Catalysis colloidal metals

N. Toshima, Y. Shiraishi, Catalysis by metallic colloids, in A. T. Hubbard (ed.) Encyclopedia of Surface and Colloid Science, Marcel Dekker, New York, 2002, 879-886. 

Catalysis of Free Radical Reactions by Colloidal Metals.117... 

Vargaftik, M.N. et al., Catalysis by metal colloids tragectories for atom assembling in Pd and Pt colloids, Kinetics Catal., 39, 740, 1998. 

In the majority of catalytic reactions discussed in this chapter it has been possible to rationalize the reaction mechanism on the basis of the spectroscopic or structural identification of reaction intermediates, kinetic studies, and model reactions. Most of the reactions involve steps already discussed in Chapter 21, such as oxidative addition, reductive elimination, and insertion reactions. One may note, however, that it is sometimes difficult to be sure that a reaction is indeed homogeneous and not catalyzed heterogeneously by a decomposition product, such as a metal colloid, or by the surface of the reaction vessel. Some tests have been devised, for example the addition of mercury would poison any catalysis by metallic platinum particles but would not affect platinum complexes in solution, and unsaturated polymers are hydrogenated only by homogeneous catalysts. 

The very soluble hexachloroplatinic acid, H2 [PtCl ] nH20, is the most useful precursor for synthetic and catalytic work. For the hydrosilation of unsaturated substrates, the catalyst of choice is chloroplatinic acid, because of its very high activity. There is currently some doubt as to whether the true catalyst is homogeneous or colloidal metal and therefore heterogeneous see Hydrosilation Catalysis). Nearly all halides andpseudohahdes (= X) form salts of the [PtXe] ion and of the [PtX4(NH3)2] type. 

The preparation of metal colloids and their use in catalysis is almost as old as the study of catalysis itself and their efficiency as catalysts is well established. In fact the high activity of colloidal metals in a number of catalytic processes has sometimes been a complicating factor in the investigation of... 

Slurry reactors are also used in other situations, such as the polymerization of ethylene or propylene. Here the slurry consists of catalyst particles and a solvent, such as cyclohexane, into which the ethylene or propylene is bubbled and dissolved. Another illustration is the Fischer-Tropsch reaction between hydrogen and carbon monoxide, where these gases are dissolved in a slurry of hydrocarbon oil and catalyst (iron) particles. Catalysis by colloidal metal particles and colloidal enzyme particles are other examples, although not always is one reactant a gas. 

In contrast, colloid chemistry has provided the colloidal dispersion of metal fine particles in water.As early as the 1950 s, colloidal dispersion of fine particles of precious metals was conducted and applied to catalyses. Although they contain fine metal particles, the size of the particles was not so uniform. In addition they were not very stable when used in solution. Thus, there still remained many problems in the reproducibility of the preparation and catalysis using metal nanoparticles. In 1976, the author s group prepared colloidal dispersions of rhodium particles protected by water-soluble polymers by reduction of rhodium(III) ions under mild conditions, i.e., reduction with refluxing alcohol in the presence of water-soluble polymers. They were applied to the catalyst for hydrogenation of olefins. In 1989, we developed colloidal dispersions of Pd/Pt bimetallic nanoclusters by the simultaneous reduction of Pd and Pt ions in the presence of poly(A-vinyl-2-pyrrolidone) (PVP). 

Moiseev n, Vargaftik MN. Clusters and colloidal metals in catalysis. Russ J Chem 2002 72 512-22. 

The literature on colloidal metals is extensive and an adequate survey covering all aspects of their physical and chemical properties would require an interdisdpli-nary authorship of some size. This chapter reflects the predilections and limitations of the author and will concentrate on the chemical aspects of metal colloid science. Suffident physical discussion will be given to support the descriptions of the structures and spectroscopic properties of colloidal metals in the context of their chemical applications to catalysis. 

The use of phosphines as stabilizers for colloidal metals is reminiscent of the stabilization of low valent transition metals. Since the latter have found extensive application in homogeneous catalysis such ligand manipulation should be a powerful method for modifying colloidal metal catalysts as well. 

The product D is produced by the transition-metal homogeneous catalysis in at least one catalytic cycle. Contributions to the rate of formation of D may also arise from stoichiometric reactions or heterogeneous catalytic reactions on suspended colloidal metal, suspended metal particles, or metal deposited on the walls of the vessel, etc. The total rate of reaction is noted (mol/s). Since the system is time dependent, and in particular due to the need to transform the catalyst precursor to intermediates, the maximum rate of product formation Pd occurs at t > to-... 

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... 

Hirai, H. and Toshima, N. In Catalysis by Metal Complexes, Tailored Metal Catalysts, Iwasawa, Y., Ed., D. Reidel Publishing Conq)any Dordrecht, 1986. Bradley, J. S. In Clusters and Colloids. From Theory to Applications, Schmid, G., Ed., VCH Weinheim, 1994. 

Historically, Michael Faraday must be credited with some of the earliest work on small particles, his gold colloid preparations are still used today over 200 years later. In 1925, Richard Zsigmondy received the Nobel Prize for his study of colloidal metal solutions. Today microclusters of metals constitute one of the most important of man s synthetic materials they are vital in heterogeneous catalysis on which 20% of the GNP of the United States is dependent. Among other numerous applications of small metal particles are thin films and coatings, latent image development, and photographic films. 

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