A Computational Study of K Promotion of CO Dissociation on Hägg carbide

The dissociation of CO is a critical step in producing long-chain hydrocarbons in the Fischer-Tropsch (FT) synthesis reaction. Although potassium (K) is known to enhance CO conversion and the selectivity to olefins of Fe-carbide FT catalysts, its precise mechanistic role remains unclear. In this work, we used density functional theory to show that K ₂O facilitates C-O bond dissociation in CO, HCO, and COH by increasing the electron density of the Fe surface atoms of Hägg carbide (χ-Fe ₅C ₂) that bind these surface intermediates. This leads to a higher electron density in anti-bonding orbitals and enhanced electron-electron repulsion between the bonding orbitals of the CO, COH, and HCO intermediates and the Fe atoms. Effective promotion of C-O bond dissociation requires K to be adjacent to the active site on the χ-Fe ₅C ₂ surface.