Passive NOx Adsorbers Research Articles

Palladium-assisted NOx storage and release on CexZr1-xO2 for passive NOx adsorber in diesel exhaust aftertreatment.

Understanding Pd effects on NOx storage and release is crucial for designing passive NOx adsorber (PNA) to control NOx emissions during diesel cold-starts. Herein, we report two oxidation states of Pd species on CexZr1-xO2 regulated by metal-support interaction. Pdδ+ (0 < δ < 2) in Pd/Ce0.25Zr0.75O2 exhibits a high affinity for O2 adsorption, which promotes the oxidation of adsorbed NO to nitrates at 100 °C. These nitrates are thermally unstable due to electron transfer from the Pd atom to the N-O bond, facilitating the decomposition of nitrates to NO2 above 200 °C. In contrast, Pd2+ in Pd/Ce0.75Zr0.25O2 prefer to NO adsorption. A large amount of adsorbed NO and nitrites accumulate on Pd2+ and Ce4+ results in high levels of NO release below 200 °C. For the potential application in PNA, Pd/Ce0.25Zr0.75O2 is recommended due to its proper NOx release temperature as well as better water and SO2 resistance.

Enhanced carbon monoxide poisoning resistance of Pd/BEA by integrating with CeO2 for low temperature NO adsorption: The role of CeO2 on oxidizing carbon monoxide to CO32−

Passive NOx adsorption (PNA) represents a promising technology for controlling NOx emissions during cold starts. However, Pd/zeolite catalysts, the latest generation of PNA materials, suffer from serious deactivation under CO concentrations, owing to the CO induced aggregation of highly dispersed Pd sites to PdO particles. In this work, we found that the resistance to CO poisoning of Pd/BEA can be enhanced by integrating with CeO2 through physical mixing. The resulting CeO2 + Pd/BEA composite catalyst demonstrated a notably higher NOx storage capacity (with a NOx/Pd ratio of 1.3) compared to Pd/BEA (0.8). This improvement can be attributed to the interaction between CeO2 and the BEA support, which promoted the formation of nitrates on CeO2 during NO adsorption at 80 °C. More importantly, CeO2 effectively shielded Pd2+ ions from reduction by CO, with 88 % of Pd2+ ions remaining after CO exposure, significantly outperforming Pd/BEA, which retained only 66 %. Spectra characterization and DFT results indicate that the defects in CeO2 strongly trapped CO (−0.973 eV), and the oxidation of trapped CO to CO32− by adsorbed oxygen (Oads) was exothermic (−1.35 eV), which reduced the exposure of Pd2+ ions to CO and inhibited the reduction and agglomeration of Pd2+ ions. While the composite sample still exhibited a slight reduction in NOx storage capacity after CO poisoning, with the NOx/Pd ratio decreasing from 1.3 to 0.9. The DFT simulation results indicate that the Ce-Oads bonds in CeO2 are weakened during the decomposition of the CO32− intermediate. This weakening could potentially facilitate the release of Oads, thereby reducing the Oads content as evidenced by spectral evidence. The decrease of Oads diminished nitrate formation on the CeO2 component in composite sample, resulting in the decline of NOx storage capacity after CO poisoning.

Coaxial 3D Printing Enabling Mn/CHA@Pd/CHA Zeolite‐Based Core–Shell Monoliths for Efficient Passive NOx Adsorbers

AbstractPd‐based zeolites are extensively used as passive NOx adsorbers (PNA) for cold‐start NOx emissions to meet stringent emission regulations. However, optimizing adsorber design to reduce Pd usage with substitution by non‐noble metals that are prone to suffer from H2O remains a significant challenge. Herein, the core–shell Mn/CHA@Pd/CHA zeolite monoliths based on non‐noble metal/zeolite core are constructed using coaxial 3D printing technology and identified as efficient passive NOx adsorbers for the first time. In the Mn/CHA@Pd/CHA monolith, the Pd/CHA shell effectively protected the Mn active sites in the core from H2O, while the integration of the Mn/CHA core not only introduced efficient storage sites but also facilitated NOx desorption, thereby achieving comparable adsorption properties and increased the NOx desorption efficiency by 35% at 350 °C compared with that of Pd/CHA monolith. Furthermore, some non‐noble metal‐based zeolites (e.g., Co/CHA, Mn/MFI, Mn/BEA) and Pd‐based zeolites (e.g., Pd/AEI) are also employed as cores and shells respectively to fabricate a series of core–shell zeolite monoliths via coaxial 3D printing, highlighting the benefits of incorporating non‐noble metals into Pd‐based zeolites for improving adsorption and desorption behaviors. This work provides a promising strategy for designing cost‐effective PNA materials and contributes to improving the exhaust after‐treatment technology.

