Comparative Study of Homogeneous and Heterogeneous

Apr 9, 2014 - Comparative Study of Homogeneous and Heterogeneous. Photocatalytic Degradation of RhB under Visible Light Irradiation with Keggin-Type M...
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Comparative Study of Homogeneous and Heterogeneous Photocatalytic Degradation of RhB under Visible Light Irradiation with Keggin-Type Manganese-Substituted Catalysts Yingjie Hua,† Guoliang Chen,‡ Xiaonan Xu,† Xiaomei Zou,† Jinyuan Liu,† Bin Wang,† Ziming Zhao,† Yan Chen,† Chongtai Wang,†,* and Xiaoyang Liu§ †

School of Chemistry and Chemical Engineering, Hainan Normal University, Haikou 571158, P.R. China Key Laboratory of Tropical Biological Resources of Ministry of Education, Hainan University, Haikou, 570228, P.R. China § State Key Laboratory of Inorganic Synthesis Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, P.R. China ‡

S Supporting Information *

ABSTRACT: This paper reported for the first time a comparative study of the photocatalytic activity of the Keggin-type Mn(II)-substituted heteropolyanion PW11O39MnII(H2O)5− (PW11Mn) and its heterogeneous system PW11Mn/D301R through the degradation of rhodamine B (RhB) under visible light irradiation. A novel photocatalytic mechanism was suggested, and an interaction between PW11Mn and RhB was scrutinized by using visible absorption spectrum and fluorescence emission spectrum. Influences of some factors such as the RhB initial concentration, the PW11Mn concentration, the PW11Mn/D301R dosage, the solution pH, and different anions existing in solution on the photocatalytic degradation rate of RhB were also examined. The stability of PW11Mn/D301R was evaluated by the cycle photodegradation of RhB in the end. The experimental results showed that 100% of RhB degradation was reached at 100 min for the PW11Mn system and 40 min for the PW11Mn/ D301R system when the solution containing 10 μmol·L−1RhB was exposed to visible light. The hydroxyl radicals were responsible for the destruction of dye. The photocatalysis mechanism was different from that of both semiconductor and Keggin parent catalysts. An electrostatic interaction and a coordination interaction between PW11Mn and RhB occurred simultaneously in an acidic aqueous solution. The coordination interaction slowed significantly the RhB degradation, but became weak obviously after PW11Mn was adsorbed onto the D301R resin. The influence of anions existing in solution on the RhB degradation followed the sequence of PO43− > SO42−> Cl−> NO3−. No matter if in neutral or in acidic aqueous solution, the photocatalyst PW11Mn/D301R was stable. tion of rhodamine B (RhB) under visible light irradiation.32−34 A high photocatalytic activity of these photocatalysts was verified. In particular, this kind of transition metal substituted derivatives exhibited a nonsensitized visible photocatalysis mechanism that is significantly different from that of the Keggin parents and other metal oxide semiconductors such as TiO2.18,34 Moreover, comparing with the Keggin parent and other semiconductor photocatalysts the Keggin-type TMSHs have a remarkable advantage, namely being simultaneously photoactive in both UV and Vis region. But the most attractive characteristic is still the adjustable visible light absorption wavelength by transforming the substituted transition metal ion in the Keggin structure, thus changing the energy gap between HOMO and substituted metal d-orbitals. Therefore, aiming to explore and prove the universality of the photocatalytic activity of the Keggin-type TMSHs when the transition metal ion is replaced, the Mn(II)-substituted heteropolyanion PW11O39Mn-

