Studies of phase transitions occurring in the system of nanocrystalline Fe/NH3/H2

Abstract

Nitriding of the iron catalyst and reduction of the obtained nitrides have been studied. Rate measurements of the reactions were conducted under atmospheric pressure and at 350 °C using a differential tubular reactor equipped with both a system to perform thermogravimetric measurements, to determine content of nitrogen in a solid phase, and a catharometric system to determine content of hydrogen in a gas phase. Mixtures of ammonia and hydrogen of a composition changing in a controlled way, were let into the reactor. Accelerations of the nitriding potential change were also varied. Ex-situ X-ray diffraction method was used to determine phase composition and mean nanocrystallites size of the solid sample. It was shown that in the ε-FeN0.33 iron nitride reduction there was a two-stage transformation. First, ε-nanocrystallites transformed into γ′ iron nitride in the order from the largest to the smallest. After complete transformation, γ′-nanocrystallites converted into the solution of nitrogen in nanocrystalline α-iron. In the reduction of nitrogen solution in ε iron nitride, ε-FeN0.48, however, there were simultaneously two mechanisms: a part of nanocrystallites of nitrogen solution in ε iron nitride underwent the two-stage transformation, while the rest underwent a one-stage transformation directly from the nitrogen solution in ε iron nitride to α-iron.