JP2009108374A - Method for producing noble metal particulate, photocatalyst used for the method and method for recovering noble metal from waste - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a noble metal particulate by which separation and recovery of the noble metal particulate can be efficiently performed by a simple operation without requiring an expensive reducing agent compared with the conventional method for producing a noble metal particulate. <P>SOLUTION: A aqueous solution containing noble metal ions is mixed with a water immiscible organic solvent containing an amphiphilic photocatalyst composed of a composite body of a polyoxo acid and a cationic surfactant, and the mixture is irradiated with light, thus, a noble metal particulate is precipitated on the boundary between the water and organic solvent into a sheet shape, so that the noble metal particulate can be produced. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for producing noble metal fine particles by photoreaction using a photocatalyst, a photocatalyst used in the method, and a method for recovering noble metal from waste using the method.

  Precious metals such as gold, platinum, silver and palladium are widely used as electrode materials for electronic devices, printed wiring materials, conductive fillers, catalyst materials, various sensor members, etc. Various manufacturing methods have been proposed for obtaining the product.

  For example, as a method for obtaining fine particles, a method of obtaining monodispersed gold fine particles by dissolving a gold-containing compound in an organic solvent, adding rosin thereto, and heating (see Patent Document 1), silver-palladium A method of forming a fine plate-like crystal using a colloid as a crystal nucleus (see Patent Document 2). After adding a sodium borohydride solution to a chloroauric acid aqueous solution, it is dispersed in an organic solvent incompatible with water. A method of forming gold fine particles by adding an octanethiol hexane solution and stirring (see Patent Document 3), adding toluene and a surfactant to a chloroauric acid aqueous solution, stirring, extracting chloroauric acid into toluene, and hydrogen A method in which a sodium borohydride solution is added to precipitate a solid, which is heat-treated, and then redissolved in toluene to precipitate all fine particles (see Patent Document 4) When the gold ion solution is mixed with the reducing agent solution to produce the gold fine particles, the reducing agent solution acting in the alkaline region and the gold ion solution are mixed in advance, and the mixed solution is then mixed with the alkaline solution. A method for producing gold fine particles having an average particle diameter of 3 nm or less by reducing gold ions (see Patent Document 5), a solution in which a polymer compound, a reducing agent and a silver salt are dissolved, at a temperature of 25 ° C. or more and 60 ° C. or less. A method for producing silver fine particles by stirring (see Patent Document 6) has been proposed so far.

  As a method for obtaining a noble metal thin film, electrolysis is performed by installing a cathode at a two-liquid phase interface formed by a noble metal electrolyte solution and a non-electrolyte organic liquid having a specific gravity larger than that of the electrolyte solution, and is deposited on the tip of the cathode. Then, a method of continuously depositing the noble metal thin film by controlling the distance between the grown noble metal thin film and the anode (see Patent Document 7), the liquid surface of the noble metal salt aqueous solution is covered with an organic liquid, and the cathode is formed at the two liquid phase interface. The method of depositing a noble metal thin film continuously by controlling the distance between the anode and the noble metal thin film that deposits and grows on the tip of the cathode (see Patent Document 8), etc. Proposed.

In addition, as a method for producing gold nanoplates, a hydrophilic polymer having a linear polyethyleneimine skeleton is dissolved in a solvent and precipitated by adding water to form a hydrogel of the hydrophilic polymer having a linear polyethyleneimine skeleton. A method of producing a gold nanoplate in which the hydrogel and gold chloride ions are mixed to form a gold plate (see Patent Document 9), a step of heating a HAuCl ₄ .3H ₂ O aqueous solution while stirring, the HAuCl _4. A step of adding a dispersion stabilizer to a 3H ₂ O aqueous solution, a step of adding a reducing agent to the aqueous solution to which the dispersion stabilizer has been added, and a step of heating and then cooling the mixture to form a gold nanoplate. A plate manufacturing method (see Patent Document 10) is known.

Thus, conventionally, noble metal fine particles are electrolyzed at the interface between the electrolyte solution and the non-electrolyte solution by adding a reducing agent to an aqueous solution containing noble metal ions to reduce the noble metal ions, or by performing electrolysis at the cathode. It is mainly manufactured by an electrolytic method in which a noble metal thin film is deposited at the tip.
However, in the above liquid phase reduction method, an expensive reducing agent must be used, and addition of a protective agent is indispensable for controlling the concentration of the reducing agent and the particle diameter of the noble metal fine particles. There is a drawback that it is difficult to control.

