JP3300053B2 - Exhaust gas purification catalyst - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying catalyst which has improved exhaust gas purifying characteristics in a low temperature range and has high heat resistance.

[0002]

2. Description of the Related Art C as an exhaust gas purifying catalyst for an engine
A three-way catalyst that simultaneously oxidizes O and HC and reduces NOx is used. It is known that this three-way catalyst has high purification efficiency when the air-fuel ratio of the engine is around 14.7, which is the stoichiometric air-fuel ratio. Have been.

On the other hand, lean burn engines have been put into practical use in order to comply with fuel regulations for engines.
Since the exhaust gas in such a lean burn system has a high concentration of oxygen, the three-way catalyst cannot effectively remove NOx.

Therefore, an active species such as Cu is supported on a metal-containing silicate as a catalyst capable of decomposing and purifying NOx in exhaust gas into N ₂ and O ₂ under an oxygen-rich atmosphere. So-called zeolite catalysts hold promise. Further, with respect to the above-mentioned zeolite catalyst, various measures such as selection of active species have been studied in order to always provide an exhaust gas purifying catalyst having excellent NOx removing ability in response to various states of exhaust gas.

For example, Japanese Patent Laid-Open Publication No. Hei 3-202157 proposes a technique for efficiently removing NOx in an oxygen-rich atmosphere. The technology disclosed in the above-mentioned publication is to provide a catalyst in which zeolite supports Cu and at least one kind of alkaline earth metal and one or more kinds of rare earth metals, thereby obtaining excellent NOx in a temperature range of 800 ° C. or less. An object of the present invention is to obtain a catalyst having a purifying ability and durability that can be used for a long time.

[0006]

SUMMARY OF THE INVENTION However, NOx
Cu ion-exchanged zeolite catalysts, which are considered to be useful catalysts for effective removal of
In spite of the fact that the NOx purification rate may exceed 0%, the NOx purification under the high oxygen concentration atmosphere by the actual operation of the lean burn engine may cause a change in the properties of the actual exhaust gas. It is inevitable that the NOx purification rate decreases.

Further, in such a Cu ion exchanged zeolite catalyst, the temperature at which the NOx purifying activity is exhibited is 350 to 40.
Since the NOx purification rate in a low temperature range decreases due to the high temperature of 0 ° C. and the tendency of the NOx purification activation temperature range to shift to a high temperature side due to actual operation, the NOx purification in a high oxygen concentration atmosphere is performed. There is a problem that the integrated value decreases.

Further, a catalyst in which Cu, an alkaline earth metal, and a rare earth metal are supported on a metal-containing silicate as described in the above-mentioned JP-A-3-202157 has low heat resistance. Although it is recognized that the purification rate is improved, there is a problem that the purification rate in a low temperature range such as 300 ° C. or less during the purification of NOx is inferior.

On the other hand, when the vehicle is equipped with a lean burn engine and the vehicle is running under repeated running and stopping conditions, the temperature of the catalyst inlet gas of the engine exhaust gas is lowered to improve the NOx purification characteristics in a low temperature range. Therefore, it is necessary for the catalyst to have improved purification characteristics in a low temperature range and have stable characteristics with high heat resistance.

Incidentally, in order to obtain excellent exhaust gas purification characteristics in a low temperature range, it is effective to use a catalyst carrying an active species such as a noble metal (white metal element) that acts in a low temperature range. Are known.

The present inventor has conducted intensive studies on an exhaust gas purifying catalyst which can be expected to improve NOx purifying characteristics in a low temperature range by supporting such a noble metal as an active species, and among the noble metals, Pt and Ir In the case of a catalyst supported on a metal-containing silicate as an active species-supporting matrix, both of which are active species, Ir has a synergistic effect of Pt and Ir, although Ir itself has a poor NOx purification activity as compared with Pt. Thus, it was confirmed that high activity was obtained in a low temperature range, and thereby excellent NOx purification characteristics were obtained.

