Catalyst for synthesis gas production used in dry reforming and method for producing synthesis gas

Abstract

To provide a catalyst for syngas production used for dry reforming, which has a high conversion rate and can maintain catalytic activity for a long time, and a method for producing a syngas.SOLUTION: A catalyst for syngas production includes reacting a source gas including methane with carbon dioxide to produce a syngas including carbon monoxide and hydrogen, wherein the catalyst is a carrier in which a sub nanoparticle composed of at least one 3d transition metal selected from Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn is supported on a carrier.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a catalyst for synthesis gas production used in dry reforming and a method for producing synthesis gas. [Background technology]

[0002] A mixed gas containing carbon monoxide and hydrogen is known as a chemical raw material from which various chemical products such as synthetic gasoline and alcohol are synthesized, and has traditionally been produced by steam reforming charcoal or coal at high temperatures.

[0003] Methane dry reforming (DRM), which synthesizes this from a mixture of methane and carbon dioxide, has recently attracted attention from the perspective of highly efficient utilization of natural gas and mitigating global warming because it can convert methane and carbon dioxide, which are the main components of natural gas and also major greenhouse gases, into chemical raw materials.

[0004] DRM is a methane reforming reaction, represented by the equation CH4 + CO2 = 2H2 + 2CO. The main reaction of DRM competes strongly with carbon deposition reactions (2CO = solid carbon + CO2 and CH4 = solid carbon + 2H2), especially in the low-temperature range. That is, in the temperature range of 550°C to 700°C, side reactions such as CO disproportionation and the production of solid carbon through methane decomposition, in other words, coking (the production of soot as a byproduct), become significant. The deposited solid carbon leads to catalyst deactivation and blockage of the raw material gas flow, resulting in decreased production efficiency and deterioration of the reactor. Suppressing carbon deposition has been one of the most important challenges in developing a DRM catalyst that is active and long-lived in the low-temperature range. In addition, under actual reaction conditions, multiple reactions compete, and in the temperature range of 500°C to 700°C, the production of water by another side reaction, such as the reverse water shift reaction (CO2 + H2 = H2O + CO), becomes significant.

[0005] Conventional DRM (Direct Response Modulation) has been carried out under high-temperature conditions exceeding 800°C to avoid blockage of catalytic reactors due to coking, which is particularly pronounced in low-temperature ranges. While many reports exist for catalytic converters, including bulk and nanoparticle catalysts, at temperatures above 700°C, these methods have drawbacks such as high fuel consumption, leading to high costs, and catalyst inactivation due to heat. Due to high fuel consumption, practical industrial-scale applications have not yet been achieved.

[0006] Furthermore, while the low-temperature range below 500°C, where solid carbon is not produced, has low fuel consumption and low costs, there are still few research examples due to the high activation energy of the substrate. The development of catalysts that can carry out reactions in this low-temperature range is highly desired.

[0007] It is known that precious metal catalysts such as platinum are active in the dry reforming reaction of methane with carbon dioxide (Non-Patent Documents 1 and 2). However, platinum is expensive and rare, and its conversion rate is significantly lower than the theoretical upper limit obtained from thermodynamic equilibrium. Furthermore, its catalytic activity decreases with prolonged use.

[0008] Base metals, particularly 3d transition metals, are useful for industrial applications due to their low cost and abundance on Earth. Among the 3d metals, nickel has been reported as a catalyst for DRMs, both in bulk form and as nanoparticles (Non-Patent Literature 3) alloyed with zirconium. However, the conversion rate is significantly lower than the theoretical upper limit derived from thermodynamic equilibrium, and the challenge remains that harsh reaction conditions such as high temperature and high pressure are required to increase reaction efficiency.

[0009] In recent years, the catalytic activity of nanoparticles composed of precious metal elements such as platinum and gold has attracted attention. Meanwhile, the synthesis of sub-nanoparticles, which have fewer atoms and smaller size than nanoparticles, has been reported. These sub-nanoparticles, which are about 1 nm in size, are composed of only a few dozen atoms or less, and due to quantum size effects, their band gap is large and discrete, and they form unique geometric structures specific to the elements. Therefore, they have aspects that cannot be captured by the conventional way of thinking about nanoparticles, which have been discussed based on simple structural characteristics such as size and shape, and are expected to exhibit catalytic activity and reaction selectivity that surpasses that of nanoparticles.

[0010] As a method for producing subnanoparticles, the present inventors have explored and developed a method using phenylazomethine dendrimers as a template (Non-Patent Documents 4, 5, Patent Documents 1-7). Phenylazomethine dendrimers form complexes with metal salts in a stepwise manner, preferentially starting from the innermost imine layer, due to an electron density gradient in which the electron density of the imine increases from the terminal imine towards the center. This makes it possible to create unicomponent systems using a single metal, such as a platinum salt, as well as binary systems using two different metal salts, where different metal species are accumulated in each layer, and even multicomponent systems of three, four, five, or more metals, enabling the preparation of dendrimer complexes with a defined number and composition of metal salts. By reducing these dendrimer complexes coordinated with metal salts, it becomes possible to form precisely controlled metal subnanoparticles with an extremely small particle size distribution. These platinum and other subnanoparticles have been reported to exhibit significantly higher oxygen reduction catalytic activity per unit mass than 3-5 nm nanoparticles. The inventors are investigating the use of platinum subnanoparticle supports, which utilize phenylazomethine dendrimers as templates, as catalysts in hydrocarbon oxidation reactions. However, dry reforming reactions using a mixed gas of methane and carbon dioxide as a substrate and subnanoparticles as catalysts have not yet been investigated. [Prior art documents] [Non-patent literature]

[0011] [Non-Patent Document 1] D. Pakhare, et al., Chem. Soc. Rev. 2014, 43, 7813-7837. [Non-Patent Document 2] T. Kobayashi et al., Chem. Eng. J. 2019, 377, 120203. [Non-Patent Document 3] Y. Wang, et al., ACS Catal. 2018, 8, 6495-6506. [Non-Patent Document 4] K. Yamamoto et al. Angew. Chem. Int. Ed. 2015, 54, 9810-9815. [Non-Patent Document 5] K. Yamamoto et al. Sci. Adv. 2017, 3, e1700101. [Patent Documents]

[0012] [Patent Document 1] Patent No. 4511125 [Patent Document 2] International Publication No. 2004 / 9076531 [Patent Document 3] Japanese Patent Publication No. 2010-18610 [Patent Document 4] Japanese Patent Publication No. 2013-159588 [Patent Document 5] Japanese Patent Publication No. 2007-23166 [Patent Document 6] Japanese Patent Publication No. 2017-087151 [Patent Document 7] Japanese Patent Publication No. 2018-145172 [Overview of the project] [Problems that the invention aims to solve]

