JPH03196841A - Catalyst for purification of exhaust gas - Google Patents

Abstract

PURPOSE:To obtain a catalyst for purification of exhaust gas having satisfactory purifying performance by forming alumina carrier layers and cerium oxide- zirconium oxide promoter layers on the base material of a columnar monolithic catalyst carrier having many pores extending in the axial direction and further forming a rhodium layer at the downstream side. CONSTITUTION:An alumina layer stabilized with the oxide of a rare earth element is formed on a columnar monolithic catalyst carrier having many pores extending in the axial direction. A layer consisting of <=0.5mol/l, in total, of <=0.1mol/l cerium oxide and zirconium oxide and a layer consisting of <=0.5 mol/l, in total, of 0.1-0.3mol/l cerium oxide and zirconium oxide are then formed at the upper stream side and downstream side, respectively. A palladium layer, a similar alumina layer and a layer consisting of <=0.5mol/l, in total, of 0.1-0.3mol/l cerium oxide and zirconium oxide are further formed and a rhodium layer is formed only at the downstream side. The resulting catalyst contains separate palladium and rhodium layers and has superior activity.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a catalyst for purifying exhaust gas. More specifically, the present invention relates to a monolithic catalyst for exhaust gas purification that has improved purification performance.

[Conventional technology]

Purification catalysts that purify gas containing toxic components discharged from internal combustion engines, especially monolithic catalysts for purifying automobile exhaust gas, are becoming increasingly demanding in terms of purification performance, durability, cost, etc. be.

Conventionally used ternary monolith catalysts consist of alumina-L stabilized by rare earth element oxides, oxides such as cerium (Ce) as co-catalyst components, and rhodium (Rh) and platinum as catalyst components. (Pt).

Cerium oxide, which is a cocatalyst, stores and releases oxygen to reduce the decrease in purification performance when the air-fuel ratio deviates from the stoichiometric air-fuel ratio. However, cerium oxide is
- At high temperatures, sintering tends to occur, leading to grain growth of catalyst components such as Rh supported on it,
Decrease catalyst activity.

Therefore, in order to prevent such grain growth, zirconium oxide is used together with cerium oxide. However, even in this case, if cerium oxide, which has the ability to store and release oxygen, is used in a high concentration, there is a problem in that the warm-up performance deteriorates and the purification characteristics at engine startup deteriorate.

In addition, Rh and pt used as catalyst components are:
Rh has the effect of purifying NOx, and pt has the effect of purifying HC-Co. Although this catalyst system has excellent purification performance, +((, Pd, which is cheaper for CO purification)
It is expected that Rh/Pd/Pt-based or Rh/Pd-based catalysts will be put to practical use. However, the biggest problem when using PrJ is that at high temperatures of 600° C. or higher, Pd reacts with Rh and the purification performance deteriorates.

Furthermore, monolithic automotive catalysts are particularly susceptible to poisoning in the upstream portion of the exhaust gas flow, and expensive precious metals may be wasted.

[Problem to be solved by the invention]

The object of the present invention is to provide a catalyst in which an alumina carrier layer and a cerium oxide-zirconium oxide promoter layer are provided on a monolithic catalyst carrier base material having a columnar shape and having a large number of pores extending in the axial direction. It is an object of the present invention to provide an exhaust gas purifying catalyst that functions effectively even when formed from a Rh/Pd system.

[Means to solve the problem]

The exhaust gas purifying catalyst that achieves the object of the present invention is formed by forming the following layers on a monolithic catalyst carrier base material having a columnar shape and having a large number of pores extending in the axial direction.

(1) Alumina layer stabilized by rare earth element oxide (2) 0.1 mol/l in the upstream part in the exhaust gas flow direction
The total of cerium oxide with the following concentration and the concentration of 0.1 to 0.3 mol/l in the downstream part and the cerium oxide concentration is 0.5 mol/p or less in each part on the upstream and downstream sides. (3) palladium layer (4) alumina layer stabilized by rare earth element oxide (5) cerium oxide with a concentration of 0.1 to 0.3 mol/l and the oxide (6) Rhodium layer formed only in the downstream portion The stabilized alumina used as a carrier is about 50% ~30
A powder of at least one of γ-alumina, δ-alumina, θ-alumina, etc., preferably γ-alumina, having a specific surface area of 0m"1g is immersed in an aqueous solution of lanthanum nitrate or the like, and It is impregnated with about 0.1 to 5 mol % of lanthanum nitrate and then calcined, and is generally used in the form of a fine powder with an average particle size of about 1 to 5 μm.

This stabilization treatment prevents transformation into α-alumina with a small specific surface area in high-temperature environments, particularly at temperatures above about 900°C, and also prevents deterioration in the purification performance of the catalyst component supported thereon. There is.

The formation of the stabilized alumina layer on the carrier substrate is carried out by conventional washcoat slurry methods.

