Modeling NOx Storage Materials: A High ... - ACS Publications

J. Phys. Chem. C 2008, 112, 9835–9846

9835

Modeling NOx Storage Materials: A High-Resolution Photoelectron Spectroscopy Study on the Interaction of NO2 with Al2O3/NiAl(110) and BaO/Al2O3/NiAl(110) T. Staudt,† A. Desikusumastuti,† M. Happel,† E. Vesselli,‡ A. Baraldi,‡ S. Gardonio,§ S. Lizzit,§ F. Rohr,| and J. Libuda*,† Lehrstuhl fu¨r Physikalische Chemie II, Friedrich-Alexander-UniVersita¨t Erlangen-Nu¨rnberg, Egerlandstrasse 3, D-91058 Erlangen, Germany, Physics Department and Center of Excellence for Nanostructured Materials, Trieste UniVersity, Via Valerio 2, I-34127 Trieste, Italy, Laboratorio Nazionale TASC INFM-CNR, in AREA Science Park, 34012 Trieste, Italy, Sincrotrone Trieste S.C.p.A., S.S. 14 Km 163.5, I-34012 Trieste, Italy, and Umicore AG & Co. KG, Rodenbacher Chaussee 4, D-63403 Hanau-Wolfgang, Germany ReceiVed: February 21, 2008; ReVised Manuscript ReceiVed: April 9, 2008

We have studied the interaction of NO2 with a single-crystal-based model NOx storage material, using highresolution 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 (BaAl2xO1+3x) formation for larger nanoparticles. These, however, are overcome by annealing in O2. In a last step, the interaction of the model NOx 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. Introduction Typically, heterogeneous catalysts are complex materials, which are optimized for maximum activity and selectivity under static reaction conditions.1 The idea of storage catalysis represents an intriguing alternative to this static concept. Their operation is based on the accumulation of a specific intermediate on a storage material and subsequent release upon a change of reaction conditions. The additional freedom provided by this concept may open up new possibilities toward tailoring selectivity and activity of catalyst materials. From the point of view of application, two types of storage catalysts have proven to be particularly successful in the field of emission control. One example are ceria-based oxygen storage compounds (OSC) which have helped to substantially improve the performance of the conventional three-way catalyst under the dynamically changing operation conditions of a combustion engine (see, e.g., refs 2 and 3). A second example from the field of exhaust gas aftertreatment are NOx storage and reduction (NSR) catalyst.4 * Corresponding author. E-mail: [email protected]. Fax: +49-9131-8528867. † Friedrich-Alexander-Universita ¨ t Erlangen-Nu¨rnberg. ‡ Trieste University and Laboratorio Nazionale TASC INFM-CNR. § Sincrotrone Trieste. | Umicore AG & Co. KG.

NSR catalysis represents one possible key concept for the reduction of NOx exhaust streams produced under lean-burn conditions.5 Lean-burn engines, i.e., combustion engines which are operated under air-rich condition, provide the advantage of substantially higher fuel and CO2 efficiency. However, they may give rise to problems with respect to the emission of toxic gases, in particular, NOx. The reason is that it is inherently difficult to catalytically reduce NOx in the strongly oxidizing exhaust gas environment under lean conditions. The NSR concept circumvents the problem by converting NOx to nitrates, primarily in the form of Ba(NO3)2 during lean operation. The lean operation cycle is followed by release and reduction of NOx during short fuel-rich operation periods. From the point of view of applied catalysis, the NSR concept has attracted considerable attention after first being suggested by Toyota Company in 1996.4 NSR catalysts made their commercial market debut in the first generation of direct injection gasoline vehicles. In 2006, NSR catalysts have also been introduced on diesel light duty applications, most notably in Daimler’s E320 Bluetech model.6 Especially in Europe, Japan, and North America, the ever-tightening emission standards and durability requirements call for improved NSR catalyst formulations.

10.1021/jp801539e CCC: $40.75  2008 American Chemical Society Published on Web 06/05/2008

9836 J. Phys. Chem. C, Vol. 112, No. 26, 2008

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Figure 1. Schematic representation of preparation procedure for the NOx model storage systems applied in this study. STM images are displayed for the three experimental situations, for which the reaction with NO2 were probed: (i) Al2O3/NiAl(110) (300 × 300 nm2); (ii) “BaO”/Al2O3/ NiAl(110), 1 Å Ba, annealed to 800 K in O2 (300 × 300 nm2); (iii) “BaO”/Al2O3/NiAl(110), 20 Å Ba, annealed to 800 K in O2 (150 × 150 nm2); see text for details.

Advances in material science together with a deeper understanding of the fundamental processes involved in NSR catalysis are a key to improve the catalyst’s performance and durability. Compared with catalysis under steady-state conditions, however, introducing the storage component adds another dimension of complexity to the catalyst material, and, as a result, the underlying reaction mechanisms and their microkinetics remain only poorly understood at the molecular level.5 There are several aspects of the storage and release process, which are controversially being discussed at present (see, e.g., refs 5 and 7–12 and references therein). These points include, for example, the chemical state and the composition of the barium containing aggregates and the resulting differences in storage activity and mechanism. Also, potential mechanisms of restructuring and deactivation including the formation of mixed oxide phases are of great interest. A lot of research has been carried out on technical NSR catalysts in real engine exhaust as well as model gas benches

simulating real exhaust conditions. This type of work mainly consists of macroscopic evaluations in terms of aging phenomena and conversion performance.9,13 Detailed microscopic and mechanistic information is very difficult to obtain in these settings due to the complexity of the catalytic systems. One strategy toward an understanding of the underlying mechanisms and kinetics at the molecular level is based on the development of single-crystal-based model catalysts.14–20 These models allow us to introduce certain aspects of real catalyst, without having to deal with their full complexity. In addition, we can take advantage of the full spectrum of surface science experimental methods. The development of model systems for NSR catalysts, however, is still in its infancy, and only a little work has been published. A first example stems from Stone, Ishii, and Bowker, who have prepared BaO particles and films on Pt(111).21 The approach may be considered an inverse model catalyst, with the oxide component supported on an active metallic substrate.

Modeling NOx Storage Materials Recently, Bowker, Nix, and co-workers presented a comprehensive study of BaO thin film layer growth and reactivity on Cu(111).22 Ordered BaO layers were prepared,23 and the reaction with CO2, H2O, and NOx was followed using X-ray photoelectron spectroscopy (XPS), IRAS (infrared reflection-absorption spectroscopy), and thermal desorption spectroscopy (TPD). The authors were able to show that several reaction steps, including the formation of BaO, hydroxides, carbonates, nitrites, and, to some extent, nitrates can indeed be observed applying this type of model approach. In a second series of model studies, Ozensoy et al. studied the formation of BaO films on Al2O3/NiAl(100)24,25 as well as the interaction of NO2 with Al2O3/NiAl(100)26,27 and BaO/ Al2O3/NiAl(100)28 at room temperature. Again, the formation of nitrates and nitrites was observed. The nitrate species formed on Al2O3 were found to be less abundant and less stable than the nitrate species formed on BaO. Upon annealing, a substantial loss of storage capacity was observed, indicating diffusion of BaO into and intermixing with the Al2O3 support. In our recent work, we have studied the formation and reactivity of BaO nanoparticles on Al2O3/NiAl(110).29–31 The growth and morphology of the BaO deposits have been characterized by STM (scanning tunneling microscopy) and the reaction with NO2 has been monitored by time-resolved IRAS, MB (molecular beam) techniques, and XPS using laboratory X-ray sources. Different elementary reaction steps were successfully monitored and different surface nitrite and nitrate species could be unambiguously identified. In spite of this success, there remain several open questions. One of the most essential issues is the chemical composition of the Ba containing particles and the relation between particle composition on the one hand and reaction mechanism and kinetics on the other. Addressing this point, Szanyi and coworkers recently performed NO2 adsorption experiments on BaO deposits supported on a thick Al2O3 film on NiAl(110), which was prepared by atomic oxygen treatment of NiAl(110).32 On the basis of IRAS spectra of adsorbed NO2, the authors indirectly concluded that there should be strong intermixing of Ba2+ and Al3+ ions under formation of a barium-aluminate-like phase. The formation of such mixed oxides (including aluminate phases) is known from real powder catalysts as well (see, e.g., refs 33–35), and may be associated with changes in the storage behavior. Normally, the formation of aluminates is associated with a negative influence on the storage efficiency. In other studies, however, high trapping efficiencies for NO2 on BaAl2O4 have been reported.36,37 Interestingly, Szanyi and co-workers suggested that contradictory results in the literature, concerning the mechanism of nitrite and nitrate formation, may be related to the formation of such mixed oxide phases.38 In order to obtain more detailed information on the composition and thermal behavior of alumina supported BaO nanoparticles, and on the influence of these factors on the mechanism and kinetics of NO2 uptake, we have performed a synchrotronbased high-resolution photoelectron spectroscopy (HR-PES) study. In this first HR-PES study on a NOx storage model system, we investigate the interaction of NO2 with Al2O3/ NiAl(110) and with BaO deposits on Al2O3/NiAl(110), with a special focus on the interaction of BaO with the underlying alumina support. 2. Experimental Section The core-level photoemission measurements have been performed at the SuperESCA beamline39,40 of ELETTRA in Trieste, Italy. The photoemission spectra were acquired with a

