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J. Phys. Chem. C 2009, 113, 2134–2140
Reaction of NO2 with a Pure, Thick BaO Film: The Effect of Temperature on the Nature of NOx Species Formed† Cheol-Woo Yi‡ and Ja´nos Szanyi*,§ Department of Chemistry and Institute of Basic Science, Sungshin Women’s UniVersity, Seoul 136-742, South Korea, and Institute for Interfacial Catalysis, Pacific Northwest National Laboratory P.O. Box 999, MSIN: K8-80, Richland, Washington 99352 ReceiVed: July 31, 2008; ReVised Manuscript ReceiVed: September 15, 2008
The adsorption and reaction of NO2 on a thick (>30 ML), pure BaO film deposited onto an Al2O3/NiAl(110) substrate were investigated in the temperature range of 300 - 660 K using temperature programmed desorption (TPD), infrared reflection absorption spectroscopy (IRAS), and X-ray photoelectron spectroscopy (XPS) techniques. The adsorption of NO2 on BaO at room temperature and the subsequent decomposition of the thus formed Ba(NOx)2 species follow the same mechanisms we have reported previously for NO2 adsorption at cryogenic temperatures. In cyclic experiments when the BaO film was exposed to NO2 at 300 K, followed by annealing to 575 K, a large amount of NOx was stored as nitrates, and no saturation was achieved even after the 10th adsorption/anneal cycle. This suggests the gradual conversion of the BaO film into barium nitrate clusters at elevated temperatures. The rate of nitrate formation increases as the sample temperature during NO2 exposure increases up to 610 K, while at even higher temperatures the amount of nitrates formed decreases. NO2 adsorption on the thick BaO film at 610 K results in the formation of strongly bound nitrates as the major NOx species. Introduction Internal combustion engines emit toxic gases, such as nitrogen oxides, sulfur oxides, and COx species. Three-way catalysts work very effectively in converting C- (CO and unburned hydrocarbons) and N-containing (NOx) pollutants from internal combustion engines that operate under stoichiometric air-to-fuel ratios. However, these catalysts are unable to perform the required NOx reduction function when the engines are operated under net lean (oxygen rich) conditions. One of the most promising catalysts to reduce harmful NOx from lean-burn automobile engine (e.g., diesel engine) emission is the NOx storage/reduction (NSR) catalyst which contains barium oxide as a NOx storage material.1-11 Therefore, the interaction of nitrogen oxides (NOx) with BaO has been attracted significant interest in both fundamental and applied research.1-11 These catalysts require cyclic engine operation, i.e., alternating lean and rich cycles. The storage of NOx into a storage material takes place during the lean engine operation phase, and the release and reduction of NOx occur during the rich operation cycle.11 The adsorption of nitrogen oxides (NO and NO2) has been extensively studied on high surface area materials.1-8 Despite the considerable research efforts,1-8 the chemical and mechanistic details of these storage and release processes are not well understood, and the nature of the active storage sites remains an open question due to the difficulties of constructing and studying adequate model systems. The two major difficulties that have hindered the study of NOx chemistry over model BaO on Al2O3/NiAl(110) model catalysts are (i) the thickening of the alumina film used as a model support and (ii) the reaction † Part of the special section “Physical Chemistry of Environmental Interfaces”. * To whom correspondence should be addressed. E-mail: janos.szanyi@ pnl.gov. ‡ Sungshin Women’s University. § Pacific Northwest National Laboratory.
