Photocatalyst for carbon dioxide reduction and method for producing methane

Abstract

To provide a photocatalyst that can efficiently reduce carbon dioxide by light irradiation, and a method for producing methane with the catalyst.SOLUTION: A photocatalyst contains a metal oxide semiconductor, and at least one selected from zero-valent metal nickel and zero-valent metal nickel-containing alloys, supported on the metal oxide semiconductor. A method for producing methane comprises the step of bringing the photocatalyst into contact with a gaseous mixture containing carbon dioxide and a hydrogen gas while irradiating the photocatalyst with light including ultraviolet light and / or visible light, thereby producing methane.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present disclosure relates to photocatalysts for carbon dioxide reduction and methods for producing methane. [Background technology]

[0002] Carbon dioxide is considered to be the main causative substance of global warming in recent years, and studies are being conducted on reducing carbon dioxide emissions or using it effectively. For example, methods are being studied for converting carbon dioxide into useful compounds using catalysts. In addition, catalysts and methods are known that can convert carbon dioxide into fuels such as methane by heating (for example, see Non-Patent Documents 1 and 2). [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] Kechao Zhao et al.;Journal of CO2 Utilization 16(2016)236-244 [Non-Patent Document 2] Xinyu Jia et al.;Applied Catalysis B:Environmental 244(2019)159-169 Summary of the Invention [Problem to be solved by the invention]

[0004] In conventional catalysts, in a method of reducing carbon dioxide by irradiating the catalyst with light such as ultraviolet light or visible light, the reduction rate of carbon dioxide, particularly the production rate of methane, is not sufficiently high, and improvement is required from the viewpoint of catalytic activity. Note that, although non-patent documents 1 and 2 disclose nickel-based catalysts, there is no consideration whatsoever of reducing carbon dioxide by light irradiation.

[0005] One object of the present disclosure is to provide a photocatalyst that can efficiently reduce carbon dioxide when irradiated with light, and in one embodiment, can convert carbon dioxide into methane. [Means for solving the problem]

[0006] The photocatalyst of the present disclosure is a catalyst for reducing carbon dioxide by irradiation with light, and includes a metal oxide semiconductor and at least one selected from zero-valent metallic nickel and an alloy containing zero-valent metallic nickel supported on the metal oxide semiconductor. Effect of the Invention

[0007] According to the present disclosure, a photocatalyst can be provided that can efficiently reduce carbon dioxide when irradiated with light, and in one embodiment, can convert carbon dioxide into methane. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] In this specification, "A to B" indicating a range of values ​​includes the values ​​A and B before and after "to" as the lower and upper limits, or the upper and lower limits. For example, "1 to 10" means a range of values ​​from 1 to 10.

[0009] The photocatalyst of the present disclosure is A catalyst for reducing carbon dioxide when irradiated with light. A metal oxide semiconductor; At least one selected from zero-valent metallic nickel and an alloy containing zero-valent metallic nickel (hereinafter also referred to as "metallic nickels") supported on a metal oxide semiconductor; Includes.

[0010] In one embodiment, in the presence of the photocatalyst of the present disclosure, a mixed gas containing carbon dioxide and hydrogen gas is passed through the photocatalyst, and the photocatalyst is irradiated with light containing either or both of ultraviolet light and visible light, thereby converting carbon dioxide into methane, which is useful as a fuel, stably over a long period of time. Hereinafter, this conversion is also referred to as the "photoreduction reaction of carbon dioxide." The photocatalyst has high catalytic activity and high selectivity to methane in the photoreduction reaction of carbon dioxide.

[0011] For this reason, for example, by irradiating the photocatalyst with sunlight while passing hydrogen gas through the air in the presence of the photocatalyst of the present disclosure, carbon dioxide in the air can be efficiently converted to methane. In this way, the concentration of carbon dioxide, which is thought to be the main cause of global warming, can be reduced using the photocatalyst of the present disclosure, hydrogen gas, and sunlight.

[0012] That is, the photocatalyst of the present disclosure is a photocatalyst for reducing carbon dioxide upon irradiation with light, and is preferably a photocatalyst for producing methane by reducing carbon dioxide upon irradiation with light.

