JP5697519B2 - Method for producing metal-supported carbon - Google Patents

Description

  The present invention relates to a method for producing metal-supported carbon by supporting a metal on a carbon material such as vertically aligned carbon nanotubes or carbon nanowalls.

  As a carbon material having a very large specific surface area, vertically aligned carbon nanotubes (also referred to as vertically aligned CNTs) having a structure in which a large number of carbon nanotubes are aligned in the vertical direction and walls made of graphene sheets are in a certain direction Carbon nanowalls having a structure lined up in a well-known manner are known. Application of these carbon materials to electrode materials of batteries such as fuel cells has been studied.

  When a carbon material as described above is to be used for a battery electrode, it is often necessary to support a metal catalyst such as platinum. However, even when a metal catalyst dispersion is applied, it is difficult to carry a sufficient metal catalyst on the vertically aligned CNTs and carbon nanowalls. Therefore, some of the present inventors have developed a method for supporting a metal catalyst as described in Patent Document 1.

  In the method described in Patent Document 1, a metal compound to be supported in advance is dissolved in a supercritical fluid such as carbon dioxide, and is brought into contact with the carbon nanowall formed on the substrate. The metal is supported by heating and precipitating the metal on the carbon nanowall. In this method, a substrate formed with carbon nanowalls is placed in a lower vessel (reaction vessel 100) and filled with supercritical carbon dioxide, while an upper vessel (stirring vessel 200) connected to the lower vessel by a valve 210 is used. After filling the supercritical carbon dioxide in which the metal compound is dissolved and making the pressure in the upper container larger than the pressure in the lower container, the valve 210 is opened and the metal compound in the upper container is contained. Supercritical carbon dioxide is sent to the lower container at once.

JP 2006-273613 A

  According to the method using the supercritical fluid of Patent Document 1, it is possible to easily carry a metal on a carbon material such as carbon nanowall. However, in the procedure of Patent Document 1, it is difficult to uniformly carry a metal on a carbon material having a certain area. In addition, extra metal may adhere, which can be costly. In order to mass-produce the metal-carrying carbon material, a method capable of uniformly carrying the metal on the carbon material is required.

As a result of examining the problems as described above, the present inventors have found a better method for supporting a metal on a carbon material. The gist of the present invention is as follows.
[1] A method for producing metal-supported carbon using a supercritical fluid, using an apparatus having a first container and a second container that is connected to the first container via a valve. ,
(A) a step of enclosing a metal compound and a supercritical fluid precursor gas in a first container, and carbon and a supercritical fluid precursor gas in a second container,
(B) Pressurize each of the first container and the second container so that the pressure in the first container is lower than the pressure in the second container, thereby bringing the supercritical fluid precursor gas into a supercritical state. Process,
(C) The method comprising the step of opening the valve to allow communication between the first container and the second container.

[2] The method for producing metal-supported carbon according to [1], wherein after the step (c), (d) the valve is closed and then the second container is depressurized so that the pressure in the first container is A step of making the pressure higher than the pressure in the second container;
(E) The method further comprising the step of opening the valve again to allow the first container and the second container to communicate with each other.
[3] The method for producing metal-supported carbon according to [2], wherein the steps (d) and (e) are repeated twice or more.
[4] The method for producing metal-supported carbon according to any one of [1] to [3], comprising heating the carbon at a temperature in the range of 150 to 450 ° C.

[5] The method for producing metal-supported carbon according to any one of [1] to [4], wherein the metal compound is selected from trimethyl (methylcyclopentadienyl) platinum and acetylacetonate platinum.
[6] The method for producing metal-supported carbon according to any one of [1] to [5], wherein the supercritical fluid precursor gas is selected from carbon dioxide and trifluoromethane.
[7] Metal-supported carbon produced by the method according to any one of [1] to [6].

  According to the method of the present invention, it is possible to produce a carbon material on which a metal catalyst is uniformly supported. Carbon materials carrying a metal catalyst, particularly vertically aligned CNTs and carbon nanowalls, are useful as electrode materials for batteries such as fuel cells.

