2685.     ---------------.  [PICHLER, H., AND MERKEL, H.]  [Chemical and Magnetochemical Investigations of Iron Catalysts Used in Fischer-Tropsch Synthesis.  II.  Magnetochemical Investigations.]  Brennstoff-Chem., vol. 31, 1950, pp. 33-42; Chem. Abs., vol. 44, 1950, p. 4223.

                    Two Fe carbides, both having the formula Fe2C, appear in various stages in the preparation and use of Fe catalysts for hydrocarbon synthesis.  The more stable form has a Curie point of 265°; the less stable, 380° (there may even be a 3d form with a Curie point near the latter).  Determinations of the C content of such carbides by the acid method give only 50% of the value determined by hydrogenation with H₂. In acid decomposition, 1/2 of the C is liberated in free form.  In normal-pressure synthesis at 220°-230°, the 2 Fe carbides are present in equivalent amounts.  In medium pressure synthesis with Cu-free catalysts, formed at temperatures above 250°, the 265° Curie-point carbide appears to predominate.  However, if the normal pressure mixture of the 2 carbides is used, the synthesis temperature can be lowered from 220° to 200° for equal throughputs, and this catalyst activity is well maintained.  Low temperatures of carbide formation and increased Cu content favor the formation of Fe₂C with the 380° Curie point; with an Fe:Cu ratio of 5:1 and a temperature below 200°, this is the predominant carbide.  An amount of Fe₃O₄ corresponding tot he specific equilibrium conditions is always found in the catalyst.  The carbide Fe₃C has not been found as a primary C reaction product in active catalysts.  While formation of the Fe₂C is a necessary condition for an active catalyst, and the activity of the catalyst is parallel to the Fe₂C content, the physical structure of the catalyst also is important.  The possible role of intermediate reactions in hydrocarbon synthesis is discussed.  (See abs. 2683).