JPH0731884A - Catalyst for purifying exhaust gas and purifying method of exhaust gas - Google Patents

Abstract

PURPOSE:To obtain a catalyst whose activity in removal of nitrogen oxide under coexistence of oxygen and steam is high even under large space velocity and durability is excellent at high temperature by compatibly carrying iridium and alkaline earth metal on the carrier of metallic carbide and/or metallic nitride. CONSTITUTION:A carrier is selected from metallic carbide such as silicon carbide and titanium carbide and metallic nitride such as titanium nitride and zirconium nitride. Iridium and alkaline earth metal such as Ca and Sr are compatibly carried on the carrier. In the catalyst obtained by such a way, higher NOx conversion and high-temperature durability are shown for exhaust gas which contains a reductive component containing hydrocarbon and oxygen excessive more than stoichiometric quantity necessary to completely oxidize the whole reductive component and nitrogen oxide.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying catalyst useful for removing nitrogen oxides (NOx) contained in exhaust gas from automobiles, boilers and the like.

[0002]

_2. Description of the Related Art Conventionally, the method for removing nitrogen oxides (NOx) contained in exhaust gas is (1) using a V ₂ O ₂ -TiO ₂ catalyst,
Selective reduction method with ammonia, (2) Pt-Rh / Al ₂ O ₃ automotive three-way catalyst method, (3) Pt / Al ₂ O ₃ , Co ₃ O
₄ , a direct decomposition method using a noble metal catalyst such as YBa ₂ Cu ₃ O _y or a metal oxide catalyst is known. However, each of these methods has drawbacks and is by no means satisfactory. In other words, since the method of (1) uses ammonia, it has problems in cost, facility and toxicity of ammonia, and it is difficult to apply it especially to small-scale migration sources of nitrogen oxides such as automobiles. . (2)
The catalysts of (3) and (3) have a problem that the NOx removal reaction hardly proceeds in the presence of oxygen in excess of the stoichiometric amount.

Therefore, in recent years, a copper ion-exchanged zeolite catalyst showing a relatively high activity even in the presence of excess oxygen (for example, JP-A-63-100919) and a copper-transition metal co-ion-exchanged zeolite catalyst (JP-A-1). Much research has been done on zeolite-based catalysts such as -310742). 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, which causes irreversible deactivation within a few hours. Further, a catalyst in which a noble metal element such as Pt, Pd, Rh, Ir, and Ru is supported on a porous metal oxide carrier such as γ-Al ₂ O ₃ , SiO ₂ , TiO ₂ , and ZrO ₂ (JP-A-3- twenty two
1144 and JP-A-3-293035) were also reported. However, these conventional supported precious metal catalysts preferentially react with hydrocarbons (HC), which should be reducing agents of nitrogen oxides (NOx), due to the strong oxidation catalytic activity of precious metals, with oxygen existing in excess. , There was a problem that the selectivity of NOx reduction could not be increased.

As described above, the conventional catalysts proposed for the selective reduction of NOx with hydrocarbons in the presence of excess oxygen have low high-temperature durability in the presence of water vapor or low NOx reduction selectivity. None of them has satisfactory performances in both selectivity and life, and has not been put to practical use.

[0005]

DISCLOSURE OF THE INVENTION The present invention has been made to solve the problems of the above-mentioned conventional catalysts, and its object is to completely reduce a reducing component containing a hydrocarbon and all the reducing components. Oxygen in excess of the stoichiometric amount required to oxidize
An object of the present invention is to provide an exhaust gas purification catalyst that exhibits a higher NOx conversion rate and higher temperature durability with respect to exhaust gas containing (O ₂ ) and nitrogen oxides (NOx).

[0006]

The present inventors have made (A) iridium and (B) an alkaline earth metal coexist on a support comprising at least one selected from metal carbides and metal nitrides. It has been found that such a catalyst solves such a problem. Hereinafter, the exhaust gas purifying catalyst of the present invention will be described in detail.

Support As the support of the catalyst of the present invention, at least one selected from metal carbides and metal nitrides is used. Examples of the metal carbide include silicon carbide, titanium carbide, boron carbide, vanadium carbide and tantalum carbide, and examples of the metal nitride include titanium nitride and zirconium nitride. These may be used alone,
You may use it in combination of 2 or more type. Of these, silicon carbide, titanium carbide or titanium nitride, or a combination of two or more thereof is preferable. The morphology of metal carbides and metal nitrides should be at least 800 in air with steam.
There is no particular limitation as long as it is stable up to ℃, preferably around 1200 ℃. As a material that can be obtained at low cost, for example, a powder having a particle diameter of about 0.1 μm to 100 μm or a commercially available product as whiskers can be used.

