Organic Hybrid

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Photoinduced Recovery of Gold Using an Inorganic/Organic Hybrid Photocatalyst Tetsuya Kida,*,† Ryuji Oshima,‡ Kazuhiro Nonaka,§ Kengo Shimanoe,† and Masamitsu Nagano‡ Department of Energy and Material Sciences, Faculty of Engineering Sciences, Kyushu UniVersity, Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan, Department of Chemistry and Applied Chemistry, Faculty of Engineering and Science, Saga UniVersity, Honjyo, Saga 840-8502, Japan, and Measurement Solution Research Center, National Institute of AdVanced Industrial Science and Technology (AIST) Kyushu, Shuku-machi, Tosu, Saga 841-0052, Japan ReceiVed: July 23, 2009; ReVised Manuscript ReceiVed: October 1, 2009

Photorecovery of gold from solution using an amphiphilic photocatalyst based on a polyoxometallate (W10O324-)/surfactant (dimethyldioctadecylammonium) hybrid was investigated. The prepared inorganic/organic hybrid dissolved in chloroform successfully photoreduced gold ions dissolved in water in a two-phase system under UV irradiation (λ > 320 nm). This resulted in the formation of gold colored films consisting of micrometer-sized hexagonal gold sheets with 〈111〉 crystal orientation at the liquid/liquid interface. The recovery can be performed at environmentally friendly pH values of 3-6. The recovery rate was dependent on the concentrations of the catalyst and a sacrificial agent (electron donor). SEM observations revealed that the catalyst has a morphology directing role in the formation of gold particles due to its preferential adsorption on gold. This makes gold nuclei formed at the interface grow into microsized large particles that can be readily collected via filtration. The hybridization of the polyoxometallate with the surfactant allows for the repeated use of the catalyst. Moreover, the addition of thiol compounds was found to significantly improve the recovery due to their covalent adsorption on gold and assistance with particle aggregation. The results show promising gold recovery feasibility using the hybrid catalyst as a new green process. Introduction The recovery and recycling of gold have become particularly important due to its scarcity and the increasing demands for electronic devices, catalysis, and biological applications. Currently, special attention has been paid to recovering gold from electronic wastes such as personal computers and cellular phones, which are considered to be new promising sources having a higher gold content than gold ores. Usually, gold is recovered from ores or wastes using a series of steps, that is, dissolution (leaching with chloride or cyanide), conditioning, and solvent extraction or ion exchange. Various extracting solvents (chelating reagents),1 activated carbon adsorbents,2 and ion-exchange resins3 have so far been used for the recovery of gold from leach solutions. Recently, biomass-originated materials have been intensively studied as cheap and environmentally benign adsorbents, including tannin,4 lignin from wood,5 and cellulose from waste paper.6 The promising feature of the recovery using these adsorbents is that adsorbed gold ions are reduced with functional groups such as amino and hydroxyl groups anchored on their surfaces to form fine gold particles, which can be collected as a solid form. However, separation of the gold particles from adsorbents requires time-consuming procedures such as dissolution, ultrasonic irradiation, or calcination. Thus, the recycling of adsorbents is rather difficult. Therefore, there is still demand for the development of a greener and cheaper process to recover gold from solution. * Corresponding author. Tel.: +81-92-586-7537. Fax: +81-92-586-7538. E-mail: [email protected]. † Kyushu University. ‡ Saga University. § National Institute of Advanced Industrial Science and Technology.

Photocatalysis using semiconductor particles (photocatalysts) has received considerable attention as an alternative green approach for the recovery of metals from wastewater that utilizes solar energy.7 UV light irradiation on semiconductor particles results in the generation of electron-hole pairs; the photogenerated electrons reduce metal ions and form elemental metal particles, making it possible to recover the metals from wastewater. Oxidation reactions by photogenerated holes take place concurrently, which in turn are utilized for the destruction of organic pollutants in wastewater. It has been demonstrated that a variety of metals such as Cu,7a,g,k Pt,7b,g Au,7b,g Ag,7e,g,h Hg,7c,d,g,i and Pb7c,f,j can be recovered by using semiconductor particles such as TiO2 and WO3 under light irradiation. One of the drawbacks associated with this method is that the recovery rate tends to decrease as the reaction proceeds because of the deposition and accumulation of recovered metal particles on active sites of the catalysts (poisoning). To overcome this problem, Papaconstantinou et al. have developed a promising method that utilizes water-soluble W- or Mo-based polyoxometallates (POMs) as homogeneous photocatalysts in place of heterogeneous photocatalyst particles.8 The method allows for the spontaneous separation of deposited metal particles from the water-soluble photocatalyst and avoids catalyst poisoning by metal deposition. However, the removal of water-soluble POM catalysts from treated solutions requires additional treatments such as extraction or drying, making recycling of the catalyst difficult. The dissolved POMs may become another source of water pollution. We have recently reported an alternative method of reducing gold ions using an amphiphilic POM/surfactant hybrid photocatalyst based on SiW12O404-.9 In this method, the inorganic/ organic hybrid photocatalyst is dissolved in an organic solution.

