JPH08131827A - Denitration catalyst and denitration method using the same - Google Patents

Abstract

PURPOSE: To provide a denitration catalyst capable of efficiently removing NOx in waste gas in a low fuel-air ratio at a high space velocity of gas by depositing at least one of silver and silver oxide on an alumina-titania carrier obtd. by incorporating titania into specified activated alumina. CONSTITUTION: At least one of silver and silver oxide is deposited on an alumina-titania carrier obtd. by incorporating titania into activated alumina to obtain the objective denitration catalyst for removal of NOx in waste gas with hydrocarbon in an atmosphere contg. excess oxygen. The activated alumina has >=120m<2> /g specific surface area and specified relation between pore radius and pore volume measured by a gaseous nitrogen adsorption method. In the relation, the total pore volume (B) of pores each having 30-100Åpore radius is >=72% of the total pore volume (A) of pores each having <300Å pore radius and the total pore volume (C) of pores each having 10-300Å pore radius is <=20% of the total pore volume (A).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a denitration catalyst for purifying exhaust gas from a mobile generation source or a stationary generation source, particularly nitrogen oxide in exhaust gas of an internal combustion engine, and more specifically to a denitration catalyst. 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 In various kinds of combustion exhaust gas discharged from an internal combustion engine such as an automobile engine or a large combustion apparatus such as a factory, nitric oxide (CO ₂ ) and nitrogen monoxide (CO ₂ ) are produced. NO) and nitrogen oxides (NOx) such as nitrogen dioxide (NO ₂ ) are contained in a considerable amount. NOx
Not only does SO and SOx adversely affect the human body, but they are also one of the causes of acid rain, which is regarded as 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 performs controlled stoichiometric combustion in the vicinity of an air-fuel ratio λ = 1, and carbon monoxide (CO) in exhaust gas is used for exhaust gas treatment. 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 is industrialized as a NOx removal 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 Patent Application Laid-Open No. 4-284848, 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 in this manner is disclosed in Japanese Patent Laid-Open No. HEI 4
There are many patent applications such as Japanese Patent Application Laid-Open No. 354536 and Japanese Patent Application Laid-Open No. 5-92124, and further, Applied Catalysis B: Environmental, 2
(1933) pp. 199-205, it is reported that the alumina catalyst supporting silver is superior to the alumina catalyst supporting Co, Cu, V and Cr in the NOx reduction performance in the presence of water vapor. ing.

However, the denitration performance of these conventional silver-supported catalysts using alumina as a carrier has not been sufficiently active as a catalyst for NOx reduction reaction by hydrocarbons in the presence of water vapor.

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
Such a phenomenon was a well-known fact in the art, as can be seen from the fact that the purification performance of Ox was reported to be significantly reduced (see “Catalyst”: 33, 61 (1991)). 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.

[0012]

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 construct 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).

After all, in any known literature concerning silver-supported alumina catalysts, nitrogen oxides in exhaust gas due to hydrocarbons can be removed with high efficiency in an oxygen-excess atmosphere in the presence of water vapor and at a high space velocity. It must be said that such a catalyst is not disclosed at all.

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

[0016]

DISCLOSURE OF THE INVENTION The present inventors have used a denitration catalyst capable of advancing the NOx reduction reaction by hydrocarbon with high efficiency even in an oxygen-excess atmosphere in which water vapor coexists, and by using the same. As a result of earnest studies on the denitration method of the above, an alumina-titania carrier containing activated alumina having a specific surface area of 120 m ² / g and a specific amount of titania is used. However, they have found that the above problems can be solved by using a catalyst in which a specific amount of silver and / or silver oxide is supported, and have completed the present invention.

That is, the present invention is a catalyst for removing nitrogen oxides in exhaust gas due to hydrocarbons in an oxygen-excess atmosphere, which comprises a pore radius and a pore volume measured by a nitrogen gas adsorption method. The relationship is that the total value of the pore volumes occupied by the pores having a pore radius of less than 300 angstrom is A, and the total value of the pore volumes occupied by the pores having a pore radius of 30 angstrom or more and less than 100 angstrom is B, Radius 1
When the total value of the pore volume occupied by pores of 00 angstrom or more and less than 300 angstrom is C, B is 72% or more of A, C is 20% or less of A, and the specific surface area is 120 m ² / g. Is a denitration catalyst formed by supporting silver and / or silver oxide on an alumina-titania carrier in which titania is contained in activated alumina, and a denitration catalyst layer for removing exhaust gas in an internal combustion engine operated at a lean air-fuel ratio. In the denitration method for exhaust gas, the denitration catalyst according to the present invention is used as a denitration catalyst constituting the denitration catalyst layer.

