JPH081014A - Denitration catalyst and denitration method using the same - Google Patents

Abstract

(57) [Summary] (Modified) [Purpose] NOx in exhaust gas of an internal combustion engine with a lean air-fuel ratio
To provide a highly efficient and highly reliable denitration method. [Structure] Using activated alumina as a carrier, tin, silver and /
Alternatively, in the catalyst supporting silver oxide, the relationship between the pore radius of activated alumina and the pore volume is 300
Let A be the total value of the pore volumes occupied by the pores of angstroms or less, and B be the total value of the pore volumes occupied by the pores of 50 angstroms or less.
When the total value of the pore volume occupied by pores in the range of 00 angstrom is C, B is 30% or more of A, and C
Is 15% or less of A, and the exhaust gas in the internal combustion engine operated at a lean air-fuel ratio is 400 to 600 at the denitration catalyst inlet.
The denitration method is carried out in the temperature range of ° C.

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 NOx removal catalyst used for purifying nitrogen oxides in exhaust gas, particularly exhaust gas of an internal combustion engine such as an automobile.
The present invention relates to a denitration catalyst capable of purifying nitrogen oxides in exhaust gas of an internal combustion engine having a lean air-fuel ratio with high space velocity and high efficiency, and a denitration method using the same.

[0002]

2. Description of the Related Art Various combustion exhaust gases emitted from an internal combustion engine such as an automobile engine are mixed with water, which is a combustion product, and carbon dioxide (CO ₂ ) and nitric oxide (NO).
And nitrogen oxides (NOx) such as nitrogen dioxide (NO ₂ ).
Is included in considerable quantity. NOx not only affects the human body and increases the morbidity rate of respiratory diseases, but it is also one of the causes of acid rain, which is a problem from the viewpoint of global environmental protection. Therefore, it is desired to develop a denitration catalyst for efficiently removing nitrogen oxides from these various exhaust gases.

An ideal method of removing NO in NOx is a method of directly decomposing NO as shown by the following reaction formula (1). The equation (1) is a reaction in which the production system on the right side is overwhelmingly dominant in terms of reaction equilibrium.

2NO = N ₂ + O ₂ (1) Japanese Patent Laid-Open No. 60-125 discloses a denitration technique that depends on this reaction.
The method described in Japanese Patent Publication No. 250 is cited. This denitration technique uses a catalyst in which Cu is supported on a zeolite by an ion exchange method, and this catalyst promotes the direct decomposition reaction of NO. However, this denitration technique has a problem that the denitration efficiency is gradually lowered because oxygen generated by the reaction of the formula (1) is preferentially attached to the catalyst active site. Further, there is a drawback that the reaction of the formula (1) is completely hindered under the condition that excess oxygen exists in the reaction system (oxygen excess atmosphere).

On the other hand, from the viewpoint of preventing global warming, an internal combustion engine of the lean burn type has recently been drawing attention. A conventional automobile gasoline engine burns in a stoichiometrically controlled manner near an air-fuel ratio λ = 1, and carbon monoxide (CO) in the exhaust gas, Mainly hydrocarbons (HC) and NOx, platinum (Pt), rhodium (Rh), palladium (Pd) and ceria (Ce)
A three-way catalyst system has been adopted in which these harmful components are simultaneously removed by contacting with an alumina catalyst containing O ₂ ). However, this three-way catalyst method has not been sufficiently effective in purifying exhaust gas in a lean burn gasoline engine of a lean burn method. Also, although the diesel engine is originally a lean burn engine,
Recently, regarding the exhaust gas, suspended particulate matter and NOx
Both of them have come under strict regulation.

Conventionally, as a method for reducing and removing NOx in an oxygen-excess atmosphere, a method has been already established in which NH ₃ which is selectively adsorbed by a catalyst even in a small amount as a reducing gas is used. It has been industrialized as a denitration catalyst for exhaust gas from boilers and diesel engines, which are fixed sources. However, this method requires a special device for the recovery treatment of unreacted reducing agent, and ammonia with a strong odor may be used for this purpose, which is a denitration technology for exhaust gas from mobile sources such as automobiles. Cannot be applied to.

