Some Thermal Reactions of the Higher Iron Carbides

Abstract

One of the undesirable side reactions of the Fischer-Tropsch synthesis is deposition of carbon. To ascertain whether thermal reactions of iron carbides might cause this deposition and to learn more about the nature of iron carbides, the transition from hexagonal close-packed (h.c.p.) iron carbide to Hägg iron carbide and the reaction of higher iron carbide with metallic iron to form cementite have been studied in a copper- and alkali-promoted iron catalyst by means of a magnetic balance. The transition from h.c.p. to Hägg iron carbide is heterogeneous, with no apparent change of the compositions of both carbides, nor any detectable solubility of one carbide in the other. The data confirm the narrow range of composition of Hägg carbide and the fact that it contains somewhat more iron than corresponds to the formula Fe2C. The synthesis of cementite from metallic iron and higher iron carbide proceeds via Hägg carbide and only above about 320°C. It does not go to completion up to 440°C; above that temperature, decomposition of Hägg carbide into cementite and free carbon is sufficiently rapid to prevent completion of the synthesis. The data show that deposition of carbon on iron catalysts during the Fischer-Tropsch synthesis is not caused by thermal reactions of iron carbides, unless h.c.p. iron carbide should contain more carbon than Hägg iron carbide. A possible explanation of the apparent stabilizing effect of copper on h.c.p. iron carbide is given.