Catalytic decomposition of methane over rare earth metal (Ce and La) oxides supported iron catalysts

Abstract

The concurrent production of carbon oxides-free hydrogen and crystalline carbon by the thermocatalytic decomposition of methane is advantageous for environmental and energy catalysis. In the present work, rare earth metal oxide-supported iron catalysts were effectively prepared via a simple co-precipitation method, and their catalytic activity for methane decomposition was evaluated for the first time. The structural, textural and redox properties of the prepared catalysts were characterized using different analytical methods. The powder X-ray diffraction patterns of the catalysts indicated the presence of CeO2 and Fe2O3 as different phases in the fresh Fe/CeO2 catalyst and La(OH)3, Fe2O3, and LaFeO3 as different phases in the fresh Fe/La2O3 catalysts. However, large amounts of cerium and lanthanum orthoferrites were identified in the reduced samples. The nitrogen sorption analysis indicated the presence of a mesoporous texture due to the homogeneous aggregation of catalyst particles. The catalysts showed high catalytic activity and stability for methane decomposition, and no deactivation was observed for a period of 360 min of time on stream. A maximum hydrogen yield of 66 ± 1% was observed for both of the catalysts at 800 °C. However, the Fe/CeO2 catalyst showed high catalytic stability, where the highest initial and final hydrogen yield was found to be almost constant for the Fe/CeO2 catalyst. However, a substantial drop in the hydrogen yield was noticed for the Fe/La2O3 catalyst during the course of the reaction. The improved surface stabilization and fine dispersion of iron nanocrystals on the ceria matrix made it highly active and stable compared to the lanthanum-based catalyst for the methane decomposition reaction. The encapsulation of metallic iron by carbon deposited on the Fe/La2O3 catalyst could be responsible for the low catalytic stability. Multi-walled carbon nanotubes with different shapes were deposited on the catalysts with respect to the support material. A set of spiral-shaped multi-walled carbon nanotubes with a double helical-shaped chain-like structure were deposited on the Fe/La2O3 catalyst, that is rarely formed in methane decomposition. The nanocarbon deposited on the Fe/CeO2 catalyst exhibited a high degree of crystallinity and graphitization, as shown by the Raman spectroscopy.