Ultrasound-Assisted Polyol Method for the Preparation ofSBA-15-Supported Ruthenium Nanoparticles and the Study

of Their Catalytic Activity on the Partial Oxidation ofMethane

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 ChemicalEngineering 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-15in situ employing a simple ultrasound-assisted polyol method. The product has been confirmed by X-raydiffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy, where ultrasoundprovides both the energy for the reduction of the Ru(III) ion and the driving force for the loading of theRu0 nanoparticles into the SBA-15 pores. An ultrasound-assisted insertion mechanism has been proposedbased on the microjets and shake-wave effect of the collapsed bubbles. The catalytic properties of theSBA-15-supported Ru nanoparticles have been tested by the partial oxidization of methane and show veryhigh activity and high CO selectivity.

Introduction

Ruthenium has been known to exhibit a very uniqueand interesting activity as a catalyst. For example,alumina-orsilica-supportedrutheniumselectively reducesnitrogen oxide to nitrogen.1,2 Zeolite-supported rutheniumis an excellent catalyst for the water-gas-shift reaction,3,4

and it is also active in the hydrogenation of carbonmonoxide.5,6 Recently, the Ru catalyst was found to havethe highest catalytic activities for ammonia synthesis.7The group III transition elements of noble metals (Ru,Rh, Ir, Pt, Pd) and non-noble metals (Co, Ni, Fe) havebeen reported as active catalysts for the partial oxidizationof methane to get synthesis gas.8 Among these elements,Ru and Rh show relative high activity and a resistanceto 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 partialoxidization of methane with Ru-related catalysts, such asLn2RuO7,10 Ru/Al2O3,11 Ru/SiO2,12 and Ru/Y2O3.8 It has

been shown that CO and H2 can be obtained by oxidizingCH4 with O2 at elevated temperatures using these Ru-related catalysts. For most of the catalytic applications,it is necessary to prepare small ruthenium nanoparticlessupported on a matrix, and several strategies have beenexplored for this purpose.13-15 For this reason, theincorporation of ruthenium into microporous or mesopo-rous materials such as zeolite, MCM-41, MCM-48, andSBA-15, employing traditional immersion and reducingprocesses, has been reported.16-18 Unlike the other noblemetals, Ru(III) is more difficult to reduce under the sameconditions as those applied to Pt, Pd, or Au.19 Therefore,the recently developed skillful preparation methods forPt, Pd, and Au nanoparticles20,21 and their introductioninto mesoporous silica-supported matrixes have not beendirectly extended to Ru.22-24 In several cases, the ruthe-nium precursors decompose, forming oxides or othercomplexes, making it difficult to understand the catalyticproperties, which are wrongly assigned to metallic ru-thenium. Indeed, the oxidation state of the metal and the

* Corresponding author. E-mail: gedanken@mail.biu.ac.il.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.

(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.; Estournes, C.; Guille, J.; Kessler,

H.; Paillaudc, J.-L.; Rosea, 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.

8352 Langmuir 2004, 20, 8352-8356

10.1021/la049290d CCC: $27.50 © 2004 American Chemical SocietyPublished on Web 08/04/2004

extent of the reduction in some preparation methods arestill controversial because of the low reactivity of Ru-(III).17,25

The polyol reducing method has been utilized to preparenoble metals or semiconductor nanoparticles.26,27 Runanoparticles were dispersed onto the surface of Al2O3particles by directly reducing a RuCl3‚H2O and Al2O3mixture in ethylene glycol (EG) at 180 °C.14 Sonic energyhas been routinely used in the field of material science formany years.28 Its chemical effects have recently comeunder investigation for the acceleration of chemicalreactions and for the synthesis of new materials29 as wellas for the generation of novel materials with unusualproperties.30 Very recently, an ultrasound method has beenreported for the preparation of SBA-15-supported Ru ina water solution, and an ultrasound-induced radicalmechanism has been proposed for it.31 However, no directevidence such as X-ray diffraction (XRD) and X-rayphotoelectron spectroscopy (XPS) has been provided toconfirm the metallic state of the resulting product, andnanoparticles have not been observed in SBA-15 pores.Herein, we report for the first time on the combination ofthe polyol and the ultrasound methods for the preparationofSBA-15-supportedRunanoparticles. Incomparisonwiththe previously reported impregnation method and therecently reported sonochemistry method, this method issimpler and results in a pure metallic state of rutheniumnanoparticles. Two different roles of sonic energy havebeen exploited in this preparation. The partial oxidizationof methane by O2 has been carried out to test the catalyticactivity of this matrix. Catalytic tests reveal that a highloading ratio sample of the SBA-15-supported Ru catalystshows high conversion and high CO selectivity for thepartial oxidization reaction.

