JPH09276702A - Catalyst for removal of nox - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a catalyst excellent in heat resistance and durability at a high temp. and capable of efficiently decomposing and removing NOx in exhaust gas discharged from an internalcombustion engine even in an atmosphere contg. excess oxygen by incorporating iridium and sulfur as catalytically active substances. SOLUTION: This catalyst for effective removal of NOx contained in exhaust gas discharged from the internal-combustion engine of an automobile, a boiler, etc., contains iridium and sulfur as catalytically active substances and the iridium has been carried on the sulfur-contg. substrate. The sulfur is preferably used in the form of sulfuric acid radicals. Sulfuric acid radical carried alumina, a sulfuric acid radical-contg. substrate compd. such as barium sulfate or a combination of the compd. with a fireproof inorg. oxide used as a catalyst carrier is used as the substrate. The pref. iridium content is 0.5-10wt.% of the amt. of the substrate.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a nitrogen oxide removing catalyst for removing nitrogen oxides in exhaust gas discharged from an internal combustion engine of an automobile, a boiler, an industrial plant or the like.

[0002]

An automobile, a boiler, nitrogen oxides contained in the exhaust gas discharged from an internal combustion engine such as an industrial plant (hereinafter, referred to as NO _X) is caused air pollution, exhaust gas There is an urgent need to remove NO _x .

Conventionally, for example, in the case of exhaust gas from a gasoline engine of an automobile, etc., the exhaust gas is treated by a so-called three-way catalyst using platinum or the like, and hydrocarbons (HC) and carbon monoxide (CO) and NO _x are simultaneously produced. Methods of removing are known. This method is extremely effective when the air-fuel ratio (hereinafter, “A / F”) is near the stoichiometric ratio (A / F = 14.6).

By the way, in recent years, lean-burn engines have been attracting attention for the purpose of improving fuel efficiency and reducing CO ₂ . However, in such an engine, the A / F becomes large (hereinafter, referred to as “excess oxygen atmosphere”), HC in exhaust gas,
Since there is oxygen in excess of the amount that completely burns unburned components such as CO, it has been difficult to reduce and remove NO _x with a normal three-way catalyst.

Further, in the case of a diesel engine, although the exhaust gas is in an oxygen excess atmosphere, the exhaust gas from the diesel engine, which is a fixed generation source such as a boiler, is
A method of removing NO _x using a reducing agent such as ammonia, hydrogen or carbon monoxide is known.

However, this method requires a separate device for adding a reducing agent and a special device for collecting and treating unreacted reducing agent, which makes the entire device complicated and large. As a result, the problem arises that it is unsuitable for an engine that is a source of movement such as an automobile.

Therefore, in order to avoid the above problems, as a catalyst for NO _X removal in an oxygen-rich atmosphere, each of the various catalysts have been proposed.

[0008]

However, in the above-mentioned prior art, NO _{X in} exhaust gas is efficiently decomposed and removed even in an oxygen excess atmosphere, and heat resistance and durability at high temperatures are excellent, and in a wide temperature range. NO that exhibits catalytic activity
The problem is that the _X removal catalyst is not yet known.

[0009] NO The _X catalyst for removing, for example, transition metal ion exchanged aluminosilicate copper ions (Japanese 60-125250, JP-Sho 63-10091
9 publication, US Pat. No. 4,297,328),
Alternatively, a metalloaluminosilicate (Japanese Patent Application Laid-Open No. 3-127)
No. 628, JP-A-3-229620), silicoaluminophosphate (JP-A-11-12488), and the like are proposed.

However, these so-called ion-exchanged zeolite catalysts have a high temperature at which NO _x can be removed, and their effect decreases at low temperatures. Further, they are inferior in heat resistance and NO when exposed to high-temperature exhaust gas. _It has a problem that the _X decomposition performance is significantly deteriorated and has not been put to practical use.

