638.    ---------------.  [CRAXFORD, S. R.]  Function of the Promoters in the Catalysts for the Fischer-Tropsch Synthesis.  Trans. Faraday Soc., vol. 42, 1946, pp. 580-585; Chem. Abs., vol. 41, 1947, p. 262.

     Determinations were made on 5 catalysts to learn which of these should be the most effective catalyst for the Fischer-Tropsch synthesis.  The 5 catalysts were:  (1) Co; (2) Co-ThO₂ 100:18; (3) Co-kieselguhr 1:1; (4) Co-Tho₂-kieselguhr 100:18:100; (5) Co-RhO₂-kieselguhr 100:21:100.  They were all prepared from solutions of the nitrates of Co and Th, with addition of kieselguhr, by precipitation with K₂CO₃ and reduction by H₂ at 375°.  Both the initial fast and the final slow rate of carbide formation by the reaction 2CO+2Co=Co₂C+CO₂ was promoted most effectively by (2) followed by (4) and (5), the latter much more slowly than (4).  ThO₂ and kieselguhr are found to be strong promoters for the reduction of Co₂C by H₂.  Of all the catalysts used, with the exception of (1), (4) gave by far the slowest rate of reduction of the carbide and, thus, has both a relatively slow rate of carbide reduction and a relatively fast rate of carbide formation, confirming the previously expressed view about the mechanism of the Fischer-Tropsch reaction according to which the rate of formation of carbide by the reaction 2Co+H₂+CO=Co₂C+H₂O must be faster than the reduction of the carbide to allow for the accumulation of surface carbide during the reaction, and the consequent exclusion of chemisorbed H₂ from the surface if oils are to be formed; (2) has a very high rate of carbide formation but is not a good Fischer-Tropsch catalyst because it shows too high a rate of carbide reduction; (b) is poor in 2 respects.  Its rate of carbide formation is slow compared with the normal active Fischer-Tropsch catalyst, and its rate of carbide reduction is high.