Direct synthesis of Ti-SBA-15 in the self-generated acidic environment ...

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Sep 27, 2013 - A series of Ti-containing SBA-15 samples were firstly prepared in the self-generated acidic condition. The samples were characterized by ...
J Porous Mater (2014) 21:63–70 DOI 10.1007/s10934-013-9748-5

Direct synthesis of Ti-SBA-15 in the self-generated acidic environment and its photodegradation of Rhodamine B Hongliang Zhang • Changjin Tang • Yuanyuan Lv • Fei Gao • Lin Dong

Published online: 27 September 2013 Ó Springer Science+Business Media New York 2013

Abstract A series of Ti-containing SBA-15 samples were firstly prepared in the self-generated acidic condition. The samples were characterized by powder X-ray diffraction, N2-adsorption, transmission electron microscopy, inductively coupled plasma, Fourier-transform infrared spectroscopy, UV–vis diffuse reflectance spectra (UV–vis) and X-ray photoelectron spectroscopy. All the samples possessed well-ordered hexagonal arrays of mesopores and Ti ions could be incorporated into the framework of SBA15. Catalytic performances of the obtained materials were evaluated in the photodegradation of Rhodamine B (RhB). Catalytic tests indicated that Ti-SBA-15 showed much higher photodegradation ability towards RhB than pure TiO2. Keywords Ti-SBA-15  Photodegradation  Self-generated acid  Incorporation

1 Introduction Titanium dioxide (TiO2) is one of the most promising semiconductor photocatalysts because of its stability and H. Zhang Analysis and Testing Central Facility, Anhui University of Technology, Maanshan 243002, People’s Republic of China H. Zhang  C. Tang  Y. Lv  L. Dong (&) Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, People’s Republic of China e-mail: [email protected] F. Gao Center of Modern Analysis, Nanjing University, Nanjing 210093, People’s Republic of China

non-toxicity. Based on the kinetic investigation of photocatalytic reactions, TiO2 nanoparticles in the anatase crystal form having both high crystallinity and large surface area [1] exhibit higher photocatalytic activity. This last property should increase the amount of surface-adsorbed substrate to enhance the capture of photogenerated electron and positive hole. For the control of the final features of the oxides, much work has been done to prepare Ti-substituted mesoporous SBA-15 because of its high surface areas, ordered framework and narrow pore size distribution [2]. Recently, many efforts have been made to prepare Ti-substituted mesoporous SBA-15 through post-synthetic grafting procedures or direct synthesis [3–7]. The post-synthesis method always forms the metal oxides in the channels or external surface of the catalysts, which would block the channels and not allow the reactant molecules to access all the reaction sites in the porous matrix [5]. Comparatively, the direct synthesis can avoid the pore blockage and provide well distribution of titanium species in the framework and surface without any decrease of mesopore size. Moreover, direct synthesis is simpler and energy saving. However, it is still a challenge to prepare Ti-substituted SBA-15 materials directly via the usual hydrothermal method remains. On the basis of synthetic conditions, this difficulty could be classified two ways. One is that SBA-15 materials have generally been synthesized under strongly acidic hydrothermal conditions that easily induce the dissociation of the Ti–O–Si bonds, if they have been formed. Another is the large difference in the hydrolysis rate between the titanium and silicon precursors. So far, several modified methods have been tried to incorporate titanium into SBA-15 and related mesoporous materials by one-pot synthesis. Those methods include microwave hydrothermal treatment [8], adding fluoride ions into the synthesis solution [9] or adjusting the pH value of the reaction system [10–13]. In the synthesis routes mentioned

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above, expensive organic titanium sources are normally applied and some of the preparations are complicated with special treatment or addition of acid/salts. Therefore, finding a facile and environmentally friendly route to directly synthesize Ti-substituted SBA-15 mesoporous material with high structural regularity and stability is of interest. In this paper, we reported an environmentally friendly method for Ti-incorporated SBA-15 in the self-generated acidic environment. This synthesis method was environmentally friendly because it avoided the use of hydrochloric acid. Finally, the resulting material was characterized by X-ray diffraction (XRD), N2-adsorption, transmission electron microscopy (TEM), inductively coupled plasma (ICP), Fourier-transform infrared spectroscopy (FT-IR), UV–vis and X-ray photoelectron spectroscopy (XPS). Their catalytic performance in the photodegradation of Rhodamine B was also evaluated.

