JP2009054922A - Catalyst - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrode catalyst for an electric double layer capacitor, a fuel cell or the like that has an excellent electrochemical initial characteristic, and excels in long-term stability of characteristics. <P>SOLUTION: Activated carbon is previously surface-oxidized by ozone, hydrogen peroxide and an oxygen-containing gas, and thereafter processed at a temperature of 200-1,300°C in the presence of ammonia, urea or its derivative. A nitrogen compound is produced on the surface of the activated carbon, this large amount of surface nitride exhibits a basic property, is chemically stable, and catalytically accelerates various chemical reactions. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a catalyst having very good electrochemical reaction efficiency, particularly a catalyst useful as an electric double layer capacitor electrode or an oxygen electrode of a fuel cell.

  Activated carbon is widely used as a catalyst and a catalyst carrier, but recently, in an electric double layer capacitor, a fuel cell, and the like, it has attracted attention as a catalyst for efficiently promoting an electrochemical reaction, that is, as an electrode. However, there are still many problems to be improved in terms of increasing the capacity of the electric double layer capacitor and its stability over time, and improving the current density and output density of the fuel cell and its stability over time.

  Conventionally, a method of increasing the capacity of an electric double layer capacitor (Patent Document 1) by activating a fibrous carbon material with alkali and further oxidizing the surface with ozone or ammonium persulfate, and a coal-based binder A method in which a mixture of nitrogen-containing organic compounds (such as melamine) is heated to 1000 ° C. and then steam-activated nitrogen-containing activated carbide is used as an oxygen electrode of a fuel cell to try to improve the output density (Patent Document 2) ) Has been devised.

Furthermore, an electric double layer capacitor (Patent Document 3) in which an antacid agent such as an alkaline compound (for example, potassium carbonate) is present on the surface of activated carbon particles has been devised.
However, in the surface oxidation by ozone or the like, the generated acidic functional group reacts with the driving electrolyte solution to deteriorate the characteristics, and there remains a problem in reliability for long-term use. Moreover, in the method of carbonizing a mixture of a coal-based binder and a nitrogen-containing organic compound and activating water vapor, an oxide is present on the surface, and the initial performance is good, but the long-term stability of the performance is lacking. Furthermore, since an alkaline compound is physically present on the activated carbon surface, there is a problem in stability for long-term use when viewed electrochemically.

JP 2006-156918 A JP 2004-330181 A JP 2006-261516 A

  An object of the present invention is to provide a catalyst that can efficiently promote an electrochemical reaction, remarkably improve initial performance, and maintain the performance for a long period of time.

  As a result of intensive studies in view of the above points, the present inventor has produced a large amount of oxides (oxygen-containing groups) on the activated carbon surface by previously oxidizing the activated carbon surface. Subsequently, ammonia, urea or When a high temperature treatment with the derivative is performed, a surface nitrogen compound is generated corresponding to the amount of the surface oxide. This large amount of surface nitrogen compounds is basic, chemically stable, catalytically accelerates various chemical reactions, dramatically accelerates electrochemical reactions involving oxygen, and has long-term functions. The present invention has been completed.

That is, the present invention
(1)
A catalyst consisting of activated carbon treated at a temperature of 200 to 1300 ° C. in the presence of ammonia, urea or a derivative thereof after surface oxidation;
(2)
The catalyst according to (1), wherein the catalyst is an electrode of an electric double layer capacitor,
(3)
The catalyst according to (1), wherein the catalyst is an oxygen electrode of a fuel cell,
(4)
The catalyst according to (1), wherein the surface oxidation of the activated carbon is oxidation with ozone, hydrogen peroxide or an oxygen-containing gas,

The activated carbon used in the present invention may be any activated carbon as long as it has been activated by chemical methods, steam activation, or the like using raw sawdust, charcoal, coke, coal, coconut shell, resin, or the like as a raw material. The shape is powder, crushed, columnar, honeycomb, fiber, etc., and the surface area is 400 m ² / g or more.

