JP2015151970A - Exhaust emission control catalyst device and exhaust emission control method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To allow for efficient NOremoval within a wide temperature range of exhaust gas.SOLUTION: A NOtrap part 3 including NOadsorption/storing material capable of adsorbing NOwhen an exhaust gas temperature is 100°C or more and 200°C or less and capable of storing NOwhen an exhaust gas temperature is 350°C or more and 450°C or less is disposed in an exhaust gas passage 2 of an engine, and a NOstoring and reducing catalyst 4 capable of absorbing NOwhen an air-fuel ratio of the exhaust gas is lean and capable of reducing NOwhen the air-fuel ratio of the exhaust gas is rich is disposed downstream from the NOtrap part in an exhaust gas flow direction.

Description

  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.

JP 2006-43541 A JP 2003-343252 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, alkaline earth metals used as NO _x storage materials are known not to exhibit NO _x storage effects when the exhaust gas temperature of about 100 ° C. to 150 ° C. is low, that is, 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 as} the exhaust gas component is externally released. May be released.

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

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 the _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. provided on the upstream side of the exhaust gas passage, downstream A NO _x storage reduction catalyst capable of storing NO _x under a lean atmosphere and reducing and purifying NO _x under a rich atmosphere is provided on the side to purify NO _x .

Specifically, 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 a lean-combustible engine, wherein an exhaust gas temperature is provided in an exhaust gas passage of the engine. the but NO _x trap portion including _the NO _x adsorption / storage material capable of inserting and extracting the NO _x when and exhaust gas temperature can adsorb NO _x is 350 ° C. or higher 450 ° C. or less at 100 ° C. or higher 200 ° C. or less reduction with placing, in exhaust gas flow direction downstream side of the NO _x trap section, the NO _x air-fuel ratio is at the rich air-fuel ratio of the exhaust gas can occlude NO _x at lean in and exhaust gas placing a NO _{x storage-and-reduction} catalysts that can, during a period from immediately after the start of the engine until the exhaust gas temperature is 200 ° C. or less, of adsorbing NO _x in the exhaust gas to the NO _x trap portion A step that, 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 , while the exhaust gas temperature is 350 ° C. or higher 450 ° C. or less, and step of storing the NO _x trap portion and _the NO _x storage reduction catalyst NO _x in the exhaust gas, in the vicinity of top dead center of the compression stroke in a combustion chamber of an engine After the main injection for supplying fuel, the fuel is injected and supplied in the expansion stroke or the exhaust stroke, and then injected to include HC in the exhaust gas so that the air-fuel ratio of the exhaust gas becomes rich, characterized in that it comprises a step of emitting by reducing purify NO _x trap portion and NO _x storage-reduction catalyst from the desorbed NO _x by _the NO _x storage-reduction catalyst.

According to the exhaust gas purification method according to the present invention, the exhaust gas temperature is provided NO _x trap portion including 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 Therefore, 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. Moreover, for placement of _the NO _x storage-reduction catalyst in the exhaust gas flow direction downstream side of the NO _x trap unit, when the exhaust gas temperature exceeds about 200 ° C. to rise immediately after the engine start, the air-fuel ratio of the exhaust gas is lean it can occlude NO _x, further air-fuel ratio of the exhaust gas by controlling so rich, to reduce and purify NO _x trap portion and _the NO _x storage from the reduced catalyst was desorbed NO _x when Can do. 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.

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, and is provided in an exhaust gas passage of the engine, and has an exhaust gas temperature. and NO _x trap portion but comprising _the NO _x adsorption / storage material of NO _x can be occluded when and exhaust gas temperature can adsorb NO _x is 350 ° C. or higher 450 ° C. or less at 100 ° C. or higher 200 ° C. or less , Provided downstream of the NO _x trap portion in the exhaust gas flow direction, can store NO _x when the air-fuel ratio of the exhaust gas is lean, and can reduce NO _x when the air-fuel ratio of the exhaust gas is rich And an NO _x storage reduction catalyst.

According to the exhaust gas purifying catalyst device according to the present invention, the exhaust gas passage of the NO _x trap portion engine comprising adsorbable _the NO _x adsorption / storage material the NO _x when the exhaust gas temperature is 100 ° C. or higher 200 ° C. or less because provided, that _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 Can be prevented. Further, since _the NO _x storage-reduction catalyst is disposed in an exhaust gas flow direction downstream side of the NO _x trap unit, when the exhaust gas temperature exceeds about rise to 200 ° C. immediately after engine start, the air-fuel ratio of the exhaust gas can occlude NO _x at lean can further when the air-fuel ratio of the exhaust gas is rich, to reduce and purify NO _x trap portion and _the NO _x storage desorbed from the reduced catalyst was NO _x. Thus, a wide range of exhaust gas temperature range, it is possible to purify efficiently NO _x.

