JPH08131829A - Denitrification catalyst and denitrifying method using the same - Google Patents

Abstract

PURPOSE: To provide a denitrification catalyst with which NOx in the exhaust gas from an inner combustion engine with a lean air-fuel ratio can be efficiently removed at an enough high gas space velocity, and to provide a denitrifying method using this catalyst showing high efficiency and high reliability to remove NOx in the exhaust gas from an inner combustion engine. CONSTITUTION: This catalyst is used to remove nitrogen oxides in exhaust gas of hydrocarbons in an oxygen-rich atmosphere. The catalyst consists of an alumina-titania carrier with deposition of silver and/or silver oxide. This alumina-titania catalyst is obtd. by adding titania to active alumina having >=120m<2> /g specific surface area, >=0.60g/cc balk density and <=1.80g/cc intrinsic density measured by a mercury pressurizing method. The denitrifying method for the exhaust gas from an inner combustion engine driven at a lean air-fuel ratio is carried out by allowing the exhaust gas to pass and touch through a denitrification catalyst layer. In this method, the catalyst is used as the catalyst constituting the denitrification catalyst layer.

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) 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. There is.

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 exemplified 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). To provide a deoxidizing catalyst that can be efficiently removed by means of a denitrification method, and a denitrification method using the catalyst with high efficiency and high reliability of NOx in exhaust gas of an internal combustion engine with a lean air-fuel ratio. The purpose is.

[0016]

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, the present invention has been completed. That is, the present invention is a catalyst used when removing nitrogen oxides in exhaust gas with hydrocarbons in an oxygen excess atmosphere, having a specific surface area of 120 m ² / g or more, and measured by mercury porosimetry. Silver is added to a titania-containing alumina carrier in which 0.1 to 15% by weight of metal is added to activated alumina having a bulk specific gravity and a true specific gravity of 0.60 g / cc or more and 1.8 g / cc or less, respectively. And / or a denitration catalyst supporting silver oxide, and in the denitration method of the exhaust gas, wherein the exhaust gas in an internal combustion engine operated at a lean air-fuel ratio is brought into contact with the denitration catalyst layer, The above-mentioned denitration catalyst according to the present invention is used as a catalyst constituting the catalyst, and the exhaust gas passing through the denitration catalyst layer is controlled at a temperature range of 200 to 600 ° C. at the denitration catalyst layer inlet. It was a denitration method.

When the denitration method of the present invention is used, the space velocity (SV) of the exhaust gas passing through the denitration catalyst layer.
It is possible to sufficiently purify the exhaust gas by removing NOx even if the NOx removal reaction is carried out at 10000 hr ^-1 or more.
It is possible to effectively reduce and remove NOx in the exhaust gas even in an oxygen excess atmosphere in which water vapor coexists.

[0018]

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

The denitration catalyst of the present invention has a specific surface area of 120 m.
² / g or more, the bulk density and the true density measured by the mercury penetration method are 0.60 g / cc or more and 1.8, respectively.
The main component of the catalyst carrier is activated alumina having an amount of 0 g / cc or less.

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. In addition, β-alumina and χ-alumina are not suitable as the denitration catalyst carrier of the present invention for almost the same reason.

Further, when the specific surface area is less than 120 m ² / g,
In addition, the catalyst in which titania is contained in activated alumina having a bulk density of less than 0.60 g / cc and silver is supported on this carrier has an insufficient denitration rate in the presence of water vapor. . Further, it was obtained when activated alumina having a specific surface area of 120 m ² / g or more and a bulk density of 0.60 g / cc or more and a true density of less than 1.80 g / cc was used. The denitrification rate of the catalyst in the presence of water vapor becomes insufficient.

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. The content of titania in the carrier thus obtained was 0.
It is preferably from 1 to 15% by weight.

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.

There is no particular limitation on the method of loading silver on the titania-containing alumina carrier thus produced,
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 ratio of silver and / or silver oxide is
It 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, firing 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 powder, 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.

By bringing such exhaust gas into contact with the denitration catalyst of the present invention, NOx is converted to N ₂ and HC is CO ₂ and H ₂ O by a reducing agent component such as HC present in a trace amount in the exhaust gas. At the same time, the reducing agent such as HC is also oxidized to 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 HC 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. Since the combustion of a certain HC becomes predominant, the reducing activity of NOx by the HC decreases, and the purification capacity deteriorates.

[0032]

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-
When the specific surface area by the nitrogen gas adsorption method and the bulk density and the true density by the mercury intrusion method were measured for each of h), the following values were obtained.

