JP3332369B2 - Engine exhaust gas purification catalyst - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a catalyst for purifying exhaust gas of an engine in which a catalyst coat layer is formed on the surface of a porous monolith carrier.

[0002]

2. Description of the Related Art The surface of a porous monolithic carrier made of a material such as cordierite is coated with hydrocarbons (H
C), promotes the oxidation of carbon monoxide (CO), etc., or promotes the reduction of nitrogen oxides (NOx), noble metal-based or other metal-based catalyst substances (for example, platinum, rhodium, etc .; An exhaust gas purifying catalyst for an engine in which a catalyst coat layer containing such a substance is simply referred to as a catalytic noble metal) is formed.

Further, there has been proposed an exhaust gas purifying apparatus in which a distribution concentration of a catalytic noble metal is partially changed according to a temperature distribution or a gas flow velocity distribution in an exhaust gas purifying catalyst (Japanese Patent Application Laid-Open No. Sho 61-46252). Gazette).

In the conventional exhaust gas purifying apparatus disclosed in the above publication, as shown in FIG. 9, a catalyst containing a catalytic noble metal on the surface of a monolithic carrier is provided in a substantially cylindrical housing 102 provided in an exhaust passage 101. An exhaust gas purifying catalyst 103 on which a coat layer is formed is disposed. In the exhaust gas purifying catalyst 103, a portion having an appropriate thickness slightly downstream from the end on the exhaust inlet side and a shaft downstream from this portion. Around the core, the distribution concentration of the catalytic noble metal is increased, and a catalytic noble metal high concentration portion 104 having a substantially T-shaped vertical cross section is formed. On the other hand, in portions other than the catalyst noble metal high concentration portion 104, the distribution concentration of the catalyst noble metal is lower than this,
In these portions, catalytic noble metal low concentration portions 105 and 106 are formed.

[0005]

However, in the conventional exhaust gas purifying catalyst 103 shown in FIG. 9, when the exhaust gas temperature is low, such as when the engine is cold, the catalytic activity becomes low and the exhaust gas purification rate decreases. There's a problem. Also,
Generally, the catalyst temperature becomes higher around the axial center of the exhaust gas purifying catalyst than at the periphery thereof. However, in the conventional exhaust gas purifying catalyst 103, the distribution concentration of the catalytic precious metal becomes higher around the axial center where the temperature becomes higher. The temperature of the shaft center becomes particularly high, and this high temperature causes the catalytic noble metal (particularly platinum)
Causes sintering (welding), and the exhaust gas purifying catalyst 1
03 suffers from thermal degradation.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned conventional problems, and can improve the exhaust gas purification rate by sufficiently increasing the catalytic activity when the exhaust gas is at a low temperature such as when the engine is cold. An object of the present invention is to provide a catalyst for purifying exhaust gas of an engine.

[0007]

The exhaust gas purifying catalyst according to the first aspect of the present invention, which has been made to achieve the above object, comprises: (i) an exhaust gas purifying action on the surface of a porous monolith carrier; An exhaust gas purifying catalyst for an engine, wherein a catalyst coat layer containing the provided catalyst substance is formed,
(Ii) The catalyst coat layer supports platinum and rhodium in a fixed amount from the end of the exhaust gas inlet to the end of the exhaust gas, and at the vicinity of the end of the catalyst coat layer on the exhaust gas inlet side, palladium than other portions of the catalyst coat layer
There is characterized in that it is formed so as to be distributed in a high concentration.

The exhaust gas purifying catalyst according to the second aspect of the present invention comprises: (i) a catalyst coat layer containing a catalytic substance having an exhaust gas purifying action formed on the surface of a porous monolithic carrier. In exhaust gas purification catalysts for engines,
(Ii) the catalyst coat layer is formed on the first coat layer, in which platinum and rhodium are carried in a fixed amount from the exhaust gas inlet end to the exhaust gas end, and on the first coat layer; And (iii) a second coat layer.
The second coating layer, is characterized in that the palladium is distributed in higher concentration than other portions of the second coating layer at an exhaust gas inlet side end portion side.

The exhaust gas purifying catalyst according to a third aspect of the present invention is the exhaust gas purifying catalyst according to the second aspect, wherein the first coat layer is a layer containing alumina and the second coat layer is an oxidized layer. It is a layer containing cerium.

