The dependence of NO2 and CO2 uptake and release on BaO content for NOx storage catalysts made using different precursor salts

Here we compare the uptake of CO2 on BaO–Al2O3 with that of NO2 for nine samples made by one, two or three incipient wetness impregnations using acetate and nitrates as the precursor salts. The amounts of CO2 and NO2 adsorbed to the point of breakthrough at 300 °C are similar, proportional to the BaO content and largely independent of the preparation method. The breakthrough of CO2 is sharp. Its desorption occurs over a wide range of temperatures. Displacement by NO2 will remove the one-quarter retained at 550 °C. Unlike the adsorption of CO2, the uptake of NO2 continues for prolonged period after initial breakthrough. It occurs with disproportionation of NO2 to gaseous NO and stored nitrate. Measurement of the overall O:N ratio in the gaseous decomposition products shows that the material stored to the point of breakthrough retains the overall stoichiometry NO2. It decomposes in two stages, initially by disproportionation to NO and nitrate followed by decomposition of the latter to NO and O2 in the ratio 4:3 starting at 450 °C. The O:N ratio of material stored after prolonged exposure corresponds to nitrate. In accordance with the literature, these species decompose to NO2 and a small amount of O2 with maximum evolution rates below 500 °C and to NO and O2 in the 4:3 ratio above that. The amount of NO2 produced is largely independent of the BaO content while that of NO is proportional to it.