[0007] In recent years, since it has been reported that unburned hydrocarbons remaining in a lean combustion gas in an oxygen excess atmosphere can be used as a reducing agent to promote the reduction reaction of NOx, various catalysts for promoting the reaction have been developed. Has been made. For example, many reports have been made that alumina or a catalyst in which a transition metal is supported on alumina is effective for NOx reduction reaction using hydrocarbon as a reducing agent. Further, in Japanese Unexamined Patent Publication No. 4-282828, there is 0.1
An example is described in which alumina or silica-alumina containing 4 wt% of Cu, Fe, Cr, Zn, Ni, V, etc. is used as a catalyst for NOx reduction.

Furthermore, by using a catalyst in which Pt is supported on alumina, the NOx reduction reaction can proceed in a low temperature range of 200 to 300 ° C.
67946, JP-A-5-68855, and JP-A-5-103949. However, when these noble metal-supported catalysts are used, there is a drawback that the selectivity of the NOx reduction reaction becomes poor because the combustion reaction of hydrocarbon as a reducing agent is excessively promoted.

The present inventors have previously found that when a catalyst containing silver as a reducing agent with hydrocarbon as a reducing agent is used under an oxygen-excessive atmosphere, NO.
It was found that the x reduction reaction selectively and preferentially proceeded, and a patent application was filed for this (Japanese Patent Laid-Open No. 4-281844).
issue). Even after that, the technique for removing NOx by using a similar NOx reduction catalyst using silver is disclosed in Japanese Patent Laid-Open No. HEI 4
Although many patent applications such as JP-A-354536 and JP-A-5-92124 are found, the denitration performance of these conventional silver-supported silver-supported catalysts is still insufficient.

On the other hand, it has been known that a catalyst using alumina as a carrier has a large space velocity dependency.
For example, at a space velocity of SV: 1000 to 10000 hr ^-1 , the NOx reduction performance is sufficiently exhibited, but S
V: N at a space velocity of 10000 hr ⁻¹ or more
As can be seen from the fact that the purification performance of Ox is significantly reduced (see “Catalyst”: 33, 61 (1991)), such a phenomenon was a well-known fact in the art. For example, in the exhaust gas treatment method disclosed in JP-A-5-92124, the contact time between the exhaust gas and the catalyst is 0.03 g. sec / cm ³ or more, preferably 0.1 g. This is the reason why it is limited to sec / cm ³ or more.

[0011]

However, in the exhaust gas treatment of a lean burn engine for vehicles such as automobiles, which is a typical internal combustion engine operated at a lean air-fuel ratio, other important factors which are indispensable for practical use are , Both the required space and the weight of the catalyst layer or the structure consisting of the supporting substrate coated with the catalyst (hereinafter, these are referred to as catalyst-containing layer in the present specification). That is, it is not practical to mount a catalyst-containing layer having a capacity of several times or more of the engine displacement when considering the engine displacement and work, and the capacity of the catalyst-containing layer is set to be equal to or less than the engine displacement. It is desirable to do so.

This means that in order to form a practical catalyst-containing layer, the space velocity of the exhaust gas passing through the catalyst-containing layer is made high (which means that the contact time becomes short), that is, Gas space velocity of 7,000 hr ⁻¹ or more,
It is preferably 10,000 hr ⁻¹ or more, that is, the contact time is 0.03 g. sec / cm ^{3 or} less, preferably 0.02 g. It means that it is required to be less than sec / cm ³ . However, the conventional silver-supported alumina catalyst using alumina as a carrier is still insufficient in denitrification performance for exhaust gas coexisting with steam at such a high space velocity (short contact time).