Conventionally, Ir catalysts for purifying exhaust gas are porous,
A metal oxide carrier having a high specific surface area and having a high degree of dispersion and a fine particle size was used (for example, KCTaylor and
JC Schlaher, J. Catal, 63 (1) 53-71 (1980)). On the other hand, in the catalyst of the present invention, the carrier is a metal carbide or a metal nitride instead of a metal oxide, and the porosity is not a high specific surface area but a low specific surface area and is non-porous. NOx in exhaust gas containing excess oxygen in the presence of water vapor
It has the unique feature of having high selectivity and long life for the removal of That is, the metal carbide and metal nitride used as a carrier in the present invention is generally 15 m ² / g or less.
It preferably has a BET specific surface area and a pore volume of 0.5 cm ³ / g or less.

Active Component In the catalyst of the present invention, (A) Ir and (B) alkaline earth metal are supported on the above carrier. The presence state of Ir carried is not particularly limited, but preferably a metal state, or
IrO, Ir ₂ O ₃ , oxide state such as IrO ₂ or complex oxide with alkaline earth metal, such as CaIrO ₃ , Ca ₂ Ir
O ₄ , Ca ₄ IrO ₅ , SrIrO ₃ , Sr ₂ Ir ₃ O ₃ , Sr ₄ IrO
₆ , an oxide state in which the valence of Ir such as BaIrO ₃ is +4, or a mixed state thereof. These Ir
Is dispersed and supported together with the alkaline earth metal on the carrier in a state where the crystallite diameter observed by the powder X-ray diffraction method is preferably 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 activity is low relative to the supported amount.

The amount of Ir supported on the catalyst is preferably 0.05 to 10.0% by weight and 0.1% by weight in terms of metal iridium.
5.0% by weight is more preferred. If the amount 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, and the selectivity of NOx reduction decreases. In the catalyst of the present invention, Mg, Ca, Sr and Ba can be used as the alkaline earth metal, and Ca, Sr and Ba are preferably used. These alkaline earth metals may be used alone or in combination of two or more. These alkaline earth metals are supported in the state of an alkaline earth metal oxide, a composite oxide of alkaline earth metal and iridium, or a mixture thereof. The amount of the alkaline earth metal supported is preferably 0.5 to 15 times, more preferably 1.0 to 5 times the atomic ratio of iridium. If the amount of the alkaline earth metal carried is too small, the effect of combining the alkaline earth metal with Ir cannot be obtained,
No improvement in heat resistance is observed. On the other hand, if the amount of the alkaline earth metal 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 low temperature activity.

[0011] Preparation method for producing 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 an alkaline earth metal is impregnated into a metal carbide or a metal nitride which is a carrier, dried, and then baked.
It is prepared by a simultaneous loading method of Ir and an alkaline earth metal element. Alternatively, first, the raw material salt of Ir is impregnated into the 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 alkaline earth metal and dried again. By firing, as a result, Ir and alkaline earth metal are coexistently supported on the carrier. Alternatively, on the contrary, various sequential supporting methods such as first carrying and fixing an alkaline earth metal element and then carrying and fixing Ir are applied.

There are no particular restrictions on the starting materials for the iridium and alkaline earth metals in the preparation of the catalyst of the present invention. Examples of the starting material for iridium include iridium trichloride (IrCl ₃ ), iridium chloride (H ₂ IrCl ₆ ), sodium iridium chloride (Na ₃ IrCl ₆ ), and the same (Na ₂ IrCl ₆ ).
, Iridium nitrate (Ir (NO ₃ ) ₄ , iridium sulfate (Ir (S
A water-soluble salt of iridium such as O ₄ ) ₂ ) is used. Further, an Ir organometallic complex such as Ir ₃ (CO) ₁₂ may be dissolved in an organic solvent such as hexane or ethanol for use.

As a starting material for the alkaline earth metal, M
Although chlorides, nitrates, nitrites, acetates, and the like of g, Ca, Sr, and Ba can be used, nitrates and nitrites are particularly preferable as the starting materials having high solubility in an aqueous solvent. In the above simultaneous loading method or sequential loading method,
The atmosphere for the firing decomposition of the Ir compound and / or the alkaline earth metal compound supported as a catalyst precursor on the carrier may be air, vacuum, an inert gas stream such as nitrogen, or a hydrogen stream depending on the type of the precursor. Moderately selected. The firing temperature is usually 300 ° C to 900 ° C, more preferably 600 ° C.
The firing temperature is usually from 10 minutes to 20 hours, preferably from 30 minutes to 5 hours. Further, the firing may be performed by combining a plurality of treatments stepwise. For example, after calcination in air at 600 ℃ ~ 800 ℃, 600 ℃ in steam flow
The reduction treatment may be carried out at ℃ to 900 ℃.

