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The concept of NOx adsorbers has been developed based on acid-base washcoat chemistry. It involves storage of NOx on the catalyst washcoat during lean exhaust conditions and release during rich operation and/or increased temperatures. Depending on the NOx release strategy, NOx adsorber systems can be classified as:
In active NOx adsorbers, stored NOx is periodically released—with a typical frequency of about once per minute—during a short period of rich air-to-fuel ratio operation, called NOx adsorber regeneration. The released NOx is catalytically converted to nitrogen, in a process similar to that occurring over three-way catalysts (TWC) widely used in stoichiometric gasoline engines. Normally, three-way catalysts are inactive in converting NOx under lean exhaust conditions, when oxygen is present in the exhaust gas. By alternating the lean storage and rich release-and-conversion phases, the applicability of the three-way catalyst has been extended to lean burn engines. The technology was first commercialized on gasoline direct injected (GDI) engines, followed by light-duty diesel engines around 2007/2009 (US Tier 2, Euro 5). NOx adsorber systems have also been introduced for NOx control from stationary natural gas turbine applications .
Due to their declining NOx reduction performance at higher exhaust temperatures, active NOx adsorbers found only very limited application on heavy-duty truck engines. Considering the trends in light-duty emission regulations, the use of active NOx adsorbers can be also expected to decline in future light-duty vehicles. Increasing focus on in-use emissions and the expected introduction of real driving emissions (RDE) testing requirements in the EU will pose a challenge for the NOx adsorber technology—high NOx conversions may be required at operating conditions outside of the regulatory test cycle, including high engine load operation.
Passive NOx adsorbers (PNA)—a more recent and simpler variant of the technology—adsorb NOx during vehicle cold start and release it when the exhaust temperature increases—without a rich regeneration—to be converted over a downstream NOx reduction catalyst. Hence, passive NOx adsorbers (or traps) are not a stand-alone NOx control technology—rather, they can be used with urea-SCR aftertreatment to improve the low temperature performance of the system. An early demonstration of PNA technology was conducted by Cummins on their 2.8 L US Tier 2 Bin 2 diesel engine developed under the US DOE ATLAS project . Passive NOx adsorbers may also find application on heavy-duty diesel engines meeting future stringent NOx limits on the order of 0.05-0.02 g/bhp-hr .
It should be noted that “part-time”, active NOx adsorbers have been also used to control cold start/low temperature NOx emissions in some light-duty diesels with urea-SCR systems. A close-coupled, actively regenerated NOx adsorber is used during cold start. Once exhaust temperatures increase, NOx is reduced over the SCR catalyst using urea. This and other configurations of emission systems with NOx adsorbers are discussed in the paper on NOx adsorber applications.
Terms. Different authors use different terms when discussing (active) NOx adsorbers, such as:
All these names are synonyms describing the same emission control technology. The term lean NOx catalyst, on the other hand, refers to the selective catalytic reduction of NOx by hydrocarbons—an entirely different technology that should not be confused with NOx adsorbers.
Definitions. We should also introduce the basic definitions related to the process of adsorption (these terms are confused in some NOx adsorber literature):
At lower temperatures, adsorption is usually caused by intermolecular forces; it is then called physical adsorption. At higher temperatures, above about 200°C, the activation energy is available to form chemical bonds; if such mechanism prevails, the process is called chemisorption.
Adsorption and desorption of NOx on the active NOx adsorber is primarily related to lean and rich cycling during vehicle operation. NOx adsorption occurs when the engine is run in a lean-burn mode. Desorption with accompanying reduction, also referred to as the regeneration, occurs during rich air-to-fuel mixture excursions. The outlet NOx concentration during an example adsorption/regeneration cycle is illustrated in Figure 1 .
It is evident that NOx absorber operation is inherently transient in character. In contrast to conventional emission control catalysts, the steady-state catalytic activity cannot be defined. This presents a major complication in evaluating NOx adsorbers, as conventional steady-state catalytic tools and techniques cannot be easily applied. The overall system performance is determined by chemical reactions confounded by storage and release processes, the effects of which are very difficult to separate. The cyclic operation also complicates comparison of literature data, as the results depend not only on the catalyst formulation and gas conditions, but also on the details of the cycle, including duration of the lean and rich phases and characteristics of the rich event.
Diesel and Gasoline Application. Quite clearly, the concept of periodic regeneration is very compatible with “partial lean burn” GDI engines, where the engine controller can easily provide an enriched mixture when needed. With electronically controlled fuel and air systems, it is also possible to periodically enrich the air-to-fuel mixture in the diesel engine in order to facilitate the regeneration of diesel NOx adsorbers. As an alternative enrichment strategy, diesel fuel can be injected directly into the exhaust system. Because of the frequent regeneration, NOx adsorbers require an unparalleled degree of integration with the engine management system, which practically limits their application to new engines and vehicles—NOx adsorbers are not particularly suitable for retrofit emission control systems.
There are many similarities between NOx adsorber catalysts used in lean burn gasoline and diesel engine applications, but there are also important differences that affect the catalyst system design. The two applications are compared in the following table and in Figure 2 .
|A/F ratio||Partially lean (lean A/F 20-35)||Continuous lean (A/F 20-55)||GDI is operated stoichiometric at high speed/load conditions.|
|Catalyst functionality||NOx adsorber + 3-way catalyst||NOx adsorber only||Due to the partial stoichiometric operation, gasoline adsorbers must be also good 3-way catalysts of wide A/F catalyst window, high HC conversion, etc.|
|Rich regeneration||Easy||Difficult||Rich operation of diesel is problematic, may cause engine oil dilution and high PM emission.|
|Fuel economy penalty||Lower||Higher|
|Operating temperature||250 - 600°C||150 - 550°C||Diesel adsorbers must be optimized for low temperature performance, which may require different formulations and higher adsorber loadings .|
|Reductants||CO, HC, H2||HC, CO, H2|
|Durability required||800 - 850°C||650 - 700°C|
Depending on the fuel quality, diesel adsorbers may also require higher sulfur tolerance than gasoline adsorbers. However, considering the great sensitivity of this technology to sulfur, NOx adsorbers require ultra low sulfur or “sulfur-free” fuels in both gasoline and diesel applications.
