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 (Fetet) and octahedral (Feoct) 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 Feoct ions of Fe3O4 and the Fetet 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.