Synthesis of well-dispersed ruthenium nanoparticles inside mesostructured porous silica under mild conditions

Microporous and Mesoporous Materials 79 (2005) 185–194 www.elsevier.com/locate/micromeso

Synthesis of well-dispersed ruthenium nanoparticles inside mesostructured porous silica under mild conditions V. Hulea a

a,*

, D. Brunel a, A. Galarneau a, K. Philippot b, B. Chaudret b, P.J. Kooyman c, F. Fajula a

Laboratoire de Mate´riaux Catalytiques et Catalyse en Chimie Organique, UMR 561, CNRS-ENSCM-UM1, Institut C. Gerhardt, FR 1878, 8 rue de l’Ecole Normale, 34 296 Montpellier Cedex 5, France b Laboratoire de Chimie de Coordination du CNRS, 205, route de Narbonne, 31077 Toulouse Cedex, France c DelftChemTech/NCH REM, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands Received 2 September 2004; received in revised form 25 October 2004; accepted 26 October 2004 Available online 6 January 2005

Abstract This study reports an efficient method for the preparation of well-dispersed ruthenium nanoparticles of controlled size loaded into micelle-templated silica (MTS). This method is based on an organometallic approach consisting in mild decomposition of Ru(COD)(COT) (COD = g4-cycloocta-1,5-diene; COT = g6-cycloocta-1,3,5-triene) embedded inside mesostructured porous silica. The role assigned to the regular channels of MTS is to host and stabilize the metal nanoparticles. Different procedures of precursor decomposition, variation of metal loadings and of MTS pore sizes were examined in order to identify the crucial parameters governing the location, size and dispersion of the nanoparticles. The composite materials were characterized using elemental analysis, thermal gravimetric analysis, X-ray diffraction, nitrogen sorption at 77 K and TEM. The advantage of this new procedure lies in the absence of particles sintering due to the Ru reduction at room temperature and the stabilization of the nanoparticles by suitable pore size of the host. Ó 2004 Elsevier Inc. All rights reserved. Keywords: Ruthenium; Ru(COD)(COT); Nanoparticles; Mesostructured porous silica; MCM-41

1. Introduction Because of their unique chemical and physical properties as compared to their bulk metal or single metal atoms, metal nanoparticles have been attracting a lot of attention, especially in the fields of catalysis, optoelectronics and microelectronics. This interest in the properties of these nano-objects has given rise to the need for the control of size, shape and monodispersity of the nanoparticles. The stabilization of nanoscale colloidal metals has been achieved previously by the use of protecting hydro*

Corresponding author. Tel.: +33 04 67 16 3464; fax: +33 04 67 16 3470. E-mail address: [email protected] (V. Hulea). 1387-1811/$ - see front matter Ó 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2004.10.041

philic surfactants during the chemical or electrochemical reduction of metal salts or organometallic complexes [1,2]. More recently, Chaudret and co-workers [3] have developed an organometallic approach to the synthesis of noble metal nanoparticles by mild decomposition of the metal complexes under dihydrogen. On the other hand, porous metal oxide supports have been used for embedding metal nanoparticles. In particular, ordered mesoporous silica (possessing well-ordered and adjustable mesopores size with a narrow pore size distribution) are valuable hosts for including individual nanoparticles of noble metals such as Pt, Ru, Pd, Au [4–11]. Different techniques have been developed to promote the formation of metal nanoparticles onto the mesoporous silica support: incipient wetness impregnation with

