Catalyst for carbon dioxide reduction electrode, carbon dioxide reduction electrode containing the catalyst, carbon dioxide reduction cell containing the electrode, and method for producing carbon monoxide using the catalyst.

Abstract

To provide a carbon dioxide reduction electrode catalyst that can be constructed using a metal cheaper than copper and that can sufficiently reduce carbon dioxide.SOLUTION: Provided is a carbon dioxide reduction electrode catalyst that contains a Zn and Al-containing layered double hydroxide and has a diffraction peak at 2θ=2° to 15° in an X-ray diffraction pattern using Cu-K α-ray.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] This disclosure relates to a catalyst for a carbon dioxide reduction electrode, a carbon dioxide reduction electrode containing the catalyst, a carbon dioxide reduction cell containing the electrode, and a method for producing carbon monoxide using the catalyst. [Background technology]

[0002] Using renewable energy sources such as solar and wind power can generate surplus electricity. One way to effectively utilize such surplus electricity is through the electrochemical reduction of carbon dioxide. The electrochemical reduction of carbon dioxide can convert carbon dioxide, which is an industrial waste product, into useful compounds such as carbon monoxide and / or chemical products, and is attracting attention from the perspective of reducing greenhouse gas emissions and the consumption of fossil fuels.

[0003] Catalysts can be used in the electrochemical reduction of carbon dioxide. By using a catalyst, the reduction reaction can be accelerated. Patent Document 1 discloses a carbon dioxide reduction electrode for reducing carbon dioxide to produce carbon monoxide, which has a catalyst layer and an auxiliary catalyst layer, wherein the catalyst layer is made of a rare earth metal or a rare earth metal complex, and the auxiliary catalyst is selected from silver, copper, or tin. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 15388 / 1988 [Overview of the project] [Problems that the invention aims to solve]

[0005] Catalysts such as those disclosed in Patent Document 1 use expensive metals such as rare earth metals, silver, copper and / or tin, which are at least copper or more expensive, and there is a need for a less expensive catalyst for carbon dioxide reduction electrodes.

[0006] The present invention has been made in view of such circumstances, and one of its objects is to provide a catalyst for a carbon dioxide reduction electrode that can be composed of a metal cheaper than copper and can sufficiently reduce carbon dioxide, a carbon dioxide reduction electrode containing the catalyst, a carbon dioxide reduction cell containing the electrode, and a method for producing carbon monoxide using the catalyst. **Means for Solving the Problems**

[0007] Aspect 1 of the present invention is a catalyst for a carbon dioxide reduction electrode, which contains a layered double hydroxide containing Zn and Al and has a diffraction peak at 2θ = 2° to 15° in an X-ray diffraction pattern using Cu-Kα rays.

[0008] Aspect 2 of the present invention is the catalyst for a carbon dioxide reduction electrode according to Aspect 1, wherein the molar ratio of Zn:Al is 1:4 to 5:1.

[0009] Aspect 3 of the present invention is a carbon dioxide reduction electrode containing the catalyst for a carbon dioxide reduction electrode according to Aspect 1 or 2.

[0010] Aspect 4 of the present invention is a carbon dioxide reduction cell containing the carbon dioxide reduction electrode according to Aspect 3.

[0011] Aspect 5 of the present invention is a method for producing carbon monoxide, which includes a step of reducing carbon dioxide using the catalyst for a carbon dioxide reduction electrode according to Aspect 1 or 2. **Advantages of the Invention**

[0012] According to an embodiment of the present invention, it is possible to provide a catalyst for a carbon dioxide reduction electrode that can be composed of a metal cheaper than copper and can sufficiently reduce carbon dioxide, a carbon dioxide reduction electrode containing the catalyst, a carbon dioxide reduction cell containing the electrode, and a method for producing carbon monoxide using the catalyst. **Brief Description of the Drawings**

[0013] [Figure 1] Figure 1 shows the SEM-EDX analysis results of catalyst a in the examples. [Figure 2] Figure 2 shows the SEM-EDX analysis results of catalyst b in the examples. [Figure 3] Figure 3 shows the SEM-EDX analysis results of catalyst c in the examples. [Figure 4] Figure 4 shows the XRD patterns of catalysts a to c in the examples. [Figure 5] Figure 5 is a schematic diagram of an experimental apparatus for verifying the effects of the embodiments of the present invention.

Mode for Carrying Out the Invention

[0014] The inventors have studied from various angles in order to realize a catalyst for a carbon dioxide reduction electrode that can be composed of a metal cheaper than copper and can sufficiently reduce carbon dioxide.

