom, A. K.; Foord, J. S.; Green, M. L. H.; Grey, C. P.; Murrel, A. J.; Vernon, P. D. F. Nature 1990, 344, 319. (11) Poirier, M. G.; Trudel, J.; Guay, D. Catal. Lett. 1993, 21, 99. (12) Moffat, J. B.; Matsumura, Y. Catal. Lett. 1994, 24, 59.
been shown that CO and H2 can be obtained by oxidizing CH4 with O2 at elevated temperatures using these Rurelated catalysts. For most of the catalytic applications, it is necessary to prepare small ruthenium nanoparticles supported on a matrix, and several strategies have been explored for this purpose.13-15 For this reason, the incorporation of ruthenium into microporous or mesoporous materials such as zeolite, MCM-41, MCM-48, and SBA-15, employing traditional immersion and reducing processes, has been reported.16-18 Unlike the other noble metals, Ru(III) is more difficult to reduce under the same conditions as those applied to Pt, Pd, or Au.19 Therefore, the recently developed skillful preparation methods for Pt, Pd, and Au nanoparticles20,21 and their introduction into mesoporous silica-supported matrixes have not been directly extended to Ru.22-24 In several cases, the ruthenium precursors decompose, forming oxides or other complexes, making it difficult to understand the catalytic properties, which are wrongly assigned to metallic ruthenium. Indeed, the oxidation state of the metal and the (13) Tu, W. X.; Liu, H. F. J. Mater. Chem. 2000, 10, 2207. (14) Miyazaki, A.; Balint, I.; Aika, K.; Nakano, Y. J. Catal. 2001, 204, 364. (15) Ko’nya, Z.; Puntes, V.; Kiricsi, I.; Zhu, J.; Ager, J., III; Ko, M.; Frei, H.; Alivisatos, P.; Somorjai, G. Chem. Mater. 2003, 15, 1242. (16) Verdonck, J. J.; Jacobs, P. A.; Genet, M.; Poncelet, G. J. Chem. Soc., Faraday Trans. 1980, 76, 403. (17) Hartmann, M.; Bischof, C.; Luan, Z. H.; Kevan, L. Microporous Mesoporous Mater. 2001, 44-45, 385. (18) Schweyer, F.; Braunstein, P.; Estourne`s, C.; Guille, J.; Kessler, H.; Paillaudc, J.-L.; Rose´a, J. Chem Commun. 2000, 1271. (19) Hirai, H. J. Macromol. Sci., Chem. 1979, A13 (5), 633. (20) Wu, M. L.; Chen, D. H.; Huang, T. C. Langmuir 2001, 17, 3877. (21) Yonezawa, T.; Imamura, K.; Kimizuka, N. Langmuir 2001, 17, 4701. (22) Kim, S. W.; Kim, M.; Lee, W. Y.; Hyeon, T. J. Am. Chem. Soc. 2002, 124, 7642. (23) Zhang, L. X.; Shi, J. L.; Yu, I.; Hua, Z. L.; Zhao, X. G.; Ruan, M. L. Adv. Mater. 2002, 14, 1510. (24) Yang, C. M.; Liu, P. H.; Ho, Y. F.; Chiu, C. Y.; Chao, K. J. Chem. Mater. 2003, 15, 275.
10.1021/la049290d CCC: $27.50 © 2004 American Chemical Society Published on Web 08/04/2004
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extent of the reduction in some preparation methods are still controversial because of the low reactivity of Ru(III).17,25 The polyol reducing method has been utilized to prepare noble metals or semiconductor nanoparticles.26,27 Ru nanoparticles were dispersed onto the surface of Al2O3 particles by directly reducing a RuCl3‚H2O and Al2O3 mixture in ethylene glycol (EG) at 180 °C.14 Sonic energy has been routinely used in the field of material science for many years.28 Its chemical effects have recently come under investigation for the acceleration of chemical reactions and for the synthesis of new materials29 as well as for the generation of novel materials with unusual properties.30 Very recently, an ultrasound method has been reported for the preparation of SBA-15-supported Ru in a water solution, and an ultrasound-induced radical mechanism has been proposed for it.31 However, no direct evidence such as X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) has been provided to confirm the metallic state of the resulting product, and nanoparticles have not been observed in SBA-15 pores. Herein, we report for the first time on the combination of the polyol and the ultrasound methods for the preparation of SBA-15-supported Ru nanoparticles. In comparison with the previously reported impregnation method and the recently reported sonochemistry method, this method is simpler and results in a pure metallic state of ruthenium nanoparticles. Two different roles of sonic energy have been exploited in this preparation. The partial oxidization of methane by O2 has been carried out to test the catalytic activity of this matrix. Catalytic tests reveal that a high loading ratio sample of the SBA-15-supported Ru catalyst shows high conversion and high CO selectivity for the partial oxidization reaction.
