Particle-Size-Dependent Interaction of NO2 with ... - ACS Publications

J. Phys. Chem. C 2009, 113, 9755–9764

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Particle-Size-Dependent Interaction of NO2 with Pd Nanoparticles Supported on Model NOx Storage Materials A. Desikusumastuti,† M. Happel,† Z. Qin,‡ T. Staudt,† Y. Lykhach,† M. Laurin,† S. Shaikhutdinov,‡ F. Rohr,§ and J. Libuda*,† Lehrstuhl fu¨r Physikalische Chemie II, Friedrich-Alexander-UniVersita¨t Erlangen-Nu¨rnberg, Egerlandstr. 3, D-91058 Erlangen, Germany, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany, and Umicore AG & Co. KG, Rodenbacher Chaussee 4, D-63403 Hanau-Wolfgang, Germany ReceiVed: January 29, 2009; ReVised Manuscript ReceiVed: March 24, 2009

Combining scanning tunneling microscopy, molecular beam methods, and time-resolved infrared reflection absorption spectroscopy, we investigate the structure and reactivity of Pd nanoparticles supported on singlecrystal-based model NOx storage materials. The latter consist of barium aluminate-like nanoparticles supported on an Al2O3 film on a NiAl(110) substrate. On these surfaces, Pd deposition under ultra-high-vacuum conditions gives rise to the growth of three-dimensional Pd particles, nucleating at the predeposited barium aluminate aggregates. The reactivity of these systems toward NO2 is tested systematically as a function of NO2 exposure and Pd particle size. At room temperature, NO, generated by dissociative adsorption of NO2, sequentially covers the following sites on the Pd nanoparticles: (i) hollow sites on (111) facets, (ii) bridge sites at particles edges and particle defects, and, finally, (iii) on-top sites at particle corners. The occupation of different sites on the particles is monitored as a function of NO2 exposure and Pd particle size. Characteristic differences in the site occupation and the coverage dependence are observed as a function of particle size. At elevated NO2 exposures, all NO-related features disappear, indicating the onset of oxidation of the Pd particles. For the Pd-containing systems, several new vibrational bands are observed. These new features are assigned to surface nitrates adsorbed on oxidized Pd particles and, tentatively, to surface nitrates adsorbed on sites that arise from the interaction between barium aluminate and oxidized Pd particles. In the limit of high NO2 exposure, these new surface nitrates coexist with surface nitrates formed on the uncovered fraction of the barium aluminate nanoparticles. 1. Introduction NOx storage and reduction (NSR) catalysts have recently been commercialized for the catalytic aftertreatment of exhaust streams of lean burn engines, both gasoline and diesel.1,2 Such lean burn engines operate at high air/fuel ratios, providing increased fuel efficiency. However, they generate oxygen-rich exhaust streams, making it difficult to catalytically reduce NOx. NSR catalysts are based on the ability of mostly bariumcontaining materials to store NOx in the form of Ba(NO3)2 under lean conditions and to release NOx during short fuel-rich operating periods, when it can be efficiently reduced.1 Commercial NSR catalysts and powder materials have been investigated intensively (see, e.g., refs 2-8), but many aspects concerning the surface species were involved; the reaction mechanism and the microkinetics remain poorly understood. Our strategy to obtain more detailed information on the underlying processes involves a surface-science-type approach.9 Single-crystal-based model catalysts are prepared, which can be studied using surface-science experimental methods, including scanning tunneling microscopy (STM) and photoelectron spectroscopy (PES). Simultaneously, these systems allow us to introduce complex features of real NSR catalysts in a wellcontrolled fashion. Yet, only few attempts have been made to * To whom correspondence should be addressed. E-mail: libuda@ chemie.uni-erlangen.de. † Friedrich-Alexander-Universita¨t Erlangen-Nu¨rnberg. ‡ Fritz-Haber-Institut der Max-Planck-Gesellschaft. § Umicore AG & Co. KG.