The NOx adsorption/reduction mechanism is illustrated in Figure 3. The catalyst washcoat combines three active components: (1) an oxidation catalyst, for example Pt, (2) an adsorbent, for example barium oxide (BaO), and (3) a reduction catalyst, for example Rh.
The overall cycle of active NOx adsorber operation is often described by the following five steps (see also Figure 1) :
(1)NO + ½O2 = NO2
(2)BaO + NO2 + ½O2= Ba(NO3)2
(3a)Ba(NO3)2 = BaO + 2NO + 1½O2
(3b)Ba(NO3)2 = BaO + 2NO2 + ½O2
(4)NO + CO = ½N2 + CO2
The above set of reactions allows for an understanding of the basic NOx adsorber chemistry, but the actual chemical and physical processes are more complex and not fully explained. A more detailed analysis should also include other reaction paths and species, for example barium carbonate and barium hydroxide which coexist with barium oxide on the catalyst surface . Describing the NOx release and reduction as two separate, consecutive processes—steps 4 and 5—is another simplification. It has been reported that the reduction of stored NOx may occur according to a Pt-catalyzed surface reaction, which does not involve the thermal decomposition of the adsorbed NOx species as a preliminary step . The exact forms of NOx storage on the surface are also being debated, with the discussion covering both the chemical (nitrates vs. nitrites) and physical (surface vs. bulk) aspects of NOx adsorption. In-depth discussion of reaction mechanisms in NOx adsorbers can be found in the literature .
NOx adsorbers also show some undesired reactivity, primarily in regards to sulfur compounds which are present in exhaust gases from both diesel and gasoline engines. Reactions of sulfur are basically equivalent to the reactions of NOx, Equation (1) - Equation (3), and can be written as follows:
(5)SO2 + ½O2 = SO3
(6)BaO + SO3 = BaSO4
First, sulfur dioxide is oxidized to sulfur trioxide over platinum, Equation (5). Then, the SO3 reacts with BaO to form barium sulfate, Equation (6). This causes gradual saturation of the barium sites with sulfur and loss of activity towards the adsorption of NO2. BaSO4 can be thermally decomposed in a process equivalent to reaction Equation (3). However, sulfates of barium or other adsorbents are more stable than the corresponding nitrates and require higher temperatures to desulfate. For this reason, sulfur deactivation is a major problem in the development of NOx adsorber systems.
A number of other reactions are possible in NOx adsorbers that can produce secondary, unregulated emissions. These are mainly reduction processes also known to occur in the three-way catalyst, which involve NO/NO2, as well as SO2, and generate products other than nitrogen—the main species include ammonia (NH3), nitrous oxide (N2O), and hydrogen sulfide (H2S).
Active NOx adsorber systems are derived from a conventional three-way catalyst which additionally incorporates a NOx sorbent. The storage components in NOx adsorbers are typically compounds of the following elements :
While most catalysts incorporate sorbents in the form of oxides, a number of defined systems, such as perovskites or metal substituted zeolites, have also been tested .
Different adsorbing elements, their chemical compounds and/or mixtures yield catalysts of different properties, such as different NOx storage capacity, thermal stability of the nitrate and its desorption temperature, susceptibility to sulfur poisoning, or sulfur desorption (desulfation) temperature. These properties are important in designing the catalyst system and its regeneration strategy, as will be discussed later. The basicity of the component is related to the NOx trapping performance by being directly related to the reaction equilibrium constants for the storage reaction. The adsorbent performance was found to decrease in the following order: K > Ba > Sr ≥ Na > Ca > Li ≥ Mg when tested at 350°C .
Alkali metals, such as potassium, exhibit superior NOx adsorption performance at high temperatures relative to barium, as illustrated in Figure 4 . If sodium, potassium, and/or cesium are incorporated into NOx adsorber washcoat containing barium, they increase NOx conversion at higher temperatures. Alkali metal oxide adsorbers (e.g., up to 50% Na in alumina washcoat) were also reported to show superior resistance to sulfur poisoning .
It may be noted that the superior high temperature performance of alkali metal sorbents is in apparent contradiction to the properties of bulk components, as barium nitrates have greater stability than alkali metal nitrates, both regarding melting point and temperature of decomposition with release of NOx—for instance, KNO3 decomposes in air at 400°C, while Ba(NO3)2 at 590°C . This indicates that the NOx reduction step may indeed be occurring without prior decomposition of the stored nitrate.
Most of commercial NOx adsorbers for both gasoline and diesel applications are based on one of the following systems:
Ba + alkali metal adsorbers provide superior NOx conversion at temperatures above 250-300°C, but exhibit a certain NOx conversion penalty at lower temperatures . Ba + alkali metal adsorbers also offer better nitrate stability, as well as better sulfur poisoning resistance than Ba-only adsorbers, with higher NOx conversion after repeated poisoning and desulfation cycles. Among all alkali metals, Cs is most effective in enhancing NOx conversion, followed by K, Na, and Li, Figure 5 .
The disadvantages of systems with alkali metals include: (1) the lower NOx conversion at low temperatures, (2) poor hydrocarbon conversion, which is lower in comparison to Ba adsorbers at all temperatures, and (3) a potential negative impact on the durability of platinum catalysts by promoting noble metal sintering.
Due to the importance of high temperature performance in spark ignited engines, the Ba + alkali metal system became the NOx adsorber of choice in gasoline applications . Adsorbers for light-duty diesel engines, which operate at lower temperatures, may avoid the necessity of using alkali metals. Ba + alkali metal formulations, however, would be likely preferred in heavy-duty diesel applications .
