Fuel synthesis catalyst and fuel synthesis system

A fuel synthesis catalyst of an embodiment for hydrogenating a gas includes at least one selected from the group consisting of; carbon dioxide and carbon monoxide, the catalyst comprising, a base material containing at least one oxide selected from the group consisting of; Al2O3, MgO, TiO2, and SiO2, first metals containing at least one metal selected from the group consisting of; Ni, Co, Fe, and Cu and brought into contact with the base material, and a first oxide containing at least one selected from the group consisting of; CeO2, ZrO2, TiO2, and SiO2 and having an interface with each of the first metals and the base material. The first metals exist on an outer surface of the base material, and on a surface of the base material in fine pores having opening ends on the outer surface of the base material and inside the base material. The first metals and the first oxide exist in the fine pores. The first metals have interfaces with the base material in the fine pores. The first metals exist inside the base material.

FIELD

Embodiments described herein relate to a fuel synthesis catalyst and a fuel synthesis system.

BACKGROUND

In recent years, renewable energy of sunlight, wind power, or the like has attracted attention as safe and clean energy, and an increase in introduction quantity thereof is expected in the future. However, such renewable energy has a low operation rate and a large output fluctuation in a short time, and has a problem in terms of stable supply. Further, if a large quantity of such renewable energy is introduced, a problem arises in that the introduced energy is not completely consumed and remains as surplus power. In this regard, there is a demand for development of techniques of storing electric power such that electric power can be stably supplied and only a necessary amount of electric power can be supplied whenever it is needed even if the introduction of such renewable energy increases. For storing electric power, in addition to a method of storing electric power in the form of electricity, a method of converting electric power into chemical energy and storing the chemical energy has been investigated. In particular, the method of storing electric power in the form of chemical energy has advantages in that the chemical power can be stored in the unit of several days to weeks or a longer span and the chemical power can be transported and used at a different place as necessary. Recently, a method of storing the electric power in the form of hydrogen attracts attention; however, methane or methanol, which is excellent in volumetric energy density as compared with hydrogen, is also a major candidate. In particular, regarding methane or the like, there are a plenty of devices capable of directly using methane or the like as a fuel and the infrastructure therefor is also established.

For example, reaction (1) in which methane is synthesized from hydrogen (H2) obtainable by electrolyzing water by renewable energy and carbon dioxide (CO2) and reaction (2) in which methanol is synthesized from hydrogen (H2) and carbon monoxide (CO) are mentioned.
CO2+4H2→CH4+2H2O  (1)
CO+2H2→CH3OH  (2)

This reaction (1) called Sabatier reaction is a reaction for reductive regeneration of CO2that is one of causes of global warming, and since the reaction is performed at a relatively low temperature of about 400° C., a great deal of research has hitherto been conducted on this reaction.

According to this reaction, the conversion of CO2and the yield of methane can be increased as the reaction approaches equilibrium and a lower temperature region; however, the reaction rate is decreased, and thus, it is more difficult to put this reaction into practical use. For this reason, when the reaction is carried out at low temperature, a noble metal-based catalyst is necessary. However, the noble metal-based catalyst is expensive. Meanwhile, when the temperature is increased to about 400° C., the reaction rate is increased and a Ni-based catalyst can be used. However, in this case, a by-product such as CO is also generated, and as a result, energy is consumed for separation of the by-product, or the like. In this regard, there is a demand for development of a non-noble metal-based catalyst having high activity at a lower temperature region and high methane yield.

As a catalyst having high activity at low temperature of the related art, for example, a Ni-based catalyst having ZrO2or CeO2as a base material has been known. It is known that a Ni catalyst supported on a CeO2base material has high reaction activity at low temperature. It is considered that these base materials have oxygen defects and help CO or CO2to be easily dissociated at lower temperature and cause the reaction with hydrogen at low temperature to be performed effectively.

However, these catalysts have metal particles supported on a base material that is powder and thus are difficult to handle without any change, and it is necessary to granulate these catalysts in a suitable size using a binder or the like. Further, there is also a problem in durability such as weak binding force between the metal particles and the base material. In particular, since methanation reaction is exothermic reaction, heat resistance to withstand a local temperature increase is also required.

DETAILED DESCRIPTION

A fuel synthesis catalyst of an embodiment for hydrogenating a gas includes at least one selected from the group consisting of; carbon dioxide and carbon monoxide, the catalyst comprising, a base material containing at least one oxide selected from the group consisting of; Al2O3, MgO, TiO2, and SiO2, first metals containing at least one metal selected from the group consisting of; Ni, Co, Fe, and Cu and brought into contact with the base material, and a first oxide containing at least one selected from the group consisting of; CeO2, ZrO2, TiO2, and SiO2and having an interface with each of the first metals and the base material. The first metals exist on an outer surface of the base material, and on a surface of the base material in fine pores having opening ends on the outer surface of the base material and inside the base material. The first metals and the first oxide exist in the fine pores. The first metals have interfaces with the base material in the fine pores. The first metals exist inside the base material.

Hereinafter, embodiments will be described using methanation reaction of CO2as an example with reference to drawings.

As a result of intensive studies on a catalyst for synthesizing one or both of hydrocarbon fuel and alcohol fuel at low temperature from CO or CO2and H2, it was found that when a composite material prepared by precipitating metal particles from the inner portion of ceramic has a structure in which an oxide capable of dissociating CO or CO2at low temperature is present to have a large contact area, a catalyst having high low-temperature activity and high reliability can be provided. In addition, it was found that when fine particles having at least one selected from the group consisting of; Fe and Co are contained in the oxide, the activity at low temperature can be further increased. Fuel synthesis catalysts of embodiments are catalysts for synthesizing hydrocarbon fuel and alcohol fuel. Therefore, in the embodiments, in terms of the configurations of the catalysts, there is no difference between a hydrocarbon fuel synthesis catalyst and an alcohol fuel synthesis catalyst. A fuel to be generated varies depending on the difference in conditions of reaction using a catalyst, for example, a difference in raw material between carbon monoxide and carbon dioxide. A fuel containing one or both of hydrocarbon fuel and alcohol fuel is synthesized by the catalysts of the embodiments.

First Embodiment

A catalyst according to a first embodiment includes a base material containing at least one selected from the group consisting of; Al2O3, MgO, TiO2, and SiO2, first metals supported on the base material and containing at least one metal selected from the group consisting of; Ni, Co, Fe, and Cu, and a first oxide brought into contact with each of the first metals and the base material to have interfaces therewith and containing at least one selected from the group consisting of; CeO2, ZrO2, TiO2, and SiO2. Such a catalyst is a fuel synthesis catalyst for hydrogenating a gas containing carbon dioxide and/or carbon monoxide.

