Atomic Structural Evolution during the Reduction of α-Fe₂O₃ Nanowires

Abstract

The atomic-scale reduction mechanism of α-Fe2O3 nanowires by H2 was followed using transmission electron microscopy to reveal the evolution of atomic structures and the associated transformation pathways for different iron oxides. The reduction commences with the generation of oxygen vacancies that order onto every 10th (303false¯0) plane. This vacancy ordering is followed by an allotropic transformation of α-Fe2O3 → γ-Fe2O3 along with the formation of Fe3O4 nanoparticles on the surface of the γ-Fe2O3 nanowire by a topotactic transformation process, which shows 3D correspondence between the structures of the product and its host. These observations demonstrate that the partial reduction of α-Fe2O3 nanowires results in the formation of a unique hierarchical structure of hybrid oxides consisting of the parent oxide phase, γ-Fe2O3, as the one-dimensional wire and the Fe3O4 in the form of nanoparticles decorated on the parent oxide skeleton. We show that the proposed mechanism is consistent with previously published and our density functional theory results on the thermodynamics of surface termination and oxygen vacancy formation in α-Fe2O3. Compared to previous reports of α-Fe2O3 directly transformed to Fe3O4, our work provides a more in-depth understanding with substeps of reduction, i.e., the whole reduction process follows: α-Fe2O3 → α-Fe2O3 superlattice → γ-Fe2O3 + Fe3O4→ Fe3O4.

The effect of Fe2O3 crystal phases on CO2 hydrogenation

“…However, the specific surface area of P-3 is less than P-2 regardless of the particle size in P-3 (16.3 nm) is smaller than P-2 (17.7 nm). This contradiction can be solved by the fact that the theoretical densities of γ-Fe 2 O 3 (5.47 g·cm -3 ) is higher than α-Fe 2 O 3 (5.27 g·cm -3 ) [28]. According to Fig 1, P-2 is a mixture of α-Fe 2 O 3 and γ-Fe 2 O 3 , while P-3 is composed of pure γ-Fe 2 O 3 .…”

“…The peak temperature corresponding to Fe 2 O 3 → Fe 3 O 4 reduction is 340°C for P-1, while it is around 310°C for P-2 and P-3 [38,39]. Because γ-Fe 2 O 3 is similar to Fe 3 O 4 in view of their crystal structure [28,40], γ-Fe 2 O 3 is more easily reduced to Fe 3 O 4 than α-Fe 2 O 3 . The weak peak at 350°C in P-3 is the reduction of α-Fe 2 O 3 which was produced from meso-stable γ-Fe 2 O 3 .…”

“…In these studies, iron oxide almost presents in α-Fe 2 O 3 [15,21,26] or Fe 3 O 4 [25] crystal phase in the as-prepared catalysts. Considering that γ-Fe 2 O 3 is one kind of iron oxide as common as α-Fe 2 O 3 [27,28], it is surprising that there are very few reports about the behavior of γ-Fe 2 O 3 in CO 2 hydrogenation. Al-Dossary et al found γ-Fe 2 O 3 coexisted with α-Fe 2 O 3 in the catalysts, but no benefit from γ-Fe 2 O 3 was disclosed [21].…”

Transmission electron microscopy analysis of some transition metal compounds for energy storage and conversion

“…Hence, investigation on the oxygen-deficient structure provides a fundamental Similarly, experimental and simulated HRTEM investigation revealed a long-range oxygen-vacancy ordering planes in α-Fe 2 O 3 nanowires and nanobelts [78,79]. The occurrence of these superstructures could be explained by the formation of oxygen-vacancy-ordered planes.…”

Solid‐State Diffusional Behaviors of Functional Metal Oxides at Atomic Scale

“…The phase of iron oxide should be carefully identified due to the several oxidation states of iron oxide. The relative height and position of the peaks provides information about the valence state of Fe and the characteristics of the Fe–O bond . The threshold energy (prepeak) and the intensity ratio of the peaks located at 532 and 542 eV are related to the O 1s transformation to the unoccupied states of O 2p for bonding with the Fe 3d orbitals, which indicates that the iron oxide was composed of Fe 3 O 4 .…”