Adsorbate-induced surface reconstruction

Perhaps the most fascinating detail is the surface reconstruction that occurs with CO adsorption (see Refs. 311 and 312 for more general discussions of chemisorption-induced reconstructions of metal surfaces). As shown in Fig. XVI-8, for example, the Pt(lOO) bare surface reconstructs itself to a hexagonal pattern, but on CO adsorption this reconstruction is lifted [306] CO adsorption on Pd( 110) reconstructs the surface to a missing-row pattern [309]. These reconstructions are reversible and as a result, oscillatory behavior can be observed. Returning to the Pt(lOO) case, as CO is adsorbed patches of the simple 1 x 1 structure (the structure of an undistorted (100) face) form. Oxygen adsorbs on any bare 1 x 1 spots, reacts with adjacent CO to remove it as CO2, and at a certain point, the surface reverts to toe hexagonal stmcture. The presumed sequence of events is shown in Fig. XVIII-28. 

When atoms, molecules, or molecular fragments adsorb onto a single-crystal surface, they often arrange themselves into an ordered pattern. Generally, the size of the adsorbate-induced two-dimensional surface unit cell is larger than that of the clean surface. The same nomenclature is used to describe the surface unit cell of an adsorbate system as is used to describe a reconstructed surface, i.e. the synmietry is given with respect to the bulk tenninated (unreconstructed) two-dimensional surface unit cell. 

The model is intrinsically irreversible. It is assumed that both dissociation of the dimer and reaction between a pair of adjacent species of different type are instantaneous. The ZGB model basically retains the adsorption-desorption selectivity rules of the Langmuir-Hinshelwood mechanism, it has no energy parameters, and the only independent parameter is Fa. Obviously, these crude assumptions imply that, for example, diffusion of adsorbed species is neglected, desorption of the reactants is not considered, lateral interactions are ignored, adsorbate-induced reconstructions of the surface are not considered, etc. Efforts to overcome these shortcomings will be briefly discussed below. 

It is well known that the catalytic oxidation of CO on certain Pt surfaces exhibits oscillatory behavior, within a restricted range of pressures and temperatures, which are coupled with adsorbate-induced surface phase transitions [16,17]. In fact, in their clean states the reconstructed surfaces of some crystallographic planes, e.g. Pt(lOO) and Pt(llO), are... 

FIG. 10 Phase diagram of the ZGB model with adsorbate induced global reconstructions of the surface. The plots show the dependence of the rate of AB production (7 ab) and the surface coverages with A and B species and 9q, respectively) with the adsorption probabihty of A species given by Fa-... 

A lattice gas model with adsorbate-induced surface reconstructions has also very recently been proposed by Kusovkov et al. [73]. This model also exhibits a rich oscillatory behavior. 

According to the data obtained with SXRS in salt solutions,519 520 at a < 0 the surface of Au(lll) forms a ( 3 x 22) structure as in a vacuum. At a > 0 the reconstruction disappears and the (1 x 1) structure is observed. On the reconstructed Au(l 11) surface there are 4.4% more atoms than on the (1 x 1) structure and on the reconstructed Au( 100) there are 24% more atoms than on the (1 x 1) structure.506,519 This phase transition shifts in the negative direction with the adsorbability of the anion. The adsorption-induced surface reconstruction of Au(l 11) electrodes has been studied in situ by second harmonic generation by Pettinger et al.521... 

Structure of the Adsorbed Adatom Layer and Adatom Induced Surface Reconstruction... 

A well-known example of adsorbate induced surface reconstruction is that of carbon on the (100) surface of nickel. Even though this surface already offers a four-fold coordination to the carbon atom in unreconstructed form, additional energy is gained by the so-called clock-anti-dock reconstruction shown in Fig. 5.8. Of course, it costs energy to rearrange the surface nickel atoms, but this investment is more than compensated by... 

FIGURE 4.3 Left The unreconstructed surface of 50% C/fccCo (100). Right The clock reconstructed surface of 50% C/fcc Co (100). The darker spheres represent cobalt atoms and the lighter ones (in the fourfold hollow sites) represent carbon atoms. (Reprinted from Ciobica. I. M.. van Santen, R. A., van Berge, P. J., and van de Loosdrecht, J., Adsorbate Induced Reconstruction of Cobalt Surfaces, Surface Science, 602, 17-28. Copyright 2008, with permission from Elsevier.)... 

The changes in local coordination of the majority specie (the substrate) at the surface (e.g. adsorbate induced reconstruction). It all depends on the detection mode and on the absolute surface sensitivity of the most surface sensitive detection mode. The way to proceed is to measure differential SEXAFS spectra on one adsorption edge of the substrate material, where the difference is made between the clean substrate and the exposed substrate, or between a bulk sensitive detection of the EXAFS, (TY, FY, high energy AEY) and a surface sensitive measure (low energy AEY). Attempts have been made on this groxmd, but final reports have not yet appeared in the literature. 

A quite different class of adsorbate-induced surface reconstruction is formed by those systems involving pseudo-(lOO) reconstruction of the outermost atomic layer this behaviour has been found to occur on fcc(lll) and (110) surfaces in several metal/adsorbate combinations. The essential driving force for such reconstructions appears to be that adsorption on a (100) surface (typically in a c(2 x 2) arrangement) is so energetically favourable that, even on a surface with a different lateral periodicity (and point-group symmetry), reconstruction of the outermost layer or layers to form this (100)-like geometry is favoured. This must occur despite the introduction of strain energy at the interface between the substrate and the... 

All these observed features underline the fundamental importance of the adsorbate-induced surface structural transformations and can be qualitatively rationalized in terms of the outlined mechanism. For example, in order to obtain oscillations at given T and p0, the CO pressure (causing the respective CO coverage) must be high enough in order to lift the surface reconstruction, but, on the other hand, also sufficiently low to permit subsequently the reactive removal of the CO adlayer by oxygen. 

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