Exploring Reversible Redox Behavior in the 6H-BaFeO_3-δ (0 < δ < 0.4) System: Impact of Fe³⁺/Fe⁴⁺ Ratio on CO Oxidation

This work is devoted to evaluating the relationship between the oxygen content and catalytic activity in the CO oxidation process of the 6H-type BaFeO_3-δ system. Strong evidence is provided about the improvement of catalytic performance with increasing Fe average oxidation state, thus suggesting the involvement of lattice oxygen in the catalytic process. The compositional and structural changes taking place in both the anionic and cationic sublattices of the catalysts during redox cycles have been determined by temperature-resolved neutron diffraction. The obtained results evidence a structural transition from hexagonal (P6₃/mmc) to orthorhombic (Cmcm) symmetry. This transition is linked to octahedra distortion when the Fe³⁺ concentration exceeds 40% (δ values higher than 0.2). The topotactical character of the redox process is maintained in the δ range 0 < δ < 0.4. This suggests that the cationic framework is only subjected to slight structural modifications during the oxygen exchange process occurring during the catalytic cycle.