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1.
This paper deals with the activity of bimetallic potassium–copper and potassium–cobalt catalysts supported on alumina for the reduction of NO x with soot from simulated diesel engine exhaust. The effect of the reaction temperature, the soot/catalyst mass ratio and the presence of C 3H 6 has been studied. In addition, the behavior of two monometallic catalysts supported on zeolite beta (Co/beta and Cu/beta), previously used for NO x reduction with C 3H 6, as well as a highly active HC-SCR catalyst (Pt/beta) has been tested for comparison. The preliminary results obtained in the absence of C 3H 6 indicate that, at temperatures between 250 and 400 °C, the use of bimetallic potassium catalysts notably increases the rate of NO x reduction with soot evolving N 2 and CO 2 as main reaction products. At higher temperatures, the catalysts mainly favor the direct soot combustion with oxygen. In the presence of C 3H 6, an increase in the activity for NO x reduction has been observed for the catalyst with the highest metal content. At 450 °C, the copper-based catalysts (Cu/beta and KCu2/Al 2O 3) show the highest activity for both NO x reduction (to N 2 and CO 2) and soot consumption. The Pt/beta catalyst does not combine, at any temperature, a high NO x reduction with a high soot consumption rate. 相似文献
2.
This paper deals with the activity of the KCu and KCo catalysts supported on beta-zeolite for the simultaneous NO x/soot removal from a simulated diesel exhaust, containing C 3H 6 as model hydrocarbon. In order to reveal the effect of potassium, the corresponding monometallic catalysts (Co/beta and Cu/beta) were analyzed and different potassium loadings were used. In addition, for comparative purpose, the performance of a platinum based catalyst (Pt/beta) was studied. All noble-free catalysts show, at 450 °C, a high activity for the simultaneous NO x/soot removal. Among them, K1Cu/beta presents the best global performance at 350 and 450 °C achieving a high soot consumption rate (comparable to platinum catalysts) and the highest NO x reduction. In contrast to platinum catalysts, K1Cu/beta has the advantage that the main reaction products are N 2 and CO 2. 相似文献
3.
Potassium-loaded lanthana is a promising catalyst to be used for the simultaneous abatement of soot and NO x, which are the main diesel-exhaust pollutants. With potassium loadings between 4.5 and 10 wt.% and calcination temperatures between 400 and 700 °C, this catalyst mixed with soot gave maximum combustion rates between 350 and 400 °C in TPO experiments, showing a good hydrothermal stability. There was no difference in activity when it was either mixed by grinding in an agate mortar or mixed by shaking in a sample bottle (tight and loose conditions, respectively). Moreover, when the K-loaded La 2O 3 is used as washcoat for a cordierite monolith, there were found no significant differences in the catalytic behaviour of the system, which implies its potentiality for practical purposes. The influence of poisons as water and SO2 was investigated. While water does not affect the soot combustion activity, SO2 slightly shift the TPO peak to higher temperature. Surface basicity, which is a key factor, was analysed by measuring the interactions of the catalytic surface with CO2 using the high frequency CO2 pulses technique, which proved to be very sensitive, detecting minor changes by modifications in the dynamics of the CO2 adsorption–desorption process. Water diminishes the interaction with CO2, probably as a consequence of an adsorption competition. The SO2 treated catalyst is equilibrated with the CO2 atmosphere more rapidly if compared with the untreated one, also showing a lower interaction. The lower the interaction with the CO2, the lower the activity. Differential scanning calorimetric (DSC) results indicate that the soot combustion reaction coexists with the thermal decomposition of hydroxide and carbonate species, occurring in the same temperature range (350–460 °C). The presence of potassium increases surface basicity shifting the endothermic decomposition signal to higher temperatures. We also found that NO2 strongly interacts with both La2O3 and K/La2O3 solids, probably through the formation of monodentate nitrate species which are stable under He atmosphere until 490 °C. These nitrate species further react with the solid to form bulk nitrate compounds. The addition of Cobalt decreases the nitrates stability and catalyses the NOx to N2 reduction under a reducing atmosphere, which is a necessary step for a working NOx catalytic trap. Preliminary studies performed in this work demonstrated the feasibility of using these catalysts to simultaneously remove NOx and soot particles from diesel exhausts. The nitrate formation is still observed during the catalytic combustion of soot in the presence of NOx, making our K/La2O3 a very interesting system for practical applications in simultaneous soot combustion and NOx storage in diesel exhausts. 相似文献
4.
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
5.
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
6.