When the denitration method of the present invention described above is used,
It is preferable that the exhaust gas passing through the denitration catalyst layer has a temperature range of 200 to 600 ° C. at the inlet of the denitration catalyst layer. And when using the denitration method of the present invention,
The space velocity (SV) of exhaust gas passing through the denitration catalyst layer is 1
Even if the NOx removal reaction is carried out at 0000 hr ^-1 or more, the exhaust gas can be sufficiently purified by NOx removal, and the NOx in the exhaust gas can be effectively reduced even in an oxygen excess atmosphere where water vapor coexists. It is possible to remove.

[0019]

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

The catalyst carrier in the denitration catalyst of the present invention has a pore volume of pores having a pore radius of less than 300 angstroms, which is obtained by the nitrogen gas adsorption method. And the total value of the volume of pores occupied by pores with a radius of 30 Å or more and less than 100 Å is B, and the total volume of pores occupied by pores with a radius of 100 Å or more and less than 300 Å is B. When the value is C, B
Is 72% or more of A, C is 20% or less of A and has a pore characteristic, and a specific surface area is 120 m ² / g. It contains titania.

The activated alumina used in the present invention is classified crystallographically into γ-type, η-type or a mixed type thereof, and these activated aluminas are generally mineralogy. Aluminum hydroxide powder or gel classified as boehmite, pseudo-boehmite, vialite and norstrandalite is heated in air or in 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 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 activated alumina are out of the above-mentioned range of the present invention, that is, the pore radius is 3
The total value B of the pore volumes occupied by the pores of 0 angstrom or more and less than 100 angstrom is the total value A of the pore volumes occupied by the pores whose pore radius is less than 300 angstrom.
Is less than 72%, and the total value C of the pore volume occupied by pores having a radius of 100 Å or more and less than 300 Å exceeds 20% of the value of A. The denitration performance of the catalyst in the presence of steam is insufficient, which is not preferable. That is, the effective alumina in the alumina-titania support for supporting silver and / or silver oxide in the catalyst of the present invention is
It is limited to activated alumina having a pore distribution such that B is 30% or more of A and C is 15% or less of A. In addition to this, the activated alumina used in the present invention preferably has a specific surface area of 120 m ² / g or more.

To prepare the titania-containing alumina carrier, titanium salt is added to an aqueous solution of activated alumina, preferably an activated alumina precursor such as aluminum hydroxide, or alkali is added to a mixed solution of aluminum salt and titanium salt. It is preferable to produce the aluminum alkoxide by hydrolyzing it by adding a titanium alkoxide or a titanium salt to the aluminum alkoxide. When a production method other than the case where the titanium salt is directly contained in the activated alumina itself, the pore distribution, the specific surface area, etc. of the activated alumina which is the main component of the carrier obtained in advance fall within the predetermined range in the present invention. It is necessary to determine raw materials, manufacturing conditions, etc. Further, the content of titania in the obtained titania-containing alumina carrier is preferably 0.1 to 15% by weight in terms of metallic titanium, based on alumina.

Regarding the effect of containing titania in the carrier, it is considered that the SMSI of silver and alumina and the synergistic effect of increasing the acidity contribute to the improvement of the denitration rate, but less than 0.1% by weight. The effect is not fully exhibited with the inclusion of, and if it exceeds 15% by weight, the effect is rather reduced.

The method for supporting silver on the titania-containing alumina carrier produced in this way is not particularly limited,
A general supporting method, for example, an adsorption method, a pore filling method, an incipient wetness method, an evaporation dryness method, a spray method, an impregnation method, a kneading method, or a combination thereof may be appropriately adopted.

The loading of silver and / or silver oxide in the catalyst is preferably in the range of 1 to 6% by weight in terms of metal, based on the activated alumina used in the carrier of the present invention. When the loading rate of silver is less than 1% by weight, satisfactory denitration activity cannot be obtained, and when it exceeds 6% by weight, the combustion reaction of hydrocarbon as a reducing agent proceeds excessively, resulting in catalytic activity and reaction selection. On the contrary, the sex declines.

