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1.
In this work, a kinetic model is constructed to simulate sulfur deactivation of the NO x storage performance of BaO/Al 2O 3 and Pt/BaO/Al 2O 3 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) SO 2 storage on NO x storage sites; (iii) SO 2 oxidation; (iv) SO 3 storage on bulk sites; (v) SO 2 interaction with platinum in the presence of H 2; (vi) oxidation of accumulated sulfur compounds on platinum by NO 2. 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 SO 2 + O 2 or SO 2 + H 2. The simulations show that the model is able to describe the main features observed experimentally. 相似文献
2.
Free energy minimization calculations are used to determine the thermodynamic equilibrium concentrations of NO x and other species in stoichiometric and lean gas mixtures over a range of temperatures and compositions. Under lean (excess N 2 and O 2) conditions, the NO decomposition (NO↔(1/2)N 2+(1/2)O 2) and NO oxidation (NO+(1/2)O 2↔NO 2) equilibria impose lower bounds on the NO x concentrations achievable by thermodynamic equilibration or NO x decomposition, and these equilibrium NO x concentrations can be practically significant. Assuming a perfect isothermal catalyst acting on a representative diesel exhaust stream collected over the federal test procedure (FTP) cycle, equilibrium NO x levels exceed upcoming California Low Emission Vehicle II (LEV-II) and Tier II NO x emissions standards for automobiles and trucks at temperatures above approximately 800 K. Consideration of a perfect adiabatic catalyst acting on the same diesel exhaust shows that equilibrium NO x values can fall below NO x emissions standards at lower temperatures, but to achieve these low concentrations would require the catalyst to attain 100% approach to equilibrium at very low temperatures. It is concluded that NO x removal based on a thermodynamic equilibrating catalyst under lean exhaust conditions is not practically viable for automotive application, and that to achieve upcoming NO x standards will require selective NO x catalysts that vigorously promote NO x reactions with reductant and do not promote NO decomposition or oxidation. Finally, the ability of a selective NO x catalyst system to reduce NO x concentrations to or below thermodynamic equilibrium values is proposed as a useful measure for selective catalytic reduction (SCR) activity. 相似文献
3.
Several hexaaluminate-related materials were prepared via hydrolysis of alkoxide and powder mixing method for high temperature combustion of CH 4 and C 3H 8, in order to investigate the effect of the concentration of the fuels, O 2 and H 2O on NO x emission and combustion characteristics. Among the hexaaluminate catalysts, Sr 0.8La 0.2MnAl 11O 19− prepared by the alkoxide method exhibited the highest activity for methane combustion and low NO x emission capability. NO x emission at 1500 °C was increased linearly with O 2 concentration, whereas water vapor addition decreased NO x emission in CH 4 combustion over the Sr 0.8La 0.2MnAl 11O 19− catalyst. In the catalytic combustion of C 3H 8 over the Sr 0.8La 0.2MnAl 11O 19− catalyst, the amount of NO x emitted was raised in the temperature range between 1000 and 1500 °C when the C 3H 8 concentration increased from 1 to 2 vol.%. It was found that NO x emission in this temperature range was reduced effectively by adding water vapor. 相似文献
4.
A model of NO x selective reduction by hydrocarbon (HC) was developed, which takes into account the adsorption and desorption of HC. The model was applied for predicting the performance of a De–NO x catalytic reactor, working under transient conditions such as a legislative driving cycle. Diesel fuel was used as a supplemental reductant. The behavior of HC and NOx reactions and HC adsorption and desorption has been simulated successfully by our numerical approach under the transient conditions of the simulated Japanese 10–15 driving cycle. Our model is expected to optimize the design of selective diesel NOx reduction systems using a diesel fuel as a supplemental reductant. 相似文献
5.
Increasingly stringent ambient air quality standards coupled with the need to improve fuel economy has drawn significant attention to the search for emission control systems for lean burn engine vehicles. Much of the focus has been on zeolite-based catalysts for the conversion of NO x to N 2 for automotive exhaust emission control. Under certain conditions, these catalysts are highly active catalysts for the reduction of NO x using hydrocarbons as the reductant. However, many of these catalysts suffer from a variety of deactivation processes such as irreversible poisoning by SO x or hydrothermal dealumination. In addition to these deactivation processes, a recent focus of our research has been on the influence of water vapor on the activity of zeolite-based catalysts at low operating temperatures. We observe a hysteresis in catalytic activity of lean NOx reduction (NO feed concentrations <100 ppm) upon increasing and decreasing temperature ramps at the low end of the operating window, that being from 100 to 300 °C using hydrocarbons as reductants. We describe these reversible influences of water vapor and the implications for this hysteresis in catalytic activity for the application of zeolite-based catalysts in lean NOx catalysis, and compare these results to the instance of using ammonia as reductant. 相似文献
6.
