640.    ---------------.   [CRAXFORD, S. R.]  Chemistry of the Fischer-Tropsch Synthesis.  Fuel, vol. 26, No. 5, 1947, pp. 119-123; Chem. Abs., vol. 42, 1948, p. 737.

      Paper read before the 11^th International Congress of Pure and Applied Chemistry.  The best catalyst for the Fischer-Tropsch synthesis reaction is 100 Co:5 ThO:S MgO:200 kieselguhr, in which the ThO and the MgO play only a minor role, the Co and the kieselguhr being the active components.  The kieselguhr has two functions both of which are important:  To insure that the reduced Co is produced and maintained in an exceedingly finely divided form; and to establish and maintain the porosity of the individual catalyst granules.  It is believed that the catalyst is not a mixture of these two constituents but rather a chemical combination of the two.  The fact that these Co-kieselguhr preparations are much more difficult to reduce than Co-carbonate alone is a further indication of chemical interaction and, in particular, of silicate formation.  The importance of the 2d function of kieselguhr is seen in connection with the removal of the synthesis products from the catalyst and with maintaining the catalyst surface in a receptive state.  Active catalysts do not consist of bulk metallic Co with the normal lattice but rather with layers of Co atoms having an arrangement based on the structure of Co silicate, ordinary Co carbide thus playing no part in the reaction.  When CO alone reacts with the reduced catalyst, the reaction is much faster than when it reacts with Co metal.  In both cases the CO is first chemisorbed on a Co atom, and the O atom is removed by reaction with a second molecule of CO, leaving a chemisorbed C atom.  In the case of bulk carbide very little CO can react in this way, and the rate is then determined by the rate of diffusion of the chemisorbed C atoms into the interior of the lattice to give Co carbide.  In the case of the catalyst, on the other hand, nearly all the Co atoms are available in the surface for reaction with CO so that the rate of reaction is fast, and the final result is an array of chemisorbed C atoms.  If CH₄ is added to the mixture of CO and H₂, not only is the normal formation of CH₄ynthesis entirely suppressed, but some of the added CH₄ enters into the reaction and polymerizes with the CH₂ groups on the catalyst surface to give higher hydrocarbons.  This effect has also been observed in synthesis in 2 stages; it is believed that this effect is due to formation of a polymerization-depolymerization equilibrium.