NOx Trapping Research Articles

Kinetics and H2O Influence on NOx Trapping and Selective Catalytic Reduction over Ce/Pd Doping Catalyst.

In this paper, the removal effects and activation energy of Ce and Pd doping on pollutants (CO, C3H6, and NO) were comparatively analyzed by using characterization methods and constructed kinetic equations. Furthermore, the problems of the water influence mechanism on the NSR process were also discussed. The results show the following: (1) Pd doping effectively improves the removal of CO (80%) and C3H6 (71%) in the low-temperature section of the catalyst (150-250 °C) compared to Ce doping, while Ce doping exhibits excellent low-temperature conversion of NO. (2) The reaction activation energy of the LaKMnPdO3 catalyst was 9784 kJ/mol, which was significantly lower than that of the LaKMnCeO3 catalyst. (3) The presence of H2O has an important enhancement effect in the storage performance of the LaKMnPdO3 catalyst for NOx but decreases the catalytic reduction of NO. It provides a solution for the effective treatment of the increasing problems of particulate matter and ozone pollution.

Mechanistic insights and application potentials for CeO2-Al2O3 passive NOx adsorber (PNA) materials

A series of CeO2/Al2O3 with Ce loading from 0.5 to 5.0 wt% were studied as PNA materials for low temperature NOx abatement. Via XRD, TEM imaging and UV–vis spectroscopy studies, it was demonstrated that CeO2 is present as nanoparticles on the Al2O3 support, and the particle size increases with increasing Ce loading. PNA performance of these materials was investigated by NOx trapping followed by TPD. The PNA chemistry was further probed with in situ NO-DRIFT spectroscopy. PNA chemistry involves a rapid direct NO trapping route that leads to surface nitrite formation, and a slower NO2 trapping route that leads to surface nitrate species. Surface nitrite decomposition releases NO at 200–300 °C, suitable for PNA application. Based on NO yields during TPD, the atomic efficiency of surface Ce was calculated, which demonstrates that dispersing CeO2 on high surface area supports is a viable strategy for improving PNA efficiency of CeO2-based materials.

Deciphering H2-induced low-temperature NOx adsorption on Ag/Al2O3: Expanding the operating temperature range of ethanol-SCR system for effective NOx abatement

Despite the advantages of the catalytic reduction of NOx by hydrocarbons, typical Ag/Al2O3 suffers from limited low-temperature activity in response to cold-start periods. In this study, the promotional effect of H2 on NOx adsorption on Ag/Al2O3 was investigated to address the low-temperature NOx emissions. H2 significantly enhanced NOx accumulation on Ag/Al2O3 below 200 °C, with desorption occurring at high temperatures. The H2-temperature-programmed reduction (H2-TPR) results showed the formation of silver oxide on metallic Ag species by H2+O2, serving as the primary site for H2-induced NOx adsorption, while the single-atom Ag species (Ag+) remained nearly unchanged. Ag/Al2O3 with low Ag loading (1–2 wt%) exhibited lower NOx adsorption capacities compared to the case with high Ag loading (4 wt%) owing to the low metallic Ag content. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies identified diverse surface-adsorbed species related to H2-induced NOx adsorption. Nitrite, bridging nitrate, and bidentate nitrate contributed to low-temperature NOx desorption, whereas monodentate nitrate contributed to high-temperature desorption. Finally, we demonstrated exceptionally-high NOx reduction activity by synergistically combining the NOx trapping ability of Ag/Al2O3 with its deNOx capability through ethanol with H2, covering a wide temperature range of 100–350 °C under transient conditions at a high space velocity of 100,000 h−1.

Metal-support interaction induced atomic dispersion and redispersion of Pd on CeO2 for passive NOx adsorption