[0013] The metal oxide semiconductor is a support that supports the metallic nickel. Examples of the metal oxide semiconductor include zirconium oxide (ZrO2), titanium oxide (TiO2), zinc oxide (ZnO), tin oxide (SnO), niobium oxide (Nb2O5), strontium titanate (SrTiO3), bismuth oxychloride (BiOCl), bismuth vanadate (BiVO4), tungsten oxide (WO3), iron oxide (Fe2O3), and potassium tantalate (KTaO3). As the metal oxide semiconductor, from the viewpoint of being able to reduce carbon dioxide well, a metal oxide semiconductor having a band gap of 2.0 to 5.5 eV at 25°C is preferable, a metal oxide semiconductor having a band gap of 3.0 to 5.5 eV at 25°C is more preferable, and a metal oxide semiconductor having a band gap of 4.5 to 5.5 eV at 25°C is even more preferable.

[0014] Among the metal oxide semiconductors, zirconium oxide, titanium oxide and zinc oxide are preferred, and zirconium oxide is more preferred, since a photocatalyst exhibiting higher catalytic activity can be obtained.

[0015] The photocatalyst of the present disclosure can include one or more metal oxide semiconductors. The metal oxide semiconductor may be either polycrystalline or single crystalline. The metal oxide semiconductor has, for example, a particulate shape.

[0016] From the viewpoint of obtaining higher catalytic activity, the arithmetic mean particle diameter of the metal oxide semiconductor is preferably 1 to 500 nm, more preferably 2 nm or more, further preferably 3 nm or more, more preferably 200 nm or less, and further preferably 100 nm or less. The mean particle diameter can be calculated by measuring the major axis diameter of 100 randomly selected metal oxide semiconductor particles in an image obtained by transmission electron microscopy (TEM) of the metal oxide semiconductor, and taking the arithmetic mean.

[0017] The specific surface area of ​​the metal oxide semiconductor is preferably 1 to 300 m from the viewpoint of obtaining higher catalytic activity. 2 / g, more preferably 10m 2 / g or more, more preferably 50m 2 / g or more, more preferably 200m 2 / g or less, more preferably 150m 2 / g or less. The specific surface area can be measured by the BET method.

[0018] The nickel contained in the metallic nickel supported on the metal oxide semiconductor is zero-valent metallic nickel (hereinafter referred to as "Ni 0 In the present disclosure, nickel supported on a metal oxide semiconductor is referred to as Ni 0 By carrying out a photoreduction reaction of carbon dioxide while maintaining the temperature at this temperature, it is possible to effectively convert carbon dioxide into methane, for example.

[0019] The metallic nickel is at least one selected from zero-valent metallic nickel and an alloy containing zero-valent metallic nickel, and is preferably zero-valent metallic nickel. Examples of other metals contained in the alloy include gold, silver, copper, iron, cobalt, and molybdenum. The alloy may be, for example, an alloy having a core-shell structure. For example, the outer shell of the core-shell structure is zero-valent metallic nickel, and the material coated with the outer shell is another metal.

[0020] The metallic nickels supported on the metal oxide semiconductor are preferably in the form of particles. In addition, in order to facilitate the photoreduction reaction of carbon dioxide, the nickel contained in the metallic nickel particles is preferably zero-valent metallic nickel on the surface of the particles, and more preferably zero-valent metallic nickel on both the surface and the inside of the particles.

[0021] From the viewpoint of obtaining higher catalytic activity, the arithmetic mean particle diameter of the metallic nickel supported on the metal oxide semiconductor is preferably 0.3 to 50 nm, more preferably 0.5 nm or more, even more preferably 1.0 nm or more, more preferably 30 nm or less, even more preferably 10 nm or less, and particularly preferably 5.0 nm or less. The average particle diameter of the metallic nickel can be calculated by measuring the major axis diameter of 100 nickel particles randomly selected in an image obtained by high angle dark field scattering (scanning transmission electron microscope) method (HAADF-STEM) or high resolution TEM (HR-TEM), and calculating the arithmetic mean.

[0022] The ratio of the arithmetic mean particle size of the metallic nickel to the arithmetic mean particle size of the metal oxide semiconductor is preferably 0.01 to 0.9, more preferably 0.05 or more, still more preferably 0.1 or more, more preferably 0.8 or less, still more preferably 0.6 or less.

