JP7723385B2 - Method for producing transition metal-supported nitrogen-containing carbon material - Google Patents

Description

The present invention relates to a method for producing a transition metal-supported nitrogen-containing carbon material. More specifically, the present invention relates to a method for producing a transition metal-supported nitrogen-containing carbon material that can be suitably used as an oxygen reduction catalyst, semiconductor, etc., and to the transition metal-supported nitrogen-containing carbon material.

Graphene-based carbon materials are inexpensive and abundant, and due to their catalytic performance, mechanical strength, electrical conductivity, and thermal conductivity, they are expected to be used in a variety of applications such as heat dissipation materials, catalysts, and electrode materials, and a great deal of research and development is being conducted on them.

For example, research and development is being conducted on a technology for modifying graphene-based carbon materials by irradiating them with microwaves.
First, it has been reported that microwave irradiation of carbon nanofibers obtained by the liquid pulse injection (LPI) method or bulky reduced graphene oxide (rGO) efficiently induces discharge, which promotes high crystallization and a decrease in defect density (e.g., Patent Literature 1 and Non-Patent Literature 1). In addition, a method of irradiating reduced graphene oxide with microwaves has been reported (e.g., Non-Patent Literature 2).

Furthermore, a method for producing a nitrogen-containing carbon material with reduced defects has been disclosed, which includes a step of irradiating a nitrogen-containing carbon material having a G-band peak in its Raman spectrum with microwaves in an atmosphere containing ammonia and/or amine (for example, Patent Document 2).
Furthermore, a method for producing a low-defect carbon material has been disclosed, which includes a step of irradiating a carbon material having a G-band peak in a Raman spectrum with microwaves while fluidizing the carbon material (for example, Patent Document 3).

Transition metal-supported carbon materials in which a transition metal is supported on a graphene-based carbon material are also expected to be used in the various applications described above.
For example, it has been shown that an Fe-supported catalyst (Fe-N-C) can be obtained by supporting iron on a nitrogen-doped carbon substrate (N-C) by chemical vapor deposition (CVD), and that the oxygen reduction reaction (ORR) performance of the obtained Fe-supported catalyst (Fe-N-C) is improved (e.g., Non-Patent Document 3).

JP 2016-145435 A Japanese Patent Application Laid-Open No. 2020-090409 Japanese Patent Application Laid-Open No. 2021-006497

Ogino, I. et al., J. Energy. Chem. 27 (2018) 1468-1474 D. Voiry et al., Science 10.1126/science.aah3398 (2016) Jingkun Li et al., J. Am. Chem. Soc. 2020, 142, 1417-1423

However, new materials with better oxygen reduction performance are desired.

The present invention was made in light of the above-mentioned current situation, and aims to provide a method for obtaining materials with excellent oxygen reduction performance.

The inventors conducted extensive research into methods for obtaining materials with excellent oxygen reduction performance, and discovered that if a nitrogen-containing carbon material that has a G-band peak in its Raman spectrum is modified by microwave irradiation and then a transition metal element is loaded onto it, the oxygen reduction performance of the resulting transition metal-loaded nitrogen-containing carbon material becomes outstanding. This led to the realization that the above-mentioned problems could be successfully solved, and resulted in the present invention.

That is, the present invention provides a method for producing a transition metal-supported nitrogen-containing carbon material, comprising the steps of: irradiating a nitrogen-containing carbon material having a G-band peak in its Raman spectrum with microwaves; and supporting a transition metal element on the modified nitrogen-containing carbon material obtained in the irradiation step.

The method for producing a transition metal-supported nitrogen-containing carbon material of the present invention makes it possible to obtain a transition metal-supported nitrogen-containing carbon material with excellent oxygen reduction performance.

1 is a photograph showing nitrogen-doped reduced graphite oxide. FIG. 1 is a schematic diagram showing a process of irradiating nitrogen-doped reduced graphite oxide with microwaves (a process of modifying the graphite by irradiation). 1 is a graph showing the pyridine-type nitrogen content and graphite-type nitrogen content of nitrogen-doped reduced graphite oxide and modified nitrogen-doped reduced graphite oxide, respectively. FIG. 1 is a schematic diagram showing a process for producing nitrogen-doped reduced graphite oxide by hydrothermal treatment of graphite oxide. 1 is a graph showing Raman spectroscopic data (Raman spectra) of nitrogen-doped reduced graphite oxide and modified nitrogen-doped reduced graphite oxide. FIG. 1 is a schematic diagram showing a process for supporting iron on nitrogen-doped reduced graphite oxide or modified nitrogen-doped reduced graphite oxide by chemical vapor deposition (CVD). 7 is a graph showing the temperature in the test tube versus time in the iron loading step shown in FIG. 6. 1 is a graph showing the pyridine-type nitrogen content and graphite-type nitrogen content of nitrogen-doped reduced graphite oxide, modified nitrogen-doped reduced graphite oxide, nitrogen-doped reduced graphite oxide with iron supported thereon, and modified nitrogen-doped reduced graphite oxide with iron supported thereon. This is a graph showing the results of evaluating the oxygen reduction reaction activity using a rotating electrode for nitrogen-doped reduced graphite oxide, modified nitrogen-doped reduced graphite oxide, nitrogen-doped reduced graphite oxide with iron supported, modified nitrogen-doped reduced graphite oxide with iron supported, and a physical mixture of an iron component and modified nitrogen-doped reduced graphite oxide. This is a graph showing the current density per mass of iron at a potential of 0.7 V evaluated using a rotating electrode for nitrogen-doped reduced graphite oxide carrying iron, modified nitrogen-doped reduced graphite oxide carrying iron, and a physical mixture of iron and modified nitrogen-doped reduced graphite oxide. 1 is a graph showing the number of water molecules adsorbed per unit surface area (nm −2 ) at 298 K versus relative humidity p/p0 after pretreatment of nitrogen-doped reduced graphite oxide, modified nitrogen-doped reduced graphite oxide, iron-loaded nitrogen-doped reduced graphite oxide, and iron-loaded modified nitrogen ^- doped reduced graphite oxide.