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
7.
The emissions of CO 2, NO x and SO 2 from the combustion of a high-volatile coal with N 2- and CO 2-based, high O 2 concentration (20, 50, 80, 100%) inlet gases were investigated in an electrically heated up-flow-tube furnace at elevated gas temperatures (1123–1573 K). The fuel equivalence ratio, φ, was varied in the range of 0.4–1.6. Results showed that CO 2 concentrations in flue gas were higher than 95% for the processes with O 2 and CO 2-based inlet gases. NO x emissions increased with φ under fuel-lean conditions, then declined dramatically after φ=0.8, and the peak values increased from about 1000 ppm for the air combustion process and 500 ppm for the O 2(20%)+CO 2(80%) inlet gas process to about 4500 ppm for the oxygen combustion process. When φ>1.4 the emissions decreased to the same level for different O 2 concentration inlet gas processes. On the other hand, NO x emission indexes decreased monotonically with φ under both fuel-lean and fuel-rich combustion. SO 2 emissions increased with φ under fuel-lean conditions, then declined slightly after φ>1.2. Temperature has a large effect on the NO x emission. Peak values of the NO x emission increased by 50–70% for the N 2-based inlet gas processes and by 30–50% for the CO 2-based inlet gas process from 1123 to 1573 K. However, there was only a small effect of temperature on the SO 2 emission. 相似文献
8.
Transient behaviour of catalytic monolith converter with NO x storage is studied under conditions typical for automobiles with lean-burn engines (i.e., diesel and advanced gasoline ones). Periodical alternation of inlet concentrations is applied—NO x are adsorbed on the catalyst surface during a long reductant-lean phase (2–3 min) and then reduced to N 2 within a short reductant-rich phase (2–6 s). Samples of industrial NO x storage and reduction catalyst of NM/Ba/CeO 2/γ-Al 2O 3 type (NM = noble metal), washcoated on 400 cpsi cordierite substrate, are used in the study. Effects of the rich-phase length and composition on the overall NO x conversions are examined experimentally. Reduction of NO x by CO, H 2 and unburned hydrocarbons (represented by C 3H 6) in the presence of CO 2 and H 2O is considered. Effective, spatially 1D, heterogeneous mathematical model of catalytic monolith with NOx and oxygen storage capacity is described. The minimum set of experiments needed for the evaluation of relevant reaction kinetic parameters is discussed: (i) CO, H2 and HC oxidation light-off under both lean and rich conditions, including inhibition effects, (ii) NO/NO2 transformation, (iii) NOx storage, including temperature dependence of effective NOx storage capacity, (iv) water gas shift and steam reforming under rich conditions, i.e., in situ production of hydrogen, (v) oxygen storage and reduction, including temperature dependence of effective oxygen storage capacity, and (vi) NOx desorption and reduction under rich conditions. The experimental data are compared with the simulation results. 相似文献
9.
We have investigated the regeneration of a nitrated or sulphated model Pt/Ba-based NO x trap catalyst using different reductants. H 2 was found to be more effective at regenerating the NO x storage activity especially at lower temperature, but more importantly over the entire temperature window after catalyst ageing. When the model NO x storage catalyst is sulphated in SO 2 under lean conditions at 650 °C almost complete deactivation can be seen. Complete regeneration was not achieved, even under rich conditions at 800 °C in 10% H 2/He. Barium sulphate formed after the high temperature ageing was partly converted to barium sulphide on reduction. However, if the H 2 reduced sample was exposed to a rich condition in a gas mixture containing CO 2 at 650 °C, the storage activity can be recovered. Under these rich conditions the S 2− species becomes less stable than the CO 32−, which is active for storing NO x. Samples which were lean aged in air containing 60 ppm SO 2 at <600 °C, after regeneration at λ=0.95 at 650 °C, have a similar activity window to a fresh catalyst. It is, therefore, important that CO 2 is present during the rich regenerations of the sulphated model samples (as of course it would be under real conditions), as suppression of carbonate formation can lead to sulphide formation which is inactive for NO x storage. 相似文献
10.
