JP4469979B2 - Photocatalyst for generating hydrogen from alcohols or aqueous solution thereof, method for producing the same, and method for producing hydrogen using the same - Google Patents

Description

  The present invention relates to a novel photocatalyst, a method for producing the same, and a method for producing hydrogen using the photocatalyst.

  Nuclear power generation has been put to practical use as a new energy source, but because of problems such as safety and waste disposal, the development of clean and safe new energy has attracted attention. Currently, environmental problems such as severe global warming caused by fossil resource constraints and mass consumption are attracting attention.

  On the other hand, the solar energy that reaches the ground in one year is enormous, equivalent to 10,000 times the annual energy consumption of mankind, and its efficient utilization research has recently become active. A typical study is photocatalysis. Photocatalysts such as ultraviolet and visible light responsive photocatalysts are attracting attention as extremely useful catalysts capable of directly producing hydrogen and oxygen as clean fuels from inexhaustible sunlight and water.

This reaction is an energy storage type reaction as shown in the following reaction formula (a), and in the photosynthesis, oxygen generation that occurs under a light reaction that requires light is none other than this decomposition reaction.
H ₂ O → H ₂ + (1/2) O ₂ (a)

  In general, when this type of photocatalyst absorbs energy that exceeds the band gap, holes and electrons are generated, and these undergo oxidation and reduction reactions to generate oxygen and hydrogen, respectively. Considering the practical application of this photocatalyst, it is essential to use sunlight as a light source. Sunlight falling on the surface of the earth has a maximum intensity of radiation in the vicinity of a wavelength of 500 nm that is visible light, and the amount of energy in the visible light region having a wavelength of about 400 to 750 nm is about 43% of the total sunlight. On the other hand, in the ultraviolet region where the wavelength is about 400 nm or less, it is less than 5% of the total sunlight.

However, although many conventional semiconductor photocatalysts are known to be able to generate both hydrogen and oxygen when irradiated with high energy ultraviolet light, the photocatalytic activity is not so high and the efficiency is low. For example, so far, hydrogen generation reactions have been investigated using titanium oxide (TiO ₂ ) simple substance systems, mixed systems of graphite and titanium oxide, mixed systems of activated carbon and titanium oxide, and mixed systems of silica and titanium oxide. However, almost no generation of hydrogen was observed.

A highly efficient photocatalyst for efficiently using sunlight is desired.
In recent years, the application of photocatalysts has been extensively studied in the field of decomposition of harmful chemical substances. For example, there are many applications such as decomposition and removal of organic substances such as pesticides and malodorous substances in water and air, or self-cleaning of a solid surface coated with a photocatalyst.

  The present invention uses a photocatalyst that efficiently absorbs ultraviolet rays contained in sunlight and the like to irradiate water and harmful chemical substances containing hydrogen-containing compounds (alcohols) with light, so that hydrogen-containing compounds (alcohols) It is intended to provide a highly efficient method for producing hydrogen or a method for detoxifying harmful substances by decomposing water or harmful substances containing hydrogen.

The present inventor has studied the above problem for a mixed system of a carbon nanostructure having a carbon network structure in the outer shell and titanium oxide, and the carbon nanostructure having a carbon network structure in the outer shell is a photocatalyst of titanium oxide. It was found to be an important factor that increases the activity. That found to be effective as a high efficiency of the photocatalyst of hydrogen generation by mixing the network structure of carbon titanium oxide TiO ₂ is conventionally known photocatalyst carbon nanostructures having a shell in the proper proportions, the Invented. The present invention includes the following inventions.

(1) A photocatalyst mainly composed of carbon nanostructures having carbon network structure in the outer shell or derivatives thereof and titanium oxide.
(2) The photocatalyst according to claim 1, wherein the carbon nanostructure is a single-wall nanotube or a multi-wall nanotube.
(3) The photocatalyst according to (1) or (2), wherein the carbon nanostructure or a derivative thereof is contained in an amount of 1 to 30 wt% based on the total weight of the carbon nanostructure or the derivative thereof and titanium oxide.
(4) A method for producing hydrogen by allowing a photocatalyst according to any one of (1) to (3) activated by light irradiation to act on an alcohol or an aqueous solution thereof.
(5) The method according to (4), wherein the photocatalyst is allowed to act on a 40 to 90 vol% aqueous solution of alcohol.

The present invention provides a photocatalyst in which the catalytic activity of titanium oxide, which is an existing photocatalyst, is dramatically increased.
The present invention also provides an efficient method for producing hydrogen.

