Electron-adatom interaction

In contrast to what has been assumed by Gomer [4], the electron-adatom interaction must not he small. This can change the instant current by an order of magnitude. 

Further studies were carried out on the Pd/Mo(l 1 0), Pd/Ru(0001), and Cu/Mo(l 10) systems. The shifts in core-level binding energies indicate that adatoms in a monolayer of Cu or Pd are electronically perturbed with respect to surface atoms of Cu(lOO) or Pd(lOO). By comparing these results with those previously presented in the literature for adlayers of Pd or Cu, a simple theory is developed that explains the nature of electron donor-electron acceptor interactions in metal overlayer formation of surface metal-metal bonds leads to a gain in electrons by the element initially having the larger fraction of empty states in its valence band. This behavior indicates that the electro-negativities of the surface atoms are substantially different from those of the bulk [65]. 

The reactivity of a surface depends on many factors. These include the adsorption energies of chemical species and their dissociation behavior, their diffusion on the surface, the adatom-adatom interactions, the active sites where a chemical reaction can occur, and the desorption behavior of a new chemical species formed on the surface. The site specificity depends on at least three factors the atomic configuration of the surface, the electronic structures of the surface, and the localized surface field. In atom-probe experiments, the desorption sites can be revealed by a timegated image of an imaging atom-probe as well as by an aiming study with a probe-hole atom-probe, the electronic structure effect of a chemical reaction can be investigated by the emitter material specificity, and the surface field can be modified by the applied field. 

Here J, JQ and Ja are the statistical sums of activated complex and gas-phase molecules and of adsorbed atom (adatom), respectively, sA and eD the adsorption and desorption activation energies, a the area of adatom localization, h Planck s constant, and f. the parameters of the activated complex-adatom and adatom-adatom interactions (e < 0 for repulsion and e > 0 for attraction), A the contribution to the complete drop of adsorption heat AQ from the electron subsystem (for a two-dimensional free-electron gas model), x = exp (ej — e) — 1, jc, = = 0), / = 1/kT (k is the Boltzmann con-... 

Figure 12.3 shows a schematic illustration of the resulting electron density of states projected onto the adatom in the Newns-Anderson model [17, 18] for two different cases. In this model, the interaction strength between the adatom wave function of one specific electronic level and the metal states is often denoted the hopping matrix element. When the hopping matrix element is much smaller than the bandwidth of the metal states, in this case the i-electrons, the interaction leads... 

The overall conclusion reached, which is of importance to catalysis, is the possibility that the strong adatom-adatom interactions may alter electronic structure for the catalyst system and that this may be more important than a simple site-blocking mechanism for poisoning. 

Catalytic activity is essentially a surface phenomenon -201) hence particularly sensitive to the presence of impurities on the surface, which are often responsible for poisoning or for modifications in the catalytic selectivity The presence of sulfur on a metallic catalyst may induce rearrangement in the surface structure resulting in modification of the electronic distiibution mostly by dative bond formation, and, for higher coverages, in strong adatom-adatom interaction. 

In the previous Sections (2.1-2.3) we summarized the experimental and computational results concerning on the size-dependent electronic structure of nanoparticles supported by more or less inert (carbon or oxide) and strongly interacting (metallic) substrates. In the following sections the (usually qualitative) models will be discussed in detail, which were developed to interpret the observed data. The emphasis will be placed on systems prepared on inert supports, since - as it was described in Section 2.3 - the behavior of metal adatoms or adlayers on metallic substrates can be understood in terms of charge transfer processes. 

The other mechanism involves atomic-size roughness (i.e., single adatoms or small adatom clusters), and is caused by electronic transitions between the metal and the adsorbate. One of the possible mechanisms, photoassisted metal to adsorbate charge transfer, is illustrated in Fig. 15.4. It depends on the presence of a vacant, broadened adsorbate orbital above the Fermi level of the metal (cf. Chapter 3). In this process the incident photon of frequency cjq excites an electron in the metal, which subsequently undergoes a virtual transition to the adsorbate orbital, where it excites a molecular vibration of frequency lj. When the electron returns to the Fermi level of the metal, a photon of frequency (u>o — us) is emitted. The presence of the metal adatoms enhances the metal-adsorbate interaction, and hence increases the cross... 

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. 

The above 1-dimensional model may be extended to 3-dimensions, in a straightforward manner, and yields a substrate whose surface is completely covered by adatoms. The TBA again leads to a difference equation and boundary conditions which can be solved directly (Grimley 1960). We do not intend to discuss the 3-dimensional case here and, instead, direct the reader to the loc. sit. articles. However, in passing, we note that, even when there is no direct interaction between the atoms in the adlayer, an important indirect interaction occurs between them via the substrate by a delocalization of the bonding electrons in directions parallel to the surface (Koutecky 1957, Grimley 1960). This topic is discussed in Chapter 8. 

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... 

If a single electron is in AO a), the unperturbed ground-state energy of the non-interacting adatom and substrate is... 

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.,... 

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