JP2013184125A - Catalyst for cleaning exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a catalyst for cleaning exhaust gas capable of improving heat resistance of a carrier itself by prompting crystal stabilization of alumina of an alumina layer as the carrier under high temperature environment so as to suppress sintering and a particle growth of Pd particles as metal catalyst particles.SOLUTION: A catalyst for cleaning exhaust gas comprises a Pd catalyst layer containing Pd particles and a Rh catalyst layer containing Rh particles formed on the Pd catalyst layer. In the Pd catalyst layer, a Pd carrier for supporting the Pd particles is a composite oxide containing AlOas a main component, and ZrO.

Description

  The present invention relates to an exhaust gas purification catalyst including at least a Pd catalyst layer on which at least Pd particles are supported.

  Conventionally, in an exhaust gas purifying catalyst, a catalyst layer in which catalyst particles such as Pt, Pd, and Rh having activity as a catalyst metal are supported on a support such as alumina is formed on the surface of a base material. Such Pt, Pd, and Rh are generally known as a three-way catalyst. Among them, Pd is known to be easily affected by olefin poisoning. Further, among these three, Pd is currently used in relatively large amounts because the market price per unit weight is low. Therefore, in recent years, since the market price of Pd tends to increase, reduction of the amount of Pd used is required.

  However, if the amount of catalyst metal (Pd) used in the exhaust gas purification catalyst is significantly reduced, the catalytic activity of the exhaust gas purification catalyst is also lowered accordingly. Thereby, there is a possibility that the exhaust gas purification performance such as oxygen absorption / release of the exhaust gas purification catalyst, low-temperature activity, and NOx purification rate may be significantly reduced. This is because the number of active sites is reduced due to the reduction of the catalyst metal, and as a result, the number of catalytic reaction sites is reduced. In view of the above, development of a catalyst structure and a support material that improve the purification efficiency of exhaust gas while reducing the amount of catalyst metal used has been made.

  By the way, as the carrier for supporting the metal catalyst particles described above, for example, various metal oxides such as silica, alumina, zeolite, diatomaceous earth, etc. are used. The catalyst performance is improved by forming it in a layer.

  For example, as a technique in which a plurality of catalyst layers are provided, Patent Document 1 discloses that at least one of rare earth metals such as Ce and La, and Pt and Pd on the surface of an inorganic carrier substrate such as cordierite. Forming a first alumina layer supporting at least one kind, and further supporting at least one kind of a metal such as Fe and Ni and a rare earth metal such as La and Rh on the surface of the first alumina layer; An exhaust gas purifying catalyst provided with two alumina layers has been proposed.

JP 59-127649 A

  However, since the exhaust gas from the engine is very high temperature (sometimes 1000 ° C. or more), in the exhaust gas purifying catalyst as in Patent Document 1, the catalyst metal is accompanied by the thermal contraction of the alumina particles as the support of the alumina layer. There may be a phenomenon in which (Pd) undergoes thermal aggregation (sintering). As a result, the Pd particles are thermally aggregated and sintered, and the grains grow, so that there is a possibility that the catalytic reaction (active site) decreases.

  The present invention has been made in view of these points, and the object of the present invention is to improve the heat resistance of the support itself by accelerating the crystal stabilization of alumina in the alumina layer as the support in a high temperature environment. Is to provide an exhaust gas purifying catalyst capable of suppressing sintering and grain growth of Pd particles as metal catalyst particles.

As a result of intensive investigations, the inventor made the catalyst layer a two-layer coat and focused on the carrier material constituting the first catalyst layer. Specifically, the first catalyst layer (lower layer) is a composite oxide in which Pd particles are used as metal catalyst particles and ZrO ₂ is added to a carrier mainly composed of Al ₂ O _3. In addition, the crystal of Al ₂ O ₃ constituting the support can be stabilized to suppress the growth of Pd particles, and the surface of Al ₂ O ₃ can be modified to further enhance the activity of Pd, which is a catalyst metal. And gained new knowledge.

  In addition, by supporting Rh particles on the carrier constituting the second catalyst layer (upper layer), the Rh particles supported in the upper layer at the interface between the upper and lower layers are converted to the first layer (lower layer) composite oxide. The inventors obtained new knowledge that the catalyst acts as a metal catalyst particle on the carrier and can extract the NOx purification performance of the catalyst metal Rh more than ever.