Correlation between Zeolitic SiO2/Al2O3 Ratio and the Low-Temperature NOx Adsorption Capacity of Pd/SSZ-13 under Realistic Exhaust Conditions

Passive NOx adsorber (PNA) is one of the important means to effectively reduce NOx emission control in the cold start of a diesel engine. A series of Pd/SSZ-13 catalysts with different SiO2/Al2O3 ratios (11, 17, and 25) were prepared using the impregnation method. Furthermore, the effect of the SiO2/Al2O3 ratio on the adsorption performance of Pd/SSZ-13 at low-temperature NOx under realistic exhaust conditions was studied. The results show that the Pd/SSZ-13 catalyst with a low SiO2/Al2O3 ratio after loading Pd has a higher specific surface area and palladium ion content. There is a negative correlation between NOx adsorption performance and the SiO2/Al2O3 ratio. After high-temperature heat treatment, the acid sites closely related to palladium species increase and palladium species will redisperse, producing more palladium ions. The palladium ions coexist in Pd/SSZ-13 in the form of Pd2+ and Pd+, among which Pd2+ is divided into two types: Z−Pd2+Z− and Z−[Pd(II)OH]+. The NOx adsorption performance of the Pd/SSZ-13 catalyst was significantly improved, and the higher the SiO2/Al2O3 ratio, the more obviously the advantage of NOx adsorption performance increased after heat treatment. The NOx adsorption kinetic model of the Pd/SSZ-13 catalyst under realistic exhaust conditions was most suitably described by the pseudo-first-order model.

Achieving High Dispersion of Pd in Small-Pore Zeolite SSZ-13: A High-Efficiency Low-Temperature NOx Adsorber.

Passive NO x adsorber (PNA) materials are primarily considered for reducing nitrogen oxide emissions during the low-temperature cold start of a motor vehicle. Pd/SSZ-13 has attracted considerable attention because of its outstanding hydrothermal stability and sulfur resistance. Optimizing the dispersion of precious metal Pd in Pd/SSZ-13 is crucial for enhancing PNA performance and nitrogen oxide adsorption capability. In this study, we prepared Pd/SSZ-13 using different methods and evaluated their influence on the NO x adsorption capability. The characterization results show that the dispersion of precious metal Pd in the Pd/SSZ-13 catalyst prepared by the quantitative ion-exchange method is as high as 92.13%, and the loading amount is as high as 98.93%. Pd predominantly exists as Pd2+, achieving near-total loading and further improving the catalyst's NO x adsorption capacity. This study offers innovative approaches and methods for applying Pd/SSZ-13 as a PNA material, serving as a reference for its further optimization and performance enhancement. Continued research into the preparation and adsorption performance of Pd/SSZ-13 materials could offer solutions to reduce motor vehicle nitrogen oxide emissions.

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.

The influence of phosphorus and CO poisoning on Pd/SSZ-13 with different Al distributions as passive NOx adsorbers

Phosphorus and CO are typical poisonous substances under realistic working conditions for Pd/SSZ-13 passive NOx Adsorbers (PNA). In this research, the Pd/SSZ-13 with different Al distributions were subject to phosphorus and CO poisoning. The characterization results indicated the resistance to both P and CO increased with the amount of paired Al on SSZ-13 support. Paired Al facilitated the formation of Pd2+Z2 which was more stable against [Pd(OH)]+ under P poisoning. The Pd2+ preferred to transform to Pd+ after poisoning while the [Pd(OH)]+ was more likely turned to agglomerated Pd. The Al-Pd-Al structure also mitigated dealumination during P poisoning. On the other hand, paired Al promoted the ion-exchange of Pd on the SSZ-13 support and led to a more homogeneous Pd distribution. This effectively alleviated the Ostwald ripening and particle migration thus preserving the performance of the catalyst under CO atmosphere. This study provided a new perspective for designing PNA materials with high resistance.

