Study on the role of atomic carbon species in cobalt-based FT synthesis catalyst

The FT synthesis reaction is a process that catalytically converts syngas into long-chain heavy hydrocarbons. The Fischer-Tropsch reaction involves CO activation, disproportionation reactions, and hydrocarbon dehydrogenation reactions that can form carbon species on the catalyst surface. The role of carbon species in the Fischer-Tropsch reaction has been a topic of controversy. This paper reports the successful construction of model catalysts with varying atomic carbon species content. The catalysts were prepared by pretreating single-crystal cobalt with HCP-Co(10-11) as the main exposed surface under different conditions. The carbon species content and existing forms of the catalyst were characterized by temperature programmed hydrogenation, Raman spectroscopy and infrared spectroscopy. The study found that the activity and CH₄ selectivity of the catalysts were closely related to the number and form of atomic carbon species introduced. With the increase of pretreatment time, the content of atomic carbon species deposited on the catalyst first increased and then decreased, and finally maintained a dynamic equilibrium, indicating that the content of atomic carbon species would remain unchanged with the increase of pretreatment time to a certain extent. The pre-treated catalyst was characterized by XRD, and no characteristic peak of CoO was found. The crystal structure is consistent with that of P-Co catalyst, and the influence of CoO and other crystal structure on the performance of FT synthesis is excluded. The P-Co-C3 catalyst, with a carbon content of 5.72%, achieved a high CO conversion rate of 72.2%, whereas the P-Co-C2 catalyst, with a carbon content of 3.01%, had a low CH₄ selectivity of 4.2%. When the carbon content of P-Co-C1 (2.76%) and P-Co-C2 (3.01%) catalysts is low, the atomic carbon species mainly exists in the form of amorphous carbon (C). The presence of amorphous carbon in atomic carbon species covers part of methane generation sites, thus inhibiting methane generation, resulting in a decrease in methane selectivity. As the carbon content of P-Co-C3 (5.72%) and P-Co-C4 (14.12%) catalysts increases, the atomic carbon species in P-Co-C3 and P-Co-C4 catalysts may mainly exist in the form of C_xH_y, and C_xH_y species plays a major role in the performance of Fischer-Tropsch reaction. It is speculated that C_xH_y may exist in the form of low carbon olefins in adsorbed state in the atomic carbon species on the catalyst. According to the olefin readsorption mechanism, olefin readsorption produced on the catalyst surface can be used as chain initiation and chain growth species, resulting in changes in activity. Using the same pretreatment conditions, the carbon deposition on the single crystal cobalt catalyst C-Co, whose main exposed crystal surface is HCP-Co (11-20), and the Fischer-Tropsch reaction performance evaluation. The carbon species on catalyst were analyzed by temperature programmed hydrogenation and infrared characterization, and it was found that atomic carbon species were deposited on catalyst C-Co. The results showed that the deposition of atomic carbon species on the C-Co catalyst inhibited the formation of CH₄ and improved the activity of the catalyst. It is further confirmed that the presence of a few atomic carbon species on cobalt-based catalysts has a certain universality to improve the performance of Fischer-Tropsch reaction.