Lean NOx Trap Catalyst Research Articles

Effect of aging on the NOx storage and regeneration characteristics of fully formulated lean NOx trap catalysts

In order to elucidate the effect of washcoat composition on lean NOx trap (LNT) aging characteristics, fully formulated monolithic LNT catalysts containing varying amounts of Pt, Rh and BaO were subjected to accelerated aging on a bench reactor. Subsequent catalyst evaluation revealed that in all cases aging resulted in deterioration of the NOx conversion as a consequence of impaired NOx storage and NOx reduction functions, while increased selectivity to NH3 was observed in the temperature range 250–450°C. Elemental analysis, H2 chemisorption and TEM data revealed two main changes which account for the degradation in LNT performance. First, residual sulfur in the catalysts, associated with the Ba phase, decreased catalyst NOx storage capacity. Second, sintering of the precious metals in the washcoat occurred, resulting in decreased contact between the Pt and Ba, and hence in less efficient NOx spillover from Pt to Ba during NOx adsorption, as well as decreased rates of reductant spillover from Pt to Ba and reverse NOx spillover during catalyst regeneration. For the aged catalysts, halving the Pt loading from 100 to 50g/ft3 was found to result in a significant decrease in overall NOx conversion, while for catalysts with the same 100g/ft3 Pt loading, increasing the relative amount of Pt on the NOx storage components (BaO and La-stabilized CeO2), as opposed to an Al2O3 support material (where it was co-located with Rh), was found to be beneficial. The effect of Rh loading on aged catalyst performance was found to be marginal within the range studied (10–20g/ft3), as was the effect of BaO loading in the range 30–45g/L.

The NOx Reduction by CO on a Pt−K/Al2O3 Lean NOx Trap Catalyst

The reduction by CO under dry conditions of NOx species stored at 350 °C onto a Pt−K/Al2O3 lean NOx trap catalyst is investigated by means of transient response methods (CO-TPSR and CO-ISC experiments) and complementary FT-IR spectroscopy. The results show that the pathway for the reduction of stored NOx by CO under dry and near isothermal conditions (and in the absence of CO2) is the same as that proposed by some of us in a previous work for a Pt−Ba/Al2O3 catalyst. In particular, (i) the reduction of stored NOx by CO occurs according to a Pt-catalyzed surface pathway that does not involve, as a first step, the release of NOx in the gas phase, is effective already at low temperature, and leads to nitrogen; (ii) the reaction scheme implies the formation of surface isocyanate species, followed by the reaction of these species with residual NOx to give nitrogen; and (iii) the reaction of NCO species with nitrates to give nitrogen is slightly slower than the reduction of nitrates to give NCO species; but on the Pt−K/Al2O3 catalyst, this last step is faster than on the Pt−Ba/Al2O3 system. As a consequence, the amount of isocyanate species present on the surface at the end of the reduction is lower for Pt−K/Al2O3 than for the Pt−Ba/Al2O3 catalyst.

Lean Gasoline Engine Reductant Chemistry During Lean NOx Trap Regeneration

Lean Gasoline Engine Reductant Chemistry During Lean NOx Trap Regeneration

Effective parameters for DME steam reforming catalysts for the formation of H 2 and CO

Recently, DME has received attention as a clean fuel and is now considered an alternative fuel for diesel engines. DME diesels need de-NOx catalysts such as LNT (Lean NOx Trap) and SCR (Selective Catalytic Reduction) systems. DME is an attractive source of hydrogen because it can be stored easily and is a good transportation fuel. Hydrogen and CO enriched gas as a reductant was used with the LNT catalyst in order to reduce NOx emissions. The steam reforming catalyst of DME was used to formation of hydrogen. It has been reported that Cu-based catalysts have high selectivity and activity in the steam reforming of DME. This research used 600 cPsi cordierite as a catalyst, which was coated with copper. The catalysts were made via a sol–gel and impregnation methods. The formation of H 2 and CO under the prepared catalysts was tested by a model gas. Experimental parameters were considered; the space velocity (SV) and concentrations of H 2O, O 2, and CO 2 were evaluated. The Cu 30%/γ-Al 2O 3 catalyst from the sol–gel method exhibited high and stable activity in the production of hydrogen from the steam reforming of DME. Both DME conversion and the selectivity of hydrogen were affected by SV and the concentrations of H 2O, O 2, and CO 2.

