NOx Storage-reduction Catalysts Research Articles

Impact of dopants on the sulfation, desulfation and NO x reduction performance of Ba-based NO x storage-reduction catalysts

The performance of a NO x storage-reduction (NSR) catalyst is strongly dependent on the relative stabilities of nitrates and sulfates on the catalyst surface. This effort studies the effects of introducing 5 mol% La, Ca, and K-dopants into a BaO phase of a model Pt/Ba/Al 2O 3 NSR catalyst. The dopants were chosen with a range of properties to affect the BaO lattice spacing and/or the number of oxygen vacancies. The resulting changes in the storage material, in turn, impact the stability of stored nitrates and sulfates, as measured by NO x conversions and desulfation temperatures. The Ca- and La-doped material shows equivalent or better NO x reduction performance between 200 and 400 °C, while the K-doped NSR catalyst showed significant decreases in performance at 200 °C but maintained the high performance at 300 and 400 °C. Following the performance measurements, the samples are then sulfated to 5.5 mg S/gcat and desulfated to 1000 °C. All NSR catalysts generally show similar desulfation behavior, but in determining the temperature of 20% sulfur removal ( T 20%), it is shown that the Ca + Ba sample has a 25 °C lower T 20% than both the Ba-only and La + Ba samples, while K + Ba is 70 °C higher. Additional Ca-based samples were than prepared at the 10, 20 and 100 mol% levels. The higher Ca-doped materials show similar NO x reduction performance, but the 5% Ca + Ba sample maintains the lowest T 20%. Interestingly, the Ca-only sample has a T 20% that is 60 °C higher than the Ba-only NSR catalyst, which indicates that the introduction of 5% Ca into the Ba-lattice has a synergistic effect in lowering the desulfation temperature.

Sulfur Poisoning and Regeneration of NOx Storage−Reduction Cu/K2Ti2O5 Catalyst

A new Cu/K2Ti2O5 catalyst has been developed recently to remove NOx through the NOx storage-reduction (NSR) process. However, its NSR performance in the presence of sulfur has not been investigated. In this article, the sulfur poisoning of the NOx storage-reduction catalyst Cu/K2Ti2O5 and the corresponding deactivation mechanisms are reported for the first time. The effect of the sulfur concentration, adsorption/ regeneration cycling tests, and temperature-programmed regeneration are studied. At low temperatures, the poisoning effect is negligible when the SO2 concentration is lower than 20 ppm, and the sulfated samples can be easily regenerated by 3.5% H2 at 550 °C. However, at high temperatures, the sulfur species are adsorbed on K + sites to form K2SO4 and, consequently, induce a structure transformation from K2Ti2O5 to K2Ti6O13 nanoparticles. The structural change is reversible, and the sulfated catalyst can be regenerated by hydrogen at 650-700 °C.

The Role of Surface Oxides in NOx Storage Reduction Catalysts

The need for greater fuel economy in automotive vehicles has driven a surge in the development of diesel and lean-burn gasoline engines. Coupled with the increase in fuel efficiency, an increase in the air to fuel ratio also results in heightened production of NOx. [1] Several different strategies have been proposed for the handling of the higher NOx levels in lean-burn automotive emissions. One such strategy is the NOx storage reduction (NSR) system whereby excess NO is further oxidized to NO2 over the exhaust catalyst, which is typically platinum, and then NO2 is stored on a carrier material, such as BaO. [2] The catalyst system operates in this fashion for some minutes until the storage material has been fully converted. The system then cycles to a rich mode, during which a reductant is injected into the exhaust stream and reacts with the stored NOx, possibly at the interface between the carrier material and the metal particle, to produce the traditional exhaust products of N2, H2O, and CO2, all in a matter of seconds. Therefore, a requirement of a successful NSR catalyst is that the catalyst must be active for both NO oxidation and NOx reduction. In recent years, a debate has emerged as to the role of metal oxides in oxidation catalysis. [3–6] Although there have been some indications that oxide surfaces may be more active in simple oxidation reactions such as CO oxidation, [7] platinum catalysts have been preferred as the metal component in the NOx storage reduction system for their combination of high activity and ability to resist deactivation through oxidation. Ribeiro et al. have studied Pt NO-oxidation catalysts for NSR applications in considerable detail. They determined that large Pt particles are more active and that single crystal Pt(111) and Pt(100) outperform disperse Pt particles. [8, 9] One possible explanation for the higher activity of the single crystal surfaces is that oxygen is bound less strongly and that the catalysts do not deactivate due to oxide formation. Reports of platinum deactivation as a NO oxidation catalyst, however, concern catalysts that have been exposed to oxygen (and NO) for long periods. [9] As NSR systems operate under a cyclic operation