Sorbent Leaching. Another issue with alkali metal materials is the high mobility of water-soluble alkali metal compounds including nitrates (the solubility of KNO3 is 13.3 g per 100 g of water at 0°C, and 246 g at 100°C). In catalysts, alkali metals can be leached by water and eventually migrate away from the adsorber washcoat. Due to their strong affinity to silicon oxide, a component of cordierite, alkali metals accumulate within the ceramic catalyst substrate, causing loss of its physical strength . Cs was found to have the highest migration intensity, followed by K and Na, as illustrated in Figure 6 . The alkali metal maps shown below were obtained by analyzing (scanning electron microscopy) aged catalyst samples of the same molar amount of metals. The Cs map shows that Cs migrated throughout both the cordierite substrate cell corner and cell wall. Lower cesium concentration was detected on the washcoat after aging. A significant amount of potassium migrated away from the washcoat and accumulated at the interface of washcoat and cordierite substrate. Little K was detected at the center region of the cell corner. In contrast to Cs, Na is concentrated at the interface of cordierite substrate and washcoat, with little Na found in middle of the cell wall and the center of the cell corner. After aging at 900°C, Cs and K containing NOx adsorbers showed skin cracking while the Na containing adsorber was free of any visible cracking.
Since alkali metals that are more mobile have also better NOx conversion effect, catalyst designers face the difficult task of optimizing their formulations to achieve the required NOx performance, while minimizing the chemical attack on the cordierite substrate. Catalysts and substrates with barrier Si coatings have been proposed to minimize the impact of alkali metals . NOx adsorbers coated on metallic substrates have also been tested . However, since material durability issues were not included in these studies, it is uncertain if metallic substrates would be sufficiently resistant to the attack from alkali metals.
Sorbent leaching is not limited to alkali metals. It also affects alkaline earth materials, including barium (the solubility of Ba(NO3)2 is 5 g per 100 g of water at 0°C, and 34.2 g at 100°C). While not necessarily destructive to the substrate, leaching of alkaline earths may result in a re-distribution and/or loss of sorbent, leading to a deactivation of the NOx adsorber.
Alumina. NOx adsorber washcoats are typically based on γ-Al2O3, which is also the main washcoat material in gasoline three-way catalysts. A common reference NOx adsorber catalyst consists of Pt-BaO/Al2O3 with a Ba loading in the range of 4-20%. One of the degradation pathways in this system is the crystallization of BaAl2O4 under lean conditions at temperatures above about 700-800°C, which results in sintering of Ba. Mixed Ba-alumina oxides have been proposed which were designed to minimize thermal deterioration of NOx adsorbers .
Ceria. Many diesel NOx adsorbers also include ceria in the catalyst washcoat. In three-way catalysts for SI engines, high loadings of ceria (often stabilized with zirconia) are typically used to provide oxygen storage capacity (OSC). By storing oxygen at lean and releasing it at rich condition, the OSC component is able to extend the three-way catalyst window. In addition to the OSC ability, the functions of ceria in diesel NOx adsorbers may also include :
A negative effect of the OSC capability in diesel applications may be an additional fuel economy penalty during regeneration due to the extra amount of enrichment necessary to react with the stored oxygen.
Other Oxides. Other common washcoat materials used in NOx adsorbers include titania (TiO2) and zirconia (ZrO2). Acidic oxides, such as TiO2, are unable to store sulfur oxides as effectively as high surface area alumina, thereby enhancing sulfur resistance of NOx adsorbers. For this reason, titania was incorporated in NOx adsorbers for the Toyota D-4 GDI engine in Japan in 1996 . Further addition of Rh/ZrO2 to BaO-based adsorbers was found to catalyze the water shift reaction with an increased generation of hydrogen and, as a result, improved desorption of sulfur . The use of titania and zirconia was further investigated with NOx adsorbers utilizing Ba + alkali metal system (Ba/K) . While the acidic TiO2 was confirmed to enhance sulfur resistance and to promote sulfur desorption at lower temperatures, it was also found to reduce the high temperature stability of KNO3, thus defying the function of alkali metals and causing poor NOx performance at higher temperatures. The more basic oxide ZrO2, on the other hand, enhanced the thermal stability of KNO3, but its sulfur poisoning resistance was far lower than that of TiO2. A complex oxide catalyst, designed to combine the sulfur resistance of titania systems with the alkali adsorber stability of zirconia systems, was developed and commercialized in 2001 on the D-4 GDI engine . Ce/Zr/Y washcoats have been introduced in commercial diesel applications by Toyota to prevent platinum sintering .
HC Traps. Nissan announced development of a NOx adsorber which incorporated HC traps—presumably zeolite-based—to enhance NOx regeneration. The catalyst—intended for a Tier 2 Bin 2 diesel car that Nissan had planned to launch in North America in 2009—had a three-layer architecture: a hydrocarbon trap layer, a NOx adsorber layer, and a NOx reduction layer at the surface. During lean operation, hydrocarbons were stored in the HC trap layer, and NOx was stored in the NOx adsorber layer. During the rich regeneration phase, the stored HCs were transformed into H2+CO gas, which in turn was used to reduce the stored NOx to nitrogen.
Noble metals have two roles in NOx adsorbers: (1) oxidation of NO to NO2, and (2) reduction of released NOx to N2 during regeneration. Platinum is the most active catalyst for NO oxidation, Equation (1). Since this step is critical for NOx storage, platinum plays an important role in adsorber catalysts. It may be noted that the Pt catalyst shows certain unexpected (and unexplained) behavior in NOx adsorbers . First, there appears to be a significant negative effect on the rate of NO oxidation over Pt due to the presence of Ba. Second, there is a strong impact of the Pt particle size, with higher Pt dispersions, or small particle sizes, not always leading to the highest rates. Rather, there is apparently a maximum in conversion attained as Pt particle size increases. Such an effect would suggest that as Pt sinters, NO oxidation would initially increase.
Rhodium is used as the NOx reducing catalyst in most adsorbers. Trimetal formulations (Pt/Pd/Rh), common in stoichiometric 3-way catalysts, can be also used in NOx adsorbers, as long as their platinum content remains relatively high. For instance, Delphi reported two trimetal formulations with precious metal loadings of 80 g/ft3 (60:15:5) and 100 g/ft3 (70:20:10) . The catalysts were designed for GDI engines, where they may be required to perform as typical 3-way catalysts (i.e., under stoichiometric conditions) over prolonged periods of time; it is not clear if palladium could offer a benefit in diesel adsorbers.
