JP2000225348A - Catalyst for cleaning exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To clean an exhaust gas containing NO, CO, and hydrocarbons efficiently. SOLUTION: The first catalyst 1 in which Ag is supported on alumina, the second catalyst 2 in which Pt is supported on zeolite by an ion exchange method, and the third catalyst in which Pt for promoting an oxidation reaction and Ir for promoting a water gas conversion reaction are supported on zeolite are arranged in turn from the upstream side in an exhaust gas flow direction.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an exhaust gas purifying catalyst.

[0002]

2. Description of the Related Art JP-A-63-049255 discloses that
The Ag catalyst layer and the zeolite catalyst layer are arranged on the upstream and downstream sides in the exhaust gas flow direction, NO in the exhaust gas is oxidized to NO ₂ by the Ag catalyst layer, and this NO ₂ is converted into N ₂ by the zeolite catalyst layer. Is described.

[0003]

The exhaust gas purifying catalyst having the Ag catalyst layer and the zeolite catalyst layer can purify NO by finally reducing NO to N _2. HC (hydrocarbon compounds) and CO
When it is contained, it is difficult to efficiently purify all of them, and it is not suitable as a catalyst when only CO or only CO and HC are discharged.

[0004] That is, the problem to be solved by the invention of this application is to efficiently purify NO, HC, and CO when they are discharged, and to solve the problem when only CO is discharged. However, the purpose is to efficiently purify it by oxidation.

[0005]

SUMMARY OF THE INVENTION The invention of this application is directed to an air-fuel ratio leaner than the stoichiometric air-fuel ratio (for example, a diesel engine) (A / F is 16 or more or 20 or more, and the O ₂ concentration in the exhaust gas is 5 or more). %) The present invention relates to an exhaust gas purifying catalyst suitable for purifying NO, HC, and CO in exhaust gas of an engine operated at a rate of at least 10%. The present invention relates to an exhaust gas purifying catalyst suitable for an exhaust gas purification system, in which three types of catalysts are provided for purifying NO, HC and CO, and a first catalyst is used for NO.
Is oxidized to NO ₂ , and this NO ₂ is converted to N ₂ by the second catalyst.
_{In this method} , HC and CO are oxidized and purified by the third catalyst, and a water gas conversion catalyst is used to oxidize and purify CO.

That is, the invention for purifying NO, HC and CO is an exhaust gas purifying catalyst provided in an exhaust passage from which these are exhausted, wherein the first catalyst oxidizes NO to NO ₂ , _{2 comprises} a second catalyst for reducing to N ₂ by reacting with HC, and a third catalyst for oxidizing HC and CO, wherein the first to third catalysts are such that the second catalyst is more than the first catalyst. Later, when the exhaust gas comes into contact with the exhaust gas,
It is disposed in the exhaust passage so as to come into contact with the exhaust gas after the catalyst.

The reason why the second catalyst comes into contact with the exhaust gas after the first catalyst is to reduce NO ₂ oxidized and generated by the first catalyst to N ₂ . The third catalyst is in contact with the exhaust gas after the second catalytic secures the HC required for the reduction reaction on the second catalyst, efficiency reduction purification of NO ₂ to the second catalyst This is to make it work well. That is, if the third catalyst contacts the exhaust gas before the second catalyst and HC is oxidized,
This is because the second catalyst has a shortage of reducing agent and is disadvantageous for NO ₂ reduction purification.

As means for making one of the two catalysts come into contact with the exhaust gas first and the other coming into contact with the exhaust gas later, the two catalysts are arranged on the upstream side and the downstream side of the exhaust passage. Alternatively, both catalysts may be provided in layers on a carrier. In the latter case, the catalyst is to be provided in a layered manner so that the catalyst to be brought into contact with the exhaust gas first is on the outside and the catalyst to be contacted later is inside.

The third catalyst preferably has a catalyst component in which a noble metal is supported on zeolite. The use of zeolite as a base material (support material) is advantageous in that it adsorbs HC in exhaust gas when the temperature of the catalyst has not risen to a level at which the catalyst exhibits activity and prevents the exhaust gas from being discharged without purification. Because it becomes. Also,
The use of a noble metal is advantageous for promoting an oxidation reaction and the like.

