Kinetics of nitrogen adsorption on Fe(111)

Abstract The dissociative adsorption of nitrogen on a clean Fe(111) surface proceeds via with a very small overall sticking coefficient and a slightly negative activation energy: σ o = 2.2 × 10 −6 exp ( E ∗ RT ) ; E ∗ = −0.8 kcal/mole (at zero coverage). The intermediate formed N2.ad is characterized by an adsorption energy of 7.5 kcal/mole and by a dipole moment of ∼0.4 D of the adsorbate complex, the negative charge being located at the nitrogen molecule. The kinetic parameters of the individual (first order) steps were determined as follows: (i)s0≈10−2 ± 0.5 (sticking coefficient into the molecular state = k 1 ); (ii) k −1 = v−1 exp ( −E RT with v−1 = 1010 ± 1s−1E1 = 7.5 kcl/mole (desorption of N2,ad); (iii) k 2 = v 2 exp (−E 2 RT ) , with v2≈107s−1, E 2 = E 1 + E ∗ = 6.7 kcal/mole (dissociation of N2,ad into 2Ns). The extremely low dissociation probability of gaseous N2 is caused by two factors, namely unusually low values for s0 and for the ratio v 2 v −1 .