JP2013086010A - Photocatalyst and production process therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a photocatalyst having an excellent catalytic activity without using rare earth elements.SOLUTION: CuOoctahedron 18 is formed by substitutionally solid-solubilizing copper ions in a part of the titanium ion sites in a crystal structure of titanium dioxide. The crystal generated by the substitution is brookite. Vacancies 20a and 20b are present in the neighborhood of the copper ions. These vacancies 20a and 20b are the sites where oxygen ions are present before the substitution. Namely, the vacancies 20a and 20b create oxygen defects.

Description

  The present invention, for example, performs purification actions such as water purification and air purification, and also includes a photoelectrode for a dye-sensitized solar cell and a photoelectrode for a hydrogen gas generation apparatus that generates hydrogen gas by decomposing water with light. The present invention relates to a photocatalyst useful for the above and a production method thereof.

  Titanium oxide is well known as a substance that functions as a photocatalyst. Specifically, when titanium oxide is irradiated with ultraviolet light, electrons in the valence band are excited to the conduction band beyond the band gap. As a result, holes are generated in the valence band and electrons are present in the conduction band. Holes and electrons stay in the valence and conduction bands without recombination, respectively. During this time, holes perform an oxidizing action and electrons perform a reducing action. Therefore, for example, when water is present around titanium oxide, water is decomposed under the action of oxidation / reduction of titanium oxide irradiated with ultraviolet light to generate hydrogen and oxygen.

  Recently, this titanium oxide (photocatalyst) has been used in the environmental field and energy field, for example, water purification by decomposition and sterilization of organic substances, air purification by decomposition and removal of NOx, photoelectrodes of dye-sensitized solar cells, optical water Application to photoelectrodes and the like for hydrogen generation by decomposition has been performed, and there is a demand for photocatalysts with higher activity and higher performance.

  For example, Patent Document 1 discloses a photocatalyst in which titanium oxide is doped with a rare earth element, and this photocatalyst exhibits photocatalytic activity not only in the ultraviolet light region but also in the visible light region.

  Patent Document 2 discloses a photocatalyst in which an electron-trapping metal made of copper or the like is exposed and fixed on the surface of a mixed layer of photocatalyst particles such as titanium oxide and a photocorrosion resistant matrix such as a thermosetting resin. Yes. According to this, even if the heat treatment at the time of manufacture is low temperature, it is excellent in photocatalytic activity.

Japanese Patent No. 4113816 Japanese Patent No. 3282184

  In the technique described in Patent Document 1, it is necessary to use a rare earth element as a dopant. However, as is well known, rare earth elements are low in the number of Clarkes and unevenly distributed on the earth, and thus are not easily available. For this reason, there is a problem that the implementation itself is not easy and the manufacturing cost increases.

  Further, according to the earnest study of the present inventor, catalytic activity is sufficient only by adding an electron-trapping metal such as copper to a layer containing titanium oxide and a thermosetting resin as described in Patent Document 2. It is hard to say that there is.

  The present invention has been made to solve the above-described problems, and an object thereof is to provide a photocatalyst excellent in catalytic activity and a method for producing the same without using rare earth elements.

To achieve the above object, the present invention provides a photocatalyst comprising titanium, copper and oxygen as constituent ions,
It is characterized by comprising an oxide in which some titanium ions in the crystal structure of titanium oxide are replaced with copper ions.

  In this configuration, energy levels derived from substituted and dissolved copper ions are formed in the band gap of titanium oxide. As a result, the band gap is narrowed, so that the photocatalytic reaction can occur not only by irradiation with ultraviolet light but also by visible light. Furthermore, copper ions capture the excited electrons generated by light irradiation and delay the recombination of the excited electrons with the holes. Accordingly, since electrons and holes that cause the photocatalytic reaction can exist for a long time, the catalytic activity can be maintained for a long time.

  The photocatalyst can be a film, but can also be a particle. In this case, the particles include crystal grains made of the oxide.

  This particle is preferably a polycrystalline body in which crystal grains having different crystal orientations are aggregated. In this case, since the crystal grains are refined, the catalyst active point increases. Therefore, excellent catalytic activity is expressed.

  The average particle diameter of the particles is generally 10 to 60 nm. The average crystal grain size of the crystal grains is generally 1 to 20 nm. In addition, these values can be calculated | required as an equivalent circle diameter based on observation results, such as a scanning electron microscope (SEM) and a transmission electron microscope (TEM).

  Moreover, it is preferable that the site | part of the oxygen ion adjacent to a titanium ion in the crystal structure of a titanium oxide becomes a void | hole in the crystal structure where the said titanium ion was substituted by the copper ion. In this case, the vacancies are oxygen defects that are deficient in oxygen. Since this oxygen defect captures excited electrons, the recombination of excited electrons and holes is further delayed. For this reason, the catalytic activity is further sustained.

  Such a substituted phase is typically a brookite phase. However, the photocatalyst is not limited to one containing only this phase, and may contain, for example, an anatase phase in which titanium ions are not replaced with copper ions.