10.1021/jp906981q CCC: $40.75  2009 American Chemical Society Published on Web 10/26/2009

Gold Recovery Using Hybrid Photocatalyst

J. Phys. Chem. C, Vol. 113, No. 46, 2009 19987

SCHEME 1: Reduction Process of Gold Ions at the Interface between Water and Chloroform Using an Amphiphilic POM/Surfactant Photocatalyst under UV Irradiationa

a (1) The POM/surfactant photocatalyst is dissolved in an aqueous-organic two-phase system, (2) UV light is irradiated to the system to reduce the POM, and (3) the photo-reduced POM reduces AuCl4- to form gold particles at the interface.

TABLE 1: Typical Experimental Conditions for Recovery of Gold Using the Hybrid Photocatalyst under UV Irradiation (λ > 320 nm) in Air at Room Temperature volume of aqueous phase/mL

HAuCl4 concentration/mol L-1

pH of aqueous phase

volume of organic (chloroform) phase/mL

catalyst concentration/g L-1

concentration of sacrificial agent (n-hexanol)/mol L-1

irradiation time/h

10

0.015

2

10

1.0

0.8

2-40

When the catalyst is photoexcited with near-visible UV light, it reduces gold ions dissolved in an aqueous solution at the liquid/liquid interface, as shown in Scheme 1. Sheet-like gold particles were produced as a result of the reduction occurring at the two-dimensional confined space. In this study, we intensively studied the feasibility of this method for the recovery of gold. One of its advantages is that the separation of catalysts from the reaction system is easier, as compared to the above recovery method developed by Papaconstantinou et al.8 Moreover, the method produces large microsized particles, which can be readily collected. Here, W10O324- was used as the POM photocatalyst because of its good activity under near-UV irradiation (300-400 nm).10 We examined the effects of several experimental conditions, including pH, concentrations of the catalyst and a sacrificial agent (electron donor), as well as the addition of thiols on the photorecovery of gold. Experimental Section Reagents. Dimethyldioctadecylammonium chloride (DODA), sodium tungstate (Na2WO4), chloroauric acid tetrahydrate (HAuCl4), ethanol, n-hexanol, 2,3-dimercapto succinic acid (DMSA), n-octadecanethiol (ODT), and 1,6-hexanedithiol (HDT) were purchased from WAKO Pure Chemical Industries, Ltd. All of these chemicals were of analytical grade and were used without further purification. Catalyst Preparation. W10O324- was prepared via acid condensation of WO42- by controlling the pH of a Na2WO4 solution (100 mL, 0.4 mmol/L) to 2.0 with hydrogen chloride under stirring. DODA (1.6 mmol) was dissolved in water (100 mL) under ultrasonic irradiation. The pH of the solution was also controlled to 2.0 with hydrogen chloride. The DODA solution was mixed with the W10O324- solution under vigorous stirring for 30 min, resulting in the formation of a white precipitate. The precipitated product, a W10O324-/DODA hybrid, was then collected by filtration, washed with copious amounts of water, and dried under vacuum at room temperature. The crystal structure of the hybrid was determined using X-ray powder diffractometry with Cu KR radiation (XRD; RINT1000, Rigaku Co., Ltd.), and the light absorption properties were measured with a spectrometer (Lambda 19, Perkin-Elmer Co., Ltd.).

