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Synthesis of novel polyoxometalate K6ZrW11O39Sn·12H2O and photocatalytic degradation aqueous azo dye solutions with solar irradiation
Accepted Manuscript Title: Synthesis of novel polyoxometalate K6 ZrW11 O39 Sn. 12H2 O and photocatalytic degradation in aqueous azo dye solutions with solar irradiationWe have edited your paper and have made a number of suggestions for improvement, which can be viewed in this TRACKED file. Look through the file, and check that none of our revisions alter the meaning or tone of what you originally wrote.–> Author: Feng Sheng Xiuhua Zhu Wei Wang Hao Bai Jiahuan Liu Pengyuan Wang Rong Zhang Liangjun Han Jun Mu PII: DOI: Reference:
S1381-1169(14)00145-9 http://dx.doi.org/doi:10.1016/j.molcata.2014.04.007 MOLCAA 9069
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date: Revised date: Accepted date:
26-12-2013 3-4-2014 6-4-2014
Please cite this article as: F. Sheng, X. Zhu, W. Wang, H. Bai, J. Liu, P. Wang, R. Zhang, L. Han, J. Mu, Synthesis of novel polyoxometalate K6 ZrW11 O39 Sn. 12H2 O and photocatalytic degradation in aqueous azo dye solutions with solar irradiation, Journal of Molecular Catalysis A: Chemical (2014), http://dx.doi.org/10.1016/j.molcata.2014.04.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis of novel polyoxometalate K6ZrW11O39Sn.12H2O and
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photocatalytic degradation of azo dye aqueous solution with solar
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irradiation
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4 Feng Shenga, Xiuhua Zhua,*, Wei Wanga, Hao Baia, Jiahuan Liua, Pengyuan Wangb, Rong
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Zhang c, Liangjun Hana, Jun Mua
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a
School of Environmental and Chemical Engineering, Dalian Jiaotong University, Dalian 116028,
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China b
College of Chemistry, Jilin University, Changchun, 130012, China
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c
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, China
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*Corresponding author: Xiuhua Zhu
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School of Environmental and Chemical Engineering, Dalian Jiaotong University, Dalian 116028,
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China
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E-mail: [email protected]
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Tel.: +86-411-84109335
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Fax: +86-411-84109335
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Synthesis of novel polyoxometalate K6ZrW11O39Sn.12H2O and
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photocatalytic degradation in aqueous azo dye solutions with solar
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irradiation[Ed 13621]
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ip t
28 Abstract
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A new environment-friendly material, K6ZrW11O39Sn·12H2O (ZrW11Sn), was synthesized by
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hydrothermal coprecipitation and characterized. The photocatalytic activities of ZrW11Sn were
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evaluated by its photocatalytic degradation of Acid Brilliant Scarlet 3R (ABS3R), Reactive Red 24
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(RR24), and Reactive Black 5 (RB5) with natural sunlight in homogeneous aqueous solutions.
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Results indicated that ZrW11Sn effectively and photocatalytically decolorized ABS3R, RR24, and
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RB5. The photocatalytic degradation of ABS3R was influenced by catalytic dosage, photolysis time,
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initial pH, and concentration. ZrW11Sn-mediated photocatalytic degradation of ABS3R was a
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pseudo first-order reaction and modeled by Langmuir–Hinshelwood-type kinetics. The effects of
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HO· (Br-) and photogenerated hole scavengers (I-) on the degradation rate of ABS3R were
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evaluated. Kinetic probes of the degradation mechanism indicated that hydroxyl radicals and
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photogenerated holes were the main oxidants in the reaction and that the oxidation of holes was
42
predominant. ABS3R solution (20 mL, 6 mg/L; initial pH, 6) with ZrW11Sn (1 g/L) was irradiated
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for 4 h under sunlight, the decoloration rate of which was over 69%. The azo structure of ABS3R
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molecules was destroyed, and NH4+, NO3-, and SO42- were detected in the irradiated solution.
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Keywords: polyoxometalate, homogeneous photocatalysis, dye, natural sunlight, kinetics
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47 48
1. Introduction
49 Polyoxometalate (POM) has gained considerable attention because of its intrinsic thermal redox
51
and photoredox activities [1]. This ion has been studied in stereoselective catalysis [2,3],
52
photocatalysis [4,5], and medicine [6]. Keggin, Dawson, Anderson, Waugh, and Silverton POM are
53
the five main types. Keggin-type POM is widely used as photocatalysts with ultraviolet (UV)
54
irradiation [1,7]. POM is a wide-bandgap material so only UV light can lead to electron transitions
55
from HOMO to LUMO [8]. UV light restricts the practical application of POM as a photocatalyst
56
because the solar energy that can impinge on the surface of the Earth is below 5%. Therefore,
57
researchers have focused on the development of photocatalysts that efficiently use sunlight [9–12].
58
This study aims to synthesize a new POM, K6ZrW11O39Sn·12H2O (ZrW11Sn), which can be
59
activated by sunlight. The annual production of dyes is estimated to be over 10,000 tons worldwide,
60
more than 4% of which is unavailable and directly discharged to the environment [13]. Some dyes
61
and their degradation products are potential carcinogens. Bodies of water can be seriously affected
62
if these effluents are improperly treated. Conventional biological and physical treatment methods
63
(i.e., adsorption, ultrafiltration, and coagulation) fail to efficiently and thoroughly handle dye
64
effluents. The traditional methods mainly transfer the contaminants from wastewater to solid wastes.
65
Advanced oxidation processes (AOPs) are attractive alternatives to nondestructive physical
66
wastewater treatment processes because of their ability to mineralize organic water contaminants
67
[14]. As one of the AOPs, photocatalytic oxidation has been studied for its effective degradation of
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azo dyes [15–17]. However, most of the photocatalysts used in these studies can only be activated
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with UV light. In consideration of this obstacle, this study aims to explore the possibility of using
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ZrW11Sn as a photocatalyst to decompose Acid Brilliant Scarlet 3R (ABS3R), Reactive Red 24
71
(RR24), and Reactive Black 5 (RB5) with natural sunlight irradiation. Azo dyes are of high concern
72
because of their degradation products (e.g., aromatic amines) that are highly carcinogenic [18].
73
Chemical structures of the dyes are shown in Scheme 1.
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Scheme 1. Chemical structure of (a) ABS3R, (b) RR24, and (c) RB5.
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78 2. Experimental procedure
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2.1. Chemicals and instruments
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ABS3R was donated by the State Key Laboratory of Fine Chemicals, Dalian University of
83
Technology (Dalian, China). RR24 and RB5 were provided by Zhejiang Longsheng Dyestuff
84
Chemical Co., Ltd. (China). The dyes were used directly without further purification.
85
Sodium tungstate, zirconium oxychloride, stannous sulfate, potassium chloride, glacial acetic
86
acid, sodium acetate, potassium bromide, potassium iodide, absolute ethanol, isopropanol, sodium
87
hydroxide, and perchloric acid were all of analytical grade and used without further purification.
88
Deionized water was used in the experiments and fully aerated by oxygen.
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The synthesized catalyst was characterized by UV–visible (UV–vis) spectrophotometer (WFZ
90
UV-2102PCS, Unico Instrument Co., Ltd., Shanghai, China), Fourier transform infrared (FTIR) 5
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spectrophotometer (IRPRESTIGE-21, Shimadzu, Japan) with KBr pellet method, full spectrum of
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inductively coupled plasma spectrometer (ICP; Optima 2000DV, Perkin-Elmer, USA), integrated
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thermal analyzer (STA449F3, Germany), and scanning electron microscope (SEM; JMS-6360LV,
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JEOL, Japan).
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95 2.2. Preparation of ZrW11Sn photocatalyst
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An aqueous solution of zirconium oxychloride (10 mL, 0.50 mol/L) was added to the aqueous
98
solution of sodium tungstate (100 mL, 0.55 mol/L). The mixed solution was uniformly stirred. Its
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pH was adjusted to 4.5–5.5 with glacial acetic acid, and the mixture was refluxed for 15–30 min at
100
70 °C. Stannous sulfate (10 mL, 0.50 mol/L, pH 5–5.5 adjusted with sodium acetate solution) was
101
then added to the previous solution under nitrogen atmosphere, and the temperature was increased
102
to 95 °C. Subsequently, aqueous KCl (20 mL, 3.35 mol/L) was added to the prepared solution and
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refluxed for 1.5 h. The solution was cooled to room temperature following the reaction, and the
104
insoluble solid was removed by filtration. Anhydrous ethanol was added to the filtered liquor with
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slow stirring, and ZrW11Sn was produced as a pale yellow solid. The solid was then filtrated by a
106
vacuum pump and recrystallized with deionized water. The product was vacuum dried in an oven
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for 4 h to remove the adsorbed water at 105 °C. ZrW11Sn was characterized by FTIR, UV-Vis, ICP,
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and SEM. Thermal stability and phase transition of the product were studied by differential
109
scanning calorimetry (DSC) and thermogravimetric analysis (TG; section 3.1).
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110 111
2.3. Photolysis experiments
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UV–Vis spectra and maximum absorption wavelengths in the visible regions of the aqueous
113
solutions of ABS3R, RB5, and RR24 were measured by a UV–Vis spectrophotometer (Fig. 1). The 6
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maximum absorption wavelengths in the visible regions of the aqueous solutions of ABS3R, RR24,
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and RB5 were 510, 590, and 538 nm, respectively. Specific concentrations and amounts of aqueous
116
dye solution and ZrW11Sn were respectively placed into ordinary glass beakers. The beakers were
117
then placed on the rooftop of the No. 1 Building at Dalian Jiaotong University (E 121°54.399´,
118
N 38°34.286´; elevation, 32 m) under sunlight between 10 a.m. and 3 p.m. The sample was
119
withdrawn at specific time intervals and analyzed. Changes in the absorbance of the dye solution
120
were measured by the spectrophotometer at their maximum absorption wavelengths in the visible
121
region. Decoloration rate (DC) was calculated as A0 − A ×100% , A0
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DC =
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(1)
where A0 and A are the absorbances of the initial dye solution and the dye solution after irradiated
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for a certain time, t (h), respectively.
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A = ε ×l ×C ,
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The Lambert–Beer Law is given as
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(2)
where A is the absorbance of the chemical solution, ε (L/mol·cm) is the molar extinction coefficient
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of the chemical, l (cm) is the optical length of the colorimetric utensil, and C (mol/L) is the
129
concentration of the chemical solution.
