Abstract
Dissociative nitrogen adsorption through N 2 ⇌ N 2, ad → 2 N s is the rate limiting step in ammonia synthesis over iron catalysts. This model study deals with the “promoter” action of potassium by studying the influence of preadsorbed K on the energetics and kinetics of molecular and atomic nitrogen adsorption on Fe(111) and Fe(100). The heat of adsorption of molecular nitrogen is locally increased to about 11 kcal/mole in the vicinity of an adsorbed K atom, which phenomenon simultaneously lowers the activation barrier for dissociation. As a consequence the effective sticking coefficient for dissociative nitrogen adsorption is markedly increased. This effect is less pronounced for Fe(111) than for Fe(100) so that the difference in activity observed with the K-free surfaces is eliminated. The increase of the N 2 adsorption energy is attributed to the enhanced electron density of the substrate (due to charge transfer from K) which enables a more pronounced “backbonding” contribution to the adsorption bond.
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