Study of Phase Transformation Processes Occurring in the Nanocrystalline Iron/Ammonia/Hydrogen System by the Magnetic Permeability Measurement Method

• Published in: Journal of physical chemistry. C 2022-05, Vol.126 (17), p.7704-7710
• Main Authors: Arabczyk, Walerian, Pelka, Rafał, Kocemba, Ireneusz, Brzoza-Kos, Agnieszka, Wyszkowski, Andrzej, Lendzion-Bieluń, Zofia
• Format: Article
• Language: English
• Subjects: C: Physical Properties of Materials and Interfaces
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.2c00807

Phase transformations over a nanocrystalline iron catalyst in the nitriding process were carried out at 350 °C and under atmospheric pressure. The process was conducted under conditions of changing nitriding potential both during the nitriding and reduction of iron nitrides. These processes were monitored by measuring the magnetic permeability and changes of mass and reaction gas composition. Four stages of nitriding were observed and ascribed to the following general schematic reactions: α-Fe → γ′-Fe4N → ε-Fe x N → ε-Fe3N7 → ε-Fe x N. The measurements of magnetic permeability revealed that the magnetic properties of the investigated samples strongly depended on the nitriding degree and can be described with linear equations of two variables. For γ′-Fe4N, the value of magnetic permeability was 1.280 times larger than for the iron catalyst, while for ε-Fe x N, it was more than 3 times smaller. However, in the case of the smallest nanocrystallites both in the process of iron nitriding and reduction of iron nitrides, the magnetic permeability did not change with the nitriding degree. It was also claimed that the composition of iron nitrides, concentrations of nitrogen in iron nitrides, and the magnetic permeability depend on the direction of the reaction (nitriding or reduction). A hysteresis phenomenon was observed.