Preparation of titania-based catalysts for formaldehyde photocatalytic oxidation from TiCl₄ by the sol–gel method

Abstract

Titania sols were prepared by acid hydrolysis of a TiCl₄ precursor instead of titanium alkoxides. The effect of acid concentration on the particle size and stability of sol was investigated. Stable titania sols with mean particle size of 14 nm could be obtained when the H⁺/Ti molar ratio was 0.5. The titania sols were modified with Pt, SiO₂, ZrO₂, WO₃ and MoO₃ to prepare a series of modified catalysts, which were used for the photocatalytic oxidation of formaldehyde at 37°C. They showed different photocatalytic activities due to the influence of the additives. Comparing with pure TiO₂, the addition of silica or zirconia increased the photocatalytic activity, while the addition of Pt and MoO₃ decreased the activity, and the addition of WO₃ had little effect on the activity. It is of great significance that the conversion of formaldehyde was increased up to 94% over the SiO₂–TiO₂ catalyst. The increased activity was partly due to higher surface area and porosity or smaller crystallite size. A comparison of our catalyst compositions with the literature in this field suggested that the difference in activity due to the addition of a second metal oxide maybe caused by the surface chemistry of the catalysts, particularly the acidity.

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.