JPH10180117A - Preparation of catalyst for exhaust gas purification - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method to restrain the growth of particles by improving the uniformity of catalyst distribution and to improve the durability of a catalyst for purifying exhaust gas in the production method of the catalyst employing a CVD method in which raw material gas, after being adsorbed temporarily on a catalyst carrier, is decomposed to prepare the catalyst. SOLUTION: Platinum acetyl acetonate which is raw material gas to be a catalyst component and aluminum acetyl acetonate which is dummy gas not to be a catalyst component are mixed, and the mixture is adsorbed to be saturated on the surface of a monolith carrier coated with activated alumina which is a catalyst carrier. Since the adsorption density of the raw material gas is reduced corresponding to the adsorption of the dummy gas, the quantity of the carried catalyst can be reduced while the uniformity of catalyst distribution being improved, and the growth of particles can be restrained.

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 a catalyst for purifying exhaust gas using a chemical vapor deposition (CVD) method.

[0002]

2. Description of the Related Art Conventionally, catalysts in which fine particles such as metal or metal oxide are supported on a catalyst carrier having a large specific surface area have been widely used. As a catalyst for purifying automobile exhaust gas, a catalyst in which a catalytic metal such as platinum, palladium or rhodium, which is a noble metal, is supported on a monolithic carrier (honeycomb carrier) coated with activated alumina is widely used. However, since these precious metals are expensive, efforts have been made to reduce the amount of precious metals used by supporting the catalyst metals uniformly and finely. Various methods have been studied.

[0003] However, in this impregnation method, when the impregnated solution is dried, the catalyst components are apt to agglomerate, so that it is difficult to carry the catalyst components uniformly and finely. For this reason, physical vapor deposition (PVD) without using a solution,
Attempts have been made to support the catalyst metal uniformly and finely by a chemical vapor deposition method (CVD method).
No. 207842 discloses a method of supporting platinum and rhodium on a carrier coated with activated alumina using a mixed gas of platinum fluoride and rhodium fluoride.
No. 7247 proposes a method in which a metal carrier coated with activated alumina is heated to a temperature not lower than the use temperature of a catalyst to support platinum and rhodium.

[0004]

In order to support a catalyst on a carrier having a complicated shape such as a monolithic carrier, it is more advantageous to use a CVD method in which a raw material flows around more than a PVD method. However, also in this CVD method, when introducing the raw material gas, many catalyst components are supported on the upstream side of the gas,
It was difficult to uniformly support the catalyst component on the entire monolithic carrier. And in the exhaust gas purifying catalyst,
Since it is used at a high temperature (850 to 900 ° C.), grain growth is likely to occur in a portion where the catalyst component is dense, which has been a cause of lowering the durability of the catalyst.

In view of the above, the present invention provides a method of manufacturing a catalyst for purifying exhaust gas using a CVD method in which a raw material gas is once adsorbed on a catalyst carrier and then the raw material is decomposed to prepare the catalyst. It is an object of the present invention to provide a method for improving the uniformity of distribution, suppressing grain growth and improving the durability of a catalyst.

[0006]

As a result of experimental studies, the present inventors have found that aluminum acetylacetonate gas (dummy gas), which does not become a catalyst component, is adsorbed on activated alumina, which is a catalyst carrier, and then platinum acetyl, which becomes a catalyst, is adsorbed. It was discovered that acetonate (source gas) hardly adsorbed. This means that in a mixed gas of a certain kind of source gas and a certain kind of dummy gas, when the dummy gas molecules are adsorbed on the surface of the activated alumina which is a catalyst carrier, the source gas molecules are not further laminated and adsorbed on the gas molecules. That is.

In addition, it has been confirmed that even when platinum acetylacetonate alone is adsorbed, no more than a certain amount of adsorption is not adsorbed. This means that, for a certain kind of source gas, the source gas is not adsorbed again on the catalyst carrier site where the source gas is adsorbed. In other words, when a certain source gas or a mixed gas of a certain source gas and a dummy gas is adsorbed, the adsorption of the gas is saturated when the gas molecules are adsorbed on the entire adsorption surface of alumina. It means that

Here, the case where the raw material gas alone is brought into a saturated state is considered. In this saturated state, the raw material gas molecules are densely adsorbed on the entire adsorption surface of the carrier, so that the catalyst distribution is naturally uniform. It becomes very large, and it is difficult to simultaneously control the amount of supported catalyst and achieve uniformity of catalyst distribution.

