Preparation and characterisation of a highly active bimetallic (Pd-Ru) nanoparticle heterogeneous catalyst

Article abstract of DOI:10.1039/a901263j

The mixed-metal carbonylate cluster [Pd₆Ru₆(CO)₂₄]^2- was used as a single-source precursor in the synthesis of a highly active hydrogenation catalyst (stoichiometry PdRu) which has been characterised by electron microscopy and X-ray absorption spectroscopy: PdRu readily hydrogenates alkenes and naphthalene (the latter predominantly to cis-decalin) under mild conditions.

Full text of DOI:10.1039/a901263j

Preparation and characterisation of a highly active bimetallic (Pd–Ru)
nanoparticle heterogeneous catalyst†
Robert Raja,ab Gopinathan Sankar,b Sophie Hermans,a Douglas S. Shephard,a Stefan Bromley,b John Meurig
Thomas*b and Brian F. G. Johnson*a
a Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, UK CB2 1EW
b Davy Faraday Research Laboratory, Royal Institution of Great Britain, 21 Albemarle Street, London, UK W1X 4BS
E-mail: dawn@n.ac.uk
Received (in Liverpool, UK) 15th February 1999, Accepted 9th July 1999
The mixed-metal carbonylate cluster [Pd6Ru6(CO)24]22 was
used as a single-source precursor in the synthesis of a highly
active hydrogenation catalyst (stoichiometry PdRu) which
has been characterised by electron microscopy and X-ray
absorption spectroscopy: PdRu readily hydrogenates al-
kenes and naphthalene (the latter predominantly to cis-
decalin) under mild conditions
The anionic molecular precursor [Pd6Ru6(CO)24]22, 1, was
+
synthesised as described10 previously; and isolated as its NEt4
salt. This particular precursor was selected for a variety of
reasons including its solubility, stoichiometry, adsorbability at,
and dispersion across, the silanol rich11,12 interior surfaces of
the mesoporous silica support (MCM-41), as well as the ease
with which it sheds its cloak of carbonyl groups during mild
thermal treatment. Of prime importance also was our wish to
design a bimetallic catalyst in which a metal (Pd) that readily
takes up hydrogen is juxtaposed with one (Ru) that has a strong
tendency to bind arenes. A Pd–Ru catalyst should therefore
function effectively in the hydrogenation of aromatic mole-
cules.
Bimetallic nanoparticle catalysts occupy a position of high
prominence in modern heterogeneous catalysis, especially for
reactions of petrochemical significance.1–3 Published reports4–9
on such catalysts, typified by Pt–Re, Ir–Sn, Pt–Ru, Ag–Ru and
Cu–Ru, reveal that enhanced catalytic performance apparently
arises from the synergy between the component elements at the
nanoscale which is absent in solid solutions of the two bulk
metals.
The methodology for its preparation and encapsulation into
the mesopores is essentially that used in our earlier work7,9 on
Ag–Ru and Cu–Ru bimetallic nanocrystals. Retention of the
structural integrity of the mixed-metal cluster carbonylate
inside the mesoporous silica was deduced from in situ
spectroscopic analysis, both infra red9 (Nujol mull) and X-ray
absorption using a specially designed cell.13,14 The respective
data sets showed the same structural features as those of the
cluster when dispersed in homogeneous solution (tetra-
hydrofuran as solvent). The characteristic IR absorption peaks
were somewhat broadened and slightly shifted (ca. 3 cm21) to
lower energy upon encapsulation. Precise structural information
(Ru–Ru, Pd–Pd and Ru–Pd distances and associated co-
ordination numbers), retrieved from EXAFS analyses for Ru
and Pd K-absorption edges, was in good accord with that
obtained from the single-crystal X-ray structure10 of Et4N+ salts
of the anion 1.
In this communication we show that a single-source, mixed-
metal cluster carbonylate precursor yields (by gentle thermol-
ysis) a uniform distribution of discrete nanoparticles (ca. 17 Å
diameter) of a Pd–Ru bimetallic catalyst encapsulated within
the pores (ca. 30 Å diameter) of mesoporous silica. This catalyst
is highly active in the hydrogenation of linear alkenes and
significantly more so than two other bimetallic catalysts also
7
prepared from single-source carbonylate precursors, Ag3Ru10
and Cu4Ru12.9 Moreover, the Pd–Ru catalyst described here is
capable of hydrogenating naphthalene under relatively mild
conditions.
When the encapsulated carbonylate 1 was heated (1024 Torr)
for 1 h, at 180 °C, the sample changed colour from brown to
Fig. 1 (a) Fourier transform of the Pd K-edge EXAFS data (effectively a
radial distribution function) for the mixed-metal carbonylate ion
[Pd6Ru6(CO)24]
22 precursor dispersed inside the mesopores of the MCM-
41 silica. (b) The corresponding transform for the dispersed precursor after
gentle thermolysis (see text). The resulting cluster (see Fig. 2) has an
average metal–metal (Pd–Ru and Pd–Pd) distance that peaks at 2.73 Å and
the average co-ordination number is 5.3.
