JP2002079094A - HIGH TEMPERATURE NOx ABSORBING AND REDUCTION TYPE CATALYST - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high temperature NOx absorbing and reduction catalyst having a high NOx absorbing capacity even after a high temperature durability test at 750 deg.C for 5 hr and capable of sufficiently absorbing NOx even in a high temperature range of 600 deg.C or higher. SOLUTION: A noble metal and an NOx occluding material are supported on a carrier which comprises a composite oxide having a magnesia/alumin spinnel structure and has a specific surface area of 50 m2/g or more. This carrier has no lattice defect and magnesia is not detected even by X-ray diffraction and the lattice constant of the carrier is near to a theoretical value. Since the basicity of the carrier is high and magnesium is uniformly contained in a crystal lattice, the reactivity with the NOx occluding material basic in the same way is low and the solid phase reaction of the NOx occluding material and the carrier is suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to relates to _the NO _x storage-reduction type catalyst, in particular has a high _the NO _x storage capacity even after high-temperature durability, high temperature NO _x storage-and-reduction type showing high _the NO _x purification rate at the high temperature range Regarding the catalyst.

[0002]

2. Description of the Related Art In recent years, carbon dioxide (CO ₂ ) in exhaust gas discharged from an internal combustion engine such as an automobile has been a problem from the viewpoint of protection of the global environment. Therefore, in order to reduce CO ₂ , which is a greenhouse gas, so-called lean burn in which lean combustion is performed in an oxygen-excess atmosphere has been put to practical use. In this lean burn, the amount of fuel used can be reduced, and the amount of CO ₂ emitted as exhaust gas can be reduced.

[0005] such as in JP-A 5-317625 Patent Publication, _the NO _x storage material and NO _x storage-and-reduction type exhaust gas purifying catalyst in which the noble metal was supported on a porous carrier such as alumina, such as alkaline earth metals is disclosed ing. According to this catalyst, by controlling such that the stoichiometric or rich side air-fuel ratio from the lean side in a pulsed manner (rich spike), NO _x is occluded in _the NO _x storage material in the lean side, it is stoichiometric or to be cleaned reacts with the reducing components such as HC and CO in the rich side, it is possible to efficiently purify NO _x even in the lean burn.

According to recent studies, SO ₂ produced by combustion of sulfur components in fuel is oxidized on a catalyst, and the SO ₂ reacts with the NO _x storage material to reduce the NO _x storage capacity. has become apparent, this is referred to as sulfur poisoning of the NO _x storage material. In order to suppress the sulfur poisoning, various solutions have been proposed in JP-A-11-76838, JP-A-10-263416 and JP-A-10-192707.

[0005]

The temperature of exhaust gas discharged from automobiles in recent years has been high due to the increase in the output of direct-injection gasoline engines or the increase in high-speed running. It is in a situation of high temperature atmosphere. However, with the conventional NO _x storage reduction catalyst, 6
There is a problem that the NO _x storage and reduction ability in a high temperature atmosphere of 00 to 700 ° C. is low. This cause is believed to contribute to _the NO _x storage material by solid-phase reaction with _the NO _x storage material and the carrier is changed to the stable complex oxide.

For example, potassium is highly alkaline and has high NO
_Although it has _x storage capacity, its NO _x storage capacity drops sharply as the incoming gas temperature rises above 500 ° C.
Above 00 ° C., it becomes almost difficult to store NO _x . Therefore, it is conceivable to increase the amount of potassium supported.
After a high-temperature durability test at about 0 ° C. for about 5 hours, it has been clarified that the NO _x storage ability is greatly reduced.

[0007] The present invention has been made in view of such circumstances, and is high even after a high-temperature durability test at 750 ° C for 5 hours.
It has _the NO _x storage ability, and to provide a NO _x storage-and-reduction type catalyst which can sufficiently occluding NO _x in the high temperature range of not lower than 600 ° C..

[0008]

The high-temperature NO _x storage-reduction catalyst according to claim 1 is characterized in that it is a carrier comprising a composite oxide having a magnesia-alumina spinel structure. It has a specific surface area of 50 m ² / g or more, and comprises a noble metal supported on a carrier and at least one NO _x occluding material selected from an alkali metal, an alkaline earth metal and a rare earth element supported on the carrier. is there.

In the above catalyst, the lattice constant of the {800} plane of the composite oxide is a theoretical value of 8.083 ± 0.02 °, and the peak of the strongest magnesia line detected by X-ray diffraction is the strongest line of magnesia-alumina spinel. And the composition ratio of magnesium (Mg) and aluminum (Al)
It is desirable that Mg / 2Al = 1 ± 0.05 (molar ratio).

