Massive Restructuring of Cobalt Catalysts Under Fischer Tropsch Conditions: Driving Force and Mechanistic Consequences

Fischer-Tropsch (FT) synthesis converts synthesis gas, a mixture of CO and H₂, to long-chain hydrocarbons and water over supported cobalt catalysts. On exposure to synthesis gas under reaction conditions, Co(0001) terraces undergo a massive reconstruction to form nano-islands of about 2 nm diameter [1]. This reconstruction creates defect sites. The strong adsorption of CO and of reaction intermediates is suggested to drive this reconstruction, but a detailed understanding of the structure of the nano-islands and of the thermodynamic driving force for this reconstruction is lacking. Using DFT, we demonstrate that the high CO coverage at the step sites is sufficient to overcome the energy penalty to create additional step sites [2]. The triangular shape of the nano-island is governed by the unusually high thermodynamic stability of square-planar carbon at the B5 step sites and by the increased stability of CO at step sites covered by carbon. Both effects result from the σ-aromaticity of the “Co₄C” subunit [3]. The shape and size of the nano-islands is moreover a strong function of the carbon chemical potential, and depends on the reaction conditions. The electron count in the aromatic system limits the carbon step coverage to 50%, leaving B5 vacancies as the active sites for the hydrogenation of CO to CH.

References.

[1] Wilson J.; de Groot C.; J. Phys. Chem., 99, 7860, 1995

[2] Banerjee A.; Saeys M.; in preparation

[3] Alexandrova A. N.; Thang Q.T.; Saeys M., submitted.