  In the present invention, titanium oxide means various titanium oxides such as anatase-type titanium oxide, rutile-type titanium oxide, and amorphous titanium oxide. Anatase-type titanium oxide alone or anatase-type / rutin containing anatase-type titanium oxide as a main component Type titanium oxide mixtures are preferred.

  In the present invention, a carbon nanostructure having a carbon network structure in the outer shell (hereinafter sometimes simply referred to as “carbon nanostructure”) is based on a network structure composed of carbon atoms similar to a graphite sheet. It is a generic term for substances with various shapes that have been rolled up or rolled up, with individual sizes on the order of nanometers. Typically, a single graphite layer is formed into a tube with a diameter on the order of nanometers. Single-walled nanotubes with a rolled shape (hereinafter sometimes abbreviated as “SWNT”), multi-walled nanotubes with a nested shape of SWNT (multi-walled nanotube, hereinafter abbreviated as “MWNT”) A), carbon nanohorns, fullerenes or their variants 1 or more, and the like. These various carbon nanostructures can be used as a single substance or as a mixture of two or more kinds. The carbon nanostructure is presumed to function as a cocatalyst for titanium oxide having photocatalytic activity.

  Of the carbon nanostructures described above, SWNT or MWNT is preferable, and SWNT is particularly preferable. These are formed by a known method such as an arc discharge method or a laser evaporation method. These carbon nanotubes may be in a state in which one or both ends are closed or in an open state. Moreover, you may have a bending structure. Usually, the tubular portion of the carbon nanotube is formed of a six-membered ring of carbon, and a cap portion that closes the end or a bent portion has a five-membered ring structure or a seven-membered ring structure.

  The carbon nanostructure derivatives described above can also be used in the present invention. Examples of such derivatives include those obtained by chemically modifying carbon nanostructures to modify the solubility in a solvent.

  The blending ratio of the carbon nanostructure or its derivative and titanium oxide is not particularly limited as long as it has a desired catalytic activity. Typically, the carbon nanostructure or its derivative is oxidized with the carbon nanostructure or its derivative. Based on the total weight with titanium, it is contained in an amount of 1 to 30 wt%, preferably 7 to 17 wt%.

  The photocatalyst of the present invention is prepared by mixing a carbon nanostructure or its derivative powder and a titanium oxide powder. In order to effectively use light, particles having a large specific surface area, that is, particles having a small particle size are preferred, and for example, particles having a particle size of 10 nm to 200 μm are preferred. In general, titanium oxide powder prepared by a solid phase reaction method has large particles and a small specific surface area, but the particle size can be reduced by grinding with a ball mill or the like. In addition to being used as a powder, it may be used after being appropriately molded or sintered. For example, fine particles can be molded and used in the form of a plate and / or a thin film. In addition, a powdered catalyst can be used by immobilizing on a suitable substrate.

Furthermore, the photocatalyst of the present invention is modified by a promoter composed of one or more components selected from the group consisting of platinum group elements such as Pt, transition metals such as Ni, NiO, IrO ₂ , RuO _{2 and the} like. Can be supported. The supporting method can be performed by a kneading method, an impregnation method, a photo-deposition method, or the like.

  Production of hydrogen by the photocatalyst of the present invention is carried out by allowing the photocatalyst of the present invention to act on alcohols (hydrogen-containing compounds) or an aqueous solution thereof or water (hereinafter sometimes collectively referred to as “reaction solution”). . It is particularly preferable to use an alcohol or an aqueous solution thereof. The concentration of the alcohol is preferably 10 to 100 vol%, particularly preferably 40 to 90 vol%. Examples of alcohols include methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, 1,2-butanediol, 1,3-butanediol, and 1,4-butanediol. In aqueous solutions of alcohols, alcohol molecules and water molecules form an association formed by hydrogen bonds, and the water molecules are connected to each other by a hydrogen bond network to form a hydrated cluster. It has been. The water used in the reaction solution is not limited to pure water, and water in which salts such as carbonate, hydrogen carbonate, iodine salt, bromine salt are mixed and dissolved may be used.

  The photocatalyst of the present invention is added to the reaction solution. The amount of catalyst added is basically selected so that incident light can be efficiently absorbed. Thus, by irradiating light to the alcohol aqueous solution to which the photolysis catalyst is added, the photocatalyst is activated and the hydrogen generation reaction proceeds. The light to be irradiated needs to have energy that exceeds the band gap of titanium oxide that is a semiconductor photocatalyst. Examples of such light include ultraviolet rays. Since the photocatalyst of the present invention has very high efficiency and can effectively use ultraviolet rays contained in sunlight, the reaction solution to which the photocatalyst is added may be irradiated with sunlight in the present invention.

