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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.

Based on a well-defined model-catalyst approach, we study the particle size dependent properties of NO x storage materials. The single-crystal based model systems are prepared on an ordered Al 2 O 3 film, on which BaO nanoparticles are grown under ultrahigh-vacuum (UVH) conditions. Particle size and density are characterized by scanning tunneling microscopy (STM). The interaction with NO 2 is probed by molecular beam (MB) methods in combination with time-resolved IR reflection absorption spectroscopy (TR-IRAS). It is found that both, the stability and the formation kinetics of alumina supported barium nitrate nanoparticles show a strong dependence on particle size. Very small BaO particles are rapidly converted into nitrates, however, the resulting aggregates exhibit a strongly reduced thermal stability. Surface and bulk nitrate and nitrate features are identified by means of vibrational spectroscopy. It is concluded that the size dependencies are related to the formation and decomposition of surface-related BaNO x species the decomposition temperature of which can be tuned over an exceptionally large temperature interval. It is suggested that the stability of these surface NO x species is strongly modified by the underlying support.

Nanocrystalline particles expose special adsorption sites close to edges and corners, giving rise to novel adsorption and reaction properties. The spectroscopic identification of these sites represents a great challenge, however. Here, we present results of a combined experimental and theoretical study on the adsorption of NO on Pd nanoparticles, using infrared reflection absorption spectroscopy (IRAS) and calculations based on densityfunctional theory (DFT). This approach facilitates identification of the adsorption sites available on the nanoparticles and reveals detailed information on their bonding properties, on the vibrational parameters of NO adsorbed on these sites, and on their sequence of occupation. With respect to all these aspects, the adsorption behavior of NO on the Pd nanoparticles notably differs from any single crystal reference data available. The IRAS studies are performed on well-defined Pd nanoparticles supported on an ordered Al 2 O 3 film on NiAl(110). The growth and structure of these particles has been characterized previously, predominately exposing (111) and a small fraction of (100) facets. Here, we systematically monitor the NO adsorption as a function of exposure in a temperature region between 100 and 300 K by means of time-resolved IRAS in combination with molecular beam (MB) dosing. We interpret these experimental data with the help of DFT calculations on the adsorption of NO on unsupported cuboctahedral Pd n clusters cut from Pd bulk and containing up to 140 atoms; for comparison, calculations of the reference adsorption complexes of NO on single-crystal Pd(111) surface have also been performed. NO molecules are shown to most favorably adsorb on hollow µ 3 -sites on (111) facets of Pd n clusters, closely followed by bridge µ 2 -sites at the edges between adjacent (111) facets. Both sites give rise to characteristic features in the vibrational spectrum and are populated sequentially. At higher coverage (and low temperature) on-top µ 1 -sites on the (111) facets begin to be occupied. At variance with the adsorption on Pd(111) surface, however, additional on-top-sites are available at the particle edges and corners, which reveal stronger NO adsorption. In spite of the strong adsorption in bridge (µ 2 ) coordination geometry at edges, our calculations predict that intermolecular repulsion between adjacent µ 2 -NO species gives rise to the formation of mixed bridge/on-top structures at high coverage. Similarly to the bridge NO at particle edges, the edge-and corner-related µ 1 -NO species reveal characteristic vibrational frequencies, allowing for direct verification of this prediction by IRAS. The present results make possible the identification and monitoring of the occupation of specific sites on Pd nanoparticles by NO during adsorption and reaction processes.

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A systematic study on the interaction of sulfur dioxide (SO2) on BaO-supported Pd nanoparticles has been carried out using suitable models and state-of-the-art density functional (DF) calculations. Detailed information concerning the structure and energetics of the different conformations of adsorbed SO2 is provided as a function of coverage together with calculated infrared reflection absorption spectroscopy (IRAS) spectra. SO2 may adsorb on Pd(111) in several conformations, some active, η2-SbOa and η1-Sb, and others inactive in IRAS, η3-SaOaOa. SO2 is found to attach stronger to Pd nanoparticle edges and corners, a fact intimately related to catalyst poisoning by site blocking. On Pd nanoparticles, SO2 is found to preferably adopt adsorption conformations that depend on the specific region on the nanoparticle, thus adding site specificity to vibrational recognition. Molecular beam experiments and IRAS have been performed on a single-crystal-based Pd/BaAl2x O1+3x /NiAl(110) model NO x storage and reduction catalyst and its individual components. SO x formation on the oxide components, evolution of a SO2 multilayer, and adsorption of SO2 on BaO or Pd nanoparticles is linked to DF calculations. The effect of cation intermixing in the oxide support and overlap of absorption bands on the unequivocal discrimination of signals are discussed.