JPH0910592A - Denitration catalyst and method using the same - Google Patents

Abstract

PURPOSE: To provide a denitration catalyst for an internal combustion engine which can remove NOx in exhaust gas in a lean air-fuel ratio internal combustion engine efficiently at a high space velocity and has high stoichiodurability and a high efficiency, high reliability denitration method for NOx exhaust gas from the internal combustion engine with the use of the catalyst. CONSTITUTION: A catalyst layer is formed with the use of a catalyst in which 0.1-10wt.% of alumina in terms of element of silver, 0.1-20wt.% of zinc, and 0.01-10wt.% of sulfur are supported on activated alumina respectively, and the catalyst is brought into contact with exhaust gas at a space velocity of 10,000-200,000h<-1> and at the temperature of a catalyst layer inlet of 300-600 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a catalyst used for purifying nitrogen oxides in exhaust gas, particularly exhaust gas of an internal combustion engine such as an automobile, and more particularly to a catalyst of lean air-fuel ratio in the exhaust gas of an internal combustion engine. The present invention relates to a catalyst capable of purifying nitrogen oxides at high space velocity with high efficiency, and a denitration method using the same.

[0002]

During various combustion exhaust gas discharged from an internal combustion engine such as an automobile engine, a combustion products water or carbon dioxide (CO ₂₎ with nitric oxide (NO) and nitrogen dioxide (NO ₂₎ And nitrogen oxides (NOx) are included. NOx not only adversely affects the human body and increases the respiratory disease morbidity, but it is also one of the causes of acid rain which is regarded as a problem from the viewpoint of global environment conservation. Therefore, development of a denitration technology for efficiently removing nitrogen oxides from these various exhaust gases is desired.

On the other hand, from the viewpoint of prevention of global warming, an internal combustion engine of a lean burn type has recently received attention. A conventional gasoline engine for automobiles has an air-fuel ratio (A / F) of 1
Combustion at a controlled stoichiometric ratio near 4.7, and carbon monoxide (CO), hydrocarbons (HC) and NOx in the exhaust gas for detoxification of the exhaust gas, Mainly platinum (Pt), rhodium (Rh), palladium (P
A three-way catalyst system in which harmful three components are simultaneously removed by contacting with an alumina catalyst containing d) and ceria (CeO ₂ ) has been adopted.

However, this three-way catalyst system must be applied to the exhaust gas purification of a lean-burn gasoline engine operated at a lean air-fuel ratio because it is absolutely necessary to operate at a stoichiometric ratio. I can't. Although the diesel engine is a lean-burn engine by nature, strict regulations are being imposed on both exhaust particulate matter and NOx with respect to its exhaust gas.

Since it has been reported in recent years that the NOx reduction reaction proceeds using unburned hydrocarbons remaining in the lean combustion gas in an oxygen excess atmosphere as a reducing agent, various catalysts for promoting this reaction have been developed and proposed. ing. For example, there are many reports that alumina or a catalyst in which a transition metal is supported on alumina is effective for a NOx reduction reaction using a hydrocarbon as a reducing agent. JP-A-4-284848 discloses that 0.1 to 4% by weight of Cu, Fe, Cr, Zn, N
It has been reported that alumina containing i, V or silica-alumina is used as a NOx reduction catalyst.

Further, when a catalyst in which Pt is supported on alumina is used, the NOx reduction reaction proceeds in a low temperature region of about 200 to 300 ° C., JP-A-4-267946.
JP-A-5-68855 and JP-A-5-103949
It is reported in the Gazette and the like. However, when these supported noble metal catalysts are used, the combustion reaction of the reducing agent hydrocarbon is excessively promoted, and a large amount of N ₂ O, which is said to be one of the causes of global warming substances, is produced, which is harmless. It has a drawback that it becomes difficult to selectively proceed the reduction reaction to N ₂ .

[0007] One of the applicants has previously found that the NOx reduction reaction selectively proceeds when a catalyst containing silver as a reducing agent is used as a hydrocarbon in an oxygen excess atmosphere.
The present technology is disclosed in Japanese Patent Application Laid-Open No. 4-281844. After the disclosure of the publication, similar NOx using a silver-containing catalyst is disclosed.
The reduction and removal technology has been disclosed in JP-A-4-354536, JP-A-5-92124, JP-A-5-92125 or JP-A-6-277454.