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subsequent reduction [4–8,12–18], ion exchange [9,19– 23], chemical vapor deposition [24–26], direct introduction of metal precursors during the synthesis of mesoporous materials [10,11,27–30]. In order to gain high performance materials, it is of paramount importance that nanoparticle size and structure have to be controlled. Unfortunately, the most accessible and used methods (impregnation or direct incorporation) lead to the uncontrolled growth of metal nanoparticles inside the pores and especially on the growth of clusters on the external surface of the support, resulting in a broad particles size distribution. Consequently, there is extensive interest in the development of new methodologies to obtain well-dispersed metal nanoparticles into the pores of mesostructured silica. This study reports an organometallic approach for the preparation of well-dispersed ruthenium nanoparticles into the pores of mesoporous silica, based on the decomposition of ruthenium complexes at room temperature under dihydrogen. The weakly bound organic ligand containing Ru(COD)(COT) complex (COD = g4-cycloocta-1,5-diene; COT = g6-cycloocta-1,3,5-triene) was chosen as ruthenium source. This accessible complex (obtained by the reaction of RuCl3 and 1,5COD in the presence of zinc dust [31]) is an ideal precursor since it contains only hydrocarbon ligands which are easily eliminated under mild conditions. The COD and COT ligands are easily reduced into cyclooctane and metallic ruthenium nanoparticles are generated (Eq. (1)) nRuðCODÞðCOTÞ þ 5nH2 ! Run þ 2nC8 H16

ð1Þ

This property was recently used for the preparation of Ru-nanoparticles either unsupported [3] or supported on polyorganophosphazenes [32] and on Al2O3 [33]. To prevent the undesired aggregation of the Ru-nanoparticles during the decomposition of the precursor complexes with H2, the processes for producing isolated metal nanoparticles are usually performed in the presence of a polymer matrix [3] or ligands (alkylamines, alkylthiols) [3,33]. In the absence of any stabilizers, precipitation of metallic powder or low metal dispersion on the supports was obtained. In the present approach, the regular channels of mesostructured porous silica of MCM-41 type [34] are used as both host and stabilizer for metal nanoparticles. One could expect that each mesopore is an ideal reactor for nanoparticles formation due to limitation of the reaction zone by the pore walls. Indeed, we investigated the effect of the pore size of the MTS, which could be crucial for such an application.

2. Experimental The host MTS type materials MS3.5, MS5 and MS8, having channel size of 3.5, 5.0, and 8.0 nm, respectively,

were synthesized according to previously reported procedure [35], using cetyltrimethylammonium bromide (CTAB) (Aldrich), pyrogenic silica (Aerosil 200V Degussa), sodium hydroxide (SDS) and deionized water. The swelling agent used for the MS5 and MS8 syntheses was 1,3,5-trimethylbenzene (TMB) (Aldrich). The reactants were added under stirring at room temperature in the following order: H2O, NaOH, CTAB, TMB, SiO2. After addition of silica, the slurries have the molar composition of: 1SiO2:0.10CTAB:0.14NaOH:32H2O for MS3.5; 1SiO2:0.10CTAB:0.26NaOH:21H2O and 0.24TMB for MS5 and 1.27TMB for MS8. The mixtures were stirred during 30 min and then autoclaved during 20 h under static conditions at 115 °C. The gels were then filtered, washed with distilled water and dried overnight at 115 °C. The calcination of the as-synthesized MSx (with x = 3.5, 5 and 8) was performed at 550 °C during 8 h under air flow. Samples with different Ru contents were prepared by loading the MTS supports with the complex Ru(COD)(COT), under inert atmosphere (argon), in tetrahydrofurane (THF) as solvent. Ru(COD)(COT) was prepared via literature procedure [31]. THF was purified by distillation over sodium benzophenone immediately before use. In a typical synthesis, 300 mg of calcined MSx was exposed under vacuum for 8 h at 150 °C in a Fischer–Porter bottle prior the addition of the Ru(COD)(COT) complex solution. Three different routes of Ru-containing MTS synthesis were then performed (Scheme 1): (i) Concentrated impregnation–dry reduction (CId) method: The impregnation uses a concentrated THF solution of Ru complex (0.29 M) and the reduction is performed on the dried impregnated solid. A solution of 90 mg Ru(COD)(COT) in 1 ml of oxygen-free anhydrous THF (0.29 M) was added to out gassed MSx samples at room temperature. The resulting yellow mixture was stirred for 8 h under argon and the wet solid was first dried under vacuum and then reduced under H2 pressure (3 bars) for 8 h at room temperature to obtain RuMxCId (d = dry) samples (x = 3.5, 5 and 8). (ii) Sequential diluted impregnation (SI) method: The sequential impregnation method uses a diluted THF solution of Ru complex (0.08 M). A solution of 180 mg Ru(COD)(COT) in 7 ml of oxygen-free anhydrous THF was added over out gassed MSx at room temperature. The resulting mixture was stirred for 8 h under argon and then the supernatant (S1) was removed by filtration. In order to eliminate the Ru(COD)(COT) placed on the external surface the solid was flushed with 2 ml of THF and then the excess liquid was removed by filtration. The wet solid was pressurized under 3 bars of H2 at room temperature for 8 h in order to decompose the ruthenium precursor. A rapid color change of the solid samples from yellow to light brown was observed, indicative of the reduction of the organometallic com-

