JPH11156209A - Waste gas purifying device and production of the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a waste gas purifying device simplified in a process for forming a catalyst-carrying layer and large in the sticking quantity and the producing method. SOLUTION: In the waste gas purifying device having the carrying layer and a catalyst carried on the carrying layer on the surface of a honeycomb base body partitioned by a straight wall and having >=400 pieces of cell spaces per 1 in<2> , the honeycomb is dipped into an alkaline solution having >=pH 13 before the formation of the inorganic material-carrying layer to have 0.25-0.5 g carrying layer per cell space per 1 liter honeycomb in one dipping.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an exhaust gas purifying apparatus for purifying nitrogen oxides in exhaust gas discharged from an internal combustion engine such as an automobile engine and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, in the course of resource saving and environmental protection, there is a social demand for operating a gasoline engine for a vehicle by lean burn. Along with this, development of a catalyst (lean NOx catalyst) for effectively purifying nitrogen oxides (NOx) in exhaust gas containing oxygen discharged from a lean burn engine has been promoted.

[0003] As to an exhaust gas purifying catalyst for a lean burn engine, for example, JP-A-6-31139 discloses a catalyst in which an alkali metal oxide and platinum are supported on a porous carrier.

Japanese Patent Application Laid-Open No. Hei 8-24643 discloses that a porous carrier comprises at least one selected from the group consisting of platinum, palladium and rhodium, and at least one selected from alkali metals, alkaline earth metals and rare earth metals. Are shown. In JP-A-8-24643, as a specific example of a catalyst material, a catalyst containing at least one of platinum, palladium and rhodium, cerium and barium, and further carrying La, Zr, Li, K, Mg, etc. Shows the catalyst.

[0005] An exhaust gas purifying apparatus using a catalyst is mounted on an automobile. In the apparatus, a layer supporting a catalyst component is provided on a honeycomb-shaped substrate to increase a reaction area between a catalyst and exhaust gas. For the support layer, an inorganic substance such as boehmite, alumina, or zeolite is used. The method of forming the support layer is a method in which the inorganic substance is adjusted to a slurry state, applied to the honeycomb, dried and fired, and then impregnated with the catalyst component in the form of a slurry. There is.

[0006]

In order to form the carrier layer, a method of dispersing an inorganic substance constituting the carrier layer in water or the like, forming a slurry, and applying the slurry to the honeycomb is used. This is because of the shape of the honeycomb forming the carrier layer. In addition, the processing method for each cell is inefficient from the viewpoint of productivity, and simultaneous processing is required. Therefore, a honeycomb supporting layer is formed using a slurry, but the only way to increase the thickness of the supporting layer is to increase the slurry viscosity. However, if the slurry viscosity is increased, the grids of the honeycomb are clogged. When clogging is used in a purification device, it must be avoided because it leads to a reduction in a reaction area and a reduction in purification performance. FIG. 3 shows a conventional process of forming a carrier layer on a honeycomb. In order to avoid clogging of the honeycomb and increase the thickness of the carrier layer, it is necessary to use a low-viscosity slurry that does not cause clogging, dry the slurry, and repeatedly apply and dry the slurry until a predetermined coating amount is reached. This contributes to an increase in the number of steps in the production of a honeycomb for a catalyst, and it has been required to apply a predetermined amount in a short time when forming a support layer.

An object of the present invention is to provide an exhaust gas purifying apparatus having a large amount of a carrier layer attached thereto and a method for producing the same.

[0008]

According to the present invention, a support layer and a catalyst supported on the support layer are provided on a surface of a honeycomb substrate having at least 400 cell spaces per square inch defined by straight walls. An exhaust gas purifying apparatus having the above-mentioned supporting layer, wherein the supporting layer has a volume of
0.25 to 0.5 per layer and per cell space
g in the exhaust gas purifying apparatus.

According to the present invention, a support layer made of an inorganic substance and a catalyst supported on the support layer are formed on a surface of a honeycomb substrate having preferably at least 400 cell spaces per square inch defined by straight walls. A method for manufacturing an exhaust gas purifying apparatus, wherein an alkaline aqueous solution is attached to the surface of the honeycomb substrate, and then the slurry of the inorganic substance is attached,
Next, there is provided a method for manufacturing an exhaust gas purifying apparatus, wherein the support layer is formed by firing.

