Methanation catalyst and method for producing methane using the same

Abstract

To provide a methanation catalyst that has a low light-off temperature T50 and exhibits superior catalytic activity at low temperatures (for example, 240°C or lower).SOLUTION: A methanation catalyst includes CeO2 fine particles with an average particle size of from 10 to 30 nm as determined by powder X-ray diffraction measurement, and NiO fine particles with an average particle size of from 20 to 50 nm as determined by powder X-ray diffraction measurement. The ratio of the average particle size of the NiO fine particles to the average particle size of the CeO2 fine particles (NiO / CeO2) ranges from 1.4 to 3.0.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a methanation catalyst and a method for producing methane using the same. [Background technology]

[0002] Methanation reactions using CO2 as a raw material have attracted attention in recent years from the perspective of reducing CO2 emissions to curb global warming, but have not yet been put to practical use. The precious metal Ru and the base metal element Ni have been investigated as catalysts that exhibit high activity in methanation reactions using CO2 as a raw material. However, because precious metal catalysts are expensive, it is desirable to use base metal elements as methanation catalysts in terms of manufacturing costs. For example, I. Sreedhar et al., Catalysis Science & Technology, 2019, Vol. 9, pp. 4478-4504 (Non-Patent Document 1) investigates a methanation catalyst in which Ni is supported on a carrier such as ceria by impregnation. Furthermore, JP 2020-32331 A (Patent Document 1) investigates a methanation catalyst consisting of a coprecipitate of ceria microparticles and nickel oxide microparticles.

[0003] However, conventional Ni-containing methanation catalysts are calcined at temperatures below 600°C to highly disperse and finely divide the Ni component, but their activity in the methanation reaction at low temperatures is not necessarily sufficient, and the light-off temperature, T 50 Because the temperature at which the CO2 conversion rate reaches 50% is high, a high temperature is required at the start of the methanation reaction, and there were also problems such as low reaction efficiency and heat recovery rate of the system during steady state operation. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2020-32331 [Non-patent literature]

[0005] [Non-Patent Document 1] I. Sreedhar et al., Catalysis Science & Technology, 2019, Vol. 9, pp. 4478-4504 Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention has been made in consideration of the problems of the prior art, and 50 The objective of the present invention is to provide a methanation catalyst that has a low catalytic activity at low temperatures (for example, 240°C or lower) and a low CO2 conversion rate (temperature at which CO2 conversion reaches 50%), and a method for producing methane using the methanation catalyst. [Means for solving the problem]

[0007] As a result of intensive research conducted by the present inventors to achieve the above object, it has been found that, when producing a methanation catalyst containing CeO2 fine particles and NiO fine particles, by calcining the particles at a specific temperature, a methanation catalyst can be obtained in which the average particle diameters of the CeO2 fine particles and the NiO fine particles and the ratio of these average particle diameters are within a specific range, and this methanation catalyst has a light-off temperature T 50 The inventors have found that the catalytic activity of the catalyst is low and excellent at low temperatures (for example, 240° C. or lower), and have thus completed the present invention.

[0008] That is, the present invention provides the following aspects.

[0009] [1] A methanation catalyst comprising CeO2 microparticles having an average particle size within the range of 10 to 30 nm as determined by powder X-ray diffraction measurement, and NiO microparticles having an average particle size within the range of 20 to 50 nm as determined by powder X-ray diffraction measurement, wherein the ratio of the average particle size of the NiO microparticles to the average particle size of the CeO2 microparticles (NiO / CeO2) is within the range of 1.4 to 3.0.

[0010] [2] The methanation catalyst according to [1], wherein the ratio of the average particle size of the NiO fine particles to the average particle size of the CeO2 fine particles (NiO / CeO2) is within the range of 1.5 to 2.5.

[0011] [3] The methanation catalyst according to [1] or [2], wherein the average particle size of the NiO fine particles is within the range of 20 to 45 nm.

[0012] [4] A method for producing methane, comprising contacting a mixed gas of carbon dioxide and hydrogen with the methanation catalyst according to any one of [1] to [3].

