JPH0675675B2 - Exhaust gas purification catalyst - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] The present invention relates to CO (carbon monoxide), HC (hydrocarbon), NOx (nitrogen oxidation) contained in exhaust gas discharged from an internal combustion engine such as an automobile engine. The present invention relates to an exhaust gas purifying catalyst for removing and purifying substances.

[Prior Art] Conventionally, an exhaust gas purifying catalyst for an automobile is generally known to be composed of a catalyst supporting layer and a catalyst metal supported on the catalyst supporting layer. Various exhaust gas purification catalysts have been developed for the purpose of efficient purification.

For example, Japanese Patent Publication No. 59-41775, Japanese Patent Publication No. 59-90695, and Japanese Patent Publication No.
58-20307 and the like disclose a technique using cerium. In these exhaust gas purification catalysts, cerium exists as an oxide, and releases or takes up oxygen (oxygen storage capacity), CO and
The purification efficiency is improved by controlling the oxidation reaction of HC and the reduction reaction of NOx.

By the way, it is known that the reaction of the above formula (1) occurs on the surface of cerium oxide particles. However, in the above conventional exhaust gas purifying catalyst, when used at a high temperature of 800 ° C. or higher, the cerium oxide may grow into grains and the surface area may decrease. Therefore, there is a problem that the purification performance is deteriorated due to the decrease of the oxygen storage capacity.

In addition, with the goal of stabilizing activated alumina,
-7537, JP-A-48-18180, JP-A-61-3351, USP
3003020, USP3951860, USP4170573 and the like disclose a technique of simultaneously using cerium and other rare earth or transition metal. For example, Japanese Examined Patent Publication No. 60-7537 discloses a complex oxide represented by the formula (2) in which cerium and lanthanum are used at the same time. It is disclosed that (0.3 ≦ x ≦ 0.5) is formed.

This exhaust gas purifying catalyst has a fluorite-type structure of a composite oxide with lattice defects having oxygen vacancies, and has durability against the oxygen storage effect. However, it was found that even with this exhaust gas purifying catalyst, although not so much as described above, grain growth occurs in cerium oxide and the purifying performance deteriorates.

[Problems to be Solved by the Invention] The present invention has been made in view of the above circumstances, and provides an exhaust gas purifying catalyst that suppresses grain growth of cerium oxide at high temperatures and prevents deterioration of purification performance. It is provided.

[Means for Solving the Problems] An exhaust gas purifying catalyst of the present invention is an exhaust gas purifying catalyst comprising a catalyst supporting layer and a catalyst metal supported on the catalyst supporting layer. Contains at least a cerium oxide, a zirconium oxide and a catalyst metal in the surface layer, at least a part of the cerium oxide and the zirconium oxide exists as a complex oxide or a solid solution, and a cerium atom of the complex oxide or the solid solution. The ratio of the number of zirconium atoms to the number (Zr / Ce) is in the range of 5/95 to 70/30.

The catalyst supporting layer supports the catalyst metal, and for example, activated alumina, zirconia, titanium oxide, etc. having a large specific surface area can be used. Generally γ-alumina,
θ-alumina or the like is used. The catalyst supporting layer is
It may be used as it is, or a carrier substrate may be used and a catalyst supporting layer may be formed on the surface of the carrier substrate.

The carrier base material may be the same as the conventional one, such as a honeycomb-shaped monolith carrier base material or a pellet-shaped carrier base material. As the material of the carrier base material, known materials such as ceramics such as cordierite, mullite, alumina, magnesia and spinel, and heat resistant metals such as ferritic steel can be used.

The catalyst metal supported on the catalyst supporting layer is platinum (P
t), rhodium (Rh), palladium (Pd), iridium (Ir), ruthenium (Ru), osmium (Os) and other precious metals, or chromium (Cr), nickel (Ni), vanadium (V), copper (Cu) ), Cobalt (Co), manganese (M
A base metal such as n) can be used as in the related art.

The greatest feature of the present invention is that the catalyst supporting layer contains cerium oxide and zirconium oxide, and at least part of the cerium oxide and zirconium oxide exists as a complex oxide or a solid solution.

Grain growth of cerium oxide easily occurs as a single oxide, and according to the research conducted by the present inventors, it is known that when heated at 1000 ° C., grains grow to a diameter of about 0.1 μm. And 1000 ℃
Even if an attempt is made to purify CO by the reaction of formula (3) after heating, the reaction rate becomes almost zero, and the oxygen storage capacity is significantly reduced.

CO + CeO ₂ → XCO ₂ + CeO _2- x (3) Therefore, as a result of intensive studies, the present inventors have found that grain growth of cerium oxide is significantly suppressed in the case of heat treatment in the presence of cerium oxide and zirconium oxide. The present invention has been completed by finding that the reaction rate of the formula (3) after heating becomes very high at 70% or more.

