JP4649746B2 - Exhaust gas purification catalyst and method for producing the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust gas purification catalyst and a method for producing the same.
[0002]
[Prior art]
In order to purify exhaust gas from engines such as automobiles and various combustors, Japanese Patent Application Laid-Open No. 7-174017 adsorbs HC (hydrocarbon) in the exhaust gas when the exhaust gas temperature is low, and the HC is removed. It is described that an HC adsorption catalyst capable of oxidizing and purifying the desorbed HC at a temperature at which separation begins is used.
[0003]
In this publication, as an HC adsorption catalyst, a layer containing Ag / ZSM-5 in which Ag is ion-supported on ZSM-5 (a kind of zeolite) is formed on a support, and Al ₂ O is formed thereon. ₃ , a catalyst layer in which Pd is supported is formed, and a catalyst layer in which Rh is supported on Al ₂ O ₃ is formed thereon. Its preparation method contains Ag / ZSM-5 slurry containing, slurry containing a catalyst powder comprising by supporting Pd on _Al 2 _{O 3,} and catalyst powder comprising by supporting Rh on _Al 2 _{O 3} A slurry is prepared, and the slurry is subjected to wash coating, drying, and firing in order. As the binder, alumina or silica sol is used.
[0004]
JP-A-11-0776826 describes that the thermal stability of a catalyst is improved by covering zeolite catalyst particles carrying an active metal component with silica fine particles made from layered silicate. . Further, as such zeolite catalyst particles, there are described particles in which Ag is supported on proton type β-type zeolite having a caivan ratio (a molar ratio of SiO ₂ to Al ₂ O ₃ ) of 40. It is described that silica sol is used as a binder for loading a catalyst on a carrier.
[0005]
[Problems to be solved by the invention]
Zeolite adsorbs HC in the exhaust gas, but when the exhaust gas temperature reaches about 150 ° C., the amount of adsorbed HC increases. The Pd catalyst works to oxidize and purify this HC, but at a temperature of about 150 ° C., the catalyst is not in an active state. Therefore, a state occurs in which HC is released into the atmosphere without being purified. On the other hand, when Ag is supported on zeolite, the release temperature of HC shifts to a high temperature side, which is advantageous for purification of HC by a catalyst.
[0006]
However, a catalyst in which the above Ag is supported on a support material and the above Pd catalyst has a problem in that the oxidation catalyst performance is lowered when exposed to a high-temperature gas. The present invention solves this problem.
[0007]
[Means for Solving the Problems]
As a result of studying the above problems, the present inventor has found that when the catalyst is exposed to a high-temperature gas, the Pd catalyst layer peels off from the support. FIG. 7 (a) shows a fresh catalyst having a layer of Ag supported on zeolite and a Pd catalyst layer supported on activated alumina layered on the pore walls of the honeycomb carrier so that the former is on the inside. (B) of the figure is a photograph after heat aging treatment. However, (a) and (b) differ in the location of the photographed honeycomb holes. Hydrated alumina was used as the binder for both layers. According to the figure, it can be seen that the Pd catalyst layer is peeled off from the support by the heat aging treatment. This exfoliation is considered to be a cause of a decrease in catalyst performance.
[0008]
Therefore, in the present invention, the Pd catalyst layer is prevented from peeling off by modifying the binder.
[0009]
That is, the present invention is arranged in an exhaust passage of a vehicle engine, and includes a first layer having Ag, a first support material and a first binder, and a second layer having Pd, a second support material and a second binder. An exhaust gas purifying catalyst formed in layers on a carrier,
The first binder is an alumina binder, and the second binder is a zirconia binder.
[0010]
According to such a configuration, when the catalyst is heated to a high temperature, when intense vibration, or spalling of the second layer when severe changes in flow velocity of the exhaust gas in contact with the catalyst is reduced, the exhaust gas purifying catalyst Improves durability.
