Patent Publication Number: US-2023149889-A1

Title: Hydrocarbon Production System

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the United States national phase of International Application No. PCT/JP2021/014083 filed Mar. 31, 2021, and claims priority to Japanese Patent Application No. 2020-065256 filed Mar. 31, 2020, the disclosures of which are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a hydrocarbon production system that produces hydrocarbon from at least water and carbon dioxide. 
     Description of Related Art 
     An example of this type of hydrocarbon production system is disclosed in JP-T-2016-522166. 
     The system disclosed in JP-T-2016-522166 includes a high temperature electrolyte (HTE) reactor (corresponding to electrolytic reaction unit of the present invention) including stacking of electrolytic single cells (corresponding to the electrolytic cell unit of the present invention) that generate either hydrogen or a synthetic raw material gas (“synthetic gas” representing a mixture of hydrogen and carbon monoxide) from water vapor and carbon dioxide, in which the synthetic gas obtained in this electrolytic single cell is converted into a desired combustible gas by a heterogeneous catalytic action. 
     Therefore, in the technique disclosed in JP-T-2016-522166, a hydrocarbon synthesis unit is provided on the downstream side of the electrolytic reaction unit, and hydrocarbons are synthesized (produced) using water and carbon dioxide as starting materials. 
     In the related art, in the electrolytic reaction unit, so-called “co-electrolysis” is carried out in which both water and carbon dioxide are electrolyzed together. A heterogeneous catalyst is used for synthesis (so-called methanation) of hydrocarbons. 
     However, although it is said that a heterogeneous catalyst is used when synthesizing a hydrocarbon, it is difficult to select this kind of catalyst, and a technique capable of stably synthesizing hydrocarbon has not yet been established. 
     Further, when producing a hydrocarbon, hydrogen and carbon monoxide are obtained from water and carbon dioxide, which are the starting materials thereof, to produce a hydrocarbon, and various components (water, carbon dioxide, hydrogen) remain in the gas containing the hydrocarbon produced in this way. However, there is no disclosure of how this type of component should be used in a process of producing the hydrocarbon. 
     SUMMARY OF THE INVENTION 
     In view of this situation, a main subject of the present invention is to obtain a hydrocarbon production system capable of efficiently producing hydrocarbon by securing hydrogen and carbon monoxide required for hydrocarbon synthesis using water and carbon dioxide as raw materials. 
     According to a first characteristic configuration of the present invention, 
     there is provided a hydrocarbon production system including: an electrolytic reaction unit that converts water into hydrogen through an electrolytic reaction or converts water and carbon dioxide into hydrogen and carbon monoxide through an electrolytic reaction; a catalytic reaction unit that converts a product generated by the electrolytic reaction unit into hydrocarbon through a catalytic reaction; and a branch path that branches a portion of an outlet component of the catalytic reaction unit. 
     The hydrocarbon production system having this configuration includes the electrolytic reaction unit and the catalytic reaction unit, and by the electrolytic reaction unit, at least hydrogen, or hydrogen and carbon monoxide, required for the hydrocarbon synthesis are obtained from water or water and carbon dioxide. Here, when only hydrogen is obtained by the electrolytic reaction unit, one or more of carbon monoxide and carbon dioxide may be supplied in front of the catalytic reaction unit. 
     Then, by the catalytic reaction unit, the hydrocarbon is obtained from one or more of the supplied hydrogen, carbon monoxide, and carbon dioxide. 
     In such a system configuration, since hydrocarbon is synthesized from one or more of the hydrogen, carbon monoxide, and carbon dioxide through the reaction by the catalytic reaction unit, water, carbon dioxide, or unreacted hydrogen is released from the catalytic reaction unit along with the hydrocarbon. Therefore, by providing the branch path for branching a portion of the outlet component of the catalytic reaction unit, useful hydrocarbons in the gas generated in the catalytic reaction unit can be taken out. Further, for example, a calorific value per unit volume, which is an important characteristic as the high-calorie gas, can be adjusted. 
     In a second characteristic configuration of the present invention, 
     the catalytic reaction unit uses the product generated by the electrolytic reaction unit and converts the product into a high-calorie gas containing at least lower saturated hydrocarbon through the catalytic reaction. 
     As described above, the catalytic reaction unit functions as a hydrocarbon synthesis reaction unit, but when the hydrocarbon generated at this unit becomes a gas containing at least the lower saturated hydrocarbon, for example, the gas can be a high-calorie gas having a calorific value of 39 MJ/Nm 3  or more. 
     As a result, by using the hydrocarbon production system according to the present invention, a very useful high-calorie gas can be constructed with a relatively simple and stable system. 
     In a third characteristic configuration of the present invention, 
     the hydrocarbon production system further includes a carbon dioxide separation unit that separates carbon dioxide from the outlet component of the catalytic reaction unit, in which the separated carbon dioxide is discharged from the branch path. 
     As will be described later, when the hydrocarbon synthesis is carried out by a catalytic reaction, hydrocarbon may be generated and carbon dioxide may remain, but this carbon dioxide is separated from the gas obtained from the catalytic reaction unit and discharged. Therefore, it is possible to increase a concentration on the hydrocarbon component side and use the discharged carbon dioxide. 
     In a fourth characteristic configuration of the present invention, 
     The hydrocarbon production system further includes a hydrogen separation unit that separates hydrogen from the outlet component of the catalytic reaction unit, in which the separated hydrogen is discharged from the branch path. 
     Unreacted hydrogen may remain in the reaction in the catalytic reaction unit, but by separating this hydrogen from the gas obtained from the catalytic reaction unit and discharging the hydrogen, it is possible to increase a concentration on the hydrocarbon component side and use the discharged hydrogen. 
     In a fifth characteristic configuration of the present invention, 
     The hydrocarbon production system further includes a water separation unit that separates water from at least one of the outlet component and an inlet component of the catalytic reaction unit, in which the separated water is discharged from the branch path. 
     The reaction in the catalytic reaction unit is basically a reaction between hydrogen and carbon monoxide, but water is generated because it contains oxygen. However, since this water is separated from the gas obtained from the catalytic reaction unit and discharged, it is possible to increase the concentration on the hydrocarbon component side and use the discharged water. In addition, unreacted water may remain in the reaction in the electrolytic reaction unit, but when this water is separated in front of the catalytic reaction unit, since the catalytic reaction in the catalytic reaction unit is likely to proceed, it is preferable to separate and discharge the water in front of the catalytic reaction unit. 
     In a sixth characteristic configuration of the present invention, the branch path is a recycle line. 
     As described above, the hydrocarbon production system according to the present invention uses water and carbon dioxide as starting materials thereof. 
     Then, when the branch path is provided in the system and this branch path is set as the recycle line, for example, by returning to a main path for hydrocarbon synthesis from that line, any one or more of recycled gases (water, carbon dioxide, and hydrogen) can be used in a useful manner. 
     Here, a recycling destination of water can be upstream of the electrolytic reaction unit. When the electrolytic reaction unit or a reverse water-gas shift reaction unit is provided, a recycling destination of the carbon dioxide can be the reverse water-gas shift reaction unit. Hydrogen can be recycled and used on an upstream side of the catalytic reaction unit which is a hydrocarbon synthesis unit. 
     In a seventh characteristic configuration of the present invention, 
     The hydrocarbon production system further includes a heavy hydrocarbon separation unit that separates heavy hydrocarbon from the outlet component of the catalytic reaction unit. 
     By providing the heavy hydrocarbon separation unit, heavy hydrocarbon which is a useful hydrocarbon component can be separated and used. 
     In an eighth characteristic configuration of the present invention, the catalytic reaction unit is provided in a plurality of stages. 
     According to this characteristic configuration, the amount of hydrocarbons synthesized can be increased by setting the catalytic reaction units including multiple stages to obtain a desired high-calorie gas. For example, by changing the reaction conditions such as a reaction temperature of each catalytic reaction unit, an appropriate reaction can be generated in each stage. 
     In a ninth characteristic configuration of the present invention, the hydrocarbon production system further includes a reverse water-gas shift reaction unit. 
     The hydrocarbon production system according to the present invention includes the electrolytic reaction unit and the catalytic reaction unit, generates at least hydrogen by the electrolytic reaction unit, and uses this hydrogen for the hydrocarbon synthesis, but the catalytic reaction unit requires one or more of hydrogen, carbon monoxide, and carbon dioxide. 
     Here, as described above, although it is possible for the catalytic reaction unit to separately supply carbon monoxide or carbon dioxide, the reverse water-gas shift reaction unit generates carbon monoxide from carbon dioxide to supply the carbon monoxide to the catalytic reaction unit, and thus, hydrocarbon synthesis can be carried out satisfactorily. 
     Furthermore, when co-electrolysis that electrolyzes both water and carbon dioxide is carried out by the electrolytic reaction unit, the electrolytic reaction tilts toward the water side and carbon monoxide may not be sufficiently generated. However, by generating a reverse water-gas shift reaction by the hydrogen generated in the electrolytic reaction unit and the unreacted carbon dioxide in the electrolytic reaction unit to increase the concentration of the carbon monoxide and supplying the carbon monoxide to the catalytic reaction unit, the hydrocarbon synthesis can be carried out satisfactorily. 
     In a tenth characteristic configuration of the present invention, 
     the electrolytic reaction unit has an electrolytic cell in which at least an electrode layer, an electrolyte layer, and a counter electrode layer are formed on a support. 
     According to this characteristic configuration, as the electrolytic cell used in the electrolytic reaction unit, for example, thin-film electrode layer, electrolyte layer, and counter electrode layer are provided on a robust support having sufficient strength even if the support is thin. Therefore, the electrolytic reaction can be effectively caused while reducing the amount of the material used to form these layers to be the electrolytic cell. As a result, it is possible to configure an electrolytic cell unit that is compact, has high performance, and has excellent strength and reliability. Metals and ceramics can be selected as constituent materials of this type of support. 
     In an eleventh characteristic configuration of the present invention, the support is a metal. 
     By adopting a metal as the support, a material cost is suppressed by ensuring the strength with an inexpensive metal material, and it is easier to process than ceramics. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram illustrating the configuration of a hydrocarbon production system. 
         FIG.  2    is a schematic diagram illustrating a configuration of an electrolytic reaction unit. 
         FIG.  3    is a diagram illustrating the configuration of a system in which the electrolytic reaction unit and a reverse water-gas shift reaction unit are integrated. 
         FIG.  4    is a schematic diagram of an electrolytic cell unit including the electrolytic reaction unit and the reverse water-gas shift reaction unit. 
         FIG.  5    is a cross-sectional view of an electrolytic cell unit used in a comparative experiment in which an electrode layer-side gas supply path is used as the reverse water-gas shift reaction unit. 
         FIG.  6    is a configuration diagram of a system equipped with a heat exchanger between the electrolytic reaction unit and the reverse water-gas shift reaction unit. 
         FIG.  7    is a diagram illustrating another configuration of a hydrocarbon production system that guides CO 2  to the reverse water-gas shift reaction unit. 
         FIG.  8    is a diagram illustrating another configuration of the hydrocarbon production system equipped with the hydrogen separation unit. 
         FIG.  9    is a diagram illustrating still another configuration of a hydrocarbon production system equipped with the water separation unit in front of a hydrocarbon synthesis reaction unit. 
         FIG.  10    is a diagram illustrating still another configuration of a hydrocarbon production system in which only water is introduced into the electrolytic reaction unit. 
         FIGS.  11 ( a )- 11 ( c )  are explanatory diagrams illustrating a preparation state of a catalyst. 
         FIGS.  12 ( a ) and  12 ( b )  are explanatory diagrams illustrating a coating/calcination state and reduction pretreatment of a catalyst. 
         FIG.  13    is a schematic diagram of an electrolytic cell unit including the electrolytic reaction unit, the reverse water-gas shift reaction unit, and the hydrocarbon synthesis reaction unit. 
         FIG.  14    is a diagram illustrating another configuration of a hydrocarbon production system. 
         FIG.  15    is a diagram illustrating another configuration of a hydrocarbon production system. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     An embodiment of the present invention will be described with reference to the drawings. 
       FIG.  1    illustrates a configuration of one form of a hydrocarbon production system  100  proposed by the inventors. 
     As illustrated in the figure, the hydrocarbon production system  100  includes an electrolytic reaction unit  10 , a first catalytic reaction unit  20 , a second catalytic reaction unit  30 , a heavy hydrocarbon separation unit  35  (illustrated as a CnHm separation unit), a water separation unit  40  (illustrated as an H 2 O separation unit), and a carbon dioxide separation unit  50  (illustrated as a CO 2  separation unit) in this order. 
     The electrolytic reaction unit  10  is a unit that electrolyzes at least a portion of an inflowing gas, the first catalytic reaction unit  20  is a reverse water-gas shift reaction unit that carries out a reverse water-gas shift reaction of at least a portion of the inflowing gas, and the second catalytic reaction unit  30  is configured to act as a hydrocarbon synthesis reaction unit that synthesizes at least a portion of the inflowing gas into hydrocarbon. Here, the hydrocarbon synthesized is mainly CH 4  (hydrocarbon having one carbon atom), but also includes other lower saturated hydrocarbons having two to four carbon atoms and the like. Further, as will be illustrated later, by appropriately selecting a catalyst used for the second catalytic reaction unit  30 , heavy hydrocarbons having a larger number of carbon atoms than the lower saturated hydrocarbons, unsaturated hydrocarbons, oxygen-containing hydrocarbons, or the like can also be synthesized. Therefore, in the present specification, the hydrocarbon is a concept including all of them, and is also collectively referred to as hydrocarbons. 
     The heavy hydrocarbon separation unit  35 , the water separation unit  40 , and the carbon dioxide separation unit  50  are units for removing at least a portion of predetermined components (CnHm, H 2 O, and CO 2  in the order of description) from the gas flowing inside. As illustrated in  FIG.  1   , the components removed and recovered by the water separation unit  40  and the carbon dioxide separation unit  50  are returned to a predetermined unit of the system via a water return path  41  and a carbon dioxide return path  51  and are reused. It is illustrated by H 2 O and CO 2  returned via both return paths  41  and  51 , respectively. Both return paths  41  and  51  are branch paths in the respective units  40  and  50 . 
     As a result, the hydrocarbon production system  100  is established as a carbon closed system that does not substantially release CO 2  to the outside of the system. 
     In the drawings, the gas flowing into each unit is illustrated in front of each unit, and the gas released from the unit is illustrated after each unit. 
     In the electrolytic reaction unit  10 , H 2 O and CO 2  as starting materials flow in and are electrolyzed internally, H 2 O is decomposed into H 2  and O 2 , and some CO 2  is decomposed into CO and O 2  and released. 
     The reaction is described as follows. 
       2H 2 O→2H 2 +O 2   (Formula 1)
 
