JPH06190285A - Production of exhaust gas purifying catalyst - Google Patents

Abstract

PURPOSE:To production an exhaust gas purifying catalyst imparted with superior NOx purification activity in a high oxygen concentration atmosphere and also improved in heat resistance. CONSTITUTION:In the production of a catalyst of a catalyst comprising a noble active seed supported on a metal-containing silicate, the above catalyst after supported is treated with sulfuric acid or the metal-containing silicate before the noble metal active seed being supported is treated with sulfuric acid. Thus since a new acid point is formed at the obtained catalyst device, the catalyst performance allowing HC to burn decompose NOx and the resistance are improved.

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.

[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.

NOx among the exhaust gas of the engine
Since there is a great concern that it will adversely affect the human body and the ecosystem, it must be prevented from being released into the atmosphere as much as possible. There are several methods for preventing the emission, but in the case of a mobile engine, it is realistic to purify with a catalyst installed in the engine exhaust system.

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, since the exhaust gas has a high oxygen concentration atmosphere due to the high air-fuel ratio, even if CO and HC can be oxidized and purified by the three-way catalyst as described above, NOx It cannot be reduced and purified.

Therefore, even in an atmosphere where the exhaust gas has a high oxygen concentration, NOx can be directly supplied or a reducing agent (for example, C
As a catalyst capable of catalytically decomposing N ₂ and O _{2 in} the presence of (O, HC, etc.), a catalyst composed of a zeolite carrying a transition metal Cu, for example, by ion exchange is considered promising.

At the same time, regarding the Cu ion-exchanged zeolite catalyst, in order to obtain a catalyst having excellent NOx purification characteristics corresponding to various states of exhaust gas, various countermeasures are taken for zeolite and active species supported on the zeolite. Is being considered.

For example, Japanese Patent Laid-Open No. 3-89942 discloses that
A technique is disclosed in which a mild acid point is formed in the zeolite by supporting a rare earth element together with Cu on the zeolite. This technique is intended to convert paraffinic hydrocarbons having low reactivity in HC into olefinic hydrocarbons having high reactivity due to the presence of the above-mentioned acid sites, thereby improving NOx decomposition activity. is there.

[0008]

However, NOx
The above-mentioned Cu ion-exchanged zeolite catalyst, which is said to be effective for effectively removing NOx, exhibits a NOx purification rate of more than 80% at the laboratory level, but under an oxygen-rich atmosphere in an actual operation under lean burn conditions, It is inevitable that the NOx purification rate will decrease due to the change in the properties of the actual exhaust gas.

Further, in general, a catalyst for purifying NOx has the following problems if the active species-supporting base material is a conventionally used zeolite. That is, when the Cayvan ratio is small, the purification activity of the catalyst is high, but at a high temperature, the activity tends to decrease due to the collapse of the crystal lattice, and when the Cayvan ratio is large, or the cation species are protons. When the catalyst is H-type, heat resistance of the catalyst is improved but purification activity is lowered.

Therefore, the inventor of the present invention, as a catalyst for NOx purification which has excellent NOx purification characteristics especially in a low temperature range and has improved heat resistance, is one in which a Pt-Ir noble metal is supported on a metal-containing silicate. Although developed, the characteristics of the metal-containing silicate itself are considered to have both a function of enhancing the activity of the catalyst in the fresh state of the catalyst and a property of improving heat resistance in order to solve the above-mentioned difficulties. Desirable for.

In view of the above, the present invention provides a catalyst containing a metal-containing silicate on which a noble metal active species is supported, by imparting the above-mentioned function and properties to the metal-containing silicate to obtain a high oxygen concentration atmosphere. It is an object of the present invention to obtain a catalyst having improved heat resistance and improved NOx purification characteristics of the catalyst.

[0012]

[Means for Solving the Problem and Its Action] The behavior of a catalyst for NOx purification in which an active species is supported on a metal-containing silicate to decompose HC by burning HC in an oxygen-rich atmosphere is as follows. At this time, the active species radicalize HC, and the radicalized HC attacks NOx, and the combustion of the HC causes NOx to become N ₂ and O _2.
It is conceivable that the HC and NOx are simultaneously removed by reducing and purifying.

