Photocatalysis of SiO2-loaded TiO2

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Abstract

SiO2 was loaded to TiO2 by sol–gel method, and the resulting SiO2/TiO2 was calcined at different temperatures. The loading of SiO2 improved photocatalytic activity of TiO2. The optimum calcination temperature was 300 °C, and the optimum amount of loaded SiO2 was 1.5–4.8 wt.%. This modified photocatalyst was tested for the degradation of nitrobenzene (NB), benzyltrimethylammonium chloride (BTMA), phenol and propionic acid. The highest effect was observed for BTMA, and attributed to electrostatic interaction between BTMA and SiO2/TiO2. Whereas simple mechanical mixing of TiO2 and SiO2 was not effective.

Graphical abstract

SiO2-loading enhanced the photocatalytic activity of TiO2 for the degradation of several substrates. The highest efficiency was observed for cationic compound, which was attributed to electrostatic interaction of negatively charged SiO2/TiO2 and cation. On the other hand simple mixing of SiO2 with TiO2 resulted in no effect.

Introduction

There have been intensive studies on TiO2 photocatalysis in the past two decades [1], [2] and its practical use are in progress for air purification and related fields. However, still several technical breakthroughs are required for water purification. Major improvement to be achieved is an increase in the degradation efficiency. One of the factors to determine the efficiency is adsorption of substrate to TiO2. Incorporation of silica [3], [4], [5], [6], [7], [8], [9], [10], alumina [4], zeolite [8], [11], [12], [13], [14] and activated carbon [8], [9], [15], [16] into TiO2 have been studied to increase the adsorption. It was demonstrated that most of the TiO2-covered adsorbents are more efficient than bare TiO2. In those works the authors aim also to support TiO2 on the larger adsorbent particle, so that the recovery of photocatalyst is facilitated. Partly because of this reason, in the most studies TiO2 was formed on the surface of adsorbent instead of vice versa. However, it is advantageous to modify existing highly efficient TiO2 to a better photocatalyst. One of the present author reported that the improved efficiency of SiO2-loaded TiO2 (SiO2/TiO2) resulted from sodium silicate. The modified catalyst was effective for cationic pollutants. In the possible reaction mechanism of SiO2-loaded TiO2 photocatalyst the substrate is concentrated on SiO2 near the surface of TiO2 and thereby increases the degradation of the substrate. This mechanism suggests that adsorbent should be finely distributed on the surface of TiO2 to achieve the high efficiency and accordingly the subtle difference in the preparation process affects the photocatalytic activity. On the other hand, it has been demonstrated that simple mechanical mixing of TiO2 and adsorbent increases the efficiency [17], [18]. The comparison between two systems, loaded TiO2 and mechanically mixing, provides some insights into the mechanism of the adsorbent effect.

In the present study SiO2 was loaded to TiO2 by sol–gel method and the optimization of this photocatalyst was studied.

Section snippets

Chemicals

The TiO2 used is TP-2 (anatase) purchased from Fujititan Co. [19]. Nitrobenzene (NB), benzyltrimethylammonium chloride (BTMA), phenol and propionic acid tested for the degradation are of reagent grade. Silica OX-50 was a product of Nihonaerosil Co.

Loading of SiO2

General procedure is as follows. 0.5–1.5 ml of TEOS (tetraethoxysilane) in 24 ml of methanol was added to 3 g TiO2 and stirred magnetically until complete evaporation of methanol. Then distilled water was added to the TiO2. After stirring for several

Results and discussion

Fig. 1 shows the effect of calcination temperature on the nitrobenzene degradation. The activity increases at higher temperature than 300 °C and the optimum was 300 °C. Similar trend was obtained for BTMA in the previous study in which the optimum temperature was 300–400 °C [20]. This temperature dependence may be attributed to the highest adsorbability of TiO2 sample calcined at 300 °C as reported for Na+ adsorption [21].

The degradation curves of nitrobenzene on SiO2/TiO2 were shown for different

Conclusion

SiO2-loading to TiO2 accelerated the degradation rate of the different substrates. The highest effect was observed for BTMA (cationic substance). The optimum amount of loaded SiO2 was 1.5–4.8 wt.%. Activity can be affected by loading method. Loaded sample calcined at 300 °C resulted in the best effect. Unbound SiO2 mechanically mixed with TiO2 was not effective at least up to 10 wt.%.

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