Method for producing reverse water gas shift catalyst

Abstract

A reverse water-gas shift catalyst (cat1) that can be used at high temperatures is obtained, along with a manufacturing method therefor. In this reverse water-gas shift catalyst (cat1), a support (cb1) having a ceria-based metal oxide or a zirconia-based metal oxide as the primary component thereof at least supports one or both of nickel and iron as a catalytic active component (ca1), and the proportion of the support (cb1) to the whole catalyst is 55 wt% or greater.

Description

[Technical Field]

[0001] The present invention relates to a reverse water gas shift catalyst and a method for producing the same, as well as to an electrolytic reaction system and a hydrocarbon production system that include a catalytic reaction section that uses this type of reverse water gas shift catalyst. [Background technology]

[0002] In the production of hydrocarbons, a technique is known in which carbon monoxide and hydrogen are used as raw material gases to obtain methane gas from these gases. Patent Document 1 discloses a system for producing carbon monoxide, which is one of the above-mentioned raw material gases.

[0003] The system disclosed in Patent Document 1 includes an electrolysis device (corresponding to the electrolysis reaction unit of the present invention) that performs electrolysis. Carbon dioxide and water are supplied to the cathode of this device, and hydrogen and carbon monoxide are produced by electrolysis. This reaction leaves unreacted water and carbon dioxide. Therefore, the hydrogen, carbon monoxide, unreacted water, and carbon dioxide output from the electrolysis device are sent to a reverse water-gas shift reactor (corresponding to the reverse water-gas shift reaction unit of the present invention), where the carbon dioxide and hydrogen are reacted to produce carbon monoxide and water.

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

[0029] of the specification). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2019-35102 Summary of the Invention [Problem to be solved by the invention]

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

[0007] CO+H2O→CO2+H2ΔH=-41 kJ / mol

[0008] The water-gas shift reaction (WGR) is a reaction widely used in processes such as hydrogen production, in which carbon monoxide reacts with water to convert it into carbon dioxide and hydrogen. However, because the reaction is exothermic, the equilibrium shifts toward carbon dioxide production as the reaction temperature decreases. Therefore, efforts have been made to develop catalysts with high low-temperature activity. Generally, iron-chromium catalysts, which are used at around 300 to 450°C, and copper-zinc catalysts, which are used at around 200°C, are known.

[0009] On the other hand, there have not been many applications requiring the reverse reaction of the water-gas shift reaction (reverse water-gas shift reaction), and therefore there has not been much development of catalysts suitable for the reverse water-gas shift reaction.

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

[0011] An object of the present invention is to provide a reverse water-gas shift catalyst that can be used at high temperatures, a method for producing the catalyst, and a hydrocarbon production system that can produce hydrocarbons using the reverse water-gas shift reaction together with the electrolysis reaction. [Means for solving the problem]

[0012] The first characteristic configuration of the present invention is as follows: The reverse water gas shift catalyst is a catalyst in which at least one of nickel and / or iron is supported as a catalytically active component on a support mainly composed of a ceria-based metal oxide or a zirconia-based metal oxide. The catalytically active component is a metal catalyst, and is supported on the support as a metal or its metal oxide.

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

[0014] When either or both of nickel and iron are used as catalytically active components, the cost per unit weight can be reduced to 1 / 1000 or less compared to platinum, and this is preferable because it allows for a reduction in cost or, for the same cost, a significantly increased amount of catalyst to be used.

[0015] Furthermore, by using a ceria-based metal oxide or a zirconia-based metal oxide as the support, it is possible to ensure resistance in high temperature ranges.

[0016] In the present invention, a reverse water-gas shift reactor is provided downstream of the electrolytic reactor (the side where the gas generated in the electrolytic reactor flows). 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 reactor, and therefore reactions can be effectively generated in both locations in approximately the same high-temperature range.

[0017] In order to obtain this reverse water gas shift catalyst, as described in the twelfth characteristic configuration of the present invention, A reverse water gas shift catalyst can be produced by at least carrying out an impregnation / support step in which a support containing a ceria-based metal oxide or a zirconia-based metal oxide as a main component is added to a solution containing nickel and / or iron, and at least one of nickel and / or iron is impregnated and supported on the support.

[0018] In order to obtain this reverse water gas shift catalyst, as described in the thirteenth characteristic configuration of the present invention, It is preferable to have at least a calcination step in which at least one or both of nickel and iron are supported as catalytically active components on a support mainly composed of a ceria-based metal oxide or a zirconia-based metal oxide, and the support is calcined at a temperature of 450°C or higher.

[0019] When using a reverse water gas shift catalyst, the reaction temperature must be relatively high to cause the desired reverse water gas shift reaction. A catalyst obtained by calcining within this temperature range can be used stably. A temperature of 450°C or higher is preferable, but temperatures of 600°C or higher or 800°C or higher are even more preferable because they enhance stability at high temperatures. For example, the catalyst can be used stably even when combined with a solid oxide electrolysis cell used at relatively high temperatures (e.g., 600°C to 800°C). Furthermore, since the cost of the calcination process increases if the calcination temperature is too high, the upper limit is approximately 1200°C.

[0020] When using this reverse water gas shift catalyst, as described in the fifteenth characteristic feature of the present invention, it is preferable to subject the reverse water gas shift catalyst to a reduction pretreatment before use in the reaction.

[0021] In many cases, at least a portion of the catalytically active components contained in a reverse water gas shift catalyst is converted into a metal oxide during the calcination process. However, by subjecting the catalyst to a pre-reduction treatment (a reduction treatment before use), the catalytically active components in an oxidized state can be reduced, allowing the catalyst to exhibit good catalytic activity.

[0022] The second characteristic configuration of the present invention is: The ratio of the carrier to the entire catalyst is 55% by weight or more. "% by weight" is synonymous with "% by mass." The same applies hereinafter.

[0023] According to this characteristic feature, since the strength of the catalyst can be increased by increasing the weight percentage of the carrier, the weight percentage of the carrier is preferably 55 wt% or more, more preferably 60 wt% or more, and even more preferably 65 wt% or more. The upper limit of this weight percentage can be, for example, 99.5 wt%. If the weight percentage exceeds this, the catalytically active components may not be sufficiently supported, and the effect as a reverse water gas shift catalyst may be difficult to obtain.

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

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

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

[0027] As will be described later, the use of stabilized zirconia can improve catalytic activity.

[0028] A fifth characteristic configuration of the present invention is The amount of the catalytically active component carried is 0.5% by weight or more.

[0029] Since this configuration allows the reverse water gas shift reaction to proceed smoothly, the amount of the catalytically active component loaded is preferably 0.5 wt% or more, more preferably 1 wt% or more, and even more preferably 5 wt% or more. Moreover, even if the amount of the catalytically active component loaded is too large, it becomes difficult to load the catalytically active component in a highly dispersed manner, making it difficult to achieve a significant improvement in catalytic activity and increasing the catalyst cost. Therefore, the amount of the catalytically active component loaded is preferably 35 wt% or less, more preferably 30 wt% or less, and even more preferably 25 wt% or less.

