Bimetallic Supported Catalysts from Single-source Precursors - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 82

EDX spectroscopy indicated that the composition of the starting nano-
particles (50% Pd-50% Au) was retained in the supported catalyst, with high
compositional uniformity. This ratio could easily be modified by changing
the amounts of salts complexed originally within the dendrimer. The cata-
lytic activity for CO oxidation was higher for the bimetallic catalyst prepared
from dendrimer-encapsulated nanoparticles than for the corresponding
Pd-only and Au-only monometallic materials.

d n 9 r 4 n g | 5

3.4 Conclusions and Future Challenges
The general concept developed in this chapter is that of using heteronuclear
coordination compounds (complexes) or clusters as precursors of supported
bimetallic nanoparticles. The precursor compound can be synthesized,
purified, and characterized in detail before being adsorbed on a given sup-
port. In some cases, the specific interactions between precursor and solid
surfaces have been studied extensively. The supported compounds are then
activated into heterogeneous catalysts, usually by thermal treatment, to re-
move the ligand shell and reduce the metallic atoms if necessary. The ad-
vantage of using organometallic (usually carbonyl) clusters over inorganic
complexes are that a preformed metal core exists that can lead to control
over size and composition of final nanoparticles if sintering can be avoided
during the activation step. In addition, the metals are usually already in a
zero-oxidation state. Many success stories have appeared in the literature
over the years, with sometimes formal proof of bimetallic nature of the
obtained nanoparticles, which are usually small and well-dispersed on the
support, together with improved catalytic performance when compared to
more conventional catalysts prepared from separated metal salts.
The botteleneck in this research area will always be large scale synthesis of
molecular well-defined transition metal complexes or clusters. Most of them
are air- and moisture-sensitive and can only be produced in small amounts.
Moreover, serendipity played an important role in obtaining new hetero-
metallic combinations and stoichiometries. Control and prediction in
cluster synthesis is not yet the rule. Bimetallic nanoparticles will play a key
role in future developments due to their relative easier synthesis. The
drawback of protected but free-standing nanoparticles being their recovery,
the immobilization of nanoparticles on a solid support will certainly con-
tinue to be a prized line of research, as initiated by Bonnemann with his
'precursor concept'. The remaining questions in this field are the nature of
nanoparticles/support interactions, as well as the need to remove the cap-
ping agents to boost the catalytic activity, with the risk of losing control over
size and composition at the nanoscale.
In the future, using ligand-stabilized colloidal nanoparticles as precursors
of tailored supported catalysts will certainly spread. Indeed, their syntheses;
although very sensitive to the experimental conditions and sometimes suf-
fering from a lack of reproducibility, are much easier than molecular clus-
ters, especially when upscaling is necessary. In addition, tuning metal

.