Preparation, characterization and sonodegradation properties of silver tyipolyphosphate catalyst

Catalysis Communications 30 (2013) 27–31

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Short Communication

Preparation, characterization and sonodegradation properties of silver tyipolyphosphate catalyst Limin Song a,⁎, Shujuan Zhang b,⁎, 1, Xiaoqing Wu c,⁎, Shuna Zhang d, 1, Haifeng Tian a, Jiayi Ye a a College of Environment and Chemical Engineering & State Key Laboratory of Hollow-Fiber Membrane Materials and Membrane Processes, Tianjin Polytechnic University, Tianjin 300387, PR China b College of Science, Tianjin University of Science & Technology (co-first institution), Tianjin, 300457, PR China c Institute of Composite Materials & Ministry of Education Key Laboratory of Advanced Textile Composite Materials, Tianjin Polytechnic University, Tianjin 300387, PR China d Zhejiang Industry Polytechnic College, Shaoxing, 312000, PR China

a r t i c l e

i n f o

Article history: Received 11 September 2012 Received in revised form 21 October 2012 Accepted 22 October 2012 Available online 1 November 2012 Keywords: Ag5P3O10 Sonodegradation Organic dye

a b s t r a c t A new catalyst of silver tyipolyphosphate (Ag5P3O10) for sonodegradation of dyes, was prepared by a hydrothermal route. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV–vis absorption spectroscopy (UV–vis), photoluminescence spectra (PL) and N2 adsorption–desorption methods. The degradation of dyes in water was investigated under ultrasonic irradiation in the presence of Ag5P3O10. The result shows Ag5P3O10 has the excellent sonodegradation activity of dyes. A large number of hydroxyl (•OH) radicals formed under ultrasonic radiation, and the content of •OH radicals increased with increased radiation time. This phenomenon provided the main basis for the high sonodegradation activity of Ag5P3O10 under ultrasonic radiation. The kinetic model and its parameters were also discussed in detail. © 2012 Elsevier B.V. All rights reserved.

1. Introduction In recent years, the sonodegradation process has attracted considerable interest due to its high decontamination efficiency [1–3]. Semiconductor sonocatalysis has emerged as a promising technique owing to its high sonoactivity [4–7]. The ability of ultrasonic radiation is well known to promote a chemical reaction that can be attributed to acoustic cavitation [1]. This acoustic cavitation can form partial high temperatures and pressures, which can result in the thermal dissociation of water to form •OH radicals. These radicals act as oxidants in chemical reactions under ultrasonic radiation. Thus, the sonodegradation oxidation is a promising method for the removal of pollutants. However, the development of new, efficient catalysts is still a major challenge. The repored catalysts with the sonodegradation oxidation mainly include TiO2, ZnO, CeO2, SnO2, ZrO2, CdS, etc. These catalysts show good the sonodegradation ability under ultrasonic radiation [8–19]. In the current paper, a new catalyst with excellent sonodegradation activity, silver tyipolyphosphate (Ag5P3O10), was synthesized by a simply hydrothermal method. This catalyst has not yet been reported to the best of our knowledge. The structure and properties of Ag5P3O10 was discussed in detail. Activity tests showed the excellent performance of Ag5P3O10 in the degradation of methylene blue (MB) and rhodamine B (RhB) molecules. Many

researchers have proposed a mechanistic kinetic model for the degradation of dyes by the sonodegradation process [8–10]. In our experiment, the degradation rate data were modeled using pseudo-first-order kinetics. The present study aimed to develop a kinetic model of the sonodegradation of dyes based on the underlying mechanism. 2. Experimental 2.1. Synthesis of catalysts All of the reagents used were of analytical purity and used without further purification. They were purchased from Tianjin Chemical Reagent Factory of China. In a typical synthesis, two separate solutions with the same volume were prepared by dissolving 0.64 g of AgNO3 and 0.28 g of Na5P3O10 into 25 mL of deionized water under vigorous stirring. Then, the two solutions were mixed and stirred for another 30 min. Afterward, the newly formed mixture was transferred into an 80 mL stainless Teflonlined autoclave and heated at 100 °C for 6 h. The resulting precipitate was collected and washed several times with absolute ethanol and distilled water. Finally, Ag5P3O10 powder was obtained by centrifugation and drying in air atmosphere at room temperature. 2.2. Characterization of catalysts

