High-Pressure CO Phases on Co(0001) and Their Possible Role in the Fischer–Tropsch Synthesis

Abstract

The cobalt catalyst used in the Fischer–Tropsch synthesis, under the conditions of the reaction, is covered by a dense adsorption layer in which CO is the main constituent. To obtain insight into the structure of this layer and possible surface phases, CO structures on a Co(0001) model have been investigated by high-pressure scanning tunneling microscopy (STM). The experiments were performed in situ, at CO pressures between 10–9 and 800 mbar and at a temperature of 300 K. Under these conditions, an adsorption–desorption equilibrium was established, reflecting the situation of the reaction. A series of CO surface phases were observed; from 10–9 to 10–7 mbar, a (√3 × √3)R30° structure; between 10–6 and 10–3 mbar, a disordered phase representing a nonstoichiometric, fluctuating (√7 × √7)R19.1° structure; and between 10–2 and 100 mbar, a (2√3 × 2√3)R30° structure. Between 100 and 800 mbar, three moiré structures were observed that were analyzed by a recently developed method. All phases were formed by intact CO molecules. A phase diagram was obtained that allowed us to extrapolate the stability regions of the CO phases to industrial Fischer–Tropsch conditions. We conclude that the reaction operates in the region of the disordered phase at a coverage between 0.43 and 0.50 monolayers of CO.