Single-step flame-made Pt/MgAl2O4 – A NOx storage-reduction catalyst with unprecedented dynamic behavior and high thermal stability

Single-step flame-made Pt/MgAl2O4 – A NOx storage-reduction catalyst with unprecedented dynamic behavior and high thermal stability

Single-step flame-made Pt/MgAl2O4 – A NOx storage-reduction catalyst with unprecedented dynamic behavior and high thermal stability

Abstract One weight percentage of Pt/MgAl 2 O 4 , without any additional classical storage component (Ba or K), was synthesized by single-step flame spray pyrolysis. The catalyst consisting of nano-sized Pt dispersed on MgAl 2 O 4 spinel showed superior dynamic performance in NO x storage-reduction at short regeneration times ( 2 O 3 reference catalyst. The better NSR performance at short regeneration times of Pt/MgAl 2 O 4 is limited to the use of hydrogen and H 2 –CO mixtures as reductants, whereas with other reductants, CO or C 3 H 6 , the NSR performance was similar for both catalysts. The efficiency of the reduction agents decreased in the order H 2 > H 2 + CO > CO > C 3 H 6 . XRD and time-resolved in-situ DRIFT investigations indicate that bulk nitrate species are formed on Pt–Ba/Al 2 O 3 , whereas on the spinel-based Pt/MgAl 2 O 4 catalyst predominantly surface species were formed. The superior dynamic performance of the spinel-based Pt/MgAl 2 O 4 is attributed to the different adsorbed NO x species and their different stability under regeneration conditions. Pt/MgAl 2 O 4 also showed higher thermal stability up to 800 °C and lower stability of sulfur-containing species.

Effect of Pt and Ba content on NO x Storage and Reduction Over Pt/Ba/Al2O3

Effects of Pt and Ba content of Pt/Ba/Al2O3 on NO x storage reduction (NSR) catalyst were investigated by means of kinetic analysis of surface nitrates using in situ FT/IR. Turnover frequencies of both storage and reduction of nitrate were significantly enhanced at higher Ba content, while those were higher at lower Pt content below 0.2 wt%.

Knowledge and know-how in improving the sulfur tolerance of deNO x catalysts

Catalytic removal of NO x under lean-burn conditions is challenging to attain the future stringent NO x emission standards. Selective catalytic reduction (SCR) of NO x with different reducing agents and NO x storage-reduction (NSR) are viewed as the two most promising technologies for NO x removal. Although a variety of catalytic systems have been developed for these processes, their practical applications remain difficult due to catalyst deactivation caused by SO 2 in the exhaust gases. Therefore, improving catalyst tolerance against SO 2 is one of the most important targets in the deNO x catalyst development. This review focuses on the knowledge and know-how that have been developed in improving the sulfur tolerance of deNO x catalysts. Ag/Al 2O 3 is the most promising catalyst for the HC-SCR of NO x . Support modification, H 2 co-feeding and some other strategies to improve the sulfur tolerance of Ag/Al 2O 3 are discussed. Some novel catalyst systems with high sulfur tolerance for NH 3-SCR, H 2-SCR and NSR are addressed, respectively. For NSR catalysts, developing novel sulfur resistant NO x storage systems and effective desulfation processes seem to be two avenues to improve their sulfur resistance.

Effects of CO 2 and steam on Ba/Ce-based NO x storage reduction catalysts during lean aging

The effects of CO 2 and steam on the morphological and chemical properties of Ba/Ce-based NO x storage reduction (NSR) catalysts during the aging were investigated. At 800 °C, BaCeO 3 formation was prevented by a CO 2 concentration as low as 5%, which shows little effect on suppressing BaAl 2O 4 and BaZrO 3 formation. CO 2 protects hexagonal BaCO 3, BaO 2, and CeO 2, to form BaCeO 3, by increasing one time higher activation energy for decarbonation, and maintaining orthorhombic BaCO 3 as the most stable storage component. Steam accelerates the particle aggregations, but it does not determine the above chemical equilibrium. In NSR reactions, although BaCeO 3 formation can be excluded in an atmosphere containing CO 2, the nitrite/nitrate bonding stabilization and the complete NO x reduction are hindered by the highly crystallized materials (induced by higher CO 2 and/or steam concentrations during aging). Particle aggregation is a major factor responsible for the deactivation of the aged Ba/Ce-based NSR catalysts.

