JPH0833845A - Catalyst for purifying exhaust gas and purifying method of exhaust gas - Google Patents

Abstract

PURPOSE:To obtain a catalyst for purifying an exhaust gas, which is obtained by coexisting and carrying iridium and nickel on a carrier composed of silicon carbide. CONSTITUTION:This catalyst for purifying the exhaust gas is capable of excellently removing the nitrogen oxides from the exhaust gas containing oxygen in excess of the stoichiometric quantity regardless of the concn. of oxygen or hydrocarbon, when compared to the conventional catalyst for purifying the exhaust gas. Furthermore, the catalyst is excellent in durability at a high temp. under the presence of moisture. Thus, the catalyst is useful for removing nitrogen oxides in an exhaust gas containing large quantity of oxygen and steam exhausted from internal combustion engine such as automobile engine, boiler or the like.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to an internal combustion engine, a boiler,
The present invention relates to a catalyst for purifying exhaust gas discharged from a gas turbine or the like, particularly an exhaust gas containing nitrogen oxides in which excess oxygen coexists, and a method for purifying the exhaust gas.

[0002]

2. Description of the Related Art Nitrogen oxides (NOx) emitted from the atmosphere of internal combustion engines and the like cause photochemical smog and acid rain. Therefore, it is urgently required to prevent the discharge of such nitrogen oxides from the viewpoint of environmental protection. In recent years, suppression of carbon dioxide (CO ₂ ) emission has been required to prevent global warming. Therefore, there is a demand for the practical application of a lean-burn engine (lean-burn gasoline engine) capable of causing a gasoline combustion reaction with an air-fuel ratio larger than the air-fuel ratio (A / F = 14.6) corresponding to the stoichiometric amount. Has been done. However, in treating the exhaust gas from this lean burn gasoline engine, the air-fuel ratio, which has been used for treating the exhaust gas of a conventional gasoline vehicle, is stoichiometric (A / F = 14.
6) NOx is converted to carbon monoxide (CO) and hydrocarbons (HC) using a Pt-Rh / Al ₂ O ₃ -based catalyst by controlling to near 6).
A three-way catalyst (TWC) method that removes at the same time is not effective. In addition, the diesel engine is a lean burn by nature,
After all, the removal of NOx is required.

Lean-burn engines such as lean-burn gasoline engines and diesel engines are collectively called lean-burn engines. As a catalyst for removing NOx in the exhaust gas of a lean burn engine, in recent years, a copper ion exchange zeolite catalyst (for example, JP-A-63-100919) and a noble metal ion exchange zeolite catalyst (for example, JP-A-1-135541). ) And other various zeolite-based catalysts have been proposed. However, these catalysts cannot be put to practical use at high temperatures of 650 to 700 ° C., because of the water vapor contained in the exhaust gas, irreversible deactivation occurs within a few hours.

As another catalyst for removing NOx, a catalyst in which a noble metal element such as Pt, Pd, Rh, Ir, Ru is supported on a porous metal oxide carrier such as alumina, silica, titania, zirconia ( JP-A-3-221144
Japanese Patent Laid-Open No. Hei 3-293035). However, this supported noble metal catalyst is a hydrocarbon (HC) which becomes a reducing agent for NOx due to the strong oxidation catalytic activity of the noble metal [in this specification, the term "hydrocarbon" means a hydrocarbon in a narrow sense]. The oxygenated hydrocarbon, which is a partial oxide thereof, such as those containing alcohols, ketones and the like], reacts preferentially with oxygen present in excess to enhance the selectivity of the NOx reduction reaction. There is a problem that there is no.

The present applicant has recently found that a catalyst in which iridium is supported on a carrier composed of at least one selected from metal carbides and metal nitrides has a reduction selectivity by hydrocarbons for NOx even in the presence of excess oxygen. It has been found that the level is increased (JP-A-6-31173). However, since the NOx removing ability of this catalyst is highly dependent on the oxygen and hydrocarbon concentrations, under high oxygen concentration conditions, in particular,
Alternatively, it may not always be possible to sufficiently remove NOx under low hydrocarbon concentration conditions.

