JP2001113172A - Exhaust gas purification catalyst - Google Patents

Abstract

(57) Abstract: _the NO _X storage reduction type suppresses sulfur poisoning of the exhaust gas purifying catalyst. In the exhaust gas purifying catalyst having a _the NO _X storage catalyst layer comprising A an alkali or alkaline earth metal, on the surface layer of _the NO _X storage catalyst layer, diffusion into _the NO _X storage catalyst layer of SO _X A barrier layer, which is a layer in which a noble metal and a transition metal are supported on an inorganic oxide that suppresses the occurrence of a noble metal. Preferably, the upper layer of the barrier layer, a noble metal is further provided with a SO _X absorption and release material layer is a zeolite layer supported.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying catalyst for purifying exhaust gas discharged from an internal combustion engine of an automobile or the like, and more particularly, to an improvement suitable for purifying exhaust gas from a lean burn engine. It relates _the NO _X storage-reduction type exhaust gas purifying catalyst that is.

[0002]

2. Description of the Related Art In recent years, from the viewpoint of protecting the earth, it has become a worldwide problem to reduce the total amount of carbon dioxide (CO ₂ ) emitted from internal combustion engines of automobiles and the like, and regulations on exhaust gas and fuel consumption have been strengthened year by year. It is getting. As a countermeasure, a lean-burn engine has been developed for the purpose of improving fuel economy, the purpose of purifying the exhaust gas, NO _X storage reduction obtained by adding a function of occluding NO _X in a lean atmosphere to a conventional three-way catalyst Three-way catalysts have been developed and have achieved some success with the above issues.

[0003] In this lean burn engine, fuel is normally burned under the condition that the air-fuel ratio (A / F) is lean (excess air), and the fuel is temporarily stoichiometric (stoichiometric air-fuel ratio) to rich (excess fuel). Burn under conditions. HC in exhaust gas
And CO is efficiently burned and removed by the action of an oxidizing atmosphere and the catalyst in the lean side, whereas, NO _X is trapped by the occluding material in the lean side, it is released in a temporary stoichiometric-rich conditions, It is reduced and purified by the action of the temporary reducing atmosphere and the catalyst. These air-fuel ratio controls and NO
By the action of _X storage type three-way catalyst, as a whole, HC simultaneously exhaust gas when improving the fuel economy, CO, can be purified efficiently NO _X.

[0004]

However, a small amount of sulfur component is contained in the fuel, which is oxidized at the time of combustion or oxidized on a catalyst to produce SO _X. Since this SO _X is acidic and the NO _X storage material is basic, SO _X reacts with the NO _X storage material to form sulfate. This sulfate has a strong bond,
_X does not readily release from the NO _X storage material, resulting in NO
The NO _X storage ability of the _X storage material is lost, and the NO _X purification ability decreases over time. This phenomenon is known as the sulfur poisoning of the NO _X storage component is one of the biggest problems in the lean-burn system using storage material.

[0005] As countermeasures for improving the sulfur-poisoning resistance of such occlusion reduction type NO _X catalyst, JP-A-8-9
No. 9034, TiO ₂ , SiO ₂ or ZrO ₂ and Al ₂
A system in which O ₃ is mixed or complexed has been proposed. Such a system is confirmed to have the effect of suppressing the attachment of SO _X to the NO _X storage material and the effect of promoting the desorption of SO _X attached to the exhaust gas due to A / F fluctuation (lean → rich). Have been. However, in order to efficiently desorb the attached SO _X, a higher temperature than normal operating conditions is required. Therefore, the poisoning of the NO _X storage material gradually progresses over a long period of use, and the NO _X purification performance is reduced. There is a problem of lowering.

[0006] Japanese Patent Application Laid-Open No. H11-156159 discloses N
In addition to O _X occlusion layer is provided with a functional layer comprising an oxide of ceria or the like for releasing it to SO _X in the exhaust gas is absorbed in the lean side stoichiometric-rich side to the exhaust gas contacting side, NO _X
A catalyst structure for suppressing SO _X from reaching the storage layer has been proposed. However, such a ceria layer lacks the ability to capture and release SO _x , so that some of the SO _x
Reach O _X occlusion layer, there is a problem that the purification performance of the underlying _the NO _X storage layer is significantly reduced in a short period of time.

