Preparation and characterization of ruthenium clusters on mesoporous supports

Microporous and Mesoporous Materials 44±45 (2001) 385±394

www.elsevier.nl/locate/micromeso

Preparation and characterization of ruthenium clusters on mesoporous supports Martin Hartmann a,*, Christian Bischof a, Zhaohua Luan b, Larry Kevan b a

Department of Chemistry, Chemical Technology University of Kaiserslautern, P.O. Box 3049, D-67653 Kaiserslautern, Germany b Department of Chemistry, University of Houston, Houston, TX 77204-5641, USA Received 15 February 2000; accepted 26 April 2000

Abstract Ruthenium-containing MCM-41, MCM-48 and SBA-15 mesoporous molecular sieves were prepared employing various methods and di€erent metal precursors, viz. RuCl3 , Ru(NH3 )6 Cl3 and Ru3 (CO)12 . The catalysts were characterized by X-ray powder di€raction, nitrogen adsorption, transmission electron microscopy and hydrogen chemisorption. The measurements reveal that small ruthenium clusters can be prepared inside the channel of the mesoporous support using ruthenium carbonyl or ruthenium hexammine chloride as precursors. Thermogravimetry coupled with mass spectrometry, UV±VIS spectroscopy and electron spin resonance spectroscopy were used to study the autoreduction of the Ru…NH3 †3‡ 6 complex, which is distinctly di€erent from the process observed on microporous materials (zeolites). Due to the low water content of the samples, the intermediate formation of [Ru(NH3 )5 OH]2‡ and Ru-red is suppressed, but the formation of nitrosyl complexes is evident from the UV±VIS spectra. A new intermediate is found, which is characterized by a sharp ESR signal with gk ˆ 1:95 and g? ˆ 2:10 accompanied by the appearance of a broad UV±VIS band centered at 706 nm, which re¯ects the green±blue color of the sample. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Ruthenium; Mesoporous molecular sieves; MCM-48; SBA-15; Electron spin resonance; H2 -chemisorption

1. Introduction Among the various transition metals used in heterogeneous catalysis ruthenium is known to catalyze a variety of reactions. Alumina- or silicasupported ruthenium selectively reduces nitrogen oxide to molecular nitrogen [1,2], zeolite-supported ruthenium is an excellent catalyst for the water±gas-shift reaction [3±5] and has speci®c ac-

*

Corresponding author. Fax: +49-631-205-4193. E-mail address: [email protected] (M. Hartmann).

tivity for the hydrogenation of carbon monoxide [6±10]. Ruthenium has recently received considerable attention in the ®eld of improved catalysts for ammonia synthesis in competition with traditional iron catalysts [11]. For many catalytic applications, the preparation of small ruthenium clusters is desired and several strategies have been explored. The incorporation of ruthenium into microporous molecular sieves and the preparation of ruthenium metal clusters have been reported. The growth of the ruthenium clusters is restricted by the size of the zeolite channel or cage [12]. Ruthenium clusters of 1 nm can be prepared in zeolite Y,

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which was found to be an active catalyst in ammonia synthesis [13,14]. Recently, we reported the incorporation of ruthenium into the mesoporous molecular sieves MCM-48 and MCM-41 [15]. Coman et al. reported the stereocontrolled hydrogenation of prostaglandin intermediates over Ru-MCM-41 catalysts [16]. The choice of a suitable precursor and optimized activation conditions also allow the controlled preparation of small ruthenium particles on mesoporous supports. ESR spectroscopy has been widely used to study ruthenium (III) species in zeolites, but interpretation of such spectra is often dicult due to their rather broad featureless nature [17±21]. The behavior of ionic ruthenium in zeolites is complex and not fully understood at this time. Ruthenium is usually introduced into the zeolite through ion exchange of Ru(NH3 )6 Cl3 in aqueous solution. 3‡ The Ru…NH3 †6 complex has been shown to undergo reduction by the amine ligands during dehydration and deammination at temperatures in excess of 573 K, although there is disagreement regarding the ®nal oxidation state of the metal and the extent of the reduction. In this paper, we report the preparation of ruthenium clusters using RuCl3 , Ru(NH3 )6 Cl3 and Ru3 (CO)12 precursors aiming at a high ruthenium 3‡ dispersion. The decomposition of the Ru…NH3 †6 precursor complex was studied in detail by thermogravimetry, ESR and UV±VIS spectroscopy to gain control over the autoreduction of Ru(III) to form ruthenium metal. 2. Experimental section 2.1. Synthesis The parent materials MCM-41, MCM-48 and SBA-15 have been synthesized in their pure silica form as described elsewhere [22±24]. The syntheses of Al-MCM-41 (nSi =nAl ˆ 15) and Al-MCM-48 (nSi =nAl ˆ 20) have been described in a previous paper [25]. The direct synthesized material (RuMCM-48) was prepared by adding a solution of RuCl3 to the synthesis gel. The ion-exchanged sample (Ru/Al-MCM-48) was obtained by stirring Al-MCM-48 (nSi =nAl ˆ 20) [25] in an aqueous so-

