Surface morphology of Hägg iron carbide (χ-Fe5C2) from ab initio atomistic thermodynamics

Ab initio atomistic thermodynamics is utilized to achieve the understanding of the surface structure and stability of χ-Fe5C2 under CO and synthesis gas (H2/CO) pretreatment conditions in catalyst activation for Fischer–Tropsch synthesis (FTS). On the basis of the computed surface free energy (γ) as a function of the carbon chemical potential (μC) by considering temperature, pressure, and H2/CO ratios, it is found that CO pretreatment favors stable carbon-rich facets, while small amount of H2 added into CO leads to a large decrease in μC and thus stabilizing carbon-poor facets. The high activity of surface carbon toward hydrogenation might explain the enhanced initial activity of the FTS catalysts activated from CO pretreatment. Moreover, under both CO and H2/CO pretreatments, either low temperature or low pressure can lead to stable carbon-rich facets.

Graphical abstractAb initio simulations demonstrate that the surface facetting of the most common iron carbide observed during Fischer–Tropsch synthesis depends on the gaseous environment.Figure optionsDownload full-size imageDownload high-quality image (72 K)Download as PowerPoint slideHighlights► Morphology of χ-Fe5C2 was investigated under CO and H2/CO pretreatment. ► CO favors stable C-rich facets, while H2/CO stabilizes C-poor facets. ► Low temperature and pressure favor C-rich facets under CO and H2/CO pretreatment. ► Very high H2/CO ratio makes χ-Fe5C2 phase unstable thermodynamically.