[0030] A sixth characteristic configuration of the present invention is In addition to the catalytically active components, copper is supported.

[0031] According to this characteristic feature, the activity as a reverse water gas shift catalyst can be improved.

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

[0033] As explained above, the main catalytic activity of the reverse water gas shift catalyst according to the present invention is due to either or both of nickel and iron, and therefore the effect of copper loading can be obtained without interfering with the effects of these components.

[0034] The eighth characteristic configuration of the present invention is The electrolytic reaction system has 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 carbon dioxide is converted into carbon monoxide using the hydrogen produced by 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 exhibits 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, thereby producing 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. Moreover, the heat (energy) from the electrolysis reaction section can be effectively utilized in the reverse water gas shift reaction section. Therefore, as shown in the eleventh characteristic configuration of the present invention, by providing a hydrocarbon synthesis reaction section in addition to an electrolysis reaction section and a reverse water gas shift reaction section, it is possible to construct a hydrocarbon production system that synthesizes hydrocarbons using the produced hydrogen and carbon monoxide.

[0036] A ninth characteristic configuration of the present invention is The electrolysis reaction section has an electrolysis 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 unit includes, for example, a thin-film electrode layer, an electrolyte layer, and a counter electrode layer on a thin but sufficiently strong, robust support. This allows for an effective electrolytic reaction while reducing the amount of material used to form these layers of the electrolytic cell. As a result, a compact, high-performance electrolytic cell unit with excellent strength and reliability can be constructed. Metals and ceramics can be selected as materials for forming this type of support.

[0038] Furthermore, as described in a fourteenth characteristic configuration of the present invention, by employing a configuration in which an impregnated support obtained through an impregnation step in which the support is impregnated with either or both of nickel and iron and disposed on at least a part of the support to form a reverse water gas shift reactor, it is possible to form both the electrolysis reactor and the reverse water gas shift reactor on the support, thereby providing a compact, high-performance electrolysis cell unit with excellent strength and reliability, and further a hydrocarbon production system including this type of electrolysis cell unit.

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

[0040] By using metal as the 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 drawings]

[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] A diagram showing the configuration of a system that integrates an electrolysis reactor and a reverse water gas shift reactor. [Figure 4] Schematic diagram of an electrolysis cell unit equipped with an electrolysis reactor and a reverse water-gas shift reactor [Figure 5] Cross-sectional view of the electrolysis cell unit used in a comparative experiment in which the electrode layer side gas supply channel was used as a reverse water-gas shift reactor. [Figure 6] Configuration diagram of a system equipped with a heat exchanger between the electrolysis reactor and the reverse water gas shift reactor. [Figure 7] Diagram showing another configuration of a hydrocarbon production system in which CO2 is introduced into a reverse water gas shift reactor. [Figure 8] FIG. 1 is a diagram showing another configuration of a hydrocarbon production system equipped with a hydrogen separation unit. [Figure 9] FIG. 10 is a diagram showing yet another configuration of a hydrocarbon production system including a water separation section before a hydrocarbon synthesis reaction section. [Figure 10] FIG. 10 is a diagram showing yet another configuration of a hydrocarbon production system in which only water is introduced into the electrolysis reaction section. [Figure 11] An explanatory diagram showing the preparation state of the catalyst [Figure 12] An explanatory diagram showing the catalyst coating and calcination state and pre-reduction treatment [Figure 13] Schematic diagram of an electrolysis cell unit equipped with an electrolysis reaction section, a reverse water-gas shift reaction section, and a hydrocarbon synthesis reaction section. DETAILED DESCRIPTION OF THE INVENTION

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

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

[0044] The electrolytic reaction section 10 is a section that electrolyzes at least a portion of the inflowing 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 inflowing 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 inflowing gas into hydrocarbons. The hydrocarbons synthesized here are primarily CH4 (a hydrocarbon with one carbon atom), but also include lower saturated hydrocarbons with two to four carbon atoms. Furthermore, as will be described later, by appropriately selecting the catalyst used in the second catalytic reaction section 30, it is possible to synthesize heavier hydrocarbons with a carbon number greater than the lower saturated hydrocarbons, unsaturated hydrocarbons, oxygenated hydrocarbons, and the like. 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 unit 35, water separation unit 40, and carbon dioxide separation unit 50 are units that remove at least a portion of predetermined components (in the order of appearance, CnHm, HO, and CO2) from the gas flowing therethrough. The components removed and recovered by the water separation unit 40 and carbon dioxide separation unit 50 are returned to predetermined parts of the system via a water return line 41 and a carbon dioxide return line 51, as shown in Figure 1, and are reused. The HO and CO2 returned via these return lines 41, 51 are shown above the two return lines. As a result, this hydrocarbon production system 100 is established as a carbon-closed system that does not substantially release CO2 outside the system.

[0046] In the figure, the gases flowing into each part are shown before each part, and the gases released from each part are shown after each part.

[0047] In the electrolysis reaction section 10, H2O and CO2 as starting materials are introduced and electrolyzed inside, with H2O being decomposed into H2 and O2, and part of the CO2 being 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 formulas 1 and 2 are also shown in the box representing the electrolytic reaction section 10 in FIG.

[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 therein, whereby CO2 is converted into CO and H2 is converted into H2O, which are then released.

[0050] The reaction is written 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 (CO2 and H2 react to produce CO and HO). CO2+H2⇔CO+H2O (Formula 3) This formula 3 is also shown in 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 shown schematically in the box.

[0051] In the second catalytic reaction section 30 (hydrocarbon synthesis reaction section), H2 and CO are flowed in, and hydrocarbons are synthesized by a catalytic reaction. For example, the reaction in which CH4 is synthesized from CO and H2 is expressed as the following equilibrium reaction, but the reaction in which CH4 is synthesized from CO and H2 is a reaction in which the reaction expressed in the following formula 4 proceeds to the right (a reaction in which CO and H2 react to produce CH4 and HO). CO+3H2⇔CH4+H2O (Formula 4) This formula 4 is also shown in the box representing the second catalytic reaction section 30 (hydrocarbon synthesis reaction section) in Fig. 1. The hydrocarbon synthesis catalyst cat2 used in the reaction is also shown schematically in this box. Furthermore, the equilibrium reaction of (Equation 3) also occurs at this site. Furthermore, depending on the type of catalyst used in the second catalytic reaction section 30, it is possible to proceed with a Fischer-Tropsch (FT) synthesis reaction or the like, and therefore various hydrocarbons such as ethane, propane, butane, pentane, hexane, paraffin, olefinic hydrocarbons, etc. can be synthesized from CO and H2.

