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1.
The performance of a model Pt/BaO/Al 2O 3 monolith catalyst was studied using H 2 as the reductant. The dependence of product selectivities on operating parameters is reported, including the durations of regeneration and storage times, feed composition and temperature, and monolith temperature. The data are explained in terms of a phenomenological model factoring in the transport, kinetic, and spatio‐temporal effects. The Pt/BaO catalyst exhibits high cycle‐averaged NOx conversion above 100°C, generating a mixture of N 2 and byproducts NH 3 and N 2O. The cycle‐averaged NOx conversion exhibits a maximum at about 300°C corresponding to the NOx storage maximum. The N 2 selectivity exhibits a maximum at a somewhat higher temperature, at which point the NH 3 selectivity exhibits a minimum. This trend conveys the intermediate role of NH 3 in reacting with stored NOx. Both N 2 and N 2O are also formed during the storage steps from the oxidation of NH x species produced during the regeneration. © 2009 American Institute of Chemical Engineers AIChE J, 2009 相似文献
2.
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
3.
The influence of a pre-treatment at 700 °C, either under a O 2/N 2 mixture or only under N 2, and followed by a hydrothermal aging at 700 °C under wet air, was studied for Pt/Ba/Al NSR model catalysts prepared by different methods: (i) successive impregnation of Ba and Pt, (ii) co-addition of Pt and Ba and (iii) barium precipitation followed by Pt impregnation. The catalysts were evaluated by NOx storage capacity measurements and were characterized by N 2 adsorption, XRD, CO 2-TPD, H 2 chemisorption and H 2-TPR. The pre-treatment under N 2 largely improves the NOx storage performance in the whole studied temperature range (200–400 °C), with or without H 2O and CO 2 in the inlet gas. The better NOx storage properties of the catalysts treated under N 2 before aging are due to: (i) a higher NO oxidation activity (mainly linked to a higher platinum dispersion), (ii) a higher number of NOx storage sites resulting from a higher barium dispersion, and consequently to (iii) a higher Pt-Ba proximity. 相似文献
5.
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
6.
In this work, we investigated the NO x storage behavior of Pt/BaO/CeO 2 catalysts, especially in the presence of SO 2. High surface area CeO 2 (110 m 2/g) with a rod like morphology was synthesized and used as a support. The Pt/BaO/CeO 2 sample demonstrated slightly higher NO x uptake in the entire temperature range studied compared with Pt/BaO/γ-Al 2O 3. More importantly, this ceria-based catalyst showed higher sulfur tolerance than the alumina-based one. The time of complete NO x uptake was maintained even after exposing the sample to 3 g/L of SO 2. The same sulfur exposure, on the other hand, eliminated the complete NO x uptake time on the alumina-based NO x storage catalysts. TEM images show no evidence of either Pt sintering or BaS phase formation during reductive de-sulfation up to 600 °C on the ceria-based catalyst, while the same process over the alumina-based catalyst resulted in both a significant increase in the average Pt cluster size and the agglomeration of a newly formed BaS phase into large crystallites. XPS results revealed the presence of about five times more residual sulfur after reductive de-sulfation at 600 °C on the alumina-based catalysts in comparison with the ceria-based ones. All of these results strongly support that, besides their superior intrinsic NO x uptake properties, ceria-based catalysts have (a) much higher sulfur tolerance and (b) excellent resistance against Pt sintering when they are compared to the widely used alumina-based catalysts. 相似文献
7.
Differences in the NO x storage-reduction (NSR) behavior of Pt/Ba/CeO 2 and Pt/Ba/Al 2O 3 have been identified and traced to their different chemical and structural properties. The results show that Pt/Ba/CeO 2 exhibits inferior NO x storage and, particularly, reduction (regeneration) activity compared to the Al 2O 3 supported catalyst. The incomplete reduction of the stored NO x-species in Pt/Ba/CeO 2 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/CeO 2 catalyst to reducing H 2 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 CeO 2 onto the surface of Pt particles. 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.
A series of MnO 2/ZrO 2 mixed oxides were prepared in reverse microemulsions for NOx adsorption and abatement. The results show that the amount of NOx adsorbed was increased with increasing MnO 2 content in various MnO 2/ZrO 2 samples. The maximum uptake value of NOx was 27.66 mg NOx/g adsorbent on the 40% Mn–Zr sample at 200 °C with NOx initial adsorption rate as 2.63 mg/(g adsorbent min). TPD results show that the complete desorption of NOx was easily obtained by heating the sample to 450 °C, and the temperature for the complete desorption can be further decreased to 210 °C by adding carbon monoxide into the argon desorption streams. Furthermore, water vapor was found to reduce NOx adsorption capacity because of its stronger competitive adsorption with NOx species. It is noteworthy that a small amount of sulfur dioxide could significantly increase the initial rate of NOx adsorption although it slightly decreased the NOx adsorption capacity. 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.
