Electron transfer from metal surfaces

In a more recent publication [36] on electron transfer from metal surfaces, further confirmation of this scheme was given, based on the results of reactions of ( + )-(S)-bromomethyledene-4-methylcyclohexane (III) with magnesium. 

The energy transfer processes during the chemisorption on metal surfaces have been generally explained in terms of electron transfer from metal to incoming species (Cox et al., 1983 Gesell et al., 1970 Prince et al., 1981). 

In addition to the acidic and basic properties mentioned previously, oxides and halides can possess redox properties. This is particularly true for solids containing transition metal ions because the interactions with probe molecules such as CO, H2, and O2 can lead to electron transfer from the surface to the adsorbed species and to the modification of the valence state of the metal centers. An important role in surface redox processes involving CO is played by the most reactive oxygen ions on the surface (e.g., those located at the most exposed positions such as corners), which can react with CO as follows ... 

The questions that needed to be answered [56] were the following Is bond-breaking a concerted or stepwise process when electrons are transferred from metal surfaces to the carbon-halogen bond Are radical anions formed as intermediates Do the n orbitals of the aromatic moiety in 18,131, and 158 complete with the a orbital of the carbon-halogen bond during the electron-transfer process from the surface, thereby causing these systems to be atypical [93d, 93e] ... 

Pd metal on supports such as AI2O3, MgO, Si02, and CaCOg, and Raney Pd, have been reported to be effective in the Heck arylation, including that promoted by use of microwaves [18a-c]. With MgO supports electron transfer from the surface of the support to that of the metal leads to anchoring of the metal to the... 

Interpretation of the first three terms in Eq.(2.229a) is straightforward. Term (1) results from the assumption that in the adsorbate no levels higher than Ef were occupied before adsorption. If such terms were occupied a correction to account for electron transfer from the adsorbate to the Fermi-level has to be introduced. Term (1) reflects electron transfer front the occupied undisturbed adsorbate levels to the Fermi level, term (2) electron transfer from a surface electronic state present before chemisorption to the Fermi level and term (3) electron transfer from the Fermi level to a surface molecule- or chemisorption- induced surface state level. In the surface molecule limit the contribution to the chemisorption energy is easy to calculate for s-type orbitals. If an orbital has Z" metal atom neighbors, the solutions cf become the solution of Ek. (2.215) ... 

Electron transfer from metal oxide surfaces to CO can be quite facile, occurring at room temperature. This process can be important as an initial CO activation step in metal oxide catalyzed reduction schemes. We have attempted to clarify what types of metal oxides interact (MO CO MO. . . CO -) with CO in this way, and what surface features these active metal oxides possess. Only MgO, CaO, SrO, BaO, and Th02 were electron transfer active. These oxides have in common the possession of both Lewis basic sites and one electron reducing site. It appears that CO is first adsorbed on Lewis base sites followed by slow migration to electron transfer reducing sites. The studies leading to this conclusion are discussed. 

For clean metal surfaces, the direction of net electron transfer can be determined from measurements of the work function variation, Acp. A positive A(p indicates net electron transfer from the surface to the adsorbed molecule (basic surface) while a negative Acp indicates electron transfer in the opposite direction (acidic surface). As with all Lewis probes, a clean metal surface may act either as a base or as an acid, depending on the nature of the interacting molecules. For example, a clean Ni(l 11) surface is a Lewis base (Acp < 0) toward carbon monoxide [301 and a Lewis acid (Acp < 0) toward acetylene and benzene [31]. However, the criterion of net electron transfer is not always sufficient to define the acid or base character of clean metal surfaces. The reason is the substantial surface charge reorganization that may accompany chemisorption on metals. Because electronic states of metal surfaces are closely spaced in energy, acid-base interactions that require electron transfer and the formation of new chemical bonds are often delocalized. 

The early studies demonstrated the possibility of initiating polymerization by intensive mechanical dispersion of certain inoiganic substances, including metals (Fe, Al, Mg, Cr, W) in vinyhc monomers. The degree of polymerization of styrene, vinyl acetate, acrylonitrile, or MMA depended on the dispersion intensity. The fresh metal surfaces play the role of catalyst and initiator. These surfaces are the sites of electron transfer from the surfaces metal atoms to the monomers to form ion-radical initiating particles. Colloidal particles of Au, Tl, and Pt were found to influence substantially the bulk and solution polymerization of styrene. - ... 