As the zeolite, those having a particularly high acidity on the surface, for example, MFI are preferable.

As the above-mentioned noble metal, it is preferable to provide a noble metal for promoting a water gas conversion reaction in addition to a noble metal for promoting an oxidation reaction. That is, since the water gas shift reaction (water gas shift reaction; CO + H ₂ O → CO ₂ + H ₂ ) occurs at a low temperature (normal temperature), when the exhaust gas temperature is low, CO in the exhaust gas is oxidized by the water gas shift reaction. The catalyst can be purified, and the catalyst temperature can be quickly raised by utilizing the fact that the reaction at normal temperature is an exothermic reaction. Therefore, at the time when the elimination of HC from the zeolite starts, it becomes possible to quickly raise the catalyst temperature to around the reaction temperature of the noble metal for promoting the oxidation reaction,
HC and CO can be efficiently oxidized and purified using the noble metal.

As the noble metal for promoting the water gas shift reaction, Ir is preferable. This is because Ir has a property of easily adsorbing CO, has a high function as a catalyst, is advantageous in improving the purification rate, and is also capable of obtaining heat resistance.
As the noble metal for accelerating the oxidation reaction, Pt is preferable from the viewpoints of low-temperature activity, improvement in purification rate, heat resistance and the like.

It is preferable that the first catalyst for oxidizing NO to NO ₂ contains Ag from the viewpoints of improvement in purification rate, heat resistance, and the like. The Ag is supported on alumina, for example. What should I do? NO above
The second catalyst for reducing ₂ to N ₂ is preferably a catalyst having a catalyst component in which a transition metal is supported on zeolite from the viewpoints of improvement in purification rate, heat resistance and the like.
In this case, as the transition metal, a noble metal, for example, Pt, or a transition metal other than the noble metal, for example, Cu can be used. As a catalyst component of the second catalyst, zeolite contains P
Those in which t is carried by ion exchange are particularly preferred.

The invention for oxidizing CO is an exhaust gas purifying catalyst in which a noble metal for promoting an oxidation reaction and a noble metal for promoting a water gas conversion reaction are supported on a base material.

With such a catalyst, CO can be oxidized and purified when the exhaust gas temperature is low by the above-described water gas conversion reaction, and the catalyst temperature can be quickly raised by this reaction. Is increased by oxidation reaction with a noble metal for promoting oxidation reaction.
O can be efficiently purified by oxidation.

The base material is preferably zeolite,
Pt is preferable as the noble metal for promoting the oxidation reaction,
Ir is preferable as the noble metal for promoting the water gas conversion reaction.

[0017]

As described above, according to the invention of this application, the first catalyst for oxidizing NO to NO ₂ and the NO ₂
A second catalyst that reduces to N ₂ by reacting with
A third catalyst for oxidizing HC and CO, and a second catalyst for the first catalyst
Since the third catalyst is disposed in the exhaust passage so as to come into contact with the exhaust gas after the catalyst and the third catalyst comes into contact with the exhaust gas after the second catalyst, the reducing agent necessary for the reduction of NO ₂ by the second catalyst is used. While securing HC, HC and CO that have not been oxidized by the second catalyst can be oxidized by the third catalyst, which is advantageous for comprehensive purification of NO, HC, and CO.

[0018] If the precious metal for accelerating the oxidation reaction and the noble metal for accelerating the water gas conversion reaction are supported on the base material as the third catalyst, HC is reduced from the time when the exhaust gas temperature is low. While preventing unreleased release, CO can be purified by the water gas conversion reaction, and the reaction heat is used to quickly raise the catalyst temperature, and the noble metal for promoting the oxidation reaction allows HC and HC to be removed. C
O can be efficiently purified. In particular,
When Pt and Ir are supported on zeolite, high CO
Purification rate can be obtained.

Therefore, even in the invention relating to a catalyst for oxidizing CO, the precious metal for accelerating the oxidation reaction and the noble metal for accelerating the water gas conversion reaction are supported on the base material, so that the CO is efficiently purified. can do.