  The photocatalyst of the present invention can also be used in various photocatalytic reactions other than the production of hydrogen. For example, the photocatalyst of the present invention can be used for decomposing and removing organic substances such as pesticides and malodorous substances in water or in the air, or for self-cleaning of a solid surface coated with a photocatalyst. The reaction form can be performed, for example, by adding a catalyst to an aqueous solution containing an organic substance and irradiating with light. Malodorous substances can also be decomposed by gas phase reaction. In the case of decomposition of an organic substance, the organic substance to be decomposed generally acts as an electron donor and is oxidatively decomposed by holes, and hydrogen is generated by electrons or oxygen is reduced.

  EXAMPLES Hereinafter, although this invention is demonstrated concretely based on an Example, this invention is not limited to these Examples.

The photocatalyst used for the material / reagent is titanium oxide (TiO ₂ , P25 manufactured by Nippon Aerosil Co., Ltd.). This powdery photocatalyst has an anatase structure of about 80% and a rutile structure of about 20%, and has a multifaceted structure with few pores. The particle size is about 20 nm and the specific surface area is about 50 m ² .

  The carbon nanotube powder used was imported from the United States (manufactured by MTR Ltd., purchased from Matsubo Co., Ltd.) and contains single-walled nanotubes (SWNT) with both ends closed at a purity of 20 to 40 wt%. To do. In this powder, as impurities considered to be mixed at the time of manufacture, amorphous (amorphous) soot, trace amounts of C-60, C-70 and fullerene having a larger carbon number, a size of about several nanometers A metal catalyst having a thickness is also included. The SWNT has a diameter of 0.7 to 1.2 nm and a length of about 10 to 30 μm. Hereinafter, this powder may be referred to as SWNT powder.

For comparison of catalyst activity, platinum having a purity of 99.98% (300 mesh manufactured by Nilaco Corporation) was used.
Alcohols (methanol, ethanol, 1-propanol) were those of special grades from Wako Pure Chemical Industries, Ltd. or Nacalai Tesque, Inc.
The water used was de-distilled deionized water twice.

Experimental Method As a reaction vessel, a Pyrex (registered trademark) glass Schlenk tube (volume 153 ml) was used. The upper end of the Schlenk tube was sealed with a silicon cap. An ultra-high pressure mercury lamp (USH-500SC (output 500 W) manufactured by USHIO INC.) Was used as an artificial light source. In order to conduct the light irradiation experiment using only ultraviolet rays, a filter (Toshiba Corporation, ATG UV-D33) that allows passage of ultraviolet rays was used, and measures such as lowering the output appropriately were taken so that the suspension did not become hot.

  A 20 ml aqueous solution of alcohol prepared to a predetermined concentration (alcohol content), pre-weighed titanium oxide powder and SWNT powder (total amount of titanium oxide and SWNT fixed to 30 mg) are placed in a Schlenk tube and covered with a silicon cap. And sealed. Sonication was applied for about 2 minutes to facilitate mixing. In order to remove dissolved oxygen in the solution and air in the Schlenk tube, bubbling with argon gas was performed for 1 hour. Bubbling means that two syringe needles (injection needles) are pierced into a lid, argon gas is introduced into the solution from one needle, and gas is released from the other needle. Then, irradiate ultraviolet rays from an ultra-high pressure mercury lamp, collect a certain amount of gas (usually 1 ml) with a syringe at regular intervals, and detect hydrogen using a gas chromatograph (manufactured by Shimadzu Corporation, GC-8AIT). It was.

