NOx Trap Research Articles

Improved NOx Storage/Release Properties of Ceria-Based Lean NOx Trap Compositions with MnOx Modification

Ceria/spinel-based lean NOx trap compositions with and without barium were modified with MnOx via incipient wetness impregnation. The effect of the MnOx layer on the aged materials (850 °C) as to the NOx storage and release properties was investigated via NOx adsorption (500 ppm NO/5% O2/balance N2) carried out at 300 °C in a dual-bed with a 1% Pt/Al2O3 catalyst placed upstream of the samples to generate sufficient amounts of NO2 required for efficient NOx storage. Subsequent temperature programmed desorption (TPD) experiments were carried out under N2 from 300 °C to 700 °C. The addition of MnOx to the barium free composition led to a slightly reduced NOx storage capacity but all of the ad-NOx species were released from this material at significantly lower temperatures (ΔT ≈ 100 °C). The formation of a MnOx layer between ceria/spinel and barium had a remarkable effect on ageing stability as the formation of BaAl2O4 was suppressed in favour of BaMnO3. The presence of this phase resulted in an increased NOx storage capacity and lower desorption temperatures. Furthermore, NOx adsorption experiments carried out in absence of the Pt-catalyst also revealed an unexpected high NOx storage ability at low NO2/NO ratios, which could make this composition suitable for various lean NOx trap catalysts (LNT) related applications.

Sulfur Poisoning Effects on Modern Lean NOx Trap Catalysts Components

In the present work, a series of different materials was investigated in order to enhance the understanding of the role of modern lean NOx trap (LNT) components on the sulfur poisoning and regeneration characteristics. Nine different types of model catalysts were prepared, which mainly consisted of three compounds: (i) Al2O3, (ii) Mg/Al2O3, and (iii) Mg/Ce/Al2O3 mixed with Pt, Pd, and Pt-Pd. A micro flow reactor and a diffuse reflectance infrared Fourier transform spectrometer (DRIFTS) were employed in order to investigate the evolution and stability of the species formed during SO2 poisoning. The results showed that the addition of palladium and magnesium into the LNT formulation can be beneficial for the catalyst desulfation due mainly to the ability to release the sulfur trapped at relatively low temperatures. This was especially evident for Pd/Mg/Al2O3 model catalyst, which demonstrated an efficient LNT desulfation with low H2 consumption. In contrast, the addition of ceria was found to increase the formation of bulk sulfate species during SO2 poisoning, which requires higher temperatures for the sulfur removal. The noble metal nature was also observed to play an important role on the SOx storage and release properties. Monometallic Pd-based catalysts exhibited the formation of surface palladium sulfate species during SO2 exposure, whereas Pt-Pd bimetallic formulations presented higher stability of the sulfur species formed compared to the corresponding Pt- and Pd-monometallic samples.

MPC-Based Optimal Control for Diesel Engine Coupled with Lean NOx Trap System

In this paper, an on-board optimal control problem for diesel engines with lean NOx trap (LNT) is investigated. First, a two-order LNT model based on mass conservation and energy conservation is co...

Global kinetic modeling of rapidly pulsed reductants for lean NOx traps: Frequency domain analysis and impact of mass transfer