2 Experimental section 2.1 Sample synthesis A typical synthesis procedure of Ti-SBA-15 was as follows: 0.75 g of nonionic triblock copolymer surfactant EO20PO70EO20 (P123, Aldrich) was dissolved in 35 ml of deionized water and stirred at 35 °C for 4 h. Then, a calculated amount of TiCl4 aqueous solution and 2.1 g tetraethyl orthosilicate (TEOS) were added and this mixture was continuously stirred at 35 °C for 24 h, finally hydrothermal in a Teflon-lined autoclave at 100 °C for 24 h. After that, the solid product was filtered, washed with deionized water and absolute ethanol for several times and finally dried in air at 100 °C for 12 h. The as-prepared product was calcined in air at 500 °C for 5 h to decompose the template. The Ti-SBA-15 was labeled as Ti(x)-SBA-15, where x denoted the nSi/nTi molar ratio in the final product. 2.2 Sample characterization The powder XRD patterns were collected on a Philips X’pert X-ray diffractometer with Cu Ka radiation (k = 0.15418 nm). Transmission electron microscopy images were taken on a JEM-2100 instrument at an acceleration voltage of 200 kV. The samples were sonicated in A.R. grade ethanol for 15 min and the resulting suspension was allowed to dry on carbon film supported on copper grids. N2-adsorption isotherms were measured at 77 K using a Micromeritics ASAP 2020 system. The samples were degassed for 160 min at 573 K in the degas port of the adsorption analyzer. The Brunauer–Emmett–Teller (BET) specific surface area was calculated using adsorption data

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in the relative pressure range from 0.04 to 0.2. The total pore volume was determined from amount adsorbed at a relative pressure of about 0.98. The pore size distribution (PSD) curves were calculated from the analysis of the desorption branch of the isotherm using the Barrett–Joyner–Halenda (BJH) algorithm. Chemical compositions were determined using a JarrellAsh 1100 inductively coupling plasma atomic emission spectrometer. The samples were completely dissolved in suitable hot acid before analysis. Fourier-transform infrared spectroscopy was carried out on a Nicolet 5700 FT-IR instrument running at 2 cm-1 resolution. UV–vis diffuse reflectance spectra (UV–vis) profiles were recorded in the range of 200 * 800 nm by a Shimadzu UV2401 spectrophotometer with BaSO4 as reference. X-ray photoelectron spectroscopy analysis was performed on a PHI 5000 Versaprobe system, using monochromatic Al Ka radiation (1,486.6 eV) operating at 15 kW. The sample was outgassed at room temperature in a UHV chamber (\5 9 10-7 Pa). All binding energies (BE) were referenced to the C 1s peak at 284.6 eV. The experimental errors were within ±0.1 eV. 2.3 Photocatalytic experiments The photocatalytic activity was tested by photodegradation of RhB in aqueous solution in a photolysis glass reactor. The UV-source was a 125 W medium pressure Hg lamp (365 nm) with a double wall jacket in which water was circulated to prevent overheating of the reaction mixture. The UV-light irradiated horizontally to the surface of the suspension and the distance from the UV source to the suspension was 10 cm. Ti-SBA-15 (5 mg) was added to a suspension of RhB (100 ml, 1 9 10-5 M) and stirred for 60 min without UV irradiation at room temperature in order to establish an adsorption–desorption equilibrium between RhB and the surface of the catalyst. Next, the solution was illuminated for 180 min with UV-light. During this illumination, the suspension (5 ml) was collected at fixed intervals (15 min) and analyzed by ultraviolet spectrophotometer (722 N from Shanghai Fine Instrument Company). The absorbance was measured at 552 nm with water as reference.

3 Results and discussion 3.1 Characterization of the structure of Ti-SBA-15 samples Figure 1a shows the small angle XRD patterns of Ti-SBA15 materials with different titanium contents. For all the

J Porous Mater (2014) 21:63–70

A

10.4

B

10.2

d100 spacing (nm)

10.0

(100)

9.8 9.6 9.4 9.2 9.0

Intensity (a.u.)

