JP2810425B2 - Exhaust gas purification catalyst - Google Patents

Description

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-60-19036,
A first coat layer made of an alumina layer containing palladium (Pd) is provided on the catalyst carrier, and a second coat made of rhodium (Rh) and palladium (Pd) and activated alumina is provided on the first coat layer. Those having a layer are generally known.

(Problems to be Solved by the Invention) However, in the exhaust gas purifying catalyst, when heat of the exhaust gas is received, Pd oxide and Rh oxide (particularly Pd oxide) of the second coat layer become Pd, It dissociates into Rh, and the catalytic performance tends to decrease due to thermal degradation of these noble metals.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an exhaust gas purifying catalyst that has improved heat resistance by preventing thermal deterioration of a noble metal.

(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 rhodium 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.

With the configuration of the first aspect described above, the noble metal Rh is
Immobilized on the cerium oxide (CeO ₂₎ particles, since it is distributed in close proximity and CeO _2, the O ₂ adsorbs O ₂ when the oxygen storage capacity effect (oxygen O ₂ concentration of CeO ₂ high Rh contributes to the catalytic reaction by releasing O ₂ when lowered)
Temperature (Rh ₂ O ₃ → Rh) can be increased to around 1000 ° C., and the heat resistance can be improved.

Furthermore, since CeO ₂ is present, the increased oxygen storage capacity effect spillover phenomenon of O ₂ (migration phenomenon of O _2),
The purification performance of HC, CO and NOx near the ideal air-fuel ratio can be improved.

Furthermore, since Rh is dispersed and provided in the first and second coat layers, it is possible to prevent the Rh particles fixed to CeO ₂ from becoming denser and to prevent the Rh particles from being too close to each other,
Even when the exhaust gas temperature is set to a high temperature, it is possible to suppress the occurrence of sintering between the Rh particles. For this reason, it is possible to suppress thermal degradation based on sintering of the Rh particles immobilized on the CeO ₂ particles.

In order to achieve the above object, according to the second aspect of the present invention, a first coat layer made of an alumina layer containing rhodium is provided on a catalyst carrier, A second coating layer comprising cerium oxide particles on which rhodium and palladium are fixed and activated alumina, and a second exhaust gas purifying catalyst.

According to the configuration of the second invention, in addition to the same effect as in the first case, Pd is also immobilized on CeO ₂ particles in the second coat layer, and Pd is distributed close to CeO ₂ . Based on the oxygen storage capacity effect of CeO ₂
It is shifted to the rich side of the air-fuel ratio (O ₂ less side), H
C and CO can be purified, and the purified air-fuel ratio range (reaction range) for HC and CO can be expanded to the rich side.

(Example) Hereinafter, an example of the present invention will be described.

In FIGS. 1 and 2, 1 (1a, 1b) represents the first and second
An exhaust gas purifying catalyst according to an embodiment of the present invention, wherein the catalyst 1
In both of the first and second embodiments, as shown in FIG.
A first coat layer 3 (3a, 3b) and a second coat layer 4 (4a, 4b) are sequentially provided on a catalyst carrier 2 (2a, 2b) having a honeycomb structure.

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 the catalyst carrier 2a. In place of this ceramic, a heat-resistant metal, a heat-resistant inorganic fiber, or the like may be used. The first coat layer 3a is made of an alumina layer containing Pd and Rh, and the second coat layer 4a is made of CeO ₂ particles impregnated with Rh and activated alumina.
The relationship between these compounding ratios will be described later.

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

That is, first, 480 g of γ-alumina, 120 g of boehmite (hydrated alumina), water 1 and 10 cc of nitric acid are mixed and stirred by a homomixer to prepare a slurry for alumina wash coat. Then, the catalyst carrier 2a having a honeycomb structure is immersed in the slurry and pulled up, and then the excess slurry is removed by high-pressure air blow, and the slurry is fired at a temperature of 600 ° C.

The alumina-coated catalyst carrier 2a is immersed in a noble metal solution obtained by dissolving a predetermined amount of palladium chloride (PdCl ₂ ) and rhodium chloride (RhCl ₃ .3H ₂ O) in 170 cc of water.
It is baked at a temperature of 00 ° C. for 2 hours to form a first coat layer (base coat) 3a.

Next, 720 g of CeO ₂ is impregnated with a rhodium solution in which a predetermined amount of rhodium chloride is dissolved, and Rh is fixed on CeO ₂ by firing or SBH (sodium borohydride NaBH ₄ ).
To this, 80 g of boehmite, 10 cc of nitric acid and water 1 are added, and mixed and stirred with a homomixer to prepare a slurry for the Rh-containing second coat layer.