⁎ Corresponding authors. Tel./fax: +86 22 83955458. E-mail addresses: [email protected] (L. Song), [email protected] (S. Zhang), [email protected] (X. Wu). 1 Co-first author. 1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.catcom.2012.10.021

X-ray diffraction (XRD) patterns were obtained with a Rigaku X-ray diffractometer with Cu Kα radiation (λ = 1.5406 Å). Field emission scanning electron microscopy (FE-SEM) images were obtained on a

L. Song et al. / Catalysis Communications 30 (2013) 27–31

field emission microscope (Hitachi, S-4800) operated at an acceleration voltage of 10 kV. Transmission electron microscopy (TEM) images were obtained on a Hitachi H-7650 transmission electron microscope (an acceleration voltage of 80 kV). The nitrogen adsorption was carried out in a Micromeritics ASAP 2020 nitrogen adsorption apparatus. UV–vis absorption spectrum and photoluminescence spectra (PL) of samples were measured on HP8453 spectrophotometer equipped with an integration sphere and Cary Eclipse photoluminescence analyzer, respectively.

7000 6000 5000

Counts (a.u.)

28

4000

**

3000 * *

2000

*

*

*

2.3. Activity measurement 1000

The sonodegradation experiments were carried out in a homemade reactor. The sonodegradation activity of samples was investigated by using MB and RhB as the model molecules. In a typical process, the experimental conditions are following: total volume of 100 mL aqueous solution, initial concentration of 5–20 mg/L dye aqueous solution, amount of 1.0 g/L sample as catalyst, temperature of room, solution acidity of pH = 7.0, irradiation time of 180 min and ultrasound of 40 kHz frequency and 80 W output power were used under continuous stirring and bubbling (air, flowing rate: 30 mL/min). At a defined time interval, the concentration of dye in the ultrasonic reaction was analyzed by using an UV–vis spectrophotometer. The removal ratio was calculated by (C0 − C)/C0, where C is the concentration of the reactant after ultrasound, C0 is the concentration of the reactant after adsorption equilibrium and before the ultrasound in the presence of catalyst. 2.4. Hydroxyl radical measurement

JCPDS file 14-0264

0 20

25

30

35

40

2 Theta/ degree Fig. 1. X-ray diffraction patterns of Ag5P3O10.

believe that the mechanism of sonocatalysis is similar to that of photocatalysis. In other words, ultrasonic radiation also causes the formation of many •OH radicals on the surface of catalysts. These •OH radicals have redox ability. Therefore, Ag5P3O10 has strong UV light absorption and may be sensitive to ultrasonic radiation, which can improve the sonodegradation efficiency of organic matters. Fig. 4 shows the N2 adsorption–desorption isotherm of the Ag5P3O10 sample. The BET specific surface area of Ag5P3O10 is 2.2 m 2/g (Table 1). The curve in Fig. 4A is a typical type IV; there were sloping adsorption and desorption branches in the hysteresis loop. According to the pore

The formation of ·OH radicals on samples under ultrasonic irradiation were investigated by a photoluminescence (PL) way. The route used terephthalic acid as reactive molecules [1]. A typical experiment was following 5 mg of samples was added to 80 mL aqueous solution (0.01 mol/L NaOH, 3 mmol/L terephthalic acid), and stirred for 30 min in dark. At a defined time interval, the concentration of solution in the system under ultrasonic irradiation was analyzed by PL (excited by 325 nm light). 3. Results and discussion 3.1. Characterization of catalysts The XRD pattern of the Ag5P3O10 sample is shown in Fig. 1. The main peak at d = 2.76 can be assigned to Ag5P3O10 (JCPDS file 14-0264), but the crystal system and lattice constants were not confirmed because Ag5P3O10 has no JCPDS record. Other peaks (d = 3.28, 3.10, 2.92, 2.62, 2.48, 2.36 and 2.27) slightly shifted to low angles compared with JCPDS file 14-0264, which may be due to the lattice distortion. However, no purity peak was found in the sample. The average crystal particle size of the as-prepared Ag5P3O10 was calculated as 39 nm by the Scherrer equation based on the strong peak of Ag5P3O10. The SEM images in Fig. 2a show the morphology of the as-prepared Ag5P3O10 sample. The irregular Ag5P3O10 particles aggregated and formed bundle shapes with sizes ranging between 1 and 2 μm. Fig. 2b shows the TEM image of the as-synthesized Ag5P3O10 sample, which was obtained after 15 min of ultrasonic irradiation. Therefore, the agglomerated particles observed in Fig. 2a had been dispersed, and the particles were spherical and about 100–150 nm in diameter. The UV-visible absorption spectra (Fig. 3A) of the Ag5P3O10 sample showed that Ag5P3O10 had strong absorption only in the ultraviolet region of 250–375 nm. Fig. 3B shows that the band gap of Ag5P3O10 was 3.40 eV, consistent with the results of the UV excitation in Fig. 3A. To date, the mechanism of organic pollutant degradation by ultrasonic radiation remains unclear. However, numerous researchers

*

Fig. 2. SEM and TEM images of Ag5P3O10.