The nitric acid industry is a source of both NO x and N 2O. The simultaneous selective catalytic reduction of both compounds using propane as a reductant has been investigated. A stacked catalyst bed with first a Co-ZSM-5 catalyst and second a Pd/Fe-ZSM-5 catalyst gives >80% conversion of N 2O and NO x above 300 °C at atmospheric pressure. At 4 bar absolute pressure (bara) the Co-ZSM-5 DeNO x catalyst shows higher NO x and propane conversion. This leaves not enough propane for the Pd/Fe-ZSM-5 DeN 2O catalyst, which causes a ‘dip’ in N 2O conversion. Reducing the space velocity (SV) of the first catalyst bed secures high NO x and N 2O conversions from 300 °C and up at 4 bara. 相似文献
7.
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
8.
Perovskite-type oxides (ABO 3) catalyzed the simultaneous removal of NO x and diesel soot particulates in the presence of oxygen, and were superior to transition metal simple oxides in the selectivity for NO x reduction. Although the catalytic activity of perovskite-type oxides depended on both A-site and B-site cations, the substitution of potassium at A sites prominently promoted the oxidation of soot and the reduction of NO x. 相似文献
9.
This study addresses the catalytic reaction of NO x and soot into N 2 and CO 2 under O 2-rich conditions. To elucidate the mechanism of the soot/NO x/O 2 reaction and particularly the role of the catalyst -Fe 2O 3 is used as model sample. Furthermore, a series of examinations is also made with pure soot for reference purposes. Temperature programmed oxidation and transient experiments in which the soot/O 2 and soot/NO reaction are temporally separated show that the NO reduction occurs on the soot surface without direct participation of the Fe 2O 3 catalyst. The first reaction step is the formation of CC(O) groups that is mainly associated with the attack of oxygen on the soot surface. The decomposition of these complexes leads to active carbon sites on which NO is adsorbed. Furthermore, the oxidation of soot by oxygen provides a specific configuration of active carbon sites with suitable atomic orbital orientation that enables the chemisorption and dissociation of NO as well as the recombination of two adjacent N atoms to evolve N 2. Moreover, carbothermal reaction, high resolution transmission electron microscopy and isotopic studies result in a mechanistic model that describes the role of the Fe 2O 3 catalyst. This model includes the dissociative adsorption of O 2 on the iron oxide, surface migration of the oxygen to the contact points of soot and catalyst and then final transfer of O to the soot. Moreover, our experimental data suggest that the contact between both solids is maintained up to high conversion levels thus resulting in continuous oxygen transfer from catalyst to soot. As no coordinative interaction of soot and Fe 2O 3 catalyst is evidenced by diffuse reflectance infrared Fourier transform spectroscopy a van der Waals type interaction is supposed. 相似文献
10.
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
11.
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
12.
Reaction activities of several developed catalysts for NO oxidation and NO x (NO + NO 2) reduction have been determined in a fixed bed differential reactor. Among all the catalysts tested, Co 3O 4 based catalysts are the most active ones for both NO oxidation and NO x reduction reactions even at high space velocity (SV) and low temperature in the fast selective catalytic reduction (SCR) process. Over Co 3O 4 catalyst, the effects of calcination temperatures, SO 2 concentration, optimum SV for 50% conversion of NO to NO 2 were determined. Also, Co 3O 4 based catalysts (Co 3O 4-WO 3) exhibit significantly higher conversion than all the developed DeNO x catalysts (supported/unsupported) having maximum conversion of NO x even at lower temperature and higher SV since the mixed oxide Co-W nanocomposite is formed. In case of the fast SCR, N 2O formation over Co 3O 4-WO 3 catalyst is far less than that over the other catalysts but the standard SCR produces high concentration of N 2O over all the catalysts. The effect of SO 2 concentration on NO x reduction is found to be almost negligible may be due to the presence of WO 3 that resists SO 2 oxidation. 相似文献
13.
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
14.
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
15.
The effect of periodic operation over Pt and Pt/Ba catalysts supported on γ-alumina under oxidizing conditions was investigated using simulated automotive exhaust gas from lean-burn combustion. The conversion of hydrocarbons and NO x was measured in cycled feedstream and steady feedstream under oxidizing conditions. The activities of the catalysts were improved in the cycled feedstream between oxidizing and reducing atmospheres under average oxidizing conditions. In particular, the NO x reduction on the Pt/Bt catalyst was higher than that on the Pt catalyst in the cycling operation. From the mass spectral analysis in streams of NOO 2C 3H 6 and NOO 2H 2 (balance He) gas, it was found that NO was oxidized and stored on the Pt/Ba catalyst under oxidizing conditions, and that the stored NO x on the catalyst was subsequently reduced to N 2 under reducing conditions. 相似文献
16.
