Adsorption on fee

Alkali metals on certain metal surfaces have a propensity to intermix with the substrate surface atoms, often producing complex surface alloy stractures. A recent review of the conplex structures formed by alkali adsorption on fee surfaces describes many of these systems [98T1]. These stractures will be discussed in chapter 4.1 in this volume here we provide a smnmary. The intermixed stractures that have been observed are all activated stractures i.e. their formation requires some thermal energy. The temperatures required to form these stractures range fi om about 200 K to 400 K. Some general features of the intermixed structures studied thus far were smnmarized in Reference [98T1] as ... 

Other data support the above picture. Hexanol adsorbs very weakly on Ag(l 10), more weakly than expected, and in any case less than on the (100) face.440 Such a poor adsorption on (110) faces has been explained in terms of steric hindrance caused by the superficial rails of atoms. Consistently, adsorption on the (110) face of Cu is vanishing small.587 Predictions based on a linear regression analysis of the data for pentanol (nine metals) give a value of-12 kJ mol 1 for Cu(l 10) and about -16 kJ mol 1 for Au(110). No data are available for polycrystalline Au, but Au(l 11) is placed in the correct position in the adsorption of hexanol.910 Thus, these data confirm the hydrophilicity sequence Hg < Au < Ag and the crystal face sequence for fee metals (111) < (100) < (110). 

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. 

Molecular modeling work performed by Sasol researchers on fee cobalt (100) shows that increased coverage of 50% atomic carbon will induce a clock type reconstruction (Figure 4.3) similar to that observed for the classic case of Ni (100).28 The adsorption energy of the carbon is stabilized by 15 kJ/mol compared to the unreconstructed surface, resulting in a more stable surface.28 The reconstruction results in a shorter distance between the carbon and cobalt but also an increase in coordination of the cobalt atoms and, thus, fewer broken bonds. The barrier for the carbon-induced clock reconstruction was found to be very small (1 kJ/mol), which suggested that the process is not kinetically hindered. The... 

Wu and coworkers studied CO adsorption on neutral and charged Au clusters. They found that the charge state influences the geometrical and electronic properties of the adsorption process. The top position is the preferable site for neutral clusters with less than 6 atoms. More recently, Phala and coworkers found that the top site is favored up to a 13 Au atoms substrate, except for 5 atoms, competing with the bridge position. The geometries considered by these authors for more than 6 atoms are extracted from bulk fee gold motifs. 

With respect to CO adsorption on neutral Aun clusters, our preliminary results show differences with previous calculations using cluster geometries extracted from the fee gold crystal. The adsorption energy of CO is minimum for the planar magic cluster Aue and shows maxima for the odd electron clusters Au5 and Au7, both with the CO absorbed on a bridge position, but the even-odd effect is not obtained for sizes larger than n=7. 

The top layer of a (111) surface actually has sixfold symmetry, but the rotational symmetry of the top layers together is threefold. Since the near surface region can influence where gases adsorb on the surface and the LEED intensities exhibit threefold rotational symmetry at normal incidence, the (111) surface will be considered to have threefold rotational symmetry. Although most of the adsorption studies have been carried out on fee and bcc crystals, there have been several studies reported on hep crystals. For hep metals the basal or (0001) plane is the surface most frequently studied by LEED investigations and it is the most densely packed plane having threefold rotational symmetry. 

A database of molecularly adsorbed species on various surfaces is also included (see Table 4.3). In all cases, the chemisorption energies have been calculated on stepped surfaces using density functional theory (see [56] for details). The metals have been modeled by slabs with at least three close-packed layers. The bcc metals are modeled by the bcc(210) surface and the fee and hep metals have been modeled by the fcc(211) surface. A small discrepancy between the adsorption on the hep metals in the fcc(211) structure is thus expected when the results are compared to the adsorption energies on the correct stepped hep structure instead. When mixing... 

The spectrum of ethyne on Cu(110) at 280 K differs from those of ethyne on Ni(110) and Pd(110) at low temperatures in showing additional absorption bands and strong and well-defined vCC and vCH absorptions (56). Both of the latter bands are broad and weak for the other two metals. In the case of Fe(110), which has the different body-centered cubic (bcc) structure, the (110) plane is nearest to close-packed so that the observed type A spectrum at 120 K may have its more usual significance as indicating a four-metal-atom site not very different from that on fee (111) planes. Adsorption on Ag(110) at 100 K gives a spectrum much less strongly perturbed relative to the spectrum of the free ethyne molecule than any of the others. This clearly denotes relatively weak 77-bonding to the surface (57, 58), in marked contrast to the copper case. 

Adsorption Site fee Hollow fee Hollow + On-Top fee Hollow + On-Top (On-Top Fixed) fee Hollow + On-Top + hep Hollow... 

No fee hollow species for NO and no bridge species for CO are found by RAIRS on the alloy. The absence of these peaks is not due to intensity transfer, but due to an absence of these species, because only one peak corresponding to the on-top species is observed at 1700 cm-1 for NO and at 2100cm-1 for CO even at very low coverages. That is, the on-top site is dominant for NO and CO adsorption on the alloy. This instability of the multi-coordination site in NO and CO adsorption is characteristic of the alloy and is interpreted as due to hybridization between the Ge 4s and the Pt 5d orbital [87, 89], i.e. s-d hybridization, in which the d hole of the metal is partially occupied by s electron of the impurity. 

