WO2015125417A1 - Catalytic device for exhaust gas purification, and method for exhaust gas purification - Google Patents

Abstract

This method includes: disposing, in the exhaust gas passage (2) of an engine, an NO_x trap (3) that includes an NO_x adsorption/occlusion material which is capable of adsorbing NO_x when the temperature of the exhaust gas is 100-200°C and which is capable of occluding NO_x when the temperature of the exhaust gas is 350-450°C; and disposing an NO_x occlusion/reduction catalyst (4) downstream from the NO_x trap in the flow direction of the exhaust gas, the catalyst (4) being capable of occluding NO_x when the air/fuel ratio of the exhaust gas is lean and being capable of reducing NO_x when the air/fuel ratio of the exhaust gas is rich.

Description

Exhaust gas purification catalyst device and exhaust gas purification method

The present invention relates to an exhaust gas purification catalyst device and an exhaust gas purification method.

Conventionally, it is known to use a lean NO _x trap catalyst (LNT catalyst) for purifying nitrogen oxides (NO _x ) contained in engine exhaust gas, particularly in lean burn gasoline engines and diesel engines. . The LNT catalyst occludes NO _x when the air-fuel ratio of the exhaust gas by _the NO _x storage material is lean. By rich purge modulating the air-fuel ratio of the engine to rich, NO _x emissions from _the NO _x storage materials, and reduction of unburnt gas and catalyst metal due to NO _x is carried out. It is known to use an alkali metal or an alkaline earth metal as the NO _x storage material. However, the alkali metal forms a glass phase at the cordierite grain boundary forming the catalyst carrier and lowers the strength of the carrier, so there is actually no such problem, for example, barium (Ba), strontium (Sr). Alkaline earth metals such as magnesium (Mg) are generally employed.

For example, Patent Document 1 discloses a lean NO _x storage catalyst that includes a CePr composite oxide and alumina in a lower layer, an Rh-doped CeZr material and alumina in an upper layer, and includes Rh, Pt, and NO _x storage materials in both layers. Is disclosed. In Patent Document 1, Ba, K, Sr, and Mg are exemplified as the NO _x storage material.

Patent Document 2 discloses an exhaust gas purification system in which a NO _x adsorbent is disposed on the upstream side in the exhaust gas flow direction and a NO _x purification catalyst is disposed on the downstream side. Patent Document 2 exemplifies ceria supported on alumina instead of the alkaline earth metal as the NO _x adsorbent.

Patent Document 3 includes a lower layer containing a NO _x adsorbent, an upper layer containing a catalyst for NO _x reduction, and an isolation layer that is provided between them and delays diffusion of NO _x from the lower layer to the upper layer. A catalyst is disclosed. In Patent Document 3, the isolation layer is made of a porous inorganic material, and this is provided between the upper layer and the lower layer, so that reaction gas diffusion is likely to occur in the pores of the isolation layer and plays a role of a so-called buffer. . Therefore, when the exhaust gas conditions change rapidly, when the NO _x adsorbed on the lower NO _x adsorbent is desorbed and diffuses into the upper layer containing the NO _x reduction catalyst, this isolation layer causes the NO _x to be absorbed. The diffusion rate of _x is moderate, and high NO _x purification performance can be maintained even in a low temperature range where the reduction reaction rate is not large.

In Patent Document 4, CePr composite oxide and alumina are included in the lower layer, ZrW composite oxide is included in at least one of the lower layer and the upper layer, catalyst metals such as Pt, Rh, and Pd are included in both layers, and Ba, Sr, and the like. A lean NO _x storage catalyst containing the NO _x storage material is disclosed.

Patent Document 5, containing _the NO _x adsorption materials such as ceria in the lower layer, NO _x storage material is an alkaline earth metal in the upper layer, and the catalyst is disclosed containing the catalyst metal is a noble metal such as Pt or Rh Yes.

JP 2006-43541 A JP 2003-343252 A JP 2004-16931 A JP 2010-188224 A JP 2009-226349 A

In Patent Document 1, as shown in FIGS. 8 and 10 of the document, although the NO _x purification performance is exhibited when the exhaust gas temperature is in a temperature range of 250 ° C. or higher, it is lower than that. it is not clear _the nO _x purification performance at temperature. In general, it is known that alkaline earth metals used as NO _x storage materials do not exhibit NO _x storage effects when the exhaust gas temperature of about 100 ° C. to 150 ° C. is low. In the catalyst of Patent Document 1, NO _x cannot be occluded in the low temperature range where the exhaust gas temperature is about 100 ° C. to 150 ° C. immediately after the engine is started, and as a result, most of the NO _x that is an exhaust gas component is externally released. May be released.

Patent Document 2 also shows that, as Example 1 of this document, when a ceria material is used as the NO _x adsorbent, the NO _x purification rate at 250 ° C. is high (see FIG. 5 and FIG. 5 of Patent Document 2). FIG. 6), the NO _x purification performance at lower temperatures is not clear.

Patent Document 3 also shows that the catalyst of the example of this document can obtain a high NO _x purification rate under the condition of 200 ° C., but the NO _x purification performance at a lower temperature is not clear.

Patent Document 4, by containing ZrW composite oxide, NO occlusion of the NO _x by _x storage material, and is set to be able to promote the purification of the released NO _x, FIG. 2 and FIG. 3 of the document As shown in FIG. 2, the NO _x purification effect is extremely low in a low temperature range lower than 200 ° C. That is, similarly to Patent Document 1, it is considered that NO _x cannot be occluded in a low temperature range, and as a result, much of the NO _x that is an exhaust gas component may be released to the outside.

In Patent Document 5, ceria capable of adsorbing NO _x even in a low temperature range of about 200 ° C. is contained in the catalyst as a NO _x adsorbent. However, as shown in FIG. _the NO _x adsorption amount at low low-temperature region than decreases.

The present invention has been made in view of the above problems, and its object is to efficiently purify NO _x in a wide exhaust gas temperature range from a low temperature range of about 100 ° C. to a high temperature range of about 450 ° C. Is to be able to do.

To achieve the above object, the present invention is an exhaust gas passage of an engine, and _the NO _x adsorption / storage material exhaust gas temperature is capable of adsorbing NO _x even at a low temperature range of about 100 ° C., under a lean atmosphere Thus, a NO _x storage reduction catalyst capable of storing NO _x and capable of reducing and purifying NO _{x in} a rich atmosphere is provided to purify NO _x .

An exhaust gas purification method according to the present invention is an exhaust gas purification method for purifying exhaust gas containing NO _x discharged from an engine capable of lean combustion,
The exhaust gas passage of the engine, NO _x adsorbs NO _x when and exhaust gas temperature can adsorb NO _x is 350 ° C. or higher 450 ° C. or less when the exhaust gas temperature is below 200 ° C. 100 ° C. or higher is capable of occluding / a NO _x trap containing storage material, the air-fuel ratio of the exhaust gas is a reducible NO _{x storage-and-reduction} catalyst for NO _x when the air-fuel ratio of and the exhaust gas can occlude NO _x is rich when the lean Placing step;
A step of adsorbing NO _x in the exhaust gas to the NO _x trap immediately after the engine is started until the exhaust gas temperature becomes 200 ° C. or less;
Until less than the exhaust gas temperature exceeds the 200 ° C. 350 ° C., a step of maintaining the air-fuel ratio of the exhaust gas lean, allowed to absorb _the NO _x storage reduction catalyst NO _x in the exhaust gas,
Storing the NO _x in the exhaust gas in the NO _x trap and the NO _x storage reduction catalyst while the exhaust gas temperature is 350 ° C. or higher and 450 ° C. or lower;
After the main injection in which fuel is injected into the combustion chamber of the engine near the top dead center of the compression stroke, the fuel is injected and supplied in the expansion stroke or the exhaust stroke, and then injected to include HC in the exhaust gas. And a step of reducing the NO _x desorbed from the NO _x storage-reduction catalyst by the NO _x storage-reduction catalyst and reducing NO _x by the NO _x storage-reduction catalyst and releasing it. To do.

According to the exhaust gas purification method according to the present invention, since the exhaust gas temperature is provided NO _x trap containing adsorbable _the NO _x adsorption / storage material of NO _x in an exhaust gas passage of the engine when the following 200 ° C. 100 ° C. or higher , even at a temperature range where _the nO _x storage material can not absorb nO _x is an alkaline-earth can trap nO _x in the exhaust gas, it is possible to prevent the outside nO _x is released. When the exhaust gas temperature rises immediately after the engine starts and exceeds about 200 ° C., NO _x can be stored in the NO _x storage reduction catalyst when the air-fuel ratio of the exhaust gas is lean, and the air-fuel ratio of the exhaust gas further increases by controlling so rich, it is possible to reduce and purify NO _x trap and _the NO _x storage desorbed from the reduced catalyst was NO _x. In this way, it is possible to efficiently purify NO _x in a wide exhaust gas temperature range.