SO 2 and NO emitted from coal-fired power plants have caused serious air pollution in China. In this study, a test system for NO oxidation using O 3 is established. The basic characteristics of NO oxidation and products forms are studied. A separate test system for the combined removal of SO 2 and NO x is also established, and the absorption characteristics of NO x are studied. The characteristics of NO oxidation and NO x absorption were verified in a 35 t·h -1 industrial boiler wet combined desulfurization and denitrification project. The operating economy of ozone oxidation wet denitrification technology is analyzed. The results show that O 3 has a high rate and strong selectivity for NO oxidation. When O 3 is insufficient, the primary oxidation product is NO 2. When O 3 is present in excess, NO 2 continues to get oxidized to N 2O 5 or NO 3. The removal efficiency of NO 2 in alkaline absorption system is low (only about 15%). NO x removal efficiency can be improved by oxidizing NO x to N 2O 5 or NO 3 by increasing ozone ratio. When the molar ratio of O 3/NO is 1.77, the NO x removal efficiency reaches 90.3%, while the operating cost of removing NO x per kilogram is 6.06 USD (NO 2). 相似文献
11.
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
12.
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
13.
A catalytic deSoot–deNO x system, comprising Pt and Ce fuel additives, a Pt-impregnated wall-flow monolith soot filter and a vanadia-type monolithic NH 3-SCR catalyst, was tested with a two-cylinder DI diesel engine. The soot removal efficiency of the filter was 98–99 mass% with a balance temperature (stationary pressure drop) of 315 °C at an engine load of 55%. The NO x conversion ranged from 40 to 73%, at a NH 3/NO x molar ratio of 0.9. Both systems were measured at a GHSV of 52 000 l/(l h). The maximum NO x conversion was obtained at 400 °C. The reason for the moderate deNO x performance is discussed. No deactivation was observed after 380 h time on stream. The NO x emission at high engine loads is around 15% lower than that of engines running without fuel additives. 相似文献
14.
NO x reduction with NO 2 as the NO x gas in the absence of plasma was compared to plasma treated lean NO x exhaust where NO is converted to NO 2 in the plasma. Product nitrogen was measured to prove true chemical reduction of NO x to N 2. With plasma treatment, NO as the NO x gas, and a NaY catalyst, the maximum conversion to nitrogen was 50% between 180 and 230 °C. The activity decreased at higher and lower temperatures. At 130 °C a complete nitrogen balance could be obtained, however between 164 and 227 °C less than 20% of the NO x is converted to a nitrogen-containing compound or compounds not readily detected by gas chromatograph (GC) or Fourier transform infrared spectrometer (FT-IR) analysis. With plasma treatment, NO 2 as the NO x gas, and a NaY catalyst, a complete nitrogen balance is obtained with a maximum conversion to nitrogen of 55% at 225 °C. For γ-alumina, with plasma treatment and NO2 as the NOx gas, 59% of the NOx is converted to nitrogen at 340 °C. A complete nitrogen balance was obtained at these conditions. As high as 80% NOx removal over γ-alumina was measured by a chemiluminescent NOx meter with plasma treatment and NO as the NOx gas. When NO is replaced with NO2 and the simulated exhaust gases are not plasma treated, the maximum NOx reduction activity of NaY and γ-alumina decreases to 26 and 10%, respectively. This is a large reduction in activity compared to similar conditions where the simulated exhaust was plasma treated. Therefore, in addition to NO2, other plasma-generated species are required to maximize NOx reduction. 相似文献
15.
