JP6112504B2 - Method for producing carbon catalyst - Google Patents

Description

  The present invention relates to a method for producing a carbon-based catalyst. More specifically, the present invention relates to a method for producing a carbon-based catalyst exhibiting oxygen reduction reaction (ORR) activity.

  Fuel cells are attracting attention as a new power source with low environmental impact because they can generate electromotive force with relatively high efficiency and clean exhaust gas. This fuel cell is classified into various types depending on the type of electrolyte used. Among them, a polymer electrolyte fuel cell (PEFC) using a proton conductive polymer electrolyte membrane (PEFC), “PEFC” may be simply referred to as “PEFC”). It can be reduced in size and weight, and operates at a temperature as low as 100 ° C. or lower. Practical use has been promoted as a household power source for cogeneration.

  The PEFC is generally constituted by a membrane-electrode assembly including an anode, a cathode, and a polymer electrolyte membrane disposed between the anode and the cathode. The anode of the PEFC is equipped with a catalyst that promotes a hydrogen oxidation reaction (HOR) that mainly oxidizes hydrogen used as a fuel, and the cathode has an oxygen reduction reaction (ORR: Oxygen) that reduces the oxidant. A catalyst that promotes reduction reaction is provided. Here, platinum catalysts exhibiting high ORR activity are widely used for cathodes with a slow reaction rate, but platinum catalysts are expensive and have low durability when used in PEFC. There is a need for new catalysts to replace platinum catalysts.

JP 2004-362802 A JP 2010-270107 A JP 2012-54157 A Japanese Patent Laid-Open No. 08-165111

  As one of platinum alternative catalysts, carbon-based catalysts represented by different element-containing carbon catalysts and the like have attracted attention and their development has been vigorously advanced. Regarding carbon materials (carbonaceous materials), when carbons with different hybrid orbitals are considered to be different component systems, it can be considered that the carbon materials themselves consist of multi-component systems mainly composed of aggregates of carbon atoms. Physical and chemical interactions can be found between these structural units. Among these, in particular, a heterogeneous element-containing carbon material obtained by substituting a heterogeneous atom for a crystal lattice point of the carbon material can be expressed as an ORR activity and can be called a heterogeneous element-containing carbon catalyst.

This heterogeneous element-containing carbon catalyst is typically heated to about 400 ° C to 1500 ° C after introducing a compound containing a heterogeneous element into a carbonaceous material such as a polymer or carbon nanotube to form a polymer or composite. It is manufactured by subjecting it to carbonization (see, for example, Patent Documents 1 to 4).
However, the above production method requires at least a step of polymerizing or combining the carbonaceous material and the heterogeneous element compound and a step of carbonization by heat treatment, and the dissimilar elements introduced by the heat treatment are desorbed. There is a problem such as. Further, industrially, a simpler method for producing a catalyst is required. Furthermore, in order to open the possibility of developing new functional properties of catalysts mainly composed of carbon materials, it is desired to develop a completely new method for producing carbon-based catalysts.

  The present invention has been created in view of the above problems, and an object thereof is to provide a simple and novel production method of a carbon-based catalyst useful as an alternative catalyst for a platinum catalyst.

  This application provides a method for producing a carbon-based catalyst as a solution to the above problems. Such a production method includes preparing, as a raw material compound, a cyclic compound having a chemical composition containing carbon and a different element other than carbon, hydrogen, and oxygen and having a ring structure at least in part, and a liquid containing the above raw material compound And polymerizing the raw material compound by generating plasma therein.

Graphite, which is a typical carbon material, has a structure in which many graphene sheets having a six-membered ring structure in which carbon atoms are sp ² bonded are stacked. Such a carbon material is known to exhibit a catalytic action against an oxidation-reduction (ORR) reaction due to a special “disturbance” in its crystal structure. Such disorder of the crystal structure may be caused by, for example, adding a dissimilar metal such as iron (Fe) or cobalt (Co) to form a nanoshell when manufacturing a carbon material, or forming a carbon (C) site in a graphene structure. It can introduce | transduce by substituting dissimilar atoms, such as nitrogen (N) and boron (B), to make dope carbon. That is, the function as an oxidation-reduction (ORR) catalyst can be expressed by appropriately introducing a different element into the carbon material and controlling the electron orbit and the crystal space.

In the method for producing a carbon-based catalyst of the present invention, plasma generated in a liquid is used as a place for producing a carbon material having the above special “turbulence”. Hereinafter, the plasma generated in the liquid may be simply referred to as “liquid plasma”. In the method for producing a carbon-based catalyst using in-liquid plasma, carbonization accompanied by a polymerization reaction of the cyclic compound is advanced in generating the carbon-based catalyst from the cyclic compound as the raw material compound. Here, in the process of generating the carbon-based catalyst, two independent processes of polymerization and carbonization are not necessary, and the generation of the carbon-based catalyst is generated in one step of generating plasma in the liquid containing the raw material compound. It can be carried out. And the carbon-type catalyst obtained by the effect | action of this plasma is equipped with the activity with respect to ORR reaction. That is, according to the present invention, a completely new method for producing a carbon-based catalyst using in-liquid plasma, which is different from existing methods, is provided.
In the present specification, a compound having a ring structure means a group of compounds in which constituent atoms are bonded cyclically in a chemical structure. Therefore, this cyclic compound includes a monocyclic compound in which one ring exists in one molecule, a polycyclic compound in which more than one ring exists, and a single element in which the elements constituting the ring are one kind. Various cyclic compounds such as cyclic compounds and heterocyclic compounds in which a ring is constituted by two or more elements are included.

