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Water-induced morphology changes in BaO/γ-Al2O3 NOx storage materials: An FTIR, TPD, and time-resolved synchrotron XRD study

Abstract:

The effect of water on the morphology of BaO/Al<inf>2</inf>O <inf>3</inf>-based NO<inf>x</inf> 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<inf>3</inf>)<inf>2</inf> 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<inf>2</inf> exposure of a H<inf>2</inf>O-containing (wet) BaO/Al<inf>2</inf>O<inf>3</inf> sample results in the formation of nitrites and bulk nitrates exclusively (i.e., no surface nitrates form). After further exposure to NO<inf>2</inf>, the nitrites completely convert to bulk nitrates. The amount of NO<inf>x</inf> 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<inf>2</inf> 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<inf>x</inf> uptake on similar storage materials. © 2007 American Chemical Society.