3- Fold site

Surface composition and structure of bimetallic nanoparticles are crucially important for their catalytic property as well as their optical property. IR measurement of CO adsorbed on surface metals (CO-IR) is utilized for this purpose. CO is adsorbed on metals not only on-top sites but also in two-fold or three-fold sites, depending on the kinds of metals and their surface structures. The dramatical changes of wavenumber of adsorbed CO occurs depending on the binding structure [177-181]. 

Figure 10.3 Adsorbed sulfur structures on Cu(100). (a, b) LEED patterns from the p(2 x 2) and ( 17 x 1) R14° structures, respectively, (c) STM image (9.3 x 9.3 nm) of the (y 17 x f17) R14° structure formed after annealing the sulfur adlayer to 1173 K. (d) High-resolution STM image (2.9x2.9nm) of (c). (e) Proposed model of the ( 17x 17) R14° structure black circles are sulfur adatoms in four-fold sites in the top layer shaded circles are sulfur adatoms which have replaced a terrace copper atom dashed circles indicate a copper atom which may be missing. (Adapted from Ref. 12).

A direct evidence of the way of tetrahedral anion adsorption at three-fold sites and the degree of hydratation is not available at present. However, a strong indication of such adsorption of sulphates is found in voltammetry on gold (14) and in our data for platinum surfaces (12). A pronounced difference between the sulphate and perchlorate adsorption effects is... 

The EARN distributions of Rh atoms ejected from 0/Rh(l 11H2 x 2) have recently been measured. In addition simulations were performed for the oxygen atoms in both of the three-fold sites and for both the 0.25 and 0.5 monolayer coverages. Experimentally the Rh atom yield decreases by about a factor of two with the oxygen adsorption. To reproduce this effect in the simulations the 0.5 monolayer coverage was required. In the experimental... 

The three hammerhead constructs display nearly identical solvent-protected (folded) sites except that HHal and RNA 6 show an additional protected site at position 15.3. This is an important site since in the hammerhead crystal structures this site in stem III is approached by domain I, part of the catalytic pocket. The authors believe that a single structure gives rise to the observed pattern of hydroxyl radical protection of nucleotides since protection at individual nucleotides varies in the same manner with Mg + concen-... 

Figure 1 shows the arrangements of metal atoms on (111), (100), and (110) surfaces of a face-centered cubic (fee) metal. It can readily be seen that different sites on a particular surface have different symmetry properties. For example, the top layer of atoms on sites 1, 2, 3, and 3 on the (111) face (where the number denotes the number of metal atoms associated with the site) exhibit 6-fold, 2-fold, 3-fold, and 3-fold rotation axes of symmetry, respectively. At this level of discrimination the point group symmetries of those sites are C6(, C2 , Cj , and Cj , respectively. However, when the arrangement of atoms in the second layer is taken into account (there is another atom under only half of the 3-fold sites) the point group symmetries of the first two sites... 

The same theoretical methods have been applied to ethylidyne absorbed in the Rh(100) surface, with the conclusion, as experimentally found, that this species will be stable on the 4-fold site (237). However, the extended-Hiickel treatment, as well as the atom-superposition and electron-delocalization molecular orbital (ASED-MO) method, predict that the CC bond length will be shorter, and hence the vCC mode higher in wavenumber on the 4-fold site, whereas experimentally... 

By analogy, hydrogen might be expected to occupy the three-fold sites on the geometrically similar Ru(001) surface also. In this case, the Ru-H loss feature(s) may be unobserved due to a low dynamic dipole moment or to a low vibrational frequency. 

The results of these neutron experiments need to be compared with singlecrystal electron energy loss data and theoretical predictions before a reliable assignment can be made of the observed excitations.19-26 Low energy electron loss data is available for H2 adsorbed upon the 100 and 111 surfaces of Ni. Tables 2 and 3 correlate the results of EELS experiments with theoretical predictions for the two surfaces. For the (110) surface it has been shown that H2 adsorption is very similar to that on the (111) surface since in both cases the preferred site has three-fold co-ordination and since the coordination about a (110) site is identical to that on a facet of a (111) plane.25 Comparisons of the results of Table 1 with those of Tables 2 and 3 show that the majority of the H atoms adsorbed by Raney nickel at coverages up to saturation occupy sites of three-fold co-ordination.3 18 The results of Kelley et al.17 suggest that in addition a small proportion of the H atoms occupy four-fold sites. 

The structure of YCu4.74Pbo.26 is shown as an example in Figure 19. The copper atoms on the 16c site build up a three-dimensional network of corner-sharing tetrahedra. This network leaves voids of coordination number 16 which are filled by the rare earth and Pb/Cu atoms on the four-fold sites. The large coordination number of this void readily explains the higher lead content on this site. The atoms occupying the 16c site (mainly copper in the RCus-xPb series) have coordination number 12 in the form of a distorted icosahedron. For more crystal chemical details on the Laves phases and related compounds we refer to review articles (Simon, 1983 Nesper, 1991 Johnston and Hoffmann, 1992 Nesper and Miller, 1993). 

Fig. 11. Inventory of 12 available three-fold sites (stars) with respect to the underlying 111 substrate. The p(4 x 4) unit cell is recalled by the black solid parallelogram. In the vacant (Ag-free) ring, 6 positions A-E are available. The two other filled oxide rings, i.e. those that contain Ag adatoms, each have three similar adsorption sites.

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