Supported nanoclusters

The importance of supported metal nanoclusters and nanoparticles in catalysis and the rough analogy between supported nanoclusters and organo-metallic cluster compounds (those with metal-metal bonds) in catalysis have motivated researchers to find connections between these two classes of materials. Thus, an obvious synthetic goal has been size-selected metal nanoclusters on supports. 

An alternative to this physical method of preparing structurally uniform metal clusters on supports involves chemistry by which molecular metal carbonyl clusters (e.g., [Rh6(CO)i6]) serve as precursors on the support. These precursors are decarbonylated with maintenance of the metal frame to give supported nanoclusters (e.g., Rh6). Advantages of this chemical preparation method are its applicability to many porous supports, such as zeolites (and not just planar surfaces) and the opportunities to use spectroscopic methods to follow the chemistry of synthesis of the precursor on the support and its subsequent decarbonylation. Zeolites, because their molecular-scale cages are part of a regular (crystalline) structure, offer the prospect of regular three-dimensional arrays of nanoclusters. 

Supported nanoclusters made from metal carbonyl clusters are emphasized here, because there are numerous characterization data on which to base the discussion. The synthetic methods are illustrated by the following examples. 

Supported nanoclusters have been prepared by decarbonylation of neutral or anionic metal carbonyl clusters on supports. The decarbonylation chemistry is not fully understood. The chemistry accompanying removal of the CO ligands from metal carbonyl clusters on metal oxides evidently involves hydroxyl groups or water on the surface of the metal oxide. 

The methods of structure determination of supported nanoclusters are essentially the same as those mentioned previously for supported metal complexes. EXAFS spectroscopy plays a more dominant role for the metal clusters than for the complexes because it provides good evidence of metal-metal bonds. Combined with density functional theory, EXAFS spectroscopy has provided much of the structural foundation for investigation of supported metal clusters. EXAFS spectroscopy provides accurate determinations of metal-metal distances ( 1-2%), but it gives only average structural information and relatively imprecise values of coordination numbers. EXAFS spectroscopy provides structure data that are most precise when the clusters are extremely small (containing about six or fewer atoms) and nearly uniform (Alexeev and Gates, 2000). 

Theory indicates that supported nanoclusters typified by Ir4 on zeolite NaX are nearly neutral. 

Supported clusters of only a few metal atoms have new catalytic properties, different from those of larger supported nanoclusters and nanoparticles. 

Industrial catalysts, connections of supported nanoclusters to, 72 Inorganic phase, bone, 132 Integrins... 

In general, the good quality of the substrate is essential to analyze in detail the properties of the supported nanoclusters. In fact, as we will discuss below, the cluster-oxide interaction is such to influence the shape and the electronic structure of the deposited cluster. A deep understanding of the cluster properties needs therefore a similarly profound knowledge of the interface bonding. Furthermore, a precise knowledge of the surface structure is essential to design realistic theoretical models. 

The schematic view of the supported nanocluster prior and after its reconstruction in the presence of CO and O2 mixture is shown in Figure 23 and 24. 

Lamas EJ, Balbuena PB. Adsorbate. Effeets on structiue and shape of supported nanoclusters a molecular dynamics study. J Ph Chem B 2003 107 11682-9. 

Kaneda and co-workers extended their initial work with palladium nanoclusters and demonstrated the synthesis of palladivun-grafted hydroxyapatite, where Pd-supported nanoclusters can be synthesised in the presence of alcohol. The nanoclustered Pd(0) species can effectively oxidise a variety of aromatic alcohols. 

Lamas, E. J. and Balbuena, P. B. 2003. Adsorbate effects on structure and shape of supported nanoclusters A molecular dynamics study, 107(42),... 

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