JP7430561B2 - Nitric acid nitrogen decomposition catalyst - Google Patents

Description

The present invention relates to a catalyst for removing nitrate nitrogen contained in a liquid, and particularly to a catalyst in which metal particles containing a Cu--Pd alloy are supported on activated carbon.

Nitrate nitrogen causes eutrophication of lakes and marshes and harms human health, so it must be removed from industrial wastewater. As a method for removing nitrate nitrogen from wastewater and the like, a chemical treatment method in which reductive decomposition is performed in the presence of a reducing agent and a catalyst is known. For example, a catalyst in which fine metal particles containing copper and palladium are supported on an inorganic carrier or a carbon carrier is used (see, for example, Patent Document 1). Patent Document 1 discloses the use of a small carrier and the use of carbon with a large specific surface area as the carrier so that the catalyst does not settle in the treated water. Furthermore, to enable repeated use, metal fine particles containing copper and palladium (average primary particle size 1-9 nm) can be supported on an inorganic carrier (particle size 5-200 nm, specific surface area 10-300 m ² /g). known (for example, see Patent Document 2).

Furthermore, in order to realize a catalyst with high activity and long life, it is known that metal particles having planar regions are supported on activated carbon (see, for example, Patent Document 3). In Patent Document 3, Pd--Cu particles are exemplified as metal particles, and phenol resin-based activated carbon is exemplified as a carbon-containing carrier.

Japanese Patent Application Publication No. 2004-097983 Japanese Patent Application Publication No. 2015-192925 Japanese Patent Application Publication No. 2010-089032

In this way, nitrate nitrogen has conventionally been decomposed using a catalyst carrying metal particles containing Pd, Cu, etc., and a reducing agent such as hydrazine. The catalyst used in this case is required not only to have high activity but also to be able to maintain high activity even after repeated use (ie, long life). However, due to the influence of the reducing agent, if metal ions or oxides are present, there is a risk that these will become metallized, and furthermore, the metals will grow into particles. Particle growth reduces specific surface area and shortens life. In addition, due to particle growth, side reactions progress, making it easier to generate ammonia.

Therefore, an object of the present invention is to realize a nitric acid nitrogen decomposition catalyst that is not easily affected by reducing agents and has a long life.

The nitric acid nitrogen decomposition catalyst of the present invention includes activated carbon having an average particle diameter of 40 to 200 μm, a pore volume of 0.4 to 1.0 mL/g, and a specific surface area of 800 to 1500 m ² /g, and a catalyst supported on this activated carbon. In addition, it contains metal particles containing a Cu-Pd alloy with an average particle diameter of 1 to 30 nm, and the amount of metal particles supported is 0.1 to 3% by mass. Furthermore, the packing density of activated carbon is in the range of 0.4 to 0.8 g/mL.

Furthermore, the sum of the Cu component and the Pd component contained in the metal particles is 95% by mass or more, and the ratio of the Pd component to the sum of the Cu component and the Pd component is 60 to 98% by mass.

Furthermore, the circularity of the metal particles was set to 0.6 to 1.0, and the circularity of the activated carbon was set to 0.8 to 1.0.

<Nitric acid nitrogen decomposition catalyst>
The nitric acid nitrogen decomposition catalyst of the present invention has metal particles having an average particle diameter of 1 to 30 nm supported on activated carbon. The metal particles include a Cu--Pd alloy. Nano-sized metal particles containing such a Cu--Pd alloy are difficult to oxidize or reduce, so even if a reducing agent is present during the decomposition of nitrate nitrogen, particle growth is difficult to occur. Furthermore, the activated carbon has an average particle diameter of 40 to 200 μm, a pore volume of 0.4 to 1.0 ml/g, and a specific surface area of 800 to 1500 m ² /g. In such a catalyst, the metal particles can be supported in a monodisperse manner inside the activated carbon, so the chances of contact between the metal particles are reduced, and drop-off and particle growth of the metal particles can be prevented. Further, it is preferable that the activated carbon is spherical and has 50 to 100 nm pores formed on its surface. When the pore diameter is within this range, metal particles are more likely to exist in the pores. In particular, it is preferable that the metal particles be supported in a monodisperse manner.

Furthermore, the life of the device is shortened due to agglomeration of fallen metal particles. The larger the activated carbon (the smaller the outer surface area), the easier the metal particles will fall off, so the particle diameter of the activated carbon is preferably 50 to 150 μm.

Furthermore, the amount of metal particles supported on this catalyst is preferably 0.1 to 3% by mass. If it exceeds 3%, the metal particles may aggregate and be supported due to the large amount of metal particles. Therefore, metal particles grow together during the nitric acid decomposition reaction, and metal particles fall off due to collisions between catalysts. This shortens the catalyst life. If it is less than 0.1% by mass, there are few metal particles, so it can be supported monodispersely, but because there are few metal particles, the catalytic activity is low, and the amount of catalyst used is large. It is sometimes difficult to separate the catalyst and use it repeatedly.

Nitrate nitrogen exists in water as NO ₃ ions and NO ₂ ions, and becomes N ₂ through a reduction reaction, but when overreduced, NH ₃ (ammonia) is produced. Fine metal particles are the active metal in this reaction.

Hereinafter, the metal particles and activated carbon that constitute the nitric acid nitrogen decomposition catalyst will be explained in detail.

As mentioned above, since the metal particles contain a Cu--Pd alloy, they are difficult to oxidize or reduce, and particle growth is difficult to occur during the nitric acid decomposition reaction. Further, the decomposition reaction of NO ₃ becomes faster, and the production of ammonia in the overreduction reaction may be suppressed. If this alloy is included, Cu and Pd may be included in the simple substance, oxide, or ion state. The sum of the Cu component and the Pd component contained in the metal particles is 95% by mass or more. This makes it possible to obtain a highly nitric acid nitrogen decomposition catalyst. If it is less than 95%, the proportion of the active metal component is small, so there is a risk that the catalyst performance will be lowered. The ratio of the Pd component to the sum of the Cu component and the Pd component is preferably 60 to 98%.

Further, other simple metals or oxides may be present in the metal particles, and examples of the other metals include fourth period transition metal elements, fifth period transition metal elements, Pt, and Au. Among them, Ti, V, Cr, Mn, Fe, Co, and Ni are particularly preferable as cocatalysts, which may increase the activity or extend the catalyst life. However, the proportion is preferably less than 5% by mass.

The average particle diameter of the metal particles is preferably 1 to 30 nm, more preferably 1 to 25 nm. When the particle size is 30 nm or more, the particle surface area is small and nitric acid decomposition is difficult to occur, and a catalyst with low activity may be obtained. Particles smaller than 1 nm are difficult and impractical to obtain.

The circularity of the metal particles is preferably 0.6 to 1.0. Within this range, most of the metal particles have a spherical or polyhedral shape, and it is possible to obtain particles that can be supported monodispersely and have high catalytic activity. In particular, ammonia production can be reduced by using polyhedral crystals.

Preferably, the metal particles are surface-treated. This allows the metal particles to maintain high dispersibility. The amount of the surface treatment agent is preferably 0.1 to 30%, more preferably 1 to 10%, based on the Pd--Cu element as carbon content. When it exceeds 30%, the dispersibility is good, but because the amount of the surface treatment agent is large, the dispersibility is high but the catalytic performance may be low. If it is less than 0.1%, the metal particles may aggregate when supported, and the catalyst may not be supported in a highly dispersed manner, resulting in a short-life catalyst.

