CO Dissociation Mechanism on Cu-Doped Fe(100) Surfaces

Periodic density functional theory calculations were carried out to investigate CO dissociation pathways on the Fe(100) surfaces covered with up to one monolayer of Cu atoms, which serve as the simple models for the Cu/Fe catalysts for higher alcohol synthesis (HAS) from syngas. For all the model catalyst surfaces, H-assisted CO dissociation was predicted to have lower energy barriers than direct CO dissociation. The difference in the energy barriers between the two dissociation pathways increases as Cu surface coverage increases, suggesting reduced contribution of direct CO dissociation on Cu-rich surfaces. A further thermodynamic analysis also reaches the same conclusion. Several reaction properties for CO dissociation, including CO physisorption and chemisorption energies, and energy barriers for direct and H-assisted CO dissociations, were found to scale linearly with Cu surface coverage, and these reaction properties were predicted to depend largely on the structure of the surface layer, which can be expected to also apply to other metal alloy catalysts. Cu doping was found to reduce the activity of the Fe­(100) surface in catalyzing direct and H-assisted CO dissociations, so CO dissociations should occur primarily on Fe-rich surfaces, leading to CH_x formation, whereas Cu-rich surfaces are potential sources for physisorbed CO molecules. This is also expected to apply to other Cu/M catalysts and is consistent with the dual site mechanism previously proposed for these bimetallic catalysts. A synergy between these two types of active sites is beneficial for the formation of higher alcohols, which may be the reason for the superior performance of the Cu/Fe catalysts for the HAS reaction.