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Jan 16, 2013 - This method has several advantages over the photorecovery method using single POMs reported by Papaconstantinou et al.(10-12) Because ...
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Efficient Photorecovery of Noble Metals from Solution Using a γ‑SiW10O36/Surfactant Hybrid Photocatalyst Tetsuya Kida,*,† Hiromasa Matsufuji,‡ Masayoshi Yuasa,† and Kengo Shimanoe† †

Department of Energy and Material Sciences, Faculty of Engineering Sciences, Kyushu University, Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan ‡ Department of Molecular and Material Sciences, Interdisciplinary Graduate School of Engineering and Science, Kyushu University, Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan S Supporting Information *

ABSTRACT: In recent years, the recovery of noble metals from waste has become very important because of their scarcity and increasing consumption. In this study, we attempt the photochemical recovery of noble metals from solutions using inorganic−organic hybrid photocatalysts. These catalysts are based on polyoxometalates such as PMo12O403‑, SiW12O404‑, and γ-SiW10O368‑ coupled with a cationic surfactant, dimethyldioctadecylammonium (DODA). The three different photocatalysts dissolved in chloroform were successful in photoreducing gold ions dissolved in water in a two-phase (chloroform/water) system under UV irradiation (λ < 475 nm). The γ-SiW10O36/DODA photocatalyst exhibited the best activity and recovered gold from solution efficiently. It was suggested that one-electron reduced γSiW10O369‑ formed by the UV irradiation reduced gold ions. As a result, large two-dimensional particles (gold nanosheets) were produced using the γ-SiW10O36/DODA photocatalyst, indicating that the reduction of gold ions occurred at the interface between chloroform and water. The γ-SiW10O36/DODA photocatalyst was able to recover metals such as platinum, silver, palladium, and copper from deaerated solutions. The selective recovery of gold is possible by controlling pH and oxygen concentration in the reaction system. chloride)21 by an ion exchange process and a layer-by-layer deposition method, respectively. Layer-by-layer deposition methods are widely used for the fabrication of ordered multilayer films with controlled thicknesses and functionality.22−24 The Langmuir−Blodgett technique has also been reported to be effective for fabricating highly ordered multilayered films containing POMs.25,26 There is potential for these POM-based inorganic−organic films to be used in a variety of applications such as electrochromic devices, magnetic devices, and photodetectors. We recently reported that POMs coupled with surfactants in the organic phase can reduce gold ions at a water/organic solution interface to produce two-dimensional structured gold particles.27 This indicates that hybridized POMs dissolved in an organic solution did not lose photocatalytic activity but showed activity at the two phase interface. This property was used successfully to recover gold from waste solutions under light irradiation.28 We demonstrated that gold was recovered effectively from solution using a W10O32/ dimethyldioctadecylammonium (DODA) hybrid dissolved in chloroform after irradiation by UV light. The reaction scheme is shown in Figure

1. INTRODUCTION Polyoxometalates (POMs) are one of the important metal oxide groups having a wide range of applications in catalysis, optics, magnetics, electronics, and medicine.1−3 In particular, POMs are highly active acid catalysts for various reactions and are used widely in industries. Their good photocatalytic activities have also attracted attention in terms of their applications in green catalytic processes. POM-based photocatalysts are used for the destruction of organic pollutants in solution,4−6 the decomposition of water into hydrogen,7−9 noble metal recovery from solution,10−12 and so on. In contrast to commonly used heterogeneous photocatalysts such as TiO2 and ZnO, POMs are soluble in aqueous solvents, allowing for homogeneous catalytic reactions to occur. Another interesting feature is that POMs are regarded as nanoscale building blocks for the creation of functional nanostructured composites or inorganic/organic hybrids.13 Anionic POMs are easily hybridized using cationic surfactants and polycations by simple electrostatic interaction. Moriguchi et al. prepared composites made of POMs and long-chain alkylammonium surfactants by simply mixing the two different chemicals.14,15 This process readily produces a wide variety of POM-surfactant complexes.16−19 Ichinose et al. fabricated composite films composed of POMs and long-chain alkylammonium amphiphiles20 or poly(alkylamine hydro© 2013 American Chemical Society

Received: November 7, 2012 Revised: January 15, 2013 Published: January 16, 2013 2128

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Figure 1. Metal recovery scheme using an amphiphilic POM/surfactant photocatalyst under UV irradiation at the interface between water and chloroform.

