JP4045588B2 - Exhaust gas purification system - Google Patents

Description

【００６１】
表１より、以下のことが分かる。
実施例１の触媒を用いて行った試験１、２では、ＮＯｘ吸蔵触媒の入口排ガス温度を常に４００℃以上に保持した場合には、硫黄被毒が抑えられている。しかし、この場合には、ＮＯｘ転化率が悪く、これは、常に温度が高いと硫黄酸化物が放出するために被毒が緩和されるだけでなく、同時に上述の▲３▼式の反応が起こりにくくなりＮＯｘ吸収量が減少するためであると考えられる。これより、本発明の排ガス浄化触媒システムでは、冷却装置につながるバイパス流路を有することが有効であるといえる。
【００６２】
また、実施例１、５〜７と比較例１との比較から、上述の▲１▼式の反応のΔＧにより、硫黄被毒の度合が大きく異なることが分かる。
即ち、ΔＧが−３５０ｋＪ／ｍｏｌ以下のＢａを含むＮＯｘ吸蔵触媒では、初期のＮＯｘ転化率は高いが、硫黄被毒の影響を受け易く、ＮＯｘ転化率の悪化率が大きい。逆に、ΔＧが−３５０ｋＪ／ｍｏｌ以上のＭｇでは、初期のＮＯｘ転化率は低いが、硫黄被毒の影響を受けにくく、ＮＯｘ転化率の悪化率が小さくなる。更に、ＮＯｘ触媒の構成元素を数種として、複合化効果により、悪化率を小さくできることも分かる（実施例５〜７）。よって、本発明の排気ガス浄化システムには、▲１▼式の反応のΔＧが−３５０ｋＪ／ｍｏｌより大きなアルカリ土類金属、希土類金属及び遷移金属をＮＯｘ吸蔵触媒に含有させることが有効であることがわかる。
【００６３】
更に、実施例１と比較例２との比較から、スラリー液の平均粒径は、５μｍとするよりも４μｍ以下までに小さくすることがより有効であることが明かである。
【００６４】
以上、本発明を好適実施例により詳細に説明したが、本発明はこれら実施例に限定されるものではなく、本発明の開示の範囲内において種々の変形が可能である。
例えば、本発明の浄化システムの大きな利点は、ＮＯｘ吸蔵触媒中に硫酸金属塩を残存させないことであり、従来は、硫酸金属塩が徐々にではあるが不可避的にＮＯｘ吸蔵触媒中に残存して行き、ついには飽和量に達してしまうが、本発明の浄化システムではかかる現象を回避することができる。
【００６５】
【発明の効果】
以上説明してきたように、本発明によれば、排ガスを所望の温度に制御した後ＮＯｘ吸蔵触媒でＮＯｘの吸収・放出を行うため、従来から知られる排気ガス浄化方法に比べ、硫黄被毒を低減するとともに、ＮＯｘ吸蔵触媒の熱劣化を防止することができる排気ガス浄化システムを提供することができる。
【００６６】
即ち、本発明によれば、上流に設置した温度予測手段で予測したＮＯｘ吸蔵触媒の入口排ガス温度に基づいて、必要時には瞬時にストイキ〜リッチ域に変動させることによって、ＮＯｘ吸蔵触媒に流入した硫黄酸化物を放出することができ、硫黄被毒の低減及びＮＯｘ吸蔵触媒を再生して使用できる。
また、ΔＧが−３５０ｋＪ／ｍｏｌ以上となるような元素を含むＮＯｘ吸蔵触媒を用いて、不純物である硫黄を比較的低温で複合硫黄酸化物とするため、ＮＯｘ吸蔵触媒に、高温でなければ除去できない金属硫酸塩の生成・蓄積を回避できるだけでなく、低温で容易に硫黄酸化物をＮＯｘとともに放出することによってＮＯｘ吸蔵触媒の熱劣化を防止でき、結果としてＮＯｘ吸蔵性能を持続できる。更に、電磁波をＮＯｘ吸蔵触媒に照射することにより、硫黄化合物となっていたペロブスカイト型複合酸化物が選択的に加熱励起され、硫黄分の放出をさらに促進することができる。
【図面の簡単な説明】
【図１】本発明の排気ガス浄化システムの一例を示すフローチャートである。
【図２】本発明の排気ガス浄化システムの一例を示す配置図である。
【符号の説明】
１ 内燃機関
２ 三元触媒
３ 温度予測手段
４ 冷却手段（クーラー）
５ ＮＯｘ吸蔵触媒
６ 三元触媒[0001]
[Technical field to which the invention belongs]
The present invention relates to an exhaust gas purification system, and more particularly, exhaust gas purification that purifies hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx) in exhaust gas discharged from an internal combustion engine. The exhaust gas purification system focusing on the NOx purification process in the system, particularly in the oxygen excess region, is suitable for the exhaust gas treatment discharged from an internal combustion engine such as an automobile (gasoline, diesel) or a boiler. Used.
