Graphene-containing composite structure and method for producing the same

Abstract

To provide a graphene-containing composite structure having high catalytic activity and a method for producing the same.SOLUTION: A composite structure comprises graphene to which an impurity-doped graphene multilayer film is given on a surface of Ni nanoparticles in which carbon is solidly dissolved. The composite structure is produced by: adjacently stacking a mixture of a nanometallic powder comprising Ni, a metal carbide heated by microwaves, and a zeolite-based catalyst, and a susceptor containing an impurity source that imparts impurities and heated by microwaves; and distributing a reaction gas containing a hydrocarbon while irradiated with microwaves.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a composite structure containing graphene having high catalytic activity, and a method for producing the same. [Background technology]

[0002] Electrochemical cells such as water electrolysis devices, fuel cells, and air secondary batteries use catalysts in their electrodes, typically platinum. However, given the depletion of metal resources, alternative catalysts are desired, and graphene-based catalysts have been proposed.

[0003] For example, Patent Document 1 discloses a catalyst for a water electrolysis electrode that uses nickel-carbon nano-onion (Ni-CNO), which is made by providing a multilayer graphene film on the surface of particulate nickel. By applying such a catalyst as a mixed solution with an ionomer to one side of a carbon cloth that constitutes a gas diffusion layer to form a water electrolysis electrode, it is possible to obtain high energy conversion efficiency and to manufacture the electrode inexpensively.

[0004] Patent Document 2 also discloses a method for producing a composite structure composed of Ni-CNO using microwave irradiation. A composition containing silicon carbide, nickel, and zeolite particles, each of which acts as a catalyst, is activated by multimode microwave irradiation, and then contacted with a hydrocarbon gas such as methane. A reaction using the carbon compounds contained in the hydrocarbon gas as a substrate proceeds, resulting in graphene being supported on the nickel surface in the composition. One mechanism for the graphene production process is described as follows: silicon carbide absorbs microwaves, generates heat, thermally activates nickel, and promotes the thermal decomposition of the carbon compounds. The template structure of the zeolite, which has pores approximately the same size as aromatic rings, induces the synthesis of a carbon structure forming a graphene skeleton, and the carbon growth effect of nickel promotes the growth of the six-membered ring carbon structure of graphene.

[0005] Non-Patent Document 1 states that defects widely present in nanomaterials become localized regions that confer catalytic activity, and that the addition (doping) of heteroatoms (impurities) as zero-dimensional point defects becomes defects that confer catalytic activity. It then discusses the experimental results of the catalytic activity of boron-doped carbon nano-onions (CNO). It states that such CNO can be obtained by heat-treating ultra-dispersed nanodiamond particles and amorphous boron. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent Publication No. 2021-46585 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-64369 [Non-patent literature]

[0007] [Non-Patent Document 1] ACS Appl. Mater. Interfaces 2021, 13, 51628-51642 Summary of the Invention [Problem to be solved by the invention]

[0008] Introducing defects into graphene can impart catalytic activity. In the Ni-CNO described above, defects are generated by forming a multilayer graphene structure by applying curvature to the Ni nanoparticles, resulting in catalytic activity. However, the catalytic activity is unstable and insufficient. Therefore, it is possible to dope impurities into graphene to further enhance its catalytic activity.

[0009] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a composite structure including doped graphene having high catalytic activity, and a method for producing the same. [Means for solving the problem]

[0010] The graphene-containing composite structure according to the present invention is characterized in that an impurity-doped graphene multilayer film is provided on the surface of Ni nanoparticles in which carbon is dissolved.

[0011] This feature allows for high catalytic activity in Ni-CNO.

[0012] In the above-described invention, the impurities may be silicon or boron. The graphene multilayer film may have a gradient in which the concentration of the impurities decreases from the interior to the surface. This feature allows stable, high catalytic activity to be obtained in Ni-CNO.

[0013] The graphene multilayer film may have an etched surface. The graphene multilayer film may have a G / D ratio of a Raman spectrum of greater than 1.30. This feature provides a new surface to the graphene multilayer film of Ni-CNO, thereby achieving stable and high catalytic activity.

