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J. Phys. Chem. C 2007, 111, 4678-4687
Water-Induced Morphology Changes in BaO/γ-Al2O3 NOx Storage Materials: an FTIR, TPD, and Time-Resolved Synchrotron XRD Study Ja´ nos Szanyi,*,† Ja Hun Kwak,† Do Heui Kim,† Xianqin Wang,† Ricardo Chimentao,‡ Jonathan Hanson,§ William S. Epling,| and Charles H. F. Peden† Institute for Interfacial Catalysis, Pacific Northwest National Laboratory, Richland, Washington 99352, Department d’Enginyeria Quimica, UniVersitat RoVira i Virgili, Tarragona, Spain, Department of Chemistry, BrookhaVen National Laboratory, Upton, New York 11973, and UniVersity of Waterloo, Waterloo, ON, Canada ReceiVed: NoVember 29, 2006; In Final Form: January 18, 2007
The effect of water on the morphology of BaO/Al2O3-based NOx storage materials was investigated using Fourier transform infrared spectroscopy, temperature programmed desorption, and time-resolved synchrotron X-ray diffraction techniques. The results of this multispectroscopy study reveal that in the presence of water surface Ba-nitrates convert to bulk nitrates and water facilitates the formation of large Ba(NO3)2 particles. The conversion of surface to bulk Ba-nitrates is completely reversible (i.e., after the removal of water from the storage material a significant fraction of the bulk nitrates reconverts to surface nitrates). NO2 exposure of a H2O-containing (wet) BaO/Al2O3 sample results in the formation of nitrites and bulk nitrates exclusively (i.e., no surface nitrates form). After further exposure to NO2, the nitrites completely convert to bulk nitrates. The amount of NOx taken up by the storage material, however, is essentially unaffected by the presence of water regardless of whether the water was dosed prior to or after NO2 exposure. On the basis of the results of this study, we are now able to explain most of the observations reported in the literature on the effect of water on NOx uptake on similar storage materials.
Introduction Today, the drive to increase fuel efficiency in internal combustion engines is coupled with the demand to decrease hazardous exhaust gas emission. Lean engine operation (higher than stoichiometric amounts of oxygen) can lead to a significant increase in fuel efficiency; however, it prevents the use of traditional three-way catalysts (TWCs) for emission control. Under lean conditions, TWCs burn most of the reducing compounds present in the exhaust gas mixture, thus starving the system of an efficient reducing agent for reaction with NOx. Therefore, eliminating NOx from these highly oxidizing environments presents a great challenge. Several new technologies have been considered for lean NOx reduction in the last two decades, although very limited practical application has been achieved. One of the more promising approaches is the NOx storage/reduction (NSR) technology (also called lean NOx traps, LNTs). Originally developed by Toyota in the early 1990s,1,2 this technology requires cyclic engine operation; in the lean cycle all of the NOx is converted to NO2 and is stored as nitrates (mostly on alkali or alkaline earth oxides), while in a subsequent short-reducing cycle this stored NOx is released and converted to N2 similarly to that in the TWC technology. Despite extensive research that has contributed to an understanding of NOx storage and reduction cycles,3 several issues remain open and debated about the operation of these systems. The effects of H2O and CO2 on the uptake of NOx over BaO/ Al2O3-based storage systems have been recognized and * Corresponding author. E-mail address:
[email protected]. † Pacific Northwest National Laboratory. ‡ Universitat Rovira i Virgili. § Brookhaven National Laboratory. | University of Waterloo.
reported.4-9 However, the extent of the effects of these compounds on the NOx uptake and the mechanisms by which these compounds influence NOx uptake are still widely debated. In particular, the effect of water seems poorly understood. Toops and co-workers6,7 suggest that water primarily influences the alumina support material by preventing NOx uptake on the alumina support itself and in this way reducing the total NOx uptake capacity of the entire system. A completely different explanation has been given by Cant and Patterson,5 who examined the effect of H2O on the temperature programmed desorption (TPD) profiles of NO2-saturated BaO/Al2O3 samples. They observed that in the presence of H2O in the purge gas (1% H2O in He) the maximum NOx desorption rate of the lowtemperature desorption feature in the TPD spectrum shifted to a higher temperature (by ∼20 K), while the total amount of NOx desorbed in the lower-temperature desorption feature decreased at the expense of the higher-temperature one. Among a number of possible explanations that they provided, Cant and Patterson5 preferred the one that suggested the formation of a Ba(OH)2 crust on top of the Ba(NO3)2 particles that slowed down the decomposition of Ba(NO3)2. Lietti and co-workers8 found an interesting dependence of NOx storage in the presence of water in the gas stream during the uptake process. They found that the presence of H2O promoted the uptake of NOx at low temperatures (2 monolayers equivalent. This BaO-free alumina surface can also adsorb NO2 as shown by the presence of small IR features (seen only as weak shoulders) at around 1270 and 1650 cm-1 (these features are more evident for the 8 wt % sample in which a higher percentage of the alumina surface is free of BaO).11 In our study on the adsorption of NO2 on γ-Al2O3,18 we have discussed extensively the assignment of this
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Figure 2. IR spectra collected during the stepwise H2O adsorption on NO2(300 K)/BaO/Al2O3 samples at 300 K. (a): 20 wt % BaO/Al2O3. (b): 8 wt % BaO/Al2O3. Each H2O was 1 Torr.
