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TiO2 nanocomposites with enhanced photocatalytic activity
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Colloids and Surfaces A: Physicochem. Eng. Aspects 316 (2008) 110–118
A novel preparation of mesoporous CsxH3−xPW12O40/TiO2 nanocomposites with enhanced photocatalytic activity Xiaodan Yu, Yingna Guo, Leilei Xu, Xia Yang, Yihang Guo ∗ Faculty of Chemistry, Northeast Normal University, Changchun 130024, PR China Received 17 April 2007; received in revised form 12 August 2007; accepted 23 August 2007 Available online 4 September 2007
Abstract Nanocomposite photocatalysts, cesium hydrogen salts of 12-tungstophosphoric acid coupled with anatase TiO2 (Csx H3−x PW12 O40 /TiO2 , x = 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0, respectively), were prepared via one-step sol–gel method followed by hydrothermal treatment at 200 ◦ C with a heating ramp of 2 ◦ C/min. Several characterization techniques, UV–vis diffuse reflectance spectroscopy (UV–vis/DRS), inductively coupled plasma atomic emission spectroscopy (ICP-AES), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) patterns, Raman scattering spectroscopy, field emission scanning electron microscopy (FESEM), and N2 adsorption/desorption analysis, were combined to confirm the structure integrity of the Keggin units in the composites and investigate the phase structure and optical absorption properties, morphology, and surface textural properties of the composites. The photocatalytic activities of Csx H3−x PW12 O40 /TiO2 were evaluated by the degradation of three model organic molecules including 4-nitrophenol (4-NP), methyl orange (MO), and rhodamine B (RB) under UV-light irradiation. For comparison, the photocatalytic activities of the parent Csx H3−x PW12 O40 and Degussa P25 were studied under the same conditions. The results showed that as-prepared nanocomposite photocatalysts were substantially more effective than the starting Csx H3−x PW12 O40 and Degussa P25. The high photoactivities of as-prepared nanocomposites could be attributed to the higher surface acidity, mesoporosity, and the synergistic effect existed between the Csx H3−x PW12 O40 and the TiO2 matrix. © 2007 Elsevier B.V. All rights reserved. Keywords: Heterogeneous photocatalysis; Anatase TiO2 ; Cesium-polyoxotungstate; Sol–gel; Hydrothermal treatment
1. Introduction To remedy the problem of organic pollutants in wastewater, semiconductor photocatalysis is a promising approach and has attracted much attention [1–10]. Among the semiconductors, TiO2 is the most extensively investigated and has been found to be capable of decomposing a wide variety of organic and inorganic pollutants in both liquid and gas phases [11–13]. However, the overall photodegradation efficiency of TiO2 is limited by the high recombination rate of photoinduced electron–hole pairs formed in the photocatalytic process. Attempts to increase the photocatalytic efficiency of TiO2 have been made by doping and coating with transition or noble metals [14], or by modifications with other semiconductors [15,16]. Among various efforts to improve the photocatalytic
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Corresponding author. Tel.: +86 431 85098705; fax: +86 431 85098705. E-mail address: [email protected] (Y. Guo).
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efficiency, it seems most interesting to incorporate polyoxometalate (POM) into the anatase TiO2 framework. For example, our group had reported a kind of nanoporous POM–TiO2 composite (POM = H3 PW12 O40 or H6 P2 W18 O40 /TiO2 ) with the anatase phase structure, which exhibited the visible-light photocatalytic activity to decompose organophosphorus pesticide such as parathion-methyl [17]. The studies indicated that the enhanced photocatalytic efficiency of POM–TiO2 compared with the parent POM and TiO2 matrix depended on a number of properties such as phase structure, surface textural properties, particle sizes, electronic structures, and synergistic effect existed between the POM and TiO2 . In order to further investigate the factors that affect the photoactivity of POM–TiO2 composites, the present work focuses on the cesium hydrogen salts of 12-tungstophosphoric acid (Csx H3−x PW12 O40 , x = 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0) and TiO2 . The acid-catalyzed properties and physicochemical properties of Csx H3−x PW12 O40 have been studied extensively, especially for Cs2.5 H0.5 PW12 O40 which has unique surface textural property and efficient acid cat-
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Scheme 1. Structures of 4-nitrophenol (A); methyl orange (B); and rhodamine B (C).
alytic property [18–20]. Furthermore, Csx H3−x PW12 O40 have been utilized as efficient heterogeneous photocatalysts for photooxidation of organic molecules under UV-light irradiation [16,21,22]. However, it is still difficult to separate them from the reaction system because of their milky dispersion in aqueous solution. The current work demonstrates more efficiently and more easily separated Csx H3−x PW12 O40 included TiO2 composite photocatalysts. The preparation method used is one-step sol–gel technique followed by hydrothermal treatment at 200 ◦ C with a heating ramp of 2 ◦ C/min. The composition and structure, phase structure, morphology, optical absorption properties, and textural properties of the composites are well characterized. The UV-light photocatalytic activities of the composites are evaluated through the photodegradation of three model organic molecules including 4-NP, MO, and RB (Their structures are illustrated in Scheme 1, respectively). Under the same conditions, the parent Csx H3−x PW12 O40 and P25 are also examined. Through the photocatalytic experiments, we find that different mole percents of Cs result in different absorption behaviors and UV-light photocatalytic activities, and Csx H3−x PW12 O40 /TiO2 shows higher photocatalytic activities than those of the corresponding parent Csx H3−x PW12 O40 . Among the six tested Csx H3−x PW12 O40 /TiO2 composites, Cs2.5 H0.5 PW12 O40 /TiO2 exhibits the highest photocatalytic activity. 2. Experimental Csx H3−x PW12 O40 were synthesized according to the literature method [23]. In a typical synthesis process, an aqueous solution of Cs2 CO3 (0.1 mol/l) was added dropwise to an aqueous solution of H3 PW12 O40 (0.08 mol/l) with Cs/P molar ratios of 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0, respectively, at room temperature under vigorous stirring. The resulting white col-
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loidal solution was aged overnight at ambient temperature and then evaporated at 50 ◦ C to dryness. The obtained samples are denoted as Csx H3−x PW12 O40 . A typical method for preparation of Csx H3−x PW12 O40 /TiO2 crystalline particles was described below. Titanium tetraisopropoxide (TTIP, 98%, 3 ml) was dissolved in 20 ml isopropyl alcohol with stirring. In another container, Csx H3−x PW12 O40 (0.3 g) was dissolved in water (0.8 ml), and then was added into TTIP solution drop by drop. The resulting mixture was adjusted to pH 1–2 by addition of 8 mol/l HCl and stirred at room temperature for 1 h. Then, the mixture was heated to 45 ◦ C until homogeneous Csx H3−x PW12 O40 /TiO2 hydrogel was formed. This hydrogel was transferred into an autoclave and heated to 200 ◦ C at a heating ramp of 2 ◦ C/min. Finally, the temperature was kept at 200 ◦ C for 1 h. After cooled to room temperature, the hydrogel was dehydrated slowly at 50 ◦ C in a vacuum for 24 h. The dried gel was washed with hot water three times and dried at room temperature. The products are labeled as Csx H3−x PW12 O40 /TiO2 . Elemental analysis was performed on a Leeman Prodigy Spec ICP-AES. UV–vis/DRS and FT-IR were recorded on a Cary 500 UV–vis–NIR spectrophotometer and a Nicolet Magna 560 IR spectrophotometer, respectively. XRD patterns were obtained on a Rigaku D/max-3c X-ray diffractometer using ˚ radiation. Raman scattering spectra were Cu K␣ (λ = 1.5418 A) recorded by a Jobin-Yvon HR 800 instrument with an Ar+ laser source of 488 nm wavelength in a macroscopic configuration. FESEM images of the composites were recorded using a XL30 ESEM FEG scanning electron microscope. The surface area of the composite catalyst was measured by BET method on an ASAP 2010M surface analyzer (the samples were outguessed under vacuum at 120 ◦ C overnight). The pore size distribution were calculated from the desorption branch of N2 -sorption isotherms using BJH method. The photoreactor was designed with an internal light source (125 W high pressure mercury lamp with main emission wavelength 313 nm) surrounded by a quartz jacket, where the suspension including the solid catalyst (0.2 g) and an aqueous 4-NP or MO/RB (90 ml, 50 mg/l) completely surrounded the light source. The suspension was ultrasonicated for 10 min and stirred in the dark for 30 min to obtain a good dispersion and to establish the adsorption–desorption equilibrium between the organic molecules and the catalyst surface. The temperature of the suspension was maintained at 30 ± 2 ◦ C by circulation water through an external cooling coil, and the system was open to air. The concentrations of the 4-NP in aqueous suspensions were monitored by a Shimadzu LC8A high pressure liquid chromatography (HPLC) equipped with a UV detector (λ = 210 nm). Before HPLC determination, the withdrawn suspensions were centrifuged and filtered with microporous membranes. Decreases in the concentrations of dyes were analyzed by a Cary 500 UV–vis–NIR spectrophotometer at λ = 553 nm for RB and λ = 464 nm for MO, respectively. Changes of total organic carbon (TOC) in reaction systems were carried out on a Shimadzu TOC500 total organic carbon analyzer. At given intervals of illumination, the samples of the reaction solution were taken out, and then centrifuged and filtrated. Finally, the filtrates were analyzed. To investigate
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Table 1 Data of ICP-AES and FT-IR for various catalysts Catalyst
Cs2.5 H0.5 PW12 O40 Cs0.5 H2.5 PW12 O40 /TiO2 Cs1.0 H2.0 PW12 O40 /TiO2 Cs1.5 H1.5 PW12 O40 /TiO2 Cs2.0 H1.0 PW12 O40 /TiO2 Cs2.5 H0.5 PW12 O40 /TiO2 Cs3.0 PW12 O40 /TiO2
Loading (wt%)
– 16.5 18.8 19.1 19.1 19.8 19.0
Molar ratio
Wavenumber (cm−1 )
Cs:P:W
P O
W O
W O W
W O W
2.5:1.1:12.0 0.5:0.9:11.9 1.0:1.1:12.0 1.5:0.9:12.2 2.0:1.1:12.0 2.5:1.2:12.1 3.1:1.0:12.0
1079 1079 1079 1080 1081 1083 1081
983 986 990 986 986 990 986
890 897 890 890 890 891 893
800 824 828 806 799 810 814
the stability of the catalysts, the photocatalytic reaction for each sample was carried out three times.