Next, the catalyst carrier 2 to which the alumina, Pd, and Rh are attached is immersed in the slurry, the slurry is attached, and the excess slurry is removed by high-pressure air blow.
By baking at a temperature of 600 ° C. for 2 hours, a second coat layer (overcoat layer) 4a is formed.

At this time, in the first coat layer 3a, Pd is 1.33 g and Rh is 0.17 g per catalyst, and in the second coat layer 4, Rh is 0.1 g per catalyst.

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 as in the case of the catalyst 1a according to the first embodiment of the first invention is used for the catalyst carrier 2b, and the first coat layer 3b is composed of an alumina layer containing Pd and Rh. The second coat layer 4b is made 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, the first coat layer 3 is formed on the catalyst carrier 2 under the same conditions as in the method for producing the catalyst 1a according to the embodiment of the invention described above.

Next, 720 g of CeO ₂ is impregnated with a mixed solution of a noble metal in which a predetermined amount of rhodium chloride and palladium chloride are dissolved, and Rh and Pd are fixed on CeO _{2 by} firing or SBH. To this, the catalyst
In the same manner as in the production method 1a, boehmite, nitric acid and water are added and mixed and stirred to prepare a slurry for the second coat layer 4b.

Next, the catalyst carrier 2bh provided with the first coat layer 4b is immersed in this slurry liquid, pulled up and air blown, and baked at a temperature of 600 ° C. for 2 hours to obtain the second coat layer 4b. Is formed on the first coat layer 3b.

At this time, the amount of the noble metal in the first coat layer 3b is
Pd is 0.667 g /, Rh is 0.133 g /, and in the second coat layer 4b, Pd is 0.667 g / and Rh is 0.133 g /.

Therefore, in the above catalyst 1a, Rh is fixed to CeO ₂ particles and distributed in a close state, so that the Rh dissociation (Rh ₂ O ₃ → Rh) temperature is based on the oxygen storage effect of CeO _2. 1
It can be raised to around 000 ° C. For this reason, heat resistance is improved.

Further, in the catalyst 1a, since Pt is not contained in each coat layer, alloying of Pt with another metal is prevented even when receiving heat of exhaust gas. On the other hand, Pt
Even if is excluded, the function of Pt up to that point is complemented by Pd carried on the second coat layer.

Further, since CeO ₂ is present in the second coat layer 4a, the oxygen storage capacity effect due to the spillover phenomenon of O ₂ is increased,
The purification performance of HC, CO, and NOx in the vicinity of the ideal air-fuel ratio can be improved.

Furthermore, since Rh is divided into the first and second coat layers 3a and 4a to avoid increasing the density of Rh immobilized on CeO ₂ particles, the Rh in the second coat layer 4a Sintering can be suppressed.

In the catalyst 1b, in addition to the action of the catalyst 1a, Pd is also immobilized on the CeO ₂ particles, so that HC and CO are purified based on the oxygen storage effect of CeO _2. The fuel ratio range (reaction range) 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 performed under the conditions.

Comparative Example 1 A predetermined amount of platinum chloride and rhodium chloride was dissolved in 1700 cc of water to make a noble metal mixed solution. The alumina-coated catalyst carrier is immersed in this noble metal mixed solution in the same manner as in the above-described production method for the catalyst 1a, and then baked to form a first coat layer.

Next, 720 g of CeO ₂ is impregnated with a Pd solution of a predetermined concentration, and the Pd is fixed on CeO _{2 by} firing or SBH. Boehmite 8
0 g, 10 cc of nitric acid and 1 of water are added to form a slurry.
The catalyst carrier having Pt and Rh attached thereto is immersed in the slurry, and air blow and baking are performed to form a second coat layer.

At this time, the amount of noble metal was 1.33 g of Pt, R
h 0.27g, Pd 1.0g.

Comparative Example 2 480 g of γ-Al ₂ O ₃ , 120 g of boehmite, 1 water and 10 c of nitric acid
c is mixed and stirred by a homomixer to form an alumina slurry, the catalyst carrier is immersed in the slurry liquid, and then air blow baked to form an alumina coat on the catalyst carrier. The alumina-coated catalyst support is immersed in a Pd solution, pulled up, and then baked 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 noble metal solution containing a predetermined amount of Pd and Rh, pulled up, and fired at a temperature of 600 ° C. for 2 hours to form a second coat layer.

At this time, the amount of the noble metal was 0.665 g of Pd in the first coat layer and Pd in the second coat layer with respect to the catalyst 1.
0.665g, Rh0.27g.