The present invention has been made on the basis of the inventor's new knowledge. An exhaust gas purifying catalyst according to the present invention is formed on a Pd catalyst layer containing Pd particles, the Pd catalyst layer, and Rh particles. An exhaust gas purifying catalyst having an Rh catalyst layer, wherein the Pd carrier carrying Pd particles in the Pd catalyst layer is a composite oxide containing Al ₂ O ₃ as a main component and containing ZrO _2. It is characterized by that.

  By promoting the crystal stabilization of alumina in the alumina layer as the support in a high temperature environment, the heat resistance of the support itself can be enhanced, thereby suppressing the sintering and grain growth of the Pd particles as the metal catalyst particles. Furthermore, the low temperature activity of the exhaust gas purifying catalyst can be improved, and the NOx purification efficiency in the exhaust gas can be further increased.

The graph which showed the result of having evaluated the activity performance of the catalyst for exhaust gas purification of Examples 1-4 and Comparative Examples 1 and 2. The graph which showed the result of having evaluated NO purification performance in an actual machine using Examples 1-4 and the exhaust gas purification catalyst of Comparative Examples 1 and 2. FIG. Graph showing the relationship between the addition amount of ZrO ₂ and a specific surface area for the composite oxide according to a reference example.

  Hereinafter, an exhaust gas purifying catalyst according to the present invention will be described based on embodiments. The exhaust gas purifying catalyst according to the embodiment refers to a ceramic honeycomb substrate whose surface is coated with a catalyst layer. The honeycomb substrate is made of, for example, a porous ceramic mainly composed of any one of alumina, zirconia, cordierite, titania, silicon carbide, silicon nitride and the like. Further, an appropriate cross-sectional shape can be applied as the cross-sectional shape constituting a part of the honeycomb substrate, and a polygonal shape such as a square can be applied in addition to a regular hexagon. Further, the outer wall portion may be, for example, a polygonal column shape in addition to the cylindrical shape.

  Here, an object of the present embodiment is to provide an exhaust gas purifying catalyst capable of exhibiting good catalytic performance even in a high temperature region. More specifically, an object of the present invention is to provide an exhaust gas purifying catalyst capable of exhibiting better catalytic performance as compared with the conventional catalyst even in a high temperature region of 1000 ° C. or higher. In order to obtain such an exhaust gas purifying catalyst, in this embodiment, the catalyst layer of the exhaust gas purifying catalyst is a two-layered catalyst layer as shown below.

  Here, the catalyst performance of the catalyst layer constituting the exhaust gas purifying catalyst is better as the size of the metal catalyst particles is smaller and the specific surface area of the carrier supporting the metal catalyst particles is larger. Furthermore, if the catalyst metal of the metal catalyst particles becomes more active (if it becomes more metalized), the catalyst performance will be good. The catalyst layer shown below is configured in view of such points.

  The catalyst layer according to the present embodiment is a porous two-layer catalyst layer composed of an upper layer coated on the surface of the honeycomb substrate and a lower layer coated on the upper layer. In general, the catalyst layer is a metal oxide that forms a matrix carrier (a carrier that supports catalyst particles), more specifically, a mixed material of zirconia and alumina, ceria and alumina, ceria-zirconia and alumina. Among them, one in which at least one kind of noble metal (catalyst metal) of platinum, rhodium and palladium is supported is known. In this embodiment, the lower layer is shown below. A carrier made of a composite oxide is used, and palladium (Pd) is used for the lower layer and rhodium (Rh) is used for the upper layer as the catalyst metal.

  Here, the lower layer is a Pd catalyst layer containing Pd particles as metal catalyst particles, and the catalyst particles are supported by a carrier (Pd carrier). In the present embodiment, a carrier made of a complex oxide is used as the Pd carrier carrying the Pd particles.

Specifically, the composite oxide constituting this carrier is a composite oxide containing Al ₂ O ₃ as a main component and containing ZrO ₂ . That is, the complex oxide here is generally a combination of two oxides of Al ₂ O ₃ and ZrO ₂ , and each Al and Zn forming an ion lattice in the gap between the closest packed oxygen. is there. Further, in such a composite oxide, ZrO ₂ is contained in an amount of at least 10% by mass, preferably 20% by mass or less, based on the whole. In addition, the composite oxide may further include at least one of La ₂ O ₃ , Y ₂ O ₃ , and Sr ₂ O ₃ . Furthermore, as another composite oxide, a composite oxide such as CeO ₂ —ZrO ₂ may be included in the Pd catalyst layer as a support.

