Microsoft Word - 379_084.doc CCHHEEMMIICCAALL EENNGGIINNEEEERRIINNGG TTRRAANNSSAACCTTIIOONNSS VOL. 39, 2014 A publication of The Italian Association of Chemical Engineering www.aidic.it/cet Guest Editors: Petar Sabev Varbanov, Jiří Jaromír Klemeš, Peng Yen Liew, Jun Yow Yong Copyright © 2014, AIDIC Servizi S.r.l., ISBN 978-88-95608-30-3; ISSN 2283-9216 DOI: 10.3303/CET1439084 Please cite this article as: Vaiano V., Sarno G., Sannino D., Ciambelli P., 2014, Photocatalytic and antistain properties of ceramic tiles functionalized with tungsten-doped TiO2, Chemical Engineering Transactions, 39, 499-504 DOI:10.3303/CET1439084 499 Photocatalytic and Antistain Properties of Ceramic Tiles Functionalized with Tungsten-Doped TiO2 Vincenzo Vaiano*, Giuseppe Sarno, Diana Sannino, Paolo Ciambelli University of Salerno, Department of Industrial Engineering, Via Giovanni Paolo II, 132, 84084, Fisciano (SA), Italy. vvaiano@unisa.it Tiles functionalized with W-doped TiO2 film has been prepared and tested in the discoloration of aqueous solution of methylene blue (MB) used as model pollutants, in presence of UV light. The influence of tungsten and its loading on tiles surface were evaluated. Photocatalytic tests, performed at controlled temperature and atmospheric pressure, have shown that the increase in the tungsten loading strongly influences the photocatalytic activity; higher content of tungsten induces an increase of the discolouration rate. Moreover, the addition of tungsten modifies the structure of titania determining a remarkably photocatalytic activity. 1. Introduction Since it was found that irradiation of TiO2 with UV-light induced redox active electron-hole pairs, TiO2 photocatalysis has been extensively used to purify water (Vaiano et al., 2014), air (Murcia et al., 2013) to degrade organic and inorganic contaminants (Gaya and Abdullah, 2008) and to inactivate microorganisms (Barudin et al., 2013). The properties of TiO2, such as thermal and chemical stability, high photocatalytic activity, low-toxicity, and low cost make TiO2 one of the most intensely investigated photocatalyst which may be utilized for the environmentally compatible treatment (Segota et al., 2011). The low quantum efficiency of titania is one of the major problems limiting its practical use (Rauf et al., 2011). In addition, titania has a relatively high band gap value of 3.2 eV, which means that it can be activated only with UV light. However for the practical applications it would be desirable to extend the band gap excitations towards the visible region, and to confer prolonged lifetime of photogenerated charge carriers. Doping of titanium dioxide with transition metal ions, such as tungsten, provides a relatively well- studied and convenient way of solving both problems described above. Doped titanium dioxide showed significantly higher photocatalytic efficiencies (Farahani et al., 2011). For air stream and wastewater purification the photocatalyst can be fixed on a substrate, e.g. tiles to promote oxidation reactions. The immobilization method is more convenient for practical use. Recently it was shown that it was possible to obtain the complete removal of NOx from air stream without deactivation of the TiO2 film deposited on ceramic tiles (Sannino et al., 2013). Moreover TiO2 on tile surface has induced the total methylene blue (MB) discoloration after many hours of irradiation. With the aim to enhance discoloration rate W-doped TiO2 film on ceramic tiles has been prepared and tested in the decolourization of aqueous solution of MB in presence of UV light. 2. Experimental 2.1 Preparation of sol-gel TiO2 films TiO2 films were deposited on porous ceramic tiles with dimensions 97 mm X 87 mm X 12 mm. The tiles were carefully cleaned prior to the process of deposition. The substrates were ultrasonically cleaned in acetone for 10 min and dried in oven at 120 °C for 30 min. For the preparation of solution (TiO2 sol), the following components were used:  Titanium (IV) isopropoxide, TIP (Ti(C3H7O)4, Sigma Aldrich) as a titanium precursor;  Ethanol, (C2H5OH, Fluka Analytical) as a solvent; 500  Glacial acetic acid (CH3COOH, Aldrich Chemistry) as a catalyst;  Acetylacetone (CH3(CO)CH2(CO)CH3, Sigma Aldrich) as a chelating agent;  Bidistilled water for gelation;  Polyethylene glycol, PEG (HO(C2H4O)nH, Mr= 5,000–7,000, Aldrich Chemistry) as an organic additive (dispersant and binder);  Ammonium (para)tungstate hydrate, ((NH4)10H2(W2O7)6 xH2O, Sigma Aldrich) as a doping agent;  Nitric acid (HNO3, Sigma Aldrich) as an acidifier. Solution