Solar thermochemical CO₂ splitting (STCDS) holds considerable potential for producing solar fuels since it is capable to harnessing abundant sunlight directly and store solar energy in the renewable fuel. This process involves two steps: The metal oxide releases its lattice oxygen under an inert environment, and then the reduced metal oxide is reoxidized by CO₂ to replenish its lattice oxygen for producing CO. The main challenge associated with STCDS is the extremely high temperature (greater than 1500 ^oC) required for the first step. The introduction of reducing agents like methane may substantially reduce the decomposition temperature by lowering the external partial pressure of oxygen and co-produce syngas, but it may also lead to carbon deposition on the catalyst surface by methane cracking, especially in the case of inexpensive and readily available iron-based catalysts. High potential to carbon deposition over metallic Fe⁰ greatly limits the improvement of methane-to-syngas selectivity and CO yield via the two-step STCDS. Herein, we found that the in-situ formed CeFe_xAl_1-xO₃ over ceria-hexaaluminate as "oxygen pool" could greatly improve carbon resistance even with presence of Fe⁰. Formation of CeFe_xAl_1-xO₃ only proceeds in ceria-hexaaluminate composite while not occurs in CeO₂-Fe₂O₃-Al₂O₃, which was probably originated from the strong interaction between CeO₂ and the adjacent Fe and Al ions in BaFe₃Al₉O₁₉ hexaaluminate structure. CeFe_xAl_1-xO₃ with outstanding oxygen ion conduction capacity was in close contact with metallic Fe⁰ exsoluted from Fe-hexaaluminate in a unique CeFe_xAl_1-xO₃/Fe⁰/hexaaluminate sandwich-like structure, which provided a convenient pathway for CeFe_xAl_1-xO₃ as “oxygen pool” to supply sufficient oxygen for in-time oxidation of carbon over adjacent Fe⁰. Consequently, the recycled ceria-hexaaluminate displayed both outstanding carbon resistance and high CH₄ conversion (~90%) with enhanced syngas yield that 105% and 72% higher than CeO₂-Fe₂O₃-Al₂O₃ oxide and Fe-hexaaluminate, respectively.

Xiaodong Wang

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