1. INTRODUCTION In recent years, in order to efficiently utilize solar energy many research interests have been being focused on the photocatalysts.1−7 Among them heteropolycompounds are a category of fast developing photocatalysts apart from the semiconductors such as TiO2, α-Fe2O3, ZnO, CdS, NaBiO3, ZrMo2O8, ZnxCd1−xS, and so on.8−23 The reason is attributed to the special geometrical and electronic structures of Keggintype heteropolyanions with a delocalized electronic distribution,24−26 which allows both electron transfer in a manner of inner-sphere electron transfer in the electrocatalytic process27−29 and a light-excited electron transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) in the photocatalytic process.8,9 In our previous works we have ever reported the photocatalysis of some Keggin-type transition metal substituted heteropolyanions (TMSHs), for example, PW11O39Fe(H2O)4−(PW11Fe), PW 1 1 O 3 9 Cr(H 2 O ) 4 − (PW 1 1 Cr), and PW 1 1 O 3 9 Cu(H2O)5−(PW11Cu). The photocatalytic activity of these photocatalysts was assessed by degradation of nitrobenzene30 and aniline31 under ultraviolet light illumination and degrada© 2014 American Chemical Society

Received: September 11, 2013 Revised: April 9, 2014 Published: April 9, 2014 8877

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Figure 1. Absorption spectrum variation of the PW11Mn solution during adsorption. Inset is the visible absorption spectrum of the PW11Mn solution (A) and IR spectra of (a) D301R resin, (b) PW11Mn/D301R, and (c) PW11Mn (B).

(H2O)5− (PW11Mn) together with its heterogeneous system PW11Mn/D301R was investigated in this study. The solid photocatalyst PW11Mn/D301R was prepared by adsorption of PW11Mn onto the surface of the D301R that is a weak alkaline anion exchange resin containing a quaternary ammonium groupR(CH3)2HN+:OH−. In our previous works, it was found that the D301R resin has a good adsorption effect toward PW11Fe and PW11Cr by ion exchange reaction.35,36 So it was chose to be an adsorbent for PW11Mn because PW11Mn has a same Keggin structure and a similar acid−base property with PW11Fe and PW11Cr. A comparative study was conducted through the photocatalytic degradation of RhB under visible light irradiation to evaluate the photocatalytic activity of the PW11Mn and PW11Mn/D301R.

tube, whereas the outer part was a Pyrex glass reactor with a volume of 1000 mL. A 200 W metal halide lamp was positioned inside the jacketed quartz tube, which was surrounded by a circulating water jacket (quartz) to cool the lamp and minimize infrared radiation. A cutoff filter was placed outside the quartz jacket to completely eliminate any radiation at wavelengths below 420 nm, and thereby, to ensure that illumination occurred only from visible light (λ > 420 nm). The solution of 250 mL was placed in the outer reactor and continuously stirred by a stirring magnet to ensure the uniformity of the solution. During irradiation, aliquots (2 mL) of solution were withdrawn at given time intervals for analysis of concentration. For the heterogeneous system of PW11Mn/D301R the degradation experiment commenced after the solution was stirred magnetically for 120 min to allow the adsorption/ desorption equilibrium in dark environment. The dark reaction was conducted by wrapping the tubes with aluminum foil to prevent exposure to light. 2.4. Analytical Methods. The concentration of RhB was analyzed by a UV−visible spectrophotometer (Unicam UV500, Canada), which recorded the temporal UV−visible spectral variations of the dyes with an absorbance peak at 554 nm. The IR spectrum was recorded using a Nicolet AVATAR360 FTIR (KBr pellet) and the Raman spectrum using an Invia Reflex (Renishaw, Britain). The fluorescence spectra of samples were recorded on a fluorescence spectrophotometer (RF-5301PC, Shimadzu, Japan) with an excitation wavelength of 352 nm, an emission wavelength of 579 nm. The total organic carbon (TOC) assays were carried out on a TOC-VCPH analyzer (Shimadzu, Japan).

