NOx Storage And Reduction Catalysts Research Articles

Advances in Designing Efficient La-Based Perovskites for the NOx Storage and Reduction Process

To overcome the inherent challenge of NOx reduction in the net oxidizing environment of diesel engine exhaust, the NOx storage and reduction (NSR) concept was proposed in 1995, soon developed and commercialized as a promising DeNOx technique over the past two decades. Years of practice suggest that it is a tailor-made technique for light-duty diesel vehicles, with the advantage of being space saving, cost effective, and efficient in NOx abatement; however, the over-reliance of NSR catalysts on high loadings of Pt has always been the bottleneck for its wide application. There remains fervent interest in searching for efficient, economical, and durable alternatives. To date, La-based perovskites are the most explored promising candidate, showing prominent structural and thermal stability and redox property. The perovskite-type oxide structure enables the coupling of redox and storage centers with homogeneous distribution, which maximizes the contact area for NOx spillover and contributes to efficient NOx storage and reduction. Moreover, the wide range of possible cationic substitutions in perovskite generates great flexibility, yielding various formulations with interesting features desirable for the NSR process. Herein, this review provides an overview of the features and performances of La-based perovskite in NO oxidation, NOx storage, and NOx reduction, and in this way comprehensively evaluates its potential to substitute Pt and further improve the DeNOx efficiency of the current NSR catalyst. The fundamental structure–property relationships are summarized and highlighted to instruct rational catalyst design. The critical research needs and essential aspects in catalyst design, including poisoner resistance and catalyst sustainability, are finally addressed to inspire the future development of perovskite material for practical application.

Recent advances in catalytic automotive emission control: Passive NO storage at low temperatures

AbstractIn the practical applications, a high portion of the NOx is emitted without purification in the cold start period of engine because the selective catalytic reduction (SCR) or NOx storage and reduction (NSR) catalysts are not warmed up. Passive NOx absorbers (PNA) were proposed earlier to store the NOx at low temperatures and release the stored NOx during warm‐up period when the SCR and NSR catalysts downstream are functional, while this concept just attracted great interests recently to meet the extra stringent requirement of governmental emissions regulations. A plant of PNA catalysts, showing different NO adsorption/release characteristics, was disclosed. This review summarizes the recent progress in the development of PNA catalysts. The characteristics of NO adsorption/release on precious metal (i.e., Pt, Pd) loading Al2O3 support catalysts, CeO2 support catalysts, and zeolite support catalysts are summarized and compared.

A comparative study on the NOx storage and reduction performance of Pt/Ni1Mg2Al1Ox and Pt/Mn1Mg2Al1Ox catalysts

A series of Ni1Mg2Al1Ox, Mn1Mg2Al1Ox, 0.5Pt/Ni1Mg2Al1Ox and 0.5Pt/Mn1Mg2Al1Ox catalysts were carefully prepared and their NOx storage and reduction (NSR) performance including NOx oxidation efficiency (NOE), NOx storage capacity (NSC), NOx conversion rate (XNO), N2 selectivity (SN2), N2O selectivity (SN2O) and NH3 selectivity (SNH3), was systematically investigated. A SO2 resistance test was also performed in the presence of 100 ppm SO2. The NOE and NSC experimental results revealed that the Ni1Mg2Al1Ox catalyst possesses a higher NSC, while the Mn1Mg2Al1Ox catalyst possesses a better NOE. With regard to XNO, 0.5Pt/Ni1Mg2Al1Ox presented higher results at 200 °C and 400 °C, while 0.5Pt/Mn1Mg2Al1Ox obtained the highest result at 300 °C, which was more than 60% for both. In addition, compared to 0.5Pt/Ni1Mg2Al1Ox, 0.5Pt/Mn1Mg2Al1Ox exhibited a relatively higher SN2 and lower SN2O and SNH3. The NOx-TPD and H2-TPSR results indicated that NOx adsorbed on Ni1Mg2Al1Ox and 0.5Pt/Ni1Mg2Al1Ox is more stable, and that NH3 can be formed in large amounts in a lower temperature range. Both Pt-containing catalysts presented a quite stable XNO in ten cycles in the presence of 100 ppm SO2, and their SN2 can be remarkably enhanced to more than 80%, which could be attributed to the reactions of NH3-SCR and SO2 + NH3. We believe this new insight can provide a new way of thinking for the development of NSR catalysts.