The catalyst of the present invention may be mixed with a suitable binder, or may be molded into a suitable fixed shape without a binder, for example, pellets, spheres, rings, honeycombs, or the like, or may be used in advance. The shaped carrier may be impregnated with Ir and a compound of alkaline earth metal. Also,
The catalyst of the present invention is a suitably shaped refractory support substrate, for example, a ceramic such as cordierite or mullite, or a surface of a metal such as stainless steel integrally formed into a honeycomb shape or a foam shape, together with a suitable binder, or Alternatively, it can be used as a coating (for example, washcoat) without a binder. At this time, the coating amount of the catalyst on the supporting substrate is not particularly limited, but is preferably 20 to 200 g / L, more preferably 60 to 120 g per unit volume of the supporting substrate.
/ L. The supported amount of Ir per unit volume of the supporting substrate is preferably 0.01 to 10.0 g / L, more preferably 0.1 to 5.0 g / L, and the supported amount of alkaline earth metal is preferably 0.5 times the atomic ratio of Ir. -15 times, more preferably 1-5 times.

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

Method of Use The catalyst of the present invention produces an exhaust gas containing a reducing component containing hydrocarbons and oxygen and nitrogen oxides in excess of the stoichiometric amount required to completely oxidize all the reducing components. By contacting the exhaust gas with the nitrogen oxides in the exhaust gas,
(N ₂ ) and water (H ₂ O) are selectively reduced and removed.
At this time, the reducing component composed of hydrocarbons and carbon monoxide (CO) in the exhaust gas is oxidized into carbon dioxide (CO ₂ ) and water (H ₂ O).

The gas space velocity is not particularly limited, but preferably the space velocity is 5,000 to 200,000 / hr, more preferably
It is in the range of 10,000 to 150,000 / hr. If the space velocity is too small, a large amount of catalyst is required, and if it is too large, NO
x The removal rate decreases. The catalyst layer inlet temperature of the exhaust gas for selective NOx reduction is in the range of 200 ° C to 700 ° C, preferably 300 ° C to 600 ° C. If the temperature is below 200 ° C, the conversion rate of NOx does not rise, and if it exceeds 700 ° C, NOx
The reduction selectivity is reduced.

In the exhaust gas targeted by the method for purifying exhaust gas of the present invention, the weight ratio A / F of the oxidizing component and the reducing component is
Is less than the reducing component (A / F = 14.7)
/F>14.7). The catalyst of the present invention has a stoichiometric amount
It exerts NOx removal performance, but the real advantage is
A / F> 17 in an oxygen-excess atmosphere. In such an oxygen-rich atmosphere, all the conventional noble metal catalysts showed remarkably insufficient NOx reduction selectivity. On the other hand, the catalyst of the present invention shows a sufficiently high NOx removal rate even in the region where the A / F 17 is exceeded and the O ₂ concentration is 3% or more.

Action NO in exhaust gas is brought into contact with the catalyst of the present invention.
x represents N ₂ and H ₂ O with HC, CO coexisting in the exhaust gas and / or HC which may be additionally added to the exhaust gas before the exhaust gas comes into contact with the catalyst layer as a reducing agent. Is reduced and decomposed into. Since the active metal Ir is stabilized on the surface of the carrier made of metal carbide or metal nitride together with the coexisting alkaline earth metal, the reaction of reducing agents such as HC and CO in the exhaust gas with NOx. It is presumed that in addition to the improved selectivity, 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.

[0020]

【Example】

(Catalyst preparation) Example 1 Commercially available SiC powder (manufactured by LONZA, BET specific surface area 15 m ² / g) 30
Wet-grinding was carried out for 5 hours with a ball mill together with a mixed solution of 4 g of 10% alumina sol and 60 mL of deionized water to obtain a slurry of SiC powder carrier. A commercially available cordierite 400-cell / inch 2 honeycomb core (diameter 1 inch, length 2.5 inches) was immersed in this slurry, and the excess slurry was removed with an air knife to remove the SiC powder from the honeycomb 1
It was adjusted to 100 g per liter, dried at 260 ° C. for 30 minutes, and then fired at 500 ° C. for 30 minutes to obtain a SiC-coated honeycomb core.

A honeycomb core coated with SiC powder is
Iridium chloride (H ₂ IrCl ₆ ) and calcium chloride (CaCl
₂・ 2H ₂ O) mixed solution (Ca: Ir = 1: 1, atomic ratio), and iridium and calcium are added to the honeycomb volume of 1
It was loaded so as to be 0.60 g and 0.13 g in terms of metal per liter. The honeycomb core was dried in a dryer at 115 ° C for 16 hours and then fired in an electric furnace at 800 ° C for 30 minutes in air. Further, reduction treatment was carried out in a 100% hydrogen stream at 700 ° C. for 2 hours to obtain the target catalyst A-1.

