Catalytically active clusters

Fig. 1.55. Schematic representation of collection zone, adlineation, and reverse spillover for the case of a gold cluster supported on a metal-oxide surface. The reactants (red spheres) might either adsorb from the gas phase in the vicinity of the cluster, within the so-called collection zone, and be directly attracted toward the catalytically active cluster. Or the adsorption might be followed by random diffusion and eventually lead to desorption back to the gas phase, if the primary adsorption places are outside the collection zone of the cluster (graphics adapted from [348])...

For the direct injection of the catalyst or one of its precursors, the choice of compounds is naturally limited to vaporizable substances. The respective transition metal carbonyls and metallocenes proved their worth here. They decompose upon heating in the CVD apparatus and release the elemental metal in the shape of small, catalytically active clusters. 

In the simplest such mechanism, polypeptides with different catalytic activities cluster closely together as subunits of a multimeric enzyme or assemble on a common scaffold (Figure 3-20b). This arrangement allows the products of one reaction to be channeled directly to the next enzyme In the pathway. The first approach Is Illustrated by pyruvate... 

Together, the results described for aRNR-AE, LAM, and PFL-AE point to the [4Fe-4S]" cluster as the catalytically active cluster, and they point to a role for this cluster in providing the electron necessary for reductive cleavage of AdoMet, either reversibly (as in the case of LAM) or irreversibly (as in the cases of the two activating enzymes), as illustrated in Figure 9. Therefore,... 

The reduction of PhCH=N—Ph to PhCH2NHPh by isopropanol is catalyzed by [Ru3(CO)i2]. a catalytically active cluster complex (17) was isolated quantita-... 

Silver bromide is a semiconductor. Absorbed light excites electrons in the conduction band and electron holes in the valence band. The holes oxidize bromide ions to traces of bromine that are dissolved in the gelatine layer. The free electrons reduce silver ions to silver atoms that form the catalytically active clusters in the halide crystallites. The reactions are ... 

Valden M, Lai X and Goodman D W 1998 Onset of catalytic activity of gold clusters on titania with the appearance of nonmetallic properties Science 281 1647... 

To explain how solid acids such as Nafion-H or HZSM-5 can show remarkable catalytic activity in hydrocarbon transformations, the nature of activation at the acidie sites of such solid acids must be eon-sidered. Nafion-H contains acidic -SO3H groups in clustered pockets. In the acidic zeolite H-ZSM-5 the active Bronsted and Tewis acid sites are in close proximity (—2.5 A). 

In what may be an example of tme cluster catalysis, [HRU3 (CO) ] shows good catalytic activity and high regioselectivity using propylene as substrate (24,25). Solvent, CO partial pressure, and temperature are important variables. In monoglyme, at 80°C and starting partial pressures for C H, ... 

Rhodacarborane catalysts have been immobilized by attachment to polystyrene beads with appreciable retention of catalytic activity (227). A 13-vertex /oj iJ-hydridorhodacarborane has also been synthesized and demonstrated to possess catalytic activity similar to that of the icosahedral species (228). Ak-oxidation of closo- >(2- P((Z [) 2 - i- > l[l-Bih(Z, results in a brilliant purple dimer. This compound contains two formal Rh " centers linked by a sigma bond and a pak of Rh—H—B bridge bonds. A number of similar dimer complexes have been characterized and the mechanism of dimer formation in these rhodacarborane clusters have been studied in detail (229). 

Organism (class) Subunit (gene) size Metal content Cluster content Catalytic activities Ref. ... 

A classical issue in transition-metal catalysis is the dependence of catalytic activity on changes in the particle size of the metal clusters in the nanosize region [14]. 

Another SBU with open metal sites is the tri-p-oxo carboxylate cluster (see Section 4.2.2 and Figure 4.2). The tri-p-oxo Fe " clusters in MIL-100 are able to catalyze Friedel-Crafts benzylation reactions [44]. The tri-p-oxo Cr " clusters of MIL-101 are active for the cyanosilylation of benzaldehyde. This reaction is a popular test reaction in the MOF Hterature as a probe for catalytic activity an example has already been given above for [Cu3(BTC)2] [15]. In fact, the very first demonstration of the catalytic potential of MOFs had aheady been given in 1994 for a two-dimensional Cd bipyridine lattice that catalyzes the cyanosilylation of aldehydes [56]. A continuation of this work in 2004 for reactions with imines showed that the hydrophobic surroundings of the framework enhance the reaction in comparison with homogeneous Cd(pyridine) complexes [57]. The activity of MIL-lOl(Cr) is much higher than that of the Cd lattices, but in subsequent reaction rans the activity decreases [58]. A MOF with two different types of open Mn sites with pores of 7 and 10 A catalyzes the cyanosilylation of aromatic aldehydes and ketones with a remarkable reactant shape selectivity. This MOF also catalyzes the more demanding Mukaiyama-aldol reaction [59]. 

The goal of this work was to prepare and characterize PtRu/MgO catalysts from cluster A which contained Pt-Ru bonds and compare with that prepared from a mixed solution of Pt(acac)2 and Ru(acac)3. The characterization methods included IR and EXAFS spectroscopy. Ethylene hydrogenation was used to test the catalytic activity of both PtRu/MgO catalysts. 

Both PtRu/MgO catalysts prepared from cluster precursor and organometallic mixture were active for ethylene hydrogenation. The apparent activation energy of the former catalyst obtained from the Arrhenius plot during -40 to -25°C was 5.2 kcal/mol and that of the latter catalyst obtained during -50 to -30°C was 6.0 kcal/mol. The catalytic activity in terms of turn over frequency (TOP) was calculated on the assumption that all metal particles were accessible for reactant gas. Lower TOP of catalyst prepared from cluster A at -40°C, 57.3 x lO" s" was observed probably due to Pt-Ru contribution compared to that prepared from acac precursors. 

Perspectives for fabrication of improved oxygen electrodes at a low cost have been offered by non-noble, transition metal catalysts, although their intrinsic catalytic activity and stability are lower in comparison with those of Pt and Pt-alloys. The vast majority of these materials comprise (1) macrocyclic metal transition complexes of the N4-type having Fe or Co as the central metal ion, i.e., porphyrins, phthalocyanines, and tetraazaannulenes [6-8] (2) transition metal carbides, nitrides, and oxides (e.g., FeCjc, TaOjcNy, MnOx) and (3) transition metal chalcogenide cluster compounds based on Chevrel phases, and Ru-based cluster/amorphous systems that contain chalcogen elements, mostly selenium. 

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