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1.
A series of 1 wt.%Pt/ xBa/Support (Support = Al 2O 3, SiO 2, Al 2O 3-5.5 wt.%SiO 2 and Ce 0.7Zr 0.3O 2, 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 CO 2-TPD. At high temperature (400 °C) in the absence of CO 2 and H 2O, 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 CO 2 decreased catalyst performances. The inhibiting effect of CO 2 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 CO 2 and H 2O were present in the inlet gas, a cumulative effect was observed at low temperatures (200 °C and 300 °C) whereas mainly CO 2 was responsible for the loss of NO x storage capacity at 400 °C. Finally, under realistic conditions (H 2O and CO 2) 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 CO 2 competition for the storage sites. 相似文献
2.
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/Al 2O 3 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 H 2 and/or CO and NO on reduced Rh and/or Pt sites. The second N 2 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 O 2. The ammonia can be formed either by hydrolysis of isocyanates or by direct reaction of NO and H 2. 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 NOx storage, since the rate of regeneration was sufficient to remove the NOx 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 NOx stored on the catalyst deteriorated from cycle to cycle until the amount of NOx stored in the lean phase matched the NOx 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. 相似文献
3.
The deactivation by sulfur and regeneration of a model Pt/Ba/Al 2O 3 NO x trap catalyst is studied by hydrogen temperature programmed reduction (TPR), X-ray diffraction (XRD), and NO x storage capacity measurements. The TPR profile of the sulfated catalyst in lean conditions at 400 °C reveals three main peaks corresponding to aluminum sulfates (550 °C), “surface” barium sulfates (650 °C) and “bulk” barium sulfates (750 °C). Platinum plays a role in the reduction of the two former types of sulfates while the reduction of “bulk” barium sulfates is not influenced by the metallic phase. The thermal treatment of the sulfated catalyst in oxidizing conditions until 800 °C leads to a stabilization of sulfates which become less reducible. Stable barium sulfides are formed during the regeneration under hydrogen at 800 °C. However, the presence of carbon dioxide and water in the rich mixture allows eliminating more or less sulfides and sulfates, depending on the temperature and time. The regeneration in the former mixture at 650 °C leads to the total recovery of the NO x storage capacity even if “bulk” barium sulfates are still present on the catalyst. 相似文献
4.
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 O 2 pulse and a subsequent NO pulse were injected into a pellet of the Pt/Ba/Al 2O 3 catalyst, the time profiles of several gas products, NO, N 2, NH 3 and H 2O, were obtained as a result of the release and reduction of NO x caused by H 2 injection. Comparing the time profiles in another analysis, which were obtained using a model catalyst consisting of a flat 5 nmPt/Ba(NO 3) 2/cordierite plate, the release and reduction of NO x on Pt/Ba/Al 2O 3 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 N 2 by H 2 pulse injection. When this H 2 pulse was injected in a large amount, NO was reduced to NH 3 instead of N 2. A only small amount of H2O was detected because of the strong affinity for alumina support. We can analyze the NOx regeneration process to separate two steps of the NOx 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 NOx release increased as temperature increase. 相似文献
5.
A multi-component NO x-trap catalyst consisting of Pt and K supported on γ-Al 2O 3 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 H 2O and CO 2 on NO x storage. The Al 2O 3 support was shown to have NO x trapping capability with and without Pt present (at 250 °C Pt/Al 2O 3 adsorbs 2.3 μmols NO x/m 2). NO x is primarily trapped on Al 2O 3 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/m 2, 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/Al 2O 3 catalyst are coordinated on the Al 2O 3 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 NOx storage capacity losses associated with carboxylate competition. 相似文献
6.
