Fischer-Tropsch synthesis carbidic intermediates

Carbenium ions, 42 115, 143 acid catalysis, 41 336 chemical shift tensors, 42 124-125 fragments in zeolites, 42 92-93 history, 42 116 superacids, 42 117 Carbide catalysts, 34 37 Carbidic carbon, 37 138, 146-147 Carbidic intermediates, 30 189-190, 194 Fischer-Tropsch synthesis, 30 196-197, 206-212... 

Carbon forms play important roles as intermediates, catalyst additives and deactivating species in Fischer-Tropsch synthesis on iron catalysts. Deactivation may be due to poisoning or fouling of the surface by atomic carbidic carbon, graphitic carbon, inactive carbides or vermicular forms of carbon, all of which derive from carbidic carbon atoms formed during CO dissociation (ref. 5). While this part of the study did not focus on the carbon species responsible for deactivation, some important observations can be made to this end. 

Here Z is a Ni surface site. The equation they derive is complex but can be simplified (see Table 4) for full-scale application. These workers point out that the same equation can be derived from a mechanism involving surface carbon as an intermediate similar to carbide theories for Fischer-Tropsch synthesis. In that case steps (b) and (c) in the above equation would be replaced by (b ) and (c ). 

The fomation of carbon on iron and iron-copper catalysts by the reaction 2C0 = C02+C has been studied by several investigators (70-73). The most significant result of this work (in so far as the Fischer-Tropsch synthesis is concerned) is the fact that neither an iron-free nor a copper-free carbon deposit was obtained. The data show that cai-bon is deposited in the crystal lattice of the catalyst and the inability to obtain a copper-free carbon deposit from tests with an iron-copper catalyst shows that iron carbonyl formation will not explain the results. It is very probable that carbon is formed from carbon monoxide b3 way of iron carbide as an intermediate. Carbidic carbon diffuses rapidly throughout the crystal lattice and subsequently decomposes to yield elemental carbon, thus disrupting the lattice structure. 

The hydrocarbon synthesis and the elemental carbon formation are competing for the available supply of carbide. Higher partial pressures of hydrogen are known to increase the rate of hydrocarbon synthesis and therefore the rate of elemental carbon formation would be expected to decrease with increase of HjrCO ratio. This explanation assumes that bulk carbide (as distinguished from surface carbide) is a necessary intermediate in the Fischer-Tropsch synthesis. As shown by the recent work of Emmett and his associates (75) using radioactive carbon as a tracer, this assumption is very probably incorrect. 

Good evidence has been obtained that heterogeneous iron, ruthenium, cobalt, and nickel catalysts which convert synthesis gas to methane or higher alkanes (Fischer-Tropsch process) effect the initial dissociation of CO to a catalyst-bound carbide (8-13). The carbide is subsequently reduced by H2to a catalyst-bound methylidene, which under reaction conditions is either polymerized or further hydrogenated 13). This is essentially identical to the hydrocarbon synthesis mechanism advanced by Fischer and Tropsch in 1926 14). For these reactions, formyl intermediates seem all but excluded. 

Surface spectroscopy, as discussed in the previous section, has recently provided evidence that concurrent with the hydrocarbon synthesis a reactive carbidic overlayer develops, and that this reactive overlayer may contain the intermediates operative in the FT synthesis. However, the claims of surface spectroscopy regarding the relevance of carbidic intermediates have had a precedent in one of the very first publications of Fischer and Tropsch... 

The role of carbides in the synthesis of hydrocarbons has been widely considered ever since the carbide theory was first postulated by Fischer and Tropsch in 1926 (20). Although recent experimental studies indicate that the carbide theory is largely incorrect, that is, that bulk-phase carbides are not intermediates in the formation of higher hydrocarbons, iron catalysts converted to Hagg carbide or cementite are usually more active than similar raw or reduced catalysts (21). (For a review of the carbide theory up to 1950, see p. 571 of reference 22.) The selectivity of carbided iron catalysts is essentially the same as that of corresponding reduced catalysts. Nitrides of iron are usually more active than reduced or carbided catalysts, and the catalyst selectivity is significantly different. 

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