A two-layer catalyst structure with separate Pt/Ba and Rh layers was claimed to minimize NOx slip during NAC regeneration . The bottom layer included Pt/Pd on alumina/ceria and Ba. The top layer included Rh supported on zirconia. The catalyst was intended for diesel engine applications. In one example, a 400 cpsi cordierite substrate was first coated with 94 g/ft3 Pt and 19 g/ft3 Pd on a washcoat composed of 2 g/in3 alumina with dispersed ceria, 1 g/in3 ceria and 400 g/ft3 Ba. The second layer consisted of 10 g/ft3 Rh supported on 0.5 g/in3 85% wt. zirconia doped with rare earth elements.
NOx adsorber catalysts can exhibit high NOx conversion efficiencies, in excess of 80-90%, as illustrated by example data in Figure 7 . The activity of NOx adsorbers covers a temperature window that extends from about 200°C to about 450°C. In diesel NOx adsorber systems, the maximum performance typically occurs in the 350-380°C range .
The lower end of the temperature window, T1, is determined by the catalyst activity in regards to the oxidation of NO to NO2, as well as NOx release and reduction (3-way function). The upper temperature, T2, is related to the thermodynamic stability of nitrates, which undergo thermal decomposition at higher exhaust temperatures—even under lean conditions.
The NOx adsorber temperature window extends over an area of low- and medium-load operation of the diesel engine. Therefore, NOx adsorbers are most suitable for light-duty, chassis-certified applications—NOx adsorber systems have been used in vehicles tested over both the European NEDC and the US FTP-75 cycles. On the other hand, the technology is not particularly suitable for heavy-duty engines or for applications where conformance testing may include high engine load operation.
Various approaches have been suggested to expand the NOx adsorber operating temperature window. In gasoline engines, the NOx adsorber performance was reported to be enhanced by a three-way catalyst positioned upstream, in a configuration typical for many GDI studies . In some diesel NOx adsorbers, NOx conversion efficiency could be improved by positioning an oxidation catalyst upstream of the adsorber .
Toyota Di-Air Concept. Evolution of intermediate chemical species during the adsorption/regeneration cycling has a major effect on the NOx conversion performance. The NOx reduction performance is enhanced by HC-based intermediate reductants that can be introduced through rapid, oscillated injection of hydrocarbons upstream of the NOx adsorber during the lean adsorption phase . The partially oxidized HCs react with intermediates generated from adsorbed NOx over the catalyst. Based on this phenomenon, Toyota proposed the Diesel NOx aftertreatment by Adsorbed Intermediate Reductants concept—abbreviated Di-Air or DiAir. The Di-Air strategy not only extended the NOx adsorber temperature window, but also improved the thermal and sulfur durability. The strategy also showed advantages during desulfation —the injection of HCs into the exhaust gas allowed to extend the rich conditions during desulfation to a wider range of engine speed and load.
High frequency hydrocarbon pulsing (through short-cycle propene injection) was also investigated with a dual layer catalyst, where a layer of a zeolite SCR catalyst was coated on top of a NOx adsorber catalyst . The short-lived intermediates generated in the NOx adsorber layer were utilized in the SCR layer for incremental NOx conversion via HC-SCR mechanisms. The light-off temperature was reduced by 50°C compared to a conventional NOx adsorber catalyst.
During the adsorption cycle, the adsorber is gradually converted into its nitrate form (e.g., barium nitrate) and the adsorption capacity becomes saturated. At this time the stored NOx needs to be released and catalytically reduced in a process called the regeneration. At lean exhaust conditions, NOx is released from barium sites at temperatures of around 450°C and above. The regeneration occurs at much lower temperatures if a short pulse of fuel rich mixture is provided. NOx adsorbers can fully regenerate at 250°C, with the onset of a partial regeneration at temperatures as low as 150°C, if the air-to-fuel ratio is maintained at λ < 1 . Therefore, the operation of the adsorber catalyst system involves continuous cycling through lean and rich fuel condition.
The designer of an adsorber system has to analyze very carefully all pertinent operation temperatures, including exhaust gas temperatures during real life duty cycle and during the emission certification test, the NOx adsorber temperature window, the rich regeneration temperature, and the lean decomposition temperature. It is important that adsorber temperatures during lean operation be below the thermal decomposition temperature of the stored nitrates. Otherwise, NOx may be released at lean, leading to a decrease in average conversion efficiency.
An example of lean/rich cycling is illustrated in Figure 8, which shows concentrations of NOx upstream and downstream of the adsorber, and the exhaust gas temperature . The data was generated on a light-duty DI diesel engine. Rich spikes were achieved by throttling the intake air, resulting in oxygen concentration decrease to below 0.4%. The 60 s storage/regeneration pattern used during the test is visible in the peaks in NOx concentration. Engine out NOx shows minimums, which are caused by lower pressures/temperatures in the combustion chamber during intake air throttling. The tailpipe NOx, on the other hand, shows maximums which represent that portion of released NOx which has not been reduced over the rhodium catalyst. The system was tested at a constant engine speed, but different, steady state torque levels resulting in stepwise changes in the exhaust temperature (red line in the graph).
At the highest tested temperature of about 330°C the adsorber catalyst achieved 95% NOx conversion efficiency. As the exhaust temperatures were lowered, the conversion gradually decreased to reach about 50% at about 200°C. The declining catalyst efficiency manifests itself by increasingly higher tailpipe NOx peaks during regeneration. Hydrocarbon break-through occurred during rich spikes at all temperatures, while no CO break-through was observed, suggesting that CO might have been a more effective NOx reductant than HCs.
To maximize NOx conversion efficiency, the storage capacity and frequency of regeneration must be optimized during the design of the adsorber system. Typical capacity of barium adsorbers in the fresh state is on the order of 2 g NOx per liter of catalyst volume . Depending on the engine emissions, catalyst size and condition, and the desired NOx reduction, regeneration must be performed approximately every 30 - 120 seconds. The duration of NAC regeneration is short, between one and a few seconds.