Experimental Section

Sample Preparation. All the chemicals were purchased fromAldrich and were used without further purification. Ultrasonicirradiation was achieved with a high-intensity ultrasonic probe(Misonix; XL sonifier, 1.13 cm diameter Ti horn, 20 kHz, 60 Wcm-2 measured calorimetrically). In a typical preparation, 0.10g of SBA-15, which was synthesized according to a well-documented procedure,32 was dispersed into 50 mL of EG, andthen 0.030 g of RuCl3‚xH2O (x < 2, FW ≈ 230) was dissolved inthe suspension, resulting in a purple solution. The mixture wasmaintained for about half an hour in order to immerse the solutioninto the mesopores. After flowing Ar for 20 min to push the airout of the flask, the mixture was irradiated by intense ultrasoundfor 4 h. During the irradiation, the flask was wrapped with alayer of soft paper in order to fully utilize the thermal energyinduced by the ultrasound irradiation, and a final temperatureof about 140 °C for the solution was detected. When the solutionwas cooled to room temperature, 5 mL of a 0.5 M HCl aqueoussolution was added, and then the mixture was allowed to standfor several hours, after which the solid was separated from thesolution by centrifugation. The solid was washed in water threetimes and then dried at room temperature in a vacuum for 1 day.

A black solid, composed of about 14% (wt) Ru (energy-dispersiveanalytical X-ray (EDAX) measurement), was obtained.

Control Experiments. To understand the preparationmechanism, three control experiments were conducted: (a)heating the same mixture of RuCl3‚H2O and SBA-15 to 145 °Cfor 4 h; (b) sonicating RuCl3‚xH2O in ethylene glycol withoutSBA-15; (c) sonicating RuCl3‚xH2O in water instead of ethyleneglycol. The solids resulting from control reactions a, b, and cwere separated, washed, and dried by the same procedure asdescribed under sample preparation above.

Study of Catalytic Properties. The partial oxidationreactions were carried out in a fixed-type quartz reactor atatmospheric pressure using 30 mg of catalyst. The gases mixedby CH4 and O2 were introduced into the reactor with a flow speedof 80 and 40 mL/min, respectively, at a temperature range of600-750 °C. Before the reaction, the catalysts were activatedunder a H2 atmosphere at 600 °C. The products were analyzedby an online high-speed gas chromatograph with a TCD detector.

Characterization. The solids obtained in the above experi-ments have been characterized by XRD, transmission electronmicroscopy (TEM), EDAX, XPS, and nitrogen adsorption mea-surements. Wide-angle XRD patterns were recorded on a RigakuX-ray diffractometer (model 2028, Co KR λ ) 1.788 92 Å). Thelow-angle XRD patterns were obtained using a Bruker D8Advance X-ray diffractometer (Cu KR ) 1.54178 Å). Morphologyand structure investigations were performed with an HitachiH-9000 transmission electron microscope. The nitrogen adsorp-tion-desorption isotherms at 77 K were measured using aMicromeritics (Gemini 2375) after the samples were dried at110 °C for 1 h. Brunauer-Emmett-Teller (BET) surface areaswere calculated from the linear part of the nitrogen adsorption-desorption plot. Pore-size distributions were calculated usingthe Barret-Joyner-Halenda model. EDAX was detected on aJEOL-JSM-840 scanning microscope. The XPS data were ac-cumulated on an AXIS HS (Kratos Analytical) electron spec-trometer system with a monochromatized Al KR standard X-raysource. The binding energies were calibrated by referencing theC1s to 285.0 eV.

Results and Discussion

Sample Preparation and Control Experiments.SBA-15 was prepared according to a documented proce-dure and has been characterized by XRD and TEM. Theresults will be compared in the following section with thoseof the Ru-loaded SBA-15 sample.