lution of RuCl3 (24 h, 70°C). The Ru/MCM-48(c) samples were prepared by stirring the all-silica parent material in a solution of Ru3 (CO)12 in pentane (24 h, 30°C) until complete evaporation of the solvent. Ru/MCM-48 and Ru/SBA-15 were obtained by impregnation of the parent materials with [Ru(NH3 )6 ]Cl3 . The Ru/Al-MCM-41 samples with di€erent ruthenium loadings have been obtained by stirring aluminum-containing MCM-41 in an aqueous solution of [Ru(NH3 )6 ]Cl3 for 12 h and subsequent evaporation of the water at 60°C. The chemical composition of the samples and the ruthenium content were determined by ICP-AES and the samples were labeled accordingly: 2.0 Ru/ Al-MCM-41 denotes a sample with a ruthenium loading of 2.0 wt.%. 2.2. Characterization The resulting materials were characterized by powder X-ray di€raction (XRD) (Siemens D5005, CuKa radiation) and nitrogen adsorption (Micromeritics ASAP 2010) at 196°C (77 K). Hydrogen adsorption isotherms were measured volumetrically at 40°C in a home-built all-steel apparatus. The samples were dehydrated at 300°C for 18 h and subsequently reduced in dry hydrogen at 400°C (p ˆ 4  104 Pa). Finally, the samples were evacuated (p < 10 4 Pa) for 2 h at 400°C to remove adsorbed hydrogen after the reduction step. The decomposition of the precursor complex Ru…NH3 †3‡ supported on various mesoporous 6 molecular sieves was monitored by thermogravimetry coupled with mass spectrometry (TG±MS) employing a SETARAM Setsys-16/MS. The UV± VIS spectra were recorded with a Perkin±Elmer Lambda 18 spectrometer in the di€use re¯ectance mode. The spectra were measured with Al-MCM41 as a standard. The ESR spectra were recorded at X-band on a Bruker EXP 300 spectrometer at 196°C. The magnetic ®eld was calibrated by a Hewlett±Packard HP 5342A frequency counter. The samples were loaded into 3 mm o.d. by 2 mm i.d. Suprasil quartz tubes. Prior to the UV±VIS and the ESR experiments, the samples were dehydrated in vacuum at room temperature and then stepwise

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heated under vacuum to 100°C, 200°C or 300°C. The transmission electron microscope (TEM) micrographs were recorded on a Hitachi H 8100. The materials were placed on a carbon grid and measured with a maximum acceleration voltage of 200 keV. 3. Results and discussion Fig. 1 displays the powder XRD patterns of ruthenium-containing MCM-41, MCM-48 and SBA-15. MCM-41 and SBA-15 exhibit the XRD patterns typically obtained for the hexagonal phase. The positions of the d1 0 0 re¯ection are characteristic for the di€erent hexagonal materials MCM-41 and SBA-15, respectively. The latter materials is prepared using an amphiphilic triblock copolymer, which typically results in materials with a pore diameter and wall thickness of about 6 nm [24]. The MCM-48 sample shows 5±8 re¯ections in the range of 2h ˆ 2±8°, which are indexed according to the cubic symmetry of the MCM-48 materials. Under a controlled pH value below 8, incorporation of ruthenium via di€erent procedures does not alter the XRD patterns, hence, indicating no signi®cant loss of sample quality. In Table 1, the chemical compositions and the results of the nitrogen adsorption experiments are given. The N2 adsorption isotherms of the ruthenium-containing samples are similar to those of the parent materials. The use of C16 TMABr and

Fig. 1. XRD patterns of (a) 4.0 Ru/Al-MCM-41, (b) 2.0 Ru/ MCM-48 and (c) 2.0 Ru/SBA-15.