The determined loadings of Csx H3−x PW12 O40 and molar ratios of Cs/P/W in the Csx H3−x PW12 O40 /TiO2 are listed in Table 1. The determined molar ratios are close to the expected values. However, the loadings of Csx H3−x PW12 O40 are lower than the expected values, suggesting that the non-trapped Cspolyoxotungstates are dropped into water during the process of hot water washing. FT-IR spectra of Cs2.5 H0.5 PW12 O40 and Csx H3−x PW12 O40 /TiO2 are shown in Fig. 1, and the characteristic IR vibration frequencies of the Keggin units are summarized in Table 1. The peaks appeared in the range from 1100 to 700 cm−1 are ascribed to the stretch vibrations of P O, W O, and W O W bonds of the Keggin units, respectively [24], indicating that the primary Keggin structures of these Cs-polyoxotungstates remain intact after formation of the composites. The retention of the Keggin structure in the composites is further confirmed by Raman scattering spectroscopy. As shown
in Fig. 2 and Table 2, the peaks appeared in the range from 900 to 1100 cm−1 are originated from the vibrations of W O W, W O, and P O bonds of the Keggin units [17]. Compared with the starting Keggin units, the shifts of the positions of IR absorption or Raman scattering peaks and the decrease of these peak intensities after formation of Csx H3−x PW12 O40 /TiO2 composites are ascribed to strong chemical interactions between the Keggin units and anatase TiO2 networks. In addition, the four Raman scattering peaks appeared in the range of 148–641 cm−1 are originated from the anatase TiO2 [25], suggesting the formation of the anatase phase structure of the composites under the current preparation method. The phase structure of the composites is further characterized by XRD analysis (Fig. 3). It indicates that Csx H3−x PW12 O40 /TiO2 nanocomposites exhibit the uniform anatase structure with characteristic 2θ values at 25.3◦ (1 0 1), 37.7◦ (0 0 4), 48.0◦ (2 0 0), 56.1◦ (2 1 1), and 62.9◦ (2 0 4), respectively (JCPDS file No. 84-1285). The key step to obtain the Csx H3−x PW12 O40 /TiO2 with an anatase phase structure at a low temperature (200 ◦ C) is to control a constant heating rate during the process of hydrothermal treatment of the Csx H3−x PW12 O40 /TiO2 hydrogel. At this relatively low temperature, thermal decomposition of polyoxotungstates is avoided totally (polyoxotungstates begin to decompose at ca. 350 ◦ C
Fig. 1. IR spectra of as-prepared (a) Cs0.5 H2.5 PW12 O40 /TiO2 ; (b) Cs1.0 H2.0 PW12 O40 /TiO2 ; (c) Cs1.5 H1.5 PW12 O40 /TiO2 ; (d) Cs2.0 H1.0 PW12 O40 /TiO2 ; (e) Cs2.5 H0.5 PW12 O40 /TiO2 ; (f) Cs3.0 PW12 O40 /TiO2 ; and (g) Cs2.5 H0.5 PW12 O40 .
Fig. 2. Raman scattering spectra of as-prepared TiO2 , Csx H3−x PW12 O40 /TiO2 , and the parent Cs2.5 H0.5 PW12 O40 .
3. Results and discussion 3.1. Photocatalyst characterizations
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Table 2 Raman shifts of Cs2.5 H0.5 PW12 O40 , TiO2 , and Csx H3−x PW12 O40 /TiO2 Sample
Raman shift (cm−1 )
Cs2.5 H0.5 PW12 O40 As-prepared TiO2 Cs0.5 H2.5 PW12 O40 /TiO2 Cs1.0 H2.0 PW12 O40 /TiO2 Cs1.5 H1.5 PW12 O40 /TiO2 Cs2.0 H1.0 PW12 O40 /TiO2 Cs2.5 H0.5 PW12 O40 /TiO2 Cs3.0 PW12 O40 /TiO2
539 (ta ) 148 (Eg b ) 149 (Eg ) 149 (Eg ) 149 (Eg ) 150 (Eg ) 150 (Eg ) 151 (Eg )
a b
906 (t) 396 (B1g b ) 402 (B1g ) 400 (B1g ) 402 (B1g ) 399 (B1g ) 401 (B1g ) 397 (B1g )
996 (t) 515 (A1g b ) 517 (A1g ) 519 (A1g ) 517 (A1g ) 518 (A1g ) 517 (A1g ) 519 (A1g )
1010 (t) 640 (Eg b ) 634 (Eg ) 640 (Eg ) 641 (Eg ) 640 (Eg ) 640 (Eg ) 636 (Eg )
995 (t) 997 (t) 995 (t) 994 (t) 995 (t) 907 (t)
1006 (t) 1006 (t) 1005 (t) 1005 (t) 1009 (t) 993 (t)
1009 (t)
Fundamental Raman vibration modes of W O, P O, and W O W bonds in Keggin units of Csx H3−x PW12 O40 . Eg , B1g , A1g , and Eg : fundamental vibration modes of Ti O and Ti O Ti bonds in anatase TiO2 .
Fig. 3. XRD patterns of as-prepared (a) anatase TiO2 ; (b) Cs0.5 H2.5 PW12 O40 /TiO2 ; (c) Cs1.0 H2.0 PW12 O40 /TiO2 ; (d) Cs1.5 H1.5 PW12 O40 /TiO2 ; (e) Cs2.0 H1.0 PW12 O40 /TiO2 ; (f) Cs2.5 H0.5 PW12 O40 /TiO2 ; and (g) Cs3.0 PW12 O40 /TiO2 nanocomposites.
[26], while the temperature for calcination of amorphous TiO2 to its anatase structure is at ca. 450 ◦ C [24]). Diffraction peaks appeared at 10.5◦ , 30.3◦ , and 35.5◦ are originated from the parent Csx H3−x PW12 O40 (JCPDS File No. 76-1815). UV–vis/DRS of the parent Cs2.5 H0.5 PW12 O40 , as-prepared TiO2 , Degussa P25, and Csx H3−x PW12 O40 /TiO2 are shown in Fig. 4. The parent Cs2.5 H0.5 PW12 O40 exhibits two absorption peaks at 190 and 260 nm, respectively, attributed to charge trans-
Fig. 4. UV–vis/DRS of P25, as-prepared TiO2 , the parent Cs2.5 H2.5 PW12 O40 , and Csx H3−x PW12 O40 /TiO2 . The absorbance values were normalized.
fer (CT) from O 2p to W 5d of the Keggin units at the W O and W O W bonds. As for the TiO2 , the absorption maximum is at 245 nm, corresponding to CT from O 2p to Ti 3d of anatase TiO2 [27]. The Csx H3−x PW12 O40 /TiO2 nanocomposites show strong and broad optical absorption in the range from 200 to 420 nm, and the red shifts are observed compared with the parent Cs2.5 H0.5 PW12 O40 , TiO2 , and P25. With respect to Csx H3−x PW12 O40 /TiO2 , the red shifts of the absorption band are apparently different with various contents of Cs. The above UV–vis/DRS results indicate that introduction of Csx H3−x PW12 O40 into TiO2 framework has an influence on coordination environment of TiO2 crystalline, resulting in the red shifts of the absorption band [28]. FESEM images of Csx H3−x PW12 O40 /TiO2 in Fig. 5 reveal that as-prepared composites consist of very small and uniform spherical particles with sizes from 16 to 20 nm. In addition, the aggregation among the particles is observed. Formation of nanometer-sized composites is closely related to the preparation method used here. The representative N2 absorption/desorption isotherm and BJH pore-size distribution plot of the Csx H3−x PW12 O40 /TiO2 are shown in Fig. 6. According to IUPAC definition, the isotherm belongs to Type IV and shows a steep hysteresis loop of type H1 at a relatively high pressure (p/p0 = 0.5–0.8), suggesting that the composite exhibits the mesoporosity. The average pore size of the composite is 5.1 nm with a narrow size distribution (Fig. 6B), implying that the pore sizes are uniform. The determined BET surface area of Cs2.5 H0.5 PW12 O40 /TiO2 is 163 m2 g−1 , higher than that of the parent Cs2.5 H0.5 PW12 O40 (135 m2 g−1 ) [29] or Degussa P25 (50 m2 g−1 ). From the above results, we conclude that the primary Keggin structure remains intact after formation of the Csx H3−x PW12 O40 /TiO2 composites. Compared with the parent Keggin unit, changes of Raman shifts, wavenumbers, and OMCT bands indicate that chemical interactions between the Keggin anions and anatase TiO2 matrix exist in the composites. These interactions are hydrogen bonding and acid basic. On the one hand, hydrogen bonding is formed between the oxygen atoms of Keggin anions and the surface hydroxyl groups (Ti OH) of anatase networks, i.e., W O· · ·HO Ti and W O· · ·HO Ti [15]. On the other hand, chemically active surface Ti OH groups are protonated in an acidic medium to form the Ti OH2 + group. The Ti OH2 + group will act as a counter
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Fig. 5. FESEM images of (A) Cs0.5 H2.5 PW12 O40 /TiO2 ; (B) Cs1.0 H2.0 PW12 O40 /TiO2 ; (C) Cs1.5 H1.5 PW12 O40 /TiO2 ; (D) Cs2.0 H1.0 PW12 O40 /TiO2 ; (E) Cs2.5 H0.5 PW12 O40 /TiO2 ; and (F) Cs3.0 PW12 O40 /TiO2 .
ion for Keggin units and yield (Ti OH2 + )(Csx H2−x PW12 O40 − ) by an acid–base reaction. Therefore, the Csx H3−x PW12 O40 particles are firmly immobilized into the interior of anatase TiO2 network.
photocatalytic tests were carried out in an aqueous solution containing oxygen (dissolved in the reaction system from air). Changes of the concentrations of 4-NP are shown in Fig. 7, and the results are summarized as follows. On stirring the sus-
3.2. Photocatalysis studies The photocatalytic activity of as-prepared Csx H3−x PW12 O40 /TiO2 was evaluated by the degradation of an aqueous 4-NP under UV-light irradiation firstly, and the starting Csx H3−x PW12 O40 and P25 were also tested for comparison. 4-NP, chosen as a probe molecule to carry out the photodegradation experiments, is one of the most refractory pollutants in industrial wastewater. Its high stability and solubility in water are the main reasons why the degradation of this compound to non-dangerous levels is a very difficult task. The present
Fig. 6. (A) N2 adsorption–desorption isotherm and (B) pore-size distribution plot on the basis of the desorption branch of the isotherm of Cs2.5 H0.5 PW12 O40 /TiO2 .