[0020]

Embodiments of the present invention will be described below with reference to the drawings.

<Structure of Exhaust Gas Purifying Catalyst> FIG. 1 shows a structure of an exhaust gas purifying catalyst suitable for purifying NO, HC, CO, and PM in exhaust gas of a diesel engine. In the figure, reference numeral 1 denotes N in exhaust gas.
A first catalyst (oxidation catalyst) for oxidizing O to NO ₂ , reference numeral 2 denotes NO ₂ oxidized by the first catalyst 1 to HC
A second catalyst (reduction catalyst) for reducing to N ₂ by reacting with N ₂ , a reference numeral 3 denotes a third catalyst for oxidizing HC and CO in the exhaust gas, and these catalysts 1 to 3 They are arranged in order from the upstream side to the downstream side in the direction A.

The first catalyst 1 is a catalyst in which Ag and a transition metal having a higher ionization tendency, for example, Ni, are supported on γ-alumina as a base material (support material). Manufactured by the company). The second catalyst 2 is a catalyst in which a transition metal, for example, Pt is supported on zeolite as a base material by ion exchange or the like, and is also supported on a honeycomb carrier. The third catalyst 3 is a noble metal for promoting an oxidation reaction, for example, P
t and a noble metal for promoting a water gas conversion reaction, for example, Ir, which is supported on a base material (for example, zeolite);
Similarly, it is carried on a honeycomb carrier.

The second catalyst 2 and the third catalyst 3 can be supported in layers on the same honeycomb carrier so that the former is on the outside and the latter is on the inside.

The third catalyst 3 is used for oxidizing and purifying only CO or for oxidizing and purifying CO and HC, and is not combined with the first and second catalysts 1 and 2. May be used alone.

[0025]

EXAMPLES Exhaust gas purifying catalysts of the following Examples 1 to 8 were prepared and their purifying performance was evaluated.

<Preparation of Exhaust Gas Purification Catalyst Example> Example 1 Preparation of First Catalyst A high specific surface area γ-alumina powder and a hydrated alumina powder (binder) were mixed at a ratio of 10: 1. Water was added to the mixed powder to prepare a slurry. A cordierite honeycomb carrier (capacity 0.7 L, cell density 4
The above-mentioned operation of immersing a cell (00 cells / square inch, cell wall thickness 6 mils), pulling it up and drying it in an oven is repeated, whereby the above-mentioned γ-alumina is added to the honeycomb carrier at 150 g / L (grams per liter of carrier). Supported.

The alumina coat layer of the honeycomb carrier was impregnated with an aqueous solution of nickel nitrate so that the Ni content was 26 g / L, followed by drying at 200 ° C. for 2 hours and firing at 500 ° C. for 2 hours. The obtained product was heated at 600 ° C. in a vacuum heating furnace at a vacuum degree of 10 ⁻⁴ to 10 ⁻⁵ Torr (O ₂ concentration: 2.0 × 10 ^{−3 to} 2.0 × 10 ⁻⁴ %). Heat treatment was performed for a time.

Next, an aqueous solution of silver acetate was impregnated into the above-mentioned coat layer carrying Ni so that the amount of Ag was 30 g / L, followed by drying at 200 ° C. for 2 hours and baking at 500 ° C. for 2 hours. . The obtained product was placed in a vacuum heating furnace for 10 hours.
Heat treatment was performed at 600 ° C. for 6 hours at a degree of vacuum of ⁻⁴ to 10 ⁻⁵ Torr. The amount of impurities in the obtained catalyst is 1% or less. This is the same for the other catalysts described below.

Preparation of Second Catalyst An aqueous solution of hexaammineplatinum tetrachloride was added to H-type MFI powder having a cubic ratio of 30 and subjected to an ion exchange treatment at 80 ° C. for about 3 hours. After cooling to room temperature, filtration was performed, and the obtained solid was washed with water, dried, and dried.
By firing at 2 ° C. for 2 hours, Pt ion-exchanged MFI (Pt ⁺ -MFI) was obtained. This was mixed with hydrated alumina, and water was added thereto to form a slurry, which was wash-coated on a cordierite honeycomb support (capacity: 1.3 L, cell density: 400 cells / square inch, cell partition thickness: 6 mil), and dried. And firing. The Pt amount of this catalyst is 0.1.
It was adjusted to 5 g / L.

