JPH07155555A - Purifying method of internal combustion engine exhaust gas - Google Patents

Abstract

PURPOSE:To reduce the generation of NOx in low temp. region and to prevent losing an effect even at a high temp. by allowing the subject gas to contact with an iridium containing catalyst in a 1st zone, and next, allowing to contact with a platinum containing catalyst in a 2nd zone. CONSTITUTION:An exhaust gas of internal combustion engine operated in a lean air/fuel ratio is allowed to contact with the iridium containing catalyst in the 1st zone and the platinum containing catalyst in the 2nd zone. The iridium catalyst placed in the 1st zone is not restricted particularly if it does not contain platinum. Iridium is preferably carried on a fire resistant metal oxide carrier such as alumina, silica, titania or zirconia, a fire resistant metallic carbide such as silicon carbide or a fire resistant metallic nitride such as titanium nitride. The catalyst in the 2nd zone essentially contains platinum as the catalytic component. As the platinum component, any of single form of metal, platinum oxide, alloys of platinum with the other metals or conjugated oxides therewith is used. The carrier of the platinum containing catalyst is not restricted particularly.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used in excess of reducing components such as hydrocarbons emitted from internal combustion engines such as gasoline lean burn engines, 2-stroke engines and diesel engines operated at a lean air-fuel ratio. The present invention relates to a method for purifying exhaust gas containing a certain amount of oxygen, and particularly to a method useful for removing nitrogen oxides.

[0002]

2. Description of the Related Art An internal combustion engine (hereinafter referred to as a lean burn engine) which operates at a lean air-fuel ratio such as a gasoline lean burn engine or a diesel engine has a conventional fossil fuel consumption amount required to perform a certain work. Compared with other gasoline engines, there is an advantage that carbon dioxide (CO ₂ ) which is a global warming substance can be reduced in emission. However, since the exhaust gas contains an excessive amount of oxygen with respect to reducing components such as hydrocarbons (HC), removal of nitrogen oxides (NO _x ) is not sufficient with conventional techniques, and it is widely used. Constraints have been added.

Conventionally, as NOx, from the viewpoint of pollution prevention, nitric oxide (NO) and nitrogen dioxide (NO ₂ ) which are mainly causes of acid rain and photochemical smog have been problematic, and nitrous oxide (N ₂ O) has been harmless. However, it has been pointed out very recently that N ₂ O may cause global warming and ozone layer depletion, and it is predicted that emission control will be necessary in the future.

In recent years, as a method of treating exhaust gas of a lean burn engine for transportation means such as automobiles, a catalyst is used for selective reduction of NOx without reacting residual HC in the exhaust gas with excess oxygen. Is considered,
Methods using various catalysts have been proposed. For example,
(1) Method using a zeolite catalyst, metallosilicate catalyst or aluminophosphate catalyst ion-exchanged with a transition metal such as Cu or Co (US Pat. No. 4,297,328)
No. 63-09119, JP-A-3-12762.
8, JP-A-3-229620, JP-A-1-11248
8), (2) A method of using a catalyst in which a noble metal such as Pd, Pt, or Rh is supported on a porous metal oxide carrier such as zeolite, alumina, silica, or titania (JP-A-3-221).
143, JP-A-3-221144) and the like. Although various characteristics are required for these catalysts, the exhaust gas may reach 700 ° C or higher in a lean burn engine. Especially, in the case of the exhaust gas of an engine for transportation vehicles such as passenger cars, buses, and trucks, The exhaust gas temperature is primarily 800 ° C to 900 ° C during operation due to severe load fluctuations.
The exhaust gas purifying catalyst is required to have heat resistance at such a high temperature because it may reach a high temperature of about ° C. Moreover,
Long-term reliability of about 100,000 to 160,000 kilometers is also required. However, the catalyst of (1) has a drawback that the effective temperature range in which NOx removal occurs is relatively high at 350 ° C. or higher, and that rapid thermal deterioration occurs at 650 ° C. or higher in water-containing exhaust gas. ) Catalyst is from 200 ℃ to 3
Although NOx conversion occurs at a relatively low temperature of 00 ° C., there is a problem that the effective temperature range is narrow and a considerable amount of N ₂ O is by-produced as a partial reduction product of NOx.

As a purification method for compensating for such a drawback of a single catalyst system to realize more efficient NOx removal, various methods of using a plurality of catalyst systems in combination in multiple stages have been proposed. For example, (1) C on the upstream side of the exhaust gas flow
A method in which a u ion-exchanged zeolite catalyst is placed and a Pt / alumina catalyst is placed downstream (JP-A-1-139145,
(JP-A-4-377713), (2) A method of arranging a Ni or Ru / alumina catalyst on the upstream side and a Pt / alumina catalyst on the downstream side (JP-A-5-76776), (3)
A method is known in which a Pt, Rh or Pd / alumina catalyst is arranged on the upstream side and Cu / zeolite is arranged on the downstream side (JP-A-5-96132). However, the method (1) uses the upstream catalyst, and the method (3) uses the downstream catalyst.
Insufficient heat resistance for each, method (2) is 300 ℃
Since the NOx removal performance is insufficient in the above high temperature range, all of them lacked practicality.

There is also known a two-stage catalyst system in which a catalyst made of rhodium or iridium is placed on the upstream side and a catalyst made of platinum or palladium is placed on the downstream side (JP-A-52-65177). However, this catalyst system is intended for exhaust gas purification of an internal combustion engine operated at an air-fuel ratio A / F = 14.6 where the reducing component and the oxidizing component in the exhaust gas are equivalent, and is excessive. It is not effective in purifying the exhaust gas of a lean burn engine that is operated in the presence of oxygen, and hardly removes NOx.

Further, there is known a method of purifying the exhaust gas of a lean burn engine by using a catalyst in which iridium and platinum are co-loaded on a metallosilicate carrier (Japanese Patent Laid-Open No. 245386/1993). However, in the catalyst used in this method, the light-off (catalyst inlet temperature versus conversion) performance curve of NOx, HC and CO becomes similar to that of the catalyst supporting only platinum, and the action effect of iridium is recognized. Absent. Its NOx removal performance is still insufficient. Probably, when the exhaust gas comes into contact with such a co-supported catalyst, it becomes a reducing agent for NOx in the exhaust gas.
Olefinic HC having a higher reactivity among Cs first interacts with platinum having a higher oxidizing ability, and as a result, N ₂ O is produced as a reduction product of NOx in a low temperature region, and olefinic HC is produced in a higher temperature region. Is presumed to have been completely oxidized.

[0008]

Therefore, the object of the present invention is to solve the above-mentioned drawbacks of the conventional method. Specifically, the exhaust gas of a lean burn engine is
Effective in a wide temperature range from 0 ℃ to 500 ℃, 200 ℃ to 3 ℃
Generation of N ₂ O is low in the low temperature range of 00 ° C, and 700
An object of the present invention is to provide a method for purifying exhaust gas that does not lose its effect even when exposed to a high temperature of ℃ or more.

[0009]

According to the present invention, as means for solving such a problem, exhaust gas of an internal combustion engine operated at a lean air-fuel ratio is brought into contact with an iridium-containing catalyst in a first zone, and then, There is provided a method for purifying exhaust gas from an internal combustion engine, which comprises contacting with a platinum-containing catalyst in a second zone downstream thereof.

First Zone Catalyst In the exhaust gas purification method of the present invention, the iridium-containing catalyst placed in the first zone upstream of the exhaust gas flow is:
There is no particular limitation as long as it does not contain platinum. Preferably, iridium is alumina, silica, titania,
It is carried on a refractory metal oxide carrier such as zirconia, a refractory metal carbide carrier such as silicon carbide, or a refractory metal nitride carrier such as titanium nitride. The method for producing these iridium-containing catalysts is not particularly limited, and impregnation method, adsorption method,
A conventional method such as an ion exchange method or a sol-gel method is applied.

