JP4874093B2 - Decomposition method of nitrogen dioxide - Google Patents

Description

The present invention relates to a method for decomposing nitrogen dioxide (NO ₂ ) in a gas mixture, for example an exhaust gas mixture of an internal combustion engine, into nitric oxide (NO).

The exhaust gas leaving the internal combustion engine comprises a mixture of contaminants including carbon monoxide (CO), unburned hydrocarbons (HC), nitric oxide (NO _x ) and particulate matter (PM). The NO _x component can comprise nitric oxide (NO) and nitrogen dioxide (NO ₂ ). The level at which these pollutants in the exhaust gas from an internal combustion engine can be released into the atmosphere is regulated by law. Such regulations can be met by engine design, engine management and / or exhaust gas aftertreatment, and typically a combination of all three measures.

A prior art exhaust mechanism primarily for treating diesel exhaust comprises an oxidation catalyst for oxidizing NO in the exhaust gas to NO ₂ and a downstream filter for capturing PM. A diesel PM treatment method using this arrangement is described in EP 0341382 or US Pat. No. 4,902,487, both of which are hereby incorporated by reference. In this method, exhaust gas containing unfiltered PM and NO, such as diesel exhaust gas, is passed over an oxidation catalyst, NO is converted to NO ₂ , PM is collected on a filter, and the collected PM is converted to NO ₂ . It burns by the reaction of. This technology is commercially available as Johnson Matthey's Continuously Regenerating Trap or CRT. By burning PM in NO ₂ , CO and NO are formed, which can be a side reaction leading to complete reduction of NO ₂ to N ₂ as described in SAE 890404.

  The advantage of this method is that diesel PM can be combusted at temperatures up to 400 ° C., whereas PM combustion in oxygen occurs above about 500 ° C. This is because diesel exhaust is generally cooler than exhaust exiting a gasoline engine, and the mechanism does not provide additional means for increasing exhaust gas temperature, the so-called “active” regeneration scheme, in oxygen. When relying solely on PM combustion, it is critical because PM accumulates on the filter and causes back pressure problems in the mechanism.

The problem with the method described in EP 0341382 is that the legislature limits the amount of NO ₂ that can be emitted into the atmosphere as exhaust emission regulations become more stringent since the publication of that application. It is about starting to discuss. For example, California Air Resources Board (CARB) is associated with up to 20% of the exhaust pipe NO _x driving cycle proposes to released as _{NO 2 (California's Diesel Risk Reduction} Program, September 2000 and Title 13, California Code of Regulations, Chapter 14, section 2706). NO ₂ is toxic and can cause headaches, dizziness, and nausea even at low concentrations. NO ₂ also has an unpleasant odor. For reaction with NO ₂ generated over the oxidation catalyst, if any PM in the filter is insufficient, or the temperature of the exhaust gas is lower than the preferred range for burning the PM in NO _2, NO ₂ May pass through the filter and undesirably be discharged into the atmosphere.

This problem is particularly acute when the internal combustion engine is used in a closed space, such as a mine where vehicles are mined, loaded and transported to the surface. Since particulate matter is generated in many mining operations, an exhaust aftertreatment mechanism comprising a filter for lowering the level of released PM is considered. Furthermore, blowing up rocks, sometimes explosive to recover the desired ore generates NO _2. Therefore, it is advantageous to improve the health and safety of miners by suppressing the release of exhaust gas containing both PM and NO ₂ into the atmosphere in a closed environment. In fact, the US Mine Safety and Health Administration has suppressed the use of diesel engine exhaust mechanism formed comprises a diesel particulate filter mechanism to increase the NO ₂ emission.

In selective catalytic reduction (SCR) with hydrocarbons (HC), HC reacts selectively with NO _{x over} O ₂ to form nitrogen, CO ₂ and water according to equation (1).
{HC} + NO _x → N ₂ + CO ₂ + H ₂ O (1)

The non-selective competitive reaction with oxygen is given by equation (2).
{HC} + O ₂ → CO ₂ + H ₂ O (2)

For causing catalyze the HC-SCR of NO _x, 2 types of preferred group (HC-SCR catalysts HC-SCR for selectively promote the desired reaction (1) is "lean NO _x catalysts" (LNC) , “DeNO _x catalyst”, “NO _x storage catalyst”, “NO _x reduction catalyst” and “non-selective catalytic reduction catalyst” (these catalysts also catalyze non-selective reactions, eg, formula (2)) So called)). These two preferred groups are platinum and copper substituted zeolites on alumina, such as Cu / ZSM-5.

