Method for producing reduction catalyst for nitrogen oxides

Abstract

To provide a reduction catalyst having high initial activity.SOLUTION: A method for producing a reduction catalyst for nitrogen oxides of the present invention includes the steps of: preparing a mixture containing a titanium-containing oxide having a sulfur content of 2.5 wt.% or more in terms of SO4, an active metal component, an alkaline earth metal ion, an acetate ion, and water; molding the mixture to produce a molded article; drying the molded article to produce a dried article; and firing the dried article at 400°C.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a reduction catalyst for nitrogen oxides.

Background Art

[0002] Exhaust gases generated from thermal power plants, waste incinerators, etc. contain nitrogen oxides that cause photochemical smog. Reduction catalysts for nitrogen oxides are installed in these facilities, and the nitrogen oxides are decomposed into harmless nitrogen and water and released into the atmosphere. As a method for manufacturing such a reduction catalyst, a method of mixing and molding a titanium-containing powder and an active metal and then firing is known (for example, Patent Document 1). This titanium-containing powder is manufactured by a thermal hydrolysis method using a sulfate solution of titanium as a raw material, and has a high specific surface area (120 m 2 / g or more). Usually, the higher the specific surface area of the catalyst, the higher the activity.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In Patent Document 1, a reduction catalyst is manufactured using a raw material of a titanium-containing oxide having a large specific surface area. This titanium-containing oxide contains a large amount of sulfuric acid (2.5% by weight or more in terms of SO4) that acts as a catalyst poison. It is presumed that sulfuric acid is thermally decomposed in the firing step during catalyst production and released as SO2 outside the catalyst, but since it is fired at a high temperature (400 °C or higher) for a long time, the specific surface area becomes small and the initial activity becomes low.

[0005] Therefore, an object of the present invention is to provide a reduction catalyst having high initial activity.

Means for Solving the Problems

[0006] It has been found that by producing a reduction catalyst by mixing acetate ions and alkaline earth metal ions with a titanium-containing oxide having a large specific surface area and sulfur content, the initial activity is increased.

Mode for Carrying Out the Invention

[0007] In the present invention, a reduction catalyst for nitrogen oxides is produced as follows. First, a mixture containing a titanium-containing oxide, an active metal component, alkaline earth metal ions, acetate ions, and water is prepared <mixing step>. Here, the sulfur content of the titanium-containing oxide is 2.5% by weight or more in terms of SO4. Next, a molded article is obtained by molding the mixture <molding step>. A dried product is obtained by drying the molded article <drying step>. The dried product is fired at 400 °C or higher <firing step>.

[0008] Since a mixture containing alkaline earth metal ions and acetate ions is used, the initial activity of the reduction catalyst is increased. It is presumed that sulfur is retained in the titanium-containing oxide by the alkaline earth metal ions, and the acetate ions assist this. Since sulfur remains in the titanium-containing oxide, it is presumed that the amount of sulfur on the surface of the reduction catalyst decreases and the initial activity of the reduction catalyst increases. Since the alkaline earth metal is in an ionic state, it is presumed that it easily traps sulfur in the titanium-containing oxide and easily retains sulfur in the titanium-containing oxide.

[0009] When acetate ions are not contained in the mixture (for example, when a water-soluble lactate of an alkaline earth metal is used and acetate ions are not used), sulfur is released from the titanium-containing oxide in the firing step. Also, in this case, the specific surface area of the reduction catalyst becomes low in the firing step. That is, by adding acetate ions, it becomes difficult for the specific surface area of the reduction catalyst to decrease in the firing step. For the above reasons, both alkaline earth metal ions and acetate ions need to be present in the mixture. Examples of the alkaline earth metal include Be, Mg, Ca, Sr, Ba, and Ra. A plurality of types of alkaline earth metal ions may be used.