It is the schematic which showed an example of the apparatus used with the method of this invention. It is a figure which shows arrangement | positioning of the CNT board | substrate 6 on the heater 5 in an Example. It is a graph which shows the relationship between arrangement | positioning of the CNT board | substrate 6, and platinum carrying amount.

  FIG. 1 is a schematic view showing an example of an apparatus used in the method of the present invention. For convenience, the method of the present invention will be described along the apparatus of FIG. 1, but the apparatus used in the method of the present invention is not limited to this. The apparatus has a first container 1 and a second container 3. In FIG. 1, the first container 1 and the second container 3 are arranged up and down, but may be arranged left and right or upside down as necessary. A stirring propeller 2 is provided in the first container 1. However, the stirring propeller 2 is not an essential constituent element, and may be provided as necessary. The first container 1 is provided with an inlet for the supercritical fluid precursor gas and the metal compound solution, and the second container 3 is provided with an inlet for the supercritical fluid precursor gas.

  The first container 1 and the second container 3 are connected via a valve 4 so as to communicate with each other. The first container 1 and the second container 3 can be pressurized to different pressures when the valve 4 is closed, but the first container 1 and the second container 3 are open when the valve 4 is opened. The container 3 has the same pressure.

  A heater 5 is provided inside the second container 3, and a carbon material 6 is placed thereon. Examples of the carbon material 6 include carbon nanotubes, particularly vertically aligned carbon nanotubes, or carbon nanowalls. The carbon material 6 is preferably in the form of a carbon material layer formed on a substrate made of silicon, quartz, SUS, aluminum or the like. The carbon material 6 is not limited to carbon nanotubes or carbon nanowalls, and may be other carbon blacks that do not have a cylindrical or wall-like structure.

  The metal compound solution poured into the first container 1 is obtained by dissolving a metal compound to be supported on carbon in an appropriate solvent. The metal supported on carbon is not particularly limited as long as it has catalytic activity. For example, noble metals such as platinum, palladium and gold, iron, ruthenium, cobalt, nickel, manganese, chromium, vanadium, titanium, niobium and molybdenum Base metals such as lead, rhodium, tungsten and iridium. As a metal catalyst, platinum is particularly excellent.

  The metal compound is not particularly limited as long as it can be deposited on the carbon and then deposited by heating or the like. The metal compound includes a metal complex and a metal compound. Specific examples of the metal complex include cyclopentadienyl metal complexes such as cyclopentadienyl ligand, methylcyclopentadienyl ligand, ethylcyclopentadienyl ligand, n-butylcyclopentadiene. Metal complexes in which a ligand having a cyclopentadienyl ring such as an aryl ligand, (trimethyl) methylcyclopentadienyl ligand, and the like, and acetylacetonate in which an acetylacetonate ligand is coordinated A metal complex is mentioned. Specific examples of the metal compound include metal chlorides, oxides, and salts with inorganic acids such as nitric acid and sulfuric acid or organic acids such as acetic acid.

  The solvent in which the metal compound is dissolved is not particularly limited, and those skilled in the art can appropriately select from hydrocarbon solvents such as hexane, cyclohexane, benzene, toluene, and other solvents.

  The supercritical fluid precursor gas is not particularly limited as long as it can be led to the supercritical state relatively easily and does not damage the metal catalyst or the carbon material. In view of handling and characteristics, it is preferable to use carbon dioxide or trifluoromethane.

  Hereinafter, the method for producing metal-supported carbon of the present invention performed using the apparatus of FIG. 1 will be described. First, a metal compound solution and a supercritical fluid precursor gas are sealed in the first container 1 with the valve 4 closed. On the other hand, a carbon material 6 is placed on the heater 5 in the second container 3 and a supercritical fluid precursor gas is sealed [step a]. Here, either the enclosure operation in the first container 1 or the enclosure operation in the second container 3 may be performed first, or both may be performed simultaneously. Further, the metal compound solution and the supercritical fluid precursor gas may be sealed in the first container 1 either first, but usually the metal compound solution is first charged.

