Efficiency of Clay-TiO2 Nanocomposites on the Photocatalytic Elimination of a Model Hydrophobic Air Pollutant

Clay-supported TiO2 photocatalysts can potentially improve the performance of air treatment technologies via enhanced adsorption and reactivity of target volatile organic compounds (VOCs). In this study, a benchtop photocatalytic flow reactor was used to evaluate the efficiency of hectorite-TiO2 and kaolinite-TiO2, two novel composite materials synthesized in our laboratory. Toluene, a model hydrophobic VOC and a common indoor air pollutant, was introduced in the air stream at realistic concentrations, and reacted under UVA (lambda(max) = 365 nm) or UVC (lambda(max) = 254 nm) irradiation. The UVC lamp generated secondary emission at 185 rim, leading to the formation of ozone and other short-lived reactive species. Performance of clay-TiO2 composites was compared with that of pure TiO2 (Degussa P25), and with UV irradiation in the absence of photocatalyst under identical conditions. Films of clay-TiO2 composites and of P25 were prepared by a dip-coating method on the surface of Raschig rings, which were placed inside the flow reactor. An upstream toluene concentration of similar to 170 ppbv was generated by diluting a constant flow of toluene vapor from a diffusion source with dry air, or with humid air at 10, 33, and 66% relative humidity (RH). Toluene concentrations were, determined by collecting Tenax-TA sorbent tubes downstream of the reactor, with subsequent thermal desorption-GC/MS analysis. The fraction of toluene removed, %R, and the reaction rate, T-r, were calculated for each experimental condition from the concentrations measured with and without UV irradiation. Use of UVC light (UV/TiO2/O-3) led to overall higher reactivity, which can be partially attributed to the contribution of gas phase reactions by short-lived radical species. When the reaction rate was normalized to the light irradiance, T-rho/l(lambda), the UV/TiO2 reaction under UVA irradiation was more efficient for samples with a higher content of TiO2 (P25 and Hecto-TiO2), but not for Kao-TiO2. In all cases, reaction rates peaked at 10% RH, with T-r values between 10 and 50% higher than those measured under dry air. However,a net inhibition was observed as RH increased to 33% and 66%, indicating that water molecules competed effectively with toluene for reactive surface sites and limited the overall photocatalytic conversion. Compared to P25, inhibition by coadsorbed water was less significant for Kao-TiO2 samples, but was more dramatic for Hecto-TiO2 due to the high water uptake capacity of hectorite.