[0052] As will be described later, the inventors have shown an example of a catalyst that uses ruthenium as its catalytically active component as the hydrocarbon synthesis catalyst cat2 to be placed in the second catalytic reaction section 30, but heavy hydrocarbons are also synthesized with a catalyst that contains iron, cobalt, or the like as its catalytically active component, and these types of heavy hydrocarbons condense as the temperature decreases and can be separated from the carrier gas. Therefore, the above-mentioned heavy hydrocarbon separation section 35 separates the hydrocarbon components separated in this manner.

[0053] The H2O produced in the water separation section 40 is separated and returned to the upstream side of the electrolysis reaction section 10 via a water return line 41 (water recycle line).

[0054] The CO2 produced in the carbon dioxide separation section 50 is separated and returned to the upstream side of the electrolysis reaction section 10 via a carbon dioxide return path 51 (carbon dioxide recycle line).

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

[0056] The above is an overview of the hydrocarbon production system 100, and the configuration and role of each part will be explained below. [Electrolytic reaction section] As previously described, the electrolytic reaction section 10 consumes the power supplied in accordance with the above formulas 1 and 2 to decompose the inflowing H2O and CO2.

[0057] FIG. 2 shows a schematic cross-sectional configuration of this electrolytic reaction section 10. The figure shows an electrolysis cell unit U in which a plurality of units are stacked to form an electrolysis stack (not shown), and this electrolysis cell unit U includes an electrolysis cell 1, which is configured to include an electrode layer 2 on one surface of an electrolyte layer 1a and a counter electrode layer 3 on the other surface. The electrode layer 2 serves as the cathode in the electrolysis cell 1, and the counter electrode layer 3 serves as the anode. Incidentally, this electrolysis cell unit U is supported by a metal support 4. Note that, here, an example is shown in which a solid oxide electrolysis cell is used as the electrolysis cell 1.

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

[0059] This electrolyte layer 1a is preferably formed by a low-temperature firing method (e.g., a wet method using firing at a low temperature without firing at a high temperature above 1100°C), a spray coating method (thermal spraying, aerosol deposition, aerosol gas deposition, powder jet deposition, particle jet deposition, cold spray, etc.), a PVD method (sputtering, pulsed laser deposition, etc.), a CVD method, etc. These film formation processes that can be used in a low-temperature range can provide an electrolyte layer 1a that is dense and has high airtightness and gas barrier properties without firing at a high temperature above 1100°C, for example. This can suppress damage to the metal support 4 and interdiffusion of elements between the metal support 4 and the electrode layer 2, thereby realizing an electrolysis cell unit U with excellent performance and durability. In particular, low-temperature firing methods and spray coating methods are preferred because they allow for low-cost elements to be realized. Furthermore, spray coating is more preferable because it is easy to obtain a dense electrolyte layer 1a that is airtight and has high gas barrier properties in a low temperature range.

[0060] Furthermore, the electrolyte layer 1a is densely structured to prevent gas leakage and exhibit high ionic conductivity. The density of the electrolyte layer 1a is preferably 90% or more, more preferably 95% or more, and even more preferably 98% or more. When the electrolyte layer 1a is a uniform layer, the density is preferably 95% or more, and more preferably 98% or more. When the electrolyte layer 1a is structured in a multi-layer structure, it is preferable that at least a portion of the layers contains a layer (a dense electrolyte layer) with a density of 98% or more, and more preferably a layer (a dense electrolyte layer) with a density of 99% or more. If such a dense electrolyte layer is included as part of the electrolyte layer 1a, it is easier to form an electrolyte layer 1a that is dense and has high airtightness and gas barrier properties, even when the electrolyte layer 1a is structured in a multi-layer structure.

[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 the electrode layer 2 is 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 electrode layer material used, thereby cutting costs, while ensuring sufficient electrode performance. The entire area where the holes (through holes) 4a are provided is covered with the electrode layer 2. In other words, the holes 4a are formed inside the area of ​​the metal support 4 where the electrode layer 2 is formed. In other words, all of the holes 4a are provided facing the electrode layer 2.

[0062] The electrode layer 2 can be made of a composite material such as NiO-GDC, Ni-GDC, NiO-YSZ, Ni-YSZ, CuO-CeO, or Cu-CeO. In these examples, GDC, YSZ, or CeO can be referred to as the aggregate of the composite material. The electrode layer 2 is preferably formed by a low-temperature firing method (e.g., a wet method using a firing process at a low temperature without firing at a high temperature above 1100°C), a spray coating method (e.g., thermal spraying, aerosol deposition, aerosol gas deposition, powder jet deposition, particle jet deposition, or cold spray), a PVD method (e.g., sputtering or pulsed laser deposition), or a CVD method. These processes, which can be used at low temperatures, can produce a good electrode layer 2 without firing at a high temperature above 1100°C. This is preferable because it prevents damage to the metal support 4 and suppresses interdiffusion of elements between the metal support 4 and the electrode layer 2, thereby realizing an electrochemical element with excellent durability. Furthermore, the use of a low-temperature firing method is even more preferable because it facilitates handling of the raw materials.

[0063] The counter electrode layer 3 can be formed as a thin layer on the surface of the electrolyte layer 1a opposite to the electrode layer 2. When the counter electrode layer 3 is a thin layer, its thickness can be, for example, about 1 μm to 100 μm, preferably 5 μm to 50 μm. This thickness reduces the amount of expensive counter electrode layer material used, thereby reducing costs, while ensuring sufficient electrode performance. Examples of materials that can be used for the counter electrode layer 3 include composite oxides such as LSCF and LSM, ceria-based oxides, and mixtures thereof. It is particularly preferred that the counter electrode layer 3 contain a perovskite-type oxide containing two or more elements selected from the group consisting of La, Sr, Sm, Mn, Co, and Fe. The electrolyte layer 1a, the electrode layer 2, and the counter electrode layer 3 are formed as thin films as will be described later, which the inventors call "thin layer formation."

[0064] As described above, the electrolysis cell unit U is of a metal support type and includes a metal support 4 as a support for the electrode layer 2. A supply channel forming member 5 that forms a U-shaped electrode layer side gas supply channel 5a is provided on the opposite side of the metal support 4 from the electrode layer 2. Furthermore, a number of holes 4a are formed in the metal support 4, penetrating the front and back surfaces. Gases (H2O and CO2) supplied via the electrode layer side gas supply channel 5a are electrolyzed and supplied to the electrode layer 2 via the many holes 4a. Furthermore, the generated gases (H2, CO) flow out from the holes 4a.

[0065] On the other hand, a supply channel forming member 6 for forming a counter electrode layer side gas supply channel 6a is also provided on the counter electrode layer 3 side. 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 carrier gas g2 (e.g., air) to this counter electrode layer side gas supply channel 6a.