Example 2 50 g of commercially available SiC powder was slurried in 2 liters of deionized water, and an aqueous solution of iridium chloride (H ₂ IrCl ₆ ) was added to this so that the solid content of iridium was 0.6% by weight in terms of metal. In addition, it was evaporated to dryness with stirring. The obtained solid was dried in a dryer at 115 ° C for 16 hours, pulverized, and then baked in an electric furnace at 800 ° C for 30 minutes in the air. The obtained iridium-supported Si
C powder 30g was put into a ball mill together with 10% of alumina sol 4g and deionized water 60 mL, this iridium an aqueous solution of calcium nitrate tetrahydrate salt as an alkaline earth metal source _{(Ca (NO 3) 2 ·} 4H 2 O): Calcium Was added in an atomic ratio of 1: 1 and wet pulverized for 5 hours to obtain a catalyst slurry. In this slurry, commercially available cordierite 400
Cell / square inch honeycomb core (1 inch diameter, length
(2.5 inch) is immersed and taken out, excess slurry is removed with an air knife, the SiC powder is adjusted to 100 g per 1 liter of honeycomb volume, dried at 260 ° C for 30 minutes, and then calcined at 500 ° C for 30 minutes. . Further, reduction treatment was carried out at 700 ° C. for 2 hours in a 100% hydrogen stream to obtain the target catalyst A-2.

Example 3 Magnesium nitrate hexahydrate (Mg) as a source of alkaline earth metal
_{(NO 3) 2 · 6H 2} O) aqueous solution using the iridium: 1 ratio of magnesium in atomic ratios: except that was adjusted to 1 in the same manner as in Example 2, the catalyst A-3 for purposes Got

Example 4 Strontium nitrate (Sr (N
The target catalyst A-4 was obtained in the same manner as in Example 2 except that an aqueous solution of O ₃ ) ₂ ) was used and the ratio of iridium: strontium was adjusted to be 1: 1 in atomic ratio.

Example 5 Barium nitrate (Ba (NO ₃ ) ₂ ) as a source of alkaline earth metal
The target catalyst A-5 was obtained in the same manner as in Example 2 except that the iridium: barium ratio was adjusted to be 1: 1 in atomic ratio by using the aqueous solution of.

Example 6 Same as Example 1 except that iridium and calcium were loaded so as to be 2.0 g and 0.43 g (Ca: Ir = 1: 1, atomic ratio) in terms of metal per liter of honeycomb volume, respectively. The desired catalyst A-6 was obtained.

Example 7 Mixing of iridium chloride (H ₂ IrCl ₆ ) and calcium nitrate tetrahydrate (Ca (NO ₃ ) ₂ .4H ₂ O) as a liquid for immersing the honeycomb core coated with SiC powder Aqueous solution (Ca: Ir = 2:
1, atomic ratio) was used in the same manner as in Example 1,
Iridium and calcium were loaded so as to be 0.60 g and 0.25 g in terms of metal, respectively, per liter of honeycomb volume, to obtain a target catalyst A-7.

Example 8 A mixture of iridium chloride (H ₂ IrCl ₆ ) and calcium nitrate tetrahydrate (Ca (NO ₃ ) ₂ .4H ₂ O) was used as a liquid for dipping a honeycomb core coated with SiC powder. Aqueous solution (Ca: Ir = 5:
1, atomic ratio) was used in the same manner as in Example 1,
Iridium and calcium were loaded so as to be 0.60 g and 0.63 g in terms of metal, respectively, per liter of honeycomb volume to obtain the target catalyst A-8.

Example 9 As a liquid for immersing a honeycomb core coated with SiC powder, a mixed aqueous solution (Sr: Ir = 10) of iridium chloride (H ₂ IrCl ₆ ) and strontium nitrate (Sr (NO ₃ ) ₂ ). Atomic ratio of 1: 1, atomic ratio) was used in the same manner as in Example 1, and iridium and strontium were loaded so as to be 0.60 g and 2.7 g in terms of metal per liter of honeycomb volume, respectively. Got

Example 10 50 g of commercially available TiC powder (manufactured by KENNAMETAL MACRO DIVISION)
Was slurried in 2 liters of deionized water, and iridium chloride (H ₂ IrCl ₆ ) aqueous solution was added to the solution to obtain a solid content of 2.0% by weight of iridium in terms of metal, and calcium nitrate tetrahydrate (Ca (NO
_{3) 2} · 4H ₂ O) aqueous solution of iridium: calcium 1 in atomic ratio: 1 so as added and evaporated to dryness with stirring. After drying the obtained solid in a dryer at 115 ° C for 15 hours,
It was crushed and fired in an electric furnace in air at 800 ° C for 30 minutes. This was coated on a cordierite honeycomb in the same manner as in Example 1. Further, reduction treatment was carried out at 700 ° C. for 2 hours in a 100% hydrogen stream to obtain the target catalyst A-10.

