Investigation of nitriding and reduction processes in a nanocrystalline iron–ammonia–hydrogen system at 350 °C

Abstract

In this paper, the series of phase transitions occurring during the gaseous nitriding of nanocrystalline iron was studied. The nitriding process of nanocrystalline iron and the reduction process of the obtained nanocrystalline iron nitrides were carried out at 350 °C in a tubular differential reactor equipped with systems for thermogravimetric measurements and analysis of gas phase composition. The samples were reduced with hydrogen at 500 °C in the above mentioned reactor. Then the sample was nitrided at 350 °C in a stream of ammonia-hydrogen mixtures of various nitriding potentials, P = pNH3/pH2(3/2). At each nitriding potential stationary states were obtained - the nitriding reaction rate is zero and the catalytic ammonia decomposition reaction rate is constant. The reduction process of the obtained nanocrystalline iron nitrides was studied at 350 °C in the stationary states as well. The phase composition of products obtained in both reaction directions (nitriding and reduction) was different despite the identical concentration of nitrogen in the nitriding mixture. The hysteresis phenomenon, occurring at the iron nitriding degree - nitriding potential system, was explained. In the single-phase areas of α-Fe(N), γ'-Fe4N or ε-Fe3-2N, a state of chemical equilibrium between the ammonia-hydrogen mixture, nanocrystalline iron surface and volume was observed. In the multi-phase areas, between the gas phase and the iron surface a state of chemical equilibrium holds, but between the gas phase and solid phase volume a state of quasi-equilibrium exists. The model of the nitriding process of nanocrystalline iron to iron nitride (γ'-Fe4N) was presented. It was found that nanocrystallites reacted in the order of their sizes from the largest to the smallest.

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

“…The value of RMP depends on the quantitative and qualitative composition of the iron sample. The literature data as well as the results of our earlier works − revealed that in the iron nitriding process not only one or two but even three phases of iron nitrides could coexist forming the iron nitride mixture. This means that the value of RMP for a given nitriding degree is a sum of multiplication of the magnetic permeability of each phase and their participation in the iron nitride mixture.…”

“…In our previous works, we reported the method of iron nitride preparation by ammonia nitriding of the nanocrystalline iron catalyst. − The kinetics of the nitriding process of NFeC was also described in our previous papers. − It was found that the concentration of nitrogen in the volume of iron nanocrystallites at a given time of the reaction depended on the ratio of nanocrystallite active surface area, S [nm 2 ], to volume, V [nm 3 ], of nanocrystallites, i.e., on the specific active surface area, A = S / V [nm –1 ].…”

Reaction Model Taking into Account the Catalyst Morphology and Its Active Specific Surface in the Process of Catalytic Ammonia Decomposition

“…The actual system consists of a set of crystallites described by the distribution density probability of their sizes characterized by the specific active surface area of the crystallite, A. It was assumed in the paper that this is a bimodal distribution consistent with experimental tests [ 33 , 38 , 47 , 49 ] and model calculations [ 44 , 45 ] for the nanocrystalline iron-ammonia-hydrogen system.…”