JP2022137740A - Method for producing photocatalyst particle for carbon dioxide reduction - Google Patents
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- 239000011941 photocatalyst Substances 0.000 title claims abstract description 65
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- CURLTUGMZLYLDI-UHFFFAOYSA-N Carbon dioxide Chemical compound O=C=O CURLTUGMZLYLDI-UHFFFAOYSA-N 0.000 title abstract description 48
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- 229910001195 gallium oxide Inorganic materials 0.000 claims abstract description 68
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/30—Hydrogen technology
- Y02E60/36—Hydrogen production from non-carbon containing sources, e.g. by water electrolysis
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Abstract
Description
本発明は、二酸化炭素還元光触媒粒子の製造方法に関する。 The present invention relates to a method for producing carbon dioxide-reducing photocatalyst particles.
半導体光触媒粒子を用いた水分解及び二酸化炭素還元技術は、エネルギー問題や環境問題を解決できる技術として注目を集めている。この光触媒粒子に助触媒として銀（Ａｇ）などからなるナノ粒子を担持させることで、光励起によって生成した電子をトラップして電荷分離を促進する効果や二酸化炭素還元生成物を選択する効果が期待できる。例えば通常の光触媒では光照射により水が水素（Ｈ２）と酸素（Ｏ２）とに分解する。これに対して銀粒子を担持した光触媒では、二酸化炭素（ＣＯ２）の還元により一酸化炭素（ＣＯ）が水素（Ｈ２）とともに生成する。このような二酸化炭素還元光触媒として、銀ナノ粒子を助触媒として担持した酸化ガリウム粒子（銀ナノ粒子担持酸化ガリウム粒子）が知られている。 Water splitting and carbon dioxide reduction technology using semiconductor photocatalyst particles is attracting attention as a technology that can solve energy problems and environmental problems. By supporting nanoparticles made of silver (Ag) or the like on these photocatalyst particles as a co-catalyst, it is possible to expect the effect of trapping electrons generated by photoexcitation to promote charge separation and the effect of selecting carbon dioxide reduction products. . For example, an ordinary photocatalyst decomposes water into hydrogen (H 2 ) and oxygen (O 2 ) by light irradiation. On the other hand, in a photocatalyst carrying silver particles, carbon monoxide (CO) is produced together with hydrogen (H 2 ) by reduction of carbon dioxide (CO 2 ). As such a photocatalyst for carbon dioxide reduction, gallium oxide particles supporting silver nanoparticles as promoters (silver nanoparticle-supporting gallium oxide particles) are known.
一酸化炭素（ＣＯ）は化学工業や産業における重要な出発物質であり、これを水素と反応させて様々な燃料や化学物質を合成することが可能である。したがって二酸化炭素還元光触媒では、一酸化炭素の生成割合、すなわちＣＯ選択率の高いことが望ましい。ここでＣＯ選択率とは、下記（１）式に表されるように、還元反応により生じる水素（Ｈ２）ガスの発生速度（発生量）と一酸化炭素（ＣＯ）ガスの発生速度の合計に対する一酸化炭素（ＣＯ）ガスの発生速度の割合である。 Carbon monoxide (CO) is an important starting material in the chemical industry and industry, and can be reacted with hydrogen to synthesize a variety of fuels and chemicals. Therefore, it is desirable that the carbon monoxide reduction photocatalyst has a high carbon monoxide generation ratio, that is, a high CO selectivity. Here, the CO selectivity is the sum of the generation rate (amount) of hydrogen (H 2 ) gas generated by the reduction reaction and the generation rate of carbon monoxide (CO) gas, as represented by the following formula (1). is the ratio of the rate of carbon monoxide (CO) gas generation to the
ところで金属ナノ粒子を担持させる方法として、含浸法や光電析法（光電着法）などの手法が従来から知られている。例えば特許文献１には二酸化炭素の還元方法に関して、ＣＯ２とＨ２Ｏと光触媒とに光を照射してＣＯ２を還元する反応によりＣＯを生成させる旨、光電着法又は含浸法で銀を酸化ガリウムに担持した触媒を用いる旨、光電着法では硝酸銀など銀前駆体を含むアルコール水溶液に酸化ガリウム粉末を入れて混合後、光照射を行って銀前駆体を還元処理する旨、含浸法では銀前駆体水溶液に酸化ガリウムを加えて攪拌し、水を除去した後に加熱乾燥し、更に空気中で焼成する旨が記載されている（特許文献１の請求項１、２及び［００１５］）。
By the way, methods such as an impregnation method and a photoelectrodeposition method (photoelectrodeposition method) have been conventionally known as methods for supporting metal nanoparticles. For example, Patent Literature 1 describes a method for reducing carbon dioxide by irradiating CO 2 , H 2 O, and a photocatalyst with light to reduce CO 2 , thereby generating CO. In the photoelectrodeposition method, gallium oxide powder is added to an alcoholic aqueous solution containing a silver precursor such as silver nitrate, mixed, and then irradiated with light to reduce the silver precursor.In the impregnation method, It is described that gallium oxide is added to an aqueous silver precursor solution, stirred, water is removed, dried by heating, and then fired in air (
また二酸化炭素還元触媒に関するものではないが、特許文献２には超音波を照射して溶媒中に１種類以上の貴金属酸化物を分散させて貴金属酸化物分散液を得る工程と、前記貴金属酸化物分散液を加熱する工程とを含むことを特徴とする貴金属ナノ材料の製造方法が開示され、溶媒に貴金属担持用の担体を更に含有させる旨、担体の表面上に高度な分散性を持って担持された状態の貴金属ナノ材料を得ることが可能であるため、これを燃料電池用触媒、材料合成用触媒等に好適に利用することが可能となる旨が記載されている（特許文献２の請求項１、６及び［００１４］）。
Further, although it does not relate to a carbon dioxide reduction catalyst,
しかしながら本発明者らが調べたところ、特許文献１で提案される含浸法や光電析法で銀担持した光触媒には、一定の効果があるものの改良の余地があることが分かった。すなわち助触媒である銀（Ａｇ）の効果を十分に発揮させるためには、その担持量をある程度に多くすることが望ましい。また銀の粒径が小さく、数十ｎｍオーダー程度であることが望まれる。粒径が大きすぎると触媒活性が失われてしまうためである。しかしながら含浸法や光電析法で作製した光触媒粒子では、銀濃度（担持量）を高くすると銀粒子が凝集して粒径が大きくなってしまう問題がある。そのため粒径の小さい銀ナノ粒子を高担持量で担持させることは困難である。 However, as a result of investigations by the present inventors, it was found that although the photocatalyst supported by silver by the impregnation method or photoelectrodeposition method proposed in Patent Document 1 has certain effects, there is still room for improvement. In other words, in order to fully exhibit the effect of silver (Ag), which is a co-catalyst, it is desirable to increase the amount of silver (Ag) supported to some extent. In addition, it is desired that the grain size of silver is small and is on the order of several tens of nanometers. This is because if the particle size is too large, catalytic activity is lost. However, the photocatalyst particles produced by the impregnation method or the photoelectrodeposition method have a problem that when the silver concentration (supported amount) is increased, the silver particles aggregate to increase the particle size. Therefore, it is difficult to support a large amount of silver nanoparticles with a small particle size.
特許文献２は貴金属ナノ材料を二酸化炭素還元光触媒に用いることを意図しておらず、ましてやナノ材料の粒径を数十ｎｍオーダーに小さくすることを目的とするものではない。実際、引用文献２では実施例において貴金属（Ｐｔ）ナノ微粒子担持球状カーボンや貴金属（Ｐｔ）ナノチューブを作製する旨、燃料電池用触媒、材料合成用触媒、医療や食品添加剤、導電性ペーストに好適である旨を教示するに過ぎない（特許文献２の［００４３］～［００６２］）。
一方で金属ナノ粒子を合成する際に、原料濃度を低くするとともに多量の有機保護剤を用いて均一且つ微細な粒子を合成する手法が知られている。しかしながらこのような手法で光触媒粒子を作製すると、光触媒粒子の収率が低いとともに有機保護剤が粒子表面に付着するという問題がある。付着した有機保護剤は触媒活性を低下させてしまうため、これを分解除去するために触媒粒子を高温焼成する必要がある。このような焼成を経た触媒粒子では、金属ナノ粒子の粒径が大きくなってしまう。そのため従来の手法では微細な銀ナノ粒子を高分散且つ高担持量で担持させることは困難であり、触媒性能、特にＣＯ選択率に優れた光触媒粒子を効率的に製造する上で限界があった。 On the other hand, when synthesizing metal nanoparticles, there is known a method of synthesizing uniform and fine particles by using a large amount of an organic protective agent while lowering the raw material concentration. However, when photocatalyst particles are produced by such a method, there are problems in that the yield of photocatalyst particles is low and the organic protective agent adheres to the particle surfaces. Since the adhering organic protective agent lowers the catalytic activity, it is necessary to calcine the catalyst particles at a high temperature in order to decompose and remove the organic protective agent. In the catalyst particles that have undergone such calcination, the particle size of the metal nanoparticles becomes large. Therefore, it is difficult to support fine silver nanoparticles with high dispersion and high loading by conventional methods, and there is a limit in efficiently producing photocatalyst particles with excellent catalytic performance, especially CO selectivity. .
本発明者らは、このような従来の問題に鑑みて検討を行い、酸化ガリウム粒子と銀供給源とに超音波を照射するという簡易な手法により、助触媒たる銀ナノ粒子を高分散且つ高担持率で析出させ、さらにクロム（Ｃｒ）処理を行うことで、改良されたＣＯ選択率を有する光触媒を得ることができるとの知見を得た。 The present inventors have conducted studies in view of such conventional problems, and have found that by a simple method of irradiating gallium oxide particles and a silver supply source with ultrasonic waves, silver nanoparticles serving as promoters are highly dispersed and highly dispersed. It has been found that a photocatalyst with improved CO selectivity can be obtained by precipitating at a supporting rate and further performing chromium (Cr) treatment.
