Abstract
Pd/CeO2 has attracted great attention owing to its unique activity for methane catalytic oxidation; however, the active sites for CH4 catalytic oxidation still remain elusive, which affects the comprehensive understanding of the catalytic mechanism. In this work, the structures of PdOx nanoparticles (NPs) loaded on octahedrons, cubes, and rods of nanocrystal CeO2 supports were systematically studied by Cs-corrected HRTEM/STEM, XPS, and Raman spectroscopy. Our results indicate that the Pd species on CeO2 supports are morphology-dependent: PdO NPs (Pe(2+)) on octahedrons, PdOx (x = 1-2) clusters (1-2 nm) on cubes, and dispersed Pd4+ ions on the CeO2 rods. Additionally, the chemical states of Pd can be tuned in oxidizing/reducing atmospheres via interactions between Pd and CeO2. Detailed studies reveal that the Pd2+ species are the active centers for the catalytic oxidation of methane. The activity of Pd-0 could be ascribed to Pd2+ produced through the gradual oxidation of Pd-0 during the CH4 oxidation. Further, Pd4+ in the CeO2 lattice is inactive for CH4 oxidation. In situ Fourier transform infrared spectroscopy results suggest that the mechanism of CH4 oxidation reaction on PdOx/CeO2 follows the Mars-van Krevelen mechanism, and adsorbed CO can be produced in CH4 decomposition over Pd2+ in the absence of gas-phase oxygen. As revealed by density functional theory calculations, the incomplete coordination of Pe ions and adjacent oxygen atoms has excellent activity in cracking the C-H bond of CH4, which leads to high methane oxidation ability.