A catalyst10according to the embodiment includes first metals11, a base material12, and a first oxide13. The catalyst10according to the embodiment is preferably a catalyst formed by the first metals11, the base material12, and the first oxide13. The first oxide13in fine pores14of the enlarged diagram is provided with diagonal lines for easy distinction from the other portions. InFIGS. 1A to 1CandFIGS. 2A to 2C, pores other than the fine pores14are not illustrated, but a large number of pores are included in the base material12and the first oxide13since they are porous. In the schematic diagrams ofFIGS. 2A, 2B, and 2C, the first metals11and the first oxide13existing on the outer surface of the base material12are referred to as first metals11A and a first oxide13A, respectively. The first metals11and the first oxide13existing in the fine pores14are referred to as first metals11B and a first oxide13B, respectively. The first metals11existing inside the base material12are referred to as first metals11C. Incidentally, existing places of the first metals11and the like are distinguished by A, B, and C in the description ofFIGS. 2A to 2C. However, in the specification, a case where the description is made without using these alphabets is also included. The outer surface indicates a surface excluding a surface portion in the fine pore among the surfaces.

In the catalyst10of the first embodiment, the first metals11are brought into contact with the base material12. The first oxide13has an interface with the first metals11. The first oxide13has an interface with the base material12. The base material12includes fine pores14. Opening ends of the fine pores14exist on the outer surface of the base material12. The first metals11and the first oxide13exist in the fine pores14. The first metals11B and the base material12are in direct contact with each other in the fine pores14, and the first metals11B have interfaces with the base material12. Further, it is preferable that the first oxide13partially or entirely cover the base material12with which the first metals11are brought into contact. A configuration in which the first metals11are bonded to the base material12to be physically brought into surface contact with the base material12is preferable.

There are differences between the catalyst10ofFIG. 1Aand the catalyst10ofFIG. 1B. The first oxide13ofFIG. 1Acovers the first metals11and the base material12; in contrast, the first oxide13ofFIG. 1Bpartially covers the first metals11and the base material12and at least some of the first metals11are exposed. InFIG. 1B, the first metals11and the base material12covered with the first oxide13are included. The ratio of coverage and partial coverage with the first oxide13can be set to an arbitrary ratio. In the catalyst10ofFIG. 1C, the first oxide13partially covers the base material12with which the first metals11are brought into contact. The catalyst10ofFIG. 1Cis different from the catalysts10ofFIG. 1AandFIG. 1Bin that some of the first metals11are not brought into contact with the first oxide13and some of surfaces, which the first metals11are not brought into contact with, of the base material12are not covered with the first oxide13. In addition, a combination of cross-sectional forms ofFIG. 1A,FIG. 1B, andFIG. 1Cmay be employed as the catalyst10of the embodiment. Incidentally, as a modified example ofFIG. 1C, a form in which some of the first metals11are not brought into contact with the first oxide13but surfaces, which the first metals11are not brought contact with, of the base material12are covered with the first oxide13and a form in which some of surfaces, which the first metals11are not brought into contact with, of the base material12are not covered with the first oxide13but the first metals11are brought into contact with the first oxide13are mentioned. The first oxide13existing on the outer surface of the catalyst10is an oxide layer and a covered layer.

As illustrated inFIG. 1A, in the cross-section, in a case where the first oxide13entirely covers the base material12with which the first metals11are brought into contact, the base material12is covered with the first metals11and the first oxide13, the first metals11have an interface between the first metals11and the base material12and an interface between the first metals11and the first oxide13, and have no exposed surface.

As illustrated inFIG. 1B, in the cross-section, in a case where the first oxide13covers the base material12with which the first metals11are brought into contact and at least some of the first metals11are exposed, the base material12is covered with the first metals11and the first oxide13, and the first metals11have exposed surfaces, an interface between the first metals11and the base material12, and an interface between the first metals11and the first oxide13. That is, inFIG. 1B, the outer surface of the catalyst10does not include the surface of the base material12but includes the surfaces of the first metals11and the first oxide13. The first metals11and the first oxide13exist inside the fine pores14having opening ends on the outer surface of the base material12. As illustrated inFIGS. 1B and 2B, the first metals11and the first oxide13are in direct contact with the base material12. More specifically, on the outer surface of the base material12, the first metals11A and the first oxide13A are in direct contact with the base material12. On the outer surface of the base material12, the first metals11A have interfaces with the base material12. In the fine pores14of the base material12, the first metals11B and the first oxide13B are in direct contact with the surfaces of the fine pores14of the base material12. In the fine pores14, the first metals11B have interfaces with the base material12and the first oxide13B has an interface with the base material12. In the fine pores14, it is preferable that the first metals11B having interfaces with the base material12have interfaces with the first oxide13B. The first metals11C are included inside the base material12. Inside the fine pores14of the base material12, the first metals11B and the first oxide13B are in direct contact with each other. A part of the first oxide13B may cover the first metals11B in the fine pores14. Further, as illustrated inFIG. 2B, the fine pores14may be connected to each other in the base material12. In addition, in the fine pores14, it is preferable that the first metals11having interfaces with the base material12have interfaces with the first oxide13.

As illustrated inFIG. 1C, in the cross-section, in a case where the first oxide13partially covers the base material12with which the first metals11are brought into contact, some of the first metals11are not brought into contact with the first oxide13, and some of surfaces, which the first metals11are not brought into contact with, of the base material12are not covered with the first oxide13, the base material12is partially covered with the first metals11and the first oxide13, and the first metals11have exposed surfaces, an interface between the first metals11and the base material12, and an interface between the first metals11and the first oxide13. Some of the first metals11have an interface with the base material12but do not have an interface with the first oxide13. That is, inFIG. 1C, the outer surface of the catalyst10includes the surfaces of the first metals11, the surface of the base material12, and the surface of the first oxide13. Further, the first metals11and the first oxide13exist inside the fine pores14having opening ends on the outer surface of the base material12. As illustrated inFIGS. 1C and 2C, the first metals11and the first oxide13are in direct contact with the base material12. More specifically, on the outer surface of the base material12, the first metals11A and the first oxide13A are in direct contact with the base material12. On the outer surface of the base material12, the first metals11A have interfaces with the base material12. In the fine pores14of the base material12, the first metals11B and the first oxide13B are in direct contact with the surfaces of the fine pores14of the base material12. In the fine pores14, the first metals11B have interfaces with the base material12and the first oxide13B has an interface with the base material12. In the fine pores14, it is preferable that the first metals11B having interfaces with the base material12have interfaces with the first oxide13B. The first metals11C are included inside the base material12. Inside the fine pores14of the base material12, the first metals11B and the first oxide13B are in direct contact with each other. Apart of the first oxide13B may cover the first metals11B in the fine pores14. Further, as illustrated inFIG. 2C, the fine pores14may be connected to each other in the base material12. In addition, in the fine pores14, it is preferable that the first metals11having interfaces with the base material12have interfaces with the first oxide13. Furthermore, the opening ends of the fine pores14may exist on the outer surface of the catalyst10.

The particle diameter of the catalyst10is preferably 2 mm or more and 10 mm or less. When the particle diameter of the catalyst10is less than 2 mm, in a case where a reaction tube is filled with the catalyst, pressure loss increases, which is not favorable. In addition, when the particle diameter of the catalyst10is more than 10 mm, the first metals existing deeply inside the catalyst are not utilized, and thus useless portions increase, which is not favorable. Regarding the particle diameter of the catalyst10, the catalyst10is subjected to slice processing and the processed sample is observed with an optical microscope. 50 particles having the clearest outline of the catalyst in the photographed image are selected, a circumscribed circle diameter ϕA1and an inscribed circle diameter ϕA2of each of 50 particles are obtained, and a value obtained from (ϕA1+ϕA2)/2 is designated as the particle diameter of each particle. Then, an average value of particle diameters of 40 particles, excluding five particles having the obtained maximum particle diameter and five particles having the obtained minimum particle diameter, is designated as the average particle diameter of the catalyst10.

