831a.    ---------------.  [EMMETT, P. H., AND KUMMER, J. T.]  Mechanism Studies of Fischer-Tropsch Synthesis.  Proc. 3d World Petrol. Cong., 1951; Fuel Abs., vol. 11, No. 2, 1852, abs. 1299; Fuel, vol. 31, 1952, p. 370.

       Tracer experiments using Fe₂C¹⁴ lead to the conclusion that most of the hydrocarbon synthesis over Fe and Co catalysts does not occur through formation of metallic carbides as intermediates.  Adsorption experiments suggest that the carbides by themselves probably are not good catalysts.  The use of radioactive CH₄ is recommended as a sensitive means of ascertaining whether or not this gas can be converted into higher hydrocarbons during synthesis.  Results on Fe and Co catalysts showed that in synthesis fewer than 0.1% of the C atoms in the higher hydrocarbons came from CH₄, even though radioactive CH₄ was present in a high concentration (about 50%).  Measurements of the free energy of formation of Fe₂C and Fe₃C show that the direct reduction of these carbides by H₂ to form olefins or paraffins by hydrocarbons with more than 6 C atoms is thermodynamically unfavorable.  This confirms the tracer experiments in indicating that the carbides of Fe and Co are not intermediates in Fischer-Tropsch synthesis.  Use of radioactive O-containing organic molecules is useful for obtaining information relative to the nature of the surface complexes serving as intermediates in the Fischer-Tropsch synthesis.  Two such experiments using radioactive EtOH as tracer have shown that either this compounds or some surface complex formed from it is capable of acting as an intermediate in the synthesis of higher hydrocarbons.  Furthermore, about 90% of the added C atoms that transform the C₂ complex to C₃ hydrocarbon add to the C in the CH₂ rather than to the C of the CH₄ group of the adsorbed alcohol.  It seems certain that at least over Fe catalysts, and probably over Co, the higher hydrocarbons are built up by successive adsorption of CO molecules on to the catalyst surface adjacent to some intermediate complex.  The CO then attaches itself to the intermediate to form momentarily an O-containing complex which in turn is hydrogenated to remove O and then either escapes from the surface as a hydrocarbon or adds another CO to form a complex containing 1 more C atom.  It seems improbable that the synthesis over either Fe or Co results from polymerization of CH₂ groups on the surface or from the reduction of carbides as intermediates.