Particularly preferred among the iridium-containing catalysts are catalyst (A) and catalyst (B) described below. These are respectively disclosed in Japanese Patent Application No. 4-20750 by the applicant of the present application.
2 and Japanese Patent Application No. 5-194260. Catalyst (A) at least 1 selected from metal carbide and metal nitride
A catalyst comprising iridium supported on a carrier composed of seeds.

Examples of the metal carbide that can be used as a carrier for the catalyst include silicon carbide, titanium carbide, boron carbide, vanadium carbide, tantalum carbide, and the like, and examples of the metal nitride include titanium nitride, chromium nitride, and nitriding. Examples include zirconium. These may be used alone or in combination of two or more. Among the above examples, silicon carbide, titanium carbide or titanium nitride, or a combination of two or more thereof is preferable.

The form of the metal carbide and metal nitride is at least 800 ° C., preferably 1
There is no particular limitation as long as it is stable up to 000 ° C. Those that can be obtained at low cost include particle sizes of 0.1 to 100μ.
It is possible to use a powder of about m 3 or a product commercially available as whiskers.

Conventionally, Ir catalysts for purifying exhaust gas are porous,
It was a dogma in the art that a metal oxide carrier having a high specific surface area and having a high degree of dispersion and a fine particle size exhibits high activity and high selectivity for NOx reduction reaction (for example,
KC Taylor and JCSchlatter, J. Catal., 63 (1) 53
-71 (1980)). However, in this catalyst, not only the carrier is a metal carbide or a metal nitride but not a metal oxide, and the low specific surface area and non-porosity of the catalyst make it more than the stoichiometric amount at high temperature and in the presence of steam. It has a unique feature that it has high activity and long life for removing NOx in the exhaust gas of lean burn engine containing oxygen.

That is, the metal carbide and the metal nitride used as the carrier generally have a BE of 15 m ² / g or less.
It preferably has a T specific surface area and a pore volume of 0.5 cm ³ / g or less. The existing state of Ir supported on the carrier is not particularly limited, but is preferably a metallic state, an oxide state such as IrO, Ir ₂ O ₃ and IrO ₂ , or a mixed state thereof. These Irs are preferably dispersed and supported on the carrier with a crystallite size of 2 to 100 nm, more preferably 5 to 20 nm. If the crystallite size is too small, the selectivity of the oxidation reaction of the reducing agents HC and CO with O ₂ becomes too high, which is not preferable. If the crystallite size is too large, the activity is low relative to the supported amount.

The amount of Ir supported on the carrier is preferably 0.005 to 10.0% by weight, more preferably 0.1 to 2.0% by weight.
If it is too small, the activity of the catalyst itself is too low, and if it is too large, the reaction of the reducing agent with oxygen is promoted, and the selectivity of NOx reduction is reduced. The method of supporting Ir on a carrier and preparing a catalyst is
The method is not particularly limited and may be a conventionally known method.

For example, iridium trichloride (IrCl ₆ ), iridium chloride (H ₂ IrCl ₆ ), sodium iridium chloride (Na ₃ IrCl ₆ ), sodium iridate (Na ₂ IrCl ₆ ), potassium iridium chloride (K ₃ IrCl ₆ ). ₆ ), the same (K ₂ IrCl ₆ ), iridium nitrate (Ir (NO ₃ ) ₄ ), iridium sulfate (Ir (SO ₄ ) ₂ ) and other soluble salts of iridium are impregnated into the carrier and dried, It is prepared by firing and decomposing.
Alternatively, it may be prepared by using an Ir organometallic complex such as Ir ₃ (CO) ₁₂ and impregnating the carrier with an organic solvent solution such as hexane and ethanol, followed by firing and decomposition. In any case,
The atmosphere of calcination decomposition of the Ir compound impregnated as a catalyst precursor in the carrier, depending on the type of precursor, in air, in vacuum,
It is appropriately selected in a stream of an inert gas such as nitrogen or in a stream of hydrogen. The firing temperature is usually preferably 300 ° C to 900 ° C, more preferably 600 ° C to 800 ° C, and the firing time is usually about 30 minutes to 10 hours.

The firing may be performed by combining a plurality of treatments stepwise. For example, after firing in air, reduction may be performed in a hydrogen stream. Furthermore, the once prepared catalyst may be calcined at 650 to 800 ° C. for 30 minutes to 5 hours in a stream containing 10 to 100% steam (the balance of air or nitrogen) to perform stabilization treatment. By such heat treatment, Ir is 5 to 20
It becomes a metal Ir or Ir oxide having a crystallite diameter of about nm.

The catalyst (A) is mixed with a suitable binder,
Alternatively, without a binder, it may be formed into a suitable fixed shape, for example, a shape such as a pellet, a sphere, a ring, a honeycomb, or the like, or a carrier which is formed into a suitable shape in advance may be impregnated with Ir. Further, the catalyst (A) is a suitably shaped refractory support substrate, for example, a ceramic such as cordierite or mullite, or a honeycomb formed of stainless steel or the like, with or without a suitable binder, on the surface thereof. , Coating (eg washcoat)
It can also be used. At this time, the coating amount on the supporting substrate is preferably 20 to 200 g / liter, more preferably 60 to 120 g / liter per unit volume of the supporting substrate,
The Ir loading amount per unit volume of the supporting substrate is preferably 0.01 to
The amount is 5.0 g / liter, more preferably 0.1 to 1.5 g / liter.

As the binder, for example, silica sol,
Conventional inorganic binders such as alumina sol and titania sol can be used. The washcoat of the catalyst powder on the refractory support substrate is, for example, silica sol and water are added to the catalyst powder and kneaded to form a thixotropic slurry, and the support substrate is immersed therein, followed by drying and firing. It can be carried out.

The catalyst (B) has a Si / Ir atomic ratio of 50 or more and 800 or less, and a Si / Al atomic ratio of
A catalyst consisting of crystalline iridium silicate that is 15 or more. This catalyst has a Si / Ir atomic ratio of 50 to 800, preferably 70 to 500, and a Si / Al atomic ratio of 15 or more, preferably
It consists of crystalline iridium silicate that is 30-1000.

The crystalline form of the crystalline iridium silicate is not particularly limited, and MFI type, MEL type, MOR type, FER type, TON
Type, MTT type, MTW type, Beta type, and the like, any crystal form may be used as long as it can stably take a high silica metallosilicate, and among them, MFI type and MEL type are particularly preferable.

[0023] Preferred compositions of the crystalline iridium silicate, for example, the following equation _{(1): xM 2 O ·} yAl 2 O 3 · IrO 2 · zSiO 2 · wH 2 O (1) [wherein, M is an alkali metal , X, y, z and w
Is 0 <x / y ≦ 5.0, 0 <y ≦ 10, 50 ≦ z ≦ 800, z
/ Y ≧ 30 and 0.01 ≦ w / z ≦ 0.5].

In the formula (1), M is Li, Na, K, Rb and
It is at least one alkali metal selected from the group consisting of Cs. M ₂ O was replaced with Si ⁴⁺ of silicate skeleton
Cation species M introduced at the ion exchange site by hydrothermal synthesis of crystalline iridium silicate or subsequent ion exchange operation in order to compensate for the +1 difference in valence of Al ^3+.
Shows M ₂ O derived from ⁺ and free alkali metal oxides encapsulated in silicate crystals other than ion exchange sites
Represents M ₂ O.

In the formula (1), the molar ratio x / y of the alkali metal oxide to Al ₂ O ₃ is 0 <x / y ≦ 5.0,
Preferably 0 <x / y ≦ 3.0. It takes more x /
In the range of y, when x / y> 1, free alkali metal oxide M ₂ O exists in the silicate crystal in addition to the alkali metal existing at the ion exchange site. x /
When y> 5.0, the catalytic activity is lowered, which is not preferable. In addition, the alkali metal ion at the ion exchange site is
^When ion-exchanged to ⁺ , 0 <x / y <1.