The carrier layer is made of an inorganic substance such as boehmite, alumina or zeolite. FIG. 2 shows a partial schematic view of a honeycomb section defined by straight walls.
21 is a honeycomb substrate, 22 to 24 are three supporting layers, 25
Is the cell space. Reference numeral 21 denotes a honeycomb of the exhaust gas purifying apparatus in which a large number of honeycomb bases are gathered. The honeycomb substrate is made of cordierite, Fe-Cr-Al alloy, or the like. The cross-sectional shape differs depending on the manufacturing method, and is a polygonal shape such as a triangle or a quadrangle. One to three carrier layers are formed on the inner surface as needed. The method of forming the support layer is to adjust the inorganic substance into a slurry, apply it to the honeycomb, dry it,
It is carried out by a method of impregnating the catalyst component which has been made into a slurry after firing, or a method of making the inorganic substance and the catalyst component into a slurry, coating the honeycomb, drying and firing. As shown in FIG. 2, the exhaust gas passing through the cell formed straight is purified by the action of a catalyst impregnated in or contained in the carrier layer.

In order to deposit a predetermined amount of the carrier layer by slurry application, it is necessary to satisfy the contradictory matters of reducing the viscosity of the slurry to prevent clogging and increasing the viscosity of the slurry to increase the adhesion. . Therefore, it was considered to increase the viscosity only in the vicinity of the honeycomb substrate while keeping the viscosity of the slurry itself low. If a material for increasing the viscosity of the slurry is applied to the surface of the honeycomb substrate, and then the honeycomb is immersed in the low-viscosity slurry, the vicinity of the honeycomb substrate is increased in viscosity and a layer is formed with a carrier layer component included in the slurry, By pulling up the honeycomb from the slurry, the low-viscosity slurry itself is discharged, and the amount of adhesion can be increased without causing clogging.

[0012] By immersing the honeycomb of the present invention in a supporting layer liquid whose composition has been adjusted so as to increase the viscosity with alkali after alkali treatment, an inorganic material layer supporting a catalyst can be easily formed in an amount of 100 g or more per liter of honeycomb. . The process of forming the carrier layer is simplified as compared with the related art. Further, the amount of the inorganic substance can be controlled by adjusting the concentration of the alkaline solution. In forming the catalyst supporting inorganic layer, it is preferable to use a liquid having a composition containing at least an inorganic substance such as alumina, boehmite, or zeolite and a hydroxide or a nitrate.

Preferably, the hydroxide uses at least one of aluminum hydroxide and magnesium hydroxide.
The nitrate is preferably magnesium nitrate. It is more effective if the solution contains ethyl alcohol. Before the step of forming the catalyst-supporting inorganic layer, a step of immersing the honeycomb in an alkaline treatment liquid having a pH of preferably 13 or more is provided. In the present invention, the honeycomb is a gasoline vehicle,
0x80mm x 160mm ellipse, 160mm long, 660cc
The diameter is preferably 80 mm and the length is preferably 80 mm, and preferably has a cell space of 400 cells or more per liter or 1 square inch.

The catalyst of the present invention is obtained by supporting a noble metal or a noble metal and an alkali metal oxide on the surface of a porous support made of an inorganic oxide.

It is desirable that the noble metal contains rhodium (Rh) and at least one selected from platinum (Pt) and palladium (Pd), and in particular, at least one selected from platinum (Pt) and palladium (Pd). It is desirable to contain one type and rhodium.

The alkali metal is sodium oxide (Na ₂
O), lithium oxide (Li ₂ O), potassium oxide (K
It is desirable that it be made of at least one selected from the group consisting of ₂ O) and bidium oxide (Rb ₂ O).

The supported amounts of the alkali metal and the noble metal are 2.5 to 27% by weight of the alkali metal, 0.05 to 0.3% by weight of the noble metal, and 0.05 to 0.3% by weight of the rhodium, based on 100% by weight of the carrier. It is desirable that platinum be 0.5 to 3% by weight and palladium be 0.5 to 15% by weight.

The carrier is desirably used in a state of being coated on a honeycomb substrate.

In the catalyst of the present invention, the one in which an alkali metal oxide is supported on a porous carrier and further a platinum metal is supported has a particularly high NOx purification rate.

The catalyst of the present invention is disposed in an exhaust gas passage of an internal combustion engine, and reduces and purifies NOx contained in exhaust gas having a lean air-fuel ratio by reaction with carbon monoxide or hydrocarbon. .

The catalyst of the present invention has a high purifying ability for NOx in exhaust gas burned lean because of the coexistence of an alkali metal, a high affinity for NOx is created, and NOx is produced on these surfaces. It is presumed that the NOx adsorbed on the NOx is reduced and the adsorbed NOx is reduced by the coexistence of the noble metal.

