JPH08112530A - Production of catalyst for purification of exhaust gas - Google Patents

Abstract

PURPOSE: To shift the temp. at which a catalyst has NOx removal activity to the high temp. side by heat-treating a Pt carried zeolite in a specified temp. range in the presence of oxygen. CONSTITUTION: A zeolite such as ferrierite or mordenite is immersed in a soln. prepd. by dissolving a Pt salt in a solvent such as water and Pt is carried on the zeolite by ion exchange or other method. The resultant Pt carried zeolite is heat-treated at 700-1,100 deg.C, preferably 750-1,050 deg.C, especially preferably 800- -1,000 deg.C in the presence of oxygen to obtain the objective Pt carried zeolite catalyst having activity at high temp. When this catalyst is used, exhaust gas is efficiently purified at about 250-300 deg.C.

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 in excess of oxygen, that is, to completely reduce reducing substances such as carbon monoxide, hydrogen and hydrocarbons contained in the exhaust gas. Nitrogen oxides (NO) in exhaust gas containing oxygen in excess of that required for oxidation.
The present invention relates to a method for manufacturing an exhaust gas purification catalyst that efficiently purifies x).

[0002]

2. Description of the Related Art Conventionally, as an exhaust gas purifying catalyst for internal combustion engines such as automobiles and factory plants, oxidation of carbon monoxide (CO) and hydrocarbons (HC) in exhaust gas and nitrogen oxides (NOx) have been performed. There are many known catalysts that purify exhaust gas by simultaneously performing reduction of). In recent years, exhaust gas that efficiently removes NOx in exhaust gas that contains an excess amount of oxygen that is necessary to completely oxidize reducing substances such as CO and HC in lean burn engines and diesel engines. Gas purification catalysts are desired.

As a catalyst for removing NOx under such an oxidizing atmosphere, for example, a catalyst in which Cu, Co or the like is supported on a zeolite carrier is known. However, although a catalyst in which Cu, Co, etc. are supported on a zeolite carrier exhibits NOx removal activity at 350 ° C. or higher, it has a problem of poor high-temperature durability. On the other hand, it is known that a catalyst in which a precious metal such as Pt is supported on the zeolite carrier or alumina carrier has relatively high temperature durability as compared with the above-described catalyst in which Cu, Co, etc. are supported on the zeolite.

[0004]

However, a catalyst in which a noble metal such as Pt is supported on an oxide such as alumina is Pt.
There is a problem that sufficient high temperature durability cannot be obtained, such as a decrease in activity due to agglomeration of particles and a decrease in surface area of the carrier. For example, in Japanese Patent Laid-Open No. 4-267946, P
It is described that heat-treating a t / alumina catalyst at 800 ° C. in the atmosphere causes the NOx removal activity at 200 to 350 ° C. to disappear.

Regarding the heat treatment of a catalyst in which a noble metal is ion-exchanged with zeolite at a high temperature, for example, Japanese Patent Laid-Open Publication No. Hei 5 (1999) -53977 is used.
JP-A-57183, Z containing ion exchanged Pd
Regarding the catalyst obtained by calcining SM-5 zeolite at a high temperature, studies on the adsorption performance of hydrocarbons are known. However, it is completely unknown about the influence of these treatments on the NOx removal performance in exhaust gas under excess oxygen.

In the prior art, the catalyst supporting Pt on zeolite has a NOx removal activity temperature of about 200 ° C., and there is a problem that sufficient activity cannot be obtained at temperatures higher than that.

NO in the exhaust gas under this oxygen excess
The calcination temperature of the Pt / zeolite catalyst prepared for removing x was conventionally 600 ° C. or lower.

[0008]

DISCLOSURE OF THE INVENTION The inventors of the present invention have made detailed studies on a method for solving the problem of the catalyst in which Pt is supported on zeolite as described above. As a result, the catalyst in which Pt is supported on zeolite is heated to 700 to 700 ° C in the presence of oxygen.
The present invention has been completed by discovering the novel fact that the temperature of NOx removal activity of the catalyst can be shifted to a high temperature by heat treatment at 1100 ° C.

That is, the present invention relates to a method for producing an exhaust gas purifying catalyst obtained by heat-treating Pt-supported zeolite at a temperature of 700 to 1100 ° C. in the presence of oxygen.

The present invention will be described in more detail below.

[0011] Zeolite to be used in the present invention are generally _{xM 2 / n O · Al 2} O 3 · ySiO 2 · zH 2 O ( where n is the valence of the cation M, x is the 0.8 to 2 The number of ranges, y is a number of 2 or more, and z is a number of 0 or more). Of these, SiO ₂ / Al ₂ O ₃
A molar ratio of 15 or more is preferable from the viewpoint of heat resistance and durability of the zeolite itself, and a zeolite having a molar ratio of about 15 to 2200 is usually used. Also, as M, Li, Na, K
Alkali metals and the like.