An object of the present invention is to solve the above-mentioned problems caused by the conventional method. NOx in exhaust gas of an internal combustion engine having a lean air-fuel ratio can be maintained at a sufficiently high gas space velocity (short contact time). An object of the present invention is to provide a denitration catalyst that can be efficiently removed, and a denitration method with high efficiency and reliability of NOx in the exhaust gas of an internal combustion engine with a lean air-fuel ratio using the catalyst. It is what

[0014]

DISCLOSURE OF THE INVENTION The present inventors have proposed a denitrification catalyst and a denitrification method using the same, which are capable of advancing the NOx reduction reaction by a hydrocarbon with high efficiency even in an oxygen excess atmosphere. As a result of intensive studies, a catalyst in which activated alumina having specific pore characteristics is used as a carrier and tin and silver and / or silver oxide are supported on the carrier can achieve the above-mentioned demand. The inventors have found that such performance can be provided and have completed the present invention.

That is, according to the present invention, in a catalyst obtained by using activated alumina as a carrier and supporting tin and silver and / or silver oxide thereon, the activated alumina is used as a pore radius measured by a nitrogen gas adsorption method. The relation with the pore volume is A, the total value of the pore volumes occupied by the pores having a pore radius of 300 Å or less is A, and the total value of the pore volumes occupied by the pores having a pore radius of 50 Å or less is B, Pore radius 1
When the total value of the pore volume occupied by pores in the range of 00 to 300 angstrom is C, B is 30% or more of A and C is 15% or less of A, and a denitration catalyst, and In a denitration method of the exhaust gas in which an exhaust gas in an internal combustion engine operated at a lean air-fuel ratio is passed through a denitration catalyst layer, the denitration catalyst constituting the denitration catalyst layer is the denitration catalyst having the above-mentioned configuration, and Exhaust gas passing through the denitration catalyst layer is 400 to 6 at the denitration catalyst layer inlet.
The denitration method is characterized in that the temperature range is 00 ° C.

According to the method of the present invention, the space velocity of the exhaust gas passing through the denitration catalyst layer is 10,000 hr.
^Even if the denitration reaction is performed at ^-1 or more, the denitration of exhaust gas can be sufficiently purified, and NOx in the exhaust gas can be effectively removed even in an oxygen atmosphere.

[0017]

The operation of the present invention will be described in detail below.

The activated alumina used in the production of the denitration catalyst of the present invention has a relationship between the pore radius and the pore volume obtained when the pores present therein are measured by the nitrogen gas adsorption method. Let A be the total value of the pore volumes occupied by the pores of 300 angstroms or less, and B be the total value of the pore volumes occupied by the pores of 50 angstroms or less in radius.
When C is the total value of the pore volume occupied by pores having a pore radius of 100 to 300 angstroms, B is 3
It has pore characteristics such that it is 0% or more and C is 15% or less of A. Further, the activated alumina is crystallographically classified into γ-type, η-type, or a mixed type thereof, and these activated aluminas are generally mineralogy of boehmite, pseudo-boehmite, and via-type. Aluminum hydroxide powder or gel classified as wright and norstrandalite is heated in air or vacuum at a heating temperature of 300 to 800 ° C, preferably 400 to 600 ° C.
It is obtained by heating and dehydrating at.

In this case, when the activated alumina as the catalyst carrier has another crystal structure, for example, α-alumina, the α-type alumina has an extremely small specific surface area and poor solid acidity. Therefore, δ-alumina is unsuitable as the denitration catalyst carrier of the present invention, and δ-alumina has a relatively small specific surface area of 100 m ² / g. It does not reach. Further, β-alumina and χ-alumina are also unsuitable as the denitration catalyst carrier of the present invention for almost the same reason.

In the present invention, when the pore characteristics of the active amemina are out of the above-mentioned range of the present invention, that is, the pore radius is 5
The total value B of the pore volumes of 0 angstroms or less is 30% or less of the total value A of the pore volumes occupied by the pores of 300 angstroms or less, and the pore radius 100
When the total value C of the pore volume occupied by the pores of ˜300 Å is 15% or more of the value of A, the denitration performance of the obtained catalyst in the presence of water vapor becomes insufficient. Not preferable. That is, in the alumina effective as a carrier in the alumina catalyst supporting tin and silver and / or silver oxide of the present invention, B is 30% or more of A,
Moreover, it is limited to activated alumina having a pore distribution such that C is 15% or less of A.

