JP4496444B2 - Photocatalyst, method for producing the same, and method for decomposing hydrogen sulfide using the photocatalyst - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention is used in the chemical industry field that produces a useful chemical substance using a photocatalyst, and in the environmental conservation field that removes malodorous substances and air pollutants using a photocatalyst. To produce sulfur and sulfur, a method for producing the photocatalyst, and the photocatalystTheHydrogen sulfideDecompositionIt is about the method.
[0002]
[Prior art]
The application of photocatalyst technology promotes various chemical reactions such as decomposition of environmental pollutants, malodorous components and bacteria, so it can be used for antibacterial tiles and antibacterial / deodorizing filters for air cleaners. Has begun.
On the other hand, there are studies aimed at obtaining hydrogen by causing photocatalysts to act on water and the like, but the application of photocatalytic technology does not stop there. It is also possible to obtain a useful chemical substance by causing a photocatalyst to act on a harmful substance. For example, it may be applied to a crude oil desulfurization process.
[0003]
FIG. 11 shows a crude oil desulfurization process which is generally performed at present.
As shown in FIG. 11, when crude oil is distilled, heavy naphtha is hydrogenated and all sulfur components contained in the crude oil are recovered as hydrogen sulfide. This hydrogen sulfide passes through a process called the Claus method and oxidizes and recovers sulfur. The Claus method is a process in which one third of hydrogen sulfide is oxidized to sulfurous acid gas, and this is reacted with the remaining hydrogen sulfide to form elemental sulfur.
[0004]
In this process, enormous energy is required not only for the catalytic reaction of sulfurous acid gas and hydrogen sulfide, but also for repeated heating and condensation. In addition, there is a problem such as costly management of sulfurous acid gas.
[0005]
A photocatalyst is added to alkaline water in which hydrogen sulfide is dissolved, and ultraviolet light is irradiated. The free water and free holes generated by the photocatalyst are absorbed by the ultraviolet light, and the alkaline water in which hydrogen sulfide is dissolved is oxidized and reduced. If a method for obtaining hydrogen and sulfur by decomposing hydrogen sulfide with a photocatalyst and producing hydrogen and sulfur can be put to practical use, hydrogen sulfide which is a harmful substance can be decomposed with much less energy, and hydrogen and sulfur which are useful substances Can be produced. That is, it contributes to solving environmental problems and can produce useful substances at low cost.
[0006]
However, conventional photocatalysts have the following problems to be solved in order to use them for the purpose of decomposing hydrogen sulfide and obtaining hydrogen and sulfur.
First, the catalytic activity is low.
Second, the photocatalyst is toxic.
Third, it uses a promoter such as a noble metal and is expensive.
[0007]
When the photocatalyst is irradiated with light, free electrons and free holes are generated, but the probability of recombination is high, and the probability that the chemical substance decomposed by the redox reaction will recombine and return to the original compound. The catalytic activity is low. For this reason, conventionally, a decrease in catalytic activity is prevented by supporting a noble metal on a part of the surface of the photocatalyst. For example, a titanium oxide TiO 2 photocatalyst carries platinum (Pt) on a part of its surface to increase the catalytic activity, but this makes the catalyst expensive.
[0008]
Fourth, the life of the catalyst is short.
When photocatalyst is irradiated with light, free electrons and free holes are generated, but due to its strong redox reaction, the catalyst itself is oxidized and reduced in addition to the target chemical substance and dissolved, resulting in loss of catalytic action. There is a problem. For this reason, a substance called a sacrificial reducing agent is generally used to prevent dissolution of the catalyst, but a combination of a photocatalyst and a sacrificial reducing agent having a practically sufficient lifetime has not yet been obtained.
[0009]
[Problems to be solved by the invention]
As described above, conventional photocatalysts have problems to be solved such as low catalytic activity, toxicity, high cost, and short life, and environment such as production of useful chemicals and removal of air pollutants. It is still insufficient for use for conservation purposes.
[0010]
  Therefore, in view of the above problems, the present invention uses a new photocatalyst having high catalytic activity, non-toxicity, low cost and long life, its production method, and its photocatalyst.TheHydrogen sulfideDecompositionIt aims to provide a method.
[0011]
[Means for Solving the Problems]
  In order to achieve the above object, the invention of the photocatalyst of the present invention is:Having a particle size of 5 to 10 nm made of zinc sulfideCompound semiconductor fine particles gather in layers,An outer shell composed of this layered assembly and a cavity, and the outer shell has a hole;In the thickness direction of this layer,Zinc (Zn) and sulfur (S), components of zinc sulfideThe photocatalyst is characterized in that the component ratio ofThePreferably, the outer shell has a capsule shape or a spherical shape.
