JP2019084527A - Z scheme-type photocatalyst system - Google Patents

Abstract

To provide a Z scheme type photocatalyst system having an oxidation photocatalyst particle and a reduction photocatalyst particle, capable of reducing carbon dioxide by using water as an electron donor in an aqueous medium by irradiation of visible light, and improving reduction performance of carbon dioxide more.SOLUTION: A photocatalyst system 1 has an aqueous medium, an oxidation photocatalyst 2 for oxidizing water, a reduction photocatalyst 3 for reducing carbon dioxide, and a mediator 4 for transferring electron between the oxidation photocatalyst particle 2 and the reduction photocatalyst particle 3, in which the mediator 4 is a metal complex compound or the reduction photocatalyst particle 3 contains a semiconductor particle and a metal complex catalyst.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a Z scheme type photocatalyst system comprising oxidation photocatalyst particles and reduction photocatalyst particles.

  As a technology to fix carbon dioxide, which is one of the solutions to global environmental problems and the exhaustion of fossil fuels, there is known a technology to reduce carbon dioxide using photocatalysis on the semiconductor surface by light irradiation to semiconductors. . Highly efficient and selective photoreduction of carbon dioxide by light irradiation is an important technology from the viewpoint of chemical conversion of solar energy and effective utilization of carbon dioxide at normal temperature and pressure, and above all, water is used as an electron source Things are expected as a clean energy generation method.

  Here, two main methods of reduction technology using a photocatalyst are a photocatalyst system using an electrode and a photocatalyst system in which particles are dispersed and suspended in a medium. The latter particle suspension type photocatalyst system is considered to be superior to the former in terms of large scale and cost in industrialization (TF Jaramillo, et al., Energy Environ. Sci., 2013, 6, 1983-2002). The two-stage excitation type (Z scheme type) photocatalyst system performs photocatalytic oxidation reaction with one semiconductor (oxidation photocatalyst) using two types of semiconductors, and performs photocatalytic reduction reaction with the other semiconductor (reduction photocatalyst). It is a photocatalyst system. So far, Z scheme type photocatalyst systems in which two types of semiconductor particles are suspended or dispersed in a medium have been proposed (patent documents 1 to 3, non-patent documents 1 to 3).

JP 2005-199187 A JP, 2013-150972, A JP 2014-46236 A

T. Kato, et al., The Journal of Physical Chemistry Letters, (US), 2015, Vol. 6, p. 1042-1047 A. Iwase, et al., The Journal of the American Chemical Society, 2016, Vol. 138, p. 10260-10264 T. Takayama, et al., Faraday Discussions, (UK), 2017, Vol. 198, p. 397-407

  In the Z scheme type photocatalyst system using two types of photocatalyst particles, unlike the photocatalyst system using an electrode, since the oxidation photocatalyst and the reduction photocatalyst are dispersed in the medium, the transfer of electrons from the oxidation photocatalyst to the reduction photocatalyst The problem was that the efficiency was low and the reaction rate was slow. Therefore, conventionally, a redox mediator that mediates electron transfer between photocatalyst particles is added to the medium (patent document 1, non-patent documents 1 to 3), or while electron transfer between photocatalyst particles is mediated, both photocatalysts are The above problem has been solved by bonding a substance for bonding or electrically approaching particles to photocatalyst particles (Patent Documents 1 and 2).

On the other hand, as described above, as a cleaner energy generation method, a technology of using water as an electron donor in an aqueous medium and reducing carbon dioxide by visible light is expected. However, the photocatalytic systems described in Patent Documents 1 to 3 and Non-Patent Document 1 generate oxygen and hydrogen by complete decomposition of water, and do not describe CO ₂ reduction reaction at all. The Z Scheme optical catalyst system described in Non-Patent Documents 2 and 3, the water in an aqueous medium using as electron donors, CO generation is realized by CO ₂ reduction reaction with visible light. However, in Non-Patent Documents 2 and 3, since the hydrogen generation reaction by reduction of hydrogen ions also occurs simultaneously, and the CO generation amount is 1/100 or less of the hydrogen generation amount, the selectivity of the CO ₂ reduction reaction is Extremely low. Also, formic acid is not detected as a CO ₂ reduction product.

Thus, the reason why the reduction of CO ₂ by visible light using water as an electron donor in an aqueous medium is hardly realized is primarily the reduction reaction of hydrogen ions (hydrogen generation in the CO ₂ reduction reaction) It is necessary to excite electrons to a higher level on the semiconductor photocatalyst to require higher (negative, zeta potential side) level electrons compared to the reaction) Second, an aqueous medium Among them, since many hydrogen ions exist in the surroundings, it is considered that the hydrogen generation reaction occurring at lower level electrons takes precedence and the CO ₂ reduction reaction becomes difficult.

  An object of the present invention is a Z scheme type photocatalyst system comprising oxidized photocatalyst particles and reduced photocatalyst particles, wherein water is used as an electron donor to reduce carbon dioxide in an aqueous medium by irradiating visible light An object of the present invention is to provide a Z scheme type photocatalytic system in which the reduction performance of carbon dioxide can be further improved.

  The photocatalyst system according to the present invention comprises an aqueous medium, an oxidation photocatalyst particle for oxidizing water, a reduction photocatalyst particle for reducing carbon dioxide, a redox mediator for transferring electrons between the oxidation photocatalyst particle and the reduction photocatalyst particle. And the redox mediator is a metal complex compound, or the reduced photocatalyst particle comprises a semiconductor particle and a metal complex catalyst.

  In a preferred embodiment, the redox mediator is a metal complex compound, more preferably a complex of at least one metal selected from metals belonging to any of Groups 6 to 12 of the periodic table. Preferably, the metal complex compound is a complex of at least one metal selected from the group consisting of cobalt, iron and copper. In another preferred embodiment, the reduced photocatalyst particles comprise semiconductor particles and a metal complex catalyst, and more preferably, the metal complex catalyst is selected from metals belonging to any of Groups 6 to 10 of the periodic table. And the level of the lower end of the conduction band of the semiconductor particle is more negative than the level of the lowest unoccupied orbital of the metal complex catalyst.

  In another preferred embodiment, the potential at the lower end of the conduction band of the semiconductor particles contained in the reduced photocatalyst particles is -0.3 V or less with respect to the potential of the standard hydrogen electrode at pH 0. In another preferred embodiment, the semiconductor particles contained in the reduced photocatalyst particles are composed of a sulfide semiconductor, and more preferably, the sulfide semiconductor contains zinc.

  In another preferred embodiment, the aqueous medium is an aqueous solution of electrolyte, and more preferably, the electrolyte is at least one selected from the group consisting of carbonate, bicarbonate and phosphate. In another preferred embodiment, the potential at the lower end of the conduction band of the oxidized photocatalyst particle is lower than the potential at the upper end of the valence band of the semiconductor particles contained in the reduced photocatalyst particle. In another preferred embodiment, the oxidized photocatalyst particle comprises a bismuth vanadate semiconductor.

  According to the Z scheme type photocatalyst system according to the present invention, by irradiating visible light, carbon dioxide can be reduced using water as an electron donor in an aqueous medium, and furthermore, reduction of carbon dioxide Performance can be further improved.

It is a figure which shows an example of a structure of the photocatalyst system which concerns on this embodiment. It is a figure which shows the other example of a structure of the photocatalyst system which concerns on this embodiment. It is a graph which shows the relationship between light irradiation time and the formic acid production amount. It is a graph which shows the relationship between light irradiation time and the production amount of carbon monoxide and oxygen. It is a graph which shows the relationship between light irradiation time and the production amount of carbon monoxide, hydrogen, and oxygen.

  Hereinafter, embodiments of the present invention will be described.

[Z scheme type photocatalyst system]
FIG. 1 shows the configuration of a Z scheme type photocatalyst system 1 (hereinafter, also simply referred to as “photocatalyst system 1”) according to the present embodiment. The photocatalyst system 1 includes an oxidation photocatalyst particle 2, a reduction photocatalyst particle 3 including the reduction semiconductor particle 3a, and a redox mediator (hereinafter, also simply referred to as a "mediator") 4. The mediator 4 transfers electrons between the oxidized photocatalyst particle 2 and the reduced photocatalyst particle 3.

The photocatalyst system 1 according to the present embodiment reduces carbon dioxide (CO ₂ ) in an aqueous medium by light irradiation. In the photocatalyst system 1, when the oxidation photocatalyst particles 2 are irradiated with visible light 5, water (H ₂ O) is oxidized by the photocatalytic reaction to generate oxygen ((1/2) O ₂ ) or hydrogen peroxide etc. At the same time, excited electrons e ⁻ move to the conduction band of the oxidized photocatalyst particle 2. The mediator 4 receives the excited electrons e ⁻ of the oxidized photocatalyst particles 2 and delivers the excited electrons e ⁻ to the valence band of the reduced semiconductor particles 3 a constituting the reduced photocatalyst particles 3. Here, when the reduced photocatalyst particle 3 is irradiated with the visible light 5, the electron e ^{− in} the valence band is photoexcited by the photocatalytic reaction to move to the conduction band of the reduced semiconductor particle 3 a, and the excited electron e ⁻ is CO ₂ It is used for the reduction reaction. This CO ₂ reduction reaction produces reduction products such as carbon monoxide (CO) and formic acid (HCOOH).

FIG. 2 shows another example of the configuration of the photocatalyst system 1 according to the present embodiment. In the photocatalyst system 1 shown in FIG. 2, the reduction photocatalyst particle 3 is configured to include the reduction semiconductor particle 3a and the metal complex catalyst 3b. In the photocatalyst system 1 shown in FIG. 2, excited electrons e ⁻ from the mediator 4 are handed over to the valence band of the reduced semiconductor particles 3 a of the reduced photocatalyst particles 3. When the reduced photocatalyst particle 3 is irradiated with the visible light 5, the electron e ⁻ is photoexcited to move to the conduction band of the reduced semiconductor particle 3a, and further to the metal complex catalyst 3b from the conduction band of the reduced semiconductor particle 3a is utilized to CO ₂ reduction in the metal complex catalyst 3b, the reduction product is produced.

In the photocatalyst system 1 of the present embodiment, in the state where two types of photocatalyst particles and the mediator 4 are suspended in an aqueous medium, water is donated by two-stage excitation type (Z scheme type) reaction using light energy. While being used as an agent, CO ₂ can be reduced to synthesize useful carbon compounds. Further, in the photocatalyst system 1 of the present embodiment, the CO ₂ reduction performance can be improved by utilizing the reduced photocatalyst particles 3 composed of the reduced semiconductor particles 3 a and the metal complex catalyst 3 b as the reduced photocatalyst. That is, in the photocatalyst system 1 of the present embodiment, the generation amount of the reduction product by the CO ₂ reduction reaction can be improved, and / or the selectivity of the CO ₂ reduction reaction to the water reduction reaction can be improved. it can. Furthermore, since the photocatalyst system 1 of the present embodiment is a particle suspension type in which photocatalyst particles are suspended, it is possible to increase the scale and to suppress the manufacturing cost as compared with a system using an oxidation catalyst electrode and a reduction catalyst electrode. Excellent in that In addition, in the photocatalyst system 1 of the present embodiment, since water is used as an electron donor, a clean energy generation method can be provided.

