JP2013155095A - Method for producing oxygen by means of light irradiation - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve oxygen-production efficiency when a powdered titanium oxide is used as a photocatalyst.SOLUTION: Oxygen-production efficiency at a light-irradiated ground is increasingly improved by adding a powdered titanium oxide to a sodium nitrate solution (plot 1). The oxygen-production efficiency is more improved by making the powdered titanium oxide carry platinum or copper oxide (plot 2, 3, 4 and 5).

Description

  The present invention relates to a method for producing oxygen with higher efficiency than before by using titanium oxide as a photocatalyst.

  A method for generating oxygen more efficiently in a heterogeneous photocatalytic system using titanium oxide has been proposed. For example, a titanium (Pt) promoter is supported on titanium oxide powder and the powder is moistened with water (Non-patent Document 1), or a titanium oxide powder supported with platinum (Pt) as a promoter is used as carbonic acid. An example is a method (Non-Patent Document 2) of increasing the oxygen generation efficiency in water splitting by suspending in an aqueous sodium solution and irradiating the system with light. However, these methods have a problem of low oxygen generation efficiency. Recently, it has been clarified that heterogeneous photocatalytic systems using silver phosphate show relatively high oxygen generation efficiency, but there is a problem that silver phosphate itself is chemically unstable (non-patented). Reference 3). Therefore, a system that can generate oxygen more stably and with higher efficiency has been desired.

  The subject of this invention is providing the method of producing | generating oxygen more efficiently than before using titanium oxide as a photocatalyst.

According to one aspect of the present invention, there is provided an oxygen generation method in which a sodium nitrate aqueous solution in which titanium oxide particles are suspended is irradiated with light.
Here, platinum or copper oxide may be supported on the titanium oxide particles, or copper element may be doped.
Moreover, you may carry | support the said platinum to the said titanium oxide particle by carrying out the heat processing of the titanium oxide particle which mixed the platinum compound with the photoelectron deposition method.
Moreover, you may carry | support the said copper oxide to the said titanium oxide particle by heat-processing the titanium oxide particle which mixed the copper compound.
The supporting of the copper oxide on the titanium oxide particles may be performed by mixing the titanium oxide particles with a copper nitrate aqueous solution and then heating in the presence of oxygen.
The supporting of the copper oxide on the titanium oxide particles may be performed by mixing the titanium oxide particles with a copper sulfate aqueous solution and then heating in the presence of oxygen.
Moreover, you may perform the said carrying | support of the said copper oxide to the said titanium oxide particle by mixing a copper oxide powder with the said titanium oxide particle, and heating in oxygen presence.

  According to the method of the present invention, oxygen can be generated even when no promoter is supported on the titanium oxide photocatalyst side. Compared with the conventionally known method of adding sodium carbonate, the efficiency is about 3 times higher. If a promoter based on platinum or copper is supported, the efficiency is further increased. When the method of the present invention is used for a photocatalyst carrying a cocatalyst based on platinum or copper, efficiency as high as 3 to 24 times or more is obtained as compared with the conventional sodium carbonate addition method. Therefore, according to the present invention, oxygen can be obtained with higher efficiency compared to the prior art. Furthermore, since the present invention has a relatively high efficiency even without a cocatalyst, the oxygen production efficiency is higher than that of the prior art. It is also possible to implement using

It is a figure which shows the oxygen production amount of the Example and comparative example of this invention. Plot 1-3 respectively _TiO 2 + NaNO ₃ aqueous solution in the figure, the result of the _TiO 2 + NaNO ₃ aqueous solution was supported CuO _x, and _TiO 2 + NaNO ₃ solution carrying Pt (Examples 1, 2, 3) There, are each _TiO 2 + _Na 2 CO ₃ aq _{6~10, TiO 2 + Na 2 CO} 3 aqueous solution was supported _{CuO x, TiO 2 + Na 2} CO 3 aq carrying Pt, TiO ₂ + pure water, and Pt It is the result of carried TiO ₂ + pure water (Comparative Examples 6 to 10). The experimental conditions were that irradiation was performed with a 300 W xenon lamp, the TiO ₂ particle size was about 50 μm or less, and most were about 0.5 to 3 μm. However, 2 and 7 are lumps in which several particles of 0.5 to 3 μm are continuous due to sintering in the manufacturing process, and the average particle size is larger than other photocatalyst particles, that is, the surface area is slightly smaller. . All use catalyst amount 0.5g. The NaNO ₃ concentration was 1.66 mol / liter, and the Na ₂ CO ₃ concentration was 1.66 mol / liter. It is a figure which shows the oxygen production amount of the Example of this invention. Plots 3 to 5 in the figure show cases where TiO ₂ supporting Pt is dispersed in an aqueous NaNO ₃ solution having concentrations of 1.66, 3.32, and 4.98 mol / liter, respectively (Examples 3, 4, and 5). ) Result. The experimental conditions were that irradiation was performed with a 300 W xenon lamp, the TiO ₂ particle size was about 50 μm or less, and most were about 0.5 to 3 μm. In all cases, the amount of catalyst used was 0.5 g.

