JPH11221471A - Photocatalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the decrease in catalytic activity accompanied by the increase in used hours, by covering a part of the surface of an oxide, by which water is decomposed at light irradiation to generate at least one of hydrogen and oxygen, with an organic polymer having a hydrophilic nature or an ion exchange capacity. SOLUTION: A part of the surface of an oxide, by which water is decomposed at light irradiation to generate at least one of hydrogen and oxygen, is covered with an organic polymer having a hydrophilic nature or an ion exchange capacity. Thereby, the adsorption of oxygen to the surface of a catalyst is blocked to prevent the decrease in catalytic activity caused by adsorption oxygen. As the organic polymer to be used, a fully fluorinated ionomer is desirable, and a fluororesin having carboxyl group or sulfo group (sulfone group) can be used. As the oxide to be used, an oxide containing monovalent copper, preferably Cu2 O, is used.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photocatalyst for decomposing water by light irradiation to generate at least one of hydrogen and oxygen.

[0002]

2. Description of the Related Art At present, energy sources mainly include:
Fossil fuels such as oil and coal and nuclear energy are used. However, fossil fuels have a finite reserve and depletion is a problem. Also, fossil fuels emit carbon dioxide, nitrogen oxides, sulfur oxides, etc. when burned, and as a result, carbon dioxide causes global warming,
Nitrogen oxides and sulfur oxides react with moisture in the air to produce nitric acid and sulfuric acid, resulting in acid rain and destruction of the global environment.

[0003] Nuclear power generation has been put into practical use to utilize nuclear energy, which is a new energy source, but has problems such as safety and waste disposal. Under such circumstances, attention has been paid to the use of hydrogen, which is a clean energy source, as one method for solving the problems of depletion of energy resources and destruction of the global environment. Hydrogen only turns into water when burned, and does not cause environmental pollution. Needless to say, it is meaningless to use fossil fuel or the like to generate this hydrogen.

[0004] For fossil fuels with finite reserves, sunlight is inexhaustible and water is abundant on the earth. Then, if hydrogen is generated by decomposing water using the energy of sunlight, each of the above-mentioned problems can be solved. As one of the means for decomposing water using the energy of sunlight, there is a photocatalyst for water decomposition. A photocatalyst is a type of semiconductor. When it absorbs energy above its band gap, it generates holes (holes) and electrons (electrons), which oxidizes water to generate oxygen, and electrons reduce water. To generate hydrogen.

[0005] Incidentally, sunlight obtained on the ground surface has a spectrum distribution having a peak near a wavelength of 500 nm. The distribution ratio is in the ultraviolet region (wavelength less than 400 nm).
Is about 5%, the visible light region (wavelength 400 nm or more, less than 750 nm) is about 43%, and the infrared light region (wavelength 750 nm or more) is about 52%. Therefore, in order to use sunlight efficiently, a photocatalyst whose bandgap energy corresponds to light energy in the visible light region or more (wavelength 400 nm or more) where the distribution ratio is large (a photocatalyst having catalytic activity in the visible light region). Is desired.

The photocatalyst having catalytic activity in the visible light region includes, for example, Cu ₂ O or a chemical formula abO ₂ (a: a metal element selected from at least one or two or more monovalent elements, or a monovalent element. A single metal element or a composite metal element, b: a metal element selected from at least one or two or more trivalent elements, or a trivalent single metal element or a composite metal element (example: CuFeO ₂ ) There is a photocatalyst represented by a particulate matter having a Delafossite crystal structure.

[0007] The monovalent composite metal element is conceptualized as a constituent element of a substance in which a part of a substance composed of a monovalent single metal element is replaced with one or more different single metal elements (monovalent). Similarly, the trivalent composite metal element is
This is a conceptualization of constituent elements of a substance in which a part of a substance composed of a trivalent single metal element is replaced by one or more different single metal elements (trivalent).

This photocatalyst is obtained by a usual solid phase method, that is, by mixing oxides of respective metal element components as raw materials at a predetermined composition ratio and firing in an inert gas atmosphere.
Can be manufactured. Alternatively, the photocatalyst is obtained by mixing oxides of respective metal element components as raw materials at a predetermined composition ratio and firing the mixture in an inert gas atmosphere to synthesize a particulate material which is a precursor of the photocatalyst. The precursor can be manufactured by pulverizing the precursor into fine particles.

