NOx Storage-reduction Research Articles

Composition-Dependent Performance of CexZr1–xO2Mixed-Oxide-Supported WO3Catalysts for the NOxStorage Reduction–Selective Catalytic Reduction Coupled Process

WO3/CexZr1–xO2 materials were evaluated as a possible NH3-selective catalytic reduction (SCR) active catalyst in a NOx storage reduction (NSR) + SCR combined system. The effect of the support composition was investigated at a constant WO3 loading (9.1 wt % of WO3). The impact of WO3 promotion over textural, structural, acid–base, and redox properties of SCR samples was characterized by means of nitrogen adsorption–desorption isotherms, XRD, NOx storage capacity, NH3 temperature programmed desporption, pyridine adsorption followed by FTIR, and H2-TPR. Catalytic activities in NH3-SCR and NH3-SCO reactions as well as corresponding kinetics parameters are also discussed. All WO3/Ce–Zr materials are active and fully selective in N2 for NOx reduction by NH3 and ammonia oxidation by O2. For the SCR reaction, the rate of NO conversion is found approximately half-order with respect to NO, and negative to nearly zero-order with respect to NH3. Tungstated ceria–zirconia materials were then associated downstream with...

Simultaneous Reduction of Particulate Matter and NOx Emissions Using 4-Way Catalyzed Filtration Systems

The next generation of diesel emission control devices includes 4-way catalyzed filtration systems (4WCFS) consisting of both NOx and diesel particulate matter (DPM) control. A methodology was developed to simultaneously evaluate the NOx and DPM control performance of miniature 4WCFS made from acicular mullite, an advanced ceramic material (ACM), that were challenged with diesel exhaust. The impact of catalyst loading and substrate porosity on catalytic performance of the NOx trap was evaluated. Simultaneously with NOx measurements, the real-time solid particle filtration performance of catalyst-coated standard and high porosity filters was determined for steady-state and regenerative conditions. The use of high porosity ACM 4-way catalyzed filtration systems reduced NOx by 99% and solid and total particulate matter by 95% when averaged over 10 regeneration cycles. A "regeneration cycle" refers to an oxidizing ("lean") exhaust condition followed by a reducing ("rich") exhaust condition resulting in NOx storage and NOx reduction (i.e., trap "regeneration"), respectively. Standard porosity ACM 4-way catalyzed filtration systems reduced NOx by 60-75% and exhibited 99.9% filtration efficiency. The rich/lean cycling used to regenerate the filter had almost no impact on solid particle filtration efficiency but impacted NOx control. Cycling resulted in the formation of very low concentrations of semivolatile nucleation mode particles for some 4WCFS formulations. Overall, 4WCFS show promise for significantly reducing diesel emissions into the atmosphere in a single control device.

Non-thermal plasma-assisted NOx storage and reduction on a LaMn0.9Fe0.1O3 perovskite catalyst

Based on its high NOx storage capacity (NSC), a study of the properties of LaMn0.9Fe0.1O3 in NOx storage-reduction catalysis was undertaken. The microstructure of the LaMn0.9Fe0.1O3 catalyst was characterized by XRD, XPS and H2-TPR techniques and compared with that of LaMnO3. The role of Fe doping in LaMnO3 on the perovskite structure and on NOx storage capacity was clarified. Though the LaMn0.9Fe0.1O3 perovskite has high NSC values even at relatively low temperature (<300°C), it shows much lower activity during lean-rich cycling compared with a traditional Pt/BaO/Al2O3 catalyst, indicating that regeneration of stored NOx is the rate limiting step for the perovskite catalyst. By employing an H2-plasma in the rich phase to assist reduction of the stored NOx, the NOx conversion is greatly improved, especially at low temperature. The results of the present study show that by combining the high NOx storage capacity of the perovskite in the lean phase with non-thermal plasma-assisted activation of the reductant in the rich phase, high NOx conversion can be obtained at low temperature.