[0010] When preparing the mixture, it is only necessary that the alkaline earth metal and acetic acid finally ionize in the mixture. When the alkaline earth metal and acetic acid are in an ionic state, they are easily dispersed in the titanium-containing oxide. Therefore, it is easy for these ions to keep sulfur homogeneously in the titanium-containing oxide. For example, when preparing the mixture, forms such as using an acetate of an alkaline earth metal as a raw material for alkaline earth metal ions and acetic acid ions, using a salt containing alkaline earth metal ions and a salt containing acetic acid ions, using a salt containing alkaline earth metal ions and acetic acid, etc. can be mentioned. When using a salt containing alkaline earth metal ions and a salt containing acetic acid ions, since these salts need to be ionized by water, these salts need to be water-soluble. Here, when 1 g of the salt is mixed with 100 g of water at 25 °C and this is stirred for one hour, if the undissolved residue is 0.01 g or less, it can be said that the salt is water-soluble. If it is a water-soluble salt, it ionizes (i.e., becomes ionized) in the mixture. When it is not desired to include an extra (other than alkaline earth metals) salt in the mixture, it is preferable to use an acetate of an alkaline earth metal as a raw material for alkaline earth metal ions and acetic acid ions. Also, acetates are more easily dissolved and ionized in water than acetic acid and are more easily dispersed in the mixture.

[0011] The ratio (M / TiO2) of the molar amount (M) of alkaline earth metal ions in the mixture to the molar amount (TiO2) of the titanium-containing oxide is preferably 0.002 or more. The activity of the reduction catalyst tends to be high. On the other hand, the lower this ratio is, the higher the ratio of the active metal component and the titanium-containing oxide in the reduction catalyst can be. Therefore, this ratio is preferably 0.06 or less, more preferably 0.04 or less, and even more preferably 0.03 or less.

[0012] Also, with respect to 100 parts by mass of the titanium-containing oxide, the content of alkaline earth metal ions (M) in the mixture is preferably 0.5 part by mass or more in terms of MO conversion (oxide conversion, in the case of Mg, in terms of MgO conversion). The activity of the reduction catalyst tends to be high. On the other hand, the lower this content is, the higher the ratio of the active metal and the titanium-containing oxide can be. Therefore, this content is preferably 5 parts by mass or less in terms of MO conversion, and more preferably 3 parts by mass or less.

[0013] The molar amount of acetate ions (AcO - ) and the ratio (AcO - / M) of the molar amount of alkaline earth metal ions (M) are preferably 0.5 or more. The activity of the reduction catalyst tends to be high. On the other hand, the lower this ratio, the lower the cost. Therefore, this ratio is preferably 2 or less.

[0014] The larger the amount of titanium-containing oxide in the mixture, the easier it is to mold the mixture. Also, the performance of the reduction catalyst tends to be high. Therefore, the titanium oxide content in the solid content of the mixture is preferably 60% by weight or more. On the other hand, in order to add other components, the titanium oxide content in the solid content is preferably 90% by weight or less. Also, the higher the specific surface area of the titanium-containing oxide, the higher the specific surface area of the reduction catalyst. Therefore, the specific surface area of the titanium-containing oxide is preferably 120 m 2 / g or more.

[0015] Examples of the active metal component include elements such as vanadium (V), chromium (Cr), nickel (Ni), copper (Cu), silver (Ag), gold (Au), palladium (Pd), yttrium (Y), cerium (Ce), neodymium (Nd), indium (In), and iridium (Ir). The active metal component becomes an oxide in the firing process and serves as the active metal of the reduction catalyst. By mixing the active metal component in an aqueous solution state with other raw materials, a reduction catalyst in which the active metal component is well dispersed can be obtained. Therefore, the activity of the reduction catalyst is increased. As the vanadium compound, vanadate, vanadium sulfate, or vanadium chloride is easily soluble in water, and it is particularly preferable to use ammonium metavanadate. It is preferable to dissolve the V oxide in an acid. By adding monoethanolamine to an aqueous solution of the V compound, V is more easily dispersed in the catalyst.

[0016] With respect to 100 parts by mass of the titanium-containing oxide, the content of the active metal component in the mixture is preferably 3 parts by mass or more (in the case of V, in terms of V2O5). With respect to 100 parts by mass of the titanium-containing oxide, when this content is up to 15 parts by mass, the higher the content of the active metal component in the reduction catalyst, the higher the activity of the reduction catalyst. On the other hand, the lower this content, the lower the cost. Therefore, this content is preferably 9 parts by mass or less with respect to 100 parts by mass of the titanium-containing oxide.