[0008]

However, the A / F of the exhaust gas discharged from the lean burn engine in the actual running condition is from the vicinity of stoichio to the lean burn excess oxygen region depending on running conditions. Although it changes continuously, the catalyst disclosed in the above publication has a drawback that the stoichio durability performance is insufficient and it is difficult to use for a long period of time. It is considered that such deterioration of the silver-alumina catalyst that occurs in the stoichio region is caused by aggregation of silver, coking on an alumina carrier, and the like.

[0009] In general, a catalyst using alumina as a carrier has a large dependency on a flow rate of a passing gas per unit volume in a catalyst layer, that is, a so-called gas space velocity (hereinafter, referred to as space velocity and represented by symbol SV). It has been known. That is, SV: 1,000 to 10,000 h ^-1
Although sufficient NOx reduction performance is exhibited at a low space velocity of a level, as reported in, for example, Catalyst, 33, 61 (1991),
SV: NO at high space velocity of 10,000h ^-1 or more
x Purification performance is significantly reduced. Generally, an exhaust gas purifying catalyst for an internal combustion engine is desired to form a relatively compact catalyst layer corresponding to the exhaust amount, but as described above, it functions only at a low space velocity of less than 10,000 h ^-1. In such a catalyst, the catalyst layer has a volume that is disproportionately large as compared with the engine displacement, so that it is not practical.

The present invention has been made to solve the above-mentioned problems of the prior art, and an object thereof is to reduce NOx in exhaust gas in an internal combustion engine having a lean air-fuel ratio to 10,
A denitration catalyst for an internal combustion engine, which can be efficiently removed at a high gas space velocity of 000 h ^-1 or more, and also has a high stoichio durability, and an exhaust gas of an internal combustion engine with a lean air-fuel ratio using the catalyst. To provide a highly efficient and highly reliable NOx removal method for NOx.

[0011]

DISCLOSURE OF THE INVENTION The inventors of the present invention have found that a NOx reduction reaction by hydrocarbons in a lean-burn region using a catalyst having stoichio-durability can proceed with high efficiency. As a result of earnest studies on the catalyst and the denitration method, the use of a catalyst containing silver, zinc and sulfur in activated alumina suppresses the agglomeration of silver even when exposed to a stoichiometric atmosphere, so that the above problems The inventors have found that the above can be solved and completed the present invention.

That is, the first invention of the present invention for solving the above-mentioned problems is a denitration catalyst characterized by supporting silver, zinc and sulfur on activated alumina, and more specifically, activated alumina. As crystallographically γ-type, η-
Type, or mixed type thereof, is used, and the supported amounts of silver, zinc, and sulfur are 0.1 to 10% by weight and 0.1 to the amount of activated alumina, respectively, in terms of elements.
It is preferably 20 to 20% by weight and 0.01 to 10% by weight.

A second aspect of the present invention is a method for denitrifying exhaust gas, which comprises contacting exhaust gas of an internal combustion engine operated at a lean air-fuel ratio with a catalyst layer at a high space velocity, The catalyst according to the first aspect of the present invention is used as the constituent catalyst, and the space velocity is 10,000 to 200,0.
00h ^-1 , preferably 10,000 to 100,000h
^-1, and the catalyst layer inlet gas temperature is preferably 300 ° C. or higher and lower than 600 ° C. in the exhaust gas denitration method.

According to the denitration catalyst and the denitration method of the present invention as described above, a high stoichiometric durability is obtained even in an oxygen excess atmosphere in which water vapor coexists, and even at a high space velocity.
Moreover, it is possible to effectively remove NOx in the exhaust gas in the lean burn region.

[0015]

The details of the present invention and the operation thereof will be described in more detail below.

First, the catalyst of the first invention will be described.

