High-temperature phase evolution in CuO/Al ₂ O ₃ oxygen carriers: insights from <i>in situ</i> quick XAS

This study investigates phase transitions in CuO/Al₂O₃ oxygen carriers during chemical looping combustion (CLC), aiming to understand performance and stability over extended redox cycles. <i>In situ</i> quick X-ray assorption spectroscopy (QXAS) was employed to track the transformations of the copper aluminate phase (Cu_<i>x</i>Al_<i>y</i>O₄) over 50 redox cycles in various oxidizing (2.5 to 21% O₂ in N₂) and reducing (H₂, CO, CH₄) environments. The study reveals that the oxygen carrier undergoes significant phase transitions, reaching a threshold where Cu_<i>x</i>Al_<i>y</i>O₄ predominantly converts to copper oxide and α-Al₂O₃, leading to irreversible structural modifications. Complementary SEM analysis further highlights morphological changes, such as particle growth prior to α-Al₂O₃ formation. This cycle-dependent phase evolution provides new insights into accelerated ageing mechanism involving the interplay between copper phase transformations and α-Al₂O₃ formation, which is critical for enhancing the durability of oxygen carriers in CLC applications.