JP3375360B2 - Method for producing exhaust gas purifying catalyst - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an exhaust gas purifying catalyst for an exhaust system of a lean burn engine of an automobile, and more particularly, a measure for expanding the active temperature range of the NOx (nitrogen oxide) purifying catalyst. Regarding

[0002]

2. Description of the Related Art C is used as a catalyst for purifying exhaust gas of automobiles.
Oxidation of O (carbon monoxide) and HC (hydrocarbons) and NOx
Generally known are three-way catalysts that simultaneously perform reduction with This three-way catalyst is one in which γ-alumina carrying Pt (platinum) and Rh (rhodium) is carried on a carrier, and the air-fuel ratio (A / F) of the engine is 14.7 which is the theoretical air-fuel ratio. A high purification efficiency can be obtained when controlled to near.

On the other hand, there is a demand to increase the ratio of the air-fuel ratio in order to reduce the fuel consumption of the engine. In this case, since the exhaust gas has a so-called lean atmosphere with excess oxygen, CO and HC can be oxidized and purified by the three-way catalyst, but NOx reduction purification is difficult.

Therefore, in recent years, research on a zeolite catalyst in which a transition metal is supported on zeolite by ion exchange has been advanced. With this zeolite catalyst, NOx can be catalytically decomposed into N ₂ and O ₂ either directly or by a coexisting reducing agent (for example, CO, HC, etc.) even in a lean atmosphere.

As an example of this, for example, Japanese Patent Laid-Open No. 1-135
Japanese Patent No. 541 discloses a zeolite catalyst in which a noble metal is supported on zeolite. According to this, by using a noble metal as an active species, it is possible to reduce and purify NOx over a wide region on the lean side from the stoichiometric air-fuel ratio.

[0006]

[SUMMARY OF THE INVENTION However, in the conventional zeolite catalysts, although certainly at the laboratory level have a relatively high purification performance (e.g., 80% is exceeded NOx purification rate), surprising for activation temperature range There is a problem of being narrow.

The reason for this is that the zeolite (catalyst base material) supported on the carrier has one function of activating HC by the acid points on the surface of the zeolite and burning it. The oxidized HC acts as a reducing agent to decompose NOx, but if the acidity of the surface of the base material is high, the activity of HC becomes too high and the decomposition is completed early, resulting in NOx reduction. This is probably because it cannot efficiently contribute to the decomposition.

The present invention has been made in view of the above problems, and an object of the present invention is to make it possible to expand the active temperature range of NOx purification when manufacturing an exhaust gas purification catalyst. is there.

[0009]

In order to achieve the above object, in the invention of claim 1, the zeolite is damaged by heat treatment in an inert gas atmosphere after the firing step. Without reducing the acidity of the zeolite surface.

[0010] Specifically, in this invention, the zeolites washcoated on a carrier, the catalyst material comprising a noble metal supported on the zeolite was dried in air, large
After baking in air , baking temperature in an inert gas atmosphere
Heat-treat at a temperature of at least 40 ° C.

[0011] The zeolite is a crystalline porous body having pores of Mi black, particularly hydrogen-type zeolite is preferred, A type, X type, Y type, ZSM-5, ferrierite,
Mordenite and the like may be used .

[0012] As the above-mentioned noble metal is suitably Pt or Ir, may be used in this addition to Rh, Pd, Ru, Os, Ag, alone or combination seen if not I a Au.

[0013] As the above SL inert gas, but N ₂ gas is preferred, can be used In addition to this example, He gas or Ar gas.

According to a second aspect of the present invention, in the first aspect of the invention, the noble metal is at least Pt and Ir, and the heat treatment is performed at a temperature of 500 ° C. or higher. In this case, other noble metals such as Rh may be used in combination.

[0015]

With the above construction, in the invention of claim 1, the catalyst material comprising the zeolite wash-coated on the carrier and the noble metal supported on the zeolite is dried in the atmosphere.
Then , the zeolite is sufficiently adhered to the surface of the carrier by firing in the air .

[0016] Later, the catalyst material by the acidity of the zeolite <br/> surface is heat treated to reduce, the acidity decreases moderately. Due to this decrease in acidity, the rate of decomposition of HC on the surface of the base material is reduced and combustion of HC is suppressed, that is, HC in a partially oxidized state is easily obtained, which results in reduction decomposition of NOx. It will proceed even at a relatively low exhaust gas temperature, and the purification temperature range will be expanded. Further, in the firing step, although the zeolite is sufficiently adhered to the surface of the carrier, the foreign molecule physically adsorbed on the surface of the base material may not be sufficiently released. On the other hand, since the heat treatment promotes the release thereof, the original activity of the catalyst can be sufficiently expressed.