2. EXPERIMENTAL SECTION 2.1. Materials. Rhodamine B (RhB) was purchased from Shanghai Chemical Reagent Company. The anionic exchange resin D301R was obtained from Nankai University. Sodium tungstate, disodium hydrogen phosphate and manganese sulfate were purchased from Sinopharm Chemical Reagent Co., Ltd. Sodium hydroxide, acetone, sulfuric acid were purchased from Guangzhou Chemical Reagent Factory. All chemicals were of analytic pure grade and used as received. The deionized and doubly distilled water was used throughout this study and the pH of the solutions was adjusted by diluted aqueous solutions of NaOH and H2SO4. 2.2. Synthesis and Preparation of Catalysts. Keggintype Manganese-substituted Sodium heteropolytungstates Na5PW11O39Mn(II)(H2O) were synthesized according to the method described in our previous reports.27−34 The heterogeneous photocatalysts PW11Mn/D301R were prepared by an adsorption process. In a typical experimental procedure, 0.1 mmol of PW11Mn together with 1 g of D301R resin was added to 100 mL of deionized water, and then the resulting solution was stirred at ambient temperature for 24 h to reach the adsorption−desorption equilibrium between PW11Mn and D301R resin. After the adsorption equilibrium, the adsorbedPW11Mn D301R was separated by filter and dried at 50 °C in an oven. The as-prepared PW11Mn/D301R was characterized using ultraviolet (UV), infrared (IR) and Raman spectroscopy. The loading amount of PW11Mn on the D301R resin was estimated to be 9.84 × 10−5 mol/g according to its saturated adsorption amount. 2.3. Photodegradation Procedures. Typical degradation experiments under visible light were performed in a photochemical reactor (XPA series, Nanjing Xujiang, China), which consisted of two parts. The inner part was a jacketed quartz

3. RESULTS AND DISCUSSION 3.1. Adsorption of PW11Mn and Characterization. Upon addition of D301R into the solution containing PW11Mn, the characteristic absorption bands of the PW11Mn solution at about 200 and 258 nm, resulting from Od → W and Ob, Oc → W charge transfer transitions that are the features of the Keggin-type heteropolyanions,37 gradually declined with increasing of time (Figure 1A), indicating that the PW11Mn in solution had been adsorbed onto the surface of the D301R resin. Inset is the Vis absorption spectrum of PW11Mn, which shows a successive absorption at the range from 400 to 800 nm and the intensity gradually weakens with increasing wavelength. The visible absorption of PW11Mn is attributed to the electron jumping from the HOMO (Od) to the d-orbitals of Mn(II), known as Od → Mn charge transfer, which offers a potential in visible photocatalysis of PW11Mn (see below). 8878

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photocatalytic activity of either PW11Mn or PW11Mn/D301R in the presence of light is dramatically enhanced, indicating that both PW11Mn and PW11Mn/D301R has a high photocatalytic activity under visible-light irradiation for the RhB degradation. But comparing the rate of RhB photodegradation in the presence of PW11Mn and PW11Mn/D301R respectively, it may be found that the photocatalytic activity of the latter is significantly higher than that of the former. This can be explained by the weak interaction between PW11Mn/D301R and RhB after PW11Mn was adsorbed onto the D301R as well as an enrichment effect of resin toward the dye molecule (see below). Insets in Figure 2 show the visible spectral changes during the photocatalytic degradation of RhB in the presence of PW11Mn (A) and PW11Mn/D301R (B) under visible light irradiation. As can be seen, the characteristic absorption band of RhB at 554 nm diminished quickly, being similar to the situation in TiO2 system.18,19 The sharp decrease in the maximum absorption implies that RhB suffered a rather facile destruction of the whole conjugated chromophore. To interpret the visible photocatalysis of PW11Mn toward RhB degradation, we believe that upon excitation of PW11Mn by light, an electron transition from the highest occupied molecular orbital HOMO(Od) to the d (Mn) orbital takes place, causing the oxidation of the H2O molecule coordinating at the Mn(II) center to generate hydroxyl radicals, thus initiating the photocatalytic process. The photocatalysis mechanism similar to that of PW11Fe34 is suggested as follows (eqs 1-5):

Infrared (IR) spectrum verified further the adsorption of PW11Mn onto the D301R resin. As shown in Figure 1B, after adsorption of PW11Mn the D301R resin exhibited four absorption peaks at the position of 816, 888, 955, and 1054 cm −1 in the fingerprint region from 700−1100 cm −1 corresponding to the characteristic stretching vibration of W−OcW, W−ObW, WOd, and P−Oa in the Keggin structure,37,38 respectively. Raman spectra gave also a similar result (see Supporting Information, Figure S1), which strengthened the IR credibility. 3.2. The Visible Photocatalysis of PW11Mn and PW11Mn/D301R toward RhB Degradation. To evaluate and compare the visible photocatalytic activity of PW11Mn and PW11Mn/D301R, the photodegradation of RhB under visible light illumination in the absence and presence of PW11Mn and PW11Mn/D301R was conducted, respectively. Figure 2 exhibits

hv

3PW11Mn II(H 2O) → 3PW11Mn I(OH)* + 3H+

(1)

