Methanation carbidic intermediates

Older IR-imwstigalions by Kolbel et at. led to the assumption of surface formyl or hydroxycarbene groups [113]. More recent measurements on supported ruthenium under synthesis conditions showed absorptions for mr ecularly adsorbed CO and for formate and CH species. As a result of deuieration experiments, however, the latter were concluded not to be reaction intermediates (113. 106], It was also shown that the production of methane and ethane continued for a significant period after CO had been removed from tlic reaction mixture and after the disappearance of all IR-observable COads species [106J. It was concluded that product formation occurs via carbidic intermediates. /n-situ IR studies at higher pressures (3 bar) revealed formation of CHjf species with absorptions in the 3000 cm region (114). 

In the absence of CO the predominant product of the hydrogenation is methane. The first of the recent proposals regarding the role of carbidic intermediates therefore related specifically to methanation (48, 56). In subsequent studies (36,37,58,61), however, it was found that smaller quantities of higher hydrocarbons, up to butane, are being coproduced (cf. Fig. 11), especially at low temperature, and/or low hydrogen pressures, stimulating the authors to propose the participation of carbidic species in chain growth. 

It thus follows that methanation proceeds via rapid formation of a carbidic intermediate CH which is converted relatively slowly to methane ... 

T he higher hydrocarbons were predominantly straight-chain ones, with a close to linear Schulz-Flory chain length distribution. Therefore, it was concluded that the carbidic intermediates CH, leading to methane [cf. Eq. (43)] were also participating in chain growth. Moreover, the extent of incorporation into the longer chains was, just as with methane, indicative of... 

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. 

We conclude, therefore, that the mechanisms of catalytic cracking reactions on nickel metal and nickel carbide are closely comparable, but that the latter process is subject to an additional constraint, since a mechanism is required for the removal of deposited carbon from the active surfaces of the catalyst. Two phases are present during reactions on the carbide, the relative proportions of which may be influenced by the composition of the gaseous reactant present, but it is not known whether the contribution from reactions on the carbide phase is appreciable. Since reactions involving nickel carbide yielded products other than methane, surface processes involved intermediates other than those mentioned in Scheme I, although there is also the possibility that if cracking reactions were confined to the metal present, entirely different chemical changes may proceed on the surface of nickel carbide. 

Mechanistic studies have mostly focussed on the 1-alkene formation by polymerization of surface CFI2 (methylenes). The formation of the CH2 species by a deoxygenation/hydrogenation sequence of adsorbed CO is not well understood as there are few convincing organometallic models, but it is usually depicted as shown in Figure 16. The path involves stepwise de-oxygenation to a surface carbide followed by sequential H transfer to make various intermediate CiHx(ad) (x = 0 - 4) species and finally methane which is liberated. 

Acetylene can be produced from coal or from hydrocarbons. The coal route, involving calcium carbide as an intermediate, was the only one practised industrially until the late 1930s, when acetylene was the basic product for the organic chemical industry. Since about 1940, coal has been supplanted by methane and other hydrocarbons. Meth e is a widely available raw materid ethane, propane and butane are more advantageously converted to the corresponding olefins than to acetylene. 

Following earlier work in which the intermediate in the formation of methane from carbon monoxide and hydrogen was found to be carbon, McCarty and Wise carried out a thorough study of the system. Four types of carbon were found to be formed from carbon monoxide on nickel at 550 50K. Chemisorbed carbon atoms reacted readily with hydrogen as did the initial layers of nickel carbide. Further deposits of the carbide, amorphous carbon, and crystalline elemental carbon were much less reactive and the kinetics of the reaction should be described by the established rate laws. Conversion of the more active to the less active forms of carbon occurred above approximately 600 K. 

We are particularly concerned about assumption (3), as surface spectroscopy has recently shown that in addition to growing chains a substantial amount of carbidic carbon develops on the catalyst surface (35-37). While only part of it might be reaction intermediate under steady state conditions, it would be converted almost completely to methane and small amounts of higher hydrocarbons when the surface is exposed to hydrogen in the absence of CO (37). 

The methanation reaction (3H2 + CO — CH4 + H20) has been thoroughly studied by Goodman and co-workers (4, 5, 71, 96) over Ni single crystals. Since the specific rates, activation energies, and pressure dependencies are very similar over Ni(100), Ni(lll), and AI203-supported Ni, the reaction is structure insensitive (71, 96). Transient kinetic studies at medium pressures combined with postreaction AES analysis on Ni(100) have identified a carbidic form of adsorbed carbon as the reaction intermediate, and graphitic carbon as a poison formed at higher temperatures (71, 96). 

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... 

Several years later Craxford and Rideal (54) published some interesting results regarding the question of formation of carbides as intermediate products of the synthesis. Higher hydrocarbons should be formed by the reaction of carbides with molecular hydrogen, while methane should be the result of a reaction of carbide with chemisorbed hydrogen (see Sec. III). 

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