In situ observation of self-assembled hydrocarbon Fischer–Tropsch products on a cobalt catalyst

Fischer–Tropsch synthesis is a heterogeneous catalytic reaction that creates approximately 2% of the world's fuel. It involves the synthesis of linear hydrocarbon molecules from a gaseous mixture of carbon monoxide and hydrogen at high pressures (from a few to tens of bars) and high temperatures (200–350 °C). To gain further insight into the fundamental mechanisms of this industrial process, we have used a purpose-built scanning tunnelling microscope to monitor a cobalt model catalyst under reaction conditions. We show that, after 30 minutes of reaction, the terraces of the cobalt catalyst are covered by parallel arrays of stripes. We propose that the stripes are formed by the self-assembly of linear hydrocarbon product molecules. Surprisingly, the width of the stripes corresponds to molecules that are 14 or 15 carbon atoms long. We introduce a simple model that explains the accumulation of such long molecules by describing their monomer-by-monomer synthesis and explicitly accounting for their thermal desorption. During the Fischer-Tropsch catalytic reaction, alkanes are synthesized from carbon monoxide and hydrogen at high pressure and temperature. Now it is shown using scanning tunnelling imaging of a cobalt surface during reaction that linear alkane product molecules of a specific length self-assemble on terraces, facilitating the desorption of new product molecules created at step sites.