Fig. 7. Photocatalytic degradation of an aqueous 4-NP (50 mg/l, 90 ml) on (A) the parent Csx H3−x PW12 O40 (0.2g) and (B) Csx H3−x PW12 O40 /TiO2 (0.2 g) under UV-light excitation.
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Fig. 8. UV–vis absorption spectra of dyes (RB and MO) after stirring the suspension in the dark for 30 min. 0.2 g catalyst was used. (A) RB (50 mg/l, 90 ml) + Csx H3−x PW12 O40 , (B) RB (50 mg/l, 90 ml) + Csx H3−x PW12 O40 /TiO2 , (C) MO (50 mg/l, 90 ml) + Csx H3−x PW12 O40 , (D) MO (50 mg/l, 90 ml) + Csx H3−x PW12 O40 /TiO2 .
pension of an aqueous 4-NP solution (50 mg/l, 90 ml) and the catalyst powder (0.2 g) in the dark for 250 min, the disappearance of 4-NP is negligible. In the absence of the catalyst, the changes of 4-NP concentrations are hardly observed. However, obvious degradation of 4-NP is founded under the simultaneous presence of the photocatalyst and UV-light irradiation. The results in Fig. 7 indicate the photocatalytic activity of the four catalysts towards the degradation of 4-NP. When the starting Csx H3−x PW12 O40 or P25 is used as the catalyst, the milky dispersion in an aqueous solution makes its separating and recycling difficult. As for the Csx H3−x PW12 O40 /TiO2 , its performance is superior to that of the parent Csx H3−x PW12 O40 or P25. For example, it only took 90 min UV-light irradiation of Cs2.5 H0.5 PW12 O40 /TiO2 to destruct 4-NP completely (Fig. 7B), whereas the conversion of 4-NP only reached 35 and 45%, respectively, under the same conditions by using Cs2.5 H0.5 PW12 O40 and P25 as the catalysts (Fig. 7A and B). With respect to Csx H3−x PW12 O40 /TiO2 or the starting Csx H3−x PW12 O40 , apparent difference in photoactivity is observed with various contents of Cs. The highest photoactivity is obtained by Cs2.5 H0.5 PW12 O40 and Cs2.5 H0.5 PW12 O40 /TiO2 . The photocatalytic activities of Csx H3−x PW12 O40 /TiO2 are further studied by dye MO and RB degradation, at the same time, the absorption behaviors of dye molecules on the start-
ing Csx H3−x PW12 O40 and Csx H3−x PW12 O40 /TiO2 are also investigated. All the experimental conditions are the same as those of 4-NP degradation, and the results are shown in Figs. 8–10. The absorption behaviors of dye molecules on the starting Csx H3−x PW12 O40 and Csx H3−x PW12 O40 /TiO2 are complex, which are related to the structures of the absorbed dye molecules, BET surface areas, and surface acidities of Csx H3−x PW12 O40 or Csx H3−x PW12 O40 /TiO2 . At first, adsorption of RB and MO on the surface of the Csx H3−x PW12 O40 or Csx H3−x PW12 O40 /TiO2 is physical and hydrogen bonding. The hydrogen bonding between RB and Csx H3−x PW12 O40 is formed by the interactions of hydroxyl groups of RB and protons of Csx H3−x PW12 O40 , while the hydrogen bonding between MO and Csx H3−x PW12 O40 is formed by the interactions of N or S atoms of MO and protons of Csx H3−x PW12 O40 . From Fig. 8 it can be seen that for all tested photocatalysts (Csx H3−x PW12 O40 and Csx H3−x PW12 O40 /TiO2 ), their order of adsorption ability to RB or MO is the same, i.e., Cs3.0 PW12 O40 > Cs2.5 H0.5 PW12 O40 > Cs0.5 H2.5 PW12 O40 > Cs1.0 H2.0 PW12 O40 > Cs1.5 H1.5 PW12 O40 > Cs2.0 H1.0 PW12 O40 . The results imply that the physical adsorption is determined by the BET surface areas of Csx H3−x PW12 O40 or Csx H3−x PW12 O40 /TiO2 . It has been reported that the surface
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area of Csx H3−x PW12 O40 monotonically decreased as the Cs content (x) increased from x = 0 to 2, whereas, the surface area sharply increased as the Cs content exceeded 2 [29]. Secondly, stronger adsorption of MO on Csx H3−x PW12 O40 is observed (Fig. 8C) compared with that of RB on Csx H3−x PW12 O40 (Fig. 8A), which results in obvious shifts of MO chromophore. Finally, the adsorption ability of Csx H3−x PW12 O40 /TiO2 is not as strong as their corresponding starting Csx H3−x PW12 O40 (Fig. 8B and D). This is due to losing of some of surface acid sites after formation of Csx H3−x PW12 O40 /TiO2 , leading to weakening of the hydrogen bonding adsorption of the composites to dye molecules. The blank experiments show that degradation of MO or RB is negligible by direct photolysis for 180 min, indicating that the sensitized photocatalysis has not occurred in current dye/Csx H3−x PW12 O40 /TiO2 /UV light system. In Figs. 9 and 10, the photocatalytic activity of Csx H3−x PW12 O40 /TiO2 to decompose MO or RB is the highest among three tested catalysts (P25, Csx H3−x PW12 O40 , and Csx H3−x PW12 O40 /TiO2 ) under UV-light irradiation, and activity of the Cs2.5 H0.5 PW12 O40 /TiO2 is the highest among Csx H3−x PW12 O40 /TiO2 . From the above photocatalytic tests we conclude that the photocatalytic activities of Csx H3−x PW12 O40 or Csx H3−x PW12 O40 /TiO2 for degrading model molecules (4-NP, MO, and RB) are also related to Cs content in the compounds or composites. The order of photocatalytic activity is Cs2.5 H0.5 PW12 O40 > Cs0.5 H2.5 PW12 O40 > Cs1.0 H2.0 PW12 O40 > Cs1.5 H1.5 PW12 O40 > Cs2.0 H1.0 PW12 O40 > Cs3.0 PW12 O40 . On the other hand, Csx H3−x PW12 O40 /TiO2 composites show higher photocatalytic activity than that of the starting Csx H3−x PW12 O40 or P25 under UV-light irradiation. Moreover, higher apparent reaction rate constant can be obtained for the photocatalytic reactions in the Csx H3−x PW12 O40 /TiO2 system (Table 3). The three tested photocatalysts (P25, the parent Cs2.5 H0.5 PW12 O40 , and Cs2.5 H0.5 PW12 O40 /TiO2 ) have extremely different mineralization ability to the above organic pollutes. During the course of photocatalytic degradation of three target molecules, temporal changes of TOC are monitored, and the results are shown in Fig. 11. It is observed that the mineralization ability of Cs2.5 H0.5 PW12 O40 /TiO2 is
Fig. 9. Photocatalytic degradation of MO (50 mg/l, 90 ml) on (A) the parent Csx H3−x PW12 O40 (0.2 g); and (B) Csx H3−x PW12 O40 /TiO2 (0.2 g) under UVlight excitation.
the strongest and the parent Cs2.5 H0.5 PW12 O40 is the weakest among three tested catalysts. The results imply that the photocatalytic activity of anatase Csx H3−x PW12 O40 /TiO2 is higher than that of the parent Csx H3−x PW12 O40 or P25, although all of the tested catalysts exhibit high activity to degrade 4-NP or dyes. The target molecules can be totally decomposed by Cs2.5 H0.5 PW12 O40 /TiO2 under UV-light irradiation for a
Table 3 UV-light photocatalytic activity of different photocatalysts for 4-NP, RB, and MO degradation Catalyst
Cs0.5 H2.5 PW12 O40 /TiO2 Cs1.0 H2.0 PW12 O40 /TiO2 Cs1.5 H1.5 PW12 O40 /TiO2 Cs2.0 H1.0 PW12 O40 /TiO2 Cs2.5 H0.5 PW12 O40 /TiO2 Cs3.0 PW12 O40 /TiO2 P25
Kapp (min−1 ) 4-NP
RB
MO
0.032 0.024 0.019 0.018 0.044 0.016 0.010
0.041 0.034 0.029 0.027 0.058 0.025 0.009
0.04 0.036 0.031 0.029 0.054 0.023 0.007
Initial concentration of 4-NP, RB, or MO 50 mg/l; the system opened to air; the temperature of the suspension 30 ± 2 ◦ C; catalyst 0.2 g; UV light (125 W HPML) irradiation time 90 min for 4-NP, and 60 min for RB or MO.
Fig. 10. Photocatalytic degradation of RB (50 mg/l, 90 ml) over P25 (0.2 g), and Csx H3−x PW12 O40 /TiO2 (0.2 g) under UV-light excitation.