Langmuir, Vol. 20, No. 19, 2004 8353 A black solid, composed of about 14% (wt) Ru (energy-dispersive analytical X-ray (EDAX) measurement), was obtained. Control Experiments. To understand the preparation mechanism, three control experiments were conducted: (a) heating the same mixture of RuCl3‚H2O and SBA-15 to 145 °C for 4 h; (b) sonicating RuCl3‚xH2O in ethylene glycol without SBA-15; (c) sonicating RuCl3‚xH2O in water instead of ethylene glycol. The solids resulting from control reactions a, b, and c were separated, washed, and dried by the same procedure as described under sample preparation above. Study of Catalytic Properties. The partial oxidation reactions were carried out in a fixed-type quartz reactor at atmospheric pressure using 30 mg of catalyst. The gases mixed by CH4 and O2 were introduced into the reactor with a flow speed of 80 and 40 mL/min, respectively, at a temperature range of 600-750 °C. Before the reaction, the catalysts were activated under a H2 atmosphere at 600 °C. The products were analyzed by an online high-speed gas chromatograph with a TCD detector. Characterization. The solids obtained in the above experiments have been characterized by XRD, transmission electron microscopy (TEM), EDAX, XPS, and nitrogen adsorption measurements. Wide-angle XRD patterns were recorded on a Rigaku X-ray diffractometer (model 2028, Co KR λ ) 1.788 92 Å). The low-angle XRD patterns were obtained using a Bruker D8 Advance X-ray diffractometer (Cu KR ) 1.54178 Å). Morphology and structure investigations were performed with an Hitachi H-9000 transmission electron microscope. The nitrogen adsorption-desorption isotherms at 77 K were measured using a Micromeritics (Gemini 2375) after the samples were dried at 110 °C for 1 h. Brunauer-Emmett-Teller (BET) surface areas were calculated from the linear part of the nitrogen adsorptiondesorption plot. Pore-size distributions were calculated using the Barret-Joyner-Halenda model. EDAX was detected on a JEOL-JSM-840 scanning microscope. The XPS data were accumulated on an AXIS HS (Kratos Analytical) electron spectrometer system with a monochromatized Al KR standard X-ray source. The binding energies were calibrated by referencing the C1s to 285.0 eV.
Experimental Section
Results and Discussion
Sample Preparation. All the chemicals were purchased from Aldrich and were used without further purification. Ultrasonic irradiation was achieved with a high-intensity ultrasonic probe (Misonix; XL sonifier, 1.13 cm diameter Ti horn, 20 kHz, 60 W cm-2 measured calorimetrically). In a typical preparation, 0.10 g of SBA-15, which was synthesized according to a welldocumented procedure,32 was dispersed into 50 mL of EG, and then 0.030 g of RuCl3‚xH2O (x < 2, FW ≈ 230) was dissolved in the suspension, resulting in a purple solution. The mixture was maintained for about half an hour in order to immerse the solution into the mesopores. After flowing Ar for 20 min to push the air out of the flask, the mixture was irradiated by intense ultrasound for 4 h. During the irradiation, the flask was wrapped with a layer of soft paper in order to fully utilize the thermal energy induced by the ultrasound irradiation, and a final temperature of about 140 °C for the solution was detected. When the solution was cooled to room temperature, 5 mL of a 0.5 M HCl aqueous solution was added, and then the mixture was allowed to stand for several hours, after which the solid was separated from the solution by centrifugation. The solid was washed in water three times and then dried at room temperature in a vacuum for 1 day.
