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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 NO x -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.

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We have studied the interaction of NO2 with a single-crystal-based model NO x storage material, using high-resolution photoelectron spectroscopy (HR-PES). As a model surface, we use an ordered Al2O3 film on NiAl(110), on which BaO nanoparticles are grown by physical vapor deposition of metallic Ba and subsequent oxidation and annealing. On the Al2O3/NiAl(110), exposure to NO2 at 300 K leads to slow formation of surface nitrite species, saturating at exposures around 100−1000 L. The surface reaction is accompanied by further oxidation of the support, leading to an increasing thickness of the alumina film. The initial surface reaction is followed by two additional very slow processes, the formation of a small amount of surface nitrates and the decomposition to aluminum nitride species. Upon annealing, the weakly bound surface nitrites and nitrates desorb at temperatures below 500 K. During preparation of the BaO nanoparticles on Al2O3/NiAl(110), intermixing of Ba2+ and Al3+ ions occurs, even at 300 K. The process is accompanied by continuing increase of the oxide film thickness. Whereas intermixing is nearly complete for small particles at 300 K, there are kinetic limitations for mixed oxide (BaAl2x O1+3x ) formation for larger nanoparticles. These, however, are overcome by annealing in O2. In a last step, the interaction of the model NO x storage materials with NO2 is probed. At an initial stage of the reaction, only the formation of surface nitrites is observed. On the BaO containing surface, nitrite formation occurs at a higher rate than on the pristine Al2O3 support. Again, the reaction is connected to an increasing thickness of the alumina layer. At exposures around 100−1000 L at 300 K, formation of surface nitrites stops and is followed by slow conversion into surface nitrates. In contrast to the pristine alumina support, decomposition to nitrides is strongly inhibited on the Ba containing model system.