8.8

(110)

8.6

(200)

32

30

28

26

24

22

20

18

16

14

12

a

Si/Ti molar ratio

b c d 1

2

3

4

2 Theta (degree)

Fig. 1 a The small angle XRD patterns of (a) Ti(30)-SBA-15, (b) Ti(25)-SBA-15, (c) Ti(19)-SBA-15 and (d) Ti(13)-SBA-15; b change of d100 spacing with different Si/Ti molar ratio







TiO2



d

Intensity(a.u.)

samples, three well-resolved peaks at (100), (110) and (200), which are characteristic of mesoporous material with 2D-hexagonal structure, can clearly be seen. The (210) peak is too weak to be recognized probably due to the incorporation of heteroatoms into the framework of SBA15, which may cause the decrease of the ordered structure [14]. Interestingly, the intensity of the (100) peak of TiSBA-15 increases with decreasing nSi/nTi ratio. This indicates that the structural order is significantly improved with the Ti incorporation into the silica framework of SBA-15. The improvement in the structural order of Ti-SBA-15 materials upon increasing the Ti content could be due to the generation of HCl from the hydrolysis of TiCl4 added in the synthesis gel, which are highly necessary for the formation of hexagonal phase through the surfactant-silica assembly, which is best represented by an S0H?X-I? pathway (nonionic polymeric surfactant (S0), halogen anions (X-) and the protonated inorganic SiO2 species (I?)). The presence of a large amount of HCl in the synthesis gel would significantly increase the interaction between the polymeric template and the silicate species. These factors would strongly stimulate the formation of more stable and small cylindrical micelles, which are critical for obtaining the well-ordered hexagonal structure. Thus the intensity of the powder XRD patterns of Ti-SBA15 materials increases upon increasing the Ti content. Figure 1b shows the change of d100 spacing with titanium content increasing. The d100 spacing of Ti-SBA-15 increases to 10.2 nm with titanium content increasing. And the shift confirms the expansion of mesopore, likely due to the increase of Ti4? incorporated into silica pore walls, which is well consistent with those previous reports [15, 16]. This is because the Ti–O bond length is larger than the Si–O bond length due to the larger radius of Ti4? (Pauling radius = 68 pm) than that of Si4? (Pauling radius = 41 pm). However, with an increase of titanium content, the d100 spacing of the Ti(13)-SBA-15 sample decreases to 9.6 nm. The reason for this phenomenon is likely that the formation of crystalline titanium oxide (TiO2) is anticipated [17]. To learn more about the state of the formed crystalline TiO2, XRD data of the corresponding wideangle region for Ti(13)-SBA-15 is shown in Fig. 2d and the broad diffraction peaks of anatase phase (typical at 2h = 25.6°, 48° and 54°) is detected, indicating a formation of TiO2 nanocrystallites. These bands are not observed for the samples with lower titanium content (Fig. 2a–c), which suggests titanium species may be incorporated into the framework or highly dispersed on the surface of SBA15. The excellent structure ordering of Ti-SBA-15 samples can be directly observed by TEM. Figure 3 shows the TEM image of Ti(30)-SBA-15. The sample Ti(30)-SBA-15 has well-ordered hexagonal arrays of mesopores with one-

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c b a 10

20

30

40

50

60

70

80

2Theta(degree)

Fig. 2 The wide angle XRD powder patterns of (a) Ti(30)-SBA-15, (b) Ti(25)-SBA-15, (c) Ti(19)-SBA-15 and (d) Ti(13)-SBA-15

dimensional channels, indicating a 2D-hexagonal (P6 mm) mesostucture, in agreement with the XRD results. The N2-adsorption isotherms of Ti-SBA-15 are shown in Fig. 4a. All isotherms are typical type IV with an H1 hysteresis loop according to the IUPAC classification [18], which is a typical adsorption for mesoporous materials with 2D-hexagonal structure. A sharp step occurs at relatively high pressure of 0.5-0.8, corresponding to capillary condensation of nitrogen within the uniform mesopores. The typical BJH pore size distributions shown in Fig. 4b indicate narrow pore size distributions for all samples, implying the well ordering of meso-structure. The structure parameters of the samples are summarized in Table 1, where a0, ABET, dp, Vp and W denote the unit cell parameter, specific surface area, pore diameter, pore

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A a

800

3

Quantity Adsorbed (cm /g STP)

1000

b

600

c

400

d

200

0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/Po)

12

B

Pore Volume (a.u.)