[0013]
  Furthermore, to produce this photocatalyst,Made of zinc oxideDissolve the oxide particles in an aqueous solution containing sulfur ions,Generated zincOf sulfide fine particlesOn the oxide particles made of zinc oxidePrecipitate and manufacture.
[0014]
This photocatalyst production method is optimally a photocatalyst production method comprising a step of mixing zinc oxide fine particles and an aqueous sodium sulfide solution, and a step of stirring this mixed solution. Furthermore, it is a method for producing a photocatalyst comprising a step of mixing zinc oxide fine particles and an aqueous sodium sulfide solution, and a step of stirring the mixture while irradiating ultraviolet rays. Furthermore, the production of a photocatalyst comprising a step of mixing zinc oxide fine particles and an aqueous sodium sulfide solution, a step of stirring while bubbling hydrogen sulfide into the mixed solution, and a step of stopping the hydrogen sulfide gas and further stirring for a predetermined time. Is the method. Furthermore, it is a method for producing a photocatalyst comprising a step of mixing zinc oxide fine particles and an aqueous sodium sulfide solution, and a step of irradiating the mixture with ultraviolet rays and bubbling hydrogen sulfide.
[0016]
  Of this inventionMade of zinc sulfidephotocatalystOf hydrogen sulfide using waterThe method comprises the following steps using the photocatalyst of the present invention.
(A) A step of dissolving hydrogen sulfide in an aqueous caustic soda solution.
(B) In the solutionMade of zinc sulfideA process of collecting hydrogen gas by adding a photocatalyst and irradiating it with ultraviolet rays.
(C) A step of recovering sulfur from the solution after the step (B).
(D) A step of reusing the solution after step (c) as the aqueous caustic soda solution of step (a).
[0017]
Next, features of the photocatalyst according to the present invention will be described.
The zinc sulfide fine particle layer, which is a photocatalyst according to the present invention, has an electric field in the layer thickness direction because the ratio of the number of zinc atoms to the number of sulfur atoms in the zinc sulfide, that is, the component ratio, changes in the thickness direction of the layer. Has occurred. For this reason, free electrons and free holes generated by light irradiation move away from each other. This reduces the recombination of free electrons and free holes, and also separates the reaction site for the oxidation reaction and the reaction site for the reduction reaction, thereby reducing the recombination of the oxidation reaction product and the reduction reaction product. .
[0018]
As a result, free electrons and free holes generated by light irradiation are effectively used for the target oxidation-reduction reaction, so that the zinc sulfide fine particle layer of the present invention has high photocatalytic activity.
[0019]
Next, features of the photocatalyst production method of the present invention will be described.
When zinc oxide fine particles as a raw material are mixed with a sodium sulfide aqueous solution and this mixed solution is stirred, zinc oxide reacts with water to become zincate ions and is eluted into the aqueous solution. This zincate ion reacts with sulfur ions in the aqueous sodium sulfide solution to become zinc sulfide. This zinc sulfide is deposited on the surface of zinc oxide particles as a raw material to form a zinc sulfide fine particle layer. When the zinc sulfide fine particle layer is formed, water penetrates the zinc sulfide fine particle layer, reaches the surface of zinc oxide as a raw material, reacts with zinc oxide, and generates zincate ions. The zincate ions diffuse through the zinc sulfide fine particle layer, reach the surface, react with sulfur ions, become zinc sulfide, and deposit on the zinc sulfide fine particle layer. As the zinc sulfide fine particle layer grows, it becomes difficult for zincate ions to diffuse through the zinc sulfide fine particle layer, and zinc sulfide having a composition deviating from the stoichiometric ratio is deposited.
[0020]
Ultraviolet irradiation is effective for increasing the formation rate of the zinc sulfide fine particle layer. That is, if zinc oxide fine particles and an aqueous solution of sodium sulfide are mixed and irradiated with ultraviolet rays, zinc oxide is photodissolved, so that zincate ions are easily eluted and the growth of the zinc sulfide fine particle layer is accelerated.
[0021]
In order to increase the production rate of the zinc sulfide fine particle layer, dissolution of hydrogen sulfide is effective. That is, when hydrogen sulfide is dissolved in a mixed solution of zinc oxide fine particles and water, a weakly acidic solution containing sulfur ions is obtained, and zinc oxide is well dissolved in the weakly acidic solution, so that the growth of the zinc sulfide fine particle layer is similarly accelerated.
[0022]
In order to increase the generation rate of the zinc sulfide fine particle layer, ultraviolet irradiation and hydrogen sulfide dissolution may be combined. That is, if zinc oxide fine particles, sodium sulfide aqueous solution and hydrogen sulfide are mixed, irradiated with ultraviolet rays and stirred, the growth of the zinc sulfide fine particle layer is further accelerated.
