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Due to the technology limitation and inferior deNOx efficiency of urea selective catalytic reduction (SCR) catalysts at low temperatures, passive NOx adsorber (PNA) for decrease of NOx, CO and hydrocarbons (HCs) during “cold start” of vehicles was proposed to meet the further tighten NOx emission regulations in future. Among them, Pd modified zeolite PNA materials have received more attention because of their excellent NOx storage capacity, anti-poisoning and hydrothermal stability and since Pd/zeolite PNA was proposed, a variety of advanced characterization methods have been applied to investigate its adsorption behavior and structure-performance relationship. The comprehension of the active sites and adsorption chemistry of Pd/zeolite PNA was also significantly improved. However, there are few reviews that systematically summarize the recent progress and application challenges in atomic-level understanding of this material. In this review, we summarized the latest research progress of Pd/zeolite PNA, including active adsorption sites, adsorption mechanism, material physicochemical properties, preparation methods, storage and release performance and structure-performance relationships. In addition, the deactivation challenges faced by Pd/zeolite PNA in practical applications, such as chemical poisoning, high temperature hydrothermal aging deactivation, etc., were also discussed at the micro-level, and some possible effective countermeasures are given. Besides, some possible improvements and research hotspots were put forward, which could be helpful for designing and constructing more efficient PNA materials for meeting the ultra-low NOx emission regulation in the future.

Pd-doped chabazite (Pd/CHA) offers unique opportunities to adsorb and desorb NOx in the target temperature range for application as a passive NOx adsorber (PNA). The ability of Pd/CHA to trap NOx emissions at low temperatures (

The potential of Ag/MgO/ $$\gamma $$ -Al2O3 (Ag/Mg–Al) as a passive NOx adsorber is evaluated, and is found to be superior as compared to Ag/ $$\gamma $$ -Al2O3. Two different adsorption sites exist on both Ag/Mg–Al and Ag/ $$\gamma $$ -Al2O3, and it is inferred that the primary role of MgO is not to provide additional adsorption sites but to decrease the Ag particle size, which results in a higher activity towards NOx adsorption. The product distribution profile with either NO or NO2 in the feed is found to be nearly the same at early times, which indicates the formation of the same surface species with either of the reactants. It is deduced that the facile conversion of nitrites to nitrates occurs in the presence of H2, but the extent of conversion decreases at longer times. A sequential configuration of the in-house Ag/Mg–Al and a commercial Cu/SSZ-13 catalyst yielded a NOx reduction efficiency of 85% at low temperatures as against 7% with the standalone Cu/SSZ-13 catalyst.

Atomically dispersed Pd in Zeolites can adsorb NO to reduce NOx emission from vehicle exhausts at low temperature. In this contribution, low temperature NO adsorption abilities of Pd/ZSM-5 and Pd/SSZ-13 are comparatively studied. Co-adsorption of NO and H2O on Pd/ZSM-5 induced PdO sintering and deactivated NO adsorption ability. On the other hand, NO adsorption ability of Pd/SSZ-13 was maintained from the 500 °C treatment under NO and H2O where PdO sintering did not occur. Therefore, Pd/SSZ-13 would be the better candidate as NO storage material to reduce NOx emission from vehicle exhaust.

The Passive NOx Adsorber (PNA) is an emerging technology to abate cold-start NOx emissions from lean combustion vehicles. Binding sites on Pd-exchanged zeolites adsorb NO at low temperature ( 120 °C) regimes, indicating competition of NO uptake with H2O. The additions of CO and C2H4 increase the lower temperature uptake and higher temperature release of NO, both features desirable for application. In contrast, H2, dodecane, and toluene have a negligible effect on the NO uptake. During NO uptake the co-generation of NO2, CO2 and CH3CHO, respectively with reductants NO, CO, and C2H4, provides evidence for the reduction of Pd2+ in the form of [Pd(OH)]+ bound to zeolitic anionic sites [Al-O-Si]− and/or PdO2 clusters. Supporting DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy) measurements identify several potential surface species that are linked to the multiple NO + NO2 desorption peaks. Two prospective mechanistic schemes are proposed that are consistent with the uptake/release and DRIFTS data.

Low-temperature operation of urea selective catalytic reduction (SCR) aftertreatment systems for the abatement of NOx from diesel engines presents new challenges related to poor catalytic activity and the urea injection temperature threshold. Physical mixtures of a NH3-SCR catalyst (e.g., Fe-/Cu-zeolite) and of a NOx storage material (e.g., BaO/Al2O3, CeO2/Al2O3) have shown promising performances in terms of overall NOx removal efficiency, as they can operate both as NOx adsorbers, trapping NOx during the cold start transient, and as SCR catalysts, reducing at higher temperatures the previously stored NOx with NH3. Herein, we extend the investigation of new Cu-CHA + BaO/Al2O3 AdSCR systems (AdSCR = adsorption + selective catalytic reduction) focusing on the reactivity between NOx and ammonia in the low-temperature window (from room temperature up to 170 °C). We find that the selective reduction of NO by NH3 over Cu-CHA is surprisingly enhanced by the presence of BaO/Al2O3 when ammonia is preadsorbed, leading to the onset of nitrogen formation already at 40 °C.