V. Hulea et al. / Microporous and Mesoporous Materials 79 (2005) 185–194 Route (i) (dry decomposition)

187

Route (ii) (sequential) MSx

MSx

outgassing outgassing Impregnation Ru(COD)(COT) in THF( 0.29M)

1st Impregnation Ru(COD)(COT) in THF(0.08 M)

Mx

Mx stirring

stirring vacuum

drying

separation

first filtrate (S1)

washing

H2

pentane

reduction (dry)

RuMxCId

separation

liquid

reduction

H2

drying

vacuum

RuMxSIa Route (iii) (wet decomposition) MSx

stirring

outgassing Impregnation Ru(COD)(COT) in THF( 0.29M)

2nd Impregnation (with S1)

2nd filtrate (S2)

separation

Mx stirring

H2

RuMxSIa

drying

vacuum

RuMxSIb

reduction(Wet)

RuMxSIb drying

vacuum

RuMxCIw

3rd Impregnation (with S2)

RuMxSIc

Scheme 1. Preparation of MTS samples containing Ru-nanoparticles.

plex. Finally, the mixture was dried under vacuum. The Ru/MTS composite thus obtained is denoted in this study as RuMxSIa (x = 3.5, 5 and 8 nm). In order to prepare samples with higher Ru loadings (denoted RuMxSIb and RuMxSIc) sequential impregnations were performed by re-engaging the filtrates (S1 and S2) recovered at the preceding step as described in Scheme 1, route ii. (iii) Concentrated impregnation–wet reduction (CIw) method: The impregnation uses a concentrated THF solution of Ru complex (0.29 M) and the reduction is performed on the wet solid. The outgassed powdered MTS was impregnated at room temperature, in a Fischer–Porter bottle with a concentrated solution (90 mg Ru(COD)(COT) in 1 ml of oxygen-free anhydrous THF). The resulting yellow mixture was stirred for 8 h under argon and then the wet solid was maintained under 3 bars of H2 for 8 h at room temperature. The resulting Ru/MTS was dried under vacuum to obtain RuMxCIw (w = wet) samples (x = 3.5, 5 and 8). The nanoparticles-loaded MTS materials were characterized using elemental analysis, thermal gravimetric analysis (TGA) (Netzsch TG 209C thermobalance), X-

ray diffraction (XRD) (Bruker AXS D8 diffractometer) and nitrogen sorption at 77 K (Micromeritics ASAP 2000). The metal nanoparticles size, dispersion and location were determined from TEM images obtained with a 300 kV Phillips CM30T electron microscope with a LaB6 filament as the source of electrons. The specific surface areas of all materials were determined from the BET equation in the p/p0 range of 0.12–0.25, the mesoporous volumes are calculated at the top of the adsorption step, the average pore diameters are calculated according to the Broekhoff and de Boer method (BdB) [36].

3. Results and discussion 3.1. Support materials The MTS type materials investigated as supports to host ruthenium nanoparticles feature the similar surface areas around 900 m2 g1 and three different pore sizes, in order to analyze the channel size effect on the metal loading, the size and the dispersion of the

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Adsorbed volume (ml STP/g)

Intensity (a.u.)

1400

MS 3.5

MS8

1200 1000

MS5

800 600

MS3.5

400 200

RuM 3.5 SIa 0 0

0.1

0.2

0.3

0.4

0

1

2

3

4

5

6

0.5

0.6

0.7

0.8

0.9

1

p/p0

RuM 3.5 SIc

7

2-Theta (˚) Fig. 1. XRD patterns of MS3.5-type materials.

