(760d) Reaction Mechanism and Microkinetics of the 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] In the carbide mechanism, originally proposed by Fischer and Tropsch, C−O activation occurs via direct CO dissociation. The carbide species then undergo hydrogenation to CH_x groups, the monomers for the growth of alkyl chains. In the carbide mechanism, C−O scission occurs before C−C coupling and the CH_x coverage needs to be sufficiently high to favor chain growth over chain termination by hydrogenation. In the CO insertion mechanism, originally proposed by Pichler and Schulz, RCH_x* groups couple with CO* before C−O scission. The FTS turnover frequency (TOF) is independent of the particle size above 10 nm, [3] and therefore the kinetically relevant steps occur on terraces or terrace-derived sites. Under FTS conditions, cobalt catalyst terraces undergo a massive reconstruction as observed by operando STM.[4] The synergistic adsorption of carbon and CO at B5 sites make it thermodynamically favorable to break up Co terraces and create B5 step sites and nano−islands. [5,6] 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.[7]

To gain insight into the rate−limiting reactions, the most abundant surface species and the factors governing the selectivity (CH₄, CO₂, Chain growth probability), a dual−site microkinetic model consisting of both the terrace and the edge sites was constructed, accounting for the catalyst structure, the coverage and the reaction conditions. Analysis of the reaction network reveals that OH−assisted C−O activation [8] is preferred over direct CO dissociation and H−assisted CO activation on both the terraces and on the edge sites. Kinetic parameters calculated at low CO coverage leads to a high CH₄ selectivity, however, kinetic parameters calculated at realistic CO coverages (~0.5 ML) lead to chain growth and a CH₄ selectivity in line with experiments. Both CH and CO coupling contribute to chain growth. A chain growth probability (α) of 0.9 was obtained with negligible selectivity to oxygenates and CO₂. The dual-site microkinetic model reinforces the complexity of FTS mechanism, at the same time provides opportunities to control activity and selectivity.

References

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