[0002]
[Prior art]
In recent years, demand for fuel-efficient vehicles has increased due to the problem of depletion of petroleum resources and global warming, and the development of lean-burn vehicles has attracted attention for gasoline vehicles.
In such lean burn automobiles, the exhaust gas atmosphere becomes an oxygen-excess atmosphere (lean) compared to the theoretical air-burning state during lean burn driving. However, when a normal three-way catalyst is used in the lean region, NOx is caused by the influence of excess oxygen. It has been known that the purification action is insufficient. For this reason, various catalysts capable of purifying NOx even when oxygen is excessive have been proposed. For example, a catalyst in which platinum (Pt) and lanthanum (La) are supported on a porous carrier (Japanese Patent Laid-Open No. 5-168860). ), A catalyst that absorbs NOx in a lean region and releases NOx during stoichiometry to purify the catalyst has been proposed. However, since sulfur is contained in the fuel and the lubricating oil, and this sulfur content is discharged into the exhaust gas as an oxide, the NOx absorbent is poisoned by the sulfur oxide (hereinafter referred to as “sulfur covered”). It is known that the NOx absorption ability is reduced.
[0003]
To prevent this sulfur poisoning,
(1) A method of disposing a sulfur trap catalyst in front of the NOx absorption catalyst, absorbing the sulfur catalyst in the lean region, and preventing sulfur oxide from flowing into the subsequent NOx catalyst (JP-A-6-58138),
(2) Based on the premise of a certain degree of sulfur poisoning, the temperature of the catalyst is increased at regular intervals, the air-fuel ratio (hereinafter referred to as “A / F”) is made rich, and the attached sulfur oxide is released. A method for recovery (Japanese Patent Laid-Open No. 7-217474),
Etc. are done.
[0004]
[Problems to be solved by the invention]
However, the conventional exhaust gas purification method as described above has the following problems.
That is, in the method (1), surplus sulfur oxides that cannot be absorbed by the sulfur trap catalyst at the time of lean flow into the NOx absorption catalyst, and the NOx absorption catalyst is subjected to sulfur poisoning.
Further, when a material capable of completely absorbing sulfur at the time of lean is used as the NOx absorption catalyst, it becomes difficult to release sulfur during stoichiometric and rich conditions.
Furthermore, the sulfur oxide released during the stoichiometric to rich time poisons the NOx absorption catalyst in the subsequent stage.
[0005]
On the other hand, in the method (2), when the NOx absorbent contains an element such as barium (Ba), the standard free energy change value (ΔG) of the generated sulfate is negatively large. Therefore, there is a problem that a high temperature of 700 ° C. or higher is required for releasing the sulfate, which causes thermal deterioration of the catalyst.
[0006]
[Means for Solving the Problems]
As a result of intensive studies to solve the above problems, the present inventors have found that the above problems can be solved by using the NOx storage catalyst after controlling the exhaust gas to a desired temperature. It came to be completed.
[0007]
That is, the exhaust gas purification catalyst system of the present invention is a system for purifying exhaust gas of a lean burn internal combustion engine in which the air-fuel ratio A / F varies between rich to stoichiometric to lean,
From the upstream side of the exhaust passage, the exhaust gas temperature predicting means, the exhaust gas cooling means and the NOx storage catalyst are sequentially arranged,
The exhaust gas temperature predicting means detects the exhaust gas temperature as needed, predicts the inlet exhaust gas temperature of the NOx storage catalyst according to the detection result, and transmits an operation signal to the exhaust gas cooling means according to the prediction result. And
The exhaust gas cooling means cools the exhaust gas in accordance with an operation signal from the exhaust gas temperature prediction means.
[0008]
In a preferred embodiment of the exhaust gas purification system of the present invention, when the A / F of the exhaust gas flowing into the NOx storage catalyst is lean, the inlet exhaust gas temperature of the NOx storage catalyst is controlled to 300 ° C. or lower. .
[0009]
Furthermore, in another preferred embodiment of the exhaust gas purification system of the present invention, when the predicted temperature of the inlet exhaust gas in the NOx storage catalyst exceeds 400 ° C., the exhaust gas A / F is adjusted to stoichiometric to rich so that the sulfur desorption is performed. It is characterized by performing separation.
[0010]
Furthermore, still another preferred embodiment of the exhaust gas purification system of the present invention is characterized in that the predicted temperature of the NOx storage catalyst inlet exhaust gas is maintained for more than 400 minutes for more than 400 ° C.
[0011]
In another preferred embodiment of the exhaust gas purification system of the present invention, the exhaust passage has a bypass passage, and the exhaust gas cooling means is installed in the bypass passage.
[0012]
Still another preferred embodiment of the exhaust gas purification system of the present invention is characterized in that a three-way catalyst is added upstream of the exhaust gas temperature predicting means.
[0013]
Furthermore, another preferred embodiment of the exhaust gas purification system of the present invention is characterized in that the exhaust gas temperature predicting means predicts the inlet exhaust gas temperature of the downstream NOx storage catalyst from the upstream three-way catalyst outlet exhaust gas temperature. .
