NOx Storage And Reduction Research Articles

NOx Storage and Reduction (NSR) Performance of Sr-Doped LaCoO3 Perovskite Prepared by Glycine-Assisted Solution Combustion

Here, we successfully synthesized Sr-doped perovskite-type oxides of La1−xSrxCo1−λO3−δ, “LSX” (x = 0, 0.1, 0.3, 0.5, 0.7), using the glycine-assisted solution combustion method. The effect of strontium doping on the catalyst structure, NO to NO2 conversion, NOx adsorption and storage, and NOx reduction performance were investigated. The physicochemical properties of the catalysts were studied by XRD, SEM-EDS, N2 adsorption–desorption, FTIR, H2-TPR, O2-TPD, and XPS techniques. The NSR performance of LaCoO3 perovskite was improved after Sr doping. Specifically, the perovskite with 50% of Sr doping (LS5 sample) exhibited excellent NOx storage capacity within a wide temperature range (200–400 °C), and excellent stability after hydrothermal and sulfur poisoning. It also displayed the highest NOx adsorption–storage capacity (NAC: 1889 μmol/g; NSC: 1048 μmol/g) at 300 °C. This superior performance of the LS5 catalyst can be attributed to its superior reducibility, better NO oxidation capacity, increased surface Co2+ concentration, and, in particular, its generation of more oxygen vacancies. FTIR results further revealed that the LSX catalysts primarily store NOx through the “nitrate route”. During the lean–rich cycle tests, we observed an average NOx conversion rate of over 50% in the temperature range of 200–300 °C, with a maximum conversion rate of 61% achieved at 250 °C.

Enhanced low-temperature NOx storage and reduction performance using Cu-loaded lean NOx trap catalysts

Lean NOx trap (LNT) used in diesel-powered vehicles exhibit poor NOx storage and reduction (NSR) performance during low-temperature operation, particularly under cold-start conditions. To overcome this limitation, Cu was added to a conventional LNT catalyst (Pt-Ba/γ-Al2O3, PBA) to improve the NOx storage and regeneration performance of the Cu-loaded Ba adsorbent (Cu-Ba/γ-Al2O3) during low-temperature operation. However, a Pt-Cu complex was formed when Pt and Cu were used together as a one-body catalyst (Pt-Cu-Ba/γ-Al2O3, PCBA), leading to decreased catalytic performance. Physical mixing was investigated as a method to prevent the catalytic performance degradation caused by the Pt-Cu complex. The physical mixture of Pt/γ-Al2O3 and Cu-Ba/γ-Al2O3 (PA + CBA) could prevent the formation of Pt-Cu complex and yielded the highest NSR performance at 150 ℃ among the synthesized LNT catalysts. PA + CBA maintained a significantly improved NSR performance in lean-rich cycling tests using CO or H2 as a reducing agent.

Comparison of Cu-CHA-Zeolites in the Hybrid NSR-SCR Catalytic System for NOx Abatement in Mobile Sources

DeNOx activity in a NSR–SCR hybrid system of two copper-containing chabazite-type zeolitic catalysts was addressed. A Pt-Ba-K/Al2O3 model catalyst was used as the NSR (NOx storage and reduction) catalyst. For the SCR (selective catalytic reduction) system, two Cu-CHA zeolites were synthesized employing a single hydrothermal synthesis method assisted with ultrasound and incorporating Cu in a 2 wt.%, 2Cu-SAPO-34 and 2Cu-SSZ-13. The prepared catalysts were characterized, and the crystallinity, surface area, pore size, HR-TEM and EDX mapping, coordination of Cu ions and acidity were compared. The NH3 storage capacity of the SCR catalysts was 1890 and 837 μmol NH3·gcat−1 for 2Cu-SAPO-34 and 2Cu-SSZ-13, respectively. DeNOx activity was evaluated for the single NSR system and the double-bed NSR–SCR by employing alternating lean (3%O2) and rich (1%H2) cycles, maintaining a concentration of 600 ppm NO, 1.5% H2O and 0.3% CO2 between 200 and 350 °C. The addition of the SCR system downstream of the NSR catalyst significantly improved NOx conversion mainly at low temperature, maintaining the selectivity to N2 above 80% and reaching values above 90% at 250 °C when the 2Cu-SSZ-13 catalyst was located. The total reduction in the production of NH3 and ~2% of N2O was observed when comparing the NSR–SCR configuration with the single NSR catalyst.