The first metals11are particulate metals (metal particles) containing at least one element selected from the group consisting of; Ni, Co, Fe, and Cu. The first metals11exist on the outer surface of the base material12and on the surface of the base material12in the fine pores14having opening ends on the outer surface of the base material12, and the first metals11exist inside the base material12. It is preferable that the first metals11exist to be interposed between the base material12and the first oxide13. Some of the first metals11may exist inside the base material12. These first metals are preferably any one of metal particles formed by a single element, metal particles in which a plurality of particles formed by a single element are mixed, alloy particles containing a plurality of elements, and particles in which metal particles and alloy particles are mixed. The first metals11are more preferably particles containing at least Ni particles. The first metals11are more preferably particles formed by at least one element selected from the group consisting of; Ni, Co, Fe, and Cu, from the viewpoint of obtaining a catalyst which is inexpensive and excellent in low-temperature activity.

The particle diameter of the first metals11is preferably in a range of from 2 nm to 200 nm. The reason for this is that particles having a particle diameter of less than 2 nm may less distribute to reaction, and when the particle diameter is more than 200 nm, the specific area of the catalyst may be decreased and adjacent particles are easily aggregated while in use. The range is more preferably 10 nm or more and 150 nm or less in terms of the average particle diameter.

Herein, the particle diameter of the first metals11and the average particle diameter thereof will be described. For measurement of the particle diameter of the first metals11, a surface layer portion of the catalyst10that is an area including at least the first metals11and includes at least the surface of the base material12is observed to obtain the particle diameter. In this case, the catalyst10is subjected to slice processing to have a form including the surface layer portion of the catalyst and the vicinity of the surface layer portion is observed by a TEM (Transmission Electron Microscope) with a magnification of 100,000 or more. For the photographing magnification, an appropriate magnification is selected depending on the size of the first metals11. In a case where the first metals11are covered with the first oxide13so that the first metals11cannot be confirmed, a sample is polished and then a surface in which the first metals11can be confirmed may be observed. Then, 50 particles having the clearest outline of the first metals11in the photographed image are selected, a circumscribed circle diameter ϕB1and an inscribed circle diameter ϕB2of each of 50 particles are obtained, and a value obtained from (ϕB1+ϕB2)/2 is designated as the particle diameter of each particle. As necessary, element specification may be performed by TEM-EDS (Transmission Electron Microscope/Energy dispersive Spectrometry). Then, an average value of particle diameters of 40 particles, excluding five particles having the obtained maximum particle diameter and five particles having the obtained minimum particle diameter, is designated as the average particle diameter of the first metals11.

The base material12is a base material containing at least one metal oxide selected from the group consisting of; Al2O3, MgO, TiO2, and SiO2. A plurality of these oxides may exist in a mixed state, but the base material12is a structural body having a particulate shape with a size suitable for practical use or a honeycomb shape. The first metals11and the first oxide13exist on the surface of the base material12, and the first metals11and the first oxide13are brought into physical contact with the base material12. It is preferable that the first oxide13exist on the surface other than the portion, in which the first metals11exist, of the surface of the base material12, but some of the surfaces of the base material12may be exposed surfaces in which neither the first metals11nor the first oxide13exists.

The base material12is formed by a so-called metal/ceramic composite material that is an integral structure with the first metals11which are in physical contact with the surface of the base material12or the first metals11which are in physical contact with the surface of the base material12and the first metals11which exist inside the base material12. It is preferable that the first metals11be directly bonded to (compounded with) the base material12and each particle of the first metals11exists on the base material12in an independently dispersed manner. It is more preferable that all of the first metals11be directly bonded to (compounded with) the base material12and each particle of the first metals11exists on the base material12in an independently dispersed manner. Some of the first metals11exist, as illustrated inFIGS. 2A to 2C, in the base material12in a precipitated manner. That is, some of the first metals11exist on the surface of the base material12and some of the first metals11exist inside the base material12. The first metals11in the base material12are also compounded with the base material12. Specifically, as illustrated inFIG. 1AandFIG. 1B, at least some of the first metals11are buried in the base material12. Such a structure can be confirmed by observing the cross-sectional portion of the catalyst by a TEM with high magnification. Since the catalyst10is obtained by reducing a compound of an easily-reducible oxide and a hardly-reducible oxide, the catalyst10has a structure in which the first metals11derived from the easily-reducible oxide are precipitated on the surface portion of the base material during reduction. Therefore, it is preferable that the first metals11, which are in direct contact with the surface of the base material12, among the first metals11have an entirely buried structure (80% or more of the total number of the first metals11).

The composite material formed by the first metals11and the base material12preferably has a porous structure. From the viewpoint of improving the catalyst activity at low temperature, a structure having both of macro pores and micro pores is preferable. Specifically, the composite material preferably has a structure having macro pores, which have a pore diameter of 200 nm or more and 10 μm or less, enabling reaction to act on the catalyst on the deep portion of ceramic and micro pores, which have a pore diameter of 2 nm or more and 30 nm or less, for providing a large reaction area. The fine pores14are micro pores having a pore diameter of 2 nm or more and 30 nm or less. The fine pores14are not locally but entirely contained in the catalyst10. It is preferable that the fine pores14form a three-dimensional network inside the base material12. When a lot of the fine pores14in which the first metals11and the first oxide13exist are contained in the catalyst10, the catalyst performance per volume of the catalyst10is improved. Since the fine pores14are supply and discharge passages for a reaction raw material and a reaction product, the catalyst10of the embodiments is preferable in terms of having excellent catalyst performance on the surface of the catalyst10and in the inside thereof (the fine pores14).

The composition of the oxide contained in the base material12is obtained by X-ray diffraction (XRD) measurement.

Further, the porous first oxide13containing at least one oxide selected from the group consisting of; CeO2, ZrO2, TiO2, and SiO2is formed on the outer surface portion of the base material of the metal/composite base material in a state where the first oxide13is brought into contact with the first metals11. These oxides have characteristics of helping CO or CO2, which has been adsorbed once, to be easily dissociated at lower temperature and play an important role of improving the catalyst activity at low temperature. The pore diameter of the first oxide13is preferably 2 nm or more and 10 μm or less. In addition, it is preferable that the first oxide13also exist in the fine pores14. It is preferable that the first oxide13existing in the fine pores14be in direct contact with the first metals11and the base material12to form an interface.

