The increased production capacity of natural/shale gas triggers the great interest in converting methane to value-added chemicals in recent years. Currently, this purpose is fulfilled via syngas intermediate (mixture of H2 and CO), which can serves as feedstock for methanol production or F-T reactions. In view of energy-intensive feature of commercial steam reforming route for syngas production, we paid our attention to sustainable two-step solar-driven thermochemical process, which utilizes the lattice oxygen of metal oxides as the oxygen source for methane oxidation. The losing oxygen can be replenished from oxidants like H2O, CO2 or air with the production of H2, CO or process heat. The great challenge of this process lies in selection of suitable metal oxides, which exhibits properties of good oxygen capacity, reactivity, selectivity and redox stability.
The Fe-based oxides attract much attention due to the inexpensive and environmental friendly features. As for the oxygen carrier, wide valence changes of Fe cations is preferred to provide more lattice oxygen, but it was always found a “seesaw” effect between the reactivity and selectivity. For example, total oxidation of methane to CO2 prevailed over the syngas production before Fe3O4 is half reduced to FeO. Although further reduction of FeO to metallic Fe0 is favorable for higher CO/CO2 ratio and oxygen capacity, serious coke deposition would occur due to methane decomposition over Fe0, leading to increased H2/CO ratio and lower CO selectivity. Thus, our goal of this study is to develop a kind of facile Fe-based oxygen carrier, which can maintain good activity, CO selectivity over wide widow of Fe valence state.
Proposed structural evolution of the material during redox reactions
In this work, we presented a highly selective and durable Fe-based perovskite oxygen carrier with formulation of La0.6Sr0.4Fe0.8Al0.2O3-δ for methane-to-syngas conversion, which showed a switch-like structure transformation between perovskite phase and Fe0@oxides composite with core-shell structure during the reaction. Such structure evolution enabled wide shuttling of Fe species from Fe4+ to Fe0, offering a satisfactory amount of lattice oxygen for methane oxidation. The direct contact channel between emerged Fe0 with methane was switched off by the oxides layer, enabling continuous deep reduction of Fe cations to donate more lattice oxygen while suppressing coke deposition. Besides, doping of Al cations not only enhanced the sintering resistance of material, but also reduced the surface adsorbed oxygen by lowering the concentration of oxygen vacancies. Thus, excellent performance with ideal syngas of H2/CO ratio of 2/1 and CO selectivity above 95% was realized, accompanied with good stability over at least 100 cycles. Importantly, except for strong oxidant O2, the intermediate with core-shell structure can regenerated to original perovskite structure by soft oxidant of H2O and CO2 with the generation of another source of syngas.
The paper in Communications Chemistry can be found here.