The lean selective catalytic reduction of NO x by methane over protonic palladium loaded ZSM-5, FER and MOR, as well as, on bimetallic Pd–Pt-HMOR was examined. Special emphasis was paid on the combined effects of water and SO 2 in the feed stream. Under dry conditions and in the absence of SO 2, the degree of NO x conversion at 450°C decreases as follows: Pd-HZSM-5>Pd-HMOR>Pd-HFER. Sulfur dioxide alone has no apparent effect on the activity for NO x reduction, but the coexistence of water and SO 2 inhibits both NO x and methane conversions. The extent of inhibition by water and SO 2 on NO x reduction is Pd-HFER>Pd-HZSM-5>Pd-HMOR. Acid mordenite doped with low levels of Pt and Pd leads to an active catalyst that is more tolerant to the presence of either water or SO 2 than the corresponding monometallic Pt- and Pd-HMOR. Nevertheless, NO x reduction is also inhibited at temperatures below 450°C when SO 2 and water are both present. TPD experiments of water over calcined samples of protonic Pd supported pentasil zeolites, Pd/γ-Al 2O 3 and Pt–Pd-HMOR with and without pretreatment in SO 2+O 2 indicate that sulfation of the surface increases water chemisorption by the support. Therefore, the observed decrease of NO x reduction on Pd-loaded zeolite catalysts when SO 2 and H 2O coexist in the feed stream may be due to enhanced water inhibition and presumably active site poisoning. 相似文献
17.
NO x sorption and reduction capacities of 12-tungstophosphoric acid hexahydrate (H 3PW 12O 40·6H 2O, HPW) were measured under representative alternating conditions of lean and rich exhaust-type gas mixture. Under lean conditions, the sorption of NO x is large and is equivalent to 37 mg of NO x/g HPW. Although a part of these NO x remains unreduced, HPW is able to reduce some of the NO x to produce N 2 by a reaction between the sorbed NO 2 and hydrocarbon (HC), but this process is slow. The addition of 1% Pt affects strongly the chemical behaviour occurring during the course of a rich operation. The NO desorption observed at the beginning of the rich phase is strongly accelerated. The direct correlation between NO 2 consumption and CO 2 production shows that the principal pathway is the reaction CO+NO 2→CO 2+NO. In a mixture of reducing gas (CO, HC, H 2), the competition is strongly in favour of CO though in its absence the reaction observed was the hydrogenation of propene to propane. 相似文献
18.
The role of plasma processing on NO x reduction over γ-alumina and a basic zeolite, NaY was examined. During the plasma treatment NO is oxidized to NO 2 and propylene is partially oxidized to CO, CO 2, acetaldehyde, and formaldehyde. With plasma treatment, NO as the NO x gas, and a NaY catalyst, the maximum NO x conversion was 70% between 180 and 230 °C. The activity decreased at higher and lower temperatures. As high as 80% NOx removal over gamma alumina was measured by a chemiluminescent NOx meter with plasma treatment and NO as the NOx gas. For both catalysts a simultaneous decrease in NOx and aldehydes concentrations was observed, which suggests that aldehyde may be important components for NOx reduction in plasma-treated exhaust. 相似文献
19.
NO x emission during the regeneration of coked fluid catalytic cracking (FCC) catalysts is an environmental problem. In order to follow the route to NO x formation and try to find ways to suppress it, a coked industrial FCC catalyst has been prepared using model N-containing compounds, e.g., pyridine, pyrrole, aniline and hexadecane–pyridine mixture. Nitrogen present in the FCC feed is incorporated as polyaromatic compounds in the coke deposited on the catalyst during cracking. Its functionality has been characterized using XPS. Nitrogen specie of different types, namely, pyridine, pyrrolic or quaternary-nitrogen (Q-N) have been discriminated. Decomposition of the coke during the catalyst regeneration (temperature programmed oxidation (TPO) and isothermal oxidation) has been monitored by GC and MS measurements of the gaseous products formed. The pyrrolic- and pyridinic-type N specie, present more in the outer coke layers, are oxidized under conditions when still large amount of C or CO is available from coke to reduced NO x formed to N 2. “Q-N” type species are present in the inner layer, strongly adsorbed on the acid sites on the catalyst. They are combusted last during regeneration. As most of the coke is already combusted at this point, lack of reductants (C, CO, etc.) results in the presence of NO x in the tail gas. 相似文献
20.
Catalysts, which were recently developed by Toyota for the control of automobile exhaust, are reviewed. (1) For use in low emission vehicles, a CeO 2-ZrO 2 solid solution (CZ) with both high oxygen storage capacity and high heat resistance was developed as a support for a high performance three-way catalyst (TWC). (2) A novel three-way catalyst named the NO x storage-reduction catalyst (NSR) was developed for automotive lean-burn engines. The NSR catalyst can store NO x in an oxidizing atmosphere and then reduce stored NO x at stoichiometric or reducing conditions. Also, it has high tolerance to sulfur poisoning which is the most stringent problem for the NSR catalyst. 相似文献
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