Metallic Pd Nanoparticles Formed by Pd−O−Ce Interaction: A Reason for Sintering-Induced Activation for CO Oxidation

Ambient-temperature CO oxidation activity of Pd/CeO 2 was found to increase by more than 20 times after thermal aging at 900 °C in air. Although the aging resulted in a significant sintering accompanied by a 92% loss of surface area from 92 to 7 m 2 ·g −1, Pd metal dispersion was preserved at a high value (0.57). The analysis using transmission electron microscopy (TEM), high-angle annular dark-field (HAADF)-scanning transmission electron microscopy (STEM), extended X-ray absorption fine structure (EXAFS), and X-ray photoelectron spectroscopy (XPS) demonstrated that the activation is driven by metal−support interactions followed by phase transformation. Owing to the formation of Pd−O−Ce bonding at the PdO/CeO 2 interface, Pd oxide species are highly dispersed into the surface structure of CeO 2 . The Pd oxide becomes unstable when the temperature reaches ≥800 °C, where thermodynamic PdO/Pd phase equilibrium is reached. Finally, the Pd−O−Ce surface moiety is fragmented into metallic Pd particles with a size of 1 to 2 nm, which provide active sites for CO oxidation.