[0014]
A preferred embodiment of the exhaust gas purification system of the present invention is characterized in that a three-way catalyst is added to the downstream side of the NOx storage catalyst.
[0015]
Furthermore, in another preferred embodiment of the exhaust gas purification system of the present invention, the NOx storage catalyst has the following reaction formula (1):
1 / 2O ₂ + SO ₂ + AO → ASO ₄ … ▲ 1 ▼
(Wherein A represents an alkaline earth metal, rare earth metal or transition metal, and any mixture thereof, where A is represented as a divalent element). (ASO ₄ ) Containing element A having a standard free energy change value (ΔG) of −350 kJ / mol or more.
In this case, the element A is preferably magnesium (Mg), lanthanum (La), manganese (Mn) or iron (Fe) and any mixture thereof.
[0016]
Furthermore, in another preferred embodiment of the exhaust gas purification system of the present invention, the NOx storage catalyst has the following general formula (2):
(Ln _1-α A _α ) _1-β BO _δ … ▲ 2 ▼
(Wherein Ln is lanthanum (La), cerium (Ce), neodymium (Nd) or samarium (Sm) and any mixture thereof, A is magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), sodium (Na), potassium (K) or cesium (Cs) and any mixture thereof, B is iron (Fe), cobalt (Co), nickel (Ni) or manganese (Mn) and any of these Wherein δ represents the amount of oxygen satisfying the valence of each element, and 0 ≦ α ≦ 1 and 0 <β <1).
[0017]
A preferred embodiment of the exhaust gas purification system of the present invention is characterized in that the NOx storage catalyst is heated by electromagnetic waves.
[0018]
Further, in the purification system of the present invention, the NOx storage catalyst contains platinum (Pt), rhodium (Rh) or palladium (Pd) and any mixture thereof, and furthermore, each component powder contained in the NOx storage catalyst. The average particle size is 4 μm or less.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the exhaust gas purification system of the present invention will be described in detail.
The exhaust gas purification system of the present invention is a system for purifying exhaust gas of a lean burn internal combustion engine in which the air-fuel ratio A / F fluctuates between rich, stoichiometric and lean, and from the upstream side of the exhaust passage, the exhaust gas temperature The predicting means, the exhaust gas cooling means and the NOx storage catalyst are sequentially arranged.
[0020]
Here, the NOx occlusion catalyst is a catalyst that absorbs NOx in exhaust gas in a lean region and releases NOx in a lean region and purifies by exhausting NOx in a lean burn internal combustion engine.
The air-fuel ratio of the internal combustion engine of the lean burn engine is typically such that A / F repeats from 10 to 14.8 (stoichi to rich) and lean (15 to 50).
[0021]
As described above, in the purification system of the present invention, the exhaust gas temperature at the inlet of the NOx storage catalyst is controlled for the following reason.
When the NOx storage catalyst is subjected to sulfur poisoning, the higher the temperature, the higher the temperature required to desorb sulfur in the catalyst. This is because when the temperature is high, the sulfate of the NOx storage material causes crystallization. Further, NOx absorption proceeds by the following reaction, and the reaction of formula (3) is more likely to occur at a lower temperature. Therefore, it is effective to use the catalyst at a relatively low temperature in order to improve NOx absorption performance.
NO + 1 / 2O ₂ → NO ₂ … ▲ 3 ▼
NO ₂ + NOx storage catalyst (complex oxide of A) → A (NO ₃ ) _n … ▲ 4 ▼
[0022]
In the exhaust gas purification system of the present invention, the exhaust gas temperature predicting means detects the exhaust gas temperature as needed, predicts the inlet exhaust gas temperature of the NOx storage catalyst according to the detection result, and according to the prediction result, An operation signal is transmitted to the exhaust gas cooling means.
The exhaust gas cooling means cools the exhaust gas in response to an operation signal from the exhaust gas temperature prediction means.
Specifically, when the exhaust gas A / F is lean, the NOx storage catalyst inlet exhaust gas temperature is preferably controlled to 300 ° C. or lower. In this case, in the NOx storage catalyst arrangement portion in the exhaust passage Since the production and accumulation of metal sulfates that can only be removed at high temperatures can be avoided, the storage performance of the NOx storage catalyst can be maintained.
[0023]
Further, when the predicted temperature of the inlet exhaust gas in the NOx storage catalyst exceeds 400 ° C., it is preferable to adjust the A / F of the exhaust gas to stoichiometric to rich, thereby performing sulfur desorption.
This is because there is a possibility that the NOx absorption amount of the NOx occlusion catalyst is always saturated in the lean region, and it is brought into contact with a relatively high temperature exhaust gas to release sulfur oxide together with NOx, thereby reducing sulfur poisoning. This is because it can be avoided.
