Gas reduction device, gas reduction method

Abstract

To provide a gas reduction apparatus capable of performing gas reduction with high efficiency, and a gas reduction method using the gas reduction apparatus.SOLUTION: A gas reduction apparatus 1 comprises: a reactor body 10; a microflow passage 20 formed in the reactor body 10; a gas inlet 30 formed in the reactor body 10 and communicating with the microflow passage 20 to introduce gas into the microflow passage 20; a liquid inlet 40 formed in the reactor body 10 and communicating with the microflow passage 20 to introduce liquid into the microflow passage 20; a cathode 50 provided on the inner surface of the microflow passage 20; an anode 60 provided on the inner surface of the microflow passage 20 and facing the cathode 50; and a gas outlet 70 formed in the reactor body 10 and communicating with the microflow passage 20 to derive a reaction gas from the microflow passage 20. The microflow passage 20 holds a gas-liquid mixed flow consisting of the gas and the liquid.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a gas reduction device and a gas reduction method using a gas reduction device. [Background technology]

[0002] The concept of using renewable energy sources such as solar power to create, store, and utilize green fuels is called "Power-to-X" (where X is Gas or Fuel, etc.) and is a technology that has attracted attention in recent years. One method to mitigate the increase of carbon dioxide in the atmosphere, a greenhouse gas, is to reduce carbon dioxide (CO2) and convert it into a useful energy source. This technology is one of the most promising technologies within the "Power-to-X" framework. Meanwhile, the need for CO2 reduction technology is also motivated by the demands for CO2 removal on the International Space Station and energy supply on Mars (where 95% of the atmosphere is CO2).

[0003] Various reduction approaches have been proposed for gas reduction, including electrochemical, photochemical, biochemical, and thermochemical methods. The technology for producing and reusing methane from carbon dioxide, a greenhouse gas, is called methanation. Methanation is a technology that utilizes natural energy such as sunlight to reduce carbon dioxide emitted from thermal power plants and other sources, converting it into gas resources such as methane. Generally, methanation utilizes a chemical reaction called the Sabatier reaction, which reacts hydrogen and carbon dioxide with a catalyst to produce methane. A thermochemical approach is used in methanation.

[0004] Compared to thermochemical methods of reducing carbon dioxide, electrochemical methods of reducing carbon dioxide offer the following advantages: (1) The electrolyte can be converted into fuel or useful chemicals and reused. The only chemicals consumed in this process are water and carbon dioxide. (2) The electrochemical reduction method can be carried out at normal pressure and room temperature. (3) The electricity required for electrochemical reduction can be obtained from renewable resources.

[0005] Despite the advantages of electrochemical reduction of carbon dioxide as described above, no practical catalytic electrodes or methanation systems for electrochemical reduction have been developed to date. The reasons for this are as follows: (1) and (2). (1) The electrochemical reduction reaction of carbon dioxide is a complex multi-electron transfer process. For example, the following electrochemical reduction reactions (A) and (B) are known (see, for example, Non-Patent Document 1). CO2 + 8H + +8e - →CH4+2H2O Electrochemical potential E0 = -0.24V···(A) CO2 + 2H + +2e - →CO+H2O Electrochemical potential E0 = -0.52V···(B) (2) The reaction has low selectivity and produces a variety of products. When gold, silver, zinc, and palladium are used as electrochemical electrodes, carbon monoxide is produced as a reaction product. On the other hand, when copper is used as an electrochemical electrode, several types of reaction products such as methane, ethylene, and ethanol are produced. Furthermore, when tin, lead, and indium are used as electrochemical electrodes, there is a reaction mechanism that produces formic acid (see, for example, Non-Patent Document 1).

[0006] When copper is used as a catalyst, various products are generally formed (see, for example, Non-Patent Documents 2 and 3). In the electrochemical reduction of carbon dioxide, methane is predominantly produced on the Cu(111) surface, and ethylene is predominantly produced on the Cu(100) surface (see, for example, Non-Patent Document 2). In addition, in the electrochemical reduction of carbon dioxide, a reduction reaction without proton transfer predominates, and carbon monoxide forms a C-C bond on the electrode surface, making it easy to form ethylene. This reaction also predominates on the surfaces of platinum, nickel, etc. Also, in the electrochemical reduction of carbon dioxide using polycrystalline Cu formed by electrolytic plating or sputter deposition, it is known that the formation of ethylene is dominant (see, for example, Non-Patent Document 3).

[0007] When examining the effects of the surfaces of electrodes with Cu nanoparticles present on the surface, electrochemically polished Cu electrodes, and Cu electrodes formed by sputter volume, it has been reported that the roughened Cu electrode surface has high selectivity for the production species for hydrocarbons (see, for example, Non-Patent Document 4). On the other hand, that report states that the following two competing reactions contribute to the electrochemical current. * CO2 + 2H + + 2e - → * CO + H2O (* indicates a molecule adsorbed on the surface) 2 * H + + 2e - → H2 That is, in order to promote the production of carbon monoxide, which is an intermediate product, hydrogen ions are required, but generally, hydrogen molecules are produced, and the production efficiency of carbon monoxide decreases. As described above, the electrochemical reduction of carbon dioxide is a promising technology, but due to technical difficulties, it has not been put into practical use.