本発明は、このような知見に基づき完成されたものであり、改良されたＣＯ選択率を有する二酸化炭素還元光触媒粒子の製造方法の提供を課題とする。 The present invention was completed based on such findings, and an object of the present invention is to provide a method for producing carbon dioxide-reducing photocatalyst particles having improved CO selectivity.
本発明は、下記（１）～（１２）の態様を包含する。なお本明細書において「～」なる表現は、その両端の数値を含む。すなわち「Ｘ～Ｙ」は「Ｘ以上Ｙ以下」と同義である。 The present invention includes the following aspects (1) to (12). In the present specification, the expression "-" includes both numerical values. That is, "X to Y" is synonymous with "X or more and Y or less".
（１）酸化ガリウム（Ｇａ２Ｏ３）粒子と銀（Ａｇ）供給源とを準備する工程、
前記酸化ガリウム粒子と前記銀供給源とを還元液に加えて反応液を作製する工程、
前記反応液に超音波を照射して、金属銀担持酸化ガリウム粒子を作製する工程、及び、
前記金属銀担持酸化ガリウム粒子にクロム（Ｃｒ）処理を行い、それにより金属銀（Ａｇ）ナノ粒子及びクロム酸銀（Ａｇ２ＣｒＯ４）ナノ粒子を担持した酸化ガリウム粒子を作製する工程を含む、二酸化炭素還元光触媒粒子の製造方法。
(1) providing gallium oxide (Ga 2 O 3 ) particles and a silver (Ag) source;
adding the gallium oxide particles and the silver source to a reducing solution to prepare a reaction solution;
A step of irradiating the reaction solution with ultrasonic waves to produce metallic silver-supported gallium oxide particles;
subjecting the gallium oxide particles supporting metallic silver to a chromium (Cr) treatment, thereby producing gallium oxide particles supporting metallic silver ( Ag) nanoparticles and silver chromate ( Ag2CrO4) nanoparticles; A method for producing carbon dioxide-reducing photocatalyst particles.
（２）前記銀（Ａｇ）供給源が酸化銀（Ａｇ２Ｏ）を含む、上記（１）の方法。 (2) The method of (1) above, wherein the silver (Ag) source comprises silver oxide (Ag 2 O).
（３）前記還元液がアルコール類を含む、上記（１）又は（２）の方法。 (3) The method of (1) or (2) above, wherein the reducing liquid contains an alcohol.
（４）前記超音波の周波数が２８～４５ｋＨｚである、上記（１）～（３）のいずれかの方法。 (4) The method according to any one of (1) to (3) above, wherein the ultrasonic wave has a frequency of 28 to 45 kHz.
（５）前記超音波の照射を１～１０時間行う、上記（１）～（４）のいずれかの方法。 (5) The method according to any one of (1) to (4) above, wherein the ultrasonic irradiation is performed for 1 to 10 hours.
（６）クロム処理前の金属銀担持酸化ガリウム粒子に、酸素含有雰囲気中１００℃以上の温度で熱処理を施す、上記（１）～（５）のいずれかの方法。 (6) The method according to any one of (1) to (5) above, wherein the metallic silver-supported gallium oxide particles before the chromium treatment are heat-treated at a temperature of 100° C. or higher in an oxygen-containing atmosphere.
（７）クロム処理を行う際に、クロム（Ｃｒ）化合物を溶解させた水溶液に前記金属銀担持酸化ガリウム粒子を浸漬して前記金属銀担持酸化ガリウム粒子の表面にクロム（Ｃｒ）化合物を析出させ、その後、クロム（Ｃｒ）化合物を析出させた金属銀担持酸化ガリウム粒子に加熱処理を施す、上記（１）～（６）のいずれかの方法。 (7) When performing the chromium treatment, the metallic silver-supporting gallium oxide particles are immersed in an aqueous solution in which a chromium (Cr) compound is dissolved to deposit the chromium (Cr) compound on the surface of the metallic silver-supporting gallium oxide particles. The method according to any one of the above (1) to (6), wherein the metallic silver-supporting gallium oxide particles on which the chromium (Cr) compound is deposited are then subjected to a heat treatment.
（８）前記加熱処理を４００～７００℃の温度で１～１０時間行う、上記（７）の方法。 (8) The method of (7) above, wherein the heat treatment is carried out at a temperature of 400 to 700° C. for 1 to 10 hours.
（９）前記金属銀（Ａｇ）ナノ粒子の平均粒子径が１０．０～５０．０ｎｍである、上記（１）～（８）のいずれかの方法。 (9) The method according to any one of (1) to (8) above, wherein the metallic silver (Ag) nanoparticles have an average particle size of 10.0 to 50.0 nm.
（１０）前記クロム酸銀（Ａｇ２ＣｒＯ４）ナノ粒子の平均粒子径が１．０～３０．０ｎｍである、上記（１）～（９）のいずれかの方法。 (10) The method according to any one of (1) to (9) above, wherein the silver chromate (Ag 2 CrO 4 ) nanoparticles have an average particle size of 1.0 to 30.0 nm.
（１１）銀成分の担持量が、金属銀換算で酸化ガリウム粒子に対して０．３～１０．０質量％である、上記（１）～（１０）のいずれかの方法。 (11) The method according to any one of (1) to (10) above, wherein the amount of silver component supported is 0.3 to 10.0% by mass in terms of metallic silver relative to the gallium oxide particles.
（１２）ＣＯ２還元光触媒性能評価試験において、前記光触媒粒子のＣＯ選択率が３０％以上である、上記（１）～（１１）のいずれかの方法。 (12) The method according to any one of (1) to (11) above, wherein the photocatalyst particles have a CO selectivity of 30% or more in a CO 2 reduction photocatalyst performance evaluation test.
本発明によれば、改良されたＣＯ選択率を有する二酸化炭素還元光触媒粒子の製造方法が提供される。 According to the present invention, a method for producing carbon dioxide reduction photocatalyst particles with improved CO selectivity is provided.
本発明の具体的な実施形態（以下、「本実施形態」という）について以下に説明する。ただし本発明は以下の実施形態に限定されるものではなく、本発明の要旨を変更しない範囲において種々の変更が可能である。 A specific embodiment of the present invention (hereinafter referred to as "this embodiment") will be described below. However, the present invention is not limited to the following embodiments, and various modifications are possible without changing the gist of the present invention.
＜＜１．光触媒粒子の製造方法＞＞
本実施形態の二酸化炭素還元光触媒粒子（以下、「光触媒」と総称する場合がある）の製造方法は、以下の工程；酸化ガリウム（Ｇａ２Ｏ３）粒子と銀（Ａｇ）供給源とを準備する工程（準備工程）、準備した酸化ガリウム粒子と銀供給源とを還元液に加えて反応液を作製する工程（混合工程）、得られた反応液に超音波を照射して、金属銀担持酸化ガリウム粒子を作製する工程（超音波処理工程）、及び得られた金属銀担持酸化ガリウム粒子にクロム（Ｃｒ）処理を行い、それにより金属銀（Ａｇ）ナノ粒子及びクロム酸銀（Ａｇ２ＣｒＯ４）ナノ粒子を担持した酸化ガリウム粒子を作製する工程（Ｃｒ処理工程）を含む。各工程の詳細について以下に説明する。
<<1. Method for producing photocatalyst particles>>
The method for producing carbon dioxide-reducing photocatalyst particles (hereinafter sometimes collectively referred to as “photocatalyst”) of the present embodiment includes the following steps; gallium oxide (Ga 2 O 3 ) particles and a silver (Ag) supply source are prepared. (preparing step), adding the prepared gallium oxide particles and silver source to the reducing solution to prepare a reaction solution (mixing step), and irradiating the obtained reaction solution with ultrasonic waves to support metallic silver. A step of producing gallium oxide particles (ultrasonic treatment step), and the obtained gallium oxide particles supporting metallic silver are subjected to chromium (Cr) treatment, whereby metallic silver (Ag) nanoparticles and silver chromate (Ag 2 CrO 4 ) A step of producing gallium oxide particles carrying nanoparticles (a Cr treatment step) is included. Details of each step are described below.
＜準備工程＞
準備工程では、酸化ガリウム（Ｇａ２Ｏ３）粒子と銀（Ａｇ）供給源とを準備する。酸化ガリウム（Ｇａ２Ｏ３）粒子は光触媒に用いられるものであれば特に限定されない。Ｇａ２Ｏ３には、α型、β型、γ型、δ型及びε型が知られているが、いずれを用いてもよい。しかしながら安定な酸化物であるβ型（β－Ｇａ２Ｏ３）が好ましい。粒子の大きさも特に限定されない。例えば粒子の平均粒子径は０．３～５．０μｍである。さらに粒子の形状も特に限定されない。例えば球状、不定形状、異方形状（ロッド又は板状等）が挙げられる。粒子がロッド状である場合、例えば長軸径が１．０～５．０μｍ、短軸径が０．３～１．０μｍのものを用いることができる。
<Preparation process>
In the preparation step, gallium oxide (Ga 2 O 3 ) particles and a silver (Ag) source are provided. Gallium oxide (Ga 2 O 3 ) particles are not particularly limited as long as they are used for photocatalysts. Ga 2 O 3 is known to be α-type, β-type, γ-type, δ-type, and ε-type, and any of them may be used. However, the β form (β-Ga 2 O 3 ), which is a stable oxide, is preferred. The particle size is also not particularly limited. For example, the particles have an average particle size of 0.3 to 5.0 μm. Furthermore, the shape of the particles is not particularly limited. For example, it may be spherical, amorphous, or anisotropic (rod-shaped, plate-shaped, etc.). When the particles are rod-shaped, for example, particles having a long axis diameter of 1.0 to 5.0 μm and a short axis diameter of 0.3 to 1.0 μm can be used.
