Design of a Reaction Protocol for Decoupling Sulfur Removal and

Mar 27, 2007 - Lean NOx traps (LNTs),1 also known as NOx adsorber or NOx ...... Poulston, S.; Rajaram, R. R. Regeneration of NOx trap catalysts. Catal...
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Ind. Eng. Chem. Res. 2007, 46, 2735-2740

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Design of a Reaction Protocol for Decoupling Sulfur Removal and Thermal Aging Effects during Desulfation of Pt-BaO/Al2O3 Lean NOx Trap Catalysts Do Heui Kim,*,† Ya-Huei Chin,† George Muntean,† Aleksey Yezerets,‡ Neal Currier,‡ William Epling,‡,| Hai-Ying Chen,§ Howard Hess,§ and Charles H. F. Peden† Institute for Interfacial Catalysis, Pacific Northwest National Laboratory, P. O. Box 999, Richland, Washington 99352, and Cummins Inc., 1900 McKinley AVenue, Columbus, Indiana 47201, and Johnson Matthey, 436 DeVon Park DriVe, Wayne, PennsylVania 19087

A novel reaction protocol was designed to decouple the effects of thermal deactivation from those due to incomplete sulfur removal during desulfation steps of Ba-based lean NOx trap catalysts. The protocol was applied to two samples: a Pt-BaO/Al2O3 model catalyst and an enhanced model sample doped with promoter species. The results obtained from the reaction protocol demonstrate that desulfation temperatures need to be maintained below those that lead to significant Pt sintering in order to prevent permanent deactivation. In addition, the modified reaction protocol allows us to compare the desulfation behavior of samples with varying degrees of sulfation, while other approaches have difficulty differentiating the effects of thermal aging from those of sulfation. We believe that this approach provides a convenient way both to assess the relative sensitivities of various catalysts to desulfation conditions, and to develop desulfation strategies that minimize the separate but often linked deactivation effects of sulfation and high temperatures. 1. Introduction Lean NOx traps (LNTs),1 also known as NOx adsorber or NOx storage-reduction (NSR) catalysts, have been introduced as a potential method to control both lean burn and diesel engine exhaust emissions. LNTs operate in lean-rich cycles where NOx is stored in a “storage” material (commonly BaO) as nitrates during a lean cycle, and then reduced to N2 during a fuel-rich cycle.2 The mechanism of NOx storage and reduction, consisting of several important reaction steps, has been the subject of numerous investigations.2-8 From this previous work, it is understood that, during lean operation, NO is oxidized to NO2 over Pt and stored on supported barium oxide as barium nitrates. In rich operation, the stored nitrate is reduced by the precious metal (typically Pt/Rh) components of the catalyst into N2 using H2, CO, and hydrocarbons as reductants. The LNT technology has been recognized as one of the most promising approaches to meet stringent NOx emission standards for diesel vehicles within the Environmental Protection Agency’s (EPA’s) 2007/2010 mandated limits. However, deactivation of the LNTs due to poisoning by SO2 and/or thermal aging remains a significant technical challenge for successful commercial application of this technology. In fact, these two deactivation routes are often coupled. In a mechanism similar to NOx storage, SO2 is first oxidized to SO3 on the Pt surface and then stored on the supported barium as BaSO4. Formation of BaSO4, which is much more stable than Ba(NO3)2, decreases the available sites for NOx adsorption, leading to the deactivation of the catalyst. Even low concentrations of SO2 (ppm levels) gradually reduce the NOx storage function of the catalyst, with the decrease in NOx storage capacity being roughly proportional to the cumulative SO2 dose to which the catalyst has been exposed.9 It has * To whom correspondence should be addressed. E-mail: [email protected]. † Pacific Northwest National Laboratory. ‡ Cummins Inc. § Johnson Matthey. | Present address: Department of Chemical Engineering, University of Waterloo, 200 University Ave., Waterloo, ON, Canada N2L 3G1.