In a preferred embodiment of the method for producing a carbon-based catalyst disclosed herein, the heterogeneous element contained in the cyclic compound contains boron (B), nitrogen (N), or both.
Carbon (C) is an element having an atomic number of 6, for example, a carbon site in a graphene sheet is relatively composed of boron (atomic number 5) or nitrogen (atomic number 7) located on both sides of the carbon in the periodic table of elements. It can exist stably and can contribute to activation of graphene effectively. Therefore, by using a cyclic compound containing boron, nitrogen or the like as a different element and forming a carbon material in a form in which the compound is polymerized, a highly active carbon-based catalyst can be produced.

In a preferred embodiment of the method for producing a carbon-based catalyst disclosed herein, the cyclic compound is characterized by having a 5-membered ring, a 6-membered ring, or both in a chemical structure.
The carbon-based catalyst obtained in the present invention is considered to have a chemical structure in which some of these are polymerized on the basis of the structure of a cyclic compound that is a raw material compound. Here, different elements such as nitrogen can exist, for example, while forming a 6-membered pyridine structure or a 5-membered pyrrole structure at the end of the graphene sheet. Therefore, it is preferable that the raw material compound used in the present invention has a 5-membered or 6-membered ring structure because a different element can be suitably introduced into the structure. Such a cyclic compound may be a cyclic compound whose ring structure is composed only of carbon, or may be a heterocyclic compound containing an element other than carbon in the ring structure.

In a preferred embodiment of the method for producing a carbon-based catalyst disclosed herein, the cyclic compound is aniline, pyridine, pyrazine and triazine, or any one or more of these derivatives. It is a feature. More preferably, the cyclic compound may include at least triazine or a derivative thereof.
According to such a configuration, a carbon-based catalyst having a configuration in which the nitrogen-containing cyclic compound is used as a monomer component and polymerized and carbonized can be obtained.

In a preferred embodiment of the method for producing a carbon-based catalyst disclosed herein, a pair of linear electrodes are disposed in the liquid, and the plasma has a pulse width of 0.1 μs to 5 μs between the electrodes and a frequency. It is generated by applying a DC pulse voltage of 10 ³ to 10 ⁵ Hz.
According to such a configuration, most of the bubbles generated in the liquid by Joule heat generated between the linear electrodes can be maintained in a stable state in the liquid without floating toward the water surface. Plasma can be generated in a stable state. Thereby, the carbon-based catalyst can be produced in a more efficient and stable state.

In a preferred aspect of the carbon-based catalyst production method disclosed herein, the plasma is glow discharge plasma.
The plasma generated in the liquid can be in the form of spark discharge, corona discharge, glow discharge, arc discharge. Especially, glow discharge plasma can be utilized for manufacture of a carbon-type catalyst as a more preferable form of plasma in liquid. Thereby, non-equilibrium low-temperature plasma can be generated, and the carbon-based catalyst can be efficiently synthesized without destroying the structure of the raw material compound.

In a preferred aspect of the carbon-based catalyst production method disclosed herein, the glow discharge plasma is formed in a gas phase generated in the liquid.
The glow discharge plasma in the liquid can be generated by applying a high-frequency voltage between the electrodes arranged in the liquid. According to such a configuration, glow discharge plasma can be steadily generated inside the gas phase generated in the liquid phase by Joule heat generated between the electrodes. That is, a liquid phase / gas phase / plasma phase interface is stably formed, and active species generated in the plasma phase are supplied to the gas-liquid interface via the gas phase, so that a carbon-based catalyst is generated with high efficiency. It becomes possible.

The carbon-based catalyst disclosed herein is a catalyst for a polymer electrolyte fuel cell, and is characterized by being produced by any one of the production methods described above.
The carbon-based catalyst described above exhibits activity for the ORR reaction, and can realize 4-electron reduction of oxygen. Therefore, such a carbon-based catalyst can be suitably used as a cathode catalyst for a polymer electrolyte fuel cell, for example.

It is a schematic diagram which shows an example of a structure of the in-liquid plasma generator used for manufacture of a carbon-type catalyst. It is the schematic diagram which illustrated the structure of the reaction field by the plasma in a liquid utilized for manufacture of a carbon-type catalyst. It is the figure which illustrated the FT-IR measurement result of (a) pyridine-carbon (p-carbon) and (b) aniline-carbon (a-carbon) which were manufactured in one Embodiment. It is the figure which illustrated the cyclic voltammogram of p-carbon manufactured in one Embodiment. It is the figure which illustrated the cyclic voltammogram of a-carbon manufactured in one Embodiment. It is the figure which illustrated other cyclic voltammograms manufactured in one Embodiment (a) p-carbon and (b) a-carbon. It is a graph which shows hydrogen (H) and nitrogen (N) content of the carbon-type catalyst (a)-(d) manufactured in one Embodiment. (A) (B) is the figure which illustrated the cyclic voltammogram of the carbon-type catalyst (a)-(d) manufactured in one Embodiment. It is the figure which illustrated the FT-IR measurement result of the carbon-type catalyst (a)-(d) manufactured in one Embodiment. It is a graph which shows hydrogen (H) and nitrogen (N) content of the carbon-type catalyst manufactured in one Embodiment.

