Monolithic Ce-Fe-Zr-O/MgO oxygen carrier for the chemical-looping partial oxidation of methane to syngas

With MgO as support, powdery Ce-Fe-Zr-O/MgO oxygen carrier was prepared by a ball milling method and monolithic Ce-Fe-Zr-O/MgO oxygen carrier was further obtained by extrusion molding method. The performances of both powdery and monolithic oxygen carriers in partial oxidation of methane to syngas via a chemical looping process were investigated. The results showed that the component of the powdery carrier for oxygen storage is Ce-Fe-Zr-O solid solution, whereas the free oxides of Ce, Zr and Fe may be segregated from the solid solution in monolithic oxygen carrier upon preparation. Two kinds of oxygen species, i.e. surface lattice oxygen and bulk lattice oxygen, can be found on both powdery and monolithic oxygen carriers; bulk lattice oxygen can selectively convert methane to CO and H₂, whereas the surface oxygen is very active for methane complete oxidation. Powdery oxygen carrier owns high content of surface lattice oxygen, which results in high concentration of H₂O and CO₂ in the product. Over the monolithic oxygen carrier, methane can be selectively oxidized to CO and H₂ due to the high content of bulk lattice oxygen. Moreover, methane decomposition occurs easily over the powdery oxygen carrier in the redox tests, resulting in a high H₂/CO mol ratio (much higher than 2.0) in the product. On the contrary, the monolithic oxygen carrier exhibits a high redox stability and can continuously produce syngas with a H₂/CO mol ratio of 2.0 during the successive redox tests.