been well established that, since BaSO4 is thermodynamically more stable than BaO or Ba(NO3)2,10 the reactivation of the catalyst to decompose BaSO4 requires exposure of the LNT catalyst to high temperatures (e.g., 600 °C). High temperature desulfation can result in performance degradation due to losses in Ba dispersion arising from the formation of a BaAl2O4 phase, the loss of alumina support surface area and porosity from a gamma to delta phase transition, and/or the sintering of Pt metal.11,12 To revert a formed BaAl2O4 phase to dispersed Ba species, a phase transformation of BaAl2O4 to BaCO3 upon water contact at room temperature has been reported, although this may not completely restore the original morphology.13,14 However, Pt sintering permanently deactivates the catalyst since this process is irreversible under typical lean NOx trap operation conditions. There are several papers regarding the desulfation of lean NOx trap catalysts.5,15-18 Liu and Anderson5 studied the effects of hydrogen, CO, and hydrocarbons as the reductants, and reported that hydrogen was the most effective for desulfation, with hydrocarbons being the least effective. Also, the presence of CO2 and H2O promotes the desulfation process.17 However, with respect to the reported activity changes, it should be pointed out that the desulfation conditions used in these studies (e.g., up to 800 °C,15 700 or 900 °C,17 and 700 °C18) were in a temperature range where thermal aging of the LNT material occurs. Therefore, the effects of sulfur removal and thermal deactivation on the LNT material were not addressed separately. Physical and chemical property changes in the material due to removal of sulfates and/or due to the required high temperatures of desulfation are unavoidable as described in the previous paragraph. Thus, there is a tradeoff in restoring the LNT storage activity via a desulfation process by removing the sulfur species while, on the other hand, potentially degrading the material during the process by exposing it to high temperatures. In fact, in most studies published to date, it is quite difficult to distinguish which of these factors are responsible for deactivation following desulfation processes. Therefore, we developed an experimental reaction protocol designed to decouple these two effects that we describe in the present publication. After

10.1021/ie061542d CCC: $37.00 © 2007 American Chemical Society Published on Web 03/27/2007

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running several lean-rich cycles in the presence or absence of SO2, the catalyst was treated at higher temperature, followed by a second evaluation of the activity without SO2. Comparison of the NOx storage activities with/without SO2 and before/after the heat treatment allowed us to estimate the independent contributions to activity changes due to the sulfur removal and to thermal deactivation. Moreover, the reaction protocol can be readily modified to investigate the desulfation ability of either lightly or heavily sulfated samples, as explained under Results and Discussion. By comparing the behavior of materials with varying composition, we can study the role of various additives on the desulfation processes, noting especially their relative sensitivities to deactivation conditions. We demonstrate that here by comparing results obtained with a “simple model” Pt/BaO/ Al2O3 and an “enhanced model” material. Such information can aid in designing more durable catalysts with improved thermal stability and/or sulfur removal ability, and also suggest optimum desulfation conditions that minimize deactivation under cyclic operation of the LNT catalyst. 2. Experimental Section Two samples in powder form were supplied by Johnson Matthey (Wayne, PA): a simple model Pt-BaO/Al2O3 sample and an “enhanced model” sample which contains Pt-BaO/Al2O3 with promoter species. The amount of Pt and Ba loaded on these samples are identical between these two samples, with concentrations of 1.5 and 14 wt %, respectively. For proprietary reasons, the detailed formulation of the “enhanced model” sample cannot be disclosed in this publication. Still, a direct comparison of the behavior of these two samples, using the reaction protocol described here, will illustrate how these methods can distinguish desulfation characteristics of differently formulated LNT catalysts. LNT performance was evaluated in a fixed-bed microcatalytic quartz reactor operated under continuous lean-rich cycling. NOx concentrations in the inlet and outlet gases were measured with a chemiluminescence NOx analyzer (Thermo Electron, 41C). NOx uptake was defined as the percentage ratio (%) of the amount of adsorbed NOx to the cumulative amount of inlet NOx during the lean period. Several sequences (typically 12) of 1 min rich and 4 min lean cycles were applied, and NOx uptake was measured during a lean cycle for 4 min after each rich cycle. Reactants consisted of a continuous flow of 200 ppm NO, 10% CO2, 10% H2O, and 0, 8, or 25 ppm SO2 balanced with He, in addition to either a rich (1330 ppm C3H6, 4% CO, and 1.33% H2) or lean (12% O2) gas mixture. The rich or lean gas mixtures were introduced into the reactor system by a four-port valve controlled with an electric actuator (Valco Instruments). All gases were controlled independently by mass flow controllers (Brooks Instruments). We measured NOx storage performance isothermally at 350 °C. Detailed reaction procedures, including the reaction protocols, will be described under Results and Discussion. Some samples were presulfated with 8 ppm SO2 in a total flow rate of 300 cm3/min for varying lengths of time to give a desired degree of sulfation (measured in grams of sulfur per liter of catalystsg/L). X-ray photoelectron spectroscopy (XPS; Kratos Axis 165 spectrometer operated at a base pressure of 1 × 10-9 Torr) was used to investigate the nature of the sulfur species deposited on the catalyst surface during reaction. Al KR X-rays at 300 W (25 kV and 12 mA) were used, and the spectrometer pass energy was 80 eV. The Al 2s signal of the alumina support was used as a BE standard at 119.3 eV to correct for charging on the substrate.

