Supported Molecular and Nano-scale Catalysts:
Precise Synthesis and Characterization in the Functioning State
Bruce C. Gates
Department of Chemical Engineering & Materials Science
University of California, Davis, USA
The typical industrial catalyst is a highly nonuniform set of nano-structures dispersed on a
porous, nonuniform support. In contrast, some supported catalysts, such as those used for olefin
polymerization, are much more nearly uniform, being molecular in character and offering the
prospective advantages of molecular catalysts in solution, such as high selectivity. Our goals were to
gain a deeper understanding of the class of oxide- and zeolite-supported metal catalysts with virtual
molecular properties. We have investigated mononuclear complexes and clusters of metals of groups
6, 7, and 8. A key to meeting the goal of structural uniformity is precise synthesis of the supported
species, by the methods of organometallic chemistry extended to surfaces. Keys to understanding of
the structures and properties of these materials are characterization by complementary spectroscopic
and microscopic techniques, even, when possible, with the samples in reactive atmospheres. The
characterization methods include vibrational, NMR, and X-ray absorption spectroscopies; TEM; and
DFT. The characterization results determine metal nuclearities, bonding of metals to supports, and
identification of intermediates bonded to the metals.
Results are presented for complexes of rhodium and of gold on oxides and zeolites and for
clusters of Ta, Re, Rh, Ir, Os, and Au on these supports. For example, complexes of Rh bonded to
dealuminated Y zeolite were prepared from Rh(C2H4)2(acac), giving supported Rh(C2H4)2 complexes
with each Rh atom bonded to two O atoms of the zeolite. NMR data demonstrate a uniformity of the
supported species nearly matching that of the precursor in the crystalline state. The brown Rh atom is
bonded to two ethylene ligands and to two (red) oxygen atoms of the zeolite support:
Supported Au complexes on zeolite NaY and on La2O3 formed from Au(CH3)2(acac) catalyze
CO oxidation at room temperature; La2O3-supported AuIII complexes are almost as active as the most
highly active supported gold catalysts. Investigation of supported Au catalysts containing mixtures of
zerovalent and cationic Au shows that the catalytic activity increases with the content of cationic Au.
These results cast doubt on attributions of catalytic activity to unique size-dependent properties of Au
nanoclusters.
Highly dispersed metal catalysts containing supported clusters of only several metal atoms
each, exemplified by Re3, Ir4, Ir6, and Rh6, were prepared by removal of CO ligands from supported
precursors (e.g., Ir4(CO)12, Rh6(CO)16) EXAFS spectra and DFT indicate metal–support-oxygen
bonding and cations at the metal–support interface, helping to stabilize the metal dispersion. The
supported clusters are not bare; rather, they are stabilized by ligands such as hydrides.
Supported catalysts prepared from precursors with oxophillic metals consist of rafts of isolated
cations, exemplified by Re3. These are quite stable and catalyze hydrocarbon conversion. Noble
metal–oxophillic metal clusters of a few atoms of the noble metal "nested" in a supported cluster of the
oxophillic metal oxide, which helps to anchor and stabilize the noble metal clusters.
These supported “molecular” catalysts are an emerging class of materials that is expected to
offer new reactivities and catalytic properties. Some of the lessons that are emerging from
understanding of their structure and bonding appear to pertain to supported catalysts generally.