Reactions of NO2 with Ba (OH) 2 on Pt (111)

Apr 9, 2010 - The amorphous and crystalline Ba(OH)2 layers were prepared by exposing a thick BaO (>20 MLE) layer on Pt(111) to H2O at 300 and 425 K,...
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J. Phys. Chem. C 2010, 114, 16955–16963

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Reactions of NO2 with Ba(OH)2 on Pt(111)† Kumudu Mudiyanselage,‡ Cheol-Woo Yi,§ and Ja´nos Szanyi*,‡ Institute for Interfacial Catalysis, Pacific Northwest National Laboratory, P.O. Box 999, MSIN: K8-87, Richland, Washington 99352, and Department of Chemistry and Institute of Basic Science, Sungshin Women’s UniVersity, Seoul 136-742, Korea (ROK) ReceiVed: February 22, 2010; ReVised Manuscript ReceiVed: March 25, 2010

The interaction of NO2 with amorphous and crystalline Ba(OH)2 supported on Pt(111) was studied in the wide pressure range of 1.0 × 10-9 to 1.0 × 10-4 Torr and compared to that with a thick (>20 monolayer equivalent (MLE)) BaO film using infrared reflection absorption spectroscopy (IRAS), temperature programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). The amorphous and crystalline Ba(OH)2 layers were prepared by exposing a thick BaO (>20 MLE) layer on Pt(111) to H2O at 300 and 425 K, respectively. The amorphous and crystalline Ba(OH)2 layers partially convert to Ba(NOx)2 (nitrites and nitrates) following their exposure to elevated NO2 pressure (∼1.0 × 10-4 Torr) at 300 K. The exposure of the crystalline Ba(OH)2/Pt(111) system to NO2 at 425 K, however, leads to the desorption of H2O and the complete conversion of the crystalline Ba(OH)2 layer to Ba(NOx)2, which consists of mainly crystalline nitrates and a small amount of nitrites. The amounts of NOx stored by BaO (>20 MLE)/Pt(111) and crystalline Ba(OH)2/Pt(111) systems upon their exposure to NO2 at 425 K are comparable. The thus-formed bulk crystalline Ba(NO3)2 phase decomposes in two steps, both releasing NO and O2, in accord with the melting/decomposition scheme for bulk Ba(NO3)2. 1. Introduction Efficient, environment-friendly vehicles need to be developed in order to both decrease energy consumption and reduce the emission of environmentally harmful gases, e.g., COx and NOx. Vehicles operated by lean-burn engines are gradually gaining popularity around the world due to their improved fuel efficiency, lower CO2 emission, etc., compared to traditional internal combustion engines. However, conventional three-way catalysts are ineffective in reducing NOx under lean-burn (i.e., highly oxidizing) conditions. Therefore, the reduction of NOx under lean-burn conditions is one of the most important and challenging tasks in catalysis research and it requires the development of new strategies to replace the conventional threeway catalyst technology. One of the most promising candidates to achieve this goal is the NOx storage and reduction (NSR) system, which can efficiently control NOx emission under leanburn conditions. However, in order to enhance the efficiency of the NOx removal process, it is necessary to understand the elementary steps involved in NSR catalysis in microscopic detail. Elementary processes involved in NSR catalysis can be studied using single-crystal-based model catalysts because these model system studies can provide the detailed microscopic information of the elementary processes in NSR catalysis without the complexity of high surface area practical catalysts. The reactions of NO2 with BaO, the active storage component in the NSR catalyst, deposited on oxide thin films, such as Al2O3/NiAl(110),1–6 as well as metal single crystals Pt(111)7–10 and Cu(111)11 have been studied previously. In all of these studies, the interaction of NOx was studied with the pristine oxide phases. However, under practical conditions, large amounts of H2O and CO2 are always present during the NOx †

Part of the “D. Wayne Goodman Festschrift”. * Corresponding author. E-mail: [email protected]. ‡ Pacific Northwest National Laboratory. § Sungshin Women’s University.

storage and reduction processes. For this reason, in the present study, we aimed at the fundamental understanding of the interaction of H2O and BaO and the reaction of the thus-formed Ba(OH)2 with NO2. A number of studies have investigated the effect of H2O on the uptake of NOx over BaO/Al2O3-based highsurface-area materials.12–19 Lietti and co-workers found that the presence of H2O promoted the uptake of NOx at low temperature (20 monolayer equivalent (MLE))/Pt(111) system. Specifically, we discuss the reactions of NO2 with amorphous and crystalline Ba(OH)2 in a wide NO2 pressure range at 300 and 425 K sample temperatures. 2. Experimental Section All of the experiments were performed in a combined ultrahigh vacuum (UHV) surface analysis chamber and elevated-

