JP2022099512A - Hydrogen combustion catalyst, and hydrogen combustion method - Google Patents

Abstract

To provide a hydrogen combustion catalyst for processing gas containing iodine and hydrogen that is more resistant to catalyst poisoning of iodine than conventional ones.SOLUTION: A hydrogen combustion catalyst of the invention has catalyst metal particles made of any noble metal of Rh, Pt, and Ru, or an alloy thereof carried by a carrier made of an inorganic oxide. The hydrogen combustion catalyst has 20 or more of the catalyst metal particles of a diameter of 20 nm or more and 200 nm or less that are observed within the visual field of 3 μm×3 μm when observed at X30,000 magnification. Preferably, the hydrogen combustion catalyst uses an inorganic oxide containing α-alumina of 50 mass% or more as the carrier. More preferably, a hydrophobic functional group of a low molecular weight of 7 carbons or less is bonded to the carrier.SELECTED DRAWING: Figure 4

Description

The present invention relates to a hydrogen combustion catalyst and a hydrogen combustion method having excellent toxicity resistance to iodine catalysts. More specifically, the present invention relates to a hydrogen combustion catalyst in which the decrease in activity due to catalyst poisoning of iodine is suppressed more than before.

In recent years, nuclear power plants have been implementing or considering the introduction of a hydrogen combustion device (igniter) as a measure to prevent hydrogen explosion in a reactor building in the event of a nuclear accident. A hydrogen combustion device is a device that oxidatively burns hydrogen (including isotope hydrogen) and changes (recombusts) it into water (steam) to reduce the hydrogen concentration in the gas. It is expected as an effective hydrogen explosion prevention measure even in the state of power loss at the time of an accident. The main configuration of the hydrogen combustion device is a catalyst layer (catalyst plate) filled with a catalyst (hydrogen combustion catalyst). As the hydrogen combustion catalyst used for this catalyst layer, a noble metal catalyst in which a noble metal such as Pt or Pd is supported as a catalyst metal on an inorganic oxide carrier is known.

Among the nuclear accidents at nuclear power plants, the core meltdown is of particular concern. When a core meltdown occurs, a large amount of hydrogen is generated due to the reaction between the fuel rod cladding tube (zirconium alloy, etc.) and water / steam, or the reaction between the dropped molten core and concrete, etc. The risk of hydrogen explosion due to this large amount of hydrogen generated increases. Therefore, the hydrogen combustion catalyst in the hydrogen combustion apparatus is required to have stable catalytic activity.

By the way, when core meltdown occurs, it is predicted that a large amount of radioactive iodine will be generated at the same time as a large amount of hydrogen. A large amount of iodine generated acts as a catalyst poisoning on the catalyst of the hydrogen combustion device, and may cause a decrease in the activity of the catalyst. The decrease in activity due to catalyst poisoning of iodine is irreversible and continues even when the reaction temperature is high. Therefore, it cannot be dealt with by adjusting the operating conditions (reaction conditions) of the hydrogen combustion device. Therefore, in the treatment of the hydrogen-containing gas containing iodine, it is necessary to equip the hydrogen combustion catalyst with iodine toxicity resistance.

The applicants of the present application disclose the hydrogen combustion catalyst of Patent Document 1 as a hydrogen combustion catalyst on which the above-mentioned noble metal catalyst particles are supported, which is provided with iodine toxicity resistance. This hydrogen combustion catalyst simultaneously supports Pt and Pd as catalyst metal particles, and further contains a certain amount of chlorine in the catalyst.

Japanese Patent No. 5780536

The above-mentioned conventional hydrogen combustion catalyst having iodine toxicity resistance can exhibit hydrogen combustion activity against a gas containing iodine and hydrogen without being easily affected by iodine. However, considering the seriousness of the impact of severe accidents such as core meltdown on the vicinity of nuclear power plant facilities, it is necessary to take measures assuming a more severe poisoned environment. In this respect, the conventional hydrogen combustion catalyst does not mean that iodine catalyst poisoning does not occur at all. According to the studies by the present inventors, it has been confirmed that when the above hydrogen combustion catalyst is exposed to a gas containing a high concentration of iodine, inactivation occurs due to catalyst poisoning.

The present invention has been made based on the above background, and in a hydrogen combustion catalyst for treating a gas containing iodine and hydrogen, the iodine catalyst poisoning resistance property is superior to that of the conventional one, which is good. Provided are those capable of maintaining hydrogen combustion activity. Then, a hydrogen combustion method to which the hydrogen combustion catalyst is applied is provided.

In order to solve the above problems, the present inventors have made a study from reviewing the manufacturing process and the composition of the catalyst particles, based on the hydrogen combustion catalyst using noble metals such as Pt and Rh as catalyst metal particles, as in the conventional case. rice field. As a result, it was found that the iodine-resistant catalyst toxicity is improved by limiting the noble metal species constituting the catalyst metal particles to a specific range and making the catalyst metal particles coarser than a predetermined particle size. I came up with the invention.

That is, the present invention relates to a hydrogen combustion catalyst in which a carrier made of an inorganic oxide is supported with a noble metal of Rh, Pt, Ru, or catalyst metal particles made of an alloy thereof, and the hydrogen combustion catalyst has a magnification of 30,000. It is a hydrogen combustion catalyst characterized in that the number of the catalyst metal particles having a particle size of 20 nm or more and 200 nm or less observed in a field range of 3 μm × 3 μm when observed at a magnification of 20 or more is 20 or more.

As described above, the hydrogen combustion catalyst according to the present invention is characterized in that the particle size of the catalyst metal particles made of a predetermined noble metal (Rh, Pt, Ru) is intentionally coarse. Not limited to hydrogen combustion catalysts, general catalysts generate a large amount of catalytic metal particles finely divided to several nm to 10 nm or less, and increase the ratio to disperse the fine particles to increase the active site. The catalytic activity is ensured. Contrary to this general finding, the hydrogen combustion catalyst according to the present invention maintains catalytic activity while improving iodine toxicity resistance by producing coarse catalytic metal particles having a particle size of 20 nm or more. .. The reason why iodine toxicity resistance is improved in coarse catalyst metal particles of 20 nm or more is not always clear. In this regard, the present inventors have stated that coarse catalyst metal particles are less likely to be adsorbed by iodine molecules than catalyst metal particles having a size of several nm, or are less susceptible to the adsorption of iodine molecules. Consider that it is because of it.

The hydrogen combustion catalyst according to the present invention will be described in detail. The hydrogen combustion catalyst according to the present invention is basically configured by supporting catalyst metal particles on an inorganic oxide carrier as in the prior art. Hereinafter, these configurations will be described.

A. Catalyst metal particles A-1. Composition of Catalyst Metal Particles The catalyst metal particles of the hydrogen combustion catalyst according to the present invention are made of any of Rh (lodium), Pt (platinum), Ru (ruthenium) noble metal or an alloy thereof. In the present invention, iodine toxicity resistance is ensured by controlling the particle size of the catalyst metal particles, but the hydrogen combustion activity is exhibited by any one of these noble metals or an alloy. The catalyst metal particles can be composed of these noble metals alone. In this respect, it is different from a catalyst that is premised on supporting both Pt and Pd, such as the hydrogen combustion catalyst described in Patent Document 1. However, two or more kinds of the above precious metals may be supported.

When two or more kinds of noble metals are supported, the catalyst metal particles made of each noble metal may be independently mixed, or the catalyst metal particles made of an alloy of these noble metals may be present. As the noble metal alloy, Rh-Pt alloy, Pt-Ru alloy, Rh-Ru alloy and the like are preferable. These catalyst metal particles are confirmed to be in a metallic state by X-ray diffraction analysis (XRD) or the like.

