Surface Modification of a Magnetic SiO2 Support and Immobilization of a Nano-TiO2 Photocatalyst on It

CHINESE JOURNAL OF CATALYSIS Volume 30, Issue 9, September 2009 Online English edition of the Chinese language journal Cite this article as: Chin J Catal, 2009, 30(9): 939–944.

RESEARCH PAPER

Surface Modification of a Magnetic SiO2 Support and Immobilization of a Nano-TiO2 Photocatalyst on It WANG Liyan1, WANG Hongxia2,*, WANG Aijie1,#, LIU Min2 1

State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (SKLUWRE, HIT), Harbin 150001, Heilongjiang, China

2

Provincial Key Lab for Nano-functionalized Materials and Excitated State, Harbin Normal University, Harbin 150025, Heilongjiang, China

Abstract: This study presented an effective method to modify the surface chemical reactivity of a SiO2/Fe3O4 support. The unmodified SiO2/Fe3O4 support was prepared by the hydrolysis and condensation of tetraethoxysilane on the surface of hydrophilic Fe3O4 nanoparticles. These were then modified by a heat treatment in an ethanol/water solution under reflux. The resulting samples were characterized by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and transmission/scanning electron microscopy. The immobilization of a TiO2 nanocatalyst on both unmodified and modified supports was performed to investigate the effects of the modification of the magnetic silica support on the loading of a TiO2 nanocatalyst and the photocatalytic activity. The loading of TiO2 and the photocatalytic activity were both improved. Key words: silicon dioxide; magnetite; refluxing; surface modification; surface chemical reactivity

Silica is one of the most common support materials in catalysis due to its thermal stability, chemical inertness, and high specific surface area. It is well known that chemical inertness is a useful property for a catalyst support. However, a completely inert silica surface is undesirable for grafting supported catalysts. Two types of structures, siloxane (Si–O–Si) and surface hydroxyl (Si–OH), generally exist on its surface [1]. The former is very inert, while the latter has chemical reactivity. Stable and highly dispersed catalysts on a silica support obtained by wet impregnation or the sol-gel process require that the hydroxyl groups on the silica surface react with the reactive precursors of the catalysts. For example, the grafting of highly dispersed TiO2 catalysts on silica can be performed by reacting titanium alkoxides, the precursor of TiO2, with the surface hydroxyls of a silica support [2–5]. Therefore, maintaining a higher concentration of surface silanols is very important for a higher silica surface chemical reactivity. Rahman et al. [6] have reported the synthesis of stable and highly dispersed silica nanoparticles and the effect of particle size on the concentra-

tion of the silanol group. They found that the concentration of the silanol group was inversely proportional to the particle size, and the silanol groups reduced during the calcination can be partly restored by exposure to air moisture. On the other hand, Shioji et al. [7] have reported the production of surface hydroxyl groups from the siloxane bridge on silica gel particles by soaking the particles in water. The number of surface hydroxyl after rehydroxylation was found to be more than that on the original surface. Many experimental and computational studies have been performed on the interactions between water and various silica materials [8–17] and demonstrated the feasibility of rehydroxylating the Si–O–Si bonds by exposing silica to water under moderate conditions. However, there have been few attempts to modify the magnetic silica support with water vapour before catalyst grafting. The use of magnetic nanoparticles as a support for catalysts has recently evolved into a promising field [18–21]. The present work focused on the modification of the chemical reactivity of the silica support containing Fe3O4 magnetic cores. In addition, a TiO2 catalyst

Received date: 20 May 2009. *Corresponding author. Tel: +86-451-88060570; E-mail: [email protected] # Corresponding author. Tel: +86-451-86282195; E-mail: [email protected] Foundation item: Supported by State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (SKLUWRE, HIT) (2008DX01-01 and 2008QN03), Foundation for University Key Teacher by the Education Office of Heilongjiang Province (1152G018), Youth Foundation of Heilongjiang Province (QC07C20), and Harbin Youth Foundation (2007RFQXG060). Copyright © 2009, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved. DOI: 10.1016/S1872-2067(08)60132-1

