Photodegradation of malachite green dye catalyzed by Keggin-type polyoxometalates under visible-light irradiation: Transition metal substituted effects

Journal of Molecular Structure 1110 (2016) 44e52

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Photodegradation of malachite green dye catalyzed by Keggin-type polyoxometalates under visible-light irradiation: Transition metal substituted effects Chun-Guang Liu*, Ting Zheng, Shuang Liu, Han-Yu Zhang College of Chemical Engineering, Northeast Dianli University, Jilin City, 132012, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 November 2015 Received in revised form 10 January 2016 Accepted 10 January 2016 Available online 13 January 2016

In the present paper, Keggin-type polyoxometalates (POMs) (NH4)3[PW12O40] and its mono-transitionmetal-substituted species (NH4)5[{PW11O39}MII(H2O)] (M ¼ Mn, Fe, Co, Ni, Cu, Zn) have been synthesized and used as photocatalyst to activate O2 for the degradation of dye molecule under visible-light irradiation. Because of the strong adsorption on the surface of POM catalyst, malachite green (MG) molecule was employed as a molecular probe to test their photocatalytic activity. The photodegradation study shows that introduction of transition metal ion leads to an increase in the degradation of MG in the following order: Mn < Fe < Co < [PW12O40]3 < Ni < Cu < Zn, which indicates that the photocatalytic activity of these POMs is sensitive to the transition metal substituted effects. Electronic structure analysis based on the density functional theory calculations shows that a moderate decrease of oxidizing ability of POM catalyst may improve the photocatalytic activity in the degradation of dye molecule under visiblelight irradiation. Meanwhile, intermediate products about the photocatalytic oxidation of MG molecule were proposed on the basis of gas chromatograph mass spectrometer analysis. © 2016 Elsevier B.V. All rights reserved.

Keywords: Polyoxometalates Photocatalyst Photodegradation Malachite green Transition metal substitution

1. Introduction In the past four decades, the photochemistry and photocatalysis of polyoxometalates (POMs) have attracted much attention because of their diverse molecular structures and unique redox properties [1e10]. It has been reported that, in general, POMs share the same photochemical characteristics of semiconductor photocatalyst [11e20]. A ligand-to-metal-charge-transfer excited state with an oxidizing capacity 2.63 V [8] versus the normal hydrogen electrode (lifetime is about 100e200 ps in water), produced after absorption of UV light, is a strong oxidant able to oxidize most organic contaminants to CO2 [21e30]. Meanwhile, the photoreduced POM species deliver the electrons to other chemical species. And thus a photocatalytic cycle can in principle take place. To data, most of the reported POMs-based photocatalyst are concentrated on UV irradiation [31e33]. However, the UV light accounts for only a small fraction (4%) of sunlight reaching the Earth surface when compared to visible light (43%).

* Corresponding author. Present address: No. 169, Changchun Road, Jilin City, PR China. E-mail addresses: [email protected], [email protected] (C.-G. Liu). http://dx.doi.org/10.1016/j.molstruc.2016.01.015 0022-2860/© 2016 Elsevier B.V. All rights reserved.

The photocatalytic behaviors of Keggin-type POMs in the degradation of dye pollutants under visible-light irradiation have been probed. The photoreaction mechanisms differ from those in the systems with UV irradiation [1,2,34e39]. It involves an electron transfer from the excited dye molecule to the Keggin-type POMs, producing the one-electron-reduced Keggin-type POMs, which can be deoxidized by H2O2 or O2 to generate free radical. High reactivity of the free radical causes the degradation of the dye pollutants [40]. Dioxygen (O2) is generally inert to most chemical reaction in its triplet electronic state. However, activated O2 always produces a violent reaction, which is difficult to control, such as the combustion of fossil fuels. It has been reported that POMs have the notable advantages in the field of remarkable controlling over the reaction of O2 among many types of molecular complexes [41]. In addition, POMs are often deployed as a physicochemical probe of some important mechanism regarding reactions in both homogenous and heterogeneous reaction media because many POMs possess reversible redox chemistries and well defined in solution. Recently, the mechanism about activation of O2 by one-electron-reduced Keggin POM anion has been established [42e49]. The results indicated that O2 was reduced by an outer-sphere electron transfer pathway to give the superoxide radical anion (O$ 2 ). In the present