[0023] In the photocatalyst of the present disclosure, the amount of metallic nickels supported is, in terms of nickel atoms, preferably 1 to 35 mass%, more preferably 2 mass% or more, even more preferably 3 mass% or more, particularly preferably 7 mass% or more, more preferably 25 mass% or less, even more preferably 18 mass% or less, particularly preferably 13 mass% or less. When the amount supported is equal to or more than the lower limit, the catalytic action of the metallic nickels is better exhibited. When the amount supported is equal to or less than the upper limit, the catalytic action of the metal oxide semiconductor is better exhibited. Therefore, when the amount supported is within the above range, better reduction performance can be obtained, specifically, a high methane production rate can be achieved.

[0024] The present inventors speculate, based on the results of FT-IR and the like, that the reason why the photocatalyst of the present disclosure exhibits the above-mentioned effects is as follows. First, carbon dioxide is adsorbed to the metal oxide semiconductor as bicarbonate. The bicarbonate is converted to formate by the action of charge separation in the metal oxide semiconductor due to the light irradiation energy, and then the formate is adsorbed to Ni 0 The formate is converted to carbon monoxide at the surface and to hydroxyl groups at the metal oxide semiconductor surface. 0 It is formed by two-electron reduction of bicarbonate at the interface with the surface. 0 is heated by the light irradiation energy, and Ni 0 It activates hydrogen gas on the surface and promotes dissociation of hydrogen gas. 0 On the surface, it is reduced to methyl and then to methane. Such an effect, particularly the high rate of methane production, cannot be obtained by, for example, convection heating alone, but can only be obtained by light irradiation. Note that the above explanation is merely speculation and does not limit the photocatalyst of the present disclosure in any way.

[0025] [Photocatalyst manufacturing method] The method for producing the photocatalyst of the present disclosure is not particularly limited as long as it can produce a metal oxide semiconductor supported with metallic nickel. II If so, Ni 0 needs to be reduced to In one embodiment, the method for producing a photocatalyst of the present disclosure includes a step of mixing a metal oxide semiconductor and a nickel-containing compound to obtain a mixture, and a step of performing a reduction treatment on the mixture.

[0026] The reduction treatment may be, for example, a liquid phase reduction method using a reducing agent, or a catalytic reduction method using hydrogen gas. Either the liquid phase reduction method or the catalytic reduction method may be used, or both may be used in combination.

[0027] The nickel-containing compound in the liquid phase reduction method is not particularly limited as long as it is soluble in the solvent, and examples thereof include nickel salts such as nickel nitrate, nickel sulfate, nickel acetate, and nickel chloride, and hydrates thereof. Among these, nickel nitrate and its hydrates are preferred. Examples of the solvent include water and organic solvents, with water being preferred. The nickel-containing compounds may be used alone or in combination.

[0028] Examples of reducing agents in the liquid phase reduction method include borohydride compounds such as sodium borohydride, potassium borohydride, and lithium borohydride, cyanoborohydride compounds, borane compounds such as dimethylaminoborane, hydrazine compounds, metallic sodium, and metallic hydrides. Among these, borohydride compounds are preferred, and sodium borohydride is more preferred. The reducing agent may be used alone or in combination with two or more other agents.

[0029] The amount of the reducing agent added is preferably 0.2 to 100 mol, more preferably 0.5 to 20 mol, per mol of nickel atom, which allows for good reduction of nickel.

[0030] A specific example of the liquid phase reduction method is a method in which a metal oxide semiconductor and a nickel-containing compound are immersed in a solvent, a reducing agent is added, and the nickel is reduced by stirring as necessary, and the solvent is removed by filtration and drying treatment. This results in zero-valent metallic nickel (Ni 0) is supported on the metal oxide semiconductor.

[0031] Since zero-valent metallic nickel is easily oxidized in air, in one embodiment, it is preferable to further carry out a treatment by a catalytic reduction method using hydrogen gas after subjecting a mixture of a metal oxide semiconductor and a nickel-containing compound to a treatment by a liquid phase reduction method.

[0032] In the catalytic reduction method, for example, hydrogen gas or a mixed gas of hydrogen gas and an inert gas (e.g., nitrogen gas, argon gas) is used. The ratio of hydrogen gas in the mixed gas is preferably 20 mol% or more, more preferably 50 mol% or more, and further preferably 80 mol% or more.

[0033] In the catalytic reduction method using hydrogen gas, the atmospheric temperature is preferably 200 to 600°C, more preferably 300°C or higher, further preferably 400°C or higher, more preferably 550°C or lower, further preferably 500°C or lower. The treatment time in the catalytic reduction method is, for example, 1 minute to 10 hours. II Ni 0 can be easily reduced to

[0034] From the viewpoint of avoiding oxidation of zero-valent metallic nickel, it is preferable to store the photocatalyst of the present disclosure under a non-oxidizing atmosphere until immediately before its use. Examples of the non-oxidizing atmosphere include an inert gas atmosphere such as nitrogen gas and argon gas, and a reducing gas atmosphere such as hydrogen gas.

