1434. HOFER, L. J. E. Preparation and Properties of Metal Carbides, With Critical Comment as to Their Significance in the Fischer-Tropsch Synthesis. Bureau of Mines Rept. of Investigations 3,770, 1944, 39 pp.; Chem. Abs., vol. 38, 1944, p. 5647.

Return to Abstracts of Literature 1250-1499

Literature Abstracts

Probable physical and chemical characteristics of the Fischer-Tropsch (abs. 1026) carbides are summarized tabularly. The kinetics of the formation of carbides of Fe, Co, and Ni were studied, and the results of various authors were correlated. 3 reactions were isolated: (1) A rapid reaction proceeding on the surface before it has been covered with carbide and reaction products; (2) a slow reaction, which may take weeks for completion. At the end of this reaction, the catalyst has a definite composition, by weight of carbide, corresponding to Fe₂C, Ni₃C, and Co₂C. Some Fe₃C may be formed. This slow reaction is identified with carbide formation in the lattice; (3) if the temperature is above a characteristic value for each metal, C is formed. It continues until the Fe is diluted to 1% of the total Fe-C mass. The Fe and any metal with it is distributed through the C in a finely divided state. Above the critical temperature mixtures of Fe and C corresponding to almost any composition can be prepared. The C-forming reaction is apparently 1^st order. All the carbides of Fischer-Tropsch catalysts are more unstable than the true refractory carbides. As Fe, Co, and Ni the carbides are unstable with respect to C and metal. For all known Fischer-Tropsch catalysts, the ratio of C-atom radius to metal-atom radius exceeds 0.58. This seems to be a condition necessary for Fischer-Tropsch activity. The carbides are more or less distorted interstitial compounds with the C inserted into the interstices of the distorted metal lattice. The C does not exist in the ionic form. It is bound to the metal by essentially intermetallic bonds. This may be true of the surface carbide. It is inconceivable that carbide formation in the body of the catalyst will not profoundly modify the properties of the catalyst. Metal-to-metal distances will change. The electrical and magnetic properties will not remain the same. H₂, dissolved in the lattice, probably will not behave the same, partly because many of the interstices in the metallic lattice are filled. The absorption of reagents and desorption of the products probably are modified by the carbides. C formation, by carrying away active centers and producing irreducible residues on the catalyst, would tend to poison it.