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1.
The NO
x
storage performance at low temperature (100–200 °C) has been studied for model NO
x
storage catalysts. The catalysts were prepared by sequentially depositing support, metal oxide and platinum on ceramic monoliths. The support material consisted of acidic aluminium silicate, alumina or basic aluminium magnesium oxide, and the added metal oxide was either ceria or barium oxide. The NO
x
conversion was evaluated under net-oxidising conditions with transients between lean and rich gas composition and the NO
x
storage performance was studied by isothermal adsorption of NO 2 followed by temperature programmed desorption of adsorbed species. The maximum in NO
x
storage capacity was observed at 100 °C for all samples studied. The Pt/BaO/Al 2O 3 catalyst stored about twice the amount of NO
x
compared with the Pt/Al 2O 3 and Pt/CeO 2/Al 2O 3 samples. The storage capacity increased with increasing basicity of the support material, i.e. Pt/Al 2O 3·SiO 2 < Pt/Al 2O 3 < Pt/Al 2O 3 · MgO. Water did not significantly affect the NO
x
storage performance for Pt/Al 2O 3 or Pt/BaO/Al 2O 3. 相似文献
2.
The potentiality of a Pt-K/Al 2O 3 catalyst in the simultaneous removal of particulate matter (soot) and NO
x
is investigated in this work by means of Temperature Programmed Oxidation (TPO) experiments and Transient Response Method
(TRM), and compared with Pt-Ba/Al 2O 3. The results point out the higher performances of K-based sample in the soot combustion as compared to the Ba-based catalyst,
and similar behaviour in the NO
x
-storage. 相似文献
3.
Mesoporous and nanosized cobalt aluminate spinel with high specific surface area was prepared using microwave assisted glycothermal method and used as soot combustion catalyst in a NO x + O 2 stream. For comparison, zinc aluminate spinel and alumina supported platinum catalysts were prepared and tested. All samples were characterised using XRD, (HR)TEM, N 2 adsorption–desorption measurements. The CoAl 2O 4 spinel was able to oxidise soot as fast as the reference Pt/Al 2O 3 catalyst. Its catalytic activity can be attributed to a high NO x chemisorption on the surface of this spinel, which leads to the fast oxidation of NO to NO 2. 相似文献
4.
The thermal ageing and reactivation of Ba/CeO 2 and Ba/Al 2O 3 based NO
x
-storage/ reduction (NSR) catalysts was studied on model catalysts and catalyst systems at the engine. The mixed oxides BaAl 2O 4 and BaCeO 3, which lower the storage activity, are formed during ageing above 850 °C and 900 °C, respectively. Interestingly, the decomposition
of BaCeO 3 in an atmosphere containing H 2O/NO 2 leads again to NO
x
-storage active species, as evidenced by comparison of fresh, aged and reactivated Pt-Ba/CeO 2 based model catalysts. This can be technically exploited, particularly for the Ba/CeO 2 catalysts, as reactivation studies on thermally aged Ba/CeO 2 and Ba/Al 2O 3 based NSR catalysts on an engine bench showed. An on-board reactivation procedure is presented, that improved the performance
of a thermally aged catalyst significantly. 相似文献
5.
NO x storage capacity, sulphur resistance and regeneration of 1wt%Pt/Ce 0.7Zr 0.3O 2 (Pt/CeZr) and 1wt%Pt/10wt%BaO/Ce 0.7Zr 0.3O 2 (Pt/Ba/CeZr) catalysts were studied and compared to a 1wt%Pt/10wt%BaO/Al 2O 3 (Pt/Ba/Al) model catalyst submitted to the same treatments. Pt/Ba/CeZr presents the best NO x storage capacity at 400 °C in accordance with basicity measurements by CO 2 TPD and Pt/CeZr shows the better performance at 200 °C mainly due to a low sensitivity to CO 2 at this temperature. For all samples, sulphating induces a detrimental effect on NO x storage capacity but regeneration at 550 °C under rich conditions generally leads to the total recovery of catalytic performance.
However, the nearly complete sulphur elimination is only observed on Pt/CeZr. Moreover, an oxidizing treatment at 800 °C leads
to partial sulphates elimination on the Pt/CeZr catalyst whereas a stabilization of sulphates on Ba containing species is
observed. 相似文献
6.
