Origin of the formation of Nanoislands on cobalt catalysts during Fischer−Tropsch synthesis

Cobalt catalysts undergo a massive reconstruction under Fischer-Tropsch conditions, resulting in the formation of uniform nanoislands. It is unclear what drives the formation of these islands, since it is highly unfavorable for clean surfaces. Using density functional theory, we show that the formation of islands and steps is driven by the embedding of carbon in an unusual square-planar form at the B5 step sites. Though carbon is not a typical oxidant for metals, it oxidizes cobalt at those sites. This strengthens CO adsorption, which further favors the formation of islands and steps. The oxidation of cobalt by carbon is predicted to be experimentally detectable as a 2 eV shift in the Co 2p binding energy

Shape and size of cobalt nano-islands formed spontaneously on cobalt terraces during Fischer-Tropsch synthesis

Cobalt-based catalysts undergo a massive and spontaneous reconstruction to form uniform triangular nanoislands under Fischer-Tropsch (FT) conditions. This reconstruction is driven by the unusual and synergistic adsorption of square-planar carbon and CO at the 4-fold edge sites of the nanoislands, driving the formation of triangular islands. The size of the nanoislands is determined by the balance between energy gain from creating C/CO-covered edges and energy penalty to create C/CO-covered corners. For carbon chemical potentials corresponding to FT conditions, triangular Co islands with 45 Co atoms (about 2 nm) are the most stable surface structure. Decreasing the carbon chemical potential and hence the stability of square planar carbon favors the formation of larger islands, until reconstruction becomes unfavorable and CO-covered terraces are thermodynamically the most stable. The predicted structure of the islands is consistent with in situ scanning tunneling microscopy images obtained for the first time under realistic FT reaction conditions on a Co(0001) surface