JP2001190964A - Photocatalyst, method for producing the same, and method for using photocatalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a new photocatalyst having high catalytic activity, no toxicity, low cost and long life, a method for producing the same, and hydrogen sulfide being decomposed by using the photocatalyst to produce hydrogen and sulfur. Provide a way. SOLUTION: A zinc sulfide fine particle layered material as a photocatalyst has an outer shell 1 composed of a zinc sulfide (ZnS) fine particle layer having a particle diameter of 5 nm to 10 nm, a cavity 2, and the outer shell 1 has a hole. 3 The layered substance of zinc sulfide fine particles has an outer shell reflecting the outer shape of zinc oxide particles as a raw material. Further, the zinc sulfide fine particle layer as the photocatalyst has a structure in which the composition ratio of zinc (Zn) and sulfur (S) changes in the layer thickness direction, and an electric field exists in the layer thickness direction, which is generated by light irradiation. The recombination between free electrons and free holes is reduced, the recombination between oxidation reaction products and reduction reaction products is also reduced, and high catalytic activity can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention is used in the chemical industry field for producing useful chemical substances using a photocatalyst, and in the environmental protection field for removing malodorous substances and air pollutants using a photocatalyst. The present invention relates to a photocatalyst for producing hydrogen and sulfur using hydrogen sulfide, a method for producing the photocatalyst, and a method for producing hydrogen and sulfur from hydrogen sulfide using the photocatalyst.

[0002]

2. Description of the Related Art The application of photocatalytic technology is to promote various chemical reactions such as decomposition of environmental pollutants, odorous components and various germs. Practical application to filters etc. has begun. On the other hand, there is research aimed at obtaining hydrogen by making a photocatalyst act on water or the like, but the application of photocatalyst 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 is conceivable to apply the present invention to a crude oil desulfurization step.

[0003] Fig. 11 shows a crude oil desulfurization step generally performed at present. As shown in FIG. 11, when distilling crude oil, heavy naphtha is hydrogenated to convert all sulfur components contained in the crude oil into hydrogen sulfide and collect it. This hydrogen sulfide is oxidized and recovered by sulfur through a process called the Claus method. The Claus method is a process in which one-third of hydrogen sulfide is oxidized into sulfur dioxide gas, and the remaining hydrogen sulfide is reacted with the sulfur dioxide gas to form elemental sulfur.

In this process, enormous energy is required not only for the catalytic reaction of sulfur dioxide and hydrogen sulfide but also for repeated heating and condensation. In addition, there is a problem that management of sulfurous acid gas is costly.

A photocatalyst is added to alkaline water in which hydrogen sulfide is dissolved, and ultraviolet light is irradiated. The free electrons and free holes generated by the photocatalyst by absorbing the light energy of the ultraviolet light redox the alkaline water in which hydrogen sulfide is dissolved. If a method for obtaining hydrogen and sulfur, that is, a method for decomposing hydrogen sulfide by a photocatalyst and producing hydrogen and sulfur can be put to practical use, hydrogen sulfide, which is a harmful substance, is decomposed with much less energy and is a useful substance. It becomes possible to produce hydrogen and sulfur. That is, it contributes to solving environmental problems and can produce useful substances at low cost.

However, the conventional photocatalyst cannot be used for the purpose of decomposing hydrogen sulfide to obtain hydrogen and sulfur.
There were the following problems to be solved. First, the catalyst activity is low. Second, the photocatalyst is toxic. Third, they use co-catalysts such as precious metals and are expensive.

When the photocatalyst is irradiated with light, free electrons and free holes are generated, but the probability of recombination is high, and the chemical substance decomposed by the redox reaction is recombined to return to the original compound. The probability is high, and the catalytic activity is low. For this reason, a reduction in catalytic activity has been conventionally prevented by supporting a noble metal on a part of the surface of the photocatalyst. For example, a titanium oxide TiO2 photocatalyst carries platinum (Pt) on a part of its surface to increase the catalytic activity, but the catalyst becomes expensive.

Fourth, the life of the catalyst is short. Irradiation of the photocatalyst with light generates free electrons and free holes. The strong redox reaction causes the catalyst itself to redox and dissolve in addition to the target chemical substance. There is a problem. For this reason, generally, a substance called a sacrificial reducing agent is used to prevent dissolution of the catalyst, but a combination of a photocatalyst and a sacrificial reducing agent having a practically sufficient life has not yet been obtained.

[0009]

SUMMARY OF THE INVENTION As mentioned above,
Conventional photocatalysts have problems to be solved, such as low catalytic activity, toxic, expensive, and short lifespan.Therefore, they cannot be used for environmental protection purposes such as production of useful chemicals and removal of air pollutants. Is still inadequate.

