JPH0256247A - Exhaust purification catalyst - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an exhaust purification catalyst for an internal combustion engine, and more particularly to an exhaust purification catalyst called a three-way catalyst.

This three-way catalyst is installed, for example, in the exhaust system of an automobile internal combustion engine, and converts the three components of exhaust gas, hydrocarbons, carbon monoxide, and nitrogen oxides, so that the oxidizing component and the reducing component are stoichiometric. It is used to simultaneously detoxify in nearly equal amounts.

[Conventional technology]

In recent years, three-way catalysts that efficiently perform oxidation and reduction have become mainstream as catalysts used for purifying exhaust gas from internal combustion engines. This three-way catalyst is provided by supporting a noble metal catalyst such as platinum (Pt), palladium (Pd), or rhodium (Rh) on a monolithic carrier having a honeycomb cross section. The monolith carrier is made of a material with a low coefficient of thermal expansion and high heat resistance, such as cordierite. After coating the monolith carrier with a porous material with a high surface area such as activated alumina or zirconia, the noble metal catalyst is supported. . As a specific example of a three-way catalyst supporting a precious metal catalyst, for example,
-20013 discloses a Pt-Pd-Rh based three-way catalyst. In this example, Pd, which tends to deteriorate performance due to poisoning, is placed in the inner layer and Pt-Rh is placed in the outer layer to further improve performance.

Since such a three-way catalyst exhibits high redox ability near the stoichiometric air-fuel ratio, the exhaust gas flowing into the catalyst is controlled to be as close to the stoichiometric air-fuel ratio as possible;
This control is performed according to the oxygen concentration in the exhaust gas detected by an oxygen concentration sensor provided upstream of the catalyst. Specifically, by adjusting the amount of fuel supplied to the internal combustion engine, the amount of intake air, the amount of air supplied to the exhaust gas, etc. according to the oxygen concentration in the exhaust gas upstream of the catalyst, the exhaust gas is kept close to the stoichiometric air-fuel ratio. It is done to keep it constant.

[Problem to be solved by the invention]

However, when an automobile internal combustion engine is cold started, the amount of supplied fuel is increased, so the air-fuel ratio in the exhaust gas becomes small (the air-fuel ratio becomes rich). i.e. carbon monoxide in the exhaust,
Since the amount of hydrocarbons increases and the temperature is low, the function of the three-way catalyst does not work sufficiently, and there is a problem that the purification rate of Co and HC in particular decreases.

Therefore, an object of the present invention is to provide a three-way catalyst that has particularly high hydrocarbon purification activity in a cold state and a rich air-fuel ratio.

[Means for Solving the Problems] Therefore, the exhaust purification catalyst of the present invention is characterized by a three-way catalyst in which a 2N catalyst layer is provided and zeolite is arranged in the inner layer.

Specifically, the configuration of the present invention is as follows.

Note that the reference numerals in FIG. 1 are used for reference.

The present invention is a catalyst for detoxifying hydrocarbons, carbon monoxide, and nitrogen oxides in the exhaust gas of an internal combustion engine. This catalyst consists of a first catalyst containing zeolite as a main component on a carrier (10).
A catalyst layer (12) and a second catalyst layer (14) whose main component is a noble metal catalyst having redox ability are provided on the catalyst layer (12).

In the configuration of the present invention described above, it is possible to use a monolithic so-called monolithic carrier, a granular pellet carrier, or the like as the carrier. The monolith carrier may be made of ceramics such as cordierite or a laminate of heat-resistant thin metal plates. As the pellet carrier, pellets of alumina or the like can be used.

The first catalyst layer is a porous layer containing zeolite as a main component. Zeolite, also called zeolite, has a chemical composition similar to feldspars or subfeldspars, and has the general formula W
-Z-0zn-Hz O (where W is Na, Ca
, , is Ba or Sr, Z is Si+Af and St/
Aj! It is a hydrous silicate represented by >1). The internal structure of zeolite consists of cage-shaped cavities and gaps such as tunnel-like pores of various diameters that interconnect these cavities. It has molecular sieving and catalytic abilities.

