Order–Disorder Transition in La_<i>x</i>Ce_2–<i>x</i>Zr₂O_7−δ Pyrochlore-fluorite Oxygen Carriers for Chemical Looping Dry Reforming of Methane

As a highly efficient and promising methane utilization technology, chemical looping dry reforming of methane (CLDRM) offers an environmentally friendly method for syngas production. The performance of the oxygen carriers (OCs) is determined by multiple properties such as thermal stability, oxygen-ion conductivity, oxygen capacity, and vacancies. In this study, a series of OCs were synthesized and characterized by adjusting the La3+/Ce3+ ratio at the A site of LaxCe2–xZr2O7−δ. Experiments were conducted in a fixed-bed reactor to study the thermal properties of the OCs. Results showed that the LaCeZr2O7−δ pyrochlore with a La3+/Ce3+ ratio of 1:1 exhibited the highest H2 production (1.01 mmol g–1), CO selectivity (58.4%), and optimal H2/CO ratio (1.97). As the La3+/Ce3+ ratio decreases, a phase transformation was observed from pyrochlore (La2Zr2O7−δ) to the defective fluorite (Ce2Zr2O7−δ), as characterized by XRD, BET, Raman, TEM/HRTEM, STEM-EDS, XPS, and H2-TPR/O2-TPO. The microstructural evolution driven by the cation antisite, anion Frenkel disorder, and defect clustering correlates well with the performance of the pyrochlore-fluorite OCs, demonstrating an optimal balance between the thermal stability and high selectivity of the pyrochlore and the oxygen capacity of the defective fluorite. This result can be viewed as an alternative method for enhancing the clean hydrogen production in chemical looping dry reforming application.