Further, the present invention is a method for producing a photocatalyst comprising an oxide containing titanium, copper and oxygen as constituent ions, wherein a part of the titanium ions in the crystal structure of titanium oxide is substituted with copper ions,
A titanium oxide precursor capable of producing titanium oxide by hydrolysis and polycondensation and a copper ion donor that is soluble in water or an organic solvent and can donate copper ions are mixed in a solvent. To obtain a mixed solution,
Obtaining an oxide in which some titanium ions in the crystal structure of titanium oxide are replaced with copper ions by hydrolyzing and polycondensing the titanium oxide precursor in the mixed solution;
Applying heat treatment to the oxide;
It is characterized by having.

  The photocatalyst described above can be easily obtained through the above process.

  In this case, it is not necessary to use rare earth elements. For this reason, the photocatalyst excellent in catalyst activity can be obtained at low cost.

  In addition, at the time of heat processing, it is suitable to hold | maintain for 1 to 180 hours by setting temperature as 400-700 degreeC.

  Moreover, titanium alkoxide can be mentioned as a suitable example of a titanium oxide precursor.

  Furthermore, when the photocatalyst is obtained as particles, the oxide may be pulverized. The photocatalyst is obtained as particles by subjecting the pulverized material obtained by this heat treatment to heat treatment.

  According to the present invention, an energy level derived from copper ions is formed in the band gap of titanium oxide because copper ions are substituted and dissolved in the crystal structure of titanium oxide. For this reason, the band gap is narrowed, and as a result, the photocatalytic reaction occurs not only by irradiation with ultraviolet light but also by visible light. In addition, since the excited electrons generated by light irradiation are captured by copper ions or oxygen defects (vacancies), the recombination of the excited electrons with the holes is delayed. Therefore, excited electrons and holes that cause a catalytic reaction can exist for a long time.

  Furthermore, since the crystal grain size of the crystal grains contained in the particles is refined, the catalyst active point increases.

  For the reasons described above, the photocatalyst is excellent in catalytic activity. Since this photocatalyst does not contain rare earth elements, it can be obtained at low cost.

It is a schematic diagram which shows the brookite phase of pure titanium oxide. FIG. 2 is a schematic diagram showing a substitution type brookite phase in which titanium ions in FIG. 1 are substituted with copper ions and vacancies as oxygen defects are generated. It is a transmission electron microscope (TEM) photograph of the photocatalyst (Example 1) which concerns on this Embodiment. It is the principal part enlarged photograph which expanded the part enclosed with the continuous line in FIG. FIG. 5 is a schematic diagram of FIG. 4. It is the table | surface which put together the preparation conditions of Examples 1-6, Comparative Examples 1-8, and Reference Examples 1 and 2. FIG. 7A to 7C are SEM photographs of Example 1, Comparative Example 1, and Comparative Example 2, respectively. 8A and 8B are TEM photographs of Comparative Example 1 and Comparative Example 2, respectively. 9A and 9B are photographs of Example 1 and Comparative Example 2, respectively, taken with a high-angle scattering annular dark field scanning transmission microscope (HAADF-STEM). 10A and 10B are diagrams showing EDS (energy dispersive X-ray) analysis measurement results of Example 1 and Comparative Example 2, respectively. It is a figure which shows the radial distribution by the Fourier transform of the copper atom periphery of Example 1 and the comparative example 2 calculated | required from the XAFS (X-ray absorption fine structure) analysis result in a Cu-k absorption edge. 2 is an X-ray diffraction profile of Example 1 and Comparative Example 2. FIG. It is a XANES (X-ray absorption edge vicinity structure) analysis result of Example 1 and metallic copper, copper oxide (I), and copper oxide (II). FIG. 14A to FIG. 14C are diagrams each showing a fitting result of a radial distribution by Fourier transform around the copper atom of Example 1 and various structural models. Examples 1-3 obtained from fitting results of radial distribution by Fourier transform around copper atoms of Examples 1-3 and a structural model in which one titanium atom of titanium oxide (brookite phase) was replaced with a copper atom It is a figure which shows the relative coordination distance and the relative coordination number with the adjacent (closest) oxygen of the copper atom in. It is the figure which showed the relationship between the amount of OH radicals of Example 1, the comparative example 1, and the comparative example 2 and the light irradiation integration time which were relatively compared by the ESR (electron spin resonance) apparatus. It is a figure which shows the measurement result of the diffuse reflection spectrum of Example 1 and Comparative Example 1. It is a figure which shows the result of having compared the crystallinity degree of Example 2, Example 6, and Comparative Example 7. FIG. 2 is an X-ray diffraction profile of Example 2, Example 6, and Comparative Example 7. FIG. 20A to 20C are SEM photographs of Example 2, Example 6, and Comparative Example 8, respectively. It is a figure which shows the amount of OH radical of Example 2, the reference example 1, the comparative example 3, and the comparative example 4 compared comparatively with the ESR (electron spin resonance) apparatus. It is a figure which shows the amount of OH radicals of Example 4, Example 5, Reference Example 2, and Comparative Example 5 that are relatively compared by an ESR device. It is a figure which shows the amount of OH radicals of Example 6 and Comparative Example 6 that are relatively compared by the ESR apparatus.

  Preferred embodiments of the photocatalyst according to the present invention will be described below in detail with reference to the accompanying drawings.

  The photocatalyst according to the present embodiment is composed of particles containing crystals in which some titanium ions in the crystal structure of titanium oxide are replaced with copper ions.

  The crystal structure of pure titanium oxide is generally either a rutile phase, an anatase phase or a brookite phase. In particular, the anatase phase is well known as a photocatalyst having a large band gap and high activity.

