Chemical looping combustion of methane, as an emerging low-carbon combustion technology, employs solid oxygen carriers to transfer oxygen between the fuel reactor and the air reactor, enabling the temporal and spatial separation of the conventional fuel combustion process without the direct contact between fuel and air. This technology offers significant advantages such as inherent CO₂ separation and near-zero NO_x emissions, making it an important technological pathway for achieving carbon neutrality. As the medium for oxygen transfer, oxygen carriers are cyclically transported between the fuel reactor and the air reactor, where their oxygen carrying capacity and cyclic stability are critical. Iron-based oxygen carriers have attracted extensive attention due to their low cost and environmental friendliness. This paper proposes a strategy that utilizes both the high iron capacity characteristics of hexaferrites (AFe₁₂O₁₉) structure and A-site cation regulation to overcome the performance bottlenecks of conventional iron-based oxygen carriers. In this work, a series of La-doped La_xSr_1-xFe₁₂O₁₉ (x = 0, 0.2, 0.4, and 0.6) oxygen carriers were synthesized via a coprecipitation method, and the effects of La substitution on phase structure, lattice oxygen mobility, redox activity, and cyclic stability were systematically investigated. The results of fixed-bed reactor evaluation show that, the oxygen carrier with an appropriate La doping (x = 0.2) achieves an O_t as high as 3.0 mmol g^-1 while maintaining 100% CO₂ selectivity, which is approximately 1.67 times the theoretical O_t of Fe₂O₃/Fe₃O₄ redox pair. After 50 redox cycles, the CH₄ conversion remains between 92% and 99%, and the O_t is maintained at 2.13 mmol·g^-1~2.30 mmol·g^-1, achieving both high O_t and high cyclic stability. Characterization results including XRD, BET, H₂-TPR, SEM, and XPS reveal that, the overall performance of the oxygen carriers was enhanced by La doping through the following mechanisms: (1) suppressing the formation of the SrFe₂O₅ impurity phase, increasing the specific surface area from 4.2 m²·g^-1 to 6.2 m²·g^-1~6.9 m²·g^-1, and promoting lattice oxygen mobility; (2) altering the structural evolution pathway during reduction, thereby promoting the preferential formation of low-valence Sr₄Fe₆O₁₂ with an average Fe valence state of +2.67, which breaks through the redox window limitation of the conventional Fe₂O₃/Fe₃O₄ system; and (3) enhancing the reversible regeneration capability and sintering resistance of the hexaferrite structure, thus enabling the Fe species to recover to a near-initial state even after 50 redox cycles. This study demonstrate that A-site La doping to regulate the structural evolution of hexaferrites provides an effective strategy to overcome the trade-off effect in iron-based oxygen carriers, which paves the way for rational design of high-performance iron-based oxygen carriers.

Key words: Chemical looping combustion, Fe-based oxygen carrier, La doping, oxygen carrying capacity, cyclic stability