[0006]

SUMMARY OF THE INVENTION The present invention has been made to solve the problems of the conventional exhaust gas treatment catalysts described above.
Its purpose is to contain a reducing component containing hydrocarbons and oxygen (O ₂ ) and nitrogen oxides (NOx) in excess of the stoichiometric amount required to completely oxidize all the reducing components. An object of the present invention is to provide an exhaust gas purifying catalyst that exhibits a higher NOx removal rate and high temperature durability in the presence of water even under low hydrocarbon concentration conditions.

[0007]

The present inventors have found that a catalyst comprising iridium and nickel coexistingly supported on a carrier made of silicon carbide solves such a problem. Hereinafter, the exhaust gas purifying catalyst of the present invention will be described in detail.

Carrier Silicon carbide is used as a carrier for the catalyst of the present invention.
Those that can be obtained at low cost, for example, have a particle size of 0.1 to 10
A commercially available powder or whiskers of about 0 μm can be used.

Conventionally, an Ir catalyst for exhaust gas purification has been used in which a metal oxide carrier having a porous surface and a high specific surface area is supported with a high dispersity and a fine particle size [eg, KC Taylor.
and JC Schlaher, J. Catal, 63 (1) 53-71 (1980)
]. On the other hand, the catalyst of the present invention has high selectivity of the conversion reaction with respect to NOx in the exhaust gas containing excess oxygen at high temperature and in the presence of steam, because the carrier is silicon carbide instead of metal oxide. It has the unique feature of having a long life. Further, the silicon carbide used as a carrier in the present invention preferably has a low specific surface area and is non-porous, specifically, a BE of 25 m ² / g or less (particularly 5 to ²⁰ m ² / g).
T. Specific surface area and 0.5 cm ³ / g or less (especially 0.2-0.8 cm ³ / g)
It is preferable to have a pore volume of.

Active ingredient In the catalyst of the present invention, Ir and Ni are coexistently supported on the above carrier. The state of existence of the carried Ir is not particularly limited. The existing state of Ir is, for example, a metallic state;
An oxide state of IrO, Ir ₂ O ₃ , IrO ₂ or the like; a state in which these states are mixed and the like can be mentioned. These Ir
Is preferably dispersed and supported together with the Ni element on the carrier. The dispersed and supported Ir preferably has a crystallite diameter observed by a powder X-ray diffraction method of 2 to 100 nm, more preferably 5 to 20 nm. If the crystallite size is too small, the oxidation reaction of HC and CO serving as a reducing agent with O ₂ proceeds undesirably. If the crystallite size is too large, the catalytic activity obtained is low relative to the amount supported. The amount of Ir supported on the carrier is preferably 0.1 to 10.0% by weight in terms of metal iridium, and 0.5 to 5.0 with respect to the whole catalyst.
Weight percent is more preferred. If it is too small, the activity of the catalyst itself is too low, and if it is too large, the reaction between the reducing agent and oxygen proceeds too much,
The NOx reduction selectivity is reduced.

Further, in the catalyst of the present invention, the state of existence of supported Ni is not particularly limited. Examples of the existing state of Ni include a metallic state; an oxide state of NiO; and a state in which these states are mixed. The supported amount of Ni is preferably 0.1 to 20 in atomic ratio with respect to iridium.
It is double, and more preferably 0.2 to 10 times. Ni
If the amount is too small, the effect of using Ni in combination with Ir cannot be obtained. On the other hand, if the amount of Ni supported is too large, the initial activity of the resulting catalyst will decrease, and the light-off temperature of the NOx conversion will shift to the high temperature side, decreasing the activity at low temperatures.

Preparation Method of Catalyst The preparation method of the catalyst of the present invention is not particularly limited, and a conventionally known method is applied. For example, a homogeneous mixed solution of a raw material salt of Ir and a raw material salt of Ni is impregnated into silicon carbide as a carrier,
After drying, firing and the like are carried out to prepare by the simultaneous loading method of Ir and Ni. Alternatively, the raw material salt of Ir is first impregnated into the above carrier, dried and baked to immobilize on the carrier as an insoluble compound of Ir or Ir metal, and then impregnated with the raw material salt of Ni, dried and baked again. As a result, Ir and Ni are coexisted and supported on the carrier. Alternatively, on the contrary, various sequential loading methods such as first loading and fixing Ni and then loading and fixing Ir are applied.

