Trend in Atomic Chemisorption Energies

When an atom with a filled level at energy approaches a metal surface it will first of all chemisorb due to the interaction with the sp electrons of the metal. Consider for example an oxygen atom. The 2p level contains four electrons when the atom is isolated, but as it approaches the metal the 2p levels broaden and shift down in energy through the interaction with the sp band of the metal. Fig. 6.28(a) and (b) show this for adsorption on jellium, the ideal free-electron metal. 

All levels below the Fermi level are filled with additional electrons, leading to a strong sp-induced bonding, which amounts to about -5 eV. The O-sp interaction cannot explain the trends in bonding through the transition metals, since the sp bands of these metals are very similar. Instead, we need to look at the d band. 

If d then = d + (2y3 /d), which leads us to the final expression for the hybridization energy 

The first term is attractive (it increases the bonding energy) while the second is repulsive (decreases the bonding energy). Hence three parameters play a role in determining the bond strength between the metal d band and the atomic adsorbate  

the interaction matrix element /3 between the wave functions of the electron on the atom and those for the d states of the metal, 

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