Furthermore, as the catalyst for NO _X removal in an oxygen-rich atmosphere, catalyst supporting iridium on a refractory inorganic oxide such as alumina is disclosed (JP-B-56
-54173, JP-B-57-13328). However, in the examples described in these publications,
Oxygen concentration in the exhaust gas not been the only shows the case of 3 volume% or less, a NO _X purification performance, durability both unknown for exhaust gases of a diesel engine or a lean burn engine comprising more oxygen .

Further, a catalyst in which iridium is supported on a base material such as zeolite or crystalline silicate has also been proposed (JP-A-6-296870, JP-A-7-80315, JP-A-7-88378). . However, as a condition of the durability test for the above catalyst, the exhaust gas is conducted only in a reducing atmosphere, and the durability and heat resistance in an oxygen excess atmosphere such as exhaust gas of a diesel engine or a lean burn engine are unknown. Is.

Further, a catalyst in which iridium is carried on a base material of metal carbide or the like has been proposed (Japanese Patent Laid-Open No. 6-3117).
No. 3, JP-A-7-31884). However, the examples in each of the above publications only show the highest NO _x removal rate, and the temperature range in which the catalyst is used is unknown.
Further, even when the light-off characteristics are shown, the NO _X purification activity is exhibited at a high temperature of 350 ° C. or higher.

As described above, a NO _X removal catalyst has been developed which efficiently decomposes and removes NO _{X in} exhaust gas even in an oxygen excess atmosphere, has excellent high temperature heat resistance, and exhibits catalytic activity in a wide temperature range. The current situation is that it has not been done.

[0015]

Means for Solving the Problems As a result of intensive studies for solving the above problems, the present inventors have found that a catalyst containing iridium and sulfur is effective in solving the above problems. The present invention has been completed and the present invention has been completed.

That is, the NO _x removal catalyst of the present invention is
In order to solve the above problems, it is characterized by containing iridium and sulfur as a catalytically active substance. The iridium is preferably supported on a base material containing sulfur. Further, the sulfur is preferably in the form of sulfate radical.

The NO _X removal catalyst of the present invention can remove NO _X in an oxygen excess atmosphere by having iridium as a catalytically active substance, and further has sulfur to improve the catalytic activity of iridium. It can be improved and shows activity in a wide temperature range in removing NO _X in an oxygen-excess atmosphere, and has excellent heat resistance and durability. In the present specification, a substance which does not have the activity of removing NO _x by itself such as sulfur, but a substance which can improve the catalytic activity of iridium having the above-mentioned activity is also defined as a catalytically active substance.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below. NO _x removal catalyst is N
As catalytically active material for the removal of O _X, and a iridium and sulfur, the iridium is supported on a substrate comprising sulfur, also is intended to include the sulfur in the form of a sulfate group.

As the above-mentioned base material, a base material compound containing a sulfur-containing sulfate group such as sulfur-containing sulfate group-bearing alumina and sulfur-containing barium sulfate is used alone, or with the above-mentioned base material compound, a refractory which is usually used as a catalyst-supporting carrier. Inorganic oxide,
For example, α-alumina, a mixture of activated alumina such as γ, δ, η, and θ, titania and the like (including a mixed sintered body), a composite oxide of each of the above refractory inorganic oxides, and the above base compound It is possible to use a mixture (including a mixed sintered body), and a mixture of aluminum phosphate, crystalline aluminosilicate, silicoaluminophosphate and the like with the above-mentioned base compound.

The content of iridium is preferably 0.5 to 10% by weight based on the base material carrying the above iridium as a catalyst component. If the amount is less than 0.5% by weight, the NO _X removal efficiency is lowered, and even if the amount is more than 10% by weight, the catalytic activity corresponding to the supported amount cannot be obtained. The iridium source is not particularly limited, but a water-soluble iridium salt such as iridium chloride and trichlorohexammineiridium is preferably used.

The method of supporting iridium on the substrate is not particularly limited, and a usual supporting method is used. For example,
(1) a method of impregnating a base material with an aqueous solution of iridium salt, and drying and firing, (2) adding an aqueous solution of iridium salt to the base material,
After mixing, a method such as reducing and supporting with a reducing agent such as hydrazine may be used.