[0013] Although the reason why the methanation catalyst of the present invention has excellent catalytic activity at low temperatures (for example, 240°C or less) is not entirely clear, the present inventors speculate as follows: When H2 is brought into contact with a methanation catalyst containing CeO2 fine particles and NiO fine particles at high temperatures, H2 is adsorbed on the catalyst surface and dissociated, forming active species H2. * This active species H * reduces NiO in the methanation catalyst to metallic Ni. When CO2 is adsorbed onto the methanation catalyst in this state, the metallic Ni becomes an active site, and CO2 is converted into CH4 through the reverse water gas shift reaction (formula (1) below) and methanation reaction (formula (2) below). CO2+H2→CO+H2O (1) CO+3H2→CH4+H2O (2) In the reactions of the above formulas (1) and (2), various types of active sites are simultaneously required, such as H adsorption / dissociation, CO adsorption, CO → CO → CO reduction, and C hydrogenation. Therefore, it is presumed that the methanation reaction of CO on a methanation catalyst is a catalytic reaction that is sensitive to the structure.

[0014] In conventional methanation catalysts containing CeO2 fine particles and NiO fine particles, the particle sizes of the CeO2 fine particles and the NiO fine particles are both small and highly dispersed, which presumably results in insufficient interaction between the CeO2 fine particles and the NiO fine particles, making it difficult to reduce NiO to metallic Ni at low temperatures (for example, 240°C or lower), resulting in low low-temperature activity in the CO2 methanation reaction (the above-mentioned formulas (1) and (2)).

[0015] On the other hand, in the methanation catalyst of the present invention, the NiO fine particles are larger than the CeO fine particles, and the CeO fine particles are finely dispersed around the NiO fine particles, thereby strengthening the interaction between them and facilitating the reduction of NiO to metallic Ni even at low temperatures (for example, 240°C or lower). Furthermore, since the particle size of the NiO fine particles is larger than that of conventional particles, the above-mentioned various types of active sites are simultaneously present, making it easier for the reactions represented by the above formulas (1) and (2) to proceed, and it is presumed that this results in higher low-temperature activity in the CO methanation reaction. [Effects of the Invention]

[0016] According to the present invention, the light-off temperature T 50 It is possible to obtain a methanation catalyst having low catalytic activity at low temperatures (for example, 240°C or lower). Furthermore, by using such a methanation catalyst, it becomes possible to produce methane in high yield from carbon dioxide and hydrogen even at low temperatures (for example, 240°C or lower). [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a graph showing X-ray diffraction spectra of the catalyst powders obtained in Examples 1 to 4 and Comparative Examples 1 to 6. [Figure 2] 1 is a graph showing the light-off temperature T50 of the catalyst powders obtained in Examples 1 to 4 and Comparative Examples 1 to 6. DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention will be described in detail below based on preferred embodiments thereof.

[0019] [Methanation catalyst] First, the methanation catalyst of the present invention will be described. The methanation catalyst of the present invention contains CeO2 fine particles and NiO fine particles.

[0020] In the methanation catalyst of the present invention, the average particle size of the CeO2 fine particles must be within the range of 10 to 30 nm. If the average particle size of the CeO2 fine particles is less than the lower limit, the CO2 adsorption site becomes smaller, and the amount of CO2 adsorbed decreases, so the light-off temperature T 50 On the other hand, if the average particle size of the CeO2 fine particles exceeds the upper limit, it becomes difficult for the CeO2 fine particles to be finely dispersed around the NiO fine particles, and the interaction between them decreases, making it difficult for the reduction of NiO to metallic Ni to proceed at low temperatures (for example, 240°C or less). 50 The light-off temperature T 50 From the viewpoint of further reducing the average particle size of the CeO2 fine particles and further improving the catalytic activity at low temperatures (for example, 240°C or lower), the average particle size of the CeO2 fine particles is preferably in the range of 10 to 25 nm, and more preferably in the range of 11 to 20 nm.