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study, a Keggin-type heteropolytungstate [PW12O40]3 and its mono-transition-metal-substituted species are used as photocatalyst to activate O2 for the degradation of dye pollutants under visible-light irradiation, and simultaneously, as well-defined probes to quantify the effects of transition metal substitution on the degradation of dye pollutants. The high water solubility of Keggin-type POM anions would impede recovery and reuse of the POM catalysts [31,49]. Many efforts have been devoted to develop methods for the synthesis of insoluble POM catalysts. Such as immobilization of POMs on active carbon and carbon fibers, impregnation of POMs on TiO2 or silica gels, intercalation of POMs into anionic clays, and the combination of POMs with appropriate counter ions to give insoluble salt [2,21,31,49,50]. The reported counter ions are concentrated on the large monovalent cations including Kþ, Csþ, and NHþ 4 [51,52]. In the present paper, the insoluble Keggin-type POMs (NH4)3[PW12O40] and its mono-transition-metal-substituted species (NH4)5[{PW11O39}MII(H2O)] (M ¼ Mn, Fe, Co, Ni, Cu, Zn) have been synthesized and used as photocatalyst to activate O2 for the degradation of dye molecules under visible-light irradiation. 2. Experimental section 2.1. Synthesis of (NH4)3[PW12O40] and (NH4)5[{PW11O39}MII(H2O)] (M ¼ Mn, Fe, Co, Ni, Cu, and Zn) The synthesis of (NH4)3[PW12O40] in accordance with methods that reported in the literature [53], sodium tungstate (100 mmol), disodium hydrogen phosphate (9.1 mmol), deionized water (200 mL) and acetic acid (60 mL) were mixing, then the mixture was stirred with a magnetic stirrer, while the temperature rose to 83  C, by replacing of tetrabutylammonium bromide with NH4Cl. NH4Cl (45 mmol) was added to the solution and formed the precipitate, then extracted the precipitate with ether get the (NH4)3[PW12O40]. (NH4)5[{PW11O39}MII(H2O)] (M ¼ Mn, Fe, Co, Ni, Cu, and Zn) were synthesized using previously reported procedures [54], by changing counter ions with NH4Cl and a series of metal nitrate or sulfate were added to give a transition metal substituted photocatalyst (NH4)5[{PW11O39}MII(H2O)] (M ¼ Mn, Fe, Co, Ni, Cu, and Zn). NH4Cl (45 mmol) was added to the solution by using acetic acid to control the pH reach to 4.8, and the suspensions were magnetically stirred at 83  C, the precipitate formed. The filter cake to obtain the mixed fluid filtered at 100  C dried, ground into powder, after calcination at 200  C for 2 h, and getting photocatalysts. Deionized and distilled water was used throughout all experiments. Manganese sulfate (MnSO4), Ferrous nitrate (Fe(NO3)2), Cobalt nitrate (Co(NO3)2), Nickel nitrate (Ni(NO3)2), Copper sulphate (CuSO4) and Zinc sulfate (ZnSO4) were obtained from Tianjin Yongda Chemical Reagent. Acetic acid (30%, analytical grade) was obtained from Traditional Chinese Medicine. All other chemicals were of analytical reagent grade and were used without further purification.

