JPH11300203A - Manufacture of catalyst for exhaust gas purification - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a trouble wherein a NOx purification performance is deteriorated due to a high temperature endurance test by a method wherein in the case where a catalyst for purifying an exhaust gas is manufactured, noble metals contain at least Pt and Pd, and Pt and Pd are simultaneously supported by using a mixed solution of a Pt compound solution and a Pd compound solution. SOLUTION: An exhaust gas purifying catalyst for purifying an exhaust gas exhausted from an internal combustion engine of an automobile or the like is composed of a base material, a coat layer comprising a porous support formed on the surface of the base material, and noble metals and a NOx absorbing material which are supported by the coat layer. In the case such the exhaust gas purifying catalyst manufactured, the noble metals should contain at least Pt and Pd, and Pt and Pd are treated so as to be simultaneously supported by using a mixed solution of a Pt compound solution and a Pd compound solution. Thereby, Pt and Pd can be supported in a condition wherein they are uniformly highly dispersed, the degree of lowering of a front surface area of the noble metals is decreased even after the temperature endurance test, and decrease of a purification performance can be suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an exhaust gas purifying catalyst for purifying exhaust gas discharged from an internal combustion engine of an automobile or the like, and more particularly, to a method for supporting at least Pt and Pd as precious metals and _x NO with storage material
_{The present} invention relates to a method for producing an occlusion reduction type exhaust gas purifying catalyst.

[0002]

2. Description of the Related Art Conventionally, as a catalyst for purifying exhaust gas of automobiles, CO and CO in exhaust gas at a stoichiometric air-fuel ratio (stoichiometric) have been used.
A three-way catalyst that purifies by simultaneously oxidizing HC and reducing NO _x is used. As such a three-way catalyst, for example, a coat layer made of γ-alumina is formed on a heat-resistant substrate made of cordierite or the like, and a noble metal such as platinum (Pt) or rhodium (Rh) is supported on the coat layer. What has been known is widely known.

On the other hand, in recent years, carbon dioxide (CO ₂ ) in exhaust gas discharged from internal combustion engines such as automobiles has become a problem from the viewpoint of protection of the global environment. As a solution, lean combustion in an oxygen-excess atmosphere has been proposed. Burn is promising. In this lean burn, the use of fuel is reduced in order to improve fuel efficiency, and the generation of CO ₂ as combustion exhaust gas can be suppressed.

[0004] From the viewpoint of exhaust gas purification, conventional gasoline internal combustion engine air-fuel ratio is combusted in the theoretical air-fuel ratio (stoichiometric), CO in the exhaust gas by a three-way catalyst, HC, the exhaust gas simultaneously oxidizing and reducing NO _x Can be purified. In lean burn contrast, the exhaust gas is an oxygen-rich atmosphere, does not exhibit purification performance for reduction and removal of the NO _x in the conventional three-way catalyst. Therefore, development of a catalyst and purification system has been desired can purify NO _x even in an oxygen rich atmosphere.

Accordingly, the applicant of the present application has previously proposed an exhaust gas purifying catalyst in which an alkaline earth metal such as Ba and Pt are supported on a porous carrier such as alumina (for example, Japanese Patent Application Laid-Open No. Hei 5-317625). Using the exhaust gas-purifying catalyst, by controlling so that the stoichiometric-rich side in a pulsed manner the air-fuel ratio from the lean side, NO _x is occluded in the alkaline earth metal (NO _x storage material) in the lean side, it order to be cleaned reacts with the reducing components such as HC and CO in the stoichiometric-rich side, it is possible to efficiently purify NO _x even in the lean burn.