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The Lambert–Beer Law states that the concentration of the dye solution is proportional to its
131
absorbance. The relationship between C of ABS3R (in mg/L; range, 2 mg/L–25 mg/L) with A is
132
given as C = 44.86 A – 0.4249
133 134 135
R2 =0.9996
(3)
C of ABS3R at different reaction times was calculated according to Eq. (3) during kinetics analysis.
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137 138
Fig. 1. UV–Vis spectra of the aqueous solutions of (a) ABS3R, (b) RB5, and (c) RR24
139 The ions in the photocatalytically degraded solution of ABS3R were analyzed. SO42- was
141
detected by BaCl2 solution. NO3-–N was detected by phenol disulfonic acid spectrophotometry.
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Ammonia nitrogen (NH3 –N and NH4+ –N) was detected by Nessler's reagent spectrophotometry.
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3. Results and discussion
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145 3.1. Characterization of ZrW11Sn
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Fig. 2 shows the FTIR and UV–Vis spectra of ZrW11Sn. Four characteristic vibration peaks can
148
be seen (Fig. 2(a)): νas(Zr–Oa; 516 cm-1), νas(W–Oc–W; 741 cm-1), νas(W–Ob–W; 818 cm-1), and
149
νas(W–Od; 953 cm-1). The peaks indicate that ZrW11Sn exhibits framework vibration of Keggin-type
150
POM [19]. Fig. 2(b) shows a characteristic peak in the UV–Vis spectrum of ZrW11Sn at 260 nm,
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and this peak is attributed to the Keggin unit O→W transitions at W–O–W bond [1,19]. The peak
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coincides with the characteristic absorption band of the other 1:11 Keggin-type POM (Kn[MZrW11
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O39 (H2O)]·χH2O (M=Ni2+, Mn2+, Cu2+, Zn2+, Fe3+, Co3+, and Cr3+)) [20]. The results indicated that
154
the synthesized ZrW11Sn was a Keggin-type POM. The absorption spectrum of ZrW11Sn also
155
shifted to the visible region relative to the polyoxometalate with no Sn (II) substitution (Fig. 2(b)).
156
This finding indicates the increase in the absorbance of ZrW11Sn in visible light. ZrW11Sn thus
157
yielded a higher photoactivity than ZrW11 with solar irradiation, as proved by contrast experiments.
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158 159
Fig. 2. (a) FTIR and (b) UV–Vis spectra of ZrW11Sn
160 161
Fig. 3 illustrates the TG–DSC curves of ZrW11Sn. The TG curve revealed the rapid weight loss of 8
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ZrW11Sn from 30 °C to 130 °C. No weight loss was observed after 300 °C. The total mass loss was
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6.6%, which corresponded to the mass of 12 crystalline waters. DSC curve showed two exothermic
164
peaks at 380 °C and 413 °C; these peaks represent the co-melting temperatures of the
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decomposition products. An endothermic peak was found in the DSC curve at 110 °C; the rate of
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loss of the crystalline water was the highest at this temperature. Other endothermic peaks were at
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614 °C and 642 °C in the DSC curve, which represents the phase transformation. The compound
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transformed from an amorphous structure in the heating process into the crystal structures of oxides
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during phase transformation. This result implies that the compound was thoroughly decomposed.
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TG–DSC curves showed that ZrW11Sn was thermally stable when the temperature was below 350
171
°C.
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Fig. 3. TG–DSC curves of ZrW11Sn
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Results from ICP elemental analysis revealed that the synthesized compound contained
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K (8.33%), Zr (3.06%), W (70.71%), and Sn (3.89%). The elemental ratios in the synthesized
177
compound were calculated. The ratio of K:Zr:W:Sn was about 6:1:11:1.
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The results indicated that the synthesized compound exhibited a Keggin structure. The molecular
179
formula of the compound is K6ZrW11O39Sn·12H2O. Fig. 4 shows the SEM images of ZrW11Sn,
180
which is not a crystal so its XRD was not provided.
181 182 183
Fig. 4. SEM images of ZrW11Sn
184 185
3.2. Contrast experiments
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To exclude the direct photolysis of dye solutions and the reaction of the dyes with the catalyst,
187
contrast experiments were carried out under two conditions: one with ZrW11Sn (20 mg) without 9
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illumination (kept in the dark for 4 h) and the other under sunlight for 4 h without the catalyst. The
189
results indicated that the ABS3R solution (10 mg/L, 20 mL) with ZrW11Sn (20 mg) failed to
190
degrade in the absence of illumination at a particular time interval, and the decoloration rate of
191
ABS3R during direct photolysis was less than 5%. In succeeding analyses, all the photocatalytic
192
experiments on aqueous ABS3R solutions were simultaneously conducted with the direct photolysis
193
experiments. The final values were recorded by subtracting the results from the direct photolysis of
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ABS3R with that of the photocatalytic reaction. Changes in the aqueous solutions of RB5 and RR24
195
were not observed in the contrast experiments.
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Aqueous solutions of ABS3R, RR24, and RB5 (6 mg/L, 20 mL; initial pH, 6) each contained
197
20 mg ZrW11Sn and were simultaneously illuminated under sunlight (average intensity, 85.4 kLux).
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Fig. 5 shows that ZrW11Sn is effective in the photocatalytic decoloration of all the azo dyes. The
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decolorizing effect follows the order ABS3R>RB5>RR24. In the following experiment, ABS3R
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was used as a model dye.
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Fig. 5. Photocatalytic decoloration of different types of dyes
3.3. Effect of irradiation time
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To study the effect of irradiation time, ABS3R solution (20 mL, 4 mg/L; initial pH, 6) containing
207
20 mg ZrW11Sn was irradiated under sunlight (average intensity, 107 kLux). The decoloration rates
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of ABS3R solution increased with the irradiation time (Fig. 6), but the extent of the increase was
209
reduced after 4 h illumination. Competition for degradation could occur between ABS3R and the
210
intermediate products because more intermediates were produced with increased reaction time. This
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result is reminiscent of the TiO2-assisted photocatalytic degradation of azo dyes [21]. The 10
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decoloration rate of ABS3R solution increased by only 8.6% from 4 h to 5 h irradiation. For work
213
efficiency, 4 h illumination time was chosen in this study.
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Fig. 7. UV–Vis absorption spectra of the ABS3R solution at different photolysis times
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Fig. 6. Effect of the photocatalysis time on ABS3R degradation
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Irradiation time (h)
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Fig. 7 shows the change in the UV–Vis absorption spectrum of the ABS3R solution with
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photolysis time. The absorption peak of ABS3R at the maximum absorption wavelength (510 nm)
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in the visible region markedly decreased with the irradiation time. The decrease implied that the azo
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structure (–N=N–) of ABS3R (Scheme 1) was destroyed and that ABS3R was degraded. However,
224
the absorption peaks in the UV (190 nm –300 nm) that represents the Keggin- structure of ZrW11Sn
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remained unchanged during irradiation. This finding indicates that ZrW11Sn was stable during the
226
photocatalytic reaction as a catalyst. The UV–Vis absorption spectra of ZrW11Sn solutions (1 g/L)
227
were identical before and after irradiation, proving the stability of ZrW11Sn. Differences in the UV
228
absorbance spectra of ZrW11Sn–ABS3R solutions failed to reveal the appearance of any new
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absorption bands relative to ABS3R or ZrW11Sn solutions alone. This result suggests that a possible
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ZrW11Sn–ABS3R complex could not represent a significant chromophore under our experimental
231
conditions. The observable change in the concentration of ABS3R is solely attributed to the known
232
photocatalytic degradation of ZrW11Sn. Furthermore, white precipitates were found in the ABS3R solution irradiated for 5 h upon the
234
addition of BaCl2 solution. This result indicates the production of SO42- ions during the reaction.
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Neither phenol disulfonic acid solution nor Nessler's reagent was added to the reaction. The color of
236
the solution all changed to yellow, suggesting the production of NO3—N, NH3–N, and NH4+–N in
237
the solution. SO42-, NH4+, and NO3- were undetected in the unirradiated ABS3R solution. The
238
molecular structure of ABS3R (Scheme 1) revealed that SO42- came from the sulfonic acid groups
239
of ABS3R; the N element in the solution was produced by the –N=N– group of ABS3R. The
240
molecular structure of ABS3R was destroyed in the photocatalytic reaction, the azo bond was
241
broken, and NH4+ was generated and further oxidized to NO3-.
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3.4. Effect of the initial concentration of ABS3R solution
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The effect of the initial concentration of ABS3R solution on the decoloration rate is shown in
245
Fig. 8. The initial pH of all the ABS3R solutions (20 mL) was 6, and all the photocatalyst doses in
246
the ABS3R solutions were 20 mg. The irradiation time was 4 h, and the average sunlight intensity
247
was 103 kLux.
248 249
Fig. 8. Effects of the initial concentration of ABS3R solution on the decoloration rate
250 251
The results show that the decoloration efficiency initially increased with the ABS3R
252
concentration at the initial stage, reached its maximum at 6 mg/L, and decreased thereafter. The 12
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253
behavior of the discoloration efficiency is attributed to the photocatalytic reaction rate r (mg/L min)
254
that can be described by the kinetic Langmuir–Hinshelwood (L–H) equation [22]. When the initial
255
concentration of the solution is low, KC << 1. The photocatalytic reaction rate is given as r = kKC =K/C,
(4)
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where C (mg/L) is the concentration of the solution, k (mg/L min) is the Langmuir rate constant, K
258
(L/mg) is the Langmuir adsorption constant, and K' (min-1) is the reaction rate constant. Eq. 4
259
indicates that the reaction rate is positively correlated with the solution concentration; therefore, the
260
decoloration rate of the dye solution increased with the initial concentration. However, the chroma
261
of the dye solution also increased with the initial concentration, the opacity of which was increased
262
and the sunlight penetration decreased [13,15]. Thus, the photocatalytic reaction could not occur in
263
the center of the reactor. As the catalyst amounts were all the same in different initial concentrations
264
of ABS3R solutions, the generated photoactivity species were also fixed, the irradiation time was
265
kept constant, and the decoloration rate of the dye solution failed to increase with the initial
266
concentration.
268 269 270
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The maximum decoloration rate was obtained because the initial concentration of the ABS3R solution was 6 mg/L, which was chosen in this study.
3.5. Effect of the photocatalyst dosage
271
The amount of catalyst in the solution mainly influenced the photocatalytic reaction. Fig. 9
272
illustrates the effect of the concentration of ZrW11Sn in ABS3R solution on the decoloration rate.
