JP2001149757A - Exhaust gas purifying catalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high temperature NOx occlusion/reduction type exhaust gas purifying catalyst exhibiting catalytic activity even at high temperature use and exhibiting an effective purifying action while suppressing the sintering of noble metal. SOLUTION: In the NOx occlusion/reduction type exhaust gas purifying catalyst having a carrier and a catalyst retaining layer formed on the carrier, the catalyst retaining layer is composed of alumina powder to which a noble metal is previously deposited, titanium oxide powder, the cerium oxide powder to which a noble metal is deposited and at least one kind of NOx occluding alkali metal salts and alkaline earth metal salts supported on the alumina powder, the titanium oxide powder and the cerium oxide powder, and at least one king of the alkali metal salts and the alkaline earth metal salts is deposited in the state dissolved in the slurry aqueous solution in which the alumina powder, the titanium oxide powder and the cerium oxide powder are disperesed.

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, and more particularly to a method for increasing the occlusion and reduction of NOx contained in exhaust gas discharged from an internal combustion engine of an automobile or the like and reducing the toxicity of sulfur. The present invention relates to a NOx storage reduction type exhaust gas purifying catalyst.

[0002]

2. Description of the Related Art Conventionally, a nitrogen oxide storage type exhaust gas purifying catalyst is prepared by bringing an exhaust gas containing nitrogen oxide into contact with the catalyst under an oxygen-excess atmosphere (lean condition) to convert the nitrogen oxide into barium or a mixture of barium and aluminum. Occluded in the composite oxide,
It is known that the exhaust gas is reduced to a reducing atmosphere (rich condition) by a method such as injection of a reducing agent, and the nitrogen oxides stored in the catalyst are reduced to nitrogen (for example, Japanese Patent Application Laid-Open No. 10-314593). No.).

Japanese Patent Application Laid-Open No. 6-142458 discloses a catalyst for simultaneously purifying hydrocarbons and nitrogen oxides, which comprises a porous carrier comprising at least one of platinum and palladium, at least one noble metal, barium and an alkali metal.
An exhaust gas purifying catalyst supporting at least one metal selected from the group consisting of iron, nickel, cobalt and magnesium is disclosed.

[0004] However, in the above-mentioned catalyst, the dispersion state of the catalytic metal and the catalytically active substance is not uniform, so that sufficient purification performance may not always be exhibited. That is, the catalytic metal may be aggregated and granulated, or a catalytically active substance that assists the catalytic metal may not be present in the vicinity of the catalytic metal, and the expected purification performance may not be exhibited.

[0005] In the above-mentioned NOx occlusion reduction type catalyst, thermal degradation is remarkable, and purifying performance is degraded due to heat history. Therefore, a catalyst which improves this point is desired.

When the alkali or alkaline earth metal is converted to NO
When x is contained in the catalyst as an occlusion material, sintering of the noble metal occurs at a high temperature, and sintering of the noble metal increases when the catalyst is used at a high temperature in a lean atmosphere.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and exhibits catalytic activity even when used at a high temperature to prevent sintering of precious metals and scattering of NOx storage materials. Temperature N showing effective and effective purifying action
It is an object to propose an Ox storage reduction type exhaust gas purifying catalyst.

[0008]

Means for Solving the Problems In order to solve the above-mentioned problems, the present inventor has sought to suppress the sintering of a noble metal exhibiting catalytic activity and to suppress the scattering of a storage material for storing nitrogen oxides. As a result of the study, the present invention has been completed.

The exhaust gas purifying catalyst of the present invention is a NOx storage reduction catalyst having a carrier and a catalyst holding layer formed on the carrier, wherein the catalyst holding layer is made of an alumina powder pre-loaded with a noble metal. , A titanium oxide powder, a cerium oxide powder supporting a noble metal, the alumina powder, the titanium oxide powder, and NOx supported on the cerium oxide.
It comprises at least one of an occluding alkali metal salt and an alkaline earth metal salt, and at least one of the alkali metal salt and the alkaline earth metal salt contains the alumina powder, the titanium oxide powder and the cerium oxide dispersed therein. Characterized by being carried in a state of being dissolved in the slurry aqueous solution.