1. This method has several advantages over the photorecovery method using single POMs reported by Papaconstantinou et al.10−12 Because the catalyst is dissolved in an organic solution, it is easy to replace the waste solution with a new solution after each recovery, making it possible to recycle the catalyst. The products are easily separated from the reaction system because of their large and sheet-like morphology. This recovery method is green and inexpensive in that no toxic chemicals are used and the catalyst can be recycled. However, the recovery speed is not high enough for practical applications. In this study, to improve the feasibility of the method for metal recovery, we studied and compared the activities of three types of POMs (PMo12O40, SiW12O40, and γ-SiW10O36) combined with DODA, for gold recovery. PMo12O40 and SiW12O40 have the well-known Keggin-type structure, while γSiW10O36 has the divacant Keggin-type structure, as shown in Figure 2. The recovery mechanism was discussed in terms of

(XRD) with Cu Kα radiation (RINT2100, Rigaku Co., Ltd.), and the light absorption properties were measured with a spectrometer (V650, JASCO Co.). 2.2. Photorecovery 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 solution containing HAuCl4 (10 mL, 15 mmol/L) was added slowly. For the recovery of copper, silver, palladium, and platinum, Cu(NO3)2, AgNO3, Pd(NO3)2, and H2PtCl6 were used, respectively. Then, nhexanol (20 μL) was added as an electron donor (sacrificial agent) in the aqueous−organic two-phase solution. To avoid the direct photoreduction of metal ions in the aqueous phase, n-hexanol with low solubility in water was used as a sacrificial agent. The solution was irradiated from the side of the beaker using a 150 W Xe lamp (PECL01, Peccell Technologies Inc.) with 310 nm optical cutoff filter for 1−40 h at room temperature in air or argon. The radiation energy of the lamp with the optical filter to the system was 0.360 mW/cm2 (340 nm) measured using an optical power meter (PD-300UV, Ophir Optronics Ltd.). The recovery percentages of gold and platinum were estimated from the concentrations of HAuCl4 and H2PtCl6 remaining in the water phase, respectively. These were measured by UV−vis absorption spectroscopy during photocatalysis. For copper, silver, and palladium, the recovery percentages were estimated by inductively coupled plasma atomic-emission spectroscopy (ICP-AES) measurements. 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 an X-ray diffractometer (RINT2100, Rigaku Co., Ltd.) using Cu Kα radiation, respectively.

Figure 2. Characteristic structures of (a) PMo12O40, SiW12O40 (Keggin-type), and (b) γ-SiW10O36 (divacant Keggin-type).

3. RESULTS AND DISCUSSION 3.1. Gold Recovery by Three Different Photocatalysts. The mixing of POMs with DODA in water readily produced white precipitates. The XRD spectra of the precipitates (Figure S1 in Supporting Information) showed peaks characteristic of layered structures as reported in the literature.15,30,31 This suggests the presence of alternating layers of POM and DODA in the precipitates. Peaks ascribable to those from POMs and DODA were seen in the Fourier transform infrared (FT-IR) spectra of the precipitates (Figures S2−S4), proving the formation of inorganic−organic hybrids. Figure 3 shows the UV−vis absorption spectra of the hybrids dissolved in chloroform. All hybrids had sensitivities to UV light, but little sensitivity to visible light. The absorption onset is estimated to be 340, 365, and 475 nm for γ-SiW10O36/DODA, SiW12O40/ DODA, and PMo12O40/DODA, respectively. The results suggest that the hybrids can act as a photocatalyst under UV light irradiation (λ < 475 nm) even in an organic solution. Figure 4 shows the concentration of gold ions in the water phase as a function of light irradiation time. The UV irradiation produced light brown particles in all cases. It was confirmed that gold ions were reduced even in the absence of catalysts,