In the Rapidly Pulsed Reductants (RPR) process, also known as Di-Air (Bisaiji et al., 2011 [1]), hydrocarbons are injected in rapid pulses ahead of a lean NOx trap (LNT) in order to improve its performance at higher temperatures and space velocities while maintaining a reasonable fuel penalty associated with injecting reductants.In this study, a 23-step global kinetic model is developed to predict the performance and product selectivity of the RPR process using propylene as the reductant over a wide range of pulsing frequencies, particularly for the temperature range of 450–600 °C. Some of the previous studies, (Bisaiji et al., 2012 [2], Perng et al., 2014 [3]) have suggested a reaction mechanism based on the formation of stable isocyanate (NCO) intermediates at high temperatures which promote the NOx conversion efficiency. However, in this study, we are checking whether a global kinetic model based on the conventional (low frequency) NOx storage and reduction mechanism is able to predict the NOx conversion improvements at higher pulsing frequencies at high temperatures, where the stability and role of previously proposed intermediates for NOx reduction is more questionable.A wide range of low frequency switching experiments were used to fit the kinetic parameters to the low frequency response of the RPR process. This model was then used to explore the high frequency performance of the RPR process. The kinetic model can predict the increase in NOx and propylene conversion as well as the change in selectivity of the reaction products, such as NH3 and CO, when the pulsing frequency is increased from conventional LNT to optimal RPR frequencies near 1 Hz corresponding to maximum NOx conversion (Reihani et al., 2018 [4]). However, in order to predict the behavior of RPR above the optimal frequency, it was required to add a model of axial pulse mixing to the kinetic model.The model results indicate that the main mechanism for improvement in NOx conversion at high frequencies and high temperatures is the more efficient utilization of storage sites. Also, the drop in NOx conversion at frequencies higher than the optimal frequency is caused by axial mixing of the reductant pulses. In addition, the model and experimental results provide insights into the importance of washcoat diffusion resistance. Particularly, the NOx reduction step with propylene becomes increasingly limited by washcoat diffusion at high temperatures and pulsing frequencies. The agreement between the model and the experimental results provides strong support for using this model to describe the RPR process.

BaTi0.8B0.2O3 (B = Mn, Fe, Co, Cu) LNT Catalysts: Effect of Partial Ti Substitution on NOx Storage Capacity

The effect of partial Ti substitution by Mn, Fe, Co, or Cu on the NOx storage capacity (NSC) of a BaTi0.8B0.2O3 lean NOx trap (LNT) catalyst has been analyzed. The BaTi0.8B0.2O3 catalysts were prepared using the Pechini’s sol–gel method for aqueous media. The characterization of the catalysts (BET, ICP-OES, XRD and XPS) reveals that: i) the partial substitution of Ti by Mn, Co, or Fe changes the perovskite structure from tetragonal to cubic, whilst Cu distorts the raw tetragonal structure and promotes the segregation of Ba2TiO4 (which is an active phase for NOx storage) as a minority phase and ii) the amount of oxygen vacancies increases after partial Ti substitution, with the BaTi0.8Cu0.2O3 catalyst featuring the largest amount. The BaTi0.8Cu0.2O3 catalyst shows the highest NSC at 400 °C, based on NOx storage cyclic tests, which is within the range of highly active noble metal-based catalysts.

Evaluation of technological solutions for compliance of environmental legislation in light-duty passenger: A numerical and experimental approach

This work includes an original approach that combines on-road experimental fuel use and emission maps and a numerical analysis to assess the impacts of fuel efficiency and NOx emission reduction technologies on a light-duty passenger vehicle. The solutions analyzed include Stop/Start, vehicle mass reduction up to 100 kg, drag coefficient reduction, as well as SCR and Lean NOx Trap systems. For this purpose, a reference EURO 6 diesel vehicle was monitored under real-world driving conditions with a PEMS and the experimental data obtained allowed building engine maps of fuel use and NOx emission to be used on AVL Cruise software. The results obtained with Cruise were firstly validated with the experimental data (with errors up to 10,6% on fuel and 17,5% on NOx) and then the solutions implemented were tested individually and simulated on 48 real-world driving cycles. The results pointed to Stop/Start, mass reduction of 100 kg and SCR as the most effective solutions in reducing fuel consumption and NOx emission up to 14,6% and 57,5%, respectively, depending on driving context. The combination of these technologies was tested and the results showed maximum fuel savings of 17% on urban context and a maximum reduction of 58% on NOx on combined driving cycles. Despite all the efforts, real-world driving presents NOx emissions not yet compliant with standards, indicating the need for new strategies in order to face the environmental challenges.

