JP2919541B2 - Nitrogen oxide removal catalyst - Google Patents

Description

The present invention relates to a catalyst for removing nitrogen oxides (hereinafter referred to as NOx) contained in exhaust gas discharged from boilers, gas turbines, diesel engines and various industrial processes. About.

In particular, the present invention relates to a catalyst that adds ammonia or the like as a reducing agent to an exhaust gas containing NOx, efficiently reduces NOx to harmless nitrogen and water at a high temperature of 400 ° C. or higher, and has excellent durability.

<Prior art> At present, as a method for removing NOx in exhaust gas, NOx can be selectively removed even from exhaust gas containing high concentration of oxygen,
In addition, since only a small amount of reducing agent is used and it is economical,
A selective catalytic reduction method using ammonia as a reducing agent has become mainstream.

Many catalysts have been proposed that support oxides such as panadium, copper, tungsten, molybdenum and iron on a carrier such as alumina, silica, zeolite or titanium oxide as a catalyst used in the selective catalytic reduction method using ammonia as a reducing agent. Among them, a catalyst containing titanium as a main component is not widely affected by SOx in exhaust gas, and has a low ability to oxidize SO _{2 in} an exhaust gas to SO ₃ . On the other hand, there are gas exhaust gas temperatures exceeding 500 ° C., such as gas turbine exhaust gas and diesel engine exhaust gas, and catalysts for treating NOx in such high-temperature exhaust gas are disclosed in JP-A-55-167044 and JP-A-5-167044. Showa 57-127426
This is already disclosed in the official gazette.

<Problems to be Solved by the Invention> However, at high temperatures, the catalyst described in the above-mentioned publication has insufficient denitration activity because the oxidation (or decomposition) reaction of ammonia occurs at the same time as the reduction of NOx. However, there is also a problem with the properties of the catalyst, and at present it is not satisfactory as a practical catalyst.

Therefore, an object of the present invention is to use a high temperature of 400 ° C. or more,
High NOx removal activity and efficient NOx over a long period
It is an object of the present invention to provide a catalyst capable of removing water.

<Means for Solving the Problems> The present inventors have conducted intensive studies to achieve the above object, and as a result, completed the present invention.

That is, the present invention relates to a catalyst for removing nitrogen oxides by reacting nitrogen oxides in exhaust gas with a reducing agent such as ammonia and catalytically reducing the nitrogen oxides, the catalyst comprising titanium (Ti) and silicon (Si). Binary oxides and / or titanium (Ti), zirconium (Zr) and silicon (Si)
The catalyst A is composed of a ternary oxide consisting of: and the oxide of tungsten (W) and / or tin (Sn) is the component of the catalyst B. Further, cerium (Ce), praseodymium (Pr)
And zeolite having a silicon oxide (SiO ₂ ) ratio of at least 8 to aluminum oxide (Al ₂ O ₃ ) carrying at least one element selected from the group consisting of neodymium and neodymium (Nd) is used as the catalyst C component. And a catalyst for removing nitrogen oxides, comprising a catalyst C component, or a catalyst A component, a catalyst B component and a catalyst C component.

 Hereinafter, the present invention will be described in detail.

The present inventors have proposed a binary composite oxide composed of titanium and silicon (hereinafter abbreviated as TiO ₂ —SiO ₂ ) and a ternary composite oxide composed of titanium, zirconium and silicon (hereinafter TiO ₂ —ZrO ₂ −). A catalyst containing SiO ₂ as a main component has been proposed in JP-A-52-122293. Also,
If titanium, phosphorus and silicon are used, a ternary composite oxide (hereinafter abbreviated as TiO ₂ —P ₂ O ₅ —SiO ₂ ) and a quaternary composite oxide (hereinafter referred to as TiO ₂ ) composed of titanium, phosphorus, zirconium and silicon ₂ -P ₂ O ₃ -ZrO ₂ -SiO ₂ )
A catalyst containing as a main component has already been proposed in JP-A-57-127426.

