JPH06106066A - Catalyst for purification of exhaust gas - Google Patents

Abstract

PURPOSE:To provide the catalyst which is improved in an exhaust gas purification characteristic in a low-temp. region and has excellent heat resistance by using metal-contg. silicate of an Na type or H type having specific Si and Me ratios respectively at the time of allowing Pt and Ir to co-exist as active species. CONSTITUTION:The first catalyst medium indispensably deposited with the Ir as the active species and the second catalyst medium indispensably deposited with the Pt are mixed and are used for an active species carrier base material. The active species carrier base material of this first catalyst medium is the metal-contg. silicate having Si and Me (the Me is metal elements exclusive of Si) as metals forming the skeleton of the crystal and is the Na type of >=70 Si/Me ratio as the ratio of the oxides and the H type of >=30 Si/Me ratio. The sintering of the active species hardly arises and the synergistic effect of the Pt and the Ir is resulted if the catalyst is formed in such a manner.

Description

Detailed Description of the Invention

[0001]

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

[0002]

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

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

Therefore, an active species such as Cu is carried 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 atmosphere of high oxygen concentration. So-called zeolite catalysts are promising. Further, with respect to the zeolite catalyst, various measures including selection of active species are being studied in order to always be an exhaust gas purifying catalyst excellent in NOx removal ability in response to various states of exhaust gas.

For example, Japanese Unexamined Patent Publication (Kokai) No. 3-202157 proposes a technique for efficiently removing NOx in an atmosphere of high oxygen concentration. The technology disclosed in the above publication is a catalyst in which Cu and one or more kinds of alkaline earth metals and one or more kinds of metals of rare earth elements are supported on zeolite, whereby excellent NOx in a temperature range of 800 ° C. or less is obtained. It is intended to obtain a catalyst having purification ability and durability capable of being used for a long time.

[0006]

However, NOx
Cu ion-exchanged zeolite catalysts, which are said to be useful catalysts for the effective removal of nitrogen, are generally 9
Despite the fact that the NOx purification rate exceeds 0%, the NOx purification in a high-concentration oxygen atmosphere by a lean-burn engine operating in a real machine is caused by fluctuations in the properties of the actual exhaust gas. It is inevitable that the NOx purification rate will decrease.

Further, in such a Cu ion-exchanged zeolite catalyst, the NOx purification activity expression temperature is 350 to 40.
The NOx purification rate in the low temperature range decreases due to the fact that it is as high as 0 ° C and that the NOx purification active temperature range tends to shift to the high temperature side due to actual machine operation. There is a problem that the integrated value decreases.

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

On the other hand, when the vehicle is equipped with a lean burn engine and the running condition is such that the vehicle is repeatedly started and stopped, the catalyst inlet gas temperature of the engine exhaust gas becomes low, so that the NOx purification characteristic is improved in the low temperature range. Therefore, it is necessary for the catalyst to have stable characteristics such that the purification characteristics in the low temperature range are further improved and the heat resistance is also excellent.

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

The inventor of the present invention has conducted extensive research into an exhaust gas purifying catalyst that can be expected to improve NOx purification characteristics in a low temperature region by supporting such a precious metal as an active species, and among the precious metals, Pt and Ir are also included. A catalyst obtained by supporting both and as active species on an active species-supporting base material, for example, a metal-containing silicate, has a synergistic effect of Pt and Ir, although Ir itself has a lower NOx purification activity than Pt. It was confirmed that high activity was brought about in the low temperature region, and thereby excellent NOx purification characteristics were obtained.

Further, Pt and I supported as active species
As a result of research on the behavior with r, in order to obtain the above-mentioned catalytic function, it was a priori that Pt and Ir coexist within a range where they can exert a synergistic effect on each other. However, if Pt and Ir are too close to each other, sintering of active species is likely to occur,
In terms of heat resistance, it was found that there is a risk of becoming negative.

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

[0014]

[Means and Actions for Solving the Problems]
The present inventor takes measures against Ir that is easily thermally deteriorated by taking measures such as supporting it on a heat-resistant active-species-supporting base material, for example, an aluminosilicate having a high silica content and a high Cavan ratio. In consideration of suppressing the occurrence of sintering and, at the same time, preventing Ir from being excessively close to Pt which is easily sintered at high temperature, Pt and Ir as active species are considered. The present invention has been completed by conducting various experiments on the carrying forms of both that enable the expression of the synergistic effect of 1.

