Bond dissociation energies, first-shell coordination

The activation energy for CO dissociation on rhodium is lower for the (100) than for the (111) surface. The (100) surface is the more open one of the two. Each rhodium atom misses four nearest neighbors in the first coordination shell as compared to a bulk atom, whereas an atom in the (111) surface misses three nearest neighbors. Hence, carbon and oxygen atoms bond more strongly with the (100) than with (111) surface and, similar to the situation in Figure 6.8, both thermodynamics and kinetics are more favorable for CO dissociation on the (100) surface. This is a manifestation of the bond order conservation principle The more the valence electrons of a metal atom at the surface become distributed in bonds with neighboring atoms, the weaker the individual bond becomes. 

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