In this method, zirconia (ZrO ₂ ) can be preferably used as the NO _x adsorption / occlusion material, and barium (Ba) is preferably used as the NO _x occlusion material contained in the NO _x storage reduction catalyst. Strontium (Sr) and magnesium (Mg) can be used.

The NO _x adsorption / occlusion material preferably contains zirconia in which yttrium (Y) is dissolved. By dissolving yttrium in zirconia, the basicity of zirconia increases, and the power to attract NO _x on the zirconia surface increases, and as a result, NO _x can be adsorbed with high efficiency.

In a preferred embodiment, the NO _x storage reduction catalyst is disposed downstream of the NO _x trap in the exhaust gas flow direction. Thereby, the NOx slipping the NO _x trap is reduced and purified is occluded in the NO _{x storage-and-reduction} catalysts, which is advantageous in NOx emissions prevention to the outside.

As described above, when the NO _x storage reduction catalyst is disposed downstream of the NO _x trap in the exhaust gas flow direction, the NO _x storage reduction catalyst includes a lower layer containing a Ce-containing oxide and an upper layer containing activated alumina. It is preferable that the upper layer and the lower layer include a NO _x storage material containing Ba, Sr, and Mg and a catalyst metal containing Rh and Pt.

Rh contained in the NO _x storage reduction catalyst as a catalyst metal contributes to the steam reforming reaction, and H ₂ is generated by this reaction. Therefore, reduction and purification of NO _x can be promoted. Therefore, it is possible to obtain _the NO _x reduction purification performance high catalyst by the inclusion of Rh. Also, since the alkaline earth metal to be _the NO _x storage material is contained, can occlude NO _x in the exhaust gas in the lean atmosphere, also eliminated the NO _x occluding under a rich atmosphere, the Rh etc. Thus, reduction and purification can be performed efficiently.

In another preferred embodiment, the NOx trap is formed between a lower layer formed on a support substrate and including activated alumina and a Ce-containing oxide, and an upper layer including activated alumina formed on the lower layer. At least one catalyst metal of Pt (platinum) and Rh (rhodium) and a NO _x storage material are supported on the lower layer, the upper layer, and the intermediate layer. In this case, the lower layer and the upper layer constitute the NOx storage reduction catalyst. Since the intermediate layer also contains the catalyst metal and the NO _x storage material, it functions as a NO _x storage reduction catalyst.

In yet another preferred embodiment, a lower layer formed on a support substrate, containing an inorganic oxide as the NOx adsorption / occlusion material, further containing a Ce-containing oxide, and formed on the lower layer and activated alumina And a laminated catalyst in which at least one catalyst metal of Pt and Rh and a NO _x storage material are supported on the lower layer and the upper layer are disposed in the exhaust gas passage. In this case, the lower layer of the stacked catalyst constitutes the NOx trap, and the upper layer constitutes the NOx occlusion reduction catalyst. Since the lower layer also contains the catalyst metal and the NO _x storage material, it functions as a NO _x storage reduction catalyst.

An exhaust gas purification catalyst device according to the present invention is an exhaust gas purification catalyst device that purifies exhaust gas containing NO _x discharged from an engine capable of lean combustion,
NO provided in the exhaust gas passage of the engine and capable of adsorbing NO _x when the exhaust gas temperature is 100 ° C. or higher and 200 ° C. or lower and capable of storing NO _x when the exhaust gas temperature is 350 ° C. or higher and 450 ° C. or lower. a NO _x trap containing _x adsorption / occlusion material;
Provided in an exhaust gas passage of an engine, and a reducible NO _{x storage-and-reduction} catalyst for NO _x when the air-fuel ratio of the exhaust gas air-fuel ratio of and the exhaust gas can occlude NO _x when the lean rich It is characterized by.

According to the exhaust gas purifying catalyst device according to the present invention, provided in an exhaust gas passage of the NO _x trap exhaust gas temperature comprises _the NO _x adsorption / storage material capable adsorbing NO _x at below 200 ° C. 100 ° C. or higher engine because it is, to prevent the _the nO _x storage material is an alkaline earth is even at a temperature range which can not occlude nO _x can trap nO _x in the exhaust gas, outside nO _x is released it can. When the exhaust gas temperature rises immediately after the engine starts and exceeds about 200 ° C., NO _x can be stored in the NO _x storage reduction catalyst when the air-fuel ratio of the exhaust gas is lean, and the air-fuel ratio of the exhaust gas further increases when the rich, it is possible to reduce and purify NO _x trap and _the NO _x storage-reduction catalyst from the desorbed NO _x. Thus, a wide range of exhaust gas temperature range, it is possible to purify efficiently NO _x.

As the NO _x adsorption / occlusion material, zirconia can be suitably used, and it is particularly preferable to contain zirconia in which yttrium is dissolved. The _the NO _x storage material contained in _the NO _x storage-reduction catalyst, can be used suitably Ba, Sr and Mg.

The NO _x adsorption / occlusion material preferably carries at least one catalyst metal of platinum (Pt), palladium (Pd), and rhodium (Rh). In this way, also it is possible to impart catalytic activity for exhaust gas purification in NO _x trap contains _the NO _x adsorption / storage material, it is possible to improve the exhaust gas purification performance.

The NO _x trap preferably includes Pt-supported alumina obtained by supporting Pt on activated alumina (Al ₂ O ₃ ). Since activated alumina has a high specific surface area, the dispersibility of Pt can be improved, and the contact property between Pt, which is a catalytic metal, and exhaust gas is improved, which is advantageous for purification of exhaust gas.

In a preferred embodiment relating to the exhaust gas purification catalyst layer, the NO _x storage reduction catalyst is disposed downstream of the NO _x trap in the exhaust gas flow direction. Thereby, the NOx slipping the NO _x trap is reduced and purified is occluded in the NO _{x storage-and-reduction} catalysts, which is advantageous in NOx emissions prevention to the outside.

As described above, when the NO _x storage reduction catalyst is disposed downstream of the NO _x trap in the exhaust gas flow direction, the NO _x storage reduction catalyst includes a lower layer containing a Ce-containing oxide and an upper layer containing activated alumina. It is preferable that the upper layer and the lower layer include a NO _x storage material containing Ba, Sr, and Mg and a catalyst metal containing Rh and Pt.

In another preferred embodiment relating to an exhaust gas purification catalyst device, the NOx trap is formed on a support substrate and includes a lower layer including activated alumina and a Ce-containing oxide, and activated alumina formed on the lower layer. And the lower layer, the upper layer, and the intermediate layer support at least one catalyst metal of Pt and Rh and a NO _x storage material. In this case, the lower layer and the upper layer constitute the NOx storage reduction catalyst. Since the intermediate layer also contains the catalyst metal and the NO _x storage material, it functions as a NO _x storage reduction catalyst.

In this embodiment, it is preferable that the intermediate layer contains activated alumina. Activated alumina has a high specific surface area, and can improve the dispersibility of _the NO _x storage material, the storage of the NO _x by _the NO _x storage material can be efficiently performed.

When zirconia is employed as the NO _x adsorption / occlusion material, the mass ratio of zirconia to the total mass of zirconia and alumina in the intermediate layer is preferably 33% by mass or more. With such a weight ratio, it is possible to efficiently perform purification of the NO _x and the storage of the NO _x by _the NO _x storage material carried on the adsorption and the alumina of the NO _x acts good balance with zirconia .

Moreover, Ba, Sr, and Mg can be suitably used as the NO _x storage material.

In still another preferred embodiment related to the exhaust gas purification catalyst device, a lower layer formed on a support substrate, containing an inorganic oxide as the NOx adsorption / occlusion material, and further containing a Ce-containing oxide, And a laminated catalyst in which at least one catalyst metal of Pt and Rh and an NO _x storage material are supported on the lower layer and the upper layer are disposed in the exhaust gas passage. . In this case, the lower layer of the stacked catalyst constitutes the NOx trap, and the upper layer constitutes the NOx occlusion reduction catalyst. Since the lower layer also contains the catalyst metal and the NO _x storage material, it functions as a NO _x storage reduction catalyst.

In this embodiment, the lower layer preferably includes activated alumina on which at least one catalyst metal of Pt and Rh and a NO _x storage material are supported.

In this way, alumina has a high specific surface area, can improve the dispersibility of the NO _x storage material and the catalyst metal, and can efficiently store and purify NO _x by the NO _x storage material.

When zirconia is used as the NO _x adsorption / occlusion material, the mass ratio of zirconia to the total mass of zirconia and alumina in the lower layer is preferably 5% by mass or more, and 35% by mass or more and 65% by mass or less. Is more preferable. With such a weight ratio, it is possible to efficiently adsorb of the NO _x with zirconia.

According to the exhaust gas purification catalyst device and the exhaust gas purification method of the present invention, NO _x can be efficiently purified in a wide exhaust gas temperature range from a low temperature range of about 100 ° C. to a high temperature range of about 450 ° C. It becomes possible.