[Oxidized photocatalyst particles]
The oxidation photocatalyst particle 2 is not particularly limited as long as it is a particle composed of a semiconductor compound (hereinafter, also described as “an oxidation semiconductor”) which exhibits a photocatalytic function by irradiation of visible light and causes an oxidation reaction of water. Here, visible light means light having a wavelength λ of 360 nm or more. The oxide semiconductor is preferably one in which the potential at the top of the valence band is more noble (positive) than the oxidation (O ₂ / H ₂ O) potential of water (1.23 V with respect to a standard hydrogen electrode (SHE)).

  Although the band gap in particular of the oxidation semiconductor which comprises the oxidation photocatalyst particle 2 is not restrict | limited, For example, 4.0 eV or less is preferable. Semiconductor particles having a band gap of 4.0 eV or less can absorb light with a wavelength λ of 300 nm to 1000 nm to form electrons and holes. The oxide semiconductor may be either an n-type semiconductor or a p-type semiconductor.

  As the oxide semiconductor, for example, oxides of various metals (including those in which the metal is partially oxidized), complex oxides (including those in which the metal is partially oxidized), nitrides (where the metal is (Including those that are nitrided), oxynitrides (including those in which metal oxides are partially nitrided, and those in which metal nitrides are partially oxidized), (Including those that are sulfurized), oxysulfides (including those in which metal oxides are partially sulfided, and those in which metal sulfides are partially oxidized), nitrofluoride (metal nitrides) Substances are partially fluorinated, including those in which metal fluoride is partially nitrided, acid fluorides (where metal oxide is partially fluorinated), metal fluoride (Including those which are partially oxidized), oxy-nitrogen fluorides (metal oxynitrides are partially Those which are fluorinated, those in which metal oxyfluorides are partially nitrided, those in which metal nitrofluorides are partially oxidized, carbides (where metals are partially carbonized) And carbon-containing oxides (including those in which the metal is partially oxidized), phosphorus compounds, silicide compounds and the like. In addition, the oxide semiconductor may be doped with at least one element of nitrogen, sulfur, carbon, phosphorus, and a metal element.

  Further specific examples of the oxide semiconductor include vanadium compounds, titanium compounds, tantalum compounds, iron compounds, zinc compounds, copper compounds and the like.

The vanadium compound is not particularly limited, and, for example, vanadium oxide (V ₂ O ₅ ), bismuth vanadate (BiVO ₄ ), Ag ₃ VO ₄ , AgVO ₃ , Bi ₄ V ₂ O ₁₁ , CuV ₂ O ₆ , Fe _{2 VO 4, Cu 3 V 2 O} 8, FeV 2 O 4 and the like. In addition, oxides of BiV, AgV, CuV, CoV, MnV, NiV, FeV, and CrV other than these may be used.

No particular limitation is imposed on the titanium compound, for _example, TiO 2, M-TiO ₂ (represents TiO ₂ of the element M doped, as the element M, N, S, F, Ni, Ru, Rh, Fe, Cu , Co etc.), SrTiO ₃ , M-SrTiO ₃ (represents SrTiO ₃ doped with the element M, and examples of the element M include Cr, Mn, Ru, Rh, Ir etc.), CaTiO ₃ , Sr ₃ Ti ₂ O ₇ , Sr ₄ Ti ₃ O ₇ , K ₂ La ₂ Ti ₃ O ₁₀ , Rb ₂ La ₂ Ti ₃ O ₁₀ , Cs ₂ La ₂ Ti ₃ O ₁₀ , CsLaTi ₂ NbO ₁₀ , La ₂ TiO ₅ , La ₂ Ti ₃ O ₉ , La ₂ Ti ₂ O ₇ , Na ₂ Ti ₆ O ₁₃ , K ₂ Ti ₆ O ₁₃ , KTiNbO _{5 and the} like.

The tantalum compound is not particularly limited. For example, Ta ₂ O ₅ , N-Ta ₂ O ₅ (nitrogen-doped tantalum oxide), TaON, Ta ₃ N ₅ , CaTaO ₂ N, SrTaO ₂ N, BaTaO ₂ N, LaTaO ₂ N, Y ₂ Ta ₂ O ₅ N, InTaO ₄ , Ni-TaO ₄ (nickel-doped TaO ₄ ), and the like.

No particular limitation is imposed on the iron compounds, for _{example, Fe 2 O 3, M-} Fe 2 O 3 ( represents _Fe 2 _{O 3} of the element M doped, as the element M, N, Zn, (N , Zn) Cu, Ni, Ti, Si, Nb, etc.), CaFe ₂ O ₄ , CuFe ₂ O ₄ , CuFeO ₂ , ZnFe ₂ O ₄ , BiFeO ₃ etc.

  The zinc compound is not particularly limited, and examples thereof include ZnS, Ni-ZnS (nickel-doped zinc sulfide), Cu-ZnS (copper-doped zinc sulfide) and the like.

The copper compound is not particularly limited, and examples thereof include Cu ₂ O, CuO, CuBi ₂ O ₄ , CuI, Cu (InGa) S ₂ , Cu (InGa) Se ₂ , CuGaS ₂ , CuGaSSe, CuGaSe _{2 and the} like. Moreover, as a copper compound, a Cu-Zn-S type compound, a Cu-Zn-Sn-S type compound, a Cu-Sn-S type compound, a Cu-In-Ga-Se type compound, Cu-In-Ga-, for example is mentioned. Examples include S-based compounds and Cu-In-Se-based compounds.

In addition, metal oxides such as WO ₃ , Bi ₂ MoO ₆ , Nb ₂ O ₅ , NiO, ZnO, SnO ₂ , ZrO ₂ , ZrO ₂ , CeO ₂ , and ZrO ₂ / CeO ₂ solid solution, InP, InAs, GaP, GaAs, GaAsP Semiconductor compounds such as GaN, GaInAsP, GaSb, CdS, CdSe, Si, SiC, Ge, SiC, and silicides of various metals (Mo, W, Ti, Co, Ni, Fe, etc.) also constitute the oxidized photocatalyst particle 2 It can be used as an oxide semiconductor.

In this embodiment, from the viewpoint of expression of higher photocatalytic activity, a compound containing at least one selected from the group consisting of vanadium, tantalum, titanium, tungsten, bismuth, and iron from the oxide semiconductor constituting the oxidation photocatalyst particle 2 It is preferable that it is an oxide containing at least one selected from the above group. Specific examples of suitable oxidizing semiconductor, _{e.g., BiVO 4, TiO 2, Fe} 2 O 3, Bi 2 WO 6, Bi 2 MoO 6, TaON, N-TiO 2 and WO _3, and the like.

  The particle size of the oxidation photocatalyst particles 2 according to the present embodiment is not particularly limited, but the average primary particle size is preferably 100 μm or less, and more preferably 10 μm or less. If the particle size of the oxidized photocatalyst particles 2 is too large, the specific surface area of the particles may be reduced, and the probability of contact between particles may be reduced. The lower limit of the average primary particle size is not particularly limited, and is, for example, 1 nm or more. The average primary particle size of the oxidized photocatalyst particles 2 can be measured by a known method, and is obtained, for example, by X-ray diffraction (XRD) measurement, transmission electron microscope (TEM) observation, scanning electron microscope (SEM) observation, etc. From the image, the average of the major axis and the minor axis can be calculated for a plurality of arbitrarily selected particle samples, and this can be averaged for the plurality of particle samples. Moreover, the shape in particular of the oxidation photocatalyst particle 2 is not restrict | limited.

Further, the specific surface area of the oxidation photocatalyst particle 2 according to the present embodiment is not particularly limited, but for example, 5 m ² / g or more is preferable. If the specific surface area of the oxidized photocatalyst particles 2 is too small, the contact probability between particles and the photocatalytic activity may decrease.

  The oxidation photocatalyst particles 2 may be semiconductor particles synthesized by a known method, or commercially available semiconductor particles may be used. The particle size may be adjusted by subjecting the oxidized photocatalyst particles 2 to processing such as grinding or polishing.

[Reduced photocatalyst particle]
The reduced photocatalyst particle 3 is configured to include the reduced semiconductor particle 3a made of a semiconductor compound (hereinafter, also described as a “reduced semiconductor”) that exhibits a photocatalytic function by irradiation of visible light and causes an oxidation reaction of water. The reduced photocatalyst particle 3 is not particularly limited as long as it contains at least the reduced semiconductor particle 3a.

(Reduced semiconductor particles)
As the reduced semiconductor constituting the reduced semiconductor particle 3a, the potential of the conduction band lower end (CBM) of the reduced semiconductor is noble (positive) than the potential of the valence band upper end (VBM) of the oxidized semiconductor constituting the oxidized photocatalyst particle 2 Some are mentioned. Thereby, a two-step photoexcitation system is such that electrons photoexcited in the oxidized photocatalyst particle 2 move to the valence band of the reduced semiconductor particle 3a via the mediator 4, and the electrons are photoexcited again in the conduction band of the reduced semiconductor particle 3a. It is formed. The reduced photocatalyst particle 3 preferably has a potential difference of 0.1 V or more obtained by subtracting the potential of the VBM of the oxidized semiconductor from the potential of CBM of the reduced semiconductor.

  The band gap in particular of the reduction semiconductor which constitutes reduction semiconductor particle 3a is not restricted, for example, 4.0 eV or less is preferred. Semiconductor particles having a band gap of 4.0 eV or less can absorb light with a wavelength λ of 300 nm to 1000 nm to form electrons and holes. Further, the reduction semiconductor may be an n-type semiconductor or a p-type semiconductor.

  As a reduced semiconductor, for example, sulfides of various metals (including those in which metals are partially sulfided), acid sulfides (in which metal oxides are partially sulfided, metal sulfides are partially contained) Oxides (including oxidized), oxides (including partially oxidized metal), complex oxides (including partially oxidized metal), nitride (partially metal) (Including those which are nitrided in nature), oxynitrides (in which metal oxides are partially nitrided, including in which metal nitrides are partially oxidized), nitrofluorides (metal nitrides) Substances are partially fluorinated, including those in which metal fluoride is partially nitrided, acid fluorides (where metal oxide is partially fluorinated), metal fluoride (Including those which are partially oxidized), oxy-nitrogen fluorides (metal oxynitrides are partially Those which are fluorinated, those in which metal oxyfluorides are partially nitrided, those in which metal nitrofluorides are partially oxidized, carbides (where metals are partially carbonized) And carbon-containing oxides (including those in which the metal is partially oxidized), phosphorus compounds, silicide compounds and the like. In addition, the reduced semiconductor may be doped with one or more elements of nitrogen, sulfur, carbon, phosphorus, and metal elements (such as silver, alkali metals, alkaline earth metals, and lanthanoids).

