Modeling NOx Storage Materials: A High-Resolution Photoelectron Spectroscopy Study on the Interaction of NO2 with Al2O3/NiAl(110) and BaO/Al2O3/NiAl(110)

We have studied the interaction of NO 2 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 Al 2 O 3 film on NiAl(110), on which BaO nanoparticles are grown by physical vapor deposition of metallic Ba and subsequent oxidation and annealing. On the Al 2 O 3 /NiAl(110), exposure to NO 2 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 Al 2 O 3 /NiAl(110), intermixing of Ba 2+ and Al 3+ 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 (BaAl 2 x O 1+3 x ) formation for larger nanoparticles. These, however, are overcome by annealing in O 2 . In a last step, the interaction of the model NO x storage materials with NO 2 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 Al 2 O 3 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.