As a method for improving the disadvantages of such a liquid phase reduction method, a method has been proposed in which titanium oxide is used as a photocatalyst and noble metal ions are reduced by excited electrons generated on the photocatalyst by light irradiation to produce noble metal fine particles. (See Non-Patent Document 1).
This method preferentially reduces metals that have a low tendency to ionize, so that even if metals with a high tendency to ionize such as cadmium and nickel coexist, it also collects precious metal ions in the order of ppm. However, in the case of a heterogeneous catalyst such as titanium oxide, the noble metal precipitates on the surface of the catalyst, so that the active site decreases as the reaction proceeds, and the repeated use becomes impossible. is there.

  In order to overcome such drawbacks, there has been proposed a method for producing noble metal fine particles by reducing noble metal ions using polyoxoacid ions as a homogeneous photocatalyst (see Non-Patent Documents 2 and 3). Then, since the polyoxo acid catalyst is used as an aqueous solution, if it is used again as a raw material containing noble metal ions after being used, it is inevitable that by-products and impurities generated in the previous time will be mixed and contaminated.

On the other hand, with regard to precious metals, the mining volume has increased in recent years as demand has increased, and since there is a concern about the depletion of natural resources that are originally low in reserves, techniques for recovering from waste and waste liquid have attracted attention.
For example, as a method for recovering from a waste liquid, an electrolytic method or an adsorption method using a chelate ion exchange resin has been known for some time, and recently, a platinum group metal ion dissolved in an acidic solution is converted to a triethylamine or the like. A method of selectively recovering using an amino compound or a precipitating agent composed of this amino compound and a heteropolyacid such as silicomolybdic acid has been proposed (see Patent Document 11).

  However, in all known methods for producing and collecting noble metal fine particles, noble metal fine particles are produced in a dispersed state in a medium, and therefore, complicated operations must be performed to separate and collect them. However, it is inappropriate as a method that can be put into practical use, and a more efficient method for producing noble metal fine particles has been desired.

JP-A-5-117726 (Claims and others) JP-A-11-106806 (Claims and others) JP 2003-193118 A (Claims and others) JP 2003-49205 A (Claims and others) JP 2006-152438 A (Claims and others) JP 2005-105376 A (Claims and others) JP-A-5-339780 (Claims and others) JP-A-6-158379 (Claims and others) JP 2006-233290 A (Claims and others) JP 2006-37221 A (Claims and others) JP 2005-194546 A (Claims and others) “Environmental Science and Technology”, 1993, Vol. 27, p. 1776-1782 “Angevante Chemi International Edition”, 2002, vol. 41, p. 1911-1914 “New Journal of Chemistry”, 2001, Vol. 25, p. 361-363

  The present invention relates to a method for producing noble metal fine particles that can be separated and recovered efficiently and more easily by a simpler operation than conventional methods for producing noble metal fine particles, and a novel photocatalyst used in the method. It was made for the purpose of providing.

  As a result of intensive research to develop a method for efficiently producing noble metal fine particles, the present inventors have used two types of water and an organic solvent as a medium, a polyoxoacid and a cationic surfactant as a photocatalyst. By using an amphiphilic material composed of a composite of the above, photoreduction of noble metal ions, noble metal fine particles are formed in the form of a sheet at the interface between water and an organic solvent, and if this is utilized, waste It has been found that the precious metal fine particles therein can be efficiently recovered, and the present invention has been made based on this finding.

  That is, the present invention mixes an aqueous solution containing noble metal ions with a water-immiscible organic solvent containing an amphiphilic photocatalyst composed of a complex of a polyoxoacid and a cationic surfactant, and irradiates with water. From a noble metal fine particle production method characterized by precipitating noble metal fine particles in the form of a sheet at the interface between an organic solvent and an organic solvent, a photocatalyst comprising a complex of a polyoxoacid and a cationic surfactant, and industrial waste containing noble metals A water-immiscible organic solvent of an amphiphilic photocatalyst consisting of a complex of polyoxoacid and cationic surfactant after eluting the noble metal to prepare an aqueous solution containing noble metal ions and adding an electron donating reagent thereto Recovery of precious metal from waste characterized by separating and recovering precious metal fine particles deposited in a sheet form at the interface between water and an organic solvent after mixing with a solution and irradiating light It is intended to provide the law.