[0010] The catalyst holding layer comprises at least one of the alumina powder, the titanium oxide powder, the cerium oxide and the alkali metal salt or alkaline earth metal salt having NOx storage properties.
It is preferred that the seeds be mechanically comminuted and mixed in the slurry so that each component is uniformly dispersed.

The noble metal supported on the alumina powder is
Preference is given to platinum and rhodium.

The noble metal supported on the cerium-containing oxide powder is preferably platinum.

The cerium oxide is preferably a cerium oxide or a composite oxide with zirconium.

The NOx-storing alkali metal and alkaline earth metal salts include barium, magnesium, lithium,
Preferably, they are sodium and potassium.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The exhaust gas purifying catalyst of the present invention is a type of catalyst that stores NOx in a lean burn engine in a lean atmosphere and reduces the stored NOx in a stoichiometric or rich atmosphere.

In a lean atmosphere, at least one of alkali metal and alkaline earth metal salts of the NOx storage material is uniformly dispersed in the catalyst holding layer together with alumina powder, titanium oxide powder carrying noble metal, and cerium oxide carrying noble metal. Thus, the configuration is such that the NOx stored by the catalyst is easily reduced in the rich atmosphere.

Since the noble metal is previously supported on the alumina powder and the cerium oxide powder, the noble metal is uniformly dispersed in the catalyst holding layer, and sintering between the noble metals can be suppressed. Also,
Since the NOx storage material is also uniformly and highly dispersed in the catalyst holding layer, the movement and scattering of the NOx storage material can be suppressed even when high-temperature exhaust gas is introduced. In addition, the reduction reaction of NOx in a stoichiometric or rich atmosphere can be promoted by the cerium oxide powder preloaded with a noble metal in a highly dispersed state and the NOx occluding material in a highly dispersed state, so that the occlusion reducibility can be enhanced. Further, the poisoning of the catalyst by sulfur is regenerated by the presence of the titanium oxide powder, and the purification performance of the catalyst can be maintained for a long time. As a result, the performance of the catalyst at a high temperature can be maintained, and the deterioration resistance performance can be improved.

Each of the above-mentioned powder components is uniformly mixed mechanically by forced pulverization and mixing with a slurry formed with a solution of the NOx storage material. Preferably, the slurry is mixed by milling or the like to make the powder particle size uniform and the noble metal is mechanically fixed to the powder particles uniformly, and then coated on the carrier to form a catalyst holding layer. As a result, each component is dispersed uniformly throughout the catalyst holding layer, unlike a case where each component is simply supported by coating or dipping, and a catalyst supporting layer in which migration of noble metals is particularly suppressed can be formed.

The noble metal of the catalyst is supported on alumina powder to maintain the heat resistance of the catalyst. The amount of the noble metal supported on alumina is preferably about 0.5 to 10% by weight to maintain the activity of the catalyst. As the noble metal, at least one of platinum and rhodium is used. When both platinum and rhodium are used in combination, Pt / Rh = 3/1 to 1
The range of 00/1 is preferable for increasing the purification efficiency.

The exhaust gas purifying catalyst of the present invention can improve sulfur resistance by blending titanium oxide powder. The amount of the titanium oxide powder is Ti / Al = 1/9 to 2
/ 1 is preferred. If the amount of the titanium oxide powder is too large, the heat resistance of the catalyst decreases, which is not preferable. The titanium oxide powder may be added alone or mixed with the cerium oxide powder by slurry addition.