Hydrotalcite as a Support for NOx Trap Catalyst

Hydrotalcite (HT)-based catalysts have been developed to improve NOx storage-reduction (NSR) activity, particularly at low temperature during the cold start period of automotive engines. The content of noble metals, including Pt and Pd, has been optimized and the effect of the K precursors on NSR activity has been extensively examined by a knowledge-based combinatorial approach using colorimetric assay. The optimum noble metal ratio was determined to be 0.5Pt/0.5Pd or 0.25Pt/0.75Pd. Pd is a more effective metal for the high performance of NSR catalyst than Pt, particularly for reducing NOx from the catalyst at 200 °C. Both CH3COOK and KNO3 are the best K precursor in view of the NSR performance at both low (200 °C) and high temperatures (350 °C). HT-based catalysts reveal excellent deNOx performance compared to Al2O3-based catalysts, mainly due to their superior redox property.

Improved NOx storage-reduction catalysts using Al2O3 and ZrO2–TiO2 nanocomposite support for thermal stability and sulfur durability

A nanocomposite of Al2O3 and ZrO2–TiO2 solid solution (AZT) was synthesized for NOx storage-reduction (NSR) catalysts Pt/Rh/Ba/K/AZT, and the effect of calcination temperature on thermal stability was investigated. The catalyst containing AZT calcined at 1073K (AZT catalyst) had a high NOx storage capacity after thermal aging. This enhanced storage capacity was attributed to the fact that ZrO2–TiO2 solid solution (ZT) was crystallized and stabilized. The solid-state reaction of potassium, which was added as a NOx storage material, was inhibited in the AZT catalyst, relative to those containing AZT calcined at a low temperature. The AZT catalyst also showed excellent NOx storage performance after sulfur aging at 973K or higher, compared with the catalyst containing physically mixed Al2O3 and ZT (physically mixed catalyst). Furthermore, AZT catalysts inhibited the solid phase reaction of potassium with support materials and kept a high ratio of active potassium, which can store NOx. Further, because the barium in the AZT catalyst prevented sulfur poisoning, the ratio of active barium in the AZT catalyst was also larger than that in the physically mixed catalyst, probably because of the low basicity and high Pt dispersion of AZT.

Effect of the Incorporation Order of Pt- and Ba-Precursors on the Structure and Catalytic Performance of NSR Catalysts

The structural properties and the performance of two NO X storage-reduction (NSR) catalysts prepared as a function of the deposition order of Pt- and Ba-precursors was investigated. The catalyst obtained by incorporating first Ba turned out to be more active mainly due to the larger regeneration extent achieved at temperatures below 350 °C. This behaviour was related to the larger external Pt/Ba interface exhibited by this catalyst.

NOx abatement for lean-burn engines under lean–rich atmosphere over mixed NSR-SCR catalysts: Influences of the addition of a SCR catalyst and of the operational conditions

Mixtures of equal amounts of a Pt–Rh/Ba/Al 2O 3 NOx storage reduction (NSR) model catalyst and Ag/Al 2O 3, Co/Al 2O 3 or Cu/ZSM-5 selective catalytic reduction (SCR) model catalyst were evaluated for the NOx removal activity under lean–rich atmosphere. NOx removal activity was increased by adding Co/Al 2O 3 or Cu/ZSM-5 to Pt–Rh/Ba/Al 2O 3 while adding Ag/Al 2O 3 had no significant influence. Experiments performed by using two catalytic beds (upstream Pt–Rh/Ba/Al 2O 3 and downstream Co/Al 2O 3 or Cu/ZSM-5) suggested that both SCR catalysts are able to reduce NOx with the NH 3 produced during the rich step on Pt–Rh/Ba/Al 2O 3. Among the studied catalysts, the Pt–RhBa/Al 2O 3 + Cu/ZSM-5 physically mixed one showed the highest activity. This catalyst mixture presented an improved performance, as compared to the NSR catalyst, regardless of the reductant used (CO and/or H 2) or of the reduction time (10, 5 or 2.5 s). The highest activity was obtained by using both CO and H 2 as reductant during the rich pulse. The addition of water in the inlet gas led to a decrease of the NOx removal activity of the catalyst mixture. Nevertheless, the NOx removal activity of the mixed Pt–RhBa/Al 2O 3 + Cu/ZSM-5 catalyst was still significantly higher than that of Pt–RhBa/Al 2O 3.

