JPH11267512A - Catalyst for purifying exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a catalyst for purifying exhaust gas which can maintain a high NOx purification ratio in an oxygen excess atmosphere by making the catalyst passes a carrier consisting of a kind of gallosilicate supporting Pt an CeO2 having a specified average crystallite diameter, and setting the SiO2 /Ga2 O3 mole ratio of the gallosilicate within a specified range. SOLUTION: As a catalyst which is deteriorated when exposed to a hydrothermic environment and in which the NOx purification capacity in an oxygen excess atmosphere is not lowered, by providing a carrier which supported Pt and CeO2 whose average crystallite diameter D is D>=13.0 nm, the catalyst is formed. The carrier to be used is formed out of a kind of gallosilicate. The gallosilicate (Ga2 O3 .SiO2 .nH2 O) is a substance in which Al which is a skeleton atom of zeolite is replaced with Ga and means a substance in which the quantity of residual Al is on an inevitable impurity level. By subjecting the gallosilicate to de-Ga treatment, a modified gallosilicate in which the SiO2 /Ga2 O3 mole ratio M is 100<=M<=1230 is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an exhaust gas purifying catalyst.

[0002]

2. Description of the Related Art The applicant of the present invention has previously proposed a catalyst for purifying exhaust gas having improved NOx (nitrogen oxide) purifying ability.
There has been proposed a zeolite carrying t and CeO ₂ (see Japanese Patent Application Laid-Open No. 8-131838).

[0003] In this case, Pt is a metal for a catalyst and exhibits an oxidizing ability and a reducing ability for exhaust gas. The oxidizing ability of Pt contributes to the oxidation reaction of HC (hydrocarbon) + O ₂ → H ₂ O + CO ₂ and CO + O ₂ → CO ₂ . Pt
At the stoichiometric air-fuel ratio, NO adsorbs NO
Contributes to a reduction reaction such as → N ₂ , while an oxidation reaction such as NO + O ₂ → NO ₂ in an oxygen-excess atmosphere,
It contributes to a reduction reaction such as NO ₂ + HC + O ₂ → N ₂ + CO ₂ + H ₂ O.

[0004] Zeolite has a function of adsorbing and concentrating HC in exhaust gas and supplying the HC to Pt. This makes it possible to increase the NOx purification rate in an oxygen-excess atmosphere. CeO ₂ is N ₂ in an oxygen-rich atmosphere.
Since it exhibits the Ox adsorption ability, the NOx concentration near Pt is increased. This also allows NO in an oxygen-excess atmosphere
x It is possible to improve the purification rate. Also CeO ₂
Has the effect of suppressing thermal degradation of the catalyst.

[0005]

As a result of various studies on the catalyst, the present inventors have found that the catalyst deteriorates when exposed to a high-temperature environment containing oxygen and water vapor, ie, a hydrothermal environment, and deteriorates in an oxygen-excess atmosphere. NOx purification ability decreases,
It was concluded that this was due to the reduced durability of the zeolite in the hydrothermal environment.

When the catalyst is in the exhaust gas temperature, that is, when the gas temperature is in a low temperature range of 150 to 300 ° C., the catalyst exhibits a relatively good NOx purification ability.
In a high temperature region exceeding 00 ° C., NOx is emitted.

It is considered that such a phenomenon occurs for the following reasons. That is, the purification of NOx in the low-temperature region is based on a mechanism of adsorbing NOx and reducing NOx. Therefore, if the adsorption amount of NOx exceeds the reduction amount of NOx, the unreduced NO
x is stored in the catalyst, and the stored NOx is discharged in the high-temperature region.

When the stored NOx is exhausted in the high-temperature region, the exhaust gas contains extra NOx in addition to the NOx originally generated by the operation of the engine. Since it is indefinite, the problem that the purification control of NOx becomes difficult has arisen.