J. Phys. Chem. C, Vol. 112, No. 26, 2008 9837 hemispherical electron energy analyzer with 150 mm mean radius, equipped with a delay-line detector.41 The background pressure in the main chamber was better than 2 × 10-10 mbar during all measurements. All data were acquired at a sample temperature of 300 K. The Ba 4d, Al 2p, O 1s, and N 1s spectra were taken at photon energies of 136, 150, 500, and 650 eV, respectively. This leads to an overall energy resolution between 50 and 200 meV. The spectra were normalized with respect to the ring current and acquisition time. All binding energies (BE) were calibrated with respect to the Fermi level. Stated BEs correspond to the peak maxima after calibration, except of those cases in which numerical procedures were applied (section 3.2, Al 2p). The tunability of photon energy allows us to enhance the photoemission cross section, in particular, for the Al 2p core level which is almost 500 times larger with respect to anodebased X-ray sources. In parallel, the high-energy resolution is a prerequisite to detect changes in the surface, interface, and bulk layer composition of the alumina thin film. The NiAl(110) sample was cleaned by several cycles of Ar+ ion sputtering followed by annealing in ultrahigh vacuum (UHV) up to a temperature of 1300 K. After preparation of the clean NiAl(110) surface, two cycles of oxidation in 2 × 10-6 mbar O2 at 550 K and subsequent annealing in UHV at 1135 K followed, in order to prepare the Al2O3 film. With respect to details of the preparation procedure, we refer to the literature.42,43 Surface cleanliness was checked by inspection of the C 1s region. Prior to the photoemission measurements, the quality of the Al2O3 film was checked by LEED (low-energy electron diffraction). The BaO nanoparticles were prepared as follows: first, Ba metal was manually cleaned under inert gas atmosphere (glovebox) and placed into a Mo crucible. In order to prevent oxidation, the crucible, filled with Ba, was covered with decane before it was mounted inside the evaporator.30 The evaporator was home-built and of resistively heated Knudsen-cell type. The crucible was installed into the UHV chamber immediately before pumping out. The calibration of the Ba source was carried out using a quartz microbalance under UHV conditions. Ba was deposited at 300 K at typical rates ranging from 0.03 to 0.065 Å s-1 (an average film thickness of 1 Å Ba corresponds to 1.6 × 1014 atoms · cm-2) and subsequently oxidized by exposure to 1 × 10-6 mbar O2, using oxidation times between 90 s (1 Å Ba) and 900 s (20 Å Ba). In a second step, the samples were annealed to 800 K in oxygen (1 × 10-6 mbar O2,) in order to decompose peroxides if formed,25 before finally being exposed to NO2. 3. Results and Discussion 3.1. Preparation and Morphology of the Model NOx Storage Systems. Before discussing the results of the photoemission measurements on the interaction of NO2 with the NOx model storage system and the pristine Al2O3 film, we briefly summarize the preparation procedure and previous results on the structural properties of the system (Figure 1; see ref 30 for details). As a model support, we use an ordered and atomically flat Al2O3 film, prepared on a NiAl(110) sample. The structure of this film and its adsorption properties have been characterized in detail43 (see also ref 16), and recently, its microscopic structure has been resolved by a combination of STM measurements and DFT (density functional theory) calculations.44 This film has been used as a model support for the growth of metal nanoparticles in numerous studies.14,45,46 For details concerning the growth of metal particles on this support, we refer to the literature.47 Ba metal is deposited onto the pristine Al2O3 film at a surface temperature of 300 K under UHV conditions and, subsequently,


Figure 2. Sequence of Al 2p photoelectron spectra of the Al2O3/ NiAl(110) model support upon interaction with NO2: (a) Stepwise exposure up to doses of 10 000 L NO2 at 300 K. The inset shows the ratio between the intensity of the spectral components originating from the NiAl substrate and the oxide (see text for discussion); (b) stepwise annealing to successively higher temperatures after exposure to 10 000 L NO2 at 300 K (all spectra taken at 300 K). The inset shows the BE of the Al 2p oxide component as a function of annealing temperature.

is exposed to large doses of oxygen (1 × 10-6 mbar) in order to ensure complete oxidation of the system. Strong interaction of the Ba deposits with the support results in the formation of a stable and uniform distribution of BaO nanoparticles. Nucleation and growth of the particles were characterized by STM measurements.30 The particle size can be varied via the amount of Ba deposited. In the present study, we focus on two different situations, corresponding to the deposition of a nominal Ba layer thickness of 1 Å and 20 Å Ba, respectively (1.6 × 1014 and 3.2 × 1015 Ba atoms · cm-2). In both cases, we observe the formation of a relatively homogeneous distribution of three-dimensional nanoparticles, nucleating both at the Al2O3 domain boundaries and on the oxide terraces. Based on the STM measurements, we can derive a particle density of (5.0 ( 1.0) × 1012 cm-2 for the 1 Å Ba deposit and (5.6 ( 1.0) × 1012 cm-2 for the 20 Å Ba deposit. According to these results and taking into account the nominal Ba layer thickness, it is possible to estimate the average number of Ba2+ ions per particle (see ref 30 for details). For 1 Å Ba deposit, we obtain an estimate of 30 ( 10 Ba2+ ions per particle, and for the 20 Å Ba deposit, the average particle contains 600 ( 200 Ba2+ ions. After comparison of these values to prior results for the growth of other metals on the same sample,16,47 it is obvious that the nucleation density is rather high. This indicates a comparably strong interaction of Ba with the support (resulting in a rather short diffusion length), as confirmed by HR-PES results presented in the following. 3.2. Interaction of NO2 with Al2O3/NiAl(110). In a first step, we investigate the interaction of NO2 with the Al2O3/ NiAl(110) model support. In Figures 2, 3, and 4, sequences of

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Figure 3. Sequence of O 1s photoelectron spectra of the Al2O3/ NiAl(110) model support upon interaction with NO2: (a) Stepwise exposure up to doses of 10 000 L NO2 at 300 K; (b) stepwise annealing to successively higher temperatures after exposure to 10 000 L NO2 at 300 K (all spectra taken at 300 K).

Figure 4. Sequence of N 1s photoelectron spectra of the Al2O3/ NiAl(110) model support upon interaction with NO2: (a) Stepwise exposure up to doses of 10 000 L NO2 at 300 K; (b) stepwise annealing to successively higher temperatures after exposure to 10 000 L NO2 at 300 K (all spectra taken at 300 K).