Different reductants have different effectiveness in reducing NOx and regenerating the catalyst surface. The most important reducing species found in engine exhaust include CO, hydrocarbons, as well as hydrogen. Propylene, C3H6, has been used as a model hydrocarbon in many NOx adsorber studies (C3H6 is not representative for diesel, but it is present in stoichiometric-burn, gasoline engine exhaust). Among the above species, H2 is the most effective reductant, while CO and C3H6 are less effective as reductants at low operating temperatures, but are comparable to H2 at elevated operating temperatures .
Various methods have been proposed, such us using reformer catalysts or plasma-based devices , to increase H2 and CO concentration in the exhaust gas to facilitate faster and more complete regeneration. Before one starts incorporating new hardware in the system, however, it should be realized that reducing species introduced to the exhaust to create a rich event may undergo substantial changes before being involved in the release and conversion of NOx. The catalyst formulation and the method of inducing the rich transient may be designed to increase concentrations of the desired species.
CO can act as a direct reductant of NOx via Equation (4). Another route of catalyst regeneration by CO is the water shift reaction, CO + H2O = CO2 + H2, where H2 again acts as the ultimate reductant. It has been shown that both the water shift and the reverse water shift reactions can occur over the Pt/Ba/Al2O3 and Pt/K/Al2O3 formulations .
Figure 9 shows the profiles of reductants along a DOC + NAC system . During the experiments, conducted with diesel engine exhaust gas, intra-catalyst gas samples were taken at 1/4, half, and 3/4 of the NOx adsorber length. The curves in the graph represent the number of moles of reductants during regeneration. The concentration of hydrogen initially decreases, to reach nearly zero at 1/4 of the catalyst length. But after that point, an H2 increase can be seen, probably due to the water shift reaction. The H2 level at the catalyst outlet is higher than in the engine-out gas.
The H2 profile depended on the method of enrichment and the duration of the cycle. Increased H2 levels were measured with 20 s cycling, but not with 60 s cycling. As more NOx was stored over the longer cycle, all of the generated hydrogen might have been used for NOx reduction.
Evolution of smaller, more reactive hydrocarbon species may be also occurring over NOx adsorber catalysts, through such processes as steam reforming, partial oxidation, or cracking of longer chain hydrocarbons.
Initial NOx adsorber research focused on NO oxidation and NOx storage and release processes, with less attention being paid to NOx reduction. It was tacitly assumed that most—if not all—of the released NOx was reduced to nitrogen. More recent studies, however, have shown that the reduction reactions in NOx adsorbers are not as selective as originally thought. Evolution of ammonia, NH3, was reported in a laboratory study with a series of Pt/Ba/γ-Al2O3 catalysts regenerated using H2 . The catalyst with the highest loading of barium (30% wt.) showed the lowest N2 selectivity of only 30%. Nitrous oxide, N2O, is another reaction product that has been detected in NOx adsorbers.
The following equations describe three different reduction paths of NO by carbon monoxide, with the formation of N2, N2O, and NH3:
(7)2NO + 2CO = N2 + 2CO2
(8)2NO + CO = N2O + CO2
(9)2NO + 5CO + 3H2O = 2NH3 + 5CO2
Under normal operation of the NOx adsorber, N2 is the desired product. The formation of N2O and NH3—both are undesired emissions—should be avoided. Since each of the above reaction paths requires a different amount of CO, the formation of N2O and/or NH3 also alters the reductant balance in the catalyst. Depending on the control strategy, if the regeneration is designed to generate N2, but instead N2O or NH3 is formed, too much or too little reductant could be delivered, respectively. Increased CO/HC emissions would occur in the first case, and incomplete regeneration with reduced storage capacity for the subsequent cycle in the other.
NOx adsorber selectivity was investigated by researchers from the US DOE Oak Ridge National Laboratory . A commercial NOx adsorber (Umicore) for a GDI application was tested in a laboratory using synthetic gas mixtures. The major components of the catalyst washcoat included alumina, ceria, zirconia, and barium. The noble metals were Pt, Pd, and Rh in the ratio 82:26:6 at a total loading of 3.99 g/dm3 (113 g/ft3). The tests included short (60 s lean + 5 s rich) and long (15 min lean + 10 min rich) cycling at 200, 300, 400, and 500°C, as well as steady flow temperature ramps using mixtures of NO/NO2 with different reductants. Results of an example long cycling experiment are shown in Figure 10.
At the beginning of the lean capture phase, most of the NOx was being trapped and almost no NOx was detected at the outlet. As the available storage sites became saturated, the breakthrough of NO and NO2 increased. An NO/NO2 peak was observed immediately following the switch to rich conditions. After the initial peak, the NOx concentration quickly dropped to a low level as the regeneration continued. Both N2O and NH3 were present among the regeneration products. N2O concentration increased rapidly after the regeneration began, and then decayed at a slower rate. The second byproduct to appear was NH3, which peaked around the time the CO breakthrough began. After CO breakthrough, the NH3 concentration slowly decayed as the CO concentration approached a steady level.
Based on the cycling and temperature ramp experiments, a number of conclusions were drawn, as follows:
In some experiments, increasing reductant concentrations caused not only increased NH3, but also increased N2O levels. This appears to be counterintuitive, as nitrogen atoms in N2O are in a less reduced state than those in N2 and NH3. Hence, adding more reductant would be expected to produce less N2O and more N2 and NH3. Authors of the study suggested that the N2O might have been formed via oxidation of ammonia.
Selectivity in Aged Catalysts. NH3 formation has also been observed in studies with diesel exhaust. One of the diesel engine studies examined the effect of catalyst aging and desulfation on NH3 formation . Ammonia levels were gradually increasing as the catalyst was aged over 9100 miles (with the exception of a few data points). In all cases, NH3 levels were higher after desulfation.
