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Bimetallic Nanoparticle Catalysts Anchored Inside Mesoporous Silica.

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tion prior to thermolysis. A loading corresponding to about 0.2
occupancy of the internal volume (127 mg PPN-2 per 427 mg
MCM-41) was used for both the microscopy and catalysis experiments (PPN = "(PPh,),).
The selected anionic bimetallic cluster 2 was fully characterized before deposition inside the mesoporous silica. According
to a single crystal X-ray analysis of [Ph,As],-2 (Figure 1) the
dianion is composed of two Ru,C(CO),, units linked by three
Bimetallic Nanoparticle Catalysts Anchored
Inside Mesoporous Silica**
Douglas S. Shephard, Thomas Maschmeyer,
Brian F. G. Johnson,* John Meurig Thomas,"
Gopinathan Sankar, Dogan Ozkaya, Wuzong Zhou,
Richard D. Oldroyd, and Robert G. Bell
There has been considerable interest in the formation, structure and further exploitation of the catalytic activity of bimetallic particles ever since Sinfelt et al.['] demonstrated the powerful,
catalytic, reforming properties of alumina-supported Ru- Cu,
Pt-Ir, and Pt-Re ensembles. With the advent[2,31of readily
preparable mesoporous solids possessing pore diameters in the
]
"singlerange 25 to 100 8, and the f e a ~ i b i l i t y [ ~of- ~inserting
site" centers that are catalytically active (such as Ti'" ions anchored through Si-0 bonds) in atomically well-defined locations inside such mesopores, the incentive for extending such
strategies to the insertion of bimetallic nanoparticles is high.
One of the key aims in this work has been the design and production of discrete, supported bimetallic particles of well-defined and tuneable atomic composition; another is to secure the
clusters to the support in such a way that they are prevented
from sintering.
In this communication we report: a) the synthesis and characterization of a novel Ag,Ru,, complex anion [Ag,Ru,,C,(CO),,C1]2- (2) from Ag' ions and [Ru,C(CO),,]~- (1); b) the
adsorption of 2 onto the inner walls of the mesoporous silica
(MCM-41), with a pore diameter of about 30 A,['] together with
spectroscopic proof that it retains its integrity; c) the subsequent
thermolytic conversion of the bound complex [Ag,Ru,,C,(CO),,:'Jl][AsPh,], (2) into discrete nanoparticles that have been
shown (by bright-field and annular dark-field high-resolution
electron microscopy and extended X-ray absorption fine structure (EXAFS)) to be firmly anchored inside the siliceous mesopores; and finally d) the performance of the nanoparticles as a
catalyst for the hydrogenation of hex-I-ene.
Several important criteria influence the choice of bimetallic
cluster precursor for the production of supported nanoparticles.
First, the protective sheath surrounding the organometallic precursor must be readily removable (for 2 mild thermolysis is
sufficient to activate and secure the bimetallic particles). Second, to ensure an even distribution of the precursor over the
surface of the support the sheath must undergo favorable interactions with the functional groups on the surface, the solvent,
and the counterion. The interaction with the surface must be
stronger than that involved both in solvation and between the
precursor species, so that aggregation into small molecular crystallites on the surface is suppressed, as these may sinter on
removal of the CO sheath. Anionic carbonyl clusters[*]fulfill
these criteria; their interaction with the MCM-41 surface is between the Si-OH group of the surface and the OC-M group,[g1
and the intermolecular coulombic repulsion prevents aggrega[*] Prof. B. F. G. Johnson, Dr. D. S. Shephard, Dr. W. Zhou
The University Chemical Laboratories
Lensfield Road, Cambridge CB2 IEW (UK)
Prof. Sir J. M. Thomas, Dr. T. Maschmeyer, Dr. G. Sankar,
Dr. R. D. Oldroyd, R. G. Bell
Day-Faraday Research Laboratories, The Royal Institution of Great Britain
21 Albemarle Street,
London, WIX 4BS (UK)
[**I
Dr. D. Ozkaya
Deptartment of Materials Science, University of Cambridge
This work was supported by a ROPA award and rolling grant from the EPSRC
(UK) to J. M. T., a EPSRC post-doctoral research award to B. F. G. J. and
a EU Fellowship (T. M.).