In the present invention, as a method for previously oxidizing the surface of activated carbon, for example, nitric acid, NOx, SOx, sulfur trioxide, sulfuric acid, chlorine dioxide, hydrogen peroxide, ozone, oxygen-containing gas (for example, air, combustion exhaust gas, etc.), etc. Examples of the oxidizing agent include a method of oxidizing in a liquid phase or a gas phase. In the case of NOx, SOx, etc., if the activated carbon is repeatedly used for a long time for flue gas denitrification (removal of NOx in exhaust gas) and flue gas desulfurization (removal of SOx in exhaust gas), the activated carbon is repeatedly oxidized, Since a large amount of oxide is generated, it can be used as it is as pre-oxidized activated carbon.
Moreover, since the activated carbon used for a long time by advanced treatment in the water purification plant has been repeatedly surface oxidized by high-concentration ozone water, it can be used as it is as pre-oxidized activated carbon. The activated carbon used in the advanced treatment varies depending on the water purification plant, but adsorbs various substances, so it is desirable to perform alkali cleaning or acid cleaning.
In the case where the oxidizing power for activated carbon is weak, such as gas phase oxidation with an oxygen-containing gas, surface oxidation is performed at a temperature of 200 ° C. or higher, preferably 250 to 850 ° C., depending on the oxygen concentration in the gas. Is efficient. The amount of these oxidizing agents used for activated carbon depends on the treatment method and oxidation conditions (eg, temperature, time, etc.), but is usually 1 mg or more, preferably 2 mg or more, more preferably in terms of oxygen atom per gram of activated carbon. Is 5 mg or more.
In the present invention, among the methods for oxidizing the surface of activated carbon, oxidation with ozone, hydrogen peroxide and an oxygen-containing gas is particularly preferable in terms of operation and efficiency.
Such oxidation generates a large amount of oxides (oxygen-containing groups) on the activated carbon surface.

  The catalyst of the present invention can be obtained by pre-oxidizing activated carbon and subsequently treating it at 200 to 1300 ° C., preferably 250 to 1200 ° C. in the presence of ammonia, urea or a derivative thereof. Oxidation of activated carbon generates a large amount of oxides (oxygen-containing groups) on the surface of the activated carbon. Depending on the amount of surface oxide, surface nitrogen compounds are generated by subsequent high-temperature treatment with ammonia, urea, or derivatives thereof. The greatest feature of the present invention is that the fact that the catalyst characteristics change greatly has been identified.

When ammonia is used in the high temperature treatment with ammonia, urea or its urea derivative, it is as follows.
That is, the contact ratio of ammonia to activated carbon is preferably 0.1 mmol or more, more preferably 0.5 to 1000 mmol, relative to 1 g of activated carbon. In this case, ammonia is mixed with an inert gas such as nitrogen gas or combustion exhaust gas, and is carried out at a temperature of 200 to 1300 ° C, preferably 250 to 1200 ° C. The contact time of ammonia with activated carbon varies depending on temperature, ammonia concentration and the like, but is 10 minutes or more, preferably 15 to 500 minutes.

  In the case of urea or a derivative thereof, as in the case of ammonia, an aqueous solution of urea or a derivative thereof is mixed with an inert gas such as nitrogen gas or combustion exhaust gas to 200 ° C. to 1300 ° C., preferably 250 to 1200 ° C. It is better to process at temperature. The contact time of urea or its derivative with activated carbon varies depending on the temperature, urea or its derivative concentration, etc., but is 10 minutes or longer, preferably 15 to 500 minutes. The contact ratio of urea or a derivative thereof to activated carbon is preferably 0.1 mmol or more, more preferably 0.5 to 1000 mmol, with respect to 1 g of activated carbon.

Moreover, when using urea or its derivative (s), you may carry out as follows. That is, after impregnating activated carbon with urea or a derivative thereof, an inert gas such as nitrogen gas is passed through this, and the temperature is 200 to 1300 ° C., preferably 250 to 1200 ° C. for 10 minutes or more, preferably 15 to 500 minutes. High temperature treatment is performed.
Examples of urea derivatives include beret, ureido, monomethylol urea, dimethylol urea and the like. The amount of urea or its derivative impregnated into activated carbon is preferably 0.1 mmol or more, particularly 0.5 mmol or more, per 1 g of activated carbon. When impregnating activated carbon with urea or a derivative thereof, urea or a derivative thereof is dissolved in water, an acid aqueous solution (for example, sulfuric acid, nitric acid, etc.), an ammonium salt aqueous solution (for example, ammonium sulfate, ammonium nitrate, etc.) It is preferable to impregnate this into activated carbon. In particular, when sulfuric acid, nitric acid, ammonium sulfate, or ammonium nitrate is used, the catalyst performance is further improved.