[0017] It is preferable to add molybdenum (Mo) to the mixture. Mo can suppress the SO2 poisoning of the reduction catalyst. The higher the Mo content in the reduction catalyst, the more SO2 poisoning can be suppressed. Therefore, with respect to 100 parts by mass of the titanium-containing oxide, the Mo content in the mixture is preferably 1 part by mass or more in terms of MoO3. On the other hand, the lower the Mo content in the reduction catalyst, the lower the cost. Therefore, with respect to 100 parts by mass of the titanium-containing oxide, the Mo content in the mixture is preferably 5 parts by mass or less in terms of MoO3.

[0018] It is preferable to add a base to the mixture. The base precipitates alkaline earth metal ions on the carrier. Examples of the alkali include sodium carbonate, soda ash, and aqueous ammonia.

[0019] It is preferable to knead the mixture. Kneading not only makes it easier to mix the mixture but also has the function of adjusting the pores of the reduction catalyst.

[0020] A reinforcing material may be added to the mixture. Examples of the reinforcing material include glass fiber, silica, and alumina. These components enhance the moldability of the molded product and the strength of the reduction catalyst. The higher the content of the reinforcing material in the solid content, the higher the strength of the reduction catalyst. Therefore, this content is preferably 5% by weight or more. On the other hand, the lower this content, the more titanium oxide, active metal component, etc. can be contained in the reduction catalyst. Therefore, this content is preferably 9% by weight or less.

[0021] The mixture is formed to obtain a formed product. For example, the mixture can be formed into a structure such as pellets or honeycombs by extrusion molding. The honeycomb structure has a large number of small holes (cells) penetrating parallel to each other. Examples of the shape of the honeycomb structure include hexagonal prisms, rectangular parallelepipeds, cylinders, etc. Examples of the shape of the cells include hexagons, quadrilaterals, circles, etc. Usually, the size (diameter) of the cells is the aperture, the space between cells is the wall, and when focusing on one cell, the distance between the centers of the left and right or upper and lower walls facing each other is called the pitch. When exhaust gas passes through the honeycomb structure, turbulence is likely to occur, so the diffusion efficiency of the exhaust gas is improved in the honeycomb structure. As a result, the denitration efficiency is increased. To make it easier to form, a forming aid may be added to the mixture. Examples of the forming aid include polyethylene oxide, crystalline cellulose, glycerin, polyvinyl alcohol, etc. Also, from the viewpoint of ease of molding, the moisture concentration in the mixture is preferably 20 to 40% by weight.

[0022] The formed product is dried to obtain a dried product. As a criterion for drying, for example, drying at 30 to 200 °C for 1 hour or more can be mentioned. Also, as a criterion for drying, drying until the weight after drying decreases by 15% or more from the weight of the formed product can also be mentioned. In this criterion, for example, when the weight of the formed product is 100%, if the weight of the dried product is 85% or less, it is considered to be dried. After drying, the dried product is calcined at 400 °C or higher to obtain a reduction catalyst. It is considered that by calcination, sulfuric acid is decomposed and released outside the reduction catalyst as SO2. It is speculated that the higher the calcination temperature or the longer the calcination time, the easier the sulfuric acid in the reduction catalyst is decomposed. The calcination time is preferably 1 hour or more. On the other hand, the lower the calcination temperature or the shorter the calcination time, the higher the specific surface area of the reduction catalyst. Therefore, the calcination temperature is preferably 700 °C or lower. Since the shorter the drying time and the calcination time, the lower the cost, the drying time is preferably 100 hours or less, and the calcination time is preferably 100 hours or less.

[0023] The reduction catalyst obtained by the above method can be used as a reduction catalyst for nitrogen oxides (NOx). NO x As a method for decomposing, NO xExamples of methods include adding and bringing into contact with a reducing agent such as ammonia to a gas containing [the substance] to cause a reduction reaction. In the production method of the present invention, alkaline earth metal ions and acetate ions are contained in the solid content. Therefore, even at a low temperature (for example, 250 °C or lower or 200 °C or lower), the initial performance of the reduction catalyst tends to be high.

[0024] Hereinafter, the production method of the reduction catalyst and the performance of the reduction catalyst will be described in detail.