273
ABS3R solutions (20 mL, 6 mg/L; initial pH, 6) with different amounts of ZrW11Sn were irradiated
274
under sunlight (average intensity, 103 kLux) for 4 h. Fig. 9 reveals that the decoloration rate of
275
ABS3R increased with the increase in catalyst concentration from 0.5 g/L to 1.5 g/L. The rate 13
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remained unchanged with the increase in catalyst concentration from 1.5 g/L to 1.75 g/L. The extent
277
of increase gradually reduced beyond 1.0 g/L. This result is attributed to the high photocatalyst
278
production that generates effective photoactivity species (·OH and h+). The decoloration rate of
279
ABS3R solution initially increased with the increase in photocatalyst concentration. However, the
280
excessive photocatalyst generated high amounts of ·OH radicals and h+. Portions of the radicals
281
recombined with each other by contact in the solution, and thus the increased extent of the
282
decoloration rate of ABS3R solution was low.
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The results indicate that the increase in catalyst dosage is ineffective in enhancing the
284
degradation efficiency. The optimum amount of ZrW11Sn should be added to avoid catalyst wastage
285
and ensure total photon absorption for efficient photomineralization dyes. The decoloration rate of
286
ABS3R increased by only 3% when the ZrW11Sn concentration increased from 1.0 g/L to 1.5 g/L.
287
In consideration of the economic factors, the optimum ZrW11Sn concentration of 1.0 g/L was
288
chosen in the experiments.
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Decoloration rate %
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65
60
55
50
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0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
C (g/L)
289 290
Fig. 9. Effect of catalyst concentration on the decoloration rate of ABS3R
291 292 293
3.6. Effect of the initial pH of ABS3R solution Experiments were carried out at various initial pH of the ABS3R solution ranging from 2–9 at a 14
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constant dye concentration (20 mL, 6 mg/L) and catalyst loading (1.0 g/L) to study the effect of the
295
initial pH of the ABS3R solution on the decolorization efficiency. Aqueous ABS3R solutions were
296
illuminated under sunlight (average intensity, 93 kLux) for 4 h. The stability of ABS3R at different
297
pH was studied prior to the tests. The UV–Vis spectra of the aqueous ABS3R solutions were not
298
affected by the changes in initial pH. This result implies that the molecular structure of ABS3R was
299
not destroyed with the change in initial pH. Fig. 10 suggests that the decoloration rate of ABS3R
300
was good (69.2%–69.8%) when the initial pH of the aqueous ABS3R solutions ranged from 4–6.
301
This result is attributed to the existence of stable 1:11 Keggin-type POM for pH ranging from
302
4.5–6.0 [19,23]. The photocatalyst was destroyed in other pH ranges; thus, the catalytic capacity
303
decreased.
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The pH value of the original aqueous ABS3R solution was about 6, which was chosen as the initial pH of the ABS3R solution in the experiments.
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309 310
Fig. 10. Effect of the initial pH of the ABS3R solution on the decoloration rate
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3.7. Kinetics of photocatalytic degradation
311
The reaction kinetics follows the L–H model for heterogeneous reaction systems, such as the TiO2
312
photocatalysis system [21,24]. Mylonas and Papaconstantinou [25] and Ozer and Ferry [26] reported
313
that the kinetics follows the L–H model for homogeneous POM reaction systems given as
314
r=−
dCt kKCt = , dt 1 + KCt
(5)
315
where r (mg/L·min) is the photocatalytic reaction rate, k (mg/L min) is the rate constant, K (L/mg)
316
is the association constant, and Ct (mg/L) is the concentration of ABS3R at reaction time t. 15
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317 318
319
When the initial concentration of ABS3R is small (KC0 << 1), the equation can be simplified to a first-order rate equation given as
⎛C ⎞ ln⎜⎜ 0 ⎟⎟ = kKt = K / t , ⎝ Ct ⎠
(6)
where C0 (mg/L) and Ct (mg/L) are the concentrations of ABS3R solution at reaction times 0 and t
321
(min), respectively. K/ (min-1) is the observed first-order rate constant.
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To study the kinetics of the photocatalytic degradation of aqueous ABS3R solutions with
323
ZrW11Sn, experiments were carried out with initial ABS3R concentrations ranging from 4 mg/L–10
324
mg/L at a constant initial pH of 6 and catalyst loading of 1 g/L. ln(C0/Ct) was plotted against the
325
irradiation time. The photocatalytic degradation was well represented by a pseudo first-order kinetic
326
model. Table 1 lists the first-order reaction kinetic equations and the correlation coefficients (R2).
M
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327
Table 1 Parameters and first-order reaction kinetic equations for the photocatalytic degradation
d
328
te
329
Developing a model for the dependence of the photocatalytic degradation rate on the
331
experimental parameters for the photolysis time is difficult because of the complex mechanism of
332
the reaction. Therefore, modeling the kinetics of photocatalysis is often restricted to the analysis of
333
the initial rate (r0) of the reaction. The model can be obtained from the initial slope of the curves
334
and the initial concentration of the reaction solutions (Eq. 4, r0=K/C0). The values of r0 are listed in
335
Table 1, in which
336 337 338
Ac ce p
330
r0 =
kKC0 . 1 + KC0
(7)
Eq. 7 can be further expressed in a linear form as 1 1 1 = + . r0 kKC0 k
(8) 16
Page 15 of 31
339 340
Fig. 11. Relationship between 1/r0 and 1/C0
341
343
The reciprocal of the initial rate 1/r0 was plotted with the reciprocal of the initial concentration
ip t
342
1/C0 (Fig. 11) to obtain the kinetics parameters k=0.2084 mg/L min and K=0.0395 L/mg.
3.8. Kinetic probes of the mechanisms of ZrW11Sn-mediated photocatalytic oxidation of ABS3R
us
345
cr
344
The mechanisms by which POM photocatalytically oxidize organic compounds in aqueous
347
solutions are critical to the potential utilities [26]. Kinetic probes were studied to determine the
348
possible role of hydroxyl radicals (·OH) and photogenerated holes (h+) in the ZrW11Sn-mediated
349
photocatalytic oxidation of aqueous ABS3R solution.
M
an
346
The functions of ·OH and h+ in the aqueous oxidation of ABS3R were tested by assessing the
351
effects on K/ from the presence of radical (KBr) [27] and hole scavengers (KI) [28], respectively.
352
Unlike in the absence of active scavenger species in the ABS3R solutions, the photocatalytic
353
decoloration rate of ABS3R decreased when KBr and KI were added to the reaction solution. This
354
result implies that the ·OH radicals and holes were the oxidants present during the
355
ZrW11Sn-mediated photocatalytic degradation of ABS3R.
te
Ac ce p
356
d
350
357
Fig. 12. ln(C0/Ct) of ABS3R solution against irradiation time for different concentrations of (a) KBr
358
and (b) KI
359 360 361
Fig. 13. Effects of KBr and KI on the observed rate constant K/
362 363
Fig. 12 shows that the kinetics of the photocatalytic degradation of ABS3R still followed the 17
Page 16 of 31
pseudo-first-order model when different concentrations of KBr and KI were added to the solution.
365
Although all the reaction rates decreased with the increasing concentrations of KBr and KI, the
366
effect using the same concentration of KBr and KI was different (Fig. 13). KI exhibited greater
367
effects on the photocatalytic decoloration rate of ABS3R than KBr (Fig. 13). The result indicates
368
that the action of the photogenerated holes relative to ·OH radicals was highly pronounced in the
369
reaction. In electron transfer, holes rather than ·OH radicals for various 1:12 series POM (Keggin
370
structure) [26] are used. However, Kim et al. [29] suggested that ·OH radicals were the sole
371
dominant photooxidant in PW12O403--mediated degradation irrespective of the kind of substrate; the
372
radicals were exclusively operated through holes for W10O324- (isopolyacid) [30].
373
is a 1:11 series POM (Keggin structure), and its photocatalytic mechanism is different from that of
374
1:12 series POM and isopolyacid. The mechanism of the primary photooxidation of POM, which is
375
operated through ·OH radicals and/or photogenerated holes, depends on the photocatalyst, the
376
substrate, and the mode of analysis.
[Ed 13622]ZrW11Sn
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d
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364
Irradiation of ZrW11Sn resulted in the formation of an O→W charge–transfer excited state at the
378
W–O–W bridge bond and produced considerable photogenerated holes and electrons. The primary
379
mechanism of the ZrW11Sn-mediated photocatalytic degradation of ABS3R was photooxidative
380
decomposition. Similar to that of other POMs [31], the photoreductive decomposition of ABS3R by
381
ZrW11Sn cannot be entirely ruled out. To deduce the exact photocatalysis mechanism, the photolysis
382
products require further analysis. The possible reaction mechanisms have been previously
383
summarized [26,27,29–32].
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384 385
4. Conclusions
386 18
Page 17 of 31
A novel polyoxometalate, ZrW11Sn, was synthesized and characterized. Irradiation of ZrW11Sn
388
under sunlight effectively photocatalytically degraded azo dyes, ABS3R, RR24, and RB5. The
389
following conclusions were drawn from the results:
390
1. The optimum initial pH of the ABS3R solution ranged from 4–6 during ZrW11Sn-mediated
391
ip t
387
photocatalytic degradation.
2. The photocatalytic decoloration rate of the ABS3R solution increased with the initial
393
concentration. However, the rate did not increase throughout the process. The optimum
394
concentration was 6 mg/L upon loading with 1.0 g/L ZrW11Sn.
us
cr
392
3. The decoloration rate of ABS3R increased with the catalyst dosage up to the optimum loading.
396
Further increasing the catalyst dose yielded no effects. The optimum concentration of ZrW11Sn
397
was 1.0 g/L based on economic factors.
M
an
395
4. The photocatalytic decoloration of ABS3R was a pseudo-first-order reaction based on
399
Langmuir–Hinshelwood kinetic model. Mathematical inferences revealed that the rate and the
400
association constants during the ZrW11Sn-mediated photocatalytic degradation of ABS3R were
401
0.2084 mg/L min and 0.0395 L/mg, respectively.
403
te
Ac ce p
402
d
398
5. Photogenerated holes and ·OH radicals during the photocatalytic degradation of ABS3R were effective active species. The holes were highly pronounced in the reaction.
19
Page 18 of 31
References
[1] D.F. Li, Y.H. Guo, C.W. Hu, L. Mao, E.B. Wang, Appl. Catal. A-Gen. 235 (2002)
ip t
11-20.