prepare model NSR catalysts and model NOx storage materials. Bowker and co-workers, for example, examined the growth of BaO on Pt(111) and Cu(111) and studied the reaction with NO2 using STM, PES, IRAS, and temperature-programmed desorption (TPD).10-12 Recently, Ozensoy et al. performed model studies on the reaction of NO2 with BaO/Al2O3/NiAl(100).13-17 Formation of nitrites and nitrates was found to take place at room temperature, both on Al2O3/NiAl(100) and on BaO/Al2O3/NiAl(100). More recently, model studies on BaO deposits on Al2O3/NiAl(110) have been carried out by Szanyi and co-workers18-20and by our group using PES, IRAS, and STM.21-26 When synchrotron-radiationbased high-resolution PES was used, it was shown that the strong interaction of BaO with the Al2O3/NiAl(110) support gives rise to the formation of barium aluminate-like nanoparticles (BaAl2xO1+3x).22 The composition of these particles depends on the preparation procedure. Here, we focus on wellannealed particles, for which we assume that they approach aluminate-like stoichiometry (BaAl2O4). It should be noted that the formation of such aluminate phases is also observed on real NSR catalysts (see, e.g., refs 2 and 27). Dense BaO model films, on the other hand, can be prepared on thick Al2O3 layers by deposition of large amounts of BaO, as recently reported by Szanyi and co-workers.18,20 Investigations on model NOx storage systems have already provided detailed insights into many aspects of the NOx uptake process. Numerous surface and bulk intermediates have been identified, mainly using IRAS, PES, and density functional theory (DFT) (see, e.g., refs 18, 20, and 22-24). The reaction

10.1021/jp9008527 CCC: $40.75  2009 American Chemical Society Published on Web 05/06/2009

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mechanism of NO2 with the different model systems has been studied and discussed in great detail. Characteristic differences have been observed as a function of material composition and reaction temperature. A recent study investigated restructuring and sintering of NOx storage materials, which are relevant to practical application.20 For a more detailed discussion, we refer to previous publications.18,20,23,26 Modeling the complete functionality of NSR catalysts, however, requires the preparation of systems that contain both the NOx storage component and the noble metal. In contrast to the extensive work on powder catalysts (see, e.g., refs 2-8), very few attempts have been made so far to study such bifunctional systems on single-crystal-based surfaces. Recently, we have reported on the growth and morphology of complete NSR model catalysts prepared by sequential co-deposition of Pd and Ba, followed by oxidation.28 It was shown that the interactions between Pd and BaO could be controlled via the preparation procedure28 and that these interactions have a direct influence on the reaction mechanism.21 In the present work, we present the results of a systematic study of the interaction of NO2 with a complete NSR model system: Pd nanoparticles supported on BaAl2xO1+3x/Al2O3/ NiAl(110). We systematically vary the Pd particle size and compare the reactivity in the presence and absence of the NOx storage component. When we employ remote-controlled TRIRAS experiments, a broad exposure range is explored (10-1-104 langmuirs; 1 langmuir corresponds to 1 × 10-6 Torr s), which allows us to follow NO2-induced processes with very different reaction probability in a single experiment. 2. Experimental Section Molecular beam/time-resolved IR reflection absorption spectroscopy (MB/TR-IRAS) experiments were performed in an ultra-high-vacuum (UHV) apparatus at the University ErlangenNuremberg. The apparatus allows up to four effusive beams and one supersonic beam to be superimposed on the sample surface. Additionally, the system is equipped with an FTIR spectrometer (Bruker IFS66/v), a beam monitor to calibrate alignment and intensity of the beams, two quadruple mass spectrometers, a vacuum transfer system, a high-pressure cell, and all necessary preparation tools. The NO2 beam (Linde, 99.0%) was generated from an effusive beam doser and modulated by a valve system. All measurements were performed by remote-controlled sequences, exposing the sample to pulses of NO2 at variable beam intensities between 2.7 × 1013 cm-2 · s-1 (equivalent pressure ) 1.2 × 10-7 mbar) and 3.2 × 1015 cm-2 · s-1 (equivalent pressure ) 1.4 × 10-5 mbar), followed by acquisition of IR spectra. The spectra were acquired at a resolution of 2 cm-1 with a typical acquisition time of 38 s. STM measurements (Micro H, Omicrometer) were performed in a separate UHV chamber at the Fritz-Haber-Institute (Berlin). The chamber is equipped with standard sample cleaning/ preparation facilities. All images were recorded using commercial Pt/Ir tips (L.O.T.-Oriel GmbH) with tunneling parameters (bias and current) as follows: (Figure 1, left column, 300 nm × 300 nm) (a) +3 V, 0.15 nA; (b) +3 V, 0.19 nA; (c) +2.6 V, 0.18 nA; (d) +2.3 V, 0.19 nA; (right column, 100 nm × 100 nm) (a) +3 V, 0.15 nA; (b) +2.8 V, 0.26 nA; (c) +2.6 V, 0.18 nA; (d) +1.5 V, 0.18 nA. For preparation of the model system, a NiAl(110) surface was cleaned by several cycles of sputtering and annealing in vacuum. The Al2O3 film was then produced by two cycles of oxidation in 10-6 mbar O2 at 550 K and UHV annealing at 1135 K. Details on the procedure may be found elsewhere.29,30 The