In the preparation of the catalyst of the present invention, there are no particular restrictions on the starting materials for iridium and Ni. Examples of the starting material of iridium include iridium trichloride (IrC).
l ₃ ), chloroiridate (H ₂ IrCl ₆ ), sodium chloroiridate (Na ₃ IrCl ₆ ), the same (Na
₂ IrCl ₆ ), iridium nitrate (Ir (N
Water-soluble salts of iridium such as O ₃ ) ₄ ) and iridium sulfate (Ir (SO ₄ ) ₂ ) are used. Also, Ir ₃ (C
O) _12, such as Ir metal carbonyl, IrCl (CO) (PPh ₃ ) ₂
The organometallic complex of Ir, etc. may be dissolved in an organic solvent such as hexane, acetone, chloroform, ethanol or the like and used. As the nickel starting material, for example, Ni chloride, nitrate, sulfate, acetate, etc. can be used. Above all,
A nitrate having a high solubility in a water solvent is particularly preferable.

In the above-described simultaneous loading method or sequential loading method, the atmosphere during the firing decomposition of the Ir compound and / or the Ni compound loaded as the catalyst precursor on the carrier depends on the kind of the precursor, in air, A vacuum, a flow of an inert gas such as nitrogen, or a flow of hydrogen is appropriately selected. The firing temperature is preferably 300 to 1000 ° C, more preferably 600 to 900 ° C. The firing time may be appropriately selected, but is usually about 10 minutes to 20 hours, preferably about 30 minutes to 5 hours. Further, the firing may be performed by combining a plurality of treatments stepwise. For example, 6 in the air
After firing at 00-800 ℃, 600-900 ℃ in hydrogen stream
May be reduced.

Exhaust Gas Purification Method According to the present invention, there is also provided an exhaust gas purification method for an internal combustion engine or the like using the above catalyst. That is, the present invention relates to an exhaust gas containing a reducing component containing hydrocarbon and an oxidizing component containing oxygen and nitrogen oxide in excess of the stoichiometric amount required to completely oxidize all the reducing component. A method for purifying the exhaust gas, which comprises contacting a gas with a catalyst-containing layer, wherein the catalyst contained in the catalyst-containing layer is the above catalyst. By this method, nitrogen oxides in the exhaust gas are selectively reductively decomposed into nitrogen (N ₂ ) and water (H ₂ O), and at the same time, hydrocarbons and carbon monoxide (CO) in the exhaust gas are removed. The reducing component is carbon dioxide (C
O ₂ ) and water (H ₂ O).

The form of the catalyst-containing layer when the catalyst of the catalyst-containing layer present invention for use in an exhaust gas purification method described above is not particularly limited. For example, the catalyst-containing layer used in the method may be composed of only the above-mentioned catalyst. In this case, usually, a method of filling the catalyst in a fixed space, a method of molding the catalyst into a predetermined shape, and the like can be considered. When the catalyst is formed into a predetermined shape, the catalyst may be mixed with an appropriate binder or formed into an appropriate fixed shape without the binder. Also, Ir
The carrier may be preliminarily formed into an appropriate shape before carrying out the supporting process of Ni and Ni. The shape of the shaped catalyst is not particularly limited, and examples thereof include a spherical shape, a pellet shape, a cylindrical shape, a honeycomb shape, a spiral shape, a granular shape, and a ring shape. The shape, size, etc. can be arbitrarily selected according to the usage conditions.

Alternatively, the catalyst-containing layer may be composed of a catalyst-coated structure obtained by coating the surface of a supporting substrate made of a refractory material with a catalyst. As the catalyst coating structure, for example, the supporting substrate is formed so as to have a large number of through holes arranged in the exhaust gas flow direction, and at least the inner surface of the through holes is coated with the catalyst. Etc. are possible.
In addition, such support substrates typically have a porosity of 60 to 90%, preferably 70 to 90%, and a square inch (5.06 c) when viewed in a cross section perpendicular to the flow direction.
30-700 per m ² ), preferably 200-600
It is preferable that individual through holes are provided. Examples of the above-mentioned support substrate include ceramics such as cordierite and mullite, and a metal such as stainless steel integrally formed into a honeycomb shape or a foam.