The inventors of the present invention impregnated the cerium oxide powder with the zirconium oxynitrate aqueous solution and the zirconium oxide powder with the cerium nitrate aqueous solution.
Heat treatment was performed at 800 ° C. for 5 hours, and the crystal morphology at that time was analyzed by X-ray diffraction. As a result, as shown in Table 9, when the cerium oxide powder was impregnated with the aqueous solution of zirconium oxynitrate, it showed a cerium oxide crystal phase and had a short lattice constant. This shows that zirconium is substituted in the cerium oxide crystal lattice and is in solid solution. Further, when the zirconium oxide powder is impregnated with the cerium nitrate aqueous solution, the zirconium oxide is divided into three phases, and the lattice constant cannot be determined exactly. This suggests the formation of a partial solid solution of zirconium oxide and cerium oxide or a complex oxide.

To form a composite oxide containing a cerium oxide and a zirconium oxide in the catalyst-supporting layer, or a solid solution, the catalyst-supporting layer is impregnated with an aqueous solution of a cerium salt and a zirconium salt at the same time or separately, and the temperature is 600 ° C. or higher. It can be performed by firing. Also, an oxide is used as one of cerium and zirconium to form a catalyst support layer. It may be carried out by mixing with activated alumina powder and firing at a temperature of 800 ° C. or higher. If the temperature is lower than these values, it is difficult to form a complex oxide or solid solution, and cerium oxide grain growth is likely to occur.

It is desirable that the whole of cerium oxide and zirconium oxide be a composite oxide or a solid solution, but it goes without saying that even if they are at least a part, the grain growth preventing effect of cerium oxide can be obtained.

Further, cerium oxide and zirconium oxide may be present inside the catalyst supporting layer or may be present in a state of being supported on the surface of the supporting layer. In particular, if it is on the surface of the supporting layer, the catalyst with exhaust gas can be easily catalyzed, and the oxygen storage capacity can be maximized.

The ratio of cerium and zirconium is such that the cerium atom and the zirconium atom supported as a complex oxide or a solid solution have an atomic ratio of the number of zirconium atoms to the number of cerium atoms of 5/95 to 70/30. It is preferable to configure. If this atomic ratio is less than 5/95, cerium oxide tends to grow grains, and if it exceeds 70/30, the oxidizing storage capacity becomes insufficient and the purification performance deteriorates.

[Operation and Effect of the Invention] In the exhaust gas purifying catalyst of the present invention, the catalyst-supporting layer contains cerium oxide, zirconium oxide, and a catalyst metal in at least the surface layer, and at least a part of the cerium oxide and zirconium oxide is It exists as a complex oxide or solid solution. Although the mechanism has not been clarified yet, the grain growth of cerium oxide is suppressed due to the presence of a complex oxide or a solid solution.

That is, according to the exhaust gas purifying catalyst of the present invention, grain growth of cerium oxide when used at high temperature is suppressed, so that the surface area of cerium oxide itself can be maintained at a sufficiently large value. Therefore, there is no problem that the oxidization storage capacity of cerium oxide is reduced, and the purification performance can be maintained at a high level for a long period of time.

[Examples] Specific examples will be described below.

(Example 1 and Comparative Example 1) 700 g of alumina sol having an alumina content of 10 wt%, 1000 g of alumina powder, and 300 g of distilled water were mixed and stirred to prepare a slurry. A honeycomb-shaped monolithic catalyst carrier substrate made of cordierite is immersed in this slurry for 1 minute and then pulled up, and the slurry in the cell is blown off by an air flow.
After drying at 150 ° C. for 1 hour, it was baked at 700 ° C. for 2 hours. This operation was repeated twice to form a catalyst supporting layer made of activated alumina.

Next, cerium nitrate (Ce (NO ₃ ) ₃ ) was 0.16 mol / and zirconium oxynitrate (ZrO (NO ₃ ) ₂ ) was 0.24 mol /.
The monolithic carrier substrate having the catalyst supporting layer formed thereon was immersed in the dissolved mixed solution for 1 minute, then pulled up, excess water was blown off, dried at 200 ° C. for 3 hours, and then baked in air at 600 ° C. for 5 hours. As a result, a monolithic support substrate (1B) having a catalyst supporting layer containing cerium oxide and zirconium oxide was obtained.

In the same manner, except that mixed aqueous solutions having different concentrations of cerium nitrate and zirconium oxynitrate are used,
Monolith carrier base materials (1B to 1E) containing cerium atoms and zirconium atoms at the values shown in Table 1 were obtained. In addition, cerium nitrate was added to 0.
The monolith carrier base material (1F) was similarly prepared except that it was immersed only in the aqueous solution containing 4 mol / mol, and the monolith carrier was similarly prepared except that it was immersed only in the aqueous solution containing 0.4 mol / zirconium oxynitrate without using the cerium nitrate aqueous solution. A base material (1G) was obtained.

