(478d) First Principle Reaction Path Analysis of the Fischer-Tropsch Synthesis Mechanism. Effect of CO Coverage On the Kinetics of the CO Insertion Mechanism

Fischer-Tropsch Synthesis (FTS), the conversion of CO and H₂ to long chain hydrocarbons, (CH₂)_n, and H₂O, involves a sequence of C-O scission, C-C forming, and C and O hydrogenation steps. Different reaction sequences have been proposed. Recent Density Functional Theory (DFT) calculations provide theoretical support for the CO insertion mechanism, where C-C bond formation via a RCH + CO step occurs before C-O scission [1]. Subsequent surface science experiments confirmed that C-O bond breaking is indeed kinetically possible on Co(0001) terraces if a C_xH_y-O moiety can be formed [2], as proposed in the CO insertion mechanism. However, on a clean Co(0001) surface the calculated CO turnover frequency (TOF) for the CO insertion mechanism is several orders of magnitude lower than experimental values.

FTS is performed under a CO partial pressure of about 7 bar. First principle phase diagram calculations indicate that the CO coverage exceeds 1/3 ML under such conditions, in agreement with experimental data [3]. DFT calculations on a CO pre-covered Co(0001) surface further show that the binding energies of the various reaction intermediates decrease by about 10 kJ/mol in the presence of 1/3 ML CO, and the overall reaction energy for the CO insertion mechanism changes from +42 kJ/mol endothermic on a clean surface to -40 kJ/mol exothermic on the CO pre-covered surface. A kinetic analysis using the first principle data furthermore shows that the CO TOF increases from 10^-8 s^-1 to 0.1 s^-1 when the high CO coverage is accounted for.

References

1. Zhuo, M.; Tan, K.F.; Borgna, A.; Saeys, M., J. Phys. Chem. C 113, 1947 (2009); Inderwildi, O. R.; Jenkins, S. J.; King, D. A., PCCP, 11, 11110 (2009)

2. Westrate, C. J.; Gericke, H. J.; Verhoeven, M. W. G. M.; Ciobîcã, I. M.; Saib, A. M.; Niemantsverdriet, J. W., J. Phys. Chem. Lett. 1, 1767 (2010)

3. Beitel, G. A.; Laskov, A.; Oosterbeek, H.; Kuipers, E. W. J. Phys. Chem. 1996, 100, 12494