In view of the above problems, the present invention provides a new photocatalyst having high catalytic activity, non-toxicity, low cost and long life, a method for producing the same, and a method for decomposing hydrogen sulfide using the photocatalyst to produce hydrogen and sulfur. An object of the present invention is to provide a method for producing the same.

[0011]

Means for Solving the Problems In order to achieve the above object, the invention of a photocatalyst according to the present invention is characterized in that compound semiconductor fine particles are aggregated in a layer and the component ratio of the compound changes in the thickness direction of the layer. Preferably, the compound semiconductor fine particles have a particle size of 5 to 10 nm. Further, the photocatalyst of the present invention has an outer shell made of the above-mentioned layered aggregate, and the outer shell has a hole. Preferably, the shape of the outer shell is capsule or spherical.

Further, the compound semiconductor is composed of a group II element and VI
Consisting of group III elements, most preferably zinc sulfide.

Further, in order to produce the photocatalyst, oxide particles of a group II element constituting the compound semiconductor are dissolved in an aqueous solution containing sulfur ions, and sulfide fine particles of the group II element are precipitated. To manufacture. Preferably, said I
The oxide particles of the Group I element are zinc oxide.

This method for producing a photocatalyst is optimally a method for producing a photocatalyst comprising a step of mixing zinc oxide fine particles and an aqueous solution of sodium sulfide, and a step of stirring this mixture. Further, the present invention provides a method for producing a photocatalyst, comprising a step of mixing zinc oxide fine particles and an aqueous solution of sodium sulfide, and a step of stirring the mixture while irradiating the mixture with ultraviolet rays. Further, a photocatalyst production process comprising the steps of mixing zinc oxide fine particles and an aqueous solution of sodium sulfide, stirring hydrogen sulfide while bubbling the mixture, and stopping hydrogen sulfide gas and further stirring for a certain period of time. Is the way. Further, a step of mixing the zinc oxide fine particles and the aqueous solution of sodium sulfide, and irradiating the mixed solution with ultraviolet rays,
And a step of bubbling hydrogen sulfide.

In the method using the zinc sulfide photocatalyst of the present invention, an aqueous sodium sulfide solution is used as a sacrificial reducing agent.

Among the methods for using the photocatalyst of the present invention, the method for decomposing hydrogen sulfide to produce hydrogen and sulfur uses the aqueous sodium sulfide solution as a sacrificial reducing agent for the photocatalyst using the photocatalyst of the present invention. Preferably, the method comprises the following steps. (A) A step of dissolving hydrogen sulfide in an aqueous solution of caustic soda. (B) a step of adding a photocatalyst to the solution, irradiating the solution with ultraviolet rays, and collecting hydrogen gas. (C) recovering sulfur from the solution after the step (b). (D) a step of reusing the solution after the step (c) as the aqueous caustic soda solution of the step (a).

Next, the 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 a ratio of the number of zinc atoms to the number of sulfur atoms of zinc sulfide in the thickness direction of the layer, that is, the component ratio changes. Has occurred. Therefore, free electrons and free holes generated by light irradiation move in directions away from each other. For this reason, recombination of free electrons and free holes is reduced, and the reaction site of the oxidation reaction is separated from the reaction site of the reduction reaction, so that the recombination between the oxidation reaction product and the reduction reaction product is also reduced. .

As a result, the free electrons and free holes generated by light irradiation are effectively used in the intended oxidation-reduction reaction, so that the zinc sulfide fine particle layer of the present invention has high photocatalytic activity.

Next, the features of the method for producing a photocatalyst of the present invention will be described. When zinc oxide fine particles as a raw material and an aqueous solution of sodium sulfide are mixed, and this mixed solution is stirred, zinc oxide reacts with water to form zincate ions and elutes into the aqueous solution. The zincate ion reacts with sulfur ions in the aqueous sodium sulfide solution to form zinc sulfide. The zinc sulfide precipitates on the surface of zinc oxide particles as a raw material, and forms 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 in the zinc sulfide fine particle layer, reach the surface, react with sulfur ions, become zinc sulfide, and precipitate on the zinc sulfide fine particle layer. As the zinc sulfide fine particle layer grows, zincate ions become difficult to diffuse through the zinc sulfide fine particle layer, and zinc sulfide having a composition deviating from the stoichiometric ratio is deposited.

Ultraviolet irradiation is effective for increasing the formation rate of the zinc sulfide fine particle layer. That is, when the zinc oxide fine particles and the aqueous solution of sodium sulfide are mixed and irradiated with ultraviolet light, zinc oxide causes photodissolution, so that zincate ions are easily eluted and the growth of the zinc sulfide fine particle layer is accelerated.