Al 2p, O 1s, and N 1s photoelectron spectra during interaction with NO2 are shown. In the top panel (A) of each figure, spectra measured at 300 K after stepwise exposure of the Al2O3/ NiAl(110) model support to NO2 up to doses of 10 000 L are displayed. For the pristine Al2O3/NiAl(110) sample, three spectral components are identified in the Al 2p region at BEs

Modeling NOx Storage Materials of 72.5, 73.6, and 75.0 eV. Each component is spin-orbit split into an Al 2p3/2 and an Al 2p1/2 peak by 0.41 eV. The BE values are given for the Al 2p3/2 component and are derived from the numerical fit (dotted line in Figure 2A; “pseudo Voigt” profiles with an intensity ratio of 2:1 and a fixed spin-orbit splitting were used for all three components, for comparison, see, e.g., refs 42 and 48). According to previous studies, the low BE component can be attributed to the NiAl substrate, the medium BE component to the interface between substrate and oxide film and the high BE component to the Al3+ ions in Al2O3 oxide film (see, e.g., refs 42 and 48). With increasing NO2 exposure, the intensity of the high BE Al 2p spectral component (originating from the oxide film) increases (see Figure 2A). In parallel with this effect, we observe a decrease of the component originating from the interface and the NiAl metal substrate. The Al 2p oxide to metal peak ratio (IO/IM) (calculated from the peak height) is plotted in the inset of Figure 2A as a function of NO2 exposure. If we exclude the contribution of photoelectron diffraction effects, the logarithm of this value (ln IO/IM) should be directly proportional to the oxide film thickness (compare, for example, ref 49). The increase in ln IO/IM illustrates that the interaction with NO2 leads to a further oxidation of the support, resulting in an increase of the Al2O3 film thickness. The oxide film growth finally appears to saturate at large exposures around 10 000 L NO2. BE shifts during NO2 exposure will be discussed below, together with the shifts in the O 1s spectra. Figure 3A displays the evolution of the O 1s spectra. The first spectrum taken at the clean Al2O3/NiAl(110) sample shows the typical asymmetric O 1s signal with a peak located at a binding energy of 531.44 eV.48,50 After high exposures of NO2, the spectrum broadens and a shoulder appears on the high BE side at about 533.5 eV. In Figure 4A, the N 1s region is displayed. The bottom-most spectrum, acquired from the pristine Al2O3/NiAl(110) surface, reveals a weak background feature only, located at a binding energy of ∼397 eV. This feature vanishes completely upon exposure to NO2. At low exposures, the formation of a peak at 404.5 eV is observed. With increasing NO2 exposure, this feature grows in intensity and, finally, saturates around 1000 L. At large exposures (10 000 L), a second weak peak around 407.5 eV starts to form, while the intensity of the feature at 404.5 eV slightly decreases and shifts to a BE of 404.36 eV. Simultaneously with the formation of the high BE peak, a broad feature occurs at a BE around 400 eV. In a conventional XPS study, Ozensoy et al. reported a BE of around 405.8 eV for a surface nitrite species (NO2-) on Al2O3/NiAl(100) and a BE of 407-408 eV for nitrates (NO3-) on the same sample.26 In our recent work, we observed a BE of 404.9 eV for surface nitrites and 408.5 eV for surface nitrates on BaO/Al2O3/NiAl(110).29 With these studies taken into account, the features at a binding energy of 404.5 and around 407.5 eV can be assigned to surface nitrite and surface nitrate species, respectively. A rough estimate of the nitrite density can be obtained by comparing the intensities of the N 1s signal to the O 1s signal of the pristine oxide film. Taking into account the photoionization cross sections and the oxygen ion density of the pristine film,44 we obtain an estimate of 8 × 1014 NO2ions · cm-2 (assuming a homogeneous distribution of nitrogen and oxygen within the surface layers). Although such estimates have to be treated with outmost care, the result shows that the Al2O3 film is not inert but relatively high nitrite coverages can be obtained. Concerning the broad feature occurring at a BE around 400 eV, there are several nitrogen-derived species which may give

J. Phys. Chem. C, Vol. 112, No. 26, 2008 9839 rise to this peak, such as, for example, Ti–NO or Ti–NHx (see refs 51–53). Due to its high thermal stability, however, we tentatively attribute this feature to a surface nitride (N3-) species in a mixed aluminum oxinitride (Al2O3-3xN2x), rather than to a molecular type of adsorbate (see discussion below). N 1s features in a similar BE range were previously reported for aluminum oxynitride films and assigned to nitrogen ions surrounded by neighboring oxygen ions.54 However, this species was not yet observed on the Al2O3/NiAl(110) or on the Al2O3/ NiAl(100) (compare refs 26 and 29). The occurrence of oxynitrides will be discussed in more detail below, in connection with annealing experiments performed on this sample. The identification of the NOx species in the O 1s region is complicated due to the overlap with the O 1s components of the Al2O3 film. After exposure to up to 10 L NO2, the O 1s features shift by -0.2 eV to a BE of 531.24 eV. The peak shifts back to 531.46 eV upon further exposure (100 L). In the limit of larger NO2 exposures, there is no significant further shift; however, we observe the appearance of a high BE shoulder. With the N 1s spectra taken into account, this shoulder can be attributed to the formation of surface nitrites (and possibly traces of nitrates). It is noteworthy that the Al 2p spectra show very similar behavior with respect to the observed BE shifts (∆BE). A negative ∆BE of -0.25 eV occurs after exposure to 10 L NO2, followed by a positive ∆BE of +0.15 eV after 100 L. Again, the ∆BE observed at higher exposure is very small. Similar to the present case, Ozensoy et al. reported large negative ∆BE (-0.8 eV) for the O 1s spectrum immediately after the Al2O3/NiAl(100) surface was exposed to a small dose of NO2.26 The effect was explained by a strong electrostatic interaction between the adsorbed nitrite and the oxide surface. They also observed a broadening of the signal at high exposures of NO2, explained by the formation of nitrates on the sample. On the basis of a purely electrostatic picture and assuming that changes in the final state contributions can be neglected, the negative ∆BE can be attributed to the generation of a negative surface charge. As the BE is referenced to the Fermi level of the NiAl substrate, the outermost oxide core levels are destabilized as a result of the corresponding changes of the electrostatic potential. Such electrostatically induced BE shifts were observed not only upon adsorption of molecules, but also upon deposition of metals on the same oxide film. Similarly, they arise as a result of charge transfer from the support to the metal particle.48,50 It should be pointed out that the negative ∆BE is compatible with the formation of negative ions on the surface, as would be expected for NO2-. Upon further reaction, it can be expected that the growing oxide film will be subject to strong structural transformations, and the resulting charge redistribution may then give rise to the inverse BE shift. The bottom panel (B) of each figure shows the effect of the stepwise annealing of a sample that was previously exposed to 10 000 L NO2 at 300 K. All photoemission spectra were acquired after the surface was briefly heated to the given temperature and subsequently cooled to 300 K. Figure 2B shows that the Al 2p oxide component successively shifts to higher BE upon annealing to increasing temperature (∆BE ) 0.4 eV after heating to 650 K). The relation between the BE of the Al 2p oxide component and the flashing temperature is plotted in the inset of Figure 2B. For the O 1s region, a similar ∆BE of +0.4 eV to a final BE of 531.86 eV is found (see Figure 3B). The BE shifts are accompanied by a slight increase of the Al 2p oxide peak intensity and an attenuation of the metal peaks, respectively.