Catalytic oxidation activity of carbon-black (CB) simulating the soot of diesel particulate matters to CO 2 over 3Pt/Al 2O 3, 3Pt5Mn/Al 2O 3 and 3Pt/30Ba–Al 2O 3 catalysts is investigated with model gases of diesel emission. In case of the large amount of CB compared to the amount of catalyst (3/1, w/w) in the mixture sample, insufficient oxygen at the point of sudden increase in the amount of CO 2 is leaded to the partial oxidation using the lattice oxygen of the catalyst. And the peaks of CO 2 after the first peak were attributed to the regional combustion of the CB, which was not in contact with catalyst particles. The fresh 3Pt5Mn was estimated to the oxidation states on the catalyst surface by XPS. For used sample at 700 °C, the BEs of Pt 4d5 was revealed to metallic state Pt(0) (314.4 eV) in a predominant levels compared with Pt(II) (317.3 eV). While BEs of Mn 2p were similar to that obtained from the fresh 3Pt5Mn. It is suggested that Pt is in charge of the roles in CB-oxidation, using the lattice oxygen of the catalyst. Two-stage catalytic system with the strategies of promoting the soot oxidation and NO x reduction, simultaneously, were composed of the CB oxidation catalyst and the diesel oxidation catalyst. The catalytic oxidation of CB was accelerated by activated oxidants and exothermic reaction resulted from the diesel oxidation catalyst, which lies in upstream of two-stage. But the system with the CB oxidation catalyst sited in the upstream showed the initiation of CB oxidation at a lower temperature than the other case. Two-stage catalytic system composed of 3Pt5Mn with CB in the upstream and DOC in the downstream showed high oxidation activity with 95% consumption rate of CB to the total loaded CB in the range of 100–500 °C during the TPR process. 相似文献
12.
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
13.
Spinel nano-Co 3O 4 was prepared by solid-state reaction at room temperature and investigated for selective catalytic reduction of NO x by NH 3 (NH 3-SCR). Although suffering from pore filling and plugging, treatment of this catalyst by SO 2 showed novel promoting effect on NH 3-SCR above 250 °C. Bulk cobalt sulfate was observed over the sulfated Co 3O 4 with XRD, which would be an active component for NH 3-SCR. The sulphated Co 3O 4 catalyst exhibited good resistance to SO 2 (500 ppm, 100 ppm) and 10% H 2O at a space velocity of about 25 000 h −1 at 300 °C, as tested for 12 h. 相似文献
14.
In this investigation, a comparative study for a NO
X
storage catalytic system was performed focusing on the parameters that affect the reduction by using different reductants (H 2, CO, C 3H 6 and C 3H 8) and different temperatures (350, 250 and 150 °C), for a Pt/BaO/Al 2O 3 catalyst. Transient experiments show that H 2 and CO are highly efficient reductants compared to C 3H 6 which is somewhat less efficient. H 2 shows a significant reduction effect at relatively low temperature (150 °C) but with a low storage capacity. We find that C 3H 8does not show any NO
X
reduction ability for NO
X
stored in Pt/BaO/Al 2O 3 at any of the temperatures. The formation of ammonia and nitrous oxide is also discussed. 相似文献
15.
A new NO x storage-reduction electrochemical catalyst has been prepared from a polycrystalline Pt film deposited on 8 mol% Y 2O 3-stabilized ZrO 2 (YSZ) solid electrolyte. BaO has been added onto the Pt film by impregnation method. The NO x storage capacity of Pt-BaO/YSZ system was investigated at 350 °C and 400 °C under lean conditions. Results have shown that the electrochemical catalyst was effective for NO x storage. When nitric oxides are fully stored, the catalyst potential is high and reaches its maximum. On the other hand, when a part of NO and also NO 2 desorb to the gas phase, the catalyst potential remarkably drops and finally stabilizes when no more NO x storage occurs but only the reaction of NO oxidation into NO 2. Furthermore, the investigation has clearly demonstrated that the catalyst potential variation versus temperature or chemical composition is an effective indicator for in situ following the NO x storage-reduction process, i.e. the storage as well as the regeneration phase. The catalyst potential variations during NO x storage process was explained in terms of oxygen coverage modifications on the Pt. 相似文献
16.
