Three-fold hollow site

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. 

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. ... 

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. 

The pathways and energy profiles for each of the three reactions investigated are displayed in Figures 5 and 6. Figure 5(a) displays the initial states of each reaction, each of which are the most stable coadsorption of the reactants in p(3x2) unit cells. In the coadsorption states shown in Figure 5(a), and when chemisorb independently, all the reactants chemisorb preferentially at three-fold hollow sites. In the initial states of the C+0 and C+N reactions, C atoms are at hep three-fold hollow sites and O or N atoms are at fee sites. In the C+H reaction it is most stable to have C atoms at fee sites and H atoms at hep sites. 

Class IV . The first reactant is at a three-fold hollow site and the second is at a next nearest neighbour bridge site. Indeed, this represents a very common structure for a transition state on a (111) metal surface. Transition states with this structure are often observed in the dissociation of diatomic molecules - reactions which are the reverse of those currently under discussion. DFT calculations have shown that the dissociation of N2, CO, and on a variety of transition metal surfaces all proceed via this... 

Figure 9. (A) and (B) Schematic illustrations of two possible real-space structures of the Pd(lll)-c(2V3x3)-rect-C6H6 adlayer. In (A), the benzene molecules occupy two-fold bridge sites in (B), the slightly tilted admolecules occupy three-fold hollow sites. (C) Real-space structural model of the Pd(lll)-( 3x3)-C6H6 adlayer all molecules are situated on three-fold hollow sites.

The structural model of the Pd(lll)-(3x3)-C6H6 adlattice is shown in Fig. 9(C), where each benzene molecule is situated at a three-fold hollow site. The image for such a model would be three spots arranged as an equilateral triangle, an expectation borne out by the results shown by the EC-STM image in Fig. 8(B). It is important to note that there ate two unoccupied three-fold hollow sites inside the molecular unit cell these ate of sufficient size to hold at least one water molecule. The existence of coadsorbed water would account for the extra (smaller and less bright) spots observed in Fig. 8(B). 

The idea of the importance of surface structure in the chemistry of bimetallic surfaces relies largely on the concept of the active site for a particular surface chemical reaction. If one reaction requires the presence of three-fold hollow sites on an fee (111) facet, while a competing reaction requires only single atomic ites, then the random dilution of the active metal by an inert second metal would rapidly deplete the number of active three-fold ensembles. By contrast, the number of single atom sites available would be depleted much less dramatically as a function of composition. Thus, the selectivity of the catalyst would vary with surface composition. 

A second structure was observed where the overlayer was rotated by only 4° with respect to the underlying Pt. The STM image (Fig. 4c) showed a periodicity of 2.7 nm which was interpreted as the distance required for Ce atoms in the bimetalhc Pt/Ce layer to return to purely three fold hollow sites on the underlying Pt surface. It appears that the Ce-Ce distance prefers to stay at -0.54 nm (as in the PtsCe) structure rather than relax to 0.554 nm which would enable each Ce atom to sit in a 3-fold hollow site on Pt(lll) in a p(2x2) stracture. This surface was also Pt terminated and very unreactive when exposed to CO or O2 even at elevated temperatures. In this case, each surface Pt atom is coordinated to 4 Pt atoms in the (0001) plane and 2 Ce and 2 Pt atoms in the layer beneath. Despite the buckling associated with the second layer Ce atoms being in different adsorption sites, the surface remains unreactive, presumably since each Pt atom is in an approximately similar environment. 

The presence of the adsorbed species seems to be responsible for the C—C bond-breaking to give CH species, perhaps because molecules impinge on the modified substrate between C2H2 molecules on xX sites and there fragment into CH units, which occupy the three-fold hollow sites, X and x, where they are bound to three Ni atoms. This would give rise to twice as many CH species per... 

CO adsorption on Ru(OOOl) shows contributions from both COl and multiple-bonded CO (COh) Wliile a certain amount of activity towards CO oxidation to CO2 was seen on the surface of polyciystalline Ru, Ru(OOOl) exhibited almost none, judged by the absence of the FTIR peak around 2350 cm . At low CO doses, the STM image showed a (V 3 x 3) R 30° CO overlayer with a coverage of 0.33 ML, similar to the structures found with UHV a further increase in CO doses produced a new c(2 x 2)-2CO stmc-ture as the saturation phase, where CO occupied both the on-top and the three-fold hollow sites, with coverage of 0.5 ML. A combined electrochemical, STM and FTIR study of CO on bare and Pt-modrfiedRu(OOOl) and Rut 1010) surfaces followed. ... 

Fig. 1. (a) Top view of the fcc( 11.1, (100) and (110) surfaces showing the surface unit cell (bold lines) and possible high symmetry adsorption sites. The adsorption sites are B bridge site, LB long-bridge site, SB short-bridge site, T top site. On the (111) surface H denotes one of two possible three-fold hollow sites the fcc-hollow or the hep-hollow. On the (100) and (110) surfaces H denotes the four-fold and two-fold hollow sites respectively. 

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