Spatially-Resolved Thermal Degradation Induced Temperature Pattern Changes along a Commercial Lean NOX Trap Catalyst

Spatially-Resolved Thermal Degradation Induced Temperature Pattern Changes along a Commercial Lean NOX Trap Catalyst

Preparation and screening of various multi-component catalysts for NOx conversion under lean-burn conditions: An active and heat-resistant RhPt–NaMn–Ce/Al 2O 3 catalyst

The present work has been undertaken to improve the thermal stability of an RhPt–NaTi/Al 2O 3 catalyst, which is one of promising lean NOx trap catalysts for practical NOx conversion. For this purpose, we mainly directed our attention to the component of Na that acts as NOx trapping sites and intended to improve the thermal stability of this Na species by using various additives. After screening of various RhPt–Naα/Al 2O 3 (α = Ti, Si, Mg, Ca, Mn, Co, Ni, La, and Nd) catalysts, Mn addition was found to be effective for improving thermal stability of the catalyst. Moreover, the modification of Al 2O 3 support with Ce additive was indicated to be effective, and the most active and thermally stable catalyst was RhPt–NaMn–Ce/Al 2O 3, which was able to function actively even after a severe heat treatment at 800 °C. The reasons for the high thermal stability of this catalyst were examined by evaluating NO oxidation activity of a model catalyst including Mn and characterizing features of the RhPt–NaMn–Ce/Al 2O 3 catalyst with FTIR of adsorbed NO and chemisorption of CO and CO 2. An important factor is that the addition of Mn prevents the Na species from undesired sintering during heat treatments, resulting in a larger surface area of exposed Na species, namely NOx trapping sites. This catalyst can also function in a commercial vehicle engine under lean-burn conditions and should be a promising catalyst for practical use.

NO x storage–reduction characteristics of Ba-based lean NO x trap catalysts subjected to simulated road aging

In order to study the effect of washcoat composition on lean NO x trap (LNT) aging characteristics, fully formulated monolithic LNT catalysts containing varying amounts of La-stabilized CeO 2 (5 wt% La 2O 3) or CeO 2-ZrO 2 (Ce:Zr = 70:30) were subjected to accelerated aging on a bench reactor. Subsequent catalyst evaluation revealed that aging resulted in deterioration of the NO x storage, NO x release and NO x reduction functions, whereas the observation of lean phase NO 2 slip for all of the aged catalysts indicated that LNT performance was not limited by the kinetics of NO oxidation. After aging, all of the catalysts showed increased selectivity to NH 3 in the temperature range 250–450 °C. TEM, H 2 chemisorption, XPS and elemental analysis data revealed two main changes which can explain the degradation in LNT performance. First, residual sulfur in the catalysts, present as BaSO 4, decreased catalyst NO x storage capacity. Second, sintering of the precious metals in the washcoat was observed, which can be expected to decrease the rate of NO x reduction. Additionally, sintering is hypothesized to result in segregation of the precious metal and Ba phases, resulting in less efficient NO x spillover from Pt to Ba during NO x adsorption, as well as decreased rates of reductant spillover from Pt to Ba and reverse NO x spillover during catalyst regeneration. Spectacular improvement in LNT durability was observed for catalysts containing CeO 2 or CeO 2-ZrO 2 relative to their non-ceria containing analog. This was attributed to (i) the ability of ceria to participate in NO x storage/reduction as a supplement to the main Ba NO x storage component; (ii) the fact that Pt and CeO 2(-ZrO 2) are not subject to phase segregation; and (iii) the ability of ceria to trap sulfur, resulting in decreased sulfur accumulation on the Ba component.

