Abstract
Methane-to-syngas conversion plays an important role in industrial gas-to-liquid technologies, which is commercially fulfilled by energy-intensive reforming methods. Here we present a highly selective and durable iron-based La0.6Sr0.4Fe0.8Al0.2O3-δ oxygen carrier for syngas production via a solar-driven thermochemical process. It is found that a dynamic structural transformation between the perovskite phase and a Fe0@oxides core–shell composite occurs during redox cycling. The oxide shell, acting like a micro-membrane, avoids direct contact between methane and fresh iron(0), and prevents coke deposition. This core–shell intermediate is regenerated to the original perovskite structure either in oxygen or more importantly in H2O–CO2 oxidant with simultaneous generation of another source of syngas. Doping with aluminium cations reduces the surface oxygen species, avoiding overoxidation of methane by decreasing oxygen vacancies in perovskite matrix. As a result, this material exhibits high stability with carbon monoxide selectivity above 95% and yielding an ideal syngas of H2/CO ratio of 2/1.
Highlights
Methane-to-syngas conversion plays an important role in industrial gas-to-liquid technologies, which is commercially fulfilled by energy-intensive reforming methods
The X-ray diffraction (XRD) patterns of as-prepared LaFeO3 and La0.6Sr0.4Fe1-xAlxO3-δ oxides indicate that all samples show pure perovskite phase and no impurities such as SrCO3, La2O3, and Al2O3 are detected (Supplementary Fig. 2)
It shows that the intensity of white line (W: 1s → 4p transition) at ca. 7130 eV notably decreases when La cations in LaFeO3 were partially substituted by Sr, which is further lowered after doping of Al (i.e., LSAF6428)
Summary
Methane-to-syngas conversion plays an important role in industrial gas-to-liquid technologies, which is commercially fulfilled by energy-intensive reforming methods. When the A site of La in LaFeO3 was partially substituted by Sr to mitigate coke formation, the syngas selectivity was reduced due to generation of oxygen vacancies associated with more surface adsorbed oxygen[21] These studies suggest a “seesaw” effect for Fe-based perovskite OCs in selective oxidation of methane, between Fe valence variation to supply more lattice oxygen, and its performance (e.g., reactivity, selectivity, and stability). This perovskite with A and B sites substituted by Sr and Al cations reduces the surface active oxygen species for CH4 overoxidation and facilitates in situ encapsulation of Fe0, originating from deep reduction of Fe4+ with the formation of Fe0@oxides composite This core–shell structure switches off the pathway for Fe0 to catalyze methane pyrolysis, and favors reformation of the original perovskite phase either in strong O2 or soft H2O–CO2 oxidant. This material exhibits redox stability in the harsh thermochemical process and syngas selectivity of above 95% with H2/CO of 2 and no coke deposition
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