Surface Sites on an fee Metal

One way of calculating the electron energy levels of the various types of surface sites is to consider each type of site as a surface complex made up of the central, active atom surrounded by the neighboring ligand atoms. In an fee metal, a bulk metal atom is surrounded by twelve near neighbor ligands as shown in both top and side views from the 111 and 100 orientations in Fig. 4.3 This is the parent species and each of the possible surface complexes can be formed by the removal of the appropriate ligands from the parent. 

The surface complex made up of an atom in a 111 plane is formed by removing ligands 10, 11 and 12 from the parent. An atom in a 100 plane is produced by removing atoms 4, 9, 10 and 12. A tetrahedral comer site is composed of the central atom and ligands , 2, and 3, while an octahedral comer has ligands 2, 3, 6 and 7. Table 4.1 lists all of the surface arrangements possible 

Site Description Number of Nearest Neighbors Ligands Designation 

The 111 plane can intersect another plane at either 60 or 120 so there are two 111-111 edge sites, labeled D and E, and two 111-100 edge sites, labeled C and F. The 90 orientation required for the intersection of two 100 planes is not possible. The seven different comer atoms range from the cubooctahedral, F, with six neighboring atoms to the tetrahedral comer, M, having only three. Here, too, the 60 and 120 plane intersections show up in the comer atom arrangements. Stmetures of these sites are depicted in Fig. 4.5.7,8 

Other views of some of these sites are illustrated with the crystal shapes shown in Figs. 4.4 and 4.6. It can be seen in the cubic crystal that the atoms on the comer are not bonded along the edges of the cube. Instead these comer atoms are bonded to three nearest neighbors and are, in reality, tetrahedral comers (M) as shown more clearly on the tetrahedral crystal. The face atoms on the cube are in a 100 plane (B). What appear to be edge atoms, though, are the surface sites shown in Fig. 4.5 as the 111-111-100-100 comer sites (H). 

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