10

a

8

b 6

4

c 2

d 0

Fig. 3 TEM images of Ti(30)-SBA-15: a in the direction perpendicular to the pore axis and b in the direction of the pore axis

volume and thickness of pore wall, respectively. The lattice pffiffiffi parameter (a0) is calculated according to a0 = 2d100/ 3 because of the ordered hexagonal pore structure of the samples and the thickness of pore wall (W) is estimated according to W = a0 - dp. With the Ti content increasing, a0, ABET and Vp increase, which may be due to the incorporation of Ti into the framework of SBA-15. All the samples have surface specific area as large as 710 m2 g-1 with the pore volume over 0.7 cm3 g-1 and the pore diameters are ca. 7.5 nm (Table 1). For the elemental compositions, the calcined Ti-SBA-15 materials with different Ti contents are characterized by ICP as listed in Table 1. Generally, in all cases, the nSi/nTi ratios of the calcined materials are larger than the input nSi/ nTi ratios in the synthesis gel. This is due to the high solubility of the Ti precursors in the hydrothermal synthesis conditions. The FT-IR spectra of various samples are given in Fig. 5. For pure silica SBA-15 (Fig. 5a), the bands at 1,097 and 810 cm-1 assigned to antisymmetric stretching vibration and symmetric stretching vibration of [SiO4] in SBA15 [19], shift to lower wavenumber as heteroatoms are introduced, while the band at 468 cm-1 assigned to rocking

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10

100

Pore Diameter (nm)

Fig. 4 a N2-adsorption isotherms and b the pore size distributions of (a) Ti(30)-SBA-15, (b) Ti(25)-SBA-15, (c) Ti(19)-SBA-15 and (d) Ti(13)-SBA-15

vibration of [SiO4] [19] is unchangeable. The same phenomenon has also been reported in the metal-substituted mesoporous materials [14], indicating a strong interaction between heteroatoms and silicon, e.g., Si–O–M bonds are formed. The greater bond length of Ti–O than that of Si–O leads to the decrease of the force constant (k) and the bigger atomic weights of Ti cause the increase of the reduced mass (l), hence the vibration frequencies calcupffiffiffiffiffiffiffiffi lated from the formula v = (1/2pc) k=l decrease. These changes are related to incorporation of Ti atoms into SBA15 framework. The band at ca. 970 cm-1, assigned to a stretching vibration of a [SiO4] unit bonded to heteroatoms, may be the evidence of the existence of framework metal ions [20]. However, silanol groups m (Si–OH) of mesoporous SiO2 can also contribute to this band because it exists in mesoporus SiO2 without metals [21]. Therefore, there are some disputes over the assignment of the IR absorption at 970 cm-1. In order to solve the problem of the assignment about the band at 970 cm-1, it is necessary to measure the IR spectra in situ at elevated temperatures.

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Table 1 Texture properties of Ti-SBA-15 materials with different titanium content nSi/nTi

Samples

d100 (nm)

a0 (nm)

ABET (m2 g-1)

dp (nm)

Vp (cm3 g-1)

W (nm)

a

Gel

Product

Ti(30)-SBA-15

25

30

9.0

10.4

705

7.5

0.67

2.9

Ti(25)-SBA-15

20

25

9.4

10.8

717

7.5

0.69

3.3

Ti(19)-SBA-15

15

19

10.2

11.8

735

7.5

0.71

4.3

Ti(13)-SBA-15

10

13

9.6

11.1

739

7.5

0.72

3.6

a

The nSi/nTi ratios in the calcined materials were calculated by the ICP-AES method

e 796

d

800

c

c

1083

b

1085

Transmittance(a.u.)