Dissolution of hydrogen sulfide is effective for increasing the rate of formation of the zinc sulfide fine particle layer. That is, when hydrogen sulfide is dissolved in a mixed liquid of zinc oxide fine particles and water, a weakly acidic solution containing sulfur ions is formed. Since zinc oxide is well dissolved in the weakly acidic solution, the growth of the zinc sulfide fine particle layer is similarly accelerated.

To increase the formation rate of the zinc sulfide fine particle layer, ultraviolet irradiation and dissolution of hydrogen sulfide 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,
Further, the growth of the zinc sulfide fine particle layer is accelerated.

By the above-described methods, a zinc sulfide fine particle layer as the photocatalyst of the present invention can be produced.

Next, the method of using the photocatalyst of the present invention will be described. When using a zinc sulfide fine particle layer as the photocatalyst of the present invention, if an aqueous sodium sulfide solution is used as a sacrificial reducing agent, sodium in the aqueous sodium sulfide solution is oxidized instead of zinc in the zinc sulfide fine particle layer. Elution of zinc in the zinc fine particle layer into the aqueous solution can be prevented, and the photocatalytic property of the zinc sulfide fine particle layer does not deteriorate.

Next, among the methods for using the photocatalyst of the present invention, the features of the 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, which are effective as sacrificial reducing agents, are generated. The zinc sulfide fine particle layer of the present invention is added to this solution, and irradiation with ultraviolet rays generates hydrogen gas and sulfur. Once the hydrogen gas and sulfur are recovered, the solution returns to caustic soda. This solution can be reused in caustic soda to dissolve hydrogen sulfide.

By using this method, hydrogen sulfide can be decomposed and hydrogen and sulfur can be obtained without requiring anything other than the energy required for the ultraviolet light source.

[0027]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGS. 1, 2 and 3, the same or corresponding parts are designated by the same reference numerals, and a preferred embodiment of a photocatalyst according to the present invention, a method for producing the same and a method for using the photocatalyst will be described. The form will be described.

In the photocatalyst of the present invention, the compound semiconductor is
The structure of a photocatalyst that is a II-VI compound semiconductor and the II-VI compound semiconductor is zinc sulfide, that is, the structure of a zinc sulfide fine particle layered material will be described with reference to FIG. FIG. 1 is a schematic structural view 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 the zinc sulfide fine particle layered material, and FIG. 1B is a schematic structural diagram of a cross section in a direction indicated by an arrow in FIG. 1A.

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), the zinc sulfide fine particle layered material which is a photocatalyst according to the present invention has an outer shell 1 composed of a zinc sulfide (ZnS) fine particle layer having a particle diameter of 5 to 10 nm; It has a cavity 2 and the shell 1 has a hole 3. The layered substance of zinc sulfide fine particles has an outer shell reflecting the outer shape of zinc oxide particles as a raw material. Feature (2): The zinc sulfide fine particle layer as the photocatalyst according to the present invention has a structure in which the component ratio of zinc (Zn) and sulfur (S) changes in the layer thickness direction.

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.

The feature (1) will be described with reference to FIG. FIG. 2A shows transmitted electrons obtained by irradiating a layered substance of zinc sulfide fine particles, which is a photocatalyst of the present invention, with an electron beam from a direction perpendicular to the cross section of FIG. 1B in the Y direction of FIG. FIG. 9 is a simulated view of a micrograph (this photograph is shown as FIG. 8). FIG.
The black portion 1 and the gray portion 1 seen in FIG. 1A correspond to the edge and the surface of the rectangular parallelepiped in FIG.

The black portion 1 and the gray portion 1 shown in FIG. 2A are minute regions EDX (Energy Disp.).
rsive X-ray Spectroscopy)
As a result of measurement using
It has been confirmed that the layer is a layer of zinc sulfide fine particles having a particle size of The difference in the intensity of the transmitted electron beam between the edge and the surface of the rectangular parallelepiped made of the same substance indicates that there is a cavity inside the rectangular parallelepiped.

That is, as shown in the schematic diagram of FIG. 1B, the zinc sulfide fine particle layered material which is a photocatalyst according to the present invention has a particle diameter of 5 to 10 nm.
It can be seen that it has an outer shell 1 made of a fine particle layer and a cavity 2, and this outer shell 1 has a hole 3.

FIG. 2B is a schematic drawing of a scanning electron microscope photograph (this photograph is shown in FIG. 9) of the zinc oxide particles 4 used as a raw material for producing the zinc sulfide fine particle layered material. As is clear from this figure, the zinc sulfide fine particle layered material as the photocatalyst according to the present invention has an outer shape reflecting the shape of zinc oxide particles used as a raw material. In this example, the shape of the rectangular parallelepiped is shown, but the shape may be a capsule or a sphere.