[0017] At this time, the noble metal active species is a noble metal because the degree of reduction in activity due to the influence of heat is small, it is possible to perform the heat treatment at a high temperature of at least the firing temperature significantly over SL action It can be anything. Also,
By performing the above heat treatment in an inert gas atmosphere, it is possible to appropriately reduce the acidity while suppressing damage to the base material due to heat.

According to the second aspect of the present invention, when NOx is adsorbed on Ir and the charge distribution of the NOx changes, when HC activated by acid points on the surface of the base material burns on Pt. The resulting partially oxidized HC attacks the NOx, which causes NOx to be decomposed. Therefore, the easier NOx is adsorbed to Ir, the higher the NOx purification rate. At this time, generally, Ir has high activity in the state of metallic Ir,
It is considered that NOx is also easily adsorbed. However, in reality, a kind of unstable redox state in which Ir oxide Ir and metal Ir are appropriately mixed together is more like NOx.
Is easily adsorbed. Therefore, Ir is subjected to a heat treatment at a temperature of 500 ° C. or higher, so that the metal Ir and the oxide Ir are mixed, and thus the NOx purification capacity is improved.

[0019]

Embodiments of the present invention will be described below with reference to the drawings.

(Example 1) Pt: Ir: Rh = 30 in weight ratio using divalent ammine acid Pt salt as Pt material, Ir chloride Ir salt as Ir material and nitric acid Rh salt as Rh material. The solvent weighed so as to be 10: 1 was mixed with alcohol and ion-exchanged water, and was modified to have H-type with a Caban ratio (SiO ₂ /
Pentasil-type zeolite of Al ₂ O ₃₎ is 70 ZSM
5 was mixed and the mixed slurry was dried to obtain a catalyst powder. Next, a catalyst slurry having an alumina binder content of 20 wt% was prepared, and the catalyst slurry was wash-coated on a cordierite honeycomb carrier (400 cells / inch ² ) so that the weight per catalyst carrier was 6 g and the catalyst was 6 g. Got the material.

With respect to the above catalyst material, as shown in FIG.
After the drying step and the firing step were sequentially performed, an aging treatment was performed in an inert gas atmosphere to obtain an exhaust gas purifying catalyst as the product of the present invention. In particular,
In the firing step, heat treatment was performed at 500 ° C. for 2 hours in the atmosphere. And in the aging process,
800 ° C in N ₂ gas atmosphere as an inert gas
Was subjected to heat treatment for 25 hours.

The NOx purification rate of the above-mentioned product of the present invention and the conventional product not subjected to the aging treatment was examined while changing the exhaust gas temperature at the inlet. The result is shown in FIG. As can be seen from FIG. 2, in the product of the present invention, the activation temperature range of 20% or more of the NOx purification rate is 220 to
This is 175 to 245 ° C, which is more than 2.3 times larger than 250 ° C.

According to this embodiment, the acidity of the surface of the base material is appropriately reduced by heating the catalyst material after drying and firing. Due to this decrease in acidity, the rate of decomposition of HC on the surface of the base material is reduced and combustion of HC is suppressed, that is, HC in a partially oxidized state is easily obtained, which results in reduction decomposition of NOx. It is considered that the purification temperature range will be expanded by starting to proceed even at a relatively low exhaust gas temperature. In addition, in the calcination step, although the zeolite is sufficiently adhered to the surface of the carrier, the catalyst poisons such as ammine and chlorine contained in the catalyst material, the HC and CO physically adsorbed on the surface of the base material, etc. Foreign molecules may not be eliminated sufficiently. On the other hand, since the desorption is promoted by the above heat treatment, it is considered that the original activity of the catalyst can be sufficiently expressed.

At this time, since the active species is a noble metal and the noble metal has a small degree of decrease in activity due to the influence of heat, the heat treatment can be performed at a high temperature of 800 ° C., for example, as described above. Can make the above-mentioned effect remarkable. Further, by performing the above heat treatment in N ₂ gas as an inert gas, it is possible to appropriately reduce the acidity while suppressing the heat damage to the base material.

Further, when NOx is adsorbed on Ir and the charge distribution of the NOx is changed, partially oxidized HC generated when HC activated by acid points on the surface of the base material burns on Pt. Attacks the above NOx, which decomposes NOx. Therefore, the easier NOx is adsorbed to Ir,
The NOx purification rate will be improved. At this time, when Ir is heat-treated at a temperature of 500 ° C. or higher, the metal Ir and the oxide Ir are mixed and NOx is easily adsorbed, so that the NOx purification performance is improved.