The weight ratio of sulfur to iridium supported is preferably 1: 5 to 50: 1. When the sulfur loading ratio becomes higher than the ratio of 50: 1, the initial activity decreases,
When the loading of sulfur becomes smaller than the ratio of 1: 5, the active temperature range becomes narrow.

The sulfur source is not particularly limited, but sulfuric acid, sulfates, sulfites, sulfides, etc. are used. As a method of adding sulfur, (1) a method of adding sulfuric acid to a base material, followed by drying and firing, and (2) using an organic solvent-soluble and / or water-soluble sulfur-containing compound among sulfates, sulfites, etc. A method of immersing a solution of the above-mentioned sulfur-containing compound in a base material, drying and firing, (3) a method of using an insoluble or slightly soluble compound among sulfates, sulfides and the like as a base material of iridium, (4) Examples thereof include a method of mixing an insoluble or slightly soluble compound among sulfates and sulfides with a base material carrying iridium, and the like.

Usually, the specific embodiment of the catalyst used in the present invention is as follows: (1) a method in which the catalyst itself is molded into a predetermined shape, for example, a spherical shape or a cylindrical shape, and (2) a three-dimensional structure. There is a method in which a carrier is coated with a catalyst component and used. Examples of the three-dimensional structure include a honeycomb monolith carrier, a foam-shaped carrier, a corrugated carrier, and the like, and those made of ceramic or metal are preferably used.

The method for preparing the catalyst will be described below. (1) When the catalyst composition itself is used as a catalyst, (a) a method in which the catalyst composition is sufficiently mixed and then formed into a columnar shape, a spherical shape or the like to form a catalyst, and (b) a catalyst-supporting base material is predetermined. The shape of
For example, a method in which the catalyst material is coated after being formed into a spherical shape or a columnar shape can be used.

(2) When using a monolithic structure or an inert inorganic carrier (hereinafter referred to as "monolithic structure"),
(A) A method in which the catalyst composition is put together in a ball mill or the like, wet-milled to form an aqueous slurry, and the monolithic structure, etc. is dipped, dried and fired. Then, it is made into an aqueous slurry, and the integrated structure or the like is dipped, dried and fired. Next, a method of immersing the monolithic structure or the like coated with the catalyst-supporting substrate in an iridium-containing aqueous solution, drying and firing, and further immersing the monolithic structure in a solution containing sulfur, drying and firing, (c) Iridium is preliminarily supported on a base material, and is further made into an aqueous slurry by a ball mill or the like, and an integrated structure or the like is immersed in this aqueous slurry to obtain an integrated structure or the like coated with an iridium-supported base material, and then a solution containing sulfur. Method of immersing in, drying and firing,
(D) Impregnation of a solution containing sulfur into a supporting base material and firing to obtain an aqueous slurry of the powder obtained by a ball mill or the like, and the monolithic structure or the like is dipped in the aqueous slurry to coat the sulfur supporting base material A method of obtaining a structure, etc., and then immersing it in an iridium-containing aqueous solution, drying and firing it, and (e) supporting iridium and sulfur on a base material in advance, forming an aqueous slurry with a ball mill, etc., and forming an integrated structure in this aqueous slurry A method of immersing a body or the like, drying and firing, (f) iridium is carried on a base material containing sulfur, and then made into an aqueous slurry by a ball mill or the like, and an integral structure or the like is immersed in this aqueous slurry and dried, Method of firing, (to) iridium is preliminarily supported on a base material, a compound containing sulfur is mixed and made into an aqueous slurry by a ball mill or the like, and the integrated structure or the like is dipped in the aqueous slurry and dried. , And a method in which firing. Among these methods, the methods (2), (A) to (G) are preferable.

When the catalyst component is coated on the monolithic structure or the like, the coating amount of the catalyst component is preferably 50 to 400 g per liter of the monolithic structure or the like. 50
When it is less than g, the catalytic activity is lowered, and when it exceeds 400 g, the catalytic activity commensurate with the supported amount cannot be obtained.