  A sulfide semiconductor is mentioned as a suitable example of a reduction semiconductor which constitutes reduction semiconductor particle 3a. The sulfide semiconductor constituting the reduced semiconductor particles 3a is not particularly limited as long as it is a reduced semiconductor composed of metal sulfide. As the sulfide semiconductor, for example, a sulfide of a group II element (group 12 element) such as cadmium (Cd) and zinc (Zn), a group V element (group 15 such as bismuth (Bi) and antimony (Sb) Element sulfides, group IV elements (group 14 elements) such as tin (Sn) and lead (Pb), copper gallium (CuGa), copper indium (CuIn), silver indium (AgIn), copper aluminum Sulfides of group I elements (group 11 elements) such as (CuAl) and silver gallium (AgGa) and group III elements (group 2 elements or group 13 elements), group I elements such as copper zinc (CuZn) and the like Sulfides of group II elements, copper gallium zinc (CuGaZn), copper indium zinc (CuInZn), silver indium zinc (AgInZn), and the like; sulfides of group II elements and group II elements and group III elements; Solid solution of La sulfides.

The sulfide semiconductor preferably contains zinc (Zn). Since Zn-containing sulfide semiconductors have relatively negative potential side (negative) CBM, the reduced semiconductor particles 3a made of Zn-containing sulfide semiconductors are reduced to CO ₂ or to the metal complex catalyst 3b. Is advantageous for the electron transfer of As a sulfide semiconductor containing Zn, a solid solution of ZnS and a metal sulfide containing a group I element and a group III element is preferable, and more specifically, for example, the composition formula is (CuGa) _1-x Zn _{2 x} S _{2 , (AgIn) 1-x Zn} 2x S 2, _{or, (CuIn) 1-x Zn} 2x S 2 ( both 0 <x <1) compound represented by is more preferable. Moreover, 20 mol% or more is preferable and, as for the content rate of Zn with respect to the total metal in the sulfide semiconductor containing Zn, 40 mol% or more is more preferable. Although the upper limit is not particularly limited, for example, the content of Zn with respect to the total amount of metals may be 99% by mass or less, and from the viewpoint of effective use of sunlight, 95% by mass or less is preferable.

The method for producing the sulfide semiconductor used as the reduced semiconductor particles 3a is not particularly limited, and may be produced by a known method. In addition, particles of a commercially available sulfide semiconductor may be used as the sulfide semiconductor. The sulfide semiconductor dissolves, for example, a metal compound constituting the sulfide semiconductor in a solvent, and after adding and stirring a solution containing a sulfur compound to the solution, centrifugation and redispersion are performed to remove the supernatant. Can be synthesized by drying it. Although it changes with metals to be used as a metal compound to be used, metal chloride, a bromide, an iodide, nitrate, a nitrite, a sulfate, an acetate, a perchlorate etc. are mentioned, for example. Further, in the case of producing a composite metal sulfide semiconductor containing a plurality of metals, sulfides of the respective metals may be used. Examples of sulfur compounds used for the synthesis of sulfide semiconductors include hydrogen sulfide, sodium sulfide hydrate, SCl ₂ , SBr ₂ , SI ₂ , thioacetic acid, thiourea, thioacetamide, thiocinamine and the like.

  Further, the sulfide semiconductor may be produced by bubbling hydrogen sulfide gas in a solution of the above metal compound, and then washing and drying the obtained precipitate and then firing. In addition, in the case of producing a solid solution of metal sulfides containing a plurality of metals, it may be produced by calcining a mixture obtained by mixing each metal sulfide at a target composition ratio. The particles of the produced sulfide semiconductor may be subjected to a treatment such as grinding or polishing to adjust the particle diameter. Furthermore, in order to support the metal complex catalyst 3b smoothly, the sulfide semiconductor may be pretreated in advance.

The reduced semiconductor constituting the reduced semiconductor particle 3a preferably has a CBM potential of -0.3 V or less relative to the potential of a standard hydrogen electrode at pH 0 (hereinafter referred to as "vs. NHE"). The CO ₂ reduction reaction requires electrons at levels more on the zeta potential side than the hydrogen generation reaction, and by using a reduced semiconductor whose CBM potential is -0.3 (vs. NHE) or less This is because the CO ₂ reduction reaction can be more advantageously promoted in the system. From the above viewpoint, the reduced semiconductor more preferably has a CBM potential of −0.5 V (vs. NHE) or less, and particularly preferably −1.0 V (vs. NHE) or less.

  The type of reduced semiconductor having a CBM potential in the above range is not particularly limited, and examples thereof include semiconductor compounds containing at least one selected from the group consisting of tantalum, iron, copper, indium and gallium, etc. An oxide semiconductor containing at least one selected is preferable. Moreover, the above-mentioned sulfide semiconductor is mentioned as a reduced semiconductor whose CBM potential is in the above range.

The tantalum compound is not particularly limited, and, for example, Ta ₂ O ₅ , TaON, Ta ₃ N ₅ , CaTaO ₂ N, SrTaO ₂ N, BaTaO ₂ N, LaTaO ₂ N, Y ₂ Ta ₂ O ₅ N, InTaO ₄ , LiTaO ₃ , KTaO ₃ , AgTaO ₃ , RbTaO ₃ , CSTaO ₃ , CSTaO ₃ , Na ₂ Ta ₂ O ₆ , K ₂ Ta ₂ O ₆ , CaTa ₂ O ₆ , SrTa ₂ O ₆ , BaTa ₂ O ₆ , NiTa ₂ O ₆ , Ca ₂ Ta ₂ O ₇ , Sr ₂ Ta ₂ O ₇ and LaTaO ₄ , and one or more elements selected from the group consisting of nitrogen, alkali metals, alkaline earth metals and lanthanoids in these compounds There doped compounds (e.g. _{N-Ta 2 O 5, Ni} -TaO 4, Na-TaO 3, La-TaO Etc.) and the like.

The iron compound is not particularly limited, but for example, a ferrite compound having a composition of Fe ₂ O ₃ and AFe ₂ O ₄ (as element A, Ca, Sr, Cu, Mg, Ni, Zn, Ba, etc. may be mentioned) And M-Fe ₂ O ₃ and M-Fe ₂ O ₄ , which are compounds doped with element M (as element M, N, Ag, Zn, Cu, Mg, Ni, Ti, Si, Nb, etc.) And the like).

The copper compound is not particularly limited, but, for example, copper oxide, an oxide of copper and another element (for example, copper aluminum oxide, copper bismuth oxide, etc.), copper iodide, Cu—In—Ga— Se-based compounds, Cu-In-Se-based compounds, etc. are mentioned, and more specifically, Cu ₂ O, CuO, CuAlO ₂ , CuBi ₂ O ₄ , CuI, Cu (InGa) Se ₂ , CuGaSe ₂ etc. Be

As a reduced semiconductor other than a sulfide semiconductor in which the potential of CBM is −0.3 (vs. NHE) or less, tantalum oxide, iron oxide (ferrite) and compounds in which they are doped with nitrogen or silver are preferable. Preferable specific examples of the oxide semiconductor include, for example, nitrogen-doped tantalum oxide (N-Ta ₂ O ₅ , CBM: about -1.3 V), nitrogen-doped iron oxide (N-Fe ₂ O ₃ , CBM: about- 0.6 V), calcium iron oxide (CaFe ₂ O ₄ , CBM: about −0.6 V), silver-doped calcium iron oxide (Ag—CaFe ₂ O ₄ , CBM: about −0.6 V) and the like.

  The method for producing reduced semiconductor particles 3a other than the sulfide semiconductor is not particularly limited, and may be produced by a known method. Alternatively, particles of a commercially available semiconductor compound may be used as the reduced semiconductor particles 3a. For example, nitrogen-doped tantalum oxide can be produced by heat treatment of tantalum oxide in an atmosphere containing ammonia gas. Ammonia is preferably diluted with a non-oxidizing gas (argon, nitrogen, etc.), for example, in a gas stream mixed with ammonia and argon so that the flow ratio is 1: 1 to 3: 1. Tantalum oxide is preferably disposed and heated. The heating temperature is preferably 500 ° C. to 900 ° C., and more preferably 550 ° C. to 850 ° C. The treatment time is preferably 6 hours or more and 15 hours or less. Tantalum oxide before doping with nitrogen may be a commercially available product, or may be prepared by subjecting a solution of a tantalum-containing compound such as tantalum chloride or tantalum alkoxide to hydrolysis treatment or the like. Other nitrogen-doped metal oxides can also be produced according to the above-mentioned production method.

(Metal complex catalyst)
The reduced photocatalyst particles 3 may be configured to include the reduced semiconductor particles 3a and the metal complex catalyst 3b. Any metal complex catalyst 3b may be used without particular limitation as long as it is a compound having a structure in which a metal and a nonmetal ligand are bonded and which exhibits a CO ₂ reduction activity by utilizing electrons. If the metal complex catalyst 3b is, for example, a compound in which the CBM level of the semiconductor compound constituting the reduced semiconductor particle 3a is +0.2 V or less with respect to the lowest unoccupied molecular orbital (LUMO) level of the metal complex catalyst 3b. Good. From the viewpoint of the efficiency of the electron transfer, it is preferable that the level of VBM of the reduced semiconductor particles 3a is more negative than the level of LUMO of the metal complex catalyst 3b.

  Examples of the metal complex catalyst 3b include complexes of at least one metal selected from metals belonging to any of Groups 6 to 10 of the periodic table and a ligand. Examples of the metal constituting the metal complex catalyst 3b include Cr, Mo, W, Ru, Re, Mn, Fe, Os, Co, Rh, Ir, Ni, Pd, Pt and the like, and Ru and Rh are preferable. . Such a complex may be mononuclear or polynuclear of two or more nuclei. Moreover, 2 or more types of metals may be contained.

The ligand in the metal complex catalyst 3b is not particularly limited. For example, as a typical main ligand, a nitrogen-containing heterocyclic compound, an oxygen-containing heterocyclic compound, an oxygen-containing compound, a sulfur-containing heterocyclic compound, etc. And CO, halogen, cyan, phosphines and the like can be mentioned as auxiliary ligands. The auxiliary ligand may be partially dissociated in contact with CO ₂ , a base, or water in the course of the reaction, and converted to a secondary ligand. The subsidiary ligand, for example _{-CO, -CO 2, -COOH, -COH} , - (CO 2) -, - OH, -OH 2 and the like. These ligands may be used alone or in combination of two or more. Although there is no restriction | limiting in particular as an element coordinated to a metal in these ligands, For example, O, N, C, P, S, Si, a halogen, etc. are mentioned. Such an element may be coordinated by one or more species.

