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(a) Cluster synthesis
Ligands
d
n
9
r
4
n
g
|
5
Metallic atoms
(b) Immobilization on
solid
(d) Supported
nanoparticles
M
1
+ M
2
Activation
(e) Catalytic
testing
M
1
+ M
2
support
support
(f) Characterization
(c) Characterization
Figure 3.5 General methodology of supported nanoparticles preparation from
molecular clusters.
This is now known since a long time, and many topic chapters and reviews
have appeared on the subject.
31,38-47
The general experimental methodology involves the following steps
(Figure 3.5): (a) synthesis of clusters with appropriate ligands to confer them
desired solubility, (b) deposition of clusters on selected support, usually by
wet impregnation where the cluster in soluble form is contacted with sus-
pended solid support, (c) characterization of the immobilized species to
confirm its intact nature or identify possible surface reaction that might
have occurred, (d) transformation of the molecular cluster into a 'naked'
activated supported nanoparticle, (e) catalytic evaluation and (f) character-
ization before and after catalysis. The characterization has relied heavily on
infrared spectroscopy and EXAFS, but also XPS, powder XRD, solid-state
NMR and TEM microscopy. This latter technique is very useful to determine
the size of the nanoparticle obtained. One needs to confirm if indeed the
obtained supported active phase is nanostructured with some memory effect
of the cluster precursor.
Clusters compared to mononuclear complexes or heterometallic dimers
have the advantage of presenting metallic atoms already assembled in a
discrete entity (the metal core). Moreover, if using carbonyl clusters, the
metal oxidation state is usually low or even zero, hence the activation step
needs only to remove the ligand layer without the need for a reducing agent.
This ligand removal step can usually be carried out thermally under mild
conditions in an inert atmosphere, therefore avoiding metal sintering and
rearrangements. This leads to much smaller nanoparticles, ideally of the
.
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