WANG Liyan et al. / Chinese Journal of Catalysis, 2009, 30(9): 939–944

1 Experimental 1.1

Preparation of the support and TiO2 catalyst

FeCl2·4H2O and FeCl3·6H2O obtained from Tianjin Kemel Chemical Reagent Ltd. Co. of China were analytical grade and used without further purification. Tetraethoxysilane (TEOS), 3-aminopropyltrimethoxysilane (APS), Ti(OC4H9)4, and Procion Red MX-5B dye were purchased from Sigma-Aldrich. First, stable Fe3O4 nanoparticles were prepared by the coprecipitation method from a mixture of FeCl2 and FeCl3 (molar ratio 1:2) by the addition of NaOH. After careful washing of the prepared Fe3O4 nanoparticles, those were refluxed in distilled water to form a stable magnetic solution of Fe3O4 nanoparticles. Second, a magnetic silica support was prepared. The coating of the Fe3O4 core with a silica shell was performed as described in the literature [22]. Freshly prepared APS solution and TEOS were added in sequence to the mixture of NH4OH (28%), H2O, ethanol, and the resulting Fe3O4 magnetic solution while the solution was mechanically stirred. The hydrolysis and condensation of TEOS onto the magnetic nanoparticles was completed in 12 h. After washing and magnetic separation, the formed SiO2/Fe3O4 support (designated as SF) was redispersed in an ethanol solution under sonication. This was further refluxed at 90 oC for 2 h, and the modified SiO2/Fe3O4 support (designated as SF-m) was obtained. Third, a magnetically separable TiO2 catalyst was prepared by dissolving 2.7 ml of Ti(OC4H9)4 in 60 ml ethanol and adding this solution dropwise into a three-necked flask containing 800 mg of SF-m support in an ethanol/water mixture. The pH was set at 5. The solution was continuously stirred and refluxed at 90 oC for 2 h. The resulting sample was then dried at 60 oC and calcined at 450 oC for 2 h to give the magnetically separable TiO2 catalyst (designated as TiO2-SF-m). Unsupported TiO2 catalyst and TiO2 catalyst supported on unmodified SF support (designated as TiO2-SF) were also synthesized for comparison purposes. 1.2

and pressed into thin wafers. X-ray photoelectron spectroscopy (XPS) studies were performed on a PHI 5700 ESCA XPS with a hemispherical analyzer and an Al KĮ radiation source (1 486.6 eV). The binding energies (BE) were referenced to the Si 2p binding energy in SiO2 at 103.3 eV. The atomic concentration ratios were calculated by correcting the intensity ratios with theoretical sensitivity factors proposed by the manufacturer. The compositions of the catalysts were analyzed using an IRIS Intrepid XSP inductively coupled plasma atomic emission spectrometry (ICP-AES). 1.3

Catalytic test

Photodegradation of Procion red MX-5B was conducted in a 100 ml round-bottom flask to evaluate the photocatalytic activity of the catalysts. The illumination sources were four TL8W UV bulbs (PHILIPS Inc.) with a wavelength of 365 nm. In the flask, 100 mg of catalyst containing 44% TiO2 (theoretical value) and aqueous solutions of Procion (50 mg, 10 mg/ml) were added. Prior to illumination, the solution was stirred in the dark to reach the adsorption/desorption equilibrium of Procion on catalyst. After a definite irradiation time, 5 ml of the suspension was collected and analyzed by a 752PC UV-Vis spectrometer.

2 Results and discussion 2.1

XRD, TEM, and SEM results

Figure 1 shows the XRD patterns of different samples. The characteristic diffraction peaks for pure Fe3O4 nanoparticles and anatase TiO2 are marked in Fe3O4 nanoparticles and unsupported TiO2 catalyst, respectively. The crystallite phase for the TiO2 in TiO2-SF-m and unsupported TiO2 catalysts was pure anatase, and no other TiO2 phase (e.g., rutile) was observed. Fe3O4 Amorphorous SiO2 Anatase TiO2

(1) Intensity

was grafted on the SiO2/Fe3O4 support, and the effects of modification were then studied by the loading of TiO2 and the photodegradation activity of Procion red MX-5B.

(2)

Characterization of the catalyst (3)

The crystal structure and microstructure of the samples were measured using a Rigaku-12KW X-ray diffractometer (XRD), a Hitachi S-4800 scanning electron microscope (SEM) with an energy dispersive spectrum (EDS) analysis facility, and a JEOL 2010 high-resolution transmission electron microscope (TEM). Fourier transform infrared (FT-IR) spectra were recorded under ambient conditions using a Nicolet 5700 instrument. The samples were mixed with KBr (1/100 by weight)

(4) 10

20

30

40

50

60

70

80

2T/( o ) Fig. 1. XRD patterns of the different samples. (1) Fe3O4 nanoparticles; (2) SF-m support (modified SiO2/Fe3O4 support); (3) Unsupported TiO2 catalyst; (4) TiO2-SF-m catalyst.