Diesel exhaust contains certain quantities of sulfur, primarily as sulfur dioxide, derived from diesel fuel and engine lubricating oil. Reduced sulfur species including hydrogen sulfide (H2S) and carbonyl sulfide (COS) may be also present in the gas during the rich excursions. In the presence of an oxidation catalyst, these compounds form stable sulfates with the NOx storage materials, Equation (5) and Equation (6). The adsorption of sulfur is preferential over the adsorption of NOx. Stable sulfates, such as Al2(SO4)3, are also formed with washcoat materials. As a result, the catalyst performance gradually declines and fewer sites are available for NOx adsorption.
Sulfur poisoning begins on the surface of the catalyst inlet and progresses deeper into the bulk of the washcoat, and in the axial direction . Sulfates derived from the known NOx storage materials are more thermally stable than the corresponding nitrates. They do not decompose at conditions that are usually encountered during adsorber operation, including both the adsorption and regeneration cycles. The problem of sulfur deactivation equally affects diesel and GDI engine applications.
Higher levels of sulfur in fuel result in faster and more severe deactivation, as shown in Figure 11 . However, even fuels with sulfur levels below 10 ppm eventually lead to NOx adsorber poisoning . Ultra low sulfur fuels are necessary for the implementation of NAC technology, but even with such fuels NOx adsorbers still require some form of desulfation mechanism.
Several strategies have been proposed to deal with sulfur poisoning of NOx adsorbers:
Sulfur poisoning is reversible (or partially reversible) and site activity can be restored by a desulfation process involving decomposition of the sulfate species. Thermal decomposition of the sulfate species under lean condition requires very high temperatures. For example, bulk BaSO4 decomposes at 1600°C . The desulfation of NOx adsorbers can be conducted at more realistic temperatures—between 500 and 725°C—when accompanied by mixture enrichment. It was reported that optimum desulfation of barium NOx adsorbers is achieved at 650°C and λ = 0.98 . The desulfation process may yield a mix of sulfur containing gases, including SO2 and H2S. The former is the desired product. The release of the latter, due to its obnoxious rotten egg odor, should be avoided.
As with NOx regeneration, H2 is a more efficient sulfate reductant than CO or propylene. The onset temperature of S release during a desulfation event is lower with H2 compared to the other species, and the amount of S that is released during a given time of desulfation is greater. However, H2 desulfation can produce a mixture of H2S and SO2, while only SO2 was detected among desulfation products when CO was used . It should be noted that H2 seems to be an effective reductant only in the presence of platinum, suggesting that H2 must spillover from Pt to reduce the sulfate species .
In theory, the desulfation of NOx adsorbers should restore their full adsorption capacity. In practice, a permanent and irreversible poisoning of some barium sites has been reported . The desulfation strategy should target those sulfur species that can be removed in an efficient manner. Morphologically different forms of stored sulfur have different impact on the NOx adsorber performance . Surface sulfates, which can be removed at relatively modest temperatures, have most impact on the NOx storage capacity. On the other hand, bulk sulfates—which may require desulfation temperatures in excess of 1000°C—have disproportionately less impact on NOx performance.
Exposure to high temperatures during repeated desulfation is a major source of permanent performance loss of the NOx adsorber due to thermal degradation of washcoat and catalyst materials . The desulfation strategy is a critical function in the NOx adsorber design. If sulfates are left in the catalyst washcoat, the NOx conversion efficiency is compromised. Frequent desulfation, on the other hand, may involve significant fuel economy penalties and accelerated thermal deterioration of the catalyst.
Desulfation Strategies. In diesel engines, the desulfation is often performed by increasing exhaust temperatures through a post-injection of fuel. Algorithms have been developed which facilitate catalyst desulfation through system integration with the engine control unit, the common-rail fuel injection system, and on-board diagnostics . Desulfation strategies for gasoline engines can also include a rapid, large amplitude modulation of the air-to-fuel ratio to create exothermal reactions increasing the catalyst temperature and minimizing H2S release . Desulfation strategies involving short A/F ratio pulses are also advantageous in the diesel engine. A desulfation through a series of short rich pulses, rather than a single, continuous rich period, allowed to minimize both H2S emission and fuel economy penalty (reported at 1% when using 10 ppm S fuel) . The pulses of λ=0.95 and 30 s duration were repeated every 250 s over a nearly 1 hour long desulfation process.
A NOx adsorber desulfation strategy was demonstrated on a diesel engine by the US DOE DECSE program . The strategy, developed for a single point on the engine map of exhaust temperature of 400°C, involved a common rail post-injection and a close-coupled warm-up catalyst. Hydrocarbons generated through the post-injection were oxidized in the warm-up catalyst to create an exotherm increasing the NOx adsorber inlet temperature to 700°C (Figure 12).
DECSE experiments to develop the strategy were conducted by operating the engine on commercial, 380 ppm S fuel for approximately two hours, until the initial NOx conversion efficiency of 80% dropped to 60% due to sulfur poisoning. At that moment, the desulfation event was triggered. The adsorber inlet temperature of 700°C was achieved 90 - 180 seconds after initiating the desulfation, depending on the post-injection quantity and the EGR rate. Restoring the NOx efficiency to the original 80% required about 5-6 minutes of desulfation. The above procedure was applied to a number of catalysts of different poisoning histories, all of which were restored to over 85% NOx reduction efficiency over the catalyst inlet operating temperature window of 300°C - 450°C. This performance level was achieved while staying within the 4% fuel economy penalty target defined for the adsorber regeneration.
A sulfur trap, also called SOx trap, is another adsorber catalyst, specifically designed to capture and store sulfur. It is placed in the exhaust system upstream of the NOx adsorber, to reduce its exposure to sulfur. Two approaches have been proposed:
The sulfur storage mechanism in NOx adsorbers and in most sulfur traps is similar. Since sulfur is stored in the form of a metal sulfate, the sulfur dioxide has to be first oxidized to SO3, Equation (5). Therefore, sulfur traps must include an oxidation catalyst, e.g. platinum, to facilitate this reaction. Also the storage systems are similar to those used in NOx adsorbers, but specifically optimized for the following features:
Typical sulfur storage systems are based on alkaline earth or alkali metal oxides and their mixtures on alumina (titania, zirconia) washcoat. A number of other materials have also been tested, including zinc, nickel, chromium, copper, and silver. These metals or their oxides are used either as stand-alone scavengers or as promoters, to modify trap performance, strengthen desired reactions, and influence the adsorption/desorption temperatures . Silver, for example, has been reported to enhance the sulfur trap selectivity towards adsorption of sulfur oxides over nitrogen oxides and to lower the regeneration temperature .