[0009]

SUMMARY OF THE INVENTION The present invention provides a method for reducing NOx in an oxygen-excess atmosphere after exposure to a hydrothermal environment as described above.
An object of the present invention is to provide the exhaust gas purifying catalyst which can maintain a high x purification performance and does not emit NOx in the high temperature region.

According to the present invention, in order to achieve the above object,
Pt-supported carrier and average crystallite diameter D is D ≧ 13.0
nm of CeO ₂ , wherein the carrier is made of gallosilicate, and the gallosilicate is SiO ₂ / Ga ₂ O _3.
An exhaust gas purifying catalyst having a molar ratio M of 100 ≦ M ≦ 1230 is provided.

Here, gallosilicate (Ga ₂ O ₃ .S
iO ₂ · nH ₂ O) is obtained by substituting Al, which is a skeletal atom of zeolite, with Ga, and has a residual Al content at an unavoidable impurity level. By subjecting such gallosilicate to a Ga removal treatment, gallosilicate having the molar ratio M, that is, a modified gallosilicate can be obtained.

In the above catalyst, Pt is, as described above,
A catalytic metal that exhibits oxidizing and reducing capabilities for exhaust gas. Also, CeO ₂ exhibits NOx adsorbing ability in an oxygen-excess atmosphere as described above. In this case, NO
The x adsorption amount correlates with the average crystallite diameter D of CeO ₂ . Therefore, when CeO ₂ having an average crystallite diameter D of D ≧ 13.0 nm is used, the adsorption amount of NOx decreases as compared with the case of using CeO ₂ with D <13.0 nm, but the adsorbed NOx is almost completely reduced. It becomes possible to purify, and thereby, occlusion of NOx can be avoided.

Further, gallosilicate having the above molar ratio M adsorbs and concentrates HC in exhaust gas,
Has a function of supplying Pt to Pt, and furthermore, exhibits excellent durability in the hydrothermal environment, so that it is possible to avoid the occurrence of problems such as burying of Pt due to blockage of pores. Is greatly suppressed and the separated Ga
And the decrease in the function of CeO ₂ can be avoided.

Therefore, even after the catalyst is exposed to the hydrothermal environment, the catalyst maintains a high NOx purifying ability in an oxygen-excess atmosphere and does not emit NOx in a high gas temperature region.

However, when the molar ratio M is M <100,
Since the amount of Ga becomes excessive and the strain of the skeleton structure becomes large, the thermal stability of gallosilicate decreases, while M> 123
At 0, the amount of Ga becomes too small and the properties of gallosilicate are close to those of SiO ₂ , so that its thermal stability is reduced.

The present invention also relates to a method for producing fresh NOx
It is an object of the present invention to provide the exhaust gas purifying catalyst, wherein the purifying temperature range is substantially matched with the NOx purifying temperature range after aging so that the NOx purifying temperature characteristic can be stabilized.

According to the present invention to achieve the above object,
There is provided an exhaust gas purifying catalyst in which Pr and Pt are carried on the gallosilicate.

In the catalyst comprising Pt-supported gallosilicate and CeO ₂ , the NOx purification temperature characteristic becomes unstable because the temperature after aging shifts to a higher temperature side as compared with the fresh NOx purification temperature range. . In this case, the NOx purification temperature range after aging is substantially constant.

As described above, when Pr is carried together with Pt on gallosilicate, this Pr has a function of suppressing the oxidizing ability of Pt. Therefore, the NOx purification temperature range in the fresh state of the catalyst containing Pr is as follows. The temperature of the catalyst which does not contain is shifted to a higher temperature than that of the
x substantially coincides with the purification temperature range. This stabilizes the NOx purification temperature characteristics of the catalyst.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of an exhaust gas purifying catalyst has a carrier supporting Pt, which is a catalytic metal, and CeO ₂ having an average crystallite diameter D of D ≧ 13.0 nm. The carrier is composed of gallosilicate and the S
iO ₂ / Ga ₂ O ₃ molar ratio M is 100 ≦ M ≦ 1230
Preferably, 130 ≦ M ≦ 1200 is set.

