JP2000325787A - Manufacture of exhaust gas cleaning catalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To inhibit the sulfur poisoning of an NOx occluding element such as an alkaline metal, an alkaline earth metal or a rare earth element beyond the present effective level and further enhance the cleaning activity by making a porous oxide carrier bearing a specific metallic oxide and a precious metal carry the NOx occluding element. SOLUTION: First a composite oxide consisting of the atoms of metals such as Ti, Zr, Si and Ce and a noble metal atom is borne by a porous oxide carrier, and then the composite oxide thus borne by the carrier is decomposed. Since the atoms of the metals such as Ti, Zr, Si and Ce and the noble metal atom are set in a close proximity to each other on the atomic level in the composite oxide, it is possible to bear the oxides of TiO2, ZrO2, SiO2 and CeO2 and the noble metal as proximity fine particles on the porous oxide carrier. In addition, an NO occluding element such as an alkaline metal, an alkaline earth metal, a rare earth element or the like is borne by the porous oxide carrier. The porous oxide carrier is preferably γ-Ale2O3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a nitrogen gas contained in an exhaust gas containing an excess amount of oxygen in excess of an amount necessary for oxidizing carbon monoxide (CO) and hydrocarbons (HC) contained in the exhaust gas. The present invention relates to a method for producing an exhaust gas purifying catalyst capable of efficiently purifying oxides (NO _x ).

[0002]

2. Description of the Related Art In a lean-burn engine, a system has been developed in which combustion is always performed under an oxygen-excess fuel lean condition, and intermittent fuel stoichiometric to rich conditions are used to reduce and purify NOx using exhaust gas as a reducing atmosphere. Has been And as the best catalysts for this system, occludes NO _x in lean atmosphere, with _the NO _x storage element that releases NO _x occluded in the stoichiometric-rich atmosphere
NO _x storage-and-reduction type exhaust gas purifying catalyst has been developed.

For example, JP-A-5-317652 proposes an exhaust gas purifying catalyst in which an alkaline earth metal such as barium (Ba) and platinum (Pt) are supported on a porous oxide carrier such as alumina. . Japanese Unexamined Patent Publication (Kokai) No. 6-31139 proposes an exhaust gas purifying catalyst in which an alkali metal such as potassium (K) and Pt are supported on a porous oxide carrier such as alumina. Further, Japanese Patent Application Laid-Open No. Hei 5-168860 proposes an exhaust gas purifying catalyst in which a rare earth element such as lanthanum (La) and Pt are supported on a porous oxide carrier such as alumina.

[0004] By using this NO _x storage-and-reduction type catalyst, by controlling so that the stoichiometric-rich side in a pulsed manner the air-fuel ratio from the lean side, the stoichiometric-rich atmosphere exhaust gas from a lean atmosphere in pulses . Thus, NO _x is occluded in _the NO _x storage element is in the lean side, because it is purified by reacting with reducing components, such as being released in the stoichiometric or rich side HC and CO, in exhaust gases from lean-burn engines can efficiently purify NO _x even. Further, HC and CO in the exhaust gas are oxidized by the noble metal and consumed in the reduction of NO _x , so that HC and CO are also efficiently purified.

However, the exhaust gas contains SO ₂ generated by burning sulfur (S) contained in the fuel, which is oxidized by a noble metal into SO ₃ in the exhaust gas in a lean atmosphere. And it is also easily converted into sulfuric acid by the water vapor contained in the exhaust gas, and these react with the NO _x storage element to generate sulfite and sulfate. The NO _x storage element that has become a sulfite or a sulfate can no longer store NO _x . In addition, even in a rich atmosphere, the sulfate of the NO _x storage element is less likely to be reduced than the nitrate, and as a result,
_The NO _x storage element in the catalyst gradually increases sulphates, there is a disadvantage that the purification performance of the NO _x is reduced during use.
This phenomenon is called sulfur poisoning of the NO _x storage element.

