JP2661383B2 - 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, and more particularly, for example, to an exhaust gas purifying an automobile under a lean burn operating condition having an air-fuel ratio relatively larger than the stoichiometric air-fuel efficiency. The present invention also 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 As an exhaust gas purifying catalyst for internal combustion engines such as automobiles and industrial plants, a catalyst which simultaneously oxidizes carbon monoxide and hydrocarbons and reduces nitrogen oxides (NOx) is used for purifying exhaust gas. 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, palladium, platinum, a precious metal such as rhodium, or a mixture of any of them supported thereon. Are known.
In particular, many proposals have been made to enhance the catalytic activity thereof, for example, in a catalyst of the type in which a noble metal or the like is dispersed on rare earth oxide-stabilized γ-alumina particles, on a particle substantially containing no rare earth oxide. 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, an exhaust gas purifying catalyst capable of reducing and removing NOx even on the lean side has been proposed (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 in, for example, Japanese Patent Publication No. 57-36015.

[0006] The zeolite catalyst ion-exchanged with a transition metal such as copper is effective for reducing NOx on the lean side, but according to the knowledge of the present inventors, in actual use conditions or in the exhaust gas regulation mode, The NOx purification rate tended to decrease. This is because, for example, when copper ions adhere to a point other than the ion exchange point of the zeolite, copper becomes a copper oxide cluster in the drying and calcining steps of the catalyst production, and this completely oxidizes hydrocarbons (HC) by the following reaction, and NOx It appears that the amount of hydrocarbons used for reduction is reduced and the NOx purification rate is reduced.

2HC → H ₂ O + CO _{2 Since} this reaction is particularly remarkable at high temperatures, there is a problem that the NOx purification rate may be further reduced under actual use conditions or the exhaust gas regulation mode. Therefore, the present inventors heat-treat the zeolite catalyst ion-exchanged with the transition metal in a gas stream containing a sulfur compound such as hydrogen sulfide, thereby converting the copper oxide clusters on the zeolite surface into copper sulfide clusters. Thus, an exhaust gas purifying catalyst with an improved NOx purification rate was proposed. However, in this catalyst, the aforementioned copper sulfide cluster blocks a part of the pores of the zeolite, so that the active sites inside the pores cannot be effectively used, and the purification rate of NOx is not yet practically sufficient. Was.

[0008]

SUMMARY OF THE INVENTION Accordingly, the present invention has been made in view of the above-mentioned problems of the prior art, and therefore, the present invention provides an exhaust gas capable of effectively purifying exhaust gas even under actual lean burn conditions. The purpose is to develop a purification catalyst.

[0009]

According to the present invention, a zeolite catalyst ion-exchanged with a transition metal selected from the group 1B and the iron group is heat-treated in a reducing gas stream containing a sulfur compound and then treated with glycol. An exhaust gas purifying catalyst is provided by washing.

[0010] The catalyst according to the present invention comprises a zeolite,
Ion exchange is performed with the above-mentioned specific transition metal (hereinafter simply referred to as “transition metal”), and this is heat-treated in a reducing gas stream containing a sulfur compound.

[0011] Zeolites are well known, the general _{formula: xM 2 / n O · Al} 2 O 3 · crystalline aluminosilicate ySiO represented by _{2 (M: Na, K,} C
(a is a metal such as Ba, n is a valence number, x and y are positive numbers) and the pore size and the like in the crystal structure are different depending on the difference of M, x and y, and their cation exchange ability and molecular sieving action are utilized. 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 with a fineness of about 5 to 10 ° slightly larger than the diameter of the NOx, CO and HC molecules to be purified. It is preferable to use one having a pore size. In the present invention, first, Cu, Co,
A metal ion in zeolite is ion-exchanged with a transition metal such as Ni or Fe. This ion exchange can be performed, for example, by treating zeolite with a metal ion solution.

Next, the zeolite ion-exchanged with the transition metal is heat-treated in a reducing 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 a transition metal ion in a reducing gas stream containing a sulfur compound according to the present invention, copper ion is used as a transition metal and this is heat-treated with a hydrogen sulfide-containing gas. The method for producing the catalyst according to the present invention will be described below with reference to FIG. The same applies to other transition metals.