The amount of Pt-supported gallosilicate and the amount of P
The amount of Pt supported on t-supported gallosilicate is the same as the amount of Pt-supported zeolite and the amount of Pt supported on Pt-supported zeolite, and the amount of Pt-supported gallosilicate (Pt / Ga) is 19 wt% <(Pt / G
a) <100 wt%, and the amount of Pt carried in the Pt-supported gallosilicate is set to 3.5 wt% <Pt <11 wt%, respectively (for details, see JP-A-8-131838).
Reference). In this case, (Pt / Ga) ≦ 19 wt%
In this case, the HC adsorption capacity of the Pt-supported gallosilicate decreases, so that the NOx (mainly NO) purification rate decreases.
(Pt / Ga) = In 100 wt%, N by CeO ₂
Since the Ox adsorption ability cannot be obtained, the NOx purification rate decreases, and the heat deterioration suppressing effect of CeO ₂ cannot be obtained, so that the heat resistance of the catalyst decreases. Further, when Pt ≦ 3.5 wt%, the Pt amount is small, so that the NOx purification rate decreases.
Even if it is set to ≧ 11 wt%, the NOx purification rate hardly changes.

The Pt-supported gallosilicate is prepared by adding Pt to gallosilicate by an ion exchange method, a vacuum plating method (PVD),
It is manufactured by being supported by an impregnation method or the like. In this case, adopting the vacuum plating method is effective in improving the NOx purification rate because Pt is supported on the gallosilicate surface and the contact between the Pt and the exhaust gas is improved.

The gallosilicate is of MFI type, and may be unmodified gallosilicate. However, in consideration of the heat resistance to high-temperature exhaust gas during operation of the engine, the unmodified gallium silicate is subjected to a degaussing treatment. The resulting modified gallosilicate is desirable.

As the Ga removal treatment for the unmodified gallosilicate, at least one treatment of an acid treatment, a steam treatment or a boiling water treatment is applied.

As the acid treatment, HCl of 0.5 to 10N is used.
The temperature of the solution is raised to 70 to 90 ° C., the unmodified gallosilicate is charged into the HCl solution, and the solution is stirred for 1 to 25 hours.

In the boiling water treatment, the unmodified gallosilicate is subjected to a water-containing treatment, and the ambient temperature around the water-containing unmodified gallosilicate is raised to 550 to 600 ° C. A method of holding the modified gallosilicate for about 4 hours is employed.

Further, as the steam treatment, unmodified gallosilicate is mixed with 750 to 900 containing about 10% of water.
For example, a method of maintaining the composition in an atmosphere of 10 ° C. for 10 to 20 hours is employed.

These acid treatment, boiling water treatment and steam treatment are applied alone or in combination of two or more, and are repeated as necessary. As a result, a modified gallosilicate was obtained, and the SiO ₂ / Ga ₂ O ₃ molar ratio M was 100.
≦ M ≦ 1230, preferably 130 ≦ M ≦ 1200
It is adjusted to. The heat-resistant temperature of such a modified gallosilicate is about 1000 ° C.

Pt, which is a metal for catalyst, is
Exerts oxidizing ability and reducing ability. The oxidation capacity of Pt is HC
+ O_Two→ H_TwoO + CO_TwoAnd CO + O_Two→ CO_TwoTo
Contributes to the oxidation reaction. The reduction capacity of Pt is the theoretical air-fuel ratio
In NO, NO is adsorbed and NO → N_TwoReduction reaction
On the other hand, NO + O _Two
→ NO_TwoOxidation reaction and NO_Two+ HC + O_Two→ N
_Two+ CO_Two+ H_TwoIt contributes to a reduction reaction such as O.

