Accepted Manuscript Photocatalytic degradation of methyl orange by PbXO4 (X = Mo, W) Yu Zhiyong, Dong Chaonan, Qiu Ruiying, Xu Lijin, Zheng Aihua PII: DOI: Reference:

S0021-9797(14)00696-1 http://dx.doi.org/10.1016/j.jcis.2014.09.047 YJCIS 19855

To appear in:

Journal of Colloid and Interface Science

Received Date: Accepted Date:

15 May 2014 14 September 2014

Please cite this article as: Y. Zhiyong, D. Chaonan, Q. Ruiying, X. Lijin, Z. Aihua, Photocatalytic degradation of methyl orange by PbXO4 (X = Mo, W), Journal of Colloid and Interface Science (2014), doi: http://dx.doi.org/ 10.1016/j.jcis.2014.09.047

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Photocatalytic degradation of methyl orange by PbXO4 (X = Mo, W)

Yu Zhiyong a*, Dong Chaonan a, Qiu Ruiying a, Xu Lijin a, Zheng Aihua b

a

Department of Chemistry, Renmin University of China, Beijing 100872, China

b

Analysis and testing center, Beijing Normal University, Beijing 100875, China

Dedicate this paper to DR PD John Kiwi

ABSTRACT PbMoO4 and PbWO4 are prepared by the simple precipitation method in this work, they show the photocatalytic activities for the degradation of methyl orange in water under the UV light illumination. In the above photocatalytic degradation processes, methyl orange concentration decreases quickly, the total organic carbon (TOC) decreases slowly; inorganic ions (SO42-, NO3-, NO2-, NH4+) can be formed and measured by the ion chromatograph; the pH value in the systems decreases gradually; a small quantity of HO· can be generated and measured by the terephthalic acid (TA) indirectly. In order to estimate the roles of active species during the above photocatalytic degradation processes, isopropanol, (NH4)2C2O4, and 1,4benzoquinone as the scavengers for HO·, h+, O2·- are introduced into the systems, respectively. Isopropanol and (NH4)2C2O4 are effective scavengers for active species HO· and h+ respectively, but 1,4-benzoquinone is not a satisfactory scavenger in all cases to capture O2·-, at least in this work. At last, PbMoO4 and PbWO4 are characterized by nitrogen sorption, DRS, SEM, TEM and XRD. Keywords: Photocatalytic activity; PbXO4 (X=Mo, W); methyl orange.

-------------------------------------------------------------------------*Corresponding author. Tex.: +86-10-62511528; fax: +86-10-62516614. E-mail address: [email protected] (Yu Zhiyong)

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1. Introduction The luminescence properties of lead molybdate (PbMoO4) and lead tungstate (PbWO4) have been studied for a long time. At the end of the 1990s, PbWO4 was selected for application as a scintillation detector in high energy physics, which resulted in many research projects dealing with different aspects of the performance characteristics of PbWO4; to a lesser extent this is also applicable to PbMoO4, which has been used for decades in optoelectronics. In recent 20 years, people have continued to study the scintillation properties of PbMoO4 and PbWO4 [1]. On the other hand, PbXO4 (X=Mo, W) can also be used as photocatalysts [2-6]; heterogeneous photocatalysis is one of effective methods to treat wasterwater with large amounts of organic pollutants [79], active species can be generated during this process. As far as the preparation methods of PbXO4 are concerned, PbMoO4 can be prepared by the solvothermal process, solid-state sintering, co-precipitation, microwave irradiation [2-5]; PbWO4 can be prepared by the hydrothermal synthesis [6]. As far as the research contents are concerned, the following contents are studied: PbMoO4 has highly dispersive valence and conduction bands, which can lead to higher mobility of the photo-generated carries [2]; the photodegradation of methyl orange in water by PbMoO4 is not easy [3-4]; a complementary combination of experimental work and first-principle calculations has been used to increase the understanding of the enhanced photocatalytic activity of PbMoO4 powders with some predominant facets [5]; the high crystallinity of PbWO4 microspheres shows an enhanced catalytic activity owing to the lower recombination rate of photo-generated electron-hole pairs [6]. But, in these works, there are no detection of inorganic ions and pH change [2-6]; there are detection of total organic carbon (TOC) only in the references [3,4]. In this work, at first, we prepare PbXO4 (X = Mo, W) by the precipitation method, then we study their photocatalytic activies on the degradation of methyl orange (MO) in water under the UV light illumination (decrease of MO concentration and TOC). At the same time, we measure the inorganic ions (SO42-, NO3-, NO2-, NH4+) and pH change in the systems. In order to estimate the roles of active species (HO·, h+, O2·-) during the above photocatalytic degradation processes, isopropanol, (NH4)2C2O4, and 1,4-benzoquinone as the scavengers are introduced into the systems, respectively. At last, PbMoO4 and PbWO4 are characterized by nitrogen sorption, DRS, SEM, TEM and XRD.