Preparation of Third Catalyst An aqueous solution of dinitrodiamineplatinum chloride and an aqueous solution of iridium trichloride are added to H-type MFI powder having a cubic ratio of 30, and the resulting mixture is spray-dried (dried) and calcined to obtain a catalyst powder. (Pt, Ir / MFI) was prepared. This is mixed with hydrated alumina, water is added thereto to form a slurry, and the slurry is wash-coated on a cordierite honeycomb support (capacity: 1.0 L, cell density: 400 cells / square inch, cell partition thickness: 6 mil), and dried. And firing. The Pt amount of this catalyst was set to 2.0 g / L, and the Ir amount was set to 1.0 g / L.

Example 2 In this example, the same Pt ion-exchanged MFI (Pt ⁺ -MFI) as the second catalyst was employed as the third catalyst, and the honeycomb carrier had a capacity of 1.0 L and the Pt amount was 0.5 g / L.
Other configurations are the same as in Example 1.

Example 3 In this example, Pt-supported MFI (Pt / MFI) was employed as the third catalyst, and the honeycomb carrier had a capacity of 1.0 L,
The Pt amount is 2.0 g / L. The Pt-supported MFI was prepared by adding an aqueous solution of dinitrodiamine platinum chloride to an H-type MFI powder having a Cibaban ratio of 30, followed by spray drying and firing. Other configurations are the same as in Example 1.

Example 4 In this example, β zeolite (Pt, Ir / β) supporting Pt and Ir was adopted as the third catalyst, and the honeycomb carrier had a capacity of 1.0 L and Pt The amount is 2.0 g / L, Ir
The amount is 1.0 g / L. This (Pt, Ir / β) was prepared by replacing the MFI in the third catalyst of Example 1 (Pt, Ir / MFI) with β zeolite. Other configurations are the same as in Example 1.

Example 5 In this example, γ-supporting Pt and Ir as a third catalyst was used.
Alumina (Pt, Ir / Al ₂ O ₃ ) is adopted, the honeycomb carrier has a capacity of 1.0 L, the Pt amount is 2.0 g / L,
The amount of Ir is 1.0 g / L. This (Pt, Ir / Al ₂ O ₃ )
It was prepared by replacing MFI in the third catalyst of Example 1 (Pt, Ir / MFI) with γ-alumina. Other configurations are the same as in Example 1.

Example 6 In this example, an MFI (Pd / MF) supporting Pd as a third catalyst was used.
The honeycomb carrier has a capacity of 1.0 L and a Pd content of 2.0 g / L. This (Pd / MFI)
Was prepared by adding an aqueous solution of palladium nitrate to H-type MFI powder having a Cayban ratio of 30 and performing spray drying and firing. Other configurations are the same as in Example 1.

Example 7 In this example, MFI (Rh / MF) supporting Rh as the third catalyst was used.
The honeycomb carrier has a capacity of 1.0 L and a Rh content of 2.0 g / L. This (Rh / MFI)
Was prepared by adding an aqueous solution of rhodium nitrate to H-type MFI powder having a Cayban ratio of 30 and performing spray drying and firing. Other configurations are the same as in Example 1.

Example 8 In this example, MFI (Ir / MF) supporting Ir as the third catalyst was used.
The honeycomb carrier has a capacity of 1.0 L and an Ir amount of 2.0 g / L. This (Ir / MFI)
Was prepared by adding an aqueous solution of iridium trichloride to H-type MFI powder having a Cayban ratio of 30, followed by spray drying and firing. Other configurations are the same as in Example 1.

<Evaluation Test of Catalyst> The purification performance of each of the above-mentioned catalysts was evaluated by an actual vehicle. The vehicle is
It is equipped with an in-line four-cylinder direct-injection diesel engine (displacement 2.0 L) with a turbocharger, and the catalyst is disposed in a portion of the exhaust pipe that passes from under the engine room to under the passenger compartment. The test is performed in the EU mode (the low speed mode E
(CER15 combined with EUDC which is a high-speed mode), and the total CO purification rate, HC purification rate, PM purification rate, and NOx purification rate were measured. The results are shown in FIG.

