JP2004230223A - Co oxidation catalyst and production method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a CO oxidation catalyst for oxidizing CO contained in a gas at low temperatures, and a production method therefor. <P>SOLUTION: The CO oxidation catalyst is composed of at least one kind of manganese oxide selected from the group of Mn<SB>2</SB>O<SB>3</SB>, Mn<SB>3</SB>O<SB>4</SB>, and MnO. Those manganese oxides are formed porously. There may be added fine grains of at least one kind of precious metal selected from the group of Pt, Ru, Pd, and alloys thereof. With this catalyst, a concentration of CO contained in a gas can be reduced efficiently, even at low temperatures of 100°C or lower. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a CO oxidation catalyst capable of removing carbon monoxide (CO) in a gas and a method for producing the same, and particularly from a reformed gas mainly containing hydrogen gas obtained by steam reforming methanol or the like. The present invention relates to a CO oxidation catalyst for removing a trace amount of CO and a method for producing the same.
[0002]
[Prior art]
Currently, automobiles use gasoline or light oil as power fuel, but they emit nitrogen oxides and other pollutants that cause air pollution. Is being actively conducted. A fuel cell uses hydrogen and oxygen to convert the energy of a chemical reaction into electric energy, thereby using the electric energy as a substitute for a fuel for driving a vehicle. The products of the chemical reaction are mainly water (steam) and carbon dioxide, so that they do not pollute the atmosphere.
[0003]
Depending on the electrolyte material (diaphragm), a fuel cell has a phosphoric acid fuel cell (hereinafter, referred to as PAFC), a molten carbonate fuel cell (hereinafter, referred to as MCFC), and a solid electrolytic fuel cell (hereinafter, referred to as MCFC). SOFC) and polymer electrolyte fuel cells (hereinafter referred to as PEFC). At present, PAFC is said to be the closest to commercialization, and is being adopted for automobiles. On the other hand, PEFC has a low operating temperature of 100 ° C. or lower and has a high energy per unit area of the electrode (energy density), so that it is suitable for a mobile power source or a small-capacity power source. It is expected to be applied to
[0004]
FIG. 1 is a diagram showing a schematic configuration of the PEFC. As shown in FIG. 1, hydrogen (H ₂ ) and air are supplied to the PEFC 10 from the outside to the separators 11A and 12A of the fuel electrode 11 (anode) and the air electrode 12 (cathode), respectively, from the diffusion layer 11B of the fuel electrode. diffused _{H 2} is the catalyst layer 11C containing the Pt particles as the catalyst, _H 2 ⁼ 2H + + 2e in the catalyst layer 11C ^- reactions occur in. The generated protons H ⁺ flow through the electrolyte membrane 13 made of a polymer film, and the electrons e ⁻ flow through the external circuit 14 and reach the air electrode 12. In the catalyst layer 12C of the air electrode 12, a reaction of 2H ⁺ + O ₂ + 2e− = 2H ₂ O occurs, and water is generated. Electric energy can be given to the load by the electrons flowing through the external circuit 14.
[0005]
Here, H ₂ is supplied to the fuel electrode 11 of the PEFC, and methane, natural gas, ethanol, methanol, or the like is used as an H ₂ generation source. For example, one method of obtaining hydrogen from methanol is a methanol steam reforming method. FIG. 2 is a block diagram illustrating the methanol steam reforming method. As shown in FIG. 2, the methanol steam reforming method comprises heating and reacting methanol and steam to generate a reformed gas containing hydrogen, and oxidizing CO contained in the reformed gas to oxidize CO contained in the reformed gas. composed of CO remover 22 to be converted to CO _2. Here, in the reformer 21, hydrogen can be extracted using a catalyst, for example, using methanol as a raw fuel, and the reaction is represented by CH ₃ OH + H ₂ O = CO ₂ + 3H ₂ (1). _Cu-ZnO-Al 2 _{O 3} is often used. Here, in the reformer 21, CO is generated not only in the reaction of (1) but also in a secondary manner. Since CO poisons the catalyst of the fuel cell and significantly lowers the power generation efficiency, it is said that PEFC needs to be reduced to 100 ppm or less. Also, CO is harmful to the human body and must be removed.