The activation of the bond that is broken occurs via a similar mechanism to that for the oxidative addition over an organometallic substrate. Dissociation leads to the formation of negatively charged adsorbate-fragment orbitals with formally oxidized metal surface atoms. Dissociation typically occurs over the top of a surface atom. The critical point in the activation of the C-H or N-H bond occurs when it is stretched sufficiently such that the empty antibonding bond orbital lowers close enough to the Fermi level to allow for back-donation and electron transfer from metal into the antibonding state of the absorbate. This is illustrated in Fig. 3.44 for the dissociation of H2 over different metal surfaces (see also ref. [4]). In the oxidative addition reaction, the reactant-molecular... 

These reactions lead to the formation (transformation) of surface carboxylate and carbonate-like species and to the two electron reduction of the (electrons that can reduce) transition metal ions located in nearest-neighbor positions. On oxidic surfaces that do not contain transition metal ions, redox reactions accompanied by electron transfer from the surface to the adsorbed molecule (or vice versa) are much less probable. 

If a paint film is to prevent this reaction, it must be impervious to electrons, otherwise the cathodic reaction is merely transferred from the surface of the metal to the surface of the film. Organic polymer films do not contain free electrons, except in the special case of pigmentation with metallic pigments consequently it will be assumed that the conductivity of paint films is entirely ionic. In addition, the films must be impervious to either water or oxygen, so that they prevent either from reaching the surface of the metal. 

In this reaction, copper metal plates out on the surface of the zinc. The blue color of the aqueous Cu2+ ion fades as it is replaced by the colorless aqueous Zn2+ ion (Figure 18.1). Clearly, this redox reaction is spontaneous it involves electron transfer from a Zn atom to a Cu2+ ion. 

Figure 7. Adsorption of an electronegative species from the gas phase onto a metal surface generates a dipolar layer due to electron transfer from the metal to the species. Adsorption of anions onto an electrode simulates the situation when the positive charge on the metal compensates for the adsorbed negative charge (zero diffuse-layer charge), and not when the charge on the metal is zero.

The common example of real potential is the electronic work ftmction of the condensed phase, which is a negative value of af. This term, which is usually used for electrons in metals and semiconductors, is defined as the work of electron transfer from the condensed phase x to a point in a vacuum in close proximity to the surface of the phase, hut heyond the action range of purely surface forces, including image interactions. This point just outside of the phase is about 1 pm in a vacuum. In other dielectric media, it is nearer to the phase by e times, where e is the dielectric constant. 

Only if one takes into account the solvent dynamics, the situation changes. The electron transfer from the metal to the acceptor results in the transition from the initial free energy surface to the final surface and subsequent relaxation of the solvent polarization to the final equilibrium value Pqj,. This brings the energy level (now occupied) to its equilibrium position e red far below the Fermi level, where it remains occupied independent of the position of the acceptor with respect to the electrode surface. 

It seems that Au ions of Au(OH) Cl4 complex, formed by the first aging at room temperature, are reduced to Au particles by electron transfer from the coordinated OH ions on the surface of hematite as a catalyst of the electron transfer. As a consequence, the essential reducing agent is water. The optimum pH to yield the maximum quantity of Au particles was ca. pH 5.9, as measured at room temperature, corresponding to the pH of the above standard system. Au ions are reduced to metallic Au by electron transfer from coordinated OH ions on the surfaces of hematite particles through their catalytic action. 

One possible mechanism for the oxidation of Mn(II) on an oxide surface is shown in Figure 2. The binding of Mn(II) to the surface may facilitate the electron transfer from Mn(II) to 02 (32). The surface groups on the metal oxide, if appropriately cooordinated, will exert a repulsive effect on the electron in the manganese dz orbital, making it easier for the 02 bound to the manganese atom to remove this electron. The nature of the products was not characterized so the overall reaction stiochiometry is unknown. 

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