Hexagonal iron carbide

Merkel (49) in iron precipitation catalysts by magnetochemical investigations. The conversion of the hexagonal Fe2C to Hagg s Fe2C is irreversible. Small amounts of copper stabilize the hexagonal iron carbide and raise the conversion temperature 50-100°C. (50). 

A thermomagnetic curve for hexagonal iron carbide is shown in Fig. 45-1. This shows two Curie points, one at 247° due to Hagg carbide, and the other at 380° apparently due to hexagonal carbide. The appearance of the 247° Curie point is surprising because x-ray lines for carbide were obtained only with considerable difficulty, but this merely illustrates the usefulness of the magnetic method for detecting a substance which neither x-ray nor chemical analysis can readily reveal. 

Fio. 45. Thermomagnetic curves, for hexagonal iron carbide, I with rising temperature, II with falling and subsequent temperature cycles. 

X-ray spectroscopy (DRON-3, cadmium cathode) showed the presence of different forms of iron (a-Fe, b-Fe), amorphous carbon, possibly iron carbide and dense forms of carbon, as well as Fe2C>3 (Figure 5). The spectrum shows signals which are characteristic for Fe-C bonds and for a hexagonal symmetry of carbon. Small angle measurements also indicated the presence of amorphous iron. The presence of amorphous iron and iron carbide may explain the low oxidation of the adsorbent particles in water. 

Wo might here deal briefly with the structure of the well-known interstitial compound iron carbide, or cementite. The largest interatomic distance in iron is 2-52A, which figure may be taken for the atomic diameter of iron, while that of carbon is 1-54A, so that the diameter ratio is 1 54/2 52 — (Mil i.e. the diameter ratio is greater than 0 59. The relatively small size of the iron atom therefore induces the formation of an interstitial structure which is more complex than the simple cubic or hexagonal types described above. ... 

From these results it is possible to bring some order out of a rather confused situation with respect to the several iron carbides. As pointed out by Hofer, the hexagonal carbide is identical with the carbide described by Pichler and Merkel as having a Curie point at 265°. This carbide can also be identified with the cubic ferric oxide saturated with potassium oxide of Lefebvre and LeClerc (Refs. 51-53). 

The identification of catalytically active species in FTS is of fundamental importance, as an improved understanding could enable the development of catalysts with increased activity and selectivity. In cobalt- and ruthenium-catalysed FTS, metallic cobalt and ruthenium function as active catalysts. However, in iron-catalysed FTS there are several distinct species generated during the reaction. Due to the lower, or similar, activation energy for iron carbide formation in comparison to carbon monoxide hydrogenation, iron-carbide formation is typically observed in FTS. The formation of several iron-carbide phases have been observed -Fe2C/8 -Fe2.2C (hexagonal... 

Carbides of iron are thermodynamically unstable with respect to decomposition to iron and carbon. When hexagonal carbide is heated in an inert atmosphere the following sequence of reactions is observed ... 

Lefebvre and LeClerc (36) carried out thermodynamic studies on catalysts of the Fischer-Tropsch synthesis. They drew attention to the significance of the specific Curie points, of various compounds, for their activity or inactivity as catalysts. They assumed that the active catalysts were cubic iron oxide and hexagonal nickel. Pichler and Merkel (37) found that the Curie point attributed by Lefebvre and LeClerc to cubic iron oxide is actually the Curie point of one special form of Fe2C. The hexagonal nickel seems to be actually a nickel carbide. 

In the case of iron catalysts, x-ray and thermomagnetic investigations confirm the work of Pichler and Merkel and show that the Fe2C with the Curie point 265°C. of Pichler and Merkel is identical with Hagg s carbide. Hofer, Cohn, and Peebles found the inflection point of the thermomagnetic curve at 247°C. The Fe2C with the Curie point 380°C. of Pichler and Merkel seems identical to a hexagonal carbide identified independently in the research laboratories of I. G. Farbenindustrie by work of Halle and Herbst (90). 

Halle and Herbst obtained a hexagonal carbide by carburization of iron-copper catalysts, and later also by carburization of copper-free catalysts (reduction and carburization at low temperatures). The x-ray pattern is not identical to that described by Hagg. On the basis of their x-ray investigations Hofer, Cohn, and Peebles believe that the carbide of Halle and Herbst is identical to the Fe2C carbide with a Curie point at 380°C. of Pichler and Merkel (see Sec. III.4.d). 

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