JPH0338894B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention deals with carbon monoxide (hereinafter referred to as CO), hydrocarbons (hereinafter referred to as HC), and nitrogen oxides (hereinafter referred to as NOx) in exhaust gas emitted from internal combustion engines, etc., especially automobiles. This invention relates to a catalyst for purifying engine exhaust gas, which is used to reduce emissions. (Conventional technology) Conventionally, CO, HC,
As a catalyst for purifying NOx, alumina (Al ₂ O ₃ ) supported on noble metals such as platinum (Pt), rhodium (Rh), and palladium (Pd) is used. In addition, in order to improve the catalytic performance of these precious metals, we have developed an oxygen storage effect (the effect of capturing oxygen from exhaust gas and contributing to the purification of the catalyst).
Catalysts have been manufactured in which cerium oxide (CeO ₂ ), which is a certain substance, is contained in an alumina coating layer along with precious metals to improve the purification rate of exhaust gas. However, catalyst components of noble metals and base metals, and oxygen storage capacity imparting agents (hereinafter referred to as OSC) such as cerium oxide
The method of coexisting and supporting the alumina coating layer has the following problems. (a) Since the OSC agent is uniformly supported on the alumina coat layer, it is not necessarily supported in areas near the pores that are likely to come into contact with exhaust gas. In other words, most of the OSC agents are carried in parts of the exhaust gas that are difficult to diffuse, and do not participate in the purification reaction, making it impossible to obtain a high purification rate. (b) The thermal instability of the OSC agent adversely affects the catalyst components and alumina due to the increased direct contact between the agent and the catalyst components. (c) Since the OSC agent and the catalyst component are supported together, the two form a compound, reducing the dispersibility of the catalyst component and reducing exhaust gas purification performance. In addition, a catalyst layer containing Pt, Pd, etc. is provided on the catalyst carrier, and in order to protect the catalyst layer, an oxide consisting of alumina or a mixture of alumina and cerium oxide (0.1 to 0.5% by weight relative to alumina) is used. A method in which a coating layer is provided has been proposed (see Japanese Patent Application Laid-Open No. 60-5230). In the case of this known technique, the coating layer is provided to protect the catalyst layer, and since the content of cerium oxide is too small, sufficient oxygen storage capacity cannot be expected. Therefore, it is possible to increase the concentration of the OSC agent in the coating layer, but in that case, the catalyst surface is exposed to exhaust gas and becomes high temperature, which promotes crystal growth of the OSC agent contained in the coating layer. sintering occurs,
This increases the particle size of the OSC agent, which may cause a problem of significantly reducing catalyst activity. (Object of the Invention) The present invention has been made in view of the above-mentioned problems, and it acts as a thermal stabilizer on a coating layer containing an OSC agent provided on the top surface of a catalyst layer containing a catalyst component made of a noble metal or a base metal. By adding oxides of lanthanum (La) or (and) neodymium (Nd), the reduction in catalyst activity due to sintering of the OSC agent at high temperatures is prevented, thereby improving purification performance. The purpose is to (Means for Achieving the Object) In the present invention, as a means for achieving the above object, a coating layer containing an OSC agent is provided on the upper surface of a catalyst layer supported on a catalyst carrier and containing a catalyst component. La and Nd act as heat stabilizers in the coating layer.
Oxides such as Here, as the catalyst component, known components such as Pt, Rh, and Pd, and mixtures of two or more thereof are used. Furthermore, cerium oxide (CeO ₂ ) is used as a component of the OSC agent. Furthermore, the thermal stabilizer (La, Nd oxide)
is added in an amount of 1 to 10% by weight. The composition of the coating layer is preferably 50 to 94% by weight of an OSC agent and the balance of activated alumina. conduct. In the simplest method, the coating layer can be formed by coating a slurry using two components: an OSC agent and hydrated alumina. An example of the thus formed exhaust gas purifying catalyst is shown enlarged in FIG. Here, reference numeral 1 indicates a catalyst carrier, 2 indicates a catalyst layer, and 3 indicates a coating layer. As the catalyst carrier 1, a honeycomb structure made of ceramics such as cordierite or various carriers made of heat-resistant metal and heat-resistant inorganic fibers are used. (Function) A catalyst having the above structure can suppress the crystal growth of cerium oxide in a high temperature state, and can maintain excellent performance against thermal deterioration. Furthermore, it exhibits extremely superior exhaust gas purification performance compared to conventional catalysts in which active catalyst components and cerium oxide are impregnated in the same layer. The reason for specifying the amount of heat stabilizer added as described above is as follows. That is, if the amount of the heat stabilizer added is 1% by weight or less, little effect of suppressing the crystal growth of cerium oxide is obtained, and no improvement in purification performance is observed. On the other hand, when the amount of heat stabilizer added exceeds 10% by weight, the effect of inhibiting crystal growth is saturated. By the way, the thermal stabilizer _{(Nd 2 O 3} or
FIG. 2 shows the results of measuring the change in temperature (° C.) at which the HC purification rate in exhaust gas was 50% with respect to the amount (wt%) added of La ₂ O ₃ /Nd ₂ O ₃ ). The catalyst components used here were composed of platinum (Pt) and rhodium (Rh) (same as those in the examples described later), and 80% by weight of cerium oxide.
