Metal microcrystallites

The sol-gel approach has been used in catalysis in four main directions preparation of inorganic oxide catalysts " entrapment of metal microcrystallites polymerization of trialkoxysilyl derivatives of metal ligands and by direct, straightforward entrapment of organometallic catalysts in sol-gel matrices ". In this Section we concentrate on the latter and compare it with covalent attachment of trialkoxy derivatives, which is fully described in Section IV. As a representative example we consider the case of ion-pair catalysts " and, in particular, RhCls/Aliquat 336 and RhCl3/[Me3N(CH2)3... 

When the oxide is formed by anodizing in acid solutions and the sample is then left to rest at the OCP, some dissolution can occur. This process has been studied by a numbers of authors,70-75 especially in relation to porous oxides [cf. Section 111(4)]. It was found that pore walls are attacked, so that they are widened and tapered to a trumpet-like shape.70 71 Finally, the pore skeleton collapses and dissolves, at the outer oxide region. The outer regions of the oxide body dissolve at higher rates than the inner ones.9,19 The same is true for dissolution of other anodic oxides of valve metals.76 This thickness dependence is interpreted in terms of a depth-dependent vacancy concentration in the oxide75 or by acid permeation through cell walls by intercrystalline diffusion, disaggregating the microcrystallites of y-alumina.4... 

Application of metal alkoxides for the preparation of niobates and tanta-lates in the form of both powders and films has been first performed by Dand-liker [475]. In our study of hydrolysis of LiOEt andNb(OEt)5 solutions in anhydrous EtOH, it was demonstrated that although the freshly prepared powder is amorphous for X-ray, it already contains microcrystallites of LiNb03 [1778]. In addition to conventional techniques, the properties ofthe powder in the course of thermal treatment were studied by the second harmonic generation ofthe laser beam, which is a sensitive technique for detection ofnoncen-trosymmetric phase the first signal was registered at 350°C. 

Microcrystallites represent highly interesting objects for research in the field of heterogenous catalysis. Big clusters can serve as model compounds for metal crystallites. Questions arising in crystal research may be answered by studying the growth and prop-... 

The CV curves obtained for carbons with preadsorbed copper shown in Figs. 45 (curves b, b, c, c ) and 46 (a-a")) exhibit only slight peaks of the Cu(II)/Cu(I) couple and broad waves due to the redox reaction of surface carbon functionalities (.see Section IV). However, preadsorbed copper enhances the peaks of the redox process in bulk solution (especially the anodic peaks for D—H and D—Ox samples), as can be seen in Fig. 46 (curves c-c"). The low electrochemical activity of samples with preadsorbed copper species observed in neutral solution is the result of partial desorption (ion exchange with Na ) of copper as well as the formation of an imperfect metalic layer (microcrystallites). Deactivation of the carbon electrode as a result of spontaneous reduction of metal ions (silver) was observed earlier [279,280]. The increase in anodic peaks for D—H and D—Ox modified samples with preadsorbed copper suggests that in spite of electrochemical inactivity, the surface copper species facilitate electron transfer reactions between the carbon electrode and the ionic form at the electrode-solution interface. The fact that the electrochemical activity of the D—N sample is lowest indicates the formation of strong complexes between ad.sorbed cations and surface nitrogen-containing functionalities (similar to porphyrin) [281]. Between —0.35 V and -1-0.80 V, copper (II) in the porphyrin complex (carbon electrode modifier) is not reduced, so there can be no reoxidation peak of copper (0) [281]. 

Photoluminescence spectra of high surface area samples of alkaline earth oxides have been reported by Tench et al. [39] and interpreted in terms of the radiative decay of surface excitons. Representative data obtained by Tench et al. [39c] with microcrystalline MgO smoke (obtained by combustion of the metal in oxygen) are reproduced in Fig. 5, together with their schematic model for surfaces of the cubic MgO microcrystallites and their modification by exposure to water vapour. 

X-rav Diffraction. X-ray diffraction patterns of metallized tubules were recorded on a Rigaku DMAXB X-ray diffractometer in the theta-two theta mode using Cu-K radiation. A small amount of an acetone suspension of metal coated tubules was dried on a silicon wafer with a small amount of adhesive cement (Duco) and the diffraction pattern was recorded in the theta-two theta mode. From the width at half height of the (111) peak, the average grain size of the nickel microcrystallites was determined using the Debye-Scherrer law. ... 

Liu, H., Penner, R.M. Electrodeposition of size monodisperse metal nano- and microcrystallites in the uncoupled Emit. J. Phys. Chem. B 104, 9131 (2000)... 

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