45

2.3. Photocatalytic reaction A 500-W xenon lamp (drawing of special lighting, Shanghai, China) as the visible light source was put in a cylindrical glass vessel with a recycling water glass jacket. A magnetic stirrer is used to make the reactants mix. The photocatalytic ability of (NH4)5[{PW11O39}MII(H2O)] was studied by adding one of the materials to an aqueous solution of organic dye including methyl orange, rhodamine B, congo red and malachite green. And recording the intensity of the characteristic absorption maxima after 30 min of exposure to xenon light. To assess the photocatalytic performance, the dispersions were prepared by addition of photocatalyst (24 mg) to a 40 mL aqueous solution containing organic dye (10 mM). Prior to irradiation, the suspensions were magnetically stirred in the dark for ca. 30 min to establish an adsorption/ desorption equilibrium of suspensions. Open source (500-W xenon light, 5 min in advance open, making stable source) into the quartz, the timing for “zero”, every 10 min sampling with ultraviolet spectrophotometer scanning. At given time intervals, 5 mL aliquot dispersions were sampled and centrifuged, the filtrates were used for record the temporal UVevisible spectral variations of the dyes through a 752 UV/Vis spectrophotometer (Jinghua Instruments). By the measured absorbance value of unknown samples, according to the standard curve can be concluded that content of dye in unknown samples. Through the experiment, find malachite green was degraded most effective. In a procedure similar to that described above, the photocatalytic degradation of malachite green was studied under visible-light conditions for 30 min. For recycling tests, the photocatalyst was separated from the dye solutions after each reaction by centrifugation at 3000 rpm, followed by the exposure of the photocatalyst to the fresh dye solutions and recording dye photodegradation by UV-vis spectroscopy.

2.4. Degradation The degradation products were separated, identified, and quantified by GC-MS with chemical standards. The gas chromatograph mass spectrometer (GC-MS) analysis was carried out on SHIMADZU GC-MS QP-2010 Ultra system. A TG-5MS capillary column (15 m  0.25 mm  0.1 mm) was used for identify the possible nonpolar intermediates and small molecule products, respectively. The initial oven temperature was 160  C, after being maintained there for 2 min, it was increased by 35  C min1 to a final temperature 320  C, and then held for 10 min. Injector was operated in the split ratio of 50: 1, after 2 min from the beginning of a run. Injector and transfer line temperatures were both 300  C. Ions were generated by a 70 eV electron beam at an ionization current of 50 mA and ion source temperature of 250  C. Mass spectra were recorded in full scan mode (m/z 30e700) for qualitative analysis.

2.2. Characterization 2.5. Computational details All the materials were examined by using Fourier transform infrared (FT-IR) spectroscopy and UVevisible spectroscopy (UVevis). IR spectra were obtained on a Fourier transform infrared (FT-IR) spectrophotometer (Shimadzu) and the samples were all supported on anhydrous KBr (spectroscopically pure) pellets. Each sample was repeated at least three times. The UVevisible absorbance analysis of aqueous colloidal suspensions was performed using a Shimadzu UV-2450 spectrophotometer with 2.5 105 M solutions of materials. Matched 10 mm optical path quartz cuvettes were used.

The molecular geometries of the series of POM anions and dye molecules were optimized by using density functional theory (DFT) B3LYP functional [55e57] at 6e31G(d) level. Considering the relativistic effects for metal atom, the LANL2DZ basis set [58e60] containing effective core potential (ECP) representations of electrons near the nuclei was applied for metal atoms in this work. The optimized geometries were characterized as energy minima at the same level. All calculations were performed using Gaussian09 program [61].