  The Pd catalyst layer is obtained by coating a slurry of a complex oxide containing Pd on the surface of a honeycomb substrate, drying and firing, thereby obtaining a Pd catalyst layer in which Pd particles are uniformly dispersed. be able to.

Thus, the lower layer is a Pd catalyst layer coated on the surface of the honeycomb substrate, and an Rh catalyst layer containing Rh particles is further formed as an upper layer on the surface of the lower Pd catalyst layer. Here, it is preferable to use a complex oxide such as CeO ₂ —ZrO ₂ as the carrier for supporting the Rh particles, but the above-mentioned general carrier material may be used.

According to the exhaust gas purifying catalyst according to the present embodiment, at least the Pd carrier supporting Pd particles in the lower Pd catalyst layer includes a composite oxide containing Al ₂ O ₃ as a main component and containing ZrO ₂ . ZrO ₂ added to the main component Al ₂ O ₃ can increase acid sites generated in Al ₂ O ₃ of the Pd catalyst layer (modifying the surface state of Al ₂ O ₃ Is possible). From the change in the electron donating property due to the generation of acid sites, the Pd of the supported Pd particles changes to a more active state (Pd is metalized).

In addition, since the Pd support constituting the lower Pd catalyst layer is a composite oxide of Al ₂ O ₃ and ZrO ₂ , the crystal of Al ₂ O ₃ constituting the Pd support can be stabilized at high temperatures. It is possible to suppress a decrease in the specific surface area of the support made of the composite oxide. Thereby, sintering of Pd particles and accompanying grain growth can be suppressed.

Furthermore, since Rh particles of the Rh catalyst layer are present at the interface between the lower Pd catalyst layer and the upper Rh catalyst layer, the Pd carrier constituting the Pd catalyst layer is present in the Rh particles present at the interface. It is thought to behave as a carrier. That is, it is possible to bring out characteristics such that the Rh particles of the Rh catalyst layer located in the upper layer are supported on Al ₂ O ₃ to which ZrO ₂ as the lower layer is added. As a result, a synergistic effect can be expected in which the Pd support composed of the lower layer ZrO ₂ —Al ₂ O ₃ composite oxide further draws out the NOx reducing ability of Rh, which is the upper layer catalyst metal.

In particular, the composite oxide containing Al ₂ O ₃ and ZrO ₂ as a main material described above contains 10% by mass or more of ZrO ₂ with respect to the composite oxide, so that the following effects can be further expected. . Specifically, by containing ZrO ₂ so as to be in such a range, the crystal of the main material Al ₂ O ₃ is stabilized at a high temperature, thereby the support of the composite oxide. Not only can the reduction of the specific surface area be suppressed, but the surface state of Al ₂ O _{3 in} the carrier can be further modified (the acid point is further increased), and the low temperature activity of the exhaust gas purifying catalyst can be improved.

Moreover, if 5% by mass or more of ZrO ₂ is contained with respect to the composite oxide of the Pd carrier, the crystal of the main material Al ₂ O ₃ can be stabilized, but ZrO ₂ is converted into composite oxide. When the amount is less than 10% by mass with respect to the product, further modification of the surface state of Al ₂ O ₃ in the carrier may not be expected. On the other hand, when ZrO ₂ exceeds 20% by mass with respect to the composite oxide, the heat resistance of the carrier itself may be lowered.

Furthermore, at least one of La ₂ O ₃ , Y ₂ O ₃ , and Sr ₂ O ₃ is further added as a composite oxide to a composite oxide containing ZrO ₂ containing Al ₂ O ₃ as a main material. Thus, the above-described effect may be further expected.