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0 WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1997
Figure 1. The molecular structure of 2 in the crystal. Top: the full dianion; bottom:
Ag,Ru,,C, core. Selected bond lengths [A] and standard deviations (in parentheses): RuO Rul2.7753( lo), RuO- Ru2 2.8580(1O ) , RuO -Ag2 2.8587(10), RuO -Agl
2.9645(10), RuO-RU~ 3.1078(10), R u I - R u ~ 2.7877(11), Rul -Ru2 2.8296(10),
Ru2-Ru3 2.8173(11), Ru2-Agl 2.8411(10), Ru2-Ru4 2.8922(11), Ru3-Ru4
2.7708(10), R ~ 4 - A g l 2.8366(10), R ~ 4 - A g 2 3.0507(10), R u S - R U ~2.7795(3 I),
Ru5-Ru7 2.8669(10), Ru5-Ag3 2.9646(10), Ru5-Ag2 3.0613(10), Ru5-Ru9
3.0756(11), Ru6-Ru8 2.7633(11), Ru6-Ru7 2.8534(10), Ru7-Ru8 2.8173(12),
Ru7-Ag3 2.8419(10), Ru7-Ru9 2.8852(10), Ru8-Ru9 2.8061(10), Ru9-Ag3
2.8279(11), Ru9-Ag2 2.9181(10), Agl-Cl(1 2.501(2), Agl-Ag2 2.8199(11),
Agl -Ag3 3.3498(10), Ag2-Ag3 2.7949(10), Ag3-CI1 2.486(2).
-
silver atoms in a triangular arrangement, which is in turn
bridged by a chlorine atom. The Ru-Ru contacts in the
bimetallic core lie in the unusually broad range between
2.7633(10) and 3.1078(10) (cf. RU,C(CO),,),['~~the two
longest distances arising between two basal ruthenium atoms
bridged by silver atoms [RuO-Ru4 3.1078(10) A, Ru5-Ru9
3.0756(10) 211. Consequently the geometries of the two pentaruthenium moieties of the anion are distorted from those of
regular square pyramids. The silver atoms occupy two distinct
environments (see Figure 1) and form an isosceles triangle with
two short sides and one long side, which is bridged by the chlorine atom [Agl-Ag2 2.8199(11) A, Ag2-Ag3 2.7949(10) A,
Agl -Ag3 3.3498(10) A].
The chemical integrity of the cluster accommodated inside the
MCM-41 was established by IR (Nujol mull) and EXAFS spectroscopies (Table 1). The major bands in the infrared spectrum
were somewhat broader and shifted to lower energy (by about
3 cm-'), which may be due to interactions with the pore wall.
The structural parameters extracted from the EXAFS analysis
(Ru and Ag K-edges) correspond well to those established from
the single crystal X-ray structure.
Activation and anchoring of adsorbed 2 on the MCM-41
support was achieved by heating the sample under dynamic
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Angew. Chem. Int. Ed. Engi. 1997, 36, No. 20
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in situ in a custom made
The K-edge EXAFS data,
corresponding Fourier transformations, and calculated fits for
Ru and Ag are given in Figure 2. The data were modeled bearing in mind the propensity of the bimetallic particle to contract
and rearrange on losing its CO sheath. The structural parameters extracted from the analysis of the EXAFS spectra are summarized in Table 1. The Ag K-edge EXAFS data establish the
bimetallic nature of the cluster, since, in addition to Ag-Ag
interactions at 2.92 A, the metal-metal distance of 2.81 k, in
between the bulk metal-metal distances for Ru-Ru (2.65 A)
and Ag-Ag (2.92 A) is consistent with a Ag-Ru contact. RuRu scattering is the dominant feature in the Ru K-edge data and
the inclusion of a Ag-Ru shell does not result in a statistically
meaningful improvement. However, we have been able to refine
one combined model to both Ru and Ag K-edge data sets simultaneously using the XFIT (for Windows 95) suite of programs." 71 This procedure clearly confirms the presence of AgRu contacts and establishes beyond doubt the bimetallic nature
of the particles. This result highlights the importance and need
of EXAFS measurements of both metals in bimetallic systems,
as concentration on the Ru data only would not have been able
to establish the bimetallic nature of the particles. In addition,
the simultaneous two edge data refinement represent a major
step forward in the structural elucidation of particles for which
one scatterer dominates.