The influence of Pt loading and dispersion on the NOx storage and reduction performance of Pt/K2CO3/Co1Mg2Al1Ox catalysts

Abstract For NOx storage and reduction (NSR) catalysts, Pt is necessary for the NO oxidation during the lean stage and the NOx reduction during the rich stage. However, due to its high price, synthesis of NSR catalysts with low Pt loading is highly demanded. In this contribution, Pt/K2CO3/Co1Mg2Al1Ox as NSR catalysts with low Pt loadings (0.15, 0.33 and 0.59 wt%) were prepared via co-precipitation and incipient wetness impregnation methods. The optimal of K2CO3 loading was determined to be 15 wt% for both samples. The influence of Pt particle size and Pt dispersion on the NOx storage capacity and the NSR cycling performance of Pt/K2CO3/Co1Mg2Al1Ox were then systematically and comparatively investigated. It was demonstrated that the samples prepared by CP method can result in higher isothermal NOx storage capacity, and higher NOx removal rate and better cycle stability during lean-rich cycling tests. By increasing the Pt loading or extending the rich period duration, the overall NOx removal rate can be promoted and became more stable. No matter what method was used for loading Pt, the N2 selectivity of all catalysts was higher than 95%. The results also indicated the samples prepared by CP method were more resistant to the effects of SO2, CO2 and H2O. CO-pulse and HR-TEM analyses confirmed that the 0.59 Pt/Co1Mg2Al1Ox (CP) possesses smaller Pt particle size (1.30–1.32 nm) and higher Pt dispersion (65.41%), while the values for 0.59 Pt/Co1Mg2Al1Ox (IWI) was 2.66–2.71 nm and 31.90%, respectively. All results indicated that the CP method, which can effectively improve Pt dispersion and reduce the Pt particle size, is superior to the traditional IWI for the preparation of highly efficient low Pt loading NSR catalysts.

NOx reduction on Ceria: Impact of lean-rich cycling

NOx storage and reduction (NSR) is a cyclic catalytic process that eliminates NOx from lean burn vehicles. Lean-rich switching can achieve a higher conversion than steady state operation. There is an active debate about the NOx reduction mechanism for NSR catalysts containing CeO2 (ceria), particularly at high temperatures and fast cycle frequency. In order to isolate the role of ceria, we examine the performance of a ceria-washcoated monolith for a wide range of operating conditions, including temperature, cycling frequency, reductant type and feed concentration of oxidants (O2, CO2 and H2O). The results reveal enhancement of NO conversion with faster cycling particularly at elevated temperatures (> 550 °C). The data are consistent with a cyclic mechanism in which oxygen vacancies are created during the rich feed through reduction by H2, CO, or C3H6 and filled during the lean feed through NO and/or O2 oxidation. An excess of O2 is detrimental to NO conversion, due to the competing oxidation of reduced ceria by O2. The data reveal that at least two types of vacancy sites participate in the cyclic redox process. Surface vacancy sites provide rapid NO reduction, while utilization of bulk vacancy sites is slowed by solid-state diffusion limitations. For a fixed duty cycle, lean/rich switching operation is superior to steady-state operation and an optimal lean/rich switching frequency exists for different reaction conditions. Fast cycling is especially favorable for stoichiometric feeds or cycle-averaged rich feeds.