Next, activated carbon will be explained. Activated carbon is used as a carrier for supporting metal particles containing a Pd-Cu alloy and having an average particle size of 1 to 30 nm. If this carrier is not present, the metal particles will aggregate or grow due to the heat generated by the nitric acid decomposition reaction, resulting in a shortened catalyst life. Activated carbon is a porous substance made of carbon, oxygen, hydrogen, calcium, etc., and is also a porous substance that has been subjected to a physical treatment such as chemical or heat. As raw materials for activated carbon, natural materials such as coconut shells, bamboo, wood, and bakasu, and petroleum-derived materials such as phenolic resin can be used. Among them, phenol resin is particularly preferable because it has a high raw material purity, so the activated carbon has a high purity, and catalyst poisoning due to impurities is unlikely to occur during the nitric acid decomposition reaction.

The average particle diameter is 40 to 200 μm, preferably 50 to 150 μm. When the average particle diameter is within this range, metal particles can be supported in a monodisperse manner, and a highly active catalyst can be obtained. If it is less than 40 um, the surface area increases, so metal particles can be supported in a monodisperse manner, but it may be difficult to separate the waste liquid from the used catalyst after the nitric acid decomposition reaction, making it difficult to repeatedly use the catalyst. When the diameter exceeds 200 μm, it is easy to separate the waste liquid and the used catalyst after the nitric acid decomposition reaction, but since the surface area is small, metal particles may aggregate and be supported. Therefore, during the nitric acid decomposition reaction, metal particles grow together, and collisions between catalysts cause metal particles to fall off, resulting in a shortened catalyst life.

The pore volume is preferably 0.4 to 1.0 mL/g. Within this range, the metal particles can be supported in a monodisperse manner, and the metal particles are less likely to grow together during the nitric acid decomposition reaction, resulting in a longer catalyst life. If the pore volume is less than 0.4 mL/g, metal particles may aggregate and be supported. Therefore, during the nitric acid decomposition reaction, metal particles grow together, and collisions between catalysts cause metal particles to fall off, resulting in a shortened catalyst life. If it exceeds 1.0 mL/g, it is possible to monodispersely support metal particles and a catalyst with a long catalyst life can be obtained, but since the activated carbon is bulky, it is difficult to separate the waste liquid and the used catalyst after the nitric acid decomposition reaction. , may not be able to be used repeatedly.

The specific surface area is preferably 800 to 1500 m ² /g. Within this range, the metal particles can be supported in a monodisperse manner, and the metal particles are less likely to grow together during the nitric acid decomposition reaction, resulting in a longer catalyst life. If the specific surface area is less than 800, the metal particles may aggregate and be supported. Therefore, during the nitric acid decomposition reaction, metal particles may grow together, or metal particles may fall off due to collisions between catalysts, resulting in a shortened catalyst life.

The packing density is preferably 0.4 to 0.8 g/mL. Within this range, the volume of the catalyst during the acid decomposition reaction is suitable, and the catalyst can be easily separated after the waste liquid is used, making it easy to use repeatedly. If it is less than 0.4 g/mL, the bulk of the catalyst is high, so it may be difficult to separate the catalyst after the acid decomposition reaction. If it exceeds 0.8 g/mL, the density may be so high that the catalyst may have poor mixability in the nitric nitrogen waste solution, resulting in a low catalytic activity.

The circularity of the activated carbon is preferably 0.8 to 1.0. Within this range, the activated carbon has a spherical shape, and when supporting metal particles, it is easy to carry them in a monodisperse manner, and it is difficult to break even when catalyst particles collide with each other during a nitric acid dispersion reaction. If it is less than 0.8, the activated carbon becomes amorphous or angular, which may cause agglomeration when supporting metal particles, and also tends to break when catalyst particles collide with each other during a nitric acid dispersion reaction.

The compressive strength is preferably 10 gf/mm ² or more. Within this range, the particles do not break during the metal particle support or during the nitric acid decomposition reaction, and the catalyst can be easily separated from the nitric acid decomposition waste liquid, making it possible to obtain a catalyst with a long catalyst life.

The activated carbon particles preferably have pores with a pore diameter of about 10 to 100 nm on the outer surface. When the pores are in this range, metal particles can be supported in the pores, and the metal particles are less likely to be detached from each other due to collisions between catalyst particles during the nitric acid decomposition reaction, resulting in a longer catalyst life.

<Method for producing nitric acid nitrogen decomposition catalyst>
The nitric acid nitrogen decomposition catalyst of the present invention includes 1) a process of activating activated carbon as necessary, 2) a process of adjusting the particle size distribution of activated carbon as necessary, 3) a process of adding organic matter or inorganic substance to activated carbon as necessary. It can be obtained through a pretreatment step, 4) a step of dispersing activated carbon in water to make it hydrophilic, if necessary, 5) a step of supporting metal particles, and 6) a drying step.

1) Activation treatment process In this process, activated carbon particles are placed in a ceramic pod and heated in an atmosphere of 100°C to 800°C under a stream of single or mixed gases such as air, nitrogen, hydrogen, carbon monoxide, and carbon dioxide. Process for ~24 hours. By this treatment, the degree of hydrophobicity, pore diameter, and pore volume can be adjusted, as well as the hydrophobicity of the activated carbon surface.

2) Particle size distribution adjustment step This step can be performed using a commonly known classification device. Preferable methods include sieving using a mesh made of SUS or nylon, adjustment by centrifugation using a cyclone, adjustment using a difference in specific gravity and sedimentation speed in a mixed solvent such as water or alcohol, and the like. Particularly, sieving is suitable because it allows a narrow particle size distribution to be obtained. The mesh size of the sieve depends on the particle size distribution, but it is preferable to use a mesh of 25 to 500 μm. By performing this sieving, activated carbon having a desired average particle size can be obtained.

3) Step of pre-treating activated carbon with organic matter or inorganic matter Activated carbon can be pre-treated with organic matter or inorganic matter if necessary. Examples of organic substances include polymers such as polyvinylpyrrolidone and polyvinyl alcohol, organic acids and acid salts such as acetic acid, oxalic acid, tartaric acid, citric acid, and polycarboxylic acids, and organometallic compounds (silane coupling agents, titanium coupling agents, etc.). ). These organic substances are dissolved in water, alcohol, etc. to a concentration of 0.1 to 5%, and stirred and mixed so that the concentration is 0.01 to 5% with respect to activated carbon. An organic substance for pretreatment can be obtained by washing this mixed liquid by a decantation method to remove unadsorbed organic substances.

As the inorganic substance, metal oxides such as silica, titania, and zirconia, and metal ions are preferable. Particularly, metal ions are suitable because some of them have a function as a co-catalyst. The metal ion is an ion of at least one metal selected from the group consisting of a fourth period transition metal element, a fifth period transition metal element, platinum, and gold. The fourth period transition metal element is preferably an element selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, and Cu, and the fifth period transition metal element is preferably an element selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, and Cu. Preferably, the element is selected from the group consisting of Tc, Ru, Rh, Pd and Ag.

The amount of metal ions added is preferably 0.1 to 100% by mass, more preferably 0.2 to 80% by mass in terms of metal elements, based on 100% by mass of activated carbon. If the amount of metal ions added is within this range, adsorption onto the surface of the carrier will be easy, and interaction and co-catalyst function will be more likely to occur when metal particles are supported. If the amount added is less than 0.1% by mass, adsorption onto activated carbon becomes difficult, and interaction with metal particles and promoter function become difficult to occur. If the amount added exceeds 100% by mass, the effect of removing unsupported metal ions in the subsequent desalination step will be insufficient, leading to problems such as the inability to carry out highly dispersed support in the step of supporting metal particles.

It is preferable that the amount of metal ions in the above-mentioned range is added to an activated carbon dispersion that has been previously dispersed in water. As the method of addition, the first method is to add a predetermined solution containing metal ions in the amount in the above range in terms of metal elements, the first method is to add a metal compound in an amount that can form metal ions in the above proportion in terms of metal elements, A second method for generating metal ions in this dispersion can be mentioned. In a first method, a solution containing metal ions can be prepared by dissolving a metal compound capable of forming metal ions in a solvent. The valence of the metal ion is not particularly limited.