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2. Experimental 2.1. Chemicals and material Methyl orange (MO, C14H14N3NaO3S, MW=327.33g/mol, its molecular structural formula seeing (a) below), terephthalic acid (TA), NaOH, Pb(NO3)2, (NH4)6Mo4O7·24H2O, Na2WO4, isopropanol (CH3CHOHCH3, IPA), (NH4)2C2O4 (AO), and 1,4-benzoquinone (BQ) are analytical reagents and used without further purification; filter membrane with 0.22 µm (micro PES, made in Membrana company, Germany); distilled water is used throughout this work. The magnetic stirring bar is made by the author, its core is a magnetic material, its crust is glass made from element boron and silicon and without element carbon. NaO3S

N N

N(CH3)2

(a)

2.2. Preparation of PbXO4 (X=Mo, W) Put the aqueous solution of Pb(NO3)2 into the aqueous solution of (NH4)6Mo4O7·24H2O or Na2WO4, we can get the different precipitates. The precipitates are purified with distilled water, then drying in air naturally.

2.3. Methyl orange adsorption test 100 mg PbXO4 and 100 mL 0.10 mM methyl orange (MO) solution are put into a beaker (the total volume is 150 mL) together, the mixture (suspension) is stirred in the dark. At different time, samples are taken and filtered by the filter membrane with 0.22 μm, MO concentrations are measured with the UV-Vis spectrophotometer (Varian Cary 50 series) at the absorption peak λ = 464 nm.

2.4. Methyl orange photodegradation process (1). 100 mg PbXO4 and 100 mL 0.10 mM MO solution are put into a beaker (the total volume is 150 mL) together, the mixture (suspension) is then put under the 30W UV lamp (30W, 253.7 nm, Kongjun houqin, Beijing, China) vertically, the distance between the UV lamp and the surface of the suspension in the beaker is 4.2 cm. The suspension is stirred in the dark for 30 minutes. Afterwards, we turn on the UV

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lamp, the photoreaction begins. At different illumination time, samples are taken and filtered for the measurement of MO concentration. (2). Repeat the step (1), samples are taken at the different illumination time and filtered for the measurement of the total organic carbon (TOC) by the TOC analyzer (TOC–VCPH, Shimadzu). In order to get accurate quantitative determination of TOC in solutions, we use methyl orange with high concentration (0.10 mM) than that commonly found in textile effluents [8,9]. (3). Repeat the step (1) four times, at 5-th (or 10-th, 15-th, 20-th) hour, 50 mL sample is taken and filtered for the measurement of inorganic ions (NO3-; NO2-; NH4+; SO42-); 50 mL sample is taken and filtered for the measurement of pH value. (4). 100 mg PbXO4 and 100 mL of (0.02 mM MO + 0.40 mM scavenger) (scavenger = (NH4)2C2O4, BQ, isoproparol) solution are put into a beaker (the total volume is 150 mL) together, the following operations are the same as the step (1).

2.5. Measurement of pH value The buffer solutions (pH=6.88; pH=4.00) are used to emend the PHS--25 meter (Lei--Ci, Shanghai INESA Scientific Instrument Co., Ltd, China). The electrode (E-201-C) is cleaned with the distilled water, the water on the surface of the electrode is wiped up with the soft clean paper, then the electrode is plunged into the fresh measured solutions.

2.6. Measurement of inorganic ions NH4+, NO3-, NO2- and SO42- are determined by an ion-chromatograph (IC) system (DX-600 and ICS2100, Dionex, USA) [10-20], which is equipped with an autosampler (sample injection volume of 100 µL). For the determination of the anions, the system is fitted with a separate column IonPac AS11 (4 i.d × 250 mm) and guard column IonPac AG11 (4 i.d × 50 mm), the mobile phase is composed of 5 mM -35 mM KOH gradient elution, and the flow rate is 1.0 mL min-1. For the determination of the cations, the system is fitted with an IonPac CS12A separation column (4 i.d × 250 mm) and guard column IonPac CG12A ( 4 i.d × 50 mm), A 20 mM of MSA (Methanesulfonic acid) solution circulating at 1.0 mL min-1 is used as the mobile phase.

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2.7. Measurement of HO· 100 mg PbXO4 and 100 mL 5×10-4 M terephthalic acid (TA) solution (TA is dissolved in 2×10-3 M NaOH in advance) are put into a beaker (the total volume is 150 mL) together, the mixture (suspension) is then put under the 30W UV lamp vertically, the distance between the UV lamp and the surface of the suspension in the beaker is 4.2 cm. The suspension is stirred in the dark for 30 minutes. Afterwards, we turn on the UV lamp, the photoreaction to catch HO· begins. At different illumination time, samples are taken and filtered, then analyzed on the RF-540 spectrofluorophotometer (Perkin Elmer LS 55).