The gas hourly space velocity when using the NO _X removal catalyst of the present invention is preferably 5,000 to 200,000 hr ⁻¹ . When it is less than 5000 hr ^-1 , the required catalyst capacity becomes too large and it is uneconomical, and when it exceeds 200,000 hr ^-1 , the NO _x purification rate decreases. NO of the present invention
The exhaust gas temperature when using the _X removal catalyst is in the range of 200 ° C. to 700 ° C., preferably 250 ° C. to 600 ° C. at the catalyst inlet. When the temperature is lower than 200 ° C, the NO _x purification capacity deteriorates below the target value, and when the temperature exceeds 700 ° C, the NO _x purification capacity falls below the target value.

[0029]

For each example of the NO _X catalyst for removing EXAMPLES The invention will be hereinafter described respectively by a method for their preparation. (Example 1) First, BET (Brunauer-Emm
ett-Teller) An iridium chloride aqueous solution containing 5 g of iridium was added to 100 g of porous powdery activated alumina having a surface area of 100 m ² , mixed and dried at 120 ° C. for 2 hours, and then at 500 ° C. for 2 hours. Firing was performed to obtain a catalyst powder composed of activated alumina having fine particles of iridium dispersed on a porous surface.

Then, this catalyst powder was wet pulverized by a ball mill to obtain an aqueous slurry. Subsequently, a commercially available cordierite honeycomb carrier (manufactured by Nippon Glass Co., Ltd., cross section per 1 inch square) was added to the above aqueous slurry. , 400 gas distribution cells, diameter 33mmφ, length 76mm
L, volume 65 ml), and then the excess aqueous slurry was blown off from the honeycomb carrier with compressed air to remove it.

Next, after drying the honeycomb carrier having the aqueous slurry on the inner surface of each cell at 120 ° C. for 2 hours,
After being immersed in a 1.5 mol / liter sulfuric acid aqueous solution, excess sulfuric acid was blown off by compressed air and dried at 120 ° C. for 2 hours to obtain a finished catalyst (A). This finished catalyst (A)
Then, 5% by weight of iridium and 5% by weight of sulfur were supported on the activated alumina as the base material.

(Embodiment 2) 1.
0.3mol instead of 5mol / L sulfuric acid aqueous solution
A completed catalyst (B) was obtained in the same manner as in Example 1 except that a 1 / liter aqueous sulfuric acid solution was used. In this completed catalyst (B), 5% by weight of iridium and 1% by weight of sulfur were supported on the activated alumina as the base material.

Example 3 An iridium chloride aqueous solution containing 5 g of iridium in Example 1 above, and 1.5
Iridium 1 instead of mol / l sulfuric acid solution
A completed catalyst (C) was obtained in the same manner as in Example 1 except that an iridium chloride aqueous solution containing g and a 6 mol / liter sulfuric acid aqueous solution were used. In this completed catalyst (C), 1% by weight of iridium and 20% by weight of sulfur were supported on the activated alumina as the base material.

(Fourth Embodiment) 1.
A completed catalyst (D) was prepared in the same manner as in Example 1 except that an aqueous solution containing 27.2 g of potassium sulfate [K ₂ SO ₄ ] was used instead of the 5 mol / liter aqueous solution of sulfuric acid. In this completed catalyst (D), 5% by weight of iridium and 5% by weight of sulfur were supported on the activated alumina as the base material.

Example 5 In place of 100 g of activated alumina in Example 1, barium sulfate [BaS] was used.
O ₄ ] was prepared in the same manner as in Example 1 except that immersion in a sulfuric acid aqueous solution was omitted, and a completed catalyst (E) was obtained. In this completed catalyst (E), 5% by weight of iridium was supported on barium sulfate as a base material, and 7.3% by weight of sulfur was contained.

(Example 6) In Example 1 above, barium sulfate [BaSO ₄ ] 3 was used to obtain an aqueous slurry.
6.4 g was added, except that the immersion in the sulfuric acid aqueous solution was omitted.
The catalyst was prepared in the same manner as in Example 1 to obtain a completed catalyst (F).
In this completed catalyst (F), 3.7% by weight of iridium was supported on the activated alumina and barium sulfate as the base material, and 3.7% by weight of sulfur was contained.