[0014] Furthermore, the manufacturing method according to the present invention is a method for manufacturing a composite structure in which an impurity-doped graphene multilayer film is applied to the surface of Ni nanoparticles that have carbon dissolved therein, and is characterized in that a mixture of a nanometal powder made of Ni, a metal carbide heated by microwaves, and a zeolite-based catalyst is adjacent to a susceptor that contains an impurity source that provides the impurities and is heated by microwaves, and a reaction gas containing hydrocarbons and carbon dioxide is circulated while irradiating microwaves.

[0015] According to this feature, a catalyst made of Ni-CNO having high catalytic activity can be stably obtained.

[0016] In the above-described invention, the susceptor may be made of silicon carbide. Furthermore, the reactive gas may contain hydrogen sulfide. Alternatively, the susceptor may be made of boron oxide. According to these features, a catalyst made of Ni-CNO with high catalytic activity can be obtained more stably.

[0017] The above-described invention may further include an etching step of etching the surface of the graphene multilayer film on the surface of the Ni nanoparticles after stopping the flow of the reaction gas. Furthermore, the etching step may be performed by gas-phase chemical etching using carbon dioxide. This feature allows for the stable production of a catalyst made of Ni-CNO with higher catalytic activity.

[0018] In the above invention, the hydrocarbon may be methane. According to this feature, a catalyst made of Ni-CNO having high catalytic activity can be obtained more stably.

[0019] In the above-mentioned invention, the metal carbide and the zeolite catalyst are respectively a molybdenum carbide and a zeolite catalyst. H type The catalyst may be ZSM-5, which makes it possible to more stably obtain a catalyst made of Ni-CNO having high catalytic activity. [Brief explanation of the drawings]

[0020] [Figure 1] This is a TEM image of Ni-CNO. [Figure 2] FIG. 2 is a cross-sectional view showing the internal configuration of the reaction tube. [Figure 3] 1 is a graph showing the relationship between the distance from the surface of a Ni nanoparticle in a graphene multilayer film and the Si concentration, measured by a STEM-EDX method. [Figure 4] This is a photoelectron spectrum obtained by X-ray photoelectron spectroscopy. [Figure 5] TEM image of graphene multilayer film after CO2 etching. [Figure 6] Raman spectrum of graphene multilayer film after CO2 etching. DETAILED DESCRIPTION OF THE INVENTION

[0021] A composite structure as one embodiment of the present invention will be described with reference to FIGS. 1 to 6. FIG.

[0022] As shown in Fig. 1, the composite structure 1 according to this example comprises a graphene multilayer film 3 doped with impurities on the surface of a Ni nanoparticle 2 containing carbon as a solid solution. That is, the composite structure has a nickel carbon nano-onion (Ni-CNO) structure in which the Ni nanoparticle 2 serves as a central core and the graphene multilayer film 3 doped with impurities is provided around the central core in the form of a shell. For example, silicon or boron can be suitably used as the impurity.

[0023] The composite structure 1 is obtained by generating defects in the graphene structure by imparting curvature to the graphene multilayer film 3 and further increasing or generating defects in the graphene structure by doping with impurities. The composite structure 1 can obtain stable high catalytic activity due to its structure having a stable number of defects.

[0024] As shown in Fig. 2, a reaction tube 10 can be used in a method for producing such a composite structure 1. Inside the reaction tube 10, catalyst layers 11 and susceptor layers 12 are alternately stacked adjacent to each other. The catalyst layer 11 contains a mixture of a nanometal powder made of Ni, a metal carbide heated by microwaves, and a zeolite catalyst. The susceptor layer 12 contains an impurity source that provides impurities and a susceptor that is heated by microwaves.

[0025] The reaction tube 10 is installed inside a single-mode or multi-mode microwave reactor with a frequency of 0.3 to 3.0 GHz, typically 2.45 GHz, so that the susceptor layer 12 and other components inside the reaction tube 10 can be heated by microwaves. A reaction gas can be introduced into the reaction tube 10 from the top, circulated through the interior, and discharged from the bottom. The reaction gas contains a hydrocarbon. Methane is a suitable hydrocarbon. To allow the reaction gas to flow through the reaction tube 10, the catalyst layer 11 and the susceptor layer 12 each have a shape that allows the reaction gas to flow through. For example, by laying each material in a pebbled form, a layer through which the reaction gas can flow can be obtained relatively easily. Because the reaction gas partially convects and diffuses inside the reaction tube 10, it is not necessarily required to flow from top to bottom; other flow directions, such as introducing the gas from the side, may also be used.

