Bulk and surface analysis of Hägg Fe carbide (Fe₅C₂): a density functional theory study

Abstract

A comprehensive density functional theory (DFT) study analysing the bulk and various low Miller index surfaces of Hägg Fe carbide (Fe(5)C(2)), considered to be the active phase in Fe-catalysed Fischer-Tropsch synthesis (FTS), has been carried out. The DFT determined bulk structure of Hägg Fe carbide (Fe(5)C(2)) is found to be in good agreement with reported monoclinic (C 2/c) XRD data, independently of whether a monoclinic (C 2/c) or triclinic ([Formula: see text]) bulk structure is used as input for calculations. Attention is focused on the construction of a surface energy stability trend with subsequent correlation with particular surface properties. It is found that a (010) Miller index plane results in the most stable surface (2.468 J m(-2)), while a (101) surface is the least stable (3.281 J m(-2)). The systematic comparison of calculated surface energies with surface properties such as the number of dangling bonds and surface atom density (within a broken bond model), as well as unrelaxed surface energies, relative ruggedness of surfaces, degree of surface relaxation upon optimization, total spin density changes of surfaces compared to the bulk, etc, result in only an approximate correlation with the surface stability trend in selected cases. From the results it is concluded that the relative surface energies fall in a narrow range and that a large number of additional surfaces may be defined, e.g. from higher Miller index planes, sharing similar surface energy values. The results serve to demonstrate the rich complexity and diverse nature of the Fe carbide phase responsible for FTS, effectively laying the foundation for further fundamental studies.

Mars-van Krevelen-like Mechanism of CO Hydrogenation on an Iron Carbide Surface

“…In order to study CO hydrogenation on iron carbides, we have chosen the (100) surface of the Fe 5 C 2 Hagg carbide, with the unit cell shifted slightly downward to obtain the carbon-terminated surface, following the work of Steynberg et al [7]. This surface, shown in Fig.…”

“…A fully reduced iron metal catalyst shows no activity for FTS, but becomes active along with its conversion into carbides [3][4][5]. Ab initio studies providing new insights into the surface chemistry of iron carbide surfaces as FTS catalysts have been published by several authors [6][7][8]. Nevertheless, a mechanistic description in terms of elementary steps along with potential energy changes has not yet been presented for the CO hydrogenation over iron carbides and forms the subject of this communication.…”

“…We do this in a molecular modelling approach with density functional theory (DFT) starting from the carbon-terminated Fe 5 C 2 (100) [7]. We acknowledge that this surface is not necessarily the prevalent one under Fischer-Tropsch conditions, and that other Fe 5 C 2 surface structures, as considered by Steynberg et al [7] and by Sorescu [8] should also receive attention. However, the carbon-terminated Fe 5 C 2 (100) surface has the highest carbon content and is therefore anticipated to exhibit the lowest reactivity towards CO.…”

First-principles kinetic modeling in heterogeneous catalysis: an industrial perspective on best-practice, gaps and needs

“…the carbide formation that forms the active phase on Fe Fischer-Tropsch catalysts. 44 For composite materials like oxides, hydrides and sulfides the ongoing reactions may also (slowly) change the surface composition by consumption of lattice oxygen, hydrogen or sulfur, as e.g. in Mars-Van Krevelen oxidation mechanisms.…”

First-Principles Calculations on Stabilization of Iron Carbides (Fe3C, Fe5C2, and η-Fe2C) in Steels by Common Alloying Elements

“…The electronic, magnetic, and structural properties of Fe 3 C; Fe 5 C 2 ; and g-Fe 2 C have been reported. [7] Formation enthalpies, [7,33] surface properties, [34] thermodynamic properties, and formation enthalpies [6] of Fe 5 C 2 have also been described. Comparable work, excluding the surface properties, has been done on g-Fe 2 C: [6,7,15] Calculations have been interpreted to show that whereas Mn and Al stabilize -Fe 2 C; Si destabilizes it.…”