In the formula (1), y is Al _{2 with} respect to IrO ₂ .
Represents a molar ratio of O ₃ , 0 <y ≦ 10, preferably 0 <y ≦
It is 5. When y> 10, the content of Ir in the crystalline iridium silicate is too low as compared with Al ₂ O ₃ , the effect of Ir addition is not exerted, and the catalytic activity is lowered.

In the formula (1), z is Si for IrO ₂ .
Represents a molar ratio of O ₂ , 50 ≦ z ≦ 800, preferably 70 ≦ z ≦
500. When z ≧ 50, in the x-ray crystal diffraction pattern, only the diffraction peaks attributed to the crystalline silicate structure are detected, and the other diffraction peaks attributed to free IrO ₂ are not detected. On the other hand, when z <50, a diffraction peak attributed to free IrO ₂ overlapping with the diffraction peak attributed to the crystalline silicate structure is detected. In this case, since the ratio of Ir outside the skeleton to Ir incorporated in the silicate skeleton is high, IrO ₂ aggregation or volatilization proceeds in a high temperature oxidizing atmosphere, and the heat resistance of the catalyst is likely to decrease. Further, when z> 800, the Ir content is too low and the catalytic activity is lowered.

Further, z is also defined by the molar ratio z / y of SiO ₂ to Al ₂ O ₃ , and the crystalline silicate has z
It is necessary that the low alumina and high silica metallosilicate satisfy / y ≧ 30, and it is preferable that z / y ≧ 300. When the Al ₂ O ₃ content is high and z / y <30, not only the NOx reduction selectivity decreases when the catalyst is used for purification of exhaust gas, but also dealumination in a high temperature steam coexisting atmosphere. Gradually progresses and the catalytic activity deteriorates, which is not preferable.

W represents H ₂ O derived from H ⁺ present at the water of crystallization and the ion exchange site in the crystal structure, and is not particularly limited and is usually in the range of 0.01 ≦ w / z ≦ 0.5, preferably The range is 0.05 ≦ w / z ≦ 0.2.

The performance of the catalyst (B) is extremely specific as compared with a metallosilicate having a similar structure in which iridium is replaced with another metal element. That is, platinum silicate, rhodium silicate or palladium silicate having a similar structure and composition has only high oxidizing activity of HC and shows almost no selectivity for reduction of NOx, whereas the above crystalline iridium silicate has CO and HC. However, it has a sufficient oxidation ability and a high NOx selective reduction ability.

H ⁺ and / or alkali metal ions at the ion exchange sites of the crystalline iridium silicate are
As taught by JP-A-3-127628 etc., Cu, Co, Fe, those ion-exchanged with transition metal ions such as Ni are inferior in hydrothermal durability of the catalyst compared to those not ion-exchanged with transition metal ions,
Not preferable.

The crystalline iridium silicate constituting the catalyst (B) is prepared according to a known method for synthesizing high silica zeolite except that an iridium source is allowed to coexist in the gel preparation step. For example, (1) an iridium source, a silicon source and an alkali metal compound, with or without addition of an aluminum source, and in the presence or absence of a suitable template agent, mixed to prepare a gel,
This is placed under hydrothermal synthesis conditions for crystallization, and then in an oxygen coexisting atmosphere at a temperature in the range of 400 ° C to 900 ° C, preferably 500 ° C.
Obtained by calcining at a temperature in the range of ℃ to 700 ℃, or
(2) An iridium source, a silicon source, an aluminum source and an alkali are mixed in the presence or absence of an appropriate template agent to prepare a gel, which is placed under hydrothermal synthesis conditions for crystallization, and then the gel is prepared. Aluminum-containing crystalline iridium silicate obtained by firing in an oxygen coexisting atmosphere is dealuminated while retaining the silicate skeleton structure.

The template agent used in the production methods (1) and (2) is appropriately selected according to the crystal form of iridium silicate to be synthesized. For MFI and MEL synthesis,
The following formula:

[0034]

[Chemical 1]

[Wherein X represents a nitrogen atom or a phosphorus atom, and R ¹ , R ² , R ³ and R ⁴ may be the same or different and are alkyl groups such as methyl, ethyl, propyl, butyl, pentyl and hexyl. , A hydrocarbon group such as an aralkyl group such as a benzyl group, and Y is an anion such as a hydroxide ion, a chloride ion, a bromide ion, or an iodide ion] or a quaternary ammonium hydroxide compound Preferred are phosphonium compounds.

In the production method (1), even when the aluminum source is not particularly consciously added to the hydrothermal synthesis gel,
Aluminum, which is an unavoidable impurity of the silicon source, is mixed in the gel, and Al ₂ in the product formula of iridium silicate
Give O ₃ minutes.

In the production method (2), first, a certain amount of an aluminum source is intentionally added to the gel during hydrothermal synthesis to carry out hydrothermal synthesis to obtain a relatively high Al ₂ O ₃ content (eg z / y).
To obtain <30) crystalline aluminoiridium silicate,
This is subjected to dealumination treatment by a suitable method while retaining the silicate skeleton structure to obtain the desired crystalline iridium silicate having a low Al ₂ O ₃ content (z / y ≧ 30).
According to the production method (2), it is possible to obtain crystalline iridium silicate having various skeleton structures which cannot be obtained by the production method (1).

As the silicon source in the production methods (1) and (2), water glass, silica sol, silica gel, fumed silica and the like are used. In addition, iridium chloride (IrCl ₃ ) and iridium chloride (H ₂ IrC) are used as iridium sources.
l ₆ ), ammonium chloroiridate ((NH ₄ ) ₃ IrC
l ₆ ), the same ((NH ₄ ) ₂ IrCl ₅ ), sodium chloroiridate (Na ₃ IrCl ₆ ), the same (Na ₂ IrCl ₆ ), potassium chloroiridate (K ₃ IrCl ₆ ), the same (K ₂ IrCl). ₆ ), iridium sulfate
(Ir (SO ₄ ) ₂ ), various iridium acids such as iridium nitrate (Ir (NO ₃ ) ₄ ) and salts thereof, organometallic complexes such as Ir ₃ (CO) ₁₂ ,
A hydroxide such as Ir (OH) ₄ and an oxide such as Ir ₂ O ₃ and IrO ₂ can be used.

As the aluminum source intentionally added in the production methods (1) and (2), aluminum nitrate, sodium aluminate, alumina sol, boehmite gel, γ
Various transition alumina such as −, θ−, δ−, η− and the like can be used. As the alkali metal compound, at least one selected from the group consisting of ammonium hydroxide and lithium, sodium, potassium, rubidium, and cesium hydroxide is used.

In the hydrothermal synthesis of the production methods (1) and (2), the raw material gel is kept at 100 ° C. to 250 ° C. for 5 hours to 200 hours under normal pressure reflux conditions or under autoreaction pressure in a closed autoclave. It is done by doing.

The reaction product is filtered, washed and dried, and then in an oxygen-containing atmosphere, preferably in air, at 400 ° C to 900 ° C.
It is preferably calcined in the temperature range of 500 ° C to 700 ° C for 2 hours to 20 hours, preferably 4 hours to 10 hours.

Through this firing step, the crystalline iridium silicate of the production method (1) and the crystalline aluminoiridium silicate of the production method (2) are obtained as a proton type, an alkali metal ion type such as Na, or a mixture type thereof. By the above operation, the crystalline iridium silicate obtained as an alkali ion type may be converted into a proton type by a normal ion exchange operation and used.