[0023]
By these methods described above, the zinc sulfide fine particle layer which is the photocatalyst of the present invention can be produced.
[0024]
Next, the method for using the photocatalyst of the present invention will be described.
When using a zinc sulfide fine particle layer that is a photocatalyst of the present invention, if a sodium sulfide aqueous solution is used as a sacrificial reducing agent, sodium in the sodium sulfide aqueous solution is oxidized in place of zinc in the zinc sulfide fine particle layer. Zinc in the zinc fine particle layer can be prevented from eluting into the aqueous solution, and the photocatalytic properties of the zinc sulfide fine particle layer do not deteriorate.
[0025]
Next, of the photocatalyst use method of the present invention, the characteristics of a method for producing hydrogen and sulfur by photolysis of hydrogen sulfide will be described.
When hydrogen sulfide is dissolved in caustic soda, sodium sulfide and water effective as a sacrificial reducing agent are produced. When the zinc sulfide fine particle layer of the present invention is added to this solution and irradiated with ultraviolet rays, hydrogen gas and sulfur are produced. If hydrogen gas and sulfur are recovered, the solution returns to caustic soda. This solution can be reused in caustic soda for dissolving hydrogen sulfide.
[0026]
If this method is used, hydrogen sulfide can be decomposed and hydrogen and sulfur can be obtained without requiring anything other than energy necessary for the ultraviolet light source.
[0027]
DETAILED DESCRIPTION OF THE INVENTION
The preferred embodiments of the photocatalyst, the method for producing the same, and the method for using the photocatalyst according to the present invention will be described below with reference to FIGS.
[0028]
Among the photocatalysts of the present invention, the structure of a photocatalyst in which the compound semiconductor is a II-VI group compound semiconductor and the II-VI group compound semiconductor is zinc sulfide, that is, a zinc sulfide fine particle layered substance will be described with reference to FIG. .
FIG. 1 is a structural schematic diagram of a zinc sulfide fine particle layered material which is a photocatalyst according to the present invention. FIG. 1A is a schematic diagram of the external structure of a zinc sulfide fine particle layered material, and FIG. 1B is a schematic diagram of a cross section in the direction indicated by the arrow in FIG.
[0029]
The zinc sulfide fine particle layered material which is a photocatalyst according to the present invention has the following features (1) and (2).
Feature (1): As shown in FIG. 1 (B), a zinc sulfide fine particle layered substance that is a photocatalyst according to the present invention comprises an outer shell 1 composed of a zinc sulfide (ZnS) fine particle layer having a particle diameter of 5 nm to 10 nm, The outer shell 1 has a cavity 2 and a hole 3.
Further, the zinc sulfide fine particle layered material has an outer shell reflecting the outer shape of the zinc oxide particles as the raw material.
Feature (2): The zinc sulfide fine particle layer which is a photocatalyst according to the present invention has a structure in which the component ratio of zinc (Zn) and sulfur (S) is changed in the layer thickness direction.
[0030]
Next, the features (1) and (2) of the zinc sulfide fine particle layered material which is a photocatalyst according to the present invention will be described with reference to FIGS.
[0031]
Feature (1) will be described with reference to FIG.
FIG. 2A shows a transmission electron photographed by irradiating an electron beam from the direction perpendicular to the cross section of FIG. 1B to the Y direction of FIG. It is a copy figure of a microscope picture (this photograph is shown as FIG. 8). A black portion 1 and a gray portion 1 seen in FIG. 2A correspond to the edges and surfaces of the rectangular parallelepiped in FIG.
[0032]
As shown in FIG. 2 (A), the black portion 1 and the gray portion 1 are measured by using a micro area EDX (Energy Dispersive X-ray Spectroscopy). As a result, both of the zinc sulfide fine particles having a particle diameter of 5 nm to 10 nm are measured. Confirmed to be a layer. The difference in the intensity of the transmission electron beam between the edge and the surface of the rectangular parallelepiped made of the same material indicates that there is a cavity inside the rectangular parallelepiped.
[0033]
That is, the zinc sulfide fine particle layered substance which is a photocatalyst according to the present invention includes an outer shell 1 composed of a zinc sulfide (ZnS) fine particle layer having a particle diameter of 5 nm to 10 nm and a cavity as shown in the schematic diagram of FIG. 2, and the outer shell 1 has a hole 3.
[0034]
FIG. 2B is a copy of a scanning electron micrograph (this photograph is shown in FIG. 9) of the raw material zinc oxide particles 4 used to produce the zinc sulfide fine particle layered material. As is clear from this figure, the zinc sulfide fine particle layered substance which is a photocatalyst according to the present invention has an outer shape reflecting the shape of the zinc oxide particles used as a raw material. In this example, the shape of the rectangular parallelepiped is shown, but it may be a capsule shape or a spherical shape.
