JP2003135964A - Catalyst for cleaning exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a high heat resistant catalyst for cleaning an exhaust gas. SOLUTION: The catalyst for cleaning the exhaust gas consists of an NOx storage material and a support composed of porous oxides consisting of zirconia and alumina in which zirconia is contained by 50-150 g per 1 L volume of the catalyst for cleaning the exhaust gas. In such a constitution, the degradation of the NOx storage material caused by forming compound oxides with the porous oxides is suppressed. It is considered that the formation of the compound oxides is suppressed by separating the porous oxides such as titania having high affinity from the NOx storage material (e.g. Ba) by the addition of zirconia or the like. Also, it is preferable that zirconia and alumina are highly dispersed in the support.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to N in exhaust gas containing oxygen in excess of that required to oxidize carbon monoxide (CO) and hydrocarbons (HC) contained in the exhaust gas.
The present invention relates to an exhaust gas purification catalyst that can efficiently purify O _x .

[0002]

2. Description of the Related Art In a lean burn engine, a system has been developed for reducing and purifying NO _x by using exhaust gas as a reducing atmosphere by intermittently burning fuel under lean oxygen conditions and intermittently making the fuel stoichiometric to rich. ,
It has been put to practical use. And as the best catalysts for this system, occludes NO _x in lean atmosphere, stoichiometric ~ _the NO _x storage-reduction type exhaust purifying catalyst is developing with _the NO _x storage material that releases occluded NO _x in a rich atmosphere Has been done.

The NO _x storage reduction catalyst is a NO _x storage material made of an alkaline earth metal, an alkali metal or a rare earth element and a noble metal supported on a porous oxide carrier such as alumina.

In this NO _x storage reduction catalyst, the air-fuel ratio of the engine is controlled from the lean side to the stoichiometric to rich side in a pulsed manner, and the exhaust gas is also pulsed from the lean side to the stoichiometric to rich side. Controlled to be used.
In other words, the exhaust gas is in the lean side to absorb NO _X in _the NO _X storage material contained in the NO _x storage-and-reduction type catalyst, when the stoichiometric-rich side from the lean side to the pulse shape, NO
_x storage material to release the NO _X is reacted with the reducing components such as HC and CO to purify. Therefore, NO _x can be efficiently purified even with the exhaust gas from the lean burn engine. Further, HC and CO in the exhaust gas are oxidized by the noble metal and are also consumed for NO _x reduction, so that HC and CO are also efficiently purified.

By the way, not only the NO _x storage reduction type catalyst,
It is preferable that the durability is improved. The NO _x storage-reduction catalyst uses NO _x due to SO _x contained in the exhaust gas.
When the occlusion material is poisoned and deteriorated, NO _x can no longer be stored. A titania carrier was used for the purpose of suppressing the poisoning deterioration. This is because titania does not adsorb SO _x . The applicant of the present invention has disclosed in
Japanese Patent Laid-Open Publication No. 1994-242, an exhaust gas of a NO _x storage reduction type, which is composed of a porous oxide containing at least ultrafine particles of titania particles and a porous oxide other than that, and which comprises a carrier carrying a noble metal and a NO _x storage material. Proposing a purification catalyst.

[0006]

However, the above-mentioned Japanese Unexamined Patent Application Publication No.
Even in the NO _x storage reduction type exhaust gas purifying catalyst disclosed in Japanese Patent Publication No. 000-246107, the NO _x purifying ability after high temperature endurance is not sufficient especially in view of the present state of air pollution in recent years and the tightening of exhaust gas regulations. There is a demand for early development of an exhaust gas purifying catalyst that exhibits a high NO _x purifying ability from the end to the end of life.

The present invention has been made in view of such circumstances, and it is an object to be solved to provide an exhaust gas purifying catalyst having high heat resistance.