10.1021/jp101589z  2010 American Chemical Society Published on Web 04/09/2010

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pressure reactor/infrared reflection absorption spectroscopy (IRAS) cell system with a base pressure of less than 2.0 × 10-10 Torr (1 Torr ) 1.3332 mbar). The UHV chamber is equipped with X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and temperature programmed desorption (TPD) techniques. The elevated-pressure cell is coupled with a commercial Fourier transform infrared (FT-IR) spectrometer (Bruker, Vertex 70). The Pt(111) single crystal (10 mm diameter, 2 mm thick, obtained from Princeton Scientific) used in these experiments was spot-welded onto a U-shaped Ta wire, and the sample temperature was measured by a C-type thermocouple spotwelded to the backside of the crystal. The Pt(111) crystal was cleaned by repeated cycles of Ar+ ion sputtering and annealing in O2 at 800 K. The cleanliness of the surface was verified with AES, XPS, and LEED. The thick (>20 MLE) BaO film was prepared by reactive layer-assisted deposition (RLAD); first, the desired amount of Ba was deposited onto a N2O4 multilayer on Pt(111) crystal at 90 K by physical vapor deposition using a resistively heated Ba doser (SAES Getters), and then, the thus formed BaNxOy layer was thermally decomposed by annealing to 1000 K. The obtained BaO film was characterized by XPS and LEISS. NO2 and H2O were purified by several cycles of freeze/pump/thaw prior to use. These reactants were introduced into the UHV chamber by pinhole dosers and delivered to the sample surface through collimating tubes. The same gas dosing system was set up in the elevated-pressure reactor/IRAS cell. This setup allows us to expose the sample to the desired coverages by adjusting the pressure in the gas manifold (back pressure), and/or the exposure time. A precision leak valve was used to introduce NO2 gas for the elevated pressure experiments. IR spectra were collected at 4 cm-1 resolution using a grazing angle of approximately 85° to the surface normal. All of the IR spectra collected were referenced to a background spectrum acquired from the clean sample prior to gas adsorption. Each spectrum presented is the average of 1024 scans, requiring a spectral acquisition time of 80 s. In cases where the sample was annealed to higher temperatures after NO2 and/or H2O exposure, the sample was cooled down to the initial temperature before the spectrum was acquired. The amorphous and crystalline Ba(OH)2 layers were prepared by exposure of the thick (>20 MLE) BaO film to H2O at 300 and 425 K, respectively. The chemistry of NO2 on the thick BaO film and on the amorphous and crystalline Ba(OH)2 layers on Pt(111) was studied by IRAS, TPD, and XPS. 3. Results and Discussion 3.1. Ba(OH)2 Formation: H2O Adsorption onto BaO (>20 MLE)/Pt(111). BaO readily reacts with H2O to form various barium hydroxide species; a number of polymorphs, several hydrated amorphous (Ba(OH)2 · 8H2O, Ba(OH)2 · 3H2O) and crystalline (R-, β-, and γ-Ba(OH)2 · H2O) hydroxides, as well as anhydrous crystalline phases of R- and β-Ba(OH)2.20 Moreover, thermal dehydration of Ba(OH)2 · 8H2O and Ba(OH)2 · 3H2O leads to the formation of β-Ba(OH)2 · H2O, the stable form of the monohydrate at ambient temperature. Dehydration of β- and γ-Ba(OH)2 · H2O results in the formation of anhydrous β-Ba(OH)2 at 378 and 388 K, respectively. Because of the existence of various Ba(OH)2 phases, before studying the reaction between NO2 and Ba(OH)2 on Pt(111), it is essential to get more insight into the nature of the Ba(OH)2 phases formed by the reaction between H2O and the thick BaO film under our experimental conditions. To this end, we performed TPD, XPS, and IRAS experiments following the exposure of the thick BaO film to various amounts of H2O at

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Figure 1. (a) Series of TPD spectra obtained following adsorption of H2O on BaO/Pt(111) as a function of exposure at 300 K. [Water exposure was varied by changing the exposure time, as indicated in Figure 1a, while keeping the gas line manifold at PH2O ∼ 8.5 Torr. The chamber pressure was increased to ∼6.0 × 10-10 (lowest exposure time) and 3.0 × 10-9 Torr (highest exposure time). Note that the pressure at the surface is different from that in the chamber during gas exposure using the pinhole doser via a collimated tube.] A series of IR spectra obtained (b) following adsorption of H2O on BaO/Pt(111) as a function of exposure at 300 K and (c) subsequent annealing to the indicated temperatures.

300 K to identify the Ba(OH)2 phases formed. Here, we discuss briefly the TPD and IRAS results, while a detailed discussion on the TPD and XPS data is provided elsewhere.9,21 A selected series of TPD spectra obtained after H2O exposure of the thick BaO film at 300 K is displayed in Figure 1a. At low water exposures, only one desorption feature is observed, and the temperature of maximum desorption rate shifts toward higher values (from 471 to 495 K) with increasing H2O exposure, while the leading edge of the desorption profiles stays constant. At higher H2O exposures (texposure > 60 s), two desorption features were observed. Near the saturation H2O exposure, the TPD spectrum shows two maxima, at ∼482 and ∼520 K. In addition,

Reactions of NO2 with Ba(OH)2 on Pt(111) the TPD spectrum collected after the saturation H2O exposure also shows a small desorption feature with maximum desorption rate at ∼420 K. Similar TPD spectra were reported by Yi et al. following the adsorption of D2O on a thick BaO (>30 MLE) film supported on Al2O3/NiAl(110).21 These dramatic changes in the TPD profile can be explained by the dehydration and phase transitions of barium hydroxide.21 XPS data (not shown here for brevity) revealed the complete conversion of BaO to Ba(OH)2 following the saturation adsorption of H2O on BaO (>20 MLE)/Pt(111) at 300 K (the complete disappearance of the characteristic O 1s of the BaO phase (∼528 eV) and the maximization of the O 1s peak belonging to the Ba(OH)2 phase (∼531 eV)) .9 A series of IRAS spectra collected following the adsorption of H2O on BaO/Pt(111) as a function of exposure at 300 K are shown in Figure 1b. At very low H2O exposures, a broad vibrational feature appears in the range 3120 - 3660 cm-1, indicating the formation of amorphous Ba(OH)2.21 As the H2O exposure increases, the intensity of this broad band monotonically increases, and at saturation, the peak maximum is positioned at 3442 cm-1. Significant changes in the IRAS spectra were observed during stepwise annealing of the water-saturated sample, as shown in Figure 1c. The IR spectrum collected after annealing the sample to 400 K shows the transformation of the broad feature into two partially resolved IR features, centered at 3416 and 3534 cm-1. Annealing to 425 K results in the appearance of a sharp vibrational feature centered at 3584 cm-1. After further annealing to 475 K, the IR spectrum displays a broad shoulder around 3141 cm-1 and a peak centered at 3534 cm-1. All of the characteristic IR features of O-H stretching vibrations completely disappear after annealing to 520 K. Even though the IRAS spectra of Ba(OH)2 film shown in Figure 1 deviate somewhat from the transmission IR spectra of bulk Ba(OH)2, peak assignments can be made based on previously reported IR data for Ba(OH)2.21,22 The sharp peak observed at 3584 cm-1 and the broad bands at 3416 and 3534 cm-1 after annealing to 425 K can be assigned to the crystalline β-Ba(OH)2 · xH2O (x ) 0, 1).21 Further annealing to 475 K leads to the loss of hydrating water molecules from the β-hydroxide phase, and the transformation to anhydrous β- and R-Ba(OH)2. Yi et al. have also shown that, upon D2O exposure of a thick BaO film on Al2O3/NiAl(110), amorphous barium hydroxide formed at room temperature, which then readily converted to crystalline Ba(OD)2 phases during annealing in UHV.21 The agreement between the results obtained in this study with those reported for the Ba(OD)2/Al2O3/NiAl(110)21 shows the identical behaviors of these two thick BaO films regardless of the supports they have been deposited onto (Pt(111) vs Al2O3 thin film), as it was expected for these thick BaO layers. The understanding of the sequence of phase transformations allows us to look into (in detail) the reactions of these different hydroxide phases, i.e., amorphous, hydrated, and anhydrous β-Ba(OH)2, with NO2. 3.2. NO2 on Amorphous Ba(OH)2/Pt(111) at 300 K. In the present study, an amorphous Ba(OH)2 layer on Pt(111) was prepared by adsorbing H2O on BaO/Pt(111) at 300 K, as described above. A series of IRAS spectra obtained following the adsorption of NO2 on amorphous Ba(OH)2/Pt(111) as a function of exposure at 300 K is shown in Figure 2a. A broad IR feature at 3442 cm-1 observed following the saturation exposure of BaO/Pt(111) to H2O at 300 K represents the νOH vibrations in amorphous Ba(OH)2. At the lowest exposure of this amorphous Ba(OH)2/Pt(111) to NO2, a peak around 1250 cm-1 appears, which can be assigned to nitrite species, as reported previously.1–3,5,6,23,24 With increasing but still very low

