Preparation of titania-based catalysts for formaldehyde photocatalytic oxidation from TiCl4 by the sol–gel method
Introduction
Photocatalytic oxidation of organic compounds in water and air has received much attention as a potential technology of pollution abatement. Several books and reviews have been published in this area [1], [2], [3], [4], [5], [6]. The removal of volatile organic compounds (VOCs) in air has been a topic of major and continuing emphasis over the last decade [7], [8], [9]. Potential application sites for air purification and decontamination technologies include completely or partially enclosed atmospheres such as those found in spacecraft, office buildings, factories and homes. Titanium dioxide (TiO2) has been extensively studied as a photocatalyst for the photocatalytic oxidation of organic compounds. A wide range of organic compounds can be oxidized to CO2 and H2O at room temperature on TiO2 catalysts in the presence of UV or near-UV illumination [10], [11], [12]. However, slow reaction rates and poor solar efficiency (maximum 5%) have hindered the commercialization of this technology. In the hope of improving reaction rates, we dedicated much of our research in this area toward synthesis of better photocatalyst.
It has been shown that the photocatalytic activity of TiO2 is influenced by the crystal structure, surface area, size distribution, porosity, band gap, and surface hydroxyl group density [13], [14], [15], [16]. Recently, noble metals (Pt, Pd, Au, Ag, etc.) deposited on TiO2 have been widely studied for the purpose of improving the latter’s photocatalytic activity [17], [18], [19], [20], [21]. An enhancement of photocatalytic activity in the noble metal modified TiO2 has been explained in term of a photoelectrochemical mechanism in which the electrons generated by UV irradiation on the TiO2 semiconductor transfer to the loaded metal particles, while the holes remain in the semiconductor, resulting in a decrease in the electron-hole recombination [6]. In contrast to metal/TiO2 photocatalysts, there have been few studies reported concerning the photocatalytic activity of metal oxide/TiO2 catalysts. Fu and co-workers have reported that the mixtures of silica or zirconia with titania had significantly higher activities than pure titania for the complete photocatalytic oxidation of ethylene [24]. Do and Papp have found that degradation rates of 1,4-dichlorobenzene on WO3/TiO2 and MoO3/TiO2 systems were enhanced by the addition of ca. 3 mol% WO3 and MoO3 in titania [22], [23]. However, Liu [25] has found that the photocatalytic activity of MoO3/TiO2 catalyst for the photooxidation of methanol was only one-fifth that of pure TiO2.
It is well known that the sol–gel technology is an advanced method to prepare the mixed-metal oxide catalysts, which provides a highly controllable preparation route with inherent advantages such as molecular-scale mixing of the constituents, homogeneity of the sol–gel product and the use of different wet-chemical preparation tailoring tools [26]. In the traditional process, most of titania colloids are obtained by the controlled hydrolysis of titanium alkoxides. However, the synthesis of titania colloids from the corresponding salts (chloride, sulfate or nitrate) is lacking, which is possibly due to the rapid hydrolysis of the inorganic salt making the formation of precipitates become more easy and uncontrolled. In our early work, it was found that the fresh precipitate produced from the hydrolysis of either alkoxide or inorganic salt could be peptized with acid to yield a stable sol under appropriate conditions. The use of inorganic salt precursor rather than organic alkoxide precursor not only can reduce the cost of synthesis, but also can avoid the use of organic solvent to decrease pollution. In this paper, aqueous titania colloids were prepared from TiCl4. A series of photocatalysts were obtained by modifying the titania colloids with Pt, SiO2, ZrO2, WO3 and MoO3. Formaldehyde was chosen as a probe for its presence and toxicity in air. The effects of the additives in titania on the structure, the photochemical properties and the photocatalytic activities were investigated by the physico-chemical methods such as X-ray diffraction (XRD), BET surface area, pore volume, UV–VIS diffuse reflectance spectroscopy (DRS) and photocatalytic oxidation of formaldehyde.
Section snippets
Preparation of catalysts
All catalysts were prepared using the sol–gel method. The titania sols were prepared via acid peptizing the precipitate of TiCl4 solution with ammonia. A 10% NH4OH solution was dropped into the TiCl4 solution and a white precipitate was obtained at pH=7. The precipitate was washed with deionized water until the absence of Cl− and NH4+ ions, then a certain amount of deionized water was added to form a suspension. By adding a correct amount 1.6 M HNO3 ([H+]/[Ti]=0.5) with strong stirring for 24 h
Preparation of sols and catalysts
In the preparation process of titania sols, the acid concentration affected the size of the sol particles. Table 1 shows that the size of the sol particles, estimated from QELS measurements, varied with the acid concentration. It was found that the sol was the most stable and its mean particle size is the smallest when the H+/Ti molar ratio was 0.5. Fig. 1a shows the size distribution of the titania sol prepared under the H+/Ti molar ratio of 0.5. The size distribution of the particles was
Conclusion
Stable titania sols can be obtained by acid hydrolysis of TiCl4 instead of titanium alkoxide when the H+/Ti molar ratio is 0.5. The modification of the titania sol with different compositions can enhance or reduce the photocatalytic activity for the oxidation of formaldehyde. The modification of the titania sol by silica has been shown to produce a better photocatalyst for the oxidation of formaldehyde. Conversion of formaldehyde at 37°C can be increased up to 94% over SiO2–TiO2 catalyst. This
Acknowledgements
The project was supported by the National Natural Science Foundation of China and Chinese Academy of Sciences. We are very grateful to Mr. S.S. Sheng for the experimental assistance and the helpful discussion.
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Present address: Research Institute of Photocatalysis, Fuzhou University, Fuzhou 350002, PR China.