  Examples of the nitrogen-containing heterocyclic compound as the main ligand include pyridine, bipyridine, diphosphonate bipyridine, phenanthroline, terpyridine, quaterpyridine, pyrrole, indole, carbazole, imidazole, pyrazole, quinoline, isoquinoline, acridine, pyridazine, Pyrimidine, pyrazine, phthalazine, quinazoline, quinoxaline and derivatives thereof and the like can be mentioned. Examples of the oxygen-containing heterocyclic compound as a main ligand include furan, benzofuran, oxazole, pyran, pyrone, coumarin, benzopyrone and derivatives thereof. Examples of the oxygen-containing compound as a main ligand include polyoxometalate and its derivatives. Examples of the sulfur-containing heterocyclic compound as a main ligand include thiophene, thionaphthene, thiazole and derivatives thereof.

Specific examples of the metal complex catalyst 3b include diphosphonate bipyridine (dpbpy), diphosphonate ethyl bipyridine (dpebpy), dicarboxybipyridine (dcbpy), bipyridine (bpy) and di (pyrrolylpropyl carbonate) bipyridine (pypcbpy) The ruthenium complex, rhenium complex, etc. which have at least one selected from the group as a main ligand are mentioned. More specifically, [Ru (dpbpy) (CO) ₂ Cl ₂ ] ²⁺ , [Ru (dpbpy) (bpy) (CO) ₂ ] ²⁺ , [Ru (dcbpy) (CO) ₂ Cl ₂ ] ^{2 +} , [Ru (pypcbpy) (CO) (MeCN) Cl ₂ ] ²⁺ , [Ru (dpebpy) (bpy) (CO) ₂ ] ²⁺ , [Ru (dcbpy) (bpy) (CO) ₂ ] ²⁺ , [Re (dcbpy) (CO) ₃ P (OEt) ₃ ] ²⁺ , [Re (dcbpy) (CO) ₃ Cl] 2 ⁺ , [Re (dcbpy) (CO) ₃ MeCN] 2 ^+, etc. . Moreover, the metal complex polymer which polymerized these may be used.

  The reduced photocatalyst particle 3 including the reduced semiconductor particle 3a and the metal complex catalyst 3b is prepared, for example, by supporting the metal complex catalyst 3b on the reduced semiconductor particle 3a. There is no particular limitation on the method of supporting the metal complex catalyst 3b, and there are no particular limitations, and for example, an adsorption supporting method, a photoprecipitation method (also referred to as photo electrodeposition method, photoelectric precipitation method), a deposition precipitation method, nanoparticle support method, physical vapor deposition The method (PVD method) etc. are mentioned. For example, a solution containing metal complex ions corresponding to metal complex catalyst 3b is impregnated with reduced semiconductor particles 3a, stirred for a predetermined time (for example, 6 hours or more, preferably 12 hours or more), and then washed and dried. The reduced photocatalyst particles 3 formed by supporting (adsorbing) the metal complex catalyst 3b on the reduced semiconductor particles 3a are obtained. Two or more of these loading methods may be used in combination. Alternatively, the metal complex catalyst 3b may be synthesized after adsorbing the above-mentioned ligand to the reduced semiconductor particles 3a.

  It is preferable that the reduced semiconductor particles 3a and the metal complex catalyst 3b be chemically bonded by a linking group. This linking group is not particularly limited as long as it is a functional group capable of chemically bonding to the reduced semiconductor particle 3a, and examples thereof include a carboxyl group, a phosphoric acid group, a sulfonic acid group, a silanol group, a mercapto group and derivatives thereof. Be Here, the linking group may have a structure in which a proton is eliminated, or a structure in which a metal and an oxygen atom are coordinated, in a state of being linked to the reduced semiconductor particle 3a. These linking groups may be used alone or in combination of two or more.

  The method of binding the reduced semiconductor particles 3a and the metal complex catalyst 3b by the linking group is not particularly limited as long as both are chemically linked via the linking group. For example, (1) a metal complex catalyst 3b in which a linking group is introduced to a ligand is adsorbed to a sulfide semiconductor, (2) a ligand in which a linking group is introduced is adsorbed to a sulfide semiconductor and then directly a metal complex catalyst Methods such as forming 3b, (3) bonding metal complex catalyst 3b to a sulfide semiconductor having a linking group introduced, and the like can be mentioned.

Although there is no restriction | limiting in particular in content of the metal complex catalyst 3b in case the reduction photocatalyst particle 3 contains the metal complex catalyst 3b, For example, 0.01 mass part or more and 50 mass parts or less with respect to 100 mass parts reduction semiconductor particle 3a Is preferably, and more preferably 0.03 parts by mass or more and 40 parts by mass or less. If the content (loading amount) of the metal complex catalyst 3 b is too small, the CO ₂ reduction activation effect by the metal complex catalyst 3 b may not be sufficiently obtained. When the supported amount of the metal complex catalyst 3b is too large, the light absorption of the reduced semiconductor particles 3a may be hindered or the catalyst activity of the reduced photocatalyst particles 3 may be reduced by acting as a recombination center. The content of the metal complex catalyst 3b in the reduced photocatalyst particle 3 can be measured, for example, by absorption spectrum measurement of a solution after adsorption or inductively coupled plasma (ICP) analysis of a complex-supported semiconductor sample.

The particle size of the reduced photocatalyst particles 3 according to the present embodiment is not particularly limited, but the average primary particle size is preferably 100 μm or less, and more preferably 10 μm or less. If the particle size of the reduced photocatalyst particles 3 is too large, the specific surface area of the particles may be reduced, and the probability of contact between the particles may be reduced. The lower limit of the average primary particle size is not particularly limited, and is, for example, 1 nm or more. The average primary particle size of the reduced photocatalyst particles 3 may be measured by the same method as the average primary particle size of the oxidation photocatalyst particles 2 described above. Further, the specific surface area of the reduced photocatalyst particle 3 according to the present embodiment is not particularly limited, but preferably 5 m ² / g or more, for example. If the specific surface area of the reduced photocatalyst particles 3 is too small, the contact probability between particles and the photocatalytic activity may decrease.

  In the photocatalyst system 1 according to the present embodiment, the contents of the oxidation photocatalyst particles 2 and the reduction photocatalyst particles 3 are not particularly limited, but from the viewpoint of the oxidation-reduction reaction of the photocatalyst system 1 as a whole, the oxidation photocatalyst particles 2 and reduction The photocatalyst particles 3 are preferably present at a mass ratio of 1:20 or more and 20: 1 or less, and more preferably 1:10 or more and 10: 1 or less.

[Redox mediator]
The mediator 4 used in the photocatalyst system 1 according to the present embodiment is not particularly limited as long as it is a compound capable of transmitting electrons between the oxidized photocatalyst particle 2 and the reduced photocatalyst particle 3. In other words, as the mediator 4, in the aqueous medium containing the photocatalyst system 1, receiving the electron e ⁻ from the oxidized photocatalyst particle 2 results in a reduced product from the oxidized body, and the reduced photocatalyst particle 3 (reduced semiconductor particle 3a Any compound can be used as long as it can return from reductant to oxidant by handing over electron e ⁻ to.

  The mediator 4 may be present in the aqueous medium in a form capable of being in contact with both the oxidized photocatalyst particle 2 and the reduced photocatalyst particle 3. Examples of the mediator 4 include a solution-based mediator dissolved or suspended in an aqueous medium, and a solid-type mediator joined to either the oxidized photocatalyst particle 2 or the reduced photocatalyst particle 3.

The compound used for the solution-based mediator is not particularly limited as long as it is a mediator that can be dissolved or suspended in an aqueous medium, and examples thereof include iron ions, iodine-based compounds and metal-containing compounds known as solution-based mediators. Examples of iron ions include Fe ³⁺ / Fe ²⁺ redox and the like. Examples of the iodine compound include iodate ion / iodide ion (IO ³⁻ / I ⁻ ), triiodide ion / iodide ion (I ³⁻ / I ⁻ ), and the like. The metal-containing compound may be a complex compound of metal. The metal (central metal) contained in the metal complex compound may be, for example, a metal belonging to any of Groups 6 to 12 of the periodic table, and specifically, Fe, Co, Cu, Cr, Mo, W, Ru, Re, Mn, Os, Rh, Ir, Ni, Pd, Pt, Zn and the like can be mentioned, with preference given to Fe, Co and Cu.

In the photocatalyst system 1 according to the present embodiment, it is preferable to use a metal complex compound as the mediator 4 because the selectivity of the CO ₂ reduction reaction to the reduction reaction of water can be further enhanced. The reason why the selectivity of the CO ₂ reduction reaction is enhanced by the use of the metal complex compound is not clear, but the metal complex compound suspended or dissolved in the aqueous medium plays a role as a mediator 4 (support for electron transfer between particles) In addition, it is possible that the ligand contained in the metal complex may create an environment in which the CO ₂ reduction reaction is likely to proceed on the surface of the reduced photocatalyst particle 3.

  The central metal of the metal complex compound includes a metal belonging to any of Groups 6 to 11 of the periodic table above, and is at least one selected from the group consisting of cobalt, iron and copper preferable. The metal complex compound may be mononuclear or polynuclear having two or more nuclei, and may contain two or more metals. There is no restriction | limiting in particular as a ligand in a metal complex compound, For example, each compound described as a ligand in description of the above-mentioned metal complex catalyst 3b is mentioned. More specifically, for example, CO, halogen, cyan, phosphine, pyridine, bipyridine, diphosphonate bipyridine, phenanthroline, terpyridine, quaterpyridine, pyrrole, indole, carbazole, imidazole, pyrazole, quinoline, isoquinoline, acridine, pyridazine, pyrimidine Pyrazine, phthalazine, quinazoline, quinoxaline, furan, benzofuran, oxazole, pyran, pyrone, coumarin, benzopyrone, polyoxometalate, thiophene, thionaphthene, thiazole and derivatives thereof and the like. In these ligands, the element to be coordinated to the metal is not particularly limited, and examples thereof include O, N, C, P, S, Si and halogen. Such an element may be coordinated by one or more species. The ligand of the metal complex compound is preferably at least one selected from the group consisting of terpyridine (tpy), bipyridine (bpy) and phenanthroline (phen).

Preferred specific examples of the metal complex compound used as a solution-based mediator include, for example, bis (2,2 ′: 6 ′, 2 ′ ′-terpyridine) cobalt ion ([Co (tpy) ₂ ] ^{3 + / 2 +} ), Bis (2,2 ′: 6 ′, 2 ′ ′-terpyridine) iron ion ([Fe (tpy) ₂ ] ^{3 + / 2 +} ), bis (2,2 ′: 6 ′, 2 ′ ′-terpyridine) copper Ions ([Cu (tpy) ₂ ] ^{2 + / 1 +} ), tris (2,2'-bipyridine) cobalt ion ([Co (bpy) ₃ ] ^{3 + / 2 +} ) and tris (1,10-phenanthroline) Cobalt ion ([[Co (phen) ₃ ] ^{3 + / 2 +} ), etc. Also, if the metal complex compound is a complex ion, it may be a counter ion which does not inhibit the reaction occurring in photocatalyst system 1. For example, any can be used.

