Bimetallic Supported Catalysts from Single-source Precursors - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 66

in a reaction catalyzed by a chiral cluster and experiments using para-
hydrogen (p-H 2 ) NMR methods have given more evidence for cluster species
acting as homogeneous catalysts. 29
Clusters have also been used in catalysis as immobilized complexes. The
most famous approach developed in this area is the so-called 'ship-in-a-bottle'
technique, where the cluster is built inside the super-cages of zeolites and
becomes entrapped due to its large size. Moreover, it has been found that
cluster compounds react at the surface of common industrial heterogeneous
catalysts supports. Indeed, inorganic oxides, such as silica and alumina,
present hydroxyl groups while carbonaceous supports also present oxygen
surface groups such as carboxylates, ketones and phenols. The nature of
surface species formed after reacting clusters with solid supports has been
mainly elucidated spectroscopically, by infra-red and EXAFS (extended X-ray
absorption fine structure spectroscopy), but also by solid-state NMR, XPS and
M¨ssbauer. 35 The main surface reactions that have been identified are (i) ion-
pairing with surface cationic centers, (ii) weak interaction with surface oxy-
genated groups, (iii) oxidative addition into an O-H bond giving surface hy-
drides, (iv) nucleophilic or electrophilic attack on a ligand of the cluster,
(v) cleavage of M-C bonds of the cluster by surface groups and formation of Si,
Al-O-M bonds, for example, or (vi) abstraction by surface groups of H atoms
bonded to the metal. The support enters the coordination sphere of metal
atoms in the clusters and can thus be considered as playing a ligand role. The
catalytic activity of the grafted molecular cluster will therefore be different to
the starting cluster applied in homogeneous phase. The support might play
electronic or steric effects and affect the selectivity. Alternatively, common
industrial supports have been found to be favoring clusters surface-mediated
syntheses. Neutral clusters are obtained on silica, while anionic clusters are
obtained on basic supports such as MgO or silica þ Na 2 CO 3 or K 2 CO 3 . 36
Usually, a suitable metal salt is impregnated on the support, followed by re-
ductive carbonylation in the solid state. This has allowed the preparation of
several immobilized homometallic Pt, Rh and Ir clusters, as well as bimetallic
clusters. These supported molecular clusters can be used as heterogenized
catalysts. 37 However, by far the most widely-spread application of cluster in
catalysis is their use as precursors of well-defined nanoparticles, which will be
developed in more details in the next section.

d n 9 r 4 n g | 5

.

3.3.4 Clusters as Precursors of Supported Bimetallic
Nanoparticle Catalysts
3.3.4.1 Introduction
This idea of using clusters as precursors of nanoparticles is the same as
developed above for heterometallic complexes: having a preformed pre-
cursor that once adsorbed on a chosen support can be treated to yield the
active phase in a controlled manner. In this case, the molecular cluster itself
is not the catalytically active phase but only a well-defined parent species.