       2CO 2 →2CO+O 2   (Formula 2)
 
     The formulas 1 and 2 are also illustrated in a box illustrating the electrolytic reaction unit  10  of  FIG.  1   . 
     In the first catalytic reaction unit  20  (reverse water-gas shift reaction unit), H 2  and CO 2  flow in, a reverse water-gas shift reaction occurs inside, CO 2  becomes CO, H 2  becomes H 2 O, and CO and H 2 O are released. 
     The reaction is described as the following equilibrium reaction, but the reverse water-gas shift reaction is a reaction (reaction proceeding in a direction in which CO 2  and H 2  react to generate CO and H 2 O) in which the reaction described by the following formula 3 proceeds to the right. 
       CO 2 +H 2 ⇔CO+H 2 O  (Formula 3)
 
     This formula 3 is also illustrated in a box illustrating the first catalytic reaction unit  20  (reverse water-gas shift reaction unit) in  FIG.  1   . A reverse water-gas shift catalyst cat 1  used in the reaction is also schematically illustrated in this box. 
     In the second catalytic reaction unit  30  (hydrocarbon synthesis reaction unit), H 2  and CO flow in, and hydrocarbon is synthesized by a catalytic reaction. For example, the reaction in which CH 4  is synthesized from CO and H 2  is described as the following equilibrium reaction, but the reaction in which CH 4  is synthesized from CO and H 2  is a reaction (reaction proceeding in a direction in which CO and H 2  react to generate CH 4  and H 2 O) in which the reaction described by the following formula 4 proceeds to the right. 
       CO+3H 2 ⇔CH 4 +H 2 O  (Formula 4)
 
     This formula 4 is also illustrated in a box illustrating the second catalytic reaction unit  30  (hydrocarbon synthesis reaction unit) in  FIG.  1   . A hydrocarbon synthesis catalyst cat 2  used in the reaction is also schematically illustrated in this box. 
     Furthermore, the equilibrium reaction of (Formula 3) also occurs at this unit. 
     Further, depending on the type of catalyst used in the second catalytic reaction unit  30 , it is possible to proceed with a Fischer-Tropsch (FT) synthesis reaction or the like. Therefore, various hydrocarbons such as ethane, propane, butane, pentane, hexane, paraffin, and olefinic hydrocarbons can be synthesized from CO and H 2 . 
     As will be described later, the inventors have illustrated an example of a catalyst using ruthenium as a catalytically active component of the hydrocarbon synthesis catalyst cat 2  disposed in the second catalytic reaction unit  30 , but heavy hydrocarbons are also synthesized in a catalyst containing iron, cobalt, or the like as the catalytically active component, and this type of heavy hydrocarbon can be condensed and separated from a transport gas as the temperature decreases. Therefore, the above-mentioned heavy hydrocarbon separation unit  35  separates the hydrocarbon component separated in this manner. 
     The generated H 2 O is separated in the water separation unit  40  and returned to the upstream side of the electrolytic reaction unit  10  via the water return path  41  (water recycle line). 
     The generated CO 2  is separated in the carbon dioxide separation unit  50  and returned to the upstream side of the electrolytic reaction unit  10  via the carbon dioxide return path  51  (carbon dioxide recycle line). 
     As a result, in this hydrocarbon production system  100 , the hydrocarbon is finally synthesized and can be supplied to the outside. 
     The above is the outline of the above-mentioned hydrocarbon production system  100 , and a configuration of each unit and a role thereof will be described below. 
     [Electrolytic Reaction Unit] 
     As illustrated above, the electrolytic reaction unit  10  decomposes H 2 O and CO 2  that flow in by consuming electric power supplied according to the above formulas 1 and 2. 
       FIG.  2    schematically illustrates a cross-sectional structure of the electrolytic reaction unit  10 . 
       FIG.  2    illustrates an electrolytic cell unit U which is stacked in multiple to form an electrolytic stack (not illustrated). The electrolytic cell unit U includes an electrolytic cell  1 , and the electrolytic cell  1  includes an electrode layer  2  on one surface of an electrolyte layer  1   a  and a counter electrode layer  3  on the other surface thereof. The electrode layer  2  serves as a cathode in the electrolytic cell  1 , and the counter electrode layer  3  serves as an anode. Incidentally, this electrolytic cell unit U is supported by a metal support  4 . Here, a case where a solid oxide electrolytic cell is used as the electrolytic cell  1  is illustrated. 
     The electrolyte layer  1   a  can be formed in the state of a thin film having a thickness of m or less. As a constituent material of the electrolyte layer  1   a , YSZ (yttria-stabilized zirconia), SSZ (scandia-stabilized zirconia), GDC (gadolinium-doped ceria), YDC (yttrium-doped ceria), SDC (samarium-doped ceria), and LSGM (strontium/magnesium-added lanthanum gallate) or the like can be used. In particular, zirconia-based ceramics are preferably used. 
     Preferably, the electrolyte layer  1   a  is formed by a low-temperature calcination method (for example, a wet method using a calcination treatment in a low temperature range that does not carry out a calcination treatment in a high temperature range exceeding 1100° C.), a spray coating method (thermal spraying method, aerosol deposition method, aerosol gas deposition method, a powder jet deposition method, particle jet deposition method, cold spray method, or the like), a PVD method (sputtering method, a pulse laser deposition method, or the like), a CVD method, or the like. These film forming processes that can be used in a low temperature range provide an electrolyte layer  1   a  that is dense and has high airtightness and gas barrier properties without using calcination in a high temperature range exceeding, for example, 1100° C. Therefore, damage to the metal support  4  can be suppressed, element mutual diffusion between the metal support  4  and the electrode layer  2  can be suppressed, and an electrolytic cell unit U having excellent performance and durability can be realized. In particular, it is preferable to use the low-temperature calcination method, the spray coating method, or the like because a low-cost element can be realized. Further, it is more preferable to use the spray coating method because the electrolyte layer  1   a , which is dense and has high airtightness and gas barrier property, can be easily obtained in a low temperature range. 
     Further, the electrolyte layer  1   a  is densely configured in order to prevent the gas leak and exhibit high ionic conductivity. A density of the electrolyte layer  1   a  is preferably 90% or more, more preferably 95% or more, and further preferably 98% or more. When the electrolyte layer  1   a  is a uniform layer, the density is preferably 95% or more, and more preferably 98% or more. When the electrolyte layer  1   a  includes a plurality of layers, it is preferable that at least a portion of the electrolyte layer  1   a  includes a layer (dense electrolyte layer) having a density of 98% or more, and it is more preferable to include a layer (dense electrolyte layer) having a density of 99% or more. In a case where the dense electrolyte layer is included in a portion of the electrolyte layer  1   a , even when the electrolyte layer  1   a  includes a plurality of layers, it is possible to easily form the electrolyte layer  1   a  that is dense and has high airtightness and gas barrier property. 
     The electrode layer  2  can be provided in a thin layer on the front surface of the metal support  4  and in a region larger than a region where holes  4   a  are provided. In the case of a thin layer, a thickness thereof can be, for example, about 1 m to 100 m, preferably 5 m to 50 m. With such a thickness, it is possible to secure sufficient electrode performance while reducing the amount of expensive electrode layer material used to reduce costs. The entire region provided with the holes (through holes)  4   a  is covered with the electrode layer  2 . That is, the hole  4   a  is formed inside the region of the metal support  4  where the electrode layer  2  is formed. In other words, all the holes  4   a  are provided facing the electrode layer  2 . 
     As the constituent material of the electrode layer  2 , for example, a composite material such as NiO-GDC, Ni-GDC, NiO—YSZ, Ni—YSZ, CuO—CeO 2 , Cu—CeO 2  can be used. In these examples, GDC, YSZ, and CeO 2  can be referred to as aggregates of the composite material. Preferably, the electrode layer  2  is formed by a low-temperature calcination method (for example, a wet method using a calcination treatment in a low temperature range that does not carry out a calcination treatment in a high temperature range exceeding 1100° C.), a spray coating method (thermal spraying method, aerosol deposition method, aerosol gas deposition method, a powder jet deposition method, particle jet deposition method, cold spray method, or the like), a PVD method (sputtering method, a pulse laser deposition method, or the like), a CVD method, or the like. These processes that can be used in the low temperature range provide an improved electrode layer  2  without using, for example, calcination in a high temperature range higher than 1100° C. Therefore, the metal support  4  is not damaged, element mutual diffusion between the metal support  4  and the electrode layer  2  can be suppressed, and an electrochemical element having excellent durability can be realized. Further, it is more preferable to use the low-temperature calcination method because the handling of the raw material becomes easy. 
     The counter electrode layer  3  can be formed in a thin layer on the surface of the electrolyte layer  1   a  opposite to the electrode layer  2 . In the case of a thin layer, a thickness thereof can be, for example, about 1 m to 100 m, preferably 5 m to 50 am. With such a thickness, it is possible to secure sufficient electrode performance while reducing the amount of expensive counter electrode layer material used to reduce costs. As the material of the counter electrode layer  3 , for example, a composite oxide such as LSCF or LSM, a ceria-based oxide, or a mixture thereof can be used. In particular, it is preferable that the counter electrode layer  3  contains a perovskite-type oxide containing two or more kinds of elements selected from the group consisting of La, Sr, Sm, Mn, Co, and Fe. 
     The electrolyte layer  1   a , the electrode layer  2 , and the counter electrode layer  3  are formed as a thin film as described later, and the inventor calls this thin layer forming. 
     As illustrated above, the electrolytic cell unit U has a metal support type, includes a metal support  4  as a support for the electrode layer  2 , and a supply path forming member  5  for forming a U-shaped electrode layer-side gas supply path  5   a  is provided on a side opposite to the electrode layer  2  in a state where the metal support  4  is interposed therebetween. Further, the metal support  4  is provided with a large number of holes  4   a  penetrating the front and back surfaces. The gas (H 2 O and CO 2 ) supplied through the electrode layer-side gas supply path  5   a  is subject to electrolysis and is supplied to the electrode layer  2  through a large number of holes  4   a . Further, the generated gas (H 2 , CO) is discharged from the hole  4   a.    
     Meanwhile, also on the counter electrode layer  3  side, a supply path forming member  6  for forming a counter electrode layer-side gas supply path  6   a  is provided. As illustrated in  FIG.  2   , the supply path forming member  6  is provided with many grooves on the counter electrode layer  3  side and is configured to supply a transport gas g 2  (for example, air) to the counter electrode layer-side gas supply path  6   a.    
     The metal support  4  supports the electrode layer  2 , the electrolyte layer  1   a , and the counter electrode layer  3  and serves as a support for maintaining the strength of the electrolytic cell  1  and the electrolytic cell unit U as a whole. In this example, the plate-shaped metal support  4  is used as the metal support, but other shapes such as a box shape and a cylindrical shape are also possible. 
     The metal support  4  may have sufficient strength to form the electrolytic cell unit U as a support, and for example, can use a support having 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. In the present embodiment, the support is made of metal, but ceramics can also be used, for example. 
     The metal support  4  has, for example, the plurality of holes  4   a  provided so as to penetrate the front surface and the back surface of the metal plate. For example, the hole  4   a  can be provided in the metal support  4  by mechanical, chemical, or optical drilling. The hole  4   a  has a function of allowing gas to pass from the back surface to the front surface of the metal support  4 . The hole  4   a  may be provided so as to be inclined in a gas advection direction (the front and back directions of the paper surface in  FIG.  2   ). 
     By using a ferrite-based stainless steel material (an example of an Fe—Cr-based alloy) as a material of a base material of the metal support  4 , a thermal expansion coefficient of the metal support  4  can be made close to those of YSZ (yttria-stabilized zirconia), GDC (gadolinium-doped ceria, also referred to as CGO), and the like used as materials for the electrode layer  2  and the electrolyte layer  1   a . Therefore, the electrolytic cell unit U is less likely to be damaged even when the low temperature and high temperature cycles are repeated. Therefore, it is preferable because the electrolytic cell unit U having excellent long-term durability can be realized. 
     The same material as that of the metal support  4  can be used for the supply path forming members  5  and  6  of the electrolytic cell unit U, and the thickness thereof can be substantially the same. 
     Although the metal support  4  and both supply path forming members  5  and  6  have conductivity, they are airtightly configured to function as a separator for separating the supply paths  5   a  and  6   a.    
     In the electrolytic cell unit U having the above configuration, in an electrolysis operation, DC power is supplied between the pair of electrode layers  2  and  3  provided with the electrolyte layer  1   a  interposed therebetween from a power supply unit (illustrated by a battery in  FIG.  2   ). In the present embodiment, as illustrated in  FIG.  2   , the case where the electrode layer  2  side is negative and the counter electrode layer  3  side is positive is illustrated. 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, H 2 O and CO 2 , which are gases to be electrolyzed, are supplied to the electrode layer  2  from an electrolytic raw material supply unit (upstream portion of the electrolytic reaction unit  10  in  FIG.  1   ), and the transport gas g 2  is supplied to the counter electrode layer side. Therefore, the reactions illustrated in the formulas 1 and 2 can be caused in the electrolytic cell  1  and the decomposed gas can be taken out. Here, regarding the supply of H 2 O, either water or water vapor may be used, or both of them may be used. Therefore, in the present invention, an electrolytic cell device is constructed which includes at least the electrolytic cell unit U, the electrolytic raw material supply unit that supplies water and/or water vapor and carbon dioxide to the electrolytic cell unit U, and the power supply unit that supplies electric power. 
     The supplied gas (H 2 O, CO 2 ) and the released gas (H 2 O, H 2 , CO, O 2 , CO 2 ) in the electrolytic reaction are illustrated above and below the electrolytic cell unit U in  FIG.  2   . However, this is for ease of understanding, and in fact, the above-mentioned electrode layer-side gas supply path  5   a  and counter electrode layer-side gas supply path  6   a  are formed so as to extend in the front and back directions of the paper surface of  FIG.  2   , and for example, the gas (H 2 O, CO 2 ) on the supply side described on an upper side of the electrolytic cell unit U in  FIG.  2    can be recovered from the front side of the paper surface, and the gas (H 2 O, H 2 , CO, O 2 , CO 2 ) on the release side described on the lower side of the electrolytic cell  1  can be recovered from the back side of the paper surface (refer to  FIG.  4    described later). In addition, in order to smoothly perform the discharge of O 2  generated in the electrolytic reaction, for example, the transport gas g 2  such as air can flow through the electrolytic cell unit U. 
     When H 2 O and CO 2  are supplied to the electrolytic reaction unit  10  to carry out the electrolysis, H 2 O has a lower electrolytic voltage than CO 2  and is easily electrolyzed. Therefore, when H 2 O and CO 2  having the same amount are temporarily supplied to the electrolytic reaction unit  10  and the electrolytic reaction is carried out, the H 2  concentration tends to be higher than the CO concentration at the outlet of the electrolytic reaction unit  10 , and unreacted CO 2  tends to remain. 
     [First Catalytic Reaction Unit (Reverse Water-Gas Shift Reaction Unit)] 
     As illustrated above, the first catalytic reaction unit  20  (reverse water-gas shift reaction unit) causes a reverse water-gas shift reaction, converts CO 2  into CO using the supplied H 2 , and converts H 2  into H 2 O. That is, in the electrolytic reaction unit  10  that supplies H 2 O and CO 2  to electrolyze, the remaining CO 2  that is not decomposed is converted into CO. 
     The reaction here is as illustrated by the formula 3, but this reaction is an endothermic reaction and is an equilibrium reaction according to the reaction temperature conditions. As a result, as described above, it is preferable that the catalyst is capable of causing the reaction represented by the formula 3 on the high temperature side (for example, 600° C. to 800° C.) as much as possible. 
     In the description of the catalyst in the present specification, a component having activity as a catalyst may be referred to as a “catalytically active component”, and a carrying body carrying the catalytically active component may be referred to as a “carrier (catalyst support)”. 
     The inventors examined various combinations of catalytically active components and carriers as described later, and found that a specific combination was suitable. 
     In a production of this type of catalyst, by executing an impregnation-supporting step of immersing the carrier (metal oxide) in a solution containing a catalytically active component (metal component), taking out the carrier, drying and heat-treating the carrier, it is possible to easily obtain a carrier-support catalyst (impregnated carrier) in which the catalytically active component is distributed on the surface of the carrier. This heat treatment is a calcination treatment. The preparation and use of the catalyst will be described with reference to  FIGS.  11 ( a )-( c ) and  12 ( a ) and ( b ) . 
     A preparation method described here is the same except that the starting material is different in the combination of various catalytically active components and carriers.  FIGS.  11 ( a )-( c )  illustrate examples of the reverse water-gas shift catalyst cat 1  and the hydrocarbon synthesis catalyst cat 2  according to the present invention. In  FIGS.  11 ( a )-( c ) , the catalytically active component of the reverse water-gas shift catalyst cat 1  is referred to as ca 1 , and the carrier thereof is referred to as cb 1 . Meanwhile, regarding the hydrocarbon synthesis catalyst cat 2 , the catalytically active component thereof is ca 2  and the carrier thereof is cb 2 . 
     As illustrated in  FIGS.  11 ( a )-( c ) , in the catalyst preparation, after an impregnation-supporting step (a) of obtaining an aqueous solution of a compound containing a metal component (which is a metal catalyst) to be the catalytically active components ca 1  and ca 2 , inputting the carriers cb 1  and cb 2  into the aqueous solution, and carrying out stirring and impregnation is executed, a drying/crushing/molding step (b) of carrying out evaporative drying, drying, and crushing and molding is executed, and thereafter, a calcination step (c) of calcinating an obtained molded product in the air is executed, and thus, the target product (cat 1 , cat 2 ) can be obtained. Therefore, this form of catalyst is also referred to as an impregnation-supported catalyst. 
     In this case, as illustrated in the example of the reverse water-gas shift catalyst cat 1  in  FIGS.  12 ( a ) and ( b ) , the catalyst can be applied to a portion where the catalyst is used and calcinated.  FIG.  12 ( a )  illustrates a coating/calcination step in which the reverse water-gas shift catalyst cat 1  is applied to the metal support  4  in which the holes  4   a  are perforated to form a coating layer  20   a , and the coating layer  20   a  is calcinated.  FIG.  12 ( b )  illustrates a reduction pretreatment step in which H 2  flows to carry out a reduction pretreatment before using the reverse water-gas shift catalyst cat 1 . 
     When the calcination treatment is carried out in air, the supported catalytically active components ca 1  and ca 2  are in a state where a part or all of them are oxidized. Before using the catalyst, a so-called reduction pretreatment may be carried out to reduce the catalytically active component in an oxidized state to sufficiently enhance the activity.  FIG.  12 ( b )  illustrates a state in which a reducing gas (typically H 2 ) is circulated on the surface of the catalyst to carry out the reduction pretreatment. 
     (Catalyst Used) 
     As the reverse water-gas shift catalyst cat 1  used for the first catalytic reaction unit  20 , the inventors have selected a catalyst that satisfies the following requirements. 
     A catalyst composed by supporting at least one or both of nickel and iron as the catalytically active component ca 1  on the carrier cb 1  containing a ceria-based metal oxide or a zirconia-based metal oxide as a main component. Here, since the strength of the catalyst cat 1  can be increased, a ratio of the carrier cb 1  to the entire catalyst is preferably 55% by weight or more, more preferably 60% by weight or more, and further preferably 65% by weight or more. Further, an upper limit of this ratio can be, for example, 99.5% by weight, but when the upper limit is more than this, the catalytically active component ca 1  cannot be sufficiently supported, and it may be difficult to obtain the effect as the reverse water-gas shift catalyst cat 1 . 
     Further, as the ceria-based metal oxide, ceria doped with at least one of gadolinium, samarium, and yttrium can also be used. 
     Further, as the zirconia-based metal oxide, zirconia stabilized by at least one of yttria and scandia can also be used. 
     Since the reverse water-gas shift reaction can proceed satisfactorily, a supported amount of the catalytically active component ca 1  is preferably 0.5% by weight or more, more preferably 1% by weight or more, and further preferably 5% by weight or more. Further, when the supported amount of the catalytically active component ca 1  is increased too much, it becomes difficult to support the catalytically active component ca 1  in a high dispersion, it is difficult to obtain a significant improvement in the catalytic activity, and the catalyst cost also increases. Accordingly, the supported amount of the catalytically active component ca 1  is preferably 35% by weight or less, more preferably 30% by weight or less, and further preferably 25% by weight or less. 
     Further, it is also preferable to add either one or both of nickel and iron to the catalytically active component ca 1  to support copper as a further catalytically active component ca 1 . In this configuration, the supported amount of copper is equal to or less than the supported amount of the catalytically active component ca 1  with either one or both of nickel and iron as a main catalytically active component ca 1 . 
     Hereinafter, test results of Examples in the case where the catalytically active component ca 1  and the carrier cb 1  are variously changed as the reverse water-gas shift catalyst cat 1  used for the first catalytic reaction unit  20  will be described. 
     As the catalytically active component ca 1 , Ni and Fe were examined and compared with Pt (platinum). 
     As the carrier cb 1 , Al 2 O 3  (alumina) was also examined using ZrO 2  (zirconia), YSZ (yttria-stabilized zirconia), GDC (gadolinium-doped ceria), and CeO 2  (ceria) as examples. 
     In the following description, Test 1 and Test 2 will be introduced, but a difference between the two tests is that in the calcination of the reverse water-gas shift catalyst cat 1 , the calcination temperature of Test 1 is set to 450° C., and the calcination temperature of Test 2 is set to a high temperature side of 600° C. to 1000° C. 
     (Test 1) 
     The test results of Examples (1 to 19) when the carrier is variously changed as the catalyst used for the first catalytic reaction unit  20  will be described. 
     As catalytically active components, Ni and Fe were examined, and Pt (platinum) was also examined. 
     As the carrier, ZrO 2  (zirconia), YSZ (yttria-stabilized zirconia), GDC (gadolinium-doped ceria), and CeO 2  (ceria) were used as examples, and Al 2 O 3  (alumina) was also examined. 
     (Catalyst Preparation) 
     In preparing the reverse water-gas shift catalyst cat 1 , an aqueous solution is obtained by quantifying and dissolving any one or both of a water-soluble nickel compound (nickel nitrate, nickel chloride, nickel sulfate, nickel ammonium sulfate, nickel acetate, nickel oxalate, nickel citrate, or the like) and a water-soluble iron compound (iron nitrate, iron chloride, iron sulfate, ammonium iron sulfate, iron acetate, iron oxalate, iron citrate, or the like) according to the composition of the target catalyst. Further, when supporting copper as another catalytically active component ca 1 , an aqueous solution is obtained by similarly quantifying and dissolving a water-soluble copper compound (copper nitrate, copper chloride, copper sulfate, ammonium copper sulfate, copper acetate, copper oxalate, copper citrate, or the like). A predetermined amount of carrier powder (ceria, zirconia, GDC, YSZ, Al 2 O 3 ) is added to the aqueous solution, stirred and impregnated, then evaporated to dryness, dried, then crushed and molded, and then calcinated in air. This impregnation is the “impregnation-supporting step” referred to in the present invention, and the result is the “impregnated carrier”. 
     The catalysts of the following examples were prepared using nickel nitrate hexahydrate, iron nitrate nonahydrate, and copper nitrate trihydrate, respectively. The catalyst using Pt was prepared using tetraammine platinum hydroxide. 
     In the above catalyst preparation, temperatures of evaporation to dryness, drying, and calcination could be carried out in a generally used temperature range, but in Test 1, the catalysts of the following examples were each at 80° C., 80° C., and 450° C. 
     Table 1 illustrates Examples 1 to 19 of the reverse water-gas shift catalyst cat 1  in the present invention. 
     A horizontal axis represents the type of carrier cb 1 , a metal supported amount (% by weight; expressed as wt. % in the table) as the catalytically active component, a CO adsorption amount (ml/g), and a BET surface area (m 2 /g). 
     Regarding the CO adsorption amount, the CO adsorption amount was measured after the catalyst was subjected to a reduction pretreatment at 350° C. for 1 hour under a hydrogen atmosphere. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Metal supported 
                 CO adsorption 
                 BET surface 
               
