JPH11123330A - Exhaust gas cleaning catalyst and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a novel catalyst which can improve the efficiency of a catalyst metal to reduce NOx in the exhaust gas of an internal combustion engine. SOLUTION: In the catalyst, at least one kind of element selected from the group consisting of iridium, platinum, rhodium, palladium, silver, cobalt, copper, nickel, iron, magnesium, and lanthanoid, in catalytically active quantities, is carried on and/or in carrier particles comprising metal oxides and/or composite oxides in an evenly dispersed state.

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 reducing NOx emitted from an internal combustion engine such as a diesel engine and a method for producing the same, and more particularly, to NOx in a normal exhaust gas temperature range of about 350 to about 500.degree. The present invention relates to the above-mentioned catalyst and a method for producing the same, which can reduce the amount of the catalyst with high efficiency.

[0002]

2. Description of the Related Art As a catalyst for reducing nitrogen oxides (NOx) contained in exhaust gas from an internal combustion engine, for example, a diesel engine together with excess air, a zeolite having exchanged ions of transition metals, for example, copper ions, is used. That is, a copper-zeolite (Cu-ZSM-5) catalyst has been proposed (see, for example, JP-A-63-283727). However, this type of zeolite catalyst tends to deteriorate over time under the influence of water vapor and sulfur compounds coexisting in the exhaust gas and the heat of the exhaust gas, and is considered to be difficult to put into practical use in terms of durability. Can be Furthermore, since the optimum activation temperature for nitrogen oxide purification is as high as 400 to 500 ° C., the nitrogen oxide purification action is extremely reduced when the exhaust gas temperature is low, such as when the engine is started or at low speed. However, even in the above-mentioned optimum activation temperature range, the NOx reduction rate is not at a sufficiently satisfactory level.

The applicant of the present application has previously proposed in Japanese Patent Application No. 8-347376 (December 26, 1996) a catalyst for purifying exhaust gas in which zeolite particles and noble metal particles are mixed. This catalyst is obtained by adding carbon particle powder to an aqueous solution of a noble metal, concentrating the resulting mixture, and subjecting the mixture to a reduction treatment to precipitate the noble metal on the carbon particles, adding zeolite particles thereto, and further adding water and an inorganic material. Adding a binder to form a slurry, applying the obtained slurry to a substrate, drying, and further firing to burn off the carbon particles and to carry the zeolite particles and the noble metal particles on the substrate in a dry state. Can be produced by a slightly more complicated method.

[0004] The applicant of the present invention has filed Japanese Patent Application No. 8-26147.
No. (Oct. 20, 1996) also proposed an exhaust gas purifying catalyst in which a coating of a mixture comprising platinum group metal particles, a metal oxide powder carrier, and a binder was applied to a porous ceramic substrate. . This catalyst can be produced by forming a slurry with platinum group metal particles (fine powder), metal oxide powder, binder and water, applying the slurry to the surface of a porous substrate (for example, honeycomb), and firing.

[0005]

DISCLOSURE OF THE INVENTION The inventors of the present invention have eagerly aimed at effectively utilizing components such as precious metals as active components of an exhaust gas purifying catalyst, further improving the activity of a catalyst, and simplifying a method for producing a catalyst. As a result of repeated studies and studies, an aqueous solution of a water-soluble compound of several specific elements selected from various elements and a carrier powder composed of a metal oxide and / or a composite oxide are intimately and uniformly mixed and dried. Then, the carrier is loaded with 0.1 to 10% by weight of the specific element (in terms of metal) based on the weight thereof (if necessary, the above mixing and drying are repeated to achieve the required loading of the specific element). In a relatively simple manner, the dried mixture is reduced in a reducing gas stream containing hydrogen gas at an elevated temperature and then treated in a nitrogen gas stream at an elevated temperature, in a relatively simple manner in the particles of the carrier and / or Catalyst in which the specific elements are attached uniformly dispersed was found that obtained. Further, it has been found that the catalyst obtained in this way can reduce NOx with high efficiency at an exhaust gas temperature of 350 to 500 ° C. In this catalyst, the specific element is not attached to the carrier in the form of particles (microscopic observation), but is attached or bound in a state of being uniformly dispersed in the carrier in the form of atoms or clusters of atoms. It is believed that the existence state and the dispersion state of the specific element contribute to the high efficiency reduction of NOx.