That is, in order to achieve the above-mentioned object, the invention of claim 1 avoids excessive proximity of Pt and Ir as active species to each other when they coexist, and the above Pt and Ir are mutually different. In order to enhance the synergistic effect on Pt, the thermal deterioration of Ir, which is carried in a state of being separated to a certain extent within a range in which they exert a synergistic effect, are respectively supported, and at the same time, thermal deterioration of Ir carried as an active species together with Pt is prevented. The present invention aims to maintain the NOx purification performance also in the active species-supporting base material itself.

[0016] Specifically, the means for solving the problems according to the invention of claim 1 is that a first catalyst body in which Ir as an active species is inevitably supported on an active species supporting base material and an active species supporting base material are active. 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 body is a metal-containing silicate having Si and Me (Me is a metal element other than Si) as metals forming a crystal skeleton, and the Si / Me ratio is as an oxide ratio. A Na type of 70 or more or an H type having a Si / Me ratio of 30 or more as an oxide ratio is adopted.

Further, the invention of claim 2 is industrially applicable by selecting and applying as the metal-containing silicate in which the ratio of Si to Me is a predetermined value or more, an aluminosilicate having a required Cavan ratio. In order to improve the usability of the above, specifically, in the structure of claim 1, the metal-containing silicate is an aluminosilicate.

Since the aluminosilicate that indispensably supports Ir, for example, an aluminosilicate having a large Cavan ratio has high heat resistance, the active species-supporting base material that indispensably supports Pt is a normal metal-containing silicate and a γ having a high specific surface area. -Alumina can be used appropriately. Above all, when γ-alumina is used as the active species-supporting base material that inevitably supports Pt, the solid acid strength of the catalyst will be increased, so that improvement in durability can also be expected.

The metal-containing silicate on which Pt and Ir are supported is preferably an aluminosilicate (zeolite) using Al as a metal forming a crystal skeleton,
If necessary, instead of Al or together with Al, Ga,
A metal-containing silicate using a metal such as Ce, Mn, or Tb as a skeleton-forming material can also be applied. As the metal-containing silicate, A type, X type, Y type, mordenite,
ZSM-5 and the like are suitable. As the metal-containing silicate, as described above, an H-type metal-containing silicate in which the cation species is H ⁺ is preferably used together with a 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)
Is 280 or more and preferably has a high specific surface area of about 340.

As described above, in the exhaust gas purifying catalyst according to the present invention, Pt and Ir as catalytically active species are basically carried on the active species carrying base materials which are different from each other, and thereafter, these Pt are carried. A catalyst body comprising an active species-supporting base material and I
An exhaust gas purifying catalyst is obtained by mixing with a catalyst body made of an active species-supporting base material carrying r.

As a result, the active species-supporting base material on which Ir is supported has a high resistance to deterioration by heat, for example, the ratio of Si and Me, in other words, the ratio of oxides such as the Cavan ratio (SiO ₂ / Since a metal-containing silicate having a large Al ₂ O ₃ ) as described above is used, heat resistance is improved and thermal deterioration is reduced. Therefore, high activation of Pt is ensured over a wide temperature range. When a γ-alumina-based support base material is used as the Ir support base material, it is difficult to obtain the above effect.

[0023]

EXAMPLES Examples of the present invention will be described below.

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

Further, Na-type Z having a Cavan ratio of 70 as well as divalent platinum ammine weighed to have a weight of 0.64 g
After mixing with 5 g of SM-5 and stirring sufficiently, 90 ° C x 3
After drying for a period of time, Pt-supported aluminosilicate powder as the second catalyst body was obtained.

These Ir-supported aluminosilicate powders and Pt-supported aluminosilicate powders were weighed in a weight ratio of 1: 1 and mixed with ion-exchanged water. A binder such as hydrated alumina was added to the mixture in an amount of about 20% by weight to form a slurry, and the slurry was wash-coated on a cordierite honeycomb (400 cells per square inch) and then dried to obtain a wash-coat amount of about. Set to 30% by weight, 500 ° C in air
The catalyst of Example 1 which is a product of the present invention was prepared by calcining for 2 hours to allow the binder to sufficiently solidify.