For example, a precursor (particulate CuF) of a photocatalyst synthesized by mixing an oxide of Cu and an oxide of Fe at a predetermined composition ratio and firing in an inert gas atmosphere.
eO ₂₎ can be produced a photocatalyst (fine particulate CuFeO ₂₎ by a further grinding.

[0010]

However, Cu ₂
Conventional photocatalysts composed of O or particulate or fine particles represented by the chemical formula abO ₂ have a function of decomposing water into hydrogen and oxygen by irradiation with visible light (catalytic activity), but have a considerable catalytic activity. There was a problem that it was low.

In addition, the conventional photocatalyst has a problem that the catalytic activity is greatly reduced as the use time increases. The present invention has been made in view of the above problems, and provides a photocatalyst having higher catalytic activity than conventional photocatalysts and capable of suppressing a decrease in catalytic activity with an increase in use time. With the goal.

[0012]

SUMMARY OF THE INVENTION Accordingly, the present invention firstly provides "a part of the surface of an oxide which decomposes water by light irradiation to generate at least one of hydrogen and oxygen, A photocatalyst coated with an organic polymer having exchangeability (Claim 1) "is provided. The present invention secondly provides a "photocatalyst according to claim 1 wherein the organic polymer is a perfluorinated ionomer (claim 2)."

[0013] The present invention thirdly provides a photocatalyst according to claim 1, wherein the organic polymer is a fluororesin having a carboxyl group or a sulfo group (sulfonic acid group) (claim 3). I will provide a. The present invention fourthly provides "a photocatalyst according to any one of claims 1 to 3, wherein the oxide is an oxide containing monovalent copper (claim 4)."

The present invention also provides a fifth aspect of the present invention wherein the oxide is Cu
_A photocatalyst according to claim 4, wherein the photocatalyst is ₂ O (claim 5). " The present invention also provides a sixth aspect in which “the oxide has a chemical formula of CuMO ₂ (M: at least one or 2
A metal element selected from at least three kinds of trivalent elements, or 3
A photocatalyst according to claim 4 (claim 6), which is a compound represented by a monovalent metal element or a composite metal element having a valency.

The present invention is also directed to a seventh aspect, wherein "M in the above chemical formula is at least one or two or more metal elements selected from the group consisting of Cr, Mn, Fe, Co, and Ga; 7. The photocatalyst according to claim 6, which is a single metal element or a composite metal element.
I will provide a. Eighth, the present invention provides a photocatalyst according to any one of claims 1 to 7, wherein the photocatalyst is in the form of fine particles having a particle size of 0.1 to 10 microns (preferably 0.1 to 1 micron). 8) ”.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The photocatalytic reaction is basically carried out in a state in which the photocatalyst is suspended in water. As a result of intensive studies by the present inventors, as the photocatalytic reaction proceeds, the generated oxygen gradually decreases. It was found that the catalyst was adsorbed on the surface of the catalyst, the surface of the catalyst was covered with adsorbed oxygen, and the catalytic activity was reduced.

That is, conventional photocatalysts (Cu ₂ O and CuFe
(O ₂ etc.) surface remains as-synthesized, and the surface is not treated in particular. Therefore, depending on the substance (photocatalyst), oxygen generated by the catalytic reaction is gradually adsorbed on the catalytic surface, and the catalytic active site is increased. It was found that the catalyst activity decreased with time. For example, Cu has a property of easily taking in oxygen, and an oxide or a composite oxide of Cu exhibits the same property as long as Cu is present near the surface.
Cu ₂ O and CuFeO _{2, which} are typical examples of conventional photocatalysts, adsorb oxygen on the catalyst surface if oxygen is present nearby.

The present inventors formed an organic polymer film having hydrophilicity or ion exchange ability on a part of the photocatalyst surface, thereby inhibiting the adsorption of oxygen on the catalyst surface,
It has been found that a decrease in catalytic activity due to adsorbed oxygen can be prevented. That is, the present inventors have found that oxygen adsorption does not occur on the photocatalyst surface on which an organic polymer film having hydrophilicity or ion exchange ability is formed, and that even on the photocatalyst surface portion where the organic polymer film is not formed, It has been found that the presence of the organic polymer film on the surface of the substrate inhibits the adsorption of oxygen, and as a result, the reduction of the catalytic activity due to the adsorbed oxygen can be prevented.