The hydrophobic property of the modified gallosilicate is enhanced by the degaussing treatment, and the specific surface area is expanded by the elimination of Ga in addition to having the basic skeleton structure of the unmodified gallosilicate. Thus, the adsorptive capacity, which is a characteristic thereof, is enhanced. Such a modified gallosilicate exhibits good adsorption capacity for HC in exhaust gas even in the presence of moisture, supplies concentrated HC to Pt, and exhibits excellent durability in a hydrothermal environment. . This makes it possible to increase the NOx purification rate in an oxygen-excess atmosphere.

CeO ₂ is polycrystalline particles in which a plurality of crystallites are aggregated as shown in FIG. 1, and the average crystallite diameter D is set to D ≧ 13.0 nm. The control of the average crystallite diameter D is performed by subjecting CeO ₂ having a predetermined average crystallite diameter D to a heat treatment in the atmosphere. In this case, CeO
The average crystallite diameter D of No. ₂ is larger after heat treatment than before heat treatment.

For calculating the crystallite diameter D _(hkl), a Schuler equation is used.
That is, D _(hkl) = 0.9λ / (β _1/2 · cos θ) was used. Here, hkl is the Miller index, λ is the characteristic X-ray wavelength (、), β _1/2 is the half-value width (radian) of the (hkl) plane,
θ is the X-ray reflection angle. Therefore, in CeO ₂ ,
The half-width β _1/2 of the (111) plane was measured from the X-ray diffraction pattern to calculate D ₍₁₁₁₎ of each crystallite, and the average crystallite diameter D was determined from them.

CeO ₂ is NOx in an oxygen-excess atmosphere.
Since it exhibits the adsorption ability, the NOx concentration near Pt is increased. This also makes it possible to improve the NOx purification rate in an oxygen-excess atmosphere.

A second embodiment of the exhaust gas purifying catalyst is one in which Pr is carried on the gallosilicate together with Pt.
Other configurations are the same as those of the first embodiment.

Since this Pr has a function of suppressing the oxidizing ability of Pt, the NOx purification temperature range in the fresh state of the catalyst containing Pr shifts to a higher temperature side than that of the first embodiment not containing Pr. , Approximately coincides with the NOx purification temperature range after aging. This stabilizes the NOx purification temperature characteristics of the catalyst.

The amount of Pr carried depends on the amount of Pt carried, and Pt / Pt /
The Pr molar ratio M ₁ is set to M ₁ ≦ 7.5. Pr is
The effect is exhibited even with a small amount of support. However, M ₁ >
In 7.5, since the NOx purification temperature range in the fresh state is on the low temperature side, the difference from the NOx purification temperature range after aging becomes large.

Hereinafter, a specific example will be described. [Example I] A. Production of Modified Gallosilicate (a) An unmodified gallosilicate having an SiO ₂ / Ga ₂ O ₃ molar ratio M of 40 was charged into a 1N HCl solution at 90 ° C., and then stirred for 3 hours. This was performed to obtain a slurry.

(B) The solid content was separated from the slurry by filtration, and the solid content was washed with pure water until the pH of the washing water became pH ≧ 4.

(C) The solid content was measured at 130 ° C., 5
A drying treatment was performed under the conditions of time, and then the solid content after drying was subjected to a baking treatment at 400 ° C. for 12 hours in the air to obtain a bulky modified gallosilicate.

(D) The bulk modified gallosilicate was pulverized to obtain a powdery modified gallosilicate. The SiO ₂ / Ga ₂ O ₃ molar ratio M of the modified gallosilicate is 10
8, which indicates that degaussing has occurred. The heat resistance temperature of the modified gallosilicate was 1000 ° C.

Next, various powdery modified gallosilicates were obtained in the same manner as described above except that the modification treatment conditions were changed. Table 1 shows examples 1 to 11 of modified gallosilicates.
, The Si content, the Ga content, the Na content, and the SiO ₂ / Ga ₂ O ₃ molar ratio M. The unmodified gallosilicate and the first exemplified modified gallosilicate are listed in Table 1 as Unmodified and Example 3, respectively.