It is a schematic diagram which shows the structure of the exhaust gas purification catalyst apparatus which concerns on Embodiment 1 of this invention. 2 is a cross-sectional view showing a configuration of a NO _x trap of the exhaust gas purification catalyst device according to Embodiment 1. FIG. 1 is a cross-sectional view showing a configuration of an LNT catalyst of an exhaust gas purification catalyst device according to Embodiment 1. FIG. Is a graph showing the relationship between the desorption quantity and temperature after ZrO ₂ of the NO _x trap. Is a graph illustrating the traps form of the NO _x for ZrO ₂ Is a graph showing the relationship between the NO _x trap amount and temperature of Pt supported ZrO _2. 6 is a graph showing the NO _x trap rates of Examples 1 to 3 of Embodiment 1 and Comparative Example 1. FIG. 5 is a perspective view and a partially enlarged view showing an exhaust gas purification catalyst according to Embodiment 2. FIG. 4 is a cross-sectional view showing a catalyst layer configuration of an exhaust gas purification catalyst according to Embodiment 2. FIG. 6 is a graph showing the NO _x trap rates of Examples 4 and 5 (Embodiment 2) and Comparative Example 2. FIG. 6 is a graph showing NO _x trap rates of Examples 6 to 8 (Embodiment 2) and Comparative Example 2. FIG. 6 is a cross-sectional view showing a catalyst layer configuration of an exhaust gas purification catalyst according to Embodiment 3. FIG. Is a graph showing the NO _x trap rate of the Examples and Comparative Examples of Embodiment 3.

Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings. The following description of the preferred embodiments is merely exemplary in nature and is not intended to limit the invention, its application, or its application.

<Embodiment 1>
(Configuration of exhaust gas purification catalyst device)
Figure 1 shows the structure of Embodiment 1 exhaust gas purifying catalyst device according to 1 of the present invention, reference numeral 2 denotes an exhaust gas passage of an engine, reference numeral 3 NO _x trap, reference numeral 4 in _the NO _x storage-reduction catalyst A lean NO _x trap catalyst (LNT catalyst) is shown. Specifically, as shown in FIG. 1, the exhaust gas purification catalyst device 1 includes a NO _x trap 3 disposed in an exhaust gas passage 2 through which exhaust gas discharged from an engine combustion chamber (not shown) passes, And an LNT catalyst 4 disposed on the exhaust gas downstream side (right side in FIG. 1).

FIG. 2 is a view showing a part of a cross section perpendicular to the exhaust gas flow direction of the NO _x trap 3. As shown in FIG. 2, the NO _x trap 3 has zirconia (ZrO ₂ ) as a NO x adsorption / occlusion material (inorganic oxide having NO x adsorption / occlusion ability) on the exhaust gas passage wall of the honeycomb carrier 30 made of cordierite. ) Including the ZrO ₂ layer 31 is provided, and the space inside the ZrO ₂ layer 31 is an exhaust gas passage. The honeycomb carrier 30 has a hexagonal cell honeycomb structure whose cell cross-sectional shape is a hexagon. In FIG. 2, for simplification of the drawing, the ZrO ₂ layer 31 is drawn only in one cell, but the ZrO ₂ layer 31 is formed in all the cells. The ZrO ₂ layer 31 may be composed of only ZrO _2, but at least one catalyst metal of Pt, Pd and Rh is supported on ZrO ₂ in order to impart catalytic performance to the NO _x trap 3. May be. The ZrO ₂ layer 31 may contain activated alumina (Al ₂ O ₃ ), and Pt may be supported on the alumina. Further, as ZrO ₂ , Y-containing ZrO ₂ in which yttrium (Y) is dissolved may be used. In this way, basic ZrO ₂ is increased, thereby improving the NO _x trapping property in ZrO _2. The function of ZrO ₂ contained in the NO _x trap 3 will be described later.

FIG. 3 is a diagram showing a part of a cross section perpendicular to the exhaust gas flow direction of the LNT catalyst 4. As shown in FIG. 3, the LNT catalyst 4 is configured by laminating two catalyst layers on the exhaust gas passage wall of a honeycomb carrier 40 made of cordierite. Note that the honeycomb carrier 40 in the LNT catalyst 4 also has a hexagonal cell honeycomb structure in which the cell cross-sectional shape is a hexagonal shape. Also in FIG. 3, the catalyst layer is applied to only one cell in order to simplify the drawing. As shown, the catalyst layer is formed in all cells. Specifically, in the LNT catalyst 4, the lower catalyst layer 41 formed so as to be in contact with the exhaust gas passage wall includes activated alumina and a Ce-containing oxide, for example, ceria, and is formed on the lower catalyst layer 41. The upper catalyst layer 42 contains activated alumina, and the lower catalyst layer 41 and the upper catalyst layer 42 are impregnated and supported with catalyst metals Pt and Rh and NO _x storage materials Ba, Sr and Mg. With this configuration, LNT catalyst 4, it is possible to absorb NO _x by the _the NO _x storage material in the exhaust gas temperature is lean atmosphere above about 250 ° C., the reduction of the NO _x by the catalyst metal in the rich atmosphere Can act. Note that the honeycomb carrier 40 in the LNT catalyst 4 also has a hexagonal cell honeycomb structure whose cell cross-sectional shape is a hexagon. Also in FIG. 3, for simplification of the drawing, the catalyst layer is drawn only in one cell, but the catalyst layer is formed in all cells.

(About ZrO ₂ )
Next, the function of ZrO ₂ contained in the NO _x trap, particularly the NO _x adsorption capacity and NO _x storage capacity will be described.

First, the performance of trapping NO _x of ZrO ₂ at each temperature when the exhaust gas temperature was gradually raised from 100 ° C. was examined. Therefore, for ZrO _2, it was measured NO trap amount and desorption amount at each temperature of ZrO ₂ using a well-known method Atsushi Nobori spectroscopy (TPD method). For this measurement, first, the ZrO ₂ layer formed on the support was pretreated at 600 ° C. for 10 minutes in a He atmosphere. Thereafter, a gas containing NO was introduced. Specifically, the composition of the evaluation gas is 1000 ppm for NO, 10% for O ₂ , and the balance for N ₂ . After the evaluation gas temperature was maintained at 100 ° C. for 30 minutes, the gas temperature was increased at 17 ° C./min. The measurement results are shown in FIG.

As shown in FIG. 4, when the gas temperature is 100 ° C., the NO outflow concentration is lower than 1000 ppm, which is the inflow concentration of NO, for about 20 minutes, so ZrO ₂ traps NO. I understand. Thereafter, the gas temperature was raised, and NO desorption from ZrO ₂ peaked at about 200 ° C. Thereafter, as the gas temperature increased, the NO desorption amount decreased, and again the NO exhaust concentration became lower than the inflow concentration of 1000 ppm, which continued to decrease to about 350 ° C. Thereafter, the desorption amount of NO increased again, and NO desorption from ZrO ₂ peaked at about 500 ° C. From this result, it was suggested that ZrO ₂ has two desorption peaks in the temperature rising process, and there are a weak trap form in which NO is desorbed at a low temperature and a strong trap form that keeps up to a high temperature. .

Next, in order to examine what the two types of NO traps are, the in-situ Fourier transform infrared spectroscopy (FT-IR) which is a well-known method is used. The state of NO _x trapped in ZrO ₂ at 0 ° C. was examined. Specifically, first, the ZrO ₂ layer formed on the carrier was pretreated at 600 ° C. for 10 minutes in a He atmosphere. Thereafter, the composition of the evaluation gas is set to 1000 ppm for NO, 2.5% for O ₂ and N ₂ for the balance, and 100 ° C., 150 ° C., 200 ° C., 250 ° C., 300 ° C., 350 ° C., 400 ° C. and 450 ° C. FT-IR was measured as the evaluation temperature. The measurement results are shown in FIG.

As shown in FIG. 5, in the 100 ° C. ~ 300 ° C., it wavenumber show a peak at about 1200 cm ^-1 to about 1330 cm ^-1, which is characteristic peak in NO _2. That is, at 100 ° C. to 300 ° C., it is considered that NO is trapped (adsorbed) in the form of NO _{2 on} the surface of ZrO _{2 in} the presence of oxygen. In addition, since the peak intensity is strongest at 100 ° C. and the peak becomes weaker as the temperature rises, in this adsorption, NO may be present on the surface of ZrO _{2 as} the temperature rises due to the influence of thermal vibration. It will be difficult.

On the other hand, in the 300 ° C. ~ 450 ° C., wavenumber peak is observed at a position of about 1250 cm ^-1 to about 1590 cm ^-1, which is NO ₃ ^- is the peak peculiar to. That is, at 300 ° C. to 450 ° C., NO is considered to be chemically trapped (occluded) in the form of NO ₃ ⁻ with respect to Zr.

These results, ZrO ₂ is a NO at low temperature range trapped by adsorption as NO _2, together with the influence of the adsorption decreases as the temperature rises, the NO NO ₃ ^- when trapped by chemical occlusion of the Conceivable. From the graph of FIG. 5, in particular, ZrO ₂ can adsorb NO with high efficiency in the temperature range of about 100 ° C. to 200 ° C., and absorbs NO with high efficiency in the temperature range of about 350 ° C. to 450 ° C. It is considered possible.