[2] D.L. Long, R. Tsunashima, L. Cronin, Angew. Chem. Int. Edit. 49 (2010) 1736-1758.
cr
[3] A. Dolbecq, E. Dumas, C. R. Mayer, P. Mialane, Chem. Rev. 110 (2010) 6009-6048.
us
[4] T.H. Li, S.Y. Gao, F. Li, R. Cao, J. Colloid. Interf. Sci. 338 (2009) 500-505. [5] C.C. Chen, Q. Wang, P.X. Lei, W.H. Ma, L.C. Zhao, Environ. Sci. Technol. 40
an
(2006) 3965-3970.
Pharmacother. 60 (2006) 349-352.
M
[6] H. Yanagie, A. Ogata, S. Mitsui, T. Hisa, T. Yamase, M. Eriguchi, Biomed.
te
1686-1694.
d
[7] Y. Yang, Y.H. Guo, C.W. Hu, C.J. Jiang, E.B. Wang, J. Mater. Chem. 13 (2003)
[8] S. Kim, J. Yeo, W. Choi, Appl. Catal. B-Environ. 84 (2008) 148-155.
Ac ce p
[9] M.D. Hernández-Alonso, F. Fresno, S. Suarez, Sci. Total. Environ. 2 (2009) 12311257.
[10] C.C. Chen, W.H. Ma and J.C. Zhao, Chem. Soc. Rev. 39 (2010) 4206–4219. [11] V.B.R. Boppana, R.F. Lobo, J. Catal. 281 (2011) 156-168. [12] S. Swetha, G. Balakrishna, Chinese. J. Catal. 32 (2011) 789-794. [13 ]E. Forgaces, T. Cserhati, G. Oros, Environ. Int. 30 (2004) 953-971. [14] V.K. Gupta, R. Jain, S. Agarwal, A. Nayak, M. Shrivastava, J. Colloid. Interf. Sci. 366 (2012 ) 135-140.
Page 19 of 31
[15] V.K. Gupta, R. Jain, A. Mittal, A.T. Saleh, A. Nayak, S. Agarwal, S. Sikarwar, Mater. Sci. Eng. C 32 (2012) 12-17. [16] A.O. Ibhadon, G.M. Greenway, Y. Yue, Catal. Commun. 9 (2008) 153-157.
ip t
[17] R. Jain, M. Mathur, S. Sikarwar, A. Mittal, J. Environ. Manage. 85 (2007) 956-964. [18] M. Neamtu, I. Siminiceanu, A. Yediler, A. Kettrup, Dyes. Pigments. 53 (2002) 93-
cr
99.
Industrial Press, Beijing, 1998. (In Chinese)
us
[19] E.B. Wang, C.W. Hu, L. Xu, Introduction of Polyoxometalates chemistry, Chemical
an
[20] W. Wang, X.H. Zhu, J.F. Liu. Chem. J. Chinese. U. 13 (1992) 735-736. [21] I.K. Konstantinou, T.A. Albanis, Appl. Catal. B-Environ. 49 (2004) 1-14.
M
[22] N.S. Deng, F.Wu, Environmental Photochemistry, Chemical Industrial Press,
d
Beijing, 2003. (In Chinese)
te
[23] W. Wang. The study of the syntheses and properties of heteropoly-tungstotianates; The Properties of lanthanide decatungstates. Dissertation for Master Degree.
Ac ce p
Changchun, Northeast Normal Universtity, 1988, pp. 34-37. (In Chinese)
[24] W. Baran, E. Adamek, A. Makowski, Chem. Eng. J. 145 (2008) 242-248. [25] A. Mylonas, E. Papaconstantinou, J. Photoch. Photobio. A. 94 (1996) 77-82. [26] R.R. Ozer, J.L. Ferry, J. Phys. Chem. B. 104 (2000) 9444-9448. [27] P. Kormali, A. Troupis, T. Triantis, A. Hiskia, E. Papaconstantinou, Catal. Today. 124 (2007) 149-155. [28] X.V. Doorslaer, P.M. Heynderickx, K. Demeestere, K. Debevere, H.V. Langenhove, J. Dewulf, Appl. Catal. B-Environ.111-112 (2012) 150-156. [29] S. Kim, H. Park, W. Choi, J. Phys. Chem. B. 108 (2004) 6402-6411.
Page 20 of 31
[30] I. Texier, J.A. Delarie, C. Giannotti, Phys. Chem. Chem. Phys. 2 (2000) 1205-1212. [31] A. Troupis, T.M. Triantis, E. Gkika, Appl. Catal. B-Environ. 86 (2009) 98-107.
Ac ce p
te
d
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an
us
cr
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[32] M.Q. Hu, Y.M. Xu, Chemosphere. 54 (2004) 431-434.
Page 21 of 31
0.25
(a ) ABS3R
0.15
0.10
cr
Absorbance
λmax= 510nm
ip t
0.20
0.00 200
300
400
500
600
700
800
an
Wavelength (nm)
us
0.05
0.12
(b ) RB5
0.10
M
λmax= 590nm
0.06
d
Absorbance
0.08
0.02
te
0.04
300
Ac ce p
0.00 200
400
500
600
700
800
Wavelength (nm)
0.12
(c ) RR24
Absorbance
0.10 0.08
λmax= 538nm
0.06 0.04 0.02 0.00
200
300
400
500 600 Wavelength (nm)
700
800
Fig. 1. UV-Vis spectra of the aqueous solutions of (a) ABS3R, (b) RB5, and (c) RR24.
Page 22 of 31
90 80
(a)
νas(Zr-Oa)
50
δ(H-O-H)
νas(W-Od)
30 20
νas (H-O-H)
νas(W-Ob-W)
10 0 4000
νas(W-Oc-W)
3500
3000
2500
2000
1500
M
2.5
(b)
500
ZrW11Sn ZrW11
d
2.0
te
1.5
Ac ce p
Absorbance
1000
an
Wavenumber (cm-1)
1.0
cr
40
ip t
60
us
Transmittance %
70
0.5
0.0 200
300
400
500
600
700
800
Wavelength (nm)
Fig. 2. (a) FTIR and (b) UV-Vis spectra of ZrW11Sn.
Page 23 of 31
100
4
98
642°C DSC
2
TG 94
110°C
o
413 C
o
380 C 200
300
400
500
600
700
800
an
100
us
1
92
90
ip t
614°C
Heat flow / (mV/mg)
96
cr
Mass / %
3
o
Temperature / C
Ac ce p
te
d
M
Fig. 3. TG–DSC curves of ZrW11Sn.
Fig. 4. SEM images of ZrW11Sn.
Page 24 of 31
80
ABS3R RB5 RR24
60 50
ip t
40 30 20 10 0
0
1
2
3
4
5
us
Irradiation time (h)
cr
Decoloration rate %
70
M
an
Fig. 5. Photocatalytic decoloration of different types of dyes.
80
40 30
te
50
d
60
Ac ce p
Decolorization rate %
70
20 10
1
2
3
4
5
Irradiation time (h)
Fig. 6. Effect of the photocatalysis time on ABS3R degradation.
Page 25 of 31
0.15
1.5
0.05
1.0 0.00
500
0.5
600
700
Wavelength (nm)
200
300
400
500
800
us
0.0
ip t
1h 2h 3h 4h 5h
0.10
Absorbance
Absorbance
2.0
ABS3R o h (ABS3R+ZrW11Sn)
cr
2.5
600
700
800
an
Wavelength (nm)
75
Decoloration rate %
Ac ce p
70
te
d
M
Fig. 7. UV–Vis absorption spectrra of the ABS3R solution at different photolysis times.
65
60
55
50
45
0
5
10
15
20
25
C (mg/L)
Fig. 8. Effects of the initial concentration of ABS3R solution on the decoloration rate.
Page 26 of 31
75
65
ip t
60
55
50
0.4
0.6
0.8
1.0
1.2
C (g/L)
1.4
1.6
1.8
2.0
us
45
cr
Decoloration rate %
70
an
Fig. 9. Effect of catalyst concentration on the decoloration rate of ABS3R
M
72 70
d
64 62
te
66
Ac ce p
Decoloration rate %
68
60 58 56
2
4
6
8
10
pH
Fig.10. Effect of the initial pH of the ABS3R solution on the decoloration rate.
Page 27 of 31
45 y = 121.48 x + 4.7978 2
R = 0.9903
35 30
ip t
1/r 0 ( L min/mg )
40
cr
25
15 0.05
0.10
0.15
0.20
us
20
0.25
0.35
an
1/C 0 (L /mg)
0.30
M
Fig. 11. Relationship between 1/r0 and 1/C0
d
2.0
te
1.6 1.4
Ac ce p
1.2
ln(C0/Ct)
(a) KBr
0 μmol/L 5 μmol/L 25 μmol/L 50 μmol/L 100 μmol/L
1.8
1.0 0.8 0.6 0.4 0.2 0.0
0
50
100
150
200
250
Irradiation time (min)
Page 28 of 31
1.8 (b) KI
0 μmol/L 5 μmol/L 25 μmol/L 50 μmol/L 100 μmol/L
1.6 1.4
ip t
ln(C0/Ct)
1.2 1.0 0.8
cr
0.6
0.2 0.0
0
50
100
150
200
250
an
Irradiation time (min)
us
0.4
Fig. 12. ln(C0/Ct) of ABS3R solution against irradiation time for different concentrations
d
M
of (a) KBr and (b) KI.
7
Ac ce p
6 -3 -1 / K *10 , min
KBr KI
te
8
5 4 3 2 1 0
0
20
40
60
80
100
Concentrations (μmol/L)
Fig. 13. Effects of KBr and KI on the observed rate constant K/.
Page 29 of 31
Highlights
ip t
A new photocatalyst, K6ZrW11O39Sn·12H2O was synthesized and characterized. ZrW11Sn showed photocatalytic activity to decolor dye with sunlight irradiation.
cr
Photocatalytic degradation of ABS3R was a pseudo first-order reaction.
Ac ce p
te
d
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an
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Photogenerated holes and ·OH were both the main oxidants in the reaction.