[0066] The metal support 4 supports the electrode layer 2, the electrolyte layer 1a, and the counter electrode layer 3, and serves as a support that maintains 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 as the metal support, but other shapes, such as a box shape or a cylindrical shape, are also possible. The metal support 4 may have sufficient strength to form the electrolysis cell unit U, and may have a thickness of, for example, 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. In this embodiment, the support is made of metal, but it may also be made of, for example, ceramics.

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

[0068] By using a ferritic stainless steel material (an example of an Fe-Cr alloy) as the base material of the metal support 4, the thermal expansion coefficient can be made closer to that of YSZ (yttria-stabilized zirconia) or GDC (gadolinium-doped ceria, also known as CGO), which are used as materials for the electrode layer 2 and the electrolyte layer 1a. This makes the electrolysis cell unit U less susceptible to damage even when subjected to repeated temperature cycles of low and high temperatures. This is preferable because it allows for an electrolysis cell unit U with excellent long-term durability.

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

[0070] The metal support 4 and both supply path forming members 5 and 6 are electrically conductive, but are configured to be airtight, so that they function as separators that separate the supply paths 5a and 6a.

[0071] During electrolysis, the electrolytic cell unit U having the above configuration supplies DC power between a pair of electrode layers 2 and 3 that are disposed on either side of the electrolyte layer 1a. In this embodiment, as shown in the figure, the electrode layer 2 side is negative and the counter electrode layer 3 side is positive. Depending on the configuration of the electrolytic cell unit U, the electrode layer 2 side may be positive and the counter electrode layer 3 side may be negative. Then, by supplying HO and CO, which are gases to be electrolyzed, to the electrode layer 2 and supplying a carrier gas g2 to the counter electrode layer side, the reactions shown in Equations 1 and 2 occur in the electrolytic cell 1, and the decomposed gas can be extracted. Here, the HO may be supplied as either water or water vapor, or both. Therefore, in the present invention, an electrolytic cell device is constructed including at least an electrolytic cell unit U, an electrolysis raw material supply unit that supplies water and / or water vapor and carbon dioxide to the electrolytic cell unit U, and a power supply unit that supplies electric power.

[0072] In FIG. 2, the gases supplied (HO, CO) and released (HO, H, CO, O, CO) during the electrolysis reaction are depicted above and below the electrolytic cell unit U. However, this is for ease of understanding. In reality, the electrode layer-side gas supply channel 5a and the counter electrode layer-side gas supply channel 6a are formed to extend in the front-to-back direction of the page in FIG. 2. For example, the supply-side gases (HO, CO) depicted above the electrolytic cell unit U in FIG. 2 can be recovered from the front side of the page, and the release-side gases (HO, H, CO, O, CO) depicted below the electrolytic cell 1 can be recovered from the back side of the page (see FIG. 4, described below). Note that a carrier gas g, such as air, can be flowed into the electrolytic cell unit U to smoothly discharge O produced during the electrolytic reaction.

[0073] When H2O and CO2 are supplied to the electrolysis reaction unit 10 and electrolyzed, H2O has a lower electrolysis voltage than CO2 and is therefore more easily electrolyzed. Therefore, if equal amounts of H2O and CO2 are supplied to the electrolysis reaction unit 10 to carry out an electrolysis reaction, the H2 concentration is likely to be higher than the CO concentration at the outlet of the electrolysis reaction unit 10, and unreacted CO2 is likely to remain.

[0074] [First catalytic reaction section (reverse water gas shift reaction section)] As described above, the first catalytic reaction unit 20 (reverse water-gas shift reaction unit) causes a reverse water-gas shift reaction, converts CO to CO using supplied H, and converts H to H O. That is, in the electrolysis reaction unit 10, which supplies H O and CO and performs electrolysis, CO that remains without being decomposed is converted to CO.

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

[0076] In describing catalysts in this specification, a component having catalytic activity may be referred to as a "catalytically active component," and a support that supports the catalytically active component may be referred to as a "support." As will be described later, the inventors have investigated various combinations of catalytically active components and supports, and have found that certain combinations are preferable. This type of catalyst can be easily produced by carrying out an impregnation process in which a carrier (metal oxide) is immersed in a solution containing catalytically active components (metal components), removed, dried, and heated. This process is called calcination. The preparation and use of this catalyst will be described with reference to Figures 11 and 12.

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

[0078] As shown in Figure 11, catalyst preparation involves preparing an aqueous solution of a compound containing metal components (metal catalysts) that will become catalytically active components Ca1 and Ca2. Then, carriers Cb1 and Cb2 are added to the aqueous solution, stirred, and impregnated in the impregnation-supporting process (a). The resulting mixture is then evaporated to dryness, dried, and then crushed and molded in the drying-crushing-molding process (b). The resulting molded body is then calcined in air in the calcination process (c), yielding the target product (Cat1 and Cat2). This type of catalyst is therefore also known as an impregnated-supported catalyst.

[0079] In this case, the catalyst can be applied to the area where it is to be used and then calcined, as shown in the example of reverse water gas shift catalyst cat1 in Figure 12. Figure 12(a) shows the application and calcination process in which the reverse water gas shift catalyst cat1 is applied to a metal support 4 having holes 4a to form a coating layer 20a, followed by calcination. Figure 12(b) shows the pre-reduction process in which H2 is passed through the reverse water gas shift catalyst cat1 before use.

[0080] When the calcination process is performed in air, the supported catalytic active components Ca1 and Ca2 are partially or completely 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 reduction pretreatment is performed by flowing a reducing gas (typically H2) over the surface of the catalyst.

[0081] (Catalyst used) The inventors selected a catalyst that satisfies the following requirements as the reverse water gas shift catalyst cat1 used in the first catalytic reaction section 20.

[0082] This catalyst is constructed by supporting at least one or both of nickel and iron as catalytically active component Ca1 on a support Cb1 primarily composed of a ceria-based metal oxide or a zirconia-based metal oxide. Here, the ratio of support 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 enhance the strength of the catalyst Cat1. The upper limit of this ratio can be, for example, 99.5% by weight, but if it exceeds this ratio, the catalytically active component Ca1 cannot be sufficiently supported, which may make it difficult to achieve the desired effect as a reverse water gas shift catalyst.

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

[0084] The zirconia-based metal oxide may be zirconia stabilized with at least one of yttria and scandia. In addition, since the reverse water gas shift reaction can be smoothly promoted, the amount of the catalytically active component Ca1 supported is preferably 0.5 wt% or more, more preferably 1 wt% or more, and even more preferably 5 wt% or more. Moreover, even if the amount of the catalytically active component Ca1 supported is too large, it becomes difficult to support the catalytically active component Ca1 with high dispersion, making it difficult to achieve a significant improvement in catalytic activity and increasing the catalyst cost. Therefore, the amount of the catalytically active component Ca1 supported is preferably 35 wt% or less, more preferably 30 wt% or less, and even more preferably 25 wt% or less.

