Towards a better understanding of the impact of VMS on photocatalytic systems
Résumé
Photocatalytic treatment of air, using TiO2 as the catalyst, is developing more and more because of increasing air pollution which is becoming a serious problem that needs to be addressed immediately. Although deactivation of the photocatalyst is among the most severe practical problems, only few works have been devoted to it, possibly because these catalysts have only recently been commercialized. Usually it is preferred to invest efforts in increasing the efficiency of the treatment rather than fighting against deactivation. In fact, it is much more complicated to carry out treatments under real conditions when compared to the model systems that are traditionally studied. Some of the potentially most important deactivation pollutants are volatile methyl siloxanes (VMS), which are becoming more and more abundant indoors and in ambient air owing to their increased use in personal care products, deodorants, skin care products and as conditioners in hair care products. Octamethylcyclotetrasiloxane (D4) is one such VMS that is highly present in the environment in concentrations ranging from 0.1-10 ľg.m-3 in ambient air and 0.1-100 ľg.m-3 indoors. In this study D4 was chosen as the VMS model, and its photocatalytic oxidation was investigated. Experiments were performed at different D4 concentrations in synthetic air under 40% relative humidity. D4 conversion was measured using TD-GC-MS analysis, while the TiO2 surface was analysed by X-ray photoelectron spectroscopy (XPS). To evaluate the mineralization, a gas chromatograph equipped with a pulsed discharge helium photoionization detector (GC-PDHID) was used. In these conditions at 250 ppbv of D4, the photocatalytic activity was found to decrease with decreasing D4 conversion, from 98% to 30% after 3 days of irradiation, revealing the deactivation of TiO2 accompanied by a yellowish coloration of the media, while total deactivation was observed after three days of irradiation at 500 ppbv of D4. Three photoproducts were identified during D4 degradation: heptamethylhydroxycyclotetrasiloxane, heptamethyl(hydroxymethyl)cyclotetrasiloxane and hexamethylcyclotetrasiloxane. XPS analyses of the media show an increase in Si/Ti ratio and the appearance of an SiOH component, which allows us to better understand the nature of the deposit responsible for the deactivation of TiO2. We also studied the photocatalysis of toluene under the same conditions over three days and a conversion close to 100% was found without any deactivation, hence leading to the confirmation that only D4 contributes to the deactivation of TiO2. Experiments on a binary mixture made up of D4 and toluene are in progress for a better understanding of the impact of VMS impact on photocatalytic systems.