Metal clusters preparation

A summary of zeolite-supported metal clusters prepared from metal salts is given in Table 2. Included here are only samples that have been characterized by EXAFS spectroscopy and incorporate extremely small clusters. The additional literature of zeolite-supported metals is reviewed elsewhere [1-5]. 

Alkali Metal Clusters Preparation of Alkali Metal Clusters... 

The outstanding features of metal clusters prepared in block copolymer micelles [81] are their high catalytic activity combined with high stability. Such micellar catalyst systems can be recovered after reaction by precipitation or ultrafiltration. In many cases high selectivity and stability have been observed. Cyclohexadiene, for instance, is selectively hydrogenated by Pd colloids just to cyclo-octene [69]. High activity and stability of such catalyst particles have been reported for the Heck-reaction with unusually high turnover numbers of... 

Table 2.22. Selected examples of metal cluster preparations via photolitic and thermolitic methods...

With the exception of direct condensation of metal vapors in micropores, the preparation of metal clusters involves at least two steps, viz., the loading of a metal precursor in the microporous material and the decomposition or reduction of the precursor yielding metal clusters. These two steps are highly interdependent and the second is often crucial for governing the final metal dispersion. The different strategies of metal cluster preparation will be examined in this section, and detailed examples of preparations for specific zeolites and metals will be given in Sect. 3. 

Special emphasis will be given to the preparation of platimun and palladium clusters in faujasites since the techniques of metal cluster preparations were first developed for these zeoUtes and later extended to other zeoUtes and other pla-timun-group metals. Case studies concerning the preparation of clusters of iron, nickel and cobalt in faujasites will not be developed since most of the references on these preparations have already been given in Sect 2. 

Studies on metal cluster preparation and characterization have been focused mainly on faujasites. There is a need to extend these investigations to other zeolite types, as well as new microporous and mesoporous materials, in order to prepare metal clusters in different geometric and electronic environments. Both medium-pore and mesoporous materials have a potential interest. Thus, conventional zeolites are not well suited for catalytic reactions involving fine chemicals in the liquid phase because of molecular size restriction and/or very low diffusivities. To take advantage of molecular constraints while tailoring the regio-, chemo- and stereoselectivity of metal-catalyzed reactions of... 

The method is clearly of potential use in preparing mixed metal clusters, e.g. (Co -t- Ni) or (Co -t-Fe), and can be extended to prepare more complicated cluster arrays as depicted below, the subrogated B atom being indicated as a shaded circle in (92). 

Many novel cluster compounds have now been prepared in this way, including mixed metal clusters. Further routes involve the oxidative fusion of dicarbon metallacarborane anions to give dimetal tetracarbon clusters such as (103) and (104) O (jjg insertion of isonitriles into inetallaborane clusters to give monocarbon meiallacarboranes such as (105) and the reaction of small ii/t/o-carboranes with alane adducts such as Et3NAlH3 to give the commo species (106) ... 

These methods may be used to prepare mixed metal clusters. Simultaneous codeposition of Ag and Cu vapors in Ar at 10-12 K yields a mixture including atomic Ag and Cu, dimers Ag, and Cu, together with AgCu. At 77 K, CuAg4 and Cu,Ag3 clusters occur . The amount of AgCu can be increased by photoexcitation with 305 nm Ag or Cu atomic radiation. The trimer AuAgCu is produced when a mixture of Au, Ag and Cu vapors is condensed at 77 K. 

The electronic, optical, and magnetic properties of metal clusters are of great current interest, but these properties have been little studied with very mixed -metal clusters. This is to some extent a reflection of the difficulty of preparing high-nuclearity examples many of these interesting properties become important upon increasing cluster size. The limited magnetic studies to date are... 

Non-ionic thiourea derivatives have been used as ligands for metal complexes [63,64] as well as anionic thioureas and, in both cases, coordination in metal clusters has also been described [65,66]. Examples of mononuclear complexes of simple alkyl- or aryl-substituted thiourea monoanions, containing N,S-chelating ligands (Scheme 11), have been reported for rhodium(III) [67,68], iridium and many other transition metals, such as chromium(III), technetium(III), rhenium(V), aluminium, ruthenium, osmium, platinum [69] and palladium [70]. Many complexes with N,S-chelating monothioureas were prepared with two triphenylphosphines as substituents. 

Ligand Combination Strategy for the Preparation of Low-dimensional Metal Cluster Materials... 

Investigations of the interaction between 3d transition metals and octahedral halide or oxide metal clusters led to the preparation of a number of novel cluster compounds such as the series AxByNbgClig (A = Li, K, Rb, Cs B=Ti, V, Mn, Cu) [33], and TizNbgOu [34]. 

Table 5 shows HDS product distributions over several catalysts prepared by using the molybdenum-nickel cluster 2. Sulfur content in decane was adjusted to 5.0 wt% in these experiments. MoNi/NaY was found to be more active than MoNi/Al203. It is to be noted that during the high temperature pretreatment the original cluster structure would have been changed. However, the high activity of the MoNi/NaY catalyst for benzothiophene HDS is probably due to the formation of active sites derived from this particular mixed metal cluster. 

When a supported metal on an oxide is prepared from an adsorbed precursor incorporating a noble metal bonded to an oxophilic metal, the result may be small noble metal clusters, each more-or-less nested in a cluster of atoms of the oxophilic metal, which is cationic and anchored to the support through metal-oxygen bonds [44,45]. The simplest such structure is modeled on the basis of EXAFS data as Re4Pt2, made from Re2Pt(CO)i2 (Fig. 6) [45]. 

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