An investigation of NOx storage on Pt–BaO–Al2O3

A series of samples containing 5-wt% or 20-wt% BaO on γ-Al2O3 with different loadings of Pt were prepared and examined for their NO2 adsorption properties using temperature programmed desorption (TPD), temperature programmed reduction (TPR), and x-ray diffraction (XRD). For calcination at 873 K or above, BaO/Al2O3 formed BaAl2O4. While carbonates were found to be unstable on the aluminate phase, NO2 reacted with the aluminate to form bulk Ba(NO3)2 and Al2O3, even at room temperature. With BaO/Al2O3, reaction to form the nitrate required slightly higher temperatures because of the need to displace CO2; however, pulsing NO2 over pure Ba(CO3) showed rapid reaction to form CO2 and NO in the gas phase, along with Ba(NO3)2, at 673 K. The decomposition temperature for Ba(NO3)2 shifted by more than 100 degrees when TPD was carried out in vacuum rather than in a carrier gas, showing that re-equilibration with the gas phase is important in the decomposition process. The addition of Pt had a minimal effect on the thermal stability of the nitrates but was essential for the reduction of the nitrate in H2. Since a relatively small amount of Pt was sufficient to cause the complete reduction of the Ba(NO3)2 phase at temperatures below 400 K, it appears that the nitrates must be extremely mobile within the Ba-containing phase. Finally, trapping studies of NO2 at 573 K, with or without 10% CO2 in the gas phase, showed no measurable difference between BaO/Al2O3 and BaAl2O4, with or without CO2.