JP3335755B2 - Exhaust gas purification catalyst - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to carbon monoxide (C) contained in exhaust gas from an internal combustion engine such as an automobile engine.
O), a hydrocarbon (HC) and a nitrogen oxide (NOx).

[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 an exhaust gas purifying catalyst for automobiles.
As such a three-way catalyst, for example, a support layer made of γ-alumina is formed on a heat-resistant carrier made of cordierite or the like, and a platinum group element such as platinum or rhodium is supported on the support layer. Are known. Also known is a three-way catalyst in which a cerium oxide (cerium oxide) having an oxygen storage effect is used in combination to increase the A / F window width.

[0003] Palladium, which is a white metal element, is excellent in the purification performance of CO and HC in an oxidizing atmosphere, but is inferior in the purification performance of NOx in a rich atmosphere and slightly leaner than the stoichiometric air-fuel ratio (stoichiometric). . Therefore, it is considered that purification of NOx is difficult only with palladium, and it is used as a three-way catalyst when used in combination with rhodium having high NOx purification performance. However, since rhodium is very expensive, it becomes an expensive exhaust gas purifying catalyst.

[0004] Therefore, only a palladium is used as a platinum group element and an exhaust gas having a low cost and excellent ternary activity is provided which has a carrier layer containing cerium oxide, barium compound, zirconium compound, lanthanum oxide, activated alumina and the like. Purification catalysts have been proposed (Japanese Patent Laid-Open Nos. 5-168926 and 5-68927).

[0005]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
Three-way catalyst combining palladium with various co-catalysts
The water gas shift reaction (C
O + H_TwoO → CO_Two+ H _Two) And steam reforming reaction (HC +
H_TwoO → CO_Two+ H_Two) To improve NOx purification performance
NOx purification performance on the lean side
Not a minute.

A three-way catalyst obtained by combining palladium with various cocatalysts shows good three-way activity in the early stage of use, but when used at a high temperature (800 ° C. or higher), palladium causes sintering and the catalytic performance deteriorates. There was a problem of doing. The present invention has been made in view of such circumstances, further improving the three-way catalyst using palladium as a catalyst component, further improving the NOx purification performance, by preventing sintering of palladium, CO, HC and NOx in the initial stage and after endurance
It is an object of the present invention to be able to stably purify.

[0007]

The exhaust gas purifying catalyst of the present invention which solves the above-mentioned problems comprises a carrier substrate, a cerium oxide powder and an activated alumina powder formed on the surface of the carrier substrate .
And Ranaru catalyst supporting layer, palladium supported on the catalyst supporting layer, it is composed of a catalytically active component consisting of alkaline earth metals and platinum supported of palladium on activated alumina
The platinum is carried on cerium oxide powder, and is characterized in that palladium is contained more in weight than platinum (Pt ≦ Pd).

As the carrier substrate, a monolithic carrier or a pellet carrier having an integral structure is used. The material may be cordierite, mullite, α-alumina, ceramics such as aluminosilicate, or Fe
-A refractory metal such as a Cr-Al alloy is exemplified. The catalyst support layer consists of cerium oxide powder and activated alumina powder.
Become.

The catalyst supporting layer supports palladium, an alkaline earth metal and platinum. As the alkaline earth metal, barium is particularly desirable, but calcium,
Strontium and radium can also be used. At the same time, cerium oxide, zirconium oxide,
It is also preferable to contain lanthanum oxide, and particularly desirable to contain cerium oxide.

The greatest feature of the present invention is that palladium is contained more than platinum in terms of weight (Pt ≦ Pd). Less palladium than platinum (Pt> Pd)
Then, the ternary activity near the stoichiometric air-fuel ratio decreases. Further, the supported amount of palladium and platinum is 1 by weight.
It is preferable that / 10 ≦ Pt / Pd ≦ 1/1. When 1/10> Pt / Pd, the NOx purification performance in a slightly lean atmosphere is lower than the stoichiometric air-fuel ratio. A particularly desirable range is 1/4 ≦ Pt / Pd ≦ 1
/ 2.