C14 TMABr for the synthesis of MCM-48 and MCM-41, respectively, results in materials with a uniform pore size of about 2:2  0:2 nm. The pore diameter of the SBA-15 materials is about 6 nm and does not decline with ruthenium incorporation.

Table 1 Chemical composition and results of the nitrogen adsorption experiments of the investigated samples Sample

nSi =nAl

Precursor

Ru loading (wt.%)

ABET (m2 g 1 )

Vp (cm3 g 1 )

dp a (nm)

MCM-48 1.0 Ru-MCM-48 1.2 Ru/Al-MCM-48 1.1 Ru/MCM-48(c) 2.0 Ru/MCM-48(c) 2.0 Ru/MCM-48

± ± 20 ± ±

± RuCl3 RuCl3 Ru3 (CO)12 Ru3 (CO)12 Ru(NH3 )6 Cl3

± 1.0 1.2 1.1 2.0 2.0

1400 1380 1280 1290 1340 1310

0.81 0.80 0.78 0.71 0.68 0.70

2.4 2.2 2.0 2.3 2.3 2.2

Al-MCM-41 1.8 Ru/Al-MCM-41

15 15

± Ru(NH3 )6 Cl3

± 1.8

1070 1020

0.69 0.63

2.4 2.0

4.0 Ru/Al-MCM-41

15

Ru(NH3 )6 Cl3

4.0

1030

0.63

2.1

SBA-15 2.0 Ru/SBA-15

± ±

± Ru(NH3 )6 Cl3

± 2.0

1000 1050

1.17 1.20

6.2 5.9

a

The pore diameters were calculated from the desorption branch of the nitrogen adsorption isotherms using the BJH model.

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3.1. TG-MS In Fig. 2, the results of TG-MS of 1.8 Ru/AlMCM-41 are displayed. The DTG curve exhibits several maxima (Fig. 2a): the ®rst weight loss is observed at 90°C. Further maxima are observed at 160°C, 280°C and 400°C. The mass spectrometry TPD (m=e ˆ 18) curve reveals that the ®rst weight loss is mainly due to desorption of physically adsorbed water. Between 160°C and 210°C, ammonia (m=e ˆ 16) is desorbed from the sample (Fig. 2b), which indicates the decomposition of the Ru…NH3 †3‡ precursor. Finally, around 300°C 6 again water formed by condensation of the hydroxyl groups is desorbed. Above 400°C also nitrogen (m=e ˆ 28) and hydrogen (m=e ˆ 2) produced by decomposition of ammonia over ruthenium metal are observed in the mass spectra (not shown) in agreement with results by Sheu et al. 3‡ [26] on Ru…NH3 †6 /NaY. In all samples, the desorption of NO is also observed around 175°C, while only in Ru/MCM-48 and Ru/SBA-15 HCl is

Fig. 2. Thermogravimetric (a) and mass spectroscopic (b) analysis of 1.8 Ru/Al-MCM-41 upon heating in a helium ¯ow.

formed around 400°C. Due to the ion exchange capacity of Ru/Al-MCM-41, the protons formed are needed to balance the negative charges from the aluminosilicate framework. Details from this work will be presented in a following publication [27]. The desorption of ammonia (determined by TG-MS, m=e ˆ 16) formed by decomposition of 3‡ Ru…NH3 †6 on di€erent mesoporous supports is displayed in Fig. 3. The desorption maximum for 1.8 Ru/Al-MCM-41 is centered at 160°C and a shoulder is visible at 185°C, while 2.0 Ru/SBA-15 shows two maxima at 174°C and 274°C and a shoulder at 225°C. The mass spectrometry TPD of 2.0 Ru/MCM-48 shows maxima at 129°C, 157°C and 175°C. The precursor ruthenium hexammine chloride loses ammonia at 240°C and 269°C. The signi®cant reduction of the decomposition temperature observed for Al-MCM-41 and MCM-48 in contrast to the unsupported precursor shows that the precursor complex is highly dispersed on

Fig. 3. TPD-MS curves (m=e ˆ 16) of 1.8 Ru/Al-MCM-41, 2.0 Ru/MCM-48 and 2.0 Ru/SBA-15 and the precursor Ru(NH3 )6 Cl3 .