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3. Results and discussion 3.1. Spectroscopic characterization FT-IR spectroscopy was employed to gather structural information of Keggin-type POM (NH4)3[PW12O40] and its mono-transition-metal-substituted species (NH4)5[{PW11O39}MII(H2O)] (M ¼ Mn, Fe, Co, Ni, Cu, Zn) synthesized in this study. It is well known that Keggin-type POM H3[PW12O40] contains a cage of tungsten atoms linked by oxygen atoms with tetrahedral phosphate group (see Chart 1). Oxygen atoms form four physically distinct bonds (PeOa, WeOt, WeObeW, and WeOceW bonds), which have distinct infrared signatures: 1080 cm1 for asymmetric stretch vibration of PeOa (Oa corresponds to oxygen atom of tetrahedral phosphate group), 987 cm1 for asymmetric stretch vibration of W ¼ Ot (Ot corresponds to the terminal oxygen atoms), 890 cm1 for bending vibration of WeObeW (Ob corresponds to oxygen atom bridging the two tungsten atoms), and 804 cm1 for bending vibration of WeOceW (Oc represents oxygen atom at the corners of the Keggin structure). For out studied system, the FT-IR spectra shows four characteristic peaks appear at 1077 cm1, 980 cm1, 890 cm1, and 804 cm1 (see Fig. 1), which are very similar to those of the H3[PW12O40] acid, indicating the intact Keggin structure after precipitation and calcination. For mono-transition-metal-substituted Keggin-type POM with Cs symmetry, the triply degenerated PeOa asymmetric stretch vibrations (about 1080 cm1) of intact Keggin structure (a[PW12O40]3) splits into two IR bands. And the W ¼ Ot, WeObeW, and WeOceW vibrations are not significantly shift when compare with that of the intact Keggin structure [62e65]. Therefore, the five characteristic peaks in the mono-transition-metal-substituted species can be conveniently followed using FT-IR spectroscopy. As clearly shown in Fig. 1, upon complexation of transition metal ions to the Keggin framework, the two PeOa asymmetric stretch vibrations appear at 1070 and 1098 cm1, respectively. And the W ¼ Ot stretch vibrations, WeObeW, and WeOceW bending vibrations appear respectively at 976 cm1, 890 cm1 and 804 cm1, which are not noticeable shift relevant to that of their parent cluster [PW12O10]3. This suggests that the transition metal atom has been successfully embedded into Keggin framework. The formation of these Keggin-type POMs was further monitored using UV visible spectroscopy. As shown in Fig. 2, the Keggintype POM anion synthesized in this study shows an absorbance maximum at ca. 212 nm (a) and a sharp peak at 263 nm, which is well in agreement with the literature [66] and can be assigned as

Ot/W and Oc/W charge transfer transitions, respectively. The introduction of transition metal ion into the Keggin framework results in different spectroscopic features, including decreased absorbance in the range from 200 nm to 300 nm and blue shift of both characteristic peaks in comparison to that of their parent cluster [PW12O40]3. It is notable that no apparent absorption peak has been found after 450 nm for all POMs studied here. 3.2. Langmuir adsorption isotherms The activity of heterogeneous catalyst normally depends on the strength of adsorption. In the present paper, the adsorption of congo red (CR), methyl orange (MO), rhodamine B (RB), and malachite green (MG) (see Chart 2) on the surface of POM catalysts in water have been compared in Fig. 3. A solid of Keggin-type POM (NH4)3[PW12O40] has been employed as an example to analyze the detail of these adsorption processes. The concentration of the adsorbate after reaching the adsorption equilibrium (Ceq) was determined by UV spectrophotometry. As shown in Fig. 3A, the absorbance of absorption spectra of MO and MG in water after reaching adsorption equilibrium indicated a significant reduction of 44.8% and 57.8%, compared to those of the substrates in water. And CR and RB exhibited very less reduction of 2.68% and 3.77%, indicating almost no adsorption on the POM catalyst for the latter two dye molecules. The adsorption isotherms of MO and MG on the POM catalyst surface have been shown in Fig. 3B. The amount of adsorbate (nad) in the number of adsorbed substrates per gram of the series of POM catalysts was calculated by the difference in the absorbance observed for solutions with and without POM catalyst powder. According to Langmuir-type adsorption model for the solideliquid interface, nad is expressed by the following Equation

nad ¼

Kceq ns 1 þ Kceq

(1)

where, ns is the amount of adsorbate at complete monolayer coverage (maximum adsorbed quantity) and K ¼ ka/kd is the equilibrium constant for surface adsorption, where ka and kd are the rate constants for adsorption and desorption, respectively. This expression can be rearranged into

ceq ceq 1 ¼ þ nad ns Kns

Chart 1. The structural diagram of Keggin-type POM (A) and its mono-transition-metal-substituted species (B).

