JP3473245B2 - Exhaust gas purification catalyst - Google Patents

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, and more particularly to an exhaust gas purifying catalyst having excellent purification efficiency of exhaust gas from an internal combustion engine such as an automobile engine and various combustors.

[0002]

2. Description of the Related Art Conventionally, three-way catalysts have been widely used as catalysts for purifying exhaust gas from automobile engines and the like. Conventional three-way catalysts mainly consist of catalysts containing alumina, which contains various components such as platinum, palladium, rhodium and other noble metals and cerium, and when the engine is operated near the stoichiometric air-fuel ratio. It shows a high purification efficiency for the exhaust gas.

On the other hand, in recent years, from the viewpoint of improving fuel efficiency and reducing carbon dioxide emissions, lean burn engines that are operated even at an air-fuel ratio higher than the stoichiometric air-fuel ratio have been receiving attention. The exhaust gas (lean exhaust gas) of such a lean burn engine has a higher oxygen content than the exhaust gas (stoichiometric exhaust gas) of a conventional engine that operates only in the vicinity of the stoichiometric air-fuel ratio. Nitrogen oxide (N
Ox) is insufficiently purified. Therefore, a new catalyst applicable to a lean burn engine that operates at a wide range of air-fuel ratios has been desired.

A metal-supported zeolite catalyst (hereinafter referred to as "zeolite-based catalyst") obtained by supporting various metal components on crystalline aluminosilicate (hereinafter referred to as "zeolite") has a high oxygen content. Even in the exhaust gas (lean exhaust gas), if hydrocarbons (HC) are present, it is noted that NOx can be purified relatively efficiently.

As the metal component, copper (Cu), cobalt (Co), silver (Ag), nickel (Ni), iron (Fe)
In addition to transition metal components such as platinum, platinum (P
t) is also known to be effective, but especially copper (Cu)
The Cu-zeolite catalyst supporting NO has a high NOx activity and has a relatively excellent NOx purification performance even under high-velocity exhaust gas conditions. Therefore, a mobile generation source such as an automobile or stationary private power generation is used. It is expected to be applied to the purification of exhaust gas from industrial engines.

However, the zeolite catalyst alone cannot sufficiently purify the exhaust gas of an engine operated in a wide air-fuel ratio from stoichiometric air-fuel to stoichiometric oxygen excess (lean). The combination was essential. This is because the zeolite-based catalyst has a poor purification performance for hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx) in the stoichiometric exhaust gas, while in the lean exhaust gas with a high oxygen content. This is because the NOx purification performance is excellent, but the ability to purify HC and CO is inferior.

Therefore, a catalyst system in which a zeolite-based catalyst and a three-way catalyst are combined in series in the exhaust gas flow direction is conceivable. For example, in JP-A-1-139145, a transition metal is provided upstream of the exhaust gas. It has been proposed to arrange a supported zeolite catalyst on the downstream side of an oxidation catalyst or a three-way catalyst.

Such a combination in which the zeolite-based catalyst is arranged on the upstream side of the exhaust gas and the three-way catalyst is arranged on the downstream side is exhaust gas purification of a lean burn engine operating at a wide air-fuel ratio from stoichiometric to lean. It is considered to be an effective catalyst system for.

[0009]

However, the conventional exhaust gas purifying catalyst system described above has a problem that the function of the three-way catalyst installed on the downstream side of the exhaust gas cannot be fully exerted in practice. Although the cause is not clear, the zeolite-based catalyst, when the state of the exhaust gas switches from lean to stoichi, stoichi to lean,
Temporarily and selectively captures a part of the gas component in the exhaust gas, which disturbs the balance between the oxidizing gas component (NOx) and the reducing gas component in the exhaust gas at the inlet of the three-way catalyst, It is generally thought to reduce function.