In addition to the metal-metal contacts, there is clear evidence of the light scatterer oxygen, in the case of Ag at a distance
of 2.30 A (close to a covalent Ag-0 bond) and in the case of Ru
at a distance of 2.53 A (most likely associated with a surface
oxygen, as such anchoring metal-0 distances have been observed in other supported catalysts and zeolitic materialsc121).
These structural features are consistent with the model of the
carrier-bound bimetallic cluster shown in Figure 3, which is
about 12 A across. It should be noted that the interatomic dis-
Table 1. Local structural parameters from Ag and R u K-edge EXAFS for MCM41-supported 2.
Absorber
0.66
1.33
2.87
0.66
2.43
2.69
2 83
3.36
0.002
0.004
0.004
0.010
2.4
1.3
1.o
3.2
1.o
3.6
1.91
1.89
2.06
2.81
2.89
3.07
0.002
0.002
0.002
0.004
0.002
0.003
0
2.0
2.2
1.o
2.81
2.92
2.29
0.006
0.010
0.003
Ru*
0
5.2
2.0
2.65
2.48
0.007
0.002
2.53
2.66
2.80
0.002
0.002
0.002
2.30
2.80
2 93
0.002
0.002
0.002
C
C
C
Ru
Ag
0
After thermolysis
Ag
Ru
Ag
Ru
Two-edge refinement
Ru
0
Ru
Ae
1.8
4.6
1.o
1.1
0
Ru
Ag
Ag
[A*]
R
Before thermolysr.~
Ag
C1
Ag
Ru
A&?
Ru
I&
N
Scatterer
1 .o
1.5
a2
[*I
Averaged environment; the Ru- Agshell is statistically not significant due to the
dominant Ru-Ru environment.
vacuum
Torr] for 1 hr at 473 K, during which the sample
turned from pink to grey. Upon thermolysis, the CO-stretching
region in the IR spectrum became featureless, and the interatomic distances as well as the coordination numbers changed
dramatically as detcrrnined by EXAFS measurements obtained
-v
4
.5
7
6
k IA-' I
x
9
I0
I4
~
I2
-
(4
10 -
8-
rlAl---+
f
-
14
Ru
Figure 2. ExAFS data for 2 after thermolysis (Ag K-edge (a) and Ru K-edge (b)),and their Fourier transforms (Ag K-edge (c) and
K-edge (d))
Solid h e s represent observed data, and dashed lines the fitted curves. The fits shown are obtained from simultaneous two-edge refinement.
AngeN,. Chrm. fnr. Ed. En@ 1997, 36, No. 20
0 WILEY-VCH Verlag GmbH, D-69451 Wemheim, 1997
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Figure 3. A possible structure of nanoparticles of the bimetallic catalyst anchored
inside MCM-41 generated from 2 by thermolysis (see text).
tances of the metal atoms are larger than expected for small
metal particles and approach those of the bulk metals. This may
be due to the surface interaction with the particle and their
bimetallic nature. Different starting clusters can yield contractions,[' 31 possibly pointing to different particle shapes (for example, spherical versus flat, yielding contraction and expansion,
respectively).
Conventional high-resolution electron microscopy (JEOL
200CX TEM microscope) of the heat-treated material yielded
the bright-field image shown in Figure 4, viewed perpendicular
Figure 4. A bright-field HRTEM micrograph of 2 after heat-treatment showing
alignment of particles (about 15 8, inddiameter) inside the channels.
to the pore axis. The uniform distribution of the bimetallic
nanoparticles, aligned along the channels, is clearly discerned.[14] A scanning transmission electron microscope
(STEM, Vicuum Generators HB.501 equipped with a field-emission gun) yielded the high-resolution, annular-dark-field (socalled 2 contrast) image (Figure 5, top left) and the bright field
image (Figure 5, top right). Electron-induced X-ray emission
maps (Figure 5, bottom) further confirm the spatial uniformity
of the distribution of the nan~particles.['~]
The catalytic performance of the activated, supported
bimetallic particles was tested for hydrogenation of hex-I-ene to
hexane. Initial experiments show a high selectivity (in excess of
99 %) and a turnover frequency of at least 6300 mol hexane per
mol [Ag,Ru,,] per hour.['61
Experimental Section
Synthesis of 2: Reaction of [Ru&(CO),J [I81 with AgBF, (3 equiv) and
(Ph,),NCI (PPN-CI, 1 equiv) or (Ph,),AsCI (TPAC1) in dichloromethane gave a
ruby red solution within 10 min. The reaction was monitored by infrared spectroscopy in the G(C0) region, which showed the growth of a new set of bands at 2053
(s), 2024 (vs), 2001 (vs), and 1830 (br s) cm-'. Addition of a small amount of
hexane/ether gave a deep red precipitate showing the same profile in the S(C0)
region. In a similar synthesis in which no C1- was added, a similar red solution was
2244
0 WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1997
OK
Si K
Ru L
AdFigure 5. High-resolution annular dark-field ( Z contrast) and bright-field scanning
transmission electron micrographs (top left and right, respectively) of adsorbed
PPN-2 in MCM-41 after heat treatment at 150 "C for 1 h. Bottom: Electron-induced
X-ray emission maps of the same sample.
