Palygorskite and SnO2–TiO2 for the photodegradation of phenol

Applied Clay Science 51 (2011) 68–73

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Palygorskite and SnO2–TiO2 for the photodegradation of phenol Lili Zhang a,⁎, Jianquan Liu a, Cao Tang a, Jinshun Lv a, Hui Zhong a, Yijiang Zhao a, Xin Wang b,⁎ a b

Jiangsu Key Laboratory for Chemistry of Low-Dimensional materials, Huaiyin Normal University, Huai'an, Jiangsu Province, 223300, PR China Materials Chemistry Laboratory, Nanjing University of Science and Technology, Nanjing 210097, PR China

a r t i c l e

i n f o

Article history: Received 11 February 2010 Received in revised form 29 October 2010 Accepted 1 November 2010 Available online 6 November 2010 Keywords: Phenol Photodegradation Palygorskite Sol–gel technique SnO2–TiO2

a b s t r a c t The photocatalytic removal of phenol was studied using palygorskite-SnO2–TiO2 composites (abbreviated as Paly-SnO2–TiO2) under ultraviolet radiation. The photocatalysts were prepared by attachment of SnO2–TiO2 oxides onto the surface of the palygorskite by in situ sol–gel technique. The products were characterized by XRD, TEM and BET measurements. SnO2–TiO2 nanoparticles, with an average diameter of about 10 nm, covered the surface of the palygorskite fibers without obvious aggregation. Compared with palygorskitetitania (Paly-TiO2), palygorskite-tin dioxide (Paly-SnO2), and Degussa P25, Paly-SnO2–TiO2 and SnO2–TiO2 exhibited much higher photocatalytic activity. The photodecomposition of phenol was as high as 99.8% within 1.5 h. The apparent rate constants (kapp) for Paly-SnO2–TiO2, TiO2, and P25 were measured. Paly-SnO2–TiO2 showed the highest rate constant (0.03435 min−1). The chemical oxygen demand (COD) of the phenol solution was reduced from 220.2 mg/L to 0.21 mg/L, indicating the almost complete decomposition of phenol. Reusability of the photocatalyst was proved. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Phenols are generally considered to be one of the important organic pollutants discharged into the environment since they cause unpleasant taste and odor in drinking water. They are considered as priority pollutants since they are harmful to organisms at low concentrations and the phenol content of drinking water should not exceed 0.002 mg/L (the Chinese standard). Sources of phenol pollution in the aquatic environment are wastewaters from the paint, pesticide, coal conversion, polymeric resin, petroleum and petrochemical industries (Ahmaruzzaman, 2008). Improper handling of these compounds and/or inadequate discharge of their wastes results in long-term deterioration of the water environment and imposes considerable risk to all life forms because of their suspected carcinogenic properties (Kidak and Ince, 2006). Traditional methods such as solvent extraction, activated carbon adsorption, and common chemical oxidation often suffer from serious drawbacks, including high cost or formation of hazardous by-products (Pattersom, 1985). Biological degradation is environmental friendly and cost effective but is usually time-consuming (Chiou and Juang, 2007). Among the methods available, heterogeneous photocatalysis has emerged as an efficient technology for the purification of air and water (Fujishima et al., 1999). Due to its many merits, such as high efficiency, chemical stability and low cost, the semiconductor TiO2 seems to be the most promising photocatalyst

⁎ Corresponding authors. Tel.: +86 517 83525312; fax: +86 517 83525311. E-mail address: [email protected] (L. Zhang). 0169-1317/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2010.11.003

for the decomposition of various organic pollutants. However, its technological application seems to be limited by several factors, such as low quantum yield when acting as an excitation source, due to its wide band gap (3.2 eV for anatase), the problem of separation and recycling of the photocatalyst from the reaction medium. Coupling TiO2 with other semiconductors was widely studied and the corresponding composite photocatalyst was very suitable because of their high quantum yield. SnO2–TiO2 nanocomposites showed greater photocatalytic activity than Degussa P25 (Yang et al., 2002; El-Maghraby et al., 2008). An alternative method is the immobilization of TiO2 particles on an inert and suitable supporting matrix (Torimoto et al., 1996; Matos et al., 2001; Lei et al., 2006; Liu et al., 2007). Supported catalysis was awarded the status of “green” chemistry because it allows easy separation of the products and permits the recycling and reuse of the catalysts, giving both operational and economical advantages (Lei et al., 2006). For example, hybrid photocatalysts, consisting of TiO2 and activated carbon, were shown to exhibit a higher rate for degradation of several organic compounds than that of unmodified TiO2 (Torimoto et al., 1996; Matos et al., 2001; Xu et al., 2005; Zhu et al., 2005). Another promising material for these systems are clay minerals, which are resistant to deterioration and commercially available in large quantities and find many industrial, catalytic and environmental applications (Shimizu et al., 2002; Mogyorosi et al., 2003; Korosoki et al., 2004; Ooka et al., 2006; Liu and Wang 2007; Guo et al., 2008). Because of their low cost, abundance in most continents of the world, pronounced adsorption properties and potential for ion exchange, clay materials are strong candidates as adsorbents. In recent years, there was an increasing interest in