2.8. Characterization of PbXO4 Measurement of the specific surface area and analysis of the porosity for the sample PbXO4 (X = Mo, W) is carried out through measuring N2 adsorption-desorption isotherms at 77K with a Quantachrome autosorb-1 system. X-ray photoelectron spectroscopy (XPS) data are obtained with an ESCALab220i-XL electron spectrometer from VG Scientific using 300W Al Kα radiation. The base pressure is about 3×10-9 mbar. All the binding energies are referenced to the C 1s line at 284.8 eV from adventitious carbon. The sample is put inside the XPS instrument for analysis, the data are treated by the attached soft (Advantage 4.15). UV–vis diffuse reflectance spectra (DRS) of the samples are recorded on a UV–vis spectrophotometer (Hitachi UV-3010) with an integrating sphere attachment for their reflectance in the range of 200 –800 nm, and BaSO4 is the reflectance standard. The catalyst powder crystal structures are measured by the powder X-ray diffraction (XRD) on the Xray diffractomer (XRD 7000, Shimadzu) with Cu target Kα radiation with generator voltage of 40 kV and tube current of 30 mA over the angle range of 2θ=10--80º. Scanning electron microscopy (SEM) (JSM-6700F high resolving power SEM, Japan) is used to measure the morphology of materials. Ttransmission electron microscopy (TEM) (JEM--2100, Japan) is used to observe the morphology, dispersion of nano materials, and to measure and estimate the size of nano materials. The samples for TEM measurement are prepared by dispersing PbXO4 powder in the absolute ethanol with the ultrasonic processing for 10 minutes, the carbon-copper grid is immersed into the above suspension, then we take the grid out of the suspension, let the grid dry naturally.

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3. Results and discussion 3.1. PbXO4 preparation According to the following equation: Pb2+ + XO42- = PbXO4 ↓, we can get the PbXO4 (X = Mo, W) in the form of precipitates, which are not soluble in water, so we can wash the precipitates with distilled water some times, then let PbXO4 dry in air naturally.

3.2. Adsorption test Adsorption behavior is a key step for the heterogeneous photoreactions. Co means the initial methyl orange concentration, C means methyl orange concentration at different adsorption time. As for the system of (100 mg PbMoO4 + 100 mL 0.10 mM MO), when the adsorption time is at 5-th, 10-th, 20-th, 30-th minute, the C/Co is 0.9787, 0.9888, 0.9888, 0.9888, respectively; as for the system of (100 mg PbWO4 + 100 mL 0.1mM MO), when the adsorption time is at 5-th, 10-th, 20-th, 30-th minute, the C/Co is 0.9888, 0.9926, 0.9926, 0.9926, respectively. We find that 30 minutes are enough to set up an adsorptiondesorption equilibrium between methyl orange molecules and the surfaces of PbXO4 (X = Mo, W). Generally speaking, the adsorption ability of PbXO4 (X = Mo, W) is weaker.

3.3. Decrease of methyl orange concentration and total organic carbon Fig.1(A)(B) show the UV–Vis spectral change of methyl orange (MO) and the decrease of methyl orange concentration and total organic carbon (TOC) in the system of (100 mg PbMoO4 + 100 mL 0.10 mM MO) under the UV light illumination respectively. During the photodegradation process, the absorption bands of methyl orange in the visible region decrease with time gradually (at t = 2h-15h), finally disappear (at t = 20h) (Fig.1(A)), which indicates the destruction of its chromophoric structure (namely, azo bond -N=N-) [8,9]. Methyl orange concentration decreases quickly, TOC decreases slowly; at t = 20-th hour, the C/Co is 0.0152; the TOC/TOCo is 0.7859 (Fig.1(B)), which means that methyl orange molecules cannot be photomineralized fully, the intermediates are stable in the solution. Similar results can be found in the Refs.[3,4]. Fig.1

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Fig.2(A)(B) show the UV–Vis spectral change of methyl orange (MO) and the decrease of methyl orange concentration and total organic carbon (TOC) in the system of (100 mg PbWO4 + 100 mL 0.10 mM MO) under the UV light illumination respectively. During the photoreaction process, the absorption bands of methyl orange in the visible region decrease with time quickly (at t = 2h-5h), finally disappear (at t =10 h, 15h, 20h) (Fig.2(A)), which indicates the destruction of its chromophoric structure (namely, azo bond N=N-) [8,9]. Methyl orange concentration decreases quickly, TOC decreases slowly, at t = 20-th hour, the C/Co is 0.00; the TOC/TOCo is 0.9016 (Fig.2(B)), which means that methyl orange molecules cannot be photomineralized fully, the intermediates are stable in the solution. Fig.2

3.4. Detection of inorganic ions Fig.3 (A)(B) show the relation between the concentration of inorganic ions and the illumination time in the above two systems respectively. Comparing Fig.3(A) with Fig.3(B), we find that the concentrations of NO3-, NO2- and NH4+ in the system (A) are higher than those in the system (B), NO2- is not found in the system (B); the concentration of SO42- in the system (B) is higher than that in the system (A). Fig.3