The photocatalytic zinc sulfide fine particle layered material used in the transmission electron micrograph was the zinc sulfide fine particle layered material prepared by the production method described in the embodiment of the photocatalyst production method of the present invention. Was added to a 0.1 molar aqueous solution of sodium sulfide (Na ₂ S) to cause a photolysis reaction by irradiation with ultraviolet rays for 50 hours.

The feature (2) will be described with reference to FIG.
FIG. 3A is a simulated view of a transmission electron micrograph (the photograph is shown in FIG. 10) of the zinc sulfide fine particle layered material that is the photocatalyst of the present invention in the process of being produced under the same conditions as in FIG. is there. FIG. 3B is a schematic structural view in the cross-sectional direction. The black portion 4 seen at the center of FIG. 3A has been confirmed to be zinc oxide as a result of measurement using the microscopic region EDX, and corresponds to the zinc oxide particles 4 in FIG. 3B.
The gray strip 2 outside the black portion 4 at the center of FIG. 3A corresponds to the cavity 2 in FIG. 3B. FIG.
It has been confirmed by measurement in the minute region EDX that the black portion 1 outside the band-like gray portion 2 in (A) is a zinc sulfide fine particle layer, and corresponds to the outer shell 1 in FIG. 3 (B). FIG.
(A) The cloud-like substance 5 distributed on the outside of the black portion 1, on the cavity 2, and on the zinc oxide particles 4 is determined to be an aggregate of zinc sulfide by a micro-area EDX analyzer. Has been confirmed by

As shown in FIG. 3A, a cavity 2 is provided between the zinc oxide particles 4 and the outer shell 1 composed of the zinc sulfide fine particle layer.
This indicates that a zinc sulfide fine particle layer is formed in the formation process described below. That is, in an aqueous solution of sodium sulfide (Na ₂ S), zinc oxide (Zn)
O) from the surface of the zinc oxide particles 4
eluting in the form of nO ^2-). The zincate ion reacts with the sulfur ion (S ²⁻ ) of the aqueous solution of sodium sulfide (Na ₂ S) to form zinc sulfide (ZnS). The zinc sulfide precipitates 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 as a raw material, reacts with the zinc oxide 4, and generates zincate ions. This zincate ion diffuses through the zinc sulfide fine particle layer,
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 form zinc sulfide, which is deposited on the zinc sulfide fine particle layer,
The zinc sulfide fine particle layer grows.

In such a production process, a part of the zinc oxide particles 4 is eluted as zincate ions and disappears, resulting in a volume loss, so that the cavity 2 as shown in FIG. Occurs.

As described above, when one component of the chemical reaction is supplied by diffusion, and the reaction product forms the diffusion layer, the supply amount of one component is gradually increased as the diffusion layer grows. And a layer having a changed chemical composition ratio is formed. That is, the zinc sulfide fine particle layer as the photocatalyst according to the present invention has a structure in which the component ratio of zinc (Zn) and sulfur (S) changes in the layer thickness direction.

The cloudy substance 5 shown in FIG. 3A, that is, the aggregate of zinc sulfide is generated as follows. When the zinc sulfide fine particle layer has grown considerably, zincate ions are less likely to diffuse through the zinc sulfide fine particle layer,
Inside the zinc sulfide fine particle layer, the pressure of the eluted zincate ions is increased, and the zinc sulfide ion breaks and ejects the zinc sulfide fine particle layer. The ejected zincate ions react with the surrounding sulfur ions to form cloud-like substances 5 which are aggregates of zinc sulfide. The hole 3 shown in FIG. 1 (B) is formed in this way.

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 in that it is formed from zinc oxide particles. That is, when ZnS is generated, a chemical reaction process of a zinc ion solution and hydrogen sulfide is generally used. On the other hand, in the present invention, zinc sulfide is generated from zinc oxide particles. Hereinafter, a specific manufacturing method will be described.

First Embodiment of Photocatalyst Production Method: Zinc oxide (ZnO) and sodium sulfide (Na
₂ S) Mix the aqueous solution and stir at room temperature. After the above processing,
Nitrocellulose membrane filter (pore size 0.1
After filtration with suction and washing with distilled water, the product is dried in a thermostat 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 (N
(a ₂ S.9H ₂ O, purity 98%) aqueous solution (100 ml) is mixed and stirred for 15 hours or more.

