Model for DissociativeH2Chemisorption on Transition Metals

Earlier calculations have shown that hydrogen atoms can be strongly absorbed on nickel and on copper surfaces although the character of the chemisorption bond is different. In the case of nickel both $3d$ and $4s$ electrons are involved, whereas for copper the bond is formed exclusively by $4s$ electrons. On the basis of this information a model is worked out for dissociative chemisorption of ${\mathrm{H}}_2$ molecules, which is experimentally found to occur on nickel but not on copper. In this model the activity of $3d$ and $4s$ electrons is studied separately. The $3d$ electrons cause an unactivated dissociative chemisorption of ${\mathrm{H}}_2$; the $4s$ electrons alone do not chemisorb an ${\mathrm{H}}_2$ molecule unless a high activation-energy barrier is surmounted.