The NO x storage-reduction catalysis under oxidizing conditions in the presence of SO 2 has been investigated on Pt/Ba/Fe/Al 2O 3, Pt/Ba/Co/Al 2O 3, Pt/Ba/Ni/Al 2O 3, and Pt/Ba/Cu/Al 2O 3 catalysts compared with Pt/Ba/Al 2O 3, Pt/Fe/Al 2O 3, Pt/Co/Al 2O 3, Pt/Ni/Al 2O 3, Pt/Cu/Al 2O 3 and Pt/Al 2O 3 catalysts. The NO x purification activity of Pt/Ba/Fe/Al 2O 3 catalyst was the highest of all the catalysts investigated in this paper after an aging treatment. That of the aged Pt/Ba/Co/Al 2O 3 and Pt/Ba/Ni/Al 2O 3 catalysts was essentially the same as that of the aged Pt/Ba/Al 2O 3 catalyst, while that of the aged Pt/Ba/Cu/Al 2O 3 and Pt/Cu/Al 2O 3 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. 相似文献
7.
A mean field model, for storage and desorption of NO x in a Pt/BaO/Al 2O 3 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/Al 2O 3; (ii) NO oxidation on Pt/BaO/Al 2O 3; (iii) NO x storage on BaO/Al 2O 3; (iv) NO x storage on Pt/BaO/Al 2O 3 with thermal regeneration and (v) NO x storage on Pt/BaO/Al 2O 3 with regeneration using C 3H 6. In this paper, we focus on the last sub-system. The kinetic model for NO x storage on Pt/BaO/Al 2O 3 was constructed with kinetic parameters obtained from the NO oxidation model together with a NO x storage model on BaO/Al 2O 3. This model was not sufficient to describe the NO x storage experiments for the Pt/BaO/Al 2O 3, because the NO x desorption in TPD experiments was larger for Pt/BaO/Al 2O 3, compared to BaO/Al 2O 3. 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/Al 2O 3. To this NO x storage model, additional steps were added to describe the reduction by hydrocarbon in experiments with NO 2 and C 3H 6. 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. 相似文献
8.
The NO x storage and reduction functions of a Pt–Ba/Al 2O 3 “NO x storage–reduction” catalyst has been investigated in the present work by applying the transient response and the temperature programmed reaction methods, by using propylene as the reducing agent. It is found that: (i) the storage of NO x occurs first at BaO and then at BaCO 3, which are the most abundant sites following regeneration of catalyst with propylene; (ii) the overall storage process at BaCO 3 is slower than at BaO; (iii) CO 2 inhibits the NO x storage at low temperatures; (iv) the amount of NO x stored up to catalyst saturation at 350 °C corresponds to 17.6% of Ba; (v) the reduction of stored NO x groups is fast and is limited by the concentration of propylene in the investigated T range (250–400 °C); (vi) selectivity to N 2 is almost complete at 400 °C but is significantly lower at 300 °C due to the formation of NO which can be tentatively ascribed to the presence of unselective Pt–O species. 相似文献
9.
The behaviour of a Pt(1 wt.%) supported on CeO 2–ZrO 2(20 wt.%)/Al 2O 3(64 wt.%)–BaO(16 wt.%) as a novel NO x storage–reduction catalyst is studied by reactivity tests and DRIFT experiments and compared with that of Pt(1%)–BaO(15 wt.%) on alumina. The former catalyst, designed as a hydrothermally stable sample, is composed of an alumina modified with Ba ions and an overlayer of ceria-zirconia. The results pointed out that during the calcination barium ions migrates over the surface of the catalyst which thus show a good NO x storage–reduction behaviour comparable with that of Pt–BaO on alumina, although Ba ions result much better dispersed. 相似文献
10.
The reduction of NO under cyclic “lean”/“rich” conditions was examined over two model 1 wt.% Pt/20 wt.% BaO/Al 2O 3 and 1 wt.% Pd/20 wt.% BaO/Al 2O 3 NO x storage reduction (NSR) catalysts. At temperatures between 250 and 350 °C, the Pd/BaO/Al 2O 3 catalyst exhibits higher overall NO x reduction activity. Limited amounts of N 2O were formed over both catalysts. Identical cyclic studies conducted with non-BaO-containing 1 wt.% Pt/Al 2O 3 and Pd/Al 2O 3 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/Al 2O 3. 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. 相似文献
11.