  Pt-based catalysts tend to work at relatively low temperatures (peak activity ~ 250 ° C) and have a relatively narrow temperature range to give HC-SCR activity, whereas zeolitic HC-SCR catalysts It has a wider temperature range than Pt-based HC-SCR catalysts and operates at higher temperatures (peak activity ~ 400 ° C).

One potential solution to this problem is described in European Patent No. 0758713, in which one embodiment, the exhaust system optionally connects a platinum-based oxidation catalyst and a diesel particulate filter (DPF) to the CRT. (Product Name) Arranged and provided with NO _x absorbent downstream of the DPF. The NO _x absorbent comprises platinum for oxidizing NO to N ₂ in lambda> 1 exhaust gas composition, rhodium for reducing NO _x to N ₂ in lambda <1 exhaust gas composition, and at least one kind Selected from alkali metals such as potassium and cesium, alkaline earth metals such as barium and calcium, and rare earth metals such as lanthanum, comprising a substance for absorbing NO ₂ and storing it as nitrate be able to. A catalyst composition comprising platinum, rhodium and NO _x absorbing material is typically referred to as a NO _x trap.

In a second embodiment, a NO _x reduction catalyst is placed downstream of the filter and uses diesel HC fuel and CO to catalyze the reduction of NO _x to N ₂ . _The NO _x reduction catalyst, a zeolite may be loaded mordenite example ZSM-5 with copper or iron ion-exchanged, or platinum. However, from EP 0 758 713, an HC reductant for reducing NO _x is introduced into the exhaust system by injecting additional fuel into the exhaust cycle or directly into the exhaust passage. It is clear that it will be introduced in In either case, the injection is always upstream of the CRT (trade name) oxidation catalyst.

In our International Patent No. WO 03/037507, we have a catalyst for oxidizing NO to NO ₂ when the exhaust gas composition is lambda> 1, such as a platinum-based catalyst, and downstream of the NO oxidation catalyst, ie An exhaust mechanism for an internal combustion engine having a CRT (trade name) structure and including an arranged filter is disclosed. The filter may comprise an oxidation catalyst, such as platinum and / or palladium, rhodium and NO _x absorbing material, such as any of the materials described in European Patent No. 0758713 mentioned above. The filter component itself of this arrangement is described in Japanese Patent No. 2722987.

We have studied methods for catalytically decomposing NO ₂ to NO and have found, very surprisingly, that relatively acidic particulate refractory oxide materials are particularly active. We have found that a relatively small amount of HC reductant is desirable to optimize the conversion. Without wishing to be bound by theory, we believe that HC forms coke on acidic materials, which promotes the decomposition of NO ₂ to NO. In order to promote such coke formation, certain metals can be included in the acidic material, and some of these metallized materials are known HC-SCR catalysts. Of course, the purpose of HC-SCR is to convert NO _x to N ₂ , which includes a C1HC: NO _x ratio of 2-6 (see our International Patent No. WO 98/40153). It is preferable within the above temperature range. On the other hand, our results show that there is a very good NO ₂ to NO conversion at a much lower temperature and C1HC: NO _x ratio than for HC-SCR, although a relatively small reduction to N ₂ takes place. Shows that can be achieved.

In a first aspect, the present invention is a method for decomposing nitrogen dioxide (NO ₂ ) in exhaust gas of a lean burn internal combustion engine into nitrogen monoxide (NO), which comprises C1 hydrocarbon: nitrogen oxide (C1HC) of exhaust gas. : adjust the NO _x) ratio from 0.1 to 2, the gas mixture, zeolites, tungsten-doped titania, silica - titania, zirconia - titania, gamma - alumina, amorphous silica - alumina, and any of them Contacting a particulate acidic refractory oxide selected from the group consisting of two or more mixtures and releasing the effluent gas into the atmosphere is provided.

  EP 0888816 discloses a catalyst for suppressing exhaust emissions, comprising three metals, copper, praseodymium and yttrium, with a hydrocarbon: nitrogen oxide molar ratio of 0.5-30. Yes.

EP 0 541 271 describes a first stage catalyst comprising a transition metal exchanged zeolite (ie Cu-ZSM5) and a first stage catalyst for treating NO _x in the exhaust exiting a lean burn gasoline fuel engine. Discloses a catalyst system comprising a second stage catalyst, which is a three-way catalyst, for treating the effluent of The engine is controlled so that the ratio of NO _x to HC in the exhaust gas is within a range of 1/3 to 3/1 (that is, minimum C ₃ H ₆ 250 ppm and NO _x 200 to 400 ppm). Only the performance of the second catalyst and the combination of the first and second stage catalysts has been assessed in the examples.

  In one embodiment, the particulate refractory oxide carries a metal or compound thereof, and the metal is selected from the group consisting of rhodium, palladium, iron, copper, and mixtures of any two or more thereof.