[0006] NO in the exhaust gas purifying catalyst_xPurification
The reaction oxidizes NO in the exhaust gas to NO _xThe first step
And NO_xNO on storage material_TwoA second step of storing ozone,
NO_xNO released from the storage material_xThe third of which is reduced over a catalyst
It is known to consist of steps. NO like this
_xTo manufacture a storage-reduction type catalyst, it is necessary to use a large amount of alumina or the like.
Prepare a slurry containing the porous carrier powder and binder,
Slurry of cordierite or honeycomb made of metal
And fired to form a coat layer. And coat
Immerse the substrate with the layer in a solution in which the noble metal compound is dissolved
Precious metal by water absorption or impregnation, etc.
NO_xNO by the water absorption method of immersion in a solution in which the storage material is dissolved
_xIt carries an occluding material. Also, NO_xOcclusion material and precious metal
Catalyst powder and binder previously supported on alumina powder, etc.
From the slurry and coat it on the honeycomb substrate.
There is also known a production method of firing and firing.

[0007]

However, in the conventional exhaust gas purifying catalyst, grain growth occurs in Pt in a lean atmosphere, and the reactivity of the second step and the third step decreases due to a decrease in the catalytic active point. , there was a problem that _the NO _x purification performance is greatly reduced when performing a high-temperature durability test.

On the other hand, Pd is known as a noble metal in which such grain growth hardly occurs in a lean atmosphere, but its oxidizing ability is inferior to Pt. Therefore, it is conceivable to use Pt and Pd together. However, since Pd covers the surface of Pt, the oxidation reaction in the first step is suppressed, and the occlusion reaction in the second step hardly occurs. Therefore Pt and Pd
When used in combination, NO _x purifying ability was a problem of a decrease in comparison with the case of Pt alone.

Further, Pd has a low NO oxidizing ability, so that Pt suppresses the NO oxidizing ability. As the amount of added Pd increases, the reactivity of the first step of oxidizing NO to NO ₂ increases. And the occlusion of NO ₂ in the second step is reduced. The present invention has been made in view of such circumstances, and Pt and Pd, by improving the method of manufacturing a catalyst supporting and _the NO _x storage material, Pd disadvantages with take maximum advantage of Pd a silencing, it is an object to a catalyst exhibiting high _the NO _x purification performance even after high-temperature durability test.

[0010]

Means for Solving the Problems The features of the method for producing an exhaust gas purifying catalyst of the present invention which solves the above-mentioned problems include a substrate, a coat layer comprising a porous carrier formed on the surface of the substrate, and a coat layer. to a supported noble metal and NO _x storage material, a process for the preparation of a more comprising an exhaust gas purifying catalyst, the noble metal comprises at least Pt and Pd, using a mixed solution of a solution of a solution of Pd compounds of Pt compound The purpose is to carry Pt and Pd simultaneously.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In a conventional method for producing an exhaust gas purifying catalyst, a coat layer is formed from a porous carrier such as alumina powder on a honeycomb-shaped or pellet-shaped substrate. The substrate having the coat layer is loaded with Pt using an aqueous solution in which dinitrodiammine platinum or the like is dissolved. To further supporting Pd is further supports Pd by using a palladium nitrate aqueous solution, and then carrying _the NO _x storage material.

As described above, since Pt and Pd are separately supported in the conventional production method, it is presumed that Pd is supported so as to surround the Pt particles on which Pd is supported, and NO oxidation characteristics of Pt are considered. Is thought to decrease. Also carry Pd first and then
There is also a method of supporting Pt, but it has been found that in the catalyst obtained by this method, the degree of dispersion of Pd and Pt is low, and it is difficult to achieve high dispersion.

Therefore, in the method for producing an exhaust gas purifying catalyst of the present invention, Pt and Pd are simultaneously supported by using a mixed solution of a Pt compound solution and a Pd compound solution. This allows Pt and
Pd is carried in a state of being uniformly and highly dispersed, and the degree of reduction in the surface area of the noble metal is reduced even after the high-temperature durability test, so that a reduction in purification performance is suppressed. Further, in the catalyst obtained by the production method of the present invention, Pt coating on Pd is reduced, and the effect of Pd, which suppresses the NO oxidizing ability of Pt, is small.
The reaction in the step proceeds smoothly, and a high NO _x purification ability is obtained.