Furthermore, Pt and I carried as active species
As a result of research on the behavior with r, it has been determined that Pt and Ir coexist within a range where they can exert a synergistic effect with each other in order to obtain the above-described catalytic function. However, if Pt and Ir are too close to each other, sintering of active species is likely to occur.
It has been found that there is a concern that the heat resistance is rather negative.

In view of the above, it is an object of the present invention to provide an exhaust gas purifying catalyst which has improved exhaust gas purifying characteristics in a low temperature range and has high heat resistance.

[0014]

Means and Action for Solving the Problems Here,
The present inventor has taken measures against Ir which is apt to be thermally degraded by supporting the Ir on a heat-resistant active species supporting base material, for example, an aluminosilicate having a large silica content and a large Caban ratio. Suppressing the occurrence of sintering, and at the same time, considering the fact that Ir made in this way can avoid excessively close to Pt which is easy to sinter at high temperature, Pt and Ir as active species are considered. Various experiments were carried out on the supporting forms of the two, which can exhibit the synergistic effect of the present invention, and the present invention was completed.

That is, in order to achieve the above object, the invention of claim 1 prevents Pt and Ir as active species from coexisting excessively close to each other when Pt and Ir coexist, so that Pt and Ir can be mutually separated. Within a range where synergistic effects are exerted, each is supported in a state of being separated to some extent, and at the same time, thermal degradation of Ir carried as an active species together with Pt is prevented, and the synergistic effect on Pt is enhanced. An active species supporting base material is used, and the active species supporting base material itself is intended to maintain NOx purification performance.

Specifically, a solution taken by the invention of claim 1 is that a first catalyst body in which Ir is indispensably supported as an active species on an active species supporting base material, and an active species supporting base material on an active species supporting base material. The above-mentioned first catalyst is intended for an exhaust gas purifying catalyst in which a second catalyst body in which Pt is indispensably supported as a seed is mixed.
The active species supporting base material of the catalyst is a metal-containing silicate having Si and Me (Me is a metal element other than Si) as a metal forming a crystal skeleton, and the Si / Me ratio is defined as an oxide ratio. The structure is such that it is an Na type having an Na type of 70 or more or an H type having an Si / Me ratio of 30 or more as an oxide ratio.

Further, the invention according to claim 2 is industrially applicable by selecting and applying a metal-containing silicate having a required Si-Me ratio in aluminosilicate as a metal-containing silicate having a ratio of Si to Me of a predetermined value or more. Specifically, in the structure of claim 1, the metal-containing silicate is an aluminosilicate.

The aluminosilicate that indispensably supports the above-mentioned Ir, for example, having a large ban ratio, has high heat resistance, so that the active species-supporting base material in which Pt is indispensably supported is usually a metal-containing silicate and a high specific surface area γ. -Alumina can be used as appropriate. In particular, when γ-alumina is used as the active species supporting base material that indispensably supports Pt, the solid acid strength of the catalyst increases, so that improvement in durability can be expected.

The metal-containing silicate supporting Pt and Ir is preferably an aluminosilicate (zeolite) using Al as a metal forming a skeleton of the crystal.
If necessary, Ga,
A metal-containing silicate using a metal such as Ce, Mn, or Tb as a skeleton forming material can also be applied. A-type, X-type, Y-type, mordenite,
ZSM-5 and the like are preferred. Further, as described above, as the metal-containing silicate, an H-type metal-containing silicate in which the cation species is H ⁺ is preferably used together with the Na-type metal-containing silicate in which the cation species is Na.

Further, γ-alumina as an active species supporting base material on which Pt is indispensably supported has a specific surface area (m ² / g)
Having a high specific surface area of 280 or more and preferably about 340 is used.

As described above, in the exhaust gas purifying catalyst according to the present invention, Pt and Ir as catalytically active species are basically supported on mutually different active species supporting base materials, and thereafter these Pt are supported. Catalytic body composed of active species-supporting matrix and I
An exhaust gas purifying catalyst is obtained by mixing with a catalyst body made of an active species supporting base material supporting r.