The cerium oxide powder previously supports a noble metal and is added to and mixed with the slurry. As the cerium oxide powder, a composite oxide formed with zirconium can be used. Ce / Zr as cerium composite oxide
(Preferable ratio is 4/1 to 1/1) and the like. Platinum is preferred as the noble metal supported on the cerium oxide powder. With the cerium oxide powder supporting platinum, the purification catalyst can improve the reproducibility of the rich pulse and improve the durability of the catalyst.

The amount of the noble metal supported on the cerium oxide powder is preferably in the range of 5 to 50 mg / g in terms of performance.

As the NOx storage material, an aqueous solution of an alkali metal salt or an alkaline earth metal salt can be used. As at least one of alkali metal salts and alkali metal salts, barium, magnesium, lithium, sodium, potassium salts (eg, carbonate, nitrate, hydrochloride, sulfate, hydroxide, acetate) and the like can be used. In particular, a mixed solution of lithium and potassium is preferably used. The aqueous solution of the alkali metal salt is mixed with the alumina powder, the titanium oxide powder, and the cerium oxide powder supporting the noble metal in the slurry,
The powder is uniformly loaded.

The amount of the NOx occluding material is preferably 0.01 to 0.5 mol / g in the catalyst holding layer.

By pre-loading the noble metal on the alumina powder or cerium oxide powder as described above, the catalyst holding layer is later immersed and supported in a noble metal solution or NOx storage material solution. The catalyst can be uniformly dispersed in the catalyst holding layer, and the durability as a catalyst can be enhanced.

The noble metal can be supported on the alumina powder and the cerium oxide powder by an ordinary method. That is, it is obtained by immersing each of the above-mentioned powders in an aqueous solution of a predetermined concentration of a noble metal salt or the like, mixing them sufficiently, and baking to dryness.

In the present invention, the mixing ratio of alumina powder, titanium oxide powder and cerium oxide powder is preferably 4: 1: 1. The amount of the NOx occluding material is 0.05 to 0.5 mol based on 100 parts by weight of the above powder.
Is preferable in order to enhance the purification performance and the durability of the catalyst.

[0028]

The present invention will be specifically described below with reference to examples.

Example 1 A mixture of a dinitrodiamine platinum solution (dissolved amount as Pt: 1.5 g) and a rhodium nitrate solution (dissolved amount as Rh: 0.1 g) was mixed with 100 g of alumina powder (particle size 0). Then, the solvent was evaporated to dryness and calcined at 170 ° C. to produce an alumina powder carrying a predetermined amount of the noble metal.

Pt-supported cerium and zirconium formed by mixing and drying 100 g of a composite oxide powder of cerium and zirconium (Ce: Zr = 5: 1 atom ratio) (dissolved amount as Pt: 2 g) a composite oxide powder 40g of titanium oxide powder 40g addition, further potassium salt of NOx-absorbing material (acetate) 20 g
And an aqueous solution 200 in which 10 g of lithium salt (nitrate) are dissolved
ml was added and mixed to form a slurry. In order to adjust the average particle size of the slurry to about 5 μm, the slurry was prepared by milling with a ball mill or the like. The resulting slurry was coated on a monolithic carrier made of cordierite or the like to prepare a catalyst of Example 1.

(Example 2) A catalyst of Example 2 was prepared in the same manner as in Example 1, except that the lithium salt of the NOx storage material in Example 1 was changed to barium acetate.

Comparative Example 1 A monolith carrier was coated as a slurry with 100 g of alumina, 40 g of titania, and 40 g of cerium composite oxide powder (same as in Example 1) to form a support layer. A 2 / 0.1 mg solution of a noble metal catalyst, Pt / Rh, was immersed and supported near the surface of the support layer, and was further coated with a solution of a NOx storage material having the same concentration as in Example 1 to obtain a catalyst of Comparative Example 1. Was.

Comparative Example 2 A catalyst of Comparative Example 2 was prepared in the same manner as in Example 1, except that the cerium composite oxide powder did not previously support Pt.