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the inner surface of the mesoporous materials under investigation. The appearance of a maximum at 270°C in 2.0 Ru/SBA-15, which is also found for bulk Ru(NH3 )6 Cl3 , indicates a signi®cantly lower dispersion of the precursor on this material. Interestingly, such a signi®cant displacement of the precursor decomposition temperature was not observed in Ru/NaHY [26]. The reasons for the observed di€erences are at present not fully understood, but it seems reasonable to assume that the decomposition of more isolated ions proceeds via a di€erent pathway than the decomposition of the bulk material. 3.2. ESR spectroscopy Fig. 4 shows a series of ESR spectra during activation of 1.8 Ru/Al-MCM-41. For the fresh sample (Fig. 4a), a broad species A with gk ˆ 2:19 and g? ˆ 1:74 is observed. This species has been

Fig. 4. ESR spectra of 1.8 Ru/Al-MCM-41 after thermal treatment at di€erent temperatures. The asterisk indicates Fe3‡ impurities.

3‡

389

reported as Ru…NH3 †6 exchanged into various zeolites [18]. Evacuation at room temperature results in a decrease of the intensity of signal A (Fig. 4b), which has completely disappeared after evacuation at 100°C (Fig. 4c). Another species is observed after activation above 100°C in vacuum accompanied by a color change from white to green, which reaches its maximum intensity at 200°C (Fig. 4e). This species B is characterized by gk ˆ 1:95 and g? ˆ 2:10. At this point, the color of the sample has turned from green into deep blue. With further increase of the temperature, the sample turns gray and the signal vanishes. A similar behavior is also found for rutheniumcontaining MCM-48 and SBA-15 prepared with ruthenium hexammine chloride as a precursor. During the activation process, the samples did not change to a wine-red color typically associated with the formation of [(NH3 )5 RuORu(NH3 )4 ORu(NH3 )5 ]6‡ , which is called Ru-red [28]. This complex is frequently observed in zeolites [29] indicating a signi®cant mobility of the ruthenium complexes in the zeolite pores. Furthermore, the ESR signal with g1 ˆ 2:58, g2 ˆ 2:24 and g3 ˆ 1:71, which is also often observed in zeolites and typically assigned to [Ru(NH3 )5 OH]2‡ , is absent in our samples. The H2 O necessary for the formation of [Ru(NH3 )5 OH]2‡ and trans[RuIV (NH3 )4 (OH)2 ]2‡ , which later form Ru-red in a basic hydrolysis reaction [29], is not provided by the more hydrophobic mesoporous molecular 3‡ sieves. Evacuation at 300°C of Ru…NH3 †6 exchanged into NaY has been reported to form Ru3‡ coordinated to the zeolite. However, the ESR signal of this paramagnetic ion has not been observed yet. The absence of an ESR signal has been suggested to be due to the formation of a Ru3‡ dimer, which is diamagnetic [19]. In Ru/Al-MCM41, we do not observe the formation of the Ru-red complex during activation, but we do see a new ESR signal after evacuation at 200°C. The g-tensor components of this species are in agreement with both a d7 (Ru‡ ) and a d5 (Ru3‡ ) electronic con®guration of the metal. The g values obtained for species B are compared in Table 2 with those obtained for ruthenium species on di€erent supports, but the remaining question concerning the oxidation state of the ruthenium is still unsolved.