The present invention will be described in detail below.
It should be noted that a combination of two or more of the individual preferred embodiments of the present invention described below is also a preferred embodiment of the present invention.

<Method of producing a transition metal-supported nitrogen-containing carbon material>
The method for producing a transition metal-supported nitrogen-containing carbon material of the present invention includes the steps of irradiating a nitrogen-containing carbon material having a G-band peak in its Raman spectrum with microwaves, and supporting a transition metal element on the modified nitrogen-containing carbon material obtained in the irradiation step.
The modification may be any physical or chemical change caused by microwave irradiation, such as reducing defects, improving the hydrophobicity of the surface, or a combination of these.
In this specification, "reducing defects" refers to a decrease in the ratio of the D band peak intensity to the G band peak intensity in a Raman spectrum ( _ID / _IG ). Fewer defects allow electrons to flow more easily, resulting in better electrical conductivity, as well as better thermal conductivity, lubricity, strength, catalytic performance, etc.
The G-band peak and the D-band peak will be described later.
Improving the hydrophobicity of the surface means reducing the number of water molecules adsorbed per unit surface area, as shown in the examples.

The mechanism by which the oxygen reduction performance of the transition metal-supported nitrogen-containing carbon material obtained by the manufacturing method of the present invention is so remarkable is unknown, but as will be described later, it is possible that microwaves moderately improve the hydrophobicity of the surface of the nitrogen-containing carbon material, making it easier for gaseous oxygen to adsorb to the surface than for liquids such as electrolytes, thereby further promoting the oxygen reduction reaction. This effect, combined with the improved oxygen reduction performance due to the transition metal being supported via nitrogen atoms, results in a synergistic improvement in oxygen reduction performance.

In the method for producing a transition metal-supported nitrogen-containing carbon material of the present invention, the irradiating step is preferably carried out in an atmosphere containing ammonia and/or an amine.
The ammonia may be used as it is in the gaseous state, or may be used by forming an aqueous solution (ammonia water) and spraying it into the atmosphere, but it is preferable to use it as it is in the gaseous state.
The amine may be a primary amine, a secondary amine, or a tertiary amine. Only one amine compound may be used, or two or more amine compounds may be used in combination. The amine may be used as it is in a gaseous state or as an aqueous solution, but among these, those that are gaseous at room temperature are preferred, for example, primary amines such as methylamine and ethylamine; secondary amines such as dimethylamine; tertiary amines such as trimethylamine, etc.

The irradiation step is preferably carried out in an atmosphere in which the partial pressure of ammonia and/or amine is 3×10 ⁴ Pa or more. The partial pressure of ammonia and/or amine is more preferably 4×10 ⁴ Pa or more, and even more preferably 5×10 ⁴ Pa or more. There is no particular upper limit to the partial pressure of ammonia and/or amine, but it is usually 1×10 ⁶ Pa or less.
The ratio of the partial pressure of ammonia and/or amine to the total pressure in the atmosphere is preferably 0.3 or more, more preferably 0.4 or more, and even more preferably 0.5 or more. There is no upper limit to the partial pressure ratio, and it is sufficient if it is 1 or less.
When ammonia and an amine are used in combination, the partial pressure of the ammonia and/or amine is the total partial pressure of the ammonia and the amine.

In another preferred embodiment of the present invention, the irradiation step is carried out while circulating ammonia and/or amine through the reaction system. The flow rate of the ammonia and/or amine can be, for example, 10 to 1,000 mL/min.

The atmosphere in the irradiation step may contain, as components other than ammonia and/or amine, active gases such as oxygen, and inert gases such as nitrogen, helium, argon, etc. Among these, inert gases are preferred.
The irradiation step can be carried out by inserting a test tube (tubular reactor) such as a quartz tube into a microwave irradiation device, placing the nitrogen-containing carbonaceous material as the raw material into the test tube, and flowing ammonia and/or amine into the test tube (see, for example, FIG. 2).

In the production method of the present invention, the microwaves irradiated in the irradiation step are electromagnetic waves having a wavelength in the range of 100 μm to 1 m.
The frequency of the microwaves is preferably within the range of 300 MHz to 300 GHz, more preferably within the range of 500 MHz to 50 GHz, and even more preferably within the range of 900 MHz to 25 GHz.