(2)

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(NH4)3 [PW12O40]

180

Ni-POM

Co-POM

1098

890

976

1070 1098

Zn-POM

976

Cu-POM

Fe-POM

160

804

140

Transmittance

1070

Mn-POM

200

47

120

804 890

100 1098

80

976

60 40 20 0 700

800

900

1000

1100

1200

700

800

900

1000

1100

1200

Wavenumber(um) Fig. 1. Infrared spectra of Keggin-type POM (NH4)3[PW12O40] and its mono-transition-metal-substituted species.

(R2 ¼ 0.997) in Fig. 3B is consistent with both dye molecules adsorbing on POM catalyst surface in a Langmuir mode. For the MG dye, the equilibrium constant and the maximum adsorbed quantity per unit mass of POM catalyst were K ¼ ~2.0  105 M1 and nad ¼ ~1.0 mol g1 in all POM catalyst studied here, which indicates that the adsorption of MG dye on the surface of the series of POM catalysts are not sensitive to the transition metal substituted effects. 3.3. Photodegradation of MG dyes under visible-light irradiation

Fig. 2. UVevisible absorbance spectra of Keggin-type POM (NH4)3[PW12O40] and its mono-transition-metal-substituted species.

Chart 2. Structural formulas of four dye molecules, where congo red and methyl orange are anions, and rhodamine B and malachite green are cation in solvent phase, respectively.

Therefore, a plot of ceq/nad against ceq should give a straight line of slope 1/ns and intercept 1/(Kns). The excellent linearity of curve

Fig. 4A shows the results of MG degradation, obtained with different experimental conditions. Control experiments in the dark or under light irradiation in the absence of catalyst show an insignificant degradation. However, the visible-light irradiation leads to rapid degradation of the dye in the present of both catalyst and O2. In a separate experiment the insignificant dye degradation was observed in the argon purged aqueous suspension of POM catalyst under visible-light irradiation. This result clearly indicates that O2, as oxidant, plays an important role in the photodegradation of MG dye. In order to improve the photocatalytic performance, we introduced various transition metal ions into Keggin POM framework. A series of mono-transition-metal-substituted Keggin POMs (NH4)5[{PW11O39}MII(H2O)] (M ¼ Mn, Fe, Co, Ni, Cu, Zn) have been used as photocatalyst to activate O2 for the degradation of MG under the same experimental conditions. As shown in Fig. 4B, introduction of transition metal ions leads to an increase in the degradation of MG in the following order: Mn < Fe < Co < [PW12O40]3 < Ni < Cu < Zn. Compared with the parent cluster, [PW12O40]3, introduction of Ni2þ, Cu2þ, and Zn2þ into the Keggin framework leads to an increased photocatalytic activity. From the point of practical application, the stability and reusability of catalysts are very important. Because Zn-POM catalyst gives the most rapid degradation of the MG among all POM-based catalyst studied here, the stability and reusability of this catalyst were examined by repetitious use of the catalyst. As shown in Fig. 5, although some activity is lost over five cycles, the catalyst did not display a significant loss of activity. The degradation rate of MG reached to ca. 72% after the Zn-POM catalyst reused for five cycles. In addition, insolubility of Zn-POM catalyst also has been considered. When the catalysts were added in the reaction system,

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A

2.0 2.0

2.0

c

b

a

d

2.0

Absorbance

1.5

1.5

1.5 1.5

1.0

1.0

1.0 1.0

0.5

0.0

0.5

500

600

700

400

500

600

500

600

400

500

600

Wavelength(nm)

B 35

40

e

30

[c]eq/nad (g/L)