[0035] [Methane production method] In one embodiment, the method for producing methane according to the present disclosure includes a step of contacting the photocatalyst according to the present disclosure with a mixed gas containing carbon dioxide and hydrogen gas, and irradiating the photocatalyst with light containing either or both of ultraviolet light and visible light to produce methane (hereinafter also referred to as "step (1)").

[0036] The amount of hydrogen gas (H2) supplied per mole of carbon dioxide (CO2) is preferably 0.1 to 40 moles, more preferably 1 mole or more, further preferably 2 moles or more, more preferably 30 moles or less, further preferably 20 moles or less. This makes it possible to reduce carbon dioxide while maintaining the nickel supported on the metal oxide semiconductor at zero valence.

[0037] The mixed gas may include an inert gas such as nitrogen gas and argon gas.

[0038] Step (1) can also be carried out in the presence of water. Water can also be a hydrogen source for reducing carbon dioxide. In one embodiment, the amount of water supplied per mole of carbon dioxide is 0.01 to 50 moles, and preferably 0.1 to 10 moles, from the viewpoint of reducing carbon dioxide while maintaining the nickel supported on the metal oxide semiconductor at zero valence.

[0039] The light irradiated to the photocatalyst in step (1) includes either ultraviolet light or visible light, or both. The ultraviolet light induces charge separation in the metal oxide semiconductor to reduce carbon dioxide to carbon monoxide, and the visible light reduces Ni 0 It is presumed that the photocatalyst heats the catalyst to activate hydrogen gas and convert carbon monoxide to methane, and therefore the light irradiated to the photocatalyst preferably includes ultraviolet light and visible light.

[0040] The light includes, for example, light having a wavelength of 200 to 800 nm. The light preferably includes at least light having a wavelength of 200 to 715 nm. The light more preferably includes at least light having a wavelength of 200 to 320 nm, light having a wavelength of more than 320 nm and not more than 520 nm, and / or light having a wavelength of more than 520 nm and not more than 715 nm.

[0041] The irradiation dose of the light is preferably 1 to 1000 mW / cm 2 and more preferably 10 mW / cm 2 More preferably, 500 mW / cm 2 The light may be, for example, sunlight.

[0042] The atmospheric temperature in step (1) is not particularly limited, and can be room temperature. In one embodiment, the atmospheric temperature in step (1) is preferably less than 200° C., more preferably 150° C. or less, even more preferably 100° C. or less, and even more preferably 50° C. or less, and is preferably 5° C. or more, more preferably 10° C. or more. Even in such a temperature range, the methane production method of the present disclosure can achieve a high methane production rate by light irradiation.

[0043] The reaction type is not particularly limited as long as the photocatalyst of the present disclosure can be efficiently contacted with the mixed gas containing carbon dioxide and hydrogen gas, and for example, the reaction can be carried out in a fixed bed, a fluidized bed, or a moving bed. The photocatalyst of the present disclosure may be used as a powder or may be molded. The reaction pressure is not particularly limited.

[0044] The product in the photoreduction reaction (hydrogenation reaction) of carbon dioxide using the photocatalyst of the present disclosure is mainly methane, but may contain by-products such as carbon monoxide and water.

[0045] By using the photocatalyst of the present disclosure, a carbon-neutral cycle can be formed in which carbon dioxide, which is irreversible due to the use of existing fossil fuels, is converted back into fuel. Therefore, the photocatalyst of the present disclosure can contribute to solving global warming and energy problems.

