Synthesis of three-dimensionally ordered macroporous LaFe_0.7Co_0.3O₃ perovskites and their performance for chemical-looping steam reforming of methane

Three-dimensionally ordered macroporous (3DOM) LaFe_0.7Co_0.3O₃ perovskite-type oxides were synthesized using a polystyrene (PS) colloidal crystal templating method. The obtained 3DOM LaFe_0.7Co_0.3O₃ perovskites were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and Brunauere-Emmette-Teller (BET) surface area. Its performance as oxygen carriers in chemical looping steam methane reforming (CL-SMR) to produce syngas (H₂ + CO) and hydrogen were investigated in a fixed-bed reactor. The size of PS spheres obviously increases as the styrene addition increases. The calcination temperature is the major factor to affect the prepared 3DOM perovskite. SEM and TEM analysis show that the samples calcined at 500, 800 and 850℃ exhibit good 3DOM structures which collapse when the sample is calcined at 900℃. XRD results suggest that the obtained 3DOM LaFe_0.7Co_0.3O₃ perovskites are pure crystalline. Two kinds of oxygen species, bulk lattice oxygen and surface adsorbed oxygen, are found to exist on the 3DOM LaFe_0.7Co_0.3O₃ perovskites. The surface oxygen contributes to the complete oxidization of methane to CO₂ and H₂O in beginning of the reaction, while the bulk lattice oxygen tends towards partial methane oxidation to H₂ and CO. In the methane conversion step, methane is partially oxidized into syngas at a H₂/CO mol ratio close to 2:1 by the 3DOM- LaFe_0.7Co_0.3O₃ in a wide range of the reactions, suggesting that the sample exhibits a good resistance to carbon deposition. In the steam oxidation step, the reduced perovskites are oxidized by steam to generate hydrogen with hydrogen productivity about 4 mmol/g oxygen carriers.