Atomically dispersed metal catalysts have attracted extensive attention in the fields of heterogeneous catalysis due to the enhanced intrinsic activity and selectivity in many energy and environment related reactions, while they suffer from sintering under harsh reaction conditions. Herein, atomically dispersed Pd/CeO2 was synthesized and investigated as a passive NOx adsorber (PNA) material. The strong interaction between CeO2 and Pd creates Pd-O-Ce linkages with 100% utilization of Pd for NOx trapping, being one of the most active oxide-based PNAs up to date. Although a sulfation treatment can substantially reduce NOx storage capacity (NSC) on the Pd/CeO2, a two-step desulfation process, including a cold plasma treatment and an oxidative calcination, can recover near 90% of its original NSC. Desulfation caused Pd aggregating into nanoparticles (NPs), while reconstruction of Pd-O-Ce moieties was achieved by prolonged oxidation at 500 °C, driving by the strong metal-support interaction between Pd and CeO2. The Pd/CeO2 system exemplifies the advantages of the metal and oxide support interaction for the atomic dispersion of Pd and redispersion of sintered Pd NPs. Monitoring this dynamic interaction enables the atomically dispersed metal structure to be modulated in controlled reaction environments, being such an example, the enhanced performance of Pd/CeO2 in PNA applications was shown in the present manuscript.

A novel ceria hollow nanosphere catalyst for low temperature NOx storage

In this work, a highly active CeO2 catalyst with hollow nanosphere morphology for low temperature NOx storage was prepared by a surfactant-assisted solvothermal reaction. The physicochemical properties of ceria samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N2 adsorption–desorption, H2-temperature programmed reduction (H2-TPR), X-ray photoelectron spectroscopy (XPS) and in situ diffused reflectance infrared Fourier transform spectroscopy (DRIFTS). The as-prepared CeO2 nanosphere possesses excellent NO oxidation capacity, smaller mesopores, better reducibility and more surface Ce3+ content. Compared with CeO2 with nanorod and nanoparticle morphologies, CeO2 nanosphere shows better intrinsic low temperature NOx trapping performance, with a wide operating temperature window (150–300 °C), high NOx adsorption capacity (NAC, 640–745 μmol/g) and high NOx storage capacity (NSC, 250–350 μmol/g). Two reaction pathways are speculated for NOx adsorption on CeO2 nanosphere, including “nitrate route” and “nitrite route”. The thermally unstable surface nitrites formed on the CeO2 nanosphere allow ceria to release more NOx during the desorption process. The present work provides a new ceria morphology for NOx traps, which may become a potential excellent NOx storage material.

Nanoarray-Based Monolithic Adsorbers for SO2 Removal

Nanoarray-based monolithic catalysts have been developed for various applications, including CO oxidation, hydrocarbon combustion, lean NOx trapping, and low-pressure CO2 hydrogenation. In this work, SO2 adsorption properties have been explored and evaluated on the cordierite honeycomb monoliths grown with zinc oxide nanoarray (ZnO), zinc oxide nanoarray washcoated by BaCO3 nanoparticles (ZnO/BaCO3), and manganese oxide nanowire array with cryptomelane structure (MnOx) at a temperature range from 50 to 425 °C. All samples show temperature-dependent SO2 adsorption behaviors. The adsorption results reveal the performance order: MnOx > ZnO/BaCO3 > ZnO, with ~ 90% SO2 adsorbed in MnOx at 425 °C. Washcoated BaCO3 contributes to the improvement of SO2 adsorption in ZnO nanoarray, and the best performance displayed in MnOx may be attributed to their high specific surface area. After regeneration, nanoarrays all exhibit good thermal stability during test-regeneration cycles. No additional phase is formed in regenerated ZnO nanoarrays (ZnO-R), while BaCO3 is converted to BaSO4 in the regenerated ZnO/BaCO3 nanoarrays (ZnO/BaCO3-R), and the sulfur species (possibly MnSO4) and Mn2O3 are found in regenerated MnOx nanoarrays (MnOx-R). It is noted that a small amount of sulfur species (possibly MnSO4) may promote the SO2 adsorption of MnOx-R at a lower temperature, while the formed Mn2O3 contributes to the deactivation of MnOx-R.