The reduction of NO x by hydrogen under lean burn conditions over Pt/Al 2O 3 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/Al 2O 3. In this case, although pure H 2 and pure CO are ineffective for NO x reduction under lean burn conditions, H 2/CO mixtures are very effective. With a realistic (1:3) H 2:CO ratio, typical of actual exhaust gas, Pd/Al 2O 3 is significantly more active than Pt/Al 2O 3, delivering 45% NO x conversion at 160 °C, compared to >15% for Pt/Al 2O 3 under identical conditions. The nature of the support is also critically important, with Pd/Al 2O 3 being much more active than Pd/SiO 2. Possible mechanisms for the improved performance of Pd/Al 2O 3 in the presence of H 2+CO are discussed. 相似文献
12.
A method to quantify DRIFT spectral features associated with the in situ adsorption of gases on a NO x adsorber catalyst, Pt/K/Al 2O 3, is described. To implement this method, the multicomponent catalyst is analysed with DRIFT and chemisorption to determine that under operating conditions the surface comprised a Pt phase, a pure γ-Al 2O 3 phase with associated hydroxyl groups at the surface, and an alkalized-Al 2O 3 phase where the surface –OH groups are replaced by –OK groups. Both DRIFTS and chemisorption experiments show that 93–97% of the potassium exists in this form. The phases have a fractional surface area of 1.1% for the 1.7 nm-sized Pt, 34% for pure Al 2O 3 and 65% for the alkalized-Al 2O 3. NO 2 and CO 2 chemisorption at 250 °C is implemented to determine the saturation uptake value, which is observed with DRIFTS at 250 °C. Pt/Al 2O 3 adsorbs 0.087 μmol CO 2/m 2and 2.0 μmol NO 2/m 2, and Pt/K/Al 2O 3 adsorbs 2.0 μmol CO 2/m 2and 6.4 μmol NO 2/m 2. This method can be implemented to quantitatively monitor the formation of carboxylates and nitrates on Pt/K/Al 2O 3 during both lean and rich periods of the NO x adsorber catalyst cycle. 相似文献
13.
The formation and stability of BaAl 2O 4 and BaCeO 3 in Pt-Ba/Al 2O 3 and Pt-Ba/CeO 2 based NO x storage-reduction (NSR) catalysts has been investigated using kinetic measurements, X-ray diffraction, thermal analysis and X-ray absorption spectroscopy. In as-prepared state, the Ba-component in the NSR catalysts was made up of amorphous BaO and BaCO 3. The formation of BaAl 2O 4 started above 850 °C, whereas the formation of BaCeO 3 was already observed at 800 °C and was faster than that of BaAl 2O 4. The stability of BaAl 2O 4 and BaCeO 3 in various liquid and gaseous atmospheres was different. BaAl 2O 4 was rapidly hydrated at room temperature in the presence of water and transformed to Ba(NO 3) 2 and γ-alumina in the presence of HNO 3, whereas BaCeO 3 was decomposed to much lower extent under these conditions. Interestingly, BaCeO 3 was transformed to Ba(NO 3) 2/CeO 2 in the presence of NO 2/H 2O at 300–500 °C. Also, the presence of CO 2 led to decomposition of barium cerate, which has important consequences for the catalyst ageing under NO x-storage conditions and can be exploited for regeneration of thermally aged NSR-catalysts. 相似文献
14.
The effect of different reducing agents (H 2, CO, C 3H 6 and C 3H 8) on the reduction of stored NO x over PM/BaO/Al 2O 3 catalysts (PM = Pt, Pd or Rh) at 350, 250 and 150 °C was studied by the use of both NO 2-TPD and transient reactor experiments. With the aim of comparing the different reducing agents and precious metals, constant molar reduction capacity was used during the reduction period for samples with the same molar amount of precious metal. The results reveal that H 2 and CO have a relatively high NO x reduction efficiency compared to C 3H 6 and especially C 3H 8 that does not show any NO x reduction ability except at 350 °C over Pd/BaO/Al 2O 3. The type of precious metals affects the NO x storage-reduction properties, where the Pd/BaO/Al 2O 3 catalyst shows both a high storage and a high reduction ability. The Rh/BaO/Al 2O 3 catalyst shows a high reduction ability but a relatively low NO x storage capacity. 相似文献
15.
