Metallic derivates dispersion

The structure of the metal particles dispersed on a silica powder support ( Aerosil 380, 70 A average silica particle diameter) has been studied by Avery and Sanders (47) using electron microscopy in both bright and dark field, to determine the extent to which the metal particles were multiply twinned or of ideal structure. Platinum, palladium, and gold were examined. These catalysts were prepared by impregnation using an aqueous solution of metal halide derivatives, were dried at 100°-150°C, and were hydrogen... 

The dispersion of metallic derivates on a carbon surface is dependent upon the surface properties of the carbon support. Edge carbon atoms which constitute the majority of the active surface area of graphitized carbon blacks may interact with the catalyst precursor or the catalyst itself thus providing a higher state of dispersion. 

EHRBURGER AND LAHAYE Dispersion of Metallic Derivates on Supports 311... 

Several methods have been proposed for preparing highly dispersed catalyst on carbon. The three most common procedures are impregnation, ionic exchange and vapor phase condensation. The former consists in wetting the carbon by a solution of an appropriate salt (11, 12) while the second is based on the exchange of carbon surface groups with cationic complexes (13, 14). In the vapor phase condensation the support is contacted with the volatile metal or metallic derivate (15, 16) When necessary, the supported precursor is then reduced into the metallic state. 

Heterogeneous catalysts are defined as solids or mixture of solids that are used to accelerate a chemical reaction without undergoing change themselves. The types of solids used in industry as heterogeneous catalysts include simple oxides, mixed oxides, metal salts, solid acids and bases, metals, and dispersed metals. Catalysts are used in a wide variety of chemical and environmental processes worldwide. The global value of fuels and chemicals produced by catalytic routes is about US 2.4-3 trillion per year. About 20% of all products produced in the United States are derived from a catalytic process of some form. As important as catalysis is to the world economy, the number of various chemicals used as a catalyst as well as the form and shape of the material vary as much as the number of processes that use catalysts. Fig. 1 is a picture of a number of various types of catalysts and illustrates the numerous possibilities of shapes and sizes. Naturally, the preparation processes of such a wide variety of products is also numerous. 

Most of the research involving the electrochemical conversion of hydrocarbons has been investigated primarily with platinum either as platinized metal or dispersed on a carbon support matrix. Early work focused upon the oxidation of the lower aliphatic hydrocarbons and partially oxygenated derivatives. Below is a table illustrating the relative activities of basic fuels investigated in basic media ... 

As long as man continues to employ metals, it is inevitable that there will be a certain amount of wastage and that this will lead to metal contamination of the environment. From a practical point of view, therefore, there can be no perfect solution to the dispersion of metals in the biosphere and the associated problem of exhaustion of metal resources. The inevitable end result of the processes at present in train is the final exhaustion of accessible reserves of metal ores and the irretrievable dispersion of the metals derived from these reserves throughout the biosphere and the best we can hope for is a marked decrease in the rate at which dispersal occurs. This can best be achieved by maximising the recycling of metals and carefully localising those metal-containing wastes which are produced, in the hope that it will eventually be possible to make use of them as a source of metals. 

Supported metal oxides can be prepared through co-precipitation, in which metal precursors to both the support and the supported metal oxide are induced to form the support material and the supported layer simultaneously. The support and the supported metal oxide are more spatially distributed than materials derived from the various deposition methods, but a fraction of the supported metal oxide may be located below the surface, leading to overall lower metal oxide dispersion. 

Recently, various sol-gel based recipes incorporating metal colloids have been shown to result in successful SERS substrates (49, 50, 52-57). Large Raman enhancements have been found for nano-sized metal particles dispersed in the resulting gel structure, in part, due to the large stabilized metal surface areas available to molecular-sized scatterers. In contrast, the sol-gel derived SiOa SERS substrate produced by the procedure described here is covered by immobilized clustered aggregates of monodispersed sized gold nano-particles that have been grown in-situ. 

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