The drying temperature is not particularly limited, and the drying is carried out in a temperature range of 80 to 120 ° C. which is usually carried out, and thereafter, baking is carried out at 300 to 800 ° C., preferably 400 to 600 ° C. If the firing temperature is lower than 300 ° C, 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 is powdery, spherical, cylindrical,
Any shape such as a honeycomb shape and a spiral shape can be adopted without particular limitation, and the size may be appropriately determined according to the usage conditions. 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.

[0031] By contacting the denitration catalyst of the present invention to such an exhaust gas, the reducing agent component NOx is present in trace amounts in the exhaust gas such as HC, in N2, HC to CO ₂ and H _{2 O} Simultaneously with the reduction, the reducing agent such as HC is also oxidized into CO ₂ and H ₂ O. HC / NOx of exhaust gas itself like exhaust gas of diesel engine
When the ratio is low, it is more preferable to adopt a method in which the catalyst of the present invention is contacted after additionally adding several hundred to several thousand ppm of fuel HC as a reducing agent component in the exhaust gas at a methane conversion concentration. It is possible to effectively purify NOx.

Although the temperature range in which the activity varies depends on the type of reducing agent, the catalyst of the present invention is used in an exhaust gas containing C ₂ or more paraffins, olefins, and aromatic HCs in an oxygen excess atmosphere at a high space velocity. In order to efficiently purify NOx, the inlet temperature of the installed catalyst layer should be 400
℃ ~ 600 ℃, in the case of C ₁₀ or more 200 ℃ ~
It needs to be 600 ° C. The reason why the inlet temperature must be adjusted in this way is that the silver and / or silver oxide-supporting titania-containing alumina catalyst according to the present invention has a minimum temperature which varies depending on the type of the reducing agent in order to exert the denitration performance. This is because HC is not activated when the temperature is lower than this, and although it slightly differs depending on the type of reducing agent, when the inlet temperature of the catalyst layer becomes higher than 600 ° C., a side reaction occurs. This is because the combustion activity of a certain HC becomes predominant and the reduction activity of NOx by the HC is reduced, so that the purification capacity is deteriorated.

[0033]

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. A. Preparation of Titanium-Containing Alumina Carrier Using 8 kinds of alumina hydrates as γ-alumina precursors having different particle size distributions, these alumina hydrates were
[Gamma] -alumina (a-
The pore distribution was measured for each of h) using a Sorptomatic made by Carlo Erba Co., which uses the pore distribution measurement method by the nitrogen gas adsorption method, and each γ-alumina (a to h) was measured. Is the total value of the pore volumes of pores having a pore radius of 300 Å or less, and the pore radius is 5
The total value of the pore volume of pores of 0 angstrom or less is B
And C is the total value of the pore volumes of the pores having a radius of 100 to 300 angstroms, A, B and C have different specific surface areas from the pore distribution having the following relationship. .

Alumina a: B is 73.2% of A, C is A
Has a pore distribution such that 2.4% of, and the specific surface area is 241m ² / g .gamma.-alumina.

Alumina b: B is 80.2% of A, C is A
Γ-alumina having a pore distribution such that the specific surface area is 219 m ² / g.

Alumina c: B is 83.4% of A, C is A
[Gamma] -alumina having a pore size distribution such that the specific surface area is 174 m < ² > / g.

Alumina d: B is 79.9% of A, C is A
Γ-alumina having a pore size distribution such that the specific surface area is 193 m ² / g.

Alumina e: B is 49.5% of A, C is A
Γ-alumina having a pore size distribution of 45.9% and a specific surface area of 241 m ² / g.

Alumina f: B is 67.5% of A, C is A
It has a pore distribution such that 22.7% of the, and the specific surface area is 266m ² / g .gamma.-alumina.

Alumina g: B is 25.0% of A, C is A
Γ-alumina having a pore size distribution of 36.9% and a specific surface area of 153 m ² / g.

Alumina h: B is 33.2% of A, C is A
Γ-alumina having a pore size distribution of 39.8% and a specific surface area of 92 m ² / g.