Smart rule-based diesel engine control strategies by means of predictive driving information

The increasing connectivity of future vehicles allows the prediction of the powertrain operational profiles. This technology will improve the transient control of the engine and its exhaust gas aftertreatment systems. This article describes the development of a rule-based algorithm for the air path control, which uses the knowledge of upcoming driving events to reduce especially [Formula: see text] and particulate (soot) emissions. In the first section of this article, the boosting and the lean [Formula: see text] trap systems of a diesel powertrain are investigated as relevant sub-systems for shorter prediction horizons, suitable for Car-to-X communication range. Reference control strategies, based on state-of-the-art engine control unit algorithms and suitable predictive control logics, are compared for the two sub-systems in a model in the loop simulation environment. The simulation driving cycles are based on Worldwide harmonized Light-duty Test Cycle and Real Driving Emissions regulations. Due to the shorter, and consequently more probable, prediction horizon and the demonstrated emission improvements, a dedicated rule-based algorithm for the air path control is developed and benchmarked in the Worldwide harmonized Light-duty Test Cycle as described in the second part of this article. Worldwide harmonized Light-duty Test Cycle test results show an improvement potential for engine-out soot and [Formula: see text] emissions of up to 5.2% and 1.2%, respectively, for the air path case and a reduction of the average fuel consumption in Real Driving Emissions of up to 1% for the lean NOx trap case. In addition, the developed rule-based algorithm allows the adjustment of the desired NOx–soot trade-off, while keeping the fuel consumption constant. The study concludes with brief recommendations for future research directions, as for example, the introduction of a prediction module for the estimation of the vehicle operational profile in the prediction horizon.

Influence of CO2 Concentration and Inlet Temperature on Adsorption Path of Lean NOx Trap

To investigate the influences of CO2 concentration and inlet temperature on the adsorption path, a new lean NOx trap (LNT) adsorption model is developed based on CHEMKIN SOFT. Both the nitrate and nitrite adsorption path are taken into account in the model. The main conclusions are drawn as follows. At temperature interval of 150℃~200℃, the Ba(NO3)2 site fraction is no more than 0.06%, meaning that the nitrite adsorption path dominates. At 300℃~350℃, two adsorption paths have equivalent occupancies. But at 400℃~550℃, nitrite adsorption path dominates again. The presence of CO2 has influence on both the nitrate adsorption path and nitrite adsorption path. And the inhibition degree of CO2 concentration on nitrate path is weaker than nitrite adsorption path. As CO2 concentration increases, the degree of inhibition weakens. As for the influence of temperature, BaO-NO2 site fraction increases with inlet temperature increasing at 150℃~200℃. And at 250℃~400℃, Ba(NO3)2 site fraction increases first and declines then. In addition, at 450℃~550℃, both nitrate and nitrite decrease as inlet temperature increases.

The Process of Diesel Particulate Filter Regeneration under Real Driving Conditions

The global automotive industry is actually facing complex challenges to ensure solutions that reduce transportation related environmental impact. Because of this, solution were created to reduce exhaust gas emission, like for example DPF [1]. This study was conducted to examine the process of Diesel particulate filter automatic active regeneration during on-road mileage accumulation of Diesel passenger cars. The Diesel test vehicles used in this study were equipped with Diesel oxidation catalysts (DOC), lean NOx trap (LNT, only one vehicle type) and two types of Diesel particulate filter: DPF and FAP (French: filtre à particules) as aftertreatment systems, and satisfied either the Euro 5 or the Euro 6 emissions standards, depending on the vehicle. The test vehicles’ mileage before testing was at least 30 000 km. The soot accumulated during normal use of the vehicles on various types of routes. Collected and processed data were obtained over a distance of 15 000 km. During soot accumulation and DPF regeneration, many parameters from the electronic control unit (ECU) were recorded, i.e. vehicle velocity, engine speed, accumulated soot mass, differential pressure and temperatures at the inlet and outlet of the DPF. The distance covered during the soot accumulation phase depended on the driving conditions, and was sometimes as high as 850 km. During the regeneration process, the temperature at the DPF inlet sometimes rose above 500 °C, and above 600 °C at the outlet, and the differential pressure sometimes reached over 250 mbar.

Lean and rich aging of a Cu/SSZ-13 catalyst for combined lean NOx trap (LNT) and selective catalytic reduction (SCR) concept

In the combined lean NOx trap (LNT) and selective catalytic reduction (SCR) concept, the SCR catalyst can be exposed to rich conditions during deSOx of the LNT.