An effective model for describing NOx adsorption and desorption on a PtPd/CeO2-ZrO2 passive NOx adsorber is presented. The kinetic parameters are evaluated from the available experimental data obtained during NOx adsorption/desorption experiments including CO2 and H2O in the feed, performed at 80, 120 and 160 °C both in the presence and in the absence of reducing agents (CO or C2H4 ). The model describes the temperature dependence of the NOx adsorption rate and capacity, the impact of CO, and dynamics of the NOx desorption events. The model predicts formation of nitrites, nitrates, and additional storage enabled in the presence of CO. Thermal decomposition of the stored NOx species results in two main desorption peaks. Nitrites are desorbed at lower temperatures while nitrates are thermally more stable. The evolution of nitrite and nitrate species in the model corresponds with the measured DRIFTS spectra of the catalyst surface. The presence of CO significantly improves the rate of NOx adsorption and storage efficiency at low temperatures, most probably due to reduction of oxidic Pt and Pd nanoparticles. The developed model captures well the observed trends and can be utilized for simulations of PNA performance under real operating conditions.

Low temperature NOx adsorbers (LTNA) adsorb the NOx emissions from diesel engines during the cold start period and then thermally release the NOx when the downstream urea SCR system is operational. A degreened monolithic core sample of a LTNA with palladium on a ceria-zirconia washcoat (Pd/CZO) was evaluated for NOx storage performance under lean conditions using a reactor-based transient temperature test with and without different reductants [i.e., ethylene (C2H4), carbon monoxide (CO), hydrogen (H2), or a 3:1 CO:H2 mix]. Relative to tests without a reductant, all four reductants increased the amount of NOx stored on the LTNA. The NOx storage performance decreased gradually during consecutive tests. Lean methane (CH4) oxidation was used to probe the average oxidation state of the PdOx in the LTNA after the transient tests. The PdOx was reduced similarly on tests with NO alone and with NO and each reductant but significantly less with the reductant alone. NO alone reduced the PdOx by reacting with the PdOx to form NO2 between 200 and 210 °C. CH4 oxidation tests were also performed after the bed temperature exceeded 185 °C on transient tests with and without the reductants (i.e., after the NOx storage period). There was little PdOx reduction with NO alone or reductant alone, but there was significant and similar PdOx reduction with NO and each reductant. This supported a mechanism where NO and the reductant interacted to form a chemical intermediate on a PdOx site, reducing the PdOx in the process. The oxidation state of the PdOx was also determined as a function of time during transient tests with and without the reductants. CO and CO/H2 reduced the PdOx more rapidly than C2H4 or H2, which correlated with the superior NOx storage performance with CO and CO/H2. It is proposed that the PdOx reduction was responsible for the gradual drop in NOx storage efficiency on consecutive tests. Consistent with that, the maximum performance could be recovered by oxidizing the catalyst between 700 and 750 °C. Future work includes a similar assessment of zeolite-based LTNAs which are more sulfur tolerant than ceria-based LTNAs.

Co-SSZ-13 served as a non-PM passive NOx adsorber (PNA) was adopted for the first time, to the best of our knowledge, as a promising alternative to platinum-based NOx adsorbers. This Co-based catalyst has a NOx storage capacity of 27.3 μmol/gcat, which is 77% of that of Pd-SSZ-13. NOx is desorbed above 250 °C, which is appropriate for use downstream of the NH3-SCR catalyst. NO adsorption on Co-SSZ-13 resulted in the formation of dinitrosyl species, which are further converted into NO+ on the zeolite framework and nitrate species on Co2+. The strong interaction between Co2+ and NO is responsible for the high NOx storage capacity. In addition, conversion among the adsorbed species increases the NO desorption temperatures, which is important for the correct operation of PNA.

Passive NOx adsorber (PNA) is one of the promising technologies for reducing NOx emissions from vehicles during cold start periods. Precious metals supported on metal oxide and zeolite have been extensively investigated as PNA materials. Hence, it is worthwhile to compare two typical PNA materials, Pd/CeO2 and Pd/SSZ-13, so as to understand the differences between them. The NOx adsorption/desorption experiments were carried out under varying NO/NO2 ratios, and the result indicated that the NOx adsorption/desorption mechanisms on Pd/CeO2 and Pd/SSZ-13 are completely different. Specifically, NO and NO2 were competitively adsorbed on Pd/SSZ-13, while the NO adsorption on Pd/CeO2 was promoted by NO2. This difference is attributed to different adsorption sites: NOx is adsorbed on the ceria surface in Pd/CeO2, whereas it is adsorbed on atomic Pd2+ sites in Pd/SSZ-13, according to DRIFTS spectra in this study. In addition, the effect of sulfur poisoning was investigated as well. Pd/SSZ-13 showed moderate sulfur tolerance, while Pd/CeO2 completely lost its PNA ability after sulfur aging.

