JP2001104784A - Catalyst for purifying exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent NOx purification performance after being exposed at a high temperature of at least 700 deg.C from being decreased and to prevent especially NOx purification performance in a low temperature region of at most 350 deg.C from being decreased. SOLUTION: This catalyst comprises a carrier with ZrO2 as the main ingredient, a strontium compound with a crystalline particle diameter of at most 40 nm carried on the carrier and a noble metal carried on the carrier. As solid phase reactivity between ZrO2 and the strontium compound is low, stabilization of the strontium compound is suppressed. In addition, the crystalline particle diameter of the strontium compd. is at most 40 nm, being extremely fine. Therefore, as the surface area is large, activity is improved and as it is suppressed to bring ZrO2 particles into contact with each other, sintering is suppressed and as sulfate salt is easily decomposed, poisoning with sulfur is also suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to relates to a catalyst for purifying exhaust gas for purifying exhaust gas of an automobile, and more particularly to a catalyst for purifying an exhaust gas of the NO _x storage reduction.

[0002]

2. Description of the Related Art In recent years, global warming due to carbon dioxide has become a problem, and reducing carbon dioxide emissions has become an issue. In automobiles, reduction of the amount of carbon dioxide in exhaust gas has become an issue, and lean burn engines have been developed in which fuel is burned lean in an oxygen-excess atmosphere. According to this lean-burn engine, the fuel consumption is improved, so that the amount of carbon dioxide emission can be suppressed.

[0003] When purifying harmful components in the exhaust gas from lean-burn engines, the reduction purification is an oxygen-rich atmosphere because NO _x becomes difficult. Therefore,
The 317,652 discloses an alkali metal with the precious metal, _the NO _x storage-reduction type exhaust gas purifying catalyst carrying _the NO _x storage material selected from alkaline earth metals and rare earth elements is disclosed. Using this NO _x storage-and-reduction type exhaust gas purifying catalyst, by controlling the mixture composition as a stoichiometric-rich atmosphere in a pulsed manner during the lean atmosphere, HC and CO
Oxidation and NO _x reduction can proceed efficiently, and high purification performance can be obtained.

[0004] That NO _x becomes oxidized is NO in the exhaust gas in a lean atmosphere, because it is occluded in _the NO _x storage material NO _x
Emission is suppressed. When the controlled to the stoichiometric-rich atmosphere in a pulsed manner, NO _x from _the NO _x storage material is released, because it is reduced by reaction with reducing components such as HC present in the exhaust gas, NO _x emissions Is suppressed. Therefore it is possible to suppress the emission of the NO _x in all the atmosphere of the lean-rich.

Japanese Patent Application Laid-Open No. 10-156145 discloses a nitrogen oxide trap in which strontium oxide is adhered to alumina, and a noble metal, zirconium and sulfate ions are further adhered thereto.

[0006]

In the [0007] However the conventional NO _x storage-and-reduction type exhaust gas purifying catalyst was reduced for a long time exposure is the _the NO _x purification performance at a high temperature above 700 ° C., in particular 3
There was a problem that the NO _x purification performance was significantly reduced in a low temperature range of 50 ° C. or lower. The following causes are considered as the reason for such a problem. (1) The crystal particles of the NO _x occluding material are sintered to reduce the surface area, and the NO _x occluding elements existing on the crystal surface are reduced, so that the NO _x purification performance is reduced. (2) The carrier reacts with the NO _x storage material to form a composite oxide, which stabilizes the NO _x storage element, thereby lowering the NO _x purification performance. (3) by reaction with SO _x and _the NO _x storage material contained in the exhaust gas sulfites and sulfates are produced, _the NO _x storage capacity of _the NO _x storage material is _the NO _x purification performance decreases to disappear.

The present invention has been made in view of such circumstances, and prevents a decrease in NO _x purification performance after being exposed to a high temperature of 700 ° C. or more, and particularly in a low temperature range of 350 ° C. or less.
It is intended to prevent a decrease in NO _x purification performance.

[0008]

Means for Solving the Problems The features of the exhaust gas purifying catalyst of the present invention that solves the above-mentioned problems include a carrier mainly composed of ZrO ₂ and a strontium compound having a crystal particle diameter of 40 nm or less supported on the carrier. And a noble metal supported on a carrier.

The carrier is LaO ₂ − composed of ZrO ₂ added with La.
Desirably containing ZrO _2, LaO ₂ -ZrO ₂ is 0.1 mol% to 10
It is desirable to contain mol% of La.