Alumina a: Specific surface area: 241 m ² / g,
Bulk density: 0.67 g / cc, true density: 1.56 g / cc Alumina b: Specific surface area: 219 m ² / g, bulk density: 0.
65 g / cc, true density: 1.49 g / cc Alumina c: specific surface area: 193 m ² / g, bulk density: 0.
69 g / cc, true density: 1.37 g / cc Alumina d: specific surface area: 174 m ² / g, bulk density: 0.
76 g / cc, true density: 1.62 g / cc Alumina e: specific surface area: 281 m ² / g, bulk density: 2.
70 g / cc, true density: 0.58 g / cc Alumina f: specific surface area: 166.7 m ² / g, bulk density:
1.00 g / cc, true density: 2.61 g / cc Alumina g: Specific surface area: 106.5 m ² / g, bulk density:
0.71 g / cc, true density: 1.59 g / cc Alumina h: specific surface area: 191 m ² / g, bulk density: 1.
06 g / cc, true density: 3.16 g / cc 117.6 g of each of the eight types of alumina hydrate that can obtain the above eight types of alumina (a to h) is 100
0.18 g of titanium tetrachloride was added to each of the solutions dispersed in 0 ml of water, and the mixture was heated to 100 to 110 ° C. with stirring to evaporate water, and then air-dried at 110 ° C., and then air. 0.1% by weight of alumina in terms of metallic titanium by calcining at 600 ° C for 3 hours
A titania-containing alumina carrier (carrier 1 to carrier 8) containing was prepared. Further, for alumina hydrate corresponding to alumina a, 4.66 g, 21.2 g and 44.8 g of titanium tetrachloride were further added to the same aqueous solution as described above, and then metal titanium was added by the same procedure as described above. A titania-containing alumina carrier containing 2.5% by weight, 10% by weight and 20% by weight 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 and Comparative Examples 1 to 4 a. Preparation of 3% Ag / Al ₂ O ₃ -0.1% TiO ₂ Catalyst Samples In Example 1, Example 2, Example 3 and Example 4,
Carriers 1, 2, 3 and 4 were used, and in Comparative Example 1, Comparative Example 2, Comparative Example 3 and Comparative Example 4, carriers 5, 6, 7 and 8 were used, and 100 g of each carrier was used.
Immerse in 1000 ml of an aqueous solution containing 6 g of silver nitrate (3.1 g in terms of metallic silver), and stir 100-1
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.

Comparative Example 5 d. Preparation of 3% Ag / Al ₂ O ₃ -20% 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 E. _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 1300 ppm, O ₂ is 5%, H ₂ O is 10
%, And the balance gas consisting of N ₂ at a space velocity of 15000 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 by a chemiluminescence type NOx meter, and the concentration of N ₂ O was measured by a gas chromatography equipped with Porapack Q column.
Each was measured using a thermal conductivity detector. 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.

[0041] Table 1 shows the catalyst layer inlet temperature 45 in the above performance evaluation test 1.
The denitration rate (%) at 0 ° C is shown.

From the results shown in Table 1, it can be seen that the catalysts of Examples 1 to 6 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.

[0043]

[Table 1] ────────────────────────────────── Carrier evaluation results Execution number Catalyst number Denitrification rate (% ) ────────────────────────────────── Example 1 3% Ag / Al₂O_Three-0.1% TiO₂ 1 88.7 Example 2 〃 2 84.5 Example 3 〃 3 82.9 Example 4 〃 4 80.3 Example 5 3% Ag / Al₂O_Three-2.5% TiO₂ 9 86.1 Example 6 3% Ag / Al₂O_Three-10% TiO₂ 10 80.5 Comparative Example 1 3% Ag / Al₂O_Three-0.1% TiO₂ 5 30.1 Comparative Example 2 〃 6 54.3 Comparative Example 3 〃 7 39.6 Comparative Example 4 〃 8 33.2 Comparative Example 5 3% Ag / Al₂O_Three-20% TiO₂ 11 13.8 Comparative Example 6 3% Ag / Al₂O_Three 12 60.2 ───────────────────────────────────b. Evaluation test 2  Next, only the space velocity in the evaluation test 1 is 60000h.
r^-1Example in the same procedure as in Evaluation Test 1 except that
The performance of the catalyst sample No. 1 was evaluated.

Table 2 shows the denitration rate (%) at the catalyst bed inlet temperature of 450 ° C. at the above space velocity. 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.

[0045]

[Table 2] ──────────────────── Evaluation results Space velocity (h ⁻¹ ) Denitration rate (%) ───────────── ──────── 60,000 90.2 ────────────────────

[0046]

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 can reduce and purify nitrogen compounds in exhaust gas, it can be said to be an excellent industrial invention.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location B01D 53/94 B01J 21/12 ZAB A B01D 53/36 102 B 102 D

Claims (4)

[Claims] 1. A catalyst used when removing nitrogen oxides in exhaust gas with hydrocarbons in an oxygen-excess atmosphere, having a specific surface area of 120 m ² / g or more and measured by a mercury injection method. The bulk density and the true density are 0.
A denitration catalyst obtained by supporting at least one of silver and silver oxide on an alumina-titania carrier in which titania is contained in activated alumina having an amount of 60 g / cc or more and 1.80 g / cc or less. 2. The denitration catalyst according to claim 1, wherein the content of titania is 0.1 to 15% by weight in terms of metal based on the active flumina. 3. 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, and a catalyst constituting the denitration catalyst layer is defined by claim 1. Item 2. A denitration method using the denitration catalyst according to Item 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.