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Synthesis of titania–silica mixed oxide mesoporous materials, characterization and photocatalytic properties
Applied Catalysis A: General 284 (2005) 193–198 www.elsevier.com/locate/apcata
Synthesis of titania–silica mixed oxide mesoporous materials, characterization and photocatalytic properties Xin Zhang a,*, Feng Zhang a, Kwong-Yu Chan b a
b
Department of Chemistry, Shantou University, Shantou, China 515063 Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China Received 14 August 2004; received in revised form 14 January 2005; accepted 21 January 2005 Available online 10 February 2005
Abstract Titania–silica mixed metal oxide materials with a mesostructure have been prepared by a novel method in which hydrolysis and condensation of titanium tetraisopropoxide (TTIP) and tetraethoxysilane (TEOS) were controlled by the pH change of acidic solution using cetyltrimethylammonium bromide (CTAB) as the structure-directing agent. The prepared materials were characterized by TEM, XRD, XPS, FT-IR and nitrogen sorption. The resulting materials showed a short-range ordered mesoporous structure with the nanoparticles (8 nm) of TiO2 dispersed uniformly on SiO2 supports. These TiO2–SiO2 mesoporous materials were also found to have a higher photocatalytic activity than that of commercial pure TiO2 nanoparticles at the same titanium loading for the degradation of methyl orange. # 2005 Elsevier B.V. All rights reserved. Keywords: Titania–silica mixed oxides; Mesostructure; Photocatalysts; Methyl orange
1. Introduction TiO2 photocatalysis has become increasingly important in recent years for environmental improvement such as the photodegradation and complete mineralization of organic pollutants [1,2]. TiO2 nanoparticles have large specific surface areas and high catalytic performance in which reactions take place on the TiO2 surface. However, their effective commercial applications are hindered by two serious disadvantages. Firstly, ultrafine powders will agglomerate into larger particles, resulting in an adverse effect on catalyst performance. Secondly, the separation and recovery of TiO2 powders from wastewater are difficult [3–5]. A considerable effort has been devoted to develop supported titanic catalysis offering high active surface area, while at the same time having easier separation and removal from water [6,7]. New synthesis methods of titania–silica mixed oxides are needed for overcoming the present disadvantages of pure titanium powders as regards their applications. * Corresponding author. Tel.: +86 754 290 2552; fax: +86 754 251 0654. E-mail address: [email protected] (X. Zhang). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.01.037
The synthesis of porous titania–silica mixed metal oxides is a challenging task, particularly when high loading and homogeneous distribution titanium species on the SiO2 support is required. Previous works mainly used mesostructure silica such as MCM-41 or SBA-15 as the support for titania coating [8–11]. However, it is difficult to prepare high titanic loading on mesoporous silica, because titania by postsynthesis grafting will choke the pore channels of mesoporous silica and decrease the surface area of mesoporous silica. Most of the proposed synthesis procedures of mesoporous titania or titania–silica mixed oxides depend critically on the control of the high reactivity of TiIV towards hydrolysis and condensation. A suitable synthesis method can be developed for obtaining TiO2 dispersing on mesoporous silica by the careful control of hydrolysis and condensation of TiIV. In this paper, we report a novel route to prepare the titania–silica mixed oxides through a careful control of the pH valve produced by the decomposition of urea. The formation mechanism of titania–silica mixed oxide mesoporous materials is discussed. The synthesized materials were characterized by transmission electron microscopy
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(TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR) and nitrogen sorption. Good composition control and pore size distribution of titania–silica mixed oxides materials were achieved. The photocatalytic performance of the prepared titania–silica mixed oxides for the degradation of methyl orange dye is reported.
model. X-ray photoelectron spectroscopy (XPS) spectra were recorded by a Quantum 2000 XPS system (Physical Electronics, Inc.) using Al ka radiation at a base pressure below 5 109 Torr. Infrared spectra were measured on KBr disks with a Thermo Nicolet (US) Avatar 360 FT-IR spectrometer. 2.1. Photocatalytic experiments