of BaO with underlying metal oxide substrates. We have previously observed the increase in the alumina film thickness when it was exposed to oxidants.12,13 Defect sites, such as boundary domains, point defects, and line defects, on the oxide film were proposed as the primary sources of water and NO2 decomposition and were shown to play critical roles in the increase of the Al-oxide film thickness by the reaction of aluminum in the NiAl(110) substrate with the atomic oxygen originating from the decomposition of water and NO2.12,13 Recent studies 14,15 by Libuda and co-workers also confirmed the thickening of the ultrathin alumina on NiAl(110) single crystal upon NO2 adsorption, and they observed the same phenomenon even with BaO (14 Å thickness)/Al2O3 (5 Å thickness)/NiAl(110) model catalyst. The thickening of the Aloxide film during NO2 adsorption makes it very difficult (i) to obtain accurate X-ray photoelectron spectroscopy (XPS) corelevel binding energies, (ii) to study the interactions between adsorbates and the surface, and (iii) to understand the behavior of the model system. Due to the difficulties with the model systems, the number of studies on BaO-containing model catalysts is very limited, and in some studies poor reproducibility was reported for NO2 adsorption. In one of the typical examples a 5-6 ML BaO film was prepared on Cu(111) and its interactions with NO2, CO2, and H2O were studied by Tsami et al.16 Temperature programmed desorption (TPD) profiles following NO2 adsorption were significantly different from the asprepared sample and from samples which were previously subjected to cycles of NO2 adsorption-desorption, i.e., the NOx chemistry on the model catalyst was strongly dependent on the history of the sample. These variations were attributed to the reaction between BaO and the underlying CuOx during NOx adsorption and desorption processes, as they formed barium cuprates. Recently, we have reported the interaction of NO2 with a thick BaO layer, prepared by reactive layer-assisted deposition
10.1021/jp806854y CCC: $40.75 2009 American Chemical Society Published on Web 10/29/2008
Reaction of NO2 with a Pure, Thick BaO Film (RLAD) of Ba onto a thick N2O4 multilayer, at liquid nitrogen temperature.10 The adsorption of NO2 on this thick BaO film at 90 K10 clearly showed that BaO reacted with NO2 to initially form nitrite-nitrate ion pairs that were proposed by the density functional theory (DFT) calculation of Schneider17 and Gro¨nbeck.18 In the thermal decomposition process the nitrite species of these ion pairs decomposed first by releasing an NO molecule and leaving an O atom on the surface. At higher temperatures the nitrates decomposed in two steps: at lower temperature as NO2 only, then, at higher temperature, as NO + O2. The formation of barium nitrate clusters was also observed on a series of model NOx storage reduction catalysts prepared by RLAD of Ba onto a thick Al2O3/NiAl(110) with BaO coverages close to those of the practical catalysts.9 These model catalysts demonstrated identical properties upon NO2 exposure as the corresponding high surface area ones. At low BaO coverages (30 ML) prepared by high temperature annealing was then thoroughly characterized (XPS, LEISS) to ensure that
Results and Discussion In order to eliminate the effect of the interaction between BaO and the underlying Al2O3 and/or NiAl substrate (i.e., the formation of barium aluminate phase which we recently reported9) on the NOx chemistry of the BaO film and to minimize the thickening of alumina film, a thick BaO film was prepared. The interaction between NO2 and the BaO film was investigated with various surface sensitive techniques at room and at elevated temperatures (300 K e T e 660 K). First, we will discuss the results of NO2 adsorption on a thick BaO film at room temperature, and then in cyclic NO2 adsorption (NO2 adsorption at 300 K and subsequent annealing at 575 K) and finally in NO2 adsorption at elevated temperatures. A. NO2 Adsorption at 300 K. The results of TPD experiments performed after NO2 adsorption at 300 K on a thick BaO film showed the same desorption profiles as those observed after adsorption at liquid nitrogen temperature,10 and therefore, they are not shown here. Briefly, in the TPD spectra following low temperature NO2 adsorption up to saturation coverage three desorption features, representing one nitrite and two nitrate decomposition processes were observed. The NO desorption feature with a maximum desorption rate below 600 K was attributed to nitrite decomposition, while nitrate decomposition occurred in two steps. At lower temperature (610-620 K) as NO2 only, and at higher temperature (above 650 K) as NO + O2 (TPD peak deconvolution (based the fragmentation patterns in our mass spectrometer and sensitivity factors) of the 30 amu signal is shown in Figure 1 of ref 10). Figure 1A shows a series of IRAS spectra as a function of NO2 exposures at room temperature on a thick BaO film. At the lowest coverage of NO2, IRAS shows broad features with
2136 J. Phys. Chem. C, Vol. 113, No. 6, 2009
Figure 1. (A) IRAS spectra for NO2 adsorption onto a thick BaO film. NO2 was dosed at 300 K. (B) IRAS spectra collected in a stepwise annealing of NO2 adsorbed on a thick BaO film. NO2 was dosed at 300 K, and each spectrum was obtained at 300 K after annealing the sample at each indicated temperature.
