(375f) Catalyst Structure and C-O Activation during Fischer-Tropsch Synthesis

G. T. Kasun Kalhara Gunasooriya,^� Arghya Banarjee,^§ Mark Saeys^�

^â? Laboratory for Chemical Technology, Ghent University, Ghent, Belgium

^§Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117576 Singapore

Fischer-Tropsch synthesis transforms synthesis gas, a mixture of CO and H₂, to long-chain hydrocarbons and water. In recent years Fischer-Tropsch synthesis has surged as an attractive route to convert natural gas, coal and biomass to clean transportation fuels. 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] Moreover, the structure and coverage of the catalyst under reaction conditions differs dramatically from the ideal clean surface.[3,4] To begin to elucidate catalyst activity and selectivity, computational catalysis requires realistic surface models and account for coverage effects under reaction conditions.

To elucidate the structure of the catalyst, the effect of carbon and CO adsorption on the stability of various surface facets was considered. Carbon atoms adsorb strongly at B5 sites (with a stability of -20 kJ/mol and more) up to a coverage of 50%. The unique stability of square-planar carbon species can be attributed to sigma-aromaticity of the â??Co₄Câ? unit.[5] The presence of square-planar carbon oxidizes the step atoms and enhances CO adsorption. The synergistic adsorption of carbon and CO make it thermodynamically favorable to break up terraces to create step sites and nano-islands.[6]

C-O activation is a crucial step FTS and several pathways have been proposed. In addition to direct CO dissociation, CO can be activated by insertion into growing hydrocarbon chains (CO insertion mechanism [2]), or through hydrogenation (hydrogen-assisted CO activation). Invariably, very high barriers are calculated for both direct C-O dissociation and for hydrogenation at the O-atom in CO* and RCO*, while hydrogenation at the C-atom leads to oxygenates. We show that the formation of OH* groups from O* is quasi-equilibrated, and the OH coverage is significant. OH* groups hydrogenate the O-atom with barriers below 100 kJ/mol, respectively, opening a novel pathway to activate C-O bonds.[7] To further elucidate the reaction mechanism, a micro-kinetic model accounting for coverage and reaction conditions is constructed. The role of OH groups as hydrogenating species is likely general and involved in several oxygenate transformation reactions.

References

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

[2] Zhuo, M., Tan, K.F., Borgna, A., Saeys, M., J. Phys. Chem. C 2009, 113, 8357

[3] Gunasooriya, G.T.K.K.; van Bavel, A.P.; Kuipers, H.P.C.E.; Saeys, M. Surf. Sci. 2015, 642, L6

[4] Wilson, J.; De Groot, C. J. Phys. Chem. 1995, 99, 7860

[5] Nandula, A.; Trinh, Q. T.; Saeys, M.; Alexandrova, A. N. Angew. Chem. Int. Ed. 2015, 54, 5312

[6] Banerjee, A.; van Bavel, A. P.; Kuipers, H. P. C. E.; Saeys, M. ACS Catal. 2015, 5, 4756

[7] Gunasooriya, G.T.K.K., van Bavel, A.P., Kuipers, H.P.C.E, Saeys, M., ACS Catal.2016, 6, 3660