Reverse water-gas shift catalyst, electrolytic reaction system, and method for converting carbon dioxide

Abstract

To obtain a reverse water-gas shift catalyst usable at high temperatures, and to obtain a method for producing it. [Solution] The reverse water-gas shift catalyst is an impregnated support in which iron is supported as the catalytic active component Ca1 on a support cb1 mainly composed of zirconia.

Description

Technical Field

[0001] The present invention relates to a reverse water gas shift catalyst and a method for producing the same, and also relates to an electrolysis reaction system and a hydrocarbon production system including a catalytic reaction section using such a reverse water gas shift catalyst.

Background Art

[0002] In the production of hydrocarbons, a technique for obtaining methane gas from carbon monoxide and hydrogen as raw material gases is known. Patent Document 1 discloses a carbon monoxide production system which is one of the above raw material gases.

[0003] The system disclosed in Patent Document 1 includes an electrolyzer (corresponding to the electrolysis reaction section of the present invention) that performs electrolysis treatment. Carbon dioxide and water are supplied to the cathode of this device, and hydrogen and carbon monoxide are generated by electrolysis. In this reaction, unreacted water and carbon dioxide remain. Therefore, hydrogen, carbon monoxide, unreacted water, and carbon dioxide sent out from the electrolyzer are sent to a reverse shift reactor (corresponding to the reverse water gas shift reaction section of the present invention), and carbon dioxide and hydrogen are reacted to generate carbon monoxide and water.

[0004] The electrolyzer operates at about 700 to 900 °C, and the reverse shift reactor operates at about 600 to 950 °C. As the catalyst, copper (Cu), nickel (Ni), etc. can be used (paragraph

[0029] of the specification).

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] The catalyst used here is also called a reverse shift catalyst, and it is a catalyst that performs the reverse reaction of the water-gas shift reaction (a reaction that produces carbon dioxide and hydrogen from carbon dioxide and water), i.e., the reverse water-gas shift reaction.

[0007] CO + H2O → CO2 + H2ΔH = -41 kJ / mol

[0008] The former, the water-gas shift reaction, is a reaction widely used in hydrogen production processes and other applications. It converts carbon monoxide into carbon dioxide and hydrogen by reacting it with water. However, because the reaction is exothermic, the equilibrium shifts towards carbon dioxide production at lower temperatures. Therefore, conventional efforts have focused on developing catalysts with high low-temperature activity. Generally, iron-chromium catalysts used at around 300-450°C and copper-zinc catalysts used at around 200°C are known.

[0009] On the other hand, there have been few applications that require the reverse reaction of the water-gas shift reaction (reverse water-gas shift reaction), so the development of catalysts suitable for the reverse water-gas shift reaction has not been extensively pursued until now.

[0010] The reverse water-gas shift reaction converts carbon dioxide into carbon monoxide and water by reacting it with hydrogen. This reaction is endothermic, and unlike the water-gas shift reaction, high temperatures are required to shift the equilibrium to the carbon monoxide production side. Therefore, catalysts that can be used at the highest possible temperatures are needed. However, from this perspective, there has been little development of high-performance catalysts suitable for the reverse water-gas shift reaction.

[0011] The object of the present invention is to obtain a reverse water-gas shift catalyst usable at high temperatures, and to obtain a method for producing the same. Furthermore, it is also to provide a hydrocarbon production system that can produce hydrocarbons using a reverse water-gas shift reaction in conjunction with an electrolytic reaction. [Means for solving the problem]

[0012] The first characteristic configuration of the present invention is, The key feature is that it is a reverse water-gas shift catalyst in which at least nickel and / or iron are supported as catalytic active components on a carrier mainly composed of ceria-based metal oxide or zirconia-based metal oxide. This catalytic active component is a metal catalyst, and is supported on the carrier as a metal or its metal oxide.

[0013] According to this characteristic configuration, by supporting either nickel and iron, or both, as catalytically active components on a support mainly composed of ceria-based metal oxide or zirconia-based metal oxide, a catalyst that exhibits high activity at relatively high temperatures is obtained, as will be explained later based on Tables 1, 2, 3, and 4. In this regard, the inventors also considered alumina as a support material, but when the firing temperature was around 450°C, they obtained clearly inferior results. The performance of this catalyst configuration was also compared with examples using platinum, an expensive precious metal, as the catalytically active component, and the catalyst according to the present invention showed comparable activity.

[0014] When using either nickel or iron, or both, as catalytic active components, the cost per unit weight can be reduced to less than 1 / 1000 compared to platinum, which is preferable because it reduces costs or, if the same cost is incurred, allows for a significant increase in the amount of catalyst used.

[0015] Furthermore, by using a ceria-based metal oxide or a zirconia-based metal oxide as the support, resistance to high temperatures can be ensured.

[0016] In this invention, a reverse water-gas shift reaction section is provided downstream of the electrolytic reaction section (on the side where the gas generated in the electrolytic reaction section is advected). By using a ceria-based metal oxide or a zirconia-based metal oxide as the support for the reverse water-gas shift catalyst, the thermal expansion coefficient can be made close to that of the material constituting the electrolytic reaction section. Therefore, reactions can be generated smoothly in both sections at almost the same high temperature range.

[0017] To obtain this reverse water-gas shift catalyst, as described in the 12th characteristic configuration of the present invention, By adding a carrier mainly composed of a ceria-based metal oxide or a zirconia-based metal oxide to a solution containing either or both of nickel and iron, and at least performing an impregnation and support step of impregnating and supporting either or both of nickel and iron on the carrier, a reverse water gas shift catalyst can be produced.

[0018] When obtaining this reverse water gas shift catalyst, as described in the 13th characteristic configuration of the present invention, It is preferable to have at least a firing step of supporting at least one or both of nickel and iron as catalytic active components on a carrier mainly composed of a ceria-based metal oxide or a zirconia-based metal oxide and firing at a temperature of 450 °C or higher.

[0019] For the use of the reverse water gas shift catalyst, in order to cause the target reverse water gas shift reaction, it is necessary to set the reaction temperature range to a relatively high temperature range. However, for a catalyst obtained by firing in this temperature range, the catalyst can be used stably. It is preferably 450 °C or higher, and more preferably 600 °C or higher and 800 °C or higher because the stability in the high temperature range can be enhanced. For example, even when combined with a solid oxide fuel cell used in a relatively high temperature range (e.g., 600 °C to 800 °C), the catalyst can be used stably. Also, if the firing temperature is too high, the cost of the firing process becomes too high, so the upper limit is about 1200 °C.

[0020] When using this reverse water gas shift catalyst, as described in the 15th characteristic configuration of the present invention, it is preferable to subject the reverse water gas shift catalyst to a pre-reduction treatment and then use it in the reaction.

[0021] The catalytic active components contained in the reverse water gas shift catalyst often become metal oxides at least partially in the firing step. However, by performing a pre-reduction treatment (a treatment of performing a reduction treatment before use), the catalytic active components in the oxidized state can be reduced and the catalytic activity can be exhibited well.

[0022] The second characteristic configuration of the present invention is The proportion of the carrier in the whole catalyst is 55% by weight or more. % by weight has the same meaning as % by mass. The same applies hereinafter.

[0023] According to this characteristic configuration, by increasing the weight ratio of the carrier, the strength of the catalyst can be increased. Therefore, the proportion of the carrier is preferably 55% by weight or more, more preferably 60% by weight or more, and still more preferably 65% by weight or more. The upper limit of this ratio can be, for example, 99.5% by weight. If it exceeds this, it may become difficult to sufficiently support the catalyst active component, and it may be difficult to obtain the effect as a reverse water gas shift catalyst.

[0024] The third characteristic configuration of the present invention is that the ceria-based metal oxide is ceria doped with at least one of gadolinium, samarium, and yttrium.

[0025] By performing the doping treatment as described later, the activity as a catalyst can be improved.

[0026] The fourth characteristic configuration of the present invention is that the zirconia-based metal oxide is zirconia stabilized with at least one of yttria and scandia.

[0027] By using stabilized zirconia as described later, the activity as a catalyst can be improved.

[0028] The fifth characteristic configuration of the present invention is that the supported amount of the catalyst active component is 0.5% by weight or more.

[0029] By adopting this configuration, the reverse water-gas shift reaction can proceed smoothly, so it is preferable that the amount of catalytically active component supported is 0.5% by weight or more, more preferably 1% by weight or more, and even more preferably 5% by weight or more. However, if the amount of catalytically active component supported is increased too much, it becomes difficult to support the catalytically active component in a highly dispersed manner, making it difficult to obtain a significant improvement in catalytic activity and increasing the catalyst cost, so it is preferable that the amount of catalytically active component supported is 35% by weight or less, more preferably 30% by weight or less, and even more preferably 25% by weight or less.

[0030] The sixth feature configuration of the present invention is: In addition to the aforementioned catalytically active component, it also contains supported copper.

[0031] This characteristic configuration can enhance the activity of the catalyst as a reverse water-gas shift catalyst.

[0032] The seventh characteristic configuration of the present invention is: The key point is that the amount of copper supported is equal to or less than the amount of the catalytically active component supported.

