Metals carbon clusters, production

In addition to these homometallic (rhodium) clusters, several hetero-metallic clusters of the type [M M CO o]2, where M and M1 are each different metals selected from the Co, Rh, Ir triad (jc = 1-11), have been described and claimed to be useful catalysts in the reaction between carbon monoxide and hydrogen to produce oxygenated products (68, 69). These complexes can be prepared from the heterometallic dodecacar-bonyl complexes, [MuM (CO)12] (M, M1 = Co, Rh, or Ir y = 1-3), by simply mixing the appropriate dodecacarbonyl species in THF under nitrogen and then adding water (70). They can be isolated by adding a suitable cation e.g., Al3+, Mg2+, Ca2+, etc. 

Homogeneous catalysts have now been reported for hydrogenation of carbon monoxide, a combustion product of coal (see Section VI,B). More effective catalysts will undoubtedly be discovered in the near future. Polynuclear or, at least, binuclear sites are favored for reduction of the triple bond in carbon monoxide (see Section VI,B), and this together with the popular parallelism to heterogeneous systems, has renewed interest in metal clusters as catalysts (see Section VI). A nickel cluster is the first catalyst reported for mild (and selective) hydrogenation of the triple bond in isocyanide (see Section VI,A). The use of carbon monoxide and water as an alternative hydrogen source is reattracting interest (see Section VI,C). 

Up to 1999, only metal atoms [1-5], metal clusters [6,7], metal nitrides [55-57], and noble gas atoms [58-60] were observed to be encaged inside C60, C70, or various sizes of higher fullerenes. The experimental evidence for carbon atoms or metal-carbon compounds (carbides) being encapsulated inside fullerenes had not yet been observed. In 2000, Shinohara et al. succeeded in the first production, isolation, and spectroscopic characterization of a scandium carbide endohedral fullerene (Sc2C2) C84. Following this, the first experimental evidence based on synchrotron X-ray diffraction was presented and revealed that the Sc carbide is encapsulated in the form of a lozenge-shaped Sc2C2 cluster inside the D2d-C84 fullerene [8]. 

Ru) is formed.547 The cluster core is made up of a square pyramid fused to an octahedron, with one carbon atom in each fragment. The isostructural mixed-metal cluster Fe3Ru5(C)2(p-PPh2)2(CO)i7 (369/Fe) is found (35%) among the products obtained from heating 325 with Fe2(CO)9 (toluene, 100 °C, 40 h).548 549... 

Similar studies in an organic solvent yielded almost the same product [66]. Nanostructured particles of amorphous carbon-activated palladium metallic clusters have been prepared (in situ) at room temperature by ultrasound irradiation of an organometallic precursor, tris-//-[dibenzylideneacetone]dipalladium [(p-CH= CH-CO-CH=CH-5 )3Pd2] in mesitylene. Characterization studies show that the product powder consists of nanosize particles, agglomerated in clusters of approximately 800 A. Each particle is found to have a metallic core, covered by a carbonic shell that plays an important role in the stability of the nanoparticles. The catalytic activity in a Heck reaction, in the absence of phosphine ligands, has been demonstrated. 

CODHs catalyse the oxidation of carbon monoxide in a reversible, two-electron process. They are homodi-meric enzymes with five metal clusters, two C-clusters which catalyse the oxidation of CO to CO2 and three typical [FeqSq] cubane clusters (Figure 15.4). In R. rubrum, electrons are transferred from the D-cluster to a membrane-associated Fe—S protein designated CooF, which transfers electrons to a hydrogenase, coupling CO oxidation with H2 production. 

The active catalytic species are iron clusters generated from the pentacarbonyl. They consist of about 40-50 iron atoms at a diameter of ca. 0.7nm. This corresponds roughly to the dimension of the smallest SWNT formed (also refer to Section 3.3.7). The iron particles generated from the catalyst attach to the outside of the carbon nanotubes and may contribute up to 7% to the overall mass of the product mixture (Figure 3.15). The metal clusters can, however, be removed by wet chemistry (Section 3.3.6) because they are not concealed deep inside the SWNTs, but usually surrounded by just two or three layers of carbon. 

At the same time cis Smalley and students at Rice University, Houston Texas, developed the laser vaporization method for production of clusters [84], a similar set-up was built at Exxon s Research Laboratory, New Jersey, USA, by the group of Kaldor and Cox [102,103]. They studied in particular transition metal clusters but also produced clusters of carbon containing up to more than hundreds of atoms as shown in the mass spectrum in Fig. 12. 

The catalytic decomposition of carbon-contaming compounds is an extensively investigated method, also known as catalytic chemical vapor deposition (CCVD). One of the advantages of this method is the potential for large-scale production at a lower energy consumption and overall cost than with other methods. The CCVD method is essentially the same as that used for a long time in the synthesis of other filamentous forms of carbon, such as nanofibers or fibrils. The CCVD method involves the catalytic decomposition of hydrocarbons or carbon monoxide on transition metal particles. The major difference with those processes that produce nanofibers is in the structure of the catalyst. To produce SWNT, the size of the metal cluster needs to be very small. Therefore, the success of a CCVD method lies in the design of the catalyst. 

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