With respect to the HC purification rate, Examples 1 to 5 all show high values, but Examples 6 to 8 show relatively low values. This is presumably because in the former five cases, the third catalyst contained Pt having a high oxidation catalyst function, whereas the latter three did not.

Regarding the CO purification rate, Example 1 (third catalyst;
(Pt, Ir / MFI) is much higher than the other examples.
This is because, in the case of Example 1, in addition to the noble metal Pt for accelerating the oxidation reaction, it has a noble metal Ir for accelerating the water gas conversion reaction, and adopts MFI as the base material. The following items are considered to be the causes of the high CO purification rate.

Even when the exhaust gas temperature is as low as 100 ° C. or less, the water gas conversion reaction (CO + H ₂ O → CO ₂ +
H ₂ ), but the conversion is promoted by the Ir.

As a result, the temperature of the third catalyst quickly rises, Pt exhibits catalytic activity, and the oxidation reaction of CO is accelerated.

At the time when the MFI starts to desorb the adsorbed HC, the catalyst usually has not yet reached the temperature range where Pt has yet shown the peak of the catalytic activity,
During the period from the start of desorption to reach the activity peak, HC is not completely oxidized and CO is generated by partial oxidation. However, since the catalyst temperature rises rapidly as described above, the period of partial oxidation is shortened, and Low generation of CO due to oxidation.

The CO generated by the partial oxidation is efficiently oxidized and removed by promoting the water gas conversion reaction by the Ir.

The high acidity of the MFI surface is advantageous for the oxidation of CO by Pt.

On the other hand, Example 2 (third catalyst; Pt ⁺ -MFI)
It is considered that the reason why the CO purification rate is low is that the above-mentioned water gas conversion reaction cannot be promoted because Ir is not provided. The low CO purification rate of Example 3 (third catalyst; Pt / MFI) is considered to be due to the same reason as in Example 2, but the higher than Example 2 is higher than that of Pt supported by ion exchange. It is considered that the supported Pt is more advantageous for the oxidation of CO, and the supported Pt in Example 3 has a larger Pt supported amount.

Example 4 (third catalyst; Pt, Ir / β) has the same Pt and Ir as in Example 1, but the supported base material is β zeolite, and the interaction with Ir and Pt on the catalytic reaction. Is inferior to MFI, and the surface acidity of β zeolite is lower than that of MFI.
It is considered that the O purification rate was not obtained. Example 5 (third catalyst; Pt, Ir / Al ₂ O ₃ ) also has Pt and Ir as in Example 1, but has a high CO purification rate because its supporting base material is γ-alumina instead of MFI. It is probable that was not obtained.

Example 6 (third catalyst; Pd / MFI) and Example 7 (third catalyst
Catalyst / Rh / MFI) is considered to have a low CO purification rate because it does not have Ir as in Examples 2 and 3.
Example 8 (third catalyst; Ir / MFI) is considered to have a low CO purification rate because the effect of promoting the water gas conversion reaction by Ir was not obtained but the effect of promoting the oxidation reaction was not obtained. Can be

Next, regarding the PM purification rate, Example 1 (third
The catalyst (Pt, Ir / MFI) is much higher than in other examples. This is probably because SOF (soluble organic matter) was decomposed. That is, PM is SOF, SOOT
(Soot) and sulfate (sulfuric acid or SO ₃ )
In the case of a diesel engine, SOF generally occupies 30 to 40%, SOOT occupies 50 to 60%, and the rest is sulfates. Among these, SOF is considered to be decomposed by the catalyst, and it is considered that this decomposition increases the PM purification rate.

Also, since the PM purification rate of Example 3 (third catalyst; Pt / MFI) is higher than that of Example 2 (third catalyst; Pt ⁺ -MFI),
It was found that carrying Pt on MFI to dryness was advantageous for the purification of PM, and that Example 1 was more advantageous than Example 3, indicating that carrying Ir was advantageous. 4 (third catalyst; Pt, Ir / β) and Example 5 (third catalyst;
Pt, Ir / Al ₂ O ₃ )
It can be seen that it is advantageous to employ MFI as the base material.