  Next, the first container 1 and the second container 3 are pressurized, and at that time, the pressure in the first container 1 is made lower than the pressure in the second container 3 [step b]. . Next, when the valve 4 is opened, the supercritical fluid flows from the second container 3 having a relatively high pressure into the first container 1 [step c]. Due to the inflow of the supercritical fluid from the second container 3, the metal compound solution that often stays in the piping near the valve 4 flows back into the first container 1 and is sufficiently mixed with the supercritical fluid. And diffused (dispersed). Thereby, it is possible to prevent the metal compound solution having a high concentration from being directly sprayed onto the carbon material 6. By opening the valve 4, the pressure in the first container 1 and the pressure in the second container 3 become equilibrium pressures. The opening of the valve 4 here is preferably relatively short, for example less than 5 minutes, in particular less than 2 minutes, in particular about 1 minute.

  Next, after a predetermined time has elapsed, the valve 4 is closed and the second container 3 is depressurized so that the pressure in the first container 1 becomes higher than the pressure in the second container 3 [step d]. The depressurization can be performed, for example, by extracting a part of the supercritical fluid precursor gas from the container. By extracting part of the gas, an effect of promoting the diffusion of the supercritical fluid in the second container 3 can be expected. The pressure difference between the first container 1 and the second container 3 after depressurizing the second container 3 is within the range of 0.1 MPa to 1 MPa, particularly within the range of 0.1 MPa to 0.5 MPa. Is preferred. However, it is necessary to prevent the pressure in the second container 3 from falling below the critical pressure of the supercritical fluid.

  Next, when the valve 4 is opened again [step e], the supercritical fluid containing the solution of the metal compound staying in the first container 1 flows into the second container 3. By this procedure, the solution of the metal compound that has reached the equilibrium pressure and remains in the first container 1 can be drawn into the second solution 3, and the material yield can be improved. The above steps d and e are preferably repeated twice or more, preferably 3 times or more, more preferably 4 times or more. By repeating these steps, the material yield is further improved. The opening of the valve 4 in step e is not particularly limited as long as it is a time sufficient for the first container 1 and the second container 3 to reach an equilibrium pressure, but for example, less than 10 minutes, particularly less than 8 minutes, Preferably it is about 5 minutes.

  Here, the carbon material 6 to which the metal compound is attached using the supercritical fluid may be heated by the heater 5 as necessary. By heating, the precipitation of the metal from the metal compound adhering to the carbon material 6 proceeds more efficiently. The heating is preferably performed at a temperature in the range of 150 to 450 ° C., particularly in the range of 300 to 400.

  EXAMPLES Hereinafter, although this invention is demonstrated in detail using an Example, this invention is not limited to these Examples.

  Using the apparatus shown in FIG. 1, platinum was supported on vertically aligned carbon nanotubes formed on a substrate. An arrangement of a substrate 6 (hereinafter referred to as a CNT substrate) 6 on which a vertically aligned carbon nanotube is formed on the heater 5 is shown in FIG. Nine CNT substrates 6 each having a size of 20 mm × 20 mm were arranged in a 100 mm × 100 mm region on the heater 5. Each substrate was numbered as shown in FIG. 2, and after carrying platinum by the following procedures, the amount of platinum carried was examined for each numbered substrate. In any of the following procedures, the time required for carrying platinum was 30 minutes, and the CNT substrate 6 was heated by the heater 5 set at 360 ° C. during that time.

