JPH06182213A - Catalyst for purification of exhaust gas - Google Patents

Abstract

PURPOSE:To provide a catalyst for purification of exhaust gas excellent in activity at a low temp. and durability and capable of efficient purification of exhaust gas from the internal-combustion engine of an automobile, etc., in a wide range of fuel-air ratio from a region close to the stoichiometric region of the exhaust gas to the loan region. CONSTITUTION:This catalyst for purification of exhaust gas has a multilayered structure and contains K and/or Ca. The multilayered structure is formed by coating a honeycomb carrier with an inorg. material based on activated alumina contg. one or more kinds of noble metals selected from the group consisting of Pt, Pd and Rh in one or more layers and further coating the upper layer with an inorg. material based on zeolite contg. one or more kinds of metals selected from among Cu, Co, Ni, Ag, Fe and Zn.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying catalyst for an internal combustion engine such as an automobile engine, and more particularly to a catalyst having a wide range of air-fuel efficiency ranging from near stoichiometric to lean, and having low temperature activity and durability. Relates to an exhaust gas purifying catalyst excellent in

[0002]

2. Description of the Related Art Conventionally, as a catalyst for purifying exhaust gas from an internal combustion engine of an automobile or the like, a catalyst obtained by supporting a noble metal component such as palladium (Pd), platinum (Pt) and rhodium (Rh) on activated alumina is generally used. Has been. This is called a three-way catalyst because it can remove hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) at once. However, this is effective only when the internal combustion engine is operated under conditions near stoichiometry, and when operating in a lean region where the oxygen content is high and fuel consumption is better, the NOx removal activity decreases.

On the other hand, it is known that catalysts composed of metal ion-exchanged zeolite and various metal oxides such as alumina, zirconia and titania are effective for removing NOx in the lean region. However, this does not provide exhaust gas purification performance in the vicinity of stoichiometry. Furthermore, even in the lean region, metal oxide catalysts are inferior in low-temperature activity, and zeolite catalysts have the disadvantages of insufficient durability (heat resistance, hydrothermal resistance, sulfur resistance, etc.). It is an obstacle.

According to Japanese Patent Laid-Open No. 1-127044,
The first layer is formed on the monolith carrier by an oxidation catalyst composed of alumina carrying a noble metal, and copper (Cu) is formed on the first layer.
By providing a second catalyst layer consisting of
N in the exhaust gas when operating the engine in the lean range
It is said that it is possible to provide a three-way catalyst in the lean region that efficiently purifies Ox, CO and HC. This is C
The low oxidation performance of the u-zeolite-based catalyst is complemented by the high oxidation performance of the alumina-supported noble metal catalyst, which is effective only in the lean region, and therefore, sufficient purification performance is not expected in the vicinity of stoichiometry. Further, in the catalyst proposed in the above-mentioned Japanese Patent Laid-Open Publication, no improvement has been made on the durability and low temperature activity of the zeolite-based catalyst.

[0005]

From such a background,
There has been a demand for a catalyst that can efficiently purify exhaust gas not only in the vicinity of the stoichiometry but also in the lean region, and that has excellent low-temperature activity and durability. Therefore, it is an object of the present invention to provide a catalyst which not only improves the durability of the zeolite-based catalyst but also improves the low temperature activity thereof, and effectively acts in the vicinity of stoichiometry and in the lean region.

[0006]

[Means for Solving the Problems] In order to achieve the above object, the inventors of the present invention have made extensive studies and devised a structure and a combination method of a three-way catalyst and a zeolite-based catalyst. As a result, platinum (Pt), palladium (Pd) ) And rhodium (Rh), and a three-way catalyst containing one or more precious metals selected from the group consisting of copper (Cu), cobalt (Co), nickel (Ni), silver (Ag), and iron (Fe). And a zeolite-based catalyst containing at least one metal selected from the group consisting of zinc (Zn), and arranging the former in the inner layer and the latter in the outer layer to form a three-way catalyst and a zeolite-based catalyst. Performance beyond what is expected can be obtained from a simple combination of catalysts, and potassium (K) and / or calcium (C
It was found that the effect is remarkably promoted by including a), and the present invention has been accomplished.

