Quantum-Chemical DFT Study of Direct and H- and C-Assisted CO Dissociation on the χ-Fe5C2 Hägg Carbide

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Abstract
Fischer-Tropsch synthesis is an important reaction catalyzed by metallic Co or carburized Fe surfaces. A crucial step in the reaction mechanism is the dissociation of the C-O bond in the carbon monoxide. We determined the rate of CO dissociation on various surfaces of the catalytically active Hägg carbide (χ-Fe5C2) using density functional theory. Candidate surfaces were seleced on the basis of their surface free energies. Preferred CO and H adsorption sites were then identified. For CO dissociation, we considered direct as well as H- and C-assisted CO dissociation pathways. CO dissociation is most facile on stepped surface sites. The presence of subsurface C atoms in lattice interstices destabilizes CO adsorption and hinders CO dissociation. CO dissociation proceeds via a direct C-O bond scission mechanism on stepped sites. On surfaces that lack steppeds surface topologies, H-assisted CO dissociation should be considered. Properly migration-corrected activation barriers were used to estimate CO dissociation rates. The most active surface is the (111 ̅) surface cut in such way that the surface exposes Fe sites in a B5 arrangement.
Funding Information
  • Netherlands Center for Multiscale Catalytic Energy Conversion

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