Page 30 of 31
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Graphical abstract
cr
Sunlight Irradiation
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Photocatalytic degradation + products (NH4 , NO3-, SO42-, H2O ……)
Ac
ce pt
ed
M
an
+
Page 31 of 31
S1381-1169(14)00145-9 http://dx.doi.org/doi:10.1016/j.molcata.2014.04.007 MOLCAA 9069
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date: Revised date: Accepted date:
26-12-2013 3-4-2014 6-4-2014
Please cite this article as: F. Sheng, X. Zhu, W. Wang, H. Bai, J. Liu, P. Wang, R. Zhang, L. Han, J. Mu, Synthesis of novel polyoxometalate K6 ZrW11 O39 Sn. 12H2 O and photocatalytic degradation in aqueous azo dye solutions with solar irradiation, Journal of Molecular Catalysis A: Chemical (2014), http://dx.doi.org/10.1016/j.molcata.2014.04.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis of novel polyoxometalate K6ZrW11O39Sn.12H2O and
1 2
photocatalytic degradation of azo dye aqueous solution with solar
3
irradiation
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4 Feng Shenga, Xiuhua Zhua,*, Wei Wanga, Hao Baia, Jiahuan Liua, Pengyuan Wangb, Rong
6
Zhang c, Liangjun Hana, Jun Mua
7
a
School of Environmental and Chemical Engineering, Dalian Jiaotong University, Dalian 116028,
us
8
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5
China b
College of Chemistry, Jilin University, Changchun, 130012, China
10
c
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, China
11
M
an
9
*Corresponding author: Xiuhua Zhu
13
School of Environmental and Chemical Engineering, Dalian Jiaotong University, Dalian 116028,
14
China
15
E-mail: [email protected]
16
Tel.: +86-411-84109335
17
Fax: +86-411-84109335
19
te
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d
12
20 21 22 23 1
Page 1 of 31
25
Synthesis of novel polyoxometalate K6ZrW11O39Sn.12H2O and
26
photocatalytic degradation in aqueous azo dye solutions with solar
27
irradiation[Ed 13621]
29
ip t
28 Abstract
cr
30
A new environment-friendly material, K6ZrW11O39Sn·12H2O (ZrW11Sn), was synthesized by
32
hydrothermal coprecipitation and characterized. The photocatalytic activities of ZrW11Sn were
33
evaluated by its photocatalytic degradation of Acid Brilliant Scarlet 3R (ABS3R), Reactive Red 24
34
(RR24), and Reactive Black 5 (RB5) with natural sunlight in homogeneous aqueous solutions.
35
Results indicated that ZrW11Sn effectively and photocatalytically decolorized ABS3R, RR24, and
36
RB5. The photocatalytic degradation of ABS3R was influenced by catalytic dosage, photolysis time,
37
initial pH, and concentration. ZrW11Sn-mediated photocatalytic degradation of ABS3R was a
38
pseudo first-order reaction and modeled by Langmuir–Hinshelwood-type kinetics. The effects of
39
HO· (Br-) and photogenerated hole scavengers (I-) on the degradation rate of ABS3R were
40
evaluated. Kinetic probes of the degradation mechanism indicated that hydroxyl radicals and
41
photogenerated holes were the main oxidants in the reaction and that the oxidation of holes was
42
predominant. ABS3R solution (20 mL, 6 mg/L; initial pH, 6) with ZrW11Sn (1 g/L) was irradiated
43
for 4 h under sunlight, the decoloration rate of which was over 69%. The azo structure of ABS3R
44
molecules was destroyed, and NH4+, NO3-, and SO42- were detected in the irradiated solution.
45
Keywords: polyoxometalate, homogeneous photocatalysis, dye, natural sunlight, kinetics
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46 47 3
Page 2 of 31
47 48
1. Introduction
49 Polyoxometalate (POM) has gained considerable attention because of its intrinsic thermal redox
51
and photoredox activities [1]. This ion has been studied in stereoselective catalysis [2,3],
52
photocatalysis [4,5], and medicine [6]. Keggin, Dawson, Anderson, Waugh, and Silverton POM are
53
the five main types. Keggin-type POM is widely used as photocatalysts with ultraviolet (UV)
54
irradiation [1,7]. POM is a wide-bandgap material so only UV light can lead to electron transitions
55
from HOMO to LUMO [8]. UV light restricts the practical application of POM as a photocatalyst
56
because the solar energy that can impinge on the surface of the Earth is below 5%. Therefore,
57
researchers have focused on the development of photocatalysts that efficiently use sunlight [9–12].
58
This study aims to synthesize a new POM, K6ZrW11O39Sn·12H2O (ZrW11Sn), which can be
59
activated by sunlight. The annual production of dyes is estimated to be over 10,000 tons worldwide,
60
more than 4% of which is unavailable and directly discharged to the environment [13]. Some dyes
61
and their degradation products are potential carcinogens. Bodies of water can be seriously affected
62
if these effluents are improperly treated. Conventional biological and physical treatment methods
63
(i.e., adsorption, ultrafiltration, and coagulation) fail to efficiently and thoroughly handle dye
64
effluents. The traditional methods mainly transfer the contaminants from wastewater to solid wastes.
65
Advanced oxidation processes (AOPs) are attractive alternatives to nondestructive physical
66
wastewater treatment processes because of their ability to mineralize organic water contaminants
67
[14]. As one of the AOPs, photocatalytic oxidation has been studied for its effective degradation of
68
azo dyes [15–17]. However, most of the photocatalysts used in these studies can only be activated
69
with UV light. In consideration of this obstacle, this study aims to explore the possibility of using
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50
4
Page 3 of 31
ZrW11Sn as a photocatalyst to decompose Acid Brilliant Scarlet 3R (ABS3R), Reactive Red 24
71
(RR24), and Reactive Black 5 (RB5) with natural sunlight irradiation. Azo dyes are of high concern
72
because of their degradation products (e.g., aromatic amines) that are highly carcinogenic [18].
73
Chemical structures of the dyes are shown in Scheme 1.
ip t
70
cr
(a)
us
74 75
Scheme 1. Chemical structure of (a) ABS3R, (b) RR24, and (c) RB5.
M
77
an
76
78 2. Experimental procedure
d
79
2.1. Chemicals and instruments
Ac ce p
81
te
80
82
ABS3R was donated by the State Key Laboratory of Fine Chemicals, Dalian University of
83
Technology (Dalian, China). RR24 and RB5 were provided by Zhejiang Longsheng Dyestuff
84
Chemical Co., Ltd. (China). The dyes were used directly without further purification.
85
Sodium tungstate, zirconium oxychloride, stannous sulfate, potassium chloride, glacial acetic
86
acid, sodium acetate, potassium bromide, potassium iodide, absolute ethanol, isopropanol, sodium
87
hydroxide, and perchloric acid were all of analytical grade and used without further purification.
88
Deionized water was used in the experiments and fully aerated by oxygen.
89
The synthesized catalyst was characterized by UV–visible (UV–vis) spectrophotometer (WFZ
90
UV-2102PCS, Unico Instrument Co., Ltd., Shanghai, China), Fourier transform infrared (FTIR) 5
Page 4 of 31
spectrophotometer (IRPRESTIGE-21, Shimadzu, Japan) with KBr pellet method, full spectrum of
92
inductively coupled plasma spectrometer (ICP; Optima 2000DV, Perkin-Elmer, USA), integrated
93
thermal analyzer (STA449F3, Germany), and scanning electron microscope (SEM; JMS-6360LV,
94
JEOL, Japan).
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91
95 2.2. Preparation of ZrW11Sn photocatalyst
cr
96
An aqueous solution of zirconium oxychloride (10 mL, 0.50 mol/L) was added to the aqueous
98
solution of sodium tungstate (100 mL, 0.55 mol/L). The mixed solution was uniformly stirred. Its
99
pH was adjusted to 4.5–5.5 with glacial acetic acid, and the mixture was refluxed for 15–30 min at
100
70 °C. Stannous sulfate (10 mL, 0.50 mol/L, pH 5–5.5 adjusted with sodium acetate solution) was
101
then added to the previous solution under nitrogen atmosphere, and the temperature was increased
102
to 95 °C. Subsequently, aqueous KCl (20 mL, 3.35 mol/L) was added to the prepared solution and
103
refluxed for 1.5 h. The solution was cooled to room temperature following the reaction, and the
104
insoluble solid was removed by filtration. Anhydrous ethanol was added to the filtered liquor with
105
slow stirring, and ZrW11Sn was produced as a pale yellow solid. The solid was then filtrated by a
106
vacuum pump and recrystallized with deionized water. The product was vacuum dried in an oven
107
for 4 h to remove the adsorbed water at 105 °C. ZrW11Sn was characterized by FTIR, UV-Vis, ICP,
108
and SEM. Thermal stability and phase transition of the product were studied by differential
109
scanning calorimetry (DSC) and thermogravimetric analysis (TG; section 3.1).
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110 111
2.3. Photolysis experiments
112
UV–Vis spectra and maximum absorption wavelengths in the visible regions of the aqueous
113
solutions of ABS3R, RB5, and RR24 were measured by a UV–Vis spectrophotometer (Fig. 1). The 6
Page 5 of 31
maximum absorption wavelengths in the visible regions of the aqueous solutions of ABS3R, RR24,
115
and RB5 were 510, 590, and 538 nm, respectively. Specific concentrations and amounts of aqueous
116
dye solution and ZrW11Sn were respectively placed into ordinary glass beakers. The beakers were
117
then placed on the rooftop of the No. 1 Building at Dalian Jiaotong University (E 121°54.399´,
118
N 38°34.286´; elevation, 32 m) under sunlight between 10 a.m. and 3 p.m. The sample was
119
withdrawn at specific time intervals and analyzed. Changes in the absorbance of the dye solution
120
were measured by the spectrophotometer at their maximum absorption wavelengths in the visible
121
region. Decoloration rate (DC) was calculated as A0 − A ×100% , A0
us
cr
DC =
an
122
ip t
114
(1)
where A0 and A are the absorbances of the initial dye solution and the dye solution after irradiated
124
for a certain time, t (h), respectively.
126
A = ε ×l ×C ,
d
The Lambert–Beer Law is given as
te
125
M
123
(2)
where A is the absorbance of the chemical solution, ε (L/mol·cm) is the molar extinction coefficient
128
of the chemical, l (cm) is the optical length of the colorimetric utensil, and C (mol/L) is the
129
concentration of the chemical solution.
Ac ce p
127
130
The Lambert–Beer Law states that the concentration of the dye solution is proportional to its
131
absorbance. The relationship between C of ABS3R (in mg/L; range, 2 mg/L–25 mg/L) with A is
132
given as C = 44.86 A – 0.4249
133 134 135
R2 =0.9996
(3)
C of ABS3R at different reaction times was calculated according to Eq. (3) during kinetics analysis.
136 7
Page 6 of 31
137 138
Fig. 1. UV–Vis spectra of the aqueous solutions of (a) ABS3R, (b) RB5, and (c) RR24
139 The ions in the photocatalytically degraded solution of ABS3R were analyzed. SO42- was
141
detected by BaCl2 solution. NO3-–N was detected by phenol disulfonic acid spectrophotometry.