2. Experimental Tetraethoxysilane (TEOS) and cetyltrimethylammonium bromide (CTAB) were obtained from Aldrich. Titanium tetraisopropoxide (TTIP) was obtained from Acros Organics. All chemicals were used as received. Titania–silica mixed oxide materials were synthesized using surfactant templating, co-hydrolysis and co-condensation of tetraethoxysilane (TEOS) and titanium tetraisopropoxide (TTIP). A slow increase of pH was caused by the decomposition of the urea added to the reactant. Titania– silica mixed oxide materials were prepared as follows: cetyltrimethylammonium bromide (CTAB) of 1 g was dissolved in 30 ml 2N HCl solution at room temperature with stirring. Then, 10 ml (5 mol/l) of urea and 3 g of TEOS were added to this mixture solution. The TEOS was first hydrolyzed for 10 min in this strongly acidic solution. Then 2 g of titanium isopropoxide was added to the partially hydrolyzed TEOS solution. The hydrolysis of titanium isopropoxide (TTIP) occurs with the slow increase of pH produced by the decomposition of urea when the solution was heated by a water bath. The titania–silica mixed oxide mesoporous materials were synthesized by the co-hydrolysis reaction of titanium tetraisopropoxide (TTIP) and tetraethoxysilane (TEOS) in the acidic solution using CTAB surfactant as the structure directing agent. Then the further condensation and aggregation of hydrate products occurred to form a mesoporous framework around the micelles. The mass ratio of TTIP to TEOS was controlled from 0.1 to 2 typically at 0.5. The rate of hydrolysis can be carefully controlled through the rate of increase of temperature. The as-synthesized materials were rinsed with deionized water and dried at 60 8C for 12 h and then calcined at 500 8C for 5 h to remove the surfactant (CTAB) and obtain titania– silica mixed oxide mesoporous materials. The titania–silica mixed oxide mesoporous materials were characterized by a JOEL 2000FX transmission electron microscope (TEM). Small-angle X-ray diffraction (XRD) measurements were performed on an X-ray diffractometer (Rigaku Rotflex D/Max-C) using Cu Ka radiation (l = 0.15064 nm). Nitrogen adsorption measurements were carried out with a Tristar 3000 Micromeritics at 196 8C. Before each analysis each sample was degassed for 2 h at 200 8C under a vacuum of about 103 Torr in the degas port of the adsorption apparatus. Surface areas were calculated by the Brunauer–Emmett–Teller (BET) method. The pore size was calculated using the Barrett–Joyner–Hatenda (BJH)
The photocatalytic activity experiments on the titania– silica mixed oxide mesoporous materials prepared for the photodegradation of methyl orange in water were carried out in a cylindrical Pyrex glass reactor containing 60 ml of methyl orange solution at concentration of 20 mg l1. The powder photocatalysts were immersed in the reactant solution and were placed perpendicularly to the UV light beam. The UV light irradiation was provided by a highpressure mercury lamp (Philips HPK 125W). The amount of the pure TiO2 nanoparticles (commercial P25, particle size: 25 nm, BET: 50 m2/g, Degussa Co., Germany) used for this experiment was kept at 30 mg and the amount of TiO2–SiO2 mesoporous powders were controlled by the same titania loading. Samples of 5 ml volume were withdrawn at fixed intervals, and the concentration of methyl orange was determined by UV-spectrophotometry at 460 nm.
3. Results and discussion Fig. 1 shows the transmission electron microscopy (TEM) image of titania–silica mixed oxide mesoporous materials prepared by surfactant templating. The morphology of these mesoporous materials possesses a short-range ordered mesoporous structure. The small-angle X-ray diffraction (XRD) patterns of titania–silica mixed oxide mesoporous materials in this study showed a reflection in the
Fig. 1. Transmission electron microscope (TEM) image of the titania–silica mixed metal oxides (mass ratio of Ti/Si is 1:2) with a mesostructure prepared by co-hydrolysis and co-condensation of titanium tetraisopropoxide (TTIP) and tetraethoxysilane (TEOS) at acidic solution using cetyltrimethylammonium bromide (CTAB) as structure-directing agent.
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indicated that the silica or silicate in the prepared samples exists as amorphous state. These results demonstrated the existence of well-dispersed titanium in the amorphous silicate framework [10,14,15]. The average particle size of TiO2 in Ti–Si mesoporous materials is 8 nm, as determined from the Scherrer equation using the broadening of the h1 0 1i anatase reflection peak [16,17]. The XRD results demonstrate that the particle size of TiO2 can be controlled by this prepared method. 3.1. Surface area and pore size distribution
Fig. 2. Small-angle X-ray diffraction (XRD) patterns for the titania–silica mixed metal oxide mesoporous materials prepared for this study.
small-angle region near 2u = 0.98 (Fig. 2), which indicated the presence of a mesoporous structure. Only one broad diffraction peak (1 0 0) appeared and no other diffraction peaks (such as 1 1 0, 2 0 0, or 2 1 0) were found at smallangle 2u values, suggesting the lack of a long-range ordered structure [12,13]. This is in agreement with the TEM results. Wide-angle X-ray diffraction (XRD) was performed for different proportions of Ti to Si materials to analyze the structure of the titania–silica mixed oxide mesoporous materials. Fig. 3 shows the XRD patterns of these Ti–Si mesoporous materials for 2u angle values of 10–708. It can be observed that no diffracted lines corresponding to SiO2 or TiO2 appeared at low proportions of Ti to Si (mass ratio of Ti/Si < 0.25 at precursor solution). The intensity of the anatase phase at 2u of 25.38 enhanced with increasing the proportion of Ti to Si (mass ratio of Ti/Si > 0.5), but there were also no observable lines in the XRD spectra corresponding to those of silica or silicate samples, which
Fig. 3. Wide-angle X-ray diffraction patterns for the titania–silica mixed metal oxide mesoporous materials prepared for this study.
