Zeolites metal cluster

The size of metal clusters and the number of atoms in the cluster can vary in a large range. The number of nuclei m can vary from 1 to thousands. In a zeolite, metal clusters can be located in different cages and defects. And the metal clusters in zeolites can be roughly classified as the following types (Figure 9.1) ... 

TEM metal clusters size and location of cluster on zeolite Metal cluster size and location can be modified by the eledron beam, due to high local temperature rises lack of resolution prevents detection of smallest metal clusters. 

High-yield catalytic conversion of glycerol into 1,2-propanediol without using an external H2 supply is possible on Pt impregnated NaY zeolites, characterized by extra-zeolitic metal clusters combined with traces of Bronsted acidity. A comparison is made with NaY zeolite supported Pt catalysts prepared by a conventional ion-exchange procedure. Both materials were characterized using SEM, XRD and CO chemisorption. 

Volu me 28 New Developments in Zeolite Science and Technology. Proceedings of the 7th International Zeolite Conference, Tokyo, August 17-22,1986 edited by Y. Murakami, A. lijima and J.W. Ward Volume 29 Metal Clusters in Catalysis... 

Oxide- and Zeolite-supported "Molecular" Metal Clusters Synthesis, Structure, Bonding, and Catalytic Properties... 

Abstract This review is a summary of supported metal clusters with nearly molecular properties. These clusters are formed hy adsorption or sirnface-mediated synthesis of metal carbonyl clusters, some of which may he decarhonylated with the metal frame essentially intact. The decarhonylated clusters are bonded to oxide or zeolite supports by metal-oxygen bonds, typically with distances of 2.1-2.2 A they are typically not free of ligands other than the support, and on oxide surfaces they are preferentially bonded at defect sites. The catalytic activities of supported metal clusters incorporating only a few atoms are distinct from those of larger particles that may approximate bulk metals. 

Oxide- and Zeolite-Supported Molecular Metal Clusters... 

Supported metal carbonyl clusters are alternatively formed from mononuclear metal complexes by surface-mediated synthesis [5,13] examples are [HIr4(CO)ii] formed from Ir(CO)2(acac) on MgO and Rh CCOlie formed from Rh(CO)2(acac) on y-Al203 [5,12,13]. These syntheses are carried out in the presence of gas-phase CO and in the absence of solvents. Synthesis of metal carbonyl clusters on oxide supports apparently often involves hydroxyl groups or water on the support surface analogous chemistry occurs in solution [ 14]. A synthesis from a mononuclear metal complex precursor is usually characterized by a yield less than that attained as a result of simple adsorption of a preformed metal cluster, and consequently the latter precursors are preferred when the goal is a high yield of the cluster on the support an exception is made when the clusters do not fit into the pores of the support (e.g., a zeolite), and a smaller precursor is needed. 

Gates BC (2005) Oxide- and Zeolite-supported Molecular Metal Clusters Synthesis, Structure, Bonding, and Catalytic Properties. 16 211-231 Gibson SE (nee Thomas), Keen SP (1998) Cross-Metathesis. 1 155-181 Gisdakis P, see Rosch N (1999) 4 109-163 Gdrling A, see Rosch N (1999) 4 109-163... 

Chemical anchoring of catalytically active metal clusters onto a support is of practical importance to stabilize catalysts against loss of activity by Ostwald ripening, i.e. metal agglomeration. Documented examples include Pt, Pd, or Rh supported on acidic oxides, in particular zeolites in their H-form. Three types of anchors have been de-... 

Stmcture determination of unknown crystals by electron diffraction was performed by several research groups, on Al-Fe alloys by Gjonnes et al. (1998), on metal-cluster compounds by Weirich et al. (2000) and on zeolites by Wagner et al. (1999). Selected area electron diffraction or electron diffraction collected by a precession technique were used and the structure factor phases were deduced by direct methods, Patterson method or from convergent beam electron diffraction. 

Rice et al. [79] were the first to demonstrate small metal clusters randomly located throughout the channels of a zeolite L crystal. Numerous channel-blocking... 

Full catalyst formulations consist of zeolite, metal and a binder, which provides a matrix to contain the metal and zeolite, as well as allowing the composite to be shaped and have strength for handling. The catalyst particle shape, size and porosity can impact the diffusion properties. These can be important in facile reactions such as xylene isomerization, where diffusion of reactants and products may become rate-limiting. The binder properties and chemistry are also key features, as the binder may supply sites for metal clusters and affect coke formation during the process. The binders often used for these catalysts include alumina, silica and mixtures of other refractory oxides. 

Palladium metal clusters confined in zeolites have been prepared from Pd (CO)y species accommodated in zeolite cages. These species can be obtained by carbon-ylation of impregnated or exchanged palladium compounds such as PdCh or [Pd(NH3)4] [47, 48]. 

Catalysts prepared from iridium neutral binary carbonyl compounds and several supports have been studied extensively. Small Ir (x = 4, 6) clusters supported on several oxides and caged in zeolite, and their characterization by EXAFS, have been prepared [159, 179, 180, 194-196]. The nuclearity of the resulting metallic clusters has been related with their catalytic behavior in olefin hydrogenation reactions [197]. This reaction is structure insensitive, which means that the rate of the reac-hon does not depend on the size of the metallic particle. Usually, the metallic parhcles are larger than 1 nm and consequently they have bulk-like metallic behavior. However, if the size of the particles is small enough to lose their bulk-like metallic behavior, the rate of the catalytic reaction can depend on the size of the metal cluster frame used as catalyst. 

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