Optimizing the lean hydrocarbon NOx trap: Sequential and dual-layer configurations

Vehicular emission control catalysts are ineffective in eliminating CO, hydrocarbons, and NOx during engine cold-start when exhaust temperatures are below 200 °C. In this study the performance of coupled low temperature NOx, n-C12H26 (C12), and C3H6 trapping, release and conversion for a series of model Lean Hydrocarbon NOx Trap (LHCNT) catalysts are examined. Pd and Pt supported on small-pore (SSZ-13) and large-pore (BEA) zeolites are selected based on the performance during transient NO and C12 uptake, release and conversion experiments. These catalysts are combined into sequential (Pt + Pd/BEA → Pd/SSZ-13; Pd/SSZ-13 → Pt + Pd/BEA) and dual-layer (Pt + Pd/BEA top, Pd/SSZ-13 bottom) configurations in an attempt to improve the trapping and conversion performance. While all three configurations trap between 75 and 100 μmolNOx/g-cat, the Pd/SSZ-13 → Pt + Pd/BEA sequential configuration is most effective in simultaneously trapping C12 and NO in the presence of H2O, resulting in excellent NO and C12 storage below 100 °C with release and/or conversion at or above 200 °C. For each configuration, C12 oxidation lights-off below 300 °C and NO oxidation achieves ∼35 % conversion in the absence of C12. Neither the presence of C12 nor the order of the sequential configuration has a significant impact on NO uptake. C12 significantly delays NO and NO2 desorption to temperatures exceeding 300 °C. The more compact dual-layer catalyst is most effective in forming NO2 as the release temperature lines up with the maximum NO conversion temperature but traps less C12 than the sequential configurations. The addition of C3H6 in the feed on the dual-layer catalyst leads to further delay in the NOx desorption as well as increased NO and C12 conversion at high temperatures. The overall findings provide guidance in the optimizing LHCNT configuration for realistic feeds.

Coupled NO and C3H6 Trapping, Release and Conversion on Pd/BEA: Evaluation of the Lean Hydrocarbon NOx Trap

Automotive emission control catalysts are not effective in eliminating CO, hydrocarbons, and NOx during engine cold-start (below 200°C). In this study the effectiveness of coupled low-temperature NOx and C3H6 trapping and conversion on a model Lean Hydrocarbon NO¬x Trap (LHCNT) consisting of a washcoated Pd/BEA monolith was examined. Transient uptake and release experiments were conducted to obtain insight regarding competitive adsorption, release temperature, conversion activity, and water impact. NO and C3H6 uptake were found to be mutually inhibitive due to competition for adsorption on Bronsted acid sites (BAS). During desorption, the existence of multiple NO sorption sites was established. Although C3H6 inhibited NO adsorption on some of these sites, it delayed NO¬x desorption to higher temperatures. Furthermore, above 200°C Pd/BEA was able to catalyze the oxidation of stored hydrocarbons and exhibited NOx reduction activity. Addition of water inhibited both NOx and C3H6 uptake, but the inhibition wa...

Investigation of an irreversible NOx storage degradation Mode on a Pd/BEA passive NOx adsorber

Passive NOx adsorbers (PNAs) are a proposed solution for cold start NOx emissions, via low temperature NOx trapping and subsequent release at a temperature where downstream NOx reduction catalysts are active. Pd/zeolites have been reported to have significant NOx storage capacities. However, their real exhaust compatibility has not been completely evaluated. In this work, Pd/BEA was used to study the mechanism of an observed PNA degradation. NO adsorption, NO and CO co-adsorption and temperature programmed desorption/oxidation were performed on a series of differently aged Pd/BEA catalysts to evaluate the NOx storage capacity and desorption temperature. An irreversible NOx storage degradation mode caused by low temperature CO exposure was observed. H2 temperature programmed reduction results indicate this irreversible degradation can be attributed to the loss of Pd2+. Additionally, STEM images revealed Pd particle migration/agglomeration after hydrothermal aging and CO/H2 exposure. Larger particles, formed during CO exposure, lead to the irreversible degradation.

Low-temperature NOx trapping on alkali or alkaline earth metal modified TiO2 photocatalyst

Low-temperature NOx trapping was demonstrated on alkali (Na, K, Rb, and Cs) or alkaline earth metal (Mg, Ca, Sr, and Ba)-modified anatase TiO2 photocatalysts (M-TiO2). The modification enhanced the NOx-trapping efficiency of bare TiO2 as well as the NOx storage capacity (NSC). The NSC depended on the modified metal species. Cs-TiO2 and Ba-TiO2 showed a comparable capacity (179 and 187 μmol g−1, respectively) with a conventional NOx-trapping catalyst, Pt/Ba/Al2O3 (176 μmol g−1). XRD and XPS revealed the modification effect on the structures and the surface properties of M-TiO2. Alkali TiO2 promoted the sintering of TiO2 and significantly increased the electron density of the TiO2 surface, while alkaline earth TiO2 suppressed the sintering and slightly changed the surface properties. The adsorption strength of NOx was evaluated from the NOx-TPD of the as-synthesized catalyst. For alkaline earth TiO2, stronger NOx adsorption increased the NOx-trapping efficiency. In addition, in-situ DRIFT measurements clearly showed the difference between adsorbed NOx species in M-TiO2. Alkaline earth TiO2 traps NOx on Ti sites, whereas alkali TiO2 traps NOx mainly on alkali sites. NOx trapping on Ti sites for alkaline earth TiO2 was effective for photocatalytic NOx trapping with high efficiency at low temperature.