NO ₂ can constitute up to about 50% NOx in the exhaust gas of an internal combustion engine. Thus, in one embodiment, the HC: NO ₂ ratio is adjusted from 0.2 to 4 .

We have found that for a defined HC: NO _x ratio, the NO ₂ conversion decreases at low temperatures. To meet the proposed CARB threshold of up to 20% of released NO _x , in one embodiment, the step of adjusting the HC: NO _x ratio is performed only when the exhaust gas temperature is above 250 ° C. Is preferred. NO ₂ conversion is possible at temperatures much lower than required for HC-SCR for similar catalysts, ie about 400 ° C. for HC-SCR over Fe-beta zeolite. Thus, it can be seen that it is slightly higher than 250 ° C. for NO ₂ conversion.

In another embodiment, the step of adjusting the HC: NO _x ratio is performed when the exhaust gas temperature is within a predetermined range to increase NO _{2 in the} exhaust mechanism. Such a temperature range usually depends on the type of engine and the load on the vehicle. Exemplary embodiments include city center buses (250-300 ° C) with heavy-duty diesel, buses that run outside the city center (up to 400 ° C), and heavy-duty diesel trucks (up to 500 ° C). Is mentioned.

Potentially, the method according to the first aspect of the invention can be used to treat gas mixtures including NO ₂ generated by any chemical, eg industrial, process. However, for the purposes of the present invention, the method is for treating a gas mixture resulting from the combustion of a hydrocarbon fuel, such as diesel fuel, gasoline fuel, natural gas (NG) or liquid petroleum gas (LPG), in an internal combustion engine. It is in.

According to a second aspect, the present invention provides an exhaust system for an internal combustion engine, a catalyst for decomposing nitrogen dioxide (NO ₂ ) into nitric oxide (NO) with a suitable reductant, and in use, Means for adjusting the C1 hydrocarbon: nitrogen oxide (C1HC: NO _x ) ratio in the exhaust gas upstream of the catalyst to 0.01-2, wherein the catalyst comprises zeolite, tungsten-doped titania, Consisting of particulate acidic refractory oxide selected from the group consisting of silica-titania, zirconia-titania, gamma-alumina, amorphous silica-alumina, and mixtures of any two or more thereof, optionally metal or Providing an exhaust mechanism carrying the compound, wherein the metal is selected from the group consisting of rhodium, palladium, iron, copper and mixtures of any two or more thereof. Provide.

In one embodiment, adjusting means, the exhaust gas C1HC: is designed NO ₂ ratio to adjust to 0.05.

  In another embodiment, the adjusting means is controlled to operate when the exhaust gas temperature exceeds 250 ° C. during use.

  In another embodiment, the regulating means is controlled to operate during use when the exhaust gas temperature is below 500 ° C.

  The control of the adjusting means can be performed in one embodiment by any suitable means comprising a processor that can optionally form part of an engine management unit (ECU).

C1HC: To control the NO _x ratio, mechanism, the following conditions in the mechanism, i.e. so as to be detected exhaust gas temperature, catalyst bed temperature, the exhaust gas mass flow, the NO 2 _(e.g. a suitable NO ₂ sensor in the exhaust gas 1) Enter one or more of the following conditions: manifold vacuum, ignition timing, engine speed, throttle position, lambda value of exhaust gas composition, amount of fuel injected into the engine, exhaust gas circulation valve position, and boost pressure It is preferable to provide one or more sensors for this purpose.

Of course, C1HC: NO _x ratio can be varied by received the or each input. For example, at low exhaust gas temperatures, a high ratio is desirable for a predetermined NO ₂ conversion, whereas at high temperatures, a low C1HC: NO ₂ ratio can be used.

  In another embodiment, the control means is operated by a stored look-up table or engine map in response to at least one of the inputs.

C1HC: be adjusted in the range defined in advance the NO _x ratio, by increasing the amount of HC or by adjusting the NO _x, for example, by adjusting the amount of exhaust gas recirculation can be carried out. When increasing the HC in the mechanism, this can be done in a number of ways, for example by adjusting the ignition timing of at least one engine cylinder, by adjusting the fuel injection timing of at least one engine cylinder, or by the engine This can be done by adjusting the air-fuel ratio.

In a special embodiment, these inputs are provided by an air exhaust gas temperature sensor and a mass flow sensor. Exhaust gas temperature is moderately correlated to the level of the NO _x exiting the engine, exhaust system, for example, CRT (trade name), it is possible to model the NO oxidation over a catalyst in, it is possible to estimate the exhaust gas NO ₂ it can. If material flow is also known, desirable to NO ₂ decomposition over the catalyst C1HC: to obtain a NO ₂ ratio, how much more of the HC fuel, for example, should be calculated or injecting diesel.