[0006]
[Patent Document 1]
JP 2000-334304 A
[Problems to be solved by the invention]
Meanwhile, as a catalyst for oxidizing CO used in the CO removing device 22, a Pt catalyst is being studied. This is because the oxidation reaction of CO + 1 / 2O ₂ = CO ₂ is promoted, and the CO concentration is reduced.
[0008]
However, the Pt catalyst does not have sufficient CO oxidation activity unless the temperature reaches about 300 ° C. Mounting a CO remover that requires such a high temperature on a portable terminal or a notebook PC has a problem in that the cooling unit and the heat insulating unit become huge, which goes against the need for compactness of the notebook PC and the like.
[0009]
JP-A-2000-334304 discloses that MnO ₂ is used as a CO oxidation catalyst. However, MnO ₂ has a problem that it is chemically unstable and corrodes a metal container.
[0010]
Therefore, the present invention has been made in view of the above problems, and an object of the present invention is to provide a CO oxidation catalyst capable of oxidizing CO contained in a gas at a low temperature and a method for producing the same.
[0011]
[Means for Solving the Problems]
According to one aspect of the present invention, there is provided a CO oxidation catalyst comprising at least one of the group consisting of Mn ₂ O ₃ , Mn ₃ O ₄ , and MnO.
[0012]
According to the present invention, manganese oxides of Mn ₂ O ₃ , Mn ₃ O ₄ , and MnO have high oxidation activity, oxidize CO to CO ₂ , and prevent the fuel cell catalyst and the like from being poisoned by CO. It is possible to suppress the emission of CO harmful to the human body into the atmosphere.
[0013]
The CO oxidation catalyst may be formed in a porous shape, and may have an average particle diameter in a range of 0.01 μm to 1.0 μm. The surface area of the CO oxidation catalyst is increased, and the oxidation activity can be further increased.
[0014]
Further, at least one kind of noble metal fine particles from the group of Pt, Ru, Pd, and alloys thereof may be further added. Since such noble metal fine particles are active even in a humid environment, when the gas containing CO contains water vapor, the manganese oxide of the present invention alone may reduce the CO removing ability. By doing so, it is possible to suppress a decrease in the CO removal ability. Further, when the temperature of the gas containing CO is, for example, a high temperature, the oxidation activity is remarkably improved, so that the CO removing ability can be improved.
[0015]
According to another aspect of the present invention, a CO oxidation catalyst comprising: a step of calcining at least one of a group of MnCO ₃ , MnO ₂ , and MnOOH; and a step of pulverizing a product in the calcining step. Is provided.
[0016]
According to the present invention, MnCO ₃ , MnO ₂ , and MnOOH are calcined to obtain Mn ₂ O ₃ , Mn ₃ O ₄ , and MnO, as well as to manganese oxide deposited by electrolysis. , Can be easily pulverized. As a result, a CO oxidation catalyst having a large specific surface area can be obtained, and wear of beads, devices, and the like used in the pulverizing step can be reduced.
[0017]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0018]
FIG. 3 is a block diagram of a fuel cell system including a CO removal device using a CO oxidation catalyst according to an embodiment of the present invention.
[0019]
Referring to FIG. 3, the fuel cell system 30 includes a reforming device 31, a CO removing device 32 filled with a CO oxidation catalyst 32A, a polymer electrolyte fuel cell 33, and the like. Methanol and water are supplied and mixed to the reformer 31 and heated by the heater 34 to generate a reformed gas in the reformer 35. The CO removal device 32 is supplied with air and reformed gas, and CO contained in the reformed gas is oxidized by the CO oxidation catalyst 32A filled in the CO removal device 32 and converted into CO ₂ . Air containing H ₂ (including a trace amount of CO ₂ ) is supplied to the fuel electrode 36 (anode) of the fuel cell, converted into H ⁺ , and reacted with oxygen at the air electrode 39 via the electrolyte membrane 38. Electrons generated at the fuel electrode 36 are supplied to the load of the external circuit 40 as electric energy. Note that a cooler 41 for adjusting the temperature of the reformed gas may be provided in the reformed gas introduction section of the CO remover 32.