               
                 Catalyst 
                 Carrier 
                 amount (wt. %) 
                 amount (Nml/g) 
                 area (m 2 /g) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Example 1 
                 Ni/ZrO 2   
                 ZrO 2   
                 Ni: 9.5 
                 1.48 
                 11.1 
               
               
                 Example 2 
                 Ni/8YSZ 
                 8YSZ 
                 Ni: 9.5 
                 1.97 
                 11.3 
               
               
                 Example 3 
                 Ni/GDC 
                 GDC 
                 Ni: 9.1 
                 3.61 
                 14.3 
               
               
                 Example 4 
                 Ni/CeO 2   
                 CeO 2   
                 Ni: 14 
                 0.47 
                 9.4 
               
               
                 Example 5 
                 Ni—Fe/CeO 2   
                 CeO 2   
                 Ni: 9.1 Fe: 0.46 
                 0.45 
                 8.9 
               
               
                 Example 6 
                 Ni—Cu/CeO 2   
                 CeO 2   
                 Ni: 9.2 Cu: 0.49 
                 0.78 
                 10.6 
               
               
                 Example 7 
                 Ni/Al 2 O 3   
                 Al 2 O 3   
                 Ni: 8.9 
                 0.65 
                 90.7 
               
               
                 Example 8 
                 Fe/ZrO 2   
                 ZrO 2   
                 Fe: 9.6 
                 0.88 
                 12.0 
               
               
                 Example 9 
                 Fe/8YSZ 
                 8YSZ 
                 Fe: 9.5 
                 0.22 
                 7.5 
               
               
                 Example 10 
                 Fe/GDC 
                 GDC 
                 Fe: 9.2 
                 0.30 
                 15.2 
               
               
                 Example 11 
                 Fe/CeO 2   
                 CeO 2   
                 Fe: 9.3 
                 0.53 
                 10.3 
               
               
                 Example 12 
                 Fe—Ni/ZrO 2   
                 ZrO 2   
                 Fe: 9.7 Ni: 0.49 
                 0.52 
                 13.0 
               
               
                 Example 13 
                 Fe—Cu/ZrO 2   
                 ZrO 2   
                 Fe: 9.7 Cu: 0.50 
                 0.21 
                 10.5 
               
               
                 Example 14 
                 Fe/Al 2 O 3   
                 Al 2 O 3   
                 Fe: 8.8 
                 0.31 
                 82.8 
               
               
                 Example 15 
                 Pt/ZrO 2   
                 ZrO 2   
                 Pt: 0.95 
                 0.95 
                 11.2 
               
               
                 Example 16 
                 Pt/8YSZ 
                 8YSZ 
                 Pt: 0.92 
                 1.18 
                 4.8 
               
               
                 Example 17 
                 Pt/GDC 
                 GDC 
                 Pt: 0.96 
                 1.10 
                 10.0 
               
               
                 Example 18 
                 Pt/CeO 2   
                 CeO 2   
                 Pt: 0.95 
                 1.17 
                 7.9 
               
               
                 Example 19 
                 Pt/Al 2 O 3   
                 Al 2 O 3   
                 Pt: 0.95 
                 1.85 
                 97.8 
               
               
                   
               
            
           
         
       
     
     (Catalytic Activity Test) 
     In the catalytic activity test, a mixed gas of 50% H 2 -50% CO 2  (a mixed gas containing H 2  and CO 2  in a ratio of 1:1 (volume ratio)) was used as a reaction gas, and the reaction temperature was changed from 600° C. to 800° C. in increments of 50° C. under the conditions in which a Gas Hourly Space Velocity (GHSV) was 10000/h. 
     Before conducting the catalytic activity test, the reduction pretreatment of the catalyst was carried out at 600° C. while flowing a hydrogen gas through the catalyst layer. As the test results, a CO 2  conversion rate (%), a CO concentration (%) at the outlet of the reaction unit, and a CH 4  concentration (%) are illustrated in Table 2. 
     The CO 2  conversion rate (%) was calculated according to the following formula based on a gas analysis result at the outlet of the catalyst layer. 
       [CH 4 concentration]+[CO concentration]/([CH 4 concentration]+[CO concentration]+[CO 2 concentration]) 
     As illustrated above, in the reverse water-gas shift catalyst cat used in the first catalytic reaction unit  20  (reverse water-gas shift reaction unit), it is desirable that the CO 2  conversion rate (%) on the high temperature side (for example, around 600 to 800° C.) is high. 
     