In another embodiment, the NO ₂ decomposition catalyst is disposed downstream of an oxidation catalyst comprising at least one PGM, preferably at least one of platinum and palladium. From EP 0 341 382 or US Pat. No. 4,902, 487, such catalysts can oxidize NO in exhaust gas to NO ₂ at temperatures up to 400 ° C. (at higher temperatures, positive The reaction is thermodynamically limited), and therefore the intended purpose of the catalyst is to do some other reaction, for example oxidation of a soluble or volatile organic fraction of diesel particulate matter, CO or diesel hydrocarbons It is known that this is the case even if it is catalyzed. However, if additional HC is introduced into the exhaust system upstream of the NO ₂ cracking catalyst, it is important that this is done downstream of the oxidation catalyst. This is a clear contradiction to the arrangement disclosed in European Patent No. 075813, where we invented that HC was injected upstream of the oxidation catalyst and the PGM oxidation catalyst was It is believed that it is intended to exotherm over the catalyst in order to obtain the advantage of further reduction with or to regenerate the diesel particulate filter. Of course, if the NO ₂ decomposition catalyst is placed downstream of the oxidation catalyst, and the oxidation catalyst generates NO ₂ and burns PM on the downstream filter, that is, a CRT (trade name) mechanism, NO _{2. The} cracking catalyst is placed downstream of the filter.

In another embodiment, the oxidation catalyst is on a particulate filter, such as a diesel particulate filter or DPF. Such an arrangement is sometimes referred to as a “catalyzed soot filter” or CSF. The catalyst can promote combustion of soot and particulate matter on the filter, i.e. reduce the combustion temperature. However, if an oxidation catalyst is present on the filter, the level of NO ₂ leaving the filter portion of the filter may be higher than the amount of NO ₂ entering the filter.

In another embodiment, an oxidation catalyst is associated with the NO _x absorbent material. In one such arrangement, the NO _x absorbing material, typically at least one compound of an alkali metal, such as potassium or cesium, at least one compound of an alkaline earth metal, such as barium, strontium or calcium, or a rare earth At least one compound of a metal, such as lanthanum or yttrium, is associated with the oxidation catalyst. In general, these compounds are oxides, but in use, these compounds should be present as nitrates (as explained below) following hydroxide, carbonate, or NO _x absorption. You can also.

In this arrangement, NO ₂ generated on the oxidation catalyst during lambda> 1 conditions is absorbed into the NO _x absorbent material and stored as nitrate. Since the NO _x absorbing material has a limited capacity to absorb NO _x , it is necessary to periodically regenerate, that is, remove the stored NO _x . In general, this is actually a matter of temporarily adjusting the lambda composition and reducing the oxygen concentration of the exhaust gas, for example by injecting additional HC fuel into the exhaust gas or reducing the air entering the combustion mixture. By doing. As a result, the resulting exhaust gas is “rich” but need not be lambda <1 composition. The nitrate forms of alkali, alkaline earth and rare earth metals have been found to be unstable in rich exhaust gases, and therefore NO _x , which is considered to be at least a mixture of NO and NO ₂ is released.

Typically, the composition comprising the NO _x absorbing material also comprises rhodium for reducing NO _x to N ₂ in the presence of the reductant. However, the NO ₂ rhodium decomposition catalyst of the present invention does not contain other PGMs commonly used as oxidation catalysts, such as platinum and / or palladium. In one arrangement, for example, the NO ₂ cracking catalyst is on a separate monolith downstream of the filter. However, in a special embodiment, a NO ₂ cracking catalyst can be placed at the downstream end of the filter.

  The filter may be a suitable substrate including a wall-flow filter of a ceramic material such as silicon carbide or cordierite. Alternatively, the device described in European Patent No. 1057519 or International Patent No. WO03 / 038248 may be used.

Examples of suitable zeolite components for the NO ₂ cracking catalyst are ZSM-5, β-zeolite, Y-zeolite or mordenite. A suitable silica to alumina molar ratio for such zeolite is 25 to 400, optionally 30 to 80.

The NO ₂ decomposition catalyst carrying a metal or a compound thereof is produced by known methods, for example by wet impregnation of at least one support material using a suitable metal salt, followed by calcination, coprecipitation or ion exchange. Can do.

  Silica-titania, zirconia-titania or tungsten-titania may be in the form of a true mixed oxide or composite oxide. “Composite oxide” as defined herein means a highly amorphous oxide material comprising an oxide of at least two elements rather than a true mixed oxide of at least two elements. To do.

