Threefold hollow site

Ethylene, C2H4, can adsorb in two modes the weaker Jt-bonded ethylene, in which the C=C double bond is above a single metal atom, or the stronger di-cr bonded ethylene in which the two C-atoms of the ethylene molecule bind to two metal atoms (Fig. 6.37). We consider the (111) metal surface. Hydrogen adsorbs dis-sociatively and is believed to reside in the threefold hollow sites of the metal. 

Final detennination of the structure was made by proposing a structural model with Cu sitting in threefold hollow sites and O atoms on atop sites with respect to the Cu atoms (Fig. 27.16). A program, FEFFIT, was used to analyze the data (Stem et al., 1995). This calculates the phase and amplitude parameters for the various backscatters. The EXAFS for the parallel polarization could be fitted six Cu-Cu interactions at a bond distance of 2.67 A and three Cu-Pt interactions at 2.6 A. For the perpendicular polarization, the data could be fitted one Cu-0 interaction at 1.96 A and three Cu-Pt interactions at 2.6 A. The Cu-Pt bond length is shorter than the sum of the metallic radii of Cu and Pt, which is 2.66 A. This indicates a Cu oxidation state different from zero, which agrees with the XANES results. 

Figure 20. Schematic representation of the structure of a copper upd monolayer on a gold (111) electrode surface. The copper atoms sit at threefold hollow sites on the gold surface and water molecules are bonded to the copper atoms.

Most recently, we have been able to obtain the in situ surface EXAFS spectrum of a half-monolayer of underpotentially deposited copper on a bulk Pt(lll) single crystal pretreated with iodine. The spectrum shown in Fig. 23 is a bit noisy (due to limited number of scans) but at least five well-defined oscillations can be observed. Preliminary data analysis indicates that the copper adatoms sit on threefold hollow sites with copper neighbors at 2.80 0.03 A. This distance is very close to the Pt—Pt distance in the (111) direction and indicates the presence of a commensurate... 

FIGURE 4.2 Representation of different carbon types on cobalt, (a) Atomic carbon/ surface carbide in a threefold hollow site, (b) CHX species located in threefold hollow sites, (c) Subsurface carbon lying in octahedral positions below the first layer of cobalt, (d) Cobalt carbide (Co2C) with an orthorhombic structure, (e) Polymeric carbon on a cobalt surface, (f) A sheet of graphene lying on a cobalt surface. The darker spheres represent carbon atoms in all the figures. 

V has been elucidated by x-ray adsorption spectroscopy [5]. Gold has an fee lattice, and the Au(lll) surface forms a triangular lattice with a lattice constant of 2.89 A. Copper atoms are smaller than gold atoms, and they adsorb in the threefold hollow sites (see Fig. 4.12), forming a triangular lattice commensurate with that of the substrate ... 

An interesting application of these principles is the prediction of CO dissociation routes on the closed-packed (111) surface of rhodium (see Fig. A.17). Two factors determine how the dissociation of a single CO molecule proceeds. First, the geometry of the final situation must be energetically more favorable than that of the initial one. This condition excludes final configurations with the C and the O atom on adjacent Rh atoms, because this would lead to serious repulsion between the C and O atoms. A favorable situation is the one sketched in Fig. A.17, where initially CO occupies a threefold hollow site, and after dissociation C and O are in opposite threefold sites. The second requirement for rupture of the CO molecule is that the C-0 bond is effectively weakened by the interaction with the metal. This is achieved when the C-O bond stretches across the central Rh atom. In this case there is optimum overlap between the d-electrons of Rh in orbitals, which extend vertically above the surface, and the empty antibonding orbitals of the CO molecule. Hence, the dissociation of CO requires a so-called catalytic ensemble of at least 5 Rh atoms [8,21,22]. 

With Ti(OOOl) + p(2 X 2) CO, preliminary results suggest that the C and O atoms occupy threefold hollow sites, the C atoms forming a p(2 X 2) array and the 0 atoms... 

Once again, further single-crystal VEELS and RAIRS work would be very advantageous for the ethyl group on other metals, specifically Ni. LEED evidence for the presence of regular arrays would enable reliable symmetry assignments to be made, and PED work could probably distinguish between on-top and threefold hollow sites. 

Further LEED work done with intensity-voltage measurements to determine the distance between the median plane of the C6 skeleton and the surface plane of the metal would seem to be important for distinguishing between the cases of on-top or threefold hollow sites for the agostic interactions of the C-H bonds. The former should require a greater metal surface to Q, plane distance. Direct distinctions between the alternative hollow-site models and the twofold bridging sites may require the use of the more recently developed photoelectron diffraction technique. 

The TPD spectra of three different adsorbate systems, corresponding to zeroth-, first- and second-order kinetics, are shown in Figure 2.9. Each trace corresponds to a different initial adsorbate coverage, as indicated in the figure. The simplest case in TPD corresponds to first-order desorption kinetics, represented by the CO/Rh(lll) series in Figure 2.9 [17, 18]. For CO coverages up to 0.5 monolayer (ML), the CO molecules do not interact on Rh(lll) and the desorption traces all fall in the same temperature range, all with the same peak maximum temperature. Hence, the rate of desorption is proportional to the surface concentration of CO. Above 0.5 ML, CO starts to populate additional sites (from vibrational spectroscopy studies we know that in addition to on-top sites also threefold hollow sites are occupied see Fig. 8.15), and a faster reaction channel for desorption opens up, as seen by the development of a shoulder at lower temperatures [18]. 

Studies of the interactions of ethylene with Pt(lll) at temperatures higher than 280 K indicate that ethylene adsorbs dissociatively and rearranges such that the C-C axis is oriented perpendicular to the surface (70). The resulting ethylidyne species prefers the threefold site on Pt(lll), as shown in Fig. 8b. DFT calculations for adsorbed ethylidyne at threefold hollow sites give an... 

Laser-induced desorption via the DIET process is a structure-sensitive phenomenon. Firstly, we describe the recent results for adsorbed NO on Pt(l 1 1), since the adsorption structure of this system has been misunderstood for a long time. Adsorbed species giving rise to the 1490 cm-1 NO stretching vibrational mode had been believed to be adsorbed at bridge sites [34, 35]. Recently it has been shown that this species is adsorbed at the threefold fee hollow site. This problem was pointed at first using LEED analysis by Materer et al. [36, 37]. A similar problem is the occupation of the fee and hep threefold hollow sites in a ratio of 50/50 described by Lindsay et al. [38] on the basis of a photoelectron diffraction investigation of NO on Ni(l 1 1) at a coverage of 0.25 monolayer. 

---