       
         
           
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Reaction temperature (° C.) 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Catalyst 
                   
                 600 
                 650 
                 700 
                 750 
                 800 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Example 1 
                 Ni/ZrO 2   
                 CO 2  conversion rate (%) 
                 31.8 
                 37.6 
                 41.6 
                 44.0 
                 46.1 
               
               
                   
                   
                 Outlet CO concentration (%) 
                 17.9 
                 23.1 
                 25.8 
                 28.0 
                 29.7 
               
               
                   
                   
                 Outlet CH 4  concentration (%) 
                 3.7 
                 0.9 
                 0.3 
                 0.1 
                 0.1 
               
               
                 Example 2 
                 Ni/8YSZ 
                 CO 2  conversion rate (%) 
                 32.7 
                 36.4 
                 37.5 
                 39.3 
                 40.7 
               
               
                   
                   
                 Outlet CO concentration (%) 
                 16.9 
                 21.1 
                 22.7 
                 23.7 
                 24.8 
               
               
                   
                   
                 Outlet CH 4  concentration (%) 
                 4.0 
                 1.3 
                 0.6 
                 0.4 
                 0.2 
               
               
                 Example 3 
                 Ni/GDC 
                 CO 2  conversion rate (%) 
                 29.9 
                 33.8 
                 36.2 
                 38.5 
                 39.6 
               
               
                   
                   
                 Outlet CO concentration (%) 
                 14.3 
                 18.8 
                 21.1 
                 22.8 
                 24.3 
               
               
                   
                   
                 Outlet CH 4  concentration (%) 
                 5.1 
                 2.0 
                 0.9 
                 0.4 
                 0.2 
               
               
                 Example 4 
                 Ni/CeO 2   
                 CO 2  conversion rate (%) 
                 34.5 
                 38.9 
                 41.9 
                 44.7 
                 47.4 
               
               
                   
                   
                 Outlet CO concentration (%) 
                 18.4 
                 23.3 
                 25.8 
                 27.9 
                 30.2 
               
               
                   
                   
                 Outlet CH 4  concentration (%) 
                 3.4 
                 0.8 
                 0.2 
                 0.1 
                 0.0 
               
               
                 Example 5 
                 Ni—Fe/CeO 2   
                 CO 2  conversion rate (%) 
                 34.1 
                 40.0 
                 42.0 
                 45.6 
                 47.7 
               
               
                   
                   
                 Outlet CO concentration (%) 
                 18.6 
                 23.6 
                 27.3 
                 29.0 
                 30.5 
               
               
                   
                   
                 Outlet CH 4  concentration (%) 
                 3.4 
                 0.8 
                 0.2 
                 0.1 
                 0.0 
               
               
                 Example 6 
                 Ni—Cu/CeO 2   
                 CO 2  conversion rate (%) 
                 35.2 
                 41.0 
                 44.2 
                 46.9 
                 48.6 
               
               
                   
                   
                 Outlet CO concentration (%) 
                 19.2 
                 24.4 
                 27.3 
                 29.1 
                 31.0 
               
               
                   
                   
                 Outlet CH 4  concentration (%) 
                 3.3 
                 0.8 
                 0.2 
                 0.1 
                 0.0 
               
               
                 Example 7 
                 Ni/Al 2 O 3   
                 CO 2  conversion rate (%) 
                 28.5 
                 33.6 
                 35.9 
                 37.4 
                 39.5 
               
               
                   
                   
                 Outlet CO concentration (%) 
                 15.4 
                 19.3 
                 21.6 
                 23.2 
                 23.9 
               
               
                   
                   
                 Outlet CH 4  concentration (%) 
                 4.6 
                 1.8 
                 0.8 
                 0.6 
                 0.4 
               
               
                 Example 8 
                 Fe/ZrO 2   
                 CO 2  conversion rate (%) 
                 39.7 
                 42.3 
                 45.1 
                 47.5 
                 49.6 
               
               
                   
                   
                 Outlet CO concentration (%) 
                 23.0 
                 25.4 
                 27.2 
                 29.3 
                 31.2 
               
               
                   
                   
                 Outlet CH 4  concentration (%) 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 Example 9 
                 Fe/8YSZ 
                 CO 2  conversion rate (%) 
                 36.3 
                 40.4 
                 43.1 
                 45.9 
                 47.5 
               
               
                   
                   
                 Outlet CO concentration (%) 
                 22.5 
                 25.4 
                 27.6 
                 29.3 
                 31.2 
               
               
                   
                   
                 Outlet CH 4  concentration (%) 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 Example 10 
                 Fe/GDC 
                 CO 2  conversion rate (%) 
                 35.8 
                 40.2 
                 42.5 
                 44.6 
                 46.8 
               
               
                   
                   
                 Outlet CO concentration (%) 
                 21.9 
                 25.2 
                 27.0 
                 28.6 
                 30.4 
               
               
                   
                   
                 Outlet CH 4  concentration (%) 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 Example 11 
                 Fe/CeO 2   
                 CO 2  conversion rate (%) 
                 37.2 
                 40.9 
                 43.5 
                 45.4 
                 48.3 
               
               
                   
                   
                 Outlet CO concentration (%) 
                 22.9 
                 25.3 
                 27.6 
                 29.6 
                 31.4 
               
               
                   
                   
                 Outlet CH 4  concentration (%) 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 Example 12 
                 Fe—Ni/ZrO 2   
                 CO 2  conversion rate (%) 
                 38.1 
                 41.6 
                 44.0 
                 46.8 
                 48.3 
               
               
                   
                   
                 Outlet CO concentration (%) 
                 23.3 
                 25.4 
                 27.6 
                 29.6 
                 31.5 
               
               
                   
                   
                 Outlet CH 4  concentration (%) 
                 0.1 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 Example 13 
                 Ni—Cu/CeO 2   
                 CO 2  conversion rate (%) 
                 36.5 
                 41.3 
                 45.1 
                 47.3 
                 49.2 
               
               
                   
                   
                 Outlet CO concentration (%) 
                 22.9 
                 25.3 
                 27.5 
                 29.6 
                 31.4 
               
               
                   
                   
                 Outlet CH 4  concentration (%) 
                 0.3 
                 0.1 
                 0.0 
                 0.0 
                 0.0 
               
               
                 Example 14 
                 Fe/Al 2 O 3   
                 CO 2  conversion rate (%) 
                 22.5 
                 27.6 
                 33.7 
                 40.0 
                 45.0 
               
               
                   
                   
                 Outlet CO concentration (%) 
                 12.7 
                 15.8 
                 20.1 
                 24.5 
                 28.6 
               
               
                   
                   
                 Outlet CH 4  concentration (%) 
                 0.1 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 Example 15 
                 Pt/ZrO 2   
                 CO 2  conversion rate (%) 
                 34.0 
                 39.8 
                 43.0 
                 45.7 
                 48.2 
               
               
                   
                   
                 Outlet CO concentration (%) 
                 19.0 
                 24.4 
                 27.0 
                 29.2 
                 31.3 
               
               
                   
                   
                 Outlet CH 4  concentration (%) 
                 3.2 
                 0.7 
                 0.2 
                 0.1 
                 0.0 
               
               
                 Example 16 
                 Pt/8YSZ 
                 CO 2  conversion rate (%) 
                 35.1 
                 40.7 
                 44.1 
                 46.4 
                 48.8 
               
               
                   
                   
                 Outlet CO concentration (%) 
                 19.3 
                 24.5 
                 27.3 
                 29.4 
                 31.3 
               
               
                   
                   
                 Outlet CH 4  concentration (%) 
                 3.2 
                 0.7 
                 0.2 
                 0.1 
                 0.0 
               
               
                 Example 17 
                 Pt/GDC 
                 CO 2  conversion rate (%) 
                 32.9 
                 38.6 
                 42.5 
                 45.2 
                 47.8 
               
               
                   
                   
                 Outlet CO concentration (%) 
                 18.6 
                 23.5 
                 26.4 
                 28.8 
                 30.6 
               
               
                   
                   
                 Outlet CH 4  concentration (%) 
                 3.3 
                 0.8 
                 0.2 
                 0.1 
                 0.0 
               
               
                 Example 18 
                 Pt/CeO 2   
                 CO 2  conversion rate (%) 
                 34.9 
                 39.4 
                 43.1 
                 45.6 
                 48.3 
               
               
                   
                   
                 Outlet CO concentration (%) 
                 19.7 
                 24.1 
                 26.5 
                 29.4 
                 31.4 
               
               
                   
                   
                 Outlet CH 4  concentration (%) 
                 2.7 
                 0.8 
                 0.2 
                 0.1 
                 0.0 
               
               
                 Example 19 
                 Pt/Al 2 O 3   
                 CO 2  conversion rate (%) 
                 35.5 
                 41.1 
                 44.7 
                 47.5 
                 49.6 
               
               
                   
                   
                 Outlet CO concentration (%) 
                 19.4 
                 24.6 
                 27.6 
                 29.7 
                 31.2 
               
               
                   
                   
                 Outlet CH 4  concentration (%) 
                 3.2 
                 0.8 
                 0.2 
                 0.1 
                 0.0 
               
               
                   
               
            
           
         
       
     
     (Test 2) 
     Hereinafter, the test results of Examples (20 to 29) of Test 2 will be described. Even in this example, 
     Ni and Fe were examined as catalytically active components, and the addition of Cu was also examined. 
     As the carrier, CeO 2  (ceria) and ZrO 2  (zirconia) are used as examples, and Al 2 O 3  (alumina) is also examined. 
     (Catalyst Preparation) 
     The reverse water-gas shift catalyst cat 1  used in Test 2 was prepared in the same manner as in Test 1 except that the calcination temperatures were changed to 600° C., 800° C., and 1000° C. 
     Table 3 illustrates the catalyst of each of Examples (20 to 29) prepared. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                   
                   
                 Calcination 
                   
                   
               
               
                   
                   
                 temperature 
                 CO adsorption 
                 BET surface 
               
               
                 Catalyst 
                 Carrier 
                 (° C.) 
                 amount (Nml/g) 
                 area (m 2 /g) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Example 20 
                 Ni/CeO 2   
                 CeO 2   
                 600 
                 0.9 
                 8.9 
               
               
                 Example 21 
                 Ni—Cu/CeO 2   
                 CeO 2   
                 600 
                 0.53 
                 9.1 
               
               
                 Example 22 
                 Fe/ZrO 2   
                 ZrO 2   
                 600 
                 0.27 
                 13.4 
               
               
                 Example 23 
                 Fe/Al 2 O 3   
                 Al 2 O 3   
                 600 
                 0.15 
                 96.0 
               
               
                 Example 24 
                 Ni/CeO 2   
                 CeO 2   
                 800 
                 0.26 
                 7.6 
               
               
                 Example 25 
                 Ni—Cu/CeO 2   
                 CeO 2   
                 800 
                 0.51 
                 7.6 
               
               
                 Example 26 
                 Fe/ZrO 2   
                 ZrO 2   
                 800 
                 0.16 
                 10.1 
               
               
                 Example 27 
                 Fe/Al 2 O 3   
                 Al 2 O 3   
                 800 
                 0.14 
                 83.5 
               
               
                 Example 28 
                 Ni/CeO 2   
                 CeO 2   
                 1000 
                 0.08 
                 5.2 
               
               
                 Example 29 
                 Fe/ZrO 2   
                 ZrO 2   
                 1000 
                 0.29 
                 8.1 
               
               
                   
               
            
           
         
       
     
     (Catalytic Activity Test) 
     In the catalytic activity test, a mixed gas containing H 2  and CO 2  in a ratio of 1:1 (volume ratio) was used as a reaction gas, and the reaction temperature was changed from 600° C. to 800° C. in increments of 50° C. under the conditions in which GHSV was 10000/h. 
     Before conducting the catalytic activity test, the reduction pretreatment of the catalyst was carried out at 600° C. while flowing a hydrogen gas through the catalyst layer. 
     As the test results, the CO 2  conversion rate (%), the CO concentration (%) at the outlet of the reaction unit, and the CH 4  concentration (%) are illustrated in Table 4. 
     
       
         
           
               
               
             
               
                   
                 TABLE 4 
               
             
            
               
                   
                   
               
               
                   
                 Reaction temperature (° C.) 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Catalyst 
                   
                 600 
                 650 
                 700 
                 750 
                 800 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Example 20 
                 Ni/CeO 2   
                 CO 2  conversion rate (%) 
                 33.8 
                 39.5 
                 42.8 
                 45.7 
                 48.1 
               
               
                   
                   
                 Outlet CO concentration (%) 
                 18.8 
                 24.2 
                 27.4 
                 29.6 
                 31.4 
               
               
                   
                   
                 Outlet CH 4  concentration (%) 
                 3.3 
                 0.8 
                 0.2 
                 0.0 
                 0.0 
               
               
                 Example 21 
                 Ni—Cu/CeO 2   
                 CO 2  conversion rate (%) 
                 34.7 
                 40.4 
                 42.9 
                 44.6 
                 46.7 
               
               
                   
                   
                 Outlet CO concentration (%) 
                 19.1 
                 24.4 
                 27.0 
                 28.6 
                 29.9 
               
               
                   
                   
                 Outlet CH 4  concentration (%) 
                 3.7 
                 0.9 
                 0.2 
                 0.1 
                 0.0 
               
               
                 Example 22 
                 Fe/ZrO 2   
                 CO 2  conversion rate (%) 
                 38.1 
                 40.4 
                 43.1 
                 45.7 
                 48.2 
               
               
                   
                   
                 Outlet CO concentration (%) 
                 23.3 
                 25.8 
                 28.0 
                 30.2 
                 31.8 
               
               
                   
                   
                 Outlet CH 4  concentration (%) 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 Example 23 
                 Fe/Al 2 O 3   
                 CO 2  conversion rate (%) 
                 24.8 
                 29.0 
                 35.3 
                 40.6 
                 45.3 
               
               
                   
                   
                 Outlet CO concentration (%) 
                 14.0 
                 16.8 
                 20.8 
                 25.3 
                 29.5 
               
               
                   
                   
                 Outlet CH 4  concentration (%) 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 Example 24 
                 Ni/CeO 2   
                 CO 2  conversion rate (%) 
                 33.9 
                 39.2 
                 42.7 
                 45.6 
                 47.1 
               
               
                   
                   
                 Outlet CO concentration (%) 
                 19.2 
                 24.6 
                 27.4 
                 29.5 
                 31.7 
               
               
                   
                   
                 Outlet CH 4  concentration (%) 
                 3.3 
                 0.7 
                 0.2 
                 0.0 
                 0.0 
               
               
                 Example 25 
                 Ni—Cu/CeO 2   
                 CO 2  conversion rate (%) 
                 34.2 
                 39.3 
                 41.9 
                 44.3 
                 46.6 
               
               
                   
                   
                 Outlet CO concentration (%) 
                 18.5 
                 23.9 
                 26.0 
                 28.7 
                 30.7 
               
               
                   
                   
                 Outlet CH 4  concentration (%) 
                 3.6 
                 0.9 
                 0.1 
                 0.1 
                 0.0 
               
               
                 Example 26 
                 Fe/ZrO 2   
                 CO 2  conversion rate (%) 
                 38.1 
                 41.2 
                 43.7 
                 46.2 
                 48.4 
               
               
                   
                   
                 Outlet CO concentration (%) 
                 23.5 
                 25.7 
                 28.1 
                 30.0 
                 31.9 
               