Hereinafter, the present invention will be described by way of examples.
<Example 1>
An exhaust gas purification catalyst was produced by the method described below.
1. Production of Support Consisting of Composite Oxide As shown below, ZrO ₂ —Al ₂ O ₃ (ZrO ₂ / Al ₂ O ₃ = 10/90 mass%) is used as a support for supporting Pd particles for the Pd catalyst layer. A composite oxide powder (carrier) comprising: First, 420 g of ion-exchanged water is put into a 2000 mL container, and 180 g of ammonium hydrogen carbonate (manufactured by Kishida Chemical Co., Ltd.) is added. Then, it stirred for 5 minutes using the stirrer and it was set as the aqueous solution A. Next, 210 g of aluminum nitrate (manufactured by Kansai Catalysts Chemical Co., Ltd.), 6.4 g of zirconium oxynitrate (manufactured by Kanto Chemical Co., Ltd.), and 3.80 g of ion-exchanged water are added to a 1000 mL beaker and stirred until they are completely dissolved. An aqueous solution B was obtained.

Next, the aqueous solution B was quantitatively introduced into the aqueous solution A at a rate of 12 g / min, and the resulting slurry was sealed in the solution and aged at 60 ° C. for 2 hours. After completion of aging, the slurry was suction filtered, and the cake obtained on the funnel was washed with 300 g of ion-exchanged water to remove impurities such as nitric acid, carbonic acid, and ammonia to obtain a composite oxide precursor. This composite oxide precursor is a metal compound mainly composed of ammonium dawsonite and containing Zr (OH) ₂ .

Subsequently, the composite oxide precursor cake was baked in an electric furnace at 1000 ° C. for 5 hours, and pulverized using a hammer mill until the particle size became 250 μm or less, as shown in Table 1. to obtain a powder of composite oxide _(ZrO 2 _-Al 2 _{O 3)} having a composition of _{ZrO 2 / Al 2 O 3 =} 10/90 mass%.

2. Formation of Catalyst Layer As a Pd catalyst layer, a carrier made of the above-mentioned composite oxide (ZrO ₂ —Al ₂ O ₃ ) powder was impregnated with a 65 g / L Pd nitrate solution to prepare a Pd-supported powder. At this time, Pd was adjusted so that the finish was 1.0 g / L.

Thereafter, CeO ₂ —ZrO ₂ composite oxide (CeO ₂ / ZrO ₂ / LaO ₂ / Y ₂ O ₃ = 30/60/5/5% by mass): 85 g / L, barium acetate: equivalent to 10 g / L, Water, Al ₂ O ₃ binder, acetic acid, thickener and the like were mixed at a predetermined ratio to obtain a slurry for the Pd catalyst layer. Then, a monolith honeycomb substrate having a capacity of 875 cc was prepared, and the slurry for the Pd catalyst layer was coated on the surface of the honeycomb substrate by a suction method and fired to form a Pd catalyst layer on the surface of the honeycomb substrate.

Next, as the Rh catalyst layer, CeO ₂ —ZrO ₂ composite oxide (Al ₂ O ₃ / CeO ₂ / ZrO ₂ / LaO ₂ / Nd ₂ O ₃ = 50/20/26/2/2 mass%): 65 g / L was prepared and impregnated with a rhodium chloride chemical solution to prepare an Rh-supported powder. At this time, Pd was adjusted so that the finish was 0.15 g / L.

Then, La added _Al 2 _{O 3} composite oxide and (4/96 wt%), water, _Al 2 _{O 3} binder, acetate, a thickener and the like, predetermined by the ratio to obtain a slurry for Rh catalyst layer. Then, the surface of the Pd catalyst layer of the honeycomb substrate obtained above is further coated with a slurry for the Rh catalyst layer by a suction method and baked to form an upper layer on the surface of the lower Pd catalyst layer. An Rh catalyst layer was formed.

<Example 2>
In the same manner as in Example 1, an exhaust gas purification catalyst was produced. Different from Example 1, as a Pd catalyst layer, instead of the particles of the composite oxide of _ZrO 2 _-Al 2 _{O 3} carrying the Pd (carrier), _ZrO 2 _-Al 2 _{O 3} shown in Table 1 This is a point using particles (carrier) of a composite oxide (ZrO ₂ / Al ₂ O ₃ / La ₂ O ₃ = 10/4/86 mass%) made of -La ₂ O ₃ .

  In order to produce a carrier composed of such a complex oxide, La nitrate is further added to the aqueous solution B described above, and the ratio of the metal oxide constituting the complex oxide is the ratio described above. What is necessary is just to adjust the addition amount of the metal ion of Zr, Al, and La.

