JPH0628730B2 - Exhaust gas purification catalyst manufacturing method - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a catalyst used for purifying exhaust gas of various combustion devices or automobiles in which rapid heat and rapid cooling conditions are severe, that is, thermal shock resistance is strongly required.

2. Description of the Related Art Conventionally, alumina, cordierite, mullite, etc. have been used as catalyst carriers, but since the specific surface area is small except for alumina, a film such as alumina is formed on the surface of the carrier.
It has been used by supporting a catalytic metal on this coating. For example, in Japanese Examined Patent Publication (Kokoku) No. 50-9749, an aqueous composition is prepared from colloidal boehmite and activated alumina particles having a large specific surface area, which is evenly adhered to a catalyst carrier.
A method of forming an activated alumina layer by heating to a temperature of ˜500 ° C. is described.

However, in terms of heat resistance, the activated alumina coating becomes agglomerated with respect to the carried metal when the temperature exceeds 1000 ° C., resulting in a rapid alpha conversion. Therefore, investigations on improving the heat resistance of activated alumina are underway.

Problems to be Solved by the Invention In the prior art, as the activated alumina deteriorates due to heat, that is, as the specific surface area of the activated alumina decreases, the catalytic performance deteriorates. Therefore, it is necessary to take measures against heat deterioration to prevent deterioration of catalyst performance.

Further, even with respect to lead poisoning resistance, a longer life is required.

Means for Solving the Problems In order to solve the above problems, the present invention comprises depositing an aqueous composition comprising titania-alumina prepared from a metal alkoxide on a carrier, followed by firing at a temperature of 400 ° C. or higher,
A coating layer is formed on the surface of the carrier, and a catalyst is supported on the coating layer to form an exhaust gas purifying catalyst.

In recent years, as environmental standards are becoming more stringent, it is desired to improve exhaust gas purification performance. Required for catalytic performance are ternary performance, heat resistance, lead poisoning resistance, and thermal shock resistance.

In the present invention, by providing a coating layer of titania-alumina, it is effective against heat deterioration. In the coating layer, the catalytic metal such as platinum is highly dispersed on the titania, and the alumina layer functions sufficiently as a porous structure. Therefore, even when the titania-alumina layer is thermally deteriorated, platinum or the like dispersed on the titania is not directly affected by the thermal deterioration of alumina. Also,
A catalyst such as platinum supported on titania acts quite effectively against Pb and S poisons and is less likely to be poisoned.

Example <Example 1> As an example of a method for preparing an aqueous composition of titania-alumina, equimolar amounts of titanium tetraethoxide and aluminum isopropoxide are added to excess pure water with stirring, and then hydrolysis can be performed. As slowly as possible
Grow the alumina gel. The obtained precipitate was filtered, and then pure water was added to make an aqueous composition of about 20 wt% titania-alumina. Similarly, an aqueous titania-alumina composition was prepared using various alkoxides. Even in that case, titania and alumina were made to be equimolar. It was also possible to use a small amount of aqueous ammonia in the hydrolysis.

The titania-alumina aqueous composition obtained as described above had a honeycomb cross-sectional area of 100 cm ² , a length of 1 cm, and a cell wall thickness of 0.2.
A honeycomb carrier with a cell diameter of 5 mm and a cell diameter of 1.0 mm × 1.0 mm is coated by the dipping method, dried at 80 ° C for 1 hour, and then dried at 500 ° C
After heat treatment for a period of time, a coating layer of titania / alumina was formed on the surface of the carrier. Next, the platinum salt was impregnated with an aqueous solution of platinum salt at a temperature of 300 ° C.
Approximately 10 mg was supported and catalyzed.

For the performance evaluation of these catalysts, carbon monoxide (CO)
The purification rate was measured under the following conditions.

Space velocity 70,000 hr ⁻¹ CO concentration 200 ppm Catalyst temperature 250 ° C. Further, as a life test, the CO purification rate after heat treatment at 800 ° C. for 1000 hours was measured in the same manner.

Table 1 shows these results. As a comparative example, performance results when titania-alumina alone is used as an aqueous composition are also shown.

As is clear from this result, the catalyst having the titania-alumina coating prepared from the metal alkoxide shows excellent performance.

Among them those prepared from titanium tetraisopropoxide _{Ti (iC 3 H 7 O)} 4 and the aluminum sec- butoxide _{A (sec-C 4 H 9} O) 3 was the most excellent. In that case, there is not much difference in performance even if a small amount of ammonia water is used. Therefore, ammonia water can be used as a gelation promoter.