Steady state and lean-rich cycling study of a three-way NOX storage catalyst: Experiments

The three-way NOx storage catalyst (TWNSC) combines components from a conventional three-way catalyst (TWC) and NOX storage and reduction (NSR) catalyst to improve NOX emissions while exploiting the higher fuel economy of lean-burn gasoline vehicles. The performance of a commercial monolithic TWNSC was studied to understand the NOX trapping and reduction performance over a range of conditions with emphasis on identifying conditions leading to optimal performance in terms of a standalone TWNSC or one coupled with a downstream selective catalytic reduction (SCR) device. Using H2, CO, and C3H6 in various combinations, the impact of cycle timing (cycle time, rich duty fraction), reductant and O2 feed concentrations, and feed temperature on NOX conversion and product (N2, NH3, N2O) selectivities was determined. Steady state experiments were conducted to assess catalyst activity and selectivity and to help interpret the phenomena observed during lean-rich switching. Cycling experiments reveal maxima in the NOX conversion and ammonia-to-NOx ratio (ANR) at distinct, intermediate cycle times. The existence of operating conditions giving these maxima depends on the reductant type, feed temperature, and O2 feed concentration. For example, a large disparity in the lean/rich ratio (stoichiometric number) of the lean and rich feeds tends to lead to a NOx conversion maximum. Where possible, the data trends are interpreted in terms of known performance features of the TWC and NSR catalysts. The study findings provide guidance for optimizing the TWNSC formulation and operation strategy.

Addition of Pd on La0.7Sr0.3CoO3 Perovskite To Enhance Catalytic Removal of NOx

Herein, we report the function of Pd species in the Pd-added La0.7Sr0.3CoO3 perovskite for removal of NOx (NO+NO2) in the lean-burn NOx traps (LNT) process. The addition of Pd significantly improves the De-NOx activity of the perovskite-based catalysts. The Pd-supported catalyst has an excellent NOx removal efficiency of 90.4%, which is higher than the Pd-doped catalyst (74.4%) and pure La0.7Sr0.3CoO3 (51.6%). Our results show that the addition of Pd species enhances both the NOx desorption behaviors to regenerate the NOx storage sites and NOx reduction ability of the catalysts in the fuel-rich period, which induces the improved NOx trapping performance in the followed lean-burn period. These achievements result in the enhanced De-NOx activity of the Pd-added catalyst in the alternative NOx storage and reduction operations. Compared with the Pd-doped catalyst, the Pd-supported catalyst possesses the higher utilization efficiency of the Pd, thus exhibiting a higher catalytic activity.

Communication—Perovskite Electrochemical System for Highly Selective NOx Reduction of Diesel Engine Exhaust

A perovskite electrochemical system was developed for selective NOx reduction of diesel engine exhaust. The system was composed of a La0.9Sr0.1CoO3±δ oxidation catalyst and a (La0.85Sr0.15)0.95MnO3±δ/Ce0.9Gd0.1O1.95 electrochemical cell with BaO nanopariticles. A selectivity of 25–35% was achieved with a NOx conversion of 65–75% in 1000 ppm NO with 8% O2 at 375°C. The superior performance of the system was suggested to be ascribed to the promotion in NO2 formation substantially activating the NOx trapping and reduction processes on the electrode.

