JPH05168860A - Purifying method of exhaust gas - Google Patents

Abstract

PURPOSE:To efficiently purify NOX in exhaust gas together with CO and hydrocarbons(HC) by bringing the exhaust gas of oxygen-excess atmosphere into contact with a catalyst of platinum and lanthanum deposited on a porous carrier. CONSTITUTION:A catalyst of platinum and lanthanum as a rare earth metal deposited on a porous carrier is used as a catalyst which simultaneously effects oxidization of CO and HC and reduction of NOX in the exhaust gas from an automobile on which a lean-burn engine is mounted. The porous carrier is alumina, zeolite, zirconia, silica alumina, silica, etc., which carries lanthanum by 0.05-1.0mol/L. By bringing the exhaust gas of oxygen-excess atmosphere into contact with this catalyst, NOX, NO and HC in the exhaust gas can be purified both in a normal driving state and in a transition state.

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 a method for completely oxidizing exhaust gas in excess of oxygen, that is, 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 the oxygen required to
x) is efficiently purified.

[0002]

2. Description of the Related Art Conventionally, carbon monoxide (CO) and hydrocarbons (H) in exhaust gas have been used as catalysts for purifying exhaust gas of automobiles.
Many exhaust gas purification three-way catalysts that purify exhaust gas by simultaneously oxidizing C) and reducing nitrogen oxides (NOx) are known. As such a catalyst, for example, a refractory carrier such as cordierite is typically coated with γ-alumina slurry and baked to carry a noble metal such as Pd, Pt, or Rh.

By the way, the performance of the exhaust gas purifying catalyst is largely influenced by the set air-fuel ratio of the engine, and on the lean side where the air-fuel ratio is lean, that is, on the lean side where the air-fuel ratio is large, the amount of oxygen in the exhaust gas after combustion increases and the 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, the oxidation 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 in which combustion is performed with an oxygen-rich mixture as much as possible during normal driving is performed, 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 catalysts that simultaneously oxidize carbon monoxide (CO) and hydrocarbons (HC) and reduce nitrogen oxides (NOx). Proposed. As such a catalyst, for example, Pt / Al in which platinum is supported on alumina
₂ O ₃ catalysts have been proposed. However, in an oxygen-excessive atmosphere, the NOx purification rate of this catalyst is at most about 30-40% (catalyst inlet temperature 275
At a temperature of A and F of 22), the purification rate was not sufficient for practical use.

[0005]

The composition of the exhaust gas emitted from the engine while the automobile is running frequently changes within the range from near the theoretical A / F value to the over lean A / F value (about 23). To do. Therefore, the present invention eliminates the above-mentioned problems of the prior art, and eliminates NO in exhaust gas in the above-described actual traveling state of an automobile or the like (continuous A / F value transition).
It is an object of the present invention to provide an exhaust gas purification method capable of efficiently purifying x, CO and HC.

[0006]

According to the present invention, exhaust gas in excess of oxygen is brought into contact with a catalyst having platinum and lanthanum supported on a porous carrier to purify nitrogen oxides in the exhaust gas. An exhaust gas purification method is provided.
As will be described below, by using the exhaust gas purification method of the present invention, CO, HC and NOx contained in exhaust gas discharged from a vehicle equipped with a lean burn engine can be steadily run (for example, 40 km / hr). Also, it is possible to purify with high efficiency in both the transient state (simulated urban driving state).

In the method of the present invention, platinum (Pt) and lanthanum (La) are used as active metals, and these are used by being supported on a porous carrier. 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 monolith made of cordierite, heat-resistant metal alloy or the like.

According to the present invention, a catalyst in which platinum and lanthanum as a rare earth metal are supported on the above-mentioned porous carrier is used. The supported amount of these metals is not particularly limited, but in order to achieve a substantially effective NOx purification rate, the supported amount of lanthanum is preferably 0.05 to 1.0 mol / L, NO described later.
x In order to maximize the adsorption capacity, it is more preferable
0.1 to 0.4 mol / L, and the amount of platinum supported is effective NO
x in order to achieve a purification rate of 0.1-10 g / L,
It is particularly preferably 0.5 to 7 g / L in order to maximize the reducing ability of NOx.