The zeolite constituting the catalyst of the present invention may be either a natural product or a synthetic product, and the manufacturing method of these zeolites is not particularly limited, but typically, it is ferrierite, mordenite, Y. , ZSM-5, Z
Zeolites such as SM-11, ZSM-12 and ZSM-20 can be used. Further, these zeolites can be used as the catalyst of the present invention as they are or after being treated with an ammonium salt, a mineral acid or the like and subjected to ion exchange into NH ₄ type or H type.

In the present invention, Pt-supported zeolite is used as a catalyst. The method for supporting Pt is not particularly limited, and a known method such as an ion exchange method or an impregnation method can be used. Pt is then used in the form of its salt solution. The salt of Pt is not particularly limited, but is usually various inorganic salts of Pt such as nitrates, sulfates, carbonates, hydrochloric acid, bromates (halides), organic acid salts such as acetates, water, etc. Oxides, oxides, and the like. Ammine complexes, amine complexes, halogeno complexes (for example, hexachloroplatinic acid) of the above various compounds, and their organic salts such as sodium salts, potassium salts, cyano complexes, and acetylacetonato complexes. Mention may be made of complexes or inorganic complexes. These Pt salts may be used alone or in combination of two or more as a mixture or complex compound.

A solvent is used when these Pt salts are made into a solution. The solvent used at this time is not particularly limited, but usually water or an organic substance such as an alcohol or a carbonyl compound can be used. These solvents may be used alone or in combination of two or more.

In the present invention, the amount of Pt supported is not particularly limited, but usually 0.0% of noble metal is added to the total weight of the catalyst.
It may be carried in an amount of from 05 to 10% by weight.

In the present invention, in the case of the ion exchange method, for example, the Pt salt prepared by dissolving a salt of Pt in a solvent such as water is used.
The catalyst is prepared by immersing zeolite in a solution of t to perform ion exchange treatment, solid-liquid separation, washing and drying. In the case of the impregnation method, P prepared by dissolving a Pt salt in a solvent is used.
The catalyst is prepared by immersing zeolite in the solution t, removing the solvent by evaporation, and drying.

Further, in the present invention, Pt-supported zeolite in the presence of oxygen is 700 to 1100 ° C., preferably 750 to 1050 ° C., more preferably 800 to 1 ° C.
It is essential to perform heat treatment at 000 ° C. The treatment temperature is preferably 800 to 1000 ° C. from the viewpoint of the heat resistance of the zeolite itself, the catalyst activation temperature and the maintenance of the catalyst activity.

The atmosphere for the heat treatment is not particularly limited, but normally, oxygen can be diluted with an inert gas such as nitrogen, carbon dioxide, helium, argon, or steam before use. Further, air can be used as it is. Of these, air is preferable from the economical viewpoint. Further, the method of existence of oxygen may be either treatment in air or under gas flow.

In the present invention, the treatment time of the catalyst in the presence of oxygen is not particularly limited, but usually 2 to 10 hours is sufficient, and the temperature may be changed stepwise during the heat treatment.

By bringing the catalyst obtained in the present invention into contact with the exhaust gas in an oxidizing atmosphere, the H in the exhaust gas is reduced.
It is possible to purify harmful substances such as C and NOx.

Here, "exhaust gas in an oxidizing atmosphere" means the amount of oxygen required to completely oxidize reducing substances such as CO, hydrogen and HC contained in the exhaust gas to convert them into water and carbon dioxide. Exhaust gas containing an excessive amount of oxygen. That is, the oxygen concentration required to completely oxidize reducing substances such as CO, hydrogen and HC contained in the exhaust gas to convert them into water and carbon dioxide gas was A, and the oxygen concentration in the exhaust gas was B. In this case, B / A is a value larger than 1, and in the exhaust gas of the present invention, B / A is usually 2 to 800. For example, in the case of exhaust gas from an internal combustion engine such as an automobile, the exhaust gas is exhausted in a large air-fuel ratio (A / F) of 17 to 23 (fuel lean region).

The catalyst obtained in the present invention is NO by promoting the reaction between HC and NOx in the exhaust gas.
x can be purified. Normally, HC remaining in the exhaust gas can be used to purify NOx, but if purification of NOx is insufficient, HC may be supplemented from the outside. In this case, the HC is not particularly limited,
The required amount of HC is preferably 0.1 to 20 times that of NOx.