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Figure 2. Series of IR spectra obtained (a) following the exposure of amorphous Ba(OH)2/Pt(111) to NO2 as a function of pressure at 300 K (the amorphous Ba(OH)2/Pt(111) system was obtained by adsorbing H2O on BaO/Pt(111) at 300 K) and (b) subsequent annealing to the indicated temperatures. (The IR spectrum obtained following the formation of the amorphous Ba(OH)2 layer is also displayed in panel a.) (The inset in panel b shows the expansion of the OH stretching region.)

NO2 exposures (total exposure ∼10 L (1 L ) 1 × 10-6 Torr s)), only the IR feature at 1250 cm-1 develops. As the total NO2 exposure exceeds 10 L, additional IR features appear at 1578 cm-1 and in the 1320-1450 cm-1 range. On the basis of the results of prior studies, the peaks in the 1320-1450 cm-1 range can be assigned to bulk nitrates.1–3,5,6,23,24 The low-intensity peak at 1578 cm-1 is similar to the one observed for surface nitrates formed on BaO/Al2O3-based high surface area material during stepwise H2O adsorption on NO2-exposed samples at 300 K.12 Therefore, most likely, the feature at 1578 cm-1 belongs to surface nitrate-like species. The transformation of this species to bulk nitrates at higher NO2 exposures also supports the assignment of the peak at 1578 cm-1 to surface species. With further increase in NO2 exposure, the intensities of all of the IR features characteristic of nitrite and nitrate species increase. After 60 s of NO2 exposure at 1 × 10-5 Torr, the IR features of the nitrites (1250 cm-1) and surface nitrates (1578 cm-1) reach their maximum intensities. At even higher NO2 doses (>10-5 Torr), the intensities of the IR bands of bulk (ionic) nitrates increase dramatically, while the features of the surface species (nitrites and surface nitrates) decrease. The intensities of the 1363 and 1419 cm-1 IR features increase continuously

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with increasing NO2 pressure and do not saturate under the experimental conditions applied (maximum NO2 pressure of 1 × 10-4 Torr). Interestingly, the nitrite feature was still present in the IR spectrum even after NO2 exposure at 1 × 10-4 Torr. This is in contrast to the observation on the reaction of NO2 with a thick BaO layer. At the elevated NO2 pressure of 1.0 × 10-4 Torr, nitrites were converted completely to nitrates, as observed previously.25 However, the presence of a relatively intense peak at 1250 cm-1 in Figure 2a clearly indicates that nitrite species are stable even at this elevated NO2 pressure (1.0 × 10-4 Torr) in the presence of H2O and Ba(OH)2 at 300 K. Lietti et al. have also observed that surface hydroxyls favored the adsorption of NOx on Pt-Ba/γ-Al2O3 in the form of nitrites, and the oxidation of nitrites to nitrates was not complete,17 similar to the results observed here. Even though these nitrite and nitrate species formed during the adsorption of NO2 at 300 K, the intensity of the broad OH stretching vibrational peak centered at 3442 cm-1 does not change significantly. During NO2 exposures when the IR bands of surface nitrate and nitrite species reached their maximum intensities, the OH stretching band intensity remained practically constant. Its intensity dropped by ∼10% as bulk nitrates formed. This observation indicates that the majority of bulk, amorphous Ba(OH)2 is unreacted with NO2 under these conditions. Even at the elevated NO2 pressure of 1.0 × 10-4 Torr, only a small fraction of the Ba(OH)2 film converted to Ba(NOx)2 at 300 K. The formation of nitrites, in particular at the initial stage of NO2 uptake, has been well documented on both model1,4,23 and practical26 BaO/Al2O3 systems. On thin BaO films (θBaO < 1 MLE) under NO2 pressures up to l × 10-7 Torr, XPS data showed the formation of nitrites exclusively,24 while on thick BaO films (θBaO > 30 MLE) and on high surface area BaO/γAl2O3 systems both nitrites and nitrates formed at low NO2 exposures.24,26 The intensities of the nitrite features in the IR spectra collected from both the model and high surface area systems were always rather low, and it was invisible in IRAS of model systems with θBaO < 1 MLE.24 On the other hand, a dramatic increase in the intensity of the nitrite feature was observed in the IR spectra collected from a BaO/γ-Al2O3 system after NO2/H2O coadsorption.12 The very large intensity of the nitrite feature observed here at low NO2 exposures of the amorphous Ba(OH)2 · xH2O/Pt(111) system is consistent with the enhancement of the intensity of the nitrite band by the presence of water reported on high surface area materials.12 The most probable explanation for this intensity enhancement is the increase in the dynamic dipole moment of adsorbed nitrite species as they strongly interact with the water present on the surface, which results in a vastly increased IR absorption cross section of this particular vibrational mode. Figure 2b shows a series of IR spectra obtained following stepwise annealing of the NO2-exposed sample (represented by the spectrum in Figure 2a collected at the highest NO2 dose) in the absence of NO2. Annealing to 350 K leads to an increase in the intensity of the broad peak centered at 1419 cm-1, and the peak is blue-shifted to 1426 cm-1. Concomitantly, the intensity of the 3442 cm-1 feature decreases. The observed drop in the intensity of the broad OH band is due to the phase transformation of the amorphous Ba(OH)2 · xH2O to a crystalline form, rather than water desorption, as we have discussed in detail previously.21 Annealing to 400 K further increases the intensity of the peak at 1426 cm-1 and shifts its peak position to 1409 cm-1. The broad O-H stretching feature changes dramatically after annealing to 400 K, due primarily to the formation of crystalline β-Ba(OH)2 · xH2O (∼3500 and 3585 cm-1), and