9840 J. Phys. Chem. C, Vol. 112, No. 26, 2008 From the changes in the relative intensities of the Al 2p components, it can be concluded that thermal decomposition of the surface nitrites and nitrates gives rise to further oxidation of the support and, therefore, to a further increasing thickness of the Al2O3 film. The parallel BE shifts in the Al 2p oxide component and O 1s regions once again indicate that they are predominately originating from electrostatic contributions as a result of changes of the surface charge distribution. Both the BE shifts and the increasing oxide thickness appear to approach saturation around annealing temperatures of 650 K. The two effects can be explained, taking the N 1s region into account (Figure 4B). A strong decrease in the intensity of both the nitrite and nitrate peaks is observed around 450 K. Both features completely vanish after heating to 550 K, indicating parallel decomposition of both corresponding species. The observed thermal behavior is in good agreement with recent studies using IR reflection-absorption spectroscopy, which also show parallel decomposition of surface nitrites and nitrates on Al2O3 around 450 K (see, e.g., ref 30). Ozensoy et al. also studied the thermal decomposition of surface nitrates and nitrites, however, on Al2O3/NiAl(100).26 Although the temperature steps applied in this study were relatively large, the observed decomposition behavior is compatible with the present results. One aspect which deserves special attention, however, is the low BE feature in the N 1s spectrum, which appears at a BE of around 400 eV (see Figure 4B). The feature remains present up to a heating temperature of 650 K. As mentioned above, this high thermal stability supports the assignment to a surface nitride (N3-) within a mixed surface aluminum oxynitride (Al2O3-3xN2x) rather than to a molecular species. The very large width of the peak is attributed to an inhomogeneous broadening due to the low degree of ordering. The intensity ratio between the N 1s components originating from NO2- and from N3- approach a value of 2:1 in the limit of large NO2 exposure (10 000 L) before annealing. Upon annealing, the N3- peak narrows and shows a slight negative ∆BE of -0.3 eV. We attribute this effect to diffusion of nitride ions into the oxide support. Prolonged diffusion finally gives rise to the bulk nitride species observed at a BE of 397 eV. The assignment of this feature to a bulk species is supported by the observation that the species remains detectable as a contamination, even after prolonged sputtering/annealing treatments. 3.3. Formation and Characterization of BaO Deposits on Al2O3/NiAl(110). In the next step, we consider the preparation and the properties of BaO deposits on the pristine Al2O3/ NiAl(110) model support. In Figure 5, photoelectron spectra of the Al 2p region are shown for two different model systems, prepared by deposition of a nominal layer thickness of 1 Å Ba (Figure 5A) and 20 Å Ba (Figure 5B), respectively (see section 3.1 for details). The corresponding Ba 4d and O 1s spectra are displayed in Figure 6. Focusing on the Al 2p region first, we start by considering the corresponding spectra of the pristine Al2O3/NiAl(110) sample (Figure 5, solid lines). As discussed in section 3.2, the Al 2p spectra consist of three components at BEs of 72.5, 73.5, and 74.9 eV (Al 2p3/2), attributed to the NiAl(110) substrate, the interface, and the Al3+ oxide ions, respectively. The thin dotted lines in Figure 2B show a corresponding fit (see section 3.2 for details). Concerning the preparation of the BaO deposits, we consider two experimental situations: first, metallic Ba (1 Å, Figure 5A; or 20 Å Ba, Figure 5B) was deposited under UHV conditions at a surface temperature of 300 K and, subsequently, exposed to large doses of O2 at 300 K in order to

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Figure 5. Photoelectron spectra of the Al 2p region upon preparation of the model NOx storage system: (a) Al 2p spectra of the pristine Al2O3/NiAl(110) (solid line), after deposition of 1 Å Ba and subsequent exposure to O2 at 300 K (dotted line), and after subsequent annealing in O2 up to 800 K (dashed line; see text for details). (b) Same as in (a), but for deposition of 20 Å Ba. The thin dotted lines show a fit for the pristine Al2O3/NiAl(110), which indicates the different components comprising the Al 2p signal.

ensure complete oxidation (2 × 10-6 mbar; duration 90 s for 1 Å Ba and 900 s for 20 Å Ba; dotted lines in Figure 5A,B). In a second step, the model surfaces were annealed in oxygen (2 × 10-6 mbar) up to a temperature of 800 K (dashed lines in Figure 5A,B). The reason for the annealing step is that in previous studies on similar systems peroxides (O22-) were observed, which decompose at these temperatures.25 Upon closer inspection of the Al 2p spectra, it is observed that, after initial oxidation at 300 K, the intensity of both the NiAl substrate component and the interface component decreases drastically for the 1 Å Ba sample. This effect is even more pronounced after deposition of 20 Å Ba. Surprisingly, the behavior of the Al 2p oxide (high BE) component is very different. For the 1 Å Ba deposit, there is hardly any change in intensity, whereas only a slight decrease is found for deposition of 20 Å Ba. Simultaneously, there are shifts of the BE of the Al 2p oxide component which however are rather small (on the order of 0.1 eV). In order to interpret the spectral changes in the Al 2p region, we have to take into account the behavior of the Ba 4d and O 1s spectra. The corresponding data are displayed in Figure 6. The Ba 4d spectrum consists of a spin-orbit-split doublet (∆BE ) 2.56 eV), with the Ba 4d5/2 component at 91.0 eV after preparation at 300 K. For the 1 Å Ba deposit (Figure 6A), there is hardly any change in intensity upon annealing in oxygen to 800 K. The corresponding BE shift is rather small as well (∆BE ) +0.1 eV). For the 20 Å deposit (Figure 6B), however, the intensity in the Ba 4d region decreases substantially upon annealing, and there is a clear shift to higher BE by +0.5 eV (Ba 4d5/2: BE ) 91.52 eV). Considering the O 1s spectra, the BE shifts are small for the 1 Å Ba deposit, both upon preparation

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Figure 6. Photoelectron spectra of the Ba 4d and the O 1s regions upon preparation of the model NOx storage system: (a) Spectra of the pristine Al2O3/NiAl(110) (solid line), after deposition of 1 Å Ba and subsequent exposure to O2 at 300 K (dotted line), and after subsequent annealing in O2 up to 800 K (dashed line; see text for details). (b) Same as in (a), but for deposition of 20 Å Ba.

at 300 K and after annealing. For the 20 Å Ba deposit, however, these shifts are more substantial and amount to approximately +0.2 eV (preparation at 300 K; Figure 6 B, dotted line) and +0.4 eV, respectively (annealing on O2 at 800 K; Figure 6, dashed line). From these observations, we come to a conclusion that may appear surprising at the first glance: upon Ba deposition and oxidation, the surface concentration of Al3+ ions remains nearly constant (1 Å Ba) or even increases (20 Å BA). Still, a high concentration of Ba2+ ions is generated in the surface layer (as indicated by the Ba 4d spectra). These findings can only be rationalized by rapid intermixing of Ba2+ and Al3+ ions, which takes place even at 300 K. Further evidence for mixed oxide formation can be derived from the observed BE shifts. In contrast to the electrostatically induced BE shifts affecting the various regions in a similar fashion (see section 3.2), we now observe different BE shifts for the Al 2p (Al3+) and Ba 4d signals. In particular for the 20 Å Ba deposit, it is evident that shifts in different direction occur upon annealing to 800 K. This observation clearly points to substantial changes in the chemical environment of the different ions. We attribute this change in environment to the intermixing of Ba2+ and Al3+ ions, resulting in the formation of mixed oxide particles (BaAl2xO1+3x). The formation of mixed Ba-Al oxides, eventually resulting in the formation of barium aluminates, is well-known from NSR powder catalysts33 and may be directly related to deactivation phenomena occurring on these systems. The formation of mixed oxide surface layers has also been suggested for NSR model systems: Szanyi and co-workers32 attributed changes in IRAS spectra observed upon exposure of BaO deposits on Al2O3/ NiAl(100) to NO2 to an intermixing of Ba2+ and Al3+ ions. They concluded that intermixing may result in the formation of barium-aluminate-like surface layers, even in the limit of very thin BaO films. In a conventional photoemission study, Ozensoy et al. performed stepwise Ba deposition and subsequent oxidation on a Al2O3/NiAl(100) samples.25 Ba 4d3/2 and Ba 4d5/2 signals located at a BE around 91.5 and 94.0 eV were reported,