The catalyst of Example 1 thus prepared was placed in a model gas corresponding to the exhaust gas composition under lean burn conditions of A / F = 22, and the NOx purification rate in the fresh state of this Example 1 was set. Was measured and the results are shown in FIG. In addition, an actual machine test was conducted in which exhaust gas from an actual machine engine was used to heat-treat Example 1, and the NOx purification rate after the actual machine test was measured. The results are also shown in FIG.

Further, Ir and Pt each having the same loading amount as in the above-mentioned Example 1 were simultaneously treated with a single metal-containing silicate (Na-type ZSM-with a Cavan ratio = 30).
A Pt-Ir-supported aluminosilicate powder supported in 5) was prepared, and thereafter, the same operation as in Example 1 was performed to prepare a catalyst of Comparative Example 1. Also for the catalyst of Comparative Example 1, as in the case of Catalyst Sample 1, the NOx purification rates in the fresh state and after the actual machine test were measured, and the results are 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 the air at 800 ° C. for 8 hours, the NOx purification rate was measured, and the average NOx purification rate after the heat treatment and 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 in separate active species-supporting base materials,
After that, the NOx purification characteristics of Example 1 according to the present invention prepared by mixing the two are 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 or reduction of the NOx purification rate unlike the NOx purification characteristics of No.

Further, according to the results shown in Table 1, in Example 1, the NOx purification rate secured a substantially excellent value even after the heat treatment such as aging at 800 ° C. × 8 hours. It can be seen that the NOx purification rate is less deteriorated by the heat treatment, and the state in which the NOx purification characteristic is significantly deteriorated as in Comparative Example 1 does not occur.

Therefore, it is apparent that Example 1 according to the present invention has excellent catalytic properties in terms of heat resistance.

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

0.64 g of divalent platinum ammine and 5 g of H
Type aluminosilicate (Cayvan 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 honeycomb. The carrier is wash-coated, and the wash-coat amount is about 30% by weight.
And a Pt-supported H-type aluminosilicate catalyst was prepared. This is referred to as Comparative Example 2.

Further, 0.186 g of iridium trichloride and 5 g of H-type aluminosilicate (Cayvan ratio = 144)
Are used, and thereafter, Ir is carried out in the same manner as in the case of Embodiment 1
A supported H-type aluminosilicate catalyst was prepared. This is referred to as Comparative Example 3.

Further, a catalyst powder made of H-type aluminosilicate on which both Pt and Ir are supported so that the molar ratio of Pt and Ir is 3: 1 is used, and thereafter, the same as in the case of Example 1 above. To prepare a P-Ir-supported H-type aluminosilicate catalyst. This is referred to as Comparative Example 4.

On the other hand, γ-alumina was prepared by the sol-gel method. At that time, 2 wt% of divalent platinum ammine was added to the sol-like γ-alumina precursor to highly disperse it, and then it was aged and dried, and calcined under an appropriate atmospheric condition of 500 to 600 ° C. for about 3 hours, A Pt-supported γ-alumina catalyst having a high specific surface area of about 340 alumina 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 in a weight ratio of 1: 1 to obtain a cordierite honeycomb carrier in the same manner as in Example 1. A [Pt-supported H-type metal-containing silicate + Pt-supported γ-alumina] catalyst was prepared by supporting the catalyst. This product is referred to as Comparative Example 6 because it does not carry Ir.

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

Further, the Pt-Ir-supporting H-type aluminosilicate powder and the Pt-supporting γ-alumina powder are in a weight ratio of 1 :.
The mixture was mixed so as to be 1, and thereafter, the same operation as in Example 1 was performed to perform [Pt-Ir-supporting H-type aluminosilicate +
Pt-supported γ-alumina] catalyst was prepared. This is Example 3 according to the present invention.

Further, Pt-Ir-supported aluminosilicate powder and Pt, which were set to 1: 1 in the above Example 3, were used.
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 used by using a model gas corresponding to the exhaust gas composition under the lean burn condition of A / F = 22.
x Purification characteristics were evaluated respectively.