Hereinafter, the manufacturing method of the carbon-type catalyst of this invention is demonstrated. Note that matters other than matters specifically mentioned in the present specification and necessary for the implementation of the present invention can be grasped as design matters of those skilled in the art based on the prior art in this field. The present invention can be carried out based on the contents disclosed in this specification and the drawings and common general technical knowledge in the field.
In the method for producing a carbon-based catalyst disclosed herein, a cyclic compound having a chemical composition containing carbon and a different element other than carbon, hydrogen, and oxygen and having a ring structure at least partially is prepared as a raw material compound. Then, by adopting a method of generating plasma in a liquid containing the cyclic compound, a carbon material obtained by polymerizing the raw material compound is generated while introducing different elements.

As a raw material compound in the present invention, as described above, a cyclic compound having (1) carbon and (2) a different element other than carbon, hydrogen, and oxygen in a chemical composition and having a ring structure at least in part is particularly used. Can be used without limitation. That is, an organic raw material compound having such a different element and structure can be used.
As the different elements, various elements other than carbon, hydrogen and oxygen can be considered. For example, a carbon-based catalyst can be made into a nanoshell, or can be doped into a carbon-based catalyst. Anything that can cause "disturbance" can be employed without any particular limitation. Examples of such different elements include transition metals typified by iron (Fe), cobalt (Co), etc., semimetals typified by boron (B), silicon (Si), and phosphorus (P), and nitrogen (N). And nonmetals represented by sulfur (S) can be considered. In order to produce a carbon-based catalyst exhibiting higher catalytic activity, it is preferable to use a cyclic compound containing either or both of boron and nitrogen as the different element.

  In addition, as described above, the compound having a ring structure means a group of organic compounds in which constituent atoms are bonded in a cyclic structure in the chemical structure. That is, it is a general term for organic compounds having a cyclic structure based on a carbon skeleton. Therefore, this cyclic compound may be a monocyclic compound in which one ring exists in one molecule or a polycyclic compound in which two or more rings exist, and the ring is constituted by one kind of element. And various cyclic compounds such as a heterocyclic compound in which a ring is composed of two or more elements. Further, it is an aromatic cyclic compound having a conjugated unsaturated ring structure (typically an aromatic hydrocarbon) or an alicyclic compound containing at least one saturated or unsaturated carbocyclic ring having no aromaticity. It's okay. A cyclic compound having at least one unsaturated bond is preferable. The number of atoms constituting such a ring structure is not limited, and may be, for example, a compound having a small ring, a medium ring, or a large ring, typically about 3 to 10 members. Cyclic compounds can be considered. Such cyclic compounds can be used as a raw material compound alone or in combination of two or more.

  Specific examples of the cyclic compound as the raw material compound include, for example, compounds having the structures shown below and derivatives thereof. Any one of these cyclic compounds may be included alone, or two or more thereof may be mixed and included.

The cyclic compounds represented by the above structural formulas (a) to (s) may have a substituent. Such substituents may be various organic functional groups, for example, hydrocarbon groups, hydroxy groups, aldehyde groups, carboxyl groups, carbonyl groups, halogeno groups, silicon-containing functional groups, sulfur-containing functional groups, as an example. Examples thereof include nitrogen-containing functional groups and phosphorus-containing functional groups. More preferably, for example, a hydrocarbon group represented by a linear, branched or cyclic alkyl group having 1 to 10 carbon atoms, a vinyl group, an aryl group, a hydroxy group, an aldehyde group, a carboxyl group, a carbonyl group, a fluoro group. , Chloro, bromo and other halogeno groups, silicon-containing functional groups such as alkylsilyl groups, sulfur-containing functional groups such as thiol groups and sulfo groups, aldehyde groups, nitro groups and other nitrogen groups Examples thereof include phosphorus-containing functional groups typified by a containing functional group and a phosphonic acid group.
Moreover, as a cyclic compound, the thing of the form which contains the cyclic compound represented by the above structural formula (a)-(s) in a part of structure may be sufficient. For example, it may be a compound such as a bipyridine compound or a phenylpyridine compound containing pyridine represented by the structural formula (g) as part of its structure. Further, for example, pyrrole represented by the structural formula (a) or thiophene represented by the structural formula (d) may be a polymer represented by polypyrrole, polythiophene, or the like in a form in which a plurality of them are bonded.
As is clear from the above, the raw material compound in the present invention can naturally contain hydrogen and oxygen unless they are different elements.

  In order to suitably produce a carbon-based catalyst having a configuration similar to a graphene sheet, it is preferable to use a cyclic compound having a 5-membered ring, a 6-membered ring, or both in the chemical structure. Alternatively, it is preferable to use both a cyclic compound having a 5-membered ring in the chemical structure and a cyclic compound having a 6-membered ring in the chemical structure as raw material compounds. In order to control the chemical structure of the carbon-based catalyst to be produced in more detail, for example, a monocyclic compound having one 5-membered or 6-membered ring structure in the chemical structure is used as the cyclic compound. It is preferable to use at least. A compound having one 6-membered ring structure is more preferable. Examples of such cyclic compounds include aniline, pyridine, pyrazine, triazine or derivatives thereof. Also, in order to produce a carbon-based catalyst exhibiting oxygen reduction activity by a four-electron reaction, a heterocyclic ring containing an element other than carbon in the ring structure rather than a cyclic compound in which the ring structure is composed of only carbon. Preference is given to using formula compounds. Examples of such heterocyclic compounds include pyridine, triazine and derivatives thereof, and these can be used alone or as a raw material compound by mixing with other cyclic compounds as described above. In particular, it is shown as a preferable example that the raw material compound contains at least triazine and a derivative thereof.

