Reduction and Oxidation of Maghemite (001) Surfaces: The Role of Iron Vacancies

Abstract

The knowledge of surface reduction and oxidation energetics in reducible oxides is essential for the design of improved catalysts for oxidation reactions. This is particularly true for iron oxides, a very attractive material system, because of the availability and biocompatibility of its constituents. In this work, by means of the density functional theory, we have thoroughly studied the γ-Fe 2 O 3 (001) maghemite surfaces, taking full account of iron vacancies beyond a mean field approach. Despite the structural similarity with the more studied magnetite Fe 3 O 4 surfaces from which maghemite differs only for the presence of iron vacancies in the octahedral sites and for the absence of reduced Fe 2+ cations, the surface properties are quite different. Our investigation shows that the presence of Fe vacancies is responsible for an increase in the surface reducibility. Also, it favors surface oxidation. The main reason is that the Fe vacancies cause a decrease of electronic charge of the surface oxygen atoms, which then become more reactive. We have considered different surface terminations and found that the reduced surfaces are more stable than the simple bulk-truncated ones. The reduction leads to a new reconstruction of the surface, which is the most stable surface termination among those investigated. We have examined the charge transfers and the modifications in the electronic structure caused by the surface reduction.

On the first evidence of exchange-bias feature in magnetically contrasted consolidates made from CoFe2O4-CoO core-shell nanoparticles

“…It is interesting to note that the ceramic produced was found to be exactly consistent with a pure magnetite while its parent powder was confirmed to be a pure maghemite (see Figure SI-3). This means that during SPS processing at temperatures as low as 500 °C, ferrous cations may be formed while oxygen atoms depart from the spinel lattice stabilizing the Fe III 2 Fe II 1 O 4 phase starting from the Fe III 2 O 3 one, and thus supporting the second hypothesis instead of the first and the third 42,43 :…”

Descriptors Affecting Methane Selectivity in CO2 Hydrogenation over Unpromoted Bulk Iron(III)-Based Catalysts

“…The structures differ only in the presence of iron vacancies in the octahedral sites and in the absence of reduced Fe 2+ cations in the maghemite phase. 47 Thus, no significant structural changes happen upon the reduction of γ-Fe 2 O 3 to Fe 3 O 4 , that is, favorable kinetically. On the other hand, α-Fe 2 O 3 belongs to the trigonal crystal system and comprises a hexagonal close-packed oxygen lattice, in which two-thirds of the octahedral sites are occupied by ferric cations.…”

Free-Radical Formation by the Peroxidase-Like Catalytic Activity of MFe2O4 (M = Fe, Ni, and Mn) Nanoparticles

“…Besides the surface, the structure and superficial plane are also important in the peroxidase-like of oxides with spinel structure, including the maghemite and the magnetite 53 , among others 3d transition metal oxides with similar structure 54 . Following, the surface reduction and oxidation kinetics in the oxide system also play fundamental roles in the catalytic activity of the metal oxide systems, as studied by Righi et al 55 . These authors showed theoretically that the vacancies are a determining factor for the oxidation/reduction reactions in the (100) plane of the maghemite, which can be related with the Haber-Weiss chain reactions through the reaction described in eq.…”

“…54 Further, the surface reduction and oxidation kinetics in the oxide system also play fundamental roles in the catalytic activity of the metal oxide systems, as studied by Righi and Magri. 55 These authors showed theoretically that the vacancies are a determining factor for the oxidation/reduction reactions in the (100) plane of the maghemite, which can be related with the Haber−Weiss chain reactions through the reaction described in eq 1. In addition, Wang et al 56 showed recently that the population of e g orbitals plays a fundamental role in the peroxidase-like catalytic activity of perovskite oxide-based nanosystems.…”