Note that the first catalyst layer may contain heat-resistant oxides such as alumina, zirconia, and rare earth oxides in addition to zeolite.

Further, a noble metal catalyst such as Pt, Pd, Rh, etc. may be supported on the first catalyst layer.

In addition to precious metals such as Pt, Pd, and Rh, the first catalyst layer contains ceria (ceat) and lanthana (La20).
．． ) may be supported.

The second catalyst layer is formed by forming a coat layer of alumina or the like on the first catalyst layer, and then supporting a precious metal catalyst such as Pt, Pd, Rh or the like in a solution on this coat layer.

One type or two or more types of these noble metals can be used, and the amount supported is preferably 0.1 to 10 g/l (weight supported per 1 tl of carrier). In this case, 0.1g
/It, the effect of the three-way catalyst is small, and 10
If it exceeds g/1, the effect of the three-way catalyst that is commensurate with the amount of supported precious metal catalyst cannot be obtained. This coat layer also contains ceria (ceoz), lantana <La201)
It can contain rare earth oxides such as zirconium oxide, etc., and zirconium oxide. This increases the heat resistance of the second catalyst layer,
Furthermore, when ceria is added, the window of the three-way catalyst (the range of air-fuel ratios that can maintain three-way purification performance centered on the stoichiometric air-fuel ratio) can be widened due to its oxygen storage ability.

Note that this exhaust gas purification catalyst is suitable for exhaust gas temperatures of 3.0 to 80.
A temperature of 0°C is desirable, and the space velocity of the exhaust flow at that time (
SV) is preferably 10000-1200 o oh-'.

[Effect]

According to the exhaust purification catalyst of the present invention described above, when the air-fuel ratio is rich in a cold state, the first catalyst layer (12
), hydrocarbons are selectively adsorbed onto the zeolite. Therefore, HC in the exhaust gas after passing through the catalyst is reduced. When the engine is warmed up, the temperature of the exhaust gas rises and the temperature of the catalyst also rises, so HC is gradually desorbed from the zeolite in the first catalyst layer (12), and the second catalyst layer (12) with increased activity is removed. 1
In 4), HCSCo is oxidized and NOx
is reduced and rendered harmless.

〔Example〕

Next, examples of the exhaust purification catalyst according to the present invention will be described together with comparative examples based on FIG.

FIG. 1 is a diagram showing a part of the flow path cross section of the monolithic catalyst, and FIG.
2.3 is a graph showing the relationship between the gas temperature entering the catalyst and the purification rate of HC, and FIG. It is a graph showing the relationship between rates.

(Example 1) As shown in FIG. 1, a honeycomb-shaped monolithic carrier 10 made of cordierite was used as a carrier to be coated with a catalyst.
was used. The monolithic carrier 10 has an apparent capacity of 1.37! with 400 square cross-sectional cells per square inch of cross-sectional area! After treating the wooden tablet, it was immersed in wash coat slurry. The slurry is zeolite 1
00 parts by weight of silica sol and 80 parts by weight of silica sol were ball milled together with water and nitric acid. Next, the monolith was pulled up from the slurry, compressed air was blown onto the cell surface to blow off the excess slurry in the cell, and it was dried (200°C).
After removing free water, it was calcined at 700°C for 1 hour.

As a result, a zeolite coat layer (first catalyst layer) 12 having a thickness of about 25 μm was obtained on the monolithic carrier 10.

Monolith carrier 1 on which this zeolite coat layer 12 is formed
After pre-treatment on the zeolite coating layer 12, the monolithic carrier is immersed in a wash coat slurry mainly composed of alumina, and then blown off, dried and immersed in the same manner as in the formation of the zeolite coat layer 12 to form alumina with a thickness of approximately 25 μm. A coat layer was obtained.

This washcoat slurry was produced by ball milling 100 parts by weight of alumina and 14 parts by weight of an aluminum nitrate aqueous solution together with water and nitric acid.

Next, platinum (
Pt), lanthanum (La) along with rhodium (Rh),
The second catalyst layer 14 was prepared by supporting cerium (Ce). The amount of these components supported is 1.5 pt per 11 carriers.
g, Rh 0.3 g, La 0.03 mol, Ce 0.
It was 3 moles.