  The above raw material compounds may be liquid at room temperature and normal pressure (typically 25 ° C., 1 atm, the same applies hereinafter). From this, as a liquid (liquid phase) containing a raw material compound, the liquid which consists of 100% of raw material compounds can be used. Moreover, the solution which such a raw material compound diluted with the suitable solvent may be sufficient. The solvent in this case is not particularly limited, and may be water, an organic solvent such as alcohol or benzene, or a mixed solvent thereof. When the raw material compound is solid at normal temperature and pressure, it may be in the form of a solution in which this is dissolved in an appropriate solvent or a dispersion in which the raw material compound is microscopically dispersed. The amount of the raw material compound contained in the solution can be appropriately adjusted.

In the method for producing a carbon-based catalyst of the present invention, in-liquid plasma generated in the liquid is used as a reaction field for polymerization and carbonization of the raw material compound. That is, by the action of active species such as positive and negative ions, electrons and radicals constituting the plasma, the polymerization reaction of the raw material compound contained in the liquid and the formation of the carbon skeleton are realized. Regardless of its specific composition and structure, the carbon-based catalyst produced in this way changes from a colorless and transparent liquid to the human eye as it undergoes treatment with plasma in liquid, and then turns brown to black. Therefore, it can be confirmed that the raw material compound typically made of an organic material is converted into inorganic carbon. This phenomenon is similar to an organic polymer material such as cellulose, in which oxygen and hydrogen are lost as the carbonization proceeds, the carbon concentration increases, and the appearance changes from brown to black.
In such polymerization and carbonization, the introduction of a different element into the carbon-based catalyst is considered to cause a special “disturbance” in the crystal structure, which has activity for oxygen reduction reaction. Such an oxygen reduction reaction can be, for example, a four-electron reaction similar to that of a platinum catalyst.

  Here, the above-mentioned plasma in the liquid as the reaction field is obtained by applying a microwave or a high frequency to a gas (gas phase) generated in the liquid to partially or completely ionize molecules constituting the gas. Can be formed. In other words, in the plasma in liquid, the gas phase surrounding the plasma phase is further surrounded by the liquid phase, and the active species such as ions, electrons and radicals constituting the plasma freely move in the limited gas phase. This is a possible state. Therefore, it exhibits physical and chemical properties that are different from gas phase plasma (typically atmospheric pressure plasma, low pressure plasma, etc.) generated in the released gas phase. For example, gas phase plasma is generated when neutral molecules constituting the gas are ionized and turned into plasma when the temperature of the gas is raised. At this time, unlike the phase transition between solid, liquid, and gas, the transition from gas to plasma occurs gradually, so that even a very small amount of ionization of constituent molecules can be sufficiently plasma. . On the other hand, the plasma in liquid is typically formed by first vaporizing the liquid by Joule heating by discharge in the liquid to form a gas phase, and further generating plasma in the gas phase. In other words, in the plasma in liquid, a high energy state called plasma is confined in the liquid (that is, the condensed phase), which realizes a closed system physics and forms a high-density plasma reaction field that is not released. . Therefore, in the present invention, by placing the cyclic compound as the raw material compound in such a high-density plasma reaction field, a carbon-based catalyst is obtained in one step without going through a plurality of steps such as a polymerization step and a carbonization step. Has realized.

  The raw material compound is also supplied as a gas phase in the gas phase plasma, but is supplied as a liquid phase in the liquid plasma. That is, in the present invention, the cyclic compound as the raw material compound is supplied to the reaction field at a high density via the liquid phase. Therefore, in the production method of the present invention, a carbon-based catalyst can be produced with high efficiency from a cyclic compound that is a raw material compound.

  The above-mentioned plasma in liquid is classified into spark discharge such as lightning, corona discharge, glow discharge, arc discharge, etc., depending on the difference in potential applied between the electrodes. When the spark discharge continuously flows, it becomes glow discharge or arc discharge. Here, glow discharge plasma (hereinafter also referred to as solution plasma) generated in the liquid has different characteristics from other liquid plasmas. For example, the arc discharge plasma is a thermal plasma having a high particle density and a local thermal equilibrium state in which the temperature of ions and neutral particles is substantially equal to the electron temperature. In contrast, glow discharge plasma is a low-temperature plasma in a non-equilibrium state in which the electron temperature is high but the temperature of ions and neutral particles is low. In addition, it is difficult to generate a continuous plasma by corona discharge, and there is a feature that a relatively large amount of oxidizing hydroxy radicals are formed together with hydrogen radicals by decomposition of water. On the other hand, glow discharge plasma has high plasma energy, and oxidizing hydroxy radicals are further decomposed to generate many reducing hydrogen radicals.

  Such glow discharge plasma can be generated relatively stably by applying a voltage with a pulse width of submicroseconds at a high repetition frequency. Therefore, the expansion / compression motion of the liquid surrounding the plasma phase and the plasma phase And a stable state can be maintained for a long time (for example, 2 hours or more). Therefore, for example, in the solution plasma, a part of the gas phase generated between the electrodes may rise from the electrodes due to buoyancy and reach the liquid surface, but most of the gas phase is constant between the electrodes. It is constantly maintained as a large gas phase. Therefore, in the solution plasma, the plasma generation state can always be controlled. Further, the solution plasma can be suitably generated even in an environment of normal temperature (typically 25 ° C.) and normal pressure (typically 1 atm). In the method for producing a carbon-based catalyst of the present invention, it is preferable to use such a controlled plasma, and a more efficient carbon-based catalyst can be produced. Whether or not the generated plasma is glow discharge plasma can be confirmed by, for example, the Townsend second coefficient obtained by plasma emission spectroscopic analysis or the like being in the range of 0.0005 to 0.005.

