Novel Silica Gel Supported TiO2 Photocatalyst Synthesized by CVD

Jun 23, 2000 - Novel Silica Gel Supported TiO2 Photocatalyst Synthesized by CVD Method. Zhe Ding .... Chemical Vapor Deposition Methods for the Contro...
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Novel Silica Gel Supported TiO2 Photocatalyst Synthesized by CVD Method Zhe Ding,† Xijun Hu,*,‡ Gao Q. Lu,† Po-Lock Yue,‡ and Paul F. Greenfield† Department of Chemical Engineering, University of Queensland, Brisbane, QLD 4072, Australia, and Department of Chemical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong Received January 31, 2000. In Final Form: April 10, 2000 TiO2 in anatase crystal phase is a very effective catalyst in the photocatalytic oxidation of organic compounds in water. To improve the recovery rate of TiO2 photocatalysts, which in most cases are in fine powder form, the chemical vapor deposition (CVD) method was used to load TiO2 onto a bigger particle support, silica gel. The amount of titania coating was found to depend strongly on the synthesis parameters of carrier gas flow rate and coating time. XPS and nitrogen ads/desorption results showed that most of the TiO2 particles generated from CVD were distributed on the external surface of the support and the coating was stable. The photocatalytic activities of TiO2/silica gel with different amounts of titania were evaluated for the oxidation of phenol aqueous solution and compared with that of Degussa P25. The optimum titania loading rate was found around 6 wt % of the TiO2 bulk concentration. Although the activity of the best TiO2/silica gel sample was still lower than that of P25, the synthesized TiO2/silica gel catalyst can be easily separated from the treated water and was found to maintain its TiO2 content and catalytic activity.

Introduction Heterogeneous photocatalysis has been widely accepted as a promising method in treating trace organic compounds in water and air.1-7 Due to its unique feature of complete mineralization of pollutants without causing secondary pollution, heterogeneous photocatalysis has been extensively studied. TiO2 in anatase form is believed to be the most efficient photocatalyst for this reaction.8,9 Pure TiO2 is typically available as a fine powder.10 There is not much of a problem when using these powders in removing pollutants for air purification, because they are packed as a fixed phase.11 It is, however, difficult to apply TiO2 powders in liquid phase. By suspending them in water, the best distribution, and hence the performance, of a catalyst can be obtained. The biggest obstacle in liquid applications is recycling the used photocatalyst. Degussa P25, of average particle size 25-30 nm, is very difficult to separate from treated water. Another problem associated with P25 is that the TiO2 powder may agglomerate in the aqueous solution thus losing its activity. Therefore, * To whom all correspondence should be addressed. Tel: +852 2358 7134. Fax: +852 2358 0054. Email: [email protected]. † University of Queensland. ‡ Hong Kong University of Science and Technology. (1) Malati, M. A. Environ. Technol. 1995, 15, 1093. (2) Serpone, N. Solar Energy Mater. Solar Cells 1995, 38, 369. (3) Oberg, V.; Goswami, D. Y.; Svedberg, G. Joint Solar Engineering Conference ASME; Klett, D. E., Hogan, R. E., Tanaka, T., Eds.; ASME: San Francisco, 1994; p 147. (4) Lu, M. C.; Roam, G. D.; Chen, J. N.; Huang, C. P. Chem. Eng. Commun. 1995, 139, 1. (5) Zhao, J.; Hidaka, H.; Takamura, A.; Pelizzetti, E.; Serpone, N. Langmuir 1993, 9, 1646. (6) Ollis, D. F.; Al-Ekabi, H. Photocatalytic Purification and Treatment of Water and Air; Elsevier Science: Lausanne, Switzerland, 1993. (7) Linsebigler, A. L.; Lu, G.; Yates, J. J. T. Chem. Rev. 1995, 95, 735. (8) Sclafani, A.; Palmisano, L.; Davi, E. J. Photochem. Photobiol. A: Chem. 1991, 56, 113. (9) Vidal, A.; Herrero, J.; Romero, M.; Sanchez, B.; Sanchez, M. J. Photochem. Photobiol. A: Chem. 1994, 79, 213. (10) Ito, S.; Inoue, S.; Kawada, H.; Hara, M.; Iwasaki, M.; Tada, H. J. Colloid Interface Sci. 1999, 216, 59. (11) Reztsova, T.; Chang, C. H.; Koresh, J.; Idriss, H. J. Catal. 1999, 185, 223.