The microwave irradiation temperature is, for example, preferably −50° C. or higher, and more preferably 0° C. or higher. The irradiation temperature is preferably 1000° C. or lower, and more preferably 500° C. or lower.
The irradiation temperature is the temperature of the atmosphere when microwave irradiation is carried out, and it is preferable that the temperature at the start of microwave irradiation is the above temperature.
The microwave irradiation time is, for example, preferably 10 seconds or more, more preferably 30 seconds or more, and even more preferably 60 seconds or more, and from the viewpoint of further improving the oxygen reduction activity, is even more preferably 180 seconds or more, and particularly preferably 240 seconds or more. The irradiation time is preferably 120 minutes or less, more preferably 90 minutes or less, and even more preferably 60 minutes or less.

In the production method of the present invention, the nitrogen-containing carbonaceous material to be irradiated with microwaves in the irradiation step may be fluidized by a gas flow.
The gas for fluidizing the carbon material is not particularly limited, and may be, for example, the above-mentioned ammonia and/or amine, an active gas such as oxygen, or an inert gas such as nitrogen, helium, or argon.

In the production method of the present invention, the G band peak in the Raman spectrum of the nitrogen-containing carbonaceous material irradiated with microwaves in the irradiation step is a peak at a Raman shift of 1550 to 1620 cm ⁻¹ , which is derived from a continuous six-membered ring structure composed of carbon atoms.
The D band peak is a peak at a Raman shift of 1270 to 1450 cm ⁻¹ that is derived from structural disorder and defects.

In this specification, a peak in a predetermined Raman shift range is one whose peak top is clearly observed within that Raman shift range relative to the baseline. For example, in the case of the G band, this means that a clear peak top exists within the range of 1550 to 1620 ^cm . Note that a peak whose top is not within the range of 1550 to 1620 ^cm but whose shoulder is within that range is not considered to be a peak with a Raman shift of 1550 to 1620 ^cm .
In this specification, the Raman spectrum is measured by the method described in the Examples.

The nitrogen-containing carbon material to be irradiated with microwaves in the above-mentioned irradiation step may be any nitrogen-containing carbon material that has a G-band peak in its Raman spectrum. Examples include nitrogen-doped graphite oxide, nitrogen-doped carbon fiber, nitrogen-doped carbon nanofiber, and nitrogen-doped carbon nanotube, and one or more of these may be used.

The nitrogen-doped graphite oxide is obtained by doping graphite oxide with nitrogen atoms. There is no particular limit to the number of layers, but it is preferably a sheet consisting of only one layer of carbon atoms, or a structure with 2 to 100 layers. Graphene with such a number of layers is also called nitrogen-doped graphene oxide. It is more preferable that the number of layers be 20 or less.

The nitrogen-doped carbon fiber has a structure obtained by doping a carbon fiber with nitrogen atoms. The carbon fiber has a structure in which a hexagonal network structure made up of carbon atoms is connected in a linear (fibrous) shape and has a diameter of more than 100 nm.
The nitrogen-doped carbon nanofibers are carbon nanofibers doped with nitrogen atoms, and have a structure in which hexagonal networks of carbon atoms are connected in a linear (fibrous) pattern, with a diameter of 1 to 100 nm.

Nitrogen-doped carbon nanotubes are carbon nanotubes doped with nitrogen atoms. Carbon nanotubes have a cylindrical (tubular) structure in which a hexagonal network structure made up of carbon atoms is connected, and may be single-walled or multi-walled carbon nanotubes.

The nitrogen-containing carbon material to be irradiated with microwaves in the irradiation step preferably has graphite oxide mixed therein and/or adsorbed thereon, and/or contains nitrogen-doped graphite oxide.
The graphite oxide mixed with and/or adsorbed on the nitrogen-containing carbon material refers to a material that does not substantially contain nitrogen atoms itself. The nitrogen-containing carbon material to which graphite oxide is mixed and/or adsorbed includes, for example, a material in which graphite oxide is mixed (adsorbed) on a nitrogen-containing carbon material other than nitrogen-doped graphite oxide. Because graphite oxide contains a large amount of ^sp2 carbon, mixing (adsorbing) it on a nitrogen-containing carbon material can increase electronic conductivity and potentially contribute to improved performance.
When graphite oxide is mixed with and/or adsorbed onto a nitrogen-containing carbon material, the mass ratio of the nitrogen-containing carbon material to graphite oxide is not particularly limited, but is, for example, preferably 1000:1 to 1:1000, more preferably 500:1 to 1:500, even more preferably 100:1 to 1:100, and particularly preferably 50:1 to 1:50.

The nitrogen-doped graphite oxide contains a large amount of ^sp2 carbon, which is thought to contribute to improved performance. The nitrogen-doped graphite oxide is preferably nitrogen-doped reduced graphite oxide.
The graphite oxide can be suitably obtained by a method that employs the oxidation method in the Hummers process and includes a step of adding permanganate to a mixed liquid containing graphite and sulfuric acid.