Overall, we can find that the concentration of inorganic ions increase with the prolongation of illumination time. At t = 20-th hour, as for the system (A), the concentrations of NO2-, NO3-, SO42-, NH4+ are 0.04 mg/L, 3.4 mg/L, 4.7 mg/L, 1.4 mg/L, respectively; as for the system (B), the concentrations of NO3-, SO42-, NH4+ are 0.16 mg/L, 6.0 mg/L, 0.92 mg/L, respectively. At this moment, as for the system (A) or (B), [NO3-] + [NO2-] + [NH4+] < 0.30 mM (1 mol methyl orange molecule contains 3 mol N atoms), the reasons include but not limited to: (i) the formation of nitrogen-containing organic intermediate(s), (ii) the formation of molecular nitrogen (N2) [14]. It is worthwhile to note that the generation of NH4+ during the photocatalytic degradation of methyl orange signifies a concurrent reduction reaction [14]. In addition, as for the system (A) or (B), [SO42-] < 0.1 mM (1 mol methyl orange molecule contains 1 mol S atom), the reasons include but not limited to: (i) the formation of sulfur-containing organic intermediate(s), (ii) the formation of SO32- rather than SO42- directly [15,19].

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3.5. pH change Dissolved CO2 makes the distilled water more acidic, the pH value of the distilled water in our lab is 6.0, the pH value of 0.10 mM methyl orange is 6.2. Fig.4 shows the pH value change with the illumination time in the two different systems. At t = 20-th hour, the pH values are 4.6 and 4.2 in the system (A) and (B), respectively, which means that the atoms S and N in the molecule of methyl orange are changed into strong acids H2SO4 and HNO3 respectively during the above photocatalytic degradation processes. Fig.4

3.6. Detection of active species by scavengers In the above photocatalytic degradation processes, it is reasonable to think that at least one kind of active species (HO·, h+, O2·-, and so on) is formed. In order to estimate the roles of active species during the photocatalytic degradation processes, isopropanol (IPA), (NH4)2C2O4 (AO), and 1,4-benzoquinone (BQ) as the scavengers for HO·, h+, O2·- are introduced into the systems, respectively [21-24]. From Fig. 5, it is easy to find that IPA has little effect on the photocatalytic activity of the catalysts (especially for PbMoO4, almost no effect), which suggests that HO· does not play an important role for the photocatalytic degradation of methyl orange. On the contrary, AO has obvious inhibition on the photocatalytic degradation of methyl orange, which suggests that h+ plays an important role for the photocatalytic degradation of methyl orange. Fig.5

In fact, a small quantity of HO· can be generated in the above photocatalytic degradation process, we can measure HO· by terephthalic acid (TA) indirectly. TA reacts with HO· to generate highly fluorescent product, 2-hydroxyterephthalic acid (TAOH), which emits photoluminescence at 426 nm on the excitation of its own 315 nm absorption band [25,26]. The intensity of the peak attributed to TAOH is known to be proportional to the amount of HO· generated. Fig. 6 shows the fluorescence intensity of two systems, it is easy to find that the fluorescence intensity increase with the illumination time under the UV light illumination. Fig.6

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Now, we analyse the role of 1,4-benzoquinone (BQ) as the scavenger for O2·-. Fig.7(A) shows the photocatalytic degradation processes of methyl orange (MO) mediated by PbMoO4. At t = 0, the absorbance of curve (a) in Fig.7(A) is less than 0.60, then the absorbance of the system decreases with illumination time. The color of 0.40 mM BQ in water is light yellow, under the experimental conditions, there is a chemical reaction to take place for BQ, because the absorbance of BQ solution varies with illumination time, seeing Fig. 7(B). At t = 0, the absorbance of curve (a) in Fig.7(B) is very little, at t = 0.5 h, the absorbance of curve (b) in Fig.7(B) is the highest, then the absorbance value of the system decreases slowly, at t = 3.5 h, the absorbance of curve (e) in Fig.7(B) is more than that at t = 0. Fig.7(C) shows the photocatalytic degradation processes of methyl orange mediated by PbMoO4 in the presence of BQ. At t= 0.5 h, the absorbance of curve (b) in Fig.7(C) is more than 0.60, then the absorbance value decreases slowly; at t = 3.5 h, the absorbance of curve (e) in Fig.7(C) is more than that at t = 0. Therefore, here, it is impossible for us to use 1,4-benzoquinine (BQ) to estimate the roles of O2·-. If we replace PbMoO4 with PbWO4, then we can get the same evaluation, seeing the Fig.8. Based on the above facts, we say that BQ is not a satisfactory scavenger in all cases to capture O2·-, at least in this work. Fig.7 Fig.8

