The dependence of NO₂ and CO₂ uptake and release on BaO content for NO_x 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.