It is known that when the sulfur content is desorbed from the NOx storage catalyst, the A / F can be increased as the temperature increases. On the other hand, if the A / F is reduced excessively, only the fuel consumption is reduced. For this reason, it is preferable to control A / F in a stoichiometric to rich range when the inlet exhaust gas temperature of the NOx storage catalyst exceeds 400 ° C. For example, when A / F is stoichiometrically controlled when the predicted exhaust gas temperature of the NOx storage catalyst is 600 ° C., and A / F = 11 (rich) when 400 ° C., instantaneously within that range. This is effective because the sulfur content can be eliminated and the fuel consumption can be prevented from being lowered by changing the temperature.
[0024]
Further, in the present invention, it is preferable that the predicted temperature of the exhaust gas at the inlet of the NOx storage catalyst exceeds 400 ° C. for 1 minute or longer, thereby facilitating desorption of sulfur oxides and preventing sulfur poisoning. Less likely to occur.
[0025]
As the exhaust gas temperature measuring means, a combination of various temperature sensors such as a thermocouple, an infrared temperature measuring device, and a thermistor and a computing device such as a personal computer and a CPU can be mentioned because of the above-described functions.
[0026]
Furthermore, as the exhaust gas cooling means, it is sufficient if the exhaust gas can be cooled, and is not particularly limited, but examples thereof include a cooler, a finned bypass flow path, and a wind guide plate.
[0027]
In the purification system of the present invention, a bypass flow path can be provided in the exhaust flow path, the exhaust gas cooling means can be installed in the bypass flow path, or the bypass flow path itself can be used as the exhaust gas cooling means.
In this case, by installing a temperature sensor as exhaust gas temperature prediction means upstream and adjusting the length of the bypass flow path, the exhaust gas temperature at the inlet of the NOx storage catalyst can be predicted from the temperature drop rate due to the flow path length. In this case, the above-mentioned arithmetic unit can be omitted.
A typical example of such a temperature drop is when the exhaust gas temperature drops at a rate of 10 ° C. per 10 cm of the bypass flow path length.
Needless to say, a switching valve can be installed at the branch point between the bypass flow path and the exhaust flow path to adjust the cooling time.
[0028]
A three-way catalyst can be added upstream of the exhaust gas temperature predicting means in the present invention. In this case, it is more preferable to predict the inlet exhaust gas temperature of the NOx storage catalyst from the outlet exhaust gas temperature of the three-way catalyst, which is effective because it is possible to efficiently remove HC and CO other than NOx.
In addition, when adding a three-way catalyst to the downstream side of the NOx occlusion catalyst, considering NOx occlusion mainly, the treatment of NOx, HC, and CO is performed before and after that, so that more exhaust gas is generated. It can be purified.
Note that not only the NOx storage catalyst described later but also the three-way catalyst has a honeycomb shape, so that the contact area between the catalyst and the exhaust gas can be increased and the pressure loss can be suppressed. In doing so, it becomes more effective.
[0029]
Next, the NOx storage catalyst will be described.
As described above, this NOx storage catalyst absorbs, releases, and purifies NOx. In the present invention, the NOx storage catalyst has the following reaction formula (1):
1 / 2O ₂ + SO ₂ + AO → ASO ₄ … ▲ 1 ▼
(Wherein A represents an alkaline earth metal, rare earth metal or transition metal, and any combination thereof, where A is represented as a divalent element). (ASO ₄ It is preferable to contain an element A having a standard free energy change value (ΔG) of −350 kJ / mol or more.
[0030]
Further, as described above, NOx occlusion proceeds as shown in the following reaction formulas (3) and (4).
NO + 1 / 2O ₂ → NO ₂ … ▲ 3 ▼
NO ₂ + NOx storage catalyst (complex oxide of A) → A (NO ₃ ) _n … ▲ 4 ▼
The reaction of reaction formula (3) is more likely to occur at lower temperatures. In addition, if the NOx storage catalyst is poisoned with sulfur at a high temperature, the sulfate of the NOx storage catalyst and sulfur, which is an impurity, crystallizes, and this crystallized sulfate cannot be desorbed unless the temperature is high. Reaction like Formula (4) becomes difficult to occur.
[0031]
In the NOx storage catalyst of the present invention, in consideration of the properties of the NOx storage catalyst, a composite oxide (ASO) ₄ ) Has a standard formation free energy change value (ΔG) of −350 kJ / mol or more, the reaction of the reaction formulas (3) and (4) is likely to proceed, and the reaction temperature is relatively low (400 to 500.degree. C.), which is an impurity, can be eliminated and is effective.
That is, the sulfur oxide can be desorbed at a low temperature, and the occlusion performance can be reactivated and maintained, so that the exhaust gas purification catalyst system of the present invention can be used for a long time.
[0032]
Further, the NOx storage catalyst is preferably used while being supported on an integral structure type carrier. Examples of the monolithic structure type carrier include a monolith carrier made of a heat resistant material, a metal carrier, and the like, and these are preferably used after being coated with a NOx storage catalyst. In particular, when purifying NOx in the exhaust gas of an automobile, it is more effective because the contact area between the catalyst and the exhaust gas can be increased and the pressure loss can be suppressed by coating the honeycomb carrier.
