Theoretical insights into the topotactic transformation of intermediate γ-Mo2N into α-Mo2C by carburization

Abstract

The carburization process of γ-Mo₂N to α-MoC_1−x under the atmosphere of CH₄ and H₂ has been investigated by using DFT calculations and ab initio molecular dynamics (AIMD) simulations. It has been shown that H₂ treatment is essential to remove the surface N atoms of γ-Mo₂N as NH₃ via a quasi-Mars-van-Krevelen (quasi-MvK) mechanism, thus creating vacancies that facilitate sequential CH₄ dissociation on the γ-Mo₂N surface. The subsequent carburization occurs through the simultaneous diffusion of surface carbon atoms from the CH₄ dissociation into the bulk and the bulk nitrogen atoms to the surface, where they are removed as NH₃. This concerted circulation clearly demonstrates the representative topotactic carburization mechanism of structural evolution from γ-Mo₂N to α-Mo₂C.