JP3361121B2 - 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.

[0002]

2. Description of the Related Art As a catalyst for purifying engine exhaust gas,
Oxidation of CO (carbon monoxide) and HC (hydrocarbons), N
A three-way catalyst that simultaneously performs reduction of Ox (nitrogen oxide) is known. A three-way catalyst is known in which Pt (platinum), Pd (palladium), and Rh (rhodium) are supported on γ-alumina, and the air-fuel ratio (A /
A high purification efficiency can be obtained when F) is near the stoichiometric air-fuel ratio of 14.7.

Among the exhaust gases of the above engine, NOx has a great concern that it has a harmful effect on the human body and the ecosystem, and therefore it has to be prevented as much as possible from being discharged into the atmosphere.
There are several methods for preventing the emission, but in the case of a mobile engine, it is realistic to purify by a catalyst installed in the latter stage of the engine.

On the other hand, in the field of automobiles, a lean burn engine, a so-called lean burn engine, has been put into practical use in order to comply with fuel regulations concerning the engine. But,
In the case of the lean burn engine, the exhaust gas is in an oxygen excess atmosphere due to the high air-fuel ratio. Therefore, although CO and HC can be oxidized and purified by the three-way catalyst as described above, NOx reduction It cannot be purified.

Therefore, even in an oxygen-excess atmosphere of exhaust gas, NOx is directly supplied or a reducing agent (for example, CO,
As a catalyst capable of catalytically decomposing N ₂ and O _{2 in} the presence of (HC etc.), a catalyst composed of a zeolite carrying a transition metal, for example, by ion exchange is widely used. Further, various proposals have been made for this transition metal-supported zeolite catalyst in order to increase the NOx purification rate and improve the activity of the catalyst.

[0006] For example, the technique disclosed in Japanese Patent Laid-Open No. 3-205157 is an example thereof. According to the technique disclosed in the above-mentioned publication, zeolite contains Cu and one or more alkaline earth metals and a rare earth metal. By supporting at least one of these, the NOx purification capacity and durability of the catalyst are improved.

[0007]

However, although NOx tends to exist as NO ₂ at 200 ° C. or lower in an oxygen-excessive atmosphere, the above-mentioned JP-A-3-20 is known.
According to the catalyst described in Japanese Patent No. 2157, the catalyst does not have a function as a catalyst for directly purifying NOx at a catalyst temperature of 200 ° C. or lower, and is poor in activity in a low temperature range during NOx purification. There is a problem.

The Cu ion exchange zeolite catalyst, which is a practical catalyst capable of effectively removing NOx, is
Generally, when the actual vehicle equipped with a lean-burn engine is used to purify NOx in an oxygen-rich atmosphere even though the NOx purification rate exceeds 90% at the laboratory level, model gas in the laboratory and exhaust gas from the actual vehicle are used. It is inevitable that the NOx purification rate will decrease due to various condition differences with the gas. Furthermore, in such a Cu ion-exchanged zeolite catalyst, the temperature at which the NOx purification function appears is 3
Since it is as high as 50 to 450 ° C., there is a problem that the total NOx purification rate becomes low when the NOx purification rate in an oxygen excess atmosphere is evaluated.

Further, when the engine is operated under a low load, the catalyst inlet gas temperature of the exhaust gas becomes low, which requires improvement of the NOx purification activity in a low temperature range. Therefore, the catalyst has a wide activation temperature range. It is necessary to have an area.

In view of the above, according to the present invention, even in the oxygen excess atmosphere of the exhaust gas and even when the catalyst inlet gas temperature is low, the purifying activity is smoothly exhibited from the low temperature range and the activity is improved. The purpose is to provide a catalyst capable of expanding the active temperature range.

[0011]

In order to achieve the above-mentioned object, the invention of claim 1 relates to a catalyst in which a metal active species-supporting base material such as zeolite supports metal active species. In order to smoothly operate the exhaust gas purifying activity from a low temperature range and improve the purifying activity, the purifying activity is improved by the synergistic effect of the precious metal having the excellent purifying activity of the above metal active species in the low temperature range and the precious metal. In addition, it is used in combination with another metal species that enables expansion of the active temperature range.