Fig. 2. N2 adsorption–desorption isotherms at 77 K of MS8, MS5 and MS3.5 materials.

3.2. RuMxCId samples

Ru-nanoparticles. The MTS prepared without TMB as swelling agent features the XRD pattern characteristic of a well ordered MCM-41 type material [34] identified by the (1 0 0), (1 1 0) and (2 0 0) diffraction peaks of the hexagonal symmetry (space group p6mm) (Fig. 1). This material presents a reversible nitrogen adsorption–desorption isotherm showing a sharp step at about 0.35 p/p0 characteristic of the filling of pores of monodisperse size of 3.6 nm. The MTS supports prepared in the presence of swelling agent exhibit a single low-angle broad diffraction peak, followed by a second order (not shown), as described previously [36]. These materials are less ordered than 3.5 nm pore size MTS, but are nevertheless two-dimensional structures. The materials featured an irreversible type-IV adsorption–desorption isotherm with a H1 hysteresis loop with a desorption step at p/p0 0.55 and 0.75, respectively (Fig. 2), characteristic of materials with 5.1 nm and 8.3 nm pore diameter. All textural parameters of MTS materials are reported in Table 1.

The RuMxCId samples prepared according to concentrated impregnation–dry reduction method (CId) were firstly analyzed by thermogravimetry in order to check the efficiency of both sorption and decomposition of the Ru(COD)(COT) complexes. In Fig. 3 is reported the curve obtained by thermogravimetric analysis for the MS8 samples before and after the CId treatment. For the calcined support MS8, only a low weight loss of 2% between 50 and 125 °C due to the elimination of the physisorbed water was observed. In the case of the RuM8CId sample, an important weight loss of 15% is observed which decomposed in two steps. The first weight loss (4.5 wt.%) could be related to the elimination of the THF (75–225 °C), while the second one (11 wt.%, in the range of 225–250 °C) could be due to the thermal oxidation in air of the Ru complex. This last loss corresponds to a ligand removal of 71% from the initial Ru(COD)(COD) complex. Elemental analysis of RuM8CId sample (13.7 wt.% C, 8.1 wt.% Ru) confirm these results: Ru(COD)(COT) complexes were only par-

Table 1 Composition and physicochemical properties of the parent MSx and RuMxSI materials Samples MS3.5 RuM3.5SIa RuM3.5SIc

Ru loading (wt.%)

Average pore size (nm)

BET surface area (m2/g)a

Vmesop, (mL/g)a

Ru XRD signalb

Ru-nanoparticles size (nm)

0 2.36 8.30

3.6 3.6 3.6

740 680 620

0.54 0.48 0.44

– ++ ++++

2.5 2.5

MS5 RuM5SIa RuM5SIc

0 3.43 12.67

5.1 5.0 4.8

950 910 780

1.09 1.01 0.74

– – +++

2.54.5 3–5

MS8 RuM8SIa RuM8SIb RuM8SIc

0 3.38 7.30 11.02

8.3 8.2 8.2 8.1

1080 935 905 850

2.02 1.43 1.25 1.07

–  + +++

3.5 5–7 6–7

a b

Standardized versus pure silica weight. The number of (+) corresponds to the intensity of XRD lines at 2h = 43° (see Fig. 6).

V. Hulea et al. / Microporous and Mesoporous Materials 79 (2005) 185–194 100

MS8 RuM8CIw 92

RuM8SIc 88

RuM8CId

84

Adsorbed volume (ml STP/g)