Hereinafter, the production method of a carbon-based catalyst of the present invention will be described in more detail by taking as an example the production of a carbon-based catalyst using solution plasma as a reaction field as a preferred embodiment of the present invention.
FIG. 1 is a diagram showing an outline of a solution plasma generator 10 for generating a solution plasma 4 in a liquid (liquid phase) 2. In this embodiment, the liquid 2 containing the raw material compound is put in a container 5 such as a glass beaker. A pair of electrodes 6 for generating plasma is disposed in the liquid 2 at a predetermined interval and is held in the container 5 via an insulating member 9. The electrode 6 is connected to an external power supply 8, and a pulse voltage of a predetermined condition is applied from the external power supply 8. As a result, the solution plasma 4 can be constantly generated between the pair of electrodes 6.

  The electrode 6 may be in various forms such as a plate electrode, a rod electrode, and a combination thereof, and the material thereof is not particularly limited. In this embodiment, a linear electrode (needle electrode) 6 made of tungsten capable of locally concentrating an electric field is used, but an electrode made of another metal material such as iron or platinum is used. You may do it. In order to suppress an excessive current that hinders electric field concentration, it is desirable that the electrode 6 exposes only the front end portion (for example, about several millimeters) and the rear portion is insulated by an insulating member 9 or the like. For example, the insulating member 9 is made of rubber or resin (for example, fluororesin). In this embodiment, the insulating member 9 has a configuration in which the electrode 6 is fixed to the container 5 and also serves as a stopper for keeping the electrode 6 and the container 5 watertight. In such an apparatus 10, the application conditions of the pulse voltage for generating the solution plasma depend on conditions such as the type and concentration of the raw material compound contained in the liquid 2, and further on the configuration conditions of the apparatus 10. As a specific example, voltage (secondary voltage): about 1 to 2 kV, frequency: about 10 to 30 kHz, and pulse width: about 0.5 to 3 μs are exemplified.

Moreover, as a raw material compound, a nitrogen-containing heterocyclic compound can be used as an example from said illustration, for example. More specifically, for example, pyridine, triazine, piperidine, pyrimidine, indole, quinoline, isoquinoline, purine and the like. For example, pyridine (100% pyridine) is prepared, and the liquid 2 containing the raw material compound in the present invention can be obtained.
The liquid 2 preferably has an electric conductivity in the range of about 300 μS · cm ^{−1 to} about 3000 μS · cm ⁻¹ in order to enable generation of stable solution plasma. If the electric conductivity is less than 300 μS · cm ⁻¹ , a large amount of electric power is required to generate the solution plasma, and it is difficult to generate the solution plasma suitably. In addition, when the electrical conductivity exceeds 3000 μS · cm ⁻¹ , the electric power supplied between the electrodes for plasma generation is consumed as an ionic current, which makes it difficult to generate plasma constantly. It is not preferable.

  Then, the solution plasma 4 is formed by applying the pulse voltage in the liquid 2 by the solution plasma generator 10. The plasma reaction field generated by the solution plasma generator 10 has a configuration as shown in FIG. 2, for example. That is, a gas phase 3 is formed in the liquid (liquid phase) 2, and a solution plasma (plasma phase) 4 is formed in the gas phase 3. This plasma reaction field is constantly maintained between the electrodes 6. In such a plasma reaction field, active species such as electrons, ions and radicals having high energy are supplied from the plasma phase 4 toward the liquid phase 2. On the other hand, from the liquid phase 2 toward the gas phase 3 and the plasma phase 4, a raw material compound (indicated by C in FIG. 2) contained in the liquid phase 2 is supplied. These contact (collision) mainly at the interface between the liquid phase 2 and the gas phase 3. For the sake of easy understanding, FIG. 2 shows a state in which each interface between the liquid phase 2 and the gas phase 3 and between the gas phase 3 and the plasma phase 4 is clearly formed in a substantially spherical shape. The interface is not necessarily limited to being clearly formed. For example, there is no critical interface between the gas phase 3 and the plasma phase 4, and this interface may have a spatial extent.

  According to the above configuration, for example, the raw material compound is polymerized and carbonized by the action of solution plasma to form a carbon-based catalyst. Although the yield varies depending on the reaction system and cannot be generally stated, for example, in the embodiment exemplified below, the carbon-based catalyst can be produced at about 1 g / 30 minutes. The carbon-based catalyst is generated, for example, in a state of being dispersed in a liquid as very fine particles. For example, the fine particles may be in the form of secondary particles having a diameter of about several μm, which are configured by aggregating primary particles having a diameter of about several nm to several tens of nm (for example, about 20 nm to 50 nm). Therefore, it can collect | recover by removing a solvent by means, such as filtration and drying, for example. The carbon-based catalyst is typically obtained in an aggregated state by removing the solvent. In such a case, for example, the carbon catalyst in an agglomerated state may be adjusted to a desired size (particle size) by means such as pulverization.

  As mentioned above, although the manufacturing method of the carbon-type catalyst was demonstrated based on suitable embodiment, this manufacturing method is not limited to this example, It can change and change an aspect suitably. For example, the liquid containing the raw material compound may contain a compound other than the raw material compound as long as the object of the present invention is not impaired. In generating solution plasma, it is not always necessary to use a needle-like electrode made of tungsten. For example, an electrode having an arbitrary shape made of another conductive material may be used. Further, solution plasma may be generated by an induction coil having a low inductance without using an electrode. Moreover, the plasma in liquid is not limited to that by solution plasma (glow discharge plasma), and for example, arc plasma in liquid may be used.

