JPH0631138A - Purifying method for exhaust gas - Google Patents

Abstract

PURPOSE:To provide the purifying method which efficiently purifies in a wide temp. range HOx, CO and HC contained in an exhaust gas in an excess oxygen atmosphere. CONSTITUTION:In simultaneously purifying carbon monoxide, hydrocarbon and nitrogen oxide contained in the exhaust gas in an excess oxygen atmosphere, the exhaust gas containing excess oxygen is brought into contact with the catalyst for purifying exhaust gas carrying alkaline-earth oxide, copper oxide and platinum on the carrier consisting of a porous body.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for purifying exhaust gas, and more particularly, to exhaust gas in the presence of excess oxygen, that is, reducing substances such as carbon monoxide, hydrogen and hydrocarbons contained in the exhaust gas. Nitrogen oxides (N) in the exhaust gas containing oxygen in excess of the oxygen required for complete oxidation.
The present invention relates to a method for efficiently purifying Ox) in a relatively high temperature range.

[0002]

2. Description of the Related Art Conventionally, as a catalyst for purifying exhaust gas of an automobile, exhaust gas is produced by simultaneously oxidizing carbon monoxide (CO) and hydrocarbon (HC) in the exhaust gas and reducing nitrogen oxides (NOx). Many three-way catalysts for purifying exhaust gas are known. 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 largely influenced by the set air-fuel ratio of the engine, and the lean air-fuel mixture, that is, the lean side where the air-fuel ratio is large, has a large amount of oxygen in the exhaust gas after combustion, resulting in oxidation. The action becomes active, and 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, and the oxidizing action becomes inactive and the reducing action becomes active. On the other hand, in recent years, in response to the demand for low fuel consumption of automobiles, lean side operation is performed in which fuel is burned with an oxygen-rich mixture as much as possible during normal driving, and a catalyst capable of sufficiently purifying NOx is also desired on the lean side. Was there.

Under such circumstances, various catalysts for purifying automobile exhaust gas in an oxygen-excess atmosphere include various catalysts that simultaneously oxidize carbon monoxide (CO) and hydrocarbons (HC) and reduce nitrogen oxides (NOx). 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 (see, for example, Nihon Kogyo Shimbun on April 11, 1991). However, this catalyst cannot be said to be a catalyst showing a practically sufficient purification rate in an oxygen excess atmosphere.

On the other hand, the present applicants have previously proposed a catalyst in which platinum and La ₂ O ₃ are supported on an alumina carrier in order to increase the NOx purification rate during steady running and transient conditions (simulated urban driving condition). (See JP-A-3-344781). However, when this catalyst was subjected to a durable treatment at a high temperature, the purification rate was lowered, and the required and sufficient purification rate was not always obtained.

Therefore, the present inventors have further proposed a catalyst in which platinum and barium oxide are supported on alumina in order to increase the NOx purification rate in a transient state (simulated urban driving state) (Japanese Patent Application No. 4- See 130904).

[0007]

[Problems to be Solved by the Invention] However, Pt-BaO
The / Al ₂ O ₃ system catalyst can effectively purify NOx in a transient state, but its optimum temperature is around 300 ℃, and Pt
-La ₂ O ₃ / Al ₂ O ₃ has an optimum temperature of around 250 ° C, which is so low that it cannot be mounted near the engine. .

That is, since the high-efficiency NOx purification temperature of the Pt-BaO / Al ₂ O ₃ catalyst is about 300 ° C, the catalyst purification device is installed at a place where the exhaust gas temperature is around 300 ° C (mainly the underfloor). There is a problem that it cannot be installed directly under the engine, for example, because there is a restriction on the installation place. When installed on the underfloor, the temperature of the gas is low, so it takes time for the catalytic metal to reach the activation temperature, and the harmful components cannot be purified at the initial stage of engine startup. If this catalyst is placed directly under the engine only for the purpose of initial purification, the exhaust gas temperature is 400 to 500 ° C (700 to 800 ° C at maximum), so initial purification is sufficiently possible, There is a problem that the components cannot be purified.

Therefore, according to the present invention, by making the exhaust gas purifying catalyst have a specific composition, NOx, CO and
An object of the present invention is to provide an exhaust gas purification method capable of efficiently purifying HC in a relatively wide temperature range, particularly in a relatively high temperature range.

