Chemical Bonding to Transition-Metal Surfaces

In this section we introduce principles of the surface chemical bond. First principle ab initio computational results are analyzed using basic quantum-chemical concepts. In this section, we analyze the adsorption of molecules. In the following section, we analyze the adsorption of atoms. The adsorption of ammonia and CO is discussed first since they are known to interact predomenantly through donation and back-donation interactions, respectively. This will subsequently lead into the analysis of the stronger bonds that form between adatoms and a surface. We note the similarities in chemical bonding of these adsorbates to surfaces, clusters and organometallic complexes, and in addition describe some of the differences. 

The primary interaction between NH3 and a metal surface is predominantly a donative one which occurs via the transfer of electrons from the doubly occupied nonbonding 2p lone-pair type orbital on N. The corresponding states on the metal depend upon the metal. 

The electron distribution (Partial Density of States, PDOS) within the 4d 2 state on one of the surface atoms of the pristine Rh(lOO) surface is shown in Fig. 3.3b(l). The electron distribution within the 4d 2 state of the Rh(lll) surface is shown in Fig. 3.3b(2). The coordination number of the surface atoms in the Rh(lOO) surface is 8 whereas that of the Rh(lll) surface is 9. The width of the d-valence electron band is smaller for the Rh atom on the (100) surface, which has the smaller number of metal neighbors. In addition, the average energy of the valence band has been shifted slightly upwards. For an atom with an s-valence-electron distribution, it can be shown that the valence bandwidth is approximately proportional to ZZV), Nn being the number of nearest-neighbor atoms. The delocalization of the electrons increases with increase in the number of nearest-neighbor atomsl l. 

Valence electron band narrowing increases the average energy of the electrons, because 

In the adsorption of ammonia on the surface, the NH3 2p lone-pair molecular orbital interacts with a transition-metal smface atom to form bonding and antibonding orbital fragments. The resulting 4d 2 electron distributions are also shown in Fig. 3.3b. 

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