JPH0957099A - Exhaust gas purification catalyst for diesel engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the increase in the amts. of SO3 and sulfates produced after a durability test as well as to keep ability to oxidize and decompose CO, etc., at a level equal to or higher than the conventional level. SOLUTION: A catalyst carrying layer 2 carrying a catalytic noble metal 20 and an alkali metal 21 is formed, an alkali metal 10 is also carried in a lower layer under the layer 2 and the concn. of the alkali metal 10 carried in the lower layer is made equal to or higher than that of the alkali metal 21 carried in the catalyst carrying layer 2. The diffusion of the alkali metal 21 in the layer 2 into the lower layer due to concn. difference is prevented and the reduction of the concn. of the alkali metal 21 in the layer 2 after a durability test is prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying catalyst for a diesel engine, and more specifically, carbon monoxide (CO), hydrocarbon (HC) and soluble organic compounds which are harmful components contained in the exhaust gas from a diesel engine. Ingredient (S
The present invention relates to an exhaust gas purification catalyst that purifies OF and reduces the emission amount of sulfate (sulfate).

[0002]

2. Description of the Related Art With respect to gasoline engines, harmful components in exhaust gas have been steadily reduced due to strict regulations on exhaust gas and advances in technology capable of coping with the regulations. However, with regard to diesel engines, due to the unique circumstances in which harmful components are emitted as particulates (carbon fine particles, sulfur-based fine particles such as sulfate, high-molecular-weight hydrocarbon fine particles, etc.), both regulations and technological progress are higher than those of gasoline engines. However, the development of an exhaust gas purification device that can reliably purify exhaust gas is desired.

[0003] As exhaust gas purifying devices for diesel engines that have been developed to date, trap type exhaust gas purifying devices using trap type exhaust gas purifying catalysts and open type exhaust gas purifying devices using open type exhaust gas purifying catalysts have been roughly classified. A purifying device is known. As a trap-type exhaust gas purifying catalyst, a ceramic plugged honeycomb body (diesel particulate filter (DP
F)) and the like are known. In the exhaust gas purifying apparatus using this exhaust gas purifying catalyst, the exhaust gas is filtered by a DPF or the like to collect particulates, and if the pressure loss increases, the particulates burned by a burner or the like are burned to burn the particulates.
And so on. Further, in order to oxidize and decompose CO, HC and SOF as well as collecting particulates, a catalyst supporting layer is formed of alumina or the like on a carrier substrate such as DPF, and platinum (Pt) or the like is supported on the catalyst supporting layer. Exhaust gas purification catalysts are also being considered.

On the other hand, as an open type exhaust gas purifying catalyst, a carrier base material made of a ceramic straight flow type honeycomb body or the like, a catalyst support layer formed of alumina or the like on the carrier base material, and the catalyst support It is known that the layer is composed of Pt and the like supported in the same manner as a gasoline engine. According to this open type exhaust gas purifying apparatus, CO and the like can be oxidized and decomposed by the catalytic action of Pt and the like.

However, the trap type having the above platinum or the like or
In the open type exhaust gas purifying device, SO on Pt₂And oxygen
Reacts with sulphate to produce the catalyst
SO in the exhaust gas₂Adsorbs SO at high temperature₂Is P
SO is oxidized by the catalytic action of t_ThreeIs discharged as
Would. Especially for diesel engines,
Sufficient oxygen gas is present in the gas, and this oxygen gas causes S
O₂Is oxidized to SO _ThreeIs easily discharged as. Soshi
SO₂Is not measured as particulates,
SO_ThreeIs measured as particulates. Also, S
O_ThreeEasily reacts with the large amount of water vapor present in the exhaust gas
Form sulfuric acid mist and are discharged as sulfate
I will end up. Therefore, in these exhaust gas purification devices, high temperature
Occasionally, the amount of particulates increases due to the emission of sulfate.
There is a big problem. From this situation, Japanese Patent Laid-Open No. Hei 4
No. 250851 discloses a catalyst such as Pt on a titania carrier.
Sulfate that supports precious metals and suppresses oxidation
Alkali metal, alkaline earth gold to suppress the emission of
Dee supporting a metal selected from genus and rare earth elements
An exhaust gas purifying catalyst for a diesel engine is disclosed.