[0013] Here, as a result of intensive research for solving the above-mentioned problems, the present inventor has found that O _{2 in} exhaust gas
In the presence of, when the HC is partially oxidized by the active species to be radicalized, if the HC is excited by the metal-containing silicate as a base material carrying the active species, the HC is radicalized in the radicalization reaction of the HC. Was further activated, and it became easier to obtain a radicalization reaction of HC suitable for decomposing NOx.

Then, as a result of various experiments conducted on various measures for forming a metal-containing silicate having a property of exciting HC so that radicalization of HC is further activated, for example, Pt-Ir active species are carried. A phenomenon in which HC is partially oxidized even at these newly formed acid points by imparting acid characteristics such that acid points in the metal-containing silicate are newly formed to the metal-containing silicate to be formed. It was found that this makes it possible to excite the HC.

At the same time, it was found that the metal-containing silicate having the above-mentioned acid characteristic that an acid point is newly formed is advantageous in terms of heat resistance of the catalyst.

The present invention has been made on the basis of the above findings, in which a metal-containing silicate carrying a noble metal active species is treated with sulfuric acid to form new acid sites on the metal-containing silicate. H depending on the acid point
It is intended to excite C and activate the radicalization reaction of HC on the catalyst.

Specifically, the solution means taken by the invention of claim 1 is directed to a method for producing an exhaust gas purifying catalyst for removing NOx in the presence of HC, and a metal-containing silicate as an active species-supporting base material. , A step of supporting an active noble metal species that decomposes NOx by burning HC, and a step of subjecting the noble metal-supporting metal-containing silicate obtained in the above step to a sulfuric acid treatment for modifying the acid characteristics of the noble metal-supporting metal-containing silicate. It is configured to include.

Further, specifically, the solution means taken by the invention of claim 2 is directed to a method for producing an exhaust gas purifying catalyst for removing NOx in the presence of HC, containing a metal as an active species supporting base material. A step of subjecting the silicate to a sulfuric acid treatment for modifying the acid characteristics of the metal-containing silicate; and burning HC on the sulfuric acid-treated metal-containing silicate to NOx.
And a step of supporting an active species of a noble metal that decomposes.

-Metal-Containing Silicate- The metal-containing silicate used in the method for producing an exhaust gas purifying catalyst according to the present invention is preferably an aluminosilicate (zeolite) using Al as a metal forming a crystal skeleton. And, if necessary, Ga together with the above Al,
An Al-containing metal-containing silicate in which a metal such as Ce, Mn, or Tb is used as a skeleton-forming material can also be used.

Examples of the metal-containing silicate are A type, X type
Type, Y type, ZSM-5, mordenite and the like are preferably used.

Although the cavern ratio in the aluminosilicate is not limited, it is advantageous to use an aluminosilicate having a large cavern ratio in order to improve the heat resistance of the catalyst.

Furthermore, regarding the metal-containing silicate represented by the aluminosilicate, the Na-type cation whose Na is the cation species has the acid characteristics such that the above-mentioned desirable excitation of HC occurs by the sulfuric acid treatment. Therefore, it is preferably used. On the other hand, the H-type metal-containing silicate in which the cation species is a proton shows a tendency different from the above-mentioned expression of the acid characteristic, and therefore its use is not very preferable.

-Regarding Active Species- As the active species supported on the metal-containing silicate, a noble metal effective for burning HC to decompose NOx into N ₂ and O ₂ is used, and contains Pt and Pt. Noble metals such as Pt-Ir-based and Pt-Ir-Rh-based active species are preferably used.

Therefore, the basic structure of the catalyst according to the production method of the present invention is particularly preferably that in which the Pt-based active species is supported on the Na-type metal-containing silicate.

-Regarding Sulfuric Acid Treatment- In order to obtain an acid property which causes preferable excitation of HC in the metal-containing silicate, the catalyst device or the metal-containing silicate is applied after or before the loading of the noble metal active species. The sulfuric acid treatment can employ a simple means of immersing the object to be treated in sulfuric acid for a certain period of time.

The concentration of sulfuric acid used for the sulfuric acid treatment by the above-mentioned immersion method is such that when the immersion time is about 30 minutes,
0.05N to 12N can be used,
Those having a concentration of 0.1 to 1 N are preferably used.