was prepared by dissolving titanium isopropoxide in ethanol. A magnetic stirrer was used to continuously mix the liquid. Then, acetylacetone and glacial acetic acid were slowly added. The resulting solution was stirred for 10 min and after that it was sonicated for 10 min. In a different flask, 4 g of polyethylene glycol (PEG), 0.62 g of ammonium tungstate and 0.5 mL of nitric acid were added to bi- distilled water under continuous stirring. The achieved solution was mixed with the first solution and stirred vigorously. The obtained clear solution was sonicated for 10 min. Tiles were impregnated with the TiO2 sol by dispersing it carefully on tiles surface. In particular a known volume of solution was dispersed on the tile to impregnate the entire surface (tile W1-1). The molar ratios of components used to prepare titania coating solutions are shown in Table 1. In the case of tile functionalized with undoped TiO2 (tile T), only ammonium tungstate was not added in the preparation. All coated substrates were dried at 120 °C for 60 min and then calcinated at 550 °C for 2 h. For the tile W1-2 the process (impregnation – calcination) was replicated two times. Table 1: Composition of coating solution and molar ratios. Component TIP C2H5OH CH3(CO)CH2(CO)CH3 CH3COOH H2O PEG (NH4)10H2(W2O7)6 xH2O Molar Ratio 1 40 1.3 0.9 20 0.0008 0.06 2.2 Characterizations of samples Samples were characterized with different techniques. Spectra were recorded with a Perkin Elmer spectrometer Lambda 35. Equivalent band gap determinations were obtained from Kubelka-Munk theory (Ciambelli et al., 2007) by plotting [F(R)*h]2 vs h and calculating the x intercept of a line through 0.5 < F(R) < 0.8. Laser Raman spectra were obtained at room temperature with a Dispersive MicroRaman (Invia, Renishaw), equipped with 514 nm diode-laser, in the range 100-1,200 cm-1 Raman shift. X-Ray fluorescence spectroscopy (XRF) was performed using a ThermoFischer ARL QUANT’X EDXRF spectrometer equipped with a rhodium standard tube as the source of radiation and with Si-Li drifted crystal detector. 2.3 Photocatalytic tests As model pollutants, dyes are commonly used, because their concentration can be easily monitored using a spectrophotometer. Among them, methylene blue (MB) has been widely studied due to its highly coloured nature and good chemical stability. The method which employs MB dissolved in water in the presence of TiO2 as thin film has been considered as a standard test for photocatalytic surfaces by the ISO (ISO 10678-2010) (International-Organization-for-Standardization, 2010). Since the substrates exhibit the tendency to adsorb the dye molecules, pre-adsorption of the surface was performed, using an aqueous solution of MB with a concentration equal to 10 ppm. 200 mL of the dye adsorption solution were placed into a glass beaker and the dye was adsorbed in the dark for 24 h. If the remaining concentration of the dye in this solution was larger than that of the solution used in the photocatalytic test, the adsorption was considered to be complete. Otherwise, the procedure was repeated for another 12 h using a fresh adsorption solution. The tests were then performed using a solution of 5 ppm. After the adsorption process of the dye was complete, tiles were placed in a plate reactor (0.20 L in volume) realized in stainless steel with a pyrex window (90 mm X 90 mm). This window was illuminated by four UV lamps (Philips, nominal power 32 W) with emission spectrum centred at 365 nm. The photoreactor is provided of temperature controlling system, consisting of a thermal exchanger within the metal body and a cooling system. Liquid samples were analysed in continuous for the first time by spectrophotometric measurement. In particular, a special assembly with a flow cuvette and an external pump for the recirculation of liquid was realized, permitting to determine the change of MB concentration, measured with a Perkin Elmer UV-Vis spectrophotometer at λ = 663 nm. A standard calibration curve was obtained for different MB concentration and allowed to convert absorbance to concentration (mg/L) units. In order to assess the photocatalytic efficiency of the TiO2 coated samples, measurements were also performed on non-coated ceramic tile. 501 3. Results and discussion 3.1 Catalysts characterization The reflectance measurements of undoped and W-doped TiO2 in powder form synthesized by drying and calcination at 550 °C of sol-gel, are reported in Figure 1. The experimental data show that the absorption properties of W-doped titania shifted