Influence of Catalyst Composition on NOX Trap Performances

In the present work the influence of the type of noble metals present in NOX storage catalysts was investigated regarding the NO oxidation, NOX storage and NO reduction performances. Monolith samples with combinations of platinum, palladium and rhodium were tested in a flow-reactor. From this study, it was concluded that platinum is necessary for NO oxidation while palladium shows a better ability to oxidize NO to NO2 at high temperature. The presence of Rh on Pt/Rh and Pt/Pd/Rh was found to have a negative effect on NO oxidation reaction. Results suggest also that the storage capacity is partially linked to the oxidation function: Surface area developed by the storage material and the Pt–Ba interaction have an important influence on NOX storage function. The rich step was not efficient to purge completely the trap and leads to accumulation of NOX and so to performances degradation which was assigned to accumulation of NOX after each cycle and storage on of Ba-bulk sites. In presence of Rh, a high oxygen storage capacity leads to high exothermic process between reducing agent and stored oxygen which improves the ability of catalyst to release NOX.

Spectrokinetic Analysis of the NOx Storage Over a Pt–Ba/Al2O3 Lean NOx Trap Catalyst

In this work the kinetics of the (reactive) lean accumulation phase of the NOx storage-reduction process is described through a detailed kinetic model, involving both the gas-phase molecules and the adsorbed species. Kinetic data have been collected following a novel approach based on simultaneous operando spectroscopic measurements and on-line pulse reactor effluent analysis. To our knowledge, this is the first time the temporal evolutions of the concentration of both the surface and the gas species are used jointly to describe the kinetic of a transient catalytic process.

Effect of water and ammonia on surface species formed during NOx storage–reduction cycles over Pt–K/Al2O3 and Pt–Ba/Al2O3 catalysts

The effect of water, in the temperature range 25-350 °C, and ammonia at RT on two different surface species formed on Pt-K/Al2O3 and Pt-Ba/Al2O3 NSR catalysts during NO(x) storage-reduction cycles was investigated. The surface species involved are nitrates, formed during the NO(x) storage step, and isocyanates, which are found to be intermediates in N2 production during reduction by CO. FT-IR experiments demonstrate that the dissociative chemisorption of water and ammonia causes the transformation of the bidentate nitrates and linearly bonded NCO(-) species into more symmetric species that we call ionic species. In the case of water, the effect on nitrates is observable at all the temperatures studied; however, the extent of the transformation decreases upon increasing temperature, consistent with the decreased extent of dissociatively adsorbed water. It was possible to hypothesize that the dissociative chemisorption of water and ammonia takes place in a competitive way on surface sites able to give bidentate nitrates and linearly bonded NCO(-) that are dislocated, remaining on the surface as ionic species.

Enhancement of NOx removal performance for (La0.85Sr0.15)0.99MnO3/Ce0.9Gd0.1O1.95 electrochemical cells by NOx storage/reduction adsorption layers

This study investigated the effect of adding a NOx adsorption layer to the cathode of an electrochemical cell on the removal of NOx from gaseous mixtures. The cathode was a composite of (La0.85Sr0.15)0.99MnO3 (LSM15) and Ce0.9Gd0.1O1.95 (CGO10). Two different kinds of adsorption layers, K–Pt–Al2O3 layer and Ba–Pt–Al2O3 layer (known as NOx storage/reduction (NSR) catalyst), were studied. The effects of the NSR adsorption layers on the electrode processes were characterized by electrochemical impedance spectroscopy (EIS). Both adsorption layers increased the reduction of NOx to N2 in an atmosphere that contained only NO. When O2 was present with NO in the atmosphere, the K–Pt–Al2O3 adsorption layer significantly enhanced the conversion of NOx to N2, but the Ba–Pt–Al2O3 adsorption layer had no effect. The selective removal of NOx under O2-rich conditions was achieved by modifying the LSM15/CGO10 cell with a suitable NSR adsorption layer. The improvement for NOx reduction by the adsorption layers was mainly contributed by the promotion of the adsorption and surface diffusion of NOx species at/near the triple phase boundary (TPB) regions of the electrode and probably the formation of a short and effective reaction path for NOx reduction. A stronger capability for oxidizing NO and/or trapping NOx under the test conditions may have contributed to the superior performance of the K–Pt–Al2O3 adsorption layer relative to the Ba–Pt–Al2O3 layer.