The first oxide13is formed to partially or entirely cover the outer surface of the composite material. The first oxide13may be formed to cover the surface portions of the first metals11on the outer surface of the catalyst10as illustrated inFIG. 1Aor may be formed in a state where the first metals11on the outer surface of the catalyst10are exposed to the surface portion as illustrated inFIG. 1B. As illustrated inFIG. 1C, a part of the base material12may have an exposed surface or may include a portion in which some of the first metals11are not brought into contact with the base material12. The first oxide13of the outer surface is preferably a porous layer-shaped product having gas diffusivity. The methanation reaction is considered to occur at a so-called three-phase interface in which CO2(or reduced CO) and hydrogen as reactive species and the first metals11as a catalyst are brought into contact with one another. The activity is improved by forming a large number of this three-phase interface. Such a porous structure enables gas to easily come and go and can increase this three-phase interface place. That is, CO2, which has been adsorbed once by the first oxide13, moved smoothly on the surface of the first metals11as a catalyst, and dissociated at low temperature, smoothly reacts with hydrogen on the surface of the first metals11, or CO2is easily reduced by oxygen defect, and thus reaction until CH4is obtained can be performed without remaining CO generated in the course of the reaction. For these reasons, even when the first oxide13has the same composition as the base material12, since the contact area with the first metals11is increased as illustrated inFIGS. 1A to 1C, an effect of further activating the reaction at low temperature can be expected.

Since the first oxide13is porous, the three-phase interface of the first metals11exists even in a case where the first metal11has or does not have an exposed surface on the outer surface of the catalyst10. Instead, since the first oxide13forms an interface with the first metals11, the first oxide13supplying CO or CO2exists in the vicinity of the first metals11so that the reaction is promoted. Further, since generated hydrocarbon or alcohol passes through porous pores of the first oxide13to be discharged to the outside of the catalyst10, the first oxide13has both functions of supplying a reaction raw material and providing a discharge passage of a product. Moreover, since the first oxide13dissociates CO or CO2at low temperature, low-temperature activity of the catalyst10using the first metals11is improved by the presence of the first oxide13.

The thickness of such a first oxide13(the thickness of the first oxide13existing on the outer surface of the catalyst10) is preferably at least 10 nm or more in terms of the average value. The reason for this is that with such a thickness of 10 nm or more, the first oxide can be formed to cover some portions of the first metals11which are precipitated by reduction. A portion, which is brought into contact with the base material12, of a general catalyst is almost dot-like; on the other hand, with the above-described configuration, a ridge portion of the first metals11can contribute to reaction. The thickness of the first oxide13is preferably thicker, but when the thickness is too thick, diffusion of gas is delayed, and thus the thickness is preferably within about 10 μm at most.

Whether the first metals11are present in the first oxide13can be analyzed by surface observation of the catalyst10by a SEM or by observation of the cross-section of the structure by a TEM. For example, the surface portion of the catalyst10is observed by a high-resolution SEM. The composition analysis of the surface portion is performed in advance by EDS or the like, a portion where the first oxide13such as CeO2exists is searched, and then the portion is enlarged to at least 10,000 times or more. Then, the acceleration voltage is changed to obtain information in the depth direction and information of the first metals11, such as Ni, located under the first oxide13, such as CeO2, can be obtained by performing photographing by a reflected electron image. The pore diameter of the first oxide13can be also obtained by the above-described method. Incidentally, also in a case where the base material12and the first oxide13are formed by the same compound, the cross-section observation described above is performed, and the structure analysis may be performed from the characteristic of the interface where crystallinity (for example grain size) varies.

In addition, the first oxide13is more preferably a solid-solubilized product with a second oxide. The second oxide more preferably contains a rare-earth oxide. The rare-earth oxide is an oxide containing at least one element selected from the group consisting of; La, Sm, Gd, and Y. Specific examples of the second oxide include an oxide such as La2O3, Sm2O3, Y2O3, Sc2O3, Gd2O3, CaO, or MgO. The reason for this is that when these oxides are solid-solubilized, the crystalline phase of the first oxide13is stabilized, oxygen defect is formed, there is a beneficent influence on dissociation behavior of CO2, and the catalyst activity at lower temperature can be improved. It is preferable that these oxides to be solid-solubilized be contained in 10 mol % or more and 60 mol % or less with respect to the number of moles of the first oxide13.

The content of the first metals11in the entire catalyst10is preferably 5% by mass or more. When the content is less than 5% by mass, the effect as the catalyst is small. In addition, the content of the first metals11is preferably 20% by mass or less. The reason for this is that when the content is more than 20% by mass, a distance between the first metals11are decreased, the first metals are easily combined or aggregated while in use (a so-called sintering phenomenon easily occurs), and thus the performance thereof are deteriorated. The content of the first metals11in the entire catalyst10is more preferably in a range of from 9.4% by mass to 16.2% by mass. The content of the first metals11in the entire catalyst10is measured by inductively coupled plasma (ICP) analysis.

Next, the method for producing the catalyst material according to the embodiment will be described.

In the following description, a catalyst having a size of 2 mm or more and 10 mm or less which is suitable for practical use is described as an example, and the embodiment is not limited thereto. For example, a material may be prepared using powder having a large specific area as a base material, and the material may be granulated in a size suitable for practical use by using an inorganic binder or by performing heat treatment.

First, a material that becomes the base material12is prepared. The material can be obtained by using powder or granulated powder of Al2O3, MgO, TiO2, SiO2, or the like and mixed powder thereof, performing molding by adding a binder or the like, and performing heat treatment under the proper conditions. Herein, in the case of using Al2O3, Al2O3of γ phase having a large specific area is preferably used. As the binder, an organic binder, an inorganic binder, or the like is appropriately selected depending on the powder to be used. Regarding molding, extrusion molding, a roll molding method, or the like can be used. The size of a molded body is preferably set to a size suitable for practical use, and is preferably about 2 mm or more and less than 10 mm. There is no particular limitation on the shape of the molded body, and the molded body can be molded or processed in an easily-handled shape such as a spherical shape, a cylindrical shape, a star shape, or a honeycomb shape.

Unless the molded body is molded or processed in a honeycomb shape, in the case of a pellet shape, since the volume of the inner portion, which does almost not contribute to reaction, of the catalyst10increases as the particle diameter of the catalyst10is larger than necessary, performance of the catalyst per volume is deteriorated. Therefore, the particle diameter of the catalyst10is preferably set to a size not exceeding 1 cm.

Next, the surface layer portion is turned into a composite oxide by using the base material12. A compound containing at least one element selected from the group consisting of; Ni, Co, Fe, and Cu is brought into contact with the porous base material12and is subjected to heat reaction, and thus at least the surface layer portion of the base material12is turned into a composite oxide. Examples of the compound include nitrate, sulfate, chloride salt, acetate, carbonate, and hydroxide which contain metal elements mentioned in the first metals11. In order to form a composite oxide up to the inner portion of the base material12, an impregnation method using a solution technique is preferably used.

For example, a case where spherical particles formed by γ-Al2O3is used as the base material12and nickel nitrate hydrate is used as a metal compound will be described as an example. The γ-Al2O3spheres are immersed in an aqueous solution of nickel nitrate, which has been dissolved in a predetermined concentration, subjected to vacuum impregnation, and dried to obtain γ-Al2O3spheres covered with nickel nitrate. The spheres are subjected to heat treatment to thermally decompose nickel nitrate, thereby forming NiO. The method of coating NiO for reacting with Al2O3of the base material12is not limited thereto. Then, the formed NiO is heated from 1000° C. to 1400° C. so that the base material12and NiO generated through thermal decomposition are reacted with each other to thereby obtain a composite base material in which a part of the base material12is turned into a NiAl2O4composite oxide.