Sample Preparation and Control Experiments. SBA-15 was prepared according to a documented procedure and has been characterized by XRD and TEM. The results will be compared in the following section with those of the Ru-loaded SBA-15 sample. Based on our experience, three key points were applied for the sample preparation. The first one is the reaction temperature; our studies show that maintaining a high temperature is essential for the reduction of RuCl3‚xH2O. Thus, during the irradiation, the flask was wrapped with a type of insulated material to utilize the thermal energy derived from the ultrasound irradiation. The second point is the addition of a diluted acid, which is essential for the separation of the product from the solution. After the irradiation, a black suspension was obtained. However, only a small amount of solid was precipitated even after a long time and use of high-speed centrifugation, and the yields of the products were very low. The mother solution still remained black. After the addition of diluted acid for a few hours, the black product precipitates completely and a transparent mother solution was obtained after the centrifugation. This phenomena can be explained as the adsorption of the broken ethylene glycol fragments and its ramifications, derived from the ultrasound irradiation and the reduction of RuCl3‚xH2O. We speculated that the function of acid here is to remove the adsorbed species on the surface of SBA-15 and then disenable the stability of the particles’ suspension. The third conclusion is the role of ethylene glycol. The control experiment shows that pure water cannot reduce RuCl3‚xH2O to Ru with the assistance of ultrasound or heat. This confirms the function of ethylene glycol as the reducing agent in this reaction,
(25) Lei, G. D.; Kevan, L. J. Phys. Chem. 1992, 96, 350. (26) Palchik, O.; Kerner, R.; Gedanken, A.; Weiss, A. M.; Slifkin, M. A.; Palchik, V. J. Mater. Chem. 2001, 11, 874. (27) Kurihara, L. K.; Chow, G. M.; Schoen, P. E. Nanostruct. Mater. 1995, 5, 607. (28) Suslick, K. S.; Choe, S. B.; Cichowlas, A. A.; Grinstaff, M. W. Nature 1991, 353, 414. (29) Li, H. L.; Zhu, Y. C.; Palchik, O.; Gedanken, A.; Palchik, V.; Slifkim, M.; Weiss, A. Inorg. Chem. 2002, 41, 637. (30) Nikitenko, S. I.; Koltypin, Y.; Palchik, O.; Felner, I.; Xu, X. N.; Gedanken, A. Angew. Chem., Int. Ed. 2001, 40, 23. (31) Zhu, S. M.; Zhou, H. S.; Hibion, M.; Honma, I. J. Mater. Chem. 2003, 13, 1115. (32) Zhao, D.; Sun, J.; Li, Q.; Stucky, G. D. Chem. Mater. 2000, 12, 275.
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Figure 2. TEM images of the pure SBA-15 (A) and as-prepared Ru/SBA-15 (B).
Figure 1. Low-angle XRD patterns of pure SBA-15 and Ru/ SBA-15 (panel a) and the wide-angle XRD pattern of the asprepared Ru/SBA-15 (panel b).
which has already been proved. However, it will be useful for us to understand the insertion mechanism later on. XRD Study. The low-angle XRD pattern of the asprepared Ru/SBA-15 sample resulting from the ultrasound-assisted preparation has been compared with that of the pure SBA-15 precursor. Figure 1a shows their lowangle XRD patterns in a 2θ range of 0.7-6°. From the comparison, we can see that the low-angle peaks of the Ru-incorporated SBA-15 are almost the same as those of its precursor. The intense diffraction peak around 2θ ∼ 0.86° in Figure 1a corresponds to the (100) reflection of the SBA-15 structure.32 The weak peaks at 2θ ∼ 1.48 and 1.72° can be indexed to (110) and (200) peaks of SBA-15. The comparison of the low-angle XRD patterns demonstrates that the periodic structure of the SBA-15 substrate was maintained after the sonication and the insertion of Ru nanoparticles. Wide-angle XRD has been measured for the Ru-incorporated SBA-15 (Figure 1b). The broad peaks at around 2θ ∼ 16° and 2θ ∼ 26.5° are typical peaks of the amorphous silica composition of the SBA-15, which also can be detected in the pure SBA-15 sample. Another two broad peaks at around 2θ ) 45 and 51° are the (100) and (101) peaks of the hexagonal phase of metallic Ru (JCPDS file no. 06-663). The broad peaks indicate the nanosized nature of the Ru particles. The XRD results imply that the mesoporous structure of SBA-15 was maintained after the ultrasound irradiation, and Ru metal was formed as a result of the ultrasound-assisted reaction. This conclusion has been clarified in the TEM measurement section. TEM Measurements. TEM measurements were carried out to study the morphologies of the SBA-15 precursor and the Ru/SBA-15 samples. Panels a and b of Figure 2 show typical images of a SBA-15 precursor and the asprepared Ru/SBA-15. Panel a clearly shows the periodic structure of the SBA-15 precursor and the pore size
Figure 3. Nitrogen adsorption-desorption isotherm plots before (A) and after (B) the insertion.
