A BaFeO 3−δ (δ ≈ 0.22) perovskite was prepared by a sol−gel method and essayed as a catalyst in the CO oxidation reaction. BaFeO 3−δ (0.22 ≤ δ ≤ 0.42) depicts a 6H perovskite hexagonal structural type with Fe in both III and IV oxidation states and oxygen stoichiometry accommodated by a random distribution of anionic vacancies. The perovskite with the highest oxygen content, BaFeO 2.78 , proved to be more active than its lanthanide-based counterparts, LnFeO 3 (Ln = La, Sm, Nd). Removal of the lattice oxygen detected in both temperature-programmed oxidation (TPO) and reduction (TPR) experiments at around 500 K and which leads to the complete reduction of Fe 4+ to Fe 3+ , i.e. to BeFeO 2.5 , significantly decreases the catalytic activity, especially in the lowtemperature range. The analysis of thermogravimetric experiments performed under oxygen and of TPR studies run under CO clearly support the involvement of lattice oxygen in the CO oxidation on these Ba-Fe perovskites, even at the lowest temperatures. Atomically resolved images and chemical maps obtained using different aberration-corrected scanning transmission electron microscopy techniques, as well as some in situ type experiments, have provided a clear picture of the accommodation of oxygen nonstoichiometry in these materials. This atomicscale view has revealed details of both the cation and anion sublattices of the different perovskites that have allowed us to identify the structural origin of the oxygen species most likely responsible for the low-temperature CO oxidation activity.

Eliminating or reducing the amount of noble metals and rare earths in catalysts is a primary issue to reach the goal of sustainable development. The substitution of noble metals by 3d metals is receiving increasing attention. Related to this strategy, ABO 3 perovskite oxides (A=lanthanide and B=Fe, Co, Mn) are being essayed as catalysts in many reactions. Substitution of La by an alkaline earth has been reported to modify the catalytic activity of such perovskites. [1] To balance the charge deficiency produced by the incorporation of divalent ions into the A sublattice, either the B element increases its oxidation state or oxygen vacancies are created or, even more, both processes occur simultaneously. In general terms, a positive effect on the catalytic activity is observed on methane, propene and hydrogen oxidation reactions within the whole temperature range, after the addition of barium in Mn, Fe and Co perovskites. On the basis of these ideas, we have considered of interest exploring the catalytic behavior of a lanthanide‐free, BaFeO 3‐δ perovskite in CO oxidation, putting emphasis on the influence of redox pre‐treatments on catalytic activity and on the possible involvement of lattice oxygen in the process. This particular process was firstly of interest in automotive pollution control devices but more recently for cutting‐edge technologies related to production of hydrogen for fuel cells. Though it is admitted that the catalytic behavior of perovskites is intimately related to the nature of their defects, studies focused on the detailed analysis of the nature of such defects at atomic scale are systematically lacking. Therefore, this contribution aims at illustrating the large potential of diffraction data combined with AC‐STEM to establish structure‐function correlations. As an illustration, we have explored the involvement of lattice oxygen in the catalytic activity of BaFeO 3‐δ perovskite for the CO oxidation. BaFeO 3‐δ (0.2<δ<0.4) depicts a 6H perovskite hexagonal structural type, with Fe both in (III) and (IV) oxidation states and oxygen nonstoichiometry accommodated by random distribution of anionic vacancies. [2] Analysis of the redox properties of 6H‐BaFeO 2.78 reveals the involvement of two types of lattice oxygen atoms in the CO oxidation over BaFeO 3‐δ . On its hand, an exhaustive study combining different diffraction techniques ( in‐situ high temperature SAED, X‐ray diffraction, powder neutron diffraction) and aberration‐corrected STEM techniques, both in imaging and spectroscopic modes, have provided us with an atomically resolved picture of the accommodation of oxygen‐non‐stoichiometry. Thus, mapping of Ba, Fe and O atomic columns, Figure 1, have allowed us identifying the distribution of all the elements in the sample in the …cchcch… sequence expected for the 6H‐polytype. No systematic differences have been observed between the hexagonal and cubic layers in BaFeO 3‐δ this meaning, in agreement to NPD and ABF results, that there is not a preferential distribution of the oxygen vacancies along the different layers. From the whole set of results obtained it has been possible to propose the role of the oxygen deficiency responsible for the low temperature CO oxidation activity of BaFeO 3‐δ .