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 (<1.4×1014 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. 114, 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.