[0010]

According to the present invention, in purifying carbon monoxide, hydrocarbons and nitrogen oxides in exhaust gas at the same time in an oxygen excess atmosphere, the oxygen excess exhaust gas is treated with a porous material. An exhaust gas purification method is provided, which comprises contacting an exhaust gas purification catalyst comprising an alkaline earth metal oxide, a copper oxide, and platinum on a carrier made of.

According to the exhaust gas purifying method of the present invention, the temperature window capable of purifying NOx in a transient state can be shifted to the high temperature side by further adding copper oxide in the catalyst system of barium oxide + platinum.

The catalyst used in the exhaust gas purification method of the present invention uses platinum (Pt), an oxide of an alkaline earth metal, and an oxide of copper (Cu) as active metals, and these are used as a porous carrier. Used by supporting.

Examples of the porous carrier that can be used in the present invention include alumina, zeolite, zirconia, silica-alumina, silica and the like. The type and physical properties of these porous carriers are not particularly limited, and any porous carrier conventionally used for catalysts can be used. Further, these porous carriers may be used by coating them on a honeycomb substrate made of cordierite, heat-resistant metal alloy or the like.

The exhaust gas purifying catalyst according to the present invention comprises platinum, an alkaline earth metal oxide and a copper oxide supported on the above-mentioned porous carrier. The supported amount of these metals is not particularly limited, but the supported amount of the oxide of an alkaline earth metal is preferably 0.05 to 10.0 mol / liter as alkaline earth metal. If the amount of the alkaline earth metal oxide supported is less than 0.05 mol / liter, it may not be possible to obtain a sufficient NOx purification rate, and if it exceeds 10.0 mol / liter, the surface area of the carrier is reduced. There is a risk. It is preferably 0.15 to 5.0 mol / liter.

The amount of platinum supported on the exhaust gas purifying catalyst used in the present invention is not particularly limited, but may be 0.1 to
It is preferably 10.0 g / liter. If the supported amount of platinum is less than 0.1 g / liter, sufficient catalytic activity may not be obtained, and if it exceeds 10.0 g / liter,
Even if the amount of platinum supported is further increased, the grain growth of platinum is promoted, and the improvement in activity is slight and expensive. Especially when the amount of platinum supported is 0.5 to 3.0 g / liter,
It is preferable in terms of activity and cost.

The amount of copper oxide supported on the exhaust gas purifying catalyst used in the present invention is not particularly limited, but it is generally preferably 0.05 to 10 mol / liter as Cu metal. If this value is less than 0.05 mol / l, the effect of adding Cu is difficult to be recognized, and conversely, if it exceeds 10 mol / l, the surface area of the carrier becomes insufficient.

The method for producing the exhaust gas purifying catalyst used in the present invention is not particularly limited and can be based on the conventional general method. For example, a substrate made of cordierite is dipped in a slurry containing alumina prepared according to a conventional method to coat the surface with alumina, and after drying and firing (for example, at a temperature of 600 to 700 ° C), dinitrodiamine platinum, chloroplatinic acid, etc. Immerse in an aqueous solution of platinum compound,
Platinum is loaded on the porous carrier by dry firing (eg, temperature 200 to 500 ° C), and further immersed in an aqueous solution of a water-soluble alkaline earth metal compound such as acetate of alkaline earth metal, and then fired (eg, temperature 600 It can be produced by supporting an alkaline earth metal at about 700 ° C).

According to the present invention, copper oxide is further supported on the carrier together with or separately from the above-mentioned oxide of alkaline earth metal. The supporting method is the same as before. For example, the copper oxide can be supported by immersing it in an aqueous solution of a copper compound such as copper acetate and copper nitrate and drying and firing (for example, at a temperature of 600 to 700 ° C.). At this time, a composite oxide is partially formed of the alkaline earth metal oxide and the copper oxide.

As an alternative method, the substrate is dipped in a slurry containing alumina and an oxide of an alkaline earth metal and / or a copper oxide to coat alumina, and after drying and firing,
It can be manufactured by immersing in an aqueous solution of a platinum compound such as dinitrodiamine platinum, chloroplatinic acid, etc., drying and firing, and supporting an oxide of an alkaline earth metal and copper and platinum. The alkaline earth metal may become a hydroxide or the like depending on the environment in which it is used.