[0012] Specifically, the invention of claim 1 is ZSM-
5 , Pt, Ir, and Rh are carried on the exhaust gas purifying catalyst.

The invention of claim 2 is the structure of claim 1, wherein the weight ratio of Pt to Ir carried is 3: 1.
Therefore, the weight of Rh carried is the same as the weight of Ir carried .

In the exhaust gas purifying catalyst of the present invention,
This is a catalyst in which Pt is indispensable when ZSM-5 carries a metal species, and Ir and Rh are carried together with Pt. As a result, each of the supported metal species exhibits a unique behavior involved in the reaction, and it is possible to improve the catalytic activity of Pt by the synergistic action of these metal species.

Platinum group elements (Ru, Rh, Pd, Os, I
r, Pt) is known to improve the low temperature activity in the catalyst as a catalytically active species, and based on this, the present inventor has found that such a platinum group element as a catalytically active species can be used as a supporting base material in various modes. NO for each catalyst supported on
x Purification characteristics were studied.

As a result, by making Pt indispensable and using Ir and Rh together with Pt to form catalytically active species, the NOx purification activity smoothly operates from a low temperature range and becomes highly activated in an oxygen excess atmosphere. It was found that the purifying characteristics described above can be obtained, and in particular, the low temperature activity becomes high .

[0017] Pt, Pd, Rh, Ir, Ru alone
As shown in Table 1, the oxidization capacity of
The order is Pt ≧ Pd> Rh≈Ir ≧ Ru.
The temperature characteristic of the conversion ability is Pt for Rh and Ir.
And Pd tend to appear on the slightly higher temperature side.

[0018]

[Table 1]

The reason why the activity is smoothly expressed from the low temperature region and highly activated depending on the combination of the catalytically active species is as follows, particularly in Ir among the platinum group elements having the above-mentioned oxidizing ability characteristics. It is considered that the phenomenon is occurring.

That is, in the present invention, I is used together with Pt.
When r is supported, the Ir interacts with Pt forming the center of the active site to bring about smooth activation of activity and high activation in the low temperature region.

Specifically, since Ir (existing as the oxide IrO ₂ ) has an oxidizing ability, HC species and CO which are oxidized as a reducing agent in the NOx purification reaction on the Pt surface coexisting with Ir described above. HC species and CO species other than the species (that is, the HC species and CO species that did not participate in the reaction on the Pt surface) are oxidized by the above-mentioned Ir (although Ir itself has a low NOx purification ability). Will be. As a result, the O ₂ adsorbed and generated in the NOx purification reaction on the Pt surface is removed, and as a result, the active catalyst surface is maintained. It is considered that activation of activity and high activity can be obtained.

Another reason is that Pt and P
Coexistence with Ir, which has an oxidation ability inferior to t,
The strong oxidizing ability of itself is suppressed, in other words, when it is diluted, it has a suitable oxidizing ability that easily produces an active HC combustion intermediate product during NOx purification. As a result, it is presumed that all of these reasons are combined to obtain smooth activation and high activity in the low temperature range as described above .

In the above ZSM-5 , the cation species is Na.
Na-type ZSM-5 having the above formula is preferably used, and in addition to this, H-type ZSM-5 having the cation species as H ⁺ can also be used.

The catalyst of the present invention can be obtained by supporting Ir and Rh together with Pt on ZSM-5 as described above. Also, about 2 as a binder for this
It is also possible to prepare a monolith type catalyst by adding an inorganic binder such as 0% by weight of hydrated alumina or silica sol and washcoating it on a suitable carrier such as a cordierite honeycomb.

The ZSM-5 may be loaded with each metal active species by any method such as an ion exchange method, an impregnation method and a coprecipitation method. The impregnation method and the coprecipitation method are more advantageous than the ion exchange method, which has a limited amount, in that a large amount of metal active species can be supported.

By the way, in the case where Pt and Ir, which are one of desirable combinations in terms of action and effect, are combined as the metal species supported on the ZSM-5 , Pt and liter per 1 liter of the obtained catalyst volume are combined. Ir total 3g
Various loading weight ratios of Pt and Ir when loaded were set as shown in Table 2. Next, the maximum NOx purification rate in the fresh state when the respective ratios were made was measured and also shown in Table 2.

