Hollow site fourfold

The Ag (100) surface is of special scientific interest, since it reveals an order-disorder phase transition which is predicted to be second order, similar to tire two dimensional Ising model in magnetism [37]. In fact, tire steep intensity increase observed for potentials positive to - 0.76 V against Ag/AgCl for tire (1,0) reflection, which is forbidden by symmetry for tire clean Ag(lOO) surface, can be associated witli tire development of an ordered (V2 x V2)R45°-Br lattice, where tire bromine is located in tire fourfold hollow sites of tire underlying fee (100) surface tills stmcture is depicted in tlie lower right inset in figure C2.10.1 [15]. 

The associative part of the adsorbing potential, Eq. (116), generates a highly localized adsorption which corresponds to the onefold, to the twofold bridging site, and to the fourfold hollow site adsorption dependence of the length L. Note that in the absence of the associative part, Eq. (119), and in the limit 0 the pore walls reduce to an array of hard spheres. 

Figure 3.2 STM images obtained with a CO-terminated tip, Vt = 70 mV and /t= 1 nA. (a) Isolated CO molecule, (b) two O atoms (adsorbed on the nearest fourfold hollow sites along the [1 1 0] direction), (c) CO and two O atoms separated by 6.1 A along the [0 0 1] direction, and (e) O-CO-O complex. Grid lines are drawn through the silver surface atoms. Scan area of (a-c) and (e) is 25 A x 25 A.

FIGURE 4.3 Left The unreconstructed surface of 50% C/fccCo (100). Right The clock reconstructed surface of 50% C/fcc Co (100). The darker spheres represent cobalt atoms and the lighter ones (in the fourfold hollow sites) represent carbon atoms. (Reprinted from Ciobica. I. M.. van Santen, R. A., van Berge, P. J., and van de Loosdrecht, J., Adsorbate Induced Reconstruction of Cobalt Surfaces, Surface Science, 602, 17-28. Copyright 2008, with permission from Elsevier.)... 

For an fee lattice a particularly simple surface structure is obtained by cutting the lattice parallel to the sides of a cube that forms a unit cell (see Fig. 4.6a). The resulting surface plane is perpendicular to the vector (1,0,0) so this is called a (100) surface, and one speaks of Ag(100), Au(100), etc., surfaces, and (100) is called the Miller index. Obviously, (100), (010), (001) surfaces have the same structure, a simple square lattice (see Fig. 4.7a), whose lattice constant is a/ /2. Adsorption of particles often takes place at particular surface sites, and some of them are indicated in the figure The position on top of a lattice site is the atop position, fourfold hollow sites are in the center between the surface atoms, and bridge sites (or twofold hollow sites) are in the center of a line joining two neighboring surface atoms. 

We can avoid this symmetry-induced trap by deliberately breaking the symmetry of our atom s coordinates. One easy way to do this is to repeat our calculations after moving the H atom a small amount (say, 0.2 A) in some arbitrary direction that does not coincide with one of the symmetry directions on the surface. What we find, if we run calculations in which we start the H atom at a point about 0.2 A away from each of the high-symmetry sites mentioned above is that the H atom relaxes to the fourfold hollow site even if it is started quite near the top and bridge sites. This shows that the top and bridge sites are not minima for this system. 

Figure 6.3 DFT calculated energies for Ag moving along the minimum energy path between two fourfold sites on Cu(100). Energies are relative to the energy of Ag in the fourfold hollow site. The reaction coordinate is a straight line in the x y plane connecting two adjacent energy minima.

It has been found that Cu electroreduction from perchloric acid solution on Au(lOO) exhibits the UPD properties [389]. X-ray surface diffraction measurements have shown that the Cu UPD layer has a primitive (1x1) structure and that the adsorbed Cu species are located in fourfold hollow sites, with a vertical distance between the Cu adlayer and the Au surface of 0.14 nm. 

In the case of Fe(lOO) + c(2 X 2)CO, the LEED analysis finds that the C and 0 atoms individually and randomly occupy fourfold hollow sites in a c(2 X 2) array, i.e., a c(2 X 2) array of unoccupied sites exists, all other sites being occupied at random by either C or 0 atoms. The average Fe-C and Fe-0 bond length is 1.93 A (C and 0 usually have very similar radii), somewhat smaller than for Fe(lOO) + p(l X 1)0 (where it is about 2.08 A) however, an expansion of the topmost substrate interlayer spacing has not been considered in this dissociative case (the bulk spacing was assumed), resulting in some uncertainty in the Fe-adsorbate bond length as well. 