The nitrogen-containing carbon material to be irradiated with microwaves in the irradiation step preferably has a nitrogen content of 1 atomic % or more, more preferably 1.5 atomic % or more, based on 100 atomic % of the total of all elements detected by XPS analysis, and preferably has a nitrogen content of 20 atomic % or less, more preferably 10 atomic % or less.
Furthermore, the nitrogen-containing carbon material preferably has an oxygen content of 20 atomic % or less, more preferably 19 atomic % or less, even more preferably 18 atomic % or less, and particularly preferably 17 atomic % or less, based on 100 atomic % of the total of all elements detected by XPS analysis.

The nitrogen-containing carbon material may further have a functional group such as a sulfur-containing group. However, the amount of elements other than carbon, hydrogen, oxygen, and nitrogen is preferably 3 atomic % or less, more preferably 1 atomic % or less, relative to 100 atomic % of the total of all elements detected by XPS analysis. It is even more preferable that the nitrogen-containing carbon material contains only carbon, hydrogen, oxygen, and nitrogen as constituent elements.
The XPS analysis is carried out under the conditions of an X-ray source of Mg-Kα and a pass energy of 10 eV.

The nitrogen-containing carbonaceous material to be irradiated with microwaves in the irradiation step preferably has a particle size in the range of 0.01 mm or more and 2.4 mm or less.
The particle size range is more preferably 0.02 mm or more and 2.0 mm or less, even more preferably 0.05 mm or more and 1.5 mm or less, and particularly preferably 0.1 mm or more and 1.0 mm or less.
The nitrogen-containing carbon material having a particle size within the above range can be obtained, for example, by a classification operation using a sieve having openings within the above particle size range.
It is also preferable that the average particle size of the nitrogen-containing carbonaceous material be within the above-mentioned preferred particle size range. For example, in the production method of the present invention, the nitrogen-containing carbonaceous material to be irradiated with microwaves in the irradiation step preferably has an average particle size of 0.01 mm or more and 2.4 mm or less. The average particle size is a volume-based average particle size measured with a laser diffraction/scattering particle size distribution analyzer.

The nitrogen-containing carbon material may be a mixture with other components when irradiated with microwaves, but the nitrogen-containing carbon material content in the mixture is preferably 90% by mass or more, more preferably 95% by mass or more, and even more preferably 99% by mass or more, and it is particularly preferable for the mixture to consist essentially of nitrogen-containing carbon material.

Doping a carbon material with nitrogen atoms to form a nitrogen-containing carbon material can be performed by adding ammonia water to the carbon material and performing a hydrothermal treatment, by calcining the carbon material under a flow of ammonia and/or amine-containing gas, or by a combination of these methods (e.g., by hydrothermally treating the carbon material and then calcining it). Examples of carbon materials to be doped with nitrogen atoms include graphite oxide, carbon nanofibers, and carbon nanotubes, with graphite oxide being preferred. Note that the graphite oxide used as the carbon material to be doped with nitrogen atoms may be reduced graphite oxide, or may be one that is reduced when doped with nitrogen atoms to become nitrogen-doped reduced graphite oxide.
When graphite oxide is used as the carbon material to be doped with nitrogen atoms, after doping with nitrogen atoms, the carbon material may be calcined under a flow of an inert gas such as nitrogen in order to further promote reduction.
When reduced graphite oxide is used as the carbon material to be doped with nitrogen atoms, the reduced graphite oxide can be obtained by reducing graphite oxide by liquid phase reduction, gas phase reduction, or the like.

The temperature of the hydrothermal treatment can be, for example, 80 to 250° C. The temperature is preferably 140 to 220° C. The time of the hydrothermal treatment is, for example, preferably 1 to 60 hours, and more preferably 2 to 18 hours.
The baking temperature can be, for example, 200 to 1000° C. The baking temperature is preferably 300 to 800° C. The baking time is, for example, preferably 10 minutes to 10 hours, and more preferably 30 minutes to 5 hours.

The modified nitrogen-containing carbon material obtained by the above-mentioned irradiating step preferably has a ratio of the peak intensity of the D band to the peak intensity of the G band in the Raman spectrum (I _D /I _G ) of 2.0 or less, more preferably 1.8 or less, and even more preferably 1.6 or less. The absolute value of the peak intensity ratio (I _D /I _G ) has no particular lower limit and may be 0.
Furthermore, the irradiation step preferably reduces the peak intensity ratio ( _I / _I ) of the modified nitrogen-containing carbonaceous material by 0.05 or more, more preferably by 0.1 or more, and even more preferably by 0.3 or more, compared to the _I /I _ratio of the starting nitrogen-containing carbonaceous material. The upper limit of the reduction in _I / _I is not particularly limited, but is usually 2 or less.
The peak intensity ratio (I _D /I _G ) can be measured by the method described in the Examples.