The method of supporting tin and silver and / or silver oxide on activated alumina in the catalyst of the present invention is particularly carried out from the viewpoint of denitration performance, except that silver is first supported on activated alumina. The method is not limited, and a general supporting method such as an adsorption method, a pore filling method, an incipient wetness method, an evaporation dryness method, a spray method, an impregnation method or a kneading method, or a combination thereof May be appropriately adopted. Further, the loading ratio of tin and silver and / or silver oxide is preferably in the range of less than 1% by weight of tin and 1 to 6% by weight of silver in terms of metal, based on the activated alumina of the present invention. When the loading rate of tin is 1% by weight or more, the denitration performance is deteriorated. On the other hand, when the loading rate of silver is less than 1% by weight, satisfactory denitration activity is not obtained and 6% by weight.
When it is above, the combustion reaction of hydrocarbon as a reducing agent proceeds excessively, and the catalytic activity and reaction selectivity are rather decreased.

Further, the drying temperature is not particularly limited, and the drying is carried out in the temperature range of 80 to 120 ° C. which is usually performed.
After that, 300 to 800 ° C., preferably 400 to 60
Bake at 0 ° C. If the firing temperature is 300 ° C. or lower, sufficient firing is not performed, and if it exceeds 800 ° C., a phase change of alumina occurs, which is not preferable.

The shape of the catalyst of the present invention is not particularly limited, such as powder, sphere, cylinder, honeycomb, and spiral, and the size can be appropriately determined according to the use conditions. Good. In particular, when purifying exhaust gas from an automobile engine, etc., since the gas space velocity is high, a fire-resistant integrated body having a large number of through holes in the exhaust gas flow direction in order to minimize pressure loss. A structure in which the channel surface of the supporting substrate is coated is suitable for use.

The catalyst of the present invention is used for CO, HC in exhaust gas.
And exhaust gas containing oxygen in excess of the stoichiometry required to completely oxidize reducing components such as H _{2 with} oxidizing components such as NOx and O ₂ , more specifically from an internal combustion engine with a lean air-fuel ratio. It is applied to the purification of NOx in the exhaust gas of.

[0025] By contacting the denitration catalyst of the present invention to such an exhaust gas, NOx by the reducing agent component present in trace amounts in the exhaust gas such as HC, is reduced to N2, CO ₂ and H _{2 O} At the same time, reducing agents such as HC are also CO
₂ and H ₂ O. When the HC / NOx ratio of the exhaust gas itself is low, such as the exhaust gas of a diesel engine, several hundred to several thousand ppm of fuel HC as a reducing agent component is additionally added to the exhaust gas as a reducing agent component. After that, if the method of contacting the catalyst of the present invention is adopted, NOx can be purified more effectively.

In order to efficiently purify NOx in exhaust gas by HC in an oxygen-rich atmosphere using the catalyst of the present invention, the inlet temperature of the installed catalyst layer is set to 400 ° C to 600 ° C.
Need to be This is because the tin and silver and / or silver oxide-supported alumina catalyst of the present invention requires a temperature of 400 ° C. or higher in order to exert the denitration performance, and HC is not activated when the temperature is lower than this. Is estimated to be Also, in this case, the inlet temperature of the catalyst layer is 600 ° C.
When the temperature becomes higher than the above, combustion of HC, which is a side reaction, becomes predominant, and the reducing activity of NOx by HC decreases, so that the purification performance deteriorates.

[0027]

The present invention will be described in more detail based on the following examples and comparative examples. However, the present invention is not limited to these examples. Preparation of carrier alumina Using a Sorptomatic made by Carlo Erba Co., which uses a pore distribution measurement method by a nitrogen gas adsorption method, the pore distribution of γ-alumina obtained by a general production method from aluminum hydroxide was measured. , The total value of the pore volumes of the pores having a pore radius of 300 Å or less is A, the total value of the pore volumes of the pores having a pore radius of 50 Å or less is B, and the pore radius is 100 to 300 Å. Assuming that the total value of the pore volume of the above-mentioned pores is C, γ-alumina having a pore distribution in which A, B and C have the following six different relationships was obtained.

Alumina a: B is 40.4% of A, C is A
Γ-alumina having a pore distribution such that it is 4.4%.