[0010]

[0011]

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be specifically described below. As shown in FIG. 1, an engine (not shown)
An exhaust passage 2 is connected to a downstream end of the exhaust manifold 1. Slightly downstream of the connection with the exhaust manifold 1, a substantially cylindrical pre-converter 3 for pre-treating exhaust gas is provided in the exhaust passage 2, and a substantially cylindrical pre-converter 3 is provided in the pre-converter 3. Deposit catalyst 4
Is arranged. In the exhaust passage 2 downstream of the pre-converter 3, hydrocarbons (HC) and carbon monoxide (CO) in the exhaust gas are converted into carbon dioxide and water, and nitrogen oxides (NOx) are converted into nitrogen. A substantially cylindrical main converter 5 is provided in the main converter 5 and, inside the main converter 5, in order from the upstream, substantially cylindrical first and second exhaust gas purifying catalysts 6, 7 having oxidation / reduction promoting action.
And a substantially cylindrical oxidation catalyst 8 having only an oxidation promoting action.

As shown in FIG. 2, the first exhaust gas purifying catalyst 6 will be described later on the surface of a honeycomb-shaped monolith carrier 10 having a large number of pores 9 extending in the axial direction. Alumina as the main component and platinum-based component
(Hereinafter simply referred to as platinum) and a rhodium-based component (hereinafter simply referred to as rhodium) as a catalyst component, a first catalyst coat layer 11 is formed, and cerium oxide (CeO ₂ ) is further formed on the first catalyst coat layer 11. ) As a main component and a palladium-based component (hereinafter simply referred to as palladium) as a catalyst component.
This is a so-called monolithic carrier catalyst on which the catalyst coat layer 12 is formed.

As shown in FIG. 3, in the vicinity of the end of the first exhaust gas purifying catalyst 6 on the exhaust gas inlet side, a portion from the end to the downstream side to a predetermined position has a second catalyst coat layer 12. Palladium is distributed at a high concentration (see FIG. 2), and this portion forms the palladium carrier layer 13. On the other hand, on the downstream side of the palladium carrier layer 13,
The second catalyst coat layer 12 (see FIG. 2) does not contain palladium at all, and this portion forms the palladium-free layer 14. The second exhaust gas purifying catalyst 7 (see FIG. 1)
Has the same configuration as the first exhaust gas purifying catalyst 6.

In the following, according to the process chart shown in FIG.
A method for manufacturing the first exhaust gas purifying catalyst 6 will be described with reference to FIG. (1) Step P1: 540 g of γ-alumina (γ-Al ₂ O ₃ )
After mixing 60 g of boehmite and 1 liter of water, 10 ml of nitric acid was added to prepare a slurry for the first catalyst coat layer 11. It is needless to say that an arbitrary amount of slurry can be produced by increasing or decreasing the amount of each material used while maintaining the above ratio. (2) Step P2: The monolith carrier 10 is immersed in the slurry. The monolith carrier 10 is made of cordierite in a cylindrical shape, and has a diameter of 25.4 mm (1 inch).
Its axial length is 50.8 mm (2 inches). In addition, 400 pores 9 extending in the axial direction of the monolith carrier 10 are formed per 6.46 cm ² (1 square inch).

(3) Step P3: The monolith carrier 10 is taken out of the slurry, and the slurry is air blown to remove the excess slurry. (4) Step P4: The air blown monolith carrier 10 is dried at 250 ° C. for 2 hours. (5) Step P5: The dried monolith carrier 10 is
The first catalyst coat layer 11 is formed on the surface of the monolith carrier 10 by firing at 2 ° C. for 2 hours. The above steps P2 to P
5, the first catalyst coat layer 11 is a monolithic carrier 1
Adjust so that it becomes 14% wt with respect to 0.

(6) Step P6: A platinum chloride solution having an appropriate concentration and a rhodium chloride solution having an appropriate concentration are prepared. (7) Step P7: The monolithic carrier 10 on which the first catalyst coat layer 11 is formed is sequentially immersed in a platinum chloride solution and a rhodium chloride solution, so that the first catalyst coat layer 11 carries platinum and rhodium. Here, the loading amounts of platinum and rhodium in the first catalyst coat layer 11 were 1.6 g / l in total, and the concentration of both solutions and the immersion time were adjusted such that the ratio of platinum to rhodium was 5: 1. To adjust. (8) Step P8: The monolith carrier 10 supporting platinum and rhodium is dried.