When the concentration of the sulfuric acid is less than 0.05 N, it is difficult to perform the sulfuric acid treatment capable of imparting the required acid characteristics to the metal-containing silicate, and conversely, when it exceeds 12 N, the sulfuric acid treatment causes Both of them are not preferable because the structure of the metal-containing silicate may collapse.

Further, when hydrochloric acid is used in the acid treatment of the metal-containing silicate, there is a concern that Cl ions in the hydrochloric acid may become a catalyst poison.
These acid treatments with hydrochloric acid and nitric acid should be avoided as they will form Ox.

According to the constitution of the present invention, in the production of a catalyst comprising a metal-containing silicate carrying a noble metal active species which decomposes NOx by burning HC, sulfuric acid is added to the catalyst device after carrying the above-mentioned noble metal active species. Since the above-mentioned metal-containing silicate is subjected to a sulfuric acid treatment before being treated or loaded with the noble metal active species, the obtained exhaust gas purifying catalyst is provided with a new effective catalyst for exciting HC in the exhaust gas. Acid points are formed.

Regarding the mechanism of formation of the above-mentioned acid points, Al outside the lattice contained in the metal-containing silicate reacts with the above sulfuric acid to form aluminum sulfate to form acid points, and in particular, noble metal active species are generated. When the sulfuric acid treatment is carried out after being supported, it is considered that the acid sites are also formed by the presence of sulfate ions on the metal-containing silicate and the noble metal active species to be supported by the sulfuric acid treatment.

[0031]

EXAMPLES Examples of the present invention will be described below.

-Manufacture of Catalytic Devices of Examples and Comparative Examples- <Example 1> Pt: divalent platinum ammine crystal as a Pt material and iridium trichloride as an iridium material were mixed with Pt:
Ir = 3: 1, and weigh each so that the amount of Pt and Ir supported is 3 g per liter of catalyst, and each weigher is thoroughly mixed and dispersed in a mixed solution of ion-exchanged water and ethanol. It was A predetermined amount of the silicate containing Na-type metal (Na / ZSM-5, Caban ratio 20) was added to the dispersion solution.
0), and the mixture was thoroughly stirred at room temperature and then evaporated to dryness. Further, it was sufficiently dried at 150 to 200 ° C. to prepare a Pt—Ir-supporting metal-containing silicate catalyst.

After adding hydrated alumina as a binder to the Pt-Ir-supporting metal-containing silicate catalyst in a weight ratio of 20%, an appropriate amount of water was added to prepare a washcoat slurry. This slurry has cordierite honeycomb carrier (400 cells per square inch)
Dip, blow off excess slurry with air blow,
After drying, a Pt—Ir-supported metal-containing silicate catalyst device was prepared by performing a wash coat (the wash coat amount was 30% by weight of the honeycomb carrier) by baking in the air at 500 ° C. for 2 hours.

Next, the Pt-Ir-supporting metal-containing silicate catalyst device was immersed in 1N sulfuric acid for 30 minutes, pulled up, and excess sulfuric acid was blown off by air blow, then 160
The catalyst device of Example 1 for purifying NOx was obtained by performing a sulfuric acid treatment by drying at 0 ° C.

<Example 2> In the sulfuric acid treatment of Example 1, the catalyst device of Example 2 for purifying NOx was used in the same manner as in Example 1 except that 0.5N sulfuric acid was used instead of 1N sulfuric acid. Obtained.

<Example 3> In the sulfuric acid treatment of Example 1, the catalyst device of Example 3 for purifying NOx was used in the same manner as in Example 1 except that 0.1N sulfuric acid was used instead of 1N sulfuric acid. Obtained.

<Example 4> In the sulfuric acid treatment of Example 1, the catalyst device of Example 4 for purifying NOx was used in the same manner as in Example 1 except that 0.05N sulfuric acid was used instead of 1N sulfuric acid. Obtained.

<Embodiment 5> In the sulfuric acid treatment in the above Embodiment 1, Embodiment 5 for NOx purification is carried out in the same manner as in Embodiment 1 except that 6N sulfuric acid is used instead of 1N sulfuric acid.
The catalyst device of

<Example 6> In the sulfuric acid treatment of Example 1, the catalyst device of Example 6 for purifying NOx was obtained in the same manner as in Example 1 except that 12N sulfuric acid was used instead of 1N sulfuric acid. .