The strontium compound is prepared by impregnating a carrier with an aqueous solution of strontium acetate, and then heating the carrier in the air at 250 ° C.
Strontium carbonate obtained by sintering at 400 ° C. to 600 ° C. in the atmosphere after firing.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The exhaust gas purifying catalyst of the present invention is Zr
A strontium compound having a crystal particle diameter of 40 nm or less is supported as a NO _x storage material on a carrier mainly composed of O ₂ . Zr
The solid-state reactivity between O ₂ and the strontium compound is, for example, Zr
O ₂ and barium compounds, Al ₂ O ₃ and strontium compounds,
Lower than solid phase reactivity of Al ₂ O ₃ and barium compounds. Therefore, even when a high temperature of 700 ° C. or more acts, a composite oxide is not easily generated, and thus stabilization of the strontium compound is suppressed.

The crystal particle size of the strontium compound is
Since the surface is very fine as 40 nm or less, the surface area is large, the amount of Sr element exposed on the surface increases, and the NO _x purification activity is improved.

[0013] and by the crystal grain size at the interface of each other ZrO ₂ particles are present the following fine strontium compound 40 nm, sintering contact each other ZrO ₂ particles is suppressed is suppressed. As a result, a decrease in the specific surface area of the carrier is suppressed, and a corresponding decrease in activity is suppressed.

Further, since the crystal particle diameter of the strontium compound is extremely fine, that is, 40 nm or less, the crystal particle diameter of strontium sulfate produced by reacting with SO _x is also extremely fine. Therefore when it is controlled to the stoichiometric-rich atmosphere in a pulsed manner, easily reproduced strontium compound having _the NO _x storage capacity is easily decomposed strontium sulfate, reduction of the NO _x purification activity is suppressed.

[0015] For these reasons, the exhaust gas purifying catalyst of the present invention falls in _the NO _x purification performance when exposed to temperatures higher than 700 ° C. is prevented. And the strontium compound is 350
℃ so high _the NO _x purification activity in the following low temperature range, 700
Even after being exposed to a high temperature of not less than ℃, the reduction of NO _x purification performance in a low temperature range of not more than 350 ° C is prevented.

As the carrier, a carrier containing ZrO ₂ as a main component is used. The carrier may be composed of only ZrO ₂ or may contain another porous oxide. A for other porous oxides
Examples include l ₂ O ₃ , SiO ₂ , and TiO ₂ . However, since the composite oxide is more reactive with higher strontium compound becomes large other porous oxide is likely to generate, preferably present ZrO ₂ is more than 20% by weight relative to the weight of all carrier More desirably, the entire carrier is composed of ZrO ₂ .

[0017] ZrO ₂ constituting the main component of the carrier, it is desirable to include LaO ₂ -ZrO ₂ consisting of ZrO ₂ which La is added.
By containing La, the NO _x storage ability is further improved, and the NO _x purification performance is further improved. Further, thermal stability is improved, and sintering is further suppressed. Therefore LaO ₂ -ZrO ₂ included
Is preferably as large as possible, and all of ZrO ₂ is LaO ₂ -ZrO ₂
It is that it is more desirable to configure the entire undesired carriers from LaO ₂ -ZrO _2. The LaO ₂ -ZrO ₂ is 0.1mol% ~
Preferably, it contains 10 mol% of La. The amount of La added is 0.1mo
the added effect is less than l% is not expressed, is added over 10 mol% activity as a carrier deteriorates is _the NO _x purification performance to decrease.

[0018] Strontium compounds, strontium carbonate, strontium oxide, strontium chloride, strontium sulfate, can be used various compounds such as strontium nitrate, higher _the NO _x storage capacity stability is high strontium carbonate is particularly preferred. Strontium sulfate or the like is converted into strontium carbonate by a reducing action in a rich atmosphere, but it is needless to say that strontium carbonate is preferably supported as strontium carbonate from the beginning.

The crystal particle diameter of the strontium compound supported is extremely fine, 40 nm or less. As a result, various effects as described above are exhibited, and the crystal particle diameter is 40 nm.
If the ratio exceeds, it is difficult to express these effects.

In order to carry the strontium compound, the carrier is impregnated with an aqueous solution of a water-soluble strontium salt such as strontium acetate, and calcined in the air. be able to. As for the firing conditions, it is preferable that after firing at 250 ° C. to 350 ° C., firing is further performed at 400 ° C. to 600 ° C. in the atmosphere. As a result, strontium carbonate having a crystal particle size of 40 nm or less can be easily and reliably dispersed and supported. In addition, the carrier powder and the strontium carbonate powder having a crystal particle diameter of 40 nm or less can be supported by solid-phase mixing and baking it in the air, but such fine strontium carbonate powder is easily aggregated and handled. Since it is also difficult, it is particularly desirable to support from an aqueous solution such as strontium acetate as described above.