The amount of γ-alumina is varied by changing the amount of Nd ₂ O ₃ or La ₂ O ₃ /Nd ₂ O ₃ added. Note that La ₂ O ₃ /Nd ₂ O ₃ =50%/50
%. In this result, the activity measurement conditions were air-fuel ratio A/F = 14.7±0.9, space velocity SV = 60000/
Hr, catalyst component: Pt/Rh=5/1 (1.2g/
), durability conditions: thermal deterioration (in the atmosphere) at 1000°C for 50 hours. According to this, it can be seen that the effect of suppressing the crystal growth of cerium oxide appears when the amount of heat stabilizer added is in the range of 1 to 10% by weight. If the amount exceeds 10% by weight, the thermal stabilizer will inhibit contact between cerium oxide and exhaust gas, resulting in decreased performance. Hereinafter, preferred embodiments of the present invention will be described. Example γ-alumina 160g, boehmite 160g, water 500g
cc. and 4 c.c. of concentrated nitric acid were mixed and stirred for 10 hours using a homomixer to obtain a slurry for coating an alumina wash. A honeycomb catalyst carrier (made of cordierite) was immersed in this slurry for about 20 seconds and then pulled out. Excess slurry was removed by high-pressure air blowing, dried at 130°C for 1 hour, and then calcined at 550°C for 1 hour. This alumina-coated catalyst carrier is coated with Pt of 200
After dipping for about 15 seconds in a mixed aqueous solution obtained by mixing chloroplatinic acid with an Rh content of 100 g/ and rhodium chloride with an Rh content of 100 g/in water and pulling it out,
It was dried at 600°C for 1 hour and then fired at 600°C for 2 hours. Precious metal content after firing is platinum (Pt) 1.0g/
, rhodium (Rh) 0.2g/. The amount of the catalyst layer 2 thus obtained was 14% by weight based on the catalyst carrier 1. Next, 50 g of lanthanum oxide (La ₂ O ₃ ) is added to 150 g of hydrated alumina and mixed using a ball mill. After adding and mixing 800 g of cerium oxide to this mixed powder, water was added and kneaded to obtain a slurry liquid for OSC agent coating. While stirring this slurry liquid, the alumina and precious metals (Pt and
After immersing the catalyst carrier with Rh) on it for about 20 seconds and pulling it out, excess slurry was removed by high-pressure air blowing, dried at 150°C for 3 hours, and then calcined at 700°C for 3 hours. As a result, 80% by weight of cerium oxide, 5% by weight of lanthanum oxide and 15
An exhaust gas purifying catalyst was obtained in which a coating layer 3 consisting of γ-alumina of % by weight was formed (first
(see figure). The amount of coating layer 3 obtained at this time is:
The amount was 30% by weight based on the catalyst carrier including the catalyst layer 2. When the change in the crystal diameter of cerium oxide with respect to temperature in the exhaust gas purifying catalyst obtained in the above example was investigated, the results shown in FIG. 3 were obtained. Note that the results of investigating a catalyst to which lanthanum oxide was not added are shown as a comparative example. According to this, it can be seen that by adding lanthanum oxide, which is a thermal stabilizer, the change in crystal diameter of cerium oxide, which is an OSC agent, with respect to temperature is reduced, and sintering is suppressed. In addition, an evaluation test was conducted comparing the purification performance of the exhaust gas purification catalyst obtained in the above example with a catalyst to which no lanthanum oxide was added (comparative example), and the results shown in Figure 4 were obtained. It was done. In addition, the activity measurement conditions are air-fuel ratio A/F = 14.7±
0.9, space velocity SV = 60000/Hr, durability conditions:
An evaluation test was conducted by thermal aging (in the air) at 1000°C for 50 hours. According to the results of the above evaluation test, the exhaust gas purification catalyst according to the example of the present invention has significantly improved purification performance compared to the comparative example as a result of suppressing sintering of cerium oxide. I understand. In this example, the content of cerium oxide in the coating layer is 80% by weight, but the cerium oxide content can be in the range of 50 to 94% by weight. Generally, as the cerium oxide content in the coating layer decreases, the catalytic performance gradually decreases, and below 50%, the catalyst performance decreases rapidly. The reason is that when the concentration of cerium oxide decreases, interaction with the active catalyst component is no longer obtained. By the way, OSC
FIG. 5 shows the results of measuring the change in CO purification rate (%) at 400° C. with respect to the CeO ₂ content (weight %) in the coating layer using CeO ₂ as the agent. In the characteristic diagram of FIG. 5, the amount of lanthanum oxide is fixed at 5% by weight, and the amount of γ-alumina is changed by changing the amount of cerium oxide. However, when cerium oxide exceeds 94% by weight, lanthanum oxide is 1% by weight. This result shows that the activity measurement conditions were changed to the air-fuel ratio A/F
= 14.7±0.9, space velocity SV = 60000/Hr, 300Hr with exhaust gas temperature 850℃ in bench engine
This is an evaluation using the catalyst after an operational durability test. According to this, when CeO ₂ falls below 50%, the CO purification rate drops significantly. On the other hand, when the content of CeO ₂ increases, the catalytic activity improves, but because the binding force of CeO ₂ itself is weak,
Physical strength (peel resistance) decreases and durability decreases. Incidentally, when a peel test was conducted by varying the content (wt%) of CeO ₂ in the coating layer using CeO ₂ as the OSC agent, 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 testing), and the test method involved heating a cylindrical test piece with a diameter of 1 inch and height of 1 inch at 600℃ for 30 minutes, then cooling it in water at 25℃, repeated three times. The method used was to thoroughly dry the film and measure the amount of peeling.