The layer of this composite oxide is formed on the surface portion of the composite base material. The concentration of unreacted Al2O3increases toward the inner portion of the base material. Since the original amount of Al2O3as the base material12is sufficiently large with respect to NiO used in coating, there is no case where NiO remains after heat treatment. The composite oxide layer varies depending on an oxide used in the base material12, and for example, in the case of the Ni-based catalyst, when the base material to be used is MgO, NixMg1-xO (0<x<1) is formed as a main composition, when the base material to be used is TiO2, NiTiO3is formed as a main composition, and when the base material to be used is SiO2, Ni2SiO4is formed as a main composition. Similarly, the same applies to other metal species, Co, Fe, and Cu.

Next, the first oxide13is formed on the composite base material having the composite oxide layer prepared as described above. Examples of the forming method include a method in which the first metals11are precipitated by reduction treatment and then are covered with the first oxide13and a method in which the first oxide13is covered on the composite oxide layer and then the first metals11are precipitated by reduction treatment. In both methods, the first metals11are precipitated by reductive precipitation.

The former performs composite oxide reduction treatment before forming the first oxide13. The latter performs composite oxide reduction treatment after forming the first oxide13. The reduction treatment is performed at a temperature range of 600 to 1100° C. in a reducing atmosphere such as hydrogen. The reducing atmosphere is not limited thereto, and heat treatment may be performed in the presence of a carbon material and in an inert atmosphere such as Ar. In a case where the reduction temperature is lower than 600° C., precipitation of the first metals11by reduction is not sufficiently performed, and in a case where the reduction temperature is higher than 1100° C., the precipitated particles are aggregated or coarsened, which is not favorable. The proper reduction temperature varies depending on a composite oxide, and for example, in the case of Co2TiO4or the like, the reduction temperature is preferably about 700° C. The proper temperature can be determined by performing thermogravimetric analysis on the composite oxide in a reducing atmosphere. The reduction time is properly about 1 minute to 1 hour. Through the above-described treatments, the composite oxide is reduced, the incorporated metal component becomes fine particles, the fine particles are precipitated to mostly the surface of the base material12and partly the inner portion of the base material12, and thus the metal-ceramic composite material is obtained. At this time, the first metals11are formed on the base material12in a state where the first metals11independently are highly dispersed.

Next, the first oxide13of at least one selected from the group consisting of; CeO2, ZrO2, TiO2, and SiO2is formed on the ceramic to which the first metals11are precipitated. This first oxide13may be a single oxide or a mixture of a plurality of oxides. In addition, since CeO2, ZrO1, or the like stabilizes the structure as the phase and forms oxygen defect, the second oxide may be solid solution in the first oxide13. At this time, the solid-solubilized product is contained in at least a part of the first oxide13or the entire of the first oxide13may be a solid-solubilized product. Such a solid-solubilized product can be produced by, for example, performing impregnation, coating, and heat calcination on an aqueous solution prepared by mixing a plurality of metal salts. It should be noted that in some aqueous solutions, a metal component such as Ni may be easily eluted. In this case, the precipitated first metals11are treated to be turned into an oxide by performing heat treatment in air, and then impregnated and covered so that a solid-solubilized product can be formed. The latter producing method is different from the former producing method only in order of treatments, and has the same treatment method as in the former producing method. Therefore, the description of the latter producing method is not provided.

Some of the first oxide13and the base material12may be reacted with each other during heat treatment. For example, CeO2as the first oxide13and Al2O3of the base material12may be reacted with each other to form CeAl2O3, but this phase itself also contributes to improvement in catalyst activity.

Second Embodiment

A catalyst according to a second embodiment includes a base material containing at least one oxide selected from the group consisting of; Al2O3, MgO, TiO2, and SiO2, first metals brought into contact with the base material and containing at least one metal selected from the group consisting of; Ni, Co, Fe, and Cu, a porous first oxide brought into contact with the first metals and the base material and containing at least one selected from the group consisting of; CeO2, ZrO2, TiO2, and SiO2, and second metals. Herein, a main component of the second metals preferably contains a metal different from a metal included in the first metals among at least one metal selected from the group consisting of; Ni, Co, Fe, and Cu.

FIGS. 3A to 3Cillustrate schematic diagrams of the cross-section of a catalyst20according to the second embodiment (FIGS. 3A to 3Cillustrating two structures similarly to the first embodiment). The basic configurations of materials other than second metals15are the same as those in the first embodiment, and thus the details thereof are not provided.FIG. 3A,FIG. 3B, andFIG. 3Care different from the cross-sectional views ofFIG. 1A,FIG. 1B, andFIG. 1Cin that the second metals15represented as black dots are included. The characteristic of the catalyst20in the second embodiment is that the first oxide13has the second metals15different from the first metals11which are formed by reductive precipitation. InFIGS. 3A to 3C, the primary particle diameter of the second metals15is smaller than the primary particle diameter of the first metals11, but the magnitude relation between the particle diameters is not limited thereto. Incidentally,FIGS. 4A, 4B, and 4Cillustrate enlarged schematic diagrams of parts ofFIGS. 3A, 3B, and 3C. The enlarged schematic diagrams ofFIGS. 4A, 4B, and 4Care also similar to the enlarged schematic diagrams ofFIGS. 2A to 2C, except that the second metals15exist in the first oxide13and the fine pores14. InFIGS. 4A to 4C, similarly toFIGS. 2A to 2C, the second metals15existing in the first oxide13A on the outer surface of the catalyst10are referred to as second metals15A, and the second metals15existing in the fine pores14are referred to as second metals15B.

InFIGS. 4A to 4C, the second metals15are contained in the catalyst10. In all ofFIGS. 4A to 4C, the second metals15A exist in the first oxide13A. The second metals15A may have interfaces with the first metals11A. Further, the second metals15B exist in the fine pores14. The second metals15B existing in the fine pores14have interfaces with the first oxide13B. It is preferable that the second metals15B existing in the fine pores14have interfaces with the first metals11B. The second metals15B existing in the fine pores14may also have interfaces with the base material12and it is preferable that the second metals15B be in direct contact with the base material12.

The second metals15described herein are particulate metals (particles) including a metal different from a metal included in the first metals among at least one metal selected from the group consisting of; Ni, Co, Fe, and Cu. The size of the second metals15is preferably 1 nm or more and 100 nm or less in terms of the primary particle diameter. The particle diameter of the second metals15is obtained in the same method as in the first metals11. Several particles may be aggregated to form secondary particles. The second metals15exist in the first oxide13, or in the first oxide13and in the fine pores14. The second metals15may exist in the first oxide13while being brought into contact with the base material12and the first metals11or may exist in the first oxide13while being separated therefrom. In addition, the second metals15may exist on the surface portion while being brought into contact with the first oxide13. However, it is more preferable that the second metals15exist in the vicinity of the first metals11since the second metals15act as a cocatalyst of the first metals11. It is more preferable that the second metals15have interfaces with the first metals11. When the second metals15exist in the first oxide13, an effect is also achieved in which the second metals15are immobilized and the aggregation of particles caused by sintering is suppressed so that durability is improved. It is more preferable that the second metals15exist also in the fine pores14. It is preferable that the second metals15have interfaces with the first metals11and the base material12since the second metals act as a cocatalyst. The second metals15have interfaces with the first metals11in the fine pores14. However, it is more preferable that the second metals15have interfaces with the first metals11and the base material12. Further, it is more preferable that the second metals15also have interfaces with the first oxide13.