Mudiyanselage et al. possibly, but to a much lesser degree, to the desorption of a small amount of water. The most significant changes in the IR spectrum occur upon annealing to 450 K. The nitrate feature centered at 1426 cm-1 and possessing a broad shoulder at 1365 cm-1 suddenly transforms into a very intense, almost symmetric (with a shoulder at around 1450 cm-1) peak centered at 1409 cm-1. At the same time, a very sharp, highly symmetric peak appears at 3574 cm-1, while the shoulder around 3500 cm-1 almost disappears. No extensive H2O desorption from the watersaturated thick BaO film was seen in the temperature range 400-450 K in the absence of NO2 exposure. The IR spectrum collected after the 450 K annealing suggests that a very highly ordered (probably mixed) nitrate/hydroxide phase forms under these conditions. The sharp features at 3574 and 1409 cm-1 are assigned to the crystalline Ba(OH)2 and crystalline Ba(NO3)2 phases, respectively. Note that these phases only exist in the presence of both nitrates and hydroxides. As the annealing temperature increases to 500 K, water desorption becomes extensive, resulting in a decrease in the intensity of the IR feature at 3574 cm-1. Concomitantly, the intensity of the 1409 cm-1 band decreases, whereas the shoulders at 1450 and 1365 cm-1 reappear. No IRAS feature is present in the νOH region following the anneal cycle at 550 K. Parallel to the complete disappearance of the sharp band at 1409 cm-1, the development of the two features characteristic of crystalline Ba(NO3)2 (1385 and 1426 cm-1) occurs. Heating the sample to even higher temperatures results in the decomposition of the crystalline Ba(NO3)2 in accord with the sequence we have discussed in detail in our prior studies.24 3.3. NO2 on Crystalline Ba(OH)2 · xH2O/Pt(111) at 300 K. A series of IR spectra obtained following the adsorption of NO2 on crystalline β-Ba(OH)2 · xH2O/Pt(111) (x ) 0, 1), prepared by adsorbing H2O on BaO/Pt(111) at 425 K, as a function of exposure at 300 K is shown in Figure 3a. The crystalline Ba(OH)2 layer shows a broad, unresolved feature (with peaks at 3398 and 3496 cm-1) and a sharp feature at 3586 cm-1, which are assigned to the β-Ba(OH)2 · xH2O phase, as described in section 3.1. At the lowest NO2 exposures (PNO2 e 1 × 10-9 Torr), a peak around 1246 cm-1 appears and is assigned to nitrite species.1–3,5,6,23,24 With increasing NO2 exposure, the intensity of this peak grows and two new features appear at 1359 and 1450 cm-1, and assigned to nitrate species.1–3,5,6,23,24 The intensities of all of these peaks increase gradually until the NO2 pressure reaches 5.0 × 10-6 Torr. At this NO2 pressure, the intensities of the nitrite band and a low intensity feature at 1581 cm-1 reach their maxima, and then, at even higher PNO2 their intensities drop as they are converted to bulk nitrates, similarly to the amorphous Ba(OH)2 system. The peaks at 1359 and 1451 cm-1 grow continuously with the NO2 pressure until saturation at PNO2 ) 1.0 × 10-4 Torr. Even though the IR spectra show dramatic changes in the 1100-1800 cm-1 spectral range, due to the formation of nitrite and nitrate species, the νOH region (3000-3700 cm-1) changes only slightly, similarly to what we have found for the adsorption of NO2 on amorphous Ba(OH)2/ Pt(111) at 300 K. Even at elevated NO2 pressures (∼1.0 × 10-4 Torr), only a fraction of the crystalline β-Ba(OH)2 · xH2O layer converts to Ba(NOx)2 at 300 K. (The presence of unreacted Ba(OH)2 is substantiated by the desorption of a very large amount of H2O in a TPD experiment after the β-Ba(OH)2 · xH2O was exposed to 1 × 10-4 Torr NO2 for 180 s.) Figure 3b shows a series of IR spectra obtained after annealing the Ba(NOx)2/ β-Ba(OH)2 · xH2O system (last spectrum in Figure 3a) at the indicated temperatures in the absence of NO2. Subsequent annealing to 350 and 400 K leads to the intensity increase of

Reactions of NO2 with Ba(OH)2 on Pt(111)

Figure 3. Series of IR spectra obtained (a) following the exposure of crystalline Ba(OH)2/Pt(111) to NO2 as a function of pressure at 300 K and (b) subsequent annealing to the indicated temperatures. The crystalline Ba(OH)2/Pt(111) system was obtained by adsorbing H2O on BaO/Pt(111) at 425 K (displayed in panel a).