In general, cordierite materials such as ceramics are often used as the honeycomb-shaped carrier, but a honeycomb-shaped carrier made of a metal material such as ferritic stainless steel can also be used. It may be formed into a honeycomb shape.
[0033]
Here, the element A contained in the NOx storage catalyst is preferably Mg, La, Mn, Fe, or any combination thereof.
Element A can also be supported on activated alumina. Specifically, it has high heat resistance and a surface area of 50 to 300 m. ² More preferably, it is supported on activated alumina of about / g.
[0034]
The NOx storage catalyst has the following general formula (2)
(Ln _1-α A _α ) _1-β BO _δ … ▲ 2 ▼
(Wherein Ln is La, Ce, Nd or Sm and any combination thereof, A is Mg, Ca, Sr, Ba, Na, K or Cs and any combination thereof, B is iron, cobalt, nickel or Manganese and any combination thereof, δ represents an oxygen amount satisfying the valence of each element, and preferably includes a complex oxide represented by 0 ≦ α ≦ 1, 0 <β <1. .
[0035]
Thereby, it becomes easier to release sulfur during stoichiometric to rich conditions, and the level of sulfur poisoning can be further reduced.
In this complex oxide, it is preferable that all of the components are complexed, but the desired NOx occlusion performance can be obtained even when some of the components are complexed.
[0036]
In addition, as a manufacturing method of the composite oxide represented by the above formula (2), an aqueous solution of metal salts (nitrate, carbonate, acetate, citrate, hydrochloride, etc.) of each component is prepared and necessary. Accordingly, there can be exemplified a method in which an aqueous solution of a precipitating agent (ammonia, ammonium carbonate or the like) is added thereto to form precipitates, and these solutions or precipitates are dried and fired to obtain composite oxide powder.
According to such a production method, at least a part of each component is compounded, and a product meeting the purpose is obtained. However, the manufacturing method of the said composite body is not necessarily limited to the said method, It is sufficient if a composite body is formed also by methods other than the above.
[0037]
Further, the complex may contain impurities contained in the component elements as long as the amount does not hinder the action.
For example, among the elements constituting this complex, barium may contain a small amount of strontium or lanthanum may contain a small amount of cerium, neodymium, samarium, etc. There is no.
[0038]
Next, the NOx storage catalyst in the present invention is preferably heated by electromagnetic waves. In this case, by irradiating the NOx storage catalyst with electromagnetic waves, the perovskite complex oxide that has been a sulfur compound is selectively heated and excited, and the release of the sulfur content is further promoted.
At this time, the noble metal does not become a heat absorber, and the heat deterioration due to sintering of the noble metal is only due to the exhaust gas temperature, which is efficient.
[0039]
The use of electromagnetic waves is preferably carried out when the exhaust gas temperature at the NOx storage catalyst inlet becomes 500 ° C. or higher. From the perovskite complex oxide that has become a sulfur compound by irradiation of the exhaust gas temperature of 500 ° C. or higher and electromagnetic waves. Removal of sulfur is easy and effective.
[0040]
Further, the NOx storage catalyst can contain Pt, Rh or Pd and a noble metal which is any combination thereof, and is preferably supported on a porous carrier such as alumina. Thereby, the function as a three-way catalyst can be exhibited and the purification | cleaning of exhaust gas can be ensured.
In order to improve the heat resistance of activated alumina, for example, ceria (CeO) having an oxygen storage function is added to add rare earth compounds such as cerium (Ce) and lanthanum (La) and to enhance the function as a three-way catalyst. ₂ ), Barium (Ba) that alleviates HC adsorption to noble metals, zirconia (ZrO) that contributes to improving the heat resistance of Rh ₂ ) Etc. may be further added.
[0041]
The average particle size of each component powder contained in the NOx storage catalyst is preferably 4 μm or less as a median diameter.
In this case, since the surface area of the catalyst is increased, the contact frequency between the catalyst component and the exhaust gas can be increased, the occlusion performance of the catalyst can be further improved, and the release of sulfur oxides easily occurs. .
[0042]
The exhaust gas purification system described above typically has a configuration such as the layout shown in FIG. 2 and can be executed according to the flowchart shown in FIG.
In FIG. 2, this purification system includes a temperature sensor 3 as an example of exhaust gas temperature prediction means, a cooling device (cooler) 4 as an example of exhaust gas cooling means, and Mg as an example of a NOx occlusion catalyst from upstream of the exhaust passage. -NOx storage catalyst 5 is arranged sequentially. The exhaust passage is connected to the internal combustion engine 1, and three-way catalysts 2 and 6 are installed on the upstream side of the temperature sensor 3 and the downstream side of the NOx storage catalyst 5, respectively, for exhaust gas purification. Completeness is achieved.
Hereinafter, the flowchart of FIG. 1 will be described in the order of steps with reference to FIG.
[0043]
In Step 1 (hereinafter simply referred to as “S1”), it is determined whether or not the temperature of the exhaust gas at the inlet of the NOx occlusion catalyst 5 is 400 ° C. or higher by the temperature sensor 3 which is an example of the upstream temperature prediction means.