  In one embodiment, the catalyst used in the exhaust system of the present invention is 0.1 to 5.0 wt% rhodium, such as 0.25 to 2.5 wt%, based on the total weight of the particulate refractory oxide. Of rhodium.

  In a particular embodiment, the catalyst consists essentially of 0.5% by weight rhodium on gamma-alumina.

In another embodiment, NO ₂ decomposition catalyst comprises 1-10 wt% copper based on the total weight of the particulate refractory oxide, for example, 2.5 to 7.5 wt% copper. When the particulate refractory oxide is a zeolite, copper can be impregnated, ion exchanged or co-precipitated on the refractory oxide.

  In a particular embodiment, the catalyst consists essentially of 5% by weight of copper on zeolite ZSM-5 and / or β-zeolite.

  In another embodiment, the catalyst comprises 1-10% iron, such as 2.5-7.5% iron by weight relative to the total weight of the particulate refractory oxide. When the particulate refractory oxide is a zeolite, iron can be impregnated, ion exchanged or co-precipitated on the refractory oxide.

  In a special embodiment, the catalyst consists essentially of 5% by weight of iron and the at least one support is zeolite ZSM-5 and / or β-zeolite.

  In another embodiment, the catalyst comprises 0.1 to 5.0 wt% palladium, such as 0.25 to 2.5 wt% palladium, based on the total weight of the particulate refractory oxide.

  In a special embodiment, the catalyst consists essentially of 2% by weight palladium on tungsten-titania.

  According to a third aspect, the present invention provides an internal combustion engine comprising the exhaust mechanism of the present invention. Such engines can be fueled by any suitable fuel, such as diesel fuel, gasoline fuel, natural gas (NG) or liquid petroleum gas (LPG), but preferably powered by diesel fuel. .

  In a fourth aspect, the present invention provides a vehicle, such as a mining vehicle, comprising the engine according to the third aspect of the present invention.

In addition to catalyzing the reduction of NO ₂ , the NO ₂ cracking catalyst described herein can also catalyze the reduction of SO ₃ to SO ₂ in exhaust gas conditions, such a reaction Can be used, for example, to reduce the amount of SO ₃ derived particulate material observed during a diesel travel cycle.

  For a better understanding of the present invention, the following examples are given by way of illustration only with reference to the accompanying drawings.

Example 1
A series of catalysts in a simulated catalytic activity test (SCAT) gas unit, NO ₂ 200 ppm, C1 diesel fuel (MK1) about 120 ppm, O ₂ 12%, CO ₂ 4.5%, H ₂ O 4.5% and SO ₂ 20 ppm NO ₂ decomposition ability in a simulated exhaust gas reaction mixture was analyzed, with the remainder being N ₂ (C1HC: NO ₂ approximately 0.6). Each catalyst was heated in the reaction mixture from 150 ° C. to 500 ° C. with a gradient of 10 ° C./min. The test catalyst was ZSM5-30 zeolite with 5 wt% copper ion exchange (5 Cu / ZSM5-30 (based on the total weight of the support)), 0.5 wt% rhodium on gamma-alumina ((total amount of particulate support). 0.5 Rh / Al ₂ O ₃ ) and 5 wt% copper ion exchanged β-zeolite-30 (relative to the total weight of the support) 5 Cu / beta-30).

For comparison, NO ₂ decomposition was measured on a blank reactor equipped with a mesh and 5Cu / β-zeolite-30 was tested with the above mixture but in the absence of diesel fuel. As another test, the 5 wt% copper ion exchanged ZSM5-30 zeolite was tested in the exhaust gas the above reaction mixture at that _time, NO100ppm the NO 2 200 ppm and _NO 2 100 ppm, i.e. NO: in 1: _NO 2 1 Replaced. The results of NO formation from NO ₂ decomposition and NO ₂ shown in Figures 1 and 2.

It can be seen that the addition of diesel fuel improves the low temperature conversion from NO ₂ to NO. The 5Cu / β-zeolite-30 catalyst was the most active catalyst and removed 100% NO ₂ at about 200 to about 350 ° C.

Be changed gaseous NO _x composition from 200PpmNO ₂ to 100 ppm NO / 100 ppm NO _2, but little effect on low temperature performance, its activity, the use of NO / _{NO 2} mixture was rapidly decreased at high temperature. It is not yet clear whether this is due to NO suppression (which probably does not occur at this high temperature) or due to the kinetic / reaction order effects associated with reducing the inlet NO ₂ concentration from 200 to 100 ppm.

Material balance for NO ₂ decomposition vs. NO formation at temperatures 300 to 400 ° C. is not correlated. For example, at 350 ° C., NO ₂ removal is 100%, but only about 150 ppm NO is seen in the gas phase (instead of the 200 ppm we expect if all NO ₂ is decomposed to NO). ). Thus, we believe that some lean NO _x reduction occurs here and use stored HC and gas phase HC to remove NO _x , but the temperature range fits this assumption. .