Figure 1. Stepwise NO2 adsorption on a 20 wt % BaO/Al2O3 sample at 300 K. (a): IR spectra. (b): Intensity of the mass spectrometer signals of masses 18, 30, and 46 during the NO2-dosing sequence. (Each NO2 dose was 1 Torr; after the introduction of each NO2 dose (first-eighth), the system was allowed to equilibrate for 2 min, and then the cell was evacuated prior to the subsequent NO2 dose. The equilibration times were 5 and 8 min for the ninth and tenth NO2 dose, respectively.)
IR band to alumina-bound N2O3 with a possible minority contribution of adsorbed NO+. The inclusion of NO+ was based on the studies of Venkov et al.19 who observed the development of a highly asymmetric band in the 1920-1980 cm-1 frequency range when they studied the adsorption of NO on alumina in the presence of O2. The IR spectra recorded during all the NO2 adsorption experiments display a small feature centered at around 1760 cm-1, which has been assigned to the ν1 + ν4 combination vibration of bulk Ba(NO3)2.20 The intensity of this IR feature closely follows those of the other bulk nitrate-related IR bands. 2. H2O Adsorption on NO2-Saturated BaO/Al2O3 Samples. These experiments were conducted on both 8 and 20 wt % BaO/ Al2O3 samples at either full or partial NO2 saturations. Both the NO2 and H2O adsorption experiments were carried out at 300 K sample temperature. Prior to the introduction of H2O, the samples were exposed to 10 Torr NO2 at room temperature and then evacuated for an extended period of time (until the base pressure in the cell dropped below 1 × 10-6 Torr). Two
series of FTIR spectra obtained during the stepwise H2O exposure of the NO2-saturated 20 (panel a) and 8 wt % BaO/ Al2O3 (panel b) samples are shown in Figure 2. The addition of water onto the NO2-saturated 8 wt % BaO/Al2O3 sample resulted in dramatic changes in the IR spectra. Specifically, the intensities of the IR bands characteristic of surface nitrates (1294 and 1582 cm-1) gradually decreased with increasing amounts of H2O introduced, while those of the bulk nitrates (1325 and 1434 cm-1) increased. After the seventh H2O dose, the intensities of peaks due to surface Ba-nitrate species almost completely diminished, while those of the bulk Ba-nitrates reached their maxima. In the spectral region where the surface nitrate features are seen (1500-1600 cm-1), new absorption bands that can be attributed to the alumina substrate-related surface nitrates become visible. Some of these bands (as we have mentioned above) can also be seen after NO2 adsorption as weak shoulders; however, they become much sharper after H2O exposure of the NO2-saturated sample. The effect of water on the surface nitrate species of alumina has been discussed in great detail previously by us21 and others.22,23 The main conclusion of our study of H2O adsorption on NO2-saturated Al2O3 was that bridge-bound surface nitrates reversibly converted to mono- and bidentate nitrates as a result of H2O exposure, but that the amount of NOx strongly held by the alumina support did not change. Similar trends were seen for the 20 wt % BaO/Al2O3 sample; however, the changes in the IR spectra were not as dramatic as we have shown for the 8 wt % one. This was due to the presence of a much larger amount of bulk Ba-nitrate on this sample prior
FTIR, TPD, and TRXRD Studies in BaO/γ-Al2O3 NOx
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Figure 3. Intensities of the mass spectrometer signals for masses 18, 30, 46 as a function of time, collected during the stepwise H2O adsorption on NO2(300K)/20 wt % BaO/Al2O3 at 300 K.