  On the other hand, in the photocatalyst according to the present embodiment, the phase in which titanium ions are replaced with copper ions mainly forms a brookite phase. Hereinafter, this phase is referred to as a “substituted brookite phase”.

In the brookite phase (orthorhombic) of pure titanium oxide, the TiO ₆ octahedrons 10 shown in FIG. In the TiO ₆ octahedron 10, six oxygen (O) ions 14 a to 14 f surround a titanium (Ti) ion 12 located at the center.

In the substitution type brookite phase, as shown in FIG. 2, a CuO ₄ octahedron 18 in which the Ti ions 12 are substituted with copper (Cu) ions 16 is formed. Here, the Ti ions 12 are tetravalent, whereas the Cu ions 16 are divalent. Therefore, in the substitutional brookite phase, 2 to 3 out of the 6 O ions 14a to 14f are released in order to maintain electrical balance. FIG. 2 shows the case where the O ions 14b and 14e are separated, but the traces of the O ions 14b and 14e thus separated become holes 20a and 20b.

The reason why the substitution brookite phase is stable is presumed to be that the ionic radius of the Cu ions 16 is appropriate for forming the CuO ₄ octahedron 18.

  The photocatalyst according to the present embodiment includes crystals in which titanium ions are not replaced with copper ions, in addition to the above-described substitutional brookite phase. That is, this crystal consists of pure titanium oxide. This titanium oxide is an anatase phase and a brookite phase.

  As described above, the photocatalyst according to the present embodiment includes a crystal composed of a substituted brookite phase, a crystal composed of an anatase phase and a brookite phase.

  The substitution type brookite phase exhibits activity as a photocatalyst even when irradiated with light having a wavelength in the visible light region, for example, 420 to 650 nm. On the other hand, the anatase phase exhibits excellent activity in the ultraviolet region. That is, a general photocatalyst (anatase-phase titanium oxide) is active in the ultraviolet light region and shows almost no activity in the visible light region, whereas the photocatalyst according to the present embodiment is from the visible light region to the ultraviolet light region. A wide range of sufficient activity.

  The reason why the substituted brookite phase exhibits excellent activity is presumed as follows.

  When Ti ions 12 (see FIG. 1) in the brookite phase are replaced with Cu ions 16 (see FIG. 2), energy levels derived from Cu ions 16 are formed in the band gap of titanium oxide. The band gap is narrowed. Accordingly, excited electrons and holes can be generated even when visible light is absorbed.

  The substituted Cu ions 16 capture electrons excited from the valence band to the conduction band by light irradiation. This delays recombination of the electrons moved to the conduction band and the holes generated in the valence band as the electrons are excited. Accordingly, the excited electrons and holes can exist in this state for a long time. As a result, the activity is expressed for a long time.

  Furthermore, in the present embodiment, as the Ti ions 12 are replaced with the Cu ions 16, the O ions 14b and 14e existing at the adjacent positions of the Ti ions 12 are separated and the holes 20a and 20b (oxygen) Defect) is formed. This oxygen defect also delays recombination of electrons and holes excited from the valence band to the conduction band. This also contributes to an excellent activity.

  The photocatalyst according to the present embodiment is composed of particles containing the above crystals. The average particle diameter obtained by observing the particles with a scanning electron microscope (SEM) is in the range of 10 to 60 nm. Note that the shape of the particles appearing in the field of view of the SEM is a substantially perfect circle. Therefore, the average particle diameter here is obtained by dividing the sum of the diameters of each particle appearing in the visual field by the number of particles.

  Here, a transmission electron microscope (TEM) photograph of the photocatalyst (particles) according to the present embodiment is shown in FIG. 3, and the part surrounded by a solid line in FIG. 3 is enlarged and shown in FIG. FIG. 5 is a schematic diagram of FIG.

  3 to 5, when the crystal grains a, b, and c are compared with each other, a lattice image is clearly shown in the crystal grain b, whereas a lattice image is displayed in the crystal grain a and the crystal grain c. It is confused. From this, it is understood that the crystal orientations of the crystal grain b, the crystal grain a, and the crystal grain c are different from each other. This reason is presumed to be because copper ions were substituted and dissolved in the crystal structure of titanium oxide.

  A photocatalyst composed of such particles exhibits excellent catalytic activity. From this, it is considered that energy levels derived from copper ions are formed in the band gap of titanium oxide.

  Accordingly, the band gap width is narrowed, and therefore it is possible to absorb not only ultraviolet light but also visible light. Furthermore, since the copper ions capture excited electrons generated by light irradiation, the timing at which the excited electrons and holes are recombined is delayed. In other words, recombination is delayed.

  In addition, the vacancies 20a and 20b in the substitution brookite phase (see FIG. 2) are traces of the separation of the O ions 14b and 14e, in other words, oxygen defects. This oxygen defect (holes 20a and 20b) also delays recombination of the excited electrons and holes.

  Since recombination is delayed in this way, excited electrons and holes that cause a catalytic reaction can exist for a relatively long time. For this reason, in the photocatalyst according to the present embodiment, the catalytic activity lasts for a long time.

  The photocatalyst particles include crystal grains having different crystal orientations (see FIGS. 3 to 5). For this reason, since a crystal grain refines | miniaturizes, a catalyst active point increases.

