Oxygen adsorption-induced morphological evolution of Hägg iron carbide at high oxygen chemical potentials

Comprehension upon the mechanism of oxidative deactivation of carbides is of significance for extending their applications, in particular, for developing Fe-based catalysts with outstanding performance in Fischer-Tropsch synthesis (FTS). Here, combining density functional theory, ab initio atomistic thermodynamics, and Wulff construction, we have investigated the interaction between oxygen and nine χ-Fe₅C₂ surfaces, as well as the morphology change of the theoretically established Fe₅C₂ model at varied oxygen chemical potential under FTS-related reaction conditions. The calculations suggest that the surface oxygen preferentially bonds to Fe sites than to C (Cs) sites. A linear relationship was discovered between the C/Fe ratio of the surface and its O coverage under typical FTS conditions. The weaker bonding strength and lower O coverage at the same oxygen chemical potential on surfaces with a higher C/Fe ratio such as (110)_0.573, (111)_0.602, and (100)_0.402 mean that C-rich surfaces are resistant to oxidation. Elevating temperature and reducing partial pressure ratio of H₂O/H₂ will destabilize the O adsorption and at the same time lead to morphological change of the particle with the antioxidant facets contributing more to the total area, which is beneficial to keep the catalyst stable. Our atomistic insights shed light on the process of deactivation caused by oxidation.