[0020]
The CO oxidation catalyst of the present invention is a catalyst composed of any one of manganese oxides of Mn ₂ O ₃ , Mn ₃ O ₄ , and MnO.
[0021]
Conventionally, it has been known that MnO ₂ is used as a CO oxidation catalyst. However, in the research on manganese oxide for many years, the present inventor has found that Mn ₂ O ₃ , Mn ₃ O ₄ , and MnO contain carbon monoxide ( CO) was found to be efficiently oxidized. That is, they found that Mn ₂ O ₃ , Mn ₃ O ₄ , and MnO were at the same level as MnO ₂ . Further, Mn ₂ O ₃ , Mn ₃ O ₄ , and MnO have a feature that the surface area can be increased because they can be more easily formed into a porous state than MnO ₂ . Therefore, a sufficient CO oxidizing ability can be ensured even with a smaller amount than that of MnO ₂ , so that the CO removing device 32 can be made compact. Further, MnO ₂ may corrode when the container is made of metal, but Mn ₂ O ₃ , Mn ₃ O ₄ , and MnO are more chemically stable, and thus do not have such a problem. Therefore, it is advantageous in that the range of selection of the container material of the CO removing device is widened.
[0022]
The manganese oxide CO oxidation catalyst of the present invention can be obtained by performing heat treatment at a predetermined temperature in an air atmosphere or an inert gas atmosphere containing oxygen, using MnCO ₃ , MnO ₂ , or MnOOH as a starting material. Specifically, when converting these starting materials to _Mn 2 _{O 3} is, 509 ℃ ~588 ℃, _Mn 3 When converting to _{O 4} is 589 ℃ ~975 ℃, 976 ℃ above when converting into MnO And heating for 1 to 5 minutes. By the heat treatment in the air, the manganese oxide of the present invention is formed in a porous state. For example, if the heating is performed for more than 10 minutes, the densification proceeds, and the porosity is lost.
[0023]
Next, the heat-treated manganese oxide is transferred to an atmosphere at room temperature, and then pulverized by a bead mill using zirconia (ZrO ₂ ) beads or the like or a jet mill. The average particle diameter after pulverization is set to 0.01 μm to 1.0 μm. If the particle size is reduced to less than 0.01 μm, the pulverization process time is significantly increased, and abrasion of the beads becomes severe, which is not economical. On the other hand, if it is larger than 1.0 μm, the oxidation activity decreases, and the amount of CO reduction decreases.
[0024]
Further, from the viewpoint of the pulverization time, it is preferable to use MnCO ₃ as a starting material. MnCO _3, compared with MnO _{2, Mn} 2 _O 3 generated, _Mn 3 _{O 4,} and it is easy grinding of MnO.
[0025]
Further, the beads may be beads of titanium oxide (TiO ₂ ), alumina (Al ₂ O ₃ ), platinum (Pt), etc. other than zirconia beads. Zirconia, titanium oxide and alumina beads are preferred in terms of hardness, and platinum is preferred in that abrasion during the pulverization step is mixed into manganese oxide and functions as a catalyst. Even when steam is contained in the reformed gas, the catalytic action of platinum does not decrease, so that a decrease in the CO oxidizing ability can be suppressed. The average particle size of the beads is preferably 0.5 mm to 5 mm. Thus, the CO oxidation catalyst of the present invention is formed.
[0026]
As the starting material MnO ₂ used in the above description, a defective MnO ₂ used for the solution layer of the manganese dry battery may be used. This is preferable from the viewpoint of lowering manufacturing costs.
[0027]
The CO oxidation catalyst of the present invention may further include at least one type of noble metal fine particles selected from the group consisting of Pt, Ru, Pd, and alloys thereof. Further, silver oxide or copper oxide may be used. Such a noble metal fine particle or a metal oxide fine particle has a CO-selective oxidizing effect, and when the reformed gas is at a high temperature of 300 ° C. or higher, the oxidizing activity of the noble metal fine particles increases. Can be converted. Even when steam is contained in the reformed gas, such noble metal fine particles do not decrease in oxidizing activity, so that it is possible to suppress a decrease in the CO removing ability of the CO oxidation catalyst of the present invention.