great efforts have been made to improve the recovery rate of the photocatalayst while maintaining its activity. Early work focused on coating TiO2 on fixed supports such as glass beads,12 sand,13 glass fibers,14,15 reactor walls,16-18 etc. The most common method in coating TiO2 on a support is the sol-gel method.19,20 Direct coating of P25 by dispersing supports in P25 suspension followed by drying and calcination was also tried.14,15,21,22 The recovery problem was somewhat overcome, but the activity was also decreased greatly because of the mass transfer limitation of pollutants to the surface of the photocatalyst. A suspension system is more advantageous in obtaining better mixing of photocatalyst with both light beams and organics in water. Therefore, this study will focus on TiO2 coating on particles that are small enough for suspension by bubbling oxygen through or stirring but large enough for efficient recovery by settling or filtration. Particles in the size range of hundreds of micrometers are suitable for this purpose. CVD is another good potential method for coating TiO2 onto particle supports. Recently, a series of works by (12) Trillas, M.; Peral, J.; Domenech, X. J. Chem. Technol. Biotechnol. 1996, 67, 237. (13) Matthews, R. W. Wat. Res. 1991, 25, 1169. (14) Peill, N. J.; Hoffmann, M. R. Environ. Sci. Technol. 1996, 30, 2806. (15) Murabayashi, M.; Itoh, K.; Kawashima, K.; Masuda, R.; Suzuki, S., Photocatalytic Purification and Treatment of Water and Air; Ollis, D. F., Al-Ekabi, H., Eds.; Elsevier Science, Lausanne, Switzerland, 1993; p 783. (16) Mills, A.; Worsley, D.; Davies, R. H. Chem. Soc., Chem. Commun. 1994, 23, 2677. (17) Brezova, V.; Jankovicova, M.; Soldan, M.; Blazkova, A.; Rehakova, M.; Surina, I.; Ceppan, M. J. Photochem. Photobiol. A: Chem. 1994, 83, 69. (18) Fernandez, A.; Lassaletta, G.; Jimenez, V. M.; Justo, A.; Gonzalez-Elipe, A. R.; Herrmann, J. M.; Tahiri, H.; Ait-Ichou, Y. Appl. Catal. B: Environ. 1995, 7, 49. (19) Yasumori, A.; Yamazaki, K.; Shibata, S.; Yamane, M. J. Ceramic Soc. Jpn. 1994, 102, 700. (20) Ichinose, H.; Katsuki, H. J. Ceramic Soc. Jpn. 1998, 106, 344. (21) Matthews, R. W. Solar Energy 1987, 38, 405. (22) Tennakone, K.; Tilakaratne, C. T. K.; Kottegoda, I. R. M. J. Photochem. Photobiol. A: Chem. 1995, 87, 177.

10.1021/la000119l CCC: $19.00 © 2000 American Chemical Society Published on Web 06/23/2000

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Figure 1. Schematic diagram of the CVD system.