As described above, the method for producing a carbon-based catalyst of the present invention is a novel method for producing a carbon-based catalyst using an unprecedented plasma reaction field, and the creation of a substance having novel characteristics or the search for a new material is performed. As well as the possibility of developing new functions of carbon catalysts containing different elements.
Next, some examples relating to the present invention will be described, but the present invention is not intended to be limited to those shown in the examples.

(Embodiment 1)
As a raw material compound, (a) pyridine (manufactured by Wako Pure Chemical Industries, Ltd., pyridine for organic synthesis (dehydration)) and (b) aniline (manufactured by Wako Pure Chemical Industries, Ltd., reagent grade 1) which are liquid at room temperature These were prepared and used as the raw material compound-containing liquids (a) and (b).
Next, plasma was generated in these raw material compound-containing liquids (a) and (b) using the following apparatus. FIG. 1 is a diagram showing an outline of a solution plasma generator 10 for generating plasma in a liquid. In this embodiment, the raw material compound-containing liquid 2 of (a) and (b) prepared above is put into a container 5 made of a glass beaker and stirred by a stirring device 7 made of a magnetic stirrer. A pair of electrodes 6 for generating plasma are disposed in the raw material compound-containing liquid 2 at a predetermined interval, and are held in the container 5 via an insulating member 9. In this embodiment, the needle-like electrode 6 capable of concentrating the electric field locally is used. The electrode 6 is composed of a tungsten wire (manufactured by Nilaco) having a diameter of 1.0 mm, the distance between the electrodes is set to 0.3 mm, and a tip portion (for example, several (only about mm) was exposed, and the rear part was insulated with an insulating member 9 made of fluororesin. In this embodiment, the insulating member 9 is configured to fix the electrode 6 to the container 5 and also serve as a stopper for keeping the water-tightness between the electrode 6 and the container 5. The electrode 6 is connected to an external power supply 8, and a pulse voltage of a predetermined condition is applied from the external power supply 8. As the external power source 8, a bipolar pulse power source (manufactured by Kurita Manufacturing Co., Ltd., MPS-R06K02C-WP1F) was used.

In this embodiment, the application conditions of the pulse voltage for generating the solution plasma are the secondary voltage: 1500 V, the pulse width: 1.0 μs, and the repetition frequency: 20 kHz. Under these conditions, the solution plasma is contained in each raw material compound-containing liquid 2. Was generated for 30 minutes.
The raw material compound-containing liquids (a) and (b) were both colorless and transparent, but became yellowish immediately after the generation of the solution plasma, turned brown after about 5 minutes, and became black and opaque after about 10 minutes. changed. This discoloration is considered to be due to polymerization of the raw material compounds pyridine and aniline to form a graphite skeleton. After treatment with such solution plasma, the solution was dried at about 100 ° C., and a black powder was obtained at the bottom of the beaker. By generating solution plasma in the raw material compound-containing liquids (a) and (b) for about 30 minutes each, about 0.74 g of black powder was obtained. The powder was pulverized in a mortar and refined and subjected to the following analysis. Hereinafter, the black powder obtained from the pyridine solution is referred to as (a) p-carbon, and the black powder obtained from the aniline solution is referred to as (b) a-carbon.

<FT-IR measurement>
For the (a) p-carbon and (b) a-carbon obtained above, Fourier transform infrared absorption spectrometry (FT-IR; manufactured by Shimadzu Corporation, FTIR8400S, control software; IRPrestig- 21) and molecular structure was identified. The obtained FT-IR spectrum is shown in FIG. In FIG. 3, (a) is a spectrum of a region of 4000 to 500 cm ⁻¹ obtained from p-carbon and (b) is obtained from a-carbon.
As shown in FIG. 3, in any of the spectra (a) and (b), it belongs to the stretching vibration of the triple bond (nitrile bond) of C and N which is not found in the raw material compound in the vicinity of 2100 to 2300 cm ^−1. Absorption was observed. Overall, (a) p-carbon has a dominant peak attributed to the triple bond of C and N, and (b) a-carbon belongs to the double bond of C and N (C = O). The peak is dominant. From these facts, it is considered that (a) p-carbon has more nitrogen (N) on the surface than (b) a-carbon.

<CV measurement 1>
The electrochemical characteristics were evaluated by the three-electrode electrochemical cell method using (a) p-carbon and (b) a-carbon made of the black powder obtained above as catalysts. In this CV measurement, the catalytic action on the reduction reaction of dissolved oxygen in the acidic solution was evaluated. A measurement catalyst electrode (working electrode) was prepared by the following procedure. Specifically, (a) p-carbon or (b) a-carbon powder (5.0 mg), Nafion (registered trademark) solution (50 μL) as a binder, and ethanol (50 μL) as a solvent are mixed, and stirred for 15 minutes by ultrasonic waves. Thus, a catalyst paste was prepared. 25 μL of this catalyst paste was dropped on the surface of the glassy carbon electrode and dried to form a catalyst layer, which was used as a measurement catalyst electrode.