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

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

The influence of xMnyCe/γ-Al2O3 on NOx catalysts on the properties of NOx storage and reduction over Pt-Ce-Ba/γ-Al2O3 catalysts

The aim of the study was to investigate the influence of Diesel Oxidation Catalysts (DOC) xMnyCe/γ-Al2O3catalysts on the properties of NOx storage and reduction (NSR) under the treatment of Pt-Ce-Ba/γ-Al2O3 catalysts. The physicochemical properties of DOC and NSR were determined with XRD, TEM and XPS. The results showed that Pt-CeO2 interaction had a great impact on the dispersion of the NSR catalyst. Furthermore, the performance of NO oxidation by DOC and NOx desorption by DOC/NSR combined system were evaluated based on the simulation experiments. Moreover, optimal operation of the single NSR catalyst and the DOC/NSR combined system were attained at 400°C, while the durations of the lean and rich periods were tL=240s and tR=30s/45s/60s, respectively. The single NSR catalyst showed that the highest NOx conversion and maximum NOx adsorption conversion were 58.0% and 74.8%. The DOC/NSR combined system allowed Pt10Ce15Ba/γ-Al2O3 catalyst storing the NO2 oxidized on the 6Mn10Ce/γ-Al2O3 catalyst, favoring the NOx storage process during the lean phase. And it also allowed the stored NOx species of NSR catalysts desorbing due to H2 oxidation by the 6Mn10Ce/γ-Al2O3 catalyst, favoring the NOx desorption process during the rich phase.

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

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

PtM/Ba/Al2O3Ce0.6Zr0.4O2: Influence of Synergetic Interactions between Transition Metal and Platinum on NOxStorage and Reduction

Not so precious: Platinum-based NOx storage and reduction (NSR) catalysts containing transition metal (Mn, Fe, or Co) have been prepared by using alumina–ceria–zirconia (ACZ) as a support. The strong interaction between Pt and transition metal induces a clear effect on NSR performance. The 0.5 wt % PtCo/Ba/ACZ shows higher NOx storage capacity (NSC) than 1.0 wt % Pt/Ba/ACZ (see figure), and Co addition allows Pt replacement in NSR catalysts.

Characteristics of low platinum Pt–BaO catalysts for NO x storage and reduction

The characteristics of low platinum Pt–BaO catalysts for NO x storage and reduction (NSR) were studied over three series of samples that have similar barium loadings (ca. 15 wt% Ba), namely, Pt–BaO/Al 2O 3, CoO x promoted Pt–BaO/Al 2O 3 and Pt–BaO/ZrO 2–Al 2O 3 catalysts. The catalysts were calcined at 800 °C and their NSR performances were evaluated under cyclic lean (1000 ppm NO and 10 vol% O 2 in Ar)/rich (1 vol% H 2 in Ar) conditions with particular attention paid to detect the possible harmful by-products, i.e., NH 3 and N 2O, during the reduction of the stored NO x under the rich condition. Lowering the Pt loading in the Pt–BaO/Al 2O 3 catalyst from 1.0 to 0.5 wt% resulted in a 15% decrease in the NO x storage capacity (NSC) under the lean condition but the N 2 selectivity under the rich condition declined dramatically (from 85% to 53%), indicating that the NO x reduction performance under the rich condition is the key to low platinum NSR catalysts. The cobalt oxides was identified as an unsuitable promoter for lowering the Pt loading; its addition into Pt–BaO/Al 2O 3 at the Pt loading of 0.5 wt% further reduced the N 2 selectivity under the rich condition although significant improvement in the NSC under lean condition could be observed. It was found instead that the use of ZrO 2–Al 2O 3 as an alternative of the Al 2O 3 support was effective in lowering the dependence on Pt loading of the Pt–BaO for the NSR catalysis since the ZrO 2–Al 2O 3 support could make the catalyst highly selective for the formation of N 2 under the rich condition, at a lower Pt loading. The overall NSR performance of a Pt–BaO/ZrO 2–Al 2O 3 catalyst with 0.5 wt% Pt was found comparable to that of its counterpart Pt–BaO/Al 2O 3 catalyst with 1.0 wt% Pt, demonstrating a viable way for developing low platinum NSR catalysts.