L. Song et al. / Catalysis Communications 30 (2013) 27–31

A

A 0.6

1.6

Quantity absorbed (cm3/g)

1.4 1.2

Absorbtion (a.u.)

29

1.0 0.8 0.6 0.4 0.2

0.5

0.4

0.3

0.2

0.1

0.0

0.0

0.0

250 300 350 400 450 500 550 600 650 700 750 800 850

0.2

B

0.4

0.6

0.8

1.0

Relative pressure (P/P0)

Wavelength (nm)

B

1.6 1.4

1.2

1.0

dV/dD (cm3/g)

(a hv)1/2

1.2 1.0 0.8 0.6

0.8

0.6

0.4

0.4 0.2 0.2 0.0

0.0 1

2

3

4

0

5

10

20

30

40

50

60

70

Pore diameter (nm)

Band gap (eV) Fig. 3. UV–vis absorption spectra of Ag5P3O10.

Fig. 4. (A) N2 adsorption–desorption isotherms and (B) pore-size distribution.

size distribution in Fig. 4B, the peaks were within the mesoporous range, indicating the mesostructure of the Ag5P3O10 sample. The Ag5P3O10 sample had a bimodal distribution with the most frequent pore diameters being 4–6 nm, as shown in Fig. 4B. The average pore size was 3.445 nm (Table 1), and the total pore volume (P/P0 = 0.99) was 0.005 cm3/g (Table 1).

and MB. In reference [20], our precious reported result shows the degradation rate of RhB is 74% over porou TiO2 under sonoradiation, which is higher than this result in the paper (60%). Aside from different materials, the main reason may assign to a high surface area of porous structure over TiO2. Fig. 7 illustrates the degradation rate as a function of the initial RhB concentration (5–20 mg/L) after 180 min of ultrasonic irradiation. The adsorption capacities within 30 min without ultrasonic irradiation were similar. The reduction rate of RhB in aqueous solution clearly depended strongly on the initial concentration. The degradation rates of RhB over Ag5P3O10 were 70%, 60%, 54%, and 43% at 5, 10, 15, and 20 mg/L RhB, respectively. A large number of •OH radicals, which act as oxidants, can be formed during the degradation process. With increased initial concentration, more RhB molecules are adsorbed on the surface of Ag5P3O10 until a saturated state is reached. Therefore, there are fewer active sites for the adsorption of OH − anions, which reduce the generation of •OH radicals. Finally, the

3.2. Sonodegradation activity of catalysts Fig. 5 shows the results under sonoradiation in the absence of Ag5P3O10. The self-decomposition rate of RhB and MB is increasing with increasing sonoradiation time without Ag5P3O10 in Fig. 5. The self-decomposition rate of RhB and MB reaches 13.4 and 23.2% after 180 min sonoradiation, respectively. It is very clear that MB moleculars are more sensitive to sonoradiation than those. Fig. 6 shows the changes in the absorption spectra of an aqueous solution of RhB and MB (10 mg/L) exposed to ultrasonic radiation for various times in the presence of Ag5P3O10 (0.1 g). The adsorption capacity of RhB and MB over Ag5P3O10 was measured by stirring for 30 min in the absence of ultrasonic radiation. The adsorption amount over Ag5P3O10 was low according to the absorption spectra. Fig. 6A shows that RhB absorption at 553 nm rapidly decreased within 180 min. Fig. 6B shows that the major absorption band of MB molecules between 290 and 665 nm sharply decreased with increased time. These results showed that Ag5P3O10 had excellent activity in the degradation of RhB

Table 1 Physical parameters of Ag5P3O10. sample

ds(nm)

SBET(m2/g)

Vp (cm3/g)

dp (nm)

Ag5P3O10

39

2.2

0.005

3.445

Definitions: ds, crystal size; SBET, BET surface area; Vp, total pore volume; dp, BJH average pore size.

