JPH07100386A - Catalyst for purification of exhaust gas - Google Patents

Abstract

PURPOSE:To obtain a catalyst hardly poisoned by S, having high durability and capable of efficient removal of NOx, CO and hydrocarbon in exhaust gas from an automobile. CONSTITUTION:A noble metal is supported on a porous carrier made of crystalline silicoaluminophosphate and an active metal selected from among alkali metals, alkaline earth metals, transition metals and lanthanoids is further supported by ion exchange to obtain the objective catalyst for removal of NOx in exhaust gas in an atmosphere contg. excess oxygen.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a catalyst for purifying exhaust gas, and more specifically, exhaust gas in excess of oxygen, that is, carbon monoxide, hydrogen and hydrocarbons contained in exhaust gas. The catalyst for efficiently purifying nitrogen oxides (NO _x ) in exhaust gas containing oxygen in excess of that required to completely oxidize the reducing substance.

[0002]

2. Description of the Related Art Conventionally, carbon monoxide (CO) and hydrocarbon (H) in exhaust gas have been used as catalysts for purifying exhaust gas of automobiles.
There are many known three-way catalysts for purifying exhaust gas that simultaneously purify C) and reduce nitrogen oxides (NO _x ) to purify exhaust gas. A typical example of such a catalyst is one in which a γ-alumina slurry is applied to a refractory carrier such as cordierite and baked to carry a noble metal such as palladium, platinum or rhodium (for example, Japanese Patent Publication No.
56-27295, etc.).

By the way, the performance of the exhaust gas purifying catalyst is greatly influenced by the set air-fuel ratio of the engine. On the lean side where the lean mixture ratio, that is, the air-fuel ratio is large, the amount of oxygen in the exhaust gas after combustion becomes large, and oxidation occurs. Active action,
The reducing action becomes inactive. On the other hand, on the rich side where the air-fuel ratio is small, the amount of oxygen in the exhaust gas after combustion becomes small, the oxidizing action becomes inactive, and the reducing action becomes active.

In recent years, in an internal combustion engine of an automobile or the like, there has been a demand for low fuel consumption from the viewpoint of energy saving. As one of the means for reducing the fuel consumption, it has been conventionally performed to burn an air-fuel mixture with excess oxygen during traveling. ing. Therefore, there has been a demand for a catalyst that can sufficiently purify NO _x even in an oxygen-excess atmosphere in which the air-fuel ratio is lean.

As catalysts for purifying automobile exhaust gas under such an oxygen-rich atmosphere, various catalysts for simultaneously oxidizing carbon monoxide and hydrocarbons and reducing nitrogen oxides have been proposed. As such a catalyst, for example, a Pt / Al ₂ O ₃ catalyst in which platinum is supported on an alumina carrier has been proposed. However, this catalyst cannot be said to be a catalyst showing a practically sufficient purification rate in an oxygen excess atmosphere. For example, even if the amount of Pt supported was increased, the NO _x purification rate was only about 30 to 40% (catalyst inlet side temperature 275 ° C., A / F = 22) during constant steady running at 40 km / hr.

[0006] In addition, during steady running and during transient states (in urban areas
NO in driving simulation state)_xTo increase the purification rate of
Platinum and La on alumina carrier₂O₃The catalyst supporting
Proposed (see Japanese Patent Application No. 3-344781). However
However, this catalyst is La₂O ₃Even if the carrying amount of the
NO at the time of delivery_xThe maximum purification rate is around 60%
If you endure it at a high temperature, the purification rate will drop to about half.
Was not enough.