Storage-reduction strategy for NOx reduction from gas turbine exhaust with W–Ti-CeOx catalyst in natural gas power plants

Traditional vanadium-based selective catalytic reduction (SCR) deNOx catalyst can hardly adapt to the gas conditions (much high NO2/NOx ratio at lower temperature) of the start-up and low loading periods for a gas turbine. Therefore, a W–Ti-CeOx catalyst with NOx storage and reduction (NSR) function was developed in this work for gas turbine exhaust NOx elimination. The experimental results reveal that W–Ti-CeOx catalyst exhibits high NO2 adsorption capacity at relatively low temperature while that is quite low for V–W/TiO2. The abundant surface Ce3+ species can be mainly responsible for its high adsorption ability owing to the reaction between NO2 and Ce3+ to form nitrate/nitrite species and NO. Meanwhile, the adsorption capacity of W–Ti-CeOx can easily regenerate at medium–high temperature and NH3-SCR reaction. Furthermore, W–Ti-CeOx also shows good NH3-SCR activity, which can fulfill the deNOx process at high temperature. The addition of W and Ti into ceria can enhance the surface acidity and redox ability, thereby increasing the SCR activity. This work proposes a novel storage-reduction strategy for NOx elimination throughout the operation of gas turbines.

Treating NOx emission of hydrogen fueled combustion engines by NOx storage and reduction catalysts: A transient kinetic study including PLIF measurements

NOx storage and reduction (NSR) catalysts are a well-known and broadly used technology to reduce NOx emissions from combustion engines, which may also be applied for hydrogen fueled engines in the future. In this study, Pt- and Pd-based NSR-catalysts were investigated in the absence and presence of water to understand how NO oxidation as well as the storage and reduction phases are influenced by the gaseous environment with H2 as a reductant. A planar channel configuration was chosen for conducting planar laser-induced fluorescence experiments during the storage phase in addition to steady-state oxidation measurements and transient lean/rich cycles in a packed bed reactor. The presence of steam significantly decreases the NO oxidation activity of both noble metal catalysts. The Pt/BaO/Al2O3 catalyst is more active during transient lean/rich cycles, however, it suffers an activity loss during repeated cycles, whereas the activity of the Pd/BaO/Al2O3 sample is slightly more stable in the wet gas feed over time. All experiments showed a strong correlation between the NO2 formation over the catalyst and its storage capability. The influence of water in the exhaust gas on the NSR-catalysts shows a strong temperature dependency on storage and reduction of NO for both catalysts containing Pt and Pd. The storage behavior is also strongly influenced by both the experimental configurations chosen revealing the significance of the interaction of intrinsic catalytic kinetics and mass transfer in the surrounding flow field.

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.

Role of Ba in an Al2O3‐Supported Pd‐based Catalyst under Practical Three‐Way Catalysis Conditions

AbstractBarium (Ba) has been widely and successfully added to three‐way catalysts to effectively control gasoline vehicle emissions. While the introduction of Ba is well‐known to have a very significant impact on three‐way catalysis (TWC) performance, the role of Ba under practical TWC conditions remains insufficiently understood. In this study, we investigated the role of Ba in a Pd‐based catalyst in a standard TWC process using a gasoline vehicle and in situ/operando infrared (IR) spectroscopy. The addition of Ba was found to reduce all of the emissions considered (NOx, CO, and non‐methane hydrocarbons) in the vehicle exhaust gas; it also suppressed supported‐metal aggregation, which is a requirement of a high‐performance catalyst. In addition, IR studies using model powder catalysts revealed that the efficient formation of intermediate surface nitrate species and their subsequent reactions with reductant gases, such as CO, H2, and C3H6 (as a hydrocarbon), play key roles in reducing emissions. This process is akin to the NOx storage and reduction (NSR) mechanism commonly used to control diesel emissions.

Mesoporous LaCoO3 perovskite oxide with high catalytic performance for NOx storage and reduction

A mesoporous LaCoO3 perovskite oxide (LaCoO3-Meso) with three-dimensionally ordered helical interwoven structure was synthesized by a nano-casting method using KIT-6 as the hard template. The obtained LaCoO3-Meso with high surface area was tested for its catalytic performance in the NOx storage and reduction (NSR) reaction and compared with a sample synthesized by the conventional sol-gel method. The LaCoO3-Meso showed a significant advantage for NOx storage, with a NOx storage capacity 2 times higher than the regular sample. LaCoO3-Meso also exhibited improved NSR catalytic performance in the 150–450 °C temperature range, especially within 350–400 °C, where the NOx conversion was raised for 40%. The results of X-ray photoelectron spectroscopy and X-ray absorption fine structure measurements suggested the presence of a high concentration of oxygen defects on the LaCoO3-Meso surface. Further results provided by temperature programmed reduction and temperature programmed desorption indicated that the oxygen defects not only increase the amount of trapped NOx, but also improve the low-temperature redox performance of the catalyst. The lower stability of NOx species adsorbed on oxygen defects promotes the NOx release step in the NSR reaction and benefits the regeneration of storage sites.