  In order to produce an electric double layer capacitor or a fuel cell electrode using the activated carbon of the present invention, a method known per se can be employed. For example, powdered activated carbon may be kneaded and molded with an electrolytic solution, or a mixture of activated carbon, binder and water may be well kneaded with a mixer, and the resulting paste-like mixture may be mixed using a roll. The sheet may be stretched under heating at about ~ 300 ° C and formed into a sheet having an appropriate thickness, for example, about 0.3 to 2 mm. The sheet electrode material can be punched into a disc shape to form a polarizable electrode.

Two or more of the obtained activated carbon moldings are stacked via a separator, housed in an outer container, and an electrolytic solution is injected therein, whereby an electric double layer capacitor unit cell can be made.
Electrolytic solutions include organic solvent-based and aqueous solutions. Propylene carbonate is generally used as the solvent for the organic solvent-based electrolytic solution, and any of various known quaternary phosphonium salts and quaternary ammonium salts can be used as the electrolyte. As the aqueous electrolyte, dilute sulfuric acid is generally used, but other inorganic acids such as tetrafluoroboric acid and nitric acid can also be used. Furthermore, an aqueous solution having an inorganic base such as potassium hydroxide, sodium hydroxide, or ammonium hydroxide as a solute can also be used conveniently. The concentration of each electrolyte can be appropriately selected within a range of 10 to 90% by weight.

  Unlike the prior art described above, the catalyst of the present invention is obtained by surface-oxidizing activated carbon in advance and treating it at a temperature of 200 to 1300 ° C. in the presence of ammonia, urea or a derivative thereof, so that a large amount of surface nitrogen compound is added to the activated carbon. In particular, the electrochemical reaction is remarkably accelerated, very good electrode material characteristics are exhibited, and the deterioration of the characteristics over time is small.

  The present invention will be specifically described below with reference to examples and comparative examples.

(1) 3 g of fibrous activated carbon A (BET specific surface area 1500 m ² / g, manufactured by Osaka Gas Co., Ltd.) is filled in a quartz glass tube of 30 mmφ, and N ₂ gas containing O ₂ -5.0 vol% is linear flow rate 5 cm. The temperature was raised to 600 ° C. over 60 minutes while circulating at / second, and kept at the same temperature for 10 minutes, and then cooled to room temperature (25 ° C.) in N ₂ gas to obtain air-oxidized activated carbon B.
(2) The same fibrous activated carbon A3g was filled into a 30 mmφ quartz glass tube, and the temperature was raised to 600 ° C. over 60 minutes while flowing N ₂ gas containing O ₂ -5.0 vol% at a linear flow rate of 5 cm / sec. . N ₂ gas containing NH ₃ -10 vol% at the same temperature was circulated at a linear flow rate of 5 cm / sec for 30 minutes, and then cooled to room temperature in N ₂ gas to obtain ammonia-treated activated carbon C.
(3) Manufacture of electrodes (method of Patent Document 1)
Fibrous activated carbon: carbon black: polytetrafluoroethylene is mixed and kneaded at a weight ratio of 85: 10: 5, filled into a 13 mm diameter mold and molded into a desk shape, and a titanium mesh as a current collector is pressure bonded. It was an electrode.
(4) Electrochemical measurement (Method of Patent Document 1)
By the three-electrode cell method, the above electrode was used as the working electrode, the activated carbon electrode on the fiber 6 times the weight of the working electrode was used, and the cell was filled with 1 M sulfuric acid electrolyte. The cell was deaerated with a vacuum pump for 12 hours, and the electrolyte was bubbled with argon gas. The electric double layer capacity was measured at a constant current of 100 mA / cm ² .
Table 1 shows the BET specific surface area and electric double layer capacity when air-oxidized activated carbon B and ammonia-treated activated carbon C were used as electrodes.

3 g of fibrous activated carbon A of Example 1 (BET specific surface area 1500 m ² / g) was filled in a ₃₀ mmφ quartz glass tube, and air containing O ₃ -200 ppm was circulated at 25 ° C. for 30 hours at a linear flow rate of 3 cm / second. As a result, ozone-oxidized activated carbon D was obtained.
₃ g of the same fibrous activated carbon A was filled in a 30 mmφ quartz glass tube, and air containing O ₃ -200 ppm was circulated at 25 ° C. for 30 hours at a linear flow rate of 3 cm / sec.
Thereafter, the N ₂ gas containing NH ₃ -10 vol% was heated to 750 ° C. while flowing at a linear flow rate of 5 cm / second, treated with ammonia at this temperature for 30 minutes, cooled to room temperature in N ₂ gas, Ammonia-treated activated carbon E was obtained.
For the ozone-oxidized activated carbon D and the ammonia-treated activated carbon E, the electric double layer capacity was measured in the same manner as in Example 1. The results are shown in Table 2.

