JPH07132226A - Catalyst for purifying exhaust gas - Google Patents

Abstract

PURPOSE:To improve both purifying performance and durability at a low temp. in the subject catalyst carrying a noble metal catalyst and a NOx absorbing metal. CONSTITUTION:This catalyst is composed of a honeycomb carrier 1, the noble metal catalyst 3 deposited uniformly in an axis direction of the carrier and the NOx absorbing metal 4 consisting of at least either alkaline earth metal or alkali metal and being deposited so as to have a concn. distribution in the axis direction of the carrier. Both NOx-absorbing function due to the NO. absorbing metal and ternary catalyst function due to the noble metal catalyst are surely exhibited.

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, and more specifically, carbon monoxide (CO) contained in exhaust gas.
Nitrogen oxides (NO) in exhaust gas that contain excess oxygen than that required to oxidize hydrocarbons and hydrocarbons (HC).
The present invention relates to an exhaust gas purification catalyst that can efficiently purify x) and a method for manufacturing the same.

[0002]

2. Description of the Related Art Conventionally, a three-way catalyst for purifying exhaust gas by simultaneously oxidizing CO and HC and reducing NOx has been used as a catalyst for purifying exhaust gas of an automobile.
As such a catalyst, for example, a catalyst in which a supporting layer made of γ-alumina is formed on a heat-resistant carrier such as cordierite and a noble metal catalyst such as Pt, Pd, Rh is supported on the supporting layer is widely known. There is.

By the way, the purification performance of such an exhaust gas purification catalyst greatly differs depending on the air-fuel ratio (A / F) of the engine. That is, on the lean side where the air-fuel ratio is large, that is, where the fuel concentration is lean, the amount of oxygen in the exhaust gas is large, and the oxidation reaction for purifying CO and HC is active, but N
The reduction reaction for purifying Ox becomes inactive. On the contrary, on the rich side where the air-fuel ratio is small, that is, where the fuel concentration is high, the amount of oxygen in the exhaust gas is small, and the oxidation reaction becomes inactive but the reduction reaction becomes active.

On the other hand, when driving an automobile, acceleration and deceleration are frequently performed in urban areas, and the air-fuel ratio frequently changes within the range from near stoichiometric (theoretical air-fuel ratio) to the rich state. In order to meet the demand for low fuel consumption in such traveling, it is necessary to operate on the lean side to supply an air-fuel mixture with excess oxygen as much as possible. Further, it is known that a fine lean pulse of about several seconds is generated in a transitional region accompanying acceleration / deceleration, and it is also known that a large amount of unpurified NOx is discharged in this lean pulse. Therefore, it is desired to develop a catalyst that can sufficiently purify NOx even on the lean side (lean pulse).

Therefore, the applicant of the present application has previously proposed a catalyst carrying an alkaline earth metal and Pt (Japanese Patent Application No. 4-1).
30904). According to this catalyst, NOx is absorbed by the alkaline earth metal and is reacted with a reducing gas such as HC to be purified, so that the lean side is also excellent in the purification performance of NOx. In the catalyst disclosed in Japanese Patent Application No. 4-130904, for example, barium is supported on a carrier as a single oxide, and it reacts with NOx to generate barium nitrate (Ba (NO ₃ ) ₂ ) to generate NOx. It is considered to adsorb.

Further, in Japanese Patent Laid-Open No. 3-106446,
An exhaust gas purifying catalyst carrying coexisting barium and Pd is disclosed.

[0007]

However, when an element having an NOx absorbing action such as an alkaline earth metal is added to the conventional three-way catalyst, the initial NOx purification performance is extremely excellent, but the durability is low. There was a problem of being bad. It was also revealed that there is a problem that the purification performance at low temperatures is low.

The present invention has been made in view of the above circumstances, and it is an object of the present invention to improve both the purification performance and durability at low temperature in an exhaust gas purification catalyst carrying a noble metal catalyst and a NOx absorbing metal. With the goal.

[0009]

The exhaust gas purifying catalyst of the present invention for solving the above-mentioned problems is a carrier which is made of a porous body and has a honeycomb passage extending in the axial direction, and is uniformly supported in the axial direction of the carrier. It is characterized by comprising a noble metal catalyst and a NOx absorbing metal which is composed of at least one of an alkaline earth metal and an alkali metal and has a concentration distribution in the axial direction of the carrier.

As the carrier made of a porous body, a monolith carrier in which a carrier body made of cordierite or the like is coated with a carrier layer made of alumina or the like, and metal flat plates and corrugated plates are alternately laminated on the carrier body made of alumina or the like. A conventional metal carrier such as a metal carrier coated with is used. Noble metal catalysts include Pt, Pd, and Rh, and the supported amount thereof is preferably 1 g to 2 g per liter of the carrier. If it is less than 1 g, the purification performance is low, and if it is more than 2 g, the effect is saturated and only the cost is increased. The noble metal catalyst is uniformly supported on the entire length of the carrier in the axial direction.