[0028]
Further, the noble metal fine particles may be supported on a conductive fine powder, for example, a conductive carbon powder. Since the substantial surface area of the noble metal particles increases, the oxidation activity can be increased, and the amount of the noble metal particles added in terms of mass can be reduced, and the cost increase due to the addition of the noble metal particles can be suppressed.
[0029]
According to the present invention, the CO in the reformed gas can be oxidized to CO ₂ and removed by these CO oxidation catalysts. Further, as described later, these oxidation catalysts have a high oxidation activity even at a relatively low temperature, for example, 80 ° C. to 100 ° C., and can efficiently oxidize CO. can do. Hereinafter, embodiments of the present invention will be described.
[Example]
First, CO oxidation catalyst samples A to I of this example were produced by the following method.
[0030]
(Sample A) Using MnCO ₃ as a starting material, the inside of a quartz tube was heated to 550 ° C. in an air atmosphere under atmospheric pressure and passed through MnCO ₃ to obtain Mn ₂ O ₃ . The residence time at this time was 5 minutes.
[0031]
Next, the obtained Mn ₂ O ₃ was cooled to room temperature and mixed with water so as to be 40% by mass to prepare a slurry. Mn ₂ O ₃ in the slurry was pulverized by a bead mill (manufactured by WAB, Switzerland, DYNO-MILL series (prototype having a capacity of about 2 L)). The beads were zirconia beads having a diameter of about 1 mm, and the slurry was supplied at a rate of 100 mL / min, and the residence time was about 10 minutes.
[0032]
The obtained slurry was poured into a cylindrical container of a CO removal device, and the water was evaporated at about 100 ° C. and dried. The particle shape of the obtained Mn ₂ O ₃ was granular, and the average particle size was about 0.1 μm. The particle diameter is determined by diluting the slurry with water, scooping it on a net-shaped sample stage, expanding it to an appropriate magnification using a transmission electron microscope, and enclosing the primary particle in the inscribed circle with the largest diameter and the circumscribed circle with the smallest diameter. The average of the diameters of the circles was regarded as the particle diameter, and the average of the particle diameters of arbitrary 20 particles was determined to obtain the average particle diameter.
[0033]
(Sample B) Using MnO ₂ as a starting material, Mn ₂ O ₃ was obtained under the same heating conditions as in Sample A. Next, pulverization was performed in the same manner as in Sample A to obtain a Mn ₂ O ₃ catalyst having an average particle size of 0.2 μm.
[0034]
(Sample C) Using MnOOH as a starting material, Mn ₂ O ₃ was obtained under the same heating conditions as in Sample A. Next, pulverization was performed in the same manner as in Sample A to obtain a Mn ₂ O ₃ catalyst having an average particle diameter of 0.1 μm.
[0035]
(Sample D) Using MnCO ₃ as a starting material, the inside of a quartz tube was heated to 700 ° C., and the residence time was set to 2 minutes to obtain Mn ₃ O ₄ . Next, pulverization was performed in the same manner as in Sample A to obtain a Mn ₃ O ₄ catalyst having an average particle diameter of 0.2 μm.
[0036]
(Sample E) Using MnO ₂ as a starting material, Mn ₃ O ₄ was obtained under the same heating conditions as in Sample D. Next, pulverization was performed in the same manner as in Sample A to obtain a Mn ₃ O ₄ catalyst having an average particle size of 0.2 μm.
[0037]
(Sample F) Using MnOOH as a starting material, Mn ₃ O ₄ was obtained under the same heating conditions as in Sample D. Next, pulverization was performed in the same manner as in Sample A to obtain a Mn ₃ O ₄ catalyst having an average particle diameter of 0.1 μm.
[0038]
(Sample G) Using MnCO ₃ as a starting material, the inside of a quartz tube was heated to 1000 ° C., and the residence time was 2 minutes to obtain MnO. Next, pulverization was performed in the same manner as in Sample A to obtain a MnO catalyst having an average particle diameter of 0.1 μm.