  For the above reasons, in the photocatalyst according to the present embodiment, high catalytic activity is exhibited over a long period of time even though the photocatalyst does not contain a rare earth element.

  Next, the manufacturing method of the photocatalyst concerning this Embodiment is demonstrated.

  The method for producing a photocatalyst according to the present embodiment includes a step of obtaining a mixture of a titanium oxide precursor and a copper ion donor, and a reaction product obtained by hydrolysis and polycondensation of the titanium oxide precursor in a solvent. And a step of subjecting the reaction product to a heat treatment.

  Hereinafter, specifically, in detail, first, a titanium oxide precursor and a copper ion donor are selected.

  Here, in order to disperse the titanium oxide precursor as uniformly as possible in the solvent described later and to mix it with the copper ion donor as much as possible, the titanium oxide precursor is diluted with an appropriate organic solvent in advance. It is preferable. As a result, it is possible to avoid a sudden increase in temperature with the progress of hydrolysis and polycondensation, and an excessive increase in reaction rate due to this, so hydrolysis and polycondensation can be avoided. There is also an advantage that the process can be carried out almost uniformly in the solvent. In particular, it is easy to obtain this advantage when a substance generated by the progress of the hydrolysis reaction of the titanium oxide precursor is selected as an organic solvent for dilution.

  The amount of the organic solvent for dilution is preferably 20 mol or more with respect to 1 mol of titanium atoms of the titanium oxide precursor. In this case, it is possible to easily prevent the reaction rate from increasing excessively, and as a result, it becomes easy to proceed the hydrolysis and polycondensation substantially uniformly.

Preferable examples of the titanium oxide precursor include titanium alkoxide. Specifically, tetramethoxy titanium (Ti (OCH ₃ ) ₄ ), tetraethoxy titanium (Ti (OC ₂ H ₅ ) ₄ ), tetrapropoxy titanium (Ti (OC ₃ H ₇ ) ₄ ), tetraisopropoxy titanium ( Ti (OCH (CH ₃ ) ₂ ) ₄ ), tetrabutoxytitanium (Ti (OC ₄ H ₉ ) ₄ ) and the like can be mentioned, but not particularly limited thereto.

  When titanium alkoxide is used as a titanium oxide precursor, an alkoxide group in the titanium alkoxide is bonded to hydrogen ions to generate an alcohol. Therefore, an alcohol may be selected as the organic solvent for dilution.

  In addition, the rate of hydrolysis decreases as the molecular weight of the alkoxide group increases. Therefore, in this case, it is easy to suppress an excessive increase in the reaction rate.

  As the copper ion donor, a copper compound containing copper ions and soluble in water or an organic solvent is selected. Furthermore, it is particularly preferable that when the heat treatment is performed, component elements other than copper are easily oxidized and vaporized and do not remain in the photocatalyst obtained after the heat treatment. Preferable examples of such copper compounds include copper bromide, copper chloride, copper carbonate, copper acetate, copper sulfate, copper nitrate, copper oxalate, and hydrates thereof.

  The copper ion donor is preferably dissolved or dispersed in water. For convenience, this solution or dispersion is referred to as an “aqueous solution”. The pH of the aqueous solution is not particularly limited.

  Hereinafter, when a diluted titanium alkoxide is selected as a titanium oxide precursor and the aqueous solution is used as an example, first, the diluted titanium alkoxide and the aqueous solution are mixed to obtain a mixed solution.

  The amount of copper atoms in the copper ion donor is such that when the total of titanium atoms in the titanium oxide precursor and copper atoms in the copper ion donor is 100 mol%, the copper atoms are 0.013 to 1.02 mol. It is preferable to set so that it is in the range of%. If it is less than 0.013 mol%, the catalytic activity is not improved so much. On the other hand, even when more copper atoms than 1.02 mol% are added, there is a tendency that surplus copper atoms that are not substituted remain.

In the mixed solution, hydrolysis of the titanium alkoxide proceeds according to the following reaction formula (1). In the reaction formula (1), R represents an alkyl group, and the same applies to the following.
_{Ti (OR) n + xH 2} O → Ti (OH) x (OR) n-x + xROH ... (1)

On the other hand, there are two reaction paths in polycondensation. Each route is shown below as reaction formulas (2) and (3).
-Ti-OH + H-O- Ti → -Ti-O-Ti + H 2 O ... (2)
-Ti-OH + RO-Ti-> Ti-O-Ti + ROH (3)

  As can be understood from the reaction formulas (2) and (3), the polycondensation involves a dehydration reaction or a dealcoholization reaction.

  Actually, before all alkoxide groups are hydrolyzed, the dehydration reaction and dealcoholization reaction shown in the reaction formulas (2) and (3), that is, polycondensation occurs. For this reason, the theoretical amount of water required for hydrolysis and polycondensation of titanium alkoxide also changes. Further, the polycondensation shown in the reaction formulas (2) and (3) is an exothermic reaction, and thus the temperature of the mixed solution rises. It is preferable to contain a large amount of water in the mixed solution in order to suppress this temperature rise and to allow the reaction to proceed substantially evenly and to provide sufficient water for hydrolysis and polycondensation to proceed. For example, it is preferable to use 100 times or more of water sufficient to produce 1 mol of titanium hydroxide from 1 mol of titanium alkoxide.