Next, for each of these monolithic carrier base materials (1B to 1G), dip them in distilled water to absorb water sufficiently and then pull up to blow off excess water, and dinitrodiamine platinum 1.0 g
It was immersed in an aqueous solution containing / for 1 hour. It was pulled up to blow off excess water, and dried at 200 ° C. for 1 hour. Further, it was immersed in an aqueous solution containing 0.1 g / rhodium chloride in the same manner, dried to support platinum (Pt) and rhodium (Rh) for catalysis, and Examples 1b to 1e and Comparative Example 1a shown in Table 1 were compared. An example
An exhaust gas purifying catalyst of 1b was obtained.

Also, using the above monolithic carrier substrate (1B-1G), an aqueous solution containing 1.5 g / palladium chloride and 0.2 g / rhodium chloride.
Palladium (Pd) and rhodium (Rh) were loaded on the respective base materials in the same manner as above using an aqueous solution containing, and Examples 1g to 1j and Comparative Examples 1c and 1c shown in Table 2 were compared.
A 1d exhaust gas purification catalyst was obtained.

Further using the monolith carrier substrate (1B ~ 1G), using an aqueous solution containing 1.0 g / dinitrodiammine platinum, an aqueous solution containing 1.0 g / palladium chloride and 0.2 g / rhodium chloride in the same manner as above. Platinum (Pt), palladium (Pd), and rhodium (Rh) are supported on each substrate to be catalyzed, and the exhaust gas purifying catalysts of Examples 1l to 1o and Comparative Examples 1e and 1f shown in Table 3 are shown. Got

For each of the obtained exhaust gas purification catalysts, 3
Attached to the exhaust system of an inline 6-cylinder engine, air-fuel ratio (A / F)
Was subjected to a 200 hour endurance test under conditions of 14.6 and an inlet gas temperature of 850 ° C. Then, for each catalyst after the durability test, the purification rate of HC, CO, and NOx was measured under the conditions of A / F = 14.6 and inlet gas temperature of 400 ° C. using the same engine as that of the durability test.

In addition, the monolithic carrier substrate (1B
~ 1G), each was heated at 1000 ℃ for 5 hours, then kept at 600 ℃ in an oxidizing atmosphere, and then kept at 600 ℃ in the state of carbon monoxide. The oxygen was stored in a pulsed manner, the CO conversion rate was determined from the amount of carbon dioxide generated according to the above equation (3), and the oxygen storage capacity was measured.

Further, the carrier base material after being heated at 1000 ° C. for 5 hours was crushed, and the particle diameter of cerium oxide was measured by an X-ray diffraction method. The results are shown together in Tables 1 to 3.

(Example 2, Comparative Example 2) A γ-alumina granular carrier (manufactured by Nikko Universal Co., Ltd.) 1 having a BET surface area of 100 to 150 m ² / g and an average pore diameter of 300 to 400 Å was used, except that the concentration was different. Each carrier base material was obtained by immersing in the same mixed aqueous solution as in Example 1 and Comparative Example 1, and similarly drying and firing. And Example 1b
.About.1e and Comparative Examples 1a to 1b were catalyzed to obtain the exhaust gas purifying catalysts of Examples 2b to 2e and Comparative Examples 2a and 2b having the configurations shown in Table 4.

The obtained exhaust gas purifying catalyst was subjected to the same purification rate measurement test as in Example 1, and the results are shown in Table 4.

(Example 3, Comparative Example 3) Each of the granular carrier base materials having cerium and zirconium atoms calcined at 600 ° C. and having not been catalyzed in Example 2 and Comparative Example 2 was subjected to an average particle size by a vibration mill. It was ground to 7 μm. And 100 parts by weight of each powder obtained,
30 parts by weight of an aqueous solution containing 40 wt% of aluminum nitrate,
100 parts by weight of water was mixed and milled for 1 hour to prepare each slurry. Example 1 using this slurry
A catalyst carrier layer was formed in the same manner on the same honeycomb carrier as above, and catalyzed in the same manner as in Examples 1b to 1e and Comparative Examples 1a to 1b, and Examples 3b to 3e and Comparative Example 3a shown in Table 5 were performed. Thus, an exhaust gas purifying catalyst of Comparative Example 3b was obtained.

The obtained exhaust gas purifying catalyst was subjected to the same purification rate measurement test as in Example 1, and the results are shown in Table 5.