[0026] When the reaction gas is introduced while irradiating the reaction tube 10 with microwaves to heat the susceptor layer 12, the impurity source is heated and decomposed, liberating impurity atoms, and the hydrocarbons in the reaction gas are decomposed by the catalyst layer 11. The decomposed carbon then captures the impurity atoms and forms a multilayered graphene film on the surface of the nanometal powder. In other words, the hydrocarbon-derived carbon that was unable to dissolve in the nucleus Ni nanoparticles 2 under the thermal environment forms layers together with the impurity dopant. This allows for the production of a composite structure 1 in which the graphene multilayered film 3 is doped with impurities.

[0027] The susceptor layer 12, which serves as a heat source due to microwave irradiation, may be disposed inside the catalyst layer 11 or mixed therewith, so that they are adjacent to each other as appropriate.

[0028] For example, a detailed description will be given of an example in which the composite structure 1 is obtained by doping silicon (Si) as an impurity.

[0029] In the catalyst layer 11, Ni powder with a particle size of 150 nm or less (for example, 20 to 150 nm) was used as the nanometal powder, and molybdenum carbide, a carbide of molybdenum, was used as the metal carbide. Here, the particle size of the nanometal powder can be determined by the following method. That is, the particles of the nanometal powder to be measured were observed with a scanning transmission electron microscope, and the minor axis and major axis of each particle were measured, and the average value was used as the particle size of that particle. In the same manner, the particle sizes of 100 particles were determined, and the average value was used as the particle size of the nanometal powder.

[0030] The zeolite catalyst used was H-type ZSM-5 (with a silicon dioxide / alumina weight ratio of 23). Silicon carbide (β-SiC) was used as the susceptor in the susceptor layer 12. Silicon carbide contained silicon, an impurity element that served as a dopant, and also served as an impurity source for supplying silicon. A quartz tube was used as the reaction tube 10. The reaction tube 10, with the catalyst layer 11 and susceptor layer 12 stacked adjacently inside, was installed in a microwave reactor. A multimode microwave generator (μ-Reactor-Ex, manufactured by Shikoku Keisoku Kogyo Co., Ltd., magnetron transmission frequency: 2.45 GHz, maximum transmission power: 1 kW) was used as the microwave reactor.

[0031] While introducing methane as a reaction gas from the top of the reaction tube 10, microwaves were irradiated to the reaction tube 10 using a microwave reactor. As a result, a composite structure 1 (hereinafter referred to as Ni-CNO) having a silicon-doped graphene multilayer film was obtained.

[0032] The obtained Ni-CNO is mixed with the catalytic components, consisting of the metal carbide and zeolite catalyst used in the catalytic layer 11, and can be separated from them. For example, separation is possible by washing using a liquid-liquid interface and acid washing. Specifically, Ni-CNO containing the catalytic components is dispersed in a mixture of distilled water and toluene at a liquid-liquid interface, and the distilled water is removed. This process is repeated several times. The Ni-CNO captured in the toluene oil phase is then filtered, washed with distilled water and ethanol, and dried. The mixture is then placed in a 30% hydrogen peroxide solution heated in a water bath and stirred for 30 minutes. The Ni-CNO can then be separated from the catalytic components by filtering and washing with distilled water and ethanol.

[0033] On the other hand, when the composite structure 1 is obtained by doping with boron (B) as an impurity, solid boron oxide (BO) is used for the susceptor layer 12, and the boron impurity is supplied from this boron oxide. Methane is used as the reaction gas, but it is preferable to mix carbon dioxide. Carbon dioxide etches the boron oxide, thereby promoting the decomposition of the boron oxide and the doping of boron. A boron-doped composite structure 1 can also be obtained in the same manner as above. Note that boron carbide or boron nitride can also be used as a susceptor for supplying boron as an impurity. Furthermore, when the reaction gas contains both a hydrocarbon (preferably methane) and carbon dioxide, the content of carbon dioxide relative to the total volume of the hydrocarbon and hydrogen sulfide is preferably 20 to 80% by volume, more preferably 40 to 60% by volume.