30

L. Song et al. / Catalysis Communications 30 (2013) 27–31

100

80

80

60

40

MB 20

RhB 0

5 mg/L 10 mg/L 15 mg/L 20 mg/L

70

stir without ultrasonic radiation

Decomposion ratio of RhB (%)

Decomposion ratio of dye (%)

No catalysts under ultrasonic radiation

60 50 40 30 20 10 0

20

40

60

80

100

120

140

160

180

200

0

20

40

60

Irradiation time (min)

80

100

120

140

160

180

200

Irradiation time (min)

Fig. 5. The decomposition ratio of dye without catalysts under ultrasonic radiation.

Fig. 7. The dependence of sonocatalytic activity on the original concentration of RhB.

degradation rate of RhB decreased with increased concentration of RhB solution.

The degradation reaction of RhB was assumed to fit first-order reaction kinetics. The rate constants were calculated from the plots of the natural logarithm of the RhB concentration versus the irradiation time. These rate constants were 0.0067, 0.0051, 0.0043, and 0.0031 min−1, respectively (Table 2). The correlation coefficients (R2) were 0.9933, 0.9993, 0.9998, and 0.9914 (Table 2). The decrease in k with increased initial RhB concentration demonstrated the competition between the intermediates and RhB for the active sites on the surface of Ag5P3O10, as proven by the deactivation of the catalyst.

3.3. Kinetics analysis The relationship between ln(Ct/C0) and the irradiation time (t) is plotted in Fig. 8 to investigate the reaction kinetics of sonodegradation.

A

2.2 2.0

RhB 0 min adsorption 60 min 120 min 180 min

Absorbtion (a.u.)

1.8 1.6 1.4

3.4. Hydroxyl radicals (·OH) and sonodegradation mechanism To determine the mechanism of the sonodegradation process, the •OH radical concentration was measured with different times until 60 min. Fig. 9 shows that the •OH radical concentration increased rapidly with increased reaction time, indicating that the Ag5P3O10 oxidation capacity gradually increased. This finding was consistent with our experimental results. According to the above conclusion and reference [1], the sonodegradation mechanism on Ag5P3O10 is following:

1.2 1.0 0.8 0.6 0.4 0.2 0.0 200

300

400

500

600

700

800

Wavelength (nm) 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

ð1Þ

•OH þ •H→H2 O

ð2Þ

2•OH→H2 O2

ð3Þ

1.4

MB 0 min adsorption 60 min 120 min 180 min

5 mg/L 10 mg/L 15 mg/L 20 mg/L

1.2 1.0

ln (C0/Ct)

Absorbtion (a.u.)

B

H2 O þ ultrasound þ catalyst→•OH þ •H

0.8 0.6 0.4 0.2

300

400

500

600

700

800

Wavelength (nm)

0.0 0

20

40

60

80

100

120

140

160

180

Irradiation time (min) Fig. 6. The UV–vis absorption spectra of solution in the process of the sonocatalytic degradation of (A) RhB and (B) MB on Ag5P3O10.

Fig. 8. The relationship between ln(C0/Ct) and irradiation time.

200

L. Song et al. / Catalysis Communications 30 (2013) 27–31

References

Table 2 Relating kinetic parameters on the sono-degradation of RhB. C0 (mg/L)

k (min−1)

R2

5 10 15 20

0.0067 0.0051 0.0043 0.0031

0.9933 0.9993 0.9998 0.9914

11000 10000 10 min 20 min 30 min 40 min 50 min 60 min

9000

Intensity (a.u.)

8000 7000 6000 5000 4000 3000 2000 1000 0 350

400

450

31

500

550

Wavelength (nm) Fig. 9. The PL signal peaks of •OH radicals at 425 nm in the presence of Ag5P3O10.

2•OH→H2 O• þ O

ð4Þ

O2 þ ultrasound þ catalyst→2O

ð5Þ

•O þ H2 O→2•OH

ð6Þ

O2 þ H•→•O2 H

ð7Þ

•O2 H þ •O2 H→H2 O2 þ O2

ð8Þ

H2 O2 þ ultrasound þ catalyst→2•OH

ð9Þ

4. Conclusion A new Ag5P3O10 catalyst for sonodegradation of dyes has been prepared by a simply hydrothermal route. This work has shown that Ag5P3O10 is an effective catalyst for the sonodegradation of organic substrates in water. The main reason is due to the production of a lot of OH radicals in solution.

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