Enhancing low-temperature NOx storage and reduction performance of a Pt-based lean NOx trap catalyst

Two lean NOx trap (LNT) catalysts, Pt/BaO/CeO2 + Al2O3 and Pt/BaO/CeO2 − Al2O3, were prepared and compared for low-temperature (< 250 °C) NOx storage and reduction performance. The influence of the form of ceria on low-temperature NOx storage and reduction performance of LNT catalysts was investigated with the focus on NOx storage capacity, NOx reduction efficiency during lean/rich cycling, product selectivity and thermal stability. Inductively coupled plasma-atomic emission spectrometry (ICP-AES), Brunner–Emmet–Teller (BET), H2-pulse chemisorption and X-ray diffraction (XRD) were conducted to characterize the physical properties of LNT catalysts. NOx storage capacity and NOx conversion efficiency were measured to evaluate NOx storage and reduction performance of LNT catalysts. Pt/BaO/CeO2 − Al2O3 catalyst exhibits higher NOx storage capacity than Pt/BaO/CeO2 + Al2O3 catalyst in the temperature range of 150–250 °C. Meanwhile, Pt/BaO/CeO2 − Al2O3 catalyst shows better NOx conversion efficiency and N2 selectivity. XRD results indicate that the thermal stability of CeO2 − Al2O3 complex oxide is superior to that of pure CeO2. H2-pulse chemisorption results show that Pt/BaO/CeO2 − Al2O3 catalyst has higher Pt dispersion than Pt/BaO/CeO2 + Al2O3 catalyst over fresh and aged samples. The improved physical properties of Pt/BaO/CeO2 − Al2O3 catalyst are attributed to enhance the NOx storage and reduction performance over Pt/BaO/CeO2 + Al2O3 catalyst.

Impacts of ambient temperature, DPF regeneration, and traffic congestion on NOx emissions from a Euro 6-compliant diesel vehicle equipped with an LNT under real-world driving conditions

A real driving emissions test procedure was introduced as a supplement to the chassis dynamometer test to diminish the discrepancy between on-road emissions and type approval certification emissions. In this study, on-road NOx emissions from a 2.2 L diesel vehicle equipped with a lean NOx trap were measured not by a portable emissions measurement system but by NOx sensors and an exhaust flow meter. This method provides a strategy for analyzing on-road NOx emissions with a measurement system that is relatively cheap, light and simple. The effects of ambient temperature, diesel particulate filter regeneration, traffic congestion, NOx conversion efficiency and uphill/downhill sections on NOx emissions were evaluated by comparing the NOx emissions characteristics using engine-out and lean NOx trap-out NOx sensors. NOx emissions in congested traffic conditions were 29% higher than those in smooth traffic conditions. NOx emissions at 33 °C were 55% higher than those at 27 °C. Additionally, NOx emissions under specific conditions with diesel particulate filter regeneration were 30% higher than those under normal conditions. The average on-road NOx emission factor for all test cases was 7.35, but this value decreased to 5.7 when an ambient temperature corrective factor (1.6) was applied for extended test conditions.

Multiobjective Optimal Design of a Lean NOx Trap and Urealess Selective Catalytic Reduction Aftertreatment System under a Control Algorithm

A lean NOx trap (LNT) followed by selective catalytic reduction (SCR) without the urea injection system (LNT-urealess SCR, LNT-pSCR) has been developed to meet the stringent regulations on NOx emission. For the LNT-pSCR system to be commercialized, optimal design and control of the system are required. It is important to minimize the capital cost while satisfying the NOx regulation standards and minimizing fuel consumption. In this study, we propose a strategy to optimally design the lengths of the LNT and pSCR and tune control parameters by solving the multiobjective optimization problem and considering control logic simultaneously. For decision makers, NOx emission and capital cost are critical factors to consider. In addition, NH3 slip and fuel loss caused by post injection must be considered. The Pareto optimal points are obtained by solving the biobjective optimization problem with respect to NOx emission and capital cost while the other factors are constrained. Among the Pareto optimal points, we su...