An overview of the production and use of ammonia in NSR + SCR coupled system for NOx reduction from lean exhaust gas

This paper gives a critical overview of the recent advances in NOx abatement in excess of oxygen based on the combination of the NOx storage–reduction (NSR) and selective catalytic reduction (SCR) processes. Ammonia may be produced during the regeneration step of NSR catalyst, by the direct reaction (NOx+H2) or/and the isocyanate route. Recent literature highlights that the ammonia production rate is higher than the ammonia reaction rate with the remaining NOx in order to form N2. In order to optimize the use of the in situ produced ammonia, a catalyst dedicated to the NOx-SCR by NH3 can be added. Zeolites are the main studied materials for this application. Catalytic reduction of NOx by NH3 relates a complex mechanism, in which the nuclearity of the active sites is still an open question. Over zeolites, the NO to NO2 oxidation step is reported as the rate-determining step of the SCR reaction, even if the first step of the reaction is ammonia adsorption on zeolite Brønsted acid sites. Thus, the addition of a NH3-SCR material to the NSR catalyst is a possible way to increase the global NOx abatement and maximize the N2 selectivity, together with the prevention of the ammonia slip.

Sn-modified NOX storage/reduction catalysts

Ba-, K-, and Ba–K-based NOX storage/reduction (NSR) catalysts were prepared, with and without Sn as a promotor. Catalysts and catalyst/soot mixtures were evaluated for NOX reduction and soot oxidation under standard NSR cycling conditions. The activity study was accompanied by dispersion measurements, TEM, XRD, and XPS characterization. It was found that modification with Sn did not significantly influence NOX adsorption, however, Sn notably enhanced soot oxidation for Ba-based systems. This was correlated to Pt oxidation state modification. Also the addition of Sn to the Pt–Ba and Pt–Ba–K catalysts lowered the onset temperature of S release. The Pt–Ba–K catalyst had higher stability against S-poisoning, likely due to the preferential adsorption of S onto K-species, partially maintaining Ba sites for NOX trapping.

Application of spaciMS to the study of ammonia formation in lean NOx trap catalysts

SpaciMS was employed to understand the factors influencing the selectivity of NOx reduction in two fully formulated LNT catalysts, both degreened and thermally aged. Both catalysts contained Pt, Rh, BaO and Al2O3, while one of them also contained La-stabilized CeO2. The amount of reductant required to fully regenerate each catalyst was first determined experimentally based on the OSC of the catalyst and the NOx storage capacity (NSC). In this way a correction was made for the change in catalyst OSC and NSC after aging, thereby eliminating these as factors which could affect catalyst selectivity to NH3. For both catalysts, aging resulted in an elongation of the NOx storage–reduction (NSR) zone due to a decrease in the concentration of NOx storage sites per unit catalyst length. In addition to decreased lean phase NOx storage efficiency, stretching of the NSR zone affected catalyst regeneration. Three main effects were identified, the first being an increase of the NOx “puff” that appeared during the onset of the rich front as it traversed the catalyst. Spatially, NOx release tracked the NSR zone, with the result that the NOx concentration peaked closer to the rear of the aged catalysts. Hence the probability that NOx could re-adsorb downstream of the reduction front and subsequently undergo reduction by NH3 (formed in the reduction front) was diminished, resulting in higher rich phase NOx slip. Second, the stretching of the NSR zone resulted in increased selectivity to NH3 due to the fact that less catalyst (corresponding to the OSC-only zone downstream of the NSR zone) was available to consume NH3 by either the NH3-NOx SCR reaction or the NH3-O2 reaction. Third, the loss of OSC and NOx storage sites, along with the decreased rate of NOx diffusion to Pt/Rh sites (as a result of Pt/Rh–Ba phase segregation), led to an increase in the rate of propagation of the reductant front after aging. This in turn resulted in increased H2:NOx ratios at the Pt/Rh sites and consequently increased selectivity to NH3.

