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SiO2 composite
Materials Letters 57 (2003) 3914 – 3918 www.elsevier.com/locate/matlet
Preparation and characterization of supported photocatalysts: HPAs/TiO2/SiO2 composite Bo Bai *, Jinglian Zhao, Xiao Feng School of Environment and Chemical Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China Received 15 September 2002; received in revised form 5 March 2003; accepted 10 March 2003
Abstract Serial novel supported photocatalysts: HPAs/TiO2/SiO2 composite, such as H3PW12O40/TiO2/SiO2, H4SiW12O40/TiO2/SiO2 and H3PMo12O40/TiO2/SiO2, were synthesized via impregnation, respectively. TG-DSC, XRD, FT-IR, SEM and BET characterized the physicochemical structures of products. The photocatalytical performance testing showed that the as-prepared composites have higher catalysis activity than that of parallel TiO2/SiO2 for the degradation of model CBW aqueous. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Composite; Photocatalyst; TiO2/SiO2; Polyoxometalates
1. Introduction Since the photocatalytic degradation of inorganic and organic compounds was introduced, the preparation of photocatalyst with high catalytic performance has attracted much attention because of its relation to solar energy conversion and environmental cleaning [1– 3]. Among the available materials, although there are several kinds of photocatalyst, such as TiO2, ZnO, CdS, ZrTiO4, etc., the TiO2 has had considerable interest due to its good characters of chemical stability and endurance [4]. Especially the composite of silicasupported TiO2 particles, it has been verified that this materials possess a larger band-gap with a higher photo-oxidation capability and has a strong oxidizing
* Corresponding author. E-mail address: [email protected] (B. Bai).
potential of the photo-generated holes. In addition, such advanced materials cannot only take advantage of both TiO2 (an n-type semiconductor and an active activity catalytic) and SiO2 (high thermal stability and excellent mechanical strength), but also extend their applications through the generation of new catalytic activity sites due to the interaction of TiO2 and SiO2 [5]. Therefore, the titania –silica composites have become one of the most attractive photocatalyst for oxidation and represent a novel class of photocatalyst. Recently, several researches [6,7] have demonstrated that the Keggin-type heteropolyacids, H8 x XM12O40nH2O, are a good ground-state oxidant or electron acceptor resulting from their unique structure and can be utilized to synergistically facilitate the photocatalytic activity. The assistant procedure resembles the plant photosynthetic reaction centers very much. Thus, according to above consideration, the supported TiO2 on the amorphous SiO2 was first prepared
0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(03)00240-4
B. Bai et al. / Materials Letters 57 (2003) 3914–3918
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via impregnation in the present work. Then, using those as-prepared composites as substrate, a spot of heteropolyacids (HPAs), such as H3PW12O40, H4Si W12O40 and H3PMo12O40, is further anchored on the surface of TiO2/SiO2. Those synthesized composites might have higher photocatalytic performances.
The photocatalytic activity of as-prepared composites was evaluated by measuring the loss of CBW from the aqueous. Its structure was showed as follows:
2. Experimental
The degradation was carried out in a 2000-ml quartz reactor. UV source comes from a 300-W highpressure Hg lamp. In each experiment, 1000 g composites in powder form were suspended in 50 mg/l CBW aqueous solution. The reactor was kept at 25 F 2 jC by the remained surrounding water temperature. A flow of air (80 ml/min) was bubbled into reactor, and the oxygen in air served as oxidant. The solid – liquid sample was separated by high-rate centrifuge at 15,000 r/min. The solution absorbance spectrum and the concentration of CBW in aqueous were measured by TU-1500 UV-Vis spectrometer.