(Comparative Example 3) In Example 1, the NOx occluding material was not mixed in the slurry but instead was immersed in a carrier layer formed by coating another alumina powder and a cerium composite oxide powder on a monolith carrier. To produce a catalyst of Comparative Example 3. (Evaluation of catalyst) The exhaust gas purification performance of the catalysts obtained in Examples 1-2 and Comparative Examples 1-3 was evaluated by the following method.

Table 1 and FIG. 1 show the results of endurance tests performed in a stoichiometric F / B (feedback) atmosphere at 800 ° C. for 50 hours using an engine having a displacement of 4 L. Evaluation was measured with a carina van made by Toyota. The evaluation mode was a vehicle evaluation mode in Europe, in which the last high-speed running was performed in an ECE mode with a simulated mode pattern, and the effect was determined based on the emission value of the EUDC unit.

[0036]

[Table 1]

As shown in FIG. 1 and Table 1, the first embodiment
It can be seen that the NOx purification performance of Example 2 is superior to that of Comparative Example 1 in which the noble metal was immersed and supported on the surface of the catalyst holding layer.

Table 2 and FIG. 2 show the results of examining the particle size of Pt after the heat resistance test under the above conditions (800 ° C. × 50 hour stoichiometric F / B atmosphere). (The measurement method was calculated from the half width of the Pt peak obtained by X-ray diffraction).

[0039]

[Table 2]

As shown in FIG. 2, Example 1 in which the noble metal of the present invention was uniformly dispersed in the supported oxide powder was compared with Comparative Example 1 in which the surface of the noble metal was coated on the supported oxide powder. Thus, it can be seen that the Pt particle size is small after the heat resistance test and the sintering of the noble metal is suppressed.

The durability test conditions were set at 800-700 ° C. ×
FIG. 3 and Table 3 show EUDC-NOx emission values when subjected to a rich (1 min) -lean (4 min) cycle atmosphere for 20 hours, and FIG. 4 and Table 4 show the Pt particle size of the noble metal.

[0042]

[Table 3]

[0043]

[Table 4]

Even after the durability test in a rich (1 min) -lean (4 min) cycle atmosphere, Example 1 of the present invention has a smaller NOx emission and a smaller Pt particle size than Comparative Example 1 in Comparative Example 1. Therefore, the effect of the noble metal being uniformly and highly dispersed in the carrier oxide powder is shown, which indicates that the durability of the catalyst at high temperatures is improved.

FIG. 5 shows an 800-liter engine with a displacement of 4 liters.
-700 ° C x 20 hours, rich (1min)-lean (4m
in) EUDC-NOx emission values when performed in a cycle atmosphere.

[0046]

[Table 5]

It can be seen that Examples 1 and 2 in which the Pt-Ce loading sites are the same have lower NOx emissions than Comparative Examples 1 and 2 in which Pt is not directly loaded on Ce.

FIG. 6 and Table 6 show Example 1 and Comparative Example 3.
About 800-700 ° C x 2 with 4L engine
The emission values of NOx and HC in the 10.15 mode when the operation was performed in a rich (1 min) -lean (4 min) cycle atmosphere for 0 hour are shown.

By making the NOx storage material highly dispersed, the emission amount is smaller and the heat resistance is improved as compared with the catalyst of Comparative Example 3 in which the NOx storage material is later loaded on the coat layer of Comparative Example 3. I understand.

[0050]

[Table 6]

FIG. 7 shows 600 ° C., 700 ° C., 800 ° C., 9
The amount of the NOx storage material remaining in the catalyst after the heat treatment at each temperature of 00 ° C. for 5 hours is shown. In Example 1 (measured by the X-ray diffraction method), it can be seen that the scattering amount of potassium in the storage material at 800 ° C. was particularly small as compared with Comparative Examples 1 and 3, and the scattering was suppressed. In other words, it can be confirmed that by making the NOx storage material highly dispersed, scattering of the storage material is suppressed and the performance of the catalyst is improved.