[0011]
As the first support material, when purifying HC in exhaust gas, it is preferable to employ zeolite that adsorbs HC at low temperatures. Thus, in the case of this combination of zeolite and alumina binder, the release temperature of HC shifts to a higher temperature side than the combination of zeolite and zirconia binder, which is advantageous for purification of HC by the Pd catalyst.
[0012]
As the zeolite, Y-type zeolite such as USY, MFI, faujasite and the like can be used, but β-type zeolite excellent in HC adsorption ability is preferable.
[0013]
The second support material is preferably a combination of activated alumina that works as a Pd cocatalyst and is advantageous for HC oxidation, and a double oxide of cerium and praseodymium.
[0014]
The amount of Ag supported per liter of the carrier is preferably 7 g or more in order to suppress HC desorption from the zeolite to a relatively high temperature.
[0015]
The first layer and the second layer are preferably supported in layers on the carrier such that the first layer is on the inside and the second layer is on the outside. Thus, when HC is desorbed from the zeolite in the first layer, the HC passes through the second layer, which is advantageous in oxidizing and purifying it with the Pd catalyst.
[0016]
The exhaust gas purifying catalyst is suitable for being disposed in the exhaust passage of a vehicle engine (gasoline engine or diesel engine) and purifying the exhaust gas.
[0017]
In addition, the present invention provides an exhaust gas in which a first layer having Ag, a first support material and a first binder, and a second layer having Pd, a second support material and a second binder are formed in layers on the carrier. A method for producing a gas purification catalyst, comprising:
The first layer is formed using an alumina binder as the first binder, and the second layer is formed using a zirconia binder as the second binder.
[0018]
As a result, a highly durable exhaust gas purifying catalyst is obtained that is less likely to peel off the second layer when heated to a high temperature, vibrated vigorously, or when the flow rate change of the exhaust gas in contact is severe.
[0019]
【The invention's effect】
As described above, according to the present invention, the first layer having Ag, the first support material and the first binder, and the second layer having Pd, the second support material and the second binder are formed in layers on the carrier. a catalyst for purifying exhaust gases is, the first binder is alumina binder, since the second binder is a zirconia binder, when the catalyst is heated to a high temperature, when intense vibration, or the The exfoliation of the second layer when the flow rate change of the exhaust gas contacting the catalyst is severe is reduced, and the durability of the exhaust gas purification catalyst is improved.
[0020]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0021]
In FIG. 1, reference numeral 1 denotes a vehicular gasoline engine, 2 denotes an intake passage thereof, and 3 denotes an exhaust passage. A catalyst container 5 is directly connected to a collecting portion of the exhaust manifold 4 constituting the exhaust passage 3, and a three-way catalyst 6 is accommodated therein, and a catalyst container 8 is interposed in the middle of an exhaust pipe 7 connected to the catalyst container 5. The adsorbing catalyst 9 is accommodated therein.
[0022]
<Structure of adsorption catalyst 9>
FIG. 2 shows a catalyst layer structure of the adsorption catalyst 9, in which a reference numeral 10 indicates a part of the honeycomb carrier, and an Ag zeolite layer (first layer) 11 and a Pd catalyst layer are formed on the carrier 10. (Second layer) 12 is formed in layers so that the former is inside and the latter is outside. In the figure, 13 is a honeycomb hole.
[0023]
The Ag zeolite layer 11 is an HC adsorbent layer in which Ag is supported on β-type zeolite as a support material. β-type zeolite preferably has a caivan ratio of 120 or more. The amount of β-type zeolite supported (the amount supported per liter of the carrier; hereinafter the same) may be, for example, about 100 to 200 g / L. The amount of Ag supported may be about 7 to 30 g / L.
[0024]
The Pd catalyst layer 12 is a catalyst layer in which Pd is supported on activated alumina as a support material and a double oxide of cerium and praseodymium. The supported amount of activated alumina may be, for example, about 50 to 100 g / L. The supported amount of the double oxide of cerium and praseodymium may be about 25 to 50 g. The supported amount of Pd may be about 2 to 8 g / L.