2.1. Preparation of composites The typical TiO2/SiO2 substrate was prepared according to the described method [8] with minor modification: the loading of titanium was set to 10.0 wt.%. The 50 ml titanium n-butoxide/ethanol solution (Vethanol/Vtitanium n butoxide = 5.0) was mixed with the amorphous silica selected previously for 3 h. The solid, after evaporating ethanol solvent, was heated at 120 jC overnight and finally calcined at 400 jC in open air for 5 h. The powder production was denoted as TiO2/SiO2. The HPA was incorporated on the surface of asprepared TiO2/SiO2 substrate by the following procedure: the 20-mg analytical grade H 3 PW 12 O 40 , H4SiW12O40 and H3PMo12O40 were, respectively, diluted in 30 ml ethanol to obtain a homogenous solution. One thousand grams of TiO2/SiO2 were added into reactor, and the slurry was stirred at room temperature for 4 h. Then, evaporation of suspension was carried out below 60 jC to remove the solvent, and the remained solid finally was mildly calcined at 125 jC for 4 h. 2.2. Characterizations of composites TG-DSC was conducted on Dupont 2950 thermal analyzer. A D/MAX-RA rotating anode X-ray diffractometer (XRD, Rigaku, Japan) was used to record the powder XRD patterns, and the diffraction patterns were taken over the 2h ranges 15 –70j. FTIR analyses were carried out on Nicolet Avatar 360 Fourier transform infrared spectrometer. Scanning electron microscopy was performed on Hitachi S-2700 to obtain the size and shape of particles. The specific surfaces (BET) and pore diameter of samples were determined by ASAP2000 apparatuses.
3. Results and discussion 3.1. DG-DSC The DT-DSC curves of as-prepared composites are shown in Fig. 1. As can be seen in Fig. 1, The TG patterns of three samples all display a weight loss (10 – 15%) in the region of 125 jC in succession. According to earlier studies [9], the protonation of surface Si – OH groups could interact with HPA as follows: H8x XM12 O40 nH2 O þ uSi OH ! ðuSi OH2 Þþ ðH7x XM12 O40 Þ nH2 O Where X is the central atom (Si4 +, P5 +, etc.), x is its oxidation state; M is the metal ion (Mo6 +, W6 +, etc.). Therefore, with a given temperature calcined, nmol water molecular and some adsorbed ethanol molecular were gradually lost in the evaluated temperatures. However, under the exposure to new water circumstances, the original HPAs still could be reconstructed. With the temperatures increased to approximately 463,
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3.3. FT-IR
Fig. 1. DT-DSC curves of samples: (a) H3PW12O40/TiO2/SiO2, (b) H3PMo12O40/TiO2/SiO2, (c) H4SiW12O40/TiO2/SiO2.
FT-IR spectra have been proven to be a powerful technique for study of surface interaction between SiO2 and TiO2, organic species. Fig. 3 shows the Frourier transformed infrared spectroscopy of as-prepared composites. On Fig. 3, the 930 and 1210 cm 1 are, respectively, assigned to the vibration of Ti–O –Si and Si– O – Si bond, which implies that the TiO2 crystallites highly dispersed on the silica surface have been formed by the Ti atoms binding to the silica via oxygen as a bridge. In addition, for three supported HPA samples, the four typical adsorption bands of heteropolyacid from 700 to 1100 cm 1 still remained, which indicates that Keggin structure of H3PW12O40, H4SiW12O40 and H3PMo12O40 was not destroyed in the anchoring procedure. Therefore, in this study, although the silica surface is very inert, the surface hydroxyls still have been utilized as adsorption/reaction sites due to their hydrophilic character. 3.4. SEM and BET
440, 372 jC, respectively, the H3PW12O40, H4SiW12 O40 and H3PMo12O40 on the TiO2/SiO2 surface began to decompose and the corresponding Keggin structure was destroyed. In contrast, it seemed that the thermal stability of H3PW12O40, H4SiW12O40 and H3PMo12 O40 on TiO2/SiO2 were slightly lower than that of the parent HPA.
SEM photographs of as-prepared composites were conducted to obtain the shape and diameter of particles. For example, Fig. 4 shows the picture of TiO2/SiO2. It can be seen that the obtained particles have smooth external surface and the diameter is within 300– 350 nm. The other three samples shared the similar surface
3.2. XRD Fig. 2 shows the XRD patterns of as-prepared composites within the 2h range of 15 – 70j. It can be seen that all samples almost share the same peaks that can be indexed as characteristic (110) reflections of TiO2, (standard anatase TiO2 appears at 2h = 25.4j, rutile TiO2 at 27.3j). This indicates that the samples consist of anatase crystallite, and no rutile or brookite crystallite existing. In addition, it should be pointed that hardly the presence of H3PW12O40, H4SiW12O40 and H3PMo12O40 crystallite phase peaks is due to the low content in samples. The peaks corresponding to amorphous SiO2 are also not present, however, in other separate experiments, it has been verified that this peak exited since the SiO2 crystallized at approximately 1200 jC.