Accordingly, the present invention provides a method in which a catalytically active amount of a specific selected metal element is supported on and / or in carrier particles comprising a metal oxide and / or a composite oxide in a state of being uniformly dispersed. An object of the present invention is to provide an exhaust gas purifying catalyst and a method for producing the same.

The present invention further provides the exhaust gas purifying catalyst described above,
It is another object of the present invention to provide a catalyst device formed into a pellet or a multi-tube through-hole honeycomb or a catalyst device coated on the surface of a refractory honeycomb substrate so that it can be mounted on an engine exhaust system.

[0008]

Thus, the present invention provides at least one element selected from the group consisting of iridium, platinum, rhodium, palladium, silver, cobalt, copper, nickel, iron, magnesium and lanthanide-based elements. Exhaust gas purification by uniformly dispersing and adhering in a carrier particle composed of a metal oxide and / or a composite oxide in an amount of 0.01 to 10% by weight based on the weight of the carrier particle Provide a catalyst.

Further, the present invention relates to iridium, platinum, rhodium, palladium, silver, cobalt, copper, nickel,
An aqueous solution of a water-soluble compound of at least one element selected from the group consisting of iron, magnesium and lanthanide elements is prepared, and the weight of the carrier relative to the carrier composed of powdered metal oxide and / or composite oxide Applying the aqueous solution in an amount to supply 0.01 to 10% by weight (in terms of metal) of the at least one element based on the above, homogeneously mixing and drying the homogeneous mixture to substantially remove free water. The dried mixture is treated at an elevated temperature in a stream containing hydrogen gas and subsequently at an elevated temperature in a stream of inert gas so that the at least one element is homogeneously and / or in the carrier particles. The present invention provides a method for producing an exhaust gas purifying catalyst dispersed and adhered to a catalyst.

The specific element is preferably a group consisting of iridium, platinum, silver, cobalt, copper, iron, samarium, praseodymium, europium, dysprosium and erbium, more preferably iridium, platinum, cobalt, copper, It is selected from the group consisting of samarium and praseodymium.

Examples of the water-soluble iridium compound used in the method of the present invention are typically iridium chloride (iridium trichloride, iridium tetrachloride and hydrates and complexes thereof) and iridium sulfate. Water-soluble compounds of the catalytically active metal element used in the method of the present invention other than the above-mentioned water-soluble iridium compounds, that is, water-soluble compounds of platinum, rhodium, palladium and lanthanide-based elements are well known, and generally, Nitrates, such as rhodium nitrate,
Cobalt nitrate as well as hydrates thereof can be used.

Examples of metal oxides and composite oxides used as carriers in the present invention include silica (SiO ₂ ), alumina (Al ₂ O ₃ ), zirconia (ZrO ₂ ), zeolites (ZSM-5), Iron (Fe ₂ O ₃ ), cobalt oxide,
Examples include gallium oxide, manganese oxide, and a composite oxide containing Fe (eg, ferrite). Preferred carriers include chromium oxide, cobalt oxide, manganese oxide, and composite oxides containing Fe. These can be used alone or as a mixture of two or more. The carrier composed of these metal oxides and / or composite oxides is used in the form of a powder, and the particle size is not critical, but is preferably several hundred microns or less, for example, in the range of 0.1 to 50 microns. is there.

