http://www.chemistrymag.org/cji/2009/114017pe.htm |
Apr.1,
2009 Vol.11 No.4 P.17 Copyright |
Li Ming, Zhong Huiqiong, Zhang Lizhen, Yang Jun
(Department of Chemistry, Jinan University, Guangzhou 510632, China）
Abstract Nanosized TiO2/SiO2
photocatalysts in pure anatase phase with crystallite size of 6 to 22 nm were prepared via
hydrothermal method using titanium sulfate. These catalysts were characterized by using
BET, XRD measurements. The experimental results show that the addition of silica gel could
effectively improve the thermal stability of TiO2 and inhibit the
transformation of anatase to rutile phase and the growth of titanium dioxide crystals
during the calcination treatment. The nanosized 40 % TiO2/SiO2-700
catalyst shows enhanced photocatalytic activity and has the highest rate constant of 0.046
min-1, comparing with 0.033 min-1 for P-25. After 90 min
irradiation of UV light, nearly no absorption band was observed in the range of 200-800 nm
in the presence of 40 % TiO2/SiO2-700 catalyst, whereas for P-25, a
broad band appeared in range of 200-360 nm. The probable reason is that TiO2/SiO2
catalyst has a relatively large collecting capability of degradation intermediates as well
as target substrate from the solution phase onto the catalyst, thereby preventing
dissolution of toxic substances in solution.
Keywords Nanosized TiO2/SiO2
composites; Titanium sulfate; Photocatalytic; Methylene blue
1 INTRODUCTION
Since the discovery of photocatalytic splitting of water on TiO2 electrodes
by Fujishima and Honda [1], there has been an explosion of research related to
the fabrication of oxide semiconductor photocatalysts for environmental application [2-3].
Among various oxide semiconductor photocatalysts, titania is believed to be the most
promising one due to its high photocatalytic activity, chemical/photoerosion stability,
low cost and without risks for the environment or humans [4-5]. The efficiency
of some commercial TiO2 in the treatment of exhaust gas and wastewater
contaminated with organic and inorganic pollutants has been fully proved. However, when
naked TiO2 are used, problems occur. Firstly, small particles trend to
agglomerate into large particles, making against on catalyst performance. Secondly, the
separation and recovery of catalyst is difficult [6]. Therefore, many papers have reported titanium dioxide
supported on various matrix, such as silica, active carbon, zeolite, etc. Among them, the
titania-silica system was regarded as a potential candidate for photocatalyst that could
be explained as a synergetic effect.
Very recently, we have investigated the synthesis of TiO2/SBA-15
composites by a post-synthesis step [7]. However, in most cases, the
preparation of silica-modified titanium dioxide usually employed Ti-alkoxide rather than
Ti-salt as a starting material. Here, we describe a facile method for the synthesis of
high photocatalytic silica-modified anatase TiO2 using titanium sulfate as
starting material.
2 MATERIALS AND METHODS
2.1 Materials
In the present work, commercial silica gel was used as the catalyst support (supplied
by Qingdao Chemical Industries). TiO2 particles were obtained from Degussa
(P-25). Ti(SO4)2 was purchase from Aldrich.
2.2 Synthesis
In a typical synthesis, 2.40 g of titanium sulfate was dissolved into 25 ml
deionized water with vigorous stirring, then a required amount of silica gel was added
into the above solution under stirring for 30 min and the mixture was further sonicated
for 30 min. After that, the mixture was transferred to a Teflon-lined autoclave, and the
hydrothermal reaction was carried at 100 oC for 2 h. The resulting products
were centrifugated, washed, dried at 80 oC overnight and calcined at a set
temperature. Throughout the subsequent discussion, the samples will be labeled by
indicating the titania content, support and calcined temperature: x % TiO2/SiO2-T,
where x % was the amount of TiO2 loading by weight and T was the calcination
temperature.
2.3 Characterization
XRD patterns of the samples were determined on a XD-2 powder X-ray diffractometer over the
scan range 2
2.4 Photocatalytic reaction
The experiments were carried out in a cylindrical double wall jacket glass reactor containing 0.050 g of catalyst sample and 100 ml of MB solution with a concentration of 20 mg/l. The mixture was first stirred for 20 min in dark at room temperature, then the reaction solution was placed perpendicularly to a 125 W medium pressure mercury lamp by water cooling, and the solution was bubbled with air under magnetic agitation. Samples were collected at regular interval and centrifugated to analyze by diffusive reflective UV-Vis spectrophotometry to determine the concentration of MB.
3 RESULTS AND ANALYSIS
3.1 XRD analysis
The XRD patterns of nanosized TiO2/SiO2 samples with different TiO2
contents and 40 % TiO2/SiO2 samples calcined at different
temperatures were shown in Fig. 1 and Fig. 2, respectively. It is seen that along with the
increase of TiO2 amount, the (101) reflection due to anatase was found to
increase.