A typical nitrogen adsorption-adsorption isotherm of the titania–silica mixed oxide mesoporous materials is presented in Fig. 4. The isotherm is a type IV curve. A steep hysteretic loop is observed from this curve, which is typical for mesoporous materials that exhibit capillary condensation and evaporation [18,19]. Fig. 4 (inset) shows the pore size distributions measured by the Barrett–Joyner–Hatenda (BJH) model. It can be observed that the pore sizes of the sample concentrate in a range of 2–10 nm and that the mean pore size is 3.2 nm. The BET surface area is 609 m2/g, and the total pore volume is 0.425 cm3/g. According to a previous report [10], the surface area (SBET) of the Ti–Si mixed oxide mesoporous materials increases while the pore size decreases with increasing Ti incorporation into the silicate framework. 3.1.1. Infrared spectra The presence of the Si–O–Ti vibration band in the titania– silica mixed oxide mesoporous framework was confirmed by FT-IR spectra. Fig. 5 shows that a strong band at 1090 cm1 and two weak bands at 808 and 472 cm1 belonged to asymmetric stretching, symmetric stretching and bending modes of bulk Si–O–Si, respectively [10,20]. The peak at 949 cm1 can be attributed to the Si–O–Ti vibration band,
Fig. 4. Nitrogen adsorption isotherms and corresponding pore size analyses for the titania–silica mixed metal oxide mesoporous materials prepared for this study (mass ratio of Ti/Si is 1:2).
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Fig. 5. FT-IR spectra of the titania–silica mixed metal oxide mesoporous materials at different proportions of Ti to Si prepared for this study.
which increases with the increase of Ti contents in silica [10,21–22]. This is the evidence of titanium incorporating into the framework of silica. XPS analyses to investigate the complicated titanium coordination states in TiO2–SiO2 mesoporous powders, we performed X-ray photoelectron spectroscopy (XPS) analyses on the titania–silica mixed oxide catalysts. Fig. 6 shows the XPS spectra in the O 1s binding energy region of the pure TiO2 and TiO2–SiO2 samples. The spectrum of pure TiO2 contained one peak appearing at 529.5 eV, which is typical for metal oxides and agrees with O 1s electron binding energy for TiO2 molecules [23]. However, the spectrum of TiO2–SiO2 sample can be fit by three Lorentzian curves appearing at 533 eV (peak 1), 532.2 eV (peak 2) and 529.7 eV (peak 3), which can be attributed to Si–O–Si (533 eV), Si–O–Ti (532.2 eV) and Ti–O–Ti (529.2 eV) components [24], respectively. XPS results show that more complicated oxygen coordination states appear in TiO2– SiO2 samples than in the pure TiO2 sample, and that more connections between titanium species and the SiO2 matrix have been established through oxygen coordination in the TiO2–SiO2 sample.
Fig. 6. XPS spectrum of the pure TiO2 and TiO2–SiO2 samples: (a) the O 1s (295.5 eV) line of TiO2, (b) peak 1 shows the O 1s (533 eV) line of Si–O–Si; peak 2 shows the O 1s (532.2 eV) line of Si–O–Ti; peak 3 shows the O 1s (529.7 eV) line of Ti–O–Ti.
3.2. Photocatalytic activity The photodegradation of methyl orange was performed in order to evaluate the photocatalytic activity of prepared titania–silica mixed oxide mesoporous materials. Two Ti–Si nanostructured samples prepared for this study and similar commercial pure TiO2 nanoparticles (P25, particle size: 25 nm, BET: 50 m2/g, Degussa Co., Germany) as reference sample were employed for comparing their photocatalytic activity value for the same method and for the same TiO2 loading. Fig. 7 shows the degradation curves of methyl orange by the two titania–silica mixed oxide catalysts (mass ratios of Ti/Si are 0.5 and 1) and pure TiO2 nanoparticles, in
Fig. 7. The photocatalytic performance of the titania–silica mixed metal oxide mesoporous materials prepared by this study and commercial pure TiO2–P25 for the degradation of methyl orange: (a) pure TiO2–P25, (b) mass ratio of Ti/Si = 1, (c) mass ratio of Ti/Si = 0.5.
X. Zhang et al. / Applied Catalysis A: General 284 (2005) 193–198
which the photocatalytic activities of titania–silica mixed oxide mesoporous samples prepared by this study were higher than that of commercial pure TiO2 nanoparticles-P25 at the same loading. The superior photocatalytic performance of Ti–Si mesoporous materials prepared by this study can be attributed to the smaller particle size (about 8 nm) of these photocatalysts compared with that of P25 (about 25 nm). It has been demonstrated that the photocatalytic activity increases when the particle size of TiO2 photocatalysts become smaller, especially below 10 nm [25]. Other advantages of titania–silica mixed oxide mesoporous materials prepared here are their larger specific surface area, higher pore structure and controlled particle size (8 nm) of TiO2 dispersed uniformly on silica supports, which can prevent agglomeration and improve photocatalytic activity. Besides the benefit of catalytic activity enhancement, these prepared materials can be separated from suspension solution easier than pure TiO2 nanoparticles. The titania– silica mixed oxide mesoporous materials prepared here sedimented quickly when the reaction finished and the stirring was stopped. However, pure TiO2 nanoparticles-P25 could not sediment and separate from suspension solution after reaction. Though the size of TiO2 particles in the titania–silica mixed oxide mesoporous