A porous oxide carrier such as alumina is
There is also a problem that the above-mentioned sulfur poisoning is promoted because of the property of easily adsorbing SO _x . Therefore, the use of a titania carrier that does not easily adsorb SO _x was recalled, and experiments were conducted. As a result, SO _x flows downstream as it is not adsorbed on titania, only SO _x in direct contact with the noble metal is revealed that since only the degree of poisoning is small is oxidized. However less active in the titania carrier, there is a problem that the initial from _the NO _x purification rate is low.

Accordingly, the applicant of the present application has disclosed in Japanese Patent Application Laid-Open No. Hei 8-099034 that TiO ₂ -Al ₂ O ₃ , ZrO ₂ -Al ₂ O ₃ and SiO ₂ -Al
_It has been proposed to use at least one composite carrier selected from ₂ O ₃ . By using such a composite carrier, the advantage of Al ₂ O ₃ increases the initial NO _x purification rate.
Also, TiO ₂ , ZrO ₂ and SiO ₂ are less likely to adsorb SO _x than Al ₂ O ₃ and the adsorbed SO _x is easier to desorb at lower temperatures than when adsorbed on Al ₂ O ₃ , NO _x storage element and SO
_The probability of contact with _x decreases. Therefore the use of the composite carrier, high _the NO _x purification rate in the initial is secured,
Since sulfur poisoning is suppressed, the NO _x purification rate after durability is improved.

[0008]

SUMMARY OF THE INVENTION The present invention relates to
Is intended to further improve the exhaust gas purifying catalyst as disclosed in 8-099034 JP, NO _x can further suppress the sulfur poisoning of the storage element, that the purification activity was further improved exhaust gas purifying catalyst Aim.

[0009]

The features of the method for producing an exhaust gas purifying catalyst of the present invention which solves the above-mentioned problems are Ti, Zr, Si
And supporting a composite compound containing at least one metal atom selected from Ce and a noble metal atom on a porous oxide carrier, and decomposing the composite compound supported on the porous oxide carrier to form TiO ₂ , ZrO ₂ , a step of supporting at least one oxide selected from SiO ₂ and CeO ₂ and a noble metal on a porous oxide carrier, and the porous oxide carrier is selected from alkali metals, alkaline earth metals and rare earth elements. a step of carrying at least one NO _x storage element is to consist of.

[0010]

DETAILED DESCRIPTION OF THE INVENTION According to the study of the present inventors, for example, the effect of suppressing SO _x adsorption by TiO ₂ is not
It has been found that it is effectively expressed at the interface with TiO ₂ . Therefore, it is desirable that the porous oxide and Ti are close at the atomic level. The SO _x, by SO ₂ in the exhaust gas is SO ₃ and SO ₄ is oxidized over a noble metal, the as Al ₂ O ₃ and NOx storage element poisoning. Therefore it is desirable for desorbing SO _x that are close to the noble metal and Ti.

However, since the exhaust gas purifying catalyst disclosed in Japanese Patent Application Laid-Open No. H08-099034 is manufactured by a method of supporting a noble metal on a composite carrier powder, the noble metal is not necessarily supported in the vicinity of Ti atoms. This is not to say that there are many precious metals that are carried in close proximity to Al atoms. Therefore, there is a large amount of TiO ₂ which does not exhibit the sulfur poisoning suppressing effect, and the sulfur poisoning suppressing effect corresponding to the amount of TiO ₂ cannot be obtained.

Therefore, in the method for producing an exhaust gas purifying catalyst of the present invention, first, a composite compound comprising at least one metal atom selected from Ti, Zr, Si and Ce and a noble metal atom is supported on a porous oxide carrier. After that, the supported composite compound is decomposed. Ti, Zr, Si and Ce in composite compounds
Since at least one kind of metal atom selected from and the noble metal atom are close to each other at the atomic level, by decomposing the composite compound supported on the porous oxide carrier, Ti
At least one oxide selected from O ₂ , ZrO ₂ , SiO ₂ and CeO ₂ and a noble metal can be supported on the porous oxide carrier as adjacent fine particles.