L. Zhang et al. / Applied Clay Science 51 (2011) 68–73

2. Experimental 2.1. Preparation Tetrabutyl titanate (Ti(OBu)4) and SnCl2·2H2O were of analytical grade and used without further purification. Palygorskite particles with an average diameter 200 meshes were provided by Jiangsu ATP Co. Ltd., Jiangsu, China. The preparation route was similar to the technique reported before (Zhang et al, 2009). The difference was that the molar ratio of SnCl2·2H2O:Ti(OBu)4 was fixed at 1:5. First, 5 g palygorskite was dispersed in 100 mL distilled water under ultrasonication for 0.5 h to remove some impurities. Then, 2.25 g SnCl2·2H2O was dissolved in 15 mL concentrated hydrochloric acid. This solution was added slowly to the palygorskite dispersion. The dispersion was thoroughly stirred by a magnetic stirrer for 0.5 h. Concentrated ammonia solution was added to hydrolyze the tin(II) salt and to deposit the Sn(OH)2 on the surface of the palygorskite fibers. The final pH value of the dispersion was adjusted to 6–7. Four hours later, the dispersion was centrifuged, washed with distilled water several times, dried at 80 °C, calcined at 300 °C for 4 h and grounded. Then 11.52 g Ti(OBu)4 was added into Paly-SnO2 powders under milling before 100 mL distilled water was added drop by drop to hydrolyze Ti(OBu)4 and deposit the Ti (hydr) oxide particles on the Paly-SnO2 fibres. After vigorously stirring for 4 h, the dispersion was centrifuged, washed with distilled water, dried at 80 °C, calcined at 300 °C for 4 h and ground (denoted PalySnO2–TiO2). For comparison, TiO2 and SnO2–TiO2 (nSnO2:nTiO2 = 1:5) were prepared by the same method and calcined at 300 °C for 4 h.

before illumination. After defined time intervals, 5 ml of the dispersion was removed and the concentration of phenol determined concentration changes. The chemical oxygen demand (COD) was measured by the closed reflux method employing potassium dichromate as the oxidant under acidic conditions. The unreacted oxidant was determined by titrating with ferrous ammonium sulfate with feroin as redox indicator (Pare et al., 2008). 3. Results and discussion 3.1. Characterization of the Paly-SnO2–TiO2 nanocomposites Fig. 1a shows the XRD patterns of palygorskite calcined for 4 h at 300 °C. The reflections at 2θ = 8.34, 25.42, 42.6 and 54.97 corresponded to palygorskite (Zhao et al., 2006). Montmorillonite and quartz were also found. The characteristic reflections of palygorskite were observed in the XRD patterns of all composites (Fig. 1b–d). In the XRD pattern of the modified palygorskites (Fig. 1b–d), the characteristic reflections of SnO2 and TiO2 as anatase were also observed. Fig. 1e shows the XRD patterns of Play-SnO2–TiO2 after 3 catalytic reactions (denoted as Paly-SnO2–TiO2 (used)), indicating the stability of the composites during the catalytic reaction. The XRD patterns of the SnO2–TiO2 hybrid oxides presented the characteristic reflections of TiO2 anatase (JCPDS 65-5714) and SnO2 (JCPDS 41-1445) and were consistent with the pattern of Paly-SnO2–TiO2. The palygorskite samples consisted of fibers (Fig. 2a), with an average diameter of about 20 nm and a length of 500–2000 nm. The

e d Intensity/a.u.