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improve the activity of photocatalysts. Higher surface acidities imply that Csx H3−x PW12 O40 or Csx H3−x PW12 O40 /TiO2 can trap more photogenerated elections on the surface of TiO2 , so that recombination of photogenerated electrons and holes may be weakened. As for the parent Csx H3−x PW12 O40 , the number of surface protons decrease as the Cs content increases from x = 0 to 2.0, but greatly increase when x is higher than 2.0 and show the highest surface acidity at x = 2.5 [31]. In addition, although the BET surface area of Cs3.0 PW12 O40 is the largest among six tested Csx H3−x PW12 O40 , its photocatalytic activity is the lowest since it has no any surface proton. However, Csx H3−x PW12 O40 /TiO2 has significantly higher photocatalytic activities than the parent Csx H3−x PW12 O40 , which is attributed to their mesoporosity and synergetic effect. The mesoporous structure of Csx H3−x PW12 O40 /TiO2 can make the diffusion of the reactants into the mesopores more rapidly. Although Cs2.5 H0.5 PW12 O40 also exhibits mesoporosity, its photocatalytic activity is much lower than that of Cs2.5 H0.5 PW12 O40 /TiO2 . This is because of the synergetic effect existing between the starting POM and TiO2 , which has been studied by us and other groups [17,32,33]. That is, POM has empty d orbits and can be used as good electron acceptors. In the POM–TiO2 photocatalytic systems, the fast photogenerated hole-electron (h+ –e− ) recombination on the surface of TiO2 can be retarded effectively by accepting e− to POM’s empty d orbits. Accordingly, the quantum efficiency of the photocatalysts is enhanced. After the reaction, it is easier to separate Csx H3−x PW12 O40 /TiO2 than the parent Csx H3−x PW12 O40 or P25 for recycling applications. The recovered catalysts are analyzed by ICP-AES and UV–vis/DRS. The loadings of Csx H3−x PW12 O40 /TiO2 maintain basically unchanged, and the structures of the catalysts are not destroyed. In addition, deactivation of the catalysts is hardly observed after three catalytic runs. 4. Conclusions
Fig. 11. Changes in TOC removal from (A) 4-NP, (B) MO, and (C) RB aqueous solutions (50 mg/l, 90 ml) with the parent Cs2.5 H0.5 PW12 O40 (), P25 (), and Cs2.5 H0.5 PW12 O40 /TiO2 (䊉) under UV-light irradiation. 0.2 g catalyst was used.
suitable period of time. However, the parent Cs2.5 H0.5 PW12 O40 or P25 can only destroy the chromophores of dyes or degrade 4-NP to other organics, and it is difficult for them to mineralize the target molecules completely. The above photocatalytic tests indicate that both the parent Csx H3−x PW12 O40 and Csx H3−x PW12 O40 /TiO2 exhibit activity to 4-NP and RB/MO degradation under UV-light irradiation. Moreover, their photocatalytic activities increase with the increase of the BET surface areas and surface acidities of the composites. Larger BET surface areas result in larger contact areas between the active sites (W O W and Ti O Ti bonds) and the target substrate [30], which is a crucial factor to
The composite photocatalysts, Csx H3−x PW12 O40 /TiO2 with different Cs contents, were prepared by a one-step sol–gel technique followed by hydrothermal treatment at 200 ◦ C with a heating ramp of 2 ◦ C/min. The materials exhibited anatase crystalline phase structure, nanometer sizes, mesoporosity, and relatively narrow band gap. The primary Keggin structure remained intact after formation of the composites. The photocatalytic activity of Csx H3−x PW12 O40 /TiO2 were higher than that of the parent Csx H3−x PW12 O40 or P25 towards decomposition of 4-NP, MO and RB in aqueous suspensions under UV-light irradiation, and Cs2.5 H0.5 PW12 O40 /TiO2 was the most active among six tested Csx H3−x PW12 O40 /TiO2 composites. Furthermore, Csx H3−x PW12 O40 /TiO2 could be separated and recovered more easily. Leakage of Csx H3−x PW12 O40 from the support and deactivation of the catalysts were hardly observed after three catalytic runs, attributed to strong interactions between the Keggin units and the TiO2 support.
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Acknowledgements This work is supported by the Program of New Century Excellent Talents in University of China (NCET-04-0311). The work is also supported by Analysis and Testing Foundation of Northeast Normal University. References [1] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental applications of semiconductor photocatalysis, Chem. Rev. 95 (1995) 69–96. [2] J.L. Mozos, J. Fraxedas, G.A. Battiston, R. Gerbasi, Designed self-soped titanium oxide thin films for efficient visible-light photocatalysis, Adv. Mater. 14 (2002) 1399–1402. [3] S. Rodrigues, K. Ranjit, S. Uma, I. Martyanov, Single-step synthesis of a highly active visible-light photocatalyst for oxidation of a common indoor air pollutant: acetaldehyde, Adv. Mater. 17 (2005) 2467–2471. [4] H. Krysova, J. Jirkovsky, J. Krysa, G. Maillhot, Metal carbides and nitrides as potential catalysts for hydroprocessing, Appl. Catal. B 40 (2003) 1–28. [5] H. Luo, T. Takata, Y. Lee, J. Zhao, K. Domen, Y. Yan, Photocatalytic acticity enhancing for titanium dioxide by Co-doping with bromine and chlorine, Chem. Mater. 16 (2004) 846–849. [6] D. Li, H. Haneda, S. Hishita, N. Ohashi, Visible-light-driven N-F-codoped TiO2 photocatalysts. 2. Optical characterization, photocatalysis, and potential application to air purification, Chem. Mater. 17 (2005) 2596–2602. [7] S. Park, H. Park, K. Yoo, J. Lee, J. Lee, Photocatalytic recovery of Ag ions from wastewater using ZnO nanopowders immobilized on microporous alumina substrates, Colloids Surf. A: Physicochem. Eng. Aspects 300 (2007) 30–34. [8] M.H. Priya, G. Madras, Photocatalytic degradation of nitrobenzenes with combustion synthesized nano-TiO2 , J. Photochem. Photobiol. A: Chem. 178 (2006) 1–7. [9] C. Karanakaran, R. Dhanalakshmi, S. Karuthapandian, Inhibition of photooxidation of iron(II) by some semiconductors, J. Photochem. Photobiol. A: Chem. 170 (2005) 233–238. [10] Y. Kurada, T. Mori, K. Yagi, N. Makihata, Y. Kawahara, Preparation of visible-light-responsive TiO2−x Nx photocatalyst by a sol–gel method: analysis of the active center on TiO2 that reacts with NH3 , Langmiur 21 (2005) 8026–8034. [11] M.R. Prairie, L.R. Evans, B.M. Stange, An investigation of titanium dioxide photocatalysis for the treatment of water contaminated with metals and organic chemicals, Environ. Sci. Technol. 27 (1993) 1776–1782. [12] J. Herrmann, Heterogeneous photocatalysis: fundamentals and applications to the removal of various types of aqueous pollutants, Catal. Today 53 (1999) 115–129. [13] O.M. Alfano, D. Bahnemann, R. Dillert, R. Goslich, Photocatalysis in water environments using artificial and solar light, Catal. Today 58 (2000) 199–230. [14] M.I. Litter, J.A. Navio, J. Photochem, Photocatalytic properties of irondoped titania semiconductors, Photobiol. A: Chem. 98 (1996) 171–181. [15] M. Yoon, J.A. Chang, Y. Kim, J.R. Choi, Heteropoly acid-incorporated TiO2 colloids as novel photocatalytic systems resembling the photosynthetic reaction center, J. Phys. Chem. B 105 (2001) 2539–2545.
[16] D.A. Friesen, L. Morello, J.V. Headleg, C.H. Langford, Factors influencing relative efficiency in photo-oxidations of organic molecules by Cs3 PW12 O40 and TiO2 colloidal photocatalysts, J. Photochem. Photobiol. A: Chem. 133 (2000) 213–220. [17] L. Li, Q. Wu, Y. Guo, C. Hu, Nanosize and bimodal porous polyoxotungstate–anatase TiO2 composites: preparation and photocatalytic degradation of organophosphorus pesticide using visible-light excitation, Microporous Mesoporous Mater. 87 (2005) 1–9. [18] Y. Izumi, M. Ono, M. Kitagawa, Silica-included heteropoly compounds as solid acid catalysts, Microporous Mater. 5 (1995) 255–262. [19] Y. Izumi, M. Ono, M. Ogawa, Acidic cesium salts of Keggin-type heteropolytungstic acids as insoluble solid acid catalysts for estenfication and hydrolysis reactions, Chem. Lett. 22 (1993) 825–826. [20] Y. Izumi, M. Ogawa, K. Urabe, Alkali metal salts and ammonium salts of Keggin-type heteropolyacids as solid acid catalysts for liquid-phase Friedel-Crafts reactions, Appl. Catal. A 132 (1995) 127–140. [21] D.A. Friesen, D.B. Gibson, C.H. Langford, Heterogeneous Cs3 PW12 O40 photocatalysts, Chem. Commun. (1998) 543. [22] D.A. Friesen, D.B. Gibson, C.H. Langford, The photooxidative degradation of N-methylpyrrolidinone in the presence of Cs3 PW12 O40 and TiO2 colloid photocatalysts, Environ. Sci. Tech. 33 (1999) 3193–3198. [23] Z. Ma, W. Hua, Y. Ren, H. He, Z. Gao, n-Butane isomerization over Cssalts of H3 PW12 O40 : a mechanistic study by 1 C MAS NMR, Appl. Catal. A. 256 (2003) 243–250. [24] S.J. Gregg, W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, 1982. [25] Y. Guo, Y. Yang, C. Hu, C. Guo, E. Wang, Y. Zhou, S. Feng, Preparation, characterization and photochemical properties of ordered macroporous hybrid silica materials based on monovacant Keggin-type polyoxometalates, J. Mater. Chem. 12 (2002) 3046. [26] L. Miao, S. Tanemura, S. Toh, Fabrication, characterization and Raman study of anatase-TiO2 nanorods by a heating-sol–gel template process, J. Crystal Growth 264 (2004) 246–252. [27] X. Jiang, T. Herricks, Y. Xia, Monodispersed spherical colloids of titania: synthesis, characterization, and crystallization, Adv. Mater. 15 (2003) 1205–1209. [28] Y. Yang, Q. Wuc, Y. Guoa, C. Hu, Efficient degradation of dye pollutants on nanoporous polyoxotungstate–anatase composite under visible-lightirradiation, J. Mol. Catal. A 225 (2005) 203–212. [29] T. Okuhara, H. Watanabe, T. Nishimura, Microstructure of cesium hydrogen salts of 12-tungstophosphoric acid relevant to novel acid catalysis, Chem. Mater. 12 (2000) 2230–2238. [30] N. Mizuno, M. Misono, Heterogeneous catalysis, Chem. Rev. 98 (1998) 199–218. [31] T. Okuhara, T. Nishimura, H. Watanabe, K. Na, M. Misono, Acid–Base Catalysis II, Kodansha/Elsevier, Tokyo/Amsterdam, 1994, p. 419. [32] Y. Yang, Y. Guo, C. Hu, C. Jiang, E. Wang, Synergistic effect of Keggintype [Xn+ W11 O39 ](12−n)− and TiO2 in macroporous hybrid materials [Xn+ W11 O39 ](12−n)− –TiO2 for the photocatalytic degradation of textile dyes, J. Mater. Chem. 13 (2003) 1684–1694. [33] G. Col´on, M.C. Hidalgo, M. Mac´ıas, J.A. Nav´ıo, Enhancement of TiO2 /C photocatalytic activity by sulfate promotion, Appl. Catal. A: Gen. 259 (2004) 235–243.