[0046] The present disclosure relates to, for example, the following [1] to [9]. [1] A photocatalyst for reducing carbon dioxide by irradiation with light, comprising a metal oxide semiconductor and at least one selected from the group consisting of zero-valent metallic nickel and an alloy containing zero-valent metallic nickel supported on the metal oxide semiconductor. [2] The photocatalyst according to [1] above, wherein the metal oxide semiconductor is at least one selected from the group consisting of zirconium oxide, titanium oxide, zinc oxide, tin oxide, niobium oxide, strontium titanate, bismuth oxychloride, bismuth vanadate, tungsten oxide, iron oxide, and potassium tantalate. [3] The photocatalyst according to the above [1] or [2], wherein the metal oxide semiconductor is zirconium oxide. [4] The photocatalyst according to any one of the above [1] to [3], wherein the amount of metallic nickel supported in the photocatalyst is 1 to 35 mass % in terms of nickel atoms. [5] The photocatalyst according to any one of the above [1] to [4], wherein the amount of metallic nickel supported in the photocatalyst is 7 to 18 mass % in terms of nickel atoms. [6] The photocatalyst according to any one of the above [1] to [5] for producing methane by reducing carbon dioxide when irradiated with light. [7] A method for producing methane, comprising the steps of contacting the photocatalyst according to any one of the above [1] to [6] with a mixed gas containing carbon dioxide and hydrogen gas, and irradiating the photocatalyst with light containing either or both of ultraviolet light and visible light to produce methane. [8] The method for producing methane according to [7] above, further comprising carrying out the above step in the presence of water. [9] The method for producing methane according to [7] or [8] above, wherein the atmospheric temperature in the above step is less than 200°C. EXAMPLES

[0047] The photocatalyst of the present disclosure will be described in more detail below based on examples.

[0048] [Preparation Example 1] 0.500 g of ZrO2 powder (JRC-ZRO-3, Catalysis Society reference catalyst; average particle size 7 nm, specific surface area 94.4 m2) was used as a support. 2 0.129 to 0.995 g of nickel (II) nitrate hexahydrate (purity >99.9%, manufactured by Wako Pure Chemical Industries, Ltd.) was added thereto.

[0049] The resulting mixture was stirred by ultrasonic waves (430 W, 38 kHz) for 20 minutes, and then stirred at 900 rpm using a magnetic stirrer for 1 hour. A solution of 0.129 to 1.04 g of sodium borohydride (purity >95%, Wako Pure Chemical Industries, Ltd.) dissolved in 20 mL of water was added thereto, and the mixture was stirred at 900 rpm using a magnetic stirrer.

[0050] The resulting suspension was then filtered using a polytetrafluoroethylene membrane filter (Merck Millipore, Omnipore, JVWP04700; pore size 0.1 μm), and the resulting solid residue was washed with 250 mL of water. The resulting powder was dried overnight at 100° C. The resulting powder is referred to as “Ni-ZrO2.”

[0051] The Ni loading was 5.0 mass%, 10 mass%, 15 mass%, or 30 mass%. The powder with a Ni loading of 5.0 mass% was light green, and the powder with a Ni loading of 10 mass%, 15 mass%, or 30 mass% was gray-black.

[0052] [Preparation Example 2] Instead of ZrO2 powder, SiO2 powder (MCM-41, Sigma-Aldrich; pore size 2 nm, specific surface area 1000 m2) was used as the support. 2 The same procedure as in Preparation Example 1 was carried out except that 100% Ni / g was used. The obtained powder is referred to as "Ni-SiO2". The obtained powder was gray in color, and the Ni supporting amount was 10 mass%.

[0053] [Preparation Example 3] 0.500 g of ZrO2 powder and 0.260 g of nickel (II) nitrate hexahydrate were mixed in water, and the water was distilled off at 80°C. The obtained powder was dried at 100°C and calcined in air at 450°C for 2 hours. The obtained powder is referred to as "NiO-ZrO2". The obtained powder was gray, and the amount of Ni supported was 10 mass%. At this stage, the powder was not treated with sodium borohydride, and the powder was calcined in air, so the nickel supported on ZrO2 was Ni. II It was.

[0054] [Reduction treatment using hydrogen gas] 20 mg of ZrO2, Ni-ZrO2, Ni-SiO2, or NiO-ZrO2 was placed in a U-shaped quartz tube with an internal volume of 46.0 mL connected to a Pyrex glass circulation system, and the tube was pumped for 10 min using both a rotary pump and a diffusion pump. -6 The catalyst samples were treated in a vacuum of 100 Pa for 1 hour. Then, 20 kPa of H2 (purity over 99.99%) was introduced into the system, and the temperature was raised to 450°C at a rate of 15°C / min and maintained at that temperature for 10 minutes. The catalyst samples treated in this way are referred to as "reduced ZrO2", "reduced Ni-ZrO2", "reduced Ni-SiO2", and "reduced NiO-ZrO2", respectively. Reduced ZrO2 was white, reduced Ni-ZrO2 was brownish black, reduced Ni-SiO2 was brownish black, and reduced NiO-ZrO2 was brownish black.

