JPH0622675B2 - Engine exhaust gas purification catalyst - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to carbon monoxide (hereinafter, referred to as CO) and hydrocarbon (hereinafter, referred to as HC) in exhaust gas discharged from an internal combustion engine or the like, particularly an automobile. Also, the present invention relates to an engine exhaust gas purifying catalyst used for reducing nitrogen oxide (hereinafter referred to as NOx).

(Prior Art) Conventionally, a noble metal such as platinum (Pt), rhodium (Rh) and palladium (Rd) is supported on alumina (Al ₂ O ₃ ) as a catalyst for purifying CO, HC and NOx in automobile exhaust gas. What has been used is used. In addition, in order to improve the catalytic performance of these precious metals, an oxygen storage capacity effect (the effect of taking in oxygen in the exhaust gas and contributing this oxygen to the purification of the catalyst)
A catalyst has been produced in which certain cerium oxide (CeO ₂ ) is contained in an alumina coating layer together with a noble metal in order to improve the purification rate of exhaust gas.

However, the method of supporting a catalyst component of a noble metal or a base metal and an oxygen storage capacity-imparting agent (hereinafter referred to as an OSC agent) such as cerium oxide coexisting in the alumina coat layer has the following problems. .

(a) Since the OSC agent is in direct contact with the catalyst component and aluminum, the instability of the OSC agent adversely affects them.

(b) Since the OSC agent is supported together with the catalyst component, they both form a compound, which reduces the dispersibility of the catalyst component and reduces the exhaust gas purification performance.

In addition, a catalyst layer containing Pt, Pd, etc. is provided on the catalyst carrier, and alumina or alumina and cerium oxide (0.1 to 0.5% by weight relative to alumina) is provided to protect the catalyst layer.
It has been proposed to provide an oxide coating layer composed of the mixture (see JP-A-60-5230). In the case of this known technique, the coating layer containing a trace amount of cerium oxide is provided for the purpose of protecting the catalyst layer, and since the concentration of cerium oxide acting as an OSC agent is extremely low, it has almost no oxygen storage effect. It was something I couldn't expect. Therefore, it is conceivable to increase the content of the OSC agent in the coating layer, but in that case, the catalyst layer is covered with the OSC agent-supporting layer having a large heat capacity (in other words, the coating layer), and Warm air may be impaired, and the exhaust gas purification performance at the time of low temperature starting may become insufficient.

(Object of the Invention) The present invention has been made in view of the above problems, and a catalyst layer containing a catalyst component composed of a noble metal or a base metal is formed on the upper surface of a coat layer containing a large amount of cerium oxide supported on a catalyst carrier. By forming, the purification performance of the catalyst (especially,
The purpose is to significantly improve the purification performance at low temperature starting).

(Means for Achieving the Purpose) In the present invention, as a means for achieving the above object, a catalyst layer containing a catalyst component is provided on the upper surface of a coat layer supported on a catalyst carrier and containing a large amount of cerium oxide. ing.

Further, the composition of the coating layer is preferably 10 to 95% by weight of cerium oxide and the balance of activated alumina, and the coating is an aqueous slurry containing cerium oxide, activated alumina, hydrated alumina and other dispersants. Perform using. As the simplest method, the coating layer can be formed by coating a slurry containing two components of cerium oxide and hydrated alumina.

As the catalyst component, known components such as Pt, Rh, Pb and mixtures of two or more thereof are used.

An example of the exhaust gas purifying catalyst thus formed is shown enlarged in FIG. Here, reference numeral 1 is a catalyst carrier,
Reference numeral 2 is a coat layer, and 3 is a catalyst layer. As the catalyst carrier 1, a honeycomb structure made of a ceramic such as cordierite or various carriers made of heat-resistant metal or heat-resistant inorganic fiber is used.

(Operation) The catalyst having the above structure shows an extremely excellent exhaust gas purification performance as compared to the conventional catalyst in which the living component and cerium oxide are impregnated in the same layer, and the lower layer is By using the cerium oxide coat layer having a large heat capacity and the catalyst layer as the upper layer, the warming property of the catalyst component is not hindered, and the deterioration of the purification performance at the low temperature start is prevented.

