Pd@CeO2 catalyst was facilely synthesized by a seed-mediated hydrothermal method,which exhibiting higher CO oxidation activity than the conventionally impregnated Pd/CeO2 catalyst.Interestingly,only doping a small amount of ZrO2 (0.5%) on the as-synthesized core/shell structure could have a significant improvement on the activity of CO oxidation.Based on the CO oxidation reaction and the corresponding characterizations,mechanism for the high CO oxidation activity was explored.Specifically,the reason for high CO oxidation activity of Pd@CeO2 could be mainly ascribed to the weaker interaction between Pd core and CeO2 shell than that of the Pd/CeO2 sample.Moreover,due to the presence of crystal defects in the Pd@CeO2 sample,doping ZrO2 on the Pd@CeO2 sample led to the formation of Ce-Zr solid solution,which led to improvement of CO oxidation activity,while no Ce-Zr solid solution was formed for the Pd/CeO2 sample.