Leboda and co-workers covered the CVD synthesis of titania/silica gel23 and titania/fumed silica.24 In addition, CVD preparations of titania/silica gel and titania/ZSM-5 were also reported by Vansant and co-workers25 and Stakheev and co-workers,26 respectively. In all these reports, titanium tetrachloride (TiCl4) was applied as the precursor for CVD reaction and a two-step synthesis procedure was used. TiCl4 was first introduced and adsorbed onto the supports, then water vapor was brought through to start the hydrolysis of those adsorbed TiCl4 species. Various techniques have been applied to study the porous structure and surface properties of the CVD and sol-gel synthesized titania/silica materials,27-29 which showed great potential for application as catalyst or catalyst supports. However, little work has been conducted in this area, particularly in the application to photocatalyst preparation.30 In this study, the CVD technique is applied to prepare TiO2 supported on porous silica gel (60-100 mesh). A different TiO2 precursor and synthesis procedure are used. The developed photocatalysts are evaluated in the photooxidation reaction of phenol in aqueous solution. Experimental Section Synthesis of the TiO2/Silica Gel Photocatalyst. The synthesis of TiO2/silica gel photocatalyst involves three steps, namely, pretreatment of the support materials, CVD reaction, and calcination. A few drops of water were first added to silica gel (Aldrich, 60-100 mesh), then the wetted silica gel was dried for 20 min at 110 °C. The prepared silica gel sample still had water adsorbed and it was ready for CVD treatment. A schematic diagram of the CVD system is shown in Figure 1. (23) Leboda, R.; Gun’ko, V. M.; Marciniak, M.; Malygin, A. A.; Malkin, A. A.; Grzegorczyk, W.; Trznadel, B. J.; Pakhlov, E. M.; Voronin, E. F. J. Colloid Interface Sci. 1999, 218, 23. (24) Gun’ko, V. M.; Zarko, V. I.; Turov, V. V.; Leboda, R.; Chibowski, E.; Holysz, L.; Pakhlov, E. M.; Voronin, E. F.; Dudnik, V. V.; Gornikov, Y. I. J. Colloid Interface Sci. 1998, 198, 141. (25) Schrijnemakers, K.; Impens, N. R. E. N.; Vansant, E. F. Langmuir 1999, 15, 5807. (26) Stakheev, A. Y.; Lee, C. W.; Chong, P. J. Bull. Korean Chem. Soc. 1998, 19, 530. (27) Stakheev, A. Y.; Shpiro, E. S.; Apijok, J. J. Phys. Chem. 1993, 97, 5668. (28) Crocker, M.; Herold, R. H. M.; Wilson, A. E.; Mackay, M.; Emeis, C. A.; Hoogendoorn, A. M. J. Chem. Soc., Faraday Trans. 1996, 92, 2791. (29) Gun′ko, V. M.; Zarko, V. I.; Turov, V. V.; Leboda, R.; Chibowski, E. Langmuir 1999, 15, 5694. (30) Lei, L.; Chu, H. P.; Hu, X.; Yue, P.-L. Ind. Eng. Chem. Res. 1999, 38, 3381.

The CVD reactor was a quartz tube with a porous quartz disk at one end. The reactor temperature was controlled by a tubular furnace. The reactor could be used either vertically or horizontally. In the vertical arrangement, a medium porosity (40-90 µm) quartz disk was used, while in the horizontal arrangement, an extra coarse porosity (150-200 µm) disk was used. Titanium tetra-isopropoxide (TTIP) was chosen as the precursor because it is relatively easier to evaporate and milder than TiCl4 in reacting with water vapor.31-33 TTIP was stored in a separate container and introduced into the CVD reactor by bubbling nitrogen through. The temperature of TTIP was kept at 80 °C using a water bath. After the silica gel was loaded into the CVD reactor, the precursor was introduced to the CVD reactor by the carrier gas (nitrogen) under vacuum. The CVD reactor was maintained at room temperature for 10 min first and then heated to 300 °C and kept at this temperature for a certain period of time. After the CVD reaction, the vacuum pump was shut down. After the pressure in the CVD system slowly returned to atmospheric, the sample was moved into a furnace to calcine at 500 °C for 3 h in air flow. Characterization of the TiO2/Silica Gel Photocatalyst. X-ray diffractometer (XRD) (Philips PW 1830, Cu KR radiation) was used to detect the anatase and rutile crystallite phase of TiO2. The sample was ground into a fine powder and mixed well before loading into the sample holder. The peak area was quantitatively measured and used to represent the TiO2 bulk concentration. The specific surface area of the sample was measured by nitrogen ads/desorption method using ASAP 2000 (Micromeritics). Before each analysis, the sample was degassed at 300 °C for 3h. The BET, BJH, and t-plot methods were used to determine the surface area, pore size distribution, and micropore area and volume, respectively. The element concentration on the surface of the TiO2/silica gel sample was measured by X-ray photoelectron spectroscopy (XPS) (Physical Electronics, PHI 5600). The particle sample was loaded onto the holder without grinding in order to obtain information on the external surface of the particles. For each sample, a fast scan detected the type of element on the surface. Then a slow scan was performed for each element, C (1s), O (1s), Si (2s), and Ti (2p). The calculated TiO2 concentration was used as the TiO2 external surface concentration. A thermogravimetric analyzer (TGA) (DuPont, 2950) was used to measure the weight loss of the silica gel before and after the (31) Babelon, P.; Dequiedt, A. S.; Mostefa-Sba, H.; Bourgeois, S.; Sibillot, P.; Sacilotti, M. Thin Solid Films 1998, 322, 63. (32) Siefering, K. L.; Griffin, G. L. J. Electrochem. Soc. 1990, 137, 814. (33) Siefering, K. L.; Griffin, G. L. J. Electrochem. Soc. 1990, 137, 1206.