Conversely, for example, even when it is desired to control the amount of supported catalyst to be small in order to suppress grain growth as in the case of an automobile catalyst, a uniform catalyst must be used unless the adsorption of a single raw material gas is saturated. Since the distribution cannot be obtained, uniformity of the catalyst distribution can be achieved, but the amount of supported catalyst cannot be controlled, which causes problems such as generation of grain growth and waste of raw material gas. Therefore, based on the above findings, the method for producing an exhaust gas purifying catalyst according to the present invention provides, in a CVD method, a raw material gas serving as a catalyst component and a gas different from the raw material gas and not serving as a catalyst component ( (Dummy gas) is mixed and adsorbed on the carrier surface.

Here, the raw material gas serving as the catalyst component is C
A compound gas containing a component (for example, a noble metal such as platinum or rhodium) that becomes a supported catalyst after being subjected to a chemical reaction after being adsorbed by the VD method, and a dummy gas that does not become a catalyst component is the above-mentioned catalyst. A compound gas that does not contain a component as a component. According to this method, it is considered that adsorption occurs by the following mechanism. According to this method, the raw material gas and the dummy gas are uniformly mixed (mixed gas) and adsorbed on the carrier. The raw material gas is not layered and adsorbed on the carrier site (adsorption point on the surface) where the dummy gas is adsorbed, and the raw material gas is not adsorbed again on the catalyst carrier site where the source gas is adsorbed. In other words, the number of adsorption points on the carrier on which the source gas is to be adsorbed is reduced by the amount of the adsorbed dummy gas.

This means that, microscopically, the adsorption points on the carrier surface are such that the site where the source gas molecules are adsorbed and the site where the dummy gas molecules are adsorbed are uniformly dispersed. Then, even if the saturated molecules are adsorbed, the raw material molecules are inhibited from adsorbing by the intervening dummy molecules, resulting in a sparse state. Then, after the chemical reaction treatment, from the viewpoint of the catalyst component, it is considered that a portion where the catalyst component is supported and a portion where the catalyst component is not supported are in a state of being uniformly dispersed.

[0012] Macroscopically speaking, it can be said that the catalyst components are uniformly distributed on the surface of the carrier only at the site where the dummy gas is adsorbed, with a gap. Therefore, it is possible to improve the uniformity of the catalyst distribution while controlling the amount of supported catalyst to be small. Further, for the following reasons, it is more preferable that the dummy gas has the same adsorption performance as the source gas. In the present invention, the adsorption performance is the same when the two gases are simultaneously adsorbed under the adsorption conditions (adsorption time, temperature, etc.) for adsorbing the source gas (the amount of adsorption of both gases per unit volume of the carrier). For example, grams per liter, g /
L) have the same number of digits (same order).

That is, according to the above estimation mechanism, the adsorption distribution of the source gas changes depending on the mixing ratio of the source gas and the dummy gas. Here, if the amounts of adsorption of the source gas and the dummy gas are extremely different, it is difficult to appropriately control the distribution according to the change of the mixing ratio. That is, when the amount of adsorption of the dummy gas is extremely small, the degree of the presence of the dummy gas cannot be changed unless the mixing ratio is significantly changed, and when the amount of adsorption of the dummy gas is extremely large, By subtly changing the mixing ratio, the degree of dummy gas intervention must be changed.

By the way, when the method utilizing the dummy gas is used, even when it is desired to suppress the grain growth by reducing the amount of supported catalyst as in the case of an automobile catalyst, uniformity of the catalyst distribution is not sacrificed. The amount of supported catalyst can be controlled. At this time, the amount of supported catalyst can be controlled by the mixture ratio of the source gas and the dummy gas. For this reason,
The amount of supported catalyst according to the desired catalyst performance can be easily realized, the grain growth can be suppressed, and the durability of the catalyst can be improved.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION As the catalyst carrier used in the present invention, a monolith carrier coated with activated alumina is preferred. Thereby, a catalytic converter having excellent warm-up performance and small pressure loss can be realized, which is suitable for an exhaust gas purifying catalyst. As the raw material gas, metal halides and organometallic compounds used in the CVD method are used. In particular, the organometallic compounds are preferably in a gas phase at a low temperature, which is preferable. It is preferable because no harmful gas or the like is generated. Here, as the metal constituting the compound, for example, a noble metal such as platinum, palladium, or rhodium suitable as a catalyst component can be mentioned. Then, after these raw material gases are adsorbed on the carrier, they are subjected to chemical reaction treatments such as thermal decomposition and hydrogen reduction, and become catalytic components such as metals, oxides and carbides on the carrier.

Although the dummy gas does not become a catalyst component as described above, it has the same adsorption performance as that of the raw material gas, and some of the dummy gas remains on the carrier when used as a catalyst and does not adversely affect the catalytic reaction. preferable. For example, when the raw material gas is an organic metal compound, an organic metal compound is also preferable, and when the raw material gas is an acetylacetonate metal complex, an acetylacetonate metal complex is also preferable. Here, as the metal constituting the compound,
For example, an inexpensive material such as aluminum is preferable.