[0008] H-type cells are known for their use in the electrochemical reduction of carbon dioxide (see, for example, Non-Patent Documents 5 and 6). In these cells, only carbon dioxide dissolved in the cathode electrolyte can be used as a reactant; therefore, the reaction rate of carbon dioxide reduction is determined by the limitation of carbon dioxide mass transfer (dissolution rate). The fact that the solubility of carbon dioxide in aqueous electrolytes is extremely low effectively determines the rate of carbon dioxide reduction. Furthermore, the presence of a solid electrolyte between the cathode and anode presents a challenge in that a large reduction current cannot be generated.

[0009] Reactors that utilize a mixture of carbon dioxide and humidified vapor are known (see, for example, Non-Patent Document 7). In these reactors, a solid electrolyte membrane such as an ion exchange membrane is used, and therefore the diffusion of hydrogen ions in the solid electrolyte becomes the rate-limiting factor in the reduction reaction of carbon dioxide. [Prior art documents] [Non-patent literature]

[0010] [Non-Patent Document 1] Zhu,DD;Liu,JL;Qiao,SZRecent advances in inorganic heterogeneous electrocatalysts for reduction of carbon dioxide.Adv.Mater.2016,28,3423-3452. [Non-Patent Document 2] Kortlever, R.; Shen, J.; Schouten, KJP; Calle-Vallejo, F.; Koper, MTMCatalysts and reaction pathways for the electrochemical reduction of carbon dioxide.J.Phys.Chem.Lett.2015,6,4073-4082. [Non-Patent Document 3] Kuhl, K.P.; Cave, E.R.; Abram, D.N.; Jaramillo, T.F. New insights into the electrochemical reduction of carbon dioxide on metallic copper surfaces. Energy Environ. Sci. 2012, 5, 7050 - 7059. [Non-Patent Document 4] Tang, W.; Peterson, A.A.; Varela, A.S.; Jovanov, Z.P.; Bech, L.; Durand, W.J.; Dahl, S.; Norskov, J.K.; Chorkendorff, I. The Importance of Surface Morphology in Controlling the Selectivity of Polycrystalline Copper for CO2 Electroreduction. Phys. Chem. Chem. Phys. 2012, 14, 76 - 81. [Non-Patent Document 5] Burdyny, T.; Smith, W.A. CO2 Reduction on Gas-Diffusion Electrodes and Why Catalytic Performance Must Be Assessed at Commercially-Relevant Conditions, Energy Environ. Sci. 2019, 12(5), 1442 - 1453. [Non-Patent Document 6] D.M. Weekes, D.A. Salvatore, A. Reyes, A. Huang and C.P. Berlinguette, Electrolytic CO2 Reduction in a Flow Cell, Acc. Chem. Res., 2018, 51, 910 - 918. [Non-Patent Document 7] L.M.Aeshala et al., Effect of solid polymer electrolyte on electrochemical reduction of CO2, Separation and Purification Technology 94(2012)131-137.

Summary of the Invention

Problems to be Solved by the Invention

[0011] The present invention has been made in view of the above circumstances, and an object thereof is to provide a gas reduction device capable of efficiently reducing a gas without using a solid electrolyte membrane, and a gas reduction method using the gas reduction device.

Means for Solving the Problems

[0012] The gas reduction device of the present invention has the following aspects. A reactor main body, a microchannel formed in the reactor main body, a gas inlet formed in the reactor main body, communicating with the microchannel, and introducing a gas into the microchannel, a liquid inlet formed in the reactor main body, communicating with the microchannel, and introducing a liquid into the microchannel, a cathode provided on the inner surface of the microchannel, an anode provided on the inner surface of the microchannel and facing the cathode, and a gas outlet formed in the reactor main body, communicating with the microchannel, and leading out a reaction gas from the microchannel, wherein the microchannel holds a gas-liquid mixed flow composed of the gas and the liquid, a gas reduction device.

Effects of the Invention

[0013] According to the present invention, it is possible to provide a gas reduction device capable of efficiently reducing a gas without using a solid electrolyte membrane, and a gas reduction method using the gas reduction device.