between 4 and 6 nm. After the ultrasound irradiation, small particles ranging from 2 to 6 nm, which are incorporated into the SBA-15 pores, are observed (panel b). The sizes of the nanoparticles inside the pores are close to the pore diameters of SBA-15. There are also a few large particles of 10-20 nm anchored on the external surface of SBA-15. We can also clearly see that the periodic structure of the SBA-15 remained after the ultrasoundassisted insertion reaction. This supports the XRD conclusion that the SBA-15 structure remains unchanged after the insertion. In the mechanism section, the TEM results will be discussed in detail and the advantage of this preparation method will be compared with the control experiments. Surface Area. The surface area of the Ru-modified SBA-15 has been compared with that of pure SBA-15. Figure 3 shows their isotherm plots, and we can see that the surface area of the Ru-modified SBA-15 was reduced from ∼550 to ∼350 m2/g. This can be explained as being due to the incorporation of Ru nanoparticles into the SBA15 mesopores. EDAX and XPS Analysis. The presence of elemental Ru and its loading ratio were studied by means of EDAX and XPS. The metallic state of ruthenium has also been demonstrated by XPS measurements. Both the EDAX and the XPS measurements show the presence of Ru. They also reveal that the loaded Ru depends on the RuCl3 concentration. The loading varied from several percentages to more than 15% (wt), by merely changing the concentration of RuCl3 in the original solution. A high-resolution XPS curve in the Ru3d region of 274296 eV was recorded, which shows an intense peak at a binding energy of 280 eV (Figure 4). This peak is characteristic of the 3d5/2 transition of metallic Ru.33,34 (33) XI-specMaster System; XPS International: Mountain View, CA, 1998 (Database).
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Figure 4. High-resolution XPS survey in the Ru3d region for as-prepared Ru/SBA-15. Figure 6. TEM image of the sample obtained by ultrasound irradiation of RuCl3 in ethylene glycol.
Figure 5. TEM image of the sample derived from heating of the RuCl3 and SBA-15 mixture.
XPS results indicate that the solid obtained in our experiment is pure Ru0 and not its oxide or a complex. XPS is known as a sensitive method for characterizing the surface layer, and it is capable of identifying the metallic or the oxide states of metals. For ruthenium, pure metal shows the d5/2 peak at 280.0 eV and the d3/2 peak around 284.0 eV, while the Ru d5/2 peak of Ru oxide undergoes a high-energy shift and appears at about 281 eV. If the sonication product is a mixture of metallic Ru and its oxides, then a broad peak at around 280-281 eV is expected. The XPS spectrum of the sonication product reveals a narrow peak centered at 280 eV. This result supports the XRD data demonstrating that the sonication yields metallic Ru. The Ru d3/2 peak is overlapped by the C1s peak around 285 eV,34 and that is the reason for the intense peak detected at this energy. The Role of Ultrasound Irradiation. To understand the mechanism and verify the role of ultrasound in the loading of Ru nanoparticles into the SBA-15 pores, three control experiments have been carried out, as described in the Experimental Section. The samples obtained in the control experiments a and b also show hexagonalstructured Ru metal patterns, meaning that RuCl3‚xH2O can be reduced by EG with heat or with irradiation of ultrasound. However, when Ru/SBA-15 was prepared only by heat (control experiment a), no obvious loading of Ru nanoparticles inside the pore of SBA-15 was observed, and big clusters were only infrequently anchored on the external surface (Figure 5). Furthermore, separate Ru aggregates could be found in the TEM images. The (34) Elmasides, C.; Kondarides, D. I.; Grunert, W.; Verykios, X. E. J. Phys. Chem. B 1999, 103, 5227.
different results obtained in these experiments indicate that ultrasound plays a critical role in the incorporation of Ru nanoparticles into the pores. The particles obtained from control experiment b had sizes in the range of 20-40 nm. These particles were agglomerated into large aggregates (Figure 6). The individual particles of their aggregates are much bigger than those observed in panel b of Figure 2. Based on the results of these control experiments, the role of ultrasound is assigned to the extreme conditions caused by the collapse of the bubble.35 First, ultrasound provides the energy for the reduction of Ru(III) to Ru0 by ethylene glycol. Second, it also offers the force for the insertion of the formed Ru nanoparticles into the SBA-15 pores in situ. Sonochemical reactions arise from acoustic cavitation phenomena: the formation, growth, and collapse of the bubbles in a liquid medium. The extremely high temperature (>5000 K), pressure (>20 Mpa), and cooling rates (>1010 K/s) attained during acoustic cavitation lead to many unique properties in the irradiated solution. Also, microjets and shock waves36 are being created near solid surfaces after the bubble collapses. We presume that the speed at which the Ru nanoparticles are thrown to the surface of SBA-15 causes the insertion of the nanoparticles into the pores, after which the growth of the small particles is restricted due to the confinement effect of the channels. At the same time, the small particles also have the chance to collide with the external surface of SBA-15 and then anchor onto it. The particles anchored on the external surface still can combine with other small particles in the solution and form a big cluster. This will explain the presence of the few large clusters on the external surface of SBA-15. When SBA-15 was absent during the ultrasound irradiation, the small particles collided with each other to form large clusters (results of control experiment b).36 When heat was applied instead of ultrasound, RuCl3‚H2O was also reduced by ethylene glycol. However, nanoparticles were not inserted into the mesopores because there was not sufficient force to push them into the pores of SBA-15, and the small particles only aggregated into a large agglomeration. Catalytic Properties. The catalytic properties of the Ru/SBA-15 complexes with different Ru loading ratios have been studied based on the partial oxidization of methane by oxygen. Table 1 shows CH4 conversion and (35) Ultrasound: Its Chemical, Physical and Biological Effect; Suslick, K. S., Ed.; VCH: Weinheim, 1988. (36) Doktycz, J. S.; Suslick, S. K. Science 1990, 247, 1067.
