Mechanistic issues in Fischer-Tropsch catalysis

Computational studies have recently generated important information regarding reaction intermediates and activation barriers of elementary reaction steps that are part of the Fischer-Tropsch synthesis. We use these results to analyze various mechanistic options that have been proposed for the Fischer-Tropsch synthesis. The computational results do not support the Pichler-Schulz chain-growth mechanism, which postulates chain growth by CO insertion. Rather, the results are in agreement with the Sachtler-Biloen mechanism, which postulates chain growth via adsorbed " C₁" species; furthermore, the Gaube chain-growth mechanism, which closely resembles that proposed by Maitlis, is found to be preferred over the initially assumed Brady-Pettit mechanism. The various elementary steps are discussed, and the values that their relative rates must assume for successful Fischer-Tropsch chain growth are outlined. Within the Sachtler-Biloen kinetics scheme, a high chain-growth probability is obtained when chain termination is rate limiting. Consequently, CO dissociation has to be facile. The " C₁" species that is incorporated into the growing chain appears to be " CH" or " CH₂" ; thus, these species must be present in high surface concentrations. Brønsted-Evans-Polanyi relationships are used to link activation energies to surface reactivity. The structure sensitivity of the elementary reaction steps, specifically, initiation, chain growth, and termination, is analyzed. On the basis of these considerations, one can understand why particular metals are suitable Fischer-Tropsch catalysts.