Example 2
CRT (trade name) exhaust gas aftertreatment mechanism described in European Patent No. 0341832, mounted on a test bench in the laboratory, ie based on platinum on aluminum to oxidize NO to NO ₂ A heavy load engine equipped with an oxidation catalyst and a downstream ceramic wall-flow diesel particulate filter was used to test the principle of the NO ₂ cracking catalyst shown in Example 1 under “real world” conditions. A diesel fuel injector was placed downstream of the filter and 400 cells per square inch (62 cells cm ₋₂ ) of ceramic monolith was coated with the 5Cu / beta-zeolite-30 catalyst of Example 1.

NO and NO ₂ sensors are used to detect the amount of these gases at various points in the exhaust system, and the detected NO ₂ amount is used to provide a C1HC: NO ₂ ratio of 0.5 and 0. The amount of diesel fuel to be injected to obtain 25 was calculated. Up to about 400 ° C., about 50% of the NOx downstream of the CRT (trade name) oxidation catalyst is NO ₂ , so these values correlate to a C1HC: NOx ratio of about 0.5 and 0.25 , respectively. Measurements were taken after adjusting the engine load to increase the temperature in the exhaust mechanism and the mechanism operated at steady state conditions.

3-5 show the results for C1HC: NO ₂ 0.5. In these list of symbols, the NO ₂ cracking catalyst is called a “clean-up” catalyst. Although good NO ₂ decomposition activity is observed, it can be seen that the% NO ₂ decomposition decreases at temperatures below about 300 ° C. At about 325 ° C., a small 7% NO _x conversion is observed (results not shown). Under the conditions used, NO ₂ / NO _x is less than 20% at any temperature except the lowest temperature (250 ° C.). The decrease in the NO ₂ / NO _x ratio after the CRT (trade name) oxidation catalyst is due to a thermodynamic equilibrium that favors NO over NO ₂ . Negligible HC slip was observed (results not shown).

For C1HC: NO ₂ 0.25 (results shown in FIGS. 6-8), good NO ₂ decomposition is still observed, but the conversion peak is 80%. Again, a small amount of NO _x conversion was observed (6.5% peak at about 325 ° C.). Under the conditions used, NO ₂ / NO _x is less than 20% at temperatures above 325 ° C. Negligible HC slip was observed.

Example 3
To investigate the effects of catalyst aging, a 5Cu / Beta-30 catalyst at 500 ° C. in air and in a gas mixture of air, 10% H ₂ O and 50 ppm SO ₂ at 400 ° C. for 63 hours (lean hydrothermal sulfur aging or LHSA), and air and _H 2 O10% in the gas mixture was subjected to aging for 162 hours at 600 ° C. (lean hydrothermal aging or LHA). These catalysts are SCAT gas test equipment containing 200 ppm NO ₂ , 100 ppm C1 diesel fuel (MK1), 12% O ₂ , 4.5% H ₂ O, 4.5% CO ₂ , 20 ppm SO ₂ and the rest In a simulated gas reaction mixture that was N ₂ , others were tested for their ability to decompose NO ₂ in the manner described in Example 1. The results are shown in FIG.

It can be seen that lean hydrothermal aging actually improves the% NO ₂ decomposition activity at low temperatures. Significantly, lean hydrothermal sulfur aging further enhances low temperature catalytic activity. Since LHSA is expected to sulphate the catalyst components, this observation suggests that catalyst coking is involved in the mechanism of catalytic activity since sulphation will increase the acidity of the catalyst. Suggests. The increased activity can increase coke on the catalyst that is derived from hydrocarbons in contact with the catalyst. To test this theory, another series of experiments was conducted.

Example 4
One way to increase the acidity of the zeolite catalyst is to change the silica to alumina molar ratio of the material. In order to investigate the theory that the NO ₂ cracking activity of the catalyst is linked to the catalyst acidity, a series of zeolite catalysts with different silica to alumina molar ratios were tested. Specifically, 5Cu / ZSM-30 (ie, ZSM5 zeolite with a silica to alumina molar ratio of 30), 5Cu / ZSM-300, 5Cu / beta-30, 5Cu / beta-300, and non-metallized beta -300 was tested with the gas mixture of Example 3 according to the procedure described in Example 1 on a SCAT gas test apparatus. The results are shown in FIGS.