Based on our experience, three key points were appliedfor the sample preparation. The first one is the reactiontemperature; our studies show that maintaining a hightemperature is essential for the reduction of RuCl3‚xH2O.Thus, during the irradiation, the flask was wrapped witha type of insulated material to utilize the thermal energyderived from the ultrasound irradiation. The second pointis the addition of a diluted acid, which is essential for theseparation of the product from the solution. After theirradiation, a black suspension was obtained. However,only a small amount of solid was precipitated even aftera long time and use of high-speed centrifugation, and theyields of the products were very low. The mother solutionstill remained black. After the addition of diluted acid fora few hours, the black product precipitates completelyand a transparent mother solution was obtained after thecentrifugation. This phenomena can be explained as theadsorption of the broken ethylene glycol fragments andits ramifications, derived from the ultrasound irradiationand the reduction of RuCl3‚xH2O. We speculated that thefunction of acid here is to remove the adsorbed species onthe surface of SBA-15 and then disenable the stability ofthe particles’ suspension. The third conclusion is the roleof ethylene glycol. The control experiment shows that purewater cannot reduce RuCl3‚xH2O to Ru with the assistanceof ultrasound or heat. This confirms the function ofethylene 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.

Ultrasound-Assisted Preparation of Nanoparticles Langmuir, Vol. 20, No. 19, 2004 8353

which has already been proved. However, it will be usefulfor us to understand the insertion mechanism later on.

XRD Study. The low-angle XRD pattern of the as-prepared Ru/SBA-15 sample resulting from the ultra-sound-assisted preparation has been compared with thatof the pure SBA-15 precursor. Figure 1a shows their low-angle XRD patterns in a 2θ range of 0.7-6°. From thecomparison, we can see that the low-angle peaks of theRu-incorporated SBA-15 are almost the same as those ofits precursor. The intense diffraction peak around 2θ ∼0.86° in Figure 1a corresponds to the (100) reflection ofthe SBA-15 structure.32 The weak peaks at 2θ ∼ 1.48 and1.72° can be indexed to (110) and (200) peaks of SBA-15.The comparison of the low-angle XRD patterns demon-strates that the periodic structure of the SBA-15 substratewas maintained after the sonication and the insertion ofRu nanoparticles. Wide-angle XRD has been measuredfor the Ru-incorporated SBA-15 (Figure 1b). The broadpeaks at around 2θ ∼ 16° and 2θ ∼ 26.5° are typical peaksof the amorphous silica composition of the SBA-15, whichalso can be detected in the pure SBA-15 sample. Anothertwo 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 thenanosized nature of the Ru particles.

The XRD results imply that the mesoporous structureof SBA-15 was maintained after the ultrasound irradia-tion, and Ru metal was formed as a result of theultrasound-assisted reaction. This conclusion has beenclarified in the TEM measurement section.

TEM Measurements. TEM measurements were car-ried out to study the morphologies of the SBA-15 precursorand the Ru/SBA-15 samples. Panels a and b of Figure 2show typical images of a SBA-15 precursor and the as-prepared Ru/SBA-15. Panel a clearly shows the periodicstructure of the SBA-15 precursor and the pore size

between 4 and 6 nm. After the ultrasound irradiation,small particles ranging from 2 to 6 nm, which areincorporated into the SBA-15 pores, are observed (panelb). The sizes of the nanoparticles inside the pores are closeto the pore diameters of SBA-15. There are also a fewlarge particles of 10-20 nm anchored on the externalsurface of SBA-15. We can also clearly see that the periodicstructure of the SBA-15 remained after the ultrasound-assisted insertion reaction. This supports the XRDconclusion that the SBA-15 structure remains unchangedafter the insertion. In the mechanism section, the TEMresults will be discussed in detail and the advantage ofthis preparation method will be compared with the controlexperiments.

Surface Area. The surface area of the Ru-modifiedSBA-15 has been compared with that of pure SBA-15.Figure 3 shows their isotherm plots, and we can see thatthe surface area of the Ru-modified SBA-15 was reducedfrom ∼550 to ∼350 m2/g. This can be explained as beingdue to the incorporation of Ru nanoparticles into the SBA-15 mesopores.

EDAX and XPS Analysis. The presence of elementalRu and its loading ratio were studied by means of EDAXand XPS. The metallic state of ruthenium has also beendemonstrated by XPS measurements. Both the EDAX andthe XPS measurements show the presence of Ru. Theyalso reveal that the loaded Ru depends on the RuCl3concentration. The loading varied from several percent-ages to more than 15% (wt), by merely changing theconcentration of RuCl3 in the original solution.