Further, the present inventors have found that by forming an organic polymer membrane having hydrophilicity or ion exchange ability on a part of the surface of the photocatalyst, it is possible to greatly improve the catalytic activity as compared with the case where it is not formed. I found it. Therefore,
The photocatalyst according to the present invention (claims 1 to 8) has hydrophilicity or ion exchange ability on a part of the surface of an oxide that decomposes water by light irradiation to generate at least one of hydrogen and oxygen. A photocatalyst coated with an organic polymer was used.

Since such a configuration is employed, the photocatalyst of the present invention (claims 1 to 8) has a higher catalytic activity than the conventional photocatalyst and suppresses a decrease in the catalytic activity due to an increase in use time. It has the effect of being able to. In the present invention, the reason that the organic polymer film is not formed on the entire surface of the photocatalyst but is formed only on a part thereof is that the photocatalytic reaction occurs on the catalyst surface and does not occur on the organic polymer film.

That is, in order to cause a photocatalytic reaction,
It is necessary to expose at least a part of the photocatalyst surface,
In other words, if the number of the exposed surfaces is too small, the catalytic active point will be lowered, and the effect of the present invention will be reduced.Therefore, the ratio of the catalyst surface forming the organic polymer film should be appropriately set. Is preferred. The setting of the ratio may be replaced by the weight of the organic polymer with respect to the weight of the photocatalyst. For example, when Nafion described later is used as the organic polymer, 4
Nafion equivalent to ６6 wt% is suitable. The thickness of the Nafion coating on the catalyst surface is not particularly problematic.

The organic polymer having hydrophilicity according to the present invention
Are the hydrophilic groups -COOH, -OH, -NH_Two, -NHCONH_Two,
-(OCH_TwoCH_Two)_n-, -SO_ThreeH, -SO_ThreeM, -OSO_ThreeH, -OSO_ThreeM, -COO
M, -NR _ThreeX (M: alkali metal or -NH_Four, R: alkyl
Group, X: halogen), phosphonic acid group, quaternary ammonium group
Organic macromolecules, including but not limited to
It is not something to be done.

Further, it has the ion exchange ability according to the present invention.
Organic polymer is an ion-exchange functional group -SO_Three
H, -COOH, phosphonic acid group, -CH_TwoN⁺(CH_Three)_ThreeCl^-, -CH_TwoN⁺(CH
_Three)_TwoC _TwoH_FourOH ・ Cl^-, -CH_TwoN (CH_Three)_Two, Quaternary ammonium groups, etc.
Organic polymers having, but are not limited to
Not something. And among these organic polymers,
Photocatalyst coated with perfluorinated ionomer (Claim 2)
Or carboxyl group (-COOH) and / or sulfo group
(Sulfonate group -SO_ThreeH) coated with fluororesin
In the case of a photocatalyst (claim 3), the effect is particularly large.
The photocatalyst has chemical resistance, oxidation decomposition resistance,
It is preferable because of its excellent resolvability.

The perfluorinated ionomer according to the present invention includes the following chemical formula:-((CF ₂ CF ₂ ) x- (CF ₂ CF) y)-((OCH ₂ C
_{FCF 3) mO- (CF 2)} nX (m = 0,1, n = 1~5, X: -COOH, -S
O ₃ H, and derivatives thereof). More specifically, for example, DuPont Bae Le fluorosulfonic acid derivative _{(Rf (OCF (CF 3)} CF 2) xOCF 2 CF 2 SO 3 Na ( trade name Nafion, Rf: fluorinated hydrocarbon group) and, improvement of this A product (perfluorocarboxylic acid or a derivative thereof, or a product obtained by introducing a carboxylic acid group or a derivative thereof into a part of perfluorosulfonic acid or a derivative thereof) is preferable.

