JPH07232087A - Manufacture of catalyst carrier - Google Patents

Abstract

PURPOSE:To provide a catalyst carrier with a metal dispersed uniformly only near the surface and a large specific surface area by dehydrating Al2O3 hydrate to pulverize it with free water on the surface being almost dried, then impregnating this dehydrated powder with an alkoxide solution of an NOx absorbing metal and hydrolyze the powder by a sol/gel method. CONSTITUTION:An Al2O3 hydrate is dehydrated, to be pulverized with free water on the surface being almost dry, and this powder is impregnated with an alkoxide solution of NOx absorbing metal, followed by hydrolysis using a sol/gel method. That is, first, alkoxide M(OR)6 of NOx absorbing metal (e.g. Ba) is formed. M is an NOx absorbing metal, and R is an alkyl group, for example, methyl, ethyl propyl or butyl. Next, this alkoxide is caused to react chemically with water contained in alumina, then the NOx absorbing metal is allowed to bond with alumina on the surface by hydrolysis. After that, a solvent is removed to dry and bake the hydrolyzed product. Finally a catalyst carrier thus obtained has an NOx absorbent immodilized stably on the surface.

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 carrier, and more particularly to a method for producing a catalyst carrier in which a large amount of metal serving as a catalyst is arranged on the surface.

[0002]

2. Description of the Related Art Air pollution has been taken up as an environmental problem, and exhaust gas has become a problem particularly with the spread of automobiles, and various regulations have been applied. For this reason, various methods such as engine improvement, reactor method, catalyst method, etc. were applied in the initial stage, but nowadays, the catalyst method that can most efficiently perform exhaust gas treatment is predominant.

Automotive catalysts are either mounted directly on the engine exhaust manifold or under the floor of the vehicle. The catalytic converter currently used is called "Molinos type", which has a large number of through holes (cells) formed in the exhaust gas flow direction, and a wash coat layer is provided on the inner surface of each cell. Has been. This washcoat layer is a substantial part of the catalyst that purifies the exhaust gas.

Noble metals such as platinum (Pt), rhodium (Rh) and palladium (Pd) are known as catalysts, and these are carriers such as alumina (Al ₂ O ₃ ) having a large surface area. Dispersed as fine particles on the surface.
The exhaust gas diffuses into the fine pores of alumina, and a catalytic reaction takes place on the surface of the catalyst. Since such a catalytic reaction is carried out on the surface of the noble metal particles, it is preferable that the noble metal particles are as small as possible.

However, in the conventional catalyst, the noble metal particles aggregate in a high temperature region of 600 ° C. or higher, and the catalyst surface area decreases. Further, at a high temperature of 1000 ° C. or higher, γ-Al ₂ O ₃ currently used as a carrier undergoes structural transition to α-Al ₂ O ₃ , the surface area is reduced, and fine pores disappear. Therefore, in order to prevent this alpha conversion, a rare earth element was added, but even with such a catalyst, its durability is 80%.
The limit is up to about 0 ° C, and satisfactory results were not obtained at higher temperatures.

In order to solve the above problems, a carrier having a magnetoplumbite type layered aluminate structure having high crystallinity has been developed. This carrier contains an alkaline earth metal oxide and an aluminum oxide, and there is no reaction between the alkaline earth metal and aluminum oxide at high temperature, and the specific surface area and activity are not reduced. However, since it has a layered aluminate structure, the activity itself of the active species is reduced, the specific surface area itself is reduced, and a sufficient catalytic function cannot be obtained.

In the absorption decomposition type lean NO _x catalyst, NO
Alkali metals, alkaline earth metals, etc., which are _x absorbents, are crystalline BaS due to oxidation of sulfur contained in fuel.
Form O ₄ . This crystalline BaSO ₄ is less likely to be decomposed even in a stoichiometric atmosphere, and therefore the catalyst NO _x
Absorption capacity of NOx decreases, and as a result, NO _x purification capacity decreases.

The conventional absorption-decomposition type lean NO _x catalyst is
By impregnating a solution of Ba salt with Al ₂ O ₃ ,
It is manufactured by coating the surface of ₂ O ₃ with Ba. In this catalyst, the bond between Ba and Al ₂ O ₃ is due to van der Waals force, and the bond force is small. Therefore, Ba with SO ₄ adsorbed is separated and diffused,
Aggregate to form crystalline BaSO ₄ . Although it is easily decomposed in stoichiometry when SO ₄ is simply adsorbed, crystalline BaSO ₄ is stable and its decomposition is difficult.
Therefore, the amount of Ba that can absorb NO _x is reduced, and the NO _x purification capability is reduced.

JP-A-4-298236 discloses a catalyst having a layered aluminate structure containing an alkali metal or an alkaline earth metal as a catalyst component. This catalyst is prepared by alkoxide method (sol-gel method) of alkali metal or alkaline earth metal alkoxide and aluminum alkoxide. In this catalyst, the above-mentioned problems are solved because the alkali metal is bonded to the alumina by the bond of hydrolysis of the alkoxide rather than the van der Waals force. However, in the above method, the alkali metal and the like are dispersed and arranged inside the carrier, and the proportion of non-functional metal increases.