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The high-temperature NO _x storage-reduction catalyst of the present invention uses a support made of a composite oxide having a magnesia-alumina spinel (MgAl ₂ O ₄ ) structure. Since this composite oxide itself has high heat resistance and is relatively easy to obtain one having a high specific surface area, it has a necessary condition as a catalyst carrier.

[0011] The high temperature NO _x storage-and-reduction type catalyst of the present invention has the greatest feature a specific surface area of the support at which a 50 m ² / g or more. As such, it has a magnesia-alumina spinel structure with a high specific surface area, no lattice defects,
No magnesia is detected by X-ray diffraction, and a complex oxide having a lattice constant close to the theoretical value is used as the carrier. And magnesium, which is an alkaline earth metal, has a strong basicity,
In such a composite oxide, since its magnesium is uniformly contained in the crystal lattice, it is considered that the reactivity with the similarly basic NO _x storage material is low. Therefore NO _x for consumption storage component and _the NO _x storage material by reaction with the carrier is suppressed, sufficient even in a high temperature range of not lower than 600 ° C. NO _x
Can be occluded. The carrier and NO after high temperature durability
_Since the reaction with the _x- occluding material hardly occurs, the NO _x storage-reduction catalyst of the present invention has excellent heat resistance and shows high NO _x storage ability even after a high-temperature durability test at 750 ° C. for 5 hours.

Such a carrier is described, for example, in JP-A-2000-12852.
It can be produced by using the production method described in JP-A-7. That is, magnesium hydroxide and aluminum hydroxide are mixed at an atomic ratio of Al: Mg = 2: 1, and are mechanically mixed and pulverized to form a mixed composition of a hydroxide containing a composite hydroxide, By heating it, a composite oxide having a magnesia-alumina spinel structure is obtained. mixture·
At the time of grinding, sufficient energy is mechanically applied to the mixture to convert at least a part of both hydroxides to a composite hydroxide and to sufficiently reduce the particle size of 80% by weight or more to 100 nm or less. To fineness. Then, by heating the obtained mixed composition at a temperature of 1100 ° C. or less, a support made of a composite oxide having a magnesia-alumina spinel structure having a specific surface area of 50 m 2 / g or more can be obtained. This carrier can also be produced by a sol-gel method or a coprecipitation method.

The carrier must have a specific surface area of at least 50 m ² / g, preferably at least 80 m ² / g, more preferably at least 100 m ² / g. It improves the dispersibility of the higher specific surface area greater _the NO _x storage materials and precious metals, exhibit both high _the NO _x storage ability in a high temperature region, high _the NO _x storage ability even after the durability is expressed.

The noble metals supported on this carrier are Pt, Rh,
It can be selected from Pd, Ru, Ir and the like. In the high temperature range of 550 ° C. or higher, there is no difference in the degree of activity by the noble metal species. However, at temperatures of 500 ° C. or lower, Pt is particularly high in NO oxidation activity, so it is preferable to use Pt. The amount of the noble metal carried is desirably 2 g or more per liter of the catalyst. Oxidation of NO becomes insufficient is less than the amount carried is 2 g / L, NO _x storage ability is lowered.

The NO _x occluding material carried on the carrier is selected from alkali metals, alkaline earth metals and rare earth elements. Examples of the alkali metal include Li, Na, K, and Cs. Alkaline earth metals refer to Group 2A elements of the periodic table, and B
a, Be, Mg, Ca, Sr and the like are exemplified. Examples of the rare earth element include Sc, Y, La, Ce, Pr, Nd, Dy, and Yb. Among them, K which has high alkalinity and does not fly even at high temperatures
Is particularly preferred.

Further loading amount of _the NO _x storage material is preferably set to 0.1 to 2 moles per liter of catalyst. If the supported amount is less than this range, the NO _x occlusion ability becomes insufficient. If the supported amount is more than this range, the activity may be reduced by covering the noble metal or accelerating the grain growth of the noble metal. In the case of K, it is desirable to support 0.3 mol or more, more preferably 0.6 mol or more, per liter of the catalyst. If it is less than 0.3 mol / L, the NO _x storage amount will drop sharply. K
The upper limit of the supported amount of is not particularly limited, but is preferably about 1 mol / L because the NO _x storage ability is almost saturated at about 1 mol / L.