  In the method of this method, the efficiency of oxygen generation is further increased by dissolving sodium nitrate in water. This method is about 3 times more efficient than the above known method even when no cocatalyst is added to the photocatalyst side, and 3 to 24 times more efficient when a promoter based on platinum or copper is added. Has been confirmed.

Since the present invention is a reaction established by a delicate balance between the photocatalyst base material (TiO ₂ ), the solution and the electronic structure constructed by the cocatalyst, does the desired reaction occur except in the combinations described in the present specification? Or, the production efficiency is greatly reduced. The use of nickel oxide NiO _x as a co-catalyst is frequently performed by those skilled in the art, but it has been found as a result of experiments that NiO _x shows little activity against a sodium nitrate solution (Comparative Example 11). ). Moreover, even if silver (Ag) is used as a promoter, the effect is low (Comparative Example 12).

In addition, it has been reported that oxygen production reactions using photocatalysts have been investigated for carbonates of K, Na, Li and sodium hydrogen carbonate, and sodium carbonate is the best among them. . Sodium hydroxide, sodium chloride, sodium sulfate, sodium phosphate, and sodium hydrogen phosphate were also investigated, but their activity has been reported to be low (Non-patent Document 1). The inventor of the present application also examined water glass (sodium silicate, Na ₂ O · nSiO ₃ , n = 2.2), but was not active (Comparative Example 13).

  However, by using the method of the present invention using sodium nitrate, as shown in Table 1, a much higher oxygen generation rate can be achieved as compared with the case of sodium carbonate (Comparative Examples 6 to 8). Has been demonstrated. Moreover, as far as the inventors of the present application have examined, sodium nitrate is the best among the nitrates (including nitric acid) (Comparative Examples 14 to 19).

  Moreover, about the density | concentration of sodium nitrate, it is not limited to what was illustrated in the Example, There exists an effect in the range to 0 to a saturated concentration.

In the following examples, a 300-mesh TiO ₂ powder was used from the viewpoint of availability and experimental workability. From the nature of the catalyst, the larger the surface area, the faster the reaction rate. Therefore, it is expected that higher oxygen generation efficiency can be achieved by using finer TiO ₂ particles.

  Below, in addition to the Example of this invention, the experimental result which performed oxygen production | generation according to a prior art is shown as a comparative example. The experiments were conducted with the experimental conditions such as irradiation conditions as uniform as possible so that these examples and comparative examples could be compared with each other.

[Example 1]
Sodium nitrate NaNO ₃ was dissolved in 220 cc of pure water, and the concentration was adjusted to 1.66 mol / liter. In the solution prepared as described above, 0.5 g of commercially available (Furuuchi Chemical, TIC-72208B, 99.99%) TiO ₂ powder (300 mesh, titanium oxide having a rutile structure) was suspended as a photocatalyst, and 300 W was added. The light from the xenon lamp was irradiated. Then, 63 micromole of oxygen was generated in 7 hours and 3 minutes.

[Comparative Example 6]
Conventionally, a method of dissolving sodium carbonate Na ₂ CO ₃ is known as a method of increasing the oxygen generation efficiency by dissolving a salt. As a comparative example, this conventional method was tested as follows. Specifically, first, sodium carbonate Na ₂ CO ₃ was dissolved in 220 cc of pure water, and the concentration was adjusted to 1.66 mol / liter. In the solution thus prepared, 0.5 g of the same TiO ₂ powder as in Example 1 was suspended and irradiated with light from a 300 W xenon lamp. As a result, only 22 micromole of oxygen was produced in 6 hours and 34 minutes, which was approximately 1/3 of Example 1.