[0013] This invention has been made in view of the above circumstances, and its main objective is to provide a catalyst for synthesis gas production used in dry reforming and a method for producing synthesis gas, which have a high conversion rate and can maintain catalytic activity for a long period of time. Among the main challenges mentioned above, another challenge is to provide a catalyst for synthesis gas production and a method for producing synthesis gas that has a high conversion rate even at low temperatures of 500°C or less. Among the main challenges mentioned above, another challenge is to provide a catalyst for synthesis gas production and a method for producing synthesis gas that can suppress aggregation even at high temperatures of 700°C or higher and maintain catalytic activity for a long period of time. Among the main challenges mentioned above, another challenge is to provide a catalyst for synthesis gas production and a method for producing synthesis gas that can suppress deactivation due to carbon deposition even in the temperature range where solid carbon is generated, and maintain catalytic activity for a long period of time. [Means for solving the problem]

[0014] As a result of diligent research to solve the above problems, the inventors of this invention discovered that by using 3d transition metal elements, which are base metals that are abundant in the environment, as sub-nanoparticle clusters and applying them as a catalyst for the CH4 dry reforming reaction, it is possible to extract catalytic function and maintain catalytic activity for a long period of time with a high conversion rate, thus completing the present invention. In other words, the synthesis gas production catalyst of the present invention is a synthesis gas production catalyst that reacts a raw material gas containing methane with carbon dioxide to produce synthesis gas containing carbon monoxide and hydrogen, and is characterized in that it is a support in which subnanoparticles made of at least one 3d transition metal selected from Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn are supported on a carrier. The present invention relates to a method for producing synthesis gas, which involves reacting a raw material gas containing methane with carbon dioxide to produce synthesis gas containing carbon monoxide and hydrogen, characterized in that the catalyst is used as the reaction catalyst. The synthesis gas containing carbon monoxide and hydrogen produced in this invention can be used as an industrially useful gas, for example, as a chemical raw material from which various chemical products such as synthetic gasoline and alcohol can be synthesized. [Effects of the Invention]

[0015] According to the present invention, the conversion rate is high and catalytic activity can be maintained for a long period of time. In particular, if the subnanoparticles are pure Ni or an alloy containing Ni, a high conversion rate can be obtained even at low temperatures of 500°C or less. In particular, by using the aforementioned sub-nanoparticles, aggregation can be suppressed even at high temperatures of 700°C or higher, and catalytic activity can be maintained for a long period of time. In particular, by using the aforementioned sub-nanoparticles, deactivation due to carbon deposition can be suppressed even in the temperature range where solid carbon is generated, and catalytic activity can be maintained for a long period of time. [Brief explanation of the drawing]

[0016] [Figure 1] (A) is the mass spectrum measured at 200-900°C after carrying out a DRM reaction using a Ni28 / silica catalyst pre-reduced with H2 at 600°C, and (B) separately shows the production of trace amounts of water. [Figure 2] (A) is the mass spectrum measured at 200-900°C after carrying out a DRM reaction using a Ni28 / silica catalyst pre-reduced with H2 at 500°C, and (B) separately shows the production of trace amounts of water. [Figure 3] (A) is the mass spectrum measured at 200-900°C after carrying out a DRM reaction using a Ni28 / silica catalyst pre-reduced with H2 at 400°C, and (B) separately shows the production of trace amounts of water. [Figure 4] These are mass spectra measured at 200-900°C after performing DRM reactions using (A) Ni bulk / silica, (B) l-Nps / silica, (C) s-Nps / silica, and (D) Ni28 / silica. The vertical axis represents the composition ratio. [Figure 5]These are mass spectra measured at 200-900°C after performing DRM reactions using (A) Ni4 / silica, (B) Ni12 / silica, (C) Ni28 / silica, and (D) Ni60 / silica. The vertical axis represents the composition ratio. [Figure 6] This shows the catalyst size dependence of the DRM reaction initiation temperature based on the mass intensity at m / z=2 corresponding to H2 measured during the DRM reaction. All catalysts were standardized to 2 wt%. [Figure 7] This shows the catalyst size dependence of the DRM reaction initiation temperature based on the mass intensity at m / z=2 corresponding to H2 measured during the DRM reaction. A sufficient amount of catalyst relative to the substrate was used to eliminate activity differences due to differences in catalyst surface area. [Figure 8] (A) shows the mass spectral intensity measured during the DRM reaction using a Ni28 / silica catalyst pre-reduced with H2 at 500°C. (B) shows the results of normalizing each spectrum by BelMASS, and (C) is a simulation of the equilibrium quantity ratio of the DRM reaction at 1 atm and an inlet supply ratio of CO2 / CH4=1, performed by the Gibbs free energy minimization algorithm in the software HSC Chemistry. [Figure 9] This shows a comparison between a plot of the conversion rate at the start of a DRM reaction using a Ni28 / silica catalyst, with the reaction temperature kept constant at 100°C intervals from 200°C to 900°C, and a theoretical curve obtained by simulation using the Gibbs free energy minimization algorithm with HSC Chemistry10 software. [Figure 10] (A) is a simulation of the equilibrium ratio of the DRM reaction, (B) is the change over time in the conversion rate of CH4 (bottom) and CO2 (top) after a 10-hour DRM reaction at 400°C and (C) at 500°C. [Figure 11] (A) shows a simulation of the equilibrium ratio of the DRM reaction, (B) shows the change over time in the conversion rate of CH4 (bottom) and CO2 (top) after a 10-hour DRM reaction at 600°C, and (C) shows the change over time in the conversion rate of CH4 (bottom) and CO2 (top) after a 10-hour DRM reaction at 700°C. [Figure 12](A) shows a simulation of the equilibrium ratio of the DRM reaction, (B) shows the change over time in the conversion rate of CH4 (bottom) and CO2 (top) after a 10-hour DRM reaction at 800°C, and (C) shows the change over time in the conversion rate of CH4 (bottom) and CO2 (top) after a 10-hour DRM reaction at 900°C. [Figure 13] These are STEM images of Ni28 / silica after a DRM reaction at (A) 400°C, (B) 500°C, (C) 600°C, (D) 700°C, (E) 800°C, and (F) 900°C for 10 hours. [Figure 14] These are profiles obtained by performing a DRM reaction at 700°C for 10 hours using Ni28 / silica and Ni nanoparticles physically mixed with silica at a concentration of 2 wt% as catalysts, followed by TG-DTA measurement under air. [Figure 15] This is a mass spectrum measured at 200-900°C during a DRM reaction using a Ni12Cu16 / silica catalyst. [Figure 16] This is a mass spectrum measured at 200-900°C during a DRM reaction using a CO28 / silica catalyst. [Modes for carrying out the invention]

[0017] The embodiments of the present invention will be described in detail below. (Catalyst for synthesis gas production) The catalyst used in the present invention is a support in which subnanoparticles made of at least one 3d transition metal selected from Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn are supported on a carrier.

[0018] Conventionally, platinum nanoparticles synthesized by reduction methods under conditions such as simple concentration control or the addition of a support have been known to have particle sizes exceeding 2 nm and represent more than 1000 elements. In contrast, in this invention, the term "sub-nano" means that the particle size is, for example, within the range of 0.5 to 2 nm, and particularly within the range of 0.8 to 1.8 nm. A polyhedral group of atoms formed by the direct bonding of some or all of several atoms is generally called a cluster, and in that sense, sub-nanoparticles are clusters.