In the production of the catalyst-coated structure, the catalyst of the present invention may be coated (eg, wash-coated) with or without a suitable binder on the surface of the above-mentioned supporting substrate. In addition, the above-mentioned supporting substrate is previously coated only with a carrier in the same manner as in the case of using a catalyst,
The support substrate coated only with the obtained carrier may be subjected to a supporting treatment with Ir and Ni to produce a catalyst-coated structure. The coating amount of the catalyst on the supporting substrate is not particularly limited, and is preferably 50 to 200 g / L, more preferably 80 to 160 g / L per unit volume of the supporting substrate. The amount of Ir carried per unit volume of the supporting substrate is preferably 0.05 to 20.0 g.
/ L, more preferably 0.3 to 10.0 g / L, and the supported amount of Ni is preferably 0.1 of Ir in terms of atomic ratio.
˜20 times, more preferably 0.2 to 10 times.

As the binder, for example, a commonly used inorganic binder such as alumina sol, silica sol, titania sol can be used. The coating of the catalyst powder on the supporting substrate can be performed, for example, by adding alumina sol and water to the catalyst powder, kneading to form a slurry, and immersing the supporting substrate in the slurry, followed by drying and firing. .

The gas space velocity of the exhaust gas flowing through the catalyst-containing layer is not particularly limited, and the space velocity is preferably 5,000.
~ 200,000 / hr, more preferably 10,000 ~ 150,000 / hr
Is. If the space velocity is too low, a large amount of catalyst is required, and if it is too high, the NOx removal rate decreases. Also,
The catalyst-containing layer inlet temperature of the exhaust gas for the selective reduction of NOx is preferably 200 to 700 ° C.,
More preferably, it is 300 to 600 ° C. This temperature is
If it is too low, the NOx conversion rate will not rise, and if it is too high, the NOx reduction selectivity will decrease.

The exhaust gas targeted by the exhaust gas purification method of the present invention is a weight ratio A / F of an oxidizing component and a reducing component.
Is preferably on the smaller reducing component side (A / F> 14.6) than the stoichiometric amount (A / F = 14.6). The catalyst of the present invention exhibits NOx removal performance even in the vicinity of the stoichiometric amount, but its main advantage is exhibited in an oxygen excess atmosphere with A / F> 17. In such an oxygen-rich atmosphere, the conventional noble metal catalysts are all extremely deficient in NO.
Only x reduction selectivity was shown. On the other hand, the catalyst of the present invention shows a sufficiently high NOx removal rate in the region of A / F> 17, for example, O ₂ concentration> 3%. Further, the catalyst of the present invention is sufficient even with A / F> 24, for example, a high oxygen concentration region of O ₂ concentration> 10%, without the additional addition of a reducing agent HC from the outside, by using only HC in the exhaust gas. It shows NOx reduction ability. Further, depending on the engine specifications, the amount of HC contained in the exhaust gas may be relatively small [THC (hydrocarbon concentration converted into methane) / NOx = about 3], but the catalyst of the present invention has such an HC concentration. A sufficient NOx reduction ability is exhibited even under conditions where the amount of oxygen is relatively small.

[0022]

By contacting with the catalyst of the present invention, NOx in the exhaust gas is converted to N by using HC and CO coexisting in the exhaust gas and optionally added HC as a reducing agent.
It is reductively decomposed into ₂ and H ₂ O. Ir which is an active metal
However, since H is stably supported on the surface of a carrier made of silicon carbide together with coexisting Ni, H in the exhaust gas is
It is presumed that the selectivity of the reaction between the reducing agent such as C and CO and NOx is improved, and the durability against heat deterioration is also improved. As a result, NOx in the exhaust gas is efficiently removed over a long period of time.

[0023]

