Iron nitrides

A process has been developed to recover niobium from ferroniobium (30). The need for this process came about when Brazil would only export niobium in the form of ferroniobium. The process starts with a hydriding step, so as to be able to cmsh the alloy. Screening precedes a nitriding step, followed by an acid leach of the iron nitrides. This leaves the niobium nitride for further processing to the pure niobium metal. 

This reaction is catalyzed by iron, and extensive research, including surface science experiments, has led to an understanding of many of the details (72). The adsorption of H2 on iron is fast, and the adsorption of N2 is slow and characterized by a substantial activation energy. N2 and H2 are both dis so datively adsorbed. Adsorption of N2 leads to reconstmction of the iron surface and formation of stmctures called iron nitrides that have depths of several atomic layers with compositions of approximately Fe N. There is a bulk compound Fe N, but it is thermodynamically unstable when the surface stmcture is stable. Adsorbed species such as the intermediates NH and NH2 have been identified spectroscopically. 

Hardness on the Mohs scale is often above 8 and sometimes approaches 10 (diamond). These properties commend nitrides for use as crucibles, high-temperature reaction vessels, thermocouple sheaths and related applications. Several metal nitrides are also used as heterogeneous catalysts, notably the iron nitrides in the Fischer-Tropsch hydriding of carbonyls. Few chemical reactions of metal nitrides have been studied the most characteristic (often extremely slow but occasionally rapid) is hydrolysis to give ammonia or nitrogen ... 

Iron Nitrides as Fischer-Tropsch Catalysts Robert B. Anderson Hydrogenation of Organic Compounds with Synthesis Gas Milton Orchin The Uses of Raney Nickel Eugene Lieber and Fred L. Morritz... 

In a preliminary report (2), Fe atoms were reacted with O2, leading to formation of FeiO ), a cyclic isosceles (C2v) species, as suggested by mixed isotope experiments. Reaction of Fe atoms with N2O resulted in formation of FeO. A feature at 887 cm, assigned to a Fe/Nj complex, is probably erroneous, and may be an iron nitride species. In the same triad, the MCD spectrum of matrix-entrapped OSO4 was studied (46). The spectrum was found to be similar to that of Mn04 in a solid lattice, and was assigned accordingly. 

Iron Nitrides as Fischer-Tropsch Catalysts Robert B. Anderson... 

Figure 5.11 A constant velocity Mossbauer experiment reveals the kinetics of the denitridation of an iron nitride in different gases at 525 K. The negative part of the time scale gives the transmission of the most intense peak of the nitride at time zero the gas atmosphere is changed to the desired gas. Denitridation occurs relatively fast in H2, but is retarded by CO, whereas the nitride is stable in an inert gas such as helium (from Hummel etal. [33]).

Key words electron diffraction, iron-based alloy, iron nitrides... 

As to efforts to carry out ammonia synthesis in a technical direction, studies along that line had been started in the B. A. S. F. after Wilhelm Ostwald had suggested such work in 1900. In laboratory experiments considerable yields of synthetic ammonia had been obtained by W. Ostwald (27). However, all attempts to reproduce these yields on a larger scale were futile, and finally Ostwald had to admit that in his original experiments, ammonia had probably been erroneously introduced into the synthesis reactor with a foreign source, presumably in form of an iron nitride, which had been formed by a previous treatment of the iron catalyst with ammonia. 

The reasoning which led the author to make this first shot in the dark regarding the usefulness of combinations of solid compounds as ammonia catalysts was as follows If we assume that a labile iron nitride is an interminate in the catalytic ammonia synthesis, every addition to the iron which favors the formation of the iron nitride ought to be of advantage. In other words, the hypothesis was used that surface catalysis acts via the formation of intermediate compounds between the catalyst and one or more of the reactants. An experimental support for this theory was the fact that a stepwise synthesis via the formation and successive hydrogen reduction of nitrides had been carried out with calcium nitrides (Haber), and cerium nitrides (Lipski). Later, the author found molybdenum nitride as being the best intermediate for such a stepwise synthesis. 

Fig. 8.1.10 Schematic illustration of the apparatus for vapor-liquid reaction. (Reprinted from J Mag Mag. Mater, 122, Iron-nitride magnetic fluids prepared by vapor-liquid reaction and their magnetic properties, 1 Nakatani et al., pp 10-14. Copyrighi 1993. with permission from Elsevier Science.)...

Mechanisms by the Method of Intermediates in Quasi-Stationary Concentrations J. A. Christiansen Iron Nitrides as Fischer-Tropsch Catalysts... 

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