CO Dissociation at Vacancy Sites on Hagg Iron Carbide: Direct Versus Hydrogen-Assisted Routes Investigated with DFT

摘要

The mechanism of activation of CO remains under debate in Fe-catalysed Fischer-Tropsch synthesis, in which iron carbides form under reaction conditions. Direct and H-assisted paths for CO activation and dissociation are investigated at carbon vacancy and non-vacancy sites on the Fe5C2(010) surface of Hagg iron carbide using density functional theory. The calculated overall energy barrier for direct and for H-assisted dissociation of CO via formation of an HCO intermediate is the same, 1.42 and 1.41 eV, respectively, but the lowest energy paths are facilitated by different vacancy sites. Furthermore, dissociation at a non-vacancy site is only marginally higher in energy by 0.1 eV. Dissociation through formation of a hydroxymethylidyne (COH) intermediate is less competitive kinetically.