to some simple electronic structure parameters of the metal catalyst. The d-band model developed by Hammer and Norskov105, 115 relates the adsorption energy of a particular molecule to the d-band position of the metal, and by relating the adsorption energies to energy barriers, catalytic trends can be simply explained. The model has been quite successful in describing trends, for example, for N2, O2, and CO adsorption for various transition metals. Though a full explanation of the d-band model is not the intention of this appendix, a brief introduction will turn out to be very useful to understand the experimental results from electrode screening studied in this chapter Figure S5.9 illustrates the main idea underlying the d-band model. When the adsorbate approaches the surface, its valence state interacts with the broad sp-states of the metal surface. After re-normalization (Figure S5.9 a and b), we notice that the interaction with the sp-state gives rise to a downshift and broadening of the electronic state of the adsorbate. Thus, energy is gained by coupling to the sp-state. The d-band model assumes that the contribution to the adsorption from the interaction between the low-lying energy sp-band and the adsorbate valence state should be the same for all transition metals since they all have the same sp-states and the variations in their electronic configuration stem from differences in the d-band states. Because Figure S5.9. Schematic illustration of the interaction between the valence level of an adsorbate and the delocalized sp-states, followed by a second interaction with the localized d-states of a transition metal surface1. a b c d d-band broad sp-band
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