Photocatalytic oxidation of 2-propanol/toluene binary mixtures at indoor air concentration levels

Photocatalytic oxidation (PCO) technology has a great potential for indoor air purification. However, the presence of humidity and different concentrations of a variety of pollutants are critical parameters that must be addressed for indoor air cleaning applications. In this work, the photodegradation over a commercial air cleaning TiO2 filter of a mixture of two common volatile organic compounds (VOCs), 2-propanol and toluene, at indoor air concentration levels was tested and the role played by water vapour on the PCO performance was discussed. Transient and steady state experiments were performed using different inlet VOCs concentrations (80–400 ppbv) and relative humidity values (RH, 0% and 60% at 25 °C) in a gas flow rate of 300 mL min−1. The adsorption, conversion and mineralization efficiencies were studied. The reaction products at ppbv levels were analysed by automated thermal desorption coupled to gas chromatography–mass spectrometry (ATD–GC–MS) and gas chromatography equipped with a pulsed discharge helium photoionization detector (GC–PDHID). Almost total conversion and mineralization of the less concentrated mixtures was achieved at 0% RH. A significant negative effect of water vapour was observed. This effect can be attributed to the weak adsorption and low solubility of toluene and to the fast desorption of 2-propanol reaction intermediates. The presence of different surface species, in the photocatalytic degradation tests performed in dry and humid conditions, suggests the occurrence of different reaction pathways depending on RH.

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► A mixture of 2-propanol and toluene, at indoor air levels, was tested by PCO.
► Almost total conversion and mineralization of the mixtures were achieved at 0% RH.
► A significant negative effect of water vapour was observed.
► Water vapour increases the number and concentration of intermediates.
► The by-products react between themselves to form new products.