Accordingly, the present invention suppresses the SO _X reaches _the NO _X storage layer by providing a barrier layer which firmly capture the SO _X, improved to maintain maximum performance of _the NO _X storage layer and to provide a _the NO _X storage-reduction type exhaust gas purifying catalyst.

[0008]

In order to solve the above problems SUMMARY OF THE INVENTION have _the NO _X storage catalyst layer containing an alkali or alkaline earth metal as _the NO _X storage material with purifying catalyst metal on a support, wherein _the NO _X storage SO _X
An exhaust gas purifying catalyst, comprising a barrier layer that suppresses the diffusion of NOx into the NO _X storage material, wherein the barrier layer is a layer made of an inorganic oxide carrying a noble metal and a transition metal.

The barrier layer specified in the present invention is a layer in which both a noble metal and a transition metal are supported on an inorganic oxide. The noble metal contained in the barrier layer oxidizes SO _X on the lean side, and the transition metal firmly captures the oxidized SO _X , thereby preventing SO _X from reaching the NO _X storage material. do. Further, the noble metal of the barrier layer acts to reduce the oxidized SO _X on the stoichiometric to rich side, and breaks the bond with the transition metal on the stoichiometric to rich side.
O _X is released from the barrier layer.

The transition metal is SO _X oxidized on the lean side.
, But on the rich side, SO
_X can be released, and thus the S
The O _X absorption capacity does not saturate, and the arrival of SO _X to the NO _X storage type catalyst layer can be suppressed stably.

In a preferred embodiment of the present invention, further comprising a SO _X absorption and release material layer on the upper layer of the barrier layer. Preferably, the SO _X absorbent material layer is a zeolite layer carrying a noble metal. In the SO _X absorbing / releasing material layer, the noble metal oxidizes SO _X on the lean side, captures it by zeolite and the like, and reduces this oxidized SO _X on the stoichiometric side to the rich side to bind with zeolite and the like. Has run out and SO _X becomes SO _X
It is released from the material layer.

The SO _X absorber layer can capture SO _X over a wider range of conditions than the barrier layer, especially over a wider temperature range. SO _X is first captured by the SO _X absorber layer. . However, since the SO _X absorbing / releasing material layer has a weaker ability to capture SO _X than the barrier layer, SO _X may pass through the SO _X absorbing / releasing material layer. Thus capture a strong bonding force to SO _X passing through the SO _X absorbing and releasing material layer.
Thus, by providing the SO _X absorbing and releasing material layer over the barrier layer, it can be more completely suppressed from reaching the _the NO _X storage material SO _X.

[0013]

FIG. 1 is a partially enlarged sectional view of an exhaust gas purifying catalyst according to the present invention. The exhaust gas purifying catalyst 1 is composed of a carrier substrate 2 and a NO.
It is composed of an _X storage catalyst layer 3. As the carrier substrate, a monolith carrier substrate made of a heat-resistant ceramic such as cordierite is preferable.

The carrier substrate 2 is covered with a NO _x storage type catalyst layer 3. This NO _X storage type catalyst layer contains an alkali or / and alkaline earth metal as a NO _X storage material together with a purification catalyst metal, and is preferably an inorganic material such as alumina, silica, zirconia, silica-alumina, or zeolite. This is a layer composed of a noble metal and an alkali or alkaline earth metal supported on an oxide porous body.

[0015] The purification catalyst metal is in the lean side, NO _X
Is oxidized to the form of NO ₃ ⁻ which is easily captured by an alkali metal or the like, and acts to reduce it on the stoichiometric to rich side. The supported amount is preferably 0.5 to 5 wt% per weight of the NO _X storage type catalyst layer 3, and the purification catalyst metal includes platinum,
Examples include gold, ruthenium, rhodium and palladium.

The NO _X storage catalyst layer contains an alkali or alkaline earth metal as a NO _X storage material.
The absorbing material captures NO _X in the lean side, stoichiometric ~
It acts to release it on the rich side. Examples of the alkali metal include lithium, sodium, potassium, rubidium, cesium, and francium. Examples of the alkaline earth metal include beryllium, magnesium, calcium, strontium, and barium. The supporting amount of the alkali or alkaline earth metal is preferably 0.05 to 0.5 mol per liter of the carrier substrate.