Fig. 1 XRD patterns of TiO2/SiO2
with different TiO2 contents. (Calcined at 700 oC)
Fig. 2 XRD patterns of as-synthesized 40% TiO2/SiO2
and calcined at different temperatures.
3.2 Textural properties
The specific surface areas, pore volumes and average particle sizes of the TiO2/SiO2 samples with different silica gel addition and 40 % TiO2/SiO2 samples calcined at different temperatures are shown in Table 1. The specific surface area of the samples decrease from 349.2 m2/g (naked SiO2) to 25.1 m2/g (100 % TiO2) with the increase of TiO2 and it is not significantly decreased when TiO2 content is not more than 20 %, which is probably due to that a little amount of TiO2 can not result in dramatically change in silica structure. Similar phenomenon was reported by Zhao et al [8], and in their work, Ti-Si mixed oxides were prepared by sol-gel one step hydrolysis method. For sample containing 40 % TiO2, with the increase of calcination temperature, the BET surface areas, pore volumes decrease and average particle sizes gradually increase from 6 nm at 500 oC to 14 nm at 900 oC. It suggests that the addition of silica can improve thermal stability of samples and inhibit the growth of titania crystallite. Similar result was reported by Chen and co-workers for silica gel supported TiO2 particles prepared by acid-catalyzed sol-gel method [9]. Contrast to this, Chen et al [10] prepared silica-doped titania photocatalyst through sol-gel method and reported that the optimum amount of silica can effectively prevent the growth of titania, but too little or too much silica cannot prevent the growth of titania grains during the heat treatment.
Table 1. Textural properties and average TiO2 crystal size of the silica-modified photocatalysts
Catalyst |
BET surface area (m2/g) |
Structure |
Pore volume |
Crystal size (Anat. nm) |
Rate constant(min-1) |
P-25 |
54.0 |
Anatase/Rutile |
- |
21 |
0.033 |
SiO2-700 |
349.2 |
- |
0.79 |
- |
- |
as-synthesized
|
334.9 |
Anatase |
1.03 |
- |
0.007 |
40% TiO2/SiO2-500 |
310.2 |
Anatase |
0.81 |
6 |
0.017 |
40% TiO2/SiO2-600 |
289.8 |
Anatase |
0.54 |
7 |
0.037 |
40% TiO2/SiO2-700 |
275.9 |
Anatase |
0.52 |
11 |
0.046 |
40% TiO2/SiO2-800 |
265.3 |
Anatase/Rutile |
0.56 |
12 |
0.030 |
40% TiO2/SiO2-900 |
235.3 |
Anatase/Rutile |
0.48 |
14 |
0.025 |
20% TiO2/SiO2-700 |
347.7 |
Anatase |
0.74 |
8 |
0.038 |
60% TiO2/SiO2-700 |
200.1 |
Anatase |
0.39 |
17 |
0.042 |
80% TiO2/SiO2-700 |
134.0 |
Anatase |
0.33 |
22 |
0.034 |
100% TiO2-700 |
25.1 |
Anatase/Rutile |
0.12 |
28 |
0.027 |
3.3 Photodegradation of methylene blue
3.3.1 Effect of TiO2 content
100 ml of methylene blue (MB) solution (0.02 g/l) was photocatalytically degraded in
the presence of 0.05 g of TiO2/SiO2 composites with different TiO2
contents and 0.02 g Degussa P-25 TiO2, the concentrations of methylene blue
versus reaction time were shown in Fig. 3. During the initial 20 min without photo
irradiation, in the presence of 40 % TiO2/SiO2-700, 21 % MB was
decolored because of the adsorption of MB onto catalyst surface. After that, the
concentration of MB remains almost unchanged, indicating that the adsorption of MB onto
catalyst reaches equilibrium after 20 min stirring in the dark. The adsorption capacities
for MB degradation decrease from 29 % to 0.4 % when TiO2 content increases from
20 % to 100 %.
Fig. 3 The photocatalytic performance of the TiO2/SiO2-700
samples with different TiO2 content. (a) 20% TiO2, (b) 40% TiO2,
(c) 60% TiO2, (d) 80% TiO2, (e) 100% TiO2, (f) P-25, (g)
40% TiO2 without photo irradiation.
