JPS63116741A - Catalyst for purifying exhaust gas - Google Patents

Abstract

PURPOSE:To prevent the lowering in purifying capacity at high temp., by forming a catalyst for purifying exhaust gas by supporting a catalytic metal by a catalyst supporting layer consisting of cerium oxide and zirconium partially present as a composite oxide. CONSTITUTION:A catalyst for purifying exhaust gas is formed by supporting a catalytic metal by a catalyst supporting layer consisting of cerium oxide and zirconium oxide partially present at least as a composite oxide or a solid solution. In this case, the cerium atom and zirconium atom supported as the composite oxide or solid solution are pref. constituted so that the atomic ratio of the number of zirconium atom to that cerium atoms is 5:95-80:20. As the catalytic metal, a noble metal such as Pt, Rh, Pd, Ir, Ru or the like can be used.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention is directed to the use of Co (-Fl? carbon) and HC contained in exhaust gas discharged from internal combustion engines such as automobile engines.
The present invention relates to an exhaust gas purifying catalyst that removes and purifies (hydrocarbons) and NOx (nitrogen oxides).

[Prior Art] Conventional catalysts for purifying exhaust gas from automobiles are generally known to consist of a catalyst support layer and a catalytic metal supported on the catalyst support layer. Various exhaust gas purification catalysts have been developed for the purpose of efficient purification.

For example, Special Publication No. 59-41775, Machima 59-906
No. 95 and Japanese Patent Publication No. 58-20307 disclose techniques using cerium.

In these exhaust gas purification catalysts, cerium exists as an oxide, releases or takes in oxygen (oxygen storage m) through the reaction shown in equation (1), and regulates the oxidation reaction of GO and l-1c and the reduction reaction of NOx. This aims to improve purification efficiency.

Ce O2;=Ce Ot-x + 202-(1) By the way, it is known that the reaction of the above formula (1) occurs on the surface of the cerium oxide particles. However, in the conventional exhaust gas purifying catalyst described above, when used at a high temperature of 800° C. or higher, the cerium oxide may grow grains and the surface area may decrease. Therefore, there was a problem in that the purification performance deteriorated due to a decrease in oxygen storage capacity.

In addition, with the goal of stabilizing activated alumina,
No. 0-7537, JP-A-48-18180, Special [61
-3531, USP30030201USP3951
860%LISP4170573 and the like disclose a technique of using cerium and other rare earths or transition metals at the same time. For example, in Japanese Patent Publication No. 60-7537, cerium and lanthanum are used at the same time, and a composite oxide Ce
≦ヱ≦0.5) is disclosed.

This exhaust gas purification catalyst has a fluorite-type structure of a composite oxide formed with lattice defects having oxygen vacancies, and has a durable oxygen storage effect. However, even in this exhaust gas purification catalyst, grain growth occurs in the cerium oxide, which is not the case with the above, and purification performance is reduced.

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

[Means for Solving the Problems] The exhaust gas purifying catalyst of the present invention is an exhaust gas purifying catalyst comprising a catalyst supporting layer and a catalytic metal supported on the catalyst supporting layer, wherein the catalyst supporting layer is made of cerium. It is characterized in that it contains an oxide and a zirconium oxide, and at least a portion of the cerium oxide and the zirconium oxide exist as a composite oxide or a solid solution.

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

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

The catalyst metal supported on the catalyst support layer is platinum (Pt
), rhodium (R1-1), palladium (Pd), iridium (Ir), ruthenium (Ru), osmium (O
Use the same metals as before, such as noble metals such as s), or base metals such as chromium (Cr), nickel (N i ), vanadium (V), copper (Cu), cobalt (CO), and manganese (Mn). be able to.

The most important feature of the present invention is that the catalyst supporting layer contains cerium oxide and zirconium oxide, and at least a portion of the cerium oxide and zirconium oxide exist as a composite oxide or a solid solution.

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

GO-1-CeO2-+ The present invention was completed by discovering that the reaction rate of formula (3) after heating at high temperature was also very high, at 70% or more.

