Photocatalytic degradation of volatile organic compounds at the gas–solid interface of a TiO 2 photocatalyst Sang Bum Kim a, * , Hyun Tae Hwang a , Sung Chang Hong b a Department of Chemical Engineering, Korea University, 1, 5-ka, Anam-dong, Sungbuk-ku, Seoul 136-701, South Korea b Department of Environmental Engineering, Kyonggi University, 94 San, Iui-dong, Paldal-ku, Suwon-si, Konggi-do 442-760, South Korea Received 25 February 2001; received in revised form 19 February 2002; accepted 19 February 2002 Abstract In the present work, photocatalytic degradation of volatile organic compounds including gas-phase trichloroeth- ylene (TCE), acetone, methanol and toluene over illuminated TiO 2 was closely examined in a batch photoreactor as a function of water vapor, molecular oxygen and reaction temperature. Water vapor enhanced the photocatalytic de- gradation rate of toluene, but was inhibitive for acetone, and, there was an optimum water vapor concentration in the TCEandmethanolremoval.Inanitrogenatmosphere,itshowedlowerphotocatalyticdegradationratethaninairand pureoxygen.Thus,itcouldbeconcludedthatoxygenisanessentialcomponentinphotocatalyticreactionsbytrapping photogeneratedelectronsonthesemiconductorsurfaceandbydecreasingtherecombinationofelectronsandholes.As fortheinfluenceofreactiontemperature,itwasfoundthatphotocatalyticdegradationwasmoreeffectiveatamoderate temperature than at an elevated temperature for each compound. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Photocatalytic degradation; TiO 2 film; Gas phase; Volatile organic compound; Batch photoreactor 1. Introduction Volatile organic compounds (VOCs) are widely used in industrial processes and domestic activities, leading to water and air pollution, even indoor work-place air pollution (EPA, 1987). Many VOCs are known to be toxic and considered to be carcinogenic. The most sig- nificant problem related to the emission of VOCs is centered on the potential production of photochemical oxides: for example, ozone and peroxyacetyl nitrate. The TiO 2 -sensitized photodegradation of organic compounds has been proposed as an alternative ad- vanced oxidation process (AOP) for the decontamina- tion of water and air. The AOP is initiated from the generation of hole–electron pairs on the semiconductor, absorbing the ultra-violet (UV) light with energy equal to or higher than the band gap energy (E g ) of semicon- ductor. Electrons and holes are photogenerated in the bulk of the semiconductor, and move to the particle surface; electrons reduce an electron acceptor such as molecularoxygen,andholescanoxidizeelectrondonors including adsorbed water or hydroxide anion to yield hydroxyl radicals. The common VOCs such as halogenated hydrocar- bons, ketones, alcohols and aromatic compounds have been widely used in many industries, and are often found in the emission flow (Shen et al., 1993). Some workers have recently examined the photocatalytic de- gradation of TCE in gas phase. To determine the ki- netics of conversion of TCE (up to 100 ppm), Dibble and Raupp (1992) systematically investigated the pho- tooxidation in air using both a fixed-bed reactor and a fluidized bed reactor. They showed that trace water Chemosphere 48 (2002) 437–444 www.elsevier.com/locate/chemosphere * Corresponding author. Tel.: +82-2-924-8362; fax: +82-2- 926-6102. E-mail address: sbkim@prosys.korea.ac.kr (S.B. Kim). 0045-6535/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII:S0045-6535(02)00101-7