Therefore, the exhaust gas-purifying catalyst of the present invention has at least one layer of an inorganic material containing activated alumina containing at least one precious metal selected from the group consisting of platinum, palladium and rhodium on a honeycomb-shaped carrier. A multi-layer structure in which an inorganic substance containing a zeolite as a main component containing at least one metal selected from the group consisting of copper, cobalt, nickel, silver, iron and zinc is coated on the upper layer It is characterized by having and containing potassium (K) and / or calcium (Ca).

[0008]

[Operation] Next, the operation will be described. The catalyst of the present invention is a combination of a zeolite-based catalyst having high NOx removal performance in a lean region of an engine and a precious metal-based three-way catalyst having high exhaust gas purification performance in the vicinity of stoichiometry, the former being the upper layer portion,
The latter is applied to the inner layer portion.

In the catalyst of the present invention, the above-mentioned combination of the three-way catalyst and the zeolite-based catalyst provides a performance higher than that expected from a simple combination of the three-way catalyst and the zeolant-based catalyst. Regarding the fact that the effect is remarkably promoted by adding Ca, its detailed mechanism is not clear, but it is considered as follows. In the lean region, the noble metal-based three-way catalyst only deteriorates the NOx removal performance.
Purification performance of O is high, and reaction heat is generated by combustion of HC and CO. This heat raises the temperature of the zeolite-based catalyst, and the NOx removal reaction proceeds even at a lower exhaust gas temperature. At this time, at the same time, a partial oxidation product of HC, which is effective for NOx reduction and removal, is produced, and it is considered that a higher reaction rate is achieved.

If the combination of the noble metal-based three-way catalyst layer and the zeolite-based catalyst layer is reversed, the above high NOx removal performance cannot be obtained. This is HC and CO
Although the effect of reaction heat due to combustion can be obtained, it is considered that the partial oxidation product of HC, which is effective in the reduction and removal reaction of NOx, is not successfully produced, or even if it is produced, it is not effectively utilized.

The effect of adding K or Ca is to weaken the complete oxidation performance of the noble metal in the three-way catalyst and to assist the production of the partial oxidation product of HC which is effective in the reduction and removal reaction of NOx. It is considered that the produced partial oxidation product is efficiently utilized by multilayering the catalyst to achieve high NOx removal performance. Another effect of K and Ca is to stabilize the metal ion which is the active site in the zeolite, and to provide stable activity at high temperature and high durability of the catalyst. This makes it possible to achieve high exhaust gas purification performance in a wider temperature range. The content of K and Ca is preferably 0.05 to 2.0% by weight as K or Ca with respect to the catalyst portion excluding the honeycomb-shaped carrier. If it is less than 0.05% by weight, the effect of the present invention is difficult to obtain,
Even if it exceeds the weight%, the effect corresponding to the added amount cannot be obtained.

Since the catalyst of the present invention has a structure in which catalyst layers having different functions are laminated, it is mainly used in a honeycomb shape. As the honeycomb carrier material, a cordierite material is generally used in many cases, but the present invention is not limited to this. It is also possible to use a honeycomb carrier made of a metal material. Further, the catalyst itself may be formed into a honeycomb shape.

Examples of the zeolite used in the present invention include, but are not limited to, mordenite, Y-type zeolite, and pentasil-type zeolite such as ZSM-5. Both naturally occurring substances and artificially synthesized substances are effective, but the silica component (SiO ₂ ) and alumina component (Al ₂
O ₃ ) ratio (SiO ₂ / Al ₂ O ₃ molar ratio) is 10 to
It is preferable that the radius of the zeolite cavity (pore) is 100 and the radius is 0.5 nm or more. The ratio of SiO ₂ / Al ₂ O ₃ is 10
When the amount is less than zeolant, dealumination phenomenon is likely to occur and the thermal stability is not sufficient, resulting in low catalyst durability. On the other hand, when the ratio exceeds 100, the amount of metal supported on zeolant is small and the catalytic activity becomes unsatisfactory. Will be enough. Further, if the zeolite pores are smaller than 0.5 nm, the pores are likely to be clogged when coking occurs, leading to deactivation.
Zeolite is hydrothermally treated for both natural and synthetic products.
It is desirable to improve the crystallinity or stabilize it by resynthesis or the like because a catalyst having higher durability can be obtained.