142
Ammonia nitrogen (NH3 –N and NH4+ –N) was detected by Nessler's reagent spectrophotometry.
ip t
140
3. Results and discussion
us
144
cr
143
145 3.1. Characterization of ZrW11Sn
an
146
Fig. 2 shows the FTIR and UV–Vis spectra of ZrW11Sn. Four characteristic vibration peaks can
148
be seen (Fig. 2(a)): νas(Zr–Oa; 516 cm-1), νas(W–Oc–W; 741 cm-1), νas(W–Ob–W; 818 cm-1), and
149
νas(W–Od; 953 cm-1). The peaks indicate that ZrW11Sn exhibits framework vibration of Keggin-type
150
POM [19]. Fig. 2(b) shows a characteristic peak in the UV–Vis spectrum of ZrW11Sn at 260 nm,
151
and this peak is attributed to the Keggin unit O→W transitions at W–O–W bond [1,19]. The peak
152
coincides with the characteristic absorption band of the other 1:11 Keggin-type POM (Kn[MZrW11
153
O39 (H2O)]·χH2O (M=Ni2+, Mn2+, Cu2+, Zn2+, Fe3+, Co3+, and Cr3+)) [20]. The results indicated that
154
the synthesized ZrW11Sn was a Keggin-type POM. The absorption spectrum of ZrW11Sn also
155
shifted to the visible region relative to the polyoxometalate with no Sn (II) substitution (Fig. 2(b)).
156
This finding indicates the increase in the absorbance of ZrW11Sn in visible light. ZrW11Sn thus
157
yielded a higher photoactivity than ZrW11 with solar irradiation, as proved by contrast experiments.
Ac ce p
te
d
M
147
158 159
Fig. 2. (a) FTIR and (b) UV–Vis spectra of ZrW11Sn
160 161
Fig. 3 illustrates the TG–DSC curves of ZrW11Sn. The TG curve revealed the rapid weight loss of 8
Page 7 of 31
ZrW11Sn from 30 °C to 130 °C. No weight loss was observed after 300 °C. The total mass loss was
163
6.6%, which corresponded to the mass of 12 crystalline waters. DSC curve showed two exothermic
164
peaks at 380 °C and 413 °C; these peaks represent the co-melting temperatures of the
165
decomposition products. An endothermic peak was found in the DSC curve at 110 °C; the rate of
166
loss of the crystalline water was the highest at this temperature. Other endothermic peaks were at
167
614 °C and 642 °C in the DSC curve, which represents the phase transformation. The compound
168
transformed from an amorphous structure in the heating process into the crystal structures of oxides
169
during phase transformation. This result implies that the compound was thoroughly decomposed.
170
TG–DSC curves showed that ZrW11Sn was thermally stable when the temperature was below 350
171
°C.
an
us
cr
ip t
162
M
172 173
Fig. 3. TG–DSC curves of ZrW11Sn
d
174
Results from ICP elemental analysis revealed that the synthesized compound contained
176
K (8.33%), Zr (3.06%), W (70.71%), and Sn (3.89%). The elemental ratios in the synthesized
177
compound were calculated. The ratio of K:Zr:W:Sn was about 6:1:11:1.
Ac ce p
te
175
178
The results indicated that the synthesized compound exhibited a Keggin structure. The molecular
179
formula of the compound is K6ZrW11O39Sn·12H2O. Fig. 4 shows the SEM images of ZrW11Sn,
180
which is not a crystal so its XRD was not provided.
181 182 183
Fig. 4. SEM images of ZrW11Sn
184 185
3.2. Contrast experiments
186
To exclude the direct photolysis of dye solutions and the reaction of the dyes with the catalyst,
187
contrast experiments were carried out under two conditions: one with ZrW11Sn (20 mg) without 9
Page 8 of 31
illumination (kept in the dark for 4 h) and the other under sunlight for 4 h without the catalyst. The
189
results indicated that the ABS3R solution (10 mg/L, 20 mL) with ZrW11Sn (20 mg) failed to
190
degrade in the absence of illumination at a particular time interval, and the decoloration rate of
191
ABS3R during direct photolysis was less than 5%. In succeeding analyses, all the photocatalytic
192
experiments on aqueous ABS3R solutions were simultaneously conducted with the direct photolysis
193
experiments. The final values were recorded by subtracting the results from the direct photolysis of
194
ABS3R with that of the photocatalytic reaction. Changes in the aqueous solutions of RB5 and RR24
195
were not observed in the contrast experiments.
us
cr
ip t
188
Aqueous solutions of ABS3R, RR24, and RB5 (6 mg/L, 20 mL; initial pH, 6) each contained
197
20 mg ZrW11Sn and were simultaneously illuminated under sunlight (average intensity, 85.4 kLux).
198
Fig. 5 shows that ZrW11Sn is effective in the photocatalytic decoloration of all the azo dyes. The
199
decolorizing effect follows the order ABS3R>RB5>RR24. In the following experiment, ABS3R
200
was used as a model dye.
204 205
te Ac ce p
201 202 203
d
M
an
196
Fig. 5. Photocatalytic decoloration of different types of dyes
3.3. Effect of irradiation time
206
To study the effect of irradiation time, ABS3R solution (20 mL, 4 mg/L; initial pH, 6) containing
207
20 mg ZrW11Sn was irradiated under sunlight (average intensity, 107 kLux). The decoloration rates
208
of ABS3R solution increased with the irradiation time (Fig. 6), but the extent of the increase was
209
reduced after 4 h illumination. Competition for degradation could occur between ABS3R and the
210
intermediate products because more intermediates were produced with increased reaction time. This
211
result is reminiscent of the TiO2-assisted photocatalytic degradation of azo dyes [21]. The 10
Page 9 of 31
212
decoloration rate of ABS3R solution increased by only 8.6% from 4 h to 5 h irradiation. For work
213
efficiency, 4 h illumination time was chosen in this study.
ip t
214
80
cr
60 50 40
us
Decolorization rate %
70
30
10
1
2
M
d
Fig. 7. UV–Vis absorption spectra of the ABS3R solution at different photolysis times
te
219
5
Fig. 6. Effect of the photocatalysis time on ABS3R degradation
217 218
4
Irradiation time (h)
215 216
3
an
20
Fig. 7 shows the change in the UV–Vis absorption spectrum of the ABS3R solution with
221
photolysis time. The absorption peak of ABS3R at the maximum absorption wavelength (510 nm)
222
in the visible region markedly decreased with the irradiation time. The decrease implied that the azo
223
structure (–N=N–) of ABS3R (Scheme 1) was destroyed and that ABS3R was degraded. However,
224
the absorption peaks in the UV (190 nm –300 nm) that represents the Keggin- structure of ZrW11Sn
225
remained unchanged during irradiation. This finding indicates that ZrW11Sn was stable during the
226
photocatalytic reaction as a catalyst. The UV–Vis absorption spectra of ZrW11Sn solutions (1 g/L)
227
were identical before and after irradiation, proving the stability of ZrW11Sn. Differences in the UV
228
absorbance spectra of ZrW11Sn–ABS3R solutions failed to reveal the appearance of any new
229
absorption bands relative to ABS3R or ZrW11Sn solutions alone. This result suggests that a possible
Ac ce p
220
11
Page 10 of 31
230
ZrW11Sn–ABS3R complex could not represent a significant chromophore under our experimental
231
conditions. The observable change in the concentration of ABS3R is solely attributed to the known
232
photocatalytic degradation of ZrW11Sn. Furthermore, white precipitates were found in the ABS3R solution irradiated for 5 h upon the
234
addition of BaCl2 solution. This result indicates the production of SO42- ions during the reaction.
235
Neither phenol disulfonic acid solution nor Nessler's reagent was added to the reaction. The color of
236
the solution all changed to yellow, suggesting the production of NO3—N, NH3–N, and NH4+–N in
237
the solution. SO42-, NH4+, and NO3- were undetected in the unirradiated ABS3R solution. The
238
molecular structure of ABS3R (Scheme 1) revealed that SO42- came from the sulfonic acid groups
239
of ABS3R; the N element in the solution was produced by the –N=N– group of ABS3R. The
240
molecular structure of ABS3R was destroyed in the photocatalytic reaction, the azo bond was
241
broken, and NH4+ was generated and further oxidized to NO3-.
d
M
an
us
cr
ip t
233
3.4. Effect of the initial concentration of ABS3R solution
Ac ce p
243
te
242
244
The effect of the initial concentration of ABS3R solution on the decoloration rate is shown in
245
Fig. 8. The initial pH of all the ABS3R solutions (20 mL) was 6, and all the photocatalyst doses in
246
the ABS3R solutions were 20 mg. The irradiation time was 4 h, and the average sunlight intensity
247
was 103 kLux.
248 249
Fig. 8. Effects of the initial concentration of ABS3R solution on the decoloration rate
250 251
The results show that the decoloration efficiency initially increased with the ABS3R
252
concentration at the initial stage, reached its maximum at 6 mg/L, and decreased thereafter. The 12
Page 11 of 31
253
behavior of the discoloration efficiency is attributed to the photocatalytic reaction rate r (mg/L min)
254
that can be described by the kinetic Langmuir–Hinshelwood (L–H) equation [22]. When the initial
255
concentration of the solution is low, KC << 1. The photocatalytic reaction rate is given as r = kKC =K/C,
(4)
ip t
256
where C (mg/L) is the concentration of the solution, k (mg/L min) is the Langmuir rate constant, K
258
(L/mg) is the Langmuir adsorption constant, and K' (min-1) is the reaction rate constant. Eq. 4
259
indicates that the reaction rate is positively correlated with the solution concentration; therefore, the
260
decoloration rate of the dye solution increased with the initial concentration. However, the chroma
261
of the dye solution also increased with the initial concentration, the opacity of which was increased
262
and the sunlight penetration decreased [13,15]. Thus, the photocatalytic reaction could not occur in
263
the center of the reactor. As the catalyst amounts were all the same in different initial concentrations
264
of ABS3R solutions, the generated photoactivity species were also fixed, the irradiation time was
265
kept constant, and the decoloration rate of the dye solution failed to increase with the initial
266
concentration.
268 269 270
us
an
M
d
te
Ac ce p
267
cr
257
The maximum decoloration rate was obtained because the initial concentration of the ABS3R solution was 6 mg/L, which was chosen in this study.
3.5. Effect of the photocatalyst dosage
271
The amount of catalyst in the solution mainly influenced the photocatalytic reaction. Fig. 9
272
illustrates the effect of the concentration of ZrW11Sn in ABS3R solution on the decoloration rate.