materials prepared here is much smaller than the size of pure TiO2 nanoparticles-P25, the small TiO2 particles on mesoporous materials can be separated easily from aqueous solutions by sedimentation or filtration because they connect with silica support to form large granules. These results demonstrate the advantage of titania–silica mixed oxide mesoporous materials prepared by this method and their potential for the application as photocatalysts. The photocatalytic activity of titania–silica mixed oxide mesoporous materials with a 0.5:1 mass ratio of Ti:Si is higher than that of 1:1 mass ratio of Ti:Si. A possible explanation is the existence of an optimal ratio of Ti:Si. When the mass ratio of Ti:Si is large, the small TiO2 particles on silica matrix will easily contact each other and aggregate to form larger particles leading to a decrease of photocatalytic activity. Formation Mechanism Discussion The titania–silica mixed oxide mesoporous materials were prepared by the co-hydrolysis reaction of titanium tetraisopropoxide (TTIP) and tetraethoxysilane (TEOS) in the acidic solution and the condensation and aggregation of the hydrated products to form a mesoporous framework around the micelles, which were formed by surfactant CTAB as structure directing agent. In order to obtain homogeneous titanium dispersion on silica support, we used two measures to overcome the obstacle that the hydrolysis rate of titanium tetraisopropoxide (TTIP) is faster than tetraethoxysilane (TEOS). First, the silicon alkoxide is hydrolyzed before the titanium alkoxide is added. Second, TEOS has higher hydrolysis rate in the initial stage of prepared process, because the least positively charged species will react fastest in a strongly acidic catalyst {The positive charge d(M) of Si (OEt)4 is +0.32, the positive charge of Ti(OEt)4 is +0.63} [26]. In this synthesis approach, the
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mechanism of the acid prepared mesostructure can be proposed to be S+XI+ {S+ = CTA+, X = Cl, and I+ = Si(OH)x+(4 x) or Ti(OH)x+(4 x)} pathway, the cooperative assembly of cationic silicate species with cationic surfactant mediated by chloride ion in strongly acidic solutions leading to formation of silica mesostructure [27]. With increasing reaction temperature, the pH of the solution increases due to the decomposition of urea and reactions with HCl. The hydrolysis of titanium isopropoxide occurs; then the condensation and aggregation of the hydrated production around the silica framework occur. The reaction can be represented by the following equation: SiðOEtÞ4 þ xH2 O ! ½SiðHOÞx þð4xÞ þ xEtOH
(1)
CTA þ Cl þ SiðOHÞþð4xÞ ! CTAþ x ðOHÞþð4xÞ x
(2)
þ
Cl Si
The decomposition of urea and reaction with HCl are described by the equations: H2 N CO H2 N ! NHþ 4 þ OCN
(3)
OCN þ Hþ þ H2 O ! CO2 þ NHþ 4
(4)
The titanium isopropoxide hydrolyzed and formed S+XI+ species as expressed by the equations: TiðOEtÞ4 þ xH2 O ! ½TiðHOÞx þð4xÞ þ xEtOH
(5)
CTAþ þ Cl þ TiðOHÞþð4xÞ ! CTAþ Cl Ti x ðOHÞþð4xÞ x
(6)
The condensation and aggregation of hydrated products can be expressed by the reactions: 2CTAþ Cl SiðOHÞþð4xÞ ! CTAþ Cl x þ þ ðHOÞþ x1 SiOSiðHOÞx1 Cl CTA
þ 2H2 O
(7)
2CTA Cl TiðOHÞþð4xÞ ! CTAþ Cl x þ þ ðHOÞþ x1 TiOTiðHOÞx1 Cl CTA þ
þ 2H2 O
(8)
or CTAþ Cl SiðOHÞþð4xÞ þ CTAþ Cl Ti x ! CTAþ Cl ðOHÞþð4xÞ x þ þ ðHOÞþ x1 SiOTiðHOÞx1 Cl CTA
þ 2H2 O
(9)
Finally, the calcination of the intermediate nanocomposite simultaneously removed the surfactant and transformed the polycondensation mesostructured component to mesostructured titania–silica mixed oxide þ CTAþ Cl ðHOÞþ x1 SiOSiðHOÞx1 Cl
CTAþ ! meso SiO2 þ H2 O þ weightloss
(10)
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References
þ CTAþ Cl ðHOÞþ x1 TiOTiðHOÞx1 Cl
CTAþ ! meso TiO2 þ H2 O þ weightloss þ
CTA Cl
þ ðHOÞþ x1 SiOTiðHOÞx1
Cl
(11)
CTAþ ! meso SiOTi þ H2 O þ weightloss
(12)
As evidenced in the XRD and FT-IR results, the particle size of TiO2 on the mesoporous silica can be controlled by increasing the rate of pH in solution, which depends on increasing the rate of temperature change. It has been found that the microstructural nature and the dispersion of the titanium dioxide are rather dependent on the preparation procedure. 4. Conclusions A novel route has been introduced to prepare the titania– silica mixed oxide mesoporous materials, in which the TiO2–SiO2 mesoporous materials were synthesized by the co-hydrolysis and condensation reaction of titanium isopropoxide and tetraethoxysilane (TEOS) in the acidic solution using surfactant CTAB as structure-directing surfactant. High surface area SBET of 609 m2/g and total pore volume of 0.425 cm3/g have been obtained. This synthetic method of titania–silica mixed oxide mesoporous materials has resulted in a high degree of homogeneous titanium dispersion and a high specific surface area. XRD, XPS and FT-IR analyses show that the crystalline phase of anatase TiO2 can incorporate the amorphous silicate framework and that the particle size of TiO2 can be controlled. The results of the activity test indicated that titania–silica mixed oxide mesoporous materials prepared by this study had better photocatalytic performance than that of the commercial pure TiO2 nanoparticles for the photodegradation of the methyl orange dye. Acknowledgment This work was supported by Science Foundation of Guangdong Province, China (990783 and 032051).