If the exhaust gas temperature is 400 ° C. or higher, sulfur oxide can be released. Therefore, in order to judge the necessity, for example, whether or not the travel distance from the previous sulfur desorption treatment is 300 km or more [S2].
When this determination is Yes, the bypass flow path leading to the cooler 4 as an example of the cooling device is closed, and the A / F of the exhaust gas is controlled to be stoichiometric to rich. This is because the NOx occlusion catalyst 5 is instantaneously exposed to the high temperature exhaust gas by closing the bypass flow path, and sulfur is released together with the occluded NOx.
[0044]
In addition, when the determination of S2 is No, that is, when the degree of sulfur poisoning is light and the NOx occlusion performance does not deteriorate, there is no need to perform sulfur release treatment, so the bypass flow path leading to the cooling device 4 is opened. The inlet exhaust gas temperature of the NOx storage catalyst is maintained at 300 ° C. or lower. This is to prevent thermal degradation of the NOx storage catalyst due to frequent release treatment, crystallization of the absorbed sulfur, and further reduction in fuel consumption.
[0045]
On the other hand, if the NOx storage catalyst inlet exhaust gas temperature is lower than 400 ° C. in S1, the bypass passage leading to the cooler 4 is opened and cooled, and the NOx storage catalyst 5 inlet exhaust gas temperature is set to 300 ° C. or lower to reduce NOx. The storage catalyst 5 can be used continuously. However, since the NOx storage catalyst 5 is always saturated in the lean region, it is determined whether the NOx storage catalyst 5 has reached saturation [S3], and when the saturation is reached, the A / F is set to the stoichiometric to rich region. Change.
In this case, it is preferable to close the bypass flow path and raise the temperature to a range of 400 to 600 ° C. at once, considering that the release treatment cannot be performed at a low temperature and the absorbed sulfur is crystallized at a high temperature. . Further, heating to this high temperature is sufficient even for a short time.
[0046]
In the present invention, by performing the flow control as described above, it is possible to efficiently purify the exhaust gas while suppressing the influence of sulfur poisoning.
[0047]
【Example】
Hereinafter, the present invention will be described in more detail by way of examples and comparative examples with reference to the drawings, but the present invention is not limited to these examples.
[0048]
In Examples 1 to 7 and Comparative Examples 1 and 2, NOx storage catalysts used in Tests 1 and 2 described later were obtained by the following operation.
Example 1
Activated alumina was impregnated with a magnesium acetate aqueous solution, dried, and then fired in air at 400 ° C. for 1 hour to obtain magnesium-supported alumina powder (powder A). The magnesium concentration of this powder was 20.0 wt%.
An aqueous Pd nitrate solution was impregnated into powder A and calcined at 400 ° C. for 1 hour in dry air to obtain Pd, magnesium-supported alumina powder (powder B). The Pd concentration of this powder was 5.0 wt%.
An aqueous Rh nitrate solution was impregnated into powder A, dried and then calcined in air at 400 ° C. for 1 hour to obtain Rh, magnesium-supported alumina powder (powder C). The Rh concentration of this powder was 2.0 wt%.
522 g of powder B, 135 g of powder C, 243 g of activated alumina powder, and 900 g of water were put into a magnetic ball mill, and mixed and ground for 2.5 hours to obtain a slurry liquid. The average particle size of this slurry liquid was 3.5 μm. This slurry liquid is attached to a cordierite monolith carrier (1.7 L, 400 cells), excess slurry in the cells is removed by air flow, dried at 130 ° C., and then baked at 400 ° C. for 1 hour. A catalyst having a layer weight of 300 g / L was obtained.
[0049]
(Example 2)
A catalyst was obtained by repeating the same operation as in Example 1 except that the magnesium acetate of the powder A was changed to iron nitrate.
[0050]
(Example 3)
A catalyst was obtained by repeating the same operation as in Example 1, except that the magnesium acetate of the powder A was changed to manganese nitrate.
[0051]
Example 4
A catalyst was obtained by repeating the same operation as in Example 1 except that the magnesium acetate of the powder A was changed to lanthanum nitrate.
[0052]
(Example 5)
An active iron powder A was impregnated with an aqueous iron nitrate solution, dried, and then fired in air at 800 ° C. for 1 hour to obtain magnesium and iron-supported alumina powder (powder D). The magnesium concentration of this powder was 20.0 wt%, and the iron concentration was 7.0% wt.
An aqueous Pd nitrate solution was impregnated into powder D, and calcined at 400 ° C. for 1 hour in dry air to obtain Pd, magnesium-supported alumina powder (powder E). The Pd concentration of this powder was 5.0 wt%.
A powder D was impregnated with an aqueous Rh nitrate solution and calcined at 400 ° C. for 1 hour in dry air to obtain an alumina-supported Rh, magnesium and iron powder (powder F). The Rh concentration of this powder was 2.0 wt%.