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

[0086] The following describes test results of examples and comparative examples in which the catalytically active component ca1 and the carrier cb1 of the reverse water gas shift catalyst cat1 used in the first catalytic reaction section 20 are changed in various ways. As the catalytically active component ca1, Ni and Fe were examined and compared with Pt (platinum). The carriers examined were ZrO2 (zirconia), YSZ (yttria-stabilized zirconia), GDC (gadolinium-doped ceria), CeO2 (ceria), and Al2O3 (alumina).

[0087] The following explanation will introduce Test Example 1 and Test Example 2. The difference between the two tests is that in the calcination of the reverse water gas shift catalyst cat1, the calcination temperature in Test Example 1 was set to 450°C, while the calcination temperature in Test Example 2 was set to a higher temperature of 600°C to 1000°C.

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

[0089] (Catalyst preparation) To prepare the reverse water gas shift catalyst cat1, an aqueous solution is prepared by dissolving either or both of 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 (iron nitrate, iron chloride, iron sulfate, iron ammonium sulfate, iron acetate, iron oxalate, iron citrate, etc.) in a measured amount, depending on the desired catalyst composition. To support copper as an additional catalytically active component ca1, a water-soluble copper compound (copper nitrate, copper chloride, copper sulfate, copper ammonium sulfate, copper acetate, copper oxalate, copper citrate, etc.) is similarly dissolved in a measured amount. A predetermined amount of support powder (ceria, zirconia, GDC, YSZ, Al2O3) is added to the aqueous solution, stirred, impregnated, evaporated to dryness, dried, crushed, molded, and calcined in air. This impregnation process is referred to as the "impregnation process" in this invention, and the resulting product is the "impregnated support." The catalysts in the following examples and comparative examples were prepared using nickel nitrate hexahydrate, iron nitrate nonahydrate, and copper nitrate trihydrate, respectively. The catalyst using Pt in the following comparative examples was prepared using tetraammineplatinum hydroxide.

[0090] The temperatures for evaporation to dryness, drying, and calcination in the above catalyst preparation can be within the commonly used temperature range, but in Test Example 1, the catalysts of 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 catalysts cat1 prepared in Examples (1 to 12) and Comparative Examples (1 to 7). The horizontal axis represents the type of carrier cb1, the amount of metal supported as a catalytically active component (weight %; expressed as wt.% in the table), the amount of CO adsorption (ml / g), and the BET surface area (m 2 / g). The CO adsorption amount was measured after the catalyst was subjected to a reduction pretreatment at 350° C. in a hydrogen atmosphere for 1 hour.

[0092] [Table 1]

[0093] (catalytic activity test) The catalytic activity test was carried out using a 50% H2-50% CO2 mixed gas (a mixed gas containing H2 and CO2 in a 1:1 (volume ratio)) as the reaction gas, with a GHSV (Gas Hourly Space Velocity) of 10,000 / h and the reaction temperature varied from 600°C to 800°C in 50°C increments. Before the catalytic activity test, the catalyst was subjected to a reduction pretreatment at 600° C. while passing hydrogen gas through the catalyst layer. As test results, the CO2 conversion rate (%), CO concentration (%) and CH4 concentration (%) at the outlet of the reaction section are shown in Table 2.

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

[0095] As mentioned above, 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 to 800°C).

[0096] [Table 2]

[0097] (Test Example 2) The test results of Examples (13 to 20) and Comparative Examples (8 and 9) of Test Example 2 will be explained below. As catalytically active components, Ni and Fe were investigated, and the addition of Cu was also investigated. The carriers used in the examples are CeO2 (ceria) and ZrO2 (zirconia), and 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 Examples (13 to 20) and Comparative Examples (8 and 9).

[0100] [Table 3]

[0101] (catalytic activity test) The catalytic activity test was carried out using a mixed gas containing H2 and CO2 in a 1:1 (volume ratio) ratio as the reaction gas, with a GHSV of 10,000 / h and the reaction temperature varied from 600°C to 800°C in 50°C increments. Before the catalytic activity test, the catalyst was subjected to a reduction pretreatment at 600° C. while passing hydrogen gas through the catalyst layer. As test results, the CO2 conversion rate (%), CO concentration (%) and CH4 concentration (%) at the outlet of the reaction section are shown in Table 4.

[0102] [Table 4]

[0103] For reference, the equilibrium values ​​(calculated values) of the CO2 conversion rate under these experimental conditions are shown in Table 4.

[0104] Iron-zirconia catalyst and iron-alumina catalyst For the iron-zirconia catalyst, the test results for calcination temperatures of 450°C, 600°C, 800°C, and 1000°C are shown in Examples 7, 15, 18, and 20, respectively. For the iron-alumina catalyst, the test results for 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, despite slight differences in metal loading, the iron-zirconia catalyst exhibits superior reverse water-gas shift reaction activity compared to the iron-alumina catalyst. Furthermore, the iron-zirconia catalyst exhibits very high catalytic activity not only at 450°C but also at higher calcination temperatures of 600°C, 800°C, and 1000°C, and the CO2 conversion rate reaches near equilibrium at all calcination temperatures.

[0105] Nickel-ceria catalyst The test results for calcination 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 has very high catalytic activity not only at a calcination temperature of 450°C, but also at higher calcination temperatures of 600°C, 800°C, and 1000°C, and the CO2 conversion rate reaches close to the equilibrium value at all calcination temperatures.

[0106] Nickel-alumina catalyst The test results when the calcination temperature was set to 450°C are shown in Comparative Example 1. The results showed that the nickel-alumina catalyst had a lower CO2 conversion rate than the nickel-ceria catalyst mentioned above.

[0107] Nickel-copper-ceria catalyst Test results for calcination temperatures of 450°C, 600°C, and 800°C are shown in Examples 6, 14, and 17, respectively. These results show that the nickel-copper-ceria catalyst tends to have a slightly lower CO2 conversion rate when the calcination temperature is increased to 600°C or 800°C, but is still superior to the iron-alumina catalyst, which has the same calcination temperature conditions as mentioned above. Furthermore, the nickel-copper-ceria catalyst, which has a calcination temperature of 450°C, achieves 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 extremely high reverse water-gas shift catalytic activity even when the calcination temperature is varied between 450°C and 1000°C. Therefore, they are useful because they can easily ensure high performance and durability even when used in combination with solid oxide electrolysis cells operated in the high-temperature range of, for example, around 600°C to 800°C.

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

[0110] Furthermore, the ceria-based metal oxide as the carrier cb1 may be ceria doped with at least one of gadolinium, samarium, and yttrium.

[0111] The zirconia-based metal oxide as the carrier cb1 may be zirconia stabilized with at least one of yttria and scandia.

[0112] Furthermore, it is also preferred to add either or both of nickel and iron to the catalytically active component ca1 and support copper as a further 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 a CO2 conversion rate (%) equal to or higher than that of a highly active but very expensive Pt catalyst at around 600 to 1000°C.

