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

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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.