The NOx purification performance for lean exhaust gas is as follows:
If the catalyst is continuously exposed to the lean exhaust gas for a long time, it gradually decreases. This is because oxygen is contained in the lean exhaust gas, and carbon monoxide or hydrocarbon, which is a NOx reducing agent, is oxidized. Therefore, lean NOx
If it is considered that the purification capacity of the fuel cell has decreased, it is desirable to switch to a stoichiometric air-fuel ratio (stoichiometric) or an excess fuel (rich), burn the fuel for several seconds to several minutes in that state, and return to lean again.

For this reason, the nitrogen oxide purification rate of the catalyst of the present invention with respect to lean exhaust gas is measured, and if the measured value falls below a set value, the air-fuel ratio of the internal combustion engine is temporarily changed to the stoichiometric air-fuel ratio ( It is desirable to switch to (stoichiometric) or excess fuel (rich) and then return to lean again.

As another method, the NOx concentration in the exhaust gas purified by the catalyst of the present invention is measured, and if the measured value falls below a set value, the operation is switched to a stoichiometric air-fuel ratio or rich combustion for a certain period of time. Is preferred.

The catalyst of the present invention has a high ability to purify not only NOx but also hydrocarbons. However, it is desirable to arrange the catalyst of the present invention and the hydrocarbon combustion catalyst in the exhaust gas flow path in order to obtain a higher hydrocarbon purification ability.
As a combination method, it is preferable to arrange a hydrocarbon combustion catalyst in the exhaust gas flow path after the exhaust gas purification catalyst according to the present invention. As the hydrocarbon combustion catalyst, a three-way catalyst or a catalyst containing Pd as a noble metal component can be used.

Trace amounts of sulfur oxides contained in combustion exhaust gas from automobile internal combustion engines poison catalytically active components, particularly alkali metals. In the present invention, adsorption of sulfur oxides on alkali metals is suppressed by containing Ti having sulfur oxide poisoning resistance. In a reducing atmosphere, it is presumed that reduction and desorption of the adsorbed sulfur oxide occur via a noble metal adjacent to the alkali metal. Therefore, even if the performance of the catalyst of the present invention is temporarily reduced due to sulfur oxides, it is possible to use a reducing atmosphere, i.e.
By heating the catalyst to 00 ° C to 800 ° C, removal of sulfur oxides and reactivation of the catalyst become possible. When the catalyst activity decreases, it is preferable to re-activate the catalyst by the reactivation.

The catalyst of the present invention is also effective for treating exhaust gas discharged from a diesel engine of a diesel vehicle. Diesel engines are operated at a high air-fuel ratio with excess oxygen, and the catalyst of the present invention exhibits excellent activity even in the presence of oxygen, so that even the exhaust gas discharged from the diesel engine can efficiently purify nitrogen oxides. can do.

The catalyst of the present invention has a temperature of 200 ° C. or more and 600 ° C.
Excellent activity in the following temperature range, especially 250 ° C
It has high activity in the temperature range of ~ 500 ° C. Therefore, it is desirable to set the temperature at which the catalyst and the gas stream are brought into contact, that is, the reaction gas temperature, within the above-mentioned temperature range.

[0029]

DESCRIPTION OF THE PREFERRED EMBODIMENTS (Experimental Example) Specifically, a change in viscosity of a liquid due to pH was used. FIG. 4 shows the relationship between aqueous solution viscosity and pH.

The solid content concentration is expressed as solid content weight / (solid content weight +
(Liquid weight). In a liquid in which alumina is dispersed in water, the higher the solid content concentration, the higher the viscosity, and the higher the alkali content, the higher the viscosity. This tendency becomes more remarkable when a hydroxide is added and an alcohol is added to the dispersion medium. Figure 5 shows 0.14M hydroxide
(Hereinafter M = mol / dm ³ ) is a relationship between the liquid viscosity and the pH when a dispersion medium in which the ratio of water to ethyl alcohol is 1: 1 is used. PH 8 and pH 1 of the liquid shown in FIGS.
Table 1 shows the viscosity of No. 2. Addition of an alcohol and a hydroxide can increase the difference in viscosity between the neutral region and the alkaline region.

[0031]

[Table 1]

FIG. 6 shows the relationship between the amount of aluminum hydroxide and the viscosity and pH. Sodium hydroxide was added to change the pH. The pH at a sodium hydroxide concentration of 0.2 M is 8.2-1.
0.5. The effect of increasing the viscosity with alkali is observed by adding 0.01 M of aluminum hydroxide. Similar effects were obtained by adding magnesium hydroxide instead of aluminum hydroxide or by adding both aluminum hydroxide and magnesium hydroxide. Further, the same effect is obtained when magnesium nitrate is added.