The influence of hydrogen-enriched gas on the performance of lean NO x trap catalyst for a light-duty diesel engine

Modern diesel engines have improved engine fuel economy and significantly reduced nitrogen oxides (NO x ) and particulate matter (PM) emissions achieved by advances in both combustion and exhaust aftertreatment technologies. Recently, it has been shown that the vehicle emissions can be further improved by several catalytic systems including fuel reformers and aftertreatment systems, such as the Lean NO x Trap (LNT). This NO x removal system, called LNT, absorbs NO x under lean exhaust gas conditions and releases NO x under rich conditions. This technology can provide high NO x conversion efficiency, but the right amount of reducing agent should be supplied into the catalytic converter under appropriate conditions. In this work, plasma reformer was used to supply a hydrogen-enriched gas as a NO x reductant. The plasma reforming is one of the most promising on-board reforming technologies, which allows reformates containing H 2 and CO to feed for LNT catalyst efficiently. Partial oxidation is induced by plasma in the fuel reformer and diesel fuel is converted into a hydrogen-enriched gas. The supplying strategy was focused on the maximization of NO x reduction efficiency varying both the total amount of hydrogen-enriched reformate and the ratio of oxygen molecules to in the reformer air-fuel mixture prior to processing at a fixed engine operating condition. The effect of exhaust gas temperature was also studied. The NO x reduction efficiency is closely connected to the amount of supplied fuel to the plasma reformer and the ratio of fuel/air feed rate. The LNT can reduce NO x efficiently with only a 2.6% fuel penalty.

Characteristics of Desulfation Behavior for Presulfated Pt-BaO/CeO2 Lean NOx Trap Catalyst: The Role of the CeO2 Support

The desulfation of presulfated Pt-BaO/CeO2 lean NOx trap catalyst was investigated by H2 TPRX (temperature programmed reaction), in situ TR-XRD (time-resolved X-ray diffraction), and in situ S K-edge XANES (X-ray absorption near edge spectroscopy) techniques. Compared with Pt-BaO/Al2O3 materials, a reductive treatment in H2 for the CeO2-supported sample up to 1073 K removes, at most, only a very small amount of sulfur species. However, the results of in situ TR-XRD measurements demonstrate that the quantity of a BaS phase formed on Pt-BaO/CeO2 is much smaller than that on Pt-BaO/Al2O3, implying that the formation of BaS crystallites, which occurs during the reduction from sulfate (SO42−) to sulfide (S2−), is significantly suppressed in the CeO2-supported catalyst. As the desulfation temperature increases under reducing conditions (in H2), in situ S XANES spectra show that, compared with alumina-supported samples, the reduction temperature for sulfates (S6+) decreases by about 150 K. Concomitantly, the for...

Study of Ammonia Formation During the Purge of a Lean NO x Trap

The aim of this work was to study the formation of ammonia during the regeneration step of a commercial lean NOxTrap catalyst in real conditions. Experiments were carried out on an engine test bench equipped with 2.2 L common rail diesel engine. The after-treatment system was composed of a commercial NSR monolith catalyst with a volume of 2.2 L. This material was composed of platinum, palladium and rhodium, as active components, with barium oxide and alumina as storage component and support, respectively. Catalytic measurements were carried out at 280 °C. Particular attention has been paid to formation of ammonia during the purge using hydrogen and CO as reducing agents. The time dependency of the extent of ammonia formation and the influence of hydrogen concentration were carefully examined. It was observed that the temperature rose in the first step of the regeneration process, after the switch in rich conditions, due to the occurrence of exothermic oxidation (H2/O2, CO/O2) accompanied with the releasing of available vacant noble metal sites which may provide a route for H2 and NO dissociation and/or subsequent surface reactions leading to the ultimate formation of ammonia. It was found that ammonia formation strongly depends on the purge duration and on the concentration of the reducing agent available during the purge.