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Figure 2. Schematic diagram of the photoreactor and the comparison of settling speeds between P25 and TiO2/SiO2 samples. (a) Full image of the photoreactor; settling of TiO2/SiO2 sample after oxygen bubbling shutoff (b) 0 s; (c) 15 s; (d) 1 min; settling of P25 after oxygen bubbling shutoff (e) 0 min; (f) 10 min; (g) 30 min. CVD reaction. The temperature was increased from 25 to 1000 °C at a ramping rate of 5 °C/min. UV-visible diffuse reflectance spectrometry (DRS) was conducted on a PU 8700 series UV-vis spectrophotometer with an integration sphere accessory and using BaSO4 as a standard reference. The samples were ground into fine powders before analysis. A slow scan at a rate of 125 nm/min was performed. The morphology of the TiO2 in the TiO2/silica gel sample was observed by transmission electron microscope (TEM) (JEOL 2010 HREM). The samples were ground first because the particles were too big for the holder. The photocatalytic activities of different samples together with Degussa P25 were determined for the oxidation of phenol in water. The photoreactor was cylindrical with a total volume of 150 mL. One UV light tube (Philips, 8W ultraviolet fluorescent tube, 365 nm) was inserted in the center as the light source. Oxygen was bubbled through from the bottom to suspend the particle samples. The digital camera image of the reactor with catalysts of both TiO2/silica gel and Degussa P25 is illustrated in Figure 2. The initial phenol concentration was 20 ppm and the concentrations of the catalysts were 10 g/L and 0.5 g/L for TiO2/silica gel samples and P25, respectively. After mixing the catalyst with the phenol solution for half an hour, the light was turned on, which was taken as the onset of the reaction. The phenol concentration was measured by a standard 4-aminoantipyrine colorometric method. Also shown in Figure 2 are the settling speed of the TiO2/silica gel and Degussa P25 catalysts. The TiO2/silica gel catalyst automatically settles by gravity after 1 min so it can be easily separated, recovered, and reused. The Degussa P25 does not

settle even after 30 min. This demonstrates the advantage of the TiO2/silica gel over Degussa P25.

Results and Discussion Effect of Synthesis Conditions on Anatase Coating. The weight loss and differential thermal analysis (DTA) of the silica gel before and after CVD reaction are shown in Figure 3. It can be seen that after the initial wetting, silica gel contains a certain amount of water, nearly half of the total weight. Water in the silica gel underwent continuous evaporation until fully dried at 100-120 °C (as confirmed from the negative peak in DTA). The further slight weight loss above 300 °C is attributed to the desorption of chemically adsorbed species on the silica gel. Therefore, during the CVD synthesis, the first 10 min deposition at room temperature is important, which allows TTIP to hydrolyze with adsorbed water on the surface of the support. From the TGA curve of silica gel after CVD reaction, it is seen that there is a similar but lower level weight loss at the beginning (25-150 °C), which arises from water desorption since the sample was kept in air for a couple of days before analysis. From 300 °C to 1000 °C, there is a continuous small weight loss (1.4 wt % of total weight), which is due to the further decomposition of residues of TTIP on the support. This is also

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Figure 5. XRD diagram of TiO2/SiO2 sample before and after calcination.