[0042]

[Table 1]

B. Production of Pt-supported gallosilicate (a) 25 g of dinitrodiammine platinum was dissolved in 1000 ml of 25% aqueous ammonia under heating to obtain a platinum solution (Pt concentration: 1.5%).

(B) 92 g of gallosilicate Example 1 was added to 533 g of the platinum solution, and then the mixture was placed in an evaporator and evaporated to dryness, thereby comprising the modified gallosilicate and Pt supported thereon. Example 1 of Pt-supported gallosilicate was obtained.

(C) Example 1 was subjected to a drying treatment in air at 130 ° C. for 1 hour, and then dried and fired in Example 1 at 400 ° C. for 12 hours in air. The amount of Pt carried in Example 1 was 8.0% by weight.

Next, using the modified gallosilicate examples 2 to 11, Pt-supported gallosilicate examples 2 to 11 were obtained in the same manner as described above. The amount of Pt carried in these Examples 2 to 11 is the same as in Example 1.

C. Producing an average crystallite size D of CeO ₂ is prepared commercially available CeO ₂ is 7.8 nm, to, 700 ° C. under atmospheric, was subjected to heat treatment for 3 hours. The average crystallite diameter D of CeO ₂ after the heat treatment is 13.3.
nm. This heat treatment suppresses CeO ₂ from absorbing NOx as nitrate.

D. Preparation of catalyst Example 1 of Pt-supported gallosilicate 30 g, CeO_Two60
g, 50 g of 20% silica sol and 180 g of pure water.
Alumina balls with a diameter of 5 mm are put into the pot and 12:00
The wet pulverization was performed to prepare a slurry. This place
Of Pt-supported gallosilicate in the first catalyst
The amount is about 33 wt%, while CeO _TwoIs about
It was 67 wt%.

A 65.4 mil cordierite honeycomb having a diameter of 25.4 mm, a length of 60 mm, and a capacity of 400 cells / in ² was immersed in the slurry, and the honeycomb was taken out of the slurry and the excess was air-jetted. Then, the slurry was dried by holding the honeycomb 2 under heating at 150 ° C. for 1 hour.
A baking treatment was performed at 12 ° C. for 12 hours to hold the solid material on the honeycomb. In this case, the retained amount of the solid material in the honeycomb, that is, the layer having the catalyst and the silica is 20%.
It was 0 g / liter. This is referred to as catalyst example 1.

Next, Examples 2 to 1 of Pt-supported gallosilicate
Using No. 1, catalyst examples 2 to 11 were obtained in the same manner as described above. The holding amounts of the honeycombs of Examples 2 to 11 are the same as those of Example 1.

E. Aging treatment The catalysts of Examples 1 to 11 were subjected to aging treatment. This aging treatment is performed by using a tubular furnace in a furnace atmosphere: 10 vol% O ₂ , 10 vol% H _{2 in} order to create a hydrothermal environment.
O and balance N ₂ ; heating temperature: 700 ° C., heating time: 2
The test was performed at 0 hours.

F. Exhaust gas assumed purification test Table 2 shows the composition of the test gas prepared to mimic the exhaust gas.

[0053]

[Table 2]

First, Example 1 of the catalyst after the aging treatment was installed in a fixed bed flow type reactor, and then Table 2 was placed in the reactor.
The test gas having the composition was subjected to space velocity S.P. V. = 5 × 10 ⁴ h
^-1 and the temperature of the test gas is 2
The temperature was raised at 0 ° C./min, and the NO purification rate was measured at a predetermined gas temperature. The same test was performed on the catalysts 2 to 11 after the aging treatment.

Table 3 shows the SiO ₂ / G for Examples 1-11.
The a ₂ O ₃ molar ratio M, the maximum NO purification rate and the gas temperature at which it is exhibited are shown. Also, FIG.
₂ is a graph showing the relationship between the ₂ / Ga ₂ O ₃ molar ratio M and the maximum NO purification rate.

