639.    ---------------.  [CRAXFORD, S. R.]  Mechanism of the Fischer-Tropsch Reaction.  Trans. Faraday Soc., vol. 42, 1946, pp. 576-580; Chem. Abs., vol. 41, 1947, p. 262.

      The theory of the Fischer-Tropsch reaction has been proposed in which hydrocarbon oils are synthesized from a suitable mixture of CO and H₂ over a Co catalyst when the Co in the catalyst surface is in the form of the carbide Co₂C, and the Co in the uncombined form gives CH₄, allows the water-gas-shift reaction to take place, and is active for the hydrogenation cracking of hydrocarbons.  The formation of higher hydrocarbons must be examined in greater detail to see whether the results of the synthesis can be accounted for by the above mechanism.  When synthesis gas (CO+2H₂) is passed over the catalyst (100 Co+6 ThO+12 MgO+200 kieselguhr) at 185°, it is found that the yield of liquid hydrocarbons and the amount of CO₂ formed, both expressed in gm. per N m.³ of synthesis gas, change with change of synthesis-gas rate.  The oil yield is a maximum for a rate of about 1 l. per gm. Co per hr. and falls off for higher and lower rates, and the amount of CO₂ formed is small for rates higher than the above but increases rapidly at smaller gas rates.  At low gas rates oil synthesis occurs only on the front part of the catalyst bed where the Co of the catalyst is largely converted to carbide and there is little chemisorbed H₂.  The rest of the catalyst bed, where oil synthesis is not taking place actively, is relatively free from carbide and therefore available for the water-gas-shift reaction.  Further, the small amount of CO₂ produced at the maximum gas rate for oil production shows that the formation of CO₂ and the oil-forming reaction do not occur simultaneously as 2CO+2H₂=CO₂+-CH₂ but the former is separate and occurs later.  The oil-forming reaction must, therefore, be CO+2H₂=H₂O+-CH₂, followed, when conditions permit, by the water-gas-shift reaction.  Variation in the oil yield with the synthesis gas rate is also explained very simply on this basis.  At a gas rate of 1 l. per gm. Co per hr. the entire catalyst is actively synthesizing oil.  At higher gas rates the oil yield decreases as the contact time of the gas with the catalyst diminishes, and the synthesis is not completed.  At lower gas rates, the synthesis is completed on the first part of the catalyst bed and the oil formed there passes over the uncarbided second portion of the bed covered by chemisorbed H₂, and is hydrogenated-cracked with formation of CH₄.