Transmittance (a.u.)

d

804

806

1089

a 810 1093

b a

970 468

970

1097

1200

800

400

-1

Wavenumber (cm ) 1200

Fig. 5 FT-IR spectra of (a) SBA-15, (b) Ti(30)-SBA-15, (c) Ti(25)SBA-15, (d) Ti(19)-SBA-15 and (e) Ti(13)-SBA-15

800

400 -1

Wavenumber(cm )

Fig. 6 FT-IR spectra of Ti(25)-SBA-15 recorded at different temperatures: (a) 298 K, (b) 373 K, (c) 473 K and (d) 573 K

According to the previous research [22], the intensity of the adsorption band (970 cm-1) for Si–OH decreased and disappeared completely by increasing temperatures, while this band for Si–O–M did not disappear at elevated temperature. Therefore, the sample Ti(25)-SBA-15 is diluted in KBr and pressed into pellets to obtain well-resolved bands in lower region. The pellet is heated in the IR cell for 20 min at the required temperature and then recorded the spectrum. The band at 970 cm-1 gradually decreases with increasing temperature and is hardly observed at 573 K (Fig. 6). Thus this band at 970 cm-1 for Ti-SBA-15 samples is due to Si–OH groups. Therefore, the band at 970 cm-1 cannot be taken as proof of metal ions incorporation into the framework of SBA-15. Diffuse-reflectance UV–vis spectroscopy is a sensitive tool that is widely used to detect the presence of framework and extraframework titanium species. The diffuse reflectance UV–vis spectra for Ti-SBA-15 samples given in Fig. 7 exhibit two bands. A band with a maximum at about 220 nm is attributed to a ligand-to-metal charge-transfer

transition in isolated TiO4 units [8]. It is generally believed to connect directly with the framework Ti4? in tetrahedral coordination and is usually used as direct proof that titanium atoms have been incorporated into the framework of a molecular sieve [8]. A second absorption band maximum at about 290 nm is observed, which corresponds to the presence of extra framework titanium species. The band near 290 nm represents a blue shift from 320 nm band for anatase TiO2. Such a blue shift has been reported for TiO2 nanoparticles through quantum size effect [23]. The band gap determined from the spectrum is ca. 3.4 eV, which is bigger than that of bulk TiO2 in anatase phase (3.2 eV). The surface chemical compositions and chemical states of the samples are investigated using XPS. The XPS peak for C 1s at binding energy of 284.6 eV is ascribed to the adventitious carbon from the XPS instrument. The relative atomic ratios of various elements are shown in Table 2. The results show that the amounts of Ti4? increase with decreasing Si/Ti ratios. However, it should be noted that

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220

Absorbance (a.u.)

290

e d c b

320

a 200

300

400

500

600

700

800

900

1000

Wave length (nm)

Fig. 7 UV–vis spectra of (a) TiO2, (b) Ti(30)-SBA-15, (c) Ti(25)SBA-15, (d) Ti(19)-SBA-15 and (e) Ti(13)-SBA-15

Table 2 Semiquantitative analysis of surface atomic concentration (% in molar ratio) over Ti-SBA-15 materials derived from XPS data Sample

Atomic concentration

Atomic ratio

Si (at.%)

Ti (at.%)

O (at.%)

Si/Ti

Ti(30)-SBA-15

26.31

0.18

68.93

146

Ti(25)-SBA-15

26.10

0.35

69.28

74

Ti(19)-SBA-15

24.80

0.38

69.25

65

Ti(13)-SBA-15

23.68

1.42

69.08

17

Fig. 8 High-resolution XPS spectra of a Ti 2p and b O 1s of (a) Ti(30)-SBA-15, (b) Ti(25)SBA-15, (c) Ti(19)-SBA-15 and (d) Ti(13)-SBA-15