Sulfur Trap Regeneration. Thermal release of sulfur under lean conditions would require temperatures beyond the thermal stability limits of existing NOx adsorber designs. On the other hand, materials have been developed that allow desulfation of sulfur traps under rich condition at temperatures as low as 300 - 350°C . For this reason, most studies have focused on rich regeneration in a manner parallel to that of the NOx adsorbers. There are two possible maintenance strategies for the sulfur trap:
In the periodic strategy the sulfur trap is regenerated at higher temperatures, thus, minimizing the re-adsorption of sulfur in the NOx adsorber. A certain fuel economy penalty would be associated with increasing the temperature of the sulfur trap. The continuous regeneration approach, while minimizing the fuel economy penalty, presents a larger material development challenge to develop scavengers that would regenerate at low temperatures and over short periods of time. Irrespective of the strategy, periodic desulfation of NOx adsorbers is likely to be required, even in the presence of sulfur traps.
The desulfation of sulfur scavengers involves release of secondary emissions, including H2S and COS . The ratio of H2S/SO2 released during regeneration increases with decreasing air-to-fuel ratio. If low air-to-fuel ratios are used for regeneration, which typically reduce the duration of regeneration, most of sulfur may be released as H2S. Even though the tendency to release sulfur as SO2 can be maximized by the selection of sulfur scavengers , the SO2 may be subsequently reduced to H2S in the downstream NOx adsorber under rich conditions. H2S production increases with the gas residence time in the catalyst, i.e., larger sulfur traps and NOx adsorbers produce a higher proportion of hydrogen sulfide. In the system shown in Figure 14, over 90% of the sulfur is released from the adsorber as H2S .
It is desirable that sulfur be released from the vehicle tailpipe as SO2 rather than H2S. If satisfactory H2S-suppressed sulfur scavengers are not developed, it may be necessary to provide an additional catalyst, downstream of the NOx adsorber, to store the hydrogen sulfide during rich regeneration periods and oxidize and release it as SO2 during lean operation. Suitable H2S scavengers, based on nickel, manganese, zinc, or iron, have been developed for the gasoline three-way catalyst  .
Formation of another secondary emission, NH3, was reported over a silver-based sulfur trap at lean condition with resulting decrease in NOx adsorber efficiency . An NH3 decomposition catalyst was installed between the sulfur trap and the NOx adsorber to alleviate the problem.
Mixture enrichment (λ < 1) on lean burn gasoline engines may require some coordination of spark advance with fuel injection (in order to minimize torque disturbances) but otherwise seems to be fairly straightforward. Generation of rich air-to-fuel mixtures in the diesel engine, which normally operates at λ > 1.3 and leaner, is more challenging. The methods of generating rich exhaust condition in diesel engines can be grouped into two categories:
The regeneration (or desulfation) of a NOx adsorber requires not only the presence of reductants, but also the absence of oxygen. This makes the process somewhat different from mixture enrichment for particulate filter regeneration or in active lean NOx catalysts, where high oxygen concentrations do not interfere with the respective processes. In NOx adsorbers, if oxygen is still present in the exhaust gas after the introduction of reductants—which is always the case with exhaust enrichment, and may happen with some in-cylinder methods—it needs to be removed through chemical reactions with the reducing species before NOx conversion can proceed. The same applies to oxygen that may be stored in catalyst washcoat, such as on oxidized ceria sites. The reactions to reduce oxygen can occur over the NOx adsorber itself and/or over an upstream oxidation catalyst. This process has certain negative aspects, including exotherms which can be damaging to the catalyst, and an additional fuel economy penalty.
In-Cylinder Enrichment. Mixture enrichment can be realized in the engine cylinder using a number of methods, including:
Electronically controlled diesel fuel injection systems, such as the common rail, offer a lot of flexibility for practical in-cylinder enrichment strategies, with post-injections of fuel commonly used to provide mixture enrichment and increase the exhaust gas temperature. Post injection of fuel can be combined with increased EGR rates and intake air throttling to reduce the exhaust oxygen concentrations.
In-cylinder enrichment strategies can be optimized to deliver the desired mix of reductants—with high levels of H2—and to minimize undesired emissions, such as PM. Figure 15 shows H2 and CO levels reported by an in-cylinder enrichment study . The maximum reductant concentrations were 4.1% H2 and 8.2% CO. The study investigated two fuel injection strategies: (1) delayed and extended main injection, and (2) post-injection. In both cases, the fuel strategies were combined with throttling to reduce intake air flow and to reduce the A/F ratio from about 30 to 12. EGR rates, 15-20% during lean operation, were increased to 30-35% during rich excursions. The delayed and extended main injection strategy produced high concentrations of H2 and CO, but also high PM emissions. The post injection produced low PM levels, but only moderate CO and H2 concentrations. Compared with lean operation, lean/rich cycling resulted in a fuel economy penalty of 2 - 5.5%, depending on the enrichment strategy.
Low temperature combustion (LTC) can produce exhaust gas compositions desired for NOx adsorber regeneration. LTC has been employed by Toyota for the regeneration of NOx adsorbers at low load and speed conditions in light-duty engines . LTC involves massive EGR, intake air throttling, and injection timing designed for smokeless combustion despite rich A/F ratio.
Exhaust Enrichment. Exhaust system enrichment is realized through injection of diesel fuel upstream of the catalyst. The injected fuel mixes with hot exhaust gas and evaporates. Upon contact with noble metal catalysts, HCs may undergo partial oxidation and steam reforming reactions, to produce a more reactive mix or reductants, including shorter chain hydrocarbons and hydrogen.