(9) Step P9: The monolith carrier 10 is fired to fix platinum and rhodium on the first catalyst coat layer 11. (10) Step P10: A second catalyst coat layer 12 is further formed on the first catalyst coat layer 11 of the monolith carrier 10. The method for forming the second catalyst coat layer 12 is the same as the method for forming the first catalyst coat layer 11 (Steps P1 to P5) except for the components of the slurry, and thus detailed description is omitted. The slurry for the second catalyst coat layer 12 is prepared by mixing 540 g of cerium oxide (CeO ₂ ), 60 g of boehmite, and 1 liter of water, and adding 10 ml of nitric acid thereto. It is a matter of course that an arbitrary amount of slurry can be prepared by increasing or decreasing the amount of each material used while maintaining the above ratio. In this step P10, the second catalyst coat layer 12 is adjusted to be 28% wt with respect to the monolith carrier 10.

(11) Step P11: A palladium chloride solution having an appropriate concentration is prepared. (12) Step P12: A water-absorbing material 16 (see FIG. 5) made of a water-absorbing material such as sponge or cloth is arranged in the impregnation vessel 15 (see FIG. 5). Absorb. Here, the water absorbing material 16 (see FIG. 5)
Is formed in a shape to engage with the end face of the monolith carrier 10. (13) Step P13: As shown in FIG. 5, the monolith carrier 10 on which the first and second catalyst coat layers 11 and 12 are formed is placed on the water absorbing material 16, and the end face of the exhaust gas inlet side is made of the water absorbing material 16. The monolith carrier 10 is pressed against the water-absorbing material 16 side (downward) at an appropriate pressure for an appropriate time for a suitable time (downward), and the second catalyst coat layer on the exhaust gas inlet side end of the monolith carrier 10 is placed. 12 is loaded with palladium. Here, the thickness of the portion where palladium is supported, that is, the thickness of the palladium supporting layer 13 in the axial direction of the monolith carrier is 5 mm.
The concentration of the palladium chloride solution and the pressing force on the monolithic carrier 10 are adjusted so that the amount of supported palladium (concentration of palladium distribution) becomes 0.5 g / l.

As described above, the palladium carrier layer 13 is formed on the monolith carrier 10 using the water absorbing material 16. According to this method, the thickness of the palladium carrier layer 13 in the axial direction of the monolith carrier is desired. Can be set with high precision and the loss rate of the palladium chloride solution can be greatly reduced. Further, by changing the shape of the contact surface of the water absorbing material 16 with the monolith carrier 10, the palladium carrier layer 13 having an arbitrary cross-sectional shape can be formed on the end surface of the monolith carrier 10. For example, as shown in FIG. 6, the palladium end face supporting layer 19 may be formed only around the axial center of the end face of the exhaust gas purifying catalyst 18.

Conventionally, such a palladium-supporting layer is formed by placing a palladium chloride solution in an impregnation vessel, inserting a monolithic carrier into the palladium chloride solution to an appropriate depth below the surface of the solution, and immersing the carrier, or using a spray. In this method, a palladium chloride solution is sprayed on the end face of the monolithic carrier. However, according to the former, there is a problem that the palladium carrier layer cannot be formed only on a part of the end face, and that the concentration of the palladium chloride solution must be changed to adjust the thickness of the palladium carrier layer.

According to the latter, the palladium-supporting layer cannot be formed only on a part of the end face, the thickness of the palladium-supporting layer cannot be easily adjusted, and the loss rate of the palladium chloride solution is high. Problem. (14) Step P14: Palladium is the second catalyst coat layer 1
The monolith carrier 10 supported on 2 is dried at 250 ° C. for 2 hours. (15) Step P15: The monolith carrier 10 is fired to fix palladium on the second catalyst coat layer 12.