Next, regarding the NOx purification rate, there is no significant difference between the examples. Therefore, disposing the third catalyst on the downstream side of the second catalyst in the exhaust gas flow direction is effective for securing the NOx purification rate.
It is understood that the use of FI and the use of Pt and Ir as the noble metals do not impair the NOx purification performance.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an arrangement configuration of an exhaust gas purifying catalyst according to an embodiment of the present invention.

FIG. 2 is a graph showing a mode purification rate of each example in which a third catalyst is different.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 1st catalyst 2 2nd catalyst 3 3rd catalyst

Continued on the front page F term (reference) 4D048 AA06 AA13 AA18 AA21 AB01 AB02 AB05 BA03X BA10X BA11X BA30X BA31Y BA32Y BA33X BA34X BA36Y BA37Y BA38X BA41X BB02 CC32 CC47 4G069 AA03 AA08 BA01A BA01B BABA BCB BC BC BC BC BC69A BC71B BC74A BC74B BC75A BC75B CA02 CA03 CA07 CA08 CA13 CA14 CA15 CA18 CC26 EA19 EB10 EB12Y EB15Y EC28 EE09 FA01 FA03 FB14 FB23 FB30 FB80 ZA01A ZA11A ZA11B ZA19B ZA32 ZF05

Claims (11)

[Claims] An exhaust gas purifying catalyst provided in an exhaust passage from which NO, HC, and CO are discharged, wherein a first catalyst that oxidizes NO to NO ₂ and a catalyst that reacts NO ₂ with HC. A second catalyst for reducing to N ₂ and HC
And a third catalyst for oxidizing CO, wherein the first to third catalysts are such that the second catalyst contacts exhaust gas after the first catalyst, and the third catalyst emits exhaust gas after the second catalyst. An exhaust gas purifying catalyst disposed in the exhaust passage so as to contact the exhaust gas. 2. The exhaust gas purifying catalyst according to claim 1, wherein the third catalyst has a catalytic component in which a noble metal is supported on zeolite. 3. The exhaust gas purifying catalyst according to claim 2, wherein the noble metal comprises a noble metal for promoting an oxidation reaction and a noble metal for promoting a water gas conversion reaction. 4. The exhaust gas purifying catalyst according to claim 3, wherein the noble metal for promoting the water gas conversion reaction is Ir. 5. The exhaust gas purifying catalyst according to claim 3, wherein the noble metal for promoting the oxidation reaction is Pt. 6. The exhaust gas purifying catalyst according to claim 1, wherein the first catalyst contains Ag. 6. The exhaust gas purifying catalyst according to claim 1, wherein the first catalyst contains Ag. 7. The exhaust gas purifying catalyst according to any one of claims 1 to 5, wherein the second catalyst has a catalyst component in which a transition metal is supported on zeolite. Exhaust gas purification catalyst. 8. The exhaust gas purifying catalyst according to claim 7, wherein the second catalyst has a catalytic component in which Pt is supported on zeolite by ion exchange. 9. An exhaust gas purifying catalyst provided in an exhaust passage from which NO, HC, and CO are exhausted, wherein the first catalyst oxidizes NO to NO ₂ and the NO ₂ reacts with HC. A second catalyst for reducing to N ₂ , and a third catalyst having a catalyst component comprising Pt and Ir supported on zeolite, wherein the first to third catalysts are such that the second catalyst is more than the first catalyst. An exhaust gas purifying catalyst disposed in the exhaust passage so that the exhaust gas contacts the exhaust gas later and the third catalyst contacts the exhaust gas later than the second catalyst. 10. An exhaust gas purifying catalyst for oxidizing CO, wherein a precious metal for promoting an oxidation reaction and a noble metal for promoting a water gas conversion reaction are supported on a base material. 11. The exhaust gas purifying catalyst according to claim 9, wherein the base material is zeolite, the noble metal for promoting the oxidation reaction is Pt, and the noble metal for promoting the water gas conversion reaction is An exhaust gas purifying catalyst that is Ir.