Accordingly, at least one selected from TiO ₂ , ZrO ₂ , and SiO ₂ and the noble metal can be carried in close proximity to each other, and the effect of suppressing the sulfur poisoning by these oxides is maximized. be able to. If CeO ₂ is supported close to the noble metal, the oxygen storage / release action of CeO ₂ can be maximized, and the purification ability in the transition region can be further improved.

The composite compound contains at least one metal atom selected from Ti, Zr, Si and Ce and a noble metal atom,
Any material can be used without particular limitation as long as it can be decomposed into at least one oxide selected from TiO ₂ , ZrO ₂ , SiO ₂ and CeO ₂ and a noble metal. For example, Ti, Zr, Si and
It can be used as a composite compound formed by reacting an alkoxide or complex of at least one metal selected from Ce with a noble metal complex with an amino acid, ethanolamine, or the like. Such a complex compound is
Organic groups are burned off by firing in the air, and TiO ₂ , ZrO ₂ , Si
It easily decomposes into at least one oxide selected from O ₂ and CeO ₂ and a noble metal.

The composition ratio of at least one kind of metal atom selected from Ti, Zr, Si and Ce and the noble metal atom in the composite compound is preferably the same as the composition ratio of the catalyst finally obtained. When the composition ratio in the catalyst finally obtained is not the same, the insufficient metal element or noble metal element can be supported on the porous oxide carrier in the same manner as in the conventional case.

In order to carry the composite compound on the porous oxide carrier, it is necessary to mix the powder of the porous oxide carrier in a solution or slurry containing the composite compound and to evaporate and dry it to carry the composite compound. it can. In some cases, it can be supported by forming a composite compound on a porous oxide carrier. The noble metal, which is a component of the composite compound, is a noble metal used in a conventional catalyst for purifying exhaust gas, such as Pt, rhodium (Rh), palladium (Pd), iridium (Ir), ruthenium (Ru), or the like. A plurality of types can be used.

As the porous oxide carrier, γ-Al ₂ O ₃ , Si
One or more selected from O ₂ , ZrO ₂ , TiO ₂ , SiO ₂ —Al ₂ O _{3 and the} like can be used. Of course, it is desirable to use a different kind of metal oxide from the composite compound. In terms of heat resistance and purification activity, γ-Al ₂ O ₃ is particularly preferable. The loading amount of the composite compound on the porous oxide carrier is
The amount of at least one metal selected from Ti, Zr, Si and Ce and the noble metal in the finally obtained catalyst may be set to a predetermined amount. When the supporting amount is insufficient, the insufficient metal element or noble metal element can be supported on the porous oxide carrier in the same manner as in the related art.

For example, when the porous oxide carrier is alumina, the molar ratio is preferably in the range of Al ₂ O ₃ / TiO ₂ = 4/1 to 1/3. Within this range, both purification performance and sulfur poisoning resistance can be achieved. If the amount of alumina is less than this, the purification performance is low and sintering of the noble metal is likely to occur. If the amount of titania is less than this, the sulfur poisoning resistance is reduced.

The amount of the noble metal carried is preferably 0.1 to 20 g in the case of Pt and Pd per liter of carrier volume,
-10 g is particularly preferred. In the case of Rh, the amount is preferably 0.01 to 10 g, particularly preferably 0.05 to 5 g. By decomposing the composite compound supported on the porous oxide carrier, TiO ₂ , ZrO ₂ , SiO ₂
In order to support at least one oxide selected from the group consisting of CeO ₂ and CeO ₂ and the noble metal on the porous oxide carrier, it is performed according to the decomposition conditions of the composite compound. In the case of the above-mentioned amine salt or ammonium salt, it can be easily decomposed by heating in an oxidizing atmosphere. Further, depending on the type of the complex compound, the compound can be decomposed by contact with a chemical such as an acid or an alkali, or can be decomposed by irradiating electromagnetic waves such as ultraviolet rays.

