Chemisorption energies

As defined in Chap. 4, the chemisorption energy is the difference between the initial and final energies of the system. Although we could use the expression (4.85) for AE, it is more convenient to work with that of App. L, with a slight modification to account for double adsorption. Specifically, we have 

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Such attractive forces are relatively weak in comparison to chemisorption energies, and it appears that in chemisorption, repulsion effects may be more important. These can be of two kinds. First, there may be a short-range repulsion affecting nearest-neighbor molecules only, as if the spacing between sites is uncomfortably small for the adsorbate species. A repulsion between the electron clouds of adjacent adsorbed molecules would then give rise to a short-range repulsion, usually represented by an exponential term of the type employed... 

More recently, Silva et a/.447,448 have found that the temperature coefficients of dEa /dT for a number of stepped Au surfaces do not fit into the above correlation, being much smaller than expected. These authors have used this observation to support their view of the hydrophilicity sequence the low 9 (rs0/97 on stepped surfaces occurs because steps randomize the orientation of water dipoles. Besides being against common concepts of reactivity in surface science and catalysis, this interpretation implies that stepped surfaces are less hydrophilic than flat surfaces. According to the plot in Fig. 25, an opposite explanation can be offered the small BEod0/dT of stepped surfaces is due to the strong chemisorption energy of water molecules on these surfaces. 

In the previous two sections we have described trends in the chemisorption energies of atoms and molecules on metallic surfaces. These express the final situation of the adsorption process. Here we consider what happens when a molecule approaches a surface. 

If we restrict ourselves to the late transition metals the trends will, as for the CO chemisorption energy, be dominated by the interaction of the antibonding orbital with the d band and the leading term is... 

Medvedev IG. 2004. To a theory of electrocatalysis for the hydrogen evolution reaction The hydrogen chemisorption energy on the transition metal alloys within the Anderson-Newns model. Russ J Electrochem 40 1123-1131. 

The change in the total electronic energy, due to the interaction of the adatom with the substrate, is called the chemisorption energy AE. In order... 

Fig. 4.1. Anionic chemisorption energy-level diagram showing transfer of j-spin electron from substrate level ek to affinity level A on adatom, while experiencing Coulomb repulsion U from j-spin electron in ionization level I.

As in (1.92), the chemisorption energy is the difference between the electronic energy of the adatom-substrate system before and after the interaction occurs, i.e.,... 

Table 5.1. Adatom charge transfer Aq and chemisorption energy AE for atomic hydrogen on Ni film of (n + l)-layers thickness on ZnO support.

Fig. 5.4. Dependence of hydrogen chemisorption energy AE (solid line) and adatom charge transfer Aq (dashed line) of 2-layer Ni film on interaction parameter 7. Reprinted from Davison et al (1988) with permission from Elsevier.

The final property of interest is the chemisorption energy AE, given by (App. L)... 

The interaction energy, AIT, is the contribution (positive or negative) to AE, due to the (indirect) interaction between the two adatoms. In other words, AIT is the difference between the chemisorption energy AE for the doubleadsorption system and the sum of the chemisorption energies A (A = a or b) for the two individual single-adsorption systems, i.e.,... 

To understand the oscillatory dependence of AE on d, it is necessary to look more closely at the interaction energy AW because, as (8.66) shows, AE is the sum of the two single-atom chemisorption energies (which are independent of d) plus AW. Hence, any effect of d on AE must arise due to AW. Alternatively, one may consider the situation in terms of the adatom wave-functions, which, as they spread out from each adatom, interfere in either a constructive or destructive fashion, thus creating oscillations in the electron density that are mirrored in the interaction. Since the wavefunctions are in or out of phase, depending on d, AE itself becomes a function of d. As d increases, the overlap of the wavefunctions decreases, and AE tends towards A eP. 

Summarizing, it is clear that the indirect interaction between adatoms has a significant effect on the chemisorption properties of the system. Most noticeably, the chemisorption energy has a damped, oscillatory dependence on the adatom separation, as first quantified in (8.1) by Grimley. Thus, multi-atom adsorption occurs preferentially with the atoms at certain sites relative to one another. 

Using perturbation theory. Hammer and Nprskov developed a model for predicting molecular adsorption trends on the surfaces of transition metals (HN model). They used density functional theory (DFT) to show that molecular chemisorption energies could be predicted solely by considering interactions of a molecule s HOMO and LUMO with the center of the total d-band density of states (DOS) of the metal.In particular. 

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