[0017] using a high-temperature NO _x storage-and-reduction type catalyst of the present invention, the NO that is contacted with the air-fuel ratio (A / F) is lean atmosphere is combusted in 15 or more exhaust gas contained in an exhaust gas NO ₂
And store it in the NO _x storage material. And by intermittently rich spike for stoichiometric-fuel excessively varies the air-fuel ratio, to reduce and purify NO _x occluded in _the NO _x storage material.

The high-temperature NO _x storage-reduction type catalyst of the present invention is 750
NO after rich spike after endurance test at 5 ° C for 5 hours
_{x The} occlusion amount can be 1200 mg / L or more at an inlet gas temperature of 600 ° C. Even after the durability test of 5 hours at 750 °, NO _x storage amount is large catalyst in such high temperature zone, absent the prior art. This is due to the solid phase reaction between _the NO _x storage material and the carrier is suppressed.

[0019]

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

Example 1 An aluminum hydroxide powder having an average particle diameter of 17 μm and a magnesium hydroxide powder having an average particle diameter of 19 μm were mixed so that the atomic ratio Al: Mg was 2: 1.
The mixture was dispersed with 2 liters of water by stirring with a propeller stirrer for 5 minutes. Next, using a medium stirring mill (Dyno mill) made of ZrO ₂ pulverized media, the stirring blade rotation speed was 4200 rpm.
For 20 hours (in this case, the acceleration applied to the grinding medium is 760 times the gravitational acceleration), and most of the aluminum hydroxide and most of the magnesium hydroxide were converted into the crystalline form of the composite hydroxide. Thereafter, the powder was dried at 150 ° C. for 10 hours to remove water, thereby obtaining a powder. (Agitator blade diameter 2R = 7.7c
m, rotation speed = 4200rpm, circumferential speed V = π x 7.7 x 4200/6
0 = 1693.3 cm / s, circular motion acceleration α = V ² / R = 744760.
35 cm / s / s, so α / g = 759.49) According to X-ray diffraction, the crystalline phase of this powder is MgAl ₂ (OH) ₈ ,
_{Mg 2 Al (OH) 10 ·} xH 2 O, Mg 4 Al 2 (OH) 14 · 3H 2 and 3 kinds of complex hydroxide of O, consist of a mixture of a small amount of Mg (OH) ₂ and unidentified phases I understood. The peak of ZrO ₂ was mixed from ZrO ₂ made grinding media was detected. As a result of a particle size analysis by a dynamic light scattering method, it was found that this powder had a particle size of 90% by weight or more and a particle size of 100 nm or less.

50 g of a powder containing the composite hydroxide as a seed component
Was placed in an alumina crucible and heat-treated at 1000 ° C. for 5 hours to obtain spinel powder. The obtained spinel powder was a MgAl ₂ O ₄ spinel having a specific surface area of 96 m ² / g and an atomic ratio Mg: Al of 1: 2. The spinel powder has a lattice constant of {800} plane of 8.083 ± 0.02% of theoretical value, and the peak of the strongest line of magnesia detected by X-ray diffraction method is 1/20 or less of the strongest line of magnesia-alumina spinel. And the composition ratio between magnesium (Mg) and aluminum (Al) was Mg / 2Al = 1 ± 0.05 (molar ratio).

This spinel powder is slurried by a standard method,
Cordierite honeycomb substrate coated with 40g /
L was formed. Next, a predetermined amount of an aqueous solution of dinitrodiaminoplatinic nitric acid having a predetermined concentration is impregnated into the coat layer, and evaporated to dryness to form Pt.
Was carried. The supported amount of Pt is 2 g per liter of the honeycomb substrate. Further, a predetermined amount of a potassium acetate aqueous solution having a predetermined concentration was impregnated into the coat layer, and evaporated to dryness to carry K. The loading amount of K is per liter of honeycomb substrate.
0.3 mol. In addition, the amount of K carried was 0.6 mol / L and 1
Three kinds of catalysts were prepared in the same manner except that mol / L was used.

Comparative Example 1 SAE Pa was used instead of spinel powder.
Pt and K were loaded in the same manner as in Example 1 except that the carrier powder described in per200-01-1196 was used. The supported amount of Pt was as high as 10 g per liter of the honeycomb substrate, and the supported amounts of K were prepared in four types: 0.3 mol / L, 0.6 mol / L, 1.5 mol / L and 2 mol / L.

