Facet-dependent reaction mechanism of Fe-based oxygen carrier for CH4 chemical looping combustion

JC Xiong and MR Dong and ZH Huang and HC Liu and HM Hou and YC Liang and JD Lu, INTERNATIONAL JOURNAL OF HYDROGEN ENERGY, 105, 1153-1163 (2025).

DOI: 10.1016/j.ijhydene.2025.01.342

Chemical looping combustion (CLC) technology decouples the traditional combustion process into two gas-solid reactions. The crystal facet effect is a crucial factor in gas-solid reactions. However, the chemical looping combustion mechanism associated with the crystal facet remains unclear. In this work, we investigated Fe2O3 crystal facet on the performance and reaction mechanism of CH4 chemical looping. Hex-, cubic and rh-like Fe2O3 with predominantly exposed (001), (012), and (104) facets were prepared, and their properties were tested in a fixed bed, respectively. The results show that the reactivity of Fe2O3 exhibits a clear facet dependence, with the order of reactivity being (104)>(012)>(001), which correlates with the high reactive oxygen content on the corresponding facet by X-ray photoelectron spectroscopy (XPS). Density functional theory (DFT) calculations further confirm that Fe2O3(104) exhibits the lowest oxygen vacancy formation energy, providing a favorable low energy barrier channel for lattice oxygen migration. The differences in crystallographic surfaces leads to an upward shift of the O-2p band of the oxygen carriers, enhancing the lattice oxygen activity. Moreover, molecular dynamics and detailed reaction pathways indicate that the rate-determining step (RDS) for Fe2O3(001) and Fe2O3(012) is the oxidation of methoxy, whereas the RDS for Fe2O3(104) is the oxidation of formate with a lower activation energy barrier. The facet-dependent oxygen mobility and reaction pathways result in a distinct catalytic performance for chemical looping combustion of CH4. This study presents a general strategy based on facet engineering to improve the performance of oxygen carriers.

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