The amount of the strontium compound to be carried is desirably in the range of 0.05 to 2.0 mol per liter of the carrier containing ZrO ₂ as a main component. 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. Even if the amount exceeds 2.0 mol, the effect is saturated and a problem occurs due to a decrease in the amount of other components.

[0022] Note that the exhaust gas purifying catalyst of the present invention may carry other NO _x storage material in addition to the strontium compound. As the other NO _x occluding material, at least one compound selected from alkali metals, alkaline earth metals, and rare earth elements can be used. Examples of the alkali metal include lithium, sodium, potassium, rubidium, cesium, and francium. In addition, examples of the alkaline earth metal include barium, beryllium, magnesium, and calcium. Examples of the rare earth element include scandium, yttrium, lanthanum, cerium, praseodymium, and neodymium. However _the NO _x storage material other than strontium compound, since easily produce a composite oxide reacts with ZrO _2, it is desirable not to use as much as possible.

As the noble metal, one or more of Pt, Rh, Pd, and Ir can be used. The supported amount is Pt and
In the case of Pd, the amount is preferably 0.1 to 20.0 g, and particularly preferably 0.5 to 10.0 g, per 120 g of the carrier. Further, in the case of Rh, 0.01 to 10 g is preferable for 120 g of the carrier, and 0.05 to 5.0 g.
Is particularly preferred.

The loading order of the strontium compound and the noble metal is not particularly limited.

[0025]

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

Example 1 ZrO ₂ powder without additives 1
20 g was impregnated with a predetermined amount of a dinitrodiammineplatinum nitric acid aqueous solution having a predetermined concentration, and evaporated to dryness to carry 2 g of Pt. Next, the entire amount of the Pt-supported ZrO ₂ powder is impregnated with a predetermined amount of a strontium acetate aqueous solution having a predetermined concentration, baked at 300 ° C. in the air for 3 hours, and then baked at 500 ° C. in the air for 5 hours, thereby converting strontium carbonate. The catalyst powder of Example 1 was prepared by supporting 0.2 mol. The crystal particle size of strontium carbonate measured by the X-ray diffraction method was 19 nm.

[0027] (Example 2) made of ZrO ₂ which La was added 1 mol%, instead of the ZrO ₂ powder without additives LaO ₂ -ZrO ₂
Example 2 was carried out in the same manner as in Example 1 except that powder was used.
Was prepared. The crystal particle diameter of strontium carbonate measured by the X-ray diffraction method was 19 nm.

Example 3 The same ZrO ₂ powder as in Example 1
20 g was impregnated with a predetermined amount of a dinitrodiammineplatinum nitric acid aqueous solution having a predetermined concentration, and evaporated to dryness to carry 2 g of Pt. Next, the whole amount of Pt-supported ZrO ₂ powder and 0.2 mol (30 g) of strontium carbonate powder having a crystal particle diameter of 30 nm are dry-mixed using a ball mill, and calcined at 500 ° C. for 5 hours in the atmosphere to carry strontium carbonate. Then, a catalyst powder of Example 3 was prepared. The crystal particle diameter of strontium carbonate measured by the X-ray diffraction method was 30 nm.

Comparative Example 1 The same ZrO ₂ powder as in Example 1
20 g was impregnated with a predetermined amount of a dinitrodiammineplatinum nitric acid aqueous solution having a predetermined concentration, and evaporated to dryness to carry 2 g of Pt. On the other hand, 0.2 mol (30 g) of strontium carbonate powder having a crystal particle diameter of 30 nm was calcined in the air at 750 ° C. for 10 hours to obtain strontium carbonate powder having a crystal particle diameter of 55 nm. Then, the whole amount of the Pt-supported ZrO ₂ powder and the whole amount of the strontium carbonate powder were dry-mixed using a ball mill, and calcined at 500 ° C. for 5 hours in the air to prepare a catalyst powder of Comparative Example 1 supporting the strontium carbonate. The crystal particle size of strontium carbonate measured by the X-ray diffraction method was 55 nm.