In the method of the present invention, a novel amphiphilic photocatalyst obtained by complexing an anionic polyoxoacid and a cationic surfactant is used, but this is insoluble in water and soluble in an organic solvent. When this is dissolved in a water-immiscible organic solvent and used, polyoxoacid ions having high hydrophilicity gather at the interface between the aqueous phase and the organic solvent phase.
Accordingly, when ultraviolet rays are irradiated in this state, the noble metal ions are reduced at the interface between the aqueous solution containing the noble metal ions and the organic solvent containing the catalyst, and the noble metal fine particles are deposited in a sheet form.

Next, the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a schematic diagram for explaining a reaction mechanism in the method of the present invention. A beaker 1 includes water 3 containing a noble metal ion M ⁺ and a photocatalyst 5 composed of a polyoxoacid and a cationic surfactant complex. An organic solvent 4 is charged. When ultraviolet rays are irradiated from the ultraviolet lamp 2 in such a state, the noble metal ions are reduced by the amphiphilic photocatalyst 5 present at the interface, and the noble metal fine particles are deposited in the form of a sheet at the interface.

Therefore, the noble metal fine particles can be easily recovered by separating and drying the noble metal fine particle sheet.
The liquid phase after collecting the noble metal fine particles consists of two phases, an aqueous phase and an organic solvent phase, but the used catalyst remains dissolved in the organic solvent phase. By separating from the phase, it can be used repeatedly.

Thus, in the conventional method using polyoxoacid as a catalyst, as described above, it was difficult to repeatedly use this, but in the method of the present invention, polyoxoacid was combined with a cationic surfactant. As a result, the catalyst can be used repeatedly because it is soluble in an organic solvent and can be easily separated from the aqueous phase containing the raw material.
Further, the sheet-like noble metal fine particles obtained as described above are used in the form of a powder in the form of a sheet or after being pulverized as desired.

  Examples of the polyoxoacid constituting the photocatalyst of the present invention include isopolyoxoacids such as polytungstic acid, polymolybdic acid, and polyvanadate, tungstosilicic acid, tungstophosphoric acid, molybdosilicic acid, molybdophosphoric acid, and vanadolinic acid. , Heteropolyacids such as vanadosilicate are used. These are used as water-soluble salts such as sodium salts, potassium salts, ammonium salts and the like.

  As the other component, the cationic surfactant, for example, the hydrophilic portion is composed of a cation such as a quaternary ammonium ion, an imidazolium ion, a pyridinium ion, a quinolinium ion, or an isoquinolinium ion, and is hydrophobic. Examples thereof include a long chain alkyl group having 10 to 20 carbon atoms, for example, a decyl group, a cetyl group, an okdadecyl group, a palmityl group, an oleyl group, or a benzyl group, but dimethyldioctadecyl ammonium chloride. Cetyltrimethylammonium bromide, stearyltrimethylammonium chloride, cetyldimethylethylammonium bromide, octadecyldimethylethylammonium bromide, methyldodecylbenzyltrimethylammonium chloride, dodecylpyridinium bromide Bromide, cetyl pyridinium chloride, octadecyl pyridinium bromide, 2-dodecyl isoquinolinium bromide is preferred.

  A complex of a polyoxo acid and a cationic surfactant can be easily produced by mixing aqueous solutions in which each is dissolved. The mixing ratio in this case is selected at a molar ratio corresponding to each charge. For example, when a complex is produced by reacting a tetravalent polyoxoacid with a monovalent cationic surfactant ion, a molar ratio of 1: 4 is selected. Since a precipitate is formed by this reaction, this is filtered off, washed thoroughly with water, and dried before use.

  Next, the photocatalyst thus obtained is used after being dissolved in a water-immiscible organic solvent. As this organic solvent, for example, chloroform, carbon tetrachloride, benzene, ethylbenzene, toluene, xylene, n-hexane, n-pentane, n-heptane, cyclohexane, methyl isobutyl ketone and the like are used. At this time, the generation state of the noble metal sheet varies depending on the specific gravity difference between the organic solvent and water. For example, if the reaction is performed using chloroform having a specific gravity greater than that of water, a noble metal sheet is generated in the aqueous phase on the upper surface of the organic solvent phase, and if the reaction is performed using n-hexane having a specific gravity smaller than that of water. A noble metal sheet is formed in the aqueous phase under the organic solvent phase.

  Next, a noble metal chloride, sulfate, nitrate, or the like is used as a source of noble metal ions. As the concentration at this time, a range of 1 to 100 mM, preferably 5 to 50 mM is selected, but a lower concentration aqueous solution can be used. The hydrogen ion concentration in this case is preferably pH 3-9. If the pH is higher than this, the polyoxoacid becomes unstable, and if the pH is lower than this, the produced noble metal fine particles may be redissolved.

