Mechanisms of CO Activation, Surface Oxygen Removal, Surface Carbon Hydrogenation, and C–C Coupling on the Stepped Fe(710) Surface from Computation

Abstract

To understand the initial steps of Fe-based Fischer–Tropsch synthesis, systematic periodic density functional theory computations have been performed on the single-atom stepped Fe(710) surface, composed by <i>p</i>(3 × 3) Fe(100)-like terrace and <i>p</i>(3 × 1) Fe(110)-like step. It is found that CO direct dissociation into surface C and O is more favored kinetically and thermodynamically than the H-assisted activation via HCO and COH formation. Accordingly, surface O removal by hydrogen via H₂O formation is the only way. On the basis of surface CH_<i>x</i> hydrogenation (<i>x</i> = 0, 1, 2, 3), surface CH_<i>x</i> + CH_<i>x</i> coupling and CO + CH_<i>x</i> insertion resulting in CH_<i>x</i>CO formation followed by C–O dissociation, surface C hydrogenation toward CH₃ formation is more favored kinetically than the formation of CH_<i>x</i>-CH_<i>x</i> and CH_<i>x</i>CO, as well as thermodynamically. Starting from CH₃, the formation of CH₄ and CH₃CO has similar barriers and endothermic reaction energies, while CH₃CO dissociation into CH₃C + O has low barrier and is highly exothermic. Therefore, turning the H₂/CO ratio should change the selectivity toward C–C formation and propagation