WANG Liyan et al. / Chinese Journal of Catalysis, 2009, 30(9): 939–944

(a)

SiO2

(b)

20 nm

20 nm

Magnetic core A (d)

(c)

A

Intensity

Ti

Si

O

Fe

Fe

1.00

3.00

7.00

5.00

9.00

Energy (keV)

Fig. 2. TEM images of the SF-m support (a) and TiO2-SF-m catalyst (b), SEM image of the TiO2-SF-m catalyst (c), and the EDS result in area A denoted in the SEM image (d).

1630

967

802

(1)

3435 1084

To investigate the effect of heat treatment in an ethanol solution on the surface properties of the SiO2/Fe3O4 support, FT-IR and XPS analysis were performed before and after the modification. Fig. 3 shows the FT-IR spectra recorded in the range of 4000–400 cm–1. The clear decrease in the intensities of the Si–O–Si bond at ~461, ~800, and ~1084 cm–1 [23–26] was observed after the SiO2/Fe3O4 support was refluxed in an ethanol/water mixture. This suggests the rupture of Si–O–Si bonds during the heat treatment in the ethanol/water mixture at 90oC for 2 h. The rehydroxylation of the Si–O–Si bonds can

461

FT-IR and XPS results

(2)

578

2.2

take place on the silica surface by exposing SiO2 to water vapour under moderate conditions [8–17] to form new surface hydroxyl groups (Si–OH). In contrast to the SF support, the bending vibration of the Si–O–H at ~967 cm–1 [6,23] decreased in the case of the SF-m support. A slight increase was observed for the peak centered at ~3435 cm–1, which was assigned to the stretching vibration of the hydroxyl (O–H) on the silica surface [6,27]. It is probably the formation of hydrogen bonds from Si–OH that caused the decreased intensity at ~967 cm–1. XPS data for SF and SF-m supports were obtained. The resulting O 1s spectra are presented in Fig. 4. The peaks can be

Transmittance

The strongest diffraction peak of anatase was at 25.26o. A broad and weak peak for the amorphous SiO2 phase around 2ș = 25o that appeared in the SF-m support was not observed in the magnetically separable TiO2-SF-m catalyst. This was because the strongest diffraction peak of anatase TiO2 and the broad and weak peak of amorphous SiO2 appeared in the same range. Figure 2 shows the TEM photographs of the samples and the corresponding EDS results. In Fig. 2(a), a core-shell structure is present in the well-dispersed SF-m support. The magnetic core (Fe3O4, dark) is surrounded by a thin shell (SiO2, bright). In Fig. 2(b), the TiO2-SF-m catalyst shows some slight aggregation, which was probably due to the calcination at 450 oC. Fig. 2(c) and (d) show that there was a homogeneous particle size and the coexistence of Ti, O, and Fe elements in the TiO2-SF-m catalyst, which suggested that the TiO2 catalyst was successfully grafted onto the SF-m support.

500

1000

1500

2000

2500

3000

3500

4000

–1

Wavenumber (cm ) Fig. 3. FT-IR spectra of SF (unmodified SiO2/Fe3O4) (1) and SF-m (2) supports.

WANG Liyan et al. / Chinese Journal of Catalysis, 2009, 30(9): 939–944

(6)

(a)

(1)

(4) (1)

(2)

(2)

Transmittance

Intensity

(5)

(3)

(b)

500

1000

1500

2000

2500

3000

3500

4000

Intensity

Wavenumber (cm–1) Fig. 5. FT-IR spectra of TiO2-SF (1) and TiO2-SF-m (2) catalysts.

