Aiming at comparing the CO oxidation activity of free PdO and Pd2+ ions, a series of PdO-CeO2/SiO2 catalysts were prepared via an impregnation method and the turnover frequencies of them were quantitatively evaluated. The catalysts were further characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), temperature-programmed reduction (TPR), Raman technique and CO chemisorption. It was found that free PdO species and Pd2+ ions could be segregated efficiently by nitric acid treatment. The PdO-CeO2/SiO2 catalyst calcined at 600 degrees C showed the highest activity. The results of CO chemisorption suggested that free PdO species had a higher activity than Pd2+ ions, and the turnover frequencies of them were 3.27 x 10(-2) s(-1) and 0.49 x 10(-2) s(-1), respectively. The activity of PdO-CeO2/SiO2 catalyst was higher than that of PdO/SiO2 catalyst. Moreover, the CO catalytic activity of PdO-CeO2/SiO2 catalyst could be assigned to the synergetic effect of free PdO and Pd2+ ions, and this effect could improve the CO catalytic activity of catalysts. (C) 2013 Elsevier B.V. All rights reserved.