Crystal facet-dependent reduction behavior of α-Fe2O3 in hydrogen atmosphere

Abstract

The reduction of α-Fe2O3 is significant in heterogeneous catalysis. However, due to the changeable crystal phase and the transformation of crystal facet, its reduction mechanism still needs further clarification. In this work, α-Fe2O3 rhombohedra, hexagonal-plate and pseudo-cubic with the facets of (104), (001) and (102) respectively were synthesized, and the evolutions of α-Fe2O3 structures during H2 reduction were investigated by multiple means including in-situ XRD. It found that the reduction of α-Fe2O3 showed a facet-dependent behavior in H2 atmosphere, that is different facets exhibited completely different rates and phase transition pathways. Specifically, the Fe2O3 (104) and Fe3O4 (220) had the fastest reduction rate in α-Fe2O3 → Fe3O4 and Fe3O4 → α-Fe, respectively, which was attributed to the higher concentration of oxygen vacancies on the corresponding crystal facets. The phase transition paths for Fe2O3 (102) and Fe2O3 (104) were α-Fe2O3 → Fe3O4 → α-Fe, while Fe2O3 (001) was α-Fe2O3 → Fe3O4 → FeO → α-Fe. Additionally, DFT calculations further revealed that Fe2O3 (104) and Fe3O4 (220) had lower interaction energy between O and Fe atoms and easily formed oxygen vacancy. Consequently, these facets presented different formation rates of H2O, leading to significant differences in the phase transition process.