Unveiling the roles of distinct active sites over Pd/SSZ-13 for low-temperature NOx adsorption

Pd-based zeolites are attractive candidates for passive NOx adsorbers to reduce NOx emission from vehicle exhausts at low temperature. However, it is not well understood the NO adsorption/desorption mechanism in the presence of H2O and CO, which are constant components in diesel exhaust. In this study, an initial Pd/SSZ-13 sample was treated by reduction in H2 and then oxidation in NO to obtain a hydrophilic Pd/SSZ-13 with mostly Z[Pd2+(OH)] sites, or by thermal aging at 750 °C to prepare a hydrophobic Pd/SSZ-13 dominated by Z2Pd2+ and Z[Pd2+(OH)] sites. The two distinct Pd/SSZ-13 catalysts were characterized by XRD, H2-TPR, 27Al MAS NMR, and in situ DRIFTS spectra of CO adsorption, etc. Furthermore, the NOx adsorption/desorption reaction experiments and in situ DRIFTS studies were carried out on the two Pd/SSZ-13 catalysts under various conditions to explore the role of Z2Pd2+ and Z[Pd2+(OH)] sites in NO adsorption in the presence of H2O or co-existence of H2O and CO. Additionally, the recovery of active Pd sites after NO desorption was studied.

Deciphering SO2 poisoning mechanisms for passive NOx adsorption: A kinetic modeling approach and development of a high-resistance catalyst

Passive NOx adsorption (PNA) is a promising technology aimed at reducing NOx emissions from vehicles during the cold start phase of the engine. This work investigated the SO2 poisoning mechanism of PNA through a combination of experimental research and kinetic modeling, leading to the development of a novel PNA sample with high resistance to SO2 poisoning. Pd/SSZ-13 samples were synthesized using different drying conditions, revealing that samples dried at room temperature showed lower degradation (10 %) compared to those dried at 80 °C (26 %). Investigation into the degradation revealed that ion-exchanged Pd sites with a hydroxyl group were more resistant to SO2 poisoning than other Pd sites. It is also found that SO2 aids in NOx storage on Pd sites, enhancing the PNA performance. A kinetic model was developed to describe the SO2 poisoning behavior and its influence on NOx storage. The model, which was verified under various conditions, effectively simulated the PNA behavior and SO2 poisoning of Pd/SSZ-13.

Condition‐dependent NOx adsorption/desorption over Pd/BEA: A combined microreactor and in situ DRIFTS study

AbstractPd/BEA is chosen as a model passive NOx adsorber (PNA) to elucidate the effect of the feed gas composition on the NOx adsorption/desorption behavior. The Brønsted acid and the partially hydrolyzed framework Al (P‐HAl(OH)) sites in HBEA adsorb NO and NO2 under dry conditions. Moreover, the performance of HBEA is not affected by CO, while CO inhibits nitrate formation and promotes NO adsorption via the Pd(NO)(CO) complexes formation over Pd/BEA. H2O inhibits NO adsorption over the Brønsted acid and P‐HAl(OH) sites, and ionic Pd is the only active site for NOx adsorption under wet conditions. Furthermore, NO adsorption over hydrated Pd (Pd2+(OH)(NO)(H2O)3) is weaker than NO adsorption over bare ionic Pd (Z2[Pd2+(NO)], Z[Pd2+(OH)(NO)]). Dehydration of Pd2+(OH)(NO)(H2O)3 forms more stable Z[Pd2+(OH)(NO)] during desorption. The NO adsorption capacity of Pd/BEA improves in the presence of CO under both dry and wet conditions by forming a stable carbonyl–nitrosyl complex.

Charge Shielding Effect of Sodium Ions in Improving The Hydrophobicity and Performance of Co/Na‐SSZ‐13 Zeolite for Passive NOx Adsorber

AbstractWith increasingly stringent regulations, the reduction of NOx emissions during vehicle cold start is a major challenge. Pd‐modified zeolites are considered as the most promising passive NOx adsorber (PNA) for cold start NOx control. Nevertheless, the scarcity and the high cost of Pd limit its practical application. Herein, a non‐precious metal modified Co/Na‐SSZ‐13 zeolite for low‐temperature NOx adsorption is reported. This one‐pot synthesized Co/Na‐SSZ‐13 exhibits an extraordinary NOx storage capacity (212 µmol gcat−1) under humid conditions (3% H2O v/v), which is more than double that of conventional Co or Pd‐modified SSZ‐13. This is attributed to the absence of the inert cobalt phyllosilicate commonly found in conventional Co‐SSZ‐13, as well as the highly dispersed Co and Na ions as the effective NOx adsorption sites in Co/Na‐SSZ‐13. Moreover, the presence of Na ions shields the negative charge generated by the Al center and weakens the local electric field strength of the Al and Co centers. This lowers the H2O adsorption energy and improves the zeolite hydrophobicity, which enhances the NO storage capacity of the Co/Na‐SSZ‐13 under humid conditions. This work highlights the Co/Na‐SSZ‐13, synthesize via a simple and high‐efficiency method, as a promising substitute for noble metal PNA materials.

