Fischer-Tropsch reaction chain growth limit

It is often proposed that "Ci" formation is also the rate-limiting step of the Fischer-Tropsch reaction. This scenario can only become true if chain growth proceeds through CO insertion, which we suggest to be unlikely. 

In order to dissociate CO, the barrier for CO activation has to be low. As illustrated in Table 3.4, the rate of dissociative CO adsorption appears to be fastest over Ru, Co and Fe, the preferred transition metals for the Fischer-Tropsch reaction. The barrier for CO dissociation, however, is very high compared with that of the chain growth or termination reactions, especially on the close-packed terraces. While the presence of steps helps to lower the CO activation barrier, it is still considered to be rate limiting. 

In this review, we limit ourselves to the mechanisms of primary product formation, which are fimdamental to Fischer-Tropsch chemistry. Using new information mainly from computational studies, we focus on two coidlicting hypotheses regarding the key reaction steps that lead to chain growth. 

Chain growth during the Fischer-Tropsch synthesis is controlled by surface polymerization kinetics that place severe restrictions on our ability to alter the resulting carbon number distribution. Intrinsic chain growth kinetics are not influenced strongly by the identity of the support or by the size of the metal crystallites in supported Co and Ru catalysts. Transport-limited reactant arival and product removal, however, depend on support and metal site density and affect the relative rates of primary and secondary reactions and the FT synthesis selectivity. 

Catalysis by Metal Ousters in Zeolites. There is an increasing interest in the use of metal clusters stabilized in zeolites. One objective of such work is to utilize the shape and size constraints inherent in these support materials to effect greater selectivities in typical metal-catalysed reactions. Much work has been concerned with carbon monoxide hydrogenation, and although the detailed nature of the supported metals so obtained is not well understood, there is clear evidence of chain limitation in the Fischer-Tropsch process with both RuY zeolites and with HY and NaY zeolites containing Fe3(CO)22- In the former case there is a drastic decline in chain-growth probability beyond C5- or C10-hydrocarbons depending upon the particle size of the ruthenium metal. 

Consecutive reactions with hydrogen or CO lead to the removal of this oxygen as H2O or CO2, resprectively. In reactions such as Fischer-Tropsch, where CO dissociation is rate-limiting, the addition of such promoters helps to enhance the activity and even the selectivity for chain growth reactions. The increase in selectivily is the result of increasing the concentration of reactive carbon atoms on the transition metal. 

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