Effect of molecular functionality on the photocatalytic oxidation of gas-phase mixtures

Developing an effective air purifier for indoor air quality (IAQ) purposes requires knowledge of the reaction rate of target compounds in the presence of other compounds that compete favorably with the target compounds for surface sites. To address this issue, the photocatalytic oxidation (PCO) of five C3 organic compounds on a sol-gel derived TiO2 thin-film catalyst was examined. Photocatalytic degradation of the five compounds was first compared in single component experiments at 50% relative humidity. The photocatalytic oxidation reaction rates proceeded in the following order: 1-propanol > propanal > propanone > propene > propane. The order of reaction rates was partially explained by considering the intermolecular forces of attraction that exist between the gas-phase molecules and the hydrated titania surface, with the lone exception of the ketone. A model that incorporates Henry's law constant and hydroxyl radical reactivity was successful at predicting PCO reactivity. Relative photocatalytic degradation was also studied using multi-component experiments. Multi-component experiments with propanone + propene, propanal + propanone, and ethanol + propanone were conducted. In each case it was observed that compounds with stronger binding energy to the photocatalyst surface displaced compounds with weaker binding energies and inhibited their further reaction until the stronger binding species was oxidized to sufficiently low levels. Further investigation of relative binding energies of compounds of interest for PCO applications should be pursued in the future through combinations of experimental studies and theoretical molecular modeling techniques.