(308c) Reaction Mechanism and Microkinetics of the Cobalt Catalyzed Fischer–Tropsch Synthesis

G. T. Kasun Kalhara Gunasooriya,^† Mark Saeys^†

^†Laboratory for Chemical Technology, Ghent University, Ghent, Belgium

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 CO₂selectivity. The detailed reaction mechanism, a complex combination of C-C bond formation, C-O scission and hydrogenation steps, remains intensely debated.[1, 2] FTS turnover frequency (TOF) is independent of the particle size above 10 nm,[3] and therefore the kinetically relevant steps occur on terraces. However under FTS conditions, cobalt catalyst surfaces undergo a massive reconstruction as observed by STM.[4] The synergistic adsorption of carbon and CO, with the high CO coverage make it thermodynamically favorable to break up Co terraces and create step sites and nano-islands.[5,6] To gain insights into the rate-limiting reactions, the most abundant surface species and the factors governing selectivity, a dual-site microkinetic model accounting for catalyst structure, coverage and reaction conditions is required.

A dual-site microkinetic model based on first principle calculations was constructed considering both the terrace sites and the edges of the self-assembled nano-islands. This model incorporates adsorption of species on both the terrace sites and the edges of the self-assembled nano-islands, and diffusion between those sites. C-O activation is a crucial step in FTS and several pathways have been proposed. Direct CO dissociation is a difficult step on Co terraces and barriers greater than 220 kJ/mol are consistently reported. 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). Hydrogenation at the O-atom in CO* and RCO* is a difficult step, while hydrogenation at the C-atom leads to oxygenates on Co terraces. In the hydroxyl-assisted route, CHOH is formed via transfer of a proton from a surface OH group to the O-atom of CO. CH-OH then undergoes a rate-limiting C-O scission step to regenerate the OH group and a CH species.[7] Moreover, the thermodynamically favorable self-assembly of nano-islands introduces edge sites on the cobalt terraces. Evaluation of various possible pathways show that a sequence involving the creation of a free site through C hydrogenation, CH migration to terraces, and direct CO dissociation is a kinetically viable step at the edges of the nano-islands. Microkinetic modeling reveals that coupled H- and OH-assisted C-O activation is preferred over direct CO dissociation and H-assisted CO activation on terraces. Moreover, both coupled H- and OH-assisted C-O activation and direct CO dissociation at the edges of the nano-islands form the CH species that initiate chain growth. However, kinetic parameters calculated at low CO coverage leads to high methane selectivity. At high CO coverage, limited stabilities of CH groups further reduce their coverage. Detailed microkinetic model again reflects the complexity of FTS mechanism providing opportunities to control activity and selectivity.

References

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