Electrodispersion

The standard deviation of the Gaussian zones expresses the extent of dispersion and corresponds to the width of the peak at 0.607 of the maximum height [24,25]. The total system variance (ofot) is affected by several parameters that lead to dispersion (Eq. 17.22). According to Lauer and McManigill [26] these include injection variance (of), longitudinal (axial) diffusion variance (of), radial thermal (temperature gradient) variance (of,), electroosmotic flow variance (of,), electrical field perturbation (electrodispersion) variance (of) and wall-adsorption variance (of ). Several authors [9,24,27-30] have described and investigated these individual variances further and have even identified additional sources of variance, like detection variance (erf,), and others... 

FIGURE I Due to the relative high amount of sample injected, electrodispersion of compound 2 has a major influence on the migration and peak shape of compound 3. (A) 0.5 mg/ml of compound 2, 2.5pg/ml of compound 3. (B) 2.5pg/ml of compound 2, 2.5pg/ml of compound 3 (with permission from reference 31). 

The width of a peak, or, in other words, the length of a solute zone, is affected primarily by diffusion phenomena leading to a broadening of the solute zone. In addition, in capillary zone electrophoresis, zone broadening can be caused by thermal effects, electrodispersion, or adsorptive effects. All these effects can be expressed as coefficients of variance cr2, adding to a total coefficient of variance of the system ofotal ... 

Another effect contributing to zone broadening is electrodispersion,... 

For high-mobility ions, electrodispersion leads to peak fronting. In this case ions diffusing out of the solute zone upstream will experience a higher electric field and are accelerated back into the solute zone (sharp trailing... 

Fig. 4 Illustration of the effect of electrodispersion that is based on local differences of the electric field along the capillary.

Neutral species remain unaffected by electrodispersion, giving rise to symmetrical peaks. An example electropherogram illustrating peak fronting and peak tailing is shown in Figure 5. 

The adsorption and oxidation of hydrazine on electrodispersed and electro-faceted Pt electrodes [27] furnish another interesting example for the influence of crystalline surface composition on the electrocatalytic properties of the electrode. 

The same dependence between concentration of M nanoparticles (Ns) on a surface of a dielectric substrate and their catalytic activity has been also found out in the investigation of an amorphous films of M nanoparticles [117], prepared by laser electrodispersion technique and deposited on Si02 dielectric surface layer of thermally oxidized Si (see Chapter 15). It has been shown that in various reactions of chlorinated hydrocarbons catalyzed by so prepared nanostructured Cu film with growth Ns the value of Y increases firstl, reaches a maximum at Ns 4 x 1012 particles/cm2, and then quickly falls. 

This section describes the method of laser electrodispersion of metals, which was used to synthesize all the catalysts whose properties are discussed in this chapter. This method proved to be exceedingly promising both for synthesis and study of model catalytic systems and for fabrication of catalysts that exhibit a record-breaking high catalytic activity. 

A new feature, on which the laser electrodispersion method is based, consists in that the process of cascade fission terminates after the daughter drops reach a nanometer size. As charged drops immersed in plasma become smaller, the electric field on their surface increases, which results in a dramatic increase in the field emission of electrons. After the size of the daughter drops decreases to several nanometers, the flow of electrons emerging from the drop surface starts to exceed that coming in from the plasma. In this case, the drops discharge and become stable, so that the fission terminates. Thus, fission of micrometer and submicrometer drops in the laser torch plasma yields a tremendous number of nanometer drops with narrow size dispersion and a small amount of residues of maternal drops that had not enough time for total fission. 

The scheme by which nanostructures are formed by laser electrodispersion is shown in Figure 15.2. In accordance with this scheme, a laser pulse causes... 

It is also worth noting that laser electrodispersion can produce nano-structured alloy coatings and composite coatings composed of particles of different metals. For these purposes, alloyed targets are used and targets made of different materials are alternately irradiated. 

By now, the possibility of deposition of granulated Cu, Ni, Pd, Pt, and Au films by laser electrodispersion has been experimentally verified. The structural parameters of the films being formed were studied by various diagnostic techniques, with the most informative results obtained with TEM, atomic-force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). 

This section describes a simple model that enables evaluation of the influence of charge effects on the catalytic activity of metallic nanostructures. Also, the results of experiments performed with nanostructured catalysts synthesized by laser electrodispersion are discussed. These results demonstrate a relationship between the catalytic activity and charge density in the... 

In fabrication of the catalysts by laser electrodispersion, thermally oxidized silicon wafers with a thickness of Si02 oxide layer of 1 pm were used as a substrate. A substrate with so thick an oxide layer can be regarded as an insulator. In some cases, wafers of crystalline (1 0 0) Si were used, which had on their surface only a thin (l-2nm) layer of a natural oxide. This layer is tunnel-transparent for electrons, and, therefore, charge exchange between supported nanoparticles and silicon is possible. 

All the metallic nanostructures deposited by laser electrodispersion on both types of silicon substrates were found to be exceedingly active in the above processes. The activity was orders of magnitude higher than that of typical supported catalysts prepared by the standard techniques. Such a high activity is presumably due not only to the small size and amorphous state of nanoparticles, but also to the influence exerted by the charge effects discussed above. 

Another specific feature of the catalytic behavior of the structures under study consists in that the chemical nature of a metal becomes a factor less important for catalysis as the surface nanoparticles density increases. This is well seen in Figure 15.14, which shows the results obtained in measurements of the activity of copper- and nickel-based catalysts in the reaction of carbon tetrachloride addition to olefins. Presented in this figure are the activities of catalysts prepared by laser electrodispersion and the conventional deposition techniques. Two important features are worth noting. First, the activity... 

Fig. 15.14. Comparison of specific catalytic activities of Ni and Cu nanoparticles deposited onto a thermally oxidized silicon by laser electrodispersion (n m 5 x 1012cm-2) with those of catalysts prepared by the method of impregnation and reduction (1% Ni/Si02, 1% Cu/Si02). Reaction of carbon tetrachloride addition to olefins.

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