Mechanistic Study of Fischer-Tropsch Synthesis: C-O Scission and Initial CH Formation Pathways for Realistic Surface Coverages

Fischer-Tropsch synthesis transforms synthesis gas, a mixture of CO and H₂, to long-chain hydrocarbons and water. In recent years, interest in Fischer Tropsch synthesis as an attractive route to convert natural gas, coal and biomass to clean transportation fuels, has surged. Supported cobalt catalysts are often preferred due to their high activity, selectivity towards long chain hydrocarbons, and low CO₂ selectivity. The detailed reaction mechanism of this complex reaction however remains intensely debated. C-O scission is a key step in the FT mechanism, and occurs either by direct cleavage of CO at defect sites, via a hydrogen-assisted pathway, or after formation of a C-C bond via CO insertion. Our kinetic study [1] shows that the propagation mechanism for the CO insertion mechanism is consistent with available experimental kinetic and selectivity data [2,3]. However, the initiation and termination steps for the CO insertion mechanism remain to be elucidated. In this presentation, we analyze various reaction paths for the formation of the chain initiating CH species on a CO saturated Co surface [1] using density functional theory. These CH species either hydrogenate to methane or initiate chain growth to long chain hydrocarbons. Next, a detailed microkinetic model is developed by combining the initiation reactions with chain growth and termination pathways.

References

1. Zhuo, M.; Borgna A.; Saeys, M.; J. Catal., 290, 217 (2013)
2. Schweicher J.; Bundhoo A.;  Kruse N.; J. Am. Chem. Soc., 134, 39, 16135 (2012)
3. Tuxen, A.; Carenco, S.; Chintapalli, M.; Chuang, C; Escudero, C.; Pach, E.; Jiang, P.; Borondics, F.; Beberwyck, B.; Alivisatos, A. P.; Thornton, G.; Pong, W.; Guo, J.; Perez, R.; Besenbacher, F.; Salmeron, M.;  J. Am. Chem. Soc., 135, 6,  2273 (2013)