JP3111491B2 - 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 for an internal combustion engine of an automobile or the like or an industrial plant, and more particularly, for example, an automobile exhaust gas under an operating condition where the air-fuel ratio is relatively larger than the stoichiometric air-fuel efficiency. The present invention relates to an exhaust gas purifying catalyst suitable for purifying a NOx-containing gas having a relatively large amount of residual oxygen in the gas.

[0002]

2. Description of the Related Art A catalyst that simultaneously oxidizes carbon monoxide and hydrocarbons and reduces nitrogen oxides (NOx) is used as an exhaust gas purifying catalyst for internal combustion engines such as automobiles and industrial plants. It is widely used as a catalyst. As such a catalyst, typically, a γ-alumina slurry is applied on a refractory carrier such as cordierite, and after firing, a palladium, platinum, one carrying a noble metal such as rhodium or any mixture thereof is supported. Are known. In particular, many proposals have been made to enhance the catalytic activity, for example, in a type of catalyst in which a noble metal or the like is dispersed on γ-alumina particles stabilized with rare earth oxides, on particles substantially containing no rare earth oxides. A catalyst in which rhodium is dispersed is disclosed in JP-A-61-11147.

[0003] However, the purification characteristics of these catalysts are greatly affected by the set air-fuel ratio of the engine. On the lean side where the air-fuel ratio is large, that is, on the lean mixture, the amount of oxygen is large even after combustion, so that the oxidizing 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 after combustion becomes small, so that the oxidizing action becomes inactive and the reducing action becomes active. Since the catalyst works most effectively near the stoichiometric air-fuel ratio (A / F = 14.6) where this oxidation and reduction can be balanced, feedback control is performed to detect the oxygen concentration in the exhaust system and maintain the mixture near the stoichiometric air-fuel ratio. Was being done.

Under such circumstances, there has been proposed an exhaust gas purifying catalyst capable of reducing and removing NOx even on the lean side (see Japanese Patent Application Laid-Open No. 1-130735). This catalyst is made of zeolite ion-exchanged with a transition metal, and is a catalyst that can efficiently purify NOx even in an oxygen-excess atmosphere where the air-fuel ratio is lean.

A catalyst for burning a carbon material comprising a zeolite type copper aluminosilicate containing a copper ion in a zeolite structure is described, for example, in Japanese Patent Publication No. 57-36015.

[0006]

The above-mentioned zeolite catalyst ion-exchanged with a transition metal such as copper is effective in reducing NOx even under a lean combustion condition. In operating conditions or emission control mode,
NOx purification rates tended to decrease. This is because, for example, when copper ions adhere to a point other than the ion exchange point of zeolite, copper becomes copper oxide in the drying and calcining steps of the catalyst production, and this completely oxidizes hydrocarbons (HC) by the following reaction and reduces NOx. The amount of hydrocarbons used for
It appears to be due to the reduced purification rate of x.

[0007] 2HC → H ₂ O + CO ₂

[0008] Since this reaction becomes particularly noticeable at high temperatures, there has been a problem that the NOx purification rate may be further lowered under actual use conditions or the exhaust gas regulation mode. In addition, there is a possibility that the zeolite structure of the zeolite catalyst may be destroyed over time, and there is a concern about the durability of the catalyst.

Accordingly, it is an object of the present invention to provide an exhaust gas purifying catalyst capable of effectively reducing and purifying NOx even under lean combustion conditions by solving the above-mentioned problems.

[0010]

According to the present invention, copper or
There is provided an exhaust gas purifying catalyst obtained by heat treating a zeolite catalyst ion-exchanged with cobalt in a gas stream containing a sulfur compound.

The catalyst according to the present invention is obtained by first converting zeolite to copper or zeolite.
Can be produced by ion exchange with cobalt and heat treatment in a gas stream containing a sulfur compound.

[0012] Zeolites are, as is well known, the general _{formula: xM 2 / n O · Al} 2 O 3 · crystalline aluminosilicate ySiO represented by _{2 (M: Na, K,} C
a, Ba, etc., where n is a valence number, x and y are positive numbers) and the pore size and other factors in the crystal structure are different depending on the difference between M, x and y, and use their cation exchange ability and molecular sieving action. It is widely used as a catalyst, molecular sieve, adsorbent, etc., and many types are commercially available.