900

96

Mass, %

189

MS5

800 700

RuM5SIa

600 500 400

RuM5SIc

300 200 100

80 50

150

250

350

450

0

550

0

Temperature, °C

tially decomposed with H2 in absence of solvent so for the CId method. The X-ray powder patterns of the resulting materials feature no additional diffraction peaks in comparison to that of the parent support. The absence of the formation of Ru nanoparticles and/or larger aggregates is observed on TEM micrographs, demonstrating the inefficiency of the dry reduction method CId for the reduction of the Ru complexes and the formation of nanoparticles. 3.3. RuMxSI materials The composition (Ru content) and the textural properties (from N2 sorption isotherms and XRD) of MSx and RuMxSI materials (obtained by sequential impregnation of diluted solution (0.08 M) of Ru(COD)(COT) complex) are summarized in Table 1. Samples with different Ru loadings (RuMxSIa, RuMxSIb and RuMxSIc) were analyzed by thermogravimetry. The letters a–c correspond to the first, second and third impregnation over the same material (Scheme 1, route ii). The curves for the RuMxSI samples show a weight loss of around 8% in the range 75–225 °C related to the elimination of THF. The example of RuM8SIc is given in Fig. 3. A MTS impregnated only with THF and evacuated at the same vacuum and temperature conditions of RuMxSI samples did not present any THF loss. It is important to note that no significant weight loss are observed at temperatures above 225 °C, so no COT and COD ligands are present in RuMxSI materials, as previously indicated for the RuMxCId materials. These results are consistent with the complete reduction of the ligands during treatment with dihydrogen when THF was not totally evacuated by distillation. The nitrogen sorption isotherms of the RuMxSI samples were investigated in order to study the Ru location inside or outside the pores. The nitrogen isotherms of Ru-containing MTS materials feature a similar shape than starting MTS materials, their mesoporous structure

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

p/p 0

Fig. 4. N2 adsorption–desorption isotherms at 77 K of MS5, RuM5SIa, RuM5SIc (standardized versus pure silica weight).

did not change during the incorporation of Ru-nanoparticles. Nevertheless, due to the additional weight corresponding to Ru nanoparticles loading, the adsorbed nitrogen volumes seriously decreased. To understand if the decrease of pore volume is only due to the extra weight brought by the Ru-nanoparticles on the solid, the nitrogen isotherms were standardized versus pure silica weight (Table 1, Figs. 4 and 5). It is noteworthy that the nitrogen adsorption isotherms standardized versus pure silica weight reveal that the surface areas and the pore volumes of Ru-nanoparticles containing MTS materials decreased as a function of the metal loading for MS5 (Fig. 4). This confirms that the Ru nanoparticles are present inside the mesopores, undergoing then a decrease of the pore volume accessible for nitrogen. Moreover the inflexion p/p0 of the desorption step decreases with the same trend, which confirms that the metal loading in SI method mainly occurs inside the channels. This trend is also observed in the case of the MS8 materials with 8 nm pore diameter although the relative effect concerning the p/p0 variation was lower mainly due to the larger pore size. On the contrary, the MS3.5 materials did not behave similarly.

Adsorbed volume (ml STP/g)

Fig. 3. TGA curves of MS8, RuM8CIw, RuM8SIc, and RuM8CId.

0.1

600 500

MS3.5

400 300

RuM3.5SIc

200 100 0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

p/p0

Fig. 5. N2 adsorption–desorption isotherms at 77 K of MS3.5 and RuM3.5SIc (standardized versus pure silica weight).