Fig. 2 Schematic diagram of the Pd6Ru6 cluster derived from an EXAFS
analysis of the Ru and Pd K-edge X-ray absorption spectra (it is difficult to
distinguish scattering by Pd atoms from that by Ru atoms).
† Electronic supplementary information (ESI) available. See http://
www.rsc.org.suppdata/cc/1999/1571.
Chem. Commun., 1999, 1571–1572
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Table 1 Hydrogenation of olefins—comparison of catalysts
Product distribution (mol%)
Substrate
(mass/g)
Reaction
time/h
Residual H2
pressure/bar
Conv. TOF/
(%)
cis-
Hex-2-ene
trans-
Hex-2-ene
Catalyst
Solvent
h21
n-Hexane
Hex-1-ene
( ≈ 50 g)
Pd6Ru6/MCM-41
Cu4Ru12/MCM-41
Ru6/MCM-41
—
—
—
4
4
4
24
4
24
24
1
8
99
56
13
19
6
4954
2805
325
277
250
196
—
68
51
14
10
6
22
30
42
36
45
33
32
9
19
45
53
48
63
67
15
13
18
16
17
Pd/MCM-41
No Catalyst
—
—
14
7
5
—
cis-
trans-
n-Dodecane Dodec-2-ene
Dodec-2-ene
Dodec-1-ene
( ≈ 50 g)
Pd6Ru6/MCM-41
Cu4Ru12/MCM-41
—
—
4
4
3
7
88
35
2202
877
63
54
29
32
7
13
trans-
cis-Decalin Decalin
Others
Naphthalene
( ≈ 8 g)
Pd6Ru6/MCM-41
Cu4Ru12/MCM-41
CH3CN
8
8
8
8
8
12
10
20
18
20
19
7
0
0.8
0
50
19
86
50
4
34
9
15
Hexadecane
Hexadecanea
CH3CN
No Reaction
—
No Reaction
2
—
100
Hexadecane
a
Reaction conditions: catalyst = 20 mg; T = 373 K; starting H2 pressure = 20 bar; solvent ≈ 55 g; 200 ppm of sulfur was added in the form of
benzothiophene.
black; the IR carbonyl stretching region gradually disappeared;
and the atomic structure of the mixed-metal cluster, as seen by
details of the XANES and EXAFS, changed dramatically (Fig.
1). High-resolution transmission electron microscopy
(HRTEM) revealed15 that the Pd–Ru bimetallic nanoparticles
were of uniform size (ca. 17 Å diameter) and spatially well
distributed within the pores of the siliceous support.
computations, we have arrived at the EXAFS model for the
structure of the bimetallic cage shown in Fig. 2.
We thank Professor C. R. A. Catlow for invaluable
assistance. We also thank the EPSRC for a rolling grant to
J. M. T., a ROPA award funding S. B. and a regular one to
B. F. G. J., the Commissioners of the 1851 Royal Exhibition for
an award to R. R., the European Commission for a grant to S. H.,
and the Royal Society and Peterhouse for a Research Fellow-
ship to D. S. S. G. S. was funded partly by J. M. T.’s rolling
grant and the CCRL Daresbury Laboratory, to whom we are
grateful.
The catalytic performance in alkene hydrogenation of the Pd–
Ru nanoparticles is compared in Table 1 with those of similarly
prepared (and sized) Cu4Ru12 bimetallic nanoparticles. The
kinetics of hydrogenation of hex-1-ene and dodec-1-ene reveal
that the Pd6Ru6 catalysts showed a higher selectivity for n-
hexane (or n-dodecane) than Cu4Ru12. The Pd6Ru6 catalyst is
more active than Cu4Ru12 for the hydrogenation of hex-1-ene
( ≈ 2 times) and dodec-1-ene ( ≈ 2.5 times). For comparison, a
monometallic Ru6 cluster was encapsulated in MCM-41 and a
Pd/MCM-41 catalyst was prepared following a literature
procedure,16 and both were tested for the hydrogenation of hex-
1-ene employing the same reaction conditions. It is very clear
from Table 1 that the bimetallic catalysts are far superior in
performance (% conversion) than their monometallic analogues
and more importantly yield a higher selectivity for hydro-
genated products, suggesting a possible synergism between the
two bimetallic nanoparticles. The Pd6Ru6 catalyst was more
effective than Cu4Ru12 in the hydrogenation of naphthalene and
higher conversions were obtained when acetonitrile was used as
a solvent. Other solvents such as hexadecane showed a lower
preference for the production of cis-decalin. The solid catalyst
may be recycled without any significant decrease in activity or
selectivity. This was done by filtering off the solvent–product
mixture and recharging the Parr reactor with fresh material17
(see Table 1 for reaction conditions). Unsurprisingly,1–3 the
introduction of ≈ 200 ppm of sulfur in the reaction mixture
completely poisons the catalyst.
Notes and references
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1998, 4, 1214.
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13 J. M. Thomas, Chem. Eur. J., 1997, 3, 1557.
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18 S. Bromley, C. R. A. Catlow et al., in preparation.
Surveys by electron-stimulated energy dispersive X-ray
emission17 of the Pd–Ru nanocatalyst particles after their use in
four consecutive test runs showed that there was no segregation
of the two components of the bimetallic catalyst. Moreover,
annular dark field (Z-contrast) high-resolution microscopy
showed15 that there was no evidence of coalescence or sintering
of the nanoparticles during catalytic use. Guided by energy-
minimisation procedures18 using density-functional theory
Communication 9/01263J
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Chem. Commun., 1999, 1571–1572