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same size as the starting cluster core. When using mixed-metal clusters,
heterometallic supported nanoparticles can be obtained with controlled
dimensions and compositions. Bimetallic particles alloyed at the nanoscale
can be obtained with metals that do not form alloys in the bulk phase. In the
present chapter, we will focus mainly on bimetallic nanoparticles obtained
from clusters and applied in heterogeneous catalysis.
d
n
9
r
4
n
g
|
5
3.3.4.2 Key Concepts
One main concept in the area of using mixed-metal clusters as precursors of
supported bimetallic nanoparticles has been to use bimetallic combinations
where one metal is very active in the catalytic reaction (usually a noble metal)
while the other has high anity for the support. Given that common sup-
ports are usually inorganic oxides, oxophilic metals are selected. The key
point is that the oxophilic metal will provide stabilization of the ensemble on
the surface and metal-support interactions, which will ensure small sizes of
the active nanoparticles. The noble metal will be highly dispersed permitting
lower loading and giving stable and reusable catalysts. For example, the
group of B. C. Gates showed that the clusters {Pt[W(CO)
3
(C
5
H
5
)]
2
(PhCN)
2
}
and {Pt
2
W
2
(CO)
6
(C
5
H
5
)
2
(PPh
3
)
2
} could be immobilized intact on MgO or
g-Al
2
O
3
.
48,49
Infrared spectroscopy and EXAFS mainly were very useful to
confirm the intact nature of the clusters compound after immobilization on
the support and to follow their fate during post-treatment. The molecular
clusters interact weakly by hydrogen bonding of their organic ligands with
surface hydroxyl groups of the supports. They could even be recovered by
extraction with an organic solvent. After thermal treatment under hydrogen
to remove the ligands, it was shown that Pt formed highly dispersed struc-
tures, stabilized by interactions with the more oxophilic metal that binds
strongly to the support.
Another point, as described above for heteronuclear complexes, is to
compare the systems obtained with mixed-metal clusters with more tradi-
tional catalysts prepared from separated metal salts or monometallic clus-
ters. This has been exemplified many times in the literature. Coming back to
the above-cited example,
48,49
the platinum dispersion and Pt-W interactions
were found to be much lower in catalysts prepared from mononuclear
analogous complexes. Another very interesting study carried out consecu-
tively by two groups concerns the Pt-Au immiscible system.
50,51
Bimetallic
Pt-Au/SiO
2
catalysts were prepared by co-impregnation of hexachloroplatinic
(H
2
PtCl
6
) and tetrachloroauric acids (HAuCl
4
) or from a Pt
2
Au
4
(C
ΒΌ
C
t
Bu)
8
cluster precursor with similar loadings. These were compared to mono-
metallic equivalents prepared by incipient wetness impregnation of the
same metal salts. The cluster-derived sample presented smaller average par-
ticle sizes (
.
3 nm) and more narrow size distribution than the two others, as
well as higher dispersion. EDXS analysis within the TEM (on a collection of
particles) showed the presence of Pt and Au in an approximate ratio of 1 : 2,
while the co-impregnated catalysts presented very large (410 nm) Au particles
B
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