Brief Description of the Drawings

[0014] [Figure 1]This is a plan view showing the schematic configuration of a gas reduction device according to one embodiment of the present invention. [Figure 2] This is a cross-sectional view along line AA in Figure 1, showing the schematic configuration of a gas reduction device according to one embodiment of the present invention. [Figure 3] This is a perspective view showing the schematic configuration of a gas reduction device according to one embodiment of the present invention. [Figure 4] This is a perspective view showing the schematic configuration of a gas reduction device according to one embodiment of the present invention. [Figure 5] This figure shows the results of analyzing the emitted gas by caschromatography after reducing carbon dioxide using a gas reduction device according to one embodiment of the present invention. [Modes for carrying out the invention]

[0015] The gas reduction apparatus and gas reduction method according to embodiments of the present invention will be described below with reference to the drawings. Note that, for convenience, the drawings used in the following description show enlarged versions of characteristic parts, and the dimensional ratios of each component may differ from those of the actual components. Furthermore, the materials, dimensions, etc., exemplified in the following description are merely examples, and the present invention is not limited to them. They can be modified as appropriate without altering the essence of the invention.

[0016] [Gas reduction device] Figure 1 is a plan view showing the schematic configuration of a gas reduction device according to one embodiment of the present invention. Figure 2 is a cross-sectional view showing the schematic configuration of a gas reduction device according to one embodiment of the present invention, along line AA in Figure 1. Figures 3 and 4 are perspective views showing the schematic configuration of a gas reduction device according to one embodiment of the present invention. As shown in Figures 1 and 2, the gas reduction device 1 comprises a reactor body 10, a microchannel 20, a gas inlet 30, a liquid inlet 40, a cathode 50, an anode 60, and a gas outlet 70.

[0017] The reactor body 10 includes a first substrate 11, a second substrate 12, a cathode substrate 13, an anode substrate 14, and an insulating member 15. The first substrate 11 and the second substrate 12 are laminated in their respective thickness directions via the cathode substrate 13, the anode substrate 14, and the insulating member 15. The cathode substrate 13 and the anode substrate 14 are arranged between the first substrate 11 and the second substrate 12 such that a portion of their surfaces (sides) along their respective thickness directions face each other. The height h of the microchannel 20 (the length along the thickness direction of the first substrate 11 and the second substrate 12 in the microchannel 20) is determined by the cathode substrate 13, the anode substrate 14, and the insulating member 15. The cathode substrate 13 and the anode substrate 14 may be integrally constructed with either the first substrate 11 or the second substrate 12, forming the microchannel 20 and the insulating member 15 on a single substrate.

[0018] As shown in Figure 1, the microchannel 20 is formed in the reactor body 10. The reactor body 10 has the microchannel 20, a gas-liquid mixer 21, and a gas-liquid inlet passage 22. The microchannel 20, the gas-liquid mixer 21, and the gas-liquid inlet passage 22 are connected in this order. The microchannel 20, gas-liquid mixer 21, and gas-liquid introduction passage 22 are formed between the cathode substrate 13 and the anode substrate 14. The cathode substrate 13 has a first recess 13A, a second recess 13B, and a third recess 13C formed in this order on the surface facing the anode substrate 14, in a direction perpendicular to the thickness direction of the cathode substrate 13. The anode substrate 14 has a first recess 14A, a second recess 14B, and a third recess 14C formed in this order on the surface facing the cathode substrate 13, in a direction perpendicular to the thickness direction of the anode substrate 14. By arranging the cathode substrate 13 and the anode substrate 14 between the first substrate 11 and the second substrate 12 such that a portion of their respective thickness-direction surfaces (sides) face each other, the first recess 13A and the first recess 14A form a microchannel 20, the second recess 13B and the second recess 14B form a gas-liquid mixer 21, and the third recess 13C and the third recess 14C form a gas-liquid introduction passage 22. Note that the cathode substrate 13 and the anode substrate 14 have a portion of their respective thickness-direction surfaces (sides) facing each other via an insulating member 15. In other words, the cathode substrate 13 and the anode substrate 14 are not in direct contact. Furthermore, while it is preferable to provide the gas-liquid mixer 21 formed from the second recess 13B and the second recess 14B in order to form the gas-liquid mixed flow described later, it does not need to be provided if the gas-liquid mixed flow is formed by introducing gas from the gas-liquid introduction passage 22. Furthermore, in this embodiment, the gas-liquid mixer 21 is configured as an extension of the microchannel 20, but any well-known configuration that promotes the gas-liquid mixed flow can be adopted.

[0019] The gas inlet 30 is formed in the reactor body 10. The gas inlet 30 is formed to penetrate the second substrate 12 in the thickness direction at a position opposite the gas-liquid introduction passage 22. The gas inlet 30 communicates with the microchannel 20 via the gas-liquid introduction passage 22 and the gas-liquid mixer 21. This allows gas to be introduced into the microchannel 20 from the gas inlet 30. The gas inlet 30 may be formed to penetrate the first substrate 11 in the thickness direction, or it may be formed to connect to the microchannel 20 from the side surfaces of the cathode substrate 13 and the anode substrate 14.