[0010]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned drawbacks of conventional catalyst carriers and provides a catalyst carrier having a large specific surface area in which a metal is uniformly dispersed only in the vicinity of the surface. is there.

[0011]

DISCLOSURE OF THE INVENTION As a result of intensive studies to solve the above problems of the catalyst carrier, the present inventors have dehydrated Al ₂ O ₃ hydrate into powder form. Al ₂
O ₃ by the structure water hydrates the alkoxide of the NO _X absorbing metal is hydrolyzed by a sol-gel method, by dehydration condensation with the OH groups on the surface of the Al ₂ O ₃ hydrate, Al ₂ O
_The present invention has been completed by finding that a carrier having a NO ₂ absorbing metal coated on only the surface of ₃ can be obtained.

That is, in the method for producing a catalyst carrier of the present invention, Al ₂ O ₃ hydrate is dehydrated to form a powder in which free water on the surface is almost dry, and the powder is impregnated with an alkoxide solution of NO _x absorbing metal. The sol-gel method is used for hydrolysis.

In the method of the present invention, Al₂O₃Hydrate
May be produced from Al alkoxide,
You may use a commercial item. This Al₂O₃A lot of hydrate
Contains water. An example is Al₂O₃Gel (Al₂O
₃-nH₂O), boehmite (Al₂O₃-H₂O), jib
Site (Al₂O₃-3H₂O), diaspore (Al ₂
O₃-H₂And hydrates such as O). This Al₂O₃
The hydrate is dried only on its surface, i.e.
Heat and dehydrate the separated water until it is almost dry, and pulverize it.
It This powdery Al₂O₃Water content of hydrate is basic
It is preferable to use only structured water. Its water content is
Depending on the structure of the mina, for example 8 to 20% by weight
Preferably. If the water content is lower than this,
NO to do after that_xAbsorbing metal alkoxide sol-gel
NO in hydrolysis by the method_xFully water absorbing metal
NO which could not be decomposed and was not hydrolyzed_x
Absorbing metal aggregates on the alumina surface. This agglomerate
The metal easily reacts with sulfur. For example, Ba is used as the metal.
If used, BaSO_FourTo form this BaSO_FourIs N
O_xSince the absorption of hydrogen is inhibited, the catalyst is likely to deteriorate. Also
If the water content is higher than 20% by weight, except for the surface of alumina
Hydrolysis also occurs, and the hydrolyzed products coagulate.
Form aggregates. As a result, the catalyst easily deteriorates
Become

The NO _x absorbing metal is an alkali metal,
Alkaline earth metals and rare earth metals are used, for example,
Barium, potassium, etc. can be used. NO _x
The method of binding the absorbing metal to the alumina is carried out by the so-called sol-gel method as described above. That is, first, the alkoxide M (OR) _n of the NO _x absorbing metal is formed. Here, M is a NO _x absorbing metal and R is an alkyl group such as methyl, ethyl, propyl or butyl. Next, this alkoxide is reacted with water contained in alumina to cause hydrolysis, and NO _{x is} formed on the surface of alumina.
Combines the absorbing metal. After that, the solvent is removed, and the catalyst carrier of the present invention is obtained by drying and calcining.

The amount of the NO _x absorbing metal that is bonded to alumina by the above-mentioned sol-gel method is preferably an amount capable of sufficiently purifying NO _x . On the other hand, if the amount is too large, the NO _x absorbing metals agglomerate with each other, and the durability deteriorates. Specifically, the amount of NO _x absorbing metal is preferably 7 to 25 mol% and more preferably 10 to 20 mol% based on alumina.

[0016]

The NO _x is absorbed and decomposed by making the catalyst carrier of the present invention thus obtained carry a noble metal such as platinum and controlling the combustion gas alternately between lean and stoichiometric. The NO _x absorbing metal in the catalyst carrier powder of the present invention is solidified on the surface of the carrier by ionic bonding, and thus is more stable than the conventional NO _x absorbent coated with the aqueous solution of the metal salt. Therefore, in the catalyst carrier of the present invention,
Ba that has adsorbed SO ₄ does not easily diffuse, and is crystalline BaS.
O ₄ is hard to generate. Therefore, even in the exhaust gas containing sulfur, the NO _x purification ability is not significantly reduced.

[0017]

EXAMPLES The present invention will be described below with reference to examples, but the present invention is not limited thereto.

