Mean field modelling of NOx storage on Pt/BaO/Al2O3

A mean field model, for storage and desorption of NO_x in a Pt/BaO/Al₂O₃ catalyst is developed using data from flow reactor experiments. This relatively complex system is divided into five smaller sub-systems and the model is divided into the following steps: (i) NO oxidation on Pt/Al₂O₃; (ii) NO oxidation on Pt/BaO/Al₂O₃; (iii) NO_x storage on BaO/Al₂O₃; (iv) NO_x storage on Pt/BaO/Al₂O₃ with thermal regeneration and (v) NO_x storage on Pt/BaO/Al₂O₃ with regeneration using C₃H₆. In this paper, we focus on the last sub-system. The kinetic model for NO_x storage on Pt/BaO/Al₂O₃ was constructed with kinetic parameters obtained from the NO oxidation model together with a NO_x storage model on BaO/Al₂O₃. This model was not sufficient to describe the NO_x storage experiments for the Pt/BaO/Al₂O₃, because the NO_x desorption in TPD experiments was larger for Pt/BaO/Al₂O₃, compared to BaO/Al₂O₃. The model was therefore modified by adding a reversible spill-over step. Further, the model was validated with additional experiments, which showed that NO significantly promoted desorption of NO_x from Pt/BaO/Al₂O₃. To this NO_x storage model, additional steps were added to describe the reduction by hydrocarbon in experiments with NO₂ and C₃H₆. The main reactions for continuous reduction of NO_x occurs on Pt by reactions between hydrocarbon species and NO in the model. The model is also able to describe the reduction phase, the storage and NO breakthrough peaks, observed in experiments.     

Experimental and microkinetic modeling of steady-state NO reduction by H2 on Pt/BaO/Al2O3 monolith catalysts

Experimental results describing the product distribution during the reduction of NO by H₂ on Pt/Al₂O₃ and Pt/BaO/Al₂O₃ catalysts are presented in the temperature range 30–500 °C and H₂/NO feed ratio range of 0.9–2.5. A microkinetic model that describes the kinetics of NO reduction by H₂ on Pt/Al₂O₃ is proposed and most of the kinetic parameters are estimated from either literature data or from thermodynamic constraints. The microkinetic model is combined with the short monolith flow model to simulate the conversions and selectivities corresponding to the experimental conditions. The predicted trends are in excellent qualitative and reasonable quantitative agreement with the experimental results. Both the model and the experiments show that N₂O formation is favored at low temperatures and low H₂/NO feed ratios, N₂ selectivity increases monotonically with temperature for H₂/NO feed ratios of 1.2 or less but goes through a maximum at intermediate temperatures (around 100 °C) for H₂/NO feed ratios 1.5 or higher. Ammonia formation is favored for H₂/NO feed ratios of 1.5 or higher and intermediate temperatures (100–350 °C) buts starts to decompose at a temperature of 400 °C or higher. The microkinetic model is used to determine the surface coverages and explain the trends in the experimentally observed selectivities.     

Reactor and in situ FTIR studies of Pt/BaO/Al2O3 and Pd/BaO/Al2O3 NOx storage and reduction (NSR) catalysts

The reduction of NO under cyclic “lean”/“rich” conditions was examined over two model 1 wt.% Pt/20 wt.% BaO/Al₂O₃ and 1 wt.% Pd/20 wt.% BaO/Al₂O₃ NO_x storage reduction (NSR) catalysts. At temperatures between 250 and 350 °C, the Pd/BaO/Al₂O₃ catalyst exhibits higher overall NO_x reduction activity. Limited amounts of N₂O were formed over both catalysts. Identical cyclic studies conducted with non-BaO-containing 1 wt.% Pt/Al₂O₃ and Pd/Al₂O₃ catalysts demonstrate that under these conditions Pd exhibits a higher activity for the oxidation of both propylene and NO. Furthermore, in situ FTIR studies conducted under identical conditions suggest the formation of higher amounts of surface nitrite species on Pd/BaO/Al₂O₃. The IR results indicate that this species is substantially more active towards reaction with propylene. Moreover, its formation and reduction appear to represent the main pathway for the storage and reduction of NO under the conditions examined. Consequently, the higher activity of Pd can be attributed to its higher oxidation activity, leading both to a higher storage capacity (i.e., higher concentration of surface nitrites under “lean” conditions) and a higher reduction activity (i.e., higher concentration of partially oxidized active propylene species under “rich” conditions). The performance of Pt and Pd is nearly identical at temperatures above 375 °C.     

