Insights Into Low-Temperature Cation Ordering in Fe-Added Ce-Zr-Based Oxides.

Area of Science:

• Materials Science
• Catalysis
• Ceramic Engineering

Background:

• Ceria-zirconia (CZ) solid solutions are key components in automotive exhaust systems due to their high oxygen storage capacity (OSC).
• The cation-ordered phase, κ-Ce₂Zr₂O₈, exhibits superior OSC but requires high-temperature processing (>1200 °C), leading to detrimental grain growth.
• Iron oxide addition has been shown to lower the ordering temperature in equimolar CZ, but its effect on Zr-rich CZ requires further investigation.

Purpose of the Study:

• To investigate the low-temperature cation ordering of Zr-rich ceria-zirconia (CZ) solid solutions by incorporating Fe₂O₃.
• To elucidate the role of the reduction atmosphere in the Fe₂O₃-assisted cation ordering process.
• To understand the mechanism of Fe₂O₃ dissolution and its impact on cation migration in CZ ceramics.

Main Methods:

• High-temperature in situ X-ray Diffraction (XRD) was employed to monitor the cation ordering.
• Experiments were conducted under controlled oxygen partial pressures during reduction.
• The influence of different reducing atmospheres on Fe₂O₃ speciation and CZ ordering was analyzed.

Main Results:

• Fe₂O₃ addition successfully promoted cation ordering in Zr-rich CZ at significantly lower temperatures than previously reported.
• A weakly reducing atmosphere favored CZ ordering, as Fe remained in an ionic state, facilitating cation diffusion.
• A strongly reducing atmosphere led to the formation of metallic Fe, which hindered the cation ordering process.

Conclusions:

• Low-temperature cation ordering of Zr-rich CZ is achievable with Fe₂O₃ addition.
• The nature of the reduction atmosphere critically influences the effectiveness of Fe₂O₃ as a promoter for cation ordering.
• Optimizing the reduction conditions is essential for leveraging Fe₂O₃-promoted ordering to enhance the performance of CZ-based catalysts.