the broad peak centered at 1405 cm-1 and the concomitant decrease of the peak at 3398 cm-1. Dramatic changes (the same trend as we have discussed for the Ba(NOx)2/amorphous Ba(OH)2 · xH2O system) in the spectrum occur upon annealing to 450 K: (i) the peak centered at 1405 cm-1 becomes a sharp, intense feature, (ii) the intensity of the broad OH stretching feature at 3398 cm-1 reduces, (iii) the peak at 3586 cm-1 completely disappears, and (iv) a new sharp peak appears at 3572 cm-1. The intensity of the feature at 3398 cm-1 further reduces upon annealing to 500 K due to the desorption of water, whereas that of the peak at 1405 cm-1 slightly increases (the sharp peaks at 1405 and 3572 cm-1 reach their maximum intensities). Annealing to 550 K leads to the complete disappearance of all the features in the OH stretching region. At the same time, the intensity of the peak at 1405 cm-1 reduces and shifts to 1419 cm-1, and a shoulder develops at around 1365 cm-1. The trends observed for the Ba(NOx)2/β-Ba(OH)2 · xH2O during annealing are identical to those observed for the amorphous, NO2-exposed Ba(OH)2 · xH2O. In both systems, the formation of a thin Ba-nitrate layer on top of the Ba-hydroxide film seems to prevent the further hydroxide-to-nitrate/nitrite conversion, most probably by dramatically slowing down the NO2 diffusion from the surface to the unreacted Ba-hydroxide. If this is indeed the case, exposing the crystalline β-Ba(OH)2 · xH2O to NO2 at elevated temperatures (e.g., 425

J. Phys. Chem. C, Vol. 114, No. 40, 2010 16959 K, where still no H2O desorption takes place) should result in the extensive conversion of the hydroxide layer to nitrates by allowing faster diffusion through the initially formed Ba(NOx)2 layer. Therefore, next we examined the interaction of the crystalline β-Ba(OH)2 · xH2O with NO2 at 425 K sample temperature at similar NO2 exposures discussed above. 3.4. NO2 on Crystalline β-Ba(OH)2 · xH2O/Pt(111) at 425 K. A crystalline β-Ba(OH)2 · xH2O layer on Pt(111) was prepared by the saturation of BaO/Pt(111) with H2O at 425 K.21 XPS analysis of the water-exposed sample revealed that the entire thick BaO layer was converted to hydroxide, as no remaining intensity of the characteristic O 1s feature of BaO was observed. A series of IR spectra as a function of NO2 exposure of the crystalline β-Ba(OH)2 · xH2O/Pt(111) at 425 K is shown in Figure 4a-c (in three different pressure ranges). At the lowest NO2 exposures (up to ∼0.25 L (the last 60 s exposure at 5 × 10-9 Torr)) of crystalline β-Ba(OH)2 · xH2O/Pt(111), a peak at 1245 cm-1 develops only. Concomitantly, the intensities of the IR features characteristic of β-Ba(OH)2 · xH2O (3424 and 3582 cm-1) gradually decrease and completely disappear in this exposure region, while the band centered at 3517 cm-1, indicative of the crystalline β-Ba(OH)2 with no hydrating water, retains its full intensity, as shown clearly by the inset in Figure 4a. With the increase in the NO2 exposure, the peak at 1245 cm-1 keeps growing and two new features appear at 1397 and 3569 cm-1 following 120 s of NO2 exposure at 5.0 × 10-9 Torr. The appearance and development of the sharp band at 1397 cm-1 seem to correlate with those of the 3569 cm-1 peak. These observations suggest that exposing the crystalline β-Ba(OH)2 · xH2O to NO2 under low pressures (5 × 10-9 Torr) at 425 K leads to the removal of the hydrating H2O molecules and the exclusive formation of nitrites. The characteristic nitrate feature (1397 cm-1) only appears after the complete disappearance of the two features representing crystalline β-Ba(OH)2 · H2O. As the NO2 exposure time at this PNO2 increases, the peak at 1241 cm-1 grows gradually, while the intensity of the 1397 cm-1 band grows at a much faster rate, accompanied by the development of the sharp peak at 3569 cm-1. The dominant IR feature after 300 s of NO2 exposure at 1 × 10-6 Torr is the nitrate band at 1401 cm-1, while the nitrite peak at 1246 cm-1 reaches its maximum intensity. At the same time, the intensity of the νOH band at 3569 cm-1 also reaches its maximum, while the shoulder at 3495 cm-1 (characteristic of β-Ba(OH)2) almost completely disappears. Keeping the sample under a NO2 pressure of 5 × 10-6 Torr results in the decrease and subsequent disappearance of the remaining IR intensity in the νOH stretching region (Figure 4c). Concomitantly, the IR feature of the nitrate species splits into two, well separated components (1396 and 1427 cm-1), characteristic of the crystalline Ba(NO3)2 in the absence of hydroxides, as we have seen previously during NO2 adsorption on BaO films.24 Exposing the sample to NO2 at even higher pressures (up to 5 × 10-5 Torr) (after no νOH vibration is seen) results in further intensity gain of the nitrate features and drop in the intensity of the nitrite band as nitrites are converted to nitrates. With the formation of crystalline Ba(NO3)2, the intensity of the ν1 + ν4 combination vibrational mode of ionic nitrates (1773 cm-1) increases as well. Figure 4d displays a series of IR spectra obtained following the annealing of the system represented by the last spectrum in Figure 4c to the indicated temperatures in the absence of NO2. Annealing to 600 and then to 650 K results in a significant decrease in the intensity of the nitrite feature (1246 cm-1), and the intensity gains and sharpening of the crystalline nitraterelated peaks (1396, 1427, and 1773 cm-1). After annealing to

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Figure 5. RAIRS (a), N 1s XPS (b), and TPD spectra (c) obtained following the formation of crystalline Ba(OH)2 and its subsequent exposure to low (150 L) and high (27000 L) NO2 exposures at 425 K.