The production method of the present invention includes a step of supporting a transition metal element on the modified nitrogen-containing carbonaceous material obtained in the above-mentioned irradiating step.
Examples of the supporting step include a method using chemical vapor deposition (CVD) or the like in which the raw materials, the modified nitrogen-containing carbon material, and the transition metal element-containing component, are placed in a non-contact state, and a method in which the raw materials are dissolved in a solvent and brought into contact with each other, and among these, the chemical vapor deposition method is preferred.
In this specification, "supporting a transition metal element" means that the transition metal element is attached to the surface of the (modified) nitrogen-containing carbon material, and may be, for example, chemically bonded via a covalent bond, an ionic bond or a coordinate bond, or physically adsorbed by an intermolecular force.

The temperature in the loading step can be, for example, 200 to 1000° C. The temperature is preferably 300 to 900° C., and more preferably 500 to 850° C. The time for the loading step is, for example, preferably 30 minutes to 60 hours, more preferably 1 to 30 hours, and even more preferably 2 to 18 hours.
The above-mentioned supporting step may be carried out in air, in the presence (under flow) of an active gas such as oxygen, or in the presence of an inert gas such as nitrogen, helium, or argon, but is preferably carried out in the presence of an inert gas.
The above-mentioned supporting step may be carried out under increased pressure, normal pressure, or reduced pressure.

The type of transition metal element in the above-mentioned supporting step is not particularly limited, but is preferably at least one selected from the group consisting of manganese, iron, cobalt, and nickel, more preferably iron and/or cobalt, and even more preferably iron.
As the transition metal element-containing component as a raw material, simple substances, oxides, hydroxides, oxyhydroxides, carbonates, sulfates, nitrates, acetates, halides, ammonium salts, oxalates, alkoxides, etc. of the transition metal elements can be used, and among these, sulfates, nitrates, and halides are preferred, halides are more preferred, and chlorides and bromides are even more preferred.
In the above-mentioned supporting step, when the raw material is dissolved in a solvent and contacted, the solvent can be water or an organic solvent capable of dissolving the raw material. If necessary, an acid such as hydrochloric acid or sulfuric acid can be used.
In the supporting step, the mass ratio of the modified nitrogen-containing carbonaceous material as the raw material to the transition metal element-containing component is preferably 100:1 to 1:10, more preferably 50:1 to 1:1, even more preferably 40:1 to 3:1, and particularly preferably 30:1 to 5:1.

The transition metal-supported nitrogen-containing carbonaceous material obtained by the manufacturing method of the present invention can be subjected to acid washing, water washing, drying, etc. as appropriate.

The transition metal-supported nitrogen-containing carbon material obtained by the production method of the present invention preferably has a BET specific surface area of 100 m ² /g or more, more preferably 150 ^{m 2} /g or more, and even more preferably 200 m ² /g or more. There is no particular upper limit to the BET specific surface area, but it is usually 1500 ^{m 2} /g or less.
The BET specific surface area can be measured by the method described in the Examples.

The transition metal-supported nitrogen-containing carbon material obtained by the production method of the present invention preferably has an absolute current density normalized by the transition metal content (mass%) at a potential of 0.7 V relative to a hydrogen electrode (RHE) of 0.01 mA/( ^cm2 ·wt%-Fe) or more, more preferably 0.1 mA/( ^cm2 ·wt%-Fe) or more, even more preferably 1.0 mA/( ^cm2 ·wt%-Fe) or more, and particularly preferably 2.0 mA/( ^cm2 ·wt%-Fe) or more. There is no particular upper limit to the absolute value of the current density, but it is usually 10 mA/( ^cm2 ·wt%-Fe) or less.
The current density can be measured by the method described in the Examples.

The transition metal-supported nitrogen-containing carbon material obtained by the production method of the present invention has a sufficiently suppressed change in surface area while sufficiently maintaining the pyridine-type nitrogen content, and has excellent oxygen reduction reaction activity, making it useful as a catalyst such as an oxygen reduction catalyst, a semiconductor, etc.
Among these, the transition metal-supported nitrogen-containing carbon material is preferably one used for an oxygen reduction catalyst.

<Transition metal-supported nitrogen-containing carbon material>
The present invention also relates to a transition metal-supported, nitrogen-containing carbon material, characterized in that a pretreated product obtained by evacuating a transition metal-supported, nitrogen-containing carbon material at a temperature of 473 K and a pressure of 0.01 Pa or less for 180 minutes has a number of adsorbed water molecules per unit surface area of 0.8 ^{nm or} less at a temperature of 298 K and a relative humidity of 25% RH.
The number of water molecules adsorbed is preferably 0.7 nm ⁻² or less, more preferably 0.65 nm ⁻² or less, and even more preferably 0.6 nm ⁻² or less.
The number of adsorbed water molecules is preferably 0.1 nm ⁻² or more, more preferably 0.2 nm ⁻² or more, and even more preferably 0.4 nm ⁻² or more, thereby providing better oxygen reduction performance.
It can be said that, under the above-mentioned sample pretreatment and measurement conditions, the measured value of the number of adsorbed water molecules does not substantially fluctuate and is uniquely determined.

The transition metal-supported nitrogen-containing carbon material of the present invention preferably has a transition metal element content of 0.01 mass % or more, more preferably 0.05 mass % or more.
The transition metal-supporting nitrogen-containing carbon material of the present invention preferably has a transition metal element content of 5 mass % or less, more preferably 1 mass % or less.
The content of the transition metal element can be measured by ICP emission spectroscopy described in the examples.