Surface reconstruction is driven by stabilization of the adsorbate after adsorption of carbon atoms on more reactive surface atoms. Ciobica et al. (74) demonstrated that an overlayer of Cads leads to the Co(lll) to Co(lOO) reconstruction on fee cobalt (the stable phase of small cobalt particles). Because of the change in metal atom density in the surface layer, the reconstruction may be associated with faceting and hence creation of step-edge sites, which are highly active in the Fischer-Tropsch reaction (5). Hence, surface reconstruction and formation of a stable carbide overlayer may actually be the processes occurring during the initial activation of the catalyst. This phenomenon has been described by Schulz (101) as self-organization. 

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. 

Structural Characteristics of the Adsorption of Oxygenated Species on Pt, PtsCo, PtsNi, PtsV, and Pt4.3Rh. Atomic Oxygen is Adsorbed on fee Hollow Sites while Hydroxyl and OOH Adsorb on Top Sites. Interactions with either Co, Ni, V or Rh are Indicated in Bold Font. Distances are in A. The d-Band Center is an Average for all the Atoms in the Slab and is Referred to the Fermi Level. 

Rb, or Cs, is only possible in part. They can be described in terms of adsorption on a substrate whose first layer contains some fraction of a monolayer of vacancies, just as in the case of the single-layer surface alloys. These vacancies are occupied with a first layer of Na. A second layer of Na (or K, Rb, or Cs in the case of the ternary alloys) is then adsorbed on the A1 vacancy layer. However, in the case of the Al(lll)—(2 x 2) multilayer surface alloys, this second layer of Na is adsorbed in fee sites on the A1 vacancy layer, and the A1 atoms ejected from the vacancy layer are readsorbed in hep sites on the vacancy layer. By contrast, in the Al(l 10)—(4 x 1)—3Na multilayer surface alloy, the second Na layer is adsorbed in sites of low symmetry on the vacancy layer, and the A1 atoms ejected from the vacancy layer do not form a part of the structure, but presumably diffuse to surface steps where they are readsorbed. 

The case of triangular lattice is particularly interesting since it corresponds to adsorption on graphite and on the (111) plane of several fee metal crystals [15,102,103,135]. The distance between adjacent potential minima for the graphite basal plane is equal to 2.46A and hence is too small to allow for their mutual occupation by even very small atoms of light noble gases. The same is true for adsorption on metals. Experimental studies have demonstrated that for rare gas atoms and simple molecules adsorbed on the graphite basal plane as well as on the (111) faces of fee crystals the ordered state corresponds to either the /3 X %/3 [102] or to the 2x2 phase [103,136,137] shown in Fig. 8. 

Figure 4.5 Simulated densities of Ar at 77.5 K (thick line) and CO2 at 195.5 K (thin line) adsorbed on an fee array of C60 molecules approximated as spherical bodies are plotted as a fimetion of distance from the sohd adsorbent. Adsorption on the outermost layer of C60 spheres produces three density peaks one at 1.4 nm in the deep (and strongly interacting) crevices located in the centers of the squares formed by four C60 molecules one at 1.8 nm in the crevices between pairs of neighboring C60 s and one at 2.1 nm for adsorbed molecules directly over a C60. Peaks at larger distances reflect structure in the adsorbed fluid, with the CO2 density decreasing to zero after second layer formation at 195.5 K because of its reladvely small amount in the simulation box compared to the Ar multilayer densities at 77.5 K.

Another interesting polymorph of carbon is fullerene. Although adsorption on individual fullerene molecules and on the surfaces offuUerene crystals is not widely studied, explicit [10] as well as mean-field [11] models are available for individual fullerene molecules. The fullerene crystal can be modeled by placing individual (model) fullerene molecules on the sites of an fee lattice, to match the symmetry and density of the real solid [12] or to match equilibrium structures obtained from computer simulations of fullerene crystals [10]. A model for a defective crystal can be obtained by removing some of the fullerene molecules [13]. 

In a subsequent study on Nj adsorption on C70, Arora et al. [36] carried out quantum mechanical calculations to predict N2 adsorption on five different known structures for Cyg. In this case it has been found that, besides the surface curvature of the Cyg molecule, an additional difference with graphite may arise from changes in the electronic configuration due to the presence of five-membered rings. The surface area, monolayer capacity, and isosteric heat of adsorption were calculated for various Cyg crystal phases [37] that are shown in Fig. 14.3 fee, deformed hexagonal-closed-packed (hep I), ideal hexagonal-closed-packed (hep II), monoclinic (mono), and rhombohedral (rh). 

Sulfur adsorption onto the surfaces of bcc metals is qualitatively different to that on fee substrates. Sulfur adsorbed onto Fc(l lO) induces lateral distortions of the Fe substrate. These movements produce a pseudo four-... 

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