La1-xSrxFeO3 perovskite-type oxides for chemical-looping steam methane reforming: Identification of the surface elements and redox cyclic performance

We report a family of perovskite-type oxides La1-xSrxFeO3 (x = 0.1, 0.3, 0.5, 0.7, 1.0) prepared by combustion method as effective redox catalysts for methane partial oxidation and thermochemical water splitting in a cyclic redox scheme. The effect of Sr-doping on the characterizations and properties of these perovskite-type oxides were studied by means of X-ray diffraction (XRD), hydrogen temperature-programmed reduction (H-2-TPR), X-ray photoelectron spectroscopy (XPS), and scanning electron microscope (SEM). All the as prepared and regenerated samples with various Sr substitutions exhibited pure crystalline perovskite structure. The oxygen carrying capacity of the La1-xSrxFeO3 perovskites was improved by doping Sr into the La-site. Besides, Sr-substitution has obvious effects on the valences of the Fe cations in the B-site and the oxygen species distribution of the La1-xSrxFeO3 perovskites. We recommend La0.7Sr0.3FeO3 as the optimal oxygen carrier in the series because it gives the maximum O-la/O-ad (O-la and O-ad stand for lattice oxygen and adsorbed oxygen species, respectively.) ratio of 3.64:1, which can be regarded as a criterion for the reactivity and selectivity of partial oxidation of methane into syngas of the oxygen carriers. Up to 80% CH4 conversion in the methane partial oxidation step and 96% of H-2 concentration in the water splitting step were achieved in ten successive redox tests conducted in a fixed bed reactor at 850 degrees C with La0.7Sr0.3FeO3 as a redox catalyst. The electronic properties of the original LaFeO3 cell and its lattice substituted by Sr were calculated based on the density functional theory method. Electronic structure analysis demonstrates that doping of Sr makes LaFeO3 more electric conductive and its electron is prone to be excited. This is in agreement with the test results that La0.7Sr0.3FeO3 exhibited better performance in chemical looping reactions. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.