in good agreement with the results from our study. We may speculate that mixed oxide formation also occurred in their studies. The present results provide direct evidence for the facile formation of mixed oxide particles (BaAl2xO1+3x). The exact stoichiometry cannot be determined on the basis of the present data. It is noteworthy, however, that in the limit of small particles (1 Å Ba deposit) nearly complete intermixing occurs even at 300 K. This conclusion follows from the observation that subsequent annealing to 800 K does not lead to further changes in the intensity or BE of the Ba 4d signal. Only the intensity changes in the Al 2p components indicate a slight increase in the oxide layer thickness. For the 20 Å deposit, however, a different behavior is observed. The initial decrease in the Al 2p signal and the subsequent increase upon annealing are due to the fact that the mixed oxide formation at 300 K is incomplete. Instead, we find an enrichment of Ba2+ ions in the surface region at 300 K and thermally activated BaAl2xO1+3x formation upon annealing to 800 K. We conclude that, for larger particles, there is a partial kinetic hindrance to mixed oxide formation at 300 K, whereas no such hindrance can be observed in the limit of small particles. Concerning the mechanism of interaction of O2 with the Ba containing particle, it was suggested that peroxide formation may play an important role during interaction with O2 (see, e.g., ref 25 and references therein). Indeed, peroxide formation was observed in several studies.25,29,55,56 Recently, Bowker et al. reported evidence for peroxide formation, combining STM and XPS experiments on an inverse NSR model catalyst.57 A feature at a BE of 533 eV was attributed to BaO2. A similar assignment was suggested be Ozensoy et al.25 Upon closer inspection of the present data, we may indeed identify a high BE shoulder around 533.5 eV for the larger BaAl2xO1+3x particles (20 Å Ba) after exposure to O2 at 300 K (see Figure 6). This shoulder vanishes upon annealing to 800 K and, therefore, may be tentatively assigned to an O22- species (compare ref 25). In addition to the high BE component attributed to peroxides, the


Figure 7. Sequence of photoelectron spectra of the Al 2p region taken upon exposure of the model NOx storage systems to NO2: (a) BaO/ Al2O3/NiAl(110) prepared by deposition of 1 Å Ba, subsequent exposure to O2 at 300 K, and subsequent annealing to 800 K in O2, exposed to increasing doses of NO2 at 300 K. (b) Same as for (a), but for model systems prepared by deposition of 20 Å Ba. The insets show the ratio between the intensity of the spectral components originating from the NiAl substrate and the oxide (see text for discussion). The dashed lines correspond to the situation before exposure to NO2.

appearance of a O 1s low BE feature below 530 eV BE was reported in several studies and, tentatively, attributed to disordered or surface BaO species.25,29 In the present study, no indication for such a low BE species was observed, suggesting that the corresponding entity may not be an inherent feature of the BaAl2xO1+3x/Al2O3 model system. 3.4. Interaction of NO2 with BaO Deposits on Al2O3/ NiAl(110). In a last step, we investigate the interaction of NO2 with the BaAl2xO1+3x mixed oxide particles on Al2O3/NiAl(110) and the thermal decomposition of the nitrite and nitrate species formed on this sample. In Figures 7, 8, and 9, sequences of photoelectron spectra of the Al 2p, O 1s, Ba 4d, and N 1s regions are displayed upon stepwise exposure of the samples to NO2 at 300 K. Again, we consider two types of model surfaces, prepared by deposition of 1 Å Ba (A) and 20 Å Ba (B), respectively. Both samples were prepared by O2 exposure at 300 K and subsequent annealing to 800 K in O2 atmosphere (see sections 3.1 and 3.3). First, we focus on the Al 2p region (Figure 7). Again, we observe a slight increase in intensity of the high BE Al 2p component (originating from the oxide) with increasing NO2 exposure. In parallel, the intensities of the NiAl substrate and interface components decrease further. The corresponding oxide to metal peak ratios (which were calculated from the corresponding peak heights) are shown in the insets in Figure 7A,B. As mentioned before, ln IO/IM shouldsin first approximationsbe a measure for the average oxide layer thickness. It is found that the oxide layer thickness further increases upon NO2 exposure.

Staudt et al. At large exposure, ln IO/IM values similar to those for NO2 exposure of the pristine Al2O3 film are found (see section 3.2). Note that the initially larger value for the oxide thickness as compared to the pristine Al2O3 film results from preoxidation after Ba deposition (see section 3.3). We suggest that the decreasing rate of oxide growth upon NO2 exposure is mainly due to the inhibition of Al3+ ions diffusion on thick oxide films, hindering further oxidation. This hypothesis is supported by the observation that the most rapid increase in thickness is observed for the very thin pristine alumina film and the slowest rate for the thick 20 Å Ba deposit. In the next step, we consider the N 1s spectra during NO2 exposure. The corresponding spectra are displayed in Figure 9 A,B. Note that the background signal around 397 eV is due to a nitride contamination in the NiAl support, which becomes increasingly difficult to remove after repeated preparation steps involving extended NO2 exposure. For both particle sizes (1 Å and 20 Å Ba deposit), the N 1s spectra are very similar. At low exposures (10 L), a signal at a BE of 404.6 (1 Å Ba) and 404.4 (20 Å Ba) appears, which grows only very slowly with increasing exposure. In the limit of large exposures (10 000 L), a faint second feature starts to appear at a BE of 407.9 eV. The observed behavior is consistent with the results of a previous study using a laboratory X-ray source. Two features were observed at BEs of 404.9 and 408.5 eV, respectively29 (the differences in BE may be due to differences in the calibration). Supported by a comparison with IR data, the two N 1s features at low and at high BE were assigned to surface nitrite and surface nitrate species, respectively. This assignment is in accord with a previous study by Bowker, Nix, and coworkers, who reported BEs of 404.0 eV for nitrite species on BaO/Cu(111) and 408.0 eV for nitrates on the same substrate.22 Similar results were reported by Ozensoy et al., who assigned a feature at 407.1 eV to nitrates on BaO/Al2O3/NiAl(100).28 In the insets in Figure 9, the intensity of the nitrite (404.5 eV BE) and nitrate (407.9 eV BE) signals (determined from the peak heights) are displayed a function of NO2 exposure. It is apparent that, at the initial stage of the reaction, only nitrites are formed on both the large and the small particles. With increasing exposure, nitrite formation first becomes very slow and, subsequently, surface nitrates appear. As shown recently, the nitrite coverage finally decreases in the limit of largest exposures.29 From this observation, it clearly follows that the initial stable intermediate of the reaction is a surface nitrite. This holds at least for the present model system consisting of mixed BaAl2xO1+3x nanoparticles and at a reaction temperature of 300 K. This nitrite may be generated either by adsorption of NO2 on a coordinatively unsaturated Ba2+ site or by adsorption and dissociation of NO2 over an O2- site. Both pathways involve oxidation of the particle surface (Ba peroxide may be formed as an intermediate, compare ref 58). Finally, the excess oxygen generated by this reaction is consumed by further oxidation of the NiAl substrate under formation of Al2O3. At a later stage of the reaction, the surface nitrites are successively converted into nitrates, which represent the final product of the reaction (compare refs 7, 55, and 59). In view of the recent controversial discussion on the mechanism of the initial steps of the NOx storage process (see ref 38 and references therein), the above observations deserve some further discussion. In several previous model studies, the initial formation of nitrites was suggested.22,29,58 This observation appears to be in agreement with some studies on NSR powder catalysts pointing into a similar direction (refs 7 and 55; compare