[0027]

[Table 2]

FIG. 1 is a graph showing the results shown in Table 2 with the Pt / Ir ratio on the horizontal axis and the NOx maximum purification rate (fresh state) on the vertical axis.

According to the results shown in FIG. 1, the effective maximum NOx purification rate is about 34% or more, and Pt /
The Ir ratio (weight ratio) is in the following range.

∞ ≧ Pt / Ir ≧ 1/3 Further, a preferable Pt / Ir ratio with which the maximum NOx purification rate exceeding 50% can be obtained is in the following range.

6/1 ≧ Pt / Ir ≧ 1/1 Therefore, when Pt and Ir are combined and carried, Pt and Ir are carried on the basis of the above Pt / Ir ratio. It can be seen that excellent NOx purification characteristics are obtained.

In the exhaust gas purifying catalyst according to the present invention, the above - mentioned ZSM-5 and Pt together with the above-mentioned respective metal species are supported in combination, so that the conventional metal-containing silicate can be obtained, as will be shown in the data in Examples described later. Compared to a catalyst in which only a noble metal active species (for example, Pt) is supported on
In the oxygen excess atmosphere, the NOx purification activity smoothly operates from the low temperature range, the highly activated purification characteristics are obtained, and the activation temperature range can be expanded.

[0033]

EXAMPLES Examples of the present invention will be described below.

Reference Examples A and B Na-type ZSM-5 (SiO ₂ / Al ₂ O ₃ = 30, the same applies hereinafter) was used from among the metal-containing silicates as the base material for supporting the metal active species. 0.4852 g (0.28 g Pt equivalent) of divalent platinum ammine crystals (57.7%) as Pt material and 0.1392 g of IrCl3 as Ir material
(Corresponding to 0.09 g Ir) was weighed, and each of these weighed materials was mixed with 15 g of the above Na-type ZSM-5 powder, and after mixing, the mixture was stirred while heating to about 50 ° C. 50 ° C
After drying at 150 ° C. for 3 hours in the air, the Pt—Ir-supported Na-type ZSM-5 catalyst was prepared by firing in air at 500 ° C. for 2 hours.

The Pt-Ir-supported Na-type ZSM
A catalyst of Reference Example A was prepared by washcoating a -5 catalyst on a cordierite catalyst carrier by a known method, followed by drying at 150 ° C for 3 hours and calcination at 500 ° C for 2 hours.

Further, TiO ₂ was used in place of the Na-type ZSM-5 in Reference Example A as the metal active species-supporting base material,
Otherwise, the same operations as in Reference Example A above were carried out to prepare a Pt-Ir-supported TiO ₂ catalyst. This Pt-
The Ir-supported TiO ₂ catalyst was used as Reference Example B.

(Example) In addition, 0.4852 g of divalent platinum ammine crystal (57.7%) as a Pt material used in Reference Example A above
(Corresponding to 0.28 g Pt) and IrCl as Ir material
_In addition to 0.1392g of ₃ (equivalent to 0.09gIr), R
Using 1.8 cc (equivalent to 0.09 gRh) of a rhodium nitrate (4.562 wt%) solution as the h material, Pt-Ir- was used in the same manner as in the preparation of Reference Example A above, except for the above. A Rh-supported Na-type ZSM-5 catalyst was prepared. This Pt-Ir-Rh-supported Na-type ZSM-5 catalyst was used as an example.

(Comparative Examples 1 to 4) Further, Ir was excluded from the metal species used in Reference Example A, and only Pt was used. That is, Na-type ZSM-
Divalent platinum ammine crystals (57.7
%) 0.4852 g (corresponding to 0.28 g Pt) is weighed and mixed, and thereafter, the same operation as in the preparation of Reference Example A is carried out to prepare a Pt-supported Na-type ZSM-5 catalyst. It was set as Comparative Example 1.