A high-resolution XPS curve in the Ru3d region of 274-296 eV was recorded, which shows an intense peak at abinding energy of 280 eV (Figure 4). This peak ischaracteristic of the 3d5/2 transition of metallic Ru.33,34

(33) XI-specMaster System; XPS International: Mountain View, CA,1998 (Database).

Figure 1. Low-angle XRD patterns of pure SBA-15 and Ru/SBA-15 (panel a) and the wide-angle XRD pattern of the as-prepared Ru/SBA-15 (panel b).

Figure 2. TEM images of the pure SBA-15 (A) and as-preparedRu/SBA-15 (B).

Figure 3. Nitrogen adsorption-desorption isotherm plotsbefore (A) and after (B) the insertion.

8354 Langmuir, Vol. 20, No. 19, 2004 Li et al.

XPS results indicate that the solid obtained in ourexperiment is pure Ru0 and not its oxide or a complex.XPS is known as a sensitive method for characterizingthe surface layer, and it is capable of identifying themetallic or the oxide states of metals. For ruthenium, puremetal shows the d5/2 peak at 280.0 eV and the d3/2 peakaround 284.0 eV, while the Ru d5/2 peak of Ru oxideundergoes a high-energy shift and appears at about 281eV. If the sonication product is a mixture of metallic Ruand its oxides, then a broad peak at around 280-281 eVis expected. The XPS spectrum of the sonication productreveals a narrow peak centered at 280 eV. This resultsupports the XRD data demonstrating that the sonicationyields metallic Ru. The Ru d3/2 peak is overlapped by theC1s peak around 285 eV,34 and that is the reason for theintense peak detected at this energy.

The Role of Ultrasound Irradiation. To understandthe mechanism and verify the role of ultrasound in theloading of Ru nanoparticles into the SBA-15 pores, threecontrol experiments have been carried out, as describedin the Experimental Section. The samples obtained in thecontrol experiments a and b also show hexagonal-structured Ru metal patterns, meaning that RuCl3‚xH2Ocan be reduced by EG with heat or with irradiation ofultrasound. However, when Ru/SBA-15 was prepared onlyby heat (control experiment a), no obvious loading of Runanoparticles inside the pore of SBA-15 was observed,and big clusters were only infrequently anchored on theexternal surface (Figure 5). Furthermore, separate Ruaggregates could be found in the TEM images. The

different results obtained in these experiments indicatethat ultrasound plays a critical role in the incorporationof Ru nanoparticles into the pores. The particles obtainedfrom control experiment b had sizes in the range of 20-40nm. These particles were agglomerated into large ag-gregates (Figure 6). The individual particles of theiraggregates are much bigger than those observed in panelb of Figure 2. Based on the results of these controlexperiments, the role of ultrasound is assigned to theextreme conditions caused by the collapse of the bubble.35

First, ultrasound provides the energy for the reduction ofRu(III) to Ru0 by ethylene glycol. Second, it also offers theforce for the insertion of the formed Ru nanoparticles intothe SBA-15 pores in situ. Sonochemical reactions arisefrom acoustic cavitation phenomena: the formation,growth, and collapse of the bubbles in a liquid medium.The extremely high temperature (>5000 K), pressure (>20Mpa), and cooling rates (>1010 K/s) attained duringacoustic cavitation lead to many unique properties in theirradiated solution. Also, microjets and shock waves36 arebeing created near solid surfaces after the bubble collapses.We presume that the speed at which the Ru nanoparticlesare thrown to the surface of SBA-15 causes the insertionof the nanoparticles into the pores, after which the growthof the small particles is restricted due to the confinementeffect of the channels. At the same time, the small particlesalso have the chance to collide with the external surfaceof SBA-15 and then anchor onto it. The particles anchoredon the external surface still can combine with other smallparticles in the solution and form a big cluster. This willexplain the presence of the few large clusters on theexternal surface of SBA-15. When SBA-15 was absentduring the ultrasound irradiation, the small particlescollided with each other to form large clusters (results ofcontrol experiment b).36 When heat was applied insteadof ultrasound, RuCl3‚H2O was also reduced by ethyleneglycol. However, nanoparticles were not inserted into themesopores because there was not sufficient force to pushthem into the pores of SBA-15, and the small particlesonly aggregated into a large agglomeration.

