Transition metal colloids

Transition metal colloids can also be prevented from agglomeration by polymers or oligomers [27,30,42,43]. The adsorption of these molecules at the surface of the particles provides a protective layer. In the interparticle space, the mobility of adsorbed molecules should be reduced decreasing the entropy and thus increasing the free energy (Fig. 2). 

Finally, the term steric stabihzation coifid be used to describe protective transition-metal colloids with traditional ligands or solvents [38]. This stabilization occurs by (i) the strong coordination of various metal nanoparticles with ligands such as phosphines [48-51], thiols [52-55], amines [54,56-58], oxazolines [59] or carbon monoxide [51] (ii) weak interactions with solvents such as tetrahydrofuran or various alcohols. Several examples are known with Ru, Ft and Rh nanoparticles [51,60-63]. In a few cases, it has been estab-hshed that a coordinated solvent such as heptanol is present at the surface and acts as a weakly coordinating ligand [61]. 

Table 1. THF-stabilized organosols of early transition metals. (Reprinted from Ref. [53], 2007, with permission from Wiley-VCH.) Solvent Stabilized Early Transition Metal Colloids...

J. S. Bradley, in G. Schmid (ed.) The Chemistry of Transition Metal Colloids, Wiley-VCH, Weinheim, 1994, 459. 

Roucoux A, Schultz J, Patin H (2002) Reduced transition metal colloids a novel family of reusable catalysts. Chem Rev 102 3757-3778 Toyooka K, Kanamitsu N, Yoshimra M (2004) PCT international application W02004/048332... 

In materials chemistry, nanoparticles of noble metals are an original family of compounds. Well-defined in terms of their size, structure and composition, zero-valent transition-metal colloids provide considerable current interest in a variety of applications. Here, the main interest is their application in catalysis. Zerovalent nanocatalysts can be generated in various media (aqueous, organic, or mixture) from two strategic approaches according to the nature of the precursor, namely (i) mild chemical reduction of transition-metal salt solutions and (ii) metal atom... 

Finally, the development of modified nanoparticles having better stability and a longer lifetime has involved interesting results in diverse catalytic reactions. Efficient activities are obtained with these transition-metal colloids used as catalysts for the hydrogenation of various unsaturated substrates. Consequently, several recent investigations in total, partial or selective hydrogenation have received significant attention. 

Reetz, M.T. and Maase, M., Redox-controlled size-selective fabrication of nanostructured transition metal colloids, Adv. Mater., 11, 773,1999. 

Bradley, J.S. et al., Surface spectroscopic study of the stabilization mechanism for shape-selectively synthesized nanostructured transition metal colloids, J. Am. Chem. Soc., 122, 4631, 2000. 

Woelk and Bargon have studied the catalytic hydrogenation of alkynes in scCOj [34], using catalysts based on transition metal colloids, carried out in the TCA setup. Under mild experimental conditions (15 bar hydrogen partial pressure, at 323 K) they have found extremely high TOFs (as high as 4 X 10 h" ... 

Roucoux, A., J. Schulz and H. Patin (2002). Reduced transition metal colloids A novel family of reusable catalysts Chemical Reviews, 102(10), 3757-3778. 

General introduction/Electronic structure of metal clusters and cluster compounds/ Clusters in ligand shells/Clusters in cages/Discrete and condensed transition metal clusters in solids/Chemistry of transition metal colloids/Perspectives G.A. Somoijai, Introduction to Surface Chemistry and Catalysis, Wiley New York, 1994... 

THF as Colloidal Stabilizer for Early Transition Metal Colloids Solvents such as THE or propylenecarbonate can act as powerful colloidal stabilizers. Small sized clusters of zerovalent early transition metals Ti, Zr, V, Nb, and Mn have been stabilized by THE after [BEtsH J-reduction of the pre-formed THF-adducts (Eq. (2.4)) [82-84, 92]. 

During the last 2-3 decades a vast amount of knowledge regarding the preparation and characterization of nanostructured transition metal colloids in the zerovalent form has accumulated [1, 2]. The emphasis has been largely on the development of methods for the control of size and, more recently, even of shape [3]. In the majority of cases application in catalysis was not pursued systematically, i.e., generally only a simple model reaction such as the hydrogenation of cyclohexene was studied. 

In contrast to nanoscaled transition metal colloids in the zerovalent form, less is known regarding the analogous metal oxides, i.e., colloidal MO (M=metal n= 1, 2, 3, etc.). Of course, such species in solid bulk form or as immobilizates on surfaces or in solid carriers have been known for a long time in heterogeneous catalysis [4], in semiconductor technology and other areas [3, 5]. More recently special methods have been developed for their preparation in constrained environments [6], in microemulsions [7], or as one- or two-dimensional nanostructures [5d, 8]. 

Schema 8.1 Electrochemical fabrication of nanosized transition metal colloids (the stabilizer is usually a tetraalkylammonium salt) [25],...

The Miilheim electrochemical method of producing R4N+X -stabUized transition metal colloids is a viable alternative to the traditional chemical process. The preparation of aqueous colloidal solutions of nanosized transition metal oxides or multimetal oxides (1-3 nm) is possible by an unusually simple procedure, namely hydrolysis of the corresponding metal salts under basic conditions in the presence of water-soluble stabiUzers. High concentrations (0.1-0.5 M) are usually possible, which is crucial for industrial applications. In many cases an in situ method for immobUization on a soUd carrier such as carbon black is possible in the absence of a stabUizer. In the rare case of IrO, the colloidal soluhon (2nm) is stable for... 

Bradley, J.S. 1994. The chemistry of transition metal colloids. In Clusters and Colloids From Theory to Applications, ed. G. Schmid, pp. 459-544. VCH Verlags gesells chaft mbH, Weinheim, Germany. 

With these considerations in mind, the goals of contemporary metal colloid research, the approaches to whidi will be covered in this chapter, may be stated as the search for synthetic methods for metal particles which are stabilized in the 1-20 nm size range, the application of characterization methods to identify the potential novelty of the partides (structural methods, electronic properties, surface chemistry), and finally the ap cation of these properties to various fields of chemistry and physics. Perhaps the field for which the renaissance in transition metal colloid chemistry has the most potential impact is catalyds. It has been... 

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