It can be seen that zeolites with a low silica to alumina molar ratio, ie, materials with higher acidity, are more active. Also, non-metallized zeolite is active in NO ₂ decomposition, whereas metallized catalysts are more active. This suggests that metal plays a role in coking. Increasing the molar ratio of silica to alumina with β-zeolite does not affect the low temperature NO ₂ decomposition performance as much as ZSM-5.

Further tests were conducted to show that the coking of the catalyst is involved in the NO ₂ decomposition activity. In this experiment, a 5Cu / beta catalyst was exposed to a gas mixture intended to induce coking for 10 hours at 300 ° C. This mixture contains 2000 ppm C1MK1 diesel fuel, 12% O ₂ , 4.5% H ₂ O, 4.5% CO ₂ , 20 ppm SO ₂ with the balance being N ₂ . The catalyst was tested in a SCAT gas test apparatus using the procedure described in Example 1 and the gas mixture of Example 3. The results are shown in FIG.

The “coked” sample was tested, then cooled and the same sample was retested to obtain “Lamp 1” and “Lamp 2”. If coke was involved in the NO ₂ decomposition reaction, it would be expected that lamp 2 would show lower activity than lamp 1 because some coke would have been consumed in lamp 1, This was confirmed by the difference in low temperature activity. At high temperatures, coke will be replenished by reaction of C1 hydrocarbons in the feed gas over the catalyst.

Coke formation on the catalyst is temperature programmed for coked 5Cu / β-zeolite-25 sample (O ₂ 11%, C1-700 ppm (MK1 diesel fuel) exposed to coke for 16 hours at 300 ° C.). Further confirmed by oxidized oxidation (TPO) analysis. The TPO analysis was performed with a temperature programmed desorption tester in 5% O ₂ and the remaining He at a heating rate of 10 ° C./min. The release of CO ₂ was monitored by a mass spectrometer. The results are shown in FIG.

Example 5
Non-zeolitic catalysts below, namely gamma - alumina on rhodium 0.5 _{wt% (0.5Rh / Al 2 O 3} ), tungsten - titania palladium on 2 _{wt% (2Pd / WO 3 -TiO 2} ), tungsten - titania itself , gamma - alumina on copper 10 _{wt% (10Cu / Al 2 O 3} ) and tungsten - titania on the copper 5 _{wt% (5Cu / WO 3 -TiO 2} ), was produced. These catalysts were tested in the manner described in Example 1 using the gas mixture described in Example 3. The results of the relationship between NO ₂ % decomposition and temperature are shown in FIGS. 14 and 15 in comparison with the 5Cu / zeolite catalyst.

FIG. 1 shows the relationship between NO ₂ conversion rate and temperature (° C.) for a NO ₂ decomposition catalyst when diesel fuel (about 120 ppm C1 (MK1)) is injected and not injected, compared with a blank reactor. It is a graph. FIG. 2 is a graph showing the relationship between NO formation from NO ₂ and temperature in the presence of diesel fuel compared to a blank reactor. FIG. 3 is a graph showing the relationship between NO ₂ decomposition and temperature on a Cu / beta-30 zeolite catalyst in HC: NO ₂ 0.5. FIG. 4 is a graph showing the relationship between% NO ₂ conversion and temperature on Cu / Beta-30 zeolite catalyst in HC: NO ₂ 0.5. FIG. 5 is a graph showing the relationship between the NO ₂ / NO _x ratio (%) and the temperature on a Cu / beta-30 zeolite catalyst in HC: NO ₂ 0.5. FIG. 6 is a graph showing the relationship between NO ₂ decomposition and temperature over Cu / beta-30 zeolite catalyst in HC: NO ₂ 0.25. FIG. 7 is a graph showing the relationship between the NO ₂ / NO _x ratio (%) and the temperature on a Cu / beta-30 zeolite catalyst in HC: NO ₂ 0.25. FIG. 8 is a graph showing the relationship between the NO ₂ / NO _x ratio (%) and the temperature on a Cu / beta-30 zeolite catalyst in HC: NO ₂ 0.25. FIG. 9 is a graph showing the relationship between% NO ₂ decomposition and temperature, comparing the activity of aged 5Cu / beta-30 zeolite catalysts. FIG. 10 is a graph showing the relationship between% NO ₂ decomposition and temperature comparing the activity of a series of 5Cu / zeolite catalysts. FIG. 11 is a graph showing the relationship between NO ₂ decomposition% and temperature, comparing the activities of two types of 5Cu / ZSM5 catalysts having zeolites with different molar ratios of silica and alumina. FIG. 12 is a graph showing the relationship between% NO ₂ decomposition and temperature comparing the activity of coked and non-coked 5Cu / Beta-30 catalysts. FIG. 13 is a graph showing temperature programmed oxidation (TPO) of a “coked” 5Cu / beta-25 zeolite catalyst. FIG. 14 is a graph showing the relationship between NO ₂ % and temperature comparing the activity of a series of non-zeolite catalysts compared to 5Cu / ZSM5-30. FIG. 15 is a graph showing% NO ₂ decomposition for a series of copper-containing non-zeolite catalysts compared to 5Cu / Beta-25.

