Photocatalytic destruction of gaseous toluene by porphyrin-sensitized TiO 2 thin films

Abstract

Photocatalytic removal of indoor level of toluene in the gas phase was performed in a tubular reactor by TiO 2-based photocatalysts under illumination of UV and visible light, respectively. Two types of TiO 2 suspensions were employed in the whole experiments: one prepared by a sol–gel route (designated as SG-TiO 2) and the other by applying a commercial Degussa P25 TiO 2 (designated as P25-TiO 2). All photocatalysts were dip coated on the inner surface of a Pyrex glass tube and followed by a post-annealing process. For visible-type photocatalysts, the above TiO 2 films were sensitized by tetrakis (4-carboxyphenyl) porphyrin (TCPP) and designated as TCPP/SG-TiO 2 and TCPP/P25-TiO 2, respectively. The photocatalytic destruction of gaseous toluene by the four catalysts was evaluated. For the UV-type photocatalysts, toluene was decomposed significantly by both types of TiO 2 films at lower toluene concentrations, [T] o. However, the overall removal efficiency (RE) was decreased dramatically as the initial concentration of was elevated to 6.5 ppm for P25-TiO 2 and 4.0 ppm for P25-TiO 2, respectively. A plot of toluene elimination capacity (EC) versus toluene loading rate indicated that the limiting EC for the SG-TiO 2 and P25-TiO 2 was 4.9 and 2.7 mg/h/m 2, respectively. In contrast, either type of dye-sensitized TiO 2 photocatalysts evaluated under illumination of visible light showed relatively low toluene RE as compared to the UV-type photocatalysts under identical conditions. After pre-soaking in HCl solution, the activity of the acid-pretreated TCPP/P25-TiO 2 [designated as TCPP/P25-TiO 2 (ACT)] was improved considerably under the identical conditions. The kinetics of photocatalytic decomposition of gaseous toluene in our study generally would follow the Langumuir–Hinshelwood (L–H) model. The rate constant k, as fitted by L–H model, for the SG-TiO 2-based catalysts is larger than that of P25-TiO 2-based catalysts while the absorption constant K, on the contrary, follows the different trend. The higher activity of SG-TiO 2 could be attributed to the smaller grain size (4.6–8.1 nm) of anatase crystals as evaluated in XRD diffraction pattern.