  During the progress of hydrolysis and polycondensation, it is preferable that the mixed solution is continuously stirred and adjusted so that the temperature of the mixed solution does not exceed 80 ° C. This is to prevent the reaction rate from increasing excessively when the temperature of the mixed solution is excessively increased.

  It is inferred that part of Ti ions is replaced with Cu ions in the process of the above reaction. That is, the obtained reaction product is considered to be a polycondensate in which a part of Ti ions is substituted with Cu ions.

  This reaction product is an aggregate. Therefore, next, the reaction product is pulverized into a pulverized product by a known method.

  Next, this pulverized product is subjected to heat treatment. The heat treatment may be performed in an inert gas atmosphere, but can be performed even in the air.

  A preferable holding temperature in the heat treatment is 400 to 700 ° C. If it is lower than 400 ° C., crystallization may not be sufficient. Such particles contribute to the insufficient catalytic activity. On the other hand, when the temperature is higher than 700 ° C., the grain growth may proceed excessively to become coarse grains. In this case, the catalyst active point on the particle surface decreases.

  In this temperature range, a preferable holding time is 1 to 180 hours. It is preferable to lengthen the holding time when the holding temperature is low, and shorten the holding time when the holding temperature is high.

  As described above, a photocatalyst composed of particles in which a part of Ti ions is substituted with Cu ions is obtained as particles. When X-ray diffraction measurement is performed on the particles, a peak attributed to the brookite phase and a peak attributed to the anatase phase appear (described later).

  Thus, in this embodiment, it is not necessary to use a rare earth element. Therefore, it is possible to obtain a photocatalyst at a low cost.

  The present invention is not particularly limited to the above-described embodiment, and various modifications can be made without departing from the scope of the invention.

  For example, in the above-described embodiment, the photocatalyst is obtained as particles, but can also be obtained as a thin film. In this case, the reaction product may be coated on the substrate by a general thin film forming technique such as dip coating or spin coating, and then the coated reaction product may be heat-treated.

  In addition, for example, a mixed solution may be prepared by adding a diluted solution of a titanium oxide precursor such as titanium alkoxide to an aqueous solution containing a copper ion donor, or adding a titanium oxide precursor and a copper ion donor to a solvent. It can be obtained by adding them individually.

[Example 1]
While stirring 2-propanol with respect to 2.31 mol of 2-propanol, 8.80 × 10 ⁻² mol of tetraisopropoxytitanium (IV) was added, and diluted tetraisopropoxytitanium liquid (hereinafter simply referred to as “diluent”). Prepared).

Meanwhile, 2.27 × 10 ⁻⁴ mol of copper nitrate (II) trihydrate (Cu (NO) ₃ ) ₂ .3H ₂ O) was added to 55.4 mol of water while stirring the water. An aqueous solution of a copper ion donor was prepared. In this case, the amount of copper atoms in copper nitrate (II) nitrate trihydrate, which is a copper ion donor, is the same as that of titanium atoms in tetraisopropoxy titanium, which is a titanium oxide precursor, and copper (II) nitrate trihydrate. When the total of copper atoms in the product is 100 mol%, it is 0.26 mol%. Hereinafter, the ratio (mol%) of the copper atom obtained in this way may be referred to as “copper donation amount”.

  Next, while continuing stirring of the aqueous solution, the diluent was added dropwise to the aqueous solution to obtain a mixed solution.

  Furthermore, hydrolysis and polycondensation of the titanium oxide precursor proceeded by maintaining the mixed solution at 70 ° C. while stirring. Thereby, a gel-like substance was obtained.

  Finally, this gel-like substance was dried, pulverized in a mortar, and then heat-treated by being held in the atmosphere at 500 ° C. for 3 hours to obtain a photocatalyst as particles. This is Example 1.

[Comparative Example 1]
A photocatalyst composed of titanium oxide particles was obtained according to Example 1 except that copper (II) nitrate trihydrate was not added. This is referred to as Comparative Example 1.

[Comparative Example 2]
Part of the titanium oxide particles obtained in Comparative Example 1 was impregnated with an aqueous solution of copper (II) nitrate trihydrate prepared so that the ratio of titanium (mol) / copper (mol) was the same as in Example 1. did. Then, it heat-processed by hold | maintaining at 300 degreeC for 3 hours in air | atmosphere. This is referred to as Comparative Example 2.

[Examples 2 and 3]
The amount of copper nitrate (II) trihydrate was 1.13 × 10 ⁻⁴ mol or 4.53 × 10 ⁻⁴ mol (copper donation amount was 0.13 mol%, 0.51 mol%). Except that, the same operation as in Example 1 was performed to obtain a photocatalyst. These are referred to as Examples 2 and 3, respectively.

[Reference Example 1]
A photocatalyst was obtained in the same manner as in Example 1 except that the amount of copper (II) nitrate trihydrate was 1.13 × 10 ⁻⁵ mol (copper donation amount was 0.013 mol%). It was. This is referred to as Reference Example 1.

[Comparative Examples 3 and 4]
1.13 × 10 ⁻⁴ mol of lanthanum nitrate (III) hexahydrate (La (NO ₃ ) ₃ .6H ₂ O), or 1.13 × 10, instead of copper (II) nitrate trihydrate ^A photocatalyst was obtained in the same manner as in Example 1 except that ^-4 mol of manganese (II) nitrate hexahydrate (Mn (NO ₃ ) ₂ .6H ₂ O) was used. These are referred to as Comparative Examples 3 and 4, respectively.