Understanding the role of C3H6, CO and H2 on efficiency and selectivity of NOx storage reduction (NSR) process

The NOx storage reduction (NSR) process is commonly envisaged for the NOx treatment of exhaust gas from lean-burn engine vehicles. NOx are firstly stored on the catalyst, which is periodically submitted to a reducing mixture for few seconds in order to reduce the stored NOx into N2. The on-board reducer is coming from the gasoline/diesel fuel and, in fact, the NSR catalyst is submitted to a mixture of hydrocarbons, CO and H2 with various compositions depending on the lean/rich step. In this study, the influence of each reducer (C3H6, CO and H2) is evaluated separately, with a special consideration to the N2O selectivity. It is demonstrated that the N2O can be emitted during both lean and rich periods, with varying ratio depending on the considered reducer and the temperature of gas. For instance, at 300°C, a high N2O selectivity is observed when C3H6 is used, and near half of the N2O emission occurs during the storage phase in lean condition.

NOx storage and reduction over flame-made M/MgAl2O4 (M = Pt, Pd, and Rh): A comparative study

Various NOx storage–reduction (NSR) catalysts with 1wt% noble metal loading (Pt, Pd, or Rh) dispersed on MgAl2O4 have been synthesized by single step flame spray pyrolysis. The as-prepared powders consisting of nonporous nanoparticles were characterized by X-ray diffraction, scanning transmission electron microscopy, nitrogen adsorption–desorption, CO chemisorption and thermogravimetric methods. Different reduction agents were applied in the fuel rich cycles: H2, CO or C3H6. Processes occurring during fuel rich and lean cycles were analyzed in detail by means of time-resolved diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and thermogravimetry combined with mass spectroscopy. With H2 as reductant, Pt/MgAl2O4 showed the best performance at short regeneration time (<50s), followed by Rh/MgAl2O4 and Pd/MgAl2O4. However, if CO or C3H6 were used as reductant, Rh/MgAl2O4 outperformed both other catalysts, especially at longer regeneration times (>50s). NOx storage on these catalysts was mainly restricted to surface species, although bulk nitrate species were identified by DRIFTS as well. The latter were not completely reduced under the experimental conditions used.

NOx storage and reduction properties of model ceria-based lean NOx trap catalysts

Three kinds of model ceria-containing LNT catalysts, corresponding to Pt/Ba/CeO2, Pt/CeO2/Al2O3 and Pt/BaO/CeO2/Al2O3, were prepared for comparison with a standard LNT catalyst of the Pt/BaO/Al2O3 type. In these catalysts ceria functioned as a NOx storage component and/or a support material. The influence of ceria on the microstructure of the catalysts was investigated, in addition to the effect on NOx storage capacity, regeneration behavior and catalyst performance during lean/rich cycling. The Pt/Ba/CeO2 and Pt/BaO/CeO2/Al2O3 catalysts exhibited higher NOx storage capacity at 200 and 300°C relative to the Pt/BaO/Al2O3 catalyst, although the latter displayed better storage capacity at 400°C. Catalyst regeneration behavior at low temperature was also improved by the presence of ceria, as reflected by TPR measurements. These factors contributed to the superior NOx storage-reduction performance exhibited by the Pt/Ba/CeO2 and Pt/BaO/CeO2/Al2O3 catalysts under cycling conditions in the temperature range 200–300°C. Overall, Pt/BaO/CeO2/Al2O3 (which displayed well balanced NOx storage and regeneration behavior), showed the best performance, affording consistently high NOx conversion levels in the temperature range 200–400°C under lean-rich cycling conditions.