In the exhaust gas purifying catalyst device according to the present invention, the NO _x adsorption / occlusion material preferably contains zirconia.

Zirconia can be suitably used as a NO _x adsorption / occlusion material because it can adsorb NO _x when the exhaust gas temperature is 100 ° C. or higher and 200 ° C. or lower.

  The exhaust gas purifying catalyst device according to the present invention 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 the exhaust gas purification catalyst device according to the present invention, it is preferable that at least one catalyst metal of platinum (Pt), palladium (Pd) and rhodium (Rh) is supported on the NO _x adsorption / occlusion material. .

In this way, also it is possible to impart catalytic activity for exhaust gas purification in NO _x trap portion that includes _the NO _x adsorption / storage material, it is possible to improve the exhaust gas purification performance.

In the exhaust gas purifying catalyst device according to the present invention, it is preferable that the NO _x trap portion includes Pt-supported alumina (Al ₂ O ₃ ).

  Since alumina has a high specific surface area, the dispersibility of Pt can be improved, and the contact property between the catalyst metal Pt and the exhaust gas is improved, which is advantageous for purification of the exhaust gas.

In the exhaust gas purification catalyst device according to the present invention, the NO _x storage reduction catalyst has a laminated structure of a lower layer containing a ceria material and an upper layer containing an alumina material, and the upper layer and the lower layer are NO _x containing Ba, Sr and Mg. An occlusion material and a catalytic metal containing Rh and Pt may be provided.

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.

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 mimetic diagram showing the composition of the exhaust gas purification catalyst device concerning the embodiment of the present invention. The configuration of the NO _x trap portion of the exhaust gas purifying catalyst device according to an embodiment of the present invention is a cross-sectional view illustrating. It is sectional drawing which shows the structure of the LNT catalyst of the exhaust-gas purification catalyst apparatus which concerns on embodiment of this invention. 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. Is a graph showing the NO _x trap rate of the catalysts of Examples 1-3 and Comparative Examples.

  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.

(Configuration of exhaust gas purification catalyst device)
FIG. 1 shows the configuration of an exhaust gas purification catalyst apparatus 1 according to an embodiment of the present invention, where reference numeral 2 is an exhaust gas passage, reference numeral 3 is a NO _x trap section, and reference numeral 4 is a lean NO _x storage reduction catalyst. A 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 section 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 diagram showing a part of a cross section perpendicular to the exhaust gas flow direction of the NO _x trap section 3. As shown in FIG. 2, the NO _x trap section 3 is configured by arranging a ZrO ₂ layer 31 containing zirconia (ZrO ₂ ) on the exhaust gas passage wall of a honeycomb carrier 30 made of cordierite. The space inside the _second 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 in order to give catalytic performance to the NO _x trap part 3, at least one catalyst metal of Pt, Pd and Rh is added to ZrO _2. It may be supported. The ZrO ₂ layer 31 may contain 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 included in the NO _x trap unit 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. 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. 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 alumina and ceria, and the upper catalyst layer 42 formed on the lower catalyst layer 41 includes alumina. The lower catalyst layer 41 and the upper catalyst layer 42 are impregnated with Pt and Rh, which are catalytic metals, and Ba, Sr, and Mg, which are NO _x storage materials. 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 part, particularly the NO _x adsorption capacity and the 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. to 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. to 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 occlude 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 part and LNT catalyst production method)
Next, a method for manufacturing of the NO _x trap portion and the LNT catalyst in the catalytic device according to the present embodiment. First, a method for manufacturing a NO _x trap part, particularly a NO _x trap part 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. This slurry is coated on a carrier, 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 carrier, and an NO _x trap part is obtained. be able to.

  Next, the manufacturing method of a LNT catalyst is demonstrated. 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. CeO ₂ may be in the form of a complex oxide (PePrO _x ) containing praseodymium or the like, for example. 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. Moreover, drying can be performed by holding at a temperature of about 100 ° C. to 250 ° C. for a predetermined time in an air atmosphere, for example, and baking is held at a temperature of about 400 ° C. to 600 ° C. for a few hours in an air atmosphere, for example. Can be done by.

The NO _x trap part 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 according to this embodiment is provided. A purification catalyst 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 the present method, first, as described above, the NO _x trap portion is provided on the upstream side of the exhaust gas passage, and the LNT catalyst is provided on the downstream side thereof. during until, adsorbing NO _x in the exhaust gas to the NO _x trap portion. 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 450 ° C. or less, allowed to absorb NO _x trap portion and LNT catalyst 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 portion and LNT catalyst.

According to such an exhaust gas purification method, the exhaust gas temperature immediately after engine start to release even when in the low temperature range, the NO _x for the NO _x trap portion can be trapped NO _x 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 portion. 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.