Promotional Effect of CO2 on Desulfation Processes for Pre-Sulfated Pt-BaO/Al2O3 Lean NOx Trap Catalysts

A combination of H2 TPRX, TR-XRD and XPS analysis has been used to investigate the effects of CO2 on the desulfation of pre-sulfated Pt-BaO/Al2O3 samples. The results demonstrate that the presence of CO2 promotes the removal of sulfur species, especially at temperatures below 500 °C, with a corresponding suppression of BaS formation, thus resulting in a lower amount of residual sulfur on the sample after desulfation.

NOx storage and reduction in model lean NOx trap catalysts studied by in situ DRIFTS

NOx storage and reduction on a model Pt/BaO/Al2O3 catalyst was studied by means of in situ DRIFTS measurements. To examine the effect of ceria addition, experiments were also conducted using Pt/BaO/Al2O3 to which Pt/CeO2 was added as a physical mixture in a 74:26 weight ratio. For the former catalyst, DRIFT spectra acquired during NO/O2 and NO2/O2 storage indicated the formation of nitrite at 200°C during the initial stages of adsorption, while increasing the adsorption temperature appeared to facilitate the oxidation of nitrite to nitrate. The ceria-containing catalyst afforded similar DRIFT spectra under these conditions, although the presence of cerium nitrates was observed at 200 and 300°C, consistent with NOx storage on the ceria phase. DRIFT spectra acquired during NOx reduction in CO and CO/H2 showed that Ba nitrate species remained on the surface of both catalysts at 450°C, whereas the use of H2-only resulted in complete removal of stored NOx. The observation of Ba carbonates when CO was present suggests that the inferior reduction efficiency of CO may arise from the formation of a crust of BaCO3 on the Ba phase, which inhibits further NOx reduction. DRIFT spectra acquired during lean-rich cycling (6.5min lean, 1.0min rich) with CO/H2 as the rich phase reductants revealed that a significant concentration of nitrates remained on the catalysts at the end of the rich phase. This implies that a large fraction of nitrate is not decomposed during cycling and thus cannot participate in NOx abatement through storage and regeneration.

Excellent sulfur resistance of Pt/BaO/CeO2 lean NOx trap catalysts

In this work, we investigated the NOx storage behavior of Pt/BaO/CeO2 catalysts, especially in the presence of SO2. High surface area CeO2 (∼110m2/g) with a rod like morphology was synthesized and used as a support. The Pt/BaO/CeO2 sample demonstrated slightly higher NOx uptake in the entire temperature range studied compared with Pt/BaO/γ-Al2O3. More importantly, this ceria-based catalyst showed higher sulfur tolerance than the alumina-based one. The time of complete NOx uptake was maintained even after exposing the sample to ∼3g/L of SO2. The same sulfur exposure, on the other hand, eliminated the complete NOx uptake time on the alumina-based NOx storage catalysts. TEM images show no evidence of either Pt sintering or BaS phase formation during reductive de-sulfation up to 600°C on the ceria-based catalyst, while the same process over the alumina-based catalyst resulted in both a significant increase in the average Pt cluster size and the agglomeration of a newly formed BaS phase into large crystallites. XPS results revealed the presence of about five times more residual sulfur after reductive de-sulfation at 600°C on the alumina-based catalysts in comparison with the ceria-based ones. All of these results strongly support that, besides their superior intrinsic NOx uptake properties, ceria-based catalysts have (a) much higher sulfur tolerance and (b) excellent resistance against Pt sintering when they are compared to the widely used alumina-based catalysts.

