Pd/Ce0.5Zr0.5O2 and Pd/Al2O3 catalysts were prepared by impregnation method and aged in different atmospheres (air, N-2, H-2 and cyclic (air-H-2)) at 1050 degrees C. The CO oxidation activities of the catalysts were evaluated. The states of palladium nanoparticles were characterized by step scanning X-ray diffraction (XRD), CO chemisorption, H-2 temperature-programmed reduction (TPR), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy (Raman). The Pd/Ce0.5Zr0.5O2 catalysts aged in air and cyclic conditions exhibit much better catalytic activities than those aged in N-2 and H-2, but the Pd/Al2O3 catalysts aged in different atmospheres show similar activities. Small PdO crystallites are observed in Pd/Ce0.5Zr0.5O2 catalysts after air and cyclic ageing, while larger Pd crystallites are found after N-2 and H-2 ageing. The sintering of Pd species is inhibited with the presence of oxygen in Pd/Ce0.5Zr0.5O2 catalysts. This is due to the strong interaction between Pd and CZ support in the form of Pd-O-Ce bond, which works during the cooling stage (from 1050 degrees C to room temperature). However, the thermal ageing deactivates the Pd/Al2O3 catalysts regardless of the ageing condition. The Pd/PdO particles grow to around 100 nm in all the aged Pd/Al2O3 catalysts, leading to the worse activity compared with the Pd/Ce0.5Zr0.5O2 catalysts. (c) 2013 Elsevier B.V. All rights reserved.