2.3 Immobilization of the TiO2 catalysts and photocatalytic activity 536

535

534

533 532 531 Binding energy (eV)

530

529

528

Fig. 4. O 1s XPS spectra of SF (a) and SF-m (b) supports. (1) H2O; (2) O2; (3) Si–OH; (4) Fe–O; (5) Si–O; (6) Total.

deconvoluted into five components with O 1s binding energy at 533.8, 533.0, 532.6, ~531.5, and 530.1 eV for SF, and 533.4, 532.9, 532.4, ~531.7, and 530.2 eV for SF-m, respectively. The O 1s peaks with binding energies at ~532.9 and ~530.1 eV can be assigned to lattice O species [28–35], which are from Si–O in SiO2 and Fe–O in Fe3O4. The other O 1s species with binding energies at ~533.4, ~532.4, and ~531.5 eV are due to surface adsorbed water, surface adsorbed oxygen, and surface Si–OH, respectively [29,30,33–35]. The integrated area (percentage) of the different surface O species and the total amount of surface O species in each sample are listed in Table 1. The results show that the amount of the surface O species was almost the same in the two supports, but the surface Si–O structures from the lattice oxygen in SiO2 was decreased in the SF-m support compared with the SF support, and the amounts of the O species in Si–OH and adsorbed water were increased. This indicates that some Si–O–Si links were ruptured by the exposure to water vapour under moderate conditions and caused the formation of new Si–OH groups. This was in good agreement with the FT-IR results. Table 1

Binding energy and content of various surface oxygen species on SF and SF-m supports and the loading of TiO2 catalyst on the two supports Eb (eV)

Sample

a

The immobilization of the TiO2 nanocatalyst was carried out on both the SF and SF-m supports using the sol-gel process. It was performed to investigate the effects of the surface modification of the SF support on the subsequent loading of the TiO2 nanocatalyst and the resulting photocatalytic activity. The loading of TiO2 on the two supports is also listed in Table 1. On the SF support, the content of the TiO2 nanocatalyst was 24.81%, while the loading on the SF-m support was as high as 41.37%, close to the possible theoretical value of 44%. Figure 5 shows the FT-IR spectra of the TiO2-SF and TiO2-SF-m catalysts. The presence of the Ti–O–Si vibration at ~965 cm–1 [2,4] indicated that the TiO2 catalysts were chemically bonded to SiO2 through a reaction between Ti–OH and Si–OH [2–5]. Therefore, the enhancement of TiO2 loading on the modified SF-m support demonstrated directly that the heat treatment of the SF support in the ethanol/water mixture under reflux was an effective method to improve the surface reactivity of the SiO2 support. This promoted the transformation of the Si–O–Si structure to Si–OH groups with chemical reactivity. Figure 6 presents the photocatalytic activity of the immobilized TiO2 catalysts. It can be seen that the photocatalytic activity of the TiO2-SF-m catalyst was clearly higher than that of the TiO2-SF catalyst. About 64% of Procion was photocatalytically degraded by TiO2-SF-m catalyst at 20 min, while on the TiO2-SF catalyst with the same reaction time, it was only

Si–O (SiO2)

Si–OH

Content (%) O (H2O)

Si–O (SiO2)

Si–OH

Surface

O (H2O)

Other O species

O species (%)

Loading of TiO2a (%)

SF

533.00

531.52

533.86

45.38

11.48

8.06

35.08

75.03

24.81

SF-m

532.91

531.72

533.37

29.78

16.46

25.24

28.52

75.10

41.37

Experimental value of TiO2 loading from ICP.

WANG Liyan et al. / Chinese Journal of Catalysis, 2009, 30(9): 939–944

1.0 0.9

c/c0

0.8 (1)

0.7 0.6 0.5 0.4

(2)

0.3 0.2

0

20

40

60

80

100

120

Time (min) Fig. 6. Photodegradation of Procion red MX-5B over TiO2-SF (1) and TiO2-SF-m (2) catalysts.

29%. In addition, the activity of the TiO2-SF catalyst at 120 min was lower than that of the TiO2-SF-m catalyst at 20 min. Thus, the photocatalytic activity of the immobilized TiO2 catalyst was greatly enhanced when the modified SF-m support was used.

3 Conclusions An effective method to improve the surface chemical reactivity of a magnetic SiO2 catalyst support was demonstrated. FT-IR and XPS results indicated that new silanols on the silica surface were formed by the rupture of Si–O–Si links on the silica surface by exposing the magnetic silica support to a refluxed ethanol/water solution. The loading of TiO2 on the modified magnetic silica support was enhanced due to the increase of surface hydroxyl groups, with which more Ti–OH can be bonded on the silica surface. The higher TiO2 loading on the modified magnetic silica support enhanced the photodegradation activity of Procion red MX-5B.

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