The effect of steam on NO
x
reduction over lean NO
x
trap (LNT) Pt–Ba/Al 2O 3 and Pt/Al 2O 3 model catalysts was investigated with reaction protocols of rich steady-state followed by lean–rich cyclic operations using
CO and C 3H 8 as reductants, respectively. Compared to dry atmosphere, steam promoted NO
x
reduction; however, under rich conditions the primary reduction product was NH 3. The results of NO
x
reduction and NH 3 selectivity versus temperature, combined with temperature programmed reduction of stored NO
x
over Pt–BaO/Al 2O 3 suggest that steam causes NH 3 formation over Pt sites via reduction of NO
x
by hydrogen that is generated via water gas shift for CO/steam, or via steam reforming for C 3H 8/steam. During the rich mode of lean–rich cyclic operation with lean–rich duration ratio of 60 /20 s, not only the feed NO,
but also the stored NO
x
contributed to NH 3 formation. The NH 3 formed under these conditions could be effectively trapped by a downstream bed of Co 2+ exchanged Beta zeolite. When the cyclic operation was switched into lean mode at T < 450 °C, the trapped ammonia in turn participated in additional NO
x
reduction, leading to improved NO
x
storage efficiency. 相似文献
7.
Transient experiments were performed to study sulfur deactivation and regeneration of Pt/BaO/Al 2O 3 and Pt/SrO/Al 2O 3 NO
x
storage catalysts. It was found that the strontium-based catalysts are more easily regenerated than the barium-based catalysts
and that a higher fraction of the NO
x
storage sites are regenerated when H 2 is used in combination with CO 2 compared to H 2 only. 相似文献
8.
Effect of additives, Ce and Mn, on the catalytic performance of Sn/Al 2O 3 catalyst prepared by sol–gel method for the selective reduction of NO x with propene under lean conditions was studied. Sn–Ce/Al 2O 3 catalysts exhibited higher activity than Sn/Al 2O 3 catalyst and the optimum Ce loading is 0.5–1%. The promoting effect of Ce is to enhance the oxidation of NO to NO 2 and facilitate the activation of propene, both of which are important steps for the NO x reduction. The presence of oxygen contributes to the oxidation of NO and shows a promoting effect. 相似文献
9.
Flow reactor experiments and X-ray photoelectron spectroscopy (XPS) measurements were used to investigate the importance of platinum oxide formation on Pt/BaO/Al 2O 3 NO
x
storage catalysts during reactions conditions. The reaction studied was NO(g) + 1/2 O 2(g) NO 2(g). During NO 2 exposure of the catalyst the NO 2 dissociation rate decreased during the reaction. This activity decrease with time was also studied with XPS and it was found to be due to platinum oxide formation. The influence of sulphur exposure conditions on the performance of the NO
x
storage catalysts was studied by exposing the samples to lean and/or rich gas mixtures, simulating the conditions in a mixed lean application, containing SO 2. The main results show that all samples are sensitive to sulphur and that the deactivation proceeds faster when SO 2 is present in the feed under rich conditions than under lean or continuous SO 2 exposure. Additionally, the influence of the noble metals present in the catalysts was investigated regarding sulphur sensitivity and it was found that a combination of platinum and rhodium seems to be preferable to retain high performance of the catalyst under SO 2 exposure and subsequent regeneration. Finally, the behaviour of micro-fabricated model NO
x
storage catalysts was studied as a function of temperature and gas composition with area-resolved XPS. These model catalysts consisted of a thin film of Pt deposited on one-half of a BaCO 3 pellet. It was found that the combination of SO 2 and O 2 resulted in migration of Pt on the BaCO 3 support up to one mm away from the Pt/BaCO 3 interface. 相似文献
10.
Pt–Ba/MeO (where MeO = Al 2O 3, CeO 2, SiO 2 and ZrO 2) NO
x
storage-reduction catalysts with Ba-loading varying from 0 wt.% to 28 wt.% were investigated concerning stability of Ba phases
and NO
x
storage-reduction efficiency. For Pt–Ba/Al 2O 3 three different Ba-containing phases with different thermal stability are distinguished based on their interaction with the
support. The relative concentration of these phases varies with the Ba-loading and NO
x
storage tests indicated that the BaCO 3 phase decomposing between 400 °C and 800 °C (LT-BaCO 3) is the most efficient Ba containing phase for NO
x
storage. Similar investigations of Pt–Ba catalysts supported on CeO 2, SiO 2 and ZrO 2 showed that the relative amount of LT-BaCO 3 phase depends also on the support material. NO
x
storage measurements confirmed a correlation between the concentration of LT-BaCO 3 and NO
x
storage efficiency. Basicity and textural properties of the support are identified as crucial parameters for efficient NO
x
storage catalysts. 相似文献
11.