Figure 1. Reaction protocol to decouple effects of thermal deactivation and sulfur removal during desulfation. The protocol contains two sequences of lean/rich cycling and a 40 min high temperature desulfation step that includes a 10 min rich period in addition to a total of 30 min under lean conditions.

3. Results and Discussion 3.1. Reaction Protocol for Decoupling Thermal Aging and Sulfur Removal Effects. Figure 1 shows the reaction protocol we used to decouple thermal aging and sulfur removal effects. First we performed a series of lean-rich cycles isothermally at 350 °C in the absence of SO2 using a total of 24 lean/rich (4 min/1 min) sequences. (Note for all future descriptions of reaction protocols to be described here that, in the absence of SO2, we typically achieve reproducible NOx uptake performance within a few lean/rich cycles.) Then the temperature was increased from 350 to 600 °C at a rate of 5 °C/min under the lean gas mixture including NO. After maintaining at 600 °C for 20 min, the rich gas mixtures were introduced, followed by holding the temperature at 600 °C for an additional 10 min under the lean gas mixture. The total exposure time at 600 °C was 40 min, including the rich period of 10 min mentioned above. After this heat treatment and desulfation cycle, the temperature was decreased to 350 °C under the lean gas mixture, and was maintained for 15 min before introducing 12 additional lean/ rich (4 min/1 min) cycles. After the last rich cycle, a NOx uptake measurement was performed. Thus, a comparison of the NOx uptake performances before and after the desulfation period at 600 °C allows us to estimate the decrease in activity arising from thermal deactivation at this temperature. To then estimate the thermal effects of higher desulfation temperatures, this experiment was repeated with the treatment temperature at 700, 800, or 900 °C, each time using a fresh catalyst. The NOx uptake during the lean period of the last pulse in the second set of lean/rich cycles at 350 °C was used for activity comparison. The same set of experiments was then repeated with a modified gas mixture in which controlled amounts of SO2 (e.g., 8 ppm, 25 ppm) were added during the first set of lean/rich cycles, i.e., prior to the thermal desulfation treatment. After finishing these cycles at 350 °C, the temperature was raised to the desired desulfation temperature (e.g., 600 °C, 700 °C) in the absence of SO2, allowing us to exclude additional sulfur effects on LNT deactivation during this desulfation process. Finally, a second lean/rich set of cycles was again performed at 350 °C. An activity measurement during these last lean/rich cycles will include possible negative thermal aging effects of the desulfation process convoluted with some recovery of activity due to the removal of sulfur. However, the earlier set of experiments performed in the complete absence of SO2 can be used to correct these latter results for the thermal deactivation effects, allowing us to then make an estimate of the changes in activity due simply to addition and removal of sulfur. Figures 2 and 3, to be discussed next, will illustrate this for the simple model Pt/BaO/ Al2O3 and “enhanced model” catalysts used here.

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Figure 2. Change in NOx uptake measured during the first 4 min of the last lean cycle of the second sequence of lean/rich cycles in the absence of SO2. Data are plotted as a function of the temperature used during the desulfation period for the simple model Pt/BaO/Al2O3 and enhanced model samples.

