Model calculations of chemisorption on transition metal clusters

Alloying of a transition metal with a non-transition metal affects the nature of the chemical bonds between the transition metal atoms in the surface and hydrogen or other atoms chemisorbed on them. The resultant changes in strength of the chemisorption bonds are due to (1) changes in delocalization of the metal electrons involved in the chemisorption bonds, (2) changes in the number of electrons available for such bonds. Our calculations have shown that the effects of alloying on chemisorption are different for monocoordinated adsorbates (ad-atom on top of metal atom) and tricoordinated adsorbates (ad-atom equidistant to three transition metal atoms) respectively. With localized bonds, alloying increases the strength of the former bond, but weakens the latter. As a result the ratio of monocoordinated to tricoordinated adsorbates, present in equilibrium at high coverage, is drastically increased by this type of alloying. The increase is much larger than expected on a geometrical basis only.