Charge distribution in adsorbates

The distribution of charges on an adsorbate is important in several respects It indicates the nature of the adsorption bond, whether it is mainly ionic or covalent, and it affects the dipole potential at the interface. Therefore, a fundamental problem of classical electrochemistry is What does the current associated with an adsorption reaction tell us about the charge distribution in the adsorption bond In this chapter we will elaborate this problem, which we have already touched upon in Chapter 4. However, ultimately the answer is a little disappointing All the quantities that can be measured do not refer to an individual adsorption bond, but involve also the reorientation of solvent molecules and the distribution of the electrostatic potential at the interface. This is not surprising after all, the current is a macroscopic quantity, which is determined by all rearrangement processes at the interface. An interpretation in terms of microscopic quantities can only be based on a specific model. 

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Figure 4.3 Two alternative ways of viewing the charge distribution in an adsorption bond. The upper part of this figure shows the dipole moment the lower part shows a partially charged adsorbate and its image charge. The dipole moments of the surrounding solvent molecules are oriented in the direction opposite to the adsorbate dipole.

Dowden (19) developed a theory of heterogeneous catalysis on the basis of electron exchanges between catalysts and adsorbates [see also Boudart (19a)]. Hauffe and Engell (20), Aigrain and Dugas (21), as well as Weisz (22), tried to relate chemisorption on semiconductors to the charge distributions in the adsorbing semiconductors. 

A similar behavior may be found when the charge distribution in the molecules is more complex. In carbon dioxide the charge distribution is of the character of a quadrupole. Lenel (40) calculated the influence of the interaction of this quadrupole with the surface of an alkali halide crystal and reached the conclusion that a substantial contribution of roughly 3 kcal./mole is to be expected from this polar interaction. Recently Drain (41) succeeded in approaching the remarkable fact that the heat of adsorption of N2 on ionic crystals is often appreciably higher than that of 02 and A, which is not the case when these gases are adsorbed on nonionic surfaces. He shows that the quadrupole of N2 may be responsible. We shall return to this problem in Sec. VI,2. 

The 0D and 0R temis always contribute, regardless of the specific electric charge distributions in the adsorbate molecules, which is why they are called nonspecific. The third nonspecific p term also always contributes, whether or not the adsorbate molecules have permanent dipoles or quadrupoles however, for adsorbent surfaces which are relatively nonpolar, the polarization energy p is small. 

Physisorption or physical adsorption is the mechanism by which hydrogen is stored in the molecular form, that is, without dissociating, on the surface of a solid material. Responsible for the molecular adsorption of H2 are weak dispersive forces, called van der Waals forces, between the gas molecules and the atoms on the surface of the solid. These intermolecular forces derive from the interaction between temporary dipoles which are formed due to the fluctuations in the charge distribution in molecules and atoms. The combination of attractive van der Waals forces and short range repulsive interactions between a gas molecule and an atom on the surface of the adsorbent results in a potential energy curve which can be well described by the Lennard-Jones Eq. (2.1). 

FIG. 31 Amphoteric poly electrolyte at charged interfaces. Schematic representation of the positive and negative charge distribution in the adsorbed layer left, stuck clays, random distribution right, dispersed clays, charge segregation. 

Specifically adsorbed anions can also significantly affect the stability and electrochemical reactivity of metal electrodes. Examples are the alteration of the potential (charge) distribution in the double layer known as Frumkin effect [248], the lifting of the surface reconstruction of Au(hkl) electrodes [249], the electrochemical deposition and dissolution of metals in the presence of anionic ligands [250], and the role of halides and sulfate on the oxygen reduction on Pt and Au [251]. 

The experimental verification of the partial charge transfer and determination of the partial charge of ions adsorbed on the electrode surface is connected with a general controversy about the pure thermodynamic interpretation of the partial charge and attempts to interpret measurements by models allowing an approximate view on the charge distribution in the double layer. 

Figure 1.5 STM images of submolecularly resolved cycio [12]thiophene 4.26 (40 x 40 A) (left), Ceo fullerene epitaxially adsorbed in a second layer [long-range order, (2x2 fj.m)] (middle) and the structural model of the 1 1 D-A complex (right, top) and charge distribution in the FIOMO. Courtesy of Elena Mena-Osteritz, University of Ulm. Middle image from E. Mena-Osteritz and P. Bauerle, Complexation of Ceo on a cyclothiophene monolayer template, Adv. Mater., 18, 447-451 (2006), Copyright Wiley-VCH Verlag GmbH Co. KGaA. Reproduced with permission...

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