[0021] In the methanation catalyst of the present invention, the average particle size of the NiO fine particles must be within the range of 20 to 50 nm. If the average particle size of the NiO fine particles is less than the lower limit, the CeO fine particles and the NiO fine particles will be highly dispersed with each other, the interaction between them will be reduced, and the reduction of NiO to metallic Ni will be difficult to proceed at low temperatures (for example, 240°C or lower). 50 On the other hand, if the average particle size of the NiO fine particles exceeds the upper limit, the amount of CeO fine particles dispersed around the NiO fine particles decreases, the interaction between them decreases, and the reduction of NiO to metallic Ni at low temperatures (for example, 240°C or lower) becomes difficult to proceed, and the light-off temperature T 50The light-off temperature T 50 From the viewpoint of further reducing the average particle size and further improving the catalytic activity at low temperatures (for example, 240°C or lower), the average particle size of the NiO fine particles is preferably in the range of 20 to 45 nm, and more preferably in the range of 22 to 40 nm.

[0022] Furthermore, in the methanation catalyst of the present invention, the ratio of the average particle size of the NiO fine particles to the average particle size of the CeO fine particles (NiO / CeO2) must be in the range of 1.4 to 3.0. If the ratio (NiO / CeO2) is less than the lower limit, it becomes difficult for the CeO2 fine particles to be finely dispersed around the NiO fine particles, and the interaction between them decreases, making it difficult for the reduction of NiO to metallic Ni to proceed at low temperatures (for example, 240°C or lower). 50 On the other hand, if the ratio (NiO / CeO2) of the average particle diameter exceeds the upper limit, the amount of CeO2 particles dispersed around the NiO particles decreases, the interaction between them decreases, and the reduction of NiO to metallic Ni at low temperatures (for example, 240°C or lower) becomes difficult to proceed, and the light-off temperature T 50 The light-off temperature T 50 From the viewpoint of further reducing the average particle size and further improving the catalytic activity at low temperatures (for example, 240°C or lower), the ratio of the average particle size (NiO / CeO2) is preferably in the range of 1.4 to 2.5, and more preferably in the range of 1.5 to 2.5.

[0023] The average particle diameters of the CeO2 microparticles and NiO microparticles are determined using the Scherrer equation based on the XRD peaks around 2θ=47° (CeO2 microparticles) and 2θ=43° (NiO microparticles) in the XRD spectrum obtained by powder X-ray diffraction measurement using CuKα as the X-ray source.

[0024] The methanation catalyst of the present invention contains CeO2 fine particles and NiO fine particles whose average particle diameters and ratios of the average particle diameters are within the predetermined ranges, and thereby the light-off temperature T50 Therefore, excellent catalytic activity is exhibited at low temperatures (for example, 240°C or lower).

[0025] In the methanation catalyst of the present invention, the content of Ce element is preferably 5 to 80 mass %, more preferably 10 to 75 mass %, and even more preferably 15 to 70 mass %, based on the total elements in the catalyst. When the content of Ce element is within the above range, the obtained methanation catalyst has a light-off temperature T 50 On the other hand, if the Ce content is less than the lower limit, the number of CO2 adsorption sites decreases, and the amount of CO2 adsorption decreases, so the light-off temperature T 50 On the other hand, if the upper limit is exceeded, the content of Ni element decreases relatively, making it difficult for CeO particles to be finely dispersed around NiO particles, reducing their interaction, and making it difficult for the reduction of NiO to metallic Ni to proceed at low temperatures (for example, 240°C or less). This leads to a decrease in the light-off temperature T 50 and the low-temperature activity tends to decrease.

[0026] Furthermore, in the methanation catalyst of the present invention, the content of Ni element is preferably 5 to 80 mass %, more preferably 10 to 75 mass %, and even more preferably 15 to 70 mass %, based on the total elements in the catalyst. When the content of Ni element is within the above range, the obtained methanation catalyst has a light-off temperature T 50 On the other hand, when the content of Ni element is less than the lower limit, it becomes difficult for CeO2 particles to be finely dispersed around NiO particles, and the interaction between them decreases, making it difficult for the reduction of NiO to metallic Ni to proceed at low temperatures (for example, 240°C or less), and therefore the light-off temperature T 50 On the other hand, if the upper limit is exceeded, the Ce element content decreases relatively, the number of CO2 adsorption sites decreases, and the amount of CO2 adsorption decreases, so the light-off temperature T50 As a result, the catalytic activity at low temperatures (for example, 240°C or lower) tends to decrease.

