Different oxidation routes for lattice oxygen recovery of double-perovskite type oxides LaSrFeCoO_6 as oxygen carriers for chemical looping steam methane reforming

Double-perovskite type oxide LaSrFeCoO_6(LSFCO) was used as oxygen carrier for chemical looping steam methane reforming(CL-SMR) due to its unique structure and reactivity. Two different oxidation routes,steam-oxidation and steam-air-stepwise-oxidation, were applied to investigate the recovery behaviors of the lattice oxygen in the oxygen carrier. The characterizations of the oxide were determined by X-ray diffraction(XRD), X-ray photoelectron spectroscopy(XPS), hydrogen temperature-programmed reduction(H_2-TPR) and scanning electron microscopy(SEM). The fresh sample LSFCO exhibits a monocrystalline perovskite structure with cubic symmetry and high crystallinity, except for a little impurity phase due to the antisite defect of Fe/Co disorder. The deconvolution distribution of XPS patterns indicated that Co,and Fe are predominantly in an oxidized state(Fe~(3+) and Fe~(2+)) and(Co~(2+) and Co~(3+)), while O 1s exists at three species of lattice oxygen, chemisorbed oxygen and physical adsorbed oxygen. The double perovskite structure and chemical composition recover to the original state after the steam and air oxidation, while the Co ion cannot incorporate into the double perovskite structure and thus form the CoO just via individual steam oxidation. In comparison to the two different oxidation routes, the sample obtained by steam-oxidation exhibits even higher CH_4 conversion, CO and H_2 selectivity and stronger hydrogen generation capacity.