[0033] As explained previously, the main catalytic activity of the reverse water-gas shift catalyst according to the present invention is due to either nickel or iron, or both. Therefore, the copper support effect can be obtained without interfering with the effects exhibited by these components.

[0034] The eighth feature configuration of the present invention is: The key feature is that it is an electrolytic reaction system having at least a reverse water-gas shift reaction section containing at least the reverse water-gas shift catalyst described above, and an electrolytic reaction section.

[0035] According to this characteristic configuration, at least water is electrolyzed in the electrolytic reaction section, and the generated hydrogen is used to convert carbon dioxide into carbon monoxide using the reverse water-gas shift catalyst according to the present invention, thereby obtaining raw materials (at least hydrogen and carbon monoxide) to be used in hydrocarbon synthesis. In this configuration, by employing the reverse water-gas shift catalyst according to the present invention, which can obtain sufficient activity at high temperatures, in the reverse water-gas shift reaction section, it is possible to efficiently produce, for example, hydrogen and carbon monoxide necessary for hydrocarbon synthesis, and to produce hydrocarbons. As will be described later, it is particularly preferable that the ratio of hydrogen to carbon monoxide can be adjusted by the reverse water-gas shift reaction. Furthermore, the heat (energy) from the electrolytic reaction section can be effectively utilized in the reverse water-gas shift reaction section. Therefore, as shown in the 11th characteristic configuration of the present invention, by providing a hydrocarbon synthesis reaction unit in addition to the electrolytic reaction unit and the reverse water-gas shift reaction unit, a hydrocarbon production system can be constructed that synthesizes hydrocarbons using the generated hydrogen and carbon monoxide.

[0036] The ninth characteristic configuration of the present invention is: The electrolytic reaction section has an electrolytic cell unit in which at least an electrode layer, an electrolyte layer, and a counter electrode layer are formed on a support.

[0037] According to this characteristic configuration, the electrolytic cell used in the electrolytic reaction section can be equipped with, for example, a thin-film electrode layer, an electrolyte layer, and a counter electrode layer on a robust support that is thin yet strong. This allows for effective electrolytic reactions while reducing the amount of material used to constitute these layers of the electrolytic cell. As a result, a compact, high-performance, and highly durable and reliable electrolytic cell unit can be constructed. Metals and ceramics can be selected as the constituent materials for this type of support.

[0038] Furthermore, as described in the 14th characteristic configuration of the present invention, by adopting a configuration in which an impregnated support obtained through an impregnation and supporting process in which either or both of the nickel and iron are impregnated and supported on the carrier is placed on at least a part of the support to form a reverse water-gas shift reaction section, both the electrolytic reaction section and the reverse water-gas shift reaction section can be formed on the support, thereby obtaining a compact, high-performance electrolytic cell unit with excellent strength and reliability, and furthermore, a hydrocarbon production system equipped with this type of electrolytic cell unit.

[0039] Furthermore, the tenth characteristic feature of the present invention is that the support is made of metal.

[0040] By using metal as a support, material costs can be reduced by ensuring strength with inexpensive metal materials, and it is easier to process than ceramics. [Brief explanation of the drawing]

[0041] [Figure 1] Diagram showing the configuration of a hydrocarbon production system. [Figure 2] Schematic diagram showing the configuration of the electrolytic reaction section. [Figure 3] This diagram shows the configuration of a system that integrates the electrolytic reaction section and the reverse water-gas shift reaction section. [Figure 4] Schematic diagram of an electrolytic cell unit equipped with an electrolytic reaction section and a reverse water-gas shift reaction section. [Figure 5] Cross-sectional view of the electrolytic cell unit used in a comparative experiment in which the electrode layer-side gas supply path was used as the reverse water-gas shift reaction section. [Figure 6] System configuration diagram showing a heat exchanger between the electrolytic reaction section and the reverse water-gas shift reaction section. [Figure 7] This diagram shows an alternative configuration of a hydrogen carbide production system that guides CO2 to the reverse water-gas shift reaction section. [Figure 8] Diagram showing an alternative configuration of a hydrocarbon production system equipped with a hydrogen separation unit. [Figure 9] A diagram showing a further alternative configuration of a hydrocarbon production system that includes a water separation unit before the hydrocarbon synthesis reaction unit. [Figure 10] A diagram showing yet another configuration of a hydrocarbon production system in which only water is introduced into the electrolytic reaction section. [Figure 11] Diagram illustrating the preparation state of the catalyst. [Figure 12] Diagram illustrating the catalyst coating and firing process, as well as the pre-treatment for reduction. [Figure 13] Schematic diagram of an electrolytic cell unit equipped with an electrolytic reaction section, a reverse water-gas shift reaction section, and a hydrocarbon synthesis reaction section. [Modes for carrying out the invention]

[0042] Embodiments of the present invention will be described with reference to the drawings. Figure 1 shows the configuration of one embodiment of the hydrocarbon production system 100 proposed by the inventors.

[0043] As shown in the figure, this hydrocarbon production system 100 is configured to include, in order, an electrolytic reaction section 10, a first catalytic reaction section 20, a second catalytic reaction section 30, a heavy hydrocarbon separation section 35 (shown as a CnHm separation section), a water separation section 40 (shown as a H2O separation section), and a carbon dioxide separation section 50 (shown as a CO2 separation section).

[0044] The electrolytic reaction section 10 is a section that electrolyzes at least a portion of the incoming gas, the first catalytic reaction section 20 is a reverse water-gas shift reaction section that performs a reverse water-gas shift reaction on at least a portion of the incoming gas, and the second catalytic reaction section 30 is configured to function as a hydrocarbon synthesis reaction section that synthesizes at least a portion of the incoming gas into hydrocarbons. Here, the hydrocarbons synthesized are mainly CH4 (carbon hydrocarbons with 1 carbon atom), but also include lower saturated hydrocarbons with 2 to 4 carbon atoms. Furthermore, as will be shown later, by appropriately selecting the catalyst used in the second catalytic reaction section 30, heavy hydrocarbons with a larger number of carbon atoms than the lower saturated hydrocarbons, unsaturated hydrocarbons, or oxygen-containing hydrocarbons can also be synthesized. Therefore, in this specification, hydrocarbons are a concept that includes all of these, and are also collectively referred to as hydrocarbons.

[0045] The heavy hydrocarbon separation section 35, the water separation section 40, and the carbon dioxide separation section 50 are parts that remove at least a portion of predetermined components (in order listed, CnHm, H2O, and CO2) from the gas flowing inside. The components removed and recovered by the water separation section 40 and the carbon dioxide separation section 50 are returned to predetermined parts of the system for reuse via the water return path 41 and the carbon dioxide return path 51, as shown in Figure 1. The H2O and CO2 returned via the respective return paths 41 and 51 are shown above them. As a result, this hydrocarbon production system 100 functions as a carbon closed system that does not release CO2 outside the system.

[0046] In the figure, the gas flowing into each part is shown before each part, and the gas released from that part is shown after it.

[0047] In the electrolytic reaction section 10, H2O and CO2 are introduced as starting materials and are electrolyzed inside, causing H2O to decompose into H2 and O2, while some of the CO2 is decomposed into CO and O2 and released.

[0048] The reaction is described as follows: 2H2O→2H2+O2 (formula 1) 2CO2→2CO+O2 (formula 2) These equations 1 and 2 are also shown inside the box representing the electrolytic reaction section 10 in Figure 1.

[0049] In the first catalytic reaction section 20 (reverse water-gas shift reaction section), H2 and CO2 are introduced, and a reverse water-gas shift reaction occurs inside, where CO2 is converted to CO and H2 is converted to H2O and released.

[0050] The reaction is described as the following equilibrium reaction, but the reverse water-gas shift reaction is a reaction in which the reaction described in Equation 3 below proceeds to the right (the reaction proceeds in the direction in which CO2 and H2 react to produce CO and H2O). CO2+H2⇔CO+H2O (Formula 3) Equation 3 is also shown inside the box representing the first catalytic reaction section 20 (reverse water-gas shift reaction section) in Figure 1. The reverse water-gas shift catalyst cat1 used in the reaction is also schematically shown inside the box.

[0051] In the second catalytic reaction section 30 (hydrocarbon synthesis reaction section), H2 and CO are introduced, and hydrocarbons are synthesized by catalytic reaction. For example, the reaction in which CH4 is synthesized from CO and H2 is described as the following equilibrium reaction, but the reaction in which CH4 is synthesized from CO and H2 is a reaction in which the reaction described in equation 4 below proceeds to the right (a reaction that proceeds in the direction in which CO and H2 react to produce CH4 and H2O). CO+3H2⇔CH4+H2O (Formula 4) Equation 4 is also shown inside the box representing the second catalytic reaction section 30 (hydrocarbon synthesis reaction section) in Figure 1. The hydrocarbon synthesis catalyst cat2 used in the reaction is also schematically shown inside this box. Furthermore, the equilibrium reaction shown in (Equation 3) also occurs in this region. Furthermore, depending on the type of catalyst used in the second catalytic reaction section 30, it is possible to carry out FT (Fischer-Tropsch) synthesis reactions, etc., so that various hydrocarbons such as ethane, propane, butane, pentane, hexane, paraffin, and olefinic hydrocarbons can be synthesized from CO and H2.