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Table 2 ESR data for paramagnetic centers on di€erent supports Support

gxx

gyy

gzz

Treatment

Reference

Zeolite Y Zeolite Y Al2 O3 MCM-41/MCM-48 SBA-15

2.0593 2.055 2.05 2.102 2.10

2.0471 2.055 2.05 2.103 2.10

1.9970 1.98 1.99 1.95 1.95

Adsorption of CO Activation in H2 S Adsorption of CO Evacuation at 200°C Evacuation at 200°C

[17] [32] [31] This work This work

An ESR signal could arise from several oxidation states of ruthenium, namely low-spin Ru‡ (4d7 ) and Ru3‡ (4d5 ) and high-spin Ru2‡ (4d6 ) ions. The ESR spectra of a high-spin Ru2‡ would be dicult to observe and is not consistent with the observed ESR spectrum in this study. A Ru0 (4d8 ) as well as a low spin Ru2‡ (4d6 ) ion would not give rise to an ESR spectrum [30]. Furthermore, the line intensity increases notably with the time of cooling to 196°C prior to the ESR measurement. Relaxation e€ects are more important in tetragonally distorted d7 electronic con®gurations than in d5 con®gurations owing to the higher degeneracy of the ground and the ®rst exited levels [31]. This would suggest the presence of Ru‡ in our samples. A similar signal has been observed under reductive conditions in the presence of H2 S in RuY zeolite (Table 2), but no assignment concerning the oxidation state has been made [32]. After evacuation at 300°C of RuHX, Lei and Kevan observe a species with g1 ˆ 2:17, g2 ˆ 2:08 and g3 ˆ 1:94, which is assigned to isolated Ru3‡ coordinated to the zeolite lattice [20]. However, it is unlikely that mesoporous materials with a high nSi =nAl ratio of 20 are able to stabilize highly charged cations such as Ru3‡ . Therefore, the sharp signal observed in this study is more likely due to coordinated or isolated Ru‡ . 3.3. UV±VIS spectroscopy Fig. 5 exhibits the UV±VIS spectra of 1.8 Ru/ Al-MCM-41 activated in vacuum. The freshly prepared sample (Fig. 5a) has a re¯ectance spectrum with a broad band centered at 300 nm 3‡ characteristic for Ru…NH3 †6 . A similar charge transfer band was observed by Alerasool for the freshly prepared complex supported on SiO2 and

Fig. 5. UV±VIS spectra of [Ru…NH3 †3‡ 6 ]/Al-MCM-41 after activation at di€erent temperatures.

the complex in solution (280 nm) and assigned to the r bonding of the NH3 ligands which are octahedrally coordinated to the centrally located ruthenium ion [33]. This complex is unstable in moist air. When the sample is kept in moist air, the color changes to pink and the formation of Ru-red [(NH3 )5 RuORu(NH3 )4 ORu(NH3 )5 ]6‡ is evident by band maxima at 390, 540 and 780 nm (not shown). When the colorless freshly prepared sample is heated in vacuum to 100°C, the color

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changes to yellow and the UV±VIS spectrum in Fig. 5b is obtained. A sharp maximum is found at 255 nm and a broad maximum is centered at 340 nm. A similar spectrum has been observed in RuNaY zeolite and assigned to a nitrosyl complex with the general formula: RuII (NO‡ )(NH3 )1;2 (OH,H2 O,Oz )4;3 [29]. It is not clear, however, whether oxygen containing ligands such as water or zeolitic oxygen (Oz ) are present in the coordination sphere of ruthenium. Upon heating to 200°C, the sample turns green and new maxima at 288, 389 and 706 nm are observed (Fig. 5c). An assignment of these bands from literature data is not unambiguous, but bands between 370 and 400 nm have been assigned to …RuII ‡ RuIII † ! p NO transitions [34]. The intensity of the broad band at 706 nm increases with prolonged treatment of the sample at 200°C accompanied by a color change to deep blue (Fig. 5d) which vanishes quickly and subsequently the sample turns gray. Due to the low water content of the hydrophobic mesoporous material, the hydrolysis of Ru…NH3 †3‡ to form 6 [Ru(NH3 )5 OH]2‡ and the subsequent formation of Ru-red are largely suppressed in our samples and the respective UV±VIS spectra were not observed. 3‡ The redox behavior of Ru…NH3 †6 in the supercages of X- and Y-zeolites has been extensively studied of over the last 20 years [21,26,28,29,35]. The thermal treatment in vacuum, oxygen, water and nitric oxide was investigated by UV±VIS, ESR, XPS and IR spectroscopy. Di€erent pathways of the ruthenium hexammine decomposition have been identi®ed. The materials investigated in this study are in several respects di€erent from the thoroughly studied zeolites. Due to the signi®cantly lower water content of the mesoporous materials under investigation, the decomposition of the precursor in vacuum does not proceed via the formation of [Ru(NH3 )5 OH]2‡ and Ru-red. The decomposition is somewhat similar to the ruthenium hexammine decomposition observed in zeolites in an oxygen atmosphere. There, the formation of Ru-red is also suppressed and nitrosyl complexes, viz. [Ru(NH3 )4 (NO)(OH2 )]3‡ and [Ru(NH3 )4 (NO)(OH)]2‡ are identi®ed by IR and UV±VIS spectra [29]. Similar UV±VIS bands are also observed in our study (Fig. 5b). As evident