to water exposure in comparison to the 8 wt % one. Therefore, the conversion of surface nitrates into bulk nitrates did not increase the intensities of the bulk Ba-nitrates as significantly as it did in the case of the 8 wt % sample. Concomitant with the sharp decrease in the intensities of surface Ba-nitrate-related IR features, sharp, alumina-related surface nitrate features appeared, although with low intensities. This observation suggests that as the surface Ba-nitrates convert to bulk nitrates, alumina sites are freed up and become available for surface nitrate formation. Figure 3 shows the results of the continuous mass spectral analysis of the gas-phase composition in the IR cell during the H2O exposure of the NO2-saturated 20 wt % BaO/Al2O3 sample. Throughout these experiments, mass fragments of 18, 30, and 46, which correspond to H2O, NO, and NO2, respectively, were followed simultaneously as a function of time. The mass 30 fragment can originate from both NO and the fragmentation of NO2 in the ionization chamber of the mass spectrometer. By calibrating the mass spectrometer, we can estimate the fraction of the mass 30 signal that originates from NO and from NO2. Figure 3 shows that upon introduction of H2O, a significant amount of NOx (mostly as NO as determined by the mass spectrometer calibration) is released from the storage material. The amount of NOx released into the gas phase gradually decreases with the number of H2O doses. (To avoid the interference of the released NOx with the subsequent H2O dose, the IR cell was evacuated for 2 min before each new water dose.) After the first two H2O doses, practically all the water was consumed within the 2 min of equilibration time; after subsequent H2O doses, measurable amounts of water were present in the IR cell after 2 min of time-on-stream. Very similar observations were made when we studied the adsorption of H2O on NO2-saturated alumina surfaces.18 There, we concluded that most of the NOx released from the surface upon H2O exposure originated from the desorption of N2O3. For the aluminasupported BaO materials studied here, we believe that the same phenomenon is responsible for the observed release of NOx into the gas phase. The incoming water strongly interacts with the storage material and replaces weakly adsorbed N2O3 molecules. Moreover, the NO/NO2 ratio observed in the mass spectrometer suggests that water actually can react with the adsorbed N2O3, forming HNOx and releasing primarily NO. In the series of IR
Figure 4. Stepwise H2O adsorption on a NO2(300 K)/(anneal at 573 K)/20 wt % BaO/Al2O3 sample at 300 K. (a): Mass spectrometer signal intensities of masses 18, 30, and 46. (b): Series of IR spectra (each H2O dose was 1 Torr).
spectra of both panels a and b of Figure 2, we can clearly see the decrease in the intensity of the broad IR band in the 19101970 cm-1 range that represents adsorbed N2O3. The other characteristic vibrations of adsorbed N2O3 cannot be distinguished due to their strong overlap with the nitrate features. One question that these observations bring up is what the possible role for the HNOx species, formed upon the interaction of N2O3 and H2O, is for NOx storage? To this end, we looked into the adsorption of H2O on NO2-saturated BaO/Al2O3 samples that were heated to 573 K in vacuum prior to H2O exposure. As noted above, NO and NO2 are released together in equal amounts during this thermal treatment, and the IR spectrum obtained after annealing the NO2-saturated sample to 573 K shows the complete absence of the IR feature at around 1940 cm-1 assigned to weakly adsorbed N2O3. Thus, this procedure ensured that no weakly held N2O3 remained on the surface of the storage material; therefore, it eliminated the possibility of the formation of HNOx. The full series of IR spectra and the corresponding traces of 18, 30, and 46 amu mass fragments collected during the course of the water exposure experiments on the thermally treated 20 wt % sample are displayed in panels b and a of Figure 4, respectively. Qualitatively identical results were obtained for the 8 wt % BaO/Al2O3 sample as well and are not shown here. With the exception of changes to the N2O3
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Figure 6. Stepwise H2O adsorption on NO2(2 Torr)/20 wt % BaO/ Al2O3 sample at 300 K. (Each H2O dose was 1 Torr.) (At 2 Torr NO2 exposure only partial saturation of the 20 wt % BaO/Al2O3 sample was achieved.)
Figure 5. A series of IR spectra collected during TPD over a H2O(300 K)/NO2(300 K)/20 wt % BaO/Al2O3 sample. (The spectrum with the thick line was taken at 640 K. This IR spectrum was identical to that obtained from the same sample in the absence of H2O. IR spectra were recorded at every 30 K temperature increase during the TPD run.)
species not present in this case, upon exposure of H2O to the 573 K-annealed NO2-saturated 20 wt % BaO/Al2O3 sample essentially identical changes are seen as for the nonannealed sample; the intensities of the IR features of surface nitrates gradually decrease with the increasing number of H2O doses, and after the last H2O dose they completely disappear. In the mass spectra, no evolution of NO or NO2 is observed. These results suggest that the HNOx species formed during the interaction of H2O and N2O3 are not responsible for the observed conversion of surface-to-bulk nitrates, and there is no NOx (NO or NO2) release during this conversion process. The next question we need to answer is whether the surfaceto-bulk nitrate conversion, caused by the exposure of the NO2saturated BaO/Al2O3 samples to H2O, is reversible? In other words, what happens to the bulk nitrates after removal of H2O from the surface? To answer this question we heated the H2Oexposed, NO2-saturated samples in a temperature-programmed fashion and collected IR spectra at every 30 K. Two spectral regions of a series of IR spectra obtained during the TPD experiments on the H2O-exposed, NO2-saturated 20 wt % BaO/ Al2O3 sample are shown in Figure 5. Similar results were obtained for the 8 wt % sample as well although they are not shown here. The two panels show that as water desorbs from the sample (evidenced by the large intensity drop in the O-H vibrational region), surface nitrates reform at the expense of the bulk nitrates. At the temperature of approximately 640 K
(indicated in Figure 5 with bold red lines), the IR spectra in the nitrate vibrational region collected from this sample are identical to those obtained at the same temperature from samples that were not exposed to water after NO2 saturation. (TPD data discussed later will show that most of the adsorbed water desorbs at sample temperature