[0114] The test was conducted under a very high GHSV condition of 10,000 / h. Therefore, it is possible to perform the reverse water-gas shift reaction with a higher CO2 conversion rate (%) by reducing the GHSV below 10,000 / h, i.e., by increasing the amount of catalyst used relative to the amount of gas to be treated.

[0115] [Combination of electrolysis reactor and reverse water gas shift reactor] In the above description, the electrolysis reactor 10 and the reverse water gas shift reactor 20 are individually provided in the order shown along the gas advection direction in accordance with the system configuration shown in FIG. The reaction in the electrolytic reactor 10 can be exothermic depending on the reaction conditions, while the reaction in the reverse water gas shift reactor 20 is endothermic. Therefore, integrating these two reactors 10 and 20 can improve the thermal efficiency of the system. Figure 3 shows the configuration of the combined and integrated reactors 10 and 20, with the integrated reactors enclosed. The reactions in this integrated reactor are shown in the same box. Essentially, the reactions represented by Equations 1, 2, and 3 are carried out. When combining and integrating the electrolytic reactor 10 and the reverse water gas shift reactor 20, it is preferable to surround them with a heat-insulating material, as this allows for efficient heat transfer between the electrolytic reactor 10 and the reverse water gas shift reactor 20. Furthermore, a heat-transfer material may be used to connect the electrolytic reactor 10 and the reverse water gas shift reactor 20 to transfer heat generated in the electrolytic reactor 10 to the reverse water gas shift reactor 20.

[0116] [Electrolysis cell unit equipped with both an electrolysis 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 reactor 20 in the electrolysis cell unit U that serves as the electrolysis reactor 10. This is because, when a solid oxide electrolysis cell that operates at around 600 to 800°C is used as the electrolysis cell 1, the reverse water gas shift catalyst cat1 of the present invention, which exhibits high activity at around 600 to 800°C, can be used in the electrolysis reactor 10 and the reverse water gas shift reactor 20 in approximately the same temperature range. In this case, too, it is sufficient that the gas that has passed through the electrolysis reactor 10 is guided to the reverse water-gas shift reactor 20 and the reverse water-gas shift reaction occurs.

[0117] An electrolysis cell unit U equipped with such a reverse water gas shift reactor 20 is shown in Figure 4. Figure 4 is a diagram illustrating the electrolysis cell unit U shown in cross section in Figure 2, including the direction of gas advection.

[0118] As shown in the figure, the cross sections of the electrolysis cell units U are basically the same. That is, this electrolysis cell unit U also includes an electrolysis cell 1 in which an electrode layer 2 and a counter electrode layer 3 are formed with an electrolyte layer 1a sandwiched therebetween, a metal support 4 that functions as a support for the electrolysis cell 1 and also functions as a separator, and supply channel-forming members 5 and 6, and is configured to form an electrode layer-side gas supply channel 5a and a counter electrode layer-side gas supply channel 6a. More specifically, as can be seen from the figure, when viewed in the gas advection direction, the metal support 4 has a hole 4a formed in a portion corresponding to the electrolysis cell 1, but no hole is formed downstream of the electrode layer 2. Therefore, the metal support 4 serves as 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] In this example, however, the reverse water gas shift catalyst cat1 described above is applied to the inner surfaces of the electrode layer side gas supply channels 5a (the inner surfaces of the supply channel-forming member 5 on the supply channel side, the surface of the metal support 4 opposite to the surface on which the electrode layer 2 is formed, and the surfaces of the plurality of holes 4a). This coating layer 20a is indicated by a thick solid line. Furthermore, the electrode layer side gas supply channels 5a extend beyond the electrolysis reaction section 10, and the coating layer 20a is also provided on this extension side.

[0120] As a result, the electrode layer side gas supply path 5a of the electrolysis cell unit U serves as an exhaust path for exhausting at least H2 generated in the electrode layer 2, and the electrolysis reaction section 10 and the reverse water gas shift reaction section 20 are integrally provided in the electrolysis cell unit U.

[0121] In this configuration, the metal support 4 functions as a separator that separates H generated in the electrode layer 2 from O generated in the counter electrode layer 3, and at least a portion of the separator on the H discharge path side serves as a reverse water-gas shift reactor 20. 2 and 4, a large number of electrolysis cell units U are stacked and electrically connected to form a so-called electrolysis cell module (not shown). Naturally, useful gases are generated across multiple layers.

[0122] Based on the concept of combining the electrolytic reaction unit 10 and the reverse water gas shift reaction unit 20 (using the electrode layer side gas supply channel 5a of the electrolytic reaction unit 10 as the reverse water gas shift reaction unit 20), the inventors conducted an experiment by storing a granular reverse water gas shift catalyst cat1 in the electrode layer side gas supply channel 5a. FIG. 5 shows a cross section of the electrolysis cell unit U used in this experiment.

[0123] A specific description will be given below with reference to Figure 5, which shows a cross-sectional view of the electrolysis cell unit U. Here, a metal-supported solid oxide electrolysis cell was used as the electrolysis cell 1. A metal substrate was fabricated as the metal support 4 by laser machining a 0.3 mm-thick ferritic stainless steel metal plate with multiple through-holes (which became the holes 4a). An electrode layer 2 and an intermediate layer 2a were laminated in this order on the metal substrate, and an electrolyte layer 1a was laminated on 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 laminated in this order on the electrolyte layer 1a to fabricate the electrolysis cell 1. The electrode layer 2 was formed using a mixture of NiO powder and GDC powder, the intermediate layer 2a was formed using GDC powder, the electrolyte layer 1a was formed using 8YSZ (8 mol% yttria-stabilized zirconia) powder, the reaction prevention layer 7 was formed using GDC powder, and the counter electrode layer 3 was formed using a mixture of GDC powder and LSCF powder. 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 electrolysis cell 1 can be improved by providing the intermediate layer 2a between the electrode layer 2 and the electrolyte layer 1a, or by providing the 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 a low-temperature firing method (e.g., a wet method using a firing process in a low-temperature range that does not involve firing at a high temperature higher than 1100°C), a spray coating method (such as a thermal spraying method, an aerosol deposition method, an aerosol gas deposition method, a powder jet deposition method, a particle jet deposition method, or a cold spray method), a PVD method (such as a sputtering method or a pulsed laser deposition method), or a CVD method. These processes that can be used in the low temperature range make it possible to obtain a good intermediate layer 2a and a good reaction-preventing layer 7 without using firing at a high temperature range, for example, higher than 1100°C. This is therefore preferable because it allows for the realization of an electrolysis cell 1 with excellent performance and durability without damaging the metal support 4. Furthermore, the use of a low-temperature firing method is even more preferable because it facilitates the handling of raw materials.

[0124] Regarding the electrolysis cell unit U obtained as described above, we investigated whether performance could be improved by installing a granular reverse water gas shift catalyst cat1 in the electrode layer side gas supply channel 5a (which also serves as an exhaust channel for the gas electrolyzed in the electrolysis reaction section 10).