391

from our TG-MS results, ammonia and NO are expelled in the course of the ruthenium complex decomposition and stepwise reduction of Ru3‡ via Ru2‡ and Ru‡ to ruthenium metal can be assumed. The simultaneous appearance of a sharp ESR signal and a characteristic UV±VIS band at 706 nm (green±blue region) was to our knowledge not observed before. However, these two experimental observations are not necessarily provoked by the same chemical species. While it is well known that Ru2‡ species display intense blue colors, Ru2‡ species are often ESR silent. The decreased stability of the Ru3‡ complexes on mesoporous supports as evident from the TG-MS curves, may as well result in a more concerted decomposition accompanied by the formation of previous not observed species. Further studies employing IR and Q-band ESR spectroscopy are underway to allow an unambiguous assignment of the novel species observed in the present study. 3.4. Hydrogen chemisorption Hydrogen chemisorption was employed to estimate the dispersion of the ruthenium and, hence, the cluster size after hydrogen reduction of the samples previously activated at 300°C. Hydrogen adsorption isotherms at 40°C of di€erent ruthenium-containing MCM-48 samples are depicted in Fig. 6. The Ru/MCM-48(c) samples prepared by impregnation with Ru3 (CO)12 adsorb a large

Fig. 6. Hydrogen adsorption isotherms at 40°C of rutheniumcontaining MCM-48 materials after dehydration at 300°C and subsequent hydrogen reduction at 400°C (p ˆ 40 kPa).

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amount of hydrogen, while the hydrogen adsorption of pure MCM-48 is negligible. Enhancing the ruthenium loading by a factor of 2 results in a roughly doubled hydrogen adsorption capacity, which con®rms a very similar metal dispersion in both samples. The samples prepared by ion exchange with RuCl3 or addition of this salt to the synthesis gel did not adsorb a large amount of hydrogen, which indicates a rather low metal dispersion. A comparison of the results of the hydrogen adsorption experiments for rutheniumcontaining MCM-48, MCM-41 and SBA-15 materials is presented in Table 3. The metal dispersions were calculated for a dissociative adsorption of hydrogen with a H:Ru ratio of 1. The ruthenium cluster sizes were computed for a cubic particle with ®ve sides available for hydrogen adsorption. Impregnation of MCM-48 with Ru3 (CO)12 results in the formation of small ruthenium clusters of about 1 nm. Similar cluster sizes were observed by Asakura et al. in their EXAFS studies on Ru/SiO2 prepared with Ru3 (CO)12 as a precursor [36]. It is most likely that the ruthenium clusters are predominantly located in the three-dimensional channel system of MCM48. The (auto)reduction of Ru(NH3 )6 Cl3 supported on Al-MCM-41 yields small ruthenium clusters in agreement with studies of Alerasool et al. on Ru/SiO2 prepared with the same precursor [33]. Presumably, the clusters are also located in the channels of Al-MCM-41. Interestingly, the cluster size found for ruthenium dispersed on SBA-15 in this study is considerably larger than the results obtained on Ru/SiO2 but smaller than the clusters found in Ru/MCM-41 [33,37]. The estimated cluster size would still allow for these