Colloids and Surfaces A: Physicochem. Eng. Aspects 316 (2008) 110–118
A novel preparation of mesoporous CsxH3−xPW12O40/TiO2 nanocomposites with enhanced photocatalytic activity Xiaodan Yu, Yingna Guo, Leilei Xu, Xia Yang, Yihang Guo ∗ Faculty of Chemistry, Northeast Normal University, Changchun 130024, PR China Received 17 April 2007; received in revised form 12 August 2007; accepted 23 August 2007 Available online 4 September 2007
Abstract Nanocomposite photocatalysts, cesium hydrogen salts of 12-tungstophosphoric acid coupled with anatase TiO2 (Csx H3−x PW12 O40 /TiO2 , x = 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0, respectively), were prepared via one-step sol–gel method followed by hydrothermal treatment at 200 ◦ C with a heating ramp of 2 ◦ C/min. Several characterization techniques, UV–vis diffuse reflectance spectroscopy (UV–vis/DRS), inductively coupled plasma atomic emission spectroscopy (ICP-AES), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) patterns, Raman scattering spectroscopy, field emission scanning electron microscopy (FESEM), and N2 adsorption/desorption analysis, were combined to confirm the structure integrity of the Keggin units in the composites and investigate the phase structure and optical absorption properties, morphology, and surface textural properties of the composites. The photocatalytic activities of Csx H3−x PW12 O40 /TiO2 were evaluated by the degradation of three model organic molecules including 4-nitrophenol (4-NP), methyl orange (MO), and rhodamine B (RB) under UV-light irradiation. For comparison, the photocatalytic activities of the parent Csx H3−x PW12 O40 and Degussa P25 were studied under the same conditions. The results showed that as-prepared nanocomposite photocatalysts were substantially more effective than the starting Csx H3−x PW12 O40 and Degussa P25. The high photoactivities of as-prepared nanocomposites could be attributed to the higher surface acidity, mesoporosity, and the synergistic effect existed between the Csx H3−x PW12 O40 and the TiO2 matrix. © 2007 Elsevier B.V. All rights reserved. Keywords: Heterogeneous photocatalysis; Anatase TiO2 ; Cesium-polyoxotungstate; Sol–gel; Hydrothermal treatment
1. Introduction To remedy the problem of organic pollutants in wastewater, semiconductor photocatalysis is a promising approach and has attracted much attention [1–10]. Among the semiconductors, TiO2 is the most extensively investigated and has been found to be capable of decomposing a wide variety of organic and inorganic pollutants in both liquid and gas phases [11–13]. However, the overall photodegradation efficiency of TiO2 is limited by the high recombination rate of photoinduced electron–hole pairs formed in the photocatalytic process. Attempts to increase the photocatalytic efficiency of TiO2 have been made by doping and coating with transition or noble metals [14], or by modifications with other semiconductors [15,16]. Among various efforts to improve the photocatalytic
∗
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efficiency, it seems most interesting to incorporate polyoxometalate (POM) into the anatase TiO2 framework. For example, our group had reported a kind of nanoporous POM–TiO2 composite (POM = H3 PW12 O40 or H6 P2 W18 O40 /TiO2 ) with the anatase phase structure, which exhibited the visible-light photocatalytic activity to decompose organophosphorus pesticide such as parathion-methyl [17]. The studies indicated that the enhanced photocatalytic efficiency of POM–TiO2 compared with the parent POM and TiO2 matrix depended on a number of properties such as phase structure, surface textural properties, particle sizes, electronic structures, and synergistic effect existed between the POM and TiO2 . In order to further investigate the factors that affect the photoactivity of POM–TiO2 composites, the present work focuses on the cesium hydrogen salts of 12-tungstophosphoric acid (Csx H3−x PW12 O40 , x = 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0) and TiO2 . The acid-catalyzed properties and physicochemical properties of Csx H3−x PW12 O40 have been studied extensively, especially for Cs2.5 H0.5 PW12 O40 which has unique surface textural property and efficient acid cat-
X. Yu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 316 (2008) 110–118
Scheme 1. Structures of 4-nitrophenol (A); methyl orange (B); and rhodamine B (C).
alytic property [18–20]. Furthermore, Csx H3−x PW12 O40 have been utilized as efficient heterogeneous photocatalysts for photooxidation of organic molecules under UV-light irradiation [16,21,22]. However, it is still difficult to separate them from the reaction system because of their milky dispersion in aqueous solution. The current work demonstrates more efficiently and more easily separated Csx H3−x PW12 O40 included TiO2 composite photocatalysts. The preparation method used is one-step sol–gel technique followed by hydrothermal treatment at 200 ◦ C with a heating ramp of 2 ◦ C/min. The composition and structure, phase structure, morphology, optical absorption properties, and textural properties of the composites are well characterized. The UV-light photocatalytic activities of the composites are evaluated through the photodegradation of three model organic molecules including 4-NP, MO, and RB (Their structures are illustrated in Scheme 1, respectively). Under the same conditions, the parent Csx H3−x PW12 O40 and P25 are also examined. Through the photocatalytic experiments, we find that different mole percents of Cs result in different absorption behaviors and UV-light photocatalytic activities, and Csx H3−x PW12 O40 /TiO2 shows higher photocatalytic activities than those of the corresponding parent Csx H3−x PW12 O40 . Among the six tested Csx H3−x PW12 O40 /TiO2 composites, Cs2.5 H0.5 PW12 O40 /TiO2 exhibits the highest photocatalytic activity. 2. Experimental Csx H3−x PW12 O40 were synthesized according to the literature method [23]. In a typical synthesis process, an aqueous solution of Cs2 CO3 (0.1 mol/l) was added dropwise to an aqueous solution of H3 PW12 O40 (0.08 mol/l) with Cs/P molar ratios of 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0, respectively, at room temperature under vigorous stirring. The resulting white col-
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loidal solution was aged overnight at ambient temperature and then evaporated at 50 ◦ C to dryness. The obtained samples are denoted as Csx H3−x PW12 O40 . A typical method for preparation of Csx H3−x PW12 O40 /TiO2 crystalline particles was described below. Titanium tetraisopropoxide (TTIP, 98%, 3 ml) was dissolved in 20 ml isopropyl alcohol with stirring. In another container, Csx H3−x PW12 O40 (0.3 g) was dissolved in water (0.8 ml), and then was added into TTIP solution drop by drop. The resulting mixture was adjusted to pH 1–2 by addition of 8 mol/l HCl and stirred at room temperature for 1 h. Then, the mixture was heated to 45 ◦ C until homogeneous Csx H3−x PW12 O40 /TiO2 hydrogel was formed. This hydrogel was transferred into an autoclave and heated to 200 ◦ C at a heating ramp of 2 ◦ C/min. Finally, the temperature was kept at 200 ◦ C for 1 h. After cooled to room temperature, the hydrogel was dehydrated slowly at 50 ◦ C in a vacuum for 24 h. The dried gel was washed with hot water three times and dried at room temperature. The products are labeled as Csx H3−x PW12 O40 /TiO2 . Elemental analysis was performed on a Leeman Prodigy Spec ICP-AES. UV–vis/DRS and FT-IR were recorded on a Cary 500 UV–vis–NIR spectrophotometer and a Nicolet Magna 560 IR spectrophotometer, respectively. XRD patterns were obtained on a Rigaku D/max-3c X-ray diffractometer using ˚ radiation. Raman scattering spectra were Cu K␣ (λ = 1.5418 A) recorded by a Jobin-Yvon HR 800 instrument with an Ar+ laser source of 488 nm wavelength in a macroscopic configuration. FESEM images of the composites were recorded using a XL30 ESEM FEG scanning electron microscope. The surface area of the composite catalyst was measured by BET method on an ASAP 2010M surface analyzer (the samples were outguessed under vacuum at 120 ◦ C overnight). The pore size distribution were calculated from the desorption branch of N2 -sorption isotherms using BJH method. The photoreactor was designed with an internal light source (125 W high pressure mercury lamp with main emission wavelength 313 nm) surrounded by a quartz jacket, where the suspension including the solid catalyst (0.2 g) and an aqueous 4-NP or MO/RB (90 ml, 50 mg/l) completely surrounded the light source. The suspension was ultrasonicated for 10 min and stirred in the dark for 30 min to obtain a good dispersion and to establish the adsorption–desorption equilibrium between the organic molecules and the catalyst surface. The temperature of the suspension was maintained at 30 ± 2 ◦ C by circulation water through an external cooling coil, and the system was open to air. The concentrations of the 4-NP in aqueous suspensions were monitored by a Shimadzu LC8A high pressure liquid chromatography (HPLC) equipped with a UV detector (λ = 210 nm). Before HPLC determination, the withdrawn suspensions were centrifuged and filtered with microporous membranes. Decreases in the concentrations of dyes were analyzed by a Cary 500 UV–vis–NIR spectrophotometer at λ = 553 nm for RB and λ = 464 nm for MO, respectively. Changes of total organic carbon (TOC) in reaction systems were carried out on a Shimadzu TOC500 total organic carbon analyzer. At given intervals of illumination, the samples of the reaction solution were taken out, and then centrifuged and filtrated. Finally, the filtrates were analyzed. To investigate
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Table 1 Data of ICP-AES and FT-IR for various catalysts Catalyst
Cs2.5 H0.5 PW12 O40 Cs0.5 H2.5 PW12 O40 /TiO2 Cs1.0 H2.0 PW12 O40 /TiO2 Cs1.5 H1.5 PW12 O40 /TiO2 Cs2.0 H1.0 PW12 O40 /TiO2 Cs2.5 H0.5 PW12 O40 /TiO2 Cs3.0 PW12 O40 /TiO2
Loading (wt%)
– 16.5 18.8 19.1 19.1 19.8 19.0
Molar ratio
Wavenumber (cm−1 )
Cs:P:W
P O
W O
W O W
W O W
2.5:1.1:12.0 0.5:0.9:11.9 1.0:1.1:12.0 1.5:0.9:12.2 2.0:1.1:12.0 2.5:1.2:12.1 3.1:1.0:12.0
1079 1079 1079 1080 1081 1083 1081
983 986 990 986 986 990 986
890 897 890 890 890 891 893
800 824 828 806 799 810 814
the stability of the catalysts, the photocatalytic reaction for each sample was carried out three times.