Fig. 2. XRD patterns of samples: (a) TiO2/SiO2, (b) H3PW12O40/ TiO2/SiO2, (c) H4SiW12O40/TiO2/SiO2, (d) H3PMo12O40/TiO2/SiO2.
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Table 1 Special surface area, pore diameter and size of sample Sample
Special surface Pore diameter Sample area (m2/g) (nm) size (nm)
TiO2/SiO2 H3PW12O40/TiO2/SiO2 H4SiW12O40/TiO2/SiO2 H3PMo12O40/TiO2/SiO2
302.5 272.3 265.5 255.6
6.4 7.2 7.8 8.0
320 336 343 356
surface areas and pore diameter of composites were found to decrease with H3PW12O40, H4SiW12O40 and H3PMo12O40 loading, which implies that some of the pores of TiO2/SiO2 were blocked by the anchored HPA molecular. However, the benefit of using the relative large surface area of amorphous SiO2 was still retained approximately 90%. 3.5. Photocatalytic performances
Fig. 3. Frourier transformed infrared spectroscopy of samples: (a) TiO2/SiO2, (b) H3PW12O40/TiO2/SiO2, (c) H4SiW12O40/TiO2/SiO2, (d) H3PMo12O40/TiO2/SiO2.
features as TiO2/SiO2 and the detail information of diameter is listed in Table 1. Besides, the BET surface areas and pore diameter of the samples are also presented on Table 1. The total
In order to detect the photocatalytic activity, the degradation of model CBW aqueous over four asprepared composite samples were carried out, respectively. The results are shown in Fig. 5. As shown in Fig. 5, the composites of TiO2/SiO2, H3PW12O40/TiO2/SiO2, H4SiW12O40/TiO2/SiO2 and H3PMo12O40/TiO2/SiO2 exhibited the variety of photocatalytic activity. Three samples supported HPA on TiO2/SiO2, all have much more activity than bare TiO2/ SiO2. Thereinto, the H3PW12O40/TiO2/SiO2 exhibits a maximum photocatalytic activity for the degradation of CBW aqueous and the H 3 PMo 12 O 40 /TiO 2 /SiO 2
50
Ca (mg/l)
45
a b c d
40 35 30 25 20 0
Fig. 4. SEM photographs of TiO2/SiO2 composites.
30
60
90 t (min)
120
150
Fig. 5. CBW concentration vs. time profile in presence of asprepared photocatalyst: (a) TiO2/SiO2, (b) H3PW12O40/TiO2/SiO2, (c) H4SiW12O40/TiO2/SiO2, (d) H3PMo12O40/TiO2/SiO2.
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showed the minimum. This order is entirely consistent with their acid strength [10]. Namely, H3 PW12 O40 > H4 SiW12 O40 > H3 PMo12 O40 In the case of TiO2/SiO2 photocatalyst, the higher photo-activity is related to two factors: smaller particle size and higher adsorption toward the organic substrate, both of which could be cooperative in making the organic molecular accessible to the active sites on the TiO2 surface [11]. With the HPA presenting the TiO2/SiO2 surface. The electron generated in TiO2 conduction band will be scavenged by the PW12O403 , SiW 12 O 404 , PMo 12 O 403 anions; this procedure restrained the electron-hole recombination. In addition, PW12O403 , SiW12O404 or PMo12O403 could also serve as an electron carrier between the TiO2 and dioxygen to facilitate the separation of hole-electron. Thus, the supported HPAs on TiO2/SiO2 indirectly accelerate the photocatalytic destruction of CBW molecular in aqueous.