their activity, structure, and adsorption properties. We also examined the activity of the hybrid photocatalysts for the recovery of copper, platinum, palladium, and silver from acid solutions.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. DODA, Na3PMo12O40·nH2O, and Na4SiW12O40·22H2O were purchased from WAKO Pure Chemical Industries Ltd. and used without further purification. γK3SiW10O36·12H2O was prepared by the alkaline hydrolysis of SiW12O404‑.29 DODA (1.2 mmol) was dissolved in water (100 mL) under ultrasonic irradiation. The DODA solution was mixed with an aqueous solution containing PMo12O40 (100 mL, 0.4 mmol/L) under vigorous stirring for 30 min, resulting in the formation of white precipitates. To synthesize SiW12O40/DODA and SiW10O36/DODA, 1.6 and 0.8 mmol of DODA were used, respectively. The precipitated product, a POM/ DODA hybrid, was collected by filtration, washed extensively with water, and dried under vacuum at room temperature. The hybrid crystal structure was measured using X-ray powder diffractometry 2129

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produce zerovalent gold is possible. For the PMo12O40/DODA catalyst, the recovery rate was improved to some extent, indicating its catalytic effects on gold recovery. The SiW12O40/ DODA catalyst worked more efficiently as a catalyst and recovered more than 90% of gold dissolved in the aqueous phase within 30 h. Furthermore, the γ-SiW10O36/DODA catalyst showed enhanced activity. Gold was completely recovered after 30 h UV irradiation. The activity of the photocatalysts is in the order of γSiW10O36/DODA > SiW12O40/DODA > PMo12O40/DODA. This is in the opposite order to the photosensitivity of these catalysts shown in Figure 3. It is important to compare the redox potentials of the POM-based photocatalysts to determine whether they can reduce gold ions thermodynamically. The redox potential of SiW12O404‑/5‑ (+0.06 V vs NHE) is more negative than that of PMo12O403‑/4‑ (+0.22 V vs NHE). On the other hand, the redox potential of γ-SiW10O368‑/9‑, which is estimated to be approximately −0.47 V versus NHE from the cyclic voltammogram data,29 is more negative than that of the other two photocatalysts. Thus, it is probable that the γSiW10O36/DODA catalyst has a higher reducing power than the SiW12O40/DODA and PMo12O40/DODA catalysts. This must be one of the reasons why γ-SiW10O36/DODA showed the best activity. Gold reduction over the γ-SiW10O36/DODA catalyst is assumed to proceed in the following manner according to literature.32−34

Figure 3. UV−vis absorption spectra of (a) PMo12O40/DODA, (b) SiW12O40/DODA, and (c) γ-SiW10O36/DODA dissolved in chloroform. Inset shows absorption spectra near absorption thresholds.

γ ‐SiW10O368 − + hυ → *γ ‐SiW10O368 −

−

Figure 4. Time dependence of AuCl4 concentration in the aqueous phase under UV irradiation (a) without and with the catalysts (b) PMo12O40/DODA, (c) SiW12O40/DODA, and (d) γ-SiW10O36/ DODA catalyst (recovery performed in ambient air).

(1)

2*γ ‐SiW10O368 − + CH3(CH 2)4 CH 2OH → 2γ ‐SiW10O36 9 − + CH3(CH 2)4 CHO + 2H+

(2)

3γ ‐SiW10O36 9 − + AuCl4 −

although the amount of gold particles collected was small. Because the precursor HAuCl4 has a UV sensitivity in the wavelength range shorter than ca. 400 nm, self-reduction to

→ Au 0 + 3γ ‐SiW10O368 − + 4Cl−

(3)

Figure 5. SEM images of the products recovered with (a) PMo12O40/DODA, (b) SiW12O40/DODA, (c) γ-SiW10O36/DODA, and (d) without a catalyst (products observed after UV irradiation for 30 h). 2130