Figure 2 shows the NOx uptake, measured during the last 4 min lean period obtained at 350 °C, after the high temperature exposures in the absence of SO2 over the simple and enhanced model samples. Since there is no SO2 in the reactants, the activity decrease is directly related to thermal degradation of the materials. The activities of both the simple and enhanced model samples remain unchanged for a 600 °C desulfation, but start to decline significantly for treatments at 700 °C or above. Thus, the thermal treatment at 600 °C, for the specific desulfation duration time and set of gas mixtures used here, does not cause significant deactivation. Clearly, however, NOx uptake activities are markedly affected by higher temperature desulfation treatments. Using in situ XRD, we have previously shown19 that metallic Pt particles in a simple model Pt/BaO/Al2O3 LNT grow exponentially with increasing treatment temperatures above 700 °C under a lean gas mixture similar to that used here. In fact, that previous work19 presents a direct correlation between the growing Pt particle size and degree of deactivation of the same two simple and enhanced model catalysts studied here. Thus, the activity decrease above a treatment temperature of 700 °C in Figure 2 can be attributed to Pt sintering, a process known to be primarily responsible for thermal degradation of LNT catalysts.2 When the samples are sulfated during the first set of lean/ rich cycles with gas mixtures containing 25 ppm SO2 for 3 h (to give the cumulative degree of sulfation of ∼3.1 g/L), there is a significant decrease in NOx uptake. This is demonstrated for both catalysts studied here by comparing the data points in parts a and b of Figure 3, with the solid squares (repeated from Figure 2) and circles obtained in the absence and presence of SO2, respectively. As in Figure 2, while all NOx uptake data are obtained at 350 °C, the activity results are plotted as a function of the temperature used during the desulfation phase (the “Desulfation” portion of the experimental protocol shown in Figure 1). For the case of the simple model sample, Figure 3a, NOx uptake increases with increasing desulfation temperatures up to 700 °C, although for 600 °C or lower desulfations it is less than the uptake measured in the absence of SO2, implying that sulfur removal is incomplete. The NOx uptake shows the maximum at 700 °C for the simple model LNT in

Figure 3. Effect of SO2 on the NOx uptake after high temperature desulfation treatment. SO2 was included in the gas mixture during the first sequence of lean/rich cycles (prior to desulfation), resulting in exposures equivalent to ∼3.1 g/L sulfur. NOx uptake performance was measured during the last 4 min lean cycle of the second sequence of lean/rich cycles (after thermal desulfation treatment) for the (a) simple model Pt/BaO/Al2O3 and (b) enhanced model samples. The square and circle symbols indicate the results of comparative experiments performed without SO2 and with SO2 present prior to thermal treatment, respectively.

the presence of SO2, with the uptake identical to those of thermally aged only samples above this temperature. Thus it appears that thermal deactivation alone is responsible for the loss of activity of the simple model LNT at or above 700 °C. In contrast, for the case of the enhanced model sample, the NOx uptake reached a maximum at 600 °C, and the uptake activity is higher than that for the model catalyst measured at this temperature. Although the enhanced model sample gave rise to a lower NOx uptake under the same sulfation condition without any desulfation treatments (compare the solid circle data points at 350 °C in Figure 3), it apparently has a superior ability to release the deactivating sulfate species at lower temperatures (i.e., 600 °C), where thermal deactivation is negligible. In other words, the enhanced model sample shows more complete desulfation than the simple model one at 600 °C, without sintering the Pt particles. These results also clearly demonstrate that the reaction protocol described here allows for the decoupling of thermal aging from sulfur adsorption and removal effects. With such results, optimum desulfation conditions with

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Figure 4. Reaction protocol to investigate the desulfation behavior of heavily sulfated samples. LNT catalysts were sulfated isothermally during lean/rich cycling up to a nominal S loading of 10 g/L. NOx uptake was regularly monitored during the lean/rich cycles (reaction protocol shown in the top of the figure) and reported with the degree of sulfation in Figure 5. A comparison of the recovery of performance using the protocol at the bottom of the figure is shown in Figure 6. Figure 6. Change in NOx uptake performance measured at 350 °C before and after two desulfation processes at 600 °C for simple model Pt/BaO/ Al2O3 (squares) and enhanced model (circles) samples.