J. Phys. Chem. C, Vol. 112, No. 26, 2008 9843

Figure 8. Sequence of photoelectron spectra of the O 1s and Ba 4d region taken upon exposure of the model NOx storage systems to NO2: (a) BaO/Al2O3/NiAl(110) prepared by deposition of 1 Å Ba, subsequent exposure to O2 at 300 K, and subsequent annealing to 800 K in O2, exposed to increasing dosed of NO2 at 300 K. (b) Same as for (a), but for model systems prepared by deposition of 20 Å Ba. The dashed curves in the O 1s region represent difference spectra between two peak-normalized spectra at different exposure (see text for discussion).

also ref 5). In contrast, there are theoretical calculations that predict the preferential formation of nitrite-nitrate pairs.60–64 Briefly, adsorption of the first NO2 should give rise to the generation of an electronic defect, which is healed by adsorption of the second NO2 molecule. The resulting NO2-/NO3- pair is expected to be strongly stabilized, even if the two ions are not located in direct proximity.64 Recently, Szanyi and co-workers have presented direct experimental evidence for the formation of such NO2-/NO3- pairs.38 Toward this aim, they utilized a special preparation procedure, which should largely prevent intermixing of Ba and Al ions, and adsorbed NO2 at cryogenic temperatures. IRAS and XPS data clearly indicated simultaneous formation of nitrites and nitrates in comparable amounts. The findings on the present model system are in clear contrast with these results. Briefly, there are two possible explanations for this discrepancy. The first is related to the formation of the BaAl2xO1+3x mixed oxide particles. We may assume that the reaction mechanism depends on the composition of the Bacontaining oxide phase, as was already suggested recently.32,38 Such a change in mechanism may also be consistent with the change in NOx storage behavior which is associated with the formation of aluminate-like phases on NSR powder catalysts.5,33 The second difference between the two studies is related to the reaction conditions. Szanyi and co-workers performed their experiments at cryogenic temperatures, initially leading to the formation of N2O4 dimers.38 We may speculate that formation of the D2h dimer (O2N-NO2) may give rise to a change in reaction mechanism. Via isomerization/disproportionation to ONO-NO2 and finally NO+NO3-(compare ref 65), a Lewis acid/base pair may be generated which may facilitate formation of a nitrate-nitrite pair. Related mechanisms have been suggested in the literature.66 Further systematic and quantitative

experiments as a function of surface composition and temperature will be required in order to finally clarify this point. Next, we come to the low BE region of the N 1s spectra. Similar as for the pristine Al2O3 support, we observe the appearance of a broad peak around 399-400 eV, which was assigned to a surface nitride (N3-) species, generated by complete decomposition of the surface nitrite, formed initially (see section 3.2). It is noteworthy, however, that this feature is substantially weaker on the Ba-containing model system than on the pure Al2O3/NiAl(110) model support, especially in the case of the large BaAl2xO1+3x particles (deposition of 20 Å Ba). In the limit of large NO2 exposure (10 000 L), the intensity ratio between the N 1s components assigned to the NO2- and the nitride are typically in the range between 5:1 and 10:1. We conclude that decomposition of nitrogen-oxo species to N3- and, consequently, the formation of aluminum oxy-nitrides is strongly suppressed on the Ba containing particles. After discussing the N 1s region, we consider the evolution of the Ba 4d spectra (see Figure 8). For the small BaAl2xO1+3x particles (1 Å Ba deposit; Figure 8A), the Ba 4d5/2 signal, located at 91.3 eV, successively shifts by -0.4 eV with increasing NO2 exposure. Surprisingly, the larger particles (20 Å Ba deposit; Figure 8B), show a somewhat different behavior. Starting from an initial BE of 91.5 eV (Ba 4d5/2), a sudden shift by -0.5 eV is observed upon the initial NO2 dose. Further dosing leads to smaller successive shifts to lower BE. It should be noted that, during the initial NO2 dose, we observed similar BE shifts in the O 1s (∆BE ) -0.5 eV) and Al 2p regions (∆BE ) -0.3 eV) (see Figures 7 and 8). Therefore, we predominately assign the initial ∆BE to the generation of a negative surface charge upon primary NO2 exposure, possibly due to rapid formation of surface nitrites (note


Staudt et al.

Figure 9. Sequence of photoelectron spectra of the N 1s region taken upon exposure of the model NOx storage systems to NO2: (a) BaO/ Al2O3/NiAl(110) prepared by deposition of 1 Å Ba, subsequent exposure to O2 at 300 K, and subsequent annealing to 800 K in O2, exposed to increasing dosed of NO2 at 300 K. (b) Same as for (a), but for model systems prepared by deposition of 20 Å Ba. The insets show the intensity of the features assigned to surface nitrates and nitrites as a function of NO2 exposure.

Figure 10. Sequence of photoelectron spectra of the N 1s region taken after exposure of the model NOx storage systems to 10 000 L NO2 and stepwise annealing to increasing temperatures (all spectra acquired at 300 L): (a) BaO/Al2O3/NiAl(110) prepared by deposition of 1 Å Ba, subsequent exposure to O2 at 300 K, and annealing to 800 K in O2. (b) Same as for (a), but for a model system prepared by deposition of 20 Å Ba.

that, due to the different ion positions in the thin film structure, electrostatically induced BE shifts may differ to some extent for the different types of ions). The effect is considerably weaker for the small particles. Further BE shifts upon continuing NO2 exposure are small and can be attributed to continuing formation of surface nitrites and, finally, surface nitrates. Similar shifts have previously observed in a conventional XPS study.29 It is noteworthy that at a later stage of the reaction nitrite and nitrate formation occurs at a very low rate. This change in kinetics may reflect the necessity of surface restructuring of the BaAl2xO1+3x particles upon reaction with NO2. This hypothesis that, at a later stage, the reaction may be associated with a massive restructuring process is supported by the following observation: in spite of the high abundance of Al3+ ions in the surface region of the larger BaAl2xO1+3x particles, IRAS experiments show evidence for the formation of nitrates on Ba2+ only, and no evidence for the characteristic bands for surface nitrates on alumina.29,30 In accord with this interpretation, Yi et al. suggested in a recent publication the formation of Ba(NO3)2 from thin barium-aluminate-like layers.32 In view of these results, we may suggest that the surface restructuring process of the BaAl2xO1+3x nanoparticles during the reaction with NO2 involves the ejection of Ba2+ ions from the surface of the mixed oxide and the formation of Ba surface nitrates. Identification of the reaction-induced effects in the O 1s spectra are slightly complicated by the overlap of the corre-

sponding peaks (see Figure 8). In addition to the shift by -0.5 eV, which occurs during the initial NO2 dose on the large BaAl2xO1+3x particles (attributed to electrostatic effects; see discussion above), we observe a slight broadening of the signals and the appearance of a high BE shoulder. Difference spectra (dashed lines in Figure 8B) reveal that the shoulder is due to a broad feature in the region between 532 and 534 eV BE. We assign the shoulder to O 1s components originating from the formation of surface nitrites and, at highest exposure, of surface nitrates. This observation is consistent with the assignment in our previous conventional XPS study.29 In addition, there are several other reports of features around 533 eV, which have been assigned to both surface nitrite and nitrate species.22,26,28 Finally, we briefly discuss the thermal decomposition of the surface nitrites and nitrates on the BaAl2xO1+3x particles. The corresponding N 1s spectra are displayed in Figure 10. Briefly, both N 1s components at 407.9 and 404.4 eV BE vanish at temperatures around 450 K. The decomposition temperatures are very similar for the smaller (1 Å Ba) and larger (20 Å Ba) particles. We conclude that the surface nitrates and nitrites generated at 300 K show similar stability for both types of particles. This behavior is in sharp contrast to the behavior of surface nitrate layers formed at elevated temperatures.30 In a recent study, we have shown that at elevated temperature and on larger particles nitrate layers are formed, which contain both ionic and surface nitrate species.30 This layers show significantly


Figure 11. Schematic representation of processes and reaction occurring on the NOx model storage systems investigated in this study (see text for discussion).