<Example 7> In the sulfuric acid treatment of Example 1, the catalyst device of Example 7 for purifying NOx was obtained in the same manner as in Example 1 except that 18N sulfuric acid was used instead of 1N sulfuric acid. .

Example 8 A silicate containing Na-type metal (Na / ZSM-5, Caban ratio 200) was immersed in 1N sulfuric acid for 30 minutes, filtered, and dried at 160 to 200 ° C. for sulfuric acid treatment. A sulfuric acid-treated Na-type metal-containing silicate was obtained.

Next, a divalent platinum ammine crystal as a Pt material and iridium trichloride as an iridium material were added,
Pt: Ir = 3: 1 and Pt and Ir were weighed so that the supported amount was 3 g per 1 liter of the catalyst, and each weighed body was thoroughly mixed in a mixed solution of ion-exchanged water and ethanol. Dispersed. This dispersion solution was added to a predetermined amount of the sulfuric acid-treated Na-type metal-containing silicate, sufficiently stirred at room temperature, and then evaporated to dryness. Furthermore, 150-200 ℃
Was sufficiently dried in order to prepare a Pt-Ir-supporting metal-containing silicate catalyst.

After adding hydrated alumina as a binder to the Pt-Ir-supporting metal-containing silicate catalyst in a weight ratio of 20%, an appropriate amount of water was added to prepare a slurry for washcoating. This slurry has cordierite honeycomb carrier (400 cells per square inch)
Dip, blow off excess slurry with air blow,
After drying, wash coating was performed in the air at 500 ° C. for 2 hours (the amount of wash coating was 30% by weight of the honeycomb carrier) to obtain a Pt—Ir-supporting metal-containing silicate catalyst device. This catalyst device was used as the catalyst device of Example 8.

<Example 9> In the sulfuric acid treatment of Example 8, the catalyst device of Example 9 for purifying NOx was used in the same manner as in Example 8 except that 0.5N sulfuric acid was used instead of 1N sulfuric acid. Obtained.

<Example 10> In the sulfuric acid treatment of Example 8, the catalyst device of Example 10 for purifying NOx was used in the same manner as in Example 8 except that 0.1N sulfuric acid was used instead of 1N sulfuric acid. Obtained.

<Example 11> In the sulfuric acid treatment of Example 8, the catalyst device of Example 11 for purifying NOx was used in the same manner as in Example 8 except that 0.05N sulfuric acid was used instead of 1N sulfuric acid. Obtained.

<Comparative Example> The same procedure as in Example 1 was repeated except that the sulfuric acid treatment step was not performed on the catalyst device obtained by wash-coating the Pt-Ir-supporting metal-containing silicate catalyst on the honeycomb carrier. A comparative catalyst device for purifying NOx was obtained.

-Evaluation of NOx purification activity of each catalytic device-In each of the catalytic devices of Examples 1 to 11 and the catalytic device of the comparative examples obtained by the method of the present invention, in a fresh state and after heating in the atmosphere. The maximum NOx purification rate and the deterioration rate of the maximum NOx purification rate after heating in the fresh state were measured to evaluate the NOx purification characteristics of each catalyst device.

The above-mentioned measurement was carried out by the atmospheric pressure fixed bed flow reactor for each of the catalyst devices of Examples 1 to 11 and Comparative Example.
A model gas almost equivalent to the exhaust gas under the lean burn condition of A / F = 22 was used. The composition of the model gas is NOx: 2000 ppm, HC: 5500 pp
mC, O ₂ : 8%, H ₂ : 650 ppm, CO: 0.2
%, CO ₂ : 10%, N ₂ : balance, and SV =
It was set to 55000 h ^-1 .

First, the NOx purification rate in the fresh state was measured by the above method, each catalyst device after the measurement was heated in the atmosphere at 800 ° C. for 8 hours, and after heating, the NOx purification rate was also determined by the above method. Was measured. Further, the deterioration rate of the NOx purification rate after heating in the fresh state was calculated.

Table 1 shows the measurement results of the catalyst devices of Examples 1 to 11 and Comparative Example.