               
                   
                   
                 Outlet CH 4  concentration (%) 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 Example 27 
                 Fe/Al 2 O 3   
                 CO 2  conversion rate (%) 
                 22.2 
                 25.2 
                 32.7 
                 38.9 
                 43.9 
               
               
                   
                   
                 Outlet CO concentration (%) 
                 13.4 
                 15.1 
                 20.0 
                 24.5 
                 29.1 
               
               
                   
                   
                 Outlet CH 4  concentration (%) 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 Example 28 
                 Ni/CeO 2   
                 CO 2  conversion rate (%) 
                 34.5 
                 40.6 
                 44.5 
                 46.3 
                 48.7 
               
               
                   
                   
                 Outlet CO concentration (%) 
                 19.1 
                 24.6 
                 28.6 
                 30.4 
                 31.8 
               
               
                   
                   
                 Outlet CH 4  concentration (%) 
                 3.5 
                 0.8 
                 0.2 
                 0.1 
                 0.0 
               
               
                 Example 29 
                 Fe/ZrO 2   
                 CO 2  conversion rate (%) 
                 37.8 
                 41.2 
                 44.5 
                 46.0 
                 48.1 
               
               
                   
                   
                 Outlet CO concentration (%) 
                 22.6 
                 25.5 
                 27.6 
                 29.5 
                 31.3 
               
               
                   
                   
                 Outlet CH 4  concentration (%) 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Reference 
                 CO 2  conversion rate (%) 
                 34.6 
                 39.9 
                 43.6 
                 46.4 
                 49.0 
               
               
                 (equilibrium value) 
               
               
                   
               
            
           
         
       
     
     For reference, an equilibrium value (calculated value) of the CO 2  conversion rate under the experimental conditions is illustrated in Table 4. 
     Iron/Zirconia Catalyst and Iron/Alumina Catalyst 
     For the iron/zirconia catalyst, the test results when the calcination temperatures are 450° C., 600° C., 800° C., and 1000° C. are illustrated in Example 8, Example 22, Example 26, and Example 29, respectively. Meanwhile, for the iron/alumina catalyst, the test results when the calcination temperatures are 450° C., 600° C., and 800° C. are illustrated in Example 14, Example 23, and Example 27, respectively. As can be seen from these results, although the metal supported amount is slightly different, the iron/zirconia catalyst is superior to the iron/alumina catalyst in the activity in carrying out the reverse water-gas shift reaction. Further, the iron/zirconia catalyst has extremely high catalytic activity not only when the calcination temperature is 450° C. but also when the calcination temperature is as high as 600° C., 800° C., and 1000° C., and regardless of the calcination temperature, the CO 2  conversion rate of the iron/zirconia catalyst reaches the vicinity of the equilibrium value. 
     Nickel/Ceria Catalyst 
     The test results when the calcination temperatures are 450° C., 600° C., 800° C., and 1000° C. are illustrated in Example 4, Example 20, Example 24, and Example 28, respectively. As can be seen from these results, the nickel/ceria catalyst has extremely high catalytic activity not only when the calcination temperature is 450° C. but also when the calcination temperature is as high as 600° C., 800° C., and 1000° C., and regardless of the calcination temperature, the CO 2  conversion rate of nickel/ceria catalyst reaches the vicinity of the equilibrium value. 
     Nickel/Alumina Catalyst 
     Example 7 illustrates the test results when the calcination temperature is 450° C. As a result, the nickel/alumina catalyst had a lower CO 2  conversion rate than the nickel/ceria catalyst described above. 
     Nickel/Copper/Ceria Catalyst 
     The test results when the calcination temperatures are 450° C., 600° C., and 800° C. are illustrated in Example 6, Example 21, and Example 25, respectively. From these results, it can be seen that the CO 2  conversion rate of the nickel/copper/ceria catalysts tends to decrease slightly when the calcination temperature thereof is as high as 600° C. or 800° C., but the calcination temperature conditions described above are superior to those of the similar iron/alumina catalyst. Further, in the nickel/copper/ceria catalyst having the calcination temperature of 450° C., the CO 2  conversion rate reaches the vicinity of the equilibrium value. 
     Usefulness as Reverse Water-Gas Shift Catalyst 
     As illustrated above, the iron/zirconia-based catalysts and the nickel/ceria-based catalysts exhibit extremely high reverse water-gas shift catalytic activity even when the calcination temperature is variously changed to 450° C. to 1000° C., and thus, for example, even when used in combination with a solid oxide type electrolytic cell used in a high temperature range of around 600° C. to 800° C., it is easy to secure high performance and durability, which is useful. 
     From the above results, as illustrated above, as the reverse water-gas shift catalyst cat 1  used for the first catalytic reaction unit  20 , the catalyst obtained by supporting at least one or both of nickel and iron as the catalytically active component ca 1  on the carrier cb 1  containing the ceria-based metal oxide or the zirconia-based metal oxide as a main component can be used. 
     Further, as the ceria-based metal oxide as the carrier cb 1 , ceria doped with at least one of gadolinium, samarium, and yttrium can also be used. 
     Further, the zirconia-based metal oxide as the carrier cb 1  can be zirconia stabilized by at least one of yttria and scandia. 
     Further, it is also preferable to add either one or both of nickel and iron to the catalytically active component ca 1  to support copper as a further catalytically active component ca 1 . 
     By using the reverse water-gas shift catalyst cat 1  in the first catalytic reaction unit  20  (reverse water-gas shift reaction unit), the reverse water-gas shift reaction can be carried out at around 600 to 1000° C. with the CO 2  conversion rate (%) equal to or higher than that of the Pt catalyst, which is highly active but very expensive. 
     Since the test of this example was carried out under a very high GHSV condition of 10000/h, by reducing the GHSV to less than 10000/h, that is, by increasing the amount of catalyst used with respect to the amount of gas to be treated, it is possible to carry out the reverse water-gas shift reaction at a higher CO 2  conversion rate (%). 
     [Combination of Electrolytic Reaction Unit and Reverse Water-Gas Shift Reaction Unit] 
     In the description so far, according to the system configuration illustrated in  FIG.  1   , the electrolytic reaction unit  10  and the reverse water-gas shift reaction unit  20  are individually provided in the order described along the advection direction of the gas. 
     The reaction of the electrolytic reaction unit  10  is an exothermic reaction depending on the reaction conditions, and the reaction of the reverse water-gas shift reaction unit  20  is an endothermic reaction. Therefore, thermal efficiency of the system can be improved by integrating the two reaction units  10  and  20 . In this way,  FIG.  3    illustrates a configuration in which the two reaction units  10  and  20  are combined and integrated, and the integration is illustrated to surround both units. In addition, a reaction when integrated in the same box in this way is illustrated. Basically, the above-mentioned formula 1, 2, and 3 are carried out. In a case where the electrolytic reaction unit  10  and the reverse water-gas shift reaction unit  20  are combined and integrated, preferably, when the units are surrounded together by a heat insulating member, heat can be efficiently transferred between the electrolytic reaction unit  10  and the reverse water-gas shift reaction unit  20 . Further, in order to transfer the heat generated in the electrolytic reaction unit  10  to the reverse water-gas shift reaction unit  20 , the electrolytic reaction unit  10  and the reverse water-gas shift reaction unit  20  may be connected using a heat transfer member. 
     [Electrolytic Cell Unit Equipped with Both Electrolytic Reaction Unit and Reverse Water-Gas Shift Reaction Unit] 
     Based on the above concept, it is preferable to provide the reverse water-gas shift reaction unit  20  in the electrolytic cell unit U which is the electrolytic reaction unit  10 . This is because when a solid oxide electrolytic cell that operates at around 600 to 800° C. is used as the electrolytic cell  1 , in the reverse water-gas shift catalyst cat 1  of the present application which can obtain high activity at around 600 to 800° C., the electrolytic reaction unit  10  and the reverse water-gas shift reaction unit  20  can be used in the same temperature range. 
     In this case as well, it is sufficient that the gas that has passed through the electrolytic reaction unit  10  is guided to the reverse water-gas shift reaction unit  20  to generate the reverse water-gas shift reaction. 
       FIG.  4    illustrates an electrolytic cell unit U provided with such a reverse water-gas shift reaction unit  20 .  FIG.  4    is a diagram illustrating the electrolytic cell unit U illustrated in cross section in  FIG.  2    including the advection direction of the gas. 
     As illustrated in  FIG.  4   , the cross sections of the electrolytic cell unit U are basically the same. 
     That is, the electrolytic cell unit U also includes the electrolytic cell  1  in which the electrode layer  2  and the counter electrode layer  3  are formed with the electrolyte layer  1   a  interposed therebetween, the metal support  4  which functions as a support thereof and also acts as a separator, and the supply path forming members  5  and  6 , and the electrode layer-side gas supply path  5   a  and the counter electrode layer-side gas supply path  6   a  are formed in the electrolytic cell unit U. More specifically, as can be seen from  FIG.  4   , when the metal support  4  is viewed in the flow direction of gas, the holes  4   a  are provided in the portion corresponding to the electrolytic cell  1 , but the hole is not provided on the downstream side of the electrode layer  2 . Therefore, the metal support  4  is a separator which effectively separates the gas which is supplied to the electrode layer  2  and released from the electrode layer  2 , and the gas which is supplied to the counter electrode layer  3  gas and is released from the counter electrode layer  3 . 
     However, in this example, the reverse water-gas shift catalyst cat 1  described above is applied to an inner surface (supply path-side inner surface of supply path forming member  5 , surface of the metal support  4  opposite to surface on which electrode layer  2  is formed, and surfaces of the plurality of holes  4   a ) of the electrode layer-side gas supply path  5   a . A coating layer  20   a  is illustrated by a thick solid line. 
     Further, the electrode layer-side gas supply path  5   a  extends beyond the electrolytic reaction unit  10 , and the coating layer  20   a  is also provided on the extension side. 
     As a result, the electrode layer-side gas supply path  5   a  of the electrolytic cell unit U is a discharge path for discharging at least H 2  generated in the electrode layer  2 , and the electrolytic cell unit U is integrally provided with the electrolytic reaction unit  10  and the reverse water-gas shift reaction unit  20 . 
     In this configuration, the metal support  4  acts as a separator that separates H 2  generated in the electrode layer  2  and O 2  generated in the counter electrode layer  3 , and at least a portion of the separator on the discharge path side of H 2  is reverse water-gas shift reaction unit  20 . 
     By stacking the electrolytic cell units U configured in this way in a right-left direction of  FIGS.  2  and  4   , a large number of electrolytic cell units U are stacked, and it is possible to form a so-called electrolytic cell module (not illustrated) in which the electrolytic cell units are electrically connected to each other. Of course, a useful gas generated can be obtained over multiple layers. 
     The inventors have stored a granular reverse water-gas shift catalyst cat 1  in the electrode layer-side gas supply path  5   a  and conducted an experiment under a concept in which the electrolytic reaction unit  10  and the reverse water-gas shift reaction unit  20  are combined with each other (the electrode layer-side gas supply path  5   a  of the electrolytic reaction unit  10  is the reverse water-gas shift reaction unit  20 ). 
       FIG.  5    illustrates a cross section of the electrolytic cell unit U used in this experiment. 
     Hereinafter, a specific description will be given with reference to  FIG.  5   .  FIG.  5    illustrates a cross-sectional view of the electrolytic cell unit U. 
     Here, as the electrolytic cell  1 , a metal-supported solid oxide electrolytic cell was used. As the metal support  4 , a metal substrate was prepared by providing a plurality of through holes (which become holes  4   a ) by applying laser processing to a ferritic stainless steel metal plate having a thickness of 0.3 mm. The electrode layer  2  and an intermediate layer  2   a  were formed in this order on the metal substrate, and the electrolyte layer  1   a  was formed on the intermediate layer  2   a  of the metal substrate so as to cover the intermediate layer  2   a . Further, a reaction prevention layer  7  and the counter electrode layer  3  were sequentially formed on the electrolyte layer  1   a  to prepare the electrolytic cell  1 . A mixture of NiO powder and GDC powder was used as the material for forming the electrode layer  2 , GDC powder was used as the material for forming the intermediate layer  2   a,  8YSZ (8 mol % yttria-stabilized zirconia) powder was used as the material for forming the electrolyte layer  1   a , GDC powder was used as the material for forming the reaction prevention layer  7 , and a mixture of and GDC powder and LSCF powder was used as the material for forming the counter electrode layer  3 . Further, the thicknesses of the electrode layer  2 , the intermediate layer  2   a , the electrolyte layer  1   a , the reaction prevention layer  7 , and the counter electrode layer  3  were about 25 m, about 10 m, about 5 m, about 5 m, and about 20 m, respectively. By providing the intermediate layer  2   a  between the electrode layer  2  and the electrolyte layer  1   a  and providing the reaction prevention layer  7  between the electrolyte layer  1   a  and the counter electrode layer  3 , the performance and durability of the electrolytic cell  1  can be improved. Moreover, preferably, the intermediate layer  2   a  and the reaction prevention layer  7  are formed by a low-temperature calcination method (for example, a wet method using a calcination treatment in a low temperature range that does not carry out a calcination treatment in a high temperature range exceeding 1100° C.), a spray coating method (thermal spraying method, aerosol deposition method, aerosol gas deposition method, a powder jet deposition method, particle jet deposition method, cold spray method, or the like), a PVD method (sputtering method, a pulse laser deposition method, or the like), a CVD method, or the like. These processes that can be used in the low temperature range provide the improved intermediate layer  2   a  and reaction prevention layer  7  without using, for example, calcination in a high temperature range higher than 1100° C. Therefore, it is preferable because the electrolytic cell  1  having excellent performance and durability can be realized without damaging the metal support  4 . Further, it is more preferable to use the low-temperature calcination method because the handling of the raw material becomes easy. 
     Regarding the electrolytic cell unit U obtained as described above, the performance improvement in the case where the reverse water-gas shift catalyst cat 1  formed in the form of particles was stored in the electrode layer-side gas supply path  5   a  (which also serves as the discharge path of the gas electrolyzed by the electrolytic reaction unit  10 ) was examined. 
     Results when Reverse Water-Gas Shift Catalyst Cat 1  is not Stored 
     An electrolytic reaction was carried out while supplying a gas containing H 2 O and CO 2  to the electrolytic cell unit U, and a ratio of H 2  to CO of an outlet gas of the electrolytic cell unit U was measured using a gas chromatograph. The results are illustrated in Table 5 below. The experimental results are described as Comparative Examples A1 and A2. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 5 
               
               
                   
                   
               
               
                   
                   
                   
                 Reaction 
                 Ratio of 
               
               
                   