3PW11Mn I(OH)* + 3H 2O → 3PW11Mn I(H 2O)* + 3HO•

Figure 2. c/c0 ∼ t curves of photocatalytic degradation of RhB: (a) RhB, irradiation; (b) RhB + PW11Mn, dark; (c) RhB + PW11Mn/ D301R, dark; (d) RhB + PW11Mn, irradiation; (e) RhB + PW11Mn/ D301R, irradiation. Inset A is the temporal visible absorption spectral changes observed for curve d, and inset B is that for curve e, [RhB] = 10 μmol·L−1; [PW11Mn] = 50 μmol·L−1; PW11Mn/D301R dosage is 100 mg.

(2)

PW11Mn I(H 2O)* + O2 + H+ → PW11Mn II(H 2O) + HOO•

(3)

PW11Mn I(H 2O)* + HOO• + H+ → PW11Mn II(H 2O) + H 2O2

the changes of RhB relative concentration as a function of irradiation time. For comparison, an identical experiment in the dark was also carried out. It can be seen from Figure 2 that, for the blank experiment without catalyst, RhB concentration under visible light illumination was almost unchanged (curve a), suggesting that RhB, which contains four N-ethyl groups at either side of the xanthene ring (Scheme 1), is relatively stable in an aqueous solution upon visible light irradiation. However, with catalysts under the same experimental conditions, the RhB concentration rapidly decayed and 100% of RhB degradation was reached at 100 min for the PW11Mn system (curve d) and 40 min for the PW11Mn/D301R system(curve e). Further compared to that observed in the dark (curve b and c), the

(4)

PW11Mn I(H 2O)* + H 2O2 + H+ → PW11Mn II(H 2O) + HO• + H 2O

(5)

The net reaction from eq 1 to eq 5 is hv

2H 2O + O2 ⎯⎯⎯⎯⎯⎯⎯→ 4HO• PW11Mn

(6)

Obviously, it is the Mn(II) center that plays a role of the primary photogenerated electron acceptor, which initiates the photocatalytic reaction, whereas the dioxygen and the consequential HOO•, H2O2 are the secondary electron acceptors that result in the oxidation of PW11MnI(H2O) and the recovery of PW11MnII(H2O), thus maintaining the photocatalytic cycle. The produced HO• radicals are responsible for the RhB destruction. Although the heteropolyanion PW11MnII is immobilized onto the surface of the D301R resin, the same photocatalysis mechanism is followed (Scheme 2). Hydroxyl radicals quenching experiments using methanol as a HO• scavenger and TOC measurements confirmed the deduction above (Figure 3). As can be seen from Figure 3, the degradation rate of RhB slowed down immediately and then terminated when methanol was added to the reaction system at