Therefore, in order not to deteriorate the function of the three-way catalyst, it is necessary to weaken the gas component capturing power of the zeolite-based catalyst. Therefore, an object of the present invention is to provide an exhaust gas purifying catalyst capable of efficiently purifying exhaust gas from an automobile engine operating at a wide air-fuel ratio from stoichiometric to lean without deteriorating the function of the three-way catalyst. To provide.

[0011]

Means for Solving the Problems As a result of research to solve the above problems, the inventors of the present invention identified the pore distribution of a zeolite-based catalyst to determine the ability of the zeolite-based catalyst to capture a specific gas component. The present invention has been achieved by finding that

The exhaust gas purifying catalyst of the present invention has a thickness of 4 nm.
Porous crystalline aluminosilicate having the following pore diameter
Comprising a porous than 40% is porous crystalline aluminosilicate of the total volume sum of the pores between the crystalline aluminosilicate particles total volume sum of the pores having a pore diameter of 1~100nm between particles It is characterized by

That is, it is considered that the pores having a diameter of 4 to 100 nm play a temporary and selective trapping force for the specific gas component, and the function of the three-way catalyst is lowered by controlling the proportion of the pores. Can be suppressed. Particularly preferably, the volume sum of pores having a diameter of 4 nm or less is
Porous crystalline alumino with a diameter of 1 nm or more and 100 nm or less
It is 50% or more of the total volume of pores between silicate particles .
This is preferable because it can effectively prevent the functional deterioration of the three-way catalyst. However, even if the ratio of the pore volume sum is increased unnecessarily, the performance is not further improved. Therefore, the more preferable range of the pore volume sum ratio is 50 to 85.
%.

The combination of the zeolite-based catalyst and the three-way catalyst of the present invention is such that the zeolite-based catalyst is on the exhaust gas upstream side and the three-way catalyst is on the downstream side. If this arrangement is reversed, the reducing gas component necessary for the NOx purification reaction of the lean exhaust gas is consumed before reaching the zeolite catalyst, and the NOx purification action is hindered.

Zeolite S in zeolite-based catalysts
The molar ratio of iO ₂ / Al ₂ O ₃ is preferably in the range of 20-60. If the molar ratio of SiO ₂ / Al ₂ O ₃ is less than 20, the thermal stability of the zeolite will be insufficient and the heat resistance and durability of the catalyst will decrease. On the contrary, when the molar ratio exceeds 60, the amount of the active metal component carried becomes small and the catalytic activity decreases.

The zeolite used in the present invention can be appropriately selected from known zeolites and used, and those of the group called pentasidi type zeolite are particularly effective. Examples of such zeolites include mordenite, ferrierite, ZSM.
5, ZSM11 and the like can be mentioned, and ZSM5 is particularly preferable in terms of heat resistance and durability.

Zeolite is preferable because it can be stabilized by increasing crystallinity by hydrothermal treatment, resynthesis, etc., and a catalyst having high heat resistance and durability can be obtained.

Various transition metal species are effective as the metal component supported on the zeolite used in the present invention.
In particular, Cu, Co, Ag, Ni and Pt are preferable. As the raw material of this metal component, inorganic acid salts, oxides, organic acid salts, chlorides, carbonates, sodium salts, ammonium salts,
Various compounds such as an ammine complex compound can be used, and the metal component can be supported on zeolite by a commonly used method such as an ion exchange method and an impregnation method. Furthermore, a method of evaporating a metal raw material at a high temperature to carry it in a vapor phase,
Supporting by a physical mixing method is also effective.

In the case of the usual ion exchange method or impregnation method, the metal raw material is often used in the form of a solution, and an acid or a base is added to the solution to adjust the pH appropriately to obtain desirable results. In some cases, the present invention is not limited by the loading method.

