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

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 (root 3 x root 3)R30 degrees structure; between 10(-6) and 10(-3) mbar, a disordered phase representing a nonstoichiometric, fluctuating (root 7 x root 7)R19.1 degrees structure; and between 10(-2) and 100 mbar, a (2 root 3 x 2 root 3)R30 degrees structure. Between 100 and 800 mbar, three moire 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.