Effect of water vapor on the by-products and decomposition rate of ppb-level toluene by photocatalytic oxidation

[Display omitted] ► Water vapor has significant influence on the PCO by-products generation of toluene. ► Maximum decomposition efficiency does not always lead to minimum by-products. ► By-products make the typical L–H model unfeasible in real application. ► Health related index (HRI) was used to evaluate the health risk of by-products. ► Health risk assessment shall be included to evaluate the PCO performance. It was found that there were unwanted by-products in the photocatalytic oxidation (PCO) of indoor ppb-level toluene, a typical volatile organic compound (VOC) in indoor air. However, up to now the control mechanism of the generation of the products has not been clear. In this study we address the problem: a titania-coated glass-plate reactor was applied to study the by-products and decomposition rate; the by-products generated under a series of water vapor concentrations were instantaneously identified by proton transfer reaction-mass spectrometer (PTR-MS); the results indicate that water vapor has a significant effect not only on the photocatalytic decomposition rate of toluene, but also on its by-products generation; the competitive adsorption mechanism between water vapor, toluene and its by-products was analyzed. By-products may make the typical Langmuir–Hinshelwood model unfeasible in real application. A health risk assessment of the by-products was also introduced. The results show that the maximum decomposition efficiency does not always lead to minimal by-products and lowest health risk. What it does show is that when evaluating the performance of photocatalytic air purification, health risks posed by the by-products should be the primary concern rather than the decomposition efficiency.