Figure 5. Change of NOx uptake performance at 350 °C during the 4 min lean period of 4 min lean/1 min rich cycles with respect to the nominal degree of sulfation (g/L) for simple model Pt/BaO/Al2O3 (square) and enhanced model (circle) samples.

minimized detrimental effects of thermal aging can be determined for LNTs of varying composition. 3.2. Modification of the Reaction Protocol To Investigate Desulfation Behavior of Heavily Sulfated Samples. In the previous example, it was demonstrated that the enhanced sample exhibited an improved ability to desulfate at lower temperatures in comparison to the model Pt/BaO/Al2O3 LNT for the same moderate level of sulfation when identical desulfation conditions are used. In this section, we describe studies aimed at determining possible variability in the NOx uptake following desulfation of heavily (up to 10 g/L) sulfated samples. All desulfations were performed at 600 °C in this set of experiments since it is clearly shown from the above-described results (e.g., Figure 2) that thermal deactivation at this temperature is negligible. The reaction protocol was modified as shown in Figure 4. In this case, the samples were sulfated with 8 ppm SO2 at 350 °C in a lean feed for 30 h. At regular periodic intervals during this 30 h sulfation process, performance measurements were made for a total time of 1 h each, again using 4 min lean/1 min rich cycles. As shown in Figure 5, the NOx uptake steadily decreases as sulfation proceeds for both samples, in good agreement with previously reported results.9 Although the enhanced model sample shows a more rapid initial decline in the NOx uptake, it

exhibits a higher activity than the model sample when total sulfation exposures exceeded ∼6 g/L. We can suggest two possible reasons for this behavior. First, this is possibly a reflection of a lower affinity for sulfur for the enhanced sample at high sulfur exposures. It is also possible that the enhanced model has a greater ability to remove sulfur species at higher sulfur loading, compared with the simple model sample. In fact, as we will show below, in general the enhanced model does remove sulfur more readily during desulfations than the simple model catalyst. Another interesting aspect of the data shown in Figure 5 is that NOx uptake actually increases during the performance measurements for high degrees (> ∼6 g/L) of sulfur exposure. We believe this result is a reflection of the fact that weaker adsorbed sulfur species are deposited onto these catalysts at high sulfur exposures. Apparently, such weakly held sulfur species can actually be at least partially removed during the rich phases of the lean/rich cycling performance measurements even at temperatures of 350 °C. To then compare desulfation ability, these heavily sulfated samples were evaluated for NOx uptake performance at 350 °C in the absence of SO2, followed by two sequences of thermal treatment (to mimic desulfation conditions) at 600 °C and activity measurements at 350 °C as described at the bottom of Figure 4. Figure 6 shows the change of NOx uptake with timeon-stream during repetitive lean/rich (4 min/1 min) cycling before and after two desulfation processes at 600 °C. The enhanced model sample demonstrates a significantly higher increase in NOx uptake following desulfation than the Pt/BaO/ Al2O3 sample, even without any treatment at high temperature (“Lean/Rich sequence 1” period in Figure 6). In particular, NOx uptake of the enhanced model sample increased slightly during the first 60 min of lean/rich cycling at 350 °C once SO2 was removed from the gas mixture. However, we are not sure whether such an increase in NOx uptake is arising from partial desulfation or sulfur redistribution under these conditions. After the first desulfation treatment at 600 °C, both samples show a marked increase in NOx uptake performance, with the enhanced model sample recovering most (88%) of its activity compared to a much more moderate (55%) recovery for the Pt/BaO/Al2O3 sample. Successive desulfation does not further improve the activity of either sample, indicating that the second desulfation

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Figure 7. Sulfur 2p X-ray photoelectron spectra of simple model Pt/BaO/ Al2O3 and enhanced model samples after completing the reaction protocol shown in Figure 4. The numbers in parentheses indicate the integrated area of the S 2p region.

Figure 8. Reaction protocol designed to investigate the sulfur removal behavior of lightly sulfated samples undergoing repetitive desulfations.