In the exhaust gas purifying catalyst of the present invention,
The catalyst supporting layer is preferably composed of cerium oxide powder and activated alumina powder, with palladium supported on activated alumina powder and platinum supported on cerium oxide powder. Further, the catalyst supporting layer has a two-layer structure of a lower layer formed on the surface of the carrier substrate and an upper layer formed on the lower layer surface, and platinum is supported on the cerium oxide powder in the lower layer and palladium on the activated alumina powder in the upper layer. It is desirable to have a supported configuration.

[0012]

In the exhaust gas purifying catalyst of the present invention, palladium is contained more in weight than platinum. Therefore, the balance between the characteristics of palladium excellent in ternary activity near the stoichiometric air-fuel ratio and the characteristics of platinum excellent in NOx purification performance in the lean atmosphere side is optimized, and the NOx purification performance near the stoichiometric air-fuel ratio and slightly in the lean atmosphere side is improved. .

The carried alkaline earth metal promotes the above-mentioned water gas shift reaction and steam reforming reaction,
The NOx purification performance on the rich atmosphere side is improved. A co-catalyst, such as cerium oxide, zirconium oxide, or lanthanum oxide, which is further added as required, has the same effect. When platinum becomes an oxide, sintering is promoted,
It is known that catalyst performance is reduced. Therefore, if platinum is supported on cerium oxide powder, cerium oxide has an oxygen storage effect, so that oxidation of platinum is prevented, and thus sintering of platinum can be prevented.

On the other hand, palladium is more stable in the form of an oxide and hardly causes sintering. Therefore, when palladium is supported on activated alumina powder, palladium is easily oxidized, and sintering is prevented. In other words, by supporting platinum on cerium oxide powder and palladium on activated alumina powder, it is possible to optimize the interaction between platinum and palladium with cerium oxide and alumina, and suppress the sintering of platinum and palladium respectively. In addition, since the probability of contact between platinum and palladium is low, alloying of both is suppressed. Therefore, deterioration of the catalyst performance is prevented, and the initial catalyst performance can be maintained for a long time.

Further, if the cerium oxide powder carrying platinum is disposed in the lower layer of the catalyst supporting layer and the activated alumina powder carrying palladium is disposed in the upper layer of the catalyst supporting layer, the cerium oxide powder is supported by the cerium oxide powder. Since the probability of contact between platinum and oxygen is further reduced and the probability of contact between oxygen and palladium carried on the activated alumina powder is further increased, sintering of platinum and palladium can be further suppressed, and catalyst performance can be reduced. Can be further prevented from decreasing. Further, since the contact probability between platinum and palladium is further reduced, alloying can be further suppressed.

[0016]

The present invention will be specifically described below with reference examples, examples and comparative examples . In the following examples, “parts” means “parts by weight” unless otherwise specified. ( Reference Example 1 ) 100 parts of activated alumina powder, 40 parts of cerium oxide powder stabilized with zirconium oxide, and lanthanum carbonate powder 30
, 40 parts by weight of an aqueous solution of aluminum nitrate and 65 parts of water were mixed to prepare a slurry for coating.

A cordierite honeycomb monolithic carrier substrate (1.3 L in volume) is immersed in water, excess water is blown off, then immersed in the slurry, taken out, and excess slurry is blown off. After drying at 250 ° C. for 1 hour, 50
It was baked at 0 ° C. for 1 hour to form a catalyst support layer mainly composed of alumina. The coating amount of the catalyst supporting layer is 190 g per liter of the carrier substrate.

The carrier thus obtained was immersed in an aqueous solution of palladium nitrate having a predetermined concentration for 1 hour, pulled up to blow off excess water, and dried at 250 ° C. The supported amount of palladium was 1.5 g per liter of the carrier. Next, the carrier supporting palladium was immersed in an aqueous solution of dinitrodiammine platinum of a predetermined concentration for 1 hour, pulled up to blow off excess water droplets, and then dried at 250 ° C. The amount of supported platinum was 1.5 g per liter of the carrier.