[0006] The applicant of the present invention is for a diesel engine in which a catalyst supporting layer made of silica, zeolite or the like carries an alkali metal together with a catalytic noble metal, thereby suppressing the emission of sulfate while maintaining the oxidative decomposition performance of CO and HC. We have proposed an exhaust gas purification catalyst (Japanese Patent Application No. 7-74).
No. 139, not known at the time of filing this application).

[0007]

However, in the above exhaust gas purifying catalyst for a diesel engine, which carries an alkali metal together with a catalytic noble metal, when a durability test is conducted in a working temperature range of 200 to 600 ° C., the production amount of sulfate is suppressed. It became clear that the effect would decrease.

The present invention has been made in view of the above conventional circumstances, and maintains the oxidation / decomposition performance of CO and the like at a level equal to or higher than the conventional one, and increases the production amounts of SO ₃ and sulfate after the durability test. The purpose is to suppress.

[0009]

The features of the exhaust gas purifying catalyst for a diesel engine of the present invention for solving the above-mentioned problems are that a first carrier layer in which an alkali metal is supported on a ceramic carrier,
A second carrier layer formed on the surface of the first carrier layer and supporting a catalytic noble metal and an alkali metal on a porous carrier is provided, and the carrier concentration of the alkali metal of the first carrier layer is defined as the carrier concentration of the alkali metal of the second carrier layer. It is equal to or more than that.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The porous carrier constituting the base material of the second supporting layer is alumina, silica, titania, zeolite,
It can be formed of a refractory inorganic oxide such as silica-alumina and titania-alumina. The refractory inorganic oxide has an average particle size of 20 μm or less and a specific surface area of 10 m ² /
It is preferably at least g. Refractory inorganic oxide has an average particle size of more than 20 μm and is 10 m ² / g
If the specific surface area is less than this, there is a possibility that sufficient decomposition performance of SOF may not be obtained. The second support layer is preferably formed of a silica-alumina refractory inorganic oxide in terms of improving the purification rate of CO and the like.

The first carrier layer can be formed by coating on a metal carrier substrate, for example. Further, the first supporting layer may be configured to also serve as a pellet carrier base material or a monolith carrier base material made of ceramic. Therefore, the material thereof may be a porous carrier similar to that of the second supporting layer or a ceramic such as cordierite, and may be the same as or different from the second supporting layer.

Typical catalytic noble metals include Pt, palladium (Pd), rhodium (Rh), ruthenium (Ru), osmium (Os) and iridium (Ir).
At least one of the above can be adopted. For example, P
The carried amount of t is 0.0 per unit volume of the exhaust gas purifying catalyst.
It is preferably from 1 to 10.0 g / L. If the amount of Pt supported is less than 0.01 g / L, sufficient oxidation / decomposition performance may not be obtained. On the contrary, even if Pt is supported in an amount of more than 10.0 g / L, the oxidation / decomposition performance is slightly improved, and the exhaust gas purifying catalyst becomes expensive. Particularly, when the supported amount of Pt is 0.1 to 3.0 g / L, it is preferable in terms of both oxidation / decomposition performance and cost.

The loading amount of Pd is preferably 0.01 to 20.0 g / L per unit volume of the exhaust gas purifying catalyst. If the supported amount of Pd is less than 0.01 g / L, there is a possibility that sufficient oxidation / decomposition performance may not be obtained. Conversely, 20.0
Even if Pd is supported in excess of g / L, the oxidation / decomposition performance is slightly improved, and the exhaust gas purification catalyst becomes expensive. Especially,
When the supported amount of Pd is 0.5 to 3.0 g / L, it is preferable in terms of both oxidation / decomposition performance and cost.