In the present invention, among the noble metals, Pd (palladium) is excluded from the constituent metals of the catalyst metal particles. Although the catalytic metal particles made of Pd can exhibit hydrogen combustion activity, it is difficult to coarsen them to 20 nm or more. Therefore, it becomes difficult to secure the toxicity of the iodine catalyst. Therefore, it can be said that the present invention is different from the hydrogen combustion catalyst described in Patent Document 1 in that Pd is excluded.

A-2. Particle size of catalyst metal particles A-2-1. Catalyst metal particles with a particle size of 20 nm or more and 200 nm or less In the hydrogen combustion catalyst according to the present invention, the presence of coarse catalytic metal particles having a particle size of 20 nm or more and 200 nm or less is essential as the catalyst metal particles composed of the above-mentioned noble metal. .. As described above, it is because the coarse catalyst particles having a diameter of 20 nm or more can exhibit the iodine catalyst toxicity resistance while having the catalytic activity (hydrogen combustion activity). The coarse catalyst metal particles essential in the present invention may include catalyst metal particles having a diameter of 40 nm or more and 50 nm or more. However, even if it exceeds 200 nm, the toxicity of the iodine catalyst cannot be particularly improved. Further, it is difficult to generate coarse catalyst metal particles exceeding 200 nm.

The hydrogen combustion catalyst according to the present invention is specified by the number of catalyst metal particles (number of particles) under predetermined observation conditions. Specifically, when the hydrogen combustion catalyst is observed at a magnification of 30,000 times, the number of catalyst metal particles having a particle size of 20 nm or more and 200 nm or less that can be observed within a visual field range of 3 μm × 3 μm is required to be 20 or more. do.

As a means for specifying the hydrogen combustion catalyst of the present invention, the observation result at a magnification of 30,000 times is applied in order to capture and measure coarse catalyst metal particles having a magnification of 20 nm or more in a wide range. As a specific means of this observation, it is preferable to apply the observation result by SEM (scanning electron microscope) at a magnification of 30,000 times. The magnification of 30,000 times in SEM observation is a relatively low magnification, and is not suitable for capturing and observing fine particles of 10 nm or less. However, according to the present inventors, fine particles of 10 nm or less are elements whose existence can be ignored from the viewpoint of iodine-resistant catalyst toxicity. Therefore, the present invention is to confirm the presence or absence of catalyst metal particles having a particle size of 20 nm or more that can exhibit iodine-resistant catalyst toxicity by SEM observation at a low magnification in a wide range.

The detailed method of SEM observation in the hydrogen combustion catalyst in the present invention is as follows. Naturally, the SEM apparatus used for SEM observation is not limited. The observation magnification is set to 30,000 times, and the acceleration voltage at this time is preferably 3 KeV or more and 15 keV or less. In the SEM observation, the catalyst metal particles are observed at least in the vicinity of the center of the visual field range of 3 μm × 3 μm, and then the catalytic metal particles are observed in the entire visual field range.

From the obtained SEM observation image, the particle size and the number of particles of the catalyst metal particles are measured. The measurement of the particle size of the catalyst metal particles in the present invention is based on the biaxial averaging method. In the biaxial averaging method, the particle size of the catalyst metal particles is calculated by averaging ((l ₁ + l ₂ ) / 2) of the major axis (l ₁ ) and the minor axis (l ₂ ) of the particles. Here, the shape of the catalyst metal particles of the hydrogen combustion catalyst according to the present invention is close to spherical, but elliptical and amorphous particles are also mixed. In particular, in the hydrogen combustion catalyst of the present invention, rod-shaped particles having a large aspect ratio may be observed. The rod-shaped catalyst metal particles are catalyst metal particles produced by close contact and fusion of a plurality of catalyst metal particles in the catalyst manufacturing process. Proximity and fusion of catalytic metal particles tend to occur, especially with the application of carriers with a small specific surface area. If the specific surface area of the carrier is small, the distance between the particles of the catalyst metal particles becomes narrow, so it is considered that the particles move and fuse due to the heat treatment in the catalyst manufacturing process to generate rod-shaped catalyst metal particles.

Regarding the counting of the number of catalyst metal particles, basically, the catalyst metal particles having a particle size of 20 nm or more by the biaxial averaging method are regarded as one particle. However, it is not preferable to count the rod-shaped catalyst metal particles as they are as one particle. This is because the rod-shaped catalyst metal particles are originally composed of a plurality of particles, and if the particle size is measured while counting the particles as one particle, an appropriate particle size distribution cannot be obtained.

Therefore, in the present invention, when measuring the number of particles and the particle size of the catalyst metal particles, first, the aspect ratio (l ₁ / l ₂ ), which is the ratio between the major axis (l ₁ ) and the minor axis (l ₂ ) of each particle, is determined. Confirm. Then, the catalyst metal particles having an aspect ratio of less than 2 are counted as 1 (pieces) as a single catalyst metal particle. Then, the particle size of the catalyst metal particles is calculated from the major axis and the minor axis. On the other hand, the catalytic metal particles having an aspect ratio of 2 or more are determined to be particles composed of a plurality of catalytic metal particles, and the number n (l ₁ / l ₂ : n) obtained by dividing the major axis by the minor axis is a natural number. Fractions less than 1 are rounded down), and the number of particles is counted as n. Then, the particle size of the divided catalyst metal particles is calculated from the divided major axis (l ₁ / n) and minor axis. Under the above conditions, the observed particle size and number of catalyst metal particles are measured, and a particle size distribution showing the number of particles for each particle size is created. When creating this particle size distribution, it is preferable that the calculated particle size value is rounded off (rounded or the like) to integrate the number of particles for each particle size in units of 10 nm.

Under the above detailed conditions, the presence or absence of catalyst metal particles having a particle size of 20 nm or more and the number of particles can be obtained in SEM observation at a magnification of 30,000 times. In the present invention, the number of observed catalyst metal particles having a particle size of 20 nm or more is set to 20 or more. This is because that. The number of the observed catalyst metal particles of 20 nm or more is preferably 40 or more, more preferably 60 or more. The upper limit of the number of catalyst metal particles having a particle size of 20 nm or more should not be limited, but is preferably 200 or less from the viewpoint of manufacturing possibility. In the particle size distribution of the catalyst metal particles under the above observation conditions in the hydrogen combustion catalyst according to the present invention, the proportion of the catalyst metal particles having a particle size of 20 nm to 30 nm tends to be relatively high. However, in a catalyst showing more preferable iodine-resistant catalyst toxicity, it has been confirmed that, in addition to the catalyst metal particles having a particle size of 20 nm to 30 nm, catalyst metal particles having a coarser particle size of 40 nm and 50 nm are also produced.

In the observation result at a magnification of 30,000 times described above, particles having a particle size of less than 20 nm may be observed. As will be described later, even in the hydrogen combustion catalyst of the present invention, the presence of catalyst metal particles having a particle size of less than 20 nm cannot be denied. However, in the SEM observation in the present invention, it is not necessary to measure the number and particle size of the catalytic metal particles having a particle size of less than 20 nm.

A-2-2. Catalyst metal particles with a particle size of less than 20 nm As described above, the hydrogen combustion catalyst according to the present invention is essential to include catalyst metal particles having a particle size of 20 nm or more, but the particle size is less than 20 nm, particularly the particle size of 1 nm or more. It does not deny the existence of fine catalytic metal particles of 10 nm or less. These fine catalyst metal particles are only inferior in iodine poisoning resistance, and even if they are deactivated by iodine poisoning, they do not adversely affect the activity of the catalyst metal particles having a particle size of 20 nm or more. Is. Further, even if the catalyst metal particles are intentionally coarsened in the manufacturing process, the formation of fine particles cannot be completely suppressed. Therefore, in the present invention, the catalyst metal particles having a particle size of less than 20 nm may be included together with the catalyst metal particles having a particle size of 20 nm.

