Effect on surface reactivity

R. J. Madix, Selected principles in surface reactivity reaction kinetics on extended surfaces and the effects of reaction modifiers on surface reactivity, in The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Vol. 4, ed. D. A. King and D. P. Woodruff, Elsevier, Amsterdam, 1982, 1. 

The state of bonding of carbon on the surface can easily be detected by AES. The carbon Auger peak shows a different fine structure for carbidic and graphitic carbon, as shown in Fig. 5a and b, respectively (32). Carbidic carbon characteristically shows three sharp peaks near 270 V the graphitic form shows a more rounded spectrum. It is therefore possible to differentiate the effects of carbidic and graphitic carbon on surface reactivity. 

The erosion effects of cavitation on solid surfaces have been extensively investigated both in terms of surface erosion [68] and corrosion [69]. The consequences of these effects on metal reactivity are important since passivating coatings are frequently present on a metal surface (e. g. oxides, carbonates and hydroxides) and can be removed by the impacts caused by collapsing cavitation bubbles. An illustration can be found with the activation of nickel powder and the determination of the change in its surface composition under the influence of cavitation by Auger spectroscopy (Fig. 3.6) [70]. 

The ability of micellar solutions and mlcroemulslons to dissolve and compartmentalize both polar and non-polar reactants has a significant effect on chemical reactivity. An Idealized representation of a typical micelle catalyzed reaction is depicted In Figure 2. Here the non-polar reactant is solubilized within the micelle while the ionic reactant is at the surface. The polar head groups of the surfactants generate a charge at the micelle surface which serves to attract an oppositely charged water soluble reactant increasing the concentration of that reactant near the micelle. The result Is an enhanced reaction rate. 

Analogous effects have also been discussed for the reactivity of different bulk metal surfaces, where the center of gravity of the d-band determines the bonding [350]. In this case, the effects on the reactivity are considerably smaller than in the case of clusters in the nonscalable size regime and are size independent. Additional factors that change the d-band structure for a particular element are strain effects and the crystallographic orientation of the respective surface plane (see Chap. 3). 

Chaimovich and coworkers have prepared large unilamellar vesicles of DODACl by a vaporization technique which gives vesicles of ca 0.5 pm diameter. These vesicles are much larger than those prepared by sonication, where the mean diameter is 30 nm, and their effects on chemical reactivity are very interesting. The reaction of p-nitrophenyl octanoate by thiolate ions is accelerated by a factor of almost 10 by DODACl vesicles (Table 2), but this unusually large effect is due almost completely to increased concentration of the very hydrophobic reactants in the small region of the vesicular surface and an increased extent of deprotonation of the thiol. There is uncertainty as to the volume element of reaction in these vesicles, but it seems that second-order rate constants at the vesicular surface are similar to those in cationic micelles or in water (Cuccovia et al., 1982b Chaimovich et al., 1984). 

Submicroscopic, colloidal aggregates can influence chemical reactivity. Aqueous micelles are the most widely studied of these aggregates, and these micelles form spontaneously when the concentration of a surfactant (sometimes known as a detergent) exceeds the critical micelle concentration, cmc (1-3). Surfactants have apolar residues and ionic or polar head groups, and in water at surfactant concentrations not much greater than the cmc, micelles are approximately spherical and the polar or ionic head groups are at the surface in contact with water. The head groups may be cationic, (e.g., trimethylammonium), anionic, (e.g., sulfate), zwitterionic (as in carboxylate or sulfonate betaines), or nonionic. The present discussion covers the behavior of ionic and zwitterionic micelles and their effects on chemical reactivity. 

The structural characterization of electrode surfaces on the mesoscopic scale is a prerequisite for the elucidation of mesoscopic effects on electrochemical reactivity. The most straightforward approach to access the mesoscopic scale is the application of scanning probes under in-situ electrochemical conditions. Three different applications of STM have been discussed, namely the structural characterization of model electrodes, the visualization of dynamic processes on the nanometer-scale, and the defined modification of electrode surfaces. 

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