The supported amount of Rh is preferably 0.01 to 1.0 g / L per unit volume of the exhaust gas purifying catalyst. If the supported amount of Rh is less than 0.01 g / L, there is a possibility that sufficient oxidation / decomposition performance may not be obtained. Conversely, 1.0g
Even if Rh is supported in excess of / L, the oxidation / decomposition performance is slightly improved, and the exhaust gas purification catalyst becomes expensive. In particular, R
When the supported amount of h is 0.05 to 0.5 g / L, it is preferable in terms of both oxidation / decomposition performance and cost.

As the alkali metal, lithium (L
i), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), francium (Fr)
At least one of the above can be adopted. The supported concentration of the alkali metal in the second support layer is preferably 0.01 to 0.1 mol / L per unit volume of the exhaust gas purifying catalyst in the case of a plurality of types of alkali metals in the total amount. If the supported concentration is less than 0.01 mol / L, sufficient oxidation / decomposition performance may not be obtained. In addition, 0.1 mol /
Even if the catalyst is loaded in excess of L, the oxidation / decomposition performance is slightly improved, and the exhaust gas purification catalyst becomes expensive. In particular, when the supported concentration is 0.01 to 0.05 mol / L, it is preferable in terms of both oxidation / decomposition performance and cost.

The greatest feature of the present invention is that the concentration of the alkali metal supported on the first carrier layer is equal to or higher than the concentration of the alkali metal supported on the second carrier layer. For example, it is considered that the melting point of potassium (K) is 63.65 ° C. and the like, and the alkali metal has a low melting point and is likely to melt and move in the catalyst use temperature range of 200 to 600 ° C., for example. Therefore, in the conventional exhaust gas purifying catalyst, during use, the alkali metal diffuses into the alumina coat layer formed by coating the carrier base material and the metal carrier made of cordierite or the like, and the sulfate suppressing effect after the durability test can be obtained. It was not the cause.

However, in the present invention, the alkali metal is also supported in the first supporting layer as the base, and the supporting concentration is equal to or higher than that of the second supporting layer in the surface layer. Therefore, even if the alkali metal in the second supporting layer easily moves due to heat during use or the durability test, the movement to the first supporting layer is suppressed by the alkali metal present in the lower first supporting layer, It is possible to suppress the diffusion due to the density difference that has occurred conventionally. Then, the alkali metal acts on the catalytic noble metal such as Pt in some form, and it becomes possible to suppress the production of SO ₃ while maintaining the oxidation / decomposition performance of CO and the like.

Since the alkali metal loading concentration in the second loading layer is preferably 0.01 mol / L or more, the alkali metal loading concentration in the first loading layer is also 0.01 mol / L.
It is desirable to set it to mol / L or more. Further, if the concentration of the alkali metal supported in the first supporting layer becomes too high, it may diffuse into the second supporting layer during use to reduce the catalytic noble metal oxidizing activity. It is preferable that the supported concentration of the alkali metal is less than twice the supported concentration of the alkali metal in the second supporting layer.

The alkali metals supported on the first and second supporting layers may be different but are preferably the same.

[0020]

EXAMPLES The present invention will be described in more detail below with reference to examples. (Embodiment 1) FIG. 1 shows a schematic configuration diagram of an exhaust gas purifying catalyst of this embodiment. This exhaust gas purifying catalyst comprises a honeycomb carrier (1) made of cordierite, a coat layer (2) coated on the surface of the honeycomb carrier (1), and K (10) supported on the honeycomb carrier (1). P carried on the coat layer (2)
It is composed of t (20) and K (21). That is, the honeycomb carrier (1) constitutes the first carrier layer, and the coat layer (2) constitutes the second carrier layer.

The method for producing the exhaust gas purifying catalyst will be described in detail below, and will replace the detailed description of the structure of the exhaust gas purifying catalyst of this embodiment. A straight flow type honeycomb carrier (1) made of cordierite (400 cells / in ² , diameter 80 mm, length 95 mm) is prepared.
This honeycomb carrier (1) was made to absorb a predetermined amount of a potassium nitrate aqueous solution having a predetermined concentration, and heat-treated at 500 ° C. for 1 hour,
0.05 mol of K (10) was carried per liter of the honeycomb carrier (1) to form a first carrying layer.