Figure 1. STM images showing different preparation steps of the model NSR catalyst: (A) pristine Al2O3/NiAl(110), (B) Ba/Al2O3/ NiAl(110) after deposition of 20 Å of Ba at 300 K, (C) BaAl2xO1+3x/ Al2O3/NiAl(110) after exposure to O2 at 300 K and subsequent annealing in O2 to 800 K, (D) Pd/BaAl2xO1+3x/Al2O3/NiAl(110) after deposition of 2 Å of Pd at 300 K (all images on the left, 300 nm × 300 nm; all images on the right, 100 nm × 100 nm).

quality of the film was controlled by LEED (low-energy electron diffraction), and complete oxidation of the surface was confirmed by the absence of CO adsorption at 100 K. For the preparation of the BaAl2xO1+3x particles, we proceeded as follows: First, the Ba metal was manually cleaned under inert gas atmosphere (glovebox) and placed into a Mo crucible. To prevent oxidation, the crucible filled with Ba was covered with decane before it was mounted in a commercial electron beam evaporator (Focus EFM3). The evaporator was installed into the UHV chamber immediately before pumping out. Under UHV conditions, the Ba source was calibrated using a quartz microbalance. During deposition, the sample was biased to the same potential as the Ba source to avoid surface defect generation by Ba ion bombardment. Ba was deposited at 300 K at typical rates of 1.0 × 1013 atoms · cm-2 · s-1 (an average film thickness of 1 Å of Ba corresponds to 1.6 × 1014 atoms · cm-2). For the present experiments, 20 Å of Ba (3.2 × 1015 atoms · cm-2) was deposited and subsequently oxidized by exposure to 6 × 10-7 mbar O2 at 300 K for 900 s, followed by heating to 800 K in oxygen atmosphere (6 × 10-7 mbar). Pd was deposited using the same type of evaporator (Focus EFM3), but evaporation was performed from a wire (Goodfel-