Example 1 91.8 g (0.45 mol) of triisopropoxyaluminum was added at 80 ° C.
After stirring and mixing in isopropyl alcohol for 5 hours with a stirrer, 26.7 g (1.49 mol) of water was added thereto to hydrolyze triisopropoxyaluminum, followed by stirring for 6 hours to obtain aluminum oxide Al ₂ O ₃
Got Then, isopropyl alcohol was evaporated at 90 ° C., and then dried at 100 ° C. for 6 hours in a drying furnace in which nitrogen was passed at 40 liter / min to obtain an Al ₂ O ₃ powder having a water content of 15% by weight. . Diisopropoxy barium 7.66 g (0.03 mol, 13.3 mol% of Al ₂ O ₃ ) in this powder
Was added and stirred in isopropyl alcohol for 24 hours with a stirrer. Then, the isopropyl alcohol was evaporated at 100 ° C., and then dried at 120 ° C. for 12 hours in a drying oven in which nitrogen was passed at 40 liter / min.

The dry powder thus obtained was heated to a maximum temperature of 1000
It was calcined in the atmosphere by holding it at 5 ° C for 5 hours. The obtained powder was impregnated with 250 ml of a dinitrodiammine Pt nitrate aqueous solution (Pt amount: 0.003 mol), and stirred with a stirrer at room temperature for 1 hour. The obtained slurry is separated into a powder and a supernatant by a centrifuge, and the powder is separated into 120
It was dried at ℃ for 12 hours and further heat-treated at 250 ℃ for 1 hour. In the powder carrier thus obtained, 0.98% by weight of Pt was supported by elemental analysis.

A slurry was prepared by adding 3 g of alumina sol, 50 g of 40% aqueous solution of aluminum nitrate and 108 g of water to 100 g of the powder carrying Pt. A cordierite honeycomb carrier (outer volume: 1 liter) was dipped in this slurry, the honeycomb carrier was coated with the slurry by a method of blowing excess slurry, and after drying at 120 ° C. for 3 hours,
A catalyst sample was obtained by firing in an electric furnace at 500 ° C for 1 hour.

Regarding the honeycomb catalyst manufactured in this way, the purification performance of the new catalyst and the model gas (lean A /
F = 18, SO ₂ = 50 ppm)) Purification performance after endurance at 600 ° C. for 24 hours was evaluated under the following conditions.

(1) Model gas composition lean CO: 0.08%, C ₃ H ₈ : 800 ppm, H ₂ : 0.03%, CO ₂ : 12.0
%, O ₂ : 4.5%, H ₂ O: 3%, NO: 1000ppm, SO ₂ : 50
ppm, N _2: balance stoichiometric _{CO: 1.05%, C 3 H} 8: 1000ppm, H 2: 0.35%, CO 2: 10.0
%, O ₂ : Fluctuation, H ₂ O: 10%, NO: 2000ppm, SO ₂ : 50pp
m, N ₂ : Remainder (2) Space velocity: 200000 h ^-1

(3) Method of measuring purification rate A stainless steel tube in which a honeycomb is set is heated in a tubular furnace, and a model gas is flown into the honeycomb while keeping the inside of the honeycomb at 300 ° C. Then, the gas after passing through the honeycomb is analyzed. The purified amount is calculated from the average amount of the gas component for 4 minutes in which lean 1 minute to stoichiometric 1 minute is repeated twice and the model gas amount flowed for 4 minutes. Thus, 30
The average purification rate of CO, HC and NO at 0 ° C. was measured.

Examples 2 to 4 and Comparative Examples 1 to ₃ The water content of Al ₂ O ₃ powder was 5, 8, 10, 20, 22 and
A catalyst carrier was produced in the same manner as in Example 1 except that the amount was 25% by weight, and Pt was supported on the catalyst carrier, and the NO _x purification rate was measured. The result is shown in FIG. When the water content of the Al ₂ O ₃ powder was 8 to 20% by weight, a sufficient NO _x purification rate was exhibited.

Examples 5-8 and Comparative Examples 4-5 The amount of diisopropoxybarium added to the Al ₂ O ₃ powder was 5 , 7, 10, ₂₀ , 25 and 27 mol% based on Al ₂ O _3.
A catalyst carrier was produced in the same manner as in Example 1, except that Pt was supported on the catalyst carrier, and N was added at the initial stage and after the durability test.
The O _x purification rate was measured. The result is shown in FIG. Al ₂
A highly durable catalyst was obtained at a Ba amount of 7 to 25 mol% with respect to O ₃ .

[0026]

INDUSTRIAL APPLICABILITY In the catalyst carrier obtained by the method of the present invention, the NO _x absorbent is stably immobilized on the surface and is evenly arranged on the surface. Therefore, NO _x purification by formation of BaSO ₄ is carried out. It is possible to suppress deterioration of performance.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the moisture content of alumina and the NO _x purification rate after endurance.

FIG. 2 is a graph showing the relationship between barium amount and purification rate.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location B01J 32/00 ZAB // B01J 23/02 ZAB A 23/58 ZAB A B01D 53/36 102 H

Claims (1)

[Claims] 1. An Al ₂ O ₃ hydrate is dehydrated to form a powder in which free water on the surface is almost dry, impregnated with a solution of an alkoxide of a NO _x absorbing metal, and hydrolyzed by a sol-gel method. And a method for producing a catalyst carrier.