As a result, the active species-supporting base material on which Ir is supported has a high ratio of Si to Me, which is highly resistant to deterioration due to heat, in other words, for example, the Cavane ratio (SiO ₂ / SiO ₂₎ converted to the oxide ratio. Since the metal-containing silicate having large Al ₂ O ₃ ) is used as described above, heat resistance is improved and thermal deterioration is reduced. Therefore, high activation of Pt is ensured over a wide temperature range. Note that when a γ-alumina-based supporting base material is used as the supporting base material for Ir, the above-described effects are hardly obtained.

[0023]

Embodiments of the present invention will be described below.

<Example 1> Iridium trichloride weighed to have a weight of 0.186 g and Na having a Caban ratio of 70
Aluminosilicate (ZSM-5) (5 g) was mixed with ethanol, sufficiently stirred at 90 ° C. for 3 hours, and dried to obtain an Ir-supported metal-containing silicate powder as a first catalyst.

Further, similarly to divalent platinum ammine weighed to be 0.64 g, Na-type Z having a Caban ratio of 70 was used.
After mixing with 5 g of SM-5 and stirring thoroughly, 90 ° C. × 3
After drying over a period of time, a Pt-supported aluminosilicate powder as a second catalyst body was obtained.

The Ir-supported aluminosilicate powder and the Pt-supported aluminosilicate powder were weighed so as to have a weight ratio of 1: 1 and then mixed using ion-exchanged water. A binder, for example, about 20% by weight of hydrated alumina is added to this mixture to form a slurry. This slurry is wash-coated on a cordierite honeycomb (400 cells per square inch) and then dried to reduce the wash coat amount to about 400%. 30% by weight and 500 ℃ in air
The above binder was sufficiently solidified by baking for 2 hours to prepare a catalyst of Example 1 of the present invention.

The catalyst of Example 1 thus prepared was placed in a model gas corresponding to the exhaust gas composition under the lean burn condition of A / F = 22, and the NOx purification rate in the fresh state of Example 1 Was measured, and the results are shown in FIG. Further, an actual machine test for heat-treating Example 1 was performed using exhaust gas from an actual engine, and the NOx purification rate after the actual machine test was measured. The results are also shown in FIG.

Further, each of Ir and Pt having the same loading as in Example 1 was simultaneously treated with a single metal-containing silicate (Na-type ZSM-
A Pt-Ir-supported aluminosilicate powder supported on 5) was prepared, and thereafter the same operation as in Example 1 was performed to prepare a catalyst of Comparative Example 1. For the catalyst of Comparative Example 1, the NOx purification rates in the fresh state and after the actual machine test were measured in the same manner as in the case of the catalyst sample 1, and each result was shown in FIG.
It was shown to.

Further, after the aging heat treatment was performed by heating the catalysts of Example 1 and Comparative Example 1 in air at 800 ° C. for 8 hours, the NOx purification rate was measured, and the average NOx purification rate after the heat treatment was measured. Table 1 shows the deterioration rate with respect to the average NOx purification rate in the fresh state.

[0030]

[Table 1]

According to the results shown in FIG. 1, Ir and Pt as active species are held on separate active species supporting base materials,
Thereafter, the NOx purification characteristics of Example 1 according to the present invention prepared by mixing the two were not different from those in the fresh state even after the actual machine test, and further, Comparative Example 1 shown in FIG. There is no shift to the low-temperature side and no decrease in the NOx purification rate unlike the NOx purification characteristics of FIG.

Further, according to the results shown in Table 1, in Example 1, even after the heat treatment such as aging at 800 ° C. for 8 hours, the NOx purification rate secured a substantially excellent value. In addition, it can be seen that the deterioration of the NOx purification rate due to the heat treatment is small, and the state in which the NOx purification characteristics are significantly reduced unlike Comparative Example 1 does not occur.

Therefore, it is clear that Example 1 according to the present invention has excellent catalytic properties from the viewpoint of heat resistance.

<Examples 2 to 4> Next, the noble metals as active species are Pt and Ir, and the manner in which these Pt and Ir are supported on the active material-supporting base material is variously changed to obtain various exhaust gases. A purification catalyst was prepared.