Co3O4 as an efficient passive NOx adsorber for emission control during cold-start of diesel engines

The Co3O4 nanoparticles, dominated by a catalytically active (110) lattice plane, were synthesized as a low-temperature NOx adsorbent to control the cold start emissions from vehicles. These nanoparticles boast a substantial quantity of active chemisorbed oxygen and lattice oxygen, which exhibited a NOx uptake capacity commensurate with Pd/SSZ-13 at 100 °C. The primary NOx release temperature falls within a temperature range of 200-350 °C, making it perfectly suitable for diesel engines. The characterization results demonstrate that chemisorbed oxygen facilitate nitro/nitrites intermediates formation, contributing to the NOx storage at 100 °C, while the nitrites begin to decompose within the 150-200 ºC range. Fortunately, lattice oxygen likely becomes involved in the activation of nitrites into more stable nitrate within this particular temperature range. The concurrent processes of nitrites decomposition and its conversion to nitrates results in a minimal NOx release between the temperatures of 150–200 °C. The nitrate formed via lattice oxygen mainly induces the NOx to be released as NO2 within a temperature range of 200–350 °C, which is advantageous in enhancing the NOx activity of downstream NH3-SCR catalysts, by boosting the fast SCR reaction pathway. Thanks to its low cost, considerable NOx absorption capacity, and optimal release temperature, Co3O4 demonstrates potential as an effective material for PNA applications.

Na Cocations and Hydrothermal Aging Cooperatively Boost the Regeneration of Phosphorus-Poisoned Pd/SSZ-13 for Passive NOx Adsorption.

Pd/SSZ-13 has been proposed as a passive NOx adsorber (PNA) for low-temperature NOx adsorption. However, it remains challenging for Pd/SSZ-13 to work efficiently when suffering from phosphorus poisoning. Herein, we report a simple and efficient strategy to regenerate the phosphorus-poisoned Pd/SSZ-13 based on the cooperation between hydrothermal aging treatment and Na cocations. It was found that hydrothermal aging treatment enabled the redispersion of Pd and P-containing species in phosphorus-poisoned Pd/SSZ-13. Meanwhile, the presence of Na cocations significantly reduced the formation of AlPO4 and retained more paired Al sites for highly dispersed Pd2+ ions, which was of great importance for the recovery of adsorption performance. To our satisfaction, the restoration ratio of the adsorption capacity of poisoned Pd/SSZ-13 was >90% after regeneration. Strikingly, the NOx adsorption activities of phosphorus-poisoned Pd/SSZ-13 with phosphorus loadings of 0.2 and 0.4 mmol g-1 almost completely recovered upon regeneration. This study demonstrates the promoting effect of Na cocations on the regeneration of phosphorus-poisoned Pd/SSZ-13 by hydrothermal aging treatment, which provides useful guidance for the design of PNA materials with excellent durability for cold-start application.

Enhancing the NOx storage and hydrothermal stability of Pd/SSZ-13 passive NOx adsorbers by regulating the Al distributions

Pd/SSZ-13 with different Al distributions were hydrothermally synthesized and utilized as passive NOx adsorbers (PNA). The characterization results indicated that more concentrated Al distribution can not only improve the ion-exchanged level of Pd but also generated more Z2Pd2+ sites at six-member rings. Thus, as revealed by NOx adsorption experiments, samples with more Z2Pd2+ sites showed high NOx storage capacity as well as stronger water resistance. Besides, 84.8 % adsorption capacity of this sample was retained after hydrothermal aging treatment. On the other side, Pd existed as [Pd(OH)]+ sites on SSZ-13 with isolated Al as the majority. These sites were vulnerable to H2O and less stable. Seriously Pd agglomerated was observed on the aged Pd/SSZ-13 with less paired Al. This study revealed that proper Al distribution on SSZ-13 could significantly improve the performance and stability of the catalyst and provided a new direction for the design of PNA catalysts.

A Study of CeSnOx and Pd/CeSnOx as Low-Temperature NOx Adsorbers with Excellent Hydrothermal Stability.

