Critical Influence of Redox Pretreatments on the CO Oxidation Activity of BaFeO_3−δ Perovskites: An in-Depth Atomic-Scale Analysis by Aberration-Corrected and in Situ Diffraction Techniques

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, LnFeO₃ (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 Fe⁴⁺ to Fe³⁺, i.e. to BeFeO_2.5, significantly decreases the catalytic activity, especially in the low-temperature 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 atomic-scale 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