3.7. N2 adsorption – desorption isotherm and pore size distribution The nitrogen adsorption-desorption isotherms of PbMoO4 and PbWO4 are shown in Fig.9(A) and Fig.10(A), respectively, it can be seen that the isotherm profiles of PbXO4 (X = Mo, W) can be categorized as the type IV. Fig.9(B) shows the Barrett-Joyner-Halenda (BJH) pore size distribution of PbMoO4 obtained from the adsorption branch, the mesopores are mainly centered at about 3.5 nm and 9.9 nm; Fig.10(B) shows the Barrett-Joyner-Halenda (BJH) pore size distribution of PbWO4 obtained from the adsorption branch, the mesopores are mainly centered at about 3.1 nm and 5.0 nm. The specific surface areas of PbMoO4 and PbWO4 are 2.2 m2/g and 3.4 m2/g, respectively [27-29]. Accordingly, the adsorption ability of PbXO4 (X = Mo, W) is weaker. Fig.9 Fig.10

3.8. XPS analysis

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The XPS allows the determination for the surface composition of very thin outermost surface layer around 2 nm with very high surface sensitivity. In order to analyze the chemical composition of photocatalysts before and after use, we have carried out the XPS measurement, the related data are listed in Table 1, the element C can be ascribed to the residual carbon from the precursor, whose peak position (C 1s) is near 284.7 eV. Fig.11 shows the XPS survey of PbMoO4 before use and after use, the peak positions of Pb 4f, O 1s, Mo 3d are near 138.5 eV, 530.5 eV, 232.4 eV, respectively. Fig.12 shows the XPS survey of PbWO4 before use and after use, we can get the similar evaluation except that the XPS peak position of W 4f is near 35.0 eV.

Table 1

Fig.11 Fig.12

3.9. Diffuse reflectance spectra of PbXO4 Fig.13 is the diffuse reflectance spectra (DRS) of PbMoO4 and PbWO4. From 200 nm to 350 nm, the absorbance of PbMoO4 is high, from 350 nm to 390 nm, the absorbance of PbMoO4 decreases very quickly, from 390 nm to 800 nm, the absorbance of PbMoO4 increases very tardily. From 200 nm to 300 nm, the absorbance of PbWO4 is high, from 300 nm to 370 nm, the absorbance of PbWO4 decreases very quickly, from 370 nm to 800 nm, the absorbance of PbWO4 increases very tardily. In addition, from 390 nm to 800 nm, the absorbances of PbMoO4 and PbWO4 are very near. Fig.13

3.10. Characterization of PbXO4 Fig.14(A,B) are the SEM images of PbMoO4 and PbWO4, respectively, it can be observed that the PbMoO4 powder agglomerates, the PbWO4 powder is more ordered relatively than PbMoO4 powder. Fig. 15(A,B) are the TEM images of PbMoO4 and PbWO4, respectively, we can observe the irregular PbMoO4 and sheet-like PbWO4. Fig.14 Fig.15

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Fig.16 shows the XRD patterns of PbMoO4 and PbWO4. PbMoO4 is a tetragonal structure, and the corresponding JCPDS number is 44-1486. The peaks at 2θ = 17.8º, 27.5º, 29.5º, 37.8º, 40.8º, 43.5º, 44.9º, 47.4º, 51.2º, 55.6º, 56.8º match well with the (101), (112), (004), (114), (105), (123), (204), (220), (116), (312), (224) crystal planes of PbMoO4 [2]. PbWO4 is a tetragonal stolzite structure, and the corresponding JCPDS number is 19-0708. The peaks at 2θ = 27.4º, 29.7º, 32.8º, 44.7º, 47.1º, 51.3º, 55.3º, 56.6º, 71.5º, 72.1º match well with the (112), (004), (200), (204), (220), (116), (312), (224), (208), (316) crystal planes of PbWO4 [6,30]. Fig.16

4. Conclusion PbMoO4 and PbWO4 are prepared by the very simple precipitation method, XRD patterns reveal that both of them have good crystal structure. They can be used as photocatalysts for the degradation of methyl orange in water under the UV light illumination, and PbWO4 shows stronger catalytic activity than PbMoO4. During the above degradation processes, methyl orange concentration decreases quickly, but the total organic carbon (TOC) decreases slowly; the inorganic ions (SO42-, NO3-, NO2-, NH4+) can be formed and measured by the ion chromatograph; the pH value in the systems decreases gradually; a small quantity of HO· can be generated and measured by the terephthalic acid (TA) indirectly. Isopropanol and (NH4)2C2O4 are effective scavengers for active species HO· and h+ respectively, but 1,4-benzoquinone is not a satisfactory scavenger in all cases to capture O2·-, at least in this work.

Acknowledgements This work is supported by The Basic Research Funds in Renmin University of China from the Central Government (No. 12XNLL03); The Fundamental Research Funds for the Central Universities, and the Research Funds of Renmin University of China (No. 11XNL011); National Natural Science Fundation of China (No. 91127039).

