Trends in Molecular Chemisorption

In diatomic molecules such as N2, O2, and CO the valence electrons are located on the 5cr, Ijt and 2jt orbitals, as shown by Fig. 6.6. [Note that the 5cr level is below the Ijt level due to interaction with the 4cr level, which was not included in the figure.] In general, the Ijt level is filled and sufficiently low in energy that the interaction with a metal surface is primarily though the 5cr and 2jt orbitals. Note that the former is bonding and the latter antibonding for the molecule. We discuss the adsorption of CO on d metals. CO is the favorite test molecule of surface scientists, as it is stable and shows a rich chemistry upon adsorption that is conveniently tracked by vibrational spectroscopy. 

Upon adsorption, there is again a strong interaction of the 5a and 2jt orbitals and the metal sp electrons, resulting, as above, in a downward shift and broadening of these two levels. Also, in this case the variation of the adsorption energy is accounted for by the interaction with the d band of the metal, which will cause the levels to split into bonding and antibonding parts. The result is shown in Fig. 6.32, which should be seen as a realistic alternative to the more qualitative representation of Fig. 6.25. 

Pt(ll 1) reveals that the contribution from the 5 r orbital to the chemisorption bond is small, whereas the 2n-d interaction clearly strengthens the bond, as only the bonding region of this orbital is occupied. [Adapted from B. Hammer, Y. Morikawa and J.K. Norskov, Phys. Rev. Lett. 76 (1996) 2141.] 

With respect to the adsorption energy, the interaction of the 5cr and 2jt orbitals with the sp band again gives a large and negative (i.e. stabilizing) contribution, E p, to the bond. The hybridization Afj.hyb can be estimated in a similar way to that in the case of atomic adsorbates. 

The contribution from the 5cr-d interaction is very similar to that of a filled state of an atom  

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