273
ABS3R solutions (20 mL, 6 mg/L; initial pH, 6) with different amounts of ZrW11Sn were irradiated
274
under sunlight (average intensity, 103 kLux) for 4 h. Fig. 9 reveals that the decoloration rate of
275
ABS3R increased with the increase in catalyst concentration from 0.5 g/L to 1.5 g/L. The rate 13
Page 12 of 31
remained unchanged with the increase in catalyst concentration from 1.5 g/L to 1.75 g/L. The extent
277
of increase gradually reduced beyond 1.0 g/L. This result is attributed to the high photocatalyst
278
production that generates effective photoactivity species (·OH and h+). The decoloration rate of
279
ABS3R solution initially increased with the increase in photocatalyst concentration. However, the
280
excessive photocatalyst generated high amounts of ·OH radicals and h+. Portions of the radicals
281
recombined with each other by contact in the solution, and thus the increased extent of the
282
decoloration rate of ABS3R solution was low.
us
cr
ip t
276
The results indicate that the increase in catalyst dosage is ineffective in enhancing the
284
degradation efficiency. The optimum amount of ZrW11Sn should be added to avoid catalyst wastage
285
and ensure total photon absorption for efficient photomineralization dyes. The decoloration rate of
286
ABS3R increased by only 3% when the ZrW11Sn concentration increased from 1.0 g/L to 1.5 g/L.
287
In consideration of the economic factors, the optimum ZrW11Sn concentration of 1.0 g/L was
288
chosen in the experiments.
te
d
M
an
283
Ac ce p
75
Decoloration rate %
70
65
60
55
50
45
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
C (g/L)
289 290
Fig. 9. Effect of catalyst concentration on the decoloration rate of ABS3R
291 292 293
3.6. Effect of the initial pH of ABS3R solution Experiments were carried out at various initial pH of the ABS3R solution ranging from 2–9 at a 14
Page 13 of 31
constant dye concentration (20 mL, 6 mg/L) and catalyst loading (1.0 g/L) to study the effect of the
295
initial pH of the ABS3R solution on the decolorization efficiency. Aqueous ABS3R solutions were
296
illuminated under sunlight (average intensity, 93 kLux) for 4 h. The stability of ABS3R at different
297
pH was studied prior to the tests. The UV–Vis spectra of the aqueous ABS3R solutions were not
298
affected by the changes in initial pH. This result implies that the molecular structure of ABS3R was
299
not destroyed with the change in initial pH. Fig. 10 suggests that the decoloration rate of ABS3R
300
was good (69.2%–69.8%) when the initial pH of the aqueous ABS3R solutions ranged from 4–6.
301
This result is attributed to the existence of stable 1:11 Keggin-type POM for pH ranging from
302
4.5–6.0 [19,23]. The photocatalyst was destroyed in other pH ranges; thus, the catalytic capacity
303
decreased.
cr
us
an
M
305
The pH value of the original aqueous ABS3R solution was about 6, which was chosen as the initial pH of the ABS3R solution in the experiments.
d
304
ip t
294
te
306 307
309 310
Fig. 10. Effect of the initial pH of the ABS3R solution on the decoloration rate
Ac ce p
308
3.7. Kinetics of photocatalytic degradation
311
The reaction kinetics follows the L–H model for heterogeneous reaction systems, such as the TiO2
312
photocatalysis system [21,24]. Mylonas and Papaconstantinou [25] and Ozer and Ferry [26] reported
313
that the kinetics follows the L–H model for homogeneous POM reaction systems given as
314
r=−
dCt kKCt = , dt 1 + KCt
(5)
315
where r (mg/L·min) is the photocatalytic reaction rate, k (mg/L min) is the rate constant, K (L/mg)
316
is the association constant, and Ct (mg/L) is the concentration of ABS3R at reaction time t. 15
Page 14 of 31
317 318
319
When the initial concentration of ABS3R is small (KC0 << 1), the equation can be simplified to a first-order rate equation given as
⎛C ⎞ ln⎜⎜ 0 ⎟⎟ = kKt = K / t , ⎝ Ct ⎠
(6)
where C0 (mg/L) and Ct (mg/L) are the concentrations of ABS3R solution at reaction times 0 and t
321
(min), respectively. K/ (min-1) is the observed first-order rate constant.
cr
ip t
320
To study the kinetics of the photocatalytic degradation of aqueous ABS3R solutions with
323
ZrW11Sn, experiments were carried out with initial ABS3R concentrations ranging from 4 mg/L–10
324
mg/L at a constant initial pH of 6 and catalyst loading of 1 g/L. ln(C0/Ct) was plotted against the
325
irradiation time. The photocatalytic degradation was well represented by a pseudo first-order kinetic
326
model. Table 1 lists the first-order reaction kinetic equations and the correlation coefficients (R2).
M
an
us
322
327
Table 1 Parameters and first-order reaction kinetic equations for the photocatalytic degradation
d
328
te
329
Developing a model for the dependence of the photocatalytic degradation rate on the
331
experimental parameters for the photolysis time is difficult because of the complex mechanism of
332
the reaction. Therefore, modeling the kinetics of photocatalysis is often restricted to the analysis of
333
the initial rate (r0) of the reaction. The model can be obtained from the initial slope of the curves
334
and the initial concentration of the reaction solutions (Eq. 4, r0=K/C0). The values of r0 are listed in
335
Table 1, in which
336 337 338
Ac ce p
330
r0 =
kKC0 . 1 + KC0
(7)
Eq. 7 can be further expressed in a linear form as 1 1 1 = + . r0 kKC0 k
(8) 16
Page 15 of 31
339 340
Fig. 11. Relationship between 1/r0 and 1/C0
341
343
The reciprocal of the initial rate 1/r0 was plotted with the reciprocal of the initial concentration
ip t
342
1/C0 (Fig. 11) to obtain the kinetics parameters k=0.2084 mg/L min and K=0.0395 L/mg.
3.8. Kinetic probes of the mechanisms of ZrW11Sn-mediated photocatalytic oxidation of ABS3R
us
345
cr
344
The mechanisms by which POM photocatalytically oxidize organic compounds in aqueous
347
solutions are critical to the potential utilities [26]. Kinetic probes were studied to determine the
348
possible role of hydroxyl radicals (·OH) and photogenerated holes (h+) in the ZrW11Sn-mediated
349
photocatalytic oxidation of aqueous ABS3R solution.
M
an
346
The functions of ·OH and h+ in the aqueous oxidation of ABS3R were tested by assessing the
351
effects on K/ from the presence of radical (KBr) [27] and hole scavengers (KI) [28], respectively.
352
Unlike in the absence of active scavenger species in the ABS3R solutions, the photocatalytic
353
decoloration rate of ABS3R decreased when KBr and KI were added to the reaction solution. This
354
result implies that the ·OH radicals and holes were the oxidants present during the
355
ZrW11Sn-mediated photocatalytic degradation of ABS3R.
te
Ac ce p
356
d
350
357
Fig. 12. ln(C0/Ct) of ABS3R solution against irradiation time for different concentrations of (a) KBr
358
and (b) KI
359 360 361
Fig. 13. Effects of KBr and KI on the observed rate constant K/
362 363
Fig. 12 shows that the kinetics of the photocatalytic degradation of ABS3R still followed the 17
Page 16 of 31
pseudo-first-order model when different concentrations of KBr and KI were added to the solution.
365
Although all the reaction rates decreased with the increasing concentrations of KBr and KI, the
366
effect using the same concentration of KBr and KI was different (Fig. 13). KI exhibited greater
367
effects on the photocatalytic decoloration rate of ABS3R than KBr (Fig. 13). The result indicates
368
that the action of the photogenerated holes relative to ·OH radicals was highly pronounced in the
369
reaction. In electron transfer, holes rather than ·OH radicals for various 1:12 series POM (Keggin
370
structure) [26] are used. However, Kim et al. [29] suggested that ·OH radicals were the sole
371
dominant photooxidant in PW12O403--mediated degradation irrespective of the kind of substrate; the
372
radicals were exclusively operated through holes for W10O324- (isopolyacid) [30].
373
is a 1:11 series POM (Keggin structure), and its photocatalytic mechanism is different from that of
374
1:12 series POM and isopolyacid. The mechanism of the primary photooxidation of POM, which is
375
operated through ·OH radicals and/or photogenerated holes, depends on the photocatalyst, the
376
substrate, and the mode of analysis.
[Ed 13622]ZrW11Sn
te
d
M
an
us
cr
ip t
364
Irradiation of ZrW11Sn resulted in the formation of an O→W charge–transfer excited state at the
378
W–O–W bridge bond and produced considerable photogenerated holes and electrons. The primary
379
mechanism of the ZrW11Sn-mediated photocatalytic degradation of ABS3R was photooxidative
380
decomposition. Similar to that of other POMs [31], the photoreductive decomposition of ABS3R by
381
ZrW11Sn cannot be entirely ruled out. To deduce the exact photocatalysis mechanism, the photolysis
382
products require further analysis. The possible reaction mechanisms have been previously
383
summarized [26,27,29–32].
Ac ce p
377
384 385
4. Conclusions
386 18
Page 17 of 31
A novel polyoxometalate, ZrW11Sn, was synthesized and characterized. Irradiation of ZrW11Sn
388
under sunlight effectively photocatalytically degraded azo dyes, ABS3R, RR24, and RB5. The
389
following conclusions were drawn from the results:
390
1. The optimum initial pH of the ABS3R solution ranged from 4–6 during ZrW11Sn-mediated
391
ip t
387
photocatalytic degradation.
2. The photocatalytic decoloration rate of the ABS3R solution increased with the initial
393
concentration. However, the rate did not increase throughout the process. The optimum
394
concentration was 6 mg/L upon loading with 1.0 g/L ZrW11Sn.
us
cr
392
3. The decoloration rate of ABS3R increased with the catalyst dosage up to the optimum loading.
396
Further increasing the catalyst dose yielded no effects. The optimum concentration of ZrW11Sn
397
was 1.0 g/L based on economic factors.
M
an
395
4. The photocatalytic decoloration of ABS3R was a pseudo-first-order reaction based on
399
Langmuir–Hinshelwood kinetic model. Mathematical inferences revealed that the rate and the
400
association constants during the ZrW11Sn-mediated photocatalytic degradation of ABS3R were
401
0.2084 mg/L min and 0.0395 L/mg, respectively.
403
te
Ac ce p
402
d
398
5. Photogenerated holes and ·OH radicals during the photocatalytic degradation of ABS3R were effective active species. The holes were highly pronounced in the reaction.