NH3 formation and utilization in regeneration of Pt/Ba/Al2O3 NOx storage-reduction catalyst with H2

The nature of H2 regeneration of a model Pt/Ba/Al2O3 LNT catalyst was investigated with specific focus on intra-catalyst formation and utilization of NH3 and its role in catalyst regeneration. In situ measurements of the transient intra-catalyst species (H2, NH3, N2, NOx) distributions at different temperatures were used to detail the reaction evolution along the catalyst axis. Comparison of the species transients identifies unique individual natures for the reductant (H2), inert product (N2) and intermediate-reductant product (NH3) which readily explain the conventional effluent species sequence as an integral effect. The data demonstrate that NH3 is created on similar timescales as the N2 product inside the catalyst, but consumed as aggressively as H2 reductant along the catalyst. This spatiotemporal NH3 behavior experimentally confirms that Intermediate-NH3 regeneration pathway is active. Analysis at 200 and 325°C indicates equivalent local NOx storage, H2 consumption and regeneration effectiveness, but differing NH3/N2 ratio, suggesting a temperature-dependence of partitioning between Direct-H2 and Intermediate-NH3 regeneration pathways. Further experimental and numerical work is needed to more clearly understand the partitioning between the possible regeneration pathways. Nevertheless, the experimental data show that intermediate NH3 plays a significant role in LNT catalyst regeneration.

Studies of Reaction Mechanism of Automobile Catalysts by in-situ Spectroscopy

Although IR and UV-Vis are conventionally used spectroscopies, in-situ analysis gives new and effective insights of the reaction mechanism of catalysts. In this manuscript, application of in-situ IR and UV-Vis to the reaction mechanism of automobile catalysts is described. The first topic is kinetic analysis of stored nitrates on NSR (NOx Storage-Reduction) catalyst by in-situ IR. The application of in-situ IR to the study on reaction mechanism of HC-SCR (Selective Catalytic Reduction of NO by hydrocarbons) over Ag/alumina catalyst is also reported. The effect of carbon number in hydrocarbon reductants is clarified. Finally, observation of morphology change in Ag species of Ag/alumina catalyst by in-situ UV-Vis is described. From the comparison of dynamic change in morphology of Ag species and catalytic activity, the mechanism of hydrogen effect on HC-SCR is clarified.

Effect of supports on formation and reduction rate of stored nitrates on NSR catalysts as investigated by in situ FT/IR

Surface dynamics of nitrates on NOx storage–reduction (NSR) catalysts are examined by means of in situ FT/IR. In the initial state of NOx storage, both nitrite and nitrates are formed on Pt–Ba/Al 2O 3 surface, and then nitrite become abundant with the increase of storage time. The initial rate of nitrate storage is faster in a flow of NO/O 2 mixture than in a flow of NO 2 without O 2. The presence of oxygen is found to be essential for the conversion of surface nitrite to nitrates. The rate and amount of nitrate formation are strongly affected by type of supports. The storage ability is in the order of MgO > Al 2O 3 > ZrO 2 > SiO 2, which is in accordance with that of basicity of the supports. On the other hand, the rate of nitrate reduction by H 2 is in the different order: Al 2O 3 > ZrO 2 > SiO 2 > MgO. The strong contribution of acid–base property of supports as a controlling factor is suggested.

Pt−Ba/Al2O3 NSR Catalysts at Different Ba Loading: Characterization of Morphological, Structural, and Surface Properties

Morphological, textural, and surface properties of several Pt−Ba/Al2O3 NOx storage reduction (NSR) catalysts at different Ba loading in the range 0−30 wt % were characterized by means of X-ray diffraction, high-resolution transmission electron microscopy, and Fourier-tranform infrared (FT-IR) spectroscopy using CO, CO2, and CH3CN as probe molecules. Upon increasing the Ba loading, the Pt exposure progressively decreased, accounting for sintering and masking of the Pt particles by Ba, whereas the interaction between Pt and the barium oxide phase increased. After a few cycles of heating in NO2 and subsequent evacuation (conditioning treatment), BaCO3 initially present evolved to Ba(NO3)2 and then decomposed into a well-dispersed nanosized BaO phase. Upon conditioning, a slight sintering of Pt is observed. However, the presence of Ba avoided a more stressed sintering, as it occurred for the Pt/Al2O3 sample. Investigation by CO2 and CH3CN adsorption followed by FT-IR spectroscopy revealed a high heterogeneity...

