Water-induced bulk Ba(NO3)2 formation from NO2 exposed thermally aged BaO/Al2O3

Phase changes in high temperature treated (>900 °C) 8 or 20 wt% BaO supported on γ-Al2O3 model lean NOx trap (LNT) catalysts, induced by NO2 and/or H2O adsorption, were investigated with powder X-ray diffraction (XRD), solid state 27Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, and NO2 temperature programmed desorption (TPD) experiments. After calcination in dry air at 1000 °C, the XRD and solid state 27Al MAS NMR results confirm that stable surface BaO and bulk BaAl2O4 phases are formed for 8 and 20 wt% BaO/Al2O3, respectively. Following NO2 adsorption over these thermally treated samples, some evidence for nanosized Ba(NO3)2 particles are observed in the XRD results, although this may represent a minority phase. However, when water was added to the thermally aged samples after NO2 exposure, the formation of bulk crystalline Ba(NO3)2 particles was observed in both samples. Solid state 27Al MAS NMR is shown to be a good technique for identifying the various Al species present in the materials during the processes studied here. NO2 TPD results demonstrate a significant loss of uptake for the 20 wt% model catalysts upon thermal treatment. However, the described phase transformations upon subsequent water treatment gave rise to the partial recovery of NOx uptake, demonstrating that such a water treatment of thermally aged catalysts can provide a potential method to regenerate LNT materials.