Coordination number surface atoms

It is noteworthy that surface carbon did not come from those CO molecules responsible for the HT peak but from sites that are able to disproportionate CO and correspond to the LT peak. Because the latter sites are important only on quite small particles, it is tempting to associate them with low coordination number surface metal atoms, the relative concentration of which increases rapidly as the particle size decreases below 2 nm (8). Thus, these atoms may be the sites responsible for the relatively weakly adsorbed state of CO. Results similar to our work were found on other Group VIII metals. In the case of a Ru/Si02 sample, Yamasaki et al. (9) have shown by infrared spectroscopy that the deposition of carbon occurs rapidly by CO disproportionation on the sites for weakly held CO. The disproportionation also occurred on a Rh/Al20 sample with 66% metal exposed so that appreciable concentrations of low coordination atoms are expected (10). 

While low coordination number sites, steps, and kinks, are the active sites for bond breaking in platinum, the atomic terrace sites with larger coordination numbers may also become active sites with unique chemistry for other elements. It will perhaps become possible to identify the bond-breaking ability of various coordination number sites of a given metal in breaking H—H, C—H, C —C, 0=0, N=N, etc., chemical bonds. By varying the atomic surface structure, which would change the relative concentrations of the different coordination number surface sites, the product distribution in surface chemical reactions may be markedly varied. 

It is often useful to consider that sites for chemisorption result from surface coordinative unsaturation, i.e., that atoms at the surface have a lower coordination number than those in bulk. Thus, for example a chromium ion at the surface of chromium oxide has a coordination number less than that of a chromium ion in the bulk. The chromium ion will tend to bind a suitable adsorptive so as to restore its coordination number. An atom in the (100) surface of a face-centered cubic metal has a coordination number of 8 vs 12 for an atom in bulk this, too, represents surface coordinative unsaturation. However, of course, there are sites to which the concept of surface coordinative unsaturation does not apply, for example, Br nsted acid sites. 

M(highc) denotes a high-coordination number surface metal atom. This is the most stable and usually the predominant state at the electrode surface and gives rise to the conventional cyclic voltammetry response. These well-embedded surface metal atoms remain in the lattice surface where, on oxidation, they lose electrons and coordinate the hydroxyl species, by Equation VIII. 

Fig. 3. (a) Face-centered cubic (fee) octahedron, m = 9 (m is the number of atoms along an edge). From Ref. 20. (b) Coordination of surface atoms versus the mean particle size d for nickel, assuming that the particle is a face-centered cubic (fee) octahedron. The symbols C - refer to atoms identified in (a). 

L. Falicov and G.A. Somoijai. Correlation between Catalytic Activity and Bonding and Coordination Number of Atoms and Molecules on Transition Metal Surfaces Theory and Experimental Evidence. Proc. Natl. Acad. Sci. USA S2i2207 (1985). 

An electrophilic character is also found for low-coordination number metal atoms at kinks and steps in metal surfaces.These sites are also known to be much more active for alkane reactions than the flat metal surfaces. 

Fig. 21.2 (a) Cubo-octahedral geometry of 2.6 nm Pt particle showing the 111 and 100 facets and coordination numbers of atoms comprising these facets, particle edges, and vertices, (b) Size dependence of dispersion and surface percentage of atoms on 111, 100 facets, and on the edges and vertices between the facets [35]... 

As was told above, coordination numbers of atoms on the surface of a crystal are less than in the bulk, and most properties of dispersed materials can be formulated and understood in terms of atomic Nc imperfection and its effect on atomic cohesion. In particular, the difference between the cohesive energy of an atom at the surface and... 

Figure 6-32. A comparison of NMR spin saturation transfer vs. irradiation frequency for CO exchanging with adsorbed CO in the presence of 7.0 nm and 2.3 nm colloidal palladium/PVP in methanol. The smaller metal particles show a greater saturation transfer at higher fields consistent with a higher incidence of low coordination number surface metal atoms (less Knight shifted) in the smaller particles. (Adapted from ref. [113].)...

Edge length Total number of atoms in the crystal Fraction of atoms on surface Relative number of atoms in different states on surface Average coordination number of atoms on the surface... 

As we will discuss in more detail in Chapter 3, the delocalization of electrons is proportional to the square root of the number of coordinating atomsl l. One would therefore expect adsorbate binding energies to increase with decreasing particle size, owing to the increased number of coordinatively unsaturated surface atoms. The reactivity of these particles with respect to cluster size will then depend the position of the adsorbate bond energy with resp>ect to the Sabatier curve maximum. 

The reactivity of a coordinatively unsaturated surface atom tends to increase with decreasing coordination number. Therefore, high-index surfaces tend to be more reactive than low-index surfaces. Steps and kinks are often the preferred sites for dissociation to occur. If dissociation leads to fragments that preferentially require a specific site or sites... 

For very small metallic particles, or clusters, crystal faces have no meaning. It is better to define surface structure with the notation introduced by Van Hardeveld and Hartog. They consider the coordination number i of a surface atom and the coordination number j of a surface active site, which was called as coordination model. Surface atom is denoted by Cj when it has i nearest neighbors. The active site is denoted by By when it has j nearest neighbors. Examples of C4, Ce and C7 atoms are shown in Fig. 2.8. Several active sites, B4, B5, Be and B7 are shown in Fig. 2.9. 

In contrast to molecular adsorption, the interaction energy of atoms as C, O, or N with a metal surface is a strong function of coordination number. Adsorbed atoms almost always prefer bonding in sites of threefold or fourfold coordination. Molecules can also adsorb on top or bridge sites. As will be seen, this has major consequences for the dissociation paths of diatomic molecules on metal surfaces. 

These correlations demonstrate that the number density of the most coordinatively unsaturated surface atoms (the corner atoms) increases sharply as a fraction of the total number of atoms, for particle sizes less than 2 nm. On the other hand, the same data demonstrate that the number density of less coordinatively unsaturated edge sites decreases gradually as particle sizes increases over 2 nm. As the particle size increases further beyond 2 nm, almost all surface atoms are located on terraces, which are the most coordinatively saturated of the surface atoms [4, 5]. 

---