[0039]
(Sample H) Using MnO ₂ as a starting material, MnO was obtained under the same heating conditions as in Sample G. Next, pulverization was performed in the same manner as in Sample A to obtain a MnO catalyst having an average particle size of 0.2 μm.
[0040]
(Sample I) Using MnOOH as a starting material, MnO was obtained under the same heating conditions as in Sample G. Next, pulverization was performed in the same manner as in Sample A to obtain a MnO catalyst having an average particle diameter of 0.1 μm.
[0041]
In the production of Samples A to I, the temperature during heating cannot be directly measured in the quartz tube, so that the optimum temperature slightly varies depending on the apparatus. However, the heating temperature may be adjusted by identifying the sample obtained after heating by X-ray diffraction.
[0042]
Next, the CO removal performance of these samples A to I and Pt fine particles (comparative example) was examined. Samples A to I and Pt fine particles (comparative example) were charged into a CO remover of a cylindrical container having a diameter of 4 cm and a height of 10 cm, respectively, and mixed gas (CO ₂ : 50%, CO: 0.05%, N ₂₎ : 49.95%) at a flow rate of 1 L / min from the gas inlet. The container was heated so that the temperature of the gas in the container became 25 ° C, 100 ° C, and 300 ° C. A CO detector was installed at the gas outlet, and the CO concentration in the discharged gas was measured. Here, the CO detector sucked out the discharged gas, lowered the temperature of the gas with a radiator or the like, and then measured the CO concentration. The CO detector uses a potentiostatic electrolysis method as a measurement principle, and has a CO concentration measurement range of 0 to 0.199% (for example, XC-341 manufactured by New Cosmos Electric Co., Ltd. )) Were used.
[0043]
FIG. 4 is a diagram showing the measurement results of the CO concentration of this example. It can be seen that when the gas temperature is 25 ° C., the CO concentration is almost the same as the concentration before the treatment. At a gas temperature of 100 ° C., samples A to I are 0.01% or less, the CO concentration is reduced to such an extent that poisoning of a catalyst such as Pt used for the fuel electrode is not a problem, and the human body is adversely affected. It can be seen that the level has no effect. On the other hand, the Pt fine particles of the comparative example have a CO concentration of 0.02% to 0.05%, which indicates that the degree of reduction of the CO concentration is lower than that of samples A to I. At a gas temperature of 300 ° C., it can be seen that Samples A to I and the Pt fine particles have the same CO removal ability. It is estimated that the oxidation activity of the Pt fine particles increased at 300 ° C. or higher.
[0044]
According to this embodiment, Mn ₂ O ₃ , Mn ₃ O ₄ , and MnO samples A to I have a high CO removal ability with respect to the Pt fine particle catalyst when the temperature of the processing gas containing CO is 100 ° C. . Therefore, it is suitable for small electric devices such as portable terminals and notebook PCs. Since the operating temperature of the PEFC is 70 ° C. to 90 ° C., the gas that has passed through the CO removal device can be directly supplied to the fuel electrode side of the PEFC. It is excellent in that it does not require adjustment. Further, at 100 ° C., the reaction of oxidizing H ₂ does not proceed, so that the selectivity of CO oxidation is excellent. Thus, by oxidizing and removing only CO in the reformed gas containing of H _2, it is excellent as a CO oxidation catalyst to supply of H ₂ to the fuel cell.
[0045]
Although the preferred embodiment of the present invention has been described in detail above, the present invention is not limited to the specific embodiment, and various modifications and changes may be made within the scope of the present invention described in the appended claims. Changes are possible.
[0046]
For example, in the fuel cell system including the CO oxidation catalyst according to the embodiment, the exhaust gas from the fuel cell 33 (shown in FIG. 3) is caused to flow through the CO oxidation catalyst of the present invention before being discharged to the atmosphere. May be. It is possible to further remove CO in the exhaust gas and suppress the adverse effect on the environment.
[0047]
Further, the CO oxidation catalyst of the present invention can be used for reducing the CO concentration in exhaust gas, in addition to the three-way catalyst in the exhaust pipe of an automobile.