100 g of each of the eight types of alumina hydrate capable of obtaining the above eight types of alumina (a to h)
0.24 g of titanium tetrachloride was added to each of the solutions dispersed in 1000 ml and stirred at 100-110.
Containing titania containing 0.3% by weight of alumina in terms of metallic titanium by heating to ℃ to evaporate the water content, further drying by ventilation at 110 ℃, and then calcination in air at 600 ℃ for 3 hours. Alumina carriers (carrier 1 to carrier 8) were prepared. Further, for alumina hydrate corresponding to alumina a, 2.04 g, 8.85 g and 17.5 g of titanium tetrachloride were further added to the same aqueous solution as described above, and then metallic titanium was added in the same procedure as described above. A titania-containing alumina carrier containing 2.5 wt%, 10 wt% and 18 wt% of alumina (carrier 9, carrier 10)
And carrier 11) was prepared. Furthermore, as an example of the conventional carrier, only alumina a containing no titania is used as the carrier 12.
Prepared as B. Preparation of Catalyst Samples About the above titania-containing alumina carriers (carriers 1 to 11), silver was loaded on each of them to prepare catalyst samples for denitration performance evaluation (Examples 1 to 7 and Comparative Examples 1 to 1).
5). In Comparative Example 6, a conventional alumina carrier (carrier 12) containing no titania was loaded with silver to prepare a catalyst sample. The details are shown below. Examples 1 to 4, Comparative Examples 1 to 4 a. Preparation of 3% Ag / Al ₂ O ₃ -0.3% TiO ₂ Catalyst Samples In Example 1, Example 2, Example 3 and Example 4,
3. Carriers 1, 2, 3 and 4, respectively, and in Comparative Example 1, Comparative Example 2, Comparative Example 3 and Comparative Example 4, carriers 5, 6, 7 and 8, respectively, were used to obtain 100 g of each carrier.
Immersion in 1000 ml of an aqueous solution containing 9 g of silver nitrate (3.1 g in terms of metallic silver), and stirring 100 to 1
By heating to 10 ° C. to evaporate water and baking in air at 500 ° C. for 3 hours, Ag / Al ₂ O ₃ —TiO _{2 was used.}
A catalyst sample was prepared. The loading ratios of Ag in these catalyst samples are all 3.0% by weight with respect to alumina in terms of metal. Example 5 b. Preparation of 3% Ag / Al ₂ O ₃ -2.5% TiO ₂ catalyst sample Ag / Al ₂ O ₃ —TiO ₂ catalyst sample was prepared in the same procedure as in Example 1 except that the carrier 9 was used as the titania-containing alumina carrier. Was prepared. The Ag loading rate of this catalyst sample was also 3.0% by weight based on the alumina, in terms of metal. Example 6 c. Preparation of 3% Ag / Al ₂ O ₃ -10% TiO ₂ Catalyst Sample Ag / Al ₂ O ₃ —TiO ₂ was prepared in the same procedure as in Example 1 except that the carrier 10 was used as the titania-containing alumina carrier.
A catalyst sample was prepared. The Ag loading rate of this catalyst sample was also 3.0% by weight based on the alumina, in terms of metal. Example 7 D. _{5.8% Ag / Al 2 O 3} -0.3% TiO 2 catalyst except that the amount of silver nitrate in the preparation of silver nitrate aqueous solution of the sample was 9.7g Example 1 and the same procedure with Ag / _Al 2 _{O 3} A TiO ₂ catalyst sample was prepared. The Ag loading rate of this catalyst sample was 5.8% by weight based on the amount of metal based on alumina. Comparative Example 5 E. Preparation of 3% Ag / Al ₂ O ₃ -18% TiO ₂ catalyst sample Ag / Al ₂ O ₃ —TiO ₂ was prepared in the same procedure as in Example 1 except that the carrier 11 was used as the titania-containing alumina carrier.
A catalyst sample was prepared. The Ag loading rate of this catalyst sample was 3.0% by weight in terms of metal, based on alumina. Comparative Example 6 f. _3% Ag / _Al 2 _O 3 except for using the carrier 12 as an alumina carrier containing no preparation titania catalyst samples of Example 1 the same procedure as in Ag / _Al 2 _{O 3}
A catalyst sample was prepared. The Ag loading rate of this catalyst sample was 3.0% by weight in terms of metal, based on alumina. C. Evaluation test of catalyst performance a. Evaluation Test 1 Using the catalyst samples of Examples 1 to 7 and Comparative Examples 1 to 6,
These catalyst samples were pressure-molded into a predetermined shape, pulverized and sized to a particle size of 250 to 500 μm, and then these sized products were filled into a stainless steel reaction tube having an inner diameter of 21 mm. It was mounted in an atmospheric fixed bed reactor. NO is 1000 ppm and C is used as model exhaust gas in this catalyst layer.
₃ H ₆ is 1000 ppm, O ₂ is 5%, H ₂ O is 10
%, The balance gas consisting of N ₂ and a space velocity of 30,000 h
Passed at r- 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 when passing through the catalyst layer of the present invention, almost NO is converted. _Since it was found that ₂ O was not produced, the denitration rate (NO conversion rate) is represented by the following formula in the present invention.