Example 11 Instead of TiC powder as a carrier, commercially available TiN powder (KINNAME
A target catalyst A-11 was obtained in the same manner as in Example 10 except that TAl MACRO DIVISION) was used.

Comparative Example 1 The same procedure as in Example 1 was carried out except that an aqueous solution of iridium chloride (H ₂ IrCl ₆ ) was used as a liquid for immersing the honeycomb core coated with SiC powder, and iridium was added per 1 liter of honeycomb. The desired catalyst B-1 was obtained by supporting the catalyst so that the amount would be 0.60 g.

Comparative Example 2 50 g of commercially available γ-Al ₂ O ₃ (KHA-24 manufactured by Sumitomo Chemical Co., Ltd.) was slurried in 2 liters of deionized water, and iridium was added to the solid content ratio of 1.0% by weight in terms of metal. Thus, an aqueous iridium chloride (H ₂ IrCl ₆ ) solution was added, and the mixture was maintained at 80 ° C. for 2 hours while stirring. The slurry was filtered, and the obtained solid was dried in an electric dryer at 115 ° C for 16 hours and then calcined in an electric furnace at 800 ° C for 30 minutes in the air. The obtained Ir-supported γ-Al ₂ O ₃ was added with 4 g of 10% alumina sol and 60 mL of deionized water.
Wet pulverization was performed with a ball mill together with the mixed solution of, to obtain a slurry of Ir-supported γ-Al ₂ O ₃ . A commercially available cordierite 400 cell / square inch honeycomb core (1 inch diameter, 2.5 inch length) was dipped in this slurry and taken out. Then, the excess slurry was removed with an air knife to remove Ir.
The supported γ-Al ₂ O ₃ was adjusted to 100 g per 1 liter of the honeycomb, dried at 260 ° C. for 30 minutes, and then calcined at 500 ° C. for 30 minutes to obtain the target catalyst B-2.

Comparative Example 3 30 g of commercially available γ-Al ₂ O ₃ was wet pulverized with 4 g of 10% alumina sol and 60 mL of deionized water in a ball mill to obtain γ-Al ₂
A slurry of O ₃ was obtained. A commercially available cordierite 400 cell / square inch honeycomb core (1 inch in diameter, 2.5 inch in length) was immersed in this slurry and taken out.
Remove excess slurry with an air knife and remove γ-Al ₂ O
₃ is adjusted to 100 g per 1 liter of honeycomb, dried at 260 ° C for 30 minutes, and fired at 500 ° C for 30 minutes to
A -Al ₂ O ₃ -coated honeycomb core was obtained.

This honeycomb core was treated with iridium chloride (H ₂
It was immersed in an IrCl ₆ ) aqueous solution, and iridium was loaded so as to be 1.0 g in terms of metal per liter of honeycomb. This honeycomb core was dried in a dryer at 115 ° C for 16 hours and then dried at 500 ° C.
It was baked for 2.5 hours. Next, this honeycomb core was immersed in an aqueous solution of barium nitrate (Ba (NO ₃ ) ₂ ) and barium was loaded so as to be 2.9 g in terms of metal per liter of honeycomb (Ba: Ir = 4: 1, atomic ratio). Then, this honeycomb core was dried in a dryer at 115 ° C for 16 hours and then calcined at 550 ° C for 4 hours to obtain a target catalyst B-3.

Comparative Example 4 The catalyst B-3 obtained in Comparative Example 3 was calcined at 600 ° C. for 6 hours,
Then, reduction treatment was performed at 500 ° C. for 1 hour in a 100% hydrogen stream. Furthermore, this calcination and the hydrogen reduction treatment are alternately performed for 2 times each.
The operation was repeated each time to obtain the desired catalyst B-4.

Comparative Example 5 As a liquid for supporting barium, barium nitrite (B
a (NO ₂ ) ₂ ) aqueous solution was used, and barium was loaded so that 12.1 g of metal was calculated per liter of honeycomb (B
Aside from a: Ir = 17: 1, atomic ratio), the same operation as in Comparative Example 3 was carried out to obtain the target catalyst B-5.