Next, silica and alumina are mixed in a weight ratio of SiO 2.
A slurry containing ₂ : Al ₂ O ₃ = 9: 1 was prepared, and a K-supported honeycomb carrier was dipped in this slurry and pulled up to blow off excess slurry, and then at 100 ° C.
And dried for 1 hour at 500 ° C. to form a coating layer (2) of 100 g per liter of the honeycomb carrier (1).

Next, the honeycomb carrier (1) on which the coat layer (2) is formed is immersed in a tetraammine hydroxide platinum solution having a predetermined concentration for 1 hour, and after it is pulled out, excess water droplets are blown off and the mixture is kept at 100 ° C. for 6 hours. After drying, heat treatment was performed at 300 ° C. for 1 hour to remove ammine and the like, and Pt (20) was supported. P
The loaded amount of t (20) is 1.5 g per liter of the honeycomb carrier (1). Then, a predetermined amount of a potassium nitrate aqueous solution having a predetermined concentration is made to absorb water and heat-treated at 500 ° C. for 1 hour to obtain 0.03 m per 1 liter of the honeycomb carrier (1).
ol K (21) was supported to form a second support layer, and the exhaust gas purifying catalyst of Example 1 was obtained. (Example 2) The loading concentration of K (10) in the honeycomb carrier (1) was 0.1 m per 1 liter of the honeycomb carrier (1).
An exhaust gas purifying catalyst of Example 2 was prepared in the same manner as in Example 1 except that it was ol. (Example 3) The loading concentration of K (10) in the honeycomb carrier (1) was 0.3 m per 1 liter of the honeycomb carrier (1).
An exhaust gas purification catalyst of Example 3 was prepared in the same manner as in Example 1 except that it was changed to ol. (Example 4) The loading concentration of K (10) in the honeycomb carrier (1) was 0.5 m per 1 liter of the honeycomb carrier (1).
An exhaust gas purifying catalyst of Example 4 was prepared in the same manner as in Example 1 except that it was ol. (Example 5) The supported concentration of K (10) in the honeycomb carrier (1) was 0.7 m per liter of the honeycomb carrier (1).
An exhaust gas purification catalyst of Example 5 was prepared in the same manner as in Example 1 except that it was changed to ol. (Example 6) The loading concentration of K (10) in the honeycomb carrier (1) was 1.0 m per 1 liter of the honeycomb carrier (1).
An exhaust gas purification catalyst of Example 6 was prepared in the same manner as in Example 1 except that it was changed to ol. (Comparative Example 1) An exhaust gas purifying catalyst of Comparative Example 1 was prepared in the same manner as in Example 1 except that K (10) was not loaded in the honeycomb carrier (1). (Test) Each of the above exhaust gas purifying catalysts was attached to the exhaust system of a diesel engine, and the incoming gas temperature was 20
An endurance test of operating at 0 ° C. for 100 hours was performed. After that, it was attached to the exhaust system of a 2.6-liter diesel engine, and was subjected to an aging treatment in which it was operated at a rotation speed of 2000 rpm and an inlet gas temperature of 500 ° C for 1 hour. The material) was analyzed and the amount of HC was measured. The results are shown in FIG. 2 in terms of PM zero% reduction temperature and HC 50% purification temperature.

The PM zero% reduction temperature is a temperature at which the amount of sulfate generation that increases as the temperature rises becomes the same value as the reduction amount of PM components such as SOF excluding sulfate, and the PM zero% reduction temperature is high. The closer to the side, the more the production of sulfate is suppressed. The HC50% purification temperature is a temperature at which 50% of HC in the exhaust gas before the catalyst is purified, and the lower the HC50% purification temperature is, the higher the oxidizing power of HC is. (Evaluation) From FIG. 2, K (10) in the honeycomb carrier (1)
It can be seen that the PM zero% reduction temperature rises as the supported concentration of P increases, and the generation of sulfate is suppressed. In particular, when the supported concentration of K (10) is 0.1 mol / L or more, the PM zero% reduction temperature is 400 ° C. or more, which is particularly effective in suppressing the formation of sulfate.