[0025] [Example 1] [Preparation of titanium oxide powder] The titanium sulfate solution obtained by the sulfuric acid method was hydrolyzed to obtain a metatitanic acid slurry. 25.0 kg of the metatitanic acid slurry was charged into a stirring tank equipped with a reflux condenser, and 30.5 kg of 15 wt% aqueous ammonia was added thereto to adjust the pH to 9.5. This was aged with stirring at 95 °C for 1 hour. This was cooled, filtered, dehydrated, and washed to obtain a cake containing 4.1 wt% sulfur in terms of SO4 in the solid content and 0.03 wt% sodium in terms of Na2O in the solid content. After drying the cake at 110 °C for 20 hours, this was calcined at 450 °C for 5 hours to obtain a titanium-containing oxide (80 wt% in terms of TiO2). The specific surface area of this titanium-containing oxide was 150 m 2 / g. The specific surface area was measured as follows. The titanium-containing oxide was pretreated at 300°C for 1 hour in an inert gas atmosphere to obtain a measurement sample. This measurement sample was put into a sample cell for measurement, and a mixed gas with a nitrogen gas concentration of 30 vol% and a helium gas concentration of 70 vol% was allowed to flow sufficiently in a measurement apparatus (MR-6 manufactured by Nippon Bell Co., Ltd.) at an atmosphere temperature of -196°C to adsorb nitrogen to the sample powder. Then, by raising the atmosphere temperature to 25°C, the nitrogen adsorbed to the sample powder was desorbed, and the desorption amount was detected by a thermal conductivity detector (TCD). The specific surface area per 1 g of the measurement sample was determined by converting the detected nitrogen desorption amount into the specific surface area using the cross-sectional area of nitrogen molecules. Also, the amount of SO4 in the titanium-containing oxide was measured as follows. Approximately 0.1 g of the sample was taken into a crucible, a combustion aid (Fe·tin (Sn)·W) was added, and it was set in an apparatus (CS844 manufactured by LECO Corporation). It was heated with a high-frequency current to generate SO2, which was detected and quantified. This value was converted to SO4. The specific surface area and the amount of SO4 in the following examples and comparative examples are also shown in Table 1.

[0026] <Preparation of Aqueous Solution of Active Metal Component> 6000 g of water, 300 g of monoethanolamine, and 1538 g of ammonium metavanadate (manufactured by Fujifilm Wako Pure Chemical Corporation: 78% by weight in terms of V2O5) were mixed, and then the temperature was raised to 80 - 90°C and stirred for 2 hours. This was cooled to 40 - 60°C, and after further adding 736 g of ammonium molybdate (manufactured by Hayashi Pure Chemical Industries, Ltd.: 81.5% by weight in terms of MoO3), it was stirred for 5 minutes. Then, 840 g of citric acid was further added to obtain an aqueous solution of the active metal component.

[0027] <Mixing Step> 630 g of calcium acetate monohydrate (32% by mass in terms of CaO) as alkaline earth metal ions and acetate ions, 19,500 g of titanium-containing oxide, and an aqueous solution of an active metal component were mixed using a kneader for 10 minutes. Then, 5000 g of aqueous ammonia (concentration: 15% by mass) was added as a base, and the mixture was further mixed using a kneader for 10 minutes. Then, 1470 g of glass fiber was added as a reinforcing material and mixed for 10 minutes. Then, 150 g of polyethylene oxide and 150 g of carboxymethyl cellulose were added as molding aids. Then, water was added while mixing until the moisture concentration of the mixture reached 30% by weight. Thereby, a mixture containing a titanium-containing oxide, an active metal component, alkaline earth metal ions, acetate ions, and water was prepared.

[0028] <Forming process, drying process, firing process> The mixture was extruded into a honeycomb shape (length: 80 mm, width: 80 mm, height: 450 mm, cells are square in the height direction (parallel to the long side) and 35 mesh × 35 mesh) to obtain a molded product. The molded product was dried at 60°C for one hour or more to obtain a dried product. The dried product was fired at 500°C for one hour or more in an air atmosphere to obtain a reduction catalyst.