Comparative Example 6 The catalyst B-5 obtained in Comparative Example 5 was subjected to calcination and hydrogen reduction in the same manner as in Comparative Example 4 to obtain the desired catalyst B-6.

Comparative Example 7 Rollmann and Valyocsik method (LORollmann and EW
ZSM-5 type zeolite (SiO ₂ / Al ₂ O ₂ = 5) synthesized according to Valyocsik, Inorg. Synthesis, 22 (1982) 67-68).
4) was wet-milled with a ball mill together with a mixed solution of silica sol and deionized water to obtain a zeolite carrier slurry.
A commercially available cordierite 400-cell / square inch honeycomb core was dipped in this slurry and taken out. Then, the excess slurry was removed with an air knife, and the zeolite was adjusted to 120 g per liter of honeycomb, and after drying Firing was performed to obtain a zeolite-coated honeycomb core.

This honeycomb core was allowed to stand at room temperature in an aqueous solution of 0.18 mol / L copper acetate hydrate (Cu (CH ₃ COO) ₂ .H ₂ O).
Immersion was carried out for a period of time to carry copper by ion exchange. This was washed with deionized water, dried in an electric drier, and further calcined in an electric furnace in air at 400 ° C. for 1 hour to obtain the target catalyst B-7.
At this time, the ion exchange rate was 139%, and the amount of supported copper was 3.1 g per liter of the honeycomb.

Performance Evaluation Example 1 For each of the catalyst A-1 of the example of the present invention and the catalyst B-7 of the comparative example, a mixed gas (A) having the following composition was used as a lean burn engine exhaust model gas in a gas space. Speed SV100,00
While flowing at 0 / hr, the catalyst layer inlet gas temperature was changed from 150 ℃ to 5
The light-off performance was evaluated by continuously measuring the CO, HC and NOx concentrations in the catalyst layer outlet gas by continuously raising the temperature to 00 ° C. The results for catalyst A-1 are shown in FIG. 1, and the results for catalyst B-7 are shown in FIG. Mixed gas composition (A) NO 1,200 (ppm) O ₂ 3.2 (%) CO 3,000 (ppm) C ₃ H ₆ 1,600 (ppm) H ₂ 1,000 (ppm) CO ₂ 10.0 (%) H ₂ O 10.0 (%) N ₂ balance

As can be seen from FIG. 1, the catalyst of the embodiment of the present invention
A-1 removed all three components of CO, HC and NOx with good removal rate in the range of 300 ℃ to 500 ℃.
Comparative example B-7 representing a Cu ion-exchanged zeolite catalyst is HC
And NOx are removed to some extent above 400 ° C, but 400 ° C
From the result, it was found that there is a drawback that a large amount of CO is generated due to the incomplete oxidation of HC between 500 ° C and 500 ° C.

Performance Evaluation Example 2 For each of the catalysts A-1 to A-11 of the example and the catalysts B-1 to B-6 of the comparative example, a mixed gas having the following composition as a lean burn engine exhaust model gas While flowing (B) at a gas space velocity SV of 38,000 / hr, the gas temperature at the inlet of the catalyst layer was adjusted.
By continuously raising the temperature from 150 ℃ to 500 ℃,
The NOx concentration in the catalyst layer outlet gas was continuously measured. Mixed gas composition (B) NO 500 (ppm) O ₂ 5.0 (%) C ₃ H ₆ 1,500 (ppm) N ₂ balance

Table 1 shows the catalysts A-1 to A-11 of the examples.
Then, Table 2 shows the maximum NOx removal rate of the catalysts B-1 to B-6 of the comparative example when fresh.

[0045]

[Table 1]

[0046]

[Table 2]

From Tables 1 and 2, silicon carbide supported Ir single catalyst B-1 and conventional supported Ir catalysts, ie, γ-alumina supported Ir single catalyst (US Pat. No. 4,039,622) B-2,
γ-alumina-supported Ir-alkaline earth catalysts B-3 and B-5,
By further repeating this hydrogen reduction and calcination, Ir-dispersed Ir-alkaline earth catalyst (G.
B.McVicker, RLGarten, anc RTKBaker, J.CATAL.,
54, 129 (1978)) Compared with both B-4 and B-6, the catalyst of the present invention is clearly superior in removing nitrogen oxides in the presence of oxygen in excess of the stoichiometric amount. You can see that

Performance Evaluation Example 3 Each of the catalyst A-1 of the example and the catalysts B-1 and B-7 of the comparative example were flown at 800 ° C. for 5 hours under a mixed gas flow of 10% H ₂ O + 90% N _2. After the aging treatment, the performance was evaluated again under the same conditions as in the performance evaluation example 2. At this time, catalysts A-1, B-1 and B-
The light-off characteristics of NOx removal during freshening and after aging of 7 are shown in FIGS. 3, 4 and 5, respectively.

