Interaction of nitrogen with iron surfaces: I. Fe(100) and Fe(111)

Abstract

The adsorption of N 2 on clean Fe(100) and Fe(111) single-crystal surfaces was studied in the temperature range 140–1000 K by means of Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), ultraviolet photoelectron spectroscopy (UPS), thermal-desorption spectroscopy (TDS) and work-function measurements (Δφ). Above room temperature, only dissociative adsorption takes place, leading to increases in work function of 0.33 and 0.25 eV on Fe(100) and (111), respectively, and is mainly identified with UPS by the appearance of a chemisorption level derived from N2p-states at about 5 eV below the Fermi level. At 500 K, the initial rate of adsorption is faster by about a factor of 20 on the (111) plane, the initial sticking coefficient, however, being very small (10 −7–10 −6) on both surfaces. The initial activation energies for adsorption are about 5 and 0 kcal/mole on Fe(100) and Fe(111), respectively, and increase with coverage in both cases. The mean activation energies for desorption were estimated to be 58(100) and 51 kcal/mole (111), so that nearly equal values for the strength of the MN bond result. A simple ordered c2 × 2 structure is formed on Fe(100) which is completed at θ = 0.5 and for which a model is proposed wherein the N atoms are located in fourfold sites on the unreconstructed Fe(100) surface, leading to a configuration similar to that in the (002) plane of (fcc) Fe 4N. Several independent observations strongly indicate that the Fe(111) surface reconstructs. A whole series of complex LEED patterns (depending on N bulk and surface concentrations and on the conditions of heat treatment) is formed with this plane which are interpreted in terms of the formation of hexagonal layers of “surface nitrides” which have a thickness of about 2 atomic layers and most probably are related to the (111) plane of Fe 4N. Desorption of N 2 (being found to be a first-order rate process) is regarded as equivalent to the decomposition of the “surface nitrides.” The close similarity to the kinetics of decomposition of (bulk) Fe 4N indicates identical mechanisms for both processes. Although the bulk solubility of N is very small under the chosen experimental conditions, this process interferes with the adsorption and desorption measurements and was analyzed in some detail, mainly by 14 N 15 N isotopic exchange. Evidence for the existence of a weakly bound (probably molecular) species was found with Fe(111) only at the lowest temperatures (140 K) and under a steady-state pressure of 4 × 10 −4 Torr of N 2. This species causes a decrease in the work function and is rapidly pumped off. Its adsorption energy is estimated to be in the range between 5 and 10 kcal/mole.