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 H, 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 CH, species is likely too low to explain the high chain growth probability. Hydrogenation lowers the C-O dissociation harrier 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 it model Co(0001) surface.