Journal of Catalysis 219 (2003) 107–116 www.elsevier.com/locate/jcat The influence of surface properties on the photocatalytic activity of nanostructured TiO 2 King Lun Yeung, a,∗ Sze Tai Yau, a A. Javier Maira, b Juan M. Coronado, b Javier Soria, b and Po Lock Yue a a Department of Chemical Engineering, the Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, SAR China b Instituto de Catálisis y Petroleoquímica, CSIC, Campus UAM, Cantoblanco, Madrid 28047, Spain Received 23 October 2002; revised 31 March 2003; accepted 10 April 2003 Abstract Nanostructured TiO 2 with 11- (P11) and 5-nm (P5) crystal sizes were prepared by a modified sol–gel method. Controlled crystallization and a pretreatment process were employed to obtain TiO 2 with different surface roughness and degree of hydroxylation, while maintaining an identical crystal (i.e., 11 or 5 nm) and aggregate (i.e., 100 nm) sizes, phase structure (i.e., anatase), and crystallinity (i.e., X-ray diffraction peak intensity). Using the photooxidation of airborne trichloroethylene as a probe reaction, we were able to identify that the hydroxyl groups on low-coordinated titanium atoms are responsible for the generation of dichloroethylene and dichloroacetaldehyde by-products. Their presence usually means lower TCE conversion and in some cases leads to catalyst deactivation.  2003 Elsevier Inc. All rights reserved. 1. Introduction Photocatalytic oxidation (PCO) is an attractive method for low-temperature remediation of volatile organic com- pounds (VOCs) in process air stream [1–3]. Using inex- pensive TiO 2 catalysts irradiated with UV light, airborne VOCs can be mineralized into carbon dioxide, water, and mineral compounds (e.g., HX, where X = Cl) at room tem- perature. The semiconductor TiO 2 catalyst generates elec- trons and holes with the absorption of UV photons and these photogenerated charges can migrate toward the cata- lyst surface where they initiate redox reactions that oxidize the adsorbed organic molecules. The low quantum yield and the possible formation of undesirable by-products are two of the most important issues in PCO technology [4]. Re- cently, new types of photocatalysts based on nanostructured TiO 2 have been developed in order to address these prob- lems. These nanometer-sized TiO 2 particles exhibited higher activity [5–7] and selectivity [7] than the commercial TiO 2 (P25, Degussa) for gas-phase PCO of VOCs. * Corresponding author. E-mail address: kekyeung@ust.hk (K.L. Yeung). Nanostructured TiO 2 with controlled crystal and ag- gregate sizes were prepared by a modified sol–gel tech- nique [6]. The anatase TiO 2 (3–20 nm) were crystallized from amorphous titania gel spheres by thermal and hy- drothermal processes. The nanostructured TiO 2 exhibited different structural, electronic, and catalytic properties, de- pending on the crystal size [6–9]. A recent study showed that the surface properties of the nanometer-sized TiO 2 cata- lysts, as well as their photocatalytic activity are affected by the crystallization conditions [10]. This paper attempts to clarify the role of the surface structure and surface hydroxyl groups on the photocatalytic properties of nanostructured TiO 2 . Two sets of nanostructured TiO 2 with crystal sizes of 11 nm (P11) and 5 nm (P5) were prepared for the study. Each set of catalysts had similar phase structure (i.e., anatase), crystal (i.e., 11 or 5 nm), and aggregate (i.e., 100 nm) sizes, crystallinity, surface area, and band-gap energy, but differed in their surface properties. Electron paramagnetic resonance (EPR) and Fourier-transformed infrared (FTIR) spectroscopies were used to obtain information on the sur- face properties of the TiO 2 samples. The photocatalytic oxi- dation of airborne trichloroethylene was chosen as the probe reaction because of the abundant literature data for this re- action system [11–22]. The photoreaction was monitored by in situ Fourier-transformed infrared spectroscopy. 0021-9517/$ – see front matter  2003 Elsevier Inc. All rights reserved. doi:10.1016/S0021-9517(03)00187-8