The formation of the carrier layer on the honeycomb utilizing the phenomenon of increasing the viscosity in the alkaline region is performed in the step shown in FIG. After the honeycomb is immersed in the alkaline liquid, the residual liquid in the honeycomb cells is blown off. It is immersed in the supporting layer forming liquid to blow off residual liquid in the honeycomb cells. Then, it is dried and fired. When the honeycomb is immersed in the carrier liquid, the alkaline liquid adhering to the honeycomb surface increases the viscosity of the carrier liquid and easily adheres to the honeycomb surface. It is desirable that the carrier layer forming liquid has a low viscosity in a neutral region and a high viscosity in an alkaline region.
For this reason, in Table 1, when the dispersion medium is only water, the viscosity in the neutral region (pH 8) is low and the honeycomb is hardly clogged, but the viscosity in the alkaline region (pH 12) is low and a sufficient amount of alumina can be obtained. I can't. In a liquid obtained by adding a hydroxide to a dispersion medium of water and ethanol, the solid content concentration is 35%, the viscosity in the neutral region (pH 8) is 35 cps, and the alkali region (pH 12).
Is 400 cps, the honeycomb is hardly clogged, and a practical amount of alumina is obtained. At a solid concentration of 40%, the viscosity in the alkaline region is 2000 cps
Therefore, alumina is more likely to adhere.

Table 2 shows the pH when the concentration of the alkali treatment was changed from 0.1 to 10M. When the concentration of sodium hydroxide was 5 M or more, it exceeded the measurement range of the pH meter. FIG. 7 shows the amount of adhesion when the liquids 5 and 6 in Table 1 were used as the carrier layer forming liquid and the concentration of the alkali treatment was changed. At pH 13 or more, the effect of attaching alumina is confirmed. FIG. 8 shows the concentration of the alkali treatment liquid and the amount of alumina attached. With liquid No. 6, when the alkali treatment is performed with a liquid having a sodium hydroxide concentration of 1 M, an adhesion amount of about 100 g can be obtained per liter of honeycomb. In this manner, the amount of the carrier layer attached can be controlled by adjusting the concentration of the alkali treatment liquid. Under the conditions shown in FIGS. 7 and 8, no clogging occurred in the honeycomb.

[0035]

[Table 2]

Example 1 Commercially available boehmite was heated to 600 ° C.
After baking for 2 hours, pulverize for 30 minutes with a grinder,
1311.5 g of boehmite powder classified by sieving to 50 μm or less, 1000 ml of pure water, 1000 ml of ethyl alcohol, and 21.8 g of aluminum hydroxide were wet-kneaded for 2 hours to prepare an inorganic slurry. Solids concentration is 40%
Met. A cordierite honeycomb having a volume of 1.7 liters was immersed in an aqueous solution having a sodium hydroxide concentration of 5 M, and then air blown to remove the residual liquid in the cells. After the honeycomb was immersed in an inorganic slurry only once to form a carrier layer, the slurry was discharged, air blown to remove the residual liquid in the cells, dried and fired at 600 ° C. to produce a honeycomb with a carrier layer. The coating amount of the carrier layer at this time was 120 g per liter of honeycomb. This is 0.3 g per cell space.

By immersing the honeycomb in the alkaline solution, it was possible to deposit 100 g or more per liter of the honeycomb by immersing the honeycomb once in the inorganic slurry.

The honeycomb in this embodiment has a cell space of 400 cells per square inch, the thickness of a straight wall is 0.2 mm, and the space is 1.1 mm square. Length is
160 mm.

Example 2 A cordierite honeycomb having a volume of 1.7 liters was immersed in an aqueous solution having a sodium hydroxide concentration of 10 M, and then immersed once in an inorganic slurry in the same manner as in Example 1 to form a carrier layer. The coating amount of the support layer was 160 g per liter of honeycomb. This embodiment also has a 400-cell space per square inch and weighs 0.4 g per cell space.

The coating amount could be adjusted by changing the concentration of the alkali treatment liquid.

Example 3 After immersing the honeycomb in an alkaline solution in the same manner as in Example 1, a slurry composed of alumina powder, magnesium nitrate and aluminum hydroxide was applied to a cordierite honeycomb (400 cells / inc ² ). After coating, drying and firing, the apparent volume of the honeycomb 1
An alumina-coated honeycomb coated with about 160 g of alumina per liter was obtained. Next, the alumina-coated honeycomb was impregnated with a sodium nitrate solution, dried at 200 ° C., and fired at 700 ° C. for 1 hour. Furthermore, a mixed solution of a dinitrodiammine Pt nitric acid solution and a Rh nitrate solution was impregnated, dried at 200 ° C., and baked at 450 ° C. for 1 hour. Finally, it was impregnated with a Mg nitrate solution, dried at 200 ° C, baked at 450 ° C for 1 hour, and then baked at 700 ° C for 5 hours. From the above, based on 100% by weight of alumina,
1.2% by weight of Na was loaded, followed by 1.6% by weight of platinum and 0.15% by weight of Rh.

