Influence of CO2 on storage and release of NOx on barium-containing catalyst

A barium-containing three-way automotive emission catalyst was submitted to a NOx storage step in flowing lean gas mixture containing 340 ppm NO and 8 vol% O-2 in helium. NOx release was carried out in the 250-550 degrees C temperature range, either in pure helium or in the presence of a 10 vol% CO2 in helium mixture. It was shown that at 450-550 degrees C all of the stored NOx on the barium trap can be released fastly in the CO2-containing gas mixture or, after a longer time, in pure helium: these data show that NOx release can occur in the absence of a reducing agent. The NOx release was not complete at 350 degrees C and did not occur at 250 degrees C. The assisting effect of CO2 as regards to NOx release was interpreted in terms of the existence of the CO2,gas + *NO2,stored \rightleftharpoons*CO2,stored +NO2,gas equilibrium, suggesting the competitive storage of CO2 and NO2 for a unique type of barium storage sites (*).