Specifically, the catalyst is placed in a reactor, and exhaust gas is introduced into the reactor to bring the catalyst into contact with the exhaust gas to reduce and purify nitrogen oxides, and then purify the nitrogen oxides. The exhaust gas may be discharged from the reactor. Further, the purifying effect can be further enhanced by using the catalyst of the present invention in combination with a known catalyst.

In the present invention, the reaction temperature of the catalyst layer at the time of purification can be usually in the range of 150 to 800 ° C.

When introducing the exhaust gas into the catalyst layer, the space velocity (SV) is not particularly limited.
The range of 0-550,000 / hour is preferable for maintaining the activity.

[0026]

The Pt-supported zeolite catalyst subjected to the specific treatment of the present invention has a high activation temperature of 250 to 30
The exhaust gas at 0 ° C. can be purified more efficiently than the conventional Pt-supported zeolite catalyst.

[0027]

EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to the following examples.

Reference Example (Preparation of Pt-supported zeolite) Na-type ZSM-5 with silica / alumina ratio of 40: 15 g
Tetraammine dichloroplatinum salt monohydrate (Pt (N
H ₃ ) ₄ Cl ₂ · H ₂ O) is the alumina (Al
135 g of an aqueous solution dissolved in an amount equal to the number of moles of ₂ O ₃ )
Was charged at room temperature and stirred for 20 hours to perform ion exchange treatment. After the treatment was completed, the solid was filtered off and the solid was thoroughly washed with pure water. 110 ° C / 12 with a dryer
A Pt-supported zeolite catalyst was obtained by drying for an hour.

As a result of chemical analysis, the Pt content in this Pt-supported zeolite catalyst was 7% by weight based on the catalyst.
It was Pt / Al ₂ O ₃ = 0.91 (molar ratio).

Example 1 (Preparation of Catalyst 1) The Pt-supported zeolite catalyst prepared in Reference Example was heated to 200 ° C./2 hours and then to 600 ° C. in the air using an electric furnace.
The catalyst 1 was prepared by heat treatment at 800 ° C. for 3 hours and further at 3 hours.

Example 2 (Preparation of Catalyst 2) The Pt-supported zeolite catalyst prepared in Reference Example was heated to 200 ° C./2 hours and then to 600 ° C. in the air using an electric furnace.
The catalyst 2 was prepared by performing heat treatment at 1000 ° C. for 3 hours and further at 1000 ° C. for 3 hours.

Comparative Example 1 (Preparation of Comparative Catalyst) The Pt-supported zeolite catalyst prepared in Reference Example was heated to 200 ° C./2 hours and then 650 ° C. in the atmosphere using an electric furnace.
Heat treatment was performed for / 3 hours, and this was used as a comparative catalyst.

Example 3 (Evaluation of activity of catalyst) Catalyst 1 was press-molded and then pulverized to be sized to 12 to 20 mesh, and 0.5 cc of the crushed catalyst was filled in a fixed pressure reaction tube at atmospheric pressure. A gas having a composition shown in Table 1 (hereinafter, referred to as a reaction gas) was heated to 600 ° C. while flowing at a rate of 166.6 ml / min, and then held for 20 minutes and then cooled, and the temperature was measured at each catalyst activity measurement temperature. Was kept constant and the catalyst activity at each temperature was measured. The activity of the catalyst 2 and the comparative catalyst was measured by the same method. Table 2 shows the initial NOx purification rate at each reaction temperature of each catalyst and the reaction temperature when the NOx purification rate becomes the highest. The NOx purification rate is a value obtained by the following equation.

NOx purification rate (%) = 100 × (NOou
t) / NOin NOin: NO concentration in the reaction gas composition NOout: Gas concentration obtained by doubling the sum of the N ₂ concentration and the N ₂ O concentration at the outlet of the atmospheric pressure fixed bed reaction tube

[0035]

[Table 1]

[0036]

[Table 2]

[0037]

According to the present invention, the Pt-supported zeolite is subjected to a specific treatment to simultaneously remove harmful substances such as hydrocarbons and nitrogen oxides in exhaust gas in an oxygen excess atmosphere. It can also be carried out in a temperature range in which the zeolite catalyst supporting Pt is not effective. INDUSTRIAL APPLICABILITY The present invention can be applied not only to internal combustion engines such as automobiles but also to purification of exhaust gas containing nitrogen oxides such as a nitric acid manufacturing plant and various combustion facilities, and is industrially very significant.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication location B01J 29/44 ZAB A 37/08 ZAB B01D 53/36 104 A

Claims (1)

[Claims] 1. A method for producing an exhaust gas purifying catalyst, which comprises heat-treating zeolite carrying Pt at a temperature of 700 to 1100 ° C. in the presence of oxygen.