An investigation of trichloroethylene photocatalytic oxidation on mesoporous titania-silica aerogel catalysts Shengli Cao a , King Lun Yeung b, * , Po-Lock Yue b a Environmental Engineering Program, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong b Department of Chemical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong Received 20 October 2006; received in revised form 27 April 2007; accepted 2 May 2007 Available online 6 May 2007 Abstract Freestanding blocks of mesoporous, titania-silica aerogel catalyst were prepared and tested for gas phase photocatalytic oxidation of trichloroethylene (TCE) in a new photoreactor where the reactant gases flow through the aerogel block. The intimate contact between the reactants and catalyst resulted in a higher conversion. TS01 aerogel was prepared by ethanol supercritical drying, and contained nanometer-sized crystalline domains of anatase TiO 2 (i.e., Ti–O–Ti sites) within an amorphous Si–O–Ti and Si–O–Si network. The TS02 aerogel prepared by CO 2 supercritical drying remained amorphous even after high temperature treatment, and analysis indicated that the Ti–O–Si is the predominant site on this catalyst. The reaction results indicated that only the TS01 aerogel containing anatase TiO 2 was active, but the formation of dichloroacetaldehyde on the amorphous Si–O–Si sites led to a rapid catalyst deactivation. However, a stable catalyst was obtained after these sites were passivated by ozone pretreatment under ultraviolet (UV) irradiation. The TS01 aerogel catalysts exhibited an average conversion of 30% and a quantum yield of 0.27 at a TCE concentration of 56 ppm, feed flow rate of 40 sccm and UV irradiation of 710 mW cm 2 . # 2007 Elsevier B.V. All rights reserved. Keywords: Nano-TiO 2 ; Sol–gel; Gas transport; Photoreactor and in situ FTIR 1. Introduction Volatile chlorinated organic compounds are major indoor air pollutants and can be found in common household items [1]. They are known to cause irritation and set off allergy. Many of these compounds have well documented neurological effects and are also suspected carcinogens [2]. Trichloroethylene (TCE) is a common solvent in dry cleaning and metal degreasing. TCE is also present in many household product including adhesives, paints and varnishes and is shown to persist in the indoor air [3]. Its removal and treatment by photocatalytic oxidation (PCO) had been investigated by several research groups [4–9]. Titanium dioxide was the most commonly used catalyst for the reaction, but other catalysts including ZnO had also been reported [10]. Dibble and Raupp [4,5] carried out the first systematic study of the photocatalytic oxidation of gas phase TCE using fixed bed and fluidized bed reactors. Complete mineralization was reported but subsequent study using infrared spectroscopy detected the formation of dichloroacetaldehyde on anatase TiO 2 [11], which was confirmed by a recent report by Yeung et al. [12]. Incomplete reaction could lead to undesirable by-products including dichloroacetyl chloride (DCAC), phosgene and carbon monoxide [6,8,9,13,14]. Trace amounts of chloroform, carbon tetrachloride, 1,1-dichloroethane, pentachloroethane, oxalyl chloride, hexachloroethane, alcohols and esters were also occasionally observed [7,15,16]. The efficiency of the PCO reaction depends not only on the catalyst but also on the reactor design. Most of the research in recent years was devoted to improving the TiO 2 catalyst by various means including metal doping [17–21] and surface modification [7,12,22–24]. In comparison, less attention was given to improving the photoreactor design. A good photoreactor must provide a good mixing between the reactant gases and the catalyst while at the same time afford the best illumination of the catalyst. The annular photoreactor with a thin layer of catalyst coated on UV-transparent quartz maximizes UVirradiation of the catalyst and is a popular photoreactor design [9,15,25]. Flat-plate [8,12,26] and packed-bed [27] reactors are also widely used. www.elsevier.com/locate/apcatb Applied Catalysis B: Environmental 76 (2007) 64–72 * Corresponding author. Tel.: +852 2358 7123; fax: +852 2358 0054. E-mail address: kekyeung@ust.hk (K.L. Yeung). 0926-3373/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2007.05.009