utilizing bentonite, kaolinite, palygorskite and Fullers earth as adsorbents. Palygorskite, a hydrated magnesium aluminum silicate with lath or fibrous morphology, has reactive –OH groups on the surface (Zhao et al., 2006; Cao et al., 2008). It has a high specific surface area and negative lattice charges, which provide a high absorption capacity. Due to its unique structure and useful textual properties, palygorskite is widely used as adsorbents, adhesives, catalysts and as catalyst supports (Melo et al., 2002; Zhu et al., 2005; Zhao et al., 2006, 2007; Chen and Wang, 2007; Huang et al, 2007; Miao et al., 2007). In this study, palygorskite was used as matrix for hybrid oxide immobilization. SnO2–TiO2 coated palygorskite composites (PalySnO2–TiO2) were prepared by the in situ hydrolysis of SnCl2 and Ti (OBu)4. Photodecomposition of phenol was used as the model system to investigate the photocatalytic activity of the palygorskite composites before and after modification by coating. The recycling properties of the photocatalyst were also studied.

2.3. Photocatalytic activity The photoreactivity of the samples was evaluated by phenol decomposition under UV irradiation using an XPA photochemical reactor with a 300 W high-pressure mercury lamp provided the irradiation. The initial concentration of phenol was adjusted to the range from 20 mg/L to 120 mg/L. The influence of catalyst loading and pH of the phenol solution were also investigated. The reaction cell (400 mL) was bubbled with air at a flow rate of 20 mL/min. The dispersion was stirred for 30 min to obtain adsorption equilibrium

c b a 10

20

30

40

60

50

2θ/deg.

400

2.2. Characterization

f 300

Intensity/a.u.

The crystalline phase was determined with an ARL/X/TRA X-ray diffractometer using Cu Kα radiation. The BET surface area was evaluated by N2 adsorption in a constant volume adsorption apparatus (Coulter SA 3100). The morphology was determined by transmission electron microscopy (TEM) using a JEOL-2100 microscope.

69

200

100

0 10

20

30

40

50

60

70

2θ/deg. Fig. 1. XRD patterns of (a) Paly, (b) Paly-SnO2, (c) Paly-TiO2, (d) Paly-SnO2–TiO2, (e) Paly-SnO2–TiO2 after 3 uses, (f) SnO2–TiO2 (☆Attapulgite ★TiO2 (anatase) ◆SnO2).

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Paly Paly-SnO2 Paly-SnO2-TiO2 Paly-TiO2 TiO2 SnO2-TiO2

Volume Absorbed(ml/g)

250

200

150

Paly

Paly-TiO2

Paly-SnO2-TiO2

100

Paly-SnO2

50

0

TiO2

SnO2-TiO2 0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure(Ps/Po) Fig. 3. N2 adsorption–desorption isotherms of composite catalysts.

Table 1 BET specific surface area of the samples and decomposition of 80 mg/L phenol within 90 min. Sample

Paly (300°C)

150 BET/m2 g−1 Removal rate / of phenol/%

Paly-SnO2– PalyPaly- PalySnO2 SnO2– TiO2 (used) TiO2 TiO2 119 /

147 99.8

135 95.8

TiO2 SnO2– P25 TiO2

202 52 59.7 25.7

98 99.8

51 73.1

palygorskite fibres (Table 1). The BET specific surface area of PalySnO2–TiO2 was higher than that of Paly-SnO2, which may be caused by the accumulation of TiO2 nanoparticles onto the surface of PalySnO2, resulting in the formation of some new quasi pores or poles (Fig. 4). The specific surface area of Paly-SnO2–TiO2 (used) showed a modest decrease after three photocatalytic cycles suggesting sufficient stability. The isotherm profiles of Paly and Paly-SnO2 were classified as type III, the profiles of SnO2–TiO2, Paly-SnO2–TiO2 and Paly-TiO2 as type IV, according to the IUPAC classification (Yang et al., 2002). The as-prepared TiO2 had a distinctly lower specific surface area (52 m2/g), the isotherm showed no obvious hysteresis loop, suggesting that this material did not have pores. 3.2. Photocatalytic activity

Fig. 2. TEM images of (a) Paly (b) Paly-SnO2–TiO2 and (c) HRTEM of Paly-SnO2–TiO2.

