In Situ Cw ESR Study on Redox Behavior and Phase Heterogeneity in A-Site-Deficient Lanthanum Iron Manganite Perovskite Catalysts

Abstract

Temperature-dependent in situ continuous wave (cw) electron spin resonance (ESR) measurements are used for investigating changes in magnetization upon reduction and reoxidation in La-deficient La_xFe_0.7Mn_0.3O₃ (x < 1) samples to advance the understanding of structural and chemical changes in these materials where A-site deficiency was shown to affect catalytic performance. For these defect-rich mixed perovskites with ferromagnetic ordering, the magnetic properties as characterized by in situ cw ESR spectroscopy are sensitive probes for structural changes in these materials. To this end, the ESR spectra observed in a redox cycle in H₂- and O₂-containing atmospheres not only show structural changes that were not observed by XRD but also provide evidence for heterogeneity in the magnetic phases, which notably depends on the La deficiency of the samples. This not only demonstrates the potential of such investigations for obtaining information complementary to other methods but also emphasizes the sensitivity of magnetic properties as probed by ESR to elucidate structural and chemical changes in such complex perovskite materials. While the XRD results lack indication for the presence of structural heterogeneity, STEM measurements provide evidence for a compositional heterogeneity between the grains but not for the presence of an additional magnetic phase, as observed by ESR for one of the samples. Importantly, the different magnetic phases exhibit distinct responses to reducing and (re)oxidizing atmospheres indicating for the sample with a lower La deficiency a facilitated reaction under reducing conditions at low temperatures but an overall higher structural stability. Both effects are expected to affect the reactivity in the redox reactions. Thus, these results provide new and complementary insights that can enhance the understanding of the effect of A-site deficiency in perovskite materials in redox reactions considered to be important for the catalytic activity of these systems.