[0034] Below, we will explain the results of a more detailed investigation of Ni-CNO obtained using silicon as a dopant.

[0035] Here, as shown in Figure 3, we investigated the impurity concentration when methane and hydrogen sulfide were mixed in a volume ratio of 50% each as the reaction gas. The amount of silicon in the Ni-CNO film from the surface of the Ni nanoparticles 2 (Ni side) to the surface of the graphene multilayer film 3 (CNO side) was measured using energy-dispersive X-ray spectroscopy (STEM-EDX) in a scanning transmission electron microscope. According to the figure, when hydrogen sulfide was included in the reaction gas (introduced gas), the amount of doped silicon increased approximately three-fold compared to when hydrogen sulfide was not included. This is because etching silicon carbide with hydrogen sulfide increased the supply of silicon as a dopant. This promotes the doping of silicon into Ni-CNO, resulting in an increased amount of silicon doped. In other words, it is also preferable to include hydrogen sulfide in the reaction gas. When the reaction gas contains both hydrocarbon (preferably methane) and hydrogen sulfide, the content of hydrogen sulfide relative to the total volume of hydrocarbon and hydrogen sulfide is preferably 20 to 80% by volume, more preferably 40 to 60% by volume.

[0036] As shown in the figure, the silicon concentration tends to decrease from the surface of the Ni nanoparticles 2 toward the surface of the graphene multilayer film 3. In this way, the graphene multilayer film 3 may have a gradient in which the impurity concentration decreases from the interior toward the surface.

[0037] It is also preferable that the graphene multilayer film 3 has an etched surface. That is, Ni-CNO with an etched surface can be obtained by adding an etching step to the above manufacturing method. Specifically, silicon-doped Ni-CNO is immersed in a hypochlorous acid solution at room temperature for several tens of hours. The etched Ni-CNO and the unetched Ni-CNO were subjected to energy analysis by X-ray photoelectron spectroscopy to obtain photoelectron spectra.

[0038] As shown in Figure 4, the photoelectron spectra obtained without etching showed almost no clear peaks, whereas with etching, a clear peak corresponding to the Si impurity was observed. It is believed that etching creates a new CNO surface, resulting in changes in its surface properties. In other words, the properties of a CNO surface formed by regular growth through carbon deposition can differ from those of an etched surface formed by erosion from chemically selected sites. As a result, etching can increase the silicon concentration at the surface, resulting in stable and higher catalytic activity. Furthermore, as mentioned above, the silicon concentration tends to increase toward the interior of CNO. Therefore, etching that moves the surface toward the interior of CNO is preferable because it increases the silicon concentration at the new surface. Furthermore, the curvature of graphene increases toward the surface of the Ni nanoparticles, increasing the number of defects per unit area. This also contributes to the enhancement of catalytic activity.

[0039] The etching process can be a known wet etching process using a hot mixed acid or the like, or can be dry etching. For example, gas-phase chemical etching using carbon dioxide may be used. In the above-described method for producing Ni-CNO, after the formation of Ni-CNO, the flow of the reaction gas is stopped, and then carbon dioxide is circulated through the reaction tube 10 and microwaves are irradiated. Then, CNO is etched from the surface by the Boudouard reaction.

[0040] As shown in Figure 5, the etching conditions were compared depending on the carbon dioxide flow time. Here, the mass of the catalyst layer 11 was approximately 20 g, and carbon dioxide was flowed at 50 mL / min. As shown in Figure 5(a), a flow time of 50 minutes resulted in 60 to 80 graphene layers of CNO remaining, with a layer thickness of approximately 50 to 100 nm. Depending on the particle size of the Ni nanoparticles, spherical particles with a diameter of approximately 100 to 200 nm were formed. Furthermore, as shown in Figure 5(b), a flow time of 150 minutes resulted in 30 to 50 graphene layers of CNO remaining, with a layer thickness of several tens of nanometers and spherical particles with a diameter of approximately 50 to 100 nm. Furthermore, as shown in Figure 5(c), a flow time of 300 minutes resulted in 10 to 30 graphene layers of CNO remaining, with a layer thickness of approximately 10 nm, thinner than the diameter of the Ni nanoparticles, and spherical particles with a diameter of approximately 20 to 100 nm. This indicates that etching is promoted by heating and circulating carbon dioxide. The particle size of Ni-CNO can be measured in the same way as the particle size of nanometal powder.