The application of a water-soluble compound of a specific selected metal element to a support composed of a metal oxide and / or a composite oxide can be performed by converting an aqueous solution of the water-soluble compound into a powdery metal oxide and / or a composite oxide. A slurry having a relatively high viscosity is added to the mixture, and the slurry is mixed or kneaded using a mixer until the slurry becomes homogeneous, and the material mixture is mixed using an evaporator (and further using a drying furnace if necessary). Several hours to several tens of hours at a temperature of 110 ° C., for example, 1
Dry for 10 to 10 hours (under reduced pressure if necessary) to substantially remove free water. If the required amount of the specific element is not adhered to the carrier in one operation, the application of the required concentration of the specific element can be achieved by repeating the above mixing (kneading) and drying operations.

Next, the dried homogeneous mixture of the specific element compound and the carrier is mixed with a hydrogen gas-containing gas, for example, about several percent or less.
Preferably 2,000 ppm to 10,000 ppm of H
In N ₂ gas containing _2, about 600 to 900 ° C., preferably at a temperature of about 5 to 10 hours to about 700 ° C., for example treated for 8 hours, subsequently the inert gas, typically in an N ₂ gas The treatment is performed at a temperature of about 600 to 900 ° C. for about 5 to 10 hours. Through this series of treatments, the specific element compound goes through several oxidation states to become metallic (for example, trivalent → 2
(Valent → monovalent → zero)), and the dispersion or distribution as an atomic state or an aggregate of atoms (cluster) on the carrier is stabilized.

If the amount of the specific selected element (in terms of metal) in the obtained catalyst is less than 0.01% by weight, a satisfactory NOx reduction effect cannot be obtained, and if it exceeds 10% by weight, the NOx reduction effect is further improved. do not do.

Since the catalyst obtained by the above treatment is generally in the form of agglomerates, it is lightly pulverized, that is, pulverized to obtain a powdery catalyst.

Examination of the catalyst by X-ray diffraction confirms the presence of a particular element. In addition, when observed with a normal optical microscope, although the carrier particles are observed, the introduced metal particles of the specific element are not visible, and therefore, it is presumed that they exist as atoms or clusters of atoms. Conceivable.

The above-mentioned powdery catalyst of the present invention is a catalyst device formed into an appropriate molded body, for example, a pellet or a multi-tube through-hole honeycomb, for mounting in an exhaust system of an engine.
Alternatively, or more practically, an aqueous slurry of the powdered catalyst is made of a refractory ceramic or metal (eg, stainless steel).
It is a catalyst device coated on the surface of the honeycomb made of. Generally, aqueous slurries having appropriate viscosities are used for the above-mentioned molding and coating, and at this time, a binder (eg, alumina sol, silica sol) is used to improve the physical strength of the molded article or coating. It is preferred that The techniques for forming and coating are well known to those skilled in the art and need not be described in further detail.

The catalyst of the present invention exhibits a high NOx reduction rate of about 30% to 46% in a wide temperature range of about 350 to 500 ° C., and shows a considerable NOx reduction rate outside the above temperature range, which is practically significant. It has excellent exhaust gas purification properties and maintains high catalytic activity for a long period of time.

The present invention will be described more specifically with reference to the following examples (and comparative examples).

Example 1 Preparation of Ir / SiO ₂ catalyst 50 parts by weight of a 10% by weight aqueous solution of iridium trichloride (IrCl ₃ = 298.58) was added to 100 parts by weight of SiO ₂ powder having an average particle size of 38 μm. Mix well with a mixer until homogeneous. This homogeneous mixture was dried at 100 to 105 ° C. for 10 hours using an evaporator. The dried mixture was placed in N ₂ gas containing 5000 ppm of H ₂ for 70 hours for 8 hours.
Treated at 0 ° C. and then at 700 ° C. for 8 hours in N ₂ gas. The heat-treated product is allowed to cool, and then the raw material SiO ₂
It was ground to the average particle size of the carrier. As a result of analysis, the powder catalyst contained 2.7% by weight of iridium (in terms of metal).