Next, in order also carrying Pt as a catalytic metal ZrO ₂ to examine whether similar tendency to the above can be seen, the Pt-supported ZrO ₂ was formed on the honeycomb support, it is attached to a model gas flow reactor The NO concentration at the gas inlet and outlet of the carrier was detected, and the NO trap amount of Pt-supported ZrO ₂ was measured based on the detected concentrations. As the honeycomb carrier, a cordierite hexagonal cell honeycomb carrier (diameter: 25.4 mm, length: 50 mm) having a cell wall thickness of 4.5 mil and a carrier capacity of 25 ml with 400 cells per square inch is used. The amount of ZrO ₂ was 100 g / L (supported amount per liter of support), and 0.5 g / L of Pt was supported on the ZrO ₂ . Method for supporting Pt to ZrO _2, and will be described later Pt supported ZrO ₂ of a method of forming the carrier.

Before this measurement, first, a lean atmosphere and a rich atmosphere were switched every minute, and a pretreatment was performed at 600 ° C. for 10 minutes. Thereafter, model gas containing NO was introduced. The gas temperatures at the time of measurement were 100 ° C., 150 ° C., 200 ° C., 250 ° C., 300 ° C., 350 ° C. and 400 ° C., and after stabilizing in a rich atmosphere at each temperature, switching to a lean atmosphere for 300 seconds. The amount of NO trap was measured. The gas composition of the lean atmosphere is 220 ppm NO, 400 ppm HC, 0.15% CO, 10% O ₂ , 6% CO ₂ , and H ₂ O 10% with the balance being N ₂ . On the other hand, the gas composition of the rich atmosphere is 220 ppm NO, 3400 ppm HC, 1.0% CO, 0.5% O ₂ , 6% CO ₂ , H ₂ O is 10%, the remainder being _{N 2.} The results of measurement under the above conditions are shown in FIG.

As shown in FIG. 6, the NO trap amount shows peaks at 200 ° C. and 350 ° C. This substantially correlates with the above two test results. Similarly to ZrO ₂ , NO is physically adsorbed to Pt-supported ZrO ₂ and is chemically adsorbed in the low temperature range. It is thought to be occluded.

(NO _x trap and LNT catalyst production method)
Next, a method for manufacturing the NO _x trap and the LNT catalyst in the catalyst device according to the present embodiment will be described. First, the NO _x trap, in particular a method for manufacturing of the NO _x trap containing Pt supported ZrO ₂ will be described.

Pt-supported ZrO ₂ can be obtained by subjecting ZrO ₂ to an evaporation to dryness method using a dinitrodiamine Pt nitric acid solution. Specifically, ion exchange water is added to the ZrO ₂ particle material to form a slurry, which is sufficiently stirred by a stirrer or the like. Subsequently, a predetermined amount of dinitrodiamine Pt nitric acid solution is dropped into the slurry while stirring, and sufficiently stirred. Thereafter, stirring is continued while heating to completely evaporate water. After evaporation, Pt-supported ZrO ₂ is obtained by baking in the atmosphere at about 500 ° C. for 2 hours.

Subsequently, a binder such as zirconyl nitrate and ion exchange water are added to the obtained Pt-supported ZrO ₂ and mixed to form a slurry. The slurry is coated on a support, dried at about 150 ° C., and then fired at about 500 ° C. for 2 hours, whereby Pt-supported ZrO ₂ can be formed on the support and an NO _x trap can be obtained. Can do.

Next, a method for producing an LNT catalyst will be described. Here, in particular, a method for producing an LNT catalyst including the catalyst layer having the above two-layer structure will be described.

First, an Al ₂ O ₃ particle material and a CeO ₂ particle material, which are constituent components of the lower catalyst layer, are mixed, and a binder such as zirconyl nitrate and water are added to the mixture and stirred to form a slurry. Instead of CeO ₂ , for example, a complex oxide (CePrO _x ) in the form of praseodymium or the like may be used. This slurry is coated on the cell walls of the honeycomb carrier, dried at about 150 ° C., and then fired at about 500 ° C. for 2 hours. Thereby, the precursor layer of a lower catalyst layer can be obtained.

Subsequently, a precursor layer of the upper catalyst layer is formed using activated alumina. Here, it may carry a catalytic metal Rh such as activated alumina, in this case, when the NO _x occluded in the lower layer is eliminated, since it is released into the exhaust gas passage through the upper layer, By supporting a large amount of catalyst metal on the upper layer, NO _x can be efficiently purified. In this case, Rh is supported on Al ₂ O ₃ by performing evaporation to dryness using an aqueous rhodium nitrate solution on the Al ₂ O ₃ particulate material that is a constituent component of the upper catalyst layer. A binder such as zirconyl nitrate and water are added to the Rh-supported Al ₂ O ₃ thus obtained, and the mixture is stirred to form a slurry. This slurry is coated on the precursor layer of the lower catalyst layer, dried at about 150 ° C., and then fired at about 500 ° C. for 2 hours. Thereby, the precursor layer of an upper catalyst layer can be obtained.

Thereafter, the precursor layers of the lower catalyst layer and the upper catalyst layer are impregnated with a mixed solution of Pt and Rh, which are catalyst metals, and Ba, Sr, and Mg, which are NO _x storage materials. After impregnation, the honeycomb carrier provided with the precursor layer is dried and fired. As a result, an LNT catalyst including a lower catalyst layer and an upper catalyst layer in which the catalyst layer and the NO _x storage material are impregnated and supported on the precursor layer is obtained. At this time, an aqueous solution of the alkaline earth metal acetate or nitrate is used as the NO _x storage material. In addition, drying can be performed, for example, by holding at a temperature of about 100 ° C. to 250 ° C. for a predetermined time in an air atmosphere, and baking is performed, for example, at a temperature of about 400 ° C. to 600 ° C. for several hours in an air atmosphere. Can be done by.

The NO _x trap obtained as described above is provided on the upstream side of the exhaust gas passage, and the LNT catalyst obtained as described above is provided on the downstream side thereof, whereby the exhaust gas purification according to this embodiment is performed. A catalytic device can be obtained.

(Exhaust gas purification method)
Next, a method for purifying exhaust gas using the exhaust gas purification catalyst device will be described. In this method, as described above, provided the NO _x trap upstream of the exhaust gas passage, also provided LNT catalyst downstream thereof, immediately after starting the engine, the exhaust gas temperature is 200 ° C. or less until, adsorbing NO _x in the exhaust gas to ZrO ₂ of the NO _x trap. Thereafter, while the exhaust gas temperature exceeds 200 ° C. and below 350 ° C., the air-fuel ratio of the exhaust gas is maintained lean, and NO _x in the exhaust gas is stored in the LNT catalyst. Thereafter, while the exhaust gas temperature is 350 ° C. or higher and 450 ° C. or lower, the NO _x trap ZrO ₂ and the LNT catalyst store NO _x in the exhaust gas. Then, after the main injection in which fuel is injected into the combustion chamber of the engine near the top dead center of the compression stroke, the fuel is injected and supplied in the expansion stroke or the exhaust stroke (rich purge) to include HC in the exhaust gas. Te, the air-fuel ratio of the exhaust gas is controlled to be in a rich state, the LNT catalyst, reduces and discharges purify NO _x desorbed from the NO _x trap and LNT catalyst.

According to such an exhaust gas purification method, even when the exhaust gas temperature immediately after engine startup is in a low temperature range, NO _x can be trapped by ZrO ₂ of the NO _x trap, so NO _x is released to the outside. Can be prevented. Further, according to the exhaust gas temperature increases, to improve the activity of the LNT catalyst, release NO _x, which have been the trapped at this timing de, in the lean atmosphere is occluded into LNT catalyst is reduced and purified in a rich atmosphere . Further the exhaust gas temperature is increased, it is possible to absorb NO _x in the NO _x trap. In this manner, NO _x in the exhaust gas can be efficiently reduced and purified in a wide temperature range of the exhaust gas temperature, and the amount of NO _x released to the outside can be reduced.
[Example]
Hereinafter, embodiments for explaining the exhaust gas purifying catalyst device according to the present invention in detail will be described. In this example, the amount of NO _x trap of the exhaust gas purification catalyst device in which the LNT catalyst is disposed downstream of the NO _x trap containing Pt-supported ZrO ₂ was measured. Further, as a comparative example, the NO _x trap rate of an exhaust gas purification catalyst device in which an LNT catalyst is disposed downstream of a NO _x trap containing Pt-supported Al ₂ O ₃ was measured.