In the preparation of the catalyst according to the invention, any of these zeolites can be used as starting material, but have a pore size of about 5 to 10 °, which is slightly larger than the diameter of the NO _X molecules to be purified. It is preferred to use one. In the present invention, first, metal ions in zeolite are ion-exchanged with Cu or Co. The ion exchange of Cu or Co can be performed, for example, by treating the zeolite with a Cu or Co ion solution. This Cu or
The zeolite ion-exchanged with Co is kneaded with a binder made of, for example, alumina sol or silica sol, and the obtained slurry is wash-coated on a suitable carrier (for example, a cordierite honeycomb carrier) and calcined to form a solid catalyst type. can do. It is also possible to knead the zeolite with a binder made of, for example, alumina sol or silica sol, wash coat the obtained slurry on a suitable carrier as described above, and then perform ion exchange with Cu or Co ions, followed by firing.

Next, the zeolite ion-exchanged with Cu or Co is heat-treated in a gas stream containing a sulfur compound according to the present invention. For example, hydrogen sulfide or the like can be used as the sulfur compound.

As an example of an apparatus for heat-treating a zeolite catalyst ion-exchanged with Cu or Co ions according to the present invention in a gas stream containing a sulfur compound , copper ions are used.
A method for producing a catalyst according to the present invention by heat-treating this with a hydrogen sulfide-containing gas will be described below with reference to FIG.

As shown in FIG. 1, a zeolite catalyst 1 ion-exchanged with copper ions is placed in, for example, an electric furnace 2 and a hydrogen sulfide-containing gas 4 (H ₂ S concentration is not particularly limited) from a hydrogen sulfide cylinder 3. However, preferably, the electric furnace is heated by a heater 5 (the temperature is not particularly limited as long as the produced sulfide is stable, but usually about 500 to 700 ° C.). ), And oxidizes copper oxide and the like adhering to sites other than the ion exchange point by ion exchange to sulfurate to form copper sulfide, thereby losing oxidation activity. In this operation, the copper ion substituted at the ion exchange point of the zeolite is stably present and remains without being sulfurized. In FIG. 1, reference numeral 6 denotes a thermocouple.

[0017]

According to the present invention, a zeolite catalyst ion-exchanged with Cu or Co is heat-treated in a gas stream containing a sulfur compound to convert a transition metal ion derived from a transition metal ion attached to a site other than the ion exchange point of the zeolite. Since the substances are converted into sulfides, the catalytic action of oxides that oxidize hydrocarbons is lost, and the NOx purification rate in the NOx purification process can be improved. If the transition metal is present in the catalyst in the form of an oxide, the structure of the zeolite may be destroyed. By converting the oxide into a sulfide, such a structure can be effectively prevented.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to the accompanying drawings, but it goes without saying that the technical scope of the present invention is not limited to the following embodiments.

Example 1 (Production Example) A powder of ZSM-5 type zeolite (manufactured by Tosoh Corporation) having a Si / Al ratio of 40 and a pore diameter of 5.5 mm was immersed in a 0.01N copper acetate aqueous solution at room temperature for several days to obtain copper. An ion-exchanged zeolite was obtained. On the other hand, alumina sol and silica sol have a Si / Al ratio of
A slurry-like binder was obtained by mixing so as to be 40, and 100 parts by weight of the above-mentioned copper ion-exchanged zeolite and 100 parts by weight of water were added to and mixed with 70 parts by weight of the binder to obtain a pH.
Was adjusted to 7.0 to 8.6 with aqueous ammonia (dilution) to obtain a slurry. This slurry was wash-coated on 0.7 liter of a cordierite honeycomb carrier (manufactured by Nippon Insulator), dried, and calcined at 600 to 650 ° C. to obtain a copper ion-exchanged zeolite catalyst (hereinafter referred to as catalyst A).

Next, put the catalyst A obtained above as a catalyst 1 in an electric furnace 2 shown in FIG. 1, H ₂ S 10 hydrogen sulfide bomb
A gas of 00 ppm (gas diluted with N ₂ ) was passed. The electric furnace 2 was adjusted so that the exhaust gas temperature at the outlet was about 600 ° C., and reacted for about 4 hours. In this way, the copper (actually copper oxide) adhered to a portion other than the ion exchange point by the ion exchange was sulfided to obtain a sulfide treatment catalyst (hereinafter, referred to as catalyst B).