[0056]

[Table 3]

[0057] Table 3, as is apparent from FIG. 2, SiO ₂
In Examples 2 to 10 in which the / Ga ₂ O ₃ molar ratio M was set to 100 ≦ M ≦ 1230, the maximum NO purification rate after the aging treatment was as high as 43% or more. When the molar ratio M is set to 130 ≦ M ≦ 1200, as in Examples 4 to 9, the maximum NO purification rate can be increased to 47% or more.

Instead of the Pt-supported gallosilicate in the catalyst, Pt having a SiO ₂ / Al ₂ O ₃ molar ratio of 300 was used.
A comparative column using a supported zeolite (the zeolite is a modified ZSM-5 zeolite) was subjected to the same aging treatment as described above, and was then subjected to the same purification test. As a result, the maximum NO purification rate was 42%. Was 260 ° C.

For example, in Example 2 of the catalyst, when CeO ₂ having an average crystallite diameter D of 8.7 nm was used, NOx emission was observed at a gas temperature of 330 ° C. or higher. [Example II] A. Production of Modified Gallosilicate (a) An unmodified gallosilicate having an SiO ₂ / Ga ₂ O ₃ molar ratio M of 40 is charged into a 3N HCl solution at 90 ° C., and then stirred for 3 hours. This was performed to obtain a slurry.

(B) The solid content was separated from the slurry by filtration, and the solid content was washed with pure water until the pH of the washing water became pH ≧ 4.

(C) The solid content was measured at 130 ° C., 5
A drying treatment was performed under the conditions of time, and then the solid content after drying was subjected to a baking treatment at 400 ° C. for 12 hours in the air to obtain a bulky modified gallosilicate.

(D) The bulk modified gallosilicate was pulverized to obtain a powdery modified gallosilicate. The SiO ₂ / Ga ₂ O ₃ molar ratio M of this modified gallosilicate is 50
0, indicating that degaussing has occurred. The heat resistance temperature of the modified gallosilicate was 1000 ° C.

B. Production of Pt-Pr-supported gallosilicate (a) 25 g of dinitrodiammine platinum was dissolved in 1000 ml of 25% aqueous ammonia under heating to obtain a platinum solution (Pt concentration 1.5%).

(B) 533 g of a platinum solution, 92 g of modified gallosilicate, 14.5 g of praseodymium acetate and 500 ml of pure water were mixed, and then the mixture was placed in an evaporator and evaporated to dryness. And Pt comprising Pt and Pr carried on the
-Example 1 of Pr-supported gallosilicate was obtained.

(C) Example 1 was subjected to a drying treatment in air at 130 ° C. for 1 hour, and then dried, and subjected to a baking treatment in air at 400 ° C. for 12 hours. The amount of Pt carried in Example 1 was 8.0 wt%,
The / Pr molar ratio M ₁ was 1.

Next, the amount of praseodymium acetate was changed to 72.
Except that it was set to 5 g, a Pt-Pr-supported gallosilicate Example 2 having a Pt / Pr molar ratio M ₁ of 0.2 was obtained in the same manner as described above. Similarly, the amount of praseodymium acetate was set to 1.5 g, and the Pt / Pr-supported gallosilicate having a Pt / Pr molar ratio M ₁ of 10 was prepared.
I got

C. Preparation of Catalyst 30 g of Pt-Pr-supported gallosilicate, 60 g of CeO ₂ having an average crystallite diameter D of 13.3 nm obtained in Step C of Example I, 50 g of 20% silica sol, and 1 g of pure water
80 g and alumina balls having a diameter of 5 mm were put into a pot, and wet-pulverized for 12 hours to prepare a slurry. The amount of Pt-Pr carrying gallosilicate in the catalyst is about 33 wt%, whereas, the amount of CeO ₂ was about 67 wt%.