Preparation of Catalyst Device for Mounting A mixture of 70 parts by weight of the above crushed catalyst, 20 parts by weight of a binder (alumina sol) and 100 parts by weight of water was stirred with a high-speed mixer to obtain a homogeneous slurry. Cylindrical honeycomb substrate made of cordierite (diameter 30 mm; volume 21
cc; cell number 400 cpi) is immersed in the above slurry, pulled up, and the excess adhered slurry is blown off with an air jet, dried (100 ° C. × 5 hours), and thus a catalyst coating having a layer thickness of 50 to 100 μm is formed on the honeycomb. It was formed to obtain a mounting catalyst device.

Example 2 Preparation of Ir / Al ₂ O ₃ catalyst According to the procedure of Example 1, a catalyst powder and a mounting catalyst were prepared by using Al ₂ O ₃ powder instead of the SiO ₂ powder of Example 1. The device was obtained.

Example 3 Preparation of Ir / Fe ₂ O ₃ Catalyst The catalyst powder and the mounting catalyst were prepared in the same manner as in Example 1 except that Fe ₂ O ₃ powder was used instead of the SiO ₂ powder of Example 1. The device was obtained.

Example 4 Preparation of Ir / ZrO ₂ catalyst Using ZrO ₂ powder instead of SiO ₂ powder of Example 1,
According to the operation of Example 1, a catalyst powder and a mounting catalyst device were obtained.

Example 5 Preparation of Ir / ZSM-5 catalyst Instead of the SiO ₂ powder of Example 1, zeolite ZSM-5 was used.
Using the powder, a catalyst powder and a mounting catalyst device were obtained according to the procedure of Example 1.

Example 6 Preparation of Ir / Cr ₂ O ₃ catalyst In the same manner as in Example 1 except that Cr ₂ O ₃ powder was used instead of the SiO ₂ powder, the catalyst powder and the mounting catalyst were mounted. The device was obtained.

Example 7 Preparation of Ir / Mn ₂ O ₃ catalyst A catalyst powder and a mounting catalyst were prepared in the same manner as in Example 1 except that Mn ₂ O ₃ powder was used instead of the SiO ₂ powder of Example 1. The device was obtained.

Example 8 Preparation of Ir / Co ₃ O ₄ catalyst A catalyst powder and a mounting catalyst were prepared in the same manner as in Example 1 except that Co ₃ O ₄ powder was used instead of the SiO ₂ powder of Example 1. The device was obtained.

Example 9 Preparation of Ir / CoFe ₂ O ₄ catalyst In the same manner as in Example 1, except that CoFe ₂ O ₄ powder was used instead of the SiO ₂ powder, a catalyst powder and a mounting catalyst were prepared. The device was obtained.

Example 10 Preparation of Ir / ZnFe ₂ O ₄ Catalyst The procedure of Example 1 was repeated, except that the ZnFe ₂ O ₄ powder was used in place of the SiO ₂ powder of Example 1. The device was obtained.

Example 11 Preparation of Ir—Co / Fe ₂ O ₃ catalyst An 8% by weight aqueous solution of iridium trichloride (IrCl ₃ = 298.58) based on 100 parts by weight of Fe ₂ O ₃ powder having an average particle size of 45 μm , cobalt nitrate _{(Co (NO 3) 2 ·} 6H
₂ O = 291.03) in 5% by weight aqueous solution
Add parts by weight and mix well with a mixer until homogeneous.
This homogeneous mixture was dried at 100 to 105 ° C. for 10 hours using an evaporator. The dry mixture, 5000P and H ₂
Treatment was performed at 700 ° C. for 8 hours in N ₂ gas containing pm, and then at 700 ° C. for 8 hours in N ₂ gas. The heat-treated product was allowed to cool, and then ground to the average particle size of the raw material Fe ₂ O ₃ carrier. As a result of analysis, it was found that 2.2% by weight
Of iridium (in terms of metal) and 1.3% by weight of cobalt (in terms of metal).