Comparative Example 2 The same ZrO ₂ powder as in Example 1
20 g was impregnated with a predetermined amount of a dinitrodiammineplatinum nitric acid aqueous solution having a predetermined concentration, and evaporated to dryness to carry 2 g of Pt. On the other hand, 0.2 mol (30 g) of strontium carbonate powder having a crystal particle diameter of 30 nm was calcined in the air at 900 ° C. for 10 hours to obtain strontium carbonate powder having a crystal particle diameter of 100 nm. Then, the total amount of the Pt-supported ZrO ₂ powder and the total amount of the strontium carbonate powder were dry-mixed using a ball mill,
After sintering for a time, a catalyst powder of Comparative Example 2 carrying strontium carbonate was prepared. The crystal particle diameter of strontium carbonate measured by the X-ray diffraction method was 100 nm.

Comparative Example 3 A catalyst powder of Comparative Example 3 carrying 0.2 mol of barium carbonate was prepared in the same manner as in Example 1 except that an aqueous solution of barium acetate was used instead of the aqueous solution of strontium acetate. The crystal particle diameter of barium carbonate measured by the X-ray diffraction method was 44 nm.

Comparative Example 4 A catalyst powder of Comparative Example 4 carrying 0.2 mol of calcium carbonate was prepared in the same manner as in Example 1 except that an aqueous solution of calcium acetate was used instead of the aqueous solution of strontium acetate. The crystal particle diameter of calcium carbonate measured by the X-ray diffraction method was 30 nm.

Comparative Example 5 A catalyst powder of Comparative Example 5 carrying 0.2 mol of magnesium carbonate was prepared in the same manner as in Example 1 except that an aqueous solution of magnesium acetate was used instead of the aqueous solution of strontium acetate. The crystal particle diameter of magnesium carbonate measured by the X-ray diffraction method was 25 nm.

Comparative Example 6 The catalyst of Comparative Example 6 supporting 0.2 mol of strontium carbonate in the same manner as in Example 1 except that Al ₂ O ₃ powder was used instead of ZrO ₂ powder containing no additive. A powder was prepared. The crystal particle size of strontium carbonate measured by the X-ray diffraction method was 50 nm.

Comparative Example 7 Same as Example 1 except that Al ₂ O ₃ powder was used instead of ZrO ₂ powder containing no additive, and barium acetate aqueous solution was used instead of strontium acetate aqueous solution. Thus, a catalyst powder of Comparative Example 7 supporting 0.2 mol of barium carbonate was prepared. The crystal particle diameter of barium carbonate measured by the X-ray diffraction method was 52 nm.

(Comparative Example 8) According to the method described in JP-A-10-156145, 90 g of Al ₂ O ₃ powder was impregnated with a predetermined amount of a strontium nitrate aqueous solution having a predetermined concentration, and evaporated to dryness.
Baked at 5 ° C for 5 hours, 36 g (0.
35 mol). Thereafter, the mixture was impregnated with a mixed aqueous solution of hexachloroplatinic acid, zirconium chloride and sulfuric acid and evaporated to dryness to carry ₂ g of Pt, 30 g of ZrO ₂ and 70 g of sulfate ions.

<Test / Evaluation> Each of the obtained catalyst powders was pelletized by a conventional method and subjected to the following tests.

First, each of the pellet catalysts was charged into a durability test apparatus, and a rich model gas of A / F = 14.0 and a lean model gas of A / F = 22.0 shown in Table 1 were alternately flowed for 4 minutes at 750 ° C. An endurance test of heating for 10 hours was performed.

[0039]

[Table 1]

Each of the pellet catalysts after the endurance test was mounted on a normal-pressure fixed-bed flow reactor, and a rich pretreatment (5 minutes) shown in FIG. 1 was performed using a lean and rich atmosphere model gas shown in Table 2. The mixture was circulated in the order of lean (20 minutes) → rich spike (3 seconds) → lean (10 minutes), and the NO _x concentration in the gas discharged from the catalyst during that time was measured. The temperature of the gas containing the catalyst is 350 ° C.

[0041]

[Table 2]

Then, from the area of the solid portion in FIG.
Saturated _the NO _x storage amount and to calculate the rich spike after _the NO _x storage amount. Table 3 shows the results. Note since _the NO _x purification performance in the actual vehicle as after the rich spike _the NO _x storage amount is large is known to be high, the _the NO _x storage amount after the rich spike
It was used as a main index of NO _x purification performance.

[0043]

[Table 3]

[0044] From Table 3, the catalyst of each example is excellent in saturation _the NO _x storage amount and the rich spike after _the NO _x storage amount are both high, NO _x purifying performance after the durability test compared to the catalyst of Comparative Example You can see that. Hereinafter, the data in Table 3 will be analyzed in more detail.