The surface layer of the NO _x storage type catalyst layer 3 is covered with a barrier layer 4. This barrier layer is a layer in which a noble metal and a transition metal are supported on an inorganic oxide. As the inorganic oxide, alumina, silica, zirconia, silica-alumina, zeolite and the like are used, and activated alumina is preferable. Examples of the noble metal include platinum, gold, ruthenium, rhodium and palladium, and preferably platinum. Transition metals are elements of groups 3A to 7A, 8 and 1B of the periodic table, and include titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zirconium, niobium, lanthanum, and cerium. Preferably, manganese, iron, cobalt, nickel, and copper are used because of easy decomposition of sulfate.

The barrier layer 4 contains at least 0.3% by weight of a noble metal per weight of the barrier layer, and preferably contains noble metal.
Contains 5-5 wt% precious metal. Also, the amount of transition metal is at least 0.3 × 10 ⁻³ mol / weight per barrier layer weight.
g, preferably 0.5 × 10 ^{−3 to} 1.0 × 10 ⁻³ mol /
g. The thickness of the barrier layer is at least 3 μm.
And preferably 10 to 20 μm.

In a preferred embodiment of the present invention, further provided with a SO _X absorbing material layer 5 on the barrier layer. This SO _X absorbing / releasing material layer is preferably a layer in which a noble metal such as platinum is supported on a zeolite such as mordenite.
The amount of noble metal should be at least 0.3 per weight of barrier layer
wt%, preferably 0.5-5 wt%, and the thickness of the barrier layer is at least 3 μm, preferably 10-20 μm.
m.

The exhaust gas purifying catalyst of the present invention includes a _the NO _X storage catalyst layer and the barrier layer, SO a preferred embodiment _X
It further comprises layers of absorbent material, which are believed to perform the following functions.

NO_XOccupants are included under lean conditions
NO by purification catalyst metal_XNO _Three ^-Oxidized to the shape of
And capture it by the action of an alkali metal or the like. This
Like, NO_XThe storage material is NO under lean conditions._XSuck
But a small amount of SO in the exhaust gas_XIncludes
Yes, for example SO_TwoIs oxidized on the surface of the noble metal to form S
O_ThreeAnd further oxidized in an atmosphere to form SO_Four ^-Form of
To generate sulfate while diffusing the occlusion layer. This sulfuric acid
Salt is harder to decompose than nitrate, under stoichiometric to rich conditions
Remains without being decomposed, and therefore, the occlusion material layer
The sulfate increases with time, and NO that can be captured by the storage material
_XThe amount will be reduced.

The barrier layer suppresses the deterioration of the performance of the NO _X storage material, and the action of the noble metal and the transition metal contained in the barrier layer causes the SO _{x to be} in front of the NO _X storage catalyst layer.
By firmly capture the _X, thereby preventing sulfur poisoning of the NO _X occluding material. That is, the barrier layer prevents SO _X from reaching the NO _X storage catalyst layer.
To provide an _X-shielding effect, which is under lean conditions, precious metal oxidizes SO _X into SO _3, it SO ₄ in an oxidizing atmosphere ^- changes to a, transition metal present in the barrier layer its SO ₄ ^- in order to capture strong. On the other hand, the stoichiometric-rich conditions, precious metal SO ₄ ^- promoting the reduction reaction with HC and CO in the exhaust gas again SO ₄ ^- are released into the exhaust gas becomes the form of SO _X.

[0023] In other words, the transition metal SO ₄ ^- one which can be firmly coupled with, the binding, _the NO _X storage-type catalytic layer _the NO _X storage material and SO ₄ ^- Unlike binding to, noble metals Since it can be cut off in the presence of a reducing atmosphere and SO _X is released into the exhaust gas, the SO _X trapping performance of the barrier layer does not saturate. Therefore, the barrier layer can provide an SO _X shielding effect stably with time to the NO _X storage type catalyst layer.