Compounds capable of producing metal ions are not particularly limited as long as they form metal ions in dispersion. For example, compounds that form Pd ions include palladium chloride, palladium nitrate, palladium sulfate, and palladium citrate. , palladium acetate, and the like. These palladium compounds can be used alone or in combination of two or more.

Table 1 shows examples of compounds that can generate other metal ions.

A metal compound that generates metal ions is usually dissolved in a solvent and added to the activated carbon dispersion. The solvent used here may be any solvent as long as it does not show reactivity with the metal and can dissolve the metal compound. Such solvents include water, alcohols (methanol, ethanol, isopropanol, n-butanol, methylisocarbinol, etc.), ketones (acetone, 2-butanone, ethyl amyl ketone, diacetone alcohol, isophorone, cyclohexanone, etc.) , amides (N,N-dimethylformamide, N,N-dimethylacetamide, etc.), ethers (diethyl ether, isopropyl ether, tetrahydrofuran, 1,4-dioxane, 3,4-dihydro-2H-pyran, etc.), glycols Ethers (2-methoxyethanol, 2-ethoxyethanol, 2-butoxyethanol, ethylene glycol dimethyl ether, etc.), glycol ether acetates (2-methoxyethyl acetate, 2-ethoxyethyl acetate, 2-butoxyethyl acetate, etc.), esters (methyl acetate, ethyl acetate, isobutyl acetate, amyl acetate, ethyl lactate, ethylene carbonate, etc.), aromatic hydrocarbons (benzene, toluene, xylene, etc.), aliphatic hydrocarbons (hexane, heptane, iso-octane, cyclohexane, etc.), halogenated hydrocarbons (methylene chloride, 1,2-dichloroethane, dichloropropane, chlorobenzene, etc.), sulfoxides (dimethyl sulfoxide, etc.), pyrrolidones (N-methyl-2-pyrrolidone, N-octyl- (2-pyrrolidone, etc.) and glycols (ethylene glycol, propylene glycol, hexylene glycol, etc.).

The temperature at which a solution containing metal ions or a metal compound is added to the activated carbon dispersion is preferably in the range of 15 to 40°C. If the addition temperature is lower than 15°C, metal ions may not be supported sufficiently, and if the addition temperature is higher than 40°C, no further improvement in supporting efficiency will be observed, which is not economically preferable.

Further, after the addition, it is preferable to stir the activated carbon dispersion while maintaining the temperature within the above range to mix thoroughly. Stirring is carried out at 15 to 40°C, usually for 5 minutes or more. It is desirable to do this for 10 minutes or more. Further, the stirring time is preferably within 3 hours, more preferably within 1 hour. In particular, when a solid metal compound is added, it is necessary to perform operations such as stirring until the metal compound is sufficiently dissolved and metal ions are generated.

After mixing, the unadsorbed metal ions are washed with excess pure water and dried at 50 to 200°C to obtain activated carbon pretreated with metal ions.

Such pretreatment strengthens the interaction between the metal particles and the activated carbon, reduces the shedding of the metal particles during use of the catalyst, and makes it possible to obtain a catalyst with a long life.

4) Step of dispersing in water The obtained activated carbon is used in a state of being dispersed in water. The concentration of activated carbon in the dispersion is preferably 1 to 50% by mass, more preferably 5 to 20% by mass. If the concentration of activated carbon is within this range, it will be easier to disperse in water, and when supporting metal particles, it will be easier to support them in a monodisperse manner. Further, it is preferable to stir the mixture after dispersing it in water, and further preferably to heat it at 40 to 100° C. while stirring. Such treatment makes the activated carbon hydrophilic and can enhance the interaction between the metal particles and the activated carbon.

5) Process of supporting metal particles (preparation process of metal particles)
First, metal particles are prepared by reducing a metal salt in a solution. The solution may be an organic solvent or water, and may contain nano-sized microbubbles. When metal particles are prepared in a solution containing nano-sized microbubbles of nitrogen or inert gas, metal particles that are resistant to oxidation and have a high alloy ratio can be obtained. This step is preferably performed under purging with an inert gas such as N ₂ gas or rare gas in order to suppress the mixing of oxidizing gases. Oxidizing gases include oxygen, ozone, carbon dioxide, nitrogen monoxide, dinitrogen monoxide, nitrogen dioxide, fluorine, chlorine, chlorine dioxide, nitrogen trifluoride, chlorine trifluoride, silicon tetrachloride, oxygen difluoride, Examples include perchloryl fluoride. The redox potential of the reaction solution in this step is preferably -50 mV or less, more preferably -100 mV or less. Further, the pH is preferably 4.0 to 11.0, more preferably 4.5 to 7.0. By reducing the metal salt within this oxidation-reduction potential and pH range, metal particles can be smoothly generated. Further, the reaction temperature is preferably 10 to 80°C.

《Microbubbles》
The microbubbles are preferably microbubbles (micro-nanobubbles) with an average bubble diameter of 40 nm to 10 μm. Such microbubbles include at least one of so-called nanobubbles with a bubble diameter of 40 to 100 nm (0.1 μm) and so-called microbubbles with a bubble diameter of 0.1 to 10 μm, and those containing both are preferred. . The upper limit of the average bubble diameter of the microbubbles is preferably 500 nm, more preferably 350 nm, and even more preferably 200 nm. Further, the lower limit of the average bubble diameter of the microbubbles is preferably 50 nm, more preferably 60 nm, and even more preferably 65 nm. When metal particles are prepared with a solution containing these microbubbles, it is easy to obtain particles containing a large amount of Pd-Cu alloy, and because the microparticles are easily adsorbed to the metal particle surface, the particles are less likely to change due to oxidation or reduction. , it is easy to obtain a long-life catalyst that is less likely to cause particle growth during the nitric acid decomposition reaction.

The content of microbubbles is preferably at least 1.0 x 10 ³ cells/mL, more preferably at least 1.0 x 10 ⁵ cells/mL, even more preferably at least 1.0 x 10 ⁸ cells/mL. The upper limit is not particularly limited, but is preferably 1.0×10 ¹¹ pieces/mL, more preferably 5.0×10 ¹⁰ pieces/mL, and even more preferably 1.0×10 ¹⁰ pieces/mL.

The average bubble diameter and the number of microbubbles are determined by analyzing the Brownian movement speed of bubbles in a liquid using nanoparticle tracking analysis (NTA). For example, it can be measured using "Nanosite NS300" manufactured by Malvern.

The gas that forms microbubbles is preferably a non-oxidizing gas. Specifically, at least one of nitrogen, hydrogen, and a rare gas is preferable.

《Metal salt》
Examples of metal salts that are raw materials for metal particles include palladium and copper chloride salts, nitrates, sulfates, organic acid salts, and the like.

《Reducing agent》
The reduction reaction in the metal particle preparation step is usually performed using a reducing agent. Examples of the reducing agent include ferrous sulfate, NaBH ₄ , hydrazine, hydrogen, alcohol, trisodium citrate, tartaric acid, sodium hypophosphite, formic acid, LiBH ₄ , LiAlH ₄ , and diborane. Among these, trisodium citrate, tartaric acid, and formic acid are preferred. These have the functions of both reducing agents and stabilizers. Therefore, the steps required to remove impurities are reduced and stability is also improved.

The amount of the reducing agent used varies depending on the reducibility of the metal salt, but is preferably 0.5 to 10 mol, more preferably 1 to 5 mol, per 1 mol of the metal salt. Here, if the amount of the reducing agent is less than 0.5 mol per mol of the metal salt, the reduction may be insufficient and desired metal particles may not be obtained. If the amount of the reducing agent exceeds 10 moles per mole of the metal salt, metal particles having a larger particle size than necessary may be produced.