FIG. 7 shows that the above-described manufacturing method basically
However, the amount of palladium carried on the palladium carrying layer 13 was set to 0.
1 to 1.0 g / l, and palladium-supporting layer 1
By changing the thickness of the monolithic carrier 3 in the axial direction within the range of 2 to 15 mm, various exhaust gas purifying catalysts 6 are produced, and the temperature at which the hydrocarbon purifying rate of these exhaust gas purifying catalysts 6 becomes 50% is obtained. (Hereinafter, referred to as HC light-off temperature) is shown. Here, the HC light-off temperature is an index indicating the low-temperature activation performance of the catalyst,
The lower the light-off temperature, the higher the low-temperature activity of the catalyst. The HC light-off temperature of the exhaust gas purifying catalyst prepared by the conventional method is also shown. The exhaust gas purifying catalyst according to this conventional method was prepared by the following method.

The first catalyst coat layer is formed by the steps P1 to P1 shown in FIG.
It is formed by the same method as in step P9. Next, an aqueous palladium chloride solution is added to the cerium oxide (CeO ₂ ) powder, kneaded, dried, and pulverized to prepare a cerium oxide (CeO ₂ ) powder carrying (fixing) palladium.

540 g of the above-mentioned cerium oxide powder carrying palladium, 60 g of boehmite and 1 l of water are mixed, and 10 ml of nitric acid is added thereto to prepare a slurry. The monolithic carrier on which the first catalyst coat layer is formed is immersed in the slurry and then dried and fired to form a second catalyst coat layer. Here, the second catalyst coat layer was adjusted to 28% by weight of the monolithic carrier, and the amount of palladium was 1.0 g.
Adjust to / l. 7 and 8, the exhaust gas purifying catalyst is shown as a PM different layer coat (conventional).

FIG. 8 shows the results of measuring the HC light-off temperature after exposing each of the exhaust gas purifying catalysts to air at 900 ° C. for 50 hours. As is clear from FIGS. 7 and 8, when the amount of supported palladium is less than 0.35 g / l, the effect of improving the low-temperature activity is low, and when it exceeds 1.0 g / l, the effect of improving the low-temperature activity is saturated. 0.35
Within the range of 1.0 to 1.0 g / l, more preferably 0.5 to 1.0 g / l.
It is preferable to set an appropriate value within the range of 0 g / l. Further, when the thickness of the palladium-supporting layer is less than 2 mm, it is difficult to adjust the thickness with high precision in manufacturing, and when it exceeds 7 mm, the effect of improving the low-temperature activity is reduced. It is more preferable to set an appropriate value within a range of 2 to 5 mm.

Here, for comparison with the present embodiment, low-temperature activity and purification rate were measured for the following three types of catalyst samples, which were basically produced by a conventional method, by a measurement method described later. Table 1 shows the measurement results of Catalyst Sample 1 As shown in FIG. 10, a single-coat alumina-based catalyst layer is formed on a cylindrical monolithic carrier having a diameter of 25.4 mm × 50 mm. This catalyst layer is composed of an A layer having a relatively low catalyst concentration and a B layer having a relatively high catalyst concentration. Each of the A and B layers is an A layer (7 mm) from the exhaust gas inlet side when viewed in the axial direction of the cylindrical carrier. ), Layer B (10 mm), Layer A (33 m
m). The noble metal catalyst compositions of the A layer and the B layer are as follows.

<Layer A> Pt 0.25 g / 1178 ml Pd 0.25 g / 1178 ml Rh 0.05 g / 1178 ml Pt / Pd / Rh = 4.5 / 4.5 / 1.0 Noble metal catalyst content 0.46 g / l <B layer> Pt 0.5 g / 1178 ml Pd 0.5 g / 1178 ml Rh 0.1 g / 1178 ml Pt / Pd / Rh = 4.5 / 4.5 / 1.0 Noble metal catalyst content 0.92 g / l

Catalyst sample 2 As shown in FIG. 11, a single-coated alumina-based catalyst layer was formed on a cylindrical monolithic carrier having a diameter of 25.4 mm × 50 mm.
(7 mm) and an A layer (43 mm). The noble metal catalyst compositions of the A layer and the B layer are the same as those of the sample 1.

