Elucidation of Active Sites for CH4 Catalytic Oxidation over Pd/CeO2 Via Tailoring Metal–Support Interactions

Abstract

Pd/CeO 2 has attracted great attention owing to its unique activity for methane catalytic oxidation; however, the active sites for CH 4 catalytic oxidation still remain elusive, which affects the comprehensive understanding of the catalytic mechanism. In this work, the structures of PdO x nanoparticles (NPs) loaded on octahedrons, cubes, and rods of nanocrystal CeO 2 supports were systematically studied by Cs-corrected HRTEM/STEM, XPS, and Raman spectroscopy. Our results indicate that the Pd species on CeO 2 supports are morphology-dependent: PdO NPs (Pd 2+ ) on octahedrons, PdO x ( x = 1–2) clusters (1–2 nm) on cubes, and dispersed Pd 4+ ions on the CeO 2 rods. Additionally, the chemical states of Pd can be tuned in oxidizing/reducing atmospheres via interactions between Pd and CeO 2 . Detailed studies reveal that the Pd 2+ species are the active centers for the catalytic oxidation of methane. The activity of Pd 0 could be ascribed to Pd 2+ produced through the gradual oxidation of Pd 0 during the CH 4 oxidation. Further, Pd 4+ in the CeO 2 lattice is inactive for CH 4 oxidation. In situ Fourier transform infrared spectroscopy results suggest that the mechanism of CH 4 oxidation reaction on PdO x /CeO 2 follows the Mars–van Krevelen mechanism, and adsorbed CO can be produced in CH 4 decomposition over Pd 2+ in the absence of gas-phase oxygen. As revealed by density functional theory calculations, the incomplete coordination of Pd 2+ ions and adjacent oxygen atoms has excellent activity in cracking the C–H bond of CH 4, which leads to high methane oxidation ability.