Hereinafter, the preparation process will be described. The coat layer is formed in the same manner as in the past by preparing a slurry containing a porous carrier powder and a binder such as alumina sol, dipping the substrate into the slurry, pulling it up, drying and firing, and the like. Can be. Pt compound solution and Pd
When Pt and Pd are simultaneously supported using a mixed solution of a compound and a solution of the compound, it is preferable that the mixed solution is impregnated into a substrate having a coat layer under reduced pressure to penetrate into the pores. As a result, the mixed solution can be absorbed into the pores of alumina as a porous carrier, so that the carrying and carrying efficiency can be improved, and Pt and Pd can be more uniformly and highly dispersed and carried.

As for the degree of pressure reduction, it is preferable that the degree of vacuum be 200 mmHg or less. If the degree of vacuum exceeds 200 mmHg, air bubbles remain inside the pores, so that it becomes difficult to absorb water to the inside of the pores of alumina, and the carrying amount is reduced. If the vacuum is 2
Within the range of 00 mmHg or less, the difference in the amount of water absorption depending on the degree of vacuum is small, and the state of loading is not affected by the degree of vacuum. Therefore, it is not necessary to use an ultra-vacuum, and an increase in man-hours and an increase in the size of the apparatus can be suppressed.

As a method for supporting Pt and Pd, water absorption of a supporting liquid is used.
Using the water absorption method to determine the loading amount of the active ingredient from the amount
Is preferred. Impregnation supported by immersion in a low concentration solution
Method, it is difficult to support the noble metal inside the coat layer
And the carrier concentration changes in the depth direction of the coat layer,
On the contrary, the degree of dispersion decreases. Also, NO_xPt and Pd occlusion materials
At the same time, it is also preferable to carry them. This makes NO _xOcclusion
Since the material is supported close to Pt and Pd, NO_xOcclusion and reduction
Performance is improved. Especially NO_xWhen K is used as the storage material
, Further improves high-temperature durability by supporting simultaneous water absorption
You. Of course, even if the impregnation method is used,
High-temperature durability is improved as compared with a catalyst that has been used.

When Ba is used as the NO _x occluding material, it is difficult to support a predetermined amount of Ba because of the low solubility of water-soluble salts such as barium nitrate. Therefore, in this case, it is preferable that Ba is first supported by an impregnation method or the like, and then Pt and Pd are simultaneously supported. The amount of Pt carried is desirably in the range of 1 to 3 g per liter of the substrate having the coat layer. If the amount of Pt carried is less than this range, the desired purification performance will not be exhibited, and if the amount of Pt carried exceeds this range, the effect will be saturated and the cost will rise.

The amount of Pd to be carried is determined according to the substrate 1 having the coat layer.
It is desirable that the amount be in the range of 1 to 4 g per liter. If the amount of Pd carried is less than this range, noble metal grains will easily grow during the high-temperature durability test, and the high-temperature durability will be reduced. If the amount of Pd carried exceeds this range, the reducing ability will decrease due to the oxygen storage ability and the CO adsorption poisoning of Pd.

The loading ratio of Pt and Pd is Pt / Pd = 1 by weight.
/ 3 to 3/1. If the loading ratio of Pd is higher than this, the NO storage performance may decrease, and if the loading ratio of Pd is lower than this, the heat resistance may decrease. Note that, in addition to Pt and Pd, other noble metals such as Rh, Ir, and Ag can be supported. Further, a base metal such as Fe, Co, Mn, and W may be supported.

As the substrate, a honeycomb-shaped substrate having a large number of passages formed of a heat-resistant ceramic such as cordierite or a metal can be used. Alternatively, pellets made of a heat-resistant ceramic such as cordierite may be used. The porous carrier constituting the coat layer can be selected from alumina, silica, zirconia, silica-alumina, zeolite and the like. One of these may be used, or a plurality of them may be mixed or combined for use.

As the NO _x occluding element constituting the NO _x occluding material, at least one element selected from alkali metals, alkaline earth metals and rare earth metals can be used. Examples of the alkali metal include lithium (Li), sodium (Na), potassium (K), and cesium (Cs). Further, the alkaline earth metal refers to a Group 2A element of the periodic table, and includes magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). Examples of the rare earth metal include lanthanum (La), cerium (Ce), praseodymium (Pr) and the like.

