Metal clusters, colloidal

Colloidal metal clusters, which offer a high surface area for better activity, have been stabilized by polymers. Thus, a homogeneous dispersion of a cobalt-modified platinum cluster was stabilized by a coordinating polymer, poly(/V-vinyl-2-pyrrolidone).74 Addition of the cobalt(II) [or iron(III)] doubled the activity and increased the selectivity from 12 to 99% when the catalyst was used to reduce cin-namaldehyde (5.23). 

Recently a technique for the preparation of catalyst particles with a narrow size distribution was developed [8], yielding colloidal metal clusters stabilized by a shell of surfactants. By adsorbing these clusters on substrate surfaces, model electrodes for dispersed electrocatalysts can be prepared [9]1 Figure 2 compares two samples prepared from different colloidal solutions of such clusters adsorbed on a gold surface. It is evident that both samples differ significantly wifii respect to their mesoscopic structure. 

The study of colloidal systems is a large field with many facets applications of these systems include optoelectronics, thin film growth, and catalysis. This is due to their exotic physiochemical properties lending credibility to the claim that these systems are an intermediate state of matter [115]. Colloidal metal clusters have also been examined as materials suitable for quantum confinement and quantum dots which may serve as models for studying single electron tunneling (SET) and... 

Tu, W., and Liu, H. (2000) Rapid synthesis of nanoscale colloidal metal clusters by microwave irradiation,/ Mater. Chem., 10(9), 2207-2211. 

Clusters of metal atoms can form colloidal suspensions. Colloidal clusters of copper, silver, and gold in glass are responsible for some of the vivid colors of stained glass in medieval cathedrals. Even aqueous suspensions of metal clusters are known (Fig. 8.45). 

Solvents such as organic liquids can act as stabilizers [204] for metal colloids, and in case of gold it was even reported that the donor properties of the medium determine the sign and the strength of the induced charge [205]. Also, in case of colloidal metal suspensions even in less polar solvents electrostatic stabilization effects have been assumed to arise from the donor properties of the respective liquid. Most common solvent stabilizations have been achieved with THF or propylenecarbonate. For example, smallsized clusters of zerovalent early transition metals Ti, Zr, V, Nb, and Mn have been stabilized by THF after [BEt3H ] reduction of the pre-formed THF adducts (Equation (6)) [54,55,59,206]. Table 1 summarizes the results. 

Schmid, G. Ligand-stabilized Giant Metal Clusters and Colloids. In Physics and Chemistry of Materials with Low-Dimensional Structures, Kluwer Academics The Netherlands, Longh, J. L. 1994 Vol. 18, pp 107. 

Living Colloidal Metal Particles from Solvated Metal Atoms Clustering of Metal Atoms in Organic Media... 

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. 

L/evelopment of sophisticated surface analytical techniques over the past two decades has revived interest in the study of phenomena that occur at the electrode-solution interface. As a consequence of this renewed activity, electrochemical surface science is experiencing a rapid growth in empirical information. The symposium on which this book was based brought together established and up-and-coming researchers from the three interrelated disciplines of electrochemistry, surface science, and metal-cluster chemistry to help provide a better focus on the current status and future directions of research in electrochemistry. The symposium was part of the continuing series on Photochemical and Electrochemical Surface Science sponsored by the Division of Colloid and Surface Chemistry of the American Chemical Society. 

After Faraday s seminal report on the preparation of transition metal clusters in the presence of stabilizing agents in 1857 [31], Turkevich [19-21] heralded the first reproducible protocol for the preparation of metal colloids and the mechanism proposed by him for the stepwise formation of nanoclusters based on nucleation, growth, and agglomeration [19] is still valid but for some refinement based on additional information available from modem analytical techniques and data from thermodynamic and kinetic experiments [32-41], Agglomeration of zero-valent nuclei in the seed or, alternatively, collisions of already formed nuclei with reduced metal atoms are now considered the most plausible mechanism for seed formation. Figure 3.1 illustrates the proposed mechanism [42],... 

Schmid, G., Large clusters and colloids. Metals in the embryonic state, Chem. Rev., 92,1709, 1992. 

Schmid, G. Maihack, V. Lantermann, F., and Peschel, S., Ligand-stabilized metal clusters and colloids properties and applications, J. Chem. Soc. Dalton Trans., 589, 1996. 

Schmid, G. and Peschel, S., Preparation and scanning probe microscopic characterization of mono-layers of ligand-stabilized transition metal clusters and colloids, New J. Chem., 22, 669, 1998. 

Colloidal nanoparticles can be employed as heterogeneous catalyst precursors in the same fashion as molecular clusters. In many respects, colloidal nanoparticles offer opportunities to combine the best features of the traditional and cluster catalyst preparation routes to prepare uniform bimetallic catalysts with controlled particle properties. In general, colloidal metal ratios are reasonably variable and controllable. Further, the application of solution and surface characterization techniques may ultimately help correlate solution synthetic schemes to catalytic activity. 

H. Bonnemann, W. Brijoux, R. Brinkmann, E. Dinjus, T. Jouben, R. Fretzen, and B. Korall, Highly dispersed metal clusters and colloids for the preparation of active liquid-phase hydrogenation catalysts, J. Mol. Catal. 74,323-333 (1992). 

This behavior differs completely from the discrete one-electron absorptions of low-nuclearity metal cluster molecules [17]. Instead, it resembles the 5d - 6s,6p interband transition of colloidal gold. This demonstrates clearly that the AU55 cluster has electronic energy levels which are closely spaced in a developing band structure, quite similar to colloidal gold. On the other hand, these electrons do not seem to show a collective behavior which would give rise to the plasma resonance. 

Schmid G (1992) Large Clusters and Colloids - Metals in the Embryonic State. Chem Rev 92 1709-1727... 

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. 

Copper, silver, and gold colloids have been prepared by Chemical Liquid Deposition (CLD) with dimethoxymethane, 2-methoxyethyl ether, and ethylene glycol dimethyl ether. The metals are evaporated to yield atoms which are solvated at low temperatures and during the warm-up process colloidal sols with metal clusters are obtained. Evaporation of the solvent was carried out under vacuum-generating metal films. These films are showing very low carbon/hydrogen content and were characterized by elemental analyses and infrared spectroscopy (Cardenas et al., 1994). 

Transition from non-metallic clusters consisting of only a few atoms to nanosized metallic particles consisting of thousands of atoms and the concomitant conversion from covalent bond to continuous band structures have been the subject of intense scrutiny in both the gas phase and the solid state during the last decade [503-505]. It is only recently that modern-day colloid chemists have launched investigations into the kinetics and mechanisms of duster formation and cluster aggregation in aqueous solutions. Steady-state and pulse-radiolytic techniques have been used primarily to examine the evolution of nanosized metallic particles in metal-ion solutions [506-508]. 

Controlled reduction of cadmium (or lead) ions on surfaces of nanosized silver (or gold) metallic particles results in the formation of double-layer colloids [532-534]. Depending on the coverage, the second layer can vary from being non-metallic clusters to quasi-metallic and metallic colloids. Growth of the second-layer particles can be monitored by absorption spectrophotometry. For... 

---