[0008]

Means for Solving the Problems As a result of intensive studies conducted by the present inventors in order to solve the above problems, a conventional exhaust gas purifying catalyst using a carrier prepared by mixing alumina and titania deteriorates at high temperature. As a cause, it was clarified that composite oxide of alumina and titania and the occlusion material at high temperature or sintering of the occlusion material was formed. According to the research conducted by the present inventors for the purpose of reducing this deterioration, it is possible to suppress the formation of a composite oxide with the storage material by reducing the amount of titania from the amount conventionally considered to be preferable, and the titania It has been found that by increasing the amount of reduction with an alkali base material such as alumina or zirconia and highly dispersing the alkali base material in the coating layer, it is possible to suppress further formation of a composite oxide with the storage material. The present inventors have made the following inventions based on this finding.

That is, the exhaust gas purifying catalyst of the present invention is
A NO _x storage-reduction type exhaust gas purifying comprising a carrier composed of a porous oxide, a noble metal supported on the carrier, and an NO _x storage material supported on the carrier containing an alkali metal or an alkaline earth metal. The catalyst is characterized in that the porous oxide is composed of zirconia and alumina, and the zirconia is contained in an amount of 50 to 150 g per 1 L of the exhaust gas purifying catalyst volume.

It is preferable that the porous oxide further contains titania, and the titania and the alumina are contained in an amount of 30 to 70 g and 100 to 170 g per 1 L of the exhaust gas purifying catalyst volume, respectively. Within this range, deterioration of the catalyst due to sintering can be prevented, and the sulfur poisoning resistance performance can be improved.

Further, it is preferable that the noble metal is rhodium, and the rhodium is supported on 30 to 100% of the entire zirconia on a mass basis.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION In the exhaust gas purifying catalyst of the present invention,
The carrier contains a NO _x storage material and zirconia and alumina which are porous oxides in the carrier, and the zirconia is contained in an amount of 50 to 150 g per 1 L of the exhaust gas purifying catalyst volume. With such a structure, deterioration of the NO _x storage material due to formation of a composite oxide with the porous oxide can be suppressed. Although details are not clear, it is considered that the addition of zirconia or the like separates the porous oxide such as titania having a high affinity from the NO _x storage material (for example, Ba) to suppress the formation of the composite oxide. . Therefore, zirconia and alumina are preferably highly dispersed in the carrier.

Furthermore, it is preferable to add 30 to 70 g of titania to the carrier per 1 L of the exhaust gas purifying catalyst volume. By using a carrier having such a structure, sulfur poisoning of the carried NO _x storage material can be effectively suppressed, and a high NO _x purifying ability is exhibited even after endurance. The reason for this is not clear, but by using titania particles having a low affinity for SO _X and forming an interface with another porous oxide, another porous oxide such as alumina is formed at the interface. It is considered that the SO _x is less likely to be adsorbed on the NO _{x and} the sulfur poisoning of the NO _x storage material existing at the interface is prevented.

Alumina is preferably contained in an amount of 100 to 170 g per 1 L of the exhaust gas purifying catalyst volume. Within this range, deterioration of the catalyst due to sintering can be further prevented. Zirconia may be added instead of alumina within the range of the preferable addition amount of alumina.

The carrier preferably contains ceria. Due to the oxygen storage / release capacity of ceria, the purification performance is further improved. Further, when ceria stabilized with zirconia (ceria-zirconia composite oxide) is used, its durability is further improved. The amount of ceria added is preferably 10 to 30 g per 1 L of the exhaust gas purifying catalyst volume.

Noble metals include Pt, Rh, Pd, and Ir.
Alternatively, one or more of Ru can be used, and Rh is particularly preferable. The supported amount is preferably 0.1 to 2.5 g, and particularly preferably 0.1 to 0.75 g in the case of Rh, per 1 L of the exhaust gas purifying catalyst volume. And Pt
And in the case of Pd, 0.1-10 g is preferable, and 2-5 g
Is particularly preferable. Further, it is preferable that the noble metal is Rh, and the Rh is supported on 30 to 100% of the entire zirconia contained in the carrier on a mass basis. By supporting Rh on zirconia, the steam reforming reaction is promoted and sulfur poisoning is suppressed.

