Iron oxides with a reverse spinel structure: impact of active sites on molecule adsorption

Abstract

Fe₃O₄ and γ-Fe₂O₃ with a reverse spinel structure have been widely applied as environmentally benign catalysts. Clarifying the catalytically active sites (CASs) of Fe₃O₄ and γ-Fe₂O₃ plays a crucial role in improving the efficiency of related reactions. However, the surface adsorption and electronic properties at the atomic level of Fe₃O₄ and γ-Fe₂O₃ are not fully understood. Here, the molecules adsorbed at the tetrahedral (Fe_tet) and octahedral (Fe_oct) sites on the Fe₃O₄ and γ-Fe₂O₃ surfaces are considered by density functional theory with the Hubbard-U method. According to the results of adsorption energy, as a whole, γ-Fe₂O₃ is more favorable to adsorb oxygen than Fe₃O₄. Furthermore, the electronic structure and periodic natural bond orbital results clearly reveal that O₂ at the octahedral sites of Fe₃O₄ and at the tetrahedral sites of γ-Fe₂O₃ obtains more charge; namely, the Fe_oct ions of Fe₃O₄ and the Fe_tet ions of γ-Fe₂O₃ act as CASs in the activation of O₂. To ensure the accuracy of the results, the adsorption of CO, H₂O, NH₃ and NO is calculated comparatively. The results suggest that Fe₃O₄ and γ-Fe₂O₃ with the same crystal structure reflect different CASs. This work provides a foundation for understanding the redox and catalytic application of iron oxides; more importantly, the different CASs of Fe₃O₄ and γ-Fe₂O₃ have not been proposed before.