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This review of structurally simple and essentially molecular metal clusters on solid supports addresses synthesis, characterization, reactivity, and catalysis. Examples of supported clusters made in high yields are Os(3), Ir(4), Ir(6), and Rh(6), and typical supports are MgO, gamma-Al(2)O(3), and zeolites. Supported clusters are synthesized by adsorption of ligated molecular metal clusters, deposition of bare size-selected metal clusters from the gas phase, and adsorption of metal complexes followed by treatment to form clusters. Some metal clusters on supports have been imaged with atomic resolution. Reactions of supported metal clusters include ligand modifications, oxidative fragmentation, and migration leading to aggregation. Reactivities and catalytic properties of the clusters (e.g., for alkene hydrogenation and epoxidation) depend on the cluster size and the support (which acts as a ligand) and are distinct from those of supported particles that resemble bulk metals. Opportunities for deeper understanding of the chemistry of supported metal clusters hinge on improvements in characterization techniques such as X-ray absorption spectroscopy and high-resolution transmission electron microscopy.

Atomic layer deposition (ALD) was employed to synthesize supported Pt–Pd bimetallic particles in the 1 to 2 nm range. The metal loading and composition of the supported Pt–Pd nanoparticles were controlled by varying the deposition temperature and by applying ALD metal oxide coatings to modify the support surface chemistry. High-resolution scanning transmission electron microscopy images showed monodispersed Pt–Pd nanoparticles on ALD Al2O3- and TiO2-modified SiO2 gel. X-ray absorption spectroscopy revealed that the bimetallic nanoparticles have a stable Pt-core, Pd-shell nanostructure. Density functional theory calculations revealed that the most stable surface configuration for the Pt–Pd alloys in an H2 environment has a Pt-core, Pd-shell nanostructure. In comparison to their monometallic counterparts, the small Pt–Pd bimetallic core–shell nanoparticles exhibited higher activity in propane oxidative dehydrogenation as compared to their physical mixture.

The conversion of anisole (a compound representative of bio-oils) in the presence of H 2 was investigated with a flow reactor operated at a temperature of 573 K and a pressure of 140 kPa with a platinum on alumina catalyst. Analysis by gas chromatographyÀmass spectrometry led to the identification of more than 40 reaction products, the most abundant being phenol, 2-methylphenol, benzene, and 2,6-dimethylphenol. The kinetically significant reaction classes were transalkylation, hydrodeoxygenation, and hydrogenation. Selectivity-conversion data were used to determine an approximate quantitative reaction network accounting for phenol, 2-methylphenol, 2-methylanisole, and 4-methylphenol as primary products. Pseudo-first-order rate constants for the formation of these products are 12, 2.8, 0.14, and 0.039 L/(g of catalyst  h), respectively. A more complete qualitative network was inferred on the basis of the observed products and the assumption that the reaction classes leading to the most abundant primary products were responsible for the minor and trace products. The removal of oxygen was evidenced by the production of benzene.

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Samples of the anatase phase of titania were treated under vacuum to create Ti(3+) surface-defect sites and surface O(-) and O(2) (-) species (indicated by electron paramagnetic resonance (EPR) spectra), accompanied by the disappearance of bridging surface OH groups and the formation of terminal Ti(3+)-OH groups (indicated by IR spectra). EPR spectra showed that the probe molecule [Re(3)(CO)(12)H(3)] reacted preferentially with the Ti(3+) sites, forming Ti(4+) sites with OH groups as the [Re(3)(CO)(12)H(3)] was adsorbed. Extended X-ray absorption fine structure (EXAFS) spectra showed that these clusters were deprotonated upon adsorption, with the triangular metal frame remaining intact; EPR spectra demonstrated the simultaneous removal of surface O(-) and O(2) (-) species. The data determined by the three complementary techniques form the basis of a schematic representation of the surface chemistry. According to this picture, during evacuation at 773 K, defect sites are formed on hydroxylated titania as a bridging OH group is removed, forming two neighboring Ti(3+) sites, or, when a Ti(4+)-O bond is cleaved, forming a Ti(3+) site and an O(-) species, with the Ti(4+)-OH group being converted into a Ti(3+)-OH group. When the probe molecule [Re(3)(CO)(12)H(3)] is adsorbed on a titania surface with Ti(3+) defect sites, it reacts preferentially with these sites, becoming deprotonated, removing most of the oxygen radicals, and healing the defect sites.