JP5840068B2 - Nitrous oxide decomposition catalyst and method for producing nitrous oxide decomposition catalyst - Google Patents

Description

  The present invention relates to a nitrous oxide decomposition catalyst, a method for producing the nitrous oxide decomposition catalyst, and a method for treating a nitrous oxide-containing gas.

Nitrous oxide (N ₂ O) contained in various combustion exhaust gases discharged from power generation gas turbines, boilers, waste incinerators, etc. and various industrial exhaust gases discharged from chemical plants, etc. is about 310 times that of carbon dioxide. Since the greenhouse effect is exhibited, the development of an efficient decomposition and removal method is desired.

As a method for decomposing and removing nitrous oxide by contacting it with a catalyst, a method using a catalyst in which ruthenium and / or rhodium and zirconium oxide are supported on hydrophobic alumina (Patent Document 1), rhodium oxide, cobalt trioxide (Co) ₂ O ₃ ), a manganese compound, and a method using a catalyst containing an alkali or alkaline earth metal compound (Patent Document 2) have been proposed. In these conventional techniques, nitrous oxide is treated at a low temperature. Therefore, it is necessary to use an expensive noble metal such as rhodium, which is not suitable for practical use.

On the other hand, as a catalyst that does not use a noble metal, there has been proposed a catalyst containing cobalt trioxide (Co ₃ O ₄ ) as a main component and containing an alkali metal and / or an alkaline earth metal (Patent Document 3). A catalyst containing a metal or an alkaline earth metal tends to cause a decrease in catalytic activity due to adsorption of carbon dioxide in exhaust gas, and is not suitable for practical use.

  Further, a nitrous oxide decomposition catalyst in which cesium and / or rubidium are blended in a specific molar ratio with cobalt oxide has been proposed (Patent Document 4), but the catalyst is poisoned by nitrogen monoxide, nitrogen dioxide or sulfur dioxide. This is not preferable for exhaust gas treatment in which the substance is easily present in the exhaust gas.

JP-A-6-142517 JP-A-6-106027 JP 2007-54717 A Japanese Patent Application No. 2010-198073

  An object of the present invention is to provide an efficient nitrous oxide decomposition catalyst and a method for treating a nitrous oxide-containing gas that efficiently decomposes and removes nitrous oxide at a low temperature without using expensive noble metals. is there.

  As a result of diligent research to achieve the above object, the present inventors have found that a nitrous oxide decomposition catalyst containing a cobalt oxide and a bismuth oxide as a catalyst component and having an optimized catalyst composition has low-temperature activity. The present invention was completed by finding that it was good and excellent in durability.

  That is, the nitrous oxide decomposition catalyst of the present invention is characterized in that it contains cobalt oxide and bismuth oxide as catalyst components, and the atomic ratio of bismuth to cobalt is 0.0005 to 0.15.

  The nitrous oxide decomposition catalyst is preferably produced by mixing cobalt trioxide obtained by firing cobalt carbonate and an aqueous solution containing bismuth nitrate, drying and firing.

The nitrous oxide decomposition catalyst of the present invention can be used for a long time at a low temperature even when the processing gas contains nitrogen oxides (NO, NO ₂ etc.), sulfur oxides (SO, SO ₂ , SO ₃ etc.) and carbon dioxide. Thus, nitrous oxide can be efficiently decomposed and removed.

  Next, the method for producing a nitrous oxide decomposition catalyst according to the present invention is produced by adding an aqueous solution containing bismuth nitrate to cobalt trioxide obtained by firing cobalt carbonate, thoroughly mixing, drying and firing. It is. By using the above production method, the nitrous oxide decomposition catalyst can be produced simply and easily.

  In addition, the method for treating a nitrous oxide-containing gas of the present invention can efficiently and stably treat nitrous oxide over a long period of time even if the nitrous oxide-containing gas contains nitrogen oxides or sulfur oxides. .

  Hereinafter, embodiments of the present invention will be described in detail. However, the present invention is not limited to the following embodiments, and various modifications can be made without departing from the scope of the present invention. .