The dealumination treatment of the production method (2) is as follows:
In order not to destroy the silicate skeleton by the treatment,
Considering the structural stability of the precursor aluminoiridium silicate crystal form, it is appropriately selected from mineral acid treatment, fluorine compound treatment, high temperature steam treatment and the like. For example, for MFI-type, MEL-type and MOR-type precursors, a method of holding at 90-100 ° C in an 8-10N hydrochloric acid aqueous solution for 1-10 hours can be applied.

In the crystalline iridium silicate of the catalyst (B), it is presumed that iridium exists mainly in the silicate skeleton by substituting Si, but some of the iridium is present outside the skeleton as fine crystals of iridium oxide IrO _2. It does not prevent existence. However, such IrO ₂ microcrystals are hardly detected by powder method X-ray diffraction. The first zone catalyst contains iridium as a catalytically active component, but other co-catalyst components that improve the performance of the iridium-containing catalyst as an exhaust gas purification catalyst may be co-loaded in addition to iridium. For example, Mg, Ca, and Ba may be supported in an amount that does not inhibit the catalytic activity of iridium (for example, the atomic ratio of added element to iridium is 1:10 to 10: 1). However, coexistence of platinum and palladium, especially platinum, among the platinum group elements is not preferable because it impairs the function of the iridium catalyst as the catalyst in the first zone.

Second Zone Catalyst The second zone catalyst must contain platinum as the catalytically active component. This platinum component is in the state of a simple metal, PtO, P
It may be in any state such as a state of platinum oxide such as tO ₂ or an alloy of platinum and another metal or a state of complex oxide. The carrier of the platinum-containing catalyst is not particularly limited, and a refractory metal oxide carrier such as alumina, silica, titania, zirconia, or a metal carbide such as silicon carbide or titanium carbide exemplified above, or a metal nitride carrier is used. Good.

The platinum-containing catalyst may carry, in addition to platinum, another promoter component which improves the performance of the platinum catalyst as an exhaust gas purifying catalyst together with the platinum component. Examples of such co-catalyst components include Mg, Ca, Ba and Sr.
Alkaline earth metals such as La, Ce, Nd, Yb, Sm
Rare earth metals such as V, Nb, Ta, W, Fe, Co, N
i, Cu, Ru, Ir, Rh, Pd, Re, Ag, Au
And other transition metal elements. These may be supported in an amount that does not inhibit the activity of the platinum catalyst (for example, the metal element of the cocatalyst: platinum (atomic ratio) is 1:10 to 10: 1).

The method for producing the platinum-containing catalyst is not particularly limited, and conventional methods such as an impregnation method, an adsorption method and an ion exchange method may be used, and an electroless white powder as disclosed in Japanese Patent Application No. 5-247428 by the applicant of the present application. It is also possible to use the gold plating method. As an example of a preferable platinum-containing catalyst, the applicant of the present invention has filed Japanese Patent Application No. 5-2.
One using the electroless plating method disclosed in 47428 is mentioned. The catalyst is selected from the group consisting of a refractory ceramic or metal monolithic substrate having through holes and a refractory metal oxide and metal carbide provided on the wall surface of the monolithic substrate in contact with the gas flow. And a platinum carrier supported on the coating layer of the carrier by electroless plating. The catalyst is usually arranged along the gas flow direction so that the exhaust gas flow passes through the through holes.

As the supporting substrate constituting the catalyst, a monolithic substrate made of refractory ceramics or metal having a through hole passing from one end face to the opposite end face is used. As the ceramic or metal monolithic substrate, for example, mullite, cordierite, silicon carbide or the like honeycomb or corrugated substrate can be used, and as the metal monolithic substrate, for example, , Austenite type, ferritic type stainless steel honeycomb substrate and the like.

As the coating layer, a refractory metal oxide and / or metal carbide is used. Examples of the refractory metal oxide and metal carbide include γ-alumina, α-alumina, magnesia, zirconia, titania, cerium oxide, tin oxide, silica, silicon carbide, titanium carbide, tungsten carbide, or a mixture thereof. Γ-alumina, zirconia, cerium oxide, silicon carbide or a mixture thereof is preferable. These are used by coating the above monolithic substrate with a binder such as silica or alumina. Hereinafter, the carrier in which the substrate is coated with the coating layer is referred to as a substrate-supporting carrier.

Further, as the active metal, platinum having an average particle size in the range of 80 to 300 nm, preferably 100 to 200 nm is preferably used, and 0.5 of the platinum is added to the monolith substrate. The range of -8 g / L, especially 1-6 g / L is preferably contained. If the platinum content is too low, the catalyst activity will decrease, and if it is too high, there will be no corresponding effect, and the catalyst preparation will not only be difficult but also economical.

The catalyst is used by, for example, impregnating and supporting 0.2% by weight of platinum chloroplatinic acid on a carrier, firing it at 600 ° C., and then coating it on a monolithic substrate. The resulting pretreated substrate-supporting carrier is usually activated with a reducing agent solution in which hydrazine is preferred. After that, for example, chloroplatinic acid (1g / L as platinum), 200mL of ammonia water
/ L, 50 wt% hydrazine solution 4 g / L. By immersing in an electroless platinum plating solution having a composition such as 4 g / L, platinum is plated on the pretreated substrate supporting carrier to support platinum, and after drying, 400 to It is manufactured by firing at 800 ° C.

The shapes of the iridium-containing catalyst in the first zone and the platinum-containing catalyst in the second zone used in the exhaust gas purification method of the present invention also have a packed bed through which the total amount of exhaust gas can pass without a significant increase in back pressure. There is no particular limitation as long as it can be formed. Each catalyst powder itself may be molded into a fixed shape such as a sphere, a pellet, or a ring, or may be preformed from a conventional refractory material such as alumina, silica, or titania as described in the preferred examples. It is also possible to coat and use a catalyst powder on a supporting substrate in the shape of a ball, pellet, ring or the like. Particularly preferably, it is made of a refractory ceramic such as cordierite, mullite, or α-alumina, or a refractory material such as stainless steel, and has an integral structure such as a honeycomb or a foam having a large number of through holes in the exhaust gas flow direction. A support substrate having a surface coated with a catalyst substance by a wash coat method or a sol-gel method is used.

In this case, separate catalyst coating structures may be provided on the upstream side and the downstream side, respectively, or the iridium-containing catalyst may be coated on the exhaust gas inlet side portion of a single supporting substrate,
The outlet side portion may be coated with a platinum-containing catalyst to be used as one catalyst structure. However, the positions of the first catalyst zone and the second catalyst zone must be clearly distinguished, and it is not preferable that the exhaust gas that does not pass through the iridium-containing catalyst first comes into contact with the platinum-containing catalyst.

[0054] In the treatment method the exhaust gas treatment method of the present invention the exhaust gas temperature of the exhaust gas at the time of contacting the exhaust gas to the catalyst, the temperature of the exhaust gas of a lean-burn engine may be used as it is, preferably Is 150 ° C to 700 ° C, more preferably 200 ° C
˜500 ° C., particularly preferably, the first catalyst zone inlet gas temperature is 300 ° C. to 500 ° C. and the second catalyst zone inlet gas temperature is 200 ° C. to 350 ° C.

In the exhaust gas purification method of the present invention, the gas space velocity (SV) for contacting the exhaust gas with the first and second catalyst zones is not particularly limited, but preferably 5,000 / hr to 150 respectively. 1,000 / hr. The ratio SV ₁ / SV ₂ of the spatial velocities of the first zone and the second zone is not particularly limited, but preferably 3> (SV ₁ / SV ₂ ).
> 0.3, more preferably 1.5> (SV ₁ / SV
₂ )> 0.5.

When NOx is insufficiently removed by only HC in the raw exhaust gas in a high oxygen concentration range like exhaust gas of a diesel engine, the exhaust gas flow from the primary source of the exhaust gas to the catalyst layer It is possible to secure a sufficiently high NOx removal rate without excessively deteriorating the fuel economy by additionally adding the minimum necessary hydrocarbon-based reducing agent from the outside on the road.