Photoreduction Measurements. The prepared amphiphilic catalyst (0.01 g) was dissolved in chloroform (10 mL) in a quartz beaker (100 mL, 5.5 cm in diameter), to which an aqueous HAuCl4 solution (10 mL, 15 mmol/L) was slowly added. n-Hexanol (8.0 mmol) was then added to the aqueous-organic two-phase solution as an electron donor. n-Hexanol, which has a low solubility in water, was chosen to avoid direct photoreduction of gold ions in the aqueous phase. In some cases, thiol compounds (3.5 × 10-4 mmol) such as DMSA, ODT, and HDT were added to the system. The solution was irradiated from the lateral side of the beaker using a 300 W Xe lamp (UXL-300D, USHIO Inc.) with a 320 nm optical cutoff filter for 1-40 h at room temperature in air. The separation distance between the light bulb and the beaker was set to 15 cm. The radiation energy of the lamp with the optical filter to the system was measured to be 1.7 (340 nm) and 0.81 (330 nm) mW/cm2 using an optical power meter (PD-300UV, Ophir Optronics Ltd.). The typical experimental conditions are summarized in Table 1. The recovery percentage of gold was estimated from the concentration of HAuCl4, measured by UV-vis absorption spectroscopy, during photocatalysis. The morphology and crystal structure of the reaction products were analyzed by field emission scanning electron microscopy (FE-SEM; JSM-6340F, JEOL Co., Ltd.) and X-ray diffractometry (XRD; RINT-1000, Rigaku, Co., Ltd.) using Cu KR radiation, respectively. Results and Discussion Catalyst Characterization. Figure 1 shows the XRD pattern of the W10O324-/DODA hybrid photocatalyst, prepared by complexization of W10O324- with DODA in water. It shows a characteristic pattern of an ordered multilayer structure, which consists of alternating organic and inorganic layers with a basal spacing of 3.5 nm. The results indicate the incorporation of W10O32 anions in organic matrices. The pattern is in good agreement with the results obtained by Moriguchi et al.,11 who reported that W10O324- is hybridized with DODA in a 1:4 molar ratio according to elemental analysis. The prepared hybrid photocatalyst was readily dissolved in various nonpolar solvents such as chloroform and toluene because of the hydrophobic nature of DODA. Figure 2 shows the UV-vis absorption spectra of the hybrid photocatalyst dissolved in chloroform. The

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Kida et al. 40 3W10O532 + AuCl4 f Au + 3W10O32 + 4Cl

Figure 1. XRD pattern of the amphiphilic photocatalyst prepared by mixing aqueous solutions containing dimethyldioctadecylammonium (DODA) chloride and W10O324-.

(2)

Figure 4 shows the concentration of AuCl4- in the aqueous phase in the presence of the hybrid catalyst as a function of UV irradiation time. The AuCl4- concentration decreased with time, and after 35 h of irradiation, AuCl4- almost completely vanished. The apparent reduction rate is much higher than that without the catalyst, indicating the catalytic role of the hybrid in reducing gold ions. Figure 5 shows the XRD patterns of the products at different reaction times. The patterns are in good agreement with that of zerovalent gold (JCPDS: 04-0784), confirming the reduction of gold ions to form gold particles. It is noted that the gold crystals formed have a preferential orientation in the direction, as manifested by the higher peak intensity at around 38° in the patterns. SEM observation was used to check the morphology of gold particles collected after different reaction times. Figure 6 shows the SEM images of the gold particles formed under UV irradiation in the presence and absence of the catalyst. The photocatalytic reduction of gold ions at the interface produced hexagonal-shaped gold particles with lateral sizes of about 10 µm. The shape of the gold particles (Figure 6a and b) is completely different from those formed in the absence of the catalyst, shown in Figure 6d. The previous study suggested that POMs self-assemble when adsorbing on the gold {111} plane and thus induce anisotropic growth of gold nuclei along other planes such as {110} and {100}.9 This idea is supported by the observed oriented growth of gold

Figure 2. UV-vis absorption spectra of (a) W10O324-/DODA and (b) W10O324- dissolved in chloroform and water, respectively.