surface of Paly-SnO2–TiO2 (Fig. 2b) was fully covered with nanoparticles about 10 nm in average and uniformly distributed, without obvious aggregation. Typical high resolution transmission electron microscopy images (HRTEM) indicated the crystallinity of the oxide particles. The circle zone in Fig. 2c shows the neighboring of TiO2 and SnO2 particles (Zhang et al., 2007). The nitrogen adsorption–desorption isotherms of palygorskite (abbreviated as Paly), Paly-SnO2, Paly-TiO2, SnO2–TiO2 and PalySnO2–TiO2 are shown in Fig. 3, and the BET specific surface areas are listed in Table 1. The surface area changed slightly after coating the

3.2.1. Reaction conditions To determine the optimal reaction conditions, the catalyst content, initial concentration and the pH of the initial phenol solution were varied (Fig. 5). The photodecomposition rate of phenol (40 mg/L) increased with the increasing amount of catalyst up to m ≤ 0.4 g and then remained constant (Fig. 5a). For economic reasons, m = 0.4 g (400 mL) was chosen as the optimal catalyst content. In general, the photodecomposition of phenol decreased with increasing of the initial

SnO2 SnCl2

Paly

Ti(OBu)4

Paly-SnO2

TiO2

Paly-SnO2-TiO2

Fig. 4. Model of the Paly-SnO2–TiO2 hybrid photocatalyst.

L. Zhang et al. / Applied Clay Science 51 (2011) 68–73

1.4

102 100

0min 10min 20min 30min 40min 50min 60min 70min 80min 90min

1.2

98

Absorbance

Photodegradation rate(%) within 60min

a

71

96 94 92 90

1.0 0.8 0.6 0.4

88

0.2

86 0.0

84

250

300

350

0.2

0.3

0.4

0.5

0.6

0.7

550

600

Thus, the photodegradation of experiments with the modified palygorskites was carried out at a catalyst content of 0.4 g/400 mL, initial phenol concentration C = 80 mg/L at pH = 5.5.

110 100 90 80 70 60 50 40 30

20

40

60

80

100

120

Concentration(mg/L)

c 100

80

3.2.2. Photocatalytic activity of the modified palygorskites Fig. 6 shows the variation of the phenol absorbance as a function of irradiation time, in the presence of Paly-SnO2–TiO2. The phenol absorbance clearly decreased with irradiation time, and the plot of maximum absorbance versus irradiation time (Fig. 7) is linear, indicating that the photocatalysis reaction is a first-order reaction. A new absorbance band was not observed in the UV–vis absorption spectrum of phenol, indicating that no intermediate product was formed and the phenol was directly decomposed into CO2 and H2O. The chemical oxygen demand (COD) tests proved this observation. The COD of the original phenol solution of 220 mg/L decreased to 0.21 mg/L after reaction with Paly-SnO2–TiO2 for 90 min, indicating almost complete decomposition of phenol, in agreement with the photospectroscopic measurements. The blank test showed that the original palygorskite did not possess any photocatalytic activity on phenol (Fig. 7). On the contrary, the phenol absorbance increased significantly during the first 40 min to a maximum. This phenomenon may be caused by the metal ions, such as Fe3+ and Al3+, released from the silicate forming complexes with phenol. The solution changed from colorless to faint purple, increasing the phenol absorbance at 270 nm.

60

40

20

0 2

3

4

5

6

7

8

9

10

Intinal pH value of phenol solution

Absorbance

Photodegradation rate(%) within 60min

500

Fig. 6. Time dependent UV–vis absorption spectra of phenol under UV light.

20

Photodegradation rate(%) within 60min

450

0.8

Catalyst loading(g)

b

400

Wabenumber(nm)

82

2.0

2.0

1.8

1.8

1.6

1.6

1.4

1.4

1.2

1.2

1.0

1.0

0.8 0.6

Fig. 5. Phenol degradation as influenced by (a) initial phenol concentration, (b) amounts of catalyst, (c) initial pH of the phenol solution.

0.4 0.2

phenol concentration (Fig. 5b). Since the photodecomposition of phenol reached 99% within 40 min at C = 20–40 mg/L, to be economic and fully exert its photocatalyst ability, C = 80 mg/L was chosen in later experiments. The photodegradation of phenol showed a maximum at pH = 5.5 (Fig. 5c).

0.8

Paly Paly-SnO2 Paly-TiO2 Paly-SnO2-TiO2 P25 TiO2 SnO2-TiO2

0.0 0

20

40

0.6 0.4 0.2 60

80

100

0.0 120

Time/min Fig. 7. Absorbance of phenol solution photocatalyzed by the modified Paly-SnO2–TiO2, TiO2 and P25.