SOx uptake and release properties of TiO2/Al2O3 and BaO/TiO2/Al2O3 mixed oxide systems as NOx storage materials

Titania was used as a promoter to obtain novel materials in the form of TiO2/Al2O3 (Ti/Al) and BaO/TiO2/Al2O3 (Ba/Ti/Al, containing 8wt% or 20wt% BaO) that are relevant to NOx storage reduction (NSR) catalysis. Two different protocols (P1, P2) were utilized in the synthesis. Ti/Al(P1) manifests itself as crystallites of TiO2 on γ-Al2O3, while Ti/Al(P2) reveals an amorphous AlxTiyOz mixed oxide. The structures of the synthesized materials were investigated via TEM, EDX, BET analysis and XPS while the catalytic functionality/performance of these support materials upon SOx and subsequent NOx adsorption were investigated with TPD and in situ FTIR spectroscopy. Ti/Al(P1, P2) revealed a high affinity towards SOx. Overall thermal stabilities of the adsorbed SOx species and the total SOx uptake of the Ba-free samples increase in the following order: TiO2(anatase)≪γ-Al2O3<Ti/Al(P1)<Ti/Al(P2). The superior SOx uptake of Ti/Al(P1, P2) support materials can be tentatively attributed to the increasing specific surface area upon TiO2 promotion and/or the changes in the surface acidity. Promotion of BaO/Al2O3 with TiO2 leads to the attenuation of the SOx uptake and a significant decrease in the thermal stability of the adsorbed SOx species. The relative SOx adsorption capacities of the investigated materials can be ranked as follows: 8Ba/Ti/Al(P1)<8Ba/Ti/Al(P2)<8Ba/Al∼20Ba/Ti/Al(P1)<20Ba/Al<20Ba/Ti/Al(P2).

Mono- and bimetallic Rh and Pt NSR-catalysts prepared by controlled deposition of noble metals on support or storage component

Mono- and bimetallic Rh and Pt based NOx storage-reduction (NSR) catalysts, where the noble metals were deposited on the Al2O3 support or BaCO3 storage component, have been prepared using a twin flame spray pyrolysis setup. The catalysts were characterized by nitrogen adsorption, CO chemisorption combined with diffuse reflectance infrared Fourier transform spectroscopy, X-ray diffraction, and scanning transmission electron microscopy combined with energy dispersive X-ray spectroscopy. The NSR performance of the catalysts was investigated by fuel lean/rich cycling in the absence and presence of SO2 (25ppm) as well as after H2 desulfation at 750°C. The performance increased when Rh was located on BaCO3 enabling good catalyst regeneration during the fuel rich phase. Best performance was observed for bimetallic catalysts where the noble metals were separated, with Pt on Al2O3 and Rh on BaCO3. The Rh-containing catalysts generally showed much higher tolerance to SO2 during fuel rich conditions and lost only little activity during thermal aging at 750°C.

NOx storage/reduction over alkali-metal-nitrate impregnated titanate nanobelt catalysts and investigation of alkali metal cation migration using XPS

We reported earlier that KNO3- and Pt-impregnated potassium titanate nanobelt (KTN) catalysts exhibit high storage capacity and excellent catalytic cycles for NOx storage-reduction (NSR) reaction. In the present study, we compared the NSR behavior of various alkali-metal-nitrate impregnated KTN catalysts, which revealed that the KNO3-impregnated catalyst exhibited superior performance. The XPS and XRD analyses of the NaNO3-impregnated KTN catalyst showed that the migration of K+ from KTN bulk to the surface nitrate salt took place more predominantly than the migration of Na+. Probably for this reason, KNO3-impregnated KTN materials exhibit the best performance for NSR catalytic cycles.