clusters to be in the channels of SBA-15. However, the TG-MS data already indicated a low dispersion of the ruthenium hexammine precursor, which in turn results in the formation of large ruthenium metal clusters. The decrease in ruthenium metal dispersion with increasing ruthenium loading from 1.8 to 4.0 wt.% in MCM-41, indicates the formation of a bimodal size distribution of ruthenium clusters in the channel system and on the outer surface of the Al-MCM-41 crystals. The use of RuCl3 as a precursor, however, does not yield highly dispersed ruthenium. This might be a consequence of the agglomeration of undissolved RuCl3 crystals on the outer surface of the MCM-48 particles during synthesis or ion exchange. The subsequent reduction results in the formation of larger ruthenium clusters most likely located on the outer surface of the MCM-48 particles. Coman et al. reported recently that using the hexammine chloride complex allows a better penetration of ruthenium into the mesopores of MCM-41 than RuCl3 and ruthenium acetylacetonate [16]. The results of the hydrogen adsorption experiments were con®rmed by powder XRD and TEM measurements. Only the samples prepared with RuCl3 as a precursor exhibit X-ray re¯ections in the range of 2h from 36° to 46° indicating ruthenium clusters with diameters larger than 4 nm [12]. 3.5. Transmission electron microscopy The determination of the ruthenium cluster size by hydrogen adsorption is widely accepted, however, in the literature some problems and inconsistencies concerning the conditions for saturation

Table 3 Metal dispersions and cluster sizes for di€erent ruthenium-containing samples Sample

Precursor

Hydrogen adsorbed (lmol g 1 )

Dispersion (%)

Cluster size (nm)

1.0 1.2 1.1 2.0 1.8 4.0 2.0

RuCl3 RuCl3 Ru3 (CO)12 Ru3 (CO)12 Ru(NH3 )6 Cl3 Ru(NH3 )6 Cl3 Ru(NH3 )6 Cl3

11 6.4 50 90 93 80 36

23 11 92 87 98 41 36

4.1 8.5 1.0 1.1 1.0 2.3 2.5

RuMCM-48 Ru/Al-MCM-48 Ru/MCM-48(c) Ru/MCM-48(c) Ru/Al-MCM-41 Ru/Al-MCM-41 Ru/SBA-15

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of a hydrogen monolayer corresponding to Ru:H ˆ 1 are reported [38]. Consequently, the reliability of hydrogen chemisorption measurements of ruthenium clusters sizes seems to be a little doubtful and should always be con®rmed by TEM. Fig. 7 shows the TEM micrographs of the 1.8 Ru/Al-MCM-41 sample. Fig. 7a shows the intact channels of the MCM-41 support, while Fig.

393

7b exhibits the same sample where the support is partially destroyed by the electron beam to allow a better visualization of the metal particles. A rather sharp particle size distribution is found. The mean particle size corresponds to 2 nm, which is somewhat larger than the cluster sizes obtained from the hydrogen chemisorption data. However, one should notice that the majority of the metal particles seem to be inside the pores. A better agreement was found for the MCM-48 samples prepared using Ru3 (CO)12 as a precursor (not shown). The mean particle size is below 1.5 nm and the metal particles are equally distributed on the support. 4. Conclusions Ruthenium metal clusters deposited inside the porous of the mesoporous molecular sieves MCM41, MCM-48 and SBA-15 can be prepared form di€erent precursors, viz. ruthenium hexammine chloride, ruthenium carbonyl and ruthenium chloride. The latter precursor yields rather large clusters (d > 4 nm) most probably located outside the channels of MCM-41 and MCM-48, while the use of Ru(NH3 )6 Cl3 and Ru3 (CO)12 results in the formation of rather small cluster (d ˆ 1±2 nm), most probably located in the channels of the mesoporous materials. The autoreduction of 3‡ Ru…NH3 †6 has been studied in detail by TG-MS, UV±VIS and ESR spectroscopy. The ammonia ligands are released from the complex between 150°C and 300°C in vacuum or ¯owing helium accompanied by a stepwise reduction of the ruthenium cations from Ru3‡ to Ru metal via Ru2‡ and Ru‡ . In contrast to the results obtained for several zeolites, an intermediate formation of [Ru(NH3 )5 OH]2‡ and Ru-red was not observed, but several NO containing species have been identi®ed. Acknowledgements

Fig. 7. TEM micrographs of 1.8 Ru/Al-MCM-41 after (a) short and (b) prolonged exposure to the electron beam.

Financial support is gratefully acknowledged by Deutsche Forschungsgemeinschaft (DFG), Deutscher Akademischer Austauschdienst (DAAD)

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