enhanced thermal stability, an effect which was tentatively attributed to a mutual stabilization of nitrate species in larger aggregates. In contrast to the molecular N 1s features at high BE, the low BE feature around 400 eV is characterized by an exceptionally high thermal stability and remains present up to temperatures of at least 750 K. As discussed in section 3.2, this behavior is consistent with the assignment to a surface nitride (N3-). Upon annealing, the nitride ions may diffuse into the Al2O3 film, leading to formation of a mixed oxy-nitride. In the present spectra, this diffusion process is reflected be a gradual broadening of the corresponding N 1s peak and a shift to lower BE. 4. Conclusions Using HR-PES, we have studied (i) the interaction of NO2 with an ordered Al2O3 film on NiAl(110); (ii) the interaction of BaO nanoparticles, prepared in UHV by Ba deposition and subsequent oxidation, with Al2O3/NiAl(110) model support; and (iii) the interaction of NO2 with BaO nanoparticles on Al2O3/ NiAl(110) as a function of particle size. A schematic representation of the processes and reactions identified is given in Figure 11. (i) Reaction of NO2 with Al2O3/NiAl(110). The reaction of NO2 with the Al2O3/NiAl(110) model support was studied at 300 K. Initially, slow formation of surface nitrites occurs. Nitrite formation saturates at exposures around 1000 L. Simultaneously, further oxidation of the support occurs, leading to an increasing

J. Phys. Chem. C, Vol. 112, No. 26, 2008 9845 thickness of the Al2O3 film. At large NO2 exposure, very slow formation of surface nitrates is observed. Simultaneously, nitride species are generated, most likely in the form of mixed aluminum oxynitrides. The surface nitrites and nitrates on Al2O3 show low thermal stability, decomposing at temperatures around 450 K. (ii) Interaction of BaO nanoparticles with Al2O3/NiAl(110). BaO were prepared by physical vapor deposition of metallic Ba onto the pristine Al2O3/NiAl(110) in UHV and subsequent oxidation by O2. It is found that rapid intermixing of Ba2+ and Al3+ ions occurs under formation of a mixed oxide BaAl2xO1+3x with increased film thickness. Whereas intermixing is nearly complete for small particles, kinetic hindrances prevent full intermixing for large BaO aggregates at 300 K. Upon annealing in oxygen, mixed oxide formation is enhanced and further growth of the oxide film occurs. (iii) Reaction of NO2 with BaO/Al2O3/NiAl(110). Upon exposure of the mixed oxide BaAl2xO1+3x particles on Al2O3/ NiAl(110) to NO2, formation of nitrites occurs, at a higher rate than for the pristine Al2O3/NiAl(110). Surface nitrite formation completely saturates at NO2 exposures around 1000 L. Afterward, surface nitrates are generated, suggesting transformation of nitrites into nitrates. In combination with previous results from vibrational spectroscopy, it is concluded that the surface nitrates and nitrites are primarily coordinated to Ba sites. During NO2 exposure, further oxidation of the support occurs, leading to an increasing thickness of the Al2O3 film and/or the BaAl2xO1+3x particles. In contrast to reaction of NO2 with the pristine Al2O3/NiAl(110), nitride formation is strongly suppressed. The surface nitrates and nitrites, formed at 300 K, are relatively weakly bound and thermally decompose in a similar temperature range as the surface species on the alumina model support. The present study shows that thin-film-based model NOx storage systems allow us to monitor numerous elementary processes on NSR systems in an ideal UHV environment. The thin film oxide support is, however, not inert toward further oxidation. Thus, the corresponding processes involving the support film and their effect on the elementary storage reactions have to be investigated in detail. In the present case, the most prominent effect is the intermixing of Ba2+ and Al3+ ions, accompanied by an increasing oxide film thickness. Although the formation of the mixed oxide may be accelerated by the thin film character of the model system, the general phenomenon of mixed oxide formation is observed on real powder catalysts as well. The present model approach will allow us to vary the composition of the Ba/Al mixed oxide phase and probe the effect of the composition on the storage behavior. Acknowledgment. The authors are particularly grateful to Zhihui Qin, Shamil Shaikhutdinov, and Hans-Joachim Freund (FHI Berlin) for providing STM images and facilities. Fruitful discussions with Henrik Gro¨nbeck concerning the spectral assignments and comparison with theory are acknowledged. We thank Herbert Pfnu¨r (University Hannover) for helpful comments concerning the deposition of barium. We acknowledge financial support of the DFG (LI 909/7-1, “Models for Nanostructured Storage Catalysts“), DAAD (PPP, Acciones Integradas Hispano-Alemanas), EU (COST D-41), the Fonds der Chemischen Industrie, and the “Zerweck Fonds” (Universita¨tsbund Erlangen-Nu¨rnberg). References and Notes (1) Thomas, J. M.; Thomas, W. J. Principle and Practice of Heterogeneous Catalysis;VCH: Weinheim, 1997.

9846 J. Phys. Chem. C, Vol. 112, No. 26, 2008 (2) Di Monte, R.; Kaspar, J. Top. Catal. 2004, 28, 47. (3) Heck, R. M.; Farrauto, R. J.; Gulati, S. T. Catalytic Air Pollution Control - Commercial Technology: John Wiley & Sons, 2002. (4) Takahashi, N.; Shinjoh, H.; Iijima, T.; Suzuki, T.; Yamazaki, K.; Yokota, K.; Suzuki, H.; Miyoshi, N.; Matsumoto, S.; Tanizawa, T.; Tanaka, T.; Tateishi, S.; Kasahara, K. Catal. Today 1996, 27, 63. (5) Epling, W. S.; Campbell, L. E.; Yezerets, A.; Currier, N. W.; Parks II, J. E. Catal. ReV. 2004, 46, 163. (6) Schommers, J.; Enderle, C.; Breitbach, H.; Lindemann, B.; Stotz, M.; Paule, M. Aachener Kolloquium Fahrzeug- und Motorentechnik; Aachen: Germany, 2006. (7) Nova, I.; Castoldi, L.; Prinetto, F.; Santo, V. D.; Lietti, L.; Tronconi, E.; Forzatti, P.; Ghiotti, G.; Psaro, R.; Recchia, S. Top. Catal. 2004, 30/31, 181. (8) Piacentini, M.; Stroebel, R.; Maciejewski, M.; Pratsinis, S. E.; Baiker, A. J. Catal. 2006, 243, 43. (9) Rohr, F.; Peter, S. D.; Lox, E.; Ko¨gel, M.; Sassi, A.; Juste, L.; Rigaudeau, C.; Belot, G.; Gelin, P.; Primet, M. Appl. Catal. B: EnViron. 2005, 56, 201. (10) Castoldi, L.; Matarrese, R.; Lietti, L.; Forzatti, P. Appl. Catal. B: EnViron. 2006, 64, 25. (11) Tonkyn, R. G.; Disselkamp, R. S.; Peden, C. H. F. Catal. Today 2006, 114, 94. (12) Dawody, J.; Tonnies, I.; Fridell, E.; Skoglundh, M. Top. Catal. 2007, 42-43, 183. (13) Amberntsson, A.; Skoglundh, M.; Ljungstrom, S.; Fridell, E. J. Catal. 2003, 217, 253. (14) Baümer, M.; Libuda, J.; Neyman, K. M.; Rosch, N.; Rupprechter, G.; Freund, H.-J. Phys. Chem. Chem. Phys. 2007, 9, 3541. (15) Chen, M. S.; Goodman, D. W. Acc. Chem. Res. 2006, 39, 739. (16) Libuda, J.; Freund, H.-J. Surf. Sci. Rep. 2005, 57, 157. (17) Freund, H.-J.; Baümer, M.; Libuda, J.; Risse, T.; Rupprechter, G.; Shaikhutdinov, S. J. Catal. 2003, 216, 223. (18) Campbell, C. T.; Parker, S. C.; Starr, D. E. Science 2002, 298, 811. (19) Zhdanov, V. P.; Kasemo, B. Surf. Sci. Rep. 2000, 39, 25. (20) Henry, C. R. Surf. Sci. Rep. 1998, 31, 231. (21) Stone, P.; Ishii, M.; Bowker, M. Surf. Sci. 2003, 537, 179. (22) Tsami, A.; Grillo, F.; Bowker, M.; Nix, R. M. Surf. Sci. 2006, 600, 3403. (23) Bowker, M.; Stone, P.; Smith, R.; Fourre, E.; Ishii, M.; de Leeuw, N. H. Surf. Sci. 2006, 600, 1973. (24) Ozensoy, E.; Peden, C. H. F.; Szanyi, J. J. Phys. Chem. B 2006, 110, 17001. (25) Ozensoy, E.; Peden, C. H. F.; Szanyi, J. J. Phys. Chem. B 2006, 110, 17009. (26) Ozensoy, E.; Peden, C. H. F.; Szanyi, J. J. Phys. Chem. B 2005, 109, 15977. (27) Ozensoy, E.; Peden, C. H. F.; Szanyi, J. J. Phys. Chem. B 2006, 110, 8025. (28) Ozensoy, E.; Peden, C. H. F.; Szanyi, J. J. Catal. 2006, 243, 149. (29) Desikusumastuti, A.; Happel, M.; Dumbuya, K.; Staudt, T.; Laurin, M.; Gottfried, J. M.; Steinru¨ck, H.-P.; Libuda, J. J. Phys. Chem. C. (in press). (30) Desikusumastuti, A.; Laurin, M.; Happel, M.; Qin, Z.; Shaikhutdinov, S.; Libuda, J. Catal. Lett. 2008, 121, 311. (31) Desikusumastuti, A.; Staudt, T.; Gro¨nbeck, H.; Libuda, J. J. Catal. 2008, 255, 127. (32) Yi, C.-W.; Kwak, J. H.; Peden, C. H. F.; Wang, C.; Szanyi, J. J. Phys. Chem. C 2007, 111, 14942. (33) Kim, D. H.; Kwak, J. H.; Szanyi, J.; Burton, S. D.; Peden, C. H. F. Appl. Catal. B: EnViron. 2007, 72, 233. (34) Casapu, M.; Grunwaldt, J.-D.; Maciejewski, M.; Baiker, A.; Wittstock, M.; Go¨bel, U.; Eckhoff, S. Top. Catal. 2007, 42-43, 3.