[0019] Subnanoparticles precisely synthesized using phenylazomethine dendrimers as templates have been confirmed to form clusters with minimal size distribution. The particle size of the subnanoparticles is preferably 1.8 nm or less, more preferably 1.5 nm or less, and even more preferably 1.3 nm or less, from the viewpoint of increasing the conversion rate. Furthermore, it is preferably 0.6 nm or more, more preferably 0.8 nm or more, and even more preferably 1.0 nm or more.

[0020] The number of elements in the sub-nanoparticles is preferably 4 or more, more preferably 12 or more, and even more preferably 20 or more, from the viewpoint of increasing the conversion rate. It is also preferably 60 or less, and more preferably 40 or less.

[0021] The constituent elements of the cluster are at least one 3d transition metal element selected from Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn. These individual elements or alloys of subnanoparticles have a high conversion rate and can maintain catalytic activity for a long time. In particular, they can suppress aggregation and maintain catalytic activity for a long time even at high temperatures above 700°C, and they can also suppress deactivation due to carbon deposition and maintain catalytic activity for a long time even in temperature ranges where solid carbon is formed. The alloy is not particularly limited as long as it consists of two or more elements, but examples include alloys consisting of two or three elements. In particular, if the subnanoparticles are pure Ni or an alloy containing Ni, a high conversion rate can be obtained even at low temperatures of 500°C or less. The alloy containing Ni is not particularly limited as long as it consists of two or more elements, but examples include those consisting of two or three elements. Ni-containing alloy Ni x M y The composition (where M represents one or more 3d transition metal elements) is not particularly limited, but for example, X:Y = 10:1 to 1:10, or 5:1 to 1:5.

[0022] The catalyst of the present invention makes it possible to suppress aggregation of subnanoparticles and other factors, thereby effectively suppressing the decrease in catalytic activity, by supporting subnanoparticles on a support. The shape of the support on which the subnanoparticles are supported is not particularly limited and can be appropriately selected according to the type of catalyst bed, and may be granular, fibrous, granular, film-like, plate-like, etc. Considering that it has a large surface area per unit weight and is suitable for catalysts, granular (powder) is preferred.

[0023] Any material that can be maintained as a support in the range of 400°C to 900°C is acceptable. For example, inorganic materials can be used. Examples of inorganic materials include silica, alumina, titania, magnesia, zirconia, iron oxide, copper oxide, glass, silica sand, talc, mica, clay, and walnut. These inorganic materials may be porous materials, and sub-nanoparticles can be supported on the surface of the pores.

[0024] The subnanoparticles can be loaded onto a carrier by, for example, dissolving the dendrimer containing the subnanoparticles in a suitable solvent such as an organic solvent, then mixing, impregnating, coating, or dropping the solution into the carrier dispersion, and finally drying it. Grinding may be performed as needed.

[0025] The amount of subnanoparticles loaded onto the support is not particularly limited, but considering factors such as increasing the loading amount to improve the reaction efficiency with the substrate, it is preferably 0.1 Wt% or more, and more preferably 0.2 Wt% or more. The upper limit is not particularly limited, but considering factors such as the reduced likelihood of cluster aggregation and the suppression of a decrease in catalyst turnover frequency due to aggregation, it is preferably 10 Wt% or less, and more preferably 5 Wt% or less.

[0026] In a preferred example of the catalyst of the present invention, the cluster is a reduced product of an organic polymer in which metal salts of its constituent elements are accumulated. This organic polymer is a template molecule having multiple sites within the molecule where metal salts accumulate, and is particularly a dendrimer.

[0027] Dendrimers are dendritic polymers with a structure that branches regularly from a center, and consist of a central molecule that forms the core and dendrons that form the side chains. The number of branching steps of the dendron portion is also called the generation. The part that branches one step from the central molecule of a dendrimer is called the first generation, and the part that branches two steps is called the second generation. In general, dendrimers are polymers that branch regularly and completely in a dendritic manner from the core, and have a spherical structure that is sparse near the center and dense near the surface, with the number of generations increasing with each repeated branching from the center. In contrast, hyperbranched polymers are polymers that have incomplete dendritic branching, unlike dendrimers which have a complete dendritic structure. In the present invention, the dendrimer may be such a hyperbranched polymer with defects in the branching units.

[0028] Dendrimers can be manufactured by methods such as the divergent method and the convergent method. The divergent method uses a molecule with multiple functional groups as a core, and branches are extended from the center outwards. The convergent method extends branches from the outside inwards, and finally attaches them to the core to form a spherical polymer. In this method, the synthesis of dendrons proceeds from the outer shell of the dendrimer inwards, and finally several dendrons are attached to the core.

[0029] Dendrimers have sites with different environments for interacting with metal salts. Here, "sites with different environments" in a dendrimer refer to sites in the dendrimer where metal salts can accumulate, and where the interactions with the metal salts are different from each other. These sites include complex-forming sites with metal salts, ionic bonding sites, and covalent bonding sites. "Complex-forming sites" refer to sites in the dendrimer that form complexes with metal salts, and are the parts that become Schiff bases. In unidirectional electron density gradient dendrimers, the strength of interactions such as complex formation strength changes stepwise from the inner layers to the outer layers, gradually weakening and forming sites with different environments. In addition, for example, if a dendrimer with an electron-donating ligand in the outermost layer has a strong coordination environment only in the outermost layer, the site with the strongest interaction will become the outermost layer, forming sites with different environments together with sites in the inner layers where the interaction is weaker.

[0030] The regions with different environments described above may also be present in the core itself. Therefore, the inner layers of the dendrimer include the core as well as the relevant region in the first generation. In a unidirectional electron density gradient dendrimer, a basicity gradient occurs from the core to the ends, so the complex formation constant is highest at the core and the closest first-generation complex formation site, and decreases stepwise towards the outside. This difference in complex formation constants acts as a driving force, causing the metal salt to accumulate stepwise from the first generation closest to the center to the second and third generations. The number of units that fill each layer is 4, 12, 28, and 60 when the core has 4 branches, 3, 9, 21, and 45 when the core has 3 branches, and 2, 6, 14, and 30 when the core has 2 branches. In a pyridyltriphenylmethane core dendrimer with one additional coordination site, the numbers are 1, 1, 3, 2, and 6. In this case, complex formation occurs first at the pyridine region of the core, and then stepwise from the inner layers to the outer layers. In this process, it is thought that in each layer, the pyridine portion of the pyridyltriphenylmethane core first forms a complex with the complex-forming site (imine portion) of the dendron bound to it, and then subsequently forms a complex with other complex-forming sites in each layer.

[0031] In the present invention, the dendrimer is preferably one that contains electron-donating bonds or atoms as complex-forming sites at the branching points of the dendritic structure. For example, dendrimers containing nitrogen atoms or oxygen atoms with lone pairs of electrons that act as electron donors are examples. Examples of nitrogen atoms to which metal salts can coordinate include nitrogen atoms in azomethine bonds (-CH=N-). In the present invention, it is preferable that the dendrimer thus contains imine moieties as complex-forming sites.

[0032] Examples of dendrimers used in the present invention include phenylazomethine dendrimers, carbazole dendrimers, carbazole-phenylazomethine dendrimers, polyamideamine dendrimers, polyalkylene imine dendrimers such as polypropylene imine dendrimers, and polyarylalkyl ether dendrimers such as polybenzyl ether dendrimers.

