High-temperature phase evolution in CuO/Al₂O₃ oxygen carriers: insights from in situ quick XAS

Area of Science:

• Materials Science
• Chemical Engineering
• Catalysis

Background:

• Chemical looping combustion (CLC) utilizes oxygen carriers for efficient fuel combustion and CO2 capture.
• The stability and performance of CuO/Al2O3 oxygen carriers are crucial for practical CLC applications.
• Understanding phase transitions during redox cycling is key to predicting and improving material lifetime.

Purpose of the Study:

• To investigate the phase transitions of CuO/Al2O3 oxygen carriers during extended chemical looping combustion (CLC) redox cycles.
• To elucidate the degradation mechanisms affecting oxygen carrier performance and stability.
• To correlate structural and morphological changes with cycle number and operating conditions.

Main Methods:

• In situ quick X-ray absorption spectroscopy (QXAS) to monitor copper speciation and phase evolution.
• Exposure to 50 redox cycles under varying oxygen (2.5-21% O2) and fuel (H2, CO, CH4) conditions.
• Scanning Electron Microscopy (SEM) for morphological analysis and particle characterization.

Main Results:

• Significant phase transitions observed in CuO/Al2O3 carriers over 50 redox cycles.
• Copper aluminate (CuxAlyO4) predominantly converted to copper oxide and alpha-Al2O3 at higher cycle numbers.
• SEM revealed particle growth and agglomeration preceding alpha-Al2O3 formation, indicating structural degradation.
• Irreversible structural modifications were linked to the interplay between copper phase changes and alumina formation.

Conclusions:

• The study reveals a critical threshold for irreversible phase transformation in CuO/Al2O3 oxygen carriers.
• Accelerated aging mechanisms involve the transformation of copper aluminate and the formation of stable alpha-Al2O3.
• These findings provide crucial insights for designing more durable oxygen carriers for enhanced chemical looping combustion applications.