Study on the combustion behavior of methane over Ce–Zr-modified Pd/Al 2O 3 catalysts

Abstract

The effects of ceria and zirconium additions to alumina-supported palladium catalysts on methane combustion behavior have been investigated. A series of Ce–Zr-modified Al 2O 3 supports and their supported Pd catalysts are prepared using impregnation method and then characterized by BET, XRD, LR, TEM, XPS techniques. The results show that the addition of Ce–Zr improves the thermal stability of alumina. The thermal stability of alumina increases with increasing the loading of Zr. The methane oxidation activity and thermal stability is found to be dependent on the Ce:Zr ratio over the Pd/Ce x Zr 1− x /Al 2O 3 catalysts. The Pd/Ce 0.2Zr 0.8/Al 2O 3 exhibits the highest activity and thermal stability for methane oxidation. The temperatures for the 90% methane conversion are 448 and 455 °C for the Pd/Ce 0.2Zr 0.8/Al 2O 3 catalysts calcined at 500 and 1100 °C, respectively. TEM results show that the particle size of Pd is not the key factor influencing the activity of Pd/Ce 0.2Zr 0.8/Al 2O 3 catalyst. An induction period is present during methane combustion reaction for the Pd/Al 2O 3 and Pd/Ce x Zr 1− x /Al 2O 3 catalysts (calcined at 1100 °C), but the lengths of induction period are obviously different and the Pd/Ce 0.2Zr 0.8/Al 2O 3 catalyst exhibits the shortest induction period. XPS results show that comparing to Pd/Al 2O 3 catalyst the addition of Ce–Zr stabilizes Pd in a high oxidation state and enhances the reoxidation of metallic Pd to the active PdO under reaction conditions. Therefore the Pd/Ce x Zr 1− x /Al 2O 3 catalysts compared to Pd/Al 2O 3 catalysts have higher thermal stability and shorter induction period.