Interaction, between chemisorbed atoms

That means that M 0 = 90 kcal/mol for Ead = 60 kcal/mol. All the data so far available indicate an appreciable decrease of this energy with increasing coverage, which must be ascribed to the operation of rather strong repulsive long-range interactions between chemisorbed oxygen atoms. 

If we move the chemisorbed molecule closer to the surface, it will feel a strong repulsion and the energy rises. However, if the molecule can respond by changing its electron structure in the interaction with the surface, it may dissociate into two chemisorbed atoms. Again the potential is much more complicated than drawn in Fig. 6.34, since it depends very much on the orientation of the molecule with respect to the atoms in the surface. For a diatomic molecule, we expect the molecule in the transition state for dissociation to bind parallel to the surface. The barriers between the physisorption, associative and dissociative chemisorption are activation barriers for the reaction from gas phase molecule to dissociated atoms and all subsequent reactions. It is important to be able to determine and predict the behavior of these barriers since they have a key impact on if and how and at what rate the reaction proceeds. 

Figure 9.6 Visual representation of the platinum oxide growth mechanism, (a) Interaction of H2O molecules with the Pt electrode occurring in the 0.27 V < < 0.85 V range, (b) Discharge of 5 ML of H2O molecules and formation of 5 ML of chemisorbed oxygen (Ochem)- (c) Discharge of the second ML of H2O molecules the process is accompanied by the development of repulsive interactions between (Pt-Pt) -Ofi m surface species that stimulate an interfacial place exchange of Ochem and Pt surface atoms, (d) Quasi-3D surface PtO lattice, comprising Pt and moieties, that forms through the place-exchange process. (Reproduced with permission...

Now, we consider H, atoms produced from hydrogen molecules adsorbed on zinc oxide under the influence of electron (ion) impact. We suppose that in this case the energy released in interaction of an electron (ion) with an adsorbed molecule is enough to break any bond between hydrogen atoms. As a consequence, Hj atoms bounce apart over the surface. Hydrogen atoms produced in this case are similar to H atoms adsorbed on the oxide surface from the gas phase at small surface coverages. In other words, they can be chemisorbed as charged particles and thus may influence electric conductivity of zinc oxide. This conclusion is consistent with the experimental results. 

This picture of chemisorbed atoms on jellium, although much too simple, illustrates a few important aspects of chemisorption. First, the electron levels of adsorbed atoms broaden due to the interaction with the s-electron band of the metal. This is generally the case in chemisorption. Second, the relative position of the broadened adsorbate levels with respect to the Fermi level of the substrate metal determines whether charge transfer between metal and adatom takes place and in which direction. 

By considering the extreme case of a crystal completely covered by a layer of foreign atoms, we have already seen in Sec. III,B that, if chemisorption involves the formation of localized electron pair bonds, some interesting interaction effects are to be expected. In this section, we approach the problem from the other extreme by considering just two atoms chemisorbed on a crystal surface. If the localized level formed by the interaction does not lie too far below the normal crystal band (or any surface band), the wave function for the localized level is damped only slowly in the crystal. Therefore, two chemisorbed atoms will be in interaction at distances when the interaction between the isolated atoms would be entirely negligible. To investigate this effect, we take the simplest model which may be expected to yield useful results 11). The crystal is represented by a straight... 

We emphasize again that these effects are not due to any direct interaction between the chemisorbed atoms. In the model used in this section, the resonance integral between the two foreign atoms is assumed to be zero at all separations. The important interaction takes place through the crystal. This interaction is large if the original localized level for a single atom... 

Some properties of palladium deposited on different amorphous or zeolitic supports were determined, including catalytic activity per surface metal atom (N) for benzene hydrogenation, number of electron-acceptor sites, and infrared spectra of chemisorbed CO. An increase of the value of N and a shift of CO vibration toward higher frequencies were observed on the supports which possessed electron-acceptor sites. The results are interpreted in terms of the existence of an interaction between the metal and oxidizing sites modifying the electronic state of palladium. 

Three types of adsorption can be considered (i) molecularly chemisorbed, (ii) atomically chemisorbed and (iii) molecularly physisorbed. In the case hydrogen is chemisorbed on platinum, a real bond between Pt and atomic H is formed, i.e. two electrons are located in a bonding orbital between Pt and H or H2. In the case hydrogen is molecularly physisorbed, the interaction between Pt and molecular H2 is electrostatic, i.e. no electrons are shared in a Pt-H bond and no dissociation takes place. 

An additional aspect of surface structure determination involves the relationship between swface structure and reactivity. The study of adsorbates on well ordered solids constitutes much of the structural work being carried out today. When an atom or molecule adsorbs on a clean substrate, its equilibrium position is determined by its interactions with the surface atoms, and its interactions with neighboring adsorbates. For physically adsorbed species, adsorbate adsorbate interactions can equal the interaction between the adsorbate and substrate and affect the structure of the overlayer. However the adsorbate adsorbate interactions are typically small when compared to the forces in chemical bonds formed upon adsorption and adsorbate-substrate interactions dominate in chemisorbed systems. 

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