A 65.4 mil cordierite honeycomb having a diameter of 25.4 mm, a length of 60 mm, and a capacity of 400 cells / in ² , was immersed in the slurry, and the honeycomb was taken out of the slurry and the excess was air-jetted. Then, the slurry was dried by holding the honeycomb 2 under heating at 150 ° C. for 1 hour.
A baking treatment was performed at 12 ° C. for 12 hours to hold the solid material on the honeycomb. In this case, the retained amount of the solid material in the honeycomb, that is, the layer having the catalyst and the silica is 20%.
It was 0 g / liter. This is referred to as catalyst example 1.

Next, using Examples 2 and 3 of Pt-Pr-supported gallosilicate, Catalyst Examples 2 and 3 were obtained in the same manner as described above. The holding amounts of the honeycombs of Examples 2 and 3 are as shown in Example 1.
Is the same as

D. Exhaust gas assumed purification test Table 4 shows the composition of the test gas prepared to mimic the exhaust gas.

[0071]

[Table 4]

First, Example 1 of the fresh catalyst was placed in a fixed bed flow type reactor, and then a test gas having the composition shown in Table 4 was placed in the reactor at a space velocity S.P. V. = 5 × 10 ⁴ h ^-1
And make the temperature of the test gas 20
The temperature was raised at a rate of ° C / min, and the NO purification rate was measured at a predetermined gas temperature. A similar test was conducted using fresh catalyst example 2,
3 was also performed.

Next, an aging treatment was applied to the fresh catalyst examples 1 to 3, and then the same purification test as described above was performed on the aged catalyst examples 1 to 3.
This aging treatment is performed by using a tubular furnace in a furnace atmosphere: 10 vol% O ₂ , 10 vol, in order to create a hydrothermal environment.
% H ₂ O and the balance N ₂ ; heating temperature: 700 ° C., heating time: 20 hours.

Table 5 shows the gas temperature which shows the fresh state and the maximum NO purification rate after aging for Catalyst Examples 1 to 3 (Pr not supported) of Catalyst I in Example I as a comparative example.

[0075]

[Table 5]

FIG. 3 shows gas temperature and N for fresh and aged catalysts of Example 1 and Comparative Example.
It is a graph which shows the relationship with O purification rate.

As is clear from Table 5 and FIG. 3, the NO purification temperature range in Example 1 of the catalyst in the fresh state shifts to a higher temperature side than that in the comparative example in the fresh state, and the NO purification temperature after aging. It can be seen that they almost match the area. In the comparative example, the NO purification temperature range has shifted to the high temperature side after aging.

From Table 5, it can be seen that Example 2 of the catalyst also shows the same tendency as Example 1, but Example 3 of the catalyst has a Pt / Pr molar ratio M
₁ is M ₁ > 7.5, so that the NO purification temperature range in the fresh state is on the low temperature side,
The difference from the O purification temperature range increases.

[0079]

According to the first aspect of the present invention, with the above-described configuration, even after being exposed to a hydrothermal environment,
It is possible to provide an exhaust gas purifying catalyst that maintains a high NOx purification rate in an oxygen-excess atmosphere and does not emit NOx in a high gas temperature range.

According to the second aspect of the present invention, in addition to the above-described effects, the NOx purification temperature range in a fresh state is made substantially equal to that after aging, so that the NOx purification temperature characteristic is improved. It is possible to provide a stabilized exhaust gas purifying catalyst.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of CeO ₂ .

FIG. 2 is a graph showing the relationship between the molar ratio M of SiO ₂ / Ga ₂ O ₃ and the maximum NO purification rate.

FIG. 3 is a graph showing a relationship between a gas temperature and a NOx purification rate.

Claims (2)

[Claims] 1. A carrier carrying Pt and an average crystallite diameter D
Has Ce ≧ ₂ with D ≧ 13.0 nm, and the carrier comprises gallosilicate, and the gallosilicate SiO 2
An exhaust gas purifying catalyst, wherein the ₂ / Ga ₂ O ₃ molar ratio M is 100 ≦ M ≦ 1230. 2. The exhaust gas purifying catalyst according to claim 1, wherein Pr and Pt are carried on the gallosilicate.