1. A metal compound solution (10 ml of a 3 wt% hexane solution of (trimethyl) methylcyclopentadienylplatinum) and CO ₂ gas are introduced into a platinum-supported first container 1 according to the method of the present invention, and pressurized to 10.2 MPa to produce CO 2. ₂ was in a supercritical state. On the other hand, CO ₂ gas was also introduced into the second container 3 and pressurized to 10.6 MPa to bring CO ₂ into a supercritical state (pressure in the first container <pressure in the second container). Next, valve | bulb 4 was opened, the 1st container 1 and the 2nd container 3 were connected, and after hold | maintaining for 1 minute, the valve | bulb 4 was closed [the 1st communication]. Further, a part of gas is released from the second container 3 with the valve 4 closed, and the pressure in the first container (10.5 MPa)> the pressure in the second container (10.2 MPa), The valve 4 was opened again to allow the first container 1 and the second container 3 to communicate with each other and held for 5 minutes [second communication]. Then, after closing valve | bulb 4 and releasing some gas from the 2nd container 3, operation which connected the 1st container 1 and the 2nd container 3 was repeated to the 7th time. Table 1 summarizes changes in the communication operation and changes in pressure in the first container 1 and the second container 3.

2. A metal compound solution (10 ml of a 3 wt% hexane solution of (trimethyl) methylcyclopentadienylplatinum) and CO ₂ gas are introduced into a platinum-supported first container 1 according to a conventional method , and pressurized to 10.6 MPa for CO ₂ It became a supercritical state. On the other hand, CO ₂ gas was also introduced into the second container 3 and pressurized to 10.2 MPa to bring CO ₂ into a supercritical state (pressure in the first container> pressure in the second container). Next, the valve | bulb 4 was opened and the 1st container 1 and the 2nd container 3 were connected, and it hold | maintained for 30 minutes.

3. Evaluation of platinum carrying amount The platinum carrying amount of each substrate was determined by measuring the weight of each of the nine CNT substrates 6. The relationship between the position of the substrate and the amount of platinum supported is summarized in the graph of FIG. In the conventional method, the platinum carrying amount of the No. 5 CNT substrate corresponding to the exit portion from the first container 1 is extremely large, and even when other substrates are compared with each other, the platinum carrying amount varies greatly. This is presumably because the metal compound solution staying in the piping of the valve 4 portion was directly sprayed and the fluid diffusibility in the second container 3 was poor. On the other hand, in the method of the present invention, there was little variation in the amount of platinum supported between the substrates as compared with the conventional method. This is presumably due to the fact that the metal compound solution was prevented from being directly sprayed by the method of the present invention, the fluid diffusibility in the second container 3 was improved, and the like.

1: First container 2: Stirring propeller 3: Second container 4: Valve 5: Heater 6: Carbon material (CNT substrate)

Claims (6)

A method for producing metal-supported carbon using a supercritical fluid, using a device having a first container and a second container that is connected to the first container via a valve,
(A) a step of enclosing a metal compound and a supercritical fluid precursor gas in a first container, and carbon and a supercritical fluid precursor gas in a second container,
(B) Pressurize each of the first container and the second container so that the pressure in the first container is lower than the pressure in the second container, thereby bringing the supercritical fluid precursor gas into a supercritical state. Process,
(C) The method comprising the step of opening the valve to allow communication between the first container and the second container. The method for producing metal-supported carbon according to claim 1, wherein after the step (c), (d) the second container is depressurized after the valve is closed, and the pressure in the first container is set to the second container. The process of making it higher than the pressure inside,
(E) The method further comprising the step of opening the valve again to allow the first container and the second container to communicate with each other.   The method for producing metal-supported carbon according to claim 2, wherein the steps (d) and (e) are repeated twice or more.   The manufacturing method of the metal carrying | support carbon of any one of Claims 1-3 including heating carbon at the temperature of the range of 150-450 degreeC.   The method for producing metal-supported carbon according to any one of claims 1 to 4, wherein the metal compound is selected from trimethyl (methylcyclopentadienyl) platinum and acetylacetonate platinum.   The method for producing metal-supported carbon according to any one of claims 1 to 5, wherein the supercritical fluid precursor gas is selected from carbon dioxide and trifluoromethane.

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