The activated alumina which is one of the main components of the denitration catalyst of the present invention is, for example, a powder or gel of aluminum hydroxide classified into boehmite, pseudo-boehmite, vialite, or norstrandalite in mineralogy. In air or vacuum at 300 to 800 ° C, preferably 400 to 7
By heat dehydration at 00 ° C., the crystallographic γ-
It is preferable from the viewpoint of denitration performance that the phase transition is made to activated alumina classified into the type, η-type or mixed type thereof. Alumina having another crystal structure, such as α-alumina, has an extremely small specific surface area and is poor in solid acidity, and is therefore unsuitable as the catalyst component of the present invention.

The denitration catalyst of the present invention comprises silver, zinc and sulfur supported on activated alumina having the above crystal structure. The state of silver, zinc and sulfur supported on the activated alumina is not particularly limited, and examples thereof include metals, alloys, metal sulfides, complex metal sulfides and their similar compounds, metal sulfates, complex metal sulfates and these. Similar compounds, and further, a mixed state thereof and the like can be mentioned. Then, the loading of the silver source and the zinc source on the activated alumina is performed on the activated alumina itself or an alumina hydrate that is a precursor of the activated alumina. The method of supporting the silver source and the zinc source on the activated alumina is not particularly limited, and there are conventionally used methods such as an adsorption method, a pore filling method, an incipient wetness method, an evaporation dryness method, and a spray method. Any of the impregnation method, the kneading method, and the physical mixing method can be adopted. Further, the method of supporting sulfur is not particularly limited, and examples thereof include treatment with a sulfate of each element or SO ₂ gas.

The forms used as the silver source and the zinc source are not particularly limited, but water-soluble salts are preferable.

For example, when drying the impregnated material, the drying temperature is not particularly limited, and is usually 8
Dry at about 0 to 120 ° C. Then 300-800
The catalyst is obtained by calcination at ℃, preferably about 400-700 ℃. If the firing temperature exceeds 800 ° C., phase transformation of alumina occurs, which is not preferable.

The amounts of silver, zinc and sulfur supported on the activated alumina are not particularly limited, but in terms of elements, 0.1 to 10% by weight and 0.1 to 10% by weight based on the amount of activated alumina, respectively.
It is preferably 20% by weight and 0.01 to 10% by weight.

The catalyst of the present invention may be in the form of powder, sphere, cylinder, honeycomb, spiral, granule, pellet, ring, etc., and is not particularly limited and may be used depending on the use conditions. The shape and size can be arbitrarily selected. In particular, when used for the purpose of purifying exhaust gas of an automobile engine, a catalyst layer is formed by coating a powdery catalyst on the surface of a support substrate having a refractory integral structure having many through holes in the exhaust gas flow direction. Those that have been used are preferably used. In this case, a powdery material which has been sieved to a certain particle size or a cylindrical catalyst which has been crushed and sieved to a certain particle size is used together with a binder to form the support substrate. A so-called usual wash coat method for coating the surface of the can be applied.

Next, the denitration method of the second invention will be described.

The catalyst of the present invention is used for CO, HC in exhaust gas.
And exhaust gas containing excess oxygen from near the stoichiometric amount required to completely oxidize reducing components such as H _{2 with} oxidizing components such as NOx and O ₂ , more specifically, exhaust gas of an internal combustion engine that reaches a lean air-fuel ratio It is applied to purify NOx in gas. When such exhaust gas is brought into contact with the catalyst of the present invention, NOx is present in the exhaust gas in a trace amount.
The reducing agent such as HC is reductively decomposed to N ₂ , CO ₂ and H ₂ O, and at the same time, the reducing agent such as HC is also oxidized to CO ₂ and H ₂ O.

Like the exhaust gas of a diesel engine,
When the HC / NOx ratio of the exhaust gas itself is low, additional HC such as automobile fuel is added to the exhaust gas so as to have a concentration of several hundreds to several thousands ppm in terms of methane conversion and adjusted to an optimum state. Then, it is contacted with the catalyst of the present invention. In this way, the method of the present invention can be applied to the exhaust gas of a diesel engine, and a sufficiently high NOx removal rate can be obtained.

When the catalyst according to the present invention is used to purify the exhaust gas of an internal combustion engine operated at an air-fuel ratio from the stoichiometric region to the lean burn region, the gas space velocity is not particularly limited. However, when it is used as a catalyst for purifying exhaust gas of a moving internal combustion engine such as an automobile,
As described above, SV 10,000 to 200,000
h ^-1, it is preferable that preferably a 10,000~200,000h ^-1.

