JP2019030848A - Exhaust emission control device - Google Patents

Abstract

To provide an exhaust emission control device indicating high exhaust emission control performance in a wide temperature region.SOLUTION: A NOx adsorbent adsorbs NOx at less than 300°C and desorbs 80% or more of the adsorbed NOx at 300°C or more and 500°C or less in an exhaust emission control device including the NOx adsorbent at a front stage of an exhaust gas purifying catalyst.SELECTED DRAWING: Figure 3

Description

  The present invention relates to an exhaust gas purification apparatus.

  Exhaust gas from gasoline vehicles, diesel vehicles, ships, factories, etc. contains NOx, which is a harmful substance, and it is necessary to purify NOx in order to suppress NOx emissions. In particular, when the exhaust gas contains oxygen in excess of the stoichiometric amount, an exhaust gas purification device capable of purifying NOx under an oxygen-excess atmosphere is required. For example, for a gasoline vehicle, a lean burn engine that makes the air-fuel ratio lean is drawing attention. Here, the air-fuel ratio represents the ratio of air to fuel in the gas. Lean burn engine exhaust gas is difficult to purify NOx with a three-way catalyst that has been conventionally used to purify the exhaust gas of a stoichiometric engine. For this reason, exhaust gas purification catalysts for lean burn engines have been studied.

  Patent Document 1 describes that NOx can be reduced and removed in an oxidizing atmosphere when a catalyst containing rhodium and aluminum and containing one or more of gallium and zirconium is used.

  Further, the exhaust gas temperature from the engine is not constant and fluctuates during operation. Therefore, there is a demand for an exhaust gas purification device that can maintain NOx purification performance even when the exhaust gas temperature is lowered.

  In Patent Literature 2, NOx emission when the exhaust gas temperature is lowered is suppressed by providing a NOx adsorbent in the front stage of the exhaust gas purification catalyst.

JP 2001-170488 A JP 2010-112292 A

  The exhaust gas purifying catalyst described in Patent Document 1 can purify NOx in an oxygen-excess atmosphere, but it can be seen that the activation temperature of most catalysts is 300 ° C. or higher. Therefore, when the exhaust gas temperature falls below 300 ° C., NOx cannot be purified by the catalyst.

  At this time, if the NOx adsorbent described in Patent Document 2 is used, NOx when the exhaust gas temperature is lowered can be adsorbed. However, since NOx is desorbed at 300 ° C. or lower, NOx adsorbed up to 300 ° C. or higher at which the catalyst is activated cannot be retained.

  An object of the present invention is to maintain high exhaust gas purification performance in a wide temperature range by providing a NOx adsorbent that desorbs NOx at 300 ° C. or higher in the previous stage of the catalyst.

  In order to achieve the above object, the present invention provides an exhaust gas purification apparatus in which a NOx adsorbent is provided in front of an exhaust gas purification catalyst, wherein the NOx adsorbent adsorbs NOx at less than 300 ° C., and 80% or more of the adsorbed NOx. Is desorbed at 300 ° C. or more and 500 ° C. or less.

  According to the present invention, it is possible to purify harmful substances, particularly NOx, with high efficiency regardless of the exhaust gas temperature.

It is a figure showing an example of the exhaust gas purification system of an engine. It is a figure showing the state of the support | carrier and active component on a NOx adsorbent surface. It is the graph which showed the NOx desorption rate in 300 degreeC or more of NOx adsorption material. It is the graph which showed the NOx adsorption amount of the NOx adsorbent.

  The exhaust gas purifying apparatus according to the present embodiment can be used for purifying exhaust gas discharged from gasoline cars, diesel cars, ships, factories, etc., particularly NOx. As an example, FIG. 1 shows an exhaust gas purification system in a gasoline vehicle. The exhaust gas purification system of FIG. 1 includes an engine 1 and an exhaust gas purification device 10 provided downstream of the engine 1. The exhaust gas purification apparatus 10 includes an exhaust gas passage 7 that carries the exhaust gas 2 from the engine 1 to the outside, a NOx adsorbent 3 provided in a part of the exhaust gas passage 7, and an exhaust gas purification catalyst 4.