In Example 1, a cordierite hexagonal cell honeycomb carrier (diameter 25.4 mm, length 50 mm) having a cell wall thickness of 4.5 mil and a carrier capacity of 25 ml with 400 cells per square inch was used. According to the above method, Pt-supported ZrO ₂ was provided to obtain a NO _x trap. Here, the amount of ZrO ₂ was 100 g / L (supported amount per 1 L of carrier), and 0.5 g / L of Pt was supported on the ZrO ₂ . Further, an LNT catalyst composed of an upper catalyst layer and a lower catalyst layer was obtained on a cordierite hexagonal cell honeycomb carrier having the same size as described above according to the above method. Here, the lower catalyst layer contains 200 g / L of Al ₂ O ₃ and 70 g / L of CePrO _x (CeO: Pr ₂ O ₃ = 9: 1, mass ratio). In which 0.4 g / L of Rh is supported on 50 g / L of Al ₂ O ₃ . Further, a mixed solution containing 4.3 g / L Pt, 0.1 g / L Rh, 25 g / L Ba, 10 g / L Sr and 5 g / L Mg is used for the lower catalyst layer and the upper catalyst layer. In use, the catalytic metal and the NO _x storage material are impregnated and supported.

Also, Example 2 differs from Example 1 only in the components contained in the NO _x trap. Specifically, in Example 2, a mixture in which Pt is supported on a mixture in which ZrO ₂ and Al ₂ O ₃ are mixed at a mass ratio of 1: 1 is used. The amount of the mixture is 100 g / L, and the supported amount of Pt is 0.5 g / L.

Further, Example 3 differs from Example 1 only in that ZrO ₂ containing yttrium (Y) is used as ZrO ₂ on which Pt is supported in the NO _x trap. Y-containing ZrO ₂ was prepared so that ZrO ₂ : Y ₂ O ₃ = 9: 1 (mass ratio).

On the other hand, Comparative Example 1 differs from Example 1 only in using Al ₂ O ₃ carrying Pt, not ZrO ₂ carrying Pt in the NO _x trap.

The NO _x traps and LNT catalysts of Examples 1 to 3 and Comparative Example 1 were attached to a model gas flow reactor, and a model gas containing NO _x components was flowed to measure the NO _x trap rate. The NO _x trap and the LNT catalyst were attached to the model gas flow reactor so that the LNT catalyst was located downstream of the NO _x trap in the exhaust gas flow direction. These catalysts were previously subjected to an aging treatment at 750 ° C. for 24 hours in an atmosphere of 2% O _{2 and} 10% H ₂ O. Thereafter, the temperature is maintained at 100 ° C. in a rich atmosphere, the model gas is introduced into the catalyst, and after 5 minutes, the model gas temperature is increased from 100 ° C. at 30 ° C./min and maintained at 200 ° C. or 300 ° C. switching the model gas in a lean atmosphere, the NOx concentration at the outlet of the LNT catalyst was measured for 120 seconds, based on the introduction amount of the measured values and NO (220 ppm), were calculated NO _x trap rate of 120 seconds.

The composition of the model gas in the lean atmosphere is 220 ppm NO, 400 ppm HC, 0.15% CO, 10% O ₂ , 6% CO ₂ , 10% H ₂ O, and the balance N ₂ is there. The composition of the rich atmosphere model gas is 220 ppm NO, 3400 ppm HC, 1.0% CO, 0.5% O ₂ , 6% CO ₂ , 10% H ₂ O and the balance N ₂ is there. The gas flow rate was 30 L / min (space velocity SV = 72000 h ⁻¹ ). The measurement results of the NO _x trap rate at 200 ° C. and 300 ° C. in Examples 1 to 3 and Comparative Example 1 are shown in FIG.

As shown in FIG. 7, when comparing the NO _x trap rates of the catalyst devices of Examples 1 to 3 and the catalyst device of the comparative example, the model gas temperature of the catalyst devices of Examples 1 to 3 is 200 ° C. and It can be seen that the NO _x trap rate at 300 ° C. is large. This is because in Examples 1 to 3, the NO _x trap contains ZrO ₂ , and as described above, the NO _x trap rate of the entire catalytic device is increased by the NO _x adsorption / storage capacity of ZrO _2. Conceivable. Further, comparing Example 1 and Example 2, it can be seen that Example 2 has a higher NO _x trap rate. This is because alumina has a high specific surface area, so the dispersibility of Pt can be improved, the contact property between Pt, which is a catalytic metal, and NO _x is improved, and oxidation to NO ₂ that is easily adsorbed is promoted. It is thought to be for this purpose. Further, comparing Example 1 and Example 3, it can be seen that Example 3 has a higher NO _x trap rate. This is because in Example 3, Y was contained in ZrO ₂ in the NO _x trap, and the inclusion of Y increased the basicity of ZrO ₂ and increased the ability to attract NO in the exhaust gas. it is conceivable that.

<Embodiment 2>
(Configuration of exhaust gas purification catalyst)
FIG. 8 shows an exhaust gas purification catalyst 11 according to Embodiment 2 of the present invention. The exhaust gas purifying catalyst 11 is disposed in the exhaust gas passage of the engine, and a catalyst layer 13 is formed on the exhaust gas passage wall of a cordierite honeycomb carrier 12. In FIG. 8, the cell cross-sectional shape of the honeycomb carrier 12 is shown as a rectangular shape, but is not limited thereto, and may be a hexagonal hexagonal cell honeycomb structure, for example.

As shown in FIG. 9, the catalyst layer 13 has a three-layer structure. Specifically, a lower layer 13 a containing activated alumina and a Ce-containing oxide is provided on the base material (exhaust gas passage wall) of the honeycomb carrier 12. An intermediate layer 13b containing zirconia (ZrO ₂ ) as a NOx adsorption / occlusion material is provided on the lower layer 13a (exhaust gas passage side). As ZrO ₂ , Y-containing ZrO ₂ in which yttrium (Y) is dissolved may be used. The intermediate layer 13b may contain activated alumina (Al ₂ O ₃ ). On the intermediate layer 13b (exhaust gas passage side), an upper layer 13c containing activated alumina is provided. The lower 13a, the intermediate layer 13b and the upper layer 13c is, Pt and Rh as a catalytic metal, and Ba as _the NO _x storage material, the Sr and Mg are impregnated carrier. The catalyst layer 13 is constituted by these three layers.

Since the intermediate layer 13b contains zirconia as a NOx adsorption / occlusion material, it constitutes a NOx trap. Since the lower layer 13a and the upper layer 13c contain a catalyst metal and a NO _x storage material, they constitute a NO _x storage reduction catalyst. Here, the intermediate layer 13b also contains a catalyst metal and a NO _x storage material, and therefore functions as a NO _x storage reduction catalyst. In particular, NO _x storage material is the alkaline earth metal may be an exhaust gas to efficiently occlude NO _x in lean atmosphere above about 250 ° C., reducing the catalytic metal of the NO _x under a rich atmosphere Act on. By these, NO _x can be reduced and purified.

(Method for producing catalyst layer)
Next, a method for manufacturing the catalyst layer according to Embodiment 2 will be described.

First, the activated alumina constituting the lower layer and the Ce-containing oxide are mixed, and a binder material such as zirconyl nitrate and ion-exchanged water are added to the mixture and mixed to form a slurry. The slurry is coated on a support, dried at about 150 ° C., and then fired at about 500 ° C. for 2 hours, whereby a lower layer precursor layer can be formed on the support. The ceria used here may be in the form of a complex oxide (CePrO _x ) containing praseodymium (Pr) or the like, for example.

Next, a binder material such as zirconyl nitrate and ion-exchanged water are added to zirconia constituting the intermediate layer and mixed to form a slurry. The slurry is coated on the lower precursor layer, dried at about 150 ° C., and then fired at about 500 ° C. for 2 hours, whereby an intermediate layer precursor layer can be formed on the lower precursor layer.

Next, an upper layer is formed using activated alumina. Here, may carry a catalytic metal Rh such as activated alumina, in this case, when the adsorption and occluded NO _x in the lower layer and the intermediate layer is eliminated, the exhaust gas passage through the upper Since it is released, it is possible to efficiently purify NO _x by supporting a large amount of catalyst metal in the upper layer. In this case, Rh is supported on the activated alumina by evaporating to dryness using an aqueous rhodium nitrate solution on the activated alumina constituting the upper layer. Specifically, ion-exchanged water is added to activated alumina to form a slurry, which is sufficiently stirred with a stirrer or the like. Subsequently, a predetermined amount of an aqueous rhodium nitrate solution is dropped into the slurry while stirring, and the mixture is sufficiently stirred. Thereafter, stirring is continued while heating to completely evaporate water. After evaporation, the activated alumina can be loaded with Rh by firing at about 500 ° C. for 2 hours in the air. A binder material such as zirconyl nitrate and ion-exchanged water are added to the Rh-supported alumina thus obtained and mixed to form a slurry. The slurry is coated on the intermediate layer precursor layer, dried at about 150 ° C., and then baked at about 500 ° C. for 2 hours, whereby an upper layer precursor layer can be formed on the intermediate layer precursor layer.

