Metal particles in zeolites

The characterization of zeolite-entrapped metal clusters by chemisorption of hydrogen or carbon monoxide provides valuable information, requiring, however, careful interpretation. When metal particles in zeolites approach monoatomic dispersion, the ratio of chemisorbed hydrogen to metal (H/M) fails to provide reliable information on metal dispersion. A first convincing example that the H/M ratio can actually decrease with increasing metal dispersion has been provided by determining the H/M ratio and the EXAFS profiles of reduced Pd/NaY and Pd/HY. It was found that the H/M ratio in Pd/HY is significantly smaller than that in Pd/NaY, particularly for small Pd particles when reduction was performed at temperatures below 500°C... 

Finally, 29xe is a very suitable and sensitive isotope for probing the pore architecture of zeolitic materials. The extended Xe electron cloud is easily deformable due to interactions between, e.g. the Xe atoms and the channel wall of a zeolitic ftamewoik, and deformation results in a large low-field shift of the Xe resonance. In addition, 129xe NMR can be used to study metal particles in zeolites, while reduction-oxidation reactions can be monitored (13). Table 2 summarizes the NMR properties of a number of nuclei which have been used in NMR investigations of zeolitic materials (11). 

B. Xu and L. Kevan, Formation of Silver Ionic Clusters and Silver Metal Particles in Zeolite Rho Studied by Electron Spin Resonance and Far-infrared Spectroscopies. J. Phys. Chem., 1991, 95, 1147-1151. 

R spectroscopy is one of the most useful techniques for the characterization of zeolitie materials. It provides information on the nature of OH-groups in the materials and on the framework vibrations. Even more useful is the IR analysis of adsorbed species. Basic probe molecules, such as pyridine, ammonia, or benzene, allow the analysis of acidic sites. CO adsorbed at low temperature also helps to analyze acidic sites, but can also be used for the analysis of noble metal particles in zeolites. 

Several studies in recent years have focussed on the preparation and characterization of metal particles in zeolites (1). Various ways have been developed to introduce metals and metal precursors including sublimation (2), adsorption (3), and ion-exchange (4). If it is desirable to remove the ligands from the precursor metal complexes many approaches can be used including high temperature reduction in hydrogen (5), photochemical degradation (6), sodium (7) or cadmium (8) reduction, H atom reduction (9), low temperature vacuum treatments (10) and plasma methods (11). 

The primary reason for preparing metal particles in zeolites is that such particles might be great catalysts. In general it is often believed that the smaller the particle size of metals the greater the surface area and consequently the greater the catalytic activity. 

We have been interested in the preparation of highly dispersed iron and cobalt metal particles in zeolites for several years (11, 18-20). It is widely known that iron readily oxidizes and is difficult to prepare as small iron particles especially in an oxidizing environment. In general we have found that if the iron starts out in an oxidized form, then it is not possible to fully reduce the iron to the metallic state (18-20). Standard methods of reduction usually lead to sintering or aggregation of the metal. 

Delafosse, D. (1986) "Dispersed metal particles in zeolite carriers", J. Chim. 

In CMRs, no separate catalyst is used and reactions take place on the membrane. Zeolite membranes can be intrinsically catalytic due to the presence of catalytic sites (Bronsted acid sites, Lewis acid sites, metal ions in cationic positions, transition metal ions in zeolite lattice positions, extra-lattice transition metal compounds in channels and cavities of a zeolite, metal particles in zeolite cavities) and their high internal surface area. 

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