Kinetic modelling of sulfur deactivation of Pt/BaO/Al2O3 and BaO/Al2O3 NOx storage catalysts

In this work, a kinetic model is constructed to simulate sulfur deactivation of the NO_x storage performance of BaO/Al₂O₃ and Pt/BaO/Al₂O₃ catalysts. The model is based on a previous model for NO_x storage under sulfur-free conditions. In the present model the storage of NO_x is allowed on two storage sites, one for complete NO_x uptake and one for a slower NO_x sorption. The adsorption of SO_x is allowed on both of these NO_x storage sites and on one additional site which represent bulk storage. The present model is built-up of six sub-models: (i) NO_x storage under sulfur-free conditions; (ii) SO₂ storage on NO_x storage sites; (iii) SO₂ oxidation; (iv) SO₃ storage on bulk sites; (v) SO₂ interaction with platinum in the presence of H₂; (vi) oxidation of accumulated sulfur compounds on platinum by NO₂. Data from flow reactor experiments are used in the implementation of the model. The model is tested for simulation of experiments for NO_x storage before exposure to sulfur and after pre-treatments either with SO₂ + O₂ or SO₂ + H₂. The simulations show that the model is able to describe the main features observed experimentally.     

Transient analysis of the release and reduction of NOx using a Pt/Ba/Al2O3 catalyst

The release and reduction of NO_x in a NO_x storage-reduction (NSR) catalyst were studied with a transient reaction analysis in the millisecond range, which was made possible by the combination of pulsed injection of gases and time resolved time-of-flight mass spectrometry. After an O₂ pulse and a subsequent NO pulse were injected into a pellet of the Pt/Ba/Al₂O₃ catalyst, the time profiles of several gas products, NO, N₂, NH₃ and H₂O, were obtained as a result of the release and reduction of NO_x caused by H₂ injection. Comparing the time profiles in another analysis, which were obtained using a model catalyst consisting of a flat 5 nmPt/Ba(NO₃)₂/cordierite plate, the release and reduction of NO_x on Pt/Ba/Al₂O₃ catalyst that stored NO_x took the following two steps; in the first step NO molecules were released from Ba and in the second step the released NO was reduced into N₂ by H₂ pulse injection. When this H₂ pulse was injected in a large amount, NO was reduced to NH₃ instead of N₂.

A only small amount of H₂O was detected because of the strong affinity for alumina support. We can analyze the NO_x regeneration process to separate two steps of the NO_x release and reduction by a detailed analysis of the time profiles using a two-step reaction model. From the result of the analysis, it is found that the rate constant for NO_x release increased as temperature increase.     

Effect of the addition of transition metals to Pt/Ba/Al2O3 catalyst on the NOx storage-reduction catalysis under oxidizing conditions in the presence of SO2

The NO_x storage-reduction catalysis under oxidizing conditions in the presence of SO₂ has been investigated on Pt/Ba/Fe/Al₂O₃, Pt/Ba/Co/Al₂O₃, Pt/Ba/Ni/Al₂O₃, and Pt/Ba/Cu/Al₂O₃ catalysts compared with Pt/Ba/Al₂O₃, Pt/Fe/Al₂O₃, Pt/Co/Al₂O₃, Pt/Ni/Al₂O₃, Pt/Cu/Al₂O₃ and Pt/Al₂O₃ catalysts. The NO_x purification activity of Pt/Ba/Fe/Al₂O₃ catalyst was the highest of all the catalysts investigated in this paper after an aging treatment. That of the aged Pt/Ba/Co/Al₂O₃ and Pt/Ba/Ni/Al₂O₃ catalysts was essentially the same as that of the aged Pt/Ba/Al₂O₃ catalyst, while that of the aged Pt/Ba/Cu/Al₂O₃ and Pt/Cu/Al₂O₃ catalysts was substantially lower than the others.