The modern three-way catalyst (TWC) is very effective for treating the hydrocarbons (HCs), carbon monoxide (CO), and nitrogen oxides (NOx) from stoichiometric gasoline engines once the TWC has achieved its minimum operating temperature (e.g., 250 to 400 °C, depending on the gas species). Likewise, the diesel oxidation catalyst (DOC), selective catalytic reduction (SCR) catalyst with urea injection, and the diesel particulate filter (DPF) are effective for treating the HCs, CO, NOx, and particulate matter (PM) emissions from diesel engines once the catalysts are warmed up, although this can require a significant length of time (e.g., 1 to 3 min) because of the relatively low exhaust temperatures from diesel engines. For both types of engines, excess fueling is often used to accelerate the heating of the catalyst system after a cold start, although this decreases the fuel economy of the vehicle. Even with excess fueling, a high portion (up to 80%) of the total vehicle emissions is emitted during the cold start period (i.e., the period before the catalysts are functional). To treat the HC emissions during this cold start period, one approach is to employ a HC trap (HCT) that can adsorb the HC emissions at low temperatures and then oxidize the stored HCs to carbon dioxide (CO2) and water (H2O) at higher temperatures. To treat the NOx emissions during the cold start period, a passive NOx adsorber (PNA) can adsorb the NOx at low temperatures. For stoichiometric gasoline applications, the PNA can then reduce the stored NOx to nitrogen (N2) at higher temperatures. On diesel engines, the PNA can release the stored NOx back into the exhaust once the downstream urea/SCR system is operational. Some adsorber technologies have the capability of adsorbing HCs and NOx simultaneously. In this review, the HC trapping and passive NOx adsorbing technologies will be discussed in separate sections. This review will describe how the current trapping technologies can be applied in vehicle exhaust systems, the material properties required for efficient HCTs and PNAs, and the exhaust conditions that can inhibit/enhance their trapping properties. First, the performance of HCTs will be discussed in terms of their physical properties (e.g., pore size, acidity, presence of metal ions) and the trapping conditions (e.g., storage temperature, space velocity, and the presence of other exhaust species such as H2O and CO2). This will be followed by in-depth coverage of the reactions occurring during HC desorption. The second part of this review will focus on the composition of various PNA formulations, the effects of the trapping conditions (e.g., temperature, space velocity, the presence of other exhaust species such as CO2, H2O, CO, and C2H4), and the effects of sulfur poisoning on their trapping performance. The effect of hydrothermal aging and the regenerability of HCTs and PNAs will also be discussed. A significant amount of literature has emerged recently regarding HCTs and PNAs; this review is primarily focused on summarizing this literature and reconciling the differences presented.

Various activation conditions such as temperature, time and the concentration of H2O during hydrothermal aging (HTA) treatment were extensively investigated to identify the proper ion-exchange conditions of Pd/SSZ-13 catalyst for low temperature NO adsorption performance. In addition, we examined how the cation in SSZ-13 (i.e. NH4-, Na- or K-SSZ-13) affects the low temperature NO adsorption performance of Pd/SSZ-13 after HTA treatment. It is found that NO adsorption ability begins to increase substantially when Pd/SSZ-13 is hydrothermally aged above 650 °C. Pd/SSZ-13 demonstrated the maximum NO adsorption performance after hydrothermal aging treatment in the range of 700–750 °C, followed by the severe decline above 850 °C. Such deactivation is attributed to the agglomeration of Pd species due to the dealumination of SSZ-13, as indicated by XRD and NMR analysis. At least 15 h of HTA treatment at 750 °C is required to provide the optimum activation of Pd/SSZ-13. Water vapor is essential component to activate Pd/SSZ-13 during the hydrothermal aging treatment. Unlike NH4-SSZ-13, Na- or K-SSZ-13 cannot be activated even if the optimum hydrothermal aging condition (750 °C for 25 h with 10% H2O) is applied. EXAFS and XRD analysis provides the evidence about the sintering of Pd species of the Pd/Na- or Pd/K-SSZ-13 catalysts rather than Pd ion exchange, indicating that the pre-existing Na or K ion inhibits the ion exchange during hydrothermal aging treatment. It can be concluded that the selection of proper hydrothermal aging conditions and zeolite form is essentially required to obtain the excellent low temperature NO adsorption performance of Pd/SSZ-13 as a passive NOx adsorber.

Pd/SSZ-13, which has ability to adsorb NO at low temperature, is potentially used as a passive NOx adsorber for cold start application. We investigate the changes in the physicochemical properties and low temperature NO adsorption ability of Pd/SSZ-13 resulting from the reduction treatments with H2 or CO at various temperatures. Treatment with H2 at 500 °C does not give rise to the decrease in NO adsorption ability. However, the ability begins to decrease gradually with the increasing reduction temperature from 600 to 900 °C. On the other hand, CO treatment at 500 °C induces a significant decrease in NO adsorption ability. Combined EXAFS, HAADF-STEM and H2-TPR results clearly demonstrate that CO treatment induced more severe sintering of Pd species than H2 one at the same reduction temperature. After re-oxidation at 500 °C, H2-treated sample generated Pd ion species, which are the active site for NO adsorption at low temperature, more abundantly than CO-treated one, well corresponding to the higher NO adsorption ability over the former sample. Such phenomenon is attributed to the facile sintering behavior due to the high mobility of Pd-carbonyl complex upon the treatment with CO. It can be concluded that the reduction treatments with H2 and CO have different effect on Pd sintering, which is regarded as the main deactivation mechanism of Pd/SSZ-13 passive NOx adsorber.