銀（Ａｇ）供給源は、銀を供給できるものである限り限定されない。具体的には、酸化物、無機金属塩及び／又は有機金属化合物が挙げられる。無機金属塩として硝酸塩、塩化物及び／又は硫酸塩などが挙げられる。銀供給源は還元液に溶解するものであってもよいし、あるいは溶解しないものであってもよい。好ましい銀供給源は酸化銀を含む。酸化銀は銀イオンと酸素イオンのみで構成されるため、ハンドリングが容易であり、廃棄物処理等の問題が無い。酸化銀には、銀の酸化数が異なるＡｇ２Ｏ、ＡｇＯ及びＡｇ２Ｏ３が知られており、いずれも使用が可能である。しかしながら入手がより容易なＡｇ２Ｏが好ましい。銀供給源の大きさも特に限定されない。例えば銀供給源の平均粒子径は０．３～３．０μｍである。 A silver (Ag) supply source is not limited as long as it can supply silver. Specific examples include oxides, inorganic metal salts and/or organometallic compounds. Inorganic metal salts include nitrates, chlorides and/or sulfates. The silver source may or may not be soluble in the reducing liquid. A preferred silver source comprises silver oxide. Since silver oxide is composed only of silver ions and oxygen ions, it is easy to handle and free from problems such as waste disposal. Ag 2 O, AgO and Ag 2 O 3 , which have different oxidation numbers of silver, are known as silver oxides, and any of them can be used. Ag 2 O is preferred, however, as it is more readily available. The size of the silver supply source is also not particularly limited. For example, the silver source has an average particle size of 0.3 to 3.0 μm.
＜混合工程＞
混合工程では、準備した酸化ガリウム粒子と銀供給源とを還元液に加えて反応液を作製する。酸化ガリウム粒子と銀供給源の配合割合は、最終的に得られる光触媒中の銀担持量が所望の値となるように調整すればよい。銀担持量が過度に少ないと助触媒の効果を十分に発揮させることが困難になる。そのためＣＯ２還元に光触媒を用いたときにＣＯガス発生速度とＣＯ選択率が低くなる。一方で銀担持量が過度に多いとＣＯガス発生速度が低下する。
<Mixing process>
In the mixing step, the prepared gallium oxide particles and silver supply source are added to the reducing liquid to prepare a reaction liquid. The mixing ratio of the gallium oxide particles and the silver source may be adjusted so that the amount of supported silver in the finally obtained photocatalyst is a desired value. If the amount of supported silver is too small, it becomes difficult to sufficiently exhibit the effect of the co-catalyst. Therefore, the CO gas generation rate and CO selectivity are low when photocatalysts are used for CO2 reduction. On the other hand, if the amount of silver carried is excessively large, the CO gas generation rate decreases.
還元液は、還元性を有する液体である限り限定されない。それ自体が還元性を有する液体であってもよく、あるいは還元性を有しない液体に還元剤を溶解させたものであってもよい。しかしながらそれ自体が還元性を有する液体であることが好ましい。また別個の還元剤を含まなくともよい。このような還元液として、毒性が低く入手が容易なエタノールやプロパノールなどのアルコール類が好ましい。またアルコール類と水との混合液も使用可能である。ただし混合液を用いる場合には、水の含有量が過度に多いと十分な還元作用を発揮させることが困難になる。したがって還元液中の水の含有量は、５０容積％以下が好ましく、２５容積％以下がより好ましい。水の含有量の下限値は特に限定されるものではなく、０容量％であってもよい。 The reducing liquid is not limited as long as it is a reducing liquid. It may be a reducing liquid itself, or may be a non-reducing liquid in which a reducing agent is dissolved. However, it is preferably a liquid which itself has reducing properties. It may also contain no separate reducing agent. Alcohols such as ethanol and propanol, which have low toxicity and are readily available, are preferable as such a reducing liquid. Mixtures of alcohols and water can also be used. However, when a mixed solution is used, if the water content is excessively high, it becomes difficult to exhibit a sufficient reducing action. Therefore, the content of water in the reducing liquid is preferably 50% by volume or less, more preferably 25% by volume or less. The lower limit of water content is not particularly limited, and may be 0% by volume.
＜超音波処理工程＞
超音波処理工程では、得られた反応液に超音波を照射して、金属銀ナノ粒子を担持した酸化ガリウム粒子を作製する。この際、反応液中の銀供給源の表面部が超音波還元されて、金属銀（Ａｇ）ナノ粒子となり、これが酸化ガリウム粒子の表面に担持される。金属銀ナノ粒子を担持した酸化ガリウム粒子が光触媒粒子になる。
<Ultrasonic treatment process>
In the ultrasonic treatment step, the obtained reaction solution is irradiated with ultrasonic waves to produce gallium oxide particles supporting metallic silver nanoparticles. At this time, the surface portion of the silver supply source in the reaction liquid is ultrasonically reduced to form metallic silver (Ag) nanoparticles, which are supported on the surfaces of the gallium oxide particles. Gallium oxide particles supporting metallic silver nanoparticles become photocatalyst particles.
金属銀ナノ粒子担持のメカニズムを、図１を用いて説明する。超音波が照射されると反応液中に粗密波が生じ、この粗密波により正負の繰り返し圧力が生じる。負圧サイクル時には蒸発により無数の微細な気泡が反応液中に生じる。正圧サイクル時に、この気泡は圧壊して強力な衝撃力を周囲に与える。この現象を超音波キャビテーションという。キャビテーションにより反応液中の酸化ガリウム粒子と銀供給源とが均一に分散されるとともに、その表面が清浄化される。またキャビテーションにより微小かつ高温高圧のホットスポットが生成する。生成したホットスポットは、銀供給源を分解還元させるとともに、反応液に作用してラジカルが発生し、このラジカルが銀供給源の分解還元を促進する。このようにして銀供給源から金属銀ナノ粒子が生成する。 The mechanism of supporting metallic silver nanoparticles will be described with reference to FIG. When ultrasonic waves are applied, compressional waves are generated in the reaction solution, and these compressional waves generate repeated positive and negative pressures. During the negative pressure cycle, evaporation causes countless fine air bubbles in the reaction solution. During a positive pressure cycle, this bubble collapses and exerts a strong impact force on the surroundings. This phenomenon is called ultrasonic cavitation. The cavitation uniformly disperses the gallium oxide particles and the silver supply source in the reaction liquid and cleans the surface thereof. In addition, cavitation generates minute, high-temperature and high-pressure hot spots. The generated hot spots decompose and reduce the silver source, and act on the reaction solution to generate radicals, which accelerate the decomposition and reduction of the silver source. Thus, metallic silver nanoparticles are produced from the silver source.
例えば固体である酸化銀（Ａｇ２Ｏ）を銀供給源に用いた場合には、酸化銀にホットスポットやラジカルが作用し、酸化銀が表面で分解及び還元されて銀ナノ粒子が析出する。この銀ナノ粒子は徐々に成長し、ある程度にまで成長すると酸化銀と銀ナノ粒子の界面応力が限界になり脱離する。あるいは超音波の作用により酸化銀から中間生成物が生成し、この中間生成物にホットスポットやラジカルが作用して銀ナノ粒子が反応液中に生成する。脱離又は生成した銀ナノ粒子は、超音波の物理的な作用により酸化ガリウム粒子表面に移動し、そこに吸着する。このようにして銀ナノ粒子を担持した酸化ガリウム粒子が得られる。超音波還元により生成した銀ナノ粒子は微小である。また有機保護剤や高温焼成が不要であるため、銀ナノ粒子を微細な状態を担持した酸化ガリウム粒子（光触媒粒子）の作製が可能である。 For example, when solid silver oxide (Ag 2 O) is used as a silver supply source, hot spots and radicals act on the silver oxide, and the silver oxide is decomposed and reduced on the surface to deposit silver nanoparticles. The silver nanoparticles grow gradually, and when they grow to a certain extent, the interfacial stress between the silver oxide and the silver nanoparticles reaches a limit and they are detached. Alternatively, an intermediate product is produced from silver oxide by the action of ultrasonic waves, and hot spots and radicals act on this intermediate product to produce silver nanoparticles in the reaction solution. The desorbed or generated silver nanoparticles move to the gallium oxide particle surface and are adsorbed there by the physical action of ultrasonic waves. Thus, gallium oxide particles supporting silver nanoparticles are obtained. The silver nanoparticles produced by ultrasonic reduction are very small. In addition, since an organic protective agent and high-temperature baking are not required, it is possible to produce gallium oxide particles (photocatalyst particles) carrying silver nanoparticles in a fine state.
光触媒粒子が銀ナノ粒子以外の銀供給源を含むと、助触媒の効果を十分に発揮させることが困難になることがある。この点、超音波処理によれば、銀供給源（酸化銀等）を殆ど含まない光触媒粒子を得ることが可能である。例えばＸ線回折パターンにおいて、酸化銀（Ａｇ２Ｏ）のピークが観察されない光触媒粒子とすることが可能である。 If the photocatalyst particles contain a source of silver other than the silver nanoparticles, it may be difficult to fully exhibit the effect of the co-catalyst. In this regard, ultrasonic treatment makes it possible to obtain photocatalyst particles that contain almost no silver source (eg, silver oxide). For example, it is possible to obtain photocatalyst particles in which no silver oxide (Ag 2 O) peak is observed in the X-ray diffraction pattern.