4. Conclusion The present work shows that TiO2/SiO2, H3PW12 O40/TiO2/SiO2, H4SiW12O40/TiO2/SiO2 and H3PMo12 O40/TiO2/SiO2 composites have been synthesized via impregnation. The physicochemical properties of composites characterized by TG-DSC, XRD, FT-IR, SEM and BET demonstrated that TiO2 with H3PW12O40,
H4SiW12O40 and H3PMo12O40 were simultaneously anchored on the surface of amorphous SiO2, and the H 3 PW 12O 40, H 4 SiW 12O 40 and H 3PMo 12 O 40 still remained their Keggin structure. By the test of their photocatalytic activity, it was verified that the obtained products of H3PW12O40/TiO2/SiO2, H4SiW 12O40/ TiO2/SiO2 and H3PMo12O40/TiO2/SiO2 exhibited a higher catalysis activity than that of parallel TiO2/ SiO2 for the degradation of model CBW aqueous. Thereinto, H3PW12O40/TiO2/SiO2 showed the best.
References [1] Z.C. Wang, J.F. Chen, X.F. Hu, Materials Letters 43 (2000) 87. [2] T. Tanaka, K. Teramura, T. Yamamoto, Journal of Photochemistry and Photobiology. A, Chemistry 148 (2002) 277. [3] Y.H. Hsien, C.F. Chang, Y.H. Chen, Applied Catalysis. B, Environmental 31 (2001) 241. [4] M. Ashokkumar, International Journal of Hydrogen Energy 33 (1998) 427. [5] X.T. Gao, I.E. Wachs, Catalysis Today 51 (1999) 233. [6] K. Kim, S. Lee, Journal of Physical Chemistry 105 (2001) 2539. [7] R.R. Ozer, J.L. Ferry, Environmental Science & Technology 35 (2001) 3242. [8] H. Chun, Y.Z. Wang, H.X. Tang, Chinese Journal of Catalysis 22 (2001) 186. [9] I.V. Kozhevnikov, K.R. Kloeststra, A. Sinnema, Journal of Molecular Catalysis. A, Chemical 114 (1999) 287. [10] I.V. Kozhenvnikov, Chemical Reviews 98 (1998) 171. [11] Y.M. Yu, W. Zheng, W.P. Liu, Journal of Photochemistry and Photobiology. A, Chemistry 122 (1999) 57.
Preparation and characterization of supported photocatalysts: HPAs/TiO2/SiO2 composite Bo Bai *, Jinglian Zhao, Xiao Feng School of Environment and Chemical Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China Received 15 September 2002; received in revised form 5 March 2003; accepted 10 March 2003
Abstract Serial novel supported photocatalysts: HPAs/TiO2/SiO2 composite, such as H3PW12O40/TiO2/SiO2, H4SiW12O40/TiO2/SiO2 and H3PMo12O40/TiO2/SiO2, were synthesized via impregnation, respectively. TG-DSC, XRD, FT-IR, SEM and BET characterized the physicochemical structures of products. The photocatalytical performance testing showed that the as-prepared composites have higher catalysis activity than that of parallel TiO2/SiO2 for the degradation of model CBW aqueous. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Composite; Photocatalyst; TiO2/SiO2; Polyoxometalates
1. Introduction Since the photocatalytic degradation of inorganic and organic compounds was introduced, the preparation of photocatalyst with high catalytic performance has attracted much attention because of its relation to solar energy conversion and environmental cleaning [1– 3]. Among the available materials, although there are several kinds of photocatalyst, such as TiO2, ZnO, CdS, ZrTiO4, etc., the TiO2 has had considerable interest due to its good characters of chemical stability and endurance [4]. Especially the composite of silicasupported TiO2 particles, it has been verified that this materials possess a larger band-gap with a higher photo-oxidation capability and has a strong oxidizing
* Corresponding author. E-mail address: [email protected] (B. Bai).