The determined loadings of Csx H3−x PW12 O40 and molar ratios of Cs/P/W in the Csx H3−x PW12 O40 /TiO2 are listed in Table 1. The determined molar ratios are close to the expected values. However, the loadings of Csx H3−x PW12 O40 are lower than the expected values, suggesting that the non-trapped Cspolyoxotungstates are dropped into water during the process of hot water washing. FT-IR spectra of Cs2.5 H0.5 PW12 O40 and Csx H3−x PW12 O40 /TiO2 are shown in Fig. 1, and the characteristic IR vibration frequencies of the Keggin units are summarized in Table 1. The peaks appeared in the range from 1100 to 700 cm−1 are ascribed to the stretch vibrations of P O, W O, and W O W bonds of the Keggin units, respectively [24], indicating that the primary Keggin structures of these Cs-polyoxotungstates remain intact after formation of the composites. The retention of the Keggin structure in the composites is further confirmed by Raman scattering spectroscopy. As shown
in Fig. 2 and Table 2, the peaks appeared in the range from 900 to 1100 cm−1 are originated from the vibrations of W O W, W O, and P O bonds of the Keggin units [17]. Compared with the starting Keggin units, the shifts of the positions of IR absorption or Raman scattering peaks and the decrease of these peak intensities after formation of Csx H3−x PW12 O40 /TiO2 composites are ascribed to strong chemical interactions between the Keggin units and anatase TiO2 networks. In addition, the four Raman scattering peaks appeared in the range of 148–641 cm−1 are originated from the anatase TiO2 [25], suggesting the formation of the anatase phase structure of the composites under the current preparation method. The phase structure of the composites is further characterized by XRD analysis (Fig. 3). It indicates that Csx H3−x PW12 O40 /TiO2 nanocomposites exhibit the uniform anatase structure with characteristic 2θ values at 25.3◦ (1 0 1), 37.7◦ (0 0 4), 48.0◦ (2 0 0), 56.1◦ (2 1 1), and 62.9◦ (2 0 4), respectively (JCPDS file No. 84-1285). The key step to obtain the Csx H3−x PW12 O40 /TiO2 with an anatase phase structure at a low temperature (200 ◦ C) is to control a constant heating rate during the process of hydrothermal treatment of the Csx H3−x PW12 O40 /TiO2 hydrogel. At this relatively low temperature, thermal decomposition of polyoxotungstates is avoided totally (polyoxotungstates begin to decompose at ca. 350 ◦ C
Fig. 1. IR spectra of as-prepared (a) Cs0.5 H2.5 PW12 O40 /TiO2 ; (b) Cs1.0 H2.0 PW12 O40 /TiO2 ; (c) Cs1.5 H1.5 PW12 O40 /TiO2 ; (d) Cs2.0 H1.0 PW12 O40 /TiO2 ; (e) Cs2.5 H0.5 PW12 O40 /TiO2 ; (f) Cs3.0 PW12 O40 /TiO2 ; and (g) Cs2.5 H0.5 PW12 O40 .
Fig. 2. Raman scattering spectra of as-prepared TiO2 , Csx H3−x PW12 O40 /TiO2 , and the parent Cs2.5 H0.5 PW12 O40 .
3. Results and discussion 3.1. Photocatalyst characterizations
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Table 2 Raman shifts of Cs2.5 H0.5 PW12 O40 , TiO2 , and Csx H3−x PW12 O40 /TiO2 Sample
Raman shift (cm−1 )
Cs2.5 H0.5 PW12 O40 As-prepared TiO2 Cs0.5 H2.5 PW12 O40 /TiO2 Cs1.0 H2.0 PW12 O40 /TiO2 Cs1.5 H1.5 PW12 O40 /TiO2 Cs2.0 H1.0 PW12 O40 /TiO2 Cs2.5 H0.5 PW12 O40 /TiO2 Cs3.0 PW12 O40 /TiO2
539 (ta ) 148 (Eg b ) 149 (Eg ) 149 (Eg ) 149 (Eg ) 150 (Eg ) 150 (Eg ) 151 (Eg )
a b
906 (t) 396 (B1g b ) 402 (B1g ) 400 (B1g ) 402 (B1g ) 399 (B1g ) 401 (B1g ) 397 (B1g )
996 (t) 515 (A1g b ) 517 (A1g ) 519 (A1g ) 517 (A1g ) 518 (A1g ) 517 (A1g ) 519 (A1g )
1010 (t) 640 (Eg b ) 634 (Eg ) 640 (Eg ) 641 (Eg ) 640 (Eg ) 640 (Eg ) 636 (Eg )
995 (t) 997 (t) 995 (t) 994 (t) 995 (t) 907 (t)
1006 (t) 1006 (t) 1005 (t) 1005 (t) 1009 (t) 993 (t)
1009 (t)
Fundamental Raman vibration modes of W O, P O, and W O W bonds in Keggin units of Csx H3−x PW12 O40 . Eg , B1g , A1g , and Eg : fundamental vibration modes of Ti O and Ti O Ti bonds in anatase TiO2 .
Fig. 3. XRD patterns of as-prepared (a) anatase TiO2 ; (b) Cs0.5 H2.5 PW12 O40 /TiO2 ; (c) Cs1.0 H2.0 PW12 O40 /TiO2 ; (d) Cs1.5 H1.5 PW12 O40 /TiO2 ; (e) Cs2.0 H1.0 PW12 O40 /TiO2 ; (f) Cs2.5 H0.5 PW12 O40 /TiO2 ; and (g) Cs3.0 PW12 O40 /TiO2 nanocomposites.
[26], while the temperature for calcination of amorphous TiO2 to its anatase structure is at ca. 450 ◦ C [24]). Diffraction peaks appeared at 10.5◦ , 30.3◦ , and 35.5◦ are originated from the parent Csx H3−x PW12 O40 (JCPDS File No. 76-1815). UV–vis/DRS of the parent Cs2.5 H0.5 PW12 O40 , as-prepared TiO2 , Degussa P25, and Csx H3−x PW12 O40 /TiO2 are shown in Fig. 4. The parent Cs2.5 H0.5 PW12 O40 exhibits two absorption peaks at 190 and 260 nm, respectively, attributed to charge trans-
Fig. 4. UV–vis/DRS of P25, as-prepared TiO2 , the parent Cs2.5 H2.5 PW12 O40 , and Csx H3−x PW12 O40 /TiO2 . The absorbance values were normalized.
fer (CT) from O 2p to W 5d of the Keggin units at the W O and W O W bonds. As for the TiO2 , the absorption maximum is at 245 nm, corresponding to CT from O 2p to Ti 3d of anatase TiO2 [27]. The Csx H3−x PW12 O40 /TiO2 nanocomposites show strong and broad optical absorption in the range from 200 to 420 nm, and the red shifts are observed compared with the parent Cs2.5 H0.5 PW12 O40 , TiO2 , and P25. With respect to Csx H3−x PW12 O40 /TiO2 , the red shifts of the absorption band are apparently different with various contents of Cs. The above UV–vis/DRS results indicate that introduction of Csx H3−x PW12 O40 into TiO2 framework has an influence on coordination environment of TiO2 crystalline, resulting in the red shifts of the absorption band [28]. FESEM images of Csx H3−x PW12 O40 /TiO2 in Fig. 5 reveal that as-prepared composites consist of very small and uniform spherical particles with sizes from 16 to 20 nm. In addition, the aggregation among the particles is observed. Formation of nanometer-sized composites is closely related to the preparation method used here. The representative N2 absorption/desorption isotherm and BJH pore-size distribution plot of the Csx H3−x PW12 O40 /TiO2 are shown in Fig. 6. According to IUPAC definition, the isotherm belongs to Type IV and shows a steep hysteresis loop of type H1 at a relatively high pressure (p/p0 = 0.5–0.8), suggesting that the composite exhibits the mesoporosity. The average pore size of the composite is 5.1 nm with a narrow size distribution (Fig. 6B), implying that the pore sizes are uniform. The determined BET surface area of Cs2.5 H0.5 PW12 O40 /TiO2 is 163 m2 g−1 , higher than that of the parent Cs2.5 H0.5 PW12 O40 (135 m2 g−1 ) [29] or Degussa P25 (50 m2 g−1 ). From the above results, we conclude that the primary Keggin structure remains intact after formation of the Csx H3−x PW12 O40 /TiO2 composites. Compared with the parent Keggin unit, changes of Raman shifts, wavenumbers, and OMCT bands indicate that chemical interactions between the Keggin anions and anatase TiO2 matrix exist in the composites. These interactions are hydrogen bonding and acid basic. On the one hand, hydrogen bonding is formed between the oxygen atoms of Keggin anions and the surface hydroxyl groups (Ti OH) of anatase networks, i.e., W O· · ·HO Ti and W O· · ·HO Ti [15]. On the other hand, chemically active surface Ti OH groups are protonated in an acidic medium to form the Ti OH2 + group. The Ti OH2 + group will act as a counter
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Fig. 5. FESEM images of (A) Cs0.5 H2.5 PW12 O40 /TiO2 ; (B) Cs1.0 H2.0 PW12 O40 /TiO2 ; (C) Cs1.5 H1.5 PW12 O40 /TiO2 ; (D) Cs2.0 H1.0 PW12 O40 /TiO2 ; (E) Cs2.5 H0.5 PW12 O40 /TiO2 ; and (F) Cs3.0 PW12 O40 /TiO2 .
ion for Keggin units and yield (Ti OH2 + )(Csx H2−x PW12 O40 − ) by an acid–base reaction. Therefore, the Csx H3−x PW12 O40 particles are firmly immobilized into the interior of anatase TiO2 network.
photocatalytic tests were carried out in an aqueous solution containing oxygen (dissolved in the reaction system from air). Changes of the concentrations of 4-NP are shown in Fig. 7, and the results are summarized as follows. On stirring the sus-
3.2. Photocatalysis studies The photocatalytic activity of as-prepared Csx H3−x PW12 O40 /TiO2 was evaluated by the degradation of an aqueous 4-NP under UV-light irradiation firstly, and the starting Csx H3−x PW12 O40 and P25 were also tested for comparison. 4-NP, chosen as a probe molecule to carry out the photodegradation experiments, is one of the most refractory pollutants in industrial wastewater. Its high stability and solubility in water are the main reasons why the degradation of this compound to non-dangerous levels is a very difficult task. The present
Fig. 6. (A) N2 adsorption–desorption isotherm and (B) pore-size distribution plot on the basis of the desorption branch of the isotherm of Cs2.5 H0.5 PW12 O40 /TiO2 .