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, 1,000 to 10,000 ppm
And the temperature of the electric furnace is raised by the heater 5 (the temperature is not particularly limited as long as the produced sulfide is stable, but usually at a temperature of about 500 to 700 ° C. and 3 to 12 ° C.). Time), copper oxide clusters and the like oxidized by adhering to sites other than the ion exchange point by ion exchange are sulfided to form copper sulfide clusters and the oxidation activity is lost. In this operation, the copper ion substituted at the ion exchange point of the zeolite is stably present and remains without being sulfurized (weak adsorption of hydrogen sulfide). In FIG. 1, reference numeral 6 denotes a thermocouple.

Copper sulfide clusters are formed on the surface of the zeolite catalyst obtained above, and the copper sulfide clusters are dissolved in glycol (for example, a pure liquid such as ethylene glycol or propylene glycol). On the other hand, ion-exchanged copper ions dissolve in acid or strong alkali solutions but do not dissolve in non-electrolyte solutions such as glycols, so this property is used to selectively wash only copper sulfide clusters near the surface. can do. The temperature for washing with glycol is not particularly limited, but it can be preferably carried out at 120 to 170 ° C. The washing with glycol can be performed, for example, by circulating the glycol from a tank 9 holding the glycol by a pump 10 through a container 7 filled with a zeolite catalyst 6 treated with a sulfur compound, as shown in FIG. Washing can be performed more efficiently by repeating the washing several times with different glycols. After washing, a desired catalyst can be obtained by calcining according to a conventional method.

[0017]

According to the knowledge of the present inventors, as described above, the conventional catalyst is used under actual operating conditions or NO in the exhaust gas regulation mode.
x The purification rate was low. This has high oxidizing ability of HC,
It was confirmed that the NOx purification rate was not improved because waste was consumed. When the exhaust gas temperature is high, the NOx purification ability of the lean NOx catalyst is considerably reduced. This is because the oxidation of HC components directly to CO ₂ is promoted at high temperatures.

On the other hand, with respect to durability, it was confirmed that, for example, in the case of a zeolite catalyst substituted with copper ions, copper oxide clusters adhering to the surface accelerated deterioration. The copper oxide cluster reacts with the zeolite skeleton, for example, to form copper aluminate and destroy the zeolite skeleton. When copper is ion-exchanged by the current ion-exchange method, copper ions in the solution are not only ion-exchanged but also form copper oxide clusters on the surface.

While the ion exchange rate is low, the formation of copper oxide clusters is relatively small, but the durability is worse than that of the one with much ion exchange. This is because while the ion exchange rate is low, the stability of the ion-exchanged copper ions is low, and the catalyst easily moves and aggregates in the zeolite during (use) operation of the catalyst to form copper oxide clusters, which accelerates the deterioration. This is because that. Therefore, in order to improve the durability, it is necessary to exchange copper ions at all ion exchange points to stabilize the copper ions and not to form copper oxide clusters on the surface.

According to the present invention, an oxide such as a copper oxide cluster is first sulfurized to form, for example, a copper sulfide cluster.
This copper sulfide cluster is dissolved in glycol and selectively washed and removed, so that pore blocking due to the copper sulfide cluster is prevented, active points inside the pores can be effectively used, and the NOx purification rate is improved. Can be done.

[0021]

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) Preparation method of zeolite catalyst A powder of ZSM-5 type zeolite (manufactured by Tosoh Corporation) having a Si / Al ratio of 40 and a pore size of 5.5% was prepared by dispersing a powder in an aqueous solution of 0.01 N copper acetate. It was immersed at room temperature for a day to obtain a zeolite in which copper ions were exchanged. On the other hand, alumina sol and silica sol were mixed so that the Si / Al ratio was 40 to obtain a slurry binder,
To 70 parts by weight of the binder, 100 parts by weight of the zeolite exchanged with copper ions and 100 parts by weight of water were added and mixed, and ammonia water (diluted) was adjusted to a pH of 7.0 to 8.6.
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).

