The Binding of Adsorbed and Absorbed Hydrogen

These repulsive interactions cannot be due to direct H-H interactions - the H-H distance even on Ni is much larger than the diameter of a H atom (0.249 and 0.15 nm) - but arises instead from an indirect interaction via the conduction electrons of the metal. The magnitude of the interaction has been calculated to be 

Weinberg and Merrill have presented a very simple and attractive model of H/Pt(lll) which, by taking account of the spatial distribution of the if-orbitals, draws attention to the differences between various metal planes. This model has been used to describe the adsorption of H on Pd. ° It is obviously important to take note of orbital geometry (similar considerations have been used to identify the most favourable sites for absorption ° ), but the fairly small differences in heats of adsorption on different planes may indicate only a small dependence on the geometry of the orbitals. Moreover, it has also been shown that the interaction between H and 

Ni is mainly between the H orbital of H and the i -band of Ni, whereas with Pd and Pt the (/-band interacts with the H Ly orbital. The simple orbital model would not distinguish between Ni, Pd, and Pt. 

The delocalization of the surface interactions has been emphasized recently by Christmann et al who introduce the concept of atomic bands , analogous to surface electronic states, to describe H adsorbed on Ni(lll). 

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