超音波処理には特別な装置を用いる必要はなく、通常の超音波発振源を備えた装置を用いればよい。例えば市販の超音波洗浄機を使用してもよい。処理も通常の条件で行えばよい。例えば超音波の周波数は２０～１００ｋＨｚであってよく、２８～４５ｋＨｚであってよい。超音波処理を同一の周波数で継続して行ってもよく、あるいは周波数発振切替モードを用いて処理の途中で周波数を切り替えてもよい。周波数切り替えの回数は１回でもよく、あるいは複数回であってもよい。周波数発振切替モード（例えば２８ｋＨｚ／４５ｋＨｚの２周波切替発振モード）で処理することで、酸化ガリウム粒子の液中での分散性をより一層に向上させることが可能になる。また超音波の出力は１０～５００Ｗであってよく、５０～２００Ｗであってよい。さらに処理時間は１～１０時間であってよい。処理時間を長くすることで、銀供給源の全てを銀ナノ粒子に変換させて酸化ガリウム粒子に担持させることが可能である。一方で処理時間を短くすることで、銀ナノ粒子の担持量を調整することが可能である。 It is not necessary to use a special device for the ultrasonic treatment, and an ordinary device equipped with an ultrasonic wave source may be used. For example, a commercially available ultrasonic cleaner may be used. The treatment may also be performed under normal conditions. For example, the frequency of ultrasound may be 20-100 kHz, and may be 28-45 kHz. Ultrasonic treatment may be performed continuously at the same frequency, or the frequency may be switched during treatment using a frequency oscillation switching mode. The number of frequency switching may be one or more. Processing in a frequency oscillation switching mode (for example, a dual frequency switching oscillation mode of 28 kHz/45 kHz) makes it possible to further improve the dispersibility of the gallium oxide particles in the liquid. Moreover, the output of the ultrasonic wave may be 10 to 500W, and may be 50 to 200W. Furthermore, the treatment time may be from 1 to 10 hours. By lengthening the treatment time, it is possible to convert all of the silver supply source into silver nanoparticles and carry them on the gallium oxide particles. On the other hand, by shortening the treatment time, it is possible to adjust the supported amount of silver nanoparticles.
超音波処理により得られた生成物（金属銀ナノ粒子を担持した酸化ガリウム粒子）は反応液中に分散又は沈殿した状態で存在する。したがって反応液から生成物を回収して、これを乾燥すればよい。回収では、ろ過や遠心分離等の公知の分離手段を用いればよい。また乾燥は、銀ナノ粒子が過度の粒成長を起こさない条件、例えば１００℃以下で行えばよい。 The product (gallium oxide particles supporting metallic silver nanoparticles) obtained by the ultrasonic treatment exists in a dispersed or precipitated state in the reaction solution. Therefore, the product may be recovered from the reaction solution and dried. For recovery, known separation means such as filtration and centrifugation may be used. Drying may be performed under conditions that do not cause excessive grain growth of the silver nanoparticles, for example, 100° C. or less.
＜熱処理工程＞
必要に応じて、後述するＣｒ処理を施す前の金属銀ナノ粒子担持酸化ガリウム粒子に、酸素含有雰囲気中１００℃以上の温度で熱処理を施してもよい。超音波担持法で作製した銀担持酸化ガリウム粒子に熱処理を施すと、粒子表面に残留したエタノール由来の有機物を除去でき、さらに銀（Ａｇ）ナノ粒子の粒径を小さくすることができる。この熱処理の作用により、ＣＯ２還元光触媒として利用した際にＣＯをより選択的に生成させることができる。
<Heat treatment process>
If necessary, the metal silver nanoparticles-supported gallium oxide particles before being subjected to the Cr treatment described later may be subjected to heat treatment at a temperature of 100° C. or higher in an oxygen-containing atmosphere. When the silver-supported gallium oxide particles prepared by the ultrasonic support method are heat-treated, ethanol-derived organic substances remaining on the particle surface can be removed, and the particle size of the silver (Ag) nanoparticles can be reduced. Due to the effect of this heat treatment, CO can be produced more selectively when used as a CO 2 reduction photocatalyst.
熱処理は、酸素を含む雰囲気中１００℃以上で行うことが好ましい。熱処理に特別な装置を用いる必要はなく、大気中での焼成が可能な一般的な電気炉を用いることができる。１００℃未満の温度では、有機物の分解や銀（Ａｇ）ナノ粒子の粒径へ与える影響が小さく、その効果を期待できない。 The heat treatment is preferably performed at 100° C. or higher in an atmosphere containing oxygen. It is not necessary to use a special apparatus for heat treatment, and a general electric furnace capable of firing in the atmosphere can be used. At a temperature of less than 100° C., the decomposition of organic substances and the effect on the particle size of silver (Ag) nanoparticles are small, and the effect cannot be expected.
＜Ｃｒ処理工程＞
Ｃｒ処理工程では、金属銀ナノ粒子を担持した酸化ガリウム粒子の上にクロム酸銀（Ａｇ２ＣｒＯ４）を生成させる。これにより金属銀（Ａｇ）ナノ粒子とクロム酸銀（Ａｇ２ＣｒＯ４）ナノ粒子とを担持した酸化ガリウム粒子を得ることができる。
<Cr treatment step>
In the Cr treatment step, silver chromate (Ag 2 CrO 4 ) is produced on the gallium oxide particles supporting the metallic silver nanoparticles. Thus, gallium oxide particles supporting metallic silver (Ag) nanoparticles and silver chromate (Ag 2 CrO 4 ) nanoparticles can be obtained.
金属銀ナノ粒子とクロム酸銀ナノ粒子を担持できる限り、Ｃｒ処理の手法は限定されない。しかしながら以下の手順で行うことが好ましい。すなわちクロム（Ｃｒ）化合物を溶解させた水溶液に金属銀担持酸化ガリウム粒子を浸漬及び乾燥して金属銀担持酸化ガリウム粒子の表面にクロム（Ｃｒ）化合物を析出させる。クロム化合物として、硝酸クロム（ＩＩＩ）、硫酸クロム水和物、塩化クロム水和物など水溶性化合物を使用すればよい。その後、クロム（Ｃｒ）化合物を析出させた金属銀担持酸化ガリウム粒子に加熱処理を施す。これによる銀（Ａｇ）とクロム（Ｃｒ）が反応してクロム酸銀（Ａｇ２ＣｒＯ４）が生成する。加熱処理は、例えば４００～７００℃の温度で１～１０時間行えばよい。 The method of Cr treatment is not limited as long as the metallic silver nanoparticles and the silver chromate nanoparticles can be supported. However, it is preferable to follow the procedure below. That is, the gallium oxide particles supporting metallic silver are immersed in an aqueous solution in which a chromium (Cr) compound is dissolved and then dried to precipitate the chromium (Cr) compound on the surface of the gallium oxide particles supporting metallic silver. As the chromium compound, water-soluble compounds such as chromium (III) nitrate, chromium sulfate hydrate, and chromium chloride hydrate may be used. After that, the gallium oxide particles supporting metallic silver on which the chromium (Cr) compound is deposited are subjected to heat treatment. Silver (Ag) and chromium (Cr) react with each other to form silver chromate (Ag 2 CrO 4 ). The heat treatment may be performed, for example, at a temperature of 400-700° C. for 1-10 hours.
＜＜２．光触媒粒子＞＞
本実施形態の二酸化炭素還元光触媒粒子は、酸化ガリウム（Ｇａ２Ｏ３）粒子と、この酸化ガリウム粒子の表面に担持された金属銀（Ａｇ）ナノ粒子及びクロム酸銀（Ａｇ２ＣｒＯ４）ナノ粒子と、を含む。この光触媒粒子では、Ａｇナノ粒子とＡｇ２ＣｒＯ４ナノ粒子とがＧａ２Ｏ３粒子表面に分散した状態で担持されている。これにより光触媒粒子の触媒性能が優れたものになる。
<<2. Photocatalyst particles>>
The carbon dioxide reduction photocatalyst particles of the present embodiment include gallium oxide (Ga 2 O 3 ) particles, metallic silver (Ag) nanoparticles and silver chromate (Ag 2 CrO 4 ) nanoparticles supported on the surface of the gallium oxide particles. particles. In this photocatalyst particle, Ag nanoparticles and Ag 2 CrO 4 nanoparticles are supported in a dispersed state on the surface of Ga 2 O 3 particles. This makes the photocatalyst particles excellent in catalytic performance.
酸化ガリウム（Ｇａ２Ｏ３）粒子は主触媒として機能するものであり、光触媒に用いられるものであれば特に限定されない。Ｇａ２Ｏ３には、α型、β型、γ型、δ型及びε型が知られているが、いずれを用いてもよい。しかしながら安定な酸化物であるβ型（β－Ｇａ２Ｏ３）が好ましい。粒子の大きさも特に限定されない。例えば粒子の平均粒子径は０．３～５．０μｍである。さらに粒子の形状も特に限定されない。例えば球状、不定形状、異方形状（ロッド又は板状等）が挙げられる。粒子がロッド状である場合、例えば長軸径が１．０～５．０μｍ、短軸径が０．３～１．０μｍのものを用いることができる。 Gallium oxide (Ga 2 O 3 ) particles function as a main catalyst, and are not particularly limited as long as they are used for photocatalysts. Ga 2 O 3 is known to be α-type, β-type, γ-type, δ-type, and ε-type, and any of them may be used. However, the β form (β-Ga 2 O 3 ), which is a stable oxide, is preferred. The particle size is also not particularly limited. For example, the particles have an average particle size of 0.3 to 5.0 μm. Furthermore, the shape of the particles is not particularly limited. For example, it may be spherical, amorphous, or anisotropic (rod-shaped, plate-shaped, etc.). When the particles are rod-shaped, for example, particles having a long axis diameter of 1.0 to 5.0 μm and a short axis diameter of 0.3 to 1.0 μm can be used.