[0033] Among these, phenylazomethine dendrimers, which use phenylazomethine as the constituent unit of a dendritic branching structure, carbazole dendrimers, which use carbazole as the constituent unit of a dendritic branching structure, and carbazole-phenylazomethine dendrimers, which have a structure combining phenylazomethine and carbazole, are preferred. These are characterized by their rigid structure due to π-conjugation, which makes them very hard, and they have sufficient space inside the molecule and many coordination sites that form complexes with metal salts, making them suitable for the precise accumulation of many metal salts from the inner layer to the outer layer at each stage of complex formation. Examples of phenylazomethine dendrimers include compounds represented by the following formula (1).

[0034] [ka]

[0035] In formula (1) above, A is the core molecular group of the phenylazomethine dendrimer, and the phenylazomethine dendrimer molecule grows a chain of units represented by B in formula (1) outward from this core molecular group. As a result, the grown phenylazomethine dendrimer molecule has a structure in which B grows radially from A at the center. The number of times B is chained is called a "generation," and the generation adjacent to the core molecular group A is considered the first generation, with the number of generations increasing outward. In formula (1) above, A is represented by the following formula

[0036] [ka]

[0037] It is represented by the structure R 2 Examples of these include monocyclic or polycyclic aromatic groups, heteroaromatic groups, porphyrin groups, phthalocyanine groups, cyclone groups, etc., which may have substituents. In the above formula (1), B is the one that forms one azomethine bond with A, as shown in the following formula.

[0038] [ka]

[0039] It is represented by the structure R 3 represents an aromatic group which may have the same or different substituents. This B constitutes a generation of phenylazomethine dendrimers, with B directly bonded to the core molecular group A being the first generation. In the above general formula (1), R 1 The following formula forms an azomethine bond to B as a terminal group:

[0040] [ka]

[0041] It is represented by the structure R 4 R represents an aromatic group which may have the same or different substituents. 1It will be located at the terminal of the radially extended structure of the phenylazomethine dendrimer molecule.

[0042] In the above formula (1), q represents the generation number through the structure of B of the phenylazomethine dendrimer, and p represents the number of terminal groups R of the phenylazomethine dendrimer 1 and p = 2 q is r.

[0043] R, which may be an aromatic group having a substituent, 2 R, 3 R, 4 are each independently, as its skeletal structure, may be a phenyl group or a related structure thereof, for example, various ones such as a phenyl group, a biphenyl group, a biphenylalkylene group, a biphenyloxy group, a biphenylcarbonyl group, a phenylalkyl group, etc. These skeletons, as substituents, include halogen atoms such as a chlorine atom, a bromine atom, a fluorine atom, alkyl groups such as a methyl group, an ethyl group, etc., haloalkyl groups such as a chloromethyl group, a trifluoromethyl group, etc., alkoxy groups such as a methoxy group, an ethoxy group, etc., alkoxyalkyl groups such as a methoxyethyl group, alkylthio groups, carbonyl groups, cyano groups, amino groups, nitro groups and other various substituents. These skeletons can optionally have one or more of these substituents.

[0044] In the core part represented by the above formula R 2 (-N=) r r is not particularly limited, but for example, integers from 1 to 4 can be mentioned. Also, q in the above formula (1) is not particularly limited, but is preferably exemplified as 2 to 6.

[0045] The phenylazomethine dendrimer represented by formula (1) above is a relatively large molecule for a monomolecule (for example, a 4th generation (q=3) phenylazomethine dendrimer has a diameter of about 2 nm), and has multiple nitrogen atoms at predetermined intervals to which metal atoms can coordinate. For this reason, the phenylazomethine dendrimer, despite its relatively large molecular size for a monomolecule, allows for the regular arrangement of multiple metal elements, one atom at a time.

[0046] The size of the phenylazomethine dendrimer can be adjusted by appropriately selecting the number of generations, the size of the aromatic group attached to the terminal, and the size of the substituents on the aromatic group attached to the terminal. By adjusting the size of the phenylazomethine dendrimer based on its structure, the size of the metal salt aggregate of the dendrimer formed using the phenylazomethine dendrimer can be adjusted.

[0047] In the production of dendrimer metal salt aggregates, the first step is to prepare a solution containing the dendrimer. The solvent used to dissolve the dendrimer and its metal salt aggregate is not particularly limited as long as it can dissolve them. Examples include chlorine-containing organic solvents such as dichloromethane, chloroform, 1,2-dichloroethane, 1,1-dichloroethane, and carbon tetrachloride; aromatic organic solvents such as benzene, toluene, xylene, chlorobenzene, anisole, and acetophenone; and organic solvents such as cyclohexanone, tetrahydrofuran, limonene, propylene glycol monoethyl ether acetate, and acetonitrile. Two or more of these may be used in combination.

[0048] The concentration of the dendrimer in the solution before mixing with the metal salt is not particularly limited, but is preferably 0.01 to 50 μmol / L, and more preferably 0.1 to 10 μmol / L.

[0049] In the next step, the metal salt is mixed with the solution to obtain a metal salt aggregate of the dendrimer in which the metal salt has accumulated. There are no particular limitations on the method of mixing the metal salt with the dendrimer solution, but examples include adding the metal salt solution dropwise to the dendrimer solution.

[0050] When a dendrimer and a metal salt are mixed, the metal element coordinates to the complex formation sites of the dendrimer and is incorporated into the interior of the dendrimer. In a unidirectional electron density gradient dendrimer, the metal element preferentially coordinates to the complex formation sites on the central side of the dendrimer, so it coordinates from the central complex formation sites to the outer complex formation sites. In a multi-element cluster containing platinum, the first metal salt mixed among the metal compounds containing each element coordinates outward from the dendrimer core or the first-generation complex formation sites in a generational order, and then another metal salt with weaker complex formation strength mixed in later generations coordinates further outward from the complex formation sites to which the first metal salt has coordinated. In other words, the metal salt coordinates outward from the dendrimer core or the first-generation complex formation sites in a generational order. Therefore, by controlling the molar ratio of the dendrimer to the metal salt, the metal element can be placed at a desired position in the dendrimer.

[0051] The metal elements in the metal salts accumulated on the dendrimers are those mentioned above as the metal species of the sub-nanoparticles. The counteranions or ligands in the metal salts are not particularly limited, but examples include halogen ions such as trifluoroacetate ions, chloride ions, bromide ions, and iodide ions, as well as trifluoromethanesulfonic acid, acetic acid, acetylacetone, acetonitrile, salen, cyclopentadiene, and tetrafluoroborate ions. When producing alloy subnanoparticles using multiple types of metal salts, simply mixing the metal salts together can lead to ion exchange between them. Furthermore, if the coordinating force is too strong, polydentate coordination occurs; conversely, if it is too weak, the complex formation breaks down, especially on the outer layer of the dendrimer. In addition, some metal salts cannot coordinate to the dendrimer without the presence of a coordinating solvent, while others cannot coordinate even with a small amount of solvent present. In such cases, it is preferable to unify the solvent for both the dendrimer and the metal salt to dichloromethane, and furthermore, to use trifluoroacetate (TFA) ions as the counteranions of the metal salts. Using TFA as the counteranion of the metal salt prevents ion exchange, suppresses polydentate coordination through appropriate steric hindrance, and offers advantages such as high solubility. Moreover, by using a single dichloromethane solvent system, the use of coordinating solvents can be minimized, and the complex formation behavior can be unified. In this case, the TFA salt is Ti III (TFA)3, V III (TFA)3, Cr III (TFA)3, Mn II (TFA)2(H2O)4, Fe II (TFA)2(H2O)4, Co II (TFA)2(H2O)4, Ni II (TFA)2(H2O)4, Cu II (TFA)2(H2O)4, Zn II (TFA)2(H2O)6 is one example.

