Effects of Co-substitution on the reactivity of double perovskite oxides LaSrFe2-xCoxO6 for the chemical-looping steam methane reforming

Abstract

The double perovskite oxides (DPOs) LaSrFe2-xCoxO6 (x = 0, 0.2, 0.4, 0.6, 0.8) were investigated as oxygen carriers for the chemical looping steam methane reforming (CL-SMR). The fresh oxides were prepared by micro-emulsion method and their physical and chemical properties were characterized by X-ray diffraction, H2-temperature programmed reduction and X-ray photoelectron spectroscopy technologies. Meanwhile, isothermal reactions for methane reforming and steam splitting were carried out in a fixed-bed reactor to determine the influences of Co-substitution on the reactivity of LaSrFe2-xCoxO6. The substitution of metal Co has no obvious effect on the crystal structure of double perovskite, but induces a certain degree of Fe/Co disorder generating oxygen vacancies and/or higher oxidation states of metal cations. Synergistic interaction between surface metal ions, such as (Fe4+/Fe5+-O2--Co2+) and (Fe3+-O2--Co3+), plays a positive effect for the dissociation of methane. The activity may be more likely to be associated with the active oxygen species in connection with Co species on the DPOs surface and abundant of syngas was generated due to the concordant of methane dissociation with the lattice oxygen diffusion. Comprehensively considered, an optimal range of the degree of Co substitution is x = 0.4–0.6 for LaSrFe2-xCoxO6, probably converting 70% of CH4 into CO and H2 with molar ratio around 2:1. At the reduced states, the ability of DPOs for steam splitting is primarily associated with the oxygen vacancies after oxygen consumption. The substitution of metal Co slightly enhances the hydrogen production capacity and resistance to carbon formation, achieving the average hydrogen yields at 2.89–3.33 mmol/g oxygen carrier and 1.46–1.61 wt% of carbon depositions.