  The nitrous oxide decomposition catalyst of the present invention is a nitrous oxide decomposition catalyst containing cobalt oxide and bismuth oxide as catalyst components, and the atomic ratio of bismuth to cobalt in the catalyst component is 0.0005 to 0.15. It is characterized by. The atomic ratio of bismuth to cobalt is 0.0005 to 0.15, more preferably 0.001 to 0.10, and particularly preferably 0.005 to 0.05. When the atomic ratio of bismuth to cobalt exceeds 0.15, the content of cobalt oxide in the catalyst is reduced, so that initial activity and long-term durability may not be sufficiently obtained, and the atomic ratio is 0.0005. If it is less than the range, the effect of adding bismuth is weakened and the reaction rate at a low temperature may be remarkably reduced, which is not preferable outside these ranges.

  The reason why the low-temperature activity is remarkably improved or the durability is greatly improved by containing cobalt oxide and bismuth oxide in the above-mentioned atomic ratio is unknown, but bismuth is dissolved in cobalt oxide. It is thought that it is an effect by doing. Similarly, a catalyst obtained by adding an alkali metal such as cesium to cobalt oxide, which is a prior art, similarly has good initial low-temperature activity. However, when nitrogen oxide is present in the gas, cesium forms cesium nitrate rapidly. It has been found that the activity decreases. On the other hand, bismuth nitrate decomposes at low temperatures, so that long-term durability can be obtained specifically without lowering the activity like cesium.

The cobalt oxide used for the catalyst may contain Co ₃ O ₄ , CoO, or Co ₂ O ₃ , but is preferably cobalt trioxide (Co ₃ O ₄ ).

  Commercially available cobalt oxide can be used as a raw material for cobalt oxide, but cobalt nitrate, cobalt chloride, cobalt acetate, cobalt hydroxide, or cobalt carbonate (such as cobalt carbonate or basic cobalt carbonate) can be used. . For example, (1) the raw material can be fired to obtain a cobalt oxide, and (2) the precipitate can be dissolved in an aqueous medium and adjusted for pH to obtain a precipitate, and then fired to obtain a cobalt oxide. Most preferred is a cobalt oxide obtained by firing cobalt carbonate. This is because a cobalt oxide having a high specific surface area can be obtained by using the method, and a nitrous oxide decomposition catalyst having a high specific surface area can be obtained by using the oxide.

  The firing temperature when cobalt carbonate is used as the raw material for the cobalt oxide is preferably 250 ° C. to 750 ° C., and more preferably 300 ° C. to 650 ° C. When the temperature is 250 ° C. or lower, cobalt carbonate is not decomposed sufficiently and it is difficult to obtain a cobalt oxide sufficiently. When the temperature is 750 ° C. or higher, particles may grow with crystal change of cobalt oxide or a decrease in specific surface area. It is not preferable.

  As raw materials for bismuth oxide, bismuth nitrate, bismuth hydroxide, bismuth acetate, bismuth citrate, bismuth carbonate, bismuth oxide can be used, and these can be dispersed in an aqueous medium and used as desired compounds. .

  Since the bismuth oxide raw material is difficult to dissolve in water, it can be dissolved in advance by adding acid or alkali to the bismuth oxide raw material. For example, when bismuth nitrate is used as a raw material, it is preferable to add and dissolve an inorganic acid such as nitric acid, sulfuric acid, hydrochloric acid and phosphoric acid, or an organic acid such as acetic acid and citric acid and use it as an acidic aqueous solution.

The specific surface area of the catalyst is more preferably 25 m ² / g or more. Especially preferably, it is 35-100 m < ² > / g. When the specific surface area of the catalyst is 25 m ² / g or less, the number of active sites of the catalyst is reduced and sufficient activity cannot be obtained. As described above, the specific surface area of the catalyst can be easily adjusted to the above range by using a material obtained by calcining at 250 to 750 ° C. using cobalt carbonate as a cobalt source.

Further, in the diffraction pattern obtained by measuring the catalyst by powder X-ray diffraction, the cobalt oxide has a crystal structure of cobalt trioxide (Co ₃ O ₄ ), and the diffraction peak derived from bismuth oxide is Preferably it is not detected. Thus, the diffraction peak derived from bismuth oxide is not detected because the bismuth oxide exists as amorphous fine particles in the vicinity of the main component cobalt oxide (ie, cobalt tetroxide). There may be a case where an oxide forms a solid solution by dissolving in cobalt oxide. In particular, it is preferable that bismuth oxide forms a solid solution by dissolving in cobalt oxide.

