The interaction of hydrazine with an Fe(111) surface

Abstract

The interaction of hydrazine with a clean and nitrogen precovered Fe(111) surface was investigated in the temperature range of 126–600 K by means of UV and X-ray photoelectron spectroscopy (PES). At temperatures below 170 K the molecular adsorption of hydrazine is followed by multilayer condensation. In going from adsorbed to condensed hydrazine the valence and core levels shift in different directions relative to the vertical gas phase ionisation energies indicating strong interactions via hydrogen bonding in the condensed phase. Dissociative adsorption of N 2H 4 was observed at temperatures above 220 K. At room temperature no difference in the photoelectron spectra following the adsorption of N 2H 4 or NH 3 was observed indicating the presence of the same surface species, predominantly being -NH 2 radicals. Preadsorbed nitrogen stabilizes N 2H 4 against decomposition. The results will be discussed in view of possible intermediates in the ammonia-synthesis reaction on iron. Simple thermochemical arguments are presented to explain the observed difference in the heterogeneous dissociation mechanism of hydrazine on transition metals. General conclusions on the mechanism of ammonia synthesis on various transition metals can also be derived from these thermodynamic considerations.