at 600 °C does not remove additional residual sulfur species. We used XPS to analyze the sulfur species of the postreaction samples, with the results shown in Figure 7. There is only one peak at 169 eV, assigned to the S 2p from sulfates (SO42-), implying that the only form of the sulfur on both samples are sulfates. The simple model Pt/BaO/Al2O3 LNT contains a more than 3 times larger amount of sulfate species on the surface relative to the enhanced model sample after the two desulfations described above. Thus, the extent of sulfur removal can be correlated directly to the recovery of NOx uptake activity on these samples. 3.3. Modification of the Reaction Protocol To Investigate Repetitive Desulfations of Lightly Sulfated Samples. In the previous example, it was found that the enhanced model sample showed superior recovery in NOx uptake performance after desulfation for heavily sulfated samples. Because practical LNT catalysts will experience frequent desulfations, we modified the reaction protocol to study the effects of repeated desulfations at 600 °C during lean/rich cycling in the presence of SO2. In this way, we probed how readily sulfur species are removed during continuous sulfation and desulfation. As shown in Figure 8, NOx uptake was measured in the presence of SO2 (8 ppm) for 2 h, followed by desulfation at 600 °C in the absence of SO2. These two steps were repeated six times such that the total amount of sulfur exposed to the materials was ∼4 g/L. For comparison purposes, similar experiments were performed without SO2 to determine the effects of thermal deactivation during these successive treatments at 600 °C. In this latter case (results not shown), NOx uptake performance remained unchanged for both samples; therefore, we can exclude any role for thermal deactivation on the results obtained during the

Figure 9. Change of NOx uptake performance with time during repeated sulfation and desulfation for simple model Pt/BaO/Al2O3 and enhanced model samples.

successive sulfation and desulfation (at 600 °C) experiments to be described next. Figure 9 shows the change of NOx uptake performance as a function of time using the reaction protocol shown in Figure 8. The break between groups of data points indicates the desulfations at 600 °C that were carried out in the absence of SO2. For the case of the simple model Pt/BaO/Al2O3 sample, continuous gradual deactivation is observed, with a slight increase in performance after each desulfation step. As already indicated by the results shown in Figure 3a, the desulfation at 600 °C was not sufficient to completely restore performance for this sample, presumably because sulfur was not fully removed under these conditions. On the other hand, the enhanced model sample shows at most a slight decrease in NOx uptake performance during this set of experiments. These results again suggest that the enhanced model sample has a significantly better ability to remove sulfur than the Pt/BaO/Al2O3 sample, and that this improved behavior exists for both low and high degrees of sulfation. In agreement with this, XPS results (not shown), obtained after this set of experiments, show that concentration of residual sulfur species (in the form of sulfate) for the Pt/ BaO/Al2O3 LNT was approximately twice that of the enhanced sample. 4. Conclusions A reaction protocol was described that enables the decoupling of thermal deactivation and sulfur removal during high temperature desulfation processes used for lean NOx trap catalysts. The results demonstrate again that desulfation temperatures need to be minimized to prevent thermal deactivation while at the same time efficiently removing the deactivating adsorbed sulfur species. Slight variations on the reaction protocol enable a comparison of NOx uptake performance for differently formulated LNT catalysts following single or multiple desulfation processes, and to compare their behaviors for variable sulfation levels. In this way, optimized desulfation strategies, which are also tailored to the specific LNT catalyst, can be developed. Acknowledgment Financial support was provided by the U.S. Department of Energy (DOE), Office of Freedom Car and Vehicle Technologies. The work was performed in the Environmental Molecular

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Sciences Laboratory (EMSL) at Pacific Northwest National Laboratory (PNNL). The EMSL is a national scientific user facility and supported by the U.S. DOE Office of Biological and Environmental Research. PNNL is a multiprogram national laboratory operated for the U.S. Department of Energy by Battelle Memorial Institute under Contract No. DE-AC0676RLO 1830. Literature Cited (1) Takahashi, N.; Shinjoh, H.; Iijima, T.; Suzuki, T.; Yamazaki, K.; Yokota, K.; Suzuki, H.; Miyoshi, N.; Matsumoto, S.; Tanizawa, T.; Tanaka, T.; Tateishi, S.; Kasahara, K. The new concept 3-way catalyst for automotive lean-burn engine: NOx storage and reduction catalyst. Catal. Today 1996, 27 (1-2), 63. (2) Epling, W. S.; Campbell, L. E.; Yezerets, A.; Currier, N. W.; Parks, J. E. Overview of the fundamental reactions and degradation mechanisms of NOx storage/reduction catalysts. Catal. ReV.sSci. Eng. 2004, 46 (2), 163. (3) Fridell, E.; Persson, H.; Westerberg, B.; Olsson, L.; Skoglundh, M. The mechanism for NOx storage. Catal. Lett. 2000, 66 (1-2), 71. (4) Prinetto, F.; Ghiotti, G.; Nova, I.; Lietti, L.; Tronconi, E.; Forzatti, P., FT-IR and TPD investigation of the NOx storage properties of BaO/ Al2O3 and Pt-BaO/Al2O3 catalysts. J. Phys. Chem. B 2001, 105 (51), 12732. (5) Liu, Z. Q.; Anderson, J. A. Influence of reductant on the regeneration of SO2-poisoned Pt/Ba/Al2O3 NOx storage and reduction catalyst. J. Catal. 2004, 228 (1), 243. (6) Szanyi, J.; Kwak, J. H.; Hanson, J.; Wang, C. M.; Szailer, T.; Peden, C. H. F. Changing morphology of BaO/Al2O3 during NO2 uptake and release. J. Phys. Chem. B 2005, 109 (15), 7339. (7) Szanyi, J.; Kwak, J. H.; Kim, D. H.; Burton, S. D.; Peden, C. H. F. NO2 adsorption on BaO/Al2O3: The nature of nitrate species. J. Phys. Chem. B 2005, 109 (1), 27. (8) Su, Y.; Amiridis, M. D. In situ FTIR studies of the mechanisim of NOx storage and reduction on Pt/Ba/Al2O3 catalysts. Catal. Today 2004, 96 (1-2), 31. (9) Amberntsson, A.; Skoglundh, M.; Ljungstrom, S.; Fridell, E. Sulfur deactivation of NOx, storage catalysts: influence of exposure conditions and noble metal. J. Catal. 2003, 217 (2), 253.