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Table 1. CH4 Conversion and CO Selectivity of the Ru Loaded SBA-15 Catalysts under Different Ru Loading Ratios (Reaction Condition: CH4/O2 ) 2:1) Ru loading ratio (wt %) 3.5
7
14
temp (°C)
CH4 conversion (%)
CO selectivity (%)
600 650 700 750 600 650 700 750 600 650 700 750
35.2 35.3 35.4 36.0 38.7 39.3 40.0 40.5 59.7 61.8 64.4 65.4
65.2 66.8 68.7 69.2 71.1 73.1 73.9 76.0 71.5 76 80.2 83.5
the CO selectivity of these complexes with different Ru loading ratios at different temperatures. From the table, we can see that both the CH4 conversions and the CO selectivity increase with the increase of the reaction temperatures and the loading ratios of Ru. The high Ru loading ratio (∼14 wt %) complex shows a more than 65% CH4 conversion and a more than 83% CO selectivity at 750 °C. The catalytic activity of 14 wt % Ru/SBA-15 keeps stable at 750 °C during 50 h of reaction. Carbon deposition measured by thermogravimetric analysis (TGA) is only 0.34% after reaction. From the catalytic results, we can also see that the conversion to products is similar for the 3.5 and 7 wt % loading, but the conversion increased when the loading increased to ∼14 wt %. We propose that perhaps at the low loadings, some Ru was first deposited on the inner surface area of the deep pores of the SBA-15. Probably, the deep pores are not easily accessible for the catalytic reaction. Once more Ru is added, it begins to deposit on the shallow surface area of the SBA-15 pores as well as on the outer surface area. This explains why the catalytic activity increases at a higher loading of Ru (Ru loading is higher than 7 wt %). The main product of this reaction is CO and H2, which is called syngas. The byproducts are CO2, H2O, and maybe
C2H4 and C2H6. The latter two gases were not detected in our experiments. More studies on the catalytic properties of this new complex are underway. Conclusions In short, we have successfully reduced RuCl3‚xH2O to Ru metal and incorporated the Ru metal nanoparticles into the pores of SBA-15 by a simple, one-pot ultrasoundassisted polyol method. Low-angle XRD and TEM show that the periodical structure of the SBA-15 remained after ultrasound irradiation. XRD and XPS results demonstrate clearly the metallic state of the Ru nanoparticles and indicate that this new method overcame the partial reduction of Ru(III) and unequivocally resulted in metallic state ruthenium nanoparticles. In this approach, the ultrasound provided the reaction not only with the thermal energy for the reduction of Ru(III) to Ru0 but also with a force for the loading of the forming Ru0 nanoparticles into SBA-15 pores. The roles of ultrasound and the reaction mechanism are different from the previously reported aqueous sonochemical method, in which an ultrasoundinduced radical mechanism has been proposed.26 The SBA15-supported complex with a Ru loading ratio of about 14% shows a high catalytic activity to the partial oxidization of methane by oxygen. Further studies of this method will be useful for the preparation of noble metal nanoparticles supported by mesoporous silica or other mesoporous materials. Acknowledgment. H. L. Li thanks the Bar-Ilan Research Authority for a postdoctoral fellowship. A. Gedanken thanks the German Ministry of Science for the support of this work through the Deutsche-Israeli program (DIP). The authors are grateful to Professor M. V. Landau, Blechner Center for Industrial Catalysis and Process Development, Chemical Engineering Department, Ben-Gurion University of the Negev, for the providing us with the SBA-15. LA049290D
