JP2788293B2 - Exhaust gas purification catalyst - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an exhaust gas purifying catalyst used for reducing CO, HC, and NOx in exhaust gas.

(Prior Art) As an exhaust gas purifying catalyst for purifying CO, HC and NOx in exhaust gas, as disclosed in JP-A-58-146441,
A first coat layer made of an alumina layer containing palladium (Pd) is provided on the catalyst carrier, and platinum (Pt) and rhodium (Rh) or Rh and active alumina are formed on the first coat layer. One provided with a second coat layer is generally known.

(Problems to be Solved by the Invention) However, in the exhaust gas purifying catalyst, when the heat of the exhaust gas is received, Rh oxide of the second coat layer is dissociated into Rh, and based on the thermal deterioration of Rh, The catalyst performance tends to decrease.

The present invention has been made in view of the above circumstances, and has as its object to prevent thermal degradation due to Rh dissociation.

(Means for Solving the Problems and Action) In order to achieve the object, according to the first aspect of the present invention, a first coat layer made of an alumina layer containing palladium is provided on a catalyst carrier. And a second coat layer made of cerium oxide particles on which rhodium is fixed and activated alumina is provided above the first coat layer. is there.

According to the configuration of the first aspect described above, since Rh is fixed to the CeO ₂ particles and CeO ₂ and Rh are distributed in close proximity to each other, the oxygen storage effect of CeO ₂ (the oxygen O ₂ concentration is high) When O ₂ is adsorbed and the O ₂ concentration is reduced, O ₂ is released and the O ₂ concentration is released to contribute to the catalytic reaction), and the Rh dissociation (Rh ₂ O ₃ → Rh) temperature is raised to about 1000 ° C. Therefore, thermal degradation due to the dissociation of Rh can be suppressed.

Moreover, in the case of the first invention, the noble metal components (Rh, Pd) are present alone in each coat layer, thereby further preventing alloying and suppressing thermal deterioration due to alloying. Can be further improved.

Further, since CeO ₂ exists in the second coat layer, O ₂
The spillover phenomenon (transition phenomenon of O ₂ ) becomes active, the oxygen storage capacity effect is enhanced, and the low-temperature activity in the air-fuel ratio A / F fluctuation region (perturbation) can be improved.

Further, in order to achieve the above object, in the second invention of the present invention, a first coat layer made of an alumina layer containing palladium is provided on the catalyst carrier, and on the first coat layer, An exhaust gas purifying catalyst is provided with a second coat layer comprising activated alumina and cerium oxide particles on which rhodium and palladium are immobilized.

According to the configuration of the second aspect, as in the case of the first aspect, the dissociation and alloying of Rh are suppressed and the low-temperature activity in the air-fuel ratio fluctuation region can be improved.
Pd on CeO ₂ and also immobilized Pd to CeO ₂ particles in the coating layer
The by where proximity distribution, be shifted to the rich side of the air-fuel ratio (O ₂ less side), HC, it is possible to purify CO, HC, and purifying the air-fuel ratio range for the CO to the rich side It can be expanded.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings.

In FIGS. 1 and 2, 1 (1a, 1b) denotes the first and second
An exhaust gas purifying catalyst according to an embodiment of the present invention, wherein the catalyst 1
As shown in FIG. 2, the first coat layer 3 (3a, 3b) and the second coat layer are formed on the catalyst carrier 2 (2a, 2b) of the honeycomb structure in both the first and second embodiments of the present invention. 4 (4a, 4b) sequentially,
It is configured to be provided.

First, the catalyst 1a according to the embodiment of the first invention will be described with reference to FIG. 3. A ceramic such as cordierite is used for a catalyst carrier 2a.
A heat-resistant metal, a heat-resistant inorganic fiber, or the like may be used. The first coat layer (base coat layer) 3a is composed of an alumina layer containing Pd dispersed therein, and the second coat layer 4a is composed of CeO ₂ particles impregnated with Rh and fixed alumina. The relationship of these compounding rates will be described later.

Such a catalyst 1a according to the first invention is manufactured, for example, as follows.