It is extremely difficult to specify the catalyst metal particles having a particle size of less than 20 nm by the above-mentioned SEM observation (magnification: 30,000 times). Therefore, it is preferable to examine the identification of the catalyst metal particles having a particle size of less than 20 nm by a TEM (transmission electron microscope). In this case, it is preferable to observe the presence or absence of fine particles and the distribution state at a magnification of 500,000 times or more and 2000,000 times or less as the magnification of TEM observation. Under this high magnification observation, the visual field range is 0.4 μm × 0.6 μm to 0.08 μm × 0.12 μm.

With respect to the present invention, the number of particles of the catalyst metal particles having a particle size of less than 20 nm, which is poorly toxic to the iodine catalyst, is not a significant factor in itself. However, when observing the fine particles, it is useful to evaluate the presence or absence of coarse particles having a particle size of 20 nm or more and the relationship with the fine particles in the evaluation of the iodine-resistant toxicity of the hydrogen combustion catalyst.

When observing the hydrogen combustion catalyst according to the present invention at high magnification by TEM, the particle size range of the catalyst metal particles to be observed is preferably 1 nm or more and 30 nm or less. Contrary to the above-mentioned SEM observation (magnification 30,000 times), TEM observation is not suitable for observation of coarse particles. Observation of catalyst metal particles having a particle size of 20 nm or more and 30 nm or less is sufficient for confirming the presence or absence of coarse catalyst metal particles and the amount thereof in TEM observation.

When the number of particles of the catalyst metal particles having a particle size of 1 nm or more and 30 nm or less is measured in the hydrogen combustion catalyst according to the present invention, the total number of particles of the catalyst metal particles having a particle size of 20 nm or more and 30 nm or less is 1 nm or more and 30 nm or less. The ratio of the catalyst metal particles to the total number of particles is preferably 1.0% or more. The ratio based on the number of catalyst metal particles of 20 nm or more and 30 nm or less is more preferably 1.5% or more.

Further, the ratio of the catalyst metal particles of 20 nm or more and 30 nm or less can be evaluated based on the volume of the catalyst metal particles. That is, in the hydrogen combustion catalyst according to the present invention, when the volumes of the catalytic metal particles having a particle size of 1 nm or more and 30 nm or less are measured, the total volume of the catalytic metal particles having a particle size of 20 nm or more and 30 nm or less is 1 nm or more and 30 nm or less. It is preferable that the ratio of the catalyst metal particles to the total volume of the catalyst metal particles is 30% or more. The volume-based ratio of the catalyst metal particles of 20 nm or more and 30 nm or less is more preferably 50% or more.

In a general catalyst such as the above-mentioned conventional hydrogen combustion catalyst (Patent Document 1), fine catalyst metal particles having a particle size of 10 nm or less are mainly used, and the number of catalyst metal particles having a coarser particle size of 20 nm or more is mainly used. The ratio is extremely low. The number ratio of the catalyst metal particles having a particle size of 20 nm or more and 30 nm or less, which is preferable in the present invention, is clearly distinguished from this conventional hydrogen combustion catalyst.

In the above observation of fine particles having a particle size of less than 20 nm by TEM, it is preferable to apply the observation conditions in the above-mentioned SEM observation to the measurement of the particle size and the number of particles. The biaxial averaging method can be applied to measure the particle size. Further, even in high-magnification observation by TEM, there is a possibility that rod-shaped particles due to fusion of a plurality of catalyst metal particles can be seen, but in that case, the number of particles and the particle size can be calculated by the above-mentioned division. The volume of the catalyst metal particles can be calculated based on the particle size measured by approximating the particles to a sphere. Then, depending on the particles, particle size, and volume of the catalytic metal particles measured and calculated, it is possible to obtain the volume-based and particle number-based ratios of the catalytic metal particles having a particle size of 20 nm or more and 30 nm or less.

B. Inorganic oxide carrier In the hydrogen combustion catalyst according to the present invention, the application of an inorganic oxide as a carrier has no risk of ignition even when local heating due to reaction heat occurs in the catalyst layer, and is a radioactive substance. This is because there is no risk of radiation damage due to. Further, the inorganic oxide carrier is often used as a carrier for various catalysts, and is excellent in cost and handleability at the time of catalyst production. In the conventional hydrogen combustion catalyst, there is an example in which a hydrophobic resin such as a styrene-divinylbenzene copolymer (SDB) is used as a carrier, but in the present invention, a resin carrier is not applied from the viewpoint of durability. This is because the resin carrier is vulnerable to heat and radiation, and there is a risk of combustion damage due to the reaction heat in the reaction layer and deterioration in the radiation environment.

B-1. Carrier composition As the inorganic oxide used as the carrier, alumina (α-alumina, γ-alumina) which is aluminum oxide (Al ₂ O ₃ ), silicon oxide (SiO ₂ : silica), and porous crystalline aluminosilicate (polycarbonate). Examples thereof include inorganic oxides such as M ^{n +} _{x / n} Al _x Si _y O _{2x + 2y} ^x -zH ₂ O: zeolite), zirconium oxide (ZrO ₂ : zirconia), and titanium oxide (TIO ₂ : titania). These inorganic oxides have been conventionally used as catalyst carriers and are excellent in porosity and heat resistance. In the present invention, the above-mentioned inorganic oxides can be used alone or in combination. A particularly preferable inorganic oxide carrier in the present invention is an inorganic oxide containing α-alumina. When an inorganic oxide containing α-alumina is used as a carrier, the content of α-alumina is preferably 50% by mass or more and 100% by mass or less with respect to the total mass of the carrier. In this case, examples of the inorganic oxide other than α-alumina include the above-mentioned inorganic oxides. For example, according to the study by the present inventors, examples of the inorganic oxide carrier containing α-alumina having high crystallinity include a mixture of silica and α-alumina. Such an inorganic oxide carrier containing α-alumina can be formed by mixing and treating α-alumina powder and powder of another inorganic oxide. Further, after supporting the catalyst metal particles on the α-alumina powder or the α-alumina powder carrier, a solution containing another inorganic oxide may be added. Specifically, as an example of the hydrophobic treatment of the catalyst described later, a silane coupling agent is added after supporting the catalyst metal particles on the α-alumina powder carrier. By this treatment, silica derived from the silane coupling agent is contained in the carrier. A catalyst having such a carrier is also a preferred embodiment of the present invention. In such a case, the content of α-alumina is preferably 50% by mass or more.

B-2. Specific surface area of the carrier The specific surface area of the inorganic oxide carrier is preferably 50 m ² / g or less. In the present invention, it is required that a predetermined number or more of catalyst metal particles having a particle size of 20 nm or more are produced. As will be described later, in the hydrogen combustion catalyst according to the present invention, the salt of the catalyst metal supported by the impregnation method is heat-treated to obtain catalyst metal particles. In order to coarsen the catalyst metal particles, it is necessary to move and bond the catalyst metal particles in close proximity during the heat treatment. By reducing the specific surface area of the carrier, the distance between the particles of the catalyst metal particles is shortened, so that the coarsening of the catalyst metal particles can be promoted by the above-mentioned movement / bonding. The specific surface area of the carrier is more preferably 30 m ² / g or less. The lower limit of the specific surface area of the carrier is preferably 0.1 m ² / g or more. This is because if the specific surface area of the carrier is too small, the effective active area as a catalyst is reduced, and the hydrogen combustion activity itself, which should be exhibited originally, may be deteriorated. The specific surface area of the inorganic oxide carrier can be measured by a gas adsorption method to which _N2 gas or the like is applied, and the value of the specific surface area can be obtained from the analysis of the measurement results by the BET multipoint method or the like.