[0048]
Further, the CO oxidation catalyst of the present invention has excellent oxidation activity at a low temperature as a CO removal device, and thus is promising as a fuel cell system for supplying electric power to a notebook PC or the like. Further, in the above-described CO removing device, the container filled with the CO oxidation catalyst is not limited to a cylindrical shape, and a partition or the like may be provided inside the container to increase the length of the gas circulation path. CO concentration can be reduced.
[0049]
In addition, the following supplementary notes are disclosed with respect to the above description.
(Supplementary Note 1) A CO oxidation catalyst containing at least one member from the group consisting of Mn ₂ O ₃ , Mn ₃ O ₄ , and MnO.
(Supplementary Note 2) The CO oxidation catalyst according to Supplementary Note 1, which is formed in a porous shape.
(Supplementary note 3) The CO oxidation catalyst according to Supplementary note 1 or 2, wherein the average particle diameter is in a range of 0.01 µm to 1 µm.
(Supplementary Note 4) The CO according to any one of Supplementary Notes 1 to 3, wherein at least one kind of noble metal fine particles is further added from a group of Pt, Ru, Pd, and an alloy thereof. Oxidation catalyst.
(Supplementary Note 5) The CO oxidation catalyst according to any one of Supplementary Note 4, wherein the noble metal fine particles are supported on conductive carbon powder.
(Supplementary note 6) The CO oxidation catalyst according to any one of Supplementary notes 1 to 3, wherein fine particles of silver oxide or copper oxide are further added.
(Supplementary Note 7) a step of heat-treating at least one of a group of MnCO ₃ , MnO ₂ , and MnOOH;
And pulverizing the product in the heat treatment step.
(Supplementary note 8) The method for producing a CO oxidation catalyst according to supplementary note 7, wherein the heat treatment is performed in an air atmosphere or an inert atmosphere.
(Supplementary Note 9) In a CO removing device in which a reformed gas is supplied and the CO concentration in the reformed gas is reduced using a CO oxidation catalyst layer,
The CO removal device, wherein the CO oxidation catalyst layer includes the CO oxidation catalyst according to any one of Supplementary notes 1 to 6.
(Supplementary Note 10) The CO removal device according to Supplementary Note 9, wherein the treatment is performed at a temperature of the reformed gas in a range of 25 ° C to 300 ° C.
[0050]
【The invention's effect】
As is apparent from the details described above, according to the present invention, since the CO oxidation catalyst is composed of Mn ₂ O ₃ , Mn ₃ O ₄ , and manganese oxide of MnO having high oxidation activity, it is contained in the gas. CO can be efficiently oxidized at a low temperature. Furthermore, since it has excellent selectivity for CO oxidation, it is possible to reduce the CO concentration without consuming H ₂ of the reformed gas, and it can be used for a CO removal device of a solid fuel cell system.
[Brief description of the drawings]
FIG. 1 is a diagram showing a schematic configuration of a PEFC.
FIG. 2 is a block diagram illustrating a methanol steam reforming method.
FIG. 3 is a block diagram of a fuel cell system including a CO removal device according to an embodiment of the present invention.
FIG. 4 is a diagram showing a measurement result of a CO concentration of an example according to the present invention.
[Explanation of symbols]
Reference Signs List 30 fuel cell system 31 reforming device 32 CO removing device 32A CO oxidation catalyst 33 fuel cell 34 heater 35 reforming reaction section 36 fuel electrode 38 electrolyte membrane 39 air electrode 40 external circuit 41 cooler

Claims (5)

A CO oxidation catalyst comprising at least one member from the group consisting of Mn ₂ O ₃ , Mn ₃ O ₄ , and MnO. The CO oxidation catalyst according to claim 1, wherein the CO oxidation catalyst is formed in a porous shape. The CO oxidation catalyst according to claim 1, wherein the average particle diameter is in a range of 0.01 μm to 1.0 μm. The CO oxidation catalyst according to any one of claims 1 to 3, further comprising at least one type of noble metal fine particles selected from the group consisting of Pt, Ru, Pd, and alloys thereof. Heat-treating at least one of the group consisting of MnCO ₃ , MnO ₂ , and MnOOH;
And pulverizing the product in the heat treatment step.

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