The transition metal-supported nitrogen-containing carbon material of the present invention can be suitably obtained by the manufacturing method of the present invention described above.

The present invention will be explained in more detail below with reference to the following examples, but the present invention is not limited to these examples. Unless otherwise specified, "parts" means "parts by mass" and "%" means "% by mass."

In the following examples and comparative examples, the analyses and evaluations were carried out as follows.
<Method for measuring BET specific surface area>
The BET specific surface area was measured using an automatic specific surface area meter (BELSORP-mini II, manufactured by Microtrac-Bell Co., Ltd.) and nitrogen as the adsorption gas. Each sample was pretreated at 250°C for 4 hours under a nitrogen stream before the adsorption measurement.

<Evaluation of the progress of defect reduction>
The degree of defect reduction in the sample was evaluated by Raman spectroscopy according to the method described in Cancado, LG et al., App. Phys. Lett. 88.16 (2006) 163106-163106.

<ICP Atomic Emission Spectroscopy (Inductively Coupled Plasma Atomic Emission Spectroscopy: ICP-AES)>
The sample, concentrated nitric acid, and hydrogen peroxide solution were placed in a TFM decomposition vessel and treated using a microwave sample pretreatment device (ETHOS One, manufactured by Milestone General Co., Ltd.). After that, 35% hydrochloric acid was added and the solution was diluted with ultrapure water. The solution was filtered, and the filtrate was analyzed using a multi-type ICP optical emission spectrometer (ICPE-9000, manufactured by Shimadzu Corporation).

<Preparation of nitrogen-doped carbon substrate (NC)>
(Preparation Example 1 [Preparation of nitrogen-doped reduced graphite oxide NrGO-1])
Reduced graphite oxide rGO-1 (liquid-phase reduced product, sample weight: 500 mg) prepared by the method of Preparation Example 2 described in JP 2021-006497 A and classified to a particle size range of 0.6 mm to 1.0 mm was nitrogen-doped by treating it at 800°C for 100 minutes in a NH ₃ flow (NH ₃ gas flow rate: 40 mL/min) to obtain nitrogen-doped reduced graphite NrGO-1 (Figure 1).

(Preparation Example 2 [Preparation of Modified Nitrogen-Doped Reduced Graphite Oxide MW-NrGO-1])
Nitrogen-doped reduced graphite oxide NrGO-1 (sample weight: 350 mg) was placed in a quartz test tube (tubular reactor). While _NH3 gas was passed from one end of the test tube to the other ( _NH3 gas flow rate: 15 mL/min), microwave irradiation was performed using a microwave oven set to an output of 700 W for 5 minutes to obtain modified nitrogen-doped reduced graphite oxide MW-NrGO-1 (Figure 2).

(Preparation Example 3 [Preparation of nitrogen-doped reduced graphite oxide NrGO-2])
20 g of a graphene oxide (GO) aqueous dispersion (non-volatile content: 2.8% by mass), 12 g of a 25% by mass aqueous ammonia solution, and 8 g of water were mixed to obtain an aqueous solution with a non-volatile content of 1.4% by mass. The aqueous solution was placed in a Teflon (registered trademark) container and heated at 180°C for 12 hours for hydrothermal treatment.
The solid obtained after the hydrothermal treatment was placed in a container, and tertiary butyl alcohol (TBA) was added. The mixture was then heated for 12 hours in a constant-temperature shaking tank at 50°C. After cooling and solid-liquid separation, fresh TBA was added. This procedure was repeated five times to perform TBA substitution.
After TBA substitution, the mixture was freeze-dried at -10°C for 2 weeks.
The mixture was carbonized by heating at 1000°C for 3 hours in a nitrogen atmosphere to obtain nitrogen-doped reduced graphite oxide NrGO-2 (Figure 4).

(Preparation Example 4 [Preparation of Modified Nitrogen-Doped Reduced Graphite Oxide MW-NrGO-2])
[Microwave Modification of Nitrogen-Doped Reduced Graphite Oxide NrGO-2]
Nitrogen-doped reduced graphite oxide NrGO-2 (sample weight: 20 mg or less) was placed in a quartz test tube (tubular reactor), and while _NH3 gas was flowing from one end of the test tube to the other ( _NH3 gas flow rate: 15 mL/min), microwave irradiation was performed using a microwave oven set to an output of 700 W for 5 minutes to obtain modified nitrogen-doped reduced graphite oxide MW-NrGO-2 (using an apparatus similar to that shown in Figure 2).
Table 1 below shows the BET specific surface area of the samples obtained in each preparation example.

From the results in Table 1, it can be said that the BET specific surface area of the modified nitrogen-doped reduced graphite oxides MW-NrGO-1 and MW-NrGO-2 is sufficiently maintained compared to before modification. Furthermore, it can be said that the modified nitrogen-doped reduced graphite oxide MW-NrGO-2 is more porous than the modified nitrogen-doped reduced graphite oxide MW-NrGO-1.