CV measurement conditions were as follows.
Working electrode: Glassy carbon and (a) p-carbon or (b) a-carbon Reference electrode: Ag / AgCl (saturated KCl)
Counter electrode: Pt
Supporting electrolyte: 0.5M sulfuric acid

That is, a three-electrode electric electrode was prepared by immersing the working electrode in a 0.5 M sulfuric acid aqueous solution maintained at 25 ° C., and using a silver electrode as a counter electrode and a silver-silver chloride electrode immersed in a saturated KNO ₃ solution as a reference electrode. A chemical cell was constructed. The electrochemical cell was connected to a potentiostat, and cyclic voltammetry (CV) measurement was performed. In the measurement, first, oxygen gas was bubbled in an electrolyte solution for 30 minutes or more to saturate oxygen, and electrode cleaning was performed for 20 cycles at a scanning speed of 100 mV / sec and a scanning range of -1.0 to 1.2 V. After confirming that the waveform was stable, the current at the time of sweeping at a scanning speed of 100 mV / sec and a scanning range of -1.0 to 1.2 V was recorded as a function of potential. The results are shown as cyclic voltammograms in FIGS. 4A and 4B. 4A is a cyclic voltammogram when p-carbon is used as a catalyst, and FIG. 4B is a cyclic voltammogram when a-carbon is used.
As is clear from FIGS. 4A and 4B, a reduction peak is observed at -0.05V when (a) p-carbon is used as a catalyst, and at 0.4V when (b) a-carbon is used as a catalyst. It was confirmed that (a) p-carbon and (b) a-carbon show ORR activity. Further, from the number of reaction electrons (n) of the catalytic reaction obtained from the gradient of the Koutecky-Levich plot based on the CV measurement result, the oxygen reduction reaction by (a) p-carbon and (b) a-carbon is represented by the following formula (1) It was revealed that any of these carbons can function as a cathode catalyst for PEFC.
O ₂ + 4H ⁺ + 4e ⁻ = 2H ₂ O (1)

<CV measurement 2>
Subsequently, the supporting electrolyte in the CV measurement 1 was replaced with 0.1 M potassium hydroxide, and the electrochemical characteristics of the cell were evaluated in the same manner under other conditions. The obtained cyclic voltammogram is shown in FIG. 5A is a cyclic voltammogram when p-carbon is used as a catalyst, and FIG. 5B is a cyclic voltammogram when a-carbon is used.
As shown in FIG. 5, when (a) p-carbon was used as a catalyst, reduction peaks were observed at -0.5 V and -1.0 V, confirming that p-carbon exhibited ORR activity. It was. From the number of reaction electrons (n) of the catalytic reaction obtained from the gradient of the Koutecky-Levich plot, the oxygen reduction reaction in this system is a two-stage oxygen represented by the following formula (2) including a two-electron system reaction. It is a reduction.
H ₂ O ₂ + 2H ⁺ + 2e ⁻ = H ₂ O
(Two-electron reaction)
O ₂ + 2H ⁺ + 2e ⁻ = H ₂ O ₂ (2)
On the other hand, when (b) a-carbon was used as a catalyst, it was found that the ORR activity was not expressed.

(Embodiment 2)
As a raw material compound-containing liquid, (c) a mixed liquid obtained by adding 1.0% by mass of triazine (1,3,5-triazine, manufactured by Tokyo Chemical Industry Co., Ltd.) to the pyridine used in Embodiment 1 above , “Pyri + tri”, etc.)) and (d) a mixed solution in which pyridine and aniline used in Embodiment 1 are mixed in equal amounts (hereinafter, referred to as “pyri + ani”, etc.). And prepared.
Under the same conditions as in the first embodiment, about 0.74 g of carbon catalyst (c) (d) was obtained from each mixed solution by generating solution plasma in the mixed solution (c) (d).

<Elemental analysis>
Elemental analysis was performed on the carbon catalysts (c) and (d) obtained above, and the contents of nitrogen (N) and hydrogen (H) in the carbon catalyst were examined. The contents of nitrogen (N) and hydrogen (H) were measured by measuring with a CHN element analyzer. The results are shown in FIG. For reference, the analysis results of (a) p-carbon and (b) a-carbon obtained in Embodiment 1 are also shown in FIG.
From FIG. 6, aniline, which is a monocyclic compound containing no nitrogen in the ring structure, is obtained by using pyridine or triazine, which is a nitrogen-containing cyclic compound containing a nitrogen element (N) in the ring structure, as a raw material compound. It was confirmed that the amount of nitrogen incorporated into the resulting carbon catalyst was larger than when using the.

<CV measurement 1>
Using the carbon catalysts (c) and (d) obtained above, the electrochemical characteristics were evaluated by the three-electrode electrochemical cell method. A measurement catalyst electrode (working electrode) was prepared by the following procedure. That is, 5.0 mg of carbon catalyst (c) (d) powder, 50 μL of Nafion (registered trademark) solution as a binder, and 50 μL of ethanol as a solvent are mixed, and stirred for 15 minutes with ultrasonic waves to obtain a catalyst paste. Prepared. 25 μL of this catalyst paste was dropped on the surface of the glassy carbon electrode and dried to form a catalyst layer, which was used as a measurement catalyst electrode.