                   
                 Electrolytic 
                 temperature 
                 H 2 /CO of 
               
               
                   
                 Inlet gas 
                 voltage (V) 
                 (° C.) 
                 outlet gas 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Comparative 
                 52%H 2 O—13%CO 2 —N 2   
                 1.2 
                 700 
                 14.2 
               
               
                 Example A1 
                 balance 
               
               
                 Comparative 
                 49%H 2 O—17%CO 2 —N 2   
                 1.2 
                 700 
                 9.9 
               
               
                 Example A2 
                 balance 
               
               
                   
               
            
           
         
       
     
     Results when the Reverse Water-Gas Shift Catalyst Cat 1  is Stored 
     As the reverse water-gas shift catalyst cat 1 , a granular catalyst obtained by supporting about 10% of Ni on the 8YSZ carrier similar to in Example 2 was stored, an electrolytic reaction was carried out while supplying a gas containing H 2 O and CO 2  to the electrolytic cell unit U, and the ratio of H 2  to CO of the outlet gas of the electrolytic cell unit U was measured using a gas chromatograph. The results are illustrated in Table 6. The experimental result is described as Example A1. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 6 
               
               
                   
                   
               
               
                   
                   
                   
                 Reaction 
                 Ratio of 
               
               
                   
                   
                 Electrolytic 
                 temperature 
                 H 2 /CO of 
               
               
                   
                 Inlet gas 
                 voltage (V) 
                 (° C.) 
                 outlet gas 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Example A1 
                 51%H 2 O—16%CO 2 —N 2   
                 1.15 
                 700 
                 5.4 
               
               
                   
                 balance 
               
               
                   
               
            
           
         
       
     
     By the above comparative experiment, in the electrolytic cell unit U in which the electrolytic cell  1  was formed in a thin layer on the metal support  4 , and the reverse water-gas shift reaction unit  20  generating CO by using CO 2  and H 2  by the reverse water-gas shift reaction was provided in the electrode layer-side gas supply path  5   a  which was the discharge path of the electrolyzed gas, it was possible to increase a composition ratio of CO to H 2  generated by electrolysis. 
     In the comparison between the electrolytic cell unit U in which the reverse water-gas shift catalyst cat 1  is not stored in the electrode layer-side gas supply path  5   a  (which is the discharge path for the electrolyzed gas) and the electrolytic cell unit U in which the reverse water-gas shift catalyst cat 1  is stored, the hydrogen/carbon monoxide ([H 2 /CO]) ratio changes from about 10 or more to about 5 at the outlet, and by combining the reaction of the electrolytic reaction unit  10  and the reaction of the reverse water-gas shift reaction unit  20 , the amount of CO that is advantageous for various hydrocarbon syntheses can be secured, which is preferable. In addition, since thermal efficiency of the hydrocarbon production system  100  can be improved by adopting a methanation reaction of CO rather than a methanation reaction of CO 2 , by combining the reaction of the electrolytic reaction unit  10  and the reaction of the reverse water-gas shift reaction unit  20 , the amount of CO can be secured, which is preferable. This is because 2 mol of H 2 O is generated when 1 mol of CO 2  is methanized, whereas 1 mol of H 2 O is generated when 1 mol of CO is methanized, and thus, the hydrocarbon production system  100  that employs the methanation reaction of CO can suppress latent heat and sensible heat loss of 1 mol of H 2 O as a whole system. 
     By appropriately adjusting the ratio of H 2 O and CO 2  introduced into the electrolytic reaction unit  10 , the reaction conditions (electrolytic voltage, reaction temperature, or the like) of the electrolytic reaction unit  10 , the reaction conditions (amount of catalyst used, GHSV, reaction temperature, or the like) of the reverse water-gas shift reaction unit  20 , or the like, the hydrogen/carbon monoxide ([H 2 /CO]) ratio at the outlet of the reverse water-gas shift reaction unit  20  can be adjusted to a value (for example, H 2 /CO=3 which is the equivalent ratio of the methanation reaction of CO, or the like) suitable for the second catalytic reaction unit  30  (hydrocarbon synthesis reaction unit) in the subsequent stage. 
     [Install Heat Exchanger between Electrolytic Reaction Unit and Reverse Water-Gas Shift Reaction Unit] 
     In the descriptions so far, the example in which the electrolytic reaction unit  10  and the first catalytic reaction unit (reverse water-gas shift reaction unit)  20  are integrated has been mainly described, however, it is possible to adopt a configuration in which both units  10  and  20  are set as separate units and a heat exchanger  11  is provided between both units  10  and  20  so that heat can be exchanged between both units. This configuration is illustrated in  FIG.  6    corresponding to  FIG.  1   . A hollow double line illustrates the heat transfer between both units. In this configuration, the temperature of each of the units  10  and  20  can be appropriately controlled. 
     The inventors have called the system including the electrolytic reaction unit  10  and the reverse water-gas shift reaction unit  20  described so far as an “electrolytic reaction system”. 
     [Second Catalytic Reaction Unit (Hydrocarbon Synthesis Reaction Unit)] 
     At least H 2  and CO flow into the second catalytic reaction unit  30  (hydrocarbon synthesis reaction unit), and hydrocarbons (methane and various hydrocarbons having two or more carbon atoms) and the like are generated by the catalytic reaction. 
     (Example Of Hydrocarbon Synthesis Catalyst) 
     As an activity test of the catalyst (hydrocarbon synthesis catalyst cat 2 ) used in the second catalytic reaction unit  30 , the inventors conducted the following evaluation test 1, evaluation test 2, evaluation test 3, evaluation test 4, and evaluation test 5. Here, the evaluation tests 1 to 4 are tests for whether or not a gas (referred to as a high-calorie gas) having a desired number of calories or more can be obtained as the gas to be produced, and the evaluation test 5 is a confirmation test for heavy hydrocarbons obtained at the same time during production. 
     As an example of the hydrocarbon synthesis catalyst cat 2 , a catalyst was prepared by variously changing the carrier and the catalytically active component. As the catalytically active component ca 2 , those obtained by adding Mo, V, Fe, Co, and the like to Ru and Ru, and Ni were examined. As the carrier cb 2 , ZrO 2 , Al 2 O 3 , SiO 2 , MgO, and TiO 2  were examined. 
     (Catalyst Preparation) 
     The preparation of the hydrocarbon synthesis catalyst cat 2  is also the method adopted as described with reference to  FIGS.  11 ( a )-( c ) and  12 ( a ) and ( b ) . 
     That is, a water-soluble ruthenium compound (ruthenium nitrate, ruthenium chloride, ruthenium sulfate, ruthenium ammonium sulfate, ruthenium acetate, ruthenium oxalate, ruthenium citrate, or the like) is quantified and dissolved according to the composition of the target catalyst to obtain an aqueous solution. Further, when molybdenum, vanadium, iron, and cobalt are supported as further catalytically active components, these water-soluble metal compounds are similarly quantified to obtain a dissolved aqueous solution. Using the aqueous solution, for example, by adding a predetermined amount of carrier particles (ZrO 2 , Al 2 O 3 , SiO 2 , MgO, TiO 2 ) to the aqueous solution to impregnate and support the catalytically active component, and carrying out necessary treatment steps such as a drying treatment, a calcination treatment, and a reduction treatment, the hydrocarbon synthesis catalyst cat 2  can be obtained. 
     Using ruthenium chloride aqueous solution, ammonium molybdate aqueous solution, vanadyl oxalate aqueous solution, iron nitrate aqueous solution, and cobalt nitrate aqueous solution, respectively, and when both ruthenium and catalytically active components other than ruthenium are supported, the catalysts of the following examples were prepared using a sequential carrier method (a two-step carrier method in which a catalytically active component other than ruthenium is first supported on a carrier and then ruthenium is supported). 
     The evaluation test 5 is described separately. 
     (Evaluation Test 1) 
     In the evaluation test 1, a mixed gas containing 12.4% CO, 24.8% CO 2 , 37.2% H 2 , 12.4% H 2 O and the balance being N 2  was used as the reaction gas, GHSV was set to 4000/h (WET base), and the activity test of the hydrocarbon synthesis catalyst cat 2  was carried out at a reaction temperature of 275° C. to 360° C. In this case, the reaction gas is an example obtained by assuming a model in which a co-electrolysis reaction between water and carbon dioxide in the electrolytic reaction unit  10  is carried out under the conditions that an electrolytic reaction rate of carbon dioxide is low, and a mixed gas of CO, CO 2 , H 2 , and H 2 O after the reverse water-gas shift reaction of carbon dioxide is carried out in the reverse water-gas shift reaction unit  20  installed in the subsequent stage is introduced into the hydrocarbon synthesis reaction unit  30  to carry out the hydrocarbon synthesis reaction. 
     The following two indicators were adopted when organizing the test results. 
     1. CO 2  removal assumed hydrocarbon conversion rate=[number of carbons in hydrocarbons in outlet gas]/[number of carbons in outlet gas−number of carbons in outlet CO 2 ] 
     This indicator is an indicator illustrating the conversion rate to hydrocarbons when CO 2  is removed from the outlet gas of the hydrocarbon synthesis reaction unit  30  obtained by the catalytic reaction, and it is preferable that this indicator is high. 
     2. C1-C4 calorific value (MJ/Nm 3 )=Σ(Nn× HN)/ENn 
     Nn [mol]: number of moles of Cn hydrocarbon in gas of catalytic reaction unit (n=1 to 4) 
     HN [MJ/m 3 (N)]: calorific value of Cn hydrocarbon in gas of catalytic reaction unit 
     [H1=39.8, H2=69.7, H3=99.1, H4=128.5] 
     This indicator is an indicator illustrating amounts of C1 to C4 components contained in the outlet gas of the hydrocarbon synthesis reaction unit  30  obtained by the catalytic reaction, and when this value exceeds 39.8, it can be confirmed that hydrocarbons such as ethane, propane, and butane are generated in addition to methane. 
     Regarding the evaluation test 1, Tables 7 and 8 illustrated below illustrate Examples B1 to B3 of the hydrocarbon synthesis catalyst cat 2  in the present invention. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 7 
               
               
                   
                   
               
               
                   
                   
                   
                 Active component 
                   
                   
               
               
                   
                   
                   
                 supported amount 
                 BET surface 
                 CO adsorption 
               
               
                   
                 Catalyst 
                 Carrier 
                 (wt. %) 
                 area (m 2 /g) 
                 amount (ml/g) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Example B1 
                 Ru/Al 2 O 3   
                 Al 2 O 3   
                 Ru: 0.4 
                 87.4 
                 0.66 
               
               
                 Example B2 
                 Ru/Mo/Al 2 O 3   
                 Al 2 O 3   
                 Ru: 0.6, Mo: 0.7 
                 88.2 
                 1.06 
               
               
                 Example B3 
                 Ru/V/Al 2 O 3   
                 Al 2 O 3   
                 Ru: 0.7, V: 1.2 
                 91.1 
                 1.20 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
             
               
                   
                 TABLE 8 
               
             
            
               
                   
                   
               
               
                   
                 Reaction temperature (° C.) 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Catalyst 
                 Indicator 
                 275 
                 310 
                 335 
                 360 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Example B1 
                 Ru/Al 2 O 3   
                 CO 2  removal assumed hydrocarbon conversion rate (%) 
                 12.4 
                   
                   
                 100.0 
               
               
                   
                   
                 C 1 -C 4  calorific value (MJ/Nm 3 ) 
                 44.9 
                   
                   
                 39.8 
               
               
                 Example B2 
                 Ru/Mo/Al 2 O 3   
                 CO 2  removal assumed hydrocarbon conversion rate (%) 
                   
                   
                 99.8 
               
               
                   
                   
                 C 1 -C 4  calorific value (MJ/Nm 3 ) 
                   
                   
                 39.9 
               
               
                 Example B3 
                 Ru/V/Al 2 O 3   
                 CO 2  removal assumed hydrocarbon conversion rate (%) 
                   
                 90.0 
               
               
                   
                   
                 C 1 -C 4  calorific value (MJ/Nm 3 ) 
                   
                 42.1 
               
               
                   
               
            
           
         
       
     
     As illustrated in Tables 7 and 8, it was confirmed that hydrocarbons could be synthesized using a catalyst in which ruthenium was supported on an alumina carrier or a catalyst in which molybdenum or vanadium was supported in addition to ruthenium as a hydrocarbon synthesis catalyst cat 2  from the mixed gas of CO, CO 2 , H 2 , and H 2 O. 
     From the above results, it was confirmed that the above-mentioned hydrocarbon production system  100  could generate a high-calorie gas having a C1-C4 calorific value of 39 MJ/Nm 3  or more. 
     (Evaluation Test 2) 
     In the evaluation test 2, a mixed gas containing 0.45% CO, 18.0% CO 2 , 71.55% H 2 , and 10.0% H 2 O was used as the reaction gas, GHSV was set to 5000/h (DRY base), and the activity test of the hydrocarbon synthesis catalyst cat 2  was carried out at a reaction temperature of about 230° C. to about 330° C. In this case, the reaction gas is an example obtained by assuming a model in which the mixed gas obtained when the co-electrolysis reaction of water and carbon dioxide is carried out in the electrolytic reaction unit  10  under the conditions that the electrolytic reaction rate of carbon dioxide is low is introduced into the hydrocarbon synthesis reaction unit  30  to carry out a hydrocarbon synthesis reaction. 
     The following two indicators were adopted when organizing the test results. 
     1. hydrocarbon conversion rate=[number of carbons in hydrocarbons in outlet gas]/[number of carbons in outlet gas] 
     This indicator is an indicator illustrating the ratio of the number of carbons converted into hydrocarbons without being converted into CO 2  among the total carbons flowing in, and it is preferable that this indicator is high. 
     2. CO 2  removal assumed hydrocarbon conversion rate=[number of carbons in hydrocarbons in outlet gas]/[number of carbons in outlet gas−number of carbons in outlet CO 2 ] 
     This indicator is an indicator illustrating the conversion rate to hydrocarbons when CO 2  is removed from the outlet gas of the hydrocarbon synthesis reaction unit obtained by the catalytic reaction, and it is preferable that this indicator is also high. 
     For the evaluation test 2, the used catalysts (Examples B4 to B16) are illustrated in Table 9, and the test results are illustrated in Table 10. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 9 
               
               
                   
                   
               
               
                   
                   
                   
                 Active component 
                   
                   
               
               
                   
                   
                   
                 supported amount 
                 BET surface 
                 CO adsorption 
               
               
                   
                 Catalyst 
                 Carrier 
                 (wt. %) 
                 area (m 2 /g) 
                 amount (ml/g) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Example B4 
                 Ru/Al 2 O 3   
                 Al 2 O 3   
                 Ru: 1.3 
                 109.8 
                 0.47 
               