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Synthesis of titania–silica mixed oxide mesoporous materials, characterization and photocatalytic properties Xin Zhang a,*, Feng Zhang a, Kwong-Yu Chan b a
b
Department of Chemistry, Shantou University, Shantou, China 515063 Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China Received 14 August 2004; received in revised form 14 January 2005; accepted 21 January 2005 Available online 10 February 2005
Abstract Titania–silica mixed metal oxide materials with a mesostructure have been prepared by a novel method in which hydrolysis and condensation of titanium tetraisopropoxide (TTIP) and tetraethoxysilane (TEOS) were controlled by the pH change of acidic solution using cetyltrimethylammonium bromide (CTAB) as the structure-directing agent. The prepared materials were characterized by TEM, XRD, XPS, FT-IR and nitrogen sorption. The resulting materials showed a short-range ordered mesoporous structure with the nanoparticles (8 nm) of TiO2 dispersed uniformly on SiO2 supports. These TiO2–SiO2 mesoporous materials were also found to have a higher photocatalytic activity than that of commercial pure TiO2 nanoparticles at the same titanium loading for the degradation of methyl orange. # 2005 Elsevier B.V. All rights reserved. Keywords: Titania–silica mixed oxides; Mesostructure; Photocatalysts; Methyl orange
1. Introduction TiO2 photocatalysis has become increasingly important in recent years for environmental improvement such as the photodegradation and complete mineralization of organic pollutants [1,2]. TiO2 nanoparticles have large specific surface areas and high catalytic performance in which reactions take place on the TiO2 surface. However, their effective commercial applications are hindered by two serious disadvantages. Firstly, ultrafine powders will agglomerate into larger particles, resulting in an adverse effect on catalyst performance. Secondly, the separation and recovery of TiO2 powders from wastewater are difficult [3–5]. A considerable effort has been devoted to develop supported titanic catalysis offering high active surface area, while at the same time having easier separation and removal from water [6,7]. New synthesis methods of titania–silica mixed oxides are needed for overcoming the present disadvantages of pure titanium powders as regards their applications. * Corresponding author. Tel.: +86 754 290 2552; fax: +86 754 251 0654. E-mail address: [email protected] (X. Zhang). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.01.037
The synthesis of porous titania–silica mixed metal oxides is a challenging task, particularly when high loading and homogeneous distribution titanium species on the SiO2 support is required. Previous works mainly used mesostructure silica such as MCM-41 or SBA-15 as the support for titania coating [8–11]. However, it is difficult to prepare high titanic loading on mesoporous silica, because titania by postsynthesis grafting will choke the pore channels of mesoporous silica and decrease the surface area of mesoporous silica. Most of the proposed synthesis procedures of mesoporous titania or titania–silica mixed oxides depend critically on the control of the high reactivity of TiIV towards hydrolysis and condensation. A suitable synthesis method can be developed for obtaining TiO2 dispersing on mesoporous silica by the careful control of hydrolysis and condensation of TiIV. In this paper, we report a novel route to prepare the titania–silica mixed oxides through a careful control of the pH valve produced by the decomposition of urea. The formation mechanism of titania–silica mixed oxide mesoporous materials is discussed. The synthesized materials were characterized by transmission electron microscopy
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(TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR) and nitrogen sorption. Good composition control and pore size distribution of titania–silica mixed oxides materials were achieved. The photocatalytic performance of the prepared titania–silica mixed oxides for the degradation of methyl orange dye is reported.
model. X-ray photoelectron spectroscopy (XPS) spectra were recorded by a Quantum 2000 XPS system (Physical Electronics, Inc.) using Al ka radiation at a base pressure below 5 109 Torr. Infrared spectra were measured on KBr disks with a Thermo Nicolet (US) Avatar 360 FT-IR spectrometer. 2.1. Photocatalytic experiments
2. Experimental Tetraethoxysilane (TEOS) and cetyltrimethylammonium bromide (CTAB) were obtained from Aldrich. Titanium tetraisopropoxide (TTIP) was obtained from Acros Organics. All chemicals were used as received. Titania–silica mixed oxide materials were synthesized using surfactant templating, co-hydrolysis and co-condensation of tetraethoxysilane (TEOS) and titanium tetraisopropoxide (TTIP). A slow increase of pH was caused by the decomposition of the urea added to the reactant. Titania– silica mixed oxide materials were prepared as follows: cetyltrimethylammonium bromide (CTAB) of 1 g was dissolved in 30 ml 2N HCl solution at room temperature with stirring. Then, 10 ml (5 mol/l) of urea and 3 g of TEOS were added to this mixture solution. The TEOS was first hydrolyzed for 10 min in this strongly acidic solution. Then 2 g of titanium isopropoxide was added to the partially hydrolyzed TEOS solution. The hydrolysis of titanium isopropoxide (TTIP) occurs with the slow increase of pH produced by the decomposition of urea when the solution was heated by a water bath. The titania–silica mixed oxide mesoporous materials were synthesized by the co-hydrolysis reaction of titanium tetraisopropoxide (TTIP) and tetraethoxysilane (TEOS) in the acidic solution using CTAB surfactant as the structure directing agent. Then the further condensation and aggregation of hydrate products occurred to form a mesoporous framework around the micelles. The mass ratio of TTIP to TEOS was controlled from 0.1 to 2 typically