Density Functional Theory Study of the CO Insertion Mechanism for Fischer−Tropsch Synthesis over Co Catalysts

Different mechanisms have been proposed for Fischer−Tropsch synthesis, the conversion of CO and H2 to long-chain alkanes. Density functional theory calculations indicate that CO activation has a barrier of 220 kJ/mol on Co(0001), and hence the concentration of surface C or CH2 species is likely too low to explain the high chain growth probability. Hydrogenation lowers the C−O dissociation barrier to 90 kJ/mol for HCO and to 68 kJ/mol for H2CO; however, CO hydrogenation has a high energy barrier of 146 kJ/mol and is +117 kJ/mol endothermic. We propose an alternative propagation cycle starting with CO insertion into surface RCH groups. The barrier for this step is 80 kJ/mol. RCHCO is subsequently hydrogenated to RCH2CHO, which undergoes C−O dissociation with a barrier of 50 kJ/mol. The hydrogenation barriers are 120 and 48 kJ/mol along the dominant reaction path. The calculated CO turnover frequency for the proposed CO insertion mechanism is 1 to 2 orders of magnitude faster the hydrogen-assisted CO activation mechanism and 4 orders of magnitude faster than direct CO activation on a model Co(0001) surface.