Structured NSR-SCR hybrid catalytic technology: Influence of operational parameters on deNOx activity

An intermediate approximation using structured reactors between the study of deNOx catalysts at laboratory scale and in motor bench has been addressed. The hybrid NSR (NOx Storage and Reduction) and SCR (Selective Catalytic Reduction) technology has been analyzed for Pt-Ba-K/Al2O3 and Cu-SAPO-34 catalysts structured in monolithic form. In order to analyze the influence of the way of driving on deNOx efficiency, operational parameters such as NO inlet concentration, space velocity in terms of total flow rate, temperature, and the concentration of reductant agent, using H2 and C3H6, have been modified. The coupled NSR-SCR configuration is more efficient than the single NSR alternative, since the former used the ammonia produced in NSR monolith to reduce the non-adsorbed NOx through the SCR process over the system placed downstream. However, NH3 production depended on NO concentration, temperature, and reductant agent content in the process, limiting the promoting effect at high temperatures and NO concentrations, and low hydrogen concentration. N2 selectivity increased when temperature increased, and NO concentration and hydrogen content decreased. On the other hand, NOx to N2 reduction efficiency of the combined NSR-SCR system was not affected by variations in space velocity, with a maximum range studied up to 90,000 h−1. Based on these results, the location of the catalytic system should be adjusted in order to work in the 250–350 °C temperature range, where conversion and N2 selectivity values were around 70% and 95%, respectively. On the other hand, the hydrogen concentration of 2% and 3% can be suitable for the efficient operation of the system, since as reductant agent increased the ammonia formation and the surface regeneration also increased. The substitution of H2 by propylene as reducing agent caused a decrease in the amount of ammonia produced in the NSR configuration, which limits the improvement of deNOx activity of the double NSR-SCR configuration, and overproduces CO2, CO and N2O through parallel reactions as reverse water gas shift or C3H6 oxidation, which could poison the system and require emission control through other technologies.

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.

Lean NOx trap catalysts with high low-temperature activity and hydrothermal stability

A combination of the Pt-BaO/CeO2 and Cu/CeO2 (PBC + CC) catalyst offers high NOx storage and reduction (NSR) performance at low temperatures; however, the PBC + CC catalyst is prone to deactivation upon hydrothermal aging. In this study, we used a reduced γ-Al2O3 support with unsaturated penta-coordinated Al3+ sites, which can anchor catalytic materials (Pt, Cu, BaO, and CeO2). The physical mixture of Pt-BaO-CeO2 and Cu-CeO2 deposited on the reduced Al2O3 (PBCrA + CCrA) was prepared and tested for the NOx removal. The PBCrA + CCrA catalyst showed little degradation after hydrothermal aging at 750 °C. XRD, HR-TEM, CO chemisorption, N2O chemisorption, and N2 physisorption results showed that catalytically active phases were barely changed even after the hydrothermal aging, particularly preventing ceria sintering. CO-TPR and XPS results demonstrated that the PBCrA + CCrA catalyst has a superior reducibility of lattice oxygen on CeO2, leading to excellent low-temperature activity.

Hybrid Technology for DeNOxing by LNT-SCR System for Efficient Diesel Emission Control: Influence of Operation Parameters in H2O + CO2 Atmosphere

The behavior and operation parameters were analyzed for the hybrid LNT-SCR (Lean NOx-Trap–Selective Catalytic Reduction) system with advanced catalyst formulations. Pt-Ba-K/Al2O3 was used as an NSR (NOx Storage and Reduction) or LNT catalyst effective in NOx and soot simultaneous removal whereas Cu-SAPO-34 with 2 wt.% of copper inside the structure was the small pore zeolite employed as the SCR catalyst. Under alternating and cyclic wet conditions, feeding volumetric concentrations of 1000 ppm of NO, 3% of O2, 1.5% of water, 0.3% of CO2, and H2 as a reductant, the NOx-conversion values were above 95% and a complete mineralization to nitrogen was registered using θ ≤ 3 (20 s of regeneration) and a hydrogen content between 10,000 and 2000 ppm in the whole temperature range tested. An excess of hydrogen fed (above 1% v/v) during the rich phase is unnecessary. In addition, in the low temperature range below 250 °C, the effect is more noticeable due to the further ammonia production and its possible slip. These results open the way to the scale up of the coupled catalytic technologies for its use in real conditions while controlling the influence of the operation map.

