Kinetics of NH 3 decomposition on iron at high temperatures

Abstract

The kinetics of NH 3 decomposition on polycrystalline iron wires between 600 and 1250 K and pressures between 0.05 and 1 Torr are measured and correlated with examination by SEM and AES. Reproducible rates are obtained in pure NH 3 which can be fit accurately at all temperatures and pressures by a Langmuir-Hinshelwood (LH) unimolecular rate expression with reaction and desorption activation energies of 49,600 and 39,600 cal/mole, respectively. Hydrogen alters the decomposition rate to a higher value which for high hydrogen pressures is independent of P H 2 and can also be fit by a LH expression. Results are interpreted in terms of surface nitride in pure NH 3 and nitride-free iron in hydrogen. Reaction in NH 3 produces much more rapid and extensive facetting than does heating in vacuum, and repeated cycling through the α-γ phase transition in NH 3 produces even more extensive facetting of the surface. This is apparently caused by preferential attack of NH 3 at dislocations induced at the surface by the phase transformation. No discontinuity in rate was observed in crossing the transition temperature.