This Nafion and its improved product are obtained by using a perfluorinated ionomer (Claim 2), a carboxyl group (-COO
H) and / or a fluororesin having a sulfo group (sulfonic acid group —SO ₃ H) (claim 3). Nafion and its improved products are excellent in chemical resistance, oxidation decomposition resistance, and reduction decomposition resistance, so they do not deteriorate even if immersed in water for a long time as in photocatalytic experiments. (Holes) and electrons (electrons) generated by the
There is no deterioration due to.

Further, since Nafion and its improved products are hydrophilic polymers, unlike hydrophobic polymers, water as a reactant can be taken in on the catalyst surface without repelling water. . That is, the organic polymer easily introduces the hydrophilic group, and the organic polymer having hydrophilicity is easily dissolved in a solvent such as water or alcohol, and is easily coated in a solution state on the surface of the photocatalyst. Is preferred.

[0027] When an inorganic substance is used, the same effect as that of the organic polymer according to the present invention may be obtained, but it takes much time and effort to coat the catalyst surface.
Even if coated, a new step is required to fix the coated material on the catalyst surface. The coating of the surface of the photocatalyst with the organic polymer according to the present invention can be performed by a photocatalyst composed of an oxide that easily adsorbs oxygen (for example, a photocatalyst composed of an oxide containing monovalent copper (Claim 4) or P This is particularly effective for a photocatalyst composed of an oxide of a type semiconductor.

Examples of the oxide containing monovalent copper include Cu ₂ O (Claim 5) and a chemical formula CuMO ₂ (M: a metal element selected from at least one or two or more trivalent elements) Or a trivalent single metal element or a composite metal element (Claim 6), wherein the trivalent composite metal element is one kind of a substance composed of a trivalent single metal element. The above is a conceptualization of the constituent elements of the substance substituted by the different single metal elements (trivalent).

The photocatalyst composed of the compound represented by the chemical formula CuMO ₂ can decompose water by irradiating visible light to simultaneously generate both hydrogen and oxygen. As described above, in a photocatalyst capable of simultaneously generating both hydrogen and oxygen, holes (holes) and electrons (electrons) generated by light irradiation effectively participate in a water decomposition reaction. This is preferable because the life and catalytic activity are hardly reduced and the stability of the catalytic function is improved.

Further, M in the above chemical formula is C
At least one, two or more metal elements selected from the group consisting of r, Mn, Fe, Co, and Ga, or a single metal element or a composite metal element (claim 7). The shape of the photocatalyst according to the present invention is preferably fine particles having a large surface area (fine particle size of 0.1 to 10 microns), and particularly preferable fine particle size is 0.1 to 1 micron so that visible light can be effectively used. Item 8).

The photocatalyst having such a fine particle size can be obtained by pulverizing a particulate photocatalyst precursor with a ball mill or a planetary mill to form fine particles. The modification generally used in the production of a photocatalyst, such as supporting a cocatalyst (for example, Pt, NiO, etc.), can also be performed on the photocatalyst according to the present invention. Further, the water used when performing the water splitting reaction according to the present invention is not limited to pure water, and water mixed with salts such as carbonates and bicarbonates may be used. Alternatively, a sacrificial reagent such as alcohol or silver ions may be used.

The photocatalyst of the present invention can be applied to not only a water decomposition reaction but also other chemical reactions (for example, a decomposition reaction of an organic substance or a reduction reaction of a noble metal ion). Hereinafter, the present invention will be described specifically with reference to examples, but the present invention is not limited thereto.

[0033]

Embodiment 1 In the photocatalyst of this embodiment, a part of the surface of a particulate oxide (Cu ₂ O) having a particle size of 10 μm or less is coated with the above Nafion which is an organic polymer having hydrophilicity and ion exchange ability. And has catalytic activity in the visible light region. (1) Method for Producing Photocatalyst of Present Example Fine particles C having a particle size of 10 μm or less were obtained by the following method.
Mottled coating of Nafion on u ₂ O surface
did. At this time, Nafion was not coated at once but was coated on the surface of Cu ₂ O in _two steps so as to form a mottled coat as uniform as possible (distributed over the entire surface of Cu ₂ O).

First, 0.5 g of fine-particle Cu ₂ O and 0.01 g of Nafion were collected, and 15 ml of isopropyl alcohol was added thereto. Then, a sample (Nafion was scattered on the surface of fine-particle Cu ₂ O) by a rotary evaporator. The pressure was reduced until the film was dried. Next, 0.01 g of Nafion and 15 ml of isopropyl alcohol were added to the obtained dried sample, and the pressure was reduced until the sample was dried with a rotary evaporator.
The photocatalyst of this example was manufactured.