[0015] When electrochemically reducing carbon dioxide to carbon monoxide, at the electrode where the carbon dioxide reduction reaction occurs (carbon dioxide reduction electrode or cathode) and the electrode where the oxidation reaction occurs (oxidation electrode or anode), reactions as shown in the following formulas (1) and (2) can occur, and as a whole, a reaction as shown in the following formula (3) can occur. Cathode: CO2 + H2O + 2e - → CO + 2OH - ···(1) Anode: 2OH - → H2O + 2e - + 1 / 2O2 ···(2) Overall: CO2 → CO + + 1 / 2O2 ···(3)

[0016] The inventors have found that the reactions of the above formulas (1) and (2), especially 2OH -As a compound capable of promoting the movement of carbon dioxide, we focused on layered double hydroxides (LDHs). Furthermore, while Cu is well known as a metal that reduces carbon dioxide, the inventors have discovered for the first time that carbon dioxide can be sufficiently reduced by constructing LDH using Zn and Al, which are cheaper than Cu but not known as metals that reduce carbon dioxide, and using them as a catalyst. At the time of filing this application, Cu was about US$10,000 / ton, Zn was about US$4,000 / ton, and Al was about US$3,000 / ton.

[0017] The details of each requirement defined in the embodiments of the present invention are shown below.

[0018] The catalyst for the carbon dioxide reduction electrode according to an embodiment of the present invention contains a layered double hydroxide containing Zn and Al, and has a diffraction peak in the 2θ = 2° to 15° range in the X-ray diffraction pattern using Cu-Kα rays. This allows the catalyst to be constructed using a metal cheaper than copper and to sufficiently reduce carbon dioxide.

[0019] In embodiments of the present invention, whether or not the catalyst for the carbon dioxide reduction electrode contains a layered double hydroxide containing Zn and Al can be confirmed as follows. First, the presence or absence of Zn and Al can be confirmed by performing elemental analysis such as EDX on the catalyst for the carbon dioxide reduction electrode. Furthermore, it is known that layered double hydroxides have a diffraction peak at a position corresponding to the (003) plane (2θ = 2° to 15°) in the X-ray diffraction pattern using Cu-Kα rays. Therefore, the presence or absence of layered double hydroxide can be confirmed by obtaining the X-ray diffraction pattern of the catalyst for the carbon dioxide reduction electrode and checking for the presence or absence of the diffraction peak of the (003) plane, which is characteristic of layered double hydroxides. It is preferable that the catalyst contains a large amount of layered double hydroxide, as this makes it easier to reduce carbon dioxide. For example, it is preferable that the X-ray diffraction pattern of the catalyst for the carbon dioxide reduction electrode has the largest diffraction peak at a position corresponding to the (003) plane.

[0020] In the embodiments of the present invention, the molar ratio of Zn:Al in the catalyst for the carbon dioxide reduction electrode is preferably 1:4 to 5:1. Thereby, it becomes easier to form a layered double hydroxide containing Zn and Al, and it becomes easier to reduce carbon dioxide. More preferably, the molar ratio of Zn:Al is 1:3 to 4:1, and even more preferably, the molar ratio of Zn:Al is 1:2 to 3:1.

[0021] In the embodiments of the present invention, it is preferable that the catalyst for the carbon dioxide reduction electrode contains a large amount of Zn and Al. Therefore, the content of Zn and Al with respect to the total mass of the catalyst for the carbon dioxide reduction electrode is preferably 10% by mass or more, more preferably 15% by mass or more, and even more preferably 20% by mass or more.

[0022] In the embodiments of the present invention, the layered double hydroxide containing Zn and Al can be represented by the following general formula (4). [Zn 2+ 1-x Al 3+ x (OH)2][A n- x / n ·yH2O] ···(4) In formula (4), A n- represents an anion, 0 < x < 1, n represents 1 or 2, and y is 0 or more.

[0023] The compound represented by formula (4) is composed of a layer consisting of Zn 2+ 1-x Al 3+ x (OH)2, which is a positively charged octahedral layer, and a layer composed of an anion represented by A n- x / n ·yH2O that compensates for the positive charge and interlayer water, and it preferably has a structure in which these layers are alternately stacked. A n- is not particularly limited as long as it is an anion with n being 1 or 2. For example, CO3 2- , Cl - , NO3 - , SO4 2-It may be one or more selected from the group consisting of and dodecyl sulfate ions. Note that the position corresponding to the (003) plane in the X-ray diffraction pattern of the layered double hydroxide using Cu-Kα rays described above is A n- Depending on the type, 2θ can vary within the range of 2° to 15°, for example, CO3 2- Then 2θ can be 11.3~12.3°, Cl - Therefore, 2θ can be 9.5~10.5°, NO3 - Then 2θ can be 9.5~10.5°, SO4 2- If it is a dodecyl sulfate ion, then 2θ can be 7.4 to 8.4°, and if it is a dodecyl sulfate ion, then 2θ can be 2.9 to 3.9°.