[0055] [Particle size] Transmission electron microscopy (TEM) was used to obtain TEM images of the catalyst samples using a model number “JEM-2100F” (manufactured by JEOL Ltd.) equipped with a field emission gun (accelerating voltage 200 kV). Images of the catalyst samples were obtained using the JEM-2100F by high angle dark field (scanning transmission electron microscopy) method (HAADF-STEM) and high resolution TEM (HR-TEM).

[0056] TEM images of reduced Ni (10% by mass)-ZrO2 showed crystals with a long axis diameter of 10 nm or less. HAADF-STEM and HR-TEM images were used to obtain the particle size distribution of Ni. The arithmetic mean particle size of Ni was approximately 2.2 nm.

[0057] [CO 2 Photoreduction test of 13 CO2( 13 C: 99.0%, 17 O: 0.1%, 18 O: 0.7%, purity >99.9%, Cambridge Isotope Lab) photoreduction test was performed using 20 mg of the catalyst sample obtained in each preparation example.13 CO2 is a mixture of reduced compounds obtained in photoreduction tests and atmospheric CO2. 12 This was used to distinguish CO2-derived reduced compounds (e.g., CH4).

[0058] 13 In the photoreduction test of CO2, 13 A mixture of CO2 and H2 (purity >99.99%) at 21.7 kPa was introduced into the reactor. 13 In the photoreduction test of CO2, 13 CO2, 2.3 kPa of H2O or D2O (D: 99.9%, chemical purity >99.5%, Cambridge Isotope Lab), and in some experiments, an additional 21.7 kPa of H2 were introduced into the reactor. Each reaction was carried out at room temperature.

[0059] Using a xenon arc lamp (SX-UID502XAM, artificial sunlight, manufactured by Ushio Inc.; 500 W), ultraviolet light and visible light were irradiated from the top and bottom of the reactor containing the catalyst sample through a Y-shaped quartz fiber light guide. The distance between the fiber light outlet (Φ=5 mm) and the catalyst sample was 20 mm. The light intensity at the center of the catalyst sample was 90.2 mW / cm. 2 The light intensity distribution for each wavelength of the xenon arc lamp was measured using a spectroradiometer (USR45DA, manufactured by Ushio Inc.) placed 20 mm from the light source.

[0060] The wavelength of the light used for irradiation was cut using a cut filter installed at the exit of each fiber light. Model numbers "UV32", "Y52" and "WR715" (thicknesses 2.0 mm, 2.0 mm and 2.5 mm, respectively; manufactured by HOYA Corporation) were used to transmit light with wavelengths λ>320 nm, λ>520 nm and λ>715 nm, respectively. For comparison, the above test was also performed under dark conditions with the reactor completely covered with aluminum foil.

[0061] 13In the photoreduction test of CO2, an online gas chromatograph mass spectrometer (GC-MS, JMS-Q1050GC, manufactured by JEOL Ltd.) was used to analyze the products. A packed column of molecular sieve "13X-S" (length 3 m, inner diameter 3 mm; manufactured by GL Sciences Inc.) was used. Helium (0.40 MPa, purity >99.9999%) was used as the carrier gas. A sampling loop (4.6 mL) made of a Pyrex (registered trademark) glass system was pumped for 10 minutes using a rotary pump and a diffusion pump connected to the GC-MS via a 1.5 m deactivated quartz glass tube (inner diameter 250 μm). -6 The tube was kept under a vacuum of 10 Pa. The quartz glass tube was kept at 120° C. to avoid gas adsorption during the analysis.

[0062] The evaluation results are shown in Tables 1 to 3.

[0063] [Table 1]

[0064] [Table 2]

[0065] [Table 3]

[0066] Table 1 shows: 13 The results of the photoreduction reaction of CO2 are shown. When ZrO2 or NiO-ZrO2 that had not been reduced with H2 was used as a photocatalyst, mainly CO was produced. When reduced ZrO2 was used, mainly CO was produced. In contrast, when reduced Ni-ZrO2 or reduced NiO-ZrO2 was used, CH4 accounted for more than 98 mol% of the products, and CO was a minor product. These results suggest that high-temperature pretreatment with H2 is important for the photoreduction of CO2 using reduced NiO-ZrO2.