[0052] As will be described later, the inventors have shown an example of a catalyst using ruthenium as its catalytically active component as the hydrocarbon synthesis catalyst cat2 to be placed in the second catalytic reaction unit 30. However, in catalysts containing iron, cobalt, or the like as catalytically active components, heavy hydrocarbons are also synthesized, and these types of heavy hydrocarbons can be separated from the transport gas as the temperature decreases. Therefore, the heavy hydrocarbon separation unit 35 separates the hydrocarbon components that are separated in this way.

[0053] The H2O generated in the water separation section 40 is separated and returned to the upstream side of the electrolytic reaction section 10 via the water return path 41 (water recycling line).

[0054] The CO2 generated in the carbon dioxide separation unit 50 is separated and returned to the upstream side of the electrolytic reaction unit 10 via the carbon dioxide return path 51 (carbon dioxide recycling line).

[0055] As a result, in this hydrocarbon production system 100, hydrocarbons are ultimately synthesized and can be supplied externally.

[0056] The above is an overview of the hydrocarbon production system 100. The following describes the configuration and role of each part. [Electrolytic reaction section] As previously shown, this electrolytic reaction unit 10 consumes the power supplied according to the above formulas 1 and 2 to decompose the incoming H2O and CO2.

[0057] Figure 2 schematically shows the cross-sectional configuration of the electrolytic reaction section 10. The figure shows an electrolytic cell unit U that is stacked in multiple layers to form an electrolytic stack (not shown). This electrolytic cell unit U comprises an electrolytic cell 1, which is composed of an electrode layer 2 on one side of an electrolyte layer 1a and a counter electrode layer 3 on the other side. The electrode layer 2 becomes the cathode in the electrolytic cell 1, and the counter electrode layer 3 becomes the anode. Incidentally, this electrolytic cell unit U is supported by a metal support 4. Note that here, an example is given in which a solid oxide type electrolytic cell is used as the electrolytic cell 1.

[0058] The electrolyte layer 1a can be formed as a thin film with a thickness of 10 μm or less. Suitable constituent materials include YSZ (yttria-stabilized zirconia), SSZ (scandia-stabilized zirconia), GDC (gadolinium-doped ceria), YDC (yttrium-doped ceria), SDC (samarium-doped ceria), LSGM (strontium-magnesium-doped lanthanum gallate), etc. Zirconia-based ceramics are particularly preferred.

[0059] The electrolyte layer 1a is preferably formed by low-temperature firing (for example, a wet method using firing in a low temperature range without firing in a high temperature range exceeding 1100°C), spray coating (such as thermal spraying, aerosol deposition, aerosol gas deposition, powder jet deposition, particle jet deposition, or cold spray), PVD (such as sputtering or pulsed laser deposition), or CVD. These low-temperature film formation processes allow for the creation of a dense, airtight, and highly gas-barrier electrolyte layer 1a without using firing in a high temperature range exceeding 1100°C, for example. This suppresses damage to the metal support 4 and inhibits elemental interdiffusion between the metal support 4 and the electrode layer 2, enabling the realization of an electrolytic cell unit U with excellent performance and durability. In particular, using low-temperature firing or spray coating is preferable because it allows for the realization of low-cost devices. Furthermore, using a spray coating method is even more preferable because a dense, airtight, and highly gas-barrier electrolyte layer 1a can be easily obtained at low temperatures.

[0060] Furthermore, the electrolyte layer 1a is densely constructed to shield against gas leaks and exhibit high ionic conductivity. The density of the electrolyte layer 1a is preferably 90% or higher, more preferably 95% or higher, and even more preferably 98% or higher. If the electrolyte layer 1a is a uniform layer, its density is preferably 95% or higher, and more preferably 98% or higher. If the electrolyte layer 1a is composed of multiple layers, it is preferable that at least a portion of it includes a layer with a density of 98% or higher (a dense electrolyte layer), and more preferably a layer with a density of 99% or higher (a dense electrolyte layer). This is because including such a dense electrolyte layer in a portion of the electrolyte layer 1a makes it easier to form a dense electrolyte layer 1a with high airtightness and gas barrier properties, even if the electrolyte layer 1a is composed of multiple layers.

[0061] The electrode layer 2 can be provided as a thin layer on the front surface of the metal support 4 in an area larger than the area where the holes 4a are provided. When it is a thin layer, its thickness can be, for example, about 1 μm to 100 μm, preferably 5 μm to 50 μm. With such a thickness, it is possible to reduce the amount of expensive electrode layer material used and lower costs while ensuring sufficient electrode performance. The entire area where the holes (through holes) 4a are provided is covered by the electrode layer 2. In other words, the holes 4a are formed inside the area on the metal support 4 where the electrode layer 2 is formed. To put it another way, all the holes 4a are provided facing the electrode layer 2.

[0062] The constituent material of this electrode layer 2 can be a composite material such as NiO-GDC, Ni-GDC, NiO-YSZ, Ni-YSZ, CuO-CeO2, or Cu-CeO2. In these examples, GDC, YSZ, and CeO2 can be called aggregates of the composite material. The electrode layer 2 is preferably formed by a low-temperature firing method (for example, a wet method using firing in a low temperature range without firing in a high temperature range higher than 1100°C), a spray coating method (such as thermal spraying, aerosol deposition, aerosol gas deposition, powder jet deposition, particle jet deposition, or cold spray), a PVD method (such as sputtering or pulsed laser deposition), or a CVD method. By using these processes that can be used in a low temperature range, a good electrode layer 2 can be obtained without using firing in a high temperature range higher than 1100°C, for example. Therefore, this method is preferable because it does not damage the metal support 4 and suppresses elemental interdiffusion between the metal support 4 and the electrode layer 2, thereby realizing an electrochemical element with excellent durability. Furthermore, using a low-temperature firing method is even preferable because it simplifies the handling of raw materials.

[0063] The counter electrode layer 3 can be formed as a thin layer on the side of the electrolyte layer 1a opposite to the electrode layer 2. When forming a thin layer, its thickness can be, for example, about 1 μm to 100 μm, preferably 5 μm to 50 μm. Such a thickness makes it possible to reduce the amount of expensive counter electrode layer material used, thereby lowering costs while ensuring sufficient electrode performance. As the material for the counter electrode layer 3, for example, composite oxides such as LSCF and LSM, ceria oxides, and mixtures thereof can be used. In particular, it is preferable that the counter electrode layer 3 contains a perovskite-type oxide containing two or more elements selected from the group consisting of La, Sr, Sm, Mn, Co, and Fe. These electrolyte layer 1a, electrode layer 2, and counter electrode layer 3 are formed as thin films, as will be described later, and the inventors refer to this as being formed in a thin layer.

[0064] As previously shown, the electrolytic cell unit U is of the metal support type, and is equipped with a metal support 4 as a support for the electrode layer 2. A supply channel forming member 5 is provided on the opposite side of the electrode layer 2 from the metal support 4, forming a U-shaped electrode layer side gas supply channel 5a. Furthermore, numerous holes 4a are provided in this metal support 4, penetrating both the front and back surfaces. The gas (H2O and CO2) supplied via the electrode layer side gas supply channel 5a is subjected to electrolysis and supplied to the electrode layer 2 through the numerous holes 4a. Furthermore, the generated gas (H2, CO) flows out from these holes 4a.

[0065] On the other hand, the counter electrode layer 3 side is also provided with a supply channel forming member 6 for forming a counter electrode layer side gas supply channel 6a. As shown in the figure, this supply channel forming member 6 has many grooves on the counter electrode layer 3 side and is configured to supply a transport gas g2 (for example, air) to this counter electrode layer side gas supply channel 6a.

[0066] The metal support 4 then supports the electrode layer 2, the electrolyte layer 1a, and the counter electrode layer 3, acting as a support to maintain the strength of the electrolytic cell 1 and the entire electrolytic cell unit U. In this example, a plate-shaped metal support 4 is used, but other shapes, such as box-shaped or cylindrical, are also possible. The metal support 4 only needs to have sufficient strength to form the electrolytic cell unit U as a support. For example, a support with a thickness of about 0.1 mm to 2 mm, preferably about 0.1 mm to 1 mm, and more preferably about 0.1 mm to 0.5 mm can be used. In this embodiment, the support is made of metal, but it can also be made of ceramics, for example.

[0067] The metal support 4 has, for example, a plurality of holes 4a that penetrate through the front and back surfaces of a metal plate. For example, the holes 4a can be provided in the metal support 4 by mechanical, chemical, or optical drilling. The holes 4a have the function of allowing gas to pass from the back surface to the front surface of the metal support 4. These holes 4a may be provided at an angle in the direction of gas advection (the front-back direction of the paper in Figure 2).

[0068] By using a ferritic stainless steel material (an example of an Fe-Cr alloy) as the base material for the metal support 4, the coefficient of thermal expansion can be made closer to that of materials such as YSZ (yttria-stabilized zirconia) and GDC (gadolinium-doped ceria, also called CGO) used as materials for the electrode layer 2 and electrolyte layer 1a. Therefore, the electrolytic cell unit U is less likely to be damaged even when subjected to repeated low-temperature and high-temperature cycles. Thus, it is preferable to realize an electrolytic cell unit U with excellent long-term durability.