First, 480 g of γ-Al ₂ O ₃ , boehmite (hydrated alumina)
120 g, water 1, and 10 cc of nitric acid are mixed and stirred by a homomixer to prepare a slurry for alumina wash coating. The catalyst carrier 2a having a honeycomb structure is immersed in this slurry, then pulled up, and the excess slurry is removed by air blow, and fired at a temperature of 600 ° C. to form an alumina coat on the catalyst carrier 2a.

The alumina-coated catalyst carrier 2a is immersed in a palladium noble metal solution obtained by dissolving a predetermined amount of palladium chloride (PdCl ₂ ) in 170 cc of water.
At a temperature of 2 hours to form a first coat layer (base coat layer) 3a.

Next, 720 g of CeO _{2 is} impregnated with a predetermined amount of rhodium chloride solution, and calcined or SBH (sodium borohydride NaB
Fix Rh on CeO ₂ at H ₄ ). To this boehmite 80
g, 10 cc of nitric acid and water 1 are added and mixed and stirred with a homomixer to prepare a slurry for the second coat layer containing Rh.

The catalyst carrier to which the alumina and Pd are adhered is immersed in the slurry to adhere the slurry, the excess slurry is removed by high-pressure air blow, and the slurry is baked at a temperature of 600 ° C. for 2 hours to form a second coat layer (overcoat). (Coat layer) 4a is formed.

At this time, the amount of noble metal is determined by using Pd of the first coat layer 3a as the catalyst 1
And the Rh of the second coat layer 4a is set to 0.27 g.

Next, the fourth example of the catalyst 1b according to the embodiment of the second invention is described.
A description will be given based on the drawings.

In the catalyst 1b, the same catalyst carrier 2b as in the case of the catalyst 1a according to the above-described embodiment of the first invention is used, and the first coat layer 3b is composed of an alumina layer containing Pd dispersed therein. The second coat layer 4b is composed of CeO ₂ particles impregnated with Rh and Pd and activated alumina.
The mixing ratio relationship will be described later.

Such a catalyst 1b according to the embodiment of the second invention is manufactured, for example, as follows.

First, 480 g of γ-Al ₂ O ₃ , 120 g of boehmite, 1 water, nitric acid
10 cc is mixed and stirred by a homomixer to prepare a slurry for alumina wash coating. The catalyst carrier 2b having a honeycomb structure is immersed in this slurry, then pulled up, and the excess slurry is removed by air blow, followed by firing at a temperature of 600 ° C. to form an alumina coat on the catalyst carrier 2b.

The alumina-coated catalyst carrier 2b is immersed in a predetermined amount of a palladium chloride noble metal solution, and then it is assembled and fired at a temperature of 600 ° C. for 2 hours to form a first coat layer.

Then, C _e O ₂ 720 g a predetermined amount of rhodium chloride, impregnated with palladium chloride solution, to fix the Rh on C _e O ₂ in a firing or SBH it (sodium borohydride write NaBH _4). To this, 80 g of boehmite, 10 cc of nitric acid and water 1 were added, and mixed and stirred with a homomixer to prepare a slurry for the second coat layer containing Rh and Pd. The alumina and the slurry
The catalyst carrier 2b to which Pd is attached is immersed, slurry is attached, excess slurry is removed by high pressure air blow, and 600 ° C.
At a temperature of 2 hours to form a second coat layer.

Therefore, in the catalyst 1a, since Rh is fixed to the CeO ₂ particles and the CeO ₂ and Rh are distributed in a close state, the Rh is dissociated based on the oxygen storage effect of CeO ₂ ( Rh ₂ O ₃ → Rh) The temperature can be increased to about 1000 ° C., and the heat resistance can be improved.

Further, since Pt is not contained in the second coat layer 4a,
Even if it receives the heat of the exhaust gas, the alloying of Pt with other metals is suppressed, and the alloying by Pt can be suppressed. On the other hand, even if Pt is removed, the function of Pt up to that time is complemented by Pd carried on the first coat layer.

In addition, in the catalyst 1a, the noble metal components (Rh, Pd) exist solely in each coat layer, whereby the alloying can be further suppressed.

Further, since CeO ₂ exists in the second coat layer 4a,
The spillover phenomenon of O ₂ becomes active, the oxygen storage capacity effect is enhanced, and the low-temperature activity in the air-fuel ratio fluctuation region can be improved.

