D-band shift model

Figure 4.16 also shows that there are additional effects due to direct interactions between adsorbates that are not described using the d band model. A large adsorbate like S, will have a sizable overlap to the valence orbitals of the incoming molecule, giving rise to a repulsion which is larger than what can be readily explained by the indirect interaction through d band shifts. 

Ruban et al. established an atomic d-band center model.91 Based on this model, they examined the impact of surroundings to the activity and how the activity can be altered. They revealed that the shift of the d-band depended on the difference in the size... 

The band shift model of Gillis-D Hamers7 is based on the observation that the peak maximum of the free silanol vibration shifts as a function of treatment temperature (figures 5.6 and 5.7). Measurements were performed using the FTIR technique, in the high resolution (0.5 cm 1) mode. 

Models of CO adsorption show that top site binding is governed by the CO HOMO (5cr orbital) donating electrons into the metal unoccupied states, with simultaneous back-donation of electrons from the metal s occupied dxz and dyz states into the CO LUMO 2tt orbital). Therefore, it follows that the standard chemisorption model, which considers shifts in the total d-band center, can be inaccurate for systems in which individual molecular orbitals, involved in bonding with the adsorbate, shift differently due to external interactions. In particular, we have shown that the formation of hybrid orbitals with the support material can lead both to downward shifts in the metal d-band center, which do not affect the adsorption of molecules to the metal surface, and to upward shifts that are vitally important. 

Figure 6.16. Illustration of the d-band model governing surface chemical bonding on transition metal surfaces. As the d-band center of a catalytic surface shifts downward more antibonding orbitals become occupied and the surface bond energy of an adsorbate (here an oxygen atom) decreases. An upward shift in the d-band center predicts strengthening of the surface bond.

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