[0035]
In addition, the zinc sulfide fine particle layered material which is the photocatalyst according to the present invention used in the transmission electron micrograph is the same as the zinc sulfide fine particle layered material produced by the production method described in the embodiment of the method for producing the photocatalyst of the present invention. .1 molar sodium sulfide (Na₂S) In addition to the aqueous solution, the photodecomposition reaction was caused by ultraviolet irradiation for 50 hours.
[0036]
The feature (2) will be described with reference to FIG.
FIG. 3A is a copy of a transmission electron micrograph (this photograph is shown in FIG. 10) obtained by photographing the zinc sulfide fine particle layered substance, which is the photocatalyst of the present invention in the process of production, under the same conditions as in FIG. is there. FIG. 3B is a structural schematic diagram in the cross-sectional direction.
The black portion 4 seen in the center of FIG. 3 (A) is confirmed to be zinc oxide as a result of measurement by the minute region EDX, and corresponds to the zinc oxide particles 4 of FIG. 3 (B). The gray strip portion 2 outside the black portion 4 seen in the center of FIG. 3A corresponds to the cavity 2 of FIG. The black portion 1 outside the strip-like gray portion 2 in FIG. 3 (A) is confirmed to be a zinc sulfide fine particle layer as measured by the microscopic region EDX, and corresponds to the outer shell 1 in FIG. 3 (B). . The microscopic region EDX analyzer indicates that the cloud-like substance 5 distributed on the outside of the black portion 1 in FIG. 3A, on the cavity 2 and on the zinc oxide particles 4 is an aggregate of zinc sulfide. It is confirmed by the measurement by
[0037]
As seen in FIG. 3A, the presence of the cavity 2 between the zinc oxide particle 4 and the outer shell 1 made of the zinc sulfide fine particle layer is generated in the formation process described below. It is shown that.
That is, sodium sulfide (Na₂S) In the aqueous solution, zinc oxide (ZnO) is transferred from the surface of the zinc oxide particles 4 to zincate ions (ZnO).^2-Elution). This zincate ion is sodium sulfide (Na₂S) Sulfur ions in aqueous solution (S^2-To zinc sulfide (ZnS). This zinc sulfide is deposited on the surface of the zinc oxide particles 4 to form a zinc sulfide fine particle layer. When the zinc sulfide fine particle layer is formed, water penetrates the zinc sulfide fine particle layer, reaches the surface of the zinc oxide 4 which is a raw material, and reacts with the zinc oxide 4 to generate zincate ions. The zincate ions diffuse in the zinc sulfide fine particle layer, and the zinc sulfide fine particle layer reaches the surface opposite to the surface facing the zinc oxide particles 4, that is, the surface of the zinc sulfide fine particle layer. On this surface, this zincate ion reacts with sulfur ions to become zinc sulfide, which is deposited on the zinc sulfide fine particle layer, and the zinc sulfide fine particle layer grows.
[0038]
Since it is such a generation process, a part of the zinc oxide particles 4 is eluted as a zincate ion and a volume defect is generated, so that a cavity 2 as shown in FIG. 3A is generated.
[0039]
As described above, when one component of a chemical reaction is supplied by diffusion and the reaction product forms this diffusion layer, the supply amount of one component gradually decreases as the diffusion layer grows. A layer with a changed chemical component ratio is formed. That is, the zinc sulfide fine particle layer that is a photocatalyst according to the present invention has a structure in which the component ratio of zinc (Zn) and sulfur (S) is changed in the layer thickness direction.
[0040]
In addition, the cloud-like substance 5 seen in FIG. 3A, that is, an aggregate of zinc sulfide is generated as follows.
At the stage where the zinc sulfide fine particle layer has grown considerably, it becomes difficult for zincate ions to diffuse through the zinc sulfide fine particle layer, so that the pressure of the eluted zincate ion increases inside the zinc sulfide fine particle layer, breaking the zinc sulfide fine particle layer. Erupt. The jetted zincate ions react with surrounding sulfur ions to become cloud-like substances 5 that are aggregates of zinc sulfide. The hole 3 shown in FIG. 1B is made in this way.
[0041]
Next, an embodiment of the method for producing a photocatalyst of the present invention will be described.
The composition of the photocatalyst of the present invention is zinc sulfide (ZnS), which is characterized by being produced from zinc oxide particles. That is, when producing ZnS, a chemical reaction process of a zinc ion solution and hydrogen sulfide is generally used. In contrast, in the present invention, zinc sulfide is generated from zinc oxide particles. A specific manufacturing method will be described below.