[0020] The porous oxide support, at least one NO _x storage element selected from alkali metals, alkaline earth metals and rare earth elements is supported. Examples of the alkali metal include lithium, sodium, potassium, rubidium, cesium, and francium. Further, the alkaline earth metal refers to a Group 2A element of the periodic table, and includes barium, beryllium, magnesium, calcium, and strontium. Examples of the rare earth element include scandium, yttrium, lanthanum, cerium, praseodymium, and neodymium.

The loading amount of the NO _x storage element is preferably in the range of 0.05 to 1.0 mol per 120 g of the porous oxide carrier. If the supported amount is less than 0.05 mol, the NO _x storage capacity is small, and the NO _x purification performance is reduced. To carry the the _the NO _x storage element, impregnated with a predetermined amount of a solution of a salt of _the NO _x storage element on a monolithic carrier having a porous oxide support powder or a porous oxide support layer, evaporated to dryness it It can be supported by the above. The step of loading the NO _x storage element may be performed before or after the loading of the composite compound, or before or after the decomposition step of the loaded composite compound.

[0022]

The present invention will be specifically described below with reference to examples and comparative examples. (Example 1) titanium tetrapropoxide, dissolved in isopropanol so as to 50g in terms of TiO _2, L
Lysine was added and allowed to react well. While stirring this solution, a predetermined amount of a predetermined concentration of an aqueous solution of platinum hydroxide was dropped, and the mixture was sufficiently reacted.

[0024] 100 g of γ-Al ₂ O ₃ powder was mixed into the suspension, stirred sufficiently, evaporated and dried to deposit a composite compound on γ-Al ₂ O ₃ . In this state, the composite compound particles 2 containing Ti and Pt are supported on the γ-Al ₂ O ₃ particles 1 as shown in FIG. Next, the composite particles were fired at 500 ° C. for 1 hour to decompose the composite compound on γ-Al ₂ O ₃ . In the obtained powder, the composite compound particles 2 were decomposed and, as shown in FIG.
TiO ₂ particles 20 and Pt particles 21 are supported on l ₂ O ₃ particles 1.

Next, the obtained powder is impregnated with a predetermined amount of a barium acetate aqueous solution having a predetermined concentration, and evaporated and dried.
For 1 hour. In the obtained catalyst powder, γ-Al ₂ O ₃ and
The weight ratio of TiO ₂ is 100: 50, 2 g of Pt is supported on the whole, and 0.2 mol of Ba is supported. The obtained catalyst powder was compacted, pulverized, and then pelletized to prepare a pellet catalyst, which was subjected to a test.

Example 2 A catalyst powder was prepared in the same manner as in Example 1 except that potassium acetate was used instead of barium acetate, and used as a pellet catalyst. In the obtained catalyst powder, the weight ratio of γ-Al ₂ O ₃ and TiO ₂ was 100: 50, and 2 g of Pt was supported on the whole and 0.2 mol of K was supported.

(Example 3) A catalyst powder was prepared in the same manner as in Example 1 except that 5 g of triethanolamine was used instead of L-lysine, and used as a pellet catalyst. In the obtained catalyst powder, the weight ratio of γ-Al ₂ O ₃ to TiO ₂ was 100: 50, and 2 g of Pt was supported on the whole and 0.2 mol of Ba was supported.

Example 4 A catalyst powder was prepared in the same manner as in Example 1 except that 5 g of triethanolamine was used instead of L-lysine and potassium acetate was used instead of barium acetate. And In the obtained catalyst powder, the weight ratio of γ-Al ₂ O ₃ and TiO ₂ was 100: 50, and 2 g of Pt was supported on the whole and 0.2 mol of K was supported.