In order to solve such a problem, a porous support is used.
Carrying alkali metal or alkaline earth metal and platinum on the body
An absorption-reduction type catalyst was proposed (Japanese Patent Application No. 4-1309).
04 specification and Japanese Patent Application No. 4-184892 specification). this
In such an absorption-reduction type catalyst, when lean, Pt
Converted NO_xAnd SO_xAre both absorbed by the absorbent.
However, when rich, NO_xIs released, but SO
_xIs not released from the absorbent due to its high decomposition temperature, therefore
Absorbent NO_xAbsorption capacity will decrease (S poisoning).
In addition, when the Ba etc. of the absorbent is exposed to 800 ° C or higher, Ba
Is stable as it reacts with alumina₂O_FourTo form
The specific surface area of the carrier is reduced, and further NO _xAbsorbs B
NO because a decreases_xPurification performance will decrease.

[0008]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned drawbacks of a catalyst for purifying exhaust gas of automobiles, is less likely to be poisoned by S, and has high durability, NO _x and C in exhaust gas.
It is an object of the present invention to provide a catalyst capable of efficiently purifying O and HC.

[0009]

The present inventor has conducted extensive studies to solve the above problems of the above-mentioned catalyst for purifying automobile exhaust gas, and as a result, uses crystalline silicoaluminophosphate (SAPO) as a carrier. By further carrying an active metal selected from the group consisting of alkali metals, alkaline earth metals, transition metals and lanthanoids as an absorbent on this carrier by ion exchange, a catalyst with high durability and low S poisoning can be obtained. Therefore, the present invention has been completed.

That is, the automobile exhaust gas purifying catalyst of the present invention is selected from the group consisting of a crystalline silicoaluminophosphate porous carrier and a noble metal, and an alkali metal, an alkaline earth metal, a transition metal and a lanthanoid by ion exchange. It is characterized in that an active metal is supported.

The lean catalyst of the present invention absorbs NO _x at the time of lean by controlling exhaust gas alternately with lean and stoichiometric, and reduces the absorbent at the beginning of stoichiometry to release NO _x , and subsequently NO _x, HC, thereby purifying the CO or the like by stoichiometry. Was then stoichiometric end sufficiently release the NO _x, performs absorbed in lean.

The crystalline silicoaluminophosphate used as a carrier of the catalyst of the present invention has uniform pores, has a diameter larger than about 3Å, is anhydrous, and has a chemical composition represented by the following formula (Si during _{x Al y P z) O 2} ( the above equation is used those represented by x + y + z = 1) .

The crystalline silicoaluminophosphate preferably has a pore size of about 5 to 10Å, which is slightly larger than the NO _x molecular size. The proportion of Si in the silicoaluminophosphate is 0.01 ≦ x <0.8, preferably 0.05-
It is 0.25. When this ratio is 0.05 or less, the solid acidity of the carrier is low, and the ability to support the active metal is reduced. On the other hand, if this ratio is 0.25 or more, the heat resistance decreases, which is not desirable.

The crystalline silicoaluminophosphate may be synthesized by a conventional method, for example, the method described in Japanese Patent Publication No. 3-72010. That is, phosphate, hydrated alumina, silica sol, etc. are used as starting materials, these are uniformly mixed, and an organic templating agent is mixed into the mixture to define the pore structure, and the mixture is stirred to be uniform. Then, a crystalline silicoaluminophosphate powder is obtained by hydrothermal synthesis.

Next, the above-mentioned crystalline silicoaluminophosphate powder is loaded with an active metal and a noble metal. Examples of the active metal include alkali metals, alkaline earth metals, transition metals and lanthanoids, preferably Ba,
Cu, Co, and Ni are mentioned. Platinum and rhodium are preferable as the noble metal.

The active metal is supported on the carrier by the so-called ion exchange method. That is, the crystalline silicoaluminophosphate powder is immersed in an aqueous solution of a salt of an active metal for a whole day and night and then washed with water repeatedly for 1 to several times, and then kept at a temperature of 500 to 700 ° C for several hours for firing. . The amount of the active metal supported is not particularly limited, but is preferably 0.5 to 3.0% by weight with respect to the crystalline silicoaluminophosphate powder. If it is less than 0.5% by weight, it may not be possible to obtain a sufficient NO _x purification rate.
If it exceeds 3.0% by weight, the surface area of the carrier may be reduced.