A

atomic ratios of Si/Ti in the final products by XPS are higher than that by the ICP-AES method (Table 1). As is known, the ICP-AES analysis is a technique to detect the whole composition of the materials, while the XPS analysis is used to detect the surface composition of the materials. Thus, it can be concluded that the Ti contents in the bulk are higher than on the surface, indicating that Ti species are incorporated into the framework of the SBA-15. Figure 8a shows the Ti 2p XPS spectra, which demonstrate a Ti 2p3/2 and Ti 2p1/2 doublet with a separation of 5.8 eV for the Ti-SBA-15 samples with varying titanium content. The position of the Ti 2p line is located at quite similar values (459.3 eV) with low titanium contents (Si/Ti = 30–19). As the Ti content further increases to Si/Ti = 13, the Ti 2p3/2 line shifts to 458.9 eV, which is close to the binding energy of Ti 2p3/2 of pure TiO2 anatase (458.6 eV) [24], indicating the presence of TiO2 in this sample, in agreement with the XRD results. Figure 8b shows O 1s XPS spectra of Ti-SBA-15 samples. Only one peak centered at 532.7 eV is seen on the Ti-SBA-15 samples with low titanium contents (Si/ Ti = 30–19), indicating that titanium ions are incorporated into the SBA-15 framework [23]. However, another peak at 529.8 eV can be seen on Ti(13)-SBA-15 samples, which is because at higher loadings anatase titanium species are formed as a second phase. Therefore, this conclusion, combined with XRD, confirms that titanium ions are successfully incorporated into the framework; however, anatase TiO2 clusters appear as a second phase with the increase of titanium content.

B

Ti 2p3/2

532.7

458.9

Ti 2p1/2

Intensity (a.u.)

Intensity (a.u.)

d

459.3

c

a

a

464

460

456

Binding Energy (eV)

123

c

b

b

468

529.8

d

452

540

536

532

528

Binding Energy (eV)

524

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Ti(13)-SBA-15 Ti(19)-SBA-15 Ti(25)-SBA-15 Ti(30)-SBA-15 TiO2

1.0

0.8

C/C0

0.6

0.4

elementary charges involved in the redox process, F is the Faraday constant, and E represents the band gap of the semiconductor. The Ti-SBA-15 samples exhibit more powerful redox ability than the pure TiO2 for its larger band gap according to the UV–vis results.

4 Conclusions Dark

0.2

0.0 -60

-30

0

30

60

90

120

150

180

Time(min)

Fig. 9 The photocatalytic activity of Ti-SBA-15 and TiO2

3.2 Photocatalytic activity The photocatalytic activities of Ti-SBA-15 and anatase TiO2 are selected to conduct the decoloration of dyes aqueous RhB solution under UV light illumination. As illustrated in Fig. 9, we plot the RhB degradation C/C0 (C and C0 are the equilibrium concentration of RhB after and before UV irradiation, respectively) versus UV-light irradiation time in the presence of various photocatalysts. The concentration of RhB decreases with irradiation time implying its degradation. RhB molecules could not be degraded at this condition under ultraviolet (UV) irradiation without catalysts. As is shown in Fig. 9, the photocatalytic activity of Ti-SBA-15 increased with decreasing Si/Ti ratio. The results in Fig. 9 obtained by using the same catalyst loading (5 mg) show that Ti(13)-SBA-15 catalyst has a similar reaction rate with the anatase TiO2. If we compare the titanium content in Ti-SBA-15 with that in TiO2, it can be thought that the titanium in Ti-SBA-15 framework acts as a more active photocatalytic site than that in TiO2 since the amount of titanium is much lower in the Ti-SBA-15 compared to TiO2 catalyst. It may be due to higher dispersion of Ti in SBA-15 structure as compared with bulk TiO2. It is proposed that the higher dispersion provides more surface active sites for the adsorption of reactants molecules and makes the photocatalytic process more efficient. Additionally, it is generally accepted that a larger band gap corresponds to more powerful redox ability. Because the photocatalytic process system can be considered similar to an electrochemical cell, the increase in band gap results in an enhanced oxidation–reduction potential on the basis of the equation [25]: DG = -zFE, where DG is the Gibbs free energy change of the redox process occurring in the system, z is a positive integer equal to the number of

The titanium-containing SBA-15 is prepared without the addition of mineral acids and the photocatalytic activity of synthesized Ti-SBA-15 materials is tested by decomposition of RhB. The results demonstrate that the titanium ions are successfully incorporated into the framework of SBA15, and TiO2 crystallites are formed and locate on the external surface of SBA-15 with the increase of titanium loading (Si/Ti = 13). The photoreaction confirms that titanium ions in the SBA-15 framework are more active for the photocatalytic reaction in water treatment. Acknowledgments Financial support from the National Natural Science Foundation of China (No. 20973091) and the National Basic Research Program of China (No. 2009CB623500) are greatly acknowledged.

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