Adatom bond energie

Let us now analyse dissociation and recombination in detail. As we saw in the previous sections dissociative adsorption is a process consisting of at least two consecutive elementary steps. Molecular adsorption proceeds dissociation. Dissociation occurs when reaction (4.4) is thermodynamically feasible. We learned in Section 4.2.1 that the surface adatom bond energy varies more than the molecular adsorption bond with variation of metal. Adsorption of atoms is favoured by metals with a partially filled d valence electron band or metals of a low work function. CO will dissociate on the first row transition metals (except... 

Vattuone L, Yeo YY, King DA (1996) Adatom bond energies and lateral interaction energies from calorimetry NO, O, and N adsorption on Ni 100. J Chem Phys 104 8096... 

In the field ion microscope, very high fields can be applied to a surface, sufficient to cause a significant distortion to the potential energy barrier experienced by an atom at the surface, resulting in desorption or field stripping. The method is used to produce the perfect surface to be imaged in the microscope and attempts have also been made to correlate the field required for desorption with the adatom bond energy to the surface [227]. 

Figure 2.54. Bethe lattice calculation result. Comparison of adatom bond energy according to Eq.(2.229), the value it calculated with the aid of Eq.(2.247). For parameters see insert Fig.(2.55).

Fig. 1.1. Chemisorption model, where aa(Pa) is adatom site (bond) energy, as(a) surface (chain) atom site energy and (3 chain bond energy.

Fig. 5.2. Schematic representation of H-Ni/ZnO system showing hydrogen adatom a of electronic energy eaa with bond energy ft attached to Ni surface atom at m = 0. Reprinted from Davison et al (1988) with permission from...

In conclusion, we have seen that alloys can exhibit a variety of interesting chemisorption properties. The chief parameters determining the behaviour of a system are the concentrations of the various layers, especially the surface one. Other important parameters are the effective electronic energy, the occupied band width, the adatom bond strength and the adatom position. 

Fig. 7.4. Chemisorption of adatom of site (bond) energy a(Pa) onto electrified chain of length m. Substrate has site (bond) energy on(P), where an = a + nT(n = 1,..., m), T being the potential gradient.

U ria-a) being the averaged self-energy of the adatom, within the HFA, and Pa the adatom-surface bond energy. From (7.52), we obtain... 

Both /-kinetics and A/-kinetics have the same thermodynamic creation energies for all configurations. For example, on a simple cubic lattice, the adatom formation energy will be As adaiom = 2s, using bond counting arguments, and thus the equilibrium adatom... 

Kinetic experiments, while useful in estimating the adatom cohesive energy of a cluster, become very complicated if pair energies at more than one bond state, or information on the inter-adatom potential, are desirable. The distance dependence of pair interaction can be more easily derived by an equilibrium experiment.173 The principle is very simple. At equilibrium, the relative frequencies of observing the two adatoms at various bond states or bond separations at a given temperature are related to their pair energies according to the Boltzmann factors. Thus... 

Adatom pair Distance of strongest bond Bond energy meV Expected dipole repulsive energy meV... 

The general conclusion from Eqs. (9)—(14) is that the molecular heat of chemisorption ( AB rapidly decreases as the gas-phase dissociation (total bond) energy DAB increases. The values of QAB are smaller than QA(QB), typically by a factor of 5-10 but sometimes even 15-20. For this reason, the periodic changes in QAB for molecules such as CO, NH3, NO, H20, C2H4, and C2H2 are expected to be small and potentially irregular, unlike the large and systematic variations in QA observed for the relevant multiply bonded adatoms A. 

The effect on the bond strength of a coadsorbate adatom B, adsorbed in the second coordination shell with respect to adatom A, is an increase of the bond energy (see Fig. 4.8). The weakened interaction between metal atoms 1 and 2, results in an increase of the M-B bond strength. 

Here, we analyze the electronic details of the surface chemical bond based on state-of-the-art DFT electronic structure calculations [5]. We address the relation between bond energy of the adatom as a function of its coordination number with the surface atoms as well as coordinative unsaturation of the surface atom, defined by its coordination number with neighbor metal atom N. We illustrate this in a detailed discussion of the chemisorption of a C atom to dilferent surfaces of Ru. In the DFT calculations, the surface is modeled as a metal slab chosen with enough metal layers that energies become independent of layer thickness. The surface atom positions and distances change slightly in contact with the adsorbate, because of the weakened metal atom-metal atom bond by interaction with the adsorbing atom. 

A perfect crystal bound by singular faces exhibits no sites for growth. A 2D nucleation process is required for the deposition of every new layer, as first noted by Gibbs and developed later by Volmer and Weber, Brandes, and Kaischew. This is because one atom deposited on the crystal face has a smaller bond energy to the crystal than a kink atom and it stays only temporarily on the surface as an adatom. A very important factor is the tendency of these surface atoms to cluster together thereby increasing their stability. Clusters of that kind are potential nuclei of a new lattice net. 

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