The present inventors impregnated a cerium oxide powder with a zirconium oxynitrate aqueous solution, and impregnated a zirconium oxide powder with a cerium nitrate aqueous solution, heat-treated each at 600"C for 5 hours, and changed the crystal form to X. It was analyzed by line diffraction. As shown in Table 9, when cerium oxide powder was impregnated with a zirconium oxynitrate aqueous solution, it showed a cerium oxide crystal phase and its lattice constant was short. It can be seen that zirconium is substituted or dissolved in the cerium oxide crystal lattice.Also, when zirconium oxide powder is impregnated with a cerium nitrate aqueous solution, it separates into three phases, and the lattice constant is not strictly determined. This suggests the formation of a partial solid solution or a composite oxide between zirconium oxide and cerium oxide.

To form a composite oxide or solid solution containing cerium oxide and zirconium oxide in the catalyst support layer, the catalyst support layer is impregnated with an aqueous solution of cerium salt and zirconium salt simultaneously or separately, and the temperature is 600°C or higher. This can be done by firing with. It is also possible to use an oxide for at least one of cerium and zirconium, mix it with activated alumina powder during catalyst-supported layered acid, and then sinter it at a temperature of 800° C. or higher. If the temperature is lower than these values, complex oxides or solid solutions are difficult to form, and grain growth of cerium oxide tends to occur.

Although it is desirable that the cerium oxide and zirconium oxide be entirely in the form of a composite oxide or a solid solution, it goes without saying that the effect of preventing the grain growth of cerium oxide can be obtained even if they are at least partially present.

Further, cerium oxide and zirconium oxide may exist inside the catalyst support layer, or may exist in a state where they are supported on the surface of the support layer.

In particular, if it is on the surface of the support layer, it can easily come into contact with the exhaust gas and exhibit a large amount of Pli element hysteresis storage, so that the catalyst performance is particularly improved.

Note that the ratio of cerium and zirconium is not particularly limited, but the atomic ratio of the number of zirconium atoms to the number of cerium atoms is 5/95 to 80 between cerium atoms and zirconium atoms supported as a composite oxide or solid solution.
/20 is preferable. When this atomic ratio is smaller than 5/95, grain growth tends to occur in cerium oxide, and when it is larger than 80/20, the oxygen storage capacity becomes insufficient and the purification performance deteriorates.

[Operations and Effects of the Invention] In the exhaust gas purifying catalyst of the present invention, the catalyst supporting layer contains cerium oxide and zirconium oxide, and at least a part of the cerium oxide and zirconium oxide is present as a composite oxide or a solid solution. Existing. Although the mechanism is not yet clear, the presence of lithium as a composite oxide or solid solution suppresses oxidation and grain growth of lithium.

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

[Example] This will be explained in detail below using examples.

(Example 1, Comparative Example 1) Alumina Silua 00Q with an alumina content of 10 wt%,
100 g of alumina powder and 300 g of distilled water were mixed and stirred to prepare a slurry.

A honeycomb-shaped monolithic catalyst carrier gutter material made of cordierite was immersed in this slurry for 1 minute, then pulled up, the slurry inside the cell was blown off with an air stream, dried at 150°C for 1 hour, and then calcined at 700°C for 2 hours. This operation was repeated twice to form a catalyst support layer made of activated alumina.

Next, Niti11Mt (Ce (NO3) 3 ) is 0°Q8mol/, l! and Oxy61! ! Zirconium acid (ZrO(NO3)t) is 0.32 mol/
J) The monolithic carrier base material on which the catalyst supporting layer was formed was immersed in the dissolved mixed aqueous solution for 1 minute, then pulled up, the excess water was blown off, and after drying at 200°C for 3 hours, it was heated to 600°C in the air.
It was baked at ℃ for 5 hours. As a result, a monolithic carrier base material (1A) having a catalyst supporting layer containing cerium oxide and zirconium oxide was obtained.

In addition, in the same manner except that mixed aqueous solutions with different concentrations of cerium nitrate and zirconium oxynitrate were used,
Monolith carrier base materials (1[3-1E) containing cerium atoms and zirconium atoms with the values shown in Table 1 were obtained.