Deactivation mechanism of Cu/Zeolite SCR catalyst under high-temperature rich operation condition

Selective catalytic reduction of NOx by urea (urea/SCR) and Lean NOx Trap (LNT) are two leading technologies for NOx reduction under lean condition. To quickly warm up SCR catalysts, considerable efforts have been made in integrating SCR onto a diesel particulate filter, so called SDPF. With the fact that the real driving emission test has taken effect, SDPF coupled with LNT might be one of most viable solutions to meet stringent emission regulations. However, desulfation process for LNT may result in a severe deactivation of SDPF in downstream. Here, we investigated the tolerance of Cu/zeolite SCR catalysts against periodic lean/rich operation condition simulating desulfation over LNT in front of SDPF. The lean/rich aging at 620 °C has led a severe deterioration of NOx conversion over Cu/SSZ-13 hydrothermally aged 800 °C, especially in high-temperature region above 400 °C. The accelerated NH3 oxidation to NO was the primary cause for the poor NOx removal activity, originated from the alteration of Cu state. Cu ions turned out to be converted to CuOx cluster upon their exposure to rich condition, which could partly be re-dispersed as Cu2+ ion after the post-treatment at 700 °C under lean condition. Cu/SSZ-13 with low Cu content revealed better activity maintenance under the high-temperature rich condition compared to higher Cu loading counterpart. Cu/LTA recently developed showed superior tolerance against rich feed due to less reducible nature of Cu+ ion therein compared to that in Cu/SSZ-13 leading to less CuOx formation.

A Data-Driven Model Predictive Control Approach to Lean NOx Trap Regeneration

Lean NOx trap (LNT) is one of the most effective after-treatment technologies used to reduce NOx emissions of diesel engines. One relevant problem in this context is LNT regeneration timing control. This problem is indeed difficult due to the fact that LNTs are highly nonlinear systems, involving complex physical/chemical processes, that are hard to model. In this paper, a novel approach for regeneration timing of LNTs is proposed, allowing us to overcome these issues. This approach, named data-driven model predictive control (D2-MPC), does not require a physical model of the engine/trap system but is based on low-complexity polynomial prediction models, directly identified from data. The regeneration timing is computed through an optimization algorithm, which uses the identified models to predict the LNT behavior. Two D2-MPC strategies are proposed, and tested in a co-simulation study, where the plant is represented by a detailed LNT model, built using the well-known commercial tool AMEsim, and the controller is implemented in matlab/simulink.

Detailed Characterization Studies of Vehicle and Rapid Aged Commercial Lean NOx Trap Catalysts

Commercial lean NOx trap (LNT) catalysts were aged and characterized to elucidate the effect of aging on their performances and examine the deactivation of a rapid-aged catalyst toward an improved correlation with respect to a vehicle-aged catalyst. Detailed characterization studies were carried out on the flow reactor with small cylindrical cores extracted from the commercial LNT catalysts. Physicochemical characterization techniques were also implemented. The catalyst evaluation revealed that aging resulted in a significant deterioration of NOx storage and reduction functions as a consequence of precious metal sintering, loss of surface area of the NOx storage and support materials, phase transitions of the adsorber compounds, and large accumulation of poison species. Among the aged samples examined, the middle (lengthwise) vehicle-aged sample showed the highest NOx conversion, while the oven-aged catalyst was the most active of the aged samples for water gas shift reaction and oxygen storage.

Redox reaction process between hydrocarbon and adsorbed NOx over lean NOx trap catalyst

In this study, we investigated the redox reaction between a hydrocarbon (HC) and adsorbed NOx over CuPtBa/Al2O3 lean NOx trap (LNT) catalyst. Experimental characterizations included temperature-programmed desorption (TPD), temperature-programmed reduction (TPR), in situ diffuse reflectance Fourier transform spectroscopy (in situ DRIFTs) and Raman spectroscopy. Density functional theory (DFT) calculations were also performed. The model C3H6 first adsorbed to the catalyst surface, and the resulting C3H6-adsorbed species reacted with adsorbed NOx species to form various intermediates such as: carboxylates, enolic compounds, carbonato complexes, Pt-carbonyls, acid anhydrides, basic hydroxyls, nitrogen-containing organic species, acyl chlorides, and ammonium salts. These intermediates participated in subsequent reactions and finally produced the effluent gases CO2, N2, NH3, and H2O. The decomposition ability of adsorbed NOx species indicated that the NOx species participated in the C3H6 oxidation in the order: adsorbed N2O4 > monodentate nitrates > free ionic nitrates > bulk free ionic nitrates. Carbonaceous materials were generated during the C3H6 oxidation process and were consumed as intermediates by gaseous NOx released upon decomposition of the adsorbed NOx species. In summary, the redox reaction between C3H6 and adsorbed NOx followed the Langmuir–Hinshelwood mechanism, and three reaction routes were proposed for the redox process over the studied catalyst. The activation energy barriers determined by DFT + U calculations for the three routes indicated that the initial dissociation of NO3− species in R(2) occurred more easily than the oxidation of C3H6 species in R(1), delivering the active oxygen species that participated in R(1) and R(3). As the reactions proceeded, the higher energy barrier for the complete dissociation of NO2* indicated that the temperature determined the further decomposition of adsorbed NOx species, and R(1) and R(3) were constrained owing to the limited surface active oxygen species from R(2).