Figure 3. TGA analysis of silica gel before and after CVD coating. (a) weight loss; (b) differential thermal analysis (DTA).

Figure 4. UV-visible DRS spectra of samples.

observed from the color change of the samples, from gray (right after CVD coating) to white (after calcination). Figure 4 shows the UV absorption of TiO2/silica gel samples before and after calcination, together with a blank silica gel, bulk anatase TiO2 sample, and Degussa P25. It is seen that silica gel is nearly transparent. There is a blue shift of TiO2/silica gel samples compared to the bulk anatase TiO2, while the absorption threshold for P25 is nearly the same as the bulk sample. This indicates that the primary particle size of anatase in TiO2/silica gel is smaller than P25, which is beneficial for the photocatalytic reaction, because they could provide a more effective surface for organic reactants and light absorption. In addition, comparing the two TiO2/silica gel samples, it is seen that there is much more absorption in the higher wavelength range for the TiO2/silica gel sample without calcination, which indicates that there are still some organic compounds left on the silica gel support after CVD reaction. Therefore, the calcination step after the CVD

deposition is necessary in order to obtain a better crystallized anatase form of TiO2. Figure 5 shows the XRD patterns of TiO2/silica gel samples before and after calcination. The peak at 2θ ) 25.3° is the anatase crystal phase. There is not much difference between the two samples. Anatase can be generated during the CVD process at the deposition temperature of 300 °C. However, the position of the peak after calcination shifts slightly to the lower angle, which further verifies that the calcination step is beneficial for obtaining better anatase crystallites and the crystal size is not significantly affected by calcination for 3 h at 500 °C. The effects of two CVD synthesis parameters, coating time and flow rate of the carrier gas, on the TiO2 bulk concentration calculated from the XRD results are demonstrated in Figure 6. It is seen that the TiO2 bulk concentration is proportional to both coating time and flow rate. Increasing the coating time to over 7 h, the increase in the TiO2 bulk concentration starts to slow and seems to reach a plateau. Because the design of the reactor did not allow the flow rate to be too high, in the range studied, the positive proportional relation applies for all flow rates. Regarding the values of TiO2 bulk concentration, for samples synthesized under the same conditions, the results could deviate 10-30% from the average value. The reproducibility is better for samples with higher titania loading rates. More interestingly, it is found that the size of the porous quartz disk also plays an important role in the titania loading rate. When a new reactor tube with a fresh porous disk is used in the horizontal arrangement, the coated titania amount is higher, which is represented in Figure 6 as open circular symbols. It is believed that during the CVD reaction, TiO2 was deposited not only on the silica gel particle supports, but also on the quartz disk and reactor wall. After using these reactor tubes for a couple of weeks, the disk became blocked. Although all other synthesis parameters are kept the same, the reaction condition is still different because of the gradual change in porosity of the disk. This might explain the variations in the results obtained under the same conditions. Once the disk is partially blocked, the flow rate will drop; thereafter, less precursor can pass through, leading to the lower titania loading rate. Two arrangements of the reactor were tried. The product from the horizontal reactor is more uniformly coated than that from the vertical reactor. The titania coated onto

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Table 1. Characteristics of SiO2/TiO2 Samples sample no

TiO2 bulk conc (wt %)

TiO2 external surface conc (wt %)

total surface area (m2/g)

mesopore size (Å)

micropore surface area (m2/g)

micropore volume (cm3/g)

micropore size (Å)

K (min-1)