The present inventors have attempted to improve the denitration activity of the above catalyst in a high temperature range of about 400 to 700 ° C., and as a result, TiO ₂ −
SiO ₂ and / or TiO ₂ —ZrO ₂ —SiO ₂ (catalyst A component) and tungsten and / or tin oxide (catalyst B component) plus cerium (Ce) and praseodymium (Pr)
A catalyst containing zeolite (catalyst C component) having a ratio of SiO ₂ to Al ₂ O ₃ of at least 8 previously supported with at least one element selected from neodymium (Nd) and neodymium (Nd) (catalyst C component) has an improved denitration activity at high temperatures Was remarkable, and it was found that the heat resistance was significantly improved.

The mechanism of the improvement of the high-temperature activity and the heat resistance of the catalyst of the present invention is not clear at present, but is presumed as follows. However, the present invention is not limited by the right or wrong of this presumption.

It is well known that zeolite has a large specific surface area and has solid acidity, and at the same time has a denitration activity itself. However, as in the present invention, zeolite is previously selected from cerium (Ce), praseodymium (Pr) and neodymium (Nd). By supporting and immobilizing at least one element on the zeolite, the acid properties of the zeolite are optimally controlled, and it is said that this usually occurs as a side reaction at high temperatures. It is considered that oxidation of NH ₃ to NOx (or decomposition to N ₂ ) is suppressed as much as possible, and as a result, the denitration activity is significantly improved.

In addition, zeolites generally not only lose their acid properties over time when exposed to high temperatures,
It is a well-known fact that the denitration activity tends to decrease because the zeolite structure itself also changes. However, as in the catalyst of the present invention, cerium (Ce), praseodymium (P
r) and at least one element selected from neodymium (Nd) are supported, and the immobilized zeolite has a reduced acidity and a suppressed change in zeolite structure even when exposed to high temperatures for a long time. Therefore, it is considered to be excellent in heat resistance.

TiO ₂ —SiO ₂ or TiO ₂ —ZrO ₂ —SiO ₂ which is a catalyst A component
If the content is less than 40% by weight, the heat resistance deteriorates, and if it exceeds 95% by weight, the oxidation (or decomposition) of NH ₃ occurs at 400 ° C. or higher, and the denitration activity decreases. Is preferably 40 to 95% by weight in the total catalyst to give a preferable result.

The composition of the catalyst A component is 40 to 95% titanium by atomic percentage,
The content of silicon and / or zirconium is preferably in the range of 5 to 60%, and the specific surface area is preferably 30 m ² / g or more, particularly preferably 50 m ² / g or more.

If the content of the catalyst B component in the same catalyst component exceeds 15% by weight, the improvement of the denitration activity cannot be expected much, and the raw material cost of the catalyst increases. Therefore, the content is preferably 0 to 15% by weight. .

The zeolite used as the catalyst C component is Si
When the O ₂ / Al ₂ O ₃ ratio is less than 8, not only does SOx in the exhaust gas react with Al ₂ O ₃ to destroy its structure, but also SiO ₂ / Al ₂ O ₃ because the denitration performance is low. The ratio is preferably 8 or more. For example, suitable zeolites include mordenite, ferrierite, ZSM-5, and the like. When the amount of at least one element selected from Ce, Pr and Nd previously supported in the catalyst C component is less than 0.1% by weight, not only the denitration performance is low but also the heat resistance is poor, and the amount is 10% by weight. If the ratio exceeds the above, not only the improvement of the denitration performance cannot be expected, but also the raw material cost of the catalyst increases, so the loading amount is preferably from 01 to 10% by weight.
The above-mentioned elements are carried as metal ions, metals or oxides, and the amount of such elements refers to those obtained from values converted as metals. When the content of the catalyst C component in all the catalyst components is less than 5% by weight, the denitration performance is low.
If it exceeds 60% by weight, the heat resistance becomes poor, and the moldability is also poor. Therefore, a range of 5 to 60% by weight gives a preferable result.

To prepare TiO ₂ -SiO ₂ used in the present invention, first, a titanium source is selected from titanium chlorides, inorganic titanium compounds such as titanium sulfate and titanium sulfate, and organic titanium compounds such as tetraisopropyl titanate. The silicon source can be selected from colloidal silica, water glass, inorganic silicon compounds such as silicon tetrachloride, and organic silicon compounds such as tetraethyl silicate. Some of these raw materials contain trace amounts of impurities and contaminants. However, this does not pose a problem as long as it does not significantly affect the physical properties of the obtained TiO ₂ —SiO ₂ .

