Theory carbide

In the present work the unassisted CO dissociation version of the carbide theory has been adopted to describe CO conversion, while the alkyl mechanism has been adopted for describing the chain growth process. Accordingly, as proposed in our previous work,10 the FT reaction pattern has been detailed as follows ... 

A complete kinetic model of FTS over a state-of-the-art Co catalyst has been derived on the basis of the carbide theory and of CH2 insertion alkyl mechanism. 

The earliest theory, advanced by Fischer and Tropsch in 1926 (84), proposed that the reaction proceeded via formation of intermediate metal carbides which react on the catalyst surface to form methylene groups. It was then suggested that these methylene groups polymerize on the surface to form hydrocarbon chains, which desorb as saturated and unsaturated hydrocarbons. In 1939 Craxford and Rideal expanded the carbide theory, proposing (85), for cobalt-based catalysts, the following reaction sequence ... 

Subsequent studies have failed to support the carbide theory, and it is now generally accepted that carbides of the type proposed by Craxford play little or no part in the Fischer-Tropsch synthesis (86, 87). It has, however, recently been suggested, by analogy with the mechanism proposed for the Haber synthesis of ammonia, that carbides formed by dissociative absorption of carbon monoxide would be expected to be readily hydrogenated and could therefore be of importance in Fischer-Tropsch synthesis over heterogeneous catalyst (88). 

This mechanism differs from the "carbide theory in that chain growth involves oxygen-containing groups and is thought to take place on the ultimate or penultimate carbon atoms of the longest chain by surface condensation reactions. 

Kinetic equation (290) is in satisfactory agreement with experiments both at r > 0 and r < 0. Equation (290) can be obtained not only from mechanism (279). Some authors have developed a carbide theory of catalytic syntheses of hydrocarbons from CO and H2 (88,89). In terms of this theory, the following mechanism involving carbon chemisorbed on nickel as an intermediate should be accepted instead of mechanism (279). 

Equations (287) are again used for the definition of, l2, and l3 in this equation via rate constants of elementary reactions of scheme (292). At lt = 0, (293) becomes (290). This corresponds to stage 2 of scheme (292), being at equilibrium. Thus the reaction kinetics does not provide a possibility for giving preference to the Eidus-Zelinskii theory or the carbide theory. 

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. 

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 ). 

Fischer and Tropsch assumed an intermediate formation of carbides (carbide theory) as mechanism of the reaction of carbon monoxide and hydrogen to higher hydrocarbons. Methane was assumed to be formed via an intermediate formation of hydrides. The competition of carbon monoxide and hydrogen in connection with the formation of carbides and hydrides was considered to be responsible for the tendency of different catalysts to form preferentially either higher hydrocarbons or methane. With this theory, Fischer and Tropsch explained why iron presented an... 

Craxford (105), who in several publications supported the carbide theory, explained the formation of carbon dioxide (in the presence of cobalt catalysts) as a secondary reaction, taking place on parts of the catalyst surface which are covered with uncarbided reduced metal atoms. This part of the catalyst could also be responsible for hydrocracking of higher hydrocarbons to methane. 

They assumed that carbon monoxide and metal react first to form carbides (formation of higher carbides is thus to be expected) and that hydrogen then reacts with carbidic carbon to form (CH2) radicals. These methylene groups polymerize to form more or less saturated hydrocarbon chains. This theory of the formation of hydrocarbons over an intermediate formation of carbides is generally referred to as the carbide theory. ... 

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. 

The assumption that carbides are intermediate products of the synthesis is not probable, at least in so far as the bulk carbide, which can be identified by thermomagnetic investigations, is concerned. The greatest part of the results, however, cannot be recognized as satisfactory proof against the carbide theory if only small parts of an irregular catalyst surface were involved in the reactions. A comparison of the behavior of the different catalysts seems to be useful. 

With the introduction of a variety of surface-sensitive instrumental techniques and the use of transient techniques utilizing isotopic tracers, the carbide theory of chain growth was revived to represent a dominant viewpoint since 1980. Brady and Pettit showed that the decomposition of diazomethane on various transition metal catalysts and in the absence of CO and H2 primarily led to ethylene. In the presence of H2, hydrocarbons up to Cjg were formed and conformed to the Schulz-Flory distribution expected for FTS (for an example of a description of Schulz-Flory distribution, see Reference 23). It was... 

JB Holt, ZA Munir. Combustion synthesis of titanium carbide Theory and experiment. J Mater Sci 21 251, 1986. 

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