(Example 4, Comparative Example 4) γ-obtained by pulverizing the granular carrier used in Example 2 above.
Alumina powder and zirconium oxide powder were blended in the composition ratio shown in Table 6 and mixed with water. A slurry was formed in the same manner as in No. 3, and a catalyst supporting layer was formed in the same manner. Then, each carrier substrate was impregnated with an aqueous cerium nitrate solution at two different concentrations and then calcined at 800 ° C. for 5 hours. After that, a catalyst metal was loaded in the same manner as in Examples 1l to 1o to obtain an exhaust gas purifying catalyst of Example 4 shown in Table 6. Comparative Example 4
Then, the cerium nitrate aqueous solution was not impregnated.

The obtained exhaust gas purifying catalyst was subjected to the same purification rate measurement test as in Example 1, and the results are shown in Table 6.

(Example 5 and Comparative Example 5) A sixth example was performed in the same manner as Example 4 and Comparative Example 4 except that cerium oxide powder was used instead of zirconium oxide powder and zirconium oxychloride was used instead of cerium sulfate.
Exhaust gas purifying catalysts of Examples 5a-5b and Comparative Example 5 shown in the table were obtained.

The obtained exhaust gas purifying catalyst was subjected to the same purification rate measurement test as in Example 1, and the results are shown in Table 6.

(Example 6, Comparative Example 6) Zirconium oxide powder, cerium oxide powder and alumina powder were added to the slurry in the composition ratios shown in Table 7. Exhaust of Examples 6a to 6b and Comparative Examples 6a to 6b in the same manner as Example 3 and Comparative Example 3 except that a mixture of water and water was used, and the firing temperature was 1000 ° C. A gas purification catalyst was obtained.

The obtained exhaust gas purifying catalyst was subjected to the same cleaning rate measurement test as in Example 1, and the results are shown in Table 7.

(Example 7 and Comparative Example 7) Table 7 shows the same as Example 1 and Comparative Example 1 except that a ferritic metal honeycomb carrier substrate containing aluminum was used instead of the cordierite honeycomb carrier substrate. Exhaust gas purifying catalysts of Examples 7a to 7b and Comparative Example 7 were obtained.

The obtained exhaust gas purifying catalyst was subjected to the same purification rate measurement test as in Example 1, and the results are shown in Table 7.

(Example 8 and Comparative Example 8) Using a ferrite metal honeycomb carrier composed of aluminum 5 wt%, chromium 20 wt% and balance iron, heat treatment was performed in a carbon dioxide atmosphere at 900 ° C for 10 minutes, and in air at 900 ° C for 1 hour. Further, the same procedure as in Example 1 was performed on the surface of the carrier substrate in the same manner as in Example 1. A catalyst supporting layer made of activated alumina is formed. Using this carrier substrate, in the same manner as in Example 1 and Comparative Example 1,
Exhaust gas purifying catalysts of Example 8 and Comparative Examples 8a to 8b shown in the table were obtained.

The obtained exhaust gas purifying catalyst was subjected to the same purification rate measurement test as in Example 1, and the results are shown in Table 8.

(Evaluation) Clearly from each table, all of the examples are superior in purification rate to the comparative examples. It is clear that this is due to the effect that cerium oxide and zirconium oxide coexist at least in part as a composite oxide or solid solution in the exhaust gas purifying catalyst of the example.

Further, it is clear from Table 1 that the exhaust gas purifying catalysts of the Examples have almost no grain growth of cerium oxide and therefore have an excellent CO conversion rate (oxygen storage capacity). It can also be read that the particle size decreases as the molar ratio of zirconium atoms to cerium atoms increases, and conversely the CO conversion rate decreases.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Nobuo Kimura, Inventor Nozomi Kimura, Aichi-gun, Aichi-gun, Nagakute, No. 41, Yokoshiro, 1st place, Toyota Central Research Institute Co., Ltd. (72) Inventor Masakuni Ozawa Nagakute, Aichi-gun, Aichi 1 in 41 Chuo Yokoido, Central Research Institute, Ltd. (72) Inventor Akio Isoya 1 in 41, Nagakute-cho, Aichi-gun, Aichi Prefecture Toyota Central Research Institute, 1 (56) References 61-197036 (JP, A) JP 61-157347 (JP, A) JP 61-93832 (JP, A) JP 61-78439 (JP, A) JP 59-209646 (JP, A) JP-A-56-87430 (JP, A) JP-A-63-88040 (JP, A) JP-A-62-282641 (JP, A) Actual development-Sho 62-1737 (JP, U)

Claims (1)

[Claims] 1. A support base material, a catalyst support layer formed on the surface of the support base material, and a catalyst metal supported on the catalyst support layer.
In the exhaust gas purifying catalyst, the catalyst-supporting layer contains cerium oxide, zirconium oxide and a catalyst metal in at least the surface layer, and at least a part of the cerium oxide and the zirconium oxide is a composite oxide or a solid solution. And a ratio of the number of zirconium atoms to the number of cerium atoms (Zr / Ce) of the complex oxide or the solid solution is in the range of 5/95 to 70/30.