EXAMPLES The present invention will be described in detail below with reference to examples. A star (*) is attached to the catalyst, the catalyst coating structure, and the catalyst-containing layer according to the comparative example. (Production of catalyst) Example 1 [Ir-Ni / SiC catalyst-coated honeycomb (A-
Production Example of 1)] (a) Support of Ir-Ni on Powder Carrier Commercially available SiC powder (manufactured by LONZA, BET specific surface area 15 m
2.6 L of deionized water was added to 130 g of ² / g, pore volume 0.54 cm ³ / g) and stirred for 20 minutes to obtain a SiC powder slurry. 160 mL of a deionized aqueous solution containing iridium chloride (H ₂ IrCl ₆ ) corresponding to 1.56 g of metal iridium, and nickel nitrate hexahydrate (Ni (NO ₃ )) corresponding to 0.48 g of metal nickel were added to this slurry. ₂ · 6H
After adding a mixed solution with 160 mL of a deionized aqueous solution containing ₂ O), the mixture was transferred onto a glass lined dish with a steam jacket, and water was evaporated with stirring for 4 hours. The residue solid thus obtained was dried for 16 hours at 105 ° C by an electric dryer and then pulverized, and the obtained pulverized product was placed in a quartz tray in an electric furnace at 800 ° C in air.
It was baked for 2 hours. Thereafter, the obtained calcined powder was further reduced in a 100% hydrogen stream at 900 ° C. for 2 hours to obtain 1.2 wt% Ir-0.4 wt% Ni [Ir / Ni = 1.
/ 1 (atomic ratio)] / SiC catalyst powder was obtained. (b) 30 g of catalyst powder obtained by washcoating (a ) on honeycomb, 60 g of deionized water and alumina sol (containing 10% by weight of Al ₂ O ₃ solid content) 4.
0 g was added, and the resulting mixture was wet-milled for 5 hours with a ball mill to obtain a catalyst slurry. A core of 1 inch in diameter and 2.5 inches in length hollowed from a commercially available 400-cell cordierite honeycomb was immersed in this slurry, a catalyst was attached to this core, and excess slurry was removed by air blowing. Dry at 300 ° C for 20 minutes, then 5
By firing at 00 ° C. for 30 minutes, an Ir—Ni / SiC catalyst-coated honeycomb (A-1) coated with 130 g of catalyst per 1 liter of honeycomb volume was obtained.

Example 2 [Ir-Ni / SiC catalyst coated ha
Production Example of Nicam (A-2)] (a) Washcoat of Silicon Carbide Powder on Honeycomb Same as Example 1 (b) except that silicon carbide powder is used instead of the catalyst powder of Example 1 (b). Then, a honeycomb core piece coated with 130 g of silicon carbide in terms of dry per 1 L of honeycomb was obtained. (b) Ir-Ni supported iridium chlorate containing 9.98 g of Ir content and Ni content 3.
Deionized aqueous solution 1000 with nickel nitrate containing 07 g
The silicon carbide-coated honeycomb core piece obtained in Example 2 (a) was immersed in mL, kept at room temperature for 3 minutes, and impregnated with a mixed solution of Ir and Ni for water absorption. The excess Ir and Ni mixed solution was removed by air blowing, and after drying, it was baked in an electric furnace in air at 800 ° C. for 2 hours. After that,
The obtained calcined product was subjected to reduction treatment at 900 ° C. for 2 hours in a 100% hydrogen stream, and Ir-Ni / SiC catalyst coated honeycomb (A-2) [1.2 wt% Ir-0.4 wt% Ni [I
r / Ni = 1/1 (atomic ratio)] / SiC] was obtained.

Example 3 [Ir-Ni / SiC catalyst coated ha
Production Example of Nicam (A-3)] Example 1 except that, in Example 1 (a), reduction treatment was performed at 700 ° C. for 2 hours in hydrogen stream instead of reduction treatment at 900 ° C. for 2 hours in hydrogen stream. Ir-Ni /
A SiC catalyst-coated honeycomb (A-3) was obtained.

Example 4 [Ir-Ni / SiC catalyst coated ha
Production Example of Nicam (A-4)] In Example 1 (a), reduction treatment was not performed at 900 ° C. for 2 hours in hydrogen gas flow, and in Example 1 (b), hydrogen gas flow was performed after firing at 500 ° C. for 30 minutes. Ir-Ni / Si in the same manner as in Example 1 except that the reduction treatment was performed at 900 ° C. for 2 hours.
A C catalyst-coated honeycomb (A-4) was obtained.

Example 5 [Ir-Ni / SiC catalyst coated ha
Production Example of Nicam (A-5)] In Example 4, the amount of SiC powder was 121 g, the amount of metallic iridium was 5.2 g, and the amount of metallic nickel was 1.60.
Ir-Ni / Si in the same manner as in Example 4 except that g was changed to g.
C catalyst coated honeycomb (A-5) [4.0 wt% Ir-
1.3 wt% Ni [Ir / Ni = 1/1 (atomic ratio)] /
SiC] was obtained.

