Threefold sites

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]. 

Hydrogen atoms on Cu( 111) can bind in two distinct threefold sites, the fee sites and hep sites. Use DFT calculations to calculate the classical energy difference between these two sites. Then calculate the vibrational frequencies of H in each site by assuming that the normal modes of the adsorbed H atom. How does the energy difference between the sites change once zero-point energies are included ... 

In the previous exercise you assumed that the H vibrations were uncoupled from the metal atoms in the surface. A crude way to check the accuracy of this assumption is to find the normal modes when the H atom and the three metal atoms closest to it are all allowed to move, a calculation that involves 12 normal modes. Perform this calculation on one of the threefold sites and identify which 3 modes are dominated by motion of the H atom. How different are the frequencies of these modes from the simpler calculation in the previous exercise ... 

Determine the activation energy for the diffusion of an Ag atom between adjacent threefold sites on Cu(lll) using the NEB method. Note that the energy of the end points in your calculation will not be exactly equal because fee and hep sites on the Cu(lll) surface are not identical. Compute the frequencies for this hopping process. 

Fig. 5 Montage image combining an STM image of the Ag oxide structure (from bottom left) superimposed over the proposed oxide structure (from top right). The numbers, n = 1-5, correspond to the symmetrically different positions within the middle silver layer sandwiched between two O layers. Agi and Ag2 have metallic character, as they are exclusively bonded to silver atoms in the substrate below, whereas Ags, Ag4, and Ags are directly bonded to oxygen inside the oxide rings and are ionic in nature. Both Ag4 and Ags sites sit above threefold sites of the underlying (111) lattice atoms, whereas Ags occupies a top site. Reprinted with permission from Bocquet et at.. Journal of the American Chemical Society, 2003, 125, 3119. 2003, American Chemical Society.

Fig. 4. Stability of carbon on different sites (A-D) on a pure nickel(l 11) surface (below) and a gold-alloyed nickel(l 11) surface (above). The probability of nucleation of graphite is determined by the stability of the adsorbed carbon atoms. The less stable the adsorbed carbon, the larger the tendency to react with adsorbed oxygen to form CO and the lower the coverage. On the pure nickel) 111) surface, the most stable adsorption configuration of carbon is in the threefold (hep) site (lower curve). The upper graph shows that carbon adsorption in threefold sites next to a gold atom is completely unstable (sites B and C), and even the threefold sites that are next-nearest neighbors (sites A and D) to the gold atoms are led to a substantial destabilization of the carbon. From Reference (79).

In an early paper, Demuth and Ibach (6) suggested that the type A spectrum of ethyne on Ni(lll) may correspond to a surface species involving four metal atoms by adsorption across the central M-M bond of the (111) unit cell of the face-centered cubic (fee) crystal. This involves interactions with the two somewhat different threefold sites on either side of the central M-M bond. This model implies that the plane of the HCCH group is perpendicular to the (111) face so that, as found experimentally, the -yCH... 

Ethylidyne occurs on the triangular threefold sites on fee (111) or hexagonal close-packed (hep) (0001) faces and is formed at lower temperatures on Pd(lll) and Pt(lll) in the presence of coadsorbed hydrogen. Its spectral signature also occurs on Ru(0001) at 330 K and on Ir(lll) at 300 K. Ni(lll) is exceptional in not giving spectroscopic evidence for the ethylidyne species derived from adsorbed ethyne (or from ethene, 1). 

In that case the prominent 864-cm 1 feature could still be evidence of v1 of molecules adsorbed on three-metal-atom sites, and indeed similarly placed features also occur in the spectra of C6D6. The controversy over the location of the v2 on threefold sites therefore continues and would... 

Another system that has received recent intensive study by both diffraction and spectroscopic methods is that of ethene on Pt(lll). In recent papers, Somorjai et al. have investigated the adsorption of ethene by diffuse LEED to give the unexpected result that the di-cr species [which on Pt(lll) gives an extreme type I spectrum] is adsorbed across threefold sites with... 

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... 

The hexagonal crystal structure of CeCus contains two inequivalent Cu sites with a ratio of 2 3. It has been found that A1 can be sub-stituted for Cu on the threefold site (19). Specific heat measurements have shown that replacing one Cu atom per formula unit by A1 leads to an extreme heavy-electron state where the cp /T ratio reaches 2.2 3/mole K, so far a record value, and no phase transition is observed above 0.15 K (18). The temperature dependence of the specific heat Cp(T) below 1 K is shown in fig. 7. Increasing the A1 content to obtain CeCu3Al2 results in a decrease of this giant y value by a factor of about two. 

A substitutional Au impurity also reduces the CO adsorption energy on Au/ Ni(lll) surfaces on the sites immediately surrounding the impurity. In this case, the incorporation of Au into the Ni(l 11) surfaces by exchanging with a Ni atom was found to be endothermic, and consequently surface alloy formation must be entropically driven. The Au atom center of mass sits about 0.5 A above the average location of the Ni atoms due to its larger size. The adsorption energy in the threefold sites that include the Au atom are only —1.18 eV, compared to —2.16 eV in neighboring three fold sites that only include Ni. On pure Ni(lll), the CO... 

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