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SYNTHESIS AND PROCESSING: MORPHOLOGICALLY SPECIFIC METHODS. 53 in reactivity have been observed (Kaldor et al., 1986). Photoionization threshold measurements are sensitive to cluster electronic structure as a function of cluster size; shifts in the photoionization thresholds induced by chemisorption provide information on the adsorbate-cluster bonding. The reaction of gas-phase platinum clusters with benzene and several hexanes have shown extensive evidence of dehydrogenation and size-dependent chemisorption (Trevor et al., 1985). Microporous Solids: Pillared Clays The intercalation of metal complexes in smectite clay minerals has recently led to the development of a new class of catalyst materials. These clays have layer lattice structures with oxyanions separated by layers of hydrated cations. The intercalation of polynuclear hydroxy metal cations and metal cluster cations can be used to form catalysts with pore sizes greatly exceeding those of typical zeolite catalysts. The size and shape of the catalyst substrate can be an important factor in determining the selectivity of the catalyst (Pinnavaia, 1983). Zeolites Zeolites are porous crystalline materials. The geometry of the pores determines the size of the molecules that can pass through the rigid structure and thereby imparts a shape-selective constraint on the chemical reactions promoted. For some time, zeolites have been recognized as promising supports for the preparation of very small metal clusters, and an extensive body of literature exists on the metal-zeolite system. Metal particles in the supercages of zeolites of faujasite are of submicron size. Their synthesis generally follows this sequence: ion exchange, calcination (to destroy the ligands), reduction in hydrogen. Almost all elements with electrochemical potential more positive than Fe(II) have been examined. For some metals, these procedures do not lead to the desired result because metal ions that lose their ligand shell tend to migrate into smaller zeolite cavities. Reduction to the metal in these locations requires high temperatures and can result in large particles at the external surface of the zeolite. Recent research has yielded ways to avoid migration of the metal particles and achieve complete reduction and high dispersion. The common element of several strategies to achieve this result is to incorporate into the same zeolite a second metal that is unreducible under the conditions where the first metal is reduced. For example, Fe4+ ions exchanged in zeolite Y appeared to increase the interaction of platinum with the zeolite framework and thereby enhance dispersion at high temperature (Tzou et al., 1986). The electronic structure and stability of transition metal ions in zeolites has been reviewed by Klier and coworkers (1977).