Interaction of NO2 with Pd NPs Supported On Model NOx low, >99.9%). Typical deposition rates were around 5.7 × 1012 atoms · cm-2 · s-1 (1 Å of Pd corresponds to 6.8 × 1014 atoms · cm-2), as calibrated using a quartz microbalance. Again, the sample was biased to avoid ion damage. 3. Results and Discussion 3.1. Nucleation and Growth of the Pd Particles on BaAl2xO1+3x/Al2O3/NiAl(110). In a first step, we study the nucleation and growth of the Pd nanoparticles on BaAl2xO1+3x/ Al2O3/NiAl(110). The corresponding behavior at low BaAl2xO1+3x and Pd coverages has been discussed recently.28 Here, we focus on the regime of larger coverage. The corresponding STM images are displayed in Figure 1. The pristine alumina film on NiAl(110) is atomically flat, and the dominating defect structures are antiphase domain (APD) boundaries, which appear as bright lines (see Figure 1a).29,30 The structure and defect structure of the film have been discussed in detail recently.31,32 It should be noted that the APD boundaries may act as preferential nucleation centers during the growth of metal nanoparticles, including Pd.33 For details on the structure of the Pd nanoparticles on pristine Al2O3/NiAl(110), we refer to the literature.9 Briefly, three-dimensional crystalline nanoparticles are formed, which reach an average particle diameter of 6 nm at a nominal Pd coverage of 4 Å. From the particle density of 1 × 1012 cm-2 (deposition at a sample temperature of 300 K), we can estimate an average number of 2700 Pd atoms per particle. Furthermore, it could be shown that these particles grow in a (111) orientation and preferentially expose (111) top and side facets (in addition to a small fraction of (100) facets).34 It should be noted that, before performing the reaction, the Pd particles on the pristine Al2O3/NiAl(110) were stabilized by O2 and CO treatment at elevated temperature, according to a procedure described in the literature.35 The stabilization leads to a thickening of the alumina film and suppresses diffusion through the film during high-temperature treatment. STM images taken after deposition of Ba on the pristine Al2O3/NiAl(110) under UHV conditions at 300 K are displayed in Figure 1b. It should be noted that the exact size of these particles is difficult to extract from STM due to inevitable tip-sample convolution effects. In contrast, the particle density determined from STM represents a more reliable quantity. Therefore, we focus mainly on particle densities in the following discussion. It is observed that Ba forms three-dimensional particles, nucleating both at the APD boundaries and on the oxide terraces. This leads to a high particle density of around 7 × 1012 cm-2. Upon oxidation and annealing (Figure 1c), the particulate structure of the system is preserved, but some minor sintering occurs. As a result, the particle density decreases slightly to a value of approximately 5 × 1012 cm-2. Figure 1d shows an STM image after subsequent deposition of 2 Å of Pd. It is found that the particle density determined from STM remains unchanged (5 × 1012 cm-2), whereas the particles themselves appear more homogeneous in size and the tunneling behavior becomes more stable. These observations suggest that the Pd particles nucleate on the BaAl2xO1+3x particles and partially cover their surface. Note that the same behavior was already observed in a previous STM study at lower Ba and Pd coverage.28 Due to the lack of atomic resolution on these nanoparticle systems, more detailed information on the structure of the Pd aggregates cannot be extracted by STM. However, in section 3.3, we show that the vibrational spectra of adsorbed NO qualitatively resemble those obtained from the larger Pd particles on Al2O3/NiAl(110) (see above). This suggests that the morphology of the Pd particles on BaAl2xO1+3x/Al2O3/

