Fluorite-Dominated High-Entropy Composite Oxide for Chemical Looping Steam Reforming of Biomass Tar

High-entropy oxides (HEOs) have recently been proposed as promising catalysts for tar cracking, a major challenge in biomass gasification. However, their catalytic performance and reaction mechanisms remain insufficiently understood. In this work, fluorite-dominated high entropy composite oxides (HECOs) with different fuel-to-oxidizer oxygen-equivalent ratios (F/O) were synthesized by the glycine-nitrate process (GNP). The as-prepared HECOs (Fe0.2Co0.2Ni0.2Mg0.2Ce0.2)O2–δ were applied as oxygen carriers (OCs) for the chemical looping steam reforming of toluene (CL-SRT), a model compound of biomass tar. Results showed that the synthesized HEOs presented abundant surface porosity and uniform elemental distribution. The higher F/O ratio and temperature improved the CL-SRT performance, while the S/C ratio effectively tuned product composition. Under the conditions of 800 °C and S/C of 2.4, the OC of F/O = 2.0 achieved the optimal reforming performance, with a syngas yield and purity of 4.01 L/g and 86.30%, respectively. The longtime life cycle performance test showed that the toluene conversion stayed at about 97.36% and syngas purity remained near 86%, with only slight carbon deposition observed. The structural characterization suggested that the dynamic exsolution of FeNi alloy may generate additional active sites for toluene conversion, contributing to the enhanced reforming performance. These results highlight fluorite-dominated HECOs as viable oxygen carriers for efficient biomass tar reforming and H2-rich syngas production.