Promotional Effects of H2O Treatment on NO x Storage Over Fresh and Thermally Aged Pt–BaO/Al2O3 Lean NO x Trap Catalysts

A simple liquid water treatment applied to fresh and thermally aged Pt(2 wt%)–BaO(20 wt%)/Al2O3 lean NOx trap catalysts at room temperature induces morphological and structural changes in the barium species as followed by XRD and TEM analysis. During the water treatment, liquid water sufficient to fill the catalyst pore volume is brought into contact with the samples. It was found that irrespective of the original barium chemical state (highly dispersed BaO or crystalline BaAl2O4), exposing the sample to this liquid water treatment promotes the formation of BaCO3 crystallites (about 15–25 nm of its size) without changing the Pt particle size. Such transformations of the barium species are found to significantly promote NOx uptake from 250 to 450 °C. The increase in the NOx uptake for the water-treated samples can be attributed to an enhanced Pt–Ba interaction through the redistribution of barium species. These results provide useful information for the regeneration of aged lean NOx trap catalysts since water is plentiful in the exhaust of diesel or lean-burn engines.

Study on the Spray Behavior and Diesel Fuel Distribution Characteristics of a Secondary Injector for a Lean NOx Trap Catalyst

A lean NOx trap (LNT) catalyst has been used to suppress NOx emission from a diesel engine. A need to control the LNT catalyst has helped introduce a hydrocarbon−lean NOx trap (HC−LNT) system. The system has a secondary injector that injects diesel fuel (HC) into an exhaust pipe. A typically high temperature (i.e., 250−350 °C) in the exhaust pipe affects the spray behavior of the secondary injector. Therefore, in this research, the spray behavior of HC injected into a transparent exhaust manifold was investigated. First, the atmospheric injection characteristic of the secondary injector was analyzed. The injector has a solid-cone-shaped structure that can concentrate the spray along the central axis. The atomization process of the injector is dominated by the liquid film breakup, which generates many droplets with broad dispersion angle. Additionally, the injector was attached in the transparent exhaust manifold, and experiments were conducted in various conditions of the exhaust gas. The results show tha...

Sequential high temperature reduction, low temperature hydrolysis for the regeneration of sulfated NO x trap catalysts

We describe a new method that minimizes irreversible Pt sintering during the desulfation of sulfated Pt/BaO/Al 2O 3 lean NO x trap (LNT) catalysts. While it is known that the addition of H 2O to H 2 promotes desulfation, we find that the significant and irreversible Pt sintering arising from the presence of water is unavoidable. Control of precious metal sintering is considered to be one of the critical issues in the development of durable LNT catalysts. The new method described here is a sequential desulfation process: the first step is to reduce the sulfates with hydrogen only at higher temperatures to form BaS, followed by a treatment of the thus reduced sample with water at low to moderate temperatures to convert BaS to BaO and H 2S. The data showed that Pt sintering was significantly inhibited due to the absence of H 2O during the desulfation at high temperatures, and also demonstrates the similar NO x uptake with the desulfated sample cooperatively with H 2 and H 2O. This study clearly revealed both positive and negative roles for water in desulfation processes and these factors must be considered when optimizing LNT operation.

A one-dimensional lean NOx trap model with a global kinetic mechanism that includes NH3 and N2O

Eaton has developed an aftertreatment system for medium- and heavy-duty diesel engines in response to the US 2010 regulations. This system consists of a fuel reformer, a lean NO x trap (LNT), and an ammonia selective catalytic reduction (SCR) catalyst in series. A transient, one-dimensional model of the system was developed to improve system performance, reduce experimental testing, and optimize system design. In this paper, the LNT portion of this model is presented. The model simulates flow, heat transfer, and chemical reactions in the LNT catalyst. A global LNT chemical kinetic mechanism was developed to simulate the key catalytic processes with the minimum number of reactions. The model can be used to predict LNT catalyst performance over a range of operating conditions and driving cycles. Simulated species concentrations and gas temperatures at the LNT outlet were compared with experimental data at three steady state engine conditions over a 13-mode test. The conditions were chosen to develop and test the model over a range of gas temperatures, space velocities, and species concentrations. The LNT model predicts species trends and magnitudes with reasonable accuracy in comparison with experimental data. The simulated LNT NO x conversion efficiency over the 13-mode test was 67 per cent, compared with 63 per cent for the experiment.