Example 12 Preparation of Ir-Sm / CoFe ₂ O ₄ catalyst [0033] Iridium trichloride (IrCl ₃ = 298.5) was used for 100 parts by weight of CoFe ₂ O ₄ powder having an average particle diameter of 35 microns.
8), a samarium nitrate (Sm (N
50 parts by weight of a 3% by weight aqueous solution of O ₃ ) ₃ .nH ₂ O) was added and mixed well by a mixer until homogeneous. This homogenous mixture is heated at 100-105 ° C in an evaporator for 1 hour.
Dried for 0 hours. The dry mixture, 5000Pp of H ₂
in an N ₂ gas was treated with 8 hours 700 ° C. containing m, then N ₂
Treated at 700 ° C. for 8 hours in gas. The heat-treated product was allowed to cool, and then pulverized to the average particle size of the raw material CoFe ₂ O ₄ support. As a result of analysis, 1.8% by weight of this powder catalyst was added.
Of iridium (in terms of metal) and 0.7% by weight of samarium (in terms of metal).

Example 13 Preparation of Pt-Sm / CoFe ₂ O ₄ Catalyst A 3% by weight aqueous solution of dinitrodiaminoplatinum was added to 100 parts by weight of CoFe ₂ O ₄ powder having an average particle diameter of 35 μm, and samarium nitrate (Sm (NO ₃ 10% by weight of ₃ · nH ₂ O)
Each 50 parts by weight of the aqueous solution was added and mixed well with a mixer until homogeneous. This homogeneous mixture was dried at 100 to 105 ° C. for 10 hours using an evaporator. The dry mixture was placed in N ₂ gas containing 5000 ppm of H ₂ for 700 hours for 8 hours.
It was treated with ° C., then treated with 8 hours 700 ° C. in N ₂ gas. The heat-treated product is allowed to cool, and then the raw material CoFe ₂
It was ground to an average particle size of the O ₄ carrier. As a result of the analysis, 1.0% by weight of platinum (in terms of metal) and 2.
It contained 7% by weight of samarium (in terms of metal).

Example 14 Preparation of Pt-Pr / CoFe ₂ O ₄ Catalyst A 100% by weight of CoFe ₂ O ₄ powder having an average particle diameter of 35 microns was treated with a 3% by weight aqueous solution of dinitrodiaminoplatinum, praseodymium nitrate (Pr (NO ₃ 50 parts by weight of a 10% by weight aqueous solution of ₃ · nH ₂ O) were added and mixed well by a mixer until homogeneous. This homogeneous mixture was dried at 100 to 105 ° C. for 10 hours using an evaporator. The dry mixture was placed in N ₂ gas containing 5000 ppm of H ₂ for 8 hours 7
Treated at 00 ° C., then at 700 ° C. for 8 hours in N ₂ gas. The heat-treated product is allowed to cool, and then the raw material CoF
It was ground to an average particle size of e ₂ O ₄ support. As a result of the analysis, the powder catalyst contained 1.1% by weight of platinum (in terms of metal) and 2.5% by weight of praseodymium (in terms of metal).

Example 15 Preparation of Cu-Sm / CoFe ₂ O ₄ catalyst A mixture of copper nitrate (Cu (NO ₃ ) ₂ .nH ₂ O) and 100 parts by weight of CoFe ₂ O ₄ powder having an average particle diameter of 35 μm was prepared. Wt% aqueous solution, samarium nitrate (Sm (NO ₃ ) ₃ .nH)
₂ O) 3 wt% aqueous solution of each 50 parts by weight were added and mixed well until homogenous with a mixer. This homogeneous mixture was dried at 100 to 105 ° C. for 10 hours using an evaporator. The dry mixture was treated at 700 ° C. for 8 hours in N ₂ gas containing 5000 ppm of H ₂ and then at 700 ° C. for 8 hours in N ₂ gas. The heat-treated product was allowed to cool, and then pulverized to the average particle size of the raw material CoFe ₂ O ₄ support. As a result of the analysis, the powder catalyst contained 2.1% by weight of copper (in terms of metal) and 0.8% by weight of samarium (in terms of metal).