CV measurement conditions were as follows.
Working electrode: Glassy carbon and carbon catalyst (c) or (d)
Reference electrode: Ag / AgCl (saturated KCl)
Counter electrode: Pt
Supporting electrolyte: 0.1M potassium hydroxide

That is, the above working electrode was immersed in a 0.1 M potassium hydroxide aqueous solution maintained at 25 ° C., and a platinum electrode as a counter electrode and a silver / silver chloride electrode immersed in a saturated KNO ₃ solution as a reference electrode, A formula electrochemical cell was constructed. The electrochemical cell was connected to a potentiostat, and cyclic voltammetry (CV) measurement was performed. The measurement was performed in a system in which oxygen was dissolved in an electrolyte solution and a system purged with an inert gas (nitrogen). First, oxygen gas or nitrogen gas is bubbled in the electrolyte solution for 10 minutes to saturate or remove oxygen, and electrode cleaning is performed for 20 cycles at a scanning speed of 100 mV / sec and a scanning range of -1.2 to 1.2 V. After confirming that the CV waveform was stable, the current at the time of sweeping at a scanning speed of 100 mV / sec and a scanning range of -1.2 to 1.2 V was recorded as a function of potential. The results are shown in FIGS. 7A and 7B as cyclic voltammograms. FIG. 7A is a cyclic voltammogram when the electrolyte solution is purged with oxygen, and FIG. 7B is a cyclic voltammogram when purged with nitrogen.
As is clear from FIGS. 7 (A) and (B), the ORR activity derived from the two-electron system reaction could be confirmed regardless of which of the carbon catalysts (c) and (d) was used. The peak potential was not significantly different from (a) p-carbon and (b) a-carbon, but the current value was higher than that of (a) p-carbon alone. ) That is, since pyrr + tri has higher ORR activity, it is considered that more element N is incorporated in the carbon structure.

<FT-IR measurement>
The carbon catalyst (c) (d) obtained above was subjected to Fourier transform infrared absorption spectrometry (FT-IR; manufactured by Shimadzu Corporation, FTIR8400S, control software; IRPreset-21) by the KBr tablet method. Molecular structure was identified. The obtained FT-IR spectrum is shown in FIG. (C) and (d) in FIG. 8 are FT-IR spectra in the region of 4000 to 500 cm ⁻¹ obtained from the carbon catalysts (c) and (d), respectively. For comparison, (a) also shows the results for p-carbon obtained using only pyridine as a raw material compound.
As shown in FIG. 8, in the spectrum of (c) in which triazine was added to (a) pyridine, the peak attributed to the triple bond of C and N was increased, and an increase in N was confirmed. On the other hand, in (d) where aniline is added to (a) pyridine, the peak attributed to the triple bond between C and N decreases, and the double bond between C and N (C═O) and aromatic C It was confirmed that the peak attributed to the —N bond was dominant.
From the above, carbon having a higher ORR activity is obtained when a heterocyclic compound having a ring structure composed of a heterogeneous element and carbon is used as a raw material compound rather than a monocyclic compound composed of a carbocyclic ring. It was confirmed that a carbon-based catalyst having a higher ORR activity can be obtained by using a heterocyclic compound containing a larger amount of different elements in the ring structure.

(Embodiment 3)
As the raw material compound-containing liquid, the triazine used in Embodiment 2 is added to the pyridine used in Embodiment 1 described above in four ways: 0.25% by mass, 0.5% by mass, 0.75% by mass, and 5% by mass. A mixed solution mixed in an addition amount was prepared. In these mixed solutions, solution plasma was generated under the same conditions as in Embodiment 1 to obtain about 0.74 g of carbon catalyst from each mixed solution.
Elemental analysis was performed on each of the obtained carbon catalysts in the same manner as in Embodiment 2 to examine the contents of nitrogen (N) and hydrogen (H) in the carbon catalyst. The results are shown in FIG. For reference, the analysis results of (a) p-carbon and (c) carbon catalyst (triazine addition amount 1.0% by mass) obtained in Embodiments 1 and 2 are also shown in FIG. .
As shown in FIG. 9, as the amount of triazine added to the raw material compound-containing liquid increases, the amount of nitrogen element (N) introduced into the resulting carbon-based catalyst also increases. It was confirmed that a heterogeneous element can be introduced into the carbon catalyst more effectively by using a heterocyclic compound containing a large amount of as a raw material compound.
Specific examples of the present invention have been described in detail above, but these are merely examples and do not limit the scope of the claims. The invention disclosed herein can include various modifications and alterations of the specific examples described above.

2 liquid (liquid phase)
3 Gas phase 4 Solution plasma (plasma phase)
5 Container 6 Electrode 7 Stirrer 8 External power supply 9 Insulating member 10 Solution plasma generator

Claims (7)

As the starting compound comprises a carbon chemical composition, and nitrogen as heterogeneous elements, providing a cyclic compound having a ring structure at least in part,
The starting compound in the liquid containing, resulting said starting compounds is polymerized by generating a glow discharge plasma, a nitrogen-containing carbon material Rukoto,
A method for producing a carbon-based catalyst, comprising:   An electrode made of a pair of metal materials is disposed in the liquid,
  The method for producing a carbon-based catalyst according to claim 1, wherein the plasma is generated between the pair of electrodes.
  The method for producing a carbon-based catalyst according to claim 1 or 2, wherein the cyclic compound has a 5-membered ring, a 6-membered ring, or both in a chemical structure.   The production of the carbon-based catalyst according to any one of claims 1 to 3, wherein the cyclic compound is aniline, pyridine, pyrazine, triazine, or one or more of these derivatives. Method.   The method for producing a carbon-based catalyst according to claim 4, wherein the cyclic compound contains at least triazine or a derivative thereof. Before SL plasma pulse width between the electrodes is in 0.1Myuesu～5myuesu, in the voltage 1KV～2kV, frequency is generated by applying a DC pulse voltage of ¹⁰ 3 ^{to 10} 5 Hz, to claim 2 The manufacturing method of the carbon-type catalyst of description. The method for producing a carbon-based catalyst according to any one of claims 1 to 6, wherein the glow discharge plasma is formed in a gas phase generated in the liquid.

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