[0024] In the compound represented by formula (4), x is preferably 0.17 to 0.80, more preferably 0.20 to 0.75, and even more preferably 0.25 to 0.67. The upper limit of y is not particularly limited, but it may be, for example, 4 or less.

[0025] The method for producing the layered double hydroxide containing Zn and Al is not particularly limited and can be produced by known methods, including the production methods described in the examples below. The catalyst for the carbon dioxide reduction electrode according to the embodiment of the present invention can be used to electrochemically reduce carbon dioxide.

[0026] A carbon dioxide reduction electrode according to an embodiment of the present invention includes the catalyst described above. In one embodiment of the present invention, the carbon dioxide reduction electrode may include the catalyst and a conductive material different from the catalyst. The conductive material may be any known conductive material, such as carbon. The shape and size of the conductive material are not particularly limited and can be appropriately selected depending on the intended use or required performance. The catalyst can be electrically connected to the conductive material.

[0027] In one embodiment of the present invention, the carbon dioxide reduction electrode may include the catalyst and a conductive substrate. The shape of the conductive substrate can be, for example, a sheet, a plate, a rod, or a mesh. The size of the conductive substrate may be, for example, a circular equivalent diameter greater than 1 mm in order to distinguish it from the conductive particles described later. Examples of conductive substrates include carbon paper (carbon sheets). The catalyst may be electrically connected to the conductive substrate. For example, the catalyst may be in direct contact with the conductive substrate, or there may be other conductive materials, such as conductive particles, between the conductive substrate and the catalyst. The conductive particles may be particles made of known conductive materials, such as carbon particles. The conductive particles may have a circular equivalent diameter of 1 mm or less, and it is preferable that the average particle size (average circular equivalent diameter) is 100 nm or less. In one embodiment of the present invention, the carbon dioxide reduction electrode may be brought into contact with a conductive substrate by mixing conductive particles and a catalyst and applying the mixture. The mixing ratio of conductive particles to catalyst can be such that the mass ratio of catalyst is 1 to 99% by mass, preferably 1 to 50% by mass.

[0028] The conductive particles and the catalyst may be mixed and then heated and sintered, or bonded together with a binder resin or the like. The binder resin is not particularly limited and any known type may be used. The binder resin content may be 1% to 100% by mass relative to the total mass of the catalyst and conductive particles, preferably 1% to 50% by mass, and more preferably 1% to 20% by mass.

[0029] A carbon dioxide reduction cell according to an embodiment of the present invention includes the carbon dioxide reduction electrode. In one embodiment of the present invention, the carbon dioxide reduction cell may include a tank that can be filled with an electrolyte, the carbon dioxide reduction electrode disposed in the tank, an oxidation electrode that functions as a counter electrode, an ion exchange membrane disposed between the electrodes, and a power supply capable of applying a voltage between the electrodes. By filling the tank of the carbon dioxide reduction cell with an electrolyte so that it is in contact with the two electrodes, supplying carbon dioxide to the tank as appropriate, and applying a voltage between the two electrodes, carbon dioxide can be reduced.

[0030] The electrolyte can be any known solution and is not particularly limited, but examples include solutions containing cations such as sodium ions and potassium ions, anions such as bicarbonate ions, carbonate ions and hydroxide ions, and water. The concentration of the electrolyte can be set as appropriate and is not particularly limited, but for example, it may be 0.01 to 1.0 mol / L. A known tank capable of filling the electrolyte may be used.

[0031] The oxidizing electrode can be constructed using known conductive materials, such as metals like platinum, palladium, or nickel, or an electrode containing a known oxygen-evolving catalyst that functions under basic conditions.

[0032] Ion exchange membranes separate gases (carbon monoxide, oxygen, etc.) that may be generated at each electrode while allowing the movement of ions (hydroxide ions, etc.). Known anion exchange membranes can be used as ion exchange membranes.

[0033] The voltage applied to the carbon dioxide reduction electrode can be, for example, -0.1 to -2.5V relative to the potential of the standard electrode. A higher voltage allows for greater reduction of carbon dioxide, but if it is too high, power consumption increases. From the viewpoint of carbon dioxide reduction amount and power consumption, the applied voltage is preferably -0.7V to -2.0V, and more preferably -0.8V to -1.4V.