[0069] The supply path forming members 5 and 6 of the electrolytic cell unit U can be made of the same material as the metal support 4, and their thickness can also be made approximately the same.

[0070] The metal support 4 and the supply channel forming members 5 and 6 are conductive, but are configured to be airtight, so they act as separators that separate the respective supply channels 5a and 6a.

[0071] The electrolytic cell unit U having the above configuration supplies DC power between a pair of electrode layers 2 and 3, which are provided with the electrolyte layer 1a in between, during the electrolysis operation. In this embodiment, as shown in the figure, the case where electrode layer 2 is negative and the counter electrode layer 3 is positive is shown. Depending on the configuration of the electrolytic cell unit U, the electrode layer 2 may be positive and the counter electrode layer 3 may be negative. Then, by supplying H2O and CO2, which are the gases to be electrolyzed, to the electrode layer 2, and supplying a transport gas g2 to the counter electrode layer, the reactions shown in Equations 1 and 2 can be caused within the electrolytic cell 1, and the decomposed gas can be extracted. Here, the supply of H2O may be either water or steam, or both. Accordingly, in the present invention, the electrolytic cell device is constructed comprising at least an electrolytic cell unit U, an electrolytic raw material supply unit that supplies water and / or steam and carbon dioxide to the electrolytic cell unit U, and a power supply unit that supplies power.

[0072] In Figure 2, the gases supplied (H2O, CO2) and released (H2O, H2, CO, O2, CO2) in the electrolytic reaction are shown above and below the electrolytic cell unit U. This is for ease of understanding; in reality, the electrode layer-side gas supply passage 5a and the counter electrode layer-side gas supply passage 6a are formed extending in the direction of the front and back of the paper in Figure 2. For example, the supplied gases (H2O, CO2) shown above the electrolytic cell unit U in Figure 2 can be recovered from the front side of the paper, and the released gases (H2O, H2, CO, O2, CO2) shown below the electrolytic cell 1 can be recovered from the back side of the paper (see Figure 4, described later). In addition, to facilitate the discharge of O2 generated in the electrolytic reaction, a transport gas g2, such as air, can be flowed into the electrolytic cell unit U.

[0073] When H2O and CO2 are supplied to the electrolytic reaction unit 10 for electrolysis, H2O has a lower electrolytic voltage than CO2 and is more easily electrolyzed. Therefore, if the same amount of H2O and CO2 are supplied to the electrolytic reaction unit 10 and the electrolytic reaction is carried out, the H2 concentration at the outlet of the electrolytic reaction unit 10 tends to be higher than the CO concentration, and unreacted CO2 tends to remain.

[0074] [First catalytic reaction section (reverse water-gas shift reaction section)] As previously shown, the first catalytic reaction unit 20 (reverse water-gas shift reaction unit) causes a reverse water-gas shift reaction to convert CO2 to CO using the supplied H2, and converts H2 to H2O. That is, in the electrolytic reaction unit 10, which supplies H2O and CO2 and performs electrolysis, any CO2 that remains without being decomposed is converted to CO.

[0075] The reaction here is as shown in Equation 3, and this reaction is an endothermic reaction, an equilibrium reaction depending on the reaction temperature conditions. As a result, it is preferable to have a catalyst that can cause the reaction shown in Equation 3 to occur at the highest possible temperature (for example, 600°C to 800°C).

[0076] In describing catalysts in this specification, a component that has catalytic activity may be referred to as a "catalytically active component," and a support that carries the catalytically active component may be referred to as a "carrier." As described later, the inventors investigated various combinations of catalytic active components and supports, and found that a particular combination was suitable. The production of this type of catalyst involves an impregnation and loading process in which a support (metal oxide) is immersed in a solution containing catalytically active components (metal components), removed, and then dried and heat-treated. This process easily yields a support-supported catalyst (impregnated support) in which the catalytically active components are distributed on the surface of the support. This heat treatment is a calcination process. The preparation and use of the catalyst will be explained with reference to Figures 11 and 12.

[0077] The preparation method described here is the same for various combinations of catalytic active components and supports, except for the starting materials. Figure 11 shows examples of the reverse water-gas shift catalyst cat1 and the hydrocarbon synthesis catalyst cat2 according to the present invention. In the figure, the catalytic active component of the reverse water-gas shift catalyst cat1 is denoted as ca1 and its support as cb1. On the other hand, for the hydrocarbon synthesis catalyst cat2, its catalytic active component is denoted as ca2 and its support as cb2.

[0078] As shown in Figure 11, in catalyst preparation, an aqueous solution of a compound containing metal components (metal catalysts) that become catalytically active components Ca1 and Ca2 is obtained. Supporters cb1 and cb2 are added to this aqueous solution, and an impregnation and supporting step (a) is performed by stirring and impregnation. Then, a drying, grinding, and molding step (b) is performed, which involves evaporation to dryness, drying, and then grinding and molding. Finally, the resulting molded body is fired in air in a firing step (c) to obtain the target products (cat1 and cat2). Therefore, this form of catalyst is also called an impregnation-supported catalyst.

[0079] In this case, as shown in Figure 12, an example of the reverse water-gas shift catalyst cat1, the catalyst can also be applied to the area where it will be used and then fired. Figure 12(a) shows the application and firing process in which the reverse water-gas shift catalyst cat1 is applied to a metal support 4 with holes 4a to form a coating layer 20a, and then fired. Figure 12(b) shows the reduction pretreatment process in which H2 is flowed through the reverse water-gas shift catalyst cat1 before use.

[0080] Furthermore, when calcination is performed in air, some or all of the supported catalytic active components Ca1 and Ca2 become oxidized. Before using the catalyst, a so-called reduction pretreatment can be performed to reduce the oxidized catalytic active components and sufficiently increase their activity. Figure 12(b) shows the state in which a reduction pretreatment is being performed by flowing a reducing gas (typically H2) over the surface of the catalyst.

[0081] (Catalyst to be used) As the reverse water-gas shift catalyst cat1 used in the first catalytic reaction section 20, the inventors have selected a catalyst that satisfies the following requirements.

[0082] A catalyst comprising a carrier cb1 mainly composed of a ceria-based metal oxide or a zirconia-based metal oxide, on which at least one or both nickel and iron are supported as catalytically active components ca1. Here, the proportion of carrier cb1 to the entire catalyst is preferably 55% by weight or more, more preferably 60% by weight or more, and even more preferably 65% ​​by weight or more, in order to increase the strength of catalyst cat1. Furthermore, the upper limit of this proportion can be, for example, 99.5% by weight, but if it is higher than this, it may become difficult to adequately support the catalytically active component ca1, making it difficult to obtain the effect as a reverse water-gas shift catalyst.

[0083] Furthermore, the ceria-based metal oxide can also be ceria doped with at least one of gadolinium, samarium, or yttrium.

[0084] Furthermore, zirconia-based metal oxides can also be zirconia stabilized with at least one of yttria or scandia. Furthermore, since the reverse water-gas shift reaction can proceed smoothly, it is preferable that the amount of catalytically active component Ca1 supported is 0.5% by weight or more, more preferably 1% by weight or more, and even more preferably 5% by weight or more. However, if the amount of catalytically active component Ca1 supported is increased too much, it becomes difficult to support the catalytically active component Ca1 in a highly dispersed manner, making it difficult to obtain a significant improvement in catalytic activity and increasing the catalyst cost. Therefore, it is preferable that the amount of catalytically active component Ca1 supported is 35% by weight or less, more preferably 30% by weight or less, and even more preferably 25% by weight or less.

[0085] Furthermore, it is preferable to add either nickel and iron, or both, to the catalytic active component ca1, and to support copper as an additional catalytic active component ca1. In this configuration, the amount of copper supported is equal to or less than the amount of either nickel and iron, or both, supported as the main catalytic active component ca1.

[0086] The following describes the test results of examples and comparative examples in which the catalytically active component Ca1 and the support CB1 of the reverse water-gas shift catalyst Cat1 used in the first catalytic reaction section 20 were changed in various ways. Ni and Fe were investigated as catalytically active components (ca1) and compared with Pt (platinum). For the support material cb1, we investigated ZrO2 (zirconia), YSZ (yttria-stabilized zirconia), GDC (gadolinium-doped ceria), CeO2 (ceria), and Al2O3 (alumina).

[0087] The following description introduces Test Example 1 and Test Example 2. The difference between the two tests lies in the calcination of the reverse water-gas shift catalyst cat1. In Test Example 1, the calcination temperature is set to 450°C, while in Test Example 2, the calcination temperature is set to a higher temperature of 600°C to 1000°C.

[0088] (Test Example 1) The test results of examples (1-12) and comparative examples (1-7) in which the support material used as the catalyst in the first catalytic reaction section 20 was changed in various ways will be described. Ni and Fe were investigated as catalytically active components and compared with Pt (platinum). As support materials, ZrO2 (zirconia), YSZ (yttria-stabilized zirconia), GDC (gadolinium-doped ceria), and CeO2 (ceria) were used as examples, while Al2O3 (alumina) was used as a comparative example.