Example 6 [Ir-Ni / SiC catalyst coated ha
Production Example of Nicam (A-6)] In Example 1 (a), the amount of commercially available SiC powder was 126
g, and the amount of metallic nickel was set to 4.80 g in the same manner as in Example 1 Ir-Ni / SiC catalyst coated honeycomb (A-6) [1.2 wt% Ir-4.0 wt% Ni [I
r / Ni = 1/10 (atomic ratio)] / SiC] was obtained.

Comparative Example 1 [Ir / SiC catalyst coated honeycomb
Production Example of (B-1 ^* )] In Example 1 (a), 1.56 g of metal iridium was used instead of the mixed solution of iridium chloride and nickel nitrate.
Example 1 except that only 160 mL of a deionized aqueous solution containing chloroiridate (H ₂ IrCl ₆ ) corresponding to
In the same manner as in Ir / SiC catalyst coated honeycomb (B-
1 ^* ) (1.2 wt% Ir / SiC) was obtained.

Comparative Example 2 [Ir / SiC catalyst coated honeycomb
Production Example of (B-2 ^* )] In Example 2 (b), instead of the mixed solution of iridium chloride and nickel nitrate, 9.98 g of metal iridium was used.
Deionized aqueous solution containing chloroiridate corresponding to
Ir / S in the same manner as in Example 2 except that 00 mL was used.
iC catalyst coated honeycomb (B-2 ^* ) (1.2 wt% Ir
/ SiC) was obtained.

Comparative Example 3 [Ni / SiC catalyst coated honeycomb
Production Example of (B-3 ^* )] In Example 1 (a), in place of the mixed solution of iridium chloride and nickel nitrate, nickel nitrate hexahydrate (Ni ( NO ₃ ) ₂・
Ni / SiC catalyst-coated honeycomb (B-3 ^* ) (0.4 wt% Ni / SiC) was obtained in the same manner as in Example 1 except that 160 mL of a deionized aqueous solution containing 6H ₂ O) was used.

Comparative Example 4 [Ir-Ni / Al ₂ O ₃ catalyst coating]
Production Example of Covered Honeycomb (B-4 ^* )] In Example 1 (a), commercially available Al was used instead of SiC powder.
₂ O ₃ powder (manufactured by Sumitomo Chemical, BET specific surface area 160 m ² /
g) I in the same manner as in Example 1 except that 130 g was used.
r-Ni / Al ₂ O ₃ catalyst coated honeycomb (B-4 ^* )
[1.2 wt% Ir-0.4 wt% Ni [Ir / Ni =
1/1 (atomic ratio)] / Al ₂ O ₃ ] was obtained.

(Performance Evaluation) In each of the following performance evaluation examples, the catalyst-containing layer was formed by filling the catalyst-coated honeycomb prepared in the above Examples. In the following description, for example, the catalyst-containing layer made of the catalyst-coated honeycomb A-1 of Example 1 is described as “catalyst-containing layer A-1”.

Performance Evaluation Example 1 [Lite on with model gas
Performance Evaluation- (1)] For each of the catalyst-containing layer A-1 of the example of the present invention and the catalyst-containing layer B-1 ^* of the comparative example, a lean burn engine model exhaust gas (A) having the following composition was used as a gas space. Speed SV60,0
While flowing at 00 / hr, the catalyst-containing layer inlet gas temperature was set to 150
The temperature is continuously raised from ℃ to 500 ℃, during which the concentration of CO, HC and NOx in the gas flowing out at the outlet of the catalyst containing layer is continuously measured to determine the light-off performance (catalyst containing layer inlet gas temperature and conversion rate). Relationship) was evaluated. Catalyst-containing layer A-1
The results are shown in FIG. 1, and the catalyst-containing layer B-
The results for 1 ^* are shown in FIG.

[0035]

As shown in FIG. 1, the catalyst-containing layer A-1 using the catalyst A-1 of Example 1 has CO, HC, and
It can be seen that all of the three NOx components can be removed from the exhaust gas with good conversion. On the other hand, as shown in FIG. 2, the catalyst-containing layer B-1 ^* using the catalyst B-1 ^* of Comparative Example 1 was composed of HC and CO.
Although it can convert satisfactorily at a temperature of 400 ° C or higher,
It can be seen that the Ox conversion rate is significantly inferior to that of the catalyst-containing layer A-1 of Example 1.