Catalyst sample 3 As shown in FIGS. 12 and 13, φ25.4 mm × 50 mm
, A first layer C ₃ based on alumina and a second layer C ₁ based on cerium oxide,
The catalyst layer of the double coat consisting of C ₂ Metropolitan is formed. Here, the catalyst layer, from the exhaust gas inlet side end portion as viewed in a carrier axial direction to 2mm is composed of a C ₁ layer and C ₃ layer, which in more rear (exhaust gas outlet side) and C ₂ layer C ₃ (48 mm). Noble metal catalyst composition of C ₁ layer and C ₃ layers are as follows. Incidentally, C ₂ layer does not contain a precious metal catalyst component.

<C ₁ layer> Pt / Pd / Rh = 4.5 / 4.5 / 1.0 Noble metal catalyst content 0.5 g / l <C ₃ layer> Pt / Rh = 5.0 / 1.0 Noble metal catalyst content 0.9 g / l Here, the low-temperature activity of the catalyst is determined by the air-fuel ratio A / F = 1.
HC, CO or NOx in exhaust gas at 4.7
The temperature at which 0% purification was possible, that is, the light-off temperature was evaluated. Of course, the lower the light-off temperature, the better the low-temperature activity. The above measurement was performed after exposing each catalyst sample to air at 900 ° C. for 50 hours.

[0032]

[Table 1]

The exhaust gas purification rate is a reduction rate of HC, CO or NOx when the exhaust gas inflow temperature is fixed at 400 ° C. As is evident from Table 1, it can be seen that samples 1 to 3 made by the conventional method basically have a high light-off temperature and poor low-temperature activity. Also, the purification rate of exhaust gas is low.

Here, for example, the light-off temperature of HC is
Although the lowest catalyst sample 3 has a temperature of 280 ° C., the catalyst according to the present invention shown in FIG. 8 has a HC light-off temperature of 220 ° C. at the highest and about 200 ° C. at the lowest. From this fact, it can be seen that the low-temperature activity of the catalyst according to the invention is very good.
In sample 2, a B layer (7 mm) having a high catalyst concentration is formed near the end of the exhaust gas inlet side, but the effect of improving the low-temperature activity is hardly recognized. This indicates that simply increasing the catalyst concentration on the exhaust gas inlet side cannot substantially achieve the effect of improving the low-temperature activity. That is, as in the catalyst according to the present invention, only by increasing the concentration of palladium at the end of the exhaust gas inlet, a good effect of improving the low-temperature activity can be obtained for the first time.

In sample 3, the catalyst layer is double-coated as in this embodiment, but the effect of improving the low-temperature activity is very small. Therefore, simply forming the catalyst layer as a double coat does not substantially improve the low-temperature activity. By forming a region with a high palladium concentration at the end of the exhaust gas inlet and forming a double coat, the low-temperature activity is improved. It can be seen that an improvement effect can be obtained.

In the present embodiment, palladium is not contained in any portion other than the palladium-supporting layer 13 (palladium-free layer 14), but palladium is distributed at a low concentration throughout the second catalyst coating layer 12. On the other hand, a palladium carrier layer may be formed at a high concentration near the end of the exhaust gas inlet. In addition, without providing two catalyst coat layers, only one catalyst coat layer containing cerium oxide (CeO ₂ ) as a main component and platinum and rhodium as catalyst components is provided. Palladium may be supported on the catalyst coat layer to form a palladium support layer.

[0037]

According to the exhaust gas purifying catalysts according to the first to third aspects of the present invention, the catalytic action for the oxidation reaction is provided near the exhaust gas inlet side end of the exhaust gas purifying catalyst. high palladium are then distributed in a high concentration. For this reason, in a portion where the palladium concentration is high, hydrocarbons, carbon monoxide and the like in the exhaust gas are strongly oxidized, and a large amount of heat is generated by this oxidation reaction. For this reason, the temperature of the exhaust gas is increased in a portion where the palladium concentration is high, and the high-temperature exhaust gas flows to the downstream exhaust gas purifying catalyst main body, thereby reducing the temperature of the downstream exhaust gas purifying catalyst main body. The activity of the entire exhaust gas purifying catalyst is increased. Therefore, its low-temperature activity is enhanced,
The exhaust gas purification rate can be improved. Further, in a portion where the palladium concentration is high, the catalyst temperature becomes high, but since palladium is excellent in heat resistance, sintering does not occur, and therefore, it is possible to effectively prevent thermal deterioration of the exhaust gas purification catalyst. .

