A series of Ba/CeO2 catalysts with different Ba loading amounts were prepared by incipient wetness impregnation.Their NOx adsorption behaviors under NO and NO+O2 conditions were investigated by in situ DRIFTS.It was found that NOx was adsorbed and stored in the form of nitrites and nitrates on both Ba and Ce sites on the surface of the catalysts.The less thermally stable BaCO3 was suggested to be the main active phase for NOx trapping.Ceria served primarily as an oxygen supplier in the absence of O2,and the reaction from nitrites to nitrates on Ba sites was the key step in this case.In the presence of O2,however,gaseous O2became the main oxygen source.The NOx adsorption capacity of the catalyst was dominated by the Ba content.Moreover,the stability of nitrites and nitrates formed on Ce sites was found to be lower than those formed on Ba sites which existed in the form of the ionic barium nitrate species. A series of Ba / CeO2 catalysts with different Ba loading amounts were prepared by incipient wetness impregnation. Their NOx adsorption behaviors under NO and NO + O2 conditions were investigated by in situ DRIFTS.It was found that NOx was adsorbed and stored in the form of nitrites and nitrates on both Ba and Ce sites on the surface of the catalysts. The less thermally stable BaCO3 was suggested to be the main active phase for NOx trapping. Crematories for as oxygen supplier in the absence of O2, and the reaction from nitrites to nitrates on Ba sites was the key step in this case. In presence of O2, however, gaseous O2became the main oxygen source. The NOx adsorption capacity of the catalyst was dominated by the Ba content. More over, the stability of nitrites and nitrates formed on Ce sites was found to be lower than those formed on Ba sites which existed in the form of the ionic barium nitrate species.