《Organic stabilizer》
In the metal particle preparation step, it is preferable to use an organic stabilizer. By adding the organic stabilizer, the organic stabilizer is adsorbed to the metal salt, the dispersibility of the metal salt is improved, and the metal salt can be reduced more smoothly. Moreover, the generated metal particles are stably dispersed in the dispersion medium.

Any organic stabilizer may be used as long as it can be adsorbed onto metal salts or metal particles to improve dispersion stability. For example, gelatin, polyvinyl alcohol, polyvinylpyrrolidone, vinyl acetate, polyacrylic acid, carboxylic acid compounds, etc. are suitable. Among these, carboxylic acid compounds having carboxyl groups that have a large interaction with metal salts and the surfaces of metal particles are preferred, and polyvalent carboxylic acid compounds are particularly preferred. Carboxylic acid compounds include anthic acid, hydroxyanthracenecarboxylic acid, hydroxynaphthoic acid, gallic acid, cresotic acid, parahydroxybenzoic acid, ortho-acetylsalicylic acid, malic acid, mandelic acid, gluconic acid, citric acid, tartaric acid, lactic acid, and benzene. Carboxylic acid, formic acid, acetic acid, butanoic acid, propionic acid, pentanoic acid, hexanoic acid, heptanoic acid, octanoic acid, nonanoic acid, decanoic acid, dodecanoic acid, tetradecanoic acid, pentadecanoic acid, hexadecanoic acid, 9-hexadecenoic acid, heptadecanoic acid , octadecanoic acid, glycolic acid, L-ascorbic acid, fumaric acid, maleic acid, adipic acid, and salts thereof. These may be used alone or in combination of two or more. Among these, citric acid or a salt thereof is preferred, and trisodium citrate is particularly preferred because of its strong interaction with metal salts and the surfaces of metal particles.

The amount of the organic stabilizer used is preferably 0.5 to 10 mol, more preferably 1 to 5 mol, per 1 mol of the metal salt. Here, if the amount of the organic stabilizer is less than 0.5 mol per mol of the metal salt, the amount of the organic stabilizer adsorbed to the metal salt will be too small, and the dispersibility of the metal salt will be insufficient. There is a risk that the reduction of the salt and the dispersibility of the metal particles will be insufficient. Conversely, if the amount of the organic stabilizer exceeds 10 mol per mol of the metal salt, the dispersibility and reducibility of the metal salt and the dispersibility of the metal particles will not be particularly improved, and the organic stabilization in the subsequent washing step will be reduced. Additional labor may be required for removal of chemical agents and wastewater treatment.

《pH adjuster》
A pH adjuster can be used so that the pH of the reaction solution in the particle preparation step is 4.0 to 11.0. Mineral acids and organic acids are suitable as the pH adjuster. Among these, organic acids having a carbon number of C ₁ to C ₃ are preferred. Note that the organic stabilizer may also function as a pH adjuster.

(Washing process)
Next, the metal particles prepared in the metal particle preparation step are washed with a washing liquid. In this step, desalting is performed and the organic stabilizer is also removed. Here, the salt is a substance other than metal particles produced by reduction treatment of a metal salt, and is present in the reaction solution as an ion. Specific examples include metal ions such as sodium and iron, boron ions, chloride ions, nitrate ions, sulfate ions, and organic acid ions.

The cleaning liquid is suitably water or alcohol, and may contain other ingredients. In addition, as the cleaning liquid, it is preferable to use at least one of a bubbling liquid that has been previously bubbled with an inert gas to remove oxygen, and a liquid containing microbubbles. ) is particularly preferably used. The details of the liquid containing microbubbles are the same as the liquid containing microbubbles (reaction liquid) used in the metal particle preparation step. Further, it is preferable that the metal particle dispersion produced after washing also contains microbubbles.

By cleaning metal particles with a cleaning solution containing microbubbles, ionization and oxidation of metal particles are prevented, dramatically increasing the storage stability of metal particle dispersions and the pot life of coating solutions using this metal particle dispersion. can be improved. Furthermore, the dispersibility of the metal particles produced is improved, and the amount of organic stabilizer in the final dispersion can be reduced. Therefore, when formed into a film, the metal particles come into more direct contact with each other, reducing the resistance at the grain boundaries, and as a result, a film with high conductivity can be formed.

Examples of the cleaning method include a decantation method and a cleaning method using an ultra membrane or a ceramic membrane. In the decantation method, for example, metal particles are recovered from the metal particle dispersion prepared in the metal particle preparation step, and the recovered metal particles are immersed in a cleaning liquid to be washed (desalted). This cleaning solution preferably contains a high concentration of organic stabilizer. Thereby, the metal particles can be appropriately aggregated, and impurities such as nitrate ions and sulfate ions can be eluted into the supernatant liquid, and these impurities can be removed. Furthermore, it is preferable to purify using an ion exchange resin. Note that the organic stabilizer used in the washing step may be the same as or different from that used in the particle preparation step.

In the present invention, impurities can be sufficiently removed even when using a cleaning solution with a relatively low concentration of organic stabilizer, and the subsequent treatment with ion exchange resin can be simplified (reduced amount of resin), resulting in efficient can produce metal particles. Moreover, it becomes possible to reduce the number of times of washing, and metal particles can be produced efficiently. By simplifying the washing process, loss of metal particles can be reduced and yield can be improved. Furthermore, the simplification of the cleaning process suppresses the induction of oxidation of the metal particles, so that the coating formed using these metal particles has high conductivity.

(Coarse particle removal process)
After the washing step, coarse particles are preferably removed by centrifugation or the like.

(Supporting process)
The metal particles obtained as described above are supported on a carrier. The supporting step consists of a step of mixing metal particles and an activated carbon dispersion to obtain a precursor dispersion of a metal particle-supported catalyst, a step of supporting the metal particles, a step of removing unsupported metal particles, and a step of drying.
In the step of obtaining a precursor dispersion of a metal particle-supported catalyst, the metal particles prepared as described above are added to activated carbon as a Pd-Cu element while adjusting the temperature of the dispersion obtained in step 4) to 15 to 40°C. 0.15 to 3.5% by mass is added and mixed to prepare a precursor dispersion of a metal particle-supported catalyst. If it is less than 0.15%, it can be supported in a highly dispersed manner, but since the amount of metal is small, the activity of the obtained catalyst may be low. If it exceeds 3.5%, agglomeration may occur during loading, and particle growth or falling off of activated carbon may occur during the nitric acid decomposition reaction.

The supporting step is preferably performed by drying with heat, removing the dispersion medium while vacuum degassing, or the like. When drying by heat, the material is left to dry at a temperature in the range of 80°C to 200°C. If the temperature exceeds 200° C., the drying speed is so fast that the metal particles may not be supported in a monodisperse manner or may not be supported inside the pores. If the temperature is lower than 80° C., the drying time becomes long, which is not economically efficient. Drying is preferably carried out under a stream of inert gas such as nitrogen or argon gas, if necessary. By performing the process under an inert gas flow, the metal particles are less likely to be oxidized. More preferably, it is dried while being vacuum degassed. By creating a vacuum, the metal particle dispersion does not easily enter the pores of the activated carbon, and can be more easily supported inside the pores.

Furthermore, when metal particles are supported on activated carbon that has been pretreated with metal ions, rearrangement of spherical particles into polyhedral crystal particles may occur.

The activated carbon carrying metal particles is subjected to a decantation method to remove unsupported metal particles. If it is not removed, unsupported metal particles may grow together during the nitric acid decomposition reaction, resulting in increased production of ammonia as a side reaction. After decantation, leave to dry at a temperature in the range of 80°C to 200°C. Preferably, it is carried out under an inert gas flow such as nitrogen or argon gas. By performing the process under an inert gas flow, the metal particles are less likely to be oxidized.