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[16] Biljana F. Abramović, Nemanja D. Banić, Daniela V. Šojić. Degradation of thiacloprid in aqueous solution by UV and UV/H2O2 treayments. Chemosphere 81 (2010) 114-119. [17] Shiyuan Ding, Junfeng Niu, Yueping Bao, Lijuan Hu. Evidence of superoxide radical contribution to demineralization of sulfamethoxazole by visible-light-driven Bi2O3/Bi2O2CO3/Sr6Bi2O9 photocatalyst. J. Hazar. Mater. 262 (2013) 812818. [18] Dunia E. Santiago, José M. Doña-Rodríguez, J. Araña, C. Fernández-Rodríguez, O. González-Díaz, J. Pérez-Peña, Adrián M.T. Silva. Optimization of the degradation of imazalil by photocatalysis: Comparison between commercial and lab-made photocatalysts. Appl. Catal. B. 138-139 (2013) 391-400. [19] Yuefei Ji, Lei Zhou, Corinne Ferronato, Arnaud Salvador, Xi Yang, Jean-Marc Chovelon. Degradation of sunscreen agent 2-phenylbenzimidazole-5-sulfonic acid by TiO2 photocatalysis: Kinetics, photoproducts and comparison to structurally related compounds. Appl. Catal. B. 140-141(2013) 457-467. [20] M. Ismail, Hasan M. Khan, Murtaza Sayed, William J. Cooper. Advanced oxidation for the treatment of chlorpyrifos in aqueous solution. Chemosphere 93 (2013) 645-651. [21] Zhihong Chen, Weilin Wang, Zhengguo Zhang, Xiaoming Fang. High-efficiency visible-light-driven Ag3PO4/AgI photocatalysts: Z-scheme photocatalytic mechanism for their enhanced photocatalytic activity. J. Phys. Chem. C 117 (2013) 19346-19352. [22] Yangming Lin, Danzhen Li, Junhua Hu, Guangcan Xiao, Jinxiu Wang, Wenjuan Li, Xianzhi Fu. Highly efficient photocatalytic degradation of organic pollutants by PANI-modified TiO2 composite. J. Phys. Chem. C 116 (2012) 57645772. [23] Haili Lin, Jing Cao, Bangde Luo, Benyan Xu, Shifu Chen. Synthesis of novel Z-scheme AgI/Ag/AgBr composite with enhanced visible light phocatalytic activity. Catal. Commu.. 21 (2012) 91-95. [24] H. Nouri, A. Habibi-Yangjeh, M. Azadi. Preparation of Ag/ZnMgO nanocomposites as novel highly efficient photocatalysys by one-pot method under microwave irradiation. J. Photochem. Photobio. A. 281 (2014) 59-67. [25] Tsutomu Hirakawa, Yoshio Nosaka. Properties of O2•- and OH• formed in TiO2 aqueous suspensions by photocatalytic reaction and the influence of H2O2 and some ions. Langmuir 18 (2002) 3247-3254. [26] Qi Xiao, Zhichun Si, Jiang Zhang, Chong Xiao, Xiaoke Tan. Photoinduced hydroxyl radical and photocatalytic activity of samarium-doped TiO2 nanocrystalline, J. Hazard. Mater. 150 (2008) 62-67. [27] Wei Chen, Fengqiang Sun, Zhimin Zhu, Zhilin Min, Weishan Li. Nanoporous SnO2 prepared by a photochemical strategy: controlling of specific surface area and photocatalytic activity in degradation of dye pollutants. Micropor. Mesopor. Mat. 186 (2014) 65-72. [28] Ting Zhu, Jun Song Chen, and Xiong Wen (David) Lou. Highly efficient removal of organic dyes from waste water using hierarchical NiO spheres with high surface area. J. Phys. Chem. C 116 (2012) 6873-6878. [29] Hailong Fei, Yuping Liu, Yingpin Li, Pingchuan Sun, Zhongyong Yuan, Baohui Li, Datong Ding, Tiehong Chen. Selective synthesis of borated meso-macroporous and mesoporous spherical TiO2 with high photocatalytic activity.

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Micropor. Mesopor. Mat. 102 (2007) 318-324. [30] Titipun Thongtem, Sulawan Kaowphong, Somchai Thongtem. Sonochemical preparation of PbWO4 crystals with different morphologies. Ceram. Inter.. 35 (2009) 1103-1108.

Fig.1 (A). UV–vis spectral change of MO in the system of (100 mg PbMoO4 + 100 mL 0.10 mM MO) as a function of illumination time. (a) t = 0 h, (b–f) illumination at t = 2, 5, 10, 15, 20 h, respectively. (B) The corresponding ratios of MO concentration and TOC value in the system (A). (a) C/Co, (b) TOC/TOCo.