low intensities at ∼1210 and 1300-1500 cm-1 assigned to bridge-bound nitrite and ionic nitrate species,1,2,5,6,8,10,22,23 respectively. There is no additional feature above 1500 cm-1 which has been assigned to the surface nitrates 8-10 on BaO/ γ-Al2O3 high surface area catalyst. With the increase of NO2 exposure, the intensities of both nitrite and nitrate features monotonically increase and eventually approach saturation. The simultaneous increase in the intensities of both ∼1240 and 1300-1500 cm-1 infrared features as a function of NOx exposure suggests the formation of nitrite and nitrate ion pairs, similarly to the low temperature NO2 adsorption on the thick BaO film.10 These IRAS results suggest a pairwise NO2 adsorption mechanism at 300 K, as nitrite-nitrate ion pairs form and are stabilized by the cooperative bonding effect. This cooperative adsorption mechanism was previously proposed in DFT studies for NO2 adsorption on MgO and BaO surfaces 17,18 and was observed experimentally in the NO2 adsorption on a thick BaO film at 90 K.10 The driving force for this cooperative enhancement is the difference in the energy spent on charge
Yi and Szanyi separation and the adsorption energy gain resulting from the stronger pairwise adsorption of the ionic species. The electron transfer between the two NO2 molecules results in a strong Lewis acid-base (NO2+-NO2-) pair formation on the BaO surface. The nitrite species are formed by bridge-bonding of NO2 molecules to two Ba2+ ions through their oxygen atoms, while the nitrate species are formed via the binding of NO2 molecules to a lattice oxygen ion through their nitrogen atom. IRAS results for stepwise annealing following saturation of NO2 adsorption at 300 K on a thick BaO film are shown in Figure 1B. The first spectrum (black solid line) was obtained after NO2 adsorption at room temperature, and it shows two characteristic IR bands at 1240 and 1300-1500 cm-1 assigned to nitrite and nitrate species, respectively. Annealing below 500 K did not show any significant change of the IRAS features and their intensities. After 575 K annealing, however, the nitrite feature at 1240 cm-1 completely disappears, whereas the features of nitrate species at 1300-1500 cm-1 remain unchanged. Further annealing at even higher temperatures results in the gradual decomposition of nitrate species. Finally, above 700 K, the nitrate infrared features completely disappear. These IRAS results are in complete agreement with the TPD results, as well as with the IRAS results obtained in the NO2 adsorption on a thick BaO film at low temperature.10 B. Cyclic NO2 Adsorption. In this section, we will discuss the reactions between NO2 and BaO in cyclic adsorption/ annealing experiments. The primary purpose of this part of the work was to determine the nature of strongly adsorbed NOx species formed under these experimental conditions that are similar to those applied in studies on other model NOx storage systems.16,24 The black line in Figure 2A is a TPD trace for the 30 amu signal obtained after exposing the clean BaO film to NO2 at 300 K at saturation coverage. The desorption features are identical to those we obtained after the saturation of the surface with NO2 at 90 K;10 three desorption peaks are seen at 590, 620, and 663 K representing NO desorption only, NO2 desorption only, and NO + O2 desorption, respectively. Next, NO2 adsorption experiments were carried out in a cyclic fashion; the BaO film was exposed to NO2 at 300 K sample temperature, followed by flashing the sample to 575 K in the absence of NO2. The annealing temperature of 575 K was chosen since it is below the onset temperature for the nitrate decomposition. In addition, as shown in Figure 1B, all the nitrite species are decomposed by flashing the NOx-saturated thick BaO film to 575 K. Hence, we focused on the more stable and strongly bound ionic NOx species that are the major species under realistic conditions. After a certain number of 300 K adsorption of NO2/575 K anneal cycles, TPD was performed to evaluate the amount of NOx stored in/on the BaO film. The results of TPD experiments are summarized in Figure 2, and the profiles of the 30 amu mass fragment are shown in Figure 2A. Prior to each TPD run, that is after the last NO2 exposure, the sample was not heated to 575 K. Specifically, the first TPD trace was obtained after NO2 adsorption at 300 K, with no subsequent annealing, and the second TPD run was carried out after two NO2 adsorption cycles, with one annealing between them. The TPD profile of the 30 amu signal, recorded from the sample that was annealed at 575 K once, and exposed to NO2 twice, was dramatically different from the one that was obtained after a single room temperature NO2 exposure (the black curve in Figure 2A). The intensity of the 590 K desorption feature decreased considerably (present only as a shoulder), while that of the feature seen at 620 K in the first run increased significantly. In addition, the temperature of maximum desorp-
