Mechanistic Insights into Cobalt-based Fischer-Tropsch Synthesis
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Fischer-Tropsch synthesis (FTS) is an alternative process of yielding transportation fuels and chemicals by converting syngas derived from natural gas, biomass and coal. Cobalt-based catalysts are favorable catalysts for the synthesis of long-chain hydrocarbons from the synthesis gas with H2/CO ratio about 2. FTS involves a large number of intermediates and hundreds of elementary steps, which make its mechanism quite complex. Hence, a combined approach of DFT calculations, kinetic isotope effect (KIE) and kinetic analysis was employed to study the mechanisms of CO activation, methane formation, chain growth, olefin to paraffin ratio and water effect on selectivity. The favorable pathways were distinguished for the CO activation, methane formation and C1+C1 coupling reaction. The adsorption energies, involving vdW interaction and entropy changes, are suggested to be the descriptor for olefin to paraffin ratio. A theory has been proposed to account for the particle size dependent water effect on selectivity. It is demonstrated that the combined approach of DFT, kinetic analysis and kinetic isotope effect analysis is a powerful method to discriminate the possible mechanisms for FTS, which may extend to other reaction systems.