Figure 4. (a-c) Series of IR spectra obtained following the formation of crystalline Ba(OH)2 and subsequent exposure of the crystalline Ba(OH)2 layer to the indicated NO2 pressures at 425 K. The inset in panel a shows the expansion in the OH stretch region of the first five spectra. (d) Series of IR spectra obtained following subsequent annealing of the system in panel c to the indicated temperatures.

700 K, the two well-resolved nitrate features at 1390 and 1428 cm-1 completely disappear and merge into a peak centered at 1418 cm-1. Further annealing to 750 K leads to the elimination of both the nitrite feature at 1250 cm-1 and the combination vibrational mode of crystalline nitrates at 1773 cm-1, as well as the shifting of the weak nitrate feature to 1464 cm-1. These observations are in accord with the collapse of the crystalline Ba(NO3)2 phase as it is thermally decomposed. Annealing to 1000 K regenerates the BaO layer, and it was confirmed by XPS. In order to confirm the NOx species identified by IR spectroscopy as well as to obtain more information on the reaction mechanisms, we performed XPS and TPD experiments. Figure 5a shows the IR spectra obtained following the formation of crystalline β-Ba(OH)2 · xH2O (green) and its subsequent

Reactions of NO2 with Ba(OH)2 on Pt(111) exposure to 150 (black) and 27000 L (red) of NO2 at 425 K. The IR spectrum obtained after the low exposure of crystalline β-Ba(OH)2 · xH2O/Pt(111) to NO2 shows peaks associated with nitrites (1243 cm-1), nitrates (1395 cm-1), and barium hydroxide (3564 cm-1), as we have discussed above. The corresponding N 1s XP spectrum (Figure 5b) exhibits peaks at 404.0 and 407.9 eV binding energies, substantiating the presence of both nitrites and nitrates, respectively.1,4,5,11,27 The higher intensity of the peak representing the nitrite species indicates the presence of a higher amount of nitrites than nitrates in this system prepared at a low NO2 exposure. Exposing the β-Ba(OH)2 · xH2O/Pt(111) sample to a much higher dose of NO2 at 425 K results in large intensity gains of the nitrate features in the IR spectrum, and a dramatic increase in the nitrate/nitrite N 1s intensity ratio. While at low NO2 exposure the N 1s XPS intensities of the nitrite species are higher than those of the nitrates, at high NO2 exposure, the amount of nitrates formed far exceeds that of the nitrites. Comparison of the IR and XP spectra obtained from these systems clearly indicates that the IR peak intensities cannot be used to obtain quantitative information about the amount of nitrites and nitrates on/in the samples, in agreement with our prior work24 and previous findings of Libuda et al.1 The O 1s XP spectrum (not shown) indicates the presence of Ba(OH)2 and Ba(NOx)2 (in about equal quantities) on β-Ba(OH)2 · xH2O after low NO2 exposure, while after a high NO2 exposure only the signal from the NOx species (nitrites and mostly nitrates) can be seen (the O 1s binding energy for the hydroxide phase is ∼531.4 eV, while that for the nitrite/nitrate phase is ∼533.4 eV). Both the IR and XP spectra of a BaO film exposed to a very high NO2 dose at 425 K indicate the complete conversion of the thick film to Ba(NO3)2, as no nitrite signature is seen either in the IR or XP spectrum. Quantitative comparison of the NOx uptake of the β-Ba(OH)2 · xH2O film at low and high NO2 exposures and a BaO film at very high NO2 exposure can be made based on the results of TPD experiments shown in Figure 5c (mass fragments 18 (H2O), 30 (NO), and 32 (O2) are displayed). Desorption of a very large amount of H2O is seen from the β-Ba(OH)2 · xH2O sample exposed to a low NO2 dose. The evolution of the NO signal follows the same trend we have discussed in detail in our prior work5,9 and does not seem to be influenced by the presence of water/hydroxide on/in the sample, in agreement with our studies on high surface area BaO/γAl2O3 systems.12 The desorption temperature of water (originating from the decomposition of unreacted β-Ba(OH)2 · H2O) is also unaffected by the presence of the nitrite/nitrate species in the sample; it is identical to that we have shown in Figure 1a (i.e., 520 K). This observation also supports the assignment of the sharp IR feature at 3574 cm-1 in Figure 2b to Ba(OH)2, since this feature disappears after annealing to 550 K, due to the decomposition of Ba(OH)2. The 30 amu desorption profile of the β-Ba(OH)2 · xH2O film exposed to a high NO2 dose shows very intense maxima at 701 and 726 K, that can be associated with the decomposition of bulk Ba(NO3)228 (the shoulder extending to temperatures below 600 K originates from the desorption of surface nitrites). The amount of water desorbing from this sample is close to the detection limit (trace at ∼560 K), and may arise from the background adsorption during sample transfer from the IR cell to the analysis chamber. For comparison, IR, N 1s XPS, and TPD spectra collected from a NO2saturated (54000 L at 425 K) BaO film are also displayed in Figure 5a, b, and c, respectively. The IR spectra of this system show the two very intense bands characteristic of highly crystalline Ba(NO3)2, with no trace of the nitrite species. This is completely supported by the N 1s XP spectrum (Figure 5b)