Next, the support on which the lower layer precursor layer, the intermediate layer precursor layer, and the upper layer precursor layer are provided is impregnated with a mixed solution of Pt and Rh, which are catalytic metals, and Ba, Sr, and Mg, which are NO _x storage materials. After impregnation, the honeycomb carrier provided with the precursor layer is dried and fired. As a result, a catalyst including a lower layer, an intermediate layer, and an upper layer in which the catalyst layer and the NO _x storage material are impregnated and supported on the precursor layer is obtained. At this time, an aqueous solution of the alkaline earth metal acetate or nitrate is used as the NO _x storage material. In addition, drying can be performed, for example, by holding at a temperature of about 100 ° C. to 250 ° C. for a predetermined time in an air atmosphere, and baking is performed, for example, at a temperature of about 400 ° C. to 600 ° C. for several hours in an air atmosphere. Can be done by.

(Exhaust gas purification method)
Next, a method for purifying exhaust gas using the exhaust gas purification catalyst device will be described. In this method, first, the catalyst obtained as described above is disposed in the exhaust gas passage of the engine. Then, immediately after the engine is started, until the exhaust gas temperature becomes 200 ° C. or lower, NO _x in the exhaust gas is adsorbed by ZrO ₂ contained in the intermediate layer of the catalyst. Thereafter, while the exhaust gas temperature exceeds 200 ° C. and below 350 ° C., the air-fuel ratio of the exhaust gas is maintained lean, and the NO _x storage material impregnated and supported with NO _x in the exhaust gas is applied to the catalyst layer. Occlude. Thereafter, while the exhaust gas temperature is 350 ° C. or higher and 450 ° C. or lower, the intermediate layer ZrO ₂ and the NO _x storage material impregnated and supported on the catalyst layer are stored with NO _x . Then, as in the first embodiment, the exhaust gas is made to contain a lot of HC by rich purge, so that the air-fuel ratio of the exhaust gas is controlled to be in a rich state, and the ZrO ₂ is controlled by the catalyst metal supported on the catalyst layer. and _the NO _x storage desorbed NO _x from material reduces and purifies to release.

According to such an exhaust gas purification method, as in the first embodiment, NO _x in the exhaust gas can be efficiently reduced and purified in a wide temperature range of the exhaust gas temperature, and the amount of NO _x released to the outside is reduced. be able to.
[Example]
Next, examples and comparative examples of the second embodiment will be described. In the examples, the NO _x trap rate of a three-layered catalyst having an intermediate layer containing ZrO ₂ as described above was measured. In addition, as a comparative example, the NO _x trap rate of a two-layered catalyst not having an intermediate layer containing ZrO ₂ was measured. This will be specifically described below.

In Example 4, a three-layered catalyst composed of a lower layer, an intermediate layer, and an upper layer was obtained on the same hexagonal cell honeycomb carrier as in the example of Embodiment 1 according to the above method. The lower layer contains 100 g / L of Al ₂ O ₃ (supported amount per 1 L of support, the same applies hereinafter) and 70 g / L of CePrO _x (CeO: Pr ₂ O ₃ = 9: 1, mass ratio). , the intermediate layer includes a ZrO ₂ of 100 g / L, the upper layer, contains _Al 2 _{O 3} of 50 g / L, Rh of 0.4 g / L is supported on _Al 2 _{O 3} . The lower, the intermediate layer and the upper layer, and Rh of Pt and 0.1 g / L of 4.3 g / L as a catalyst metal, NO _x as storage material 25g / L Ba, 10g / L of Sr and 5 g / L Mg is impregnated and supported.

Further, Example 5 differs from Example 4 only in that ZrO ₂ containing Y is used as ZrO ₂ of the intermediate layer. Y-containing ZrO ₂ was prepared so that ZrO ₂ : Y ₂ O ₃ = 9: 1 (mass ratio).

On the other hand, Comparative Example 2, as compared with Example 4, without a middle layer, lower layer 200 g / L of _Al 2 _{O 3} and 70 g / L of _{CePrO x (CeO: Pr 2 O} 3 = 9: 1 , Mass ratio).

The catalysts of Examples 4 and 5 and Comparative Example 2 were attached to a model gas flow reactor, and a model gas containing NO _x component was flowed to measure the NO _x trap rate. These catalysts were previously subjected to an aging treatment at 750 ° C. for 24 hours in an atmosphere of 2% O _{2 and} 10% H ₂ O. Thereafter, the temperature is maintained at 100 ° C. in a rich atmosphere, the model gas is introduced into the catalyst, and after 5 minutes, the model gas temperature is increased from 100 ° C. at 30 ° C./min and maintained at 175 ° C. or 300 ° C. The model gas was switched to a lean atmosphere, the NO _x concentration at the catalyst outlet was measured for 120 seconds, and the NO _x trap rate for 120 seconds was calculated based on the measured value and the amount of NO introduced (220 ppm).

[Correction 04.02.2015 based on Rule 91]
Note that the composition and gas flow rate of the model gas in each of the lean atmosphere and the rich atmosphere are the same as those in the first embodiment. The measurement results of the NO _x trap rate at 175 ° C. and 300 ° C. in Examples 4 and 5 and Comparative Example 2 are shown in FIG.

Fig As shown in 10, when comparing the NO _x trap rate of the catalyst and of Comparative Example 2 catalysts of Examples 4 and 5, towards the catalyst of Example 4 and 5, the model gas temperature is 175 ° C. and 300 ° C. It can be seen that the NO _x trap rate is large. This is considered because Examples 4 and 5 have an intermediate layer containing ZrO ₂ , and as described above, the NO _x trap rate as a whole catalyst increased due to the NO _x adsorption / storage capacity of ZrO _2. . Further, comparing Example 4 and Example 5, it can be seen that Example 5 has a higher NO _x trap rate. This is presumably because, in Example 5, ZrO ₂ contained Y, and the inclusion of Y increased the basicity of ZrO ₂ and increased the ability to attract NO in the exhaust gas.

Next, the NO _x trap rate when ZrO ₂ and Al ₂ O ₃ were contained in the intermediate layer of the catalyst was examined. Therefore, according to the above method, the catalyst containing only ZrO ₂ in the intermediate layer as in Example 6 to prepare a catalyst comprising ZrO ₂ and _Al 2 _{O 3} in the intermediate layer as in Example 7,8.

Specifically, the catalyst of Example 6 includes 67 g / L of Al ₂ O ₃ and 70 g / L of CePrO _x (CeO: Pr ₂ O ₃ = 9: 1, mass ratio) in the lower layer, and the intermediate layer. 123 g / L of ZrO ₂ is contained, 50 g / L of Al ₂ O ₃ is contained in the upper layer, and 0.4 g / L of Rh is supported on the Al ₂ O ₃ . The lower, the intermediate layer and the upper layer, and Rh of Pt and 0.1 g / L of 4.3 g / L as a catalyst metal, NO _x as storage material 25g / L Ba, 10g / L of Sr and 5 g / L Mg is impregnated and supported.

Further, Example 7 is different from Example 6 in that the intermediate layer contains 89 g / L of ZrO ₂ and 44 g / L of Al ₂ O ₃ . That is, the mass ratio of ZrO ₂ to the total mass of ZrO ₂ and Al ₂ O ₃ in the intermediate layer was 67% by mass.

Also, Example 8 differs from Example 6 in that the intermediate layer contains 44 g / L ZrO ₂ and 89 g / L alumina. That is, the mass ratio of ZrO ₂ to the total mass of ZrO ₂ and Al ₂ O ₃ in the intermediate layer was 33% by mass.

FIG. 11 shows the results of measuring the NO _x trap rate using Examples 6 to 8 and Comparative Example 2 in the same manner as described above.

As shown in FIG. 11, when comparing the NO _x trap rates of the catalysts of Examples 6 to 8 and the catalyst of Comparative Example 2, the catalysts of Examples 6 to 8 have model gas temperatures of 175 ° C. and 300 ° C. It can be seen that the NO _x trap rate is large. This, as described above, by _the NO _x adsorption / storage capacity of ZrO _2, presumably because NO _x trap rate of the entire catalyst is increased.

Further, comparing Example 6 and Example 7, it can be seen that Example 7 has a higher NO _x trap rate. In Example 7, in the intermediate layer, a mixture of ZrO ₂ and Al ₂ O ₃ is impregnated with the catalyst metal and the NO _x storage material. Thus, Al having a large specific surface area in addition to ZrO ₂ is used. by including ₂ O _3, and improves the dispersibility of _the NO _x storage material, by contact between _the NO _x storage material and NO _x can be improved is considered that NO _x trap rate is increased. Moreover, when Example 6 and Example 8 are compared, it can be seen that Example 8 has a smaller NO _x trap rate. This is presumably because the content ratio of ZrO _{2 in} the intermediate layer was reduced and the NO _x adsorption / occlusion effect of ZrO ₂ was reduced.