In order to carry the NO _x occluding material on the coat layer, the substrate having the coat layer is made to absorb a solution of the NO _x occluding material compound at a predetermined concentration, and then dried and fired as in the conventional case. be able to. This NO _x storage material supporting step includes Pt and
It may be carried out before or after the step of carrying Pt, or as described above, simultaneously with carrying Pt and Pd.

The amount of the NO _x occluding material varies depending on the type of the NO _x occluding material.
It can be 0.1 to 0.3 mol. If the loading amount of the NO _x occluding material is less than this range, sufficient NO _x purification ability cannot be obtained, and if it exceeds this range, heat resistance and poisoning resistance decrease, which is not preferable. Although the upper limit of this range is larger than the upper limit of the amount of NO _x supported in the conventional catalyst, the above-described problem is caused even when the NO _x storage material is supported in a larger amount than the conventional amount, according to the production method of the present invention. Is suppressed. Therefore NO _x
It is possible to increase occluding NO _x just by increasing the storage material, thereby improving _the NO _x purification performance.

[0024]

The present invention will be specifically described below with reference to examples and comparative examples. (Example 1) 30 parts by weight of a powder obtained by mixing γ-alumina powder and ceria-zirconia composite oxide powder (Ce: Zr = 1: 1) at a weight ratio of 1: 1 and alumina sol (alumina concentration (60% by weight) and 30 parts by weight of pure water were mixed to prepare a slurry.

Next, a honeycomb substrate made of cordierite is prepared, immersed in the above slurry, pulled up, and after blowing off excess slurry, dried at 120 ° C. and baked at 500 ° C. for 1 hour. A coat layer was formed on the surface to obtain a monolith carrier. The coat layer is formed in an amount of 135 g per liter of the honeycomb substrate. With respect to this monolithic carrier, pure water was absorbed under a reduced pressure of 200 mmHg, and the amount of water absorption was measured from the increase in weight. Based on the amount of water absorption, the concentrations of dinitrodiammine platinum, palladium nitrate, potassium nitrate, and lithium nitrate were determined so that the supported amounts would be the values described below, and mixed aqueous solutions were prepared by dissolving these at the respective concentrations. .

Then, the dried monolithic carrier is applied with a vacuum
A predetermined amount of the mixed aqueous solution was absorbed under a reduced pressure of 200 mmHg, dried at 250 ° C. for 20 minutes, and calcined at 500 ° C. for 1 hour to prepare a catalyst of this example. In this catalyst, 2 g of Pt and 3 g of Pd were calculated for 1 liter of the monolithic carrier in terms of metal.
0.2 mol of K and 0.1 mol of Li are supported. (Example 2) For the same monolithic carrier as in Example 1,
Pure water was absorbed under a reduced pressure of 200 mmHg, and the amount of water absorption was measured from the increase in weight. Based on the amount of water absorption, the concentration was determined so that the amount of barium nitrate carried would be the value described below, and an aqueous barium nitrate solution of that concentration was prepared.

Then, the dried monolithic carrier is evacuated to a vacuum degree.
A predetermined amount of a mixed aqueous solution of barium nitrate was absorbed under a reduced pressure of 200 mmHg, dried at 250 ° C for 20 minutes, and calcined at 500 ° C for 1 hour to obtain a Ba-supported monolithic carrier. Was. Next, determine the respective concentrations so that the amount of dinitrodiammine platinum, palladium nitrate and lithium nitrate carried becomes the value described below,
These were dissolved at the respective concentrations to prepare mixed aqueous solutions.