The sample B obtained by subjecting the above conventional product (sample A) to a heat treatment at 800 ° C. for 25 hours in the atmosphere and the sample B obtained in the H ₂ gas atmosphere at 800
For the sample C obtained by performing the heat treatment at 25 ° C. for 25 hours, the air-fuel ratio (A /
When F) was evaluated for the respective NOx purification characteristics under the condition of A / F = 22, as shown in FIG. 3, a reduction in the purification capacity was recognized. It is considered that this is because the acidity is not lowered in the atmosphere or the H ₂ gas atmosphere as in the N ₂ gas atmosphere. Especially in sample C,
Protons and OH adsorbed on the base material due to the reducing atmosphere
It is considered that even the ions react with each other and are desorbed, so that they are easily damaged by heat. However, the above sample B was subjected to 25 at 800 ° C. in an N ₂ gas atmosphere.
In Sample D obtained by performing the heat treatment for a long time, a high purification ability was recognized in a wide temperature range from a low temperature region to a high temperature region. This shows that the effect of adding the heat treatment is present even in the case where deterioration occurs due to long-term use, and the high purifying ability is caused by the occurrence of an appropriate oxidation state of Ir. it seems to do.

The NOx of the same heat treatment in a He gas atmosphere instead of the above N ₂ gas
When the purification rate was examined, although a slight decrease (about 2 to 5%) was observed at the maximum purification rate compared with the case of N ₂ gas,
It has been found that a purifying characteristic close to that of N ₂ gas can be obtained.

(Example 2) Pt and Ir were supported so that Pt: Ir = 3: 1 by weight ratio, and H-type zeolite ZS with a Caban ratio of 200 was carried.
M-5 was loaded on a cordierite honeycomb carrier at a rate of 6 g per liter of the carrier, and a catalyst material obtained by sequentially performing the drying step and the calcination step shown in Table 1 below was used to prepare samples 1 to 6. Were produced respectively. That is, Samples 1 to 3 above were obtained by performing the drying process together in the atmosphere,
In the firing step, Sample 1 was similarly performed in the atmosphere, Sample 2 was performed in the N ₂ gas atmosphere, and Sample 3 was performed in the H ₂ gas atmosphere. On the other hand, Samples 4 to 6 are those in which the drying process was performed in an N ₂ gas atmosphere. In the firing process, Sample 4 was in the air, Sample 5 was in an N ₂ gas atmosphere, and Sample 6 was H. These were performed in _two gas atmospheres.

[0029]

[Table 1]

Further, Rh is added to the above Pt and Ir to obtain P
Samples 7 to 9 were prepared by sequentially performing the drying step and the calcination step shown in Table 1 above on the same catalyst material with t: Ir: Rh = 30: 10: 1. That is, the sample 7 is subjected to both the drying and firing steps in the air, and the sample 8 is subjected to the drying step in an N ₂ gas atmosphere while the firing step is performed in the air, and the sample 9 is dried and fired. Both of the above processes were carried out in an N ₂ gas atmosphere. In the firing process, each sample 1-9
Was subjected to a heat treatment at 500 ° C. for 2 hours.

With respect to each of the samples 1 to 9 described above, the space velocity (SV) was SV = 55000 in the fresh state and the state where the sample was heat-treated in the air at 800 ° C. for 8 hours.
Each NOx purification rate at the time of h ⁻¹ was examined. The results are shown in Table 2 below.

[0032]

[Table 2]

NOx in the fresh state shown in Table 2 above
As can be seen from the value of the purification rate, the sample 1 and the sample 7 in which both processes were both performed in the air showed the highest NOx purification rate in each catalyst material group. On the other hand, the sample 4 in which the drying step was performed in the N ₂ gas atmosphere was slightly inferior to the sample 1 in comparison, and the sample 2 in which the firing step was performed in the N ₂ gas atmosphere was considerably deteriorated. From this, it can be seen that even if the treatment in the N ₂ gas atmosphere is performed at a temperature of 500 ° C. or less in the drying step or the firing step, a sufficient effect cannot be obtained. In addition, the NOx purification rate after the heat treatment in the atmosphere was determined by using the dry firing process under N ₂
Samples 2 and 5 which were performed in a gas atmosphere and samples 5 and 6 and 9 in which both the drying and firing steps were performed in an atmosphere other than the atmosphere were all deteriorated. Although 9 was also improved, the level of the NOx purification rate itself was low. From this, it is understood that even if the heat treatment after the firing step is performed in the atmosphere, a sufficient effect cannot be obtained.

