Three fold hollows

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. 

As with any system, there are complications in the details. The CO sticking probability is high and constant until a 0 of about 0.5, but then drops rapidly [306a]. Practical catalysts often consist of nanometer size particles supported on an oxide such as alumina or silica. Different crystal facets behave differently and RAIRS spectroscopy reveals that CO may adsorb with various kinds of bonding and on various kinds of sites (three-fold hollow, bridging, linear) [307]. See Ref 309 for a discussion of some debates on the matter. In the case of Pd crystallites on a-Al203, it is proposed that CO impinging on the support... 

Figure 2. The BeisXe D3/1 cluster used to model chemisorption into the four-fold hollows. The Be 13X3 cluster used to model chemisorption into the three-fold hollows is generated by a 60° rotation of the six adsorbate atoms about the vertical axis. The D3d metal cluster is formed by rotating the bottom three metal atoms by 180° about the vertical axis.

SCHEME 1.4 Depiction of deuterium attacking from the subsurface through a three-fold hollow site. The second (lower) layer of metal atoms is not shown. 

Studies of Ag on Au(lll)87 yield very similar results in terms of the structure of the deposited monolayer (i.e., the silver atoms are bonded to three surface gold atoms and are located at three-fold hollow sites forming a commensurate layer) with again strong interaction by oxygen from water or electrolyte (perchlorate). 

Mixture of on-top (ri ) and two-fold bridge (though the three-fold hollow site is not excluded)... 

Table 13.5 lists the ABads values obtained. On the basis of these calculations, the phenyl isocyanide should adsorb on the on-top sites on Ag and in the three-fold hollow sites on Au. On Au, phenyl isocyanide was calculated to adsorb on the three-fold hollow site with a slight rehybridization of the N atom, which causes a slight hit (27 °) from the surface normal. However, the energy difference between the most stable structure and the one with a linear C=N-C unit is only 0.3 kcalmoT. ... 

On Pt, the mode of isocyanide adsorption depends on the nature of the Pt substrate and perhaps on the nature of the isocyanide. On Pt(lll), CH3NC adsorbs by both T (low coverage) and p,2-T h (high coverage) modes. On Pt nanoparticles, only T adsorption is observed for n-dodecyl isocyanide, but on Pt nanoparticle electrodes evidence suggests that DMPI adsorbs by T (on-top), X2-T t (two-fold bridge) and X2-tl T T (three-fold hollow) modes. 

Structures of SA alkanethiol monolayers generated on gold substrates are better understood than the chemistries involved in their formation. Electron diffraction and FTIR measurements, together with computer simulations, have provided a picture of a SA monolayer in which the axes of the alkyl chains of the surfactants are tilted approximately 30° with respect to the surface normal of the substrate and the sulfur atoms reside in the three-fold hollows of the gold (111) surface (Fig. 25) [68, 207-210, 230, 240]. The use of scanning tunneling micro-... 

Pt(l 11) surface were performed. Tables 4 and 5 list chemisorption energies for C, N, 0, and H and CHx(x=i- 3)> NHx(x=i- 3), OHx(x=i->2) on the four high symmetry sites of Pt(lll). From Table 4 we see that the atomic adsorbates have a strong preference for adsorption at three-fold hollow sites, with the exception of H which has a very smooth potential energy surface (PES). Table 5 reveals that the molecular adsorbates do not exhibit quite the same preference for adsorption at three-fold hollow sites, binding preferentially at a variety of sites CH and NH favour adsorption at fee three-fold hollow sites CH2, NH2 and OH at bridge sites and CH3, NH3 and H2O at top sites. 

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