The specific surface area of the inorganic oxide carrier is determined by the particle size of the inorganic oxide particles and the state of the pores on the surface. Therefore, in the present invention, the particle size of the inorganic oxide carrier is preferably 0.1 μm or more and 10 μm or less. Examples of the inorganic oxide carrier that easily develops the above-mentioned suitable specific surface area include α-alumina. Therefore, an inorganic oxide containing α-alumina in an amount of 50% by mass or more and 100% by mass or less with respect to the total mass of the above-mentioned carrier and having the specific surface area is a particularly preferable carrier.

The shape of the inorganic oxide carrier is not particularly limited. The shape of the carrier is generally one in which inorganic oxide particles are formed into cylindrical or spherical pellets. It is also generally known that an appropriate support such as a honeycomb or a net is coated with inorganic oxide particles and the coating layer is used as a carrier.

B-3. Hydrophobicization treatment on the carrier In the hydrogen combustion catalyst according to the present invention, it is preferable that the inorganic oxide carrier is hydrophobized. In the hydrogen combustion apparatus equipped with the hydrogen combustion catalyst of the present invention, water in the atmosphere (water mist, water vapor) and water generated by the hydrogen combustion reaction come into contact with the catalyst. These waters cover the active sites of the catalytic metal and cause a decrease in catalytic activity. In addition, a reaction between hydrogen and iodine in the gas to be treated may occur, and at this time, hydrogen iodide (HI) is generated. This hydrogen iodide reacts with the above-mentioned water to become hydrogen iodide acid. Hydrogen iodide acid will erode the inorganic oxide carrier and cause damage to the catalyst. For these reasons, the inorganic oxide carrier of the hydrogen combustion catalyst is preferably hydrophobized so as to be able to effectively discharge water.

As the hydrophobicized inorganic oxide carrier, a carrier surface to which a hydrophobic small molecule functional group having 1 or more and 7 or less carbon atoms is imparted is preferable. Examples of the hydrophobic low molecular weight functional group include an alkyl group, a vinyl group, an epoxy group, an acrylic group and the like. The reason why the carbon number of the low molecular weight functional group is 1 or more and 7 or less is that the functional group having more than 7 carbon atoms tends to lose the hydrophobicity when the temperature of the catalyst becomes high, thereby reducing the catalytic activity. This is because there is a risk. Preferred specific examples of the hydrophobic small molecule functional group include a methyl group, an ethyl group and a propyl group. These alkyl groups are excellent in the effect of lowering the polarity of the surface of the carrier, and can effectively promote the discharge of water molecules from the carrier. At least one of these functional groups may be added to the carrier, but it may have a plurality of functional groups.

The hydrophobic small molecule functional group does not contain a hydroxyl group (OH). This is because the hydrogen iodide acid generated by the above-mentioned reaction between hydrogen and iodine may generate hydrogen iodide acid by reacting with a hydroxyl group, and in this case as well, there is a risk of carrier erosion.

It is presumed that the low molecular weight functional group that makes the above-mentioned inorganic oxide carrier hydrophobic is present on the surface of the inorganic oxide. In the hydrogen combustion catalyst of the present invention, as a method of observing and confirming the presence of a hydrophobic functional group, an infrared absorption spectrum when the catalyst is irradiated with infrared rays may be measured. Fourier Transform Infrared Spectroscopy (FT-IR) is well known as a method for measuring an infrared absorption spectrum. When analyzing a catalyst containing a hydrophobic functional group by this measuring method, an infrared absorption peak is observed in the vicinity of 2900 to 3000 cm ^-1 , which confirms the presence of the functional group.

C. The amount of the catalyst metal particles supported The amount of the catalyst metal particles made of the noble metal described above supported on the inorganic oxide carrier is 0.1% by mass to 5.0% by mass based on the total mass of the catalyst. Is preferable. This carrying amount is the content of all the noble metals in the catalyst, and is not only the amount of the noble metal constituting the catalyst metal particles having a particle size of 20 nm or more, but also the amount of the noble metal constituting the fine catalyst metal particles having a particle size of 20 nm or less. Including quantity.

When the hydrogen catalyst according to the present invention is applied and laminated on a support such as a metal honeycomb, the total amount of the catalyst metal supported should be adjusted to 0.5 to 10 g / L per volume of the structure. preferable. However, this condition is based on the premise that the amount of the noble metal supported is 0.1% by mass to 5.0% by mass with respect to the total mass of the catalyst coated on the support.

D. Method for Producing Hydrogen Combustion Catalyst According to the Present Invention Next, a method for producing the hydrogen combustion catalyst according to the present invention will be described. The hydrogen combustion catalyst according to the present invention can be manufactured basically by the same method as the conventional hydrogen combustion catalyst. That is, a method is used in which a solution of a metal compound of a metal (Rh, Pt, Ru) constituting the catalyst metal particles is contacted and impregnated with an inorganic oxide carrier, and then an atomic metal is supported by heat treatment. Then, as a preferred form of the catalyst, a hydrophobic small molecule functional group is imparted to the catalyst.

As the metal compound used in the step of impregnating the inorganic oxide carrier with the metal compound solution, rhodium chloride, rhodium nitrate or the like is preferable as the Rh compound. As the Pt compound, platinum compounds such as platinum chloride acid, didinitroplatinum diamine, tetraammine platinum dichloride, hexaammine platinum tetrachloride, potassium chloride platinum acid, and platinum chloride are preferable. As the Ru compound, ruthenium chloride, ruthenium nitrate and the like are preferable. As the solvent for solving these metal compounds, in addition to an organic solvent such as water or alcohol, a mixed solvent of water and an organic solvent can be used.

The metal compound concentration of the metal compound solution is adjusted according to the amount of the catalyst metal supported. Preferably, the concentration of the solution is 0.04 to 15.5% by mass. Further, when the inorganic oxide carrier is impregnated with the metal compound solution, the amount of the metal compound solution is preferably 60% to 150% of the water absorption amount of the inorganic oxide carrier. If an excessive amount of liquid is impregnated with respect to the amount of water absorbed, the metal concentration on the carrier tends to be uneven. If the metal concentration is non-uniform, a region where coarse particles are not formed may occur, and it may not be possible to impart uniform catalytic activity to the catalyst after production. Therefore, it is preferable to limit the amount of liquid at the time of impregnation.

When a plurality of catalyst metal particles are supported, each metal compound solution may be adsorbed individually, and the order thereof does not matter. Further, a mixed solution obtained by mixing each compound solution may be adsorbed on the carrier. As a method for adsorbing the solution, the carrier may be immersed in the solution, or the solution may be dropped onto the carrier.

After impregnating the inorganic oxide carrier with the above metal compound solution, catalyst metal particles are generated as metal atoms by heat treatment. In the production of the hydrogen combustion catalyst of the present invention, drying is performed by evaporating and volatilizing the solvent before heat treatment. It is preferable to carry out the treatment. In this drying treatment, coarse catalyst metal particles (20 nm or more and 100 nm or less) are effectively formed, so that it is preferable to make the drying conditions appropriate.

As a condition for the drying treatment before the heat treatment, it is preferable that the inorganic oxide carrier after impregnation has a relatively low temperature of 50 ° C. or higher and 100 ° C. or lower. When the inorganic oxide carrier impregnated with the metal compound solution is heated and dried, the water (solvent) does not evaporate uniformly from the surface of the carrier, and easily evaporates such as corners of irregularities on the surface of the carrier. Evaporates preferentially from (drying point). Then, the metal compound solution around the drying point moves to the vicinity of the drying point and dries. The cycle of movement of the metal salt solution and drying at this drying point causes the metal compound to aggregate near the drying point. By setting the drying temperature to a relatively low temperature, it is possible to adjust the progress rate of the cycle to promote aggregation of the metal compound and form coarse catalytic metal particles.