Fig.2 (A) UV–vis spectral change of MO in the system of (100 mg PbWO4 + 100 mL 0.10 mM MO) as a function of illumination time. (a) t = 0 h, (b–f) illumination at t = 2, 5, 10, 15, 20 h, respectively. (B) The corresponding ratios of MO concentration and TOC value in the system (A). (a) C/Co, (b) TOC/TOCo.

Fig.3. Evolution of inorganic ions with illumination time in the different systems. (A): (100 mg PbMoO4 + 100 mL 0.10 mM MO), (B): (100 mg PbWO4 + 100 mL 0.10 mM MO). (a) SO42-, (b) NO3-, (c) NO2-, (d) NH4+.

Fig.4. Change of pH of the solution with illumination time. (A): (100 mg PbMoO4 + 100 mL 0.10 mM MO), (B): (100 mg PbWO4 + 100 mL 0.10 mM MO).

Fig.5 Photocatalytic degradation of methyl orange (MO) in the two different systems with various scavengers. (A) catalyst = PbMoO4; (B) catalyst = PbWO4. (a) 100 mg catalyst + 100 mL 0.02 mM MO; (b) 100 mg catalyst + 100 mL of (0.02 mM MO + 0.40 mM IPA); (c) 100 mg catalyst + 100 mL of (0.02 mM MO + 0.40 mM AO).

Fig.6 Fluorescent spectra of the two systems under the UV illumination. (A) 100 mg PbMoO4 + 100 mL 5 ×10-4 M TA, (B) 100 mg PbWO4 + 100 mL 5 ×10-4 M TA. (a) t = 0.5 h, (b) t = 1 h, (c) t = 2 h, (d) t = 3 h.

Fig.7 Photocatalytic degradation of methyl orange (MO) and 1,4-benzoquinine (BQ) in different systems. (A) 100 mg PbMoO4 + 100 mL 0.02 mM MO; (B) 100 mL 0.40 mM BQ; (C) 100 mg PbMoO4 + 100 mL of (0.02 mM MO + 0.40 mM BQ). (a) t = 0, (b) t = 0.5h, (c) t = 1.5h, (d) t = 2.5h, (e) t = 3.5h.

Fig.8 Photocatalytic degradation of methyl orange (MO) and 1,4-benzoquinine (BQ) in different systems. (A) 100 mg PbWO4 + 100 mL 0.02 mM MO; (B) 100 mL 0.40 mM BQ; (C) 100 mg PbWO4 + 100 mL of (0.02 mM MO + 0.40 mM BQ). (a) t = 0, (b) t = 15 min, (c) t = 30 min, (d) t = 45 min, (e) t = 60min.

Fig.9 (A) N2 adsorption-desorption isotherm and (B) BJH pore size distribution of PbMoO4. Fig.10 (A) N2 adsorption-desorption isotherm and (B) BJH pore size distribution of PbWO4.

Fig.11 (A) XPS survey spectra of PbMoO4 before use, (B) XPS survey spectra of PbMoO4 separated from the system of (100 mg PbMoO4 + 100 mL 0.10 mM MO) at illumination t = 20-th hour. Fig.12 (A) XPS survey spectra of PbWO4 before use, (B) XPS survey spectra of PbWO4 separated from the system of (100 mg PbWO4 + 100 mL 0.10 mM MO) at illumination t = 20-th hour.

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Fig.13 Diffuse reflectance spectra of (a) PbMoO4 and (b) PbWO4. Fig.14 (A) SEM image of PbMoO4 , (B) SEM image of PbWO4. Fig.15 (A) TEM image of PbMoO4, (B) TEM image of PbWO4. Fig.16 XRD patterns of PbMoO4 and PbWO4

1.5

a b c

1

0.5

e f

0 264

364

464 564 wavelength (nm)

b

ratio

Abs.

d

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

A 664

a B 0

5

10 time (h)

15

20

Fig.1

15

1.5

1

a

b

0.8 1 Abs.

A

0.5

d,e 264

B

0.4 0.2

c 0

ratio

0.6

b

a

0 364

464 564 wavelength (nm)

664

0

5

10 time (h)

15

20

Fig.2

16

6

6

A

C (mg/L)

4

a

4

C (mg/L)

a

B

b

2

2

d c

0 0

5

10 15 time (h)

d b

0 20

0

5

10 15 time (h)

20

Fig.3

17

8

a

4

b

pH

6

2 0 0

10 time (h)

20

Fig.4

18

1

1

A

0.8

0.8

c

0.4

0.4

a,

0.2

0.2

0

0 0

1

c

0.6 C/Co

C/Co

0.6

B

2 3 time (h)

4

a b

0

15

30 45 time (min)

60

Fig.5

19

56

d

70 A

c

Intensity

42 28 14

56 42

b a

0

B

Intensity

70

28 14

d c b a

0 360 400 440 480 520 560 wavelength (nm)

360 400 440 480 520 560 wavelength (nm)

Fig.6

20

1

A

0.8 0.6

0 264

364

at 464 from up to b,c,d,

0.8 Abs.

0.6

0.4 0.2 0

464 564 664 wavelength (nm)

1

at 464 from up to b,c,d,

Abs.