<Example 3>
In the same manner as in Example 1, an exhaust gas purification catalyst was produced. Different from Example 1, as a Pd catalyst layer, instead of the particles of the composite oxide of _ZrO 2 _-Al 2 _{O 3} carrying the Pd (carrier), _ZrO 2 _-Al 2 _{O 3} shown in Table 1 This is a point using particles (carrier) of a composite oxide (ZrO ₂ / Al ₂ O ₃ / Y ₂ O ₃ = 10/4/86 mass%) composed of —Y ₂ O ₃ .

  In order to produce a carrier composed of such a complex oxide, nitric acid Y is further added to the aqueous solution B described above, and the ratio of the metal oxide constituting the complex oxide becomes the ratio described above. What is necessary is just to adjust the addition amount of the metal ion of Zr, Al, and Y.

<Example 4>
In the same manner as in Example 1, an exhaust gas purification catalyst was produced. Different from Example 1, as a Pd catalyst layer, instead of the particles of the composite oxide of _ZrO 2 _-Al 2 _{O 3} carrying the Pd (carrier), _ZrO 2 _-Al 2 _{O 3} shown in Table 1 This is a point using particles (carrier) of a composite oxide (ZrO ₂ / Al ₂ O ₃ / Sr ₂ O ₃ = 10/4/86 mass%) composed of —Sr ₂ O ₃ .

  In order to produce a carrier composed of such a complex oxide, Sr nitrate is further added to the aqueous solution B described above, and the ratio of the metal oxide constituting the complex oxide becomes the ratio described above. What is necessary is just to adjust the addition amount of the metal ion of Zr, Al, and Sr.

<Comparative Example 1>
In the same manner as in Example 1, an exhaust gas purification catalyst was produced. The difference from Example 1 is that, as the Pd catalyst layer, La ₂ O ₃ —Al ₂ shown in Table 1 is used instead of the composite oxide particles (support) composed of ZrO ₂ —Al ₂ O ₃ supporting Pd. This is a point using composite oxide particles (support) made of O ₃ (La ₂ O ₃ / Al ₂ O ₃ = 10/90 mass%).

<Comparative example 2>
In the same manner as in Example 1, an exhaust gas purification catalyst was produced. The difference from Example 1 is that the surface of the honeycomb base material is covered only with the lower Pd catalyst layer, and the upper layer Ph catalyst layer is not covered.

(An endurance test)
The prepared exhaust gas-purifying catalyst was set directly under the C8_4.6L engine, and an endurance test was performed at a bed temperature of 1000 ° C. for 50 hours under a composite pattern in which the A / F was changed cyclically.

(Engine bench evaluation)
<Light-Off test>
Using the exhaust gas purifying catalysts of Examples 1 to 4 and Comparative Examples 1 and 2 after the endurance test, the engine was warmed at A / F = 14.6, and the exhaust gas temperature of the engine was controlled with a heat exchanger. The temperature is raised at 20 ° C./min using the exhaust gas. Next, when the temperature was raised from 200 ° C. to 500 ° C., the temperature (T50) ° C. of the exhaust gas purifying catalyst when 50% of the engine output gas was purified was measured. The result is shown in FIG.

<HOT-NOx test>
In order to measure the purification performance of NOx in the exhaust gas, the exhaust gas of A / F = 14.1 and A / F = 15.1 is switched at intervals of 2 minutes (that is, the A / F is changed from rich to lean in a rectangular shape). The average Nox emission amount (ppm) when held in rich (A / F = 14.1) was measured. The catalyst inlet gas temperature was fixed at 550 ° C. The result is shown in FIG.

〔Results and Discussion〕
As shown in FIG. 1, with respect to the exhaust gas purifying catalyst of Comparative Example 1 using a composite oxide containing Al ₂ O ₃ as a main component and LaO ₂ as a carrier for Pd particles, Al ₂ O ₃ is the main component. As a catalyst for exhaust gas purification using a composite oxide containing ZrO ₂ as a carrier for Pd particles, the low-temperature activity performance was excellent. In addition, as shown in FIG. 2, the NOx purification performance was substantially the same for the exhaust gas purification catalysts of Examples 1 to 4 and Comparative Example 1.