Reaction of NO2 with a Pure, Thick BaO Film
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Figure 3. IRAS spectra for NO2 adsorption on a thick BaO film as a function of the number of NO2 adsorption at room temperature and annealing to 575 K cycles.
Figure 2. TPD data for NO2 adsorption on a thick BaO film as a function of the number of room temperature NO2 adsorption and 575 K anneal cycles. (A) 30, (B) 32, and (C) 46 amu.
tion rate of this latter peak shifted to a higher value (∼647 K). The increase in the number of cycles of NO2 adsorption at room temperature/575 K annealing resulted in further intensity gain, as well as a gradual shift to higher temperatures of this desorption profile. After 10 cycles of adsorption/annealing, only one main desorption feature can be seen centered at 680 K with shoulders at both lower and higher temperatures. Other mass fragment traces (32 and 46 amu) were also collected simultaneously with the 30 amu one in order to help in rationalizing the TPD results of these cyclic NO2 uptake experiments. TPD profiles of the 32 (O2) and 46 (NO2) amu fragments are displayed in Figure 2, panels B and C, respectively. The desorption feature of the 46 amu fragment closely follows the main desorption feature for the 30 amu one in the temperature range of 620 and 680 K. Taking into account the fragmentation pattern of our mass spectrometer, it is evident that the main desorption feature of the mass 30 signal at 620-680 K is the result of the fragmentation of desorbed NO2 in the ionization chamber of the mass spectrometer. On the other hand, the desorption profiles of 32 amu fragment perfectly overlap with the high temperature tail of the 30 amu signal (the shoulder in the high temperature side). Similarly to the results of the low temperature NO2 adsorption experiments,10 in the room temperature adsorption cases this latter feature arises from the decomposition of nitrate species as NO + O2. The low temperature shoulder on the 30 amu signal is the result of the decomposition of the small amount of nitrite present. These TPD results clearly show that with increasing number of cyclic NO2 exposure/anneal process the intensity of the lowest temperature desorption peak (the decomposition of nitrite species) decreases while that of the central peak (the decomposition of nitrate species) increases. IRAS spectra, collected in a parallel cyclic experiment, are shown in Figure 3. The bottom spectrum (black line) was collected after the first room temperature NO2 exposure/575 K annealing experiment. This spectrum is very similar to that collected after annealing the BaO film to 575 K following NO2 saturation at either 9010 or 300 K (see Figure 1B). It indicates that nitrites formed by cooperative adsorption of NO2 are removed to a great extent by annealing the sample to 575 K after the first exposure, while the nitrates remained practically
2138 J. Phys. Chem. C, Vol. 113, No. 6, 2009 unchanged. Increasing the number of adsorption/anneal cycles resulted in the gradual intensity gain of these nitrate features. After the ninth cycle the major nitrate band is centered at 1416 cm-1, with shoulders toward lower wavenumbers (1385 and 1365 cm-1). From the results of the NO2 uptake experiments at 90 K, 300 K and the cyclic experiments, it is evident that the amount of strongly held NOx formed on/in the BaO film is much higher when the sample is annealed to 575 K after each NO2 exposure prior to the subsequent NO2 exposure at 300 K. The results of NO2 uptake experiments at both 90 K and at 300 K suggest that under these conditions the surface is saturated with nitrite-nitrate ion pairs formed by the cooperative adsorption of NO2. On the other hand, when the sample is annealed to 575 K between each 300 K NO2 exposure, the nitrate species that decompose as NO2 only at higher temperature form preferentially. It is also evident from the results of Figure 3 that the amount of nitrites present on the surface also increases with the number of room temperature NO2 adsorption/575 K anneal cycles. This is most probably due to the increasing roughness of the BaO phase. As Ba-nitrates form, they agglomerate to larger