A cocatalyst layer consisting of cerium oxide and zirconium oxide is generally formed in this order on such a stabilized alumina support layer.

Cerium oxide is added in an amount of 0.1 mol/l or less, preferably 0.05 to 0.08 mol/l, in the upstream part, and 0.1 to 0.3 mol/l, preferably 0 in the downstream part. .12~
Each is supported at a concentration distribution of 0.25 mol/l.

Regarding the amount of cerium oxide supported, the second flow side part is 0.1
If it exceeds 0.1 mol/l, the warm-up performance will deteriorate and the purification efficiency will deteriorate.
If it is less than 0.3 mol/l, the purification efficiency will be deteriorated due to insufficient 02 storage capacity, while if it is more than 0.3 mol/l, the specific surface area of the activated alumina will decrease, and the efficiency of supporting the precious metal as a catalyst component will decrease.

Cerium oxide can be supported at different concentration distributions on the upstream portion (approximately 5 to 10% of the length from the end) and downstream portion of the catalyst support by calcination of cerium nitrate, cerium carbonate, etc. Using an aqueous solution of a compound that gives , dry,
By firing at about 300 to 800°C for about 10 minutes to 10 hours, 0.1 mol/l or less of cerium oxide is added to the upstream part and 0.1 to 0.3 mol/l of cerium oxide to the downstream part. This is done by making it carry.

In addition, zirconium oxide is supported by zirconium nitrate,
Using an aqueous solution of a compound such as zirconium carbonate that yields zirconium oxide by firing, a carrier partially supported with cerium oxide at different concentrations is uniformly immersed in this aqueous solution, dried, and heated at about 300 to 800°C for about 10 minutes. This is carried out by firing for 10 minutes to 10 hours, and the amount supported is such that the total amount of cerium oxide supported is 0.000% on the upstream and downstream sides.
The concentration is selected to be less than or equal to 5 mol/l, generally the same concentration in each part.

Next, a palladium catalyst is supported on this cerium oxide and zirconium oxide supported carrier,
A similar method is applied to support this. That is,
Immersion in an aqueous solution of a compound that gives palladium by calcination, such as dinitrodiammine palladium, palladium nitrate, palladium chloride, etc., drying and calcination (approximately 150 to 4
0.04 to 2.0 depending on
Palladium is supported in an amount of g/l.

After this, cerium oxide with a concentration of 0.1 to 0.3 mol/l is added, and the total of this cerium oxide concentration is 0.5 mol/l.
A layer consisting of zirconium oxide having the following concentration is uniformly formed on the palladium catalyst supporting surface by the same method as described above.

In each layer in which both cerium oxide and zirconium oxide are supported, a concentration ratio of cerium oxide to zirconium oxide of about 1 to 3 is generally used.

Finally, immersion in an aqueous solution of a compound that gives rhodium by calcination, such as rhodium chloride, is carried out only on the downstream part, followed by drying and calcination (approximately 150-400 m
℃, about 10 minutes to 3 hours), there 0.004 to
The desired catalyst is prepared by supporting rhodium in an amount of 1.0 g/Q.

Regarding the supported amounts of palladium and rhodium, which are catalysts, if the supported amount is less than this, the purification performance of the catalyst will be insufficient.On the other hand, even if more than this is used, no further purification performance can be expected, and the cost of expensive precious metals will increase. In addition to this, these precious metals may react with each other, either alone or with each other, resulting in a reduction in purification efficiency.

〔Effect of the invention〕

The exhaust gas purifying catalyst according to the present invention has the following characteristics.

(1) Since the palladium catalyst and the rhodium catalyst are supported separately from each other, there is no risk of deterioration in purification performance due to interaction with rhodium, which is generally a problem when using a palladium catalyst.

(2) Rhodium is not supported on the upstream side, which is most affected by poisoning, and the amount of expensive rhodium used can be reduced.

(3) When a monolithic catalyst carrier is used, the temperature on the upstream side is lower than that on the downstream side, so the purification efficiency deteriorates when the internal combustion engine temperature is low, such as when starting the engine. However, with the catalyst of the present invention, Since the cerium oxide concentration on the upstream side, especially the cerium oxide concentration on the lower layer side, is kept low, the warm-up characteristics are also improved.

(4) As shown in the results of 50% purification temperature below.

Even after the durability test, it maintains excellent catalytic activity.

〔Example〕

Next, the present invention will be explained with reference to examples.

Example: To γ-alumina powder with a specific surface area of 175+++”/g, an aqueous solution of lanthanum nitrate was added at a concentration of 1% to 99 mol% of alumina.
After impregnating the lanthanum with a mol% amount of lanthanum and drying, sequentially calcining at 600°C for 5 hours and 880°C for 3 hours, and pulverizing it to obtain stabilized alumina fine powder with an average particle size of 2 μm or less. I got it.