As the reducing agent to be added, a C _{2 to} C ₁₈ saturated or unsaturated hydrocarbon or an oxygenated hydrocarbon which is a partial oxidation product thereof is used. Among them, C _{2 to} C ₁₀ unsaturated hydrocarbons such as ethylene, propylene, butene, hexene, octene, toluene, xylene or cumene are particularly preferable. Further, it is possible to use the fuel itself of a general-purpose internal combustion engine such as gasoline, kerosene, light oil, methanol and ethanol as a reducing agent for additional addition. These hydrocarbons are added after being vaporized in advance, or are spray-added as a liquid, and are supplied to the catalyst layer in a mixed state with raw exhaust gas.

[0058]

In general, the exhaust gas of a lean burn engine contains 3 to 15% of O ₂ and several hundred to several thousand ppm of NOx as well as several hundred to several thousand ppm of CO and several hundred to several thousand ppm of HC. It HC is an unburned component of fuel, a thermally decomposed component of a fuel molecule, a partially oxidized component of a fuel molecule, etc., and its molecular structure does not consist of a single molecular species.
For example, alkenes such as ethylene, propylene, butene and pentene, alkanes such as ethane, propane, butane and hexane, aromatic H such as benzene, toluene and xylene.
It contains Cs and the like in a certain degree of composition. As many components as possible of HC composed of such a wide range of components,
Effective use of NOx as a reducing agent in the presence of excess oxygen is essential for increasing the NOx removal rate.

The iridium-containing catalyst, particularly the iridium-supported metal carbide catalyst, which is arranged upstream in the method of the present invention, is
Among the HCs, it is particularly effective for the selective reduction of NOx by unsaturated hydrocarbons containing olefins and alkylaromatic HCs, and N ₂ O is hardly produced as a by-product in the reduction reaction. Further, a platinum-containing catalyst disposed on the downstream side, particularly a catalyst in which platinum is supported on a porous carrier such as zeolite or alumina, is compared in the reaction of olefinic HC and NOx in the presence of excess oxygen at 150 ° C to 250 ° C. The reduction reaction of NOx occurs in a relatively low temperature range to mainly produce N ₂ O. However, even in the presence of the platinum catalyst, a system in which unsaturated HC does not exist as the HC species, that is, paraffinic H
Selectively reduced decomposed into N ₂ and H ₂ O the NOx without generating little N ₂ O in the reaction system between C and NOx.

Therefore, by using the two-stage catalyst system of the present invention, NOx selective reduction by olefinic or alkylaromatic unsaturated HC which is relatively easily oxidized on the upstream side occurs, and then downstream thereof. On the side, a selective reduction reaction of NOx by paraffinic HC, which is relatively hard to be oxidized, occurs. In this way, a wide variety of HC species contained in the exhaust gas can be effectively utilized as a reducing agent for NOx to the maximum extent, and N ₂ O by-product in the NOx reduction reaction by the supported platinum catalyst is generated. As a result of being suppressed, purification of the NOx-containing exhaust gas is achieved with high efficiency.

[0061]

[Examples] The following are production examples, examples and comparative examples.
The present invention will be described in more detail. However, the present invention is not limited to the following examples. In the specification of the present invention,
The NOx conversion rate, NOx removal rate and N ₂ O production rate are defined as follows. CO and HC conversion is NOx
The conversion rate is defined in the same way as the definitional expression. NOx removal rate = NOx conversion -N ₂ O formation rate

Production Example 1 Production of Iridium-Supported Silicon Carbide Coated Honeycomb Catalyst (1) (a) Silicon Carbide (SiC) Powder (BET1 manufactured by Lonza Co., Ltd.)
To 30 gr of 5 m ² / g), 2.0 gr of 30% silica sol and 50 mL of deionized water were added and kneaded with a ball mill for 16 hours. 1. The obtained slurry was cut out from a commercially available 400-cell cordierite honeycomb to have a diameter of 2. A 54 cm long, 6.35 cm long core piece is dipped and pulled up, then excess slurry is removed by air blow, dried and then baked at 500 ° C for 30 minutes to make a honeycomb 1L.
A silicon carbide-coated honeycomb coated with 100 gr of solid content per (liter) was obtained. (B) Deionized aqueous solution 100 of iridium chloride (H ₂ IrC1 ₆ ) containing 0.27 gr of iridium (Ir)
Immerse the honeycomb core obtained in (a) in 3 mL at room temperature for 3
It was held for a minute and impregnated with an iridium solution having a water absorption rate. Excess iridium solution was removed by air blow, dried and then calcined in air at 800 ° C for 30 minutes, then reduced in a 100% hydrogen stream at 700 ° C for 2 hours, and 0.6 gr / L iridium-supported silicon carbide honeycomb catalyst (1) Got

[Production Example 2] Crystalline iridium silicate-coated honeycomb catalyst (2)
Production iridium chloride (H ₂ IrCl ₆ ; Ir content 38.
0%) in 1000 mL of 8.2 g deionized water solution,
97% sodium hydroxide (NaOH) 3 with vigorous stirring
0.9g and tetrapropyl ammonium bromide (TP
AB) 190 g and 1000 mL deionized water aqueous solution 3
Drip in 0 minutes and then add 97% sulfuric acid (H ₂ SO ₄ ) 2
9.7 g of 200 mL aqueous deionized water solution was added dropwise over 10 minutes. Further, 1000 g of 30% colloidal silica was added dropwise at a constant rate over 30 minutes to adjust the pH of the gel to 11.2.
˜11.8.

The obtained gel was placed in a stainless steel autoclave, covered, and the internal temperature was adjusted to 1 after 6 hours with stirring.
After raising the temperature to 70 ° C and maintaining the stirring at 170 ° C for 100 hours under self-reaction pressure, allowing to cool to room temperature, taking out the contents, filtering, washing with deionized water, and drying at 110 ° C for 16 hours. Repeated. The obtained crystals are crushed, and the temperature is raised from room temperature to 5 in air.
The temperature was raised up to 40 ° C. at a temperature rising rate of 60 ° C./Hr, kept at 540 ° C. for 4 hours, and allowed to cool to obtain 296 gr of crystalline iridium silicate powder. This powder was coated on a honeycomb core in the same manner as in (a) of Production Example 1, dried, and then fired at 500 ° C. for 1 hour to obtain 100 dry weight conversion per 1 L of honeycomb volume.
Thus, g of iridium silicate-coated honeycomb catalyst (2) was obtained.

Production Example 3 Production of Platinum-Supported Alumina-Coated Honeycomb Catalyst (3) 120 gr of activated alumina having a BET specific surface area of 150 m ² / g and an average particle diameter of 30 μ was put in a mixer, and 4.8 gr of platinum was contained while stirring. 30 mL of an amine aqueous solution of platinum hydroxide was dropped little by little to uniformly support the dispersion. Then add 25 mL of 25% acetic acid aqueous solution little by little and add 4% Pt.
A supported alumina powder was obtained. 120 gr of this powder, 15 gr of alumina sol (alumina content 20% by weight), and 30 gr of deionized water were charged in a ball mill pot and wet-ground for 16 hours to obtain a slurry. Production Example 1 of this slurry
In the same manner as in (a) above, the honeycomb was coated, dried and then fired in air at 500 ° C. for 1 hour to obtain a 4 g / L Pt-supported alumina-coated honeycomb catalyst (3).

Production Example 4 Production of Platinum-Supported Alumina-Coated Honeycomb Catalyst (4) (a) BET Specific Surface Area 150 m ² / g, Average Particle Diameter 30
120 gr of activated alumina having μ and chloroplatinic acid (H
0.06 gr of ₂ PtC1 ₆ ) as platinum, 15 gr of alumina sol (alumina content 20% by weight), and 30 gr of deionized water were charged in a ball mill pot and wet-ground for 16 hours to obtain a slurry. Using this slurry, the honeycomb was coated with alumina in the same manner as in (a) of Production Example (1), dried, and then baked in air at 600 ° C. for 30 minutes to obtain an alumina-coated honeycomb having a dry weight of 100 gr / L.