Example 2 (Preparation Example) A cobalt ion exchanged zeolite catalyst (referred to as catalyst A ') was prepared in exactly the same manner as in Example 1 except that a mixture of cobalt acetate and cobalt nitrate (1: 1) was used instead of copper acetate. A sulfurization treatment catalyst (referred to as catalyst B ') was obtained in the same manner as in Example 1 except that the temperature of the exhaust gas at the outlet of the electric furnace 2 was set to about 700 ° C.

Example 3 (Evaluation Example) Using the catalysts A and A '(control example) and B and B' (invention example) prepared in Examples 1 and 2, the purifying activity of these catalysts was evaluated in 10 modes. (Catalyst capacity: 0.7 liter, vehicle: 1.6 liter lean burn, running mode: 10 modes). The results were as shown in Table 1 below.

[0023]

[Table 1]

As is evident from the results in Table 1, the catalysts B and B 'according to the present invention have a significantly reduced hydrocarbon (HC) purification rate and a reduced oxidizing ability. Is shown in

Next, a similar test was performed in 10 modes while increasing the catalyst capacity from 0.7 liter to 1.7 liter. The results were as shown in Table 2.

[0026]

[Table 2]

As is evident from the results in Table 2, when the capacity of the catalyst was increased, the NOx purification rate was increased in the catalysts B and B 'of the present invention, whereas the NOx purification rate was increased in the control catalysts A and A'.
No substantial increase in purification rate was observed.

Example 4 (Evaluation example) Using catalysts A and A '(control) and B and B' (invention) prepared in Examples 1 and 2, the catalyst capacity was increased to 0.7 liter, and The performance of the exhaust gas purification catalyst during the lean operation was evaluated.

The results are as shown in FIG. As shown in Fig. 2, at high speed, the catalyst inlet gas temperature to the exhaust catalyst system is 6
The temperature rises from 00 to 700 ° C., and the oxidation reaction of hydrocarbons becomes more remarkable. In the catalysts A and A ′ (control catalysts), the NOx purification rate decreases remarkably with increasing temperature. On the other hand, in the catalysts B and B '(the catalyst of the present invention), a decrease in the NOx purification rate was not so much recognized.

[0030]

According to the present invention, as described above,
Heat treatment of zeolite catalyst ion-exchanged with copper or cobalt in a gas stream containing sulfur compounds such as hydrogen sulfide,
For example, in order to stabilize by exchanging copper or cobalt oxide such as copper oxide or cobalt oxide present at a site other than the ion exchange point of zeolite with sulfide such as copper sulfide or cobalt sulfide, such as copper oxide or cobalt oxide The problem that the hydrocarbons in the exhaust gas are oxidized by the oxidizing ability of the oxides and the purification rate of NOx in the exhaust gases is reduced can be solved effectively by converting the oxides into sulfides. On the other hand, copper oxide and cobalt oxide have an adverse effect of destroying the structure of the zeolite, so that transition metals such as ion-exchanged copper are clogged and pores are blocked, and the catalytic activity is lost.

The sulfurization treatment of the present invention stabilizes copper oxide, cobalt oxide, and the like into copper sulfide, cobalt sulfide, and the like, so that the structural destruction of zeolite is prevented and the durability of the catalyst is greatly improved.

[Brief description of the drawings]

FIG. 1 is a drawing showing an example of an apparatus for heat-treating a zeolite catalyst ion-exchanged with copper or cobalt in an air stream containing a sulfur compound according to the present invention;

FIG. 2 is a graph showing test results of Example 4.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Catalyst 2 ... Electric furnace 3 ... Hydrogen sulfide cylinder 4 ... Hydrogen sulfide containing gas 5 ... Heater 6 ... Thermocouple

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Shigta Inoue 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Corporation (56) References JP-A-3-157126 (JP, A) (58) Investigated Field (Int.Cl. ⁷ , DB name) B01J 21/00-37/36 B01D 53/86

Claims (1)

(57) [Claims] An exhaust gas purifying catalyst obtained by heat-treating a zeolite catalyst ion-exchanged with copper or cobalt in a gas stream containing a sulfur compound.