[0020] The liquid inlet 40 is formed in the reactor body 10. The liquid inlet 40 is formed to penetrate the second substrate 12 in the thickness direction at a position opposite the gas-liquid inlet passage 22. The liquid inlet 40 communicates with the microchannel 20 via the gas-liquid inlet passage 22 and the gas-liquid mixer 21. This allows liquid to be introduced into the microchannel 20 from the liquid inlet 40. The liquid inlet 40 may be formed to penetrate the first substrate 11 in the thickness direction, or it may be formed to connect to the microchannel 20 from the side surfaces of the cathode substrate 13 and the anode substrate 14.

[0021] The cathode 50 is provided on the inner surface 20a of the microchannel 20. More specifically, the cathode 50 is provided on the inner surface 13a of the first recess 13A of the cathode substrate 13.

[0022] The anode 60 is provided on the inner surface 20a of the microchannel 20. More specifically, the anode 60 is provided on the inner surface 14a of the first recess 14A of the anode substrate 14.

[0023] The gas outlet 70 is formed in the reactor body 10. The gas outlet 70 is formed to penetrate the second substrate 12 in the thickness direction at a position opposite the microchannel 20, and is in communication with the microchannel 20. This allows the reaction gas generated by the reduction of gas from the microchannel 20 to be discharged to the outside of the microchannel 20 via the gas outlet 70. The gas outlet 70 may be formed to penetrate the first substrate 11 in the thickness direction, or it may be formed to connect to the microchannel 20 from the side surfaces of the cathode substrate 13 and the anode substrate 14.

[0024] "First base material, second base material" The materials of the first substrate 11 and the second substrate 12 are not particularly limited as long as they do not degrade due to the liquid or gas introduced into the microchannel 20, or the reaction gas generated by the reduction of the gas, but examples include silicon, glass, sapphire, polymer materials such as acrylic, etc.

[0025] "Cathode base material" The material of the cathode substrate 13 is not particularly limited as long as it does not degrade due to the liquid or gas introduced into the microchannel 20, or the reaction gas generated by the reduction of the gas, and can form the cathode 50. Examples include silicon, germanium, aluminum, and titanium.

[0026] The cathode 50 has a portion 51 formed by thinning or porousning the inner surface 20a of the microchannel 20, i.e., the inner surface 13a of the cathode substrate 13, into a nanowire shape, in order to increase the surface area of ​​the electrode, and a metal film formed on the portion 51, or a supported reducing catalyst metal 52. If silicon or the like is used as the material of the cathode substrate 13, the electrode may be formed by doping with phosphorus, arsenic, etc.

[0027] The structure of the part of the inner surface 20a of the microchannel 20 that is thinned or porous is not particularly limited, but for example, if the cathode substrate 13 is made of silicon, porous silicon can be used. Porous silicon can be formed by metal-assisted etching of a silicon substrate using an AgNO3 catalyst in a hydrofluoric acid solution (see NVToan, T Ono et al. IEEE transaction on Nanotechnology, 2017).

[0028] The reducing catalyst metal is not particularly limited as long as it can be used to reduce gases such as carbon dioxide and can be supported on the finely wired or porous portion of the inner surface 13a of the cathode substrate 13, and is appropriately selected depending on the hydrocarbon to be produced. Examples include at least one metal selected from copper, silver, iron, nickel, platinum, rhodium, gold, silver, zinc, palladium, tin, lead, indium, titanium, tungsten, and molybdenum, or alloys or oxides of these metals. Among these, copper is used when producing methane as an energy substance because of its excellent carbon dioxide reduction efficiency.

[0029] If the cathode 50 has a portion 51 formed by thinning or porousning the inner surface 13a of the cathode substrate 13, and a reducing catalyst metal 52 supported on that portion 51, the surface area becomes larger. As a result, the contact efficiency between the gas and the reducing catalyst metal 52 in the microchannel 20 increases, and the gas reduction efficiency increases. In addition, because the cathode 50 has the above configuration, the ratio of the surface area of ​​the reducing catalyst metal 52 to the volume of the gas becomes larger, thus increasing the efficiency of the gas reduction reaction by the reducing catalyst metal 52.

[0030] "Anode base material" The material of the anode substrate 14 is not particularly limited as long as it does not degrade due to the liquid or gas introduced into the microchannel 20, or the reaction gas generated by the reduction of the gas, and can form the anode 60. Examples include silicon, germanium, and aluminum.

[0031] The anode 60 comprises an anode substrate 14 and an anode film 61 formed on the inner surface 14a of the anode substrate 14.

[0032] The material of the anode film 61 is not particularly limited, but examples include metals such as nickel, iron, and platinum, and carbon. A metal that generates little oxygen is preferred for the anode film 61. If silicon or the like is used for the anode substrate 14, the electrode may be formed by doping with boron, phosphorus, etc.