(B) 1.0 gr of chloroplatinic acid as platinum
/ L, 200 mL / L of ammonia water, and 4 gr / L of 50 wt% hydrazine aqueous solution were used to prepare a platinum electroless plating solution. (C) Add the alumina-coated honeycomb obtained in (a) above to 5
After being immersed in 300 mL of a wt% hydrazine aqueous solution at room temperature for 15 minutes and taken out, the electroless platinum plating solution 16 of (b) 16
Immerse in 0 mL (containing 0.16 gr of platinum) at 30 ° C. for 6 hours, take out 110 ° C., dry for 12 hours, then in air 70
It was fired at 0 ° C. for 5 hours to obtain a 5 gr / L platinum-supported alumina-coated honeycomb (4).

[Production Example 5] Pt-Rh-supported alumina-coated honeycomb catalyst (TWC)
Production 120 ml of activated alumina having a BET specific surface area of 150 m ² / g and an average particle diameter of 30 μ was put in a mixer and 30 mL of an amine aqueous solution of platinum hydroxide containing 2.0 gr of platinum was dripped little by little and uniformly dispersed and supported. . Then, an aqueous rhodium nitrate solution containing 0.41 gr of rhodium 15
After adding mL little by little to disperse and support uniformly, 25%
10 mL of acetic acid was dropped little by little, and 1.7% Pt-0.3 was added.
4% Rh-supported alumina powder (Pt / Rh weight ratio = 5 /
1) was prepared. This was coated on a honeycomb in the same manner as in Production Example 3, dried, and then baked in air at 500 ° C. for 1 hour,
A Pt—Rh-supported alumina-coated honeycomb catalyst (5) was obtained.

Production Example 6 Production of Pt-Ir-Supported Alumina-Coated Honeycomb Catalyst In 100 mL of a deionized aqueous solution of iridium chloride (H ₂ IrC1 ₆ ) containing 1.18 gr of iridium (Ir),
The Pt-supported alumina-coated honeycomb prepared in Production Example 3 was immersed and kept at room temperature for 3 minutes to impregnate it with an iridium solution having a water absorption rate. After removing the excess solution by air blowing, it was dried, calcined in air at 700 ° C. for 30 minutes, and further reduced in a 100% hydrogen stream at 800 ° C. for 2 hours to give 4 g / L Pt-
1.2 g / L Ir-supported alumina-coated honeycomb catalyst (6) was obtained.

Production Example 7 Production of Cu Ion Exchange / ZSM-5 Coated Honeycomb Catalyst Rormann and Valyosky's Method (LDRollmann and
EWValyocsik, Inorg. Synthesis., 22 (1982) P.67〜
68) to obtain H-type aluminosilicate ZSM-5 powder (Si / A1 ratio = 38). 100 g of this powder was slurried in 2000 mL of 0.03 M copper acetate deionized water, stirred at room temperature for 16 hours, filtered, washed and dried,
1.2 wt% Cu ion exchange / ZSM-5 powder (Cu ²⁺ conversion ion exchange rate 9 after firing in air at 500 ° C. for 30 minutes
5%) was obtained. Cu ion exchange / Z by coating on honeycomb
An SM-5 coated honeycomb catalyst (7) was obtained.

[Example 1] Lean burner with (upstream Ir catalyst + downstream Pt catalyst)
Purification of engine exhaust model gas Iridium-supported silicon carbide-coated honeycomb catalyst (1) 1 / on the upstream side of the gas flow in a stainless steel reaction tube with an inner diameter of 30 mm
2 pieces of the platinum-supported alumina catalyst (3) cut on the downstream side were cut to a length of 2, and glass wool was wrapped around each side to prevent gas blow-through and to be filled. In the reaction tube, NO 1,200 ppm (hereinafter, gas component concentration is shown by volume concentration) as an exhaust model gas of a gasoline lean burn engine, and 800 ppm of propylene + 8 propane as HC.
00 ppm, CO 1,000 ppm, H ₂ 1,000
ppm, O ₂ 3.2%, CO ₂ 10%, H ₂ O 10
%, And the balance of N ₂ was 16.1 L / mi
Flow rate of n (upstream catalyst SV 60,000 / hr, downstream catalyst SV 60,000 / hr, SV3 of entire catalyst layer)
Flow rate at a flow rate of 10,000 / hr) and the catalyst layer inlet gas temperature is 1
While continuously raising the temperature from 50 ° C. to 500 ° C. at a temperature raising rate of 30 ° C./min, NO _x , CO, and
The concentrations of HC and N ₂ O are respectively NO _x meter, CO meter,
By measuring with an HC meter and an N ₂ O meter, the conversion rate of NO _X , CO, and HC and the production rate of N ₂ O are obtained, and NO _X , CO,
The catalyst layer inlet gas temperature dependence (light-off performance) on the HC conversion rate and N ₂ O production rate was evaluated. However,
When a two-stage catalyst system placed on the upstream side and the downstream side is used as in the present embodiment, the catalyst inlet gas temperature means the inlet gas temperature of the catalyst placed in the upstream first zone.

As shown in FIG. 1, NO _X is 270 ° C. to 43 ° C.
It shows a high conversion rate of 40% or more over a wide temperature range of 0 ° C, and the conversion rates of CO and HC are 180 ° C and 25 ° C, respectively.
The conversion rate from 0 ℃ to 500 ℃ is 98 respectively.
% And 97%. The maximum production rate of N ₂ O in the temperature range of 200 to 300 ° C was 16%.

[Example 2] Lean burn with (upstream Ir catalyst + downstream Pt catalyst)
Purification of engine exhaust model gas In Example 1, an Ir silicate catalyst (2) of the same size was used on the gas inlet side instead of the catalyst (1), and a Pt-supported catalyst of the same size ((1) instead of the catalyst (3) on the outlet side. 4)
HC, CO, N in the same manner as in Example 1 except that
The O _X light-off performance and N ₂ O formation rate of the evaluation.

As shown in FIG. 2, NO is 280 ° C. to 470 ° C.
It shows a high conversion rate of 40% or more over a wide temperature range of ℃, and the conversion rates of CO and HC are 220 ℃ and 260 respectively.
Conversion rate rises from ℃ to 500% at 500 ℃
And 97%. The maximum production rate of N ₂ O in the temperature range of 200 to 300 ° C was 14%.

Comparative Example 1 Lean Burn Engine Exhaust Model Gas Using Ir Catalyst Only
In Example 1 of purifying gas, instead of placing the catalyst (1) having a half length on the gas inlet side and the catalyst (3) having a half length on the outlet side, the original (6 (0.35 cm) long Ir supported catalyst (1) only,
In the same manner as in Example 1 to evaluate HC, CO, and light-off performance of the NO _X.

As shown in FIG. 3, the maximum NO _x at 380 ° C.
Conversion of 60%, NO _X conversion in NO _X conversion 23% to obtain but 300 ° C. at 500 ° C. was only 11%. Further, although the CO conversion rate rises from 310 ° C., the HC conversion rate rises only near 340 ° C., and even at 500 ° C., it was 73%, which was insufficient. This is presumed to be due to insufficient reactivity between propane and NO _x of HC species in the exhaust model gas. However, the maximum N ₂ O production rate was as low as 8%.

