Adsorption sites twofold-bridge

For the infrared spectra there is, of course, no impact mechanism available for exciting additional features from not completely symmetrical modes, and none of the in-plane ca. 1395 (pi3), 1275 (v9), or 1147 cm 1 (vU) or vv) modes would be allowed if the MSSR applied strictly to parallel adsorption on (111) or (100) facets. They would, however, all become allowed on a Cs site, such as would arise from adsorption on twofold bridges. The infrared spectra of alternative monosubstituted or ortho-disubstituted benzenes (the most likely dissociatively adsorbed species) would give rise to two additional strong bands between 1400 and 1620 cm-1, and so the observed spectrum is again seen to be consistent with nondissociative adsorption. 

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. 

In a considerable number of cases both sets of modes have been observed in on-specular VEEL spectra, and the deduction has been made that the symmetry of the surface complex is Cs (or less) (145,146,151,152,160,162). The question remains whether this implies a twofold bridged adsorption site or a neighbor-induced asymmetry within an essentially C3 site, as already described. However, there are examples of species on Pt(lll) (150), Ni(ll 1) (117), and Cu(lll) (161) surfaces for which MSSR as applied to VEEL spectra clearly indicates C3v symmetry of the surface complex, without significant differences in the other frequencies as observed off-specular. These favorable cases may arise from particularly regular arrays of adsorbed species, the presence of which could very profitably be confirmed by LEED. We deduce that the CH3 adsorption sites are intrinsically C3v as far as the bare surface is concerned, i.e., on-top or threefold hollow in nature with the threefold axis of the CH3 group perpendicular to the surface. 

Carbon monoxide chemisorption on Ni 7 9 11 represents an interesting case with which to check these concepts since comparable studies have been performed on Ni 001 and Ni lll and since a number of other experimental methods have been applied to this system. Electron energy loss spectroscopic (EELS) studies performed at 150 K suggest that the initial adsorption occurs in threefold and twofold bridge sites along the step edge. Beyond this point, the CO molecules begin to occupy terrace sites. (12)... 

The surface composition and availability of certain adsorption sites are not the only factors that determine how CO binds to the surface rather, interactions between CO and co-adsorbed molecules also play an important part. The RAIRS study conducted by Raval et al. [35] showed how NO forces CO to leave its favored binding site on palladium (see Fig. 8.10). When only CO is present, it occupies the twofold bridge site, as the infrared frequency of about 1930 cm-1 indicates. However, if NO is co-adsorbed, then CO leaves the twofold site and ultimately appears in a linear mode with a frequency of approximately 2070 cm-1. Raval and colleagues [35] attributed the move of adsorbed CO to the top sites to the electrostatic repulsion between negatively charged NO and CO, which decreases the back-donation of electrons from the substrate into the In orbitals of CO. In this interpretation, NO has the opposite effect that a potassium promoter would have (see Chapter 9 and the Appendix). 

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). 

Similar results were obtained for Pt(lll) four temperature-dependent adsorption states of O2 were also formed. The two molecularly chemisorbed forms identified were (i) a superoxo-type (O2) species bonded at twofold bridge sites and characterized by vqo of 870 cm" (108 meV), and (ii) a peroxolike (O ) species more strongly coordinated (vqo = 690 cm" (86meV)) at threefold... 

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. 

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