at 0.5. The rate of hydrolysis can be carefully controlled through the rate of increase of temperature. The as-synthesized materials were rinsed with deionized water and dried at 60 8C for 12 h and then calcined at 500 8C for 5 h to remove the surfactant (CTAB) and obtain titania– silica mixed oxide mesoporous materials. The titania–silica mixed oxide mesoporous materials were characterized by a JOEL 2000FX transmission electron microscope (TEM). Small-angle X-ray diffraction (XRD) measurements were performed on an X-ray diffractometer (Rigaku Rotflex D/Max-C) using Cu Ka radiation (l = 0.15064 nm). Nitrogen adsorption measurements were carried out with a Tristar 3000 Micromeritics at 196 8C. Before each analysis each sample was degassed for 2 h at 200 8C under a vacuum of about 103 Torr in the degas port of the adsorption apparatus. Surface areas were calculated by the Brunauer–Emmett–Teller (BET) method. The pore size was calculated using the Barrett–Joyner–Hatenda (BJH)
The photocatalytic activity experiments on the titania– silica mixed oxide mesoporous materials prepared for the photodegradation of methyl orange in water were carried out in a cylindrical Pyrex glass reactor containing 60 ml of methyl orange solution at concentration of 20 mg l1. The powder photocatalysts were immersed in the reactant solution and were placed perpendicularly to the UV light beam. The UV light irradiation was provided by a highpressure mercury lamp (Philips HPK 125W). The amount of the pure TiO2 nanoparticles (commercial P25, particle size: 25 nm, BET: 50 m2/g, Degussa Co., Germany) used for this experiment was kept at 30 mg and the amount of TiO2–SiO2 mesoporous powders were controlled by the same titania loading. Samples of 5 ml volume were withdrawn at fixed intervals, and the concentration of methyl orange was determined by UV-spectrophotometry at 460 nm.
3. Results and discussion Fig. 1 shows the transmission electron microscopy (TEM) image of titania–silica mixed oxide mesoporous materials prepared by surfactant templating. The morphology of these mesoporous materials possesses a short-range ordered mesoporous structure. The small-angle X-ray diffraction (XRD) patterns of titania–silica mixed oxide mesoporous materials in this study showed a reflection in the
Fig. 1. Transmission electron microscope (TEM) image of the titania–silica mixed metal oxides (mass ratio of Ti/Si is 1:2) with a mesostructure prepared by co-hydrolysis and co-condensation of titanium tetraisopropoxide (TTIP) and tetraethoxysilane (TEOS) at acidic solution using cetyltrimethylammonium bromide (CTAB) as structure-directing agent.
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indicated that the silica or silicate in the prepared samples exists as amorphous state. These results demonstrated the existence of well-dispersed titanium in the amorphous silicate framework [10,14,15]. The average particle size of TiO2 in Ti–Si mesoporous materials is 8 nm, as determined from the Scherrer equation using the broadening of the h1 0 1i anatase reflection peak [16,17]. The XRD results demonstrate that the particle size of TiO2 can be controlled by this prepared method. 3.1. Surface area and pore size distribution
Fig. 2. Small-angle X-ray diffraction (XRD) patterns for the titania–silica mixed metal oxide mesoporous materials prepared for this study.
small-angle region near 2u = 0.98 (Fig. 2), which indicated the presence of a mesoporous structure. Only one broad diffraction peak (1 0 0) appeared and no other diffraction peaks (such as 1 1 0, 2 0 0, or 2 1 0) were found at smallangle 2u values, suggesting the lack of a long-range ordered structure [12,13]. This is in agreement with the TEM results. Wide-angle X-ray diffraction (XRD) was performed for different proportions of Ti to Si materials to analyze the structure of the titania–silica mixed oxide mesoporous materials. Fig. 3 shows the XRD patterns of these Ti–Si mesoporous materials for 2u angle values of 10–708. It can be observed that no diffracted lines corresponding to SiO2 or TiO2 appeared at low proportions of Ti to Si (mass ratio of Ti/Si < 0.25 at precursor solution). The intensity of the anatase phase at 2u of 25.38 enhanced with increasing the proportion of Ti to Si (mass ratio of Ti/Si > 0.5), but there were also no observable lines in the XRD spectra corresponding to those of silica or silicate samples, which
Fig. 3. Wide-angle X-ray diffraction patterns for the titania–silica mixed metal oxide mesoporous materials prepared for this study.
A typical nitrogen adsorption-adsorption isotherm of the titania–silica mixed oxide mesoporous materials is presented in Fig. 4. The isotherm is a type IV curve. A steep hysteretic loop is observed from this curve, which is typical for mesoporous materials that exhibit capillary condensation and evaporation [18,19]. Fig. 4 (inset) shows the pore size distributions measured by the Barrett–Joyner–Hatenda (BJH) model. It can be observed that the pore sizes of the sample concentrate in a range of 2–10 nm and that the mean pore size is 3.2 nm. The BET surface area is 609 m2/g, and the total pore volume is 0.425 cm3/g. According to a previous report [10], the surface area (SBET) of the Ti–Si mixed oxide mesoporous materials increases while the pore size decreases with increasing Ti incorporation into the silicate framework. 3.1.1. Infrared spectra The presence of the Si–O–Ti vibration band in the titania– silica mixed oxide mesoporous framework was confirmed by FT-IR spectra. Fig. 5 shows that a strong band at 1090 cm1 and two weak bands at 808 and 472 cm1 belonged to asymmetric stretching, symmetric stretching and bending modes of bulk Si–O–Si, respectively [10,20]. The peak at 949 cm1 can be attributed to the Si–O–Ti vibration band,
Fig. 4. Nitrogen adsorption isotherms and corresponding pore size analyses for the titania–silica mixed metal oxide mesoporous materials prepared for this study (mass ratio of Ti/Si is 1:2).
