Pichler-Merkel Curie point

Pichler and Merkel (24) investigated the composition of iron catalysts at various stages of pretreatment and synthesis by chemical and thermo-magnetic analysis. Copper-free iron catalysts, carburized at 325°C. before medium-pressure synthesis, were virtually completely transformed to a ferromagnetic higher iron carbide with a Curie point of 265°C., whose formula corresponded to approximately Fe2C. 

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). 

X-ray diffraction studies on iron samples carburized with carbon monoxide were carried out by Hagg (124), Halle and Herbst (125), Hofer, Cohn, and Peebles (126), Kolbel, Ackerman, Juza, and Tentschert (127), and by Eckstrom and Adcock (128). Hagg prepared his carburized iron samples below 260°C., following a method described by Bahr and Jessen (129), and determined x-ray diffraction patterns, according to Hofer, Cohn, and Peebles, identical to the Fe2C, with a Curie point at 265°C. of Pichler and Merkel. 

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). 

When two or more substances are present in a mixture, their thermomagnetic curves are additive, as shown in Fig. 39. The resolution of these curves depends on the amounts of substance present, their specific magnetizations, and the spacing of their Curie points. Figure 40 shows one of the Pichler-Merkel catalysts in which two carbides are clearly shown. 

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). 

---