Improved activity of magnetite oxygen carrier for chemical looping steam reforming by ultrasonic treatment

Magnetite with high content of Fe3O4 is a promising low-cost oxygen carrier candidate for co-production of syngas and pure hydrogen via chemical looping steam reforming (CLSR). The present work investigated the effect of ultrasonic treatment on the structure of magnetite by using X-ray diffraction (XRD) technology and Energy Dispersive Spectrometer (EDS) mapping technologies, and the results were correlated to the activity of magnetite oxygen carrier for selective oxidation of methane and water splitting. The evolutions of phases and compositions of magnetite during the reaction with methane were also studied to discuss the reaction mechanism. The results showed that ultrasonic treatment significantly improves the activity of magnetite oxygen carrier for methane selective oxidation by destroying the dense structure of magnetite and promoting the interdiffusion of different elements (e.g., Fe, Al and Mg) that could enhance the interaction between the active species (Fe oxides) and inert spaces (Al and Mg oxides). The syngas production increased from 10.29 to 13.15 mmol/g and hydrogen production increased from 4.93 to 5.43 mmol/g after the ultrasonic treatment, and both the produced hydrogen and syngas show high purity (ca. 96.5% for syngas and ca. 98.9% for H2). During the reaction between methane and magnetite, the reduction of iron oxides firstly occurred in the boundary between the iron oxides and the inert components (e.g., Al/Mg oxides), which then diffused to the adjacent areas and created numbers of smaller Fe islands. The channels around the Fe islands provide pathways for methane diffusion, improving the further reduction of oxygen carrier.