金属銀（Ａｇ）ナノ粒子は、助触媒として機能するものであり、その平均粒子径がｎｍオーダーである。金属銀ナノ粒子を微細にすることで、光触媒の触媒性能が高くなる。これに対して銀ナノ粒子がｎｍオーダーより大きいと、触媒活性などの助触媒としての機能が失われたり、酸化ガリウム粒子の表面活性点が少なくなったりする恐れがある。 Metallic silver (Ag) nanoparticles function as co-catalysts and have an average particle size of nm order. By making the metallic silver nanoparticles finer, the catalytic performance of the photocatalyst is enhanced. On the other hand, if the silver nanoparticles are larger than the nanometer order, there is a possibility that the function as a co-catalyst such as catalytic activity may be lost, or the number of surface active sites of the gallium oxide particles may decrease.
金属銀（Ａｇ）ナノ粒子の平均粒子径は１０．０～５０．０ｎｍであることが好ましい。平均粒子径を１０．０ｎｍ以上にすることで、Ａｇ濃度（担持量）を低くしたり、有機表面保護剤を添加したりする必要無く、銀ナノ粒子を形成することが可能になる。一方で平均粒子径を５０．０ｎｍ以下にすることで、触媒活性などの助触媒としての機能が失われたり、酸化ガリウム粒子の表面活性点が少なくなったりするという問題を防ぐことが可能になる。平均粒子径は１０．０～３０．０ｎｍがより好ましい。なお平均粒子径は、光触媒粒子を、透過型電子顕微鏡（ＴＥＭ）を用いて観察することで求めることができる。一例として、目視又は画像解析ソフトウエアにより銀ナノ粒子の粒径分布を求め、この粒径分布から平均値を算出するという手法が挙げられる。 The average particle size of the metallic silver (Ag) nanoparticles is preferably 10.0-50.0 nm. By setting the average particle diameter to 10.0 nm or more, it becomes possible to form silver nanoparticles without the need to lower the Ag concentration (supported amount) or add an organic surface protective agent. On the other hand, by setting the average particle size to 50.0 nm or less, it is possible to prevent problems such as loss of co-catalyst function such as catalytic activity and reduction of surface active sites of gallium oxide particles. . More preferably, the average particle size is 10.0 to 30.0 nm. The average particle size can be obtained by observing the photocatalyst particles with a transmission electron microscope (TEM). As an example, there is a method of determining the particle size distribution of silver nanoparticles by visual inspection or image analysis software, and calculating the average value from this particle size distribution.
クロム酸銀（Ａｇ２ＣｒＯ４）ナノ粒子は、金属銀ナノ粒子がもつ助触媒の機能を補助する働きがある。すなわちクロム酸銀から生じるクロム（Ｃｒ）イオンは、触媒として働く際に二酸化炭素（ＣＯ２）と反応して炭酸塩を形成する。したがって光触媒粒子がクロム酸銀ナノ粒子を備えることで、炭酸塩及びこれから生じる炭酸イオンを助触媒たる銀ナノ粒子の近傍に存在させることができ、その結果、二酸化炭素の還元を効果的に促進させることが可能となる。なおＡｇ２ＣｒＯ４ナノ粒子は、金属銀を担持させたＧａ２Ｏ３粒子を硝酸クロム（ＩＩＩ）水溶液と混合した後に熱処理することで、これを生成させることができる。 Silver chromate (Ag 2 CrO 4 ) nanoparticles have a function of assisting the co-catalyst function of metallic silver nanoparticles. That is, chromium (Cr) ions originating from silver chromate react with carbon dioxide (CO 2 ) to form carbonates when acting as a catalyst. Therefore, by providing the photocatalyst particles with silver chromate nanoparticles, carbonate and carbonate ions generated therefrom can be present in the vicinity of the silver nanoparticles serving as cocatalysts, and as a result, the reduction of carbon dioxide can be effectively promoted. becomes possible. Ag 2 CrO 4 nanoparticles can be produced by mixing Ga 2 O 3 particles supporting metallic silver with an aqueous solution of chromium (III) nitrate and then heat-treating the mixture.
クロム酸銀（Ａｇ２ＣｒＯ４）ナノ粒子の平均粒子径は１．０～３０．０ｎｍであることが好ましい。平均粒子径を１．０ｎｍ以上にすることで、凝集することなく安定して粒子を担持させることができるという効果がある。一方で平均粒子径を３０．０ｎｍ以下にすることで、活性点を増やすことで触媒性能を向上させることができるという効果がある。平均粒子径は１．０～２０．０ｎｍがより好ましい。クロム酸銀ナノ粒子の平均粒子径は、金属銀ナノ粒子と同様の手法で測定すればよい。 Silver chromate (Ag 2 CrO 4 ) nanoparticles preferably have an average particle size of 1.0 to 30.0 nm. By setting the average particle size to 1.0 nm or more, there is an effect that the particles can be stably supported without agglomeration. On the other hand, by setting the average particle size to 30.0 nm or less, there is an effect that the catalytic performance can be improved by increasing the number of active sites. More preferably, the average particle size is 1.0 to 20.0 nm. The average particle size of silver chromate nanoparticles may be measured by the same method as for metallic silver nanoparticles.
銀（Ａｇ）成分の担持量は、金属銀換算で酸化ガリウム（Ｇａ２Ｏ３）粒子に対して０．３～１０．０質量％であることが好ましい。ここで、銀成分の担持量は、金属銀ナノ粒子及びクロム酸銀ナノ粒子のそれぞれに含まれる銀（Ａｇ）の合計量である。担持量が過度に少ないと助触媒の効果を十分に発揮させることが困難になる。そのためＣＯ２還元に光触媒を用いたときにＣＯガス発生速度とＣＯ選択率が低くなってしまう。一方で担持量が過度に多いとＣＯガス発生速度が低下する。銀供給源の配合量や超音波処理条件を制御することで担持量を調整することができる。銀ナノ粒子の担持量は０．５質量％以上であってよく、１．０質量％以上であってよく、３．０質量％以上であってよく、５．０質量％以上であってもよい。また担持量は７．５質量％以下であってよく、５．０質量％以下であってよく、３．０質量％以下であってよく、１．０質量％以下であってもよい。 The supported amount of silver (Ag) component is preferably 0.3 to 10.0% by mass in terms of metal silver with respect to gallium oxide (Ga 2 O 3 ) particles. Here, the supported amount of silver component is the total amount of silver (Ag) contained in each of the metallic silver nanoparticles and the silver chromate nanoparticles. If the supported amount is excessively small, it becomes difficult to sufficiently exhibit the effect of the co-catalyst. Therefore, the CO gas generation rate and CO selectivity are low when photocatalysts are used for CO2 reduction. On the other hand, if the supported amount is excessively large, the CO gas generation rate decreases. The supported amount can be adjusted by controlling the blending amount of the silver supply source and the ultrasonic treatment conditions. The amount of silver nanoparticles supported may be 0.5% by mass or more, may be 1.0% by mass or more, may be 3.0% by mass or more, or may be 5.0% by mass or more. good. Further, the supported amount may be 7.5% by mass or less, 5.0% by mass or less, 3.0% by mass or less, or 1.0% by mass or less.
本実施形態の光触媒粒子は、触媒性能、特にＣＯ選択率が優れている。例えばＣＯ２還元光触媒性能評価試験においてＣＯ選択率が３０％以上である。そのためＣＯ２還元により発生するＣＯガス量の割合を高くすることが可能になる。ＣＯ選択率は４０％以上であってよく、５０％以上であってよく、６０％以上であってよく、７０％以上であってもよい。ＣＯ選択率の上限は特に限定されるものではないが、典型的には９０％以下、より典型的には８０％以下である。 The photocatalyst particles of the present embodiment are excellent in catalytic performance, particularly in CO selectivity. For example, the CO selectivity is 30% or more in the CO 2 reduction photocatalyst performance evaluation test. Therefore, it becomes possible to increase the ratio of the amount of CO gas generated by CO 2 reduction. The CO selectivity may be 40% or higher, 50% or higher, 60% or higher, or 70% or higher. Although the upper limit of the CO selectivity is not particularly limited, it is typically 90% or less, more typically 80% or less.
ＣＯ２還元光触媒性能評価試験は公知の評価装置を用いて行えばよい。評価装置の一例を図２に示す。評価装置（２）は槽（４）とこの槽（４）内部に設けられた高圧水銀（Ｈｇ）ランプ（６）とから構成されている。槽（４）の内部には評価用溶液（２２）が入れられる。また槽（４）はガス導入管（８）、ガス排出管（１０）、ｐＨ計（１２）、ゴム栓（１４）及びスターラー（１６）を備えている。ガス導入管（８）の先端にはバブリングフィルター（１８）が設けられている。水銀ランプ（６）は、その周囲に流れる冷却水（２０）によって冷却される。 The CO 2 reduction photocatalyst performance evaluation test may be performed using a known evaluation device. An example of an evaluation device is shown in FIG. The evaluation device (2) consists of a tank (4) and a high-pressure mercury (Hg) lamp (6) provided inside the tank (4). An evaluation solution (22) is placed inside the tank (4). The tank (4) is also equipped with a gas inlet tube (8), a gas outlet tube (10), a pH meter (12), a rubber stopper (14) and a stirrer (16). A bubbling filter (18) is provided at the tip of the gas introduction pipe (8). The mercury lamp (6) is cooled by cooling water (20) flowing around it.