The Fe-compound on the aged Pt/Ba/Fe/Al₂O₃ catalyst has played a role in decreasing the sulfur content on the catalyst after exposure to simulated reducing gas compared with the Pt/Ba/Al₂O₃ catalyst without the Fe-compound. XRD and EDX show that the Fe-compound inhibits the growth in the size of BaSO₄ particles formed on the Pt/Ba/Fe/Al₂O₃ catalyst under oxidizing conditions in the presence of SO₂ and promotes the decomposition of BaSO₄ and desorption of the sulfur compound under reducing conditions.     

Comparative study of structural properties and NOx storage-reduction behavior of Pt/Ba/CeO2 and Pt/Ba/Al2O3

Differences in the NO_x storage-reduction (NSR) behavior of Pt/Ba/CeO₂ and Pt/Ba/Al₂O₃ have been identified and traced to their different chemical and structural properties. The results show that Pt/Ba/CeO₂ exhibits inferior NO_x storage and, particularly, reduction (regeneration) activity compared to the Al₂O₃ supported catalyst. The incomplete reduction of the stored NO_x-species in Pt/Ba/CeO₂ seems to be caused by a faster and more profound reoxidation of Pt particles during the lean period as evidenced by in situ X-ray absorption spectroscopy. Interestingly, the reduction activity could be significantly improved by a pre-reduction step at mild conditions. Exposure of the Pt/Ba/CeO₂ catalyst to reducing H₂ atmosphere in the temperature range 300–500 °C lead to a moderate increase of Pt particle size which beneficially influenced the regeneration activity. In contrast, pre-reduction at temperatures above 500 °C was unfavorable and resulted in a severe decrease of the regeneration activity, probably due to migration of the partially reduced CeO₂ onto the surface of Pt particles.     

Quantified NOx adsorption on Pt/K/gamma-Al2O3 and the effects of CO2 and H2O

A multi-component NO_x-trap catalyst consisting of Pt and K supported on γ-Al₂O₃ was studied at 250 °C to determine the roles of the individual catalyst components, to identify the adsorbing species during the lean capture cycle, and to assess the effects of H₂O and CO₂ on NO_x storage. The Al₂O₃ support was shown to have NO_x trapping capability with and without Pt present (at 250 °C Pt/Al₂O₃ adsorbs 2.3 μmols NO_x/m²). NO_x is primarily trapped on Al₂O₃ in the form of nitrates with monodentate, chelating and bridged forms apparent in Diffuse Reflectance mid-Infrared Fourier Transform Spectroscopy (DRIFTS) analysis. The addition of K to the catalyst increases the adsorption capacity to 6.2 μmols NO_x/m², and the primary storage form on K is a free nitrate ion. Quantitative DRIFTS analysis shows that 12% of the nitrates on a Pt/K/Al₂O₃ catalyst are coordinated on the Al₂O₃ support at saturation.

When 5% CO₂ was included in a feed stream with 300 ppm NO and 12% O₂, the amount of K-based nitrate storage decreased by 45% after 1 h on stream due to the competition of adsorbed free nitrates with carboxylates for adsorption sites. When 5% H₂O was included in a feed stream with 300 ppm NO and 12% O₂, the amount of K-based nitrate storage decreased by only 16% after 1 h, but the Al₂O₃-based nitrates decreased by 92%. Interestingly, with both 5% CO₂ and 5% H₂O in the feed, the total storage only decreased by 11%, as the hydroxyl groups generated on Al₂O₃ destabilized the K–CO₂ bond; specifically, H₂O mitigates the NO_x storage capacity losses associated with carboxylate competition.     