J. Phys. Chem. C, Vol. 114, No. 40, 2010 16961 that displays only one feature at ∼407.0 eV. The TPD trace of 30 amu shows increased intensities for the peaks at 701 and 726 K, and no shoulder at lower temperature. These observations indicate that, at 425 K and at very high NO2 exposure, the entire thick (>20 MLE) BaO film was converted to crystalline Ba(NO3)2. (Note that this conversion is a kinetically controlled process.1,25 In our previous study24 on the reaction of NO2 with thick BaO films (>30 MLE) under comparable reaction conditions (high NO2 exposure achieved by dosing at 1 × 10-4 Torr) at 300 K, we were not able to convert the entire oxide film to Ba(NO3)2. This kinetic control was also highlighted by our previous results6 that showed the effect of sample temperature on the amount of nitrite/nitrate formed at identical NO2 exposures.) (The TPD spectra in Figure 5c collected after partial conversion of the hydroxide layer (thick solid lines) were recorded at the same mass spectrometer sensitivity as all of the other traces. Under those conditions, due to desorption of a large amount H2O from this particular sample, the 18 amu signal was out of range.) One issue that still has not been addressed so far is the kinetics of conversion of the oxide vs the hydroxide to nitrates/nitrites. Higher rates of conversion to nitrates have been reported for the oxide on high surface area, alumina-supported systems.17 3.5. Comparison of Ba(NOx)2 Formation on BaO and β-Ba(OH)2 · xH2O at 425 K. In order to investigate the kinetics of the Ba(NOx)2 formation process on BaO and crystalline β-Ba(OH)2 · xH2O, a series of IRAS spectra were collected as a function of time (28 spectra in 36 min) during the adsorption of NO2 (PNO2 ) 1.0 × 10-6 Torr) at 425 K. Figure 6a and b shows contour plots of the IRAS spectra in the frequency range 1150-1550 cm-1 for the adsorption of NO2 on BaO and β-Ba(OH)2 · xH2O, respectively. (A contour plot, showing the changes in the intensities of OH stretching vibrational features during NO2 exposure of the hydroxide sample, is displayed in Figure 6c.) The corresponding integrated IR peak intensities of nitrites (1240-1260 cm-1) and nitrates (1390-1430 cm-1) are displayed in Figure 6d. On these two systems (oxide and hydroxide), initially both the nitrate and nitrite peak intensities increase at a fast rate with exposure time. However, the intensity increase in the first 6 min of NO2 exposure of the BaO film is significantly faster than that of the barium hydroxide film. This observation is in concert with results reported on aluminasupported oxide and hydroxide systems that demonstrated a faster initial reaction with NO2 over the oxide.17 On the oxide system, however, after the initial very high rate of nitrite/nitrate formation, a sudden decrease in the slope of the integrated IR intensity with respect to exposure time plot is observed. This slope change is correlated with the rate of nitrite formation reaching its maximum. After about 9 min of NO2 exposure, the intensity of the nitrate features increases gradually with exposure time, while that of the nitrite peak decreases slowly. Very different behavior is observed for the case of the β-Ba(OH)2 · xH2O system upon NO2 exposure. Initially, the intensities of both nitrite and nitrate features increase fast, although not as fast as those seen for BaO. However, the intensity of the nitrite feature does not reach a maximum, and it continuously increases with exposure time. Concomitantly, the intensity of the nitrate feature increases at a much faster rate than what we have seen for the BaO layers, and keeps increasing through the entire 36 min of adsorption. These results can probably be explained by the different morphology changes that occur during the NO2 uptake on the oxide vs the hydroxide systems. In a previous study,12 we have shown that the presence of water results in dramatic morphology changes in the NO2-

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Mudiyanselage et al. exposed BaO/γ-Al2O3 systems. Very large Ba(NO3)2 crystals formed upon the exposure of NO2/BaO/γ-Al2O3 systems to H2O at 300 K. Furthermore, assuming that the nitrite species during NO2 exposure are only present in these systems as surface species (i.e., no bulk nitrites form), we can rationalize the experimental results by the changes in morphology phenomenon. Surface nitrites and nitrates form on BaO at a very high rate at the beginning of the NO2 exposure, manifested in the fast intensity gains of both nitrite and nitrate IR features. As the surface (and near surface) layer(s) saturates with NOx, the rate of nitrate formation will be limited by the conversion of the bulk oxide to nitrate. In the absence of significant morphology change, this should be a slow, diffusion-limited process. The intensity of the nitrite feature reaches its maximum, since the impinging NO2 molecules oxidize the surface nitrites to nitrates, and the formation of “new” surface sites where nitrites could form (by morphology changes) is limited. The fact that nitrites are present on the surface even at the completion of the experiment (36 min) indicates that oxide surface sites continuously open up and adsorb NO2, but this process is rather slow. In contrast, the morphology change in the β-Ba(OH)2 · xH2O system during NO2 adsorption is much more pronounced. This fast morphology change opens up an increasingly larger number of sites for NO2 adsorption, and the amount of nitrites formed increases continuously. This results in a much faster nitrate formation rate on the hydroxide than over the oxide. Therefore, higher NOx uptake is observed on β-Ba(OH)2 · xH2O at the completion of the experiment (after 36 min of NO2 exposure), as shown by the NO (30 amu) TPD traces in the inset of Figure 6d. The data seem to suggest that, as large nitrate crystallites form in the presence of H2O, more and more surface sites open up and are ready to react with the incoming NO2 molecules. Although this mechanism is consistent with our experimental results, it needs to be substantiated by scanning probe microscopy, or inert noble gas adsorption experiments. 4. Conclusions

Figure 6. Contour plots of RAIR spectra recorded during NO2 exposure (PNO2 ) 1.0 × 10-6 Torr) of BaO (a) and crystalline β-Ba(OH)2 · xH2O (b, c) as a function of time at 425 K (28 spectra were collected for each system for 36 min). (d) Integrated IR peak areas of nitrites and nitrates formed on BaO and crystalline β-Ba(OH)2 · xH2O as a function of time, obtained from the RAIR spectra shown in panels a and b. (The inset in panel d displays the NO TPD traces collected after completion of the IR experiments (panels a and b).)