Low-Temperature Pd/Zeolite Passive NOx Adsorbers: Structure, Performance, and Adsorption Chemistry

Pd/zeolite passive NOx adsorber (PNA) materials were prepared with solution ion-exchange between NH4/zeolites (Beta, ZSM-5, and SSZ-13) and PdCl2 solutions. The nature of Pd (dispersion, distribution, and oxidation states) in these materials was characterized with Na+ ion exchange, TEM imaging, CO titration with FTIR, and in situ XPS. The NOx trapping and release properties were tested using feeds with different compositions. It is concluded that multiple Pd species coexist in these materials: atomically dispersed Pd in the cationic sites of zeolites and PdO2 and PdO particles on the external surfaces. While Pd is largely atomically dispersed in ZSM-5, the small pore opening for SSZ-13 inhibits Pd diffusion such that the majority of Pd stays as external surface PdO2 clusters. NOx trapping and release are not simple chemisorption and desorption events but involve rather complex chemical reactions. In the absence of CO in the feed, cationic Pd(II) sites with oxygen ligands and PdO2 clusters are reduced by N...

Synthesis of Pt/K2CO3/MgAlOx\u2013reduced graphene oxide hybrids as promising NOx storage\u2013reduction catalysts with superior catalytic performance

Pt/K2CO3/MgAlOx–reduced graphene oxide (Pt/K/MgAlOx–rGO) hybrids were synthesized, characterized and tested as a promising NOx storage and reduction (NSR) catalyst. Mg–Al layered double hydroxides (LDHs) were grown on rGO via in situ hydrothermal crystallization. The structure and morphology of samples were thoroughly characterized using various techniques. Isothermal NOx adsorption tests indicated that MgAlOx–rGO hybrid exhibited better NOx trapping performance than MgAlOx, from 0.44 to 0.61 mmol · g−1, which can be attributed to the enhanced particle dispersion and stabilization. In addition, a series of MgAlOx–rGO loaded with 2 wt% Pt and different loadings (5, 10, 15, and 20 wt%) of K2CO3 (denoted as Pt/K/MgAlOx–rGO) were obtained by sequential impregnation. The influence of 5% H2O on the NOx storage capacity of MgAlOx–rGO loaded with 2 wt% Pt and 10% K2CO3 (2Pt/10 K/MgAlOx–rGO) catalyst was also evaluated. In all, the 2Pt/10 K/MgAlOx–rGO catalyst not only exhibited high thermal stability and NOx storage capacity of 1.12 mmol · g−1, but also possessed excellent H2O resistance and lean–rich cycling performance, with an overall 78.4% of NOx removal. This work provided a new scheme for the preparation of highly dispersed MgAlOx–rGO hybrid based NSR catalysts.

Advantages of MgAlOx over γ-Al2O3 as a Support Material for Potassium-Based High-Temperature Lean NOx Traps

MgAlOx mixed oxides were employed as supports for potassium-based lean NOx traps (LNTs) targeted for high-temperature applications. Effects of support compositions, K/Pt loadings, thermal aging, and catalyst regeneration on NOx storage capacity were systematically investigated. The catalysts were characterized by XRD, NOx-TPD, TEM, STEM-HAADF, and in situ XAFS. The results indicate that MgAlOx mixed oxides have significant advantages over conventional γ-Al2O3 supports for LNT catalysts, in terms of high-temperature NOx trapping capacity and thermal stability. First, as a basic support, MgAlOx stabilizes stored nitrates (in the form of KNO3) to much higher temperatures in comparison to mildly acidic γ-Al2O3. Second, MgAlOx minimizes Pt sintering during thermal aging, which is not possible for γ-Al2O3 supports. Notably, combined XRD, in situ XAFS, and STEM-HAADF results indicate that Pt species in the thermally aged Pt/MgAlOx samples are finely dispersed in the oxide matrix as isolated atoms. This strong me...

New Insights into the N2O Formation Mechanism over Pt-BaO/Al2O3 Model Catalysts Using H2 As a Reductant

The N2O formation mechanism was investigated over a Pt-BaO/Al2O3 catalyst applied on light-duty diesel vehicles using H2 as a reductant in the absence and presence of H2O. In the absence of H2O, N2O forms mainly at the initial phase of lean NOx trapping; while in the presence of H2O, N2O appears mainly at the beginning of the rich reduction phase. In the lean period, N2O is formed via the gaseous NO/O2 reacting with the adsorbed H and NH3 that are formed during the previous rich period. The N2O formation in the rich period is insignificant in the absence of H2O but is greatly enhanced by the presence of H2O. The amount of N2O formed is proportional to the H2O level in the feed, and its formation is favored at low temperatures. Our FTIR data show that H2O enhances the rate of nitrite/nitrate reduction during the rich regeneration, which increases the amount of released NOx, an oxygen source for N2O formation. Our temperature-programmed experiments indicate that H2O competes with NH3 for adsorption sites on Pt surface. This competitive adsorption may increase the NH3 desorption rate at low temperatures in the rich phase and make Pt surface more accessible to NO.