Post injection is commonly used for triggering regeneration of exhaust aftertreatment devices such as diesel particulate filters and NOx adsorber catalysts. Post injection is also a method to reduce engine-out particulate matter (PM) emissions while maintaining efficiency, to reduce or potentially eliminate exhaust aftertreatment. This study explores post injection in a light duty multi-cylinder turbodiesel engine to reduce PM. The impact of post injection dwell and duration on soot reduction and reactivity are investigated in this study. Soot samples were collected from the engine exhaust and subjected to analyses of oxidative reactivity and nanostructure of the primary soot particles. Close-coupled post injections conditions reduce soot emissions by 11–21%, while long-dwell post injection conditions reduce soot emissions by 28–33%. The reaction rate constants of soot collected from close-coupled post injection conditions decrease by 10–13% compared to baseline condition, while the reaction rate constants of soot collected from long dwell post injection conditions increase by 37–39% compared to the baseline condition. The difference in soot reactivity indicates fuel from the post injection interacts with the main injection flame. Moreover, with the increase of injection dwell and post injection size, soot surface oxygen content and amorphous carbon content increase. This explains the change in reactivity of soot from different injection dwells. Primary soot particle and particle aggregate sizes do not vary much with post injection. Soot from post injection conditions show shrinking-core-type oxidation without graphene layer rearrangement.

Passive NOx adsorbers have been proposed to help address cold start NOx emissions. Although the original concept was proposed earlier, it is not until the last few years that more significant efforts focused on understanding the fundamentals of this technology. Several groups of potential PNAs have been evaluated for low temperature NOx storage and subsequent release, and the laboratory-scale experimental testing results appears to be promising. This review will summarize the currently reported materials for PNA application, their unique behaviors when operating under different conditions, systematically evaluate and compare their performances and discuss the relevant reaction chemistry. The discussion around each specific topic is based on literature reported experimental data from testing and characterizing potential PNA candidates and will put more emphasis on the recent advances in the fundamental understanding of this technology. Since at this stage the mechanism of the adsorption and desorption processes is not fully understood, performance evaluation and reaction chemistry will be discussed independently. Unresolved issues are listed at the end of each section as an attempt to summarize the critical unknowns within a specific research topic.

We show that replacing alkaline (NaOH) for alkaline-earth metal (Sr(OH)2 as an example) in the synthesis of SSZ-13 zeolite with Si/Al~10 produces SSZ-13 zeolite material with novel, advantageous properties. Its NH4-form ion-exchanges higher amount of Co(II) ions than the conventional one: this is the consequence of increased number of Al pairs in the structure induced by the +2 charge of Sr(II) cations in the synthesis gel that force two charge-compensating AlO4- motives to be closer together. We characterize the +2 state of Co(II) ions in these materials with infra-red spectroscopy and XANES measurements. They can be used for NOx pollutant adsorption from ambient air: the ones derived from SSZ-13 with higher Al pair content contain more cobalt(II) and thus, perform better as ambient-air NOx adsorbers before reaching full saturation capacity. Notably, Co(II)/SSZ-13 material with increased number of Al pairs is significantly more hydrothermally stable than its NaOH-derived analogue. Loading 1.7 wt% Pd into Co-SSZ-13 synthesized in the presence of Sr(II) produces a passive NOx adsorber (PNA) material that can be used for NOx adsorption from simulated diesel engine exhaust. The critical issue for these applications is hydrothermal stability of Pd-zeolites. Pd/SSZ-13 synthesized in NaOH media loses most of its PNA capacity after ~800 ⁰C hydrothermal aging in the flow of air and steam (10 hours in 10% H2O/air flow). The 1.7 wt% Pd/Co/SSZ-13 material with Si/Al ~10 does not lose its PNA capacity after extremely harsh aging at 850 and 900 ⁰C (10 hours in 10% H2O/Air flow) and loses only ~55% capacity after hydrothermal aging at 930 ⁰C. It shows considerably enhanced stability compared with previous record for Pd/FER, Pd/SSZ-39 and Pd/BEA materials that could survive hydrothermal aging no higher than 820 ⁰C. We herein reveal a new, simple, and scalable strategy for making remarkably (hydro)thermally stable metal-zeolite materials/catalysts with a number of useful applications.

The majority of harmful atmospheric CO and NOx emissions are from vehicle exhausts. Although there has been success addressing NOx emissions at temperatures above 250 °C with selective catalytic reduction technology, emissions during vehicle cold start (when the temperature is below 150 °C), are a major challenge. Herein, we show we can completely eliminate both CO and NOx emissions simultaneously under realistic exhaust flow, using a highly loaded (2 wt %) atomically dispersed palladium in the extra-framework positions of the small-pore chabazite material as a CO and passive NOx adsorber. Until now, atomically dispersed highly loaded (>0.3 wt %) transition-metal/SSZ-13 materials have not been known. We devised a general, simple, and scalable route to prepare such materials for PtII and PdII . Through spectroscopy and materials testing we show that both CO and NOx can be simultaneously completely abated with 100 % efficiency by the formation of mixed carbonyl-nitrosyl palladium complex in chabazite micropore.