               
                 Example B5 
                 Ru/SiO 2   
                 SiO 2   
                 Ru: 1.0 
                 212.3 
                 0.13 
               
               
                 Example B6 
                 Ru/MgO 
                 MgO 
                 Ru: 1.3 
                 24.7 
                 0.15 
               
               
                 Example B7 
                 Ru/TiO 2   
                 TiO 2   
                 Ru: 1.2 
                 64.7 
                 0.71 
               
               
                 Example B8 
                 Ru/Al 2 O 3   
                 Al 2 O 3   
                 Ru: 2.3 
                 114.5 
                 0.97 
               
               
                 Example B9 
                 Ru/MO/Al 2 O 3   
                 Al 2 O 3   
                 Ru: 1.4, Mo: 1.5 
                 131.4 
                 0.47 
               
               
                 Example B10 
                 Ru/V/Al 2 O 3   
                 Al 2 O 3   
                 Ru: 1.2, V: 2.1 
                 108.3 
                 0.45 
               
               
                 Example B11 
                 Ru/Mo/Al 2 O 3   
                 Al 2 O 3   
                 Ru: 2.5, Mo: 1.7 
                 115.5 
                 1.24 
               
               
                 Example B12 
                 Ru/V/ZrO 2   
                 ZrO 2   
                 Ru: 1.1, V: 1.4 
                 46.4 
                 0.62 
               
               
                 Example B13 
                 Ru/V/Al 2 O 3   
                 Al 2 O 3   
                 Ru: 1.2, V: 3.9 
                 118.0 
                 0.63 
               
               
                 Example B14 
                 Ru/V/TiO 2   
                 TiO 2   
                 Ru: 1.2, V: 1.4 
                 57.2 
                 1.19 
               
               
                 Example B15 
                 Ru/Mo/TiO 2   
                 TiO 2   
                 Ru: 1.2, Mo: 1.2 
                 58.1 
                 1.21 
               
               
                 Example B16 
                 Ni/Al 2 O 3   
                 Al 2 O 3   
                 Ni: 13.0 
                 95.7 
                 0.01 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
             
               
                   
                 TABLE 10 
               
               
                   
                   
               
             
            
               
                   
                 Reaction temperature (° C.) 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Catalyst 
                 Indicator 
                 233 
                 249 
                 257 
                 273 
                 274 
                 276 
                 277 
                 278 
               
               
                   
               
               
                 Example B4 
                 Ru/Al 2 O 3   
                 Hydrocarbon conversion rate (%) 
               
               
                   
                   
                 CO 2  removal assumed hydrocarbon 
               
               
                   
                   
                 conversion rate (%) 
               
               
                 Example B5 
                 Ru/SiO 2   
                 Hydrocarbon conversion rate (%) 
               
               
                   
                   
                 CO 2  removal assumed hydrocarbon 
               
               
                   
                   
                 conversion rate (%) 
               
               
                 Example B6 
                 Ru/MgO 
                 Hydrocarbon conversion rate (%) 
                   
                   
                   
                   
                   
                   
                   
                 4.2 
               
               
                   
                   
                 CO 2  removal assumed hydrocarbon 
                   
                   
                   
                   
                   
                   
                   
                 78.0 
               
               
                   
                   
                 conversion rate (%) 
               
               
                 Example B7 
                 Ru/TiO 2   
                 Hydrocarbon conversion rate (%) 
                   
                   
                   
                   
                   
                 64.2 
               
               
                   
                   
                 CO 2  removal assumed hydrocarbon 
                   
                   
                   
                   
                   
                 99.9 
               
               
                   
                   
                 conversion rate (%) 
               
               
                 Example B8 
                 Ru/Al 2 O 3   
                 Hydrocarbon conversion rate (%) 
               
               
                   
                   
                 CO 2  removal assumed hydrocarbon 
               
               
                   
                   
                 conversion rate (%) 
               
               
                 Example B9 
                 Ru/Mo/Al 2 O 3   
                 Hydrocarbon conversion rate (%) 
                 14.2 
                   
                   
                 74.6 
               
               
                   
                   
                 CO 2  removal assumed hydrocarbon 
                 99.8 
                   
                   
                 100.0 
               
               
                   
                   
                 conversion rate (%) 
               
               
                 Example B10 
                 Ru/V/Al 2 O 3   
                 Hydrocarbon conversion rate (%) 
                   
                   
                 74.4 
               
               
                   
                   
                 CO 2  removal assumed hydrocarbon 
                   
                   
                 100.0 
               
               
                   
                   
                 conversion rate (%) 
               
               
                 Example B11 
                 Ru/Mo/Al 2 O 3   
                 Hydrocarbon conversion rate (%) 
                   
                 87.1 
               
               
                   
                   
                 CO 2  removal assumed hydrocarbon 
                   
                 100.0 
               
               
                   
                   
                 conversion rate (%) 
               
               
                 Example B12 
                 Ru/V/ZrO 2   
                 Hydrocarbon conversion rate (%) 
                   
                   
                   
                   
                   
                   
                 87.8 
               
               
                   
                   
                 CO 2  removal assumed hydrocarbon 
                   
                   
                   
                   
                   
                   
                 100.0 
               
               
                   
                   
                 conversion rate (%) 
               
               
                 Example B13 
                 Ru/V/Al 2 O 3   
                 Hydrocarbon conversion rate (%) 
                   
                   
                   
                   
                 78.8 
               
               
                   
                   
                 CO 2  removal assumed hydrocarbon 
                   
                   
                   
                   
                 99.9 
               
               
                   
                   
                 conversion rate (%) 
               
               
                 Example B14 
                 Ru/V/TiO 2   
                 Hydrocarbon conversion rate (%) 
               
               
                   
                   
                 CO 2  removal assumed hydrocarbon 
               
               
                   
                   
                 conversion rate (%) 
               
               
                 Example B15 
                 Ru/Mo/TiO 2   
                 Hydrocarbon conversion rate (%) 
               
               
                   
                   
                 CO 2  removal assumed hydrocarbon 
               
               
                   
                   
                 conversion rate (%) 
               
               
                 Example B16 
                 Ni/Al 2 O 3   
                 Hydrocarbon conversion rate (%) 
               
               
                   
                   
                 CO 2  removal assumed hydrocarbon 
               
               
                   
                   
                 conversion rate (%) 
               
               
                   
               
            
           
           
               
               
            
               
                   
                 Reaction temperature (° C.) 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Catalyst 
                 Indicator 
                 287 
                 289 
                 299 
                 302 
                 308 
                 309 
                 317 
                 331 
               
               
                   
               
               
                 Example B4 
                 Ru/Al 2 O 3   
                 Hydrocarbon conversion rate (%) 
                   
                   
                 78.6 
                   
                   
                 80.8 
                   
                 82.0 
               
               
                   
                   
                 CO 2  removal assumed hydrocarbon 
                   
                   
                 99.9 
                   
                   
                 99.9 
                   
                 99.9 
               
               
                   
                   
                 conversion rate (%) 
               
               
                 Example B5 
                 Ru/SiO 2   
                 Hydrocarbon conversion rate (%) 
                   
                   
                   
                 3.7 
               
               
                   
                   
                 CO 2  removal assumed hydrocarbon 
                   
                   
                   
                 46.9 
               
               
                   
                   
                 conversion rate (%) 
               
               
                 Example B6 
                 Ru/MgO 
                 Hydrocarbon conversion rate (%) 
               
               
                   
                   
                 CO 2  removal assumed hydrocarbon 
               
               
                   
                   
                 conversion rate (%) 
               
               
                 Example B7 
                 Ru/TiO 2   
                 Hydrocarbon conversion rate (%) 
               
               
                   
                   
                 CO 2  removal assumed hydrocarbon 
               
               
                   
                   
                 conversion rate (%) 
               
               
                 Example B8 
                 Ru/Al 2 O 3   
                 Hydrocarbon conversion rate (%) 
                 88.4 
               
               
                   
                   
                 CO 2  removal assumed hydrocarbon 
                 100.0 
               
               
                   
                   
                 conversion rate (%) 
               
               
                 Example B9 
                 Ru/Mo/Al 2 O 3   
                 Hydrocarbon conversion rate (%) 
               
               
                   
                   
                 CO 2  removal assumed hydrocarbon 
               
               
                   
                   
                 conversion rate (%) 
               
               
                 Example B10 
                 Ru/V/Al 2 O 3   
                 Hydrocarbon conversion rate (%) 
               
               
                   
                   
                 CO 2  removal assumed hydrocarbon 
               
               
                   
                   
                 conversion rate (%) 
               
               
                 Example B11 
                 Ru/Mo/Al 2 O 3   
                 Hydrocarbon conversion rate (%) 
               
               
                   
                   
                 CO 2  removal assumed hydrocarbon 
               
               
                   
                   
                 conversion rate (%) 
               
               
                 Example B12 
                 Ru/V/ZrO 2   
                 Hydrocarbon conversion rate (%) 
               
               
                   
                   
                 CO 2  removal assumed hydrocarbon 
               
               
                   
                   
                 conversion rate (%) 
               
               
                 Example B13 
                 Ru/V/Al 2 O 3   
                 Hydrocarbon conversion rate (%) 
               
               
                   
                   
                 CO 2  removal assumed hydrocarbon 
               
               
                   
                   
                 conversion rate (%) 
               
               
                 Example B14 
                 Ru/V/TiO 2   
                 Hydrocarbon conversion rate (%) 
                   
                   
                   
                   
                 81.7 
               
               
                   
                   
                 CO 2  removal assumed hydrocarbon 
                   
                   
                   
                   
                 99.9 
               
               
                   
                   
                 conversion rate (%) 
               
               
                 Example B15 
                 Ru/Mo/TiO 2   
                 Hydrocarbon conversion rate (%) 
                   
                 75.2 
               
               
                   
                   
                 CO 2  removal assumed hydrocarbon 
                   
                 99.8 
               
               
                   
                   
                 conversion rate (%) 
               
               
                 Example B16 
                 Ni/Al 2 O 3   
                 Hydrocarbon conversion rate (%) 
                   
                   
                   
                   
                   
                   
                 26.7 
               
               
                   
                   
                 CO 2  removal assumed hydrocarbon 
                   
                   
                   
                   
                   
                   
                 96.2 
               
               
                   
                   
                 conversion rate (%) 
               
               
                   
               
            
           
         
       
     
     (Evaluation Test 3) 
     In the evaluation test 3, a mixed gas (H 2 /CO=3) containing H 2  and CO in a ratio of 3:1 (volume ratio) was used as the reaction gas, the GHSV was set to 2000/h, and the activity test of the hydrocarbon synthesis catalyst cat 2  was carried out at the reaction temperature of 235° C. to about 330° C. In this activity test, a catalyst (Examples B17 and B18) in which iron or cobalt was supported on a titania carrier in addition to ruthenium was used. In this case, the reaction gas is an example obtained by assuming a model in which a mixed gas obtained by adding carbon monoxide to hydrogen obtained by electrolyzing water in the electrolytic reaction unit  10 , and a mixed gas of hydrogen and carbon monoxide obtained by separating water and carbon dioxide as needed from the gas obtained by carrying out a co-electrolysis reaction of water and carbon dioxide are introduced into the hydrocarbon synthesis reaction unit  30  to carry out the hydrocarbon synthesis reaction. 
     The results of the evaluation test 3 are illustrated in Table 11. 
     
       
         
           
               
               
             
               
                   
                 TABLE 11 
               
             
            
               
                   
                   
               
               
                   
                 Reaction temperature (° C.) 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Catalyst 
                 Indicator 
                 235 
                 250 
                 278 
                 327 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Example B17 
                 2 wt. % Ru/2 
                 CO 2  removal assumed hydrocarbon conversion rate (%) 
                 11.6 
                   
                 99.9 
                 99.9 
               
               
                   
                 wt. % Fe/TiO 2   
                 C 1 -C 4  calorific value (MJ/Nm 3 ) 
                 59.0 
                   
                 47.1 
                 42.16 
               
               
                 Example B18 
                 2 wt. % Ru/2 
                 CO 2  removal assumed hydrocarbon conversion rate (%) 
                   
                 99.0 
               
               
                   
                 wt. % Co/TiO 2   
                 C 1 -C 4  calorific value (MJ/Nm 3 ) 
                   
                 46 
               
               
                   
               
            
           
         
       
     
     As illustrated in Table 11, it was confirmed that hydrocarbons can be synthesized from a mixed gas containing H 2  and CO using a catalyst in which ruthenium and iron or cobalt are supported on a titania carrier as a hydrocarbon synthesis catalyst cat 2 . 
     It was confirmed that the above-mentioned hydrocarbon production system  100  can generate a high-calorie gas having a C1-C4 calorific value of 39 MJ/Nm 3  or more. 
     (Evaluation Test 4) 
     In the evaluation test 4, a mixed gas containing 0.5% CO, 20.0% CO 2 , and 79.5% H 2  was used as the reaction gas, the GHSV was set to 5000/h (DRY base), and the activity test of the hydrocarbon synthesis catalyst cat 2  was carried out at a reaction temperature of about 260° C. to about 290° C. In this case, the reaction gas corresponds to a mixed gas obtained by separating and removing water from the reaction gas used in the evaluation test 2. 
     Table 12 illustrates the test results for the evaluation test 4. 
     