  When the mediator 4 is a solution-based mediator, the content of the solution-based mediator contained in the aqueous medium may be, for example, 0.005 mM (mmol / L) or more relative to the total amount of the aqueous medium, 0.01 mM (mmol) / L) or more is preferable. The upper limit of the content of the solution-based mediator may be the saturation concentration in the aqueous medium.

  The solid mediator is not particularly limited as long as it is a compound that can intervene in the electron transfer between the oxidized photocatalyst particle 2 and the reduced photocatalyst particle 3, and preferred examples thereof include, for example, reduced graphene oxide (RGO And other carbon-based materials. RGO may be prepared by a known method for reducing graphene oxide. For example, RGO bonded to the oxidized photocatalyst particle 2 can be prepared by irradiating visible light to a suspension in which graphene oxide and the oxidized photocatalyst particle 2 are suspended and performing a photoreduction reaction of graphene oxide. Graphene oxide as a raw material of RGO is also not particularly limited, and for example, graphene oxide prepared by a known method such as Hummers method of oxidizing graphite using potassium permanganate as an oxidizing agent may be used.

  When the mediator 4 is a solid-type mediator, the amount of use of the solid-type mediator is, for example, 0.05% by mass or more and 50% by mass or less based on the total amount of the oxidation photocatalyst particles 2 or the reduction photocatalyst particles 3 to be joined. The content is preferably 0.5% by mass or more and 20% by mass or less.

[Aqueous medium]
As the aqueous medium used in the present embodiment, any medium containing water as a main component can be used appropriately. The aqueous medium contains water and the mediator 4, and may contain, for example, an electrolyte, and may contain a water-soluble organic solvent such as methanol and ethanol.

The electrolyte contained in the aqueous medium is a compound generally used as a supporting electrolyte in the electrolytic reaction of water and can be used without particular limitation as long as it does not inhibit the action of the photocatalyst system 1, for example And oxo acids and oxo acid salts. In the present embodiment, the aqueous medium is preferably an aqueous solution of electrolyte. The reason is not clear, but the inclusion of the electrolyte in the aqueous solvent tends to further enhance the selectivity of the CO ₂ reduction reaction and the generation amount of the CO ₂ reduction product. In the present specification, for the sake of convenience, the term “electrolyte” also includes an electrolyte ionized into positive ions and negative ions in an aqueous medium.

  As an oxo acid which comprises an electrolyte, carbonic acid, phosphoric acid, nitric acid, a sulfuric acid, boric acid, halogen oxo acids (hypochlorous acid, chloric acid, perchloric acid etc.) etc. are mentioned, for example. As a cation which comprises an oxo acid salt, alkali metals, such as sodium and potassium, are mentioned, for example. Specific examples of the electrolyte include carbonates such as sodium carbonate and potassium carbonate, bicarbonates such as sodium hydrogen carbonate and potassium bicarbonate, phosphates such as potassium phosphate and sodium phosphate (dihydrogen phosphate, And hydrogen sulfates such as sodium sulfate and potassium sulfate; sodium perchlorate; and perchlorates such as potassium perchlorate. The electrolyte used for the aqueous medium is preferably at least one selected from the group consisting of carbonates, bicarbonates and phosphates.

  The concentration of the electrolyte in the aqueous medium is not particularly limited. For example, the molar concentration of the electrolyte relative to the total amount of the aqueous medium is preferably 0.001 M (mol / L) or more and 2 M or less, more preferably 0.01 M or more and 1 M or less. When the concentration of the electrolyte is in the above range, the electron transfer between the photocatalyst and between the catalyst and the reactant is excellent.

[Formation of photocatalyst system]
The photocatalyst system 1 which concerns on this embodiment can be obtained by mixing an aqueous medium, the oxidation photocatalyst particle 2, the reduction photocatalyst particle 3, and the mediator 4. For example, the photocatalyst system 1 according to the present embodiment is formed by mixing the oxidized photocatalyst particles 2 and the reduced photocatalyst particles 3 and adding them to the aqueous medium containing the mediator 4.

  When the photocatalyst system 1 thus formed is irradiated with visible light 5, as described above, reduction products such as formic acid and carbon monoxide are generated from carbon dioxide in water by the reduction reaction in the reduction photocatalyst particles 3 At the same time, the oxidation reaction in the oxidation photocatalyst particles 2 generates oxygen gas. In addition, in the photocatalyst system 1 according to the present embodiment, the reduced semiconductor particles 3a or the metal complex catalyst 3b in the reduced photocatalyst particles 3 are selected, and the catalyst reaction occurs in an appropriate environment, and not only formic acid and carbon monoxide. It is also possible to synthesize useful carbon compounds such as ethanol from carbon dioxide.

  The photocatalyst system 1 according to the present embodiment can also utilize light including ultraviolet light and ultraviolet light. The photocatalyst system 1 of the present embodiment can obtain sufficient photocatalytic activity in visible light with a wavelength λ longer than 360 nm, so that light energy such as sunlight can be used more effectively.

  Hereinafter, the present invention will be more specifically described based on examples and comparative examples, but the present invention is not limited to the following examples.

[Preparation of reduced semiconductor particles]
Preparation Example A-1
Copper sulfide (II) (Cu ₂ S, manufactured by High Purity Chemical Laboratories, Inc.), gallium sulfide (III) (Ga ₂ S ₃ , manufactured by High Purity Chemical Laboratories, Inc.), and zinc sulfide (ZnS, Inc. The high purity chemical laboratory make) was mixed in the quantity which becomes the molar ratio Cu: Ga: Zn = 0.5: 0.6: 1. The resulting mixture is sealed in a quartz tube and crystallized by firing at 1073 K for 10 hours to obtain reduced semiconductor particles composed of a composite metal sulfide semiconductor represented by the composition formula (CuGa) _0.5 ZnS ₂ The

Preparation Example A-2
Preparation Example A- except that copper (II) sulfide, gallium (III) sulfide and zinc sulfide are mixed in an amount such that the molar ratio is Cu: Ga: Zn = 0.3: 0.36: 1.4. According to the method of 1, a reduced semiconductor particle composed of a composite metal sulfide semiconductor represented by the composition formula (CuGa) _0.3 Zn _1.4 S ₂ was obtained.

Preparation Example A-3
Preparation Example A- except that copper (II) sulfide, gallium (III) sulfide and zinc sulfide are mixed in an amount such that the molar ratio is Cu: Ga: Zn = 0.1: 0.12: 1.8. According to the method of 1, a reduced semiconductor particle composed of a composite metal sulfide semiconductor represented by the composition formula (CuGa) _0.1 Zn _1.8 S ₂ was obtained.

Preparation Example A-4
Silver nitrate (I) (AgNO ₃ , manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.), indium nitrate (III) hydrate (In (NO ₃ ) ₃ · 3.9H ₂ O, manufactured by High Purity Chemical Laboratory Co., Ltd.), and nitric acid Zinc hydrate (Zn (NO ₃ ) ₂ · 6H ₂ O, manufactured by Wako Pure Chemical Industries, Ltd.) in an amount such that the molar ratio is Ag: In: Zn = 0.22: 0.22: 1.56 Hydrogen sulfide gas was bubbled in the mixed aqueous solution contained. The formed precipitate is washed and dried, and then crystallized by baking for 5 hours at 1123 K under a nitrogen stream to form a reduced semiconductor composed of a composite metal sulfide semiconductor represented by the composition formula (AgIn) _0.22 Zn _1.56 S ₂ I got the particles.

Preparation Example A-5
Preparation Example A- except that copper (II) sulfide, gallium (III) sulfide and zinc sulfide are mixed in an amount such that the molar ratio is Cu: Ga: Zn = 0.8: 0.96: 1.4. According to the method of 1, a reduced semiconductor particle composed of a composite metal sulfide semiconductor represented by the composition formula (CuGa) _0.8 Zn _1.4 S ₂ was obtained.

Preparation Example A-6
Copper chloride (CuCl, purity 99.9%, manufactured by Wako Pure Chemical Industries, Ltd.), indium (III) nitrate hydrate (In (NO ₃ ) ₃ · 3.9H ₂ O, purity 99.99%, high Purity Chemical Research Institute) and zinc nitrate hydrate (Zn (NO ₃ ) ₂ · 6H ₂ O, purity 99.0%, Wako Pure Chemical Industries, Ltd.), the molar ratio of Cu: In: Zn = 0 Hydrogen sulfide gas was bubbled in the mixed aqueous solution containing the amount of .09: 0.09: 1.8. The formed precipitate is washed and dried, and then crystallized by baking for 5 hours at 1123 K under a nitrogen stream to form a reduced semiconductor composed of a composite metal sulfide semiconductor represented by the composition formula (CuIn) _0.09 Zn _1.8 S ₂ I got the particles.

Preparation Example A-7
Copper (II) sulfide and gallium sulfide (III), the molar ratio of Cu: Ga = 1: except that mixing in an amount of 1.2, according to the method of Preparation A-1, Table by a composition formula CuGaS ₂ Thus, reduced semiconductor particles composed of the composite metal sulfide semiconductor obtained are obtained.

Preparation Example A-8
5 g of tantalum chloride (manufactured by Wako Pure Chemical Industries, Ltd.) was dissolved in 100 ml of ethanol, and a 5% aqueous ammonia solution was added to make up to 300 ml. The solution was stirred for 5 hours to synthesize tantalum oxide (Ta ₂ O ₅ ) powder. The white powder was heat-treated at 500 ° C. for 5 hours in the atmosphere, and then treated at 575 ° C. for 5 hours under a mixed flow of ammonia and argon (ammonia gas: 0.4 L / min, argon gas: 0.2 L / min). Then, reduced semiconductor particles composed of a nitrogen-doped tantalum oxide semiconductor (N-Ta ₂ O ₅ ) were obtained as a yellow crystal powder.

[Preparation of reduced photocatalyst particles containing reduced semiconductor particles and metal complex catalyst]
According to the following Preparation Examples B-1 to B-8, reduced photocatalyst particles composed of reduced semiconductor particles supporting a metal complex catalyst were prepared. The supported amount of the metal complex catalyst in the reduced photocatalyst particles prepared in Preparation Examples B-1 to B-8 was calculated by measuring the absorbance of the supernatant solution after adsorption.