J. Phys. Chem. C, Vol. 113, No. 22, 2009 9757 NiAl(110) is comparable to that on pristine Al2O3/NiAl(110). Concerning the particle size, we can derive a rough estimate on the basis of the nucleation and growth behavior. For this purpose, we assume that Pd nucleation exclusively occurs on the BaAl2xO1+3x particles leading to a Pd particle density of 5 × 1012 cm-2. We assume that the Pd particle density is nearly constant within the range of the nominal Pd coverage (1-8 Å) investigated in this study. This assumption is justified by the observation that, for most of the noble metal/oxide systems, nucleation occurs in the initial stages of deposition before the particle density remains nearly unchanged over a substantial coverage range (compare ref 33). On the basis of this assumption and the nominal Pd coverage, we obtain an estimate of the average number of Pd atoms per particle. To obtain a rough estimate of the particle size, we may assume the formation of hemispherical particles. With this assumption, we obtain the following characteristic quantities for the systems studied in this work: (1, nominal Pd coverage 1 Å) Pd atoms per particle NPd ≈ 140, average particle size dPd ≈ 2.0 nm; (2, nominal Pd coverage 2 Å) NPd ≈ 270, dPd ≈ 2.5 nm; (3, nominal Pd coverage 4 Å) NPd ≈ 540, dPd ≈ 3.2 nm; (4, nominal Pd coverage 8 Å) NPd ≈ 1100, dPd ≈ 4.0 nm. 3.2. NO2 Adsorption on Pd Nanoparticles on Al2O3/ NiAl(110). A schematic picture of the model NSR catalyst is shown in Figure 2. By means of systematic IRAS experiments, the numerous NO-derived species formed on its surface (see Figure 2) will be identified step by step. As a first simple reference case, we start by investigating the interaction of NO2 with Pd nanoparticles on Al2O3/NiAl(110) in the absence of Ba-containing deposits. Toward this aim, we perform a combined TR-IRAS/MB experiment. Briefly, the sample surface is exposed to a sequence of pulses of NO2; each pulse is followed by the acquisition of an IR spectrum. By choosing appropriate pulse intensities and durations, we probe an exposure range between 10-1 and 104 langmuirs systematically. The experimental data for the NO stretching frequency region are displayed in Figure 3. At the lowest exposure, bands appear at 1530 and 1630 cm-1, which blue shift to 1570 and 1668 cm-1 with increasing NO2 exposure. At exposures around 1 langmuir, an additional weak feature around 1750 cm-1 appears. With increasing exposure, all three bands decrease in intensity. They finally disappear completely between 5 and 10 langmuirs. Meanwhile, a new absorption feature at around 1545 cm-1 is observed. This band remains present up to the largest exposures but, finally, becomes somewhat obscured by new intense bands at 1255, 1297, 1595, and 1652 cm-1, growing rapidly at exposures exceeding 1000 langmuirs. Most of the absorption features can be assigned on the basis of previous work on NO and NO2 adsorption on Pd singlecrystal surfaces,36-41 Pd nanoparticles,42,43 Al2O3 thin films,17,25 Al2O3 powders,44,45 alumina-supported NSR catalysts,3,45-49 and model catalysts.18-20,23-25 First, we focus on the low-exposure region. Here, we have to take into account that NO2 dissociates on transition-metal surfaces, such as Pd at 300 K (compare refs 50-52), leading to co-adsorption of molecularly adsorbed NO and atomic oxygen. NO adsorption on Pd nanoparticles has previously been investigated by IRAS.42,43 Comparing with DFT, we have recently provided a detailed assignment of the observed bands.42 Briefly, the absorption feature at 1530 cm-1 was assigned to NO adsorbed at hollow sites on (111) facets of the Pd crystallites. The bands at 1630 and 1750 cm-1 can be attributed to particle-specific sites that do not occur on singlecrystal surfaces, namely, bridge-bonded NO at (111)/(111) edges and on-top NO at particle corners, respectively. It should be

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Figure 2. Schematic summary of the model NSR catalyst with possible NO-derived surface species. Corresponding assignments of vibrational bands are discussed in this work.

noted that the bridge sites at the edges show a substantially enhanced adsorption energy in comparison to bridge sites on the (111) facets. As a result, adsorption at these edge sites can nearly compete with the most favorable hollow sites on the facets, and they are populated already at relatively low coverage. Similarly, the adsorption energy for on-top NO at the corners is much larger than that for on-top adsorption at the (111) facets. Whereas the latter are not populated at 300 K, on-top adsorption at particle edges can be observed under the same conditions.42 The strong blue shift as a function of coverage is the result of coverage-dependent dipole interactions between adjacent NO, electronic NO-NO interaction via the Pd support, and the influence of co-adsorbed O (compare refs 43, 53, and 54; see also ref 55). Comparing the spectral features after NO2 exposure corresponding to bridging (1668 cm-1) and on-top (1755 cm-1) NO to the NO-derived bands generated by NO adsorption on the same system (1657 and 1730 cm-1), we can attribute the additional blue shift to the electronic effect of co-adsorbed oxygen, leading to reduced π-backbonding and, consequently, an increased blue shift (compare refs 43, 56, and 57). The complete loss of the NO adsorption capacity at larger NO2 exposure is the result of the increasing oxygen coverage, giving rise to replacement of NO. It should be noted that, at elevated temperature, NO2 very efficiently leads to surface oxidation of Pd(111).50 In contrast to close-packed single-crystal surfaces, the kinetic hindrance to formation of oxides and surface oxides is substantially reduced on small supported particles.58,59 Thus, it appears likely that large NO2 exposure leads to oxidation or surface oxidation of the Pd nanoparticles. In this respect, the new absorption band at 1545 cm-1, which increases in intensity with increasing NO2 exposure, deserves particular attention. It should be noted that this band is not observed on the pristine Al2O3/NiAl(110). We tentatively assign the feature to a surface nitrate species on oxidized Pd. The same feature is observed for Pd/BaAl2xO1+3x/Al2O3/NiAl(110) and is discussed in more detail in section 3.4. Finally, we note that the two pairs of bands appearing at 1255/ 1297 and 1595/1652 cm-1 at exposures exceeding 1000 lang-