Pitch 2 kg as a binder, a small amount of water, and 500 g of melamine were added to 10 kg of finely pulverized 100-200 mesh coconut shell carbide, mixed and kneaded, and pressed into a cylindrical shape. This molded product was carbonized at 600 ° C. for 1 hour and further activated at 850 ° C. in the presence of water vapor to obtain activated carbon F having a BET specific surface area of 1050 (m ² / g). 30 g of this activated carbon F was filled in a 55 mmφ quartz glass tube, and air containing O ₃ -200 ppm was circulated at 25 ° C. at a linear flow rate of 3 cm / second for 300 hours to obtain ozone-oxidized activated carbon G. Further, 15 g of this activated carbon G was filled in a quartz glass tube of 5 mmφ, and the temperature was raised to 750 ° C. while N ₂ gas containing NH ₃ -10 vol% was circulated at a linear flow rate of 5 cm / sec. Ammonia treatment was performed for minutes, cooling to room temperature in N ₂ gas, ozone treatment after ammonia oxidation, and activated carbon H (present invention) was obtained.
Further, 30 g of activated carbon F was filled into a 55 mmφ quartz glass tube, and the temperature was raised to 750 ° C. while N ₂ gas containing NH ₃ -10 vol% was circulated at a linear flow rate of 5 cm / sec. This was treated with ammonia for 30 minutes, cooled to room temperature in N ₂ gas, and treated with ammonia to obtain activated carbon I. Each of these activated carbons F, G, H and I was pulverized to produce an oxygen electrode with a particle size of 100 microns or less. Its characteristics were evaluated.
(1) Production of electrode Each of the above activated carbon fine powders is suspended in a sedimentation solution, allowed to settle on a rock wool diaphragm, and dried to prepare an oxygen electrode having an activated carbon deposition amount of 20 mg / cm ² . In order to support this electrode, a tantalum porous plate and a polypropylene net are used on the electrolyte side.
(2) Electrochemical measurement Measure the initial current density when the above electrode is inserted into 1 M sulfuric acid at 60 ° C. and a voltage of 700 mV is applied to the hydrogen electrode at an oxygen pressure of 2 × 10 ⁴ N / m ^2. did. Under these conditions, the current density after 50 hours was also measured, and the change in current density with time was examined to evaluate the oxygen electrode characteristics. The results are shown in Table 3.

30 g of 10/30 mesh crushed coconut shell activated carbon J (BET specific surface area 1100 m ² / g) was filled in a quartz glass tube of 55 mmφ, and N ₂ gas containing O ₂ -5.0 vol% was drawn at 450 ° C. Air-oxidized activated carbon K was prepared by flowing for 10 minutes at a flow rate of 5 cm / sec. 15 g of this activated carbon K was filled in a quartz glass tube of 55 mmφ, and the temperature was raised to 800 ° C. while N ₂ gas containing NH ₃ -10 vol% was circulated at a linear flow rate of 5 cm / sec, and circulated at this temperature for 30 minutes. after cooled to room temperature in N ₂ gas, it was then ammonia treatment after air oxidation to give the activated carbon L.
Each of these activated carbons J, K and L was pulverized to a particle size of 100 microns or less. An oxygen electrode was produced in the same manner as in Example 3, and the characteristics of the oxygen electrode were evaluated.

Into a 20 L enamel container, 100 g of crushed bituminous coal-based activated carbon M (BET specific surface area of 1250 m ² / g) activated with water vapor and 10 L of water and 10 L of water were added, and 8% by weight of hydrogen peroxide was stirred at room temperature. 4 L of water was added dropwise at 15 mL / min. Stirring was continued for 5 hours after the hydrogen peroxide solution was added dropwise. The activated carbon sample after hydrogen peroxide oxidation was drained and dried at 110 ° C. to obtain hydrogen peroxide-oxidized activated carbon N.
This hydrogen peroxide-oxidized activated carbon N30 g was filled in a quartz glass tube of 55 mmφ, and the temperature was raised to 750 ° C. while flowing N ₂ gas containing NH ₃ -10 vol% at a linear flow rate of 5 cm / sec. Ammonia treatment was performed for 30 minutes, and the mixture was cooled to room temperature in N ₂ gas. After the hydrogen peroxide oxidation treatment, ammonia-treated activated carbon O was obtained.
Each of these activated carbons M, N, and O was pulverized to a particle size of 100 microns or less. An oxygen electrode was produced in the same manner as in Example 3, and the characteristics of the oxygen electrode were evaluated. The results are shown in Table 5.