Performance of NO x traps after high-temperature treatments in different redox environments was studied. Two types of treatments were considered: aging and pretreatment. Lean and rich agings were examined for a model NO x trap, Pt–Ba/Al 2O 3. These were done at 950 °C for 3 h, in air and in 1% H 2/N 2, respectively. Lean aging had a severe impact on NO x trap performance, including HC and CO oxidation, and NH 3 and N 2O formation. Rich aging had minimal impact on performance, compared to fresh/degreened performance. Deactivation from lean aging was essentially irreversible due to Pt sintering, but Pt remained dispersed with the rich aging. Pretreatments were examined for a commercially feasible fully formulated NO x trap and two model NO x traps, Pt–Ba/Al 2O 3 and Pt–Ba–Ce/Al 2O 3. Pretreatments were done at 600 °C for 10 min, and used feed gas that simulated diesel exhaust under several conditions. Lean pretreatment severely suppressed NO x, HC, CO, NH 3 and N 2O activities for the ceria-containing NO x traps, but had no impact on Pt–Ba/Al 2O 3. Subsequently, a relatively mild rich pretreatment reversed this deactivation, which appears to be due to a form of Pt–ceria interaction, an effect that is well known from early work on three-way catalysts. Practical applications of results of this work are discussed with respect to NO x traps for light-duty diesel vehicles. 相似文献
16.
A systematic mechanistic study of NO storage and reduction over Pt/Al 2O 3 and Pt/BaO/Al 2O 3 is carried out using Temporal Analysis of Products (TAP). NO pulse and NO/H 2 pump-probe experiments at 350 °C on pre-reduced, pre-oxidized, and pre-nitrated catalysts reveal the complex interplay between storage and reduction chemistries and the importance of the Pt/Ba coupling. NO pulsing experiments on both catalysts show that NO decomposes to major product N 2 on clean Pt but the rate declines as oxygen accumulates on the Pt. The storage of NO over Pt/BaO/Al 2O 3 is an order of magnitude higher than on Pt/Al 2O 3 showing participation of Ba in the storage even in the absence of gas phase O 2. Either oxygen spillover or transient NO oxidation to NO 2 is postulated as the first steps for NO storage on Pt/BaO/Al 2O 3. The storage on Pt/Ba/Al 2O 3 commences as soon as Pt–O species are formed. Post-storage H 2 reduction provides evidence that a fraction of NO is not stored in close proximity to Pt and is more difficult to reduce. A closely coupled Pt/Ba interfacial process is corroborated by NO/H 2 pump-probe experiments. NO conversion to N 2 by decomposition is sustained on clean Pt using excess H 2 pump-probe feeds. With excess NO pump-probe feeds NO is converted to N 2 and N 2O via the sequence of barium nitrate and NO decomposition. Pump-probe experiments with pre-oxidized or pre-nitrated catalyst show that N 2 production occurs by the decomposition of NO supplied in a NO pulse or from the decomposition of NOx stored on the Ba. The transient evolution of the two pathways depends on the extent of pre-nitration and the NO/H 2 feed ratio. 相似文献
17.
Novel NO x storage-reduction (NO xSR) catalysts prepared by Pt and/or Cu impregnation of Mg–Al (60:40) hydrotalcite (HT)-type compounds show better performances in NO x storage than Pt–Ba/Al 2O 3 Toyota-type NO xSR catalysts at reaction temperatures lower than 250 °C. The presence of Pt or Cu considerably enhances the activity, with the former more active. The nature of the HT source, however, also influences performance. The co-presence of Pt and Cu slightly worsens the low temperature activity, but considerably promotes the resistance to deactivation after severe hydrothermal treatment and in the presence of SO 2. This effect is attributed to both the possibility of formation of a Pt–Cu alloy after reduction, and the modification of the HT induced during the deposition of Cu. The overall Pt–Cu/HT performances are thus superior to those of the Pt–Ba/Al 2O 3 Toyota-type NO xSR catalysts. 相似文献
18.
