Gaseous Photocatalytic Oxidation of Formic Acid over TiO2: A Comparison between the Charge Carrier Transfer and Light-Assisted Mars-van Krevelen Pathways

Under light illumination, it is usually considered that photocatalytic oxidations of organics such as volatile organic compounds over semiconductors are driven by the transfer of photogenerated carriers. Some studies also proposed that the photocatalytic oxidations might take place according to the light-assisted Mars-van Krevelen (MvK) pathway that involved the participation of lattice oxygens under aerobic conditions. Based on the concept of the MvK mechanism, the current work first gives an elaboration on the light-assisted MvK pathway and its intrinsic difference from the charge carrier transfer pathway. We then examined which one of these two mechanisms is responsible for the the photocatalytic oxidation of formic acid over TiO2. Comprehensive experiments, including apparent kinetics, online electric conductances, vacuum electric conductances, online optical transmittances, and first principle calculations, were carried out to discuss this problem. The results showed that the photocatalytic oxidations of formic acid over TiO2 dominantly follow the charge carrier transfer pathway at both low and elevated temperatures, the light-assisted MvK mechanism could not play a major role, and there was also no transition from the charge carrier transfer to the light-assisted MvK mechanism with an increase of reaction temperature.