<Test / Evaluation> The catalysts of the examples and the comparative examples were respectively placed in an evaluation apparatus, and the lean gas and the rich spike model gas shown in Table 1 were used.
The NO _x storage amounts after the rich spike at 00 ° C. and 700 ° C. were measured, respectively. The results are shown in FIGS.
The NO _x occlusion amount after the rich spike refers to the occlusion amount until the saturated state is reached after the rich spike is introduced into the saturated catalyst for 3 seconds.

For each catalyst, a durability test was conducted at 750 ° C. for 5 hours using the lean and rich model gases shown in Table 2, and then NO _x after rich spike was carried out in the same manner as described above.
The amount of occlusion was measured. The results are shown in FIGS.

[0026]

[Table 1]

[0027]

[Table 2]

Example 1 in which the amount of supported K is 0.6 mol / L
And the catalyst of Comparative Example 1 in which the amount of supported K is 1.5 mol / L, NO _x was absorbed at 400 ° C. in a model gas shown in Table 3 until the NO _x was saturated. A NO _x thermal desorption test was conducted in which the temperature was raised to 700 ° C. in a model gas at a rate of 10 ° C./min. The continuously measure the concentration of NO _x in the gas leaving between them, and the results are shown in Figure 5. This test was performed on the catalyst at the initial stage and after the durability test.

[0029]

[Table 3]

As can be seen from FIGS. 1 and 2, the catalyst of Example 1 was compared with Comparative Example 1 when the amount of K supported was the same.
NO _x after the rich spike in a high temperature range of 600 to 700 ° C.
The amount of occlusion is much higher. However, even with the catalyst of Comparative Example 1, the amount of NO _x stored after the initial rich spike can be made equal to that of Example 1 by increasing the amount of K carried. As can be seen from FIG. 5, the NO _x holding power also shows the same performance as that of the first embodiment.

However, it is not preferable to increase the amount of supported K, since the growth of Pt grains is promoted or the Pt is covered with K, which may lower the catalytic activity.

On the other hand, when the characteristics after the durability test are compared, as can be seen from FIGS. 3 and 4, even if the amount of supported K is increased in the catalyst of Comparative Example 1, the NO _x storage amount after the rich spike was not reduced. It is far from the catalyst of Example 1. Further, from FIG. 5, the NO _x holding power is also significantly reduced.

That is, with the catalyst of Example 1, 5
Since the reaction between the carrier and K of the spinel structure is suppressed even after the durability test time, reduction of the NO _x holding force is suppressed, so that entering at gas temperature 600 ° C. after 1200 mg / L or more of the rich spike It is considered that the NO _x storage amount was expressed. While in the catalyst of Comparative Example 1 is considered to K majority of the solid phase reaction between the carrier and K during the durability test has lost _the NO _x storage capacity.

Example 2 Among the catalysts of Example 1, those having a K loading of 0.6 g / L were used as the catalyst of Example 2.

(Example 3) The same configuration as in Example 2 except that 2 g / L of Pd was used instead of Pt.

(Example 4) The structure is the same as that of Example 2 except that 2 g / L of Rh is used instead of Pt.

<Test / Evaluation> The catalysts of Examples 2 to 4 were respectively placed in an evaluation device, and the temperature was raised to 700 ° C. at a rate of 10 ° C./min while flowing the lean model gas shown in Table 1. Was continuously measured. FIG. 6 shows the results. The NO oxidation rate is determined by comparing NO and NO _x (NO + N
After measuring the amount of O ₂ ) and calculating the amount of NO ₂ from these differences, the value is calculated as 100 × NO ₂ amount / (NO amount + NO ₂ amount).

As can be seen from FIG. 6, there is no difference between noble metal species in the high temperature range of 550 ° C. or higher. However, below 500 ° C., the NO oxidation rate is Pt>Rh> Pd, and it is clear that Pt has particularly high NO oxidation activity.

(Example 5) Among the catalysts of Example 1, those having a K loading of 0.3 mol / L were used as the catalyst of Example 5.

Comparative Example 2 A catalyst in which 2 g / L of Pt and 0.3 mol / L of K were supported on a honeycomb substrate having a γ-Al ₂ O ₃ coat layer was used as a catalyst of Comparative Example 2.

(Comparative Example 3) Pt was added to a honeycomb substrate having a coat layer made of spinel powder having a specific surface area of several m ² / g.
A catalyst supporting 0.3 mol / L of g / L and K was used as a catalyst of Comparative Example 3. The spinel powder used had a magnesia peak detected as a result of X-ray diffraction.