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The simplest mechanism is that γ-SiW10O368‑ is photoexcited by UV light to form *γ-SiW10O368‑ initially. This intermediate reacts with sacrificial agents (electron donors) to form oneelectron reduced γ-SiW10O369‑. Gold ions in the water phase are reduced by γ-SiW10O369‑ to form gold particles at the water/ chloroform interface. This is thermodynamically possible because the redox potential of γ-SiW10O368‑/9‑ is much more negative than that of the AuCl4−/Au0 pair (0.99 V vs NHE). Figure 5 shows the morphologies of gold particles collected with and without the catalyst after the reaction for 30 h. In the absence of the catalyst (Figure 5d), the gold recovered was mainly spherical in shape. Some sheetlike particles are present but in small amounts. When the PMo12O40/DODA or SiW10O36/DODA catalysts were used, many sheetlike particles appeared together with small irregular-shaped particles (Figure 5a,b). In particular, the γ-SiW10O36/DODA catalyst produced well-defined sheetlike particles, larger than 10 μm in lateral size (Figure 5c). The particle thickness was approximately 500 nm. The formation of two-dimensional particles indicates clearly that the reduction of gold ions occurred at the interface between chloroform and water by the assistance of the catalyst and sacrificial agents. In contrast, the formation of spherical particles in the absence of catalysts suggests the occurrence of the self-reduction of gold ions in the water phase. The XRD spectra of the products shown in Figure 6 confirmed the formation of gold particles without any

with (111) orientation when the PMo12O40 and γ-SiW10O36 catalysts were used. This is in good agreements with the SEM results. To understand the formation mechanism of the sheetlike gold particles, the amounts of adsorbed photocatalysts on gold particles were analyzed by ICP-AES measurements. This provides the adsorptivity of the catalysts onto gold. Before the analyses, collected gold particles were washed thoroughly with distilled water. The presence of Mo (or W) on collected gold particles was confirmed, as shown in Table 2. It was found Table 2. Adsorbed Amount of Catalysts on Gold Particles Estimated from ICP-AES Analyses catalyst

adsorbed amount/mmol per 1 mol gold collected

PMo12O40/DODA SiW12O40/DODA γ-SiW10O36/DODA

7.8 5.2 12.8

that the adsorbed amount of the photocatalyst on gold was highest for the γ-SiW10O36 catalyst, which produced gold nanosheets preferentially (see Figure 5d). It is known that γSiW10O36 has four nucleophilic surface oxygen atoms, which are formed after the removal of two WO6 octahedra from SiW12O40.35 The presence of these oxygen sites in γ-SiW10O36 would facilitate its adsorption on gold. It is proposed that the high adsorptivity of γ-SiW10O36 onto gold effectively fixes gold nuclei at the water/organic solution interface, making it possible for the nuclei to grow into large nanosheets. Growth can be achieved by electron transfer from the photoreduced γSiW10O36 to AuCl4− via gold nuclei. The observed stronger adsorption behavior of γ-SiW10O36 onto gold and its most negative redox potential are probably responsible for its highest activity for gold recovery among the catalysts tested. 3.2. Efficient Gold Recovery by Stirring. In this method, the reaction is essentially limited to the two-dimensional interface between water and organic solution. It is readily expected that this confinement of the reaction area leads to a slow reaction speed and the resulting poor efficiency. To improve the efficiency of the gold recovery, we stirred the reaction system to increase the reaction area and studied this effect on gold recovery. Figure 7 shows the concentration of gold ions in the water phase with and without stirring as a function of light irradiation time. In this case, to suppress the vaporization of the organic phase, chloroform was replaced with dodecane having a higher vapor point (215 °C) than chloroform (61.2 °C). The recovery speed was improved successfully by stirring, and the recovery was almost complete

Figure 6. XRD spectra of the products recovered with (a) PMo12O40/ DODA, (b) SiW12O40/DODA, (c) γ-SiW10O36/DODA, and (d) without a catalyst (products analyzed after UV irradiation for 30 h).

detectable impurity phases. It is noteworthy that the relative intensity of the (111) diffraction peak to other peaks increased when gold ions were reduced by the photocatalysts. The estimated relative intensities of the (111) diffraction peak to the (200) diffraction peak for the PMo12O40 and γ-SiW10O36 catalysts were 2 times higher than that without catalysts, as shown in Table 1. We confirmed that the sheetlike gold particles obtained by the present method is (111) oriented by TEM-electron diffraction measurements. Thus, the XRD results indicate the preferential formation of sheetlike gold particles Table 1. Estimated Relative Intensities of the (111) Diffraction Peak to the (200) Diffraction Peak of Gold Particles Recovered with Different Catalysts catalyst

relative intensity (I111/I200)

none PMo12O40/DODA SiW12O40/DODA γ-SiW10O36/DODA

1.4 3.3 2.5 3.5

Figure 7. Time dependence of AuCl4− concentration in the aqueous phase under UV irradiation (a) without and (b) with stirring (γSiW10O36/DODA catalyst used for gold recovery). 2131