Next, comparative catalysts as comparative examples with respect to the above-mentioned completed catalysts (A) to (F) will be described based on their manufacturing methods. Comparative Example 1 A comparative catalyst (X) was prepared in the same manner as in Example 1 except that the immersion in sulfuric acid was omitted. In this comparative catalyst (X), 5% by weight of iridium was supported on the activated alumina as the base material.

Comparative Example 2 100 g of commercially available ZSM-5 type zeolite (SiO ₂ / Al ₂ O ₃ = 40) and 4 parts of pure water
After stirring the mixture mixed with 00 g for 2 hours at 98 ° C., 600 ml of a 0.2 mol / liter copper ammine complex aqueous solution was slowly added dropwise at 80 ° C. to the mixture.

Then, the zeolite having the copper ammine complex was filtered from the mixture and thoroughly washed, and then at 120 ° C.
For 24 hours to obtain a zeolite catalyst powder. This zeolite catalyst powder was wet-pulverized by a ball mill to obtain an aqueous slurry. Thereafter, in the same manner as in Example 1, a comparative catalyst (Y) was obtained using the aqueous slurry. This comparative catalyst (Y) had 5.6% by weight of copper supported on the zeolite.

Next, Examples 1 to 6 and Comparative Examples 1 and 2
For each of the catalysts (A) to (F), (X), and (Y) prepared in 1., a model gas (A / F = 21) simulating the exhaust gas of a lean burn engine in which the exhaust gas is in an oxygen excess atmosphere
(Equivalent to) was used to evaluate the performance of catalytic activity.

(Evaluation method) Diameter 34.4 mmφ, length 3
After filling each catalyst into a 00 mm stainless steel reaction tube, a reaction gas having the following composition was supplied with a space velocity of 50,000 hr.
^-1 under the conditions of ^-1 , the catalyst bed inlet temperature from 150 ℃ to 5
The NO _x purification rate was measured by continuously raising the temperature to 00 ° C., and the light-off performance of each catalyst was evaluated.

(Reaction Gas Composition) Nitric oxide (NO) 300 ppm Propylene (C ₃ H ₆ ) 3000 ppm (Methane conversion) Carbon monoxide (CO) 0.18% by volume Hydrogen (H ₂ ) 600 ppm Steam (H ₂ O) ) 10% by volume carbon dioxide (CO ₂ ) 10% by volume oxygen (O ₂ ) 7% by volume nitrogen (N ₂ ) balance Also, as a result showing the evaluation of each catalyst, the maximum NO _x purification rate and the catalyst bed inlet at that time are shown. The temperatures are shown in Table 1, respectively.

[0043]

[Table 1]

Next, Examples 1 to 6 and Comparative Examples 1 and 2
Heat resistance and durability were tested for each of the catalysts (A) to (F), (X), and (Y) prepared in. In this test method, each catalyst was charged into a multi-converter to form each packed catalyst bed.

Then, the exhaust gas of a commercially available gasoline lean burn engine was passed through with an air-fuel ratio (A / F) adjusted to 21 and a space velocity (SV) of 160,000 hr ⁻¹ and a catalyst bed temperature of 700 ° C. Aged under conditions for 20 hours. Then, performance evaluation was performed on each of the packed catalyst beds by the evaluation method. The results are shown in Table 1.

Among these results, the catalysts (A), (E),
The light-off performances of (X) and (Y) at the initial time (Fresh) and after the endurance test (Aged) are shown in FIGS. 1 to 4, respectively. 1 to 4, the initial state (Fresh)
The result is shown by the solid line, and the result after the durability test (Aged) is shown by the broken line.

First, as is clear from the results shown in Table 1, the catalysts (A) to (F) of the present invention are the catalysts of the comparative example (X),
Compared with (Y), NO _X removal in an oxygen excess atmosphere
It can be seen that it can be performed from a lower temperature (around 300 ° C) to a wide temperature range. In addition, almost no decrease in catalyst activity was observed after the aging test (Aged), indicating that the catalyst has sufficient heat resistance and durability.

