Adatom chemisorption energy

From chemisorption theory we know that adatom adsorption energies wiU decrease in a row of the periodic system of the group VIII metals when the position of the element moves to the right. The rate of hydrogenation of Cads vviU decrease with increasing adsorption energy of Cads and hence wiU decrease in the same order with element position in the periodic system. 

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 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 the above formulas we have calculated the total chemisorption energy AE for a single fluorine atom adsorbed on top of a carbon atom, as well as the charge transfer from the substrate SWCNT to the adatom, which is... 

Generally, the bonding of adatoms other than hydrogen to a metal surface is highly coordination-dependent, whereas molecular adsorption tends to be much less discriminative. For the different metals the bond strength of an adatom also tends to vary much more than the chemisorption energy of a molecule. Atoms bind more strongly to surfaces than molecules do. Here we will discuss the quantum chemical basis of chemisorption to the transition metal surfaces. We will illustrate molecular chemisorption by an analysis of the chemisorption bond of CO [3] in comparison with the atomic chemisorption of a C atom. 

Finally, we wish to briefly consider the case of a surface impurity. A physical quantity of interest is the energy gained in an adatom chemisorption process. We consider a simplified but workable model, which gives at least a guideline on how to face more complicated situations. ... 

Using Eq. (5.13) we can now calculate the defect-induced change in the density of states and hence the energy change in the adatom chemisorption process. The above results must be appropriately generalized if interactions of a more complicated nature are to be considered, but it is still possible to calculate physical quantities of interest, such as change of energy and ionic-ity. 

So-called underpotential deposited species arise when an electrochemical reaction produces first, on a suitable substrate adsorbent metal, a two-dimensional array or in some cases two-dimensional domain structures (cf. Ref 100) at potentials lower than that for the thermodynamically reversible process of bulk crystal or gas formation of the same element. The latter often requires an overpotential for initial nucleation of the bulk phase. The thermodynamic condition for underpotential deposition is that the Gibbs energy for two-dimensional adatom chemisorption on an appropriate substrate must be more negative than that for the corresponding three-dimensional bulk-phase formation. Underpotential electrochemisorption processes commonly involve deposition of adatoms of metals, adatoms of H, and adspecies of OH and O. 

The calculations can also be used to analyze the electronic differences that cause the dependence law of the chemisorption energy of adatoms as a function of metal atom coordination N. ... 

Regardless of the exact extent (shorter or longer range) of the interaction of each alkali adatom on a metal surface, there is one important feature of Fig 2.6 which has not attracted attention in the past. This feature is depicted in Fig. 2.6c, obtained by crossploting the data in ref. 26 which shows that the activation energy of desorption, Ed, of the alkali atoms decreases linearly with decreasing work function . For non-activated adsorption this implies a linear decrease in the heat of chemisorption of the alkali atoms AHad (=Ed) with decreasing > ... 

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