[0089] (Catalyst preparation) In preparing the reverse water-gas shift catalyst cat1, an aqueous solution is obtained by quantifying and dissolving one or both of the following, according to the desired catalyst composition: a water-soluble nickel compound (nickel nitrate, nickel chloride, nickel sulfate, nickel ammonium sulfate, nickel acetate, nickel oxalate, nickel citrate, etc.) and a water-soluble iron compound (ferrous nitrate, iron chloride, iron sulfate, ferric sulfate, iron ammonium sulfate, iron acetate, iron oxalate, iron citrate, etc.). Furthermore, when supporting copper as a further catalytically active component ca1, an aqueous solution is obtained by similarly quantifying and dissolving a water-soluble copper compound (copper nitrate, copper chloride, copper sulfate, copper ammonium sulfate, copper acetate, copper oxalate, copper citrate, etc.). A predetermined amount of carrier powder (ceria, zirconia, GDC, YSZ, Al2O3) is added to the aqueous solution, stirred, impregnated, evaporated to dryness, dried, then crushed, molded, and calcined in air. This impregnation is the "impregnation and supporting process" referred to in this invention, and the resulting product is the "impregnated supported material". The catalysts in the following examples and comparative examples were prepared using nickel nitrate hexahydrate, iron nitrate nonahydrate, and copper nitrate trihydrate, respectively. Furthermore, the catalyst using Pt in the comparative examples below was prepared using tetraammineplatinum hydroxide.

[0090] While the temperatures used for evaporation to dryness, drying, and calcination in the catalyst preparation described above can generally be within the commonly used temperature range, in Test Example 1, the catalysts in the following examples and comparative examples were prepared at 80°C, 80°C, and 450°C, respectively.

[0091] Table 1 shows the reverse water-gas shift catalyst cat1 for each of the prepared examples (1-12) and comparative examples (1-7). The horizontal axis represents the type of carrier cb1, the amount of metal supported as a catalytic active component (weight %), the amount of CO adsorbed (ml / g), and the BET surface area (m²). 2 It is written as / g). The amount of CO adsorbed was measured after the catalyst was subjected to a reduction pretreatment at 350°C under a hydrogen atmosphere for 1 hour.

[0092] [Table 1]

[0093] (Catalyst activity test) The catalyst activity test was conducted using a mixed gas of 50% H2-50% CO2 (a mixed gas containing H2 and CO2 in a 1:1 volume ratio) as the reaction gas, under conditions of a GHSV (Gas Hourly Space Velocity) of 10,000 / h, while varying the reaction temperature from 600°C to 800°C in 50°C increments. Prior to conducting the catalyst activity test, the catalyst was pre-treated for reduction at 600°C while hydrogen gas was passed through the catalyst layer. As a result of the test, the CO2 conversion rate (%), along with the CO concentration (%) and CH4 concentration (%) at the outlet of the reaction section, are shown in Table 2.

[0094] The CO2 conversion rate (%) was calculated based on the gas analysis results at the catalyst layer outlet according to the following formula. [CH4 concentration] + [CO concentration] / ([CH4 concentration] + [CO concentration] + [CO2 concentration])

[0095] As previously mentioned, it is desirable that the reverse water-gas shift catalyst cat1 used in the first catalytic reaction section 20 (reverse water-gas shift reaction section) has a high CO2 conversion rate (%) at high temperatures (for example, around 600-800°C).

[0096] [Table 2]

[0097] (Test Example 2) The following describes the test results for the examples (13-20) and comparative examples (8,9) of Test Example 2. In this example as well, Ni and Fe were investigated as catalytically active components, and the addition of Cu was also considered. CeO2 (ceria) and ZrO2 (zirconia) are used as examples of support materials, while Al2O3 (alumina) is used as a comparative example.

[0098] (Catalyst preparation) The reverse water-gas shift catalyst cat1 used in Test Example 2 was prepared in the same manner as in Test Example 1, except that the calcination temperature was changed to 600°C, 800°C, and 1000°C.

[0099] Table 3 shows the catalysts prepared in the examples (13-20) and comparative examples (8,9).

[0100] [Table 3]

[0101] (Catalyst activity test) The catalyst activity test was conducted using a mixed gas containing H2 and CO2 in a 1:1 volume ratio as the reaction gas, under conditions of GHSV at 10,000 / h, while varying the reaction temperature from 600°C to 800°C in 50°C increments. Prior to conducting the catalyst activity test, the catalyst was pre-treated for reduction at 600°C while hydrogen gas was passed through the catalyst layer. As a result of the test, the CO2 conversion rate (%), along with the CO concentration (%) and CH4 concentration (%) at the outlet of the reaction section, are listed in Table 4.

[0102] [Table 4]

[0103] For reference, Table 4 shows the equilibrium value (calculated value) of the CO2 conversion rate under these experimental conditions.

[0104] Iron-zirconia catalyst and iron-alumina catalyst The test results for the iron-zirconia catalyst at calcination temperatures of 450°C, 600°C, 800°C, and 1000°C are shown in Examples 7, 15, 18, and 20, respectively. On the other hand, the test results for the iron-alumina catalyst at calcination temperatures of 450°C, 600°C, and 800°C are shown in Comparative Examples 2, 8, and 9, respectively. As can be seen from these results, although the amount of metal supported differs slightly, the iron-zirconia catalyst exhibits superior activity in the reverse water-gas shift reaction compared to the iron-alumina catalyst. Furthermore, the iron-zirconia catalyst exhibits very high catalytic activity not only at calcination temperatures of 450°C, but also at higher calcination temperatures of 600°C, 800°C, and 1000°C, and the CO2 conversion rate reaches near the equilibrium value at all calcination temperatures.

[0105] Nickel-ceria catalyst The test results for firing temperatures of 450°C, 600°C, 800°C, and 1000°C are shown in Examples 4, 13, 16, and 19, respectively. As can be seen from these results, the nickel-ceria catalyst exhibits very high catalytic activity not only at firing temperatures of 450°C, but also at higher firing temperatures of 600°C, 800°C, and 1000°C, and the CO2 conversion rate reaches near the equilibrium value at all firing temperatures.

[0106] Nickel-alumina catalyst Comparative Example 1 shows the test results when the firing temperature was set to 450°C. These results show that the nickel-alumina catalyst had a lower CO2 conversion rate compared to the nickel-ceria catalyst mentioned earlier.

[0107] Nickel-copper-ceria catalyst The test results for firing temperatures of 450°C, 600°C, and 800°C are shown in Examples 6, 14, and 17, respectively. From these results, it can be seen that the nickel-copper-ceria catalyst shows a slight decrease in CO2 conversion rate when the firing temperature is high, such as at 600°C or 800°C, but it is superior to the iron-alumina catalyst under the same firing temperature conditions mentioned earlier. Furthermore, the nickel-copper-ceria catalyst fired at 450°C shows a CO2 conversion rate close to the equilibrium value.

[0108] Usefulness as a reverse water-gas shift catalyst As shown above, iron-zirconia and nickel-ceria catalysts exhibit very high reverse water-gas shift catalytic activity even when the calcination temperature is varied, such as from 450°C to 1000°C. Therefore, they are useful because they make it easier to ensure high performance and durability even when used in combination with solid oxide type electrolytic cells used in high-temperature ranges such as around 600°C to 800°C.

[0109] As shown above, the results indicate that, as previously stated, the reverse water-gas shift catalyst cat1 used in the first catalytic reaction section 20 can be a catalyst comprising a carrier cb1 mainly composed of a ceria-based metal oxide or a zirconia-based metal oxide, with at least one or both of nickel and iron supported as catalytically active components ca1.

[0110] Furthermore, the ceria-based metal oxide used as the support cb1 can also be ceria doped with at least one of gadolinium, samarium, or yttrium.

[0111] Furthermore, the zirconia-based metal oxide used as the support cb1 can also be zirconia stabilized with at least one of yttria or scandia.

[0112] Furthermore, it is preferable to add either nickel or iron, or both, to the catalytically active component Ca1, and to support copper as an additional catalytically active component Ca1.

[0113] Furthermore, by using the reverse water-gas shift catalyst cat1 in the first catalytic reaction section 20 (reverse water-gas shift reaction section), the reverse water-gas shift reaction can be carried out at around 600-1000°C with a CO2 conversion rate (%) equivalent to or higher than that of a highly active but very expensive Pt catalyst.

[0114] Furthermore, since the test was conducted under very high GHSV conditions of 10,000 / h, it is possible to perform the reverse water-gas shift reaction with a higher CO2 conversion rate (%) by reducing the GHSV to less than 10,000 / h, that is, by increasing the amount of catalyst used relative to the amount of gas being treated.