Through these steps, a nitric acid nitrogen decomposition catalyst can be obtained in which the Pd--Cu element of the metal particles is 0.1 to 3% by mass based on the activated carbon.

<Method for treating nitrate nitrogen-containing water>
Using the catalyst of the present invention, nitrate nitrogen in nitrate nitrogen-containing water is subjected to reductive decomposition treatment. Nitrate nitrogen-containing water refers to an aqueous solution containing nitrate nitrogen, and includes, for example, domestic wastewater and industrial wastewater. Further, nitrate nitrogen refers to NO ₃ , NO _{2 ,} etc., and usually exists as an ion in an aqueous solution. The concentration of nitrate nitrogen in the nitrate nitrogen-containing water is preferably 100 to 100,000 ppm, more preferably 300 to 60,000 ppm in terms of N. The nitrate nitrogen-containing water may also contain substances other than nitrate nitrogen. Examples include inorganic substances such as NH ₃ , ClO ₃ , ClO ₂ , ClO, Na, Cl, Fe, and Ni, and organic substances such as citric acid, oxalic acid, EDTA, and EDDS.

Furthermore, before adding the catalyst, the pH of the nitric nitrogen-containing water is preferably adjusted to a range of 6 to 12, more preferably 7 to 11. By setting the pH within such a range, it is possible to prevent elution of metal particles of the catalyst, a decrease in catalyst activity, and an increase in the amount of NH ₃ as a by-product.

The catalyst of the present invention is preferably added so that the metal particles in the catalyst are 0.00001 to 0.5% by mass, and 0.0001 to 0.1% by mass in nitric nitrogen-containing water. It is more preferable to add it to. When the amount added is within this range, more sufficient catalytic activity can be obtained and it is also economically preferable.

The contact time between the catalyst and the nitrate nitrogen-containing water depends on the amount of the nitrate nitrogen-containing water, the concentration of nitrate nitrogen before treatment, the target concentration of nitrate nitrogen after treatment, and the impurities in the nitrate nitrogen-containing water ( Although it varies depending on the concentration of organic substances and metals), the content of metal particles in the catalyst, the particle size of the catalyst, etc., it is generally 20 hours or less, and usually 3 to 15 hours is preferable.

Further, the temperature during contact is preferably in the range of 20°C to 100°C, more preferably 40°C to 80°C. When the temperature at the time of contact is within this range, more sufficient catalytic activity can be obtained and the progress of deterioration of the catalyst can be further delayed.

The contact between the nitric nitrogen-containing water and the catalyst is preferably carried out in the presence of a reducing agent. As the reducing agent, formic acid, hydrazine, sodium borohydride, sodium hypophosphite, quinone, hydroquinone, hydrogen gas, etc. can be used. The amount of the reducing agent added is preferably 1 to 3 mol times, more preferably 1 to 2 mol times the amount of nitrate nitrogen in the nitrate nitrogen-containing water.

In the treatment method of the present invention, the treatment equipment is not particularly limited as long as the catalyst can be brought into contact with nitric nitrogen-containing water, and examples include complete mixing tank type, flow type, multistage type, batch type, and other treatment equipment. It will be done.

Examples of the present invention will be specifically described below.

[Example 1]
As the activated carbon, "Taiko A100FB" (Futamura Chemical), which is made from carbon derived from phenol resin, was used. Table 1 shows the physical properties of activated carbon. 100 g of this activated carbon was dispersed in 900 g of pure water to prepare an aqueous dispersion of activated carbon (concentration 10% by mass).

Next, 122 g of ferrous sulfate as a reducing agent was dissolved in 219 g of a trisodium citrate aqueous solution (concentration 30% by mass). To 341 g of this solution, 39 g of a palladium (II) nitrate aqueous solution (concentration 20% by mass) was added at room temperature, and then 10 g of a copper (II) nitrate aqueous solution (concentration 20% by mass) was added. This solution was thoroughly mixed to prepare a dispersion of Pd--Cu particles. Thereafter, 100 g of pure water was added to the solid obtained by centrifugation, and further, in order to remove the reducing agent, 100 g of trisodium citrate aqueous solution (concentration 30% by mass) was added and stirred for 1 hour. To the solid matter recovered from this solution by centrifugation, 100 g of pure water was added and stirred. Further, an amphoteric ion exchange resin SMNUPB was added to this dispersion to remove impurities. After separating the ion exchange resin, coarse particles were removed by centrifugation (10,000 G for 30 minutes) to obtain a Pd--Cu colloidal solution. The resulting solution had a Pd-Cu equivalent concentration of 3%. The Pd component ratio in the metal particles of this solution was 78% with respect to the "sum of Pd component and Cu component".

26.66 g of this Pd--Cu colloidal solution was added to 1000 g of the above-described aqueous dispersion of activated carbon, and the mixture was stirred for 10 minutes. The pH of the resulting mixture was 6.5.

This mixed solution was dried in a nitrogen atmosphere at a temperature of 105° C. for 24 hours to obtain a catalyst in which metal particles containing a Pd—Cu alloy were supported on activated carbon.

The physical properties of activated carbon, metal particles, and catalyst were measured and evaluated using the following methods. The same procedure was carried out for subsequent Examples and Comparative Examples. Table 1 shows the results regarding activated carbon and metal particles, and Table 2 shows the results regarding the catalyst.

(1) Average particle diameter Photographed with a scanning electron microscope (S-5500 manufactured by Hitachi, Ltd.), the maximum diameter of arbitrary 100 particles was measured, and the average was taken as the average particle diameter. The average particle diameter of the activated carbon was 76 μm, and the average particle diameter of the metal particles was 3 nm.

(2) Circularity (sphericity coefficient)
The area and perimeter of 100 arbitrary particles were measured using an image analysis method from images taken with a scanning electron microscope (S-5500 manufactured by Hitachi, Ltd.), and the circularity was calculated from the following formula.
Circularity = 4π x area ÷ perimeter ²
The maximum value of circularity is 1, and the more complex the figure, the smaller the value. The closer the circularity is to 1, the more truly spherical the particle is. The average value of the circularity of 100 particles was taken as the sphericity coefficient. The circularity of both activated carbon and metal particles was 0.95.

(3) Average pore diameter Observe the surface of any particle from an image taken with a scanning electron microscope (S-5500 manufactured by Hitachi, Ltd.), measure the long diameter of 10 arbitrary pores on the surface, The longest diameter of the largest pore was defined as the surface pore diameter of the particle, and the average of the surface pore diameters of the 10 particles was defined as the average pore diameter. The average pore diameter of the activated carbon was 30 nm.

(4) Pore volume After the sample was treated in an electric furnace at 300°C for 1 hour, it was measured by mercury porosimetry using a porosimeter (PM33GT-17 manufactured by Quantachrome). As a result of measurement, the pore volume of the activated carbon was 0.55 mL/g, and the pore volume of the catalyst was 0.54 mL/g.

(5) Specific surface area After the sample was treated in an electric furnace at 300° C. for 1 hour, the specific surface area was measured by the BET method using a specific surface area measuring device (Macsorb HM model-1220 manufactured by Mountec). As a result of measurement, the specific surface area of the activated carbon was 895 m ² /g, and the specific surface area of the catalyst was 896 m ² /g.

(6) Packing density After the sample was treated in an electric furnace at 300° C. for 1 hour, it was filled into a 100 mL graduated cylinder. The weight at this time was measured to calculate the packing density (filling weight per 1 mL). As a result of measurement, the packing density of activated carbon was 0.69 g/mL, and the packing density of catalyst was 0.69 g/mL.

(7) Compressive fracture strength Compressive strength was measured using a micro compression tester (MCT-W500 manufactured by Shimadzu Corporation). The activated carbon sample was compressed and loaded, and the load at which the sample broke was measured. This measurement was performed on five samples, and the average value was taken as the compressive fracture strength. The compressive fracture strength of the activated carbon was 41.7 gf/mm ² , and the compressive fracture strength of the catalyst was 41.7 gf/mm ² .