In the water purification plant, 8/30 mesh crushed bituminous coal-based activated carbon P (BET specific surface area 650 m ² / g) used in the advanced treatment for 2 years was used in the following experiments.
The activated carbon P is in contact with water having an ozone concentration of about 0.25 mg / L at a superficial velocity of about 6 / hour for 2 years, undergoes ozone oxidation, and surface oxide is converted to about 200 mg in terms of oxygen atom per gram. It was generated. 30 g of this activated carbon I is filled into a 55 mmφ quartz glass tube, heated to 850 ° C. while flowing N ₂ gas containing NH ₃ -10 vol% at a linear flow rate of 5 cm / sec, and ammonia is kept at this temperature for 30 minutes. treated, and cooled to room temperature in N ₂ gas, to obtain a ammonia treatment of activated carbon Q.
Each of these activated carbons P and Q was finely pulverized to a particle size of 100 microns or less. An oxygen electrode was produced in the same manner as in Example 3, and the characteristics of the oxygen electrode were evaluated. The results are shown in Table 6.

Regarding the activated carbon of Example 6, the characteristics of the electric double layer capacitor were evaluated by the method according to Patent Document 3.
That is, activated carbon particles: carbon black: a sheet-like electrode having a molding density of 0.85 g / cm ³ and a thickness of 145 microns was prepared. This sheet was affixed to both sides of a strip-shaped current collector made of an aluminum foil using a conductive adhesive to form a positive electrode and a negative electrode, which were laminated together with a separator to form a wound element. These elements were inserted into an aluminum cylindrical container having a diameter of 40 mm and a length of 120 mm, and the terminal portion was sealed. The electrolyte was injected into a cylindrical capacitor cell that was vacuum-dried at 200 ° C., and a voltage of 2.7 V was applied at 65 ° C. for 6 hours, and then the initial capacitance was measured by constant current discharge at 30 A. The electrolytic solution is a triethylmethylammonium tetrafluoroborate 1.8 mol / L solution.
Moreover, the electrostatic capacity after 50 hours was measured while applying a constant voltage of 2.7 V at 65 ° C. The results are shown in Table 7.

The following experiment was conducted on 10 mmφ cylindrical lignite activated carbon R (BET specific surface area 550 m ² / g) used for about 6 months in flue gas desulfurization. The activated carbon was repeatedly oxidized with SOx, and its surface contained 165 mg of oxide per gram of oxygen per gram of oxygen. 30 g of this activated carbon R is filled in a 55 mmφ quartz glass tube, heated to 850 ° C. while flowing N ₂ gas containing NH ₃ -10 vol% at a linear flow rate of 5 cm / sec, and ammonia is kept at this temperature for 30 minutes. treated, and cooled to room temperature in N ₂ gas, to obtain a ammonia treatment of activated carbon S.
Each of these activated carbons R and S was finely pulverized to a particle size of 100 microns or less. An oxygen electrode was produced in the same manner as in Example 3, and the characteristics of the electric double layer capacitor were evaluated in the same manner as in Example 7. The results are shown in Table 8.

  The catalyst according to the present invention is highly efficient in electrochemical reaction, the initial performance is remarkably high, and the performance can be maintained over a long period of time, so as an electrode of an electric double layer capacitor, an electrode of a fuel cell, etc. It can be used advantageously.

Claims (4)

  A catalyst comprising activated carbon which has been surface oxidized and then treated at a temperature of 200 to 1300 ° C. in the presence of ammonia, urea or a derivative thereof.   The catalyst according to claim 1, wherein the catalyst is an electrode of an electric double layer capacitor.   The catalyst according to claim 1, wherein the catalyst is an oxygen electrode of a fuel cell.   The catalyst according to claim 1, wherein the surface oxidation of the activated carbon is an oxidation with ozone, hydrogen peroxide or an oxygen-containing gas.