muirs have previously been attributed to the two components of the asymmetric NO stretching mode νas(NO3-) of surface nitrates on Al2O3/NiAl(110).25 On the basis of the intensity ratio between the low-frequency and the high-frequency components, it was suggested that these nitrates preferentially adsorb in bridging geometry.23 The splitting into two bands in each region could be explained by the presence of two adsorption sites, for example, on terraces and on defects. 3.3. NO2 Adsorption on Pd Nanoparticles on BaAl2xO1+3x/ Al2O3/NiAl(110). In the next step, we have studied the interaction of NO2 with Pd nanoparticles supported on BaAl2xO1+3x/ Al2O3/NiAl(110). Similar to the Pd nanoparticles supported on the pristine Al2O3/NiAl(110) (see section 3.2), we have systematically probed the full exposure range between 10-1 and 104 langmuirs by MB/TR-IRAS experiments. In addition, the particle size was varied via the nominal Pd coverage (see section 3.1). The corresponding data are summarized in Figure 4 for a nominal Pd coverage of 1 Å, in Figure 5 for 2 Å of Pd, in Figure 6 for 4 Å of Pd, and in Figure 7 for 8 Å of Pd. As the behavior turns out to be rather complex, the discussion is organized as follows: First, we investigate the general dependence on the NO2 exposure choosing a nominal coverage of 4 Å of Pd. In section 3.4, we will then address particle-size-dependent effects. Similar as for adsorption on Pd/Al2O3/NiAl(110), the interaction of NO2 with Pd/BaAl2xO1+3x/Al2O3/NiAl(110) can be divided into a low-exposure regime (8 Å) Pd layer on BaAl2xO1+3x/ Al2O3/NiAl(110). A detailed discussion of the spectra for Al2O3/NiAl(110) and BaAl2xO1+3x/Al2O3/NiAl(110) has been given elsewhere.23-25

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Figure 10. Particle-size-dependent changes during the final stages of NO2 exposure: IR reflection absorption spectra of the NO stretching frequency region are displayed after exposure of (iv) Pd/BaAl2xO1+3x/ Al2O3/NiAl(110) to large doses of NO2 at 300 K. For comparison, corresponding spectra are displayed for NO2 exposure of (i) pristine Al2O3/NiAl(110), (ii) Ba-free Pd/Al2O3/NiAl(110) (4 Å Pd), (iii) Pdfree BaAl2xO1+3x/Al2O3/NiAl(110), and (v) a thick Pd layer on BaAl2xO1+3x/ Al2O3/NiAl(110).

The spectra for the Pd-containing systems were briefly considered in sections 3.2 and 3.3. Now, we focus on particle-sizedependent effects. First, it should be noted that the same principal features are observed for all particle sizes: The most prominent bands are those at 1333 and 1468 cm-1, with some weaker features at 1270, 1548, 1622, and 1660 cm-1. The comparison with Pdfree BaAl2xO1+3x/Al2O3/NiAl(110) shows that the bands at 1333 and 1468 cm-1 can be assigned to surface nitrates on BaAl2xO1+3x particles.23-25 However, the assignment of the other features is not straightforward, and a closer comparison reveals some remarkable details. The band around 1550 cm-1 was tentatively attributed to a surface nitrate species on oxidized Pd particles (see sections 3.2 and 3.3). There are several arguments that support this hypothesis. First, the feature appears on all systems that contain Pd, including Ba-free Pd/Al2O3/ NiAl(110) and the thicker Pd film. As pointed out, Pd singlecrystal surfaces are easily oxidized by NO2.50 The kinetic hindrance that prevents PdO formation at low temperature is substantially reduced on nanoparticle systems.58,59 Therefore, we assume either that a surface oxide is formed on the Pd particles or that the smaller nanoparticles may be oxidized completely, at least in the limit of highest NO2 exposures. Similar as for other surface nitrates25 and in sharp contrast to the NO-related species, the band at 1550 cm-1 does only show minor coverage-dependent shifts. Also, the frequency observed