[Brief description of the drawings]

FIG. 1 includes an exhaust gas purifying catalyst according to the present invention.
It is a schematic diagram which shows the longitudinal cross section of the exhaust gas purification apparatus of an engine.

FIG. 2 is an explanatory cross-sectional view of a first (second) exhaust gas purifying catalyst.

FIG. 3 is an explanatory longitudinal sectional view of a first (second) exhaust gas purifying catalyst.

FIG. 4 is a process chart showing a manufacturing process of a first (second) exhaust gas purifying catalyst.

FIG. 5 is an explanatory longitudinal sectional view of a water absorbing material and a monolithic carrier arranged in an impregnation container.

FIG. 6 is a schematic view of an exhaust gas purifying catalyst in which a palladium carrier layer is formed only around a shaft center.

FIG. 7 shows the characteristics of the catalyst for purifying exhaust gas produced by the method according to the present invention and the catalyst for purifying exhaust gas produced by the conventional method with respect to the thickness of the palladium-supported layer and the amount of palladium carried at the HC light-off temperature. FIG.

FIG. 8 shows a catalyst for purifying exhaust gas shown in FIG.
FIG. 4 is a graph showing characteristics of the HC light-off temperature with respect to the thickness of a palladium-supported layer and the amount of palladium carried after being exposed to air for 50 hours.

FIG. 9 is an explanatory vertical cross-sectional view of a conventional exhaust gas purifying catalyst.

FIG. 10 is a schematic diagram of a conventional catalyst sample 1 made for comparison of performance with the catalyst according to the present invention.

FIG. 11 is a schematic view of a conventional catalyst sample 2 made for comparison of performance with the catalyst according to the present invention.

FIG. 12 is a schematic view of a conventional catalyst sample 3 made for comparison of performance with the catalyst according to the present invention.

FIG. 13 is a schematic diagram showing a configuration of a catalyst layer of the conventional catalyst sample 3 shown in FIG.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Exhaust manifold, 2 ... Exhaust passage, 3 ... Pre-converter, 5 ... Main converter, 6, 7 ... First and second exhaust gas purifying catalysts, 9 ... Pores, 10 ... Monolith carrier, 11
... First catalyst coat layer, 12 ... Second catalyst coat layer, 13 ...
Palladium carrying layer, 14: palladium-free layer, 15: impregnated container, 16: water absorbing material.

────────────────────────────────────────────────── ─── Continuation of the front page Examiner Yoshihisa Seki (56) References JP-A-58-150436 (JP, A) JP-A-62-68542 (JP, A) JP-A-4-1188053 (JP, A) Japanese Patent Application Laid-Open No. 5-49929 (JP, A) Japanese Utility Model Application No. 55-130739 (JP, U) (58) Fields investigated (Int. Cl. ⁷ , DB name) B01J 21/00-38/74 B01D 53/94 F01N 3/28

Claims (3)

(57) [Claims] 1. An exhaust gas purifying catalyst for an engine, comprising: a catalyst coating layer containing a catalytic substance having an exhaust gas purifying action formed on a surface of a porous monolithic carrier. Platinum and rhodium are carried in a fixed amount from the inlet side end to the outlet side end, and the catalyst coat layer is closer to the exhaust gas inlet side end than the other parts of the catalyst coat layer. the catalyst for purifying exhaust gases of the engine, wherein a beam is formed to be distributed in a high concentration. 2. An exhaust gas purifying catalyst for an engine, comprising: a catalyst coating layer containing a catalytic substance having an exhaust gas purifying action formed on a surface of a porous monolithic carrier. It comprises a first coat layer on which platinum and rhodium are carried in a fixed amount from the inlet side end to the outlet side end, and a second coat layer coated on the first coat layer. Te, the second coating layer, for purifying exhaust gases of the engine, characterized in that the palladium is distributed in higher concentration than other portions of the second coating layer at an exhaust gas inlet side end portion side catalyst. 3. The catalyst for purifying exhaust gas of an engine according to claim 2, wherein the first coat layer is a layer containing alumina, and the second coat layer is a layer containing cerium oxide.