1 2 3 4 5 6 7 silica gel

1.91 2.38 2.74 4.12 6.08 6.34 9.61 s

5.22 8.02 14.3 21.6 23.6 30.0 41.0 s

271 286 280 259 272 275 265 294

139 138 138 137 137 140 136 139

28.3 22.4 22.5 21.5 22.9 16.7 19.6 26.9

0.0100 0.006 80 0.007 03 0.007 03 0.007 44 0.004 41 0.005 77 0.008 84

7.07 6.07 6.25 6.54 6.50 5.28 5.89 6.57

0.0025 0.0119 0.0143 0.0178 0.0264 0.0437 0.0119 s

Figure 6. Effect of CVD synthesis parameters on the TiO2 bulk concentration. (a) Effect of coating time; (b) effect of flow rate.

small particles by CVD is a diffusion controlled process. In the horizontal reactor, the support layer is thinner and the interfacial area is higher, resulting in a more uniform coating. The TiO2 bulk concentration obtained in the vertical reactor is lower than that in the horizontal reactor. Effect of TiO2 Amount on Photocatalytic Activity. Seven samples with various TiO2 bulk concentrations were selected for catalytic activity comparison. The samples are numbered in the order of their titania loading from low to high. The characteristics of these samples together with the original silica gel sample were listed in Table 1. It is seen that compared with the original silica gel sample, the surface area of TiO2/silica gel dropped a little and as the titania loading increases, the surface area decreases slightly. Comparing with the total surface area, this difference is small. For all the samples, mesopore surface area is dominant and less than 10% of total surface area comes from the micropore surface area. There is no clear relation between the TiO2 bulk concentration and the ratio of micropore surface area to total surface area or the average micropore size. Therefore, coating titania onto silica gel by CVD did not bring a dramatic change

in the micropore structure of the silica gel. Considering that the maximum TiO2 bulk concentration is 9.61%, this result is reasonable. In addition, the average mesopore size decreases slightly upon the increase of TiO2 bulk concentration. Again, compared with the mesopore size, this reduction is very small. These results imply that the titania coating is mainly on the external surface of the silica gel support and the layer of titania on the internal pore surface should be very thin. This is proved by the XPS results, which show that the TiO2 external surface concentration is always much higher than its bulk concentration (Table 1). There might also be some titania particles coated on the mesopores of the silica gel and the coating is either few layers or loosely packed agglomerates of titania particles that would not block the pores. For the photocatalytic reaction, the external surface is more important, since the reaction is light excited and the external surface has much more chance to receive light.34 Although from Figure 4 it is seen that silica gel has little absorption in the near UV light range, the absorption of light by TiO2 particles on the external surface might prevent the light from penetrating the silica gel to reach the TiO2 particles located in the mesopores. Therefore, this coating result is beneficial when applying TiO2/silica gel as the photocatalyst. Furthermore, a large surface area is also useful, as it provides more space for both free radicals and organic compounds, leading to better oxidation activity. The morphology of the TiO2/silica gel samples is shown in Figure 7. For samples with various titania concentrations, the titania crystal size is very similar, all between 10 and 20 nm. This explains the blue shift observed in Figure 4. Since the samples were ground before the TEM analysis, it is difficult to distinguish the TiO2 coating on the external surface and that on the internal surface. However, it is clearly observed that there are two types of combination between the silica gel support and small TiO2 particles. In one case (Figure 7a), there is a thick TiO2 layer, around 200 nm, on the surface of the silica gel support, while in another case (Figure 7b), there is only a thin TiO2 layer or small TiO2 agglomerate on the surface of silica gel. It is believed that the thick layer reflects the external surface, while the thin TiO2 layer reflects the internal surface. There are also a lot of isolated TiO2 particles (Figure 7c), which are possibly separated particles as a result of the grinding. The thickness of the TiO2 layer on the external surface would be around several hundred nanometers. In addition, from Figure 7b, it is seen that some TiO2 agglomerates are located in the porelike corner of the silica gel support, which might be the inner part of the silica gel. The photocatalytic activity of the TiO2/silica gel samples together with P25 is illustrated in Figure 8 for the oxidation of phenol. The first-order initial reaction rate (34) Ding, Z.; Zhu, H. Y.; Lu, G. Q.; Greenfield, P. F. J. Colloid Interface Sci. 1999, 209, 193.