In order to efficiently proceed the purification of NOx in the oxygen excess atmosphere with the above-mentioned high space velocity using the catalyst of the present invention, the catalyst layer inlet gas temperature is set to 300 ° C. or more and less than 600 ° C. Preferably. This is because the catalyst containing silver, zinc and sulfur in the alumina according to the present invention requires a temperature of 300 ° C. or higher in order to exert the denitration performance, and when the temperature is lower than this, HC is hard to be activated. Presumed. Further, the catalyst layer inlet temperature is 6
When the temperature becomes higher than 00 ° C., combustion of HC, which is a side reaction, becomes dominant, and the NOx reduction activity relatively decreases.

[0028]

EXAMPLES The present invention will be described in more detail with reference to Examples and Comparative Examples. However, the present invention is not limited to the following examples.

(Example 1) [Preparation of catalyst] 300 g of commercially available boehmite powder (structured water 27.7%), silver nitrate 7 g, zinc nitrate hexahydrate 70 g
After dipping in a 500 ml aqueous solution of 1.33 g of sulfuric acid and sulfuric acid for 24 hours, the mixture was heated with stirring to evaporate water.

Next, after air drying at 110 ° C., 70 in air
It was calcined at 0 ° C. for 3 hours to obtain a catalyst (1). The contents of silver, zinc and sulfur in terms of element are 2.0%, 6.6% and 0.2% with respect to alumina, respectively.

[Performance Evaluation Example 1] The obtained catalyst (1) was pressure-molded and then pulverized to adjust the particle size to 250 to 500 µ. This sized catalyst (1) was filled in a stainless steel reaction tube having an inner diameter of 21 mm and mounted in a fixed-pressure atmospheric reactor.

NO was added to this catalyst layer as model exhaust gas.
_{500ppm, C 3 H 6 500ppm,} O 2 5%, H 2
A mixed gas consisting of 10% O and the balance N _{2 was used} to produce a space velocity of 3
It was passed at 10,000 h ^-1 .

The catalyst layer inlet gas temperature is set to a predetermined temperature in the range of 300 to 600 ° C., and the denitration rate is calculated by the following equation 1 using the analysis value at the time when the reaction tube outlet gas composition becomes stable at each predetermined temperature. I asked for.

NO and NO ₂ in the reaction tube outlet gas were measured by a chemiluminescence type NOx meter, and the N ₂ O concentration was P
The measurement was performed using a gas chromatograph-thermal conductivity detector equipped with an orapack Q column. As a result, N ₂ O and NO ₂ were scarcely produced by any of the catalysts of the present invention.

Equation 1 Table 1 shows the obtained results. The maximum denitration rate Cmax (%) obtained in Performance Evaluation Example 1 is shown in Table 1.

[0036] [Performance Evaluation Example 2] The catalyst (1) was exposed to the stoichiometric conditions shown in Table 2 below, and then evaluated again under the model gas conditions of Performance Evaluation Example 1. The obtained results are shown in Table 1 as the maximum denitration rate Cmax (%) of Performance Evaluation Example 2.

[0037] [Performance Evaluation Example 3] Using the above catalyst (1), the space velocity was set to 7
The performance of the catalyst was evaluated in the same manner as in Performance Evaluation Example 1 except that the catalyst performance was set to 10,000 h ^-1 .

Table 3 shows the maximum denitration rate C at the above space velocity.
Indicates max (%).

[0039] The catalyst (1) of this example showed very excellent denitration performance even at a higher space velocity. (Example 2) to (Example 3) Catalyst (2) (Example 2) was prepared in the same manner as in Example 1 except that the contents of zinc were changed to 8% and 3%, respectively. A catalyst (3) (Example 3) was obtained.

Next, performance evaluation examples 1 and 2 were carried out using these catalysts. The obtained results are shown in Table 1.

(Example 4) In the same manner as in Example 1 except that the sulfur content in Example 1 was changed to 1.5%,
A catalyst (4) was obtained.