Further, a carrier (Pt / Pd = 1/1) supporting palladium and platinum is immersed in an aqueous solution of barium acetate having a predetermined concentration, and is lifted to blow off excess water droplets.
Dried at 250 ° C. The supported amount of barium was 30 g per liter of the carrier. ( Reference Example 2 ) The configuration is the same as that of Reference Example 1 , except that 1.0 g of platinum and 2.0 g of palladium (Pt / Pd = 1/2) are supported per liter of the monolithic carrier. ( Reference Example 3 ) The configuration is the same as that of Reference Example 1 except that 0.75 g of platinum and 2.25 g of palladium (Pt / Pd = 1/3) are supported per liter of the monolithic carrier. ( Reference Example 4 ) The configuration is the same as that of Reference Example 1 except that 0.5 g of platinum and 2.5 g of palladium (Pt / Pd = 1/5) are supported per liter of the monolithic carrier. ( Reference Example 5 ) The configuration is the same as that of Reference Example 1 except that 0.27 g of platinum and 2.7 g of palladium (Pt / Pd = 1/10) are supported per liter of the monolith carrier. ( Reference Example 6 ) A catalyst supporting layer was formed using the same slurry as in Reference Example 1 except that lanthanum carbonate was removed, and 0.75 g of platinum and 2.25 g of palladium were used per liter of the monolith carrier.
Reference Example 1 except that (Pt / Pd = 1/3) was carried
This is the same configuration as. ( Reference Example 7 ) A catalyst-supporting layer was formed using the same slurry as in Reference Example 1 except that cerium oxide powder not stabilized with zirconium oxide was used, and 0.75 g of platinum was used per liter of the monolith carrier. , 2.25 g of palladium (Pt / P
(d = 1/3) The configuration is the same as that of Reference Example 1 except that it is supported. (Comparative Example 1) The configuration is the same as that of Reference Example 1 except that platinum was not supported and 3.0 g of palladium was supported per liter of the monolith carrier. (Comparative Example 2) The structure was the same as that of Reference Example 1 except that palladium was not supported and 3.0 g of platinum was supported per 1 L of the monolith carrier. (Comparative Example 3) Barium was not supported, and 0.75 g of platinum and 2.25 g of palladium per liter of the monolith carrier (Pt / P
(d = 1/3) The configuration is the same as that of Reference Example 1 except that it is supported. (Comparative Example 4) The structure was the same as that of Reference Example 1 except that 2.0 g of platinum and 1.0 g of palladium (Pt / Pd = 2/1) were supported per liter of the monolithic carrier. (Comparative Example 5) The structure was the same as that of Reference Example 1 except that 0.19 g of platinum and 2.81 g of palladium (Pt / Pd = 1/15) were supported per liter of the monolithic carrier. ( Example 1 ) Fig. 1 is a schematic cross-sectional view in which main parts of an exhaust gas purifying catalyst of this example are enlarged. The exhaust gas purifying catalyst includes a cordierite honeycomb monolithic carrier substrate 1, a catalyst supporting layer 2 formed on the surface of the carrier substrate 1, Pd and Pt supported on the catalyst supporting layer 2, Similarly, the catalyst supporting layer 2 is composed of lanthanum oxide and barium (not shown).

The catalyst supporting layer 2 is composed of activated alumina powder and cerium oxide powder, Pd is supported on activated alumina powder, and Pt is supported on cerium oxide powder.
This exhaust gas purifying catalyst was produced as follows. First, cerium oxide powder to which zirconium is added is mixed with an aqueous solution of dinitrodiammineplatinum having a predetermined concentration, filtered, dried at 150 ° C. for 5 hours, and calcined at 500 ° C. for 1 hour to carry platinum, and to 40 g of cerium oxide powder, Thus, Pt-supported ceria powder supporting 0.75 g of Pt was obtained.