NO x decomposition, storage and reduction over novel mixed oxide catalysts derived from hydrotalcite-like compounds

Effective control and removal of nitrogen oxides (NO x ) emission from vehicles exhausts under lean-burn condition is one of the most important targets in scientific research of environmental protection. A comprehensive introduction of NO x storage and reduction (NSR), the most promising lean-NO x control technology, is given including the sum-up of NSR materials, catalytic activity and related reaction mechanisms. Emphasis is put on the novel multifunctional NSR catalysts, derived from hydrotalcite-like compounds, with characteristic of simultaneous NO x strorage–decomposition–reduction. Finally, future research directions in the area of lean-NO x control based on mixed oxide catalysts derived from hydrotalcite-like materials is also proposed.

Thermally stable Pt/K2Ti2O5 as high-temperature NOx storage and reduction catalyst

Pt/K2Ti2O5 is developed as a high-temperature NOx storage and reduction (NSR) catalyst. K2Ti2O5, in place of alkali metals and γ-Al2O3, works as both support and NOx adsorber; and, unlike alkali-doped NSR catalysts that tend to deactivate at temperature higher than 500°C, K2Ti2O5 shows stable NOx adsorption in a high temperature range of 500–600°C. The NOx storage and reduction performance over Pt/K2Ti2O5 is studied by various means, including temperature-programmed adsorption, isothermal storage, and lean–rich cycling experiments. A maximum value of adsorbed NOx species (close to 1.2mmol/g catalyst) is achieved at 550°C. Temperature-programmed de-sorption reveals that the adsorbed NOx on Pt/K2Ti2O5 is highly stable, with a peak at 720°C. Thermal stability investigations indicate Pt/K2Ti2O5 has a much higher stability than Pt–K/TiO2. Mechanism studies reveal that KNO3-like compound is formed after NOx adsorption, and that NOx storage and reduction process on K2Ti2O5 is accompanied by a structure transformation between K2Ti2O5 and K2Ti6O13.

NO x storage behavior and sulfur-resisting performance of the third-generation NSR catalysts Pt/K/TiO 2–ZrO 2

The NO x storage and reduction (NSR) catalysts Pt/K/TiO 2–ZrO 2 were prepared by an impregnation method. The techniques of XRD, NH 3-TPD, CO 2-TPD, H 2-TPR and in situ DRIFTS were employed to investigate their NO x storage behavior and sulfur-resisting performance. It is revealed that the storage capacity and sulfur-resisting ability of these catalysts depend strongly on the calcination temperature of the support. The catalyst with theist support calcined at 500 °C, exhibits the largest specific surface area but the lowest storage capacity. With increasing calcination temperature, the NO x storage capacity of the catalyst improves greatly, but the sulfur-resisting ability of the catalyst decreases. In situ DRIFTS results show that free nitrate species and bulk sulfates are the main storage and sulfation species, respectively, for all the catalysts studied. The CO 2-TPD results indicate that the decomposition performance of K 2CO 3 is largely determined by the surface property of the TiO 2–ZrO 2 support. The interaction between the surface hydroxyl of the support and K 2CO 3 promotes the decomposition of K 2CO 3 to form –OK groups bound to the support, leading to low NO x storage capacity but high sulfur-resisting ability, while the interaction between the highly dispersed K 2CO 3 species and Lewis acid sites gives rise to high NO x storage capacity but decreased sulfur-resisting ability. The optimal calcination temperature of TiO 2–ZrO 2 support is 650 °C.