[0125] Results without reverse water gas shift catalyst cat1 An electrolysis 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 5 below. The experimental results are shown as Comparative Examples A1 and A2.

[0126] [Table 5]

[0127] Results when reverse water gas shift catalyst cat1 is installed A granular catalyst obtained by supporting approximately 10% Ni on an 8YSZ carrier similar to that used in Example 2 was used as the reverse water gas shift catalyst cat1. An electrolysis reaction was carried out while supplying a gas containing HO and CO to the electrolytic cell unit U, and the ratio of H to CO in the gas at the outlet of the electrolytic cell unit U was measured using a gas chromatograph. The results are shown in Table 6. This experimental result is referred to as Example A1.

[0128] [Table 6]

[0129] The above comparative experiments demonstrated that the electrolytic cell unit U, in which the electrolytic cell 1 was formed as a thin layer on the metal support 4 and the reverse water-gas shift reactor 20, which produced CO using CO and the H by the reverse water-gas shift reaction, was provided in the electrode layer-side gas supply path 5a, which served as the discharge path for the electrolyzed gas, was able to increase the composition ratio of CO to H produced by electrolysis.

[0130] Comparing an electrolysis cell unit U that does not include the reverse water gas shift catalyst cat1 in the electrode layer-side gas supply channel 5a (which serves as the discharge channel for electrolyzed gas) with an electrolysis cell unit U that does include the catalyst cat1, the hydrogen / carbon monoxide ([H2 / CO]) ratio at the outlet is approximately 10 or more and approximately 5. This is because combining the reactions in the electrolysis reactor 10 and the reverse water gas shift reactor 20 ensures an amount of CO advantageous for synthesizing various hydrocarbons, which is preferable. In addition, adopting the CO methanation reaction rather than the CO2 methanation reaction can improve the thermal efficiency of the hydrocarbon production system 100, and combining the reactions in the electrolysis reactor 10 and the reverse water gas shift reactor 20 ensures an amount of CO. This is because methanation of 1 mole of CO2 produces 2 moles of HO, whereas methanation of 1 mole of CO only produces 1 mole of HO. Therefore, the hydrocarbon production system 100 that employs the CO methanation reaction can reduce the loss of latent heat and sensible heat equivalent to 1 mole of HO throughout the system. By appropriately adjusting the ratio of H2O and CO2 introduced into the electrolytic reaction unit 10, the reaction conditions of the electrolytic reaction unit 10 (electrolysis voltage, reaction temperature, etc.), and the reaction conditions of the reverse water gas shift reaction unit 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 unit 20 can be adjusted to a value suitable for the downstream second catalytic reaction unit 30 (hydrocarbon synthesis reaction unit) (for example, H2 / CO=3, which is the equivalent ratio for the CO methanation reaction).

[0131] [A heat exchanger is installed between the electrolysis reactor and the reverse water gas shift reactor] In the explanations above, we have mainly described an example in which the electrolysis reaction section 10 and the first catalytic reaction section (reverse water gas shift reaction section) 20 are integrated together. However, it is also possible to use a configuration in which the two sections 10 and 20 are separate sections and a heat exchanger 11 is provided between the two sections 10 and 20 to allow heat transfer between the two sections. This configuration is shown in Figure 6, which corresponds to Figure 1. The hollow double lines indicate heat transfer between the two sections. With this configuration, the temperature of each section 10 and 20 can be appropriately controlled.

[0132] The inventors call the system consisting of the electrolysis reactor 10 and the reverse water gas shift reactor 20 described above an "electrolysis reactor system."

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

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

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

[0136] (Catalyst preparation) The hydrocarbon synthesis catalyst cat2 was also prepared by the method described in FIGS. That is, an aqueous solution is prepared by dissolving a measured amount of a water-soluble ruthenium compound (such as ruthenium nitrate, ruthenium chloride, ruthenium sulfate, ruthenium ammonium sulfate, ruthenium acetate, ruthenium oxalate, or ruthenium citrate) according to the desired catalyst composition. Furthermore, when supporting molybdenum, vanadium, iron, or cobalt as an additional catalytically active component, the water-soluble metal compounds are similarly measured and dissolved in the aqueous solution. Using this aqueous solution, a predetermined amount of carrier particles (ZrO, Al, O, SiO, MgO, or TiO) is impregnated with the catalytically active component, for example, and then subjected to necessary treatments such as drying, calcination, and reduction to obtain the hydrocarbon synthesis catalyst cat2. The catalysts in the following examples were prepared using an aqueous ruthenium chloride solution, an aqueous ammonium molybdate solution, an aqueous vanadyl oxalate solution, an aqueous iron nitrate solution, and an aqueous cobalt nitrate solution, respectively, and when supporting both ruthenium and a catalytically active component other than ruthenium, they were prepared using a sequential loading method (a two-stage loading method in which a catalytically active component other than ruthenium is first loaded on a 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, 12.4% H2O, and the remainder N2 was used as the reactant gas, and an activity test of the hydrocarbon synthesis catalyst cat2 was performed at a GHSV of 4000 / h (wet basis) and a reaction temperature between 275°C and 360°C. Note that the reactant gas in this case is an example of an assumed model in which a co-electrolysis reaction of water and carbon dioxide is carried out in the electrolysis reaction unit 10 under conditions where the electrolysis reaction rate of carbon dioxide is low, and then the reverse water gas shift reaction of carbon dioxide is carried out in the reverse water gas shift reaction unit 20 installed downstream, and the resulting mixed gas of CO, CO2, H2, and H2O is introduced into the hydrocarbon synthesis reaction unit 30 to carry out the hydrocarbon synthesis reaction.

[0138] The following two indicators were used to organize the test results.

[0139] 1. Estimated hydrocarbon conversion rate for CO2 removal = [number of carbon atoms in hydrocarbons in outlet gas] / [number of carbon atoms in outlet gas - number of carbon atoms in 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 it is preferable that this index is high.

[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 to 4) HN [MJ / m 3 (N)]: Calorific value 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 to C4 components contained in the outlet gas of the hydrocarbon synthesis reaction section 30 obtained by the catalytic reaction, and if this value exceeds 39.8, it can be confirmed that hydrocarbons such as ethane, propane, and butane are produced in addition to methane.