The apparent rate
constants of different photocatalysts were calculated and listed in Table. 1. It is seen
that the photocatalytic activities obey the following order: 40 % TiO2/SiO2-700
> 60 % TiO2/SiO2-700 > 20 % TiO2/SiO2-700
> 80 % TiO2/SiO2-700 > 100 % TiO2-700, indicating
that the addition of silica significantly improves the photodegradation ability. It is
observed that the 40 % TiO2/SiO2-700 exhibits the highest
rate constant of 0.046 min-1. It is generally accepted that the photocatalytic
activity of composite catalysts depends on the adsorption of silica gel or other carriers
and activity of titanium dioxide, the amount of them certainly influences on it. When they
reached to a suitable ratio and quantity, the catalyst should show excellent
photocatalytic capacity [11-12]. In our present work, an optimum amount
addition of 60 % silica can reach an optimum synergetic effect between supports and
titanium dioxide, and effectively improve the adsorption ability and photocatalytic
activity of catalysts, but too much or too less silica addition will lower the
photoactivity of catalysts.
3.3.2 Effect of calcination temperature
The effect of calcination temperature on the 40 % TiO2/SiO2
samples for MB degradation is presented in Fig. 4. The photoactivity of 40 % TiO2/SiO2
samples obeys the following order: 700 oC >600 oC >800 oC
>900 oC >500 oC. The samples calcined at 500 oC and
600 oC have higher BET surface area than those calcined at higher temperatures,
which suggests besides BET surface the crystallinity
of catalysts also plays an important role in determination of photocatalytic activity. As
shown in Fig. 2, it can be seen that the sample calcined at 500 oC has the
highest BET surface area and smallest particle size, but its crystallinity is worse than
those calcined at higher temperatures, which is probably the reason for its low
photoactivity.
Fig. 4 The photocatalytic activity of the 40% TiO2/SiO2
samples calcined at different temperature.(a) As-synthesized, (b) 500 oC, (c)
600 oC, (d) 700 oC, (e) 800 oC, (f) 900 oC
Fig. 5 The UV-Vis spectrum of MB solution by
photocatalysis under UV-irradiation after 90 min.
3.3.3 UV-Vis spectra
In order to compare the degradation ability of 40 % TiO2/SiO2-700
with P-25, the UV-Vis spectra of methylene blue solutions were measured after 90 min UV radiation in the presence of catalysts, and results
are shown in Fig. 5. It is seen that the blank MB
solution exhibits a broad absorption band in the range of 530-700 nm. In the presence of
the 40 % TiO2/SiO2, the intensity of the 660 nm absorption band
decreased rapidly under UV light irradiation and almost disappeared after 90 min, and
there is almost no absorption band in the region of 250-400 nm, while P-25 has a broad
absorption band at 300 nm. This indicates that 40 % TiO2/SiO2-700
has excellent degradation ability for MB. The good explanation is that 40 % TiO2/SiO2-700
has a relatively high capability of collection of degradation intermediates as well as
target substrate from the solution phase onto the catalyst, thereby preventing dissolution
of toxic substances in solution, whereas when P-25 was used, reaction intermediates mostly
dissolved in the solution phase [13].
4 CONCLUSIONS
Nanosized TiO2/SiO2 catalysts were prepared by hydrothermal
method. The experimental results show that the addition of silica gel could effectively
improve the thermal stability of TiO2/SiO2 composite and inhibit the
transformation of anatase to rutile phase and the growth of titanium dioxide crystals
during the calcination treatment. The nanosized 40 % TiO2/SiO2-700
catalyst shows excellent photocatalytic activity for MB degradation, its activity is
higher than that of P-25 with the same TiO2 amount. After 90 min irradiation of
UV light, nearly no absorption band was observed in the UV-Vis spectra in the presence of
40 % TiO2/SiO2-700 catalyst, whereas for P-25, a broad band appeared
in range of 200-360 nm.
ACKNOWLEGMENT
We gratefully acknowledge financial support from the Team Project of Guangdong
Province Natural Science Foundation (Grant No 05200555).
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李明，钟慧琼，张丽珍，杨骏
(暨南大学化学系，广州510632)
摘要 采用水热法经硫酸钛合成晶粒大小为6 -22 nm的纯锐钛矿相TiO2/SiO2光催化剂，并用XRD和BET对催化剂进行了表征。结果表明：硅胶的加入有效地提高TiO2/SiO2催化剂的热稳定性,抑制了热处理过程中TiO2由锐钛矿相向金红石相的转变和晶粒的长大。40 % TiO2/SiO2-700催化剂显示了降解亚甲基蓝的光催化活性的增强，其反应速率常数是0.046 min-1，而P-25催化剂反应速率常数是0.033 min-1。紫外灯照射90 min后，40 % TiO2/SiO2-700催化剂存在下，废水溶液在200 -800 nm范围内几乎观察不到吸收带，而P-25在200 -360 nm范围内有吸收带。可能的原因是TiO2/SiO2催化剂在催化作用过程中，对溶液中的降解中间物和目标物有相对强的吸附能力，因而防止了有毒物质在溶液中的分散。
关键词 纳米TiO2/SiO2复合物，硫酸钛，光催化，亚甲基蓝