New approach to enhance the NOx storage performance at high temperature using basic MgAl2O4 spinel support

With the purpose of improving the NOx storage performance of NOx storage and reduction (NSR) catalysts above 673K, a new approach using a support with a higher basicity than γ-Al2O3 was proposed and exemplified by using the MgAl2O4 spinel as the support for an NSR catalyst containing the potassium NOx storage material. The NOx storage amounts on the fresh and thermally-aged K/Pt/MgAl2O4 catalysts at 873K were apparently higher than those on the K/Pt/Al2O3 catalysts. The higher potassium basicity resulting from the interaction between the basic MgAl2O4 support and the potassium NOx storage material enhanced the stability of the stored NOx over the fresh and thermally-aged K/Pt/MgAl2O4 catalysts, which leads to their competitive NOx storage amounts versus those of the K/Pt/Al2O3 catalysts. These results indicated a breakthrough that improved the performance of the NSR catalysts at high temperature.

Identifying critical factors in the regeneration of NOx-trap materials under realistic conditions using fast transient techniques

NOx storage and reduction (NSR) catalysts are considered to be one of the most promising solutions to meet the upcoming NOx exhaust emission regulations. However, despite the high level of activity in this area, it is surprising that most studies have been conducted under conditions that are far from realistic conditions. In the present work a fast transient apparatus has been developed to study NSR catalysts under realistic temporal conditions, i.e. where the storage time was 1–2min and the regenerative rich time was of the order of 1s. To exemplify the value of this methodology, the performance of a 1%Pt/17.5%Ba/γ-Al2O3 NSR catalyst was studied as a function of the temperature, rich phase duration, reductant concentration, the type of reductant and the effect of an inert gas purge. From these results it is clearly found that excess reductant to regenerate the trap at short regeneration times is advantageous and that hydrogen is a better reductant than CO. This is especially true at low temperatures where the rate of reduction of released NOx is not sufficiently fast to allow for complete regeneration of the trap. For a given quantity of reductant, a long, low concentration rich pulse regenerates the catalyst more effectively than a short, high concentration pulse. Readsorption of NOx released during the rich phase but not reduced quickly enough can be a significant cause of loss of trap performance. This study underlines the importance of the regeneration step in the NSR process and highlights the need to investigate these systems using fast transient techniques capable of reproducing the short regeneration times used in real systems.

Performance of Co-containing NO x storage and reduction catalysts as a function of cycling condition

Co-containing NO x storage and reduction (NSR) catalysts were studied for their NO x storage and NO x conversion behavior. It was found that addition of 5% Co increased the NO x storage capacity of a 1% Rh/15% Ba catalyst by 50% and that of a 1% Pt/15% Ba catalyst by 100%. This promotional effect was attributed to the high oxidizing ability of Co, present in the form of Co 3O 4, providing extra oxidation sites for NO to NO 2 conversion and more contact area for NO 2 spillover to Ba storage sites. Empirical models were developed to predict the NO x conversion capacity of the catalyst as a function of total cycle time and the fraction spent in the lean phase. Using these models, it was found that 5% Co also improved the performance of 1% Pt/15% Ba and 1% Rh/15% Ba catalysts at higher lean fractions, allowing substantial improvement in overall fuel efficiency. The reduction capacity of Co was much less than that of noble metals, such that noble metals were still necessary (although at a reduced loading) for regenerating the catalyst in the rich phase. Along these lines, it was found that a 0.25% Pt/5% Co/15% Ba-containing catalyst showed better performance than a 1% Pt/15% Ba-containing catalyst. This implies that the cost of NSR catalysts can be substantially reduced by replacing some of the Pt with Co as the active oxidizing metal in NSR catalyst formulations.

Model NSR catalysts: Fabrication and reactivity of barium oxide layers on Cu(1 1 1)

The growth of barium oxide on a Cu(1 1 1) substrate, formed by the deposition of barium and its subsequent oxidation, yields stable BaO films which expose predominantly the BaO(1 0 0) surface. The interaction of the oxide films with common components of motor-vehicle exhaust gases (CO 2, H 2O, NO x ) has been studied using surface analytical techniques, including X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD) and reflection IR spectroscopy (RAIRS). The spectroscopic identification of Ba(OH) 2, BaCO 3 and Ba(NO 2) 2 phases is discussed, and the relative stabilities and decomposition mechanisms of these materials when supported on Cu(1 1 1) is revealed by a combination of TPD and XPS. BaO is shown to be resistant to reaction with pure NO and NO/O 2 mixtures, but exposure to NO 2 leads to the rapid formation of barium nitrite. The formation of the nitrite is proposed to be the first-step in the production of barium nitrate, which has previously been shown to be the main phase involved in NO x storage and reduction (NSR) catalysis.