V. Hulea et al. / Microporous and Mesoporous Materials 79 (2005) 185–194

The p/p0 desorption step of the standardized isotherms (Fig. 5) does not change after the Ru-nanoparticles loading, suggesting that the loading does not occur inside the channels, but only on the outer surface of the material. The decrease of pore volume between RuMxSIc samples and the parent MSx (for x = 5 and 8) are 3 and 9 times, respectively, compared to MS3.5. This trend suggests an easier internal Ru loading for the MTS materials having a higher pore size. Note that several authors have previously reported that nanoparticles of ruthenium can be easily encapsulated even into the low diameter pores of mesostructured silica. Thus, using anionic ruthenium cluster carbonylates as precursors, Zhou et al. [37] incorporated nanoparticles of ruthenium into the 3 nm diameter pores of MCM-41 mesoporous silica. It seems that in our case the Ru complexes used are too large to enter in the pores of 3.5 nm and Ru-nanoparticles are mainly formed outside the pore of MS3.5. In the case of the MS3.5 materials, the pore volume slightly decreases even though the pore size did not change significantly during the metal loading (Fig. 5). Similar results were recently reported concerning the effect of impregnation of a [Ru(bpy)3]2+ complex [38] or of surfactant Ru complexes used to directly template mesoporous silicates [39,40]. It was assumed that the adsorbed molecules aggregate in the cylindrical mesopore and suppress the adsorption of nitrogen into the mesopore. We can also suggest that the lower the pore size, the more pore openings are blocked by larger Ru aggregates which could be formed on the external surface by Ru complex reduction. Furthermore, powder X-ray diffraction of the Rucontaining materials has been investigated to confirm or rule out this assumption. As shown in Fig. 1, for the samples having an initial pore size of 3.5 nm, the low angle XRD patterns (2h = 0.7–7°) of Ru-containing MTS materials were not affected by the metal incorporation, indicating that the initial structure was preserved. In order to identify the Ru clusters possibly formed on the MTS surface, the XRD data recorded at higher angles (2h = 7–50°) were used. As can be seen in Fig. 6, the X-ray diffraction patterns of RuM8SI samples were similar to that of the parent MS8 except the appearance of a new diffraction line at around 43°, assigned to Ru crystallites [40,41], especially for RuM8SIc obtained after three impregnation. At a relatively low Ru loading level, as 2 wt.% Ru (RuM8SIa sample), this line is barely visible, but it becomes prominent for RuM8SIc containing 8 wt.% Ru. These results suggest that the fraction of large Ru particles (named in this study crystallites), which are not located in the mesopores, increases and the intensity of the metallic diffraction line increases with increasing loading. We note that a large band centered between 20° and 25° is assigned to the ‘‘amorphous’’ silica nature

MS8

Intensity (a.u.)

190

+

RuM8SIa

+++ RuM8SIc

5

10

15

20

25

30

35

40

45

50

2-Theta (˚)

Fig. 6. X-ray powder diffraction patterns of MS8, RuM8SIa and RuM8SIc; the number of (+) corresponds to the intensity of XRD line.

of the walls. A pronounced behavior is pointed out for the MS5 series. Indeed, this trend is stressed by the Xray diffraction patterns of the MS3.5 materials, which reveal the characteristic diffraction line of Ru crystallites already after the first impregnation step. This evidences the previous assumption about the possible pore blocking of the mesopore entrances for 3.5 nm materials, which will be stronger for the materials possessing the smaller pore size, revealing lower accessibility to the large metal complex used in our study by diffusion limitation. The higher intensity of XRD peaks corresponding to Ru crystallites has been indicated in Table 1 with a higher number of (+) sign. Direct examination of all as-synthesized samples by transmission electron microscopy clearly confirmed the different conclusions previously stated. Typical TEM pictures of Ru-containing MSx named RuMxSIa and RuMxSIc are shown in Fig. 7. The TEM image of RuM3.5SIa shows only a few small individual metal nanoparticles (2.5 nm) incorporated inside the mesopores. Most of the metal particles are agglomerated outside the pores as larger dark spots. In the case of the RuM3.5SIc (not shown), more individual metal nanoparticles are observed inside the pores as compared to sample RuM3.5SIa, but also a lot of larger crystallites are formed on the external surface of the materials. Hence, this result confirms the possible pore blocking by the agglomerated larger particles previously suggested to explain the decrease of the standardized mesopore volume. In the case of the RuM5SIa sample containing 3 wt.% of Ru, many individual nanoparticles (2.5–4.5 nm) are present throughout the material, mainly inside the mesoporous structure. The average Ru-nanoparticle size and the number of individual nanoparticles appear to be slightly larger for RuM5SIc, containing 13 wt.% of Ru, than for

V. Hulea et al. / Microporous and Mesoporous Materials 79 (2005) 185–194

191

Fig. 7. TEM image of (a) RuM3.5SIa, (b) RuM5SIa, (c) RuM5SIc.