  In addition to cobalt oxide and bismuth oxide, the catalyst may be added with a compound of at least one metal element selected from the group consisting of groups 8 to 11 of the periodic table, if necessary. As the metal element, ruthenium, rhodium, iridium, palladium, platinum, iron, nickel, copper, and silver are preferable. The amount of the component to be added is preferably 0.01 to 20% by mass, and preferably 0.1 to 10% by mass, when the total of the cobalt oxide and the bismuth oxide is 100% by mass. More preferred.

  The catalyst can be used by supporting cobalt oxide and bismuth oxide on a porous inorganic oxide or mixing with a porous inorganic oxide. In this case, the amount of cobalt oxide and bismuth oxide is preferably 0.1 to 50% by mass and more preferably 1 to 20% by mass when all the catalyst components are 100% by mass. preferable. As the porous inorganic oxide, an oxide of one or more elements selected from the group consisting of aluminum, silicon, titanium, cerium, magnesium and zirconium can be used.

  Next, although the typical manufacturing method of a nitrous oxide decomposition | disassembly catalyst is shown below, unless it is contrary to the main point of this invention, it is not limited to the following manufacturing method. (1) A method of sufficiently mixing a cobalt oxide precursor such as cobalt oxide, carbonate, hydroxide or the like and a bismuth compound, and firing if necessary (mixing method), (2) an aqueous solution of cobalt oxide and bismuth compound Or mixing with aqueous dispersion, adjusting pH and filtering if necessary, drying and baking (impregnation method), (3) mixing cobalt aqueous solution or aqueous dispersion with bismuth compound aqueous solution or aqueous dispersion. If necessary, a method of adjusting pH and filtering and drying and baking (coprecipitation method or deposition method) can be used, but an impregnation method in which an aqueous solution or an aqueous solution of a bismuth compound is mixed with cobalt oxide is preferable. .

  More preferably, the method for producing the nitrous oxide decomposition catalyst of the present invention is produced by adding an aqueous solution of bismuth nitrate to cobalt trioxide obtained by calcining cobalt carbonate, thoroughly mixing, drying and calcining. Is. In the above method for producing a nitrous oxide decomposition catalyst, it is more preferable to add nitric acid as an acid in order to dissolve bismuth nitrate. By using the above production method, a nitrous oxide decomposition catalyst in which bismuth oxide is uniformly dispersed in cobalt oxide can be produced easily and easily.

  In the above catalyst production method, the drying conditions after mixing the cobalt oxide and the bismuth aqueous solution are not particularly limited, but considering the productivity, the drying temperature is 80 to 200 ° C. and the drying time is 1 to 20 hours. Is preferred. This is because if the drying temperature is less than 80 ° C. or the drying time is less than 1 hour, drying may be insufficient and the catalyst performance may be adversely affected. Moreover, it is because it is unpreferable from a viewpoint of energy efficiency or production efficiency to make drying temperature higher than 200 degreeC, or to make drying time longer than 20 hours.

  Further, the firing conditions of the catalyst after the drying can be appropriately changed, and are not particularly limited. However, it is preferably fired at 300 to 700 ° C. for 1 to 10 hours in an air atmosphere. This is because when the firing temperature is less than 300 ° C. and the firing time is less than 1 hour, the bismuth oxide in the cobalt oxide may not be uniformly dispersed and a predetermined performance may not be obtained. . Also, if the calcination temperature exceeds 700 ° C. or the calcination time exceeds 10 hours, it is not preferable because the specific surface area of the catalyst may decrease or the performance may be deteriorated by sintering due to heat load.