[Example 2]
Sodium nitrate NaNO ₃ was dissolved in 220 cc of pure water, and the concentration was adjusted to 1.66 mol / liter. In the solution thus prepared, 0.5 g of a photocatalyst in which copper oxide CuO _x is supported on the same TiO ₂ powder as in Example 1 by an impregnation method (details will be described later) is suspended, and light from a 300 W xenon lamp is suspended. Irradiated. As a result, 67 micromole of oxygen was generated in 6 hours and 56 minutes.

The support on the co-catalyst CuO _x was 480 ° C. obtained by adding (1) an aqueous solution in which 1.61 g of copper nitrate (Cu (NO ₃ ) ₂ .3H ₂ O) was dissolved and added to 799 g of TiO ₂ powder. And then heat-treated in air for 2 hours, followed by further heat treatment in air at 680 ° C. for 2 hours. However, the heat treatment conditions and the charged amounts have a wide range, and are not limited to these supporting conditions. Copper nitrate is known to have several hydrates and anhydrides, any of which may be used. In addition to the method of supporting CuO _x , (2) TiO ₂ powder mixed with copper oxide (copper oxide such as CuO or Cu ₂ O) powder in a mortar at about 400 to 700 ° C. in the air. It may be supported by heat treatment, or (3) TiO ₂ powder mixed with copper sulfate (CuSO ₄ or its hydrate) aqueous solution is heat-treated at 400 to 700 ° C. in air. May be supported.
Even if a compound of copper oxide and titanium oxide is formed on the surface of titanium oxide, there is an effect. For example, a substance in which a part of titanium atoms of titanium oxide is replaced with copper atoms (doped) may be supported. In any case, it is preferable that a portion where the number of Cu atoms is distributed at a ratio of about 1 to 15% with respect to the number of Ti atoms is present in the photocatalyst carrying the promoter. In (2) and (3), the same amount as that of (1) in the amount charged when the promoter is supported (the ratio of the number of Cu atoms to the number of Ti atoms) exhibits substantially the same performance as (1).

  In any case, since heat treatment is performed at a relatively high temperature, sintering occurs, and the surface area of the catalyst powder is smaller than in the case of Example 1 and Comparative Example 6 described above. Therefore, if the same surface area is compared, the amount of oxygen produced in this example should be larger, and therefore the photocatalyst used in Example 2 is much more efficient if the particle size is the same as in Example 1. It is considered to be.

[Comparative Example 7]
Sodium carbonate Na ₂ CO ₃ was dissolved in 220 cc of pure water to adjust the concentration to 1.66 mol / liter. In the solution thus prepared, 0.5 g of a photocatalyst in which copper oxide CuO _x is supported on the same TiO ₂ powder as in Example 1 by the same process as in Example 2 is suspended and irradiated with light from a 300 W xenon lamp. Then, only about 3 micromole of oxygen was generated even after irradiation for about 7 hours. Since CuO _x is supported under the same conditions as in Example 2, this comparative example can be directly compared with Example 2.

[Example 3]
Sodium nitrate NaNO ₃ was dissolved in 220 cc of pure water, and the concentration was adjusted to 1.66 mol / liter. A photocatalyst in which platinum Pt was supported on the same TiO ₂ powder as in Example 1 by the photo-deposition method in the solution thus prepared was prepared.

0.5 g of the photocatalyst prepared as described above was suspended in the NaNO ₃ aqueous solution prepared above, and irradiated with light from a 300 W xenon lamp. As a result, 136 micromole of oxygen was generated in 5 hours and 24 minutes. This is a generation efficiency approximately 7.5 times that of Comparative Example 6. In this photocatalyst, platinum was supported by the photo-deposition method, and thus no heat treatment was performed. Therefore, sintering does not occur in the TiO ₂ powder. Therefore, since the surface area of the catalyst powder is the same as in Example 1 and Comparative Example 6, the results of Example 3 can be directly compared with these Examples and Comparative Examples.

  The above-mentioned “photodeposition method” is also called “photoplating”, and was specifically performed as follows.