Fig. 2. STM image (78 A x 76 A) of nitrogen atom adsorbates on an Fe(l 00) surface. Because the nitrogen adsorbates deplete the LDOS at the Fermi level, the nitrogen atoms are imaged as depressions in accord with the Tersoff-Hamann model. From the STM image it is concluded that nitrogen atoms adsorb in fourfold hollow sites on Fe(l 0 0). This is just one of many examples illustrating how the STM contrast may depend on the details of the LDOS around an adsorbate and produce a somewhat counterintuitive picture. Adapted from Reference (dd).

Figure 36. Schematic representation of the influence of an adsorbed electronegative modifier in a fourold hollow site (black circle) on the neighboring sites. Neighboring fourfold hollow sites which are strongly (+) or less strongly (-) influenced are indicated [260],...

In this study four different adsorption sites have been considered, namely the twofold-bridge, the threefold hollow, the diagonal fourfold hollow and the aligned fourfold hollow sites, see Fig. 3. 

To study the adsorption on the twofold-bridge and threefold hollow sites we used a Cu9(5,4) cluster as shown in Fig. 4(a), where the numbers inside brackets indicate the number of metal atoms in the first and second layers respectively. To study the adsorption on the aligned-fourfold-hollow and diagonal-fourfold-hollow sites we used the same cluster but on an inverted position, that is, we used a Cu9(4,5) cluster as shown in Fig. 4(b). 

The difference in stability between the diagonal fourfold-hollow site and the aligned fourfold-hollow site is only 9.2 kJ.moT. This difference is too small to allow us to conclude unambiguously which is the most stable adsorption mode. This served as a motivation to the calculation of the vibrational frequencies for acetylene adsorbed on these two most stable adsorption sites. The results obtained, along with the available experimental results, are presented in Table 4. 

TABLE 4. Calculated vibrational fiequencies of acetylene adsorbed on the diagonal fourfold hollow and aligned fourfold hollow sites. Ref [23]... 

CO stretching frequency is in many cases characteristic of the binding site, allowing one to differentiate between adsorption on three and fourfold hollow sites, bridge sites, on-top sites, steps, etc. (17-19). One should keep in mind, however, that this differentiation may not always be possible, in particular, when strong adsorbate-adsorbate interactions occur or when coadsorption of multiple species occurs (199,200). 

Again, the doubly dehydrogenated bitartrate phase is considered to represent the catalytically active state, because its overall low coverage allows reactant species to approach the surface, and also because the adsorption conditions of low coverage and high temperature match those found by catalytic studies to be the most effective [6, 7, 9]. Periodic DFT calculations on the bitartrate/Ni(110) phase [22] confirm, as was observed on Cu(llO), that the bitartrate molecule is located above the fourfold hollow site and bonds via both carboxylate groups, with each of the four oxygen atoms located at on-top sites (Fig. 5.9). 

Partial reaction of the Cu(110)-p(2x l)-0 surface covered by 0.24 ML oxygen with SOj generates a sulfite in coexistence with adsorbed oxygen (Fig. 11.9) [18]. The sulfite moieties occupy fourfold hollow sites, which are also the sites occupied by added Cu atoms in the Cu(110)-p(2xl)-O structure (Fig. 11.9). TPRS after exposure of SO on the Cu(110)-p(2x l)- 0 surface only showed the production of SOj and a very small amount of SO 0, suggestive of a dominant monodentate... 

Fig. 11.9 STM image following co-dose of SO and onto Cu(l 10)-p(2x l)-0. As indicated in the imet, the Cu atoms comprising the (1 x 1) parent lattice are seen to sit in-between bright features in the p(2xl) structure, suggesting that the species imaged by the STM in the p(2xl)-0 overlayer are the added Cu atoms (white arrows). These two images indicate that the CuSO moieties occupy fourfold hollow sites. A structural model accompanying the boxed area is shown. Reprinted with permission from [18]. Copyright 2003 Elsevier...

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