[0115] [Combination of electrolytic reaction section and reverse water-gas shift reaction section] In the explanation so far, we have described a configuration in which the electrolytic reaction section 10 and the reverse water-gas shift reaction section 20 are individually provided in the order listed, along the gas advection direction, according to the system configuration shown in Figure 1. The reaction in the electrolytic reaction section 10 is exothermic depending on the reaction conditions, while the reaction in the reverse water-gas shift reaction section 20 is endothermic. Therefore, the thermal efficiency of the system can be increased by integrating these two reaction sections 10 and 20. Figure 3 shows the configuration when these two reaction sections 10 and 20 are combined and integrated, with both parts enclosed to indicate that they are integrated. The reaction when they are integrated in this way is also shown in the same box. Basically, the reactions described in equations 1, 2, and 3 above will occur. When combining and integrating the electrolytic reaction section 10 and the reverse water-gas shift reaction section 20, it is preferable to enclose them together with an insulating material so that heat can be efficiently transferred between the electrolytic reaction section 10 and the reverse water-gas shift reaction section 20. Alternatively, a heat-transferring material may be used to connect the electrolytic reaction section 10 and the reverse water-gas shift reaction section 20 in order to transfer the heat generated in the electrolytic reaction section 10 to the reverse water-gas shift reaction section 20.

[0116] [Electrolytic cell unit equipped with both an electrolytic reaction section and a reverse water-gas shift reaction section] Based on the above concept, it is preferable to provide a reverse water-gas shift reaction section 20 in the electrolytic cell unit U, which is the electrolytic reaction section 10. This is because, when a solid oxide type electrolytic cell operating at around 600 to 800°C is used as the electrolytic cell 1, the reverse water-gas shift catalyst cat1 of the present invention, which obtains high activity at around 600 to 800°C, allows the electrolytic reaction section 10 and the reverse water-gas shift reaction section 20 to be used in similar temperature ranges. In this case as well, it is sufficient if the gas that has passed through the electrolytic reaction section 10 is guided to the reverse water-gas shift reaction section 20 to generate a reverse water-gas shift reaction.

[0117] Figure 4 shows an electrolytic cell unit U equipped with such a reverse water-gas shift reaction section 20. Figure 4 is a diagram showing the electrolytic cell unit U, which was shown in cross-section in Figure 2, including the direction of gas advection.

[0118] As shown in the figure, the cross-section of the electrolytic cell unit U is basically identical. In other words, this electrolytic cell unit U is also composed of an electrolytic cell 1 in which an electrode layer 2 and a counter electrode layer 3 are formed with an electrolyte layer 1a in between, a metal support 4 that functions as a support for the electrolytic cell and also acts as a separator, and supply path forming members 5 and 6, which are configured to form a gas supply path 5a on the electrode layer side and a gas supply path 6a on the counter electrode layer side. To explain in more detail, as can be seen from the figure, when the metal support 4 is viewed in the direction of gas advection, holes 4a are provided in the part corresponding to the electrolytic cell 1, but no holes are provided downstream of the electrode layer 2. Therefore, the metal support 4 becomes a separator that effectively separates the gas supplied to and released from the electrode layer 2 from the gas supplied to and released from the counter electrode layer 3.

[0119] However, in this example, the reverse water-gas shift catalyst cat1 described earlier is applied to the inner surface of the electrode layer-side gas supply passage 5a (the supply passage-side inner surface of the supply passage forming member 5, the surface of the metal support 4 opposite to the surface on which the electrode layer 2 is formed, and the surface of the multiple holes 4a). This coating layer 20a is shown by a thick solid line. Furthermore, the electrode layer-side gas supply passage 5a extends beyond the electrolytic reaction section 10, and the coating layer 20a is also provided on this extended side.

[0120] As a result, the electrode layer-side gas supply passage 5a of the electrolytic cell unit U is configured as a discharge passage for discharging at least H2 generated in the electrode layer 2, and the electrolytic reaction section 10 and the reverse water-gas shift reaction section 20 are integrated into the electrolytic cell unit U.

[0121] In this configuration, the metal support 4 acts as a separator that separates H2 generated in the electrode layer 2 from O2 generated in the counter electrode layer 3, and at least a portion of the H2 discharge path side of this separator is designated as the reverse water-gas shift reaction section 20. By stacking the electrolytic cell units U configured in this manner in the left-right direction as shown in Figures 2 and 4, a large number of electrolytic cell units U are stacked and electrically connected to form a so-called electrolytic cell module (not shown). Naturally, the useful gas produced can be obtained over multiple layers.

[0122] Based on the concept of combining the electrolytic reaction section 10 and the reverse water-gas shift reaction section 20 (making the electrode layer-side gas supply passage 5a of the electrolytic reaction section 10 the reverse water-gas shift reaction section 20), the inventors conducted experiments by housing granular reverse water-gas shift catalyst cat1 in the electrode layer-side gas supply passage 5a. Figure 5 shows a cross-section of the electrolytic cell unit U used in this experiment.

[0123] The following explanation will be given with reference to Figure 5. Figure 5 shows a cross-sectional view of the electrolytic cell unit U. Here, a metal-supported solid oxide electrolytic cell was used as the electrolytic cell 1. As the metal support 4, a metal substrate was fabricated by laser processing a 0.3 mm thick ferritic stainless steel metal plate to create multiple through holes (which become holes 4a). On this metal substrate, an electrode layer 2 and an intermediate layer 2a were sequentially laminated, and an electrolyte layer 1a was laminated on top of the intermediate layer 2a of the metal substrate so as to cover the intermediate layer 2a. Furthermore, a reaction prevention layer 7 and a counter electrode layer 3 were sequentially laminated on top of the electrolyte layer 1a to fabricate the electrolytic cell 1. The material used to form the electrode layer 2 was a mixture of NiO powder and GDC powder, the material used to form the intermediate layer 2a was GDC powder, the material used to form the electrolyte layer 1a was 8YSZ (8 mol% yttria-stabilized zirconia) powder, the material used to form the reaction prevention layer 7 was GDC powder, and the material used to form the counter electrode layer 3 was a mixture of GDC powder and LSCF powder. Furthermore, the thicknesses of the electrode layer 2, intermediate layer 2a, electrolyte layer 1a, reaction prevention layer 7, and counter electrode layer 3 were approximately 25 μm, 10 μm, 5 μm, 5 μm, and 20 μm, respectively. The performance and durability of the electrolytic cell 1 can be improved by providing an intermediate layer 2a between the electrode layer 2 and the electrolyte layer 1a, or by providing a reaction prevention layer 7 between the electrolyte layer 1a and the counter electrode layer 3. The intermediate layer 2a and reaction prevention layer 7 are preferably formed by low-temperature firing (for example, a wet method using firing in a low-temperature range without firing in a high-temperature range higher than 1100°C), spray coating (such as thermal spraying, aerosol deposition, aerosol gas deposition, powder jet deposition, particle jet deposition, and cold spray), PVD (such as sputtering and pulsed laser deposition), or CVD. These processes, which can be used in low-temperature ranges, allow for the acquisition of a good intermediate layer 2a and reaction-preventing layer 7 without using firing at high temperatures, for example, above 1100°C. Therefore, it is preferable because it enables the realization of an electrolytic cell 1 with excellent performance and durability without damaging the metal support 4. Furthermore, using a low-temperature firing method is even preferable because it simplifies the handling of raw materials.

[0124] Regarding the electrolytic cell unit U obtained as described above, we investigated how to improve its performance when a granular reverse water-gas shift catalyst cat1 is housed in the electrode layer-side gas supply passage 5a (which also serves as the discharge passage for the gas produced by electrolysis in the electrolytic reaction section 10).

[0125] Results when the reverse water-gas shift catalyst cat1 is not installed. An electrolytic reaction was carried out by supplying a gas containing H2O and CO2 to an electrolytic cell unit U, and the ratio of H2 to CO in the outlet gas of the electrolytic cell unit U was measured using a gas chromatograph. The results are shown in Table 5 below. These experimental results are described as Comparative Examples A1 and A2.

[0126] [Table 5]

[0127] Results when the reverse water-gas shift catalyst cat1 is housed inside As the reverse water-gas shift catalyst cat1, a granular catalyst obtained by supporting approximately 10% Ni on an 8YSZ support similar to that in Example 2 was placed inside the electrolytic cell unit U. An electrolytic reaction was carried out while supplying a gas containing H2O and CO2 to the electrolytic cell unit U, and the ratio of H2 to CO in the outlet gas of the electrolytic cell unit U was measured using a gas chromatograph. The results are shown in Table 6. These experimental results are described as Example A1.

[0128] [Table 6]

[0129] The comparative experiments described above showed that in an electrolytic cell unit U in which the electrolytic cell 1 is formed in a thin layer on a metal support 4, and a reverse water-gas shift reaction section 20 that generates CO using CO2 and the aforementioned H2 by a reverse water-gas shift reaction is provided in the electrode layer-side gas supply passage 5a, which serves as the discharge passage for the electrolyzed gas, it was possible to increase the composition ratio of CO to H2 generated by electrolysis.