0.5

0.5

f

35

25

30

20

25

15

20 15

10

10

5

5 0

0

5

10

15

20

25

30

0

5

10

15

20

25

30

[c]eq (uM) Fig. 3. (A) UV absorption spectra of (a) malachite green (b) methyl orange (c) rhodamine B and (d) congo red, black lines indicate the spectra of dyes in water at room temperature, red lines indicate the spectra after reaching the adsorption equilibrium in water. (B) Adsorption isotherm of malachite green (e) and methyl orange (f) on the surface of POM catalyst, the linear form according to the Langmuir model. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 4. (A) Degradation kinetics of MG in different reaction system. Initial concentrations: MG (10 uM); POM catalyst (0.6 g/L, pH ¼ 5.77). (a) MG/O2/POM/hn; (b) MG/Ar/POM/hn; (c) MG/O2; (d) MG/O2/hn; (e) MG/O2/POM; (B) the percentage photodegradation of MG express as reduction in the intensity of absorbance at 616 nm.

the mixed system is stirred in dark for 4 h. Then, the catalysts were filtered, and the filtrate was irradiated under visible light. A

comparison shows that the absorbance of MG dye in filtrate is almost constant before and after irradiation, such as, the

C.-G. Liu et al. / Journal of Molecular Structure 1110 (2016) 44e52

Fig. 5. The recycling tests of catalysts in the catalytic system.

absorbance is 1.398 and 1.392 for the 5th recycling test, respectively. This indicates that the degradation phenomenon of MG was not observed, and thus it suggests that the catalyst is not dissolve in the reaction system. All results indicate that this catalyst is considerably stable during the photodegradation of the MG dyes. there was no leaching of Zn-POM catalyst during the cycles. In general, the full reaction mechanism about degradation of dye pollutants in the presence of POM photocatalyst under visiblelight irradiation has been reported as the following [1]:

Dye þ POM/½Dye  POM

(3)

i h hv ½Dye  POM!½Dye  POM/ Dyeþ$  POM

(4)

h

i i h Dyeþ$  POM þO2 / Dyeþ$  POM  O$ 2 /POM þ degradation products

(5)

49

The initial step is the formation of an electron transfer (ET) complex between dye and POM surface (Eq. (3)). The dye molecule was excited to a higher energy state, and then an electron of the excited dye molecule migrates to the LUMO of the POM catalyst, forming the one-electron-reduced POM and dyeþ$ radical. Finally, the one-electron-reduced POMs are slowly re-oxidized by O2, generating the O$ 2 radicals via an outeresphere mechanism (Eq. þ$ (5)) [40]. The interaction between the active O$ 2 and the dye radicals led to the efficient degradation and partial mineralization of the dye. Obviously, this process would be significantly affected by the nature of LUMO of the series of POM anions. We will start by discussing the nature of LUMO of these POM anions based on DFT calculations. An important character of the transition metal center is that, they have the capability of adopting different spin states as a function of the ligand environment. The molecular geometries of these POM anions in various spin states were optimized without any restriction on the symmetry at B3LYP/ 6-31G(d) levels (LANL2DZ basis sets on metal atoms) in this work. According to unrestricted DFT calculations, the Mn, Fe, and NiPOMs are found to have a high-spin sextet, quintet, and triplet ground state, respectively. And the CoPOM prefers the quartet ground state. The ground state of Cu and ZnPOMs were doublet and singlet state, respectively. The LUMOs of these POM anions in their ground state have been listed in Fig. 6. It can be found that the LUMOs of these POM anions are mainly delocalized over the tungsten atoms with some antibonding participation of oxygen p orbitals. We did not find large contributions from the transition metal center in these molecular orbitals. This result indicates that the one-electron-reduced center should be the tungsten and oxygen atom. The oxidizing power of these POM anions can be rationalized by the analysis of their LUMO energies. According to our DFT calculations, the relative energy of the LUMO increases in the order Mn < Fe < Co < Ni z Cu z Zn (see Fig. 7). This result indicates that introduction of Mn, Fe, Co, and Ni can effectively tune their LUMO energy, and thus the oxidizing power. By contrast, the relative energy of LUMO is almost constant for the Ni, Cu, and Zn atoms. The relative energy of the LUMO in ZnePOM is ~1 eV higher than that of MnePOM, which indicates MnePOM is a more powerful oxidizing agent than the corresponding ZnePOM. And thus MnePOM can be

Fig. 6. The frontier molecular orbitals of all transition metal substituted POM anions studied here.