Performance Evaluation Example 2 (Depending on the oxygen concentration of the NOx conversion rate)
Existence evaluation) Oxygen concentration is 3.2%, 5.0%, 7.5, respectively.
%, 10.0% and 14.0%, except that 5 kinds of model exhaust gas having the same composition as the above model exhaust gas (A) were used, and the catalyst containing layer was prepared in the same manner as in Performance Evaluation Example 1. The light-off performance of A-1 and the catalyst-containing layer B-1 ^* was evaluated, and the light-off performance curve was obtained. The results of plotting the maximum NOx conversion rate (maximum value of the light-off performance curve) of the light-off performance curve at each oxygen concentration obtained are shown in FIG.
Shown in From FIG. 3, the catalyst-containing layer A-1 has an oxygen concentration of 3.2.
% (In this case, A / F = 17) to oxygen concentration 14%
High maximum NO over (A / F = 38 equivalent in this case)
It can be seen that the x conversion is retained. On the other hand, the maximum NOx conversion rate of the catalyst-containing layer B-1 ^* sharply decreases as the oxygen concentration increases, and in particular, it is significantly reduced in the oxygen concentration range of 5% or more. The catalyst of the present invention has an oxygen concentration of 3.2% (A / F = 17 equivalent) to 14.0.
It was shown that the NOx conversion rate was high and the oxygen concentration dependency was small over a wide oxygen concentration range up to 100% (A / F = 38).

Performance Evaluation Example 3 [Lite on with model gas
Performance evaluation- (2)] For each of the catalyst-containing layers A-1 to A-6 using the catalyst of the example and the catalyst-containing layers B-1 ^{* to} B-4 ^* using the catalyst of the comparative example, Lean-burn gasoline engine model exhaust gas (B) of composition is flowed at a gas space velocity SV38,000 / hr while the catalyst-containing layer inlet gas temperature is changed from 150 ° C to 7 ° C.
The temperature was continuously raised to 00 ° C., and the NOx concentration of the gas flowing out at the outlet of the catalyst-containing layer was continuously measured during that period.

[0039]

In Table 1, catalyst-containing layers A-1 to A-6 and B-
The maximum NOx conversion rate is shown for 1 ^{* to} B-4 ^* . From the results of Table 1, the catalyst-containing layers A-1 to A-6 of the examples are
Conventional supported Ir catalyst, supported Ni catalyst or Ir-Ni / A
using l ₂ O ₃ catalyst, the catalyst-containing layer of the Comparative Example (B-
1 ^* , B-3 ^* or B-4 ^* ),
It can be seen that the conversion of nitrogen oxide is excellent in the presence of oxygen in excess of the stoichiometric amount.

[0041]

[Table 1]

Performance Evaluation Example 4 [Dependence of NOx conversion rate on HC concentration]
Existence evaluation] Propylene concentrations are 500 ppm and 750 p, respectively.
pm, 1,000 ppm, 1,250 ppm and 1,5
Except for the difference from 00ppm, the above model exhaust gas (B)
The light-off performances of the catalyst-containing layer A-2 and the catalyst-containing layer B-2 ^* were evaluated in the same manner as in Performance Evaluation Example 3 except that 5 types of model exhaust gas having the same composition as the above were used. Maximum N of the light-off performance curve at each propylene concentration obtained
The result of plotting the Ox conversion rate (maximum value of the light-off performance curve) is shown in FIG. From FIG. 4, the catalyst-containing layer A-2 has a propylene concentration of 1,500 ppm (THC / NOx =
It is understood that a high maximum NOx conversion rate is maintained from 9) to 500 ppm (THC / NOx = 3). On the other hand, the maximum NOx conversion rate of the catalyst-containing layer B-2 ^* is
It can be seen that the propylene concentration sharply decreases as the propylene concentration decreases, and particularly in the concentration range of the propylene concentration of 1,200 ppm or less. The catalyst of the present invention is THC / NO
It was shown that the NOx conversion was high and the HC concentration dependency was small over a wide propylene concentration range in which the x ratio was 9 to 3.

