Emerging Potential of Eu 2 O 2 SO 4 in Reversible Oxygen Storage: A Comparative Study with Pr 2 O 2 SO 4

Rare-earth oxysulfates (RE 2 O 2 SO 4, where RE = lanthanides) have emerged as promising candidates for high-capacity oxygen storage, particularly in moderate- to high-temperature applications. In this study, a detailed comparative investigation was conducted on a well-studied praseodymium oxysulfate system and the relatively unexplored europium analogue. Both materials were synthesized via a precipitation method and systematically reduced to their corresponding oxysulfides (RE 2 O 2 S) using H 2 /N 2 flow. Redox and structural behaviors were systematically analyzed by using temperature-programmed reduction (TPR), thermogravimetric oxidation, X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and X-ray absorption near-edge spectroscopy (XANES). Both systems exhibit a reversible monoclinic to hexagonal phase transformation during redox cycling, along with exceptionally high oxygen storage capacities. Eu 2 O 2 SO 4 demonstrated a marginally earlier reduction onset compared to Pr 2 O 2 SO 4, attributed to its distorted oxysulfate lattice observed in Raman spectra and its surface multivalence (Eu 3+ /Eu 2+ ) as revealed by XPS. The presence of a dominant oxygen vacancy peak in the O 1s XPS spectrum of Eu 2 O 2 S suggests an enhanced surface defect concentration, correlating with faster reoxidation behavior. This comprehensive study elucidates the redox mechanisms in rare-earth oxysulfates and positions Eu 2 O 2 SO 4 as a promising yet underexplored oxygen storage material. The findings pave the way for future optimization strategies to further optimize these materials for versatile redox and oxygen storage applications across a wide temperature range.