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Fig. 7. Relative energies and composition of the lowest unoccupied orbitals for all transition metal substituted POM anions studied here (only the a-LUMO orbital energy has been listed here for the open-shell systems).

Fig. 8. GC-MS TIC chromatograms obtained from full scan mode (m/z 30e700) of the reaction extracts from the reaction supernatant.

more easily reduced to give one-electron-reduced species than ZnePOM. Meanwhile, one-electron-reduced species of MnePOM is more difficultly re-oxidized than that of ZnePOM. Therefore, as shown in Fig. 4B, the ZnePOM gives a more rapid degradation of the MG than that of MnePOM under the same experimental conditions (74% vs. 90%). In general, a special ability of most POMs is that, it is able to accept one or more several electrons with minimal structural changes, which makes POMs excellent candidates as “electron reservoirs”. But at the same time, the release of electron from the reduced POM is going to be not favoring. Thus, a moderate decrease of oxidizing ability of POM catalyst may improve the photocatalytic activity in the degradation of dye under visible-light irradiation, but it should be noted that, as far as possible not to affect its ability to accept electron in step 2. 3.4. Formation of intermediate products In the process of degradation, parts of organic acid and inorganic acid were detected in the GC analysis. Fig. 8 shows the total ion

chromatograms (TICs) of the reaction extracts from the reaction supernatant. And Fig. 9 indicates the mass spectrograms of the fragment peaks from the precursor ion m/z 301 at 6.92 min. On the basis of these data, the degradation products which had low-molecular-weights were considered to be generated after the cleavage of the double bond and benzene ring of the MG molecule. The supposed structures of these products are shown in Fig. 10. Weinstock have proposed that O2 can be reduced by one-electron$ reduced Keggin POM anion to give O$ 2 /HO2 [46e49]. Thus, we $ believed that the main oxidative species is O$ 2 /HO2 in the photocatalytic degradation of MG under visible irradiation. The degradation products were proposed to be generated from direct oxidation on the double bond and benzene ring of the MG molecule. As shown in Fig. 10, the MG molecule was initially oxidized to dimethylnitrosamine and quinones. Then, further oxidation occurred to form few low-molecular-weight quinones and ketones. The structure of quinones and ketones was subsequently destroyed, the low-molecular-weight quinones were oxidized to give the ringopening products, which proposes a further oxidative pathway to form low-molecular organic and inorganic acids.

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Fig. 9. Mass spectrograms of the fragment peaks from the precursor ion m/z 274 at 5.34 min.

Fig. 10. Proposed reaction scheme for the degradation of malachite green by the Keggin-type POM photocatalysts.

4. Conclusion Our study on the photodegradation of dye molecules catalyzed by Keggin-type POMs (NH4)3[PW12O40] and its transition-metalsubstituted species (NH4)5[{PW11O39}MII(H2O)] (M ¼ Mn, Fe, Co, Ni, Cu, Zn) under visible-light irradiation may present a fundamental understanding of the effects about transition metal substitution on their photocatalytic activity. The photodegradation experiment shows that the photocatalytic performance of these POMs is sensitive to the transition-metal-substituted effects. The degradation of dye molecule increases in the following order: Mn < Fe < Co < [PW12O40]3 < Ni < Cu < Zn. Electronic structure analysis shows that a moderate decrease of oxidizing ability of POM catalyst may improve the photocatalytic activity in the degradation of dye under visible-light irradiation. The degradation products about the photocatalytic oxidation of MG molecule were proposed on the basis of GC-MS analysis. Acknowledgments The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (21373043). References [1] C.C. Chen, Q. Wang, P.G. Lei, W.J. Song, W.H. Ma, J.C. Zhao, Environ. Sci. Technol. 40 (2006) 3965e3970. [2] P.G. Lei, C.C. Chen, J. Yang, W.H. Ma, J.C. Zhao, L. Zang, Environ. Sci. Technol. 39 (2005) 8466e8474.

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