Thermal decomposition of dispersed and bulk-like NOx species in model NOx trap materials

The thermal decomposition of bulk Ba(NO3)2, Ba(NO3)2 impregnated on γ-Al2O3, and the release of NO2 stored on BaO/γ-Al2O3 are investigated by thermogravimetric analysis (TGA) and infrared spectroscopy (IR) as a function of baria loading. Two distinct weight loss events at ∼434 °C and ≥545 °C are observed on the γ-Al2O3 supported samples, corresponding to the decomposition of dispersed and bulk-like Ba(NO3)2 to BaO, respectively. Decomposition of dispersed phase having a temperature of 150 °C lower than bulk Ba(NO3)2 is attributed to a strong interaction between dispersed BaO and alumina. Bulk-like phase shows a similar decomposition characteristics to bulk Ba(NO3)2 as BaO loading increases to 31.8%. The decomposition products vary from predominantly NO2 at low temperature (≤434 °C) to NO at high temperature (≥545 °C), consistent with the trend expected from the NO2 ↔ NO + 1/2O2 thermal equilibrium. As the total baria loading is increased, the amount of dispersed phase saturates at a baria loading of 14% (w/w) while the bulk-like phase increases without reaching saturation. Both phases can be regenerated on 15 min exposure of corresponding BaO/Al2O3 to NO2 at room temperature. The release of these regenerated nitrates mimics the decomposition characteristics of impregnated Ba(NO3)2 on γ-Al2O3. X-ray powder diffraction and IR spectra show that the stored NO2 species gradually convert to a crystalline, bulk-like Ba(NO3)2 phase when aged at room temperature. These results are useful for optimizing BaO loading strategies for lean NOx trap catalyst.