[0052]

[Table 7]

[0053]

As described above, in the exhaust gas purifying catalyst of the present invention, the noble metal of the catalyst metal is pre-loaded with alumina powder and cerium powder and uniformly dispersed in the catalyst holding layer together with the NOx storage material. Sintering can be prevented, and scattering of the NOx storage material at high temperatures can be prevented. As a result, the heat resistance of the exhaust gas purifying catalyst can be improved.

Further, by supporting the noble metal on the cerium oxide powder, the reduction reaction in a stoichiometric or rich atmosphere is promoted to enhance the reducibility of the stored NOx, and the uniformly dispersed titanium oxide powder is formed by sulfur. Since the poisoning of the catalyst is regenerated, the durability of the catalyst can be improved.

[Brief description of the drawings]

FIG. 1 shows the catalysts of Example 1 and Comparative Example 1 at 800 ° C. 50
It is a bar graph which shows the discharge amount of NOx after the heat resistance test of time.

FIG. 2 shows the catalysts of Example 1 and Comparative Example 1 at 800 ° C. 50
It is a graph which shows the particle size of the catalyst noble metal Pt after the heat resistance test of time.

FIG. 3 shows 800 ° C.-7 of the catalysts of Example 1 and Comparative Example 1.
It is a bar graph which shows the NOx discharge amount after the heat test in the rich-lean cycle of 00 degreeC 20 hours.

FIG. 4 shows a temperature of 800 ° C.-7 of the catalysts of Example 1 and Comparative Example 1.
It is a bar graph which shows the particle size of catalytic noble metal Pt of NOx after the heat test in a rich-lean cycle of 00 ° C for 20 hours.

FIG. 5 is a bar graph showing NOx emission amounts according to differences in Pt—Ce carrying sites in Example 1, Example 2, Comparative Example 1, and Comparative Example 2.

FIG. 6: 800 ° C. to 700 ° C. in Example 1 and Comparative Example 3
After a heat test in a 20-hour rich-lean cycle,
It is a bar graph which shows the emission amount of NOx and HC in 10.15 mode.

FIG. 7 is a graph showing the relationship between the catalyst-containing gas temperature and the amount of NOx storage material scattered in Example 2, Comparative Examples 1 and 3.

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Claims (6)

[Claims] 1. A NOx occlusion reduction type catalyst having a carrier and a catalyst holding layer formed on the carrier, wherein the catalyst holding layer comprises alumina powder on which a noble metal is previously supported, titanium oxide powder, A cerium oxide powder carrying a noble metal, and the alumina powder, the titanium oxide powder, at least one of an alkali metal salt of NOx storage ability and an alkaline earth metal salt carried on the cerium oxide, An exhaust gas characterized in that at least one of the alkali metal salt and the alkaline earth metal salt is supported in a state of being dissolved in an aqueous slurry solution in which the alumina powder, the titanium oxide powder, and the cerium oxide are dispersed. Purification catalyst. 2. The catalyst holding layer comprises at least one of the alumina powder, the titanium oxide powder, the cerium oxide, and the alkali metal salt or alkaline earth metal salt having NOx storage properties.
2. The exhaust gas purifying catalyst according to claim 1, wherein the seeds are mechanically comminuted and mixed in a slurry, and each component is uniformly dispersed. 3. The noble metal supported on the alumina powder,
The exhaust gas purifying catalyst according to claim 1, which is platinum and rhodium. 4. The exhaust gas purifying catalyst according to claim 1, wherein the noble metal supported on the cerium oxide is platinum. 5. The exhaust gas purifying catalyst according to claim 1, wherein the cerium oxide is a composite oxide with cerium oxide or zirconium. 6. The NOx-storing alkali metal or alkaline earth metal salt of at least one of barium, magnesium, lithium, sodium and potassium.
An exhaust gas purifying catalyst according to claim 1.