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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.

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.

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.

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.

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.

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.

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.

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.

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.

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.