The effects of thermal aging and H 2O treatment on the physicochemical properties of BaO/Al 2O 3 (the NO x storage component in the lean NO x trap systems) were investigated by means of X-ray diffraction (XRD), BET, TEM/EDX and NO 2 TPD. Thermal aging at 1000 °C for 10 h converted dispersed BaO/BaCO 3 on Al 2O 3 into low surface area crystalline BaAl 2O 4. TEM/EDX and XRD analysis showed that H 2O treatment at room temperature facilitated a dissolution/reprecipitation process, resulting in the formation of a highly
crystalline BaCO 3 phase segregated from the Al 2O 3 support. Crystalline BaCO 3 was formed from conversion of both BaAl 2O 4 and a dispersed BaO/BaCO 3 phase, initially present on the Al 2O 3 support material after calcinations at 1000 and 500 °C, respectively. Such a phase change proceeded rapidly for dispersed
BaO/BaCO 3/Al 2O 3 samples calcined at relatively low temperatures with large BaCO 3 crystallites observed in XRD within 10 min after contacting the sample with water. Significantly, we also find that the change
in barium phase occurs even at room temperature in an ambient atmosphere by contact of the sample with moisture in the air,
although the rate is relatively slow. These phenomena imply that special care to prevent the water contact must be taken during
catalyst synthesis/storage, and during realistic operation of Pt/BaO/Al 2O 3 NO x trap catalysts since both processes involve potential exposure of the material to CO 2 and liquid and/or vapor H 2O. Based on the results, a model that describes the behavior of Ba-containing species upon thermal aging and H 2O treatment is proposed. 相似文献
12.
Three model catalysts (Pt/Al 2O 3, Pt/TiO 2, Pt/V 2O 5/TiO 2) were examined in regard to their NO 2 formation ability under a changing lean gas composition. The results show that the NO to NO 2 oxidation function as well as the NO
x
reduction under lean gas conditions is affected by a change in the lean gas atmosphere. The NO oxidation activity also decreased with time, for Pt/Al 2O 3 and Pt/TiO 2, and a possible explanation may be platinum oxide formation. This deactivation was not observed for Pt/V 2O 5/TiO 2. 相似文献
13.
The transient reactivity and surface phenomena of storage and conversion of NO
x
species on Pt(1%)–Me/Al 2O 3 catalysts, where Me = Ba, Ce and Cu, were studied by the RWF (rectangular wavefront) method. The Me component has a relevant influence on the processes of surface storage and transformation. The reduction of NO
x
by propene in the presence of oxygen is promoted by adding Cu to a Pt/Al 2O 3 catalyst, while cerium promotes transient conversion of NO in the absence of propene, but inhibits the reduction of NO
x
in the presence of propene. Copper is suggested to be a promising element to add together with Ba for new NO
x
storage-reduction catalysts due to its capacity to act both as a storage element and as promoter for NO
x
reduction. 相似文献
14.
The effect of various parameters on the NO
x
conversion over NO
x
storage and reduction catalysts supported on alumina was investigated. The Pt/BaO/Al 2O 3 catalyst exhibited a higher NO
x
reduction activity than the Pt/Al 2O 3 catalyst under the static and cycling conditions. The activity of Pt/BaO/Al 2O 3 catalyst was improved in the cycled feedstream. The Pt/SrO/Al 2O 3 was found to have as high activity as Pt/BaO/Al 2O 3 for NO
x
reduction. In order to achieve effective reduction of NO
x
, NO
x
storage in the form of Me(NO 3) 2 (Me = Ba or Sr) is more favorable than other nitrates and the rich condition should be chosen in such a way that the sorption capacity can be fully regenerated at a fast rate and the inhibition effect by strongly adsorbed molecules derived from C 3H 6 and CO can be minimized. 相似文献
15.
Au/Al 2O 3 · xH 2O and Au/TiO 2/Al 2O 3 · xH 2O ( x = 0–3) catalysts were prepared by assembling gold nanoparticles on neat and TiO 2-modified Al 2O 3, AlOOH, and Al(OH) 3 supports, and their catalytic activity in CO oxidation was tested either as synthesized or after on-line pretreatment in
O 2–He at 500 °C. A promotional effect of TiO 2 on the activity of gold catalysts was observed upon 500 °C-pretreatment. The catalyst stability as a function of time on
stream was tested in the absence or presence of H 2, and physiochemical characterization applying BET, ICP-OES, XRD, TEM, and 27Al MAS NMR was conducted. 相似文献
16.
The temperature window of NO
x
consumption lies between 140 and 500 °C. The 0.5 wt%Co/Al 2O 3 catalyst exhibits a total consumption of NO
x
between 300 and 350 °C at a space velocity of 50 000 h −1. The presence of acetonitrile and methylnitrite can explain the difference between N 2 formation and NO x consumption at T< 400 °C. The Co 2+, in octahedral site, has been shown to coordinate two NO molecules. 相似文献
17.