[0042]
First embodiment of photocatalyst production method:
Zinc oxide (ZnO) and sodium sulfide (Na₂S) Mix aqueous solution and stir at room temperature. After the above treatment, the membrane is filtered with suction through a nitrocellulose membrane filter (pore size: 0.1 μm) and washed with distilled water, and then dried in a constant temperature bath at 60 ° C.
For example, in the case of 5 g of zinc oxide (purity 99.999%, particle size of about 1 μm), 0.1 mol sodium sulfide (Na₂S • 9H₂O, purity 98%) It is optimal to mix 100 ml of aqueous solution and stir for 15 hours or more.
[0043]
Next, the process in which the zinc sulfide fine particle layer that is the photocatalyst of the present invention is formed by this method will be described.
Zinc oxide particles (ZnO) and sodium sulfide (Na₂S) When an aqueous solution is mixed and this mixed solution is stirred, zinc oxide reacts with water, and zincate ions (ZnO₂ ^2-) In the form of sulfur ions (S^2-) To form zinc sulfide (ZnS), which precipitates on the surface of the zinc oxide particles to form a zinc sulfide fine particle layer. Subsequently, zincate ions generated by the reaction of zinc hydroxide that has permeated the zinc sulfide fine particle layer diffuse into the zinc sulfide fine particle layer that has already been generated, reach the surface of the zinc sulfide fine particle layer, and react with sulfur ions. Thus, zinc sulfide is formed and deposited on the zinc sulfide fine particle layer. As the zinc sulfide fine particle layer grows, it becomes difficult for zincate ions to diffuse through the zinc sulfide fine particle layer, and the zinc sulfide fine particles having a composition with a shifted component ratio of zinc sulfide are deposited. A layered material is formed.
[0044]
Second embodiment of photocatalyst production method:
Put zinc oxide and aqueous sodium sulfide solution in a fused quartz container and irradiate with ultraviolet rays. For example, in the case of 50 mg of zinc oxide (purity 99.999%, particle size of about 1 μm), 5 ml of a 0.1 molar sodium sulfide aqueous solution is mixed, and irradiation with a 500 W ultrahigh pressure mercury lamp for 1 hour or more is optimal.
After the above treatment, the membrane filter is subjected to suction filtration / distilled water washing and dried in a thermostatic bath at 60 ° C.
[0045]
Next, the process in which the zinc sulfide fine particle layer that is the photocatalyst of the present invention is formed by this method will be described. The process of forming the zinc sulfide fine particle layer is the same as that in the first embodiment of the photocatalyst production method. However, when irradiated with ultraviolet rays, zinc oxide is photodissolved and zincate ions are likely to be eluted. The zinc sulfide fine particle layer grows faster.
[0046]
Third embodiment of photocatalyst production method:
Mix zinc oxide particles and distilled water to form hydrogen sulfide (H₂S) Stirring while bubbling with gas, stop hydrogen sulfide gas after a certain time, and further stir for a certain time.
For example, in the case of 2 g of zinc oxide (purity 99.999%, particle size of about 1 μm), 50 ml of distilled water is mixed, bubbled at a hydrogen sulfide gas flow rate of 50 ml / min for about 1 hour, and after stopping the gas, about 12 Stirring over time is optimal.
After the above treatment, the membrane filter is subjected to suction filtration / distilled water washing and dried in a thermostatic bath at 60 ° C.
[0047]
Next, the process in which the zinc sulfide fine particle layered material which is the photocatalyst of the present invention is generated by this method will be described. The process of forming the zinc sulfide fine particle layer is the same as that in the first embodiment of the photocatalyst production method. However, when hydrogen sulfide is dissolved in a mixture of zinc oxide particles and water, a weakly acidic solution containing sulfur ions is formed. Since zinc oxide dissolves well in weakly acidic solutions, the growth of the zinc sulfide fine particle layer is accelerated.
[0048]
Fourth embodiment of photocatalyst production method:
Zinc oxide and sodium sulfide aqueous solution are put into a fused quartz container, bubbled with hydrogen sulfide gas, and simultaneously irradiated with ultraviolet rays. For example, in the case of 2 g of zinc oxide, 50 ml of a 0.1 molar sodium sulfide aqueous solution is mixed and bubbled at a hydrogen sulfide gas flow rate of 50 ml / min. After the above treatment, the membrane filter is subjected to suction filtration / distilled water washing and dried in a thermostatic bath at 60 ° C.
[0049]
In this method, the process of generating the zinc sulfide fine particle layered material which is the photocatalyst of the present invention is the same as that of the first embodiment of the photocatalyst production method. However, the second and third photocatalyst production methods are the same. As described in the embodiment, since zinc oxide is further dissolved and the supply of sulfur ions is increased, the growth of the zinc sulfide fine particle layer is further accelerated.