The method for supporting the noble metal is not particularly limited, and for example, the above-mentioned ion exchange method or so-called impregnation method may be used. There is no particular limitation on the amount of this noble metal supported, but the amount of crystalline noble metal supported on crystalline silicoaluminophosphate powder
It is preferably 1.0 to 8.0% by weight. If it is less than 1.0% by weight, sufficient catalytic activity may not be obtained, and if it exceeds 8.0% by weight, even if the supported amount of the noble metal is further increased, the activity is slightly improved and it becomes expensive. Because it is only.

The powdery catalyst obtained as described above may be used as it is, or a binder such as alumina sol or silica sol may be added to the catalyst powder to form a desired shape, or water may be added to form a slurry. As a shape, it may be applied on a refractory substrate in the shape of a honeycomb or the like and used.

[0019]

The reason why the absorbent of the catalyst according to the present invention is less likely to be poisoned with S is not clear, but it is considered as follows. In the catalyst of the present invention, the absorbent is SAPO.
Ions are exchanged near the solid acid points in the pores of
The ions of the absorbent are carried in individual units (size). In this supported state, the negative charge of the solid acid point is strongly connected to the positive ion charge of the absorbent. Therefore, the SO _x gas in the exhaust gas flows into the catalyst and is oxidized by Pt at the time of leaning to become SO ₄ ²⁻ , and even if it comes near the positive ions of the absorbent, only weak adsorption occurs. Therefore, when rich, SO ₄ ²⁻ is reduced and released from the absorbent, and as a result, S poisoning becomes difficult. On the other hand, in the conventional case, since the catalyst carrier is not a micropore style like SAPO, the absorbent is not ion-exchanged and the bond with the catalyst carrier is not strong. For this reason, the SO when lean
₄ ²⁻ is strongly bound to the element of the absorbent and cannot be sufficiently reduced when rich, and S poison cannot be recovered.

[0020]

The present invention will be described in more detail with reference to the following examples, but the present invention is not limited thereto.

Silico aluminophosphate porous carrier
61.2 g of the synthetic aluminum isopropoxide of was dissolved in 120 g of water, 31.9 g of 85% by weight phosphoric acid was added and the mixture was stirred at room temperature until homogeneous. To this mixture was added 20.0 g of 20 wt% silica sol, then 117 ml of 40 wt% tetraethylammonium hydroxide, and the mixture was stirred at room temperature for 4 hours until uniform. This mixture was placed in an autoclave, subjected to hydrothermal synthesis at 250 ° C. for 72 hours, cooled, recovered by centrifugation, and thoroughly washed with water. The hydrothermal compound was dried at 120 ℃ and then calcined in air at 600 ℃ for 3 hours to give a specific surface area of 550c.
SAPO (No. 1) having m ² / g and a pore size of 4Å was obtained.

In the same synthesis method as SAPO (No. 1),
Di-n-propylammonium was used instead of 40 wt% tetraethylammonium hydroxide, and the specific surface area was 350 cm ^2.
SAPO (No. 2) having a pore size of 6 g / g and a pore size of 6Å was obtained.

Further, in the same synthetic method as SAPO (No. 1), a specific surface area of 300 cm ² / g was obtained by using triethylamine instead of 40% by weight of tetraethylammonium hydroxide.
SAPO (No. 3) having a pore size of 8Å was obtained.

Catalyst Preparation Each of the above SAPO powders (Nos. 1, 2 and 3) was added to a 0.2N barium acetate aqueous solution whose pH was adjusted by adding aqueous ammonia, and ion exchange was performed at room temperature for 24 hours. , Ba were carried. Then, washing and drying were performed, and then this SAPO powder was added to a dinitroamine Pt nitric acid aqueous solution (Pt concentration 4.5 wt%) and stirred at room temperature for 1 hour to impregnate and carry Pt. This slurry was separated into solids and supernatant by centrifugation, and the solids were collected at 120 ° C.
Was dried for 12 hours and then calcined at 250 ° C. for 1 hour. By elemental analysis of the obtained sample, SAP
2.0 wt% Pt and 5.0 wt% Ba based on O powder
Was carried.

