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Abstract: In Fischer–Tropsch synthesis (FTS), the prepared catalyst hematite (α-Fe2O3) originates from goethite (α-FeOOH) dehydration in calcinations process. The structure of α-FeOOH and its topotactic transformation into α-Fe2O3 under 300–900K have been modeled within the framework of density functional theory. Molecular dynamics simulations show that the transformation begins preferentially on the surface and the dehydration is associated with the formation of voids. The loss of H2O in α-FeOOH does not modify the morphology extremely with respect to that of the original α-FeOOH. The transformation process involves hydrogen migration with the formation of adsorbed water, followed by desorption of the water molecule. The calculated barriers for hydrogen migration (118kJ/mol) and water desorption (82kJ/mol) suggest a direct reaction path without the formation of intermediates. [Copyright &y& Elsevier]

Abstract: Kinetic Monte Carlo simulations were carried out for Fischer–Tropsch synthesis in a wide range of industrially relevant reaction conditions. The macroscopic performance of different reaction conditions obtained from simulations quite agrees with experimental trends. In all examined conditions, H and CO are dominant species on catalyst surface. The rest intermediates only exist in small amount, and are controlled by H and CO coverages. Activity and selectivity for Fischer–Tropsch synthesis and water gas shift reaction are determined by surface H/CO ratio. Reaction conditions can directly change surface coverage of H, CO, and vacant sites, and consequently exert an influence on macroscopic performance of iron catalyst. [Copyright &y& Elsevier]

The structure and stability of seventeen facets of the Hägg carbide phase (χ-Fe 5 C 2 ) under the consideration of K 2 O promotion have been analyzed utilizing density functional theory approach and ab initio atomistic thermodynamics. On the basis of different interaction strengths of these facets with K 2 O promoter, the morphology of the χ-Fe 5 C 2 phase under the variation of K 2 O content has been predicated. At the surface Fe/K atomic ratio of 30, the morphology of the χ-Fe 5 C 2 phase can be modified apparently. Among the most exposed (1 1 1), (1 0 0), ( 1   1   1 ¯ ), (5 1 0) and ( 4 ¯   1   1 ) facets, the ( 1   1   1 ¯ ), (5 1 0) and ( 4 ¯   1   1 ) facets have more open surface structures as well as provide more surface Fe atoms and less carbon atoms. However, all these mostly exposed facets do not favor CO direct dissociation at low CO coverage. [ABSTRACT FROM AUTHOR]

Abstract: 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 (μ C) 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 μ C 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. [Copyright &y& Elsevier]