The second metals15exhibit higher catalyst activity particularly in a case where the second metals15are combined with Ni as the first metals11. In addition, the proportion of the first metals11and the second metals15in the entire catalyst20is preferably 5% by mass or more and 20% by mass or less. The reason for this is as follows. When the proportion is less than 5% by mass, the effect as the catalyst20is small. When the proportion is more than 20% by mass, although there is no serious problem in the catalyst activity, the aggregation of metal particles occurs by sintering or the like in the case of long-term use, and thus the performance is deteriorated. From the viewpoint of efficiently utilizing the catalyst20without waste, the proportion is more preferably in a range of from 5% by mass to 15% by mass.

A particle diameter of the first metals11in which a cumulative volume curve in particle size distribution is 16% is preferably larger than a particle diameter of the second metals15in which a cumulative volume curve in particle size distribution is 84%. That is, the size of the first metals11is larger than the size of the second metals15such that the particle size distribution of the first metals11and the particle size distribution of the second metals15do almost not overlap. When such a relation is satisfied, it is not easily inhibited that the second metals15are contained in the catalyst20, and this contributes to improvement in catalyst activity. It is considered that the first metals11act as a cocatalyst when the second metals15smaller than the first metals11in size exist in the vicinity of the first metals11. For example, when Ni with high hydrogenation activity is used as the first metals11and Fe with hydrogenation activity lower than that of Ni is used as the second metals15, catalyst activity is further improved, which is preferable. When the size of the second metals15is adjusted to be finer than the size of the first metals11such that the above particle size distribution is satisfied, the effect of improving catalyst activity with respect to the amount of the second metals15added becomes more significant, which is preferable.

As the method for producing the catalyst20containing the second metals15, for example, a method is mentioned in which a mixed oxide layer is formed on a composite oxide such that the oxide of the second metals15that is an easily-reducible oxide is contained in the first oxide13and then reduction treatment is performed.

Next, with reference to the schematic diagram ofFIG. 5, a fuel synthesis apparatus (system) using the catalyst of the embodiment will be described. The fuel synthesis apparatus includes a reaction column X provided with the fuel synthesis catalyst, a first raw material supply line L1configured to supply one or both of carbon dioxide and carbon monoxide to the reaction column, a second raw material supply line L2configured to supply hydrogen to the reaction column X, and a recovery unit Y configured to recovery a fuel generated by reacting one or both of the carbon dioxide and the carbon monoxide with the hydrogen using the catalyst in the reaction column X.

The reaction column X and the recovery unit Y are connected via a product supply line L3, and the fuel generated in the reaction column X moves from the reaction column X to the recovery unit Y through the product supply line L3. It is preferable that impurities in the product be removed in the recovery unit Y. The fuel generated in reaction column contains one or both of hydrocarbon fuel and alcohol fuel. In addition, although not illustrated in the drawing, the fuel synthesis apparatus is preferably configured such that unreacted carbon dioxide or carbon monoxide and hydrogen are separated from each other in the recovery unit Y and the separated components are sent to the reaction column X to be used for reaction. In addition, the recovery unit Y may further include a unit configured to consume fuel, for example, generate power using the fuel.

The temperature of reaction of the carbon dioxide or carbon monoxide with the hydrogen using the catalyst is preferably 250° C. or higher but 400° C. or lower. When the temperature is too low, the catalyst activity is lowered, which is not favorable. In addition, when the temperature is too high, deterioration of the catalyst is accelerated and energy necessary for reaction is required much more, which is not favorable. From these points of view, the temperature of reaction of the carbon dioxide or carbon monoxide with the hydrogen using the catalyst is more preferably 300° C. or higher but 350° C. or lower. At such a low temperature, a catalyst of the related art has low catalyst activity and thus is not suitable for practical use; on the other hand, the catalyst of the embodiment has excellent low-temperature activity and thus is suitable for practical use even at low-temperature conditions.

In addition, as illustrated in the schematic diagram ofFIG. 6, the power generation and the fuel synthesis apparatus (system) may be combined. Hydrogen generated by performing water electrolysis using electric power (surplus power) generated by sunlight or wind power can be used as hydrogen used in the fuel synthesis. In addition, when carbon dioxide generated by thermal power generation or the like is used as carbon dioxide used in fuel synthesis, fuel synthesis that is carbon-neutral and uses renewable energy can be performed.

EXAMPLES

Hereinafter, specific examples will be mentioned and the effect thereof will be described. However, embodiments are not limited to these examples.

A catalyst material was prepared under the conditions as described below.

γ-Al2O3spherical particles having a particle diameter of 2 to 4 mm were immersed in an aqueous solution of nickel nitrate (Ni(NO3)2.6H2O) and subjected to impregnation treatment for 2 hours under reduced pressure in a vacuum desiccator. A sample was taken out and subjected to dry treatment, and then heat treatment was performed at 500° C. for 1 hour in air with an electrical furnace so that nickel nitrate was thermally decomposed to obtain NiO. Further, the NiO was reacted with Al2O3of the base material by increasing the temperature to 1200° C. and performing calcination treatment for 2 hours so that some of Al2O3of the base material became NiAl2O4that is the composite oxide. Next, this calcined sample was subjected to reduction treatment at 1000° C. for 10 minutes in hydrogen so that first metals of Ni were precipitated from NiAl2O4portion. Further, the sample subjected to reduction was immersed in an aqueous solution of cerium nitrate (Ce(NO3)3.6H2O) and after the resultant solution was dried, heat treatment was performed at 500° C. for 1 hour so that a CeO2layer was formed as the first oxide on the catalyst surface. The catalyst performance evaluation test was carried out using this sample. As a result of evaluation of the composition of this sample by ICP emission spectrometry, the content of Ni was 9.4% by mass.

For the test, a fixed-bed flow type reaction apparatus was used. A reaction tube having an inner diameter of 42.8 mm was charged with the catalyst mixed with Al2O3spheres (average size of 3 mm) unrelated to the reaction, hydrogen reduction was performed at 400° C. for 1 hour, and then a gas prepared by mixing CO2and H2at a flow ratio of 1:4 was supplied at a space velocity of 6000/h. The CO2conversion and methane yield at from 250° C. to 400° C. were obtained by analyzing the outlet gas after the reaction by micro gas chromatograph. The methane yield was calculated by the following equation.
Methane yield=CO2conversion×methane selectivity   (2)

Further, for some samples, identification of the constituent phase by XRD and measurement of micro pore distribution by a nitrogen adsorption method and a mercury intrusion method were carried out. In addition, the microstructure of the sample was observed by a SEM.

Comparative Example 1

In Example 1, a sample not subjected to coating treatment using cerium was prepared, and the catalyst performance evaluation test was performed.