[0130] In a comparison between an electrolytic cell unit U that does not house the reverse water-gas shift catalyst cat1 in the electrode layer-side gas supply passage 5a (which serves as the discharge passage for the electrolyzed gas) and an electrolytic cell unit U that does house it, the hydrogen / carbon monoxide ([H2 / CO]) ratio at the outlet decreases from approximately 10 or more to approximately 5. This is preferable because combining the reaction in the electrolytic reaction section 10 and the reaction in the reverse water-gas shift reaction section 20 ensures a favorable amount of CO for various hydrocarbon synthesis processes. In addition, adopting the methane reaction of CO rather than the methane reaction of CO2 improves the thermal efficiency of the hydrocarbon production system 100. Therefore, combining the reaction in the electrolytic reaction section 10 and the reaction in the reverse water-gas shift reaction section 20 ensures a favorable amount of CO. This is because methane reaction of 1 mole of CO2 produces 2 moles of H2O, whereas methane reaction of 1 mole of CO only produces 1 mole of H2O. Thus, the hydrocarbon production system 100 that employs the methane reaction of CO can suppress the loss of latent and sensible heat equivalent to 1 mole of H2O across the entire system. Furthermore, by appropriately adjusting the ratio of H2O to CO2 introduced into the electrolytic reaction section 10, the reaction conditions of the electrolytic reaction section 10 (electrolysis voltage, reaction temperature, etc.), and the reaction conditions of the reverse water-gas shift reaction section 20 (amount of catalyst used, GHSV, reaction temperature, etc.), the hydrogen / carbon monoxide ([H2 / CO]) ratio at the outlet of the reverse water-gas shift reaction section 20 can be adjusted to a value suitable for the subsequent second catalytic reaction section 30 (hydrocarbon synthesis reaction section) (for example, H2 / CO=3, which is the equivalent ratio for the methane reaction of CO).

[0131] [A heat exchanger is installed between the electrolytic reaction section and the reverse water-gas shift reaction section.] The above explanation mainly described an example in which the electrolytic reaction section 10 and the first catalytic reaction section (reverse water-gas shift reaction section) 20 are integrated. However, it is also possible to treat the two sections 10 and 20 as separate parts and install a heat exchanger 11 between them to allow heat exchange between the two sections. This configuration is shown in Figure 6, corresponding to Figure 1. The double lines with holes indicate heat transfer between the two sections. In this configuration, the temperature of each section 10 and 20 can be appropriately controlled.

[0132] The inventors refer to the system consisting of the electrolytic reaction unit 10 and the reverse water-gas shift reaction unit 20 described above as an "electrolytic reaction system."

[0133] [Second catalytic reaction section (hydrocarbon synthesis reaction section)] In this second catalytic reaction section 30 (hydrocarbon synthesis reaction section), at least H2 and CO are introduced, and hydrocarbons (methane and various hydrocarbons with 2 or more carbon atoms) are produced by catalytic reaction.

[0134] (Example of a hydrocarbon synthesis catalyst) As an activity test of the catalyst (hydrocarbon synthesis catalyst cat2) used in this second catalytic reaction section 30, the inventors conducted the following evaluation tests 1, 2, and 3.

[0135] As an example of hydrocarbon synthesis catalyst cat2, catalysts were prepared by changing the support and catalytically active component in various ways. For the catalytically active component ca2, Ru, Ru with Mo, V, Fe, Co, etc., and Ni were investigated. For the support cb2, ZrO2, Al2O3, SiO2, MgO, and TiO2 were investigated.

[0136] (Catalyst preparation) The hydrocarbon synthesis catalyst CAT2 was also prepared using the method described in Figures 11 and 12. Specifically, according to the desired catalyst composition, a water-soluble ruthenium compound (such as ruthenium nitrate, ruthenium chloride, ruthenium sulfate, ammonium ruthenium sulfate, ruthenium acetate, ruthenium oxalate, or ruthenium citrate) is quantified and dissolved to obtain an aqueous solution. Furthermore, when supporting molybdenum, vanadium, iron, or cobalt as additional catalytic active components, water-soluble metal compounds of these compounds are similarly quantified and dissolved to obtain an aqueous solution. Using this aqueous solution, the catalytic active components are impregnated onto a predetermined amount of carrier grains (such as ZrO2, Al2O3, SiO2, MgO, or TiO2), and necessary processing steps such as drying, calcination, or reduction are performed to obtain the hydrocarbon synthesis catalyst cat2. The catalysts used in the following examples were aqueous solutions of ruthenium chloride, ammonium molybdate, vanadyl oxalate, iron nitrate, and cobalt nitrate, respectively. When both ruthenium and other catalytic active components were supported, the catalysts were prepared using a sequential loading method (a two-step loading method in which the catalytic active components other than ruthenium are first supported on the support, and then ruthenium is loaded).

[0137] (Evaluation Test 1) In Evaluation Test 1, a mixed gas containing 12.4% CO, 24.8% CO2, 37.2% H2, and 12.4% H2O, with the remainder being N2, was used as the reaction gas. The GHSV was set to 4000 / h (WET basis), and the activity test of the hydrocarbon synthesis catalyst cat2 was conducted at a reaction temperature between 275°C and 360°C. In this case, the reaction gas is an example of a model in which a co-electrolytic reaction of water and carbon dioxide is carried out in the electrolytic reaction section 10 under conditions where the electrolytic reaction rate of carbon dioxide is low, and a reverse water-gas shift reaction of carbon dioxide is carried out in the reverse water-gas shift reaction section 20 installed afterwards. The mixed gas of CO, CO2, H2, and H2O is then introduced into the hydrocarbon synthesis reaction section 30 to carry out the hydrocarbon synthesis reaction.

[0138] In compiling the test results, the following two indicators were adopted.

[0139] 1. Expected hydrocarbon conversion rate for CO2 removal = [Number of carbon atoms in hydrocarbons in the outlet gas] / [Number of carbon atoms in the outlet gas - Number of carbon atoms in the outlet CO2] This index indicates the conversion rate to hydrocarbons when CO2 is removed from the outlet gas of the hydrocarbon synthesis reaction section 30 obtained by the catalytic reaction, and a high index is preferable.

[0140] 2.C1-C4 calorific value (MJ / Nm 3 ) = Σ(Nn × HN) / Σ Nn Nn [mol]: Number of moles of Cn hydrocarbons in the catalytic reaction gas (n=1~4) HN [MJ / m 3 (N): Heat output of Cn hydrocarbons in the catalytic reaction gas [H1 = 39.8, H2 = 69.7, H3 = 99.1, H4 = 128.5] This index indicates the amount of C1-C4 components contained in the outlet gas of the hydrocarbon synthesis reaction section 30 obtained by the catalytic reaction. If this value exceeds 39.8, it can be confirmed that hydrocarbons other than methane, such as ethane, propane, and butane, are also being produced.

[0141] Regarding Evaluation Test 1, Tables 7 and 8 below show Examples B1 to B3 of the hydrocarbon synthesis catalyst cat2 in the present invention.

[0142] [Table 7]

[0143] [Table 8]

[0144] As shown in Tables 7 and 8, it was confirmed that hydrocarbons can be synthesized from a mixed gas of CO, CO2, H2, and H2O using a hydrocarbon synthesis catalyst cat2, which includes a catalyst in which ruthenium is supported on an alumina support, or a catalyst in which molybdenum or vanadium is supported in addition to ruthenium. Based on these results, the C1-C4 calorific value produced by the above hydrocarbon production system 100 is 39 MJ / Nm³. 3 It was confirmed that the above high-calorie gases can be produced.

[0145] (Evaluation Test 2) In Evaluation Test 2, a mixed gas containing 0.45% CO, 18.0% CO2, 71.55% H2, and 10.0% H2O was used as the reaction gas, and the activity test of the hydrocarbon synthesis catalyst cat2 was performed at a reaction temperature between approximately 230°C and 330°C with a GHSV of 5000 / h (DRY basis). In this case, the reaction gas is an example of a model in which a mixed gas obtained when a co-electrolytic reaction of water and carbon dioxide is carried out in the electrolytic reaction section 10 under conditions where the electrolytic reaction rate of carbon dioxide is low is introduced into the hydrocarbon synthesis reaction section 30 to carry out the hydrocarbon synthesis reaction.

[0146] In compiling the test results, the following two indicators were adopted.

[0147] 1. Hydrocarbon conversion rate = [Number of carbon atoms in hydrocarbons in the outlet gas] / [Number of carbon atoms in the outlet gas] This index indicates the proportion of carbon atoms in the total incoming carbon that are converted to hydrocarbons without being converted to CO2, and a high index is preferable.

[0148] 2. Expected hydrocarbon conversion rate for CO2 removal = [Number of carbon atoms in hydrocarbons in the outlet gas] / [Number of carbon atoms in the outlet gas - Number of carbon atoms in the outlet CO2] This index indicates the conversion rate to hydrocarbons when CO2 is removed from the outlet gas of the hydrocarbon synthesis reaction section obtained by the catalytic reaction, and a high index is preferable.

[0149] For Evaluation Test 2, the catalysts used (Examples B4 to B16) are shown in Table 9, and the test results are shown in Table 10.