0.64 g of divalent platinum ammine and 5 g of H
Type aluminosilicate (Caban ratio = 144) was thoroughly mixed with ethanol and dried at 90 ° C. for 3 hours to obtain a catalyst powder, and an alumina binder was added to the catalyst powder to prepare a slurry without cordierite. Wash coat the carrier, wash coat amount is about 30% by weight
And heated to prepare a Pt-supported H-type aluminosilicate catalyst. This is referred to as Comparative Example 2.

Also, 0.186 g of iridium trichloride and 5 g of H-type aluminosilicate (Cavan ratio = 144)
, And thereafter, in the same manner as in Example 1 above,
A supported H-type aluminosilicate catalyst was prepared. This is referred to as Comparative Example 3.

Further, a catalyst powder composed of H-type aluminosilicate, in which both Pt and Ir are supported at a molar ratio of 3: 1, is used according to Example 1 above. Thus, a Pt-Ir supported H-type aluminosilicate catalyst was prepared. This is referred to as Comparative Example 4.

On the other hand, γ-alumina was prepared by a sol-gel method. At that time, 2% by weight of divalent platinum ammine was added to the sol-like γ-alumina precursor to make it highly dispersed, then aged, dried, and calcined under an appropriate atmosphere condition of 500 to 600 ° C. for about 3 hours to obtain γ. -A Pt-supported γ-alumina catalyst having a high specific surface area of about 340 was prepared. This is referred to as Comparative Example 5.

Next, the Pt-supported aluminosilicate powder and the Pt-supported γ-alumina powder were mixed at a weight ratio of 1: 1 and the mixture was applied to a cordierite honeycomb carrier in the same manner as in Example 1 above. By carrying, a [Pt-supported H-type metal-containing silicate + Pt-supported γ-alumina] catalyst was prepared. This is referred to as Comparative Example 6 because it does not carry Ir.

The Ir-supported H-type aluminosilicate powder and Pt-supported γ-alumina powder were mixed at a weight ratio of 1: 1.
Then, the same operation as in Example 1 was performed to prepare an [Ir-supported H-type aluminosilicate + Pt-supported γ-alumina] catalyst. This was designated as Example 2 according to the present invention.

Further, the weight ratio of Pt-Ir-supported H-type aluminosilicate powder and Pt-supported γ-alumina powder is 1: 1:
1 and then the same operation as in Example 1 was performed to obtain [Pt-Ir-supported H-type aluminosilicate +
[Pt-supported γ-alumina] catalyst was prepared. This was designated as Example 3 according to the present invention.

Further, the Pt-Ir-supported aluminosilicate powder which was 1: 1 in Example 3 was
A catalyst was prepared in which the mixing ratio with the supported γ-alumina powder was changed to 3: 1, and this was designated as Example 4 according to the present invention.

For the catalysts of Examples 2 to 4 and Comparative Examples 2 to 6 according to the present invention, NO was determined using a model gas corresponding to the exhaust gas composition under the lean burn condition of A / F = 22.
Each of the x purification characteristics was evaluated.

The evaluation method was as follows.
After performing the aging heat treatment by heating at 8 ° C. × 8 hours, the aging heat treatment is performed by the method of measuring the NOx purification rate for the model gas, and the average NOx purification rate after the heat treatment and the deterioration rate with respect to the average NOx purification rate in a fresh state are determined. The measurement was performed, and the results are shown in Table 2.

[0045]

[Table 2]

According to the results shown in Table 2, Comparative Examples 2 to
As compared with the respective catalysts of No. 6, each of the catalysts of Examples 2 to 4 according to the present invention has a practical NOx purification rate even after the heat treatment, and the deterioration of the NOx purification rate by the heat treatment is reduced. You can see that

In Example 3, the heat treatment was performed at 800 ° C. for 8 hours using the exhaust gas from the actual engine, and the NOx purification rate after such an actual test was measured. As shown in FIG.
In the above measurement, a model gas corresponding to the exhaust gas under the lean burn condition of A / F = 22 was used.