Further, generally, the catalytic performance gradually decreases as the cerium oxide content in the coating layer decreases, and sharply decreases at 10% or less. The reason is that when the concentration of cerium oxide is reduced, it becomes impossible to interact with the active ingredient. By the way, FIG. 2 shows the results of measurement of changes in the CO purification rate (%) at 350 ° C. with respect to the CeO ₂ content (% by weight) in the coat layer. The catalyst used in the test is supposed to have Pt of 1.0 g / l and Rh of 0.2 g / l supported on a carrier having a coat layer formed by mixing CeO ₂ and alumina. These results were measured after the activity measurement conditions were air-fuel ratio A / F = 14.7 ± 0.9, space velocity SV = 60000 / Hr, and heat aging at 1100 ° C. for 50 hours in the atmosphere. According to this, when CeO ₂ becomes 10% or less, C
The O purification rate has dropped significantly.

On the other hand, when the content of cerium oxide is high, the catalytic activity is improved, but since the binding force of cerium oxide itself is weak,
Physical strength (peeling resistance) is reduced and durability is reduced. By the way, the content of CeO ₂ in the coating layer (% by weight)
When a peeling test was conducted by changing the above, the results shown in Table-1 were obtained. Here, peeling amount = (coat adhesion amount before test−coat adhesion amount after test) / (coat adhesion amount before test), and as a test method, a diameter of 1 inch,
Heat a 1-inch-high cylindrical test piece at 600 ℃ for 30 minutes,
Next, after repeating the procedure of cooling in water at 25 ° C three times,
A method of sufficiently drying and measuring the amount of peeling was adopted.

From the results of the above peel test, it is understood that the CeO ₂ content in the coat layer is preferably 95% or less.

When the adhesion ratio of the coating layer to the catalyst carrier is 3% by weight or less, the concentration is too low, and the performance of the catalyst component in the upper layer does not improve, so that the catalyst performance sharply decreases and exceeds 40% by weight.
It does not contribute to the improvement of the catalyst performance, the physical strength of the coat layer is lowered, and peeling easily occurs. Taking this into consideration, it is desirable that the coating layer has a deposition rate of 3 to 40% by weight.

Preferred embodiments of the present invention will be described below.

Example 400 g of cerium oxide, 100 g of boehmite, and 500 cc of water were mixed and stirred with a homomixer to obtain a slurry for cerium oxide coating. In this slurry, after immersing the honeycomb catalyst simple substance (made by Cordierite) and pulling it out, excess slurry is removed by high-pressure air blow, and 330 to 550 ° C. is placed in an electric furnace set at a heating rate of 200 ° C./Hr. This was baked for 1 hour at a temperature of 82% by weight of cerium oxide and 18% by weight of activated alumina.
It was possible to coat a coating layer consisting of 20% by weight on the catalyst carrier.

Next, 160 g of γ-alumina, 160 g of boehmite, 500 cc of water, and 4 cc of concentrated nitric acid were mixed and stirred by a homomixer for 10 hours to obtain an alumina washcoat slurry. The above cerium oxide-coated catalyst carrier was immersed in this slurry and pulled up, and then the excess slurry was removed by high-pressure air blow, and the mixture was calcined at 200 to 600 ° C. at a temperature rising rate of 200 ° C./Hr. Next, this is converted to platinum and rhodium, immersed in a mixed aqueous solution of chloroplatinic acid / rhodium chloride having a concentration of 1.0 g / l and 0.2 g / l, respectively, and pulled up, and then dried at 200 ° C. for 1 hour, and 550 Calcination was performed for 2 hours. The noble metal contents after firing were platinum (Pt) 1.0 g / l and rhodium (Rh) 0.2 g / l. The catalyst layer was 14% by weight based on the weight of the catalyst carrier.

The purification performance of the exhaust gas purifying catalysts obtained in the above examples was compared with a conventionally known catalyst, that is, a catalyst obtained by mixing a catalytically active component and cerium oxide. The results shown in FIGS. 5 to 5 are obtained. Here, the comparative example was prepared as follows.