potential of the photo-generated holes. In addition, such advanced materials cannot only take advantage of both TiO2 (an n-type semiconductor and an active activity catalytic) and SiO2 (high thermal stability and excellent mechanical strength), but also extend their applications through the generation of new catalytic activity sites due to the interaction of TiO2 and SiO2 [5]. Therefore, the titania –silica composites have become one of the most attractive photocatalyst for oxidation and represent a novel class of photocatalyst. Recently, several researches [6,7] have demonstrated that the Keggin-type heteropolyacids, H8 x XM12O40nH2O, are a good ground-state oxidant or electron acceptor resulting from their unique structure and can be utilized to synergistically facilitate the photocatalytic activity. The assistant procedure resembles the plant photosynthetic reaction centers very much. Thus, according to above consideration, the supported TiO2 on the amorphous SiO2 was first prepared
0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(03)00240-4
B. Bai et al. / Materials Letters 57 (2003) 3914–3918
3915
via impregnation in the present work. Then, using those as-prepared composites as substrate, a spot of heteropolyacids (HPAs), such as H3PW12O40, H4Si W12O40 and H3PMo12O40, is further anchored on the surface of TiO2/SiO2. Those synthesized composites might have higher photocatalytic performances.
The photocatalytic activity of as-prepared composites was evaluated by measuring the loss of CBW from the aqueous. Its structure was showed as follows:
2. Experimental
The degradation was carried out in a 2000-ml quartz reactor. UV source comes from a 300-W highpressure Hg lamp. In each experiment, 1000 g composites in powder form were suspended in 50 mg/l CBW aqueous solution. The reactor was kept at 25 F 2 jC by the remained surrounding water temperature. A flow of air (80 ml/min) was bubbled into reactor, and the oxygen in air served as oxidant. The solid – liquid sample was separated by high-rate centrifuge at 15,000 r/min. The solution absorbance spectrum and the concentration of CBW in aqueous were measured by TU-1500 UV-Vis spectrometer.
2.1. Preparation of composites The typical TiO2/SiO2 substrate was prepared according to the described method [8] with minor modification: the loading of titanium was set to 10.0 wt.%. The 50 ml titanium n-butoxide/ethanol solution (Vethanol/Vtitanium n butoxide = 5.0) was mixed with the amorphous silica selected previously for 3 h. The solid, after evaporating ethanol solvent, was heated at 120 jC overnight and finally calcined at 400 jC in open air for 5 h. The powder production was denoted as TiO2/SiO2. The HPA was incorporated on the surface of asprepared TiO2/SiO2 substrate by the following procedure: the 20-mg analytical grade H 3 PW 12 O 40 , H4SiW12O40 and H3PMo12O40 were, respectively, diluted in 30 ml ethanol to obtain a homogenous solution. One thousand grams of TiO2/SiO2 were added into reactor, and the slurry was stirred at room temperature for 4 h. Then, evaporation of suspension was carried out below 60 jC to remove the solvent, and the remained solid finally was mildly calcined at 125 jC for 4 h. 2.2. Characterizations of composites TG-DSC was conducted on Dupont 2950 thermal analyzer. A D/MAX-RA rotating anode X-ray diffractometer (XRD, Rigaku, Japan) was used to record the powder XRD patterns, and the diffraction patterns were taken over the 2h ranges 15 –70j. FTIR analyses were carried out on Nicolet Avatar 360 Fourier transform infrared spectrometer. Scanning electron microscopy was performed on Hitachi S-2700 to obtain the size and shape of particles. The specific surfaces (BET) and pore diameter of samples were determined by ASAP2000 apparatuses.
3. Results and discussion 3.1. DG-DSC The DT-DSC curves of as-prepared composites are shown in Fig. 1. As can be seen in Fig. 1, The TG patterns of three samples all display a weight loss (10 – 15%) in the region of 125 jC in succession. According to earlier studies [9], the protonation of surface Si – OH groups could interact with HPA as follows: H8x XM12 O40 nH2 O þ uSi OH ! ðuSi OH2 Þþ ðH7x XM12 O40 Þ nH2 O Where X is the central atom (Si4 +, P5 +, etc.), x is its oxidation state; M is the metal ion (Mo6 +, W6 +, etc.). Therefore, with a given temperature calcined, nmol water molecular and some adsorbed ethanol molecular were gradually lost in the evaluated temperatures. However, under the exposure to new water circumstances, the original HPAs still could be reconstructed. With the temperatures increased to approximately 463,
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3.3. FT-IR
Fig. 1. DT-DSC curves of samples: (a) H3PW12O40/TiO2/SiO2, (b) H3PMo12O40/TiO2/SiO2, (c) H4SiW12O40/TiO2/SiO2.
