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 (lamda(max) = 365 nm) or UVC (lamda(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-Ti02 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 approximately 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, Tr, 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, Tr/Ilamda,the UV/TiO2 reaction under UVA irradiation was more efficient for samples with a higher content of TiO2 (P25 and Hecto-TiO2), but notfor Kao-TiO2. In all cases, reaction rates peaked at 10% RH, with Tr values between 10 and 50% higherthan 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.
Facultad de Química, Universidad Nacional Autónoma de México, Circuito Exterior, Ciudad Universitaria, Coyoacán, C.P. 04150, México City, México.
Daria
D
Kibanova
Javiera
J
Cervini-Silva
Hugo
H
Destaillats
D000393:Air Pollutants / Q000302:isolation & purification* / Q000528:radiation effects; D000538:Aluminum Silicates / Q000737:chemistry*; D002384:Catalysis / Q000528:radiation effects; D000077215:Clay; D006813:Humidity; D057927:Hydrophobic and Hydrophilic Interactions; D053761:Nanocomposites / Q000737:chemistry* / Q000528:radiation effects; D055668:Photochemical Processes / Q000528:radiation effects; D014025:Titanium / Q000737:chemistry*; D014050:Toluene / Q000302:isolation & purification* / Q000528:radiation effects; D014466:Ultraviolet Rays; D014835:Volatilization / Q000528:radiation effects
D000393:Air Pollutants; D000538:Aluminum Silicates; C009495:titanium dioxide; D014050:Toluene; D014025:Titanium; D000077215:Clay