Interaction of NO2 with Pd NPs Supported On Model NOx is comparable with those of other nitrates on noble metal oxides (see overview in ref 78). However, the split νas(NO3-) mode should give rise to a second absorption band at lower frequency, similar to the nitrate species on BaAl2xO1+3x and Al2O3. Indeed, a close inspection of the spectra reveals that there is an additional weak band appearing at 1270 cm-1 for all Pd-containing systems. We tentatively attribute this feature to the lowfrequency component of νas(NO3-) of a surface nitrate on oxidized Pd. Again, the frequency range is consistent with other transition and noble metals and oxides.78 If we take into account the intensity ratio between the two components and follow the arguments outlined previously,23 we may speculate that the polarization of the two components as derived from the intensity ratio should point to a preferential adsorption in bridging geometry. However, it should be noted that this assignment relies on the assumption of a relatively flat surface, whereas substantial roughness is introduced by the growth of the Pd and BaAl2xO1+3x nanoparticles. Therefore, such assignments should be treated with care in the present case. It should also be noted that there is an additional weaker band around 1620 cm-1, the intensity behavior of which shows a similar trend as for the bands at 1550 and 1270 cm-1. We may tentatively attribute this feature to a second surface nitrate species on oxidized Pd. Finally, we consider the band at 1660 cm-1 for NO2 adsorption on Pd/BaAl2xO1+3x/Al2O3/NiAl(110) This feature nearly coincides with the surface nitrate band on pristine Al2O3/ NiAl(110) at 1652 cm-1. However, the band is not visible for Pd-free BaAl2xO1+3x/Al2O3/NiAl(110), and it does not follow the decrease of the BaAl2xO1+3x-related nitrates (1333 and 1467 cm-1) with increasing Pd coverage. These observations suggest that it cannot be assigned to a nitrate on Al3+. On the other hand, it cannot be associated with a nitrate species on oxidized Pd either as its intensity does not increase with Pd coverage and it completely disappears in the case of the thick Pd film. In fact, the band is only observed in the presence of both Pd and BaAl2xO1+3x, which suggests that it may be associated with the interaction of Pd (or PdO) and BaAl2xO1+3x and may possibly be related to a nitrate species at mixed oxide or particle boundary sites. 4. Conclusions We have studied the particle-size-dependent interaction of NO2 with model NSR catalysts using STM, molecular beams, and TR-IRAS. The model systems comprise Pd nanoparticles and BaAl2xO1+3x nanoparticles supported on Al2O3/NiAl(110). 4.1. Structural Characterization by STM. Upon deposition under UHV conditions, Pd grows in the form of threedimensional nanoparticles nucleating on the predeposited BaAl2xO1+3x particles on Al2O3/NiAl(110). The Pd particle size can be varied with the amount of Pd deposited on the support. Particles from approximately 140 atoms/particle (2 nm, 1 Å nominal Pd coverage) to approximately 1100 atoms/particle (4 nm, 8 Å nominal Pd coverage) were studied and compared to reference systems, including Ba-free Pd/Al2O3/NiAl(110). 4.2. NO2 Adsorption and Reaction at Low NO2 Exposure. At the initial stages of exposure at 300 K, NO2 adsorbs dissociatively on the Pd nanoparticles, giving rise to the formation of molecularly adsorbed NO and co-adsorbed oxygen. These adsorbed NO species can be assigned to specific adsorption sites on the Pd nanoparticles. The NO sequentially occupies hollow on-top sites on (111) facets and bridge edge sites at (111)/(111) edges. With increasing coverage, the co-adsorbed oxygen replaces NO from the edge sites and forces NO to occupy specific on-top sites, for example, at particle edges. Upon

