Ln/K oxide catalysts (Ln = La, Sm, Y) supported on the alumina substrate have been prepared and their catalytic activities of diesel soot oxidation were tested by TPR system. The results showed that all La/K, Sm/K and Y/K catalysts showed a similar change of catalytic activity with the variation of K content. Nevertheless, Y/K catalyst was more active for diesel soot oxidation with a soot ignition temperature of about 350 degrees C at a loose soot/catalyst contact mode. XRD, TG/DSC analyses were employed to investigate the structures and thermal behaviors of catalysts. It was found that the oxides would react with KNO3 to form the solid solution and the K+ saturation level. was related to the ionic radii of the Ln(3+). The Ln/K catalysts showed a physical CO2 absorption ability that was further enhanced at higher temperature. However, when the temperature was above a critical temperature, the absorption ability was reduced. The CO2 absorption characteristics was believed to be diffusion controlled and was attributable to the lattice defects (oxygen vacancies) caused by the solid solution formation. The dynamic CO2 transportation within the catalyst could play an important role on the catalytic oxidation process. (C) 2007 Elsevier B.V. All rights reserved.