[Examples 4 and 5]
A photocatalyst was obtained in the same manner as in Examples 1 and 2 except that the holding time during the heat treatment was 72 hours. These are designated as Examples 4 and 5, respectively.

[Reference Example 2]
The amount of copper (II) nitrate trihydrate was 9.06 × 10 ⁻⁴ mol (the same operation as in Example 4 was carried out except that the amount of copper donation was 1.02 mol%). This is referred to as Reference Example 2.

[Comparative Example 5]
A photocatalyst was obtained in the same manner as in Comparative Example 1 except that the holding time during the heat treatment was 72 hours. This is referred to as Comparative Example 5.

[Example 6]
A photocatalyst was obtained by performing the same operation as in Example 2 except that the holding temperature during the heat treatment was 600 ° C. This is Example 6.

[Comparative Example 6]
A photocatalyst was obtained in the same manner as in Comparative Example 1 except that the holding temperature during the heat treatment was 600 ° C. This is referred to as Comparative Example 6.

[Comparative Example 7]
A photocatalyst was obtained by performing the same operation as in Example 2 except that the holding temperature during the heat treatment was 400 ° C. This is referred to as Comparative Example 7.

[Comparative Example 8]
A photocatalyst was obtained by performing the same operation as in Example 2 except that the holding temperature during the heat treatment was 700 ° C. This is designated as Comparative Example 8.

  The production conditions of Examples 1 to 6, Comparative Examples 1 to 8, and Reference Examples 1 and 2 are collectively shown in FIG.

  Among these, Example 1 and Comparative Examples 1 and 2 were observed with a scanning electron microscope (SEM). The SEM photographs are shown in FIGS. 7A to 7C. In any case, the average particle diameter was about 20 nm, which was substantially equivalent to each other.

  Further, TEM observation was performed on Example 1 and Comparative Examples 1 and 2. 3 and 4 are TEM photographs of Example 1. FIG. As described above, in Example 1, the crystal grain b in which the lattice image of titanium oxide clearly appears, and the crystal grain a and the crystal grain c having a disordered crystal structure are recognized. The crystal grain size of each of the crystal grains a to c is about 5 to 10 nm, and is smaller than the average particle diameter and fine. That is, fine crystal grains having different crystal orientations exist inside one particle of about 20 nm.

  The remaining TEM photographs of Comparative Examples 1 and 2 are shown in FIGS. 8A and 8B. In this case, a crystal structure with different crystal orientations observed in Example 1 was not recognized. Moreover, the crystal grain size of the crystal grains was about 20 nm, similar to the average particle diameter.

  Furthermore, the high-angle scattering annular dark field scanning transmission microscope (HAADF-STEM) was observed for Example 1 and Comparative Example 2. The photograph obtained by each is shown to FIG. 9A and FIG. 9B, respectively. 3 and FIG. 8B are different views.

  In FIG. 9A, high brightness particles (copper particles) were not observed. On the other hand, in FIG. 9B, as indicated by arrows in FIG. 9B, it was recognized that particles with high brightness (copper particles) were present on the surface of the particles.

  Therefore, EDS (energy dispersive X-ray) analysis measurement was performed for Example 1 and Comparative Example 2. Each EDS spectrum is shown in FIGS. 10A and 10B. Note that the EDS spectrum in FIG. 10A is for the whole particle, and FIG. 10B is for the particle numbered “5” among the high-brightness particles in FIG. 9B.

  As can be seen by comparing FIG. 10A and FIG. 10B, the copper peak was not recognized in Example 1, whereas the copper peak was recognized in Comparative Example 2. From this, it was confirmed that the high brightness particles are copper.

  Further, for Example 1 and Comparative Example 2, the radial distribution by Fourier transform around the copper atom was obtained from the XAFS (X-ray absorption fine structure) analysis result at the Cu-k absorption edge. The results are also shown in FIG. From FIG. 11, the radial distribution is clearly different between Example 1 and Comparative Example 2 in which copper particles are present on the surface of the titanium oxide particles. This means that the peripheral structure of the copper atom in each of Example 1 and Comparative Example 2 is fundamentally different.

  Next, XRD (X-ray diffraction) measurement was performed for Example 1 and Comparative Example 2. The diffraction profile is also shown in FIG. In any case, a peak attributed to the brookite phase of titanium oxide and a peak attributed to the anatase phase appear. From this, it can be seen that Example 1 and Comparative Example 2 have the same crystal phase structure.

In FIG. 13, the XANES (X-ray absorption edge vicinity structure) analysis result of Example 1 and a reference material (metal copper, copper oxide (I), and copper oxide (II)) is shown. In Example 1, it can be seen that the energy absorption edge is chemically shifted to a higher energy side than copper metal or copper oxide (I), and is close to copper oxide (II). This indicates that in Example 1, copper exists in the state of divalent copper ions (Cu ²⁺ ).

  Next, a radial distribution by Fourier transform around the copper atom of Example 1 obtained from the XAFS analysis result at the Cu-k absorption edge, and a structure in which one titanium ion of titanium oxide (brookite phase) is replaced with a copper ion Fitting with a model, fitting with a structural model in which one titanium ion of titanium oxide (anatase phase) was replaced with copper ion, and fitting with a structural model of copper (II) oxide were examined. The results are shown in FIGS. 14A to 14C.

