Adsorbate-metal interactions

The Gibbs energy of adsorption is a measure of adsorbate-metal interactions. Its values depend, however, on the choice of standard states for the chemical potentials of the components involved in the process. Therefore AG° values determined for different systems can only be compared if they refer to the same standard-state conditions. AG° values of adsorption of thiourea (TU) on several metallic electrodes, calculated for the most often used standard states, are presented in Table 1. 

Before we perform the integration, it is, however, important to note that the effect of the perturbation is to introduce some extra states below the Fermi level 5N. These are states that, because of the adsorbate-metal interaction event, have been removed... 

In the first linear free energy approach (Model 2a. 1), the energy of the electron(s) transferred on anion adsorption is the only potential dependent term in the adsorption free energy. This approach assumes that the electric field caused by the applied potential (or excess surface charge) has no interaction with the adsorbate or influence on the adsorbate-metal interaction. Two methods to approximate the impact of the electric field effect on elementary surface processes that do not require explicitly charging the metal surface (Models 2a.2 and 2a.3) will be discussed. The dependence of a species energy within an electric field F) is... 

The dipole density profile p (z) indicates ordered dipoles in the adsorbate layer. The orientation is largely due to the anisotropy of the water-metal interaction potential, which favors configurations in which the oxygen atom is closer to the surface. Most quantum chemical calculations of water near metal surfaces to date predict a significant preference of oxygen-down configurations over hydrogen-down ones at zero electric field (e.g., [48,124,141-145]). The dipole orientation in the second layer is only weakly anisotropic (see also Fig. 7). 

The orientational structure of water near a metal surface has obvious consequences for the electrostatic potential across an interface, since any orientational anisotropy creates an electric field that interacts with the metal electrons. Hydrogen bonds are formed mainly within the adsorbate layer but also between the adsorbate and the second layer. Fig. 3 already shows quite clearly that the requirements of hydrogen bond maximization and minimization of interfacial dipoles lead to preferentially planar orientations. On the metal surface, this behavior is modified because of the anisotropy of the water/metal interactions which favors adsorption with the oxygen end towards the metal phase. 

Indirect ( through the metal ) interaction due to the redistribution of electrons in the metal. In this case an electropositive promoter decreases the work function of the surface and this in turn weakens the chemisorptive bond of electropositive (electron donor) adsorbates and strengthens the chemisorptive bond of electronegative (electron acceptor) adsorbates. 

Underpotential Deposition of Metal Atoms Because of the energy of interaction between a foreign substrate and the adsorbed metal atoms formed by discharge, cathodic discharge of a limited amount of metal ions producing adatoms is possible at potentials more positive than the equilibrium potential of the particular system, and also more positive than the potential of steady metal deposition. 

Interactions between adsorbed metal ions and complex organic molecules have been observed in other systems. For example, IR results Indicated that coraplexation occurred between sulfolane (C4 H8S02) and either Cu or NI montmorillonites, with the result that... 

Hence, if adsorbed molecules interact only weakly and nonspecifi-cally with the metal surface, the term AG° B - M) can be treated as being independent of the nature of the metal. If, simultaneously one assumes that for organic compounds the 5 - 5 interactions are approximately the same, then... 

The ion-water interactions are very strong Coulomb forces. As the hydrated ion approaches the solution/metal interface, the ion could be adsorbed on the metal surface. This adsorption may be accompanied by a partial loss of coordination shell water molecules, or the ion could keep its coordination shell upon adsorption. The behavior will be determined by the competition between the ion-water interactions and the ion-metal interactions. In some cases, a partial eharge transfer between the ion and the metal results in a strong bond, and we term this process chemisorption, in contrast to physisorption, which is much weaker and does not result in substantial modification of the ion s electronic structure. In some cases, one of the coordination shell molecules may be an adsorbed water molecule. hi this case, the ion does not lose part of the coordination shell, but some reorganization of the coordination shell molecules may occur in order to satisfy the constraint imposed by the metal surface, especially when it is charged. 

It was not until recently that Chen and Goodman probed the influence of the oxide support material on the intrinsic properties at the metal surface. By covering a titania support with one or two flat atomic layers of gold they eliminated, direct adsorbate-support interactions as well as particle size and shape effects. Their results definitively showed that the electronic properties at the metal surface changed due to charge transfer between the support and the metal. Furthermore, their comparison of one- and two-layer films highlighted the dependence of these effects on the thickness of the metal slab. 

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