In this study, the reactivity of well-characterized diesel soot samples is investigated by thermogravimetry under different kinds of oxidizing atmospheres (20% O 2 or 10% O 2 + 700 ppm NO 2) either under catalyzed or non-catalyzed conditions. Whatever the atmosphere used, the catalyst Pt/ceria-zirconia was able to lower significantly the ignition temperature of soot, but the catalytic effect was found to be more pronounced when the oxidation process was assisted by NOx. This is due mainly to the efficiency of both catalyst components (the noble metal and the OSC material) in recycling the NO released after attack of the soot by NO 2. By contrast, the NO 2 is of very limited use in the absence of catalyst under our experimental conditions. The global kinetic parameters representative of the carbonaceous matrix oxidation are determined using a methodological approach combining thermogravimetric experiments and non-linear multivariate regression. The kinetic parameters obtained are consistent both with the literature results and the postulated mechanistic pathways for soot oxidation assisted or not by NOx. 相似文献
17.
The effects of regeneration-phase CO and/or H 2, and their amounts as a function of temperature on the trapping and reduction of NO X over a model and a commercial NO X storage/reduction catalyst have been evaluated. Overall, for both catalysts, their NO X removal performance improved with each incremental increase in H 2 concentration. For the commercial sample, using CO at 200 °C, beyond a small amount added, was found to decrease performance. The addition of H 2 to the CO-containing mixtures resulted in improved performance at 200 °C, but the presence of the CO still resulted in decreased performance in comparison to activity when just H 2 was used. With the model sample, the presence of CO resulted in very poor performance at 200 °C, even with H 2. The data suggest that CO poisons Pt sites, including Pt-catalyzed nitrate decomposition. At 300 °C, H 2, CO, and mixtures of the two were comparable for trapping and reduction of NO X, although with the model sample H 2 did prove consistently better. With the commercial sample, H 2 and CO were again comparable at 500 °C, but mixtures of the two led to slightly improved performance, while yet again H 2 and H 2-containing mixtures proved better than CO when testing the model sample. NH 3 formation was observed under most test conditions used. At 200 °C, NH 3 formation increased with each increase in H 2, while at 500 °C, the amount of NH 3 formed when using the mixtures was higher than that when using either H 2 or CO. This coincides with the improved performance observed with the mixtures when testing the commercial. 相似文献
18.
The NO x storage and reduction (NSR) catalysts Pt/K/TiO 2–ZrO 2 were prepared by an impregnation method. The techniques of XRD, NH 3-TPD, CO 2-TPD, H 2-TPR and in situDRIFTS were employed to investigate their NO x storage behavior and sulfur-resisting performance. It is revealed that the storage capacity and sulfur-resisting ability of these catalysts depend strongly on the calcination temperature of the support. The catalyst with theist support calcined at 500 °C, exhibits the largest specific surface area but the lowest storage capacity. With increasing calcination temperature, the NO x storage capacity of the catalyst improves greatly, but the sulfur-resisting ability of the catalyst decreases. In situ DRIFTS results show that free nitrate species and bulk sulfates are the main storage and sulfation species, respectively, for all the catalysts studied. The CO 2-TPD results indicate that the decomposition performance of K 2CO 3 is largely determined by the surface property of the TiO 2–ZrO 2 support. The interaction between the surface hydroxyl of the support and K 2CO 3 promotes the decomposition of K 2CO 3 to form –OK groups bound to the support, leading to low NO x storage capacity but high sulfur-resisting ability, while the interaction between the highly dispersed K 2CO 3 species and Lewis acid sites gives rise to high NO x storage capacity but decreased sulfur-resisting ability. The optimal calcination temperature of TiO 2–ZrO 2 support is 650 °C. 相似文献
19.
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
20.
This article discusses the performance of ZrO 2-supported size-selected Pt nanoparticles for the decomposition of methanol, ethanol, 2-propanol, and 2-butanol. The potential of each alcohol for the production of H 2 and other relevant products in the presence of a catalyst is studied in a packed-bed mass flow reactor operating at atmospheric pressure. All the alcohols studied show some decomposition activity below 200 °C which increased with increasing temperature. In all cases, high selectivity towards H 2 formation is observed. With the exception of methanol, all alcohol conversion reactions lead to catalyst deactivation at high temperatures ( T > 250 °C for 2-propanol and 2-butanol, T > 325 °C for ethanol) due to carbon poisoning. However, long-term catalyst deactivation can be avoided by optimizing reaction conditions such as operating temperature. 相似文献
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