(8) Composition of metal particles The Pd-Cu colloidal solution was analyzed by EXAFS (R-XAM Looper manufactured by RIGAKU). As a result, it was confirmed that Pd--Cu alloy was contained.

(9) Proportion of Pd component A Pd-Cu colloidal solution was dissolved in a 61% by mass nitric acid aqueous solution and diluted with pure water. The masses of Pd and Cu elements in the solution were measured using an ICP inductively coupled plasma emission spectrometer (SPS1200A manufactured by Seiko Electronic Industries, Ltd.). From the obtained results, Pd amount/[Pd amount+Cu amount]×100 was calculated to determine the ratio of the Pd component.

(10) Amount of surface treatment agent in metal particles (amount of carbon in surface treatment agent)
The Pd-Cu colloidal solution was dried at 105°C and tungsten oxide was added as an auxiliary agent. This was high-frequency fired to produce _CO2 , and the amount of CO2 was measured. The amount of carbon [C amount] in the surface treatment agent was determined from this amount of CO ₂ . From the obtained results, ([C amount]/[Pd amount+Cu amount])×100 was calculated, and the ratio of carbon amount [C amount] to Pd-Cu amount was calculated. Note that [Pd amount+Cu amount] is the value obtained in the above (9). The amount of carbon affects the degree of dispersion when supported, and the larger the amount, the more effective the degree of dispersion and life will be.

(11) Amount of metal particles supported on catalyst A sample (catalyst) was calcined at 600°C and then melted with an alkaline melting agent. This was dissolved in a 28% by mass aqueous hydrochloric acid solution and diluted with pure water. The mass of the metal element contained in the catalyst was measured using an ICP inductively coupled plasma emission spectrometer (SPS1200A manufactured by Seiko Electronic Industries, Ltd.). If metals other than Pd and Cu components are included, the supported amount is calculated without including those metals.

(12) Supporting state of metal particles A photograph of the sample (catalyst) was taken with a scanning electron microscope (Hitachi S-5500) (magnification: 300,000 times), and a range of 10,000 nm ² was determined from the photographic projection diagram obtained. Measure the number of metal particles (t) existing within (100 nm square), the number (s) of metal particles that have no other metal particles within a radius of 1 nm, and measure the number of metal particles (s) that exist within a radius of 1 nm. The proportion m of metal particles that do not occur was determined. {m=(s/t)×100}. This measurement was performed at 50 locations on the catalyst, and the average value was taken as the dispersibility M [%].

Furthermore, the inside of the pores at 10 arbitrary locations was observed to confirm whether metal particles were supported within the pores.

Next, the nitrate nitrogen decomposition performance of the catalyst according to this example was evaluated as follows. The results are shown in Table 4.

First, 200 g of a sodium nitrate solution with a nitrate ion concentration of 4.5 mol/L was placed in a 1 L separable flask. A catalyst in an amount of 0.02 g in terms of metal particles was added thereto, and the system was stirred with a magnetic stirrer while purging with argon, and the temperature was adjusted to 80°C. Thereafter, 1.2 mol of hydrazine was added over 3 hours, and the nitrate ion concentration after the addition was measured and calculated by spectrophotometry.

《Nitric acid decomposition rate》
The conversion rate of nitrate ions at that time [((initial nitrate ion concentration−nitrate ion concentration after completion of reaction)/initial nitrate ion concentration)×100 (mol%)] was defined as the nitric acid decomposition rate.

《 _NH3 production amount》
Furthermore, after the addition of hydrazine, the gas phase was sampled, and the amount of NH ₃ was measured using a gas chromatograph. This was defined as the amount of NH ₃ produced.

《Waste liquid separation》
After adding hydrazine, the mixture was allowed to stand for 1 hour and the supernatant waste liquid was collected. At that time, the following criteria were used to determine the amount recovered without catalyst contamination.
◎: More than 80% of waste liquid was recovered.
○: More than 60% to less than 80% of waste liquid was recovered.
△: More than 40% to less than 60% of waste liquid was recovered.
×: Less than 40% of waste liquid was recovered.

"lifespan"
The above-mentioned decomposition reaction was repeated, and the number of times until the nitric acid decomposition rate was less than 70% or the waste liquid separability evaluation became "x" was defined as the life span.

《Crack of catalyst》
The catalyst after one use was photographed using a scanning electron microscope (S-5500 manufactured by Hitachi, Ltd.), and the number of cracked particles out of 100 arbitrary particles was measured and judged based on the following criteria.
○: Number of broken pieces is less than 10 △: Number of broken pieces is 10 or more to less than 30 ×: Number of broken pieces is 30 or more

《Supported amount ratio》
The catalyst used once was calcined at 600°C and melted with an alkaline melting agent. This melt was dissolved in a 28% by mass aqueous hydrochloric acid solution, and further diluted with pure water. The amount of elements contained in the catalyst was measured using an ICP inductively coupled plasma emission spectrometer (SPS1200A manufactured by Seiko Electronic Industries, Ltd.), and the supported amount ratio [=(metal particle supported amount after one use)/(before use) amount of metal particles supported)] was determined.

[Example 2]
The activated carbon used in Example 1 was passed through a sieve with an opening of 50 μm, and the particles under the sieve (particles that passed through the sieve) were collected and used as the activated carbon of this example. A catalyst was produced in the same manner as in Example 1 except that this activated carbon was used.

[Example 3]
The activated carbon used in Example 1 was passed through a sieve with an opening of 50 μm, and the particles on the sieve (particles that did not pass through the sieve) were collected and used as the activated carbon of this example. A catalyst was produced in the same manner as in Example 1 except that this activated carbon was used.

[Example 4]
The activated carbon used in Example 1 was fired at 800° C. for 1 hour under a nitrogen stream with an oxygen concentration of 0.1% or less. A catalyst was produced in the same manner as in Example 1 except that the activated carbon thus obtained was used.

[Example 5]
In this example, the following Pd-Cu colloid solution was used. First, ultrapure water and N ₂ are brought into contact with a swirl flow type bubble generator (HYK-20-SD manufactured by Ligaric), and N ₂ micro-nano bubble water (average bubble diameter 70 nm, number of bubbles 240 million/ mL, pH 5.79 (25° C., same hereinafter), electrical conductivity 1.17 μS/cm, dissolved oxygen concentration (DO) 1.70 ppm, and oxidation-reduction potential (ORP) 330 mV). Using this N ₂ micro-nano bubble water, 122 g of ferrous sulfate as a reducing agent was dissolved in 219 g of a trisodium citrate aqueous solution (concentration 30% by mass). To 341 g of this solution, 39 g of a palladium (II) nitrate aqueous solution (concentration 20% by mass) was added at room temperature, and then a copper (II) nitrate aqueous solution (concentration 20% by mass) prepared in the same manner using _N2 micro-nano bubble water was added. 10g was added. Thereafter, the mixture was heated to 50°C and stirred for 10 hours. In this way, a dispersion of Pd--Cu particles was prepared. Thereafter, 100 g of pure water was added to the solid material obtained by centrifugation, and further, 100 g of trisodium citrate aqueous solution (concentration 30% by mass) was added and stirred for 1 hour. To the solid matter recovered from this solution by centrifugation, 100 g of pure water was added and stirred. Further, an amphoteric ion exchange resin SMNUPB was added to this dispersion to remove impurities. After separating the ion exchange resin, coarse particles were removed by centrifugation (10,000 G for 10 minutes) to obtain a Pd--Cu colloidal solution. The Pd-Cu equivalent concentration of this solution was 3%, and the Pd component ratio of the metal particles was 78% with respect to the "sum of Pd component and Cu component".

