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Three Pd/CeO2 catalysts were,respectively,prepared by reduction-deposition and impregnation method (IMP) to investigate the effect of chemical state of Pd on CO oxidation.Two kinds of surface Pd species,namely PdO and Pdδ+ (2 ＜ δ ≤ 4) in PdxCe1-xO2,were identified in all Pd/CeO2 samples although their relative ratios in each sample were different.Surface PdxCe1-xO2 species were found to be very active for CO oxidation,and it could act as a channel by which active oxygen species can be transferred from CeO2 to Pd species for CO oxidation.Our results reveal that the preparation method can severely influence the chemical state of Pd which can further determine the activity for CO oxidation.