  (1) When the temperature of the exhaust gas 2 flowing through the exhaust gas passage 7 is higher than the temperature at which the exhaust gas purification catalyst 4 is activated, the exhaust gas purification device 10 purifies the NOx contained in the exhaust gas 2 by the exhaust gas purification catalyst 4 to the outside. Discharge. However, (2) when the temperature of the exhaust gas 2 is lower than the temperature at which the exhaust gas purification catalyst 4 is activated, NOx contained in the exhaust gas 2 cannot be purified by the exhaust gas purification catalyst 4, so that NOx is directly discharged to the outside. The In the present embodiment, this problem is solved by providing the NOx adsorbent 3 above the exhaust gas purification catalyst 4.

  In the case of (2) above, if NOx is adsorbed by the NOx adsorbent 3 instead of the exhaust gas purification catalyst 4, the amount of NOx discharged to the outside can be suppressed. Here, when the temperature of the exhaust gas 2 rises, the exhaust gas purification device 10 desorbs the NOx adsorbed by the NOx adsorbent 3, carries it to the downstream exhaust gas purification catalyst 4, and purifies it by the exhaust gas purification catalyst 4. In order to realize this system, the temperature of the exhaust gas 2 when desorbing the NOx adsorbed by the NOx adsorbent 3 needs to be higher than the temperature at which the exhaust gas purification catalyst 4 is activated.

As the exhaust gas purification catalyst 4, when the engine 1 is a stoichiometric engine, a three-way catalyst is used for exhaust gas purification, and the engine 1 is an air / fuel ratio (14.7 or more) that is leaner than the theoretical air / fuel ratio. In the case of a lean burn engine that operates at a low temperature, an exhaust gas purification catalyst for a lean burn engine is used for exhaust gas purification. For purification of exhaust gas from a lean burn engine, a catalyst containing any of Mn, Fe, Co, Cu, Ga, In, Sn, Rh, Pd, Ag, Ir, and Pt as an active component, and Al ₂ O as a carrier ₃ , a catalyst containing any of SiO ₂ , TiO ₂ , ZrO ₂ , and MFI can be used.

  However, many of the exhaust gas purification catalysts 4 have an activation temperature of 300 ° C. or higher. Therefore, the NOx adsorbing material 3 needs to adsorb NOx at a temperature lower than 300 ° C. and to desorb most of the adsorbed NOx after reaching 300 ° C. or higher. Further, when the temperature is 500 ° C. or higher, the reducing agent hydrocarbons and the like burn, so even if NOx is desorbed from the NOx adsorbent 3, the exhaust gas purification catalyst 4 cannot purify NOx.

  At this time, the ratio of the desorption amount at 300 ° C. or higher to the total NOx desorption amount is preferably 80% or higher. Here, the ratio of the desorption amount at 300 ° C. or more to the total NOx desorption amount is the ratio of the NOx amount desorbed at 300 ° C. or more to the NOx amount desorbed at 150 ° C. or more and 500 ° C. or less. is there.

  FIG. 2 is a conceptual diagram showing the structure of the NOx adsorbent 3 in the present invention.

The NOx adsorbent 3 may be one in which at least one adsorbing component 6 selected from cobalt (Co) and manganese (Mn) is supported on a carrier 5 containing cerium oxide (CeO ₂ ). When Co or Mn is supported, oxidation of NO contained in NOx to NO ₂ is promoted. Since NO ₂ is desorbed at a higher temperature than NO, the ratio of NOx desorbed at a higher temperature can be increased as the ratio of NO ₂ is increased. Since Co and Mn have the same effect of promoting oxidation to NO ₂ , the NOx adsorbent 3 only needs to have either Co or Mn.