The evaluation method is as follows.
After performing the aging heat treatment by heating at ℃ × 8 hours, it is performed by the method of measuring the NOx purification rate for the model gas, the average NOx purification rate after heat treatment and the deterioration rate with respect to the average NOx purification rate in the fresh state 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 catalyst of No. 6, each of the catalysts of Examples 2 to 4 according to the present invention secured a practical NOx purification rate even after the heat treatment, and the deterioration of the NOx purification rate due to the heat treatment was reduced. I know that

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

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

The maximum NOx purification rate after the actual machine test is
The higher value than that in the fresh state is that Pt and Ir are supported on the metal-containing silicate in the form of a compound, and under lean burn conditions in an actual machine, even if the exhaust gas has a high oxygen concentration, Since Pt and Ir are activated by acting on the compound as a reducing gas, the exhaust gas purification rate is increased, which may cause a difference in behavior between the actual engine exhaust gas and the aging model gas. It is presumed that, as a result, the measured value of the maximum NOx purification rate became larger after the actual vehicle test than in the fresh state.

<Embodiment 5> This embodiment 5 is carried out in the case where other noble metal such as Rh is added as the active species in addition to Pt and Ir.

In place of Pt in the Pt-supported γ-alumina catalyst (Comparative Example 5) obtained by highly dispersing Pt in γ-alumina, a Rh-supported γ-alumina catalyst in which 4% by weight of Rh was supported was used. Prepared. In addition, a Pt-Ir-supported γ-alumina catalyst in which Pt-Ir was supported in place of Pt was prepared.

Next, the Pt-Ir-supporting H-type aluminosilicate catalyst was mixed with the powder and the Rh-supporting γ-alumina catalyst powder in a weight ratio of 1: 1 and an alumina binder was added to this mixture. It was made into a slurry, and was loaded on a cordierite honeycomb carrier so that the amount of washcoat was about 30% by weight, and then heated [P
A 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 in a weight ratio of 1: 1.
A [Pt-Ir-supporting H-type metal-containing silicate + Pt-Ir-supporting [gamma] -alumina] catalyst was prepared in the same manner as in, and this was designated as Example 5.

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

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

[0056]

[Table 3]

According to the results shown in Table 3, when noble metals, especially Ir, are supported on the metal-containing silicate,
The effect of improving the activity due to the synergistic effect with Pt is remarkable, and it is clearly supported that the above effect is difficult to be obtained when it is supported on the alumina-based carrier. Also, R
By adding h, it is possible to reduce the deterioration due to the heat treatment, but it is understood that it does not contribute so much to the improvement of the NOx purification rate.

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

0.186 g of iridium trichloride was weighed. In addition, Na-type aluminosilicates having a Cayvan ratio of 30, 70 and 200 and H having a Cayban ratio of 30 and 200 are used.
5 g of each type of aluminosilicate was weighed. Then, the above iridium trichloride and one of the above metal-containing silicates were mixed with ethanol, sufficiently stirred and dried to obtain 5 types of Ir-supported aluminosilicate powders.

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

The above Ir-supported aluminosilicate powder and Pt-supported aluminosilicate powder were mixed in a weight ratio of 1 :.
The mixture was mixed so as to be 1 and a solvent was added to form an integrated slurry, which was then heated and dried. Each of these was wash-coated on a honeycomb carrier and supported, and five kinds of catalyst samples according to the present invention having different cation species and Cavan ratio in an aluminosilicate carrying Ir were prepared.

For each of these catalyst samples, A / F = 2
The NOx purification characteristics of each of the NOx purification characteristics were evaluated using the 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, regarding the Na type and the H type of the aluminosilicate carrying Ir in each of these catalyst samples, the Al content was slightly higher or lower (Cayvan ratio) and N was higher.
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, Pt and Ir are separated for improving heat resistance within a range where Ir exerts an effective synergistic effect on Pt. As a catalyst according to the gist of the present invention, in which the Ir is supported on a metal-containing silicate, the ratio of Si to Me and the ratio of the oxides, for example, the Cavan ratio are large. Since it is supported by the above-mentioned materials, NO that is practically excellent even after heat treatment
It can be seen that the x purification rate is obtained, and the deterioration rate of the NOx purification rate before and after the heat treatment is also low.

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

[0066]

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

[Brief description of drawings]

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

FIG. 2 is a graph showing the NOx purification characteristics of Comparative Example 1 related to Example 1 above.

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

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

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

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takashi Takemoto 3-1, Shinchi, Fuchu-cho, Aki-gun, Hiroshima Mazda Motor Corporation

Claims (2)

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