Preferred methods for preparing TiO ₂ —SiO ₂ include the following methods.

A method in which titanium tetrachloride is mixed with a silica sol, ammonia is added to form a precipitate, and the precipitate is washed, dried and calcined at 300 to 650 ° C.

Add an aqueous solution of titanium tetrachloride sodium silicate,
After reacting to form a precipitate, which was washed and dried,
Baking at 650 ° C.

A method in which ethyl silicate [(C ₂ H ₅ O) ₄ Si] is added to a water-alcohol solution of titanium tetrachloride to cause a hydrolysis reaction to form a precipitate, which is washed, dried and calcined at 300 to 650 ° C.

A method in which ammonia is added to a water-alcohol solution of titanium oxychloride (TiOCl ₂ ) and ethyl silicate to form a precipitate, which is washed, dried and calcined at 300 to 650 ° C.

Among the above preferred methods, a particularly preferred method is preferred, and this method is specifically carried out as follows.
That is, the compound of the titanium source and the silicon source is TiO ₂
The molar ratio of SiO ₂ and SiO ₂ is set to a predetermined amount, and titanium and silicon are converted to oxides at a concentration of 1 to 100 g / in an acidic aqueous solution or sol, and the temperature is kept at 10 to 100 ° C.

Ammonia water is dripped into it as a neutralizing agent with stirring,
A coprecipitated compound consisting of titanium and silicon is formed at pH 2 to 10 for 10 minutes to 3 hours, filtered, washed well, dried at 80 to 140 ° C. for 1 to 10 hours, and dried at 450 to 700 ° C. for 1 to 10 hours.
After firing for a time, TiO ₂ —SiO ₂ is obtained.

TiO ₂ —ZrO ₂ —SiO ₂ is prepared in the same manner as TiO ₂ —SiO _2, and zirconium sources include inorganic zirconium compounds such as zirconium chloride and zirconium sulfate and organic compounds such as zirconium oxalate. Can be selected from among the reactive zirconium compounds. That is, TiO ₂ —ZrO ₂ —SiO ₂ can be easily prepared by treating the zirconium compound together with the titanium compound in the same manner as described above. The amount of zirconium is converted to ZrO ₂ with respect to the total amount of TiO ₂ + ZrO ₂ + SiO ₂
It is preferably in the range up to 30% by weight.

Next, starting materials of other catalyst components used together with TiO ₂ —SiO ₂ and TiO ₂ —ZrO ₂ —SiO ₂ are appropriately selected from oxides, hydroxides, ammonium salts, oxalates, halogen compounds and the like.

Further, the preparation of the zeolite pre-loaded with cerium, praseodymium and / or neodymium used in the present invention may be performed by using nitrate, sulfate, hydroxide, ammonium salt, oxalate, halide, etc. of Ce, Pr, Nd. It is carried out by adding an aqueous solution to a hydrogen-type zeolite, supporting the solution by an ion exchange method, a dipping method, or a mixing and kneading method, and then drying and, if necessary, firing.

As an example of a method for preparing the catalyst of the present invention, zeolite is added to an aqueous solution containing cerium nitrate, and after well mixed and kneaded.
It is dried at a temperature in the range of 80 ° C. to 140 ° C., and if necessary, calcined at a temperature in the range of 300 ° C. to 800 ° C. in an atmosphere of an inert gas such as nitrogen or air for 1 hour to 10 hours. Next, this cerium-supported zeolite was obtained by the method described above.
A solution obtained by dissolving ammonium paratungstate in an aqueous solution containing monoethanolamine and a forming aid are added to TiO ₂ —SiO ₂ powder, mixed, kneaded, and formed into a honeycomb shape by an extruder.

After drying the molded product in the temperature range of 50 to 120 ° C, 400 to 700
° C temperature range, preferably 500 ° C to 650 ° C.
The catalyst can be obtained by calcining in a stream of air for up to 10 hours, preferably 2 to 6 hours.