Then, the dried Ba-supported monolithic carrier is
A predetermined amount of the mixed aqueous solution was absorbed under a reduced pressure of 200 mmHg, dried at 250 ° C. for 20 minutes, and calcined at 500 ° C. for 1 hour to prepare a catalyst of this example. In this catalyst, 2 g of Pt, 3 g of Pd, 0.1 mol of Ba, and 0.1 mol of Li are supported per liter of the monolithic carrier in terms of metal. (Comparative Example 1) For the same monolithic carrier as in Example 1,
Pure water was absorbed under a reduced pressure of 200 mmHg, and the amount of water absorption was measured from the increase in weight. Based on the amount of water absorption, the respective concentrations were determined so that the amounts of dinitrodiammineplatinum, potassium nitrate, and lithium nitrate had the values described below, and mixed aqueous solutions were prepared by dissolving these at the respective concentrations.

Then, the dried monolithic carrier is applied with a vacuum
A predetermined amount of the mixed aqueous solution was absorbed under a reduced pressure of 200 mmHg, and dried and calcined in the same manner as in Example 1 to prepare a catalyst of this comparative example. In this catalyst, 2 g of Pt, 0.2 mol of K, and 0.1 mol of Li were calculated for 1 liter of the monolith support in terms of metal.
Molar supported. (Comparative Example 2) For the same monolithic carrier as in Example 1,
Pure water was absorbed under a reduced pressure of 200 mmHg, and the amount of water absorption was measured from the increase in weight. Based on the amount of water absorption, the respective concentrations were determined so that the amounts of dinitrodiammineplatinum, potassium nitrate and lithium nitrate had the values described below, and mixed aqueous solutions were prepared by dissolving the respective concentrations.

Then, the dried monolithic carrier is evacuated to a vacuum degree.
A predetermined amount of the mixed aqueous solution was absorbed under a reduced pressure of 200 mmHg, and dried and calcined in the same manner as in Example 1 to prepare a catalyst of this comparative example. In this catalyst, 4 g of Pt, 0.2 mol of K, and 0.1 mol of Li were calculated for 1 liter of the monolith support in terms of metal.
Molar supported. (Comparative Example 3) For the same monolithic carrier as in Example 1,
Pure water was absorbed under a reduced pressure of 200 mmHg, and the amount of water absorption was measured from the increase in weight. Based on the amount of water absorption, the respective concentrations were determined so that the amounts of palladium nitrate, potassium nitrate, and lithium nitrate had the values described below, and mixed aqueous solutions were prepared by dissolving these at the respective concentrations.

Then, a vacuum degree is applied to the dried monolithic carrier.
A predetermined amount of the mixed aqueous solution was absorbed under a reduced pressure of 200 mmHg, and dried and calcined in the same manner as in Example 1 to prepare a catalyst of this comparative example. In this catalyst, 3 g of Pd, 0.2 mol of K, and 0.1 mol of Li were converted to 1 liter of the monolith support in terms of metal.
Molar supported. (Comparative Example 4) For the same monolithic carrier as in Example 1,
Pure water was absorbed under a reduced pressure of 200 mmHg, and the amount of water absorption was measured from the increase in weight. Based on the water absorption, the respective concentrations were determined so that the amounts of dinitrodiammineplatinum, palladium nitrate, potassium nitrate, and lithium nitrate had the values described below, and respective aqueous solutions were prepared. Note that potassium nitrate and lithium nitrate were mixed aqueous solutions.

Then, the dried monolithic carrier is applied with a vacuum
First, a predetermined amount of an aqueous solution of dinitrodiammine platinum was absorbed under a reduced pressure of 200 mmHg, and dried and calcined in the same manner as in Example 1 to carry Pt. Next, a predetermined amount of an aqueous solution of palladium nitrate was absorbed under a reduced pressure of 200 mmHg and dried and calcined in the same manner as in Example 1 to carry Pd. Further vacuum
Under a reduced pressure of 200 mmHg, a predetermined amount of a mixed aqueous solution of potassium nitrate and lithium nitrate was absorbed, dried and calcined in the same manner as in Example 1 to carry K and Li, thereby preparing a catalyst of this comparative example. In this catalyst, 2 g of Pt, 3 g of Pd, 0.2 mol of K and 0.2 mol of Li were calculated for each liter of the monolithic carrier in terms of metal.
0.1 mol is carried.