As a method for controlling the pore size distribution of the zeolite-based catalyst, (1) a method of pulverizing zeolite particles under wet or dry conditions to control the particle size, (2) at least 6
A method of sintering particles by firing at a high temperature of 00 ° C. or higher,
(3) A method of mixing fine powders of various oxides, and (4) a method of controlling the particle size by coating the surface of the catalyst with various oxides. The various oxides used in the above methods (3) and (4) are preferably those that are as inert as possible and do not adversely affect the catalyst performance, such as alumina (Al ₂ O ₃ ), silica (SiO ₂ ), titania ( Fine powders of TiO ₂ ), silicon carbide (SiC), silicon nitride (Si ₃ N ₄ ) and the like are preferably used. The above method is effective alone, but if the methods (1) to (4) are appropriately combined, a greater effect can be obtained.

The catalyst disposed on the downstream side of the exhaust gas flow has a so-called three-way catalyst function, and is obtained by supporting a noble metal component on a refractory inorganic material. The noble metal contained in the catalyst contains at least one element selected from the group consisting of Pt, Pd, and Rh. Further, as the refractory inorganic material, alumina, silica, silica-alumina,
A general catalyst carrier such as magnesia, zirconia or titania can be used, but is not limited thereto. Alumina is a preferred catalyst carrier because it has a large specific surface area and is excellent in heat resistance, so that it can support precious metals in a highly dispersed manner and can effectively utilize less precious metals.

The catalyst of the present invention is preferably used in the form of a honeycomb. In this case, usually, the zeolite-based catalyst of the present invention is applied to a honeycomb-shaped carrier for use. Generally, a cordierite material is widely used as the honeycomb material, but the material is not limited to this, and a honeycomb carrier made of a metal material is also effective, and the catalyst powder itself is formed into a honeycomb shape. You can also Furthermore, by making the shape of the catalyst honeycomb, the contact area between the catalyst and the exhaust gas is increased and pressure loss is also suppressed, so there is vibration and a large amount of exhaust gas is processed in a limited space. This is extremely advantageous when used as an automobile catalyst that is required to

[0023]

The present invention will be described in more detail below with reference to the following examples and comparative examples, but the present invention is not limited thereto. Example 1 (1) Aqueous ammonia was added dropwise to a copper nitrate aqueous solution having a zeolite catalyst concentration of 0.1 mol to adjust the pH to 8.2. SiO ₂ / Al in this solution
_The solid-liquid was separated by adding NH ₄ type ZSM5 powder having a molar ratio of ₂ O ₃ of about 37, stirring well, and then filtering. By repeating the stirring and filtering operation three times,
A ZSM5 zeolite catalyst cake carrying Cu ion exchange was obtained. This cake is dried at 120 ° C. for 2 hours using a dryer.
It was dried for 4 hours or more and then calcined in an air atmosphere at 630 ° C. for 8 hours to obtain Cu-ZSM5 catalyst powder carrying 4.6% by weight of Cu.

The Cu-ZSM5 catalyst powder thus obtained, aluminum sol, silica sol and water were placed in a magnetic ball mill pot and mixed and pulverized for 6 hours to form a slurry. The addition amounts of alumina sol and silica sol were Al ₂ O ₃ and SiO ₂ , respectively, and Cu excluding adsorbed water was used.
8% by weight and 4% by weight with respect to the ZSM5 catalyst powder.

The obtained slurry was applied to 0.8 L of a cordierite honeycomb carrier having about 400 flow paths per 1 square inch cross section and dried with hot air at 150 ° C.
It was baked at 450 ° C. for 1 hour. Cu-Z on honeycomb carrier
The coating amount of the SM5 catalyst powder was about 150 g / L. The honeycomb catalyst is dipped in a colloidal silica solution and then dried at 100 ° C. using a dryer, and then 500 ° C. in an electric furnace.
For 30 minutes, and the honeycomb-shaped Cu-ZSM5 of the present invention
A catalyst was obtained. The amount of impregnated and supported colloidal silica is SiO.
₂ was 6 g / L.