[0027] The methanation catalyst of the present invention may consist only of CeO2 fine particles and NiO fine particles, or may further contain fine particles of at least one metal oxide selected from the group consisting of ZrO2 and rare earth metal oxides (hereinafter referred to as "other metal oxide fine particles"). The inclusion of such other metal oxide fine particles such as ZrO2 fine particles improves the thermal stability of the CeO2 fine particles and improves the heat resistance of the methanation catalyst.

[0028] The content of the other metal oxide fine particles is preferably 2 to 20 mol %, and more preferably 3 to 15 mol %, of the metal elements constituting the other metal oxide fine particles relative to 100 mol % of all metal elements contained in the methanation catalyst. If the content of the other metal oxide fine particles is less than the lower limit, the heat resistance of the methanation catalyst may not be sufficiently improved. On the other hand, if the content exceeds the upper limit, the interaction between the CeO2 fine particles and the NiO fine particles tends to be difficult to obtain.

[0029] (Methanation catalyst manufacturing method) The methanation catalyst of the present invention can be produced, for example, by preparing a catalyst precursor by coprecipitation using a precursor solution containing Ce ions and Ni ions, and then calcining the catalyst precursor at a predetermined temperature. Alternatively, the catalyst can be produced by impregnating Ni ions into CeO fine particles, which have been adjusted to a predetermined average particle size, to prepare a catalyst precursor, and then calcining the catalyst precursor at a predetermined temperature.

[0030] The calcination temperature of the catalyst precursor is not particularly limited as long as it is a temperature at which the average particle diameters of the CeO2 fine particles and the NiO fine particles and the ratio of these average particle diameters are within the predetermined ranges, but is preferably 625 to 775°C, more preferably 625 to 750°C, and even more preferably 650 to 725°C. If the calcination temperature is below the lower limit, at least one of the average particle diameters of the CeO2 fine particles and the NiO fine particles and the ratio of these average particle diameters tends to be smaller than the predetermined ranges, so the light-off temperature T 50 On the other hand, if the calcination temperature exceeds the upper limit, the average particle size of the NiO fine particles tends to be larger than the predetermined range, and the light-off temperature T 50 As a result, the catalytic activity at low temperatures (for example, 240°C or lower) tends to decrease.

[0031] The firing time is not particularly limited as long as the average particle diameters of the CeO2 fine particles and the NiO fine particles and the ratio of these average particle diameters are within the predetermined ranges, but is preferably 1.5 to 8.0 hours, more preferably 2.0 to 6.0 hours, and even more preferably 3.0 to 5.0 hours. If the firing time is less than the lower limit, at least one of the average particle diameters of the CeO2 fine particles and the NiO fine particles and the ratio of these average particle diameters tends to be smaller than the predetermined ranges, so the light-off temperature T 50 On the other hand, if the calcination temperature exceeds the upper limit, the average particle size of the NiO fine particles tends to be larger than the predetermined range, and the light-off temperature T 50 As a result, the catalytic activity at low temperatures (for example, 240°C or lower) tends to decrease.

[0032] An example of a method for preparing the catalyst precursor by coprecipitation is a method in which a precipitant is added to a precursor solution containing Ce ions and Ni ions to generate a catalyst precursor (a coprecipitate containing a Ce compound and a Ni compound).

[0033] The precursor solution containing Ce ions and Ni ions is not particularly limited, and examples thereof include a precursor solution in which a Ce salt and a Ni salt are dissolved in a solvent. Furthermore, in order to improve the heat resistance of the resulting methanation catalyst, a salt of at least one metal selected from the group consisting of Zr and rare earth metals (hereinafter referred to as "other metal") may be added to this precursor solution. Examples of the Ce salt, Ni salt, and other metal salt include nitrates, acetates, chlorides, etc. of these metals. The solvent is not particularly limited as long as it dissolves the Ce salt, Ni salt, and other metal salt and forms a coprecipitate by adding a precipitant. Examples include water, ethanol, methanol, etc., and mixed solvents of water with ethanol, methanol, etc.