[0052] In the present invention, the metal salt may be a metal salt that directly coordinates to different sites in the environment of the dendrimer, or it may be a compound in which an organic cation or proton, which acts as a counter-anion to the metal salt, coordinates to and accumulates at the site.

[0053] By reducing the metal salt aggregate of this dendrimer, sub-nanoparticles of that metal element can be produced. The reduction of dendrimer metal salt aggregates can be carried out in solution, for example, using a reducing agent that has a reducing effect on metal salts and can reduce them to a zero-valent state. When reducing in solution, examples of reducing agents include sodium borohydride, sodium cyanoborohydride, hydrogen, hydrazines, lithium aluminum hydride, diisobutylaluminum hydride, lithium borohydride, tetra-n-butylammonium borohydride, methylammonium borohydride, lithium triethylborohydride, borane complexes, sodium triacetoxyboro, zinc borohydride, lithium tributylborohydride, potassium tributylborohydride, Schwartz reagent, Stryker reagent, tributyltin hydride, sodium hydride, lithium hydride, calcium hydride, benzophenone ketyl radicals, metal naphthalenides, and hydrogen peroxide. By reducing the metal salt aggregates of the dendrimer in this way, sub-nanoparticles of a size corresponding to the number of aggregated metal salts can be prepared as being incorporated into the dendrimer. The catalyst of the present invention is obtained by supporting such sub-nanoparticles on a support as described above. Dendrimer metal salt compound aggregates can be reduced in the gas phase by, for example, supporting them on a carrier and calcining the carrier under a hydrogen gas atmosphere, or a mixed gas atmosphere of hydrogen, nitrogen, or argon.

[0054] (Method of producing synthesis gas) Next, a method for producing synthesis gas using the synthesis gas catalyst of the present invention will be described. The present invention relates to a method for producing synthesis gas, which involves reacting a raw material gas containing methane with carbon dioxide to produce synthesis gas containing carbon monoxide and hydrogen, and uses the catalyst described above as the reaction catalyst. The dry reforming reaction of methane is a reaction that can convert greenhouse gases such as CO2 and methane into useful gases such as hydrogen and CO. Because the dry reforming reaction of methane is endoergonic, theoretically, the difference in energy between reactants and products, i.e., the reaction enthalpy, must be supplied as energy such as heat. Furthermore, the actual energy diagram forms a peak as shown below, indicating that additional energy is required.

[0055] [ka]

[0056] The peak formed by this excess activation energy can be reduced by a catalyst, and by using sub-nanometer catalysts that are extremely reactive compared to conventional materials, this excess activation energy can be reduced to as close to zero as possible, especially with materials like Ni.

[0057] Reducing the size from bulk to sub-nanometer clusters significantly improves catalytic activity. This is thought to be due to the emergence of active sites associated with structural changes in Ni particles, as explained using Ni particles as an example. Previous reports have shown that, compared to planar sites, these step-like Ni sites facilitate the interaction of multiple Ni atoms with the carbon atom of methane, thereby promoting the activation of methane, which is the rate-limiting step in the dry reforming reaction of methane. Therefore, it is expected that clusters of extremely small size, too small to maintain a crystalline structure, can efficiently activate the substrate by possessing such highly activated reaction sites and coordination-unsaturated sites.

[0058] Also, Ni before and after hydrogen reduction 28 Then, using Ni bulk, we compared the XANES spectrum and the EXAFS spectrum. In the XANES spectrum, we observed changes in white lines, pre-edge peaks, and shoulder peaks, and in the EXAFS spectrum, we observed the positions of the obtained peaks to determine the Ni 28 It was confirmed that it was sufficiently reduced. Furthermore, by analyzing the EXAFS spectrum, Ni 28 The coordination number of the Ni atoms inside was calculated. The coordination number of Ni atoms in bulk Ni is 12, but according to previous reports, the coordination number in nanoparticles of about 3 nm is 10 to 8. In comparison, Ni 28The coordination number within the cluster was 6-7, which is even smaller than that of conventional nanoparticles. This result suggests that even smaller particles than conventional nanoparticles are being formed, and that each atom is activated in a more coordination-unsaturated state. Furthermore, a more detailed analysis of the obtained EXAFS spectrum revealed that the Ni cluster has even shorter Ni-Ni bonds than the bulk and nanoparticles, and that a very small coordination number of 6-7 was calculated. From previous reports, Ni 28 The cluster is considered most stable in a structure similar to that of a part of an aecosahedron, and since the coordination number of atoms within it is between 5 and 8, the calculated coordination number of 6-7 is considered reasonable. Therefore, the EXAFS analysis results suggest that sub-nanoclusters are even smaller particles than conventional nanoparticles, and that each individual atom is activated in a more coordination-unsaturated state.

[0059] The synthesis gas catalyst of the present invention is typically subjected to a reduction treatment beforehand when used in synthesis gas production, as described later. This reduction treatment increases the degree of reduction of the supported Ni, bringing it closer to a metallic state and thereby improving the catalytic activity. The reduction treatment method can be appropriately optimized depending on the catalyst composition, etc., but it is usually performed in a reducing gas. The reducing gas is not particularly limited, but hydrogen is preferably used. The reducing gas may be mixed with an inert gas such as nitrogen, helium, or argon, and is usually used after diluting the reducing gas with an inert gas. The temperature for the reduction treatment is not particularly limited, but it is preferably 500°C or higher, more preferably 600°C or higher, from the viewpoint of promoting the reduction reaction. Furthermore, from the viewpoint of energy cost and suppression of aggregation, it is preferably 900°C or lower. The reduction time can be adjusted as needed for the supported subnanoparticles to be reduced, depending on the amount and composition of catalyst used, but it is typically 30 minutes or longer.

[0060] The raw material gas containing methane may include other gas components, such as natural gas or coke by-products, in addition to methane gas alone, considering that these can be effectively utilized as raw material gases. While not particularly limited, other gas components include hydrocarbons with 1 to 5 carbon atoms, such as ethane, propane, and butane.

[0061] The reaction temperature during synthesis gas production is not particularly limited, but from the viewpoint of raw material conversion rate and reaction rate, it is preferably 400°C or higher, more preferably 500°C or higher. Furthermore, from the viewpoint of fuel consumption and suppression of thermal inactivation, it is preferably 1000°C or lower, more preferably 900°C or lower.