It is best that the particles of titania and alumina are evenly dispersed. Next, there is a case where alumina grows first and titania grows later, which is a state in which titania is coated on porous alumina. The worst is when titania grows first and alumina grows thereafter.

The carrier used here is a carrier in which lime aluminate and fused silica in a weight ratio of 3: 1 are kneaded with water, extruded into a honeycomb shape, dried, and then heat treated at 1000 ° C. The carrier that can be used is not limited to this, and may be a carrier composed of cordierite or mullite. However, in consideration of the adhesion between the coating film and the carrier, a honeycomb carrier having porosity and a low expansion coefficient is desirable.

Example 2 Regarding the catalyst using the titania-alumina coating film prepared from titanium tetraisopropoxide and aluminum sec-butoxide, which showed excellent characteristics in Example 1,
The heat treatment temperature of the coating was examined. The results are shown in Table 2.

From this result, if the heat treatment temperature is too low, the platinum carried later will be buried in the coating, resulting in a shorter life. Further, even if the temperature is too high, the characteristics as titania-alumina cannot be exhibited. Therefore, the optimum temperature is 500 to 600 ° C.

Example 3 As a method for preparing an aqueous titania-alumina composition from a metal alkoxide, the case of thermally decomposing a metal alkoxide will be described.

By using titanium tetraisopropoxide and aluminum sec-butoxide and introducing them into a reaction furnace at 400 ° C., fine titania-alumina particles were obtained. The titania-alumina particles were used as a 30 wt% aqueous composition to form a film on the same honeycomb-shaped carrier as in Example 1,
Comparison was made with the case of No. 4 in Example 1. The performance evaluation method is the same as that of the first embodiment.

As is clear from this result, hydrolysis is superior to pyrolysis as a method for preparing titania-alumina.

Example 4 A honeycomb carrier having a titania / alumina coating film prepared using titanium tetraisopropoxide and aluminum sec-butoxide, which showed excellent performance in Example 1, was evaluated for its performance as an automobile catalyst. did.

The carrier has a honeycomb cross-sectional area of 100 cm ² , a length of 8 cm, a cell wall thickness of 0.25 mm, and a cell diameter of 1.0 mm × 1.0 mm.

After impregnating the above carrier with an aqueous cerium nitrate solution, 500
20 g of cerium oxide per carrier was supported by a method of heat treatment at ℃, then impregnated with an aqueous solution containing a platinum salt and a rhodium salt, and subjected to a reduction treatment under a hydrogen atmosphere at 300 ℃, 1 g of platinum per carrier, 5 rhodium
g was supported.

The performance test was performed by using a V-4 engine of 1.5, and installing a catalyst at a place where the exhaust gas temperature at the stoichiometric air-fuel ratio was 450 ° C. and measuring.

The initial characteristics of the catalyst are shown in the figure. In order to further test the catalyst life, the temperature of the catalyst layer is 800 ° C and the space velocity is 6000.
An endurance test was conducted using leaded gasoline for 100 hours at 0 hr ^-1 . Then, the subsequent performance was measured as in the initial stage.

The evaluation method is hydrocarbon (HC), CO, nitric oxide (N
The air-fuel ratio salt having a purification rate of O) of 80% was measured as the window width.

As a result, the initial window width is 0.24,800.
It was 0.19 after the heat treatment at 200C for 200 hours.

From these results, it can be seen that the catalyst of the present invention has sufficient performance for automobiles and maintains durability against Pb and S poisoning.

Effect of the Invention The catalyst obtained by the present invention has excellent heat resistance and lead poisoning resistance, and can be widely used as a catalyst for various combustion equipment and automobiles.

[Brief description of drawings]

The figure shows the performance of the catalyst of the example of the present invention as an automobile catalyst.

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Claims (2)

[Claims] 1. A titania prepared from a metal alkoxide.
After uniformly depositing an aqueous composition of alumina on a carrier, it is fired at a temperature of 400 ° C. or higher to form a titania-alumina film on the surface of the carrier, and a platinum group metal or platinum is formed on the film layer. A method for producing an exhaust gas-purifying catalyst, which comprises supporting a group metal and a rare earth metal oxide. 2. The method for producing an exhaust gas purifying catalyst according to claim 1, wherein the metal alkoxide is ethoxide or isopropoxide of titanium and isopropoxide or sec-butoxide of aluminum.