In Example 1, the reduction treatment of the composite oxide and the formation of the CeO2layer were performed in the reverse order, that is, impregnation and coating treatment with cerium nitrate was performed before the reduction treatment at 1000° C., the resultant product was subjected to heat treatment at 500° C., and then reduction treatment was performed at 1000° C. for 10 minutes. The catalyst performance evaluation test using the obtained sample was performed. In the case of preparation in this order, the covered CeO2layer was reacted with Al2O3as the base material at the time of reduction to form CeAlO3.

A sample was prepared in the same manner as Example 1, except that γ-Al2O3spherical particles in which micro pores having a size of several nm were increased in amount and relatively large pores having a size of about 1 μm were also contained were used as the base material, and the catalyst performance evaluation test was performed. As a result of evaluation of the composition of this sample by ICP emission spectrometry, the content of Ni was 16.2% by mass.

A sample was prepared in the same procedures as in Example 1, except that a mixed aqueous solution of cerium nitrate and samarium nitrate (Sm(NO3)3.6H2O) at a molar ratio of 1:0.2 was used in the formation treatment of the CeO2layer, and the catalyst performance evaluation test was performed.

A sample was prepared in the same procedures as in Example 1, except that a mixed aqueous solution of cerium nitrate and yttrium nitrate (Y(NO3)3.6H2O) at a molar ratio of 1:0.2 was used in the formation treatment of the CeO2layer, and the catalyst performance evaluation test was performed.

A sample was prepared in the same procedures as in Example 1, except that a mixed aqueous solution of cerium nitrate and gadolinium nitrate (Gd(NO3)3.6H2O) at a molar ratio of 1:0.2 was used in the formation treatment of the CeO2layer, and the catalyst performance evaluation test was performed.

A sample was prepared in the same procedures as in Example 1, except that a mixed aqueous solution of cerium nitrate and lanthanum nitrate (La(NO3)3.6H2O) at a molar ratio of 1:0.2 was used in the formation treatment of the CeO2layer, and the catalyst performance evaluation test was performed.

A sample was prepared in the same procedures as in Example 1, except that a mixed aqueous solution of cerium nitrate and lanthanum nitrate (La(NO3)3.6H2O) at a molar ratio of 1:0.4 was used in the formation treatment of the CeO2layer, and the catalyst performance evaluation test was performed.

A sample was prepared in the same procedures as in Example 1, except that the concentration of cerium nitrate was changed to twice that of cerium nitrate in the formation treatment of the CeO2layer, and the catalyst performance evaluation test was performed.

An aqueous solution of zirconium oxynitrate (ZrO(NO3)2.2H2O) was used for forming the ZrO2layer instead of the formation of the CeO2layer, instead of cerium nitrate. Since a phenomenon that Ni of metal was eluted in the aqueous solution of zirconium oxynitrate was observed, Ni precipitated by reduction was subjected to oxidation treatment in advance to form NiO, and then impregnation and coating treatment was performed. A sample was prepared in the same procedures as in Example 1 except the above-described procedures, and the catalyst performance evaluation test was performed.

A sample was prepared in the same procedures as in Example 5, except that a mixed aqueous solution of zirconium oxynitrate and samarium nitrate at a molar ratio of 1:0.2 was used for forming the ZrO2layer instead of the CeO2layer, and the catalyst performance evaluation test was performed.

A sample was prepared in the same procedures as in Example 10, except that a mixed aqueous solution of zirconium oxynitrate and yttrium nitrate at a molar ratio of 1:0.2 was used for forming the ZrO2layer, and the catalyst performance evaluation test was performed.

A sample was prepared in the same procedures as in Example 10, except that a mixed aqueous solution of zirconium oxynitrate and gadolinium nitrate at a molar ratio of 1:0.2 was used for forming the ZrO2layer, and the catalyst performance evaluation test was performed.

A sample was prepared in the same procedures as in Example 10, except that a mixed aqueous solution of zirconium oxynitrate and lanthanum nitrate at a molar ratio of 1:0.2 was used for forming the ZrO2layer, and the catalyst performance evaluation test was performed.

A sample was prepared by changing the order of the reduction treatment and the oxide layer formation treatment in Example 10, and the catalyst performance evaluation test was performed.

A sample was prepared in the same procedures as in Example 5, except that a mixed aqueous solution of zirconium oxynitrate and cerium nitrate at a molar ratio of 1:1 was used for forming the ZrO2layer, and the catalyst performance evaluation test was performed.

A sample was prepared in the same procedures as in Example 1, except that hydrolysis of tetraethyl orthosilicate (TEOS) and condensation reaction were used for forming the SiO2layer instead of the CeO2layer, and the catalyst performance evaluation test was performed.

A sample was prepared in the same procedures as in Example 1, except that titanium tetraethoxide (Ti(OCH2H5)4) was used and hydrolysis and heat treatment were performed for forming the TiO2layer instead of the CeO2layer, and the catalyst performance evaluation test was performed.

Comparative Example 2

An aqueous solution of nickel nitrate was subjected to vacuum impregnation by using a cylindrically-formed MgO porous body as the base material. After drying, the MgO surface layer portion and the Ni component were reacted with each other by heat treatment at 1200° C. for 2 hours to form a solid-solubilized product of MgO and NiO (NixMg1-xO, 0<x<1). The solid-solubilized product was subjected to reduction treatment at 1000° C. for 10 minutes in a hydrogen atmosphere to obtain a catalyst material having Ni particles on the surface layer portion. The catalyst performance evaluation test on this material was performed.

A catalyst material having Ni particles on the surface layer portion of Comparative Example 2 was further immersed in an aqueous solution of cerium nitrate and subjected to vacuum impregnation, and after drying the catalyst material, heat treatment was performed at 500° C. for 1 hour to form a CeO2layer. The catalyst performance evaluation test on this sample was performed.

A sample was prepared in the same procedures as in Example 10, except that zirconium oxynitrate was used in a ZrO2layer and a ZrO2layer was formed, and the catalyst performance evaluation test was performed.

Comparative Example 3

An aqueous solution of nickel nitrate was subjected to vacuum impregnation by using a spherically-formed TiO2porous body having a particle diameter of 2 to 4 mm as the base material. After drying, the TiO2surface layer portion and Ni component were reacted with each other by heat treatment at 1300° C. for 2 hours to form a NiTiO3layer. The NiTiO3layer was subjected to reduction treatment at 800° C. for 10 minutes in a hydrogen atmosphere to obtain a catalyst material having Ni particles on the surface layer portion. The catalyst performance evaluation test on this material was performed.

A catalyst material having Ni particles on the surface layer portion of Comparative Example 3 was further immersed in an aqueous solution of cerium nitrate and subjected to vacuum impregnation, and after drying the catalyst material, heat treatment was performed at 500° C. for 1 hour to form a CeO2layer. The catalyst performance evaluation test on this sample was performed.

Comparative Example 4

An aqueous solution of nickel nitrate was subjected to vacuum impregnation by using a spherically-formed SiO2porous body having a particle diameter of 2 to 4 mm as the base material. After drying, the TiO2surface layer portion and Ni component were reacted with each other by heat treatment at 1400° C. for 1 hour to form a Ni2TiO4layer. The Ni2TiO4layer was subjected to reduction treatment at 1000° C. for 10 minutes in a hydrogen atmosphere to obtain a catalyst material having Ni particles on the surface layer portion. The catalyst performance evaluation test on this material was performed.