RuM5SIa. The distribution of Ru over the mesoporous material seems to be a little more homogeneous than for RuM5SIa, but some Ru crystallites are also present at the external surface. The RuM8SI materials revealed comparable features as for RuM5SI materials with a slight increase of the individual Ru nanoparticles size after each impregnation step. Indeed, RuM8SIa revealed many individual Ru nanoparticles of ca. 3–5 nm throughout the whole material and mainly inside the pores. TEM images of RuM8SIc and RuM8SIb (not shown) show that larger particles (6–7 nm) are formed, mainly inside the mesoporous material, and Ru loading increases from 3 wt.% to 11 wt.% for the first and the third impregnation, respectively. At the third impregnation (RuM8SIc) crystallites of Ru are present at the outer surface. The size of the Ru crystallites (20–30 nm) also increases with the number of impregnation. The distribution of Ru-nanoparticles over MTS material was much more homogeneous for RuM8SIc than for RuM5SIc. There could be several reasons for this difference. The higher homogeneity in Ru nanoparticles distribution observed in the MS8 materials compare to the MS5 materials could result from a better diffusivity of the Ru complexes induced by larger pore diameter.

The increase of the internal Ru-nanoparticles size as a function of the successive impregnations, particularly for the two MTS possessing the larger pore size, suggests a possible catalytic effect of the nanoparticle surface during the complex decomposition under dihydrogen. The presence of confined solvent (THF) inside the channels would be crucial to favor both local dihydrogen concentration and the complex molecule diffusion to the activated nanoparticle and to allow their reactive encounter. Unfortunately, the successive impregnation leads also to the formation of Ru crystallites at the outer surface due probably to the nucleation of the clusters from the particles at the mouth of the pore opening. 3.4. RuMxCIw samples The role of the confined THF solvent in the reduction efficiency of the Ru complexes prompted us to investigate again the CI method, but this time using wet conditions during the hydrogenation step (method CIw). The thermogravimetric analysis (Fig. 3) of the RuMxCIw samples confirms that the Ru complexes were totally reduced upon dihydrogen treatment under mild (room temperature) and wet conditions. Table 2 reports the compositions and the textural characteristics of the RuMxCIw.

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Table 2 Composition and textural properties of RuM3.5CIw and RuM8CIw materials Ru loading (wt.%)

Average pore size (nm)

MS3.5 RuM3.5CIw

0 10.12

3.6 3.5

MS8 RuM8CIw

0 11.51

8.3 8.2

a b

Vmesop, (mL/g)a

Ru XRD signalb

Ru-nanoparticles size (nm)

740 690

0.54 0.50

– +++

3.3 ± 0.2

1080 910

2.02 1.51

– +

3.5 ± 0.5

BET surface area (m2/g)a

Standardized versus pure silica weight. The number of (+) corresponds to the intensity of XRD line.

It must be noted that the Ru-containing MTS materials prepared according to the impregnation method with concentrated Ru (COD)(COT) complex solution possess nearly the same Ru-loading at 10 wt.% whatever the pore size is. In spite of such a similar loading between RuM3.5CIw and RuM8CIw, the features of the materials are different. The pore size of the MS3.5 was not modified and the standardized surface area and pore volume have barely decreased. For MS8, the pore size has significantly decreased simultaneously with the standardized pore volume and surface area. Furthermore, XRD patterns of the different RuMxCIw revealed differ-

ent features. An important diffraction line at ca. 2h = 43° ascribed to Ru-crystallites is present in the diffractogram of the RuM3.5CIw sample, but is absent for the RuM8CIw material. Hence, the slight decrease in the porosity of the MS3.5 upon Ru loading is consistent with the conclusion drawn for the MS3.5 treatment according to the SI method. The TEM image of RuM3.5CIw clearly reveals isolated Ru-nanoparticles of monodisperse size (2.5 nm) inside the mesopores of the MS3.5 materials but also larger particles (4–5 nm) on the external surface indicating that clustering has occurred (Fig. 8a).

Fig. 8. Transmission electron micrograph of (a) RuM3.5CIw, (b) RuM8CIw, (c) RuM5CIw.

V. Hulea et al. / Microporous and Mesoporous Materials 79 (2005) 185–194

193

Scheme 2. Formation of well-dispersed Ru-nanoparticles inside the mesopores.