According to the results shown in FIG. 3, after the actual machine test, the temperature at which the maximum NOx purification rate is shifted slightly lower than that in the fresh state, and the exhaust gas purification activation temperature range is It can be seen that there is no shift and that a wide active temperature range is maintained.

The maximum NOx purification rate after the actual test was
Pt and Ir are supported on the metal-containing silicate in the form of a compound, which shows higher values than those in the fresh state. Since Pt and Ir are activated by acting as a reducing gas for the compound, the exhaust gas purification rate is increased, so that a difference in behavior between the actual engine exhaust gas and the aging model gas may occur. As a result, it is estimated that the measured value of the maximum NOx purification rate is larger after the actual vehicle test than in the fresh state.

<Embodiment 5> This embodiment 5 is a case where another noble metal, for example, Rh is added in addition to Pt and Ir as active species.

Instead of Pt in the Pt-supported γ-alumina catalyst obtained by highly dispersing Pt in γ-alumina (Comparative Example 5), a Rh-supported γ-alumina catalyst in which 4% by weight of Rh is supported is used. Prepared. Further, a Pt-Ir-supported γ-alumina catalyst in which Pt-Ir was supported instead of the above-mentioned Pt was prepared.

Next, the powder and the Rh-supported γ-alumina catalyst powder were mixed with the Pt-Ir-supported H-type aluminosilicate catalyst at a weight ratio of 1: 1 and an alumina binder was added to the mixture. The slurry was applied to a cordierite honeycomb carrier so as to have a wash coat amount of about 30% by weight, and heated to obtain [P
t-Ir supported H-type metal-containing silicate + Rh supported γ-alumina] catalyst was prepared. This is referred to as Comparative Example 7.

The Pt-Ir-supported aluminosilicate catalyst powder and the Pt-Ir-supported γ-alumina were mixed at a weight ratio of 1: 1.
[Pt-Ir-supported H-type metal-containing silicate + Pt-Ir-supported γ-alumina] catalyst was prepared by the same procedure as described above.

With respect to Example 5 and Comparative Example 7 according to the present invention, NOx purification characteristics were evaluated using a model gas corresponding to an exhaust gas composition under a lean burn condition of A / F = 22.

The evaluation method is the same as in Examples 2 to 4 above. Table 3 shows the evaluation results. In Table 3, the evaluation results of Example 2 (I-supported H-type metal-containing silicate + Pt-supported γ-alumina) are also shown for reference.

[0056]

[Table 3]

According to the results shown in Table 3, Ir among the noble metals, particularly when Ir is supported on a metal-containing silicate,
The effect of synergistic effect with Pt, such as improvement of activity, is remarkable, and it is clearly supported that the above effect is hardly obtained when supported on an alumina-based supporting base material. Also, R
It can be seen that the addition of h reduces the deterioration due to the heat treatment but does not contribute much to the improvement of the NOx purification rate.

Next, in relation to each of the above examples, the relationship between the type of metal-containing silicate that indispensably supports Ir as an active species and the catalytic properties obtained thereby was examined.

0.186 g of iridium trichloride was weighed. Further, a Na-type aluminosilicate having a cubane ratio of 30, 70 and 200 and an H-type aluminosilicate having a cubane ratio of 30 and 200 were used.
5 g of each type of aluminosilicate was weighed. Then, the above-mentioned iridium trichloride and one of the above-mentioned metal-containing silicates were respectively mixed using ethanol, sufficiently stirred, and dried to obtain five types of Ir-supported aluminosilicate powders.

Also, 0.64 g of divalent platinum ammine and N
5 g of a-type aluminosilicate (Caban ratio = 30) was mixed, sufficiently stirred, and dried to obtain a Pt-supported aluminosilicate powder.

Each of the Ir-supported aluminosilicate powder and the Pt-supported aluminosilicate powder were mixed in a weight ratio of 1:
Then, the mixture was adjusted to 1 and a solvent was added to form an integrated slurry, which was then heated and dried again. These were respectively supported on a honeycomb carrier by wash coating, and five types of catalyst samples according to the present invention were prepared in which aluminosilicate supporting Ir had a different cation type and different carbane ratio.