As the NO _x storage material, at least one selected from alkali metals, alkaline earth metals and rare earth elements can be used. Above all, the alkalinity is high and N
It is preferable to use at least one of an alkali metal and an alkaline earth metal having a high Ox storage capacity.

Examples of the alkali metal include lithium, sodium, potassium, rubidium, cesium and francium. The alkaline earth metal means an element of Group 2A of the periodic table, and examples thereof include barium, beryllium, magnesium, calcium, strontium and the like. Further, as rare earth elements, scandium, yttrium, lanthanum,
Cerium, praseodymium, neodymium, dysprosium,
An example is ytterbium.

The amount of the NO _x storage material carried is preferably in the range of 0.3 to 1.0 mol per 1 L of the exhaust gas purifying catalyst volume. When the supported amount is within this range, the NO _x storage capacity can be sufficiently contained without being saturated, and the NO _x purification performance is improved.

In order to produce the carrier used for the above-mentioned catalyst for purifying exhaust gas of the present invention, porous oxide powders such as zirconia and alumina may be simply dry mixed, but these powders are used as a surfactant. It is desirable to form the carrier from a slurry formed by mixing with water in the presence of By highly dispersing zirconia and alumina with a surfactant, the sintering suppressing ability is improved.

A surfactant may be added to the dispersion liquid in which these powders are mixed, or the porous oxide powder may be dispersed in an aqueous surfactant solution. As the surfactant, in addition to anionic, cationic and nonionic surfactants, metal soaps such as sodium stearate, polyvinyl alcohol, polyvinylpyrrolidone, polyacrylamide, polymeric surfactants such as polyacrylic acid, etc. Can be used.

In order to form a catalyst from the slurry obtained as described above, a honeycomb-shaped carrier substrate is coated with the slurry, dried and fired to form a supporting layer, and the supporting layer is formed of a noble metal and NO. _{x The} occlusion material may be supported by a standard method. It is also possible to produce a pellet catalyst by carrying out extrusion molding and then cutting it into pellets, drying and firing it, and then carrying a noble metal and a NO _x storage material. Further, when preparing the above slurry, a noble metal-supported powder in which a noble metal is previously supported on a part or all of the porous oxide powder may be used.

When the exhaust gas purifying catalyst of the present invention is operated at an air-fuel ratio (A / F) of 18 or more and is brought into contact with exhaust gas from a lean burn engine which is intermittently in a fuel stoichiometric or rich atmosphere, the fuel lean In the atmosphere, NO contained in the exhaust gas is oxidized on the catalyst to NO _x ,
It is stored in the NO _x storage material. When the fuel stoichiometric-rich atmosphere is intermittently generated, NO _x is released from the NO _x storage material, which is the HC or C in the exhaust gas on the catalyst.
It reacts with O and is reduced.

At this time, zirconia, alumina, etc.
It is possible to suppress the deterioration of the NO _x storage material. Thereby, the durability is remarkably improved, and the high NO _x purification ability can be maintained for a long period of time.

[0025]

Example (Catalyst 1) 50 parts by mass of zirconia (Rh / ZrO ₂ ) carrying 1% by mass of Rh, 100 parts by mass of alumina as a porous oxide and 90 parts of zirconia.
Parts by mass, 30 parts by mass of titania, and 20 parts by mass of a ceria / zirconia composite oxide (CeO ₂ / ZrO ₂ ).
Appropriate amount of water and binder sol (aluminum nitrate solution, etc.)
And are mixed to form a slurry.

The preparation of Rh / ZrO ₂ was carried out by dispersing zirconia in an appropriate amount of water and then adjusting the amount of Rh to 1 relative to zirconia.
An aqueous solution of rhodium chloride having a predetermined concentration was added so that the content of the mixture was 1% by mass, the mixture was stirred for 1 hour, filtered, dried at 110 ° C. for about 10 hours, and then baked at 250 ° C. for 2 hours.