評価試験は次のようにして行えばよい。純水、炭酸水素ナトリウム（ＮａＨＣＯ３）及びサンプル（光触媒粒子）を混合して評価用溶液（２２）を作製する。この評価用溶液（２２）を評価装置（２）の槽（４）に入れて、スターラー（１６）で撹拌する。二酸化炭素（ＣＯ２）ガス（３０）をガス導入管（８）から吹き込み、それと同時に高圧Ｈｇランプ（６）からＵＶ光を評価用溶液（２２）に照射する。所定時間照射した後に、発生したガス（３２）を、ガス排出管（１０）を通してガスクロマトグラフィー（３４）に導入し、そこで分析する。この分析により水素（Ｈ２）、酸素（Ｏ２）及び一酸化炭素（ＣＯ）の発生速度（生成量）を求める。得られた発生速度を用いて、下記（１）式に基づきＣＯ選択率を算出する。 An evaluation test may be performed as follows. Pure water, sodium hydrogen carbonate (NaHCO 3 ) and a sample (photocatalyst particles) are mixed to prepare an evaluation solution (22). This evaluation solution (22) is placed in the tank (4) of the evaluation device (2) and stirred with a stirrer (16). Carbon dioxide (CO 2 ) gas (30) is blown from the gas inlet pipe (8), and at the same time UV light is irradiated from the high pressure Hg lamp (6) to the solution for evaluation (22). After irradiation for a given period of time, the evolved gas (32) is introduced through the gas outlet (10) into the gas chromatograph (34) and analyzed there. By this analysis, the generation rate (production amount) of hydrogen (H 2 ), oxygen (O 2 ) and carbon monoxide (CO) is determined. Using the obtained generation rate, the CO selectivity is calculated based on the following formula (1).
銀ナノ粒子の粒子サイズと電子遷移状態は、光触媒の触媒性能、例えばＣＯ選択率に影響を及ぼすことが予想される。このことを金属銀ナノ粒子担持酸化ガリウム粒子（光触媒粒子）のＣＯ２還元のメカニズム（図３）に基づき説明する。図３に示されるように、エネルギーｈνをもつ光が粒子に照射されると、酸化ガリウム粒子（半導体光触媒粒子）中に電子（ｅ－）と正孔（ｈ＋）とが生じる。この際、金属銀ナノ粒子（助触媒）は電荷分離（電子ｅ－と正孔ｈ＋の分離）を促進する。正孔（ｈ＋）が周囲の水分（Ｈ２Ｏ）と反応する結果、下記（２）式に示す反応が右方向に進み、酸素（Ｏ２）とプロトン（Ｈ＋）とが生成される。一方で電子（ｅ－）が二酸化炭素（ＣＯ２）及びプロトン（Ｈ＋）と反応する結果、下記（３）及び（４）式に示す反応が右方向に進み、一酸化炭素（ＣＯ）と水（Ｈ２Ｏ）と水素（Ｈ２）が生成する。また下記（２）から（４）式の反応を合わせると、原理的には下記（５）式に示す反応が進む。 The particle size and electronic transition state of silver nanoparticles are expected to affect the catalytic performance of photocatalysts, such as CO selectivity. This will be explained based on the mechanism of CO 2 reduction of gallium oxide particles (photocatalyst particles) supported by metallic silver nanoparticles (Fig. 3). As shown in FIG. 3, when particles are irradiated with light having energy hν, electrons (e − ) and holes (h + ) are generated in gallium oxide particles (semiconductor photocatalyst particles). At this time, the metallic silver nanoparticles (promoter) promote charge separation (separation of electrons e − and holes h + ). As a result of the reaction of the holes (h + ) with the surrounding moisture (H 2 O), the reaction shown in the following formula (2) proceeds rightward to generate oxygen (O 2 ) and protons (H + ). . On the other hand, electrons (e − ) react with carbon dioxide (CO 2 ) and protons (H + ). Water (H 2 O) and hydrogen (H 2 ) are produced. Further, when the reactions of the following formulas (2) to (4) are combined, in principle, the reaction shown in the following formula (5) proceeds.
上記（３）式の反応と（４）式の反応が同じ程度で起こると、上記式（５）に示されるようにＣＯ選択率（ＣＯ発生速度／（Ｈ２発生速度＋ＣＯ発生速度））は一定である。しかしながら実際には、これらの反応が同じ程度に起こるとは限らない。上記（３）式の反応が優先的に起こることで、ＣＯ選択率が高くなる。 If the reaction of formula (3) and the reaction of formula (4) occur to the same extent, then as shown in formula (5) above, the CO selectivity (CO generation rate/(H generation rate + CO generation rate)) is constant. In practice, however, these reactions do not always occur to the same extent. The preferential reaction of the formula (3) increases the CO selectivity.
上記（３）式の反応では、二酸化炭素（ＣＯ２）が炭酸塩種（carbonate species）として触媒表面に吸着し、光照射により反応中間体であるギ酸塩種（formate species）に変化した後に水分子と相互作用して一酸化炭素（ＣＯ）になるとの報告がある。またこの報告では銀助触媒が反応中間体の生成を促進することが示唆されている。したがって金属銀ナノ粒子（助触媒）による電荷分離の作用及び反応中間体生成の作用を高めることで、上記（３）式の反応が優先的に起こり、ＣＯ選択率がより一層に改善されると期待される。この点、本実施形態の光触媒粒子は、担持銀ナノ粒子が微細で特有の電子遷移状態を有するものになっているが故に、これらが複合的に作用して電荷分離の作用及び反応中間体生成の作用が高くなっているのではないかと推察している。 In the reaction of formula (3) above, carbon dioxide (CO 2 ) is adsorbed on the surface of the catalyst as carbonate species, and is converted to formate species, which is a reaction intermediate by irradiation with light, and then water There are reports that it interacts with molecules to become carbon monoxide (CO). This report also suggests that the silver promoter promotes the formation of reaction intermediates. Therefore, by enhancing the action of charge separation and the action of generating reaction intermediates by the metallic silver nanoparticles (promoter), the reaction of the above formula (3) occurs preferentially, and the CO selectivity is further improved. Be expected. In this regard, in the photocatalyst particles of the present embodiment, since the supported silver nanoparticles are fine and have a unique electronic transition state, these act in combination to effect charge separation and generate reaction intermediates It is speculated that the effect of
このような製造方法を採用することで、触媒性能、特にＣＯ選択率が優れた触媒粒子を簡易に得ることが可能である。その詳細な理由は不明であるが、超音波還元により生成した担持銀ナノ粒子が微細で特有の電子遷移状態を有するためと推測している。すなわち超音波還元処理で生成した銀ナノ粒子（助触媒）は粒子サイズが小さい。また超音波処理時に発生した高温且つ高圧のホットスポットやラジカルの作用によって特有の電子遷移状態になっていると考えられる。実際、超音波により生じたホットスポットは５０００℃近くの高温であるとの報告があり、このような高温のホットスポットが瞬間的にでも作用することで、電子遷移状態が変化することは容易に予想される。そしてこの微細な粒子サイズと特有の電子遷移状態とが複合的に作用して優れたＣＯ選択率をもたらすと推測している。 By adopting such a production method, it is possible to easily obtain catalyst particles excellent in catalytic performance, particularly in CO selectivity. Although the detailed reason is unknown, it is speculated that the supported silver nanoparticles produced by ultrasonic reduction have fine and unique electronic transition states. That is, the silver nanoparticles (promoter) produced by the ultrasonic reduction treatment have a small particle size. In addition, it is considered that the unique electronic transition state is caused by the action of high-temperature and high-pressure hot spots and radicals generated during the ultrasonic treatment. In fact, it has been reported that hot spots generated by ultrasonic waves have a high temperature of nearly 5000 ° C. It is easy to change the electronic transition state by the instantaneous action of such a high temperature hot spot. is expected. It is speculated that this fine particle size and the unique electronic transition state act in combination to bring about excellent CO selectivity.
またこのような製造方法によれば、金属銀ナノ粒子を高分散且つ高担持率で析出させることが可能になるとともに、金属銀ナノ粒子がもつ助触媒の機能を補助するクロム酸銀（Ａｇ２ＣｒＯ４）ナノ粒子を酸化ガリウム粒子の表面に担持することができる。これにより炭酸イオンを助触媒たる銀ナノ粒子の近傍に存在させることができ、その結果、二酸化炭素の還元を効果的に促進させることが可能となる。さらにこのような製造方法によれば、限定されるものではないが、反応液に溶解しない酸化銀などの化合物を固体状態のまま銀供給源に用いることが可能である。反応液に溶解しない化合物を用いた場合には、反応液がアニオン等の有害物を含んでおらず、廃液処理が容易である。 In addition, according to such a production method, it is possible to precipitate metallic silver nanoparticles with high dispersion and high loading, and silver chromate (Ag 2 CrO 4 ) nanoparticles can be supported on the surface of the gallium oxide particles. This allows carbonate ions to exist in the vicinity of the silver nanoparticles serving as promoters, and as a result, it is possible to effectively promote the reduction of carbon dioxide. Furthermore, according to such a production method, it is possible to use a compound such as silver oxide, which is not dissolved in the reaction solution, as a silver supply source in a solid state, although it is not limited thereto. When a compound that does not dissolve in the reaction liquid is used, the reaction liquid does not contain harmful substances such as anions, and waste liquid treatment is easy.
