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 (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−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 ∼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, %<i>R</i>, and the reaction rate, <i>T</i>_r, were calculated for each experimental condition from the concentrations measured with and without UV irradiation. Use of UVC light (UV/TiO₂/O₃) 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, <i>T</i>_r/<i>I</i>_λ, 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 <i>T</i>_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.