Comparative Example 1 After mixing 50 g of TiO ₂ powder, 100 g of γ-Al ₂ O ₃ powder, and ion-exchanged water, a predetermined amount of a platinum hydroxide aqueous solution having a predetermined concentration was dropped, and the mixture was evaporated and dried. Solidify Pt
A supported powder was prepared. Next, the Pt-supported powder is impregnated with a predetermined amount of a barium acetate aqueous solution having a predetermined concentration, and after evaporating to dryness.
It was baked at 00 ° C. for 1 hour. In the obtained catalyst powder, γ-Al ₂
The weight ratio of O ₃ to TiO ₂ is 100: 50, and 2 g of Pt is supported on the whole and 0.2 mol of Ba is supported.

The obtained catalyst powder was compacted, pulverized and then pelletized to prepare a pellet catalyst, which was subjected to a test. Comparative Example 2 A catalyst powder was prepared in the same manner as in Comparative Example 1 except that potassium acetate was used instead of barium acetate, and was used as a pellet catalyst. In the obtained catalyst powder, γ-A
The weight ratio of l ₂ O ₃ to TiO ₂ is 100: 50, and the total Pt is 2 g
Supported, and 0.2 mol of K is supported.

<Test / Evaluation> Each of the pellet catalysts of Examples and Comparative Examples was placed in a laboratory reactor, and model exhaust gas having the composition shown in Table 1 was introduced at a gas space velocity of 100,000 h- ¹ . . At a catalyst bed temperature of 350 ° C., a lean gas and a rich gas were repeatedly flown at one-minute intervals, and the initial NO _x purification rate when switching from the rich gas to the lean gas was measured. Table 3 shows the results.

[0031]

[Table 1] Each of the pellet catalysts of Example and Comparative Example was placed in a laboratory reactor, and a model exhaust gas having the composition shown in Table 2 was subjected to a lean gas 55 seconds at a catalyst bed temperature of 600 ° C. and a gas space velocity of 100,000 h ^−1. -A rich gas was repeatedly introduced at a cycle of 5 seconds for 4 hours, and each was subjected to a sulfur poisoning durability test. Then, for each of the catalysts after the sulfur poisoning durability test, the NO _x purification rate was measured in the same manner as described above, and the results are shown in Table 3.

[0032]

[Table 2]

[0033]

[Table 3] From Table 3, the catalyst according to the embodiment showed higher _the NO _x purification rate even after the sulfur poisoning endurance test as compared to the catalyst of Comparative Example, which carries the Ti and Pt as a composite compound, and then decomposed to
This is considered to be the effect carried as TiO ₂ and Pt.

[0034]

According to the method for producing an exhaust gas purifying catalyst of the present invention, sulfur poisoning can be further suppressed, and an exhaust gas purifying catalyst excellent in durability can be reliably produced.

[Brief description of the drawings]

FIG. 1 is a schematic explanatory view showing a composite particle produced in one embodiment of the present invention and a state after firing.

[Explanation of symbols]

1: γ-Al ₂ O ₃ particles 2: composite compound particles 20: TiO ₂ particles 21: Pt particles

Continued on the front page F-term (reference) 4D002 AA12 AC10 BA03 BA04 DA01 DA04 DA11 DA21 EA08 EA13 FA01 4D048 AA02 AA06 AA13 AA18 BA03X BA07X BA08X BA14X BA15X BA19Y BA30X BA31Y BA32Y BA33Y BA41X BB01 BA03 ABABA BAB BAA BAB BAA BAB BAA BAA BC01B BC03A BC03B BC08A BC08B BC13A BC13B BC43A BC69A BC75A BC75B CA02 CA03 CA12 CA13 CA14 CA15 DA06 EA02Y ED07

Claims (1)

[Claims] A step of supporting a composite compound containing at least one kind of metal atom selected from titanium, zirconium, silicon and cerium and a noble metal atom on a porous oxide carrier; and supporting the composite compound supported on the porous carrier. Decomposing the composite compound to support at least one oxide selected from titania, zirconia, silica and ceria and a noble metal on the porous oxide carrier; at least one a step of carrying _the NO _x storage element, method of manufacturing the exhaust gas purifying catalyst characterized by comprising a selected from the class metals and rare earth elements.