As another method, a method in which a TiO ₂ —SiO ₂ powder and a zeolite powder supporting cerium are formed in a honeycomb shape in advance and impregnated with an aqueous solution containing tungsten to support the honeycomb is supported. Further, it is also possible to use a carrier. As the carrier, for example, alumina, silica, silica-
Alumina, bentonite, diatomaceous earth, silicon carbide, titania, zirconia, magnesia, cordierite, mullite, pumice, inorganic fibers, and the like can be used.For example, zeolite in which TiO ₂ —SiO ₂ and cerium are supported on granular silicon carbide It can be prepared by a method in which powder and other catalyst components are made into a slurry state and supported by an impregnation method. Of course, the method for preparing the catalyst of the present invention is not limited to these methods.

The shape of the catalyst is not limited to the above-described honeycomb shape, and may be appropriately selected from a columnar shape, a cylindrical shape, a plate shape, a ribbon shape, a corrugated plate shape, a pipe shape, a donut shape, a lattice shape, and other integrally molded shapes. .

The composition of the exhaust gas to be treated using the catalyst of the present invention is usually SOx 0 to 3000 ppm, oxygen 1 to 20 volume%, carbon dioxide gas 1 to 15 volume%, steam 5 to 15 volume%, dust
0.01~30g / Nm ³ and NOx are those which contain the degree of (mainly NO) 20~1000ppm. Although ordinary boiler exhaust gas falls within this range, the gas composition is not particularly limited. This is because the catalyst of the present invention can treat special exhaust gas such as NOx-containing exhaust gas not containing SOx and NOx-containing exhaust gas containing a halogen compound.

Although the treatment conditions vary depending on the type and properties of the exhaust gas, the amount of ammonia (NH ₃ ) added is preferably 0.5 to 3 parts with respect to 1 part of NOx. For example, in the boiler exhaust gas composition, most of NOx is NO, so NO and NH ₃
A molar ratio of about 1: 1 is particularly preferred. This is because care must be taken not to discharge excess NH ₃ as unreacted components. Further, when it is necessary to suppress unreacted NH ₃ as much as possible, it is preferable to use a molar ratio of NH ₃ / NOx of 1 or less. Next, the reaction temperature is 300-700 ° C, especially 400-650
° C is preferred and the space velocity is between 1000 and 100000 hr ^-1 , especially 30
A range from 00 to 30000 hr ^-1 is preferred. The pressure is not particularly limited, but is preferably in the range of 0.01 to 10 kg / cm ² .

The type of the reactor is not particularly limited, but a usual fixed bed, moving bed, fluidized bed, or other reactor can be used.

<Examples> Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

Example 1 TiO ₂ —SiO ₂ was prepared by the method described below.

11.4 kg of titanium tetrachloride (TiCl ₄ ) is gradually dropped into water 80 with ice cooling and stirring, and then Snowtex-0 (containing 20 to 21% by weight as silica sol SiO ₂ manufactured by Nissan Chemical Co., Ltd.) 4.5 k
g was added. While maintaining the temperature at about 30 ° C. and stirring well, ammonia water was gradually added dropwise until the pH reached 7, and the mixture was left to stand and aged for 2 hours.

The TiO ₂ -SiO ₂ gel thus obtained was filtered, washed with water 12
It was dried at 0 ° C. for 10 hours, washed with water, and fired at 600 ° C. for 3 hours. The composition of the obtained powder is TiO ₂ / SiO ₂ as oxide.
= 4 (molar ratio) and the BET surface was 180 m ² / g. The powder obtained here is hereinafter referred to as TS-1.

Next, Ce was supported on the hydrogen-type zeolite by the method described below. Cerium nitrate _{(Ce (NO 3) 3 .6H} 2 O) 9.30g of H ₂ O150
g) and hydrogenated mordenite (TSZ-600H
OA) Stir and knead well with a kneader together with 297 g. afterwards
It was dried at 120 ° C. for 3 hours and fired at 600 ° C. for 3 hours. The Ce content in the obtained zeolite was 1% by weight. The powder obtained here is hereinafter referred to as Ce-M.

21 ml of monoethanolamine was mixed with 210 ml of water, and 58.3 g of ammonium paratungstate was added and dissolved to obtain a uniform solution. Next, this solution was mixed with the above powder TS-
1650 g and Ce-M 300 g were mixed and kneaded while adding an appropriate amount of water with a kneader in addition to a kneader.Then, the diameter was 4 mm and the length was 5 with an extruder.
It was formed into a pellet of mm. Then dried at 60 ° C, 60
Calcination was carried out at 0 ° C. for 5 hours under flowing air. TS-1 obtained the finished catalyst, Ce-M, and the content of WO ₃ is 65%, respectively, by weight%, 30%, was 5%.