A catalyst was produced in the same manner as in Example 1 except that this Pd--Cu colloidal solution was used.

[Example 6]
In this example, the following Pd-Cu colloid solution was used. 122 g of ferrous sulfate as a reducing agent was dissolved in 219 g of a trisodium citrate aqueous solution (concentration 30% by mass). To 341 g of this solution, 45 g of palladium (II) nitrate aqueous solution (concentration 20% by mass) was added at room temperature, and then 5 g of copper (II) nitrate aqueous solution (concentration 20%) was added to form a dispersion of Pd-Cu particles. Prepared. Thereafter, 100 g of pure water was added to the solid material obtained by centrifugation, and further, 100 g of trisodium citrate aqueous solution (concentration 30%) was added and stirred for 1 hour. To the solid matter recovered from this solution by centrifugation, 100 g of pure water was added and stirred. An amphoteric ion exchange resin SMNUPB was added to this dispersion to remove impurities. After separating the ion exchange resin, coarse particles were removed by centrifugation (10,000 G for 10 minutes) to obtain a Pd--Cu colloidal solution. The Pd--Cu equivalent concentration of this solution was 3%, and the Pd component ratio of the metal particles was 95% with respect to "the sum of the Pd component and the Cu component."

A catalyst was produced in the same manner as in Example 1 except that this Pd--Cu colloidal solution was used.

[Example 7]
In this example, the following Pd-Cu colloid solution was used. 122 g of ferrous sulfate as a reducing agent was dissolved in 219 g of a trisodium citrate aqueous solution (concentration 30% by mass). To 341 g of this solution, 29 g of palladium (II) nitrate aqueous solution (concentration 20% by mass) was added at room temperature, and then 25 g of copper (II) nitrate aqueous solution (concentration 20%) was added to prepare a dispersion of Pd-Cu particles. did. Thereafter, 100 g of pure water was added to the solid material obtained by centrifugation, and further, 100 g of trisodium citrate aqueous solution (concentration 30%) was added and stirred for 1 hour. 100 g of pure water was added to the solids recovered from this solution by centrifugation and stirred. An amphoteric ion exchange resin SMNUPB was added to this dispersion to remove impurities. After separating the ion exchange resin, coarse particles were removed by centrifugation (10,000 G for 10 minutes) to obtain a Pd--Cu colloidal solution. The Pd-Cu equivalent concentration of this solution was 3%, and the Pd component ratio of the metal particles was 62% with respect to the "sum of Pd component and Cu component".

A catalyst was produced in the same manner as in Example 1 except that this Pd--Cu colloidal solution was used.

[Example 8]
In the same manner as in Example 1, 26.66 g of a Pd--Cu colloid solution was added to 1000 g of an aqueous dispersion of activated carbon, and the mixture was stirred for 10 minutes. The pH of the resulting mixture was 6.5. Here, 100 g of a palladium nitrate aqueous solution (concentration 1%) was mixed with this liquid mixture, and the mixture was stirred for 1 hour. This mixed solution was dried in a nitrogen atmosphere at a temperature of 105° C. for 24 hours. The obtained powder was dispersed in 1000 g of pure water, left to stand for 1 hour, and then the supernatant was removed three times. The pH at that time was 4.8. This mixed solution was dried in a nitrogen atmosphere at a temperature of 105° C. for 24 hours to obtain a catalyst on which metal particles containing a Pd—Cu alloy were supported.

[Example 9]
In this example, the following Pd--Cu colloid solution was used. 122 g of ferrous sulfate as a reducing agent was dissolved in 219 g of a trisodium citrate aqueous solution (concentration 30% by mass). To 341 g of this solution, 39 g of palladium (II) nitrate aqueous solution (concentration 20% by mass) was added at room temperature, and then 10 g of copper (II) nitrate aqueous solution (concentration 20%) was added to prepare a dispersion of Pd-Cu particles. . Thereafter, 100 g of pure water was added to the solid material obtained by centrifugation, and further, 100 g of trisodium citrate aqueous solution (concentration 30% by mass) was added and stirred for 1 hour. To the solid matter recovered from this solution by centrifugation, 100 g of pure water was added and stirred. Coarse particles were removed from this dispersion by centrifugation (10,000 G for 30 minutes) to obtain a Pd--Cu colloidal solution. This solution had a Pd-Cu equivalent concentration of 3%. The Pd component ratio in the metal particles was 78% with respect to the "sum of Pd component and Cu component".

1000 g of the activated carbon aqueous dispersion prepared in Example 1 was further heated to reflux at 100° C. for 24 hours with stirring. To this was added 26.66 g of the Pd--Cu colloidal solution of this example, and the mixture was stirred for 10 minutes. From this point on, a catalyst was produced in the same manner as in Example 1. In this example, the pH of the mixed solution after addition of the Pd--Cu colloidal solution was 8.5.

[Example 10]
In this example, a nickel nitrate aqueous solution was used instead of the palladium nitrate aqueous solution added to the mixed liquid in Example 8. That is, in the same manner as in Example 1, 26.66 g of a Pd--Cu colloidal solution was added to 1000 g of an aqueous dispersion of activated carbon, and the mixture was stirred for 10 minutes. The pH of the resulting mixture was 6.5. Here, 100 g of nickel nitrate aqueous solution (concentration 1%) was mixed with this mixed solution and stirred for 1 hour. This mixed solution was dried in a nitrogen atmosphere at a temperature of 105° C. for 24 hours. The obtained powder was dispersed in 1000 g of pure water, left to stand for 1 hour, and then the supernatant was removed three times. The pH at that time was 5.1. This mixed solution was dried in a nitrogen atmosphere at a temperature of 105° C. for 24 hours to obtain a catalyst on which metal particles containing a Pd—Cu alloy were supported.

[Example 11]
The activated carbon used in Example 1 was fired at 800° C. for 1 hour under a nitrogen stream at an oxygen concentration of 0.1% or less to obtain the activated carbon of this example. 100 g of this activated carbon was dispersed in 900 g of pure water, and heated under reflux while stirring at 100° C. for 24 hours to prepare an aqueous dispersion of activated carbon (concentration 10% by mass). 26.66 g of the Pd-Cu colloidal solution prepared in Example 9 was added to 1000 g of this aqueous dispersion, and the mixture was mixed and stirred for 10 minutes. The pH of the mixed solution at this time was 6.5.

This mixed solution was dried using a rotary evaporator under conditions of 100 mmHg or less and a bath temperature of 80°C, and further dried at a temperature of 105°C in a nitrogen atmosphere for 24 hours. As a result, a catalyst on which metal particles containing a Pd--Cu alloy were supported was obtained.

[Example 12]
An activated carbon aqueous dispersion and a Pd-Cu colloidal solution were prepared in the same manner as in Example 1, and 100g of the Pd-Cu colloidal solution was added to 1000g of the activated carbon aqueous dispersion and stirred for 10 minutes. At this time, the pH of the mixed solution was 6.0. This mixed solution was dried in a nitrogen atmosphere at a temperature of 105° C. for 24 hours to obtain a catalyst on which metal particles containing a Pd—Cu alloy were supported.

[Example 13]
An aqueous dispersion of activated carbon and a Pd-Cu colloidal solution were prepared in the same manner as in Example 1, and 6.67g of the Pd-Cu colloidal solution was added to 1000g of the aqueous dispersion of activated carbon, followed by stirring for 10 minutes. At this time, the pH of the mixed solution was 6.7. This mixed solution was dried in a nitrogen atmosphere at a temperature of 105° C. for 24 hours to obtain a catalyst on which metal particles containing a Pd—Cu alloy were supported.