[0052]

[Table 1]

According to the results shown in Table 1, Examples 1 to 6 (obtained by carrying out sulfuric acid treatment after supporting active species on a metal-containing silicate) and Examples 8 to 11 obtained by the method of the present invention. The NOx purification characteristics of each catalyst device (those obtained by supporting active species on the metal-containing silicate previously treated with sulfuric acid) are compared with the NOx purification characteristics of the catalyst device of the comparative example (those not including the sulfuric acid treatment step). As a result, the NOx purification rate in the fresh state is improved, and the excellent NOx purification rate is maintained even after heating. From this, it is clear that the catalyst for purifying NOx manufactured by including the sulfuric acid treatment step according to the present invention has NOx purifying activity and heat resistance for efficiently decomposing NOx in exhaust gas.

The catalyst device of Example 7 has a lower NOx purification rate and heat resistance than the catalyst device of the comparative example is that sulfuric acid having a high concentration exceeding 12N is used. This is because the sulfuric acid treatment facilitates the destruction of the structure of the metal-containing silicate. Therefore, the sulfuric acid treatment using an excessively high concentration of sulfuric acid may reduce the NOx purification activity of the resulting catalyst, especially after heating, which is not preferable.

-Solid acid characteristics of each catalyst device- Among the catalyst devices of the examples according to the present invention, each catalyst device of Examples 1 to 7 in which a metal-containing silicate is loaded with active species and then treated with sulfuric acid. Was measured for solid acid properties. At the same time, the solid acid characteristics of the catalyst device of the comparative example were measured.

These solid acid characteristics were measured by the so-called Johnson method, which is a method for measuring the acid strength using a Hammett indicator and the acid amount in the acid strength region, and the measurement results are shown in Table 2.

[0057]

[Table 2]

According to the results shown in Table 2, the acid strength Ho
However, the acid amount in the range of −5.6 <Ho ≦ −3.0 is such that the acid amount increases / decreases almost according to the concentration of sulfuric acid used in the sulfuric acid treatment step during catalyst production.

Therefore, by correlating each acid amount of the catalyst device of each example shown in Table 2 with the evaluation result of the NOx purification activity shown in Table 1 above, Examples 1 to 6 in Table 2 above are made to correspond.
The amount of acid in the catalyst device of No. 2 is preferably such that the radicalization of HC by active species in the NOx purification catalyst is activated.
It is judged that the acid amount is such that a new acid point that causes the excitation of C can be formed.

In particular, the catalysts of Examples 1 to 3 (concentration of 1 to
The catalyst treated with sulfuric acid by 0.1 N sulfuric acid) has an acid amount that gives solid acid characteristics such that an acid site where appropriate HC excitation and radicalization occur in the catalyst is obtained. It is considered that the excellent NOx purification activity is obtained and the heat resistance is improved.

[0061]

As described above, according to the method for producing an exhaust gas purifying catalyst according to the first and second aspects of the present invention, a sulfuric acid treatment is applied to the catalyst device after supporting the noble metal active species on the metal-containing silicate. Since the above-mentioned metal-containing silicate before being subjected to the treatment or supporting the noble metal active species is subjected to a sulfuric acid treatment in advance, the obtained exhaust gas purifying catalyst has a new acid point effective for exciting HC in the exhaust gas. Thus, excellent NOx purification activity in the exhaust gas purification catalyst can be obtained, and heat resistance can be improved. Further, since the sulfuric acid treatment is performed by immersing the catalyst device or the metal-containing silicate in a sulfuric acid solution having an appropriate concentration, the manufacturing process of the exhaust gas purifying catalyst having such characteristics is remarkably labor-saving.

 ─────────────────────────────────────────────────── ─── 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 method for producing an exhaust gas purifying catalyst for removing NOx in the presence of HC, comprising a noble metal active species for decomposing NOx by burning HC on a metal-containing silicate as an active species-supporting base material. And a step of subjecting the noble metal-supported metal-containing silicate obtained in the above step to a sulfuric acid treatment for modifying the acid characteristics of the noble metal-supported metal-containing silicate, the exhaust gas purifying catalyst comprising: Production method. 2. A method for producing an exhaust gas purifying catalyst for removing NOx in the presence of HC, which comprises treating a metal-containing silicate as an active species-supporting base material with a sulfuric acid treatment for modifying the acid characteristics of the metal-containing silicate. And a step of causing the sulfuric acid-treated metal-containing silicate to carry a precious metal active species that decomposes NOx by burning HC, the method for producing an exhaust gas purifying catalyst.