Shaping and Stabilizing the Active Phase: The Role of Carbon Surface Defects in Carbon-Supported Co Fischer–Tropsch Synthesis Catalysts

Carbon supports offer a promising alternative to conventional oxide supports for cobalt-based Fischer-Tropsch synthesis (FTS) catalysts. However, unlike well-studied oxide systems (e.g., Co/Al₂O₃, Co/TiO₂), the fundamental interactions between cobalt nanoparticles (Co NP's) and unfunctionalized carbon surfaces remain poorly understood, largely due to the structural and chemical diversity of carbon materials. Establishing a universal "baseline" interaction for Co/C interfaces has therefore remained elusive. In this work, we investigated Co anchoring mechanisms on two carbon black model supports that differ by a factor of 20 in surface defect (chemisorption) site density but exhibit otherwise similar properties. On this basis, Co-based catalysts were synthesized using size-controlled colloidal Co nanoparticles and conventional incipient wetness impregnation. Employing high-resolution SEM and HAADF STEM imaging, we could show that Co NP sintering occurs predominantly via nanoparticle migration and coalescence during catalyst reduction, with negligible additional growth under FTS conditionsimplying that Co NP anchoring is established in the reduction step. Combined in situ XANES/XRD experiments during reduction, coupled with off-gas analysis by online mass spectrometry, showed that Co phase transformations coincided with significant CO₂ and CH₄ evolution. This was attributed to carbothermal reduction and carbon hydrogasification at the Co/C interface, which appeared to correlate with the density of carbon surface defect (chemisorption) sites. We hypothesize that carbon gasification at the Co/C interface is directly linked to the immobilization of Co NP, as it generates highly reactive "dangling bonds" at the Co/C interface, which act as anchoring points. Overall, the defect-rich carbon support stabilized Co nanoparticles more effectively than its defect-poor counterpart, resulting in most cases in higher FTS activity. Our results imply carbon gasification-mediated anchoring as a "baseline" interaction for Co/C catalysts and suggest that the chemisorption site densityas measurable by simple TPD or TPOcan serve as a practical descriptor for designing more stable carbon-supported FTS catalysts.