[Example 14]
In this example, "Taiko CG" (Futamura Chemical), which is made from carbon derived from coconut shells, was used as activated carbon. This activated carbon was passed through a sieve with an opening of 50 μm, and the particles on the sieve were collected to obtain the activated carbon of this example. A catalyst was produced in the same manner as in Example 1 except that this activated carbon was used.
After adding the Pd--Cu colloidal solution to the activated carbon aqueous dispersion, the pH of the mixture was 4.5.

[Comparative example 1]
In this comparative example, a colloidal solution of metal particles was used as a catalyst without using a carrier. First, 200 g of a sodium nitrate solution with a nitrate ion concentration of 4.5 mol/L is placed in a 1 L separable flask. 0.67 g of the Pd--Cu colloid solution prepared in Example 1 is added to this solution. That is, the Pd--Cu colloid solution was added in an amount equivalent to 0.02 g in terms of metal particles. This liquid mixture was stirred with a magnetic stirrer under an argon purge and the temperature was adjusted to 80°C. Thereafter, 1.2 mol of hydrazine was added over 3 hours.

[Comparative example 2]
``Granular Shirasagi W2C Amorphous '' (Osaka Gas Chemical), which is made from carbon derived from coconut shells, was used as the activated carbon. This activated carbon was passed through a sieve with an opening of 50 μm, and the particles under the sieve were collected to obtain the activated carbon of this comparative example. A catalyst was produced in the same manner as in Example 1 except that this activated carbon was used.

[Comparative example 3]
``Granular Shirasagi C2C Spherical '' (Osaka Gas Chemical), which is made from carbon derived from coconut shells, was used as activated carbon. This activated carbon was passed through a sieve with an opening of 50 μm, and the particles under the sieve were collected to obtain the activated carbon of this comparative example. A catalyst was produced in the same manner as in Example 1 except that this activated carbon was used.

[Comparative example 4]
The activated carbon used was "Taiko A100FB" (Futamura Chemical), which is made from carbon derived from phenolic resin. This activated carbon was fired at 800° C. for 5 hours under a nitrogen stream at an oxygen concentration of 0.1% or less to obtain activated carbon of this comparative example. A catalyst was produced in the same manner as in Example 1 except that this activated carbon was used.

[Comparative example 5]
An aqueous dispersion of activated carbon and a Pd--Cu colloidal solution were prepared in the same manner as in Example 1. 1.667 g of Pd--Cu colloidal solution was added to 1000 g of this aqueous dispersion of activated carbon, and the mixture was stirred for 10 minutes. The pH of the resulting mixture was 6.7. This mixed solution was dried in a nitrogen atmosphere at a temperature of 105° C. for 24 hours to obtain a catalyst on which metal particles containing a Pd—Cu alloy were supported.

[Comparative example 6]
An aqueous dispersion of activated carbon and a Pd--Cu colloidal solution were prepared in the same manner as in Example 1. 166.7 g of Pd--Cu colloidal solution was added to 1000 g of this aqueous dispersion of activated carbon, and the mixture was mixed and stirred for 10 minutes. The pH of the resulting mixture was 5.8. This mixed solution was dried in a nitrogen atmosphere at a temperature of 105° C. for 24 hours to obtain a catalyst on which metal particles containing a Pd—Cu alloy were supported.

[Comparative Example 7]
In this comparative example, the following Pd colloid solution was used instead of the Pd--Cu colloid solution of Example 1. First, 122 g of ferrous sulfate as a reducing agent was dissolved in 219 g of a trisodium citrate aqueous solution (concentration 30% by mass). To 341 g of this solution, 100 g of an aqueous palladium (II) nitrate solution (concentration 20% by mass) was added at room temperature and thoroughly mixed to prepare a dispersion of Pd particles. Thereafter, 100 g of pure water was added to the solid material obtained by centrifugation, and further, 100 g of trisodium citrate aqueous solution (concentration 30% by mass) was added and stirred for 1 hour. To the solid matter recovered from this solution by centrifugation, 100 g of pure water was added and stirred. Further, an amphoteric ion exchange resin SMNUPB was added to remove impurities. After separating the ion exchange resin, coarse particles were removed by centrifugation (10,000 G for 10 minutes) to obtain a Pd colloid solution. The Pd concentration of this solution was 3%, and the Pd component ratio of the metal particles was 100%.

26.66 g of the Pd colloid solution of this comparative example was added to 1000 g of the aqueous dispersion of activated carbon prepared in Example 1, and the mixture was stirred for 10 minutes. The pH of this mixture was 4.1.

This mixed solution was dried in a nitrogen atmosphere at a temperature of 105° C. for 24 hours to obtain a catalyst on which Pd particles were supported.

[Comparative example 8]
In this comparative example, the following Pd--Cu colloidal solution was used instead of the Pd--Cu colloidal solution of Example 1. First, 122 g of ferrous sulfate as a reducing agent was dissolved in 219 g of a trisodium citrate aqueous solution (concentration 30% by mass). To 341 g of this solution, 60 g of palladium(II) nitrate aqueous solution (concentration 20% by mass) was added at room temperature, and then 10g of copper(II) nitrate aqueous solution (concentration 20% by mass) was added, and the mixture was thoroughly mixed to form Pd-Cu particles. A dispersion was prepared. Thereafter, 100 g of pure water was added to the solid material obtained by centrifugation, and further, 100 g of trisodium citrate aqueous solution (concentration 30% by mass) was added and stirred for 1 hour. 100 g of pure water was added to the solids recovered from this solution by centrifugation and stirred. Further, an amphoteric ion exchange resin SMNUPB was added to this dispersion to remove impurities. After separating the ion exchange resin, coarse particles were removed by centrifugation (10,000 G for 10 minutes) to obtain a Pd--Cu colloidal solution. The Pd--Cu concentration of this solution was 3%. The Pd component ratio of the metal particles was 20% with respect to the "sum of Pd component and Cu component".

26.66 g of the Pd--Cu colloidal solution of this comparative example was added to 1000 g of the activated carbon suspension prepared in Example 1, and mixed and stirred for 10 minutes. The pH of the resulting mixture was 5.5. This mixed solution was dried in a nitrogen atmosphere at a temperature of 105° C. for 24 hours to obtain a catalyst on which Pd—Cu particles were supported.

[Comparative Example 9]
In this comparative example, a granular sponge copper catalyst containing 30% by weight of aluminum was used. A granular sponge copper catalyst was prepared according to Example 1 of JP-A-2012-148219. That is, alloy grains were prepared with a weight ratio of copper and aluminum of 70:30, and the alloy grains were immersed in a sodium hydroxide solution to obtain a granular sponge copper catalyst.

Claims (6)

Activated carbon having an average particle diameter of 40 to 200 μm, a pore volume of 0.4 to 1.0 mL/g, a specific surface area of 800 to 1500 m ² /g, and an average pore diameter of 10 to 80 nm ;
Containing metal particles with an average particle diameter of 1 to 30 nm containing a Cu-Pd alloy supported in the pores of the activated carbon,
The supported amount of the metal particles is 0.1 to 3% by mass,
A nitric acid nitrogen decomposition catalyst, wherein the ratio of the Pd component to the sum of the Cu component and the Pd component contained in the metal particles is 60 to 98% by mass . The nitric acid nitrogen decomposition catalyst according to claim 1, wherein the activated carbon has a packing density of 0.4 to 0.8 g/mL. The nitric acid nitrogen decomposition catalyst according to claim 1 or 2, wherein the sum of the Cu component and the Pd component contained in the metal particles is 95% by mass or more. The nitric acid nitrogen decomposition catalyst according to any one of claims 1 to 3, wherein the metal particles have a circularity of 0.6 to 1.0. The nitric acid nitrogen decomposition catalyst according to any one of claims 1 to 3, wherein the activated carbon has a circularity of 0.8 to 1.0. The nitric acid nitrogen decomposition catalyst according to any one of claims 1 to 3, which has a compressive strength of 10 gf/mm ² or more.