Electronic spillover

The tight binding framework discussed here is general, although the specific calculations may incorporate some differences or simplifications with respect to the basic method. For instance, Guevara et al.21 have pointed out the importance of the electron spillover through the cluster surface. These researchers incorporated this effect by adding extra orbitals with s symmetry outside the surface. This development will be considered later in some detail. 

This effect is formally called electron spillover, and can be as far as 0.1 to 0.2 nm from the metal edge (Fig. 6.71).49 Since the electronic gas is the active part in the metal (the moving part), electrons are considered the main factor responsible for the properties of the metal. 

Carbon is the usual support because of its high conductivity and relative high resistance to corrosion synergetic effects such as electronic spillover is possible in supported electrocatalysts. 

Here, as = -Vrs is the charge density on the -S groups of a hypothetical close-packed monolayer of thiolate anions, P is the distance of the center of charge of the sulfur atoms from the metal surface, d = p + y is the length of the thiol molecule, and p and y are the dielectric constants accounting for the distortional polarization in the (0 < x< P) and (psurface dipole potential due to electron spillover and Xs is that due to any polar groups of the thiol molecule. For simplicity, the small potential difference (4/ across the diffuse layer is disregarded. 

Here P and y are the thickness of the hydrocarbon tail and of the polar head region of the lipid monolayer, Sp and eY are the corresponding distortional dielectric constants, %e and Xm are the surface dipole potentials due to the electron spillover and to the oriented polar heads, and fa is the potential difference across the diffuse layer. At ion concentrations that are not exceedingly low, fa can be disregarded as a good approximation. Moreover, the orientation of the polar heads of the lipid film is hardly affected by changes in aM. The differential capacity C of the electrode can, therefore, be written ... 

Related to these matters has been the question whether two-component electrode metals (dual-site model) could lead to an electrocatalyst surface that exhibited catalytic properties better than either of its components. Qualitative ideas about electron spillover between one component and another at microcrystal grain boundaries, or transfer of the chemisorbed intermediate from one site to another, could suggest the possibility of such an effect. However, a quantitative theoretical analysis of this question by Parsons (149), based on his treatment of chemisorption effects at single metals (23) having various AG ,, h values, showed that, for practical applications, almost no... 

AuNPs inserted between the electrode surface and redox metalloproteins therefore both work as effective molecular linkers and exert eflfident electrocatalysis. Recent considerations based on resonance turmeling between the electrode and the molecule via the AuNP as a mechanism for enhanced interfadal ET rates suggest that electronic spillover rather than energetic resonance is a hkely origin of the effects (J. Kleis et al., work in progress). Even slightly enhanced spillover compared with a planar Au(lll) surface is enough to enhance the ET rate by the observed amount over a 10-15 A ET distance. 

We also mentioned earlier the oscillatory nature of the density as a function of metal-surface distance. If the electron density is high the oscillation decreases by the electron-electron interactions. We will revert to this later. Because of the electron spillover at z > 0, negative charges will not be balanced by the positive background. As a result a surface-dipole layer develops. The potential due to this double layer can be calculated with Poisson s equation ... 

In the models discussed in the previous sections the metal electrode has a constant contribution to the equilibrium constant(s) of the adsorption process(es). However, the role of the metal is more complex, especially in the case of solid electrodes. Up to now two effects have been analyzed in some detail electron spillover from the metal into the electrolyte solution, and the heterogeneity of the electrode surface. 

Figure 20. Effect of electron spillover (- - -) on the adsorption isotherms predicted by Guidelli s et al three-dimensional model for a polar monomeric solute pointing towards the metal (A) and away from the metal (B) at = -...

Because in many electrochemical reactions, particularly in electroorganic ones, radicals are formed as reactive intermediates, ESR has been applied frequently to studies of the mechanism and the kinetics of these reactions [594-596]. Although possible, NMR spectroscopy has been used infrequently and only in very recent experiments, mainly because of the considerably larger experimental effort [597]. With NMR spectroscopy, information about surface structure, surface diffusion and electron spillover from the metal electrode onto an adsorbate can be obtained. So... 

As already mentioned, the distribution of electrons at the jellium surface entails a surface dipole moment. Therefore, at the pzc a water molecule situated within the electronic tail experiences a positive field that tends to orient the molecule with its oxygen end toward the metal surface. Conversely, the water molecules tend to enhance the electronic spillover. As a result, the work function for jellium in contact with water is lower than in the vacuum. Model calculations for this effect have been performed by several authors [34, 37, 39-41]. While the results depend on the details of the model, it is generally agreed that the larger the overall change in the dipole potential, the higher the electronic density. 

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