Influence of CO2 and H2O on NOx storage and reduction on a Pt–Ba/γ-Al2O3 catalyst

In this paper, the effect of CO₂ and H₂O on NO_x storage and reduction over a Pt–Ba/γ-Al₂O₃ (1 wt.% Pt and 30 wt.% Ba) catalyst is shown. The experimental results reveal that in the presence of CO₂ and H₂O, NO_x is stored on BaCO₃ sites only. Moreover, H₂O inhibits the NO oxidation capability of the catalyst and no NO₂ formation is observed. Only 16% of the total barium is utilized in NO storage. The rich phase shows 95% selectivity towards N₂ as well as complete regeneration of stored NO. In the presence of CO₂, NO is oxidized into NO₂ and more NO_x is stored as in the presence of H₂O, resulting in 30% barium utilization. Bulk barium sites are inactive in NO_x trapping in the presence of CO₂·NH₃ formation is seen in the rich phase and the selectivity towards N₂ is 83%. Ba(NO₃)₂ is always completely regenerated during the subsequent rich phase. In the absence of CO₂ and H₂O, both surface and bulk barium sites are active in NO_x storage. As lean/rich cycling proceeds, the selectivity towards N₂ in the rich phase decreases from 82% to 47% and the N balance for successive lean/rich cycles shows incomplete regeneration of the catalyst. This incomplete regeneration along with a 40% decrease in the Pt dispersion and BET surface area, explains the observed decrease in NO_x storage.     

Pt/Al_2O_3与Pt/MgO-Al_2O_3催化剂制备及其催化氢能载体甲基环己烷脱氢反应性能

以碱共沉淀法制备Mg-Al水滑石,然后采用浸渍法负载活性组分Pt,经焙烧、氢气还原得到Pt/Al_2O_3与Pt/Mg O-Al_2O_3催化剂,采用XRD、N2吸附-脱附、FT-IR、H2-TPR和Py-IR等分析Mg O的加入对Pt/Al_2O_3催化剂结构性能的影响,并在甲基环己烷连续脱氢反应中对比两种催化剂活性。结果表明,Pt/Mg O-Al_2O_3催化剂比表面积小于Pt/Al_2O_3催化剂,且表面基本无酸性活性中心,但表现出与Pt/Al_2O_3催化剂相同的脱氢活性。在Pt负载质量分数2%、催化剂用量0.5 g、甲基环己烷0.1 m L·min-1纯样进料和325℃反应10 h后,原料平均转化率79.9%,脱氢产物只有甲苯,对应的产氢速率192.8 mmol·(g-metal·min)-1,表现出优良的脱氢活性。     

Insight into the key aspects of the regeneration process in the NOx storage reduction (NSR) reaction probed using fast transient kinetics coupled with isotopically labelled NO over Pt and Rh-containing Ba/Al2O3 catalysts

For the first time, the coupling of fast transient kinetic switching and the use of an isotopically labelled reactant (¹⁵NO) has allowed detailed analysis of the evolution of all the products and reactants involved in the regeneration of a NO_x storage reduction (NSR) material. Using realistic regeneration times (ca. 1 s) for Pt, Rh and Pt/Rh-containing Ba/Al₂O₃ catalysts we have revealed an unexpected double peak in the evolution of nitrogen. The first peak occurred immediately on switching from lean to rich conditions, while the second peak started at the point at which the gases switched from rich to lean. The first evolution of nitrogen occurs as a result of the fast reaction between H₂ and/or CO and NO on reduced Rh and/or Pt sites. The second N₂ peak which occurs upon removal of the rich phase can be explained by reaction of stored ammonia with stored NO_x, gas phase NO_x or O₂. The ammonia can be formed either by hydrolysis of isocyanates or by direct reaction of NO and H₂.

The study highlights the importance of the relative rates of regeneration and storage in determining the overall performance of the catalysts. The performance of the monometallic 1.1%Rh/Ba/Al₂O₃ catalyst at 250 and 350 °C was found to be dependent on the rate of NO_x storage, since the rate of regeneration was sufficient to remove the NO_x stored in the lean phase. In contrast, for the monometallic 1.6%Pt/Ba/Al₂O₃ catalyst at 250 °C, the rate of regeneration was the determining factor with the result that the amount of NO_x stored on the catalyst deteriorated from cycle to cycle until the amount of NO_x stored in the lean phase matched the NO_x reduced in the rich phase. On the basis of the ratio of exposed metal surface atoms to total Ba content, the monometallic 1.6%Pt/Ba/Al₂O₃ catalyst outperformed the Rh-containing catalysts at 250 and 350 °C even when CO was used as a reductant.     

