TRMC, XPS, and EPR Characterizations of Polycrystalline TiO 2 Porphyrin Impregnated Powders and Their Catalytic Activity for 4-Nitrophenol Photodegradation in Aqueous Suspension Giuseppe Mele,* ,² Roberta Del Sole, ² Giuseppe Vasapollo, ² Giuseppe Marcı `, ‡ Elisa Garcı `a-Lo ` pez, ‡ Leonardo Palmisano, ‡ Juan M. Coronado, § Marı ´a D. Herna ´ ndez-Alonso, § Cosimino Malitesta, | and Maria Rachele Guascito | Dipartimento di Ingegneria dell’InnoVazione, UniVersita ` di Lecce, Via Arnesano, 73100 Lecce, Italy, Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, UniVersita ` di Palermo, Viale delle Scienze, 90128 Palermo, Italy, Instituto de Cata ´ lisis y Petroleoquı `mica, CSIC, c/ Marie Curie, 2, Cantoblanco, Madrid 28049, Spain, and Dipartimento di Scienza dei Materiali, UniVersita ` di Lecce, Via Arnesano, 73100 Lecce, Italy ReceiVed: December 17, 2004; In Final Form: April 18, 2005 Characterization of polycrystalline TiO 2 bare or porphyrin impregnated powders, used as photocatalysts for the degradation of 4-nitrophenol (4-NP) in aqueous suspension, was performed by time-resolved microwave conductivity (TRMC) measurements and electronic paramagnetic resonance (EPR) and X-ray photoelectron (XPS) spectroscopies. The presence of porphyrin sensitizers, as the metal-free or Cu [5,10,15,20-tetra (4- tert-butylphenyl)] porphyrin, impregnated onto the TiO 2 surface improved the photocatalytic activity of the bare TiO 2 . TRMC measurements indicate that the number and lifetime of the photoinduced excess charge carriers increase in the presence of the macrocycles, and EPR and XPS spectroscopies support the mechanistic hypotheses based on the photoreactivity experiments. Introduction Phthalocyanines and porphyrins are considered to be very attractive photo- and radiosensitizers that can find extensive application in chemical technology, ecology, medicine, and electronics. In recent years, they have also received increasing attention because of their biological implications for many photocatalytic reactions. On the other hand, there is nowadays a growing interest in the development of a new class of TiO 2 based hybrid organic/ organometallic/inorganic materials where porphyrins, phthalo- cyanines, and their metal complexes are used as surface modi- fiers of TiO 2 in various morphologies and crystalline forms. 1-5 At the same time, photodegradation of organic pollutants in aqueous media is of topical importance especially in view of the possible development of environmentally sustainable pro- cesses and deserves the attention of both academic and industrial research. The photoactivity of phthalocyanine and porphyrin com- pounds as sensitizers for a variety of oxidation reactions of phenol derivatives is known, and some mechanistic aspects have been also investigated. 6,7 In particular, metal phthalocyanines have been shown to be efficient photosensitizers and catalysts for many reactions. 8 It is also well-known that polycrystalline TiO 2 powders repre- sent a class of innovative photocatalysts widely used for the oxidative degradation of various organic pollutants in water. 9-13 Moreover, the modified TiO 2 systems, doped or loaded with some metallic or organometallic species, can present an enhanced photoactivity compared with the corresponding bare TiO 2 samples, depending on the type of metal, reaction, and experimental conditions used. 14-17 Recently, the photocatalytic activity of TiO 2 powders im- pregnated with lipophilic metal-free or Cu porphyrins used as sensitizers for the decomposition of 4-nitrophenol (4-NP) has been investigated. 18,19 4-NP photocatalytic degradation was chosen as a probe reaction because it is well-known in the literature. 13,20 The presence of the sensitizers was to be found beneficial, and in particular, the Cu porphyrin complex was shown to be the best one. 18,19 The main aim of the present paper is to establish through X-ray photoelectron spectroscopy (XPS) and electronic para- magnetic resonance (EPR) techniques the role of copper related to its oxidation state when the Cu porphyrin TiO 2 impregnated samples were used as photocatalysts for the above photoreaction and to justify the beneficial role that different copper species could play in these processes. Indeed, XPS studies on Cu(II) macrocyclic compounds including copper phthalocyanines and copper porphyrins show Cu2p spectra consisting of two doublets separated by ∼20 eV due to the spin-orbit splitting. 21-25 The presence of a high binding energy satellite accompanying each component of the most intense doublet distinguishes Cu(II) species from the Cu- (I) one, and consequently, this technique is suitable to obtain information on the oxidation states of copper. Similarly, EPR spectroscopy, on the other hand, has been widely used for the study of Cu(II) porphyrins, since this * Corresponding author. Fax: 00 39 0832 297279. Phone: 00 39 0832 297281. E-mail: giuseppe.mele@unile.it. ² Dipartimento di Ingegneria dell’Innovazione, Universita ` di Lecce. ‡ Universita ` di Palermo. § Instituto de Cata ´lisis y Petrolequı ´mica. | Dipartimento di Scienza dei Materiali, Universita ` di Lecce. 12347 J. Phys. Chem. B 2005, 109, 12347-12352 10.1021/jp044253g CCC: $30.25 © 2005 American Chemical Society Published on Web 06/02/2005