Urea-SCR, the selective catalytic reduction using urea as reducing agent, has been investigated for about 10 years in detail and today is a well established technique for DeNO x of stationary diesel engines. It is presently also considered as the most promising way to diminish NO x emissions originating from heavy duty vehicles, especially trucks. The paper discusses the fundamental problems and challenges if urea-SCR is extended to mobile applications. The major goal is the reduction of the required catalyst volume while still maintaining a high selectivity for the SCR reaction over a wide temperature range. The much shorter residence time of the exhaust gas in the catalyst will lead to higher secondary emissions of ammonia and isocyanic acid originating from the reducing agent. Additional problems include the control strategy for urea dosing, the high freezing point of urea, and the long term stability of the catalyst. 相似文献
17.
The effect of hydrogen on the selective catalytic reduction of NO x with n-octane, methylcyclohexane and toluene over a 2 wt.% Ag/Al 2O 3 was investigated. Diesel fuels contain, in addition to straight hydrocarbons, varying amounts of cyclic and aromatic compounds, which have a detrimental effect on the Ag/Al 2O 3 catalyst activity. The results showed that the NO to N 2 conversion was significantly promoted, independent of the nature (straight, cyclic or aromatic) of the hydrocarbon, in the presence of 1 vol.% H 2. The role of hydrogen is connected to a faster oxidation of the hydrocarbons and to enhanced formation of amines, which react with activated NO to form N 2. 相似文献
18.
A catalytically assisted low NO x combustor has been developed which has the advantage of catalyst durability. Combustion characteristics of catalysts at high pressure were investigated using a bench scale reactor and an improved catalyst was selected. A combustor for multi-can type gas turbine of 10 MW class was designed and tested at high-pressure conditions using liquefied natural gas (LNG) fuel. This combustor is composed of a burner system and a premixed combustion zone in a ceramic type liner. The burner system consists of catalytic combustor segments and premixing nozzles. Catalyst bed temperature is controlled under 1000°C, premixed gas is injected from the premixing nozzles to catalytic combustion gas and lean premixed combustion is carried out in the premixed combustion zone. As a result of the combustion tests, NO x emission was lower than 5 ppm converted at 16% O 2 at a combustor outlet temperature of 1350°C and a combustor inlet pressure of 1.33 MPa. 相似文献
19.
The behavior of the selective catalytic reduction of nitrogen oxides (NO x) assisted by a dielectric barrier discharge was investigated. The principal function of the dielectric barrier discharge in the present system is to generate ozone, which is continuously fed to a chamber where the ozone and NO-rich exhaust gas (NO forms the large majority of NO x) are mixed. In the ozonization chamber, a part of NO contained in the exhaust gas is oxidized to NO 2, and then the mixture of NO and NO 2 enters the catalytic reactor. The ozonization method proposed in this study was found to be more energy-efficient for the oxidation of NO to NO 2 than the typical nonthermal plasma process. The degree of NO oxidation was approximately equal to the amount of ozone added to the exhaust gas, implying that the decomposition of ozone into molecular oxygen was relatively slow, compared to its reaction with NO. When the exhaust gas was first treated by ozone to produce a mixture of NO and NO 2, a remarkable enhancement in the catalytic reduction of nitrogen oxides was observed. Neither NO 3 nor N 2O 5 was formed in the present system, but small amounts of ozone and N 2O (less than 5 ppm) were detected in the outlet gas. 相似文献
20.
Catalytic oxidation was initially associated with the early development of catalysis and it subsequently became a part of many industrial processes, so it is not surprising it was used to remove hydrocarbons and CO when it became necessary to control these emissions from cars. Later NOx was reduced in a process involving reduction over a Pt/Rh catalyst followed by air injection in front of a Pt-based oxidation catalyst. If over-reduction of NO to NH 3 took place, or if H 2S was produced, it was important these undesirable species were converted to NOx and SOx in the catalytic oxidation stage. When exhaust gas composition could be kept stoichiometric hydrocarbons, CO and NOx were simultaneously converted over a single Pt/Rh three-way catalyst (TWC). With modern TWCs car tailpipe emissions can be exceptionally low. NO is not catalytically dissociated to O 2 and N 2 in the presence of O 2, it can only be reduced to N 2. Its control from lean-burn gasoline engines involves catalytic oxidation to NO 2 and thence nitrate that is stored and periodically reduced to N 2 by exhaust gas enrichment. This method is being modified for diesel engines. These engines produce soot, and filtration is being introduced to remove it. The exhaust temperature of heavy-duty diesels is sufficient (250–400 °C) for NO to be catalytically oxidised to NO 2 over an upstream platinum catalyst that smoothly oxidises soot in the filter. The exhaust gas temperature of passenger car diesels is too low for this to take place all of the time, so trapped soot is periodically burnt in O 2 above 550 °C. Catalytic oxidation of higher than normal amounts of hydrocarbon and CO over an upstream catalyst is used to give sufficient temperature for soot combustion with O 2 to take place. 相似文献
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