  Further, praseodymium (Pr) may be further supported as the adsorbing component 6. When Pr is supported, the NOx adsorption force becomes strong, so the amount of NOx adsorption can be increased. Pr has an effect different from Co and Mn.

The amount of adsorbed component is preferably in the range of 5 to 20 mol% with respect to CeO ₂ contained in the carrier 5. (Mole% here represents the content ratio of each component in terms of mole. For example, the loading amount of the B component with respect to the A component is 5 mol%. Regardless of this, it means that A is 100 and B is supported at a ratio of 5 in terms of mole.)
When the amount of adsorbed component is less than 5 mol%, the effect of adding the adsorbed component is not sufficiently exhibited, and the NOx desorption ratio at 300 ° C. or higher may be reduced or the NOx adsorbed amount may be reduced. Even if it is more than 20 mol%, the surface exposure amount of the adsorbing component 6 may be reduced, or the other adsorbing component may be covered with the adsorbing component, and the NOx adsorption performance may not be sufficiently developed.

Further, even in a part of the NOx adsorbing material 3, if there are portions where the adsorbed component amount is 5 to 20 mol% with respect to CeO ₂ contained in the carrier 5, the above effect can be efficiently expressed. For example, the following cases (1) and (2) can be considered as examples of the form of the NOx adsorbent 3. (1) There is plate-like or granular CeO ₂ as a layer not containing the adsorbing component 6, and the carrier 5 containing the adsorbing component 6 is present thereon. (2) The carrier 5 containing the adsorbing component 6 is present on the surface of the granular CeO ₂ not containing the adsorbing component 6. In any case of (1) and (2), it is preferable that there is a portion where the adsorbing component 6 is 5 to 20 mol% with respect to CeO ₂ contained in the carrier 5 containing the adsorbing component 6. .

Adsorbate 6 are preferably in contact with the carrier 5 containing CeO _2, state adsorbed component 6 to the surface of the support 5 containing CeO ₂ is provided is preferable.

As the adsorbing component 6, metals such as Fe, Ni, Cu, La, Nd, and Gd can be used in addition to Co, Mn, and Pr. Fe, Ni, and the like promote the oxidation of NO to NO ₂ , so that the adsorbed NOx can be maintained up to a high temperature. In addition, since La, Nd, etc. have strong NOx adsorption power, the amount of NOx adsorption can be increased.

In addition to being a NOx adsorption point, the carrier 5 is considered to play a role of enhancing the dispersibility of the adsorption component 6. The support 5 contains CeO _2, and the specific surface area of CeO ₂ is preferably 50 to 300 m ² / g for NO adsorption performance. Also preferred the particle size of CeO ₂ to NO adsorption performance When 0.5 to 15 m.

The support 5 is most preferably CeO _2, but other metal oxides such as CoO, NiO, CuO, MgO, hydrotalcite, Al ₂ O ₃ , ZrO ₂ , TiO ₂ , SiO ₂ , composite oxides, etc. It can also be used.

  The shape of the NOx adsorbent 3 can be applied in various shapes depending on the application. It can be applied in the form of pellets, plates, powders, honeycombs, etc. The honeycomb-shaped NOx adsorbent 3 is obtained by coating an adsorbent powder carrying various components on a honeycomb structure made of various materials such as cordierite and stainless steel. In this case, the adsorbing component 6 may be carried after the carrier 5 is coated on a substrate such as a honeycomb structure, or the substrate 5 may be coated with the carrier 5 carrying the adsorbing component 6.

  As the preparation method of the NOx adsorbent 3, any of physical preparation methods such as impregnation method, kneading method, coprecipitation method, sol-gel method, ion exchange method, vapor deposition method, and preparation method using chemical reaction can be applied. As starting materials for the adsorbing component 6, various compounds such as nitric acid compounds, acetic acid compounds, complex compounds, hydroxides, carbonic acid compounds, organic compounds, metals and metal oxides can be used.

  EXAMPLES Hereinafter, although this invention is demonstrated with a specific example, this invention is not restrict | limited by these Examples.