Then, performance evaluation examples 1 and 2 were carried out using this catalyst. The obtained results are shown in Table 1.

(Example 5) A catalyst (5) was obtained in the same manner as in Example 1 except that the content of silver was changed to 3%.

Then, performance evaluation examples 1 and 2 were carried out using this catalyst. The obtained results are shown in Table 1.

(Comparative Example 1) A catalyst (6) was obtained in the same manner as in Example 1 except that the contents of sulfur and zinc were changed to 0%.

Then, performance evaluation examples 1 and 2 were carried out using this catalyst. The obtained results are shown in Table 1.

(Comparative Example 2) A catalyst (7) was obtained in the same manner as in Example 4 except that the content of zinc was changed to 0%.

Then, performance evaluation examples 1 and 2 were carried out using this catalyst. The obtained results are shown in Table 1.

Comparative Example 3 A catalyst (8) was obtained in the same manner as in Example 1 except that the contents of silver and sulfur were changed to 3.6% and 0%, respectively.

Then, performance evaluation examples 1 and 2 were carried out using this catalyst. The obtained results are shown in Table 1.

Comparative Example 4 A catalyst was prepared based on Example 9 of JP-A-6-277454.

46 g of zinc nitrate hexahydrate and 280 g of alumina nitrate nonahydrate were dissolved in 1.5 l of water. To this aqueous solution, 650 g of a 7% by weight aqueous ammonia solution was added with vigorous stirring to obtain a precipitate. About 1 of this precipitate
After aging for 24 hours, this was filtered and washed. The precipitate thus obtained is dried at 110 ° C. for about 1 day and then 6
The carrier was obtained by baking at 00 ° C. for 6 hours. The composition of the obtained carrier was ZnO: Al ₂ O ₃ = 25: 75 (% by weight, calculated as oxide). 38 g of this carrier was added to a 200 ml aqueous solution containing 1.26 g of silver nitrate, evaporated to dryness and baked to obtain a catalyst (9) carrying 2% by weight of silver.

Then, performance evaluation examples 1 and 2 were carried out using this catalyst. The obtained results are shown in Table 1.

The results of the performance evaluation example 1 in Table 1 show that the catalysts (1) to (5) of the examples of the present invention and the catalysts (6) to (6) of the comparative examples.
(8) showed a very excellent denitration performance as compared with the catalyst (9) of the comparative example. However, in the results of Performance Evaluation Example 2, only the catalysts (1) to (5) of the example showed excellent denitration performance, and it was found that the catalyst of the present invention has excellent stoichioresistance.

[0055]

As described above, according to the denitration catalyst and the denitration method according to the present invention, in a lean combustion region where water vapor coexists, even at a high space velocity even after holding in a stoichiometric state, nitrogen is converted at a high conversion rate. Since oxides can be reduced and purified, it is particularly useful for purification of nitrogen oxides discharged from an internal combustion engine operated at a lean air-fuel ratio.

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Hiroyuki Ikeda Ichikawa City, Chiba Chugoku 3-18-5 Sumitomo Metal Mining Co., Ltd. Central Research Laboratory

Claims (5)

[Claims] 1. A denitration catalyst comprising activated alumina supporting silver, zinc and sulfur. 2. The amount of silver, zinc and sulfur supported is 0.1 to 10% by weight, 0.1 to 20% by weight and 0.01 to 10% by weight based on the amount of activated alumina, respectively, in terms of elements. The denitration catalyst according to claim 1, wherein the denitration catalyst is present. 3. A denitration method for exhaust gas, which comprises contacting exhaust gas of an internal combustion engine operated at a lean air-fuel ratio with a catalyst layer at a high space velocity, wherein the catalyst constituting the denitration catalyst layer is a catalyst constituting the denitration catalyst layer. A denitration method using the described denitration catalyst. 4. The space velocity is 10,000 to 200,0.
00h ^-1 , the catalyst layer inlet gas temperature is 300 ° C or higher, 600
4. The denitration method according to claim 3, wherein the temperature is lower than ℃. 5. The space velocity is 10,000 to 100,0.
The denitration method according to claim 3 or 4, wherein the denitration is set to 00h- ¹ .