Stability of Carbon on Cobalt Surfaces in Fischer–Tropsch Reaction Conditions: A DFT Study

We establish a thermodynamic model correlating the carbon deposition and desorption energies on cobalt surfaces with the temperature, the CO/H 2 ratio, and the Anderson–Schulz–Flory (ASF) distribution in the Fischer–Tropsch (FT) synthesis reaction. Density functional theory (DFT) calculations predict that the surface sites of Co particles, whether located on terraces or steps, are covered by carbon species in FT reaction conditions unless extremely high H 2 /CO ratios are used. The graphene-covered Co(111) surface is the most stable system. At low ASF coefficient, the Co(111) surface undergoes a strong reconstruction associated with the insertion of C in subsurface sites. At high ASF coefficient, the Co(111) surface is covered by oligomeric C species which may be seen as either long chain alkane or graphene precursors. This theoretical work gives crucial insights into the link between the composition and structures of Co surfaces in FT reaction conditions. Furthermore, it highlights that the existence of purely metallic sites, whether located on terraces or steps, is questioned in long-run FT synthesis.