In the present embodiment, Nafion equivalent to 4 wt% is coated in a mottled state with respect to fine particle Cu ₂ O. (2) Evaluation of catalytic activity The catalytic activity of the produced photocatalyst was evaluated by using a closed-circulation catalytic reactor and generating hydrogen and oxygen using pure water as a reaction solution as shown below. Was.

First, 0.5 g of the produced photocatalyst was added to 30 parts of pure water.
The mixture was placed in 0 ml, and irradiated with light from the outside while stirring with a magnetic stirrer. At this time, 50
A 0W xenon lamp and a reaction tube made of Pyrex glass were used. Further, light having a wavelength of 460 nm or less was cut by a filter, and light irradiation was performed. Next, measurement (detection and quantification) of the generated hydrogen and oxygen was performed by gas chromatography. As a result of the measurement, generation of hydrogen and oxygen was recognized, and the catalytic activities were hydrogen: 14.8 μmol / h and oxygen: 7.4 μmol / h.

For comparison, when the catalytic activity of fine particle Cu ₂ O (conventional photocatalyst) not coated with Nafion was measured, hydrogen was 1.0 μmol / h and oxygen was 0.5 μmol / h. Was. That is, it was confirmed that the photocatalyst of this example had a considerably higher catalytic activity in the visible light region than the conventional photocatalyst.

In addition, when the photocatalyst of this example and the conventional photocatalyst were evaluated for a decrease in catalytic activity with use,
Compared with the conventional photocatalyst, it was confirmed that the photocatalyst of the present example showed a remarkably small decrease in catalytic activity with an increase in the use time.

[0039]

Embodiment 2 The photocatalyst of this embodiment coats a part of the surface of a particulate oxide (CuFeO ₂ ) having a particle size of 10 μm or less with Nafion which is an organic polymer having hydrophilicity and ion exchange ability. And has catalytic activity in the visible light region. (1) Method for Producing the Photocatalyst of the Example First, Cu ₂ O and Fe ₂ O ₃ were mixed at a predetermined composition ratio and fired in an inert gas atmosphere (solid phase method) to obtain a photocatalyst. A precursor (particulate CuFeO _x ) was synthesized.

Here, considering that Cu ₂ O volatilizes during the synthesis, the stoichiometric amount (amount based on stoichiometric ratio: 2.36) is considered.
g) increased by almost 10%. That is, Cu ₂ O: 2.
60 g and 2.64 g of Fe ₂ O ₃ were weighed and placed in an alumina boat at 1050 ° C. for 1 hour in a nitrogen atmosphere.
The firing was performed for 0 hours. Thereafter, the synthesized precursor was pulverized in a mortar into fine particles having a particle size of 10 μm or less, thereby producing fine-particle CuFeO ₂ .

Next, the fine particle C was prepared by the following method.
The surface of uFeO ₂ was mottled with Nafion. At this time, Nafion was not coated at once but was coated on the surface of CuFeO _{2 in two} steps so as to form a mottled coat as uniform as possible (distributed over the entire surface of CuFeO ₂ ). First, 0.5 g of fine-particle CuFeO ₂ and 0.01 g of Nafion were collected, and 15 ml of isopropyl alcohol was added thereto. Then, a sample was prepared using a rotary evaporator (the surface of fine-particle CuFeO ₂ was mottled with Nafion. Was reduced to dryness.

Next, Nafion 0.1 was added to the obtained dried sample.
After adding 01 g and 15 ml of isopropyl alcohol,
The photocatalyst of this example was manufactured by reducing the pressure with a rotary evaporator until the sample was dried. In this example, 4 wt% of Nafion was applied to the fine particle CuFeO ₂ in a mottled state. (2) Evaluation of catalytic activity When the catalytic activity was evaluated in the same manner as in Example 1,
The photocatalyst of the present example has a considerably higher catalytic activity in the visible light region than the conventional photocatalyst (fine-particle CuFeO ₂ not coated with Nafion), and the decrease in the catalytic activity with an increase in use time is extremely small. It was confirmed that.