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Fig. 5. FT-IR spectra of the titania–silica mixed metal oxide mesoporous materials at different proportions of Ti to Si prepared for this study.
which increases with the increase of Ti contents in silica [10,21–22]. This is the evidence of titanium incorporating into the framework of silica. XPS analyses to investigate the complicated titanium coordination states in TiO2–SiO2 mesoporous powders, we performed X-ray photoelectron spectroscopy (XPS) analyses on the titania–silica mixed oxide catalysts. Fig. 6 shows the XPS spectra in the O 1s binding energy region of the pure TiO2 and TiO2–SiO2 samples. The spectrum of pure TiO2 contained one peak appearing at 529.5 eV, which is typical for metal oxides and agrees with O 1s electron binding energy for TiO2 molecules [23]. However, the spectrum of TiO2–SiO2 sample can be fit by three Lorentzian curves appearing at 533 eV (peak 1), 532.2 eV (peak 2) and 529.7 eV (peak 3), which can be attributed to Si–O–Si (533 eV), Si–O–Ti (532.2 eV) and Ti–O–Ti (529.2 eV) components [24], respectively. XPS results show that more complicated oxygen coordination states appear in TiO2– SiO2 samples than in the pure TiO2 sample, and that more connections between titanium species and the SiO2 matrix have been established through oxygen coordination in the TiO2–SiO2 sample.
Fig. 6. XPS spectrum of the pure TiO2 and TiO2–SiO2 samples: (a) the O 1s (295.5 eV) line of TiO2, (b) peak 1 shows the O 1s (533 eV) line of Si–O–Si; peak 2 shows the O 1s (532.2 eV) line of Si–O–Ti; peak 3 shows the O 1s (529.7 eV) line of Ti–O–Ti.
3.2. Photocatalytic activity The photodegradation of methyl orange was performed in order to evaluate the photocatalytic activity of prepared titania–silica mixed oxide mesoporous materials. Two Ti–Si nanostructured samples prepared for this study and similar commercial pure TiO2 nanoparticles (P25, particle size: 25 nm, BET: 50 m2/g, Degussa Co., Germany) as reference sample were employed for comparing their photocatalytic activity value for the same method and for the same TiO2 loading. Fig. 7 shows the degradation curves of methyl orange by the two titania–silica mixed oxide catalysts (mass ratios of Ti/Si are 0.5 and 1) and pure TiO2 nanoparticles, in
Fig. 7. The photocatalytic performance of the titania–silica mixed metal oxide mesoporous materials prepared by this study and commercial pure TiO2–P25 for the degradation of methyl orange: (a) pure TiO2–P25, (b) mass ratio of Ti/Si = 1, (c) mass ratio of Ti/Si = 0.5.
X. Zhang et al. / Applied Catalysis A: General 284 (2005) 193–198
which the photocatalytic activities of titania–silica mixed oxide mesoporous samples prepared by this study were higher than that of commercial pure TiO2 nanoparticles-P25 at the same loading. The superior photocatalytic performance of Ti–Si mesoporous materials prepared by this study can be attributed to the smaller particle size (about 8 nm) of these photocatalysts compared with that of P25 (about 25 nm). It has been demonstrated that the photocatalytic activity increases when the particle size of TiO2 photocatalysts become smaller, especially below 10 nm [25]. Other advantages of titania–silica mixed oxide mesoporous materials prepared here are their larger specific surface area, higher pore structure and controlled particle size (8 nm) of TiO2 dispersed uniformly on silica supports, which can prevent agglomeration and improve photocatalytic activity. Besides the benefit of catalytic activity enhancement, these prepared materials can be separated from suspension solution easier than pure TiO2 nanoparticles. The titania– silica mixed oxide mesoporous materials prepared here sedimented quickly when the reaction finished and the stirring was stopped. However, pure TiO2 nanoparticles-P25 could not sediment and separate from suspension solution after reaction. Though the size of TiO2 particles in the titania–silica mixed oxide mesoporous materials prepared here is much smaller than the size of pure TiO2 nanoparticles-P25, the small TiO2 particles on mesoporous materials can be separated easily from aqueous solutions by sedimentation or filtration because they connect with silica support to form large granules. These results demonstrate the advantage of titania–silica mixed oxide mesoporous materials prepared by this method and their potential for the application as photocatalysts. The photocatalytic activity of titania–silica mixed oxide mesoporous materials with a 0.5:1 mass ratio of Ti:Si is higher than that of 1:1 mass ratio of Ti:Si. A possible explanation is the existence of an optimal ratio of Ti:Si. When the mass ratio of Ti:Si is large, the small TiO2 particles