[0025]
Each of the layers 11 and 12 includes a binder, hydrated alumina is used as the binder of the Ag zeolite layer 11, and zirconia is used as the binder of the Pd catalyst layer. The binder loading amount may be about 10 to 30% by mass of the supporting material loading amount.
[0026]
<Example>
Hereinafter, specific examples will be described.
[0027]
The carrier 10 is made of cordierite having a cross-sectional area of about 6.45 cm ² (1 square inch) with 400 cells, a wall thickness separating adjacent cells of about 0.15 mm (6 millimeters), and a capacity of 1.3 L. A honeycomb carrier was adopted.
[0028]
The Ag zeolite layer 11 is formed by impregnating and supporting Ag in a mixed layer of β-type zeolite and a hydrated alumina binder having a boehmite structure. The amount of β-type zeolite supported was 160 g / L, and the amount of hydrated alumina binder supported was 20% by mass of the amount of β-type zeolite supported. β-type zeolite has a Siban ratio of 300 and an Ag loading of 10 g / L.
[0029]
The Pd catalyst layer 12 is a mixed layer of a catalyst powder in which Pd is supported on a support material (a mixture of activated alumina (γ-alumina) and a double oxide of cerium and praseodymium) and a zirconia binder. The supported amount of activated alumina is 70 g / L, the supported amount of double oxide of cerium and praseodymium is 35 g / L, the supported amount of zirconia binder is 16 g / L, and the supported amount of Pd is 5.7 g / L. As the zirconia binder, zirconyl acetate was employed. Zirconium hydroxide or zirconium acetate may be employed.
[0030]
The method for producing the adsorption catalyst 9 is as follows.
[0031]
-Formation of zeolite layer-
β-type zeolite and hydrated alumina binder are mixed in the above-mentioned ratio of supported amount, water and nitric acid are added thereto, and the mixture is stirred with a disperser to obtain a slurry. Nitric acid was 30 g per kg of β-type zeolite.
[0032]
The carrier is coated with a predetermined amount of slurry by repeating the operation of immersing the honeycomb carrier in the slurry and pulling it up and removing the excess slurry by air blow at room temperature.
[0033]
Thereafter, the honeycomb carrier is heated from room temperature to 500 ° C. at a substantially constant heating rate over 1.5 hours and held at that temperature for 2 hours (drying / firing).
[0034]
-Preparation of Pd-supported catalyst powder-
Activated alumina powder and powder of double oxide of cerium and praseodymium are mixed in the above-mentioned ratio of supported amount, and a solution obtained by dissolving palladium nitrate in ion-exchanged water is added dropwise to this and dried at 500 ° C. By calcining, a Pd-supported catalyst powder is obtained.
[0035]
-Formation of Pd catalyst layer-
The Pd-supported catalyst powder and the zirconia binder are mixed, and water and nitric acid are added to the Pd-supported catalyst powder in the same manner as in the case of the zeolite layer, and mixed and stirred with a disperser to obtain a slurry. The slurry is coated on the zeolite layer of the honeycomb carrier in the same manner, and the same drying and firing treatment is performed.
[0036]
-Impregnation support of Ag solution-
Silver nitrate is dissolved in ion-exchanged water, and this is impregnated in the coating layer of the honeycomb carrier on which the Pd is supported. Thereafter, the honeycomb carrier is heated from room temperature to 200 ° C. at a substantially constant heating rate over 1.5 hours, held at that temperature for 2 hours (dried), and substantially until 200 ° C. to 500 ° C. The temperature is raised over a period of 4 hours at a constant rate of temperature rise and held at that temperature for 2 hours (firing).
[0037]
<Adsorption and release characteristics of HC>
FIG. 3 shows HC adsorption / release characteristics when a hydrated alumina binder is used as the binder for the Ag zeolite layer, and FIG. 4 shows HC adsorption / release characteristics when a zirconia binder is used as the binder for the Ag zeolite layer. . However, in each case, only the Ag zeolite layer is formed on the support, the Pd catalyst layer is not formed, the Ag loading is 7.5 g / L, and the binder amount is 20% by mass of the β-type zeolite loading. It is. From the start of the test to 600 seconds, nitrogen gas having a temperature of 80 ° C. and a benzene concentration of 500 ppm was passed through the catalyst, and thereafter the temperature was gradually increased with the benzene concentration being 0 ppm.