COMPARATIVE EXAMPLE This comparative example relates to a catalyst in which iridium metal is present in the form of particles (visible by an optical microscope) on particles of a powdery Al ₂ O ₃ carrier.

A compounding ratio of iridium particles having a particle size of 1.0 μm and alumina powder having a particle size of 1.0 to 10 μm, wherein 2.7% by weight of iridium particles and 97.3% by weight of alumina powder. And mixed with alumina sol (binder)
And water in a weight ratio of mixture: alumina sol: water = 10
0: 20: 100, and sufficiently stirred to form a slurry. The porous honeycomb substrate made of cordierite of Example 1 having a porosity of 10% was immersed in the slurry, pulled up, and subjected to high pressure toward the substrate. Excess slurry was blown off by blowing air, and the substrate was dried at 100 to 150 ° C for 2 to 3 hours, and subsequently calcined at 500 to 600 ° C for 3 to 5 hours in an air atmosphere. The thickness of the coating layer of the obtained catalyst was 90 μm.

NOx Reduction Test The NOx reduction performance of the catalytic converters mounted on the honeycomb substrate obtained in each of the above Examples and Comparative Examples was measured under the following gas flow conditions using a simulated gas having the following composition. Simulated gas composition NO 1,000 ppm C ₃ H ₆ 1.360 ppm O ₂ 10% SO ₂ 20 ppm H ₂ O (steam) 4% (volume) Space velocity (SV) = 50,000 h ⁻¹ Temperature = 300 ° C.

The measured average NOx (actually, N
O) The reduction rate was as follows. Catalyst NOx reduction rate% Example 1 28 Example 2 32 Example 3 40 Example 4 30 Example 5 34 Example 6 38 Example 7 39 Example 8 37 Example 9 45 Example 10 46 Example 11 42 Example Example 12 41 Example 13 32 Example 14 30 Example 15 33 Comparative Example 24 The results are shown graphically in FIG.

[Brief description of the drawings]

FIG. 1 is a graph of a NOx reduction rate of a catalyst.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI B01J 23/745 B01J 35/04 301H 23/75 B01D 53/36 102B 29/40 102H 35/04 301 102A 102C B01J 23/74 301A 311A

Claims (6)

[Claims] At least one element selected from the group consisting of iridium, platinum, rhodium, palladium, silver, cobalt, copper, nickel, iron, magnesium and a lanthanide element is metal oxide and / or composite oxide. An exhaust gas purifying catalyst, which is uniformly dispersed and adhered to and / or on carrier particles made of a material in an amount of 0.01 to 10% by weight based on the weight of the carrier particles. 2. The exhaust gas purifying catalyst according to claim 1, wherein the metal oxide is at least one selected from silica, alumina, zirconia, zeolite, iron oxide, cobalt oxide, chromium oxide and manganese oxide. 3. The exhaust gas purifying catalyst according to claim 1, wherein the composite oxide is a composite oxide containing iron. 4. An exhaust gas purifying catalyst device formed by molding the exhaust gas purifying catalyst of claim 1 into a pellet shape or a multi-tube perforated honeycomb shape. 5. An exhaust gas purifying catalyst device comprising the exhaust gas purifying catalyst according to claim 1 coated on the surface of a refractory ceramic or metallic honeycomb. 6. An aqueous solution of a water-soluble compound of at least one element selected from the group consisting of iridium, platinum, rhodium, palladium, silver, cobalt, copper, nickel, iron, magnesium and a lanthanide element is prepared. At least one element based on the weight of the carrier of the metal oxide and / or complex oxide carrier
An amount of the above aqueous solution that supplies 1 to 10% by weight (in terms of metal) is applied and mixed homogeneously, and the homogeneous mixture is dried to substantially remove free water, and the dried mixture is placed in a stream containing hydrogen gas. And at a temperature in an inert gas stream, whereby the at least one element is contained in and / or on the carrier particles made of metal oxide and / or composite oxide. A method for producing an exhaust gas purifying catalyst which is uniformly dispersed and adhered.