YCeZrO Ternary Oxide Solid Solution Supported Nonplatinic Lean-Burn NOx Trap Catalysts Using LaCoO3 Perovskite as Active Phase

A series of nonplatinic ceria-based oxides supported catalysts LaCoO3/K2CO3/S (S=CeO2, Ce0.75Zr0.25O2 or 5%Y/Ce0.75Zr0.25O2) were prepared by successive impregnation and employed for lean-burn NOx trapping. It is found that the doping of Zr or YZr into CeO2 facilitates the formation of CeZrO binary or YCeZrO ternary solid solutions, increasing the specific surface area and improving the redox property of ceria. The results of EXAFS and H2-TPR reveal that the LaCoO3 in the solid solution supported catalysts possesses higher dispersion and better reducibility than that in CeO2 supported one. The result of O2-TPD shows that the surface active oxygen species are remarkably increased after loading LaCoO3 on the supports. The YCeZrO ternary solid solution supported catalyst containing 5 wt % K2CO3 exhibits the best performance for NO oxidation and reduction at 350 °C, showing a high NO-to-NO2 conversion (66.5%) at lean condition, and a very high NOx reduction percentage (98.2%) and an extremely high NOx-to-N2 s...

A series of ceria supported lean-burn NOx trap catalysts LaCoO3/K2CO3/CeO2 using perovskite as active component

A series of ceria supported perovskite-based lean-burn NOx trap (LNT) catalysts 10 wt.% LaCoO3/xK(2)CO(3)/CeO2 (denoted as L/xK/C, x = 1, 3, 5, 8 wt.%) were prepared by successive impregnation. They display excellent performance for NOx storage and reduction, especially the one with 3% K2CO3. The formation of supported perovskite LaCoO3 on CeO2 is confirmed by XAFS characterization. The results of XPS and O-2-TPD reveal that the supported perovskite LaCoO3 contains considerable amounts of surface adsorbed oxygen, which are responsible for NO to NO2 oxidation during NOx storage. The catalyst L/3K/C exhibits fast and complete NOx trapping in lean condition, showing little NOx leak for long time, over which a particularly high NOx reduction percentage (97.7%) and NOx to N-2 selectivity (98.5%) are simultaneously achieved in cyclic lean/rich atmospheres. FT-IR and CO2-TPD results demonstrate the diversity of K species including -OK groups, dispersed surface K2CO3 species and bulk or bulk-like K2CO3 species. The dispersion and states of K species show big impact on NOx storage pathways. In situ DRIFTS results indicate that on L/xK/C NOx is stored as diverse nitrates without evident nitrite species detected during NOx adsorption and storage, which further verifies their excellent oxidation ability. (C) 2014 Elsevier B.V. All rights reserved.

Effects of Ti-doping on the NOx storage and the sulfur resistance of the BaFe1 − xTixO3 − y perovskite-type catalysts for lean-burn exhausts

Doping with Ti in the BaFeO3−x perovskite resulted in the significantly improved NOx adsorbability and the sulfur resistance, as compared with the BaFeO3−x. The Ti loading and residual carbonate could enhance the NOx storage capacity of these catalysts. However, during the sulfation, the sulfates were readily formed over the undissolved titanium oxides and the carbonates on perovskite. The complete Ti-doping in perovskite would reduce the amount of the residual carbonate, as well as the undissolved titanium, consequently leading to the relatively smaller NOx storage capacity (NSC) but the further improved sulfur resistance. Among these Ti incorporated perovskites, the BaFe0.8Ti0.2O3−y perovskite prepared by the impregnation method had the most complete Ti-doping degree and the highest sulfur resistance: only 5.1% of the NSC dropped (from 1000 to 949μmolg−1) after the sulfation pretreatment at 400°C. It is a possible NOx trapping material with the advantages of the high NOx storage capacity and sulfur tolerance for lean-burn exhausts.