Experiment 1
The amount of hydrogen generated per hour when the mixing ratio of titanium oxide (TiO ₂ ) powder and SWNT powder was changed in a 50 vol% alcohol aqueous solution was measured. In this experiment, with the total amount of the catalyst fixed at 30 mg, the mixing ratio of both the TiO ₂ powder and the SWNT powder was changed from 0 wt% (in this case, including only TiO ₂ ) to 100 wt% (in this case, only the SWNT powder) Variously). The amount of 50 vol% alcohol aqueous solution was 20 ml. The alcohol used is methanol, ethanol or 1-propanol. The results are shown in FIG. 1 (in the figure, the hydrogen generation amount is expressed as a hydrogen gas generation rate (unit: μmol / h)). From FIG. 1, it can be seen that a rapid increase in the amount of hydrogen generation can be seen by adding the SWNT powder to the TiO ₂ powder. The hydrogen generation amount is in the range of 10 to 50 wt% as SWNT powder content (assuming that the SWNT purity in SWNT powder is 30 wt%, SWNT content is 3 to 23 wt% based on the total weight of titanium oxide and SWNT). It is particularly high in the range of 20 to 40 wt% (7 to 16 wt%) and highest in the range of 25 to 40 wt% (9 to 14 wt%). On the other hand, when the SWNT powder content exceeded 70 wt% (41 wt%), the hydrogen generation rate tended to decrease. After 1 hour, the amount of hydrogen gas at the SWNT powder content with the largest amount of hydrogen generation was 32 μmol in methanol. Although hydrogen gas was generated in any alcohol aqueous solution used, the order of alcohols with the largest hydrogen generation amount under the present experimental conditions was methanol>ethanol> 1-propanol. Most notably, even when only TiO ₂ powder is added or when only SWNT powder is added, hydrogen gas is hardly generated, but when both are mixed, a surprising amount of hydrogen is generated. is there. It is considered that the SWNT powder functions as a strong promoter for TiO ₂ .

Experiment 2
30 mg of TiO ₂ powder / SWNT powder mixture (SWNT powder content: 25 wt%) as a catalyst is added to 20 ml of alcohol aqueous solutions of various concentrations described below, and the hydrogen generation reaction proceeds by light irradiation to generate the amount of hydrogen generated per hour. (Μmol / h) was measured. Methanol, ethanol or 1-propanol was used as the alcohol. The alcohol concentration in the aqueous alcohol solution was set to 0 vol% (in this case, only water), 20 vol%, 40 vol%, 60 vol%, 80 vol%, or 100 vol% (in this case, only alcohol). The results are shown in FIG.

  Even when methanol, ethanol, or 1-propanol is used, when alcohol is added to water, a rapid increase in the amount of hydrogen generated is observed. When the concentration of alcohol is 40 to 90 vol%, the amount of hydrogen generated is increased. You can see that it increases. It can also be seen that a significant amount of hydrogen is generated with alcohol alone.

Experiment 3
Reaction in 20 ml of 50 vol% methanol aqueous solution using a mixture of TiO ₂ powder and platinum (Pt) powder (total weight 30 mg) or a mixture of TiO ₂ powder and SWNT powder (total weight 30 mg) as a photocatalyst. The dependence of the amount of hydrogen gas generated on irradiation time was compared. The results are shown in FIG. From FIG. 3, it can be seen that the amount of hydrogen generated from the system using SWNT is sufficiently comparable to the system using platinum. It is known that platinum is the most excellent promoter for TiO ₂ . This result shows for the first time that SWNT has the same function as the most excellent promoter. Platinum is a noble metal and expensive. Carbon nanotubes such as SWNT are expected to be mass-produced in the future and the price will drop rapidly. In addition, since carbon nanotubes with considerably low purity were used this time, it is expected that the generation amount and efficiency of hydrogen gas will be further improved by using carbon nanotubes with much higher purity than this time. The Therefore, in view of practical use, it can be said that carbon nanotubes such as SWNT are far more advantageous than platinum.

It is a figure which shows the hydrogen gas generation | occurrence | production speed | velocity when using various alcohol (methanol, ethanol, or 1-propanol) aqueous solution and changing the SWNT content in a photocatalyst. It is a figure which shows the hydrogen gas generation | occurrence | production speed | velocity | rate when various alcohol (methanol, ethanol, or 1-propanol) aqueous solution is used and the density | concentration of alcohol is changed. Additive to TiO ₂ powder as (cocatalyst) is a diagram showing an irradiation time dependency of the amount of hydrogen gas generated when using platinum (Pt) powder or SWNT powder.

Claims (3)

A photocatalyst for generating hydrogen from an alcohol or an aqueous solution thereof, the main component of which is a carbon nanostructure having a carbon network structure in the outer shell or a derivative thereof and titanium oxide , wherein the carbon nanostructure is a single unit. The photocatalyst that is a single-walled nanotube or a multi-walled nanotube, and the carbon nanostructure or derivative thereof is contained in an amount of 1 to 30 wt% based on the total weight of the carbon nanostructure or derivative thereof and titanium oxide . An alcohol or an aqueous solution thereof, a method for producing hydrogen by the action of a photocatalyst according to claim 1 which is activated by light irradiation. The method according to claim 2 , wherein the photocatalyst is allowed to act on a 40 to 90 vol% aqueous solution of alcohol.

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