Performance Evaluation Example 5 [Light with model exhaust gas
Tooff Performance Evaluation- (3)] In Performance Evaluation Example 3, the catalyst-containing layer A-5 having the lowest NOx conversion rate among the examples and the catalyst-containing layer B-having the highest maximum NOx conversion rate among the comparative examples. 2 ^{* was} subjected to a heat resistance test under high temperature conditions. Catalyst-containing layer A
About 5 and the catalyst-containing layer ^{B-2 *, 10% H} 2 O +
After aging treatment at 700 ° C. and 800 ° C. for 5 hours under a mixed gas flow of 90% air, the NOx conversion rate was evaluated in the same manner as in Performance Evaluation Example 3. The results are shown in Table 2. The results when no aging treatment is performed (fresh) are also shown. From Table 2, the catalyst-containing layer B-
In 2 ^* , the NOx conversion rate decreased from 73% to 56% by the aging treatment at 800 ° C. On the other hand, the catalyst-containing layer A-5 has NOx even after the same 800 ° C aging treatment.
The conversion rate was 61%, which was almost the same as 63% when fresh, indicating that the NOx removal ability was maintained. Therefore, it was shown that the catalyst of the present invention is also excellent in high temperature durability in the presence of steam.

[0044]

[Table 2]

[0045]

The exhaust gas purifying catalyst of the present invention differs from conventional exhaust gas purifying catalysts in that the exhaust gas containing oxygen in excess of the stoichiometric amount changes the concentration of oxygen and hydrocarbons. In other words, it shows a high conversion rate of nitrogen oxides. Further, this catalyst is also excellent in durability at high temperature in the presence of water. Therefore, the exhaust gas purifying catalyst of the present invention is effective for removing nitrogen oxides in exhaust gas containing a large amount of oxygen and water vapor from internal combustion engines such as automobile engines and boilers. In particular, this catalyst is useful as an exhaust gas treatment catalyst for a lean burn engine for a vehicle used under conditions where load changes are severe.

[Brief description of drawings]

FIG. 1 is a diagram showing light-off characteristics of CO, HC and NOx with respect to a lean burn engine model exhaust gas (A) of a catalyst containing layer A-1.

FIG. 2 is a diagram showing light-off characteristics of CO, HC and NOx with respect to a lean burn engine model exhaust gas (A) of a catalyst containing layer B-1 ^* .

FIG. 3 is a diagram showing the oxygen concentration dependence of the maximum NOx conversion rates of the catalyst-containing layer A-1 and the catalyst-containing layer B-1 ^* .

FIG. 4 is a diagram showing the propylene concentration dependence of the maximum NOx conversion rates of the catalyst-containing layer A-2 and the catalyst-containing layer B-2 ^* .

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[Procedure amendment]

[Submission date] August 31, 1994

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0009

[Correction method] Change

[Correction content]

Conventionally, an Ir catalyst for purifying exhaust gas has been used, which is supported on a metal oxide carrier having a high porosity and a high specific surface area with a high degree of dispersion and a fine particle size [eg, K. C. Ta
Yor and J. C. Schlaher, J .; C
atal, 63 (1) 53-71 (1980)]. On the other hand, in the catalyst of the present invention, the carrier is not a metal oxide,
Since it is silicon carbide, it has the unique feature of exhibiting high selectivity and long life of the conversion reaction with respect to NOx in the exhaust gas containing excess oxygen in the presence of steam at high temperature. further,
The silicon carbide used as a carrier in the present invention preferably has a low specific surface area and is non-porous, and specifically 25
B. of m ² / g or less (particularly 5 to ²⁰ m ² / g). E. FIG. T.
Specific surface area and 0. 8 cm ³ / g or less (especially 0.2 to 0.8
It is preferred to have a pore volume of cm ³ / g).

Claims (4)

[Claims] 1. An exhaust gas purifying catalyst comprising iridium and nickel coexistingly supported on a carrier made of silicon carbide. 2. A catalyst-coated structure for purifying exhaust gas comprising the support substrate made of a refractory material and the catalyst according to claim 1 coated on the surface of the support substrate. 3. Exhaust gas containing a reducing component containing hydrocarbons and an oxidizing component containing oxygen and nitrogen oxides in excess of the stoichiometric amount required to completely oxidize all the reducing components. In the method for purifying the exhaust gas, which comprises contacting the catalyst with a catalyst-containing layer, the catalyst contained in the catalyst-containing layer is the catalyst according to claim 1. 4. The exhaust gas purifying method according to claim 3, wherein the catalyst-containing layer comprises the catalyst-coated structure according to claim 2.