Atomistic Insights into Early-Stage Oxidation of χ-Fe5C2 and the Dependence on the Surface Structure

Iron carbides play a crucial role in many industrial fields, such as the steel industry, catalysis, carbon material production, and so on. One of the main challenges is that iron carbides are highly susceptible to being oxidized under service conditions, which can significantly reduce the performance and retard their industrial applications. However, the complex chemical structure of iron carbide materials and the dynamic nature of the Fe–C–O system under reactive conditions, such as the Fischer–Tropsch synthesis condition, hindered the understanding of the oxidation process. In this paper, structure evolution of the early-stage oxidation of χ-Fe5C2 was investigated through reactive force field molecular dynamics simulations. The impacts of surface structure, oxygen coverage, temperature, and particle size were systemically explored. It was found that the oxidation rate was not only related to the surface carbon content but also to the homogeneity of the subsurface. Meanwhile, the dynamic structure transformation during oxidation was analyzed by various correlation functions. The mechanism of counter-direction migration of surface atoms during the oxidation process was illustrated, where carbon atoms migrate to the bulk region and iron atoms migrate to the surface. Our study reveals the atomistic dynamic mechanism of χ-Fe5C2 oxidation, which offers crucial insights for catalyst oxidation and iron carbide materials.