Catalytic Properties. The catalytic properties of theRu/SBA-15 complexes with different Ru loading ratioshave been studied based on the partial oxidization ofmethane by oxygen. Table 1 shows CH4 conversion and

(34) Elmasides, C.; Kondarides, D. I.; Grunert, W.; Verykios, X. E.J. Phys. Chem. B 1999, 103, 5227.

(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.

Figure 4. High-resolution XPS survey in the Ru3d region foras-prepared Ru/SBA-15.

Figure 5. TEM image of the sample derived from heating ofthe RuCl3 and SBA-15 mixture.

Figure 6. TEM image of the sample obtained by ultrasoundirradiation of RuCl3 in ethylene glycol.

Ultrasound-Assisted Preparation of Nanoparticles Langmuir, Vol. 20, No. 19, 2004 8355

the CO selectivity of these complexes with different Ruloading ratios at different temperatures. From the table,we can see that both the CH4 conversions and the COselectivity increase with the increase of the reactiontemperatures and the loading ratios of Ru. The high Ruloading ratio (∼14 wt %) complex shows a more than 65%CH4 conversion and a more than 83% CO selectivity at750 °C. The catalytic activity of 14 wt % Ru/SBA-15 keepsstable at 750 °C during 50 h of reaction. Carbon depositionmeasured by thermogravimetric analysis (TGA) is only0.34% after reaction. From the catalytic results, we canalso see that the conversion to products is similar for the3.5 and 7 wt % loading, but the conversion increased whenthe loading increased to ∼14 wt %. We propose thatperhaps at the low loadings, some Ru was first depositedon the inner surface area of the deep pores of the SBA-15.Probably, the deep pores are not easily accessible for thecatalytic reaction. Once more Ru is added, it begins todeposit on the shallow surface area of the SBA-15 poresas well as on the outer surface area. This explains whythe 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, whichis called syngas. The byproducts are CO2, H2O, and maybe

C2H4 and C2H6. The latter two gases were not detected inour experiments. More studies on the catalytic propertiesof this new complex are underway.

Conclusions

In short, we have successfully reduced RuCl3‚xH2O toRu metal and incorporated the Ru metal nanoparticlesinto the pores of SBA-15 by a simple, one-pot ultrasound-assisted polyol method. Low-angle XRD and TEM showthat the periodical structure of the SBA-15 remained afterultrasound irradiation. XRD and XPS results demonstrateclearly the metallic state of the Ru nanoparticles andindicate that this new method overcame the partialreduction of Ru(III) and unequivocally resulted in metallicstate ruthenium nanoparticles. In this approach, theultrasound provided the reaction not only with the thermalenergy for the reduction of Ru(III) to Ru0 but also with aforce for the loading of the forming Ru0 nanoparticles intoSBA-15 pores. The roles of ultrasound and the reactionmechanism are different from the previously reportedaqueous sonochemical method, in which an ultrasound-induced radical mechanism has been proposed.26 The SBA-15-supported complex with a Ru loading ratio of about14% shows a high catalytic activity to the partial oxi-dization of methane by oxygen. Further studies of thismethod will be useful for the preparation of noble metalnanoparticles supported by mesoporous silica or othermesoporous materials.

Acknowledgment. H. L. Li thanks the Bar-IlanResearch Authority for a postdoctoral fellowship. A.Gedanken thanks the German Ministry of Science for thesupport of this work through the Deutsche-Israeliprogram (DIP). The authors are grateful to Professor M.V. Landau, Blechner Center for Industrial Catalysis andProcess Development, Chemical Engineering Department,Ben-Gurion University of the Negev, for the providing uswith the SBA-15.

LA049290D

Table 1. CH4 Conversion and CO Selectivity of the RuLoaded SBA-15 Catalysts under Different Ru Loading

Ratios (Reaction Condition: CH4/O2 ) 2:1)

Ruloading ratio

(wt %)temp(°C)

CH4conversion

(%)

COselectivity

(%)

3.5 600 35.2 65.2650 35.3 66.8700 35.4 68.7750 36.0 69.2

7 600 38.7 71.1650 39.3 73.1700 40.0 73.9750 40.5 76.0

14 600 59.7 71.5650 61.8 76700 64.4 80.2750 65.4 83.5

8356 Langmuir, Vol. 20, No. 19, 2004 Li et al.