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Figure 8. Photocatalytic activities of P25 and TiO2/SiO2 samples in the degradation of phenol in water.

constants (K) for TiO2/silica gel samples are given in Table 1. It can be seen that there is an optimum loading rate to obtain the highest activity, which is around 6 wt % of the TiO2 bulk concentration. When further increasing the loading, the titania layer becomes thicker and it is more difficult for light to penetrate and for organic compounds to diffuse through. Furthermore, with a high loading rate, the small titania crystals tend to agglomerate which also has an adverse effect on the performance of the catalyst. From Table 1, it is seen that the relation between the activity and the TiO2 bulk concentration or TiO2 external surface concentration follows a similar trend. However, the activities of samples 5 and 6 are significantly different, although they have similar TiO2 bulk concentrations. The improvement in the activity for sample 6 compared with sample 5 is believed to be due to the difference in the TiO2 external surface concentrations (23.6% for sample 5 and 30% for sample 6). For a similar bulk concentration, a higher external surface concentration indicates that more titania particles are dispersed over the external surface, which could provide more active sites for the reaction, particularly for light absorption. Therefore, the photocatalytic activity of TiO2/silica gel sample is more related to TiO2 external surface concentration than to TiO2 bulk concentration, which also verifies that the external surface coating of TiO2 has a stronger effect on the activity than the internal coating. From Figure 8, it is seen that the activity of the best TiO2/silica gel sample is still lower than that of P25. Good performance of P25 is attributed to its superior dispersion in aqueous solution, which could be seen from Figure 2, high crystallinity and a trace Fe3+ doping (0.012 wt. %),35 which is a surface impurity introduced during the synthesis process. On the other hand, P25 is difficult to separate from the solution. The TiO2 coating prepared by the CVD method is very stable. There is little decrease in titania amount after several photocatalytic reactions followed by flushing with water for 2 days. The main advantage of the TiO2/silica gel photocatalyst compared with Degussa P25 is in the aspect of photocatalyst recycling. From Figure 2, it is clear that the TiO2/silica gel sample can be easily settled and removed from water. Moreover, there is a great potential to further improve Figure 7. TEM images of TiO2/SiO2 samples. (a) Thick TiO2 layer; (b) small TiO2 agglomerate; (c) detached TiO2 particles.

(35) Zhang, Z.; Wang, C.-C.; Zakaria, R.; Ying, J. Y. J. Phys. Chem. B: Chem. 1998, 102, 10 871.

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the photocatalytic activity of TiO2/silica gel by metal doping, which will be studied in the future. Conclusions TiO2/silica gel catalysts with various amounts of titania loading were synthesized by the CVD method. The anatase crystallite size for all the TiO2/silica gel samples is quite uniform in the range of 10-20 nm. The amount of titania loading is directly proportional to the flow rate and the coating time in the CVD treatment. Apart from these two parameters, the degree of pore openness of the porous quartz disk in the CVD reactor is also found to be important for the final titania loading. Reducing the pore openness of the disk will result in a reduced amount of titania being coated. There is little change on the pore structure and surface area of silica gel during the CVD process. The final product still has over 250 m2/g surface area and the

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average pore size is round 137 Å. Most of the TiO2 particles are deposited on the external surface of the silica gel and the adhesion of TiO2 particles on the surface is very strong. The photocatalytic activity is related to the titania amount and the optimum loading of titania is around 6 wt % of the TiO2 bulk concentration. The activity of the best TiO2/ silica gel sample is nearly half of that of P25. The TiO2/ silica gel particles have an advantage in that they can be easily separated and recycled from the treated water by filtering or settling. Acknowledgment. The authors are grateful for financial support from the Research Grants Council of Hong Kong (Project No. HKUST6035/98P) and the Australian Research Council. LA000119L