The activated alumina powder was mixed with an aqueous solution of palladium nitrate having a predetermined concentration, filtered, dried at 150 ° C. for 5 hours, and calcined at 500 ° C. for 1 hour to carry palladium. To 2.25
g of Pd-supported alumina powder was obtained. A slurry was prepared by mixing 40 parts of Pt-supported ceria powder, 100 parts of Pd-supported alumina powder, 30 parts of lanthanum carbonate powder, 65 parts of a 40 wt% aluminum nitrate aqueous solution and 80 parts of water. Then, the cordierite-based monolithic carrier substrate 1 (volume 1.3)
L) is immersed in water, excess water is blown off, and then immersed in the slurry.
After drying at 50 ° C for 1 hour, baking at 500 ° C for 1 hour, Pd
And a catalyst-carrying layer 2 carrying Pt and lanthanum oxide. The coating amount of the catalyst supporting layer 2 is the monolithic carrier substrate 1
Of the monolithic carrier substrate 1
0.75 g of platinum and 2.2 palladium per L
5 g (Pt / Pd = 1/3) is uniformly supported.

Then, the obtained carrier was immersed in an aqueous solution of barium acetate having a predetermined concentration, pulled up to blow off excess water droplets, and dried at 250 ° C. to carry barium. The supported amount of barium is 30 per liter of the monolith carrier volume.
g. ( Example 2 ) FIG. 2 is an enlarged sectional view of a main part of an exhaust gas purifying catalyst of this example.
Shown in The exhaust gas purifying catalyst includes a cordierite honeycomb monolithic carrier substrate 1, a catalyst supporting layer 2 formed on the surface of the carrier substrate 1, Pd and Pt supported on the catalyst supporting layer 2, Similarly, the catalyst support layer 2 is composed of lanthanum oxide and barium (not shown).

The catalyst-carrying layer 2 is composed of a lower layer 20 formed on the surface of the carrier substrate 1 and an upper layer 21 formed on the surface of the lower layer 20. The lower layer 20 and the upper layer 21 are each composed of a cerium oxide powder and an active layer. And alumina powder. Pd is carried on the activated alumina powder in the upper layer 21, and Pt is carried on the cerium oxide powder in the lower layer 20.

This exhaust gas purifying catalyst was produced as follows. First, 40 parts of Pt-supported cerium oxide powder prepared in the same manner as in Example 1 and 100 parts of activated alumina powder
Was mixed with 30 parts of lanthanum carbonate powder, 65 parts of a 40 wt% aluminum nitrate aqueous solution and 80 parts of water to form a slurry. And cordierite-based monolithic carrier substrate 1
(1.3 L) was immersed in water, blown off excess water, immersed in this slurry, taken out, blown off the excess slurry, dried at 250 ° C. for 1 hour, and baked at 500 ° C. for 1 hour. A lower layer 20 carrying Pt was formed.

Next, 100 parts of Pd-supported alumina powder prepared in the same manner as in Example 1 , 40 parts of cerium oxide powder,
A slurry was prepared by mixing 30 parts of lanthanum carbonate powder, 65 parts of a 40 wt% aluminum nitrate aqueous solution and 80 parts of water. Then, the carrier substrate 1 having the cerium oxide layer 22 is immersed in water, the excess water is blown off, then the slurry is immersed in the slurry, the excess slurry is blown off, the excess slurry is blown off, dried at 250 ° C. for 1 hour, and dried at 500 ° C. By firing for 1 hour, an upper layer 21 carrying Pd was formed.

In this exhaust gas purifying catalyst, 0.75 g of platinum and 2.25 g of palladium (Pt / Pd = 1/3) are supported per liter of the monolith carrier substrate.
Then, the obtained carrier is immersed in an aqueous solution of barium acetate having a predetermined concentration, and is lifted to blow off excess water droplets.
It was dried at ℃ to support barium. The supported amount of barium is 30 g per liter of monolithic carrier volume. (Evaluation of Purification Performance) Each of the above exhaust gas purifying catalysts was attached to an exhaust system of an engine having a displacement of 2 L, and was operated for 100 hours at an exhaust gas temperature of 800 ° C. and an A / F of 14.6 (stoichiometric). A durability test was performed.

