JP5483723B2 - Nitrous oxide decomposition catalyst and purification method of gas containing nitrous oxide using the same - Google Patents

Description

  The present invention provides a nitrous oxide decomposition catalyst that exhibits high activity even at low temperatures and is less susceptible to the influence of carbon dioxide contained in a gas containing nitrous oxide, and a gas containing nitrous oxide using the same. It relates to a purification method.

Nitrous oxide (N ₂ O) contained in various combustion exhaust gases discharged from power generation gas turbines, boilers, garbage incinerators, and various industrial exhaust gases discharged from chemical plants, etc. is carbon dioxide (CO ₂ ). In addition to showing a higher greenhouse effect (nitrogen oxide has a warming potential of about 310 times that of carbon dioxide), it decomposes in the stratosphere to produce nitric oxide (NO), which is involved in ozone layer destruction. A method for efficiently decomposing and removing it is desired.

Examples of the method for decomposing and removing nitrous oxide include, for example, cobalt oxide mainly composed of cobalt tetroxide (Co ₃ O ₄ ) and alkali metal and / or alkaline earth metal, and alkali metal and / or alkaline earth. There has been proposed a method (Patent Document 1) using a catalyst having a compounding amount of a metal group of 0.01 to 15% by weight.

  The catalyst used in Patent Document 1 shows activity even in a low temperature range around 200 to 300 ° C. as compared with a conventional catalyst. However, Patent Document 1 discloses alkali metals and / or alkaline earths that can exhibit higher performance in this low temperature range and are less susceptible to the influence of carbon dioxide contained in a gas containing nitrous oxide. There is no disclosure of the optimum amount of metal.

JP 2007-54714 A

  When the present inventors conducted an experiment for decomposing nitrous oxide contained in various exhaust gases with respect to the catalyst of Patent Document 1, the catalytic activity is reduced when carbon dioxide is contained in the exhaust gas. I found out. Therefore, in the present invention, nitrous oxide decomposition that exhibits high activity even at a low temperature and that can efficiently decompose and remove nitrous oxide without being affected by carbon dioxide contained in the gas containing nitrous oxide. An object was to provide a catalyst and a method for purifying a gas containing nitrous oxide using the catalyst.

  As a result of intensive studies, the present inventors have found that the above problems can be solved by using a catalyst having the following composition, and have completed the present invention.

  That is, the nitrous oxide decomposition catalyst according to the present invention is a catalyst for decomposing nitrous oxide in a gas containing carbon dioxide and nitrous oxide, and contains cobalt oxide and cesium and / or rubidium. The molar ratio of cesium and rubidium to cobalt is 0.0005 to 0.05.

  The carbon dioxide contained in the gas is preferably 0.001 to 30% by volume. The molar ratio of cesium and rubidium to cobalt is preferably 0.0025 to 0.03. Furthermore, the cobalt oxide is preferably a cobalt oxide obtained by firing a precursor containing cobalt at 200 to 700 ° C.

  A method for purifying a gas containing nitrous oxide, comprising bringing a gas containing carbon dioxide and nitrous oxide into contact with the nitrous oxide decomposition catalyst to decompose nitrous oxide contained in the gas, is also provided. Included in the invention. This gas may be a gas after being treated with a denitration catalyst.

  The nitrous oxide decomposition catalyst of the present invention exhibits high activity even at low temperatures, and efficiently decomposes and removes nitrous oxide without being affected by carbon dioxide contained in the gas containing nitrous oxide. Can do. Therefore, by using the nitrous oxide decomposition catalyst of the present invention, it has become possible to efficiently purify nitrous oxide even for a gas containing nitrous oxide containing carbon dioxide.

Cs / Co is a graph showing the relationship between the molar ratio and the N ₂ O decomposition rate of the (%) (reaction temperature: 300 ° C.).

  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 may be made without departing from the scope of the present invention. .

  First, the nitrous oxide decomposition catalyst of the present invention will be described in detail.

  The nitrous oxide decomposition catalyst of the present invention is a catalyst for decomposing nitrous oxide in a gas containing carbon dioxide and nitrous oxide, and contains cobalt oxide and cesium and / or rubidium. The molar ratio of rubidium to cobalt is 0.0005 to 0.05.

Since the combination of cobalt oxide and Cs and / or Rb is superior in decomposition performance of N ₂ O compared to combinations of Na and K, Cs and / or Rb was selected in the present invention.

  The raw material that can be used to form the cobalt oxide (hereinafter may be referred to as “Co raw material”) is not particularly limited as long as it can become cobalt oxide in the catalyst preparation process. Cobalt salts such as cobalt, cobalt chloride, cobalt acetate, and cobalt carbonate, cobalt oxide, and cobalt hydroxide can be used, but cobalt carbonate is preferably used.