[0041] As shown in Figure 6, the Raman spectra of CNO were measured at the three different flow times mentioned above, and the peak at 1,350 cm -1 The peak intensity ratio (G / D ratio) of the D-band, a nearby vibrational mode, to the G-band, a vibrational mode similar to the Raman-active mode of graphite, was calculated to evaluate the amount of defects introduced. The G / D ratio increased with increasing carbon dioxide flow time. These results indicate that dry etching can also change the surface properties of CNO. To achieve stable, high catalytic activity, it is desirable to etch the CNO Raman spectrum to a G / D ratio of greater than 1.10, preferably greater than 1.30, and more preferably greater than 1.50. The upper limit of the G / D ratio is not particularly limited, but is, for example, 2.50 or less.

[0042] Although the embodiments of the present invention and modifications based thereon have been described above, the present invention is not necessarily limited to these examples. Furthermore, those skilled in the art will be able to find various alternative embodiments and modifications without departing from the spirit of the present invention or the scope of the appended claims. [Explanation of symbols]

[0043] 1 Composite structure 2. Ni nanoparticles 3. Graphene multilayer film 10 Reaction tube 11 Catalyst layer 12 Susceptor layer

Claims

1. A composite structure having an average particle size of 200 nm or less, in which an impurity-doped graphene multilayer film is provided on the surface of Ni nanoparticles having carbon as a solid solution, the impurity is silicon or boron; The graphene multilayer film has a gradient in which the concentration of the impurities decreases from the inside toward the surface.

2. The graphene-containing composite structure according to claim 1 , wherein the graphene multilayer film has an etched surface.

3. 3. The graphene-containing composite structure according to claim 2, wherein the graphene multilayer film has a G / D ratio in a Raman spectrum of greater than 1.

30.

4. A method for producing a composite structure having an average particle size of 200 nm or less, comprising providing a graphene multilayer film doped with impurities on the surface of Ni nanoparticles having carbon dissolved therein, A method for producing a composite structure containing graphene, comprising: placing a mixture of nanometal powder made of Ni having an average particle size of 150 nm or less, metal carbide heated by microwaves, and a zeolite-based catalyst adjacent to a susceptor made of silicon carbide and containing an impurity source that provides the impurities, and heated by microwaves; and flowing a reaction gas containing hydrocarbons and hydrogen sulfide through the mixture while irradiating microwaves.

5. A method for producing a composite structure having an average particle size of 200 nm or less, comprising providing a graphene multilayer film doped with impurities on the surface of Ni nanoparticles having carbon dissolved therein, A method for producing a composite structure containing graphene, comprising: placing a mixture of nanometal powder made of Ni having an average particle size of 150 nm or less, metal carbide heated by microwaves, and a zeolite catalyst adjacent to a susceptor made of boron oxide and containing an impurity source that provides the impurities, and heated by microwaves; and flowing a reaction gas containing hydrocarbons through the mixture while irradiating microwaves.

6. 6. The method for producing a graphene-containing composite structure according to claim 4, further comprising an etching step of etching the surface of the graphene multilayer film on the surface of the Ni nanoparticles after stopping the flow of the reaction gas.

7. The method for producing a graphene-containing composite structure according to claim 6, wherein the etching step is performed by gas-phase chemical etching using carbon dioxide.

8. The method for producing a composite structure containing graphene according to claim 4 or 5, wherein the hydrocarbon is methane.

9. The method for producing a graphene-containing composite structure according to claim 4 or 5, wherein the metal carbide and the zeolite catalyst are molybdenum carbide and H-type ZSM-5, respectively.

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