[0023] (b) placed in a catalyst A obtained in the sulfur compound-treated zeolite catalysts A as a catalyst 1 in an electric furnace 2 shown in FIG. 1, the hydrogen sulfide bomb H ₂ S 1000 ppm gas (N
₍ A gas diluted by ₂ ) was passed in an amount sufficient to sulphide the entire amount of copper adhering to portions other than the ion exchange point of the catalyst. The electric furnace 2 is prepared so that the outlet exhaust gas temperature is about 600 ° C.
The reaction was performed for about 4 hours. In this way, the copper (actually, a copper oxide cluster) adhered to a portion other than the ion exchange point by ion exchange was sulfided, and a sulfurization treatment catalyst (hereinafter, referred to as catalyst B) was obtained.

(C) Glycol washing of catalyst B for sulfidation Using ethylene glycol from which water has been removed by distillation, sulfuric acid adhering to catalyst B is passed through catalyst B which has been subjected to H ₂ S treatment by the above method in the washing apparatus shown in FIG. The copper cluster was dissolved. The washed catalyst B was taken out, and fresh ethylene glycol was replaced in the glycol tank 9 again for secondary washing. After the second washing, drying under reduced pressure was performed for about 3 hours, and then drying was performed at 150 ° C. in N ₂ for 1 day. Further, the catalyst was calcined at 600 ° C. in air to obtain a catalyst (hereinafter referred to as catalyst C). In this way, copper ions were ion-exchanged at the ion exchange points inside the zeolite, and copper sulfide clusters as surface impurities were removed.

Example 2 (Preparation Example) A cobalt ion-exchanged zeolite catalyst (catalyst D), a sulfidation catalyst (catalyst E) and a catalyst E were prepared in exactly the same manner as in Example 1 except that cobalt acetate was used instead of copper acetate. A glycol-washed catalyst (catalyst F) was prepared.

Example 3 (Evaluation Example) Catalysts A, B, D and E prepared in Examples 1 and 2 (Control Example)
The purification activity of these catalysts was evaluated in 10 modes using C and F (Examples of the present invention) (catalyst capacity: 0.71 liter, vehicle: 1.6 liter lean burn, running mode: 10
mode). The results were as shown in Table 1 below.

[0027]

[Table 1]

As is evident from the results in Table 1, in the case of the catalyst B or E, compared with the catalyst A or D, the purification rate of HC in the 10 mode was significantly reduced, and the oxidizing ability was clearly reduced. Further, although the purification rate of HC in the catalysts C or F is still smaller than that in the catalysts A or D, the degree of the reduction is not as large as that of the catalysts B or E, and the purification rate of NOx is greatly improved. When comparing catalyst C or F with catalyst B or E, catalyst C or F
Have significantly improved both the HC purification rate and the NOx purification rate. The catalyst C or F has a high activity even if the capacity is small. In the catalyst B or E, inactive copper sulfide exists near the surface, but in the catalyst C or F, since it is removed, the inside of the pores is reduced. It seems that the ion exchange point has been effectively used.

Since a specific HC component remains in the catalyst C, the exhaust gas is cracked by an oxidation catalyst in advance to reduce the HC component, and then the lean NOx catalyst of the present invention is installed. Rates have risen further.

[0030]

According to the present invention, as described above,
A zeolite catalyst ion-exchanged with a transition metal selected from the group 1B and iron such as Cu or Co is heat-treated in a gas stream containing a sulfur compound such as hydrogen sulfide, for example, at a site other than the ion exchange point of the zeolite. The oxides of transition metals such as copper oxide clusters are exchanged for sulfides such as copper sulfide clusters, and then the pores closed by the copper sulfide clusters for washing and dissolving and removing the sulfides and the like with glycol. The active points in the area can be used effectively, and the purification rate of NOx can be improved by the increase in the active points.

[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 a transition metal in an air stream containing a sulfur compound according to the present invention.

FIG. 2 is a drawing showing an example of an apparatus for washing a zeolite catalyst treated with a sulfur compound with glycol.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Catalyst 2 ... Electric furnace 3 ... Sulfur-hydrogen cylinder 4 ... Sulfur-hydrogen containing gas 5 ... Heater 6 ... Thermocouple 7 ... Catalyst 8 ... Container 9 ... Glycol tank 10 ... Pump

Claims (1)

(57) [Claims] An exhaust gas purifying catalyst comprising a zeolite catalyst ion-exchanged with a transition metal selected from the group 1B and the iron group, heat-treated in a reducing gas stream containing a sulfur compound, and then washed with glycol.