Scheme 1. Molecular Structure of the Rhodamine B

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results in a significant different photocatalytic efficiency. For the former, only N-deethylation took place in the visible photocatalytic degradation of RhB.18 But for the latter, a complete ring-opening degradation was reached. Apart from the novel photocatalysis mechanism, the well-defined Keggin structure and the light absorption property in both UV and Vis region make PW11Mn or PW11Mn/D301R better than other semiconductor or Keggin−parent photocatalysts. 3.3. Effect of Catalyst Dosage. The dosage of photocatalyst is an important parameter affecting the degradation rate of organic pollutants in the photocatalytic process. Figure 4 gives the variation of the RhB relative concentration as a function of the catalyst concentration or dosage under visible light irradiation. In the homogeneous system, the degradation rate of RhB increases with increasing of the PW11Mn concentration in the early stage, but decreases after the concentration of PW11Mn reaches 50 μmol·L−1, thus the value is the best concentration under the experimental conditions. In the heterogeneous system, a variable trend in RhB concentration similar to the homogeneous system was observed. The best PW11Mn/D301R dosage is 100 mg, but compared to the homogeneous system the influence of catalyst dosage increasing over the best value is relatively weak. Existence of the best catalyst value implies that there are two opposite factors at least in the reaction system. Our previous works indicated that there is an interaction between RhB and PW11Fe.34 This phenomenon is considered to occur in the same way in the PW11Mn system. Figure 5 shows the fluorescence quenching effect of PW11Mn toward RhB. It can be seen that the fluorescence absorption peak intensity of RhB at 570 nm gradually descends with increasing of the PW11Mn concentration. The apparent association constant K and association number n between RhB and PW11Mn were estimated to be 1.88 × 103 L·mol−1 and 1 from the linear relationship of ln((F0/F) − 1) with ln[PW11Mn] (see inset in Figure 5 and eq 7).34

Scheme 2. Schematic Diagram of the Photocatalysis of PW11Mn/D301R

a reaction time of 10 min (Figure 3A, curve b), suggesting that hydroxyl radicals are produced as a result of the secondary chemical events after the photocatalyst is excited by visible light irradiation. Decay in TOC during RhB degradation further proved the existence of hydroxyl radicals because only the HO• could completely destroy RhB and mineralize it.34 Clearly, the photocatalysis mechanism of PW11Mn/D301R is different from that of the usual semiconductor and the Keggintype unsubstituted parent catalysts. For the system consisting of a usual semiconductor photocatalyst such as TiO2, WO3 etc.,1,9,19 photogenerated electron−hole pairs are produced upon excitation of catalysts by light. The photocatalytic efficiency is closely related to the electron hole separation, and affected by the recombined rate of electrons and holes. For the system constituted by a Keggin-type unsubstituted parent catalyst, e.g., PW12O403− (PW12), SiW12O404− (SiW12) etc.,8,18 species excited by visible light are the dye molecules, not the heteropolyanions and the photogenerated electrons divert from the dye molecules to the catalysts under the visible light excitation. But for the system of PW11Mn or PW11Mn/D301R, PW11Mn is excited under the visible light irradiation and an excited state of PW11MnI(OH)* is generated instead of electron hole pair. The molecule H2O coordinated with Mn center plays an important role as a photogenerated electron donor and the Mn(II) center acts as a primary receptor of the photogenerated electrons. Therefore, the Mn(II) together with H2O in PW11Mn constitute the photoactive center. As for PW12 and PW11Mn, difference in the photocatalysis mechanism

⎛F ⎞ ln⎜ 0 − 1⎟ = ln K + n ln[L] ⎝F ⎠

(7)

Besides the interaction of RhB with PW11Mn, increasing of the PW11Mn concentration facilitates the reaction 8, leading to the quenching of hydroxyl radicals, thus to the decline of the RhB degradation rate.

Figure 3. Effect of a hydroxyl radical scavenger on RhB degradation (A): (a) RhB+ PW11Mn/D301R; (b) 80 μL methanol was added to the reaction system at the reaction time of 10 min and Changes in solution TOC during RhB degradation (B); [RhB] = 10 μmol·L−1; PW11Mn/D301R dosage is 100 mg; 200 W metal halide lamp. 8880

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Figure 4. Influences of PW11Mn concentration (A) and PW11Mn/D301R dosage (B) on the photocatalytic degradation rate of RhB in aqueous solution containing 10 μmol·L−1 RhB.