produced; however, the resulting compound has proved difficult to isolate and
characterize. Positive fast atom bombardment (FAB) mass spectrometry also
showed only peaks assignable to the presence of PPN or TPA', whilst negative
FAB mass spectrometry of the PPN salt gave broad peaks at m / z 2718,2550,2178,
and 2011. The highest peak was tentatively assigned to {[Ru,C(CO),,],Ag,CI.
PPN}- (calcd 2717), whilst the peak at 2178 corresponds to {[Ru,C(CO),,],Ag,CI}~ (calcd 2178). Elemental analysis gave 31.18% C, 1.20% H,
0.00% N (calcd for C,,H,,Ag,As,CI,O,,Ru,,
gives 31.81% C, 1.37% H).
Crystals of the TPA salt of 2 suitable for a single crystal X-ray diffraction study were
grown from a solution mixture of dichloromethane/ethylacetate into which pentane
was allowed to diffuse at about 253 K. Crystal data for [Ag,Ru,,C,M = 3039.63, triclinic, space
(CO),,CI][Ph,As],: C,,H,,Ag,As,CI,O,,Ru,,,
group Pi, a =14.6991(5), b = 26.1473(8), c =12.7729(4) A, CL =102.146(2),
3
/ = 112.439(3), y = 82.522(2)", U = 4429.1(2)
pFrlfd= 2.272 gcm-', Z = 2,
Mo,, radiation, 1 = 0.71069 A, p = 3.206 mm-', T = 153(2) K. Data were collected on a Rigaku AFC7 image plate diffractometer for an RS 3000 coated, rapidly
cooled crystal ofdimensions 0.40 x 0.30 x 0.10 mm, mounted directly from solution
(T. Kottke, D. Stalke, J. Appl. Crystallogr. 1993, 26, 615) by the Q/w method
(3' <2 6 < 50"). Of a total of 23085 reflections collected, 13750 were independent
(Rin,= 0.0642). The structure was solved by direct methods (SHELXTL PLUS,
SHELXL-93, Gottingen Univeristy, 1993) and refined by full-matrix least-squares
analysis on FZ with R,(F>4u(F)) = 0.0603 and wR,(all data) = 0.1724. H atoms
were placed in calculated positions and allowed to refine riding on their C atoms.
.
Largest peak/hole in final difference map: 3.6771 - 2.085 e k 3Crystallographic
data (excluding structure factors) for the structure(s) reported in this paper have
been deposited with the Cambridge Crystallographic Data Centre as supplementary
publication no. CCDC-100617. Copies of the data can be obtained free of charge on
application to The Director, CCDC, 12 Union Road, Cambridge CB2 IEZ, UK
(fax: int. code + (1223)336-033; e-mail: deposit@chemcrys.cam.ac.uk).
Adsorption of PPN-2 into MCM-41: PPN-(2) was slurried in ether with MCM-41
(3: 10 m/m)for 411 h in which time the white mesoporous silica became pink in color.
The ether was removed by filtration and the solid dried under high vacuum
[0.01 mmHg] at ambient temperature.
HRTEM of activated 2: The dry sample ground in air was deposited on a copper
grid with a carbon film, transferred to a Jeol TEM-200CX electron microscope
operating at 200 kV, and kept in the microscope vacuum overnight. The images were
recorded at 49 000-fold magnification.
The EXAFS data were analyzed with the XFIT (for Windows 95) [17] and
EXCURVE suites of programs. The simultaneous refinement of two edges was
performed with XFIT (WIN95). The S,"parameters were taken from the single-edge
refinements and held constant. The u2 parameters were constrained to be equal for
the silver and ruthenium atoms. Two E,, parameters were included in the model, one
for silver and one for ruthenium. All parameters (coordination numbers, distances,
E,, and uz values) were refined simultaneously. The precise fitting protocol and
theoretical underpinning will be published elsewhere.
