The effect of water on the morphology of BaO/Al 2 O 3 -based NO x storage materials was investigated using Fourier transform infrared spectroscopy, temperature programmed desorption, and time-resolved synchrotron X-ray diffraction techniques. The results of this multispectroscopy study reveal that in the presence of water surface Ba-nitrates convert to bulk nitrates and water facilitates the formation of large Ba(NO 3 ) 2 particles. The conversion of surface to bulk Ba-nitrates is completely reversible (i.e., after the removal of water from the storage material a significant fraction of the bulk nitrates reconverts to surface nitrates). NO 2 exposure of a H 2 O-containing (wet) BaO/Al 2 O 3 sample results in the formation of nitrites and bulk nitrates exclusively (i.e., no surface nitrates form). After further exposure to NO 2, the nitrites completely convert to bulk nitrates. The amount of NO x taken up by the storage material, however, is essentially unaffected by the presence of water regardless of whether the water was dosed prior to or after NO 2 exposure. On the basis of the results of this study, we are now able to explain most of the observations reported in the literature on the effect of water on NO x uptake on similar storage materials.
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Szanyi et al. (2007) studied this question.
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