[0033] "Microchannels" The width w of the microchannel 20, that is, the distance w between the cathode substrate 13 and the anode substrate 14 when the microchannel 20 is viewed from above, is preferably 0.1 mm to 2.0 mm, and more preferably 0.1 mm to 1.0 mm, depending on the applied voltage. If the width w of the microchannel 20 is less than 0.1 mm, the conductance of the gas flow becomes small, and the processing capacity decreases. On the other hand, if the width w of the microchannel 20 exceeds 2.0 mm, the reduction efficiency may decrease. If the width w of the microchannel 20 is 2.0 mm or less, the efficiency of the gas reduction reaction can be increased.

[0034] The height h of the microchannel 20 is preferably 0.1 mm to 2.0 mm, and more preferably 0.1 mm to 1.0 mm. If the height h of the microchannel 20 is less than 0.1 mm, the conductance of the gas flow will be small, and the processing capacity will decrease. If the height h of the microchannel 20 is 2.0 mm or less, the efficiency of the gas reduction reaction can be increased. Furthermore, the length of the microchannel 20 is arbitrary; the longer it is, the more liquid can be processed. The microchannel 20 is not limited to a straight shape; it may also be a folded channel. Furthermore, the microchannel 20 can be configured to circulate liquid. For example, the end point of the microchannel 20 or the liquid outlet 90 may be connected to the liquid inlet 40 or the upstream of the starting point of the microchannel 20 to circulate the liquid.

[0035] "Gas inlet" The inner diameter of the gas inlet 30 is not particularly limited and can be adjusted as appropriate according to the width and height of the gas-liquid inlet passage 22. A gas inlet pipe 31 is connected to the gas inlet 30, which connects the gas inlet 30 to a gas cylinder or the like, which is a gas supply source.

[0036] "Liquid inlet" The inner diameter of the liquid inlet 40 is not particularly limited and can be adjusted as appropriate according to the width and height of the gas-liquid inlet passage 22. A liquid inlet pipe 41 is connected to the liquid inlet 40, which connects the liquid inlet 40 to a liquid supply source.

[0037] "Gas outlet" The inner diameter of the gas outlet 70 is not particularly limited and can be adjusted as appropriate according to the width and height of the microchannel 20. A gas outlet pipe 71 is connected to the gas outlet 70, which connects the gas outlet 70 to a gas recovery unit that contains the reaction gas generated by the reduction of gas within the microchannel 20.

[0038] "Gas selective permeation filter" The gas reduction device 1 of this embodiment preferably includes a gas selective permeable filter 80 provided at the gas outlet 70 to remove impurities contained in the reaction gas discharged from the microchannel 20. The gas selective permeable filter 80 is not particularly limited as long as it selectively permeates only the reaction gas and does not allow moisture to pass through. An example of the gas selective permeable filter 80 is a porous body made of a hydrophobic material. An example of a hydrophobic material is polytetrafluoroethylene. By providing the gas selective permeable filter 80, only the desired reaction gas can be recovered from the microchannel 20. That is, impurities contained in the reaction gas are removed by the gas selective permeable filter 80. The discharged gas may be passed through a gas separation membrane to extract and utilize only the useful gas components. For example, methane can be separated using a methane separation membrane for separating methane.

[0039] "Liquid outlet" In this embodiment, the gas reduction device 1 is preferably equipped with a liquid outlet 90 connected to a microchannel 20 for discharging the reaction liquid from the microchannel 20. The liquid outlet 90 is formed in the reactor body 10. The liquid outlet 90 is formed at a position opposite the microchannel 20, penetrating the second substrate 12 in the thickness direction, and communicating with the microchannel 20. This allows the reaction liquid to be discharged from the microchannel 20 to the outside of the microchannel 20 via the liquid outlet 90. A liquid outlet pipe 91 is connected to the liquid outlet 90, which connects the liquid outlet 90 to a liquid recovery section that contains the reaction liquid discharged from the microchannel 20.

[0040] "Voltage application means" In this embodiment, the gas reduction device 1 preferably includes a voltage applying means 100 for applying a voltage to the cathode 50 and the anode 60. By applying a voltage to the cathode 50 and the anode 60 using the voltage applying means 100, the gas in the microchannel 20 can be reduced. There are no particular restrictions on the voltage applied by the voltage application means 100, but for example, it is sufficient to apply -10V to -1V, preferably around -5V.

[0041] The gas reduction apparatus 1 of this embodiment may have two or more reactor bodies 10. In that case, it is more preferable to have two or more reactor bodies 10 arranged in parallel. Because the microchannels 20 formed in the reactor body 10 are small, the amount of gas that can be processed at one time in one microchannel 20 (processing volume) is small. Therefore, by providing two or more reactor bodies 10, the processing volume per unit time can be improved. This reactor can be integrated and mass-produced using semiconductor processes, and for example, by integrating or arranging 1000 or more reactors in parallel, it is possible to configure a reactor that obtains a processing speed 1000 times or more than that of a single reactor body 10.