Claims (18)

A method for decomposing nitrogen dioxide (NO ₂ ) in exhaust gas of a diesel internal combustion engine into nitric oxide (NO),
Adjusting the C1 hydrocarbon: nitrogen oxide (C1HC: NO _x ) ratio of the exhaust gas to 0.1-2,
The gas mixture is a particulate acidic refractory oxide selected from the group consisting of zeolite, tungsten-doped titania, silica-titania, zirconia-titania, amorphous silica-alumina, and mixtures of any two or more thereof. And releasing the effluent gas into the atmosphere. The particulate refractory oxide carries a metal or a compound of the metal,
The method according to claim 1, wherein the metal is selected from the group consisting of rhodium, palladium, iron, copper, and a mixture of any two or more thereof. The _{C1HC: NO x (x = 2} ) ratio is adjusted from 0.2 to 4, the method according to claim 1 or 2. The C1HC: NO step of adjusting the _x ratio is done at above 250 ° C., carried out at up to 500 ° C. If necessary, the method according to any one of claims 1 to 3. An exhaust mechanism for a diesel internal combustion engine,
A catalyst for decomposing nitrogen dioxide (NO ₂ ) into nitric oxide (NO) in a suitable reductant, and C1 hydrocarbon: nitrogen oxide (C1HC: NO in exhaust gas upstream of the catalyst during use _x ) means for adjusting the ratio to 0.1-2,
The catalyst is a particulate acidic refractory oxide selected from the group consisting of zeolite, tungsten-doped titania, silica-titania, zirconia-titania, amorphous silica-alumina, and mixtures of any two or more thereof. Optionally carrying a metal or a compound of said metal,
An exhaust mechanism, wherein the metal is selected from the group consisting of rhodium, palladium, iron, copper and a mixture of any two or more thereof.     The exhaust mechanism according to claim 5, wherein the at least one zeolite is ZSM-5, β-zeolite, Y-zeolite or mordenite. The modulate means, C1HC in the flue gas: a NO ₂ ratio formed by employed to adjust from 0.2 to 4, the exhaust mechanism according to claim 5 also 6.     8. The apparatus according to any one of claims 5 to 7, wherein the adjusting means is controlled to operate when in use, the exhaust gas temperature exceeds 250C, and optionally up to 500C. The method according to item.     The exhaust according to any one of claims 5 to 8, wherein the adjusting means comprises control means comprising a processor and, if necessary, a processor which is part of an engine management unit (ECU). mechanism. The control means has the following inputs:
Exhaust gas temperature, catalyst bed temperature, exhaust gas material flow rate, NO _{2 in} the exhaust gas, manifold vacuum, ignition timing, engine speed, throttle position, lambda value of the exhaust gas composition, amount of fuel injected into the engine, exhaust gas position of circulation valve, and the boost pressure in response to one or more of the C1HC: one which regulates NO _x ratio, exhaust system according to claim 9.     11. An exhaust system according to claim 10, wherein the control means is activated by a stored look-up table or engine map in response to at least one of the inputs. Means for adjusting the C1HC: NO _x ratio;
Means for injecting a reductant into the exhaust gas,
Means for adjusting the ignition timing of at least one engine cylinder;
Means for adjusting the fuel injection timing of at least one engine cylinder;
The exhaust mechanism according to any one of claims 5 to 11, comprising at least one of means for adjusting an air-fuel ratio of the engine and adjustment of an exhaust gas recirculation speed. The exhaust mechanism according to any one of claims 5 to 12, wherein the NO ₂ decomposition catalyst is arranged downstream of an oxidation catalyst comprising at least one PGM, preferably at least one of platinum and palladium. . The exhaust mechanism according to claim 13, comprising a particulate matter filter between the oxidation catalyst and the NO ₂ decomposition catalyst. The exhaust mechanism according to claim 14, wherein the NO ₂ decomposition catalyst is disposed at a downstream end of the particulate matter filter. Dependent on claim 12 or claim 12, wherein the reductant injection means injects the reductant into the exhaust mechanism upstream of the NO ₂ decomposition catalyst and downstream of all PGM oxidation catalysts. The exhaust mechanism according to any one of claims 13 to 15.     A diesel internal combustion engine comprising the exhaust mechanism according to any one of claims 5 to 16.     A vehicle comprising the engine according to claim 17, for example, a mining vehicle.

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