The zeolitic catalyst in the catalyst of the present invention comprises the above zeolite carrying one or more metals selected from the group consisting of Cu, Co, Ni, Ag, Fe and Zn. As the raw material of the metal, various ones such as nitrate, acetate, chloride or ammine complex compound are used, and a method of forming an aqueous solution and supporting it by an ion exchange method, an impregnation method or a kneading method, vaporizing the metal raw material, and vaporizing Various methods can be used, such as a method of supporting by contacting with zeolite in a phase, and a method of supporting by simply physically mixing the oxide, carbonate or the like of the metal. In addition, as a method of adding K and Ca to the catalyst, nitrate,
A method in which various forms such as acetate, chloride or carbonate are made into an aqueous solution and added by an impregnation method or a kneading method can be adopted. In the case of a zeolite catalyst, the ion exchange method can also be used. Further, the effect can be obtained by physically mixing the salt. The types of metals supported on zeolite include Cu, Co, Ni, Ag, and Fe.
And Zn. Any of the zeolites supporting these has an ability to reduce and remove nitrogen oxides in the lean region in the presence of hydrocarbons, and thus can be used as a catalyst constituting the present invention. However, it is Cu and Co that exhibit relatively high activity among the metals, and other metals are preferably used in combination with Cu and Co.

[0015]

EXAMPLES The present invention will be described in detail below with reference to examples, comparative examples and test examples. Example 1 Activated alumina powder 1000 containing γ-alumina as a main component
Palladium is 1.38 wt% with respect to g,
An aqueous dinitrodi-amminepalladium solution was added, and the mixture was stirred well and dried in a dryer at 120 ° C. for 8 hours. This was fired in an air stream at 400 ° C. for 2 hours. 1400 g of this activated alumina supporting palladium was 936 g of ceria (cerium oxide: CeO ₂ ), 320 g of activated alumina containing γ-alumina as a main component, nitric acid boehmite sol (10% by weight of boehmite suspension in 10% by weight of HN).
2212 g of the sol obtained by adding O ₃ ) was put into a ball mill pot and pulverized for 8 hours to obtain a slurry. This slurry was applied to a cordierite honeycomb carrier (0.9 L) having about 400 channels per square inch cross section, dried in a dryer at 120 ° C for 2 hours, and then in an air stream at 400 ° C. It was baked for 2 hours. The powder coating amount at this time was 85 g.

Next, similarly, 1000 g of activated alumina powder
On the other hand, an aqueous rhodium nitrate solution was added so that the rhodium content was 1% by weight, and the mixture was thoroughly stirred, then dried and fired to produce an alumina powder carrying rhodium. This powder 44
4 g was put into a ball mill pot together with 319 g of activated alumina powder and 637 g of nitric acid acidic boehmite sol, and pulverized for 8 hours to obtain a slurry. This slurry was applied to a honeycomb coated with the alumina-supported Pd catalyst layer so that the coating amount was 30 g, and similarly dried and fired.

On the other hand, the SiO ₂ / Al ₂ O ₃ molar ratio is about 3
Cu (as Cu) by an ion exchange method using 0.1N copper nitrate aqueous solution to Na type ZSM-5 zeolite of 0.
After supporting about 3% by weight, it was dried in a dryer at 120 ° C. for 24 hours. Then, the powder of Cu-ZSM-5 catalyst was obtained by baking at 500 degreeC in the atmosphere for 2 hours with an electric furnace. 1800 g of this powder was mixed with silica sol (solid content 20
%) 1170 g and water 1170 g were put in a ball mill pot and pulverized for 8 hours to obtain a slurry. This slurry was applied to the above honeycomb carrier already coated with the three-way catalyst so that the coating amount was 130 g, and similarly dried and fired.

As described above, 85 g of the alumina-supported Pd catalyst layer was used as the first inner layer, 30 g of the alumina-supported Rh catalyst layer was used as the second inner layer, and 1 Cu-ZSM-5 catalyst layer was used as the outer layer.
A catalyst having a three-layer structure coated with 30 g was obtained. The catalyst layer including the first inner layer and the second inner layer is a three-way catalyst that can remove HC, CO, and NOx in exhaust gas at the same time when the engine is operated under stoichiometric conditions. The catalyst was dipped in a mixed aqueous solution of potassium nitrate and calcium nitrate to obtain K and Ca.
Was carried on the supported catalyst layer excluding the honeycomb by 0.1 wt% and 0.5 wt% respectively, dried in a dryer at 120 ° C. for 4 hours, and then calcined in an electric furnace at 500 ° C. for 2 hours. The catalyst (1) of Example 1 was obtained.