(2) Three-way catalyst Dinitrodiamminepalladium aqueous solution was added to activated alumina containing γ-alumina as a main component, Pd was supported by the impregnation method, and then dried at 120 ° C. for 8 hours using a dryer, and then air was used. It was calcined in an air stream at 450 ° C. for 2 hours to obtain Pd-activated alumina carrying 1.4% by weight of Pd. Cerium oxide (CeO ₂ ), activated alumina and nitric acid acidic boehmite sol were added to and mixed with this Pd-activated alumina, and pulverized in a magnetic ball mill pot for 8 hours to obtain a slurry. 0.9 L of the honeycomb carrier was coated with this slurry in the same manner as the above zeolite-based catalyst, dried at 120 ° C, and 400 ° C.
It was baked in. The coating amount of the catalyst was about 100 g / L.

Then, an aqueous rhodium nitrate solution was added to the activated alumina, and Ph was carried by the impregnation method, and then dried with a drier.
It was dried at 20 ° C. for 8 hours and calcined in an air stream at 450 ° C. for 2 hours to obtain Ph-activated alumina supporting 0.9% by weight of Rh. A nitric acid-acidic boehmite sol was added to and mixed with this Ph-activated alumina, and pulverized in a magnetic ball mill pot for 8 hours to obtain a slurry. The slurry was coated on the Pd-activated alumina-coated honeycomb, dried at 120 ° C., and calcined at 400 ° C. to obtain a three-way catalyst. The coating amount of the Ph-activated alumina is
It was 42 g / L.

The above Cu-ZSM5 catalyst and the three-way catalyst are arranged in series and installed in a catalytic converter, and [0.8L
+0.9 L] = 1.7 L of tandem catalyst (1) was obtained.

Example 2 In Example 1, the Cu-ZSM5 catalyst was replaced by Pt-ZSM.
Tandem catalyst (2) in the same manner except that the catalyst was changed to 5
Got However, the method for producing the Pt-ZSM5 catalyst powder is as follows. Using a dinitrodiammine platinum aqueous solution, N ₂ having a SiO ₂ / Al ₂ O ₃ molar ratio of about 42 was obtained by a spray impregnation method.
H ₄ type ZSM5 powder was loaded with platinum. This was dried using a dryer at 120 ° C. for 24 hours or more, and then baked in an electric furnace at 700 ° C. for 6 hours in an air atmosphere. A catalyst powder was obtained. This Pt-ZSM5 catalyst powder was coated on the honeycomb carrier in the same manner as in Example 1. At this time, the coating amount on the cordierite carrier was about 120 g / L, and the impregnated and supported amount of colloidal silica was 6 g / L as SiO ₂ as in Example 1.

Example 3 In Example 1, the Cu-ZSM5 catalyst powder was calcined at 73
A tandem catalyst (3) was obtained in the same manner except that the temperature was set to 0 ° C. for 6 hours and impregnation with colloidal silica was not performed.

Example 4 In Example 1, the Cu-ZSM5 catalyst powder was calcined at 68
6 hours at 0 ° C, and impregnated and supported amount of colloidal silica is 4
A tandem catalyst (4) was obtained in the same manner except that g / L was used.

Example 5 In Example 1, Cu-ZSM5 catalyst powder was calcined at 70
5 hours at 0 ° C, and impregnated and supported amount of colloidal silica is 6
A tandem catalyst (5) was obtained in the same manner except that g / L was used.

Comparative Example 1 In Example 1, the Cu-ZSM5 catalyst powder was calcined at 50 times.
The temperature was set to 0 ° C. for 4 hours, and at the time of slurry production, only alumina sol was added without adding silica sol. The amount added as Al ₂ O _3, Cu-ZSM5, excluding the adsorbed water
It was 12% by weight based on the catalyst powder. Further, a honeycomb-shaped Cu-ZSM5 catalyst was obtained in the same manner as in Example 1 except that impregnation and loading of the silica component using the colloidal silica sol solution was not carried out. Further, in the same manner as in Example 1, a 1.7 L [= 0.8 L + 0.9 L] tandem catalyst (6) in which a Cu-ZSM5 catalyst and a three-way catalyst were incorporated in a catalytic converter was obtained.