[A] Heat resistance (thermal deterioration) 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 condition Activity evaluation Air-fuel ratio A / F; 14.7 ± 0.9 (1.0 Hz) Space velocity SV; 60,000H ^-1 catalyst capacity; 24 ml Aging condition Heating in air at 1000 ° C for 6 hours Test result The result of the above test is shown in Fig. Got content. This seventh
According to the contents of the figure, 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. ) Are both shown with Comparative Example 1 (characteristic line C _1. Hereinafter, the same.), Shows with Comparative Example 2 (characteristic line C _2. Hereinafter, the same. Purification performance from a low exhaust gas temperatures as compared to) This indicates that the heat resistance has been improved. This is based on the oxygen storage capacity effect of CeO _2, dissociation of Rh (Rh ₂ O ₃
→ Rh) temperature, Pd dissociation (PdO → Pd) temperature is increased, alloying is suppressed except for Pt, and Rh is dispersed in the first and second coat layers 3 and 4 to increase the density in the second coat layer. This is probably because the sintering between Rhs due to the conversion was avoided.

[B] HC, CO, NOx purification performance test HC, CO, NOx purification performance test was performed under the following test conditions.

Test conditions The catalysts 1a and 1b and the catalysts according to Comparative Examples 1 and 2 each have a capacity of 24 ml.

Before performing the purification performance test, aging is performed by heating in the air at 900 ° C for 50 hours.

The exhaust gas temperature at the catalyst inlet is 500 ° C.

The space velocity is 60000H ^-1 .

As a result of such a test, the catalysts 1a and 1b are shown in FIG. 8, the catalyst according to Comparative Example 1 is shown in FIG.
The catalyst according to the above has the contents shown in FIG. According to this result, both the catalysts 1a and 1b have improved purification performance as compared with the catalysts of Comparative Examples 1 and 2, and this is due to the improvement in heat resistance and the oxygen storage capacity effect of CeO ₂ described above. It is considered that the improvement based on the above results in the improvement of the catalyst performance.

In addition, the catalyst 1b showed that the purification air-fuel ratio range for HC and CO expanded to the rich side of the air-fuel ratio with respect to HC and CO as compared to the catalyst 1a (see FIG. 8). 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.

[C] Test for examining the effect of the noble metal on the purification performance (activity) in each coat layer Test conditions The test conditions are the same as those in the aforementioned purification performance test.

Regarding purification performance, CO purification performance characteristic line and NOx
The cross point (intersection point)% with the purification performance characteristic line was used as an index. This is because the catalyst needs to have both functions (performance) of the oxidation function and the reduction function. This is because the focus was on CO and NOx, which is an index of reduction performance, and comprehensive evaluation of purification performance was conducted based on the cross points between the two.

First, in the catalyst 1a, the Rh of the second coat layer 4a is kept constant (0, 1 g /), and the Pd / Rh ratio of the first coat layer 3a is changed (the sum of Pd and Rh in the first coat layer 3a). Is 1.5g /
And the CO / NOx cross point (%) were examined.

As a result, the contents shown in FIG. 11 were obtained. According to this result, even if the Pd / Rh ratio is smaller than 7.8 / 1 (same as the ratio in the catalyst 1a, indicated by a black circle), the purification performance does not change (even if Rh is made rich), And the Pd / Rh ratio to 7.8
When it was larger than / 1 (when Pd was made rich), the purification performance decreased.

Next, in the catalyst 1a, the Pd / Rh of the first coat layer 3a
Constant ratio (the sum of Pd and Rh is unified to 1.5g /) (7.8 / 1)
Then, the Rh of the second coat layer 4a was changed, and the CO / NOx cross point (%) was examined.

As a result, the content shown in FIG. 12 was obtained. According to FIG. 12, the Rh amount of the second coat layer 4a was 0.1 g / (the catalyst 1
Same as Rh of a. Shown by black circles. ) Purification performance decreases as the temperature decreases, and when the Rh amount is set to 0.1 g / g or more, the saturation condition occurs for a while.
Purification performance began to decline.

From this and the results in FIG. 11, the total amount of Rh was
It is more advantageous from the viewpoint of heat resistance (from the viewpoint of preventing sintering of Rh) from dispersing the amount of Rh in the first and second coat layers while avoiding the increase in the density, rather than bringing the density only to the coat layer 4a. It can be understood that

Next, in the catalyst 1b, the first and second coat layers 3b, 4
pd / Rh in b is constant 5/1), first and second coat layers 3b, 4b
, The CO / NOx cross point (%) was examined by changing the ratio of the amount of the noble metal in the first coat layer 3b to the amount of the noble metal in the second coat layer 4b while keeping the total noble metal amount constant (1.6 g /).

As a result, the contents shown in FIG. 13 were obtained. According to FIG. 13, when the ratio between the amount of noble metal in the first coat layer 3b and the amount of noble metal in the second coat layer 4b is 1/1 (the same as the ratio of the above-described catalyst 1b, indicated by black circles). The purification performance gradually decreased even if the ratio was increased or decreased from this peak point. This is because, when the above ratio is increased, the amount of noble metal in the second coat layer 4b decreases and the number of active sites decreases, and when the above ratio is decreased, the noble metal becomes denser. It is thought to be due to sintering. In particular, the latter was described in the description of the results in FIG. 12,
This can be supported by the results shown in FIG.

