Insight into the Rate-Determining Step and Active Sites in the Fischer–Tropsch Reaction over Cobalt Catalysts

Most studies on the Fischer–Tropsch reaction assume that the dissociation of the C–O bond is crucial in determining the overall reaction rate. However, recent experimental results show that a hydrogenation step is crucial in the overall kinetics. At low pressures, which are typically used in academic research, the structure-independent termination by hydrogenation dominates the reaction rate. This is reflected in a particle-size- and structure-independent apparent activation energy and is confirmed by kinetic modeling of transient experiments. At higher (i.e., industrially more relevant) pressures, both the availability of appropriate dissociation sites and the removal of adsorbates by hydrogenation appear to limit the rate. This results in comparable degrees of rate control for CO dissociation and hydrogenation. At low pressures, the locus of termination by hydrogenation has been studied by selective site blocking of planar sites with graphene and by using nanoparticles exposing specific crystal planes. It is found that the termination runs mainly on planar surfaces, while corrugated surfaces contribute to chain growth, which follows the ASF distribution. Migration of CH x species between these two sites is foreseen. A shortage of stepped sites needed for monomer formation limits the yield of both methane and longer hydrocarbons. We propose that as long as the number of stepped CO dissociation sites is sufficient, the overall rate is dominated by the structure-insensitive hydrogenation. Otherwise, the turnover frequency follows the occurrence of CO dissociation sites.