Lean NOx reduction with CO + H2 mixtures over Pt/Al2O3 and Pd/Al2O3 catalysts

The reduction of NO_x by hydrogen under lean burn conditions over Pt/Al₂O₃ is strongly poisoned by carbon monoxide. This is due to the strong adsorption and subsequent high coverage of CO, which significantly increases the temperature required to initiate the reaction. Even relatively small concentrations of CO dramatically reduce the maximum NO_x conversions achievable. In contrast, the presence of CO has a pronounced promoting influence in the case of Pd/Al₂O₃. In this case, although pure H₂ and pure CO are ineffective for NO_x reduction under lean burn conditions, H₂/CO mixtures are very effective. With a realistic (1:3) H₂:CO ratio, typical of actual exhaust gas, Pd/Al₂O₃ is significantly more active than Pt/Al₂O₃, delivering 45% NO_x conversion at 160 °C, compared to >15% for Pt/Al₂O₃ under identical conditions. The nature of the support is also critically important, with Pd/Al₂O₃ being much more active than Pd/SiO₂. Possible mechanisms for the improved performance of Pd/Al₂O₃ in the presence of H₂+CO are discussed.     

Pt–Ba–Al2O3 for NOx storage and reduction: Characterization of the dispersed species

Pt–Ba–Al₂O₃ active and selective for NO_x storage and selective reduction to N₂ has been prepared and tested. Characterization of the parent Al₂O₃, Pt–Al₂O₃ and Ba–Al₂O₃ materials, as well as of Pt–Ba–Al₂O₃ catalyst in the oxidized, reduced and sulphated state has been performed by FT-IR spectroscopy of low-temperature adsorbed carbon monoxide and of adsorbed acetonitrile. XRD, TEM and XPS analyses have also been performed. Evidence for the predominance of Ba species, which are highly dispersed on the alumina support surface, and may be carbonated or sulphated, has been provided. Competitive interaction of Pt and Ba species with the surface sites of alumina has also been found.     

Pd–Pt/Al2O3 bimetallic catalysts for the advanced oxidation of reactive dye solutions

The Pd–Pt/Al₂O₃ bimetallic catalysts showed high activities toward the wet oxidation of the reactive dyes in the presence of 1% H₂ together with excess oxygen. Palladium was believed to act as a co-catalyst to spillover the adsorbed H₂ onto the surface of the oxidized Pt surface, and thereby the reducibility of the Pt increased greatly. The organic dye molecule adsorbed on the reduced Pt surface more easily than the oxidized Pt surface under the competition with excess oxygen, which is an essential step for the catalytic wet oxidation (CWO). The Pd–Pt/Al₂O₃ catalysts also produced H₂O₂ from H₂/O₂ mixture, and the hydroxyl radical was formed through the subsequent decomposition of H₂O₂. Additional oxidation of the reactive dyes was obtained with hydroxyl radical. The high activities of the Pd–Pt/Al₂O₃ catalysts were believed to be due to the combined effects of the faster redox cycle resulting from the increased reducibility of Pt surface and the additional oxidation of the reactive dyes with hydroxyl radical.     