In the present work, we report on two passive NOx adsorber (PNA) material candidates: the novel support CeSnOx with and without Pd loading. The NOx adsorption and storage capacities of fresh and hydrothermally aged CeSnOx and Pd/CeSnOx were investigated. The results show that CeSnOx exhibits a rather large NOx uptake and storage capacity (28.9 μmol/g), while the loading of Pd on CeSnOx can further increase the storage capacity to 37.6 μmol/g and affect the desorption temperature of NOx. It was found that the NOx desorption temperature of Pd/CeSnOx was compatible with the efficient operating window of selective catalytic reduction (SCR) catalysts. After a hydrothermal aging treatment at 800 °C for 12 h, the NOx adsorption and storage capacities of CeSnOx and Pd/CeSnOx increased, indicating excellent hydrothermal stability. The interaction of Pd with CeSnOx, the state of Pd species, and the structure of CeSnOx and Pd/CeSnOx are studied by combination of the characterization results.

Revealing the Roles of Cu/Ba on Ce-Based Passive NOx Adsorbers

At present, passive NOx adsorbers (PNAs) represent one of the most effective technologies for addressing NOx emissions from diesel engines during cold-start periods. Conventional PNAs, which primarily consist of noble metals (such as Pt, Pd, and Ag) loaded on metal oxides or zeolites, share the common drawback of high production costs. Consequently, developing low-cost PNAs with outstanding NOx storage performance remains a significant challenge. In this study, a series of CuxBa5Ce adsorbents were synthesized using the impregnation method, and a monolithic adsorbent was employed to evaluate NOx storage and release performance. Techniques such as XRD, UV-Vis DRs, H2-TPR, XPS, and in situ DRIFTs confirmed the crucial roles of Cu and Ba in NOx storage and release. Specifically, the incorporation of Cu into CeO2 enhanced NOx storage performance. Moreover, in the Cu3Ba5Ce adsorbent, the addition of Ba not only introduced new storage sites and altered the stability of NOx adsorption species but also helped prevent the aggregation of CuO, thereby prolonging the complete NOx storage duration and satisfying desorption temperature requirements. The Cu3Ba5Ce adsorbent exhibited the most favorable NOx storage performance, including a complete NOx storage time of 135 s and a NOx storage efficiency exceeding 50% at 80 °C over a 10 min period. While PNAs loaded with noble metals, such as Pd/CeO2 and Pt/CeO2, exhibited NOx storage efficiencies below 50% after adsorbing for 5 min at 80 °C. Therefore, this research offered a crucial strategy for developing non-noble-metal-loaded, Ce-based PNAs.

Exploring the framework of small pore zeolites for passive NOx adsorption

The use of a passive NOx adsorber (PNA) for NOx storage during the cold-start period is crucial for eliminating NOx emissions from the diesel exhaust. Palladium (Pd)-loaded zeolites have garnered significant attention owing to their high potential as PNA. In this study, small-pore zeolites with different frameworks were synthesized, and their structural characteristics and passive NOx adsorption abilities were investigated. Five zeolites, with CHA, AEI, AFX (low- and high- Si/Al ratio), and LEV framework structures, were synthesized by the hydrothermal conversion of faujasite-type (FAU) zeolite. Pd (1 wt%) was impregnated in combination with hydrothermal treatment. NOx adsorption and desorption behaviors were studied based on breakthrough NO adsorption and subsequent temperature-programmed desorption (TPD) experiments. Pd-loaded CHA, AEI, and LEV zeolites and high-silica AFX zeolites adsorbed approximately 93.88, 107.93, 97.57 and 106.53 μmol-NO g−1, respectively. On the other hand, the incorporation of Pd into AFX zeolite with a low Si/Al ratio led to significant hydrothermal damage and resulted in lower levels of adsorption and desorption capabilities. This demonstrates that hydrothermally stable small-pore zeolites on Pd-incorporation have great potential for passive NOx adsorption. The zeolite with different framework exhibited the different temperature range of NOx release. CHA, AEI, and high-silica AFX zeolites desorbed NOx in the low-temperature region (T < 250 °C), whereas the LEV zeolite showed a relatively high NOx desorption amount in the middle-temperature region (250 °C < T < 350 °C).