Further, in this drying treatment, it is more preferable to heat the inorganic oxide carrier impregnated with the metal compound solution in a state of vibration, shaking, and rotation. By drying the inorganic oxide carrier under a moving state rather than a stationary state, the movement and aggregation of the above-mentioned metal compound are promoted, and coarse catalyst metal particles are effectively formed. In the above-mentioned drying treatment performed at a relatively low temperature, the drying treatment time is preferably 1 hour or more and 24 hours or less. This is to sufficiently evaporate and volatilize the solvent while treating at a relatively low temperature. In this drying treatment, it is preferable to sufficiently dry the inorganic oxide carrier impregnated with the metal compound solution. As a specific standard, it is preferable to dry the inorganic oxide carrier before the water-containing drying treatment until it shows a weight loss rate of 95% or more and 99.5% or less.

Then, by heat-treating the inorganic oxide carrier on which the metal compound after the drying treatment is supported, atomic metals are precipitated and catalytic metal particles are generated. Further, when two or more kinds of metal atoms are supported, alloying may occur in the process of this heat treatment. As for the heat treatment conditions, the heat treatment temperature is preferably 300 to 700 ° C. The reason why the temperature is 300 ° C. or higher is that the carried metal compound is decomposed into catalytic metal particles, and the particle size thereof is coarsened. Further, when the heat treatment is performed at 700 ° C. or higher, the metal particles may be excessively coarsened. The heat treatment time is preferably 0.5 to 10 hours. This is because sufficient catalytic metal is not produced in less than 0.5 hours. Further, the heat treatment for 10 hours or more is not preferable in terms of catalyst production efficiency, energy cost and the like because there is no difference in the effect.

Further, it is necessary to perform this heat treatment under an appropriate reducing atmosphere. A suitable heat treatment atmosphere includes a mixed gas atmosphere containing 3 to 100% by volume of hydrogen and the balance being an inert gas (nitrogen, argon, etc.). Regarding this mixed gas, the lower limit of the hydrogen concentration is set to 3% by volume because the decomposition of the metal compound does not easily proceed in an atmosphere where the hydrogen concentration is too low.

By the above heat treatment, a hydrogen combustion catalyst having a high proportion of coarse catalyst metal particles specified in the present invention can be obtained. Then, by imparting a hydrophobic small molecule functional group to this hydrogen combustion catalyst, a hydrophobic hydrogen combustion catalyst can be obtained.

In the hydrophobic treatment of the catalyst, a method of immersing the hydrogen combustion catalyst produced above in a solution of a compound having a low molecular weight functional group such as an alkyl group having 7 or less carbon atoms at the terminal is preferable. Thereby, a functional group can be imparted to the surface of the inorganic oxide carrier.

As the compound for the hydrophobic treatment, a silane inorganic surface modifier is preferable, and as a silane inorganic surface modifier having a low molecular weight hydrophobic functional group at the terminal, trimethylmethoxysilane, trimethylethoxysilane, trimethylchlorosilane, dimethyl Dimethoxysilane, dimethyldiethoxysilane, dimethyldichlorosilane, methyltrimethoxysilane, methyltriethoxysilane, methyltrichlorosilane, triethylmethoxysilane, triethylethoxysilane, triethylchlorosilane, diethyldimethoxysilane, diethyldiethoxysilane, diethyldichlorosilane, Ethyltrimethoxysilane, ethyltriethoxysilane, ethyltrichlorosilane, tripropylmethoxysilane, tripropylethoxysilane, tripropylchlorosilane, dipropyldimethoxysilane, dipropyldiethoxysilane, dipropyldichlorosilane, propyltrimethoxysilane, propyl Either triethoxysilane or propyltrichlorosilane is preferable. Compounds having a propyl group include not only linear compounds but also branched compounds.

As a specific method of the hydrophobizing treatment, the catalyst is immersed in a solution in which the above-mentioned compound containing a functional group is dissolved in a solvent. After that, the catalyst is taken out from the solution, and washed and dried as appropriate. In the hydrogen combustion catalyst according to the present invention, it is preferable that the catalyst is completely substituted with hydroxyl groups on the surface of the carrier. The amount of the compound to be mixed in the solution can be calculated from the coating area (m ² / g) specified for each compound, the weight (g) of the carrier and the specific surface area (m ² / g) ( (Carrier weight x carrier specific surface area) / coating area of the compound), approximately 1.0 to 100 g of the compound is used with respect to 100 g of the carrier. The amount of the solution (solvent) is preferably such that the carrier is completely immersed.

As described above, in the present invention, the hydrophobization treatment is preferably performed after the hydrogen combustion catalyst is manufactured (after the catalyst metal particles are supported on the carrier). It is possible to impart a functional group to the inorganic oxide carrier itself by treating the inorganic oxide carrier before the production of the catalyst with a silane inorganic surface modifier or the like. However, in the catalyst manufacturing process of the present invention, as described above, the temperature up to 700 ° C. is assumed as the temperature at which the inorganic oxide carrier carrying the metal compound is heat-treated, and the heat treatment is performed at 400 ° C. or higher. There is also. Since the hydrophobic group may be damaged at such a high temperature, it is preferable to perform the hydrophobic treatment after the catalyst is manufactured.

E. Method for Treating Hydrogen-Containing Gas Using Hydrogen Combustion Catalyst According to the Present Invention The hydrogen combustion catalyst according to the present invention described above is suitable for a hydrogen combustion process in which a hydrogen-containing gas containing iodine is treated. In this hydrogen combustion method, the hydrogen-containing gas is passed through the hydrogen combustion catalyst, whereby hydrogen in the hydrogen-containing gas is burned. The reaction temperature in the present invention is preferably 50 to 300 ° C. Further, since the hydrogen combustion catalyst of the present invention can continue the combustion reaction even at a relatively low temperature, the hydrogen combustion reaction can be continued even at a relatively low reaction temperature of 50 ° C. or lower.

The hydrogen combustion catalyst of the present invention is excellent in iodine resistance to toxicity, and is effective against a hydrogen-containing gas having an iodine concentration of 0.01 ppm or more. Regarding the upper limit of the iodine concentration of the gas to be treated, even if it is 30 ppm, the decrease in activity due to catalyst poisoning is suppressed.

Further, the hydrogen combustion catalyst according to the present invention can suppress the influence of water produced by the hydrogen combustion reaction and water in the atmosphere by subjecting the hydrogen combustion catalyst to a hydrophobic treatment. In the hydrogen combustion method of the present invention, even if the hydrogen-containing gas to be treated contains water corresponding to the saturated water vapor amount at the reaction temperature, it can be effectively treated.

As described above, in the hydrogen combustion catalyst according to the present invention, the proportion of coarse particles (20 nm or more and 100 nm or less) is increased as the particle size distribution of the catalyst metal composed of Rh, Pt, Ru and alloys thereof. It is a thing. As a result, the hydrogen combustion catalyst according to the present invention has excellent catalytic toxicity to iodine and can continuously burn hydrogen without significantly reducing its activity. Further, the catalyst according to the present invention is subjected to a hydrophobic treatment to suppress a decrease in activity due to moisture.