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reaction system affects the metal recovery. To determine the effect of oxygen on metal recovery, the reaction system was deaerated by bubbling argon for several minutes. Figure 9

within 12 h. In contrast to the previous case, no gold nanosheets were formed, but fine gold nanoparticles were visible in the product. The increase in reaction area hindered the growth of gold nuclei into large nanosheets. The results obtained obviously indicate the effectiveness of this methodology for gold recovery. One drawback of this modified method is that the resultant fine nanometer sized gold particles are difficult to collect. However, we found that the addition of dithiol compounds such as 1,2-ethanedithiol induces their aggregation. This allows for their collection from the reaction system by a simple filtration procedure. As shown above, stirring the reaction system improved the rate of reaction. We discuss this effect in more detail. It is noted that the hybrids dissolved in chloroform assemble at the interface between water and chloroform because of the strong affinity of POMs for water. This is supported by the fact that two-dimensional gold nanosheets were formed at the interface using the hybrids, as shown in Figure 5. Separately, we confirmed that the POM/DODA hybrids were stable during the reaction and that dissolution of POMs into the aqueous phase did not occur. Accordingly, the gold reduction should occur preferentially at the two phase interface even when the reaction system is stirred. The aqueous phase is not the main reaction space. The question remains: How does the reaction interface change if the system is stirred? The formation of the emulsion should be taken into consideration when the two phase reaction system is stirred. In this case, the area of the reaction interface depends on the emulsion droplet size. We expect that, as the droplet size decreases, the rate of reaction improves because of an increase in the reaction interface area. It is thus possible that the rate of reaction is further improved by controlling the stirring speed to increase the reaction interface area. Controlling the emulsion droplet size by optimizing the combination of water and organic solutions or by addition of an appropriate additive such as alcohol would also be effective in increasing the reaction rate. 3.3. Recovery of Other Noble Metals with the SiW10O36 Catalysts. We next studied the feasibility of using the γ-SiW10O36/DODA catalyst for the recovery of other metals such as copper, silver, palladium, and platinum. Figure 8 shows the dependence of the metal ion concentrations in water on light irradiation time. The concentrations of platinum and copper ions were unchanged even after 30 h light irradiation, while silver, palladium, and gold were recovered efficiently from solution. In these cases, experiments were performed under an air atmosphere. It is suspected that oxygen dissolved into the

Figure 9. Dependence of metal concentrations in the deaerated aqueous phase containing (a) Cu2+, (b) PtCl62‑, (c) Ag+, (d) Pd2+, and (e) AuCl4− on light irradiation time in the presence of the γ-SiW10O36/ DODA catalyst (system deaerated using argon).

shows the dependence of metal ion concentrations on light irradiation time when deaerated water and chloroform were used. The concentrations of copper and platinum ions were decreased by irradiating UV light to the reaction system. However, the recovery percentages of gold, silver, and palladium were not significantly influenced by using deaerated solvents. It has been reported that photoreduced POMs soon react with molecular oxygen to form superoxides.32,36,37 Thus, in the presence of oxygen, γ-SiW10O36 possibly loses its reduction activity according to the following reaction: γ ‐SiW10O36 9 − + O2 → γ ‐SiW10O368 − + O2− +

2+

(4)

and AuCl4−/Au 2+

The redox potentials of Ag /Ag, Pd /Pd, are more positive than those of PtCl62‑/Pt and Cu /Cu, as shown in Figure 10. Thus, the reaction of Ag+, Pd2+, and AuCl4− with γ-SiW10O369‑ is thermodynamically more favorable than that of Cu2+ and PtCl62‑. This accounts for the observed results that the recovery of gold, palladium, and silver was not significantly influenced by oxygen dissolved in the reaction system.

Figure 8. Dependence of metal concentrations in the aqueous phase containing (a) Cu2+, (b) PtCl62‑, (c) Ag+, (d) Pd2+, and (e) AuCl4− on light irradiation time in the presence of the γ-SiW10O36/DODA catalyst (recovery performed in ambient air).