[0042] In addition to the cathode 50 and anode 60, the gas reduction device 1 of this embodiment may also be equipped with a reference electrode. Because the gas reduction apparatus 1 of this embodiment is equipped with a reference electrode, voltage fluctuations associated with the structure of the reactor body 10 can be suppressed, and a stable voltage for the reduction reaction can be applied.

[0043] "Gas-liquid mixed flow" The gas reduction device 1 of this embodiment forms a gas-liquid mixed flow by introducing gas from the gas inlet 30 and liquid from the liquid inlet 40. The gas-liquid mixed flow in this embodiment may be any of the following: a bubble flow, a slug flow, a floss flow, an annular flow, a wave flow, or a spray flow. Preferably, it is a slug flow in which gas and liquid are alternately formed within the microchannel 20. In the gas reduction device 1 of this embodiment, the gas phase and the liquid phase are formed between the electrodes in the microchannel 20, which is characterized by efficient gas reduction.

[0044] "Introduction of liquids and gases" The gas reduction device 1 of this embodiment can form a gas-liquid mixed flow by introducing a liquid through the liquid inlet 40 and a gas through the gas inlet 30. The gas inflow velocity from the gas inlet 30 should be between 0.01 sccm and 100 sccm, and preferably between 0.1 sccm and 10 sccm. For example, when reducing carbon dioxide to produce methane, a gas-liquid mixed flow is formed by introducing liquid from the liquid inlet 40 into the microchannel while continuously introducing gas from the gas inlet 30. Once the gas-liquid mixed flow fills, forms, and is maintained within the microchannel, a voltage is applied to initiate the reduction reaction.

[0045] According to the gas reduction apparatus 1 of this embodiment, for example, when copper is used for the cathode and cathode catalyst, carbon dioxide can be reduced almost entirely to methane, demonstrating highly efficient gas reduction. The gas reduction apparatus 1 of this embodiment is equipped with a cathode 50 and an anode 60 and does not have a separator for separating the gas, so a large reduction current density can be obtained between the cathode 50 and the anode 60. Therefore, the efficiency of the gas reduction reaction is increased. Furthermore, since this method does not use solid electrolyte membranes such as ion exchange membranes, which were conventionally used in hydrocarbon generation reactions, the diffusion of hydrogen ions in the solid electrolyte does not become a rate-limiting factor in the reduction reaction of carbon dioxide, resulting in high reduction efficiency.

[0046] The reason why the gas reduction device 1 of this embodiment achieves high gas reduction efficiency is currently under investigation, but it is suspected to be due to the following factors. In this embodiment, the gas reduction device 1 reduces a gas-liquid mixed flow by applying a voltage while the cathode 50 and anode 60 are facing each other in the microchannel 20. In this situation, since gas is introduced into a narrow space called a microchannel and a gas-liquid mixed flow is continuously formed, it is thought that various reactions are promoted in the micro-reaction field by the rapid diffusion of reactants produced by the reaction, as well as by the constant pressure fluctuations associated with the gas-liquid mixed flow and the gas generation associated with the reaction. In addition to this, it is thought that the reaction is further accelerated and high reduction efficiency is achieved by the combined effects of electrode surface effects such as thinning and porosification of the cathode surface, reaction hot spots occurring locally at the electrode, and temperature fluctuations such as heat fluctuations associated with the reaction.

[0047] [Gas reduction method] A gas reduction method according to one embodiment of the present invention is a gas reduction method using the gas reduction apparatus of the above-described embodiment, comprising: a step of introducing liquid into the microchannel from the liquid inlet (hereinafter referred to as the "first step"); a step of introducing gas into the liquid in the microchannel from the gas inlet to form a gas-liquid mixed flow (hereinafter referred to as the "second step"); a step of applying a voltage to the cathode and the anode to reduce the gas in the liquid (hereinafter referred to as the "third step"); and a step of dischargeing the reaction gas generated by the reduction of the gas from the microchannel through the gas outlet (hereinafter referred to as the "fourth step"). The liquid introduced into the microchannel may be continuously discharged from the liquid outlet.

[0048] The gas reduction method of this embodiment will be described below with reference to Figures 1 to 4.

[0049] "First step" In the first step, liquid is introduced into the microchannel 20 from the liquid inlet 40 of the reactor body 10 via the gas-liquid inlet passage 22 and the gas-liquid mixer 21.

[0050] The liquid is not particularly limited as long as it can dissolve or disperse the gas to be reduced in the microchannel 20, but examples include a liquid containing at least one selected from water, ethanol, and carboxylic acids. Specifically, examples of liquids include electrolytes. Examples of electrolytes include aqueous solutions of salt, liquids containing ethanol, and liquids containing carboxylic acids. Note that water, ethanol, and carboxylic acids (R-COOH) act as sources of hydrogen in the reduction reaction.