The figure which shows the particle size distribution of the α-alumina carrier used in this embodiment. The figure explaining the SEM image (30000 times) of the hydrogen combustion catalyst of Example 2 of this Embodiment, and the measuring method about the rod-shaped particles. Photographs showing SEM observation images (30000 times) of hydrogen combustion catalysts of Examples 1, 2 and 4 and Comparative Examples 1 and 3 of this embodiment. The figure which shows the particle size distribution of the catalyst metal particle of the hydrogen combustion catalyst of Examples 1, 2 and 4 and Comparative Examples 1 and 3 of this Embodiment obtained by SEM observation (30000 times). Photographs showing TEM observation images (500,000 times) of hydrogen combustion catalysts of Examples 1, 2 and 4 and Comparative Examples 1 and 3 of this embodiment. The figure which shows the particle size distribution (particle size-the number of particles) of the catalyst metal particles of the hydrogen combustion catalyst of Examples 1, 2 and 4 and Comparative Examples 1 and 3 of this Embodiment obtained by TEM observation (500,000 times). It is a figure which shows the particle size distribution (particle size-volume ratio) of the catalyst metal particles of the hydrogen combustion catalyst of Examples 1, 2 and 4 and Comparative Examples 1 and 3 of this Embodiment obtained by TEM observation (500,000 times). The figure which shows the infrared absorption spectrum of the hydrogen combustion catalyst of Example 5 and Comparative Examples 1 and 3 of this Embodiment.

Hereinafter, the best embodiment of the present invention will be described. In this embodiment, α-alumina and γ-alumina are used as the inorganic oxide carrier, and Rh, Pt, and Ru are supported on the α-alumina and γ-alumina to produce a hydrogen combustion catalyst. In addition, the produced hydrogen combustion catalyst that had been hydrophobized was also produced. Then, for each catalyst, the hydrogen combustion activity due to the influence of iodine was evaluated.

Example 1 (Pt / α-Al ₂ O ₃ ) : As an inorganic oxide carrier, 100 g of commercially available α-alumina powder was prepared. FIG. 1 shows the particle size distribution of this α-alumina carrier measured by observation with a scanning electron microscope. The α-alumina carrier of this example had a peak particle size distribution near 400 nm, and the average particle size was about 450 nm. The specific surface area of this α-alumina carrier was about 1 m ² / g.

Then, 25 g of an ethanol chloride solution (Pt concentration 4% by mass, equivalent to 1 g of platinum, 70% of the amount of water absorbed by the carrier) was added to this inorganic oxide carrier and impregnated. Then, a drying treatment was performed. In the drying treatment, the carrier impregnated with the metal compound solution was placed in an evaporator flask, and the flask was immersed in warm water at 70 ° C. to 80 ° C. with the flask tilted 45 degrees. The inside of the flask is 50 mbar. The pressure is reduced to. Then, it was heated while slowly rotating for 8 hours (rotation speed 40 rpm). After this drying treatment, the carrier was placed in a column and 3% by volume hydrogen gas (N ₂ balance) was circulated at 300 ° C. for 2 hours for reduction to produce a hydrogen combustion catalyst.

Example 2 (Rh / α-Al ₂ O ₃ ) : To 100 g of the same α-alumina as in Example 1, 25 g of a rhodium chloride solution (Rh concentration 4% by mass, equivalent to 1 g of rhodium) was added and impregnated. Then, the drying treatment was performed under the same conditions as in Example 1. After the drying treatment, the carrier was placed in a column and 100% by volume hydrogen gas was circulated at 400 ° C. for 2 hours for reduction to produce a hydrogen combustion catalyst.

Example 3 (Ru / α-Al ₂ O ₃ ) : To 100 g of the same α-alumina as in Example 1, 50 g of ruthenium chloride solution (Ru concentration 2% by mass, equivalent to 1 g of ruthenium, 145% of carrier water absorption) is added and impregnated. I let you. Then, the drying treatment was performed under the same conditions as in Example 1. After the drying treatment, the carrier was placed in a column and 100% by volume hydrogen gas was circulated at 600 ° C. for 4 hours for reduction to produce a hydrogen combustion catalyst.

Example 4 (Pt / γ-Al ₂ O ₃ ) : In this example, γ-alumina was used as the inorganic oxide as a carrier. The average particle size of this γ-alumina carrier was about 450 nm. The specific surface area was 50 m ² / g. Then, a hydrogen combustion catalyst was produced by supporting Pt in the same manner as in Example 1.

Example 5 (Rh / Hydrophobicization α-Al ₂ O ₃ ) : A catalyst obtained by hydrophobizing the hydrogen combustion catalyst produced in Example 2 was produced. The hydrophobization treatment was carried out by adding a mixture in which 22.7 g of methyltrimethoxysilane, 60 g of pure water and 60 g of ethanol were uniformly dissolved in 100 g of the catalyst, shaking and stirring. After 1 day, it was taken out, washed with pure water, and dried at 200 ° C. to prepare a hydrogen combustion catalyst with hydrophobization treatment. The weight increase of the catalyst by this hydrophobization treatment was about 1 g, and the content of α-alumina in the carrier was 99% by mass with respect to 100% by mass of Example 2.

Comparative Example 1 (Pt / Hydrophobicized SiO ₂ ) : As a comparative example for each of the above Examples, a catalyst corresponding to the above-mentioned conventional hydrogen combustion catalyst was produced. As a carrier, 100 g of a silica carrier (average particle size 0.3 μm, specific surface area 230 m ² / g) was prepared and treated with hydrophobicity. The hydrophobization treatment was carried out by adding a mixed solution of 40 g of methyltrimethoxysilane, 50 g of pure water and 50 g of ethanol uniformly to the carrier, shaking and stirring. After 1 day, it was taken out, washed with pure water, and then dried at 200 ° C.

Next, 25 g of a platinum chloride ethanol solution (Pt concentration 4% by mass, equivalent to 1 g of platinum) was added to the silica carrier subjected to the hydrophobic treatment and impregnated. Then, it was dried at 120 ° C. in an explosion-proof dryer to evaporate ethanol. At this time, the catalyst is allowed to stand still. Then, it was placed in a column, and 3% by volume hydrogen gas (N ₂ balance) was circulated at 230 ° C. for 2 hours and reduced to produce a hydrogen combustion catalyst.

Comparative Example 2 (PtPd / Hydrophobicized SiO ₂ ) :
A hydrogen combustion catalyst in which a PtPd alloy was supported as catalyst metal particles on the same hydrophobicized silica carrier as in Comparative Example 1 was produced. A solution of 25 g of an ethanol chloride solution (Pt concentration 4% by mass, equivalent to 1 g of platinum) and a solution of 6.25 g of palladium chloride solution (Pd concentration 8% by mass, equivalent to 0.5 g of palladium) are mixed with a hydrophobicized silica carrier. It was added in a liquid state and impregnated (the carrying ratio of platinum and palladium was 1: 1 in terms of molar ratio). Then, it was dried and heat-treated under the same conditions as in Comparative Example 1 to produce a hydrogen combustion catalyst.

Comparative Example 3: (Pd / α-Al ₂ O ₃ ) : A hydrogen combustion catalyst in which Pd was supported as catalyst metal particles on the same α-alumina carrier as in Example 1 was produced. 100 g of the α-alumina carrier was impregnated with a solution of 12.5 g of a palladium chloride solution (Pd concentration: 8% by mass, equivalent to 1 g of palladium). Then, while rotating the catalyst under the same conditions as in Example 1, the catalyst was dried at 70 ° C. and heat-treated at 400 ° C. to produce a hydrogen combustion catalyst.

[Observation of catalyst metal particles, measurement of particle size distribution]
The hydrogen combustion catalysts of Examples and Comparative Examples produced above were observed by SEM and TEM, and the particle size distribution of the catalyst metal particles was examined.

[SEM observation]
SEM observation was performed on each hydrogen combustion catalyst, and the presence or absence of catalyst metal particles of 20 nm or more and the number of particles were measured. As for the observation conditions, a visual field range of 3 μm × 3 μm was observed and imaged at a magnification of 30,000 times (acceleration voltage 15 keV). In the obtained SEM image, the major axis and the minor axis of the visible catalytic metal particles (having about 10 nm or more) were measured, and the particle size was measured by the biaxial averaging method.