In addition, 0.4 m of cerium nitrate was used without using the zirconium oxynitrate aqueous solution.

The monolith carrier base material (1F) was immersed in the same manner except that it was immersed only in an aqueous solution containing 1/ρ, but the monolithic carrier base material (1F) was immersed only in an aqueous solution containing 0°4 mol/N of zirconium oxynitrate without using an aqueous cerium nitrate solution. A monolith carrier base material (1G) was obtained.

Next, each of these monolithic carrier substrates (1△~IG>) was immersed in distilled water to absorb sufficient water, then pulled out to blow off excess moisture, and soaked in an aqueous solution containing 1.0a/fJ of dinitrodiammine platinum for 1 hour. It was immersed in an aqueous solution containing rhodium chloride at 0.1o/1 and dried for 1 hour at 200°C. Platinum (Pt) and rhodium (Rh
) was catalyzed by supporting Examples 1a to 1 shown in Table 1.
Exhaust gas purifying catalysts of Comparative Example 1a and Comparative Example 1b were obtained.

Further, using the monolithic carrier base material (IA~1G>), using an aqueous solution containing 1.50/ρ of palladium chloride and an aqueous solution containing 0.2Q/fJ of rhodium chloride, palladium was added to each base material in the same manner as above. (Pd) and rhodium (Rh) were supported, Examples 1f to 1j shown in Table 2
And Comparative Example 1C1 Exhaust gas purifying catalyst of Comparative Example 1d was obtained.

Furthermore, using the above monolithic carrier base materials (1A to 1G), an aqueous solution containing dinitrodiammine platinum at 1.0G/41, an aqueous solution containing palladium chloride at 1.0g/11, and an aqueous solution containing rhodium chloride at 0.20/ρ. , Platinum (Pt) and Palladium (Pt) were added to the glue materials in the same manner as above.
(j) and rhodium (Rh) are supported and catalyzed,
Exhaust gas purifying catalysts of Examples 1 to 1o, Comparative Examples 1e and 1f shown in Table 3 were obtained.

For each of the obtained exhaust gas purification catalysts, 31
Attached to the exhaust system of an in-line 6-cylinder engine, the air-fuel ratio (A/F
) is 14.6, large scum fA degree is 200 under the condition of 850℃
A time durability test was conducted. Then, for each catalyst after the durability test, using the same engine as the durability test,
F-14,6, Hc, C under the condition of large gas temperature 400℃
o, NOX purification rate was measured.

In addition, the monolith carrier base material (1) before supporting the catalyst metal
A to IG) were heated at 1000°C for 5 hours and then held at 600°C in an oxidizing atmosphere.
While keeping the temperature at
3) Oxygen storage capacity was measured by determining the C₂ conversion rate from the generated carbon dioxide according to the formula.

Furthermore, the carrier base material after being heated at 1000° C. for 5 hours was pulverized, and the particle size of cerium oxide was measured by X-ray diffraction. These results are shown in Tables 1 to 3.

(Example 2, Comparative Example 2) 1 g of γ-alumina granular carrier (manufactured by Yakitori Universal Co., Ltd.) with a BET surface area of 100 to 150 m2/9 and an average pore diameter of 300 to 400 angstroms was used, except that the concentrations were different. Each carrier base material was immersed in the same flooded aqueous solution as in Example 1 and Comparative Example 1, dried and fired in the same manner as in Example 1 and Comparative Example 1.
is. And Examples 18 to 1e and Comparative Examples 1a to 1b
Examples 2a~ with the configurations shown in Table 4 were catalyzed in the same manner as
Exhaust gas purifying catalysts of Comparative Example 2d, Comparative Example 2a, and Comparative Example 2b 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 4.