Alkaline earth metals include Ca, Sr,
Ba or the like is used, and alkali metals such as Li and N are used.
a, K, etc. are used. One of these may be used, or two or more of them may be coexistently supported. These metals are NO
Since it has an x-absorbing effect, it is called a NOx absorbing metal. This NOx absorbing metal is carried with a concentration distribution in the axial direction of the carrier. The amount of the NOx absorbing metal supported is preferably 0.1 to 0.5 mol per liter of the carrier. 0.
When it is less than 1, no NOx absorption effect is obtained, and when it is more than 0.5, there is a problem such as coating with a noble metal or reduction in heat resistance of alumina.

The concentration distribution may be carried out, for example, in a structure in which it is carried only in the upstream half or in the downstream half, in a structure having a predetermined width only in the central portion in the axial direction, or as it goes from the upstream side to the downstream side. Various modes can be selected according to the purpose, such as a structure in which the amount is small or large. It is preferable that the NOx-absorbing metal-supported portion is about 50%.

The NOx absorbing metal is generally supported in the form of an oxide.

[0014]

The present inventors have conducted extensive studies on the cause of the above-mentioned problems in the exhaust gas purifying catalyst in which the alkaline earth metal and the precious metal catalyst are co-loaded, and as a result, the alkaline earth metal reduces the active sites of the precious metal catalyst. As a result, it was clarified that the catalyst activity at low temperature decreased.

Further, at a high temperature, there occurs a phenomenon that the alkaline earth metal and alumina react with each other to reduce the specific surface area, which directly affects the coexisting and supported noble metal catalyst, so that the durability may be reduced. it was thought. Therefore, in the present invention, the noble metal catalyst is uniformly loaded in the axial direction, and the NOx absorbing metal is loaded with a concentration distribution in the axial direction. Therefore, direct contact between the NOx absorbing metal and the noble metal catalyst can be avoided in the portion where the NOx absorbing metal is not supported, or in the portion where the NOx absorbing metal is supported in a small amount, and the reduction of the active site of the noble metal catalyst is prevented. Then, the noble metal catalyst performs the oxidation reaction of HC and CO and the reduction reaction of NOx similarly to the conventional three-way catalyst.

On the other hand, in the portion where the NOx absorbing metal is carried, NOx in the exhaust gas is absorbed by the NOx absorbing metal under lean conditions, and at the same time the NOx absorbed under stoichiometric or rich conditions is released. The precious metal catalyst reacts with HC and CO for reduction and purification. Also N
Even if the Ox absorbing metal reacts with the alumina, in the portion where the NOx absorbing metal is not supported or in the portion where the amount of NOx absorbing metal supported is small, the direct influence on the noble metal catalyst is avoided, so the heat resistance is improved. As a result, durability is improved.

[0017]

EXAMPLES The present invention will be specifically described below with reference to Examples and Comparative Examples. (Embodiment 1) FIG. 1 shows a schematic sectional view of an exhaust gas purifying catalyst according to an embodiment of the present invention, and FIG. 2 shows an enlarged sectional view of a main part thereof. This exhaust gas purifying catalyst comprises a carrier base 1, an alumina coat layer 2 formed on the surface of the carrier base 1, a noble metal catalyst 3 made of Pt and Rh supported on the alumina coat 2, and a carrier base. The axial length from the end face of the material 1 is 25
% Consisting of barium supported only in the% portion (L)
and x-absorbing metal 4.

The method for producing this exhaust gas purifying catalyst will be described below. In the following examples, "parts" means "parts by weight" unless otherwise specified. Alumina powder 10
0 parts and alumina sol (alumina content 10 wt%) 1
0 parts, 60 parts of 40 wt% aluminum nitrate aqueous solution, 40 parts of cerium oxide and 30 parts of water were mixed to prepare a coating slurry.

The cordierite honeycomb carrier substrate 1 (volume 1.7 l) was dipped in water, and after blowing off excess water, it was dipped in the above-mentioned coating slurry. After taking it out, the excess slurry was blown off. 600 after drying at ℃ for 20 minutes
The alumina coating layer 2 was formed by firing at 1 ° C. for 1 hour. The coating amount of the alumina coat layer 2 is 1 volume of the honeycomb carrier.
It is 160 g per liter.

The honeycomb carrier obtained as described above was dipped in a dinitrodiammine platinum aqueous solution of a predetermined concentration, pulled up to blow off excess water drops, and then dried at 250 °. Then, it was dipped in a rhodium chloride aqueous solution of a predetermined concentration, pulled up, blown off excess water droplets, and then dried at 250 °. As a result, 1.5 g of Pt and 0.3 g of Rh were uniformly loaded per liter of the honeycomb carrier.