  The cesium (Cs) and / or rubidium (Rb) contained in the nitrous oxide decomposition catalyst of the present invention may be in the form of carbonate or hydroxide in addition to the oxide. Here, “and / or” means that two components may be contained simultaneously or only one of them may be contained.

  The raw material that can be used to form Cs and Rb in the above form (hereinafter sometimes referred to as “Cs and / or Rb raw material”) is one that can become Cs and Rb in the above form in the catalyst preparation process. For example, salts such as nitrates, carbonates, acetates, oxides and hydroxides can be used.

The molar ratio of Cs and Rb (total amount of Cs and Rb: the amount of the element when only one of them is included) to Co is 0.0005 to 0.05. 0.0025 or more is preferable, 0.005 or more is more preferable, 0.03 or less is preferable, and 0.02 or less is more preferable. By setting the molar ratio of Cs and Rb to Co in the above range, a nitrous oxide decomposition catalyst that exhibits high activity even at low temperatures and is less susceptible to the influence of CO ₂ contained in a gas containing nitrous oxide. Can be provided. On the other hand, when the molar ratio is less than 0.0005, the effect of adding Cs and Rb is small, and sufficient nitrous oxide decomposition performance cannot be obtained. The surface of the component cobalt oxide is covered with Cs and Rb, and the activity cannot be sufficiently exhibited. Here, the “molar ratio” means the amount of oxide, carbonate or hydroxide and cobalt oxide of Cs and Rb contained in the nitrous oxide decomposition catalyst, and the number of moles of Cs, Rb and Co, respectively. Then, the ratio of the number of moles is calculated.

The decomposition performance of the nitrous oxide decomposition catalyst of the present invention increases as the specific surface area increases. Therefore, the specific surface area is preferably 10 m ² / g or more, more preferably 30 m ² / g or more.

  The shape of the nitrous oxide decomposition catalyst of the present invention is not particularly limited, and a columnar shape, a cylindrical shape (pellet shape), a ring shape, a spherical shape, a plate shape, a honeycomb shape, and other integrally formed ones are appropriately selected. can do. The catalyst can be molded by a general molding method such as a tableting molding method or an extrusion molding method.

  The nitrous oxide decomposition catalyst of the present invention can also be used by being supported on a carrier having a predetermined shape appropriately selected from the above shapes. The carrier is not particularly limited. For example, in addition to silica, alumina, zirconia, titania and the like, a composite oxide containing two or more elements of Si, Al, Zr, Ti and the like is used. Can be used.

  Next, a method for preparing the nitrous oxide decomposition catalyst of the present invention will be described.

  The method for preparing the nitrous oxide decomposition catalyst of the present invention is not particularly limited, and a conventionally known method can be used. For example, it can be prepared by adding a predetermined Cs and / or Rb raw material to a Co raw material, drying, and firing. Also, the nitrous oxide decomposition catalyst of the present invention is prepared by mixing Cs and / or Rb components prepared in a separate process with cobalt oxide prepared through a process such as firing from a Co raw material other than cobalt oxide. You can also

  The addition method is not particularly limited, but when the Cs and / or Rb raw material is insoluble in a solvent (usually water), it can be added to the Co raw material or cobalt oxide by a physical mixing method (kneading). When the Cs and / or Rb raw material is water-soluble, it can be added by impregnating (mixing) an aqueous solution of the Cs and / or Rb raw material with the Co raw material or cobalt oxide.

  The drying conditions can be appropriately changed depending on the catalyst preparation conditions and the like, and are not particularly limited. However, from the viewpoint of sufficient drying while considering productivity, the drying conditions are 80 to 200 ° C. for 1 to 20 hours. It is preferable to dry. If the drying temperature is less than 80 ° C. or the drying time is less than 1 hour, drying may be insufficient and the performance of the prepared catalyst may deteriorate, and the drying temperature may be higher than 200 ° C. or the drying time may be longer than 20 hours. This is because even if the length is long, the drying efficiency does not increase, which is disadvantageous in terms of energy cost and production efficiency.

  Further, the firing conditions can be appropriately changed depending on the catalyst preparation conditions and the like, and are not particularly limited, but the Co raw materials other than the cobalt oxide (that is, the precursor containing cobalt) without causing a decrease in activity. From the viewpoint that can be sufficiently converted to cobalt oxide, it is preferable to calcine at 200 to 700 ° C. for 1 to 10 hours. If the calcination temperature is less than 200 ° C. or the calcination time is less than 1 hour, the conversion to cobalt oxide is not sufficient, and if the calcination temperature is higher than 700 ° C. or the calcination time is longer than 10 hours, the specific surface area decreases. This is because the catalyst performance may decrease.