An example of measuring the particle size of the catalyst metal particles by SEM observation will be described. FIG. 2 is an SEM image (30000 times) of the catalyst of Example 2 (Rh / α-Al ₂ O ₃ ). In the SEM image of FIG. 2, the white contrast points on the carrier are the catalytic metal particles. In measuring the particle size distribution, the number and particle size (biaxial averaging method) of particles recognizable by this 30,000 times SEM image were measured. Most of the catalyst metal particles have an aspect ratio of less than 2. However, rod-shaped particles with an aspect ratio of 2 or more can also be seen (see FIG. 2). For example, in FIG. 2, rod-shaped particles having a major axis of 150 nm and a minor axis of 50 nm are observed. For such rod-shaped catalyst metal particles having an aspect ratio of 2 or more, as described above, the numerical value obtained by dividing the major axis by the minor axis was used as the number of particles, and the particle size was calculated for each. In the example of FIG. 2, the number of particles was 3, and the particle size was 50 nm for each. Then, a particle size distribution was created based on the measured number and particle size of the catalyst metal particles. At this time, the numerical value of the particle size less than 10 nm was rounded off to obtain the particle size distribution for each 10 nm.

[TEM observation]
TEM observation was carried out for each hydrogen combustion catalyst, and the distribution state of the catalyst metal particles of 30 nm or less was observed. As for the observation conditions, a visual field range of 0.4 μm × 0.6 μm was observed and imaged at a magnification of 500,000 times (acceleration voltage 100 keV). In the obtained TEM image, the particle size of the visible catalyst metal particles was measured. Here, catalyst particles of 1 nm or more and 30 nm or less were measured in units of 1 nm. Then, a particle size distribution of catalyst particles of 1 nm or more and 30 nm or less was created. Regarding the particle size distribution of the child, a distribution map of particle size-number of particles and a distribution map of particle size-volume ratio (total volume ratio of catalytic metal particles belonging to each particle size) were created.

The SEM images of Examples 1, 2 and 4 and Comparative Examples 1 and 3 are shown in FIG. 3 as an example of the results of SEM observation (30000 times) performed for each hydrogen combustion catalyst. Further, FIG. 4 shows the particle size distribution of the catalyst metal particles in Examples 1, 2 and 4 and Comparative Examples 1 and 3 measured based on this SEM image. As can be seen from FIG. 4, in the hydrogen combustion catalysts of Examples 1, 2 and 4, catalyst metal particles having a particle size of 20 nm or more were clearly generated, and it was confirmed that 20 or more coarse particles were generated in each of them. To. In these catalysts, in addition to the catalyst metal particles having a particle size of 20 nm, many catalyst metal particles having a particle size of 30 to 40 nm are found. In particular, in the catalyst to which α-alumina is applied as a carrier (Examples 1 and 2), the formation of catalyst metal particles having a particle size of 50 nm or more has also been confirmed.

On the other hand, in the case of the conventional hydrogen combustion catalyst of Comparative Example 1, catalyst metal particles having a particle size of 20 nm or more are also generated, but the number thereof is extremely small. The hydrogen combustion catalyst of Comparative Example 1 is mainly composed of catalytic metal particles having a particle size of about 10 nm in SEM observation, and no catalytic metal particles having a particle size of 30 nm or more are observed.

Further, the hydrogen combustion catalyst of Comparative Example 3 has the same carrier (α-alumina carrier) and the same production method as in Example 1 and the like, and Pd is applied as the catalyst metal particles. As can be seen from FIG. 4, in this catalyst, the number of catalyst metal particles having a particle size of 20 nm or more is as small as less than 20. From this, it is considered that the metal species constituting the catalyst metal particles are important in addition to the production method in the formation of coarse catalyst metal particles having a particle size of 20 nm or more.

Next, as a result of TEM observation (500,000 times), TEM images of Examples 1, 2 and 4 and Comparative Examples 1 and 3 are shown in FIG. Further, the particle size distributions of the catalyst metal particles in Examples 1, 2 and 4 and Comparative Examples 1 and 3 measured based on the TEM image are shown in FIGS. 6 (particle size-number of particles) and FIG. 7 (particle size-volume ratio). show. As can be seen from FIG. 5, the formation and dispersion of fine catalyst metal particles having a particle size of 10 nm or less are confirmed in any of the catalysts. However, in the catalyst of the example, catalyst metal particles having a particle size of 20 nm or more are confirmed even by TEM observation, whereas in the comparative example, coarse particles having a particle size of 20 nm or more are not observed.

When the particle size distribution (particle size-number of particles) confirmed by TEM observation in FIG. 6, the catalyst of each example has a total number of catalytic metal particles of 20 nm or more and 30 nm or less, and the total number of particles is 1 nm or more and 30 nm or less. The ratio of particles to the total number of particles is 2% or more. In the comparative example, no catalyst metal particles having a diameter of 20 nm or more are observed. The presence of the catalyst metal particles having a diameter of 20 nm or more becomes clearer by looking at the volume fraction of each catalyst metal particle (FIG. 7). In the catalyst of each example, the ratio of the total volume of the catalyst metal particles having a particle size of 20 nm or more and 30 nm or less to the total volume of the catalyst metal particles having a particle size of 1 nm or more and 30 nm or less is 50% or more.

The SEM and TEM observation results and particle size distribution of the catalyst metal particles of Example 3 and Comparative Example 2 (not shown) showed the same tendency as described above. Table 1 summarizes the particle size distributions of the catalyst metal particles of each hydrogen combustion catalyst by the above SEM observation and TEM observation.

[Confirmation of hydrophobic functional groups]
Infrared absorption spectra of Example 5 and Comparative Examples 1 and 3 in which the catalyst was hydrophobized were measured by FT-IR analysis. The FT-IR analysis used the diffuse reflection method. The catalyst was pulverized in an agate mortar and sufficiently pulverized, then KBr powder having about twice the volume of the catalyst was added and mixed until uniform. After placing it on an aluminum dish to form a smooth surface, it was set in an analyzer and the infrared absorption spectrum was measured at room temperature and in the atmosphere.

FIG. 8 shows the infrared absorption spectra of the hydrogen combustion catalysts of Example 5 and Comparative Examples 1 and 3. In any of the catalysts, an absorption peak derived from the methyl group, which is a hydrophobic functional group, is observed in the vicinity of 2900 to 3000 cm ^-1 . It can be said that the presence or absence of the hydrophobic treatment in the hydrogen combustion catalyst according to the present invention can be confirmed by measuring the infrared absorption spectrum.

[Hydrogen combustion activity test]
An evaluation test of hydrogen combustion activity was carried out for each hydrogen combustion catalyst produced in Examples 1 to 5 and Comparative Example 1. In this evaluation test, the catalyst is exposed to iodine-containing gas in advance to perform iodine poisoning treatment that causes iodine poisoning, and then the catalytic activity is determined by a hydrogen combustion test using a hydrogen-containing gas as the test gas using the catalyst. evaluated.

The iodine poisoning treatment was carried out by bubbling a carrier gas in an iodine aqueous solution (iodine concentration 80 mg / L) to generate an iodine-containing gas, and circulating the iodine-containing gas in a column filled with a hydrogen combustion catalyst. In this iodine poisoning treatment, two types of gases, air and a hydrogen mixed gas (1% by volume hydrogen + 0.5% by volume oxygen + nitrogen balance), were used as carrier gases for bubbling iodine water. The latter treatment with iodine poisoning with a hydrogen mixed gas is a test for evaluating hydrogen combustion activity in a low oxygen atmosphere. Hydrogen is difficult to burn (oxidize) in an oxygen-deficient atmosphere. Therefore, the superiority or inferiority of the catalytic activity can be confirmed more clearly by performing the iodine poisoning treatment with the hydrogen mixed gas. In the iodine poisoning treatment, the flow rate of the carrier gas bubbling iodine water was set to 300 mL / min, the catalyst amount was set to 10 cc, and the treatment time was set to 15 hours.