Catalytic oxidation of HCN over a 0.5% Pt/Al2O3 catalyst

The adsorption of HCN on, its catalytic oxidation with 6% O₂ over 0.5% Pt/Al₂O₃, and the subsequent oxidation of strongly bound chemisorbed species upon heating were investigated. The observed N-containing products were N₂O, NO and NO₂, and some residual adsorbed N-containing species were oxidized to NO and NO₂ during subsequent temperature programmed oxidation. Because N-atom balance could not be obtained after accounting for the quantities of each of these product species, we propose that N₂ and was formed. Both the HCN conversion and the selectivity towards different N-containing products depend strongly on the reaction temperature and the composition of the reactant gas mixture. In particular, total HCN conversion reaches 95% above 250 °C. Furthermore, the temperature of maximum HCN conversion to N₂O is located between 200 and 250 °C, while raising the reaction temperature increases the proportion of NO_x in the products. The co-feeding of H₂O and C₃H₆ had little, if any effect on the total HCN conversion, but C₃H₆ addition did increase the conversion to NO and decrease the conversion to NO₂, perhaps due to the competing presence of adsorbed fragments of reductive C₃H₆. Evidence is also presented that introduction of NO and NO₂ into the reactant gas mixture resulted in additional reaction pathways between these NO_x species and HCN that provide for lean-NO_x reduction coincident with HCN oxidation.     

Influence of BaO on Pd/Al2O3-based catalysts in C2H4 and CO oxidation as well as in NO reduction

In this study, Pd/Al₂O₃ and Pd/BaO/Al₂O₃ metallic monoliths were used to investigate the effect of BaO in C₂H₄ and CO oxidation as well as in NO reduction. A FT-IR gas analyser was used to study the activity of the catalysts. Several activity experiments carried out with dissimilar feedstreams revealed that BaO enhances CO and C₂H₄ oxidation as well as NO reduction reactions in rich conditions. This effect is due to BaO, which causes a decrease in the ethene poisoning of palladium. In lean conditions BaO is present in the form of Ba(OH)₂ which reacts with oxidised NO releasing water. Therefore, NO was stored during the lean reaction.     

The role of lanthana loading on the catalytic properties of Pd/Al2O3-La2O3 in the NO reduction with H2

The role of La₂O₃ loading in Pd/Al₂O₃-La₂O₃ prepared by sol–gel on the catalytic properties in the NO reduction with H₂ was studied. The catalysts were characterized by N₂ physisorption, temperature-programmed reduction, differential thermal analysis, temperature-programmed oxidation and temperature-programmed desorption of NO.

The physicochemical properties of Pd catalysts as well as the catalytic activity and selectivity are modified by La₂O₃ inclusion. The selectivity depends on the NO/H₂ molar ratio (GHSV = 72,000 h⁻¹) and the extent of interaction between Pd and La₂O₃. At NO/H₂ = 0.5, the catalysts show high N₂ selectivity (60–75%) at temperatures lower than 250 °C. For NO/H₂ = 1, the N₂ selectivity is almost 100% mainly for high temperatures, and even in the presence of 10% H₂O vapor. The high N₂ selectivity indicates a high capability of the catalysts to dissociate NO upon adsorption. This property is attributed to the creation of new adsorption sites through the formation of a surface PdO_x phase interacting with La₂O₃. The formation of this phase is favored by the spreading of PdO promoted by La₂O₃. DTA shows that the phase transformation takes place at temperatures of 280–350 °C, while TPO indicates that this phase transformation is related to the oxidation process of PdO: in the case of Pd/Al₂O₃ the O₂ uptake is consistent with the oxidation of PdO to PdO₂, and when La₂O₃ is present the O₂ uptake exceeds that amount (1.5 times). La₂O₃ in Pd catalysts promotes also the oxidation of Pd and dissociative adsorption of NO mainly at low temperatures (<250 °C) favoring the formation of N₂.     