[0067] When reduced ZrO2 was used, the CO production rate was almost constant up to 18 hours of reaction time, and then decreased by 14% at 28 hours of reaction time. When reduced Ni (10 mass%)-ZrO2 was used, 13 CH4: 320 μmol / h g cat The CH4 production rate was stable for 2 days at a production rate of 100%, and decreased slightly after 48 to 49 hours of reaction. The decrease in the CH4 production rate after 48 hours of reaction when reduced Ni (10 mass%)-ZrO2 was used was not due to the inactivation of the photocatalyst, but due to the increase in the CH4 production rate after 48 hours of reaction. 13 This is thought to be because almost all of the CO2 was consumed.

[0068] Carbon-containing products (CO and CH4) obtained using reduced ZrO2 or reduced Ni (5.0-30 mass%)-ZrO2 12 The proportion of C is 2.0 to 4.3 mol %, 13 CO2 contained in reaction gas 12 This was larger than the proportion of C (1.0 mol%) adsorbed in the catalyst sample before the photoreduction reaction test. 12 It is thought to be caused by CO2.

[0069] Table 2 shows the effect of the wavelength of the irradiated light when reduced Ni (10 mass%)-ZrO2 was used. The production rates of carbon-containing products when irradiated with light having a wavelength of more than 320 nm, more than 520 nm, and more than 715 nm were 21%, 5.6%, and 0.086%, respectively, of the rate when irradiated with light without wavelength cutting. The production rate of carbon-containing products under dark conditions was 0.024% of the rate when irradiated with light without wavelength cutting. Therefore, the contribution of the wavelength of the irradiated light to the photoreduction of CO2 was 79% (λ≦320 nm), 15% (320 nm<λ≦520 nm), 5.5% (520 nm<λ≦715 nm), 0.06% (λ>715 nm), and 0.02% (thermal reaction).

[0070] The rate of carbon-containing product formation in the dark decreased after 10 h of reaction, likely due to the oxidation of the metallic nickel surface by CO2.

[0071] Table 3 shows the results of the photoreduction reaction of CO2 when water was used as the hydrogen supply substance. With the addition of water and H2 to the reactor, Ni 0 The state is maintained, 13 CO2 is 13 The conversion to CH4 was stable over a period of 2 days. When reduced Ni (10 mass%)-ZrO2 was used, the overall formation rate of carbon-containing products was 130 μmol / h g cat It was.

[0072] 13 Under the conditions of CO2 (2.3 kPa), H2 (21.7 kPa) and D2O (2.3 kPa), 13 CH4 is the main product, 13 CH4, 13 CH3D, 13 CH2D2, 13 CHD3 and 13 CD4 was produced stably for two days. Isotopic labeling of hydrogen gas (D2) confirmed that both water and hydrogen gas contributed to the photoreduction of carbon dioxide to methane. Specifically, 8.9% of the hydrogen atoms in methane were deuterium derived from DO, and the methane produced contained H and D in accordance with the H:D ratio introduced.

Claims

1. A semiconductor, At least one selected from zero-valent metallic nickel and an alloy containing zero-valent metallic nickel supported on the semiconductor; A photocatalyst for reducing carbon dioxide by light irradiation, comprising: The photocatalyst, wherein the semiconductor is at least one selected from the group consisting of zirconium oxide, titanium oxide, zinc oxide, tin oxide, niobium oxide, strontium titanate, bismuth oxychloride, bismuth vanadate, tungsten oxide, iron oxide, and potassium tantalate.

2. The photocatalyst of claim 1 , wherein the semiconductor is zirconium oxide.

3. 3. The photocatalyst according to claim 1, wherein the amount of the metallic nickel supported in the photocatalyst is 1 to 35 mass % in terms of nickel atoms.

4. The photocatalyst according to any one of claims 1 to 3, wherein the amount of the metallic nickel supported in the photocatalyst is 7 to 18 mass % in terms of nickel atoms.

5. The photocatalyst according to any one of claims 1 to 4, for producing methane by reducing carbon dioxide upon irradiation with light.

6. A method for producing methane, comprising the steps of contacting the photocatalyst according to any one of claims 1 to 5 with a mixed gas containing carbon dioxide and hydrogen gas, and irradiating the photocatalyst with light containing either or both of ultraviolet light and visible light to produce methane.

7. The method for producing methane according to claim 6, further comprising carrying out the process in the presence of water.

8. The method for producing methane according to claim 6 or 7, wherein the atmospheric temperature in the step is less than 200°C.

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