19
Page 18 of 31
References
[1] D.F. Li, Y.H. Guo, C.W. Hu, L. Mao, E.B. Wang, Appl. Catal. A-Gen. 235 (2002)
ip t
11-20.
[2] D.L. Long, R. Tsunashima, L. Cronin, Angew. Chem. Int. Edit. 49 (2010) 1736-1758.
cr
[3] A. Dolbecq, E. Dumas, C. R. Mayer, P. Mialane, Chem. Rev. 110 (2010) 6009-6048.
us
[4] T.H. Li, S.Y. Gao, F. Li, R. Cao, J. Colloid. Interf. Sci. 338 (2009) 500-505. [5] C.C. Chen, Q. Wang, P.X. Lei, W.H. Ma, L.C. Zhao, Environ. Sci. Technol. 40
an
(2006) 3965-3970.
Pharmacother. 60 (2006) 349-352.
M
[6] H. Yanagie, A. Ogata, S. Mitsui, T. Hisa, T. Yamase, M. Eriguchi, Biomed.
te
1686-1694.
d
[7] Y. Yang, Y.H. Guo, C.W. Hu, C.J. Jiang, E.B. Wang, J. Mater. Chem. 13 (2003)
[8] S. Kim, J. Yeo, W. Choi, Appl. Catal. B-Environ. 84 (2008) 148-155.
Ac ce p
[9] M.D. Hernández-Alonso, F. Fresno, S. Suarez, Sci. Total. Environ. 2 (2009) 12311257.
[10] C.C. Chen, W.H. Ma and J.C. Zhao, Chem. Soc. Rev. 39 (2010) 4206–4219. [11] V.B.R. Boppana, R.F. Lobo, J. Catal. 281 (2011) 156-168. [12] S. Swetha, G. Balakrishna, Chinese. J. Catal. 32 (2011) 789-794. [13 ]E. Forgaces, T. Cserhati, G. Oros, Environ. Int. 30 (2004) 953-971. [14] V.K. Gupta, R. Jain, S. Agarwal, A. Nayak, M. Shrivastava, J. Colloid. Interf. Sci. 366 (2012 ) 135-140.
Page 19 of 31
[15] V.K. Gupta, R. Jain, A. Mittal, A.T. Saleh, A. Nayak, S. Agarwal, S. Sikarwar, Mater. Sci. Eng. C 32 (2012) 12-17. [16] A.O. Ibhadon, G.M. Greenway, Y. Yue, Catal. Commun. 9 (2008) 153-157.
ip t
[17] R. Jain, M. Mathur, S. Sikarwar, A. Mittal, J. Environ. Manage. 85 (2007) 956-964. [18] M. Neamtu, I. Siminiceanu, A. Yediler, A. Kettrup, Dyes. Pigments. 53 (2002) 93-
cr
99.
Industrial Press, Beijing, 1998. (In Chinese)
us
[19] E.B. Wang, C.W. Hu, L. Xu, Introduction of Polyoxometalates chemistry, Chemical
an
[20] W. Wang, X.H. Zhu, J.F. Liu. Chem. J. Chinese. U. 13 (1992) 735-736. [21] I.K. Konstantinou, T.A. Albanis, Appl. Catal. B-Environ. 49 (2004) 1-14.
M
[22] N.S. Deng, F.Wu, Environmental Photochemistry, Chemical Industrial Press,
d
Beijing, 2003. (In Chinese)
te
[23] W. Wang. The study of the syntheses and properties of heteropoly-tungstotianates; The Properties of lanthanide decatungstates. Dissertation for Master Degree.
Ac ce p
Changchun, Northeast Normal Universtity, 1988, pp. 34-37. (In Chinese)
[24] W. Baran, E. Adamek, A. Makowski, Chem. Eng. J. 145 (2008) 242-248. [25] A. Mylonas, E. Papaconstantinou, J. Photoch. Photobio. A. 94 (1996) 77-82. [26] R.R. Ozer, J.L. Ferry, J. Phys. Chem. B. 104 (2000) 9444-9448. [27] P. Kormali, A. Troupis, T. Triantis, A. Hiskia, E. Papaconstantinou, Catal. Today. 124 (2007) 149-155. [28] X.V. Doorslaer, P.M. Heynderickx, K. Demeestere, K. Debevere, H.V. Langenhove, J. Dewulf, Appl. Catal. B-Environ.111-112 (2012) 150-156. [29] S. Kim, H. Park, W. Choi, J. Phys. Chem. B. 108 (2004) 6402-6411.
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[30] I. Texier, J.A. Delarie, C. Giannotti, Phys. Chem. Chem. Phys. 2 (2000) 1205-1212. [31] A. Troupis, T.M. Triantis, E. Gkika, Appl. Catal. B-Environ. 86 (2009) 98-107.
Ac ce p
te
d
M
an
us
cr
ip t
[32] M.Q. Hu, Y.M. Xu, Chemosphere. 54 (2004) 431-434.
Page 21 of 31
0.25
(a ) ABS3R
0.15
0.10
cr
Absorbance
λmax= 510nm
ip t
0.20
0.00 200
300
400
500
600
700
800
an
Wavelength (nm)
us
0.05
0.12
(b ) RB5
0.10
M
λmax= 590nm
0.06
d
Absorbance
0.08
0.02
te
0.04
300
Ac ce p
0.00 200
400
500
600
700
800
Wavelength (nm)
0.12
(c ) RR24
Absorbance
0.10 0.08
λmax= 538nm
0.06 0.04 0.02 0.00
200
300
400
500 600 Wavelength (nm)
700
800
Fig. 1. UV-Vis spectra of the aqueous solutions of (a) ABS3R, (b) RB5, and (c) RR24.
Page 22 of 31
90 80
(a)
νas(Zr-Oa)
50
δ(H-O-H)
νas(W-Od)
30 20
νas (H-O-H)
νas(W-Ob-W)
10 0 4000
νas(W-Oc-W)
3500
3000
2500
2000
1500
M
2.5
(b)
500
ZrW11Sn ZrW11
d
2.0
te
1.5
Ac ce p
Absorbance
1000
an
Wavenumber (cm-1)
1.0
cr
40
ip t
60
us
Transmittance %
70
0.5
0.0 200
300
400
500
600
700
800
Wavelength (nm)
Fig. 2. (a) FTIR and (b) UV-Vis spectra of ZrW11Sn.
Page 23 of 31
100
4
98
642°C DSC
2
TG 94
110°C
o
413 C
o
380 C 200
300
400
500
600
700
800
an
100
us
1
92
90
ip t
614°C
Heat flow / (mV/mg)
96
cr
Mass / %
3
o
Temperature / C
Ac ce p
te
d
M
Fig. 3. TG–DSC curves of ZrW11Sn.
Fig. 4. SEM images of ZrW11Sn.
Page 24 of 31
80
ABS3R RB5 RR24
60 50
ip t
40 30 20 10 0
0
1
2
3
4
5
us
Irradiation time (h)
cr
Decoloration rate %
70
M
an
Fig. 5. Photocatalytic decoloration of different types of dyes.
80
40 30
te
50
d
60
Ac ce p
Decolorization rate %
70
20 10
1
2
3
4
5
Irradiation time (h)
Fig. 6. Effect of the photocatalysis time on ABS3R degradation.
Page 25 of 31
0.15
1.5
0.05
1.0 0.00
500
0.5
600
700
Wavelength (nm)
200
300
400
500
800
us
0.0
ip t
1h 2h 3h 4h 5h
0.10
Absorbance
Absorbance
2.0
ABS3R o h (ABS3R+ZrW11Sn)
cr
2.5
600
700
800
an
Wavelength (nm)
75
Decoloration rate %
Ac ce p
70
te
d
M
Fig. 7. UV–Vis absorption spectrra of the ABS3R solution at different photolysis times.
65
60
55
50
45
0
5
10
15
20
25
C (mg/L)
Fig. 8. Effects of the initial concentration of ABS3R solution on the decoloration rate.
Page 26 of 31
75
65
ip t
60
55
50
0.4
0.6
0.8
1.0
1.2
C (g/L)
1.4
1.6
1.8
2.0
us
45
cr
Decoloration rate %
70
an
Fig. 9. Effect of catalyst concentration on the decoloration rate of ABS3R
M
72 70
d
64 62
te
66
Ac ce p
Decoloration rate %
68
60 58 56
2
4
6
8
10
pH
Fig.10. Effect of the initial pH of the ABS3R solution on the decoloration rate.
Page 27 of 31
45 y = 121.48 x + 4.7978 2
R = 0.9903
35 30
ip t
1/r 0 ( L min/mg )
40
cr
25
15 0.05
0.10
0.15
0.20
us
20
0.25
0.35
an
1/C 0 (L /mg)
0.30
M
Fig. 11. Relationship between 1/r0 and 1/C0
d
2.0
te
1.6 1.4
Ac ce p
1.2
ln(C0/Ct)
(a) KBr
0 μmol/L 5 μmol/L 25 μmol/L 50 μmol/L 100 μmol/L
1.8
1.0 0.8 0.6 0.4 0.2 0.0
0
50
100
150
200
250
Irradiation time (min)
Page 28 of 31
1.8 (b) KI
0 μmol/L 5 μmol/L 25 μmol/L 50 μmol/L 100 μmol/L
1.6 1.4
ip t
ln(C0/Ct)
1.2 1.0 0.8
cr
0.6
0.2 0.0
0
50
100
150
200
250
an
Irradiation time (min)
us
0.4
Fig. 12. ln(C0/Ct) of ABS3R solution against irradiation time for different concentrations
d
M
of (a) KBr and (b) KI.
7
Ac ce p
6 -3 -1 / K *10 , min
KBr KI
te
8
5 4 3 2 1 0
0
20
40
60
80
100
Concentrations (μmol/L)
Fig. 13. Effects of KBr and KI on the observed rate constant K/.
Page 29 of 31
Highlights
ip t
A new photocatalyst, K6ZrW11O39Sn·12H2O was synthesized and characterized. ZrW11Sn showed photocatalytic activity to decolor dye with sunlight irradiation.
cr
Photocatalytic degradation of ABS3R was a pseudo first-order reaction.
Ac ce p
te
d
M
an
us
Photogenerated holes and ·OH were both the main oxidants in the reaction.
Page 30 of 31
ip t
Graphical abstract
cr
Sunlight Irradiation
us
Photocatalytic degradation + products (NH4 , NO3-, SO42-, H2O ……)
Ac
ce pt
ed
M
an
+
Page 31 of 31