Below, the Example for demonstrating in detail the exhaust-gas purification catalyst apparatus which concerns on this invention is shown. In this example, the NO _x trap amount of the exhaust gas purification catalyst device in which the LNT catalyst is disposed downstream of the NO _x trap portion 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 on the downstream side of the NO _x trap portion containing Pt-supported Al ₂ O ₃ was measured.

Specifically, in Example 1, a cordierite hexagonal cell honeycomb carrier (diameter: 25.4 mm, long length) having a cell wall thickness of 4.5 mil and a carrier capacity of 25 ml with 400 cells per square inch. 50 mm) was provided with Pt-supported ZrO ₂ according to the above method to obtain a NO _x trap part. 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.

Further, Example 2 is different from Example 1 only in the components contained in the NO _x trap part. 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 part. Y-containing ZrO ₂ was prepared so that ZrO ₂ : Y ₂ O ₃ = 9: 1 (mass ratio).

On the other hand, the comparative example differs from Example 1 only in using Al ₂ O ₃ supporting Pt, not ZrO ₂ supporting Pt in the NO _x trap portion.

The NO _x trap portions and LNT catalysts of Examples 1 to 3 and the comparative example were each 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. The NO _x trap part 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 part 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, and the space velocity was SV = 72000 h ⁻¹ . The measurement results of the NO _x trap rates at 200 ° C. and 300 ° C. of Examples 1 to 3 and the comparative example 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. It can be seen that the NO _x trap rate at 300 ° C. is large. This is because in Examples 1 to 3, ZrO ₂ was contained in the NO _x trap part, and as described above, the NO _x trap rate of the entire catalyst device increased due to the NO _x adsorption / storage capacity of ZrO _2. it is conceivable that. 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 part, and the inclusion of Y increased the basicity of ZrO ₂ and increased the ability to attract NO in the exhaust gas. It is believed that there is.

1 exhaust gas purifying catalyst device 2 exhaust gas passage 3 NO _x trap portion 4 lean NO _x trap catalyst (LNT catalyst, NO _x storage-reduction catalyst)
30, 40 Honeycomb carrier 31 ZrO ₂ layer 41 Lower catalyst layer 42 Upper catalyst layer

Claims (9)

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 with arranging the NO _x trap section including a suction / storage material, said the NO _x trap exhaust gas flow direction downstream side of the unit, the air-fuel ratio of the exhaust gas is storable in and exhaust gas NO _x when the lean Disposing a NO _x storage reduction catalyst capable of reducing NO _x when the air-fuel ratio is rich;
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 portion,
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,
Between the exhaust gas temperature is 350 ° C. or higher 450 ° C. or less, and step of storing the NO _x trap portion and the NO _x NO _x occlusion reduction catalyst to the exhaust gas,
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 controlled so that the air-fuel ratio becomes rich state, and a step of releasing the NO _{x storage-and-reduction} catalyst by said NO _x trap portion and _the NO _x storage from the reducing catalyst desorbed _the NO _x reduction purification to An exhaust gas purification method characterized by the above. _2. The exhaust gas purification method according to claim 1, wherein zirconia (ZrO ₂ ) is used as the NO _x adsorption / occlusion material. 3. The exhaust gas purification method according to claim 1, wherein the NO _x storage reduction catalyst contains a NO _x storage material containing barium (Ba), strontium (Sr), and magnesium (Mg). 4. 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 portion including _the NO _x adsorption / storage material,
Than said NO _x trap portion provided in the exhaust gas flow direction downstream of the NO _x air-fuel ratio is at the rich air-fuel ratio of the exhaust gas can occlude NO _x at lean in and exhaust gas reducible An exhaust gas purification catalyst device comprising an NO _x storage reduction catalyst. The exhaust gas purification catalyst device according to claim 4, wherein the NO _x adsorption / occlusion material contains zirconia. Wherein _the NO _x adsorption / storage component, the exhaust gas purifying catalyst according to claim 5, characterized in that it comprises a zirconia yttrium (Y) was dissolved. The said _the NO _x adsorption / storage material, either platinum (Pt), palladium (Pd) and rhodium at least one catalytic metal of (Rh) is as claimed in claim 4 to 6, characterized in that it is carried The exhaust gas purifying catalyst device according to Item 1. The exhaust gas purification catalyst apparatus according to any one of claims 4 to 7, wherein the NO _x trap part includes Pt-supported alumina (Al ₂ O ₃ ). The NO _x storage reduction catalyst has a laminated structure of a lower layer containing a ceria material and an upper layer containing an alumina material, and the upper layer and the lower layer are a catalyst containing NO _x storage material containing Ba, Sr and Mg, and Rh and Pt. The exhaust gas purifying catalyst device according to any one of claims 4 to 8, comprising a metal.

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