Response surface study of the performance of lean NOx storage catalysts as a function of reaction conditions and catalyst composition

NOx storage and reduction (NSR) catalysts containing Pt, Ba and Fe were studied as a function of reaction conditions and catalyst composition using response surface methodology combined with high-throughput experimentation. The concentrations of the reactant gases and the reactor temperature were varied to probe their effect on catalyst performance, as quantified by lean NOx storage and N2O production. An empirical model relating the catalyst performance to five reaction condition variables and three metal weight loading variables has also been developed. It was found that the temperature and the concentrations of the reducing agents, i.e. carbon monoxide and ethylene, had the strongest effect on the lean NOx storage. It was also found that the Pt and Ba weight loadings had a much greater effect than Fe weight loadings on the performance of NSR catalysts. This model provides insight about the factors controlling the NOx conversion by NSR catalysts and also predicts the optimum catalyst composition for given reaction conditions and vice versa. As an additional study, the relationship between sulfur poisoning, nitrous oxide production, and exotherm generation was also explored.

The low-temperature performance of NOx storage and reduction catalyst

The catalytic performance and the behavior of NOx storage and reduction (NSR) over a model catalyst for lean-burn gasoline engines have been mainly investigated and be discussed based on the temperature and reducing agents use in this study. The experimental results have shown that the NOx storage amount in the lean atmosphere was the same as the NOx reduction amount from the subsequent rich spike (RS) above the temperature of 400°C, while the former was greater than the latter below the temperature of 400°C. This indicated that when the temperature was below 400°C compared with the NOx storage stage, the reduction of the stored NOx is somehow restricted. We found that the reduction efficiencies with the reducing agents decrease in the order H2>CO>C3H6 below 400°C, thus not all of the NOx storage sites could be fully regenerated even using an excessive reducing agent of CO or C3H6, which was supplied to the NSR catalyst, while all the NOx storage sites could be fully regenerated if an adequate amount of H2 was supplied. We also verified that the H2 generation more favorably occurred through the water gas shift reaction than through the steam reforming reaction. This difference in the H2 generation could reasonably explain why CO was more efficient for the reduction of the stored NOx than C3H6, and hinted as a promising approach to enhance the low-temperature performance of the current NSR catalysts though promoting the H2 generation reaction.

Soot oxidation over NO x storage catalysts: Activity and deactivation

Soot oxidation activity and deactivation of NO x storage and reduction (NSR) catalysts containing Pt, K, and Ba supported on Al 2O 3, are studied under a variety of reaction conditions. K-containing catalysts decrease soot oxidation temperature with O 2 alone and the presence of Pt further enhance the activity due to synergetic effect. The active species responsible for synergism on Pt/K–Al 2O 3 are unstable and cannot be regenerated. Soot oxidation temperature decreases by about 150 °C with NO + O 2 exhaust feed gas and under lean conditions NSR system acts as catalysed soot filter (CSF). The reactions that are mainly responsible for decreasing soot oxidation temperature are: (i) soot oxidation with NO 2 followed by NO recycles to NO 2, and (ii) soot oxidation with O 2 assisted by NO 2. Only a part of the stored NO x that is decomposed at high temperatures under lean conditions is found to be useful for soot oxidation. NO x storage capacity of NSR catalysts decreases upon ageing under soot oxidising conditions. This will lead to a decreased soot oxidation activity on stored nitrate decomposition. Pt/K–Al 2O 3 catalyst is more active, but least stable compared with Pt/Ba–Al 2O 3.