       
         
           
               
               
             
               
                   
                 TABLE 12 
               
             
            
               
                   
                   
               
               
                   
                 Reaction temperature (° C.) 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Catalyst 
                 Indicator 
                 257 
                 258 
                 292 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Example B4 
                 Ru/Al 2 O 3   
                 Hydrocarbon conversion rate (%) 
                   
                   
                 87.4 
               
               
                 (DRY) 
                   
                 CO2 removal assumed hydrocarbon 
                   
                   
                 100.0 
               
               
                   
                   
                 conversion rate (%) 
               
               
                 Example B9 
                 Ru/Mo/Al 2 O 3   
                 Hydrocarbon conversion rate (%) 
                 82.5 
               
               
                 (DRY) 
                   
                 CO2 removal assumed hydrocarbon 
                 100.0 
               
               
                   
                   
                 conversion rate (%) 
               
               
                 Example B10 
                 Ru/V/Al 2 O 3   
                 Hydrocarbon conversion rate (%) 
                   
                 84.7 
               
               
                 (DRY) 
                   
                 CO2 removal assumed hydrocarbon 
                   
                 100.0 
               
               
                   
                   
                 conversion rate (%) 
               
               
                   
               
            
           
         
       
     
     From the above results, comparing the test results of the evaluation test 2 and the evaluation test 4, it was found that by separating water in the stage before the second catalytic reaction unit  30  (hydrocarbon synthesis reaction unit), in the hydrocarbon synthesis reaction unit  30 , the hydrocarbon conversion rate can be improved even at a lower reaction temperature, and the activity of the catalyst can be improved. Therefore, the configuration of the hydrocarbon production system  100  illustrated in  FIG.  9   , which will be described later in another embodiment, is also effective. 
     (Evaluation Test 5) 
     Catalyst Preparation 
     Even in this evaluation test 5, the hydrocarbon synthesis catalyst cat 2  was prepared by the method adopted as described in  FIGS.  11 ( a )-( c ) and  12 ( a ) and ( b ) . 
     In the catalyst cat 2 , the catalytically active components were ruthenium and iron, and the carrier was titania. A ruthenium chloride aqueous solution was used in the step of supporting the ruthenium component, and an iron nitrate aqueous solution was used in the step of supporting the iron component. In addition, as illustrated above, aqueous solutions of other compounds containing ruthenium and iron can also be used. 
     The specific preparation method is as follows. 
     A titania carrier as a metal oxide carrier was impregnated with an iron-containing aqueous solution having a predetermined concentration, and iron was supported on the titania carrier. This iron-supported titania was impregnated with a ruthenium chloride aqueous solution having a predetermined concentration. Then, a wet reduction treatment was carried out using a hydrazine aqueous solution to obtain iron-ruthenium-supported titania as a hydrocarbon synthesis catalyst cat 2 . 
     The amount of iron and ruthenium supported on the hydrocarbon synthesis catalyst cat 2  can be arbitrarily set depending on the concentration of each aqueous solution and the impregnation conditions. However, the hydrocarbon synthesis catalyst cat 2  in the evaluation test 5 was iron-ruthenium-supported titania (2 wt. % Ru/2 wt. % Fe/TiO 2 ) having an iron content (supported amount) of 2% by weight and a ruthenium content (supported amount) of 2% by weight using a spherical titania carrier having a diameter of 2 to 4 mm. 
     Reduction Pretreatment 
     In the test, the reduction pretreatment illustrated in  FIG.  12  ( b )  was carried out. 
     Specifically, while flowing a nitrogen gas containing 10% hydrogen as a hydrogen-containing gas through the catalyst storage unit serving as the hydrocarbon synthesis reaction unit  30 , the temperature of the catalyst unit was maintained at 350° C., and a pre-use hydrogen treatment (reduction pretreatment) of the catalyst was carried out for 1 hour. 
     Here, the hydrogen-containing gas used in the reduction pretreatment can be reduced in hydrogen consumption by using a mixed gas of hydrogen and nitrogen in this way. Therefore, in this reduction pretreatment, for example, it is preferable to use a mixed gas of hydrogen and nitrogen having a hydrogen concentration of 20% by volume or less, and in the present embodiment, it was decided to carry out the reduction pretreatment of the hydrocarbon synthesis catalyst cat 2  using a hydrogen-containing gas containing hydrogen containing 10% by volume of hydrogen and the balance being a nitrogen gas. 
     Test Conditions 
     In the evaluation test 5, a mixed gas (H 2 /CO=3) containing H 2  and CO in a ratio of 3:1 (volume ratio) was used as the reaction gas, and the hydrocarbon synthesis catalyst cat 2  was tested under the condition of GHSV=2000/h. Incidentally, the reaction pressure in this test was set to 0.6 MPa. 
     Heavy Hydrocarbon Confirmation Test 1 
     A hydrocarbon synthesis test was carried out at a reaction temperature of 275° C. for about 80 hours using the hydrocarbon synthesis catalyst cat 2  (2 wt. % Ru/2 wt. % Fe/TiO 2 ) subjected to the above reduction pretreatment. As a result, it was confirmed by gas analysis using a gas chromatograph that C2-C6 hydrocarbons such as ethane, propane, and butane were continuously generated at the outlet of the catalytic reaction unit, with methane (CH 4 ) as the main component. 
     Meanwhile, when wax-like precipitate obtained at the air-cooled section (section corresponding to the heavy hydrocarbon separation unit  35 ) provided under the catalytic reaction unit was analyzed, it could be identified as a linear higher aliphatic hydrocarbon (average chain length: C26) from the IR and C13 NMR analysis. 
     Further, when the precipitate on the hydrocarbon synthesis catalyst cat 2  was extracted with a solvent and analyzed by MS, peaks (maximum intensity of 352) were obtained for every 14 mass units in 268 to 604. From this result, it was found that higher hydrocarbons of C19 to C43 (mostly C25) were precipitated on the hydrocarbon synthesis catalyst cat 2 . 
     Heavy Hydrocarbon Confirmation Test 2 
     Using the hydrocarbon synthesis catalyst cat 2  (2 wt. % Ru/2 wt. % Fe/TiO 2 ) in the same manner as in the above test, the reduction pretreatment was carried out at 550° C., and the hydrocarbon synthesis test was carried out at a reaction temperature of about 325° C. for 270 hours. As a result, it was confirmed by gas analysis using a gas chromatograph that C2-C6 hydrocarbons such as ethane, propane, and butane were continuously generated at the outlet of the catalytic reaction unit, with methane (CH 4 ) as the main component. Meanwhile, in the air-cooled section (section corresponding to the heavy hydrocarbon separation unit  35 ) provided under the catalytic reaction unit, it was confirmed that wax-like precipitates (linear higher aliphatic hydrocarbons (average chain length: C18)) were obtained. Further, it was found that higher hydrocarbons of C19 to C60 (mostly C29) were precipitated on the hydrocarbon synthesis catalyst cat 2 . 
     Production of Hydrocarbons with Another Hydrocarbon Synthesis Catalyst 
     Further, another hydrocarbon synthesis catalyst cat 2  (2 wt. % Ru/2 wt. % Co/TiO 2 ) different from the above-mentioned hydrocarbon synthesis catalyst cat 2  (2 wt. % Ru/2 wt. % Fe/TiO 2 ) was used, the reduction pretreatment was carried out at 450° C., and the hydrocarbon synthesis test was carried out at a temperature of 250° C. to 270° C. for about 380 hours. As a result, it was confirmed by gas analysis using a gas chromatograph that hydrocarbons such as ethane, propane, and butane were continuously generated at the outlet of the catalytic reaction unit, with methane (CH 4 ) as the main component. Meanwhile, in the air-cooled section (section corresponding to the heavy hydrocarbon separation unit  35 ) provided under the catalytic reaction unit, it was confirmed that wax-like precipitates (linear higher aliphatic hydrocarbons (average chain length: C27)) were obtained. Further, it was found that higher hydrocarbons of C23 to C60 (mostly C37) were precipitated on the hydrocarbon synthesis catalyst cat 2 . 
     From the evaluation test 5, it was confirmed that when the hydrocarbon synthesis catalyst cat 2  according to the present invention was used, a lower saturated hydrocarbon could be produced and a heavy hydrocarbon could also be produced. 
     From the above results, as illustrated above, a catalyst in which at least ruthenium is supported as the catalytically active component ca 2  on the metal oxide carrier cb 2  can be used in the second catalytic reaction unit  30  (hydrocarbon synthesis reaction unit). Further, it is preferable to support at least one of molybdenum, vanadium, iron, and cobalt as the catalytically active component ca 2 . 
     It was found that, preferably, the hydrocarbon synthesis catalyst cat 2  was a catalyst in which at least ruthenium was supported on the metal oxide carrier cb 2 , the supported amount of ruthenium was 0.1% by weight or more and 5% by weight or less, and at least one of molybdenum, vanadium, iron, and cobalt as the catalytically active component ca 2  was supported on the metal oxide carrier cb 2  in addition to ruthenium. 
     Here, the supported amount of at least one of the molybdenum, vanadium, iron, and cobalt can be 0.2% by weight or more and 6% by weight or less. 
     Further, in hydrocarbon synthesis catalysts cat 2 , the adsorption amount of carbon monoxide of the highly active catalyst was 0.4 ml/g or more. 
     [Heavy Hydrocarbon Separation Unit] 
     When the gas reaching the heavy hydrocarbon separation unit  35  is cooled, the heavy hydrocarbons contained in the gas released from the hydrocarbon synthesis reaction unit  30  are condensed and the heavy hydrocarbons can be taken out to the outside. For example, in the hydrocarbon synthesis reaction unit  30  using the 2 wt. % Ru/2 wt. % Fe/TiO 2  catalyst, when a mixed gas (H 2 /CO=3) containing H 2  and CO in a ratio of 3:1 (volume ratio) was introduced and the reaction was carried out at 275° C., a linear higher aliphatic hydrocarbon having an average chain length of 26 carbon atoms could be extracted from the heavy hydrocarbon separation unit  35 . Moreover, when the reaction was carried out at 325° C., a linear higher aliphatic hydrocarbon having an average chain length of 18 carbon atoms could be extracted from the heavy hydrocarbon separation unit  35 . 
     [Water Separation Unit] 
     A condenser is arranged in the water separation unit  40 , and the gas containing H 2 O flowing in is adjusted to a predetermined temperature and pressure to be condensed and water is taken out to the outside. 
     [Carbon Dioxide Separation Unit] 
     For example, PSA is arranged in this unit  50 , and the gas containing CO 2  flowing in is adsorbed to the adsorbent under a predetermined temperature and pressure to separate CO 2 , the separated CO 2  is separated from the adsorbent, and thus, CO 2  is favorably separated. The separated CO 2  can be returned to the front of the electrolytic reaction unit  10  and reused via the carbon dioxide return path  51 . 
     It is also possible to use PSA or the like to make the carbon dioxide separation unit and the water separation unit the same separation unit. 
     Another Embodiment 
     
         
         
           
             (1) In the above embodiment, CO 2  separated in the carbon dioxide separation unit  50  is returned to the front of the electrolytic reaction unit  10 . However, in the hydrocarbon production system  100  according to the present invention, since the conversion of CO 2  to CO is mainly performed by the reverse water-gas shift reaction unit  20 , a return destination of CO 2  may be in front of the reverse water-gas shift reaction unit  20 . This configuration is illustrated in  FIG.  7   . 
           
         
       
    
     (2) In the above embodiment, H 2  in the gas obtained from the hydrocarbon synthesis reaction unit  30  is not particularly described. However, a hydrogen separation unit (described as H 2  separation unit in the drawing)  60  that separates H 2  using a hydrogen separation membrane or the like may be provided to separate H 2  and use H 2  separately. This configuration is illustrated in  FIG.  8   . In this example, the return destination of H 2  separated by the hydrogen separation unit  60  may be provided in front of the reverse water-gas shift reaction unit  20  so that H 2  is used for the reverse water-gas shift reaction. In this case as well, hydrogen is separated from the hydrogen separation unit  60  into a branch path. 
     (3) In the above embodiment, the water separation unit  40  is provided on the lower side of the hydrocarbon synthesis reaction unit  30 . However, as illustrated in  FIG.  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 the water separation unit  40  is to facilitate the hydrocarbon synthesis reaction. 
     (4) In the above embodiment, an example in which both H 2 O and CO 2  are supplied to the electrolytic reaction unit  10  and subjected to the electrolysis reaction is illustrated. However, as illustrated in  FIG.  10   , a system may be used in which only H 2 O is supplied to the electrolytic reaction unit  10  to be subjected to the electrolysis reaction. In this case, the carbon consumed in the hydrocarbon synthesis is input to the reverse water-gas shift reaction unit  20  as carbon dioxide. 
     (5) In the above embodiment, an example in which a solid oxide electrolytic cell is used as the electrolytic cell  1  in the electrolytic reaction unit  10  is illustrated. However, as the electrolytic cell  1 , an alkaline type electrolytic cell, a polymer film type electrolytic cell, or the like may be used. 
     (6) In the above embodiment, the electrolytic reaction unit  10  and the first catalytic reaction unit  20  are integrated. However, in addition to the reaction units  10 . 20 , the second catalytic reaction unit  30  may be integrated. A configuration example in this case is illustrated in  FIG.  13   . Incidentally, in  FIG.  13   , a reference numeral  30   a  indicates a coating layer of the hydrocarbon synthesis catalyst cat 2 . 
     Also, in the case of this configuration, each of the reaction units  10 ,  20 , and  30  can be configured on the metal support  4  and the supply path forming member  5 , and the metal support  4  is supposed to act as a separator for separating the generated hydrocarbon and oxygen. 
     (7) In the above embodiment, the hydrocarbon production system  100  is provided with the reverse water-gas shift reaction unit  20 . However, as illustrated in  FIG.  14   , by electrolyzing water in the electrolytic reaction unit  10  without using the reverse water-gas shift reaction unit  20  and adding one or more selected from carbon monoxide and carbon dioxide in the stage before the hydrocarbon synthesis reaction unit  30 , a high-calorie gas can be obtained. 
     As described above, when the hydrocarbon production system (which becomes a high-calorie gas production system) according to the present invention is constructed by the electrolytic reaction unit  10  and the hydrocarbon synthesis reaction unit  30 , the hydrocarbon synthesis reaction units may be provided in a plurality of stages.  FIG.  15    illustrates an example in which the hydrocarbon synthesis reaction unit  30  has two stages ( 30   a ,  30   b ). By doing so, different hydrocarbon synthesis catalysts cat 2  can be disposed in different hydrocarbon synthesis reaction units  30  and operated in different temperature ranges. 
     (8) In the above embodiment, an example of synthesizing a hydrocarbon such as methane in the hydrocarbon synthesis reaction unit  30  is illustrated. However, depending on how the hydrocarbon synthesis catalyst used in the hydrocarbon synthesis reaction unit  30  is selected, it is also possible to synthesize a chemical raw material from hydrogen and carbon monoxide introduced into the hydrocarbon synthesis reaction unit  30 . 
     REFERENCE SIGNS LIST 
     
         
         
           
               1 : Electrolytic cell 
               1   a : Electrolyte layer 
               2 : Electrode layer 
               3 : Counter electrode layer 
               4 : Metal support (support/separator) 
               4   a : Hole 
               5 : Supply path forming member (separator) 
               6 : Supply path forming member (separator) 
               10 : Electrolytic reaction unit 
               20 : First catalytic reaction unit (reverse water-gas shift reaction unit) 
               20   a : Coating layer 
               30 : Second catalytic reaction unit (hydrocarbon synthesis reaction unit) 
               40 : Water separation unit 
               50 : Carbon dioxide separation unit 
               60 : Hydrogen separation unit 
             U: Electrolytic cell unit 
             cat 1 : Reverse water-gas shift catalyst 
             ca 1 : Catalytically active component (metal component) 
             cb 1 : Carrier (metal oxide) 
             cat 2 : Hydrocarbon synthesis catalyst 
             ca 2 : Catalytically active component (metal component) 
             cb 2 : Carrier (metal oxide)