particles which process opens up more surface BaO sites that can react with NO2 in subsequent cycles. What is not clear at this point is the mechanism of the formation of nitrate species. There are two probable mechanisms: (i) the cooperative adsorption mechanism for the formation of nitrite-nitrate pairs from the same way as we have seen on the clean surface and (ii) the direct interaction between NO2 and barium peroxides (we have recently reported that barium peroxide species10 formed during the annealing NO2exposed thick BaO films at temperatures where the nitrites decomposed). The TPD profiles displayed in Figure 2 did not show the saturation of the 650 K feature, rather indicated a gradual increase in its intensity with increasing number of adsorption/annealing cycles. This suggests either the change of film morphology, i.e. that barium nitrate clusters are formed during the annealing process9 or diffusion of the nitrate species into the thick BaO film. Bowker and co-workers have recently reported significant morphology change with scanning tunneling microsocopy (STM) for BaO/Pt(111) prior to and after NO and O2 adsorption at 573 K.25 They observed a significant expansion of the particle size, and explained it by the formation of nitrates that has larger unit cell than BaO. However, in their STM image after NO/O2 treatment,25 one can also observe the formation of large barium nitrate cluster from several small clusters. This kind of morphology change may be expected to cause the creation of empty BaO sites which can react with NO2 and form nitrite/nitrate ion pairs in subsequent room temperature NO2 exposures. In addition, note that a small amount of nitrite (∼1250 cm-1 IRAS feature in Figure 3, and the low temperature shoulder in Figure 1A) is always present as well in these cyclic NO2 uptake experiments, as it identified by the low intensity nitrite feature in both TPD and IRAS. This nitrite species may be formed in the cooperative adsorption of NO2 on the NOxfree, empty BaO surface, which, in turn, formed during either the decomposition of weakly bound ionic NOx species or the change of the surface morphology. The formation of Ba(NO3)2 cluster in the annealing step and the agglomeration of Ba(NO3)2 clusters at a rate proportional to the sample temperature has been observed in high surface area BaO/γ-Al2O3 systems8). Nonetheless, we could not completely rule out the formation of nitrate species by the direct interaction between NO2 and the peroxides. The O 1s XPS data in our previous publication,10 discussed for the low temperature NO2 adsorption and the subsequent stepwise annealing, also revealed that the decom-
Yi and Szanyi position of the nitrite species results in the formation of BaO2. Therefore, it is possible that in the next NO2 adsorption step the incoming NO2 will interact with a surface that is partially nitrated and also possesses a certain amount of barium peroxides. Due to the much higher reactivity of peroxides in comparison to their ordinary oxides, it is possible to form the nitrates by the direct reaction between BaO2 and NO2. In this case, after the annealing step, bare BaO2 sites are created by the decomposition of nitrite species, which opens up the possibility to form the nitrate species directly during the subsequent NO2 exposure. In this experimental setup, we could not differentiate and identify the two different nitrate species that decomposed to NO2 only and NO + O2. However, the data of our cyclic NO2 uptake experiments indicate that no saturation of either desorption feature was achieved even after ten cycles of adsorption/ annealing. Furthermore, the intensities of features representing nitrates in both the TPD and IRAS spectra are much higher after the cyclic experiments than saturation NO2 exposures at either 90 or 300 K. It is tempting to assign the lower temperature nitrate decomposition feature to Ba(NO3)2 clusters based on the results of the cyclic experiment. Recent reports from Schwank and co-workers 26,27 have shown that NO2 and NO are the primary product of low temperature (LT) and high temperature (HT) decomposition of Ba(NO3)2, respectively. In addition, they concluded that larger aggregates (bulk-like particles) decomposed to NO and O2 above melting point of Ba(NO3)2 (HT decomposition) while small aggregates or dispersed particles decomposed to NO2 at temperatures much lower than the melting point of Ba(NO3)2 (LT decomposition).26,27 Therefore, it is evident that barium nitrate clusters are formed, and we could argue that the intensities of TPD and IRAS features did not