[0038]
According to FIGS. 3 and 4, the HC release temperature tends to be lower on the high temperature side when hydrated alumina is used than when the zirconia binder is used. Therefore, it can be said that it is preferable to use hydrated alumina as a binder in the Ag zeolite layer.
[0039]
<Evaluation of actual HC adsorption and purification characteristics>
Next, for the case of using hydrated alumina as the binder of the Ag zeolite layer and the case of using zirconia, a catalyst having a two-layer structure in which a Pd catalyst layer is formed on the Ag zeolite layer is prepared. The catalyst was accommodated in the catalyst container 8 of the exhaust pipe 7 as shown in FIG. 1, and the amount of HC discharged through the catalyst was measured and compared. Both test catalysts have the same configuration as the previous examples except that the type of binder of the Ag zeolite layer is different. That is, in all cases, the amount of Ag supported is 10 g / L, the amount of binder in the Ag zeolite layer is 20% by mass of the amount of β-type zeolite supported, and the binder in the Pd catalyst layer is hydrated alumina.
[0040]
As for the three-way catalyst 6, the number of cells per cross-sectional area of about 6.45 cm ² (1 square inch) is 900, the wall thickness separating adjacent cells is about 0.05 mm (2 milliinches), and the capacity is three-way catalyst 6. A 1.0 L cordierite honeycomb carrier was employed, and a catalyst layer in which Pt, Pd and Rh were supported on activated alumina was formed on the carrier. These noble metals were made to have a mass ratio of Pt: Pd: Rh = 1: 30: 2 and the total supported amount of these noble metals was 12.5 g / L.
[0041]
The three-way catalyst was preliminarily aged at 1100 ° C. for 24 hours in an air atmosphere. Further, the aging of the HC adsorption catalyst is performed in a temperature state (15 minutes) at a maximum of 880 ° C. with respect to the HC adsorption catalyst after being incorporated in the exhaust passage of the engine as shown in FIG. The engine was operated so that 25 thermal cycles were alternately repeated for 45 minutes). Thereafter, the HC emission amount was measured by combining this HC adsorption catalyst and the three-way catalyst.
[0042]
The result is shown in FIG. There is no difference between the catalyst using the hydrated alumina binder in the Ag zeolite layer for 0 to 51 seconds and the catalyst using the hydrated alumina binder for 51 to 125 seconds. Emissions are low.
[0043]
When the exhaust gas temperature is low from 0 to 51 seconds, HC in the exhaust gas is adsorbed by the zeolite, and no difference in the HC emission amount is recognized. The difference in the binder does not affect the HC adsorption characteristics. This is probably because of this.
[0044]
The 51 to 125 seconds when the exhaust gas temperature rises is when HC adsorbed on the zeolite is released, and the amount of HC emission when the hydrated alumina binder is used is small. This is probably because the amount of HC released after the activity of the Pd catalyst was increased increased, and the amount of HC released from the zeolite and purified by the Pd catalyst was increased.
[0045]
<Influence of binder of Pd catalyst layer>
Next, hydrated alumina is adopted as the binder for the Ag zeolite layer, and a catalyst using hydrated alumina and a catalyst using zirconia are prepared as the binder for the Pd catalyst layer. The test catalyst is shown in FIG. The amount of HC discharged through the catalyst after being accommodated in the catalyst container 8 of the exhaust pipe 7 was measured and compared. Both test catalysts have the same configuration as in the previous examples except that the type of binder in the Pd catalyst layer is different. The test vehicle used was a vehicle different from the previous HC emission measurement test.