[0034] In the precursor solution, the Ce ion content is preferably such that the Ce element content in the resulting methanation catalyst is 5 to 80 mass %, more preferably 10 to 75 mass %, and even more preferably 15 to 70 mass %, based on the total mass of all elements in the catalyst. When the Ce ion content is within the above range, the light-off temperature T 50 On the other hand, if the Ce ion content is less than the lower limit, the number of CO2 adsorption sites decreases, and the amount of CO2 adsorbed decreases, resulting in a methanation catalyst with a low light-off temperature T 50 On the other hand, if the upper limit is exceeded, the content of Ni element decreases relatively, making it difficult for CeO particles to be finely dispersed around NiO particles, reducing their interaction, and making it difficult for the reduction of NiO to metallic Ni to proceed at low temperatures (for example, 240°C or less). This leads to a decrease in the light-off temperature T 50 and the low-temperature activity tends to decrease.

[0035] In the precursor solution, the content of Ni ions is preferably 5 to 80 mass %, more preferably 10 to 75 mass %, and even more preferably 15 to 70 mass %, of the total amount of Ni elements in the resulting methanation catalyst, relative to the total amount of elements in the catalyst. 50 On the other hand, if the content of Ni ions is less than the lower limit, it becomes difficult for CeO particles to be finely dispersed around NiO particles, and the interaction between them decreases, making it difficult for the reduction of NiO to metallic Ni to proceed at low temperatures (for example, 240°C or less), resulting in a methanation catalyst with even better catalytic activity at low temperatures (for example, 240°C or less). 50 On the other hand, if the upper limit is exceeded, the Ce element content decreases relatively, the number of CO2 adsorption sites decreases, and the amount of CO2 adsorption decreases, so the light-off temperature T 50 As a result, the catalytic activity at low temperatures (for example, 240°C or lower) tends to decrease.

[0036] Furthermore, the content of other metal ions in the precursor solution is preferably such that the content of metal elements constituting the other metal oxide fine particles in the resulting methanation catalyst is 2 to 20 mol %, more preferably 3 to 15 mol %, relative to 100 mol % of all metal elements contained in the methanation catalyst. If the content of the other metal ions is below the lower limit, the heat resistance of the methanation catalyst may not be sufficiently improved. On the other hand, if the content exceeds the upper limit, the interaction between the CeO2 fine particles and the NiO fine particles tends to be difficult to obtain.

[0037] Examples of precipitants used in the coprecipitation method include sodium-containing precipitants such as sodium hydroxide and sodium carbonate, and sodium-free precipitants such as ammonium hydrogen carbonate. When a sodium-containing precipitant is used, it is preferable to remove the sodium by washing the resulting coprecipitate with water or the like.

[0038] [Method for producing methane] Next, the methane production method of the present invention will be described. The methane production method of the present invention is a method for producing methane by bringing a mixed gas of carbon dioxide and hydrogen into contact with the methanation catalyst of the present invention. By using the methanation catalyst of the present invention, methane can be produced in high yield from carbon dioxide and hydrogen even at low temperatures (for example, 250°C or lower). [Example]

[0039] The present invention will be described in more detail below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0040] Example 1 Nickel nitrate hexahydrate (manufactured by Wako Pure Chemical Industries, Ltd.) and cerium nitrate hexahydrate (manufactured by Wako Pure Chemical Industries, Ltd.) were dissolved in ion-exchanged water to prepare a nickel ion (NiO) catalyst, so that the nickel oxide (NiO) content and the ceria (CeO) content in the resulting catalyst would be 50% by mass and 50% by mass, respectively. 2+ ) and cerium ions (Ce 3+ A precursor aqueous solution containing Ni was prepared. 2+ and Ce 3+ A precipitant solution containing 1.3 equivalents of sodium carbonate (Na2CO3) was added dropwise over 30 minutes, and the mixture was heated at 70°C for 1 hour and then left to stand overnight to form a coprecipitate. After that, filtration and washing with warm water at 60°C were repeated 7 times to remove the Na2CO3 from the coprecipitate. + The resulting purified product was dried at 110°C for 12 hours and then calcined in air at 650°C for 3 hours to obtain a NiO-CeO2 coprecipitated catalyst powder (Ni content: 39.3 mass% (based on Ni element)).

[0041] Example 2 A NiO-CeO2 coprecipitated catalyst powder (Ni content: 39.3 mass % (based on Ni element)) was obtained in the same manner as in Example 1, except that the calcination temperature was changed to 700°C.