Example 2 11.4 kg of titanium tetrachloride, zirconium oxychloride [ZrOCl _2.
8H ₂ O was repeated except for using 1.2kg and Snowtex -0.34kg was prepared TiO ₂ -ZrO ₂ -SiO ₂ analogously as described in Example 1. The composition of the obtained powder was TiO ₂ : ZrO ₂ : SiO ₂ = 80:
At a ratio of 5:15 (molar ratio), the BET surface area was 210 m ² / g. The obtained powder was called TZS-1, and Example 1 was performed using this TZS-1.
A catalyst having the same composition as described above was prepared. The content of TZS-1, Ce-M and WO _{3 in} the obtained finished catalyst was expressed by weight%.
65% and 30% respectively. 5%.

Examples 3 to 10 Using the TiO ₂ —SiO ₂ powder (TS-1) obtained in Example 1, the composition of WO ₃ , SnO ₂ and Ce-supported zeolite, and the elements pre-supported on the zeolite were Pr or Nd. And a catalyst was prepared according to Example 1. The obtained catalyst composition is as follows in terms of% by weight.

<Catalyst activity test> The denitration rate of each of the catalysts of Examples 1 to 10 was determined by the following method.

20 ml of the catalyst was charged into a quartz reaction tube having an inner diameter of 16 mm charged into an electric furnace, and a synthesis gas having the following composition was introduced into the catalyst layer. The NOx concentration in the gas from the inlet to the outlet of the reactor was measured by a chemiluminescent NOx meter (BCL-77A type) manufactured by Yanagimoto Seisakusho, and the denitration rate was calculated according to the following equation.

Reaction gas conditions Gas volume 3.33Nl / min Space velocity (SV) 10000Hr ^-1 NH ₃ / NOx (molar ratio) 1.0 Gas composition NOx 100ppm O ₂ 15% SO ₂ 200ppm H ₂ O 10% N ₂ remaining As shown in Table 1, durability tests were performed on Examples 1, 5, 6, and 7 by the following method.

Using the same apparatus and the same gas for which the denitration rate was determined, heat exposure was performed at 600 ° C under circulation using the same gas, and the change in the denitration rate at 500 ° C with elapsed time was examined. Table 2 shows the obtained results.

────────────────────────────────────────────────── ─── Continued from the front page Examiner Eiko Ebihara (56) References JP-A-63-12350 (JP, A) JP-A-57-127426 (JP, A) JP-A-52-122293 (JP, A) Kaihei 2-83039 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) B01J 21/00-38/74 B01D 53/56

Claims (2)

(57) [Claims] 1. A catalyst for removing nitrogen oxides by reacting nitrogen oxides in exhaust gas with a reducing agent such as ammonia and catalytically reducing the nitrogen oxides, the catalyst comprising titanium (Ti) and silicon (Si). A binary oxide and / or a ternary oxide composed of titanium (Ti), zirconium (Zr) and silicon (Si) is used as a catalyst A component, and tungsten (W)
And / or tin (Sn) oxide as catalyst B component,
Furthermore, the ratio of silicon oxide (SiO ₂ ) to aluminum oxide (Al ₂ O ₃ ), which previously supports at least one element selected from cerium (Ce), praseodymium (Pr) and neodymium (Nd), is 8 or more. Zeolite as catalyst C
A catalyst for removing nitrogen oxides, which comprises the catalyst A component and the catalyst C component, or the catalyst A component, the catalyst B component, and the catalyst C component as components. 2. The catalyst A component has a content of 40 to 95% by weight, the catalyst B component has a content of 0 to 15% by weight and the catalyst C component has a content of 5 to 60% by weight. (Ti) 40 to 95%, silicon (S) and / or zirconium (Zr) in the range of 5 to 60%, and cerium (Ce), praseodymium (Pr) and neodymium (Nd) in the catalyst C component The catalyst for removing nitrogen oxides according to claim 1, wherein the carried amount of at least one element selected from the group consisting of (a) and (b) is 0.1 to 10% by weight.