Comparative example 100g activated alumina, 400g cerium oxide, 4cc concentrated nitric acid, 700c water
c was mixed and stirred with a homomixer to obtain a slurry for coating alumina and cerium oxide. Using this slurry, the honeycomb catalyst carrier was coated with alumina and cerium oxide in the same manner as in the above example. The amount of the coating layer deposited was 25% by weight based on the catalyst carrier, and the ratio of cerium oxide in the coating layer was 82% by weight.

A catalyst was prepared by further adhering platinum (Pt) 1.0 g / l and rhodium (Rh) 0.2 g / l to this catalyst carrier in the same manner as in the above example. The catalyst layer was 14% by weight based on the weight of the catalyst carrier.

The activity measurement conditions were air-fuel ratio A / F = 14.7 ± 0.9, space velocity SV = 60000 / Hr, and measurement was performed after heat aging at 1100 ° C. in the atmosphere for 10 hours.

According to the results of the above-mentioned evaluation test, the exhaust gas purifying catalyst according to the example of the present invention has significantly improved purifying performance as compared with the conventionally known catalyst, that is, the comparative example. In particular, the catalyst activity is good in the low temperature region, because the cerium oxide coat layer is located under the catalyst layer, which improves the warming property of the catalyst layer.

(Effects of the Invention) As described above, according to the present invention, the active catalyst components (Pt, Rh, Rd) are formed on the coating layer containing a high concentration (that is, 10 to 95% by weight) of cerium oxide (CeO ₂ ). Since the catalyst layer containing () etc. is formed, the active catalyst component and cerium oxide do not form a compound, and the exhaust gas is exhausted as compared with the conventional mixture of the active catalyst component and cerium oxide. There is an excellent effect that the purification performance is improved and the active catalyst component reacts effectively from the low temperature range of the exhaust gas, and the purification performance at the cold start can be remarkably improved.

Further, since the coat layer containing cerium oxide, which has relatively poor heat resistance, is formed below the catalyst layer, there is an effect that the thermal deterioration of the coat layer can be reduced.

Furthermore, the content of cerium oxide in the coating layer is set to 10
When the content is up to 95% by weight, the oxygen storage capacity effect of cerium oxide is significantly increased, and the effect of further improving the exhaust gas purification performance is obtained.

[Brief description of drawings]

FIG. 1 is an enlarged view showing an example of an exhaust gas purifying catalyst according to the present invention, and FIG. 2 is an exhaust gas with respect to a CeO ₂ content (% by weight) in a coat layer in the exhaust gas purifying catalyst according to the present invention. Characteristic diagram showing changes in CO purification rate (%) in gas,
FIG. 3, FIG. 4 and FIG. 5 are characteristic diagrams showing the results of the purification performance evaluation tests of the example of the present invention and the comparative example. 1 ... Catalyst carrier 2 ... Coat layer 3 ... Catalyst layer

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification number Office reference number FI Technical display location B01J 35/04 301 L 7821-4G 37/02 ZAB 7821-4G 301 L 7821-4G (72) Invention Person Kazunori Ihara 3-1, Shinchi, Fuchu-cho, Aki-gun, Hiroshima Prefecture Mazda Co., Ltd. (72) Inventor Shigeru Yazaki 1-56-26 Matsubara, Matsubara 1-56-26, Matsubara Setagaya-ku, Tokyo (72) Inventor Yasutaka Yoshino 1970-221 Nara-cho, Midori-ku, Yokohama-shi, Kanagawa (56) References Japanese Patent Laid-Open No. 61-78439 (JP, A)

Claims (1)

[Claims] 1. A coat layer of alumina containing 10 to 95% by weight of cerium oxide, which is supported on a catalyst carrier and acts as an oxygen storage capacity-imparting agent, and platinum, rhodium and palladium which are provided on the coat layer. An exhaust gas purifying catalyst for an engine, comprising a catalyst layer containing at least one type of catalyst component selected from the group.