522 g of powder B, 135 g of powder C, 243 g of activated alumina powder, and 900 g of water were put into a magnetic ball mill, and mixed and ground for 2.5 hours to obtain a slurry liquid. The average particle size of this slurry liquid was 3.5 μm. This slurry solution is attached to a cordierite monolith carrier (1.7 L, 400 cells), excess slurry in the cells is removed with an air flow, dried at 130 ° C., and then baked at 400 ° C. for 1 hour. A catalyst having a layer weight of 300 g / L was obtained.
[0053]
(Example 6)
A catalyst was obtained by repeating the same operation as in Example 5, except that the iron nitrate of the powder D was changed to manganese nitrate.
[0054]
(Example 7)
A catalyst was obtained by repeating the same operation as in Example 5 except that the iron nitrate of powder D was lanthanum nitrate.
[0055]
(Comparative Example 1)
A catalyst was obtained by repeating the same operation as in Example 1 except that powder A magnesium acetate was changed to barium acetate.
[0056]
(Comparative Example 2)
A catalyst was obtained by repeating the same operation as in Example 1 except that the average particle size of the slurry liquid was 5 μm.
[0057]
The following tests were conducted for each catalyst obtained by the above operation.
(Test 1)
A catalyst was attached to the exhaust system of an engine with a displacement of 4400 cc, the exhaust gas temperature at the inlet of the NOx storage catalyst was set to 650 ° C., and the durability was measured by operating for 50 hours (refer to the data of “first cycle” in Table 1).
[0058]
Using gasoline containing 300 ppm of sulfur, a catalyst was mounted on the exhaust system of a 2000 cc engine, and the EC + EUDC mode was repeated 100 cycles. During this mode, the exhaust gas temperature at the inlet of the NOx storage catalyst was controlled to be 400 ° C. or more for 1 minute or more.
The total conversion in this mode was determined and evaluated from the first cycle and the 100th cycle. Moreover, the deterioration rate was calculated | required from the reduction ratio of the conversion rate of 1st cycle and 100th cycle. The results are shown in Table 1.
[0059]
(Test 2)
Using the catalyst of Example 1, the test was performed by repeating the same operation as in Test 1 except that the NOx storage catalyst inlet exhaust gas temperature in the mode was always set to 400 ° C. or higher. The results are shown in Table 1.
[0060]
[Table 1]

[0061]
Table 1 shows the following.
In tests 1 and 2 performed using the catalyst of Example 1, sulfur poisoning is suppressed when the exhaust gas temperature at the inlet of the NOx storage catalyst is always kept at 400 ° C. or higher. However, in this case, the NOx conversion rate is poor. This is because not only is the poisoning mitigated because the sulfur oxide is released when the temperature is always high, but at the same time the reaction of the above formula (3) occurs. This is considered to be because the amount of NOx absorbed is reduced. From this, it can be said that it is effective to have a bypass channel connected to the cooling device in the exhaust gas purification catalyst system of the present invention.
[0062]
Further, from comparison between Examples 1 and 5 to 7 and Comparative Example 1, it can be seen that the degree of sulfur poisoning varies greatly depending on ΔG of the reaction of the above-mentioned formula (1).
That is, in the NOx storage catalyst containing Ba with ΔG of −350 kJ / mol or less, the initial NOx conversion rate is high, but it is easily affected by sulfur poisoning, and the deterioration rate of the NOx conversion rate is large. Conversely, Mg with ΔG of −350 kJ / mol or more has a low initial NOx conversion rate, but is less susceptible to sulfur poisoning, and the NOx conversion rate deterioration rate becomes small. Further, it can be seen that the deterioration rate can be reduced due to the composite effect by using several kinds of constituent elements of the NOx catalyst (Examples 5 to 7). Therefore, in the exhaust gas purification system of the present invention, it is effective that the NOx storage catalyst contains an alkaline earth metal, a rare earth metal and a transition metal having a ΔG of the reaction of the formula (1) larger than −350 kJ / mol. I understand.
[0063]
Furthermore, it is clear from comparison between Example 1 and Comparative Example 2 that it is more effective to reduce the average particle size of the slurry liquid to 4 μm or less than to 5 μm.
[0064]
As mentioned above, although this invention was demonstrated in detail by the preferable Example, this invention is not limited to these Examples, A various deformation | transformation is possible within the range of the indication of this invention.
For example, the major advantage of the purification system of the present invention is that the metal sulfate salt does not remain in the NOx storage catalyst. Conventionally, the metal sulfate salt is gradually but inevitably left in the NOx storage catalyst. In the end, the saturation amount is reached, but this phenomenon can be avoided in the purification system of the present invention.
[0065]
【The invention's effect】
As described above, according to the present invention, since NOx is absorbed and released by the NOx storage catalyst after the exhaust gas is controlled to a desired temperature, sulfur poisoning is reduced as compared with conventionally known exhaust gas purification methods. It is possible to provide an exhaust gas purification system that can reduce and prevent thermal deterioration of the NOx storage catalyst.