Next, the process of forming a zinc sulfide fine particle layer as the photocatalyst of the present invention by this method will be described.
Zinc oxide particles (ZnO) and sodium sulfide (Na ₂ S)
When the aqueous solution is mixed and this mixture is stirred, zinc oxide reacts with water, elutes in the form of zincate ions (ZnO ₂ ^2- ), and reacts with sulfur ions (S ^2- ) of the sodium sulfide aqueous solution. It becomes zinc sulfide (ZnS) and 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 penetrating the zinc sulfide fine particle layer diffuse through the already generated zinc sulfide fine particle layer, reach the surface of the zinc sulfide fine particle layer, and react with sulfur ions. It becomes zinc sulfide and deposits 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 fine particles having a composition in which the component ratio of zinc sulfide is shifted are precipitated. Layered material forms.

Second Embodiment of Photocatalyst Production Method: Zinc oxide and an aqueous solution of sodium sulfide are placed in a fused quartz container and irradiated with ultraviolet rays. For example, zinc oxide (99.999 purity)
%, Particle size of about 1 μm), 50 mg is best mixed with 5 ml of a 0.1 molar sodium sulfide aqueous solution and irradiated with a 500 W ultra-high pressure mercury lamp for 1 hour or more. After the above treatment, the resultant is subjected to suction filtration and distilled water washing with a membrane filter, and dried in a thermostat at 60 ° C.

Next, the process of forming a zinc sulfide fine particle layer as the photocatalyst of the present invention by this method will be described. The process of forming the zinc sulfide fine particle layer is the same as in the case of the first embodiment of the photocatalyst manufacturing method. However, when ultraviolet irradiation is performed, zinc oxide undergoes photodissolution and zincate ions are easily eluted. As a result, the growth of the zinc sulfide fine particle layer is accelerated.

Third Embodiment of Photocatalyst Production Method: A mixture of zinc oxide particles and distilled water is stirred while bubbling with hydrogen sulfide (H ₂ S) gas. Stir for hours. For example, in the case of 2 g of zinc oxide (purity 99.999%, particle size about 1 μm), 50 ml of distilled water is mixed, and the flow rate of hydrogen sulfide gas is 50 ml / min.
After bubbling for about 1 hour and stopping the gas, stirring for about 12 hours or more is optimal. After the above treatment, the resultant is subjected to suction filtration and distilled water washing with a membrane filter, and dried in a thermostat at 60 ° C.

Next, the process of forming a layered material of fine particles of zinc sulfide as the photocatalyst of the present invention by this method will be described. The process of forming the zinc sulfide fine particle layer is the same as that of the first embodiment of the photocatalyst manufacturing 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. On the other hand, zinc oxide dissolves well in a weakly acidic solution, so that the zinc sulfide fine particle layer grows faster.

Fourth Embodiment of Photocatalyst Manufacturing Method: A zinc oxide and an aqueous solution of sodium sulfide are put in a fused quartz container, bubbling is performed with hydrogen sulfide gas, and ultraviolet irradiation is performed at the same time. For example, when 2 g of zinc oxide is used, 50 ml of a 0.1 molar sodium sulfide aqueous solution is mixed, and a hydrogen sulfide gas flow rate of 50 ml is mixed.
/ Min, and at the same time, irradiation with a 500 W ultra-high pressure mercury lamp for 1 hour or more is optimal. After the above treatment, the resultant is subjected to suction filtration and distilled water washing with a membrane filter, and dried in a thermostat at 60 ° C.

In this method, the process of forming the layered material of the zinc sulfide fine particles, which is the photocatalyst of the present invention, is the same as that of the first embodiment of the photocatalyst manufacturing method, except that the second and third photocatalysts are used. As described in the embodiment of the manufacturing method, the dissolution of zinc oxide is further accelerated, and the supply of sulfur ions is also increased, so that the growth of the zinc sulfide fine particle layer is further accelerated.

Next, the photocatalyst performance of the photocatalyst of the present invention, that is, the photocatalyst of a layered substance of fine particles of zinc sulfide (named as a photocatalyst of stratified ZnS nanoparticle) will be described. FIG. 4 is a performance comparison result in which the photocatalyst of the present invention is subjected to photolysis of an aqueous sodium sulfide solution under the same apparatus and under the same conditions as the conventional photocatalyst, and the amount of hydrogen gas generated is compared.