After impregnating CeO ₂ powder (manufactured by the first rare element) with a Co nitrate solution as an adsorbing component, drying was performed at 120 ° C., followed by baking at 600 ° C. for 1 hour. The obtained powder was press-molded and then crushed and sized to 0.5 mm to 1 mm. Thus, an adsorbent A containing 10 mol% of Co in terms of element with respect to CeO ₂ was obtained.

The obtained adsorbent was subjected to a NOx adsorption performance test under the following conditions. A 3 mL capacity adsorbent was packed into a quartz glass reaction tube. This reaction tube was introduced into an electric furnace, and the heating was controlled so that the gas temperature introduced into the reaction tube was 150 to 500 ° C. In the reaction tube, model gas was introduced at a space velocity of 60000 h ⁻¹ assuming exhaust gas when the automobile engine is performing lean burn operation. The composition of the model gas was NOx: 150 ppm, O ₂ : 10%, N ₂ : residual.

First, as a pretreatment, N ₂ was passed through the adsorbent at 200 ° C. for 1 hour, and then model gas was introduced at 150 ° C. The NOx adsorption amount was measured while maintaining the model gas temperature at 150 ° C. until the NOx concentration at the adsorbent outlet became constant at 150 ppm. Next, the temperature was raised to 500 ° C. at 10 ° C./min, and the NOx desorption amount was measured. The total amount of NOx desorbed at 150 ° C. or more and 500 ° C. or less is defined as the NOx total desorption amount. At this time, the NOx desorption ratio at 300 ° C. or higher was calculated by the following equation.

NOx desorption ratio (%) above 300 ° C
= NOx desorption amount (mol / L) above 300 ° C / NOx total desorption amount (mol / L)
× 100 ... Formula (1)

Except that a Mn nitrate solution was used instead of the Co nitrate solution as an impregnation component, an adsorbent B containing 10 mol% of Mn in terms of element with respect to CeO ₂ was obtained by the same operation as Example 1. The obtained adsorbent B was subjected to a NOx adsorption performance test in the same manner as in Example 1.

An adsorbent containing 10 mol% Co and 10 mol% Co in terms of element with respect to CeO ₂ by the same operation as in Example 1 except that a Ni nitrate Pr solution was used in addition to the Co nitrate solution as the impregnation component. C was obtained. The obtained adsorbent C was subjected to a NOx adsorption performance test in the same manner as in Example 1.

[Comparative Example 1]
An adsorbent D containing only CeO ₂ was obtained by the same operation as in Example 1 except that the impregnation component was not used. A NOx adsorption performance test was performed on the obtained adsorbent D in the same manner as in Example 1.

[Comparative Example 2]
Except that a Ni nitrate Pr solution was used instead of the Co nitrate solution as an impregnation component, an adsorbent E containing 10 mol% Pr in terms of element with respect to CeO ₂ was obtained in the same manner as in Example 1. A NOx adsorption performance test was performed on the obtained adsorbent E in the same manner as in Example 1.

[Test results]
The results of the NOx desorption ratios at 300 ° C. or higher for the adsorbents according to Examples 1 to 3 and the adsorbents according to Comparative Examples 1 and 2 are shown in Table 1 and FIG. The adsorbents according to Examples 1 to 3 have a higher NOx desorption ratio at 300 ° C. or higher than the adsorbents according to Comparative Examples 1 and 2, and both are 80% or higher. The reason for this is considered that the addition of Co and Mn promoted the oxidation of NO to NO ₂ , and the adsorbed NOx could be maintained at a high temperature.

  In Comparative Example 2 in which Pr was added, the numerical value of the NOx desorption ratio (%) at 300 ° C. or higher is low. This is because Pr has an effect of increasing the amount of NOx adsorbed because the NOx adsorbing force becomes strong, but has no effect of holding the adsorbed NOx to a high temperature.

