Chemisorption orbitals

The catalytic significance of Fig. 9.12 is that it represents the differences in the effective work functions that a molecule experiences upon adsorption at different positions on the surface. As explained in the Appendix, a low work function of the substrate enhances the capability of the substrate to donate electrons into empty chemisorption orbitals of the adsorbate. If such an orbital is antibonding with respect to an intramolecular bond of the adsorbed molecule, the latter is weakened due to a higher electron occupation. 

The decreased CO stretch frequencies are easily rationalized as the result of adsorption on sites with a lower work function, as explained in the Appendix. The effect of a lower work function is that all orbitals of CO shift downward with respect to the Fermi level of the substrate. This shift of the occupied CO levels to higher binding energy has been observed in UPS spectra (see Fig. 3.20), while the shift of the unoccupied part of the 2tt -derived chemisorption orbital has been observed in inverse photoemission [33, 34, 45], The overall effect is that the bond between the metal and the CO becomes stronger while at the same time the intramolecular CO bond is weakened. 

Figure A.14 Energy diagram for the adsorption of an atom on a d-metal. Chemisorption is described with molecular orbitals constructed from the d-band of the metal and atomic orbitals of the adatom. The chemisorption bond in b) is weaker, because the antibonding chemisorption orbital is partially filled (compare Fig. A.5).

If the antibonding chemisorption orbital lies entirely above the Fermi level, it remains empty and a strong chemisorption bond results (Fig. A. 14a). 

Intermediate cases in which the antibonding chemisorption orbital is broadened across the Fermi level can also arise (Fig. A. 14b). In such cases the antibonding orbital is only partially filled and the atom A will be chemisorbed, though with a weaker chemisorption bond than in Fig. A. 14a. 

Figure A.15 Energy diagram for the adsorption of a simple diatomic molecule on a d-metal. Chemisorption orbitals are constructed from both the bonding and the antibonding levels of the molecule. As the latter becomes partially occupied, the intramolecular bond of the adsorbate has been activated.

The first interaction, between CO 5c and Rh d,2, gives two new molecular orbitals. The bonding orbital has mostly 5<7 character, and it is customary to call it 5c. However, the level is lower than in free CO. UPS spectra reveal this shift immediately (see the spectrum of CO/Fe in Fig. 3.20) and indicate that its energy is close to that of the CO 1ft level. The antibonding chemisorption orbital has mainly d,2 character and is shifted upwards in energy. If the latter falls below the Fermi level, the 5c - d,2 interaction is entirely repulsive. For CO/Rh(100) the calculations indicate that the dz2 level falls across the Fermi level, such that the repulsion is partially relieved. 

The simple picture for the chemisorption of an atom on a metal with d-electrons in Figure A. 14 arises as follows. First, we construct molecular orbitals from the atomic orbital of the adsorbate atom and the entire d-band. This produces a pair of bonding and antibonding chemisorption orbitals. Second, these new orbitals are broadened and perhaps shifted by the interaction with the free electron s-band of the metal. 

Second, interaction (b) gives a bonding orbital, which can either be above or below the Fermi level. If it is fully occupied, the adsorbate molecule dissociates, but if it is only partially occupied then it will contribute to bonding between A2 and the surface, while at the same time the bond A-A in the chemisorbed molecule is weakened. This situation, which is shown in Figure A.15, very much resembles the adsorption of CO on transition metals, although the chemisorption orbitals are much more complex than the simple block-shaped bands of Figure A.15. 

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