これに対して、従来から提案されている含浸法や光電析法では金属銀ナノ粒子を高分散且つ高担持率で担持させることは困難である。例えば、特許文献１には銀の担持量に関して、光電着法による場合には酸化ガリウムに対して０．２～２質量％、含浸法による場合には０．０５～２％である旨、最適範囲より多い場合には銀ナノ粒子の粒子径が大きくなり触媒活性などの効果が失われたり、酸化ガリウムの表面活性点を減らしたりする旨が記載されている（特許文献１の［００１４］及び［００１５］）。 On the other hand, it is difficult to support metallic silver nanoparticles with high dispersion and high loading rate by the conventionally proposed impregnation method and photo-electrodeposition method. For example, in Patent Document 1, regarding the amount of silver supported, it is 0.2 to 2% by mass with respect to gallium oxide when using the photoelectrodeposition method, and 0.05 to 2% when using the impregnation method. It is described that if the amount is more than the range, the particle size of the silver nanoparticles becomes large and the effect such as catalytic activity is lost, or the surface active sites of gallium oxide are reduced ([0014] of Patent Document 1 and [0015]).
また特許文献１で提案される含浸法や光電析法では、硝酸銀を溶解させた水溶液などの前駆体溶液を用いている。このような前駆体溶液に含まれる硝酸イオンなどのアニオンは大気汚染の原因となる有害物質である。したがって含浸法や光電析法では有害物質を無毒化するための廃液処理が必要である。ただし本実施形態の製造方法は、反応液に溶解する化合物を用いることを排除するものではない。そのような場合であっても、金属銀ナノ粒子を高分散且つ高担持率で析出させる効果が得られる。 Further, in the impregnation method and the photoelectrodeposition method proposed in Patent Document 1, a precursor solution such as an aqueous solution in which silver nitrate is dissolved is used. Anions such as nitrate ions contained in such a precursor solution are harmful substances that cause air pollution. Therefore, the impregnation method and the photoelectrodeposition method require waste liquid treatment to detoxify harmful substances. However, the production method of this embodiment does not exclude the use of a compound that dissolves in the reaction solution. Even in such a case, it is possible to obtain the effect of depositing metallic silver nanoparticles with high dispersion and high loading.
また特許文献２で提案される製造方法では超音波処理は貴金属酸化物を分散させるために行われるに過ぎず、超音波処理後の加熱工程で貴金属酸化物の還元を図っている（特許文献２の［００３８］）。したがって超音波処理により銀供給源（酸化銀等）の還元を行う本実施形態の方法とは明確に異なる。本実施形態の製造方法では、超音波処理後の加熱を行わなくとも、十分に還元された銀ナノ粒子を担持させることが可能である。
In addition, in the production method proposed in
本実施形態を、以下の例によってさらに具体的に説明する。しかしながら本発明は以下の実施例に限定されるものではない。 The present embodiment is further specifically described by the following examples. However, the invention is not limited to the following examples.
（１）光触媒粒子の作製
［実施例１］
超音波処理により銀ナノ粒子を還元生成させて光触媒粒子（金属銀ナノ粒子担持酸化ガリウム粒子）を作製した。酸化ガリウムに対する銀濃度（担持量）を３．０質量％にした。具体的には以下のようにしてサンプルを作製した。
(1) Production of photocatalyst particles [Example 1]
Photocatalyst particles (metallic silver nanoparticles-supported gallium oxide particles) were produced by reducing silver nanoparticles by ultrasonic treatment. The silver concentration (supported amount) with respect to gallium oxide was set to 3.0% by mass. Specifically, samples were produced as follows.
＜準備工程＞
酸化ガリウム粒子（株式会社高純度化学研究所、Ｇａ２Ｏ３）と酸化銀（富士フィルム和光純薬株式会社、Ａｇ２Ｏ）とを準備した。酸化ガリウム粒子は、純度が９９.９９％であり、平均粒子径は、長径が約３μｍ、短径が約１μｍであった。また酸化銀は、純度が９９％であり、１次粒子径が約２μｍの凝集体であった。
<Preparation process>
Gallium oxide particles (Kojundo Chemical Laboratory Co., Ltd., Ga 2 O 3 ) and silver oxide (Fujifilm Wako Pure Chemical Industries, Ltd., Ag 2 O) were prepared. The gallium oxide particles had a purity of 99.99% and an average particle size of about 3 μm in major axis and about 1 μm in minor axis. The silver oxide was an aggregate having a purity of 99% and a primary particle size of about 2 μm.
＜混合工程＞
準備した酸化ガリウム粒子（１ｇ）と酸化銀（３２ｍｇ）とを還元液（５０ｍＬ）に添加した。還元液としてエタノール（富士フィルム和光純薬株式会社）を用いた。これにより反応液を作製した。
<Mixing process>
The prepared gallium oxide particles (1 g) and silver oxide (32 mg) were added to the reducing solution (50 mL). Ethanol (Fuji Film Wako Pure Chemical Industries, Ltd.) was used as the reducing liquid. A reaction solution was thus prepared.
＜超音波処理工程＞
得られた反応液を超音波装置（本多電子株式会社、ＷＴ―１００―Ｍ）に入れて超音波処理を施した。超音波処理は、２８ｋＨｚと４５ｋＨｚの２周波切替発振とし、出力１００Ｗの条件で行った。また処理時間を０～３時間の間で変えた。この際、反応液の温度を４０℃に維持した。この処理により、反応液中の酸化銀（Ａｇ２Ｏ）を還元して銀（Ａｇ）に変化させた。次いで処理により生成した生成物をろ過した後、エタノール（１０ｍＬ）を用いて洗浄し、大気中６０℃で０．５時間の条件で乾燥して、金属銀ナノ粒子担持酸化ガリウム粒子を得た。
<Ultrasonic treatment process>
The obtained reaction solution was placed in an ultrasonic device (WT-100-M, Honda Electronics Co., Ltd.) and subjected to ultrasonic treatment. The ultrasonic treatment was performed under the conditions of 28 kHz and 45 kHz switching oscillation and an output of 100 W. Also, the treatment time was varied between 0 and 3 hours. At this time, the temperature of the reaction solution was maintained at 40°C. By this treatment, silver oxide (Ag 2 O) in the reaction solution was reduced and changed to silver (Ag). Next, the product produced by the treatment was filtered, washed with ethanol (10 mL), and dried in the atmosphere at 60° C. for 0.5 hours to obtain gallium oxide particles supporting metallic silver nanoparticles.
＜熱処理工程＞
得られた金属銀ナノ粒子担持酸化ガリウム粒子を大気中２００℃で５分間熱処理した。
<Heat treatment process>
The obtained metal silver nanoparticle-supported gallium oxide particles were heat-treated in air at 200° C. for 5 minutes.
＜Ｃｒ処理工程＞
含浸法によりＣｒと銀を反応させＡｇ２ＣｒＯ４を生成させて光触媒粒子を作製した。まず熱処理した金属銀ナノ粒子担持酸化ガリウム（１．０３ｇ）を超純水（２０ｍＬ）及び硝酸クロム（ＩＩＩ）水溶液（０．１４Ｍ、１．９８ｍＬ）と混合した。この際、Ａｇ：Ｃｒ＝１：１（モル比）になるようにＣｒ仕込み量を調整した。次いで得られた混合溶液を、ウォーターバスを用いて８０℃で１時間加熱した。加熱した混合溶液を大気中８０℃で３時間乾燥し、得られた乾燥物を４５０℃で２時間焼成した。これにより光触媒粒子を得た。
<Cr treatment step>
Photocatalyst particles were produced by reacting Cr and silver by an impregnation method to produce Ag 2 CrO 4 . First, heat-treated gallium oxide supporting metallic silver nanoparticles (1.03 g) was mixed with ultrapure water (20 mL) and chromium (III) nitrate aqueous solution (0.14 M, 1.98 mL). At this time, the charging amount of Cr was adjusted so that Ag:Cr=1:1 (molar ratio). The resulting mixed solution was then heated at 80° C. for 1 hour using a water bath. The heated mixed solution was dried in air at 80° C. for 3 hours, and the resulting dried product was calcined at 450° C. for 2 hours. Photocatalyst particles were thus obtained.
［比較例１］
光電析法（光電着法）によりＣｒ処理を行って光触媒粒子を作製した。まず熱処理した金属銀ナノ粒子担持酸化ガリウム（１．０３ｇ）を超純水（１Ｌ）及び硝酸クロム（ＩＩＩ）水溶液（０．１４Ｍ、１．９８ｍＬ）と混合した。この際、Ａｇ：Ｃｒ＝１：１（モル比）になるようにＣｒ仕込み量を調整した。次いで得られた混合溶液にアルゴン（Ａｒ）ガスを吹き込んでガス置換した。その後、アルゴンガスを３０ｍＬ／分の流量で吹き込みながら、４００Ｗ高圧ＨｇランプでＵＶ光を混合溶液に１．５時間照射して水酸化クロムを金属銀ナノ粒子担持酸化ガリウム上に析出させた。光照射後の溶液をろ過して粉末を回収した。回収した粉末を室温で乾燥して光触媒粒子を得た。
[Comparative Example 1]
Photocatalyst particles were produced by performing Cr treatment by photoelectrodeposition (photoelectrodeposition). First, heat-treated gallium oxide supporting metallic silver nanoparticles (1.03 g) was mixed with ultrapure water (1 L) and chromium (III) nitrate aqueous solution (0.14 M, 1.98 mL). At this time, the charging amount of Cr was adjusted so that Ag:Cr=1:1 (molar ratio). Next, argon (Ar) gas was blown into the resulting mixed solution to replace the gas. After that, while blowing argon gas at a flow rate of 30 mL/min, the mixed solution was irradiated with UV light from a 400 W high-pressure Hg lamp for 1.5 hours to deposit chromium hydroxide on the gallium oxide supporting metallic silver nanoparticles. The powder was recovered by filtering the solution after light irradiation. The recovered powder was dried at room temperature to obtain photocatalyst particles.
