TiO2-based Materials for Applications in Oxidative Photocatalysis

ROBERTO CANDAL

Niagara, Buffalo, NY, USA

Congreso; The 14th International Conference on TiO2 Photocatalysis; 2009

Redox Technology

TiO2 is undoubtedly the most popular and effective material for photocatalysis. However the direct use of TiO2 in the form of powder is not always possible. The need to use the photocatalyst in different environments and situations has lead to the development of materials with TiO2 in their composition or on their surface (in the form of TiO2 coating on selected supports). Besides, the limited range of light absorption by TiO2, restricted to UVC, is being overcome by doping with different species, and by developing new and sophisticated materials. In our laboratory, in collaboration with research groups of different countries, we have work on the design of different materials combined with TiO2 for photocatalytic application. Some of those materials are: TiO2 coated on plastics In a first approach, TiO2 was coated on UVC transparent plastics in an attempt to improve the use of the light in photocatalytic reactors for air purification, and to reduce the weight of the support. Different plastics were tested; polyethylene terephtalate (PET) and cellulose acetate were the more appropriate due to transparency and resistance to UVC. TiO2 was incorporated on the surface of the plastics using a stable TiO2 aqueous based sol, synthesized as reported by Anderson et al (1). The adherence of the sol to the plastic was improved by a previous impregnation with PDDA, followed by coating with a SiO2 sol layer. The performance of the photocatalyst was determined by following the degradation of trichloroethylene in air. The stability of the materials against UVC was tested in an accelerated weathering chamber. These works were done in collaboration with the group of Dr. Benigno Sánchez Cabrero (CIEMAT, Madrid, Spain) (2, 3). In a second approach, in order to prepare a cheap, light and versatile material for the preparation of disposable bags for water decontamination, TiO2 was attached to the surface of polyethylene. In this case, polyethylene films were firstly coated with TiO2 suspended in a polystyrene-ethyl acetate solution. After dried at 60 ºC, the polystyrene was strongly attached onto the polyethylene surface. The TiO2 exposed at the surface display photocatalytic activity. The activity was improved by spraying the material with a TiO2 sol. The photocatalytic activity of the system allows the degradation of dyes and bacteria. These works were done in collaboration with the group of Dr. Juan Rodriguez, UNI, Lima, Perú (4). TiO2 coated on tiles Self cleaning tiles were made by spraying TiO2 aqueous sols on glassy tiles, followed by firing at different temperatures. The photocatalytic activity was determined through the kinetic of decoloration of crystal violet deposited on the tiles surface. The antibacterial activity was also measured using Escherichia coli as indicator microorganism. The effect of different synthesis parameters, as TiO2 amount, firing temperature, type of sol, was determined. TiO2 doped with W(VI) In an attempt to modify the TiO2 surface to improve the adsorption of positive moieties, TiO2 was doped with W(VI). The point of zero charge decreased with the amount of W(VI), while the surface area increased. The presence of W(VI) also increased the stability of the anatase phase at high firing temperatures. The adsorption of crystal violet (CV) –a positively charged dye-, increased with the surface area but there was not a direct effect of W(VI) on its adsorption. The degradation rate of CV was affected by both, the increment in surface area and the deleterious effect on the photocatalytic activity, produced by the presence of W(VI). The maximum rate was obtained with c.a. 2% of W(VI). The reaction mechanism was also affected by W(VI). In pure TiO2, degradation of CV at pH to 4.0 leaded to the formation of demethylated intermediates and fuchsine basic. In the presence of W(VI), at the same pH, demethylation was less important and direct degradation of the aromatic rings was observed. This effect may be cause by inhibition of superoxide formation due to the presence of W(VI) (an electron tramp), followed by an increment in the contribution of OH· in the reaction mechanism. At pH 9.0, the demethylation mechanism predominated in all the cases. The modification of the reaction path by incorporation of transition metal cations on TiO2, as well as pH control, have important implications in the aim of achieving more specific cleavage of undesired aromatic structures (5). TiO2 doped with N Doping of TiO2 with non-metals, especially nitrogen (N-TiO2) or sulfur (S-TiO2), is one of the newly developments realized to extend the absorption of light by TiO2 into the visible range. Recently, in collaboration with Prof Hector Mancilla (Facultad de Cs. Químicas, Universidad de Concepción, Chile), we have tested the photocatalytic activity of N-TiO2 and S-TiO2 for the degradation of flumequine (a well known antibiotic used in salmon farms). N and S doping, slightly but clearly, increased the oxidative photocatalytic activity under simulated solar light. This effect was attributed to the enhanced generation of superoxide by reduction of adsorbed O2. The electrons promoted from surface states, close to the valence band, to the conduction band by visible light illumination were responsible for the reduction of adsorbed O2. N-TiO2 can be easily prepared from TiO2 and urea. However, the synthesis conditions must be carefully controlled to obtain reproducible results. The doping mechanism and the different steps and intermediates produced during the synthesis are no very well known. To clarify these aspects, a systematic study of N-TiO2 synthesis via urea was done. Several intermediates as biuret, cyanuric acid and cyanates were produced during the firing of urea and TiO2 precursors. Anatase was the only detected crystalline phase; the crystallization process took place at c.a. 375 ºC, after elimination of most of the nitrogenated intermediates. A deep yellow color is reached when only cyanates are detected by FTIR. References 1)      B.L. Bischoff, M.A. Anderson, Chem. Mater. 7 (1995) 1772-1778. 2)      B. Sánchez, J. M. Coronado, R. Candal, R. Portela, I. Tejedor, M. A. Anderson, D. Tompkins, T. Lee; Applied Catalysis B: Environmental 66 (2006) 295-301. 3)      R. Portela, B. Sánchez, J. M. Coronado, R. Candal, S. Suárez; Catalysis Today, 129 (2007) 223-230. 4)      S. Ponce, E. Carpio, J. Venero, W. Estrada, J. Rodriguez, C. Reche, R. Candal; J. Adv. Oxd. Technology, 12 (2009), 81-86. 5)      N. Couselo, F. S. García Einschlag, R. Candal, M. Jobbágy; J. Phys. Chem. C 2008, 112, 1094-1100. 6)      J. Nieto, J. Freer, D. Contreras, R.J. Candal, E.E. Sileo, H.D. Mansilla; J. Haz. Mat., 155 (2008) 45–50.