Next, a barium acetate aqueous solution having a predetermined concentration is prepared, and a portion (L) of 25% of the axial length is immersed from one end face of the above honeycomb carrier carrying Pt and Rh, and after drying 60
The catalyst for exhaust gas purification of Example 1 was obtained by firing at 0 ° C. for 1 hour. In this 25% portion, 0.3 mol of Ba is loaded per liter of the honeycomb carrier. (Embodiment 2) Ba is made to have an axial length of 5 from one end face of the honeycomb carrier.
The structure is the same as that of Example 1 except that it is carried on the 0% portion. (Comparative Example 1) Example 1 except that Ba was not carried.
It has the same configuration as. (Comparative Example 2) The configuration is the same as that of Example 1 except that Ba is uniformly loaded on the entire honeycomb carrier. (Evaluation) With respect to the above four kinds of exhaust gas purifying catalysts, the low temperature activity and the transient performance were evaluated on the engine bench at the initial stage and after the endurance. The test method is as follows, and the results are shown in Table 1.

Low temperature activity: A / F = 14.6 (stoichiometric)
Was maintained, and the 50% purification temperature of each of HC, CO, and NOx when the exhaust gas temperature was raised to 200 to 400 ° C. was measured. Transient performance: Maintain exhaust gas temperature at 400 ° C, A / F =
After holding 14.6 (stoichi) for 30 seconds, A / F =
After changing to 16.0 (lean) and holding the lean state for 10 seconds, the A / F = 14.6 (stoichi) was returned again and held for 20 seconds, and the NOx purification rate (average) for 1 minute was measured.

Durability test: Attached to the exhaust system of an engine with a displacement of 2 liters, exhaust gas temperature 800 ° C., A / F = 14.6
After running for 50 hours under the condition of (stoichi), the above test was conducted. With respect to the exhaust gas purifying catalysts of the examples, two kinds of evaluation were performed, respectively, in the case where the Ba-supported portion was located on the upstream side of the exhaust gas and when it was located on the downstream side.

[0024]

[Table 1] From the initial data of Table 1, the exhaust gas purifying catalyst of the example is superior to the comparative example 2 in low-temperature activity, and is equivalent to the conventional three-way catalyst of the comparative example 1. Further, the NOx purification rate shows a high value equivalent to that of Comparative Example 2.

After endurance, the example has a low temperature activity equivalent to that of the comparative example 1, and is superior to the comparative example 2 in the low temperature activity. Further, it is clear that in Comparative Example 2, the NOx purification rate is greatly reduced, whereas in the Example, the degree of reduction is small and the durability is excellent. That is, as is clear from the above evaluation, when Ba is carried over the entire length, the NOx absorption performance is excellent, but after the endurance, the activity of the noble metal catalyst decreases due to the coating of active sites with Ba and sintering, etc. Inferior to. But Ba
By partially supporting, the degree of decrease in NOx absorption performance after endurance is reduced while maintaining the action of the noble metal catalyst, and the degree of decrease is equivalent to that in Comparative Example 1.

When comparing the upstream side and the downstream side at the Ba loading position, the NOx purification rate is higher on the upstream side. Further, the NOx purification rate of Example 2 is higher than that of Example 1. Therefore, it can be seen from Table 1 that it is preferable that the carrying position of Ba is on the upstream side and Ba is 50% rather than 25% of the axial length.

[0027]

As described above, according to the exhaust gas purifying catalyst of the present invention, the NOx absorbing metal is carried to provide excellent NOx purifying performance on the lean side as well.
Since the reduction of the active site of the noble metal catalyst is prevented by the distribution and support of the absorbing metal, it exhibits high catalytic activity even at low temperature, and is excellent in heat resistance and durability.

[Brief description of drawings]

FIG. 1 is a schematic sectional view of an exhaust gas purifying catalyst according to an embodiment of the present invention.

FIG. 2 is an enlarged schematic cross-sectional view of a main part of FIG.

[Explanation of symbols]

1: Honeycomb carrier substrate 2: Alumina coat layer 3: Noble metal catalyst 4: NOx
Absorbing metal

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location B01J 35/04 ZAB 8017-4G 301 L 8017-4G B01D 53/36 104 A

Claims (1)

[Claims] 1. A carrier comprising a porous body having a honeycomb passage extending in the axial direction, a noble metal catalyst uniformly supported in the axial direction of the carrier, and at least one of an alkaline earth metal and an alkali metal. An exhaust gas purifying catalyst, comprising: a NOx absorption metal supported with a concentration distribution in the axial direction.