In purifying the exhaust gas in the exhaust gas purifying method according to the present invention, the place where the catalyst is installed in the exhaust passage of the lean burn engine is not particularly limited. For example, under the floor of the vehicle immediately below the engine, the start catalyst. Alternatively, it is preferably installed under the floor on the downstream side of the three-way catalyst.

The purification method according to the present invention can be carried out by a usual method, and the space velocity for introducing the exhaust gas into the catalyst layer is
(SV) is not particularly limited, for example, 300,000 ~ 10,000hr ^-1
Is preferable, and the temperature of the catalyst layer is not particularly limited, but a temperature of about 200 to 500 ° C. is preferable.

[0022]

In accordance with the present invention, a catalyst in which an oxide of an alkaline earth metal and a copper oxide are supported on a porous carrier is used, and the catalyst is installed in the exhaust passage of an internal combustion engine for lean combustion. As shown, CO, HC and HOx
Can be purified with high efficiency.

[0023]

EXAMPLES The present invention will be described in more detail below with reference to examples, but it goes without saying that the scope of the present invention is not limited to these examples. In the following examples, "parts" are parts by weight unless otherwise specified.

A ) Preparation of Alumina Slurry 100 parts of alumina powder was mixed with alumina sol (alumina content
70 parts by weight, 10 parts by weight of 40% by weight aluminum nitrate aqueous solution and 30 parts by weight of water were added and mixed with stirring to prepare a coating slurry.

B) Coating and firing A cordierite honeycomb carrier is dipped in water to blow off excess water, then dipped in the slurry obtained in the above a) and taken out, and then blown off the excess slurry. , Temperature 80
It was dried at 0 ° C for 20 minutes and then calcined at 600 ° C for 1 hour. The coating amount of alumina was 120 g per liter of honeycomb volume.

C) Support of Pt The honeycomb obtained as described above is immersed in an aqueous solution of dinitrodiamine platinum having a predetermined concentration prepared so as to obtain the Pt support amount shown in Table 1, and dried at 250 ° C. A catalyst having a supported amount of Pt shown in Table 1 was prepared.

The platinum-supported carrier was immersed in a mixed aqueous solution of Cu acetate and Ba acetate having a predetermined concentration prepared so that the amounts of supported Ba and Cu shown in Table 1 were obtained, dried and calcined at 600 ° C. for 1 hour,
The catalyst of the present invention shown in Table 1 was obtained.

[0028] Table 1: Preparation catalytic responsible lifting weight Example No. Cu (mol / L) Ba (mol / L) Pt (g / L) 1 * 1 0.01 0.1 2 2 * 2 0.1 0.1 2 3 * 2 0.2 0.1 2 4 ^{* 1} 0 0.1 2 ───────────────────────── ^{* 1} : Comparative example ^{* 2} : Inventive example

Evaluation The purification performance of each of the above catalysts was evaluated under the following conditions.

Investigation of transient characteristics A heat exchanger and a catalyst were installed in the exhaust passage of a lean burn engine (1.6 liters), and A / F20 ← → 14.6 was changed in a pulse shape for 1 minute each, and the NOx purification rate was changed every 50 ° C. of incoming gas temperature. The measurement result is shown in FIG. It can be seen from FIG. 1 that the catalyst according to the present invention can shift the temperature window in the transient state to the high temperature side by adding Cu. Therefore, the catalyst used in the present invention can be mounted near the engine, and the warm-up property at the time of starting the engine can be improved.

[0031]

According to the present invention, as described above, the reason for adding the copper oxide to the oxide of alkaline earth metal and platinum to platinum as the catalyst metal is unknown, but high efficiency NOx is obtained.
It was found that the purification temperature shifted to the high temperature side. Therefore, by disposing the catalyst immediately below the engine, it is possible to purify harmful components at the time of engine start and steady state.

[Brief description of drawings]

FIG. 1 is a graph showing the evaluation results of catalysts in Examples 1 to 4.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Koichi Kasahara 7800 Chihama, Daito-cho, Ogasa-gun, Shizuoka Cataler Industry Co., Ltd. Within the corporation

Claims (1)

[Claims] 1. When simultaneously purifying carbon monoxide, hydrocarbons and nitrogen oxides in exhaust gas under an oxygen-excess atmosphere, the oxygen-excess exhaust gas is oxidised to a carrier made of a porous body by alkaline earth metal oxidation. And a catalyst for purifying exhaust gas, which carries platinum oxide and platinum.