(Example 2) In this example, the catalyst was prepared in the same manner as in Example 1, except that the thickness of the zeolite co-1 layer, which was the first catalyst layer, was 10 μm.

(Comparative Example 1) In this comparative example, the second catalyst layer in Example 1 was not formed,
The thickness of the zeolite coat layer, which is the first catalyst layer, is approximately 50 μm.
m.

(Comparative Example 2) In this comparative example, the first catalyst layer in Example 1 was not formed with a zeolite coat layer, and the thickness of the second catalyst layer was approximately 50 μm.
m.

(Comparative Example 3) In this comparative example, the noble metal catalyst layer of Example 1 was used as the first catalyst layer formed on the monolith substrate, and the zeolite coat layer was used as the second catalyst layer formed thereon.

(Evaluation Results 1) The purification rates of Example 1.2 and Comparative Example 1.2.3 described above were measured and evaluated.

The above-mentioned catalyst was installed in the exhaust system of a four-cylinder engine with a displacement of 1.6N, and the catalyst activity was examined. The exhaust gas temperature is
The temperature was varied from 100°C to 500°C, and the HC purification rate was measured. The air-fuel ratio (A/F) is 13.5 (rich burn), and the space velocity (S V) of exhaust gas is
was approximately 60,000 h-1.

As is clear from FIG. 2, Example 1 according to the present invention.
Catalyst No. 2 shows an extremely high HC purification rate from the low temperature range (near 100 to 200 degrees Celsius) under conditions of excess fuel at low temperatures, such as during cold startup, indicating that it has excellent low-temperature activity. . Although purification performance decreases as the temperature of the entering gas increases, it shows high purification activity in higher temperature ranges. The reason why the purification rate once decreases between the low-temperature region and the high-temperature region is thought to be due to a slight deviation occurring after HC desorption occurs at a temperature exceeding 200°C until the oxidation reaction becomes active. , the time in between is short,
Not a problem.

(Example 3) As shown in Fig. 1, a honeycomb-shaped monolithic carrier made of cordierite was used as a carrier for coating the catalyst.
was used. The monolithic carrier 10 has an apparent capacity of 1.3 I! with 400 square cross-sectional cells per square inch of cross-sectional area. After pretreatment with water, the sample was immersed in a washcoat slurry. The slurry is zeolite 1
00 parts by weight of silica sol and 80 parts by weight of silica sol were ball milled together with water and nitric acid. Next, the monolith was pulled up from the slurry, compressed air was blown onto the cell surface to blow off the excess slurry in the cell, and it was dried (200°C).
After removing free water, it was calcined at 700°C for 1 hour.

As a result, a zeolite coat layer (first coat layer) with a thickness of about 25 μm was obtained on the monolithic carrier 10.

After the above coating layer was pretreated with water, it was immersed in a dinitrodiamine platinum aqueous solution and left to stand for 24 hours.
5 g of pt was ion exchanged.

After that, excess water was blown off and dried at 250°C for 1 hour, and then immersed in an aqueous rhodium chloride solution and left for 24 hours to support 1j! The first catalyst layer 12 was formed by ion-exchanging 0.15 g of Rh per sample.

Next, the monolithic carrier was immersed in a washcoat slurry mainly composed of alumina, and was blown off, dried, and immersed in the same manner as when forming the zeolite coat layer, to a thickness of approximately 25 μm.
An alumina coat layer (second coat layer) of m was obtained. This washcoat slurry was produced by ball milling 100 parts by weight of alumina and 14 parts by weight of an aluminum nitrate aqueous solution together with water and nitric acid.

Next, platinum (
pt) and rhodium (Rh) were supported to form the second catalyst layer 14. The supported amount of these components is Pt per 12 carriers.
was 0.75 g, and Rh was 0.15 g.

The total amount of noble metals supported on the first catalyst layer and the second catalyst layer is 1.5 g of Pt and 0.3 g of Rh per carrier 11.
It was g.

Next, the above carrier is immersed in a ceria (CeOz) aqueous solution,
By blowing off and drying, 0 per carrier 11
．． 3 mol of Ce was held once.