Figure 10. Redox potentials of the POMs and metal ions (volts vs NHE). 2132

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Figure 11. SEM images of (a) silver, (b) palladium, (c) platinum, and (d) gold particles recovered using γ-SiW10O36/DODA catalyst (products observed after UV irradiation for 30 h).

Figure 12. pH effect on the metal recovery from solutions containing (a) AuCl4−, (b) Pd2+, and (c) Ag+ in the presence of the γ-SiW10O36/DODA catalyst (recovery performed in ambient air).

palladium crystals leads to the preferential formation of spherical or irregular shaped silver and palladium particles. It is important to know and find optimal pH values for efficient metal reduction when the developed method is applied to metal recovery from real waste solutions. Figure 12 shows the concentrations of Ag+, Pd2+, and AuCl4− in water at different pH as a function of light irradiation time. The solution pH was adjusted with HNO3 for AuCl4− and H2SO4 for Ag+ and Pd2+. In all cases, the reaction was influenced by changing pH. In particular, the reduction of palladium ions was retarded significantly in a highly acidic solution. For gold recovery, no large sheetlike particles were observed in the product. The

Figure 11 shows the morphology of silver, platinum, palladium, and gold collected after the reactions. The formation of sheetlike particles was observed for platinum and gold, suggesting that the reduction of platinum proceeds in a similar manner to that of gold. However, sheetlike particles were not confirmed for silver and palladium. One possible reason is that POMs may not adsorb effectively on specific planes of silver and palladium crystals to form sheetlike particles. It has been reported that self-assembled POMs adsorb onto the gold {111} plane.38 This can help the anisotropic growth of gold nuclei along other planes such as {110} and {100} to form gold sheets. Thus, a weak adsorption of POMs to silver and 2133

