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

• Published in: Journal of catalysis Vol. 294; pp. 47 - 53
• Main Authors: Zhao, Shu, Liu, Xing-Wu, Huo, Chun-Fang, Li, Yong-Wang, Wang, Jianguo, Jiao, Haijun
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier Inc 01.10.2012; Elsevier; Elsevier BV
• Subjects: Ab initio atomistic thermodynamics; Carbides; carbon; Carbon chemical potential; Catalysis; catalysts; Chemistry; energy; Exact sciences and technology; Fischer–Tropsch synthesis; General and physical chemistry; hydrogen; Hydrogenation; Hägg carbide; Morphology; Surface free energy; Surface morphology; synthesis gas; temperature; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; Thermodynamics; Ab initio atomistic thermodynamics; Carbon chemical potential; Hägg carbide; Surface free energy; Surface morphology; Fischer–Tropsch synthesis; Structure stability; Activation; Iron carbide; Synthesis gas; Hydrogenation; Surface structure; Thermodynamics; Pretreatment; agg carbide; Low temperature; Fischer Tropsch synthesis; Carbon; Free energy; Heterogeneous catalysis; Fischer-Tropsch synthesis; Morphology; Low pressure; Chemical potential; Thermodynamic properties; Catalyst; Surface energy
• ISSN: 0021-9517; 1090-2694
• DOI: 10.1016/j.jcat.2012.07.003

Ab initio simulations demonstrate that the surface facetting of the most common iron carbide observed during Fischer–Tropsch synthesis depends on the gaseous environment. [Display omitted] ► 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. 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.