PW11O39Mn II(H 2O)5 − ⇌ PW11O39Mn II(OH)6 − + H+ (9)

Obviously, variation of the solution pH will affect the existing form of PW11Mn in the solution, leading to changes in RhB degradation rate. Figure 6 shows the photocatalytic degradation of RhB at different pH in the presence of PW11Mn (A) and PW11Mn/D301R (B). It can be seen that the degradation percentage of RhB increases with increasing of the solution pH in the homogeneous system, but just opposite in the heterogeneous system. Also, the degradation rate of RhB in the heterogeneous system is much larger than that in the homogeneous system. This may be explained by the interaction between catalyst and RhB. There are strong interactions between PW11Mn and RhB in the homogeneous system, especially in a solution with relatively low pH value. For example, the maximum visible absorption peak of RhB is shifted by 25 nm from original 554 to 579 nm in a solution of pH 2.5 in the presence of PW11Mn (Figure 7A). Whereas no obvious change in absorption peak position and shape of RhB takes place in the absence of PW11Mn when the solution pH varies (Figure 7B), indicating that the change of the visible absorption peak of RhB is due to interaction of RhB with PW11Mn. The interaction, including electrostatic interaction and complexing interaction, strengthens with increasing of the PW11Mn concentration at a low pH value (Figure 8A), which differs somewhat from the situation in a neutral aqueous solution where only an electrostatic interaction exists.34 On the basis of the fluorescence quenching effect of PW11Mn toward RhB in the solution with changing PW 11 Mn concentration (Figure 8B), the apparent association constant

Figure 5. Changes of the fluorescence intensity of RhB with PW11Mn concentration. The inset is the relationship between ln(F0/F − 1) and ln[PW11Mn].

PW11Mn I(H 2O)* + HO• + H+ → PW11Mn II(H 2O) + H 2O

(8)

After PW11Mn was adsorbed onto the surface of the D301R resin, forming a heterogeneous PW11Mn/D301R system, the interaction between RhB and PW11Mn becomes weak, which is in favor of the improvement of the RhB degradation rate. However, when the dosage of the increased PW11Mn/D301R reaches a certain extent, the quenching effect of PW11Mn/ D301R toward hydroxyl radicals becomes dominated, so the PW11Mn/D301R dosage has a best value. 3.4. Effect of pH. There is an acid−base equilibrium in the solution of PW11Mn as follow:

Figure 6. Influence of pH on the photocatalytic degradation rate of RhB under visible light irradiation: (A) RhB + PW11Mn; (B) RhB + PW11Mn/ D301R. Inset is the temporal visible absorption spectral changes observed for pH 2.5 solution. [RhB] = 10 μmol·L−1; [PW11Mn] = 50 μmol·L−1; PW11Mn/D301R dosage is 100 mg. 8881

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Figure 7. Visible spectra of RhB at different pH: (A) 10 μmol·L−1 RhB + 50 μmol·L−1 PW11Mn; (B) 10 μmol·L−1RhB.

Figure 8. Changes of visible spectra of RhB with PW11Mn concentration (A) and Changes of fluorescence spectra intensity of RhB with PW11Mn concentration (B). [RhB] = 10 μmol·L−1; pH = 2.5. Inset is the relationship between ln(F0/F − 1) and ln[PW11Mn].

K and association number n between RhB and PW11Mn in an acidic solution of pH 2.5 were estimated to be 1.63 × 107 L· mol−1 and 1.5 from the linear relationship of ln((F0/F)−1) with ln[PW11Mn] (see inset in Figure 8B and eq 7). Clearly, the values of K and n are much larger compared to those in a neutral aqueous solution, suggesting that interaction between RhB and PW11Mn in an acidic aqueous solution is much stronger. Apart from an electrostatic interaction, complexation of RhB with PW11Mn occurs through a coordination of N atom in N,N-diethyl group with Mn(II) center in PW11Mn, forming a complex of PW11Mn-RhB. Therefore, the degradation rate of RhB became slow in an acidic aqueous solution. However, after PW11Mn was adsorbed onto the D301R resin the interaction between PW11Mn and RhB was greatly weakened. A higher acidity in this case is more conducive to the reactions of eqs 3−5, leading to a faster degradation rate of RhB. In addition, the surface of D301R resin has also an enrichment effect on the dye molecule via adsorption, which facilitates the degradation of RhB. 3.5. Effect of Different Ions. Generally, there are some different ions in actual wastewater, so we investigated the effect of some common anions on the visible photocatalytic degradation of RhB in the heterogeneous system composed of PW11Mn/D301R, as shown in Figure 9. It can be seen that NO3−, Cl−, and SO42− affect the photocatalytic degradation of RhB indeed. The degradation rate of RhB declined obviously under existences of these ions. The affecting extent follows the sequence: SO42− > Cl− > NO3−. This effect may be attributed to the competitive adsorption of the ions with RhB on the resin surface and their quenching of hydroxyl radicals.39,40 As for quenching effect on hydroxyl radicals, PO43− behaves more remarkable (Figure 10). Because of quenching of hydroxyl radicals by PO43− the dominating species resulting in RhB degradation are hydroperoxy radicals that only cause N-