<Test / Evaluation> Using the catalysts of Example 5 and Comparative Examples 2 and 3, a durability test was performed in the same manner as in Example 1 and Comparative Example 1. Then, using the lean gas and rich spike model gas shown in Table 1, the inlet gas temperature was 600 ° C and
The NO _x storage amount after the rich spike at 700 ° C. was measured. FIG. 7 shows the results.

FIG. 7 shows that the catalysts of Comparative Examples 2 and 3 had a remarkably low NO _x storage amount in a high temperature range, while the catalyst of Example 5 was stable from a low temperature to a high temperature despite the high temperature durability test. It is clear that the obtained NO _x storage ability was exhibited. It is clear that this is due to the carrier, and the catalyst of Example 5 shows high NO _x storage ability because of its large specific surface area, its lattice constant is close to the theoretical value, and also in X-ray diffraction. It is clear that this is due to the use of a composite oxide having a spinel structure in which no magnesia peak is detected.

[0044]

According to the high-temperature NO _x storage-reduction catalyst of the present invention, high NO _x storage capacity is exhibited even in a high temperature range of 600 ° C. or higher, and its characteristics are not significantly degraded even after high-temperature durability. Thus the NO _x in the exhaust gas from the engine exhaust gas temperature becomes high, it is possible to purify a long period of time efficiently.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the amount of K carried at an initial gas temperature of 600 ° C. and the amount of NO _x stored after a rich spike.

FIG. 2 is a graph showing the relationship between the amount of K carried at an initial gas temperature of 700 ° C. and the amount of NO _x stored after a rich spike.

FIG. 3 is a graph showing the relationship between the amount of K carried and the amount of NO _x stored after rich spike at an inlet gas temperature of 600 ° C. of the catalyst after durability.

FIG. 4 is a graph showing the relationship between the amount of K carried and the amount of NO _x stored after a rich spike at an inlet gas temperature of 700 ° C. of the catalyst after durability.

[5] gas temperature entering the NO _x temperature-programmed desorption test with NO _x
It is a graph which shows the relationship with a density.

FIG. 6 is a graph showing a relationship between a noble metal species and a NO oxidation rate at each temperature.

FIG. 7 is a graph showing the results of measuring the NO _x occlusion amount after rich spikes at the respective temperatures after the durability test in the catalysts of Example 5 and Comparative Examples 2 and 3.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F01N 3/08 F01N 3/10 A 3/10 3/28 301C 3/28 301 301P B01D 53/36 102B ( 72) Inventor Chiwa Ando 1-41, Chuchu-ji, Yokomichi, Nagakute-cho, Aichi-gun, Aichi F-term in Toyota Central R & D Laboratories Co., Ltd. BA03X BA03Y BA10X BA14X BA14Y BA15Y BA30X BA30Y BA31X BA33X BB02 EA04 4G066 AA02B AA13B AA16C AA20C AA66C AD10B BA07 BA26 BA32 BA36 BA38 CA28 BC02 BC02 BC02 BC02 BC02A02 CA03 CA07 CA13 DA06 EA19 EC24 FA02 FB14 FB16 FB19

Claims (6)

[Claims] 1. A carrier comprising a composite oxide having a magnesia / alumina spinel structure, wherein the carrier has a specific surface area of 50 m ² / g or more, and a noble metal supported on the carrier and a carrier supported on the carrier. A high-temperature NO _x storage-reduction catalyst, comprising at least one kind of NO _x storage material selected from alkali metals, alkaline earth metals and rare earth elements. 2. The composite oxide has a lattice constant of {800} plane of a theoretical value of 8.083 ± 0.02 °, and the peak of the strongest line of magnesia detected by the X-ray diffraction method is the peak of the strongest line of magnesia-alumina spinel. 1/20 or less, and the composition ratio of magnesium (Mg) and aluminum (Al) is Mg / 2
Al = 1 ± 0.05 high temperature NO _x storage-and-reduction type catalyst according to claim 1, characterized in that the (molar ratio). 3. After the endurance test at 750 ° C. for 5 hours,
NO after touch spike _x When the storage amount is 600 ° C
2. The composition according to claim 1, wherein the amount is 1200 mg / L or more.
High temperature NO_x Storage reduction catalyst. 4. A high-temperature NO _x storage-and-reduction type catalyst according to claim 1, wherein the _the NO _x storage material including at least potassium. 5. The high-temperature NO _x storage-reduction catalyst according to claim 4, wherein the amount of the supported potassium is 0.3 mol / L or more. 6. The high-temperature NO according to claim 1, wherein the noble metal is platinum and is supported at 2 g / L or more.
_x Storage-reduction catalyst.