FIG. 9 is a conceptual diagram of a fuel injection type automobile gasoline engine using the catalyst of this embodiment.

The air mixed with gasoline in the intake pipe 8 burns in the cylinder by electric ignition. The exhaust gas generated by the combustion is discharged outside the system through the exhaust pipe 19 and the exhaust gas purifying catalyst 20 of the present invention. By controlling the fuel injection valve 13 and the ignition device in the control unit 15, the combustion state in the cylinder is controlled to an arbitrary state of a stoichiometric air-fuel ratio (stoichiometric), an excess fuel state (rich), and an excess air state (lean). Is done.

Here, the exhaust gas discharged from the engine 7 contains harmful components such as HC, CO, NOx and the like. Therefore, these harmful components must be detoxified and then discharged out of the system. No.

For this purpose, an exhaust gas purifying catalyst 20 for purifying exhaust gas by using a catalytic action is provided in the exhaust pipe 19. According to the present invention, lean exhaust gas can be newly purified in addition to the conventional stoichiometric rich combustion exhaust gas purification, and the combustion state of the combustion system shown in FIG. 9 can be arbitrarily set. In addition, the combustion system shown in FIG. 9 can be operated stably by improving the heat resistance and the SOx poisoning resistance. In FIG. 9, reference numerals 1 and 2 denote an air cleaner, an intake port 2, a throttle valve 5, a fuel tank 9, a fuel pump 10, a fuel pump 11, a fuel damper 12, a fuel filter 12, a power distribution device 16, respectively. Reference numeral 18 denotes a throttle sensor.

The above-described catalyst was arranged in the exhaust passage of a vehicle equipped with a lean burn engine (1.6 liter), and the vehicle was driven in a city driving mode, and the purification rates of CO, HC and NOx were measured. The initial purification rate of the catalyst of this embodiment is 100% CO, H
C was 96% and NOx was 92%.

[0047]

According to the present invention, the step of forming the carrier layer is simplified, and the coating amount of the inorganic substance can be controlled by adjusting the alkali solution concentration.

[Brief description of the drawings]

FIG. 1 is a view showing a process of forming a carrier layer on a honeycomb according to the present invention.

FIG. 2 is a partial schematic view of a honeycomb.

FIG. 3 is a view showing a conventional process of forming a carrier layer on a honeycomb.

FIG. 4 is a graph showing the relationship between the viscosity and the pH of an aqueous alumina dispersion solution.

FIG. 5 is a graph showing the relationship between the pH and the viscosity of a hydroxide-added alumina-dispersed ethanol aqueous solution having different solid contents.

FIG. 6 is a graph showing the relationship between the hydroxide concentration and the viscosity of an aqueous solution of ethanol dispersed in alumina.

FIG. 7 is a diagram showing the relationship between the pH of an alkali treatment liquid and the amount of alumina attached.

FIG. 8 is a diagram showing the relationship between the concentration of an alkali treatment solution and the amount of alumina attached.

FIG. 9 is a conceptual diagram of a fuel injection type automobile gasoline engine.

[Explanation of symbols]

7 ... engine, 8 ... intake pipe, 13 ... fuel injection valve, 15 ...
Control unit, 19: exhaust pipe, 20: exhaust gas purification catalyst, 21: honeycomb substrate, 22, 23, 24: catalyst support layer, 25: void.

Claims (2)

[Claims] 1. An exhaust gas purifying apparatus comprising: a support layer on a surface of a honeycomb substrate having at least 400 cell spaces per square inch defined by straight walls; and a catalyst supported by the support layer, The exhaust gas purifying apparatus according to claim 1, wherein the supporting layer has a honeycomb having a volume of 1 liter and has a layer thickness of 0.25 to 0.5 g per one cell space. 2. A method for producing an exhaust gas purifying apparatus comprising: forming a support layer made of an inorganic substance and a catalyst supported by the support layer on a surface of a honeycomb substrate having a cell space defined by straight walls; After attaching the alkaline aqueous solution to the substrate surface, attaching the slurry of the inorganic substance,
A method for producing an exhaust gas purifying apparatus, comprising forming the carrier layer by firing.