Catalytic performance of Sn/Al 2O 3 catalysts prepared by impregnation (IM) and sol–gel (SG) method for selective catalytic reduction of NO x by propene under lean burn condition were investigated. The physical properties of catalyst were characterized by BET, XRD, XPS and TPD. The results showed that NO 2 had higher reactivity than NO to nitrogen, the maximum NO conversion was 82% on the 5% Sn/Al 2O 3 (SG) catalyst, and the maximum NO 2 conversion reached nearly 100% around 425 °C. Such a temperature of maximum NO conversion was in accordance with those of NO x desorption accompanied with O 2 around 450 °C. The activity of NO reduction was enhanced remarkably by the presence of H 2O and SO 2 at low temperature, and the temperature window was also broadened in the presence of H 2O and SO 2, however the NO x desorption and NO conversion decreased sharply on the 300 ppm SO 2 treated catalyst, the catalytic activity was inhibited by the presence of SO 2 due to formation of sulfate species (SO 42−) on the catalysts. The presence of oxygen played an essential role in NO reduction, and the activity of the 5% Sn/Al 2O 3 (SG) was not decreased in the presence of large oxygen. 相似文献
19.
In this paper, the effect of CO 2 and H 2O on NO x storage and reduction over a Pt–Ba/γ-Al 2O 3 (1 wt.% Pt and 30 wt.% Ba) catalyst is shown. The experimental results reveal that in the presence of CO 2 and H 2O, NO x is stored on BaCO 3 sites only. Moreover, H 2O inhibits the NO oxidation capability of the catalyst and no NO 2 formation is observed. Only 16% of the total barium is utilized in NO storage. The rich phase shows 95% selectivity towards N 2 as well as complete regeneration of stored NO. In the presence of CO 2, NO is oxidized into NO 2 and more NO x is stored as in the presence of H 2O, resulting in 30% barium utilization. Bulk barium sites are inactive in NO x trapping in the presence of CO 2·NH 3 formation is seen in the rich phase and the selectivity towards N 2 is 83%. Ba(NO 3) 2 is always completely regenerated during the subsequent rich phase. In the absence of CO 2 and H 2O, both surface and bulk barium sites are active in NO x storage. As lean/rich cycling proceeds, the selectivity towards N 2 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. 相似文献
20.
The NO x storage behavior of a series of Pt-Ba/Al 2O 3 catalysts, prepared by wet impregnation of Pt/Al 2O 3 with Ba(Ac) 2, has been investigated. The catalysts with Ba loadings in the range 4.5–28 wt.% were calcined at 500 °C in air and subsequently exposed to NO pulses in 5 vol.% O 2/He atmosphere. Catalysts were characterized by means of thermogravimetry (TG) combined with mass spectroscopy (MS) and XRD before and after exposure to NO pulses. Characterization of the calcined catalysts corroborated the existence of three Ba-containing phases which are discernible based on their different thermal stability: BaO, LT-BaCO 3 and HT-BaCO 3. Characterization after NO x exposure showed that the different Ba-containing phases present in the catalysts possess different reactivity for barium nitrate formation, depending on their interfacial contact. The different Ba(NO 3) 2 species produced upon NO x exposure could be distinguished based on their thermal stability. The study revealed that during the NO x storage process a new thermally instable BaCO 3 phase formed by reaction of evolved CO 2 with active BaO. The fraction of Ba-containing species that were active in NO x storage depended on the Ba loading, showing a maximum at a Ba loading of about 17 wt.%. Lower and higher Ba loading resulted in a significant loss of the overall efficiency of the Ba-containing species in the storage process. The loss in efficiency observed at higher loading is attributed to the lower reactivity of the HT-BaCO 3, which becomes dominant at higher loading, and the increased mass transfer resistance. 相似文献
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