Further, with respect to NO _X, at least a portion of the NO _X by the action of the noble metal contained in the barrier layer NO ₃ ^- no binding force but is oxidized to a form of the transition metal to catch it firmly . Accordingly, the barrier layer, by capturing SO _X preferentially relative to NO _X, is to act as a barrier layer against _the NO _X storage material.

As described above, SO _X is repeatedly adsorbed and desorbed by the upper barrier layer of the NO _X storage type catalyst layer, and SO _X
Capture efficiency without saturating, the diffusion into _the NO _X storage material is suppressed, it is possible to suppress stable over time poisoning of _the NO _X storage catalyst layer.

In a preferred embodiment of the present invention, further provided with a SO _X absorbing material layer 5 on the barrier layer. This SO _X absorbing and releasing material layer, a noble metal contained in the SO _X absorbing and releasing material layer under rich conditions oxidizes SO _X into SO _3, it SO ₄ under an atmosphere of an oxidizing ^- for changes to a, The capture of this by the zeolite prevents SO _X from reaching the barrier layer. On the other hand, the stoichiometric-rich conditions, SO ₄ by the catalytic action of the noble metal ^- may be released into the exhaust gas in the form of HC and CO again SO _X by promoting the reaction in the exhaust gas.

Here, this SO_XThe material layer is mainly
Due to differences in components between the transition metal and zeolite, etc.
SO layer than layer_XOf the barrier layer, and therefore the barrier layer
RIMO SO_XLow trapping efficiency but wider temperature than barrier layer
SO in range_XCan be captured. This SO_XAbsorbing material layer
By providing it on the exhaust gas side of the rear layer, the SOI of the barrier layer can be reduced. _X
At the same time as the burden of capturing is reduced, SO_XCan be captured
The temperature range expands and SO_XHigher capture efficiency
Can be done.

In addition, since both the barrier layer and the SO _x absorbing / releasing material layer contain a noble metal, NO _x in the exhaust gas
When but to reach _the NO _X storage catalyst layer through both the barrier layer and the SO _X absorbing and releasing material layer, NO _X is captured easily NO ₃ at a high rate ^- are oxidized to form, and therefore, N
O _X storage efficiency is also increased.

It should be noted that the present inventors described the prior art described above.
It is equipped with a layer made of oxide such as ceria, which does not contain precious metals.
In the obtained catalyst configuration, SO_XLow capture efficiency
And the layer is SO_XSignificant SO after saturation_XAbsorption and release
No effect has been confirmed. This is
SO for rear alone_XTo SO_ThreeLow catalysis
From SO_XSO that is easily captured_Four ^-Is not converted to
In addition, in the case of ceria alone, the SO
_Four ^-SO_XHas a low reducing effect on _XRelease
It is considered that the efficiency of the emission is low.

[0030]

EXAMPLES Example 1 A cylindrical monolithic carrier having a diameter of 30 mm and a length of 50 mm was used.
Forming a washcoat layer of active Al ₂ O ₃ ,
By immersing in an aqueous solution of Pt dinitrodiamine and then drying, 2 g of Pt was supported per liter of catalyst volume.
Next, using an aqueous barium acetate solution, 0.2 mol of Ba was supported per liter of catalyst volume, and calcined at 500 ° C. for 1 hour. Next, separately, ion-exchanged water and binder, P
A slurry containing an active Al ₂ O ₃ powder carrying t and Co and a stabilizer was prepared, wash-coated on the monolith catalyst, dried and fired to form a SO _X barrier layer. At this time, the amount of supported Co per 180 g of activated alumina in the SO _X barrier layer was 0.1 mol (3.3 wt.
%), Pt was 2 wt%, and the coating amount was about 30 g.

Example 2 In the same manner as in Example 1, Pt and Ba were placed on a monolithic carrier.
Was formed, and a barrier layer composed of Pt, Mn, and Al ₂ O ₃ was formed on the surface of the NO _X storage catalyst layer. At this time, 2 g of Pt and 0.2 mol of Ba were supported per liter of catalyst volume on the NO _x storage catalyst layer,
The barrier layer contained 2 wt% Pt, 0.1 mol Mn, and the coating amount was about 30 g.

