Effect of Key Parameters on the Photocatalytic Oxidation of Toluene at Low Concentrations in Air under 254 + 185 nm UV Irradiation
The effect of key experimental parameters on the removal of toluene under 254 + 185 nm irradiation was investigated using a benchtop photocatalytic flow reactor. Toluene was introduced at low concentrations between 10 and 500 ppbv, typical of indoor environments, and reacted on TiO2-coated Raschig rings. Two different TiO2-coated rings were prepared: in one case, by dip-coating using a P25 aqueous suspension and, on the other, using an organic/inorganic sol-gel method that produced thin films of mesoporous anatase. Flow rates in the photoreactor varied between 4 L min-1 and 125 mL min-1, leading to residence times in the range 100 ms < tau < 2 s. For these conditions, toluene removal efficiencies were between 30 and 90percent, indicating that the system did not achieve total conversion in any case. For each air flow rate, the conversion of toluene was significantly higher when the reactor length was 10 cm, as compared with 5 cm; however, only marginal increases in conversions were achieved in the two reactor lengths at equal residence time and different concentration of toluene, suggesting that that the reactor is effectively behaving as an ideal reactor and that the reaction is first-order in the concentration of toluene. Experiments were carried out between 0 and 66percent relative humidity (RH), the fastest reaction rate being observed at moderately low humidity conditions (10percent RH), with respect to both dry air and higher humidity levels. Formaldehyde was formed as a partial oxidation byproduct at low and at high residence times (240 and 960 ms), although higher formaldehyde molar yields (up to 20percent) were observed at low tau (240 ms) and moderate humidity conditions (10 and 33percent), suggesting that both tau and RH can be optimized to reduce the formation of harmful intermediates. Toluene removal efficiency increased with the TiO2 thickness (i.e., mass) until a maximum value of 500 nm, beyond which the removal efficiency decreased. This should be attributed to limitations on the rates of toluene decomposition imposed by radiation transport through the film and/or internal diffusion resistances, which can take place within the porous photocatalytic films.