Pd-promoted ternary oxides of the type Mn-Ce-Zr and Mn-Pr-Zr were characterized and evaluated for low temperature NOx storage applications such as diesel vehicle cold starts. While X-ray diffraction data were in all cases consistent with the formation of solid solutions, Mn/Zr and Mn/Ce(Pr) ratios found by XPS were consistently higher than the bulk values, indicative of an enrichment of Mn at the surface of the solids. Both sets of Pd-promoted mixed oxides showed remarkably high NOx storage efficiency in the range 80–160 °C, while a 1.8% Pd/Mn(27)-Ce(7)-Zr catalyst showed excellent NOx storage in simulated cold start experiments. Moreover, ramping the temperature to 370 °C in these experiments, simulating higher speed operation, resulted in near complete purging of stored NOx from the catalyst. NOx storage efficiency in isothermal storage experiments was found to improve with increasing Mn content for the 1%Pd/Mn(x)-Ce(7)-Zr series (x = 9, 18, 27 wt.%), DRIFTS measurements showing that relative to Ce-Zr mixed oxides, Mn incorporation favored NOx storage as nitrate. During temperature programmed desorption (TPD) two main desorption events were observed, corresponding to decomposition of nitrites (up to 200 °C), followed by loss of nitrates (⁓200–400 °C). Nitrates stored on Pd/Mn-Ce(Pr)-Zr mixed oxides desorbed during TPD at lower temperatures than for CeO2-ZrO2 mixed oxides, a finding attributed to the lower basicity of Mn compared to Ce. Hydrothermal aging of 1.8%Pd/Mn(27)-Ce(7)-Zr at 700 °C reduced NOx storage efficiency, although the catalyst was still able to store significant amounts of NOx. However, catalyst sulfation led to a large decrease in NOx storage efficiency and the efficiency could not be completely recovered with lean or rich desulfations at high temperatures. This project was funded by the National Science Foundation and the U.S. Department of Energy (DOE) under award no. CBET-1258742.

Model Pd/SSZ-13 with high dispersion of Pd ions (0.1 and 1 wt % Pd) was synthesized. The material was characterized with Fourier transform infrared (FTIR) and cryo-scanning transmission electron microscopy. Adsorption of NO leads to the formation of Pd(II)–NO and Pd(I)–NO complexes as well as NO+ species that replace residual H+ (extra-framework) sites. These nitrosyl species have notable thermal stability, with resistance to decomposition under high vacuum at 200 °C. Addition of molecular oxygen to NO-containing stream improves NOx storage of the Pd/H-SSZ-13. In particular, addition of O2 to NO slightly increases the amount of Pd(II)–NO complex with νNO at ∼1865 cm–1, whereas the low frequency νNO band at 1805 cm–1, assigned to Pd(I)–NO, decreases in intensity. Simultaneously, polydentate nitrate species appear in small amounts, contributing to the high temperature NOx release stage during a passive NOx adsorber (PNA) cycle. The concentration of NO+ (characterized by the broad infrared band centered at 2...

The uptake of NO in the presence of C12H26 (dodecane) and H2O over a Pd/SSZ-13 washcoated Passive NOx Adsorber (PNA) monolith is reported. When a co-feed containing C12H26 and NO is supplied to an unsaturated (with C12H26) sample, the NO uptake is unaffected but during the subsequent temperature ramp the release of trapped NO is delayed from 175°C to over 220°C. The release delay is beneficial for PNA performance as the primary NOx aftertreatment technology, Selective Catalytic Reduction (SCR), is not operated below 200°C. However, pre-saturation of C12H26 followed by the same NO and C12H26 co-feed results in a decrease in the NO uptake compared to the NO-only feed. We conjecture that C12H26 pre-adsorbed on the exterior surface of the sample blocks NO access to the pores, decreasing the number of available sites for NO uptake. Oxidation of C12H26 leads to the generation of partial oxidation product CO at lower temperatures ( [1] ] is upgraded to include C12H26 storage, release, and conversion. Through a systematic combination of judicious experiments, model tuning, and validation, we provide evidence for the underlying NO uptake and release mechanisms in the presence of C12H26.

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...

An Ag/MgO/ γ -Al2O3 catalyst is developed, which can perform the dual role of a passive NOx adsorber (PNA) at low temperatures and a NOx reduction catalyst at higher temperatures. The effect of H2O and CO2, as well as various reductants such as H2, NH3, and C3H6 on its deNOx performance is investigated. The NOx storage is comparable to that obtained by Pd-based PNAs, and occurs through a spillover mechanism. The presence of H2 remarkably increases the low-temperature NOx adsorption and binding strength, and is attributed to the significant formation of nitrates as opposed to the predominant formation of nitrites in the absence of H2. Moreover, the positive effect of H2 is not related to the H2-SCR activity of the catalyst or an enhancement in the NO oxidation activity. C3H6 inhibits NOx storage, but the presence of H2 promotes it considerably. A significantly high NH3-SCR activity with negligible N2O formation is measured at 150 °C, while C3H6-SCR activity is exhibited at higher temperatures. However, the presence of NH3 does not affect the binding of NOx on the various adsorption sites, which is primarily governed by the presence of H2.

