The Interaction of Nitrogen with the (111) Surface of Iron at Low and at Elevated Pressures

New adsorption results have been obtained for the N2/Fe(111) system using partial and total pressures from 10−4to 500 Torr and temperatures in the range 393–578 K. They show that the initial, dissociative chemisorption probability is, within the accuracy of the measurements, independent of the gas temperature in contrast to molecular beam results corresponding to much higher gas molecule energies. This result suggests that the dissociative chemisorption of N2proceeds at thermal energies via a precursor-mediated process rather than a direct, activated process. It also confirms the validity of the adsorption results of F. Bozso, G. Ertl, M. Grunze, and M. Weiss (J. Catal.49, 18, 1977) obtained using low-pressure exposures. However, the appearances of a 5×5 LEED pattern and TPD spectra with two peaks, a sharp one and a broader one, show that a new chemisorption state is created during exposures at nitrogen pressures ≥50 Torr and temperatures above 570 K. The stability of the new state increases strongly with the concentration of surface nitrogen and disappears rapidly when the concentration decreases below a critical value. It is then transformed into the well-known (33× 33R30° surface structure, which also disappears rapidly below a lower critical surface concentration. After adsorption saturation at ∼0.85 monolayer (ML) the coverage can be increased to ∼1.1 ML by segregation. Bulk thermodynamic estimates indicate that the 5×5 surface state can be formed under industrial ammonia synthesis conditions. However, the presence of strongly bound hydrogen containing surface species may prevent its formation.