50 ml of methanol was added to 220 ml of pure water, and 5.5 g of TiO ₂ powder was added to this solution and suspended. Hexachloroplatinic acid _{(H 2 PtCl 6 · 6H 2} O) 1.0g was prepared which was dissolved in pure water 100 g, and this solution was added 7.37ml in suspension. This was irradiated with light from a 300 W xenon lamp for about 2 to 3 hours. However, the amount of platinum carried is not limited to this.

In addition, platinum can be supported by mixing a platinum compound such as hexachloroplatinic acid and titanium oxide powder and heating in air at about 400 to 700 degrees Celsius for 1 to 2 hours.

[Example 4]
Sodium nitrate NaNO ₃ was dissolved in 220 cc of pure water to adjust the concentration to 3.32 mol / liter. In addition, a photocatalyst in which platinum Pt was supported on the same TiO ₂ powder as in Example 1 by the photo-deposition method in the same process as in Example 3 was prepared in the solution thus prepared.
0.5 g of the photocatalyst prepared as described above was suspended in the NaNO ₃ aqueous solution prepared above, and irradiated with light from a 300 W xenon lamp. As a result, about 450 micromole of oxygen was generated in about 5 hours. This is a generation efficiency of about 20 times that of Comparative Example 6. In this photocatalyst, platinum was supported by the photo-deposition method, and thus no heat treatment was performed. Therefore, sintering does not occur in the TiO ₂ powder. Therefore, since the surface area of the catalyst powder is the same as in Examples 1 and 3 and Comparative Example 6, the results of Example 4 can be directly compared with these Examples and Comparative Examples.

[Example 5]
Sodium nitrate NaNO ₃ was dissolved in 220 cc of pure water, and the concentration was adjusted to 4.98 mol / liter. In addition, a photocatalyst in which platinum Pt was supported on the same TiO ₂ powder as in Example 1 by the photo-deposition method in the same process as in Example 3 was prepared in the solution thus prepared.
0.5 g of the photocatalyst prepared as described above was suspended in the NaNO ₃ aqueous solution prepared above, and irradiated with light from a 300 W xenon lamp. As a result, about 350 micromole of oxygen was generated in about 5 hours. This is a generation efficiency about 20 times that of Comparative Example 6. In this photocatalyst, platinum was supported by the photo-deposition method, and thus no heat treatment was performed. Therefore, sintering does not occur in the TiO ₂ powder. Therefore, since the surface area of the catalyst powder is the same as in Examples 1, 3, 4 and Comparative Example 6, the results of Example 5 can be directly compared with these Examples and Comparative Examples. From the results of Examples 3 to 5, it was found that there is an optimum value for the concentration of sodium nitrate. However, the concentration depends on the type of promoter and so on, and can be appropriately adjusted according to the production conditions of the photocatalyst. desirable.

[Comparative Example 8]
Sodium carbonate Na ₂ CO ₃ was dissolved in 220 cc of pure water to adjust the concentration to 1.66 mol / liter. In the solution thus prepared, 0.5 g of a photocatalyst in which platinum Pt was supported on the same TiO ₂ powder as in Example 1 by the same process as in Example 3 was suspended, and irradiated with light from a 300 W xenon lamp. As a result, oxygen was hardly generated even after irradiation for about 7 hours. As in Example 3, since platinum was supported by the photo-deposition method, no heat treatment was performed. Therefore, no sintering has occurred. Therefore, since the surface area of the catalyst powder is the same as in Examples 1, 3, 4, 5 and Comparative Example 6, the results of Comparative Example 8 can be directly compared with these Examples and Comparative Examples.

[Comparative Example 9]
In 220 cc of pure water, 0.5 g of the same TiO ₂ powder as in Example 1 was suspended and irradiated with light from a 300 W xenon lamp. As a result, oxygen was hardly generated even after irradiation for about 7 hours. Since the powder surface area in this comparative example is the same as that of Examples 1, 3, 4, 5 and Comparative Examples 6 and 8, it can be made relatively directly with them.