[0051] "The second step" In the second step, gas is introduced into the liquid in the microchannel 20 from the gas inlet 30 of the reactor body 10. By introducing gas into the liquid in the microchannel 20, a gas-liquid mixed flow is generated within the microchannel 20.

[0052] The gas is not particularly limited as long as it can form a gas-liquid mixed flow in the liquid within the microchannel 20, but examples include carbon dioxide, carbon monoxide, nitrogen, nitrogen oxides, etc.

[0053] "The third step" In the third step, a voltage is applied to the cathode 50 and anode 60 by the voltage application means 100 to reduce the gas in the liquid within the microchannel 20. When a gas-liquid mixed flow is generated within the microchannel 20, the gas phase and liquid phase reduce the substance (mainly gas) on the reduction catalyst metal 52 of the cathode 50, and as a result, a reduced gas (reaction gas) is produced.

[0054] In the gas reduction method of this embodiment, a reduction voltage lower than -1V (for example, -3V to -5V) is applied to the cathode 50 to induce an electrochemical reduction reaction in the microchannel 20. In this case, not only an electrochemical reaction but also a non-Faraday reaction can occur in the microchannel 20. In other words, an electrochemical reaction can yield a reaction rate higher than that limited by the reduction current.

[0055] For example, if the gas contains carbon dioxide and the liquid is water, methane is produced by reducing the carbon dioxide, as shown in the reaction equation (1) below. 8H + +CO2→CH4+2H2O···(1)

[0056] Furthermore, if the reducing catalyst metal 52 is iron, the gas contains nitrogen, and the liquid is an aqueous electrolyte, ammonia is produced by reducing the nitrogen, as shown in the reaction equation (2) below. N2+6H + →2NH3···(2)

[0057] "The fourth step" In the fourth step, the reaction gas generated by the reduction of the gas is discharged from the microchannel 20 via the gas outlet 70 of the reactor body 10. In this process, if the product is a liquid, the reaction liquid generated by the reduction of the gas is discharged from the microchannel 20 via the liquid outlet 90.

[0058] In the gas reduction method of this embodiment, the gas reduction reaction occurs in a gas-liquid mixed flow within a small, closed space, which is a microchannel 20. As a result, the gas diffuses at high speed, and a gas reduction reaction occurs that would not occur in a larger space.

[0059] In the gas reduction method of this embodiment, a large reduction current can accelerate the gas reduction reaction by causing localized heating (a hot spot phenomenon where high-temperature areas occur) and gas generation (pressure increase). Furthermore, in reactions with negative enthalpy, the reaction is accelerated at high pressure. Also, at high pressure, the solubility of the gas in the liquid increases within the microchannel 20. As a result, the efficiency of the gas reduction reaction increases.

[0060] In the gas reduction method of this embodiment, the gas reduction reaction takes place within a small, closed space called a microchannel 20, so the ion product of the electrolyte (hydrogen ions (H)) is reduced. + ) and hydroxide ions (OH - An enhancement of the product of ) is also expected. In other words, a larger number of hydrogen ions than in equilibrium water can exist, which may be useful for reduction.

[0061] In the gas reduction method of this embodiment, a solvent that removes oxide films may be added to the liquid in the microchannel 20 to prevent the reduction catalyst metal 52 from being oxidized by oxygen. For example, hydrogen fluoride may be mixed into the electrolyte at a concentration of 0.1% by mass or less. [Examples]

[0062] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0063] [Examples] (Construction of a gas reduction device) A reactor body with a microchannel (channel cross-sectional area 1 mm × 0.4 mm, channel length 10 mm) was fabricated as described below.

[0064] Two silicon substrates (cathode substrate and anode substrate) were joined together at their respective sides via an insulating material. Each substrate has a recess formed on its opposite surface, perpendicular to the thickness direction of the substrate. By arranging the two substrates so that their sides face each other, microchannels were formed by the recesses in each substrate.

[0065] Next, using photolithography, the portion of the two substrates' sides that would not have electrodes formed was protected with photoresist. Next, a silicon substrate with microchannels formed on it was immersed for 11 to 12 minutes in a mixed solution of 0.5 mol / L silver nitrate and 0.14 mol / L hydrogen fluoride to form silver nanoparticles on the inner surface of the microchannels.

[0066] Next, a silicon substrate on which silver nanoparticles were formed was immersed in a solution consisting of 20 mL of hydrogen peroxide, 50 mL of hydrogen fluoride (50%), and 10 mL of DI water. Porous silicon was then formed on the inner surface of the microchannels by a localized electrochemical reaction of silver. Next, the silicon substrate was washed with DI water and methanol.

[0067] Next, copper was deposited (supported) onto porous silicon by electroless plating to form a cathode. Catalyst treatment was performed for 5 minutes, an activator treatment for 8 minutes, and an electroless copper plating process for 15-20 minutes. Finally, the silicon substrate was washed with DI water.

[0068] Next, nickel was deposited by electroless plating on a portion of the inner surface of the microchannel that faced the surface where copper had been deposited, thereby forming an anode.