[0045] Table 1 shows the catalyst layer inlet temperature 30 in the above performance evaluation test 1.
The maximum denitration ratio Cmax (%) and the corresponding temperature Tmax (° C) between 0 ° C and 600 ° C are shown.

From the results shown in Table 1, it can be seen that the catalysts of Examples 1 to 7 of the present invention show remarkably high denitration performance even at a high space velocity as compared with the catalysts of Comparative Examples 1 to 6.

[0047]

[Table 1] ──────────────────────────────────── Carrier evaluation results Implementation number Catalyst number Cmax ( %) Tmax (C) ──────────────────────────────────── Example 1 3% Ag / Al _{2 O 3 -0.3% TiO 2 1} 88.9 500 example 2 〃 2 88.4 500 example 3 〃 3 84.2 500 example 4 〃 4 87.3 500 example _{5 3% Ag / Al 2 O} 3 -2.5% TiO 2 9 86.5 500 Example 6 3% Ag / Al ₂ O ₃ -10% TiO ₂ 10 83.2 500 Example 7 5.8% Ag / Al ₂ O ₃ -0.3% TiO ₂ 1 84.1 450 Comparative Example 1 3% Ag / Al ₂ O ₃ -0.3% TiO ₂ 5 73.7 500 Comparative Example 2 〃 6 74.1 500 Comparative Example 3 〃 7 53.9 550 Comparative Example 4 〃 8 57.3 550 Comparative Example 5 3% Ag / Al ₂ O ₃ -18% TiO ₂ 11 52.7 550 Comparative Example _{6 3% Ag / Al 2 O} 3 12 62.8 500 ───────── ────────────────────────── b. Evaluation test 2 Next, only the space velocity in the evaluation test 1 was set to 60000h.
The performance of the catalyst sample of Example 1 was evaluated by the same procedure as in Evaluation Test 1 except that r ^-1 was used.

Table 2 shows the maximum denitration rate Cmax (%) in the above space velocity between the inlets of the catalyst layer and 300 to 600 ° C. The results in Table 2 show that the catalyst of Example 1 of the present invention exhibits an excellent denitration rate even at a shorter contact time, that is, at a higher space velocity.

[0049]

[Table 2] ─────────────────────── Evaluation results Space velocity (h ⁻¹ ) Cmax (%) ─────────── ───────────── 60,000 91.5 ───────────────────────

[0050]

As described above, when the catalyst according to the present invention is used and the denitration of exhaust gas is performed by the denitration method of the present invention, a high conversion is performed in an oxygen excess atmosphere in the presence of water vapor and at a high space velocity. Since it is possible to reduce and purify nitrogen compounds in exhaust gas at a high rate, it can be said that this is an excellent industrial invention.

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

Claims (4)

[Claims] 1. A catalyst used when removing nitrogen oxides in exhaust gas by hydrocarbons in an oxygen-rich atmosphere, comprising a pore radius and a pore volume measured by a nitrogen gas adsorption method. The relation is that the total value of the pore volumes occupied by the pores having a pore radius of less than 300 angstrom is A, and the total value of the pore volumes occupied by the pores having a pore radius of 30 angstrom to less than 100 angstrom is B. Radius 10
When the total value of the pore volume occupied by pores of 0 to less than 300 angstrom is C, B is 72% or more of A, and C
Is 20% or less of A, and the specific surface area is 120 m ² /
A denitration catalyst in which at least one of silver and silver oxide is supported on an alumina-titania carrier in which titania is contained in activated alumina having a weight of at least g. 2. The denitration catalyst according to claim 1, wherein the content of titania is 0.1 to 15% by weight in terms of metallic titanium with respect to the activated alumina. 3. A denitration catalyst for forming the denitration catalyst layer in the denitration method for exhaust gas, wherein exhaust gas in an internal combustion engine operated at a lean air-fuel ratio is brought into contact with the denitration catalyst layer through contact. A denitration method using the denitration catalyst according to claim 2. 4. The denitration method according to claim 3, wherein the space velocity of the exhaust gas passing through the denitration catalyst layer is 10,000 hr ⁻¹ or more.