[Table] From the results of the above peel test, CeO ₂ in the coating layer
It can be seen that the content is preferably 95% by weight or less. In the case of this example, considering the amount of lanthanum oxide added, it is preferably 94% by weight or less. If the adhesion ratio of the coating layer to the catalyst carrier is less than 5% by weight, the surface of the catalyst layer cannot be effectively coated, resulting in a rapid decline in catalyst performance, and if it exceeds 40% by weight, active components and exhaust gas Since contact with gas is inhibited, catalyst performance rapidly decreases. Taking this into consideration, it is desirable that the adhesion ratio of the coating layer be 5 to 40% by weight. Note that the thickness of the coating layer is preferably 20 to 40 μm. Note that in the above example, cerium oxide (CeO ₂ ) is used as the OSC agent, but nickel oxide (NiO), molybdenum oxide (MoO ₃ ), which has almost the same oxygen storage capacity as cerium oxide, ), iron oxide (Fe ₂ O ₃ or FeO) may be added. (Effects of the Invention) As described above, according to the present invention, a catalyst layer containing at least one active catalyst component selected from the group consisting of platinum, palladium, and rhodium;
Since the coating layer containing cerium oxide is formed separately, the active catalyst component and cerium oxide do not form a compound, compared to the conventional case where the active catalyst component and cerium oxide are mixed. This has the excellent effect of improving exhaust gas purification performance. Furthermore, since the coating layer contains 1 to 10% by weight of at least one oxide of lanthanum and neodymium that acts as a thermal stabilizer, crystal growth of cerium oxide at high temperatures is suppressed and sintering is prevented. This is effectively prevented, and has the effect of ensuring stable purification performance even at high temperatures. Furthermore, since the upper surface of the catalyst layer is coated with a coating layer containing cerium oxide, the catalyst layer is easily placed in a reducing atmosphere, which has the advantage that the NOx purification performance in exhaust gas is further improved.

[Brief explanation of drawings]

FIG. 1 is an enlarged view showing an example of the exhaust gas purification catalyst according to the present invention, and FIG. 2 is a diagram showing the amount of heat stabilizer added (wt%) in the exhaust gas purification catalyst according to the present invention. Figure 3 is a characteristic diagram showing the change in temperature (°C) at which the HC purification rate is 50%.
A characteristic diagram showing changes in the crystal diameter (Å) of CeO ₂ with respect to heating temperature in Examples and Comparative Examples of the present invention,
FIG. 4 is a characteristic diagram showing the results of the purification performance evaluation test of the example of the present invention and the comparative example, and _FIG. FIG. 2 is a characteristic diagram showing changes in CO purification rate (%) in exhaust gas with respect to content (weight %). 1...Catalyst carrier, 2...Catalyst layer, 3...Coating layer.

Claims (1)

[Claims] 1. A catalyst layer supported on a catalyst carrier and containing at least one catalyst component selected from the group consisting of platinum, palladium, and rhodium, and cerium oxide provided on the catalyst layer and acting as an oxygen storage ability imparting agent. and an alumina coating layer containing lanthanum and neodymium, the coating layer containing 1 to 10% by weight of an oxide of at least one of lanthanum and neodymium. catalyst.