As can be seen from the comparison between FIG. 3 and FIG. 4, the catalyst A-1 of the example of the present invention is aged in the presence of water vapor as compared with the catalyst B-1 of the comparative example in which iridium is supported alone. The subsequent NOx removal rate was also clearly superior. Further, as can be seen from FIG. 5, the known Cu ion-exchange ZSM-5 catalyst (catalyst B-7 of the comparative example) shows some high activity in the fresh state, but the activity is not increased by heat aging at 800 ° C. in the presence of steam. Remarkably decreased, and the temperature range showing activity was shifted to the high temperature side. On the other hand, in the catalyst A-1 of the example, the temperature showing the highest NOx removal rate did not change before and after aging. Therefore, the catalyst A-1 of the example of the present invention is the catalyst of the comparative example.
It can be seen that the high-temperature durability in the presence of water vapor is remarkably superior to that of B-7.

[0050]

As described above, the exhaust gas purifying catalyst of the present invention removes nitrogen oxides in the presence of oxygen and water vapor and at a high space velocity as compared with the conventional exhaust gas purifying catalyst. It is a catalyst with high activity and excellent durability at high temperature. Therefore, the exhaust gas purifying catalyst of the present invention is effective for removing the nitrogen oxides contained in the exhaust gas of automobiles, boilers, etc. in which oxygen and water vapor coexist.

[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 exhaust model gas of catalyst A-1 of an example.

FIG. 2 shows CO, HC, and NOx light-off characteristics of a catalyst B-7 of a comparative example with respect to a lean-burn engine exhaust model gas.

FIG. 3 is a diagram showing NOx light-off performance for lean burn engine exhaust model gas before and after heat aging at 800 ° C. for 5 hours of catalyst A-1 of the example.

FIG. 4 is a diagram showing NOx light-off performance for a lean burn engine exhaust model gas before and after heat aging at 800 ° C. for 5 hours of catalyst B-1 of a comparative example.

FIG. 5 is a graph showing NOx light-off performance of a catalyst B-7 of a comparative example before and after heat aging at 800 ° C. for 5 hours with respect to lean burn engine exhaust model gas.

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[Procedure amendment]

[Submission date] October 28, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] 0002

[Correction method] Change

[Correction content]

[0002]

_2. Description of the Related Art Conventionally, the method for removing nitrogen oxides (NOx) contained in exhaust gas is (1) using a V ₂ O ₅ -TiO ₂ catalyst,
Selective reduction method with ammonia, (2) Pt-Rh / Al ₂ O ₃ automotive three-way catalyst method, (3) Pt / Al ₂ O ₃ , Co ₃ O
₄ , a direct decomposition method using a noble metal catalyst such as YBa ₂ Cu ₃ O _y or a metal oxide catalyst is known. However, each of these methods has drawbacks and is by no means satisfactory. In other words, since the method of (1) uses ammonia, it has problems in cost, facility and toxicity of ammonia, and it is difficult to apply it especially to small-scale migration sources of nitrogen oxides such as automobiles. . (2)
The catalysts of (3) and (3) have a problem that the NOx removal reaction hardly proceeds in the presence of oxygen in excess of the stoichiometric amount.

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Active Component In the catalyst of the present invention, (A) Ir and (B) alkaline earth metal are supported on the above carrier. The presence state of Ir carried is not particularly limited, but preferably a metal state, or
IrO, Ir ₂ O ₃ , oxide state such as IrO ₂ or complex oxide with alkaline earth metal, such as CaIrO ₃ , Ca ₂ Ir
O ₄ , Ca ₄ IrO ₆ , SrIrO ₃ , Sr ₂ Ir ₃ O ₈ , Sr ₄ IrO
₆ , an oxide state in which the valence of Ir such as BaIrO ₃ is +4, or a mixed state thereof. These Ir
Is dispersed and supported together with the alkaline earth metal on the carrier in a state where the crystallite diameter observed by the powder X-ray diffraction method is preferably 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 activity is low relative to the supported amount.

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There are no particular restrictions on the starting materials for the iridium and alkaline earth metals in the preparation of the catalyst of the present invention. Examples of the starting material for iridium include iridium trichloride (IrCl ₃ ), iridium chloride (H ₂ IrCl ₆ ), sodium iridium chloride (Na ₃ IrCl ₆ ), and the same (Na ₂ IrCl ₆ ).
, Iridium nitrate (Ir (NO ₃ ) ₄ ), iridium sulfate (Ir
A water-soluble salt of iridium such as (SO ₄ ) ₂ ) is used.
Further, an Ir organometallic complex such as Ir ₃ (CO) ₁₂ may be dissolved in an organic solvent such as hexane or ethanol for use.