FT-IR spectra have been proven to be a powerful technique for study of surface interaction between SiO2 and TiO2, organic species. Fig. 3 shows the Frourier transformed infrared spectroscopy of as-prepared composites. On Fig. 3, the 930 and 1210 cm 1 are, respectively, assigned to the vibration of Ti–O –Si and Si– O – Si bond, which implies that the TiO2 crystallites highly dispersed on the silica surface have been formed by the Ti atoms binding to the silica via oxygen as a bridge. In addition, for three supported HPA samples, the four typical adsorption bands of heteropolyacid from 700 to 1100 cm 1 still remained, which indicates that Keggin structure of H3PW12O40, H4SiW12O40 and H3PMo12O40 was not destroyed in the anchoring procedure. Therefore, in this study, although the silica surface is very inert, the surface hydroxyls still have been utilized as adsorption/reaction sites due to their hydrophilic character. 3.4. SEM and BET
440, 372 jC, respectively, the H3PW12O40, H4SiW12 O40 and H3PMo12O40 on the TiO2/SiO2 surface began to decompose and the corresponding Keggin structure was destroyed. In contrast, it seemed that the thermal stability of H3PW12O40, H4SiW12O40 and H3PMo12 O40 on TiO2/SiO2 were slightly lower than that of the parent HPA.
SEM photographs of as-prepared composites were conducted to obtain the shape and diameter of particles. For example, Fig. 4 shows the picture of TiO2/SiO2. It can be seen that the obtained particles have smooth external surface and the diameter is within 300– 350 nm. The other three samples shared the similar surface
3.2. XRD Fig. 2 shows the XRD patterns of as-prepared composites within the 2h range of 15 – 70j. It can be seen that all samples almost share the same peaks that can be indexed as characteristic (110) reflections of TiO2, (standard anatase TiO2 appears at 2h = 25.4j, rutile TiO2 at 27.3j). This indicates that the samples consist of anatase crystallite, and no rutile or brookite crystallite existing. In addition, it should be pointed that hardly the presence of H3PW12O40, H4SiW12O40 and H3PMo12O40 crystallite phase peaks is due to the low content in samples. The peaks corresponding to amorphous SiO2 are also not present, however, in other separate experiments, it has been verified that this peak exited since the SiO2 crystallized at approximately 1200 jC.
Fig. 2. XRD patterns of samples: (a) TiO2/SiO2, (b) H3PW12O40/ TiO2/SiO2, (c) H4SiW12O40/TiO2/SiO2, (d) H3PMo12O40/TiO2/SiO2.
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Table 1 Special surface area, pore diameter and size of sample Sample
Special surface Pore diameter Sample area (m2/g) (nm) size (nm)
TiO2/SiO2 H3PW12O40/TiO2/SiO2 H4SiW12O40/TiO2/SiO2 H3PMo12O40/TiO2/SiO2
302.5 272.3 265.5 255.6
6.4 7.2 7.8 8.0
320 336 343 356
surface areas and pore diameter of composites were found to decrease with H3PW12O40, H4SiW12O40 and H3PMo12O40 loading, which implies that some of the pores of TiO2/SiO2 were blocked by the anchored HPA molecular. However, the benefit of using the relative large surface area of amorphous SiO2 was still retained approximately 90%. 3.5. Photocatalytic performances
Fig. 3. Frourier transformed infrared spectroscopy of samples: (a) TiO2/SiO2, (b) H3PW12O40/TiO2/SiO2, (c) H4SiW12O40/TiO2/SiO2, (d) H3PMo12O40/TiO2/SiO2.
features as TiO2/SiO2 and the detail information of diameter is listed in Table 1. Besides, the BET surface areas and pore diameter of the samples are also presented on Table 1. The total
In order to detect the photocatalytic activity, the degradation of model CBW aqueous over four asprepared composite samples were carried out, respectively. The results are shown in Fig. 5. As shown in Fig. 5, the composites of TiO2/SiO2, H3PW12O40/TiO2/SiO2, H4SiW12O40/TiO2/SiO2 and H3PMo12O40/TiO2/SiO2 exhibited the variety of photocatalytic activity. Three samples supported HPA on TiO2/SiO2, all have much more activity than bare TiO2/ SiO2. Thereinto, the H3PW12O40/TiO2/SiO2 exhibits a maximum photocatalytic activity for the degradation of CBW aqueous and the H 3 PMo 12 O 40 /TiO 2 /SiO 2
50
Ca (mg/l)
45
a b c d
40 35 30 25 20 0
Fig. 4. SEM photographs of TiO2/SiO2 composites.