[0034] A method for producing carbon monoxide according to an embodiment of the present invention includes a step of reducing carbon dioxide using the above-mentioned catalyst for a carbon dioxide reduction electrode. In one embodiment of the present invention, carbon monoxide may be produced by reducing carbon dioxide using the above-mentioned carbon dioxide reduction electrode. In another embodiment of the present invention, carbon monoxide may be produced by reducing carbon dioxide using the above-mentioned carbon dioxide reduction cell.

[0035] In embodiments of the present invention, it is preferable that carbon dioxide can be reduced to carbon monoxide (CO) in greater quantities. For example, it is preferable that the Faraday efficiency of CO be 10% or more, and more preferably 20% or more. In embodiments of the present invention, in addition to CO, hydrogen (H2) and other gases may be generated from the reduction electrode, but it is preferable that the amount of other gases such as hydrogen be less. In one embodiment of the present invention, the ratio of the Faraday efficiency of CO to the sum of the Faraday efficiencies of CO and H2 generated from the reduction electrode is preferably 0.30 or more, and more preferably 0.40 or more. [Examples]

[0036] The embodiments of the present invention will be described in more detail below with reference to examples. The embodiments of the present invention are not limited by the following examples, and can be implemented with appropriate modifications within the scope that is consistent with the spirit described above and below, and all such modifications are included within the technical scope of the embodiments of the present invention.

[0037] A catalyst for the carbon dioxide reduction electrode was prepared as follows: Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, manufactured by Wako Pure Chemical Industries, special grade) and aluminum nitrate notahydrate (Al(NO3)3·9H2O, manufactured by Wako Pure Chemical Industries, special grade) were dissolved in 50 mL of distilled water so that the concentration of Zn was 0.06 mol / L and the concentration of Al was 0.03 mol / L (so that the molar ratio of Zn:Al was 2:1) to obtain a Zn-Al mixed aqueous solution. After stirring this mixed aqueous solution at room temperature for 1 hour, the layered double hydroxide of formula (4) above, A n- =CO3 2- To obtain the compound, it was added dropwise to 200 mL of 0.3 mol / L sodium carbonate (Na2CO3) aqueous solution, yielding a white precipitate. At this time, 2.0 mol / L sodium hydroxide (NaOH) aqueous solution was added dropwise as needed to maintain the pH at 10. The precipitate was then left at room temperature for 17 hours. After standing, it was filtered, washed with distilled water, and the precipitate was recovered. This was dried in an 80°C oven for 24 hours, and the dried sample was crushed in a mortar to obtain a powdered catalyst for carbon dioxide reduction electrodes (referred to as catalyst a). Similarly, catalysts b to f were prepared by adjusting the molar ratio of Zn:Al to 3:1, 4:1, 1:2, 1:3, and 1:4.

[0038] SEM-EDX analysis was performed on the carbon dioxide reduction electrode catalysts obtained by the method described above. As an example, the SEM-EDX analysis results for catalysts a to c are shown in Figures 1 to 3. Figures 1 to 3 show the surface SEM image (left), the Zn mapping diagram by EDX (center), and the Al mapping diagram by EDX (right) for catalysts a to c, respectively. As shown in Figures 1 to 3, Zn and Al were uniformly distributed in almost the same positions. The molar ratios of Zn and Al for catalysts a to c were calculated to be 0.67:0.33, 0.77:0.23, and 0.79:0.21, respectively, which were found to be approximately equal to the addition ratios (2:1, 3:1, and 4:1). Furthermore, for catalysts a to f, the content of Zn and Al relative to the total mass of the catalyst was at least 20% by mass.

[0039] Powder XRD patterns using Cu-Kα radiation were obtained for the carbon dioxide reduction electrode catalysts obtained by the above method. For example, catalysts a-c (and for reference, ZnO and Co-Al layered double hydroxide (A) were obtained. n- The XRD patterns of carbonate ions are shown in Figure 4. As shown in Figure 4, a peak characteristic of layered double hydroxides, the (003) plane, was observed at 2θ = 11.3 to 12.3°. For catalysts b and c, in addition to the peak attributable to layered double hydroxides, a peak attributable to ZnO was observed. This is thought to be due to the high Zn ratio in the catalyst. Similarly, the peak characteristic of layered double hydroxides, the (003) plane, was observed in the XRD patterns of catalysts d to f.