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Paly-SnO2-TiO2

3.0

ln(C0/C)

2.5 2.0 P25 1.5 1.0 TiO2

0.5 0.0

Photodecomposition rate of phenol/%

120 3.5

Paly-SnO2-TiO2

80

60

40

20

0 0

20

40

60

80

P25

100

100

1

2

3

Cycle times

Time/min

Fig. 9. Reproducibility of Att-SnO2–TiO2 and P25 for phenol photodegradation at comparable rates within 90 min for three cycles (catalyst dosage 1 g/L, 80 mg/L phenol).

Fig. 8. ln(C0/C) as a function of irradiation time for Paly-SnO2–TiO2.

The catalytic activity of Paly-SnO2 was similar to that of the original palygorskite. After SnO2 coating, possibly less metal ions were released and ions from the silicate were reduced. Also, the positive surface charges the surface of SnO2 particles improve the absorption of phenol molecules, which may be a further reason that the absorbance curve for Paly-SnO2 was below that of the pure palygorskite. As for Paly-TiO2, the phenol absorbance also increased during the first 10 min. Because the solution pH was bpHzpc ≈ 6.8 [Vargas and Nú˜nez, 2005], the outer surface of the Paly-TiO2 also contains positive charges. Paly-TiO2 presented a larger specific surface area, so that higher amounts of phenol could be adsorbed. Paly-TiO2 anatase showed about 60% decomposition of phenol within 100 min, compared to about 26% with pure TiO2. Paly-SnO2–TiO2 nanocomposites showed excellent photocatalytic activity (Table 1 and Fig. 7). This can be attributed to the anatase structure as well as to the sensitization by coupling with SnO2. The different conduction bands of SnO2 and TiO2 cause efficient separation of the photoinduced electron-hole pairs and high quantum yield (Yang et al, 2002; El-Maghraby et al., 2008). Also, the large specific surface area and good dispersibility may promote the photoactivity. The different micropore structures may also be of influence responsible. SnO2–TiO2 showed a slightly higher activity than that of Paly-SnO2–TiO2 and a distinctly higher activity than TiO2 and P25, due to the effects mentioned above. lnðC0 = C Þ = kapp × t The apparent first-order kinetic equation was used to fit the experimental data (Fig. 8), where C is the concentration of phenol remaining in the solution at time t, and C0 is the initial concentration at t = 0 (Matos et al., 2001). The apparent rate constant kapp of PalySnO2–TiO2, TiO2 and P25 is reported in Table 2. Paly, Paly-TiO2 and Paly-SnO2 did not follow the first-order kinetics. The apparent rate constant of Paly-SnO2–TiO2 was significantly enhanced compared with TiO2 and P25 (Zhao et al., 2007), indicating that the coating of palygorskite with titania–tindioxide enhanced the photoactivity. The photodecomposition of phenol over Paly-SnO2–TiO2 was only slightly reduced after repeated reactions (Fig. 9). Table 2 Apparent rate constants and Regression coefficient (R) data for each sample. Sample

Paly-SnO2–TiO2

TiO2

P25

Apparent rate constant kapp (min−1) Regression coefficient R

0.035 0.9943

0.003 0.9880

0.016 0.993

4. Conclusions SnO2–TiO2 particles were deposited on palygorskite fibres by an in situ sol–gel technique, and the SnO2–TiO2 particles on the surface of palygorskite had an average size about 10 nm without obvious aggregation. Photodecomposition of phenol was used as the model system to investigate the photocatalytic activity of the modified palygorskites. The pure palygorskite was not photocatalytically active. Compared with Paly-TiO2, Paly-SnO2 and Degussa P25, SnO2–TiO2 and Paly-SnO2–TiO2 exhibited much higher photocatalytic activity, reaching 99.8% phenol decomposition within 1.5 h. The COD mensuration was employed to illuminate the overall decomposition of phenol. Experiments proved that the Paly-SnO2–TiO2 composite photocatalyst could be used repeatedly.

Acknowledgments The authors thank the Program for Jiangsu Higher Institutions Key Basic Research Projects of Natural Science (10KJA430005 and 07KJA15012), the Natural Science Foundation of Jiangsu Province (BK2010289) and China (20975043), the Impellent Industrialization Program of College Research Achievement (JH08-31), the College Natural Science Foundation (07KJB430010) of Jiangsu Province, and the Technological Research Foundation of Huai'an City (HAG08072) for the financial support.

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