Fig. 7. Photocatalytic degradation of an aqueous 4-NP (50 mg/l, 90 ml) on (A) the parent Csx H3−x PW12 O40 (0.2g) and (B) Csx H3−x PW12 O40 /TiO2 (0.2 g) under UV-light excitation.
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Fig. 8. UV–vis absorption spectra of dyes (RB and MO) after stirring the suspension in the dark for 30 min. 0.2 g catalyst was used. (A) RB (50 mg/l, 90 ml) + Csx H3−x PW12 O40 , (B) RB (50 mg/l, 90 ml) + Csx H3−x PW12 O40 /TiO2 , (C) MO (50 mg/l, 90 ml) + Csx H3−x PW12 O40 , (D) MO (50 mg/l, 90 ml) + Csx H3−x PW12 O40 /TiO2 .
pension of an aqueous 4-NP solution (50 mg/l, 90 ml) and the catalyst powder (0.2 g) in the dark for 250 min, the disappearance of 4-NP is negligible. In the absence of the catalyst, the changes of 4-NP concentrations are hardly observed. However, obvious degradation of 4-NP is founded under the simultaneous presence of the photocatalyst and UV-light irradiation. The results in Fig. 7 indicate the photocatalytic activity of the four catalysts towards the degradation of 4-NP. When the starting Csx H3−x PW12 O40 or P25 is used as the catalyst, the milky dispersion in an aqueous solution makes its separating and recycling difficult. As for the Csx H3−x PW12 O40 /TiO2 , its performance is superior to that of the parent Csx H3−x PW12 O40 or P25. For example, it only took 90 min UV-light irradiation of Cs2.5 H0.5 PW12 O40 /TiO2 to destruct 4-NP completely (Fig. 7B), whereas the conversion of 4-NP only reached 35 and 45%, respectively, under the same conditions by using Cs2.5 H0.5 PW12 O40 and P25 as the catalysts (Fig. 7A and B). With respect to Csx H3−x PW12 O40 /TiO2 or the starting Csx H3−x PW12 O40 , apparent difference in photoactivity is observed with various contents of Cs. The highest photoactivity is obtained by Cs2.5 H0.5 PW12 O40 and Cs2.5 H0.5 PW12 O40 /TiO2 . The photocatalytic activities of Csx H3−x PW12 O40 /TiO2 are further studied by dye MO and RB degradation, at the same time, the absorption behaviors of dye molecules on the start-
ing Csx H3−x PW12 O40 and Csx H3−x PW12 O40 /TiO2 are also investigated. All the experimental conditions are the same as those of 4-NP degradation, and the results are shown in Figs. 8–10. The absorption behaviors of dye molecules on the starting Csx H3−x PW12 O40 and Csx H3−x PW12 O40 /TiO2 are complex, which are related to the structures of the absorbed dye molecules, BET surface areas, and surface acidities of Csx H3−x PW12 O40 or Csx H3−x PW12 O40 /TiO2 . At first, adsorption of RB and MO on the surface of the Csx H3−x PW12 O40 or Csx H3−x PW12 O40 /TiO2 is physical and hydrogen bonding. The hydrogen bonding between RB and Csx H3−x PW12 O40 is formed by the interactions of hydroxyl groups of RB and protons of Csx H3−x PW12 O40 , while the hydrogen bonding between MO and Csx H3−x PW12 O40 is formed by the interactions of N or S atoms of MO and protons of Csx H3−x PW12 O40 . From Fig. 8 it can be seen that for all tested photocatalysts (Csx H3−x PW12 O40 and Csx H3−x PW12 O40 /TiO2 ), their order of adsorption ability to RB or MO is the same, i.e., Cs3.0 PW12 O40 > Cs2.5 H0.5 PW12 O40 > Cs0.5 H2.5 PW12 O40 > Cs1.0 H2.0 PW12 O40 > Cs1.5 H1.5 PW12 O40 > Cs2.0 H1.0 PW12 O40 . The results imply that the physical adsorption is determined by the BET surface areas of Csx H3−x PW12 O40 or Csx H3−x PW12 O40 /TiO2 . It has been reported that the surface
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area of Csx H3−x PW12 O40 monotonically decreased as the Cs content (x) increased from x = 0 to 2, whereas, the surface area sharply increased as the Cs content exceeded 2 [29]. Secondly, stronger adsorption of MO on Csx H3−x PW12 O40 is observed (Fig. 8C) compared with that of RB on Csx H3−x PW12 O40 (Fig. 8A), which results in obvious shifts of MO chromophore. Finally, the adsorption ability of Csx H3−x PW12 O40 /TiO2 is not as strong as their corresponding starting Csx H3−x PW12 O40 (Fig. 8B and D). This is due to losing of some of surface acid sites after formation of Csx H3−x PW12 O40 /TiO2 , leading to weakening of the hydrogen bonding adsorption of the composites to dye molecules. The blank experiments show that degradation of MO or RB is negligible by direct photolysis for 180 min, indicating that the sensitized photocatalysis has not occurred in current dye/Csx H3−x PW12 O40 /TiO2 /UV light system. In Figs. 9 and 10, the photocatalytic activity of Csx H3−x PW12 O40 /TiO2 to decompose MO or RB is the highest among three tested catalysts (P25, Csx H3−x PW12 O40 , and Csx H3−x PW12 O40 /TiO2 ) under UV-light irradiation, and activity of the Cs2.5 H0.5 PW12 O40 /TiO2 is the highest among Csx H3−x PW12 O40 /TiO2 . From the above photocatalytic tests we conclude that the photocatalytic activities of Csx H3−x PW12 O40 or Csx H3−x PW12 O40 /TiO2 for degrading model molecules (4-NP, MO, and RB) are also related to Cs content in the compounds or composites. The order of photocatalytic activity is Cs2.5 H0.5 PW12 O40 > Cs0.5 H2.5 PW12 O40 > Cs1.0 H2.0 PW12 O40 > Cs1.5 H1.5 PW12 O40 > Cs2.0 H1.0 PW12 O40 > Cs3.0 PW12 O40 . On the other hand, Csx H3−x PW12 O40 /TiO2 composites show higher photocatalytic activity than that of the starting Csx H3−x PW12 O40 or P25 under UV-light irradiation. Moreover, higher apparent reaction rate constant can be obtained for the photocatalytic reactions in the Csx H3−x PW12 O40 /TiO2 system (Table 3). The three tested photocatalysts (P25, the parent Cs2.5 H0.5 PW12 O40 , and Cs2.5 H0.5 PW12 O40 /TiO2 ) have extremely different mineralization ability to the above organic pollutes. During the course of photocatalytic degradation of three target molecules, temporal changes of TOC are monitored, and the results are shown in Fig. 11. It is observed that the mineralization ability of Cs2.5 H0.5 PW12 O40 /TiO2 is
Fig. 9. Photocatalytic degradation of MO (50 mg/l, 90 ml) on (A) the parent Csx H3−x PW12 O40 (0.2 g); and (B) Csx H3−x PW12 O40 /TiO2 (0.2 g) under UVlight excitation.
the strongest and the parent Cs2.5 H0.5 PW12 O40 is the weakest among three tested catalysts. The results imply that the photocatalytic activity of anatase Csx H3−x PW12 O40 /TiO2 is higher than that of the parent Csx H3−x PW12 O40 or P25, although all of the tested catalysts exhibit high activity to degrade 4-NP or dyes. The target molecules can be totally decomposed by Cs2.5 H0.5 PW12 O40 /TiO2 under UV-light irradiation for a
Table 3 UV-light photocatalytic activity of different photocatalysts for 4-NP, RB, and MO degradation Catalyst
Cs0.5 H2.5 PW12 O40 /TiO2 Cs1.0 H2.0 PW12 O40 /TiO2 Cs1.5 H1.5 PW12 O40 /TiO2 Cs2.0 H1.0 PW12 O40 /TiO2 Cs2.5 H0.5 PW12 O40 /TiO2 Cs3.0 PW12 O40 /TiO2 P25
Kapp (min−1 ) 4-NP
RB
MO
0.032 0.024 0.019 0.018 0.044 0.016 0.010
0.041 0.034 0.029 0.027 0.058 0.025 0.009
0.04 0.036 0.031 0.029 0.054 0.023 0.007
Initial concentration of 4-NP, RB, or MO 50 mg/l; the system opened to air; the temperature of the suspension 30 ± 2 ◦ C; catalyst 0.2 g; UV light (125 W HPML) irradiation time 90 min for 4-NP, and 60 min for RB or MO.
Fig. 10. Photocatalytic degradation of RB (50 mg/l, 90 ml) over P25 (0.2 g), and Csx H3−x PW12 O40 /TiO2 (0.2 g) under UV-light excitation.