[0050]
Next, the photocatalytic performance of the photocatalyst of the present invention, that is, the zinc sulfide fine particle layered material photocatalyst (named as a stratified ZnS nanoparticle photocatalyst) will be described. FIG. 4 is a performance comparison result of comparing the amount of hydrogen gas generated by photodecomposing a sodium sulfide aqueous solution under the same apparatus and under the same conditions as a conventional photocatalyst in order to show the performance of the photocatalyst of the present invention.
[0051]
The apparatus used for the performance comparison is shown in FIG. As shown in FIG. 5, this apparatus stores a photoreaction portion 6 made of quartz glass, a hydrogen determination portion 7 for quantifying generated hydrogen, and a sodium sulfide aqueous solution 8 corresponding to the volume of generated hydrogen gas. Thus, a solution reservoir 9 that prevents an increase in hydrogen pressure, a 500 W mercury lamp 10 for irradiating ultraviolet rays, a lens L for condensing the ultraviolet rays 11, and a reflecting mirror 13 for reflecting the ultraviolet rays 11 and irradiating the photocatalyst 12 thereon. It consists of and. At the start of the photolysis reaction, the entire system is filled with the sodium sulfide aqueous solution 8, a certain amount of photocatalyst 12 is precipitated at the bottom of the photoreaction portion 6, the generated gas recovery port 14 is closed, and the 500 W mercury lamp 10 is turned on. The amount of hydrogen generation is measured at a fixed amount of time in the hydrogen determination portion 7.
[0052]
Conventional photocatalysts used for comparison were ZnS (purity 99.999%), CdS (purity 90%), CdSe (purity 99.99%) and TiO.₂(Purity 95%). The photocatalyst of the present invention was produced by the method described in the third embodiment of the photocatalyst production method. The amount of photocatalyst used is 50 mg each. The aqueous sodium sulfide solution has a 0.1 molar concentration of 140 ml.
[0053]
This reaction is represented by the following formula.
Na₂S + H₂O → ← 2Na⁺+ HS^-+ OH^-
HS^-+ 2h⁺  → S + H⁺
2H⁺  + 2e^-  → H₂↑
Where e^-, H⁺Represents free electrons and free holes generated by photocatalysis by light irradiation. “→ ←” represents a chemical equilibrium reaction.
[0054]
As is clear from the graph showing the performance comparison in FIG. 4, the zinc sulfide fine particle layered material (stratified ZnS nanoparticle photocatalyst) which is the photocatalyst of the present invention has much higher catalytic activity than the conventional photocatalyst.
[0055]
As described above, the reason why the catalytic activity is high is that, as described above, since the component ratio of zinc sulfide in the zinc sulfide fine particle layer changes in the layer thickness direction, space charges are generated, and an electric field is generated in the layer thickness direction. This is because. This electric field causes the free electrons and free holes to move away from each other, reducing the recombination of free electrons and free holes, and also separating the reaction site for the oxidation reaction and the reaction site for the reduction reaction. Since the recombination between the product and the reduction reaction product is also reduced, the catalytic activity is increased.
[0056]
Next, the method for using the photocatalyst of the present invention, in which an aqueous sodium sulfide solution is used as a sacrificial reducing agent for the zinc sulfide fine particle layered material (stratified ZnS nanoparticle photocatalyst) that is the photocatalyst of the present invention, will be described.
[0057]
FIG. 6 is a characteristic diagram showing the lifetime of the zinc sulfide fine particle layered material which is the photocatalyst of the present invention. When the apparatus shown in FIG. 5 is used, the photocatalyst of the present invention is added to a 0.1 molar sodium sulfide aqueous solution and irradiated with ultraviolet rays, and the generation of hydrogen gas is completed, that is, all the sulfur ions in the aqueous solution are reduced. When (S) is reached, after measuring the amount of hydrogen gas, the hydrogen gas is discharged, a sodium sulfide aqueous solution is newly added, and photolysis is continued. In FIG. 6, black circles indicate the time when a new sodium sulfide aqueous solution is added, and the numerical value above this indicates the amount of the added sodium sulfide aqueous solution.
[0058]
As is apparent from FIG. 6, the catalytic activity of the zinc sulfide fine particle layered material, which is the photocatalyst of the present invention, does not change even when used for 40 hours. Although not shown in FIG. 6, no change in the photocatalytic activity was observed even after 50 hours of use.