Example 2 The same Na-type ZSM-5 zeolite as in Example 1 was added with 0.1N.
Ion exchange method using calcium nitrate aqueous solution
After supporting 0.2% by weight of Ca (as Ca), it was dried in a dryer at 120 ° C. for 24 hours. Then, about 3% by weight of Cu (as Cu) was loaded by an ion exchange method using a 0.2N copper nitrate aqueous solution, and then dried in a dryer at 120 ° C. for 24 hours. After that, 2 hours at 500 ℃ in the atmosphere with an electric furnace
It was calcined for a time to obtain a Cu-ZSM-5 catalyst powder containing Ca. A catalyst having a three-layer structure was obtained in the same manner as in Example 1 except that this zeolite catalyst powder was used. This catalyst was immersed in an aqueous potassium nitrate solution to carry K, and then a catalyst (2) was obtained in the same manner as in Example 1. The K content in this catalyst is
It was 0.14% by weight with respect to the catalyst excluding the honeycomb.

Example 3 The same Na-type ZSM-5 zeolite as in Example 1 was mixed with 0.05
0.1% by weight of Ca (as Ca) was loaded by an ion exchange method using an aqueous solution of N-calcium acetate, and then dried in a dryer at 120 ° C. for 24 hours. Then, using a mixed aqueous solution of 0.2N copper nitrate and cobalt nitrate (Cu / Co atomic ratio = 7/3), Cu and Co were ion-exchanged.
About 3% by weight (as Cu and Co), and dried in a dryer at 120 ° C. for 24 hours. Then, C containing Ca by firing in an electric furnace at 500 ° C. for 2 hours in the atmosphere
A powder of u-Co-ZSM-5 catalyst was obtained. Using this zeolite catalyst powder, a catalyst having a three-layer structure was obtained in the same manner as in Example 1 except that it was dipped in a mixed aqueous solution of potassium nitrate and calcium nitrate to carry K and Ca, and then, as in Example 1. Similarly, a catalyst (3) was obtained. The K and Ca contents in this catalyst were 0.14% by weight and 0.11% by weight, respectively, with respect to the catalyst portion excluding the honeycomb.

Example 4 In Example 3, Na-type ZSM-5 zeolite was mixed with Si.
A catalyst (4) was obtained in the same manner except that the H-type mordenite having an O ₂ / Al ₂ O ₃ molar ratio of about 33 was used.

Example 5 The same as Example 1 except that a dinitrodiammineplatinum aqueous solution was used in place of the dinitrodiamminepalladium aqueous solution, and the catalyst was prepared so that platinum was 1% by weight with respect to 1000 g of activated alumina powder. And then the catalyst (5)
Got

Example 6 A catalyst (6) was obtained in the same manner as in Example 3, except that nickel nitrate was used instead of cobalt nitrate.

Example 7 A catalyst (7) was obtained in the same manner as in Example 3 except that silver nitrate was used instead of cobalt nitrate.

Example 8 A catalyst (8) was obtained in the same manner as in Example 3 except that iron nitrate was used instead of cobalt nitrate.

Example 9 A catalyst (9) was obtained in the same manner as in Example 3, except that zinc nitrate was used instead of cobalt nitrate.

Comparative Example 1 A Cu-ZSM-5 catalyst was obtained in the same manner as in Example 1 and made into a slurry, and this was coated on the cordierite honeycomb carrier in an amount of 2
It was applied so as to be 40 g. 120 ° C in a dryer, 2
After drying for an hour, it was calcined in an air stream at 400 ° C. for 2 hours to obtain a catalyst (10).