  The shape of the nitrous oxide decomposition catalyst of the present invention is not particularly limited, and can be appropriately selected from a cylindrical shape, a ring shape, a spherical shape, a plate shape, a honeycomb shape, and other integrally formed ones. The catalyst can be molded by a general molding method such as a tableting method or an extrusion method. In the case of a spherical shape, the average particle diameter is usually 1 to 10 mm. In the case of a honeycomb shape, it is manufactured by an extrusion molding method or a method of winding a sheet-like element, and the shape of the gas passage port (cell shape) may be any of a hexagonal shape, a quadrangular shape, a triangular shape, or a corrugation shape. . The cell density (number of cells / unit cross section) is usually 25 to 800 cells / in 2. The catalyst component may be extruded or supported on a ceramic carrier such as cordierite having a predetermined shape or a metal carrier.

  Next, a method for treating a nitrous oxide-containing gas uses the nitrous oxide decomposition catalyst of the present invention, and nitrous oxide is removed without adding a reducing agent such as hydrocarbon, carbon monoxide, hydrogen or ammonia. It can be decomposed directly into nitrogen and oxygen. In addition, even if the nitrous oxide-containing gas contains nitrogen oxides or sulfur oxides, the use of the nitrous oxide decomposition catalyst of the present invention can suppress a decrease in activity due to poisoning and can stably treat for a long time. it can.

  The nitrous oxide concentration of the nitrous oxide-containing gas is 1 to 50000 ppm, more preferably 5 to 5000 ppm. When the nitrous oxide concentration is less than 1 ppm, efficient treatment is difficult, and when it exceeds 50,000 ppm, it is preferable to treat by a method other than the catalytic method.

When the nitrous oxide-containing gas contains other nitrogen oxides, the total of the NO concentration and the NO ₂ concentration is preferably less than 1000 ppm, more preferably less than 500 ppm. When sulfur oxide is contained, the concentration of sulfur oxide is preferably less than 500 ppm, and more preferably less than 100 ppm. In addition, the nitrous oxide-containing gas may contain nitrogen, oxygen, carbon dioxide, carbon monoxide, water, hydrogen, ammonia, and the like.

Moreover, the reaction temperature in the said processing method is 200-700 degreeC, Preferably it is 250-450 degreeC, More preferably, it is preferable that it is 300-400 degreeC. If the reaction temperature is less than 200 ° C, nitrogen oxides or sulfur oxides coexisting in the treatment gas may not be stably treated for a long time due to accumulation in the catalyst. If it exceeds 700 ° C, the exhaust gas is heated. In order to do so, a large amount of fuel is required, which causes a problem of economy. The space velocity (SV) is 1,000 to 50,000 hr ⁻¹ , preferably 2,000 to 20,000 hr ⁻¹ . Furthermore, the reaction pressure in the processing method of the present invention is 0.1 to 2 MPa, preferably 0.1 to 1 MPa.

  The treatment method of the nitrous oxide-containing gas includes various combustion exhaust gases such as gas turbines for power generation, boilers, waste incinerators, sewage sludge incinerators, and industrial exhaust gases emitted from chemical plants that produce adipic acid, nitric acid, etc. Can be applied to.

  The invention is further illustrated by the following examples, which illustrate advantageous embodiments of the invention.

(Example 1)
Commercially available basic cobalt carbonate (manufactured by Nacalai Tesque) in an air atmosphere was calcined at 400 ° C. for 2 hours to 20 g of cobalt trioxide, 1.21 g of bismuth nitrate (manufactured by Wako Pure Chemical Industries) and nitric acid ( An aqueous solution containing 2.54 g (manufactured by Koyo Pure Chemical Co., Ltd.) is added and mixed well as a paste, dried in a 120 ° C. dryer for 5 hours, and then fired at 500 ° C. for 2 hours in an air atmosphere. Thus, a catalyst having a Bi / Co atomic ratio of 0.01 / 1 was obtained.

(Example 2)
The same as Example 1 except that 2.43 g of bismuth nitrate and 5.10 g of nitric acid were added to 20 g of cobalt trioxide obtained by calcining the basic cobalt carbonate of Example 1 in an air atmosphere at 600 ° C. for 2 hours. Thus, a catalyst having a Bi / Co atomic ratio of 0.02 / 1 was obtained.

(Example 3)
A catalyst having a Bi / Co atomic ratio of 0.03 / 1, in the same manner as in Example 1, except that an aqueous solution containing 3.64 g of bismuth nitrate and 7.64 g of nitric acid was added to 20 g of cobalt trioxide of Example 1. Got.