characteristic light absorption spectrum for W10O324- shows a peak around 260 nm due to the ligand to metal charge transfer (LMCT). This is well fitted to the spectrum of W10O324dissolved in water (Figure 2b). The threshold absorption wavelength is judged to be near 400 nm for the hybrid photocatalyst. These results suggest that the hybrid can act as a photocatalyst under UV light irradiation (λ < 400 nm) even in an organic solution. Photocatalytic Recovery of Gold. Figure 3 shows photographs of the reaction system before (a) and immediately after (b) UV irradiation (λ > 320 nm). The upper solution (aqueous phase), where AuCl4- is dissolved, turned transparent after UV irradiation, while the lower solution (organic phase, chloroform) turned blue, accompanied by the formation of gold colored films at the interface. This phenomenon is explained as follows: The color change of the organic phase corresponds to the formation of W10O325- by photoreduction of W10O324- with alcohol. The photoreduced W10O325- transferred electrons to gold ions to form gold particles at the interface. This was followed by the oxidation of W10O325- to form W10O324-. Oxidation with atmospheric oxygen can also take place. The above catalytic cycle can be expressed by the following reaction equations:

2W10O324-+ CH3(CH2)4CH2OH + hν f 2W10O532 + CH3(CH2)4CHO + 2H+ (1)

Figure 3. Photographs of the two-phase reaction system (a) before and (b) immediately after UV irradiation for 20 h. Inset of (b) shows the overhead image of the reaction system. The lower solution turned colorless by the oxidation of W10O325- with atmospheric oxygen within 30 min after the test.

Figure 4. Time dependence of AuCl4- concentration in the aqueous phase under UV irradiation in the (a) presence and (b) absence of the hybrid W10O324-/DODA photocatalyst.



Figure 5. XRD patterns of the products after UV irradiation for different periods.

Figure 7. Time dependence of AuCl4- concentration in the aqueous phase at different pH values under UV irradiation in the presence of the hybrid catalyst.

Figure 6. SEM images of the products after UV irradiation for (a) 5, (b) 10, and (c) 20 h in the presence of the catalyst. Part (d) shows the images of the products after UV irradiation for 20 h in the absence of the catalyst.

Figure 8. Dependence of the sacrificial agent (n-hexanol) concentration on gold recovery after UV irradiation for 25 h. The other experimental conditions were the same as those presented in Table 1.

crystals shown in Figure 5. The SEM images (Figure 6a and b) also indicate the presence of irregularly shaped particles, which may be mainly responsible for the diffraction from {110} and {100} planes. Such particles are possibly formed via direct photoreduction of gold ions at the interface or in the water phase. On the other hand, after UV irradiation for 35 h, heavy aggregation of particles occurred on recovery from the reaction system (Figure 6c). This probably resulted from the highly anisotropic shape of the sheet-like gold particles. Effects of pH. It is known that the stability of POMs is strongly dependent on the pH of the solution. In an alkaline solution, POMs tend to decompose to form their constituting small oxyanions. On the other hand, in a highly acidic solution, the reduction of gold ions competes with the dissolution of gold particles. Thus, it is quite important to study the pH dependence of gold recovery. Figure 7 shows the concentration of AuCl4in aqueous phases with different pH values as a function of UV irradiation time. The recovery was always the lowest at pH 1. However, no significant decreases in the reduction rate were observed at pH values of 2, 4, or 6. These results suggest that the present recovery process is not significantly influenced by solution pH and can be carried out at environmentally friendly pH values.

Effects of Sacrificial Agent and Photocatalyst. We next examined the effects of the concentrations of the sacrificial agent (n-hexanol) and the photocatalyst in the organic phase on the gold recovery. Figure 8 shows the dependence of the recovery percentage of gold after 25 h of UV irradiation on the n-hexanol concentration. The recovery rate increased with an increasing n-hexanol concentration in the organic phase but was almost saturated at concentrations higher than 0.5 mol L-1. The role of n-hexanol is to donate electrons to W10O324-, producing a reduced form of the POM under UV irradiation according to reaction 1. It is thus probable that at lower concentrations, n-hexanol was rapidly consumed as the reaction cycles proceeded, decreasing the rates of reactions 1 and 2. In contrast, at higher concentrations, sufficient amounts of electrons can be supplied to the catalyst, thereby keeping the recovery rate constant. Figure 9 shows the effect of catalyst concentration on the recovery percentage of gold after UV irradiation for 25 h. Similarly, the recovery rate increased with an increasing catalyst concentration and was saturated at concentrations higher than 2.0 mol L-1. It is suggested that increasing the concentration of the catalyst assists in the formation of a reduced form of the POM and speeds up the gold recovery, as in the case above. To examine the role of the catalyst in more detail, the morphology of the products formed with different catalyst

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Figure 9. Dependence of the catalyst concentration on gold recovery after UV irradiation for 25 h. The other experimental conditions were the same as those presented in Table 1.