[Comparative Example 2] Lean burn engine exhaust model gas using only Pt catalyst
In Example 1 of purifying gas, instead of placing the catalyst (1) having a half length on the gas inlet side and the catalyst (3) having a half length on the outlet side, the original (6 (0.35 cm) long Pt-supported catalyst (3) only, except
In the same manner as in Example 1 to evaluate HC, CO, and light-off performance of the NO _X. As shown in FIG. 4, the conversion rates of CO and HC rise at 170 ° C. and 240 ° C., respectively, and reach 500
The conversions at ° C were 98% and 99%, respectively. The NO _X, shows 62% maximum NO _X conversion at a catalyst inlet temperature 280 ° C., in a reduced 500 ° C. slowly NO _X conversion toward 350 ° C. to 500 ° C. 9
It only shows%. N ₂ maximum production rate of O is higher than 2 times Te obtained comparing to Example 1 at 35% at 260 ° C. N ₂ O
The production rate was shown.

Comparative Example 3 Lean Burn Engine with Pt-Rh / A1 ₂ O ₃ Catalyst
In the purification example 1 of the exhaust gas model gas, instead of placing the catalyst (1) having a length of 1/2 on the gas inlet side and the catalyst (3) having a length of 1/2 on the outlet side, instead of originally, (6.35 cm) long Pt-Rh / A1 ₂ O ₃ catalyst (5) was used in the same manner as in Example 1 except that HC, CO, N
O _X of the light-off performance was evaluated. C as shown in FIG.
The conversion rates of O and HC rose at 160 ° C. and 220 ° C., respectively, and the conversion rates at 500 ° C. were 98% and 99%, respectively. Regarding NO _X , the maximum NO _X conversion rate is 39% at a catalyst inlet temperature of 230 ° C, but 250 ° C
The NO _x conversion rate drops sharply from 50 to 300 ° C.
It only shows 7% at 0 ° C. Maximum incidence of N ₂ O with 27% in the 200 ° C., N from NO _X conversion rate
_The NOx removal rate after subtracting the ₂ O production rate was low. As described above, it is apparent that the conventional three-way catalyst supported by Pt-Rh coexistence has insufficient NOx removal efficiency in the presence of excess oxygen.

Comparative Example 4 Lean Burn Engine with Pt—Ir / A1 ₂ O ₃ Catalyst
In the purification example 1 of the exhaust gas model gas, instead of placing the catalyst (1) having a length of 1/2 on the gas inlet side and the catalyst (3) having a length of 1/2 on the outlet side, instead of originally, (6.35 cm) length of Pt-Ir / Al ₂ O ₃ catalyst (6) was used in the same manner as in Example 1, except that HC, CO, N were added.
O _X of the light-off performance was evaluated.

As shown in FIG. 6, the conversion rates of CO and HC rose at 160 ° C. and 240 ° C., respectively, and the conversion rates at 500 ° C. were 98% and 99%, respectively. N
Regarding O _X , the maximum NO _X at the catalyst inlet temperature of 270 ° C
The conversion rate is 45%, but the NO _x conversion rate drops sharply from 300 ° C. to 400 ° C., and shows only 9% at 500 ° C. The maximum production rate of N ₂ O was 30% at 260 ° C., and the NO x removal rate obtained by subtracting the N ₂ O production rate from the NO _x conversion rate was low. Thus, the known Pt-I
It is clear that the rx coexisting supported catalyst has an insufficient NOx removal efficiency in the presence of excess oxygen.

[Comparative Example 5] (Replacement with upstream Cu / ZSM-5 catalyst + downstream Pt catalyst)
Instead of placing the catalyst (1) having a length of 1/2 on the gas inlet side and the catalyst (3) having a length of 1/2 on the outlet side in Example 1 for purifying an engine exhaust model gas. In addition, the length C is half the length at the gas inlet side.
u / ZSM-5 catalyst (7), except filled with 1/2 of the length of the Pt catalyst (3) on the downstream side, in the same manner as in Example 1, HC, CO, NO _X in the light-off performance Was evaluated.

As shown in FIG. 7, the conversion rates of CO and HC rise at 170 ° C. and 240 ° C., respectively, and the conversion rates at 500 ° C. are 98% and 96%, respectively.
For NO _x , catalyst inlet temperatures of 260 ° C and 460 ° C
In the vicinity, the maximum values of NO _X conversion of 36% and 55% are shown, but the NO _X conversion is 10 from 320 ° C to 400 ° C.
It fell to the% range. The maximum production rate of N ₂ O was 22% at 240 ° C, and the NOx removal rate was low.

Thus, Cu / zeolite catalyst and Pt
In a known combination catalyst system with a supported catalyst, 250 ° C to 3 ° C.
It is clear that the NOx removal performance in the medium temperature range of 50 ° C is insufficient.

Example 3 Thermal Aging of (Upstream Ir Catalyst + Downstream Pt Catalyst)
Purification of later lean burn engine exhaust model gas The Ir catalyst (1) and Pt catalyst (3) used in Example 1 were taken out from the model gas reactor at 800 ° C. under air flow containing 10% H ₂ O, After thermal aging for 5 hours, the model gas reactor was again charged with upstream Ir catalyst and downstream Pt.
The catalysts were charged in this order and evaluated in the same manner as in Example 1.

FIG. 8 shows the light-off performance. Even after thermal aging at 800 ° C., the light-off performance of NO _x , HC, and CO was hardly reduced, and a NO _x conversion rate of 40% or more was given in a wide temperature range from 280 ° C. to 470 ° C. Thus, the method of the present invention provides an exhaust gas purification method having excellent heat resistance.

[Comparative Example 6] Therma of (upstream Cu / ZSM-5 catalyst + downstream Pt catalyst)
Lean burn engine exhaust model gas after aging
Purification of No. 5 upstream Cu / ZSM-5 catalyst used in Comparative Example 5 (7)
And Pt catalyst (3) on the downstream side were taken out from the model gas reactor, subjected to thermal aging at 800 ° C. for 5 hours in an air flow containing 10% H ₂ O, and then again into the model gas reactor to the upstream Cu / ZSM. -5 catalyst (7) and downstream Pt catalyst (3) were charged in this order and evaluated in the same manner as in Comparative Example 5.

FIG. 9 shows the light-off performance. The deterioration of the upstream Cu / ZSM-5 catalyst due to thermal aging at 800 ° C. was remarkable, and the NO _x conversion rate in the medium to high temperature range of 300 ° C. or higher was 20% or less, which was extremely insufficient.

[0088]

The exhaust gas purification method of the present invention is (1) 200 ° C. to 50 ° C. for exhaust gas of a lean burn engine.
It is effective against engine exhaust gas in a wide temperature range of 0 ° C, and (2) HC species in exhaust gas are used as a maximum reducing agent, and (3) N in the low temperature range of 200 ° C to 300 ° C.
The generation of ₂ O is small, and (4) the catalyst used has heat resistance to a high temperature of 700 ° C or higher even in the presence of water,
Exhaust gas purification can be performed stably and efficiently over a long period of time.

[Brief description of drawings]

FIG. 1 is a graph showing the purification performance of lean burn engine exhaust model gas obtained in Example 1.

FIG. 2 is a graph showing the purification performance of lean burn engine exhaust model gas obtained in Example 2.

FIG. 3 is a graph showing the purification performance of the lean burn engine exhaust model gas obtained in Comparative Example 1.

FIG. 4 is a graph showing the purification performance of lean burn engine exhaust model gas obtained in Comparative Example 2.

5 is a graph showing the purification performance of the lean burn engine exhaust model gas obtained in Comparative Example 3. FIG.

FIG. 6 is a graph showing the purification performance of the lean burn engine exhaust model gas obtained in Comparative Example 4.

7 is a graph showing purification performance of lean burn engine exhaust model gas obtained in Comparative Example 5. FIG.

FIG. 8 is a graph showing the purification performance of the lean burn engine exhaust model gas obtained in Example 3.

9 is a graph showing the purification performance of lean burn engine exhaust model gas obtained in Comparative Example 6. FIG.