Next, in the catalyst 1b, the amount of noble metal in each of the coat layers 3b and 4b was made constant (0.8 g /), and the Pd / Rh of each of the coat layers 3b and 4b was changed.
The CO / NOx cross point (%) was examined while changing the ratio while maintaining the same ratio.

As a result, the contents shown in FIG. 14 were obtained. According to the contents, when Pd became rich from the boundary of Pd / Rh = 5/1 (same as the above-mentioned catalyst 1b, indicated by black circles), the purification performance was reduced.

[D] Test for examining the effect of the ratio of Rh of the first coat layer and Rh of the second coat layer on heat resistance (stability of low-temperature activity) In the catalyst according to the first invention, a catalyst similar to that of the catalyst 1a was used. By adjusting, Rh of the first coat layer 3a and Rh of the second coat layer 4a are adjusted.
And the 50% purification temperature was examined.

 At this time, the total amount of Rh is 0.27 g /.

The test conditions are the same as the test conditions for the heat resistance test.

As a result, the content shown in FIG. 15 was obtained. According to this content, Rh is dispersed in the first and second coat layers 3a and 4a under the condition that a certain purification rate (50%) is secured, and Rh is used alone in the first coat layer 3a. It was shown that the purification temperature (HC50% purification temperature) was lower than the case where it was dependent on the above, and that the former case had higher heat resistance (stability of low-temperature activity) than the latter case. . This is considered to be because, as in the case of the description of FIGS. 12 and 13, it is possible to avoid the increase in the density of Rh in the second coat layer 4a and to suppress sintering even when the exhaust gas temperature is increased. .

(Effects of the Invention) As described above, in the first invention, improvement in heat resistance due to an increase in the dissociation temperature of a noble metal, improvement based on an oxygen storage effect, and Rh particles immobilized on CeO ₂ particles It is possible to suppress thermal degradation based on sintering between them. As a result, the catalyst performance is significantly improved.

Further, in the second invention, not only the above effects of the first invention can be obtained, but also the purification air-fuel ratio range (reaction range) for HC and CO can be expanded to the rich side.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing an exhaust gas purifying catalyst according to one embodiment of the first and second inventions, and FIG. 2 is an exhaust gas purifying catalyst according to one embodiment of the first and second inventions. FIG. 3 is a diagram conceptually showing the catalyst according to the first invention, FIG. 4 is a diagram conceptually showing the catalyst according to the second invention, FIG. 5 is a comparative example 1 FIG. 6 is a diagram conceptually showing a catalyst according to Comparative Example 2, and 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 the purification performance of the catalyst according to the first and second embodiments of the invention, FIG. 9 is a characteristic diagram showing the purification performance of the catalyst according to Comparative Example 1, and FIG. FIG. 11 is a characteristic diagram showing a purification performance of the catalyst according to Comparative Example 2, and FIG. 11 is a cross point of CO / NOx-Pd / Rh of the first coat layer.
FIG. 12 is a characteristic diagram of the catalyst according to the first invention showing the ratio relationship, FIG. 12 is a characteristic diagram of the catalyst according to the first invention showing the relationship between the CO / NOx cross point and the Rh amount of the second coat layer, FIG. 13 is a CO / NOx cross point—a characteristic diagram of the catalyst according to the second invention showing the relationship of the amount of noble metal in the first coat layer / the amount of noble metal in the second coat layer. FIG. FIG. 15 is a characteristic diagram of the catalyst according to the second invention showing the relationship of the Pd / Rh ratio, and FIG.
FIG. 4 is a characteristic diagram showing a relationship of a coating layer (Rh amount). 1,1a.1b ... catalyst 2,2a, 2b ... catalyst carrier 3,3a, 3b ... first coat layer 4,4a, 4b ... second coat layer

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Kazunori Ihara 3-1 Shinchi, Fuchu-cho, Aki-gun, Hiroshima Mazda Co., Ltd. (72) Kenji Okubo 3-1 Shinchi, Fuchu-cho, Aki-gun, Hiroshima Mazda Inside the corporation

Claims (2)

(57) [Claims] 1. A first coat layer comprising a rhodium-containing alumina layer is provided on a catalyst carrier, and a first coat layer comprising rhodium-fixed cerium oxide particles 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 a rhodium-containing alumina layer is provided on a catalyst carrier, and a first coat layer comprising rhodium and palladium-immobilized cerium oxide particles and active alumina is provided above the first coat layer. An exhaust gas purifying catalyst, wherein a second coat layer is provided.