Soot particles and NO are the main pollutants emitted by diesel engines. The K/La2O3 catalyst is active for soot combustion and it is able to adsorb NO,; therefore, it could work as NO trap. In this work, the effect of the chemical state of K on the catalytic activity is addressed. Both, the influence of the precursor used in the preparation and the effect of NO in the gas phase, are studied. Potassium nitrate, carbonate. and hydroxide were used to prepare the K/La2O3 catalysts. The catalytic activity for soot combustion was studied by TPO. XRD and FTIR were used to characterize the different crystalline phases found in the catalyst. Pulses of CO2 and CO2-TPD were useful to understand the effect of each precursor on the catalytic activity. The analysis of CO2 adsorption-desorption dynamics made it possible to determine that the strength of the interaction between this molecule and the catalyst depends on the K precursor, and follows the order: KOH > KNO3 > K2CO3. The presence of La(OH)(3) leads to a strong and irreversible interaction between the catalyst and the CO2, while the interaction is reversible with the dehydroxilated catalyst. The shape of the TPO profiles is related to the basicity of the surface, which according to the dynamics of the CO2 adsorption-desorption is significantly affected by the potassium precursor used during the preparation, the thermal treatment, and the treatment with gases normally found in the diesel exhaust, such as the NO. (C) 2011 Elsevier B.V. All rights reserved.