[0045] (1) Example 1 after impact endurance test of the crystal grain size, Example 3, the relationship between the crystal grain size and rich spike after _the NO _x storage amount of strontium carbonate in the catalyst of Comparative Example 1 and Comparative Example 2 Is shown in FIG.

FIG. 2 shows that when the crystal particle diameter of strontium carbonate is about 40 nm or less, it is practically sufficient even after the durability test.
It can be seen that _the NO _x purification activity.

(2) Influence of NO _x storage element FIG. 3 shows the NO _x storage amount after rich spike of the catalysts of Example 1, Comparative Example 3, Comparative Example 4 and Comparative Example 5 after the durability test.

FIG. 3 clearly shows that Sr is the most preferable as the NO _x storage element. In the case of Ba, Example 1
It is considered that the NO _x purification performance was lower than that of Sr because the crystal particle diameter of barium carbonate was large even when the barium carbonate was manufactured in the same manner as described above. In the case of Ca and Mg, the crystal grain size of the carbonate salts are equivalent to the case of Sr, Ca and Mg is low is _the NO _x purification performance compared to a low basicity when compared to the Sr Sr It is considered that

(3) Influence of Carrier FIG. 4 shows the NO _x storage amounts after rich spike of the catalysts of Examples 1, 2 and Comparative Example 6 after the durability test.

FIG. 4 shows that the ZrO ₂ carrier has higher NO _x purification performance than the Al ₂ O ₃ carrier, and that the LaO ₂ -ZrO ₂ carrier composed of ZrO ₂ added with La is particularly preferable. it is obvious. In the case of ZrO ₂ or LaO ₂ -ZrO ₂ carriers, Al ₂
It is considered that the crystal particle diameter of strontium carbonate is smaller than that of O ₃ , and the surface has a large number of Sr atoms effective for occlusion of NO _x , thereby exhibiting high NO _x purification performance.

[0051] (4) from the difference Table 3 and JP-A 10-156145 and JP-the catalysts Examples 1 to 3, it saturated _the NO _x storage amount in comparison with the catalyst of Comparative Example 8 and the rich spike after _the NO _x storage amount There are many. It is apparent that this is the effect of using the carrier containing ZrO ₂ as a main component and carrying it as fine strontium carbonate in Examples 1 to 3. That is, Comparative Example 8
In the catalyst of (1), although the carrier contains ZrO ₂ but carries Al ₂ O ₃ as a main component, it is judged that the NO _x purification performance in a low temperature range of 350 ° C. or less after the durability test is low.

[0052]

According to the catalyst of the present invention, 700 ° C.
There is almost no decrease in NO _x purification performance even when exposed to temperatures above 350 ° C even after exposure to temperatures above 700 ° C
The following shows the high _the NO _x purification performance at low temperature range, is excellent in durability.

[Brief description of the drawings]

1 is an explanatory view for explaining the evaluation method of the saturated _the NO _x storage amount and the rich spike after _the NO _x storage amount.

FIG. 2 is a graph showing the relationship between the crystal particle diameter of strontium carbonate and the NO _x storage amount after rich spike.

FIG. 3 is a graph showing a relationship between NO _x storage element species and NO _x storage amount after rich spike.

4 is a graph showing the relationship between the carrier species and rich spike after _the NO _x storage amount.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor ▲ Taka ▼ Naoki Hashi 41, Chukku Yokomichi, Oku-cho, Nagakute-cho, Aichi-gun 1 Toyota Central Research Laboratory Co., Ltd. No. 1 Toyota Motor Co., Ltd. (72) Inventor Yuichi Sobue 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Co., Ltd. F-term (reference) 4D048 AA06 BA08X BA08Y BA14X BA14Y BA18X BA18Y BA30Y BA41X BA41Y BA42X BA42Y BB17 4G069 AA01 AA03 AA12 BA01B BA05A BA05B BB04A BB04B BB06A BB06B BB16B BC42A BC42B BC51A BC51B BC69A BC71B BC72B BC74B BC75B CA03 CA13 EB18X EB18Y EB19

Claims (3)

[Claims] 1. A carrier comprising zirconia (ZrO ₂ ) as a main component, a strontium compound supported on the carrier having a crystal particle diameter of 40 nm or less, and a noble metal supported on the carrier. Exhaust gas purification catalyst. 2. The carrier is composed of ZrO ₂ to which La is added.
LaO ₂ -ZrO exhaust gas purifying catalyst according to claim 1, characterized in that it comprises a _2. 3. The method according to claim 1, wherein the LaO ₂ —ZrO ₂ is 0.1 mol% to 10 mol% of La.
The exhaust gas purifying catalyst according to claim 2, comprising: