Ambient-temperature CO oxidation activity of Pd/CeO2 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 m2·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/CeO2 interface, Pd oxide species are highly dispersed into the surface structure of CeO2. 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.

The influence of the redox behavior of Rh/AlPO4 on automotive three-way catalysis (TWC) was studied to correlate catalytic activity with thermal stability and metal–support interactions. Compared with a reference Rh/Al2O3 catalyst, Rh/AlPO4 exhibited a much higher stability against thermal aging under an oxidizing atmosphere; further deactivation was induced by a high-temperature reduction treatment. In situ X-ray absorption fine structure experiments revealed a higher reducibility of Rh oxide (RhO x ) to Rh, and the metal showed a higher tolerance to reoxidation when supported on AlPO4 compared with Al2O3. This unusual redox behavior is associated with an Rh–O–P interfacial linkage, which is preserved under oxidizing and reducing atmospheres. Another effect of the Rh–O–P interfacial linkage was observed for the metallic Rh with an electron-deficient character. This leads to the decreasing back-donation from Rh d-orbitals to the antibonding π* orbital of chemisorbed CO or NO, which is a possible reason for the deactivation by high-temperature reduction treatments. On the other hand, surface acid sites on AlPO4 promoted oxidative adsorption of C3H6 as aldehyde, which showed a higher reactivity toward O2, as well as NO, compared with carboxylate adsorbed on Al2O3. A precise control of the acid–base character of the metal phosphate supports is therefore a key to enhance the catalytic performance of supported Rh catalysts for TWC applications.