saturate even after the 10th NO2 adsorption/anneal cycle since the thick BaO film is gradually being converted to barium nitrate and then the nitrate species agglomerate during the annealing procedure. This explanation is in accord with the STM observations of Bowker et al.,28 we have mentioned before. Since here we are looking at a BaO film reacting with NO2, due to the large difference between the unit cell size of BaO and Ba(NO3)2, the surface nitrate concentration may increase significantly faster than that of the bulk one. In addition, annealing to 575 K results in the agglomeration of barium nitrate species. In fact, we observed the increase in the intensities of both nitrate desorption features, but the rates of increase were different for these two species (the intensity of the lower temperature desorption feature (NO2 desorption) increased faster than the higher temperature one (NO + O2 desorption)). C. NO2 Adsorption at Elevated Temperatures. In order to investigate the stability of ionic NOx species as a function of the sample temperature, we performed NO2 adsorption on the thick BaO film at elevated sample temperatures. TPD spectra obtained after the adsorption/reaction of NO2 on a thick BaO film as a function of adsorption temperature are summarized in Figure 4. The same amount of NO2 (the dosage is the same as that can saturate the surface of the BaO film at room temperature) was dosed onto the clean BaO film at the indicated temperature and then TPD was carried out. Both the integrated intensity and the temperature of maximum desorption rate of the TPD curve gradually increase as the sample temperature during NO2 adsorption increases. The highest amount of NOx desorbed was observed after NO2 adsorption at 610 K. However, TPD data for NO2 adsorption at sample temperatures above 610 K show a gradual decrease in the intensity of the desorption feature as the adsorption temperature increases. The amount of
Reaction of NO2 with a Pure, Thick BaO Film
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Figure 4. TPD data (30 amu signal) for NO2 adsorption on BaO thick film as a function of adsorption temperature. The amount of NO2 dosed is the same in each experiment.
NOx species stored on/in BaO film is influenced by two factors. At moderate sample temperatures (500-610 K) the Ba(NO3)2 particles formed by the reaction between NO2 and BaO agglomerate at a rate proportional to the sample temperature (i.e., the higher the sample temperature the higher the rate of agglomeration). As the agglomeration rate increase the area of unreacted BaO surface also increases leading to increased NO2 uptake. On the other hand, further increasing the adsorption temperature leads to a temperature regime that is above the onset of nitrate decomposition temperature (above 610 K), and the probability of NO2 adsorption on/reaction with the BaO surface decreases. This ultimately leads to a gradual decrease in NO2 uptake as the sample temperature increases from 610 to 660 K. XPS experiments were also performed on the BaO film exposed to NO2 at elevated temperatures. Panels A and B of Figure 5 show XPS spectra in the N 1s and O 1s core-level regions, respectively, after the clean BaO surface was exposed to NO2 at 610 K and then annealed to the indicated temperatures. After NO2 adsorption at 610 K, the N 1s XPS spectrum shows a large nitrate and a very small nitrite features, centered at 407.5 and 403.6 eV, respectively. From results presented in the previous sections, it is expected that nitrate species are more stable than nitrite species at higher temperatures. Annealing the sample to 655 K results in a significant loss of the N 1s feature representing the nitrite species and a small loss in the intensity of the nitrate feature. Concomitantly, a loss in the intensity of the oxygen 1s XPS feature in ionic NOx species was observed at 534.5 eV. After 675 K annealing, the nitrogen 1s region of the XPS spectrum shows the complete disappearance of ionic NOx species. In the O 1s region, however, a small shoulder was observed at 533 eV that has been assigned to barium peroxide previously.10,25,29 Since the amount of nitrites formed is small when the NO2 adsorption is carried out at 610 K sample temperature, only a small shoulder of peroxide feature was observed in the XPS spectrum. Conclusion In this study, the interaction of NO2 with a chemically pure, thick BaO film supported on Al2O3/NiAl(110) was examined
Figure 5. XPS spectra of N 1s (A) and O 1s (B) regions. Thick BaO film was exposed to NO2 at 610 K and flashed to the indicated temperatures.
