A First-Principles Study of Oxygenates on Co Surfaces in Fischer−Tropsch Synthesis

Extensive density function theory calculations are performed to study the mechanism of the formation of aldehyde and alcohol on Co surfaces in Fischer−Tropsch synthesis, a challenging issue in heterogeneous catalysis. Three possible pathways for the production of formaldehyde and methanol on flat and stepped Co(0001) surfaces are investigated: (i) CO + 4H → CHO + 3H → CH 2 O + 2H → CH 3 O + H → CH 3 OH; (ii) CO + 4H → COH + 3H → CHOH + 2H → CH 2 OH + H → CH 3 OH; and (iii) the coupling reactions of CH 2 + O → CH 2 O and CH 3 + OH → CH 3 OH. It is found that these pathways are generally favored at step sites, and the preferred mechanism is pathway (i) via CHO. Furthermore, the three traditional chain growth mechanisms in Fischer−Tropsch synthesis are semiquantitatively compared and discussed. Our results suggest that the two mechanisms involving oxygenate intermediates (the CO-insertion and hydroxycarbene mechanisms) are less important than the carbene mechanism in the production of long chain hydrocarbons. However, the CO-insertion mechanism may be responsible for the production of long-chain oxygenates.