30
60
90 t (min)
120
150
Fig. 5. CBW concentration vs. time profile in presence of asprepared photocatalyst: (a) TiO2/SiO2, (b) H3PW12O40/TiO2/SiO2, (c) H4SiW12O40/TiO2/SiO2, (d) H3PMo12O40/TiO2/SiO2.
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showed the minimum. This order is entirely consistent with their acid strength [10]. Namely, H3 PW12 O40 > H4 SiW12 O40 > H3 PMo12 O40 In the case of TiO2/SiO2 photocatalyst, the higher photo-activity is related to two factors: smaller particle size and higher adsorption toward the organic substrate, both of which could be cooperative in making the organic molecular accessible to the active sites on the TiO2 surface [11]. With the HPA presenting the TiO2/SiO2 surface. The electron generated in TiO2 conduction band will be scavenged by the PW12O403 , SiW 12 O 404 , PMo 12 O 403 anions; this procedure restrained the electron-hole recombination. In addition, PW12O403 , SiW12O404 or PMo12O403 could also serve as an electron carrier between the TiO2 and dioxygen to facilitate the separation of hole-electron. Thus, the supported HPAs on TiO2/SiO2 indirectly accelerate the photocatalytic destruction of CBW molecular in aqueous.
4. Conclusion The present work shows that TiO2/SiO2, H3PW12 O40/TiO2/SiO2, H4SiW12O40/TiO2/SiO2 and H3PMo12 O40/TiO2/SiO2 composites have been synthesized via impregnation. The physicochemical properties of composites characterized by TG-DSC, XRD, FT-IR, SEM and BET demonstrated that TiO2 with H3PW12O40,
H4SiW12O40 and H3PMo12O40 were simultaneously anchored on the surface of amorphous SiO2, and the H 3 PW 12O 40, H 4 SiW 12O 40 and H 3PMo 12 O 40 still remained their Keggin structure. By the test of their photocatalytic activity, it was verified that the obtained products of H3PW12O40/TiO2/SiO2, H4SiW 12O40/ TiO2/SiO2 and H3PMo12O40/TiO2/SiO2 exhibited a higher catalysis activity than that of parallel TiO2/ SiO2 for the degradation of model CBW aqueous. Thereinto, H3PW12O40/TiO2/SiO2 showed the best.
References [1] Z.C. Wang, J.F. Chen, X.F. Hu, Materials Letters 43 (2000) 87. [2] T. Tanaka, K. Teramura, T. Yamamoto, Journal of Photochemistry and Photobiology. A, Chemistry 148 (2002) 277. [3] Y.H. Hsien, C.F. Chang, Y.H. Chen, Applied Catalysis. B, Environmental 31 (2001) 241. [4] M. Ashokkumar, International Journal of Hydrogen Energy 33 (1998) 427. [5] X.T. Gao, I.E. Wachs, Catalysis Today 51 (1999) 233. [6] K. Kim, S. Lee, Journal of Physical Chemistry 105 (2001) 2539. [7] R.R. Ozer, J.L. Ferry, Environmental Science & Technology 35 (2001) 3242. [8] H. Chun, Y.Z. Wang, H.X. Tang, Chinese Journal of Catalysis 22 (2001) 186. [9] I.V. Kozhevnikov, K.R. Kloeststra, A. Sinnema, Journal of Molecular Catalysis. A, Chemical 114 (1999) 287. [10] I.V. Kozhenvnikov, Chemical Reviews 98 (1998) 171. [11] Y.M. Yu, W. Zheng, W.P. Liu, Journal of Photochemistry and Photobiology. A, Chemistry 122 (1999) 57.