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, alumina or other porous carrier prepared according to a conventional method is immersed in an aqueous solution of a lanthanum compound such as lanthanum nitrate or an alumina slurry containing a lanthanum compound, and dried and calcined (for example, at a temperature of 60
(0 to 700 ° C.) to allow La to be carried on the porous carrier, which is further immersed in an aqueous solution of a platinum compound such as dinitrodiamine platinum or chloroplatinic acid and dried and baked (for example, at a temperature of 200 to 500).
℃) and can be manufactured.

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, start catalyst or three-way catalyst. It is preferably arranged under the floor on the downstream side of the catalyst.

In the purification method according to the present invention, the space velocity (SV) for introducing the exhaust gas into the catalyst layer is not particularly limited, but is preferably in the range of 300,000 to 10,000 hr ^-1 , for example.
The temperature of the catalyst layer is also not particularly limited, but is preferably 200 to 500 ° C.

[0012]

According to the present invention, when a catalyst in which lanthanum and platinum are supported on a porous carrier is used and is installed in the exhaust passage of an internal combustion engine which burns lean, as shown in the following examples, when the vehicle is running, CO, HC and NOx can be purified with high efficiency.

The structure of the catalyst according to the present invention is such that lanthanum oxide (La ₂ O ₃ ) is covered on the surface of a porous carrier such as alumina, and platinum is carried thereon, and the catalyst having such a structure is used when the vehicle is running. It is assumed that the following changes will act on the composition change of the engine exhaust gas.

When the vehicle is stopped: The A / F is near the theoretical A / F and the exhaust gas contains little NOx. NOx adsorbed on lanthanum oxide on the catalyst moves to Pt (reverse spillover) and reacts with the reducing gas (CO, H ₂ , HC) in the exhaust gas, and NOx is reduced to N ₂ . At this time, the reducing gas is also purified. When the NOx adsorbed on the lanthanum oxide disappears, the reducing gas is adsorbed on Pt.

During vehicle acceleration: The A / F continuously changes from near the theoretical A / F to lean (A / F = 23), and a large amount of NOx exists in the exhaust gas. On the catalyst, NOx in the exhaust gas is reduced to N ₂ by the reducing gas adsorbed on Pt. At this time, the reducing gas is also purified.

During steady running: A / F is lean (A / F = 2
In the 3) state, NOx in the exhaust gas is less than during acceleration,
HC is increased and NOx is reduced to N ₂ by HC. At this time, HC is also purified. Also, N does not react
Ox is adsorbed on lanthanum oxide on the support.

During deceleration: A / F is automatic (A / F
T) is near theoretical A / F, and manual (M /
The T) vehicle has an oxidizing atmosphere (fuel cut), and the amount of NOx in the exhaust gas is small. On the catalyst, in the case of the A / T vehicle, the state is the same as when the vehicle is stopped, and in the case of the M / T vehicle, NOx is adsorbed and retained by lanthanum oxide.

[0018]

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" means "parts by weight" unless otherwise specified.

Example 1 Pt / lanthanum oxide / alumina monolith catalysts having different loadings were prepared and their NOx purification activities were compared. a) Preparation of alumina slurry To 100 parts of alumina powder, alumina sol (alumina content rate)
10 parts by weight) 70 parts, 15 parts by weight of 40% by weight aluminum nitrate aqueous solution and 30 parts of water were added and mixed with stirring to prepare a coating slurry.

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

The alumina-coated honeycomb carrier was immersed in an aqueous solution of lanthanum nitrate having a predetermined concentration prepared so as to obtain the amount of La shown in Table 1, dried, and calcined at 600 ° C. for 1 hour. Catalysts with La loadings shown in 1 to 12 were obtained.

C) Support of Pt The La-supported monolith obtained as described above was immersed in an aqueous solution of dinitrodiamine platinum having a predetermined concentration prepared so as to obtain the amount of supported Pt shown in Table 1, and Catalyst No.
Catalysts having Pt loadings shown in 1 to 12 were prepared. As comparative catalysts, catalysts of Nos. 13 to 15 which did not carry La were also prepared in the same manner as in the above-mentioned examples.