CeO ₂ / ZrO ₂ was prepared by mixing ceria powder and zirconium oxynitrate solution at a ceria: zirconia mass ratio of 85:15, stirring and drying.
Firing was performed.

A monolith carrier is coated with this slurry (formation of a coat layer). Then, dry at 250 ° C for 1 hour,
Firing was performed at 500 ° C. for 1 hour.

At this time. The coating composition was 100 g of alumina, 50 g of Rh / ZrO ₂ , 90 g of zirconia, and 20 g of CeO ₂ / ZrO ₂ per 1 L of the catalyst volume.

Thereafter, the monolith carrier having the coat layer formed thereon is immersed in a mixed solution of barium acetate, potassium acetate and lithium acetate for 1 minute and then dried to obtain barium, potassium and lithium at 10% by mass, 2.2% by mass and 0% by mass, respectively. .3
It was immersed in the prepared aqueous solution for 1 minute so as to be a mass% and then dried to obtain a target catalyst.

(Catalysts 2 to 6) Catalysts 2 to 6 were prepared so that each composition had the loading amount shown in Table 1. Each catalyst was prepared in the same manner as catalyst 1.

[0032]

[Table 1]

(Heat resistance test) For each catalyst,
Using a 4-liter engine, stoichiometric F / B atmosphere, catalyst bed temperature 800 ° C, gas space velocity 60000h
Durability was performed for 50 hours under the condition of ^-1 . After that, displacement 1.
The performance of each catalyst was evaluated using an 8 L lean burn engine. The performance evaluation is judged by the NO _X storage amount, and the temperature of the gas containing the catalyst is changed to 300 ° C, 350 ° C, 400 ° C,
Introduce a rich spike when the output gas is stable, and then calculate RS by the total of the input gas amount and the output gas amount.
The NO _X storage amount was measured. In addition, the particle size (BaCO ₃ and BaAlTi
O) was measured by XRD. The results are shown in Table 2.

[0034]

[Table 2]

As shown in Table 2, the catalysts 1 to 5 having a large zirconia supported amount (total zirconia) were compared with the catalyst 6 having a small zirconia supported amount (total zirconia) in the NO _x storage amount with an increase in the gas temperature entering the catalyst. The decrease was suppressed, and it was revealed that the catalyst has excellent heat resistance. Catalyst 6
The NO _X storage amount of was also smaller in absolute value than the catalysts 1 to 5.

It is considered that this is because the reactivity of Ti, Al and Ba can be suppressed by adding zirconia (ie, Zr element) to the catalyst. This is also clear from the values of the particle diameters of BaCO ₃ and BaAlTiO after high temperature durability shown in Table 1. That is, catalyst 5, catalyst 1
Therefore, as the amount of catalyst 4 and zirconia supported decreases,
The amount of BaCO ₃ particles in the catalyst changed to BaAlTiO particles is large (that is, the NO _X storage material is deteriorated).

Therefore, the thermal durability of the catalyst can be improved by increasing the amount of zirconia supported. The effect of improving the heat resistance due to the increase in the supported amount of zirconia is sufficiently exhibited by the catalyst 4 having a supported amount of zirconia (total zirconia) of 50 g / L. From the viewpoint of improving the heat resistance, the supported amount of zirconia is increased. It has been clarified that it is sufficient if the total (zirconia) is 50 g / L or more.

(Sulfur poisoning endurance test) For each of the catalysts 1 to 6, a lean burn engine with a displacement of 1.8 L was used, and the temperature of the gas containing the catalyst was 550 ° C. and the gas space velocity was 5
Durability was performed for 20 hours under the condition of 0000 h ^-1 . At this time, since the durability is performed using gasoline with an S content of 500 ppm, the accelerated S durability test can be achieved in a short time. Then, the performance of each catalyst was evaluated using a lean burn engine with a displacement of 1.8 L. The performance evaluation is judged by the NO _X storage amount,
The temperature of the gas containing the catalyst was changed to 300 ° C., 350 ° C., and 400 ° C., and the measurement was performed by the same method as the performance evaluation test after the heat resistance test. The results are shown in Table 3.