These external larger particles could also explain the loss of standardized mesopore volume and surface area by pore blocking taking into account the nearly constant pore size. Nevertheless, the size of such larger particles is not so important as those observed for the sample RuM3.5SIc possessing nearly the same Ru loading. This result shows that the Ru-nanoparticles already present at the surface of the support used in RuM3.5SIc preparation may be considered as activating sites for the formation of larger Ru crystallites. In the case of the MS8 materials, even though the porosity modification is well in line with an important internal loading, it is noteworthy that no larger particles were detected by TEM (Fig. 8b) although such clustering was observed during the sequential method with a comparable Ru loading (RuM8SIc) of around 10 wt.%. For RuM5CIw, the TEM image (Fig. 8c) shows Runanoparticles of very homogeneous size (3.3 ± 0.2 nm) inside the pores of MS5 (5 nm pore diameter). A similar result is observed for MS8 with 8.3 nm pore size (RuM8CIw), which can accept up to 11.5 wt.% of well-dispersed Ru-nanoparticles with a homogeneous size in the range of 3.5 ± 0.5 nm (Fig. 8b). Such a result would be explained by the stabilizing role played by the mesoporous walls, which prevents aggregation (Scheme 2) and avoids getting larger nanoparticles. This effect, which may be due to the presence of silanols in a constrained environment, is very similar to that observed for the reaction in heptanol [42]. In the latter case, the nanoparticles are monodisperse (3 nm), well dispersed in the solvent and adopt the hexagonal compact (hcp) structure of bulk ruthenium. The silanols may act as similar coordinating groups as heptanol. Moreover, the texture of the silica support and the impregnation method play an important role in the resulting Ru-nanoparticles size. Opposed to particle growth when using the successive impregnation (SI method), in the CI method as well as in the first impregnation step for the SI method, both the amount and the size of Ru-nanoparticles result from a rapid reduction of the complex by dihydrogen with Ru–Ru formation from activated complexes (partially hydrogenated) and incorporated in a constrained volume of the channels possessing well-defined size. In the case of MS3.5, the size of the nanoparticles is strongly

limited by both diffusion and constrained environment. On the contrary, in the case of MS8, the size of the nanoparticles would not be limited by these later effects, but the chemical surface properties would play a role for preventing the particle agglomeration. The MTS possessing a pore size of 5 nm seems to be the best compromise to obtain the best monodisperse Ru nanoparticles of 3 nm inside the channels up to a Ru loading of ca. 12 wt.%.

4. Conclusion Well-dispersed ruthenium nanoparticles have been stabilized into the pores of mesostructured silica by using an organometallic approach. The advantage of this procedure based on the decomposition under H2 at room temperature of organometallic precursor Ru(COD)(COT) is the absence of particle sintering resulting from the mild reduction conditions and suitable pore size of the host. The mesoporous walls of MTS materials, as well as the silanol groups play an important role in the formation of well-dispersed Ru nanoparticles of uniform size. The size of the Ru nanoparticles and their location significantly depend on the mesopore size, the metal loading and the impregnation method. Thus, the ruthenium was mainly aggregated on the external surface of the 3.5 nm pore size MTS and only a few nanoparticles were found inside the pores. In the case of 5 and 8 nm pore size MTS, well-dispersed nanoparticles of 3 nm were found inside the pores. The distribution of Ru-nanoparticles was more homogeneous for metal loading lower than 3 wt.% in the case of the sequential impregnation method using a diluted THF solution of metal precursor. At higher loading, some external aggregation takes place on the external surface of MTS. In the case of the impregnation of MTS achieved in one step and using a more concentrated THF solution of metal precursor, very monodisperse Ru-nanoparticles were obtained inside the channels possessing pore size of 5 or 8 nm with a loading of Ru up to 12 wt.%. Concentrated impregnation allows to form well-dispersed Ru-nanoparticles of 3 nm as soon as the silica host presents pore larger than 5 nm.

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Acknowledgment V. Hulea thanks CNRS for a research-associate position. The authors thank Marie-France Driole for the preparation of the parent MTS samples.

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