Figure 10. SEM images of the products after UV irradiation for 25 h in the presence of the catalyst at different concentrations: (a) 0.25, (b) 0.75, (c) 2.0, and (d) 5.0 g L-1.

concentrations was observed by SEM. Figure 10 shows SEM images of the products formed after 25 h of UV irradiation in the presence of different concentrations of the catalyst. At lower concentrations, aggregates of irregularly shaped particles were mainly formed, while at higher concentrations large sheet-like particles more than 1 µm in lateral size were formed in aggregates. Micrometer-sized wires were also formed at concentrations higher than 5.0 g L-1, together with spherical and irregularly shaped particles. There have been a large number of reports on the preparation of various gold nanostructures such as nanowires,12 nanosheets or nanoplates,13 and nanorods14 using organic templating agents. In particular, the use of surfactants such as polyvinylpyrrolidone (PVP) is reported to successfully produce gold nanosheets and nanowires.13b Thus, the observed morphology changes of the obtained gold particles with an increasing catalyst concentration clearly indicate the morphology directing role of the amphiphilic catalyst. Because the large sheet-like particles tend to self-assemble into aggregates due to their high anisotropic shape as observed in the SEM images, the morphology-dictating properties of the developed catalyst make it effective for collecting gold particles in the present process. Recycling of Catalyst. To examine the feasibility of the present photorecovery process, the reusability of the catalyst

Kida et al.

Figure 11. Recovery percentages of gold after UV irradiation for 25 h without an optical cutoff filter versus the number of reaction cycles. At the sixth cycle, the recovery test was performed without the catalyst. The other experimental conditions were the same as those presented in Table 1.

was tested. In the recycling test, an aqueous solution of the system was replaced with a fresh one containing AuCl4- after gold recovery and filtration of products. The next photorecovery test was then performed using the original chloroform solution containing the catalyst. Figure 11 shows the recovery percentages of gold after UV irradiation for 25 h versus the number of reaction cycles. The recovery rate remained constant in the second cycle, but decreased by about 20% after that. One of the possible reasons for this is a decrease in the catalyst concentration in the chloroform solution due to adsorption of the catalyst on the gold particles that were filtered. Other possible reasons are the consumption of sacrificial agents and degradation of the hybrid catalyst after UV irradiation for long periods, although the details of this have yet to be elucidated. Nevertheless, the recovery rates after recycling were still higher than those obtained without the catalyst (sixth cycle). The apparent turnover numbers (TONs), defined as the ratio of the amount of gold ions reduced to that of W10O324- contained in the hybrid catalyst, of the reactions after 25 h were estimated to be 57-71, indicating that the reactions proceeded catalytically. Effects of Thiols. In the present process, the reaction space is limited to a two-dimensional confined area. Accordingly, the reaction rate is essentially lower than that of homogeneous catalytic reactions. Thus, the process needs further modification. It is well-known that thiol compounds covalently adsorb on gold and form self-assembled monolayers (SAMs).15 Utilizing the self-assembly adsorption of thiols on gold, a large number of methods have been developed to prepare gold nanoparticles functionalized with a wide variety of surface ligands.16 Of these, hydrophilic17 and hydrophobic thiol18 compounds have been used to make gold nanoparticles dispersible in water and organic solvents, respectively. Such thiol ligands are also used to transfer gold nanoparticles from water to an organic solvent or vice versa (spontaneous phase transfer).19 It is expected that hydrophobic thiols effectively attract gold nuclei at the interface and accelerate the recovery of gold. Hence, the addition of thiol additives to the reaction system was investigated to improve the recovery. Three thiol compounds, n-octadecanethiol (ODT), 1,6-hexanedithiol (HDT), and 2,3-dimercapto succinic acid (DMSA), were used. Their chemical structures are displayed in Figure 12. Figure 13 shows the concentration of AuCl4- in the aqueous phase as a function of UV irradiation time in the presence of



Figure 12. Chemical structures of (a) n-octadecanethiol (ODT), (b) 1,6-hexanedithiol (HDT), and (c) 2,3-dimercaptosuccinic acid (DMSA) used as additives for gold recovery.