[0062] Among the above, a preferred reaction temperature for synthesis gas production is 350-550°C, particularly 400-500°C. In the temperature range of 550°C to 700°C, side reactions such as CO disproportionation and methane decomposition, which generate solid carbon, become significant. Since this solid carbon poisons the catalyst, it is desirable to avoid catalytic reactions in this temperature range. Also in the temperature range of 500°C to 700°C, another side reaction, the reverse water shift reaction, which generates water, becomes significant. In this invention, particularly when the subnanoparticles are pure Ni or an alloy containing Ni, catalytic reactions can be made to proceed at low temperatures. By reducing the catalyst size from bulk to subnanoparticles, the activity is significantly improved, and Ni 28 Then the reaction initiation temperature matches the simulation value obtained from thermodynamic equilibrium calculations, Ni 28 As the size is reduced, the reaction sites become more active. Low temperatures below 500°C result in low fuel consumption and are suitable for industrialization. By lowering the activation energy of the substrate with the catalyst of this invention, the reaction can proceed even in this low temperature range.

[0063] Another example of a preferred reaction temperature for synthesis gas production, as mentioned above, is 700°C or higher, particularly 700-900°C. In this temperature range, the formation of solid carbon and the generation of water through the reverse water shift reaction, a side reaction, are suppressed. In particular, by using the subnanoparticles of the present invention as a catalyst, aggregation can be suppressed even at high temperatures of 700°C or higher, and the decrease in catalytic activity due to heat can be further suppressed, thus maintaining catalytic activity for a long period of time. Furthermore, the subnanoparticles have an extremely small particle size of about 1 nm, and because they are well dispersed and supported on the carrier, they not only exhibit high catalytic activity but also inhibit the growth of crystalline carbon on the particles, further suppressing inactivation due to carbon deposition, and maintaining catalytic activity for a long period of time.

[0064] In the method for producing synthesis gas of the present invention, the reaction pressure is not particularly limited, but from the viewpoint of productivity, it is preferably 0.01 MPa or higher, more preferably 0.1 MPa or higher. Furthermore, from the viewpoint of reducing the cost of the manufacturing equipment required for the high-pressure reaction, it is preferably 10 MPa or lower, more preferably 1 MPa or lower.

[0065] In the method for producing synthesis gas of the present invention, the reaction type is not particularly limited, and for example, a fixed-bed reactor, a fluidized-bed reactor, a moving-bed reactor, a suspension-bed reactor, etc. can be used, and a fixed-bed reactor is preferred.

[0066] In the method for producing synthesis gas of the present invention, when the reaction is carried out in a fixed-bed reactor, the gas space velocity (GHSV) is not particularly limited, but is preferably 1000 mL·g. -1 h -1 The above is comfort 10000mL·g -1 h -1 That is all. Furthermore, preferably 1,000,000 mL·g -1 h -1 Below is a comfortable 100,000 mL·g -1 h -1 The following applies:

[0067] In the synthesis gas production method of the present invention, the molar ratio of carbon dioxide to methane in the raw material gas is not particularly limited, but from the standpoint of suppressing a decrease in catalytic activity due to carbon deposition and reactor clogging, the molar ratio of carbon dioxide to methane (CO2 / CH4) is preferably 0.1 or higher, more preferably 1 or higher. Furthermore, from the standpoint of reducing the recycling burden due to an increase in unreacted CO2 and the process economy, it is preferably 10 or lower, more preferably 5 or lower.

[0068] In the method for producing synthesis gas of the present invention, when reacting a raw material gas containing methane with carbon dioxide, it is not prohibited to introduce other gases in addition to the raw material gas containing methane, as long as it does not hinder the effects of the present invention. [Examples]

[0069] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. 1. Preparation of the catalyst The synthesis was performed according to the following scheme.

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[0071] Under a glove box atmosphere, a solution of fourth-generation phenylazomethine dendrimer (DPA G4) was dissolved, and an equivalent amount of TFA metal salt solution corresponding to the number of elements in the target cluster was added to form a metal salt complex. For the synthesis of nickel clusters, Ni was used as the TFA salt. II (TFA)2(H2O)4, nickel and copper alloy cluster synthesis is required for Ni II (TFA)2(H2O)4 and Cu II (TFA)2(H2O)4, Co is used in the synthesis of cobalt clusters. II (TFA)2(H2O)4 was used, and chloroform was used as the solvent.

[0072] After stirring, the reaction was reduced with a methanol solution to which sodium boro (NaBH4) had been added. The reaction solution was mixed with a support dispersed in methanol, and the dendrimer complex accumulated and was supported on the support. The supported catalyst was filtered through a membrane filter, washed with methanol, and then vacuum dried. Silica was used as the support.

[0073] STEM images revealed subnanoparticles, which are clusters of uniform size, and it was also observed that each cluster is composed of multiple atoms. Atomic-resolution observation confirmed the presence of subnanoparticles with the desired number of atoms. Furthermore, EDS analysis confirmed that these clusters were composed of the target metal element.

[0074] When the particle size distribution was obtained from the STEM image, the synthesized subnanoparticles were, for example, Ni, as seen in the histogram. 28 Ni4 is 1.06±0.21nm, Ni4 is 0.74±0.24nm, Ni 12 is 0.92±0.22nm, Ni 60 The value was 1.31±0.15 nm. Also, Co 28 The wavelength is 1.2±0.2nm. Ni 12 Cu 16 The value was 1.1 ± 0.3 nm.

[0075] 2. Investigation of the pretreatment reduction temperature of the Ni catalyst in the CH4 dry reforming reaction using Ni clusters. For the catalytic reaction, a BelCAT (MicrotracMRB), an atmospheric pressure flow-type catalyst analyzer, was used, and for product identification, a BelMASS (MicrotracMRB), a quadrupole mass spectrometer, was used.

[0076] Ni supported at 4 wt% on the silica support Aerosil (registered trademark) 28 After removing water from the catalyst by pretreatment at 200°C for 5 hours with 100 mg of catalyst, the catalyst was sufficiently reduced by hydrogen treatment (He:H2=40:10) at 600°C for 30 minutes.

[0077] At a heating rate of 5°C / min, the temperature rises from 200°C to 900°C using CH4:CO2 = 2 sccm:2 sccm (GHSV:30000 ming). -1 h -1 The catalytic reaction was carried out by flowing a mixed gas of ) through the system.

[0078] The obtained mass spectra showed that the spectra corresponding to CO2 and CH4 at m / z=44 and m / z=16, respectively, decreased from around 400°C, while the spectra corresponding to H2 and CO at m / z=2 and m / z=28, respectively, increased at around the same temperature (Figure 1A). Simultaneously, the formation of water at m / z=18 was confirmed around 500°C to 600°C (Figure 1B, also Figures 2B and 3B). This is thought to be due to a side reaction, the reverse water-water shift reaction. Furthermore, when a blank test was conducted using a silica support without clusters, no consumption of reaction gases or generation of the target gas was observed. From these results, Ni 28 It was found to exhibit activity in the CH4 dry reforming reaction.

[0079] In the DRM reaction, the Ni catalyst needs to be reduced in order to activate the substrate CH4 on the catalyst. 28 O x It produces water through hydrogen treatment and Ni 28 Therefore, Ni 28 The catalyst pretreatment reduction temperature was investigated using [a specific method / tool]. Similar mass spectra were obtained even when the hydrogen treatment temperature was 500°C (Figure 2A). However, when the hydrogen treatment temperature was 400°C, the DRM reaction did not proceed until 530°C, and a rapid reaction initiation behavior was observed at temperatures above that (Figure 3A).

