Modeling NOx storage materials:: On the formation of surface nitrites and nitrates and their identification by vibrational spectroscopy

We have studied the reaction of NO2 with BaO nanoparticles supported on an ordered Al2O3 thin film on NiAl(110). Combining chemical analysis by X-ray photoelectron spectroscopy (XPS) and vibrational spectroscopy by infrared reflection absorption spectroscopy (IRAS), performed in combination with molecular beam (MB) techniques, the sequence of appearance of various nitrogen-oxo surface intermediates and their spectral properties are identified. The initial intermediates at 300 K are surface nitrites (NO2-), which are preferentially oriented parallel to the surface. Whereas formation of nitrites is rapid even at 300 K, conversion of nitrites into surface nitrates (NO3-) occurs at a very low rate. After surface nitrate formation, no further reaction is observed. At higher temperature (500 K), conversion into surface nitrates is more facile and is followed by formation of ionic nitrates. All three nitrogen-oxo species can be clearly identified via their characteristic vibrational spectra. No spectroscopic evidence for the formation of other NOx-derived surface species is found under the reaction conditions applied in this study. The results suggest that (i) conversion of the surface nitrite into the surface nitrate and (ii) formation of the ionic nitrate are the rate-controlling steps in the storage process.