Perovskite-Based Catalysts as Efficient, Durable, and Economical NOx Storage and Reduction Systems

Diesel engines operate under net oxidizing environment favoring lower fuel consumption and CO2 emissions than stoichiometric gasoline engines. However, NOx reduction and soot removal is still a technological challenge under such oxygen-rich conditions. Currently, NOx storage and reduction (NSR), also known as lean NOx trap (LNT), selective catalytic reduction (SCR), and hybrid NSR–SCR technologies are considered the most efficient control after treatment systems to remove NOx emission in diesel engines. However, NSR formulation requires high platinum group metals (PGMs) loads to achieve high NOx removal efficiency. This requisite increases the cost and reduces the hydrothermal stability of the catalyst. Recently, perovskites-type oxides (ABO3) have gained special attention as an efficient, economical, and thermally more stable alternative to PGM-based formulations in heterogeneous catalysis. Herein, this paper overviews the potential of perovskite-based formulations to reduce NOx from diesel engine exhaust gases throughout single-NSR and combined NSR–SCR technologies. In detail, the effect of the synthesis method and chemical composition over NO-to-NO2 conversion, NOx storage capacity, and NOx reduction efficiency is addressed. Furthermore, the NOx removal efficiency of optimal developed formulations is compared with respect to the current NSR model catalyst (1–1.5 wt % Pt–10–15 wt % BaO/Al2O3) in the absence and presence of SO2 and H2O in the feed stream, as occurs in the real automotive application. Main conclusions are finally summarized and future challenges highlighted.

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.

Fast cycling NOx storage and reduction: Modeling and analysis of reaction pathways, transport and reductant effects

NOx Storage and Reduction (NSR) conducted at higher cycling frequency enables enhanced conversion of NOx emitted by lean burn gasoline and diesel vehicles. Ting et al. (2018) reported data using a Pt/Rh/BaO/CeO2/Al2O3 showing that stored NOx utilization is primarily responsible for enhanced NOx conversion during fast cycling. In this study a low-dimensional, two-phase monolith reactor model comprising a network of reactions with global kinetics using C3H6 as the reductant is developed. Kinetic parameters not available from the literature are estimated through a fit of NSR with conventional cycling and anaerobic rich feed devoid of CO2 and H2O, while the model is validated for NSR spanning a range of cycle times for a rich feed containing CO2 H2O, and O2. The model predicts most of the data features, including transient and cycle-averaged species concentrations and temperature. The model elucidates differences between C3H6 and H2 during both conventional and fast cycling. For example, the higher NOx conversion obtained with C3H6 during conventional cycling is a result of a higher temperature rise with H2 which adversely impacts NOx storage, while the higher conversion obtained during fast cycling is attributed to the faster diffusional flux of H2, which results in a larger fraction of NOx released during the rich feed (“NOx puff”). The reaction network includes a two-step HC-intermediate pathway involving vinyl isocyanate (C2H3NCO) as the proposed surface intermediate. This pathway predicts the observed double peaks of N2, while requiring substantially more C3H6 to reduce stored NOx to N2. As a result, the vinyl isocyanate pathway is shown to be less efficient than the conventional NSR pathway.

Strontium doping and impregnation onto alumina improve the NOx storage and reduction capacity of LaCoO3 perovskites

Here we report the effects of strontium doping of perovskites (La1-xSrxCoO3) on NOx storage and reduction (NSR) as well as the effects of supporting the perovskite onto an alumina support. The NSR capacity of La0.7Sr0.3CoO3 improved with strontium doping: this material displayed a good balance between NO oxidation capacity and adsorption sites accessibility. Increased accessibility was mainly due to the strontium oxide segregates. In addition, different loadings of La0.7Sr0.3CoO3 perovskite (10, 20, 30, 40 and 50%) were impregnated onto alumina in order to increase the exposed surface area of the perovskite. This had the effect of increasing the NSR capacity of the perovskite. The results of X-Ray diffraction, UV–vis–NIR spectroscopy, N2 adsorption-desorption, electron microscopy, and temperature programmed techniques, demonstrated that the cobalt ions preferably formed cobalt aluminate (CoAl2O4) in the case of low perovskite loadings (<20%). Meanwhile, a well-developed perovskite phase was observed with the higher loadings (>30%). The specific NO oxidation rate per gram of perovskite increased dramatically with the incorporation onto an alumina support. 30% LSCO/Al2O3 sample had an oxidation rate of 138 μmol min-1 (g LSCO)-1 at 350 °C, more than double the rate of the bulk La0.7Sr0.3CoO3 (49 μmol min−1 (g LSCO)−1). Likewise, the 30% LSCO/Al2O3 sample had a higher NOx storage capacity than its bulk counterpart at 400 °C: 306 vs. 115 μmol (g LSCO)−1. The higher oxidation capacity of the alumina-supported samples also facilitated the diffusion of the intermediate compounds from oxidation to adsorption sites. Impregnating alumina with perovskite could be used to improve the efficiency of perovskite mediated NOx removal in automobile applications. Furthermore, adding palladium onto optimum alumina-supported perovskite sample, i.e. 1.5% Pd-30% LSCO/Al2O3, resulted in a clear improvement in NOx storage and reduction capacity. This last sample demonstrated a nitrogen yield as high as 65%, an improvement over the model Pt-based NSR catalyst.

