Cluster immobilization

Reetz, M.T., Quaiser, S.A., Breinbauer, R., and Tesche, B., A New Strategy in Heterogeneous catalysis the design of cortex catalysts/catalysis/clusters/ immobilization/ surface chemistry, Angew. Chem. Int. Ed. Engl., 34, 2728,1995. 

In the past five years, the field of metal-zeolite chemistry has developed to the point where a number of metal atomic and small cluster guests have been synthesized and unequivocally characterized by spectroscopic and crystallographic techniques. The silver-zeolite A, X and Y systems in particular have yielded a wealth of valuable structural and site information pertaining to silver guests in the size range of one to six atoms (3). The spectroscopy of these silver-zeolites can therefore be probed in detail and compared with the corresponding data for silver atoms and silver clusters immobilized in weakly interacting rare gas solid supports (4). 

Choi, K.-M., Mizugaki, T., Ebitani, K., Kaneda, K. Nanoscale palladium cluster immobilized on a Ti02 surface as an efficient catalyst for liquid-phase Wacker oxidation of higher terminal olefins. Chem. Lett. 2003, 32, 180-181. 

Recendy, a spatially resolved model has also been applied to study the 1-MeV Kr irradiation in Fe-Cr F-M alloys and the commercial NF616 steels [81]. The in situ irradiation experiments showed that the interstitial cluster density saturates at around 10 dpa and the saturation is insensitive to temperature when the temperature is below 300°C [83,84]. In addition, interstitial clusters were found to hop for a few tens of nanometers and then remain immobile for some time when the irradiation beam is on. However, the clusters are immobile when the beam is off. It is likely that impurities trapped in the clusters make the clusters immobile, while the irradiation may detrap the impurities so that the clusters become mobile until they are trapped by impurities again [85]. 

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). 

Deposition of Cold Nanoparticles and Clusters onto Carbons and Polymers 3.2.2.1 Cold Colloid Immobilization (Cl)... 

A somewhat similar approach has been nsed for the formation of nanosize wires stretching between gold electrodes [34]. Lambda-DNA was positioned between two electrodes, with immobilized oligonncleotides complementary to lambda-DNA sticky ends. Silver (Ag ) ions were deposited on the stretched DNA bridges, followed by rednction of absorbed ions to metallic silver with hydroquinone. The resulting silver clusters formed on DNA strands were found to be 100 nm in diameter and were capable of condncting the electric current. 

By swelling with aqueous electrolyte, cations (and, to lesser extent, also anions) penetrate together with water into the hydrophilic regions and form spherical electrolyte clusters with micellar morphology. The inner surface of clusters and channels is composed of a double layer of the immobilized —SO3 groups and the equivalent number of counterions, M+. Anions in the interior of the clusters are shielded from the —SOJ groups by hydrated cations and water molecules. On the other hand, anions are thus... 

Fig. 2.20 The Gierke model of a cluster network in Nafion. Dimensions are expressed in nm. The shaded area is the double layer region, containing the immobilized —SO3 groups with corresponding number of counterions M+. Anions are expelled from this region electrostatically...

Common to all encapsulation methods is the provision for the passage of reagents and products through or past the walls of the compartment. In zeolites and mesoporous materials, this is enabled by their open porous structure. It is not surprising, then, that porous silica has been used as a material for encapsulation processes, which has already been seen in LbL methods [43], Moreover, ship-in-a-bottle approaches have been well documented, whereby the encapsulation of individual molecules, molecular clusters, and small metal particles is achieved within zeolites [67]. There is a wealth of literature on the immobilization of catalysts on silica or other inorganic materials [68-72], but this is beyond the scope of this chapter. However, these methods potentially provide another method to avoid a situation where one catalyst interferes with another, or to allow the use of a catalyst in a system limited by the reaction conditions. For example, the increased stability of a catalyst may allow a reaction to run at a desired higher temperature, or allow for the use of an otherwise insoluble catalyst [73]. 

Immobilization of these clusters on Pt(hkl) surfaces, employing bifunctional linkers with one pyridyl- and one thiol-based anchoring group, was achieved [334, 335]. The terminal pyridyl anchor ensures a strong chemical bond to the... 

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