[0066]
That is, according to the present invention, the sulfur flowing into the NOx storage catalyst is instantaneously changed from the stoichiometric to rich region when necessary based on the inlet exhaust gas temperature of the NOx storage catalyst predicted by the temperature prediction means installed upstream. Oxides can be released, sulfur poisoning can be reduced, and NOx storage catalysts can be regenerated and used.
In addition, the NOx storage catalyst containing an element that has an ΔG of −350 kJ / mol or more is used to convert sulfur, which is an impurity, into a composite sulfur oxide at a relatively low temperature. In addition to avoiding the generation and accumulation of metal sulfates that cannot be performed, it is possible to prevent thermal deterioration of the NOx storage catalyst by easily releasing the sulfur oxide together with NOx at a low temperature, and as a result, the NOx storage performance can be maintained. Furthermore, by irradiating the NOx storage catalyst with electromagnetic waves, the perovskite complex oxide, which has been a sulfur compound, is selectively heated and excited to further promote the release of the sulfur content.
[Brief description of the drawings]
FIG. 1 is a flowchart showing an example of an exhaust gas purification system of the present invention.
FIG. 2 is a layout view showing an example of an exhaust gas purification system of the present invention.
[Explanation of symbols]
1 Internal combustion engine
2 Three-way catalyst
3 Temperature prediction means
4 Cooling means (cooler)
5 NOx storage catalyst
6 Three-way catalyst

Claims (14)

A system for purifying exhaust gas of a lean burn internal combustion engine in which an air-fuel ratio A / F varies between rich, stoichiometric, and lean,
From the upstream side of the exhaust passage, the exhaust gas temperature predicting means, the exhaust gas cooling means and the NOx storage catalyst are sequentially arranged,
The exhaust gas temperature predicting means detects the exhaust gas temperature as needed, predicts the inlet exhaust gas temperature of the NOx storage catalyst according to the detection result, and transmits an operation signal to the exhaust gas cooling means according to the prediction result. And
The exhaust gas purifying system, wherein the exhaust gas cooling means cools the exhaust gas in accordance with an operation signal from the exhaust gas temperature predicting means. 2. The exhaust gas purification system according to claim 1, wherein when the exhaust gas A / F is lean, an inlet exhaust gas temperature of the NOx storage catalyst is controlled to 300 ° C. or less. 3. The sulfur desorption is performed according to claim 1, wherein when the predicted temperature of the inlet exhaust gas in the NOx storage catalyst exceeds 400 ° C., the A / F of the exhaust gas is adjusted to stoichiometric to rich, thereby performing sulfur desorption. Exhaust gas purification system. The exhaust gas purification system according to claim 3, wherein the predicted temperature of the exhaust gas at the inlet of the NOx storage catalyst exceeds 400 ° C for 1 minute or longer. The exhaust gas purification system according to any one of claims 1 to 4, wherein the exhaust passage has a bypass passage, and the exhaust gas cooling means is installed in the bypass passage. . The exhaust gas purification system according to any one of claims 1 to 5, wherein a three-way catalyst is added upstream of the exhaust gas temperature predicting means. The exhaust gas purification system according to claim 6, wherein the exhaust gas temperature predicting means predicts an inlet exhaust gas temperature of the NOx storage catalyst from an outlet exhaust gas temperature of the three-way catalyst. The exhaust gas purification system according to any one of claims 1 to 7, wherein a three-way catalyst is added downstream of the NOx storage catalyst. The above NOx storage catalyst has the following reaction formula (1):
1 / 2O ₂ + SO ₂ + AO → ASO _{4 (} 1)
(Wherein A represents at least one selected from the group consisting of alkaline earth metals, rare earth metals, and transition metals, provided that A is expressed as a divalent element). The exhaust gas according to any one of claims 1 to 8, comprising an element A having a free energy change value (ΔG) of a composite oxide (ASO ₄ ) of -350 kJ / mol or more. Gas purification system. 10. The exhaust gas purification system according to claim 9, wherein the element A is at least one selected from the group consisting of magnesium, lanthanum, manganese and iron. The NOx storage catalyst has the following general formula (2)
(Ln _1−α A _α ) _1−β BO _{δ (} 2)
(Wherein Ln is at least one element selected from the group consisting of lanthanum, cerium, neodymium and samarium, and A is at least one selected from the group consisting of magnesium, calcium, strontium, barium, sodium, potassium and cesium) Species element, B is at least one element selected from the group consisting of iron, cobalt, nickel and manganese, δ indicates the amount of oxygen satisfying the valence of each element, 0 ≦ α ≦ 1, 0 <β The exhaust gas purification system according to any one of claims 1 to 10, further comprising a composite oxide represented by <1. The exhaust gas purification system according to any one of claims 1 to 11, wherein the NOx storage catalyst is heated using electromagnetic waves. The exhaust gas purification system according to any one of claims 1 to 12, wherein the NOx storage catalyst contains at least one kind of noble metal selected from the group consisting of platinum, rhodium and palladium. The exhaust gas purification system according to any one of claims 1 to 13, wherein an average particle size of each component powder contained in the NOx storage catalyst is 4 µm or less.

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