Chemical poisoning by zinc and phosphorous of Pt/Ba/Al2O3 NOx storage catalysts

The effects of phosphorous and zinc on the performance of Pt/Ba/Al2O3 catalysts were investigated through wet impregnation of ammonium phosphate and zinc acetate aqueous solutions. Six different sample combinations were studied; 1 wt-% P, 1 wt-% Zn, 1 wt-% P with 1 wt-% Zn, 2 wt-% P, 2 wt-% Zn, 2 wt-% P with 2 wt-% Zn. NOx storage and reduction (NSR) activity and NO2 temperature programmed desorption (TPD) profiles were measured before and after impregnation of P and Zn. Samples containing P performed significantly worse than samples only containing Zn in both NSR activity and TPD measurements. The increased NOx slip during lean phase in activity measurements for P-poisoned samples is mainly related to an increased slip of NO2. This was found for both NO and NO2 in the gas feed during lean phase and suggests that it is mainly the storage component that is poisoned and not the noble metal. Furthermore, the combination (1 wt-% P and 1 wt-% Zn) proved to result in slightly worse performance than only 1 wt-% P, however this was not the case for samples containing 2 wt-%, where the addition of zinc reduced the negative effect of phosphorous. Measurements from NO2-TPD experiments showed that NOx release at low temperature was not affected by the addition of P, while desorption in the temperature range 425–475 °C was significantly reduced. It can therefore be concluded that the poisoning mainly is related to barium NOx storage sites and not to alumina sites. Moreover, X-ray diffraction measurements indicate that some of the barium species are affected by phosphorous. Images from scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDX) mapping were in line with the results seen in both the activity tests and NO2-TPD experiments. Phosphorous was concentrated at the same position as barium in the observed images, whereas zinc was more evenly distributed over the surface. For the sample with both 2 wt-% P and 2 wt-% Zn, X-ray photoelectron spectroscopy measurements indicate that Zn and P have a low interaction and this suggests that most of the zinc and phosphorous are separated. However, STEM-EDX showed agglomerates of some zinc and phosphorous, which could be zinc phosphates. This is a plausible explanation of the decreased deactivation observed after introducing 2 wt-% Zn compared to the sample only containing 2 wt-% P.

Effects of four-mode hydrothermal aging on three-way catalysts for passive selective catalytic reduction to control emissions from lean-burn gasoline engine

Passive selective catalytic reduction (SCR) is a promising approach for the control of NOX emissions in lean burn gasoline exhausts. It requires ammonia (NH3) to be produced over a three-way catalyst (TWC) during periods of fuel-rich operation for the reduction of NOX during periods of fuel-lean operation. Previous research has shown the viability of this system but has not examined the effects of hydrothermal degradation. This work is focused on evaluating the effects of hydrothermal aging on the TWC in a passive SCR system. Two catalysts were studied: a Pd-TWC, and a NOX storage and reduction (NSR) TWC. Samples were aged at 920 °C for 100 h using a four-mode hydrothermal aging procedure. This causes the catalyst to be oxidized and reduced, as it would in a real system. The effects of aging were evaluated using simulated exhaust under both steady state and lean-rich cycling conditions. Hydrothermal aging caused significant changes in catalyst activity, leading to a decrease in low temperature conversion of carbon monoxide (CO) and propane (C3H8) on both catalysts, and degradation of oxygen storage and NOX storage components. However, the catalysts maintained their activity for NOX conversion and NH3 production, showing sufficient activity for the operation of a passive SCR with an optimum projected fuel consumption of 92–98% compared to stoichiometric operation.

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.