  Hereinafter, the purification method of the gas containing nitrous oxide according to the present invention will be described.

  The method for purifying a gas containing nitrous oxide according to the present invention is characterized in that a gas containing carbon dioxide and nitrous oxide is brought into contact with the nitrous oxide decomposition catalyst to decompose nitrous oxide contained in the gas. To do.

In the purification method, N ₂ O is directly decomposed into N ₂ and O ₂ , and a gas containing nitrous oxide is used without using a reducing agent such as hydrocarbon, CO, H ₂ or NH _3. Can be processed.

Examples of the gas containing nitrous oxide include gases containing various nitrous oxides discharged from fixed combustion apparatuses such as fluidized bed boilers, sewage sludge incinerators, chemical plants that produce adipic acid, glyoxal, nitric acid, and the like. It is done. Moreover, the gas after processing with a denitration catalyst may be sufficient. These gases usually contain CO ₂ .

The concentration of N ₂ O in the gas containing nitrous oxide that can be used in the purification method is preferably 0.01 to 10% by volume, more preferably 0.02 to 0.5% by volume. This is because if the N ₂ O concentration is less than 0.01% by volume, a sufficient removal rate cannot be obtained, and if it exceeds 10% by volume, many catalysts are required to reduce N ₂ O, which is inefficient.

The amount of CO ₂ in the gas containing nitrous oxide is preferably 0.001 to 30% by volume, more preferably 0.005 to 10% by volume, and still more preferably 0.01 to 1% by volume.

The gas containing nitrous oxide may contain N ₂ , O ₂ , CO, H ₂ O, H ₂ , NH ₃ , NOx, SOx, etc. as components other than CO ₂ .

Conventionally, when a nitrous oxide decomposition catalyst that has been used contains CO _{2 in} a gas containing nitrous oxide, the decomposition performance thereof may be greatly reduced. On the other hand, if the nitrous oxide decomposition catalyst of the present invention is used, the mechanism is not yet elucidated, but as can be seen from the results of the examples described later, CO ₂ is contained in the gas containing nitrous oxide. Even if it is, it is possible to decompose and remove N ₂ O with high decomposition activity without being affected by it. Therefore, according to the catalyst of the present invention, with respect to the gas containing nitrous oxide contained in industrial exhaust gas is usually CO _2, it is not necessary to remove CO _2, it is possible to purify directly, production This leads to cost savings and can be expected to make a significant contribution to industry.

In the purification method, nitrous oxide space velocity of the gas containing (SV) is preferably 1000 hr ^-1 or more, more preferably 2000 hr ^-1 or more, preferably 50000Hr ^-1 or less, 20000Hr ^-1 or less is more preferable. If the SV exceeds 50000 hr ⁻¹ , a sufficient removal rate cannot be obtained. If the SV is less than 1000 hr ⁻¹ , the removal rate is not greatly improved, whereas the amount of catalyst increases and is inefficient.

  The reaction pressure of the gas containing nitrous oxide is preferably 0.1 to 4 MPa, more preferably 0.1 to 2 MPa.

Furthermore, the reaction temperature for decomposing N ₂ O is preferably 100 ° C. or higher, more preferably 150 ° C. or higher, and further preferably 250 ° C. or higher. Moreover, 700 degrees C or less is preferable, 500 degrees C or less is more preferable, and 400 degrees C or less is further more preferable. This is because if the reaction temperature is less than 100 ° C., a sufficient removal rate cannot be obtained, and if the reaction temperature exceeds 700 ° C., the catalyst may be thermally damaged and performance degradation may be significant.

  Next, the present invention will be described in detail with reference to examples and comparative examples. However, the present invention is not limited to these examples, and all modifications may be made without departing from the spirit described above and below. It is included in the technical scope of the present invention.

Example 1
An aqueous solution containing 0.47 g of cesium nitrate was added to 25 g of commercially available cobalt carbonate (manufactured by Nacalai Tesque), and the mixture was stirred and heated at 100 ° C. for 1 hour until moisture sufficiently evaporated with a hot stirrer. After drying at 120 ° C. for 2 hours, calcination was carried out at 400 ° C. for 4 hours to obtain a nitrous oxide decomposition catalyst (Cs / Co (molar ratio) = 0.01).

Examples 2-5
A nitrous oxide decomposition catalyst was obtained in the same manner as in Example 1 except that the amount of cesium nitrate was changed as shown in Table 1.

Example 6
A nitrous oxide decomposition catalyst was obtained in the same manner as in Example 1 except that 0.47 g of cesium nitrate was changed to 0.71 g of rubidium nitrate (Rb / Co (molar ratio) = 0.02).

