Abatement of diesel-exhaust pollutants:: NOx storage and soot combustion on K/La2O3 catalysts

Potassium-loaded lanthana is a promising catalyst to be used for the simultaneous abatement of soot and NOx, which are the main diesel-exhaust pollutants. With potassium loadings between 4.5 and 10 wt.% and calcination temperatures between 400 and 700degreesC, this catalyst mixed with soot gave maximum combustion rates between 350 and 400degreesC in TPO experiments, showing a good hydrothermal stability. There was no difference in activity When it was either mixed by grinding in an agate mortar or mixed by shaking in a sample bottle (tight and loose conditions, respectively). Moreover, when the K-loaded La2O3 is used as washcoat for a cordierite monolith, there were found no significant differences in the catalytic behaviour of the system, which implies its potentiality for practical purposes. The influence of poisons as water and SO2 was investigated. While water does not affect the soot combustion activity, SO2 slightly shift the TPO peak to higher temperature. Surface basicity, which is a key factor, was analysed by measuring the interactions of the catalytic surface with CO2 using the high frequency CO2 pulses technique, which proved to be very sensitive, detecting minor changes by modifications in the dynamics of the CO2 adsorption-desorption process. Water diminishes the interaction with CO2, probably as a consequence of an adsorption competition. The SO2 treated catalyst is equilibrated with the CO2 atmosphere more rapidly if compared with the untreated one, also showing a lower interaction. The lower the interaction with the CO2, the lower the activity. Differential scanning calorimetric (DSC) results indicate that the soot combustion reaction coexists with the thermal decomposition of hydroxide and carbonate species, occurring in the same temperature range (350-460degreesC). The presence of potassium increases surface basicity shifting the endothermic decomposition signal to higher temperatures. We also found that NO2 strongly interacts with both La2O3 and K/La2O3 solids, probably through the formation of monodentate nitrate species which are stable under He atmosphere until 490degreesC. These nitrate species further react with the solid to form bulk nitrate compounds. The addition of Cobalt decreases the nitrates stability and catalyses the NOx to N-2 reduction under a reducing atmosphere, which is a necessary step for a working NOx catalytic trap. Preliminary studies performed in this work demonstrated the feasibility of using these catalysts to simultaneously remove NOx and soot particles from diesel exhausts. The nitrate formation is still observed during the catalytic combustion of soot in the presence of NOx, making our K/La2O3 a very interesting system for practical applications in simultaneous soot combustion and NOx storage in diesel exhausts. (C) 2002 Elsevier Science B.V. All rights reserved.