FIG. 3 is a graph showing the pyridine-type nitrogen content and the graphite-type nitrogen content of nitrogen-doped reduced graphite oxide and modified nitrogen-doped reduced graphite oxide, respectively.
From the results in Figure 3, it can be said that the content of pyridine-type nitrogen, which has two bonds with carbon and is thought to contribute to the formation of catalytically active sites for the oxygen reduction reaction, is sufficiently maintained in the modified nitrogen-doped reduced graphites MW-NrGO-1 and MW-NrGO-2 compared to before modification.

5 is a graph showing Raman spectroscopy data (Raman spectra) of the samples (nitrogen-doped reduced graphite oxide and modified nitrogen-doped reduced graphite oxide) obtained in each preparation example. Table 2 below shows the ratio of the D band peak intensity to the G band peak intensity in the Raman spectrum of the samples obtained in each preparation example ( _ID / _IG ).

From the results in FIG. 5 and Table 2, a G band peak and a D band peak were observed in all samples.
Furthermore, in the modified nitrogen-doped reduced graphite oxide MW-NrGO-2, the ratio of the peak intensities (I _D /I _G ) is significantly reduced compared to before modification, demonstrating the progress of defect reduction. In addition, it is believed that the modified nitrogen-doped reduced graphite oxide MW-NrGO-1 also has progressed in defect reduction compared to before modification.
The modified nitrogen-doped reduced graphite oxide MW-NrGO-2 has a larger 2D band peak at around 2700 cm ⁻¹ than before modification, suggesting that the number of regularly stacked layers of graphite oxide has decreased.

<Iron loading on nitrogen-doped carbon substrate (NC) by chemical vapor deposition (CVD)>
Example 1 (Preparation of iron-supported modified nitrogen-doped reduced graphite oxide Fe-MW-NrGO-1)
FIG. 6 is a schematic diagram showing a process for supporting iron on nitrogen-doped reduced graphite oxide or modified nitrogen-doped reduced graphite oxide by chemical vapor deposition (CVD).
The raw materials, modified nitrogen-doped reduced graphite oxide MW-NrGO-1 (30 mg) placed in a boat and iron(II) chloride tetrahydrate FeCl ₂ ·4H ₂ O (2 mg) placed in a boat, were placed in a quartz test tube (tubular reactor). N ₂ gas was flowed from one end of the test tube to the other at 650 mL/min, and the reaction was performed at 750°C for 3 hours to obtain iron-loaded modified nitrogen-doped reduced graphite oxide Fe-MW-NrGO-1 (see, for example, Figure 6).
FIG. 7 is a graph showing the temperature in the test tube versus time in the iron loading step shown in FIG.

(Comparative Example 1 [Preparation of iron-supported nitrogen-doped reduced graphite oxide Fe—NrGO-1])
Iron-supported nitrogen-doped reduced graphite Fe—NrGO-1 was obtained in the same manner as in Example 1, except that nitrogen-doped reduced graphite NrGO-1 (30 mg) was used as the raw carbon material instead of the modified nitrogen-doped reduced graphite MW-NrGO-1 (30 mg).

Example 2 (Preparation of a Physical Mixture of Iron and Modified Nitrogen-Doped Reduced Graphite Oxide, Fe-MW-NrGO-1A)
A boat containing a physical mixture of modified nitrogen-doped reduced graphite oxide MW-NrGO-1 (30 mg) and iron(II) chloride tetrahydrate FeCl ₂ ·4H ₂ O (2 mg) was placed in a tubular furnace and heat-treated at 750 °C for 3 hours while flowing nitrogen at 50 ml/min to obtain a physical mixture of iron and modified nitrogen-doped reduced graphite oxide, Fe-MW-NrGO-1A.

FIG. 8 is a graph showing the pyridine-type nitrogen content and graphite-type nitrogen content of nitrogen-doped reduced graphite oxide, modified nitrogen-doped reduced graphite oxide, nitrogen-doped reduced graphite oxide with iron supported thereon, and modified nitrogen-doped reduced graphite oxide with iron supported thereon.
In the nitrogen-doped reduced graphite oxide Fe-NrGO-1 (Comparative Example 1) on which iron is supported, and the modified nitrogen-doped reduced graphite Fe-MW-NrGO-1 (Example 1) on which iron is supported, it can be said that the content of pyridine-type nitrogen is sufficiently maintained compared to before iron was supported.

The iron element content of iron-loaded, nitrogen-doped reduced graphite oxide Fe-NrGO-1 (Comparative Example 1) and iron-loaded, modified nitrogen-doped reduced graphite oxide Fe-MW-NrGO-1 (Example 1) was measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES). The results are shown in Table 3 below.