Example 4
Example except that an aqueous solution containing 0.36 g of bismuth nitrate and 0.76 g of nitric acid was added to 20 g of cobalt trioxide obtained by baking the basic cobalt carbonate of Example 1 at 700 ° C. for 2 hours in an air atmosphere. In the same manner as in Example 1, a catalyst having a Bi / Co atomic ratio of 0.003 / 1 was obtained.

(Example 5)
In the same manner as in Example 1, except that an aqueous solution containing 7.04 g of bismuth nitrate and 14.78 g of nitric acid was added to 20 g of basic cobalt carbonate not previously calcined in Example 1, the Bi / Co atomic ratio was 0. 075/1 catalyst was obtained.

(Comparative Example 1)
In Example 1, a catalyst was obtained in the same manner as in Example 1 except that an aqueous solution containing 0.39 g of potassium nitrate was added instead of bismuth nitrate. In the catalyst, the atomic ratio of K / Co was 0.02 / 1.

(Comparative Example 2)
In Example 1, a catalyst was obtained in the same manner as in Example 1 except that bismuth nitrate and nitric acid were not added.

(Comparative Example 3)
In Example 1, Bi / Co was prepared in the same manner as in Example 1 except that 20 g of commercially available cobalt trioxide (manufactured by High-Purity Chemical Laboratory) was added with an aqueous solution containing 36.44 g of bismuth nitrate and 76.44 g of nitric acid. A catalyst having an atomic ratio of 0.3 / 1 was obtained.

(Measurement of specific surface area)
The specific surface areas of the catalysts of Examples 1 to 5 and Comparative Examples 1 to 3 were measured by the BET method using nitrogen gas.

(Catalytic activity test)
The catalysts of Examples 1 to 5 and Comparative Examples 1 to 3 were subjected to catalytic activity test according to the following procedure. After each catalyst powder was pressure-molded, it was crushed into granules and classified to obtain a catalyst in the range of 0.6 to 1.18 mm. 1 ml of the catalyst was filled into a SUS reaction tube having an inner diameter of 10 mm, and the following synthesis gas was introduced into the reaction tube at a space velocity of 10,000 hr ⁻¹ and a reaction temperature of 350 ° C., after 1 hour and after 20 hours. The nitrous oxide concentrations on the inlet side and outlet side of the reaction tube were measured with a gas chromatograph (manufactured by Shimadzu Corporation, GC8A, column: porapakQ), and the nitrous oxide decomposition rate was determined by the following formula 1 and shown in Table 1.

<Composition of synthesis gas>
_{N 2 O: 300ppm, CO 2} : 300ppm, NO: 50ppm, SO 2: 50ppm, O 2: 16 _vol%, H 2 O: 10 vol%, _{N 2:} balance

  According to the present invention, a nitrous oxide decomposition catalyst having high activity at a low temperature can be provided without using an expensive noble metal. Even if the nitrous oxide-containing gas contains nitrogen oxides or sulfur oxides, it can be treated stably and can be expected to be used in various industrial applications.

Claims (6)

A nitrous oxide decomposition catalyst comprising cobalt oxide and bismuth oxide as a catalyst component, wherein the atomic ratio of bismuth to cobalt is 0.0005 to 0.15. ^2. The nitrous oxide decomposition catalyst according to claim 1, wherein the catalyst has a specific surface area of 25 m ² / g or more. _3. The nitrous oxide decomposition catalyst according to claim 1 , wherein the cobalt oxide is tribasic cobalt tetraoxide (Co ₃ O ₄ ). And tricobalt tetroxide obtained by firing the cobalt carbonate (Co 3 _{O 4),} any one of claims 1 to 3, characterized by mixing dried and calcined to an aqueous solution containing bismuth nitrate A process for producing the described nitrous oxide decomposition catalyst. The method for producing a nitrous oxide decomposition catalyst according to claim 4, wherein the calcination temperature of the cobalt carbonate is 250C to 750C. A method for treating a nitrous oxide-containing gas, wherein the nitrous oxide-containing gas is treated using the nitrous oxide decomposition catalyst according to any one of claims 1 to 3.