Further, as is clear from the comparison between FIGS. 1 and 2 and FIG. 3, the catalyst for NO _x removal of the present invention contains only iridium and sulfur, so that only iridium is supported. As compared with the case of the catalyst (X) of Comparative Example 1, the activity at high temperature is also improved, and NO _x purification in a wide temperature range can be realized.

Further, as is clear from FIG. 4, the copper ion-exchanged zeolite catalyst known as a catalyst for NO _x removal in an oxygen excess atmosphere showed a marked decrease in activity after the durability test. On the other hand, as shown in FIGS. 1 and 2, the catalyst for NO _x removal of the present invention showed almost no decrease in activity even after the durability test. Therefore, the NO of the present invention
_{The X} removal catalyst has sufficient heat resistance and durability as compared with the catalyst (Y) of Comparative Example 2.

As described above, the NO _x removal catalyst of the present invention omits expensive metal carbide or metal nitride and is cheaper than the conventional catalyst in which iridium is supported on metal carbide or metal nitride. Since it has the same NO _X removal activity as the conventional catalyst by using a compound having various sulfate groups, the cost can be reduced as compared with the above-mentioned conventional catalyst.

By the way, the sulfuric acid-supporting base material such as SO ₄ / ZrO ₂ as the carrier of the denitration catalyst described in JP-A-7-80315 is a substance called solid superacid.
The solid superacid is obtained by immersing sulfuric acid in a hydroxide such as zirconium, filtering, drying, and calcining the hydroxide in advance, and using the solid superacid as a carrier. Such a denitration catalyst takes a lot of time to prepare a denitration catalyst, for example, by calcining a carrier in advance.

However, the catalyst for removing NO _{x of} the present invention does not require that the supporting form of the sulfuric acid radical is a solid superacid, and the above-mentioned effect can be obtained by supporting sulfuric acid on a metal oxide such as alumina afterwards. Therefore, the labor of preparation thereof can be reduced as compared with the above-mentioned conventional publication.

[0053]

As described above, the catalyst for removing NO _{x of the} present invention is active in a wide temperature range in the removal of NO _x in an oxygen-excess atmosphere because it has a structure containing iridium and sulfur. In addition, since it has excellent heat resistance and durability, the exhaust gas becomes an oxygen excess atmosphere and the temperature fluctuation range of the exhaust gas is wide, and
It has an effect that it can be effectively used for an internal combustion engine such as a lean burn engine.

[Brief description of drawings]

FIG. 1 is a graph showing the NO _X light-off performance of an NO _X removal catalyst (A) of Example 1 of the present invention with respect to a model exhaust gas after an initial test and after a durability test.

FIG. 2 is a graph showing the NO _X light-off performance of the NO _X removal catalyst (E) of Example 5 of the present invention with respect to model exhaust gas, after the initial test and after the durability test.

FIG. 3 is a graph showing the NO _X light-off performance of the catalyst (X) of Comparative Example 1 against model exhaust gas after the initial test and after the durability test.

FIG. 4 is a graph showing the NO _X light-off performance of the catalyst (Y) of Comparative Example 2 against the model exhaust gas after the initial and endurance tests.

 ──────────────────────────────────────────────────続 き Continuation of the front page (71) Applicant 395016659 65 CHALLENGER ROAD R IDGEFIELD PARK, NEW JERSEY 07660 U.S.A. S. A. (72) Inventor Akihisa Okumura 1 at 992 Nishioki, Akihama, Aboshi Ward, Himeji City, Hyogo Prefecture, Nihon Catalyst Co., Ltd. ) Inventor Makoto Horiuchi 1 992 Nishioki, Okihama, Aboshi-ku, Himeji-shi, Hyogo Prefecture Nippon Shokubai Co., Ltd.

Claims (3)

[Claims] 1. A catalyst for removing nitrogen oxides, which contains iridium and sulfur as catalytically active substances. 2. The catalyst for removing nitrogen oxides according to claim 1, wherein iridium is supported on a base material containing sulfur. 3. The catalyst for removing nitrogen oxides according to claim 1, wherein the sulfur is in the form of sulfate.