In an alternative exhaust enrichment strategy, the injected fuel may be processed using an on-board fuel reformer. The fuel reformer is a catalyst that promotes partial oxidation reactions to transform diesel HCs into syngas-like mixture with increased concentrations of H2, CO and lighter HCs. Regeneration of a NOx adsorber through the injection of syngas (CO+H2) into the exhaust system was demonstrated on a light duty vehicle . An on-board fuel reformer was incorporated into a NOx adsorber system designed to achieve US EPA Tier 2 Bin 5 emissions . The system showed a relatively high fuel economy penalty of 6%, with the majority of reformate gas used to reduce exhaust oxygen concentration to stoichiometry.
It was also proposed that hydrogen-rich gas (CO+H2) be produced from diesel fuel by electric discharge (plasma) continuously applied to flowing fuel/air mixture in a device termed the “diesel plasmatron reformer” .
One of the most important aspects of utilizing NOx adsorber technology is to establish engine operating conditions that would facilitate a satisfactory level of NOx conversion through proper adsorber regeneration and desulfation, while minimizing the associated fuel economy penalty. This optimization is achieved by defining a lean/rich modulation strategy, while paying close attention to resulting concentrations of NOx and the reducing gases, as well as to exhaust temperatures. More fuel-rich modulations typically result in faster and more complete regeneration of the adsorber and, thus, in higher average NOx conversion efficiencies.
The DECSE program estimated that an 80% peak NOx conversion efficiency could be achieved at a fuel economy penalty of less than 4% . While the 4% figure provides a useful reference number on the fuel penalty, optimization of the engine-catalyst system is critically important to achieve high NOx reductions. This is illustrated in Figure 16, taken from the same DECSE study. Two NOx conversion curves were generated using two different engine calibrations. In both cases the fuel penalty was kept below 4% and identical rich/lean timing was used, but different NOx conversions were seen. It is clear that the regeneration has to be carefully tuned to match the catalyst requirement; otherwise, quantities of fuel may be wasted through unproductive mixture enrichment.
The regeneration and desulfation cycles in high efficiency systems require a closed-loop control, based on the concentrations of NOx and oxygen, temperature, and other parameters. The feedback signals must be provided by sensors, including NOx sensors and A/F ratio sensors. The control system, integrated with the engine control module, has to determine the regeneration and desulfation parameters (timing, duration, A/F ratio, ...) and the enrichment strategy depending on the process variables and the engine operating conditions.
Model-based approaches are typically employed for the control of NOx adsorber systems . Models, such as for the untreated NOx emissions and the NOx storage capacity, are stored in the control unit. Based on the output of the models, driving conditions, and input from sensors, software modules control NOx adsorber regeneration and desulfation. A catalyst protection block has the authority to override any intervention in order to protect the catalyst(s) from exposure to excessive temperatures.
The concept of a passive NOx adsorber (PNA) has been proposed to enhance the low temperature NOx performance in urea SCR emission systems . The PNA is a NOx storage device that adsorbs NOx at low temperatures, such as during the cold start segment of the FTP-75 test. Once exhaust temperatures increase, the stored NOx is released and reduced to nitrogen over a downstream SCR catalyst using urea.
Catalyst Systems. The challenge in developing passive NOx adsorbers for diesel engines is the right choice of the adsorbent, with the desired temperature window. To provide a good cold start performance, NOx adsorption should start at low exhaust temperatures, preferably around 100°C. The NOx desorption temperature should correlate with the light-off temperature of the SCR catalyst—around 250°C. Since barium materials used for active NOx adsorbers release NOx at lean conditions at temperatures around 450°C and higher, they are clearly not suitable for passive NOx adsorbers.
A Pd/zeolite catalyst was developed for the PNA application by Johnson Matthey . Palladium is deposited via wet impregnation (using Pd nitrate) over a small pore size zeolite, such as chabazite (CHA), followed by drying and calcination. The metal is deposited both inside and outside of the zeolite pores. In one example, the Pd loading was 1% relative to the mass of the zeolite.
After hydrothermal aging at 750°C, the Pd/CHA catalyst had a NOx storage capacity of 0.28 g NO2/L at 80°C, and 0.45 g NO2/L at 170°C. The above storage was calculated with reference to a monolith containing a catalyst loading of 3 g/in3. After sulfation (exposure to 100 ppm SO2 at 300°C), the catalyst NOx storage at 100°C decreased from 0.41 to 0.28 g NO2/L.
Solvay (former Rhodia) developed ceria-based adsorbents doped with barium , which can release NOx at low temperature ranges and may be used in PNA applications. Pd/CeO2 systems were also suggested as a possible NOx adsorbent for such low temperature applications .
Performance. Figure 17 shows the NOx storage capacity of the PNA and the NOx conversion efficiency of a state-of-the-art SCR catalyst. There is no active regeneration and the adsorbent should release the stored NOx within a temperature range that correlates with the SCR catalyst light-off. The data in Figure 17 is based on the evaluation results of a PNA-SCR emission system considered for use in the ATLAS engine developed by Cummins .
The prototype ATLAS engine was a 2.8 L, 4 cylinder unit rated at 210 hp, targeting US EPA Tier 2 Bin 2 emission standards (NOx = 0.02 g/mi), intended for pickup truck applications. Tested on an engine emitting 0.4 g/mi NOx, a 0.9 L passive adsorber was able to store 65% of NOx emitted during the first 180 s of the FTP-75 test. The PNA was the dCSC (cold start catalyst) by Johnson Matthey.
The PNA in Figure 17 can adsorb and store NOx up to about 150°C. Its NOx desorption temperature, however, is not matching particularly well the low temperature activity of the SCR catalyst (the grayed area in the chart). Bulk release of the stored NOx occurs at temperatures of around 150-200°C, when the SCR catalyst activity is still low. To achieve the emission target, the PNA should have a higher desorption temperature and/or the SCR catalyst should have higher activity at low temperatures. A close-coupled location of the SCR catalyst was also considered, as well as the use of gaseous ammonia (Amminex system) instead of urea.