FIG. 5 shows an apparatus used for performance comparison. FIG.
As shown in the figure, the apparatus comprises a photoreaction section 6 made of quartz glass and a hydrogen quantification section 7 for quantifying generated hydrogen.
And a solution reservoir 9 for preventing an increase in hydrogen pressure by storing a sodium sulfide aqueous solution 8 corresponding to the volume of the generated hydrogen gas.
And a 500 W mercury lamp 10 for ultraviolet irradiation, and ultraviolet 11
And a reflecting mirror 13 for reflecting the ultraviolet light 11 and irradiating the photocatalyst 12 with it. At the start of the photolysis reaction, the entire system is filled with the aqueous sodium sulfide solution 8, a certain amount of the photocatalyst 12 is precipitated at the bottom of the photoreaction portion 6, the generated gas recovery port 14 is closed, and the 500W mercury lamp 10 is turned on. In the hydrogen determination section 7, the amount of generated hydrogen is measured at every constant irradiation time.

The conventional photocatalysts used for comparison are ZnS (purity 99.999%), CdS (purity 90%), CdSe
(Purity 99.99%) and TiO ₂ (purity 95%). The photocatalyst of the present invention was manufactured by the method described in the third embodiment of the photocatalyst manufacturing method. The amount of photocatalyst used is 50 mg each. The aqueous sodium sulfide solution has a 0.1 molar concentration of 140 ml.

This reaction is represented by the following formula. _{Na 2 S + H 2 O →} ← 2Na + + HS - + OH - HS - + 2h + → S + H + 2H + + 2e - → H 2 ↑ Here, ^e -, h ⁺ is a photocatalyst by light irradiation Represents free electrons and free holes in which. “→ ←” represents a chemical equilibrium reaction.

As is clear from the graph showing the performance comparison of FIG. 4, the zinc sulfide fine particle layered material (the stratified ZnS nanoparticulate photocatalyst), which is the photocatalyst of the present invention, has much higher catalytic activity than the conventional photocatalyst. high.

The reason why the catalytic activity is high is that, as described above, since the zinc sulfide component ratio of the zinc sulfide fine particle layer changes in the layer thickness direction, space charge is generated, and an electric field is generated in the layer thickness direction. Is occurring. This electric field causes 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 sites for oxidation and reduction from oxidation reactions. Since the recombination of the product and the reduction reaction product is also reduced, the catalytic activity is increased.

Next, the method of using the photocatalyst of the present invention using an aqueous solution of sodium sulfide as a sacrificial reducing agent for the zinc sulfide fine particle layered material (Stratified ZnS nanoparticle photocatalyst) which is the photocatalyst of the present invention will be described.

FIG. 6 is a characteristic diagram showing the life of the zinc sulfide fine particle layered material which is the photocatalyst of the present invention. Using the apparatus shown in FIG. 5, the photocatalyst of the present invention was added to a 0.1 molar sodium sulfide aqueous solution, and the mixture was irradiated with ultraviolet light to completely generate hydrogen gas, that is, when all the sulfur ions in the aqueous solution were reduced and the sulfur When (S) is reached, the hydrogen gas is discharged after measuring the amount of the hydrogen gas, and a new aqueous solution of sodium sulfide is added to continue the photolysis. In FIG. 6, the black circles indicate the point in time at which a new sodium sulfide aqueous solution is added, and the numerical value above this indicates the amount of the added sodium sulfide aqueous solution.

As is clear from FIG. 6, the photocatalyst of the present invention, the zinc sulfide fine particle layered material, does not change its catalytic activity even after being used for 40 hours. Although not shown in FIG. 6, no change in photocatalytic activity was observed even after use for 50 hours or more.

The reason why the lifetime is long is that ZnS + 2h ⁺ → Zn ²⁺ + S, as shown in the following equation, oxidizes sodium sulfide by free holes rather than the dissolution reaction in which zinc sulfide shown below is oxidized by free holes. Na ₂ S + 2h ⁺ → 2Na ⁺ + S In place of zinc sulfide, sodium sulfide is oxidized to prevent the zinc sulfide fine particle layered material from being eluted.

That is, when an aqueous solution of sodium sulfide (Na ₂ S) is used as a sacrificial reducing agent for a photocatalyst composed of a layered material of zinc sulfide fine particles, the life of the photocatalyst is prolonged.

As is well known, zinc sulfide (ZnS) has no toxicity. Therefore, the zinc sulfide fine particle layered material (Stratified ZnS) which is the photocatalyst of the present invention is used.
Nanoparticulate photocatalyst) is non-toxic.

Since the zinc sulfide fine particle layered material as the photocatalyst of the present invention does not use a noble metal as described above,
It is cheap.

Next, a method for producing hydrogen and sulfur by decomposing hydrogen sulfide using the layered material of fine particles of zinc sulfide as the photocatalyst according to the embodiment of the photocatalyst use method of the present invention will be described. .