Comparative Example 2 1000 activated alumina powder containing γ-alumina as a main component
An aqueous dinitrodiammine platinum solution was added so that platinum was 1.4% by weight with respect to g, and the mixture was stirred well and dried in a dryer at 120 ° C. for 3 hours. 400 ° C in an air stream
The powder was calcined for 2 hours to obtain a powder of a platinum catalyst supporting activated alumina. 1400 g of this powder was added to 1256 g of activated alumina containing γ-alumina as a main component, and nitric acid-acidified boehmite sol 22.
20 g was put into a ball mill pot and pulverized for 8 hours to obtain a slurry. This slurry was applied to the cordierite honeycomb carrier, dried in a dryer at 120 ° C. for 2 hours, and then fired in an air stream at 400 ° C. for 2 hours. The amount of powder applied at this time was 120 g. To this upper layer, 130 g of the Cu-ZSM-5 catalyst obtained in Example 1 was similarly applied, and the oxidation catalyst was arranged in the inner layer portion and the zeolite catalyst was arranged in the outer layer portion.
A layered catalyst (11) was obtained.

Comparative Example 3 A catalyst (12) was obtained in the same manner as in the preparation of the catalyst (1) of Example 1, except that K and Ca were not added.

Comparative Example 4 On the assumption that the catalyst (1) of Example 1 was prepared, the order of applying each catalyst to the cordierite honeycomb carrier was changed to Cu-
A three-layer structure catalyst having a ZSM-5 catalyst as an inner layer, an alumina-supported Pd catalyst as an upper layer, and an alumina-supported Rh catalyst as an upper layer was obtained. In addition, as in the first embodiment, K
And Ca were added to obtain a catalyst (13).

Test Example By the bench evaluation using the following engine exhaust gas, the NOx removal performance before and after the rapid durability treatment of the catalysts of Examples 1 to 9 and Comparative Examples 1 to 4 (catalyst inlet temperature 250 to 3)
Table 2 shows the maximum NOx removal rates at 00 ° C and 400 to 500 ° C. Table 3 shows stoichiometry (air mileage 14.
6) NOx removal performance (catalyst inlet temperature 300 to 4) of the catalysts of Example 3 and Comparative Examples 1 and 4 (before the rapid endurance treatment) near 6)
The maximum NOx removal rate at 00 ° C) is shown.

[0032]

[Table 1] Performance evaluation conditions Catalyst capacity: 0.9 L Space velocity: Approximately 25000-35000 h ^-1 Catalyst layer inlet temperature: 200-500 ° C Engine: Sunny (Nissan Motor Co., Ltd.) displacement 160
0cc (SCV) engine Fuel: Unleaded regular gasoline Average air fuel consumption (A / F): Approx. 20 THC (total hydrocarbons) / NO ratio = 7.8 Rapid endurance treatment conditions Catalyst inlet exhaust gas temperature: 570 ° C Average fuel consumption : About 18 Processing time: 80h

[0033]

[Table 2]

[0034]

[Table 3]

According to Tables 2 and 3, the catalyst of the present invention is
It works effectively in the vicinity of the stoichiki and in the lean region, and it is highly efficient and N
Remove Ox. Further, the high NOx removal activity is maintained even after the rapid durability treatment, which shows that it is extremely effective.

[0036]

EFFECT OF THE INVENTION The noble metal-based three-way catalyst containing one or more noble metals selected from the group consisting of Pt, Pd and Rh is applied to the inner layer portion and the zeolite catalyst containing the above metal is applied to the upper layer. In addition, the catalyst of the present invention containing K and / or Ca can efficiently purify the exhaust gas from the internal combustion engine with a wide air-fuel ratio from the vicinity of stoichiometry to the lean region, and is excellent in low-temperature activity and durability. It is possible to provide a vehicle with extremely low pollution and good fuel efficiency.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Naoki Kachi 2 Takaracho, Kanagawa-ku, Yokohama-shi, Kanagawa Nissan Motor Co., Ltd.

Claims (1)

[Claims] 1. A honeycomb carrier is laminated with one or more layers of an inorganic material containing activated alumina containing at least one precious metal selected from the group consisting of platinum, palladium and rhodium as a main component, and then coated on the upper layer thereof. Having a multilayer structure formed by applying an inorganic substance containing a zeolite as a main component containing one or more metals selected from the group consisting of copper, cobalt, nickel, silver, iron and zinc, and potassium (K)
And / or calcium (Ca) is contained, The exhaust gas purification catalyst characterized by the above-mentioned.