Intermediate hypothesis, carbide

Experiments have recently been completed by Kummer, DeWitt, and Emmett (75), using C as a tracer in the synthesis on an iron catalyst. The results are inconclusive. If the total catalyst surface is uniformly active in the synthesis, the results show that only a small fraction of the reaction proceeds by way of the carbide. However, if only occasional active patches of the surface are participating in the synthesis, then it is possible to interpret the results as indicating that all of the reaction proceeds by way of the carbide. The carbide intermediate hypothesis for the mechanism of the synthesis on iron catalysts, however, is probably incorrect. Thus, the results of recycle operations at low temperatures on iron catalysts show that alcohols are formed earlier in the synthesis than olefinic hydrocarbons. 

The hypothesis of formation of oxygenated compounds as intermediate products was rejected by Eidus on the basis of experiments on the conversion over cobalt of methyl and ethyl alcohols and formic acid which were found to form carbon monoxide and hydrogen in an intermediate step of the hydrocarbon synthesis (76). Methylene radicals are thought to be formed on nickel and cobalt catalysts (76) by hydrogenation of the unstable group CHOH formed by interaction of adsorbed carbon monoxide and hydrogen, while on iron catalysts methylene radicals are probably formed by hydrogenation of the carbide (78,81). Carbon dioxide was found to interact with the alkaline promoters on the surface of iron catalysts as little as 1 % potassium carbonate was found to occupy 30 to 40% of the active surface area. The alkali also promotes carbide formation and the synthesis reaction on iron (78). 

A great deal of research work was performed in connection with the question of whether or not the carbide theory presents the correct explanation of the conversions occurring on the catalyst surface during the hydrocarbon synthesis. Many experiments seemed to support the hypothesis of Fischer and co-workers, while results of other more recent investigations makes intermediate formation of carbides more and more unlikely. 

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