Further, in the catalyst 2b, not only the dissociation of Rh and the alloying of Pt can be suppressed, but also the second coat layer
In 4b, Pd is also immobilized on CeO ₂ particles to bring CeO ₂ and Pd close to each other, whereby the purification air-fuel ratio range for HC and CO can be expanded to the rich side.

Next, in order to confirm the performance of the catalysts 1a and 1b obtained as described above, a certain amount was compared with Comparative Examples 1 (see FIG. 5) and Comparative Example 2 (see FIG. 6) as described below. Various tests were conducted under the following test conditions.

Comparative Example 1 A noble metal mixed solution in which Pt and Rh were dissolved was prepared, and an aluminum-coated catalyst carrier 1a was immersed in the noble metal mixed solution in the same manner as in the production method for the above-described catalyst 1a, and then calcined. Do.

Next, 720 g of CeO ₂ is impregnated with a palladium chloride solution having a predetermined concentration, and Pd is fixed on CeO _{2 by} firing or SBH. Add 80g of boehmite, 10cc of nitric acid and 1 of water to this,
Make a slurry. Immerse the catalyst carrier with Pt and Rh attached to this slurry, pull it up, air blow,
Baking is performed to form a second coat layer.

At this time, the amount of the noble metal was 1.33 g of Pt, 0.2
7 g and Pd 1.0 g.

Comparative Example 2 γ-Al ₂ O ₃ 480 g, boehmite 120 g, water 1, nitric acid 10 cc
Are mixed and stirred with a homomixer to produce an alumina slurry.
The catalyst carrier is immersed in this slurry liquid, then pulled up, air blown and fired to form an alumina coat on the catalyst carrier. Then, the catalyst support on which the alumina coat is formed is immersed in a Pd solution, pulled up, and then removed.
Firing at a temperature of 600 ° C. for 2 hours to form a first coat layer.

Next, the catalyst carrier on which the first coat layer is formed is immersed in a Rh solution, and then pulled up, and is then heated at 600 ° C. for 2 hours.
Baking for a time forms a second coat layer.

At this time, the amount of noble metal was 1.33 g of Pd and Rh was 1
0.27g.

[A] Heat resistance (thermal degradation) test Under the following test conditions, the HC purification rate was measured by changing the exhaust gas temperature at the catalyst inlet, and the heat resistance was examined.

Test conditions Catalysts 1a and 1b, catalysts according to comparative examples 1 and 2 (FIG. 5, FIG.
(See the figure), the volume is 24 ml.

Before performing the test, aging is performed by heating in an atmosphere at 1000 ° C. for 6 hours.

Air-fuel ratio A / F: 14.7 ± 0.9 (1.0Hz), space velocity is 6000
0H ^-1 .

As a result of such a test, the contents shown in FIG. 7 were obtained. According to FIG. 7, the catalyst 1a (shown by a characteristic line fa; the same applies hereinafter) and the catalyst 1b (shown by a characteristic line fb.
the same. ), Both show with Comparative Example 1 (characteristic line c _1. Hereinafter, the same.), Shows with Comparative Example 2 (special line c _2. Hereinafter, the same. In a low exhaust gas temperature compared to) shows the purification performance , The heat resistance was improved. this is,
Based on the oxygen storage capacity effect of CeO ₂ , Rh dissociation (Rh ₂ O ₃ → R
h) The temperature can be raised to about 1000 ° C and Pt
It is considered that alloying was suppressed except for. In particular, in the case of the first invention, since the noble metal is solely provided in each of the coat layers 3a and 4a, the heat resistance is further improved, which is apparent from FIG. It is.

In addition, a purification performance evaluation test was performed for each of the catalysts when the aging was not performed (fresh).

The results are also shown in FIG. 7, and based on these results and the test results after aging, catalysts 1a and 1b
However, it can be understood that the low-temperature activity is improved and stable as compared with Comparative Examples 1 and 2 (the characteristic line of each catalyst in the case where aging is not performed is indicated by the reference character of each catalyst characteristic line in the case where aging is performed). Is marked with a “′” symbol).