Figure 9. Effects of different ions on the visible photocatalytic degradation of RhB in the solution containing 10 μmol·L−1 RhB + 100 mg PW11Mn/D301R: (a) no ion added; (b) 0.1 mol·L−1 NO3−; (c) 0.1 mol·L−1 Cl−; (d) 0.1 mol·L−1 SO42−. The inset is the temporal visible adsorption spectra of RhB in the presence of 0.1 mol·L−1 SO42−.

deethylation.18 Therefore, an obvious hypochromatic shift from 554 to 497 nm was observed, as shown in Figure 10. 3.6. Stability Evaluation. Estimating the stability and reusability of catalyst is indispensable for the evaluation of its practical applications.5 Therefore, the durability of PW11Mn/ D301R in the reaction was investigated by the RhB degrading experiment repeated six times. As shown in Supporting Information Figure S2, RhB was completely decomposed in each circulation and little loss of activity was observed after six recycles no matter in neutral also in acidic (pH2.5) aqueous solution. Furthermore, no absorption at 258 nm was detected using UV spectroscopy for the extracting solution in which PW11Mn/D301R was immersed under stirring for 3 days (see Supporting Information, Figure S3). All of these results suggest that the PW11Mn/D301R photocatalyst is stable. 8882

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Figure 10. Effect of PO43− on the visible photocatalytic degradation of RhB in the solution containing 10 μmol·L−1 RhB+100 mg PW11Mn/D301R: (A) 0.1 mol·L−1 PO43; (B) 0.5 mol·L−1 PO43−; (c) 1.0 mol·L−1 PO43−. (5) Yu, K.; Yang, S. G.; He, H.; Sun, C.; Gu, C. G.; Ju, Y. M. Visible Light-Driven Photocatalytic Degradation of Rhodamine B over NaBiO3: Pathways and Mechanism. J. Phys. Chem. A 2009, 113, 10024−10032. (6) Sahoo, P. P.; Sumithra, S.; Madras, G.; Row, T. N. G. Synthesis, Characterization, and Photocatalytic Properties of ZrMo2O8. J. Phys. Chem. C 2009, 113, 10661−10666. (7) Li, W. J.; Li, D. Z.; Chen, Z. X.; Huang, H. J.; Sun, M.; He, Y. H.; Fu, X. Z. High-Efficient Degradation of Dyes by ZnxCd1‑xS Solid Solutions under Visible Light Irradiation. J. Phys. Chem. C 2008, 112, 14943−14947. (8) Lei, P. X.; Chen, C. C.; Yang, J.; Ma, W. H.; Zhao, J. C.; Zang, L. Degradation of Dye Pollutants by Immobilized Polyoxometalate with H2O2 under Visible-Light Irradiation. Environ. Sci. Technol. 2005, 39, 8466−8474. (9) Hiskia, A.; Mylonas, A.; Papaconstantinou, E. Comparison of the Photoredox Properties of Polyoxometalates and Semiconducting Particles. Chem. Soc. Rev. 2001, 30, 62−69. (10) Chambers, R.; Hill, C. Comparative Study of Polyoxometalates and Semiconductor Metal Oxides as Catalysts. Photochemical Oxidative Degradation of Thioethers. Inorg. Chem. 1991, 30, 2776− 2781. (11) Androulaki, E.; Hiskia, A.; Dimotikali, D.; Minero, C.; Calza, P.; Pelizzetti, E.; Papaconstantinou, E. Light Induced Elimination of Mono- and Polychlorinated Phenols from Aqueous Solutions by PW12O403‑. The Case of 2,4,6-Trichlorophenol. Environ. Sci. Technol. 2000, 34, 2024−2028. (12) Ozer, R.; Ferry, J. Photocatalytic Oxidation of Aqueous 1,2Dichlorobenzene by Polyoxometalates Supported on the NaY Zeolite. J. Phys. Chem. B 2002, 106, 4336−4342. (13) Yue, B.; Zhou, Y.; Xu, J.; Wu, Z.; Zhang, X.; Zou, Y.; Jin, S. Photocatalytic Degradation of Aqueous 4-Chlorophenol by Silicaimmobilized Polyoxometalates. Environ. Sci. Technol. 