Example 3 The same operation as in Example 1 was repeated except that the catalyst volume was 2
g of Pt and 0.2 mol of Ba were formed on the NO _x storage catalyst layer, and the surface thereof was formed from 0.1 mol of Fe, 2 wt% of Pt, and Al ₂ O ₃ per liter of catalyst volume. A barrier layer was formed. The coating amount of the barrier layer was about 30 g per liter of catalyst volume.

Example 4 In the same manner as in Example 1, Pt and Ba were placed on a monolithic carrier.
Was formed, and a barrier layer composed of Pt, Ni, and Al ₂ O ₃ was formed on the surface of the NO _x storage catalyst layer. At this time, 2 g of Pt and 0.2 mol of Ba were supported per liter of catalyst volume on the NO _x storage catalyst layer,
The barrier layer contained 2 wt% Pt, 0.1 mol Ni, and the coating amount was about 30 g.

Example 5 In the same manner as in Example 1, Pt and Ba were placed on a monolithic carrier.
Was formed, and a barrier layer made of Pt, Cu, and Al ₂ O ₃ was formed on the surface of the NO _x storage catalyst layer. At this time, 2 g of Pt and 0.2 mol of Ba were supported per liter of catalyst volume on the NO _x storage catalyst layer,
The barrier layer contained 2 wt% Pt, 0.1 mol Cu, and the coating amount was about 30 g.

Example 6 In the same manner as in Example 1, Pt and Ba were placed on a monolithic carrier.
Was formed, and a barrier layer composed of Pt, Zn, and Al ₂ O ₃ was formed on the surface of the NO _x storage catalyst layer. At this time, 2 g of Pt and 0.2 mol of Ba were supported per liter of catalyst volume on the NO _x storage catalyst layer,
The barrier layer contained 2 wt% Pt and 0.1 mol Zn, and the coating amount was about 30 g.

Example 7 In the same manner as in Example 1, Pt and Ba were placed on a monolithic carrier.
Was formed, and a barrier layer composed of Pt, Ce, and Al ₂ O ₃ was formed on the surface of the NO _x storage catalyst layer. At this time, 2 g of Pt and 0.2 mol of Ba were supported per liter of catalyst volume on the NO _x storage catalyst layer,
The barrier layer contained 2 wt% Pt, 0.1 mole Ce, and the coating amount was about 30 g.

Example 8 In the same manner as in Example 1, Pt and Ba were placed on a monolithic carrier.
Was formed, and a barrier layer composed of Pt, Co, and Al ₂ O ₃ was formed on the surface of the NO _X storage catalyst layer. At this time, 2 g of Pt and 0.2 mol of Ba were supported per liter of catalyst volume on the NO _x storage catalyst layer,
The barrier layer contained 2 wt% of Pt and 0.1 mol of Co, and the coating amount was about 30 g.

Next, a slurry containing mordenite powder preliminarily loaded with 2 wt% of Pt, ion-exchanged water, a binder, and a stabilizer is prepared, and this slurry is wash-coated to form a third layer of SO on the barrier layer. _{An X} absorbing / releasing material layer was formed. Pt / Mor (mordenite supporting platinum)
The coating amount of the layer was 20 g per liter of catalyst volume, and Pt was 2 wt%.

Comparative Example 1 A cylindrical monolithic carrier having a diameter of 30 mm and a length of 50 mm was
Forming a washcoat layer of active Al ₂ O ₃ ,
2 g of Pt was supported per liter of catalyst volume. Next, using an aqueous barium acetate solution, 0.2 mol of Ba was supported per liter of catalyst volume, and calcined at 500 ° C. for 1 hour.

Comparative Example 2 In the same manner as in Example 1, Pt and Ba were placed on a monolithic carrier.
Was formed, and a barrier layer made of Pt, Mg, and Al ₂ O ₃ was formed on the surface of the NO _x storage catalyst layer. At this time, 2 g of Pt and 0.2 mol of Ba were supported per liter of catalyst volume on the NO _x storage catalyst layer,
The barrier layer contained 2 wt% of Pt and 0.1 mol of Mg, and the coating amount was about 30 g.