Uniform SSZ-39 (Si/Al ratio ∼12) crystals with an average size of about one micron were synthesized and used to support Pd (0.7–3 wt%) for PNA. The as-synthesized materials were characterized by FTIR, XRD, Helium Ion Microscopy, HAADF-STEM imaging, 27Al, 29Si, and 1H-29Si CP solid state NMR spectroscopic techniques. FTIR studies with CO and NO probe molecules reveal that the majority of Pd is dispersed as isolated Pd(II) and Pd(II)−OH centers and as such is suitable as a low-temperature passive NOx adsorber. Pd(II)-NO, Pd(II)(OH)(NO), and Pd(II)(CO)(NO) complexes form in this material during PNA. Comparison to Pd/SSZ-13 (Si/Al ∼12) shows the superior hydrothermal stability of this new material, surviving hydrothermal aging up to 815 °C in 10 % H2O/Air for 16 h without a significant loss of activity. The SSZ-39 crystal structure remains intact during hydrothermal aging up to 1000 °C confirmed by XRD and HAADF-STEM imaging/EDS mapping. However, changes to the framework, as evidenced by high-field 27Al NMR, during such severe hydrothermal treatment significantly alter the NOx release profiles. Besides PNA, this hydrothermally stable material (3 wt% Pd on SSZ-39; Si/Al ∼12) can be used as a robust methane combustion catalyst under practically relevant conditions (i.e., GHSV∼300 L/g*hr). This catalyst shows minimal deactivation after both hydrothermal aging at 750 and 800 °C and prolonged time on stream (105 h) at 425°C. In contrast, both 3 wt% Pd/alumina and 3 wt% Pd/SSZ-13 lose a significant portion of their activity under such conditions, marking an improvement over current technology.

A series of Co-based oxides were synthesized and investigated as passive NOx adsorbers (PNAs). The morphology variation of Co3O4 oxide from nanoparticle to flake shape enhanced the NOx storage efficiency (NSE) and improved the NOx desorption capability. The enhanced NSE of the flake-like sample is related to its abundant surface oxygen species and large surface area. DRIFTS and CO2-TPD experimental results indicated that Co3O4-flake and Co7Ni3Ox-flake samples with moderate basicity could store NOx as nitrite species, which facilitated NOx desorption at lower temperatures.

Pt- and Pd-promoted CexZr1–xO2 mixed oxides were characterized and investigated for passive NOx adsorber applications. X-ray diffraction analysis revealed a phase transition from tetragonal to cubic with increasing cerium content in CexZr1–xO2, while H2 and CO chemisorption data in all cases indicated average Pt and Pd particle sizes of close to 2 nm. H2-temperature programmed reduction (TPD) measurements revealed a shift of the Pt reduction peak to higher temperature with increasing Ce content, consistent with a corresponding increase in the degree of Pt oxidation. According to microreactor data, doping Ce into the ZrO2 lattice resulted in a significant improvement in low temperature (80–160 °C) NOx storage efficiency. Diffuse reflectance infrared Fourier transform spectroscopy measurements on Pt/CexZr1–xO2 showed that as Ce content increased, relatively more nitrite species were generated during NOx storage. However, oxidation of nitrite to nitrate during subsequent NOx-TPD—increasing the concentration ...

Pd supported on various zeolites has been found to be able to adsorb NO at temperatures below 200 °C with high storage efficiency and capacities in a gas feed which simulate diesel engine exhaust conditions. Highly dispersed Pd at the exchange sites of the zeolites are believed to be the active site for the low temperature NO storage. Different framework structures of the zeolites significantly affect not only the NO storage capacity but also the NOx desorption temperature. These materials show high potential for low temperature NOx emission control.

Pd/HZSM-11 prepared by the incipient wetness impregnation method was applied as a passive NOx adsorber (PNA) at low temperatures. The NO storage and release properties of Pd/HZSM-11 have been investigated by cyclic operation. It was found that the NO storage capacity of Pd/HZSM-11 at 100 °C significantly increased with repeated adsorption-release cycles. The NOx storage capacity increased by ~49% between the first and 5th adsorption-release cycle. Encouraged by these results, the fresh sample was pretreated in the feeding reaction gas for 2 h. This pretreatment resulted in highly enhanced NO storage capacity, comparable to the storage capacity of the sample after 5 adsorption-release cycles. The nature of Pd species in the sample has been characterized by XPS, NH3-TPD, N2 adsorption, and FTIR with CO titration. Pretreatment by O2 or NO + O2 at 500 °C led to highly dispersed Pd (II) species in the zeolite that were responsible for the capture of NO at low temperatures, thereby contributing to the enhanced NO storage capacity.

A combined experimental and modeling study of the passive NOx adsorber (PNA) is presented that advances the understanding and prediction of the effects of various operating parameters and material properties of H-ZSM-5 and Pd/H-ZSM-5. Experiments reveal the NOx uptake process to be kinetically limited. Two microkinetic schemes are developed to explain the NOx uptake and release and NO2 generation data. Both schemes are incorporated into a 1 + 1 D monolith model to predict and validate NOx uptake and temperature programmed desorption (TPD) data. Scheme I involves Z - [ P d O H ] + , Z - P d 2 + Z - a n d Z - P d + with Z - P d + as the strongest NO binding site. Scheme II involves reduction of PdO2 to PdO and nitrate formation. Model tuning utilizes a combination of infrared spectroscopy measurements and density functional theory (DFT) estimates of energy barriers. The model is validated at different conditions including feed parameters and Pd loading.