[0029] <Initial activity (reduction catalyst activity)> The initial activity was measured as follows. The reduction catalyst was cut into a rectangular parallelepiped (length: 21 mm, width: 21 mm, height: 250 mm, cells are square in the height direction (parallel to the long side) and 5 mesh × 5 mesh) to obtain an evaluation sample. This was filled into a flow-type reactor. A gas (gas composition: NO x = 116 ppm, NH3 = 116 ppm, SO x = 0 ppm, O2 = 6.1%, H2O = 21.45%, N2 = balance) was passed through the reactor under the condition of SV: 14200 h -1 and brought into contact with the evaluation sample at 200°C. The value (denitration rate η) calculated using the following formula * from the NOx concentration in the gas before and after the reduction catalyst was contacted with NOx was defined as the denitration rate η O .

[0030] Denitration rate η (%) = [NO in the gas before contact x (ppm) - NO in the gas after contactx (ppm)] / NO in the pre-contact gas x (ppm) × 100 ······ *

[0031] Furthermore, substitute the denitration rate η into the following formula *2 to obtain the reaction rate constant K of the catalyst (initial reaction rate constant K O ). The higher this value, the higher the (initial) activity of the catalyst. The initial reaction rate constants K of the following Examples and Comparative Examples o are also listed in Table 1 together. o K = -V s × ln(1 - (〔η / 100〕 / α)) ······ *2 Here, V s is the area velocity (m 3 / (m 2 × h)), and α is the ratio of the NH3 concentration C NH3 to the NO x concentration C NOX in the pre-contact gas (C NH3 / C NOX ). Note that the concentration of NO in the gas x was measured using a chemiluminescence nitrogen oxide analyzer (manufactured by Anatech Yanaco Co., Ltd., ECL - 88AO).

[0032] Example 2 This example is the same as Example 1 except for the following differences. That is, 1073 g of magnesium acetate was used instead of calcium acetate monohydrate in the mixing step.

[0033] Example 3 This example is the same as Example 1 except for the following differences. That is, 357 g of barium acetate was used instead of calcium acetate monohydrate in the mixing step.

[0034] Example 4 This example is the same as Example 1 except for the following differences. That is, the addition amount of calcium acetate monohydrate in the mixing step was doubled, and an additional 1073 g of magnesium acetate was added.

[0035] Example 5​ This example is the same as Example 1 except for the following differences. That is, glass fiber was not added in the mixing step.

[0036] 〔Comparative Example 1〕 This comparative example is different from Example 1 in the following points. That is, calcium acetate monohydrate and glass fiber were not added in the mixing step.

[0037] 〔Comparative Example 2〕 This comparative example is different from Example 1 in the following points. That is, glass fiber was not added in the mixing step. In the mixing step, a titanium-containing oxide prepared by the following method was used instead of the titanium-containing oxide obtained in Example 1. A titanium sulfate solution obtained by the sulfuric acid method was hydrolyzed to obtain a metatitanic acid slurry. 25.0 kg of the metatitanic acid slurry was charged into a stirred tank equipped with a reflux condenser, and 30.5 kg of 15 wt% aqueous ammonia was added thereto to adjust the pH to 9.5. This was aged with stirring at 95 °C for 1 hour. This was cooled, filtered, dehydrated, and washed to obtain a cake containing 4.1 wt% sulfur in the solid content in terms of SO4 and 0.03 wt% sodium in the solid content in terms of Na2O. After drying the cake at 110 °C for 20 hours, this was calcined at 550 °C for 5 hours to obtain a titanium-containing oxide (80 wt% in terms of TiO2). The specific surface area of this titanium-containing oxide was 90 m 2 / g.

[0038] 〔Comparative Example 3〕 This comparative example is different from Example 1 in the following points. That is, 1120 g (17.8 wt% in terms of CaO) of calcium lactate was used instead of calcium acetate monohydrate.

[0039]

Table 1

Claims

1. sulfur to SO ４ preparing a mixture containing a titanium-containing oxide containing 2.5% by weight or more of sulfur in terms of SO, an active metal component, ions of an alkaline earth metal, acetate ions, and water A step of obtaining a molded article by molding the mixture, A step of obtaining a dried article by drying the molded article, and A method for producing a catalyst for reducing nitrogen oxides, comprising a step of firing the dried article at 400 °C or higher.

2. A method for reducing nitrogen oxides at 200 °C or lower using the catalyst obtained by the production method of Claim 1.

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