<Embodiment 3>
The present embodiment is a modification of the second embodiment, and as shown in FIG. 12, the catalyst layer 13 has a two-layer structure. Specifically, a lower layer 13 d containing zirconia (ZrO ₂ ) as a NOx adsorption / occlusion material and a Ce-containing oxide is provided on the base material (exhaust gas passage wall) of the honeycomb carrier 12. The lower layer 13d may further contain activated alumina (Al ₂ O ₃ ). Further, as ZrO ₂ , Y-containing ZrO ₂ in which yttrium (Y) is dissolved may be used. On the lower layer 13d (exhaust gas passage side), an upper layer 13e containing activated alumina is provided. The lower layer 13d and the upper layer 13e, Pt and Rh as the catalyst metal and NO _x Ba as storage material, the Sr and Mg are impregnated carrier. The catalyst layer 13 is constituted by these two layers.

Since the lower layer 13d contains zirconia as a NOx adsorption / occlusion material, it constitutes a NOx trap. Since the upper layer 13e contains the catalyst metal and the NO _x storage material, the NO _x storage reduction catalyst is configured. Since the lower layer 13d also contains the catalyst metal and the NO _x storage material, it has NO _x storage reduction catalytic ability. In particular, NO _x storage material is the alkaline earth metal may be an exhaust gas absorbs NO _x in lean atmosphere above about 250 ° C., the catalyst metal acts on the reduction of the NO _x under a rich atmosphere To do. By these, NO _x can be reduced and purified.

(Method for producing catalyst layer)
Next, the manufacturing method of the catalyst layer of this Embodiment 3 is demonstrated.

First, zirconia and ceria constituting the lower layer are mixed, and a binder material such as zirconyl nitrate and ion-exchanged water are added to the mixture and mixed to form a slurry. The slurry is coated on a support, dried at about 150 ° C., and then fired at about 500 ° C. for 2 hours, whereby a lower layer precursor layer can be formed on the support. The ceria used here may be in the form of a complex oxide (CePrO _x ) containing praseodymium (Pr) or the like, for example.

Next, an upper layer is formed using activated alumina. Here, may carry a catalytic metal Rh such as activated alumina, in this case, when the lower layer by adsorption and occluded NO _x is desorbed, is the discharged into the exhaust gas passage through the upper Therefore, by keeping carries many catalytic metal layer, it can be purified efficiently NO _x. In this case, Rh is supported on the activated alumina by evaporating to dryness using an aqueous rhodium nitrate solution on the activated alumina constituting the upper layer. Specifically, ion-exchanged water is added to activated alumina to form a slurry, which is sufficiently stirred with a stirrer or the like. Subsequently, a predetermined amount of an aqueous rhodium nitrate solution is dropped into the slurry while stirring, and the mixture is sufficiently stirred. Thereafter, stirring is continued while heating to completely evaporate water. After evaporation, the activated alumina can be loaded with Rh by firing at about 500 ° C. for 2 hours in the air. A binder material such as zirconyl nitrate and ion-exchanged water are added to the Rh-supported alumina thus obtained and mixed to form a slurry. This slurry is coated on the lower precursor layer, dried at about 150 ° C., and then fired at about 500 ° C. for 2 hours, whereby an upper precursor layer can be formed on the lower precursor layer.

Next, the carrier on which the lower precursor layer and the upper precursor layer are provided is impregnated with a mixed solution of Pt and Rh, which are catalytic metals, and Ba, Sr, and Mg, which are NO _x storage materials. After impregnation, the honeycomb carrier provided with the precursor layer is dried and fired. Thereby, a catalyst including a lower layer and an upper layer in which the catalyst layer and the NO _x storage material are impregnated and supported on the precursor layer is obtained. At this time, an aqueous solution of the alkaline earth metal acetate or nitrate is used as the NO _x storage material. In addition, drying can be performed, for example, by holding at a temperature of about 100 ° C. to 250 ° C. for a predetermined time in an air atmosphere, and baking is performed, for example, at a temperature of about 400 ° C. to 600 ° C. for several hours in an air atmosphere. Can be done by.

(Exhaust gas purification method)
Next, a method for purifying exhaust gas using the exhaust gas purification catalyst device will be described. In this method, first, the catalyst obtained as described above is disposed in the exhaust gas passage of the engine. Then, immediately after the engine is started, until the exhaust gas temperature becomes 200 ° C. or lower, NO _x in the exhaust gas is adsorbed by ZrO ₂ contained in the lower layer of the catalyst. Thereafter, while the exhaust gas temperature exceeds 200 ° C. and below 350 ° C., the air-fuel ratio of the exhaust gas is maintained lean, and the NO _x storage material impregnated and supported with NO _x in the exhaust gas is applied to the catalyst layer. Occlude. Thereafter, while the exhaust gas temperature is between 350 ° C. and 450 ° C., NO _x is occluded in ZrO ₂ and the NO _x occlusion material impregnated and supported in the catalyst layer. Then, after the main injection in which fuel is injected into the combustion chamber of the engine near the top dead center of the compression stroke, the fuel is injected and supplied in the expansion stroke or the exhaust stroke (rich purge) to include HC in the exhaust gas. Te, the air-fuel ratio of the exhaust gas is controlled to be in a rich state, by the catalyst metal supported on the catalyst layer, release and reduction purification of NO _x desorbed from ZrO ₂ and NO _x storage material.

According to such an exhaust gas purification method, as in the second embodiment, NO _x in the exhaust gas can be efficiently reduced and purified in a wide temperature range of the exhaust gas temperature, and the amount of NO _x released to the outside is reduced. be able to.
[Example]
Next, examples and comparative examples of the third embodiment will be described. In Examples, the NO _x trap rate of a two-layered catalyst having a lower layer containing ZrO ₂ and an upper layer containing activated alumina was measured. As a comparative example, the NO _x trap rate of a two-layered catalyst that does not contain ZrO ₂ in the lower layer was measured. This will be specifically described below.

In Examples, a catalyst having a two-layer structure composed of a lower layer and an upper layer was obtained on the same hexagonal cell honeycomb carrier as in the example of Embodiment 1 according to the above method. The lower layer contained 70 g / L of CePrO _x (CeO: Pr ₂ O ₃ = 9: 1, mass ratio), ZrO ₂ and Al ₂ O ₃ (supported amount per 1 L of support). Here, catalysts of Examples 9-1 to 9-6 having different mass ratios of ZrO ₂ and Al ₂ O ₃ contained in the lower layer were prepared. Table 1 shows the mass ratio of ZrO ₂ and Al ₂ O ₃ contained in the lower layer in Examples 9-1 to 9-6. In the lower layer, ZrO ₂ and Al ₂ O ₃ are contained in a total of 200 g / L.

The upper layer in the catalysts of Examples 9-1 to 9-6, contains _Al 2 _{O 3} of 50 g / L, Rh of 0.4 g / L is supported on _Al 2 _{O 3.} Further, the lower and upper, and Rh of Pt and 0.1 g / L of 4.3 g / L as a catalyst metal, NO _x as storage material 25 g / L Ba, Mg and Sr and 5 g / L of 10 g / L Is impregnated.

On the other hand, the catalyst of the comparative example is different from the catalyst of each of the above examples in that the lower layer does not contain ZrO ₂ as shown in Table 1.

Further, a catalyst using ZrO ₂ containing Y as ZrO ₂ contained in the lower layer was produced. Specifically, the catalysts of Examples 10-1 to 10-6 in which the mass ratio of Y-containing ZrO ₂ and Al ₂ O ₃ in the lower layer is the same as that of Examples 9-1 to 9-6, respectively. Was made. Y-containing ZrO ₂ was prepared so that ZrO ₂ : Y ₂ O ₃ = 9: 1 (mass ratio).

Each catalyst of each of the above Examples and Comparative Examples was attached to a model gas flow reactor, and a model gas containing a NO _x component was flowed to measure the NO _x trap rate. These catalysts were previously subjected to an aging treatment at 750 ° C. for 24 hours in an atmosphere of 2% O _{2 and} 10% H ₂ O. Thereafter, the temperature is maintained at 100 ° C. in a rich atmosphere, the model gas is introduced into the catalyst, and after 5 minutes, the model gas temperature is increased from 100 ° C. at 30 ° C./min and maintained at 175 ° C. or 300 ° C. The model gas was switched to a lean atmosphere, the NO _x concentration at the catalyst outlet was measured for 120 seconds, and the NO _x trap rate for 120 seconds was calculated based on the measured value and the amount of NO introduced (220 ppm).

Note that the composition and gas flow rate of the model gas in each of the lean atmosphere and the rich atmosphere are the same as those in the first embodiment. FIG. 13 shows the measurement results of the NO _x trap rates at 175 ° C. and 300 ° C. of Examples 9-1 to 9-6, 10-1 to 10-6 and the comparative example.

As shown in FIG. 13, the catalyst of each example (that is, when ZrO ₂ / Al ₂ O ₃ is 5/95 to 100/0) and the catalyst of the comparative example (that is, ZrO ₂ / Al ₂ O ₃ are When the NO _x trap rate is compared with the case of 0/100, it can be seen that the NO _x trap rate is higher when the model gas temperatures are 175 ° C. and 300 ° C. in the catalyst of the example. This is presumably because the catalyst of each example contained ZrO ₂ in the lower layer, and as described above, the NO _x trap rate as a whole catalyst increased due to the NO _x adsorption / storage capacity of ZrO ₂ .

