A new mechanism for K promotion of surface reactions: N2 on K-precovered Fe(111)

Abstract

The adsorption of N₂ on K‐precovered Fe(111) at 74 K has been studied with low energy electron diffraction (LEED), thermal desorption spectroscopy (TDS), and high resolution electron energy loss spectroscopy (HREELS). The presence of low precoverages of K (1⁴ cm⁻²) dramatically enhances the sticking coefficient of α₁‐N₂, the π‐bonded precursor to dissociation, and causes an increase in the maximum population of this species. We conclude that the effects of K on this system are primarily mediated by long range interactions; we have modeled the nonlocal K‐induced changes of the adsorption and desorption of N₂ for temperatures ≤430 K and found that by slightly decreasing the γ‐N₂ adsorption energy and increasing the α₁‐N₂ adsorption energy we can quantitatively account for both the increase of the α₁‐N₂ sticking coefficient at 74 K and the increase of the dissociative sticking coefficient at 430 K previously reported by Ertl, Lee, and Weiss [Surf. Sci. 1 1 4, 527 (1982)]. The promoted α₁ state has an N–N stretch frequency less than 20 cm⁻¹ lower than that of unpromoted α‐N₂, as expected for a weak long range interaction with K, indicating that the N–N bond is not appreciably perturbed. This is significant, as the catalytic effects of K have been previously attributed to bond weakening in the dissociation precursor. At higher K precoverages, local N₂–K interactions dominate, characterized by broad N–N vibrations at 1600–1800 cm⁻¹. The HREELS spectra of H₂ and H₂O, common vacuum contaminants, are also reported for adsorption on a K‐precovered surface at 83 K.