Different zeolite-based materials have been commercialized as efficient catalysts for the selective catalytic reduction (SCR) of NO using ammonia as a reducing agent. However, these materials show limited catalytic activity at low exhaust temperatures, which are achieved during warming up of the engine (>cold start> conditions). To overcome this limitation, inclusion of a passive NO adsorber (PNA) as a new component of the emission-control system has been proposed. The PNA can adsorb NO species during the cold start at low temperatures and, afterwards, permits their release at higher temperatures, at which point the SCR catalyst can complete the NO reduction. In this review, we discuss the most relevant PNA materials, including those based on amorphous oxides and zeolites, together with fundamental understanding of the NO adsorption-desorption mechanisms associated with PNA materials. The ability to control the dispersion and nature of the metals, the reducibility of the amorphous oxide supports, or the physicochemical properties of the zeolites can be key parameters in the design of more active and stable PNA materials., This work has been supported by Umicore, the European Union (ERC-AdG-2014-671093 (SynCatMatch)), Spanish Government-MINECO (Severo Ochoa; SEV-2016-0683 and MAT2015-71261-R) and by Fundación Ramón Areces through a research contract of the “Life and Materials Science” program.

Low-temperature operation of urea/NH3-SCR converters, required by recent emission regulations, is challenging due to technological limits related to poor catalytic activity, difficult urea decomposition and undesired ammonium nitrate formation. Herein, we present a novel concept of a catalytic device, which adds a NOx storage functionality to a typical SCR catalyst. This enables capture and storage of NOx in lean exhaust streams already at room temperature, with significant efficiencies under dry conditions, as well as direct reduction of the stored NOx at higher temperatures. The proposed system (AdSCR = adsorption + selective catalytic reduction) is intrinsically different from the well-known existing NOx storage reduction (LNT) + SCR systems. In fact, it is free of PGM materials and does not require lean–rich cycling, being operated just like an SCR catalyst. It has therefore potential advantages both in terms of costs and flexibility of the aftertreatment configuration: in principle, it can be integrated into conventional SCR converters, leading to improved deNOx efficiencies without additional modifications of the aftertreatment architecture.

Future emission standards are becoming increasingly stringent as California will phase in LEV III regulations starting in 2015 and EPA will implement Tier 3 regulations starting in 2017. Current vehicles emit greater than 50% of targeted tailpipe emissions during the first minute or two following “cold-start” mainly due to the ineffectiveness of existing catalytic converters at low temperatures (

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.

The lean burn technology has been paid attention recently by more and more people because the economics and emission of the gasoline engine can been improved extensively by lean burn technology. However, the reduction of NOx emission is quite small because of the rich oxygen. The different combination schemes of NOx adsorber and Three Way Catalyst were adopted in this paper and experiments of reduction NOx emission have been studied in an modified EFI 16 valves lean burn gasoline engine . The experiment results show that the scheme of the position of Three Way Catalyst is before the NOx adsorber Catalyst was the best scheme of reduction NOx emission in lean burn gasoline engine.

Spatial- and time-resolved temperature distributions over a Pt/Ba/Al2O3 model NOX storage/reduction (NSR) catalyst were measured using infra-red thermography. The heat generated during regeneration was correlated to surface nitrate reduction, thereby revealing the concentration of surface nitrates at specific locations along the catalyst. The results demonstrate that there is more nitrate formation at upstream positions relative to downstream, or from front to back of the catalyst, with short trapping times. However, as more NOX was trapped on the catalyst during longer trapping times, it was found that the largest amount of NOX was trapped slightly downstream of the inlet, evolving to a local maximum in amount trapped. Applying infrared (IR) thermography to this system resulted in a spatially resolved calorimetry method via the correlation of temperature to the distribution of sorbed nitrate species along the catalyst.

Commercially available lean NOx catalyst and lean NOx adsorber samples were tested in a synthetic exhaust gas bench with DME as reducing agent. The LNA catalyst gave a very high NOx conversion (> 80 %) over a wide temperature range (275–450°C), and with low concentration of by-products such as NH3, N2O and methanol. The Cu-zeolite based LNC catalyst, however, showed practically no NOx conversion.

Diesel engine particle emissions during transient operations, including emissions during FTP transient cycles and during active regenerations of a NOx adsorber, were studied using a fast Engine Exhaust Particle Sizer (EEPS). For both fuels tested, a No. 2 certification diesel and a low sulfur diesel (BP-15), high particle concentrations and emission rates were mainly associated with heavy engine acceleration, high speed, and high torque during transient cycles. Averaged over the FTP transient cycle, the particle number concentration during tests with the certification fuel was 1.2e8/cm3, about four times the particle number concentration observed during tests using the BP-15 fuel. The effect of each engine parameter on particle emissions was studied. During tests using BP-15, the particle number emission rate was mainly controlled by the engine speed and torque, whereas for Certification fuel, the engine acceleration also had a strong effect on number emission rates. The effects of active regenerations of...

A method to quantify DRIFT spectral features associated with the in situ adsorption of gases on a NOx adsorber catalyst, Pt/K/Al2O3, is described. To implement this method, the multicomponent catalyst is analysed with DRIFT and chemisorption to determine that under operating conditions the surface comprised a Pt phase, a pure γ-Al2O3 phase with associated hydroxyl groups at the surface, and an alkalized-Al2O3 phase where the surface –OH groups are replaced by –OK groups. Both DRIFTS and chemisorption experiments show that 93–97% of the potassium exists in this form. The phases have a fractional surface area of 1.1% for the 1.7 nm-sized Pt, 34% for pure Al2O3 and 65% for the alkalized-Al2O3. NO2 and CO2 chemisorption at 250 °C is implemented to determine the saturation uptake value, which is observed with DRIFTS at 250 °C. Pt/Al2O3 adsorbs 0.087 μmol CO2/m2and 2.0 μmol NO2/m2, and Pt/K/Al2O3 adsorbs 2.0 μmol CO2/m2and 6.4 μmol NO2/m2. This method can be implemented to quantitatively monitor the formation of carboxylates and nitrates on Pt/K/Al2O3 during both lean and rich periods of the NOx adsorber catalyst cycle.