[B] Test for examining the effect of the ratio of Pd of the first coat layer to Rh of the second coat layer on low-temperature activity In the catalyst according to the first invention, the Pd of the first coat layer 3a
The effect of the ratio of Rh to the second coat layer 4a on the 50% purification temperature of HC and NOx (an index indicating the stability of low-temperature activity) was examined.

 The test conditions are the same as the test conditions described above.

As a result of these test conditions, the contents shown in FIG. 8 were obtained.
According to this content, for both NC and NOx, in the ratio of Pd of the first coat layer 3a and Rh of the second coat layer 4a, as the Rh amount increases, the 50% purification temperature deteriorates, and the Pd / Rh ratio decreases. Than 5/1
Even when Pd became rich, the 50% purification temperature hardly changed.

 In FIG. 8, black circles indicate the values of the catalyst 1a.

[C] Purification performance test for HC and CO Under the following conditions, the purification performance of HC and CO was tested using the catalysts 1a and 1b and the catalysts of Comparative Examples 1 and 2.

Test conditions Activity evaluation Exhaust gas temperature at catalyst inlet: 400 ° C Space velocity SV: 60000H ^-1 Catalyst capacity: 24 ml Aging condition Heating in air at 1000 ° C for 6 hours As a result, the contents shown in FIG. 9 were obtained. According to the content, the catalysts 1a and 1b have higher purification performance than the comparative examples 1 and 2, and the purification air-fuel ratio range of the catalyst 1b is expanded to the rich side of the air-fuel ratio as compared with the catalyst 1a. This is considered to be due to the effect of CeO _{2 on} the oxygen storage ability and the effect of Pd on the second coat layer 4b.

(Effects of the Invention) As described above, in the first invention, the dissociation temperature is increased by the effect of the oxygen storage capacity of CeO ₂ and the effect of Rh, thereby suppressing dissociation and suppressing alloying. In addition, thermal deterioration can be suppressed, and the low-temperature activity in the air-fuel ratio fluctuation region can be improved by the oxygen storage effect of CeO ₂ .

Further, in the second invention, in addition to the same effect as the first invention, the purification air-fuel ratio range for HC and CO is enriched by the effect of CeO ₂ on oxygen storage capacity and Pd. Can be expanded to the side.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing an exhaust gas purifying catalyst according to the first and second embodiments of the present invention, and FIG. 2 is a diagram showing an exhaust gas purifying catalyst according to the first and second embodiments of the present invention. FIG. 3 is a diagram conceptually showing a catalyst according to the first invention, FIG. 4 is a diagram conceptually showing a catalyst according to the second invention, and FIG. 5 is a diagram showing a comparative example 1. FIG. 6 is a diagram conceptually showing a catalyst, FIG. 6 is a diagram conceptually showing a catalyst according to Comparative Example 2, FIG. 7 is a characteristic diagram showing a relationship between HC purification rate and exhaust gas temperature at a catalyst inlet, FIG. 8 is a characteristic diagram showing a relationship between 50% purification temperature-Pd / Rh, and FIG. 9 is a characteristic diagram showing a relationship between purification ratio and air-fuel ratio. 1, 1a, 1b: catalyst 2, 2a, 2b: catalyst carrier 3, 3a, 3b: first coat layer 4, 4a, 4b: second coat layer

Continuing from the front page (72) Inventor Kazunori Ihara 3-1 Shinchi, Fuchu-cho, Aki-gun, Hiroshima Prefecture Mazda Corporation (72) Inventor Kenji Okubo 3-1 Shinchi, Fuchu-cho, Aki County, Hiroshima Prefecture Mazda Corporation

Claims (2)

(57) [Claims] 1. A first coat layer comprising an alumina layer containing palladium is provided on a catalyst carrier, and a first coat layer comprising cerium oxide particles having rhodium immobilized thereon and activated alumina is provided above the first coat layer. An exhaust gas purifying catalyst, comprising a two-coat layer. 2. A first coat layer comprising an alumina layer containing palladium is provided on a catalyst carrier, and the first coat layer comprises cerium oxide particles on which rhodium and palladium are immobilized and activated alumina. Second
An exhaust gas purifying catalyst, comprising a coat layer.