2002, 36, 1325− 1329. (14) Yoon, M.; Chang, J.; Kim, Y.; Choi, J.; Kim, K.; Lee, S. Heteropoly Acid-Incorporated TiO2 Colloids as Novel Photocatalytic Systems Resembling the Photosynthetic Reaction Center. J. Phys. Chem. B 2001, 105, 2539−2545. (15) Troupis, A.; Hiskia, A.; Papaconstantinou, E. Photocatalytic Reduction and Recovery of Copper by Polyoxometalates. Environ. Sci. Technol. 2002, 36, 5355−5362. (16) Troupis, A.; Hiskia, A.; Papaconstantinou, E. Synthesis of Metal Nanoparticles by Using Polyoxometalates as Photocatalysts and Stabilizers. Angew. Chem., Int. Ed. 2002, 41, 1911−1914. (17) Mandal, S.; Selvakannan, P.; Pasricha, R.; Sastry, M. Keggin Ions as UV-Switchable Reducing Agents in the Synthesis of Au Core-Ag Shell Nanoparticles. J. Am. Chem. Soc. 2003, 125, 8440−8441. (18) Chen, C.; Zhao, W.; Lei, P.; Zhao, J. C.; Serpone, N. Photosensitized Degradation of Dyes in Polyoxometalate Solutions versus TiO2 Dispersions under Visible-Light Irradiation: Mechanistic Implications. Chem.Eur. J. 2004, 10, 1956−1965. (19) Chen, C.; Lei, P.; Ji, H.; Ma, W.; Zhao, J. C. Photocatalysis by Titanium Dioxide and Polyoxometalate/TiO2 Cocatalysts. Intermediates and Mechanistic Study. Environ. Sci. Technol. 2004, 38, 329−337. (20) Guo, Y.; Wang, Y.; Hu, C.; Wang, E.; Zhou, Y.; Feng, S. Microporous Polyoxometalates POM/SiO2: Synthesis and Photcata-

4. CONCLUSIONS The visible photocatalyst PW11Mn/D301R, prepared by adsorption of Keggin type Mn(II)-substituted heteropolyanion PW11Mn onto D301R resin, possesses a higher photocatalytic activity compared to PW11Mn in the degradation of RhB. At the same time, PW11Mn/D301R can avoid the troublesome separation of catalysts in the homogeneous system and be used in a wide pH range, thus has a potential in the degradation of aquatic organic pollutants utilizing solar energy.

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ASSOCIATED CONTENT

S Supporting Information *

Raman spectroscopy characterization of PW11Mn adsorption onto the surface of the D301R resin, cycle photodegradation of RhB to survey the stability of PW11Mn/D301R, and UV spectra of the extracting solution immersed-PW11Mn/D301R to characterize the adsorption stability of PW11Mn on the D301R resin. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-898-32888491. E-mail [email protected] (C.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Natural Science Foundation of China (Grant Nos. 21161007), the Key Science and Technology International Co-operation Foundation of Hainan Province, China (Grant No. 2012-GH004), the Key Science and Technology Project of Hainan Province (Grant No.ZDXM 20130088), the Science Research Foundation of University of Hainan Province, China (Grant No. Hjkj 201215), and the Innovative Programs for National College Students, China (Grant No. 201211658042).

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REFERENCES

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