on silica matrix will easily contact each other and aggregate to form larger particles leading to a decrease of photocatalytic activity. Formation Mechanism Discussion The titania–silica mixed oxide mesoporous materials were prepared by the co-hydrolysis reaction of titanium tetraisopropoxide (TTIP) and tetraethoxysilane (TEOS) in the acidic solution and the condensation and aggregation of the hydrated products to form a mesoporous framework around the micelles, which were formed by surfactant CTAB as structure directing agent. In order to obtain homogeneous titanium dispersion on silica support, we used two measures to overcome the obstacle that the hydrolysis rate of titanium tetraisopropoxide (TTIP) is faster than tetraethoxysilane (TEOS). First, the silicon alkoxide is hydrolyzed before the titanium alkoxide is added. Second, TEOS has higher hydrolysis rate in the initial stage of prepared process, because the least positively charged species will react fastest in a strongly acidic catalyst {The positive charge d(M) of Si (OEt)4 is +0.32, the positive charge of Ti(OEt)4 is +0.63} [26]. In this synthesis approach, the
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mechanism of the acid prepared mesostructure can be proposed to be S+XI+ {S+ = CTA+, X = Cl, and I+ = Si(OH)x+(4 x) or Ti(OH)x+(4 x)} pathway, the cooperative assembly of cationic silicate species with cationic surfactant mediated by chloride ion in strongly acidic solutions leading to formation of silica mesostructure [27]. With increasing reaction temperature, the pH of the solution increases due to the decomposition of urea and reactions with HCl. The hydrolysis of titanium isopropoxide occurs; then the condensation and aggregation of the hydrated production around the silica framework occur. The reaction can be represented by the following equation: SiðOEtÞ4 þ xH2 O ! ½SiðHOÞx þð4xÞ þ xEtOH
(1)
CTA þ Cl þ SiðOHÞþð4xÞ ! CTAþ x ðOHÞþð4xÞ x
(2)
þ
Cl Si
The decomposition of urea and reaction with HCl are described by the equations: H2 N CO H2 N ! NHþ 4 þ OCN
(3)
OCN þ Hþ þ H2 O ! CO2 þ NHþ 4
(4)
The titanium isopropoxide hydrolyzed and formed S+XI+ species as expressed by the equations: TiðOEtÞ4 þ xH2 O ! ½TiðHOÞx þð4xÞ þ xEtOH
(5)
CTAþ þ Cl þ TiðOHÞþð4xÞ ! CTAþ Cl Ti x ðOHÞþð4xÞ x
(6)
The condensation and aggregation of hydrated products can be expressed by the reactions: 2CTAþ Cl SiðOHÞþð4xÞ ! CTAþ Cl x þ þ ðHOÞþ x1 SiOSiðHOÞx1 Cl CTA
þ 2H2 O
(7)
2CTA Cl TiðOHÞþð4xÞ ! CTAþ Cl x þ þ ðHOÞþ x1 TiOTiðHOÞx1 Cl CTA þ
þ 2H2 O
(8)
or CTAþ Cl SiðOHÞþð4xÞ þ CTAþ Cl Ti x ! CTAþ Cl ðOHÞþð4xÞ x þ þ ðHOÞþ x1 SiOTiðHOÞx1 Cl CTA
þ 2H2 O
(9)
Finally, the calcination of the intermediate nanocomposite simultaneously removed the surfactant and transformed the polycondensation mesostructured component to mesostructured titania–silica mixed oxide þ CTAþ Cl ðHOÞþ x1 SiOSiðHOÞx1 Cl
CTAþ ! meso SiO2 þ H2 O þ weightloss
(10)
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References
þ CTAþ Cl ðHOÞþ x1 TiOTiðHOÞx1 Cl
CTAþ ! meso TiO2 þ H2 O þ weightloss þ
CTA Cl
þ ðHOÞþ x1 SiOTiðHOÞx1
Cl
(11)
CTAþ ! meso SiOTi þ H2 O þ weightloss
(12)
As evidenced in the XRD and FT-IR results, the particle size of TiO2 on the mesoporous silica can be controlled by increasing the rate of pH in solution, which depends on increasing the rate of temperature change. It has been found that the microstructural nature and the dispersion of the titanium dioxide are rather dependent on the preparation procedure. 4. Conclusions A novel route has been introduced to prepare the titania– silica mixed oxide mesoporous materials, in which the TiO2–SiO2 mesoporous materials were synthesized by the co-hydrolysis and condensation reaction of titanium isopropoxide and tetraethoxysilane (TEOS) in the acidic solution using surfactant CTAB as structure-directing surfactant. High surface area SBET of 609 m2/g and total pore volume of 0.425 cm3/g have been obtained. This synthetic method of titania–silica mixed oxide mesoporous materials has resulted in a high degree of homogeneous titanium dispersion and a high specific surface area. XRD, XPS and FT-IR analyses show that the crystalline phase of anatase TiO2 can incorporate the amorphous silicate framework and that the particle size of TiO2 can be controlled. The results of the activity test indicated that titania–silica mixed oxide mesoporous materials prepared by this study had better photocatalytic performance than that of the commercial pure TiO2 nanoparticles for the photodegradation of the methyl orange dye. Acknowledgment This work was supported by Science Foundation of Guangdong Province, China (990783 and 032051).
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