Catalytic Combustion of Methane over CeO2-MOx (M=La3+, Ca2+) Solid Solution Promoted Pd/γ-Al2O3 Catalysts

Solid solution CeO2-MOx (M=La3+, Ca2+) promoted Pd/γ-Al2O3 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 Mn+ ions incorporated into CeO2 lattice and solid solutions formed in Pd/γ-Al2O3-CeO2-MOx catalysts. The formation of solid solution was confirmed by the change in lattice parameters of CeO2 and the shift of 2θ angles as compared with pure CeO2. A strain formed in the O2− sub-lattice of CeO2 was revealed by Raman analyses, which decreased the intensity of the Raman-active band at around 463 cm−1 owing to the F2g symmetric stretching of Ce−O bond. The appearance of a new band at 615 cm−1 (in the case of Pd/γ-Al2O3-CeO2-CaO) and a shoulder at 320 cm−1 (in the case of Pd/γ-Al2O3-CeO2-La2O3) was also confirmed. Ionic Pdδ+ species were formed in the catalysts, which exhibited higher binding energies (0.5−0.6 eV higher for Pd 3d5/2) 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-Al2O3-CeO2-La2O3 catalyst under an hourly space velocity of 50000 h−1.