Alumina b: B is 74.7% of A, C is A
? -Alumina having a pore distribution such that it is 2.4%.

Alumina c: B is 57.8% of A, C is A
Γ-alumina having a pore distribution such that it is 3.9%.

Alumina d: B is 32.1% of A, C is A
Γ-alumina having a pore size distribution such that it is 10.8%.

Alumina e: B is 14.0% of A, C is A
Γ-alumina having a pore distribution such that it is 45.9%.

Alumina f: B is 31.0% of A, C is A
? -Alumina having a pore distribution such that it is 22.7%. A. Preparation of Catalyst Sample Using each of the above-mentioned alumina as a carrier, tin and silver were supported on the carrier to prepare a catalyst sample for denitration performance evaluation. In Examples 1 to 5 and Comparative Examples 1 and 2, and Comparative Example 4, Sn / Ag / Al ₂ O ₃ catalyst was used, and in Comparative Example 3, A was used.
A g / Al ₂ O ₃ catalyst was prepared. Examples 1 to 5, Comparative Example 1, Comparative Example 2 and Comparative Example 5 a. Preparation of Sn / Ag / Al ₂ O ₃ Catalyst Samples Alumina a, Alumina b, Alumina c and Alumina d are shown in Example 1, Example 2, Example 3 and Example 4, respectively, and Comparative Example 1 and Comparative Example. 2 is alumina e and alumina f, respectively, and in Example 5, alumina a
In Comparative Example 4, alumina b was used, and in each Example and Comparative Example, 100 g of alumina was immersed in a 1,000 ml aqueous solution containing 4.9 g of silver nitrate, and heated to 100 to 110 ° C. with stirring. Then, the water was evaporated and the mixture was calcined in air at 500 ° C. for 3 hours to first obtain an Ag / Al ₂ O ₃ catalyst. Next, this catalyst was immersed in an aqueous solution of stannous chloride having different concentrations, and the same procedure was followed to obtain a Sn / Ag / Al ₂ O ₃ catalyst sample. The loading ratios of Sn and Ag in these catalysts are shown in Examples 1 to 4.
In Comparative Examples 1 and 2, the amounts are 0.01% and 3%, respectively, in terms of metal, based on alumina. In Example 5, 0.5% and 3%, respectively, and in Comparative Example 4, 1%, respectively. 0.0% and 3%. Comparative Example 3 b. Ag / Al supporting only Ag without supporting Sn
Preparation of ₂ O ₃ catalyst sample An Ag / Al ₂ O ₃ catalyst sample supporting 3% of Ag was obtained by the same procedure as in Example 1 except that alumina a was not used to support tin. B. Evaluation test of catalyst performance a. Evaluation Test 1 Using the catalyst samples of Examples 1 to 5 and Comparative Examples 1 to 4,
These catalyst samples were pressure-molded into a predetermined shape, pulverized and sized to a particle size of 250 to 500 μm. It was mounted in a pressure fixed bed reactor. NO 1,000 pp was added to this catalyst layer as model exhaust gas.
m, propylene 1,300 ppm, O ₂ 5%, balance N
_The mixed gas consisting of ₂ was passed through at a space velocity of 15,000 hr ⁻¹ .

Regarding the composition of the gas at the outlet of the reaction tube, NO and NO
The concentration of ₂ was measured using a chemiluminescence type NOx meter, and the concentration of N ₂ O was measured using a gas chromatography thermal conductivity detector equipped with Porapack Q column. Set the catalyst layer inlet temperature to a predetermined temperature in the range of 300 to 600 ° C,
The value at the time when the gas composition at the outlet of the reaction tube became stable at each predetermined temperature was taken as the measured value.

When the model exhaust gas passes through the catalyst layer, NO in the reaction gas is converted into NO ₂ , N ₂ O and / or N ₂ , but the reaction gas is heated to the catalyst layer inlet temperature 30.
Since it was found that N ₂ O was hardly generated when the catalyst layer of the present invention was passed at 0 ° C. or higher, the denitration rate (NO conversion rate) in the present invention is represented by the following formula.