The active metal-supported SAP thus obtained
A slurry was prepared by adding alumina sol, aluminum sulfate and water to O porous carrier powder. Next, a cordierite honeycomb carrier was immersed in the slurry, and the slurry was wash-coated by a method of blowing off the excess slurry. It is dried at 120 ° C for 3 hours and then 500
It was calcined for 1 hour at 0 ° C. to obtain an absorption decomposition type lean NO _x catalyst.

Evaluation of catalyst S poisoning amount The powder before coating on the monolith was subjected to a durability test (lean, 600 ° C. × 10 hours) with the following model gas, and the S poisoning amount was measured by FT-IR ( Fourier transform infrared spectroscopy) and the results are shown in Table 1. Model gas composition Lean: NO C ₃ H ₆ CO CO ₂ O ₂ H ₂ O SO ₂ N ₂ 1000ppm 800ppm 0.08% 12.0% 4.5% 10% 200ppm Residual space velocity SV = 100,000 h ^-1 It was calculated from the following formula.

[Equation 1] The weight of S is converted from SO ₂ and the term “absorbent” means B here.
a.

Specific Surface Area The specific surface area before and after the above durability test was measured by an ordinary method, and the results are shown in Table 2.

[0028] Absorption-resolved lean NO _x catalyst purification rate monolithic shape,
The purification performance of the fresh catalyst and the catalyst after the model gas durability test (lean, 600 ° C x 50 hours) were measured, and the results are shown in Table 3. This purification rate is the following model gas, lean 2 minutes,
The stoichiometry is repeated for 2 minutes to control NO _x , HC and C
The average purification rate of O was measured at a measurement temperature of 350 ° C. Model gas composition lean: NO C ₃ H ₆ CO CO ₂ O ₂ H ₂ O SO ₂ N ₂ 1000ppm 800ppm 0.08% 12.0% 4.5% 10% 200ppm balance stoichiometry: NO C ₃ H ₆ CO CO ₂ O ₂ H ₂ O SO ₂ N ₂ 2500ppm 1000ppm 0.5% 14.0% 0.3% 10% 200ppm Remainder space velocity SV = 100,000 h ^-1

Comparative Example Pt was supported on γ-alumina having a specific surface area of 150 cm ² / g and then impregnated with an aqueous barium acetate solution to support Ba. Then, a monolith was coated in the same manner as in the example to obtain a catalyst. For this catalyst, the S poisoning amount, the specific surface area and the purification rate were measured in the same manner as above, and the results are shown in Tables 1, 2 and 3, respectively.

[0030]

[Table 1]

[0031]

[Table 2]

[0032]

[Table 3]

[0033]

EFFECT OF THE INVENTION In the absorption-reduction type catalyst of the present invention, absorption
The agent is ionic in the vicinity of the solid acid points in the pores of the SAPO carrier.
It has been exchanged, and it absorbs the negative charge of the solid acid point.
It is tightly bound by the positive ion charge of. This
Therefore, SO in exhaust gas_xWhen lean, platinum etc.
Oxidized by precious metals in SO_Four ^2-Becomes an absorbent plastic
Only weak adsorption occurs near the swion. Obey
SO when rich _Four ^2-Is reduced and absorbed
Since it is released, it can be seen in conventional absorption-reduction type catalysts.
S poisoning is prevented.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location B01D 53/36 102 H

Claims (1)

[Claims] 1. A catalyst for removing NO _x in exhaust gas in an oxygen excess atmosphere, comprising a crystalline silicoaluminophosphate (SAPO) porous carrier and a precious metal, and an alkali metal or alkaline earth by ion exchange. An exhaust gas purifying catalyst, which carries an active metal selected from the group consisting of group metals, transition metals, and lanthanoids.