Table 1: Preparation catalyst ────────────────────────────────── Supported catalyst No. La (mol / L) Pt (g / L) 1 0.04 2.0 2 0.1 2.0 3 0.2 2.0 4 0.7 2.0 5 1.2 2.0 6 1.5 2.0 7 0.2 0.2 8 0.2 0.5 9 0.2 1.0 10 0.2 5.0 11 0.2 7.5 12 0.2 12.0 13 0.0 0.2 14 0.0 2.0 15 0.0 7.5 ─── ──────────────────────

Evaluation The purification performance of the above catalyst was evaluated under the following conditions. a) Steady activation The above-mentioned No.
Each catalyst 1 to 15 (1.3 liters) was installed at a position 1.6 m downstream of the exhaust manifold, and the engine was set to A / F = 22.
The engine output was adjusted so that the temperature on the inlet side of the catalyst would be 275 ° C. Then, the purification rates of CO, HC and NOx under these conditions were examined by measuring the gas concentrations before and after the catalyst with an exhaust gas analyzer. The results are shown in Table 2.

Table 2 ─────────────────────────── Purification rate (%) Catalyst No. NOx HC CO 1 37 84 85 2 39 85 86 3 40 85 85 4 39 86 88 5 32 84 86 6 30 83 85 7 25 80 79 8 30 84 81 9 40 86 85 10 43 86 87 11 43 86 85 12 43 86 86 13 ^* 15 76 75 14 ^* 30 82 80 15 ^* 35 85 82 ─────────────────────────── ^* : Comparative example

B) Activity in A / F change (transient activity) A / F = 14.5 (stoichi) and A / F in a lean burn engine.
= 18 (lean) was changed in a 6-minute cycle. A / F = 1
The exhaust gas temperatures on the catalyst inlet side of the engine at 4.5 and 18 were 250 ° C and 275 ° C, respectively. The throttle opening at that time is fully closed when A / F = 14.5, A / F =
At 18, the manifold negative pressure was 400 mmHg. The experimental results were obtained by measuring the NOx emission amount at the catalyst inlet side and the NOx emission amount after passing through the catalyst with an exhaust gas analyzer, and obtaining the NOx amount reduced from the difference between the two, and converting it into a gas amount, then N
It was defined as the Ox purification rate. The results are shown in Table 3.

Table 3 ─────────────────── Catalyst No. NOx purification rate (%) 1 84 2 92 3 89 4 85 5 76 6 70 7 68 8 85 9 87 10 91 11 92 12 92 13 ^* 53 14 ^* 75 15 ^* 81 ─────────────────── ^* : Comparative example

From the above results, the catalyst and method according to the present invention are NO emitted from vehicles equipped with lean burn engines.
It is clear that x, CO and HC are efficiently purified. In particular, the La loading amount of 0.05 to 1.0 mol / L and the Pt loading amount of 0.5 to 7.0 g / L are preferable.

[0029]

In the actual driving, the automobile is currently used by repeatedly accelerating and decelerating. According to the present invention, by controlling the electronic state of Pt by lanthanum oxide with respect to the exhaust gas discharged under such a condition, the reaction activity of HC and NOx can be improved. By utilizing the NOx adsorption characteristic, the NOx purification rate during acceleration / deceleration can be greatly improved. As a result, CO and HC in exhaust gas with excess oxygen
The nitrogen oxides in the exhaust gas can be efficiently purified under sufficient purification.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kiyoshi Nakanishi 1st Toyota-cho, Toyota-shi, Aichi Prefecture Toyota Motor Co., Ltd. (72) Inventor Satoshi Iguchi 1st Toyota-cho, Toyota-shi, Aichi Toyota Motor Co., Ltd. ( 72) Inventor Tetsuro Kihara 1 Toyota Town, Toyota City, Aichi Prefecture, Toyota Motor Co., Ltd. (72) Inventor Hideaki Muraki 41, Nagakute Town, Nagakute Town, Aichi County, Aichi Prefecture 1 Toyota Central Research Institute Co., Ltd. (72 ) Inventor Mariko Adachi No. 41, Yokocho, Nagakute-machi, Aichi-gun, Aichi Prefecture, 1 Yokouchi Central Research Co., Ltd. (72) Inventor, Master Tateishi 7800, Chihama, Oto-gun, Shizuoka Prefecture, Cataler Industry

Claims (1)

[Claims] 1. A method for purifying exhaust gas, which comprises purifying nitrogen oxides in exhaust gas by bringing the exhaust gas in excess of oxygen into contact with a catalyst having platinum and lanthanum supported on a porous carrier.