A thick BaO (>20 MLE) layer readily reacts with H2O to form amorphous Ba(OH)2 at 300 K, which, in turn, converts to crystalline hydroxides (β-Ba(OH)2 · H2O and β-Ba(OH)2) upon annealing. Crystalline β-Ba(OH)2 can also be prepared by exposing a BaO (>20 MLE)/Pt(111) system to H2O at 425 K. Amorphous and crystalline Ba(OH)2 partially convert to a Ba(NOx)2 layer (nitrites and nitrates) following their exposure to elevated NO2 pressure (1.0 × 10-4 Torr) at 300 K. The water formed during this partial conversion process stays on/in the hydroxide layer at 300 K. Exposure of the crystalline β-Ba(OH)2 layer to elevated NO2 pressure at 425 K leads to its complete conversion to crystalline Ba(NO3)2 with a small amount of nitrites still present on the surface. The water formed during this conversion process desorbs from the surface at 425 K. The nitrite species formed from the reaction between NO2 and Ba(OH)2 cannot be converted completely to nitrates at elevated NO2 pressure most likely due to the higher stability of nitrite species in the presence of surface hydroxyl groups. In contrast, the exposure of a thick BaO layer to elevated NO2 pressure leads to the formation of crystalline nitrates. The initial rate of Ba(NO3)2 formation is faster on BaO than on crystalline β-Ba(OH)2 · xH2O under the specific experimental conditions applied (1.0 × 10-6 Torr NO2 pressure, 425 K), but with the total NOx uptake on the crystalline β-Ba(OH)2 · xH2O is much higher. This is most likely due to the fundamental differences in the morphology of the crystalline β-Ba(OH)2 · xH2O and BaO systems during their reactions with NO2. However, both BaO

Reactions of NO2 with Ba(OH)2 on Pt(111) and crystalline Ba(OH)2 can completely be converted to Ba(NOx)2 at 425 K at high NO2 exposures administered under elevated-pressure conditions. Acknowledgment. We gratefully acknowledge the US Department of Energy (DOE), Office of Basic Energy Sciences, Division of Chemical Sciences, for the support of this work. The research described in this paper was performed at the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the DOE Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is operated for the US DOE by Battelle Memorial Institute under contract number DE-AC05-76RL01830. C.-W.Y. also acknowledges the support of this work by Sungshin Women’s University Research Grant of 2010. References and Notes (1) Desikusumastuti, A.; Happel, M.; Dumbuya, K.; Staudt, T.; Laurin, M.; Gottfried, J. M.; Steinruck, H.-P.; Libuda, J. J. Phys. Chem. C 2008, 112, 6477. (2) Desikusumastuti, A.; Laurin, M.; Happel, M.; Qin, Z.; Shaikhutdinov, S.; Libuda, J. Catal. Lett. 2008, 121, 311. (3) Desikusumastuti, A.; Staudt, T.; Gro¨nbeck, H.; Libuda, J. J. Catal. 2008, 255, 127. (4) Staudt, T.; Desikusumastuti, A.; Happel, M.; Vesselli, E.; Baraldi, A.; Gardonio, S.; Lizzit, S.; Rohr, F.; Libuda, J. J. Phys. Chem. C 2008, 112, 9835. (5) Yi, C.-W.; Kwak, J. H.; Szanyi, J. J. Phys. Chem. C 2007, 111, 15299. (6) Yi, C.-W.; Szanyi, J. J. Phys. Chem. C 2009, 113, 2134. (7) Bowker, M.; Stone, P.; Smith, R.; Fourre, E.; Ishii, M.; de Leeuw, N. H. Surf. Sci. 2006, 600, 1973.

J. Phys. Chem. C, Vol. 114, No. 40, 2010 16963 (8) Bowker, M.; C, M.; Hall, M.; Fourre, E.; Grillo, F.; McCormack, E.; Stone, P.; Ishii, M. Top. Catal. 2007, 42-43, 341. (9) Mudiyanselage, K.; Yi, C. W.; Szanyi, J. Langmuir 2009, 25, 10820. (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) Szanyi, J.; Kwak, J. H.; Kim, D. H.; Wang, X.; Chimentao, R.; Hanson, J.; Epling, W. S.; Peden, C. H. F. J. Phys. Chem. C 2007, 111, 4678. (13) Cant, N. W.; Patterson, M. J. Catal. Lett. 2003, 85, 153. (14) Corbos, E. C.; Courtois, X.; Bion, N.; Marecot, P.; Duprez, D. Appl. Catal., B 2007, 76, 357. (15) Epling, W. S.; Campbell, G. C.; Parks, J. E. Catal. Lett. 2003, 90, 45. (16) Hendershot, R. J.; Vijay, R.; Snively, C. M.; Lauterbach, J. Appl. Surf. Sci. 2006, 252, 2588. (17) Lietti, L.; Forzatti, P.; Nova, I.; Tronconi, E. J. Catal. 2001, 204, 175. (18) Scholz, C. M. L.; Gangwal, V. R.; de Croon, M. H. J. M.; Schouten, J. C. Appl. Catal., B 2007, 71, 143. (19) Toops, T. J.; Smith, D. B.; Epling, W. S.; Parks, J. E.; Partridge, W. P. Appl. Catal., B 2005, 58, 255. (20) Lutz, H. D.; Eckers, W.; Christian, H.; Engelen, B. Thermochim. Acta 1981, 44, 337. (21) Yi, C.-W.; Szanyi, J. J. Phys. Chem. C 2009, 113, 15692. (22) Lutz, H. D.; Eckers, W.; Schneider, G.; Haeuseler, H. Spectrochim. Acta, Part A 1981, 37, 561. (23) Desikusumastuti, A.; Staudt, T.; Happel, M.; Laurin, M.; Libuda, J. J. Catal. 2008, 260, 315. (24) Yi, C.-W.; Szanyi, J. J. Phys. Chem. C 2009, 113, 716. (25) Mudiyanselage, K.; Yi, C.-W.; Szanyi, J. J. Phys. Chem. C, in preparation. (26) Epling, W. S.; Campbell, L. E.; Yezerets, A.; Currier, N. W.; Parks, J. E. Catal. ReV.sSci. Eng. 2004, 46, 163. (27) Schmitz, P.; Baird, R. J. Phys. Chem. B 2002, 106, 4172. (28) Chen, X.; Schwank, J.; Li, J.; Schneider, W. F.; Goralski, C. T., Jr.; Schmitz, P. J. Appl. Catal., B 2005, 61, 164.

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