Comparative Example 3 The same operation as in Comparative Example 1 was performed, except that the diameter was 30 mm and the length was 50 mm.
Was formed on the cylindrical monolithic carrier of the formula (1), and a wash coat layer of active Al ₂ O ₃ was formed.
g of Pt. Next, using an aqueous barium acetate solution, 0.2 mol of Ba was supported per liter of catalyst volume, and calcined at 500 ° C. for 1 hour. Next, in the same manner as in Example 8, Pt / Mor was applied to the surface of this NO _X storage catalyst layer.
A layer was formed. The coating amount of Pt / Mor was 20 g per liter of catalyst volume, and Pt was 2 wt%.

Evaluation Method Examples 1 to 9 and Comparative Examples 1 to 3 were evaluated for sulfur poisoning resistance in the model gases shown in Table 1. Specifically, the gas temperature entering the catalyst was set to 400 ° C., and the rich gas for 30 seconds and the lean gas for 60 seconds were alternately introduced into the catalyst. After 1 hour, the sulfur content adhering to the NO _X storage material of each catalyst was reduced. Was quantitatively analyzed. Table 2 shows the results.

[0043]

[Table 1]

[0044]

[Table 2]

As can be seen from Table 2, Examples 1 to 5
Al ₂ O ₃ wash-coated on monolith substrate with N
O _X occlusion material of Pt supported as Ba and purification catalyst metal as, is a catalyst coated with Al ₂ O ₃ containing Pt and a transition metal as a barrier layer on the surface of the storage layer, both of which are barrier layer In comparison with the conventional catalyst (Comparative Example 1) containing no NO, the sulfur content adhering to the NO _X storage material after the test was clearly smaller.

Examples 6 and 7 are catalysts coated with Al ₂ O ₃ containing a rare earth metal as a barrier layer, which are different from those of Examples 1 to 5, but have not obtained very good results. This is considered to be due to the ease of decomposition of the sulfate of the element contained in the barrier layer. On the other hand, Example 9 in which a mordenite layer having an excellent SO _X absorption / desorption function was further formed on the barrier layer showed even better results. This is considered to be due to the double effect of the barrier layer and the SO _X absorbing / emitting material layer.

[0047]

According to the exhaust gas purifying catalyst of the present invention, SO _X is prevented from reaching the NO _X storage catalyst layer,
A decrease in the performance of NO _X storage reduction is suppressed.

[Brief description of the drawings]

FIG. 1 is a partially enlarged sectional view of an exhaust gas purifying catalyst of the present invention.

FIG. 2 is a partially enlarged sectional view of an exhaust gas purifying catalyst according to a preferred embodiment of the present invention.

[Explanation of symbols]

1 ... an exhaust gas purifying catalyst 2 ... support substrate 3 ... NO _X occluding catalyst layer 4 ... barrier layer 5 ... SO _X absorbing material layer

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (reference) B01J 29/22 B01J 33/00 ZABA 33/00 ZAB B01D 53/36 102B 102H F-term (reference) 4D048 AA02 AA06 AB02 BA03X BA09X BA15X BA28X BA30X BA30Y BA31Y BA32Y BA33Y BA34Y BA35X BA35Y BA36X BA37X BA38X BB02 CC46 EA04 4G069 AA03 AA08 BA01B BA07A BA07B BB15B BC01A BC08A BC13B BC31B BC35B BC43B BC62B BC67BCA13 BC69BBC13B

Claims (3)

[Claims] 1. A catalyst containing an alkali or alkaline earth metal as an NO _X storage material together with a purification catalyst metal on a carrier.
_{An X} storage catalyst layer, and a barrier layer for suppressing diffusion of SO _{X to} the NO _X storage material on the surface of the NO _X storage catalyst layer, wherein the barrier layer carries a noble metal and a transition metal. An exhaust gas purification catalyst comprising a layer made of an inorganic oxide. 2. The exhaust gas purifying catalyst according to claim 1, further comprising an SO _X absorbing / releasing material layer above the barrier layer. 3. The exhaust gas purifying catalyst according to claim 2, wherein the SO _X absorbing / desorbing material layer is a zeolite layer carrying a noble metal.