Bulk and surface analysis of Hägg Fe carbide (Fe(5)C(2)): a density functional theory study

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.