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The Photocatalytic Effects of Tio2 Impregnated with Transition Metals for Removal of Organic Compounds in Liquid Phase

Won-Mook Lee1, Chul Woo Lee2, and Kwang Joo Kim2. (1) Department of Chemical Engineering, Hanbat National University, Daejeon, South Korea, (2) Chemical Engineering, Hanbat National University, Daejeon, South Korea

1. Backgrounds and experiments Recently, it is reported that TiO2 particles with metal deposition on the surface is more active than pure TiO2 for photocatalytic reactions in aqueous solution because the deposited metal provides reduction sites which in turn increase the efficiency of the transport of photogenerated electrons (e-) in the conduction band to the external system, and decrease the recombination with positive hole (h+) in the balance band of TiO2, i.e., less defects acting as the recombination center[1,2,3]. The catalytic converter contains precious metals, mainly platinum less than 1 wt%, partially, Pd, Re, Rh, etc. on cordierite supporter. Thus, in this study, solutions leached out from wasted catalytic converter of automobile were used for precious metallization source of the catalyst. The TiO2 were prepared with two different methods i.e., hydrothermal method and a sol-gel method. The prepared titanium oxide and commercial P-25 catalyst (Deagussa) were metallized with leached solution from wasted catalytic converter or pure H2PtCl6 solution for modification of photocatalysts. They were characterized by UV-DRS, BET surface area analyzer, and XRD[4]. Band gap energies of the catalysts were calculated from adsorption edge wavelength equation [Eg(eV)= 1240/l(nm)]. The prepared photocatalysts were tested to know the reactivity and quantum efficiency in the aqueous solution with trichloroethylene(TCE) as a reactant in photocatalytic batch reactor. Also these results were compared the reactivity to the case of P25 catalyst. 2. Results and discussion Modified hotocatalysts were prepared using commercial P25 and synthesized TiO2. The modification was carried out in two different methods, i.e. platinization with H2PtCl6 solution and metallization with leached solution from wasted catalytic converter. The basic structure of TiO2 wasn't changed by platinization and metallization under this preparation conditions. Particle size of modified TiO2 catalysts were about 30nm bigger than P25 based catalyst. Also the surface areas of P25 based catalysts were larger than those of TiO2 based catalysts prepared by sol-gel method. The band gap energy changes of the catalysts are shown in Table 1. The band gap energies of P25 and TiO2 without precious metals were about 3.0 eV, which corresponds to 400 nm radiation energy. It is interesting that band gap energy is decreased with the increase of platinum content. For P25-600R and TiO2-600R which were doped with 600 ppm of platinum using leached solution from wasted automobile catalytic converter on the commercial P25 and synthesized TiO2 respectively, it was revealed that the lowest band gap energy of 1.8 eV corresponds to 700nm of absorption wavelength of photon. Lower band gap energy of P25-500R and TIO2-500R is due to increased transition metal contents as the catalysts which prepared using leached solution contains 0.3 wt% Al, 0.7 wt% Fe, 0.2 wt% Mg, 0.03 wt% Ce, respectively and trace amount of Rh, Si, Zr and La. It is reasonable that UV diffuse reflectance spectra of modified photocatalysts were varied and their band gap energy decrease on increase of loading amounts of transition metals. The basic structure of the catalysts was not changed on the conditions of modification from the XRD patterns. From the ICP analysis, it was observed that impregnated concentration of transition metals on the surface of TiO2 were consistent with leached solution concentration. The photocatalytic decomposition of TCE was carried out and the results were shown in Fig. 1. Photocatalytic reaction has been found to be less sensitive to the conditions such as the concentration of trichloroethylene, and the stoichiometric decomposition (Cl2C=CHCl + 3/2O2 + H2O 2CO2 + 3HCl) proceeds with fairly good reproducibility by prepared photocatalysts. Table1. The Band gap energy changes of modified photocatalysts Catalyst Band gap energy (eV) Wavelength (nm) P25 3.05 407 P25/500 2.60 478 *P25-500R 1.76 705 P25-1000 1.91 648 TiO2 3.07 403 TiO2-500 2.99 414 *TiO2-500R 1.80 690 TiO2-1000 2.51 495 *R: prepared from wasted catalyst

The photocatalytic reactivity for TCE decomposition was increased by platinization and the photocatalytic activity of the catalysts prepared with leached solution from wasted automobile catalyst was similar to that of the catalysts modified with H2PtCl6.

Fig. 1. Catalytic performance of modified P-25 catalysts and prepared TiO2 catalysts for photocatalytic decomposition of TCE (0.1g/L) 3. Conclusion Modified photocatalysts were prepared using commercial and synthesized TiO2. The modification was carried out in two different methods, i.e. platinization with H2PtCl6 solution and metallization with leached solution from wasted catalytic converter. They were characterized by UV-DRS, BET, and XRD and tested their catalytic performance for decomposition and oxidation of TCE in liquid phase. The band gap energy of modified catalysts decreased down to 1.6 eV, and the basic structure and physical properties of the catalysts were not changed during modification process. All of the synthesized TiO2 were anatase structure but commercial TiO2 were contained 30% rutile structure. However, the catalytic activity of modified catalysts using two different TiO2 were almost the same in this reaction conditions. The photocatalytic activity of the catalysts prepared with leached solution from wasted automobile catalyst was similar to that of the catalysts modified with H2PtCl6. Although platinization on the surface doesn't affect the structure of TiO2, band gap energy decreases with increasing platinum amount. The photocatalytic activity of the catalysts platinized by leached solution is 50% higher than that of pure TiO2 for photocatalytic decomposition of trichloroethylene. The modified P-25 catalyst (RP-25) showed the highest activity for the photocatalytic decomposition of TCE.

References [1]. D. Bahnemann, D. Bockelmann and R. Goslish, Solar Energy Materials, 24 (1991) 564. [2]. M. R. Prairie, L. R. Evans, B. M. Stange and S. L. Martinez, Environ. Sci. Technol., 27 (1993) 1776. [3]. R.Mathews et. al., J. Chem. Soc., 80 ( 1984) 457. [4] Byung-Yong Lee, Sang-Hyuk Park, Sung-Chil Lee, Misook Kang, Chang-Ho Park, Suk-Jin Choung, Korean J. Chem. Eng., 20 (2003) 812 Contact address : wmlee@hanbat.ac.kr