Optimizing Hydrogen Production Capacity of Plasmatron Diesel Reformer

To curb exploding nitrogen oxides emission, Plasmatron converter offers a simple technique to convert diesel fuel into hydrogen rich gas. The H2 rich gas can be used for NOx trap applications, improving lean burn and reducing burn duration. In the present work, a plasmatron device is fabricated using G.I Pipe and electrode that produces low current non-thermal plasma. Two design of the device considering fuel inlet process is examined. The diesel reformate obtained is tested for hydrogen content by Gas Chromatography (GC) at four different fuel flow rates. The effectiveness of the designs is tested with respect to H2 content in the reformate and its corresponding fuel flow rate and found that atomized diesel spray using fuel injector is effective in comparison to fuel nozzle. GC results shows hydrogen quantity by volume percentage in the reformate is optimum within the range of 0.18g/s to 0.22g/s of diesel flow rate.

Gender: Fitness to plead for medical identity?

A fully 3D unsteady Computational Fluid Dynamics (CFD) approach coupled with heterogeneous reaction chemistry is presented in order to study the behavior of a single square channel as part of a Lean NOx Traps. The reliability of the numerical tool has been validated against literature data considering only active BaO site. Even though the input/output performance of such catalyst has been well known, here the spatial distribution within a single channel is investigated in details. The square channel geometry influences the flow field and the catalyst performance being the flow velocity distribution on the cross section non homogeneous. The mutual interaction between the flow and the active catalyst walls influences the spatial distribution of the volumetric species. Low velocity regions near the square corners and transversal secondary flows are shown in several cross-sections along the streamwise direction at different instants. The results shed light on the three-dimensional characteristic of both the flow field and species distribution within a single square channel of the catalyst with respect to 0-1D approaches.

Elucidating the mechanism of fast cycling NOx storage and reduction using C3H6 and H2 as reductants

A focused experimental study describes fast cycling NOx storage and reduction (NSR) with H2 and C3H6 as reductants on a Pt/BaO/CeO2/Al2O3 lean NOx trap (LNT) catalyst for emission control of lean burn vehicles. The results provide insight and deeper understanding of the impact of cycle frequency on the LNT performance and underlying NSR mechanism, with particular focus on whether a new NOx reduction pathway emerges when hydrocarbon reductant is used. Differentiation is made between two NOx conversion enhancement mechanisms—the conventional NSR mechanism with improved NOx storage utilization and a hydrocarbon-intermediate mechanism. A comparison of cycle-average NOx conversion using H2 and C3H6 as reductants at the same lean/rich stoichiometry level provides revealing information. Both aerobic + anhydrous and anaerobic + anhydrous propylene-containing feeds enable an estimate of the contribution of H2-generating water-gas shift and propylene steam reforming. The observed NOx conversion enhancement of up to 45% (absolute) with 10× faster cycling is nearly independent of reductant type and is attributed to improved NOx storage utilization. Exothermic heat effects with temperature rise as high as 275 °C and significant temperature spatial gradients are encountered with H2 during slow cycling, which makes H2 an unexpectedly less effective reductant than C3H6. The hydrocarbon-intermediate pathway is shown to be responsible for a smaller fraction of the NOx conversion enhancement encountered during fast cycling.