Here, by heating the monolithic carrier on which the raw material gas is adsorbed in the air, the raw material is decomposed and becomes a catalyst component. At this time, for example, when aluminum acetylacetonate is used as a dummy gas, resublimation or Although it decomposes on the carrier, even if it decomposes on the carrier to form an oxide, it is the same component (alumina oxide) as the carrier and does not adversely affect the catalytic performance.

[0018]

The present invention will be described below in more detail with reference to Examples and Comparative Examples. (Embodiment 1) FIG. 1 shows the configuration of an apparatus for producing the catalyst of this embodiment. In FIG. 1, a first quartz boat 1 loaded with platinum acetylacetonate (A), which is an organometallic complex serving as a catalyst raw material, is used as a raw material gas, and a second quartz boat 2 is loaded with aluminum acetylacetonate (B), which is a dummy gas. And the monolith carrier 3 is a SUS cylindrical case 4
Installed inside.

On the lower surface of the first quartz boat 1, a first raw material heater 5 for heating the catalyst raw material and a second quartz boat 2
A second source heater 6 for heating the dummy gas source is provided in the case 4 on the lower surface of the case. further,
A carrier heater 7 is provided near the circumferential surface of the case 4 so that the entire case 4 is uniformly heated.

The monolith carrier 3 is made of a heat-resistant ceramic, has a cylindrical shape with a total diameter of 71 mm and a length of 60 mm, and has a cell density of 400 in 1 inch square.
Cell exists. Then, the activated alumina is further coated with 120 g per 1 L (L: liter) of the carrier volume of the monolith carrier 3. A tube 8 and a tube 9 are connected to each circular surface of the case 4 so that a carrier gas (nitrogen gas) flows through the case 4 from the tube 8 and is exhausted from the tube 9 together with excess gas. It has become.

The exhaust gas purifying catalyst of this embodiment is manufactured as follows based on the above-mentioned device configuration. While vacuuming the entire device with a rotary pump (not shown),
The carrier heater 7 is controlled so that the temperature of the monolith carrier 3 becomes 130 ° C. While flowing a carrier gas (nitrogen gas) heated to 170 ° C. at a flow rate of 20 liters / min, each of the raw material heaters 5 was heated to 180 ° C. for platinum acetylacetonate (A) and 150 ° C. for aluminum acetylacetonate (B). , 6 to sublimate and adsorb the organometallic complex on the monolithic carrier 3 for 1-2 hours.
Here, the weight ratio of platinum acetylacetonate (A) to aluminum acetylacetonate (B) was 1: 2.

Subsequently, the monolithic carrier 3 on which the organometallic complex is adsorbed is taken out and heated to 200 ° C. in a high-temperature bath to decompose the raw materials to obtain a platinum catalyst. At this time, the supported amount of platinum was approximately 1.5 g / L. (Example 2) The same apparatus as in Example 1, except that the weight ratio of platinum acetylacetonate to aluminum acetylacetonate was set to 4: 1 in the apparatus,
A platinum catalyst was prepared by the method. At this time, the supported amount of platinum was about 3 g / L. (Comparative Example 1) A platinum catalyst was prepared by the same apparatus and method as in Example 1 except that aluminum acetylacetonate serving as a dummy gas was not loaded into the quartz boat 2 in order to eliminate the effect. At this time, the supported amount of platinum was approximately 1.5 g / L. Comparative Example 2 A catalyst was prepared by an impregnation method using a dinitrodiammine platinum solution as a starting material. The same monolithic carrier as used in Example 1 was used, and the amount of supported platinum was 1.
The solution concentration was adjusted to 5 g / L, the monolith carrier was immersed in the solution for 2 hours, dried at 100 ° C. for 1 hour, and then baked at 800 ° C. for 2 hours.

FIG. 2 shows the radial direction ((a) in FIG. 2) and the length direction ((FIG. 2)) of the columnar monolith catalysts produced in Example 1, Example 2 and Comparative Example 1. The result of measuring the platinum distribution of b)) using X-ray fluorescence spectroscopy is shown. That is, in Example 1, platinum was approximately 1.5 g / L in the entire carrier, and in Example 2, platinum was approximately 3 g / L in the entire carrier.
g / L with a uniform loading. On the other hand, Comparative Example 1
Then, the same amount of platinum as in Example 1 was used in the length direction of 0 mm to 20 m.
m are supported in a concentrated manner at the carrier site, that is, the upstream portion of the gas. The state is that the supported amount gradually decreases from the uppermost stream (0 mm, about 6 g / L), and platinum is not supported at a portion downstream of the length direction of 20 mm.