Titanium-doped nanocomposite of Al2O3 and ZrO2–TiO2 as a support with high sulfur durability for NOx storage-reduction catalyst

A nanocomposite of Al2O3 and a ZrO2–TiO2 solid solution (AZT) doped with Ti (Ti–AZT) was synthesized as a support for a NOx storage-reduction (NSR) catalyst in order to achieve sulfur durability. Ti–AZT maintained the original structure of AZT after the Ti doping step and exhibited lower basicity. Energy dispersive X-ray spectroscopy revealed that the Ti concentration on the Al2O3 particles was more than 10at.% on average for a sample containing 3.3at.% of doped titanium. Doped titanium was homogeneously distributed on the surface without the formation of discrete TiO2 particles. SO2-temperature programmed desorption at less than 823K indicated that the catalyst containing Ti–AZT had larger sulfur desorption than that containing AZT. After sulfur aging tests, the Ti–AZT catalyst provided a large amount of NOx storage. The improved sulfur durability in the NSR catalyst resulted from the presence of TiO2 as a Al2O3–TiO2 solid solution in Ti–AZT.

Carbonate Formation and Stability on a Pt/BaO/γ-Al2O3 NOX Storage/Reduction Catalyst

There has been recent debate regarding the role or influence of BaCO3 species on the performance or operation of Pt/BaO/Al2O3 model NOX storage/reduction (NSR) catalysts. This influence is primarily regarded as negative, but the extent of its impact is not clear. For this reason, the formation and stability of barium carbonate species on a Pt/BaO/Al2O3 model NSR catalyst were characterized using Fourier transform infrared (FTIR) spectroscopy. The catalyst sample was exposed to CO2, CO, and CO + O2 at various temperatures, from 300 to >500 K. Bidentate carbonate species readily form under all conditions, while at higher temperatures, unidentate species were also observed and likely formed from bidentate species as a result of a change in their coordination to the oxide surface. Reaction of COX species with residual hydroxide species on the catalyst led to the formation of bicarbonates, and when the sample was exposed to CO at low temperature, formate species were also formed. These formate species decomposed at elevated temperatures and contributed to the formation of carbonates. H2O exposure resulted in the agglomeration of various COX-containing phases to larger particles.

NO X storage/reduction catalyst performance with oxygen in the regeneration phase

NO X storage/reduction (NSR) catalysts operate over a cycle where the catalyst is periodically regenerated with a reductant-rich gas mixture. In this study, the effects of including O 2 during this regeneration phase on the performance, NO X reduction to N 2, of a model NSR catalyst and a commercial NSR catalyst were investigated. The increased temperature associated with reductant oxidation when oxygen was available resulted in improved performance at temperatures below 375 °C for the model sample, but resulted in decreased performance at higher test temperatures. The improved performance primarily originated from increased trapping in the subsequent lean phase but was also improved by decreased byproduct formation during the rich phase under some conditions. The improved performance measure is based on an increase in the lean-phase time before a regeneration phase would need to be triggered, with the increase on the order of 10 s for 288 and 375 °C operating temperatures. The decreased trapping performance at higher temperatures is due to decreased nitrate stability, while the increased trapping performance at lower temperature is due to improved oxidation kinetics. Similar effects were observed with the commercial sample, although the presence of surface oxygen storage resulted in an already significant temperature increase with no O 2 added in the regeneration gas composition. The results demonstrate that the time between regenerations can be increased significantly by tuning or optimizing the amount of reductant and oxygen added during the regeneration phase.

Impact of copper on the performance and sulfur tolerance of barium-based NOx storage-reduction catalysts

The presence of sulfur in automotive exhaust is known to be detrimental to lean-NOx traps as SO2 is oxidized to SO3 that competes with NO2 for sites on the trap and is difficult to remove. In this study the effect of adding Cu to the prototypical Pt–BaO/γ-Al2O3 formulation on the system's tolerance for sulfur was investigated. It was found that in the absence of sulfur, Cu decreases the performance in terms of both NOx storage capacity and reduction of NOx to N2 during regeneration. In the presence of SO2, Cu provides a significant improvement in sulfur tolerance so that, after sulfur exposure, the storage capacity of the Cu-modified material can exceed that of the baseline material. The sulfur tolerance afforded by Cu is attributed to a moderation in the activity for SO2 oxidation resulting from the formation of a Pt–Cu bimetallic phase. The propensity for NO oxidation is also modified, but to a lesser effect. Evidence for the bimetallic phase is provided by temperature-programmed reduction (TPR) and electron microscopy. The impact of SO2 on the Cu-modified material is greater during the regenerative reduction cycle. In this case, the results suggest that sulfur blocks Pt and possibly Cu sites and that the sulfur is not removed by oxidation during the subsequent storage cycle. Hence, activity lost during the reduction cycle is not restored. In contrast, sulfur that blocks Pt sites on the baseline material during the reduction cycle is subsequently oxidized and desorbs from the Pt, restoring the activity. However, some of the resulting SO3 reacts with the BaO to form BaSO4, and there is a partial loss of storage capacity.