(Comparative Example 5) With respect to the same monolithic carrier as in Example 1, pure water was absorbed under a reduced pressure of 200 mmHg, and the amount of water absorption was measured from the increase in weight. Based on the water absorption, the respective concentrations were determined so that the amounts of dinitrodiammineplatinum, palladium nitrate, potassium nitrate, and lithium nitrate had the values described below, and respective aqueous solutions were prepared. Note that potassium nitrate and lithium nitrate were mixed aqueous solutions.

Then, a predetermined amount of an aqueous solution of palladium nitrate was first absorbed in the dried monolithic carrier under a reduced pressure of 200 mmHg, and dried and calcined in the same manner as in Example 1 to obtain Pd.
Was carried. Next, a predetermined amount of an aqueous dinitrodiammine platinum solution was absorbed under a reduced pressure of 200 mmHg, and dried and calcined in the same manner as in Example 1 to carry Pt. Further vacuum
Under a reduced pressure of 200 mmHg, a predetermined amount of a mixed aqueous solution of potassium nitrate and lithium nitrate was absorbed, dried and calcined in the same manner as in Example 1 to carry K and Li, thereby preparing a catalyst of this comparative example. In this catalyst, 3 g of Pd, 2 g of Pt, 0.2 mol of K and 0.2 mol of Li were calculated for 1 liter of the monolithic carrier in terms of metal.
0.1 mol is carried.

(Test / Evaluation) Example 1 and Comparative Examples 1 to 5
Each of the catalysts obtained in the above was subjected to a high-temperature durability test in which the catalyst was heated at 800 ° C. for 50 hours in exhaust gas. Then, the specific surface areas of the noble metals after the high-temperature durability test were measured, and the relative values when the specific surface area of the catalyst of Comparative Example 1 was 1 are shown in Table 2.

Each catalyst after the heat endurance test was placed in an evaluation device, and the lean side model exhaust gas and the rich side model exhaust gas shown in Table 1 were alternately repeated every two minutes, and the flow rate was 2 L / min.
, The NO _x occlusion amount and the NO oxidation rate were measured.
The incoming gas temperature was 300 ° C., the space velocity SV was 42800 h ⁻¹ , and the results are shown in Table 2.

[0037]

[Table 1]

[0038]

[Table 2]

As can be seen from Table 2, in the catalysts of Comparative Examples 1 and 2 in which only Pt was supported as the noble metal, Pt grain growth occurred after heat endurance, and although the NO oxidation rate after high temperature endurance was high, the NO _x storage amount was high. It can be seen that it is lower. In the catalyst of Comparative Example 3 carrying Pd alone, although sintering showed a high specific surface area even after a high temperature durability is suppressed, NO _x storage amount and NO oxidation rate is very low.

On the other hand, the production methods of the first and second embodiments
The catalysts obtained from the catalysts of Comparative Examples 4 and 5 even after endurance at high temperatures
Higher specific surface area than NO_xExtremely large storage capacity
And the NO oxidation rate is close to that of Comparative Example 1 in which only Pt is supported.
High NO _xHave purification ability
I understand. This supported Pt and Pd simultaneously from a mixed solution
The effect is clear.

[0041]

According to the method of manufacturing the exhaust gas purifying catalyst of the Effect of the Invention] The present invention, grain growth of noble metal is suppressed by the high-temperature durability test, a stable and durable with superior _the NO _x purification performance catalyst And can be easily manufactured.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Tetsu Iguchi 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Corporation

Claims (1)

[Claims] 1. A method for producing an exhaust gas purifying catalyst comprising: a base material; a coat layer formed of a porous carrier formed on the surface of the base material; and a noble metal and a NO _x storage material supported on the coat layer. A method for producing an exhaust gas purifying catalyst, wherein the noble metal contains at least Pt and Pd, and uses a mixed solution of a Pt compound solution and a Pd compound solution to simultaneously support Pt and Pd.