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improve the activity of photocatalysts. Higher surface acidities imply that Csx H3−x PW12 O40 or Csx H3−x PW12 O40 /TiO2 can trap more photogenerated elections on the surface of TiO2 , so that recombination of photogenerated electrons and holes may be weakened. As for the parent Csx H3−x PW12 O40 , the number of surface protons decrease as the Cs content increases from x = 0 to 2.0, but greatly increase when x is higher than 2.0 and show the highest surface acidity at x = 2.5 [31]. In addition, although the BET surface area of Cs3.0 PW12 O40 is the largest among six tested Csx H3−x PW12 O40 , its photocatalytic activity is the lowest since it has no any surface proton. However, Csx H3−x PW12 O40 /TiO2 has significantly higher photocatalytic activities than the parent Csx H3−x PW12 O40 , which is attributed to their mesoporosity and synergetic effect. The mesoporous structure of Csx H3−x PW12 O40 /TiO2 can make the diffusion of the reactants into the mesopores more rapidly. Although Cs2.5 H0.5 PW12 O40 also exhibits mesoporosity, its photocatalytic activity is much lower than that of Cs2.5 H0.5 PW12 O40 /TiO2 . This is because of the synergetic effect existing between the starting POM and TiO2 , which has been studied by us and other groups [17,32,33]. That is, POM has empty d orbits and can be used as good electron acceptors. In the POM–TiO2 photocatalytic systems, the fast photogenerated hole-electron (h+ –e− ) recombination on the surface of TiO2 can be retarded effectively by accepting e− to POM’s empty d orbits. Accordingly, the quantum efficiency of the photocatalysts is enhanced. After the reaction, it is easier to separate Csx H3−x PW12 O40 /TiO2 than the parent Csx H3−x PW12 O40 or P25 for recycling applications. The recovered catalysts are analyzed by ICP-AES and UV–vis/DRS. The loadings of Csx H3−x PW12 O40 /TiO2 maintain basically unchanged, and the structures of the catalysts are not destroyed. In addition, deactivation of the catalysts is hardly observed after three catalytic runs. 4. Conclusions
Fig. 11. Changes in TOC removal from (A) 4-NP, (B) MO, and (C) RB aqueous solutions (50 mg/l, 90 ml) with the parent Cs2.5 H0.5 PW12 O40 (), P25 (), and Cs2.5 H0.5 PW12 O40 /TiO2 (䊉) under UV-light irradiation. 0.2 g catalyst was used.
suitable period of time. However, the parent Cs2.5 H0.5 PW12 O40 or P25 can only destroy the chromophores of dyes or degrade 4-NP to other organics, and it is difficult for them to mineralize the target molecules completely. The above photocatalytic tests indicate that both the parent Csx H3−x PW12 O40 and Csx H3−x PW12 O40 /TiO2 exhibit activity to 4-NP and RB/MO degradation under UV-light irradiation. Moreover, their photocatalytic activities increase with the increase of the BET surface areas and surface acidities of the composites. Larger BET surface areas result in larger contact areas between the active sites (W O W and Ti O Ti bonds) and the target substrate [30], which is a crucial factor to
The composite photocatalysts, Csx H3−x PW12 O40 /TiO2 with different Cs contents, were prepared by a one-step sol–gel technique followed by hydrothermal treatment at 200 ◦ C with a heating ramp of 2 ◦ C/min. The materials exhibited anatase crystalline phase structure, nanometer sizes, mesoporosity, and relatively narrow band gap. The primary Keggin structure remained intact after formation of the composites. The photocatalytic activity of Csx H3−x PW12 O40 /TiO2 were higher than that of the parent Csx H3−x PW12 O40 or P25 towards decomposition of 4-NP, MO and RB in aqueous suspensions under UV-light irradiation, and Cs2.5 H0.5 PW12 O40 /TiO2 was the most active among six tested Csx H3−x PW12 O40 /TiO2 composites. Furthermore, Csx H3−x PW12 O40 /TiO2 could be separated and recovered more easily. Leakage of Csx H3−x PW12 O40 from the support and deactivation of the catalysts were hardly observed after three catalytic runs, attributed to strong interactions between the Keggin units and the TiO2 support.
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Acknowledgements This work is supported by the Program of New Century Excellent Talents in University of China (NCET-04-0311). The work is also supported by Analysis and Testing Foundation of Northeast Normal University. References [1] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental applications of semiconductor photocatalysis, Chem. Rev. 95 (1995) 69–96. [2] J.L. Mozos, J. Fraxedas, G.A. Battiston, R. Gerbasi, Designed self-soped titanium oxide thin films for efficient visible-light photocatalysis, Adv. Mater. 14 (2002) 1399–1402. [3] S. Rodrigues, K. Ranjit, S. Uma, I. Martyanov, Single-step synthesis of a highly active visible-light photocatalyst for oxidation of a common indoor air pollutant: acetaldehyde, Adv. Mater. 17 (2005) 2467–2471. [4] H. Krysova, J. Jirkovsky, J. Krysa, G. Maillhot, Metal carbides and nitrides as potential catalysts for hydroprocessing, Appl. Catal. B 40 (2003) 1–28. [5] H. Luo, T. Takata, Y. Lee, J. Zhao, K. Domen, Y. Yan, Photocatalytic acticity enhancing for titanium dioxide by Co-doping with bromine and chlorine, Chem. Mater. 16 (2004) 846–849. [6] D. Li, H. Haneda, S. Hishita, N. Ohashi, Visible-light-driven N-F-codoped TiO2 photocatalysts. 2. Optical characterization, photocatalysis, and potential application to air purification, Chem. Mater. 17 (2005) 2596–2602. [7] S. Park, H. Park, K. Yoo, J. Lee, J. Lee, Photocatalytic recovery of Ag ions from wastewater using ZnO nanopowders immobilized on microporous alumina substrates, Colloids Surf. A: Physicochem. Eng. Aspects 300 (2007) 30–34. [8] M.H. Priya, G. Madras, Photocatalytic degradation of nitrobenzenes with combustion synthesized nano-TiO2 , J. Photochem. Photobiol. A: Chem. 178 (2006) 1–7. [9] C. Karanakaran, R. Dhanalakshmi, S. Karuthapandian, Inhibition of photooxidation of iron(II) by some semiconductors, J. Photochem. Photobiol. A: Chem. 170 (2005) 233–238. [10] Y. Kurada, T. Mori, K. Yagi, N. Makihata, Y. Kawahara, Preparation of visible-light-responsive TiO2−x Nx photocatalyst by a sol–gel method: analysis of the active center on TiO2 that reacts with NH3 , Langmiur 21 (2005) 8026–8034. [11] M.R. Prairie, L.R. Evans, B.M. Stange, An investigation of titanium dioxide photocatalysis for the treatment of water contaminated with metals and organic chemicals, Environ. Sci. Technol. 27 (1993) 1776–1782. [12] J. Herrmann, Heterogeneous photocatalysis: fundamentals and applications to the removal of various types of aqueous pollutants, Catal. Today 53 (1999) 115–129. [13] O.M. Alfano, D. Bahnemann, R. Dillert, R. Goslich, Photocatalysis in water environments using artificial and solar light, Catal. Today 58 (2000) 199–230. [14] M.I. Litter, J.A. Navio, J. Photochem, Photocatalytic properties of irondoped titania semiconductors, Photobiol. A: Chem. 98 (1996) 171–181. [15] M. Yoon, J.A. Chang, Y. Kim, J.R. Choi, Heteropoly acid-incorporated TiO2 colloids as novel photocatalytic systems resembling the photosynthetic reaction center, J. Phys. Chem. B 105 (2001) 2539–2545.
[16] D.A. Friesen, L. Morello, J.V. Headleg, C.H. Langford, Factors influencing relative efficiency in photo-oxidations of organic molecules by Cs3 PW12 O40 and TiO2 colloidal photocatalysts, J. Photochem. Photobiol. A: Chem. 133 (2000) 213–220. [17] L. Li, Q. Wu, Y. Guo, C. Hu, Nanosize and bimodal porous polyoxotungstate–anatase TiO2 composites: preparation and photocatalytic degradation of organophosphorus pesticide using visible-light excitation, Microporous Mesoporous Mater. 87 (2005) 1–9. [18] Y. Izumi, M. Ono, M. Kitagawa, Silica-included heteropoly compounds as solid acid catalysts, Microporous Mater. 5 (1995) 255–262. [19] Y. Izumi, M. Ono, M. Ogawa, Acidic cesium salts of Keggin-type heteropolytungstic acids as insoluble solid acid catalysts for estenfication and hydrolysis reactions, Chem. Lett. 22 (1993) 825–826. [20] Y. Izumi, M. Ogawa, K. Urabe, Alkali metal salts and ammonium salts of Keggin-type heteropolyacids as solid acid catalysts for liquid-phase Friedel-Crafts reactions, Appl. Catal. A 132 (1995) 127–140. [21] D.A. Friesen, D.B. Gibson, C.H. Langford, Heterogeneous Cs3 PW12 O40 photocatalysts, Chem. Commun. (1998) 543. [22] D.A. Friesen, D.B. Gibson, C.H. Langford, The photooxidative degradation of N-methylpyrrolidinone in the presence of Cs3 PW12 O40 and TiO2 colloid photocatalysts, Environ. Sci. Tech. 33 (1999) 3193–3198. [23] Z. Ma, W. Hua, Y. Ren, H. He, Z. Gao, n-Butane isomerization over Cssalts of H3 PW12 O40 : a mechanistic study by 1 C MAS NMR, Appl. Catal. A. 256 (2003) 243–250. [24] S.J. Gregg, W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, 1982. [25] Y. Guo, Y. Yang, C. Hu, C. Guo, E. Wang, Y. Zhou, S. Feng, Preparation, characterization and photochemical properties of ordered macroporous hybrid silica materials based on monovacant Keggin-type polyoxometalates, J. Mater. Chem. 12 (2002) 3046. [26] L. Miao, S. Tanemura, S. Toh, Fabrication, characterization and Raman study of anatase-TiO2 nanorods by a heating-sol–gel template process, J. Crystal Growth 264 (2004) 246–252. [27] X. Jiang, T. Herricks, Y. Xia, Monodispersed spherical colloids of titania: synthesis, characterization, and crystallization, Adv. Mater. 15 (2003) 1205–1209. [28] Y. Yang, Q. Wuc, Y. Guoa, C. Hu, Efficient degradation of dye pollutants on nanoporous polyoxotungstate–anatase composite under visible-lightirradiation, J. Mol. Catal. A 225 (2005) 203–212. [29] T. Okuhara, H. Watanabe, T. Nishimura, Microstructure of cesium hydrogen salts of 12-tungstophosphoric acid relevant to novel acid catalysis, Chem. Mater. 12 (2000) 2230–2238. [30] N. Mizuno, M. Misono, Heterogeneous catalysis, Chem. Rev. 98 (1998) 199–218. [31] T. Okuhara, T. Nishimura, H. Watanabe, K. Na, M. Misono, Acid–Base Catalysis II, Kodansha/Elsevier, Tokyo/Amsterdam, 1994, p. 419. [32] Y. Yang, Y. Guo, C. Hu, C. Jiang, E. Wang, Synergistic effect of Keggintype [Xn+ W11 O39 ](12−n)− and TiO2 in macroporous hybrid materials [Xn+ W11 O39 ](12−n)− –TiO2 for the photocatalytic degradation of textile dyes, J. Mater. Chem. 13 (2003) 1684–1694. [33] G. Col´on, M.C. Hidalgo, M. Mac´ıas, J.A. Nav´ıo, Enhancement of TiO2 /C photocatalytic activity by sulfate promotion, Appl. Catal. A: Gen. 259 (2004) 235–243.