Correlation between zeolitic particle size and the low-temperature NOx storage capacity of Pd/ZSM-11 passive NOx adsorber

The effect of zeolitic particle size on the NOx adsorption and desorption was investigated by synthesizing ZSM-11 zeolites with different particle sizes (0.5∼0.8, 1∼2, and 5∼6 μm) for doping Pd as passive NOx adsorbers (PNAs). Several characteristic techniques of XRD, HAADF-STEM, H2-TPR, CO-DRIFT, XPS and NH3-TPD were applied to clarify the essential association of the PNAs performance of Pd/ZSM-11 zeolites with the support zeolite properties and Pd active species. It was found that both the efficient Pd(II) species for assisting NOx adsorption-release and the acid amount (especially the strong acid sites) for facilitating Pd dispersion have positive correlation with the decrease of zeolite particle size. Differently, in addition to the active Pd species, meso-macroporosity properties are another key factors to determine the NOx adsorption ability of PNAs. The meso-macroporosity properties of Pd/Z11-1.5 show a negative correlation with the decrease of zeolite particle size while Pd/Z11-0.7's show the positive correlation with the decrease of zeolite particle size. As a consequence, Pd/Z11-1.5 with higher Pd(II) content but weaker meso-macroporosity properties than Pd/Z11-5.0 shows comparable adsorption capacity to Pd/Z11-5.0, while Pd/Z11-0.7 with both high Pd(II) content and good meso-macroporosity properties not only exhibits the highest NOx storage capacity, but also can release the NOx at the lowest temperature of less than 300 °C, indicating to be a good candidate for purifying NOx at the actual exhaust temperature of diesel vehicles.

Novel layered double hydroxide-based passive NOx adsorber: Synergistic effects of Co and Mn on low-temperature NOx storage and regeneration

Passive NOx adsorber (PNA) is an efficient strategy for reducing vehicular NOx emission during cold-start period, and current studies on PNA development are limited to materials containing precious metals (Pt, Pd, and Ag, etc.). Herein, Co–Mn–Mg–Al mixed metal oxides were prepared from layered double hydroxides (LDHs) and utilized as a new type of PNA. Although Co–Mg–Al oxides exhibited excellent NOx adsorption capabilities because Co species promoted the generation of active oxygen species, their NOx adsorption performance deteriorated substantially with decreasing adsorption temperature below 150 °C and they required high temperatures over 400 °C to release the adsorbed NOx. Conversely, Mn–Mg–Al oxides retained their adsorption performance regardless of adsorption temperature and allowed for NOx desorption at relatively low temperatures by decreasing the thermal stability of the adsorbed NOx species. Notably, Co–Mn–Mg–Al mixed oxides exhibited high NOx adsorption performance at low temperatures between 50 and 150 °C and were passively regenerated via thermal treatment within a temperature range of 200–350 °C owing to the synergistic effects of Co and Mn. The developed LDH-based PNA materials presented excellent cyclic NOx adsorption performances for long-term operation, and dynamic tests indicated that NOx adsorbed onto Co–Mn–Mg–Al mixed oxides was effectively released with increasing temperature even in the presence of NO and O2.

Effects of alkali and alkaline earth metals on the activity and stability of Pd/SSZ-13 for passive NOx adsorption

Pd/SSZ-13 has been proposed as a passive NOx adsorber (PNA) to effectively alleviate NOx emission during vehicle cold start. However, the chemical poisons which are derived from biodiesel and lubricant oil are challenging for the practical application of PNA technology. In this paper, Pd/SSZ-13 was poisoned by alkali/alkaline earth metals to investigate the effects of chemical poisons on the adsorption performance of Pd/SSZ-13. It was found that the alkali/alkaline earth metals led to the decrease of surface area and Brønsted acid sites of Pd/SSZ-13. Strikingly, the 27Al MAS NMR demonstrated that the alkali/alkaline earth metals could effectively prevent Pd/SSZ-13 from dealumination during hydrothermal aging treatment. The H2-TPR results showed that more than 70% of Pd2+ ions were lost in the Na- and K-impregnated Pd/SSZ-13, while almost complete loss of Pd2+ ions was observed in the Mg- and Ca-impregnated Pd/SSZ-13, which resulted in the deactivation of Pd/SSZ-13. Further, the DRIFTS results showed that Mg2+ and Ca2+ ions exhibited preferential substitution for Pd2+ ions, while Na+ and K+ ions preferentially replaced the H+ ions. This work provides insights into the effects of alkali/alkaline earth metals on Pd/SSZ-13, expecting to attract attention to design PNA materials with resistance to chemical poisons.