[0069] Next, a thin film polyimide (thin film adhesive) was applied to one surface of each of the two glass substrates. The silicon substrate, on which the cathode and anode were formed as described above, was then sandwiched between the two glass substrates to obtain a reactor body in which the two glass substrates and the silicon substrate were laminated. The two glass substrates and the silicon substrate are laminated via a thin film polyimide (adhesive layer). This formed a microchannel with a cathode and anode within the reactor body.

[0070] One of the glass substrates used had a gas inlet, a liquid inlet, and a gas outlet formed on it.

[0071] Next, a gas inlet pipe was connected to the gas inlet, a liquid inlet pipe to the liquid inlet, and a gas outlet pipe to the gas outlet. A gas selective permeation filter made of porous polytetrafluoroethylene was also placed at the gas outlet to obtain a gas reduction device.

[0072] (Reduction of carbon dioxide by reduction reaction) An electrolyte solution prepared by adding 0.1 mol / L sodium bicarbonate (NaHCO3) to DI water was used as the reducing liquid. The reducing liquid was introduced into the microchannel of the gas reduction device from the liquid inlet beforehand. At room temperature (25°C), a flow control device flows carbon dioxide from the gas outlet into the reducing liquid in the microchannel at a flow rate of 3 sccm, with a voltage of -5V and an average current of -12mA at -300mA / cm². 2 The current density was applied to the cathode to promote the reduction reaction of carbon dioxide. The gas produced by the reduction reaction of carbon dioxide was extracted and analyzed by gas chromatography using a GC-2014A manufactured by SIMAZU Corporation. The results are shown in Figure 5. As shown in Figure 5, 100% of the gas produced by the reduction was methane. [Industrial applicability]

[0073] According to the present invention, a gas reduction device is obtained that does not require a solid electrolyte membrane, can perform gas reduction with high efficiency, and can generate only specific types of gases. [Explanation of Symbols]

[0074] 1. Gas reduction device 10 Reactor body 20 Microfluidic Channels 30 Gas inlet 40 Liquid inlet 50 cathode 60 Anode 70 Gas outlet 80 Gas Selective Permeation Filter 90 Liquid outlet 100 Voltage application means

Claims

1. The reactor body and A microchannel formed in the reactor body, A gas inlet is formed in the reactor body, communicates with the microchannel, and introduces gas into the microchannel, A liquid inlet is formed in the reactor body, communicates with the microchannel, and introduces liquid into the microchannel; A cathode provided on the inner surface of the aforementioned microchannel, An anode is provided on the inner surface of the microchannel, facing the cathode, A gas outlet is formed in the reactor body, communicates with the microchannel, and discharges the reaction gas from the microchannel, Equipped with, The microchannel is a gas reduction device that maintains a gas-liquid mixed flow consisting of the gas and the liquid.

2. The gas reduction apparatus according to claim 1, wherein the cathode comprises a portion of the inner surface of the microchannel that is thinned or porous, and a reducing catalyst metal supported on the portion.

3. The gas reduction apparatus according to claim 2, wherein the reduction catalyst metal is at least one metal selected from copper, silver, iron, nickel, platinum, rhodium, gold, silver, zinc, palladium, tin, lead, indium, titanium, tungsten, and molybdenum, or an alloy or oxide of these metals.

4. The gas reduction apparatus according to claim 1, further comprising a liquid outlet connected to the microchannel and for discharging a reaction liquid from the microchannel.

5. The gas reduction apparatus according to claim 1, further comprising a gas selective permeation filter provided at the gas outlet, which selectively permeates the reaction gas discharged from the microchannel and removes impurities contained in the reaction gas.

6. The gas reduction apparatus according to claim 1, wherein the width and height of the microchannel are 0.1 mm to 2.0 mm.

7. The gas reduction apparatus according to claim 1, wherein the gas inflow velocity from the gas inlet is 0.01 sccm to 100 sccm.

8. The gas reduction apparatus according to claim 1, further comprising a voltage applying means for applying a voltage to the cathode and the anode.

9. The gas reduction apparatus according to claim 1, comprising two or more reactor bodies.

10. A gas reduction method using a gas reduction apparatus according to any one of claims 1 to 9, A step of introducing the liquid into the microchannel from the liquid inlet, The process involves introducing the gas into the liquid in the microchannel from the gas inlet, and forming the gas-liquid mixed flow in the microchannel. A step of applying a voltage to the cathode and the anode and reducing the gas in the liquid, A step of releasing the reaction gas generated by the reduction of the gas from the microchannel through the gas outlet, A gas reduction method having the following characteristics.

11. The gas reduction method according to claim 10, wherein the gas contains carbon dioxide, the liquid is a liquid containing at least one selected from water, ethanol, and carboxylic acids, the cathode contains copper, and the reaction gas contains methane.

Images