[0059]
The reason for such a long life is that, rather than the dissolution reaction in which zinc sulfide shown in the following formula is oxidized by free holes,
ZnS + 2h⁺→ Zn²⁺+ S
Since the oxidation reaction in which sodium sulfide is oxidized by free holes is more likely to occur,
Na₂S + 2h⁺→ 2Na⁺+ S
This is because sodium sulfide is oxidized in place of zinc sulfide to prevent the zinc sulfide fine particle layered material from eluting.
[0060]
That is, sodium sulfide (Na₂If the S) aqueous solution is used as a sacrificial reducing agent for a photocatalyst composed of a zinc sulfide fine particle layered material, the life of the photocatalyst is extended.
[0061]
Zinc sulfide (ZnS) is not toxic as is well known. Therefore, the zinc sulfide fine particle layered material (stratified ZnS nanoparticle photocatalyst) which is the photocatalyst of the present invention is not toxic.
[0062]
In addition, the zinc sulfide fine particle layered material which is the photocatalyst of the present invention is inexpensive because no precious metal is used as described above.
[0063]
Next, a method for producing hydrogen and sulfur by decomposing hydrogen sulfide using the zinc sulfide fine particle layered material, which is the photocatalyst of the present invention, according to an embodiment of the photocatalyst using method of the present invention will be described.
[0064]
This method comprises the following steps.
(A) A step of dissolving hydrogen sulfide in an aqueous caustic soda solution.
(B) A step of adding a photocatalyst to the solution after the step (a), irradiating with ultraviolet rays, and recovering hydrogen gas.
(C) A step of recovering sulfur from the solution after the step (B).
(D) A step of reusing the solution after step (c) as the aqueous caustic soda solution of step (a).
[0065]
This method will be described with reference to FIG. 7 schematically showing the configuration of the above steps.
In FIG. 7, 15 is an alkali dissolution tank for holding the caustic soda aqueous solution 16 and dissolving hydrogen sulfide gas by bubbling or the like, and is a part for performing the step (a). 17 holds a caustic soda aqueous solution 16 in which hydrogen sulfide is dissolved by the step (a), holds a photocatalyst 12, and has a transparent bottom so that the photocatalyst 12 can be irradiated with ultraviolet rays 11 from the outside, and is generated. This is a photocatalytic reaction tank in which the hydrogen gas to be collected is recovered, and is a part for performing the step (b). Reference numeral 18 denotes a sulfur recovery tank that recovers sulfur (S) from the solution 16 that has undergone the photodecomposition reaction in the photocatalytic reaction tank 17 by the process (b), and is a part that performs the process (c). Reference numeral 10 denotes a light source in which the photocatalyst 12 is disposed so as to be irradiated with ultraviolet rays from outside the photocatalytic reaction tank 17, and 11 is ultraviolet rays.
[0066]
Next, the operation in the configuration of FIG. 7 will be described.
Step (a). When hydrogen sulfide is dissolved in the alkali dissolution tank 15, the sodium hydroxide aqueous solution 16 becomes the sodium sulfide aqueous solution 16 by the reaction shown in the following formula.
2NaOH + H₂S → ← 2Na⁺+ HS^-+ H₂O + OH^-
2Na⁺+ HS^-+ OH^-  → ← Na₂S + H₂O
[0067]
Step (b). When this sodium sulfide aqueous solution 16 is transferred to the photocatalytic reaction tank 17 and irradiated with the photocatalyst 12 by the ultraviolet rays 11 from the light source 10, free electrons and free holes are generated, and the sodium sulfide aqueous solution 16 is oxidized and reduced by the reaction shown in the following formula. , Produces hydrogen gas and sulfur. Hydrogen gas is recovered as soon as it is generated.
Na₂S + H₂O → ← 2Na⁺+ HS^-+ OH^-
HS^-+ 2h⁺  → S + H⁺
2H⁺  + 2e^-  → H₂↑
[0068]
Steps (c) and (d). When the redox reaction is completed, that is, when all the sulfur ions are reduced to sulfur, the sodium sulfide aqueous solution 16 becomes a caustic soda aqueous solution 16 containing sulfur as is apparent from the above formula. This caustic soda aqueous solution 16 is transferred to the sulfur recovery tank 18, and sulfur is recovered. This caustic soda aqueous solution 16, which is free of sulfur, is returned to the alkali dissolution tank 15 and used again as the caustic soda aqueous solution 16 in the step (a).
[0069]
Therefore, according to the embodiment of the method of using the photocatalyst of the present invention, the method of decomposing hydrogen sulfide and producing hydrogen and sulfur using the zinc sulfide fine particle layered substance which is the photocatalyst of the present invention is described above. As described above, hydrogen sulfide, which is an environmentally hazardous substance, is decomposed without requiring any energy other than that required for ultraviolet light sources, and without generating any harmful substances, producing hydrogen and sulfur, which are useful substances. can do.