  In general, when a reduction reaction is performed using a photocatalyst, an oxidation reaction corresponding to the reduction reaction is required. Therefore, it is necessary to add an electron-donating reagent, that is, an electron donor. In the method of the present invention, an organic solvent is an electron donor. Therefore, noble metal fine particles can be obtained without adding an electron donor.

  However, by adding an electron donor, the photoreduction rate of polyoxoacid ions is greatly improved, and the generation rate of noble metal fine particles is increased. The amount of electron donor added at this time is preferably about the same as the number of moles of noble metal ions used.

  As the electron donor, a primary or secondary alcohol, an organic acid or a salt thereof is used. Preferred electron donors are methanol, ethanol, 2-propanol, oxalic acid, sodium salicylic acid, and the like. In addition, waste liquid containing organic substances can be used.

  In the method of the present invention, ultraviolet rays are irradiated while slowly mixing the organic solvent solution containing the photocatalyst prepared as described above and the aqueous solution containing the noble metal ions. As the ultraviolet rays at this time, it is necessary to select a wavelength region having energy higher than the band gap of the photocatalyst. This wavelength region is preferably 400 nm or less.

  The sheet of noble metal fine particles formed at the interface between the organic phase and the aqueous phase by this ultraviolet irradiation is collected and separated on a filter paper, for example, and dried at room temperature or while heating. In this way, noble metal fine particles having a particle diameter of about 0.1 to 5 μm are obtained.

Next, in order to recover noble metals from industrial waste containing noble metals such as silver, gold, platinum, etc. by the method of the present invention, first, these noble metals are dissolved with an acid to prepare an acid solution. After adding the electron donating reagent, a solution prepared by dissolving an amphiphilic photocatalyst composed of a complex of polyoxoacid and a cationic surfactant in a water-immiscible organic solvent is slowly added, mixed, and irradiated with light. By this treatment, noble metal fine particles are deposited in the form of a film at the interface between the aqueous phase and the organic phase.
Therefore, if this membrane is separated, washed with water and dried, the desired noble metal can be recovered.

  According to the present invention, by using a novel photocatalyst, a noble metal can be efficiently recovered from an aqueous solution containing a small amount of noble metal ions by a simple operation. In addition, the photocatalyst used at this time has an advantage that it can be used repeatedly without a significant decrease in efficacy.

  Next, the best mode for carrying out the present invention will be described by way of examples, but the present invention is not limited to these examples.

Dissolve 0.1 mmol of dimethyldioctadecyl ammonium chloride (hereinafter abbreviated as DODA) in 10 ml of water, and separately add 10 ml of water adjusted to pH 2 with 0.045 mmol of sodium 12-tungstosilicate [(SiW ₁₂ O ₄₀ ) Na ₄ ]. Two types of aqueous solutions were prepared by dissolving in 2%.
Next, when these two aqueous solutions were mixed at room temperature and stirred for 10 minutes, a precipitate was formed. The precipitate was filtered off, washed with water until the by-product sodium chloride was completely removed, filtered, and dried under vacuum at 30 ° C. for 60 minutes to obtain a SiW ₁₂ O ₄₀ ⁴⁻ / DODA complex photocatalyst. 200 mg was obtained as a white powder.

Decathungstic acid (W ₁₀ O ₃₂ H ₄ ) aqueous solution was prepared by adding hydrochloric acid to 0.1 M sodium tungstate (Na ₂ WO ₄ ) aqueous solution and adjusting the pH to 2.
Next, 0.8 mmol of dimethyl dioctadecyl ammonium chloride was dissolved in 100 ml of water adjusted to pH 2 with hydrochloric acid, and mixed with 100 ml of an aqueous solution of decatungstic acid prepared as described above to a concentration of 2 mM at room temperature. . The produced precipitate was sufficiently washed with water and then dried at 50 ° C. under vacuum to obtain 500 mg of W ₁₀ O ₃₂ ⁴⁻ / DODA complex photocatalyst as a white powder.

The XRD pattern of the composite photocatalyst produced in Examples 1 and 2 is shown in FIG. In these patterns, a sharp peak was observed on the low angle side near 2θ = 2 °, and this composite photocatalyst was confirmed to have a layered structure in which DODA and inorganic layers were alternately arranged.
Next, the absorbance of the composite photocatalyst in Examples 1 and 2 is shown in FIG. As can be seen from FIG. 3, the absorbance increased from around 360 to 400 nm, and these composite photocatalysts can utilize light having a shorter wavelength.