[0040] A carbon dioxide reduction electrode (referred to as electrode A) containing catalyst a was prepared as follows. Carbon paper A (EC-TPI-060T, manufactured by ElectroChem Inc.) was prepared as a conductive substrate. Five parts by mass of catalyst a, five parts by mass of carbon black A (VULCAN® XC72, manufactured by Cabot, average particle size 50 nm) as conductive particles, and an anion exchange resin solution (solid content concentration 10% by mass) as a binder resin solution were mixed at a ratio of 4 μL per 10 mg of the total mass of catalyst and conductive particles, and the mixture was applied to the conductive substrate. Then, the coated material was pressed at 10 MPa for 30 seconds, cut to 1 cm × 1 cm, and a carbon dioxide reduction electrode (referred to as electrode A) was obtained.

[0041] An experiment was conducted to electrochemically reduce carbon dioxide using electrode A as described above. Figure 5 is a schematic diagram of the experimental apparatus for verifying the effects of the embodiment of the present invention. As shown in Figure 5, the electrolyte 2 is filled in the tank 1. The working electrode (carbon dioxide reduction electrode) 3, counter electrode (oxidation electrode) 4, reference electrode 5, and ion exchange membrane 6 are arranged in the electrolyte 2. The working electrode side of the tank 1 is configured to allow the introduction of gas such as CO2. A 0.1 mol / L aqueous solution of potassium bicarbonate (KHCO3) was used as the electrolyte 2, electrode A as the working electrode 3, a Pt electrode as the counter electrode 4, an Ag / AgCl electrode as the reference electrode 5, and an anion exchange membrane (AHA type, manufactured by Astom) as the ion exchange membrane 6.

[0042] To saturate the electrolyte on the working electrode side with carbon dioxide, carbon dioxide gas was flowed through the electrolyte at the working electrode side at a flow rate of 30 mL / min for 15 minutes. Subsequently, a constant voltage was applied to the electrode while flowing carbon dioxide gas through the electrolyte at the working electrode side at a flow rate of 10 mL / min. The gas generated during the 30 minutes from 10 minutes to 40 minutes after the voltage was applied was collected and analyzed by gas chromatography (GC). The applied voltage was obtained by converting the potential measured at reference electrode 5 to the hydrogen electrode potential (RHE) and varying it from -1.0 to -1.4V. In a gas chromatograph, hydrogen and oxygen were quantified using a TCD (thermal conductivity detector), and carbon monoxide and carbon dioxide were quantified using a FID (flame ionization detector). The volume concentrations were calculated from the carbon monoxide and hydrogen peak areas obtained by GC, using calibration curves prepared with standard gases. The Faraday efficiency (%), representing the contribution of the flowing electrons to the reaction, was calculated from the volume concentration, the current during measurement, and the flow rate of carbon dioxide gas.

[0043] As shown in Table 1, electrodes B to F were obtained by changing the catalyst, conductive particles, and / or conductive substrate from electrode A. Specifically, catalyst a was replaced with catalysts b to f. For conductive particles, carbon black B (Denka Black (powder), average particle size 35 nm) was sometimes used instead of carbon black A. For conductive substrates, carbon paper B (Toray Industries, TGP-H-090) was sometimes used instead of carbon paper A. The above carbon dioxide reduction experiment was performed by replacing electrode A with electrodes B through F, and the Faraday efficiency of each gas was determined. The results are shown in Table 1. In Table 1, "-" indicates that the experiment was not performed.

[0044] [Table 1]

[0045] As shown in Table 1, in all cases of Test No. 1 to 6, carbon dioxide was sufficiently reduced to carbon monoxide (specifically, a Faraday efficiency of 20% or more for CO was obtained at a measurement potential of -1.0V (vs RHE)). [Explanation of Symbols]

[0046] 1 tank 2 Electrolyte 3 Working electrode 4. Opposite 5 Reference pole 6. Ion exchange membrane

Claims

1. A catalyst for carbon dioxide reduction electrodes, comprising a layered double hydroxide containing Zn and Al, and exhibiting diffraction peaks in the 2θ = 2° to 15° range in an X-ray diffraction pattern using Cu-Kα rays.

2. The catalyst for a carbon dioxide reduction electrode according to claim 1, wherein the molar ratio of Zn to Al is 1:4 to 5:

1.

3. A carbon dioxide reduction electrode comprising the catalyst for carbon dioxide reduction electrode according to claim 1 or 2.

4. A carbon dioxide reduction cell comprising the carbon dioxide reduction electrode described in claim 3.

5. A method for producing carbon monoxide, comprising the step of reducing carbon dioxide using the carbon dioxide reduction electrode catalyst described in claim 1 or 2.

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