[0039]

[Table 3]

As is clear from Table 3, catalysts 1 to 4 and 6 have high sulfur poisoning resistance. It is considered that this is because the titania particles having a low affinity for SO _X are arranged with good dispersibility, and thus high sulfur poisoning resistance can be maintained. It has been clarified that it is preferable to support 30 g / L or more, because the addition of titania exerts sufficient sulfur poisoning resistance performance even with the addition amount of catalyst 1 (30 g / L).

(Heat Resistance Test and Sulfur Poisoning Durability Test) From the results of the heat resistance test and the sulfur poisoning durability test, the amount of zirconia supported increased from the viewpoint of improving the heat resistance, and the titania from the viewpoint of improving the sulfur poisoning resistance. It was revealed that it is desirable to increase the supported amount of each of the catalysts. However, in order to keep the warm-up performance of the catalyst above a certain level, the supported amount of the entire catalyst should be as small as possible. Therefore, referring to Tables 2 and 3 for the purpose of effectively utilizing the limited amount of supported catalyst, the supported amounts of zirconia are from catalyst 1 (supported amount of zirconia 140 g / L) to catalyst 5 (supported amount of zirconia 170 g / L). Since the effect is saturated in the vicinity of (1), it becomes clear that the supported amount of zirconia is preferably about 50 to 150 g per 1 L of the catalyst.

Similarly, the amount of titania supported increases from catalyst 3 (titania supported amount 70 g / L) to catalyst 4 (titania supported amount 120 g / L), and further catalyst 6 (titania supported amount 140 g / L). The effect is saturated, and the amount of titania supported is 30 to 70 g per 1 L of the catalyst.
It became clear that the degree is preferable.

[0043]

As described above, the catalyst for purifying exhaust gas of the present invention increases the supported amount of zirconia,
It is possible to suppress deterioration of the NO _X storage material due to heat, and it is possible to provide an exhaust gas-purifying catalyst having high heat resistance. Further, by increasing the amount of titania supported, it is possible to suppress deterioration of the catalyst due to sulfur, and it is possible to provide an exhaust gas purifying catalyst having high sulfur resistance.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Yoshinori Sato             7800 Chihama, Daito Town, Ogasa County, Shizuoka Prefecture Stock Association             Inside the company caterer F-term (reference) 3G091 AA12 AB06 AB09 BA01 BA11                       BA14 BA39 CB02 DA01 DA02                       FA14 FB03 FB10 FB11 FB12                       FC08 GA06 GA07 GA16 GA20                       GB01X GB02Y GB04Y GB05W                       GB06W GB07W GB10X GB16X                 4D048 AA06 AB02 BA03X BA07X                       BA08X BA19X BA33X BA41X                       BA42X BB02                 4G069 AA03 AA08 BA01B BA04B                       BA05B BB04B BB06B BC03B                       BC04B BC13B BC43B BC71B                       CA03 CA08 CA13 DA06 EA19                       ED06 FB04 FB06 FB07

Claims (3)

[Claims] 1. A NO _x storage reduction comprising a support made of a porous oxide, a noble metal supported on the support, and an NO _x storage material supported on the support containing an alkali metal or an alkaline earth metal. Type exhaust gas purifying catalyst, wherein the porous oxide comprises zirconia and alumina, and the zirconia is contained in an amount of 50 to 150 g per 1 L of the exhaust gas purifying catalyst volume. . 2. The porous oxide further contains titania, and the titania and the alumina are 30 to 70 g and 100 to 1 per 1 L of the exhaust gas purifying catalyst volume, respectively.
The exhaust gas-purifying catalyst according to claim 1, which contains 70 g. 3. The exhaust gas purifying catalyst according to claim 1, wherein the noble metal is rhodium, and the rhodium is supported on 30 to 100% of the entire zirconia on a mass basis.