Abs.

0.2

B

0.8 0.6

a b c d e

0.4

1

264

364

464 564 664 wavelength (nm)

C

0.4 0.2 0 264

364

464 564 664 wavelength (nm)

Fig.7

21

1

A

0.8 0.6

Abs.

Abs.

c e

0 264

264

364

464 564 664 wavelength (nm)

C

Abs.

0.6

0

464 664 wavelength (nm)

at 464 from up to c,d,e,

0.8

0.2

d

1

0.8

B

0.4

b

0.2

at 464 from up to c,d,e,

0.6

a

0.4

1

0.4 0.2 0 264

464 664 wavelength (nm)

Fig.8

22

A

Vol. Abs. (cm3/g)

6

0.0025

B

0.002

dV/dD (cm3/g/nm)

8

0.0015

4

0.001

2

0.0005

0

0

0

0.2 0.4 0.6 0.8 1 Relative pressure (P/Po)

0

20 40 60 80 Pore size (nm)

Fig.9

23

A

Vol. Abs. (cm3/g)

9 6 3 0 0

0.2 0.4 0.6 0.8 1 Relative pressure (P/Po)

0.003 0.0025 0.002 0.0015 0.001 0.0005 0 dV/dD (cm3/g/nm)

12

B

0

20

40 60 80 Pore size (nm)

Fig.10

24

25

45 Pb

O

15

Mo

10

5

O

36

C

t=

Intensity (X10^4)

Intensity (X10^4)

20

Pb

27

Mo

18

C

9

A

0

0 0 200 400 600 80010001200 Binding energy (eV)

t= B

0 200 400 600 80010001200 Binding energy (eV)

Fig.11

25

Pb

30

O

Pb

C

20

W

10

0

O C

20

Intensity (X10^4)

Intensity (X10^4)

30

W

10

t=0 A

0

0 200 400 600 80010001200 Binding energy (eV)

t= B 0 200 400 600 80010001200 Binding energy (eV)

Fig.12

26

1.2

Abs.

0.9 0.6 0.3

a b

0 200 300 400 500 600 700 800 wavelength (nm) Fig.13 Diffuse reflectance spectra of (a) PbMoO4 and (b) PbWO4.

27

A

B

Fig.14

28

A

B

Fig.15

29

Intensity

PbMo

PbW

10 20 30 40 50 60 70 80 2θ (º) Fig.16

Table 1 XPS peak position of PbXO4 at different systems. ---------------------------------------------------------------------------------------Peak

Binding energy (eV) FWHM (eV) Surface composition(at.%)

---------------------------------------------------------------------------------------PbMoO4(before use) C 1s

285.13

2.94

50.47

O 1s

530.88

1.61

34.41

Pb 4f

138.91

1.49

8.12

Mo 3d

232.6

1.27

6.99

PbMoO4 (100 mg PbMoO4 +100 mL 0.1 mM MO, illumination t = 20h) C 1s

284.82

1.81

28.96

O 1s

530.51

1.51

50.83

Pb 4f

138.45

1.42

8.12

Mo 3d

232.22

1.22

12.1

284.91

2.39

52.03

O 1s

530.52

1.67

33.77

Pb 4f

138.44

1.44

8.01

W 4f

35.01

1.33

6.19

PbWO4 (before use) C 1s

PbWO4 (100 mg PbWO4 +100 mL 0.1 mM MO, illumination t = 20h) C 1s

284.72

2.09

53.56

30

O 1s

530.48

1.57

32.62

Pb 4f W 4f

138.31

1.46

7.74

34.81

1.32

6.08

--------------------------------------------------------------------------

Graphical abstract

6

6

A

B

C (mg/L)

a

4

C (mg/L)

a

4

b

2

2

d c

0 0

5

10 15 time (h)

d b

0 20

0

5

10 15 time (h)

20

Evolution of inorganic ions with illumination time in the different systems. (A): (100 mg PbMoO4 + 100 mL 0.10 mM MO), (B): (100 mg PbWO4 + 100 mL 0.10 mM MO). (a) SO42-, (b) NO3-, (c) NO2-, (d) NH4+.

Highlights ► Photocatalysts PbMoO4 and PbWO4 are prepared by the precipitation method. ► Inorganic ions (SO42-, NO3-, NO2-, NH4+) are measured by ion chromatograph. ► HO· can be detected by terephthalic acid indirectly which is generated in the above process. ► 1,4-benzoquinone is not a satisfactory scavenger in all cases to capture O2·-.

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Photocatalytic degradation of methyl orange by PbXO4 (X=Mo, W).

PbMoO4 and PbWO4 are prepared by the simple precipitation method in this work, they show the photocatalytic activities for the degradation of methyl o...
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