助剂Ba对Pd/Al_2O_3和Pt/Al_2O_3催化剂的C1-C3烷烃催化燃烧性能的影响

通过浸渍法制备了Al_2O_3负载的Pd和Pt催化剂,考察催化剂的甲烷、乙烷和丙烷催化燃烧活性,以及助剂Ba对催化性能的影响。对于Pd/Al_2O_3催化剂,加入Ba使活性物种PdO颗粒变大和还原温度升高,形成更稳定的PdO活性物种,是Pd-Ba/Al_2O_3催化剂活性提升的主要原因。对于Pt/Al_2O_3催化剂,加入Ba助剂使活性物种Pt0含量降低,PtO_x与Al_2O_3载体相互作用增强,使PtO_x物种更难被还原为Pt~0,导致Pt-Ba/Al_2O_3催化剂活性降低。Pd和Pt催化剂催化烷烃氧化反应活性规律一致:丙烷乙烷甲烷。Pd/Al_2O_3催化剂有利于C—H键活化,Pt/Al_2O_3催化剂有利于C—C键活化。Pt/Al_2O_3催化剂对C1-C3烷烃氧化活性的差别明显大于Pd/Al_2O_3催化剂。Pt/Al_2O_3催化剂对碳比例高的烷烃活性更高。     

Impact of support oxide and Ba loading on the NOx storage properties of Pt/Ba/support catalysts: CO2 and H2O effects

A series of 1 wt.%Pt/_xBa/Support (Support = Al₂O₃, SiO₂, Al₂O₃-5.5 wt.%SiO₂ and Ce_0.7Zr_0.3O₂, x = 5–30 wt.% BaO) catalysts was investigated regarding the influence of the support oxide on Ba properties for the rapid NO_x trapping (100 s). Catalysts were treated at 700 °C under wet oxidizing atmosphere. The nature of the support oxide and the Ba loading influenced the Pt–Ba proximity, the Ba dispersion and then the surface basicity of the catalysts estimated by CO₂-TPD. At high temperature (400 °C) in the absence of CO₂ and H₂O, the NO_x storage capacity increased with the catalyst basicity: Pt/20Ba/Si < Pt/20Ba/Al5.5Si < Pt/10Ba/Al < Pt/5Ba/CeZr < Pt/30Ba/Al5.5Si < Pt/20Ba/Al < Pt/10BaCeZr. Addition of CO₂ decreased catalyst performances. The inhibiting effect of CO₂ on the NO_x uptake increased generally with both the catalyst basicity and the storage temperature. Water negatively affected the NO_x storage capacity, this effect being higher on alumina containing catalysts than on ceria–zirconia samples. When both CO₂ and H₂O were present in the inlet gas, a cumulative effect was observed at low temperatures (200 °C and 300 °C) whereas mainly CO₂ was responsible for the loss of NO_x storage capacity at 400 °C. Finally, under realistic conditions (H₂O and CO₂) the Pt/20Ba/Al5.5Si catalyst showed the best performances for the rapid NO_x uptake in the 200–400 °C temperature range. It resulted mainly from: (i) enhanced dispersions of platinum and barium on the alumina–silica support, (ii) a high Pt–Ba proximity and (iii) a low basicity of the catalyst which limits the CO₂ competition for the storage sites.     

Water-induced bulk Ba(NO3)2 formation from NO2 exposed thermally aged BaO/Al2O3

Phase changes in high temperature treated (>900 °C) 8 or 20 wt% BaO supported on γ-Al₂O₃ model lean NO_x trap (LNT) catalysts, induced by NO₂ and/or H₂O adsorption, were investigated with powder X-ray diffraction (XRD), solid state ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, and NO₂ temperature programmed desorption (TPD) experiments. After calcination in dry air at 1000 °C, the XRD and solid state ²⁷Al MAS NMR results confirm that stable surface BaO and bulk BaAl₂O₄ phases are formed for 8 and 20 wt% BaO/Al₂O₃, respectively. Following NO₂ adsorption over these thermally treated samples, some evidence for nanosized Ba(NO₃)₂ particles are observed in the XRD results, although this may represent a minority phase. However, when water was added to the thermally aged samples after NO₂ exposure, the formation of bulk crystalline Ba(NO₃)₂ particles was observed in both samples. Solid state ²⁷Al MAS NMR is shown to be a good technique for identifying the various Al species present in the materials during the processes studied here. NO₂ TPD results demonstrate a significant loss of uptake for the 20 wt% model catalysts upon thermal treatment. However, the described phase transformations upon subsequent water treatment gave rise to the partial recovery of NO_x uptake, demonstrating that such a water treatment of thermally aged catalysts can provide a potential method to regenerate LNT materials.