Charge pools

Successful systems have used colloidal platinum as an efficient catalyst for the multi-electron reduction process by which hydrogen is produced. The platinum acts as a charge pool in that electrons from one-electron processes are trapped, to be later delivered to the substrate in a concerted manner, thus avoiding formation of high-energy intermediates (Figure 12.12). 

Clusters, as possible catalytic reactors, are perfectly dispersed in solutions. They are thus suitable systems for observing, under quasi-homogeneous conditions by time-resolved techniques, the kinetics of catalyzed electron transfer, which would be inaccessible on a solid catalyst. It was demonstrated that the reaction of radiation-induced free radicals COT and (CH3)2COH catalyzed by metal clusters started by the storage of electrons on clusters as charge pools and that electrons were then transferred pairwise to water-producing molecular hydrogen [22,75]. 

Dendrimers possessing a large number of chemically identical redox units can be defined as charge pooling devices only if they satisfy some conditions. Their redox units should be... 

Ir , concerning the electron transfer from radiation-induced free radicals such as COj or (CH3)2C OH to a substrate, after the complete reduction of the metal ions, has been demonstrated. Electrons donated from the radicals are first stored on clusters as charge pools and then are transferred again for example pairwise to water producing molecular hydrogen ... 

Purification of the hydrothermally treated brewery s spent grain also resulted in a pool containing charged xylan-material (9). Most likely, the recovered xylan-fragments were still partly substituted with (4-0-methyl-)glucuronic acid residues. However, the charged pool represented less than 1 % of the material obtained after hydrothermal treatment of brewery s spent grain and was not studied in more detail. 

The results of several studies were interpreted by the Poole-Erenkel mechanism of field-assisted release of electrons from traps in the bulk of the oxide. In other studies, the Schottky mechanism of electron flow controlled by a thermionic emission over a field-lowered barrier at the counter electrode oxide interface was used to explain the conduction process. Some results suggested a space charge-limited conduction mechanism operates. The general lack of agreement between the results of various studies has been summari2ed (57). 

Further down, ca 75 cm below the electrode tips, the mix is hot enough (2200—2500°C) to allow the lime to melt. The coke does not melt and the hquid lime percolates downward through the relatively fixed bed of coke forming calcium carbide, which is Hquid at this temperature. Both Hquids erode coke particles as they flow downward. The weak carbide first formed is converted to richer material by continued contact and reaction with coke particles. The carbon monoxide gas produced in this area must be released by flowing back up through the charge. The process continues down to the taphole level. Material in this area consists of soHd coke wetted in a pool of Hquid lime and Hquid calcium carbide at the furnace bottom. 

Besides its temperature dependence, hopping transport is also characterized by an electric field-dependent mobility. This dependence becomes measurable at high field (namely, for a field in excess of ca. 10d V/cm). Such a behavior was first reported in 1970 in polyvinylcarbazole (PVK) [48. The phenomenon was explained through a Poole-ITenkel mechanism [49], in which the Coulomb potential near a charged localized level is modified by the applied field in such a way that the tunnel transfer rale between sites increases. The general dependence of the mobility is then given by Eq. (14.69)... 

Enantioenriched alcohols and amines are valuable building blocks for the synthesis of bioactive compounds. While some of them are available from nature s chiral pool , the large majority is accessible only by asymmetric synthesis or resolution of a racemic mixture. Similarly to DMAP, 64b is readily acylated by acetic anhydride to form a positively charged planar chiral acylpyridinium species [64b-Ac] (Fig. 43). The latter preferentially reacts with one enantiomer of a racemic alcohol by acyl-transfer thereby regenerating the free catalyst. For this type of reaction, the CsPhs-derivatives 64b/d have been found superior. 

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