(Example 4) In this example, the catalyst was prepared in the same manner as in Example 3, except that Pd supported on the first and second catalyst layers was replaced with Pd.

(Example 5) This example is similar to Example 3, but the catalyst is not supported with Ce.

(Comparative Example 4) In this comparative example, the first catalyst layer in Example 3 was formed.
The thickness of the second catalyst layer was about 50 μm, Pt was 1.5 g,
Rh was set at 0.3 g and Ce was set at 0.3 mol.

(Evaluation Results 2) The purification rates of Examples 3.4.5 and Comparative Example 4 described above were measured and evaluated.

The above catalyst has an exhaust i1.6j! The device was installed in the exhaust system of a four-cylinder engine, and the catalyst activity was investigated. The exhaust gas temperature was varied from 100°C to 500°C, and the HC purification rate was measured. The air-fuel ratio (A/F) is 13.5 (rich burn), and the space velocity of exhaust gas (S V
) was about 60,000 h-1. As is clear from FIG. 3, Example 3.
The catalyst No. 4.5 shows an extremely high HC purification rate from the low temperature range (around 100-200°C) and has excellent low-temperature activity under conditions of excess fuel at low temperatures such as during cold startup. I understand.

As the temperature of the incoming gas increases, the purification performance decreases, but in the higher temperature range, both HC1C01NOx show high purification activity. The purification rate decreases once between the low-temperature region and the high-temperature region when the temperature exceeds 200°C.
This is thought to be due to the fact that there is a slight lag between the time when desorption occurs and the time when the oxidation reaction becomes active, but the time period is so short that it does not pose a problem.

In addition, as a result of comparing the CO purification rates in Example 3.4.5 and Comparative Example 4, it was found that the entering gas temperature was 150°C140°C.
The respective purification rates at 0°C are 0% and 0% in Comparative Example 4, respectively.
While it was 86%, in Example 3, it was 43% and 87%.
%, Example 4: 40%, 85%, Example 5: 1
0%, 85% and according to Example 3.4.5, CO
It can be seen that the low-temperature activity of is also improved.

Furthermore, in Example 3.4 in which a rare earth oxide is added, the 0□ storage effect of Ce is applied to HC at low temperatures, and HC is converted to Cog and rendered harmless through an oxidation reaction with the noble metal. Since the reaction heat generated at this time accelerates the catalytic reaction, the low-temperature activity is improved compared to Example 5.

Although specific embodiments of the present invention have been described above, the present invention is not limited to these embodiments, and includes various embodiments within the scope of the claims. .

〔Effect of the invention〕

As described above, according to the exhaust gas purification catalyst of the present invention, since two catalyst layers are provided and zeolite is arranged in the inner layer, the catalyst has high hydrocarbon purification activity even in a cold state and at a Lynch air-fuel ratio. A three-way catalyst is obtained.

In addition, in this exhaust purification catalyst, even when HC in the exhaust gas increases during accelerated fuel increase, the zeolite adsorbs HC and can prevent a temporal increase in HC.

[Brief explanation of the drawing]

1 to 3 are drawings for explaining an embodiment of the exhaust purification catalyst according to the present invention, and FIG. The figure shows Example 1.2 and Comparative Example 1.2.
．． 3 is a graph showing the relationship between the gas temperature entering the catalyst and the HC purification rate of No. 3. FIG. 3 is a graph showing the relationship between the gas temperature entering the catalyst and the HC purification rate in Example 3.4.5 and Comparative Example 4. 10---Support 12--1--First catalyst layer 14--Second catalyst layer Applicant Toyota Motor Corporation 10TE Jiji = 9 2nd rA Demonstration example 1 Catalyst person') 77's Degree 3 Figure 1 Bubble Greeting 3 Temperature of the kiss that enters the snout, t c'

Claims (1)

[Claims] A catalyst for detoxifying hydrocarbons, carbon monoxide and nitrogen oxides in the exhaust gas of internal combustion engines, comprising a first catalyst layer mainly composed of zeolite on a carrier and a redox ability on the first catalyst layer. An exhaust purification catalyst characterized by being provided with a second catalyst layer containing a precious metal catalyst as a main component.