[0042] Example 3 A NiO-CeO2 coprecipitated catalyst powder (Ni content: 39.3 mass % (based on Ni element)) was obtained in the same manner as in Example 1, except that the calcination temperature was changed to 725°C.

[0043] Example 4 A NiO-CeO2 coprecipitated catalyst powder (Ni content: 39.3 mass % (based on Ni element)) was obtained in the same manner as in Example 1, except that the calcination temperature was changed to 750°C.

[0044] (Comparative Example 1) A NiO-CeO2 coprecipitated catalyst powder (Ni content: 39.3 mass % (based on Ni element)) was obtained in the same manner as in Example 1, except that the calcination temperature was changed to 500°C.

[0045] (Comparative Example 2) A NiO-CeO2 coprecipitated catalyst powder (Ni content: 39.3 mass % (based on Ni element)) was obtained in the same manner as in Example 1, except that the calcination temperature was changed to 600°C.

[0046] (Comparative Example 3) A NiO-CeO2 coprecipitated catalyst powder (Ni content: 39.3 mass % (based on Ni element)) was obtained in the same manner as in Example 1, except that the calcination temperature was changed to 800°C.

[0047] Comparative Example 4 A NiO-CeO2 coprecipitated catalyst powder (Ni content: 39.3 mass % (based on Ni element)) was obtained in the same manner as in Example 1, except that the calcination temperature was changed to 900°C.

[0048] (Comparative Example 5) A NiO-CeO2 coprecipitated catalyst powder (Ni content: 39.3 mass % (based on Ni element)) was obtained in the same manner as in Example 1, except that the calcination temperature was changed to 550°C.

[0049] (Comparative Example 6) Nickel nitrate hexahydrate (manufactured by Wako Pure Chemical Industries, Ltd.) was dissolved in ion-exchanged water to obtain nickel ions (Ni 2+An aqueous solution containing was prepared. To this aqueous solution, while stirring, 10 g of cerium oxide (manufactured by Kojundo Chemical Laboratory Co., Ltd., "PC") was added so that the content of nickel oxide (NiO) in the resulting catalyst was 40% by mass and the content of ceria (CeO2) was 60% by mass. Then, it was evaporated to dryness while stirring on a hot stirrer. The obtained dried product was calcined in air at 500 °C for 3 hours to obtain a NiO / CeO2 catalyst powder (Ni content: 31.4% by mass (based on Ni element)) in which NiO fine particles were supported on CeO2 fine particles.

[0050] <Measurement of average particle diameters of CeO2 fine particles and NiO fine particles> The X-ray diffraction pattern of the obtained catalyst powder was measured using a sample horizontal multi-purpose X-ray diffractometer (XRD, "UltimaIV" manufactured by Rigaku Corporation). The results are shown in Figure 1. Based on the XRD peak near 2θ = 47° of the obtained XRD spectrum, the average particle diameter of CeO2 fine particles was determined using the Scherrer equation, and based on the XRD peak near 2θ = 43°, the average particle diameter of NiO fine particles was determined using the Scherrer equation. Furthermore, the ratio of the average particle diameter of NiO fine particles to the average particle diameter of CeO2 fine particles (NiO / CeO2) was calculated. The results are shown in Table 1.

[0051] <Catalyst performance evaluation test> First, 0.26 g of the obtained catalyst powder was filled into a stainless steel reaction tube with an inner diameter of 6 mm to prepare a catalyst bed, and it was attached to a temperature-programmed desorption mass spectrometer (TPD-MS, "TP-5000" manufactured by Hemmi Calculator Co., Ltd.). While flowing a mixed gas of H2 (20%) + He (80%) through this catalyst bed at a pressure of 0.1 MPa and a flow rate of 40 ml / min, the temperature of the catalyst bed was raised from room temperature to 300 °C at a rate of 10 °C / min, and then held at 300 °C for 30 minutes for pre-reduction treatment. After that, the temperature of the catalyst bed was lowered to room temperature.