［比較例２］
Ｃｒ処理を行わなかった。それ以外は実施例１と同様にして光触媒粒子を得た。
[Comparative Example 2]
No Cr treatment was performed. Photocatalyst particles were obtained in the same manner as in Example 1 except for the above.
（２）光触媒粒子の評価
実施例１、比較例１及び２で得られたサンプルについて、各種特性の評価を以下のとおり行った。
(2) Evaluation of Photocatalyst Particles Various characteristics of the samples obtained in Example 1 and Comparative Examples 1 and 2 were evaluated as follows.
＜Ｘ線回折（ＸＲＤ）＞
サンプルをＸ線回折法により分析して、その結晶相を調べた。分析条件は以下のとおりにした。
<X-ray diffraction (XRD)>
A sample was analyzed by X-ray diffraction to determine its crystalline phase. The analysis conditions were as follows.
‐Ｘ線回折装置：Ｂｒｕｋｅｒ Ｊａｐａｎ、Ｄ８ ＤＩＳＣＯＶＥＲ Ｖａｒｉｏ－１
‐線源：ＣｕＫα１ ｍｏｎｏｃｈｒｏｍａｔｅｄ
‐管電圧：４５ｋＶ
‐管電流：４０ｍＡ
‐スキャン速度：０．３°／分
‐スキャン範囲（２θ）：２０～６０°
-X-ray diffractometer: Bruker Japan, D8 DISCOVER Vario-1
- Source: CuKα1 monochromated
- Tube voltage: 45kV
- Tube current: 40mA
- Scanning speed: 0.3°/min - Scanning range (2θ): 20-60°
＜ＴＥＭ／ＥＤＳ分析＞
サンプルを、透過型電子顕微鏡（ＴＥＭ；ＨＩＴＡＣＨＩ、ＨＦ－２２００）を用いて観察した。観察は、透過電子像で加速電圧２００ｋＶの条件で行った。
<TEM/EDS analysis>
The samples were observed using a transmission electron microscope (TEM; HITACHI, HF-2200). Observation was performed with a transmission electron image under the condition of an acceleration voltage of 200 kV.
＜ＣＯ２還元光触媒性能＞
サンプルのＣＯ２還元光触媒性能を図２に示す評価装置を用いて評価した。まず超純水（１Ｌ）、ＮａＨＣＯ３（０．１Ｍ）及び光触媒粒子（０．５ｇ）を混合して評価用溶液を作製した。次にこの評価用溶液を評価装置の槽に入れ、二酸化炭素（ＣＯ２）ガスを３０ｍＬ／分の流量で吹き込みながら、４００Ｗ高圧ＨｇランプでＵＶ光を照射した。１時間照射後に発生したガスをガスクロマトグラフィー（島津製作所、ＧＣ－８Ａ）を用いて分析して、Ｈ２、Ｏ２及びＣＯ発生速度を求めた。そして下記（１）式に基づきＣＯ選択率を算出した。
< CO2 reduction photocatalyst performance>
The CO2 reduction photocatalytic performance of the samples was evaluated using the evaluation apparatus shown in FIG. First, ultrapure water (1 L), NaHCO 3 (0.1 M) and photocatalyst particles (0.5 g) were mixed to prepare a solution for evaluation. Next, this solution for evaluation was placed in a tank of an evaluation apparatus, and UV light was irradiated with a 400 W high-pressure Hg lamp while blowing carbon dioxide (CO 2 ) gas at a flow rate of 30 mL/min. The gas generated after 1 hour of irradiation was analyzed using gas chromatography (Shimadzu Corporation, GC-8A) to determine the generation rate of H 2 , O 2 and CO. Then, the CO selectivity was calculated based on the following formula (1).
（３）評価結果
＜Ｘ線回折（ＸＲＤ）＞
実施例１、比較例１及び２について得られたＸ線回折（ＸＲＤ）パターンを、銀（Ａｇ）及びクロム酸銀（Ａｇ２ＣｒＯ４）のパターンと併せて図４（ａ）及び（ｂ）に示す。いずれのサンプルでもＧａ２Ｏ３がメインピークとして見られたが、詳細に解析すると差異が見られた。具体的には、実施例１ではＡｇ２ＣｒＯ４のピークが確認できる一方で、比較例１及び２ではＡｇ２ＣｒＯ４のピークが検出されなかった。またピークの重なりにより正確な判断は困難であるが、比較例１及び２ではＡｇのピークが検出されたのに対し、実施例１ではＡｇのピーク強度が弱くなっていた。
(3) Evaluation results <X-ray diffraction (XRD)>
The X-ray diffraction (XRD) patterns obtained for Example 1, Comparative Examples 1 and 2 , together with the patterns of silver (Ag) and silver chromate (Ag2CrO4), are shown in Figures 4 (a) and (b). shown in Ga 2 O 3 was seen as the main peak in all samples, but a difference was seen when analyzed in detail. Specifically, in Example 1, an Ag 2 CrO 4 peak was observed, while in Comparative Examples 1 and 2, no Ag 2 CrO 4 peak was detected. Although it is difficult to make an accurate judgment due to overlap of peaks, in Comparative Examples 1 and 2, Ag peaks were detected, whereas in Example 1, the Ag peak intensity was weak.
＜ＴＥＭ／ＥＤＳ観察＞
実施例１及び比較例１のサンプルについて得られたＳＴＥＭ写真のそれぞれを図５及び図６に示す。実施例１のＴＥＭ像から、Ｇａ２Ｏ３柱状粒子と数ｎｍのサイズの粒子が観察された。またＥＤＳ分析により数ｎｍのサイズ粒子中にＡｇ、Ｃｒ及びＯが検出された。このことから数ｎｍのサイズの粒子はＡｇ２ＣｒＯ４であることが分かった。これに対して比較例１のＴＥＭ像では、数ｎｍのサイズの粒子は観察されなかった。２０ｎｍ程度のＡｇナノ粒子は検出され、ＣｒはＡｇ上に存在しているようであった。
<TEM/EDS Observation>
STEM photographs obtained for the samples of Example 1 and Comparative Example 1 are shown in FIGS. 5 and 6, respectively. From the TEM image of Example 1, Ga 2 O 3 columnar particles and particles with a size of several nm were observed. Also, Ag, Cr and O were detected in particles with a size of several nanometers by EDS analysis. From this, it was found that the particles with a size of several nm were Ag 2 CrO 4 . In contrast, in the TEM image of Comparative Example 1, particles with a size of several nm were not observed. Ag nanoparticles as small as 20 nm were detected, and Cr appeared to reside on the Ag.
＜ＣＯ２還元光触媒性能＞
実施例１、比較例１及び２について得られたＣＯ２還元光触媒性能（ガス発生速度及びＣＯ選択率）を表１に示す。含浸法で作製したサンプル（実施例１、比較例２）の性能を対比するに、Ｃｒ処理を施したサンプル（実施例１）は、ＣＯ選択率がＣｒ未処理サンプル（比較例２）とほとんど同じであるにも関わらず、ＣＯ生成速度が２．５倍程度に高かった。一方で、光電析法で作製したサンプル（比較例１）は、Ｃｒ未処理のサンプル（比較例２）に比べてＣＯ生成速度は増加するもののＨ２生成速度も増加した。そのためＣＯ選択率は大幅に悪化した。
< CO2 reduction photocatalyst performance>
The CO 2 reduction photocatalytic performance (gas generation rate and CO selectivity) obtained for Example 1 and Comparative Examples 1 and 2 are shown in Table 1. When comparing the performance of the samples prepared by the impregnation method (Example 1 and Comparative Example 2), the Cr-treated sample (Example 1) has almost the same CO selectivity as the Cr-untreated sample (Comparative Example 2). Despite being the same, the CO production rate was about 2.5 times higher. On the other hand, the sample (Comparative Example 1) prepared by the photo-electrodeposition method showed an increased rate of CO production, but also an increased rate of H 2 production compared to the sample (Comparative Example 2) not treated with Cr. Therefore, the CO selectivity deteriorated significantly.
２ 評価装置
４ 槽
６ 水銀（Ｈｇ）ランプ
８ ガス導入管
１０ ガス排出管
１２ ｐＨ計
１４ ゴム栓
１６ スターラー
１８ バブリングフィルター
２０ 冷却水
２２ 評価用溶液
３０ ＣＯ２ガス
３２ 発生ガス
３４ ガスクロマトグラフィー
2 evaluation device 4 tank 6 mercury (Hg)
Claims (12)
前記酸化ガリウム粒子と前記銀供給源とを還元液に加えて反応液を作製する工程、
前記反応液に超音波を照射して、金属銀担持酸化ガリウム粒子を作製する工程、及び、
前記金属銀担持酸化ガリウム粒子にクロム（Ｃｒ）処理を行い、それにより金属銀（Ａｇ）ナノ粒子及びクロム酸銀（Ａｇ２ＣｒＯ４）ナノ粒子を担持した酸化ガリウム粒子を作製する工程を含む、二酸化炭素還元光触媒粒子の製造方法。 providing gallium oxide ( Ga2O3 ) particles and a silver (Ag) source;
adding the gallium oxide particles and the silver source to a reducing solution to prepare a reaction solution;
A step of irradiating the reaction solution with ultrasonic waves to produce metallic silver-supported gallium oxide particles;
subjecting the gallium oxide particles supporting metallic silver to a chromium (Cr) treatment, thereby producing gallium oxide particles supporting metallic silver ( Ag) nanoparticles and silver chromate ( Ag2CrO4) nanoparticles; A method for producing carbon dioxide-reducing photocatalyst particles.
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