(Evaluation of oxygen reduction reaction activity using a rotating electrode)
The catalytic activity was evaluated by linear sweep voltammetry (LSV) using a rotating disk electrode in _a 0.5 M _H2SO4 aqueous solution in which oxygen had been dissolved until saturated at room temperature. Each sample was pretreated and dried by heating at 80°C for 12 hours under vacuum (-0.1 MPa on a negative pressure gauge). 2.5 mg of the carbon material (catalyst) was weighed out and added to 25 μL of a 5% by mass Nafion dispersion, 300 μL of ethanol, and 300 μL of distilled water to prepare a catalyst ink. The catalyst ink was sonicated at 120 kHz for 30 minutes while ice-cooled, then an appropriate amount was drawn up using a micropipette. 4 μL of the ink was applied as a drop to a glassy carbon electrode in a rotating disk electrode apparatus and dried to prepare a working electrode (0.082 mg/ ^cm2 of catalyst was applied to the electrode). The electrode was rotated at 1600 rpm, and the potential was scanned at a 1 mV/s sweep rate from 0.05 to 1.05 V vs. The RHE was swept over a sweep range, and the current was recorded as a function of potential.
The ORR measurement results for each sample are shown in Figures 9 and 10. Compared to the nitrogen-doped reduced graphite oxide NrGO-1, the modified nitrogen-doped reduced graphite MW-NrGO-1 and the iron-loaded nitrogen-doped reduced graphite Fe-NrGO-1 each exhibited reduced overpotentials. Furthermore, the physical mixture of iron and modified nitrogen-doped reduced graphite Fe-MW-NrGO-1A and the iron-loaded modified nitrogen-doped reduced graphite Fe-MW-NrGO-1 each exhibited significantly reduced overpotentials. In particular, the iron-loaded modified nitrogen-doped reduced graphite Fe-MW-NrGO-1 exhibited a significant reduction in overpotential, resulting in a significant improvement in the absolute value of the current density at a potential of 0.7 V vs. the hydrogen electrode (RHE).
Thus, it is believed that a sample obtained by modifying a nitrogen-containing carbon material having a G-band peak in the Raman spectrum by microwave irradiation and then supporting a transition metal element thereon can sufficiently reduce the activation energy required for oxygen reduction.

(Evaluation of surface hydrophobicity of substrate and catalyst)
11 is a graph showing the number of water molecules adsorbed per unit surface area (nm −2 ) versus relative humidity p/p0 at 298 K after pretreatment of nitrogen-doped reduced graphite oxide, modified nitrogen-doped reduced graphite oxide, iron-loaded nitrogen-doped reduced graphite, and iron-loaded modified ^nitrogen -doped reduced graphite. Table 4 below shows the BET specific surface area of each of the nitrogen-doped reduced graphite oxide, modified nitrogen-doped reduced graphite oxide, iron-loaded nitrogen-doped reduced graphite, and iron-loaded modified nitrogen-doped reduced graphite.

From the comparison in Figure 11 between the modified nitrogen-doped reduced graphite MW-NrGO-1 and the nitrogen-doped reduced graphite NrGO-1 before modification, and the comparison between the modified nitrogen-doped reduced graphite Fe-MW-NrGO-1 with the iron-doped nitrogen-doped reduced graphite Fe-NrGO-1, it was found that microwave irradiation improves the surface hydrophobicity of carbon materials.
The hydrophobicity of the surface of the nitrogen-containing carbon material, moderately improved by microwave irradiation, may affect the oxygen reduction performance. That is, moderately improved surface hydrophobicity may make it easier for gaseous oxygen to adsorb to the surface than for liquids such as electrolytes, thereby further promoting the oxygen reduction reaction. This effect, combined with the oxygen reduction performance improvement effect of supporting the transition metal via nitrogen atoms, is thought to synergistically improve the oxygen reduction performance. Note that if the surface hydrophobicity of the carbon material is improved too much by a modification method other than microwave irradiation, it may become more difficult for the carbon material to receive protons necessary for oxygen reduction from the liquid acid component, resulting in a risk of reduced oxygen reduction performance.

Claims (6)

Irradiating a nitrogen-containing carbon material having a G-band peak in a Raman spectrum with microwaves;
The method for producing a transition metal-supported nitrogen-containing carbon material comprises a step of supporting a transition metal element on the modified nitrogen-containing carbon material obtained in the irradiating step. The method for producing a transition metal-supported nitrogen-containing carbon material according to claim 1, characterized in that the irradiation step is carried out in an atmosphere containing ammonia and/or an amine. The method for producing a transition metal-supported nitrogen-containing carbon material according to claim 1 or 2, characterized in that the nitrogen-containing carbon material irradiated with microwaves in the irradiation step is one in which graphite oxide is mixed and/or adsorbed, and/or contains nitrogen-doped graphite oxide. The method for producing a transition metal-supported nitrogen-containing carbon material according to any one of claims 1 to 3, characterized in that the transition metal element is at least one selected from the group consisting of manganese, iron, cobalt, and nickel. The method for producing a transition metal-supported nitrogen-containing carbon material described in any one of claims 1 to 4, characterized in that the transition metal-supported nitrogen-containing carbon material is used as an oxygen reduction catalyst. A transition metal-supported nitrogen-containing carbon material, characterized in that a pretreated product obtained by evacuating a transition metal-supported nitrogen-containing carbon material at a temperature of 473 K and a pressure of 0.01 Pa or less for 180 minutes has a number of adsorbed water molecules per unit surface area of 0.7 nm ^or less at a temperature of 298 K and a relative humidity of 25% RH.