A Carbon-Induced Surface Reconstruction on an Operating Fischer-Tropsch Catalyst

Heterogeneous catalysts are often assumed to undergo substantial chemical and morphological changes under reaction conditions. However, obtaining information on the state of a catalyst surface at the usually applied high pressures is difficult. For the Co catalyst used in the industrial Fischer-Tropsch synthesis, a widely held view is that the surface experiences a carbon-induced roughening that is essential for its activity. We have tested this idea by performing the reaction on a Co(0001) surface with a sacrificial carbide layer. In situ scanning tunneling microscopy experiments at ∼1 bar synthesis gas and 473 K show that, at the increased C coverage, the surface does not undergo the predicted roughening. Instead, a unique carbon-induced reconstruction forms, consisting of triangular partial stacking faults in which C atoms occupy the fault lines. Density functional theory calculations reveal that the reconstruction is more stable than the roughened state. For the detectable range of C₁ to C₄ reaction products, gas chromatography shows no effect of the reconstruction, consistent with the view that the activity is primarily determined by the density of atomic steps, which remains unchanged. These findings can serve as a new basis for describing the Co particles of the industrial catalyst.