On the other hand, K (10) in the honeycomb carrier (1)
Since the 50% purification temperature of HC tends to increase as the supported concentration of HC increases, the supported concentration of K (10) becomes HC5.
It can also be seen that it is desirable to set the 0% purification temperature to less than 0.5 mol / L, which is less than 230 ° C. As described above, when the concentration of K (10) loaded in the honeycomb carrier (1) is too high, the purifying activity of HC is lowered because K in the coat layer (2) is reduced.
It is speculated that this is because (10) diffuses and the concentration of K in the coat layer (2) becomes too high, and the activity of Pt is inhibited.

Therefore, under the conditions of the above embodiment, the optimum loading concentration of K (10) loaded on the honeycomb carrier (1) is 0.1 to 0.5 mol / L. (Embodiment 7) FIG. 3 shows a schematic configuration diagram of an exhaust gas purifying catalyst of the present embodiment. This exhaust gas purification catalyst is a metal carrier (3)
An alumina coating layer (4) coated on the surface of the metal carrier (3), a catalyst supporting layer (5) formed on the surface of the alumina coating layer (4), and K supported on the alumina coating layer (4). (40) and Pt supported on the catalyst supporting layer (5)
(50) and K (51). That is, the alumina coat layer (4) constitutes the first support layer and the catalyst support layer (5) constitutes the second support layer.

The method of manufacturing the exhaust gas purifying catalyst will be described in detail below, and will replace the detailed description of the structure of the exhaust gas purifying catalyst of this embodiment. A metal carrier (3) having a volume of 0.9 L is immersed in an alumina slurry, pulled up to blow off excess slurry, and then dried at 100 ° C. for 1 hour,
It heat-processed at 500 degreeC for 1 hour, and formed the alumina coating layer (4). The coating amount of the alumina coat layer (4) is 50 g per liter of the metal carrier (3).

The metal carrier (3) having the alumina coat layer (4) was made to absorb a predetermined amount of a potassium nitrate aqueous solution having a predetermined concentration and heat-treated at 500 ° C. for 1 hour to obtain 0.03 mol per liter of the metal carrier (3). K (40) was supported to form a first support layer. Next, a slurry containing silica and alumina in a weight ratio of SiO ₂ : Al ₂ O ₃ = 9: 1 was prepared, and a metal carrier (3 having an alumina coat layer (4) carrying K (40) was prepared. ) Is soaked in this slurry, pulled up to blow off excess slurry, dried at 100 ° C. for 1 hour, and calcined at 500 ° C. for 1 hour to obtain 100 g of catalyst carrier layer (5) per liter of metal carrier (3). Was formed.

Next, the metal carrier (3) on which the catalyst supporting layer (5) is formed is immersed in a tetraammine hydroxide platinum solution having a predetermined concentration for 1 hour, and after it is pulled up, excess water droplets are blown off and the temperature is raised at 100 ° C. for 1 hour. After drying for an hour, heat treatment was performed at 300 ° C. for 1 hour to remove ammine and the like, and Pt (50) was supported. P
The loaded amount of t (50) is 1.5 g per liter of the metal carrier (3). Then, a predetermined amount of an aqueous potassium nitrate solution having a predetermined concentration is made to absorb water and heat-treated at 500 ° C. for 1 hour to obtain 0.03 mol per liter of the metal carrier (3).
K (51) of Example 2 was supported to form a second supporting layer, and the exhaust gas purifying catalyst of Example 7 was obtained. (Example 8) The supported concentration of K (40) in the alumina coat layer 4 was 0.05 m per liter of the metal carrier (3).
An exhaust gas purifying catalyst of Example 8 was prepared in the same manner as in Example 7 except that it was changed to ol. (Example 9) The supported concentration of K (40) in the alumina coat layer 4 was 0.1 mo per 1 liter of the metal carrier (3).
An exhaust gas purification catalyst of Example 9 was prepared in the same manner as in Example 7 except that the amount was changed to 1. (Example 10) The supported concentration of K (40) in the alumina coat layer 4 was 0.2 m per liter of the metal carrier (3).
Example 10 Example 10 is the same as Example 7 except that
The exhaust gas purifying catalyst of was prepared. (Example 11) The loading concentration of K (40) in the alumina coat layer 4 was 0.3 m per liter of the metal carrier (3).
Example 11 Similar to Example 7 except that
The exhaust gas purifying catalyst of was prepared. (Comparative Example 2) The supported concentration of K (40) in the alumina coat layer 4 was 0.01 m per liter of the metal carrier (3).
An exhaust gas purifying catalyst of Comparative Example 2 was prepared in the same manner as in Example 7 except that it was changed to ol. (Comparative Example 3) An exhaust gas purifying catalyst of Comparative Example 3 was prepared in the same manner as in Example 7, except that K (40) was not supported in the alumina coat layer 4. (Test / Evaluation) Each of the above exhaust gas purifying catalysts was tested in the same manner as in Example 1, and the results are shown in FIG. From FIG. 4, it can be seen that as the supported concentration of K (40) in the alumina coat layer (4) increases, the PM zero% reduction temperature rises, and the generation of sulfate is suppressed. Especially K (4
If the supported concentration of 0) is 0.03 mol / L or more, P
The M zero% reduction temperature is 400 ° C. or higher, which is particularly effective in suppressing the formation of sulfate.