[0150] [Table 9]

[0151] [Table 10]

[0152] (Evaluation Test 3) In Evaluation Test 3, a mixed gas containing H2 and CO in a 3:1 (volume ratio) (H2 / CO=3) was used as the reaction gas, the GHSV was set to 2000 / h, and the activity test of hydrocarbon synthesis catalyst cat2 was conducted at a reaction temperature between 235°C and approximately 330°C. In this activity test, catalysts (Examples B17, B18) were used, in which iron or cobalt was supported in addition to ruthenium on a titania support. In this case, the reaction gas is an example of a model in which a mixed gas obtained by adding carbon monoxide to hydrogen obtained by electrolysis of water in the electrolytic reaction section 10, or a mixed gas of hydrogen and carbon monoxide obtained by separating water or carbon dioxide as needed from a gas obtained by a co-electrolytic reaction of water and carbon dioxide, is introduced into the hydrocarbon synthesis reaction section 30 to carry out the hydrocarbon synthesis reaction.

[0153] The results of evaluation test 3 are shown in Table 11.

[0154] [Table 11]

[0155] As shown in Table 11, it was confirmed that hydrocarbons can be synthesized from a mixed gas containing H2 and CO using a catalyst, cat2, which consists of ruthenium and iron or cobalt supported on a titania support.

[0156] The above hydrocarbon production system 100 produces a C1-C4 calorific value of 39 MJ / Nm³. 3 It was confirmed that the above high-calorie gases can be produced.

[0157] From the results above, as previously shown, a catalyst in which at least ruthenium is supported on a metal oxide support cb2 as the catalytically active component ca2 can be used in this second catalytic reaction section 30 (hydrocarbon synthesis reaction section). Furthermore, it is preferable to support at least one of molybdenum, vanadium, iron, and cobalt as the catalytically active component ca2.

[0158] Furthermore, it was found that the hydrocarbon synthesis catalyst cat2 is preferably a catalyst in which at least ruthenium is supported on a metal oxide support cb2, and the amount of ruthenium supported is preferably 0.1% by weight or more and 5% by weight or less. It was also found that, in addition to ruthenium, at least one of molybdenum, vanadium, iron, and cobalt is further supported on the metal oxide support cb2 as a catalytically active component ca2.

[0159] Here, the amount of at least one of the molybdenum, vanadium, iron, and cobalt supported can be 0.2% by weight or more and 6% by weight or less.

[0160] Furthermore, among these hydrocarbon synthesis catalysts (CAT2), the carbon monoxide adsorption capacity of the highly active catalysts was 0.4 ml / g or higher.

[0161] [Heavy hydrocarbon separation section] The gas reaching the heavy hydrocarbon separation section 35 is cooled, causing the heavy hydrocarbons contained in the gas released from the hydrocarbon synthesis reaction section 30 to condense, allowing the heavy hydrocarbons to be extracted to the outside. For example, in the hydrocarbon synthesis reaction section 30 using the 2wt.%Ru / 2wt.%Fe / TiO2 catalyst shown in Example B17 above, when a mixed gas containing H2 and CO in a 3:1 (volume ratio) (H2 / CO=3) was introduced and the reaction was carried out at 275°C, a straight-chain higher aliphatic hydrocarbon with an average chain length of 26 carbon atoms was extracted in the heavy hydrocarbon separation section 35. When the reaction was carried out at 325°C, a straight-chain higher aliphatic hydrocarbon with an average chain length of 18 carbon atoms was extracted in the heavy hydrocarbon separation section 35.

[0162] [Water separation section] This water separation unit 40 is equipped with a condenser, which adjusts the incoming H2O-containing gas to a predetermined temperature and pressure, condenses it, and extracts the water to the outside.

[0163] [Carbon dioxide separation section] For example, PSA is placed in this section 50, and CO2 is separated from the incoming CO2-containing gas by adsorption onto the adsorbent under predetermined temperature and pressure, and the separated CO2 is desorbed from the adsorbent, thereby effectively separating the CO2. The separated CO2 can be returned to the front of the electrolytic reaction section 10 via the carbon dioxide return path 51 and reused. Furthermore, by using PSA or similar methods, the carbon dioxide separation unit and the water separation unit can be made into a single separation unit.

[0164] [Another embodiment] (1) In the above embodiment, the CO2 separated in the carbon dioxide separation unit 50 was returned to the front of the electrolytic reaction unit 10. However, in the hydrocarbon production system 100 according to the present invention, the conversion of CO2 to CO is mainly carried out in the reverse water-gas shift reaction unit 20, so the return destination of the CO2 may be set to the front of the reverse water-gas shift reaction unit 20. This configuration is shown in Figure 7.

[0165] (2) In the above embodiment, H2 in the gas obtained from the hydrocarbon synthesis reaction section 30 was not specifically mentioned, but a hydrogen separation section (indicated as H2 separation section in the figure) 60 is provided to separate H2 using a hydrogen separation membrane or the like, and H2 may be separated and used separately. This configuration is shown in Figure 8. In this example, the return destination of the H2 separated in the hydrogen separation section 60 may be set before the reverse water-gas shift reaction section 20 and used for the reverse water-gas shift reaction.

[0166] (3) In the above embodiment, the water separation unit 40 is provided downstream of the hydrocarbon synthesis reaction unit 30, but as shown in Figure 9, the water separation unit 40 may be provided between the reverse water-gas shift reaction unit 20 and the hydrocarbon synthesis reaction unit 30. The main function of this water separation unit 40 is to facilitate the hydrocarbon synthesis reaction.

[0167] (4) In the above embodiment, an example was shown in which both H2O and CO2 are supplied to the electrolytic reaction unit 10 for the electrolysis reaction. However, as shown in Figure 10, a system in which only H2O is supplied to the electrolytic reaction unit 10 for the electrolysis reaction may also be used. In this case, the carbon consumed in hydrocarbon synthesis is introduced into the reverse water-gas shift reaction unit 20 as carbon dioxide.

[0168] (5) In the above embodiment, an example was shown in which a solid oxide type electrolytic cell was used as the electrolytic cell 1 in the electrolytic reaction section 10, but an alkaline type electrolytic cell or a polymer film type electrolytic cell may also be used as the electrolytic cell 1.

[0169] (6) In the above embodiment, the electrolytic reaction section 10 and the first catalytic reaction section 20 are shown as an integrated configuration. However, it is also possible to integrate a second catalytic reaction section 30 in addition to these reaction sections 10 and 20. An example of this configuration is shown in Figure 13. Incidentally, in this figure, 30a represents the coating layer of the hydrocarbon synthesis catalyst cat2. In this configuration as well, each of these reaction units 10, 20, and 30 can be configured on a metal support 4 and a supply channel forming member 5, and this metal support a acts as a separator that separates the generated hydrocarbons from oxygen.

[0170] (7) In the above embodiment, an example was shown in which hydrocarbons such as methane are synthesized in the hydrocarbon synthesis reaction unit 30. However, depending on how the hydrocarbon synthesis catalyst used in the hydrocarbon synthesis reaction unit 30 is selected, it is also possible to synthesize chemical raw materials from hydrogen and carbon monoxide introduced into the hydrocarbon synthesis reaction unit 30. [Explanation of Symbols]

[0171] 1 electrolytic cell 1a Electrolyte layer 2 electrode layer 3 Counter electrode layer 4. Metal support (support / separator) 4a hole (through hole) 5. Supply channel forming member (separator) 6. Supply channel forming member (separator) 10 Electrolytic reaction section 20 First catalytic reaction section (reverse water-gas shift reaction section) 20a Coating layer 30 Second catalytic reaction section (hydrocarbon synthesis reaction section) 40 Water separation section 50 Carbon dioxide separation section 60 Hydrogen separation section U Electrolytic Cell Unit Cat1 Reverse Water Gas Shift Catalyst Ca1 catalytic active component (metal component) cb1 carrier (metal oxide) Cat2 Hydrocarbon Synthesis Catalyst ca2 Catalytic active component (metal component) cb2 support (metal oxide)

Claims

1. A reverse water-gas shift catalyst is an impregnated support in which iron is supported as a catalytic active component on a carrier mainly composed of zirconia.

2. The reverse water-gas shift catalyst according to claim 1, wherein the proportion of the support to the entire catalyst is 55% by weight or more.

3. The reverse water-gas shift catalyst according to claim 1 or 2, wherein the amount of the catalytically active component supported is 0.5% by weight or more.

4. In addition to the catalytically active component, the reverse water-gas shift catalyst according to any one of claims 1 to 3, wherein copper is supported.

5. The reverse water-gas shift catalyst according to claim 4, wherein the amount of copper supported is equal to or less than the amount of the catalytically active component supported.

6. An electrolytic reaction system comprising at least a reverse water-gas shift reaction section containing at least the reverse water-gas shift catalyst described in any one of claims 1 to 5, and an electrolytic reaction section.

7. The electrolytic reaction system according to claim 6, wherein the electrolytic reaction section has an electrolytic cell unit in which at least an electrode layer, an electrolyte layer, and a counter electrode layer are formed on a support.

8. The electrolytic reaction system according to claim 7, wherein the support is made of metal.

9. A method for converting carbon dioxide, comprising contacting carbon dioxide with a reverse water-gas shift catalyst according to any one of claims 1 to 5 at a temperature of 600°C to 800°C to carry out a reverse water-gas shift reaction.

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