Roles of Pt and BaO in the Sulfation of Pt/BaO/Al2O3 Lean NOx Trap Materials: Sulfur K-edge XANES and Pt LIII XAFS Studies

The roles of barium oxide and platinum during the sulfation of Pt−BaO/Al2O3 lean NOx trap catalysts were investigated by S K edge XANES (X-ray absorption near-edge spectroscopy) and Pt LIII XAFS (X-ray absorption fine structure). All of the samples studied [Al2O3, BaO(x; x = 8 or 20 wt %)/Al2O3, Pt(2.5 wt %)/Al2O3, and Pt(2 wt %)-BaO(x; x = 8 or 20 wt %)/Al2O3] were pre-sulfated prior to the X-ray absorption measurements. It was found that barium oxide itself has the ability to directly form barium sulfate even in the absence of Pt and gas-phase oxygen. In the platinum-containing samples, the presence of Pt−O species plays an important role in the formation of sulfate species. For the case of the BaO(8)/Al2O3 sample, where the barium coverage is about 0.26 ML, both baria and alumina phases are available for sulfation. S XANES results show that barium sulfates are formed preferentially over aluminum sulfates. When oxygen is absent from the gas phase, the sulfation route that involves Pt−O is eliminated after the initially present Pt−O species are completely consumed. In this case, formation of sulfates is suppressed unless barium oxide is also present. Pt LIII XAFS results show that the first coordination sphere around the Pt atoms in the Pt particles is dependent upon the gas mixture used during the sulfation process. Sulfation under reducing environments (e.g., SO2/H2) leads to formation of Pt−S bonds, while oxidizing conditions (e.g., SO2/O2) continue to show the presence of Pt−O bonds. In addition, a reducing environment was found to cause Pt sintering in greater extent than an oxidizing one. This result explains why samples sulfated under reducing conditions had lower NOx uptakes than those sulfated under oxidizing conditions. Therefore, our results provide needed information for the development of optimum practical operation conditions (e.g., sulfation or desulfation) for lean NOx trap catalysts that minimize deactivation by sulfur.

A Study on the Optimal Injection Conditions for an HC-LNT Catalyst System with a 12-Hole Type Injector

NOx catalytic converter systems periodically require rich or stoichiometric operating conditions to reduce NOx. The HC (hydrocarbon) concentration in a diesel engine is typically so low that the HC is not sufficient for NOx conversion. It was proposed that a rich air fuel ratio in a diesel engine could be realized via post fuel injection or supplemental fuel injection into the exhaust gas. A new method that optimizes the control of an external HC injection to diesel exhaust pipes for HC type LNT (Lean NOx Trap) catalyst systems has been developed. The external injection has other benefits: it can be controlled independently without disturbing engine control, it can be adapted to various layouts for exhaust systems, it has no oil dilution problems, among other benefits. In this study, the concentration and amount of HC can be controlled via control of the external injection. This research investigated the spray behavior of hydrocarbons injected into the transparent exhaust pipe. From this experiment, we obtained useful information about the optimal injection conditions for the HC-LNT catalyst system with an MPI injector.

Effect of Ceria on the Storage and Regeneration Behavior of a Model Lean NO x Trap Catalyst

In this study the effect of ceria addition on the performance of a model Ba-based lean NOx trap (LNT) catalyst was examined. The presence of ceria improved NOx storage capacity in the temperature range 200–400 °C under both continuous lean and lean-rich cycling conditions. Temperature-programmed experiments showed that NOx stored in the ceria-containing catalyst was thermally less stable and more reactive to reduction with both H2 and CO as reductants, albeit at the expense of additional reductant consumed by reduction of the ceria. These findings demonstrate that the incorporation of ceria in LNTs not only improves NOx storage efficiency but also positively impacts LNT regeneration behavior.