The results of the NOx adsorption amount of the adsorbent according to Example 3 and the adsorbents according to Comparative Examples 1 and 2 are shown in Table 2 and FIG. Compared with the adsorbent D to which no adsorbing component is added, the NOx adsorption amount decreases in the adsorbent E to which Pr is added as a single adsorbing component. The reason for this is thought to be that the NOx adsorption points on the CeO ₂ surface, which is the carrier, were coated by the addition of the adsorption component. On the other hand, the NOx adsorption amount increased in Example adsorbent 3 to which Co and Pr were simultaneously added as adsorbing components. This is presumably because NO ₂ oxidized from NO by Co was adsorbed not only to the adsorption point on CeO ₂ but also to Pr having a strong adsorption force.

1 Engine 2 Exhaust gas 3 NOx adsorbent 4 Exhaust gas purification catalyst 5 Carrier 6 Adsorbed component 7 Exhaust gas passage 10 Exhaust gas purification device

Claims (13)

An exhaust gas passage for transporting exhaust gas from the engine to the outside,
A NOx adsorbent provided in the exhaust gas passage;
An exhaust gas purification catalyst provided in the exhaust gas passage downstream of the NOx adsorbent;
The exhaust gas purification apparatus, wherein the NOx adsorbent adsorbs NOx at less than 300 ° C and desorbs 80% or more of the adsorbed NOx at 300 ° C or more and 500 ° C or less. The exhaust gas purification apparatus according to claim 1,
The exhaust gas purifying device is activated when the temperature of the exhaust gas reaches 300 ° C or higher to purify NOx. The exhaust gas purification apparatus according to claim 1 or 2,
The exhaust gas purifying apparatus, wherein the exhaust gas contains oxygen in excess of the stoichiometric amount. The exhaust gas purification apparatus according to any one of claims 1 to 3,
The NOx adsorbent is composed of a carrier containing CeO ₂ and a first metal provided on the carrier,
The exhaust gas purifying apparatus, wherein the first metal contains Co or Mn. In the exhaust gas purification apparatus according to claim 4,
The exhaust gas purification apparatus, wherein the NOx adsorbent further contains Pr. In the exhaust gas purification apparatus according to claim 4,
Exhaust gas purifying apparatus, wherein the amount of the first metal contained in the NOx adsorbent is from 5 to 20 mol% with respect to CeO _2. The exhaust gas purification apparatus according to claim 5,
The amount of the first metal contained in the NOx adsorbent is from 5 to 20 mol% with respect to CeO _2,
Exhaust gas purifying apparatus, wherein the amount of Pr contained in the NOx adsorbent is from 5 to 20 mol% with respect to CeO _2. An exhaust gas passage for transporting exhaust gas from the engine to the outside,
A NOx adsorbent provided in the exhaust gas passage;
An exhaust gas purification catalyst provided in the exhaust gas passage downstream of the NOx adsorbent;
The NOx adsorbent is composed of a carrier containing CeO ₂ and a first metal provided on the carrier,
The exhaust gas purifying catalyst, wherein the first metal contains Co or Mn. The exhaust gas purification apparatus according to claim 8,
The exhaust gas purifying device is activated when the temperature of the exhaust gas reaches 300 ° C or higher to purify NOx. The exhaust gas purification apparatus according to claim 8 or 9,
The exhaust gas purifying apparatus, wherein the exhaust gas contains oxygen in excess of the stoichiometric amount. The exhaust gas purification apparatus according to claim 10,
The exhaust gas purification apparatus, wherein the NOx adsorbent further contains Pr. The exhaust gas purification apparatus according to claim 10,
Exhaust gas purifying apparatus, wherein the amount of the first metal contained in the NOx adsorbent is from 5 to 20 mol% with respect to CeO _2. The exhaust gas purification apparatus according to claim 11,
The amount of the first metal contained in the NOx adsorbent is from 5 to 20 mol% with respect to CeO _2,
Exhaust gas purifying apparatus, wherein the amount of Pr contained in the NOx adsorbent is from 5 to 20 mol% with respect to CeO _2.

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