Next, the samples 1, 5 and 7 were subjected to a heat treatment at 800 ° C. for 8 hours in a N ₂ gas atmosphere after the firing step, and the same NOx purification rate was examined. The results are shown in Table 3 below.

[0035]

[Table 3]

As can be seen from Table 3 above, after the heat treatment, the NOx purification rate is 800 ° C. at 8:00 in each atmosphere.
It is improved by 20 to 30% as compared with the case of performing the heat treatment during the period . That is, the temperature during the heat treatment in the N ₂ gas atmosphere is preferably 500 ° C. or higher, which is the firing temperature of the catalyst material , and the active metal, particularly Ir, is not activated only by performing the N ₂ gas treatment without heating. Conceivable.

Further, the sample 1 of the present invention which has been subjected to the heat treatment, the untreated product which has not been subjected to the heat treatment, the Cu ion-exchanged zeolite catalyst, and P as the active species.
A three-way catalyst carrying t and Rh and a three-way catalyst obtained by subjecting the three-way catalyst to the heat treatment are evaluated for NOx purification ability in the case where the temperature of exhaust gas is generated from low temperature to high temperature. Therefore, the NOx purification rate in the FTP mode (US emission test mode) was tested. The results are shown in Table 4 below. The gasoline engine used had a displacement of 1.3 liters and had an air-fuel ratio (A / F)
Is A / F = 16 to 24, and the capacity of each catalyst is 1.3 liters.

[0038]

[Table 4]

As can be seen from Table 4 above, in the product of the present invention, N
The Ox purification rate is nearly 1.7 times higher than that of the untreated product and 1.5 times or more higher than that of the Cu ion-exchanged zeolite catalyst. The reason for this is that in the FTP mode, the exhaust gas temperature is 300 to 40 except when starting the vehicle.
Since the product of the present invention has a characteristic of changing in the range of 0 ° C., it is considered that the NOx purification rate is improved by expanding the active temperature range of NOx purification to the extent that most of the above range can be covered. The NOx purification rate of the heat-treated three-way catalyst of this example is improved by 14% as compared with the non-treated three-way catalyst.

[0040]

As described above, according to the first aspect of the present invention, the zeolite wash-coated on the carrier is noble.
The catalytic material metal, which are supported and dried in atmospheric
After firing in mind, to reduce the acidity of the zeolite surface
With As at a firing temperature or higher in an inert gas atmosphere
By heat treatment with, it is possible to reduce the acidity of the surface of the zeolite while protecting the catalyst base material, the combustion of HC on the surface of the base material is appropriately suppressed,
As a result, the temperature range in which NOx is reduced and decomposed can be expanded.

According to the second aspect of the present invention, at least Pt and Ir are contained in the noble metal as an active species carried by the zeolite.
And the heat treatment temperature was set to 500 ° C. or higher, the adsorption of NOx to Ir was increased, and H in Pt was increased.
Since the reductive decomposition of NOx due to the combustion of C can be promoted, the NOx purification rate can be improved in a wide temperature range in combination with the effect of the invention of claim 1.

[Brief description of drawings]

FIG. 1 is a process chart showing a method for manufacturing an exhaust gas purifying catalyst according to a first embodiment of the present invention.

[FIG. 2] NOx of the product of the present invention and the conventional product in Example 1.
It is a characteristic view which shows a purification characteristic.

3 is a characteristic diagram showing NOx purification characteristics of samples A to D in Example 1. FIG.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Makoto Kyogoku 3-3 Shinchi, Fuchu-cho, Aki-gun, Hiroshima Mazda Co., Ltd. (56) References JP-A-4-197447 (JP, A) (58) Survey Areas (Int.Cl. ⁷ , DB name) B01J 21/00-37/36 B01D 53/86

Claims (2)

(57) [Claims] 1. A carrier is wash-coatedZeorai
ToAnd theZeoliteCatalyst consisting of precious metals supported on
MaterialIn the method of manufacturing the exhaust gas purifying catalyst used, The above catalyst material in the atmosphere DriedIn the atmosphereBurned
rear, NonIn an active gas atmosphereFiring temperature of the above catalyst material
At temperatures aboveExhaust gas characterized by heat treatment
Method for producing purification catalyst. 2. A process according to claim 1, the exhaust gas purifying catalyst, wherein the noble metal is at least comprises Pt and Ir, the exhaust gas purification The heat treatment, characterized in that the dividing line at a temperature above 500 ° C. For producing catalyst for automobile.