100 parts by weight of the thus obtained stabilized alumina fine powder, 25 parts by weight of a commercially available aluminum nitrate aqueous solution, 12 parts by weight
In the wash coat slurry obtained by mixing 2 parts by weight of N nitric acid and 400 parts by weight of water while grinding, a volume of 0
．． 8 Q. After soaking a monolithic cordierite support with a cell density of 400 cells/in2, the excess slurry was blown off with compressed air, dried, and fired at 700°C for 1 hour to form a film of approximately 15 μι. A thick alumina coat was formed on the monolithic support (coat weight 165 g/Ω).

This alumina-coated monolithic carrier was immersed in a 1M cerium nitrate aqueous solution and dried, and then the monolithic carrier (length 122 mm) was soaked in a 1.7 cerium nitrate aqueous solution, leaving a 10 mm length portion. soak, dry, and finally
．． The entire body was immersed in an 88M aqueous zirconium nitrate solution, dried, and fired at 600°C for 3 hours to give a concentration of 0.06 mol/l in the length of 1oIl11, and 0.06 mol/l in other parts.
Cerium oxide was supported on each at a concentration of 12 mol/l, and zirconium oxide was further supported on the whole at a concentration of 0.05 mol/l.

Next, this support carrying cerium oxide and zirconium oxide was immersed in an acidic nitric acid aqueous solution of 9mM dinitrodiammine palladium, dried, and then calcined at 200°C for 1 hour to support palladium at a concentration of 1.5 g/ρ on the support. I let it happen.

On this palladium catalyst, a film with a thickness of about 15 cm was applied in the same manner as above.
After forming an alumina coat of μm by a method using wash coat slurry, it was immersed in a 1.7 cerium nitrate aqueous solution, dried, immersed in a 0.88 M zirconium nitrate aqueous solution, dried, and heated at 600°C for 3 hours. Firing was performed sequentially to support cerium oxide at a concentration of 0.12 mol/a and zirconium oxide at a concentration of O, OS mol/Ω.

Finally, in a 2mM rhodium chloride aqueous solution, the 10m
The sample was immersed and dried, leaving a length of m, and then calcined at 200°C for 1 hour to obtain a heterogeneous catalyst on which rhodium was supported at a concentration of 0.27 g/Q.

Comparative Example 1 The inner surface of an integral support made of cordierite contains alumina as the main component, cerium oxide at a concentration of 0.12 mol/l, platinum at 1.5 g/l, and rhodium at a concentration of 0.3 g/Q. Wash coat layer with a film thickness of approximately 25 μm (coat amount 1
A heterogeneous catalyst was prepared in which 40g was marked with 0.

Comparative Example 2 In Comparative Example 1, instead of platinum, a heterogeneous catalyst supporting palladium at the same concentration was used! Made by Ill.

The following measurements were performed on the heterogeneous or homogeneous catalysts obtained in the above Examples and Comparative Examples.

Purification rate: Each catalyst is installed in the exhaust system of the engine, and from the time the engine starts, the catalyst inlet temperature is measured and HC, CO
Each purification rate of 1NOx (by gas chromatography:
The unit illusion was measured. The results obtained are:

It is shown in Table 1 below.

Table 1 Co 28 10 9
NOx 31 13 1
2300℃ HC59403I C0724233 NOx 71 50 3
g350℃ HC988583 CO96838O NOx 90 80
6950% purification temperature: After each catalyst was aged at 800°C for 20 hours in engine exhaust gas with a concentration of 0□, it was installed in a laboratory reactor and 0.7% CO10, 23% H2, 0,65 %0
2.1600ppm C3H, 1200pp+w N
While heating a gas consisting of Ox, 10% CO, 10% H, 0 and the remainder N2 at a heating rate of 5°C/min, the space velocity (F/V) 100, 0
00 into a reactor equipped with a catalyst, and the purification rates of HC, Co, and NOx at that time were measured at each temperature. Table 2 below shows the temperature at which each of these components is purified by 50% (
Unit: °C).

La 1 friend mail H.C. O Ox Table 2 Loss of money ↓ 294 352 290 320 286 322 le m 56 24 28

Claims (1)

[Scope of Claims] 1. An exhaust gas purifying catalyst comprising the following layers formed on a monolithic catalyst carrier base material having a columnar shape and having a large number of pores extending in the axial direction. (1) Alumina layer stabilized by rare earth element oxide (2) 0.1 mol/l in the upstream part in the exhaust gas flow direction
The total of cerium oxide with the following concentration and the concentration of 0.1 to 0.3 mol/l in the downstream part and the cerium oxide concentration is 0.5 mol/l or less in each part on the upstream and downstream sides. (3) palladium layer (4) alumina layer stabilized by rare earth element oxide (5) cerium oxide with a concentration of 0.1 to 0.3 mol/l and the oxide A layer consisting of zirconium oxide at a concentration such that the total concentration with the cerium concentration is 0.5 mol/l or less (6) Rhodium layer formed only in the downstream part