(645g) Mechanistic Insights into Cobalt-Catalyzed Fischer-Tropsch Synthesis

Fischer-Tropsch synthesis (FTS) transforms synthesis gas, a mixture of CO and H₂, to long-chain hydrocarbons and water. Supported cobalt catalysts are often preferred due to their high activity, selectivity towards long-chain hydrocarbons, and low CO₂ selectivity. The detailed reaction mechanism, a complex combination of C-C bond formation, C-O scission and hydrogenation steps, remains intensely debated. [1, 2] To gain insights into the rate-limiting reactions, the most abundant surface species and the factors governing selectivity (CH₄, CO₂, Chain growth probability), we use density functional theory (DFT) to develop a dual-site microkinetic model consisting of both terrace sites and B5 step sites, accounting for catalyst structure, coverage and reaction conditions. Analysis of the reaction network reveals that CO activates at the B5 step sites and the carbon atom at B5 step site undergoes hydrogenation to form CH species which then diffuses to terraces sites for chain growth. Interestingly, kinetic parameters calculated at low CO coverage leads to high CH₄ selectivity, however, kinetic parameters calculated at realistic CO coverages (~0.5 ML) [3] lead to chain growth and CH₄ selectivities in line with experiments. Both carbide (CH+CH) and CO insertion (CH₂+CO) pathways contribute to chain growth. A chain growth probability (α) value of 0.9 was obtained with negligible selectivity to oxygenates and CO₂. The detailed dual-site microkinetic model reinforces the complexity of FTS mechanism, at the same time providing opportunities to control activity and selectivity.

[1] R.A. van Santen, A.J. Markvoort, I.A.W. Filot, M.M. Ghouri, E.J.M. Hensen, Phys. Chem. Chem. Phys., 15 (2013)17038

[2] G.T.K.K. Gunasooriya, A.P. van Bavel, H.P.C.E. Kuipers, M. Saeys, Surf. Sci 642 (2015) L6−L10

[3] G.T.K.K. Gunasooriya, A.P. van Bavel, H.P.C.E. Kuipers, M. Saeys, ACS Catal. 6 (2016) 3660−3664.