This method comprises the following steps. (A) A step of dissolving hydrogen sulfide in an aqueous solution of caustic soda. (B) A step of adding a photocatalyst to the solution after the step (a), irradiating the solution with ultraviolet rays, and collecting hydrogen gas. (C) recovering sulfur from the solution after the step (b). (D) a step of reusing the solution after the step (c) as the aqueous caustic soda solution of the step (a).

This method will be described with reference to FIG. 7, which schematically shows the structure of the above steps. In FIG. 7, reference numeral 15 denotes an alkali dissolution tank for holding a caustic soda aqueous solution 16 and dissolving hydrogen sulfide gas by bubbling or the like.
This is the part where the process is performed. 17 holds the caustic soda aqueous solution 16 in which hydrogen sulfide is dissolved in the step (a),
The photocatalyst 12 is held, and ultraviolet light 1
1 is a photocatalytic reaction tank having a transparent bottom surface so as to be able to irradiate 1, and collecting the generated hydrogen gas;
This is a part for performing the step (b). Reference numeral 18 denotes the solution 1 in which the photolysis reaction has been completed in the photocatalyst
It is a sulfur recovery tank that recovers sulfur (S) from 6,
This is the part where the step (c) is performed. 10 is the photocatalyst 12
Is a light source arranged to irradiate ultraviolet light from outside the photocatalytic reaction tank 17, and 11 is ultraviolet light.

Next, the operation in the configuration of FIG. 7 will be described. Step (a). In the alkali dissolution tank 15,
When hydrogen sulfide is dissolved, the aqueous caustic soda solution 16 becomes an aqueous sodium sulfide solution 16 by a reaction represented by the following equation. _{2NaOH + H 2 S → ← 2Na} + + HS - + H 2
^{O + OH - 2Na + + HS} - + OH - → ← Na 2 S + H 2 O

Step (b). When the aqueous sodium sulfide solution 16 is transferred to the photocatalyst reaction tank 17 and the photocatalyst 12 is irradiated with the ultraviolet light 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 a reaction represented by the following formula. Produces hydrogen gas and sulfur. Hydrogen gas is recovered as it is generated. _{Na 2 S + H 2 O →} ← 2Na + + HS - +
^{OH - HS - + 2h + →} S + H + 2H + + 2e - → H 2 ↑

Steps (c) and (d). When the oxidation-reduction reaction is completed, that is, when all the sulfur ions are reduced to sulfur, the sodium sulfide aqueous solution 16 containing sulfur as shown in the above formula is used.
Becomes This caustic soda aqueous solution 16 is supplied to a sulfur recovery tank 18.
Then, the sulfur is recovered, and the aqueous caustic soda solution 16 having no sulfur is returned to the alkali dissolving tank 15 and used again as the aqueous caustic soda solution 16 in the step (a).

Therefore, according to the embodiment of the method for using a photocatalyst of the present invention, the method of decomposing hydrogen sulfide using the zinc sulfide fine particle layered material as the photocatalyst of the present invention to produce hydrogen and sulfur is used. As described above, hydrogen sulfide, which is an environmentally harmful substance, is decomposed without requiring any other energy than the energy required for an ultraviolet light source and without generating any harmful substance, and hydrogen, which is a useful substance, is decomposed. Sulfur can be produced.

[0070]

As will be understood from the above description, the photocatalyst of the present invention has high catalytic activity as a photocatalyst, has no toxicity, is inexpensive and has a long life. The use of the method of the present invention for decomposing hydrogen sulfide to produce hydrogen and sulfur can contribute to solving environmental problems, and can also provide practical effects such as production of useful substances at low cost.

[Brief description of the drawings]

FIG. 1 is a schematic structural view showing the structure of a photocatalyst of the present invention.

FIG. 2A is an electron micrograph of a photocatalyst structure of the present invention, and FIG. 2B is an electron micrograph of zinc oxide particles as a raw material.

FIG. 3A is a simulated view of an electron micrograph showing a process of producing a photocatalyst according to the present invention, and FIG. 3B is a schematic structural view thereof.

FIG. 4 is a performance comparison diagram of the photocatalyst of the present invention and a conventional photocatalyst.

FIG. 5 is a configuration diagram of an apparatus used for performance comparison of FIG. 4;

FIG. 6 is a characteristic diagram showing the life of the photocatalyst of the present invention.

FIG. 7 is a process diagram of a method for decomposing hydrogen sulfide to produce hydrogen and sulfur according to an embodiment of the present invention.

FIG. 8 is an electron micrograph of the photocatalyst of the present invention corresponding to the schematic diagram shown in FIG. 2 (A).

FIG. 9 is an electron micrograph of zinc oxide particles as a raw material of the photocatalyst of the present invention corresponding to the schematic diagram shown in FIG. 2 (B).

