DOI: 10.1002/cssc.200900262 Photocatalytic Oxidation of Phenolic Compounds by Using a Carbon Nanotube-Titanium Dioxide Composite Catalyst Clµudia Gomes Silva and Joaquim Luís Faria* [a] Introduction The exponential growth of the human population and the in- tensification of agricultural and industrial activities have result- ed in a continuous increase in the demand for Earth’s limited supply of fresh water. The protection of natural water resour- ces and the development of new technologies for (waste)water treatment are key environmental issues for the 21st century. Advanced oxidation processes (AOPs), including the Fenton and photo-Fenton processes, ozone and/or peroxide photoly- sis, and semiconductor photocatalysis, [1–4] are considered prom- ising and competitive solutions for the abatement of numer- ous hazardous compounds in wastewater streams. AOPs are based on nonselective, highly reactive hydroxyl radicals, able to drive oxidation processes that lead to the complete elimina- tion and full mineralization of a wide range of organic and in- organic pollutants. The importance of photocatalytic processes that use semiconductor catalysts for wastewater treatment has increased, because they are among the most effective break- down technologies. [5, 6] What contributes to this status is that these processes require only mild operating conditions and in many cases result in total mineralization of the pollutants, without any additional waste disposal problems. TiO 2 has been extensively employed as photocatalyst in wastewater treatment by oxidative degradation. [5, 7] The photo- catalytic activity of TiO 2 largely depends on its microstructure and physical properties [8–10] as well as on the incorporation of some other metal ions, adsorbents, or supports. [11–13] It has been reported that carbon materials have some beneficial ef- fects on the photocatalytic activity of TiO 2 by inducing syner- gies or cooperative effects between the metal oxide and carbon phases. [14–17] Since they were discovered, [18] carbon nanotubes (CNTs) have been the focus of various studies owing to their unique structural, electronic, and mechanical properties. In the field of catalysis carbon nanotubes constitute a promising alternative catalyst support, competing with acti- vated carbon. [19] Some recent works have emphasized the preparation of CNT/TiO 2 nanocomposite catalysts, aiming at a synergetic combination of their intrinsic properties and there- by enhancing performance to meet new requirements im- posed by advanced applications in distinct fields, such as opto- electronics, solar energy utilization, and heterogeneous photo- catalysis. [16, 20–26] In the present work, TiO 2 and a multiwalled CNT/TiO 2 com- posite catalyst (CNT-TiO 2 ) are produced by an acid-catalyzed sol–gel technique starting from an alkoxide precursor. The cat- alysts are used for the photocatalytic degradation of four para- substituted phenols (4-chlorophenol, 4-aminophenol, 4-hydrox- ybenzoic acid, and 4-nitrophenol) under near-UV to visible irra- diation. Phenol and phenolic derivatives are common com- pounds in industrial wastewaters, [27–29] being refractory and re- calcitrant species in conventional biological treatment process- es. The heterogeneous photocatalytic process using the hybrid CNT-TiO 2 catalyst is characterized by studying the effect of sev- eral operational parameters, such as catalyst loading, pH of the reaction medium, addition of hydrogen peroxide, and sub- strate concentration, in the photoefficiency of degradation of 4-chlorophenol as model compound. The effect of the elec- tronic nature of the substituent group on the photoreactivity of the para-substituted phenols using both TiO 2 and CNT-TiO 2 catalysts is also reported. A nanostructured multiwalled carbon nanotube (CNT) and tita- nium dioxide composite catalyst is prepared by a modified acid-catalyzed sol–gel method. Pure anatase TiO 2 and the CNT- TiO 2 composite are tested in the photocatalytic degradation of four para-substituted phenols: 4-chlorophenol, 4-aminophenol, 4-hydroxybenzoic acid and 4-nitrophenol. The effect of several operational parameters on the photoefficiency of the compo- site catalyst is studied by using 4-chlorophenol as model com- pound, namely catalyst loading, pH of the medium, hydrogen peroxide concentration, substrate concentration. A relationship between the Hammett constant of each para-substituted phe- nolic compound and its degradability by the photocatalysts is found. The effect of the carbon phase in the catalyst is as- cribed to its photosensitizer action. A clear beneficial effect is observed for the degradation of 4-aminophenol and 4-chloro- phenol. For the molecules with stronger electron-withdrawing (deactivating) groups, such as 4-hydroxybenzoic acid and 4-ni- trophenol, no synergy effect is observed. [a] Dr. C. G. Silva, Prof. J. L. Faria Laboratório de Catµlise e Materiais Laboratório Associado LSRE/LCM Departamento de Engenharia Química—Faculdade de Engenharia Universidade do Porto (Portugal) Fax: (+ 351) 225 081 449 E-mail : jlfaria@fe.up.pt ChemSusChem 2010, 3, 609 – 618  2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 609