Efficiency Of Clay-tio_2 Nanocomposites On The Photocatalytic Elimination Of A Model Hydrophobic Air Pollutant

摘要

Clay-supported TiO_2 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-TiO_2 and kaolinite-TiO_2, 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 (λ_(max) = 365 nm) or UVC (λ_(max) = 254 nm) irradiation. The UVC lamp generated secondary emission at 185 nm, leading to the formation of ozone and other short-lived reactive species. Performance of clay-TiO_2 composites was compared with that of pure TiO_2 (Degussa P25), and with UV irradiation in the absence of photocatalyst under identical conditions. Films of clay-TiO_2 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 ~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, %B, and the reaction rate, Tr, were calculated for eachrnexperimental condition from the concentrations measured with and without UV irradiation. Use of UVC light (UV/TiO_2/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/lλ,the UV/TiO_2 reaction under UVA irradiation was more efficient for samples with a higher content of TiO_2 (P25 and Hecto-Ti0_2), but not for Kao-Ti0_2. 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-Ti0_2 samples, but was more dramatic for Hecto-TiO_2 due to the high water uptake capacity of hectorite.