Supported Metal-cluster Compounds

Catalysis by Supported Metal-cluster Compounds. Further work has been reported recently on methods of chemically binding cluster compounds to supports and on the characterization of the resulting materials by various spectroscopic techniques. For example, the reaction of Rh6(CO)i6 with amine- and phosphine-modified silicas has been examined by infrared spectroscopy and has shown that cluster breakdown occurs giving L Rh(CO)2 and Lfn I (CO), where L comprizes the surface attached ligands. This behaviour is similar to that observed with Rh4(CO)i2 on unmodified silica where cluster breakdown occurs readily, particularly in the presence of traces of water and/or oxygen.  

---

Another emerging area of investigation is the behavior of supported metal cluster compounds. " " For research purposes, these materials provide well-defined models. However, they have not yet led to any major developments in industrial applications. 

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. 

There has been interest in using metal cluster compounds as precursors to heterogeneous catalysts on oxide supports.21 It is already known that metal alloys dispersed on supports are heterogeneous catalysts and they are widely used in petroleum refining. 

There are no known examples of supported clusters dispersed in crystallo-graphically equivalent positions on a crystalline support. Thus, no structures have been determined by X-ray diffraction crystallography, and the best available methods for structure determination are various spectroscopies (with interpretations based on comparisons with spectra of known compounds) and microscopy. The more nearly uniform the clusters and their bonding to a support, the more nearly definitive are the spectroscopic methods however, the uniformities of these samples are not easy to assess, and the best microscopic methods are limited by the smallness of the clusters and their tendency to be affected by the electron beam in a transmission electron microscope furthermore, most supported metal clusters are highly reactive and... 

Supported mixed metal catalysts are also prepared by other means such as the deposition of bimetallic colloids onto a support O and the decomposition of supported bimetallic cluster compounds.208 The photocatalytic codeposition of metals onto titania was also attempted with mixed results.209 with a mixture of chloroplatinic acid and rhodium chloride, very little rhodium was deposited on the titania. With aqueous solutions of silver nitrate and rhodium chloride, more rhodium was deposited but deposition was not complete. In aqueous ammonia, though, deposition of both silver and rhodium was complete but the titania surface was covered with small rhodium crystallites and larger silver particles containing some rhodium. With a mixture of chloroplatinic acid and palladium nitrate both metals were deposited but, while most of the resulting crystallites were bimetallic, the composition varied from particle to particle.209... 

Adams et al. [78,79] have reported a series of synthesis of mixed-metal cluster compounds. One example, Pt2Ru4(CO)is, is depicted in Figure 1(b). This mixed cluster compound was investigated to study the effect of Pt-Ru nanoparticles developed after the precursor annealing on carbon [80]. In line with the spectroscopic and microscopic measurements, the authors demonstrated that mixed Pt-Ru nanoparticles, with an extremely narrow size distribution (particle size 1.4nm), reflect an interaction that depends on the nature of the carbon support. Furthermore, as revealed by EXAFS, the Pt-Pt, Pt-Ru, and Ru-Ru coordination distances in the precursor (2.66, 2.64, and 2.84 A) [79] changed to 2.73, 2.70, and 2.66 A, respectively, on the mixed-metal nanoparticles supported on carbon black, with an enhanced disorder [80]. Furthermore, some metal segregation could be... 

Other approaches use metal-atom synthesis (metal-vapor synthesis, MVS [91, 92]) to produce labile complexes that are absorbed in porous supports and form catalytically active metal cluster compounds. 

The implication of transition metal clusters in catalytic reactions has been addressed by several workers (10-15). The use of cluster compounds as homogeneous catalysts has already been reviewed in general (9,16-21), and special aspects have also been treated (22-29). There are also a considerable number of reviews on supported metal clusters as heterogeneous catalysts (17,29-38). The present review considers exclusively catalytic applications of transition metal clusters in a homogeneous phase. 

The other disadvantage of the metal cluster compound decomposition is that the ligands, which have to be removed to produce the metal particle, and the organic solvent used to impregnate the support by the precursor compound may remain on the surface and thus interfere later in the catalytic reaction. The problem of the solvent can sometimes be avoided by using volatile metal cluster compounds (65b). 

The preceding paragraphs have introduced the various preparation methods leading eventually to supported metal particles. These methods fall obviously into two categories depending on whether the metal is basically in its zerovalent state (decomposition of metal cluster compound, chemical deposition, ion implantation, and vapor-phase deposition) or in an oxidized state (coprecipitation, impregnation, and ion ex-change)(Table I). 

Supported metal clusters play an important role in nanoscience and nanotechnology for a variety of reasons [1-6]. Yet, the most immediate applications are related to catalysis. The heterogeneous catalyst, installed in automobiles to reduce the amount of harmful car exhaust, is quite typical it consists of a monolithic backbone covered internally with a porous ceramic material like alumina. Small particles of noble metals such as palladium, platinum, and rhodium are deposited on the surface of the ceramic. Other pertinent examples are transition metal clusters and atomic species in zeolites which may react even with such inert compounds as saturated hydrocarbons activating their catalytic transformations [7-9]. Dehydrogenation of alkanes to the alkenes is an important initial step in the transformation of ethane or propane to aromatics [8-11]. This conversion via nonoxidative routes augments the type of feedstocks available for the synthesis of these valuable products. 

For more practical purposes, therefore, one should take recourse to metal particles as produced by other means, in particular on supports or in matrices. The advantage is the availability of macroscopic amounts of sample the disadvantage is that interaction with the supporting medium must be assessed. A great variety of synthetic methods exists, of which we can mention only a few. Metal clusters can be produced by aerosol techniques, by vapor deposition, by condensation in rare-gas matrices, by chemical reactions in various supports, e.g. zeolites, SiOi, AI2O3, or polymer matrices. Many different metal-nonmetal composites, such as the ceramic metals (cermets) have been obtained with metal particles with sizes varying from nanometers upward. In alternative approaches, metal particles are stabilized by chemical coordination with ligand molecules, as in metal colloids and metal cluster compounds. 

Metal bromides, 4 322-330 Metal can food packaging, 18 37-39 Metal-carbene complexes, 26 926 Metal-carbon compounds, 4 648, 650 Metal-carbon eutectic fixed points, 24 454 Metal carbonyl catalysts, supported, 16 75 Metal carbonyl complexes, 16 73 Metal carbonyls, 15 570 16 58-78 bonding and structure of, 16 59-64 from carbon monoxide, 5 12 in catalysis, 16 72-75 economic aspects of, 16 71 health and safety aspects of, 16 71 heteronuclear, 16 69-71 high nuclearity, 16 66-69 high nuclearity carbonyl clusters, 16 64-66... 

---