Preparation Example B-1
In accordance with the method described in Inorganic Chemistry, 1995, Vol. 34, No. 24, p. 6145-6157, [Ru (CO) ₂ Cl ₂ ] n and diphosphonate bipyridine in methanol under a stream of N ₂ Ruthenium complex having a diphosphonate bipyridine ligand ([Ru (4,4'-diphosphonate-2,2'-bipyridine) (CO) ₂ Cl ₂ ] ²⁺ ) (below) by refluxing at 80 ° C. for 3 hours "Ru-dpbpy" was synthesized. Subsequently, 5 mL of a methanol solution of Ru-dpbpy was added to 200 mg of the reduced semiconductor particles synthesized in Preparation Example A-1, and the metal complex catalyst was adsorbed on the surface of the reduced semiconductor particles by stirring for 16 hours. The obtained mixture was filtered, washed with methanol, and dried to obtain particles of a semiconductor (composition formula (CuGa) _0.5 ZnS ₂ ) supporting 0.06 mass% of Ru-dpbpy.

Preparation Example B-2
According to the method of Preparation Example B-1, 0.03% by mass of Ru-dpbpy is used except that the reduced semiconductor particles synthesized in Preparation Example A-1 are replaced with the reduced semiconductor particles synthesized in Preparation Example A-2. The particles of the supported semiconductor (composition formula (CuGa) _0.3 Zn _1.4 S ₂ ) were obtained.

Preparation Example B-3
0.03% by mass of Ru-dpbpy according to the method of Preparation Example B-2, except that the reduced semiconductor particles synthesized in Preparation Example A-2 are replaced with the reduced semiconductor particles synthesized in Preparation Example A-3 There was obtained particles of the supported semiconductor (composition formula _{(CuGa) 0.1 Zn 1.8 S 2} ).

Preparation Example B-4
0.05% by mass of Ru-dpbpy according to the method of Preparation Example B-1, except that the reduced semiconductor particles synthesized in Preparation Example A-4 are used instead of the reduced semiconductor particles synthesized in Preparation Example A-1 The particles of the supported semiconductor (composition formula (AgIn) _0.22 Zn _1.56 S ₂ ) were obtained.

Preparation Example B-5
0.19% by mass of Ru-dpbpy according to the method of Preparation Example B-1 except that the reduced semiconductor particles synthesized in Preparation Example A-1 are used instead of the reduced semiconductor particles synthesized in Preparation Example A-1 The particle of the nitrogen-doped tantalum oxide semiconductor (N-Ta ₂ O ₅ ) supported is obtained.

Preparation Example B-6
Similar to Preparation Example B-1, according to the method described in Inorganic Chemistry, 1995, Vol. 34, No. 24, p. 6145-6157, a ruthenium complex having a diphosphonate bipyridine ligand and a bipyridine ligand ([[ Synthesis of Ru (4,4'-diphosphonate-2,2'-bipyridine) (2,2'-bipyridine) (CO) ₂ ] ²⁺ (hereinafter also referred to as "Ru- (dpbpy) (bpy)") did. Then, according to the method of Preparation Example B-2, 0.03 mass% of Ru- (dpbpy) (m) was used except that a methanol solution of Ru- (dpbpy) (bpy) was used instead of the Ru-dpbpy methanol solution. bpy) resulting in particles of the supported semiconductor (composition formula _{(CuGa) 0.1 Zn 1.8 S 2} ).

Preparation Example B-7
Similar to Preparation Example B-1, according to the method described in Inorganic Chemistry, 1995, Vol. 34, No. 24, p. 6145-6157, a ruthenium complex ([Ru (4,4 '-dicarboxyl-2,2'-bipyridine (CO) ₂ Cl ₂ ] ²⁺ ) (hereinafter also described as "Ru-dcbpy") was synthesized. Then, according to the method of Preparation Example B-2, a semiconductor on which 0.03 mass% of Ru-dcbpy was supported (composition formula except for using a methanol solution of Ru-dcbpy instead of the methanol solution of Ru-dpbpy) (CuGa) was obtained _0.1 Zn _1.8 S ₂₎ of the particles.

Preparation Example B-8
According to the method described in Journal of Material Chemistry A, 2015, Vol. 3, p. 13283-13290, [Ru (CO) ₂ Cl ₂ ] n and di (pyrrolylpropyl carbonate) bipyridine were dissolved in N _{2 in} methanol. A ruthenium complex having a di (pyrrolylpropyl carbonate) bipyridine ligand ([Ru {4,4'-di (1H-pyrrolyl-3-propyl carbonate)-) by refluxing at 80 ° C. for 3 hours under air flow] 2,2′-bipyridine} (CO) ₂ Cl ₂ ] ²⁺ ) (hereinafter also referred to as “Ru-pypcbpy”) was synthesized. Then, a semiconductor on which 0.06 mass% of Ru-pypcbpy is supported according to the method of Preparation Example B-1 (composition formula, except that a methanol solution of Ru-pypcbpy is used instead of the methanol solution of Ru-dpbpy) (CuGa) was obtained _0.1 Zn _1.8 S ₂₎ of the particles.

[Preparation of oxidation photocatalyst particles]
Preparation Example C-1
<Synthesis of bismuth vanadate semiconductor (BiVO ₄ ) particles>
20 mmol of bismuth nitrate hydrate (Bi (NO ₃ ) ₃ .5 H ₂ O, manufactured by Kanto Chemical Co., Ltd.), and 10 mmol of vanadium pentoxide (V ₂ O ₅ , manufactured by Wako Pure Chemical Industries, Ltd.) were mixed with 0. The mixture was added to 100 mL of a 5 mol / L aqueous nitric acid solution, and the obtained mixture was stirred at room temperature for 72 hours. The formed particles were filtered, washed with water, and dried at 120 ° C. to obtain oxidized photocatalyst particles made of bismuth vanadate semiconductor (BiVO ₄ ).

[Preparation of mediator]
Preparation Example D-1
Cobalt (II) hexahydrate (Co (NO ₃ ) ₂ · 6H ₂ O, manufactured by Wako Pure Chemical Industries, Ltd.) 1.5 mmol, and 2,2 ′: 6 ′, 2 ′ ′-terpyridine (Tokyo Kasei Kogyo Co., Ltd.) 3.0 mmol of Industrial Co., Ltd.) was dissolved in methanol, stirred and concentrated under reduced pressure, and then the obtained residue was recrystallized to obtain [Co (2,2 ′: 6 ′, 2 ′ ′-terpyridine)) ₂ ] (NO ₃ ) The mediator represented by ₂ was synthesized.

Preparation Example D-2
Iron (II) sulfate heptahydrate (FeSO ₄ · 7 H ₂ O, manufactured by Wako Pure Chemical Industries, Ltd.) 1.5 mmol, and 2,2 ': 6', 2 "-terpyridine (manufactured by Tokyo Chemical Industry Co., Ltd.) 3.0 mmol was dissolved in methanol, stirred and concentrated under reduced pressure, and then the obtained residue was recrystallized to obtain [Fe (2,2 ′: 6 ′, 2 ′ ′-terpyridine) ₂ ] SO ₄ The mediator represented by was synthesized.

Preparation Example D-3
Copper sulfate (II) pentahydrate (CuSO ₄ · 5 H ₂ O, manufactured by Wako Pure Chemical Industries, Ltd.) 3.0 mmol, and 2,2 ′: 6 ′, 2 ′ ′-terpyridine (manufactured by Tokyo Chemical Industry Co., Ltd.) 3.0 mmol was dissolved in methanol, stirred and concentrated under reduced pressure, and then the obtained residue was recrystallized to obtain [Cu (2,2 ′: 6 ′, 2 ′ ′-terpyridine) ₂ ] SO ₄ The mediator represented by was synthesized.

Preparation Example D-4
Graphite oxide ("CMX-40" manufactured by Japan Graphite Industry Co., Ltd.) was oxidized according to the Hummers method (Journal of American Chemical Society, 1958, Vol. 80, p. 1339) to prepare graphene oxide (GO). A suspension was obtained by adding 0.05 g of the obtained graphene oxide and 1 g of the oxidized photocatalyst particles obtained in Preparation Example C-1 to 40 mL of an aqueous methanol solution (50% by volume). The suspension was irradiated with visible light for 3 hours while flowing Ar gas, thereby preparing a complex of reduced type graphene oxide (RGO), which is a mediator, and oxidized photocatalyst particles.

Preparation Example D-5
Cobalt (II) heptahydrate (CoSO ₄ · 7 H ₂ O, manufactured by Wako Pure Chemical Industries, Ltd.) 2.5 mmol, and 7.5 mmol of 2,2'-bipyridine (manufactured by Wako Pure Chemical Industries, Ltd.) Dissolve in ethanol, stir and concentrate under reduced pressure. Next, the obtained residue was recrystallized to synthesize a mediator represented by [Co (2,2′-bipyridine) ₃ ] SO ₄ .

[Production of photocatalytic system, evaluation of photocatalytic performance by test tube method]
Example 1
The mediator obtained in Preparation Example D-1 was added to an aqueous electrolyte solution containing NaHCO ₃ at a concentration of 0.25 M to a concentration of 0.02 mM to prepare an aqueous medium, and 4 mL of the aqueous medium obtained Was placed in a glass test tube with a volume of 8 ml. Next, 10.7 mg of the reduced photocatalyst particles obtained in Preparation Example B-1 and 5.3 mg of the oxidized photocatalyst particles obtained in Preparation Example C-1 are added to a test tube, and the photocatalyst system according to the present embodiment is used. A sample solution was prepared.

The evaluation test of the photocatalytic performance by the test tube method was performed with respect to the sample solution obtained above. CO ₂ gas was bubbled into the obtained sample solution for 15 minutes to saturate the sample solution, and the test tube was sealed with a rubber stopper. The test tube was attached to a merry-go-round rotating apparatus. Using a xenon lamp (manufactured by Ushio Inc.) as a light source while stirring the sample solution with a stirrer, the visible light (λ> 390 nm) that has passed through the heat ray absorption filter (“SCF-1” manufactured by Asahi Glass Co., Ltd.) The sample solution was irradiated for 16 hours. After light irradiation, the gas component contained in the gas phase portion in the test tube is analyzed and quantified using a gas chromatograph ("GC-2014" manufactured by Shimadzu Corporation), and the compound contained in the liquid phase portion is ion chromatography ( It analyzed and quantified using Thermo Fisher SCIENTIFIC company "DIONEX ICS-2100").

Example 2
The mediator obtained in Preparation Example D-1 was added to an aqueous electrolyte solution containing NaHCO ₃ at a concentration of 0.1 M to a concentration of 0.02 mM to prepare an aqueous medium, and 4 mL of the aqueous medium obtained Was placed in a glass test tube with a volume of 8 ml. Next, 8.0 mg of the reduced photocatalyst particles obtained in Preparation Example B-2 and 8.0 mg of the oxidized photocatalyst particles obtained in Preparation Example C-1 are added to the test tube, and the photocatalyst system according to the present embodiment is used. A sample solution was prepared. The photocatalytic performance of the obtained sample solution was evaluated according to the method of Example 1.

Example 3
A sample which is a photocatalyst system according to the present embodiment according to the method of Example 1, except that the reduced photocatalyst particle obtained in Preparation Example B-2 is used instead of the reduced photocatalyst particle obtained in Preparation Example B-1 The solution was prepared, and the photocatalytic performance of the obtained sample solution was evaluated.