[0070]
【The invention's effect】
As understood from the above description, the photocatalyst of the present invention has high catalytic activity as a photocatalyst, is not toxic, is inexpensive and has a long life. If the method for decomposing hydrogen sulfide and producing hydrogen and sulfur according to the present invention is used, it can contribute to the solution of environmental problems and can produce useful effects such as being able to produce useful substances at a low cost.
[Brief description of the drawings]
FIG. 1 is a structural schematic diagram showing the structure of a photocatalyst of the present invention.
2A is a copy of an electron micrograph of a photocatalytic structure of the present invention, and FIG. 2B is a copy of an electron micrograph of zinc oxide particles as a raw material.
FIG. 3A is a copy of an electron micrograph showing the process of producing a photocatalyst according to the present invention, and FIG. 3B is a structural schematic diagram thereof.
FIG. 4 is a performance comparison diagram of the photocatalyst of the present invention and a conventional photocatalyst.
5 is a configuration diagram of an apparatus used for performance comparison in FIG. 4;
FIG. 6 is a characteristic diagram showing the lifetime of the photocatalyst of the present invention.
FIG. 7 is a process diagram of a method for producing hydrogen and sulfur by decomposing hydrogen sulfide according to an embodiment of the present invention.
FIG. 8 is an electron micrograph of the photocatalyst of the present invention corresponding to the copy shown in FIG.
FIG. 9 is an electron micrograph of zinc oxide particles that are the raw material of the photocatalyst of the present invention corresponding to the copy shown in FIG. 2 (B).
FIG. 10 is an electron micrograph showing a photocatalyst being generated corresponding to the copy shown in FIG.
FIG. 11 is a process diagram of desulfurization of crude oil according to the prior art.
[Explanation of symbols]
1 Outer shell made of zinc sulfide fine particle layer
2 cavity
3 holes
4 Zinc oxide particles
5 Cloudy material
6 Photoreactive part
7 Hydrogen determination part
8 Sodium sulfide aqueous solution
9 Solution reservoir
10 500W mercury lamp
11 UV
12 Photocatalyst
13 Reflector
14 Generated gas recovery port
15 Alkali dissolution tank
16 Caustic soda aqueous solution, sodium sulfide aqueous solution, or caustic soda aqueous solution containing sulfur
17 Photocatalytic reactor
18 Sulfur recovery tank

Claims (9)

Compound semiconductor fine particles made of zinc sulfide having a particle diameter of 5 to 10 nm are gathered in layers , and have an outer shell and a cavity made of the layered aggregate , and the outer shell has holes. A photocatalyst characterized in that the component ratio of zinc (Zn) and sulfur (S) , which are components of the zinc sulfide , changes in the thickness direction of the layer. The outer shape of the photocatalyst with the outer shell, characterized in that it is a capsule-shaped or spherical shape, according to claim 1 of the photocatalyst. A method for producing a photocatalyst, wherein oxide particles made of zinc oxide are dissolved in an aqueous solution containing sulfur ions, and the generated zinc sulfide is deposited on the oxide particles made of zinc oxide. The method for producing a photocatalyst according to claim 3 , comprising a step of mixing zinc oxide fine particles and an aqueous sodium sulfide solution and a step of stirring the mixed solution. The method for producing a photocatalyst according to claim 3 , comprising a step of mixing zinc oxide fine particles and an aqueous sodium sulfide solution, and a step of stirring the mixture while irradiating with ultraviolet rays. 4. The method according to claim 3 , comprising: a step of mixing zinc oxide fine particles and an aqueous sodium sulfide solution; a step of stirring hydrogen sulfide in the mixed solution; and a step of stopping hydrogen sulfide gas and further stirring for a predetermined time. A method for producing a photocatalyst. The method for producing a photocatalyst according to claim 3 , comprising a step of mixing zinc oxide fine particles and an aqueous sodium sulfide solution, and a step of irradiating the mixture with ultraviolet light and bubbling hydrogen sulfide. Consisting subsequent steps, the method for decomposing hydrogen sulfide with a photocatalyst consisting of zinc sulfide of claim 1, wherein.
(A) a process of dissolving hydrogen sulfide in an aqueous caustic soda solution,
(B) adding a photocatalyst made of zinc sulfide to the solution, irradiating with ultraviolet rays, and recovering hydrogen gas;
(C) recovering sulfur from the solution after step (b);
(D) A step of reusing the solution after step (c) as the aqueous caustic soda solution of step (a). 9. The method for decomposing hydrogen sulfide using a photocatalyst according to claim 8, wherein the outer shape of the photocatalyst is a capsule shape or a spherical shape.

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