Further, when Examples 9-1 to 9-6 containing ZrO ₂ in the lower layer were compared, the mass ratio of ZrO ₂ and Al ₂ O ₃ in the lower layer was changed from 5:95 (Example 9-1) to 50:50. As the amount of ZrO ₂ was increased up to (Example 9-4), that is, the NO _x trap rate increased. Further, as the mass ratio of ZrO ₂ and Al ₂ O ₃ in the lower layer was changed from 50:50 (Example 9-4) to 95: 5 (Example 9-6), that is, as the amount of ZrO ₂ was increased, NO _x trap rate was reduced. Thus, when the mass ratio of ZrO ₂ to Al ₂ O ₃ was 50:50 (Example 9-4), the highest NO _x trap rate could be obtained. This is considered to be because the NO _x adsorption / storage capacity of ZrO ₂ and the improvement of the dispersibility of the NO _x storage material due to the high specific surface area of Al ₂ O ₃ act in a balanced manner.

Further, when Examples 10-1 to 10-6 (Zr—Y—O) using Y-containing ZrO ₂ and Examples 9-1 to 9-6 (ZrO ₂ ) were compared, the results at 175 ° C. were compared. The NO _x trap rate was almost the same, but the NO _x trap rate at 300 ° C. was higher in Examples 10-1 to 10-6 using Y-containing ZrO ₂ than in Examples 9-1 to 9-6. The result was high. This is presumably because the inclusion of Y increases the basicity of ZrO ₂ and increases the ability to attract NO in the exhaust gas.

1 Exhaust gas purification catalyst device 2 Exhaust gas passage 3 NO _x trap 4 Lean NO _x trap catalyst (LNT catalyst = NO _x storage reduction catalyst)
12 honeycomb carrier 13 catalyst layer 13a lower layer 13b intermediate layer 13c upper layer 13d lower layer 13e upper layer 30, 40 honeycomb carrier 31 ZrO ₂ layer 41 lower catalyst layer 42 upper catalyst layer

Claims (20)

1. An exhaust gas purification method for purifying exhaust gas containing NO _x discharged from an engine capable of lean combustion,
   The exhaust gas passage of the engine, the exhaust gas temperature is possible occlude NO _x when and exhaust gas temperature can adsorb NO _x is 350 ° C. or higher 450 ° C. or less when: 200 ° C. 100 ° C. or higher NO _x and NO _x trap containing adsorbent / occluding material, and the air-fuel ratio can reduce the NO _x to NO _x when the air-fuel ratio of the storable in and the exhaust gas is rich when is lean NO _{x storage-and-reduction} catalyst in the exhaust gas Placing a step;
   Between immediately after the engine start to the exhaust gas temperature is 200 ° C. or less, a step of adsorbing NO _x in the exhaust gas to the NO _x trap,
   Between the to less than the exhaust gas temperature exceeds the 200 ° C. 350 ° C., a step of maintaining the air-fuel ratio of the exhaust gas lean, allowed to absorb NO _x in the exhaust gas to the NO _{x storage-and-reduction} catalysts,
   Wherein between the exhaust gas temperature is 350 ° C. or higher 450 ° C. or less, and step of storing the NO _x in the exhaust gas to the NO _x trap and NO _x storage-reduction catalyst,
   After the main injection in which fuel is injected into the combustion chamber of the engine near the top dead center of the compression stroke, the fuel is injected and supplied in the expansion stroke or the exhaust stroke, and then injected to include HC in the exhaust gas. and controls so that the air-fuel ratio becomes rich state, it and a step of releasing the _the NO _x storage the reductive catalytic NO _x trap and _the NO _x storage from the reduced catalyst desorbed _the NO _x reduction purification to An exhaust gas purification method characterized by the above.
2. In claim 1,
   The NO _x storage reduction catalyst is disposed downstream of the NO _x trap in the exhaust gas flow direction.
3. In claim 1,
   The NOx trap is provided in an intermediate layer formed between a lower layer formed on a support substrate and containing activated alumina and a Ce-containing oxide, and an upper layer containing activated alumina formed on the lower layer,
   At least one catalyst metal of Pt and Rh and NO _x storage material are supported on the lower layer, the upper layer, and the intermediate layer,
   It said lower layer and said upper layer constitutes the NOx storage reduction catalyst functions as a NOx storage reduction catalyst because also contains the _the NO _x storage material and the catalytic metal the intermediate layer.
4. In claim 1,
   In the step of disposing the NOx trap and the NOx occlusion reduction catalyst in the exhaust gas passage, a lower layer formed on a carrier base material, containing an inorganic oxide as the NOx adsorption / occlusion material, and further containing a Ce-containing oxide, A laminated catalyst formed on the lower layer and containing activated alumina, wherein the lower layer and the upper layer carry at least one catalyst metal of Pt and Rh and a NO _x storage material. Placed in the aisle,
   Wherein the lower layer of the multilayer catalyst constitutes the NOx trap, the upper layer constitutes the NOx storage reduction catalyst functions as a NOx storage reduction catalyst from the lower also contains the _the NO _x storage material and the catalytic metal.
5. In claim 1,
   Zirconia is used as the NOx adsorption / occlusion material.
6. In claim 1,
   As the NOx storage material of the NO _{x storage-and-reduction} catalyst, Ba, use Sr and Mg.
7. An exhaust gas purification catalyst device that purifies exhaust gas containing NO _x discharged from a lean-combustible engine,
   Provided in an exhaust gas passage of the engine, it is possible to occlude NO _x when the exhaust gas temperature is below 450 ° C. possible and exhaust gas temperature is 350 ° C. or higher adsorbing NO _x at below 200 ° C. 100 ° C. or higher and NO _x trap containing _the NO _x adsorption / storage material,
   An NO _x storage reduction catalyst provided in the exhaust gas passage of the engine, capable of storing NO _x when the air-fuel ratio of the exhaust gas is lean, and capable of reducing NO _x when the air-fuel ratio of the exhaust gas is rich An exhaust gas purifying catalyst device comprising:
8. In claim 7,
   The NO _x storage-reduction catalyst is disposed downstream of the NO _x trap in the exhaust gas flow direction.
9. In claim 8,
   The NO _x storage reduction catalyst has a laminated structure of a lower layer containing a Ce-containing oxide and an upper layer containing activated alumina, and the upper layer and the lower layer contain NO _x storage material containing Ba, Sr and Mg, and Rh and Pt. Containing catalytic metal.
10. In claim 7,
    The NOx trap is provided in an intermediate layer formed between a lower layer formed on a support substrate and containing activated alumina and a Ce-containing oxide, and an upper layer containing activated alumina formed on the lower layer,
    At least one catalyst metal of Pt and Rh and NO _x storage material are supported on the lower layer, the upper layer, and the intermediate layer,
    It said lower layer and said upper layer constitutes the NOx storage reduction catalyst functions as a NOx storage reduction catalyst because also contains the _the NO _x storage material and the catalytic metal the intermediate layer.
11. In claim 10,
    The intermediate layer includes activated alumina.
12. In claim 11,
    The intermediate layer includes zirconia as the NO _x adsorption / occlusion material,
    The mass ratio of the zirconia to the total mass of the zirconia and the alumina in the intermediate layer is 33% by mass or more.
13. In claim 7,
    A laminated catalyst is disposed in the exhaust gas passage;
    The laminated catalyst is formed on a support substrate, includes an inorganic oxide as the NOx adsorption / occlusion material, further includes a Ce-containing oxide, and an upper layer is formed on the lower layer and includes activated alumina. Comprising at least one catalyst metal of Pt and Rh and a NO _x storage material supported on the lower layer and the upper layer,
    Wherein the lower layer of the multilayer catalyst constitutes the NOx trap, the upper layer constitutes the NOx storage reduction catalyst functions as a NOx storage reduction catalyst from the lower also contains the _the NO _x storage material and the catalytic metal.
14. In claim 13,
    The lower layer includes activated alumina on which at least one catalyst metal of Pt and Rh and a NO _x storage material are supported.
15. In claim 14,
    The lower layer includes zirconia as the NO _x adsorption / occlusion material,
    The mass ratio of the zirconia to the total mass of the zirconia and the activated alumina in the lower layer is 5% by mass or more.
16. In claim 14,
    The lower layer includes zirconia as the NO _x adsorption / occlusion material,
    The mass ratio of the zirconia to the total mass of the zirconia and the activated alumina in the lower layer is 35% by mass or more and 65% by mass or less.
17. In claim 7,
    The NOx adsorption / occlusion material includes zirconia.
18. In claim 7,
    Wherein _the NO _x adsorption / storage material comprises zirconia yttrium is dissolved.
19. In claim 7,
    Wherein the _the NO _x adsorption / storage material, Pt, at least one catalytic metal of Pd and Rh.
20. In claim 7,
    The NO _x trap includes Pt-supported alumina.

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