[0141] Regarding evaluation test 1, Tables 7 and 8 shown below show Examples B1 to B3 of the hydrocarbon synthesis catalyst cat2 of 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, as the hydrocarbon synthesis catalyst cat2, a catalyst in which ruthenium is supported on an alumina carrier, or a catalyst in which molybdenum or vanadium is supported in addition to ruthenium. From the above results, it can be seen that the hydrocarbon production system 100 described above produces a C1-C4 calorific value of 39 MJ / Nm 3 It was confirmed that the above high calorie gases could 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 reactant gas, and an activity test of the hydrocarbon synthesis catalyst cat2 was carried out at a GHSV of 5000 / h (DRY basis) and a reaction temperature between approximately 230°C and approximately 330°C. Note that the reactant gas in this case is an example of an assumed model in which a mixed gas obtained when a co-electrolysis reaction of water and carbon dioxide is carried out in the electrolysis reaction section 10 under conditions where the electrolysis reaction rate of carbon dioxide is low is introduced into the hydrocarbon synthesis reaction section 30 to carry out the hydrocarbon synthesis reaction.

[0146] The following two indicators were used to organize the test results.

[0147] 1. Hydrocarbon conversion rate = [number of hydrocarbon carbons in the outlet gas] / [number of carbons in the outlet gas] This index indicates the proportion of carbon atoms that are converted into hydrocarbons without being converted into CO2 out of the total carbon atoms that are introduced, and it is preferable that this index is high.

[0148] 2. Estimated hydrocarbon conversion rate for CO2 removal = [number of carbon atoms in hydrocarbons in outlet gas] / [number of carbon atoms in outlet gas - number of carbon atoms in 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 it is preferable that this index is also high.

[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 at a volume ratio of 3:1 (H2 / CO = 3) was used as the reactant gas, and the activity of the hydrocarbon synthesis catalyst cat2 was tested at a GHSV of 2000 / h and a reaction temperature between 235°C and approximately 330°C. In this activity test, a catalyst (Examples B17 and B18) in which iron or cobalt in addition to ruthenium was supported on a titania support was used. Note that the reactant gas in this case 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 electrolysis reaction section 10, or a mixed gas of hydrogen and carbon monoxide obtained by separating water or carbon dioxide as needed from the gas obtained by the co-electrolysis reaction of water and carbon dioxide, is introduced into the hydrocarbon synthesis reaction section 30 to perform 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 in which ruthenium and iron or cobalt are supported on a titania carrier as the hydrocarbon synthesis catalyst cat2.

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

[0157] From the above results, as previously shown, a catalyst in which at least ruthenium is supported on a metal oxide carrier 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 that at least one of molybdenum, vanadium, iron, and cobalt is supported as the catalytically active component ca2.

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

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

[0160] Furthermore, among such hydrocarbon synthesis catalysts cat2, the highly active catalysts had carbon monoxide adsorption amounts of 0.4 ml / g or more.

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

[0162] [Water separation section] The water separation section 40 is provided with a condenser, which adjusts the temperature and pressure of the inflowing gas containing H2O to a predetermined level, condenses the gas, and extracts the water to the outside.

[0163] [Carbon dioxide separation section] For example, a PSA is provided in this section 50, which separates CO2 from the incoming CO2-containing gas by adsorbing it onto an adsorbent under a predetermined temperature and pressure, and then desorbs the separated CO2 from the adsorbent, thereby effectively separating CO2. The separated CO2 can be returned to the front of the electrolysis reaction section 10 via the carbon dioxide return path 51 and reused. It is also possible to use PSA or the like to form the carbon dioxide separation section and the water separation section into the same separation section.

[0164] [Another embodiment] (1) In the above embodiment, the CO2 separated in the carbon dioxide separation unit 50 is returned to the upstream side of the electrolysis unit 10. However, in the hydrocarbon production system 100 according to the present invention, the conversion of CO2 to CO is mainly performed in the reverse water-gas shift reactor 20. Therefore, the CO2 may be returned to the upstream side of the reverse water-gas shift reactor 20. This configuration is shown in FIG.

[0165] (2) In the above embodiment, no particular mention was made of H in the gas obtained from the hydrocarbon synthesis reaction section 30. However, a hydrogen separation section (shown as H separation section in the figure) 60 that separates H using a hydrogen separation membrane or the like may be provided to separate H for separate use. This configuration is shown in FIG. 8. In this example, the H separated in the hydrogen separation section 60 may be returned to the upstream side of 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 section 40 is provided downstream of the hydrocarbon synthesis reaction section 30. However, as shown in Fig. 9, the water separation section 40 may be provided between the reverse water gas shift reaction section 20 and the hydrocarbon synthesis reaction section 30. The main function of this water separation section 40 is to facilitate the hydrocarbon synthesis reaction.

[0167] (4) In the above embodiment, both HO and CO are supplied to the electrolysis reaction unit 10 for the electrolysis reaction. However, as shown in Fig. 10, a system may be used in which only HO is supplied to the electrolysis reaction unit 10 for the electrolysis reaction. In this case, carbon consumed in the hydrocarbon synthesis is input to 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 electrolysis cell was used as the electrolysis cell 1 in the electrolysis reaction section 10. However, the electrolysis cell 1 may also be an alkaline electrolysis cell or a polymer membrane electrolysis cell.

[0169] (6) In the above embodiment, the electrolytic reaction section 10 and the first catalytic reaction section 20 are integrated into one structure. However, it is also possible to integrate the second catalytic reaction section 30 in addition to the reaction sections 10 and 20. An example of this structure is shown in FIG. 13. In this figure, 30a denotes a coating layer of the hydrocarbon synthesis catalyst cat2. In this configuration, each of the reaction sections 10, 20, and 30 can be constructed on a metal support 4 and a supply path forming member 5, and this metal support a functions as a separator that separates the produced hydrocarbons from oxygen.

[0170] (7) In the above embodiment, an example was shown in which hydrocarbons such as methane were synthesized in the hydrocarbon synthesis reaction section 30. However, depending on how the hydrocarbon synthesis catalyst used in the hydrocarbon synthesis reaction section 30 is selected, chemical raw materials can also be synthesized from hydrogen, carbon monoxide, etc. introduced into the hydrocarbon synthesis reaction section 30. [Explanation of symbols]

[0171] 1 electrolysis cell 1a Electrolyte layer 2 electrode layer 3. Counter electrode layer 4 Metal support (support / separator) 4a hole (through hole) 5. Supply path forming member (separator) 6 Supply path 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 Electrolysis Cell Unit Cat 1 reverse water gas shift catalyst ca1 Catalytically active component (metal component) cb1 Carrier (metal oxide) Cat2 hydrocarbon synthesis catalyst ca2 Catalytic active component (metal component) cb2 carrier (metal oxide)

Claims

1. A method for producing a reverse water gas shift catalyst, comprising at least an impregnation step of adding a support which is a ceria-based metal oxide or a zirconia-based metal oxide to a solution containing either or both of nickel and iron, and impregnating the support with at least either or both of nickel and iron; and a calcination step of calcining the impregnated support obtained through the impregnation step at a temperature of 450°C or higher.

2. A method for producing a reverse water gas shift catalyst, comprising at least a calcination step of supporting at least one or both of nickel and iron as catalytically active components on a support that is a ceria-based metal oxide or a zirconia-based metal oxide, and calcining the support at a temperature of 450°C or higher.

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