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under visible-light irradiation. J. Mol. Catal. A: Chem. 2005, 225, 203− 212. (7) Yamase, T. Water splitting by photoirradiation of alkylammonium polytungstates in homogeneous solutions and detectable paramagnetic species. Inorg. Chim. Acta 1983, 76, L25−L27. (8) Muradov, N.; T-Raissi, A. Solar production of hydrogen using “self-assembled” polyoxometalate photocatalysts. J. Sol. Energy Eng. 2006, 128, 326−330. (9) Zhang, Z.; Lin, Q.; Zheng, S.-T.; Bu, X.; Feng, P. A novel sandwich-type polyoxometalate compound with visible-light photocatalytic H2 evolution activity. Chem. Commun. 2011, 47, 3918−3920. (10) Troupis, A.; Hiskia, A.; Papaconstantinou, E. Photocatalytic reduction and recovery of copper by polyoxometalates. Environ. Sci. Technol. 2002, 36, 5355−5362. (11) Gkika, E.; Troupis, A.; Hiskia, A.; Papaconstantinou, E. Photocatalytic reduction and recovery of mercury by polyoxometalates. Environ. Sci. Technol. 2005, 39, 4242−4248. (12) Troupis, A.; Hiskia, A.; Papaconstantinou, E. Reduction and recovery of metals from aqueous solutions with polyoxometallates. New J. Chem. 2001, 25, 361−363. (13) Liu, S.; Tang, Z. Polyoxometalate-based functional nanostructured films: Current progress and future prospects. Nano Today 2010, 5, 267−281. (14) Moriguchi, I.; Hanai, K.; Hoshikuma, A.; Teraoka, Y.; Kagawa, S. Novel multibilayer film incorporating electro- and photo-chemically active decatungstate anion. Chem. Lett. 1994, 23, 691−694. (15) Moriguchi, I.; Orishikida, K.; Tokuyama, Y.; Watabe, H.; Kagawa, S.; Teraoka, Y. Photocatalytic property of a decatungstatecontaining bilayer system for the conversion of 2-propanol to acetone. Chem. Mater. 2001, 13, 2430−2435. (16) Kurth, D. G.; Lehmann, P.; Volkmer, D.; Colfen, H.; Koop, M. J.; Muller, A.; Chesne, A. D. Surfactant-encapsulated clusters (SECs): (DODA)20(NH4)[H3Mo57V6(NO)6O183(H2O)18], a case study. Chem.Eur. J. 2000, 6, 385−393. (17) Zhang, T.; Spitz, C.; Antonietti, M.; Faul, C. F. J. Highly photoluminescent polyoxometaloeuropate-surfactant complexes by ionic self-assembly. Chem.Eur. J. 2005, 11, 1001−1009. (18) Bu, W.; Li, W.; Li, H.; Wu, L.; Tang, A.-C. Surfactantencapsulated polyoxometalloeuropate: polarized Eu3+ emission in the highly ordered self-organizing film. J. Colloid Interface Sci. 2004, 274, 200−203. (19) Kida, T.; Iemura, K.; Kai, H.; Nagano, M. Preparation of molybdenum, vanadium, and tungsten oxide films from self-assembly deposited polyoxometalate-alkylamine layered hybrid films. J. Am. Ceram. Soc. 2007, 90, 618−621. (20) Ichinose, I.; Asai, T.; Yoshimura, S.; Kimizuka, N.; Kunitake, T. Two-dimensional arrangement or polynuclear metal complexes in the interlayer of multibilayer cast films. Chem. Lett. 1994, 23, 1837−1840. (21) Ichinose, I.; Tagawa, H.; Mizuki, S.; Lvov, Y.; Kunitake, T. Formation process of ultrathin multilayer films of molybdenum oxide by alternate adsorption of octamolybdate and linear polycations. Langmuir 1998, 14, 187−192. (22) Moriguchi, I.; Fendler, J. H. Characterization and electrochromic properties of ultrathin films self-assembled from poly(diallyldimethylammonium) chloride and sodium decatungstate. Chem. Mater. 1998, 10, 2205−2211. (23) Liu, S.; Kurth, D. G.; Mohwald, H.; Volkmer, D. A thin-film electrochromic device based on a polyoxometalate cluster. Adv. Mater. 2002, 14, 225−228. (24) Liu, S.; Kurth, D. G.; Bredenkotter, B.; Volkmer, D. The structure of self-assembled multilayers with polyoxometalate nanoclusters. J. Am. Chem. Soc. 2002, 124, 12279−12287. (25) Clemente-León, M.; Agricole, B.; Mingotaud, C.; GómezGarcía, C. J.; Coronado, E.; Delhaes, P. Toward new organic/inorganic superlattices: Keggin polyoxometalates in Langmuir and Langmuir− Blodgett films. Langmuir 1997, 13, 2340−2347. (26) Clemente-León, M.; Coronado, E.; Delhaes, P.; Gómez-García, C. J.; Mingotaud, C. Hybrid Langmuir−Blodgett films formed by alternating layers of magnetic polyoxometalate clusters and organic

reason for this observation is not yet clear. The interference of POM adsorption on metal nuclei by coexisting anions and protons is considered to be one of the reasons. Another possibility is that metal dissolution competes with metal reduction in highly acidic solutions. Solution pH should therefore be controlled for the efficient recovery of each metal. Nevertheless, the observed different pH dependence of metal recovery is advantageous for the selective recovery of gold from solutions containing various metal ions.

4. CONCLUSION The photocatalytic activities of three hybrid photocatalysts, γSiW10O36/DODA, SiW12O40/DODA, and PMo12O40/DODA, for the reduction of gold ions under UV irradiation were examined. The γ-SiW10O36/DODA photocatalyst dissolved in chloroform exhibited the best activity and completely recovered gold ions from an aqueous solution within 30 h. The photorecovery measurements coupled with ICP-AES analyses suggest that γ-SiW10O36 adsorbed more strongly on the gold nuclei and induced their two-dimensional growth. The photorecovery rate of gold was improved significantly by stirring the reaction system because of an increase in the area of the reaction interfaces. It was also found that the γ-SiW10O36/ DODA photocatalyst is able to recover platinum, palladium, silver, and copper from acidic solutions in the absence of oxygen.

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ASSOCIATED CONTENT

S Supporting Information *

XRD measurements and FT-IR spectra of samples synthesized by mixing POMs and DODA (dimethyldioctadecylammonium). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +81-92-583-7537. Fax: +81-92-583-7538. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported financially by a Grant-in-Aid (No. 21710082) for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

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REFERENCES

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