[0052] Next, a mixed gas of H2 (40%) + CO2 (10%) + He (50%) (H2 / CO2 = 4.0) was passed through the catalyst bed after the reduction pretreatment at a pressure of 0.1 MPa and a flow rate of 40 ml / min (SV = 6000 / h), while the catalyst bed was heated from room temperature to 400°C at a rate of 5°C / min. During this time, the CO2 and CH4 concentrations in the catalyst outlet gas were measured, and the CO2 conversion rate at each catalyst temperature was calculated. Based on the results obtained, the light-off temperature T 50 The temperature at which the CO2 conversion rate reaches 50% and the maximum CO2 conversion rate were determined. These results are shown in Table 1 and Figure 2.

[0053] [Table 1]

[0054] As shown in Table 1, it was confirmed that the NiO-CeO2 coprecipitated catalyst powders (Examples 1 to 4) calcined at temperatures in the range of 650 to 750°C had average particle sizes of CeO2 microparticles and NiO microparticles, as well as the average particle size ratio (NiO / CeO2), all within the specified ranges.

[0055] On the other hand, it was found that the NiO-CeO2 coprecipitated catalyst powder (Comparative Example 1) calcined at 500°C had an average particle size of the CeO2 fine particles and NiO fine particles, as well as an average particle size ratio (NiO / CeO2) that was smaller than the predetermined range.Furthermore, it was found that the NiO-CeO2 coprecipitated catalyst powder (Comparative Example 2) calcined at 600°C had an average particle size of the CeO2 fine particles that was smaller than the predetermined range.

[0056] It was also found that the NiO-CeO2 coprecipitated catalyst powder (Comparative Example 3) calcined at 800°C had an average particle size of NiO fine particles larger than the specified range, and the NiO-CeO2 coprecipitated catalyst powder (Comparative Example 4) calcined at 900°C had an average particle size of CeO2 fine particles and NiO fine particles larger than the specified range.

[0057] Furthermore, it was found that the NiO-CeO2 coprecipitated catalyst powder calcined at 550°C (Comparative Example 5) had an average particle size ratio (NiO / CeO2) smaller than the specified range, and the NiO / CeO2 catalyst powder calcined at 500°C (Comparative Example 6) had an average particle size ratio (NiO / CeO2) larger than the specified range.

[0058] As shown in Table 1 and FIG. 2, the catalyst powders (Examples 1 to 4) in which the average particle diameters of the CeO2 fine particles and the NiO fine particles and the ratio of the average particle diameters (NiO / CeO2) were all within the predetermined ranges had a light-off temperature T 50 It was confirmed that the temperature was low at 240°C or less, and that the catalyst had excellent low-temperature activity in the methanation reaction of CO2.

[0059] On the other hand, the catalyst powders (Comparative Examples 1 to 6) in which either the average particle size of the CeO2 fine particles or the NiO fine particles or the ratio of the average particle sizes (NiO / CeO2) did not satisfy the predetermined ranges had a light-off temperature T 50 In particular, the catalyst powders (Comparative Examples 3 and 4) in which the average particle size of NiO fine particles was larger than the predetermined range had a light-off temperature T 50 It was found that the low-temperature activity in the methanation reaction of CO2 was very poor. [Industrial Applicability]

[0060] As described above, according to the present invention, the light-off temperature T 50 It is possible to obtain a methanation catalyst having low catalytic activity at low temperatures (for example, 240°C or lower). Therefore, the method for producing methane of the present invention uses such a methanation catalyst, and is therefore useful as a method for producing methane from carbon dioxide and hydrogen in high yield even at low temperatures (for example, 240°C or lower).

Claims

1. CeO having an average particle size in the range of 10 to 30 nm as determined by powder X-ray diffraction measurement ２ and NiO fine particles having an average particle diameter in the range of 20 to 50 nm as determined by powder X-ray diffraction measurement, ２ The ratio of the average particle size of the fine particles (NiO / CeO ２ ) is in the range of 1.4 to 3.

0.

2. The average particle size of the NiO fine particles and the CeO ２ The ratio of the average particle size of the fine particles (NiO / CeO ２ 2. The methanation catalyst according to claim 1, wherein the ratio of β-amino- and β-amino-p-total to β-amino-p-total is in the range of 1.5 to 2.

5.

3. 3. The methanation catalyst according to claim 1, wherein the average particle size of the NiO fine particles is within a range of 20 to 45 nm.

4. A method for producing methane, which comprises contacting a mixed gas of carbon dioxide and hydrogen with the methanation catalyst according to claim 1.

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