Surface atomic configuration

Earlier we described the catalytic reaction as a series of consecutive steps at the surface, in which adsorbate and adsorbate-surface bonds are formed and/or broken on the reaction path towards the product molecule. The forces between surface atoms and adsorbate atoms responsible for rearrangement of the chemical bond are similar to those responsible for strong adsorption (E > 10 kcal/nx)l). The adsorption process dominated by such interaction is called chemisorption. Even on a single crystal metal surface, several adsorption modes are conceivable and for dissociation of a diatomic molecule many different reaction paths can be envisioned. However, usually only one particular surface atom configuration is preferred to lead to the idea of catalytic active site. If catalysis of a molecule is studied that has several reaction possibilities, some desirable and others not, a selective reaction usually requires a particular surface atom composition and rearrangement. 

Langmuirian view, the active catalytic surface is comprised of a uniform distribution of static sites that do not interact with one another. This is sharply contrasted by the Taylor view, which proposes vacancies and topologically unique surface atom configurations as the centers of reactivity. The Langmuirian idea of a catalytically reactive surface leads to the ensemble effect that ascribes the changes in the selectivity for an alloy surface to the dilution of multi-atom surface ensembles in the alloy induced by mixing inert components into the active surface. In this view, the selectivity of a particular reaction depends predominantly on the number of reactive surface atoms that participate in elementary reaction events. 

Extended x-ray absorption fine stmcture measurements (EXAFS) have been performed to iavestigate the short-range stmcture of TbFe films (46). It is observed that there is an excess number of Fe—Fe and Tb—Tb pairs ia the plane of the amorphous film and an excess number of Tb—Fe pairs perpendicular to film. The iacrease of K with the substrate temperature for samples prepared by evaporation is explained by a rearrangement of local absorbed atom configurations duting the growth of the film (surface-iaduced textuting) (47). 

Part of a 15-nm long, 10 A tube, is given in Fig. 1. Its surface atomic structure is displayed[14], A periodic lattice is clearly seen. The cross-sectional profile was also taken, showing the atomically resolved curved surface of the tube (inset in Fig. 1). Asymmetry variations in the unit cell and other distortions in the image are attributed to electronic or mechanical tip-surface interactions[15,16]. From the helical arrangement of the tube, we find that it has zigzag configuration. 

Here, we address the more general question of the relative stability of monomers, dimers and triangular trimers on the (111) surface of FCC transition metals of the same chemical species as a function of the d band filling Nd. All possible atomic configurations of the systems are considered monomers and dimers at sites N and F, triangles with A and B borders at sites N and F (Fig. 4). The d band-filling includes the range of stability of the FCC phase (Nd > 7.5e /atom). The densities of states are obtained from... 

Figure 4. Possible atomic configurations for monomers, dimers and triangular trimers on (111) FCC surfaces at normal (N) and fault (F) sites. Trimers of type A have their center above an atom in the surface layer, for type B the center is at an adsorption site...

In any case, it is interesting to note that catalytic efficacy has been observed with nano- or mesoporous gold sponges [99-101, 145] suggesting that neither a discrete particle nor an oxide support is actually a fundamental requirement for catalysis. An alternative mechanism invokes the nanoscale structural effect noted in Section 7.2.2, and proposes that the catalytic effect of nanoscale gold structures is simply due to the presence of a large proportion of lowly-coordinated surface atoms, which would have their own, local electronic configurations suitable for the reaction to be catalyzed [34, 49,146] A recent and readily available study by Hvolbaek et al. [4] summarizes the support for this alternate view. 

Figure 6.3 Potential energy surface for colinear reaction AB + C —> A + BC (a) 2-D topographical representation (b) 3-D representation (c) potential energy along reaction coordinate (d) atomic configurations along reaction coordinate...

Under the present conditions, we propose that the -CN group in the nitrile molecules acts in a similar fashion. The nitrile molecules are adsorbed on a fraction of the Pt surface atoms, and their presence forces the co-adsorbed CO molecules on the remaining sites to assume the bridging configuration. When the potential is increased, part of the driving force for this conversion is removed and the CO molecules convert back to their linear configuration. This conversion is irreversible when the potential is lowered a second time. The reason for this irreversibility is uncertain and should be explored further. 

One of the oldest mechanisms of interaction between adsorbed reactant and adsorbed TA has been proposed by Klabunovskii and Petrov [212], They suggested that the reactant adsorbs stere-oselectively onto the modified catalyst surface. The subsequent surface reaction is itself nonstere-ospecific. Therefore, the optically active product is a result of the initial stereoselective adsorption of the reactant, which in turn, is a consequence of the interactions between reactant, modifier, and catalyst. The entities form an intermediate chelate complex where reactant and modifier are bound to the same surface atom (Scheme 14.4). The orientation of the reactant in such a complex is determined by the most stable configuration of the overall complex intermediate. The mechanism predicts that OY only depends on the relative concentrations of keto and enol forms of the reactant,... 

Sachtler [195] proposed a dual-site mechanism in which the hydrogen is dissociated on the Ni surface and then migrates to the substrate that is coordinated to the adsorbed dimeric nickel tartrate species. In their model, adsorption of modifier and reactants takes place on different surface atoms in contrast to Klabunovskii s proposal. Adsorbed modifier and reactant are presumed to interact through hydrogen bonding (Scheme 14.5). The unique orientation of adsorbed modifier molecules leads to a sterically favored adsorbed reactant configuration to achieve this bonding. 

---