Successive Dissociation of CO, CH4, C2H6, and CH3CHO on Fe(110): Retrosynthetic Understanding of FTS Mechanism

To understand the mechanisms of Fe-based Fischer-Tropsch synthesis in a retrosynthetic way, successive dissociation of CH4, C2H6, and CH3CHO on the Fe(110) p(5 X 5) surface has been systematically computed using density functional theory. The successive dissociation of CH4 [CH4 -> H + CH3 -> 2H + CH2 -> 3H + CH -> 4H + C] and C2H6 [C2H6 -> H + CH3CH2 -> 2H + C2H4 -> 3H + CHCH2 -> 4H + CHCH -> 4H + CH + CH] has close apparent barrier (0.69 and 0.65 eV, respectively) and is highly exothermic (-2.47 and -3.17 eV). The CH3CHO successive dissociation is barrierless and favors the minimum energy path of CH3CHO -> H + CH3CO -> 2H + CH2CO -> 3H + CHCO -> 4H + CH + CO. All of these successive dissociations result in the formation of surface species H, C, and CO, which are related with the initial steps of the adsorption and activation of H-2 and CO. It is particularly noted that the computed results that ethylene dissociates easily into acetylene and acetylene dissociates reversibly into CH + CH agree with that experimentally observed. Under the consideration of gaseous H-2 and surface carbonaceous species, the formation of CH4 and C2H6 has very low apparent barriers and is favored thermodynamically, in agreement with the experiment from carbon deposited on Fe(110) surface and H-2 gas. This indicates that surface coverage plays a very important and decisive role in determining reaction mechanisms.