Therefore, like the exhaust gas purification catalysts of Examples 1 to 4, by using a composite oxide containing ZrO ₂ as a main component and containing ZrO ₂ as the carrier of the Pd catalyst layer, Al ₂ O ₃ crystals can be obtained. It is considered that the addition of ZrO ₂ increases the acid sites on the surface of Al ₂ O ₃ to increase the activity of Pd, which is a catalytic metal, while stabilizing and suppressing sintering of Pd. It is thought that the low temperature activity of the catalyst was improved.

  Further, as shown in FIG. 2, for the exhaust gas purifying catalyst having only the Pd catalyst layer of Comparative Example 2, the Pd catalyst layer is used as the lower layer and the Rh catalyst layer is provided as the upper layer. The catalyst had excellent low temperature activity performance. Further, as shown in FIG. 2, the exhaust gas purification catalysts of Examples 1 to 4 were superior in NOx purification performance of the exhaust gas purification catalyst as compared with the exhaust gas purification catalyst of Comparative Example 2.

Therefore, as in the exhaust gas purifying catalysts of Examples 1 to 4, the Pd catalyst layer is the lower layer and the Rh catalyst layer is provided on the upper layer, so that one layer of Rh particles supported by the upper layer is formed at the upper and lower layer interfaces. It is considered that the particles acted as metal catalyst particles supported on an alumina (composite oxide) support containing ZrO ₂ in the eye (lower layer). As a result, it is considered that the support made of the lower layer ZrO ₂ —Al ₂ O ₃ composite oxide is derived from the NOx reduction ability of Rh, which is the upper layer catalyst metal.

<Reference example>
In the same manner as in Example 1, a powder (carrier) made of a composite oxide composed of ZrO ₂ —Al ₂ O ₃ was produced. Specifically, the aqueous solution B described above is prepared, and a carrier powder made of a composite oxide is prepared so that the content of ZrO ₂ is 0% by mass, 5% by mass, 10% by mass, and 20% by mass. did.

  Next, after carrying out the above-described durability test using these carriers (however, the bed temperature was 1100 ° C.), these specific surface areas were measured by the BET method based on JIS Z 8830: 2001. The result is shown in FIG. Further, the specific surface areas of the carriers for Examples 1 to 4 and Comparative Example 1 described above were measured before and after the durability test. The results are shown in Table 2.

〔Results and Discussion〕
As shown in FIG. 3, the _Al 2 _{O 3,} by using a composite oxide _ZrO 2 _-Al 2 _{O 3} which ZrO ₂ was added, decrease in the specific surface area at high temperatures is considered to have been suppressed. As a result, it is considered that Al ₂ O ₃ crystals are stabilized, Pd sintering is suppressed, and the catalytic activity of the exhaust gas purifying catalyst is increased.

On the other hand, as shown in Table 2, the specific surface areas of Examples 1 to 4 after the durability test were smaller than those of Comparative Example 1. However, as described above, from the results of FIG. 1, the low temperature activity of the exhaust gas purifying catalysts of Examples 1 to 4 was superior to that of Comparative Example 1. Therefore, in the case of the exhaust gas purifying catalyst of Examples 1-4, the composite oxide constituting the carrier, than _La 2 _{O 3} by adding addition of ZrO _{2, Al} 2 _{O 3} It is thought that the activity of Pd, which is a catalytic metal, could be further increased by increasing the acid sites on the surface.

  Although the embodiment of the present invention has been described in detail above, the specific configuration is not limited to this embodiment, and even if there is a design change within a scope not departing from the gist of the present invention, they are not limited to this embodiment. It is included in the invention.

Claims (3)

A Pd catalyst layer containing Pd particles;
An exhaust gas purification catalyst having an Rh catalyst layer containing Rh particles formed on the Pd catalyst layer,
In the Pd catalyst layer, the Pd carrier supporting Pd particles is a composite oxide containing Al ₂ O ₃ as a main component and containing ZrO ₂ . _2. The exhaust gas purification catalyst according to claim 1, wherein the composite oxide contains 10 mass% or more of ZrO ₂ . The composite in _{oxide, La 2 O 3, Y 2} O 3, Sr 2 O 3 catalyst for purifying an exhaust gas according to claim 1 or 2, characterized in that it contains at least one kind further.

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