On the other hand, K (4 in the alumina coat layer (4)
As the carrier concentration of 0) increases, the HC50% purification temperature tends to increase, so the carrier concentration of K (40) is H
C50% purification temperature is less than 250 ° C. 0.2 mol /
It can also be seen that it is desirable to set it to less than L. Therefore, under the conditions of the above example, the loading concentration of K (40) loaded on the alumina coat layer (4) was 0.0.
3 to 0.2 mol / L is optimal.

Although only K is used as the alkali metal in the above embodiment, the same effect as that of this embodiment can be obtained by using alkali metals other than K alone or in combination. Also, regarding the catalytic precious metal, Pt
However, it is needless to say that the same effect can be obtained with other catalytic noble metals.

[0032]

EFFECTS OF THE INVENTION That is, according to the exhaust gas purifying catalyst for a diesel engine of the present invention, since the diffusion of the alkali metal present together with the catalytic noble metal is prevented, the same sulfate production suppressing effect and oxidizing activity as those in the initial stage are obtained even after the durability test. Is obtained, and deterioration of catalyst performance is prevented.

[Brief description of drawings]

FIG. 1 is a schematic sectional view illustrating a configuration of an exhaust gas purifying catalyst according to a first embodiment of the present invention.

FIG. 2 is a graph showing the relationship between the supported concentration of K (10) in the honeycomb carrier (1) and the PM zero% reduction temperature and the HC 50% purification temperature.

FIG. 3 is a schematic cross-sectional view illustrating the structure of an exhaust gas purifying catalyst according to a seventh embodiment of the present invention.

FIG. 4 is a graph showing the relationship between the supported concentration of K (40) in the alumina coat layer (4) and the PM zero% reduction temperature and the HC 50% purification temperature.

[Explanation of symbols]

1: Honeycomb carrier (first carrier layer) 2: Coat layer (second carrier layer) 3: Metal carrier 4: Alumina coat layer (first carrier layer) 5: Catalyst carrier layer (second carrier layer)

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Norio Taguchi 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Automobile Co., Ltd. (72) Inventor Koji Sakano 1 41, Nagakute, Nagakute Town, Aichi County, Aichi Prefecture (72) Inventor Koichi Kasahara 7800 Chihama, Daito-cho, Ogasa-gun, Shizuoka Cataler Industry Co., Ltd. (72) Norihiko Aono 7800 Chihama, Daito-cho, Ogasa-shi, Shizuoka Cataler Industrial Co., Ltd. In the company

Claims (1)

[Claims] 1. A first carrier layer having an alkali metal carried on a ceramic carrier, and a second carrier layer having a catalytic noble metal and an alkali metal carried on a porous carrier formed on the surface of the first carrier layer, the first carrier layer comprising: 1. An exhaust gas purification catalyst for a diesel engine, wherein the supported concentration of alkali metal in the first supporting layer is equal to or higher than the supported concentration of alkali metal in the second supporting layer.