Direct conversion of methane to synthesis gas using lattice oxygen of CeO 2–Fe 2O 3 complex oxides

Abstract

Three kinds of complex oxides oxygen carriers (CeO 2–Fe 2O 3, CeO 2–ZrO 2 and ZrO 2–Fe 2O 3) 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, H 2-TPR and Raman. The XRD measurement shows that Fe 2O 3 particles well disperse on ZrO 2 surface and Ce–Zr solid solution forms in CeO 2–ZrO 2 sample. For CeO 2–Fe 2O 3 sample, only a small part of Fe 3+ 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 ZrO 2–Fe 2O 3 and CeO 2–ZrO 2 samples, respectively by H 2-TPR experiments. On the other hand, CeO 2–Fe 2O 3 shows a rather high reduction peak ascribed to the consuming of H 2 by bulk CeO 2, indicating high lattice oxygen content in CeO 2–Fe 2O 3 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. ZrO 2–Fe 2O 3 sample shows evident methane combustion during the reducing of Fe 2O 3, and then the methane conversion is strongly enhanced by the reduced Fe species through catalytic cracking of methane. CeO 2–ZrO 2 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 H 2/CO ratio of 2) can be obtained over CeO 2–Fe 2O 3 oxygen carrier through gas–solid reaction at 800 °C. It is proposed that the dispersed Fe 2O 3 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 CeO 2–Fe 2O 3 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 CeO 2–Fe 2O 3 oxygen carrier, and that may be responsible for its favorable successive redox cycles performance.