Photocatalytic oxidation of toluene at indoor air levels (ppbv): Towards a better assessment of conversion, reaction intermediates and mineralization

We report here a new analytical methodology for the investigation of toluene photocatalytic removal at indoor-relevant concentration level (ppbv). Experiments were performed using an annular flow-through reactor with TiO2 as photocatalyst, toluene as a model VOC and under different ranges of relative humidity (RH: 0-70%), inlet concentration (20-400 ppbv) and flow rate (70-350 mL min(-1)). Analysis of reaction intermediates was conducted using an automated thermal desorption technique coupled to GC-MS instrument (ATD-GC-MS) whereas a GC coupled to pulsed discharge helium ionization detector (GC-PDPID) was used for the first time for on-line measurements of CO and CO2 at ppbv level. Under these conditions, toluene conversion was up to 90-100% with a slight influence of inlet concentration and RH, whereas flow rate was found to be a prevalent factor. Mineralization (%) varied from 55 to 95% and has shown to be strongly inhibited by the increase of RH whereas flow rate and inlet concentration exhibited a negligible effect. The reaction intermediates were found to be different according to the RH level: in absence of water vapor, traces of low molecular weight carbonyls (formaldehyde, methyl glyoxal, etc.) were detected and quantified in the gas phase whereas at RH 40%, hydroxylated intermediates such as cresols and benzyl alcohol were observed. On the basis of identification results, a reaction mechanism was proposed involving mainly direct hole oxidation at dry conditions and hydroxylation by OH radicals at high RH level. (C) 2008 Elsevier B.V. All rights reserved.