[0080] The above results suggest that the catalyst was reduced by CH4 at 530°C, and the DRM reaction proceeded simultaneously. From these results, Ni 28 It was decided to perform a hydrogen reduction treatment at 500°C before using the silica support as a catalyst for the DRM reaction.

[0081] 3. Size dependence of Ni catalyst on DRM reaction To investigate the size dependence of Ni catalytic activity for DRM reactions, Ni4 and Ni 12 Ni 28 Ni 60 DRM reactions were performed using samples in which commercially available Ni nanoparticles (5-20 nm: s-Nps, 100 nm: l-Nps) and commercially available Ni bulk (>100 μm) were each added at 2 wt% relative to the silica support Aerosil®. Ni4, Ni 12 Ni 28 Ni 60 For the first two materials, each was supported on a silica carrier, while for the second two materials, commercially available Ni nanoparticles and commercially available Ni bulk were physically mixed with silica. The hydrogen reduction treatment temperature was standardized to 500°C based on the results of section 2, and other conditions were the same as in section 2. The reaction conditions were the same as in step 2, and the temperature at which the conversion rate of CH4 reached 1% was evaluated as the reaction start temperature.

[0082] The results of gas analysis during the actual catalytic reaction are shown in Figures 4A-4D and 54A-4D. Using the m / z=2 mass spectra corresponding to H2 obtained from each result, the catalyst size dependence of the DRM reaction activity was investigated. Since a sufficient amount of catalyst was added relative to the substrate, differences in catalytic activity due to differences in catalyst surface area were eliminated.

[0083] Figures 4A-4D show that in the bulk sample, the temperature increased from 200°C at a rate of 5°C per minute, and around 650°C, a decrease in methane and CO2 and the generation of hydrogen and CO were observed, indicating that the catalytic reaction started at 650°C. With larger nanoparticles (l-Nps), the catalytic reaction started around 500°C, and with smaller nanoparticles (s-Nps), it started around 400°C. In other words, as the size of the catalyst increased, from nanoparticles to bulk, the reaction initiation temperature shifted to an even higher temperature.

[0084] Figures 5A-5D show that when Ni clusters were used as catalysts, the reaction started at around 300°C, confirming that the catalytic reaction proceeded at an even lower temperature than with bulk or nanoparticles. Each Ni sub-nanoparticle catalyst promoted the DRM reaction, and among them, Ni in particular... 60 The reaction initiation temperature when using silica support is Ni 28 The results were almost the same as when using silica support. In contrast, Ni 12 As the size of the silica support, Ni4 / silica support, and sub-nanoparticles decreased, the reaction initiation temperature shifted to higher temperatures.

[0085] The reaction initiation temperature was investigated by applying various sizes of Ni catalysts as described above and conducting temperature increase tests (Figure 6). All catalysts were tested at a uniform concentration of 2 wt%. By reducing the catalyst size from bulk to nanoparticles and then to sub-nanoparticles, the DRM reaction activity is improved, and among them, Ni 28 / Silica support and Ni 60 The silica support showed the highest activity. The spectra of both were in close agreement with the theoretical curve obtained from chemical equilibrium calculations (J. CO2Util. 2013, 1, 37-42.), suggesting that the activation energy of the DRM reaction was significantly reduced. In the case of Ni catalysts, it is known that step sites are more effective in activating CH4 than flat sites. Therefore, by reducing the catalyst size to the sub-nanometer region, the number of step sites in the particles increases, and Ni 28 It is thought that the activation energy of the reaction is lowered to the absolute minimum. On the other hand, Ni 12 When the Ni4 and catalyst size were further reduced, the reaction initiation temperature actually increased. Possible reasons for this include the fact that if the catalyst is too small, there is insufficient space to activate the substrate and allow for further reaction, or the active sites for the reaction are Ni4 or Ni 12 Ni 28 It is thought that they are relatively abundant in that area.

[0086] In Figure 7, to eliminate the difference in activity due to differences in catalyst surface area, the amount of catalyst was increased until the change in the heating curve disappeared, and the catalyst size dependence of activity was evaluated using a sufficient amount of catalyst relative to the substrate. An inverse volcano-like relationship was observed between the reaction initiation temperature and catalyst size. Similar to Figure 6, Ni in particular... 28 The reaction proceeded at the lowest temperature when using [specific method / library], and this temperature was almost identical to the simulation result obtained when the activation energy was set to 0.

[0087] 4. Quantification of gases in DRM reactions Ni supported at 2 wt% on the silica support Aerosil (registered trademark) 28 For 100 mg of catalyst, water was removed by pretreatment at 200°C for 5 hours under Ar, and then the catalyst was sufficiently reduced by hydrogen treatment at 500°C for 30 minutes. This measurement was performed by heating from 200°C to 900°C at a heating rate of 5°C / min using CH4:CO2=2sccm:2sccm(GHSV:30000ming). -1 h -1 The catalytic reaction was carried out by flowing a mixed gas of (Figure 8A). The obtained spectra were normalized using BelMass, and CO2, CH4, CO, and H2 were quantified individually (Figure 8B).

[0088] The resulting plots showed good agreement with the simulation results obtained from chemical equilibrium calculations using the software HSC Chemistry (Chemistry Software Ltd) (Figure 8C). From these results, the synthesized Ni 28 The catalyst was shown to successfully significantly reduce the activation energy of the DRM reaction.

[0089] 5. Ni 28 Temperature dependence of DRM activity in In heating experiments, especially in the high-temperature range, several hours have passed since the start of the reaction, making it difficult to conduct detailed studies on conversion rates, etc. Therefore, Ni 28The DRM reaction was carried out using a constant reaction temperature of 200°C to 900°C, and the conversion rate at the start of the reaction was calculated.

[0090] The conversion rates of the substrates methane and CO2 obtained at each temperature showed good agreement with the simulations obtained from chemical equilibrium calculations (Figure 9). From these results, Ni 28 The catalyst exhibited high catalytic activity at various temperatures, demonstrating success in reducing the activation energy of the reaction to nearly zero compared to conventional catalysts.

[0091] 6. Ni for DRM reaction 28 Catalyst durability In heating experiments, the temperature is increased from the low-temperature side, and especially at the high-temperature side, several hours have passed since the reaction was initiated. Therefore, if condensation or other issues occur during the reaction, the results obtained at the high-temperature side become unreliable. Therefore, the next consideration is Ni 28 By using [a specific method] and carrying out the reaction for 10 hours while keeping the reaction temperature constant, the true catalytic activity at that temperature from the start of the reaction was evaluated from the change in the conversion rate of CH4 and CO2 over time, and the durability of the catalyst was evaluated from the change over 10 hours (Figure 10-12).