Figure 13. Time dependence of AuCl4- concentration in the aqueous phase under UV irradiation in the presence and absence of thiols.

the thiol compounds. The apparent recovery rate increased in the presence of ODT and the recovery was almost completed within 25 h, which is shorter than the time taken without ODT (35 h). The recovery rate was also increased using HDT, alkanedithiol, and was further increased by the coaddition of water-soluble DMSA. It is noteworthy that the addition of such a small amount of the thiols (3.5 × 10-4 mmol) improved the recovery, although it is difficult to recycle the thiols. Figure 14 shows representative SEM images of gold particles recovered after UV irradiation for 1-15 h in the presence of the thiols. The morphologies of the particles are not significantly different from those obtained without the thiols. However, the ratio of hexagonal particles to other particles was apparently increased, particularly when both HDT and DMSA were added into the system. In the photocatalytic reduction of AuCl4-, the hybrid catalyst that adsorbs on gold nuclei is able to transfer electrons to AuCl4- through the gold nuclei. Gold can serve as an electron reservoir due to its large work function (ca. 5 eV). This indirect electron transfer from the hybrid catalyst to AuCl4- makes gold nuclei grow into the observed large hexagonal crystals. Thus, it is likely that hydrophobic ODT and HDT attract gold nuclei at the interface and assist in the adsorption of the hybrid catalyst, resulting in the increase in the reduction rate of AuCl4-, as expected. In addition, dithiols of DMSA and HDT may also serve as cross-linkers of gold nuclei formed at the interface and accelerate further catalyst adsorption, thereby increasing the

Figure 14. SEM images of the products obtained after UV irradiation for different times in the presence of the catalyst and thiols: (a) 5 h, ODT, (b) 15 h, ODT, (c) 1 h, ODT/DMSA, (d) 5 h, ODT/DMSA, (e) 5 h, HDT, (f) 15 h HDT, (g) 5 h, HDT/DMSA, and (h) 15 h, HDT/ DMSA.

reduction rate. However, the detailed mechanism of how thiols work in photocatalytic gold recovery is not fully understood and is currently under investigation. The addition of thiols offers another benefit to the present recovery process. Figure 15 shows photographs of gold particles formed at the interface in the presence of HDT and DMSA after UV irradiation for 15 h. Note that the particles are heavily

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Kida et al. and a joint research program between Saga University and the National Institute of Advanced Industrial Science and Technology (AIST), Kyushu. References and Notes

Figure 15. Photographs of gold particles recovered using HDT and DMSA after UV irradiation for 15 h. Inset shows the photograph of the gold aggregates collected with tweezers.

aggregated and easily collected without filtration, as shown in the image where particles are collected with tweezers. This observation supports the above idea that the thiols attract gold particles at the interface and induce their aggregation. Conclusion An inorganic/organic hybrid photocatalyst has been developed for the photorecovery of gold from solution. Mixing two solutions containing W10O324- and dimethyldioctadecylammonium chloride (DODA) led to the formation of an inorganic/ organic hybrid. UV-vis absorption measurements suggested that the hybrid has photosensitivity to UV light (λ < 400 nm) even in an organic solution. Irradiation of a two-phase system with UV light (λ > 320 nm), where AuCl4- and the hybrid photocatalyst are dissolved in water and chloroform, respectively, produced gold colored films at the interface and completely recovered gold from the water phase within 35 h. XRD and SEM analyses revealed that large hexagonal gold particles with orientation were formed, together with spherical and irregularly shaped gold particles. The pH of the water phase had no significant influence on the gold recovery in the pH range of 2-6. Increases in the concentrations of the catalyst and the sacrificial agent (electron donor) improved the recovery rate. The catalyst was successfully recycled in the present process, although a decrease in the recovery rate was observed to a certain degree. The addition of thiol compounds significantly improved the recovery rate and allowed for straightforward collection of gold particles formed at the interface, due to their covalent adsorption on gold particles and assistance in particle aggregation. The obtained results indicate the effective use of the developed inorganic/organic hybrid photocatalyst in recovering gold from solution under light irradiation. The present method can potentially be applied to the recovery of other noble metals as well. Although the recovery rate is still insufficient as a practical recovery process, we believe that optimized combinations of various types of polyoxometallates (POMs) and organic counterparts with visible light-sensitive functional groups would achieve substantial improvements in the present process. Acknowledgment. This work was financially supported by a Grant-in-Aid (no. 21710082) for Scientific Research from the Ministry of Education, Science, Sports, and Culture of Japan

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