using various surface sensitive techniques in order to elucidate the effect of sample temperature on the nature of NOx species formed during NO2 adsorption. NO2 adsorption on the clean, thick BaO film at 300 K sample temperature leads to the formation of nitrite-nitrate ion pairs by the cooperative adsorption mechanism, in an identical manner we have shown previously for NO2 adsorption at 90 K. Cycles of NO2 adsorption at 300 K followed by anneal at 575 K resulted in the formation of large amount of nitrates, and the intensities of features characteristic of nitrates in both TPD and IRAS were not saturated even after ten cycles NO2 adsorption/annealing. These observations indicate the gradual conversion of the thick BaO film into barium nitrate clusters at elevated temperatures. NO2 adsorption at elevated temperatures (up to 610 K) leads to the formation of nitrates with a rate proportional to the sample temperature. The agglomeration of nitrates, formed on the BaO surface, into Ba(NO3)2 partricles increases with increasing sample temperature opening up new, unreacted BaO sites for reaction with NO2. Increasing the sample temperature above 610 K, however, results in a gradual decrease in the amount of nitrates formed, due to the onset of nitrate decomposition at these high temperatures.
2140 J. Phys. Chem. C, Vol. 113, No. 6, 2009 Acknowledgment. We gratefully acknowledge the U.S. Department of Energy (DOE), Office of Basic Energy Sciences, and 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 No. DE-AC05-76RL01830. This work was also supported by the Sungshin Women’s University Research Grant of 2008. References and Notes (1) Broqvist, P.; Gronbeck, H.; Fridell, E.; Panas, I. J. Phys. Chem. B 2004, 108, 3523. (2) Fanson, P. T.; Horton, M. R.; Delgass, W. N.; Lauterbach, J. Appl. Catal., B 2003, 46, 393. (3) Hess, C.; Lunsford, J. H. J. Phys. Chem. B 2002, 106, 6358. (4) Mahzoul, H.; Brilhac, J. F.; Gilot, P. Appl. Catal., B 1999, 20, 47. (5) Nova, I.; Castoldi, L.; Lietti, L.; Tronconi, E.; Forzatti, P.; Prinetto, F.; Ghiotti, G. J. Catal. 2004, 222, 377. (6) Prinetto, F.; Ghiotti, G.; Nova, I.; Lietti, L.; Tronconi, E.; Forzatti, P. J. Phys. Chem. B 2001, 105, 12732. (7) Su, Y.; Amiridis, M. D. Catal. Today 2004, 96, 31. (8) Szanyi, J.; Kwak, J. H.; Kim, D. H.; Burton, S. D.; Peden, C. H. F. J. Phys. Chem. B 2005, 109, 27. (9) Yi, C. W.; Kwak, J. H.; Peden, C. H. F.; Wang, C.; Szanyi, J. J. Phys. Chem. C 2007, 111, 14942. (10) Yi, C. W.; Kwak, J. H.; Szanyi, J. J. Phys. Chem. C 2007, 111, 15299.
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