10 is an electron micrograph showing a photocatalyst in the course of generation corresponding to the mimic diagram shown in FIG. 3 (A).

FIG. 11 is a diagram of a crude oil desulfurization process according to the prior art.

[Description of Signs] 1 Outer shell composed of zinc sulfide fine particle layer 2 Cavity 3 Hole 4 Zinc oxide particle 5 Cloud material 6 Photoreactive part 7 Hydrogen quantitative part 8 Sodium sulfide aqueous solution 9 Solution reservoir 10 500W mercury lamp 11 Ultraviolet 12 Photocatalyst 13 Reflection Mirror 14 generated gas recovery port 15 alkali dissolving tank 16 caustic soda aqueous solution or sodium sulfide aqueous solution,
Or caustic soda aqueous solution containing sulfur 17 photocatalytic reaction tank 18 sulfur recovery tank

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C02F 1/32 B01D 53/36 ZABJ (72) Inventor Tsunetori Yanagisawa 01 Aoba Aramaki Aoba-ku, Aoba-ku, Sendai City, Miyagi Prefecture. Tohoku University Campus (72) Inventor Takeo Arai 01 Aoba Aramaki, Aoba Ward, Sendai City, Miyagi Prefecture Tohoku University Campus (72) Inventor Shuhei Sakima Aramaki Aoba 01 Aoba Ward, Aoba Ward, Sendai City, Miyagi Prefecture Tohoku Univ. Welfare 01 Aoba, Aramaki, Aoba-ku, Sendai-shi, Miyagi F-term (reference) 4D037 AA15 AB13 BA18 BB09 4D048 AA22 AB01 AB03 BA16X BA46X BB12 BB16 BB17 EA01 4G069 AA02 AA02 AA03 AA08 AA09 BC48AB BCB BCB BCBA BCB BC BD08C BD09A BD10A CA02 CA03 CA07 CA10 CA17 CB81 DA05 EA01X EA01Y EA02X EA02Y EB10 EB18X EB18Y EC29 FB50

Claims (14)

[Claims] 1. A photocatalyst wherein compound semiconductor fine particles are aggregated in a layer, and the component ratio of the compound changes in the thickness direction of the layer. 2. The method according to claim 1, wherein the compound semiconductor fine particles are 5 to 10
2. The photocatalyst according to claim 1, having a particle size of nm. 3. An outer shell comprising the layered assembly,
2. The shell according to claim 1, wherein said shell has a hole.
The photocatalyst as described. 4. An outer shape of the photocatalyst having the outer shell is a capsule shape or a spherical shape.
The photocatalyst as described. 5. The compound semiconductor according to claim 1, wherein the compound semiconductor is a II-VI group compound semiconductor.
The photocatalyst as described. 6. The photocatalyst according to claim 1, wherein the compound semiconductor is zinc sulfide. 7. I constituting a II-VI compound semiconductor
A method for producing a photocatalyst, comprising dissolving oxide particles of a group I element in an aqueous solution containing sulfur ions and depositing and generating a sulfide of a group II element formed on the oxide particles. 8. The method for producing a photocatalyst according to claim 7, wherein the oxide particles of the group II element are zinc oxide. 9. The method for producing a photocatalyst according to claim 8, comprising a step of mixing the zinc oxide fine particles with an aqueous solution of sodium sulfide, and a step of stirring the mixture. 10. The method for producing a photocatalyst according to claim 8, comprising a step of mixing the zinc oxide fine particles and an aqueous solution of sodium sulfide, and a step of stirring the mixture while irradiating the mixture with ultraviolet rays. 11. A method comprising: mixing zinc oxide fine particles and an aqueous solution of sodium sulfide; stirring hydrogen sulfide while bubbling the mixture; stopping hydrogen sulfide gas, and further stirring for a certain period of time. Item 10. A method for producing a photocatalyst according to Item 8. 12. A step of mixing zinc oxide fine particles and an aqueous solution of sodium sulfide, and irradiating the mixed liquid with ultraviolet rays.
9. The method for producing a photocatalyst according to claim 8, comprising a step of bubbling hydrogen sulfide. 13. A method for using a photocatalyst, wherein an aqueous solution of sodium sulfide is used as a sacrificial reducing agent for the photocatalyst comprising zinc sulfide according to claim 6. 14. The method for using a photocatalyst according to claim 13, comprising the following steps. (A) dissolving hydrogen sulfide in caustic soda aqueous solution,
(B) adding a photocatalyst to the solution, irradiating ultraviolet rays to recover hydrogen gas, (c) recovering sulfur from the solution after the step (b), (d) after the step (c) Recycling the solution as the aqueous solution of caustic soda in the step (a).