Hydrogenation of hex-I-ene: A 200mL teflon-lined autoclave equipped with a
magnetic follower was charged with catalyst (20 mg). hex-1-ene (3.0 mL), and H,
(65 atm). The ensemble was heated to 393 K for 4 h, after which the vessel was
cooled to ambient temperature, and the contents were analyzed by 'HNMR to
reveal greater than 99% conversion to n-hexane.
Received: March 12, 1997 [Z10233IE]
German version: Angew. Chem. 1997, 109,2337-2341
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Angew. Chem. Int. Ed. Engl. 1997,36, No. 20
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-
Keywords: clusters * electron microscopy
EXAFS spectroscopy . heterogeneous catalysis mesoporosity
-
[11 J. H. Sinfelt. Bimeto/lic Cufulysts, Wiley, New York, 1983, and references
therein; J. H. Sinfelt. Int Rev. Phys. Chem. 1988, 7, 281.
[2] J. S . Beck, J. C. Vartuli, Curr. Opinion Solid State Muter. Sci. 1996, I , 76.
131 D. A. Antonelli, J. Y. Ying, Curr. Opinion Colloid Sci., 1996, 1, 523.
[41 T. Maschmeyer. F. Rey, G . Sankar, J. M. Thomas, Nature, 1995, 378, 159.
[5] R. D. Oldroyd, J. M . Thomas, T. Maschmeyer, P. A. MacFaul, D. W Snelgrove. K. U . Ingold, D. D. M. Wagner, Angew. Chem. 1996,108,2966; Angew,.
Chem. In!. Ed. Engl. 1996, 35, 2787.
[6] J. M. Thomas. Furczduv Discuss. 1996. 105, 1
[7] D. Ozkaya, J. M. Thomas, W. Zhou, unpublished.
[S] M. A. Beswick, Dissertation, University of Cambridge, 1992, and references
therein; M. A. Bes.vick. J. Lewis, P. R. Raithby, M. C. Ramirez de Arellano,
Angew. Cheni. 1997. 108, 303; Angew. Chem. Inf. Ed. Engl. 1997, 36, 291
191 S. J.Taverner. J. H.clark, G. W.Gray, P. A.Heath, D. J.Macquarrie, J Chem.
Soc. Chem. Commtm. 1997, 1073
[ l o ] C. R. Eady, B. F Ci Johnson, J. Lewis, J Orgunornet. Chem. 1979,57, C82.
[ I l l P. A. Barrett. G. Sknkar, C. R. A. Catlow, J. M. Thomas, J PhysChem. 1996,
100, 8977.
A
~ Chem
~ I n / ~Ed EngL
~
1997.36,
~
. No
20
G
[I21 M. Vaarkamp, J. T. Miller, F. S. Modea, D. C. Koningsberger, J. Catul. 1996,
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results.
~ 3 Unpublished
1
[I41 The sample was unstable under the electron beam, and therefore images were
recorded at low magnification. In this case no structural fringes from the
particles could be discerned. and the size of the black dots (about 15 %.)do not
necessarily correspond to the size of the real particles.
[I51 The Z-contrast image is formed from electrons scattered through wide angles
(50 to 250 mrad). Such an image is formed from electron beams that have
undergone Rutherford scattering, in which the intensity of the signal is proportional to Z 2 ,where Zi s the atomic number of the scattering atom. A. V. Crewe,
J. Wall, J. Mol. Biol. 1970, 48, 375.
[I61 This compares very favorably with a TOF of 250 h- ' given in E. Linder,
M. Haustein, R. Fawzi, M. Steinmann, P. Wegner, Organometallics1994, 13,
5021 for homogeneous ruthenium hydrogenation catalysts under similar conditions.
[17] XFIT (for Windows95). P. J. Ellis, H C.Freeman, J Si.nchroton Rudiution
1995, 2, 190.
[18] B. F. G. Johnson, J. Lewis, W. J. H. Nelson. J. N. Nicholls, J Puca. P. R. Raithby, M. J. Rosales, M. Schroder, M. D. Vargas. J Chem Sac. Dalton Trans.
1983. 2447.
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