It should be noted that fine particle Cu having a particle size of 10 μm or less
GaO_Two, CuCrO_Two, CuMnO _Two, CuCoO_TwoEach of
Each photocatalyst whose surface is coated with Nafion in a mottled pattern
The same procedure was followed to produce each photocatalyst.
When evaluated, the conventional photocatalyst (coated with Nafion
Not much) but significantly higher catalytic activity in the visible light region
And the decrease in catalytic activity with the increase in use time
It was confirmed to be extremely small.

[0044]

As described above, the present invention (Claims 1 to 5)
The photocatalyst of 8) has a higher catalytic activity than the conventional photocatalyst, and has an effect of suppressing a decrease in the catalytic activity due to an increase in use time. Photocatalyst in which the organic polymer according to the present invention is coated with a perfluorinated ionomer (Claim 2)
When the photocatalyst is coated with a fluororesin having a carboxyl group (—COOH) and / or a sulfo group (sulfonic acid group —SO ₃ H) (claim 3), the above effect is particularly enhanced, and the photocatalyst is: Excellent resistance to chemicals, oxidation and decomposition.

The photocatalyst surface according to the present invention is coated with an organic polymer by a photocatalyst composed of an oxide which easily adsorbs oxygen (for example, a photocatalyst composed of an oxide containing monovalent copper (Claim 4). ) Or a photocatalyst composed of a P-type semiconductor oxide). For example, Cu
_The present invention is particularly effective for a compound (claim 6) represented by ₂ O (claim 5) or a chemical formula CuMO ₂ (M: a single metal element or a composite metal element selected from trivalent elements). is there.

Further, M in the above chemical formula is, for example, a single metal element or a composite metal element selected from the group consisting of Cr, Mn, Fe, Co, and Ga.
The present invention is particularly effective in these cases. The photocatalyst composed of the compound represented by the chemical formula CuMO _{2 according} to the present invention can decompose water by irradiation with visible light and simultaneously generate both hydrogen and oxygen.

As described above, in a photocatalyst capable of simultaneously producing both hydrogen and oxygen, holes (holes) and electrons (electrons) generated by light irradiation effectively participate in a water decomposition reaction. Therefore, the catalyst life and the catalyst activity are hardly reduced, and the stability of the catalyst function is improved. When the photocatalyst (particulate matter) of the present invention is formed into a particulate form having a particle size of 0.1 to 10 μm (preferably 0.1 to 1 μm), the surface area of the photocatalyst becomes large and the irradiated visible light is effectively used. (Claim 8). that's all

──────────────────────────────────────────────────の Continuation of the front page (51) Int.Cl. ⁶ Identification code FI // B01J 37/02 101 C04B 35/00 K (72) Inventor Yoshikazu Hara 4259 Nagatsuda-cho, Midori-ku, Yokohama-shi, Kanagawa Tokyo industry Within the Institute for Resource Chemistry

Claims (8)

[Claims] 1. A photocatalyst in which a part of the surface of an oxide that decomposes water by light irradiation to generate at least one of hydrogen and oxygen is coated with an organic polymer having hydrophilicity or ion exchange ability. . 2. The photocatalyst according to claim 1, wherein the organic polymer is a perfluorinated ionomer. 3. The photocatalyst according to claim 1, wherein the organic polymer is a fluororesin having a carboxyl group or a sulfo group (sulfonic acid group). 4. The photocatalyst according to claim 1, wherein the oxide is an oxide containing monovalent copper. 5. The photocatalyst according to claim 4, wherein said oxide is Cu ₂ O. 6. The oxide is represented by a chemical formula CuMO ₂ (M: a metal element selected from at least one or two or more trivalent elements, or a trivalent single metal element or a composite metal element). The photocatalyst according to claim 4, wherein the photocatalyst is a compound. 7. In the above chemical formula, M is Cr, Mn, F.
7. The photocatalyst according to claim 6, wherein the photocatalyst is at least one or two or more metal elements selected from the group consisting of e, Co, and Ga, or a single metal element or a composite metal element. . 8. A particle size of 0.1 to 10 microns (preferably 0.1 to 10 microns).
(1 micron) in the form of fine particles.
8. The photocatalyst according to any one of 7.