The amount of the divalent platinum ammine mixed in Comparative Example 1 was 0.4852 g to 0.216 g (0.
(Corresponding to 12 g Pt), but in the same manner as in Reference Example A, except that a Pt-supported Na-type ZSM-5 catalyst was prepared.
And

Further, Pt was excluded from the metal species used in Reference Example A, and only Ir was used. That is, N
15 g of a-type ZSM-5 powder and 0.18 of IrCl ₃
6 g (corresponding to 0.12 g Ir) was weighed, added and mixed, and thereafter, the same operation as in Reference Example A was performed to prepare an Ir-supported Na-type ZSM-5 catalyst, which was designated as Comparative Example 3.

In addition, Pt-Rh was obtained by removing Ir from the metal species used in the above examples. That is, N
1. A solution of 0.4852 g (corresponding to 0.28 g Pt) of divalent platinum ammine crystal (57.7%) and rhodium nitrate (4.562% by weight) was added to 15 g of a-type ZSM-5 powder.
8 cc (corresponding to 0.09 g Rh) was weighed, added and mixed, and thereafter, the same operation as in Reference Example A was performed to perform Pt-R.
A h-supported Na-type ZSM-5 catalyst was prepared and used as Comparative Example 4.

(Measurement of NOx Purification Rate) Each of the catalysts of Reference Examples A and B, Examples and Comparative Examples 1 and 2 was used as a catalyst sample, and each catalyst sample was mounted in an atmospheric fixed bed reactor. The model gas in an oxygen excess atmosphere corresponding to lean burn with an engine air-fuel ratio of 22 is SV =
The NOx purification rate in the fresh state was measured by circulating the above catalyst samples at 55000 h ^−1, and the purification characteristics are shown in FIG. 2 (reference examples A and B, examples) and FIG.
(Comparative Examples 1 and 2) (Z in the description of FIG. 2 and subsequent figures represents Na-type metal-containing silicate).

According to the results shown in FIGS. 2 and 3, P
Compared to Comparative Examples 1 and 2 in which only t is supported, Pt and I
It is clear that the catalysts of Reference Examples A, B, and Example in which r and R are inevitably carried have a greatly improved NOx purification rate and that the purification activity operates smoothly from a low temperature range. In particular, the examples are excellent in low temperature activity.

Further, with respect to the catalysts of Comparative Examples 3 and 4 in which Pt and Ir do not coexist, the NOx purification rate was measured in the same manner as the catalysts of the above Reference Examples A, B and Examples.
The NOx purification rate was similar to that of Comparative Examples 1 and 2 shown in FIG. This indicates that the coexistence of Pt and Ir as the metal species makes it possible to obtain excellent NOx purification characteristics as in the catalysts of Reference Examples A, B and Examples.

[0045]

As described above, according to the exhaust gas purifying catalyst of the present invention , ZSM-5 Pt, Ir and Rh are used.
Since the exhaust gas is carried in the low temperature region, the purification activity of the exhaust gas smoothly operates, so that the low temperature activity is improved and the purification rate is improved, so that the practicality in an actual vehicle is significantly improved .

[0046] In addition, the use of inexpensive materials activates precious metals.
Higher economics by having the same effect as using seeds
Can be turned on.

[Brief description of drawings]

FIG. 1 is a graph showing a Pt—Ir carrying amount ratio in the present invention.

FIG. 2 is a graph showing NOx purification characteristics of catalysts of Examples in Reference Examples and Examples according to the present invention.

FIG. 3 is a graph showing NOx purification characteristics of a comparative catalyst.

─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Shuji Iwakuni Shinchi 3-1, Fuchu-cho, Aki-gun, Hiroshima Mazda Co., Ltd. (56) References JP-A-4-63140 (JP, A) JP-A-4 -78442 (JP, A) JP 5-293380 (JP, A) JP 5-146642 (JP, A) JP 5-103985 (JP, A) JP 63-185453 (JP, A) ) JP-A-51-105993 (JP, A) JP-A-3-98644 (JP, A) JP-A-3-293035 (JP, A) JP-A-1-135541 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) B01J 21/00-37/36 B01D 53/86

Claims (2)

(57) [Claims] 1. An exhaust gas purifying catalyst, wherein ZSM-5 carries Pt, Ir and Rh. 2. The supported weight ratio of Pt to Ir is 3: 1.
And the supported weight of Rh is the same as the supported weight of Ir.
The exhaust gas purifying catalyst according to claim 1, wherein the amount is an amount .