Staudt et al. (35) Casapu, M.; Grunwaldt, J.-D.; Maciejewski, M.; Wittrock, M.; Go¨bel, U.; Baiker, A. Appl. Catal. B: EnViron. 2006, 63, 232. (36) Hodjati, S.; Bernhardt, P.; Petit, C.; Pitchon, V.; Kiennemann, A. Appl. Catal. B: EnViron. 1998, 19, 221. (37) Hodjati, S.; Bernhardt, P.; Petit, C.; Pitchon, V.; Kiennemann, A. Appl. Catal. B: EnViron. 1998, 19, 209. (38) Yi, C.-W.; Kwak, J. H.; Szanyi, J. J. Phys. Chem. C 2007, 111, 15299. (39) Baraldi, A.; Barnaba, M.; Brena, B.; Cocco, D.; Comelli, G.; Lizzit, S.; Paolucci, G.; Rosei, R. J. Electron Spectrosc. Relat. Phenom. 1995, 76, 145. (40) Baraldi, A.; Comelli, G.; Lizzit, S.; Kiskinova, M.; Paolucci, G. Surf. Sci. Rep. 2003, 49, 169. (41) Lacovig, A.; Sala, A.; Gardonio, S.; Baraldi, A.; Lizzit, S. (in preparation). (42) Jaeger, R. M.; Kuhlenbeck, H.; Freund, H.-J.; Wuttig, M.; Hoffmann, W.; Franchy, R.; Ibach, H. Surf. Sci. 1991, 259, 235. (43) Libuda, J.; Winkelmann, F.; Baümer, M.; Freund, H.-J.; Bertrams, T.; Neddermeyer, H.; Mu¨ller, K. Surf. Sci. 1994, 318, 61. (44) Kresse, G.; Schmid, M.; Napetschnig, E.; Shishkin, M.; Ko¨hler, L.; Varga, P. Science 2005, 308, 1440. (45) Schauermann, S.; Hoffmann, J.; Johańek, V.; Hartmann, J.; Libuda, J.; Freund, H.-J. Angew. Chem., Int. Ed. 2002, 41, 2532. (46) Johańek, l.; Schauermann, S.; Laurin, M.; Gopinath, C. S.; Libuda, J.; Freund, H.-J. J. Phys. Chem. B 2004, 108, 14244. (47) Baümer, M.; Freund, H.-J. Prog. Surf. Sci. 1999, 61, 127. (48) Libuda, J.; Frank, M.; Sandell, A.; Andersson, S.; Bru¨hwiler, P. A.; Baümer, M.; Martensson, N.; Freund, H.-J. Surf. Sci. 1997, 384, 106. (49) Alexander, M. R.; Thompson, G. E.; Zhou, X.; Beamson, G.; Fairley, N. Surf. Interface Anal. 2002, 34, 485. (50) Andersson, S.; Bru¨hwiler, P. A.; Sandell, A.; Frank, M.; Libuda, J.; Giertz, A.; Brena, B.; Maxwell, A. J.; Baümer, M.; Freund, H.-J.; Martensson, N. Surf. Sci. 1999, 442, L964. (51) Braic, M.; Balaceanu, M.; Vladescu, A.; Kiss, A.; Braic, V.; Epurescu, G.; Dinescu, G.; Moldovan, A.; Birjega, R.; Dinescu, M. Appl. Surf. Sci. 2007, 253, 8210. (52) Florea, M.; Silvy, R. P.; Grange, R. Appl. Catal. A: General 2005, 286, 1. (53) Esaka, F.; Furuya, K.; Shimada, H.; Imamura, M.; Matsubayashi, N.; Kikuchi, T.; Ichimura, H.; Kawana, A. Surf. Interface Anal. 1999, 27, 1098. (54) Malengreau, F.; Hautier, V.; Vermersch, M.; Sporken, R.; Caudano, R. Surf. Sci. 1995, 330, 75. (55) Hess, C.; Lunsford, J. H. J. Phys. Chem. B 2002, 106, 6358. (56) Hess, C.; Lunsford, J. H. J. Phys. Chem. B 2003, 107, 1982. (57) Bowker, M.; Cristofolini, M.; Hall, M.; Fourre, E.; Grillo, F.; McCormack, E.; Stone, P.; Ishii, M. Top. Catal. 2007, 42-43, 341. (58) Schmitz, P. J.; Baird, R. J. J. Phys. Chem. B 2002, 106, 4172. (59) Sedlmair, C.; Seshan, K.; Jentys, A.; Lercher, J. A. J. Catal. 2003, 214, 308. (60) Broqvist, P.; Panas, I.; Fridell, E.; Persson, H. J. Phys. Chem. B 2002, 106, 137. (61) Schneider, W. F.; Hass, K. C.; Miletic, M.; Gland, J. L. J. Phys. Chem. B 2002, 106, 7405. (62) Miletic, M.; Gland, J. L.; Hass, K. C.; Schneider, W. F. J. Phys. Chem. B 2003, 107, 157. (63) Broqvist, P.; Gro¨nbeck, H.; Fridell, E.; Panas, I. J. Phys. Chem. B 2004, 108, 3523. (64) Gro¨nbeck, H.; Broquist, P.; Panas, I. Surf. Sci. 2006, 600, 403. (65) Hadjiivanov, K. I. Catal. ReV. - Sci. Eng. 2000, 42, 71. (66) Venkov, T.; Hadjiivanov, K.; Klissurski, D. Phys. Chem. Chem. Phys. 2002, 4, 2443.

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