For each of these catalyst samples, A / F = 2
Each NOx purification characteristic was evaluated using a model gas corresponding to the exhaust gas composition under the lean burn condition of No. 2, and the results are shown in Table 4. Further, in each of these catalyst samples, the Na content and H content of the Ir-supported aluminosilicate in each of the catalyst samples were slightly changed in the Al content (the magnitude of the Cayban ratio) and the N content.
The relationship between the Ox purification rate and the deterioration rate was analyzed and shown in FIGS. 4 and 5, respectively.

[0063]

[Table 4]

According to the results shown in Table 4 and FIGS. 4 and 5, within the range where Ir exerts an effective synergistic effect on Pt, the above Pt and Ir are separated to improve heat resistance. As a catalyst according to the gist of the present invention, the above-mentioned Ir is supported on a metal-containing silicate, and the ratio of Si to Me and the ratio of oxides, such as Caban ratio, are large. NO, which is practically excellent even after heat treatment
It can be seen that the x purification rate was obtained, and the deterioration rate of the NOx purification rate before and after the heat treatment was also low.

That is, it is apparent that the excellent NOx purification characteristics as described above can be obtained when the Ca-van ratio is 70 or more in the case of Na-type aluminosilicate and 30 or more in the case of H-type aluminosilicate.

[0066]

As described above, according to the exhaust gas purifying catalysts according to the first and second aspects of the present invention, Si and Me (where Me is a metal element other than Si) in which Ir as an active species is indispensably supported. ) With an oxide ratio of 70 or more
Catalyst body composed of a metal-containing silicate such as an aluminosilicate of a type or 30 or more H-type, and an active species-supporting base material that indispensably supports Pt as an active species, for example, an aluminosilicate with a small Cayban ratio or a high ratio Surface area γ-
Since the catalyst is a mixture of alumina and the second catalyst body made of alumina, the Pt and Ir are separable within a range that exerts a synergistic effect on each other, whereby sintering of active species is suppressed. Therefore, thermal degradation of Pt and Ir can be reduced while ensuring purification performance. Further, the shift of the active temperature range due to thermal degradation can be suppressed.

[Brief description of the drawings]

FIG. 1 is a graph showing NOx purification characteristics of Example 1 according to the present invention.

FIG. 2 is a graph showing NOx purification characteristics of Comparative Example 1 relating to Example 1;

FIG. 3 is a graph showing NOx purification characteristics of Example 3 according to the present invention.

FIG. 4 is a graph showing a relationship between a Cayban ratio and catalytic characteristics in an exhaust gas purifying catalyst according to the present invention.

FIG. 5 is a graph showing the relationship between the Cayban ratio and the catalyst characteristics in the exhaust gas purifying catalyst according to the present invention.

Continuation of front page (72) Inventor Takashi Takemoto 3-1 Shinchi, Fuchu-cho, Aki-gun, Hiroshima Prefecture Inside Mazda Co., Ltd. (56) References JP-A-4-176337 (JP, A) JP-A-1-139144 ( JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B01J 21/00-37/36 B01D 53/86

Claims (2)

(57) [Claims] 1. A first catalyst body in which Ir is indispensably supported as an active species on an active species supporting base material, and a second catalyst body in which Pt is indispensably supported as an active species on an active species supporting base material. An exhaust gas purifying catalyst mixed with a medium, wherein the active species supporting base material of the first catalyst body is Si and Me (Me
Is a metal-containing silicate having a metal element other than Si) and a metal forming a skeleton of the crystal.
Na or Si with a / Me ratio of 70 or more as an oxide ratio
An exhaust gas purifying catalyst, wherein the catalyst is an H-type catalyst having a / Me ratio of 30 or more as an oxide ratio. 2. The exhaust gas purifying catalyst according to claim 1, wherein the metal-containing silicate is an aluminosilicate.