Sorption of SO2 over a commercial SOx adsorber was studied at a laboratory scale using a fixed bed reactor. The adsorbent material contains mainly alumina, barium oxide and precious metals. Sulphates formed by SO2 sorption are reduced by carbon monoxide or hydrogen in the presence of water. The main species emitted during the reduction was H2S. The species COS was emitted at a very low level when carbon monoxide was used as a reducing agent. The total removal of sulphur was not possible in the absence of water, the sulphur remaining bonded to platinum (or palladium) sites. The role of water is to hydrolyse these sulphides, leading to H2S or COS. The species SO2 was also emitted when an aged catalyst was used, without a loss of the catalytic reduction efficiency. After a first sequence adsorption–regeneration, the adsorption capacity was not affected and the second regeneration was more efficient. Such a SOx adsorber can be placed upstream a NOx adsorber to avoid poisoning of this latter by SO2.

Apart from the NOx adsorber catalyst, which has already been introduced in series for direct injection spark ignition engines, SCR technology represents an interesting solution for reducing NOx emissions from diesel vehicles. However, this requires a reductant such as urea as an additional operating medium. From today’s point of view, a wide-scale provision of the frequently discussed aqueous urea solution does not appear feasible in the short term. Therefore, the use of a solid reductant such as ammonium carbamate, with its considerably lower space requirements, presents an interesting alternative for individual motor vehicles. This article describes the solid SCR system developed by FEV Motorentechnik GmbH, as well as the first vehicle tests.

The use of an NOx adsorber catalyst, which has already become a standard component in direct-injection spark-ignition engines, poses a special challenge in diesel engines. This article presents the results of experiments on an NOx adsorber catalyst in a passenger car diesel engine carried out by the Lehrstuhl fur Verbrennungskraftmaschinen of the RWTH Aachen in cooperation with FEV Motorentechnik GmbH.

International audience; To ensure overall optimization of engine performance (NOx, particulates, efficiency), the use of a NOx after-treatment system appears necessary to meet the future Euro IV emissions standards and after. Among the known means, the NOx trap should offer an efficiency of the same order than the reduction by urea, without having the same constraints. The trap operates on the basis of the alternation of operating phases with a lean mixture during which NOx is stored and rich phases during which nitrates are destored and treated. Important aspect of the technology, NOx adsorbers are very sensitive to sulfur poisoning and future fuel standards are unlikely to be sufficient to prevent the system from requiring periodic desulfation procedures. The present paper presents results obtained on a light duty application, on a one hand, on a heavy duty case on a second hand. In the light duty front, the purpose of the work presented first, was to study the effects of low fuel sulfur content such as 50 ppm and 10 ppm on the NOx adsorber efficiency for a Diesel application. Through this study, the influence of the substrate cell geometry has also been assessed. The use of a 10 ppm sulfur fuel is not enough to maintain, at a high level, the NOx adsorber performance during a 40 000 km aging test. The desulfation criterion (efficiency loss of 30%) is reached after the first 16 000 km. However, the desulfation operation is not enough to recover the initial catalyst performance and the poisoning velocity increases as the catalyst ages. The hexagonal cell substrate catalyst is less sensitive to sulfur poisoning than a square cell substrate catalyst so that its desulfation frequency is much lower. With a 10 ppm sulfur fuel, 4 desulfation operations were achieved with a hexagonal cell catalyst as opposed to 11 with a square cell catalyst during the 40 000 km aging test. In the heavy duty application, the investigation, using a single-cylinder engine, demonstrated that the best NOx emission/fuel consumption trade-off are obtained with destorage richer than stoichiometry. These destorage conditions could be obtained in various configurations (injection timing, EGR, post-injection). The various adjustments tested show the advantage of producing a large amount of carbon monoxide to obtain fast destorage and efficient treatment of the trapped nitrates. With a method like this, the extra fuel consumption resulting from the succession of rich/lean phases is around 3% for an emission objective of NOx < 3. 5 g/kW·h. By mounting a second trap in parallel, an oxidation catalyst, or using EGR, this result can be significantly improved. The gradual storage of sulfur contained in fuel limits the trapping capacity of the NOx system, necessitating periodic desulfation. Various configurations have been tested to purge the sulfates. Mass spectrometer exhaust gas analysis makes it possible to quantify COS, H2S and SO2 emissions during this phase. Nevertheless, the tests have shown the limits of the possibilities in a single cylinder installation.

One possible way to reduce NO x in lean exhausts is by using NO x trap catalysts. This paper addresses storage of NO x on such catalysts at temperatures below the catalyst light-off. Experiments carried out on commercial samples in synthetic exhausts revealed a large capacity for storage of NO x when NO2 was added at temperatures below 150°C. In contrast, when NO was added instead, no storage took place. CO was found to decrease the storage by reacting with NO2 and forming NO and CO2. Propene inhibited the reaction between NO2 and CO and therefore gave rise to larger NO x storage when CO was present. The paper concludes with a discussion of a possible mechanism for the storage of NO x at low temperatures.