Role of the Exposed Pt Active Sites and BaO2 Formation in NOx Storage Reduction Systems: A Model Catalyst Study on BaOx/Pt(111)

BaOx(0.5 MLE - 10 MLE)/Pt(111) (MLE: monolayer equivalent) surfaces were synthesized as model NOx storage reduction (NSR) catalysts. Chemical structure, surface morphology, and the nature of the adsorbed species on BaOx/Pt(111) surfaces were studied via X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and low-energy electron diffraction (LEED). For θBaOx < 1 MLE, (2 × 2) or (1 × 2) ordered overlayer structures were observed on Pt(111), whereas BaO(110) surface termination was detected for θBaOx = 1.5 MLE. Thicker films (θBaOx ≥ 2.5 MLE) were found to be amorphous. Extensive NO2 adsorption on BaOx(10 MLE)/Pt(111) yields predominantly nitrate species that decompose at higher temperatures through the formation of nitrites. Nitrate decomposition occurs on BaOx(10 MLE)/Pt(111) in two successive steps: (1) NO(g) evolution and BaO2 formation at 650 K and (2) NO(g) + O2(g) evolution at 700 K. O2(g) treatment of the BaOx(10 MLE)/Pt(111) surface at 873 K facilitates the BaO2 formatio...

Use of the differential charging effect in XPS to determine the nature of surface compounds resulting from the interaction of a Pt/(BaCO3 + CeO2) model catalyst with SO x

Changes in the chemical composition of the surface of a Pt/(BaCO3 + CeO2) model NOx storage-reduction catalyst upon its interaction with SOx (SO2 (260 Pa) + O2 (2600 Pa) + H2O (525 Pa)) followed by regeneration in a mixture of CO (2100 Pa) with H2O (525 Pa) were studied by X-ray photoelectron spectroscopy (XPS). Model catalyst samples were prepared as a thin film (about several hundreds of angstrom units in thickness) on the surface of tantalum foil coated with a layer of aluminum oxide (∼100 A). It was found that the Pt/BaCO3 and Pt/CeO2 catalyst constituents acquired different surface charges (differential charging) in the course of photoelectron emission; because of this, it was possible to determine the nature of surface compounds formed as a result of the interaction of the catalyst with a reaction atmosphere. It was found that barium carbonate was converted into barium sulfate as a result of reaction with SOx on the surface of BaCO3 at 150°C. As the treatment temperature in SOx was increased to 300°C, the formation of sulfate on the surface of CeO2 was observed. The sulfatization of CeO2 was accompanied by the reduction of Ce(IV) to Ce(III). The regeneration reaction of the catalyst treated in SOx at 300°C resulted in the consecutive decomposition of cerium(III) sulfate at ≤500°C and then barium sulfate at 600–700°C. Upon the decomposition of BaSO4, a portion of sulfur was converted into a sulfide state, probably, because of the formation of BaS.

Mechanism Study of NO Catalytic Oxidation over MnOx/TiO2 Catalysts

The MnOx/TiO2 catalysts have been proved very active in the catalytic oxidation of NO to NO2, but the mechanism of this heterogeneous reaction was still not clear. In this study, through a systematic in situ diffuse reflectance infrared Fourier transform spectroscopy investigation, the main pathway of NO oxidation over MnOx/TiO2 was revealed. Experimental results indicated that the NO was first coordinated to Mn sites, forming nitrosyls, which were readily oxidized to nitrates by lattice oxygen, and then the nitrates were decomposed to NO2 at high temperatures. However, when SO2 was present, the formation of nitrates was significantly hindered because the SO2 would permanently occupy the active Mn sites as sulfates. We believe that this revealed the mechanism of NO oxidation and SO2 deactivation would provide useful guidance for the development of better catalysts toward fast selective catalytic reduction, NOx storage-reduction, continuously regenerating trap, and NOx wet scrubbing.

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.