Direct conversion of methane to synthesis gas using lattice oxygen of CeO2–Fe2O3 complex oxides

Three kinds of complex oxides oxygen carriers (CeO2–Fe2O3, CeO2–ZrO2 and ZrO2–Fe2O3) were prepared and tested for the gas–solid reaction with methane in the absence of gaseous oxidant. These oxides were prepared by co-precipitation method and characterized by means of XRD, H2-TPR and Raman. The XRD measurement shows that Fe2O3 particles well disperse on ZrO2 surface and Ce–Zr solid solution forms in CeO2–ZrO2 sample. For CeO2–Fe2O3 sample, only a small part of Fe3+ has been incorporated into the ceria lattice to form solid solutions and the rest left on the surface of the oxides. Low reduction temperature and low lattice oxygen content are observed over ZrO2–Fe2O3 and CeO2–ZrO2 samples, respectively by H2-TPR experiments. On the other hand, CeO2–Fe2O3 shows a rather high reduction peak ascribed to the consuming of H2 by bulk CeO2, indicating high lattice oxygen content in CeO2–Fe2O3 complex oxides. The gas–solid reaction between methane and oxygen carriers are strongly affected by the reaction temperature and higher temperature is benefit to the methane oxidation. ZrO2–Fe2O3 sample shows evident methane combustion during the reducing of Fe2O3, and then the methane conversion is strongly enhanced by the reduced Fe species through catalytic cracking of methane. CeO2–ZrO2 complex oxides present a high activity for methane oxidation due to the formation of Ce–Zr solid solution, however, the low synthesis gas selectivity due to the high density of surface defects on Ce–Zr–O surface could also be observed. The highly selective synthesis gas (with H2/CO ratio of 2) can be obtained over CeO2–Fe2O3 oxygen carrier through gas–solid reaction at 800 °C. It is proposed that the dispersed Fe2O3 and Ce–Fe solid solution interact to contribute to the generation of synthesis gas. The reduced oxygen carrier could be re-oxidized by air and restored its initial state. The CeO2–Fe2O3 complex oxides maintained very high catalytic activity and structural stability in successive redox cycles. After a long period of successive redox cycles, there could be more solid solutions in the CeO2–Fe2O3 oxygen carrier, and that may be responsible for its favorable successive redox cycles performance.