Electron spillover

Solvent displacement, and isotherms. 954. 955 Solvent excess entropy at the interface, 912 Solvent interactions, 923, 964 Soriaga, M., 1103, 1146 Specifically adsorbed ions, 886 Spectrometer, 797 Spikes, electrodeposition. 1336 Spillover electrons, of metal, 889 Spiral growth, electrodeposition, 1316, 1324, 1326, 1324,1328 s-polarized light, 802 Srinivasan, S 1439,1494 Standard electrode potential American convention, 1354 convention, 1351 rUPAC convention, 1355 prediction of reactions, 1359 the zinc-minus and copper-plus convention, 1352... 

Fig6.69. Effectofthechargeofthemetal on the spillover electrons and on the metal capacity of the double layer. 

Fig. 6.71. The jellium model of the metal electrode. The positive background charge abruptly disappears at the jellium edge while spillover electrons can be found beyond the edge (shaded area). The continuous line represents the profile of electrons in the interfacial region. The positions of the ion cores are indicated by the arrows.

Electron tail ( spillover electrons, Fig. 3.17) can be also involved in bonding of adsorbed molecules (atoms). Adsorption is discussed further in Chapter 10. 

Similar observations confirming the reversible Na spillover-backspillover mechanism of electrochemical promotion have been made by Lambert and coworkers using AES (Auger Electron Spectroscopy).61... 

Figure 7.9. Schematic representation of the density of states N(E) in the conduction band of two transition metal electrodes (W and R) and of the definitions of work function O, chemical potential of electrons p, electrochemical potential of electrons or Fermi level p, surface potential x, Galvani (or inner) potential (p and Volta (or outer) potential for the catalyst (W) and for the reference electrode (R). The measured potential difference UWr is by definition the difference in p q>, p and p are spatially uniform O and can vary locally on the metal surfaces 21 the T terms are equal, see Fig. 5.18, for the case of fast spillover, in which case they also vanish for an overall neutral cell Reprinted with permission from The Electrochemical Society.

Modern theories of electronic structure at a metal surface, which have proved their accuracy for bare metal surfaces, have now been applied to the calculation of electron density profiles in the presence of adsorbed species or other external sources of potential. The spillover of the negative (electronic) charge density from the positive (ionic) background and the overlap of the former with the electrolyte are the crucial effects. Self-consistent calculations, in which the electronic kinetic energy is correctly taken into account, may have to replace the simpler density-functional treatments which have been used most often. The situation for liquid metals, for which the density profile for the positive (ionic) charge density is required, is not as satisfactory as for solid metals, for which the crystal structure is known. 

Finite resolution and partial volume effects. Although this can occur in other areas of imaging such as MRS, it is particularly an issue for SPECT and PET because of the finite resolution of the imaging instruments. Resolution is typically imaged as the response of the detector crystal and associated electron to the point or line source. These peak in the center and fall off from a point source, for example, in shapes that simulate Gaussian curves. These are measures of the ability to resolve two points, e.g. two structures in a brain. Because brain structures, in particular, are often smaller than the FWHM for PET or SPECT, the radioactivity measured in these areas is underestimated both by its small size (known as the partial volume effect), but also spillover from adjacent radioactivity... 

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. 

Electronic Properties of Transition Metal Clusters Consideration of the Spillover in a Bulk Parametrization. 

The rate enhancement observed for submonolayer Cu deposits may relate to an enhanced activity of the strained Cu film for this reaction due to its altered geometric and electronic properties. Alternatively, amechansim whereby the two metals cooperatively catalyze different steps of the reaction may account for the activity promotion. For example, dissociative Hj adsorption on bulk Cu is unfavorable due to an activation barrier of approximately 5 kcal/mol . In the combined Cu/Ru system, Ru may function as an atomic hydrogen source/sink via spillover to/from neighboring Cu. A kinetically controlled spillover of Hj from Ru to Cu, discuss above, is consistent with an observed optimum reaction rate at an intermediate Cu coverage. 

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. 

Through the jellium model of the metal we have explained the effect of the metal electrons on the interfacial properties. We also know that the spillover of electrons creates a separation of charges at the metal edge, and consequently, a surface potential. However, what is the magnitude of this surface potential How important is its contribution to the total potential drop in the interfacial region ... 

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. 

As the particle size decreases, the ratio between the number of atoms at the surface to those in the bulk increases with a parallel decrease in the average coordination number for the metal atom, which is also expected to be a factor of electrocatalysis. It has been calculated for Pt that the minimum size of a crystallite (cluster) for all atoms to be on the surface is 4 nm, corresponding to a specific surface area of 280 m2g-1 [322] (note that this is larger than the critical particle size where absorption of H atoms disappears on Pd) [333]. It is also interesting that dispersed catalysts can in turn influence the electronic properties of the support so that an interesting combination of sites with varied properties can result [330]. At low catalyst loadings, spillover of intermediates is also possible. 

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