Metal clusters, preparation methods

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

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). 

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. 

As can be seen in table 1, with different preparation methods and active metals, the average size of the copper particle for the catalysts A and D were 20.3 nm and 50.0 nm. While those of the catalysts B and C were 51.3 nm and 45.4 run, respectively. CuO, non-supported metal oxide, made by impregnation is sintered and cluster whose particle size was 30 pm. The water-alcohol method provided more dispersed catalysts than the impregnation method. 

A review of preparative methods for metal sols (colloidal metal particles) suspended in solution is given. The problems involved with the preparation and stabilization of non-aqueous metal colloidal particles are noted. A new method is described for preparing non-aqueous metal sols based on the clustering of solvated metal atoms (from metal vaporization) in cold organic solvents. Gold-acetone colloidal solutions are discussed in detail, especially their preparation, control of particle size (2-9 nm), electrophoresis measurements, electron microscopy, GC-MS, resistivity, and related studies. Particle stabilization involves both electrostatic and steric mechanisms and these are discussed in comparison with aqueous systems. 

There are several preparative methods for the production of bare metal clusters including the fast flow reactor (PER), the fast flow tube reactor (FTR), the SIDT (24), the GIB (23), and a supersonic cluster beam source (SCBS) (198). Essentially, all of these methods are similar. The first process is to vaporize the metal sample producing atoms, clusters, and ions. Laser vaporization is generally favored although FAB or FIB may be used. The sample is located in a chamber or a tube and so vaporization generally takes place in a confined environment. An inert gas such as helium may be present in the vaporization source or may be pulsed in after the ionization process. 

The preparation and reactions of metal cluster ions containing three or more different elements is an area with a paucity of results. The metal cyanides of Zn, Cd (258), Cu, and Ag (259) have been subjected to a LA-FT-ICR study and the Cu and Ag complex ions reacted with various reagents (2,256). The [M (CN) ]+ and [M (CN) +11 ions of copper, where n = 1-5, were calculated to be linear using the density functional method. The silver ions were assumed to have similar structures. The anions [M (CN) +1 of both copper and silver were unreactive to a variety of donor molecules but the cations M (CN) H + reacted with various donor molecules. In each case, where reactions took place, the maximum number of ligands added to the cation was three and this only occurred for the reactions of ammonia with [Cu2(CN)]+, [Cu3(CN)2]+, [Ag3(CN)2]+, and [ Ag4(CN)3]+. Most of the ions reacted sequentially with two molecules of the donor with the order of reactivity being Cu > Ag and NH3 > H2S > CO. 

In this section, two methods used to prepare dendrimer-encapsulated metal nanoclusters are discussed direct reduction of dendrimer-encapsulated metal ions and displacement of less noble metal clusters with more noble elements. 

Type 3 metal complexes involve the physical interaction of a metal complex, chelates, or metal cluster with an organic polymer or inorganic high molecular weight compound. The preparation of type 3 compounds differs from those of type 1 and type 2, as they are ultimately achieved through the use of adsorption, deposition by evaporation, microencapsulation, and various other methods. 

A related approach to the preparation of highly dispersed supported bimetalhc catalysts involves the reaction of metal complexes with supported metal clusters or particles. The method is based on the idea that by careful choice of the metal complex and control of the reaction parameters it may be possible to cause the metal complex to react selectively with the supported metal but not with the support surface [13]. Because this approximation to the subject is the main focus of this chapter, it is thoroughly developed in the following sections. 

In heterogeneous catalysis by metal, the activity and product-selectivity depend on the nature of metal particles (e.g., their size and morphology). Besides monometallic catalysts, the nanoscale preparation of bimetallic materials with controlled composition is attractive and crucial in industrial applications, since such materials show advanced performance in catalytic processes. Many reports suggest that the variation in the catalyst preparation method can yield highly dispersed metal/ alloy clusters and particles by the surface-mediated reactions [7-11]. The problem associated with conventional catalyst preparation is of reproducibility in the preparative process and activity of the catalyst materials. Moreover, the catalytic performances also depend on the chemical and spatial nature of the support due to the metal-support interaction and geometrical constraint at the interface of support and metal particles [7-9]. 

The color of the colloidal solutions of gold depends on the size of colloidal particles (clusters). Several methods have been used for the preparation of such clusters (for a review see [578]). Since the size of clusters may change from one to several hundred angstroms, their electronic structure may vary between that of single atoms and the solid metal. 

The structural relationship between the molecular and solid-state compounds has been a hot issue in inorganic chemistry for some time (25-27). The extrusion (or excision) from preformed solid-state cluster compounds is one of the major synthetic methods of the preparation of cluster complexes (26). Use of cluster complexes as precursors to solid-state cluster compounds is the reverse reaction of excision. Both reactions utilize the structural similarity of the metal cluster units. The basic cluster units of polyhedra (deltahedra) or raft structures are triangles, and both molecular and solid-state clusters with octahedral, tetrahedral, and rhomboidal cores have been reported. Similarity of other properties such as electronic structures based on the cluster units is also important. The present review is concerned with the syntheses and structures of the cluster complexes of the group 6 metals and with their relationships to solid-state chemistry. 

The method of choice is given here for the preparation of these mixed-metal cluster complexes and it can be extended to other phosphine ligands.3,4 A second route to these complexes starting with M C12(PR3)2 and Na[Mo(CO)3(f/5-C5H5)] 2DME is also known,3,4 but it is often of lesser value giving generally lower yields... 

Several synthetic methods are now available for the preparation of mixed-metal clusters.1 However, when particular clusters are desired, two main points of concern for their synthesis often remain the availability and price of the precursors, and the yield of the reaction. Mixed-metal clusters containing ruthenium have attracted considerable interest mainly because of the variety of structural and bonding types encountered, and of their potential for homogeneous and heterogeneous catalysis.2... 

One of them is the reduction of simple metal salts in the presence of CO or similar ligands, the most general starting reaction in organometallic chemistry. The method has been used to prepare all basic metal clusters, and during the last five years improved procedures have been described for Ru3(C0)j2 and Os3(CO)j2 121, 227,... 

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