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SYNTHESIS AND PROCESSING: MORPHOLOGICALLY SPECIFIC METHODS. 51 HIGH-SURFACE-AREA MATERIALS Most of the synthesis methods discussed previously produce initially high-surface-area particulate materials that if left uncompacted may yield materials exhibiting novel catalytic properties or may be suitable for physical and/or chemical separation purposes. An example is the high-surface-area Co-WC produced by reductive pyrolysis or by lithographic etching of multilayer structures. Methods already in use for the synthesis of supported metal catalysts are well known. Metals are normally dispersed as small particles on the surface of catalyst supports in order to increase the metal surface exposed per unit mass of metal. For a dispersion exceeding 50 percent (more than 50 percent of the metal atoms on the surface of particles), the metal particle size must be less than 2 nm in diameter. Metal clusters of interest as catalysts are generally smaller than 10 nm and are commonly 1 to 5 nm. Commercial catalyst preparation generally consists of a series of unit operations, which depend on the type of catalyst being prepared. For example, precipitation may be used for the manufacture of oxide-based catalysts, gel formation for making silica and alumina, and coating for the preparation of catalysts with low surface area. Procedures used for the preparation of supported metal catalysts with submicron-sized particles include impregnation, ion exchange, and adsorption-precipitation. The role of the catalyst support in determining catalytic properties has received wide recognition in the past few years. Catalyst supports are selected on the basis of their effectiveness in dispersing and âanchoringâ the metal particles, their chemical and thermal stability, and their stability in the reaction environment. Interactions between the support and the catalyst particles can influence the catalytic properties of the metal particles. Other factors are the surface area of the support, its pore structure, the affinity of the support for catalyst poisons, crush strength, and attrition resistance. Typical supports are alumina, silica, silica-alumina, zeolites, titania, magnesia, and carbon. Typical configurations for catalyst supports are powders, pellets, spheres, tablets, granules, extrudates, and monoliths. Molecular-level information of the details involved in the synthesis is not readily available for highly dispersed systems with particles of interest buried within the porous support. Widely used as well as new catalyst-preparation techniques of interest for preparing submicron-sized metal particles are summarized in the following sections.
