Sachtler-Biloen mechanism

According to the Sachtler-Biloen mechanism, the Fischer-Tropsch reaction is initiated through CO adsorption followed by CO dissociation. Experimental evidence for the involvement of an oxygen-free intermediate exists it was observed that predeposited C is incorporated into the product during Fischer-Tropsch synthesis when CO was included in the feed gas (3). It is important to distinguish whether during the Fischer-Tropsch s)mthesis CO dissociation is strictly monomolecular or instead involves a reaction with Hads to produce an intermediate "HCO" formyl species that in a subsequent reaction decomposes to "CH" and Oads-Another question is how the rates of CO dissociation, chain growth, and termination depend on the catalyst surface structure. Thus, it is essential to know the relative values of the rate constants for these three elementary reactions. 

To initiate chain growth, a "Ci" surface species may be required as in the Sachtler-Biloen mechanism, but the rate of formation of such species may be low. This scenario is comparable to conventional polymerization catalysis, in which initiation is usually the rate-limiting step. Assuming the generation of "Ci" species to be rate determining contrasts the Pichler-Schulz reaction scheme from the Sachtler-Biloen scheme, in which the slow step of the reaction is the termination. Because of the structure sensitivity of the CO dissociation reaction, and also because of the expected structure sensitivity of the chain-growth reaction, the Pichler-Schulz mechanism requires unique sites. The rate of CO insertion and consecutive steps should be fast compared with the rate of CO dissociation. Of course, the rate of termination should be low compared with that of chain growth. 

The first one is the carbide mechanism (Fig. 10), which was initially advanced by Fischer and Tropsch in 1925 and later further developed by Sachtler, Biloen and others. Therefore it is also known as Sachtler-Biloen mechanism. The main steps of this mechanism involve CO cleavage to a surface carbide (CHj as the monomer for chain propagation, hydrogenation (eventually to methane), and the formation and coupling of surface hydrocarbyl (C Hj species, from which 1-alkenes are produced by p-elimi nation. 

Biloen P, Sachtler WMH. Mechanism of hydrocarbon synthesis over Fischer-Tropsch catalysts. Adv Catal. 1981 30 165-216. 

Recombination of CH. fragments is an essential step to initiate the chain-growth reaction according to the Sachtler-Biloen carbide mechanism. In a series of elegant papers, Cheng et al. (31-33) reported on the structure dependence as well as on the metal dependence of this class of reactions. Activation energies for CH. —CH recombination on flat and stepped surfaces of cobalt are listed in Table 4. 

Mechanism of Hydrocarbon Synthesis over Fischer Tropsch Catalysts P. Biloen and W. M. H. Sachtler Surface Reactions and Selectivity in Electro-catalysis... 

The first hypothesis of a Fischer-Tropsch mechanism, as formulated by Sachtler and Biloen, is illustrated in Scheme 1. This mechanism is characterized by an initial C—O bond cleavage and chain growth through addition of species. 

SCHEME 1 Fischer-Tropsch mechanism proposed by Sachtler and Biloen (7). 

A characteristic of the Pichler-Schulz mechanism (presented in Scheme 2) is that the chain-growth reaction does not require high "Ci coverage but is favored by a high CO coverage. In contrast to the Biloen-Sachtler... 

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