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As a starting material for the alkaline earth metal, M
g, Ca, Sr and Ba metal chlorides, nitrates,
Although nitrite, acetate, etc. can be used, nitrate and nitrite are particularly preferable as a starting material having high solubility in an aqueous solvent. In the simultaneous loading method or the sequential loading method, the atmosphere for the firing decomposition of the Ir compound and / or the alkaline earth metal compound supported as the catalyst precursor on the carrier is, in air, in vacuum, depending on the type of the precursor. Medium, in a stream of an inert gas such as nitrogen, in a stream of hydrogen, etc. are appropriately selected. The firing temperature is usually 300 ° C to 900 ° C, more preferably 600 ° C to 800 ° C, and the firing time is usually 10 minutes to 20 ° C.
It is about time, preferably about 30 minutes to 5 hours. Further, the firing may be performed by combining a plurality of treatments stepwise. For example, after reduction in air at 600 ° C to 800 ° C, reduction treatment may be performed at 600 ° C to 900 ° C in hydrogen stream.

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Example 10 Commercially available TiC powder (manufactured by KENNAMETAL MACRO DIVISION) 50
Slurry g in 2 liters of deionized water, and add an aqueous solution of iridium chloride (H ₂ IrCl ₆ ) to the solid content of 2.0% by weight of iridium in terms of metal, calcium nitrate tetrahydrate (Ca
The _{(NO 3) 2 · 4H 2} O) aqueous solution of iridium: calcium 1 in atomic ratio: 1 so as added and evaporated to dryness with stirring. After drying the obtained solid in a dryer at 115 ° C for 15 hours,
It was crushed and fired in an electric furnace in air at 800 ° C for 30 minutes. This was coated on a cordierite honeycomb in the same manner as in Example 1. Further, reduction treatment was carried out at 700 ° C. for 2 hours in a 100% hydrogen stream to obtain the target catalyst A-10.

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Example 11 Instead of TiC powder as a carrier, commercially available TiN powder (KENNAM
A target catalyst A-11 was obtained in the same manner as in Example 10 except that ETAL MACRO DIVISION) was used.

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Comparative Example 7 Rollmann and Valyocsik method (LORollmann and EW
ZSM-5 type zeolite (SiO ₂ / Al ₂ O ₃ = 5) synthesized according to Valyocsik, Inorg. Synthesis, 22 (1982) 67-68).
4) was wet-ground with a ball mill together with a mixed solution of silica sol and deionized water to obtain a slurry of zeolite carrier. A commercially available cordierite 400 cell / inch 2 honeycomb core was dipped in this slurry and taken out.
Remove excess slurry with an air knife and adjust the zeolite to 120 g per liter of honeycomb,
After drying, it was fired to obtain a zeolite-coated honeycomb core.

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From Tables 1 and 2, silicon carbide supported Ir single catalyst B-1 and conventional supported Ir catalysts, ie, γ-alumina supported Ir single catalyst (US Pat. No. 4,039,622) B-2,
γ-alumina-supported Ir-alkaline earth catalysts B-3 and B-5,
By further repeating this hydrogen reduction and calcination, Ir-dispersed Ir-alkaline earth catalyst (G.
B.McVicker, RLGarten, and RTKBaker, J.Catal.,
54, 129 (1978)) Compared with both B-4 and B-6, the catalyst of the present invention is clearly superior in removing nitrogen oxides in the presence of oxygen in excess of the stoichiometric amount. You can see that

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FIG. 2 is a diagram showing light-off characteristics of CO, HC and NOx with respect to a lean burn engine exhaust model gas of a catalyst B-7 of a comparative example.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI Technical display location B01J 27/24 ZAB A 9342-4G 29/068 ZAB A 9343-4G B01D 53/36 102 H

Claims (5)

[Claims] 1. An iridium (A) on a carrier comprising at least one selected from a metal carbide and a metal nitride,
(B) An exhaust gas purifying catalyst co-loaded with an alkaline earth metal. 2. The exhaust gas purifying catalyst according to claim 1, wherein the alkaline earth metal is at least one selected from the group consisting of Ca, Sr, and Ba. 3. The metal carbide is silicon carbide.
Or the exhaust gas purifying catalyst according to 2. 4. The exhaust gas purifying catalyst according to claim 1, which is molded into a fixed shape or coated on a refractory supporting substrate. 5. An exhaust gas containing a reducing component containing hydrocarbons and oxygen and nitrogen oxides in excess of the stoichiometric amount required to completely oxidize all the reducing components.
Contacting the catalyst according to any one of 4 above,
A method for purifying the exhaust gas.