(660a) Photocatalytic Degradation of Methylene Blue on Multifunctional Titanosilicate ETS-10

Semiconductor photocatalysis has attracted increasing attention in recent years as an alternative to traditional physical, chemical, or biological technologies for environmental remediation. The photocatalyst market is expected to reach US $30 billion in the near future [1]. Engelhard titanosilicate ETS-10 is a microporous (4.9 Å x 7.6 Å) zeotype material. The well-defined monatomic -O-Ti-O-Ti-O- chains in ETS-10 with a band gap energy of ~4.03 eV make this semiconductor material a promising candidate for photocatalytic applications. ETS-10 has been reported to exhibit shape selectivity in the photodegradation of phenol, 1,3,5-trihydroxybenzene, and 2,3-dihydroxynaphthalene [2]. The photocatalytic activity of ETS-10 could also be improved by introducing transition metal ions such as Cr (III), Co (II), and Ag (I) into its framework structure [3].

In this investigation, photocatalytic degradation of methylene blue (MB) was used as a model reaction to evaluate the photocatalytic activity of ETS-10 in a stirred batch reactor system under UV irradiation. A 500 mg/L suspension of the photocatalyst was used in all the experiments. The initial concentration of MB was 10 mg/L and the MB concentration change during the photocatalysis process was determined from the absorbance maximum at ~664 nm in its UV-vis spectrum. pH was found to be a key factor in this type of reaction as it affected both the surface charge of the ETS-10 and the ionization of the cationic MB dye. The highest photocatalytic activity of the as-synthesized ETS-10 was achieved in the pH range of 4.0-5.2; for which the pseudo-first-order reaction rate constant was determined to be ~0.043 min^-1 m^-2. Transition metal ions such as V(IV), Cr (III), Mn (II), Fe (III), Co (II), Cu (II), Ag (I), and Au(I) were introduced into ETS-10 through isomorphous substitution or ion-exchange [3]. Among all these modified ETS-10 crystals, silver exchanged ETS-10 exhibited the highest photocatalytic activity; the reaction rate constant was ~0.191 min^-1 m^-2, which was 4-5 times higher than that obtained using the as-synthesized EST-10 and two times higher than that obtained using the commercial Degussa P25 TiO₂ under the same operating conditions. The enhanced activity may be attributed to the ability of these transition metal ions to scavenge electrons on the ETS-10 particle surface, thereby reducing the undesired electron-hole recombination [4]. The presence of hydrogen peroxide (H₂O₂) in the ETS-10 photocatalysis system resulted in a synergistic effect. The photocatalytic degradation of MB on the as-synthesized ETS-10 was improved 3-5 times through adjusting the H₂O₂ concentration. The optimum H₂O₂ dosage was determined to be ~9 wt.%, at which the pseudo-first-order reaction rate constant was ~0.188 min^-1 m^-2. Currently, an optical fiber reactor system is being designed and optimized. The photocatalytic activity of the modified ETS-10 crystals will be tested under the optimized operating conditions in this optical fiber reactor and compared with the traditional immobilized photocatalytic reactor systems.

[1] Estimation by the National Institute for Materials Science in Japan, http://www.mext.go.jp/a_menu/shinkou/sangaku/07091398/4_6.pdf.

[2] P. Calza, C. Pazé, E. Pelizzetti, and A. Zecchina, Chem. Commun., (2001) 2130.

[3] S. Uma, S. Rodrigues, I.N. Martyanov, and K.J. Klabunde, Micropor. Mesopor. Mater., 67 (2004) 181.

[4] Marta I. Litter, Appl. Catal. B: Environ., 23 (1999) 89-114.