Comparative Example 1
A catalyst was obtained in the same manner as in Example 1 except that potassium nitrate was used instead of cesium nitrate (K / Co (molar ratio) = 0.02).

Comparative Example 2
A catalyst was obtained in the same manner as in Example 1 except that an aqueous solution containing cesium nitrate was not added (Co ₃ O ₄ only).

  The compositions of the catalysts obtained in Examples 1 to 6 and Comparative Examples 1 and 2 are summarized in Table 1.

  The nitrous oxide decomposition performance of the catalysts obtained in Examples 1 to 6 and Comparative Examples 1 and 2 was evaluated by the following method.

1 ml of a catalyst formed into granules and classified to 0.6 to 1.18 mm (specific surface area of each catalyst is shown in Table 1) was charged into a SUS reaction tube having an inner diameter of 10 mm. The catalyst was pretreated at 500 ° C. for 30 minutes under N ₂ atmosphere.

As reaction gases, N ₂ O: 300 ppm (0.03% by volume), O ₂ : 16% by volume, CO ₂ : about 300 ppm (about 0.03% by volume), H ₂ O: 10% by volume, balance gas: N ₂ was used. The SV of the reaction gas was 10,000 hr ⁻¹ .

  An evaluation test was conducted by flowing a reaction gas through the reaction tube filled with the catalyst at a predetermined reaction temperature under normal pressure. The results are shown in Table 2.

The concentration of N ₂ O in the gas at the inlet side and the outlet side of the catalyst layer was measured with a gas chromatograph (manufactured by Shimadzu Corporation, GC-8A, column: porapakQ), and the decomposition rate of N ₂ O was calculated by the following equation. .

Decomposition rate of _{N 2 O (%) = 100} × (N 2 O concentration in the inlet side - N ₂ O concentration at the outlet side) / (N ₂ O concentration in the inlet side)

FIG. 1 shows the relationship between Cs / Co (molar ratio) and N ₂ O decomposition rate (%) (reaction temperature: 300 ° C.).

<Influence of CO ₂ >
The influence of the presence or absence of CO ₂ in the reaction gas on the catalyst decomposition performance was evaluated by the following methods using the catalysts obtained in Example 1 and Comparative Example 1.

As an evaluation method, the test was performed in the same manner as described above except that a reaction gas not containing CO ₂ was used. The results are shown in Table 3 together with the results in Table 2.

As can be seen from the results in Table 2, in the purification method using the catalysts obtained in Examples 1 to 6, N ₂ O can be decomposed and removed at a high decomposition rate even at a low reaction temperature such as 350 ° C. or 300 ° C. did it. On the other hand, in the purification method using the catalyst obtained in Comparative Examples 1 and 2, only a low decomposition rate was obtained.

Further, as can be seen from the results in Table 3, the purification method using the catalyst of the present invention was able to efficiently decompose and remove N ₂ O even at low temperatures without being adversely affected by CO ₂ . On the other hand, in the purification method using a catalyst using potassium, the decomposition performance of the catalyst has been greatly reduced due to the presence of CO _{2 in the} reaction gas.

According to the present invention, it is possible to provide a nitrous oxide decomposition catalyst that exhibits high activity at low temperatures and is not easily affected by CO ₂ contained in a gas containing nitrous oxide. By using this catalyst, even for a gas containing nitrous oxide contained in industrial exhaust gas is usually CO _2, it is possible to efficiently purify N ₂ O, expected to contribute greatly to the industry it can.

Claims (6)

  A catalyst for decomposing nitrous oxide in a gas containing carbon dioxide and nitrous oxide, containing cobalt oxide and cesium and / or rubidium, and having a molar ratio of cesium and rubidium to cobalt of 0. A nitrous oxide decomposition catalyst characterized by being 0005 to 0.05.   The nitrous oxide decomposition catalyst according to claim 1, wherein carbon dioxide contained in the gas is 0.001 to 30% by volume.   The nitrous oxide decomposition catalyst according to claim 1 or 2, wherein the molar ratio of cesium and rubidium to cobalt is 0.0025 to 0.03.   The nitrous oxide decomposition catalyst according to any one of claims 1 to 3, wherein the cobalt oxide is a cobalt oxide obtained by firing a precursor containing cobalt at 200 to 700 ° C.   A gas containing carbon dioxide and nitrous oxide is brought into contact with the nitrous oxide decomposition catalyst according to any one of claims 1 to 4 to decompose nitrous oxide contained in the gas. A gas purification method comprising:   The method for purifying a gas containing nitrous oxide according to claim 5, wherein the gas is a gas after being treated with a denitration catalyst.

Images