─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] January 27, 1994

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0004

[Correction method] Change

[Correction content]

In recent years, as a method of treating exhaust gas of a lean burn engine for transportation means such as automobiles, a catalyst is used for selective reduction of NOx without reacting residual HC in the exhaust gas with excess oxygen. Is considered,
Methods using various catalysts have been proposed. For example,
(1) Method using a zeolite catalyst, metallosilicate catalyst or aluminophosphate catalyst ion-exchanged with a transition metal such as Cu or Co (US Pat. No. 4,297,328)
No. 63-09119, JP-A-3-12762.
8, JP-A-3-229620, JP-A-1-11248
8), (2) A method of using a catalyst in which a noble metal such as Pd, Pt, or Rh is supported on a porous metal oxide carrier such as zeolite, alumina, silica, or titania (JP-A-3-221).
143, JP-A-3-221144) and the like. Although various characteristics are required for these catalysts, the exhaust gas may reach 700 ° C or higher in a lean burn engine. Especially, in the case of the exhaust gas of an engine for transportation vehicles such as passenger cars, buses, and trucks, The exhaust gas temperature is primarily 800 ° C to 900 ° C during operation due to severe load fluctuations.
The exhaust gas purifying catalyst is required to have heat resistance at such a high temperature because it may reach a high temperature of about ° C. Moreover,
Long-term reliability of about 100,000 to 160,000 kilometers is also required. However, there are catalyst cormorants have the effective temperature range removal of NOx occurs is relatively high and 350 ° C. or higher, and occurs rapid thermal degradation at 650 ° C. or higher in water content exhaust gases disadvantages of (1), (2 ) Catalyst is from 200 ℃ to 3
Although NOx conversion occurs at a relatively low temperature of 00 ° C., there is a problem that the effective temperature range is narrow and a considerable amount of N ₂ O is by-produced as a partial reduction product of NOx.

[Procedure Amendment 2]

[Document name to be amended] Statement

[Name of item to be corrected] 0029

[Correction method] Change

[Correction content]

W represents the proportion of H ₂ O derived from H ⁺ existing at the water of crystallization and the ion exchange site in the crystal structure, and is not particularly limited, and usually 0.01 ≦ w / z ≦ 0.5. The range is preferably 0.05 ≦ w / z ≦ 0.2.

[Procedure 3]

[Document name to be amended] Statement

[Correction target item name] 0048

[Correction method] Change

[Correction content]

[0048] As the base substance that make up the catalyst, refractory ceramic or metal monolith substrate having a through hole passing into end surface opposite from the one end surface is used. As the ceramic or metal monolithic substrate, for example, mullite, cordierite, silicon carbide or the like honeycomb or corrugated substrate can be used, and as the metal monolithic substrate, for example, , Austenite type, ferritic type stainless steel honeycomb substrate and the like.

[Procedure amendment 4]

[Document name to be amended] Statement

[Correction target item name] 0049

[Correction method] Change

[Correction content]

As the coating layer, a refractory metal oxide and / or metal carbide is used. Examples of the refractory metal oxide and metal carbide include γ-alumina, α-alumina, magnesia, zirconia, titania, cerium oxide, tin oxide, silica, silicon carbide, titanium carbide, tungsten carbide, or a mixture thereof. Γ-alumina, zirconia, cerium oxide, silicon carbide or a mixture thereof is preferable. These are used by coating the above monolithic substrate with a binder such as silica or alumina. Hereinafter, those coated with the <br/> covering layer of the support on the substrate is referred to as a carrier coated substrate.

[Procedure Amendment 5]

[Document name to be amended] Statement

[Correction target item name] 0050

[Correction method] Change

[Correction content]

Further, as an active metal, a transmission electron microscope is used.
The average particle size observed with a mirror (SEM) is 80 to 300 n.
m using platinum, preferably in the range of 100 to 200 nm, for the monolith substrate,
Platinum in the range of 0.5 to 8 g / L, especially 1 to 6 g / L
It is preferable to contain it. If the platinum content is too low, the catalyst activity will decrease, and if it is too high, there will be no corresponding effect, and the catalyst preparation will not only be difficult but also economical.

[Procedure correction 6]

[Document name to be amended] Statement

[Correction target item name] 0051

[Correction method] Change

[Correction content]

The catalyst is used, for example, by impregnating and supporting 0.2% by weight of platinum chloroplatinic acid on a carrier, calcining at 600 ° C., and then exposing the monolithic substrate. The resulting pretreated carrier harvested substrate is usually activated with a reducing agent solution in which hydrazine is preferred. After that, for example, chloroplatinic acid (1g / L as platinum), 200mL of ammonia water
/ L, 50 wt% hydrazine solution 4 g / L by immersing it in an electroless platinum plating solution having a composition such that platinum is plated on the pretreated carrier coating layer to support platinum,
After drying, it is baked at 400 to 800 ° C. to be manufactured.

[Procedure Amendment 7]

[Document name to be amended] Statement

[Correction target item name] 0056

[Correction method] Change

[Correction content]

[0056] As the exhaust gas of de I over diesel engine, the case is only HC of raw exhaust gas in a high oxygen concentration range inadequate removal of NOx, the exhaust gas to reach the catalyst layer from the primary source of the exhaust gas It is possible to secure a sufficiently high NOx removal rate without excessively deteriorating the fuel economy by externally adding the minimum necessary hydrocarbon-based reducing agent on the flow path.

[Procedure Amendment 8]

[Document name to be amended] Statement

[Name of item to be corrected] 0070

[Correction method] Change

[Correction content]

[0070]

Production Example 7 Production of Cu Ion Exchange / ZSM-5 Coated Honeycomb Catalyst Rorman and Valyosky's Method (LD Roll
mann and E.I. W. Valyocsik, In
org. Synthesis. , 22 (1982) P.I.
67-68) according to H-type aluminosilicate ZSM-
5 powders (Si / Al ratio = 38) were obtained. 100 of this powder
Gr 0.03 M copper acetate deionized water solution 2000
Slurry in mL, stir at room temperature for 16 hours, filter, wash, dry, and bake in air at 500 ° C for 30 minutes to 1.2.
A wt% Cu ion exchange / ZSM-5 powder (Cu ²⁺ conversion ion exchange rate 95%) was obtained. Cu coating on honeycomb
Ion exchange / ZSM-5 coated honeycomb catalyst (7) was obtained.

[Procedure Amendment 9]

[Document name to be amended] Statement

[Name of item to be corrected] 0083

[Correction method] Change

[Correction content]

Thus, Cu / zeolite catalyst and Pt
In the known combination catalyst system with a supported catalyst, 300 ° C to 4 ° C.
It is clear that the NOx removal performance in the medium temperature range of 00 ° C is insufficient.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location B01D 53/36 102 H

Claims (7)

[Claims] 1. Exhaust gas of an internal combustion engine operated at a lean air-fuel ratio is contacted with an iridium-containing catalyst in a first zone,
Then, a method for purifying exhaust gas from an internal combustion engine, which comprises contacting with a platinum-containing catalyst in a second zone downstream thereof. 2. The method according to claim 1, wherein the iridium-containing catalyst in the first zone is a catalyst comprising iridium supported on a carrier comprising at least one selected from the group consisting of metal carbides and metal nitrides. 3. The iridium-containing catalyst in the first zone is a catalyst composed of crystalline iridium silicate.
the method of. 4. The method according to claim 1, 2 or 3, wherein the platinum-containing catalyst in the second zone has platinum supported on the surface of a porous carrier by electroless plating. 5. The exhaust gas of claim 1, wherein the exhaust gas is a gasoline lean burn engine exhaust gas.
The method described in the section. 6. The method according to claim 1, wherein the exhaust gas is a diesel engine exhaust gas. 7. The method of claim 6, wherein hydrocarbons are added to the exhaust gas before the exhaust gas is introduced into the first zone.