[0036] Table 1 shows the catalyst layer inlet temperature 40 in the above performance evaluation test 1.
The maximum denitration ratio Cmax (%) between 0 ° C and 500 ° C and the temperature Tmax (° C) at that time are shown.

From the results shown in Table 1, it can be seen that the catalysts of Examples 1 to 5 of the present invention exhibit remarkably high denitration performance even at a high space velocity in a lower temperature region than the catalysts of Comparative Examples 1 to 4.

[0038]

[Table 1] ────────────────────────────────── Catalyst denitration rate Execution number Catalyst activated alumina 400 ℃ 450 ℃ 500 ℃ ─────────────────────────────────── Example 1 0.1% Sn / 3% Ag / Al ₂ O ₃ a 79.2 93.7 59.5 Example 2 〃 b 82.1 97.5 63.2 Example 3 〃 c 80.9 96.2 61.7 Example 4 〃 d 78.3 93.7 59.2 Example 5 0.5% Sn / 3% Ag / Al ₂ O ₃ a 76.7 81.2 72.7 Comparative Example 1 0.1% Sn / 3% Ag / Al ₂ O ₃ e 16.8 30.1 61.6 Comparative Example 2 〃 f 13.4 27.5 75.0 Comparative Example 3 3% Ag / Al ₂ O ₃ a 50.9 99.5 88.6 Comparative Example 4 1.0% Sn / 3 % Ag / Al ₂ O ₃ b 52.7 62.0 15.3 ────────────────────────────────── b. Evaluation test 2 Next, only the space velocity in the evaluation test 1 was changed to 50,000.
hr except for changing the ^-1 and 70,000Hr ^-1 evaluated the performance of the catalyst samples of Example 1 in the same manner as Evaluation Test 1.

Table 2 shows the maximum denitration rate Cmax (%) between the catalyst layer inlets 400 to 500 ° C. and the temperature Tmax (° C.) at that time at the above space velocity. The results of Table 2 show that the catalyst (1) of Example 1 of the present invention had a shorter contact time,
That is, it can be seen that excellent denitration rate is exhibited even at a higher space velocity.

[0040]

[Table 2] ───────────────────────────── Catalyst denitration rate Space velocity (h ⁻¹ ) 400 ° C 450 ° C 500 ° C ─ ──────────────────────────── 50,000 80.1 92.1 60.3 70,000 82.5 98.7 69 3 ──────────────────────────────

[0041]

As described above, when the exhaust gas is denitrated by the denitration method of the present invention using the catalyst of the present invention, a high conversion rate is obtained in an oxygen excess atmosphere in which water vapor coexists and at a high space velocity. Since it is possible to reduce and purify nitrogen compounds in exhaust gas, it can be said that the invention is highly practical.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location B01D 53/94 B01J 23/50 ZAB M 23/62 ZAB M 23/66 ZAB A B01D 53/36 102 D 104 A

Claims (3)

[Claims] 1. A catalyst comprising activated alumina as a carrier, and tin and at least one of silver and silver oxide supported on the activated alumina, the activated alumina being used as a carrier and having a fine pore radius and a fine diameter measured by a nitrogen gas adsorption method. The relationship with the pore volume is the pore radius 3
Let A be the total value of the pore volumes occupied by the pores of 00 angstroms or less, and B be the total value of the pore volumes occupied by the pores of 50 angstroms or less.
A denitration catalyst in which B is 30% or more of A and C is 15% or less of A, where C is the total value of the pore volume occupied by the pores in the range of ˜300 Å. 2. A denitration catalyst for forming the denitration catalyst layer in a denitration method for the exhaust gas, wherein exhaust gas in an internal combustion engine operated at a lean air-fuel ratio is passed through the denitration catalyst layer. Exhaust gas, which is a denitration catalyst and passes through the denitration catalyst layer, is 400 at the inlet of the denitration catalyst layer.
A denitration method characterized by being in a temperature range of up to 600 ° C. 3. The denitration method according to claim 2, wherein the space velocity of the exhaust gas passing through the denitration catalyst layer is 10,000 hr ⁻¹ or more.