The NO
x
storage performance at low temperature (100–200 °C) has been studied for model NO
x
storage catalysts. The catalysts were prepared by sequentially depositing support, metal oxide and platinum on ceramic monoliths. The support material consisted of acidic aluminium silicate, alumina or basic aluminium magnesium oxide, and the added metal oxide was either ceria or barium oxide. The NO
x
conversion was evaluated under net-oxidising conditions with transients between lean and rich gas composition and the NO
x
storage performance was studied by isothermal adsorption of NO2 followed by temperature programmed desorption of adsorbed species. The maximum in NO
x
storage capacity was observed at 100 °C for all samples studied. The Pt/BaO/Al2O3 catalyst stored about twice the amount of NO
x
compared with the Pt/Al2O3 and Pt/CeO2/Al2O3 samples. The storage capacity increased with increasing basicity of the support material, i.e. Pt/Al2O3 · SiO2 < Pt/Al2O3 < Pt/Al2O3 · MgO. Water did not significantly affect the NO
x
storage performance for Pt/Al2O3 or Pt/BaO/Al2O3. 相似文献
18.
Pt/Al 2O 3 and Pt/BaO/Al 2O 3 catalysts (1 wt% Pt, 10 wt%BaO) were sulfated under conditions simulating a real NSR catalyst operation. Comparative TPR
and XPS studies of sulfur removal from Pt/Al 2O 3 and Pt/BaO/Al 2O 3 catalysts indicate that the sulfur removal from Al 2O 3 surface precedes reductive decomposition of BaSO 4 (250–400 °C). Barium sulfate decomposition started with further increase in desulfation temperature at the point of surface
atomic ratio Ba:S = 1 (~450 o). Simultaneously, an intensive formation of sulfide species on the catalyst surface was observed. Thermodynamic analysis
of the desulfation process allows us to hypothesize that barium sulfide formation may hinder sulfur removal under reducing
conditions. 相似文献
19.
The NO
x
adsorption mechanism on Pt/BaO/Al 2O 3 catalysts was investigated by performing NO
x
storage/reduction cycles, NO 2 adsorption and NO + O 2 adsorption on 2%Pt/(x)BaO/Al 2O 3 ( x = 2, 8, and 20 wt%) catalysts. NO
x
uptake profiles on 2%\Pt/20%BaO/Al 2O 3 at 523 K show complete uptake behavior for almost 5 min, and then the NO
x
level starts gradually increasing with time and it reaches 75% of the inlet NO
x
concentration after 30 min time-on-stream. Although this catalyst shows fairly high NO
x
conversion at 523 K, only ~2.4 wt% out of 20 wt% BaO is converted to Ba(NO 3) 2. Adsorption studies by using NO 2 and NO + O 2 suggest two different NO
x
adsorption mechanisms. The NO 2 uptake profile on 2%Pt/20%BaO/Al 2O 3 shows the absence of a complete NO
x
uptake period at the beginning of adsorption and the overall NO
x
uptake is controlled by the gas–solid equilibrium between NO 2 and BaO/Ba(NO 3) 2 phase. When we use NO + O 2, complete initial NO
x
uptake occurs and the time it takes to convert ~4% of BaO to Ba(NO 3) 2 is independent of the NO concentration. These NO
x
uptake characteristics suggest that the NO + O 2 reaction on the surface of Pt particles produces NO 2 that is subsequently transferred to the neighboring BaO phase by spill over. At the beginning of the NO
x
uptake, this spill-over process is very fast and so it is able to provide complete NO
x
storage. However, the NO
x
uptake by this mechanism slows down as BaO in the vicinity of Pt particles are converted to Ba(NO 3) 2. The formation of Ba(NO 3) 2 around the Pt particles results in the development of a diffusion barrier for NO 2, and increases the probability of NO 2 desorption and consequently, the beginning of NO
x
slip. As NO
x
uptake by NO 2 spill-over mechanism slows down due to the diffusion barrier formation, the rate and extent of NO 2 uptake are determined by the diffusion rate of nitrate ions into the BaO bulk, which, in turn, is determined by the gas phase
NO 2 concentration. 相似文献
20.
The NO
x
storage process over Ba/Al 2O 3 and Pt–Ba/Al 2O 3 NSR catalysts has been analyzed in this study by performing experiments at 350 °C with NO 2 and NO/O 2 mixtures using different complementary techniques (Transient Response Method, in situ FT–IR and DRIFT spectroscopies). The collected data suggest that over the Pt–Ba/Al 2O 3 catalyst the NO
x
storage process from NO/O 2 mixtures occurs forming at first nitrite species, which progressively evolve to nitrates. In addition, a parallel nitrate formation via disproportionation of NO 2 (formed upon NO oxidation) cannot be excluded. 相似文献
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