Journal of Molecular Catalysis A: Chemical 305 (2009) 147–154 Contents lists available at ScienceDirect Journal of Molecular Catalysis A: Chemical journal homepage: www.elsevier.com/locate/molcata Effect of key operational parameters on the photocatalytic oxidation of phenol by nanocrystalline sol–gel TiO 2 under UV irradiation  Cláudia Gomes Silva, Joaquim Luís Faria ∗ Laboratório de Catálise e Materiais (LCM), Laboratório Associado LSRE/LCM, Departamento de Engenharia Química, Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, s/n 4200-465 Porto, Portugal article info Article history: Available online 30 December 2008 Keywords: Heterogeneous photocatalysis Titanium dioxide Nanomaterials Sol–gel catalysts Kinetics abstract Nanocrystalline TiO 2 materials produced by an acid-catalyzed sol–gel method are used as catalysts in the photocatalytic degradation of phenol under ultraviolet light. Materials with different crystalline and mor- phological properties are obtained by controlling the temperature used in the calcination step. Induced light conversion and adsorption have opposite dependencies on the light intensity. The operational parameters (nature of TiO 2 crystal phase, catalyst concentration, pH and initial phenol concentration) have the expected influence in the efficiency of the photocatalytic degradation process. The effect of two different co-oxidants (H 2 O 2 and Na 2 S 2 O 8 ) in the photocatalytic process is also described. A modified Langmuir–Hinshelwood kinetic model is used considering a pseudo-steady state approach in order to explain the dependence of both, the kinetic rate and adsorption equilibrium constants, on light intensity. Hydroquinone and catechol are the main intermediates of the photocatalytic reaction, as result from the reaction of phenol with photogenerated hydroxyl radicals. A possible degradation pathway is advanced. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Heterogeneous photocatalysis is nowadays recognized as a strategic area of growing importance in what concerns the develop- ment of sustainable technologies for energy production and storage [1], green chemical synthesis [2] and water [3] and air [4] treatment. It can be used together with other techniques for soil remediation [5] and chemical control [6]. The photocatalytic process starts with the irradiation of a semiconductor material by light with sufficient energy to excite the electrons form the valence to the conduc- tion band generating extremely reactive electron/hole (e - /h + ) pairs which migrate to the surface of the semiconductor where they can react with adsorbed species. Positively charged holes react with adsorbed water molecules or hydroxyl anions leading to the formation of the hydroxyl radical, HO • . The surface electrons are transferred to adsorbed oxygen originating the superoxide radical anion, O 2 •- , which may undergo protonation to the hydroperoxyl radical, HO 2 • . Some other adsorbed compounds can react with the surface holes and/or electrons resulting in oxidation and/or reduc- tion products, respectively. Although radical species like HO 2 • and O 2 •- are able to oxidize most of the organic compounds, hydroxyl radical is regarded as  Paper submitted by occasion of the Symposium in Honor of Eric Derouane. ∗ Corresponding author. Tel.: +351 225 081 645; fax: +351 225 081 449. E-mail address: jlfaria@fe.up.pt (J.L. Faria). the major responsible species for the complete oxidation of organic pollutants [7,8]. Titanium dioxide (TiO 2 ) has been extensively employed as a photocatalyst in wastewater treatment by oxidative degradation [9–12]. The most popular form of this material is the commer- cially available ready-to-use P 25 from Evonik Degussa, consisting of roughly 75:25 anatase to rutile weight ratio of TiO 2 . Titanium dioxide can be prepared by both liquid and gas phase processes. For laboratory purposes, the sol–gel method is one of the most used techniques to synthesize films, powders and membranes [13–15]. The sol–gel method has many advantages over other production techniques, including ease of processing, control over the compo- sition, purity and homogeneity of the obtained materials [16]. In the present work nanocrystalline sol–gel TiO 2 catalysts were produced, characterized and used in the photocatalytic degradation of phenol under ultraviolet irradiation. Several studies about photo- oxidation of phenol have already been published [17–19], especially because phenol and phenolic derivatives are commonly found in industrial wastewaters [20–22]. These compounds are normally refractory and recalcitrant limiting the benefits of the preferred bio- logical treatment process. Therefore, phenol is a very useful model pollutant for photocatalytic studies. Kinetics and mechanism of the photocatalytic oxidation of phe- nol in a slurry reactor (where the catalyst is suspended in the liquid phase) are here investigated. Besides of being very simple, slurry (or immersion) reactors are very popular, because of the easy handling and inexistence of mass transfer limitations. The major drawback of these reactors relies on the fact that due to geometry diversity, 1381-1169/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.molcata.2008.12.015