[Comparative Example 10]
In 220 cc of pure water, 0.5 g of a photocatalyst in which platinum Pt was supported in the same TiO ₂ powder as in Example 1 by the same process as in Example 3 was suspended, and irradiated with light from a 300 W xenon lamp. As a result, oxygen was hardly generated even after irradiation for about 7 hours. Since the powder surface area in this comparative example is the same as those in Examples 1, 3, 4, and 5 and Comparative Examples 6, 8, and 9, it can be directly compared with them.
The oxygen generation amount (time accumulation value) of Examples 1 to 3 and Comparative Examples 6 to 10 described above is shown in FIG. Moreover, in FIG. 2, the oxygen generation amount (time accumulation value) of Examples 3-5 is shown. 1 and 22, the horizontal axis represents the elapsed time from the start of light irradiation, and the vertical axis represents the time cumulative value of the oxygen generation amount corresponding to the elapsed time.

In addition to the above-described Examples and Comparative Examples, experiments for additional Comparative Examples were also conducted with the same experimental conditions. Specifically, experiments using substances other than platinum and copper oxide as cocatalysts supported on TiO ₂ (Comparative Examples 11 and 12), and experiments using water glass instead of sodium nitrate (Comparative Example 13) In addition, experiments in which the cation of sodium nitrate was replaced with other than Na (Comparative Examples 14 to 19) were performed and verified, and in each case, the oxygen generation rate was significantly lower than that of the Examples, or an experiment of 6 hours or more. However, it was confirmed that the occurrence could not be observed. Further, in experiments using silver phosphate synthesized by reacting silver oxide and phosphoric acid solution (Ag 3 _{PO 4)} (Comparative Example 20), not reach its efficiency Examples 3-5, also, phosphoric acid The chemical instability of silver was confirmed (silver metal was deposited). These results are summarized in Table 1 below.

In the examples and comparative examples listed above, the concentration of nitrate groups in the aqueous solution having nitrate groups was adjusted to 1.66 mol / liter. However, Example 4 is 3.32 mol / liter, and Example 5 is 4.98 mol / liter. The amount of TiO ₂ powder was 0.5 g, and the amount of pure water was 220 ml. In the “photocatalyst system” column, () represents a photocatalyst. In Comparative Example 13, the concentration of Na ₂ O · 2.2SiO ₃ (water glass) was 0.76 mol / liter.

  According to the present invention, light energy such as sunlight can be stored in the form of simple oxygen, and therefore it is expected to be used in the field of utilizing natural energy.

“Photochemical Energy Conversion-Fundamentals and Applications-” Editor Masao Kaneko IP Publishing, pp. 289-290, and p. 294, published on June 10, 1997. K. Sayama and H. Arakawa, "Remarkable Effect of Na2CO3Addition on Photodecomposition of Liquid Water into H2 and O2from Suspension of Semiconductor Powder Loaded with Various Metals", Chem. Lett. Vol. 21, No. 2, pp. 253-256, (1992). Zhiguo Yi, Jinhua Ye, Naoki Kikugawa, Tetsuya Kako, Shuxin Ouyang, Hilary Stuart-Williams, Hui Yang, Junyu Cao, Wenjun Luo, Zhaosheng Li, Yun Liu and Ray L. Withers, "An orthophosphate semiconductor with photooxidation properties under visible- light irradiation ", Nature Materials, Vol. 9, Issue 7, pp. 559-564, (2010).

Claims (7)

  An oxygen generation method in which light is applied to an aqueous solution of sodium nitrate in which titanium oxide particles are suspended.   The oxygen generation method according to claim 1, wherein the titanium oxide particles are supported with platinum or copper oxide, or doped with copper element.   The oxygen generation method according to claim 2, wherein the platinum is supported on the titanium oxide particles by a photo-deposition method or by heat-treating the titanium oxide particles mixed with a platinum compound.   The oxygen generation method according to claim 2, wherein the supporting of the copper oxide on the titanium oxide particles is performed by heat-treating the titanium oxide particles mixed with a copper compound.   The oxygen generation method according to claim 4, wherein the supporting of the copper oxide on the titanium oxide particles is performed by mixing the titanium oxide particles with an aqueous copper nitrate solution and then heating in the presence of oxygen.   The oxygen generation method according to claim 4, wherein the supporting of the copper oxide on the titanium oxide particles is performed by mixing the titanium oxide particles with a copper sulfate aqueous solution and then heating in the presence of oxygen.   The oxygen generation method according to claim 4, wherein the supporting of the copper oxide on the titanium oxide particles is performed by mixing the titanium oxide particles with copper oxide powder and then heating in the presence of oxygen.