(Example 3, Comparative Example 3) The granular carrier base materials of Example 2 J3 and Comparative Example 2, which have cerium and zirconium atoms and have been calcined at 600°C and have not yet been catalyzed, were processed into average particles using a vibration mill. Diameter 7μ
It was ground to m. and each powder ioom obtained
Part a, 30 parts by weight of an aqueous solution containing 4 Qwt% of aluminum nitrate, and 100 parts by weight of water were mixed and milled for 1 hour to prepare respective slurries. Using this slurry, a catalyst support layer was formed on the same honeycomb carrier as in Example 1 in the same manner as in Example 1, and catalyzed in the same manner as in Examples 1a to 1e and Comparative Examples 1a to 1b. Exhaust gas purifying catalysts of Examples 3a to 3d and Comparative Examples 3a and 3b 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 5.

(Example 4, Comparative Example 4) γ-
Alumina powder and zirconium oxide powder were blended in the composition ratio shown in Table 6, mixed with water to form a slurry in the same manner as in Example 3, and a catalyst support layer was formed in the same manner. After impregnating each carrier base material with cerium nitrate aqueous solution at two concentrations,
It was baked at 00°C for 5 hours. Thereafter, a catalyst metal was supported on Example 1 in the same manner as in ~1o, and Examples 4a to 4 shown in Table 6 were
The exhaust gas purifying catalyst b was obtained. In Comparative Example 4, li
l! The cerium I acid 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, Comparative Example 5) A sixth example was prepared 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 nitrate.
Exhaust gas purifying catalysts of Examples 5a to 5b and Comparative Example 5 shown in the table were (q).

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 VA6', Comparative Example 6) Example 3 except that a slurry containing zirconium oxide powder, cerium oxide powder, and alumina powder in the composition ratio shown in Table 7 and a mixture of water were used.
, Examples 6a to 6 shown in Table 8 1 were prepared in the same manner as Comparative Example 3.
b. One exhaust gas purifying catalyst of Comparative Examples 6a to 6b was used.

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 7, Comparative Example 7) Same as Example 1 and Comparative Example 1 except that a ferritic metal honeycomb carrier base material containing aluminum is used instead of the cordierite honeycomb carrier base material, as shown in Table 7. Examples 7a to 7b1 An exhaust gas purifying catalyst of Comparative Example 7 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 7.

(Example 8, Comparative Example 8) A ferritic metal honeycomb carrier consisting of 5 wt% aluminum, 2'Ow t% chromium, and the balance iron was heated at 900°C for 10 minutes in a carbon dioxide atmosphere, and then at 900°C in air.
C. for 1 hour, and then in the same manner as in Example 1, a catalyst supporting layer made of activated alumina as shown in Table 8 (hereinafter referred to as blank space) as in Example 1 was formed on the surface of the carrier base material.

Exhaust gas purifying catalysts of Examples 88 to 8b and Comparative Examples 8 to 8b shown in Table 8 were obtained in the same manner as in Example 1 and Comparative Example 1 using the second carrier base material.

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) It is clear from each table that all Examples are superior in purification rate compared to Comparative Examples. This is clearly due to the effect of the exhaust gas purifying catalyst of the example in which cerium oxide and zirconium oxide coexist at least in part as a composite oxide or solid solution.

Furthermore, from Table 1, in the exhaust gas purification catalysts of the examples, there is almost no grain growth of cerium oxide, and therefore the CO conversion rate (
It is clear that the oxygen storage capacity is layered.

It can also be seen that as the molar ratio of zirconium atoms to cerium atoms increases, the particle diameter decreases, and conversely, the co conversion rate decreases.

Claims (2)

[Claims] (1) In an exhaust gas purifying catalyst comprising a catalyst support layer and a catalyst metal supported on the catalyst support layer, the catalyst support layer contains cerium oxide and zirconium oxide, and the catalyst support layer contains cerium oxide and zirconium oxide. An exhaust gas purifying catalyst characterized in that at least a part of the zirconium oxide exists as a composite oxide or a solid solution. (2) The cerium atoms and zirconium atoms supported as the composite oxide or solid solution have an atomic ratio of the number of zirconium atoms to the number of cerium atoms of 5/95 to 80.
/20 Claim 1
Exhaust gas purification catalyst described in Section 1.