Hollow sites

Studies to determine the nature of intermediate species have been made on a variety of transition metals, and especially on Pt, with emphasis on the Pt(lll) surface. Techniques such as TPD (temperature-programmed desorption), SIMS, NEXAFS (see Table VIII-1) and RAIRS (reflection absorption infrared spectroscopy) have been used, as well as all kinds of isotopic labeling (see Refs. 286 and 289). On Pt(III) the surface is covered with C2H3, ethylidyne, tightly bound to a three-fold hollow site, see Fig. XVIII-25, and Ref. 290. A current mechanism is that of the figure, in which ethylidyne acts as a kind of surface catalyst, allowing surface H atoms to add to a second, perhaps physically adsorbed layer of ethylene this is, in effect, a kind of Eley-Rideal mechanism. 

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. 

FIG. 5 Schematic of site parameters and interactions employed for the hollow-bridge site model of Te on W(IOO). Also depicted are the six hollow sites (squares) and adjacent bridge sites (small open circles) allowed in one strip in the construction of the transfer matrix. (Reprinted from Ref. 37 with permission from Elsevier Science.)... 

More recent calculations by a relatively new technique have in part modified the above sequence.438 The proposed order for the (100) face is Ag < Au < Cu. This confirms the position of Cu in Table 28, but Au still appears to be more reactive than Ag. There are also other interesting aspects. The most adsorbing position has been found to be the top or bridge rather than the hollow site. On the other hand, the adsorption energy has been calculated to be -31.8 kJ mol-1 for Cu, which is the same value as that found for water on Hg by other authors.890... 

Figure 5.55. (a) CuJ4 cluster used to model the Cu(100) surface. Oxygen has been adsorbed on the central 4-fold hollow site, (b) Pt25 cluster used to model the Pt(111) surface. Oxygen has been adsorbed on the central 3-fold hollow site. The position of the adsorbed ions (or point charges) is also shown.83,84 Reprinted with permission from the American Chemical Society. 

Definitions of the most common adsorption sites are shown in Fig. 5.5. They are named on-top site, bridge sites (long or short bridge), and hollow sites, which may be three-fold or four-fold in character. In case of three-fold adsorption on the fcc(lll) surface it is also necessary to distinguish between hep and fee sites, having an atom just below the site or not. 

The number of adsorption sites on a surface per unit or area follows straightforwardly from the geometry. Consider, for example, adsorption on a four-fold hollow site on the fee (100) surface. The number of available sites is simply the number of unit cells with area (ja /2) per m, where a is the lattice constant of the fee lattice. Note that the area of the same (100) unit cell on a bee (100) surface is just a, a being again the lattice constant of the bcc lattice. 

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. 

The adsorption of sulfur at platinum,40 rhodium,41 rhenium42 and ruthenium43 has been studied predominantly at fcc(lll) and hcp(0001) surfaces and shows many similar characteristics. Adsorption is initially into fee hollow sites of the fee metals and hep sites of the hep metals at higher coverages, mixed site occupancy occurs. A (2 x 2) structure is the first to be recorded appearing in the... 

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.

Figure4.4 c(2 x 2)A structure. Left panel structural model. Right top panel corresponding simulated STM image (VB = + 1.30V, / = 0.04nA). The protrusions correspond to oxygen couples, whereas the depressions are the hollow sites surrounded by O—H complexes. Right bottom panel simulated current profiles along [00 1] at decreasing (light blue to red) tip-surface distances. (Reprinted with permission from Ref. [18].)...

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