Catalytic Combustion of Methane over CeO 2-MO x (M=La 3+, Ca 2+) Solid Solution Promoted Pd/ γ-Al 2O 3 Catalysts

Abstract

Solid solution CeO 2-MO x (M=La 3+, Ca 2+) promoted Pd/ γ-Al 2O 3 catalysts were prepared by a deposition-precipitation method. The structural properties were investigated by X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy. The results showed that M n+ ions incorporated into CeO 2 lattice and solid solutions formed in Pd/ γ-Al 2O 3-CeO 2-MO x catalysts. The formation of solid solution was confirmed by the change in lattice parameters of CeO 2 and the shift of 2θ angles as compared with pure CeO 2. A strain formed in the O 2− sub-lattice of CeO 2 was revealed by Raman analyses, which decreased the intensity of the Raman-active band at around 463 cm −1 owing to the F 2 g symmetric stretching of Ce−O bond. The appearance of a new band at 615 cm −1 (in the case of Pd/ γ-Al 2O 3-CeO 2-CaO) and a shoulder at 320 cm −1 (in the case of Pd/ γ-Al 2O 3-CeO 2-La 2O 3) was also confirmed. Ionic Pd δ+ species were formed in the catalysts, which exhibited higher binding energies (0.5−0.6 eV higher for Pd 3 d 5/2) than that of normal PdO. The catalysts showed high activity and stability for low temperature methane combustion. 10% and 100% conversions of methane could be obtained at temperatures of 254 and 340 °C, respectively, over Pd/ γ-Al 2O 3-CeO 2-La 2O 3 catalyst under an hourly space velocity of 50000 h −1.