Edge and kink sites

Fig. 4. Butterfly cluster compounds useful for molecular modeling of intermediates at edge and kink sites on an lr(/00) surface.

Fig. 7. High-resolution electron micrograph showing Rh clusters scattered on trace, edge, and kink sites of the (III) surface of spinel-type AIjO, crystallines.

Figure 13.4.2 The atomic structure of several high-Miller-index stepped surfaces, showing terraces, step edges, and kink sites. [From G. A. Somorjai in Photocatalysis— Fundamentals and Applications, N. Serpone and E. Pelizzetti, Eds., Wiley, New York, 1989, p. 265, with permission.]...

Relation (2.23) imphes that in alloys such as Ni with Cu, there is substantial enrichment at the surface where the low surface energy component, or a complex or a phase on a metal oxide, segregates to the surface. This enrichment preferably occurs at the more coordinatively unsaturated step edge and kink sites. 

The above arguments illustrate the importance of edge and kink sites in catalysis. As a consequence, reconstruction phenomena that change also the edge and kink site distribution can have a large effect not only on the overall rate of a catalytic reaction but also on its selectivity. The latter occurs when competing elementary reaction steps have... 

Adsorption experiments with CO have conclusively shown that CO adsorbs weakly on the MgO terrace but more strongly on edges and corners sites. Using QM cluster calculations, Petterson et al.I l predicted CO adsorption values of 8, 18 and 48 kJ/mol for the terrace, corner and edge sites, respectively. The adsorption of CO at terrace, edge, and kink sites was found to lead to an upward shift in the CO stretching frequency by - -9, +27 and +50 cm , respectively. These results are consistent with the generally accepted experimental data on this system of Wichtendahl et al. . 

As a consequence, several concepts and strategies have been developed for improved stability. They encompass a chemical stability approach, such as surface termination of Si by hydrogen or CH3 [41 6]. However, since even highly ordered single crystal surfaces are characterized by atomic terraces, step edges, and kink sites, solvolytic splitting of backbonds can occur that results in either terrace dissolution or in initiation... 

However, this assumption is not necessarily justified. Even for a well-faceted nanoparticle there are a number of nonequivalent adsorption sites. For example, in addition to the low-index facets, the palladium nanoparticle exhibits edges and interface sites as well as defects (steps, kinks) that are not present on a Pd(l 1 1) or Pd(lOO) surface. The overall catalytic performance will depend on the contributions of the various sites, and the activities of these sites may differ strongly from each other. Of course, one can argue that stepped/kinked high-index single-crystal surfaces (Fig. 2) would be better models (64,65), but this approach still does not mimic the complex situation on a metal nanoparticle. For example, the diffusion-coupled interplay of molecules adsorbed on different facets of a nanoparticle (66) or the size-dependent electronic structure of a metal nanoparticle cannot be represented by a single crystal with dimensions of centimeters (67). It is also shown below that some properties are merely determined by the finite size or volume of nanoparticles (68). Consequently, the properties of a metal nanoparticle are not simply a superposition of the properties of its individual surface facets. 

An energy-level model for this mechanism, proposed hy Gerischer et al. [123], is shown in Fig. 30. The energy levels associated with Si - H2 groups at kink sites are assumed to be located just above the valence hand edge and hence sites for hole capture. A hole trapped at the kink site oxidizes one of the Si - H groups to release a... 

Figure 2 The (a) mass- and (b) surface-averaged distribution of atoms on the (111) and (100) crystal faces and on the edges and corner sites of a cubo-octahedral cluster model. (From Ref. 4.) Mass-averaged and surface-averaged distributions are based on calculations using cubo-octahedron cluster model and represent number of different crystallographic planes divided by the (a) mass or (b) the surface area of the cluster (at the corresponding particle size). Hence (e- -c) in (a) represents edge and kink positions and (100) (111) the normal cubic crystal planes.

Catalytic sequences generally occur at an atomic scale on the defect sites, i.e. terrace sites, edge sites and kink sites, of the active phase nanoparticles. Often the true active sites are generated during the catalytic reaction by surface modification in the presence of the reactants and products. In these cases, the surface nature... 

When atoms are deposited onto or removed from the surface of a solid, this process often preferentially occurs at higher energy sites on the surface of the solid, such as at step edges or kink sites. Atoms at these sites have more dangling/ unsatisfied bonds and thus are more susceptible to reaction. 

---