[0092] The results of the studies at 400°C and 500°C are shown in Figures 10B and 10C. This temperature range is considered difficult to drive the reaction even with highly reactive catalysts, and there are very few studies that evaluate catalytic activity at this temperature. In the low temperature range of 400°C and 500°C, Ni was found after 10 hours of reaction for both CH4 and CO2. 28 Ni showed stable catalyst durability even in that temperature range. 28 The compounds exhibited extremely high reactivity, showing conversion rates for both CH4 and CO2 that were almost identical to those predicted in the calculation simulation (Figure 10A). Furthermore, their ability to maintain these conversions for 10 hours demonstrated high durability.

[0093] Next, the results of the studies at 600°C and 700°C are shown in Figures 11B and 11C. In this temperature range, the conversion rate of CH4 to CO2 rises to around 50% in the simulation (Figure 11A). The results showed conversion rates that were almost the same as the simulation at both 600°C and 700°C, confirming high catalytic activity. At 700°C, a slight decrease in the conversion rate was observed over time.

[0094] Finally, the results of the studies at 800°C and 900°C are shown in Figures 12B and 12C. At this temperature range, the conversion rate increases further, but ultimately Ni 28 The conversion rates at both 800°C and 900°C were almost the same as those in the simulation (Figure 12A), confirming high catalytic activity. A slight decrease in the conversion rate was observed over time at both 800°C and 900°C. At 900°C, a decrease in the conversion rate was observed between 2 and 5 hours after the start of the reaction for both CH4 and CO2.

[0095] The time-dependent changes in the conversion rates of CH4 and CO2 both showed excellent durability in the low-temperature range, but some inactivation was observed, particularly in the high-temperature range of 900°C. To investigate the cause of this, the Ni used as a catalyst at each reaction temperature was examined. 28 STEM observations were performed after the reaction (Figures 13A-13F).

[0096] In the low-temperature range of 400°C to 600°C, where catalyst deactivation was not observed to a significant degree, no noticeable aggregation of particles was seen. 28 It was confirmed that the material was dispersed and supported on the silica support while maintaining its size from before the reaction (Figures 13A, 13B, 13C).

[0097] On the other hand, Ni used in high-temperature ranges 28When observing the STEM image, aggregation was gradually seen starting from 700 °C (Figure 13D). After the reaction at 800 °C (Figure 13E), aggregates of various sizes up to 4 nm were confirmed. After the reaction at 900 °C (Figure 13F), many aggregates with a size of about 4 nm were confirmed. The particle size distributions after the reaction for 10 hours were 1.08 ± 0.17 nm at 400 °C, 1.13 ± 0.17 nm at 500 °C, 1.12 ± 0.17 nm at 600 °C, and 1.25 ± 0.23 nm at 700 °C, while they were 1.66 ± 0.52 nm at 800 °C and 2.65 ± 0.61 nm at 900 °C. Aggregation suddenly started from 800 °C, the peak of the histogram collapsed, and it shifted to the right.

[0098] Thus, Ni 28 showed excellent durability at both 400 °C and 500 °C, and it was observed that the clusters were well-dispersed while maintaining their original size even after the reaction. Ni-Zr NPs / SiO2, which are 10-nm nanoparticles prepared by the precipitation method, have been reported to have durability at low temperatures (400 °C, 450 °C) (Non-Patent Document 3), but the conversion rates at 400 °C are 2.0 for CH4 and 2.0 for CO2. On the other hand, the conversion rates of Ni 28 / SiO2 were 4.3 for CH4 and 7.9 for CO2. Pt nanoparticles (Non-Patent Document 2) did not reach the theoretical values (Equilibrium) obtained from the thermodynamic equilibrium at each temperature. The conversion rate when carried out at 400 °C is much higher than the results shown by the previously reported nanoparticles also carried out at 400 °C, indicating the usefulness of the Ni clusters synthesized in this example as a catalyst. The cause of the slight inactivation in the high-temperature region may be due to the aggregation of the clusters. However, even in the low-temperature region where the reaction progress was difficult with conventional nanoparticle catalysts such as 400 °C and 500 °C, Ni 28 advanced the reaction with high reactivity.

[0099] 7. By-products durability of Ni in the DRM reaction 28 Typical factors for catalyst deactivation in the DRM reaction include interparticle aggregation and carbon poisoning of the catalyst. This solid carbon is generated by the decomposition of methane (CH4→C + 2H2) and the disproportionation of CO (2CO→C + CO2). Therefore, TG-DTA measurements were used to investigate the presence or absence of solid carbon on the catalyst after the reaction.

[0100] Ni prepared at 2 wt% 28 / silica support and Ni nanoparticles (Ni l-NPs) physically mixed with silica at 2 wt% were each used as catalysts for a 10 h DRM reaction at 700 °C, and then TG-DTA measurements were performed under air (Figure 14). For Ni nanoparticles, a large weight loss attributed to the combustion of carbon was observed near 600 °C, suggesting carbon deposition on NiNPs. On the other hand, for Ni 28 no significant changes were observed in either the TG curve or the DTA curve, suggesting that the amount of carbon deposition on Ni 28 is extremely small.

[0101] According to previous reports (J. Zhang et al., Appl. Catal. B 2015, 176, 513 - 521.; M. Akri et al., Nat. Commun. 2019, 10, 5181.), it has been found that reducing the catalysts to the limit and dispersing them well can prevent the growth of crystalline carbon on the catalysts. From the above results, it is suggested that the extremely small particle size of about 1 nm and their good dispersion and loading not only exhibit high catalytic activity but also prevent the growth of crystalline carbon on the particles and deactivation due to carbon deposition.

[0102] 8. Expansion of Metal Elements Ni 12 Cu 16 / silica catalysts were used for the DRM reaction, and mass spectra were measured at 200 - 900 °C (Figure 15). The reaction start temperature shifted to the higher temperature side compared to the case of the Ni 28 / silica catalyst, and the reaction started from around 400 °C. Co 28 The DRM reaction was carried out using a silica catalyst, and the mass spectrum was measured at 200-900°C (Figure 16). The reaction initiation temperature was Ni 28 Compared to the case with a silica catalyst, the temperature shifted to the higher end, but the reaction started at around 700°C.

Claims

1. A synthesis gas catalyst for producing synthesis gas containing carbon monoxide and hydrogen by reacting a source gas containing methane with carbon dioxide, wherein the catalyst is a support in which subnanoparticles made of at least one 3d transition metal selected from Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn are supported on a carrier. The particle size of the aforementioned subnanoparticles is 0.6 to 1.8 nm. The aforementioned sub-nanoparticles are reduced products of dendrimers in which metal salts of constituent elements are accumulated. The dendrimer in question contains complex formation sites at the branching points of the dendritic structure and is selected from phenylazomethine dendrimers, which use phenylazomethine as the constituent unit of the dendritic branching structure; carbazole dendrimers, which use carbazole as the constituent unit of the dendritic branching structure; and carbazole-phenylazomethine dendrimers, which have a structure combining phenylazomethine and carbazole, and is a catalyst for synthesis gas production.

2. The catalyst for synthesis gas production according to claim 1, wherein the sub-nanoparticles are Ni element or Ni alloy.

3. A method for producing synthesis gas containing carbon monoxide and hydrogen by reacting a raw material gas containing methane with carbon dioxide, wherein the catalyst described in claim 1 or 2 is used as the reaction catalyst.

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