Test Example The pore size distribution and the catalytic performance of the exhaust gas purifying catalysts obtained in Examples 1 to 5 and Comparative Example 1 were examined by the following method.

The fineness of a zeolite-based catalyst supporting a metal component
Pore size distribution measuring device; Shimadzu (Micromeritex) Asap 2400 measuring method; Constant volume method pore distribution data analysis by N ₂ gas adsorption; BJH method sample pretreatment method; Degassing at 250 ° C for about 15 hours processing ^(10-3 tons or less) analysis data; from the adsorption side of the data, plots the cumulative adsorption pore volume against pore diameter, from which the volume sum V ₄ of the following pore diameter 4 nm, a diameter 1nm or more 100nm The total volume V ₁₀₀ of the following pores is calculated, and [V ₄ / V ₁₀₀ ] × 1
00 (%) was calculated. The results are shown in Table 1.

Example of Catalyst Performance Test The tandem type catalysts of the above-mentioned Examples and Comparative Examples were mounted on the exhaust system of a vehicle equipped with an inline 6-cylinder 2L engine, respectively.
After driving for 40 seconds at F = 14.6 (stoichi), A / F
The A / F switching operation mode of 20 seconds at = 21 (lean) was repeated 20 times. The purification rates of HC, CO, and NOx in the exhaust gas during this switching movement were calculated by the following formulas, and the results are shown in Table 1. The catalyst inlet average temperature during the mode switching operation was 380 ° C.

[Equation 1]

[0037]

[Table 1]

[0038]

The catalyst for purifying automobile exhaust gas of the present invention comprises:
By specifying the pore distribution of the zeolite-based catalyst, it is possible to suppress the ability of the zeolite-based catalyst to supplement a specific gas component, so that it can be operated over a wide range of air-fuel ratios from stoichiometric to lean without degrading the function of the three-way catalyst. Since the exhaust gas of the engine to be used can be purified with high efficiency, it is possible to provide a vehicle with less environmental pollution and excellent economy (fuel consumption).

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification symbol FI F01N 3/10 F01N 3/28 301J 3/28 301 B01D 53/36 102H 102B 102C 104A (56) Reference JP-A-1-135542 (JP, A) JP-A-6-171915 (JP, A) JP-A-4-27706 (JP, A) JP-A-2-139040 (JP, A) JP-A-1-139145 (JP, A) (JP-A) 58) Fields investigated (Int.Cl. ⁷ , DB name) B01J 21/00-38/74 B01D 53 / 86,53 / 94 F01N 3 / 10,3 / 28

Claims (3)

(57) [Claims] 1. A porous binder having a pore diameter of 4 nm or less.
The total volume of pores between crystalline aluminosilicate particles is 1 to 1
Porous crystalline aluminosilicate with a pore diameter of 00 nm
An exhaust gas purifying catalyst comprising a porous crystalline aluminosilicate which accounts for 40% or more of the total volume of pores between the acid salt particles . 2. A porous binder having a pore diameter of 4 nm or less.
The total volume of pores between crystalline aluminosilicate particles is 1 to 1
Porous crystalline aluminosilicate with a pore diameter of 00 nm
A catalyst containing 40% or more of the total volume of the pores between the acid salt particles is placed upstream of the exhaust gas flow, and a catalyst containing the porous crystalline aluminosilicate is placed downstream of the exhaust gas flow. An exhaust gas purifying catalyst, which is arranged on the side. 3. The exhaust gas purifying catalyst according to claim 1, wherein the particles have a pore diameter of 4 nm or less.
The total volume sum of the pores between the exhaust gas purifying catalyst, which comprises 50% or more than 85% of the total volume sum of the pores between the particles having a pore diameter of 1 to 100 nm.