In a 100 ml volume beaker, 0.01 g of the composite photocatalyst produced in the same manner as in Example 1 was dissolved in 20 ml of chloroform, and 20 ml of a 7.5 mM chloroauric acid aqueous solution was gradually added to the upper surface of the solution. Subsequently, the mixture was irradiated with ultraviolet rays for 5 hours from above the beaker using a 100 W high pressure mercury lamp in the atmosphere at room temperature.
After completion of the reaction, a golden sheet-like product was observed at the interface between the organic phase and the aqueous phase. A scanning electron micrograph of this product is shown in FIG. 4, and a transmission electron micrograph is shown in FIG.
From the electron diffraction pattern, this product was confirmed to be gold oriented in the <111> direction.

In a 100 ml volume beaker, 0.01 g of the composite photocatalyst produced in the same manner as in Example 2 was dissolved in 20 ml of chloroform, and 7.5 mM chloroauric acid and sodium oxalate at the same concentration were contained on the upper surface of the solution. 20 ml of an aqueous solution was gradually added. Next, using a 100 W high-pressure mercury lamp, the mixture was irradiated with ultraviolet rays for 5 hours at room temperature in the atmosphere from the top of the beaker.
After completion of the reaction, particles were formed in a sheet form at the interface between the organic phase and the aqueous phase. A scanning electron micrograph of the product is shown in FIG. This confirmed that the product was composed of gold particles having a particle diameter of about 1 μm.

In a 100 ml volume beaker, 10 mg of the composite photocatalyst obtained in Example 2 was dissolved in 20 ml of chloroform, and 20 ml of an aqueous solution containing sodium oxalate equivalent to 7.5 mM silver nitrate was gradually added to the upper surface of the solution. .
Next, using a 100 W high-pressure mercury lamp, the mixture was irradiated with ultraviolet rays for 5 hours at room temperature in the atmosphere from the top of the beaker.
After the reaction was completed, silver particles were formed in a sheet shape at the interface between the organic phase and the aqueous phase. A scanning electron micrograph of the product thus obtained is shown in FIG.

  Since the noble metal contained in a trace amount in the aqueous solution can be efficiently separated by a simple operation, it is suitable as a means for recovering the noble metal from the industrial waste liquid.

The schematic diagram for demonstrating the reaction mechanism in this invention method. The XRD pattern of the composite photocatalyst obtained in Examples 1 and 2. The graph which shows the relationship between the irradiation light wavelength of the composite photocatalyst obtained in Example 1 and 2, and a light absorbency. 4 is a scanning electron micrograph of gold particles obtained in Example 3. FIG. 4 is a transmission electron micrograph of gold particles obtained in Example 3. FIG. 4 is a scanning electron micrograph of gold particles obtained in Example 4. FIG. 6 is a scanning electron micrograph of silver particles obtained in Example 5. FIG.

Explanation of symbols

1 Beaker 2 UV lamp 3 Water 4 Organic solvent 5 Photocatalyst

Claims (7)

  Mixing an aqueous solution containing noble metal ions with a water-immiscible organic solvent containing an amphiphilic photocatalyst composed of a complex of polyoxoacid and cationic surfactant, and irradiating with light, the interface between water and the organic solvent A method for producing noble metal fine particles, comprising depositing noble metal fine particles in a sheet form.   2. The method for producing noble metal fine particles according to claim 1, wherein the noble metal ion is an ion of at least one kind of noble metal selected from gold, silver, platinum, palladium, rhodium, iridium and ruthenium.   The polyoxoacid is at least one of polytungstic acid, polymolybdic acid, polyvanadate, tungstosilicate, tungstophosphoric acid, molybdosilicic acid, molybdophosphoric acid, vanadolinic acid, and vanadosilicic acid. Method for producing noble metal fine particles.   The method for producing noble metal fine particles according to claim 1, 2 or 3, wherein the aqueous solution containing the noble metal ions further contains an electron donating reagent.   The method for producing noble metal fine particles according to claim 4, wherein the electron donating reagent is at least one selected from primary or secondary alcohols, organic acids and salts thereof.   A photocatalyst comprising a complex of a polyoxoacid and a cationic surfactant.   An amphiphilic photocatalyst consisting of a complex of polyoxoacid and cationic surfactant after eluting precious metal from industrial waste containing precious metal to prepare aqueous solution containing precious metal ions, adding electron donating reagent to it A method for recovering noble metal from waste, comprising: mixing with a water-immiscible organic solvent solution, and irradiating with light; and separating and recovering noble metal fine particles deposited in a sheet form at the interface between water and the organic solvent.