  14A to 14C, the radial distribution of Example 1 and the structural model in which one titanium oxide of titanium oxide in the brookite phase is replaced with copper ions are in good agreement with each other. It can be seen that there is little agreement with the model. From this, in Example 1, it is recognized that the copper ion is substituted and dissolved in the brookite phase of titanium oxide.

  Further, the radial distribution by Fourier transform around the copper atoms in Examples 1 to 3 obtained from the XAFS (X-ray absorption fine structure) analysis result at the Cu-k absorption edge, and the titanium ion 1 of titanium oxide (brookite phase) 1 Fitting with a structural model in which one was replaced with copper ions was examined, and the relative coordination distance and the relative coordination number of copper atoms with adjacent oxygen in Examples 1 to 3 were determined from the results. The results are also shown in FIG.

  As is clear from FIG. 15, in any of Examples 1 to 3, the relative coordination distance and the relative coordination number with adjacent oxygen are equal to each other. The relative coordination distance is equivalent to the average coordination distance between the titanium atom and the adjacent oxygen in titanium oxide (brookite phase), but the relative coordination number is the adjacent oxygen coordination of the titanium atom in titanium oxide (brookite phase). It is less than the number. This indicates that the photocatalysts according to Examples 1 to 3 have oxygen defects adjacent to the substituted and dissolved copper ions.

  From the above results, in Example 1, copper is solid-dissolved in titanium oxide, and for this reason, oxygen defects are generated, and in Comparative Example 2, copper particles simply adhere on the titanium oxide particles. It turns out that it is in a state. Therefore, as shown in FIGS. 3 to 5, it is presumed that fine crystal grains having different crystal orientations are generated when copper dissolves.

Next, with respect to Example 1 and Comparative Examples 1 and 2 in the above-described photocatalyst, OH radicals were generated by irradiating excitation light having a maximum wavelength of 352 nm for 30 seconds, 60 seconds, 90 seconds, or 120 seconds with an integration time. The amount of OH radicals at this time was measured with an ESR apparatus, and the amount of OH radicals generated per unit weight was relatively compared for each photocatalyst irradiated with excitation light for each integration time. The results are also shown in FIG. By performing this relative comparison, the magnitude of the catalytic activity of each photocatalyst can be compared.

  The measurement of the amount of OH radicals was made by adding 2 ml of 0.147 mol / liter spin trapping agent (5,5-dimethyl-1-pyrroline-N-oxide; DMPO) to 4 mg of photocatalyst, and The measurement sample was stirred with a magnetic stirrer at 200 rpm.

As can be seen from FIG. 16, in Example 1, OH radical generation amount when the irradiation integrated time of 30 seconds is most often, also, OH radicals generated amount even 60 to 120 seconds of light irradiation integration time It has not decreased so much. On the other hand, in Comparative Example 1, the amount of OH radicals generated is small even when the light irradiation integration time is short. In Comparative Example 2, the amount of OH radicals generated is relatively large when the light irradiation integration time is 30 seconds, but is smaller than that of Example 1, and when the light irradiation integration time is 60 to 120 seconds, OH The amount of radical generation is significantly reduced.

  From the above results, it is clear that Example 1 has higher photocatalytic activity than Comparative Examples 1 and 2, and the catalytic activity lasts for a relatively long time. As described above, since the crystal phase structure itself is the same in Example 1 and Comparative Example 2, the difference in catalytic activity depends on whether or not copper ions are in solid solution and in the vicinity of the copper ions. This is considered to be based on whether or not there is a defect and whether or not a fine crystal having a different crystal orientation is included.

  Further, for Example 1 and Comparative Example 1, diffuse reflection spectrum measurement was performed using an ultraviolet-visible spectrophotometer. The results are shown in FIG. As can be seen from FIG. 17, in Example 1, in addition to the absorption of ultraviolet light, absorption was also observed in the wavelength range of visible light of 420 to 650 nm. On the other hand, in Comparative Example 1, almost no absorption was observed in the visible light wavelength region.

  Apart from the above, the crystallinity of Example 2, Example 6, and Comparative Example 7 was examined. The comparison results are shown in FIG. The crystallinity is 100% for the commercial titanium oxide powder (anatase phase) based on the peak intensity measured by XRD, and 0% for the blank measured without a sample. Relative value of the case.

  In Examples 2 and 6 where the heat treatment was performed at 500 ° C. and 600 ° C., the crystallinity exceeded 90%, but the crystallinity of Comparative Example 7 where the heat treatment was performed at 400 ° C. was 85%. . Since it is not preferable that the crystallinity is further reduced, it can be said that the heat treatment temperature is desirably 400 ° C. or higher.

  FIG. 19 shows the XRD measurement results of Example 2, Example 6, and Comparative Example 7.

  20A to 20C are SEM photographs of Example 2, Example 6, and Comparative Example 8. FIG. In Comparative Example 8 in which the heat treatment was performed at 700 ° C., since the particles were coarser than those in Example 2 and Example 6, it is presumed that the catalytic activity point was decreased. Since it is not preferable that the catalyst active point is excessively reduced, the heat treatment temperature is desirably set to 700 ° C. or lower.