A catalyst material having Ni particles on the surface layer portion of Comparative Example 4 was further immersed in an aqueous solution of cerium nitrate and subjected to vacuum impregnation, and after drying the catalyst material, heat treatment was performed at 500° C. for 1 hour to form a CeO2layer. The catalyst performance evaluation test on this sample was performed.

The performance test results of the catalysts of Examples and Comparative Examples are collectively presented in Table 1.

In Example 1, after the reductive precipitation treatment of the catalyst, an aqueous solution of cerium nitrate (Ce(NO3)3.6H2O) and iron nitrate (Fe(NO3)3.9H2O) were mixed at a molar ratio of 10:1, the mixed solution was subjected to impregnation and dried, and then heat treatment was performed at 500° C. for 1 hour to form a mixed layer of Fe2O3—CeO2on the surface of the catalyst. The catalyst performance evaluation test was performed using this sample. Fe2O3was reduced in the reduction treatment before the test and the covered layer became the CeO2layer having Fe fine particles dispersed therein.

In Example 1, after the reductive precipitation treatment of the catalyst, an aqueous solution of cerium nitrate (Ce(NO3)3.6H2O) and cobalt nitrate (Co(NO3)2.6H2O) were mixed at a molar ratio of 10:1, the mixed solution was subjected to impregnation and dried, and then heat treatment was performed at 500° C. for 1 hour to form a mixed layer of CoO—CeO2on the surface of the catalyst. The catalyst performance evaluation test was performed using this sample. CoO was reduced in the reduction treatment before the test and the covered layer became the CeO2layer having Co fine particles dispersed therein.

As clearly seen from Tables (Table 1A and Table 1B), it was found that the first metals and the base material and the first oxide, which are integrated with the first metals, were allowed to simultaneously exist and thus the methane yield was improved.

Particularly, in a case where the oxide layer (the first oxide) was formed after reduction treatment, including Example 1, it was found that the covered oxide was formed without a change in composition thereof and methane was generated at a higher yield. In addition, although not shown in Table 1, it was clear that the generation of CO, which was observed in Comparative Example 1, was almost not observed and CO2was rapidly reduced to CH4.

Further, in the case of the Al2O3base material, the crystalline phase was changed from a γ type of low-temperature type to an α type at the stage of forming the NiAl2O4layer by sintering. According to this, it was found that the specific area was decreased; however, when pores having a size of about 1 μm were introduced as described in Example 3, the metal component could be impregnated and precipitated in a wider range, and the first metals formed in the deep portion could also contribute to reaction so that the activity was further improved.

FIG. 7shows a photographed image of the typical microstructure of the catalyst according to the embodiment.FIG. 7is obtained by observing the structure of the surface portion of the catalyst prepared in Example 2 and is a reflected electron image photographed in such a manner that an acceleration voltage was set to slightly high and thus the structure of the inner portion of the sample was further captured. In the drawing, particulate white spots are Ni particles of metal. The size of Ni particles was about 30 to 60 nm and was sufficiently larger than the size of Ni particles of a general catalyst. In this way, the Ni particles each independently exist on the surface portion of the catalyst and have a structure in which the particles are highly dispersed. This structure is considered to improve durability. In addition, a Ce-containing layer was formed on the surface portion of this particle and in the vicinity thereof. In this case, CeO2subjected to coating treatment at the time of reduction treatment was reacted with Al2O3of the base material to form a CeAlO3layer. From a result of catalyst performance test, activity is improved as compared with the case of having no layer. That is, it was found that even when the oxide layer and the base material as described above were reacted with each other to form a compound, there is no problem in improvement of catalyst activity and the effect is achieved.

The sample described in Example 1 in which the oxide layer was formed after the other treatments were performed was also observed with a SEM, but the structure thereof was the same phase as illustrated inFIG. 1B.

Regarding the content of Ni, the test was also performed on a sample having a Ni supported amount of less than 5 wt %, but although there was a CeO2layer, the catalyst for changing the dissociated CO2into methane was not sufficient and the performance thereof was deteriorated. In contrast, when the Ni supported amount was increased, the activity was improved, but the activity almost reached to the peak in an area having a Ni supported amount of more than 25% by mass. When the supported amount increases, there is a concern of deterioration in performance caused by combining or growth of Ni particles while in use. For this reason, the supported amount of the first metals is preferably set to 20% by mass or less. Further, regarding the first oxide, in addition to CeO2, the same effect was also recognized in SiO2, TiO2, and ZrO2. Regarding to material that is coated with rare-earth oxide and is heat-treated at the same time, solid-solubilization into CeO2and ZrO2and improvement in catalyst performance at low temperature were confirmed.

Furthermore, the catalyst performance test was performed on a sample in which a component such as Co, Fe, or Cu was added in advance instead of Ni and the component was precipitated by reduction at the same time of precipitation of Ni. As a result of the test, in all cases, the methane yield was lowered as compared with a case where Ni was added alone. In addition, remaining CO, which is generated when CO2is not completely turned into CH4, was observed. In contrast, in the catalysts using Co particles or Fe particles together with Ni particles as the first metals, the methane yield is improved. When the oxide layer such as CeO2was provided in the catalysts, it was confirmed that the catalyst activity is improved.

The performance evaluation results of the catalysts in which Fe and Co were added as the second metals (Example 23 and Example 24) are presented in Table 1. The methane yield at the low temperature region is improved as compared with the case of using only the first metals. On the other hand, in another test, the system in which Fe and Co are simply supported on Al2O3did almost not exhibit the catalyst activity. The reason for this is found that this catalyst activity is a specific phenomenon occurring when the second metals are combined with the first metals. It is considered that fine particles of Fe and Co play a role of assisting the catalyst performance of Ni, that is, act as a cocatalyst. Further, the catalysts of Examples were observed with TEM-EDX to confirm that the first metals, the first oxide, and the second metals exist on the surface of the base material in the fine pores. In addition, some of the fine pores were connected with each other and unevenness having a size of 5 μm or more and 20 μm or less was confirmed on the outer surface of the catalyst. A mapping result of EDX in which it is considered that some of the first metals have interfaces with the first oxide or the first oxide and the second metals as well as the base material was obtained. It is considered that low-temperature activity is improved by improving catalyst characteristics when a lot of catalyst activity points exist in the fine pores.

It was confirmed that the catalysts of Examples are excellent in activity at low temperature without use of a noble metal in a catalytic metal (metal particles). In addition, since the metal particles are configured to be held by both the base material and the oxide layer, the metal particles are less likely to drop off even in high-temperature environment or long-term use. For this point of view, the catalysts of Examples (embodiments) are catalysts that are excellent in terms of all aspects of inexpensiveness, high low-temperature activity, and durability, which are required for practical use. In addition, although methane is generated using the catalyst in Examples, similarly, methanol is generated by changing a raw material from carbon dioxide to carbon monoxide. Further, these catalysts are expected to exhibit higher performance even under increased pressure generally used for increasing the reaction rate.

In the specification, some elements are described only by chemical symbols.