In the hydrogen combustion test, the column is filled with the hydrogen combustion catalyst after the above-mentioned iodine poisoning treatment, a reaction gas containing hydrogen is circulated there, and the composition of the reaction gas on the column outlet side is analyzed to determine the hydrogen combustion rate (catalyst). Activity) was measured. The hydrogen burning rate was calculated as "(hydrogen concentration before reaction-hydrogen concentration after reaction) / hydrogen concentration before reaction x 100". As the introduced reaction gas, a hydrogen mixed gas having a hydrogen concentration of 1000 ppm (air balance) was used. Other test conditions were as follows.
-Reaction gas flow rate: 800 mL / min
-Reaction temperature: room temperature, 100 ° C
-Test method: The temperature rise rate was set to 10 ° C./min, and the reaction gas was analyzed after holding for 1 hour after reaching a predetermined reaction temperature.

The results of the above hydrogen combustion test are shown in Table 2. In the hydrogen combustion test of the present embodiment, in order to confirm the initial activity of each catalyst, a hydrogen combustion catalyst not subjected to iodine poisoning treatment was also carried out.

With reference to Table 2, the magnitude of the effect of iodine poisoning treatment on catalytic activity can be confirmed. The hydrogen combustion catalyst of Comparative Example 1 shows a hydrogen combustion rate of 98% at room temperature and 99% at 100 ° C. in a state without iodine poisoning, but hydrogen is treated by iodine poisoning treatment using air as a carrier gas. The combustion rate decreased to 15% (reaction temperature 100 ° C.). Then, when a harsher iodine poisoning treatment using a hydrogen mixed gas as a carrier gas was performed, the hydrogen burning rate was 0% (no activity) even when the reaction temperature was 100 ° C.

Comparing the results of each of the above comparative examples with the results of the hydrogen combustion catalysts of Examples 1 to 5, the catalysts of these Examples have a small decrease in the hydrogen combustion rate when subjected to iodine poisoning treatment. ing. Even with the catalysts of these examples, the hydrogen combustion activity is basically not exhibited by the iodine poisoning treatment when the reaction temperature is low (room temperature), but the hydrogen combustion activity can be exhibited by setting the reaction temperature to 100 ° C. Even if it is subjected to iodine poisoning treatment using a hydrogen mixed gas as a carrier gas, it will not be completely inactivated. It can be said that the iodine-resistant catalyst toxicity was obtained by forming coarse catalyst metal particles (particle size of 20 nm or more) in the hydrogen combustion catalyst of each example.

In particular, the hydrogen combustion catalyst (Rh / α-alumina carrier + hydrophobic treatment) of Example 5 is subjected to iodine poisoning treatment using a hydrogen mixed gas as a carrier gas, by setting the reaction temperature to 100 ° C. It exhibits extremely high activity with a hydrogen burning rate of 88%. It can be seen that the catalyst of Example 5 is a catalyst obtained by hydrophobizing the catalyst of Example 2, and that the hydrophobicization exhibits an improvement in initial activity and an improvement in iodine resistance catalyst toxicity.

However, according to the study by the present inventors, the hydrophobizing treatment is not always essential for the hydrogen combustion catalyst. Even if the catalyst of Example 2 is subjected to iodine poisoning treatment using a hydrogen mixed gas as a carrier gas, hydrogen combustion activity is exhibited by setting the reaction temperature from room temperature to 100 ° C. Therefore, when a similar hydrogen combustion test was conducted with the reaction temperature set to 150 ° C. only in Example 2, it was confirmed that a hydrogen combustion rate of 35% or more was exhibited. Considering this point, it is confirmed that the desired application can be achieved by adjusting the reaction temperature even when the hydrophobizing treatment is not performed. It can be said that the hydrophobization treatment is effective for applications and environments where severe iodine poisoning is expected.

As described above, it was confirmed that high iodine-resistant catalyst toxicity can be imparted to the hydrogen combustion catalyst by forming coarse catalyst metal particles having a particle size of 20 nm or more. Here, when the catalyst metal particles and the carrier types, which are elements other than the particle size of the catalyst metal particles, are confirmed, as the metal species of the catalyst metal particles, Rh is an iodine-resistant catalyst toxicity viewpoint from the comparison of Examples 1 to 3. Therefore, it can be said that it is a particularly preferable precious metal. In particular, when Pd is applied, the catalyst becomes poor in initial activity and iodine resistance catalyst toxicity. This is considered to be due to the fact that Pd is a noble metal that is difficult to coarsen, as can be seen with reference to FIGS. 4, 5 and 6. As for the type of carrier, it is confirmed that the application of α-alumina is preferable from the comparison between Examples 1 and 4.

As described above, the hydrogen combustion catalyst according to the present invention is a catalyst in which catalyst poisoning due to iodine is suppressed more than in the prior art in the combustion of hydrogen gas. The catalyst according to the present invention exhibits more effective hydrogen combustion activity by hydrophobizing treatment.

The hydrogen combustion catalyst according to the present invention can be applied to various devices for hydrogen combustion. In particular, it is useful as a catalyst mounted on a hydrogen combustion device (hydrogen recombination device) in a nuclear power plant.

The hydrogen combustion catalyst according to the present invention is also effective for burning deuterium (D2) and tritium (T), which are isotopes of hydrogen. These isotope hydrogens may be handled in nuclear-related facilities such as fusion power generation facilities, and can be applied to hydrogen combustion equipment installed in them.

Claims (8)

In a hydrogen combustion catalyst in which a carrier made of an inorganic oxide is supported with catalyst metal particles made of a noble metal of Rh, Pt, or Ru or an alloy thereof.
A hydrogen combustion catalyst characterized in that the number of the catalyst metal particles having a particle size of 20 nm or more and 200 nm or less observed in a visual field range of 3 μm × 3 μm when observed at a magnification of 30,000 times is 20 or more. When the number of particles of the catalyst metal particles having a particle size of 1 nm or more and 30 nm or less is measured, the ratio of the total number of catalyst metal particles having a particle size of 20 nm or more and 30 nm or less to the total number of particles of the catalyst metal particles having a particle size of 1 nm or more and 30 nm or less. The hydrogen combustion catalyst according to claim 1, which is 1% or more. When the volume of the catalytic metal particles having a particle size of 1 nm or more and 30 nm or less is measured, the ratio of the total volume of the catalytic metal particles having a particle size of 20 nm or more and 30 nm or less to the total volume of the catalytic metal particles having a particle size of 1 nm or more and 30 nm or less is 30%. The hydrogen combustion catalyst according to claim 1 as described above. The hydrogen combustion catalyst according to any one of claims 1 to 3, wherein the carrier contains 50% by mass or more of α-alumina. The hydrogen combustion catalyst according to claim 4, wherein the specific surface area of the carrier is 0.1 m ² / g or more and 50 m ² / g or less. The hydrogen combustion catalyst according to any one of claims 1 to 5, wherein a hydrophobic low molecular weight functional group having 7 or less carbon atoms is bonded to the carrier. The hydrogen combustion catalyst according to claim 6, wherein the hydrophobic small molecule functional group is a methyl group or an ethyl group. A method of passing a hydrogen-containing gas through the hydrogen combustion catalyst according to any one of claims 1 to 7 to burn hydrogen in the hydrogen-containing gas.
The hydrogen-containing gas contains 0.01 ppm or more of iodine.
A hydrogen combustion method in which hydrogen is burned at a reaction temperature of 10 ° C. or higher and 300 ° C. or lower.

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