J. Phys. Chem. C, Vol. 113, No. 22, 2009 9763 further NO2 exposure, the increasing O coverage leads to complete desorption of molecularly adsorbed NO. As a result of the capture zone effect, the NO2 exposure required to fully saturate a given type of site on the Pd particles strongly depends on structural parameters of the system, that is, the particle size and the particle density. Characteristic shifts are observed for the NO-related bands as a function of particle size, which, in part, can be related to dipole-coupling effects on larger particles. A schematic summary of the bands and assignment used in this work is provided in Figure 2. 4.3. NO2 Adsorption and Reaction at High NO2 Exposure. With increasing NO2 exposure, chemisorbed NO on Pd is fully replaced by the increasing coverage of atomic oxygen. Subsequently, several new bands appear in the vibrational spectrum, which are attributed to nitrogen-oxo species on oxidic surface areas. Besides the characteristic bands of surface nitrites and nitrates on BaAl2xO1+3x, there are several new features, which are observed neither on the pristine Al2O3/NiAl(110) nor on the Pd-free BaAl2xO1+3x/Al2O3/NiAl(110). The most prominent one is a band at 1550 cm-1, which, together with a weak absorption feature at 1270 cm-1, is assigned to a surface nitrate on oxidized Pd, presumably in bridging geometry. In addition, there are indications of a second nitrate species, which are associated with the interaction between oxidized Pd and BaAl2xO1+3x, for example, at the particle boundaries. Acknowledgment. This project was financially supported by the Deutsche Forschungsgemeinschaft (DFG). We also acknowledge the additional support of the DFG within the Excellence Cluster “Engineering of Advanced Materials” in the framework of the excellence initiative. Furthermore, we acknowledge the financial support of the Fonds der Chemischen Industrie, the DAAD (PPP, Acciones Integradas HispanoAlemanas), the European Union (COST D-41), and the “Zerweck Fonds” (Universita¨tsbund Erlangen-Nu¨rnberg). The authors are grateful to Hans-Joachim Freund (FHI Berlin) for providing the STM facilities. The support of Karsten Meyer/ Matthias Moll/Marc Ga¨rtner/Carola Vogel (University Erlangen) and Friederike Jentoft/Robert Schlo¨gl (FHI, Berlin) is acknowledged with respect to the required local glovebox facilities. The authors thank Konstantin M. Neyman and Henrik Gro¨nbeck for fruitful cooperations and discussions. References and Notes (1) 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. (2) Epling, W. S.; Campbell, L. E.; Yezerets, A.; Currier, N. W.; Parks, J. E., II. Catal. ReV. 2004, 46, 163. (3) Nova, I.; Castoldi, L.; Prinetto, F.; DalSanto, V.; Lietti, L.; Tronconi, E.; Forzatti, P.; Ghiotti, G.; Psaro, R.; Recchia, S. Top. Catal. 2004, 30/31, 181. (4) Piacentini, M.; Stroebel, R.; Maciejewski, M.; Pratsinis, S. E.; Baiker, A. J. Catal. 2006, 243, 43. (5) 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 2005, 56, 201. (6) Castoldi, L.; Matarrese, R.; Lietti, L.; Forzatti, P. Appl. Catal., B 2006, 64, 25. (7) Tonkyn, R. G.; Disselkamp, R. S.; Peden, C. H. F. Catal. Today 2006, 114, 94. (8) Dawody, J.; Tonnies, I.; Fridell, E.; Skoglundh, M. Top. Catal. 2007, 42-43, 183. (9) Libuda, J.; Freund, H.-J. Surf. Sci. Rep. 2005, 57, 157. (10) Stone, P.; Ishii, M.; Bowker, M. Surf. Sci. 2003, 537, 179. (11) Tsami, A.; Grillo, F.; Bowker, M.; Nix, R. M. Surf. Sci. 2006, 600, 3403. (12) Bowker, M.; Cristofolini, M.; Hall, M.; Fourre, E.; Grillo, F.; McCormack, E.; Stone, P.; Ishii, M. Top. Catal. 2007, 42-43, 341. (13) Ozensoy, E.; Peden, C. H. F.; Szanyi, J. J. Catal. 2006, 243, 149.

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