Example 4
According to the method of Example 3, a sample solution which is a photocatalyst system according to the present embodiment is prepared according to the method of Example 3, except that the purified aqueous solution containing no electrolyte is used instead of the aqueous electrolyte solution containing NaHCO _3. The photocatalytic performance of the obtained sample solution was evaluated.

Example 5
A sample which is a photocatalyst system according to the present embodiment according to the method of Example 2, except that the reduced photocatalyst particles obtained in Preparation Example B-3 are used instead of the reduced photocatalyst particles obtained in Preparation Example B-2. The solution was prepared, and the photocatalytic performance of the obtained sample solution was evaluated.

Example 6
Using the reduced photocatalyst particles obtained in Preparation Example B-4 instead of the reduced photocatalyst particles obtained in Preparation Example B-2, and replacing the aqueous electrolyte solution containing NaHCO ₃ at a concentration of 0.1 M A sample solution which is a photocatalyst system according to the present embodiment is prepared according to the method of Example 2 except that an aqueous electrolyte solution containing Na ₂ CO ₃ at a concentration of 0.1 M is used, and a photocatalyst of the obtained sample solution is prepared. The performance was evaluated.

Example 7
According to the method of Example 3 except that the mediator obtained in Preparation Example D-2 is used instead of the mediator obtained in Preparation Example D-1, a sample solution which is a photocatalyst system according to the present embodiment is prepared. The photocatalytic performance of the obtained sample solution was evaluated.

Example 8
A sample solution which is a photocatalyst system according to the present embodiment is prepared according to the method of Example 3 except that the mediator obtained in Preparation Example D-3 is used instead of the mediator obtained in Preparation Example D-1 The photocatalytic performance of the obtained sample solution was evaluated.

Example 9
Four mL of an aqueous electrolyte solution containing NaHCO ₃ at a concentration of 0.25 M was placed in a glass test tube having a volume of 8 ml. Next, 10.7 mg of the reduced photocatalyst particles obtained in Preparation Example B-2 and 5.3 mg of a complex of reduced graphene oxide (RGO) and the oxidation photocatalyst obtained in Preparation Example D-4 are provided in a test tube. The sample solution was added to prepare a photocatalyst system according to the present embodiment. The photocatalytic performance of the obtained sample solution was evaluated according to the method of Example 1.

Example 10
A sample which is a photocatalyst system according to the present embodiment according to the method of Example 2, except that the reduced semiconductor particles obtained in Preparation Example A-2 are used instead of the reduced photocatalyst particles obtained in Preparation Example B-2. The solution was prepared, and the photocatalytic performance of the obtained sample solution was evaluated.

Example 11
Preparation for using mediators obtained in Preparation Example D-5 in place of the resultant mediator in D-1, and a NaHCO ₃ instead of NaHCO ₃ in an aqueous electrolyte solution containing a concentration of 0.25M 0 According to the method of Example 10 except that an aqueous electrolyte solution containing 1 M concentration was used, a sample solution which is a photocatalyst system according to the present embodiment was prepared, and the photocatalytic performance of the obtained sample solution was evaluated.

Example 12
Using the reduced semiconductor particles obtained in Preparation Example A-2 instead of the reduced photocatalyst particles obtained in Preparation Example B-2, and replacing the aqueous electrolyte solution containing NaHCO ₃ at a concentration of 0.1 M A sample solution which is a photocatalyst system according to the present embodiment is prepared and obtained according to the method of Example 2 except that an aqueous electrolyte solution containing K ₂ HPO ₄ and KH ₂ PO ₄ each at a concentration of 0.05 M is used. The photocatalytic performance of the sample solution was evaluated.

Example 13
A sample which is a photocatalyst system according to the present embodiment according to the method of Example 2, except that the reduced semiconductor particles obtained in Preparation Example A-1 are used instead of the reduced photocatalyst particles obtained in Preparation Example B-2. The solution was prepared, and the photocatalytic performance of the obtained sample solution was evaluated.

Example 14
A sample which is a photocatalyst system according to the present embodiment according to the method of Example 2, except that the reduced semiconductor particles obtained in Preparation Example A-5 are used instead of the reduced photocatalyst particles obtained in Preparation Example B-2. The solution was prepared, and the photocatalytic performance of the obtained sample solution was evaluated.

Example 15
A sample which is a photocatalyst system according to the present embodiment according to the method of Example 2, except that the reduced semiconductor particles obtained in Preparation Example A-6 are used instead of the reduced photocatalyst particles obtained in Preparation Example B-2. The solution was prepared, and the photocatalytic performance of the obtained sample solution was evaluated.

Example 16
A sample which is a photocatalyst system according to the present embodiment according to the method of Example 2, except that the reduced semiconductor particles obtained in Preparation Example A-7 are used instead of the reduced photocatalyst particles obtained in Preparation Example B-2. The solution was prepared, and the photocatalytic performance of the obtained sample solution was evaluated.

Example 17
A sample which is a photocatalyst system according to the present embodiment according to the method of Example 2, except that the reduced semiconductor particles obtained in Preparation Example A-8 are used instead of the reduced photocatalyst particles obtained in Preparation Example B-2. The solution was prepared, and the photocatalytic performance of the obtained sample solution was evaluated.

Example 18
A sample which is a photocatalyst system according to the present embodiment according to the method of Example 1, except that the reduced photocatalyst particles obtained in Preparation Example B-5 are used instead of the reduced photocatalyst particles obtained in Preparation Example B-1 The solution was prepared, and the photocatalytic performance of the obtained sample solution was evaluated.

Example 19
A sample which is a photocatalyst system according to the present embodiment according to the method of Example 1, except that the reduced photocatalyst particle obtained in Preparation Example B-6 is used instead of the reduced photocatalyst particle obtained in Preparation Example B-1 The solution was prepared, and the photocatalytic performance of the obtained sample solution was evaluated.

Example 20
A sample which is a photocatalyst system according to the present embodiment according to the method of Example 1, except that the reduced photocatalyst particles obtained in Preparation Example B-7 are used instead of the reduced photocatalyst particles obtained in Preparation Example B-1 The solution was prepared, and the photocatalytic performance of the obtained sample solution was evaluated.

Example 21
A sample which is a photocatalyst system according to the present embodiment according to the method of Example 1, except that the reduced photocatalyst particles obtained in Preparation Example B-8 are used instead of the reduced photocatalyst particles obtained in Preparation Example B-1 The solution was prepared, and the photocatalytic performance of the obtained sample solution was evaluated.

Comparative Example 1
Using the reduced semiconductor particles obtained in Preparation Example A-2 instead of the reduced photocatalyst particles obtained in Preparation Example B-1, and using a mediator in an aqueous electrolyte solution containing NaHCO ₃ at a concentration of 0.25 M A sample solution was prepared according to the method of Example 1 except that it was not added, and the photocatalytic performance of the obtained sample solution was evaluated.

Comparative Example 2
A sample solution was prepared according to the method of Example 9, except that the reduced semiconductor particles obtained in Preparation Example A-2 were used instead of the reduced photocatalyst particles obtained in Preparation Example B-2, and a sample solution obtained. The photocatalytic performance of was evaluated.

Comparative Example 3
The sample solution was prepared according to the method of Example 3 except that the oxidized photocatalyst particles were not added to the test tube, and the photocatalytic performance of the obtained sample solution was evaluated.

Comparative Example 4
A sample solution was prepared according to the method of Example 3 except that the mediator was not added to the aqueous electrolyte solution containing NaHCO ₃ at a concentration of 0.25 M, and the photocatalytic performance of the obtained sample solution was evaluated.

Reference Example 1
When carrying out the evaluation test of the photocatalytic performance, the photocatalyst of the sample solution prepared in Example 3 was evaluated according to the evaluation test of Example 1, except that Ar gas was used instead of CO ₂ gas as a gas to be ventilated in the sample solution. The performance was evaluated.

Reference Example 2
When conducting the evaluation test of photocatalytic performance, the photocatalytic performance of the sample solution prepared in Example 3 was evaluated according to the evaluation test of Example 1 except that the reaction was carried out for 16 hours in the dark room without light irradiation. .

The composition of the photocatalyst system is shown in Table 1 for each of the above Examples, Comparative Examples and Referential Examples, and the results of product analysis after the light irradiation test by the test tube method are shown in Table 2. Table 2 shows the amounts of formic acid (HCOOH), carbon monoxide (CO) and hydrogen (H ₂ ) detected in Examples, Comparative Examples and Reference Examples. In addition, CO ₂ reduction selectivity was calculated from the amount of formic acid, carbon monoxide and hydrogen produced in each photocatalyst system. The calculated CO ₂ reduction selectivity is shown in Table 2. The CO ₂ reduction selectivity is an index for evaluating the selectivity of the CO ₂ reduction reaction, and the CO ₂ reduction products (formic acid and carbon monoxide) with respect to the amount of total reduction products (formic acid, carbon monoxide and hydrogen) formed It is calculated as the ratio of the amount of The higher the CO ₂ reduction selectivity, the more the photocatalytic system shows the selectivity of the CO ₂ reduction reaction to the reduction reaction of water.

  Further, the turnover numbers (TON) of formic acid, carbon monoxide and hydrogen are shown in Table 2 for Examples and Comparative Examples using the reduced photocatalyst particles containing the metal complex catalyst. The turnover number is, for each product, the number of moles of the product when using a reduction photocatalyst not supported with a metal complex catalyst from the number of moles of the product when a reduction photocatalyst with a metal complex catalyst supported is used. It is a value obtained by dividing the subtracted value by the number of moles of the metal complex catalyst contained in the sample solution. That is, since the turnover number represents the number of molecules of each reduction product generated from one metal complex catalyst molecule, the turnover number is an index indicating the net activity of the metal complex catalyst.

In Tables 1 and 2, “N—Ta ₂ O ₅ ” represents nitrogen-doped tantalum oxide. "[Co (tpy) ₂ ] ^{3 + / 2 +} " represents bis (2,2 ': 6', 2 "-terpyridine) cobalt ion." [Fe (tpy) ₂ ] ^{3 + / 2 +} " Represents a bis (2,2 ': 6', 2 "-terpyridine) iron ion." [Cu (tpy) ₂ ] ^{2 + / 1 +} "is a bis (2,2 ': 6', 2" -Terpyridine) represents a copper ion "RGO" represents a reduced graphene oxide obtained in Preparation Example D-4. "[Co (bpy) ₃ ] ^{3 + / 2 +} " represents tris (2,2'-bipyridine) cobalt ion.