The low-temperature activity and purification performance of each exhaust gas purification catalyst after the durability test were evaluated, and the results are shown in Table 1. The evaluation of the low-temperature activity was performed by raising the exhaust gas temperature to 250 to 450 ° C. while maintaining A / F = 14.6 (stoichiometric).
The 50% purification temperature of each of HC, CO, and NOx at that time was measured and evaluated. The purification performance is to maintain the exhaust gas temperature at 400 ° C., and to improve the purification rates of HC, CO, and NOx at A / F = 14.6 (stoichiometric) and A / F = 14.8 (slightly leaner than stoichiometric). It was measured and evaluated.

[0028]

[Table 1]

In Reference Examples 1 to 5 in which the Pt / Pd ratio was changed,
Comparative Example 1 (Pd only), Comparative Example 5 (Pt / Pd = 1/1)
The NOx purification rate is higher than that of 5), both the 50% purification temperature and the NOx purification rate are superior to Comparative Example 2 (Pt only), and has performance equal to or higher than Comparative Example 6 (Pt / Rh). are doing. In other words, it can be seen that high purification performance can be obtained by supporting more palladium than platinum and desirably 1/10 ≦ Pt / Pd.

The effect of adding barium and lanthanum as co-catalysts is apparent from the comparison between Reference Examples 3 , 6 and Comparative Example 3. Comparative Example 3 having no barium has a 50% purification temperature and CO, NOx. It is clear that the purification rate is inferior, and the purification performance is improved by the addition of barium. Further, the exhaust gas purifying catalysts of Examples 1 and 2 in which platinum is supported on cerium oxide powder and palladium is supported on activated alumina powder have improved purification performance as compared with Reference Example 3 having the same composition. It can be seen that selective loading is effective. Also implemented
Since Example 2 is superior to Example 1 , it is clear that it is preferable to support platinum in the lower layer.

[0031]

According to the exhaust gas purifying catalyst of the present invention, high NOx purifying performance can be obtained in each of the A / F rich, stoichiometric and slightly lean regions. Also platinum,
Since the interaction between each of palladium, cerium oxide and activated alumina is optimized, the sintering and alloying of platinum and palladium are suppressed, and the high-temperature durability is also improved.

[Brief description of the drawings]

FIG. 1 is an enlarged sectional view of a main part of an exhaust gas purifying catalyst according to a first embodiment of the present invention.

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

[Explanation of symbols]

1: carrier base material 2: catalyst support layer 20: lower layer
21: Upper layer

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Shinji Matsuura 7800, Chihama, Oto-cho, Ogasa-gun, Shizuoka Prefecture Inside (72) Inventor Yuki Sato 7800, Chihama, Oto-cho, Ogasa-gun, Shizuoka Cataler (56) References JP-A-4-161249 (JP, A) JP-A-1-143642 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B01J 21 / 00-37/36 B01D 53/86

Claims (3)

(57) [Claims] 1. A carrier substrate, a catalyst supporting layer formed on the surface of the carrier substrate and comprising cerium oxide powder and activated alumina powder , and palladium, alkaline earth metal and platinum supported on the catalyst supporting layer. And palladium supported on the activated alumina powder
Is an exhaust gas purification catalyst supported on the cerium oxide powder and containing more palladium than platinum in terms of weight (Pt ≦ Pd). 2. The exhaust gas purifying catalyst according to claim 1, wherein the carried amount of palladium and platinum is in a range of 1/10 ≦ Pt / Pd ≦ 1/1 by weight ratio. 3. The catalyst supporting layer comprises a lower layer formed on the surface of the carrier substrate and an upper layer formed on the surface of the lower layer. Platinum is supported on the cerium oxide powder in the lower layer, and palladium is active in the upper layer. The exhaust gas purifying catalyst according to claim 1, which is supported on alumina powder.