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SYNTHESIS AND PROCESSING: MORPHOLOGICALLY SPECIFIC METHODS. 52 Impregnation and Ion Exchange. Two techniques commonly used to prepare supported metal catalysts are impregnation and ion exchange, which yield submicron-sized catalyst particles in the form of small metal crystallites dispersed on the porous surface of preformed supports. For impregnation, the support in either a wet or dry state is treated with a solution containing a compound of the desired catalytic constituent. The liquid is removed by drying, leaving catalytic materials on the surface of the support. Supports such as alumina or silica that have a high surface-to-volume ratio stabilize the dispersion of the metal over a large area. Research studies on catalyst preparation have provided understanding of the impregnation process and how to control the amount and location of the metal particles on the support (D'Aniello, 1981; Komiyama, 1985). The strength of the interaction of the metal-containing anion and cation with the support influences the penetration of the metal into the support matrix. Preparation of supported catalysts by impregnation produces a distribution of metal particle sizes. As a result, precise information on the consequences of molecular structure is not available for highly dispersed systems. Other catalyst-preparation techniques that make use of preformed supports include vapor-phase methods in which metal atoms, preformed metal clusters, or a vapor-phase compound of the metal such as a carbonyl complex are deposited on the support. Decomposition of Metal Carbonyls Metal carbonyls can also be used to produce small supported metal particles. Thermal decomposition of metal carbonyls on hydroxylated supports, such as alumina, results in the formation of highly dispersed oxide species. These catalysts are of interest as model systems to probe support effects and as a means to prepare materials with precisely controlled characteristics. Provided that the temperature is kept below 400 K, metal carbonyls can be used to form zero valent subcarbonyl species, which are analogs of metal carbonyl clusters with one of the CO ligands replaced by the interaction of the metal with the support. These catalysts have been studied as potential replacements for homogeneous catalysts. Bare Metal Clusters Recently, a significant development in the preparation of new materials with potential significance to the field of catalysis is the ability to prepare bare metal clusters of variable size. These clusters are generally prepared by the vaporization of a metal into a gas, supersonic expansion to promote nucleation, introduction of a reactant gas if desired, and finally mass selection. Cluster-size-dependent variations