As shown in Examples 1 to 21, in a photocatalyst system containing an oxidized photocatalyst particle, a reduced photocatalyst particle and a mediator, and in which the mediator is a metal complex compound or the reduced photocatalyst particle contains a reduced semiconductor particle and a metal complex catalyst It has been found to reduce CO ₂ in an aqueous medium to form formic acid and CO. In addition, except for Example 18, generation of hydrogen was confirmed as a by-product. The photocatalyst system of each example has approximately high CO ₂ reduction selectivity. Further, for example, in the photocatalyst system of Example 1～10,13,19～21, more CO ₂ reduction product (formic acid and carbon monoxide) is generated.

On the other hand, in the photocatalyst system of Comparative Example 1 in which the mediator is absent and the reduced photocatalyst particles do not contain the metal complex catalyst, the hydrogen generation reaction predominantly proceeds, and the amount of CO ₂ reduced product is small (total 0.01 μmol), and the CO ₂ reduction selectivity was a remarkably low value of 0.05. In addition, the hydrogen generation reaction proceeds in the same manner in the photocatalyst system of Comparative Example 2 in which the mediator is not a metal complex compound and the reduced photocatalyst particle does not contain a metal complex catalyst, and the amount of CO ₂ reduced product is similarly produced. (Total 0.02 μmol), and the CO ₂ reduction selectivity was a remarkably low value of 0.04.

In the photocatalyst system of Comparative Example 3 to which no oxidation photocatalyst was added, a certain amount of formic acid and carbon monoxide were produced (formic acid: 0.45 μmol, CO: 1.41 μmol). However, as apparent from the temporal evaluation described later, in Comparative Example 3, the mediator is not reduced due to the absence of the oxidized photocatalyst particle, and the reductant of the mediator (the electron donor for the reduced photocatalyst particle) is consumed completely. As a result, the reduced photocatalyst particles can not receive electrons. Therefore, it is considered that oxidation photocatalyst particles are indispensable for promoting continuous CO ₂ reduction.

  Formic acid and carbon monoxide were also produced in the photocatalyst system of Comparative Example 4 in which no mediator was used. Since the oxidized photocatalyst particles and the reduced photocatalyst particles are suspended and mixed in the solution, it is considered that the contact between the oxidized photocatalyst particles and the reduced photocatalyst particles causes electron transfer between the particles and the reaction proceeds. However, the electron transfer efficiency between particles is lower compared to the case of using a mediator, and the amount of formic acid produced is about 1/9 or less, and the amount of CO produced is about 1 compared to Example 3 which differs only in the presence or absence of the use of a mediator. It showed a low value of / 23 or less.

In Reference Example 1 in which argon was bubbled instead of CO ₂ , the formation of formic acid and carbon monoxide was not detected. This indicates that the formic acid and carbon monoxide generated in the photocatalyst system according to this embodiment are derived from the reduction reaction of aerated CO ₂ . Moreover, in the reference example 2 which does not irradiate light, a product was not able to be detected at all. From this, it was found that this catalytic reaction proceeds by light.

As apparent from each example and each comparative example, it comprises an aqueous solvent, an oxidized photocatalyst particle, a reduced photocatalyst particle and a mediator, the mediator is a metal complex compound, or the reduced photocatalyst particle is a reduced semiconductor particle and a metal complex catalyst In the Z scheme type reaction using light energy, the generation amount of CO ₂ reduction product increases or the selectivity of CO ₂ reduction reaction increases, and the CO ₂ reduction performance is further improved. It turned out to do.

In addition, in the particle suspension type photocatalyst system according to the present embodiment, it has been found that the mediator in the aqueous solvent or in one of the semiconductor particles supports the electron transfer between the particles and plays a large role in improving the catalyst activity. The In particular, as is apparent from the comparison of Examples 3, 7 and 8 with Example 9, and the comparison of Examples 10-11 with Comparative Example 2, a photocatalyst system using a metal complex compound which is a solution type mediator. It turned out that it tends to have the selectivity of CO ₂ reduction reaction higher than the photocatalyst system using RGO which is a solid type mediator. Further, as is clear from the comparison between Examples 1 to 9 and 18 to 21 and Examples 10 to 17 and Comparative Examples 1 and 2, in the photocatalyst system according to the present embodiment, the reduced photocatalyst particles are the reduced semiconductor particles and the metal It has been found that the inclusion of the complex catalyst significantly increases the amount of CO ₂ reduction product, particularly the amount of formic acid.

[Temporal evaluation of photocatalytic reaction by test tube method]
(Examples 22 and 23, Comparative Example 5)
The sample solution obtained in Example 1 was subjected to a light irradiation test in which the light irradiation time was 1 hour, 2 hours, 4 hours, 8 hours and 16 hours, and each product was analyzed and quantified. The photocatalytic performance was evaluated according to the method of Example 1 (Example 22). Further, the photocatalytic performance was evaluated according to the method of Example 22 except that the sample solutions obtained in Example 3 and Comparative Example 3 were used as Example 23 and Comparative Example 5, respectively.

Figure 3 shows the time course of formic acid production amount by visible light CO ₂ reduction in each photocatalyst systems. The horizontal axis shows the light irradiation time, and the vertical axis shows the amount (μmol) of formic acid produced. The photocatalyst system of Example 22 and 23 according to the present _embodiment, formic acid production amount increased with time. On the other hand, in the photocatalyst system of Comparative Example 5 in which the oxidation photocatalyst particles are not used, although formic acid is generated smoothly until 4 hours of light irradiation, the amount of generated formic acid stagnated after that. This is because, although the Co ²⁺ mediator functions as an electron donor at the initial stage of the reaction, the electron-donating Co ³⁺ mediator can not receive electrons because the oxidized photocatalyst particle is absent, and the Co ³⁺ mediator is accumulated. Become. The graph shown in FIG. 3 indicates that in Comparative Example 5, the reduction reaction does not proceed after the Co ²⁺ mediator is completely consumed. From the above results, it has been confirmed that the redox mediator is essential for realizing the continuous visible light CO ₂ reduction reaction in an aqueous solvent.

[Evaluation of photocatalytic reaction using flow reactor]
In order to analyze and quantify the generation amount of oxygen (O ₂ ) associated with the oxidation reaction among the Z scheme type reactions, photocatalytic reaction evaluation was performed using a flow reaction apparatus shown below.

Example 24
The mediator obtained in Preparation Example D-1 was added to an aqueous electrolyte solution containing NaHCO ₃ at a concentration of 0.25 M to a concentration of 0.02 mM to prepare an aqueous medium, and 150 mL of the obtained aqueous medium Was placed in a 170 mL glass container. Next, 0.4 g of the reduced photocatalyst particles obtained in Preparation Example B-1 and 0.2 g of the oxidized photocatalyst particles obtained in Preparation Example C-1 are added to the container, and a sample solution which is a photocatalyst system according to the present embodiment Was prepared. The sample solution in the container was irradiated with visible light (λ> 420 nm) while aerating CO ₂ gas at a flow rate of 15 mL per minute. The gas component contained in the gas phase component in the container was analyzed and quantified using a gas chromatograph ("GC-8A" manufactured by Shimadzu Corporation) each time the irradiation time passed a predetermined time.

Example 25
It is a photocatalyst system according to the present embodiment according to the method of Example 24 except that the reduced semiconductor particles obtained in Preparation Example A-2 are used instead of the reduced photocatalyst particles obtained in Preparation Example B-1 A sample solution was prepared, and analysis and quantification of the gas component contained in the gas phase component in the container were performed each time the irradiation time of visible light to the sample solution passed a predetermined time.

Figure 4 shows the time course of CO and O ₂ generation amount from the Z scheme photocatalyst system of Example 24. Further, FIG. 5 shows time-dependent changes in CO, H ₂ and O ₂ production amounts from the Z scheme type photocatalyst system of Example 25. It was confirmed that, with visible light irradiation, CO as a CO ₂ reduction product, H _{2 as a by-} product (only in Example 25), and O _{2 as} an oxidation product of water increase with time. That is, in the photocatalyst systems of Examples 24 and 25, it is suggested that the visible light CO ₂ reduction reaction proceeds using water as an electron source. In the photocatalyst system of Example 25, the CO ₂ reduction selectivity after 6 hours from the start of light irradiation showed a high value of 0.72.

  1 photocatalyst system (Z scheme type photocatalyst system), 2 oxidation photocatalyst particle, 3 reduction photocatalyst particle, 3a reduction semiconductor particle (semiconductor particle), 3b metal complex catalyst, 4 mediator (redox mediator), 5 visible light.

Claims (14)

An aqueous medium,
Oxidation photocatalyst particles that oxidize water,
Reduced photocatalyst particles for reducing carbon dioxide,
A redox mediator for transferring electrons between the oxidized photocatalyst particle and the reduced photocatalyst particle;
The redox mediator is a metal complex compound, or the reduced photocatalyst particles include semiconductor particles and a metal complex catalyst.
Z scheme type photocatalyst system.   The Z scheme photocatalytic system according to claim 1, wherein the redox mediator is a metal complex compound.   The Z scheme photocatalytic system according to claim 2, wherein the metal complex compound is a complex of at least one metal selected from metals belonging to any of Groups 6 to 12 of the periodic table.   The Z scheme photocatalytic system according to claim 2, wherein the metal complex compound is a complex of at least one metal selected from the group consisting of cobalt, iron and copper.   The Z scheme type photocatalyst system according to any one of claims 1 to 4, wherein the reduced photocatalyst particle comprises a semiconductor particle and a metal complex catalyst.   The Z-scheme type photocatalyst system according to claim 5, wherein the metal complex catalyst is a complex of at least one metal selected from metals belonging to any of Groups 6 to 10 of the periodic table.   The Z scheme type photocatalyst system according to claim 5 or 6, wherein the level of the lower end of the conduction band of the semiconductor particle is more negative than the level of the lowest unoccupied orbital of the metal complex catalyst.   The Z scheme according to any one of claims 1 to 7, wherein the potential of the lower end of the conduction band of the semiconductor particles contained in the reduced photocatalyst particles is -0.3 V or less with respect to the potential of a standard hydrogen electrode at pH 0. Type photocatalyst system.   The Z scheme type photocatalyst system according to any one of claims 1 to 7, wherein the semiconductor particles contained in the reduced photocatalyst particles are made of a sulfide semiconductor.   The Z scheme photocatalytic system according to claim 9, wherein the sulfide semiconductor contains zinc.   The Z scheme type photocatalyst system according to any one of claims 1 to 10, wherein the aqueous medium is an aqueous solution of electrolyte.   The Z scheme photocatalytic system according to claim 11, wherein the electrolyte is at least one selected from the group consisting of carbonate, bicarbonate and phosphate.   The Z scheme type according to any one of claims 1 to 12, wherein the potential at the lower end of the conduction band of the oxidized photocatalyst particle is lower than the potential at the upper end of the valence band of the semiconductor particles contained in the reduced photocatalyst particle. Photocatalyst system.   The Z scheme type photocatalyst system according to any one of claims 1 to 13, wherein the oxidation photocatalyst particle comprises a bismuth vanadate semiconductor.