  Furthermore, for Example 2, Reference Example 1, Comparative Example 3 and Comparative Example 4, the amount of OH radicals was measured with an ESR apparatus in the same manner as above except that the light irradiation integration time was 90 seconds. The result of the relative comparison is shown in FIG.

  The amount of OH radicals generated in Example 2 is significantly larger than the amount of OH radicals generated in Comparative Example 3 using a lanthanum ion donor instead of a copper ion donor and Comparative Example 4 using a manganese ion donor. . From this, it can be seen that excellent catalytic activity is exhibited by dissolving copper ions in solid solution.

  In addition, the amount of OH radicals generated in Reference Example 1 (copper donation amount 0.013 mol%) in which the copper content was reduced as compared with Example 2 was compared with the amount of OH radicals generated in Comparative Example 3 and Comparative Example 4. Many are recognized as having excellent catalytic activity. However, the amount of OH radicals generated is slightly smaller than in Example 2 where the amount of copper donation is large. Therefore, the copper donation amount is more preferably 0.013 mol% or more.

  Moreover, also in Example 4, Example 5, Reference Example 2, and Comparative Example 5, the amount of OH radicals was measured with an ESR apparatus with the light irradiation integrated time being 120 seconds. The result of the relative comparison is shown in FIG.

  The amount of OH radicals generated in Examples 4 and 5 is significantly larger than the amount of OH radicals generated in Comparative Example 5 in which no copper ion donor is used, and it is clear that the catalyst activity is excellent. is there.

  Further, the amount of OH radicals generated in Reference Example 2 (copper donation amount of 1.02 mol%) compared to Example 4 and Example 5 was larger than the amount of OH radicals generated in Comparative Example 5, It is clear that the catalyst activity is excellent. However, the catalytic activity is lower than in Examples 4 and 5 where the amount of copper donation was reduced. From this, it can be seen that even if the copper donation amount is excessively increased, the solid solution amount of copper ions is saturated, and for this reason, the catalytic activity is saturated. Therefore, the copper donation amount is more preferably 1.02 mol% or less.

  Further, for Example 6 and Comparative Example 6, the amount of OH radicals was measured with an ESR apparatus with the light irradiation integration time of 10 seconds. The result of the relative comparison is shown in FIG.

  The amount of OH radicals generated in Example 6 in which the copper donor amount was 0.13 mol% and the heat treatment temperature was 600 ° C. was the same as that of Comparative Example 6 in which the copper ion donor was not used and the heat treatment temperature was 600 ° C. The amount is significantly larger than the amount generated. Also from this, it is clear that excellent catalytic activity is manifested by dissolving copper ions.

DESCRIPTION OF SYMBOLS 10 ... TiO ₆ octahedron 12 ... Ti ion 14a-14f ... O ion 16 ... Cu ion 18 ... CuO ₄ octahedron 20a, 20b ... Hole

Claims (11)

A photocatalyst containing titanium, copper and oxygen as constituent ions,
A photocatalyst comprising an oxide in which a part of titanium ions in a crystal structure of titanium oxide is substituted with copper ions.   2. The photocatalyst according to claim 1, wherein the photocatalyst is made of particles, and the particles include crystal grains made of the oxide.   3. The photocatalyst according to claim 2, wherein the particle is a polycrystalline body in which crystal grains having different crystal orientations are aggregated.   The photocatalyst according to claim 2 or 3, wherein the average particle diameter of the particles is 10 to 60 nm, and the average crystal particle diameter of the crystal grains is 1 to 20 nm.   5. The photocatalyst according to claim 1, wherein sites of oxygen ions adjacent to titanium ions in the crystal structure of titanium oxide are vacancies in the crystal structure in which the titanium ions are replaced with copper ions. The photocatalyst characterized by becoming.   The photocatalyst according to any one of claims 1 to 5, wherein a phase in which a part of titanium ions in the crystal structure is replaced with copper ions has a brookite phase structure.   7. The photocatalyst according to claim 6, wherein the photocatalyst includes an anatase phase in which titanium ions are not substituted with copper ions in addition to the phase forming the brookite phase structure. A method for producing a photocatalyst comprising an oxide in which titanium, copper, and oxygen are included as constituent ions, and a part of the titanium ions in the crystal structure of titanium oxide are replaced with copper ions,
A titanium oxide precursor capable of producing titanium oxide by hydrolysis and polycondensation and a copper ion donor that is soluble in water or an organic solvent and can donate copper ions are mixed in a solvent. To obtain a mixed solution,
Obtaining an oxide in which some titanium ions in the crystal structure of titanium oxide are replaced with copper ions by hydrolyzing and polycondensing the titanium oxide precursor in the mixed solution;
Applying heat treatment to the oxide;
A process for producing a photocatalyst, comprising:   The method for producing a photocatalyst according to claim 8, wherein the heat treatment is held at a temperature of 400 to 700 ° C for 1 to 180 hours.   10. The method for producing a photocatalyst according to claim 8, wherein titanium alkoxide is used as the titanium oxide precursor.   The manufacturing method according to any one of claims 8 to 10, wherein the oxide is pulverized to obtain a pulverized product, and the pulverized product is subjected to the heat treatment to obtain a photocatalyst as particles. A method for producing a photocatalyst.

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