[0046]
As for the three-way catalyst 6, the number of cells per cross-sectional area of about 6.45 cm ² (1 square inch) is 600, the wall thickness separating adjacent cells is about 0.10 mm (4 millimeters), and the capacity A 1.0 L cordierite honeycomb carrier was employed, and a catalyst layer in which Pt, Pd and Rh were supported on activated alumina was formed on the carrier. These noble metals were made to have a mass ratio of Pt: Pd: Rh = 1: 30: 2, and the total supported amount of these noble metals was 10 g / L.
[0047]
The three-way catalyst was separately subjected to bench aging (thermal degradation treatment) in advance. Further, the aging of the HC adsorption catalyst is performed in a temperature state (15 minutes) at a maximum of 880 ° C. with respect to the HC adsorption catalyst after being incorporated in the exhaust passage of the engine as shown in FIG. The engine was operated so that 25 thermal cycles were alternately repeated for 45 minutes). Thereafter, a three-way catalyst was combined with the HC adsorption catalyst, and the HC emission amount was measured.
[0048]
The result is shown in FIG. In the case of 0 to 51 seconds when the exhaust gas temperature is low, no difference is observed in the HC emission amount as in the case of FIG. However, during 51 to 125 seconds when the exhaust gas temperature is high, the catalyst using the zirconia binder has less HC emission than the catalyst using the hydrated alumina binder. This is presumably because the Pd catalyst layer partially peeled off when using the hydrated alumina binder. It should be noted that there is no difference in the results for 0 to 51 seconds when HC is adsorbed on zeolite during this period, and whether or not the Pd catalyst layer has peeled has a substantial effect on HC adsorption. This is because there is not.
[Brief description of the drawings]
FIG. 1 is a view showing an arrangement of an exhaust gas purifying catalyst in a vehicle engine according to the present invention.
FIG. 2 is a cross-sectional view showing an exhaust gas purifying catalyst structure according to the present invention.
FIG. 3 is a graph showing the HC adsorption / release characteristics when a hydrated alumina binder is used as the binder of the Ag zeolite layer.
FIG. 4 is a graph showing HC adsorption / release characteristics when a zirconia binder is used as a binder of an Ag zeolite layer.
FIG. 5 is a graph showing the effect of the difference in the binder of the Ag zeolite layer on the HC emission amount examined with an actual vehicle.
FIG. 6 is a graph showing the effect of the difference in the binder of the Pd catalyst layer on the HC emission amount examined with an actual vehicle.
7A is a micrograph of a part of a fresh exhaust gas purifying catalyst, and FIG. 7B is the same photo after heat aging treatment.
[Explanation of symbols]
1 Engine 2 Intake passage 3 Exhaust passage 6 Three-case catalyst 9 Adsorption catalyst 10 Carrier 11 Ag zeolite layer 12 Pd catalyst layer

Claims (6)

A first layer having Ag, a first support material and a first binder, and a second layer having Pd, a second support material and a second binder are formed in layers on the carrier , arranged in the exhaust passage of the vehicle engine. An exhaust gas purifying catalyst,
The exhaust gas purifying catalyst, wherein the first binder is an alumina binder and the second binder is a zirconia binder. The exhaust gas purifying catalyst according to claim 1,
The exhaust gas purifying catalyst, wherein the first support material is zeolite. The exhaust gas purifying catalyst according to claim 2,
The exhaust gas purifying catalyst, wherein the second support material is activated alumina and a double oxide of cerium and praseodymium. The exhaust gas purifying catalyst according to claim 3,
An exhaust gas purifying catalyst, wherein the amount of Ag supported per liter of the carrier is 7 g or more. The exhaust gas purifying catalyst according to claim 3,
The exhaust gas purifying catalyst, wherein the first layer and the second layer are supported on the carrier in layers so that the first layer is on the inside and the second layer is on the outside. Production of an exhaust gas purifying catalyst in which a first layer having Ag, a first support material and a first binder, and a second layer having Pd, a second support material and a second binder are formed in layers on the carrier. A method,
A method for producing an exhaust gas purifying catalyst, wherein the first layer is formed using an alumina binder as the first binder, and the second layer is formed using a zirconia binder as the second binder.

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