Here, since the aluminum acetylacetonate, which is a dummy gas, is the same aluminum compound as the surface of the carrier (alumina), the exact amount supported is not known even by elemental analysis such as X-ray fluorescence spectroscopy. However, it can be said that it has the same adsorption performance (the same order of adsorption amount) as platinum acetylacetonate as a raw material gas for the following reasons.

That is, it can be said that the amount of platinum carried on the carrier portion in the length direction of 0 mm is almost saturated adsorption in Example 1, Example 2 and Comparative Example 1. Therefore, the effect of the mixing ratio of each example is not significant. Can be compared by the amount of platinum carried at this site.
As shown in FIG. 2A, the amount of platinum carried at this portion is about 6 g / L in Comparative Example 1, about 1.5 g / L in Example 1, and about 3 g / L in Example 2. / L. On the other hand, the weight ratio of platinum acetylacetonate to aluminum acetylacetonate was 1: 2 in Example 1 and 4: 1 in Example 2, and the amount of supported platinum was roughly increased or decreased according to the weight ratio. And that the amounts of adsorption of both gases to the catalyst carrier are of the same order. Therefore, it is possible to control the amount of catalyst carried by the weight ratio.

In Examples 1 and 2, the entire surface of the carrier in the radial and length directions was
Since platinum acetylacetonate and aluminum acetylacetonate were adsorbed at a constant ratio, the amount of supported platinum was controlled and the supported platinum was uniformly distributed. As described above, according to the method of the present invention in which the CVD method is performed using the dummy gas, the conventional CV without using the dummy gas is used.
Compared with Method D, control of the amount of supported catalyst can be achieved without sacrificing the uniformity of catalyst distribution.

Table 1 shows the conversion rates of carbon monoxide (CO) and the particle diameter (nm) of the catalyst (platinum) produced in Example 1, Comparative Example 1 and Comparative Example 2.
This was measured after performing an endurance test of heating at 900 ° C. for 80 hours in the atmosphere. The purification rate of CO was measured under the following conditions. The condition is that the catalyst temperature is 400
° C, 450 ° C, 500 ° C, space velocity (SV) = 50,
000 / hr, air-fuel efficiency: (A / F) = 14.6.
The particle size of the catalyst was measured by TEM (transmission electron microscope) observation.

As is evident from Table 1, the catalyst produced in Example 1 has a high CO purification rate after durability since the grain growth of the supported platinum is suppressed. It has better durability than the catalyst produced according to Example 2.

[Brief description of the drawings]

FIG. 1 is a schematic view of an apparatus used for producing a catalyst in Example 1 of the present invention.

2 (a) is a distribution diagram in the monolith radial direction of the amount of catalyst carried in Examples 1 and 2 and Comparative Example 1, and (b) is a distribution diagram of the amount of catalyst carried in the monolith length direction in each of the above examples.

FIG. 3 is a table showing a CO purification rate and a catalyst particle size after an endurance test in Example 1 and Comparative Examples 1 and 2.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... 1st quartz boat, 2 ... 2nd quartz boat, 3 ... Monolith support, A ... Platinum acetylacetonate, B ... Aluminum acetylacetonate.

Claims (7)

[Claims] In a method for producing a catalyst using a chemical vapor deposition method, a raw material gas serving as a catalyst component and a dummy gas not serving as a catalyst component are mixed and adsorbed on a catalyst carrier, and thereafter, the adsorbed raw material gas is used. Wherein the raw material gas is not adsorbed to the catalyst carrier site where the dummy gas is adsorbed, and is supplied to the catalyst carrier site where the source gas is adsorbed. A method for producing an exhaust gas purifying catalyst, which has a characteristic of not being adsorbed again. 2. The method for producing an exhaust gas purifying catalyst according to claim 1, wherein the amount of adsorption of the source gas and the amount of adsorption of the dummy gas are in the same order. 3. The method for producing an exhaust gas purifying catalyst according to claim 1, wherein the catalyst carrier is a monolith carrier coated with activated alumina. 4. The method according to claim 1, wherein the source gas and the dummy gas are:
The method for producing an exhaust gas purifying catalyst according to any one of claims 1 to 3, wherein the method is an organometallic compound. 5. The method according to claim 1, wherein the source gas and the dummy gas are:
The method for producing an exhaust gas purifying catalyst according to any one of claims 1 to 4, wherein the method is an acetylacetonate metal complex compound. 6. The method according to claim 1, wherein the source gas is a noble metal complex compound of acetylacetonate, and the dummy gas is aluminum acetylacetonate.
6. The method for producing an exhaust gas purifying catalyst according to any one of the above items 5 to 5. 7. The method for producing an exhaust gas purifying catalyst according to claim 1, wherein the chemical reaction treatment is a thermal decomposition treatment.