(10) Lietti, L.; Forzatti, P.; Nova, I.; Tronconi, E. NOx storage reduction over Pt-Ba/gamma-Al2O3 catalyst. J. Catal. 2001, 204 (1), 175. (11) Jang, B. H.; Yeon, T. H.; Han, H. S.; Park, Y. K.; Yie, J. E. Deterioration mode of barium-containing NOx storage catalyst. Catal. Lett. 2001, 77 (1-3), 21. (12) Uy, D.; O’Neill, A. E.; Li, J.; Watkins, W. L. H. UV and visible Raman study of thermal deactivation in a NOx storage catalyst. Catal. Lett. 2004, 95 (3-4), 191. (13) Graham, G. W.; Jen, H. W.; Theis, J. R.; McCabe, R. W. Leaching of Ba2+ in NOx traps. Catal. Lett. 2004, 93 (1-2), 3. (14) Kim, D. H.; Chin, Y. H.; Kwak, J. H.; Szanyi, J.; Peden, C. H. F. Changes in Ba phases in BaO/Al2O3 upon thermal aging and H2O treatment. Catal. Lett. 2005, 105 (3-4), 259. (15) Elbouazzaoui, S.; Corbos, E. C.; Courtois, X.; Marecot, P.; Duprez, D. A study of the deactivation by sulfur and regeneration of a model NSR Pt/Ba/Al2O3 catalyst. Appl. Catal., B: EnViron. 2005, 61 (3-4), 236. (16) Kim, D. H.; Szanyi, J.; Kwak, J. H.; Szailer, T.; Hanson, J.; Wang, C. M.; Peden, C. H. F. Effect of barium loading on the desulfation of PtBaO/Al2O3 studied by H-2 TPRX, TEM, sulfur K-edge XANES, and in situ TR-XRD. J. Phys. Chem. B 2006, 110 (21), 10441. (17) Poulston, S.; Rajaram, R. R. Regeneration of NOx trap catalysts. Catal. Today 2003, 81 (4), 603. (18) Rohr, F.; Peter, S. D.; Lox, E.; Kogel, M.; Sassi, A.; Juste, L.; Rigaudeau, C.; Belot, G.; Gelin, P.; Primet, M. On the mechanism of sulphur poisoning and regeneration of a commercial gasoline NOx-storage catalyst. Appl. Catal., B: EnViron. 2005, 56 (3), 201. (19) Kim, D. H.; Chin, Y.-H.; Muntean, G.; Yezerets, A.; Currier, N. W.; Epling, W. S.; Chen, H.-Y.; Hess, H.; Peden, C. H. F. Relationship of Pt particle size to the NOx storage performance of thermally aged Pt/BaO/ Al2O3 lean NOx trap catalysts. Ind. Eng. Chem. Res. 2006, 45, 8815.

ReceiVed for reView November 30, 2006 ReVised manuscript receiVed February 26, 2007 Accepted February 28, 2007 IE061542D