Photocatalytic degradation of Chromotrope 2R using nanocrystalline TiO 2 /activated-carbon composite catalysts Wendong Wang 1 , Cla ´udia Gomes Silva, Joaquim Luı ´s Faria * Laborato ´rio de Cata ´lise e Materiais, Departamento de Engenharia Quı ´mica, Faculdade de Engenharia da Universidade do Porto, Rua Dr. Roberto Frias s/n 4200-465 Porto, Portugal Available online 30 June 2006 Abstract Composite catalysts made of nanocrystalline TiO 2 and carbon were prepared by a modified sol–gel method over activated carbon (AC). The composite catalysts were characterized by N 2 adsorption–desorption isotherm, TG, diffuse reflectance UV–vis spectroscopy, XRD and SEM. The photocatalytic activity was tested on the degradation of Chromotrope 2R (C2R) in aqueous medium under UV radiation. The composite catalysts exhibited higher activities than commercial Degussa P25 alone and the photocatalytic process was more efficient than the pure photolytic degradation. A modified Langmuir–Hinshelwood approach was used to study the kinetics and to determine the adsorption equilibrium constant and the reaction rate constant. Two different mechanisms are proposed and discussed in order to explain the observed synergy. # 2006 Published by Elsevier B.V. Keywords: Photocatalysis; UV photodegradation; Titanium dioxide; Activated carbon; Composite catalyst 1. Introduction Titanium dioxide (TiO 2 ) is generally chemically and biologically inert, photoactive and inexpensive, which are the underlying reasons for the fact that this material is probably one of the most extensively used photocatalytic supports for solving environmental problems, especially in what concerns the purification of wastewater [1–3]. Titania exists as anatase, rutile and brookite crystalline forms. Anatase possesses the best photocatalytic properties and is commonly used as photo- catalyst or photocatalytic support, normally mixed with rutile as in commercial Degussa P25 titania. The TiO 2 /UV system has been widely investigated in association with heterogeneous photocatalysis, in a process whereby UV irradiation of the semiconductor generates electron/hole (e  /h + ) couples ready to initiate redox chemistries. In the absence of any electron acceptor, the photogenerated electrons are sufficiently reduc- tive to reduce water to hydrogen. If oxygen is present, the superoxide radical ion (O 2  ) can be readily formed. The photogenerated holes are very oxidizing and can oxidize water to form hydroxyl radical (HO  ). Both of the radicals are very reactive and strongly oxidizing, capable of totally mineralizing most of the organic pollutants. Anatase can be produced by several low temperature methods based on the alkaline hydrolysis of a titanium (IV) starting material, followed by calcination at temperatures up to 500 8C. Above 700 8C anatase is readily converted to rutile with a significant loss of specific surface area and photocatalytic activity. One of the most preferred routes to the production of TiO 2 anatase is the sol–gel technique from an alkoxide precursor. This route provides a versatile approach to synthesize anatase TiO 2 with tailored morphological features, through adjustment of the operation conditions, such as different solvent removal procedures [4], surfactant or polymer modification [5], UV illumination [6] and ultrasonication [7]. The photocatalytic activity of the produced TiO 2 powder greatly depends upon its microstructure and the associated physical properties. A number of reports [4–12] have investigated the relationship between the synthesis methods and the properties of the resulting TiO 2 nanoparticles, including surface area, particle size, pore volume and pore size distribution, crystal structure and crystallinity, phase composition, thermal stability and band-gap energy. Biphasic TiO 2 and carbon composite catalysts are believed to exhibit cooperative effect or synergy between the metal www.elsevier.com/locate/apcatb Applied Catalysis B: Environmental 70 (2007) 470–478 * Corresponding author. Tel.: +351 225 081 645; fax: +351 225 081 449. E-mail address: jlfaria@fe.up.pt (J.L. Faria). 1 Present address: Department of Materials Science and Engineering, Uni- versity of Science and Technology of China, Hefei 230026, People’s Republic of China. 0926-3373/$ – see front matter # 2006 Published by Elsevier B.V. doi:10.1016/j.apcatb.2005.11.034