from UV to visible region, determining a decrease of band-gap values from 3.2 eV (the typical band-gap of undoped TiO2) to about 3.0 eV. This change in band-gap is attributed to the presence of tungsten in the crystal structure phase. In Figure 2 it is reported the comparison between XRF results of raw tile, tile T, tile W1-1 and tile W1-2. The surface concentration of titanium and tungsten in the functionalized samples (Figure 2) increases by increasing the number of preparation steps, reaching the value of about 11.5 % and 0.26 % for titanium and tungsten respectively (tile W1-2). Moreover the decrease of silicon and calcium concentration indicated that TiO2 is built up as thin film on the tile surface. The result obtained with UV-vis analysis is confirmed by the Raman spectra of the tiles (Figure 3). In the range 100 – 900 cm-1 (Figure 3a) functionalized tiles display bands at 144, 396, 514 and 637 cm-1 and a weak shoulder at 195 cm-1, due to the Raman-active fundamental modes of titania in anatase phase (Ciambelli et al., 2008). In the range 700 – 1,100 cm-1 (Figure 3b), samples T and W1-1 show signals at 970 and 1,020 cm-1 due to the chemical composition of raw tile. These bands disappeared in the case of tile W1-2, indicating that a double step of the preparation process is able to induce the formation of a W-doped TiO2 layer that covers the entire surface of the support. Moreover, for the same tile, the band at about 800 cm-1 is due to the structure of TiO2 in anatase phase.   Figure 1: UV-Vis DRS spectra for TiO2 and W-doped TiO2 as powders 3.2 Photocatalytic activity tests Discoloration of MB as a model reaction was studied to investigate the photocatalytic activities of tiles under UV irradiation. The changes in the concentration of MB recorded in continuously mode are shown in Figure 4. Control experiment carried out on raw tile showed that MB concentration decreases very slowly throughout all the irradiation time, reaching a final value of about 4.6 ppm. During photolysis test, MB is almost stable and its concentration decreased only of about 3 % after 48 h of irradiation. 502 Figure 2: XRF analysis of surface for raw tile and functionalized tiles Figure 3: Raman spectra of functionalized and raw tile: (a) in the range 100 – 900 cm-1, (b) in the range 700 – 1,100 cm-1 As it can be seen for the tile functionalized with undoped titania (tile T) MB concentration decreases progressively, reaching a discoloration of about 63 % in 48 h of irradiation. At fixed irradiation time, with the same titanium loading (5 %), the presence of W-doped TiO2 thin film (tile W1-1) showed a higher discoloration rate. By increasing W loading (tile W1-2), the photocatalytic activity is strongly enhanced, reaching the almost complete disappearance of MB within 48 h. 503 4. Conclusions In this work W-doped TiO2 was deposited on ceramic tile by coating method and photocatalytic properties were analyzed in presence of UV light. The characterization results of functionalized tiles showed that the coating method is able to induce the formation of W-doped TiO2 on the tile surface. Photocatalytic tests on MB removal in liquid solution, performed at controlled temperature and atmospheric pressure, have shown that the increase in the tungsten loading strongly influences the photocatalytic activity; higher content of tungsten induces an increase of the discolouration rate. Moreover, the addition of tungsten modifies the structure of titania determining a remarkably photocatalytic activity. In particular, the comparison of the tiles with the same titania load shows that the presence of tungsten allows to reach a MB discoloration after 48 h of about 80 %, higher than that one obtained without the doping agent (about 40 %). Figure 4: MB discoloration as a function of UV irradiation time Acknowledgements The authors wish to thank “POR Campania Rete di Eccellenza FSE. Progetto “Tecnologie e monitoraggio ambientale per la sostenibilità delle Aree Vaste” (TEMASAV), CUP B25B09000090009”. References Barudin N.H.A., Sreekantan S., Thong O. M., Sahgal G., 2013. Antibacterial Activity of Ag-TiO2 Nanoparticles with Various Silver Contents. In: Umar, A. A., SallehALLEH, M. M. & YAHAYA, M. (eds.) Isesco Conference on Nanomaterials and Applications 2012. Stafa-Zurich, Switzerland: Trans Tech Publications Ltd. Ciambelli P., Sannino D., Palma V., Vaiano V., Bickley R. I., 2008. Reaction mechanism of cyclohexane selective photo-oxidation to benzene on molybdena/titania catalysts. 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